Keywords: structure dynamics, steady state photocrystallohraphy, CW vs pulse laser, timeresolved photocrystallography, photoinduced linkage isomers

حرا عل ی

, क ृ पाली, فريال and my lovely travelling friends Гаухар Ахметова, Зарина Туртаева

you for believing in me for this fascinating project and giving me the chance to participate in various experiments and different synchrotrons. Although beamtimes were sometimes long and difficult, you have never lost your enthusiasm and optimism. Thank you for all the curious and stimulating discussions, your guidance, support and all your patience during these three years of my Ph.D. Working with you has been a pleasure. I am incredibly thankful to Sébastien Pillet for insightful discussions and feedback that pushed me to sharpen my scientific thinking ability.

I am grateful for all your suggestion and guidance. Also, I would like to sincerely thank you for helping me during the COVID-19 pandemic period in the very beginning days of my Ph.D. I am immensely grateful to my thesis committee. Thanks to Panĉe Naumov, Katarzyna N. Jarzembska, Claire Laulhé, Emilie Gaudry and Sébastien Pillet for the exhaustive reading of my thesis, for the very interesting questions and discussions during the day of my defense, and for their comments and suggestions to improve this manuscript.

I am also grateful to our collaborators Peter Klüfers group at LMU Munich for performing theoretical calculations and providing high quality crystal samples. Without the proper samples, my experiments would certainly not have been possible. Thanks to Philippe Gros and Frederique Favier for accepting to be part of my "comité de suivi" each year.

Thanks to all the members of CRM2 in discussions of experimental data or simply for the nice day to day conversations during lunch or coffee breaks. Thanks to fantastic CRISP group.

Thanks to Theo Woike for interesting ideas whose passion and curiosity to science are contagious. A very special thanks to Krzysztof, for all our experimental work from February to July 2022. I remember those summer days when sitting inside the diffractometer chamber and aligning the laser beam was horrible. Thank you for helping me in lifting heavy optical tables. And yes, after having a nice single crystal optical absorption spectrum, our  très bien, c'est très très beau  jokes!! Of course, my thesis work would never be possible without the help of Emmanuel Wenger.

Thank you, Emmanuel, for walking me through different diffractometers and data treatment techniques. Thank you for all those small to large magical tips and tricks I learned from you and are written nowhere. A very special thanks to the liveliest person Elodie Tailleur, for all the long insightful discussions on STOE and X-area. Thanks for all the bon courage. Thanks to Paul Allé, the person who developed synchronization work for the in-house time resolved set-up, for the explanatory answers to my curious questions and details of experimental setup.

I would like to thank El-Elumi Bendeif for the restless nights we were working together during Helium measurements. Thanks to Maxim and Pierrick Durand for the instructive talks. Thanks to Claude Lecomte for the support as being the mentor of my Master advisor Maqsood Ahmed.

I would like to acknowledge and thank to all the local contacts in the beamtimes in which I have participated. Special thanks to Claire Lauhlé at CRISTAL SOLEIL for her support during and after the experiments. Thanks to Marco Cammarata at ID09 ESRF and collaborators Piotr, Radik and Kasia from structural dynamics research group and my colleague Artem Mikhailov for the sleepless nights while working at the beamtime.

Special thanks to all the PhD students of CRM2 past and present. To Vedran Vukovič, Julien,

Introduction générale en français

Ce travail de thèse a été réalisé dans le cadre de la photocristallographie sur des complexes métalliques nitrosylés, avec l'ambition de contribuer au développement et à l'application de méthodes permettant l'analyse structurale des isomères de liaison photoinduits (PLI) à différentes échelles de temps. Plusieurs nouveaux résultats ont été obtenus qui ouvrent certaines perspectives, comme discuté dans ce que suit.

D'une part, de nouveaux types de PLI ont été détectés et caractérisés de manière approfondie pour trois différents composés métal-nitrosyl, un complexe {MNO} 6 , un {MNO} 8 , et un {M(NO)2} 10 . D'autre part, des progrès ont été réalisés dans la génération et l'analyse de PLI par irradiation laser pulsée.

Le PLI découvert dans le complexe dinitrosylruthénium {Ru(NO)2} 10 La troisième nouvelle structure PLI rapportée concerne la famille bien connue des composés {MNO} 6 . Dans [Ru(NH3)5(NO)]Cl3•H2O, les structures des isomères liés κO (isonitrosyle, MS1) et κ 2 N,O (latéral, MS2) sont rapportées, avec la particularité que pour MS2 deux configurations structurelles différentes sont adoptées, les MS2 dites éclipsées et décalées, qui ont été prédites par la théorie il y a 20 ans. Cet exemple a également montré les problèmes que pose la détermination de la structure des états mixtes GS+MS1 ou GS+MS2 dans un composé fortement désordonné. La stratégie adoptée, consistant à affiner les structures dans le groupe spatial à symétrie supérieure Pnma, au lieu du groupe spatial probablement réel Pna21, s'est avérée fructueuse, avec toutefois une précision limitée concernant les paramètres structurels de MS1 et MS2. Dans ces composés {MNO} 6 , l'existence de deux PLI conduit en fait à une compétition entre l'occupation de l'un ou l'autre, ce qui complique en outre l'analyse structurale. Enfin, nous avons tenté d'étudier la photogénération de PLI dans le composé prototype des systèmes {MNO} 6 , Na2[Fe(CN)5(NO)]•2H2O (SNP), avec une résolution temporelle d'environ 100 ps. Le SNP présente le même type de processus en deux étapes pour la génération de MS1 que celui présenté ci-dessus, mais avec une efficacité accrue pour la première étape de GS à MS2 et une efficacité réduite pour l'étape de MS2 à MS1, ce qui le rend approprié pour ce type d'étude. La préparation minutieuse de l'expérience à résolution temporelle à la ligne de lumière CRISTAL sur le synchrotron SOLEIL, a permis de répondre à toutes les contraintes expérimentales, par une simulation extensive des cartes de photodifférence en fonction de la population de l'espèce PLI attendue MS2 et de la couverture de l'espace réciproque. Cela a permis de réaliser une expérience de RX réussie en termes de résolution temporelle envisagée et en ce qui concerne l'exhaustivité nécessaire des données pour générer des cartes de photodifférence significatives après une excitation pulsée avec des impulsions laser x femtosecondes. Ces cartes de photodifférence n'ont pas révélé de signatures claires d'un PLI généré dans cette expérience, de sorte que pour l'instant, la question de savoir si la structure latérale du ligand nitrosyle dans l'état MS2 est déjà adoptée après 100 ps reste sans réponse.

Cependant, la procédure établie pour la préparation et la réalisation des expériences s'est avérée fiable, de sorte que d'autres expériences, éventuellement avec des complexes ayant des efficacités de photoexcitation plus élevées, peuvent être envisagées.

xi [START_REF] Planck | Über irreversible Strahlungsvorgänge[END_REF], which sets an order of length/duration for the minimum possible changes in nature, that can be described by current theories.

Table of Contents

List of Tables

This value is, indeed, far away from what we could be able to distinguish or even perceive. A human eye can perceive the dynamic processes of nature occurring on time scales of around 50 ms and is therefore limited with respect to the detection of the transition states in dynamic processes. For example, the fastest wing beats of hummingbirds (around 80 beats per second) are out of reach of the human eye. Other examples of unobserved transition states in dynamic physical processes are the blossoming of flowers, the motion of a fast-running horse, moving clouds, a fast-moving fan, raindrops falling, a bullet passing through an apple, and other objects, etc. In the same way, the transition states in chemistry, such as the mechanisms of fast chemical reactions and biological changes, can only be detected if accessed on their real-time scale. Therefore, to detect and fully describe the rapid changes in the dynamic process, we need to make use of fast detection (e.g., shutter cameras) systems within their time scale. For example, the fall of liquid droplets has been collected at 47000 fps using a high-speed video camera [START_REF] Xu | Drop Splashing on a Dry Smooth Surface[END_REF].

The ultimate achievement of the studies of fundamental dynamics is the femtosecond (1 fs = 10 -15 s) to some hundreds of attosecond time scale (1 as = 10 -18 s) resolution (Figure 1.1). In the past decades, tremendous progress has been made in exploring the dynamics of the structure of matter using the electron (~pm) and X-ray (~Å) diffraction methods. The pioneering work of A. Zewail on femtochemistry was awarded the Nobel prize in chemistry in 1999 [START_REF] Zewail | 4D electron microscopy: imaging in space and time[END_REF][START_REF] Zewail | Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond[END_REF]. In the case of molecular dynamics, the molecule inside the eye, i.e., rhodopsin, responds to the light by twisting around one single bond in 200 fs [START_REF] Kukura | Structural Observation of the Primary Isomerization in Vision with Femtosecond-Stimulated Raman[END_REF][START_REF] Ernst | Microbial and Animal Rhodopsins: Structures, Functions, and Molecular Mechanisms[END_REF], thereby triggering the process of vision by the eye in three dimensions. Observing the very act of the atoms in chemistry, of breaking and making bonds on their actual time and length scale, is the wellspring of femtochemistry [START_REF] Zewail | Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond[END_REF].

The ability to design the key properties in molecular systems by using light grasps huge perspectives for the field of solid-state molecular materials. The design of solid-state materials whose properties and functions can be altered in a controlled manner by the application of light is a primary goal of modern solid-state chemistry. The applicative potential of photosensitive materials is wide-ranging, from the generation of new smart materials to data storage devices.

The global photonics and optoelectronic industry were worth $597 billion and $154.47 billion in 2020 and is estimated to increase more in the future. In the optoelectronics industry, photosensitive materials have applications in optical data storage, holographic data storage [START_REF] Th | New information storage elements on the basis of metastable electronic states[END_REF][START_REF] Haussühl | Nitroprussides, a new group of materials for holographic information storage on the basis of metastable electronic states[END_REF][START_REF] Brinkman | The future of solid-state electronics Bell[END_REF], optical, thermo-optic, and magneto-optic switches [START_REF] Kusz | X-ray structure study of the light-induced metastable states of the spin-crossover compound [Fe(mtz)6](BF4)2[END_REF], organic non-linear optic materials in the telecommunication and optoelectronic industries [START_REF] Long | Organometallic Compounds for Nonlinear Optics-The Search for Enlight-enment![END_REF], luminescent photonic materials governing the photonic devices such as optical display technology, light-emitting diodes and potential use in sunscreens [START_REF] Kimbrough | The Photochemistry of Sunscreens[END_REF] (Figure 1.2). The cornerstone of such systems is to manipulate the structure and properties in a controlled manner, for example, in molecular photo-switches by reversible photo-isomerization between two states, commonly referred to as photoinduced linkage isomers (PLI) [START_REF] Coppens | Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes[END_REF]. The notion of utilizing photoinduced linkage isomers with "maximum or near to maximum photoconversion" has motivated to design molecular devices for data storage applications. Considering a molecular data storage application analogous to an optical disc, in which the dominant isomer, either ground state (GS) or metastable state (MS), could encode the state of a binary digit (bit) in microscopic areas on a thin film. Laser is used to photoactivate an area of binary digits GS (0)→MS (1), reset by localized heating, and read with an appropriate probe (Figure 1.3). In order to write data, controlled-complete photoconversion after laser irradiation with preferably low power, is ideally required. And in order to maintain the data integrity the generated metastable state or PLI should be stable at or close to room temperature so that no cooling systems are required. In the end, a discriminatory absorption change at a well-defined wavelength or a change in refractive index [START_REF] Goulkov | Pulse recording of thermal and linkage isomer gratings in nitrosyl compounds[END_REF] could be used to read-out the region of the recording surface of dominant isomers GS/MS. With the growing emphasis on solid-state materials that have two or more accessible states which differ in energy and exhibit bistability, attention has turned to transition metal complexes that contain ambidentate ligands that can switch between linkage isomeric forms when activated by external stimulus of light. Suitable ligands that show promise in this area include nitrosyls, nitro groups, and coordinated sulfur dioxide molecules, each of which can coordinate to a metal center in more than one bonding mode [START_REF] Coppens | Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes[END_REF][START_REF] Cole | Single-crystal X-ray diffraction studies of photo-induced molecular species[END_REF][START_REF] Hatcher | Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal X-Ray Diffraction Studies 21st Century Challenges in Chemical Crystallography I Structure and Bonding[END_REF][START_REF] Coppens | The New Photocrystallography[END_REF].

At a molecular level, optical excitation converts molecules from the ground state (GS) to an electronically excited state (ES nth* ) triggering possible further structural and electronic changes. The analysis of such materials in their short-lived excited states or metastable states has been aided by the development of X-ray photocrystallographic techniques [START_REF] Coppens | The dramatic development of X-ray photocrystallography over the past six decades[END_REF], allowing the full three-dimensional structure of a single crystal of a complex to be determined. The technique effectively brings the dimension of "time" to the crystallographic experiment and brings us closer to being able to watch solid-state processes occur in real-time. On different time scales, different processes and phenomena can be observed, as schematically illustrated in (Figure 1.1). Understanding the physical mechanism underlying the observed photo-induced structural changes has become a central concern in the community of solid-state materials. The main questions regarding these types of photoinduced structural changes can be listed as follows:

• What is the main driving force in these light-induced structural changes?

• How strong is the interplay between the ground state GS to photoinduced linkage isomers (PLI)?

• What are the different time scales associated with PLI states as a function of temperature?

• What methods need to be developed? How to better comprehend the whole structure dynamics of PLI?

X-ray photocrystallography (static and time-resolved) is a very promising technique, with the capability to directly follow the complete structural dynamics within the limit of the relevant time scales according to the lifetime of the PLI as a function of temperature, to shed light on these questions. When a photosensitive molecule, e.g., a transition-metal nitrosyl complex, is irradiated by light, the molecules in the energetic ground state (GS) absorb energy and are transferred into an excited state configuration ES * …ES nth* . The optical excitation induces an electronic transition, e.g. by promoting an electron from metal-centered orbitals to ligandcentered orbitals (metal-to-ligand charge transfer, MLCT), which then triggers the further structural and electronic changes according to the available structural and electronic relaxation and transformation pathways. Good evidence of the increasing interest in these photoswitchable systems is given by a large amount of research works on the transition metal nitrosyl complexes over the last two decades [START_REF] Coppens | The dramatic development of X-ray photocrystallography over the past six decades[END_REF][START_REF] Carducci | Diffraction Studies of Photoexcited Crystals: Metastable Nitrosyl-Linkage Isomers of Sodium Nitroprusside[END_REF][START_REF] Coppens | Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes[END_REF][22][START_REF] Woike | Structures on Different Time Scales[END_REF][START_REF] Hatcher | Photocrystallographic Studies on Transition Metal Nitrito Metastable Linkage Isomers: Manipulating the Metastable State Acc[END_REF]. Therefore, studies on these metal nitrosyl complexes will help to gain insight into the structural dynamics of photoswitchable molecules and the underlying physical phenomena.

Within this overall framework, the scope of this thesis is threefold:

First of all, steady-state photocrystallography of representatives of three different kinds of nitrosyl-complex families are studied, such as (a) a pentacoordinated {MNO} 8 * mono-nitrosyl complex undergoing bent-to-bent photoisomerization of the NO ligand, (b) a hexacoordinated {MNO} 6 mono-nitrosyl complex exhibiting three different isomeric configurations with two MS2 (=side-on NO) configurations, and (c) a {MNO} 10 dinitrosyl complex presenting a synchronous double-linear to double-bent excitation and relaxation of the two NO ligands. The investigation of new compounds exhibiting PLI is motivated by the fact that they might exhibit novel types of PLI, possibly better suited for applications, as well as to deepen the overall understanding of the PLI phenomenon.

The second aim of the thesis work is to explore the possibilities and implications of using *{MNO} n is the Enemark-Feltham notation [START_REF] Enemark | Principles of structure, bonding, and reactivity for metal nitrosyl complexes Coordination[END_REF], where n is the number of d-electrons if NO + is assumed to be the ligand.

I thus addressed here several fundamental questions:

• What is the main difference between CW and pulsed laser irradiation sources? Can one expect better stability and/or more photoinduced population after excitation with a pulsed laser source compared to CW laser source?

• How does pulsed laser irradiation overshadow the refined population of metastable states? What is the damage threshold of a particular crystal upon irradiation with a pulsed laser?

• What are the critical defining parameters to generate PLI with the maximum achievable population: fluence (Q) value or energy of the photons?

The last aim of the thesis is to study the time-resolved structural dynamics using SNP, a prototype system in mono-nitrosyl complexes. In this case, the attention centered on the structural determination of the side-on nitrosyl metastable state (MS2) in time-resolved mode.

This test experiment was performed on the CRISTAL beamline at the SOLEIL synchrotron.

The thesis is divided into six chapters:

• The remainder of this first chapter serves as an introduction to photocrystallography on different time scales. A brief explanation of the type of photoinduced linkage isomers (PLI) that are investigated that leads to the main topic of the nitrosyl complex and their different coordination families and the resultant outcome of the metastable states.

• In chapter two, I introduce the experimental setups used in the development of my thesis. Firstly, I describe the complementary spectroscopy techniques followed by excitation sources. Secondly, steady-state X-ray diffraction (XRD setups followed by the transient pump-probe optical absorption spectroscopy and the single crystal optical absorption spectroscopy are described. Finally, the last part describes the CRISTAL beamline at the SOLEIL synchrotron where the time-resolved experiment was performed.

• In chapter three, the steady-state photocrystallographic results of three representatives of type {MNO} 6,8 and {M(NO)2} 10 are presented.

• In chapter four, I present the results obtained from the comparative analysis of the CW and pulsed laser irradiation sources and their respective photoinduced products. I also describe here the possibilities and outcomes of the one-pulse single-shot experiment.

• In chapter five, I show the time-resolved experiment, our objective, and expectation of the results and the results obtained.

• To conclude, the main task of this work is summarized and followed by a discussion of its limitations and future perspectives.

Photocrystallography

The field of photocrystallography owes much to the work of Gerhard Schmidt and coworkers in the fifties and sixties at the Weizmann Institute of Science [START_REF] Rabinovich | Part XV. The solid-state photochemistry of p-quinones[END_REF][START_REF] Schmidt | Photodimerization in the solid state[END_REF]. Schmidt [START_REF] Schmidt | Photodimerization in the solid state[END_REF] first conducted a setup of X-ray crystallography to study the photoinduced structural changes in crystals and proposed topochemical factors driving solid-state photochemical reactions and later extended by Boldyrev [START_REF] Boldyreva | The concept of the "reaction cavity": A link between solution and solid-state chemistry[END_REF] and Ohashi et al [START_REF] Ohashi | Dynamic motion and various reaction paths of cobaloxime complexes in crystalline-state photoreaction[END_REF]. Their topochemical postulate states that the crystal lattice's restrictions ensure that the reaction will follow a minimal energy route with the least amount of atomic movement feasible to sustain the crystalline environment [START_REF] Cohen | Topochemistry. Part I[END_REF][START_REF] Cohen | Topochemistry. Part II. The photochemistry of trans-cinnamic acids[END_REF][START_REF] Schmidt | Topochemistry. Part III. The crystal chemistry of some transcinnamic acids[END_REF].

Although, due to the technological restrictions of time, Schmidt could only investigate the ground state (GS) structure. Recent developments, however, have allowed us to determine the photoinduced structural changes by single crystal X-ray diffraction (SCXRD), and ultimately the term photocrystallography was developed in the 1990s to diversify this new area of research [START_REF] Coppens | Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes[END_REF][START_REF] Cole | Single-crystal X-ray diffraction studies of photo-induced molecular species[END_REF][START_REF] Coppens | The New Photocrystallography[END_REF][START_REF] Carducci | Diffraction Studies of Photoexcited Crystals: Metastable Nitrosyl-Linkage Isomers of Sodium Nitroprusside[END_REF]22,[START_REF] Hatcher | Photocrystallographic Studies on Transition Metal Nitrito Metastable Linkage Isomers: Manipulating the Metastable State Acc[END_REF][START_REF] Cole | X-Ray Diffraction of Photolytically Induced Molecular Species in Single Crystals Models, Mysteries and Magic of Molecules[END_REF][START_REF] Naumov | Direct Observation of Dynamic Solid-State Processes with X-Ray Diffraction[END_REF][START_REF] Naumov | Chemical X-Ray Photodiffraction: Principles, Examples[END_REF][START_REF] Pillet | Time-resolved structural analysis: probing condensed matter in motion[END_REF].

The basic principle of optical excitation

X-ray photocrystallography was developed at the forefront of the recent advances in the field of SCXRD [START_REF] Coppens | The dramatic development of X-ray photocrystallography over the past six decades[END_REF][START_REF] Carducci | Diffraction Studies of Photoexcited Crystals: Metastable Nitrosyl-Linkage Isomers of Sodium Nitroprusside[END_REF]. In its broadest sense, X-ray photocrystallography employs XRD data collection of crystal samples irradiated with light, aimed to analyze the changes in their structure caused by photoexcitation either during the process of irradiation or immediately after irradiation or after the system has relaxed back to the ground state. In practice, ultraviolet, visible, and/or infrared light is used to excite a single crystal or powder sample, and, therefore, diffraction data is collected before, during, and/or after excitation. Depending on the irradiation time, type of irradiation source (lamp, LED, CW, pulsed LASER), and stability of the photoexcited state(s) at a particular temperature, the analytical X-ray beam is adjusted to collect a sufficient amount of data, which would then provide meaningful structural information in the excited states. After irradiation, the molecules in the crystal, promoted from the equilibrium When a photo-switchable molecular complex (here the molecules of interest are metal complexes that undergo linkage isomerization by stimulus of light) is irradiated with a certain specific wavelength of light (that correspond to the energy difference between two electronic states), a photon of energy hν is absorbed, and the energy is transferred to the particular electron, inducing the transition GS→GS*, whereby the electron is excited from lower-lying energy level GS to higher-lying energy levels, i.e., excited state GS* and some excited vibrational states. This first optical excitation occurs in the fs time scale (Figure 1 From this relaxed, excited state, there are two pathways: one is an irreversible path where it's converted to a final photoproduct, and the second path and which is the center of interest of the thesis, is to recover back to the GS via thermally activated relaxation completing the whole photocycle. Another pathway for molecules to cope with the absorbed energy from photons is to emit a photon via fluorescence. Fluorescence is a relaxation process that can occur on a wide range of time scales from fs-ns and is most often observed between the first excited electronic state and ground state GS*→GS. Therefore, this path for electrons to dissipate energy is in competition with internal conversion. The energy of the fluorescent photon is always less than the exciting photons (since energy is lost in the internal conversion and vibrational relaxation processes).

Yet another relaxation path a molecule may adopt is the dissipation of the energy via the intersystem crossing. This relaxation path is adopted when the system electron changes spin multiplicity from an excited singlet state to an excited triplet state. These transitions are governed by electronic selection rules and are thus often forbidden. However, by coupling vibrational factors into the selection's rule, the transition becomes weakly allowed and able to compete with the timescale of fluorescence. Intersystem crossing leads to several interesting routes back to the ground electronic state, namely, phosphorescence.

The main steps illustrated in the general scheme Figure 1 techniques, allowing the complete three-dimensional structure determination of a single crystal photo-switchable molecular complex. This technique successfully brings the "time" dimension to the photocrystallographic experiment and enables us to see closely the very happening of excited states after irradiation with light and following their subsequent relaxation paths taking place in real-time.

Types of photocrystallographic methods

Depending on the lifetime of the excited state, different methods of measurement need to be applied.

Steady-state photocrystallography

If the light-induced structural changes are long-lived and suitable for X-ray diffraction XRD, the excited state(s) structures can be determined with steady-state photocrystallography. In the course of steady-state photocrystallography, the crystal sample is irradiated using LED, continuous or pulsed laser source. After irradiating the crystal sample, XRD data collection is performed for actual structural analysis (ex-situ), or irradiation and data collection of the excited state for structure determination are performed simultaneously on the goniometer (insitu). In the simplest case of the steady-state photocrystallography, the single crystal is irradiated directly on the goniometer with the light source of a specific wavelength to generate the metastable or excited state, and is irradiated until the saturation is achieved, and then the excitation source is removed, i.e., excited state is usually produced in-situ, and X-ray diffraction data is collected for the corresponding excited state (Figure 1.6). Typically, the XRD data collection of excited state species is performed at low temperatures (100 K or even below) to extend the lifetime of ES ranging from minutes to infinite time scale. The photoinduced product/yield/magnitude/population produced in such an experiment is considered a key parameter for the success of structural characterization, which depends on the particular crystal sample case, the diffraction resolution, and the deterioration of the crystallinity during irradiation.

In the case of pseudo-steady state photocrystallography, the ES species with a lifetime ranging from seconds to milliseconds is studied. This method requires an effective means of irradiation for the crystal during the X-ray data collection of the single crystal. In some cases, depending on the crystal type, continuous irradiation during the data collection helps to achieve the study of a short-lived ms-s excited state. In practice, the crystal is continuously irradiated. Once a photo-stationary situation is reached, i.e., the population of the photoinduced species is stable over time; the X-ray data collection starts under continued light irradiation.

Time-resolved photocrystallography

In the case of short-lived transient excited state species, a stroboscopic or pump-probe photocrystallographic method is used. The shorter the lifetime of the photoinduced species, the more complicated the photocrystallographic procedure will become.

With the advancement in newly developed technologies, electronically gated detectors with fast readout times can be synchronized with the excitation pump laser pulses to realize a pumpdetection synchronization. Since the X-rays are continuous, it is thus possible to probe excited states multiple times in one cycle i.e. performing a pump-multiprobe experiment (Figure 1.6).

Overall, by probing multiple excited states after single pulse to measure different time delays of the excited state reduces the overall time duration of the experiment. The readout time of the detector is in few ms timescale and to probe excited states with shorter life time, timeresolved pump probe experiments are used with high brilliance synchrotron X-ray sources.

In time resolved pump-probe experiments, laser irradiation (pump) and X-ray pulses (probe) are synchronized to arrive at the single crystal in a specified sequence of time (Figure 1.6). The pump-probe experiments typically require the use of synchrotron radiation sources to dispose of a sufficiently high X-ray flux within the short probe pulses. It is simply due to the fact that the X-ray flux for a laboratory X-ray source is too low to produce sufficient incident X-ray intensity for a feasible measurement. In the case of sub-picosecond timescales, the development of "single-shot" diffraction methods is required, where the whole diffraction pattern is obtained in a single X-ray image after irradiation with a single laser pulse shot via a "diffract-and-destroy" approach [START_REF] Chapman | Femtosecond X-ray protein nanocrystallography[END_REF]. The requirement of a very strong X-ray pulse limits this method. Nevertheless, attempts are being made to counteract this limitation either by the use of Laue diffraction for molecular systems [START_REF] Kamiński | Optimizing the accuracy and precision of the single-pulse Laue technique for synchrotron photo-crystallography[END_REF] or by accessing even higher levels of X-ray flux, i.e., X-ray Free Electron Laser (X-FEL) facilities.

Instrumental considerations for time-resolved structural studies 1.5.1 Pump-probe approach

The fundamental objective of the time-resolved single crystal X-ray diffraction experiments is to directly observe the evolution of excited/transition states as a function of time by collecting complete X-ray structures averaged over a suitably short time window after pulsed excitation.

Time-resolved experiments are typically designed in a pump-probe sequence. In a general design of a pump-probe experiment, a crystal is excited to ES with short laser pulses 'pump' at t = 0 and is allowed to relax back to GS in a decay time before the next laser pulse arrives. The measurement 'probe' e.g., X-rays is synchronized with the pump pulses in such a way data is collected at a fixed time delay t. The pump pulses produce a small population of ES, which is recorded by the subsequent probe over a short time frame in accordance with the intended time resolution. However, the recorded intensities of the ES population from the X-ray probe are mostly weak. In consequence, to collect reasonable statistics of signal-to-noise ratio, the pump-probe cycle must be repeated periodically (n number of cycles). The data collection in different crystal orientations, collection of most sensitive reflections from GS to ES, symmetry of the crystal, diffraction power of the crystal, and many other factors can help in possible complete data collection of the ES. The three-dimensional structure of ES at each unit of time can then be combined into three-dimensional molecular movies to visualize the complete structure dynamics at their real-time scale, which is the primary advantage of this approach.

X-ray Sources

In time-resolved photocrystallographic experiments, the choice of X-ray source primarily depends on the required flux necessary for obtaining statistically significant results. The typical brilliance of laboratory-based sources: sealed tube anode source is ~10 9 ph/s.mm 2 , and rotating anode source ~10 10 ph/s.mm 2 . Synchrotron radiation sources have brilliance of the order of magnitude ~10 24 ph/s.mm 2 . The very high brilliance of X-ray free electron laser XFEL is ~10 33 ph/s.mm 2 . Higher flux X-ray sources like synchrotron and XFEL [START_REF] Bilderback | Review of third and next generation synchrotron light sources[END_REF][START_REF] Bergmann | X-Ray Free Electron Lasers: Applications in Materials[END_REF], enable to collect data with a higher signal-to-noise ratio in a short duration of the experiment than low-flux X-ray sources e.g., typical X-ray tubes on laboratory diffractometer. A short experiment reduces the total exposure of the crystal sample to the irradiation source, helping in prevention of crystal damage with laser or radiation damage and saving overall experiment time. Sometimes, and especially in the case of investigation of cyclic processes, one can compensate lower flux by increasing the number of measurement cycles.

Detectors for time-resolved photocrystallography

Different types of detectors can be used depending on the type of time-resolved experiment.

For example, when an X-ray beam is chopped or pulsed, a conventional integrating detector such as CCD or an image-plate (IP) detector might be efficient. However, a fast detector is necessarily required when a selection within a sequence of X-ray pulses is needed, such as selecting a single X-ray bunch out of a multi-bunch mode of the synchrotron. For this purpose, a silicon avalanche photodiode (APD) with temporal response in the nanosecond regime and an X-ray streak camera with temporal response in the sub-picosecond time scale would be appropriate [START_REF] Chang | Demonstration of a sub-picosecond x-ray streak camera[END_REF][START_REF] Liu | An accumulative x-ray streak camera with sub-600-fs temporal resolution and 50-fs timing jitter[END_REF].

Recent advancements in the gated single-photon-counting pixel area detectors (PAD) opened new horizons in the field of pump-probe time-resolved X-ray experiments [START_REF] Ejdrup | Picosecond time-resolved laser pump/X-ray probe experiments using a gated single-photon-counting area detector[END_REF][START_REF] Fertey | Diffraction studies under in situ electric field using a large-area hybrid pixel XPAD detector[END_REF][START_REF] Nüske | Time-resolved x-ray scattering from laser-molten indium antimonide[END_REF][START_REF] Allé | Comparison of CCD, CMOS and Hybrid Pixel x-ray detectors: detection principle and data quality[END_REF]. These detectors are gated with the nanosecond time (ns) scale resolution, faster than the separation between the individual X-ray bunches at the synchrotron radiation sources. To conduct inhouse time-resolved experiments in millisecond (ms) or microsecond (s) timescale, X-ray tubes of laboratory experiments can be used with gated detectors [START_REF] Fertey | Diffraction studies under in situ electric field using a large-area hybrid pixel XPAD detector[END_REF][START_REF] Allé | Comparison of CCD, CMOS and Hybrid Pixel x-ray detectors: detection principle and data quality[END_REF][START_REF] Wenger | XPAD X-ray hybrid pixel detector for charge density quality diffracted intensities on laboratory equipment[END_REF][START_REF] Casaretto | In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector[END_REF]. One of the significant limitations of such detectors is their read-out time.

Excitation Sources

The pump-probe time-resolved experiments are performed at the shortest possible time scale.

Typically, the choice of excitation sources used for these experiments is a pulsed laser with pulse widths ranging from ns to ps. LEDs are also used for excitation for experiments with relatively longer pulse duration and lesser intensity [START_REF] Hatcher | LED-pump-X-ray-multiprobe crystallography for sub-second timescales[END_REF].

Types of photoactive ligands in transition metal complexes

Chemically similar, structurally distinct coordination compounds concerning the connectivity of the metal to the ligand are called linkage isomers. c) In the case of the ambidentate ligand NO2 -, the possible binding modes for the nitrite ligand in a transition metal complex are explained in [START_REF] Chattopadhyay | Linkage isomerism in 4-(2-aminoethyl)morpholine (L) complexes of nickel (II) nitrite: X-ray single crystal structure of trans-[NiL2(NO2)2[END_REF][START_REF] Warren | Reversible 100 % Linkage Isomerization in a Single-Crystal to Single-Crystal Transformation: Photocrystallographic Identification of the Metastable [Ni(dppe)(η 1 -O NO)Cl] Isomer Angew[END_REF]. Further work on the NO2 -was carried out mainly by the Raithby group [START_REF] Hatcher | Metastable Linkage Isomerism in [Ni(Et4dien)(NO2)2]: A Combined Thermal and Photocrystallographic Structural Investigation of a Nitro/Nitrito Interconversion Angew[END_REF][START_REF] Warren | Photoactivated linkage isomerism in single crystals of nickel, palladium and platinum di-nitro complexesa photocrystallographic investigation[END_REF][START_REF] Bajwa | On the appearance of nitrite anion in [PdX(OAc)L2] and [Pd(X)(CN)L] syntheses (X = OAc or NO2): photocrystallographic identification of metastable Pd(η 1 -ONO)(CN)PPh3[END_REF][START_REF] Hatcher | Thermal and photochemical control of nitro-nitrito linkage isomerism in single-crystals of [Ni(medpt)(NO2)(η 2 -ONO)[END_REF][START_REF] Hatcher | Raising the (metastable) bar: 100% photo-switching in [Pd(Bu4dien)(η 1 -NO2)] + approaches ambient temperature[END_REF][START_REF] Hatcher | Understanding solid-state photoswitching in [Re(OMe2-bpy)(CO)3 (η 1 -NO2)] crystals via in situ photocrystallography[END_REF][START_REF] Kutniewska | Photocrystallographic and spectroscopic studies of a model (N,N,O)donor square-planar nickel(II) nitro complex: in search of high-conversion and stable photoswitchable materials[END_REF].

d) The crystallographic evidence for linkage isomerism in dinitrogen fragment is comparatively less studied, and the first single crystal dinitrogen PLI was reported in [START_REF] Fomitchev | The First Crystallographic Evidence for Side-On Coordination of N2 to a Single Metal Center in a Photoinduced Metastable State[END_REF].

However, in recent days a variety of dinitrogen complexes have been reported for their use in catalytic nitrogen fixation [START_REF] Tanaka | Interplay between Theory and Experiment for Ammonia Synthesis Catalyzed by Transition Metal Complexes[END_REF][START_REF] Nishibayashi | Development of catalytic nitrogen fixation using transition metaldinitrogen complexes under mild reaction conditions[END_REF][START_REF] Yang | Transition Metal-dinitrogen Complex Embedded Graphene for Nitrogen Reduction Reaction[END_REF].

Photoinduced linkage isomers in transition metal nitrosyl complexes

The nitrosyl ambidentate ligand is an interesting case of a photo-switchable ligand due to its particular bonding properties as well as the competition between the photoinduced linkage isomerism of NO [START_REF] Coppens | Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes[END_REF][START_REF] Enemark | Principles of structure, bonding, and reactivity for metal nitrosyl complexes Coordination[END_REF][START_REF] Stepanenko | Ruthenium-nitrosyl complexes as NO-releasing molecules, potential anticancer drugs, and photoswitches based on linkage isomerism[END_REF] and NO release [START_REF] Rose | Photoactive ruthenium nitrosyls: Effects of light and potential application as NO donors Coordination[END_REF][START_REF] Giglmeier | Nitric-oxide photorelease and photoinduced linkage isomerism on solid [Ru(NO)(terpy)(L)]BPh4 (L = glycolate dianion) Dalton Trans[END_REF]. The formation of PLI in the solid crystalline state is also accompanied by the change in color and refractive index, i.e., photochromic and photorefractive properties which can be exploited for optical applications [START_REF] Goulkov | Pulse recording of thermal and linkage isomer gratings in nitrosyl compounds[END_REF][START_REF] Schaniel | Photoinduced Nitrosyl Linkage Isomers Uncover a Variety of Unconventional Photorefractive Media[END_REF]. The first photocrystallographic explanation for the structure of PLI in the {MNO} 6 family of mononitrosyl complexes was provided by Coppens and co-workers in sodium nitroprusside (SNP) [START_REF] Carducci | Diffraction Studies of Photoexcited Crystals: Metastable Nitrosyl-Linkage Isomers of Sodium Nitroprusside[END_REF]. The prototypical nitroprusside [Fe(CN)5NO] 2-allows for the illustration of the particular bonding properties of the NO ligand to the transition metal and gives an explanation for the nature of two metastable linkage isomers, isonitrosyl MS1, and side-on MS2, after light irradiation (example E1 in Table 1. (l-NO-κN)2 → (a-NO-κN)2

Photocrystallographic
In {MNO} 8 type complexes, the occupation of the d(z 2 ) or d(x 2 -y 2 ) metal orbital change the bonding scheme, compared to the one described above for SNP. First, the electron pair in the d(z 2 ) orbital competes as a donor pair in a back bond with d(xz). Second, this occupied orbital imposes Pauli repulsion with the ligand's lone pairs, which affects GS and MS1, or the occupied NO(π) bonds, which affects MS2. As a result, the GS is, in general, a bent MNO configuration with angles of 120-140° (example E2 in Table 1.1), except for some rare linear MNO configurations, for which the trans-to-NO position is weakened or left empty (example E3 in Table 1.1). Only two types of PLI are produced after irradiation of the bent-GS. In the first case of {MNO} 8 type of species, the bent nitrosyl in GS converted to bent isonitrosyl in the excited state as reported in the example of [PtCl(NH3)4(NO)]Cl2 [START_REF] Schaniel | Generation of one light-induced metastable nitrosyl linkage isomer in [Pt(NH3)4Cl(NO)]Cl2 in the red spectral range[END_REF] in E2. Here, the number of photoinduced linkage isomers is restricted by the absent metastability of the linear form. DFT calculations [performed by P. Klüfers from LMU Munich] show that on scanning the Pt-N-O angle from the bent GS towards linearity, the energy rises without reaching a local minimum. As a result, there is a single PLI, the bent-κO form, which is derived from the bent-κN GS by the N/O interchange within the nitrosyl [START_REF] Schaniel | Generation of one light-induced metastable nitrosyl linkage isomer in [Pt(NH3)4Cl(NO)]Cl2 in the red spectral range[END_REF]. In the second case of {MNO} 8 center, the bent nitrosyl in the GS converted to linear nitrosyl in the metastable state in [CoCl2(NO)(PMePh2)2] [79] example E3. Note that the case of E3 is actually widened by the change in spin multiplicity after irradiation [START_REF] Popp | Bent and Linear {CoNO} 8 Entities: Structure and Bonding in a Prototypic Class of Nitrosyls[END_REF]. As a result, {MNO} 8 species establish this scenario in mostly pentacoordinate or (5+1)-coordinate species with a sixth ligand at a greater distance. From the family of {MNO} 8 type of complexes, a {CoNO} 8 complex has been studied photo-crystallographically in this thesis (Chapter 3).

Introducing a second NO ligand and thus producing dinitrosyl complexes changes the bonding configuration and in consequence, also the energy landscape of the various isomers.

{M(NO)2} 8 dinitrosyl complexes can be composed of a square-planar {MNO} 8 mononitrosyl and a NO + acceptor ligand. As has been shown by previous work in the CRM2 laboratory [81],

the nitrosyl ligand in the square planar core is linearly bonded and seems to be largely inert in PLI experiments, while the second nitrosyl along z forms different PLI. The photoswitching of a single nitrosyl has been observed in a series of {Ru(NO)2} 8 dinitrosyl of various [RuX(NO)2(PR3)2]BF4 (X = Cl, Br; PR = PPh, PCy, PCyp) complexes. In a dinitrosyl, one more geometrical parameter has to be accounted for in a bent isomer. The nitrosyl ligand in the bent bonding mode can be inclined either away from (anti) or towards (syn) the inert nitrosyl.

On moving from anti to syn and back, the active nitrosyl passes a point of linear bonding.

Regarding energy, the double-linear structure is found as a transition or an intermediate state in electron-poor systems. In electron-rich systems, the double-linear configuration is the GS.

In this case, photoexcitation may switch one nitrosyl to the bent mode. Hence, even though the nitrosyls' possible bonding modes follow the same linear/bent pattern for {MNO} 8 and {M(NO)2} 8 geometrically, energetically, it creates a significantly different energetic situation.

As shown above, activation walls between the bent and the linear isomers are low to nonexistent in the case of an {MNO}

]; (3) The SPY-5-configured syn-bent-GS of [RuCl(NO)2(PCy3)2]
BF4 switches to the anti-bent isomer [START_REF] Casaretto | Photo-induced linkage NO isomers in the dinitrosyl complex [Ru(NO)2(PCy3)2Cl](BF4) identified by photocrystallography and IR-spectroscopy[END_REF]; (4) for the same substance, a switch of the SPY-5 syn-bent-GS shows a single syn-bent-κO switch (example E4 in Table 1.1) [START_REF] Schaniel | Evidence for a photoinduced isonitrosyl isomer in ruthenium dinitrosyl compounds[END_REF]. In summary, {Ru(NO)2} 8 dinitrosyl have, until now, been found with a single switchable nitrosyl ligand [START_REF] Stepanenko | Ruthenium-nitrosyl complexes as NO-releasing molecules, potential anticancer drugs, and photoswitches based on linkage isomerism[END_REF]. If the GS contains a bent nitrosyl, it is preferred as the switched ligand.

{M(NO)2} 10 types of complexes possess two more electrons, amounting to ten d-electrons if one considers the nitrosyl ligand as NO + . This changes the bonding situation and the photophysics again. Up to now, no results on PLI in this type of complex have been reported. While all the previously described photoinduced phenomena affect a single nitrosyl ligand, in this thesis work, we have, for the first time, found a concerted switching of two nitrosyls in the complex [Ru(NO)2(PPh3)2] as detailed in Chapter 3.

Conditions and considerations for successful generation and structure determination of PLI

Developing useful devices for real applications is complementary to control properties and, therefore, the functioning of photo-switchable molecular materials. In tuning the properties of materials, it is essential first to understand the factors that favor or prevent the generation of PLI.

(1) Hard Considerations: First, there are some necessary conditions for PLI to occur a) On the one hand, the sample must absorb the exciting light, i.e., the excitation wavelength must match the energy gap between the GS and GS* of the material.

b) On the other hand, the PLI must correspond to a minimum on the (ground state) potential energy surface i.e., ES must be separated by an activation barrier from the GS. c) However, these two conditions are insufficient for the generation of PLI, as the system might simply relax back from GS* to GS without passing via the PLI.

There must exist a "suitable" path toward the PLI state. In terms of energy, this

implies that the excited state potential has to have a minimum at or near the saddle point of the ground state energy surface between GS and PLI [START_REF] Schaniel | Necessary conditions for the photogeneration of nitrosyl linkage isomers[END_REF] to • Photoconversion fraction: Ideally, the photoconversion factor is 100%. In this case the PLI structure can be measured and analyzed like a standard GS structure, avoiding notably the problems of partial occupations, and therefore a much higher precision and accuracy on the PLI structural parameters could be obtained.

• Optical wavelength control: If the previous condition is not fulfilled at 100%, one needs to know the photoconversion fraction as a function of applied excitation wavelength and temperature.

• Structureproperty: Ideally, the X-ray setup would allow for in-situ measurement of the properties, e.g., the change in absorption or in the refractive index of the crystal under study. How far is this thesis work from the ideal conditions mentioned above?

✓ Regarding thermal stability, the progress of this thesis work is at 230K (Chapter 3 and Chapter 4).

✓ In terms of photoconversion, this thesis is based on the nitrosyl (mono and dinitrosyl) complexes. Therefore, in mono-nitrosyl and dinitrosyl complexes, we could achieve 50% photoconversion. Also, a reported example in our group of mononitrosyl is rereported in the thesis with pulsed laser irradiation where the photoconversion is 90%.

✓ In the case of optical wavelength control, a systematic series of tests are performed using LEDs and lasers of a series of different wavelengths to thoroughly check which amongst them is the optimal wavelength to achieve the maximum photoconversion. In this case, we are almost close to the ideality. The red dot in Figure 1.7 represents the work presented in this thesis and how close we are to the ideality.

✓ In the case of structure and optical absorption, both the structural characterization of the excited states and the optical absorption measurements can be performed on the same sample, thus enabling a better insight into structure-property relationship and thus helping to tailor devices for applications.

X-ray photocrystallography provides a unique possibility to determine metastable state geometries accurately. If the population of PLI is 100%, the quality of the determined structures will be that of standard (GS) structures. However, if the PLI population is less than 100%, additional factors influence the achievable quality of PLI structures. An important factor is whether the structure of the photoinduced excited state is sufficiently distinct from the GS structure or not. If this is the case, reasonably good PLI structure quality can be expected, even for relatively small populations. Further, the occupancy can be refined from the X-ray measurement with reasonable accuracy in this case. If the GS and PLI structures are close, as e.g. the case for the linear Fe-N-O (GS) and Fe-O-N (MS1) in SNP, the PLI structure cannot be refined with good accuracy, and especially the occupancy (population) of the PLI fraction will be affected by larger errors and strongly correlated to other parameters (notably atomic displacement parameters ADPs) [87]. As can be seen from the above considerations, the knowledge of the population of the PLI and GS fractions inside the crystal is essential for a successful analysis of the photocrystallographic experiment. This is actually a key issue because there are only few examples of photoswitchable complexes with (almost) 100% conversion, principally NO2 compounds [START_REF] Warren | Reversible 100 % Linkage Isomerization in a Single-Crystal to Single-Crystal Transformation: Photocrystallographic Identification of the Metastable [Ni(dppe)(η 1 -O NO)Cl] Isomer Angew[END_REF]. For metal nitrosyl compounds, there are two known examples with >90% population, [RuCl(NO)(py)4]PF6 [87] and

[RuF(NO)(py)4](ClO4)2 [START_REF] Mikhailov | Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes[END_REF].

Experimental Techniques

This thesis chapter describes the experimental setups used for the investigations presented in the following chapters. Most of the experiments were performed in-house except for the timeresolved X-ray diffraction experiments at the CRISTAL beamline on the synchrotron SOLEIL.

Photocrystallography aims to understand the structure-property relationship in photosensitive materials. It relies on an ensemble of experimental methods, each of which contributes a piece to a complete understanding of the photoinduced processes, as shown in Figure 2.1:

1) Variable-temperature Infrared spectroscopy under irradiation (Photo-IR) yields information on the vibrational changes upon photoexcitation. This method also allows for obtaining a quick overview of the photo-response of material by scanning different excitation wavelengths. Thereby essential information on the optimal wavelength for generating PLI, the number of PLI and their population, and the stability of PLI (vs temperature) can be obtained. All the above-mentioned experimental methods require the use of light sources for the excitation of the samples. Thus, before presenting the methods, a short overview of the different light sources used during this thesis work is given.

Photo-excitation sources

Lamp

A white light source is used as a probe beam for the in-situ single-crystal optical absorption The general mounting of these lasers includes a Z-stage for vertical adjustment and a combination of λ/2 waveplate and polarizer to adjust the laser power and polarization. The beam is guided by a set of mirrors towards the sample position and, if necessary, is focused or defocused by an appropriate choice of lenses.

Pulsed Laser

Two different types of pulsed lasers were used for the excitation of the samples, one for the Photo-XRD and one for the transient absorption spectroscopy and ns-infrared spectroscopy see Tunable Nd: YAG pulsed laser EKSPLA NT342 is a peripheric device (outside diffractometer cabinet)the beam is guided by a set of mirrors in the following order: M1'-M4' (peripheric mirrors), M1-M5 (cabinet mirrors). Afterward, the beam is focused onto the sample by lens 1 (L1). The beam power can be adjusted directly from the laser interface; however, for convenience, /2 wave plate and polarizer are mounted between M4 and L1 for precise power regulation. If additional beam diagnostic is necessary, a beam splitter (BS, removable/ exchangeable with mirror 5, M5) guiding the laser beam to the photodiode connected to the oscilloscope can be mounted. nanoseconds at a frequency of 10 Hertz. The emitted wavelength is 1064 nm. It is equipped with a frequency-doubling crystal (532 nm) and a frequency-tripling crystal (355 nm). The pulses emitted have an energy per pulse of 650 mJ at 1064 nm, 300 mJ at 532 nm, and 160 mJ at 355 nm. This laser is used to pump an OPO, which allows recombination (with interactions of nonlinear type inside a crystal of beta-borate of barium BBO, β-BaB2O4) to emit monochromatic and coherent laser pulses with a wavelength continuously adjustable between 400 and 700 nm (with an idler signal between 800 and 2500 nm). A remote control allows for emitting single laser pulses in order to perform single-shot experiments.

Infrared spectroscopy with CW and ns-pulsed laser

Infrared absorption (IR) spectroscopy is a well-established analytical technique used to characterize and verify the nature, behavior, and surrounding of molecules by the measurement of the interaction of infrared radiation with materials under investigation, either by absorption, emission, or reflection. Each absorption band corresponds to a vibration mode of the molecule under study, giving information about the bonds in the molecule or complex under investigation. In the analysis of IR spectra of metal nitrosyl complexes, in particular, the characteristic band of N-O bond is particularly sensitive to structural changes, that's why infrared spectroscopy is a technique of choice for the study of nitrosyl complexes, which is the scope of the thesis.

In the thesis work, IR measurements were performed at different temperatures between T = 10-300 K using a Nicolet 5700 FT-IR spectrometer with a resolution of 2 cm -1 . The crystal sample was finely grounded, mixed with KBr, and pressed into a pellet. The KBr pellets were then bonded onto the cold finger of the cryostat using silver paste. Sample was kept in a vacuum (P with f = 10 Hz of a metal nitrosyl complex for single-shot pulsed laser experiment.

=

Uv-visible absorption spectroscopy

UV/Vis spectroscopy yields insight into the electronic structure of PLI, especially concerning the optimal wavelength range for the population and depopulation of the linkage isomers.

UV/Vis spectroscopy was performed using a CARY 4000 spectrometer in the wavelength range 900-350 nm with a resolution of 2 nm. In some cases, in addition, a Shimadzu 3600 spectrometer was used, giving access to the range 3000-190 nm. Sample preparation was as for IR experiments, and the temperature was 100 K in the same cryostat as used for IR experiments. GS is measured first and then irradiated with LED or laser lights in the wavelength range of 365 -1050 nm (for example, see Chapter 3). Generally, in nitrosyl complexes, the GS bands decrease, and new bands of MS arise. The spectrum of the GS alone, however, does not allow us to assert with certainty which wavelength is best suited for the generation of PLI. As a matter of fact, if an absorption band of the PLI is present in the same spectral range as those of the GS, the photo-switching might occur in forward (GS to MS) and backward direction (MS to GS), and it results in an equilibrium population (photo-stationary state), given by the corresponding conversion efficiencies of the forward and backward reaction. Therefore, the knowledge of the absorption spectra of all metastable states will help understand the resulting populations and yield insight into the mechanism of PLI generation.

Differential scanning calorimetry

DSC measurements allow for the determination of the activation energy EA and, thus, the determination of the energy barrier separating the PLI from the GS [START_REF] Schaniel | Evidence of four light-induced metastable states in iron-nitrosyl complexes[END_REF]. In the thesis work, DSC experiments were performed on a DSC1 Mettler-Toledo instrument equipped with a highsensitivity DSC HSS8 sensor. The crystal sample (m = X mg of the powder; while X=given value) was placed in a standard aluminum crucible (40 μL). In some cases, the crystal sample was homogeneously grounded and used in powdered form in such a way that it formed a thin layer covering the whole surface of the crucible. Irradiation of the sample is performed at 120-150 K through a glass window as shown in Figure 2.5, using laser light (max = X nm as determined in the above two experiments) with light Intensity (Iirr = X mWcm -2 as determined in the above two experiments) for (tirr = X min as determined in the above two experiments).

For the detection of the enthalpy release during thermal relaxation of the photoinduced states, the sample was then heated from 150 K to 300 K at a heating rate of β = 4 K min -1 . As a reference measurement (GS), the sample was measured using the same protocol but without light irradiation. The difference between irradiated and non-irradiated heat flow was evaluated using the equation:

d𝐻(𝑡) d𝑡 = ∆𝐻 tot Z exp (- 𝐸 A 𝑘 B 𝑇 - 𝑍 𝛽 ∫ exp (- 𝐸 A 𝑘 B 𝑇 ′ ) d𝑇 ′ 𝑇 𝑇 0 ) (2.1)
This assumes an Arrhenius-like behavior with EA as the activation energy and Z as the frequency factor [START_REF] Schaniel | Evidence of four light-induced metastable states in iron-nitrosyl complexes[END_REF]. Htot denotes the total enthalpy released during the decay, and kB is the Boltzmann constant.

In summary, using the thermal relaxation measurement of PLI, the activation energies EA can be determined by separating the different states. Note that observation of only one decay indicates the presence of a single PLI state. Additionally, the determined released enthalpy ∆𝐻 tot is proportional to the energy difference between the minima of GS and MS. Knowing the population of the MS, one can therefore deduce the energetic position of the MS with respect to the GS. The goal of this setup is to probe the optical response of a sample undergoing laser perturbation.

The sample was mounted on the sample holder, which is in direct contact with the thermocouple Peltier sensor, to ensure the temperature was down to 278.15-313.15 K. The output of the Surelite II-10 Nd-YAG/OPO (technical features of laser described in Table 2.1) pulsed laser (pump) beam is focused on the crystal sample using a system of optical lenses and mirrors (optics features are detailed in supplementary information) in order to irradiate the surface Apulse of the sample. The pulsed laser light can be polarized by a Glan polarizer (if necessary). A CW probe beam is aligned by mirrors and focused by lenses on a spot size Aprobe < Apulse on the sample. The transmitted signal of the probe beam is guided by a mirror (M8) and a lens on the fast photodiode (200 MHz Femto Messtechnik Si diode), which is connected to the 1.5 GHz LECROY WavePro 715Zi oscilloscope (see the technical features in the supplementary information Table B1). A system of filters was placed in front of the photodiode in order to absorb and thus filter the wavelength of the pump beam. The sample here is a nicely polished plate single crystal glued on a copper plate with 300 μm pinhole opening.

Later on, this setup was installed on the goniometer and validated by repeating a similar type of experiment in Chapter 5. This setup on the goniometer will serve as an important component for our next step toward an in-house time-resolved experiment.

The pump wavelength is chosen according to the results of IR and UV/Vis spectroscopic results

for optimal excitation of PLI. The probe wavelength is chosen according to the results of UV/Vis experiments on PLI absorption bands, allowing to identify the most sensitive spectral range for the detection of the PLI. The change in the optical density or intensity transmitted by the sample from GS to PLI as a function of time is given by

𝐼(𝑡) = 𝐼 0 + ∆𝐼𝑒 -𝑡 𝜏 ⁄ , (2.2)
where I0 is the transmitted light intensity before the laser pulse arrives on the sample, ΔI is the photoinduced change in amplitude, and τ is the time constant of the relaxation. With this information, we can, therefore, determine the lifetimes  of PLI and their respective evolution kinetics. Measurement as a function of temperature also gives information about the rate of relaxation back to GS, which will aid in deducing the thermally activated processes, i.e., activation energy EA of the PLI(s). Above all, this information is essential to define the measurement conditions for the time-resolved photocrystallographic experiment.

Single Crystal optical absorption spectroscopy

The tungsten lamp in the designed system, as discussed above, allows for optical absorption (OA) measurements in 400-800 nm range, up to ca. 2.7 optical densities (slightly varying in different spectral ranges). The white light source (technical features described in appendix Table B2) is focused slightly behind the crystal, so the diameter of the spot illuminating the crystal is ca. 1 mm big, while the spot size of the detection system (technical features described in appendix Table B3) is from 20 to 4 times smaller and varies from ca. 50 μm to 250 μm depending on the number of optical fiber bundles (appendix Table B4) connected to the USB4000 spectrometer (1 or 6) (appendix Table B4). The set-up is shown in Figure 2.8. The qualitative OA measurements can be conducted for arbitrarily chosen crystal orientation, e.g., plate-shaped crystal can be oriented perpendicularly to the light for higher spectrum quality, and in most cases, it will correspond to one of the main lattice planes (100) (010) or (001) perpendicular to the light. For quantitative analysis, proper orientation of the crystal towards linearly polarized light might be necessary, e.g., for crystals showing linear dichroic effect [START_REF] Ronda | Protein crystal microspectrophotometry[END_REF] and depending on the crystal system of a sample averaging over few measurements along few directions may be required.

Photocrystallography

The XRD measurements in this thesis work were conducted on three different in-house diffractometers and the beamline CRISTAL at SOLEIL. First, a short overview of the three diffractometers is given in their standard configuration, then the corresponding laser installations on them are described.

• The Rigaku Supernova diffractometer with Atlas CCD detector and double micro source Cu/Mo was used. The data treatment, in this case, was done with CrysAlis [91] and structure solution with Olex2 [START_REF] Dolomanov | OLEX2 : a complete structure solution, refinement and analysis program[END_REF]. Temperature control was done either with a nitrogen or helium blower (Figure 2.9).

• The Bruker D8 diffractometer with PHOTON III CMOS detector and double micro source Mo/Ag was used. Data treatment was done with the APEX software suite and SADBS [START_REF] Bruker | APEX3 Crystallography Software Suite[END_REF]. Temperature control was done either with a nitrogen or helium blower (Figure 2.10).

• The STOE-Cegitek diffractometer with Mo microfocus source and RebirX pixel detector was used (Figure 2.11). Data integration was done with XAREA software suite [START_REF] Stoe | X-Area[END_REF].

Steady-state photocrystallography (a) Steady-state photocrystallography with CW laser

In steady-state photocrystallography with a continuous wave (CW) laser irradiation source, the intensity of the laser and irradiation time were used accordingly as determined in the IR and UV-Vis spectroscopic techniques. In my thesis work, I mostly used a small optical table with the possibility to adjust precisely the height of the laser on the sample position. In front of the laser, lenses are mounted in order to adjust beam size to irradiate the sample homogeneously and also to adjust power density (mW/cm 2 ). In the case of Bruker D8 Venture diffractometer equipped with PHOTON III CMOS detector, this table with CW laser can be placed inside the diffractometer chamber, which is big enough Figure 2.10. For the SuperNova microfocussource diffractometer equipped with an ATLAS detector, due to the smaller space inside the where laser beams are adjusted with the help of λ/2 waveplate and polarizer for intensity, guided with mirrors (including the vertical direction in order to reach the height of the center of the diffractometer) and lenses onto the sample position. A helium 10 K experiment was also performed for a complex with excited state stability at this temperature on the D8. The geometry of Hybrid Pixel Area Detector (HPAD)

We will use for our experiment RebirX 540S Hybrid Pixel Area Detector (HPAD) (Table 2 Power losses and stability of the pump beam

We performed an analysis by placing the photodiode at the EKSPLA NT342 pulsed Laser output, to all the thirteen optical mirrors (characteristic features in appendix), and at the crystal at the end of the optical path. The test wavelength selected for this purpose was 420 nm, f = 10

Hz. With our laser, the output pulse energy in the visible range is ~90 mJ. After the first two mirrors, the attenuation of the intensity of the beam along the optical path is of the order of 8%.

After the 6 th mirror, power loss is even more significant, ~40%. The measured power at the 8 th mirror ⎯ just before the crystal was 58 mW, which can further be attenuated with λ/2 plate (in 400-800 nm and for IR range 1064 nm) and a polarizer which is placed at the end of the optical path to fix the polarization of the pump beam with respect to the crystal orientation. In order to focus the laser on the crystal, a plano-convex lens is placed on an XYZ translation stage, allowing fine tune the focal point of the lens with respect to the crystal. In the thesis work, a lens of f = 140 mm was used, with a focal point preferably behind the crystal so that any beam damage to the crystal can possibly be reduced. In order to center the pump beam first, a steel ball was used. To determine the effective pump beam size and to illuminate the crystal with maximum laser power, a 500 m pinhole was used before placing the crystal for the laser irradiation. It is, therefore, important to understand the importance of the peak power of the pump pulse on the crystal, as this will define the population rate of the metastable states generated by this pulse.

Analysis tools in photocrystallography

Photodifference maps

In X-ray photocrystallography, the first step in the analysis of the data is, in general, to inspect so-called photodifference maps (2D or 3D), a useful tool for the first unbiased look to qualitatively assess the photoinduced structural changes. The excitation by light source leads to induce changes in electron density and hence modifies structure. Therefore, a photodifference map is calculated by using 'common' independent reflections between the ground state GS and the photo-irradiated state PLI by the inverse Fourier transform of the difference while using the phases obtained from the refinement of the reference GS structural model as represented in Figure 2.12. As a first approximation, the phases for the photoirradiated state (composed of GS and PLI fractions) are assumed to be the same as those of the pure ground state and are thus calculated from the ground state structural model.

The difference in electron density for the photodifference maps can be expressed as:

𝐹 𝑝ℎ𝑜𝑡𝑜𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑜𝑏𝑠 (ℎ𝑘𝑙) = 𝐹 𝑃𝐿𝐼 𝑜𝑏𝑠 (ℎ𝑘𝑙) -𝐹 𝐺𝑆 𝑜𝑏𝑠 (ℎ𝑘𝑙) (2.3) ∆ 𝑝ℎ𝑜𝑡𝑜𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑜𝑏𝑠 (ℎ𝑘𝑙) =  𝑃𝐿𝐼 𝑜𝑏𝑠 (ℎ𝑘𝑙) - 𝐺𝑆 𝑜𝑏𝑠 (ℎ𝑘𝑙) (2.4) ∆𝜌(𝒓) = 1 𝑉 ∑[|𝐹 𝐸𝑆 𝑜𝑏𝑠 (𝑯)| -|𝐹 𝐺𝑆 𝑜𝑏𝑠 (𝑯)|]𝑒 𝑖𝜑 𝐺𝑆 𝑐𝑎𝑙𝑐 𝑒 -𝑖2𝜋𝑯.𝒓 𝑛 ℎ𝑘𝑙 (2.5)
Where F obs are the observed structure factors obtained from the GS and ES data sets, and one uses the phases from the GS model, for each measured reflection at reciprocal vector H.

Accurate Bragg peak intensity measurements are required for this Fourier summation to avoid any bias or artifacts in the photodifference maps.

Figure 2.12 : The photodifference maps (2D) (left) is equal to the difference between observed GS (middle)

and the photo-irradiated state (right) while using the phases from the reference GS structural model.

Once the photodifference maps have been generated and analyzed, the structural hypothesis can be proposed for the PLI. For example, an isonitrosyl configuration can be qualitatively deduced by looking at the depletion of electron density at the position of oxygen (red) and, therefore, accumulation of electron density at nitrogen of nitrosyl (blue), which leads to a clear indication that the new configuration is isonitrosyl (Figure 2.12). Since the X-ray diffraction experiment is performed on a single crystal in the photo-stationary state, which therefore contains molecules in the ground state and molecules in the PLI, the population needs to be refined. For nitrosyl PLI the assumption holds that GS and PLI molecules are spatially randomly distributed in the crystal sample. Accordingly, the structural model of the photogenerated metastable state is determined and refined, and their occupancies are refined quantitatively with the constraint PGS + PPLI = 1.

The photodifference maps, on the other hand, can be used to simulate the expected changes due to a specific PLI, using 𝐹 𝑝ℎ𝑜𝑡𝑜𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑐𝑎𝑙𝑐 (ℎ𝑘𝑙) = 𝐹 𝑃𝐿𝐼 𝑐𝑎𝑙𝑐 (ℎ𝑘𝑙) -𝐹 𝐺𝑆 𝑐𝑎𝑙𝑐 (ℎ𝑘𝑙). These simulations allow for the preparation of time-resolved photocrystallographic experiments either in-house or at the synchrotron if the expected changes due to a specific PLI, using photodifference maps are determined as a function of (a) metastable state population or (b) crystal orientation and (c) determining the proper configuration to achieve maximum possible coverage of Ewald sphere (see Chapter 5).

Screening the most sensitive reflections

Another useful tool in photocrystallography is the search and analysis of selected Bragg reflections, that are most sensitive to the photoinduced structural changes. A most sensitive reflection is a Bragg reflection common to both data sets (GS and excited sate ES), which undergoes the largest change in intensity from GS to ES:

∆𝐼 𝐼 𝐺𝑆 = (𝐼 𝐸𝑆 -𝐼 𝐺𝑆 ) 𝐼 𝐺𝑆 (2.6)
The physical origin of this intensity difference is the change in electron density when going from GS to ES: Generally, the heaviest atom in the structure contributes the most to the diffraction, and therefore, the changes can be best seen in the photodifference map (Figure 2.12) on the central heaviest atom. As shown in the general schematic Figure 2.13 the sum of structure factor Fhkl and phases hkl can also be expressed as a complex number of all the contributors both from GS to photo-irradiated state 𝐹 𝐺𝑆 = 𝐹 1 + 𝐹 2 + 𝐹 3 + 𝐹 4 ; 𝐹 𝐸𝑆 = 𝐹 1 + 𝐹 2 + 𝐹 3 + 𝐹 4 and therefore the difference in photodifference maps can be calculated 𝐹 𝑝ℎ𝑜𝑡𝑜𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑜𝑏𝑠 (ℎ𝑘𝑙) = 𝐹 𝐸𝑆 𝑜𝑏𝑠 (ℎ𝑘𝑙) -𝐹 𝐺𝑆 𝑜𝑏𝑠 (ℎ𝑘𝑙).

∆𝜌(𝒓) = 𝜌 𝐸𝑆 (𝒓) -𝜌 𝐺𝑆 (𝒓) (2.7)
The criteria for the most sensitive reflection from GS to ES is:

-The higher the difference between FES and FGS, the more sensitive that common reflection is.

-Suitable reflections are of strong or moderately strong intensity, as they present low statistical errors. However, too strong reflections, with intensity close to saturation, are not suitable. Weaker reflections presenting large differences are of interest but need to

be measured with sufficient statistics to be useful. Very interesting cases are those reflection which are zero (or forbidden) in one of the states but not in the other.

The intensity change of this particular reflection is the direct consequence of linkage isomerism GS/ES. After screening the most sensitive reflection, a data collection strategy can be devised in such a way that the target reflection is present in the frames, which can then be repeated to increase the multiplicity of the target reflection. This tool for the analysis of the most sensitive reflections will be very useful for time-resolved photocrystallography. Pump-probe diffraction @ CRISTAL Time-resolved experiments can be performed thanks to the inherent pulsed nature of the synchrotron X-ray beam. For these investigations, SOLEIL has four alternative operating modes: hybrid mode, single bunch mode, eight bunch mode, and low alpha mode. All of them contain an isolated bunch or a bunch that is at least 147 ns apart from its nearest neighbors.

The current carried by the isolated bunch determines how many photons are released by each isolated pulse at a beamline. The CRISTAL beamline accomplishes the following tasks:

-Hybrid mode (5 mA + 445 mA, 5 mA in the isolated bunch) // pulse duration: 62 ps;

FWHM// 3.7×10 5 The test experiment in Chapter 5 was performed with 8-bunch mode.

In-house time-resolved photocrystallography setup

Our in-house time-resolved setup is developing. We use EKSPLA NT342 pulsed Laser for the pump and for the probe a Mo microfocus tube. The general schematic of the in-house timeresolved setup is shown in Figure 2.16. The detection system is a RebirX 540S Hybrid Pixel Area Detector (HPAD). For the time-resolved experiment, the detector is triggered by an external signal to be able to synchronize it with the incoming pulses, which is in progress.

Steady-state photocrystallography

As outlined in Chapter from investigations performed on dinitrosyl-ruthenium and dinitrosyl-iron complexes [81,[START_REF] Iam | {FeNO} 7 -Type Halogenido Nitrosyl Ferrates: Syntheses, Bonding, and Photoinduced Linkage Isomerism[END_REF] in the CRM2 laboratory.

In view of studying the photoisomerization process with time-resolved diffraction, we chose to study representatives of three of these {MNO} n and {M(NO)2} n types in my thesis. As a prerequisite for the study of time-resolved processes and transient structures, the corresponding GS and PLI structures, as well as their generation conditions, must be known with good accuracy, as they will serve as reference points for any investigation of the dynamics.

This chapter describes the results of a systematic steady-state photocrystallographic study of the three following complexes: A major problem in the photocrystallographic studies is the fact that MS1 and MS2 states are stable only at low temperatures and that their population is below 50%, i.e., one has to deal with a mixture of GS and MS in the crystalline solid, which complicates structure refinement.

• {MNO} 6 representative: [Ru(NH3)5(NO)]Cl3•H2O • {MNO} 8 representative: [Co(fpin)(NO)(phen)] • {M(NO)2} 10 representative: [Ru(NO)2(PPh3)2]
Further research has thus focused on improving these aspects, and ruthenium nitrosyl isomers MS1 =400-500nm and MS2 =1000-1100nm respectively and DFT analysis [START_REF] Coppens | Crystallography of molecular excited states. Transition-metal nitrosyl complexes and the study of transient species[END_REF][START_REF] Silva | Metastable excited state and electronic structure of [Ru(NH3)5NO] 3+ and [Ru(NH3)4(OH)NO] 2+[END_REF][115][START_REF] Yamaletdinov | The Effect of trans Ligands in the NO-Linkage Reverse Isomerization for Ruthenium-Nitrosyl-Tetraammine Complexes: A DFT Study[END_REF][START_REF] Sizova | DFT study of metastable linkage isomers of six-coordinate ruthenium nitrosyl complexes[END_REF].

Notwithstanding this significant amount of experimental and theoretical evidence of PLI in ruthenium ammine (tetraamine and pentaamine) compounds, there are almost no reports on the structure (by XRD) of the PLI in these systems. A structural model for MS1 has been reported in [START_REF] Mikhailov | Combining photoinduced linkage isomerism and nonlinear optical properties in ruthenium nitrosyl complexes[END_REF], but to our knowledge, no structural model has been reported for MS2 yet.

The reason for this lack of XRD study of PLI in these systems is twofold, on the one hand, the populations of MS1 and MS2 are lower than in the ruthenium pyridine compounds, and on the other hand, the ammine ligands are often subject to disorder, which severely complicates photocrystallographic studies. Moreover, the generation of MS2 requires a twostep irradiation procedure at low temperatures, further complicating the analysis.

To address these issues and to propose structural models for MS1 and MS2 in ruthenium pentaamine mononitrosyl complex, we report an in situ experimental photocrystallographic study of [Ru(NH3)5(NO)]Cl3•H2O and highlight the MS1, two conformational geometries of MS2 experimentally characterized for the very first time along with the treatment of disordered NO ligand.

Experimental

[Ru(NH3)5(NO)]Cl3•H2O crystals were obtained from our collaborators from University of Bern (K. W. Krämer). The synthesis followed the method reported in the literature [START_REF] Gleu | Ruthen-Ammine. XI. Nitroso-Pentammine des Rutheniums[END_REF], and large crystals were grown by slow evaporation from the solution. The photocrystallographic experiment was carried out in two consecutive steps on the Rigaku diffractometer (see Chapter

2). The crystal structure of the ground sate was first derived as a reference, followed by a measurement of the same single crystal sample after laser light exposure to determine the crystal structure of the photo-induced metastable states MS1 and MS2. The corresponding crystallographic data are given in Table 3.1.

As discussed below, the crystal structure of [Ru(NH3)5(NO)]Cl3•H2O in the ground state, and in the MS2 state as well, presents severe disorder so that defining the exact space group is a delicate task (centrosymmetric Pnma or non-centrosymmetric Pna21 space group in the present case), and a matter of choice with respect to the goal of the present study. Hereafter, in order to comparatively discuss the GS, MS1, and MS2 crystal structures, we consider that the most appropriate space group is centrosymmetric Pnma. A complete discussion is given in the appendix C1, based on additional X-ray diffraction experiments using Cu Kα radiation, in order to correctly assign the space group and especially distinguish between centrosymmetric Pnma and non-centrosymmetric Pna21 space groups for [Ru(NH3)5(NO)]Cl3•H2O. As argued below, we opt for the Pnma space group. Accordingly, the measured data were merged with SORTAV [START_REF] Blessing | Data Reduction and Error Analysis for Accurate Single Crystal Diffraction Intensities[END_REF] in Laue point group mmm to give 2414 unique reflections. The GS crystal structure was solved and refined in Pnma space group using Olex2 (version 2-1.3) [START_REF] Dolomanov | OLEX2 : a complete structure solution, refinement and analysis program[END_REF]. A riding model [120] was used for the hydrogen atoms attached to the amine Nsp3.

After collecting the GS data, a new crystal was used for the determination of the photoinduced states. The metastable state MS1 was generated by irradiating the crystal at 100 K with CW laser at 422 nm wavelength (P = 60 mW) for 45 min in the complete dark. The complete diffraction data were collected by using the same strategy as used for the GS, and the structure was solved and refined in Pnma space group using Olex2 (version 2-1.3) [START_REF] Dolomanov | OLEX2 : a complete structure solution, refinement and analysis program[END_REF]. Subsequently, the MS2 state was generated by optical transfer from the state MS1 with irradiation at 1064 nm (P = 100 mW) for 107 min; data were then collected on the MS2 state, and data were collected for MS2. More details on data collection and refinement results for the three measured states are summarized in Table 3.1.

Results and discussion

Ground state structural analysis

In order to determine and investigate the structural reorganization upon formation of MS1 and Relevant structural parameters are collected in Table 3.1 and Table 3.

Structural analysis of the metastable state MS1

The photodifference map shows several important features, which correspond to electron density displacement between the ground state and the photo-induced MS1 state, that is to say We, therefore, attribute it to two nitrogen atoms of the PLI, labeled N1A and N1B. Therefore, to model the qualitative observations of PLI in the photodifference map (Figure 3.4), we introduced two positions for the nitrogen atom (N1A and N1B) but only one position for the oxygen atom (O1A). The refinement of this model with two NO orientations for MS2, with a shared O1A position, was performed using Olex2 (Version 2-1.3) [START_REF] Dolomanov | OLEX2 : a complete structure solution, refinement and analysis program[END_REF]. Keeping in mind that there is also the N(5)H3 present sharing this ligand position with coordinates very close to those of N1 of GS, we proceeded stepwise. In the very beginning, N1A-O1A PLI was refined freely without introducing N1B. This allowed checking the occupancy of the commonly shared O1A so that the occupancy of two nitrogen N1A and N1B could be deduced. The refined occupancy O1A was 0.054 [START_REF] Zewail | Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond[END_REF]. During the course of the subsequent refinement, we then fixed

Structural analysis of the metastable state MS2

(a) (b)
the occupancy of O1A to 0.0542 while those of N1A and N1B to 0.0271 each, and further restrained the bond distances of N1A-O1A and N1B-O1A. The atomic displacement parameters of N1A-O1A, N1B-O1A were also constrained to be equal to that of N1-O1. The refinement converged to the final model depicted in the Figure 3.5 with to R=0.033, wR=0.068.

We can distinguish two clearly separated positions for the N1A-O1A and N1B-O1A groups, which we label in the following MS2X and MS2Y, respectively. One NO group (N1A-O1A)

is almost parallel to the N2-Ru-N4 direction, while the other NO group (N1B-O1A) is along the diagonal between the equatorial ligands. The dihedral angle between the planes O1ARu1N1B of MS2X and O1ARu1N1B of MS2Y is 53.46. The most relevant distances of MS2 with two geometrical isomers MS2X (O1A-N1A) and MS2Y (O1A-N1B) are presented in Table 3.1 and Table 3.

Discussion

According to the literature on metal nitrosyls, the well-known side-on linkage isomer had been structurally characterized in Fe [START_REF] Carducci | Diffraction Studies of Photoexcited Crystals: Metastable Nitrosyl-Linkage Isomers of Sodium Nitroprusside[END_REF] and Ru nitrosyl complexes [87]. Two MS2 are not many reported structures of MS2 species, we might expect that this phenomenon also occurs in other complexes. Another significant result of this study is that we could show that by treating the system in a higher symmetry space group, the structures of the PLI could be determined, even though the presence of disorder limits the precision and imposes some constraints.

Photoinduced linkage isomerism in a {CoNO} 8 complex: [Co(fpin)(NO)(phen)]

Introduction

The {M(NO)} 8 type of mono nitrosyl complexes is less studied than {MNO} 6 with respect to PLI. There are two reports on PLI in hexacoordinated {PtNO} 8 complexes [START_REF] Schaniel | Generation of one light-induced metastable nitrosyl linkage isomer in [Pt(NH3)4Cl(NO)]Cl2 in the red spectral range[END_REF]121] where from a bent Pt-N-O in GS a single bent PLI in the isonitrosyl Pt-O-N configuration was found. For {CoNO} 8 complexes, there is no report on PLI up to now. There is an ongoing discussion about the different possible ground state configurations for these complexes, between a bent and a linear Co-N-O configuration, also linked to their magnetic state [START_REF] Popp | Bent and Linear {CoNO} 8 Entities: Structure and Bonding in a Prototypic Class of Nitrosyls[END_REF].

Marroux et al. [START_REF] Marroux | Spin Changes Accompany Ultrafast Structural Interconversion in the Ground State of a Cobalt Nitrosyl Complex[END_REF] reported ultrafast intersystem crossing at room temperature in the pentacoordinated [CoCl2(NO)(PMePh2)2] accompanied by isomerization between bent and linear Co-N-O. Tang et al. [START_REF] Tang | Electron Transfer and Geometric Conversion of Co-NO Moiety in Saddled Porphyrins: Implications for Trigger Role of Tetrapyrrole Distortion[END_REF] reported the bent-to-linear conversion of the Co-N-O moiety in saddled porphyrins. To extend the knowledge about PLI in {M(NO)} 8 type mono nitrosyl complexes, photocrystallographic analysis on the cobalt nitrosyl complex [Co(fpin)(NO)(phen)] is performed. Since for this case, there is now prior knowledge, the PLI properties are determined by infrared and UV/Vis spectroscopy, before characterizing the structure by X-ray photocrystallography.

Experimental methods

Synthesis

Powder and single crystal samples of [Co(fpin)(NO)(phen)] (II) were obtained from Peter Klüfers' group at LMU Munich. The synthesis of II is described in [START_REF] Popp | Bent and Linear {CoNO} 8 Entities: Structure and Bonding in a Prototypic Class of Nitrosyls[END_REF].

IR and UV-Vis spectroscopy

IR measurements were performed at different temperatures between T = 10 and 300 K, and the sample was kept in a vacuum inside a closed cycle cryostat, using a Nicolet 5700 FT-IR spectrometer with a resolution of 2 cm -1 as has been described in Chapter 2. UV-Vis measurements were performed at different temperatures between T = 10 and 300 K on samples prepared for IR and using the same cryostat and following the same procedures for sample irradiation. Two spectrophotometers, a CARY 4000 operating in the range 900-200 nm and a Shimadzu 3600 operating in the range 1750-350 nm were used to cover the whole spectral range.

Results and discussion

Infrared spectroscopy

Recording IR spectra after irradiation at different wavelengths yields information on the optimal wavelength for the maximum population of PLI, by measuring the decrease of the GS bands. During the recording of spectra after irradiation, we observed a progressive decrease of the amplitude of the PLI band at 1779 cm -1 and an increase of the GS band at 1708 cm -1 as illustrated in Figure 3.7. To interpret this behaviour, we recall that the PLI is generated by irradiation with 590 nm, through the KBr windows of the cryostat at 10 K, but this population sequence is performed outside of the IR spectrometer, and thus the sample is not exposed to the IR probing beam of the spectrometer. Once the irradiation is stopped, the cryostat is moved into the IR spectrometer to record the spectra.

UV/Vis spectroscopy

UV-Vis spectroscopy yields information on the electronic structure of the different isomers and allows identifying the optimal spectral ranges for switching between isomers. In the GS we observe bands at 600 nm (2.066 eV) and 410 nm (3.024 eV), as illustrated in Figure 3.8 (left panel). After irradiation with yellow light (589 nm), the GS bands decrease, as evident from the 600 nm band. New bands arise at 490 nm (2.530 eV) and 447 nm (2.774 eV).

Irradiation in the blue spectral range, e.g., with 473 nm, and in the infrared spectral range, e.g., with 1064 nm, transfers the system back to the GS. To detect the absorption band in the infrared spectral range responsible for this back-transfer and the depopulation observed during the IR experiments, we measured another set of spectra using more concentrated samples. Furthermore, we investigated the thermal stability of the PLI state by measuring absorption spectra after irradiation at different temperatures. Figure 3.9 shows the results. At 60 K the viable population of PLI is already significantly smaller than at 10 K; heating to 70 K results in the complete vanishing of the PLI state, as can be seen from the disappearance of the two bands at 447 nm and 490 nm, and the recovery of the 600 nm GS band. Given that the recording of a spectrum takes about 3 minutes, this indicates that the lifetime of PLI at 60 K is of this order of magnitude, i.e., a few minutes.

Photocrystallography

Single-crystal X-ray diffraction data were collected on a BRUKER D8 Venture diffractometer equipped with PHOTON III CMOS detector, using Mo Kα radiation (λ = 0.71073 Å); a Helijet cryostream was used to keep the temperature at 5 K. A single crystal was mounted on a microgripper loop using vacuum grease. In the first step, diffraction data were collected at 5 K in the ground state. The data integration and reduction were performed using APEX3 software (Version 2019.11) [START_REF] Bruker | APEX3 Crystallography Software Suite[END_REF]. A numerical absorption correction was applied using crystal shape and face indexing. The GS structure was solved in triclinic crystal system with space group P-1 using the SHELXT [123] program and refined using the SHELXL [123] program in the OLEX2-1.3 suite [START_REF] Dolomanov | OLEX2 : a complete structure solution, refinement and analysis program[END_REF]. A riding model [120] was used for the hydrogen atoms. All the hydrogen atoms were placed using riding model. The crystallographic information is selected in the Table 3.3.

Given that the PLI in II is stable below 60 K, we collect SC-XRD data at 10 K. Figure 3.10

shows the structure of the GS at 10 K. Another single crystal was mounted on a glass fiber using vacuum grease. The sample was then irradiated at 10 K with a laser diode of 590 nm (P = 1.6 mW) for 60 min in the complete dark until the photo-stationary state was reached. The sample is continuously rotated during the irradiation. According to the infrared results, at this temperature and irradiation wavelength, the metastable state of PLI can be selectively populated with a sufficiently high population to enable a meaningful photocrystallographic experiment. Complete diffraction data were collected in the photo-stationary state; no spacegroup change occurred with respect to the ground state. Like the GS structure, the photoirradiated state was solved in triclinic crystal system with space group P-1 and refined by using First, we note that the conversion from GS to PLI-state is not complete. We thus need to introduce two NO ligands, N3O3 for GS and N3AO3A for PLI, and consider the GS and PLI like a disorder over two positions, with a total occupancy of 1. Second, the NO ligand can adopt either an isonitrosyl or a nitrosyl configuration. Therefore, we need to test both models and compare the results. As can be seen from Table 3. actually present electron density, leading to an increase of the ADP (smearing out). In the present case, there is almost no difference between the two models, each providing quite reasonable ADPs, identical within experimental errors (Table 3.4). This is probably due to the rather low population. Nevertheless, the ADPs of the isonitrosyl model seems to better match our expectations. Figure 3.12 shows exemplarily the final model for the PLI configuration of the Co-O3A-N3A. As a matter of fact, both the isonitrosyl and nitrosyl structural models (Figure 3.13) for the PLI result in almost identical parameters (Table 3.4 and Table 3. results alone, we cannot decide whether the configuration is isonitrosyl or nitrosyl. If we compare the IR results with the previously reported PtNO complexes, we observe that the shift of the NO stretching vibration by 71 cm -1 occurs in the same direction (towards higher energies). These earlier results were supported by DFT calculations yielding a bent isonitrosyl Pt-O-N for the PLI, so it might be speculated that in the CoNO we also are confronted with a bent isonitrosyl PLI. Therefore, DFT calculations should be performed in order to confirm or refute this interpretation. Such calculations would also be helpful to investigate another aspect of the cobalt nitrosyl complex, its possible photomagnetic behaviour. Looking at the UV-Vis spectra, we find the absorption band of the PLI is very far shifted into the near-infrared spectral range. This is typical for triplet states, so that possibly the observed PLI is accompanied by a change in the magnetic state. To confirm this, photomagnetic measurements should be performed. In the end, there comes the question of different population. In X-ray diffraction measurement only 5-6% population were obtained, while in the IR experiments, up to 50% could be achieved. There are two possible explanations for this difference. First, the single crystal used for XRD is much bigger than the diluted finely ground powder in the KBr pellet used for IR. Hence the exciting light might simply not penetrate far enough into the crystal, and PLI generation is limited to the surface layers of the crystal. Second, the extreme sensitivity of the PLI state towards infrared light, which depopulates the PLI very rapidly. Even the probe beam of the IR spectrometer was enough to depopulate the PLI in a few minutes. We cannot exclude that in the XRD experiments, some light or other radiation sources lead to a depopulation of the PLI during the measurement, which lasts several hours.

Conclusion

We could not unambiguously identify a PLI configuration for a {CoNO} 8 complex. While it is evident that it corresponds to a bent NO configuration, X-ray photocrystallography does not

give a conclusive answer concerning the question of whether it is in the isonitrosyl configuration or in the nitrosyl configuration. Notwithstanding this ambiguity, the characteristic structural parameters as well as the population of this PLI can be determined with sufficient accuracy. We found an exceptionally high sensitivity for population and depopulation of PLI, as well as some indications that we might deal with a photomagnetic effect. For further validation of the singlet to triplet spin state change, photomagnetic measurements as well as theoretical calculations, will be necessary. In terms of bonding scheme within the {MNO} 8 species, II behaves like the majority of these compounds, i.e., by adopting a bent structure with an angle in the range 120-140° for both the GS and the PLI. 1.1 of Chapter 1). In the example [RuCl(NO)2(PCy3)2]BF4 two of the three PLI structures could be determined by X-ray photocrystallography; in both cases, only the already bent NO undergoes PLI. In PLI-1 the NO switches from a syn-bent NO in GS to an anti-bent NO in the PLI [START_REF] Casaretto | Photo-induced linkage NO isomers in the dinitrosyl complex [Ru(NO)2(PCy3)2Cl](BF4) identified by photocrystallography and IR-spectroscopy[END_REF]. In PLI-2 the syn-bent NO of GS switches to an anti-bent ON in the PLI [START_REF] Schaniel | Evidence for a photoinduced isonitrosyl isomer in ruthenium dinitrosyl compounds[END_REF]. Thus, overall, for {Ru(NO)2} 8 dinitrosyls, up to now, only one of the NO ligands has been found to undergo PLI. In particular, we note that the second NO ligand is not involved in the PLI (besides minor structural adjustments concomitant with the overall relaxation of the molecule).

PLI in the ruthenium

{M(NO)2} 10 species possess two more electrons, which is expected to affect the photophysical behavior. On the example of dinitrosyl-bis(triphenylphosphane)ruthenium, [Ru(NO)2(PPh3)2]

(III), we explore the consequences of this change in electronic configuration on the type of PLI that can be formed.

Experimental Section

Powder and single crystals of (III) were obtained from the group of P. Klüfers at LMU (Daniel Beck). Description of the synthesis is elaborated in [START_REF] Hasil | Pas de Deux of an NO Couple: Synchronous Photoswitching from a Double-Linear to a Double-Bent Ru(NO)2 Core under Nitrosyl Charge Conservation[END_REF]. Infrared and UV/Vis spectroscopy at 100 K as well as DSC experiments from 150 K to 300 K at a heating rate of β = 4 K min -1

were performed as described in Chapter 2.

Results and Discussion

Infrared spectroscopy

An important question that arises in dinitrosyl complexes is whether the photoinduced linkage isomerization involves both nitrosyls ligands coupled as a single PLI state or as a double PLI of two independent nitrosyl groups, thereby possibly leading to multiple PLI. In the GS, the NO ligands have stretching modes associated with the bands at 1612 cm -1 and 1657 cm -1 at 100 K Figure 3.14, in good agreement with the results of [126], which reported the ν(NO) bands at 1615 cm -1 and 1665 cm -1 . Considering the fact that two oscillating N-O bonds are, however, shared by a common ruthenium atom-the ν(NO) vibrations are coupled in symmetric and asymmetric modes resulting in νsym(NO) and νasym(NO). For the tetrahedral dinitrosyl complex [Ru(NO)2{Fe(C5H4PPh2)2}] [START_REF] Arulsamy | Group 8 and 10 hyponitrite and dinitrosyl complexes[END_REF] these NO stretching frequencies were reported at ν(NO) = 1630, 1673 cm -1 . Five-fold coordinated dinitrosyl ruthenium complexes exhibiting vacant octahedron and trigonal bipyramidal type structures have been reported by Gallien [START_REF] Gallien | Synthesis[END_REF], with νsym(NO) and νasym(NO) ranging from 1794-1842 cm -1 and 1681-1776 cm -1 respectively.

In the first step, the optimal spectral range for photo-excitation was determined by systematically testing photoexcitation in the wavelength range of 365-735 nm. The maximal photo-excitation was reached between 556 nm and 590 nm (Figure 3.14). for the PLI, we obtain the difference in vibrational frequencies of ν = -147cm -1 and ν = -167 cm -1 . From the decrease of the area of the GS bands at 1612 cm -1 and 1657 cm -1 , the total PLI population is found to be 50%. The PLI state generated by irradiation in the yellow-green spectral range can be transferred back (partially) to the GS by irradiation with red light. Figure 3.15 illustrates this effect for the subsequent irradiation with 556 nm and 660 nm. Both bands of PLI at 1445 cm -1 and 1510 cm -1 decrease, and correspondingly, both bands of the GS at 1612 cm -1 and 1657 cm -1 increase synchronously, indicating a single PLI state. To investigate in more detail whether we deal with a single or a multiple PLI, the population behavior was monitored as a function of irradiation fluence Q = I t for the irradiation wavelength 556 nm. Figure 3.17 shows the corresponding spectra, where a synchronous decrease of the two GS bands is observed together with the increase of the two PLI bands.

Analysis of the area

Details of the UV/Vis spectroscopic investigation

Upon irradiation with 590 nm, we observe that the GS bands (at 540 nm and 450 nm) decrease and new bands of MS arise (clearly visible at 485 nm, probably also a weak one above 600 nm). This is consistent with the observation of the maximum population by irradiation between 556-590 nm (Figure 3.19).

Photocrystallography

Single-crystal X-ray diffraction data were collected on a SuperNova microfocus-source diffractometer (Chapter 2), at 100 K. A single crystal was mounted on a micro-gripper loop using vacuum grease. In the first step, diffraction data were collected at 100 K in the ground state. Another single crystal was mounted on a glass fiber using vacuum grease for the measurement of PLI. The sample was then irradiated at 100 K with a laser diode of 590 nm (P = 1.6 mW) for 60 min in the complete dark until the photo-stationary state was reached, the sample is continuously rotated during the irradiation. According to the infrared results, at this temperature and irradiation wavelength, the metastable PLI-state can be populated with a sufficiently high population to enable a meaningful photocrystallographic experiment. Relevant data are given in Table 3.6.

Parameters of the photocrystallographic investigation: the GS

The crystal structure of [Ru(NO)2(PPh3)2] complex consists of distorted tetrahedral conformation Figure 3.21 in which two trans phosphine ligands and two apical nitrosyl groups are centered with ruthenium atom. A structural model was constructed based on the qualitative information obtained from the photodifference maps considering two molecular entities, the unreacted GS species and the PLI state (keeping in mind that from IR spectroscopy, about 50% population of PLI are expected).

Both the nitrosyl ligands are considered as make part of one PLI state. This implies that the structural change to be modeled on one molecule involves new positions for both NO ligands, which then necessarily exhibit the same population. In this respect, several refinement strategies corresponding to the four different possible combinations of κNand κO-bonded nitrosyls were applied to consider the subtracted electron density from the total occupancy of GS in order to obtain the most relevant three-dimensional structural model in PLI state.

Nomenclature and main characteristics of the tested models with agreement factors are listed in Table 3.6 and Table 3.7. an intermolecular distance of 2.431 Å that has been significantly decreased to 2.360 Å after photo-excitation. This suggests that the interactions in the PLI state get slightly stronger in contrast to the interactions in the GS, which might help stabilize the PLI configuration.

Correspondingly, the hydrogen bonding moderately gets stronger in C33-H33•••O1 ii and C27-H27•••O2 iii from 2.473 Å and 2.657 Å in the GS to 2.416 Å and 2.652 Å, respectively, in the excited state. The newly introduced moieties O1B, O2B are stabilized by forming the C18--

H18•••O2B iv (dH18•••O2B = 2.570 Å), C27--H27•••O2B iii (dH27•••O2B = 2.638 Å), C21- H21•••O2B v (dH21•••O2B = 2.641Å) and C33-H33•••O1B ii (dH33•••O1B = 2.

Discussion and comparison with DFT calculations

In a recent work, Ghosh and Conradie classified dinitrosyl structures in terms of electronic and geometrical parameters [129]. III follows their rules for an {M(NO)2} 10 (M = 4d or 5d element) species with its large N-Ru-N angle and the 'repulso' arrangement of the nitrosyls (which means that the slight deviation of the Ru-N-O function from linearity results in an outward tilt of both NO ligands). Due to the only small deviation of the RuNO angle from 180°, III is an example of a dinitrosyl with two linear MNO functions. For short, III is an (l-NO-κN)2 species.

As illustrated above from the X-ray photocrystallographic results the (l-NO-κN)2 GS of III turns to the metastable (a-NO-κN)2 double anti-bent isomer upon irradiation with light in the green-yellow spectral range (590 nm). Contrary to previously observed PLI events in dinitrosylruthenium complexes we thus find a double linkage isomer, i.e., both NO ligands undergo a significant structural rearrangement upon light irradiation. This synchronous photoswitch of two NO ligands is supported by the results obtained by infrared spectroscopy.

The synchronous decrease/increase of ν(NO)sym/ν(NO)asym at 1657/1612 cm -1 for GS and viceversa at 1510/1445 cm -1 for the PLI, upon population/depopulation with green/red light as well as the synchronous behavior during thermal relaxation are strong evidence for a single PLI involving the synchronous switch of both NO ligands. Moreover, the DSC signal is also indicative of a single PLI, as it can be fitted with a single Arrhenius-type decay, with an activation energy EA of 0.63 [START_REF] Wendel | Physical Implications of a Fundamental Period of Time[END_REF] eV and a frequency factor Z of 8.0(3) • 10 11 s -1 .

To investigate the conceivable isomers of III, extensive DFT calculations using different functionals and levels of theory were performed by P. Klüfers [START_REF] Hasil | Pas de Deux of an NO Couple: Synchronous Photoswitching from a Double-Linear to a Double-Bent Ru(NO)2 Core under Nitrosyl Charge Conservation[END_REF]. Many variants of PLI were investigated. As a result, the experimentally found PLI state, with its double anti-bentbonding, is the most stable of the non-GS isomers. The related double syn-bent isomer is a local minimum as well but of markedly less stability. Table 3.8 gives the key parameters obtained from these calculations in comparison to the experimentally determined values. Table 3.8 : Mean distances and angles along the path GS-TS-MS. Calculated metrical and spectroscopic values from a BP86/def2-TZVP+D3+CPCM(toluene) approach, energies include a ZORA correction [START_REF] Hasil | Pas de Deux of an NO Couple: Synchronous Photoswitching from a Double-Linear to a Double-Bent Ru(NO)2 Core under Nitrosyl Charge Conservation[END_REF].

Note the limited precision of the MS exp metrical values due to the ≈1:1 disorder of GS and MS in the photocrystallographic experiment. Table 3.8 also includes the parameters of the transition state (TS) between the GS (Figure 3.21)

GS

and the MS (Figure 3.24). An animation of the vibration (Figure 3.26) with the most negative frequency of -835 cm -1 confirms the concerted motion of both nitrosyl ligands. Moreover, it is the entire Ru(NO)2 moiety, including the Ru atom which swings between the statesin agreement with the photodifference map in Figure 3.23. Certainty that we have in fact detected the correct TS comes from another experimental method, the above-mentioned DSC investigation. The calculated EA entries in Table 3.8, the difference between the TS and the MS energies, are close to the measured activation wall of the decay of the MS. The DFT calculations provide additional valuable information concerning the mechanism for the photoswitching as well as the nature of the electronic configuration that helps stabilize the PLI. The electronic origin of the characteristics of the radiationless MS-to-GS transition, in particular the concerted nitrosyl motion, becomes evident from a look at the changes of the frontier MOs. Figure 3.27 shows the correspondence of the significant orbitals in terms of a CASSCF(8,8) approach with a small active space (see [START_REF] Hasil | Pas de Deux of an NO Couple: Synchronous Photoswitching from a Double-Linear to a Double-Bent Ru(NO)2 Core under Nitrosyl Charge Conservation[END_REF] for details). Three of the four doubly occupied orbitals exhibit a completely conservative change. Without changing the symmetry of the orbitals' interaction, the nitrosyl ligand rotates (from GS to MS: the upper one counterclockwise, the lower one clockwise; a so-called disrotatory motion) from the linear to the bent position (MOs 158, 160, 161). MO159 is an exception. If the nitrosyls perform their disrotatory motion in MO159 with its Ru-4dxz contribution, the bond in the GS turns to an antibond in the MS and vice-versa. As a consequence, the Ru-4dxz orbital is a largely metalonly MO in the TS (loss of a Ru-NO backbond). It is this balance of a symmetry-allowed transition of three out of four metal d-orbitals and a symmetry-forbidden transition of the fourth that we see as the origin of the transition's characteristics, specifically the radiation-less decay of the MS. On the one hand, the conservative part assures a not too high energetic wall towards the PLI state. On the other hand, there is in fact, an activation barrier that allows the isolation of the metastable isomer. The latter point makes a difference to most {MNO} 8 systems that lack this barrier (e.g., the platinum example mentioned in Chapter 1). A closer look at the conservative part unravels another decisive prerequisite for the symmetry-allowed motion of the nitrosyls: their turn has to be synchronous to keep track of a low-energy path of the hypersurface.

The loss of a Ru-NO bond in the TS matches with this proposed mechanism of the GS-to-MS transition. The excitation by green light i.e., 556nm, corresponds well with the calculated HOMO→LUMO excitation as the lowest-energy transition at 522 nm (2.376 eV).

Conclusion

In conclusion, we found, for the first time, a synchronous photo-induced linkage isomerization of two nitrosyl ligands of a dinitrosyl complex. Due to the high population of about ≈ 50% population of the metastable (up to 230 K) excited state, reliable experimental verification of the MS succeeded and showed the formation of a double anti-bent, κN-bonded MS from the originally double-linear GS. Specifically, the synchronicity of the MS's formation and decay is without precedence in PLI research. Computation unraveled the origin of the synchronous transformation. Only the synchronous disrotatory motion of the pair of nitrosyl ligands, pictorially the pas de deux of a nitrosyl couple, allows a smooth transition free of too many bond ruptures. Two factors are decisive for the transition path. The first is a symmetryconserving MO motion for most frontier orbitals. In electron-rich {MNO} n species, this fact is common and usually prohibits an activation barrier between bent and linear isomers, thus leaving only the more stable of the two 'ground-state candidates' isolable. As a peculiarity of III and as a second factor, one of the metal's d orbitals experiences a bond-to-antibond transition on the motion of the two nitrosyls with the necessity to break one Ru-NO bond in the transition state. As a result, there is an activation barrier that results in the two stable isomers, the double-linear GS and the double-bent PLI. Moreover, due to the symmetryconserving first factor, the motion has to be synchronous and disrotatory. This type of motion might find application in molecular motors since low doses of yellow light can switch it on, thereby inducing a movement in one direction.

Steady-state photocrystallography with pulsed laser Irradiation

The Chapter is divided into two parts: In the case of the nitrosyl (NO) photoinduced linkage isomers, the earliest studies were performed with neutron diffraction by Woike and coworkers [START_REF] Rüdlinger | Ground-and light-induced metastable states of sodiumnitroprusside[END_REF][START_REF] Rüdlinger | Light-induced structural changes in sodiumnitroprusside (Na2(Fe(CN)5NO).2D2O) at 80[END_REF] and later by X-ray diffraction by Coppens and coworkers [102], but they did not find the linkage isomer model.

The pioneering work in steady-state photocrystallography giving evidence for the existence of two photoinduced linkage isomers in SNP (Na2[Fe(CN)5(NO)]H2O) was carried out by

Coppens in 1997 [START_REF] Carducci | Diffraction Studies of Photoexcited Crystals: Metastable Nitrosyl-Linkage Isomers of Sodium Nitroprusside[END_REF]. In this work, CW irradiation of the single crystal was used for the generation of both type of PLIs i.e. 488 nm for MS1 and 1064 nm for MS2. Coppens and his coworkers expanded the work to a variety of metal nitrosyl complexes that exhibit the formation of PLIs in a steady state [START_REF] Coppens | Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes[END_REF][START_REF] Fomitchev | X-ray Diffraction Analysis of Geometry Changes upon Excitation: The Ground-State and Metastable-State Structures of K2[Ru(NO2)4 (OH)(NO)[END_REF]. In a parallel series of metal nitrosyl complexes, The population of photoswitched molecules depends on several factors, notably the crosssections for the forward GS to PLI process and the backward PLI-to-GS process at the excitation wavelength (in steady-state conditions). In an ideal case, there would be only the forward process, and the switching efficiency would be one if the quantum yield is considered one. Under these conditions, one would need one photon to switch one molecule, and we can thus estimate the minimum number of photons required in order to switch 100% of the molecules in a given crystal volume. The number of photons nphotons in a single laser pulse can be obtained from the following formula:

𝐸 𝑝𝑢𝑙𝑠𝑒 𝑙𝑎𝑠𝑒𝑟 = 𝑛 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 ℎ𝑐  (4.1)
with the Planck constant h = 6.62607015×10 -34 Js, c = 299792458 ms -1 the speed of light, and λ the wavelength of the laser light. In general, we use (in terms of surface) only a fraction of a laser pulse, for which we determine the energy density according to the formulas given earlier.

We thus have:

𝑒 𝑝𝑢𝑙𝑠𝑒 𝑙𝑎𝑠𝑒𝑟 = 𝐸 𝑝𝑢𝑙𝑠𝑒 𝑙𝑎𝑠𝑒𝑟 A (4.2)
with A the reference surface used for determining the energy density. If we now shine the laser pulse on a crystal with surface area Acrystal, the energy contained in this area of the laser pulse is given by:

𝐸 𝑝𝑢𝑙𝑠𝑒 𝑙𝑎𝑠𝑒𝑟 𝑜𝑛 𝑐𝑟𝑦𝑠𝑡𝑎𝑙 = 𝑒 𝑝𝑢𝑙𝑠𝑒 𝑙𝑎𝑠𝑒𝑟 𝐴 𝑐𝑟𝑦𝑠𝑡𝑎𝑙 (4.3)
So that we end up with the number of photons entering on a crystal with surface Acrystal

𝑛 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑜𝑛 𝑐𝑟𝑦𝑠𝑡𝑎𝑙 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 = 𝑒 𝑝𝑢𝑙𝑠𝑒 𝑙𝑎𝑠𝑒𝑟 𝐴 𝑐𝑟𝑦𝑠𝑡𝑎𝑙  ℎ𝑐 (4.4)
In order to estimate whether this number is sufficient for excitation of a majority of molecules present in the probed crystal volume Vcrystal, one should compare this number to the number of molecules Zmolecules in the crystal.

𝑍 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 = 𝑉 𝑐𝑟𝑦𝑠𝑡𝑎𝑙 𝑍 𝑢𝑛𝑖𝑡 𝑐𝑒𝑙𝑙 𝑉 𝑢𝑛𝑖𝑡 𝑐𝑒𝑙𝑙 (4.5)
where Zunit cell is the number of photoswitchable molecules per unit cell, and Vunit cell and is the unit cell volume. So, we should make sure that nphotons on crystal surface > Zmolecules in order to have a reasonable chance of detecting some molecules in the photoswitched configuration, primarily if we use only a single laser pulse. In steady-state conditions, when the photoswitched molecules remain in their configuration, one can otherwise calculate the number of pulses needed to obtain a sufficient high photon number. This is important, as we also have to consider some serious drawbacks of the pulsed laser excitation. Especially, irradiation with a pulsed laser with its high peak intensity can lead to significant heating of the crystal, with all kinds of after-effects (rapid crystal damage, photo-bleaching, etc.).

Generally, the criteria considered for obtaining the maximum photoconversion are crystal absorbance max, crystal thickness d, proper choice of excitation wavelength irr , laser power P, irradiation time t, and photoconversion efficiency. The maximum pulsed laser power used for the generation of PLI is a critical parameter to handle due to the dissipation of heat inside the crystal induced by the laser pulse. This effect gets even more pronounced when repeated laser pulses are used to achieve the maximum population of the excited state i.e., saturation of the photo-stationary state, resulting in the degradation of the sample with time. It is, nevertheless, very difficult and itself often a question of empirical finding to estimate the incident laser intensity that a 'particular crystal' can tolerate.

To address this issue to some extent, a systematic study was performed with optical absorption spectroscopy to determine the acceptable tuning range of the power of the pulsed laser. A drastic illustration of the damage that can be done by pulsed laser irradiation on a crystal is shown in goniometer equipped with an ATLAS CCD detector as has been described in Chapter 3. Unit cell determination and data reduction was performed using X-AREA program [START_REF] Stoe | X-Area[END_REF]. Structure solution and refinement was performed by using Olex2-1.5-alpha [START_REF] Dolomanov | OLEX2 : a complete structure solution, refinement and analysis program[END_REF].

For the irradiation and generation of PLI in I & II, an optical parametric oscillator (OPO) Nd-YAG pulsed laser system (Ekspla NT340 series) was used as the pump source. The short pulse duration of 5-10 ns and the ability to tune its output from 210 nm to 1800 nm have provided a wide selection range. For our experiment, we used  = 420 nm as reported with CW laser [START_REF] Mikhailov | Combining photoinduced linkage isomerism and nonlinear optical properties in ruthenium nitrosyl complexes[END_REF]. The beam was aligned on the sample position by using a 500 m pinhole, and the maximum power at the sample position with this setup was determined with a pulse-power

Optical absorption spectroscopy of single crystal

If the photoinduced species have different absorption spectra compared to the ground state molecules in the visible region, the progress of the conversion from ground state to photoinduced species can be monitored by using optical spectroscopy. The characterization by Optical Absorption (OA) spectra of the single crystal will aid in revealing the differences/similarities in the CW and pulsed laser irradiation procedures and therefore help to analyze the structure-property relationships. The alignment of the lamp source spot size was done in such a way that the spot size of the lamp on the crystal is smaller than the crystal surface (see Figure 4.5) in order to ensure that only light transmitted through the crystal is captured by the optics. The waveband, power focus, and aperture can be controlled by putting filters, lenses, and pinholes on the optical benches, as illustrated in Figure 4.4. The proper alignment of all elements on the optical path from the lamp to the fiber entrance is critical to ensure good absorption spectra. Slight offsets from the optical axis will create losses at the fiber entrance and less sensitivity in the OA measurement.

The spectrophotometer is further connected to a computer to record live spectra of the crystal at 100 K. Diffractometer's camera was used to record the live images of the crystal after irradiation with the pulsed/CW laser. For further details of the methodology of single crystal optical absorption spectroscopy, see also the set up illustrated in Chapter 2. For the generation of PLI, the crystal is irradiated with the light of the pulse laser, aligned by using a set of mirrors followed by a /2 plate, a polarizer, and lenses to focus and fine-tune the incident spot of the pulse laser. The experiment is then conducted as follows. After excitation with one or several laser pulses, the optical absorption spectra of the PLI are recorded with the lamp probe beam. Then a picture of the crystal is taken with the camera in order to monitor the photoinduced changes and possible damage on the surfaces, and finally, X-ray diffraction data are collected to determine the photoinduced structural changes.

Comparative photocrystallographic study of I with CW and pulsed irradiation

As a first step, steady-state analysis was performed in a "classical" manner, i.e., by irradiation of the crystal with continuous wave irradiation. For the population of MS1 a CW laser of 420nm wavelength was used with an average power of P = 120 mW on a surface of A = 0.7807 cm 2 , for a total Q value of 1200 J/cm 2 . In order to follow the population of MS1, several absorption spectra have been recorded as a function of Q (see Figure 4.7). For each time point also, a photo of the crystal has been taken to illustrate the color change and spot potential damage or other sources of problems (e.g., icing, which leads to diffusion of light and hence affects the OA spectra). As can be seen from Figure 4.6, the crystal color changes from yellow to green, and correspondingly, in the OA spectra the GS bands at 450 nm decrease, and new bands of MS1 at 700 nm arise. This is consistent with the published report of Mikhailov et al., [START_REF] Mikhailov | Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes[END_REF]. Note that the OA spectra for this case are limited in the GS to the spectral range 450-850 nm, due to the strong absorption of GS below 450 nm. With increasing population of MS1, and decreasing GS, the band at 450 nm decreases, and spectra can be measured down to 400 nm.

Although the measurement conditions are not as clean as in a two-beam spectrophotometer, we can try to calculate the absorption coefficient for MS1 from the observed increase of the band at 700 nm. The absorbance A of the crystal is given by A = αd, with α the absorption coefficient and d the thickness of the crystal. Correcting for the baseline at A = 0.15 (it is not at zero due to reflection and scattering effects on the crystal surface), we observe an absorbance A = 0.85 -0.25 = 0.60 in the final spectrum for Q = 1200 J/cm 2 . Knowing the thickness d = 0.02 cm of the crystal, we obtain αMS1(700 nm; Q = 1200 J/cm 2 ) = 30 cm -1 . If we know the population of MS1 we can then calculate the "true" absorption coefficient of MS1, since GS does not absorb at this wavelength. However, we need to be aware of the limited accuracy of the method, e.g., shifts in the baseline or errors in the determination of crystal thickness linearly modify α. After achieving a value of Q = 1200 J/cm 2 X-ray diffraction data was collected, and photodifference maps were calculated (see Table 4.1). The maps show the typical features for the isonitrosyl PLI, a depletion of electron density on the GS positions O1 and O2 (two independent molecules in the unit cell), and some additional electron density around the N1 and N6 positions. The corresponding structural model Table 4.2 was refined, resulting in a population of 71(2)% for molecule 1 and 92(2)% for molecule 2, in good agreement with published data [START_REF] Mikhailov | Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes[END_REF]. J/cm 2 . The photodifference map (Table 4.3) shows the typical features of the side-on nitrosyl configuration, a depletion on the GS NO positions, and new electron density peaks at about 90° orientation with respect to the GS structure. The structural refinement yielded an MS2 structure with a population of MS2 of 17( 5)% for molecule 1 and no refinable population for molecule 2 (Table 4.4), as reported by [START_REF] Mikhailov | Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes[END_REF].

Table 4.3 : Photodifference map for I after generation of MS2 with 940 nm, Q = 923 J/cm 2 . The contours level for both the maps is 0.2 eÅ -3 . In CW, MS2 with the contours level for both the maps is 0.

Mol. 2 NO refineable MS2!

The results of the structural refinements and corresponding statistics are given in Table 4.5 and in comparison with GS and MS1. We find that the structures agree with those published [START_REF] Mikhailov | Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes[END_REF].

Another crystal of [Ru(Py)4(NO)F](ClO4)2 was used for the study of MS1 and MS2 generated by pulsed laser irradiation. For the population of MS1 we used the same wavelength as for the CW study, 420 nm, but this time using the EKSPLA pulsed laser (f = 10 Hz) 5 ns pulse duration, and adjusted the average power to P = 2.5 mW. This corresponds to an average intensity on the crystal of I = 35 mW/cm 2 .

Irradiation was performed for a total duration of 570 minutes, yielding a total fluence of For the population of MS2 (transfer starting from MS1), we used the same wavelength as for the CW study, 940 nm, and EKSPLA pulse laser (f = 10 Hz), with adjustment of average power to P = 2 mW. This corresponds to intensity on the crystal of I = 28mW/cm 2 (d of spot size=0.3 cm). Irradiation was performed for a total duration of 120 minutes, yielding a total fluence of

Q = can fit P(Q) = Pmax(1-exp(-Q/Q0),
Q = 202 J/cm 2 .
The structural analysis follows the usual procedure. From the photodifference maps we can again see clearly the features of the side-on nitrosyl configuration, this time with clear differences for both of the two molecules in the unit cell. The corresponding refinement statistics of the MS2 model and its parameters are given in Table 4.8. Table 4.4 illustrates the final result for the structural model of MS2. For molecule 1, the obtained structure corresponds to the one published [START_REF] Mikhailov | Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes[END_REF]. For molecule 2, we also find an MS2 side-on structure with similar population as for molecule 1. There is, thus a significant difference to the CW results, where no MS2 was found for molecule 2. The most probable origin for this difference is the fact that we use two different light sources for the generation of MS2. In the CW experiment, starting from MS1, we transfer by LED light, while for the pulsed experiment, we use the laser line from the OPO. The LED light is not monochromatic, contrary to the laser light, which has a comparatively narrower bandwidth. Looking at the emission spectrum of the Thorlabs M940L3 LED, we find quite a large bandwidth centered around 940 nm, with a rather long tail in the high energy side, down to almost 800 nm. It seems that for I, the two molecules of the unit cell have slightly different absorption properties, which leads to the depopulation of molecule 2 when irradiated with the broad-band LED. When using the narrower laser line at 940 nm, MS2 can be produced in both molecules. If one would do a systematic study of this transfer as a function of wavelength (using the OPO for finely tuning the wavelength) one could therefore probably detect the differences in the absorption spectra of the two molecules.

As we have seen now that in I MS1 and MS2 can be generated by pulsed irradiation, I might be an appropriate candidate for a time-resolved diffraction study. This compound has a low symmetry and therefore, it will be hard to collect complete data sets in time-resolved mode, which would then allow the calculation of photodifference maps. Identifying the most sensitive Bragg reflections, i.e. the reflections with the most important change in intensity, might then still allow following the evolution of the structure as a function of time. There is no published information on the absorption properties of MS1 and MS2 for [Ru(NH3)5(NO)]Cl3•H2O. Some spectra have been collected by D. Schaniel on single crystals, and the results are illustrated in Figure 4.9 for a c-cut. The GS band is found at 465 nm, MS1 exhibits two bands, at 610 nm and 825 nm, while for MS2 a very strong absorption is observed at 470 nm and a second band at 750 nm. Clearly, the bands of MS1 are rather weak, it will thus be challenging to detect them in our setup.

Continuous Wave laser Irradiation Experiment

The XRD experiments under CW irradiation have already been described in chapter 3, based on measurements on the Supernova diffractometer. Here, only OA spectra were collected on the STOE-STADIVARI setup. Irradiation was performed with the 420 nm LASER at an average power of 100 mW (on the surface of 0.7807 cm 2 ). The total exposure time was 45 minutes, resulting in Q = 346 J/cm 2 . The transfer of MS1 to MS2 was performed with 980 nm LED, at an average power of 75 mW for a total exposure of 40 min resulting Q = 230 J/cm 2 .

The spectra are shown in Figure 4.11 and Figure 4.12. During the generation of MS1 with blue light, we observe the decrease of the GS band at 465 nm and the increase of the band around 610 nm. Beyond 700 nm, the spectra for this crystal are not of very good quality, so one cannot confirm the presence of the second band of MS1. The transfer of MS1 to MS2 was investigated on another single crystal, first MS1 was produced with 420 nm irradiation as before, and then transfer to MS2 was done by irradiation with 980 nm (Figure 4.12). In this crystal, the OA spectra are of very good quality, and one can even measure over the GS band at 465 nm, which clearly decreases when populating MS1. For MS1 again, the band at 610 nm is clearly visible.

Upon transfer, to MS2 the absorption rises very strongly, and the optical density is too high below 620 nm, in agreement with the above-reported measurement collected on a two-beam spectrophotometer. As for MS1, the MS2 bands arising around 730 nm and the very strong absorption in the blue-green agree with the previous findings.

Pulsed laser Irradiation Experiment

First, the GS was measured, the results are given in Figure 4.11 and Figure 4.12. For the population of MS1 we used the same wavelength as for the CW study, 420 nm, but this time from the EKSPLA pulse laser (f = 10 Hz) using the power P = 2.67 mW. This corresponds to an intensity on the crystal of I = 38 mW/cm 2 . Irradiation was performed for a total duration of 90 minutes, yielding a total fluence of Q = 205 J/cm 2 . As can be seen from the spectra, there is in the first spectrum increase at 750 nm, before the expected band around 610 nm appears more clearly in the subsequent spectra. Overall the absorption increases between 520 and 850 nm, but its rather featureless above 700 nm. Probably there is some increase/modification in the baseline. However, compared to the spectra obtained previously, such a featureless increase might be expected, as the two bands at 610 and 820 nm are weak and of approximately the same amplitude.

Moreover, in our setup, we are limited to 850 nm, and the near-infrared range 800-850 nm is certainly the most affected by scattering phenomena due to icing, as the detection sensitivity is also limited in this range. The OA spectra collected after the generation of MS2 (transfer from MS1 with 940 nm, 2.5 mW, 240 min, Q = 504 J/cm 2 ) are shown in Figure 4.12. The absorption in the blue-green spectral range is again too strong to be measured below 700 nm, but clearly, a band at 730-750 nm arises, in agreement with the previous CW measurements. in between the two "expected" positions from the CW measurements. Experimental evidence for this two-step process of generation of MS1 in [Ru(Py)4(NO)F](ClO4)2 has been given by [START_REF] Mikhailov | Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes[END_REF] using a combination of low-temperature spectroscopy after CW irradiation and room-temperature transient absorption and infrared spectroscopy after pulsed laser irradiation. These results can be summarized in an energy scheme for the [Ru(NH3)5(NO)]Cl3•H2O complex (Figure 4.14). From this scheme it becomes clear that MS2 needs to exhibit a certain lifetime for that the second step can take place. If MS2 relaxes before the second photon arrives, MS1 cannot be populated. Two possibilities occur when exciting with nanosecond laser pulses: i) the two steps are generated within one laser pulse (5 ns long), since MS2 is occupied within a few picoseconds, there is, in principle, the possibility that during the 5 ns long excitation pulse, both steps occur; ii) the two steps are generated by subsequent laser pulses, in this case the lifetime of MS2 needs to be longer than the time delay between two laser pulses, i.e. 100 ms if one uses 10 Hz repetition rate of the laser. We measured CW and pulsed generation of MS1 and its transfer to MS2 at 150 K, 170 K, 230 K, 250 K, 260 K and 270 K. At 150 and 170 K both states have a sufficiently long lifetime time for detection by optical spectroscopy, and as can be seen from Figure 4.15, both for CW and pulsed irradiation MS1 is produced by blue light irradiation. At 230 K still, both irradiation methods result in the population of MS1, as evidenced by the absorption band at 610 nm. At 250 K there is a hint of this band in the CW irradiation, while the pulsed irradiation spectra do not show any sign of this MS1 band. At this temperature the lifetime of MS2 is approximately 1 s (as measured in the TA experiments), and the lifetime of the MS1 is of the order of 1000 s, still longer than the measurement time. At 260 K and 270 K the lifetime of MS2 drops well below one second and therefore at these temperatures MS1 cannot be populated (Table 4.15).

Ru1-O1A-N1A O1A-Ru1-N1A - - 165 (2) 171.8(6) 173.6(10)
Also, the lifetime of MS1 is of the order of a few minutes at 260 K which is the time needed for the collection of OA single crystal spectra. These experiments are in agreement with earlier

CW MS1

T=260 K T=270 K

Conclusion and Perspectives

The main conclusion of this part is that with pulsed irradiation, the same type and amount of PLI can be generated as in CW. The most important consideration is the effective irradiance Q value of a particular complex under study. The number of photons required to achieve saturation population is the same in CW and pulse irradiation, and hence one can either increase intensity or irradiation time in order to reach the needed Q-value.

Part II

One-pulse single-shot steady-state photocrystallography

Studies of photoinduced linkage isomerization carried out in part I of the Chapter show that PLI's can be generated by using either CW or pulsed irradiation source types with very similar properties of photoexcitation, relaxation, product of photoconversion, and type of photoswitching. This opens the possibility of tuning the optical excitation flux density of the irradiation source to control the physical and chemical properties of materials that can undergo photo-switching. My objective, here in part II of this Chapter, was to possibly squeeze all the photons that a particular single crystal metal nitrosyl complex needed to reach its maximum photoconversion level, i.e., Q value needed for saturation population, into one single pulse. If one can achieve the maximum possible photoconversion with only one pulse in a single shot, this is for sure interesting for applications since the photochromic and photorefractive changes scale with the population of the PLI. But, the main purpose of this experiment was to investigate the effect of such a single-shot experiment on the sample properties, especially the achievable population, in order to apply it later for our in-house time-resolved photocrystallographic experiments [START_REF] Casaretto | In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector[END_REF]. This technique can also be combined together with other ultrafast techniques [START_REF] Chergui | Ultrafast Photophysics of Transition Metal Complexes[END_REF][START_REF] Chergui | Photoinduced Structural Dynamics of Molecular Systems Mapped by Time-Resolved X-ray Methods[END_REF]. Such one-shot-laser-pulse excitations are extensively investigated in samples exhibiting a photoinduced phase transition, see e.g. [START_REF] Tokoro | One-shot-laser-pulse-induced demagnetization in rubidium manganese hexacyanoferrate[END_REF][START_REF] Tokoro | Optical switching between bistable phases in rubidium manganese hexacyanoferrate at room temperature[END_REF] Single-shot one-pulse laser excitation experiments are challenging to perform and have their pros and cons.

Pros: Success in single-shot one-pulse excitation up to saturation will help to see the real-time structure dynamics in time-resolved and ultrafast mode.

Cons: thermal-induced effects are known to compete with actual photoinduced product. High flux-density and short laser pulse can lead to rapid crystal damage [START_REF] Garman | Radiation Damage in Macromolecular Crystallography Protein Crystallography Methods in Molecular Biology[END_REF], sample heating, inhomogeneous irradiation, and a more biased result of refined photoconversion due to thermal smearing of ADP's. In some cases, one pulse irradiation starts heating and several other photoinduced processes (e.g. photorelease) may occur in competition to the aimed PLI generation, which ultimately results in low photoexcitation product or sometimes photobleaching.

To avoid the constraint of pump pulse laser damage, Raithby group and co-workers [START_REF] Hatcher | LED-pump-X-ray-multiprobe crystallography for sub-second timescales[END_REF] used pulsed LED (P = 23 mW) with pulse lengths of ms. As in this case, the pulse length is relatively long and the average power relatively low, this will on the one hand not allow for investigation of fast processes, and cost, on the other hand, a longer time to collect a complete threedimensional crystal structure in ms-to-s time-resolved photocrystallography. I tried to solve this challenge by opting for the single-shot one-pulse excitation so that later I could use this study to monitor the photoinduced process in real-time. For the success of the single-shot onepulse excitation photo-crystallography experiment, I opt for the following methodology:

(a) I choose two compounds (one with high excitation efficiency and a second with low excitation efficiency) on which I already have a lot of information about their tolerance level towards irradiation sources with pulse and CW laser as described in Part I. This information will pave the way to really understanding the appropriate range of pulse laser power and length to reach maximum photoconversion without damaging/swelling or photobleaching the crystal. It is to be noted here that this cannot be used for another type of crystal unless systematically determined, as each crystal has its characteristic damage threshold towards pulse laser power.

(b) Secondly, I repeated measurements on different crystals by using a range of fluence to validate and avoid the ambiguity in thermal-induced heating and actual photoconversion product, i.e., population. This helped to better tune the appropriate pulse laser energy within the required need of the experiment.

(c) Thirdly, a comparison of ADP's before one pulse, after one pulse, and after bleaching all the photoinduced population, i.e., relaxed GS, will help to better compare the photoinduced against the ground state. A comparison of the most sensitive Bragg reflection is made showing the highest difference in intensity from ground state to excited state.

(d) Approximate calculation of number of photons with respect to number of molecules in a crystal volume under study in order to better assess the photoconversion efficiency.

(e) Lastly, the validation of the single shot ns-pulse photocrystallography experimental results validation with the help of ns-infrared spectroscopy.

The dilemma of precision and accuracy in the population of Excited states

What are the number of molecules in the excited state and the number of molecules in the ground state? With different experimental tools, the results are different.

The population of the photoexcited molecules in the infrared spectroscopy is different compared to the population recorded in the absorption spectroscopy, and it even varies by measuring the optical absorption of the crystal on the goniometer and also with the refined PLI's population with the SC-XRD. What is then the real population globally? How close is this precision of population to accuracy in different experimental techniques? The overall refinement result by SC-XRD is biased due to the fact that in a diffraction measurement obtained after in-situ irradiation of the crystal, the results give a spatially and/or temporally averaged picture between the excited state and ground state. It is also due to the fact that absorption coefficient of X-rays is different compared to the absorption coefficient of ultraviolet, visible and infrared light in solid state. And therefore, it results in different populations of excited state in SC-XRD, UV-Vis, and IR spectroscopy. Therefore, careful attention is required to discern the real population of a PLI from different experiments, especially very minor contributions of a small factor (like icing, the temperature of the crystal, grease, crystal thickness, crystal polarization axis with respect to irradiation plane, back scattering in OA, different pellet thickness as well as human and experimental errors) can contribute to the real results.

Experimental section

Photocrystallography

The single crystals of [Ru(NO)(Py)4F](ClO4)2 I and [Ru(NH3)5(NO)](Cl)3•H2O II complexes were preferably selected for the experiment. The Ru(NO)(Py)4F](ClO4)2 complex [88] I has high efficiency for photoinduced population ~85% and changes color after irradiation, and is, therefore, an interesting candidate for that a small change after one pulse irradiation will be possibly detectable. The [Ru(NH3)5(NO)](Cl)3•H2O II complex has relatively moderate/lower excitation efficiency and can undergo ~50% photoconversion. Data collection was performed in a similar way as described using STOE Stadivari diffractometer equipped with RebirX detector, using Mo K radiation ( = 0.71073 Å) at 100 K. Unit cell determination and data reduction were performed using X-AREA program [START_REF] Stoe | X-Area[END_REF]. Structure solution and refinement were performed by using Olex2-1.5-alpha [START_REF] Dolomanov | OLEX2 : a complete structure solution, refinement and analysis program[END_REF].

Excitation source: For exciting the crystal sample EKSPLA NT342 pulse laser with a pulse duration of 5 ns was used with a wavelength of 420 nm at a frequency of 10 Hz. Beam focusing and set up is the same as shown in part I of this chapter.

Infrared spectroscopy

Infrared spectroscopy of both the [Ru(NO)(Py)4F](ClO4)2 I and [Ru(NH3)5(NO)](Cl)3•H2O II complexes was performed using Nicolet 5700 FT-IR spectrometer with a resolution of 2 cm -1 at 100 K. The crystal samples were finely grounded, mixed with KBr, pressed into pellet, and finely bonded onto the cold finger of the cryostat using silver paste. For more details on the experimental section, see Chapter 2.

Excitation source: For exciting the crystal Surelite II-10 model, Nd-YAG solid-state laser pumped by a flash lamp, completed by an Optical Parametric Oscillator (OPO, model SLOPO +) with pulse duration of 5 ns was used with a wavelength of 420 nm at a frequency of 10 Hz.

Beam alignment was done by using mirrors, see the experimental set up in Chapter 2.

Low excitation efficiency: one pulse single-shot steady-state photocrystallography in I

Single crystal GS and one pulse shot data collection was performed at 100 K using  = 420 nm (corresponding to 2.952 eV) as reported with CW laser [START_REF] Mikhailov | Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes[END_REF]. The beam was aligned by using 500 m pinhole (Figure 4.20), and the peak power position of the pulse laser was determined at the sample position with power meter to P = 45 mW and I = 23 W/cm 2 . In one second, we have thus 2.3 J/cm 2 average fluence on the sample, and since the repetition rate is f = 10 Hz, i.e., there are 10 pulses in 1s, we obtain 2.3 J/cm 2 in one single laser pulse. The crystal was irradiated with one pulse (irradiation time = 5 ns). Table 4.16 shows these parameters in comparison to those used for the generation of the saturation population, discussed in part I of this chapter Figure 4.2.

(a) Damage threshold of I towards one-shot ns pulsed laser

One laser flash photoinduced conversion is often known to compete with the thermal-induced effects or internal heating of samples. To perform single-pulse one shot experiment particularly in single crystals, the first crucial step is to convert the saturation fluence value into one pulse in such a way that crystals have the ability to absorb this much power by ns-pulse laser with maximum photoconversion without too much laser heating, is a very tricky job. However, a systematic trial mostly helps to end up with a choice of a conclusive laser power range that can be used to excite the single crystal within a good compromise of laser damage and saturation fluence. Here, in the present case of I single crystal, I end up choosing the P = 45 mW with fluence of 2.3 J/cm 2 as an appropriate choice in the light of successive trials of a series of different fluences and resulted data collections.

(b) Single shot one pulse Photocrystallography of I

From the single crystal X-ray diffraction data of GS and PLI after single-shot with ns pulse laser, photodifference maps were calculated (Figure 4.16) for visualization of the light-induced changes in electron density and for the identification of the possible light-induced structural changes from GS to one ns-pulse shot PLI. From the inspection of calculated photodifference maps for Molecules 1 and 2, it can be seen that after one ns-pulse irradiation, the most pronounced difference in electron density can be observed by a shift of the heaviest central Ru atoms in the direction opposite to GS nitrosyl i.e. N1O1 in Molecule 1 and N6O2 in the Molecule 2. In addition, for both molecules, there is a depletion of the GS positions of the nitrosyl ligand. For molecule 1 (left panel in Figure 4.16), it can be observed also some positive From the above observations, there are two possibilities, either population of MS1 or the population of MS2. This might be expected in a one-pulse experiment since in the two-step excitation towards MS1, some molecules might remain in MS2 as they "see" only one photon.

It is then a question of the relative efficiency of the two steps. Conclusively, it can be reasoned by carefully looking at the most probable model. Therefore, another refinement was done, assuming no PLI at all, i.e. just refining a GS model to the excited state data. The three models are compared in Table 4. [START_REF] Coppens | The New Photocrystallography[END_REF]. In terms of refinement statistics, they are equivalent, but the MS2 model shows a somewhat higher residual density. The forced GS model compares quite well with the pure GS, only an overall small increase of the ADPs of N and O is observed. Contrary to the relaxed GS, measured after quenching to room temperature and back to 100 K, which shows a rather strong increase of all these ADPs (doubling). The MS1 refinement looks reasonable compared to the previously obtained results for this case, only the Ru1-O1B-N1B

and Ru2-O2B-N6B angles are smaller by 20° and 10°, respectively. The ADPs of N1B/O1 and N6B/O2 are a bit smaller than in the GS, which could indicate an overestimation of the MS1 population. In the MS2 model the resulting Ru1-N1A distance of 2.29(3) Å is rather long compared to the 2.11(2) Å obtained in the previous pulsed measurement of MS2 in I. Also, ADPs of N1B/O1B are strongly increased. Overall, the XRD is rather inconclusive, the forced GS and MS1 models seem to fit the data, even though MS1 population is probably overestimated. The MS2 model seems to be the least probable.

The second possibility can be by looking at the number of photons arriving on the sample in comparison to the number of molecules present in the sample. In the present case, this possibility also favors one or the other model. In the given conditions, about 2 photons arrive for 6 molecules (see Table 4.16). Therefore, if the conversion efficiency is 1, it could be expected at best 33% of switched molecules. For the two-step process, 2 photons are required to switch from GS to MS1, so approximately 17% of molecules can be switched. This is a bit less than the 27(3)% and 22(3)% refined MS1 model (see Table 4.17 and Table 4.18). So, either the conversion efficiency is close to 1, or this model, accidentally, is able to describe the structure. For the MS2 model, only about 3.6% of one of the molecules in the unit cell are switched, so 1.8% of all the molecules in the crystal. For switching GS to MS2 we need one photon, so in this case the switching efficiency would be about 5%. For comparison, in the steady-state experiment leading to saturation, about 170 photons were necessarily required to switch one molecule to MS1 (Table 4.5) and in this case, about 90% were switched. So, the overall efficiency for the GS to MS1 process would be about 0.6%. Given this reasoning, the second model with GS and MS2 seems more probable.

Table 4.17 : Crystallographic parameters of GS, one pulse PLI and relaxed GS in I.

GS ONE Pulse PLI GS after ONE Pulse RELAXED Crystal data

Chemical formula 2(C 20 H 20 FN 5 ORu)•4(ClO 4 )

Crystal system, space group Triclinic, P-1

Temperature (K) 100 a, b, c (Å) 10.4720(5), 16.4511(9), 16.5125(9) 10.4685(14), 16.4511(18), 16.5555(19) 10.5154(10), 16.7751(18), 16.6657(17)
U eq N1, U eq N1B 0.0138(

U eq O1, U eq O1B 0.0222(

(c) Relative intensity variation of most sensitive Bragg peaks in I

The structural response of a crystal can be investigated by the relative intensity variation of Bragg peaks from GS to PLI generated after irradiation with one pulse. This is especially useful if one knows the model of the PLI, e.g. from a previous CW measurement. It allows to identify the Bragg peaks that are most affected by the structural reorganization and thus offers an alternative approach to check whether possible PLIs have been generated or not. Amongst the most intense reflections, one finds 230 and 2-42, which increase by 1.6% and 3.8%, respectively. The reflection -124 decreases by 2.6%. The reflection 200 decreased by 0.6%, which can be compared to the observed decrease in the steady-state experiment discussed before, where a decrease of 11% was observed for a population of ~85%. So, this could indicate a possible population of about 8% for MS1 in the present experiment. However, to confirm this one needs to check more of the sensitive reflections (see appendix D for tables with the most intense reflections). Some of them, like the 230 fit into the scheme, while others like the -124

show opposite changes, decreasing in the current experiment and increasing in the steady-state experiment.

(d) Infrared spectroscopy of I with ns pulse laser

In order to investigate in more detail, the question what type of PLI is produced with a single laser pulse, infrared spectroscopy at low temperature after pulsed laser irradiation was performed. As we have seen in Chapter 3, IR spectroscopy gives an unambiguous answer concerning the type of PLI, as GS, MS1, and MS2 vibrational bands have very distinct positions. For I, one expects the NO stretching vibration for MS1 at 1762/1768 cm -1 and the one for MS2 at 1565/1580 cm -1 . Figure 4.18 shows the result of I, after irradiation with pulses of 420 nm wavelength, an average power of 22 mW (for 0.81 cm 2 surface) at 10 Hz repetition rate. This corresponds to an average fluence of 27 mJ/cm 2 and thus 2.7 mJ/cm 2 per pulse. The irradiated surface of the KBr pellet is 1.326 cm 2 . In one pulse, there are thus about 7.57 x 10 15 photons arriving on the sample, which contains 1 mg of I, corresponding to 9.8 x 10 17 molecules (see Table 4.20). So, there is about 1 photon for 100 molecules, and with one pulse one could hope, in best case to populate 1% (if efficiency is 1). The IR spectra (Figure 4.18)

shown that after 4000 pulses 6.1% of the molecules have been transferred out of GS, either to MS1 or to MS2, as evidenced by the corresponding vibrational bands. In this case, there are about 30 photons per molecule available. Assuming only a population of MS1, and a two-step process, we get 15 photons/molecule resulting in 6.1% population, i.e., the efficiency is about 0.4% for generating MS1. If there was only MS2 produced, the efficiency would be 0.2%.

Looking at difference spectra, one can observe traces of MS2 after about 20-30 pulses, while MS1 appears after about 60-80 pulses. It seems thus that the first MS2 is populated before MS1 appears, in agreement with the two-step model for the generation of MS1. Comparing the estimated switching efficiencies for the MS1 (0.4%) and MS2 (0.2%) models with those from SC-XRD, where 100% was estimated for MS1 and 5% for MS2, they are closer to the MS2 model.

Overall, it seems that a relatively low population of MS2 can be generated in I upon singlepulse excitation, which does not allow free refinement of the structure from the measured Xray data set. The refined MS1 population in the XRD data is not supported by the IR experiments, even though a certain population of MS1 cannot be excluded. It can be concluded that I is not ideal candidate for time-resolved measurement, since it needs a relatively high Q value for generation of a significant population. Moreover, the crystal symmetry is relatively low, requiring long X-ray data collections. We should therefore search other candidates with lower Q value (higher photoconversion efficiency). As a second test case for the single-shot photocrystallographic experiment, a crystal of [Ru(NH3)5(NO)](Cl)3.H2O II complex was selected. This II complex has also a high efficiency for photoinduced population of MS1 ~50%; however, the color change in MS1 is not very pronounced, so that a small change after one pulse irradiation will probably not be detectable.

Single crystal GS and one pulse shot data collection were performed using three different

(b) Photocrystallography of II

In the one-pulse photocrystallographic experiment, I tried three different crystals Figure 4.21 of molecule II for one pulse experiment, using three different irradiation conditions as shown below.

(1) Crystal 1: P = 10 mW, 500 m beam spot, one pulse

(2) Crystal 2: P = 7 mW, 500 m beam spot, one pulse

(c) The relative intensity variation of most sensitive Bragg peaks in II

The most sensitive common reflection from GS to PLI after one pulse are 205, 040, and 322 in crystals 1, 2, and 3 (Table 4.26 and Figure 4.25). We note however that for 322 in crystal 1 the change is +15%, in crystal 2 it is -21%, and in crystal 3 it is +5%.

Results and Discussions

The fundamental objective of this Chapter of my thesis is first to understand the steady-state photocrystallography with CW irradiation source for photoexcitation. Secondly, reproduce the same possible photoexcited states and photoconversion with pulsed laser as an irradiation source. This second successful step will then pave the way of reaching to single-pulse one-shot steady-state photocrystallography. The best understanding can only be achieved, however, by rigorously using the same system of nitrosyl complex crystals in all three steps (Figure 4.27). Both the results from photocrystallography and infrared spectroscopy and calculations of incident photons per molecule in the crystal in the complex I with single pulse experiment indicate that MS2 could possibly be generated in low quantities, probably even a small amount of MS1. But it also became clear that I is not an ideal choice in these single-shot one-pulse experiments due to the rather high Q value needed for the population of the PLI.

For II, the photocrystallography and spectroscopic measurements are contradictory concerning the achieved population. The calculations of photons per molecule in both cases show that more photons are required for practical photoconversion, and in the present case, the number of molecules with respect to the incident photons is not compatible. Therefore, I am not sure about the refined population ending up always more than 30% (MS1) even by using different crystals and repeating experiments systematically in quite a lot of trials by interplaying the intensity and number of pulses. According to pulsed-IR experiment, the crystal should be excited with more power than the chosen power for irradiation. But as it is shown above, the maximum damage threshold towards pulse laser of II is 12 mW while using 500 micron pinhole.

However, in order to obtain good quality structures, one must consider the sensitivity of a particular crystal towards pulse laser power, as has been clearly shown by the different trials that I performed. Concerning the problematic of MS1 population, one can recall that the determination of this structure is always problematic, since the NO positions of GS overlap with the ON positions of MS1, and that in the case of II the situation is complicated in addition by the disorder with the NH3 group. As a further important result, one can state the MS2 in II is hard to investigate, as it is immediately re-excited forward to MS1.

Is one pulse single laser-shot photoconversion possible for all kinds of metal-nitrosyl complexes? It is very difficult to control many parameters at once to perform one pulse single laser-shot steady-state photocrystallographic experiments. I found a few important methodological criteria after spending quite a lot of time performing one pulse experiments.

These general criteria will help to select a candidate metal nitrosyl complex for these special kinds of experiments with at least some successful outcomes.

The results obtained from one-pulse experiments show that by using high laser power, one can obtain the PLI but not the maximum photoconversion of ~85% PLI as the population reported with CW. In addition to the fact that one cannot increase power infinitely due to the damage of the crystals, this might also be due to the effect that only one face of the crystal is facing the incident laser pulse, and hence the irradiation is not homogeneous in the crystal.

In summary of the above discussion, in both the cases of I and II, it is difficult to trust the population achieved and to say it equivocally. After one pulse irradiation, thermal-induced effects can bias the refinement and population determination, e.g. by inducing disorder, increased ADPs, etc., even though I tried to eliminate these effects by systematically checking the crystal tolerance towards pulse laser power without compromising on the saturation flux as discussed above.

Conclusion and perspectives

I think the fundamental element is the choice of nitrosyl complex candidate. It would be best option if the choice of the candidate can only go to one photo-induced linkage isomer. This will simplify the complexity compared to the cases I worked with as discussed above with two metastable states. Ideally, this PLI has a significant structural difference compared to GS.

Secondly, I learned from experiments that the choice of PLI candidate should have a very low Q-value, i.e., high photo-excitation efficiency. In this case, a lesser number of photons will be needed to excite the required number of molecules. This will also aid in preventing the photoinduced thermal heating, particularly in single pulse experiments.

I find out that the most critical thing in the whole excitation is the number of photons to number of molecules ratio. This should be considered more carefully, apart from other factors of saturation, etc. to better estimate experiment success.

In complex, I, the two-step process for MS1 generation was confirmed as proposed in the literature [153]. Given the relatively high damage threshold and the fact that the photodifference maps show traces of MS1/MS2, this system seems a possible candidate for time-resolved experiments, even though Q-value is relatively high.

In complex II, the fact that I could only find MS1 by ns pulsed irradiation and not traces of MS2, indicates that the MS2 is depopulated quickly by the irradiation wavelength necessary for population of MS1. I can see MS2 with one shot ns-pulse laser in transient optical absorption measurement (Chapter 5), due to its extremely strong absorption coefficient in the blue-green spectral range. This leads to two results:

-This complex is not a good candidate for in-house time-resolved measurements with the available ns-pulse laser.

-Perhaps one needs fs pulsed laser to really see the MS2, since in this case according to the population hypothesis given in Figure 4.14, the laser pulse has passed before the complex has relaxed into the MS2 minimum (some picoseconds).

Time resolved pump probe photocrystallography 5.1 Pump-probe photocrystallography

The type of photocrystallographic experiment depends on the lifetime () of the photoactivated species, which can range from hours to femtoseconds timescale. The experiment and the chemical process should be brought onto a common scale so that the photoexcited state can be captured. If the photoexcited state has a sufficiently long lifetime, up to minutes or even hours, this means that a standard data collection can be done to structurally characterize it. If the lifetime of the photoactivated species is shorter than this standard data collection time, there is a need to bring together the two on a similar time scale. This can be achieved by slowing down the relaxation of the photoactivated state by effectively trapping it, e.g., with cryo or chemical trapping methods, thereby freezing it long enough to permit its analysis. This method has been successfully applied in the previous chapters, allowing for the study of the various linkage isomers as metastable states trapped at low temperature. Though sometimes, this trapping method might cause changes to the natural progress of the reaction under study, and more generally, it does not allow capturing truly transient states. It is, therefore, not adapted to study the dynamics of reactions and understand the underlying mechanisms. Another approach is to shorten the time scale of the measurement by applying time-resolved X-ray diffraction, using fast detectors or a pump-probe approach [START_REF] Pillet | Time-resolved structural analysis: probing condensed matter in motion[END_REF]. One of the aims of the project of which this thesis work is part of is to evaluate the feasibility of such time-resolved X-ray diffraction experiments, on the one hand, on a dedicated laboratory setup with the help of fast detectors and the other hand, on synchrotron sources by exploiting the natural time structure of these facilities.

Before going into the details of the measurements, a short reminder of the background of the investigated {MNO} 6 mononitrosyl complexes with composition [ML5NO] n± (M=transition metal, L=ligand such as F,Cl,Br, CN, etc), that have been studied for a long time due to their ability to form linkage isomers upon irradiation with light in the visible spectral range [START_REF] Coppens | Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes[END_REF].

Amongst these compounds, Na2[Fe(CN)5NO]•2H2O (sodium nitroprusside, SNP) has received special attention, since the photoinduced linkage isomers were discovered in this compound [START_REF] Carducci | Diffraction Studies of Photoexcited Crystals: Metastable Nitrosyl-Linkage Isomers of Sodium Nitroprusside[END_REF][START_REF] Hauser | On optical dispersion in transparent molecular systems: I. Müssbauer resonance observation of a new kind of isomeric molecular states generated by polarized light[END_REF]. Furthermore, SNP can be produced in large quantities and perfect single crystals can be grown easily and subsequently cut and polished in any orientation desired. The photoinduced linkage isomers in SNP shown in Figure 5.1(left) are stable at low temperatures and can thus be studied by diffraction and spectroscopic methods, therefore their structural and electronic properties at low temperature are well known [START_REF] Carducci | Diffraction Studies of Photoexcited Crystals: Metastable Nitrosyl-Linkage Isomers of Sodium Nitroprusside[END_REF][START_REF] Schaniel | Light-induced absorption changes by excitation of metastable states in Na2[Fe(CN)5NO]2H2O single crystals[END_REF]. The study of these photoinduced states at room temperature (RT) started in 2005. Using transient absorption spectroscopy after nanosecond pulsed laser excitation, it was concluded that MS2 in SNP can be generated at room temperature with a lifetime of about 300 ns [START_REF] Schaniel | Transient kinetics of lightinduced metastable states in single crystals and aqueous solutions of Na2[Fe(CN )5NO]•2H2[END_REF]. The generation of MS2 occurs on a much faster time scale, and femtosecond UV/Vis and infrared spectroscopy measurements were performed to study the mechanism [START_REF] Schaniel | Ultrafast reversible ligand isomerisation in Na2[Fe(CN)5NO]•2H2O single crystals[END_REF][START_REF] Gallé | Unraveling the mechanism of NO ligand photoisomerism by time-resolved infrared spectroscopy[END_REF].

Objectives (vs reality)

One of the ambitious goals at the start of my Ph.D. project was to measure the structure of the MS2 state at different time points after the pulsed laser excitation. Using the laboratory setup with millisecond time resolution, one could follow the relaxation from the MS2 minimum back to the GS. In contrast, with the faster time resolution available at synchrotrons, one could reach time resolutions of a few picoseconds, thereby capturing the very beginning of the existence of MS2 (as a relaxed state). Going even faster, using XFEL sources, one could dream of making a movie of the NO rotation occurring on the sub-picosecond time scale.

Reality sometimes upsets the nicest plans (and a small creature named SARS-CoV-2 had its fair share on that). To make a long story short, in the following, there will be no nice movies about NO rotation nor complete structures of MS2 at the mentioned time resolutions. However, these experiments need a thorough preparation and their planning and execution is instructive, even if the expected result is not (yet) achieved. Therefore, in the following, the two experimental concepts and corresponding setups are shortly introduced, followed by the results of the necessary preparatory investigations and simulations, for which the compounds SNP (best-known system) and [Ru(NH3)5NO]Cl3•H2O (studied in the previous chapters) were chosen. Some conclusions and perspectives from the first time-resolved diffraction experiments conducted with a time resolution of ~100 ps on single crystals of SNP at the beamline CRISTAL@SOLEIL are drawn.

Pump-probe experiment at the beamline CRISTAL@SOLEIL

In order to perform a pump-probe XRD experiment with 100 ps time resolution, one needs to carefully address the following points As can be seen from these points, some of them will lead to quite strong constraints, e.g. the geometry at the beamline and the time structure of the X-ray pulses, while others offer some flexibility, such as the choice of a suitable sample.

Let's first have a look at the beamline characteristics and especially the setup for pump-probe XRD experiments (Figure 5.2), in a second step, look into the choice of sample and the preparatory work. The experimental set up for the CRISTAL beamline is shown schematically in Figure 5.3, highlighting the temporal structure imposed by the electron bunches in the synchrotron storage (a) Choose a sample that is available in sufficient quantities and of which crystals of good quality are available.

(b) The higher the symmetry, the shorter the measurement time and the better the chances of measuring a complete data set (with some redundancy).

(c) The sample should have a high photoconversion rate, i.e., number of excited state molecules should be maximum.

(d) The lifetime of the photoinduced state should be adapted to the time scheme of the setup, and ideally, the system should be excitable at room temperature.

Considerations (a) and (b) rapidly lead to the conclusion that SNP is a suitable system, as crystals are of good quality and available in (almost) unlimited quantity. They can be prepared Second, we consider the geometrical constraints. Figure 5.7 shows the crystal plates mounted on the goniometer head. Due to the particular geometry of the setup, with the laser from the top (from the right in Figure 5.7) and the X-rays at 90° with respect to direction of laser beam, we need to consider spatial overlap between X-ray pulse and laser pulse. We need to ensure that the laser spot size is always bigger than the X-ray spot size. The latter depends significantly on the incident angle θ, due to the grazing incidence geometry. For example, the original Xray beam (not projected onto sample) has a size of 30x500 µm². This implies that at an angle of θ = 45° we need a laser beam size of 150x1500 µm², at θ = 10° 550x1500 µm², and at θ = 5° 1000x1500 µm² (with a factor of 3 error margin). So, ideally, we avoid too much rotation of the crystal, in order to guarantee that the sample volume probed by X-rays is always smaller than that excited by the laser. As can be seen from Finally, we also checked whether the light polarization influences the achievable population of the MS2 state (Figure 5.8 right). Using the transient absorption setup in our laboratory (see Chapter 2), we observe that the same photoinduced absorption (measured at 785 nm) is produced, independent of the polarization (pulse laser at wavelength 410 nm). At the same time, this experiment shows that indeed MS2 is populated, as the lifetime of the observed signal corresponds well with that expected for MS2. We can estimate a population of 0.1% from an observed photoinduced absorption of 1 cm -1 at 785 nm, at the used pulse energy of approximately 12 mJ/cm 2 . We used c-cuts for this experiment, as those were used for the experiment at SOLEIL, as discussed in the following. The geometry of the experiment affects the achievable coverage of reciprocal space, considering the limited angular movements that are possible because of the laser irradiation as well as the angular coverage by the detector. For this purpose, we studied different orientations of SNP crystals, and tested the corresponding geometries on the laboratory diffractometer (Bruker D8). As conclusion, c-cut crystals offer the best overall solution, yielding a sufficient reciprocal space coverage and, at the same time acceptable irradiation conditions.

Last but not least, we address the issue of the necessary data quality to be able to detect small populations of the MS2 species (or any other structural modification upon LASER excitation).

For this purpose, X-ray diffraction measurements have been performed at T=100 K in the GS and MS2 of SNP in order to get complete X-ray diffraction data sets. The results were then used to simulate the various measurement conditions that result from the setup at CRISTAL beamline (incomplete data sets, noise, etc.), by first screening the most sensitive Bragg reflections, which exhibit the highest GS to MS2 intensity differences, which could be used in the time-resolved photocrystallographic experiment, if one follows those selected reflections as a function of time. In the second step, the resulting simulated photodifference maps are analyzed, in order to choose the best possible measurement conditions.

GS and MS2 states X-ray diffraction in steady-state conditions

A photocrystallographic X-ray diffraction measurement was performed at T =100 K on the Supernova Atlas CCD diffractometer (see Chapter 2 for complete description) in order to determine structures of GS and MS2 on the samples that were going to be used at SOLEIL.

MS2 was generated by the following procedure: An LED of 420 nm with P = 50 mW was used for t = 5 min irradiation time at 100 K for the generation of MS2. This investigation, yields the structural parameters of GS and MS2 as well as the population of MS2 under these irradiation conditions. The color of the crystal changes upon irradiation, which is an indication of the generation of MS2 (Figure 5.9). phenomenon with time, which should then correspond to the lifetime of the MS2 state. This approach has been applied in the study of Casaretto et al. [START_REF] Casaretto | In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector[END_REF].

In conclusion, reflections 240, 008, 305, and 303 seem most appropriate as they exhibit the most pronounced changes. In addition, it is favorable to choose some reflections that show a positive difference and some that show a negative difference upon MS2 generation. In this manner, one can exclude potential systematic errors due to, e.g., sample degradation.

Photo-difference maps as a function of the metastable state population

The second possibility is to simulate the photodifference maps expected from the population of the MS2 state, under various conditions, in order to identify the minimum necessary conditions for successful time-resolved experiments. Amongst the key parameters that will affect the visibility of a structural modification in a photodifference map we find the following:

a) The population of the MS2 state. The higher the population, the more significant the differences in the photodifference map should be.

b) The completeness of the data. The photodifference map is a Fourier map. Incomplete data will thus necessarily affect the quality of the map and may even create artifacts that are not related to the photo-induced changes. c) Noise, systematic errors and other limitations of a real experimental setup. For the time being, we will consider that we are note limited by these effects.

a) Photodifference maps as a function of MS2 population

The first point can be addressed easily. By using the structure factors of mixed states (GS+MS2), the predicted photo-difference maps can be computed for various simulated population levels of MS2. Before describing the result, a note on the methodology. In an experiment, the lattice parameters change between the GS and GS+MS2 data sets. This leads to corresponding effects in the photodifference maps. When doing simulation, one can get rid of this effect by using strictly one set of lattice parameters and only calculated structure factors.

b) Photodifference maps as a function of data completeness

For addressing the second point, we consider first the experimentally obtained photodifference maps (see above) and then limit the data set to a selection of hkl values. The selection is made according to the geometry that will be encountered at the setup on the CRISTAL beamline, i.e., a plate-like single crystal for which only a few angular positions can be measured. Therefore, for the preparation of the experiment on the CRISTAL beamline, we tried to use the similar geometry after considering all the constrains on the lab diffractometer to determine the possibly achievable population of MS2. A perpendicularly oriented crystal was used as depicted in Figure 5.15 on a special goniometer head that can allow collecting data at different chi positions (since in the lab diffractometer, horizontal detector movement is allowed) shown in Figure 5.16. Therefore, the photodifference maps were calculated from the data set of few frames collected at two different chi positions i.e. 0 and 5 and a third one following the addition of these two data set i.e., full data photodifference maps. The photodifference maps in Figure 5.17 The photodifference maps in Figure 5.17 clearly show the signature of the MS2 state, although the details differ to some extent. In the photodifference map from Chi=0 position, there is no depletion on the Fe position, contrary to what is typically observed due to the shift of the central metal atom, and only yields 57% completeness of the Ewald sphere by covering only 1031 independent reflections (see Table 5.5). In the case of photodifference maps obtained from Chi=5 position data set, the peak intensity at the MS2 position is comparatively higher than in the Chi=0 position data set. With the data set from Chi=5 position 70% completeness of the Ewald sphere was achieved by covering double the 2159 independent reflections, which is twice the obtained from Chi=5 position data (see Table 5.5). It is to be noted here that the Chi=5 position data set covers (001) reflections. This data set covers 008 reflection which is one of the most sensitive reflections from GS to MS2 (see Table 5.4), and therefore, in summary Chi=5 position data is a better choice. As a conclusion to this preparatory work, we can state that the population of MS2 has a bigger effect on its visibility on the photodifference maps than the coverage for the chosen configuration. As a matter of fact, the generation of photoinduced linkage isomers in all configurations can be seen quite easily, if a sufficient amount of population /photoconversion is achieved.

Results of measurement at SOLEIL

We performed a time resolved diffraction experiment with a time resolution of 150 μs, that is the gate was set to this value and pump-probe cycles were performed at a laser repetition rate of 33 Hz. The temperature was 200 K, resulting in a lifetime of 3 ms of MS2, so that MS2 has relaxed before the next laser pulse arrives. By choosing the c-cut of the SNP crystal and the geometry indicated in Figure 5.7 by gluing the crystal with its face perpendicular to the laser beam, which should guarantee the homogeneous irradiation, we succeed in collecting a data set of 53% completeness up to sin(θ/λ) = 0.764695 Å -1 (θ = 30°). The completeness at θ = 26° is 70%. According to our simulations, this should be sufficient to detect the photoinduced MS2 configuration. Figure 5.20 illustrates the collected data at the example of one image, where we need to carefully integrate the useful Bragg reflections, making sure that they do not fall partially into a dead zone of the detector, such as the inter-chip and inter-module zones, or Figure 5.21 shows the experimental photodifference maps obtained from this experiment. We note that there is mainly positive electron density on the N and O positions of nitrosyl as well as on the Fe central atom. Cleary, this photodifference map does not match with the changes expected from the photoexcitation of MS2. What can be the possible reasons for this difference? One the one hand it is possible that the population generated in our experiment was not sufficient to be detected. On the other hand, it is possible that laser induced damage leads to sample degradation over time, leading to low data quality of the data set of the photoexcited state. Since the experimental photodifference maps obtained from the time-resolved experiments do not allow us to conclude on the presence of MS2, we performed additional measurements by following single Bragg reflections as a function of time. Figure 5.22 shows the result for the 240 reflection at four time points : (i) at negative delays, that is, before the laser pulse arrives on the crystal, (ii) at positive delays, i.e. immediately after the laser pulse has arrived on the crystal, the time resolution is then given by the gate width of the detector, in the present case it is 30 microseconds, (iii) at longer time delays of 150 microsecond and 1 millisecond, still with the same gate width. One observes that the intensity of this particular Bragg peak decreases after the laser excitation, and there is some recovery towards the initial state at the longer time delays. The initial decrease of the intensity of this particular Bragg reflection is expected upon excitation of MS2 from our simulation, as has been shown in the previous study by Casaretto et al [START_REF] Casaretto | In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector[END_REF] after nanosecond pulsed laser excitation. The measurement was performed at a temperature of 200 K, where the MS2 lifetime is of the order of a few milliseconds (see Figure 5.5), so we cannot expect a full recovery of the GS at 1 ms. Overall the observation would thus be compatible with a MS2 model. Of course, measuring only one reflection that decreases upon laser excitation is not sufficient to validate the MS2 model. Moreover, we noted that during the experiment, the laser spot on the sample was slightly misaligned, exciting only about half of the surface probed by the X-rays, so the measurement conditions were not optimal. However, as for the previous full data set collection, this experiment proves the viability of the approach and that, in principle, the study of such linkage isomers, even though they exhibit low populations of photoinduced species, is possible.

Conclusion

Our experiments have shown that in principle the measurement of a sufficiently complete diffraction data set in pump-probe mode at the 100-picosecond time scale is possible. However, the problems inherent with the stability of molecular crystals under prolonged laser & X-ray irradiation and the necessity for rather long exposures in order to get sufficient statistics did not allow for the observation of light-induced structural changes.

PART II

In-house time resolved photocrystallography

As we have seen above from the simulation and preparation of the time-resolved X-ray diffraction experiment at the CRISTAL beamline at the synchrotron SOLEIL, we can expect to see a structural reconfiguration in a photodifference map if a minimum amount of population is achieved and if sufficient reciprocal space coverage is guaranteed. At CRM2 we dispose of a setup allowing for time-resolved diffraction experiments at the millisecond time scale that has been previously described in the thesis of Nicolas Casaretto [81] and has been used to follow the evolution of selected Bragg reflections after pulsed LASER excitation [START_REF] Casaretto | In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector[END_REF]. With see Appendix E. Furthermore, one needs to ensure that the goniometer rotation speed is adapted to the time scheme of the X-ray measurement, especially when applying the continuous scan mode [START_REF] Casaretto | In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector[END_REF], where one has to make sure that there are enough points measured (enough data sampling) within the rocking curve of a Bragg reflection for later peak intensity integration and profile fitting.

Experimental validation

Results from transient absorption spectroscopy

An important parameter for planning of the time resolved experiment and successful synchronization is the lifetime of the photoinduced state. As we have seen for SNP above, the PLI exhibit an Arrhenius like behavior, so if one determines the activation energy and frequency factor, (e.g. from DSC) one can theoretically predict the lifetime for each temperature. Even better would be to measure the lifetime directly on the sample on the

Conclusion and Future Perspectives

Overall, we have seen that careful preparation of the time-resolved experiment, including extensive simulations of expected structural changes and corresponding photodifference maps and an intelligent choice of the investigated sample, allow in principle to perform a successful experiment on a time scale of 100 ps. The main issue that remains is the photoinduced population which needs to be as high as possible, a requirement that is difficult to achieve due to the limited damage threshold of the investigated samples. In this respect, the two {MNO} 8 and {M(NO)2} 10 samples investigated in chapter three hold some promise. They show only one PLI, with a sufficiently distinct structure compared to GS, and they exhibit a relatively high photoexcitation efficiency (low Q-value). However, concerning their other optical properties and especially lifetimes, some preparatory work is still needed before one can envision a timeresolved X-ray diffraction experiment.

Conclusions and perspectives

This thesis work was performed in the framework of photocrystallography on metal nitrosyl complexes, with the ambition to contribute to the development and application of methods allowing direct structural analysis of photoinduced linkage isomers (PLI) on different time scales. Several new results have been obtained that open some perspectives, as discussed in the following.

On the one hand, novel types of PLI have been detected and thoroughly characterized for three different metal-nitrosyl compounds, an {MNO} 6 , an {MNO} 8 , and an {M(NO)2} 10 complex.

On the other hand, progress has been made in the generation and analysis of PLI by pulsed laser irradiation.

The PLI discovered in the {Ru(NO)2} 10 dinitrosylruthenium complex [Ru(NO)2(PPh3)2] is a phenomenal result, as this complex shows a PLI of a special kind: a synchronous photoswitching of the two NO ligands from the double-linear κN-bonded Ru(NO)2 core in the ground state (GS) to a double-bent κN-bonded Ru(NO)2 core in the PLI. Moreover, this PLI shows a relatively good thermal stability up to 230 K, a good photoconversion efficiency of about 50% using light in the green-yellow spectral range, and a rather high excitation efficiency with a low Q value of 14 J/cm 2 . Computations of our colleagues from LMU Munich showed that the synchronous disrotatory motion of the pair of nitrosyl ligands, pictorially the pas de deux of a nitrosyl couple, occurs both in the formation as well as in the relaxation of the PLI.

Such concerted motion might be useful in applications based on molecular rotors or motors, and possibly other complexes with this property exist and should be searched for. Furthermore, synchronous motion is a subject of high interest for time-resolved studies on the corresponding time scales that should be pursued further. The fact that a double-linear to the double-bent transformation of the nitrosyl ligand can be induced by light in this complex also offered the opportunity to address a long-standing issue in coordination chemistry concerning the quest of M n (NO + )/M n+2 (NO -) valence tautomerism in the course of an MNO linear/bent transition. As a matter of fact, the calculations on the experimentally verified isomers show that the oxidation states (OSs) as well as the nitrosyls' charge, do not change significantly in the course of the linear/bent transition, which leaves the OSs unaltered at the Ru -II (NO + )2 level. Hence, there is no justification to assign NO + ligands to the linear but NO -to the bent isomer.

A second important new result is the observation of a bent-to-bent photoswitching of the NO ligand, most probably from the κN-bonded GS to κO-bonded PLI, in a {CoNO} 8 complex, [Co(fpin)(NO)(phen)]. This case thus resembles the previously reported photoswitching in {PtNO} 8 complexes, but with some significant differences. The low thermal stability, with decay around 60 K, as well as the appearance of an absorption band in the infrared indicate that possibly the PLI phenomenon is coupled to a photomagnetic effect. This needs to be investigated by corresponding measurements and calculations in the future and would open interesting possibilities for applications where magnetic properties can be switched by light. Furthermore, like in the previous example, the photoswitching is rather efficient, yielding populations of the order of 50% at high excitation efficiency with Q value of 90 J/cm 2 .

The third novel PLI structure reported concerns the well-known family of {MNO} 6 compounds. In [Ru(NH3)5(NO)]Cl3•H2O, the structures of the κO-bonded (isonitrosyl, MS1) and κ 2 N,O (side-on, MS2) isomers are reported, with the particularity that for MS2 two different structural configurations are adopted, the so-called eclipsed and staggered MS2, which was predicted by theory 20 years ago. This example also showed the problematics of structure determination of mixed GS+MS1 or GS+MS2 states in a severe disorder compound.

The strategy adopted to refine the structures in the higher symmetry space group Pnma, instead of the probably real space group Pna21, proved successful, with, however limited accuracy concerning the structural parameters of MS1 and MS2. In these {MNO} 6 compounds, the existence of two PLI leads actually to a competition between the occupation of one or the other, which in addition complicates structural analysis. From a fundamental point of view, the question of the photoisomerization mechanism in these systems is of interest, as the current working hypothesis is that MS1 is populated in a two-step process from GS via MS2. The presented results support this mechanism, with the remarkable finding that for collected after single-shot excitation and most probably also in the X-ray diffraction measurements. In view of potential time-resolved studies after single-shot excitation, [Ru(NO)(Py)4F](ClO4)2 is thus a more appropriate candidate than [Ru(NH3)5(NO)]Cl3•H2O, since the structural change of MS2 with respect to GS is much bigger than the one for MS1. However, the photoexcitation efficiency is not optimal with a rather high Q value of 1200 J/cm 2 for MS1. Future work should investigate more systematically the optimal Q value to favor the GS to MS2 step in the GS-MS2-MS1 process when using pulsed excitation. Overall, these results highlight the necessity of finding photoswitchable complexes with high photoexcitation efficiency, i.e. lowest possible Q value for the highest possible population, in order to be able to perform successful single-shot experiments and later time-resolved studies. The two newly reported PLI systems {CoNO} 8 and {Ru(NO)2} 10 are thus interesting candidates for this purpose.

Finally, the attempt to study the photogeneration of PLI in the prototype compound for {MNO} 6 systems, Na2[Fe(CN)5(NO)]•2H2O (SNP), with a time resolution of about 100 ps was reported. SNP exhibits the same type of two-step process for MS1 generation as discussed above, but with increased efficiency for the first GS to MS2 step and decreased efficiency for the MS2 to MS1 step, rendering it suitable for this type of study. The careful preparation of the time-resolved experiment at the beamline CRISTAL on the synchrotron SOLEIL addressed all experimental constraints by extensive simulation of photodifference maps as a function of population of the expected PLI species MS2 and reciprocal space coverage. This allowed us to perform a successful XRD experiment in terms of the envisioned time resolution and with respect to the necessary completeness of data to generate meaningful photodifference maps after pulsed excitation with femtosecond laser pulses. These photodifference maps did not reveal clear signatures of a PLI generated in this experiment so for the time being, the question whether the side-on structure of the nitrosyl ligand in the MS2 state is already adopted after 100 ps remains unanswered. However, the established procedure for preparing and conducting the experiments has proven reliable, so further experiments, possibly with complexes having higher photoexcitation efficiencies, can be envisioned. Pnma with complete disorder of the nitrosyl ligand. The authors reported that the measured data could not allow deciding on the possible unequal occupancy of the two orientations.

B.2 Optics for Single Crystal Optical Absorption

One efficient way to correctly assign space group, and detect the presence or absence of inversion symmetry (distinguish between Pnma and Pna21 in the present case), is by making use of anomalous dispersion effects in X-ray scattering. Appropriate methodologies have been introduced which enable a precise determination of absolute structures [START_REF] Flack | Absolute structure and absolute configuration[END_REF][169][170]. Following these, an appropriate choice of X-rays wavelength is made in order to enhance resonance scattering factors, which lead to significant Bijvoet intensity differences (∆𝐼  = 𝐼(ℎ𝑘𝑙) -𝐼(ℎ ̅ 𝑘 ̅ 𝑙 ̅ )).

For that purpose, we collected diffraction data at two different wavelengths (Mo K radiation, 6), O2B = 0.0278 [START_REF] Kukura | Structural Observation of the Primary Isomerization in Vision with Femtosecond-Stimulated Raman[END_REF]. However, we note that in this model, the ADP of O2B is larger than that of N2B, even though the O2B is the inner atom of the Ru-O2B-N2B PLI configuration. This is a typical sign of a wrong assignment of the atom species in NO linkage isomers. The oxygen has a higher number of electrons than the nitrogen. So, if an oxygen atom is wrongly placed on a nitrogen position, the overestimation (model versus data) of the electron density is somewhat compensated by increasing the ADP value, leading to a smearing of the electron density of the O2B atom. The inverse situation is observed at the outer position, occupied by the N2B, where the model underestimates the experimental electron density and therefore leads to a smaller ADP value. An in-depth discussion of this issue is available in [87].

C.4.3 N1O1 and N2O2 as bent isonitrosyl: (a-NO-κO) 2

In this model of the PLI the bent isonitrosyl configurations were used for both N1O1 and N2O2.

The GS configuration for N1O1 was assigned as Ru-N1A-O1A and in PLI the configuration is Ru-N1B-O1B, while the configuration for N2O2 in GS Ru-N2A-O2A and in PLI as Ru-

E.2: Details on the Cegitek RebirX HPAD detector and the IPI mode

The hybrid pixel detectors are composed of hundreds of thousands of independent point detectors that detect individual X-ray photons. The sensor layer is hybridized to the electronics so that the photons are counted one by one. Hence, this type of detector is named as hybrid pixel detector, photon counting detector, hybrid photon counting detector, or HPAD. Once the acquisition of frames is done, the counter values of the 537,600 pixels for each electronic chip are transferred to the electronic board with the minimum readout time tR =1.7 ms. Once the reading is complete, the images can be processed on the board within the detector independently of what happens on the chip, allowing the chip to begin a fresh acquisition while the previous image is being processed. The electronic board including an FPGA (Field Programmable Gate Area) and SRAM fast memory inside the detector enables the management of the pixels' electronics as well as fiber-optic communication with the computer. This custom firmware configures the FPGA's electrical architecture to allow controlling and reading of all pixels, as well as multiplexing, summing, and storing images in n distinct channels (with this set up it is possible to measure up to 100 channels but off course due to less exposure time, intensity signal will be very low) in synchronization with the gate signal. This is known as the Image Plus Image mode (IPI). Following that, a full acquisition as a series of n pump-probe cycles was defined. Collection/acquisition and stacking of all the images occurs when n frames are gathered during each cycle, independently summed (in the FPGA) with the frames measured during the previous cycles, and stored in the SRAM memory. As a result, the measurement output is essentially n separate summed pictures. The stacking processing in the FPGA takes tS = 3.7 ms. The stacking of images can begin while the next image is being exposed to X-rays since it is independent of the chip's photon counting process. The read time tR is the blind time tb between two subsequent picture observations (1.7 ms). The end of the gate initiates the reading-stacking process, however before reading a new image, the FPGA must have completed stacking of the previous images. This requires a minimum sample duration of the read time tR plus the stacking time tS, which adds out to 1.7 + 3.7 = 5.4 ms.

The master computer that drives the goniometer and dedicated electronic to produce the gate signals drives the RebirX detector computer in slave mode for the time-resolved X-ray

  [Ru(NO)2(PPh3)2] est certainement le nouveau résultat le plus spectaculaire, car ce complexe présente un PLI d'un type particulier : un photoswitch synchrone des deux ligands NO du noyau Ru(NO)2 à double liaison κN linéaire dans l'état fondamental (GS) à un noyau Ru(NO)2 à double liaison κN courbé dans la PLI. De plus, ce PLI présente une stabilité thermique relativement bonne jusqu'à 230 K, une bonne efficacité de photoconversion d'environ 50% en utilisant la lumière dans la gamme spectrale vert-jaune, et une efficacité d'excitation plutôt élevée avec une faible valeur de fluence Q de 14 J/cm 2 . Les calculs de nos collègues de la LMU de Munich ont montré que le mouvement disrotatoire synchrone de la paire de ligands nitrosyle, illustré par le pas de deux d'un couple nitrosyle, se produit aussi bien dans la formation que dans la relaxation du PLI. Un tel mouvement concerté pourrait être utile dans des applications basées sur des rotateurs ou des moteurs moléculaires, et il est possible que d'autres complexes ayant cette propriété existent et doivent être recherchés. De plus, le mouvement synchrone est un sujet de grand intérêt pour les études résolues en temps sur les échelles de temps correspondantes, qui devraient être poursuivies. Le fait qu'une transformation double-linéaire à double-courbé du ligand nitrosyle puisse être induite par la lumière dans ce complexe, a également offert l'opportunité d'aborder une question de longue date en chimie de coordination concernant la recherche de la tautomérie vii de valence M n (NO + )/M n+2 (NO -) au cours d'une transition linéaire/courbé MNO. En fait, les calculs sur les isomères vérifiés expérimentalement montrent que les états d'oxydation (OS) ainsi que la charge des nitrosyles ne changent pas de manière significative au cours de la transition linéaire/courbé, ce qui laisse les OS inchangés au niveau du Ru -II (NO + )2. Par conséquent, il n'est pas justifié d'attribuer des ligands NO + à l'isomère linéaire et NO -à l'isomère coudé. Un deuxième résultat, peut-être moins spectaculaire mais tout de même important, est l'observation d'une photocommutation courbé-courbé du ligand NO, très probablement du GS lié κN au PLI lié κO, dans un complexe {CoNO} 8 , [Co(fpin)(NO)(phen)]. Ce cas ressemble donc à la photocommutation précédemment rapportée dans les complexes {PtNO} 8 , mais avec quelques différences significatives. La faible stabilité thermique, avec une décroissance autour de 60 K, ainsi que l'apparition d'une bande d'absorption dans l'infrarouge indiquent que le phénomène PLI est peut-être couplé à un effet photomagnétique. Cette hypothèse doit être étudiée à l'avenir par des mesures et des calculs correspondants, et ouvrirait des possibilités intéressantes pour des applications où les propriétés magnétiques peuvent être commutées par la lumière. De plus, comme dans l'exemple précédent, la photocommutation est plutôt efficace, donnant des populations de l'ordre de 50% à haute efficacité d'excitation avec une valeur Q d'environ 10 J/cm 2 .

  photocristallographie aux rayons X ainsi que la spectroscopie infrarouge après une irradiation à une seule impulsion ne sont pas assez sensibles pour détecter les petites quantités de MS2 générées au cours de ce processus. Ces expériences à une seule impulsion ont révélé un critère important pour la préparation d'expériences de diffraction des rayons X résolues dans le temps : il faut soigneusement peser le nombre de photons disponibles dans une seule impulsion par rapport au nombre de molécules photosensibles présentes dans le monocristal, et en outre prendre en compte les efficacités de conversion entre les différents états PLI possibles.

.

  ................................................................................................................................... 1.1 Context, motivation, and outline of the thesis ................................................................. 1.2 Photocrystallography ....................................................................................................... 1.3 The basic principle of optical excitation .......................................................................... 1.4 Types of photocrystallographic methods ....................................................................... 1.4.1 Steady-state photocrystallography .......................................................................... 1.4.2 Time-resolved photocrystallography ...................................................................... 1.5 Instrumental considerations for time-resolved structural studies .................................. 1.5.1 Pump-probe approach ............................................................................................. 1.5.2 X-ray Sources .......................................................................................................... 1.5.3 Detectors for time-resolved photocrystallography .................................................. 1.5.4 Excitation Sources .................................................................................................. 1.6 Types of photoactive ligands in transition metal complexes ......................................... 1.7 Photoinduced linkage isomers in transition metal nitrosyl complexes .......................... 1.8 Conditions and considerations for successful generation and structure determination of PLI........................................................................................................................................ .................................................................................................................................. 2.1 Photo-excitation sources ................................................................................................ 2.1.1 Lamp ....................................................................................................................... 2.1.2 LEDs and CW laser ................................................................................................ 2.1.3 Pulsed Laser ............................................................................................................

2. 9 . 2 4 . 5 4 . 8 FiguresFigure 1 .

 9245481 photocrystallography.......................................................................................................... 4.1.1 Experiment Methodology: .................................................................................... 4.2 Comparative photocrystallographic study of I with CW and pulsed irradiation ......... 4.3 Comparative photocrystallographic study of II with CW and pulsed irradiation ........ 4.3.1 Results and discussions of I & II with CW and pulsed steady-state photocrystallography and optical absorption mapping: ................................................. 4.4 Conclusion and Perspectives ........................................................................................ 4.5 One-pulse single-shot steady-state photocrystallography ............................................ 4.6 The dilemma of precision and accuracy in the population of Excited states ............... 4.7 Experimental section .................................................................................................... 4.8 Low excitation efficiency: one pulse single-shot steady-state photocrystallography in I ............................................................................................................................................ 4.9 High excitation efficiency: one pulse single-shot steady-state photocrystallography in II ............................................................................................................................................ 4.10 Results and Discussions ............................................................................................. 4.11 Conclusion and perspectives ...................................................................................... ................................................................................................................................ 5.1 Pump-probe photocrystallography ............................................................................... 5.2 Objectives (vs reality) ..................................................................................................

1 : 2 Figure 1 . 2 : 3 Figure 1 . 3 : 4 Figure 1 . 4 : 9 Figure 1 . 5 : 10 Figure 1 . 6 : 14 Figure 1 . 7 : 24 Figure 2 . 1 : 27 Figure 2 . 2 : 30 Figure 2 . 3 : 32 Figure 2 . 4 : 32 Figure 2 . 5 : 35 Figure 2 . 6 : 37 Figure 2 . 7 : 37 Figure 2 . 8 : 38 Figure 2 . 9 : 40 xviFigure 2 . 10 : 40 Figure 2 . 11 : 41 Figure 2 . 12 : 44 Figure 2 . 13 : 46 Figure 2 . 14 : 48 Figure 2 . 15 : 50 Figure 2 . 16 : 51 Figure 3 . 1 : 56 Figure 3 . 2 : 58 Figure 3 . 3 : 60 Figure 3 . 4 : 62 xviiFigure 3 . 5 : 64 Figure 3 . 6 : 69 Figure 3 . 7 : 70 Figure 3 . 8 : 72 Figure 3 . 9 : 72 Figure 3 . 10 : 75 Figure 3 . 11 : 76 Figure 3 . 12 : 80 Figure 3 . 13 : 80 Figure 3 . 14 : 84 Figure 3 . 15 : 85 Figure 3 . 16 : 85 xviiiFigure 3 . 17 : 86 Figure 3 . 18 : 87 Figure 3 . 19 : 88 Figure 3 . 20 : 89 Figure 3 . 21 : 91 Figure 3 . 22 : 92 Figure 3 . 93 Figure 3 . 24 : 95 Figure 3 . 25 : 96 Figure 3 . 26 : 99 Figure 3 . 27 : 100 xixFigure 4 . 1 : 105 Figure 4 . 2 :Figure 4 . 3 : 109 Figure 4 . 4 :Figure 4 . 5 :Figure 4 . 6 : 113 Figure 4 . 7 : 114 Figure 4 . 8 : 122 Figure 4 . 9 : 125 Figure 4 . 10 : 127 Figure 4 . 11 : 127 Figure 4 . 12 : 128 Figure 4 . 13 : 130 Figure 4 . 14 : 135 xxFigure 4 . 15 :Figure 4 . 16 : 147 Figure 4 . 17 :Figure 4 . 18 : 154 Figure 4 . 19 : 155 Figure 4 . 20 : 156 Figure 4 . 21 : 157 Figure 4 . 22 :Figure 4 . 23 :Figure 4 . 24 : 159 Figure 4 . 25 : 162 Figure 4 . 26 : 164 Figure 4 . 27 : 165 Figure 5 . 1 :Figure 5 . 2 :Figure 5 . 3 :Figure 5 . 4 :Figure 5 . 5 :Figure 5 . 6 :Figure 5 . 7 :Figure 5 . 8 :Figure 5 . 9 :Figure 5 . 10 :Figure 5 . 11 :Figure 5 . 12 :Figure 5 . 13 :Figure 5 . 14 :Figure 5 . 15 :Figure 5 . 16 :Figure 5 .Figure 5 . 18 :Figure 5 . 19 :Figure 5 . 20 :Figure 5 . 21 :Figure 5 .Figure 5 . 23 :Figure 5 . 24 :Figure 5 . 25 :

 12123134149151016141724212722302332243225352637273728382940210402114121244213462144821550216513156325833603462356436693770387239723107531176312803138031484315853168531786318873198832089321913229239332495325963269932710041105424310944454611347114481224912541012741112741212841313041413541541614741741815441915542015642115742242342415942516242616442716551525354555657585951051151251351451551655185195205215523524525 Figure 1.1 : Temporal scales of various chemical and physical processes in solution and crystalline solids from seconds to femtoseconds. ...................................................................... 2 Figure 1.2 : Potential applications of photosensitive materials. ................................................ 3 Figure 1.3 : Photoinduced linkage isomers in data storage application. .................................... 4 Figure 1.4 : General schematic representing the irradiation of a crystal by the application of light. ........................................................................................................................................... 9 Figure 1.5 : The general schematic of photocycle after irradiation with light. ........................ 10 Figure 1.6 : Methods of photocrystallography as a function of excited state lifetimes ........... 14 Figure 1.7 : Schematic representing the ideal needs to control the properties in tailoring the device (red dots represent the progress of this thesis in comparison to ideality) .................... 24 Figure 2.1 : A schematic summarizing the associated experimental techniques in photocrystallography to better understand the structure-property relationships. ..................... 27 Figure 2.2:Tunable Nd: YAG Pulse laser EKSPLA NT342 (left panel); Nd-YAG solid-state laser (Surelite II-10 model) (right panel) ................................................................................. 30 Figure 2.3: Infrared spectrometer measurements during irradiation with continuous wave laser of a metal nitrosyl complex. ..................................................................................................... 32 Figure 2.4 : Infrared spectrometer measurement during irradiation with ns-pulsed laser (5 ns pulse duration) with f = 10 Hz of a metal nitrosyl complex for single-shot pulsed laser experiment................................................................................................................................ 32 Figure 2.5: DSC measurements showing crystal irradiation of a metal nitrosyl complex using 1064nm CW laser. ................................................................................................................... 35 Figure 2.6 : Schematic pump-probe optical absorption spectroscopy on the optical table. ..... 37 Figure 2.7: Experimental setup of transient pump-probe optical absorption spectroscopy. .... 37 Figure 2.8: Single crystal optical absorption spectroscopy before and after photoexcitation . 38Figure 2.9: Steady-state photocrystallography SuperNova microfocus-source diffractometer equipped with an ATLAS detector (left panel); An optical table top for irradiation (right panel). .................................................................................................................................................. 40

Figure 1 . 1 :

 11 Figure 1.1 : Temporal scales of various chemical and physical processes in solution and crystalline solids from seconds to femtoseconds.

Figure 1 . 2 :

 12 Figure 1.2 : Potential applications of photosensitive materials.

Figure 1 . 3 :

 13 Figure 1.3 : Photoinduced linkage isomers in data storage application.

  photocrystallographic and single-crystal optical absorption experiments to assess the parameters influencing the photoexcitation efficiency when using a pulsed and CW laser irradiation source. And following that, finding a way to squeeze this into a one-pulse singleshot steady-state photocrystallographic experiment respecting the crystal tolerance threshold.

  state or ground state (GS) to intermediate perturbed transition state(s), either irreversibly convert to a final photoproduct or reversibly recover back to the initial GS, thereby completing a whole photocycle, as schematically depicted in Figure 1.4.

Figure 1 . 4 :

 14 Figure 1.4 : General schematic representing the irradiation of a crystal by the application of light.

Figure 1 . 5 :

 15 Figure 1.5 : The general schematic of photocycle after irradiation with light.

Figure 1 . 6 :

 16 Figure 1.6 : Methods of photocrystallography as a function of excited state lifetimes

2 in

 2 1). The metal-nitrosyl bond in SNP is established through the interplay of three interactions: two Fe-to-NO π-backbonds from the low-spin-d 6 metal's occupied d(xz) and d(yz) atomic orbitals (AO) (z is along the Fe-NO axis) to the empty N-Oπ* molecular orbital (MO), and the NO-to-Fe donor bond from the N-atom's lone pair to the iron's empty d(z 2 ) AO. For GS and MS1 this results in a linear M-N-O and M-O-N configuration, essentially due to the fact that the N(O)-atom's lone pair can interact with the empty d(z 2 ) of the iron. In the MS2 isomer, the occupied d(xz) cloverleaf at Fe forms a threecenter backbond with the in-plane NO(π*). The second backbond of local π-symmetry from Fe-d(yz) to the empty NO-π*(y) MO completes the three-orbital scheme for MS2. On rotation of the NO ligand from the GS via MS2 to MS1, the orbital overlap collapses between the three minima, which results in sufficient activation barriers along the path as a prerequisite for populating the local MS2 and MS1 minima. In this thesis, the nature of photoinduced linkage isomers in {MNO} 6 family of complexes is investigated in Na2[Fe(CN)5(NO)]•2H2O in Chapter 5, [Ru(NH3)5(NO)]•Cl3.H2O in Chapter 3 and Chapter 4 and [RuF(Py)4(NO)]•(ClO4)Chapter 4.

E1: 8 E2:

 8 Na2[Fe(CN)5(NO)]•2H2O: l-NO-κN → l-NO-κO ('MS1'), l-NO-κN → NO-κ 2 N,O ('MS2') {MNO} [PtCl(NH3)4(NO)]Cl2: bent-NO-κN → bent-NO-κO E3: [CoCl2(NO)(PMePh2)2]: singlet bent-NO-κN → triplet l-NO-κN {M(NO)2} 8 E4: [RuCl(NO)2(PCy3)2]BF4: s-NO-κN → a-NO-κN s-NO-κN → s-NO-κO E5: [RuBr(NO)2(PCyp3)2]BF4: (l-NO-κN)2 → a-NO-κN/l-NO-κN {M(NO)2} 10 E6: [Ru(NO)2(PPh3)2]:

( 2 )

 2 photoconversion, a high Q or a high I value is needed. On the contrary, a high Q or I value might also induce unwanted effects, like photodegradation, and thus reduce the overall photoconversion efficiency.

Figure 1 . 7 :

 17 Figure 1.7 : Schematic representing the ideal needs to control the properties in tailoring the device (red dots represent the progress of this thesis in comparison to ideality)

2 ) 6 )

 26 Photo-XRD captures the structure of possible transient species and short-lived excited states.6) Variable-temperature transient absorption spectroscopy after pulsed excitation yields information about the lifetime of short-lived excited and transient states. It also complements the knowledge gained from DSC about activation energy of PLI (at a higher temperature) and confirms whether PLI can be generated under the same irradiation conditions as found in the low-temperature Photo-UV/Vis measurements.

Figure 2 . 1 :

 21 Figure 2.1 : A schematic summarizing the associated experimental techniques in photocrystallography to better understand the structure-property relationships.

8 ) 2 . 1 . 2

 8212 spectroscopy on the X-ray diffractometer. The 100 W Quartz Tungsten Halogen (QTH) lamp is mounted into the Oriel 77501 housing with an optic fiber connector. The light is collected by housing optics and guided by optic fiber (Ø 200 μm) to the lens tube, where an achromatic collimating lens (L2, fixed) and sample-focused lens (L3) are installed to focus the beam on the single crystal mounted on the goniometer. If necessary, L3 can be easily exchanged or removed without setup destabilization. (Figure 2.LEDs and CW laser A series of different Constant Wavelength (CW) Lasers with various wavelengths are used for the generation of PLI at low temperatures as well as for the probing of the transient absorption after pulsed excitation (422nm, 514 nm, 685 nm coherent OBIS laser, (max Poutput = 40 mW), 785 nm (max Poutput = 100 mW), Nd:YAG 1064 nm). LED lights in the wavelength range of 365 -1050 nm from Thorlabs LP series are used. They are equipped with collimating lenses generating a rather homogenous beam of 10-30 mm diameter at the focal position.

Figure 2 . 2 .

 22 Figure 2.2. Their specifications are listed in Table 2.1.

Figure 2 . 2 :

 22 Figure 2.2:Tunable Nd: YAG Pulse laser EKSPLA NT342 (left panel); Nd-YAG solid-state laser (Surelite II-10 model) (right panel)

  ~ 10 -5 bar) inside a closed-cycle cryostat. Ground state was measured and then irradiated through KBr windows with LED and CW or pulsed laser light in the wavelength range of 365 -980 nm. The power of LED light used during the experiments is different depending on the type of sample. Infrared spectroscopic measurements during irradiation with continuous wave is shown in Figure 2.3, and with pulsed laser light is shown in Figure 2.4. The maximum population at the particular wavelength max is determined as a function of irradiation time until saturation of PLI is achieved. By integrating the area under the characteristic band of the ground state in GS (AGS) and the photoinduced state (APLI), we can also deduce the population of metastable states 𝑃𝑜𝑝. 𝑃𝐿𝐼 = 𝐴 𝐺𝑆 -𝐴 𝑃𝐿𝐼 𝐴 𝐺𝑆. After the population, the relaxation of the PLI is measured by following the increase in temperatures to determine the stability of the PLI at a particular temperature.

Figure 2 . 3 :

 23 Figure 2.3: Infrared spectrometer measurements during irradiation with continuous wave laser of a metal nitrosyl complex.

Figure 2 . 4 :

 24 Figure 2.4 : Infrared spectrometer measurement during irradiation with ns-pulsed laser (5 ns pulse duration)

Figure 2 . 5 :

 25 Figure 2.5: DSC measurements showing crystal irradiation of a metal nitrosyl complex using 1064nm CW laser.

Figure 2 . 6 :

 26 Figure 2.6 : Schematic pump-probe optical absorption spectroscopy on the optical table.

Figure 2 . 7 :

 27 Figure 2.7: Experimental setup of transient pump-probe optical absorption spectroscopy.

Figure 2 . 8 :

 28 Figure 2.8: Single crystal optical absorption spectroscopy before and after photoexcitation

Figure 2 . 9 :

 29 Figure 2.9: Steady-state photocrystallography SuperNova microfocus-source diffractometer equipped with an ATLAS detector (left panel); An optical table top for irradiation (right panel).

Figure 2 .

 2 Figure 2.10: Steady-state photocrystallographic experiment Bruker D8 Venture diffractometer equipped with PHOTON III CMOS detector.

Figure 2 .

 2 Figure 2.11: Steady-state photocrystallography with pulsed laser irradiation.

Figure 2 . 13 :

 213 Figure 2.13 : General schematic of Argand diagram representing difference in structure factor and phases going from GS to ES.

2. 10 Time-resolved photocrystallography 2 . 10 . 1

 102101 photons have energy ranging from Terahertz to hard X-rays. With the help of optical systems, each of the 29 beamlines can choose and shape the beam afterward to their necessary wavelengths according to the experimental technique. The CRISTAL beamline works in hard X-rays between 6 -30 keV energy range. A silicon monochromator allows one to choose the wavelength with a resolution of energy ∆𝐸 𝐸 ⁄ = 2.410 -4 and a photon flux of 10 13 photons/s in the case of 8 keV[START_REF]Cristal Soleil beamlines: Cristal[END_REF].

Figure 2 . 14 :

 214 Figure 2.14: Schematic overview of the main synchrotron components (Source: synchrotron SOLEIL).

  A 6-circle diffractometer (Figure2.15) with kappa geometry is installed at the beamline. The geometry of the diffractometer allows controlling the orientation of the sample. The detector arm of the diffractometer can be rotated perpendicular to the incident X-ray beam, both on the horizontal and vertical axis. With the help of a cryostat, the sample temperature can be set down to 5 K. The laser pump beam is installed with a repetition rate f = 1000 Hz with pulse energy of 5 mJ, a central wavelength of 800 nm, and 35 fs pulse duration. The illuminated sample area by the pump beam is 3.5×1.0 mm 2 and 1.7×0.5 mm 2 by the probe X-ray beam. The synchronization of pump (laser) and probe (X-ray) pulses is done on a 352 MHz signal, and temporal overlap is achieved with a fast photodiode. The beamline is equipped with the 2D hybrid pixel detector XPAD3.2 as a detection system. The detector is made up of 8 modules composed of 7 chips (1 module= 7chips of 120×80 pixels; 1 pixel=130×130 m 2 ), with an entire area of 560×960 pixels. For time-resolved experiments, the detector is triggered by an external signal able to synchronize it with the incoming pulses. In the triggered mode, the number of counts/pixel/second is lowered to a few photons/pixel/s, implying that numerous images must be collected per delay time to provide sufficient Intensity. The pump-probe diffraction temporal resolution at the CRISTAL beamline allows to examine structural dynamics in the picosecond time scale range with a reciprocal spatial resolution of Q ~ 5×10 4 Å -1 . It is primarily used to investigate the structural phase transitions, thermal processes, photoinduced linkage isomers, and strain wave propagation at these temporal time scales.

Figure 2 .

 2 Figure 2.15: Six-circle diffractometer at CRISTAL beamline SOLEIL.

  Single crystal optical absorption spectroscopy and transient pump-probe optical setups are installed and validated (Figure 2.8, Figure 2.6, Figure 2.7). These developments are briefly explained in Chapter 5.

Figure 2 . 16 :

 216 Figure 2.16 : Overall experimental set up of in-house pump-detection synchronization for time resolved photo-crystallography at ms timescale.

3. 1

 1 photocrystallographic studies in 1994[102], Pressprich et al. also interpreted the photoinduced states by small changes in bond lengths as done in the previous neutron photocrystallography.Only later, with the work of Fomitchev et al.[START_REF] Fomitchev | X-ray Diffraction Analysis of Geometry Changes upon Excitation: The Ground-State and Metastable-State Structures of K2[Ru(NO2)4 (OH)(NO)[END_REF] on K2[Ru(NO2)4(OH)(NO)] and finally with the study of Carducci et al. on SNP[START_REF] Carducci | Diffraction Studies of Photoexcited Crystals: Metastable Nitrosyl-Linkage Isomers of Sodium Nitroprusside[END_REF] the nowadays established picture of the isonitrosyl (MS1) and side-on (MS2) configuration for NO in these compounds was experimentally confirmed, as later also verified by neutron photocrystallography, again for SNP[START_REF] Schaniel | Structure of the light-induced metastable state SII in Na2[Fe(CN)5NO]•2H2O[END_REF][START_REF] Schaniel | Neutron diffraction shows a photoinduced isonitrosyl linkage isomer in the metastable state S I of Na2[Fe(CN)5NO]•2D2[END_REF].

  complexes have been found particularly interesting because amongst them, the compounds exhibiting the highest thermal stability and population of PLI have been discovered[88,106- 108]. One of the rare examples of nearly complete conversion to PLI is [Ru(py)4Cl(NO)] 2+, where a population of 92% of the MS1 isonitrosyl isomer can be obtained at 80 K upon irradiation with blue light[87]. Besides the ruthenium pyridine nitrosyl complexes, the family of ruthenium amine nitrosyl complexes shows similar qualities in terms of thermal stability.The presence of PLI has been reported in [Ru(NH3)4(H2O)NO]Cl3•H2O, trans-[Ru(NH3)4(OH)NO]Cl2 [107] and trans-[Ru(NH3)4FNO]SiF6 [109]. The metastable states in [Ru(NH3)5(NO)]Cl3•H2O complex were studied by DSC [110], revealing the existence of two long-lived metastable states with the activation energies EA(MS1) = 0.73(3) and EA(MS2) = 0.66(3) eV respectively, IR [111,112] evidencing the generation of two metastable linkage

  MS2 in [Ru(NH3)5(NO)]Cl3•H2O, one has to select the most appropriate and relevant structural model. As outlined in detail in the appendix, rigorously speaking, the crystal structure of [Ru(NH3)5(NO)]Cl3•H2O corresponds to the non-centrosymmetric space group Pna21 with inversion twinning and non-equal contributions from the two twin components, leading to apparent structural disorder. In order to enable a structural analysis of the PLI, we consider as an acceptable and necessary approximation the structural description of [Ru(NH3)5(NO)]Cl3•H2O in the centrosymmetric space group Pnma, that is to say, considering a structural disorder with equal 0.5 fractions of the two orientations.The ground state structure has been derived from X-ray diffraction using Mo Kα radiation in the Pnma space group. All non-hydrogen atoms were refined anisotropically, while the hydrogen atoms attached to the Nsp3 atoms of the amine ligands were refined using a riding model. The ground state crystal structure in Pnma space group is built from [Ru(NO)(NH3)5] 3+ octahedral cations located on a mirror plane perpendicular to the crystallographic b direction, one water molecule, and three chloride anions, two of which exhibit positional disorder.

  2; the fragmented structure of the [Ru(NO)(NH3)5] 3+ cation and crystal packing are given in Figure 3.1.

Figure 3 . 1 :

 31 Figure 3.1 : (a,b) Ground state molecular structure (with atomic labeling) showing the positional disorder on the nitrosyl ligand and (c) crystal packing of [Ru(NO)(NH 3 ) 5 ]Cl 3 •H 2 O (only one component of the structural disorder is displayed). Intermolecular hydrogen bonds are depicted as dashed lines. Displacement ellipsoids are drawn at the 50% probability level. In the crystal packing, the [Ru(NO)(NH3)5] 3+ units are stacked in the direction of the crystallographic X-axis at y=1/4 and y=3/4, and connected by a network of O-H…Cl and N-H…Cl hydrogen bonds through the water molecule and chloride ions.

  electron density redistribution upon laser light exposure. Two very strong positive (in blue) and negative (in red) peaks of electron density differences are located close to the Ru position, which evidences a shift of electron density, and, therefore, a slight displacement of the Ru atom from GS to MS1 in the direction opposite to the disordered NO groups (Figure3.2). The atomic shift occurs in the crystallographic mirror plane (the m plane perpendicular to the crystallographic b direction), and still preserves the local symmetry. A clear peak of negative electron density difference occurs at the O1 position, while at the N1 position, we observe a small negative electron density difference surrounded by stronger positive electron density differences. This is characteristic of a nearly 180° rotation of the nitrosyl ligand in the photoinduced state[START_REF] Carducci | Diffraction Studies of Photoexcited Crystals: Metastable Nitrosyl-Linkage Isomers of Sodium Nitroprusside[END_REF]87,[START_REF] Fomitchev | X-ray Diffraction Analysis of Geometry Changes upon Excitation: The Ground-State and Metastable-State Structures of K2[Ru(NO2)4 (OH)(NO)[END_REF][START_REF] Mikhailov | Combining photoinduced linkage isomerism and nonlinear optical properties in ruthenium nitrosyl complexes[END_REF], and can therefore be considered as a signature of photo-induced linkage isomerism associated to the formation of an isonitrosyl Ru-O1-N1 configuration in MS1.

Figure 3 . 2 :

 32 Figure 3.2 : Photo-difference maps of Ruthenium pentaamine complex revealing the largest positive and negative regions located close to the atomic positions, which are affected by light irradiation. (a)MS1 was produced after laser irradiation of 422nm (P=60mW) for 45 minutes. Countours@0.1eÅ -3 .

Figure 3 . 3 :

 33 Figure 3.3 : MS1configuration of Ruthenium pentaamine complex solved in Pnma space group. Only the MS1 configuration is shown.

Figure 3 .

 3 Figure 3.4 (a) shows the photodifference map obtained from the data set collected after irradiation with 1064 nm laser 100 mW (transferring from the previously generated MS1). We observe well-defined electron-deficient regions (red) centered on the position of the heaviest atom 'Ru' as well as on the N1 and O1 positions of the nitrosyl ligand of GS leading to the

Figure 3 . 4 :

 34 Figure 3.4 : Photo-difference map of [Ru(NO)(NH3)5]Cl3•H2O (a) after transferring MS1 into MS2 with 1064 nm (100 mW) CW for laser irradiation for 107 min. Selected section (red rectangle) out of the MS2 Photo-difference maps ofRuthenium pentaamine complex in Pnma space group (Ru1-N1-O1 plane) Countours@0.1eÅ -3 (b) for which slices (Countours@0.05eÅ -3 ) perpendicular to the Ru1-N1-O1 direction reveals the split position of the side-on linkage isomer.

2 .

 2 With respect to the GS (Ru1-N1=1.835(6)Å), the Ru-N distances are increased in MS2 (Ru1-N1A=2.46(5); Ru1-N1B=2.537 Å) and also the Ru-O bond is longer (Ru-O1A=2.41(4) Å) in MS2 compared to MS1 (Ru-O1A=1.934(14) Å). Moreover, nitrosyls of MS2 O1A-N1A & O1A-N1B distances are hardly changed ~1.11 Å. The N1A-Ru1-O1A and N1B-Ru1-O1A bond angles are almost identical ~26.3(7). In the refinement, it was necessary to fix the occupancy of the two nitrogen to be equal and their sum to that of the commonly shared O1A occupancy. This way, the population of N1A and N1B, was fixed to 2.71% each, a net of 5.42%, while O1A population was 5.42%. Considering the symmetry (disorder with NH3 over two sites), this corresponds to a population of MS2 of 10.84% and, consequently, 89.16% molecules in GS. The most important intermolecular interactions involving the nitrosyl ligand of MS2 are depicted in the appendix. Considering the crystal environment of the molecule, interactions possibly participating in the stabilization of the photoinduced states, N1A forms N1A•••Cl interactions with an average distance of 3.0965 Å while N1B contributes in N1B•••Cl interactions with an average distance of 2.782 Å. The O1A have an O1A•••Cl interaction of 2.734 Å. O1A is involved in O1A•••O1A and O1•••O1A interactions with 2.740 and 2.887 Å distances, respectively. This indicates that it might be the crystal environment around O1A, that leads to a single position for this atom, while the nitrosyl of nitrogen is more prone to

3 . 1 . 4 Conclusion

 314 Photocrystallographic investigations of [Ru(NH3)5(NO)]Cl3•H2O confirmed the nature of the MS1 and MS2 isomers in {MNO} 6 complexes, i.e., the isonitrosyl and side-on configurations for MS1 and MS2, respectively. While MS1 bares no surprises, the investigation of MS2 revealed for the first time experimental evidence for eclipsed and staggered configuration in the same complex, as was predicted by DFT[115]. The fact that these two structurally welldistinct configurations were not previously detected by spectroscopy or calorimetry is most probably due to their almost identical structural characteristics (bond lengths and angles), their only difference being a rotation of the NO ligand around the original Ru-N-O axis. Since there

Figure 3 . 6 (

 36 left panel) illustrates the results obtained for II in the range 1650 -1850 cm - 1 . The υ(NO) stretching vibration of the GS found at 1708 cm -1 decreases after irradiation, while a new band, corresponding to the υ(NO) stretching vibration of the PLI arises at 1779 cm -1 . The maximum population of about 50% for PLI is obtained for irradiation with wavelengths in the spectral range 590-620 nm. Performing the same procedure at different temperatures yields information on the stability of the PLI.

Figure 3 . 6 (

 36 right panel) shows the spectra recorded between 10 and 100 K. Above 70 K no significant amount of PLI is detected (data collection time after stopping irradiation is about 1 minute).

Figure 3 . 6 :

 36 Figure 3.6 : Infrared spectra in the range 1650 -1850 cm -1 after irradiation at 10 K with different wavelengths (left panel) and at different temperatures (right panel).

Figure 3 . 7 (

 37 left panel) illustrates the spectra as a function of time after starting the recording in the IR spectrometer. In order to have good statistics, we record and merge 20 IR scans, which take about 1 minute. Plotting the sequence of six repeated 20 scan recordings, one clearly observes the vanishing of the PLI band and complete recovery of the GS band. In order to identify the origin of the depopulation back to GS, we performed another experiment: we irradiated the sample precisely as before outside the IR spectrometer for the same time but then simply waited 3 minutes before starting the recording. Figure3.7 (right panel) shows that in this case, we obtain on the first 20 scans exactly the same population as in the previous measurement. If the depopulation would be due to thermal relaxation, we should have observed the spectrum corresponding to the 3-minute waiting time (that is, spectrum Scan 61-80 in the left panel). Clearly, the depopulation is induced by the probing beam of the IR spectrometer. This indicates that the PLI state absorbs in the infrared region, as will be further elucidated in the following by UV-Vis spectroscopy.

Figure 3 . 7 :

 37 Figure 3.7 : Infrared spectra in the range 1650 -1850 cm -1 after irradiation with 590 nm at 10 K as a function of measurement time (left panel) and after an initial waiting time (right panel).

Figure 3 . 8 (

 38 right panel) shows the resulting difference spectra between irradiated and GS, clearly revealing a broad absorption band with a maximum of 1100 nm (1.127 eV).

Figure 3 . 8 :

 38 Figure 3.8 : (left) UV-Vis spectra in the range 350-900 nm in the GS (black line), and after sequential irradiation with 589 nm (red line), 1064 nm (blue line), 589 nm (green line), and 473 nm (pink line). (right) UV-Vis difference spectra (irradiated -GS) in the range 500-1750 nm.

Figure 3 . 9 :

 39 Figure 3.9 : UV-Vis spectra in the range 350-900 nm: (left) at 10 K in the GS (black line) and after irradiation with 589 nm (red line), and at 60 K in the GS (blue line) and after irradiation with 589 nm (green line). (right) GS and PLI at 60 K (black and red line) and subsequent heating to 70 K (blue line).

OLEX2- 1 . 3

 13 suite[START_REF] Dolomanov | OLEX2 : a complete structure solution, refinement and analysis program[END_REF]. All non-hydrogen atoms were refined anisotropically. A riding model[120] was used for the hydrogen atoms, constraining the isotropic displacement parameters to be 1.2 times those of the attached C atom. The unit-cell parameters change only slightly from a = 8.4842(8) Å, b =8.7672(8) Å and c = 13.2798(10) Å in GS to a = 8.4986(6), b =8.7877(6) and c = 13.2646(8) in the photo-induced state. The angles only slightly change from 94.744(2), 90.253(2), 93.610(3) in GS to 94.681(2), 90.191(2), 93.522(2) in photo-induced state. The unit-cell volume expands from 982.40(15) in GS to 985.44(11) Å 3 in the photo-induced state. Numerical absorption correction was performed. Relevant data are given in Table 3.3.

Figure 3 . 10 : 3 . 2 . 3 . 3 . 2

 31032332 Figure 3.10 : GS structure of II at T = 10 K.

Figure 3 .

 3 Figure 3.11 : 2D Photodifference map (right panel) with Contours@0.2e/Å 3 ; 3D Photodifference map (left panel) with Contours@0.7e/Å 3 .

  4 both models converge with almost identical agreement factors (R = 0.042/0.042, wR2 = 0.142/0.141 and GoF = 1.14/1.14), yielding populations of about 5-6% for PLI. Inspecting the ADPs might give a clue whether the isonitrosyl or nitrosyl model should be favoured. As shown in earlier works on nitrosyl linkage isomers, the wrong atom assignment leads to over-or underestimated ADPs, e.g., if we assign N to a position actually occupied by O, the electron density of the model underestimates the experimental electron density. To compensate, the ADPs will be decreased. The opposite behaviour is encountered when placing an O on an N position; the model overestimates the

  5). The most significant difference is the Co-N/O distance with 1.56(4) Å and 1.64(4) Å and the corresponding Co-N-O and Co-O-N angles of 143(6)° and 148(8)°. Based on the X-ray

Figure 3 .Figure 3 . 13 :

 3313 Figure 3.12 : PLI isonitrosyl (H2)

  As outlined in Chapter 1, {Ru(NO)2}8 dinitrosyls have been studied extensively in a collaboration between the group of P. Klüfers at LMU and the CRM2 laboratory. The focus was on a family of bis-phosphane salts of the formula [Ru(NO)2(PR3)2X]BF4 (X=Cl, Br, I; PR = PPh, PCy, PCyp, P i Pr), for which two structure types occur: (i) vacant octahedron (vOC-5) with one (strongly) bent and one (almost) linear NO ligand, (ii) a trigonal bipyramid (TBPY-5) with two almost linear NO ligands [124]. Concerning PLI, the vOC-5 type compounds exhibit up to three different PLI [82], while the TBPY-5 type compounds exhibit only one PLI [83]. In both cases, only one of the NO ligands is photoactive and adopts different structural conformations. For the TBPY-5 type example [RuBr(NO)2(PCyp3)2]BF4 [83], one of the linear NO ligands is significantly bent in the PLI, the Ru-N-O angle being decreased from 166° in GS to 114° in the PLI away from the other NO ligand, thus representing an anti-bent isomer (Example E5 in Table

Figure 3 .

 3 Figure 3.14 : IR spectra of [Ru(NO) 2 (PPh 3 ) 2 ] at 100 K and after irradiation with light of different wavelengths, indicating that the maximum population can be obtained using 556 nm wavelength.

Figure 3 .

 3 Figure 3.15 illustrates the effect of light irradiation on the example of 556 nm wavelength. BothGS bands at 1612 cm -1 and 1657 cm -1 decrease, while two new bands arise at 1445 cm -1 and 1510 cm -1 . Assuming ν(NO)sym/ν(NO)asym at 1657/1612 cm -1 for GS and at 1510/1445 cm-1

Figure 3 .

 3 Figure 3.15 : IR spectra of [Ru(NO) 2 (PPh 3 ) 2 ] at 100 K in the ground state (GS, black line) and after irradiation with light of 556 nm wavelength (green line) and after subsequent irradiation with 660 nm (red line).

Figure 3 . 16 :

 316 Figure 3.16 : IR spectra of [Ru(NO) 2 (PPh 3 ) 2 ] at 100 K and after irradiation with light of 556 nm wavelength and at different temperature upon heating indicating the decrease of the PLI bands around 210-220 K and the transfer back to GS.

Figure 3 .

 3 Figure 3.17 : (left) IR spectra of [Ru(NO) 2 (PPh 3 ) 2 ] at 100 K as a function of fluence Q = I t using 556 nm wavelength illustrating increase of two PLI bands at 1445 cm -1 and 1510 cm -1 and decrease of GS bands at 1612 cm -1 and 1657 cm -1 . (right) Corresponding difference spectra.

  A of these four bands as a function of Q shows a mono-exponential behavior 𝐴 = 𝐴 0 (1 -𝑒𝑥𝑝(-𝑄/𝑄 0 )) with the same effective irradiance Q0 = 1.2(1) Jcm -2 for all bands (Figure 3.18).

Figure 3 . 18 :

 318 Figure 3.18 : Band areas of PLI and GS bands integrated over spectral ranges as indicated in brackets, as a function of fluence Q = I t. The solid line corresponds to mono-exponential fit.

Figure 3 .

 3 Figure 3.19 : UV/Vis spectra in the range 850-370 nm, before and after irradiation with 590 nm with the corresponding difference spectrum, indicating the decrease of GS bands and increase of MS bands.

3. 3 . 3 . 3

 333 Details of the calorimetric investigationDSC measurements allow for the determination of the activation energy and, thus determination of the energy barrier separating the PLI from the GS. Figure3.20 shows the result of the corresponding measurement and analysis supposing an Arrhenius-like behavior, yielding activation energy of 0.63[START_REF] Wendel | Physical Implications of a Fundamental Period of Time[END_REF] eV for PLI to GS relaxation. Note that we observe only one decay indicative of a single PLI state.

Figure 3 . 20 :

 320 Figure 3.20 : Difference of heat flow (black line) measured before and after light irradiation. Fit assuming an Arrhenius-like decay, yielding an activation energy of 0.63(1) eV.

  It is noteworthy that due to high photosensitivity of the sample, illumination was carried out in absolute darkness to get the highest possible MS population. Complete diffraction data were collected in the photo-stationary state; no space-group change occurred with respect to the ground state. The unit-cell parameters change only slightly from a = 9.22200(11) Å, b =36.4379(4) Å and c = 9.90412(12) Å in GS to a = 9.2775(5) Å, b = 36.4176(16) Å and c = 9.9290(5) Å in the photo-induced state. The angles only slightly change from β = 111.0381(13)° in the GS to β = 111.867(6)° in photo-induced state. The unit-cell volume expands from 3106.24(7) Å 3 in the GS to 3113.3(3) Å 3 in the photo-induced state. 36255 reflections were measured up to a maximum resolution of sinθ λ -1 = 0.759 Å -1 , and merged to 10613 unique reflections (Rint= 0.046). Numerical absorption correction was performed.

Figure 3 . 21 :

 321 Figure 3.21 : Structure of [Ru(NO) 2 (PPh 3 ) 2 ] complex with selected bond distances (Å) and bond angles () in the ground state showing distorted tetrahedral geometry.

Figure 3 . 22 : 3 . 3 . 4 . 2

 3223342 Figure 3.22 : Selected intermolecular interactions involving the nitrosyl ligands in the ground state of [Ru(NO) 2 (PPh 3 ) 2 ].

Figure 3 .

 3 Figure 3.23 : (Left) Photodifference map of III after irradiation with 590 nm light at 100 K; blue: positive values, red: negative values; contour-lines every 0.5 e Å -3 . (Right) comparison of GS and MS structure illustrating the (l-NO-κN) 2 GS to the metastable (a-NO-κN) 2 MS configurational change.

  double bent nitrosyl conformation (a-NO-κN)2. The other models are presented in the appendix. Two NO conformations were used to refine the appropriate atomic positions, one corresponds to the GS conformation Ru-N1A-O1A (bond angle of 169.4(5), dRu-N1A = 1.885(5) Å, dNIA-O1A = 1.172(5) Å), Ru-N2A-O2A (bond angle of 174.2(5), dRu-N2A = 1.763(5) Å, dN2A-O2A = 1.154(6) Å) with an occupancy factor of 53.6(4)%, and the other corresponds to the excited-state conformation Ru-N1B-O1B (bond angle of 119.1(5), dRu-N1B = 1.732(4) Å, dN1B-O1B = 1.212(6) Å), Ru-N2B-O2B (bond angle of 135.2(5), dRu-N2B = 1.835(5) Å, dN2B-O2B = 1.262(6) Å) with an occupancy factor of 46.4(4)%, respectively. The final refined model for the PLI is depicted in Figure 3.24. When the refinement is performed without constraining the occupancies of the two NO groups in the PLI to the same value, one obtains 46.5(3)% for N1BO1B and 59.3(2)% N2BO2B. However, no improvement in agreement values is obtained, and ADP values of involved atoms (N1A, N1B, O1A, N2A, N2B, O2A, O2B) increase by 0.002 to 0.003 Å 2 compared to those given in Table 3.7.

Figure 3 . 24 :

 324 Figure 3.24 : Structural model of the PLI state upon photo-excitation at T = 100 K.

  680 Å). Besides these the C•••N and C•••O interactions are also formed with distances ranging from 3.161 Å to 3.401 Å and 3.218 Å to 3.4875 Å, respectively.

Figure 3 . 25 :

 325 Figure 3.25 : Selected intermolecular interactions involving the nitrosyl ligands in the PLI state compared to the GS of [Ru(NO) 2 (PPh 3 ) 2 ].

Figure 3 . 26 :

 326 Figure 3.26 : The transition state (TS), which connects the double-linear GS with the double-bent MS) of III. For an animation of the concerted motion see video.

Furthermore, also the

  next excitations in the visible part of the spectrum resemble the transition of one electron from the Ru-NO bonding to the Ru-NO antibonding regime (Figure 3.27 left), thus eliminating one of the bonds. Illumination of samples of III thus causes the loss of one Ru-NO-backbond-related MO, which initiates the motion of the nitrosyl ligand.

Figure 3 . 27 :

 327 Figure 3.27 : From bottom to top: the four orbitals that are doubly occupied in the major configuration of the GS (left) and the MS (right) in a CASSCF(8,8) approach (weight of the major 22220000 configurations in both the GS and MS: 0.76; leftmost: MO numbers, occupation and assignment with z up, x right and y into the plane for the GS; almost the same values were obtained for the MS).

Part 1 :Part 2 : 4 . 1

 1241 photocrystallography

Figure 4 . 1 :

 41 Figure 4.1 : The comparison of CW laser energy density with pulsed laser with its temporal shape.

Figure 4 . 2 (

 42 left panel). Using P = 45 mW with the pinhole of 500 m (corresponding to 23 W/cm 2 and thus 2.3 J/cm 2 in a single pulse), the crystal degrades visibly after the second pulse Figure4.2 (left panel). The corresponding optical absorption spectra are shown in Figure4.2 (right panel), although, given the macroscopic destruction of the crystal, the interpretation of these spectra is difficult, as the scattering of light on the damaged surface will occur, changing the baseline considerably.

Figure 4 . 2 :

 42 Figure 4.2 : Snapshots of the [Ru(Py) 4 (NO)F](ClO 4 ) 2 crystal and corresponding optical absorption spectra, recorded after irradiation with maximum pulse laser power.

Figure 4 . 3 :

 43 Figure 4.3 : Structures of [Ru(Py) 4 (NO)F](ClO 4 ) 2 I and [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O II, selected for the CW and pulsed laser irradiation comparative in-situ photocrystallographic study.

  Single crystal data collection was performed at 100 K for I [Ru(Py)4(NO)F](ClO4)2 and II [Ru(NH3)5(NO)]Cl3•H2O complexes in GS and PLI state. For I all the CW and ns-pulse steadystate experiments were collected using the STOE Stadivari diffractometer equipped with RebirX detector, using Mo K radiation (=0.71073 Å). For II the pulse experiments were performed on the STOE Stadivari diffractometer, whereas the CW experimental series of [Ru(NH3)5(NO)]Cl3•H2O data collection was performed using Supernova four-circle

  meter to P = 45 mW and I = 23 W/cm 2 (f = 10 Hz) for 420 nm. The power can be adjusted directly on the laser output. After alignment of the pulsed laser and determination of the peak power, the pinhole was removed and a block shaped, clean (without vacuum grease on either face of the crystal to avoid any loss in penetration of the laser beam or back scattering) crystal was mounted using super glue. The schematic of the in-house set up is shown in Figure4.4.

Figure 4 . 4 :

 44 Figure 4.4 : Schematic in-house CRM2 set up for the photo-structural and optical characterization of the single crystal.

Figure 4 .

 4 Figure 4.4 illustrates the technical design of the experimental setup that was constructed inhouse. The experimental set up comprises the UV-Vis source lamp QTH 100 W with the lamp output power ~500-750 mW (I0) to probe the absorption of the crystal in the spectral range 350-900 nm (while the crystal mounted on the goniometer is kept at a temperature of 100 K by the cryostream). The transmitted light (It) going through the single crystal is collected with adapted optics, capturing the whole transmitted beam and focusing into a fiber, which is connected to the ocean optics USB4000 spectrophotometer (fiber output power ~10-15 mW).

Figure 4 . 5 :

 45 Figure 4.5 : Crystal choice for the optical absorption spectral mapping (left panel) and the spot size of the transmitted light (right panel) from the crystal to the spectrophotometer detector.

Figure 4 . 6 :Figure 4 . 7 :

 4647 Figure 4.6 : Illustration of color change in I upon irradiation with 420 nm, inducing the photoconversion from GS to MS1.

Table 4 . 1 : 2

 412 Photodifference map for I after generation of MS1 with 420 nm, Q = 1200 J/cm 2 . The contours level for both the maps is 0.2 eÅ -3 . population of MS1 is 71(2)% for molecule 1 and 92(2)% for molecule 2 (b). Using pulsed laser irradiation population of MS2 is 88(3)% and 86(3)% for molecule 1 and 2, respectively. In a second step, MS2 was generated from MS1 by irradiation with 940 nm wavelength from a CW source (Thorlabs LED M940L3), with P = 200 mW for t = 60 min, yielding Q = 923

  3 eÅ -3 . (a) population of MS2 molecule 16% (b) population 15% of MS2 in the second independent molecule. of MS2 is 17(5) % for molecule 1 and zero for molecule 2 (b) Using pulsed laser irradiation population of MS2 is 13.1(5) % and 12.4(5) % for molecule 1 and 2, respectively. (a) MS2 with CW (b) MS2 with pulsed Mol. 1

Figure 4 . 8 :

 48 Figure 4.8 : The population of MS1 as a function of Q. The line is a fit P(Q) = P max (1-exp(-Q/Q 0 ) with Q 0 = 450(200) J/cm 2 and P max = 91(16)%.

4. 3

 3 Comparative photocrystallographic study of II with CW and pulsed irradiationAs a second example, [Ru(NH3)5(NO)]Cl3•H2O is investigated in the same manner as the above described [Ru(Py)4(NO)F](ClO4)2. Compared to the latter, [Ru(NH3)5(NO)]Cl3•H2O crystallizes in an orthorhombic space group, which allows faster data collection, an important criterion in view of the goal of performing time-resolved measurements. As a drawback, this compound shows disorder (see Chapter 3) and a somewhat lower population. An interesting point is the existence of two configurations for MS2, found in the CW measurements described in Chapter 3, and the effect of pulsed laser irradiation on this point is worthwhile to investigate.

Figure 4 . 9 :

 49 Figure 4.9 : Absorption spectra of GS, MS1, and MS2 in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O (D. Schaniel, private communication).

Figure 4 .Figure 4 . 11 :Figure 4 . 12 :

 4411412 Figure 4.10 : GS, MS1 and MS2 crystal irradiation of II.

Figure 4 . 13 :

 413 Figure 4.13 : MS1 and MS2 photodifference maps for [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O after CW and pulsed generation of MS1 and MS2. The contours level for both the maps is 0.2eÅ -3 and for CW 0.15 eÅ -3 .

Finally, as for

  the [Ru(Py)4(NO)F](ClO4)2 compound, we identify the most sensitive reflections of II, in view of a potential time-resolved experiment. The results calculated from the CW and pulsed experiments are shown in the appendix D. Despite the qualitative agreement of the structural models found, there are some quite significant differences in the most sensitive In the ruthenium nitrosyl compounds and especially for [Ru(Py)4(NO)F](ClO4)2 and [Ru(NH3)5(NO)]Cl3•H2O there is the hypothesis that the MS1 state is generated in a two-step process. The excitation of a molecule with one photon leads to the population of MS2, which is immediately transferred to MS2 by absorption of a second photon of the same energy (wavelength). This mechanism has been postulated early by Gorelsky & Lever [115] from DFT calculations in order to explain the non-observation (at that time) of MS2 in the [Ru(NH3)5(NO)] 3+ cations. Later MS2 was found in the population via MS1 by infrared light [110] for this compound. The same hypothesis was put forward for [Ru(Py)4(NO)Cl] 2+ by calculations [151,152] and based on low-temperature optical spectroscopy [153].

Figure 4 . 14 :

 414 Figure 4.14 : Scheme illustration population and relaxation of MS1 and MS2 in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O .MS1 is generated in a two-step process via MS2.

Figure 4 . 15 :

 415 Figure 4.15 : Lifetime of MS1 and MS2 as a function of temperature in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O. For MS2 the two curves show the calculations according to activation energy and frequency factor known from DSC measurements and from transient absorption (TA) measurements.

  and more recently by Collet group and co-workers on thin film samples of rubidium manganese hexacyanoferrate microcrystals [158]. I performed a systematic series of single-shot pulse laser photocrystallographic experiments using the two previously investigated metal nitrosyl complexes [Ru(NO)(Py)4F](ClO4)2 complex (I) and [Ru(NH3)5(NO)](Cl)3•H2O (II) of {MNO} 6 family as described in part I of this Chapter. For exciting the sample, EKSPLA NT340 pulsed laser (for characteristic features, see chapter 2) with a wavelength of 420 nm and a pulse duration of 5 ns was used. A systematic test was performed to determine the most suitable laser power that can allow performing a one-pulse single-shot experiment for achieving saturation and at the same time avoiding crystal damage with pulse laser for both the nitrosyl complexes I and II single crystals.

Figure 4 . 17 :

 417 Figure 4.17 : Structure of MS1 in Mol.1 and 2 of I after one pulse irradiation. The population of the first molecule in MS1 is 27(2)%; while the population of MS1 is the second independent molecule is 25(3)% after irradiation with one pulse.

Figure 4 . 18 :

 418 Figure 4.18 : IR spectra of [Ru(NO)(Py) 4 F](ClO 4 ) 2 after ns-pulse laser irradiation with 420 nm with f =10 Hz; P = 22 mW at 100 K. (Top left) Increase of MS1 and MS2 bands. (Top right) GS and final PLI state after 4000 pulses. (Bottom left) zoom on MS2 region (Bottom right) zoom on MS1 region

crystals at 100 K

 100 . The systematic test for tuning maximum power without damaging crystal with pulse laser shows that less than P = 12 mW would be the best choice for this particular crystal see Figure 4.19. The pulse laser beam was aligned with the 500 micron pin hole (Figure 4.20).

Figure 4 . 19 :

 419 Figure 4.19 : Crystal damage with pulse laser.

Figure 4 . 20 :

 420 Figure 4.20 : Pinhole used for the alignment of pulse laser beam size (Left panel). Alignment of 420 nm pulses laser irradiation through the pinhole (Right panel).

Figure 4 . 22 :

 422 Figure 4.22 : Crystal 1 structural model of MS1after 1 pulse irradiation with P = 10 mW.

Figure 4 . 23 :

 423 Figure 4.23 : Crystal 2 structural model of MS1 after 1 pulse irradiation with P =7 mW.

Figure 4 . 24 :

 424 Figure 4.24 : Crystal 3 structural model of MS1after 10 pulses irradiation with P = 1 mW.

Figure 4 . 25 :

 425 Figure 4.25 : Most sensitive common reflections from GS to PLI in II Crystal 1, 2 and 3.

Figure 4 .

 4 Figure 4.26 : IR spectra of [Ru(NH 3 ) 5 (NO)](Cl) 3• H 2 O after ns-pulse laser irradiation with 420 nm with f = 10 Hz; P = 22 mW at 100 K. (Left) Increase of MS1 band. (Right) GS and final PLI state after 600-54000 pulses.

Figure 4 . 27 :

 427 Figure 4.27 : Schematic of the concept applied in the thesis work from steady state to single-shot to timeresolved in-house photocrystallagraphy.

Figure 5 .

 5 1 (right) summarizes the conclusions of these studies, indicating that after the ultrafast excitation the MS2 minimum is reached within a few picoseconds. More recently time-resolved X-ray diffraction experiments were performed on a laboratory source with millisecond time resolution[START_REF] Casaretto | In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector[END_REF]. On the example of a few Bragg reflections, the relaxation of the MS2 configuration was evidenced on this time scale at a temperature of 150 K, where the lifetime of MS2 is of the order of 10 ms.

Figure 5 . 1 :

 51 Figure 5.1 : (left) structures of GS, MS1, and MS2 in SNP. (right) Excitation scheme for MS2 deduced from ultrafast optical spectroscopy at room temperature, Figures adapted from [162,163].

1 .

 1 The time structure of the experiment: laser repetition rate, pulse duration, and wavelength, X-ray repetition rate and pulse duration (and detector characteristics), and a lifetime of transient and/or metastable states of the photoswitching sample 2. The geometry of the experiment: angle between laser and X-ray beam, reciprocal space coverage (detector size, sampledetector distance, X-ray energy, goniometer properties, space group of investigated crystal) 3. The expected data quality: statistics and noise (X-ray flux, sample damage), completeness, photoconversion rate (population of photoinduced species, laser power, and damage threshold of investigated sample)

Figure 5 . 2 :

 52 Figure 5.2 : Photo of setup at CRISTAL for time resolved diffraction mode depicting geometry of laser (pump) and X-rays (probe) on the crystal sample.

Figure 5 . 3 :

 53 Figure 5.3 : Schematic of the pump probe experimental setup in CRISTAL beamline at SOLEIL.

Figure 5 . 4 :

 54 Figure 5.4 : Temporal characteristics and synchronization of the X-ray and LASER beam with respect to the sample response time, and XRD measurement.

Figure 5 . 5 :

 55 Figure 5.5 : Lifetime of MS2 in SNP as a function of temperature.

Figure 5 . 7 ,

 57 plate-like crystals of approximately 100 µm thickness are mounted with the plate surface perpendicular to the laser beam, ensuring homogeneous penetration of the laser in the crystal. Penetration of light in the crystal is always an issue in photocrystallographic experiments, so we calculated the penetration depth for 400 nm in SNP from the known absorption coefficients[START_REF] Schaniel | Optical absorption spectroscopy in the metastable state SI of Na2[Fe(CN)[END_REF] using the Lambert-Beer law for different light polarization directions. Along the a-axis (b-axis is equivalent) at 50 µm about 80% of the incoming light has been absorbed (Figure5.8 left).

Figure 5 . 6 :

 56 Figure 5.6 : Systematic tests of pulse laser power threshold test of SNP crystal at CRISTAL beamline SOLEIL performed by Claire LAUHLÉ. The good agreement of pulse laser energy was found between 10-15 mJ/cm 2 .

Figure 5 . 7 :

 57 Figure 5.7 : Photo of SNP crystals and mounting on the goniometer head. laser will come from the right, while X-rays from the top (small angle with crystal surface).

Figure 5 . 8 :

 58 Figure 5.8 : (left) Calculated penetration depth of 400 nm light into SNP. (right) Transient absorption measurements show light-induced absorption upon pulsed laser excitation as a function of the polarization of incident laser light.

Figure 5 .

 5 10 shows the resulting photodifference map calculated for MS2, corresponding to an experimentally determined (from the structural refinement) population of 4.6(2)% for MS2. The crystallographic parameters for GS and MS2 of SNP are listed in Table 5.2 and Table 5.3. The structural model is shown in Figure 5.11.

Figure 5 . 9 :

 59 Figure 5.9 : GS crystal of SNP (Left panel); After photoirradiation (MS2+GS) (Right panel).

Figure 5 . 10 :

 510 Figure 5.10 : The photodifference map F photodifference obs

Figure 5 . 11 :

 511 Figure 5.11 : GS and MS2 crystal structure of SNP.

Figure 5 . 12 :

 512 Figure 5.12 : The most sensitive reflections from the experimental data with the change in intensity from ground state GS to excited state ES (Left panel). Simulated data for most sensitive reflections in MS2 from 1%, 5%, 10% with the change in intensity from ground state GS to excited state ES (Right panel).

Figure 5 .

 5 Figure 5.13 illustrates this effect on the example of the measured data on SNP.

Figure 5 . 13 :

 513 Figure 5.13 : Photodifference maps between GS and MS2 of SNP calculated in the Fe-N4b-Ob plane using (a) observed structure factors; (b) calculated structure factors; (c) calculate structure factors with the same lattice parameters. Contours@ 0.1 e/Å 3 for each case (negative differences are shown in red and positive differences are shown in blue).

Figure 5 .Figure 5 . 14 :

 5514 Figure 5.14 depicts (a) 1% (b) 5% (c) 10% maps population for a full data Contours@ 0.1 e/Å 3 .It is evident that MS2 configuration in all the cases, unambiguously in the 10% and 5% cases shows a clear depletion of the GS positions and the addition of new electron density peaks at

  on the left panel shows the result of full data set, the middle corresponds to the Chi=0 position, while the top right is the result of the Chi=5 position.

Figure 5 . 15 :

 515 Figure 5.15 : The crystal orientation used in the laboratory set up for preparation of proper goniometer orientation.

Figure 5 . 16 :

 516 Figure 5.16 : Goniometer allowing rotation from Chi=0 to Chi=5.

Figure 5 . 21 :

 521 Figure 5.21 : Photodifference map of SNP from SOLEIL data set.

Figure 5 .

 5 Figure 5.22 Comparison of scan over Bragg peak at different time delays.

  respect to the setup described by Casaretto et al., some important changes occurred in the last three years. First, in 2018 a new detector -RebirXwas installed, which has a thicker Silizium detector layer (1000 μm compared to the previous 300 μm of XPAD) and therefore an improved detection efficiency at the X-ray wavelength 0.71 Å of the Mo source. Second, at the end of 2020, a new goniometer was installed (necessary due to the breakdown of the old goniometer).The new STADIVARI goniometer from STOE has now integrated the RebirX detector, and so for classical (non-time resolved) diffraction measurements, the data collection is automatized, and data treatment is performed with XAREA software from STOE (as detailed in the previous chapter). However, for the special time-resolved mode with the particular capacities of the RebirX detector (the so-called IPI mode, see below), these changes require a complete reconfiguration of the setup, in particular the synchronization of the LASER pulse with the goniometer movements and data collection via RebirX detector. This work is not yet finished, and therefore during my thesis I could not perform in-house time-resolved XRD measurements. Moreover, in autumn 2021 a new pulse LASER has been installed, the EKSPLA LASER used for the steady-state measurements described in the previous chapter. In the following I will therefore quickly describe the principles of the time-resolved measurement in this setup, since these differ slightly from the synchrotron type measurements, and give some results of preparatory optical measurements on the compound [Ru(NH3)5(NO)]Cl3•H2O.

Figure 5 .

 5 Figure 5.23 shows two photos of the setup in order to illustrate the beam path of the optical pump and optical probe LASER light. The pump LASER (EKSPLA NT342) has already been

Figure 5 . 24 :

 524 Figure 5.24 : Temporal characteristics of pump-probe measurement using the HPAD detector with IPI mode.

  diffractometer. For this purpose, we installed the transient optical absorption spectroscopy setup (see above). Beforehand, such a transient absorption experiment was performed on a separate optical table (Chapter 2), on a single crystal of [Ru(NH3)5(NO)]Cl3•H2O The experiment was performed as a function of temperature over the range of 10-40°C (temperature adjusted by a Peltier element). As seen in the previous chapters [Ru(NH3)5(NO)]Cl3•H2O is a {MNO}6 type mononitrosyl complex with two metastable states MS1 and MS2 apart from GS.These metastable states have their own distinct UV-visible absorption spectrum (Chapter 4) and the absorption does not show a strong dependence on the polarization axis of light with respect to crystal axes. The GS shows an absorption band at 465 nm. MS1 exhibits two rather weak bands at 610 nm and 825 nm. MS2 exhibits a strong broad absorption at the GS position, i.e. around 465 nm, reaching out until 700 nm, as well as a second band at 750 nm. For the transient absorption measurement, we used the excitation wavelength of 422 nm (like for the pulsed experiments presented in Chapter 4) and for the probe wavelength we chose 532 nm. At this wavelength MS2 absorbs strongly, while the GS shows limited absorption and MS1 almost no absorption. Figure 5.25 shows the time trace of the transmitted light intensity (recorded via SI photodiode and oscilloscope, see Chapter 2). At time t0, when the pulse laser arrives, the transmitted intensity decreases instantly, before relaxing back to the initial value. The curve of the transmitted light intensity It can be fitted by a mono-exponential function of the type It = I0+ΔIexp(-t/τ), where I0 is the transmitted light intensity before the laser pulse arrives on the sample, ΔI is the photoinduced change in amplitude, and τ is the time constant of the relaxation (τ = lifetime of photoinduced state). Since for the time being we are only interested in the temporal characteristics, the absolute values of the light induced absorption change will not be discussed [161]. Performing this measurement for a series of temperatures, and plotting the log of time constant as a function of inverse temperature, one obtains an Arrhenius plot and can hence determine the activation energy and frequency factor for the observed photoinduced state. For [Ru(NH3)5(NO)]Cl3•H2O we obtain EA(slope)=0.60(2) eV and ln(Z)(intercept)=27.9(8) and thus Z = 1.3(1.6) x 10 12 s -1 . Comparison with published results EA = 0.66(3) eV and Z = 5x10 12 s -1 from DSC measurements [110] show good agreement. By using the Arrhenius equation, we can determine the MS2 life time, temperature T, e.g. for T = 300 K we obtain τ = 9.2 ms.

Figure 5 .

 5 Figure 5.25 : (left) Single time trace of transmitted light intensity after pulsed excitation (pump at 422 nm, probe at 532 nm). (right) Arrhenius plot for time constants measured as function of temperature in the range 10-40°C.

Fiber

  (NH3)5(NO)]Br3•H2O is isostructural to [Ru(NH3)5(NO)]Cl3•H2O. The crystal structure has been derived by Paulat et al [167], and refined in the centrosymmetric space group

=0. 71073

 71073 Å) and (Cu K radiation, =1.5406 Å), in order to enhance resonance scattering effects due to Ru (measured by the amplitude of the real and imaginary parts of dispersion factors in the atomic scattering factor 𝑓 = 𝑓 0 + 𝑓 ′ + 𝑖𝑓′′) with the Cu K radiation (Cu K 𝑓 ′ (𝑅𝑢) = 0.08 𝑒, 𝑓 ′′ (𝑅𝑢) = 3.33 𝑒) with respect to Mo K radiation (Mo K  𝑓 ′ (𝑅𝑢) = -1.23 𝑒, 𝑓 ′′ (𝑅𝑢) = 0.84 𝑒)) Accordingly, the Cu K data will be used to analyse the anomalous dispersion effects and derive the correct symmetry and disorder contributions, while the Mo K data will be used to derive the final structural model. As a matter of fact, the Mo K radiation allows collecting more independent reflections for the same 2𝜃 range than using Cu K radiation with reduced X-ray absorption effects, leading to a more accurate structural model. The intrinsic resonant scattering effects in a specific crystal structure, and therefore the resolving power for absolute structure determination, can be quantified through the 𝐹𝑟𝑖𝑒𝑑𝑖𝑓 𝑠𝑡𝑎𝑡 parameter [171]. For [Ru(NH3)5(NO)]Cl3•H2O, calculated 𝐹𝑟𝑖𝑒𝑑𝑖𝑓 𝑠𝑡𝑎𝑡 values are 720 and 190 for Cu K and Mo K radiation respectively, which strengthen the appropriateness of our Cu Kα diffraction measurement.A careful analysis of the cumulative probability distribution [172] and low-order moments of the normalized intensities of Cu Kα data set indicates that the structure is probably centrosymmetric. The crystal structure has been refined in the non-centrosymmetric space group Pna21, in order to evaluate the refined (using the TWIN/BASF commands for inversion twin refinement) and post-refinement values of the Flack parameter x, as well as the Parson's parameter z and Hooft's parameter y (calculated using PLATON). The results of the refinement are a value of x = 0.48[START_REF] Kukura | Structural Observation of the Primary Isomerization in Vision with Femtosecond-Stimulated Raman[END_REF] (inversion twin refinement), x = 0.48[START_REF] Kukura | Structural Observation of the Primary Isomerization in Vision with Femtosecond-Stimulated Raman[END_REF] (post-refinement), y = 0.482[START_REF] Th | New information storage elements on the basis of metastable electronic states[END_REF], and z = 0.479[START_REF] Goulkov | Pulse recording of thermal and linkage isomer gratings in nitrosyl compounds[END_REF], which are very consistent, and would suggest a centrosymmetric structure in Pnma space group with a complete disorder of the nitrosyl N1-O1 group and amine group N4 (with equal occupation 0.5/0.5 on each site). The calculated observed Fourier map in the Ru-N1-N4 plane only partially confirms this conclusion. As a matter of fact, two distinct electron density peaks are observed on each of the two positions (which are related by a mirror plane in Pnma), corresponding to a superposition of amine N4 and nitrosyl N1-O1. However, the height of the peaks corresponding to the terminal O atoms is slightly different, suggesting an unequal occupation of the two sites, and therefore a non-equal structural disorder. Refining the NO/NH3 population leads to occupancies of 0.44(3)/0.56(3).

Figure C. 1 :

 1 Figure C.1: (a) Fobs map contours@2 eÅ -3 , (b) Residual map contours@0.4 eÅ -3 , (c) GS structure in Pna2 1 Cu K ( = 1.54 Å) radiation source.

Figure C. 2 :

 2 Figure C.2: Reconstruction of reciprocal space with respect to the Pna2 1 setting (a = 11.8258(2) Å, b = 14.0936(2) Å, c = 6.85720(10) Å). Strong diffuse scattering features are evidenced by red arrows.

Figure C. 3 :

 3 Figure C.3: Fobs Fourier map in the Ru-N1A-N1B plane calculated using the Cu Kα diffraction data and assuming Pna2 1 space group.

Figure C. 4 :

 4 Figure C.4: Ground state structure of Ruthenium pentaamine complex solved in Pnma space group. (a) F obs map (Countours@2eÅ -3 ) in O1-Ru1-N3 plane (b) F obs map (Countours@2eÅ -3 ) in O1'-Ru1-N3' plane (c) Ground state structure of Ruthenium pentaamine complex solved in Pnma space group.

Figure C. 5 :

 5 Figure C.5: Sheldrick's E statistics favoring the centrosymmetric choice of space group.

Figure C. 6 :

 6 Figure C.6: Cumulative probability distribution N(z) of GS data set favoring the centric distributions.

Figure C. 7 :

 7 Figure C.7: Overlay of the GS (in blue) and MS1 (in red) state for the [Ru(NO)(NH 3 ) 5 ] 3+ cation.

Figure C. 8 :Figure C. 9 :

 89 Figure C.8: Selective intermolecular interactions and their bond distances involving N1A, N1B, O1 and O1A atoms of Ruthenium pentaamine complex.

Figure C. 10 :C. 3 [

 103 Figure C.10: Overall Schematic and crystal color change in MS1 after irradiation with ultraviolet and subsequent irradiation with infrared laser for generation of MS2 in Ruthenium pentaamine complex.

Figure C. 12 :C. 4

 124 Figure C.12: [Ru(NO) 2 (PPh 3 ) 2 ] crystal irradiation from GS to ES with 590 nm.

Figure C. 13 :

 13 Figure C.13: Ortep view of N1O1 as bent and N2O2 as isonitrosyl. Ellipsoids are plotted at 50% probability level.

O2B- N2B .

 N2B The results of the structural refinement lead to a refined population of PLI N1BO1B = 40.0(4)%, N2BO2B = 35.2(15)%, agreement statistic factors of R = 0.049 and wR2 = 0.088, respectively. When restraining the population of both the PLI to the same value, the structural refinement results in a population of 39.6(4)% for the PLI and of 60.4(4)% for the GS with agreement statistics R = 0.049, wR2 = 0.088. The refinement leads further to reasonable values for the atomic displacement parameters (in Å 2 ) N1B = 0.0291[START_REF] Brinkman | The future of solid-state electronics Bell[END_REF], O1B = 0.0262(7), N2B = 0.0238(6), O2B = 0.0272[START_REF] Kukura | Structural Observation of the Primary Isomerization in Vision with Femtosecond-Stimulated Raman[END_REF]. As in the previous models with one isonitrosyl configuration we observe an ADP value for O2B which is too high and an ADP of N1B which is too low, pointing again to a probably wrong assignment N/O of this group. For the N1BO1B group the effect is less pronounced, but compared to GS the value of the ADP of O1B is still too high.

Figure C. 15 :

 15 Figure C.15: Ortep view of N1O1 and N2O2 as isonitrosyl. Ellipsoids are plotted at 50% probability level.

Figure D. 2 :

 2 Figure D.2: Systematic MS1 generation steps until saturation to 85% population in [Ru(Py) 4 (NO)F](ClO 4 ) 2 .

Figure E. 4 :

 4 Figure E.4: Transient optical absorption measurement of a crystal plate of [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O before and after 10 pulses.

  2.2 Infrared spectroscopy with CW and ns-pulsed laser ...................................................... Differential scanning calorimetry .................................................................................. 2.5 Pump-probe optical absorption spectroscopy ................................................................ 2.6 Single Crystal optical absorption spectroscopy ............................................................. 2.7 Photocrystallography ..................................................................................................... 2.8 Steady-state photocrystallography ................................................................................. 2.9 Analysis tools in photocrystallography .......................................................................... 2.9.1 Photodifference maps..............................................................................................

2.3 Uv-visible absorption spectroscopy ............................................................................... xii 2.4

Table 1 .

 1 1 : Schematics for GS and MS in {M(NO)m} n with m=1,2 and n=6,8,10. ................. 19

Table 2 .

 2 1 Selected parameters of pulsed laser 1 and 2 ........................................................... 29

Table 2 .

 2 2 : Selected parameters of XPAD RebirX540S detector ............................................ 42

Table 3 .

 3 1 : Crystallographic data and refinement details of [Ru(NO)(NH3)5]Cl3•H2O in Pnma space group. ............................................................................................................................. 57

Table 3 .

 3 2 : Selected bond distances, bond angles and thermal ellipsoids of [Ru(NO)(NH3)5]Cl3•H2O in Pnma space group. ..................................................................... 65

Table 3 .

 3

3 : Crystallographic data and refinement details of [Co(fpin)(NO)(phen)] II in ground state and photo-irradiated state. ............................................................................................... 74

Table 3.4 : Refinement details of II in ground state and photo-irradiated state. ..................... 78 Table 3.

5 : Comparison of selected bond distances (Å) and bond angles (º) for the GS and PLI. .................................................................................................................................................. 79 Table 3.6 : Crystallographic parameters of III [Ru(NO)2(PPh3)2] in GS and PLI. ................ 90 Table 3.7 : Refinement details of [Ru(NO)2(PPh3)2] in the ground state (data up to sinθ/λ = 0.997 Å -1 ) and the photo-irradiated state; refinement with Shelxl. ......................................... 94 Table 3.8 : Mean distances and angles along the path GS-TS-MS. Calculated metrical and spectroscopic values from a BP86/def2-TZVP+D3+CPCM(toluene) approach, energies include a ZORA correction [125]. Note the limited precision of the MSexp metrical values due to the ≈1:1 disorder of GS and MS in the photocrystallographic experiment. ........................ 98 Table 4.1 : Photodifference map for I after generation of MS1 with 420 nm, Q = 1200 J/cm 2 . The contours level for both the maps is 0.2 eÅ -3 . .................................................................. 115

Table 4 .

 4 2 : Structural models for MS1 for the two independent molecules of the unit cell. (a)

	Using CW irradiation, the population of MS1 is 71(2)% for molecule 1 and 92(2)% for
	molecule 2 (b). Using pulsed laser irradiation population of MS2 is 88(3)% and 86(3)% for
	molecule 1 and 2, respectively.

.............................................................................................. 116

Table 4 .

 4 3 : Photodifference map for I after generation of MS2 with 940 nm, Q = 923 J/cm 2 . The contours level for both the maps is 0.2 eÅ -3 . In CW, MS2 with the contours level for both xxv the maps is 0.3 eÅ -3 . (a) population of MS2 molecule 16% (b) population 15% of MS2 in the second independent molecule. ............................................................................................... 117

Table 4 .

 4 4 : Structural models for MS2 for the two independent molecules of the unit cell. (a) , respectively. ............................................................................................................. 118

	Using CW irradiation, population of MS2 is 17(5) % for molecule 1 and zero for molecule 2
	(b) Using pulsed laser irradiation population of MS2 is 13.1(5) % and 12.4(5) % for molecule
	1 and 2

Table 4 .

 4 5 : Data collection parameters and refinement statistics for all data sets measured in [Ru(Py)4(NO)F](ClO4)2 upon CW and ns-pulsed LASER irradiation. ................................. 119

Table 4 .

 4 6 : Main structural parameters obtained from refinement of GS and MS1 and MS2 for

various data sets. .................................................................................................................... 121 Table 4.7 : Most sensitive common reflections after irradiation with CW and pulse in I. .... 124 Table 4.8 : Data collection parameters and refinement statistics for GS, MS1, and MS2 data sets in [Ru(NH3)5(NO)]Cl3•H2O ............................................................................................ 129 Table 4.9 : Structure of MS1 and MS2 in II from CW and pulsed irradiation sources. ........ 131 Table 4.10 : Main structural parameters obtained from the refinement of GS and MS1 and MS2, for the various data sets in [Ru(NH3)5(NO)]Cl3•H2O. MS1 was generated by irradiation with pulsed laser light (420 nm, f = 10 Hz; t = 240 min; P = 2.67 mW; I = 38 mW/cm 2 ). ........... 132 Table 4.11 : OA absorption spectra at different temperatures after pulsed and CW irradiation in II ........................................................................................................................................ 137 Table 4.12 : OA absorption spectra at 150 and 170 K temperatures after CW irradiation in II ................................................................................................................................................ 139 Table 4.13 : OA absorption spectra at 230 K to monitor population and depopulation dynamics of MS1 after pulsed irradiation in II ...................................................................................... 139 Table 4.14 : Comparative OA absorption spectra at 170 K of transfer from MS1 to MS2 while using 980 nm and 1064 nm after CW irradiation in II .......................................................... 140 Table 4.15 : OA single crystal spectra of MS1 after CW irradiation with 422 nm at 260 K and 270 K. ..................................................................................................................................... 140

Table 4 .

 4

16 : Comparison of excitation parameters between the previously describe measurement with low-intensity pulses to obtain saturation and the single-shot experiment. The number of photons is calculated according to Eq. (4.4) ........................................................ 146 xxvi

Table 4 .

 4 17 : Crystallographic parameters of GS, one pulse PLI and relaxed GS in I. .......... 149

Table 4 .

 4 18 : Selected bond distances and angles of GS, one pulse PLI and relaxed GS of I.150

Table 4 .

 4 19 : Most sensitive reflections from GS to One-pulse irradiation. ........................... 152

Table 4 .

 4 20 : Estimation of number of photons and number of molecules in the pellet of I for one pulse experiment (Molar mass of I MI=665.38 g/mol). .................................................. 154

Table 4 .

 4 21 : Comparison of number of photons and number of molecules in a crystal of saturation fluence and the crystal 1,2,3 used of ns-pulse photocrystallography. ................... 156

Table 4 .

 4 22 : Crystallographic parameters in Crystal 1 of II. ................................................. 159

Table 4 .

 4 23 : Selected bond distances and bond angles in Crystal 1 of II. ............................. 160

Table 4 .

 4

24 : Crystallographic parameters in Crystal 2 and 3 of II. ...................................... 161 Table 4.25 : Selected bond distances and bond angles in Crystal 2 and 3 of II. .................... 161 Table 4.26 : Selected most sensitive common reflections from GS to PLI in II Crystal 1, 2 and 3.............................................................................................................................................. 163 Table 4.27 : Comparison of Fluence with respect to number of photons in pellet after irradiation with ns-pulse laser in IR spectroscopy. .................................................................................. 164 Table 5.1 : Main characteristics of XPAD3.2 detector .......................................................... 172 Table 5.2 : Selected crystallographic parameters of GS and MS2 of SNP. ........................... 181 Table 5.3 : Selected bond distances and bond angles of GS and MS2 of SNP. .................... 182 Table 5.4 : Most sensitive reflection in SNP from GS to MS2.............................................. 184 Table 5.5 : Full data, chi=0 and to Chi=5 to see the effect different orientations on data completeness. ......................................................................................................................... 190 Table 5.6 : Percentage completeness as a function of crystal to detector distance. ............... 193 Table 5.7 : SNP ground state data set collected at SOLEIL and its comparison to Supernova diffractometer. ........................................................................................................................ 195 Introduction 1.

1 Context, motivation, and outline of the thesis

  Over many millennia, humankind has created the abstract concept of time as a way of monitoring and organizing events in the present, past, and future in the earliest endeavors.

Therefore, the notion of time is profoundly ingrained in our knowledge of the natural world. In an attempt to demonstrate what time is, we may define it as a measure of change, or more accurately, as the period required for anything to evolve from a state X to a state Y. In this dynamic process, what is then the shortest scale of time where these changes start to become significant? From a physical standpoint, quantum mechanics and the theory of relativity answer this challenging question. The shortest imaginable duration of time scale is provided by the socalled Planck time tp = 5.39•10 -44 s [1], corresponding to the time the light needs to travel one Planck length lp = 1.616255(18)•10 -35 m

  .5).

	Once an electron is excited, there are multiple ways to dissipate this absorbed energy. The first
	way to dissipate this absorbed energy would be through vibrational relaxation, i.e., non-
	radiative relaxation. This process is very fast and occurs on the fs-ps time scale. It is, therefore,
	extremely likely to follow this relaxation pathway immediately after excitation. If vibrational
	energy levels of neighboring electronic energy levels strongly overlap, it opens other relaxation
	ways, i.e., an internal conversion. In internal conversion, a transition takes place from one
	vibrational level of one electronic state to another relatively lower-lying vibrational energy
	level in another electronic state, i.e., GS*→ES*. Internal conversion occurs in the fs to ps time
	scale. After the internal conversion GS*→ES* the excited electron relaxes to ES, i.e., relaxed,

excited state via vibrational relaxation. The vibrational relaxation occurs in the ps-ns time scale.

Table 1 .

 1 1 : Schematics for GS and MS in {M(NO)m} n with m=1,2 and n=6,8,10.

	{M(NO)m} n	GS	MS	Exemples E1 -E6
	{MNO} 6

Table 2 .

 2

	Parameters	Pulsed laser 1	Pulse laser 2
		EKSPLA NT342 pulsed Laser Surelite II-10 pulsed Laser
	Wavelength range	192-2600 nm	410-2500 nm
	Max. frequency	20 Hz	10 Hz
	Max. energy (visible spectral range)	~90 mJ (visible spectral range)	~40 mJ

1 Selected parameters of pulsed laser 1 and 2

  .2).

	Its sensor is a silicon layer of 1000 µm thickness providing about 80% detection efficiency for
	the wavelength of Kα radiation of molybdenum. RebirX 540S HPAD consists of eight modules
	separated by blind zones due to the connectivity that we call intermodal areas (560 x 960)
	pixels wide, i.e., 537600 pixels per chip. The dimensions of a pixel are 130 x 130 μm 2 .
	Table 2.2 : Selected parameters of XPAD RebirX540S detector
	Dynamic range	32 bits
	Counting rate	7 x 10 5 ph/sec/pix
	Energy range	7-35 keV
	Threshold range	5-30 keV
	Frame rate	up to 700 fps
	Pixel size	130 x 130 μm 2
	Pixels number	537600 (560 x 960)
	Active area	15 x 7.5 cm 2
	Sensor thickness	1000 μm
	Modules	8

  1, metal-nitrosyl complexes can adopt different electronic configurations that are classified based on the Enemark-Feltham notation as {M(NO) n }, where M denotes the transition metal and n the number of metal-d electrons if the NO ligand is considered as NO + (see scheme Table 1.1). More recently also, dinitrosyl complexes, {M(NO)2} n , came into the focus of PLI research, the essential structural contributions coming

Table 3 .

 3 1 : Crystallographic data and refinement details of [Ru(NO)(NH 3 ) 5 ]Cl 3 •H 2 O in Pnma space group.

		GS	MS1	MS2
	Crystal data
	Chemical formula		H 15 N 6 ORu•3(Cl)•H 2 O
	Crystal system, space group		Orthorhombic, Pnma
	Temperature (K)		100
	a, b, c (Å)	11.81231(19),	11.8318(2),	11.83192(19),
		6.84105(15),	6.87236(16),	6.84943(15),
		14.0907(2)	14.0641(2)	14.1000(2)
	V (Å 3 )	1138.64(4)	1143.59(4)	1142.69(4)
	Z		4
	Radiation type		Mo K
	Diffractometer	SuperNova microfocus diffractometer equipped with ATLAS detector
	 (mm -1 )	2.062	2.04	2.05
	Crystal size (mm)	0.118×0.188×0.237 0.118×0.188×0.237	0.131×0.180×0.214
	Data collection
	T max , T min	1.00, 0.551	1.00, 0.564	1.00, 0.538
	No. of measured, independent and observed [I > 2(I)] reflections	32472, 2241, 2095 34194, 2255, 2120	34177, 2246, 2106
	R int	0.031	0.030	0.040
	(sin /) max (Å -1 )	0.764	0.765	0.765
	Refinement
	R[F 2 > 2(F 2 )], wR(F 2 ), S	0.033, 0.081, 1.25 0.031, 0.076, 1.18	0.033, 0.080, 1.16
	No. of parameters	101	85	87
	 max ,  min (e Å -3 )	0.73, -1.22	1.07, -0.95	0.96, -1.00
	Refined population of PLI
	N1O1		0.274(16)[54.8%]¹	0.447 (5)[89.4%]¹
	N1AO1A		0.226 (16)[45.2%]¹
	N1AO1A			0.0265(5)[5.3%]¹
	N1BO1A			0.0265(5)[5.3%]¹

¹Owing to the disorder and molecular point symmetry in the considered Pnma space group, the refined site occupancy corresponds to half the real population of GS (N1O1), or MS (N1AO1A and N1BO1A) states. The real population is given in brackets.

Table 3 .

 3 2 : Selected bond distances, bond angles and thermal ellipsoids of [Ru(NO)(NH 3 ) 5 ]Cl 3 •H 2 O in Pnma

			space group.
				MS2X	MS2Y
		GS	MS1	Staggered	Eclipsed
				(N1AO1A)	(N1BO1A)
	Ru1-N1	1.883(18)	1.776(11)	1.835(6)	1.835(6)
	Ru1-O1A	-	1.934(14)	2.41(4)	2.41(4)
	Ru1-N1A	-	-	2.46(5)	-
	Ru1-N1B	-	-	-	2.537
	Ru1-N5	1.963(18)	2.034(7)	1.998(6)	1.998(6)
	Ru1-N4	2.098(3)	2.091(3)	2.093(3)	2.093(3)
	Ru1-N3	2.091(3)	2.095(3)	2.100(3)	2.100(3)
	Ru1-N2	2.084(3)	2.077(2)	2.084(2)	2.084(2)
	N1-O1	1.069(19)	1.147(11)	1.120(7)	1.120 (7)
	N1A-O1A	-	1.145(14)	1.11(2)	-
	N1B-O1A	-	-	-	1.12(2)
	O1A-Ru1-N1A	-	-	26.3(7)	-
	O1A-Ru1-N1B	-	-	-	25.98
	Ru1-N1-O1	173.1(8)	179.2(18)	171.8(6)	171.9(6)
	Ru1-O1A-N1A	-	165(2)	-	-
	U eq (Ru1)	0.02343(9)	0.02364(8)	0.02489(7)	0.02489(7)
	U iso * (N1)	0.0255(16)	0.0236(10)*	0.0280(12)*	0.0280(12)*
	U iso * (N1A)	-	0.0245(10)*	0.0280(12)*	-
	U iso * (N1B)	-	-	-	0.0280(12)*
	U iso * (O1)	0.0365(9)	0.0245(10)*	0.0358(9)*	0.0358(9)*
	U iso * (O1A)	-	0.0236(10)*	0.0358(9)*	0.0358(9)*
	U iso * (N5)	0.032(2)	0.0356(16)*	0.0295(12)*	0.0295(12)*
	*The refinement of ADPs as isotropic.

Table 3 .

 3 3 : Crystallographic data and refinement details of [Co(fpin)(NO)(phen)] II in ground state and photoirradiated state.

	Crystal data	Ground State	PLI_Isonitrosyl
	Chemical formula	C 18 H 8 CoF 12 N 3 O 3
	M r	601.20
	Crystal system, space group	Triclinic, P-1
	Temperature (K)	5
	a, b, c (Å)	8.4842(8), 8.7672(8), 13.2798(10) 8.4986(6), 8.7877(6), 13.2646(8)
	, ,  (°)	94.744(2), 90.253(2), 93.610(3)	94.681(2), 90.191(2), 93.522(2)
	V (Å 3 )	982.40(15)	985.44(11)
	Z	2
	Radiation type	Mo K
	 (mm -1 )	1.01
	Crystal size (mm)	0.27 × 0.25 × 0.17
	Data collection
	Diffractometer	Bruker D8 Venture PHOTON III CMOS
	No. of measured, independent and	23360, 4461, 3759	28552, 5897, 4749
	observed [I > 2(I)] reflections
	R int	0.052	0.0498
	(sin /) max (Å -1 )	0.650	0.715
	Refinement
	R [F 2 > 2(F 2 )], wR(F 2 ), S	0.040, 0.144, 1.07	0.042, 0.141, 1.14
	No. of parameters	334	341
	H-atom treatment	H-atom parameters constrained
	 max ,  min (e Å -3 )	0.65, -0.69	0.84, -0.90
	3.2.3.3.1 Ground state crystal structure
	In the GS we find a bent Co-NO configuration with distances of 1.830(2) Å for Co1-N3,
	1.161(3) Å for N3-O3 and a Co-N3-O3 angle of 122.07(19)°.

Table 3 .

 3 4 : Refinement details of II in ground state and photo-irradiated state.

		Ground state	PLI nitrosyl	PLI isonitrosyl
			Hypothesis 1(H1)	Hypothesis 2(H2)
	Refinement strategy		N3O3 & N3AO3A both refined as Nitrosyl	N3O3 Nitrosyl & N3AO3A refined as isonitrosyl
	N3-O3 configuration		Nitrosyl	Nitrosyl
	N3A-O3A configuration		Nitrosyl	Isonitrosyl
	R [F 2 > 2(F 2 )], wR(F 2 ), S	0.040, 0.144, 1.07	0.042, 0.142, 1.14	0.042, 0.141, 1.14
	 max ,  min (e Å -3 )	0.65, -0.69	0.84, -0.90	0.84, -0.90
	Refined population
	of PLI N3O3 N3AO3A	-	94.8(4)% 5.2(4)%	94.1(4)% 5.9(4)%
	Ueq(Co1)	0.00571(13)	0.00786(10)	0.00786(10)
	Ueq(N3)	0.0108(5)	0.0076(5)	0.0081(5)
	Ueq(N3A)		0.0076(5)	0.0136(4)
	Ueq(O3)	0.0121(4)	0.0140(4)	0.0136(4)
	Ueq(O3A)	-	0.0140(4)	0.0081(5)
	Ueq(N1)	0.0067(4)	0.0080(4)	0.0080(4)
	Ueq(N2)	0.0075(4)	0.0072(3)	0.0072(3)
	Ueq(O1)	0.0076(4)	0.0079(3)	0.0079(3)
	Ueq(O2)	0.0074(4)	0.0072(3)	0.0072(3)

Table 3 .

 3 5 : Comparison of selected bond distances (Å) and bond angles (º) for the GS and PLI.

		Ground state	Photo-irradiated state	Photo-irradiated state
			(isonitrosyl)	(nitrosyl)
			Hypothesis 2 (H2)	Hypothesis 1(H1)
	Co1-N3	1.830(2)	1.873(3)	1.869(2)
	Co1-O3A	-
			1.64(4)
	Co1-N3A	-		1.56(4)
	Co1-N1	1.933(2)	1.934(2)	1.934(2)
	Co1-N2	1.944(2)	1.943(2)	1.943(2)
	Co1-O1	1.8678(18)	1.8715(16)	1.8716(16)
	Co1-O2	1.8613(18)	1.8692(16)	1.8692(16)
	N3-O3	1.161(3)	1.143(3)	1.146(3)
	N3A-O3A		0.78(2)	0.77(2)
	Co1-N3-O3	122.07(19)	122.6(2)	122.62(18)
	Co1-O3A-N3A	-	143(6)	-
	Co1-N3A-O3A	-	-	148(7)
	N1-Co1-N2	83.10(9)	83.17(8)	83.17(8)
	O1-Co1-O2	86.81(8)	86.95(7)	86.96(7)

Table 3 .

 3 6 : Crystallographic parameters of III [Ru(NO) 2 (PPh 3 ) 2 ] in GS and PLI.

	GS	PLI

Table 3 .

 3 7 : Refinement details of [Ru(NO) 2 (PPh 3 ) 2 ] in the ground state (data up to sinθ/λ = 0.997 Å -1 ) and the photo-irradiated state; refinement with Shelxl.

		Ground state		Photo-irradiated state
	Refinement strategy	(l-NO-κN) 2	(a-NO-κN) 2 (a-NO-κN)	(a-NO-κO)	(a-NO-κO) 2
				(a-NO-κO)	(a-NO-κN)
	N1A-O1A configuration		nitrosyl	nitrosyl	nitrosyl	nitrosyl
	N1B-O1B configuration		nitrosyl	nitrosyl	isonitrosyl	isonitrosyl
	N2A-O2A configuration		nitrosyl	nitrosyl	nitrosyl	nitrosyl
	N2B-O2B configuration		nitrosyl	isonitrosyl	nitrosyl	isonitrosyl
	No. of refined parameters	381	379	392	392	392
	R[F 2 > obs*(F 2 )]	0.066	0.049	0.048	0.048	0.049
	wR2[F 2 > obs*(F 2 )]	0.132	0.088	0.088	0.086	0.088
	S	1.26	1.10	1.09	1.11	1.11
	 max ,  min (e Å -3 )	1.09, -4.29	0.70, -0.82	1.04, -0.82	0.75, -0.79	0.98, -0.79
	Refined population of PLI
	N1AO1A N2AO2A (GS)		53.6(4)%	54.6(4)%	59.3(4)%	60.4(4)%
	N1BO1B N2BO2B (PLI)		46.4(3)%	45.4(4)%	40.7(4)%	39.6(4)%
	U eq (Ru)	0.01243(2)	0.01622(5)	0.01622(5)	0.01623(5)	0.01623(5)
	U eq (P1)	0.01099(5)	0.01368(10) 0.01370(11)	0.01371(10)	0.01370(11)
	U eq (P2)	0.01170(6)	0.01336(10) 0.01333(10)	0.01332(10)	0.01333(11)
	U eq (N1A)	0.0181(2)	0.0206(6)	0.0207(6)	0.0262(7)	0.0262(7)
	U eq (O1A)	0.0272(3)	0.0265(8)	0.0268(8)	0.0279(9)	0.0291(10)
	U eq (N1B)		0.0206(6)	0.0207(6)	0.0279(9)	0.0291(10)
	U eq (O1B)		0.0265(8)	0.0268(8)	0.0262(7)	0.0262(7)
	U eq (N2A)	0.0188(2)	0.0201(6)*	0.0278(6)*	0.0203(6)*	0.0272(6)*
	U eq (O2A)	0.0351(4)	0.0291(6)*	0.0223(6)*	0.0294(6)*	0.0238(6)*
	U eq (N2B)		0.0201(6)*	0.0223(6)*	0.0203(6)*	0.0238(6)*
	U eq (O2B)		0.0291(6)*	0.0278(6)*	0.0294(6)*	0.0272(6)*
	We describe hereafter in detail what we consider the model best fitting the data, namely a

Table 4 .

 4 5 : Data collection parameters and refinement statistics for all data sets measured in [Ru(Py) 4 (NO)F](ClO 4 ) 2 upon CW and ns-pulsed LASER irradiation.

	[Ru(Py)4(NO)F](ClO4)2	GS	CW MS1 CW	ns-pulsed MS1 Pulse	CW MS2 CW	ns-pulsed MS2 Pulse
	Crystal data
	Chemical formula		2(C 20 H 20 FN 5 ORu)•4(ClO 4 )
	Crystal system, space group			Triclinic, P-1
	Temperature (K)			100
		10.4720(5),	10.5338(5),	10.5583(10),	10.4672(11),	10.4832(11),
	a, b, c (Å)	16.4511(9),	16.4775(9),	16.4988(15),	16.522(2),	16.4774(15),
		16.5125(9)	16.4316(8)	16.3871(15)	16.4946(19)	16.5948(15)
		66.231(4),	65.862(4),	65.494(7),	66.199(9),	65.819(7),
	, ,  (°)	87.646(4),	87.597(4),	87.581(8),	87.064(9),	82.857(8),
		83.134(4)	82.329(4)	82.504(7)	82.220(9)	87.089(8)
	V (Å 3 )	2584.7(2)	2579.1(2)	2575.0(4)	2585.9(5)	2594.7(5)
	Z			2
	Radiation type			Mo K
	 (mm -1 )			0.88
	Data collection
	Diffractometer	STOE STADI-VARI equipped with RebirX 540S Hybrid Pixel Area Detector (HPAD)
	No. of measured, independent and	57215, 22415,	55851, 22403,	44984, 19079,	56653, 22500,	45461, 18603,
	observed [I > 2(I)] reflections	20306	20257	16465	20180	13909
	R int	0.071	0.032	0.035	0.024	0.031
	(sin /) max (Å -1 )	0.830	0.860	0.830	0.860	0.88
	Refinement
	R[F 2 > 2(F 2 )], wR(F 2 ), S	0.056, 0.173, 1.12	0.050, 0.104, 1.12	0.045, 0.123, 1.11	0.038, 0.089, 1.12	0.039, 0.124, 1.10
	No. of parameters	688	692	728	685	720
	 max ,  min (e Å -3 )	1.49, -2.85	1.43, -2.11	1.04, -2.32	1.56, -1.75	1.14, -1.57

  as illustrated in Figure 4.8 with Q0 = 450(200) J/cm 2 and

	Pmax = 91(16)%. U eq N6, N6B	0.0137(5)	0.0171(4)*, 0.0213(5)*		0.0140(9), 0.0224(10)	0.0152(2) -	0.0186(6)*, 0.0308(6)*
	Table 4.6 : Main structural parameters obtained from refinement of GS and MS1 and MS2 for various data U eq O2, O2B 0.0213(5)*, 0.0224(10), 0.0241(3) 0.0308(6)*, 0.0223(5) 0.0171(4)* 0.0140(9) -0.0308(6)*
	Population PLI(Mol. 1)			71(2)%	sets.	88(3)%	17(5)%	13.1(5)%
	GS(Mol. 1)			29(2)%		12(3)%	83(5)%	86.9(5)%
	PLI(Mol. 2)		CW	92(2)%	Pulsed 86(3)%	CW	Pulsed 12.4(5)%
	GS(Mol. 2)	GS	MS1 CW 8(2)%	MS1 Pulse 14(3)%	MS2 CW Not Found!	MS2 Pulse 87.6(5)%
	Ru1-N1	1.756(3)	1.871(10)		1.76(7)	1.757(2)	1.766(3)
	Ru1 -O1B	-	1.837(4)		1.859(7)	2.101(4)	2.11(2)
	Ru12-N6	1.762(3)		1.79(2)		1.81(6)	1.7533(14)	1.770(3)
	Ru2 -O2B	-	1.842(4)		1.848(7)	-	2.05(2)
	Ru1-F1	1.914(2)	1.9100(14)		1.9070(19)	1.9267(11)	1.9206(17)
	Ru2-F2	1.938(2)	1.9015(14)		1.9024(19)	1.9192(11)	1.9169(16)
	N1-O1	1.148(4)	1.149(11)		1.30(10)	1.157(3)	1.137(4)
	N1B-O1B	-	1.159(5)		1.142(12)	1.088(12)	1.13(3)
	N6-O2	1.155(4)	1.135(17)		1.05(8)	1.1412(19)	1.149(4)
	N6B-O2B	-	1.151(4)		1.167(11)	-	1.14(3)
	Ru1-N1-O1	178.4(3)		177.868		170.931	178.790	177.727
	Ru1-O1B-
	N1B	-		178.504		177.77	-	-
	O1B-Ru1-	-		-		-	30.7	31.4(8)
	N1B
	Ru2-N6-O2	177.6(3)		168.733		163.876	177.284	179.282
	Ru2-O2B-
	N6B	-		174.524		178.688	-	-
	O2B-Ru12-	-		-		-	-	32.5(9)
	N6B
	U eq Ru1, Ru2	0.01140(6), 0.01129(6)	0.01440(4), 0.01430(4)		0.01403(5), 0.01394(5)	0.01307(3), 0.01240(3)	0.01646(5), 0.01688(5)
	U eq N1, N1B	0.0138(5)	0.0156(4)*, 0.0207(5)*		0.0154(9), 0.0253(11)	0.0215(4), 0.0216(18)*	0.0170(5)*, 0.0170(5)*
	U eq O1, O1B	0.0222(5)	0.0207(5)*, 0.0156(4)*		0.0253(11), 0.0154(9)	0.0215(4), 0.0216(18)*	0.0292(6)*, 0.0292(6)*

Table 4 .

 4 [START_REF] Ernst | Microbial and Animal Rhodopsins: Structures, Functions, and Molecular Mechanisms[END_REF] gives a selection of the most sensitive reflections based on the above discussed experiments. There is a number of strong reflections with significant relative intensity changes, up to 26% between GS and MS1, and the correspondence between CW and pulsed, results are in general, quite good for MS1 (see Table4.7). For MS2 the correspondence is not very good, but this is certainly due to the presence of a second MS2 in the pulsed experiments. The few reflections that disagree between the two experiments have quite large errors (see Appendix D).

Table 4 .

 4 7 : Most sensitive common reflections after irradiation with CW and pulse in I.

	[Ru(Py)4(NO)F](ClO4)2	CWMS1	PulseMS1	CWMS2	PulseMS2
	No .	h k l	IGS	IMS1	(IMS1-IGS)/IGS	IMS1	(IMS1-IGS)/IGS	IMS2	(IMS2-IGS)/IGS	IMS2	(IMS2-IGS)/IGS
	1	2 0 0	88866.2 3	82127.1 8	-0.0758 3	82011.5 6	-0.0771 3	90368.9 6	0.0169 1	81430.5	-0.0836 7
	2	0 0 2	78258.2 2	74202.4 1	-0.0518 3	73042.3 1	-0.0666 5	82960.7 6	0.0600 9	59661.7 8	-0.2376 3
	3	-2 0 2 55669.1	66992.4	0.2034	67784.5 7	0.2176 3	54956.0 2	-0.0128 1	64604.1 7	0.1605
	4	1	-1	3	51131.2 5	41931.8 2	-0.1799 2	41014.8 4	-0.1978 5	52831.3 9	0.0332 5	498.628 9	-0.9902 5
	5	-2 1 3	48503.3 7	35780.4 5	-0.2623 1	34298.0 8	-0.2928 7	50261.2	0.0362 4	55672.2 6	0.1478

Table 4 .

 4 8 and Table 4.10, and are also in agreement with the CW results.

	For MS1, Ru-O1A

Table 4 .

 4 8 : Data collection parameters and refinement statistics for GS, MS1, and MS2 data sets in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O

	[Ru(NH3)5(NO)]Cl3•H2O	GS	CW MS1 CW	Pulse ns MS1 Pulse	CW MS2 CW	Pulse ns MS2 Pulse
	Crystal data
	Chemical formula	H 15 N 6 ORu•3(Cl)•0.5(H 4 O 2 )
	Crystal system, space	Orthorhombic, Pnma
	group
	Temperature (K)	100
	a, b, c (Å)		11.833(2),	11.8318(2),	11.873(3),	11.83192(19),	11.853(3),
				6.9231(11),	6.87236(16,	6.9651(15,	6.84943(15),	6.9322(15),
				14.216(3)	14.0641(2)	14.184(3)	14.1000(2)	14.223(3)
	V (Å 3 )			1164.6(4) 1143.59(4) 1173.0(4)	1142.69(4)	1168.6(4)
	Z			4
	Radiation type		Mo K
	Data collection
	Diffractometer		SuperNova microfocus-source diffractometer equipped with an ATLAS
				detector [For CW]
				STOE STADI-VARI equipped with RebirX 540S Hybrid Pixel Area
				Detector (HPAD) [For pulsed ns]
	No.	of	measured,	14686,	34194,	13820,	34177, 2246,	13798, 1829,
	independent	and	1591, 1495	2255, 2120	1826, 1716	2106	1699
	observed [I > 2(I)]
	reflections
	R int			0.0384	0.030	0.0396	0.040	0.0373
	(sin /) max (Å -1 )	0.671	0.765	0.703	0.765	0.703
	Refinement
	R[F 2 > 2(F 2 )], wR(F 2 ), S 0.039,	0.031,	0.042,	0.033, 0.080,	0.047, 0.159,
				0.102, 1.20	0.076, 1.18	0.108, 1.17	1.16	1.29
	No. of parameters	101	85	86	85	86
	 max ,  min (e Å -3 )	1.28, -0.86 1.07, -0.95 1.42, -1.02 0.96, -1.00	1.76, -0.94

Table 4 .

 4 10 : Main structural parameters obtained from the refinement of GS and MS1 and MS2, for the various data sets in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O. MS1 was generated by irradiation with pulsed laser light (420 nm, f = 10 Hz; t = 240 min; P = 2.67 mW; I = 38 mW/cm 2 ).

	[Ru(NH 3 ) 5 (NO)]Cl 3		CW	Pulsed ns	CW	Pulsed ns
	•H 2 O	GS	MS1 CW	MS1 Pulse	MS2 CW	MS2 Pulse
	Ru1-N1	1.82(3)	1.776(11)	1.73(2)	1.835 (6)	1.860(10)
	Ru1-O1A	-	1.934(14)	1.97(4)	2.45(3)	2.553
	Ru1-N2	2.096(3)	2.077(2)	2.091(4)	2.084(2)	2.123(5)
	Ru1-N5	2.04(2)	2.034(7)	2.047(12)	1.998(6)	2.027(11)
	N1-O1	1.14(2)	1.147 (11)	1.21(2)	1.120(7)	1.28(13)
	N1A-O1A	-	1.145(14)	1.13(2)	1.11(2)	1.12(4)
	Ru1-N4	2.107(4)	2.091(3)	2.117(5)	2.093(3)	2.120(6)
	Ru1-N1-O1 175.280	179.2(18)	177

Table 4 .

 4 12 : OA absorption spectra at 150 and 170 K temperatures after CW irradiation in II

	CW MS1 transfer to MS2
	T=150 K	T=170 K

Table 4 .

 4

13 : OA absorption spectra at 230 K to monitor population and depopulation dynamics of MS1 after pulsed irradiation in II Pulse MS1 T=230 K Population Relaxation

Table 4 .

 4 14 : Comparative OA absorption spectra at 170 K of transfer from MS1 to MS2 while using 980 nm and 1064 nm after CW irradiation in II CW MS2 transfer using 980 nm and 1064 nm for comparison @T=170 K

CW MS2 transfer using 980 nm CW MS2 transfer using 1064 nmTable 4 .

 4

15 :

OA single crystal spectra of MS1 after CW irradiation with 422 nm at 260 K and 270 K.

Table 4 .

 4 16 : Comparison of excitation parameters between the previously describe measurement with lowintensity pulses to obtain saturation and the single-shot experiment. The number of photons is calculated

	according to Eq. (4.4)
	[Ru(NO)(Py)4F](ClO4)2 Single crystal	Saturation	One Pulse
	Fluence 𝑄 = 𝐼 × 𝑡	1200 J/cm 2	2.3 J/cm 2
	Power	2.5 mW	45 mW
	Irradiation time t	570 minutes	5 ns
	Number of pulses	342000	1
	Number of photons/cm 2	2.53 × 10 21	4.853 × 10 18
	Number of photons on a 0.02 x 0.02 cm 2 surface	1.01 × 10 18	1.94 × 10 15
	Number of molecules in 0.02 x 0.02 x 0.02 cm 3	6.19× 10 15	6.19× 10 15
	Crystal Irradiation	Homogeneous	~One face
	Photoconversion	~85%	~24.5% (MS1); 2.3%
			(MS2 Model)

Table 4 .

 4 18 : Selected bond distances and angles of GS, one pulse PLI and relaxed GS of I.

					ONE Pulse PLI	GS after ONE
		GS	Refined as MS1	Forced GS Refined as	Modelled as MS2	Pulse RELAXED
	R[F 2 > 2(F 2 )],	0.056, 0.173,	0.042,	0.135,	0.042, 0.135,	0.043, 0.137, 1.12 0.054, 0.149,
	wR(F 2 ), S	1.12	1.12		1.12		1.18
	No. of parameters 688	677		687	683	769
	 max ,  min (e Å -3 ) 1.49, -2.85	1.53, -2.87	1.52, -2.88	2.09, -2.97	0.68, -1.64
	Ru1-N1	1.756 (3)	1.747(6)		1.771(2)	1.769(3)	1.748(3)
	Ru1-N1A	-	-		-	2.29(3)	-

Table 4 .

 4 19 : Most sensitive reflections from GS to One-pulse irradiation.

I:

Screening most sensitive reflections from GS to single-shot Pulse laser excited state

	No.	h	k	l	IGS	Iexc(One Pulse PLI)	(Iexc-IGS)/IGS	Relaxed GSIexc	(Irelaxed GS-IGS)/IGS
	1	2	0	0	88866.23	88297.47	-0.0064	73387.35	-0.17418
	2	2	3	0	84875.62	86297.41	0.01675	79364.58	-0.06493
	3	2	-4	2	64963.51	67445.8	0.03821	48026.59	-0.26071
	4	-2	2	0	63423.79	63246.62	-0.00279	35210.61	-0.44484
	5	-1	2	4	61359.85	59748.66	-0.02626	51714.72	-0.15719

Table 4 .

 4 20 : Estimation of number of photons and number of molecules in the pellet of I for one pulse experiment (Molar mass of I M I =665.38 g/mol).

	[Ru(NO)(Py)4F](ClO4)2 Pellet	One Pulse	4000 Pulses
	Fluence 𝑄 = 𝐼 × 𝑡	2.2 mJ/cm 2	8.8 J/cm 2
	Power	22 mW	22 mW
	Irradiation time t	5 ns	400 s
	Number of pulses	1	4000
	Number of photons/cm 2	5.71 × 10 15	2.28 × 10 19

High excitation efficiency: one pulse single-shot steady-state photocrystallography in II (a) Damage threshold of II toward one pulse laser

Table 4 .

 4 21 : Comparison of number of photons and number of molecules in a crystal of saturation fluence and the crystal 1,2,3 used of ns-pulse photocrystallography.

	[Ru(NH3)5(NO)](Cl)3•H2O Saturation with	Crystal One	Crystal Two	Crystal Three
		pulsed laser
	Fluence 𝑄 = 𝐼 × 𝑡	205 J/cm 2	0.509 J/cm 2	0.3566 J/cm 2	0.509 J/cm 2
	Power	2.67 mW	10 mW	7 mW	1 mW
	Laser beam spot size Area	30m [0.070cm 2 ]	[0.0019625cm 2 ]	[0.0019625cm 2 ]	[0.0019625cm 2 ]
		3mm diameter	by using 500m	by using 500m	by using 500m
			pinhole	pinhole	pinhole
	Intensity	205 W/cm 2	5.095 W/cm 2	3.567 W/cm 2	0.509 W/cm 2
	Irradiation time t	90 minutes	5 ns	5 ns	1 s
	Number of pulses	54000	1	1	10
	Number of photons on a	1.7 × 10 17	4.3 × 10 14	3.0 × 10 14	4.3 × 10 14
	0.02 x 0.02 cm 2 surface
	Number of molecules in	2.7 × 10 16	2.7 × 10 16	2.7 × 10 16	2.7 × 10 16
	0.02 x 0.02 x 0.02 cm 3
	Crystal Irradiation	Homogeneous	~One face	~One face	~One phi rotation
	Photoconversion	~50%	~38(3)%	30(4)%	40.6(19)%

Table 4 .

 4 22 : Crystallographic parameters in Crystal 1 of II.

		GS	One Pulse PLI	Relaxed GS after One Pulse
			crystal 1
	Crystal data
	Chemical formula	H 15 N 6 ORu•3(Cl)•0.5(H 4 O 2 )
	Crystal system, space group	Orthorhombic, Pnma
	Temperature (K)	100
	a, b, c (Å)	11.833(2),	11.910(2),	11.919(2), 6.8738(16), 14.251(4)
		6.9231(11),	6.8620(16),
		14.216(3)	14.194(3)
	V (Å 3 )	1164.6(4)	1160.0(4)	1167.6 (5)
	Z	4
	Radiation type	Mo K
	Data collection
	Diffractometer	STOE STADI-VARI

Table 4 .

 4 23 : Selected bond distances and bond angles in Crystal 1 of II.

		GS	One Pulse PLI_	Relaxed GS after One Pulse
			crystal01
	Ru1-N1	1.82(3)	1.75(2)	1.90(2)
	Ru1-O1A	-	1.99(5)	-
	Ru1-N2	2.096(3)	2.087(5)	2.115(4)
	Ru1-N5	2.04(2)	2.01(2)	1.965(18)
	N1-O1	1.14(2)	1.18(2)	1.07(2)
	N1A-O1A	-	1.11(6)
	Ru1-N4	2.107(4)	2.104(6)	2.109(5)
	Ru1-N1-O1	175.280	178.0(17)	173.1(9)-
	Ru1-O1A-N1A	-	162(4)
	U eq (Ru1)	0.02458(17)	0.02595(16)	0.02389(12)
	U eq (N1)	0.027(3)	0.024(2)*	0.032(3)
	U eq (N1A)	-	0.0244(16)*
	U eq (O1)	0.0389(13)	0.0244(16)*	0.0351(14)
	U eq (O1A)	-	0.024(2)*
	U eq (N5)	0.033(4)	0.034(3)	0.032(3)
	U eq (N4)	0.0365(10)	0.069(3)	0.059(2)
	U eq (N3)	0.0600(18)	0.0344(12)	0.0425(9)
	U eq (N2)	0.0414(8)	0.0436(11)	0.0356(12)
	Population
	GS		62(3)%	-
	PLI		38(3)%

Table 4 .

 4 24 : Crystallographic parameters in Crystal 2 and 3 of II.

		GS	One Pulse PLI	One pulse PLI Crystal 3
			crystal 2	10 pulses 1 mW
	Crystal data
	Chemical formula	H 15 N 6 ORu•3(Cl)•0.5(H 4 O 2 )
	Crystal system, space group	Orthorhombic, Pnma
	Temperature (K)	100
	a, b, c (Å)	11.833(2),	11.858(3), 6.919(2),	11.823(2), 6.905(2), 14.172(4)
		6.9231(11),	14.213(3)
		14.216(3)
	V (Å 3 )	1164.6(4)	1166.1(5)	1157.0(5)
	Z	4
	Radiation type	Mo K
	Data collection
	Diffractometer	STOE STADI-VARI
	No. of measured, independent and	14686, 1591,	3550, 1380, 1371	8015, 3523, 3221
	observed [I > 2(I)] reflections	1495
	R int	0.0384	0.024	0.040
	(sin /) max (Å -1 )	0.671	0.650	0.915
	Refinement
	R[F 2 > 2(F 2 )], wR(F 2 ), S	0.039, 0.102,	0.046, 0.124, 1.30 0.070, 0.186, 1.17
		1.20
	No. of parameters	101	90	90
	 max ,  min (e Å -3 )	1.28, -0.86	0.77, -1.11	3.44, -2.19

Table 4 .

 4 25 : Selected bond distances and bond angles in Crystal 2 and 3 of II.

		GS	One Pulse PLI_	One pulse PLI crystal 3 after 10
			crystal 2	pulses 1mW
	Ru1-N1	1.82(3)	1.75(2)	1.760(12)
	Ru1-O1A	-	1.98(8)	1.90(2)
	Ru1-N2	2.096(3)	2.089(5)	2.095(4)
	Ru1-N5	2.04(2)	2.05(2)	2.065(13)
	N1-O1	1.14(2)	1.19(2)	1.160(13)
	N1A-O1A	-	1.19(10)	1.171(18)

Table 4 .

 4 26 : Selected most sensitive common reflections from GS to PLI in II Crystal 1, 2 and 3. one expects the NO stretching vibration for MS1 at 1799 cm -1 and the one for MS2 at 1570 cm -1 . Figure4.26 shows the result after irradiation with pulses of 420 nm wavelength, an average power of 22 mW (for 0.81 cm 2 surface) at 10 Hz repetition rate as used for I (see above). The IR spectra (Figure4.26) show that after 54000 pulses 11% of the

				Crystal01	Crystal02	Crystal03
	No. h k l	IGS	IMS1	(IMS1-IGS)/IGS	IMS1	(IMS1-IGS)/IGS	IMS1	(IMS1-IGS)/IGS
	1	2 0 5 54605.454 27746.7642	-0.492	35625.241	-0.348	42240.032	-0.226
	2	0 4 0 45483.922 43071.9384	-0.053	38776.777	-0.147	40504.461	-0.109
	3	3 2 2 42568.809 49265.9753	0.157	33450.106	-0.214	44739.061	0.051
	4	1 2 8 24059.546 15895.5360	-0.339	22747.185	-0.055	18104.106	-0.248
	5	3 1 1 19832.892 24059.0190	0.213	--	--	17126.669	-0.136
	(d) Infrared spectroscopy with ns pulsed laser
	As in the case of [Ru(NO)(Py)4F](ClO4)2 IR spectroscopy after pulsed excitation was
	performed on [Ru(NH3)5(NO)](Cl)3•H2O, in order to investigate in more detail the population
	mechanism. For II, molecules have been transferred out of GS, principally to MS1, as evidenced by the
	corresponding vibrational band at 1799 cm -1 . The band of MS2, expected at 1570 cm -1 is not
	visible in these spectra, indicating a very low population of less than 0.1%. We can calculate
	as before, for the case of I, that there are about 16 photons per molecule available. Assuming
	only population of MS1, and a two-step process, we get 8 photons/molecule resulting in 11%
	population, i.e. the efficiency is about 1.4% for generating MS1. Since in this case we do not
	observe any trace of the MS2 state, this indicates that the efficiency for excitation out of MS2
	is very high (implying a very large absorption cross section).

Table 4 .

 4 27 : Comparison of Fluence with respect to number of photons in pellet after irradiation with ns-pulse

	laser in IR spectroscopy.
	[Ru(NH 3 ) 5 (NO)](Cl) 3• H 2 O Pellet	One Pulse	54000 Pulses
	Fluence 𝑄 = 𝐼 × 𝑡	2.2 mJ/cm 2	118 J/cm 2
	Power	22 mW	1180 W
	Irradiation time t	100 ms	90 min
	Number of pulses	1	54000
	Number of photons/cm 2	5.71 × 10 15	2.28 × 10 19
	Number of photons on a 1.326 cm 2 surface	7.67 × 10 15	3.02 × 10 19
	Number of molecules in pellet	1.9 × 10 18	1.9 × 10 18
	Pellet Irradiation	~One face	~One face
	Photoconversion	0	11%

Table 5 . 1

 51

	: Main characteristics of XPAD3.2 detector
	Main features of XPAD3.2
	Sensor	Si-500 m
	Maximal count rate	2×10 5 photons s -1 pixel -1
	Maximal count rate	1.2×10 7 photons s -1 mm -2
	Pixel size(m)	130×130
	Number of pixels	560×960 = 537600
	Detection surface (cm)	7.5×15
	Time-resolved possibilities	Pump-probe

Table 5 . 2 :

 52 Selected crystallographic parameters of GS and MS2 of SNP.

	SNP	Ground state	Metastable state 2
	Crystal data
	Chemical formula	C 5 H 4 FeN 6 Na 2 O 3
	M r	297.97
	Crystal system, space group	Orthorhombic, Pnnm

Table 5 .

 5 3 : Selected bond distances and bond angles of GS and MS2 of SNP.

		Ground state	MS2
	Fe-N4A	1.6542(7)	1.6642(8)
	Fe-N4B	⎯	2.05(2)
	Fe-OB	⎯	2.18(2)
	N4A-OA	1.1276(9)	1.1330(11)
	N4B-OB	⎯	0.82(3)
	Fe-N4A-OA	176.03(7)	176.03(8)
	Fe-N4B-OB	⎯	88(2)
	Fe	0.00586(4)	0.00608(4)
	OA	0.01759(13)	0.01840(19)
	OB	⎯	0.011(4)*
	N4A	0.00875(10)	0.00886(16)
	N4B	⎯	0.011(4)*

Table 5 .

 5 4 : Most sensitive reflection in SNP from GS to MS2.

	No.	h	k	l	Iexc	 2	IGS	I=Iexc-IGS	I/IGS
	1	2	4	0	31320.26	36.94208	31682.47	-362.212	-0.01143
	2	0	0	8	37196.75	17.64	31051.69	6145.063	0.1979
	3	1	2	4	29680.92	9.89795	29651.63	29.28063	9.87E-04
	4	1	3	4	19745.36	5.67726	19422.91	322.4547	0.0166
	5	0	2	2	19618.57	7.36254	18178.43	1440.141	0.07922
	6	1	2	1	17862.3	6.09004	17524.68	337.6196	0.01927
	7	3	0	5	18624.99	6.5173	16103.33	2521.658	0.15659
	8	1	4	5	13105.53	3.93546	13205.92	-100.384	-0.0076
	9	1	4	3	11989.57	3.74074	12450.86	-461.284	-0.03705
	10	2	4	8	12572.02	3.60089	12191.32	380.698	0.03123

Table 5 .

 5 5 : Full data, chi=0 and to Chi=5 to see the effect different orientations on data completeness.

		Full data	Chi=0	Chi=5
	Completeness	88.5%	57.5%	70.7%
	(sin /)max (Å -1 )	0.863	0.714	0.851
	No. of measured, independent and observed [I > 2s(I)] reflections	24144, 2160, 1408	10604, 1031, 847	13541, 2159, 1367
	-h→ h	-10→6	-6→6	-10→10
	-k→ k	-16→14	-13→13	-16→16
	-l→ l	-26→4	-20→0	-26→26
	max, min	37.8°, 2.2°	30.5°, 2.2°	37.5°, 2.15°
	R[F 2 > 2s(F 2 )], wR(F 2 ), S	0.054, 0.174, 1.03	0.044, 0.126, 1.09	0.055, 0.180, 1.04

The next step is to simulate MS2 population from the data sets result for the three abovediscussed cases i.e., full data, Chi=0, Chi=5 deg into 1, 5, 10% populations resulting in 9 combinations (Figure

5

.18). The MS2 position is clearly visible for 5%, and 10% population in all configurations, slightly less pronounced for the Chi=0 and 5% case. For 1% on the other hand, in all configurations, the MS2 positions are invisible, only depletion is observed in the GS positions at the chosen contour level of 0.1 e/Å 3 .

Table 5 .

 5 6 : Percentage completeness as a function of crystal to detector distance.

		Crystal to detector	Crystal to detector
		distance 50 mm	distance 150 mm
	Completeness	98.5%	50.8%
	(sin /)max (Å -1 )	0.889	0.834
	No. of measured, independent and observed [I > 2s(I)] reflections	17212, 3216, 2001	12302, 1074, 926
	-h→ h	-11→10	-9→8
	-k→ k	-21→21	-17→17
	-l→ l	-5→26	-1→24
	max, min	39.2°°, 2.2°	36.3°, 3.4°
	R[F 2 > 2s(F 2 )], wR(F 2 ), S	0.029, 0.075, 0.94	0.034, 0.104, 1.10

Table 5 .

 5 7 : SNP ground state data set collected at SOLEIL and its comparison to Supernova diffractometer.

	SNP	Ground state, Supernova	Ground state, SOLEIL
	Crystal system, space group	Orthorhombic, Pnnm
	Temperature (K)	100	200
	a, b, c (Å)	6.1029(2), 11.7823(5),	6.1809(8), 11.9043(15),
		15.4483(9)	15.6292(16)
	V (Å 3 )	1110.83(9)	1150.0(2)
	Z	4
	Radiation type	Mo K	Synchrotron
	Wavelength	0.71073	0.67142
	Crystal size (mm)	0.73×0.26×0.23
	Data collection
	Diffractometer	Agilent SuperNova with Atlas	CRISTAL	6	circles
		2 CCD	diffractometer
	Absorption correction	Empirical (using intensity	none
		measurements)
	No. of measured, independent and	48941, 3061, 2884	2735, 995, 647
	observed [I > 2(I)] reflections
	R int	0.047	0.160
	(sin /) max (Å -1 )	0.862	0.862
	Refinement
	R[F 2 > 2(F 2 )], wR(F 2 ), S	0.022, 0.065, 1.06	0.047, 0.047, 1.29
	No. of reflections	3061	995

Table : B

 : .2 Selected features of the White light module

			400-2400 nm (6 bundle leg)
	Core diameter		200 μm
	Glass filter		Schott GG435, Ø40mm, thickness
	White light source		633 QTH (Quartz Tungsten 3 mm
			Lamp)-Newport
	Power		100 W
	Colour Temperature Table B.4: Features of fiber optic spectrometer	3300 K
	Flux Optic spectrometer	3600 1 m Ocean Optics USB4000
	Lamp housing Detector	Oriel 77501 Toshiba TCD1304AP linear CCD array
	Optic fiber Detector Range	Ocean Optics QP-200-2-SR-BX 179-886 nm
	Wavelength range Grating	1	200-1100 nm
	Core diameter Slit size	200 μm 25 μm
	Fiber Connector Pixels number	3648	Thorlabs SM1SMA
	Lense tubes Dynamic range	Thorlabs SM1L30C 3.4 x 10 6
	Lens type		Achromatic Plano-Convex Lense,
			f =50 mm; Ø1" (fixed), Thorlabs
			AC254-050-AB-ML
	Table B.3 Technical features of Light detecting module
	Objective		Rodenstock Apo-Rodagon N 1:4,
			f =150 mm
	Beam Expander		Achromatic Galilean Thorlabs
			GBE20-A
	Expansion		20x
	Coating		AR 400-650 nm
	Fiber connector		Thorlabs SM1SMA
	Optic fiber		Thorlabs Y-bundle (6+1)
	Wavelength range		250-1200 nm (1 bundle leg)

Table D .

 D 2: Selective crystallographic parameters of stepwise MS1 generation and its transfer to MS2 in [Ru(Py) 4 (NO)F](ClO 4 ) 2 .

			0.01129(6)	83.134(4) 0.01218(5) 82.699(8) 0.01416(6) 82.621(9) 0.01394(5) 82.504(7)	87.089(8) 0.01688(5)
	V (Å 3 ) Z U eq N1, N1B	2 0.0138(5)	2584.7(2) 0.0131(6)*, 2583.5(5) 0.0122(7)*, 2571.4(5) 0.0161(7)* 0.0187(8)* 0.0253(11) 2575.0(4) 0.0154(9),	2594.7(5) 0.0170(5)*, 0.0170(5)*
	Radiation type  (mm -1 ) U eq O1, O1B Data collection U eq N6, N6B	Mo K 0.0222(5) 0.88 0.0137(5)	0.0161(7)*, 0.0131(6)* 0.0131(6)*, 0.0182(7)*	0.0187(8)*, 0.0122(7)* 0.0132(7)*, 0.0198(9)*	0.0253(11), 0.0154(9) 0.0140(9), 0.0224(10)	0.0292(6)*, 0.0292(6)* 0.0186(6)*, 0.0308(6)*
	Diffractometer No. of measured, independent and U eq O2, O2B 0.0223(5) STOE STADI-VARI 53578, 53578, 45419, 45419, 0.0182(7)*, 0.0198(9)*, 45182, 19141, 0.0224(10), 44984, 19079, 0.0308(6)*, 45461, 18603, 0.0131(6)* 0.0132(7)* 0.0140(9) 0.0308(6)*
	observed [I > 2(I)] reflections Population	44684		37155	16843	16465	13909
	R int	(Mol. 1)PLI		0.071	0.034 32(3)%	0.037 58(3)%	0.035 88(3)%	0.031 13.1(5)%
	(sin /) max (Å -1 ) GS		0.830	0.833 68(3)%	0.833 42(3)%	0.830 12(3)%	0.88 86.9(5)%
	Refinement (Mol. 2)PLI				21(3)%	55(4)%	86(3)%	12.4(5)%
	R[F 2 > 2(F 2 )], wR(F 2 ), S GS		0.056, 0.173, 79(3)% 0.042, 0.117, 45(4)% 0.054, 0.140, 14(3)% 0.045, 0.123,	0.039, 0.124, 87.6(5)%
				1.12		1.12	1.10	1.11	1.10
	No. of parameters		688		676	737	728	720
	 max ,  min (e Å -3 )		1.49, -2.85	1.24, -2.01	1.44, -2.46	1.04, -2.32	1.14, -1.57
			GS			MS1A	MS1B	MS1C	MS2
		Ru1-N1	1.756(3)		1.791(8)	1.822(11)	1.76(7)	1.766(3)
		Ru12-N6	1.762(3)		1.773(7)	1.797(13)	1.81(6)	1.770(3)
		Ru1-F1	1.914(2)		1.9193(18)	1.904(2)	1.9070(19)	1.9206(17)
		Ru2-F2	1.938(2)		1.9184(18)	1.904(2)	1.9024(19)	1.9169(16)
		N1-O1	1.148(4)		1.158(9)	1.177(15)	1.30(10)	1.137(4)
		N1B-O1B	-			1.182(17)	1.171(10)	1.142 (12)	1.13(3)
		N6-O2	1.155(4)		1.148(8)	1.152(17)	1.05(8)	1.149(4)
		N6B-O2B	-			1.19(3)	1.139(12)	1.167(11)	1.14(3)
		Ru1-N1-O1	178.4(3)		170.1(8)	161.8(5)	170.931	177.727
		Ru1-O1B-N1B	-			161.719	166.3(11)	177.77	-
		O1B-Ru1-N1B	-			-		-	-	31.432
		Ru2-N6-O2	177.6(3)		178.038	178.7(12)	163.876	179.282
		Ru2-O2B-N6B	-			156.492	176.7(9)	178.688	-
		O2B-Ru2-O2B	-			-		-	-	32.506
		U eq Ru1, Ru2	0.01140(6),		0.01215(5),	0.01388(6),	0.01403(5),	0.01646(5),

Table D .

 D 3: Selected common reflections from GS to MS1 and MS2 in [Ru(Py) 4 (NO)F](ClO 4 ) 2 after irradiation with CW and pulsed laser. All the selective reflections are sorted descending order on the basis of GS

							intensities.
						[Ru(Py) 4 (NO)F](ClO 4 ) 2
	No.	h	k	l	IGS	IMS1	(IMS1-IGS)/IGS	IMS1	(IMS1-IGS)/IGS	IMS2	(IMS2-IGS)/IGS	IMS2
	1	2	0		88866.23 82127.18	-0.07583	82011.56	-0.07713	90368.96 0.01691 81430.5
	2	2	3		84875.62 88061.75 0.03754 89082.49 0.04957 88907.85 0.04751 18325.06
	3	0	0		78258.22 74202.41	-0.05183	73042.31	-0.06665	82960.76 0.06009 59661.78
	4	2	-4		64963.51 64152.28	-0.01249	59723.54	-0.08066	66331.74 0.02106 309.0986
	5	-2 2		63423.79 66861.24 0.0542 62909.52	-0.00811	64922.38 0.02363 54839.1
	6	-1 2		61359.85 62152.88 0.01292 68537.25 0.11697 60366.13	-0.01619	362.434
	7	-2 3		59786.07 52798.16	-0.11688	50966.18	-0.14752	58239.01	-0.02588	45979.32
	8	1	-3		58183.76 59133.89 0.01633 60313.07 0.0366 59158.74 0.01676 29419.69
	9	1	4		57430.54 61350.09 0.06825 62182.7 0.08275 56893.79	-0.00935	6869.244
	10	-2 0		55669.1	66992.4	0.2034 67784.57 0.21763 54956.02	-0.01281	64604.17
	11	0	4		54643.36 50261.74	-0.08019	47132.54	-0.13745	55471.18 0.01515 7350.284
	12	-1 3		53826.37 58730.61 0.09111 64832.97 0.20448 54616.95 0.01469 54092.01
	13	1	-1		51131.25 41931.82	-0.17992	41014.84	-0.19785	52831.39 0.03325 498.6289
	14	-1 1		50424.5 48973.78	-0.02877	48372.86	-0.04069	52161.54 0.03445 6086.013
	15	0	4		50151.14 52541.07 0.04765 50948.12 0.01589 50783.03 0.0126 28550.42
	16	3	-2		48509.45 45087.64	-0.07054	45167.22 -0.0689 48031.28	-0.00986	8373.366
	17	-2 1		48503.37 35780.45	-0.26231	34298.08	-0.29287	50261.2 0.03624 55672.26
	18	3	-2		45751.58 41215.54	-0.09915	40781.18	-0.10864	44228.53	-0.03329	312.9573
	19	-2 2		45353.96 38528.15 -0.1505 37632.93	-0.17024	46265.17 0.02009 16365.5
	20	1	6		45212.07 48578.67 0.07446 49302.69 0.09048 44133.23	-0.02386	5872.601

Table D .

 D 4: Calculations of most sensitive reflections in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O (GS and MS1 after pulsed irradiation). All the selective reflections are sorted descending order on the basis of GS intensities.

	2	2	0	5	48618.79	12.12641	54605.45	-5986.66	-0.10963
	3	0	4	0	46150.68	11.86114	45483.92	666.7606	0.01466
	4	3	2	2	48479.85	0.73943	42568.81	5911.04	0.13886
	5	3	0	2	28816.15	25.85214	27492.53	1323.624	0.04814
	6	1	2	8	25174.07	6.37361	24059.55	1114.528	0.04632
	7	2	0	0	22242.74	6.25E-04	22611.5	-368.758	-0.01631
	8	2	0	6	18980.85	1.64224	21735.78	-2754.93	-0.12675
	9	3	1	1	21428.48	0.5776	19832.89	1595.589	0.08045
	10	1	1	5	18577.44	13.86371	18953.47	-376.025	-0.01984
	11	1	0	8	16072.08	1.26518	18389.96	-2317.89	-0.12604
	12	6	0	3	18045.17	5.29046	17612.53	432.6388	0.02456
	13	1	2	3	17907.26	1.45781	16043.21	1864.046	0.11619
	14	0	2	6	17311.66	2.12722	15794	1517.66	0.09609
	15	6	1	0	14478.8	0.26389	15784.61	-1305.8	-0.08273
	16	6	2	3	14992.58	1.28732	15377.19	-384.608	-0.02501
	17	2	1	3	17399.43	0.31607	15325.3	2074.129	0.13534
	18	5	0	5	12113.16	0.03614	15014.39	-2901.23	-0.19323
	19	9	0	1	14952.25	0.65238	14475.87	476.384	0.03291
	20	6	3	0	14045.95	2.2491	14412.72	-366.775	-0.02545
	No.	h	k	l	IMS1	 2	IGS	I=IMS1-IGS	I/IGS
	1	0	2	0	73532.9	0.00144	64405.86	9127.036	0.14171

Table D .

 D 5: Calculations of most sensitive reflections in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O (GS and MS2 after pulsed irradiation). All the selective reflections are sorted descending order on the basis of GS intensities.

	15	6		0	14629.22	5.42238	15784.61	-1155.39	-0.0732
	16	6		3	13759.29	5.3361	15377.19	-1617.9	-0.10521
	17	2		3	16901.2	4.28863	15325.3	1575.895	0.10283
	18	5		5	13344.62	8.45065	15014.39	-1669.77	-0.11121
	19	9		1	13586.05	9.82258	14475.87	-889.821	-0.06147
	20	6		0	13048.06	11.0909	14412.72	-1364.66	-0.09468
	No.	h	k	l	IMS2	 2	IGS	I=IMS2-IGS	I/IGS
	1	0	2	0	97666.63	4.89162	64405.86	33260.76	0.51642
	2	2	0	5	68531.86 279.4782	54605.45	13926.4	0.25504
	3	0	4	0	51689.7	128.4369	45483.92	6205.783	0.13644
	4	3	2	2	56479.23 13.60429	42568.81	13910.42	0.32678
	5	3	0	2	35070.02	9.2562	27492.53	7577.49	0.27562
	6	1	2	8	26416.91 30.77254	24059.55	2357.365	0.09798
	7	2	0	0	28530.25 1.43E+00	22611.5	5918.752	0.26176
	8	2	0	6	18224.68 83.77924	21735.78	-3511.11	-0.16154
	9	3	1	1	23745.79	7.85176	19832.89	3912.901	0.19729
	10	1	1	5	21669.64	4.97602	18953.47	2716.166	0.14331
	11	1	0	8	17522.9	10.09841	18389.96	-867.061	-0.04715
	12	6	0	3	16765.51 13.88308	17612.53	-847.017	-0.04809
	13	1	2	3	17906.48	3.50963	16043.21	1863.269	0.11614
	14	0	2	6	17659.94	7.91691	15794	1865.934	0.11814

Table D .

 D 6: Calculations of most sensitive reflections in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O (GS and MS1 after single pulse irradiation in crystal 1). All the selective reflections are sorted descending order on the basis of GS intensities.

Crystal01:

Screening most sensitive reflections from GS to Excited state

	No.	h	k	l	IGS	Iexc	I=Iexc-IGS	I/IGS
	1		0	5	54605.454	27746.7642	-26858.690	-0.492
	2		4	0	45483.922	43071.9384	-2411.984	-0.053
	3		2	2	42568.809	49265.9753	6697.166	0.157
	4		2	8	24059.546	15895.5360	-8164.010	-0.339
	5		1	1	19832.892	24059.0190	4226.127	0.213
	6		1	5	18953.469	9085.9596	-9867.510	-0.521
	7		0	8	18389.964	11402.6945	-6987.269	-0.380
	8		0	3	17612.528	19430.8266	1818.299	0.103
	9		2	3	16043.212	16458.6320	415.420	0.026
	10		2	6	15794.005	16338.2592	544.255	0.034
	11		1	0	15784.606	17318.8495	1534.244	0.097
	12		2	3	15377.190	16124.7331	747.543	0.049
	13		0	5	15014.385	15174.3225	159.937	0.011
	14		0	1	14475.868	14729.3419	253.474	0.018
	15		3	0	14412.723	16853.1545	2440.432	0.169
	16		0	4	13963.109	15414.1661	1451.057	0.104
	17		2	1	13935.732	15323.2461	1387.514	0.100
	18		2	0	13764.592	14220.9441	456.352	0.033
	19		1	2	13411.724	14588.1224	1176.398	0.088
	20		1	1	12681.553	14384.5721	1703.020	0.134

Table D .

 D 7: Calculations of most sensitive reflections in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O (GS and MS1 after single pulse irradiation in crystal 2). All the selective reflections are sorted descending order on the basis of GS intensities.

Crystal02:

Screening most sensitive reflections from GS to Excited state

	No.	h	k	l	IGS	Iexc	I=Iexc-IGS	I/IGS
	1	2	0	5	54605.454	35625.241	-18980.213	-0.348
	2	0	4	0	45483.922	38776.777	-6707.145	-0.147
	3	3	2	2	42568.809	33450.106	-9118.703	-0.214
	4	1	2	8	24059.546	22747.185	-1312.361	-0.055
	5	3	1	1	19832.892	--	--	--
	6	1	1	5	18953.469	--	--	--
	7	1	0	8	18389.964	16115.465	-2274.499	-0.124
	8	6	0	3	17612.528	17192.218	-420.310	-0.024
	9	1	2	3	16043.212	--	--	--
	10	0	2	6	15794.005	14104.151	-1689.853	-0.107
	11	6	1	0	15784.606	14721.794	-1062.812	-0.067
	12	6	2	3	15377.190	14898.741	-478.449	-0.031
	13	5	0	5	15014.385	13806.885	-1207.501	-0.080
	14	9	0	1	14475.868	12226.211	-2249.656	-0.155
	15	6	3	0	14412.723	14665.525	252.802	0.018
	16	6	0	4	13963.109	13850.842	-112.267	-0.008
	17	3	2	1	13935.732	13918.714	-17.018	-0.001
	18	2	2	0	13764.592	--	--	--
	19	9	1	2	13411.724	13018.582	-393.143	-0.029
	20	1	1	1	12681.553

Table D .

 D 8: Calculations of most sensitive reflections in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O (GS and MS1 after single pulse irradiation in crystal 3). All the selective reflections are sorted descending order on the basis of GS intensities.

Crystal03:

Screening most sensitive reflections from GS to Excited state

	7	1	0	8	18389.964	17126.538	-1263.425	-0.069
	8	6	0	3	17612.528	15204.123	-2408.405	-0.137
	9	1	2	3	16043.212	16209.287	166.076	0.010
	10	0	2	6	15794.005	11951.803	-3842.202	-0.243
	11	6	1	0	15784.606	15784.606	12.868	0.001
	12	6	2	3	15377.190	15763.104	385.913	0.025
	13	5	0	5	15014.385	13988.952	-1025.433	-0.068
	14	9	0	1	14475.868	13868.454	-607.414	-0.042
	15	6	3	0	14412.723	15034.757	622.034	0.043
	16	6	0	4	13963.109	13340.504	-622.605	-0.045
	17	3	2	1	13935.732	15447.432	1511.701	0.108
	18	2	2	0	13764.592	13855.691	91.099	0.007
	19	9	1	2	13411.724	13482.647	70.922	0.005
	20	1	1	1	12681.553	8765.622	-3915.931	-0.309
	No.	h	k	l	IGS	Iexc	I=Iexc-IGS	I/IGS
	1	2	0	5	54605.454	42240.032	-12365.422	-0.226
	2	0	4	0	45483.922	40504.461	-4979.462	-0.109
	3	3	2	2	42568.809	44739.061	2170.252	0.051
	4	1	2	8	24059.546	18104.106	-5955.440	-0.248
	5	3	1	1	19832.892	17126.669	-2706.223	-0.136
	6	1	1	5	18953.469	15539.093	-3414.376	-0.180

Table D .

 D 9: Selected ADPs in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O (GS and forced GS model after single pulse irradiation in crystal 1, 2, 3)

	Selective ADP'S	GS (Actual)	Crystal 1(Forced refinement as pure GS)	Crystal 2(Forced refinement as pure GS)	Crystal 3(Forced refinement as pure GS)
	U eq (Ru1)	0.02458(17)	0.0259(2)	0.02222(17)	0.0227(2)
	U eq (N1)	0.027 3)	0.034(4)	0.025(3)	0.024(3)*
	U eq (O1)	0.0389(13)	0.0398(18)	0.0377(16)	0.0369(18)*
	U eq (N5)	0.033(4)	0.037(5)	0.030(4)	0.033(3)*
	U eq (N4)	0.0365(10)	0.066(3)	0.058(2)	0.056(3)
	U eq (N3)	0.0600(18)	0.0432(11)	0.0393(9)	0.0376(11)
	U eq (N2)	0.0414(8)	0.0343(13)	0.0331(12)	0.0350(16)

Table E .

 E 2: Transient optical absorption measurement at different temperature in [Ru(NH 3 ) 5 (NO)]Cl 3 •H 2 O after one ns-pulse laser shot.

	No.	T(s)	k=1/t(s -1 )	lnK	T(C)	T(K)	KBT(J)	KBT(ev)	1/KBT(ev)
		0.03974	25.1636	3.2254	7	280.16	3.8710 -21	2.41610 -2	41.3842
		0.03924	25.4842	3.23806	10	283.16	3.9110 -21	2.44 10 -2	40.9458
		0.02806	35.6379	3.57341	14	287.16	3.9610 -21	2.4810 -2	40.3754
		0.02	50	3.91202	18	291.16	4.0210 -21	2.5110 -2	39.8207
		0.01467	68.1663	4.22195	22	295.16	4.0710 -21	2.5510 -2	39.2811
		0.01068	93.633	4.53938	26	299.16	4.1310 -21	2.5810 -2	38.7559
		0.00756	132.275	4.88488	30	303.16	4.1810 -21	2.6210 -2	38.2445
		0.00523	191.205	5.25334	34	307.16	4.2410 -21	2.6510 -2	37.7465
		0.00395	253.165	5.53404	38	311.16	4.2910 -21	2.6810 -2	37.2612
	10	0.00285	350.877	5.86044	42	315.16	4.3510 -21	2.7210 -2	36.7883

Ru1-O1A -1.833(14) -2.39(3) -

(3) Crystal 3: P = 1 mW, 500 m beam spot, ten pulses We note, that for these parameters, we obtain relatively limited photon numbers, resulting in approximately 1 photon for 100 molecules in the crystal. We would thus expect a correspondingly low population of PLI. Figure 4.21 : Crystal of II selected for ns-pulse SC-XRD.In Crystal 1, after a single pulse laser shot (5ns pulse duration) of fluence of Q = 0.509 J/cm 2 , 38% photoconversion is achieved with SC-XRD. The saturated photoconversion for II is 50% (Figure4.22). Crystallographic details are given in Table 4.22 and Table 4.23.

Acknowledgments

First and foremost, I would like to express my deepest gratitude to my doctorate advisor, Dominik Schaniel, whose professional support throughout my Ph.D. was invaluable. Thank

attacks by nucleophiles making facile σ interactions, thereby relaxing in two proximal yet welldefined positions. Schaniel and Woike extended the study [START_REF] Schaniel | Generation of one light-induced metastable nitrosyl linkage isomer in [Pt(NH3)4Cl(NO)]Cl2 in the red spectral range[END_REF][START_REF] Casaretto | Multiple light-induced NO linkage isomers in the dinitrosyl complex [RuCl(NO)2 (PPh3)2]BF 4 unravelled by photocrystallographic and IR analysis[END_REF][START_REF] Casaretto | Photo-induced linkage NO isomers in the dinitrosyl complex [Ru(NO)2(PCy3)2Cl](BF4) identified by photocrystallography and IR-spectroscopy[END_REF][START_REF] Schaniel | Evidence for a photoinduced isonitrosyl isomer in ruthenium dinitrosyl compounds[END_REF]87,[START_REF] Mikhailov | Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes[END_REF][START_REF] Schaniel | Neutron diffraction shows a photoinduced isonitrosyl linkage isomer in the metastable state S I of Na2[Fe(CN)5NO]•2D2[END_REF]108,[START_REF] Schaniel | Photogeneration of metastable side-on N2 linkage isomers in [Ru(NH3)5N2]Cl2[END_REF][START_REF] Giglmeier | Nitric-oxide photorelease and photoinduced linkage isomerism on solid [Ru(NO)(terpy)(L)]BPh4 (L = glycolate dianion) Dalton Trans[END_REF][START_REF] Cormary | Structural Influence on the Photochromic Response of a Series of Ruthenium Mononitrosyl Complexes[END_REF][START_REF] Kostin | Photocrystallographic, Spectroscopic, and Calorimetric Analysis of Light-Induced Linkage NO Isomers in [RuNO(NO2)2-(pyridine)2OH][END_REF][134][START_REF] Mikhailov | Combining photoinduced linkage isomerism and nonlinear optical properties in ruthenium nitrosyl complexes[END_REF]. In these reported steady-state photocrystallographic studies of metal nitrosyls (mostly Ru or Fe complexes) the source of irradiation used was "CW" laser or LED.

Another ambidentate ligand that has been studied photocrystallographically is sulfur dioxide (SO2). The first photocrystallographic evidence of metal sulfur dioxide photoinduced linkage isomerism was studied on trans-[Ru(NH3)4Cl(SO2)]Cl by using 300-500 nm irradiation [START_REF] Kovalevsky | Light-Induced Metastable Linkage Isomers of Ruthenium Sulfur Dioxide Complexes[END_REF][START_REF] Bowes | Photocrystallographic structure determination of a new geometric isomer of [Ru(NH3)4(H2O)(η 1 -OSO)[END_REF]136]. Photocrystallographic studies of metal SO2 complexes were extended by the Cole group [START_REF] Cole | η 2 -SO 2 Linkage Photoisomer of an Osmium Coordination Complex[END_REF][START_REF] Cole | Single-Crystal Optical Actuation Generated by 100% SO2 Linkage Photoisomerization in a Ruthenium-Based Coordination Complex The[END_REF][START_REF] Cole | Structural Capture of η1-OSO to η 2 -(OS)O Coordination Isomerism in a New Ruthenium-Based SO2-Linkage Photoisomer That Exhibits Single-Crystal Optical Actuation[END_REF][START_REF] Cole | In situ synthesis, crystallisation, and thin-film processing of single crystals of trans-[Ru(SO 2)(NH3)4(H2O)[END_REF][START_REF] Cole | Light-Induced Macroscopic Peeling of Single-Crystal Driven by Photoisomeric Nano-Optical Switching[END_REF][START_REF] Cole | Systems Approach of Photoisomerization Metrology for Single-Crystal Optical Actuators: A Case Study of[END_REF][START_REF] Cole | Nanooptomechanical Transduction in a Single Crystal with 100% Photoconversion The[END_REF][141][START_REF] Cole | Assigning Optical Absorption Transitions with Light-Induced Crystal Structures: Case Study of a Single-Crystal Nanooptomechanical Transducer[END_REF]. In all these studies, the irradiation source used for the generation of photoinduced linkage isomers from ground state was "CW" laser or LED.

The case of the NO2 ligand was studied extensively by Raithby and Hatcher [START_REF] Bajwa | On the appearance of nitrite anion in [PdX(OAc)L2] and [Pd(X)(CN)L] syntheses (X = OAc or NO2): photocrystallographic identification of metastable Pd(η 1 -ONO)(CN)PPh3[END_REF][START_REF] Hatcher | Raising the (metastable) bar: 100% photo-switching in [Pd(Bu4dien)(η 1 -NO2)] + approaches ambient temperature[END_REF][START_REF] Hatcher | Understanding solid-state photoswitching in [Re(OMe2-bpy)(CO)3 (η 1 -NO2)] crystals via in situ photocrystallography[END_REF][START_REF] Kutniewska | Photocrystallographic and spectroscopic studies of a model (N,N,O)donor square-planar nickel(II) nitro complex: in search of high-conversion and stable photoswitchable materials[END_REF][START_REF] Hatcher | Metastable Linkage Isomerism in [Ni(Et4dien)(NO2)2]: A Combined Thermal and Photocrystallographic Structural Investigation of a Nitro/Nitrito Interconversion Angew[END_REF][START_REF] Hatcher | Thermal and photochemical control of nitro-nitrito linkage isomerism in single-crystals of [Ni(medpt)(NO2)(η 2 -ONO)[END_REF]. In these in-situ photocrystallographic studies, the irradiation source used for the generation of PLI from ground state was "CW" laser or LED.

Overall, in almost all steady-state photocrystallographic studies of the various types of photoactive-ligands NO, SO2 & NO2, CW irradiation sources (LED, Laser) were used for the photoconversion from ground state to excited state. Pulsed laser has been used typically for time-resolved photocrystallographic studies [START_REF] Pillet | Time-resolved structural analysis: probing condensed matter in motion[END_REF][START_REF] Coppens | Static and time-resolved photocrystallographic studies in supramolecular solids[END_REF][START_REF] Hallmann | Time-Resolved X-ray Diffraction of the Photochromic α-Styrylpyrylium Trifluoromethanesulfonate Crystal Films Reveals Ultrafast Structural Switching[END_REF][START_REF] Lorenc | Successive Dynamical Steps of Photoinduced Switching of a Molecular Fe(III) Spin-Crossover Material by Time-Resolved X-Ray Diffraction[END_REF][START_REF] Coppens | Time-resolved synchrotron diffraction and theoretical studies of very short-lived photo-induced molecular species[END_REF][149][150]. For metal nitrosyl compounds, one time-resolved XRD study was reported by Casaretto et al. [START_REF] Casaretto | In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector[END_REF], but in which only the intensities of a few Bragg reflections were recorded as a function of time after pulsed laser excitation (with millisecond time resolution).

In this Chapter of the thesis, the results from the steady-state photocrystallographic analysis performed using a "pulsed irradiation source" (instead of classical CW irradiation sources) for photoexcitation, along with a comparative analysis of the results obtained from the CW irradiation source are presented. The objective of this comparison of the results obtained by the two types of irradiation sources is to see (a) is the photoexcitation mechanism different using pulsed laser compared to CW laser? (b) is the fluence 𝑄 = 𝐼 × 𝑡 required to achieve saturation of PLI population smaller due to the high energy density of the pulsed laser (faster photoconversion until saturation) than when using a CW laser? (c) what is the difference in the population and relaxation pathway of PLI using pulse compared to CW irradiation source? To be able to address these questions, X-ray photocrystallography using a ns-pulsed laser has been combined with in-situ optical absorption spectroscopy of the single crystal in order to track real-time changes from GS to PLI.

In order to compare the effect of CW or pulsed irradiation on our samples, let's have first a look at the output of CW and pulsed lasers. As schematically illustrated in Figure 4.1, a CW laser provides a constant power output Pavg (given in Watts), so if one wants to calculate the total energy seen by the sample, one needs to multiply with the time during which the sample was irradiated 𝐸 𝐶𝑊 = 𝑃 𝑎𝑣𝑔 × 𝑡 𝑖𝑟𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 . If in addition one knows the spot size of the laser beam (area A), one can calculate the energy density (Joules per square meters) as 𝑒 𝐶𝑊 = 𝐸 𝐶𝑊 𝐴 ⁄ . However, one has to pay attention since in general, laser beams do not exhibit a flattop profile but rather Gaussian shapes; the energy density is not homogeneous over the spot size. For calculating the (spatial) peak energy density, one would thus need to consider the laser beam profile. We will not consider this aspect of spatial beam profile further here since in general, we use expanded laser beams much larger than the sample size so that we can consider the spatial beam profile on the sample position as flat-top, which results in a constant energy density over the irradiated area.

In the case of a pulsed laser the energy delivered by the laser is concentrated in (short) pulses, which are thus arriving on the sample at a given frequency f (repetition rate of laser) and also with a temporal peak shape (Figure 4.1), most often considered to be of Gaussian type with a full width at half maximum (FWHM) corresponding to the so-called pulse duration τpulse. The average power Pavg is then linked to the pulse energy by 𝑃 𝑎𝑣𝑔 = 𝐸 𝑝𝑢𝑙𝑠𝑒 × 𝑓. E.g. for a 10 Hz repetition rate and a pulse energy of 10 mJ, one obtains an average power of 100 mW. The peak power of the pulse depends on the temporal peak shape. For a rectangular peak shape, it is simply the pulse energy divided by the pulse duration 𝑃 𝑝𝑒𝑎𝑘 𝑟𝑒𝑐𝑡 = 𝐸 𝑝𝑢𝑙𝑠𝑒 𝜏 𝑝𝑢𝑙𝑠𝑒 ⁄ . For a pulse with Gaussian shape one has 𝑃 𝑝𝑒𝑎𝑘 𝑔𝑎𝑢𝑠𝑠 = 2√log(2) /𝜋 × 𝐸 𝑝𝑢𝑙𝑠𝑒 𝜏 𝑝𝑢𝑙𝑠𝑒 ⁄ . Note that this is due to the Gausssian peak shape in time, we still consider the spatial peak shape as flat-top since we place the sample in the expanded beam, leading to a homogeneous (spatial) energy density.

1197 J/cm 2 . One minute of pulsed laser irradiation corresponds to 600 laser pulses, so after 570 minutes the crystal has seen 342'000 laser pulses. Three X-ray data sets, MS1A-MS1C, were collected for MS1 after 60, 280, and 570 minutes, corresponding to Q values of 126, 588, and 1197 J/cm 2 (see in Appendix MS1A, MS1B; Here, in the main text only the final MS1 structure is described). The OA spectra were collected more frequently (13 time points in total), in order to follow the increase of the MS1 population by this method (Figure 4.7). Unfortunately, in this case the OA spectra are not of very good quality, there is a significant change in the baseline, so that it is not useful to calculate the absorption coefficient. This is partly due to the long experiment time over 2 days, as the intermediated X-ray data collection took several hours. However, from the OA spectra, we can still observe a continuous increase of the band at 700 nm, characteristic of MS1.

Having collected three data sets at different Q values, we can analyze the structure of the PLI at these points, which should correspond to different populations. We consider the final measurement at Q = 1197 J/cm 2 for comparison with the CW measurement. Table 4.1 shows the corresponding photodifference maps, which nicely illustrate the evolution of the features typical for the isonitrosyl PLI: a depletion of electron density on the oxygen position of NO and an accumulation around the nitrogen position of NO. The corresponding structural models have been refined, and the results are listed in Table 4.2. The refined population amounts to 88(3)% and 86(3)% for molecules 1 and 2, respectively. Compared to the structural models obtained from CW we observe thus similar populations of MS1 for both independent molecules in the unit cell. The Ru1-O1B and Ru12-O2B distances are 1.837(4) Å and 1.842(4) Å in the case of CW irradiation, and 1.859(7) Å and 1.848 [START_REF] Ernst | Microbial and Animal Rhodopsins: Structures, Functions, and Molecular Mechanisms[END_REF] Å in the case of pulsed irradiation. The corresponding Ru-O-N angles show small differences. The GS values for these distances and angles are different from the pure GS. This is mainly due to the fact that the population of GS is too low compared to the PLI population, and therefore the accuracy is comparatively lower [87].

The two other data sets for MS1 collected after 60 and 280 minutes of irradiation, corresponding to Q = 126 and 588 J/cm 2 , were refined accordingly. [Ru(Py)4(NO)F](ClO4)2 the pulsed experiment showed that the transfer to MS2 for the two independent molecules in the unit cell depends on the wavelength, and the pulsed laser light offers an opportunity to study this effect in more detail. For [Ru(NH3)5(NO)]Cl3•H2O we could not distinguish the two MS2 configurations for MS2 found in the CW experiment, but this is mainly due to inferior data quality in the pulsed experiment. Overall, we confirm that pulsed excitation produces the same MS1 and MS2 structures as CW irradiation at low temperatures.

This is an important result as it shows that by nanosecond laser excitation, the same PLI are generated than with CW laser irradiation. Hence, time-resolved studies will be addressing the same PLI and yield insight in the corresponding generation and relaxation pathways. From a methodological point of view, we have seen that the OA spectra allow to quickly (in a few minutes the crystal is aligned and the spectrum collected) obtain information whether population of the crystal has succeeded or not. This is a big advantage, because X-ray data collection takes many hours, in particular for low-symmetry crystals.

Can we use these two results to gain more insight into the population mechanism of PLI by doing steady-state experiments?

studies of the population process using CW irradiation [START_REF] Schaniel | Long-lived light-induced metastable states in trans-[Ru(NH3)4(H2O)NO]Cl3•H2O and related compounds[END_REF] in [Ru(NH3)5(NO)]Cl3•H2O and CW and pulsed irradiation in [Ru(Py)4(NO)F](ClO4)2 [START_REF] Mikhailov | Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes[END_REF]. Contrary to the latter, the pulsed irradiation in [Ru(NH3)5(NO)]Cl3•H2O does not allow to populate MS1 at higher temperatures compared to CW irradiation. This is most probably due to a very efficient second step, from MS2 to MS1, in the population process. The very strong absorption of MS2 in the blue-green spectral range is a strong indicator for this hypothesis. This is again confirmed by the OA spectra collected after transfer from MS1 to MS2 by using two different wavelength 980 nm and 1064 nm (Table 4.14). For both transfers, we observed a strong increase of the absorption starting at 750 nm and saturating at 600 nm. Below 600 nm the absorption is too strong to be measured with the present set-up.

Once MS1 is generated at a relatively high temperature, one can follow its relaxation with time. ring. In the 8-bunch injection mode, there are 8 packets of electrons circulating in the ring (and each time producing X-rays in the undulator of the CRISTAL beamline) within 1.18 μs. This results in a time delay between two X-ray pulses of 147.5 ns, while the X-ray pulse duration is 87 ps. The energy of the X-ray beam can be tuned from 5-30 keV, and for our experiment is set to 18.5169 keV. The beamline is equipped with a 6-circle goniometer, on which the sample can be installed with a standard goniometer head (Figure 5.7). The XPAD detector (size

) can be placed with either a vertical or horizontal configuration at distances ranging from 80 mm to 1000 mm and angles θ from 5° to 40° with respect to incident beam in the vertical configuration. On this beamline, the light from the femtosecond laser arrives from the top, since the setup is optimized for studying photoinduced effects on oriented samples in grazing incidence mode, e.g., for investigating photoinduced strain [START_REF] Juvé | Ultrafast light-induced shear strain probed by time-resolved x-ray diffraction: Multiferroic BiFeO 3 as a case study[END_REF]. The laser system is a regenerative sapphire amplifier providing 800 nm pulses with a duration of 25 fs at a repetition rate of 1 kHz. The frequency of laser pulses hitting the sample can then be reduced by an optical chopper at harmonics of this frequency, e.g., in our experiment, it is set to 33 Hz, corresponding to a delay between two laser pulses of 30 ms. The wavelength of the laser pulse can be adjusted to 400 nm, by doubling the frequency of the fundamental (800 nm) by second harmonic generation in a BBO crystal. First preliminary fluence tests allowed us to determine the damage threshold of sodium nitroprusside crystals; clear damage was detected with fluences from 20 mJcm -2 . For the experiment, the laser fluence was chosen accordingly; 12 mJcm -2 pulse -1 (Figure 5.6) was used in our experiment in order to avoid too much damage.

The beamline is equipped with an XPAD3.2 detector (Table 5.1). The fast-electronic shutter of this detector allows for selecting a single X-ray pulse out of the 8-packet bunch arriving every 1.18 μs. Typical data collection consisted of two different positions of the detector at  angles of 37° and 47°, which allows for the detection of a θ angle of [5-55°], when placing it at a distance of 80 mm. The electronics of the detector also allows for setting a defined time delay between the laser pulse and the detection of the X-ray pulse, e.g., one can choose to measure the diffraction due to a single X-ray pulse 150 μs after the laser pulse.

in different orientations with different dimensions. Further, SNP crystallizes in the orthorhombic space group Pnnm, and it is thus not necessary to collect diffraction data over the full sphere, but (in principle) sufficient to collect data over a quarter of the sphere.

Concerning the photoswitching properties, MS2 can be generated at room temperature with short laser pulses, and the lifetime as a function of temperature is perfectly known, which allows for defining the data collection strategy (see below). The only drawback is that the photoconversion rate for SNP is rather low, upon excitation with a single laser pulse only 1-2% population can be expected.

We first choose the temporal characteristics of the data collection strategy, as schematically illustrated in Figure 5.4. In order to minimize laser induced damage of the crystal, we set the laser repetition rate to 33 Hz, corresponding to 30 ms delay between the pulses. Within this time delay, via the gated detector, we can choose the X-ray gate duration and, thus the effective time resolution of the experiment. Selecting one single X-ray pulse of 87 ps, we would thus obtain the maximum achievable time resolution. We can also extend the X-ray detection window by effectively opening the detector for a longer time, e.g., 10 μs, and integrating over the corresponding 80 X-ray pulses. As long as we integrate data below 1 ms, the goniometer rotation (in order to collect properly integrated intensities) will not affect the outcome since it is set to 1°/s. Since we know the lifetime of MS2 in SNP, we can then adjust the temperature so that MS2 has relaxed back to GS, before the next laser pulse hits the sample. As can be seen from Figure 5.5, setting the temperature between 210 and 240 K allows for tuning the lifetime of MS2 from 1 ms down to 50 μs (at room temperature, it is approximately 300 ns). So, if we choose 210 K, the MS2 state will have relaxed to 1/e of its initial population after 1 ms, ensuring a complete relaxation to GS after 100 ms (before the next laser pulse arrives). For this situation, we can collect X-ray data up to 50 μs without significantly affecting the initial population (its reduced to 95% of the initial value). If such a "slow" experiment succeeds, we can stepwise reduce the time window until reaching the single pulse X-ray scheme and the optimal time resolution of 87 ps.

Simulations of photoinduced structural changes in the diffractions data for the preparation of the time-resolved measurement

Structure factors for different metastable state populations can be derived using the structural models of the MS1 and MS2 states obtained from the above-described photo-stationary observations. Technically, one can compute numerous parameters in different scenarios based on the structural parameters derived from these observations, that will then allow us to prepare the time-resolved XRD measurement. In a first step, we can immediately analyze the Bragg reflections that are most affected by the MS2 generation. This method was already used by Casaretto et al. for the in-house millisecond time-resolved XRD experiment [START_REF] Casaretto | In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector[END_REF]. Second, since we aim not only to follow single Bragg reflections to possibly confirm a known structural model, but to determine a full (possibly unknown) structure from a complete data set, we simulate photodifference maps using the experimental constraints of the setup (described above) under different conditions. As we know, photodifference maps are a good tool to identify structural changes upon irradiation without prior knowledge (or bias due to the expected change) of the photoinduced state.

Screening of most sensitive reflections as a function of the metastable state population

As a first possibility, we screen the Bragg reflections of the two data sets collected in GS and GS+MS2 state (see above) for the most significant differences. Based on the structural model determined for MS2, we can then modify the population of MS2, in order to check the minimum necessary accuracy on the measured differences ΔI = Iexc-IGS and statistics ΔI/ IGS that would allow conclusion for one or another model (Figure 5.12).. However, one needs to be aware that measuring only a few Bragg reflections that correspond to model expectations will not be sufficient to unambiguously conclude whether the proposed model is the only possible one. Ideally, in this case, one follows another parameter, e.g., following the relaxation of the Figure 5.18 shows that, again for the lowest populations, the position of MS2 is hard to detect, while for 5 and 10% it becomes rather clearly visible. These photodifference maps are associated to the MS2 state population, which was estimated to be 4.6(2)% during the refining process.

c) Photodifference maps as a function of crystal to detector distance

As the last test, we modified the crystal-to-detector distance, which also resulted in a modification of the reciprocal space coverage. Here, we choose two cases, i.e., crystal to detector distances 50 and 150mm. The results are shown in Figure 5.19. As expected for the smaller distance D50, the reciprocal space coverage is 98% with 3216 independent reflections and therefore resulting a relatively quite significant MS2 population in photodifference maps (Table 5.6). In the case of the largest distance D150 data set, the reciprocal space coverage is only about 50% encountering 1074 independent reflections, which is around 1/3 rd of that obtained from D50 data set. The photodifference maps obtained from this data set are the least significant, even though the signature of the MS2 can be identified. overlap with dead and/or hot pixels. Clearly the data quality obtained from such kind of experiment is lower than a standard in-house diffraction experiment, as can be seen from the rather large Rint value of 16% (see Table 5.7). The main difference between the synchrotron and the in-house time-resolved measurement is the nature of the X-ray beam and the corresponding detection technique. At the synchrotron, the X-ray beam is pulsed and therefore enables measurements by the classical pump-probe technique, i.e., an X-ray pulse arrives on the sample after a well-defined delay after the exciting LASER pulse. Then, over many cycles, sufficient statistics for an as high as a possible number of Bragg reflections is integrated into the detector. In the in-house setup, the X-ray beam is continuous. In this case, it is the detector that defines the time resolution. The RebirX detector is specially conditioned for pump-probe cycling measurements by its particular electronic architecture [START_REF] Allé | Comparison of CCD, CMOS and Hybrid Pixel x-ray detectors: detection principle and data quality[END_REF][START_REF] Casaretto | In-house time-resolved photocrystallography on the millisecond timescale using a gated X-ray hybrid pixel area detector[END_REF]. The temporal characteristics of the time-resolved XRD experiment are then as follows (Figure 5.24). The pump laser, working e.g., at 10 Hz, excites the sample. As Ru(NH3)5(NO)]Cl3•H2O the second step from MS2 to MS1 is extremely efficient due to a very high absorption coefficient of MS2 at the wavelengths used for photoswitching. However, overall the excitation efficiency is well below that of the above-mentioned compounds, with Q values of the order of 400 J/cm 2 , while the photoconversion efficiency remains good with approximately 50% of MS1 generated.

Crystal to detector distance 50 mm Crystal to detector distance 150 mm

On the example of [Ru(NH3)5(NO)]Cl3•H2O it was shown that in steady-state conditions, the pulsed irradiation by ns-laser pulses leads to the same PLI as the previously used continuous wave (CW) irradiation by lasers or LEDs. This was confirmed for both the direct generation of MS1 from GS by blue light as well as for the generation of MS2 out of MS1 by infrared light.

The two-step mechanism for the generation of MS1 from GS via MS2 was also verified by temperature-dependent measurements using single-crystal optical absorption spectroscopy.

One-shot single-pulse experiments showed that the MS2 species involved in the two-step generation of MS1 could only be detected by transient absorption spectroscopy due to the high absorption cross-section of MS2 at the corresponding wavelengths. X-ray photocrystallography as well as infrared spectroscopy after single-pulse irradiation, are not sensitive enough to detect the small amounts of MS2 generated during this process. These single-shot experiments revealed an important criterion for the preparation of time-resolved Xray diffraction experiments: one must carefully weigh the number of photons available in a single pulse with respect to the number of photoswitchable molecules present in the single crystal, and in addition, consider the conversion efficiencies between the different possible PLI states. The choice of crystal with only one possible PLI will lead to better success in such experiments.

In order to confirm these findings, a second compound of the {MNO} 6 family, [Ru(NO)(Py)4F](ClO4)2, was investigated by ns-pulsed irradiation, both under steady-state and in single-shot conditions. Again, the results confirm that MS1 and MS2 are generated in the same way as under CW irradiation so that this seems to be a general property of these compounds. For this case the conversion efficiencies in the two-step process, from GS to MS2 and from MS2 to MS1 are modified, the second step becoming less efficient than for [Ru(NH3)5(NO)]Cl3•H2O, which results in a detectable amount of MS2 in the infrared spectra The concentration of the excited state species grows exponentially:

The whole process of excitation and relaxation can be described by the following equation: