Dr Emmanuel Allard

Dr Jad Rabah

Prof Emanuel Cadot

Dr Clément Falaise

Prof Miomandre Fabien

Dr Julie Pham

Anam Fatima

Hélène Fensterbank

Karen Wright

• Gotard Burdzinski

Gilles Clavier

Michel Sliwa

Thomas Pino

Rachel Méallet-Renault

Karine Steenkeste

Minh-Huong Ha-Thi

Selective population of triplet excited states in heavy-atom-free BODIPY-C 60 based molecular assemblies

Keywords: ultrafast spectroscopy, time-resolved spectroscopy, photoinduced electron transfer, energy transfer, artificial photosynthesis, solar energy conversion, photovoltaics

Understanding the mechanism of the relaxation pathways upon photoexcitation is a decisive step for improving photoactive systems toward applications in solar energy conversion. In this context, our work provides fundamental insights into the complex photophysical processes (inter-or intramolecular electron, energy transfer, etc.) in organic and inorganic photosensitizers coupled with electron acceptors or photocatalytic platforms depending on their potential applications in photovoltaics or artificial photosynthesis. On the one hand, we studied intramolecular photoinduced electron/or energy transfer processes within all organic

[BODIPY]boron-dipyrromethene-[C60] fullerene-based covalently-linked donor-acceptor systems. On the other hand, we investigated excited state dynamics of a new class of noble-metal-free inorganic photosensitizers (i.e., metal halide clusters) This research work would not have been possible without the assistance and guidance of several individuals who contributed and extended their valuable assistance in the preparation and completion of this study in various ways.

Foremost, it is a genuine pleasure to express my deep gratitude to my amiable supervisors for bringing the weight of their knowledge and considerable experience to this project. I am grateful to Dr. Minh Huong Ha-Thi for her patient guidance, encouragement

in combination with different polyoxometalates (POMs) as photocatalytic platforms. Moreover, combining the best of the two, covalently linked organic-inorganic hybrid systems that utilize the electron affinity of the POM for catalytic purposes while using the organic chromophore (i.e., BODIPY) as a photosensitizer was photophysically investigated. Fast and ultrafast time-resolved spectroscopic methods, including femtosecond and nanosecond time-scale transient absorption, and time-resolved emission allowed the monitoring of different photoinduced processes within these complex photoactive systems in solution. The fundamental understanding of charge separation and relaxation dynamics within these donor-acceptor systems are particularly important for developing artificial lightconversion devices.

Title: Spectroscopie résolue en temps sur des systèmes photoactifs : vers la conversion de l'énergie solaire Mots clés: spectroscopie ultrarapide, spectroscopie résolue en temps, transfert d'électron photoinduit, transfert d'énergie, photosynthèse artificielle, conversion de l'énergie solaire Résumé: Comprendre les mécanismes de relaxation suite à la photoexcitation est une étape décisive pour améliorer les systèmes photoactifs impliqués dans la conversion de l'énergie solaire. Dans ce contexte, nos travaux permettent de progresser sur notre compréhension des processus photophysiques complexes (transfert d'électron inter ou intramoléculaire, transfert d'énergie, etc.) dans les photosensibilisateurs organiques et inorganiques couplés à des accepteurs d'électrons ou des plateformes photocatalytiques selon sur leurs applications potentielles dans le photovoltaïque ou la photosynthèse artificielle. D'une part, nous avons étudié les processus de transfert d'électrons/ou d'énergie photoinduits intramoléculaires au sein de systèmes covalents donneurs-accepteurs organiques [BODIPY] bore-dipyrrométhène-[C60] fullerène. D'autre part, nous avons étudié la dynamique de l'état excité d'une nouvelle classe de photosensibilisateurs à base des composés inorganiques sans métaux nobles (agrégats inorganiques d'halogénures métalliques) en combinaison avec différents polyoxométalates (POM) comme plateformes photocatalytiques. Combinant le meilleur des deux, les propriétés photophysiques des assemblages covalents hybrides organiquesinorganiques qui utilisent l'affinité électronique du POM comme catalyseur avec le chromophore organique BODIPY comme photosensibilisateur ont été étudiées. Des méthodes spectroscopiques rapides et ultrarapides, telles que l'absorption transitoire à des échelles femtoseconde et nanoseconde et l'émission résolue dans le temps ont permis de suivre différents processus photoinduits au sein de ces systèmes photoactifs. La compréhension de la dynamique de séparation et de recombinaison des charges au sein de ces systèmes donneur-accepteur est particulièrement importante pour le développement de dispositifs de conversion de l'énergie solaire.

I'm obliged to Dr. Michel Sliwa for hosting me in his Lab (LASIR, Université de Lille), allowing me to perform time-resolved emission experiments, and helping me with the data analysis. I also thank him for being a member of my thesis committee. During our thesis committee meetings, his thoughtful remarks and in-depth conversations greatly helped me advance in my thesis work.

I cannot express enough thanks to Prof. Gotard Burdzinski at Quantum Electric Lab, Adam Mickiewicz University, Poznań, for investing his time and energy in the project and allowing us to perform femtosecond transient absorption spectroscopy in his lab. I greatly benefited from his extensive knowledge of the subject and his incredible style of patiently answering even my most naïve questions. I thank him for being an email away to answer my queries about the femtosecond data analysis and interpretation. This work would not have been completed without his guidance and support. I owe a great deal of gratitude to Dr. Sabina Brazevic, Prof.

Burdzinski's Ph.D. student at the time, for treating me with utmost kindness and generosity during each of my visits to Poznań and for helping me in the lab.

I thank all my friends and colleagues at ISMO, especially Daniel, Ning, Nishith, Ozan, Joffrey, Olivia, Jean Baptiste, Pan, Hai Linh, Arindam, Riccardo, Rosemond, Loise, and anyone else I may have forgotten to mention. Their support and encouragement during my thesis writing made it a little easier. The fun moments that I shared with Daniel will be treasured for a lifetime.

I am thankful to my friends here and in India for being a constant source of support and love.

A special thanks to my wonderful friends Benazir Khurshid and Alaul Asif for always being by my side and for being a family away from home.

And last, but by no means least, a big shout-out to my adoring parents and my sisters, Tabish appiya and Mehvish, for their unconditional love and unwavering encouragement. I owe it all to you. Many thanks!

Table of Contents

Résumé

Compte tenu de deux des problèmes les plus urgents auxquels l'humanité est confrontée au XXIe siècle, à savoir la pollution de l'environnement et la crise énergétique, il est essentiel de rechercher des sources d'énergie propres et renouvelables pour un développement sociétal durable. Dans ce contexte, l'énergie solaire reste la plus prometteuse des différentes sources d'énergie renouvelables et son utilisation pour la production directe de carburants a fait l'objet de nombreuses recherches. Des efforts considérables ont été déployés pour créer des systèmes photosynthétiques artificiels (PA) efficaces inspirés du remarquable photosystème naturel. La photosynthèse artificielle suit le cours de la photosynthèse naturelle, c'est-à-dire l'absorption des photons, la séparation des charges et la production de carburant.

Un système PA idéal devrait avoir une séparation de charge rapide et efficace pour minimiser la perte de l'énergie des photons absorbés et une recombinaison lente pour assurer le transport efficace des charges. Ainsi, la compréhension des processus fondamentaux (absorption de photons, transfert d'énergie, transfert d'électron, etc.) au sein des systèmes photoactifs est une étape décisive vers le développement de systèmes PA performants. Cette thèse vise à étudier les processus photoinduits dans différents systèmes photoactifs (assemblages donneurs-accepteurs liés chimiquement et systèmes multicomposants).

Le premier thème de la thèse porte sur l'étude des processus photoinduits intramoléculaires dans des assemblages donneur-accepteur (D-A) organiques à base de BODIPY-C60. Une nouvelle triade BODIPY-C60-distyrylBODIPY et les deux dyades de référence (BODIPY-C60 et distyrylBODIPY-C60) ont été étudiées en utilisant la spectroscopie résolue en temps de l'échelle de temps femtoseconde à l'échelle de temps nanoseconde. La photophysique de la triade est particulièrement intéressante car elle absorbe dans tout le domaine visible, forme un état excité triplet de longue durée de vie sur le distyrylBODIPY (280 µs), et subit un transfert d'électrons photoinduit (PET) quelles que soient les longueurs d'onde d'excitation (excitation de BDP ou DSBDP). Néanmoins, des états de séparation de charges de courte durée de vie (quelques dizaines de ps) limitent l'utilisation de ces systèmes dans les applications photovoltaïques. Par conséquent, une nouvelle génération d'assemblages D-A, qui incorpore un donneur supplémentaire, le ferrocène (Fc), a été conçue pour former des systèmes donneurphotosensibilisateur-accepteur (D-P-A) : ceci permet un transfert séquentiel d'électrons et un plus grand éloignement des charges opposées conduisant ainsi à la stabilisation la paire de charges séparées (400 ps). Cette approche rend les assemblages D-P-A plus appropriés pour leur utilisation dans le développement de systèmes photovoltaïques.

La deuxième partie de la thèse explore les processus photoinduits intermoléculaires dans les clusters inorganiques d'halogénures métalliques utilisés comme photosensibilisateurs et les polyoxométalates (POM) jouant le rôle du catalyseur. Des études photophysiques (spectroscopies stationnaires et résolues en temps à l'échelle nanoseconde) ont été menées sur le cluster [Mo6I8Cl6] 2-en présence de différents

Preface

With an ever-increasing global population and the indiscriminate use of fossil fuel reserves to power our lives, the Earth is heating up significantly, a phenomenon known as global warming.

Consequently, we have already started feeling the effects of forest fires, shrinking water table, and extreme weather conditions. Every summer is hotter than the last 1 . Now that we have caused the problem, what can be the solution? The future of planet Earth will rely on renewable energy and therefore, renewable energy resources, particularly solar energy, are being sought more than ever before. Hand in hand with this, technologies to convert CO2 to green fuel must be developed. Naturally, CO2 (the major contributing greenhouse gas) is flushed from the atmosphere through photosynthesis 2 . Plants, microorganisms, and bacteria absorb sunlight and split water molecules into hydrogen and oxygen 3 . While the oxygen is released into the atmosphere for us to breathe, hydrogen is used to reduce the absorbed CO2 into the biomass, in the form of organic molecules 2,4 . Despite the high efficiencies of the early events of photosynthesis (light capture and electron transfer), on a year average only 0.1 percent of the total sunlight (100 000 TW) arriving on our planet is converted into energy by natural photosynthesis 4 .

Therefore, a complementary approach of utilizing solar output guided by natural photosynthesis is topical among the scientific community as a long-term solution, called artificial photosynthesis 5 . The concept of artificial photosynthesis has been around for more than a century. The prediction of exploiting "the best-kept secret of plants" to capture solar energy (using photochemical devices) was made by Italian chemist Giacomo Ciamician in his visionary article published in Science magazine in 1912 6 . In his words: ''On the arid lands there will spring up industrial colonies without smoke and without smokestacks; forests of glass tubes will extend over the plains and glass buildings will rise everywhere; inside of these will take place the photochemical processes that hitherto have been the guarded secret of the plants, but that will have been mastered by human industry which will know how to make them bear even more abundant fruit than nature, for nature is not in a hurry and mankind is''.

Ciamician's concept of producing "solar fuel" from renewable and abundant materials such as water and sunlight (though not all the components used in artificial photosynthesis are green/renewable), is gaining traction. While his dream is still a long way from being realized, given the advances in the approach and its enormous potential, artificial photosynthesis is likely to address our global clean energy dilemma in the near future.

CHAPTER 1. Introduction 1.1 Natural and Artificial photosynthesis

The conversion of light energy by plants into chemically usable forms is a fascinating scientific process that is also of paramount significance to humanity. Our oxygen-rich atmosphere is created through photosynthesis, which also provides most of our food, energy, and raw material needs. Photosynthesis is a biochemical process by which plants, microorganisms, and bacteria store solar energy 7 . Naturally, plants use CO2 and H2O as raw materials to create a usable form of carbon (sugar) and oxygen. This natural process of converting sunlight to energy rich products is divided into four steps as mentioned below 5 , and schematically pesented in Fig. 1.1:

Light harvesting: It is simply the absorption and concentration of sunlight. To initiate photosynthetic reactions, photosynthetic antenna proteins (also known as light harvesting pigment-protein complexes) absorb light energy from the sun. The light energy that has been collected is then directed to the reaction center (the place where charge separation takes place).

Light capture, excitation energy migration, regulation, and photoprotection are all the functions of antennas.

Charge separation: Excitation energy from the captured solar energy is transferred to the photosystem (PS) reaction centers (PSI and PSII) which consists of donor and acceptor units.

In the reaction center, the chlorophyll a in PSII (680 nm) absorbs photon energy which excites electrons, that are transferred to the acceptor unit (Plastoquinon; Pq) thus creating positive and negative charges. Meanwhile, the PSI (700 nm) generates a driving force for extracting electrons through electron transport chain (carries electrons from PSII to PSI).

Water splitting: The charge separation in PSII (680 nm) triggers the splitting of water (due to the positive charge on chlorophyll a) into hydrogen ions and oxygen, two highly energetic electrons thanks to the photon's energy.

Fuel production: The water splitting reaction creates a charge gradient and hence electron and protons generated by PSII pass through the PSI (700 nm) to create another gradient and produce NADPH. Then, in a dark process called Calvin cycle, the products of the light reaction (i.e. NADPH) and CO2 from atmosphere produce carbohydrate-a stable and storable form of energy. Natural photosynthesis has inspired many researchers to devise an artificial photosynthetic process that can convert solar energy to fuels and valuable chemicals through the assemblies of organic and inorganic nanomaterials 8,9 . The conversion of solar energy directly to electrical energy is termed photovoltaics whilst the conversion of solar energy to chemical energy is called photosynthesis; both research fields are vital in the search for solar-based energy production. Basically, artificial photosynthesis follows the same key steps as that of natural photosynthesis, that is, light capture followed by charge separation and finally fuel production through water splitting 5 .

Understanding how artificial photosynthesis works at the molecular level is vital in order to achieve potential efficiency. To reach this aim, typical artificial photosynthetic systems are developed. They are composed of an array of light-harvesting moieties, known as antennae or photosensitizers/chromophores, that ensure efficient collection of light in the entire spectral range useful for fuel production (400 nm to beyond 1000 nm) and convert solar-energy to electrochemical energy/photocurrent 10 . Next, the high electromotive force generated by the photoexcitation of the antenna leads to a charge-separated state at the reaction center. The radical cations/holes can then be introduced to the water oxidation catalyst (WOC), to oxidize water into oxygen. Further, radical anions/electrons of the charge-separated state can be used by reduction catalyst (RC) to reduce protons (H + ) or CO2 for fuel generation (Fig. 1.2).

Therefore, an efficient artificial photosynthetic system requires an antenna/photosensitizer, a reaction center, and oxidation and/or reduction catalysts. Electron donors frequently serve as antennae or photosensitizers in simplified molecular light-conversion devices, whereas acceptors can be used directly as RCs. It is to be noted that in artificial systems, achieving the charge-separated (CS) state, which has a sufficient lifetime for successive charge-separation stages in competition with the charge-recombination (CR) process to the ground state, is crucial for efficient energy conversion. A short lifetime of the charge-separated state would lead to their fast recombination, which is deleterious for charge transport.

Since the entire reaction is very complicated and only a handful of artificial systems are known to produce O2 and H2 simultaneously, a useful strategy is to divide the overall process into its two half reactions, the oxidative O2 evolving reaction and the reductive H2 evolving (or CO2 reducing) reaction. Before delving further into the specifics and design rationale of efficient artificial photosynthetic systems, it is crucial to comprehend the basic photophysical concepts involved, namely light absorption, emission, photoinduced electron and energy transfer mechanisms. The introduction to these photophysical processes therefore is the focus of the sections that follow.

Photophysical Concepts in Artificial Photosynthesis

Light-absorption and Excited-state deactivation

The process by which light is absorbed by matter and converted into energy is known as light absorption. One molecule absorbs one photon in the simplest and most common interaction between molecules and light through electric dipole interaction 11,12 . A molecule can thus be promoted from the electronic ground state to an electronically excited state by the absorption of a photon. This phenomenon is due to the excitation of an electron from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO).

Experimentally, the efficiency of light absorption at a wavelength λ by an absorbing medium is characterized by the absorbance A(λ) or transmittance T(λ), given by the Beer-Lambert's Law where 𝐼 𝜆 [START_REF]Current Potential (V) vs. Fc 0/+ [PW 11 VO 40 ] 4[END_REF] and 𝐼 𝜆 are the respective incident and transmitted intensities of light, 𝜀(𝜆) is the molar absorption coefficient (commonly expressed in L mol -1 cm -1 ), c is the concentration of absorbing species (in mol L -1 ) and l is the absorption path length (thickness of the absorbing medium, in cm).

When a photon is absorbed, the molecule usually gains some vibrational energy in addition to being transferred into an excited electronic state. According to the Franck-Condon principle, photon absorption is nearly an instantaneous process. However, since nuclei are enormously heavy in comparison to electrons, thus, during light absorption, which occurs in femtoseconds, the internuclear distance in the excited electronic state remains the same as it was in the initial ground state before the time of electronic transition. Therefore, the most probable transitions occur "vertically" between the states keeping the same nuclear coordinates. Potential energy diagrams, such as the one shown in Fig. 1.3, are the best way to illustrate this 13 . On this diagram of energy (ordinate) versus distance between the nucleii of a diatomic molecule (abscissa), the transition is vertical, promoting the electron from the lowest vibrational state (ν=0) of a molecule in the ground state to the zero or higher vibrational states (ν= 0, 1, 2, 3, ..n) of the excited state (e.g. S1 or S2) of the molecule. In polyatomic molecules, potential energy curve is rather transformed to potential energy surface (PES), that associates energy with each geometry of the molecule, meaning that each geometry is associated it a unique energy. PES shows that each electronic state of a molecule has a separate potential energy surface and the separation between these surfaces yields the electronic spectrum. Because electronically excited states contain a high amount of energy, they must be deactivated quickly (shown by downward arrows in Fig. 1. 3). Deactivation can occur via states of the original molecule that leads back to the ground state (photophysical processes) or through the formation of other species (photochemical processes).

Photophysical processes including light absorption and non-radiative relaxations, can be described using a Perrin-Jablonski diagram (Fig. 1.4) 14 . The singlet electronic states are denoted by S0 (ground state) S1, S2, S3… (excited states) and triplet states are denoted by T1,T2,T3...

Vibrational levels, associated with each electronic state, are denoted by thin lines with lowest one indicated by ν = 0. Light absorption usually excites the molecule in a high vibrational level, which is regarded as 'hot' species. These vibrational hot states then dissipate their excess vibrational energy by interaction with surrounding molecules via a non-radiative process called vibrational relaxation (VR). However, if the vibrational energy levels of one electronic state strongly overlap with the vibrational states of another electronic state of the same multiplicity, there is a possibility that the excited molecule can transit from a vibration level in one electronic state to another vibration level in a lower electronic state. This isoenergetic process is called internal conversion (IC). It is also indicated as a purple wavy horizontal arrow on a Jablonski diagram (Fig. 1.4), between two vibrational levels in different electronic states with the same multiplicity.

Another pathway for molecules to deal with energy received from photons is to emit a photon. The molecule in the excited state can relax back to the ground state (S1→S0) by giving off some light, termed as fluorescence (a radiative process, indicated on a Jablonski diagram as a straight green line), which occurs at a longer wavelength than that of absorption due to vibrational relaxation in the excited state. Therefore, compared to absorption bands, fluorescence bands are red shifted (Stokes' shift), i.e., emission bands are lower in energy as compared to the absorption bands. Apart from a few exceptions, fluorescence emission occurs from S1 state (Kasha's rule) 15,16,17 and therefore its characteristics do not depend on the excitation wavelength. Another pathway of excited state deactivation from S1 is intersystem crossing (ISC) to the T1 triplet state represented by a blue wavy horizontal arrow on the Jablonski diagram. ISC is yet another non-radiative transition process between two iso-energetic electronic states having different miltiplicities. For instance, a molecule in the 0th vibrational level of the excited state transits to the isoenergetic vibrational level of the triplet state (T1), which by vibrational relaxation comes to the lowest vibrational level of the T1 state. ISC is a forbidden process (crossing between electronic states of different multiplicities), but the large enough coupling between the orbital magnetic moment and the spin magnetic moment makes it possible. The T1 state relaxes to the ground state by emitting a photon, but because that would involve a change in the spin state, it is not allowed and thus takes longer than an allowed transition (from S1 to ground state). This is termed phosphorescence, denoted by a red vertical arrow in the Jablonski diagram.

Furthermore, when the energy gap between S1 and T1 is minimal and T1 lifetime is long, reverse intersystem crossing T1→S1 can take place. Because the final emission is from the S1 state, the spectral distribution is same as the normal fluorescence but with a much longer decay time constant (molecules remain in the triplet state before emitting from S1). This reverse intersystem crossing is called delayed fluorescence.

Excited state reaction mechanisms

A molecule in the excited state D* can interact with another molecule A (quencher), in competition with the above-mentioned intrinsic deactivation processes. It is to be noted that the interaction can be inter or intra molecular, depending on whether the molecules are covalently bound or free in solution, affecting the emission features of the excited state. This phenomenon, called quenching, mostly involves fast energy or electron transfer processes (fast enough to compete other radiative and non-radiative processes) from donor to acceptor unit.

a. Photoinduced Energy Transfer (EnT)

Photoinduced energy transfer, also known as excitation energy transfer, is an important photophysical phenomenon whereby the excitation energy of the donor molecule is transferred to the acceptor. Notably, photoexcitation energy transfer is a pivotal step in photosynthetic systems. This phenomenon can be primarily classified into (a) Förster resonance energy transfer, FRET 11,18,19 and (b) Dexter energy transfer mechanism 11,13 . Both of these are nonradiative photoinduced processes, which involve some coulombic interactions and/or intermolecular orbital overlap between the donor and acceptor moiety.

The coulombic interaction mainly consists of long-range 'through-space' dipole-dipole coupling, leading to FRET. FRET can be predicted when the emission spectrum of the donor at least partially overlaps with the absorption spectrum of the acceptor unit. This implies that several vibronic transitions in the donor resonate with that of the acceptor (Fig. 1.5a). The efficiency of FRET is a function of several physical parameters: the distance between donor and acceptor, the overlap integral between the donor emission spectrum and the acceptor absorption spectrum, and their mutual molecular orientation, and hence dipole orientation.

On the other hand, interactions involving intermolecular orbital wave function overlap of the donor and acceptor molecules, leading to the electron exchange, are termed Dexter energy transfer or electron exchange mechanisms. In this case of energy transfer, an electron from D* jumps to the LUMO of A with simultaneous exchange of an electron from A to D (Fig. 1.5b).

This energy transfer mechanism requires the two molecules to be in proximity with each other, operating within 2 nm. It should be noted that even at short distances, coulombic interaction is dominant in the case of allowed transitions on D and A. Exceptionally, for forbidden transitions on D and A between triplet and singlet states (e.g., 3 D * + 1 A → 1 D + 3 A*), the coulombic interaction is negligible, and the electron exchange mechanism comes into play. But it is only operative at short distances because it requires molecular orbital overlap.

b. Photoinduced Electron Transfer (PET)

Light driven inter and intra-molecular electron transfer are the most fundamental processes in nature which find potentially useful applications including charge separation processes in solar cells, optical devices and chemical and biological sensors. PET is of course in competition with other radiative and non-radiative deactivation processes in the excited state of the molecular system and therefore, it leads to the decrease of both excited state lifetimes and emission quantum yields of the chromophore. Photoinduced electron transfer can be divided broadly into two categories 11,20 : (i) intermolecular electron transfer, when the donor and acceptor units are separate entities, and (ii) intramolecular electron transfer, in which the donor and acceptor units belong to the same molecule. While intermolecular PET always occurs via through-space mechanism, intramolecular electron transfer generally occurs via through-bond mechanism in molecules. In rare case scenarios, intramolecular PET can also occur via through-space mechanism when the donor and the acceptor are in favorable proximity. Further, PET can occur via two main pathways, according to the following reactions: Nevertheless, the comparable levels of HOMO and LUMO of the donor and acceptor molecules respectively, are important to control the photoinduced electron transfer process. The aspect of charge separation (CS) was focused in the 1960s by Weller and co-workers who put forward that the driving force of the charge separation (ΔGCS) can be estimated using redox potentials of donor and acceptor molecules 21 , according to the following Rehm-Weller's equation (eq.2) :

ΔGCS = E 0 (D/D •+ ) -E 0 (A/A •-) -ΔE0,0 +wel (eq.2)
where E 0 (D •+ /D) and E 0 (A/A •-) are the typical oxidation and reduction potentials of the donor and acceptor, respectively, ΔE0,0 is the excitation energy associated with the 0,0 optical transition that correspond to the mid-point of the absorption and emission, and wel is the electrostatic work term. If the free energy change is negative, the process of electron transfer between the redox counterparts is thermodynamically feasible. In other words, the stronger the driving force higher will be the electron transfer rate.

In contrast, Rudolph A. Marcus proposed a theory in 1956 that demonstrates that even a strong driving force can lead to a slow charge separation rate. The theory puts forward a method for calculating rates of electron transfer in a solution where an electron can move or jump from one chemical species (D) to another chemical species (A). It treats the dependence of the free energy of activation (ΔG ⧧ ) on standard free energy difference between the reactants and the products (-ΔG 0 ) through a parabola which can be mathematically expressed as the equation below 22 (eq.3):

𝛥𝐺 ⧧ = (𝜆+𝛥𝐺 0 ) 2 4𝜆 ≈ (𝜆-𝑛𝐹𝛥𝐸 0 ) 2 4𝜆 (eq.3)
where ΔG 0 can be related to the redox potentials and replaced with the term -nFΔE 0 , where n is number of electron tranferred in a redox reaction, F is Faraday's constant (C/mol), λ is reorganization energy. The parameters ΔG°, ΔG ⧧ are all energy terms with units of kJ/mol (or kcal/mol). ΔE 0 is the cell potential. λ is in eV and ΔE° is in V; multiplying by nF converts to kJ/mol.

The equation introduces an important new term, the "reorganization energy", λ. Marcus defines it as the energy input required to change the structure of solvated reactants to match that of solvated products prior to electron transfer. It can be divided into an outer shell component (λout) that involves shifs of the orientation of the surrounding solvent shell and an inner shell component (λin) that involves alterations of bond lengths and angles. The two elements are complementary and always positive: λ = λin + λout. Since Equation 3 is a second order polynomial in ΔE 0 , the ΔG ⧧ curve is a parabola. It should be noted that the Marcus theory can also be depicted with lnkET (rate of electron transfer) vs ΔE 0 curves, the red dashed curve in Fig. 1.7. Three inferences can be drawn from the curves in the Fig. 1.7: ii)

The minimum of the curve (ΔG 0 = -λ): When the free energy of the spontaneous reaction is equal and opposite to the reorganisation energy, the reaction reaches to its minimum ΔG ⧧ and thus highest kET, and, iii) Right side of the curve (ΔG 0 < λ): When the redox reaction gets more spontaneous, the ΔG ⧧ becomes higher, implying the slower rate of redox reaction. This right part of the parabola is referred to as the 'inverted region'.

The physical significance of the Marcus theory can be understood in the terms of the charge separation and recombination rates. In order to achieve efficient or fast forward electron transfer with relatively slower recombination, the charge separation process should be in the top region, while the charge recombination process should lie in the inverted region. Taking both into account, it can be claimed that reorganization energy at least equal to the free energy of the reaction would make the forward electron transfer process faster and a small reorganization energy compared to the free energy would entail a slower backward electron transfer process.

The aforementioned master theories have been applied to study and design a large number of donor-acceptor systems. Both bimolecular systems (leading to intermolecular PET) and covalently linked assemblies (leading to intramolecular PET) have been studied over the years.

The following sections cover the available knowledge about inter-and intramolecular PET.

Focus on photoinduced inter and intramolecular electron transfer

Photoinduced intermolecular electron transfer in D-A systems

Intermolecular charge transfer occurs when the donor and acceptor are well separated from each other in the ground state, and depending upon the relative concentrations of D and A, the following two common photoinduced quenching processes can be considered 9,20 : static and dynamic quenchings which are detailed below.

a. Static Quenching

Static quenching can occur when quencher molecule is in large excess such that prior to excitation, the D and A are close enough to have significant interaction. In this case, no mutual interaction during the excited state lifetime is required and therefore, it is referred to as static quenching. Here, no apparent change in the lifetime of the excited state is observed, however the intensity of the emission decreases. There are two possible ways in which a static quenching can occur: a) the existence of a sphere-of-effective quenching or b) the formation of a ground state non-fluorescent complex. Briefly, sphere-of-action quenching, also called Perrin's quenching, requires the quencher to be located inside the sphere surrounding the chromophore with no actual physical association (Fig. 1.8a). Therefore, a portion of the quenchers located inside the sphere will decrease the emission intensity while those remaining outside will have no effect on the chromophore. As a result, the population of the excited state available for fluorescence (emission intensity) will decrease but the fluorescence decay after photoexcitation will remain unchanged. In the possibilty of ground-state complex formation, the chromophore forms a ground state non-fluorescent complex, which obviously decreases the emission intensity, however again the population of the uncomplexed excited chromophore results in unaffected fluorescent lifetime (Fig. 1.8b).

b. Dynamic quenching

Dynamic quenching occurs when quencher is not in large excess and when the diffusive encounters of D* and A is possible during the excited state lifetime, thus decreasing the lifetime of the excited state. This kind of intermolecular interaction is termed dynamic quenching or collisional quenching. Since here the quenching occurs only after excitation, the benchmark of a dynamic quenching process is the change in the excited state lifetime of the chromophore.

Stern-Volmer analysis to characterize the quenching:

The Stern-Volmer approach is used to obtain quantitative information about fluorescence quenching by plotting the I0/I versus [Q], where the terms I0 and I refer to the measured fluorescence intensity for a fixed amount of fluorophore in the absence and in the presence of quencher [Q], respectively. A linear trend is commonly expected. Stern-Volmer plots yield an intercept of 1 on the y-axis and a slope which gives the Stern-Volmer constant (ksv). If the excited -state lifetime in the absence of a quencher is known, the quenching constant, kq can be calculated, according to the following equation 4:

𝐼 0 𝐼 = 1 + 𝑘 𝑞 𝜏 0 [𝑄] = 1 + 𝑘 𝑠𝑣 [𝑄] (eq.4)
In the case of purely dynamic quenching:

An equivalent decrease in the intensity and the lifetime is observed and it follows the classical stern volmer model (i.e., I0/I = τ0/τ =1+ksv[Q])

In the case of purely static quenching:

Stern-Volmer plot yields a linear variation in terms of emission intensities (i.e., I0/I = 1+ksv[Q]), however, as already explained above there will be no change in excited state emission lifetimes (i.e., τ0/τ = 1). Static and dynamic quenching can be distinguished by their different temperature and viscosity dependencies, or, more preferably, by lifetime measurements. For example, higher temperatures cause faster diffusion and thus more collisional quenching, but also typically the dissociation of weakly bound complexes, resulting in less static quenching.

In the case of ''combined static and dynamic'' quenching:

Stern-Volmer plot gives a positive deviation (upward curvature) from the classical linear behaviour and therefore, a modified form of Stern-Volmer equation is used, which is given by the product of the static and dynamic quenching terms, as given below (eq. 5):

𝐼 0 𝐼 = (1 + 𝑘 𝑠 [𝑄])(1 + 𝑘 𝐷 [𝑄]) (eq.5)
This modified Stern-Volmer equation is of second order in [Q], accounting for the upward curvature. The value of the slope (ks. kD) provides apparent quenching rate constant, knowing the dynamic quenching constant (kD), the value of static quenching constant ks can be deduced.

Photoinduced intramolecular electron transfer in D-A systems

The need for high concentration of molecules to induce an intermolecular elctron transfer can be eliminated by covalently linking the donor and acceptor molecules. The importance of photoinduced intramolecular electron transfer processes in the development of light harvesting devices has significantly inspired the design and study of chromophore-acceptor and donorchromophore-acceptor model systems 9,20 . In principle, donor and acceptor molecules can be attached to each other directly by carbon-carbon bonds or by means of a linker bridge (covalent or non-covalent), in a flexible or a rigid manner. In fact, the nature of the linker also plays an important role in the charge transfer and recombination rates 23,24,25 .

A rigid spacer or linking group between the donor and acceptor keeps the distance and orientation constant. In contrast, a donor-acceptor pair connected by a flexible linker can interact at various distances and orientations, resulting in changes in the separation distance between the donor and the acceptor 25,26 . Because the motions of flexible chains are influenced by the nature and length of the chain, as well as solvent viscosity and temperature, interpretations of the experimental results are frequently complicated. In solution, electron transfer can occur at maximum distances when the chain is fully extended or at closer distances when a donor and acceptor adopt a compact geometry. There may also be several intermediate configurations between the extended and folded geometries 26,27 .

Within D-A systems, the step of electron transfer from the photoexcited donor to the acceptor forming the ion pair is termed as charge separation or forward electron transfer (cs), while the step of relaxation of the ion-pair is termed as charge recombination or back electron transfer (cr). The quantum efficiency of electron transfer can be estimated from the equation below (eq.6):

𝜙 𝑐𝑠 = 𝑘 𝑐𝑠 /(𝑘 𝑐𝑠 + 𝑘 𝑑 ) (eq.6)
where kcs is the rate of forward electron transfer and kd is the rate constant of radiative and nonradiative competitive relaxation processes. In order to maximize the efficiency of electron transfer, the rate constants of other competitive relaxation processes must be diminished. Also, in deciding the efficiency of electron transfer, the redox potentials of the donor and the acceptor entities plays a key role. Higher the difference between the oxidation and reduction potentials of the donor and acceptor units respectively, higher will be the driving force for the charge separation 28,29 .Moreover, the rate of electron transfer decreases with increasing the separation distance between donor and acceptor 30 .

Light harvesting D-A systems: a puzzle of compounds

Photoinduced electron transfer (PET) is one of the most investigated reactions from both fundamental frame of reference as well as for practical applications. In this regard, several types of donor-acceptor systems, selectively incorporating chromophores and redox active units are of major importance in the development of light-harvesting devices. Despite tremendous progress in the development of key components for an artificial photosynthetic system, the production of low-cost and efficient photosensitizers continues to be a significant bottleneck.

Therefore, the following sections introduces the pre-requisites of a quintessential photosensitizer and further a broad classification into organic and inorganic PSs is presented.

Photosensitizers

Photoinduced electron transfer employ photosensitizers that absorb photons and undergoes an electronic transition from ground (HOMO, valence band) state to excited state (LUMO, conduction band) 31 . Photosensitizers have been widely used for various photochemical reactions such as photodynamic therapy, dye sensitized solar cells (DSSCs) and solar fuel production 32,33,34,35,36 . Since, the performance of a photosynthetic reaction is primarily dependent on the light-conversion efficiency (e.g., turnover frequency and quantum yield) of a photosensitizer, the design rationale and engineering of photosensitizing material is critical for the development of an artificial photosynthetic process 31,36 . While designing an efficient photosensitizer, the following pre-requisites should be considered 5 :

a.) High photochemical stability b.) High absorption coefficient in broad spectral range c.) Favorable oxidation and reduction potentials d.) Long excited state lifetime e.) Synthetic tractability ease or facile synthesis

The following sections provide an overview of the various types of photosensitizers that have been researched so far for creating light harvesting devices, broadly categorized into organic or inorganic classes.

a. Organic Photosensitizers

Classical organic photosensitizers in donor-acceptor systems are usually based on xanthenes 37 , porphyrins or other cyclic tetrapyrroles 38,39,40 and phthalocyanines 41,42 .

Xanthene is a hydrosoluble fluorescent dye that absorbs in the range of 500-600 nm. Xanthene dyes (Fig. 1.9) for instance fluorescein and its halogenated derivatives (eosin Y, erythrosine B, rose bengal and phloxine B) have been applied for photochemical hydrogen production 43 , DSSCs 44 and also for biocatalyzed artificial photosynthesis as a molecular photoelectrode 45 .

Xanthenes show high quantum yields and high molar absorption coefficients (100 000 M -1 .cm - 1 ), however, tuning their photophysical properties is quite challenging 46 .

Eosine Y Erythrosine B Rose bengal

Fig. 1.9 Chemical structures of frequently used Xanthene dyes.

Among the chromophores that have been used as molecular components in artificial photosynthetic systems, porphyrinoids, the ubiquitous molecular building blocks employed by nature in natural photosynthesis, have been the preferred and obvious choice, due to their intense optical absorption and rich redox chemistry. Porphyrins represent a class of molecules with four pyrrole rings linked by methine carbon bridges 47 (Fig.1.10). Among various organic photosensitizers, porphyrins have been most extensively studied as light collecting and electron transfer units due to their resemblance with chlorophyll natural pigments 48,49 . Porphyrins absorb on a large visible area : they show intense Soret band at 400-450 nm with absorption coefficient in the order of 10 5 M -1 cm -1 at 400 nm and less intense Q bands at 500-650 nm 50 . In order to increase the absorption cross section of these molecules, several modification approaches have been reported. One of the methods involves the structural modification of the porphyrin backbone in order to decrease the energy gap between the HOMO and LUMO, thus broadening the absorption and increasing the intensity of the Q band 51,52 . This can be done either by adding pi-extended chromophores to the porphyrin ring or by making fused porphyrins. Examples of pi-extended chromophores have been reported by Diau et al. who synthesized a family of acene derivatives ranging from benzene to pentacene linked to the porphyrin via an acetylene group 53 .

Another approach is to synthesize arrays of multiple chromophores that results in complexes with large extinction coefficient as demonstrated by Davila et al. 54 Within the large family of porphyrinoid systems, phthalocyanines (Pcs) enjoy a privileged position as they possess in fact unique physicochemical properties which render these macrocycles valuable building blocks in materials science 55 . Phthalocyanines are analogues of porphyrins which consists of four pyrrole rings linked by nitrogen atoms and conjugated with benzene rings 56 (Fig. 9). Phthalocyanines exhibit architectural flexibility and have been studied

for their physical, chemical, and spectroscopic properties. The lowest energy absorption bands, the Q bands, originate from π-π* transitions that show an intense absorption at the red end of the visible spectrum, from 620 to 720 nm with molar extinction coefficient in the order of 10 M -1 cm -157 , 58 . A second less intense band referred to as B-band appears as a broad band between 300 and 400 nm. Due to their high molar extinction coefficient in the near infrared region and its promising photophysical and chemical properties, Pc have attracted considerable attention for their application in the field of photoelectronic devices and photocatalytic water splitting 59 .

b. Inorganic Photosensitizers

Inorganic photosensitizers, especially transition metal complexes (TMCs), possess intricate and rich photophysical properties that make them attractive options as photosensitizers for light harvesting devices 63 . More importantly, their intrinsic property to populate triplet excited state due to the heavy atom effect, observed to be longer lived than singlet state that allows efficient electron transfer quenching to occur 64 . In view of this, there have been numerous reports on TMCs as inorganic photosensitizers typically based on Ru(II), Pt(II) and Ir(III) complexes for many different applications such as photovoltaics and photoinduced charge separation 65,66 , luminescent molecular probes 67 as well as triplet-triplet annihilation upconversion (TTA-UC) 68,69 . However, transition metal complexes, for example, [Ru(bpy)3] 2+ bpy stands for 2,2'bipyridine) and [Ir(ppy)3] (ppy stands for 2-phenylpyridine) complexes (Fig. 1.12) show weak absorption in the visible range. This weak absorption is the result of weakly allowed S0→ 1 [MLCT] (MLCT=metal-to-ligand charge transfer) resulting in the singlet excited state 70 .

Further, the triplet excited state is not long lived in these complexes (<10 µs) since the 3

[MLCT]

→ S0 intersystem crossing is accelerated due to the heavy atom effect. This pose a significant drawback where a long lived triplet excited state is desired for their applications, TTA-UC and luminescent oxygen sensing 71 for example. On account of these aspects, TMCs complexes with relatively long-lived triplet state are more thoroughly studied and an interesting design strategy has been proposed to lengthen the lifetime of the triplet state. These include the introduction of the organic chromophore into the complex, that enhances the molar absorption coefficient of the compound and establish an equillibrium between the intrinsic 3 [MLCT] and the intra ligand 3 [IL] excited states of the attached chromophore, thus elongating the triplet lifetime at the same time 72,73,74 . [Ru(pyrenyl-phen)3][PF6]2 is a representative example obtained by this approach, wherein the pyrene-localized 3 [IL] energy level is comparable with 3 [MLCT] level, thus upon photoexcitation, an equilibrium between the two states enhanced the lifetime of the triplet state (148±8 µs) 75 . Another example to enhance the lifetime of the triplet state is by direct metalation of light harvesting ligand. For instance, cyclometalated iridium and platinum complexes that showed long-lived triplet state coupled with intense absorption of visible light were developed for applications in luminescent oxygen sensing and TTA-UC 76,77 . Despite these improvements in the photophysical properties of classical inorganic photosensitizers, it is indisputable that these complexes comprise of noble metal centers, and therefore, the limited supply of nobel metals would not sustain a low-cost and widescale production of artificial photosynthetic systems. Nevertheless, the quest for photochemical energy storage has enabled an appreciable interest in the electron transfer study of transition metal complexes incorporating non-noble metals. The long-lived triplet lifetime of coordination complexes allows efficient electron transfer making them promising candidates for solar energy conversion 78,79 . In this regard, the development of non-noble transition metal based clusters is of growing interest, and have been reported to exhibit exceptional optical and electronic properties like strong photoluminescence, large

Stokes shifts and long emission lifetimes 80,81 . Therefore, it is interesting to explore the photosensitizing ability and electron donating properties of this new class of inorganic complexes for potential applications in energy and photocatalysis.

Model D-A systems based on the previous photosensitizers

To build the artificial photosynthetic reaction center state, the choice of the donor-acceptor pair is crucial. CS lifetimes can be finely controlled by the λ value (see section 1.2.2) which is determined by the choice of the D and A pair, the type of linkage between the donor and acceptor molecules, and also the solvent. Extensive efforts have so far been made to achieve a long-lived CS state by tuning these factors.

In this aspect, intramolecular photoinduced electron transfer has been widely studied with derivatives of porphyrins and phthalocyanines linked to acceptor moities, particularly quinone and fullerene derivatives 26,82,83,84,85,86,87 (Fig. 1.13). Especially, fullerenes have been considered

as excellent electron acceptors with low electron transfer reorganization energies because of their massive pi-electron systems and rigid, restricted structure made up exclusively of carbon aromatic rings 88 . The chlorophyll-like macrocyclic photosensitizers, however, are complicated for having difficult derivitization and purification processes that provide low product yields. As Besides, PET for photocatalytic splitting of water has been explored in all inorganic multicomponent donor-acceptor systems. It involves at least three components, with the significant benefit that each component can be modified synthetically to be optimized for its Further, organic photosensitizers have also been coupled with polyoxometalates to give rise to organic-inorganic hybrid systems. This approach makes it possible to utilize the electron affinity of the POM for catalytic purposes while using the organic residue as a photon collector. Odobel et al. and Harriman et al. were the pioneer researchers in this field who studied photoinduced electron transfer in perylene and porphyrin covalently linked to POMs 90,91,92 .

Very recently, this stategy was extended to covalently link BODIPY chromophores to the POM units 93 . A few examples of organic-inorganic hybrids are illustrated in Fig. 1.14. Odobel et al., 2009 Harriman et al., 2009 Izzet et al., 2018 Fig. 1.14 Examples of covalently linked organic-inorganic D-A hybrids.

The donor-acceptor system families are discussed in detail in the corresponding chapters of this thesis.

Objective of the dissertation

The exploration of novel photoactive systems with relatively different spectral and electrochemical properties is highly attractive since the constructed new systems could exhibit novel photochemical properties useful for energy harvesting and optoelectronic applications.

Particulary, development of cost-effective and efficient photosensitizers for the purpose of building efficient light-conversion devices is one of the crucial bottlenecks and is topical among scientific community. Therefore, the objective of this thesis is to explore the photoinduced electron and energy transfer (-inter or intra PET) processes upon excitation of organic and inorganic photosensitizers coupled with electron acceptors or photocalatytic platforms, depending on their potential applications (photovoltaics or artificial photosynthesis). The photoinduced processes in different D-A systems in competition with the radiative and nonradiative processes are explored using fast and ultrafast time-resolved spectroscopic methods.

Understanding the mechanism of the relaxation pathways upon photoexcitation is a decisive step for the improvement of these photoactive systems toward applications in solar energy conversion. The output voltage of TAC is thus the function of delay time Δt between the laser and sample emission pulse. The TAC output pulse is effectively an analog pulse of a height corresponding to a measured time of a single start-stop sequence. For further processing, the pulse height will be measured by a digital pulse height measuring device, an analog-to-digital converter (ADC).

Following this

The ADC resolution determines how many discrete time values are possible. All possible measured TAC pulse amplitudes will therefore be put into different time bins. The width of the time bin is the ratio of the full-time range and the resolution of the ADC in channels. It is the time resolution, usually given in picoseconds/channel or nanoseconds/channel.

Photodetectors can detect single photons with intrinsically high gain. For statistical reasons, it is important to ensure no more than one single photon event per light flash is detected. Multiphoton events will affect the histogram statistics and will yield erroneous measurement results.

To ensure that only one photon per light flash is detected, the photon rate is kept low in comparison to the rate of the exciting lamp, usually 5% or lower. with polarizers at a magic angle. Fluorescence decays were then collected until a maximum of 10,000 counts using a PicoHarp 300 TCSPC system (bin time = 4 ps).

In both cases, data were analyzed using the Globals software (Globals Unlimited, the University of Illinois at Urbana-Champaign, Laboratory of Fluorescence Dynamics), which uses a nonlinear least-squares method (Levenberg Marquardt algorithm).

Transient absorption spectroscopy

The technique, also called flash photolysis, is applied to follow the dynamics of the transient species involved in a photochemical reaction. This technique employs an excitation (pump) pulse to promote a fraction of molecules to an electronically excited state. Another pulse (probe) of low intensity, a white light continuum, is directed to the sample having a delay time τ relative to the pump pulse. In this way, the differential absorption spectrum is obtained, which is principally the difference between the absorption of molecules in the excited state and the absorption of molecules in the ground state. The classical pump-probe setup established at ISMO (Institut des Sciences Moléculaire d'Orsay), has been previously described elsewhere 2 and is schematically shown in Fig. An optical parametric oscillator (OPO) is also a light source based on the optical gain from parametric amplification in a nonlinear crystal of Beta Barium Borate (BBO). The output of OPO depends on the angle of the BBO crystal. Upon reaching the phase matching condition between the pump beam and crystal, the pump beam is converted to both a signal beam (410-690 nm) and an idler beam (730-2000 nm) by means of the non-linear optical phenomenon to fulfill energy conversion law:

hv pump = hv signal + hv idler A charge-coupled device (CCD) is an integrated circuit, an array of photodiodes (light-sensitive pixels) that collects photons. Photons incident on this surface generates a charge that can be read by electronics by changing the electrical bias of pixels. This charge can be converted to a voltage by storing the charge on the capacitor. Each voltage represents a certain amount of charge accumulated on each pixel. Finally, the voltage is transformed into a digital signal (Nbit digital representation). An intensified charge-coupled device (ICCD) uses an image intensifier that is coupled to the CCD chip to increase the sensitivity down to a single photon level. An intensified CCD camera allows image acquisition at very low light levels over a wide spectrum and at relatively high speed.

To excite the sample, the excitation beam from the EKSPLA laser is passed through the neutral density filters (NDF) to control the power of the excitation pulses, which are aligned by Iris; I1 and focused using a lens (L1), reflected using a mirror to enter the sample cell. The power of the pulse should be sufficient to promote a significant population of molecules in the excited state without photodegradation. For flash photolysis experiments, the cuvette of dimensions 10 mm x 4 mm was used. The white-light continuum laser beam (probe) of 2 kHz is sent via optical fibre through a transparent optics (O.T.) and reflected by two mirrors, which is then collected and and aligned by means of I2 and focused by a lens (L2) to the hole of the rotating optical chopper. The speed of the chopper allows cutting the frequency of the probe laser to 20

Hz. The beam is sent to the beam splitter (B.S.) through another lens (L3) which directs one part of the probe beam to the reference while the other is sent to the sample cell. The transmitted light from the sample enters the center of the optical fiber coupler that goes to the ICCD camera.

The reference beam allows the measurement of the intensity and spectral characteristics of white light to decrease the impact of energy fluctuations. Before starting the measurements, to ensure the synchronization between the pump and probe pulse, a part of the beam from the beam splitter can be sent to the photodetector that sends the signal to the oscilloscope. The schematic representation of the nanosecond TA set-up built at ISMO is shown in Fig. 2.3.

Timing control and synchronization

Since the delay time between the pump and probe is of the nanosecond timescale domain, the synchronization is done by electronics rather than any translational stage to delay the probe.

Another device used in this set-up is GINI (Impulse generator for numerical information) which is a home-built pulse generator (a master clock) consisting of 16 channels. It controls the triggering of the camera, laser, and other devices in addition to their synchronization by the Lab View program to achieve the most optimized parameters for the pump-probe experiment. The function generator can produce a minimum delay of 4 ns with jitter < 4 ns. Considering the temporal width of the pump laser pulse (~3-4 ns), that of the probe laser (< 1 ns), and the inherent jitter, our time resolution is basically the width of the pump laser. This value can be estimated approximately from the very fast rise of the excited state of [Ru(bpy)3] 2+ . The size and locus of the probe beam, the diameter of which is set aside at approximately 1 mm, and that of the pump pulse are adjusted in such a way that there is spatial overlap between the two.

Therefore, the energy that is being reported in the results and discussion section is the energy that is essentially used during the measurements.

The in-house Lab View program coordinates with the software Winspec to directly download the spectra recorded by it. It takes the average value of the spectra recorded with and without the pump pulse for the sample cell and a reference cell and performs corresponding calculations in order to visualize the differential absorption profile in real time.

In summary, the master clock operates at 2 kHz. The probe laser output of 2 kHz is cut down to 20 Hz by an optical chopper whose trigger signal is resulted in the master clock. A repetition rate of 10 Hz was generated to trigger the pump lasers and the intensifier. The time delay of the two laser pumps is governed by the computer via the pulse generator. For example, if one needs to measure N number of spectra, 12 spectra, for instance, taking into account the frequency of the camera, that is 20 Hz, the gate of CCD should be open for about 600 ms. The timing diagram shown in Fig. 5.4 represents the sequence of pump pulse (10 Hz), meaning there is an on-andoff sequence of pump every 100 ms, CCD (20 Hz), and probe pulse (20 Hz) after being cut by chopper. Thus, CCD captures the image every 50 ms, starting from time zero when the probe and pump temporally overlap (Δt = 0), followed by when the pump is off and on, one after the other. In this way, CCD records spectra alternately with and without the pump, and using equation 1, difference absorption spectra are obtained.

b. Femtosecond TA set-up

The femtosecond transient absorption spectra were obtained using a commercially available set-up (Ultrafast systems, Helios) housed in the Quantum Electric Lab at Adam Mickiewicz University in Poznan. The full description of this setup used for our study has been stated elsewhere 3 . Briefly, the short-pulse titanium-sapphire oscillator (Mai-Tai, Spectra Physics, 70 fs) of the ultrafast laser system is amplified by a high-energy titanium-sapphire regenerative amplifier (Spitfire Ace, Spectra Physics, 100 fs). The output fundamental pulse of 800 nm is divided into two pulses with different energy using a beam splitter. The stronger energy pulse travels through the OPO to create the pump pulse at a particular wavelength for the excitation of the sample. The weaker pulse with lower energy generates probe pulses, a white-light continuum in the UV-Vis-NIR range (300 to 1400 nm), using a CaF2 plate (330-660 nm), a sapphire plate (440-780 nm) and a YAG crystal (830-1400 nm). Initially a pump pulse excites the sample and then, the delayed probe pulse passes through the same excited cross-section of the sample to create the transient absorption (TA) spectra which is then collected by using a spectrometer. The delay time between the pump and the probe pulse was controlled by using a In the present studies, the samples were measured at the pump energy of 1-2 μJ per pulse (beam size of 250 μm FWHM). The spectra were recorded in 2 mm path-length quartz cell under continuous stirring using a Teflon-coated bar. All measurements were performed in argon degassed solutions at room temperature. All the samples were scanned bi-directionally to ensure data reproducibility. The Fig. 2.6 shows the pump beam in violet and the probe beam in red colour. In order to prevent rotational diffusion effects, the pump beam was passed through an achromatic depolarizer (D). Nevertheless, this does not eliminate rotational diffusion entirely as in the case of linearly polarized pumps and probes under magic angle conditions. The depolarized pump was then passed through a neutral density filter (NDF1) that reduces the energy of the excitation pulses. An optical chopper (Ch) chops the frequency of the pump beam to 500 Hz, which is then focussed on to the sample with the help of the lens (L1= 0.5 m). The probe beam is sent to the delay line and then to NDF2, which reduces the energy of 800 nm pulse (this affects the stability and shape of the intensity spectrum of the generated white light pulses). Next, three crystals are available to obtain the supercontinuum spectrum: calcium fluoride CaF2 (330 -660 nm), sapphire Al2O3 (430 -780 nm) or YAG (830 -1390 nm). The parabolic mirror (PM) focusses the beam on the sample. The transmitted probe light from the sample is then sent to the multichannel detector by a using a set of two lenses (L2= f = 0.25 m and L3= f = 0.05 m) and NDF3 (to regulate the intensity of the probing beam pulses and prevent saturation of the multi-channel detector). Covalently linked organic donor-acceptor (D-A) systems have largely been investigated owing to their contribution to the design and development of organic materials for a number of applications as in photovoltaics 1 , light emitting diodes 2 and transistors 3 , etc. In particular, covalently linked donor-acceptor assemblies have gained considerable attention for application in solar energy conversion, aiming to understand and mimic the natural photosynthetic reaction center 4 . By carefully choosing the electron donor-acceptor pairs, taking into account the mutual orientation, the flexible or rigid nature of the linker as well as the redox and excited state properties of the constituent units, photoinduced electron transfer (PET) processes in covalently linked D-A molecules can be improved 5 . In designing a quintessential photovoltaic system, the aim is to generate charge-separated pairs fast and efficient enough to compete with other detrimental radiative and non-radiative processes. Also, the charge-separated pair must be longlived to enable the transport of the charges to the respective electrodes of the photovoltaic cell, because fast recombination would again be an unproductive pathway toward electricity generation. To achieve this goal, an important strategy is to employ donor-acceptor units with small reorganization energies that can undergo facile single-step electron transfer resulting in the formation of a donor radical cation and of an acceptor radical anion 6 .

Various donor-acceptor architectures have been constructed, mostly with tetrapyrroles, such as porphyrins and phthalocyanines, due to their close resemblance with natural photosynthetic pigments. However, synthetic photovoltaic or optoelectronic molecules can potentially employ a wide range of other chromophores and electron-accepting units in D-A systems. For acceptors, fullerenes and their derivatives are quite popular choice owing to their remarkable electron accepting properties 6 . C60 fullerene exhibits very promising feature as an electron acceptor, which comes from the fact that C60 in the ground state can accept reversibly up to six electrons 6 .

According to the theoretical calculations, the highly symmetric C60 molecule has three fold degenerate LUMOs (Lowest Unoccupied Molecular Orbital) and therefore it shows six oneelectron reduction waves in cyclic voltametry (CV) 7 .

Additionally, due to the symmetrical shape and large size of the fullerene framework, C60 fullerenes require only little energy for the adjustment of the generated state (e.g., excited or reduced state) to the new solvent environment. In other words, C60 tends to show low reorganization energy in polar solvents which is crucial for their employment as electron acceptors in D-A dyads. Indeed, as explained earlier, the reorganization energy of the donor and acceptor governs the electron transfer rate constant in a dyad. These facts suggest that fullerenes i.e., Buckminster (will be referred to as C60 hereafter) might serve as useful electron acceptor moieties in multicomponent photosynthesis mimics. In a donor-acceptor dyad, a photoexcited donor can donate an electron to the fullerene unit leading to the formation of the fullerene-based radical anion and the oxidized form of the donor. The C60 radical anion shows a characteristic absorption band in the near IR region at about 1080 nm. In the case of C60 derivatives, methanofullerenes and fulleropyrolidines, the absorption of C60 anion is slightly blue shifted to 1010 nm and 1040 nm, respectively 6 . Gust et al. in 1994 reported the first porphyrin-C60 dyad that exhibited rapid PET in both polar and non-polar solvents 8 . An interesting and pioneering set of covalently linked donor-acceptor dyad or polyad assemblies comprised of zinc-porphyrins (ZnP) linked to pyrrolidino ormethanofullerene derivatives have been further reported 9,10,11,12,13 . In many of these examples, a rapid intramolecular electron transfer predominantly occurs via photoexcitation of ZnP unit to yield charge-separated state in high yields. Sakata et al. also compared the ZnP-fullerene dyad with ZnP-benzoquinone to highlight the effect of small reorganization energy of the acceptor on the charge separation and recombination rates 14 . The work exemplifies the advantage of using fullerene derivatives as electron acceptors in place of small ones (such as quinone), that tend to have high reorganization energy. Ohkubo and co-workers reported a charge-separated state lifetime of 120 s of a porphyrin-C60 system in a frozen polar solvent 15 .

This can be subject to some scrutiny due to its unusually long duration which seems to be unrealistic. It may have been overestimated or misinterpreted, probably due to the use of a nonoptimal solvent.

Molecular structures of some studied ZnP-C60 dyads are presented in Fig. 3.1.1. Gust et al., 1994 D'Souza et al., 2002 Ohkubo et al., 2004 Fig. 3.1.1 Few examples of ZnP-fullerene dyads studied in literature 9,8,15 .

Another well explored class of covalently linked D-A systems is phthalocyanines-fullerene (Pc-C60) assemblies. Linssen, Hanack, and Hirsch reported the first molecular system containing a Pc unit covalently attached to a fullerene moiety in 1995 16 , but it was not until several years later that Torres and colleagues presented photoinduced electron transfer events in covalently linked Pc-C60 dyad 17 (Fig. 3.1.2). A few years later, a thorough examination of the photophysical properties of a Pc-C60 dyad supported the existence of PET events in such covalently linked dyads, providing some fresh insight into the stabilization of the chargeseparated radical ion pairs and the influence of the redox active transition-metal centers on the photoperformance of the Pc based dyad systems in solution 18 . Since then the group of Torres has explored the dynamic properties of several covalently linked Pc-C60 systems 19,20,21 . All these reports highlight the lifetimes of the charge -separated state in the nanosecond timescale in solution and up to millisecond range in solid state.

Torres et al., 2003 Gouloumis et al., 2006 Fig. 3.1.1 Examples of Pc-fullerene dyads studied in literature 21,20 .

Although there are notable advancements in such systems regarding the development of molecular light-conversion devices, the main drawback of these complex assemblies continues to be their extremely time-consuming synthesis, which necessitates laborious multistep reactions and challenging purification techniques that frequently give low yields.

In order to address this problem and create elegant organic donor-acceptor complexes, several researchers have been searching for "non-porphyrin" dye replacements. One such dye, borondifluorodipyrromethene (BODIPY) has been championed as such a molecule, and is based on the half-porphyrin motif but chelated by a difluoroboron moiety to maintain overall structural rigidity (Fig. 3.1.3). The dye originally prepared way back in 1968 by Triebs and Kreuzer 22 and developed as a laser dye has over the past few decades grown in popularity. The BODIPY core has eight reactive positions that can be used to modulate its optical properties (Figure 3.1.3): the two Boron positions, the two α-positions, the four β-positions, and the meso-position 23 . BODIPYs tend to show intense absorption and emission in the visible range of solar spectrum characterized by high absorption coefficient (ε > 50 000 M -1 . cm -1 ), high fluorescence quantum yields (ΦF > 0.5), relatively long excited state lifetimes (in nanosecond range), and excellent photostability 24,25,26 . The representative absorption and emission spectra of a classic BODIPY dye is shown in Fig. 3.1.3 27 . Moreover, slight and facile modifications of the BODIPY core structure tune the photophysical and photochemical properties. In addition, BODIPYs show good solubility in many common organic solvents 28 . Depending on the redox potential of the substituent unit, the excited state of BODIPY can act as electron donor or acceptor 29,30 . The aforementioned remarkable photophysical properties make the BODIPY-based D-A systems attractive for diverse applications in optoelectronics such as Dye sensitized solar cells (DSSCs) 31 , 32 electrochemiluminescence materials 33 , organic light-emitting diodes (OLED) 34 , and particularly for photoinduced electron transfer reactions 35,36,37 and triplet photosensitizer 38 .

The following section outlines the BODIPY-fullerene based D-A systems that have been photophysically investigated for artificial light-conversion purposes, which is the core of this dissertation.

Covalently linked BODIPY-fullerene based D-A systems:

There have been numerous studies on covalent and non-covalent supramolecular BODIPY-C60 based donor-acceptor systems. Even though non-covalent linkage is considered nature's choice of assembling the different redox and photoactive entities in the active centers of photosynthetic systems, covalent attachement of donor-acceptor entities to control the distance and orientation has been a popular choice (as evident from the above reports) for artificial photosynthetic devices. As a result, this section will be limited to the discussion of the covalently linked BODIPY-C60 D-A assemblies. Molecular structures of a few interesting BODIPY-C60 systems are shown in Fig. 3.1.4. The very first photophysical investigations on a closely spaced BODIPY-C60 dyad were reported by Ziessel and coworkers in 2009 39 . The dyad is linked through a diphenylacetylene rigid linker connected from one side at the meso-position of the BODIPY and from the other to the fullerene C60 through a pyrrolidine unit. The photophysical investigations revealed that the photoinduced processes between the two redox active units were highly dependent on the solvent polarity. Briefly, upon photoexcitation in non-polar solvents (such as toluene), singlet energy transfer from BODIPY to fullerene was dominant and then followed by an ISC to populate the triplet excited state of fullerene. However, in polar solvents (DCM and benzonitrile), a charge separated state was observed due to the PET from the excited BODIPY to the fullerene unit with lifetimes around 430 and 160 ps in benzonitrile and DCM, respectively. Ziessel et al., 2009 Amin et al., 2012 Iagatti et al., 2016 Fig. 3.1.3 A few examples of the reported BDP-C60 based D-A systems 35,37,39 .

Inspired by these studies, number of researchers have examined different BODIPY-C60 based dyad and multiads for photoinduced electron and energy transfer processes 35,36,39,40,41 . For examples, Amin and colleagues have reported the photodynamics of a BF2-chelated azadipyrromethene-C60 dyad (Fig. 3.1.4) wherein the ultrafast electron transfer from photoexcited aza-BODIPY to C60 generated a charge separated state with 200 ps lifetime 35 . In a BODIPY-C60 adduct (no linker between the two units) published by Iagatti et al. in 2016 (Fig. 3.1.4), upon selective excitation of BODIPY unit, the charge transfer from photoexcited BODIPY to C60 took place in sub-ps time scale and the recombination lasted for less than 1 ns 36 .

Besides, in some of the aforementioned reports, the PET from excited BODIPY to C60 is followed by the population of the triplet state localized either on C60 or on the BODIPY unit.

This phenomenon opens up new possibilities for the use of BODIPY-C60 based covalent assemblies as organic triplet photosensitizers. Infact, the population of triplet excited state of the chromophores upon excitation finds interesting applications in different fields such as, triplet-triplet annihilation (TTA), upconversion (UC) 42,43,44 , photodynamic therapy (PDT) 45,46 and photocatalytic reactions 47,48 . Since the triplet state of the photosensitizer is populated, efficient ISC is an important factor for designing triplet photosensitizers. Besides, an ideal triplet photosensitizer should exhibit strong absorption in the UV and visible range alongwith a long-lived triplet excited state 49 .

Conventionally, transition metal (Ru(II), Pt(II) and Ir(III)) complexes have been studied as triplet photosensitizers owing to their high phosphorescence quantum yield (due to heavy atomeffect of transition metal) but their application is limited by weak absorption of visible light 50,51 coupled with short triplet lifetime (<10 µs) 52 . Besides, transition metal complexes are usually toxic and not cost-effective. Apart from transition metals, iodine or bromine substituted organic chromophores, BODIPY and naphthalene diimide (NDI) for instance, are also used to induce the ISC by heavy atom effect 53,54,55 .

So far, the development of heavy atom-free organic triplet photosensitizer with pre-determined intersystem crossing has remained a challenge 52,40 . A slight modification in the core structure of a known chromophore may lead to the disappearance of the ISC property 56 . To address this problem, visible light harvesting units, usually BODIPYs are linked to a spin convertor (acceptor) 52 . When the chromophore is photoexcited, singlet energy transfer from excited chromophore to the spin-convertor leads to the population of its singlet-excited state. Then, the triplet state of the acceptor will be populated by ISC. This approach provides a way to design molecular triplet photosensitizers with predictable ISC. In this context, C60 fullerene is widely used as spin convertor for its high triplet yield and rate of ISC process 6 . Many examples of BODIPY-C60 based dyads developed and studied for application as organic heavy-atom-free triplet photosensitizers 39,56,57,58 can be cited.

Additionally, as the solar spectrum encompasses visible to infrared ranges, special emphasis has been placed on extending the visible light absorption characteristics of chromophores into at least the near-infrared area 59,60,61 . Chromophores with large extinction coefficients in the visible region of the solar spectrum are a pretty clever and straightforward approach 62 . The extension of the delocalized π-system is a potential method for improving the absorption properties in the long wavelength region. However, this strategy is also accompanied by changes in the electronic properties, most notably the HOMO and LUMO levels and redox potentials 62 . Alternatively, panchromatic light harvesting, which involves complementing the absorption characteristics of the various chromophores should be taken into consideration. In this regard, many multi-chromophoric donor-acceptor covalent supramolecular systems based on multiple photo-and redox-active species have been extensively studied to mimic and comprehend the mechanistic details of sequential energy and electron transfer reactions in natural photosynthesis to convert and store solar energy 61,60-65 (Fig. Aside from these, an example of all covalently linked multiple-BODIPYs-C60 triad system (BODIPY-azaBODIPY-C60 triad, Fig. The results of the above-mentioned studies inspired us to further investigate the photoinduced processes in covalently-linked multichromophoric-fullerene compounds, particularly, BODIPY-C60 based assemblies, which can add-up to the fundamental understanding of charge separation and recombination processes crucial to design efficient light conversion devices.

From a synthetic point of view, the classical approach (i.e., the direct addition of C60 core in the final step of the synthesis) of most of the aforementioned BODIPY-C60 assemblies suffers from the drawback of unwanted multi-adducts formations due to the multifunctional nature of the fullerene core. As a result, low product yields are obtained for desired BODIPY-C60 assembly.

To circumvent this issue, Emmanuel Allard and co-workers synthesized BODIPY-C60 based assembly using an indirect method. They have used the famous copper-catalyzed alkyl-azide cycloaddition (CuAAC) ''click chemistry'' approach to attach the BODIPY-azide with fullerene platform bearing two clickable alkyne moieties (one unprotected and one protected)

on either side of it (Fig. 3.1.6). This strategy allowed the design of a novel BODIPY-C60

(referred to as BDP-C60 in the text) dyad in which the BODIPY and C60 are linked via a triazole function. The synthesis is described in the report by Rabah et al 71 . and not as a terminal group as reported in all the previous panchromatic triads. The approach not only extended the absorption range of the BDP-C60 dyad in the NIR range stemming from the complementary absorption of the two BODIPY units, but multiple photoinduced events were also expected due to the non-rigid linker that could lead to the formation of the charge separated state upon photoexcitation.

A reference distyryl-BODIPY-C60 dyad (DSBDP-C60) was also synthesized by clicking the distyryl BODIPY unit with the C60 platform (Fig. 3.1.8). Article #1 is dedicated to the photoinduced processes observed in the nanosecond time scale, which mainly explores the selective localization of the triplet excited states in the two dyads and the triad. Also, the potential application of these systems in triplet-triplet annihilation upconversion (TTA-UC) has been illustrated.

Article #2 deciphers the ultrafast photoinduced electron and energy transfer processes within these systems leading to the selective population of the triplet states, thus providing valuable fundamental insights for the development of artificial light-conversion devices.

Introduction

Triplet photosensitizers have gained much attention due to their wide range of applications in photocatalysis [1, 2], photodynamic therapy [3, 4] and triplet-triplet annihilation (TTA) upconversion [5, 6]. A quintessential triplet photosensitizer should show broadband visible light absorption with a high absorption coefficient (> several tens of thousands M -1 cm -1 ), high triplet formation quantum yield, hypotoxicity and long-lived triplet excited states [7, 8]. Conversely, classical organic triplet photosensitizers containing transition metal atoms, able to populate triplet-excited states suffer from the drawbacks of short lifetimes (typically several microseconds) and weak absorption in the visible range [9, 10], in addition to being expensive. Other frequently explored triplet photosensitizers consist of heavy halogen atom (Br, I) substituted organic chromophores, resulting in shortened triplet excited state lifetimes on account of increased spin-orbit coupling [11-13]. Meanwhile, the development of heavy-atom-free organic triplet photosensitizers remains a challenge, mainly due to the difficulty in predicting their inter-system crossing (ISC) ability upon derivitization [7, 14, 15]. Moreover, the conventional triplet photosensitizers are mono-chromophoric, limiting their absorption in the entire visible spectral range. To deal with the above mentioned deleterious effects, one of the important strategies for devising heavy-atom-free triplet organic photosensitizers makes use of multi-chromophores in conjunction with an electron spin convertor to serve a dual purpose: wide spectral absorption with an efficient ISC [7, 8]. Along these lines, fullerene (C 60 ) derivatives are frequently employed as efficient spin convertors due to their high ISC efficiency, rounding-up to unity [16]. Even so, fullerene itself cannot be used as a model photosensitizer owing to its weak visible light absorption, but the existence of a lowlying singlet excited state can be exploited to realize singlet energy transfer from organic chromophores to C 60 [15, 17]. The choice of organic chromophores has been greatly directed towards boron-dipyrromethene (BODIPY) in light of its photophysical properties (strong visible-light absorption, photostability, high fluorescence quantum yields) and easy functionalization that in turn facilitates absorption tunability over the whole visible range [18-20]. Consequently, BODIPY-C 60 based molecular motifs are of particular interest for the formation of a triplet excited state upon photoexcitation of a chromophore entity.

A number of BODIPY-C 60 based molecular combinations have been reported, featuring the mechanistic traits of triplet state formation depending upon the polarity of the solvent and triplet energy levels [21-25]. In non-polar solvents, singlet energy transfer from BODIPY to fullerene populates the singlet excited state of C 60 , which then undergoes an ISC to populate the triplet-excited state of C 60 [15, 21, 22, 26]. Further, a back triplet energy transfer can occur from C 60 to triplet BODIPY in what is known as ''ping-pong'' energy transfer [15, 23]. In polar solvents, charge recombination may be responsible for triplet population of either the BODIPY or C 60 [21, 24]. A formerly reported broadband light-absorbing triad and tetrad (450-650 nm) demonstrated excitation energy funneling to C 60 , followed by an ISC to populate the C 60 tripletexcited state, finally resulting in the population of the triplet excited state of BODIPY with a lifetime of about 132.9 μs [23]. However, the authors did not characterize the efficiency of the triplet-excited state formation. Another recently published supramolecular triad containing two BODIPY units and a fullerene enabled the absorption of panchromatic light but its ability to induce a tripletexcited state was not explored [27]. Despite these previous reports, to our knowledge, panchromatically absorbing heavy-atom-free BODIPY-C 60 -based molecular systems have barely been investigated for their triplet excited state properties.

The present work reports photophysical investigations of a novel BODIPY-fullerene-distyryl BODIPY panchromatic light-absorbing triad (BDP-C 60 -DSBDP) and its associated dyads (BODIPY-fullerene; BDP-C 60 and distyryl BODIPYfullerene; DSBDP-C 60 ) (Scheme 1). The synthesis of the triad, following a new synthetic approach of grafting two differently functionalized BODIPY cores on two clickable arms of a fullerene platform using alkyne-azide click chemistry, was reported previously [28]. The presence of two chromophore units enables broad absorption of the system in the visible region. The population of a triplet excited state upon visible excitation in the two isolated dyads and the triad was confirmed by employing nanosecond time-resolved spectroscopy. For the BDP-C 60 dyad, excitation at 525 nm of the BDP unit populated the triplet excited state localized on the C 60 moiety. However, in the DSBDP-C 60 dyad, excitation of the DSBDP unit allowed the population of a long-lived triplet excited state localized on the DSBDP moiety. Moreover, the BDP-C 60 -DSBDP triad results in the population of long-lived triplet state on the DSBDP, regardless of selective excitation of the two chromophore units, while leveraging the panchromatic light absorption at the same time. To demonstrate the potential application of this long-lived triplet state photosensitizer in these systems, triplet-triplet annihilation up-conversion, occurring upon energy transfer from the triplet states to a triplet acceptor/annihilator, is shown.

3 2 Materials and methods

Materials

All the molecular systems (BDP, DSBDP, methanofullerene platform, BDP-C 60 , DSBDP-C 60 and BDP-C 60 -DSBDP triad) were synthesized at Institut Lavoisier de Versailles following the procedures described by Allard et al. [28].

Spectroscopic grade benzonitrile, toluene and ethyl iodide were purchased from Sigma-Aldrich and used without further purification.

Steady-state and time-resolved fluorescence measurements

Steady-state state absorption measurements were carried out using UV-2600, a UV-Vis spectrophotometer from SHIMADZU. 10 × 10 mm 2 quartz cells were used for steady state measurements. Emission and excitation spectra were recorded using Scientific Fluoromax PLUS (Horiba Jobin-Yvon) under right angle configuration with a bandwidth of 1 nm in excitation and 2 nm in emission.

To minimize the error in the estimation of fluorescence quantum yields, three different concentrations were measured (absorbance < 0.1 to avoid reabsorption and inner filter effects). Briefly, the integral area of the emission of a sample molecule and its corresponding reference at each concentration was plotted as a function of the respective absorbance at the excitation wavelength. The slopes obtained by the linear regression analysis were used to calculate the fluorescence quantum yield of the molecule using Eq. ( 1):

where η is the refractive index of the solvent and subscripts S and R denote the sample molecule and reference respectively, R is the fluorescence quantum yield of the reference. Using this method, an error of 10% was estimated in the calculation of fluorescence quantum yields.

Fluorescence decay curves were obtained using the same set-up previously described by Ghose et al. [29] with two different excitations: (i) 495 nm femtosecond excitation pulses (< 200 fs, FWHM, 4 MHz) obtained with a Ti:sapphire laser (Coherent Chameleon Ultra II, 80 MHz) coupled to a pulse picker (4 MHz) and a harmonic generator (SHG/THG, APE) and (ii) 635 nm picosecond excitation pulse (Picoquant laser diode) emitting with a pulse width < 90 ps (FWHM) and a repetition rate of 8 MHz. Photon counting was performed using a PicoHarp 300 TC-SPC system (PicoQuant) with a bin time of 4 ps. The IRF (instrumental response function) of the setup was measured using a LUDOX colloidal solution and measurements were performed using a FluoTime 200 spectrometer (4 nm bandpass) and polarizers were set at magic angle. Solutions with an absorbance below 0.1 (1 cm path length) at excitation wavelengths were used. The emission decays were collected until 10,000 counts at the maximum. They were analyzed with a weighted sum of exponential decay convolved by the IRF using the FluoFit software (Pico-Quant). The quality of the fits was assessed by the value of χ 2 (< 1.1) and the residuals autocorrelation function. The ( 1) amplitude weighted mean lifetime of the molecules were obtained using: τ m = ΣA i τ i /ΣA i.

F = slope S slope R X S R 2 X R ,

Transient absorption spectroscopy

All the measurements were performed under argon saturation using a quartz cuvette of dimensions 10 mm × 4 mm. For laser flash photolysis, an optical parametric oscillator (OPO) pumped by a pulsed nanosecond Nd:YAG laser (Quanta-Ray INDI series) was used to excite the samples. The pump laser was operated at 10 Hz. The resulting pulse width was 4 ns and the energy was about 1-2 mJ/pulse. A white light super continuum laser (STM-2-UV LEUKOS) was used as a probe light for transient absorption measurements. The probe laser was operated at a repetition rate of 20 Hz. The probe beam was split by means of a beam splitter that directs one part of beam to the sample cell while the other is directed towards the reference, to monitor spectral energy distribution fluctuations. The transmitted light from the sample enters into the center of the optical fiber (roundto-linear) bundle that goes to a spectrograph (SPEX 270 M, Jobin-Yvon) for analysis. The dispersed white light from the spectrograph is detected by an intensified charge coupled device (ICCD) camera PIMAX-4 (Princeton Instrument). The spectrograph simultaneously records the reference and sample spectra. The ICCD area vertically receives a bundle of 18 optical fibers of which 9 are from the sample and 9 from the reference. ICCD records the spectra with the same frequency as the probe (20 Hz). Upon varying the delay time between pump and probe, the transient absorption spectra with and without pump can be recorded using Eq. ( 2

Calculation of triplet quantum yields

The triplet states have been quantified using the relative actinometry method [30], using an aqueous solution of tris(2,2'-bipyridine)ruthenium(II) [Ru(bpy) 3 ] 2+ as a reference. Transient absorption changes upon decreasing the excitation power were recorded for the reference and corresponding dyad and triad molecules, under exactly the same experimental conditions (λ exc , optical path, absorbance etc.).

Transient absorption decays at 455 nm for [Ru(bpy) 3 ] 2+ (maximum of ground state bleach) and the triplet state absorption maxima (710 nm for BDP-C 60 dyad and 710 nm for DSBDP-C 60 dyad and BDP-C 60 -DSBDP triad) were 3) given below, triplet quantum yields were obtained where T is the triplet whose quantum yield is to be calculated and Φ ref is the formation quantum yield of [Ru(bpy) 3 ] 2+ (Φ ref ~ 1), Δ ref is the absolute value of molar absorption coefficient for the [Ru(bpy) 3 ] 2+ reference ground state bleach ( Δ 455 = -10,100 M -1 cm -1 ) [31, 32], Δ T is the value of molar absorption coefficient for 3 C 60 * (16,100 M -1 cm -1 ) [33] or 3 DSBDP* (58,000 M -1 cm -1 ).

Results and discussion

Steady-state absorption and fluorescence studies

The absorption spectra of the BDP-C 60 -DSBDP triad and its related dyads in benzonitrile are shown in Fig. 1a. Their photophysical parameters are summarized in Table 1. The absorption spectra of the parent compounds BDP-N 3 and DSBDP-N 3 and the methanofullerene platform (C 60 ) in benzonitrile are presented in the supplementary information (Fig. SI-4). The fullerene building block shows typical absorption spectrum of methanofullerene with an intense band at 330 nm and a small sharp band at 428 nm [34]. The main absorption band of the BDP located at 527 nm corresponds to the S 0 -S 1 transition in phenyl-substituted BDPs [35, 36] whereas a weaker and broader feature at 380 nm corresponds to the S 0 -S 2 transition [36] (Fig. SI-4). The intense and large absorption band of DSBDP peaking at 637 nm is attributed to the S 0 -S 1 transition. This band is significantly red shifted and broadened as compared to the BDP band due to the extended conjugation of the BODIPY core [19]. The UV bands with a maximum at 345 nm for DSBDP can be associated with the π-π distyryl subunits along with a shoulder attributed to the S 0 -S 2 transition of the BODIPY backbone.

The spectra of the BDP-C 60 and DSBDP-C 60 are very similar to those observed in the BDP and DSBDP moieties with in addition the expected contribution of the methanofullerene, i.e., a strong band in the UV region at 330 nm and a small sharp band at 428 nm (Fig. 1a). The absorption of ( 3)

Φ T = slope T × Φ ref × Δ ref slope ref × Δ T , 69 1 3
Table 1 Absorption and emission data for BDP and DSBDP systems in benzonitrile Φ F : Fluorescence quantum yield with 10% error, τ m : mean fluorescence lifetime with 10% error a Ref. [37] b λ exc = 495 nm, λ em = 535 nm, quantum yield measurement of BODIPY moiety with Rhodamine 6G used as standard (λ exc = 495 nm, ϕ = 95% in ethanol) c λ exc = 635 nm, λ em = 650 nm, quantum yield measurement of DSBDP moiety with Oxazine 1 used as standard (λ exc = 605 nm, ϕ = 11% in ethanol) d λ exc = 495 nm, λ em = 650 nm, quantum yield measurement of DSBDP moiety with [Ru(bpy) 3 ] 2+ used as standard (λ exc = 495 nm, ϕ = 4.2% in water) This relates to the limited electronic coupling between the BDPs and the fullerene due to the absence of conjugation, as already observed in a similar BDP-C 60 dyad [38].

Compound λ abs , nm (ε, M -1 cm -1 ) λ em , nm Φ F , % τ m , ns
Upon exciting the parent BDP in benzonitrile, a strong fluorescence band centered at 537 nm was obtained with a high quantum yield of 73% (Fig. 1b). However, the BDP emission intensity was critically reduced by over 98% in the BDP-C 60 dyad giving a quantum yield of 1.5%. This observation is characteristic of excited state energy transfer and/or electron transfer events from the BDP antenna to C 60 . Indeed the energy transfer from BDP to C 60 must be taken into consideration due to the spectral overlap between BDP emission and C 60 absorption. Likewise, DSBDP-N 3 excited at 605 nm gave an intense fluorescence band at 650 nm with a fluorescence quantum yield (Φ F ) of 85%, which was also greatly quenched by 92% in the dyad where DSBDP was attached to C 60 (Φ F = 7.2%). In addition, upon exciting the DSBDP-C 60 -BDP triad at 495 nm, corresponding mainly to the absorption of the BDP chromophore, the emission of the BDP subunit at 537 nm was quenched by over 99% (Φ F = 0.4%), with the appearance of a more intense emission band at around 650 nm. This band was assigned to DSBDP emission by comparison with the reference fluorescence spectrum of DSBDP (red curve Fig. 1b), suggesting the occurrence of energy transfer from 1 BDP* to DSBDP. In addition, the excitation spectra of the two dyads almost completely corresponds to their respective BODIPY absorption spectra and the excitation spectra of pristine BODIPYs, indicating that the BODIPY moieties are responsible for the emission of the dyads (Fig. 2a, b and Fig. SI-6). However, the excitation spectrum of the triad observed at the emission of DSBDP moiety (λ em = 670 nm) clearly shows the contribution of the BDP moiety (λ abs = 525 nm) in addition with that of DSBDP (λ abs = 637 nm), confirming the energy transfer process from the BDP to DSBDP (Fig. 2c). Also, the (Table 1). However, the decays of the dyads and triad are no longer monoexponential, with the contribution of several short components. The mean lifetimes were significantly reduced in the two dyads and the triad (Table 1), indicating 1 3 some rapid electron and/or energy transfer events taking place in the excited state of the BODIPY moieties. A similar lifetime was obtained for DSBDP emission either for the DSBDP-C 60 dyad or the triad regardless of the excitation wavelength, suggesting a similar deactivation pathway from 1 DSBDP* in the two systems. However, we observed a decrease in lifetime of BDP emission in the triad as compared to that of the BDP-C 60 dyad, due to the additional deactivation pathway of energy transfer from BDP to DSBDP in the triad.

Nanosecond time-resolved spectroscopy

To decipher the production of the triplet excited states in the dyads and triad, time-resolved nanosecond transient absorption was performed upon laser pulsed excitation. First, the transient absorption (TA) measurements were studied in reference compounds (BDP-N 3 , DSBDP-N 3 and C 60 platform) in benzonitrile. Transient absorption bands of the excited species for BDP-N 3 and DSBDP-N 3 decay on a nanosecond timescale with a lifetime of a few nanoseconds as obtained by TCSPC measurements (Table 1, and 710 nm characteristic of the C 60 triplet state [16] with a lifetime of (9 ± 1) µs.

Next, a pulsed laser beam at 525 nm was used to excite the BDP moiety of the BDP-C 60 dyad. The TA spectra show positive transient bands at about 400 and 710 nm which are attributed to absorption of the 3 C 60 * state and decay with a lifetime of (7 ± 1) μs (Fig. 4a). This observation was confirmed by measurements in aerated solution: the bands of 3 C 60 * decay with a very short lifetime of about 400 ns, indicating the effect of oxygen in quenching the triplet excited state (Fig. SI-11a). As already reported in a similar BDP-C 60 dyad [38], the formation of a 3 C 60 * triplet state may be explained by an energy transfer from 1 BDP* to C 60 followed by an intersystem crossing to form 3 C 60 *. Scheme 2 summarizes the energy level and the lifetime of the triplet state in BDP-C 60 . The triplet formation quantum yield of 3 C 60 * was then determined by applying laser pulse relative actinometry. For this, [Ru(bpy) 3 ] 2+ has been used as a reference due to its well defined triplet formation quantum yield close to unity [32]. A triplet formation quantum yield of 40 ± 8% was obtained for 3 C 60 * in the BDP-C 60 dyad (Fig. SI-1a and2). An analogous yield of 3 C 60 * formation (24%) has been reported for a C 60 -TTF 2. The details of the quantum yield calculation are given in the Sect. 2.

Similarly, DSBDP-C 60 was subjected to 635 nm excitation to exclusively excite the DSBDP unit. Upon excitation, the TA spectra (Fig. 4b) showed a bleaching band at 637 nm corresponding to the depletion of the DSBDP-N 3 ground state. In addition, the positive absorption bands at around 380, 500 and 700 nm can be attributed to 3 DSBDP* [14]. The 3 DSBDP* formation was confirmed by measurements in an aerated solution which revealed a strong decrease in the excited state lifetime to 400 ns due to oxygen quenching (Fig. SI-11b). By performing transient absorption measurements on parent DSBDP-N 3 in ethyl iodide, the 3 DSBDP* signature could also be witnessed (Fig. SI-12), by virtue of the heavy-atom effect. The presence of the 3 C 60 * in DSBDP-C 60 could not be excluded since its absorption is in the same spectral range, but the contribution of this species should be very low as compared to that of 3 DSBDP*. Indeed, the transient decays of DSBDP-C 60 can be satisfactorily fitted with a monoexponential function with a lifetime of 250 µs without any contribution of the 7 µs lifetime of 3 C 60 *. Assuming that ground state depletion leads to a 100% population of the triplet state and the absorption of the triplet state is negligible in the bleaching region (the ground state absorption band overlaps with that of bleaching at 637 nm (Fig. SI-13)), the differential molar absorption coefficient of 3 DSBDP* was estimated at a maximum of 58,000 M -1 cm -1 . A minimum value of 37 ± 8% was then obtained for the quantum yield of the triplet-excited state formation of 3 DSBDP* (Fig. SI-1b and SI-3b). The triplet excited state formation of the DSBDP can be explained by an initial thermodynamically feasible electron transfer from 1 DSBDP* to C 60 ( ΔG CS = -0.44 eV estimated from the redox potentials of DSBDP and C 60 ) [28] followed by charge recombination which populates the 3 DSBDP*, which was already observed in a DSBDP-C 60 dyad by Fukuzumi, Ng et al [14]. Another way to populate the DSBDP triplet state could be an initial energy transfer from 1 DSBDP* to C 60 followed by ISC from 1 C 60 * to 3 C 60 * and a second energy transfer from 3 C 60 * to DSBDP. This ping-pong intramolecular energy transfer between DSBDP and C 60 moieties was reported by Huang et al. for a DSBDP-C 60 dyad [15]. Femtosecond transient absorption will be performed in a next step to decipher the formation of triplet states in these assemblies.

In the case of the triad, selective excitation of the DSBDP unit (λ exc = 635 nm) leads to triplet-excited state formation on the DSBDP unit, as observed in the DSBDP-C 60 dyad (Fig. SI-14) with a similar lifetime in argon-saturated solution. The 3 DSBDP* formation was confirmed by measurements in an aerated solution which revealed a strong decrease in the excited state lifetime to 400 ns due to oxygen quenching (Fig. SI-11c). Additionally, upon photoexcitation of the BDP unit at 525 nm, the localization of the triplet state was again detected on DSBDP (Fig. 5 and Fig. SI-11d). This observation confirms the excitation energy transfer from 1 BDP* to DSBDP. The triplet bands in all cases follow similar kinetics with a time constant of around (280 ± 20) µs. Using the method previously described (Fig. SI-1b and SI-3c), a quantum yield of (20 ± 4)% for 3 DSBDP* formation was obtained in the triad. The decrease in the quantum yield value in the triad as compared to that in the dyad can be explained by the non-selective excitation of the DSBDP chromophore at 420 nm, which leads to more de-excitation pathways in the triad.

Remarkably, in all the events where DSBDP plays the role of the photoactive unit, the triplet state is exclusively localized on the DSBDP unit and not on C 60 . It should be noted that direct excitation of the DSBDP reference compound does not lead to the formation of the DSBDP triplet state. As in the case of the DSBDP-C 60 dyad, the formation of the triplet state localized on the DSBDP in the triad upon excitation of the DSBDP may be explained either by an initial charge transfer from 1 DSBDP* to C 60 followed by charge recombination to populate the 3 DSBDP* [14] or by a ping-pong energy transfer between the DSBDP and C 60 moieties [15, 23]. When the BDP moiety was excited in DSBDP-C 60 -BDP, a similar spectral signature was observed for the DSBDP-C 60 and the triad, suggesting the occurrence 1 3

of the same processes involving the DSBDP preceded by an energy transfer from 1 BDP* to DSBDP. Unlike BDP-C 60 dyad, no visible signature of 3 C 60 * was observed. As in the DSBDP-C 60 dyad, we cannot exclude the formation of 3 C 60 * in the triad but due to the lower energy of 3 DSBDP* [15], it is probably the interaction between the 3 C 60 * and DSBDP on a sub-nanosecond timescale which provides a channel to the formation of 3 DSBDP* in the triad.

Application as organic triplet photosensitizer for TTA up-conversion

Triplet-triplet annihilation up-conversion (TTA-UC) was first carried out with the BDP-C 60 dyad that exhibits a high triplet state quantum yield in addition to a strong visible light absorption. Perylene was used as a triplet annihilator due to the comparable energy levels of 3 C 60 * (1.56 eV) [21] and the triplet of perylene (1.53 eV) [40] and therefore a possible triplet-triplet energy transfer (TTET) between the two could be observed.

The mixture of BDP-C 60 dyad and perylene was excited at 525 nm, corresponding to the absorption of the BDP moiety, under argon saturation at room temperature in toluene. It should be noted that perylene does not absorb at this wavelength. The respective decay and evolution of triplet C 60 and triplet perylene was monitored using transient absorption spectroscopy. Transient absorption spectra of the mixture (Fig. 6a) initially show the appearance of the 3 C 60 * triplet absorption band at 710 nm as already described in the dyad experiment. The decrease of this band occurs simultaneously with the increase of a new band at around 485 nm, corresponding to the excited state absorption of triplet perylene [41], suggesting an efficient TTET process from 3 C 60 * triplet to the triplet state of perylene. This triplet state absorption band of perylene further decays over longer delay times. Kinetic analyses (Fig. SI-15) show a shortened decay time for 3 C 60 * at 710 nm (3.0 μs ± 0.2 μs) as compared to that of the dyad alone (25 µs ± 3 μs) in toluene (Fig. SI-16), and this decay time is similar to the rising time of the triplet state of perylene at 485 nm, confirming again a TTET process. The triplet perylene further decays with a time constant of (68 ± 7) μs (Fig. SI-15).

To witness up-converted fluorescence, time-resolved emission of the dyad-perylene mixture was recorded (Fig. 6b). Monitoring the emission kinetics at 490 nm gives one rising time constant of (4.0 ± 0.2) μs and one decay time constant of (21 ± 2) μs for the delayed fluorescence of perylene. The rising time is related to the TTET and TTA processes whose kinetics are diffusion-limited. The decay time, which is in the same range as the triplet state lifetime of perylene, corresponds to the TTA process between two triplet molecules [42]. This lifetime is significantly higher compared to the 3.8 ns reported for perylene alone [40] and can unambiguously be assigned to the delayed fluorescence due to TTA up-conversion. Figure 7 resumes all the triplet-triplet annihilation processes leading to the delayed fluorescence of perylene.

A similar experiment was performed with the dyad and triad containing DSBDP with perylene, but no TTET was observed, due to the lower triplet state energy of the DSBDP as compared to that of perylene. To evaluate the TTA-UC in the triad, the DSBDP monomer was employed as an annihilator. The triad-DSBDP solution in toluene was excited at 525 nm to attain selective excitation of the BDP unit. Transient absorption spectra of the mixture show the expected formation of 3 DSBDP* (Fig. SI-17), but the lifetime of 3 DSBDP* was reduced to 45 ± 5 µs as opposed to (230 ± 28) µs for the triad alone in toluene (Fig. SI-18), implying a TTET/TTA process between 3 DSBDP* (triad) and 3 DSBDP* (monomer). Further, similarly to the delayed fluorescence observed for perylene, fitting the time resolved fluorescence kinetics at 675 nm corresponding to the emission of 1 DSBDP*, gives one rising time of (6 ± 2) µs and one decay time of (29 ± 5) µs (Fig. 8). This long decay time compared to the prompt fluorescence lifetime of the DSBDP monomer (4.46 ns, Fig. SI-8) clearly points towards the TTA-UC leading to the delayed fluorescence of DSBDP. Also, control experiments performed on the DSBDP monomer or triad (Fig. SI-19) in the same experimental conditions do not reveal any delayed fluorescence. These findings confirm that UC occurred due to indiscernible processes of TTA between 3 DSBDP* (triadmonomer) and/or 3 DSBDP* (monomer-monomer).

Conclusion

We have photophysically studied a novel broadband lightcapturing BODIPY-fullerene-based triad and its control dyads, synthesized by a click chemistry approach. The two differently functionalized BODIPY units mounted on the two arms of a C 60 core results in an extended UV visible absorption spectrum (300-700 nm), due to the cumulative absorptions of the two BODIPYs. The photophysical properties of the dyads and triad were studied by steady-

Supplementary Information

The online version contains supplementary material available at https:// doi. org/ 10. 1007/ s43630-022-00241-z. Delay Time, ms

Introduction

Understanding photoinduced processes is of particular importance for the design of molecular systems for diverse applications such as artificial photosynthesis, photovoltaics, molecular electronics and photonics [1-9]. A large number of elegant a e-mail: gotardb@amu.edu.pl (corresponding author) b e-mail: rachel.meallet-renault@universite-paris-saclay.fr (corresponding author) c e-mail: karine.steenkeste@universite-paris-saclay.fr (corresponding author) d e-mail: minh-huong.ha-thi@universite-paris-saclay.fr (corresponding author) donor-photosensitizer-acceptor (D-P-A) and photosensitizer-acceptor (P-A) systems have been designed and extensively studied [6-12]. Apart from the classical chromophores such as porphyrins and phthalocyanines, the use of BODIPYs as photosensitizer or electron/energy donor is promising in the light of its photophysical properties. Their attractive properties including strong visible-light absorption, photo-stability, high fluorescence quantum yields and facile optical and electrochemical tunability [5, 13, 14] are crucial to build innovative D-A systems. Remarkably, BODIPYs can be employed as electron/energy donor or acceptor based on the counter molecular entity [15, 16]. Furthermore, fullerenes and their derivatives have also been largely studied in artificial photosynthetic systems due to their outstanding electrochemical properties and low reorganization energy in electron transfer processes [17, 18]. Over the years, scores of reported BODIPY-C 60 based 0123456789().: V,-vol 123 Scheme 1. Structures of the BDP-C60-DSBDP triad and the two reference BDP-C60 and DSBDP-C60 dyads under investigation and simplified scheme of the processes leading to the formation of the 3 DSBDP* state in the triad donor-acceptor systems have illustrated the photoinduced electron transfer leading to the charge-separated state (BODIPY •+ -C 60 •-) [19-24].

Certainly, broad-spectrum light capture is vital for the realization of artificial photosynthesis and, therefore, much efforts have been made to cover the entire visible range absorption of photosensitizers. For this purpose, Obondi and coworkers have investigated a series of bis(styryl)-BODIPY-C 60 assemblies featuring a wide-band light capture on account of extended conjugation of the BODIPY core [15]. Another strategy is to use multichromophoric systems as reported in the subphthalocyanine-azaBODIPY-C 60 triad in which competitive ultrafast electron transfers were observed due to the V-configured system [25]. D'Souza and coworkers have also reported broadband capturing, BODIPY-Porphyrin-C 60 triads and demonstrated different photoinduced processes depending on the excitation wavelengths [26, 27]. Several panchromatic supramolecular systems containing multiple BODIPY chromophores linked to C 60 moiety capable of mimicking the photosynthetic 'antenna-reaction centre' were developed [28, 29]. However, to the best of our knowledge, very few reports are available on covalently linked multiple BODIPY-fullerene systems highlighting ultrafast photoinduced processes [29]. Recently, Allard et al. synthesized a novel broadband absorbing multiple BODIPY-C 60 covalently linked triad (300-750 nm) having a distinct design rationale [30]. The two different BODIPYs, a BODIPY (BDP) and a more conjugated distyryl-BODIPY (DSBDP), are linked to the two sides of the methanofullerene platform using a facile stepwise or one-pot synthesis process to build the BDP-C 60 -DSBDP triad (Scheme 1). The selective population of the triplet excited state in this triad and the two reference dyads (BDP-C 60 and DSBDP-C 60 ) has recently been reported by our group using nanosecond transient absorption spectroscopy [31]. In the triad, the localization of the long-lived triplet excited state (280 μs) on 3 DSBDP*, regardless of the excitation wavelength makes this system interesting in the field of wide-band light-capturing heavy atom free triplet photosensitizer. However, the early steps of the relaxation pathways leading to the formation of the triplet states in the two dyads and the BDP-C 60 -DSBDP triad (Scheme 1) were not investigated.

In the present study, we have systematically investigated the photoinduced electron and energy transfer processes in the two reference dyads and in the triad. The flexible linker between the BODIPY and fullerene units allows various possibilities of donor-acceptor distances in different conformations, leading to a complex dynamics of photoinduced processes. Combining the results of ultrafast spectroscopy, steady-state and timeresolved fluorescence studies, we present herein the complete relaxation pathways in these systems. Details of photoinduced electron and energy processes, in particular the formation of radical ion-pairs in this multichromophoric donor-acceptor compound are important for designing strategies for artificial photosynthetic systems.

Materials and methods

Materials

All the molecular systems (BDP, DSBDP, methanofullerene platform, BDP-C 60 , DSBDP-C 60 and BDP-C 60 -DSBDP) were synthesized in the Institut Lavoisier de Versailles following the procedures described by Allard et al. [30]. Spectroscopic grade benzonitrile and dichloromethane were purchased from Sigma-Aldrich and used without further purification. 123

Steady-state absorption and fluorescence spectroscopy

The UV-Visible ground-state absorption spectra were recorded with a UV-2600 spectrophotometer (Shimadzu). Emission spectra were measured using Scientific Fluoromax PLUS (Horiba Jobin-Yvon). The concentrations of the samples used for absorption and emission measurements were in the micro-molar range and the spectra were recorded in 1 cm optical path quartz cells.

Time-correlated single-photon counting

Time-resolved fluorescence measurements were performed using the set-up previously described in the literature [32]. The BODIPY moieties in the dyads and the triad were excited using two excitation wavelengths (495 or 635 nm). Using a Ti:sapphire laser (Coherent Chameleon Ultra II, 80 MHz) coupled to a pulse picker (4 MHz) and a harmonic generator (SHG/THG, APE), 495 nm femtosecond excitation pulses (< 200 fs, FWHM, 4 MHz) were employed to excite the BDP-C 60 dyad or the triad. The DSBDP-C 60 dyad or the triad were excited using a 635 nm picosecond excitation pulse (Picoquant laser diode) emitting with a pulse width < 90 ps (FWHM) and a repetition rate of 8 MHz. Fluorescence emission was recorded at two specific wavelengths, 535 nm and 655 nm, corresponding to the emission of BDP and DSBDP units respectively, using a FluoTime 200 spectrometer (4-nm bandpass) with polarizers at a magic angle. Fluorescence decays were then collected until a maximum of 10,000 counts was attained using a PicoHarp 300 TCSPC system (bin time = 4 ps). The analyses of emission decay data were done with a weighted sum of exponential decays convolved by the IRF signal measured in a lightscattering Ludox colloidal solution, using the FluoFit software (PicoQuant). The full width at half maximum (FWHM) of IRF at 495 and 635 nm excitations was equal to 40 ps and 90 ps, respectively, which can be considered as the time-resolution of the measurements. Solutions with an absorbance below 0.1 (1 cm pathlength) at excitation wavelengths were used. The quality of the fits was assessed by the value of χ 2 (< 1.1), the weighted residuals and their autocorrelation function.

Spectroelectrochemistry

Spectroelectrochemical measurements were performed with the help of an optically transparent thin-layer electrochemical cell (OTTLE) connected with a Palm-Sens4 potentiostat. The cell employs a platinum mesh as a working electrode, a platinum wire as a counter electrode and a silver wire as a quasi-reference electrode. 0.1 M of hexafluorophosphate tetrabutylammonium (TBAPF 6 ) was used as a supporting electrolyte in dichloromethane. The spectra were recorded in Varian 5000 Cary spectrophotometer. The concentration of DSBDP in dichloromethane solution was 0.2 mM.

Femtosecond transient absorption

Femtosecond UV-Vis-NIR spectra were recorded using a commercial set-up (Ultrafast systems, Helios) [33]. The fundamental 800 nm beam was split into two beams to generate (a) a pump (λ exc = 525 or 640 nm) in the optical parametric amplifier and (b) probe pulses, a white-light continuum in the UV-Vis-NIR range (300 to 1400 nm), using either a CaF 2 plate (330-660 nm) or a sapphire plate (440-780 nm) or a YAG crystal (830-1400 nm). The pump energy was 2 μJ per pulse (beam size of 250 μm FWHM). The spectra were recorded in a 2 mm path-length quartz cell under continuous stirring using a Teflon-coated bar. All measurements were performed in argon degassed solutions at room temperature. All the samples were scanned bi-directionally to ensure data reproducibility. The transient absorption data were corrected for the chirp of the white-light continuum using the Surface Xplorer program, supplied by the Ultrafast systems. The instrument response function (IRF) was around 200 fs (FWHM). Notably, due to the strong benzonitrile solvent contribution within 2 ps time windows, a precise assignment of the shortest lifetime close to 1 ps could not be ascertained.

Results and discussion

Steady-state absorption and emission

The steady-state absorption and emission studies of the triad and the two reference dyads have recently been reported [31]. Briefly, the BDP-C 60 and DSBDP-C 60 dyad absorption spectra show maxima at 535 and 637 nm identical to those of the parent BDP and DSBDP moieties, respectively (Fig. 1a,b). Furthermore, a strong band at 330 nm and a small sharp band at 428 nm correspond to the absorption of the methanofullerene platform. The absorption spectrum of the BDP-C 60 -DSBDP triad (Fig. 1c) overlaps well with the sum of the absorption spectra of BDP, DSBDP and C 60 [31]. Therefore, these observations infer a negligible communication between the two BODIPY photosensitizers and the C 60 acceptor units in the electronic ground state [31]. The ground-state absorption of the triad shows a good coverage in the entire visible range due to the presence of two differently functionalized BODIPY units along with the C 60 moiety (Fig. 1c).

Upon excitation at 495 nm and 605 nm, respectively, the two BDP-C 60 and DSBDP-C 60 dyads show a significant quenching of the fluorescence intensities as compared to their parent BODIPYs (quantum yield decrease of 98 and 92%, respectively). These observations suggest efficient energy and/or electron transfer events from the BODIPY excited state to the fullerene moiety [31]. The steady-state fluorescence spectrum of the triad after the excitation of the BDP unit (λ exc = 495 nm) reveals two emission bands with maxima at 537 nm and 650 nm corresponding to the emission of BDP and DSBDP, respectively. The intensity of the 537 nm emission band of the BDP in the triad is more strongly reduced (quantum yield decrease of over 99%) as compared to the BDP-C 60 dyad. The significant emission quenching of BDP points out the photoinduced processes similar to those observed in the case of the BDP-C 60 dyad with an additional energy transfer from 1 BDP* to DSBDP as previously demonstrated [31]. Furthermore, this was confirmed by recording the excitation spectrum of the triad at the emission of DSBDP moiety (λ em = 670 nm). The excitation spectrum revealed the contribution of the BDP moiety (λ abs = 525 nm) in addition to that of DSBDP (λ abs = 637 nm) (SI Fig. 1) [31].

Spectroelectrochemical measurements (SEC)

The electrochemical properties of the dyads and the triad were previously studied by cyclic voltammetry [30]. To help assign the spectroscopic transient signals, spectroelectrochemical measurements were performed on DSBDP and BDP. The measurements were carried out in dichloromethane since these compounds display an entirely reversible oxidation process in this solvent. Spectral changes owing to one-electron oxidation of BDP recorded at varying oxidation potentials (1 to 1.3 V) are shown in Fig. 2a. The SEC spectra of oxidized BDP radical cation (BDP •+ ) reveal a new band at 415 nm. Furthermore, the SEC absorption bands for one-electron oxidized DSBDP radical cation (DSBDP •+ ) show up at 429, 458, 542 nm and an intense band at 680 nm upon gradually increasing the oxidation potential from 0.7 to 1.1 V (Fig. 2b). Also, the SEC absorption spectra of one electron reduced fullerene moiety (C 60 •-) was previously reported, showing a sharp absorption band at about 1035 nm [24].

Time-resolved emission

To probe the kinetics of photophysical processes from excited BODIPYs to C 60 , time-correlated single-photon counting studies were performed upon photoexcitation of BODIPY units in the two dyads and the triad. The fluorescence decay profiles are presented in Fig. 3 and the corresponding lifetimes are detailed in Table 1. In benzonitrile, the excitation of BDP and DSBDP units leads to a monoexponential fluorescence decay giving a lifetime of 4.57 and 4.46 ns, respectively [31], typical of the BODIPY singlet excited state [34]. However, the fluorescence decays of BDP and DSBDP in the two dyads are more complex and are satisfactorily fitted with three exponentials. Multiple time constants obtained in a similar reported BODIPY-C 60 dyad were attributed to the presence of different conformations due to the flexible linker between donor-acceptor moieties [24]. The flexibility allows for different donor-acceptor distances and therefore photoinduced processes (electron and/or energy transfer) of different kinetics are possible in such assemblies. The two short lifetimes of the order of hundred picoseconds with high contributions in the two dyads can be attributed to the energy or electron transfer processes in conformers with relatively short donor-acceptor distances. Furthermore, the long lifetime of several nanoseconds with a very small contribution (less than 1%) can be due to an extended conformer with negligible interaction between the BOD-IPY and C 60 units, as previously reported in a flexible porphyrin-fullerene dyad [35].

Upon exciting the BDP unit in the BDP-C 60 -DSBDP triad (λ exc = 495 nm), the decay of the BDP unit (λ em = 535 nm) can be satisfactorily fitted with a four exponential model. The three short components can be related to electron or energy transfer processes from 1 BDP* to C 60 as in the BDP-C 60 dyad, in addition to a supplement excitation energy transfer from 1 BDP* to DSBDP in a conformer with a short BDP-DSBDP distance. Furthermore, a very low contribution of the nanosecond lifetime (< 1%) can be again associated with an extended conformer with a long distance between BDP and both the DSBDP and C 60 moieties. On the other hand, the decay of the DSBDP unit (λ em 123 1). These observations indicate that 1 DSBDP* once formed after excitation energy transfer from 1 BDP* in the triad or from a direct excitation follows the same relaxation pathways as that of 1 DSBDP* in the DSBDP-C 60 dyad.

Femtosecond transient absorption spectroscopy

Femtosecond transient absorption (TA) spectra were recorded to characterize the early photophysical events taking place in the BDP-C 60 -DSBDP triad and the two associated dyads (BDP-C 60 and DSBDP-C 60 ) upon selective excitation of chromophore units. At the outset, TA spectra were measured for BDP-C 60 and DSBDP-C 60 dyads by selectively exciting the BDP and DSBDP units at 525 and 640 nm, respectively, in benzonitrile.

Then, the BDP-C 60 -DSBDP triad TA spectra were recorded at the same selective excitation wavelengths.

BDP-C 60 dyad

Femtosecond transient absorption spectra of the BDP-C 60 dyad were recorded over a range of 330-1450 nm, detailed in four different time windows until 2460 ps (Fig. 4). During very short pump-probe delays from -0.3 ps to 0.4 ps, as depicted in Fig. 4a, the rising of positive signals at 344 nm, 427 nm in visible and at 1278 nm in the NIR region is coupled with a negative broad absorption band at about 495-550 nm. This negative transient absorption band is associated with the depopulation of the BDP ground-state and BDP stimulated emission (inverted absorption and emission spectra in black and grey curves, respectively, are shown in Fig. 4a). These spectral features are characteristic of the formation of 1 BDP* as already observed in a formerly reported BODIPY-C 60 dyad [24]. Moreover, the rising of another transient absorption band at 1045 nm is associated with the formation of C 60 •-(C 60 •-signature obtained by SEC shown in Fig. 4a; pink curve) [24], suggesting an ultrafast electron transfer within the IRF, from the Franck-Condon zone of 1 BDP* to C 60 . Then, from 0.5 to 100 ps, the increase and narrowing of the C 60 •-band in the NIR region can be explained by a vibrational cooling of C 60 •-. Furthermore, a decay in 1 BDP* signals is coupled with the rise of a new band at 916 nm which is attributed to the 1 C 60 * absorption (the 1 C 60 * reference spectrum is shown as a black curve in Fig. 4c) [36]. This is related to an energy transfer from 1 BDP* to C 60 .

The third temporal window from 100 to 300 ps exhibits a subsequent decrease in 1 BDP* absorption together with the ground-state recovery and a second rise of the band at 916 nm of 1 C 60 * while the band at 1045 nm related to C 60 •-remains constant (Fig. 4c). It should be noted that the absorption spectra of both the species ( 1 C 60 * and C 60 •-) largely overlap in the NIR region. Thus, this behavior suggests a charge recombination process causing the depopulation of C 60 •-with a simultaneous population of the 1 C 60 * due to a second energy transfer.

In the final time window above 300 ps, the bands for 1 BDP* and 1 C 60 * decay, and a new band rises at 710 nm which is attributed to 3 C 60 * (see the dark green reference spectrum in Fig. 4d) [24, 31]. The formation of 3 C 60 * is explained by an inter-system crossing from the 1 C 60 *.

Kinetic traces at some specific wavelengths representing the transient species described above are presented in SI Fig. 2. The chosen probe wavelengths at 427, 490, 550 and 1278 nm correspond to 1 BDP*. 916 and 1045 nm represent 1 C 60 * and C 60 •-while 710 nm corresponds to the triplet excited state of C 60 [18, 31]. These kinetic traces can be satisfactorily fitted by a four 123 and DADS in Fig. 5b) is attributed to the ultrafast formation of C 60 radical anion. Then, the time constant of about 5 ps obtained by global fitting is interpreted as vibrational cooling of C 60 •-, initially formed with excess energy. Furthermore, the 42 ps and 197 ps time components appear as decay times in the fitting of the time traces corresponding to the 1 BDP* and as rising times in the fitting of those of 1 C 60 *. These time constants are also close to the fluorescence lifetimes obtained by TCSPC (40 and 230 ps, Fig. 2 and 123 Table 1). Therefore, these observations can be associated with energy transfer processes from 1 BDP* to C 60 occurring in two BDP-C 60 conformers. From the DADS shown in Fig. 5b, the 42 ps time constant can also be related to the charge recombination (positive band of C 60 •-at 1045 nm and of BDP •+ at 415 nm). Finally, the 1.4 ns time constant is attributed to the intersystem crossing (ISC) from 1 C 60 * to 3 C 60 * (positive band of 1 C 60 * at 916 nm and a negative band of 3 C 60 * at 710 nm in DADS). The population of 3 C 60 * was confirmed by nanosecond transient absorption measurements (SI Fig. 3a) in agreement with the recent report [31]. Figure 6 summarizes the photoinduced electron or energy transfer processes in the BDP-C 60 dyad. Different photoinduced processes can be related to different families of conformers that were already observed for a similar dyad [24]. It is to be noted that the analyses were complex since C 60 •-and 1 C 60 * show very similar absorption features in the NIR region. Therefore, the electron/energy transfer processes in each family of conformers highlight the dominant relaxation pathways. The ultrafast electron transfer process (< 1 ps) is associated with a conformer with the shortest distance between BODIPY and C 60 units (named Conformer 1). Furthermore, the two energy transfer processes occurring in about 40 and 200 ps are related to the two conformers (named Conformers 2 and 3, respectively), with intermediate donor-acceptor distances. In addition, it should be reminded that a conformer with a long distance between BDP and C 60 (named conformer 4) with nanosecond fluorescence lifetime was observed due to negligible electron and energy transfer processes.

DSBDP-C 60 dyad

First, femtosecond transient absorption measurement was performed for the parent DSBDP upon excitation at 640 nm (SI Fig. 4). Similar transient spectra were observed at all delay times corresponding to only one excited state 1 DSBDP*. The transient spectra show two negative bands at 640 and 720 nm which correspond to the ground-state bleach and the stimulated emission, respectively, as well as excited-state absorption bands at 455, 855 and 1000 nm. The fitting of kinetic traces (SI Fig. 4) gives two time constants of (0.9 ± 0.1) ps and (4 ± 1) ns, which are attributed to the vibrational cooling followed by the relaxation of the singlet-excited state.

Upon exciting DSBDP unit of the DSBDP-C 60 dyad at 640 nm, the femtosecond transient absorption spectra also exhibit the rise of bands at 455, 855 and 1000 nm until 0.4 ps (Fig. 7a). These spectral features are the same as the spectrum of the parent DSBDP at 0.4 ps (dashed, blue curve in Fig. 7a), therefore, supporting the selective excitation of the DSBDP unit at 640 nm and the formation of 1 DSBDP*.

From 0.4 ps to 20 ps, the decay of the 1 DSBDP* band at 455 nm is accompanied by the concomitant rise of new bands at about 687 and 1045 nm. The band at 1045 nm corresponds to the absorption of the C 60 radical anion as previously shown in the BDP-C 60 dyad study. The one at 687 nm is attributed to the DSBDP radical cation (DSBDP •+ ), since its absorption corresponds to the DSBDP •+ signature obtained by spectroelectrochemistry (DSBDP •+ SEC in Fig. 7b; dashed, brown curve) and the chemical oxidation of DSBDP-C 60 dyad with FeCl 3 .6H 2 O (SI Fig. 5). This observation suggests the photoinduced electron transfer from 1 DSBDP* to C 60 .

In the third time regime from 20 to 300 ps (Fig. 7c), charge-separated state bands at 687 and 1045 nm decay, indicating the charge recombination process. Moreover, the bands at 455, 855 and 1000 nm continue to decrease and can be related to additional photoinduced energy transfer processes from 1 DSBDP* to C 60 . However, the rising band of 1 C 60 * cannot be observed due to the spectral overlap of the two decay species 1 DSBDP* and C 60 •-with that of the rising of 1 C 60 * in NIR (800-1000 nm).

In the final time panel above 300 ps (Fig. 7d), the transient bands corresponding to 1 DSBDP* continue to decay. An additional shoulder is still observed above 1000 nm that could be assigned to the spectrum of 1 C 60 *. At the last delay time, transient bands at about 710 and 1050 nm are observed. As shown by the reference spectra of 3 C 60 * obtained from femtosecond TA measurement of pristine C 60 -platform at 2.4 ns and of 3 DSBDP* from nanosecond TA at 5 ns in Fig. 7d, these bands could be assigned to the formation of both 3 C 60 * and 3 DSBDP* at the end of probed time window. The formation of 3 C 60 * is explained by inter-system crossing from 1 C 60 *.

Nanosecond transient absorption measurement with a selective excitation of DSBDP unit at 640 nm revealed transient absorption bands corresponding to 3 DSBDP* (SI Fig. 3b, red curve) and the kinetics could be fitted mono-exponentially without any contribution of 3 C 60 * lifetime. These observations have already been detailed previously [31]. Therefore, although a mixture of 3 DSBDP* and 3 C 60 * was observed at the end of the femtosecond measurement window, the final excited state is 3 DSBDP* populated from an energy transfer from the higher energy 3 C 60 *. These findings point out the ping-pong energy transfer between C 60 and DSBDP as already observed by Huang et al. [37].

As with the BDP-C 60 dyad, the kinetic fitting was performed at several specific wavelengths corresponding to the absorption of the transient species. The attribution of the transient species relative to these wavelengths is summarized in SI Table 2 andtheir 9). It should be noted that, unlike the BDP-C 60 dyad, no ultrafast electron transfer (< 1 ps) from 1 DSBDP* to C 60 was observed. This can be explained by the fact that the electron transfer is less thermodynamically favorable in the DSBDP-C 60 dyad (ΔGcs = -0.54 eV versus -0.70 eV for the BDP-C 60 dyad).

The DADS of the second time constant (87 ps) presents the decay of the charge-separated state species corresponding to the charge recombination. Furthermore, in the same DADS, a decay of 1 DSBDP* is also observed. It can be related to a parallel process of energy transfer in a second conformer with an intermediate distance between DSBDP and C 60 (named Conformer 2, Fig. 9). This time component is in a decent agreement with the fluorescence lifetime of 1 DSBDP* (τ 1 = 150 ps, Fig. 3. and Table 1).

The DADS of the last time constant of 874 ps exhibits a decay feature of 1 DSBDP* with typical absorption at 455 nm. This decay time agrees well with the results of the individual fit (SI. Table 2) as well as with the fluorescence lifetime of 730 ps (Fig. 3 and Table 1). Simultaneously, a negative band in DADS at about 916 nm corresponds to the rise of 1 C 60 *, indicating a second energy transfer from 1 DSBDP* to C 60 in a more extended conformer (named Conformer 3). In addition, the DADS also presents a rising of 3 C 60 * and 3 DSBDP* features at the 700 nm region, which are in agreement with the rising component in the individual fits (see 687 nm fit of SI Table 2). This can be related to the formation of both 3 C 60 * and 3 DSBDP* by the ping-pong mechanism as previously explained. Figure 9 puts forward all the dominant photoinduced processes in the DSBDP-C 60 dyad.

BDP-C 60 -DSBDP triad

The femtosecond TA spectra of the DSBDP-C 60 -BDP triad at the excitation wavelength of 525 nm corresponding to the absorption of BDP are shown in Fig. 10. In the first time window from -0.3 to 0.4 ps (Fig. 10a), in addition to the rising of the bands corresponding to the 1 BDP* and the C 60 •-as in the BDP-C 60 dyad, TA spectra of the triad also show the appearance of weaker bands of 1 DSBDP* (ground-state bleach (GSB) at 650 nm) due to the direct excitation of DSBDP at 525 nm (ε (DSBDP, 525 nm) = 4000 M -1 .cm -1 ). Since no ultrafast electron transfer (< 1 ps) from DSBDP to C 60 was observed for the DSBDP-C 60 dyad, the observation of C 60 •-reveals an ultrafast electron transfer from the Franck-Condon zone of 1 BDP* to C 60 , similar to the BDP-C 60 dyad (Fig. 11).

In the second time window from 0.4 to 30 ps, a rapid recovery of BDP GSB with the concomitant increase of DSBDP GSB and the rise of 1 DSBDP* absorption bands at 455, 855 and 1000 nm are detected (Fig. 10b). These observations reveal the rapid excitation energy transfer from 1 BDP* to DSBDP. Then, 1 DSBDP* once formed follows the same sequence of photophysical events as observed in the DSBDP-C 60 dyad. Briefly, the bands at 687 and 1045 nm corresponding to DSBDP •+ and C 60 •-continues to rise, due to electron transfer from 1 DSBDP* to C 60 as observed in the DSBDP-C 60 dyad. The charge-separated state (DSBDP •+ -C 60 •-) recombines in the third time-period from 30 to 300 ps (Fig. 10c) as evident by the decay of C 60 •-at 1045 nm and DSBDP •+ at 687 nm. Further, the absorption bands of 1 DSBDP* in the visible and NIR part decay due to the energy transfer process to C 60 as already described for the DSBDP-C 60 dyad. In the same time window, the recovery of the GSB of BDP suggests an energy transfer process to C 60 consistent with the observations in the isolated BDP-C 60 dyad.

Finally, in the fourth time panel above 300 ps, as in the DSBDP-C 60 dyad, a second energy transfer in a more extended conformer causes a decay of 1 DSBDP* bands along with a recovery of GSB of DSBDP. The TA spectra of last delay times show the formation of a mix of 3 C 60 * and 3 DSBDP* which was previously observed for the DSBDP-C 60 dyad. In the triad also, the final excited state is 3 DSBDP* with the lifetime of (280 ± 20) μs, which was confirmed by nanosecond transient absorption measurements (SI Fig. 3b, green curve) [31].

Kinetic analyses at specific wavelengths converged with a four-exponential model (SI Fig. 7). The analyses of the transient decays is sophisticated for the triad due to the overlap of different transient spectra ( 1 BDP*, 1 DSBDP*, BDP •+ , DSBDP •+ , C 60 •-, 1 C 60 *, 3 C 60 * and 3 DSBDP*) and the occurring of simultaneous photophysical processes (electron/energy transfer) in similar timescales. The time constants obtained from the fit of several probe wavelengths corresponding principally to some transient species are detailed in SI Table 3. Moreover, the global fitting of kinetic traces converged with four time constants of < 1 ps, 7, 68 and 595 ps. The DADS associated with the shortest time constant of < 1 ps shows a rising component at the absorption of C 60 •-, which is attributed to the ultrafast electron transfer from 1 BDP* to C 60 in a short-distance conformer between BDP and C 60 , as observed in BDP-C 60 dyad. Moreover, an increase in DSBDP GSB at 650 nm is observed and can be assigned to an additional ultrafast energy transfer from 1 BDP* to DSBDP. This rising component is also obtained in the individual fit at the GSB band of DSBDP at 660 nm.

Furthermore, the time constant of 7 ps could be related to two simultaneous processes. First, an additional increase in the DSBDP GSB band (positive band at 650 nm) is observed due to a second energy transfer from 1 BDP* to DSBDP, which was previously demonstrated in steady-state spectroscopy [31]. Second, a simultaneous rising of the C 60

•-band (negative band 123 at 1045 nm) reveals an electron transfer from 1 DSBDP* to C 60 .

Next, the DADS of 68 ps presents positive bands at 687 nm (DSBDP •+ absorption) and 1045 nm (C 60 •-) which could be assigned to the decay of the chargeseparated state (DSBDP •+ -C 60 •-). It should be noted that similar time constants for charge separation and recombination were obtained in the DSBDP-C 60 dyad. Moreover, at the same time, the decay of BDP GSB at 525 nm is related to an energy transfer from 1 BDP* to C 60 which is consistent with a time constant of about 50 ps obtained by individual fitting of the kinetic trace at 550 nm (BDP GSB). It should be reminded that 42 ps was obtained in the BDP-C 60 dyad for the same process. This time component is also found in the fluorescence lifetime of 1 BDP* (Table 1) upon excitation at 495 nm. Also, the fluorescence decay of the triad shows a lifetime of 270 ps for DSBDP, which can be correlated to an energy transfer process from 1 DSBDP* to C 60 . This component was also obtained in the individual fitting of the kinetic traces corresponding to the 1 DSBDP* (455 nm) and GSB DSBDP (660 nm) (SI Table 3), confirming the energy transfer process with a time constant of about 200 ps.

Subsequently, the last time constant of 595 ps is attributed to two simultaneous processes. A second energy transfer from 1 DSBDP* to C 60 is evident by the decays of the 1 DSBDP* band at 455 nm and DSBDP GSB at 660 nm in the DADS. This time constant also agrees with the second fluorescence lifetime of DSBDP in the triad (880 ps, Table 1). The DADS also indicate the population of the triplet state of both 3 C 60 * and 3 DSBDP* at about 710 nm, revealing the ISC of 1 C 60 * and the ping-pong energy transfer between C 60 and DSBDP as explained in Sect. 3.4.2.

Figure 12 summarizes all these photoinduced processes observed in the triad after excitation of BDP at 525 nm. The flexibility of the linkers leads to the presence of different families of conformers and consequently to multiple parallel processes. The accurate determination of the time constants for all these processes is impossible. The values shown in Fig. 12 are indicative and were obtained by the mean of different time-resolved techniques used in this study.

Furthermore, femtosecond transient absorption measurements were also performed upon excitation of the DSBDP unit at 580 nm in the BDP-C 60 -DSBDP triad. The femtosecond transient absorption spectra (SI Fig. 8) along with the individual and global fit are presented in SI Fig. 9 and SI Fig. 10, respectively. Almost identical spectral and kinetic features are observed as compared to those of the DSBDP-C 60 dyad. This indicates similar relaxation pathways as that of the dyad when 1 DSBDP* is formed either by direct excitation or by an energy transfer from 1 BDP*.

Conclusion

We have elucidated the mechanistic details of complex photophysical processes within a broadband visiblelight-absorbing BDP-C 60 -DSBDP triad together with the two reference BDP-C 60 and DSBDP-C 60 dyads. The photophysical characterization using ultrafast transient absorption and time-resolved emission methods revealed the presence of multiple conformations due to the flexible linker between the two BODIPY and C 60 units. In the BDP-C 60 dyad, an ultrafast electron transfer leads to the formation of a radical ion pair in a short BDP-C 60 distance conformer. Additionally, two energy transfer processes from 1 BDP* to C 60 were observed in two other conformers. In the DSBDP-C 60 dyad, a rapid electron transfer and two energy transfer processes were also observed in conformers of different donor-acceptor distances. In the case of the triad, additional energy transfer processes from 1 BDP* to DSBDP were observed upon excitation of the BDP unit. Furthermore, 1 DSBDP*, once formed, followed the same relaxation pathways as that of the DSBDP-C 60 dyad. The present triad in which the BODIPYs act as a lightharvesting antenna as well as electron donor contributes to the development of artificial molecular systems capable of mimicking photosynthetic "antenna-reaction center". For all these D-A systems with a flexible linker, a rapid photoinduced electron transfer from BODIPY to C 60 was evidenced in conformers with close proximity of the D and A moieties, with a short lifetime of the charge-separated state. This will be taken into account for the design of the next generations of BODIPY-based D-A systems. Works are under progress in our laboratory to extend the lifetime of the charge-separated state by adding supplementary electron donor as ferrocene. With the scarce reporting on the ultrafast processes in covalently linked flexible systems, the intriguing photophysics of this broadband energy harvesting triad will provide useful fundamental insights for the development of covalently linked donor-acceptor assemblies for solar energy applications. 1. Time constants obtained by fitting femtosecond transient absorption kinetics at specific wavelengths s in BDP-C60 dyad, λexc = 525 nm. (r): rising time, (d): decay time; a ultrafast electron transfer from 1 BDP* to C60 from Franck-Condon zone b vibrational cooling of C60• -from Franck Condon region, c energy transfer from 1 BDP* to C60, d second energy transfer from 1 BDP* to C60, e formation of triplet excited state 3 C60 by ISC. # Note: Time constant of <1 ps could not be retrieved with a precision due to strong signal from benzonitrile. Following these early reports, several publications highlighted the cascade PET mostly in porphyrins linked to quinones or imide derivatives 77,78,79 . As mentioned previously, fullerenes can accelerate charge separation while retarding recombination in D-A systems due to their low reorganization energy (λ): many molecular architectures have thus been reported with fullerenes as acceptor moieties in place of quinones 11,80,81 . Besides, additional donor components such as amines 82 , tetrathiofulevene 83 , and ferrocene 4,84,85 have frequently been covalently linked to photosensitizers to realize cascade PET. Among these, owing to its unique photochemical and electrochemical properties, ferrocene (Fc) is commonly used as an electron donor species. Indeed, ferrocene can undergo a simple, reversible one-electron oxidation reaction (0.4 V vs. SCE in acetonitrile) 4 . Numerous ferrocene-fullerene-based D-A systems have been reported in the literature showing a stabilized charge-separated state with a lifetime in the order of nanoseconds 86,87,88 . However, these systems suffer from weak absorption in the visible spectral range, which necessitates the incorporation of a chromophoric unit in the framework. In this perspective, BODIPYs owing to their remarkable photophysical properties To get the mono and bis-Fc-BDP-C60 triad (α or β positioned), the strategy of the ''postfunctionalization'' of the Fc-BDP dyad with the C60 unit was followed. In this dissertation, we have focussed on β positioned mono and bis-Fc-BDP-C60 triad along with their reference dyads (mono and bis-Fc-BDP). We have photophysically characterized them. Future efforts within the cooperation of the two laboratories will examine the α-positioned analogs. The molecular structures of mono and bis-Fc-BDP-C60 triads are presented in Fig. 3.2.3.

SI Table

The results of this work are presented in this sub-chapter, which highlights photoinduced processes upon excitation of the BODIPY unit within Fc-BDP-C60 conjugates. Steady state absorption and emission, time-resolved emission, and ultrafast transient absorption spectroscopies were used to characterize the charge transfer dynamics within these systems. weaker band at 400 nm typical of the BODIPY-centered π-π* S0→S1 and S0→S2 transitions, respectively 24 (see Annexe A, Fig. A1a). The spectra of the dyad and the triad with single conjugated ferrocene exhibit absorption bands at 525 and 400 nm, similar to their reference compound (BDP-EtPh) (Fig. 3.2.4a). However, the 525 nm band is broadened as compared to BDP-EtPh due to the overlap of the intramolecular charge transfer (ICT) band from Fc to BDP 92 . In the triad, additional absorption features are observed: a strong band at 330 nm and a small sharp band at 428 nm corresponding to C60 platform (Fig. 3.2.4a) 93 . It is to be noted that the absorption of Fc moiety gives negligible contribution due to its low molar absorption coefficient (61 M -1 cm -1 at 322 nm) 94 .

In the case of bis-ethynylphenyl-BODIPY (EtPh-BDP-EtPh) which serves as the reference compound for bis-Fc-BDP analogs, π-π* S0→S1 and S0→S2 absorption bands appear at 575 and 400 nm, respectively (see Annexe, Fig. A1a). Compared to the BDP-EtPh, both S0→S1 and S0→S2 transition bands are bathochromically shifted by 35 nm due to the extended πconjugation of the additional ethynyl phenyl group. In line with this, bis-Fc-BDP dyad and the triad show similar red-shifted S0→S1 and S0→S2 transition bands (575 nm) as compared to that of mono-Fc-BDP analogs (50 nm and 20 nm, respectively), owing to the more significant extended π-congugation when the second ferrocene was functionalized on the Fc-BDP core (Fig. 3.2.4a). Moreover, this band is comparatively broader than the one in mono-Fc-BDP compounds. These findings can be attributed to an additional ICT between the second Fc donor and the BDP acceptor. These results demonstrate the robust electronic interaction between the BDP and Fc entities, which became even more evident after the BDP backbone was modified to include two ferrocene donor units.

Upon exciting the S0→S1 transition band of the BODIPY unit, intense emission is observed for the reference compounds BDP-EtPh and EtPh-BDP-EtPh at about 575 and 610 nm, respectively with fluorescence quantum yield values ϕF of more than 70 % (see Annexe A, Fig. A1b).

However, strong quenching of fluorescence was observed for mono-Fc-BDP dyad and its corresponding fullerene triad (Fc-BDP-C60), reflected by the small ϕF values of 2.2 and 1.4%, respectively (Fig. 3.2.4b). This strong emission quenching can be attributed to the photoinduced processes between the excited state of BDP and the Fc and C60 units. In the case of bis-ferrocene substituted analogs, the emission quenching was so significant that no emission bands were visible, as compared to the mono ferrocene substitutes. Fluorescence decay of the reference compound can be fitted by a monoexponential function with a lifetime of 3.4 ns. were satisfactorily fitted with three exponentials yielding one very short principal time-constant of several picoseconds (> 96%), which is below the instrumental response function. This short component indicates some rapid electron and/or energy transfer events taking place in the excited state of the BODIPY moiety. Further, the greater contribution of this short component in the fluorescence decay of bis-Fc-BDP-C60 triad as compared to that of mono-Fc-BDP dyad is in line with the lower fluorescence quantum yield of the former, indicating additional deactivation pathways due to the added C60 moiety in the triad.

Photoinduced processes will be further elucidated by femtosecond transient absorption. Table 1 summarizes the time-constants obtained by TCSPC. Femtosecond TA spectra were recorded to investigate the relaxation pathway in the Fc-BDP conjugates and their corresponding fullerene triad and tetrad in benzonitrile by selectively exciting the BDP unit (λexc = 580 nm). Following 3 ps, the TA bands corresponding to the BDP radical anion (440, 550 and 1100 nm) decay to simultaneously form two new TA bands at 450 and 650 nm. These latter bands can be attributed to the triplet excited state bands localized on BDP 37 . It implies that the Fc ), which recombines in 26 ps to populate the triplet of the bis-Fc-BDP dyad. The latter then decays to the ground state in about 400 ps. In NIR region, the probe wavelength of 1150 nm is related to the overlapping absorption of 1 BDP* and BDP .-, and the decay at this wavelength could be satisfactorily fit with two time-constants. The first time-constant of about 2 ps is attributed to the decay of 1 BDP* while 25 ps corresponds to the recombination of the chargeseparated state (Fc .+ -BDP .-). Further, the femtosecond TA spectra and fitting of the kinetic traces of mono-Fc-BDP dyad are presented in Fig. A2 and A3 and the fitting parameters are presented in Table A2 (Annexe A).

The overlap of the kinetics of the mono and bis-Fc dyads at specific wavelengths corresponding to the 1 BDP* and BDP anion, is presented in Fig. A4 These observations suggest that although the addition of two Fc units on the BDP moiety leads to a broader absorption of the visible light due to extended conjugation and enhances the electronic coupling between Fc and BDP, it has no significant effect on the charge-separation and relaxation dynamics. The relaxation pathway of the mono or bis-Fc-BDP dyad is summarized in Fig. 3.2.9. The energy of the charge separated state (Fc .+ -BDP .-) was calculated using the electrochemical data reported in the thesis of Dr. Jad Rabah (Fc 0/+ = 0.17 and BDP 0/- = -1.51) 95 . The fitting of kinetic traces at the characteristic probe wavelength of 550 nm (BDP anion absorption and ground state bleach of BDP) converged with three time constants: 3 ps, 25 ps and 400 ps (Fig. Notably, the short lifetime of about 2 ps (see Table A2) corresponding to the electron transfer from Fc to excited BDP is close to the short emission lifetime (τ = 9 ps) obtained by TCSPC (see Fig. 3.2.5 and table 3.2.1). An overlap of the kinetics of the mono and bis-Fc-BDP-C60 triads at specific wavelengths corresponding to the 1 BDP*, BDP and C60 anions is presented in Fig. A7. The overlap exhibits very similar dynamic features in the two analogs, suggesting the photoinduced processes remain indifferent of the number of β substituent ferrocenes. The relaxation pathways upon photoexcitation of mono or bis-Fc-BDP-C60 triad are summarized schematically in Fig. 3.2.12. The energy of the charge separated state (Fc .+ -BDP .-) was calculated using the electrochemical data reported in the thesis of Dr. Jad Rabah (Fc 0/+ = 0.11 and BDP 0/-= -1.03) 95 .

Conclusion

Photoinduced electron transfer processes in β-positioned mono and bis Fc-BDP-C60 triads along with their corresponding Fc-BDP dyads are reported here. Due to the flexible linker between BDP and C60, the triads exhibit electron transfer in two conformers depending on the relative distance between Fc-BDP and BDP-C60. In one conformer, electron transfer from Fc to BDP is preferred leading to a short-lived ion-pair, which recombines via populating 3 BDP* and finally 3 Fc*. However, the charge separated pair Fc-BDP .+ -C60 .-formed by an ultrafast electron transfer from BDP to C60 in another conformer with short distance between BDP and C60 undergoes a rapid charge arrangement to ultimately form the charge separated state Fc .+ -BDP-C60 .-with a lifetime of about 400 ps, which is much longer than that of the isolated BDP-C60 dyad recently reported by our group with a lifetime of only 40 ps. Therefore, adding one or two Fc moieties on the BDP-C60 core has resulted in the lengthening of the charge separated state by over ten times due to electron/charge shift mechanism.

Therefore, in Section 3.2, we have investigated a second family of molecules, in which one or two ferrocene units were attached at the β position of the BDP-C60 dyad (the same dyad from the Section 3.1). Photophysical investigations were carried out by using femtosecond TA and TCSPC on β-mono and bis Fc-BDP-C60 triads along with the corresponding mono and bis-Fc BDP dyads. The work reveals that firstly, the addition of one or two ferrocenes broadens the absorption of the BDP unit due to the extended conjugation and the overlapping charge transfer band. And secondly, electron transfers in the two conformers (from Fc to BDP to form Fc .+ -BDP .--C60 ; and from BDP to C60 to form Fc-BDP .+ -C60 .-) are followed by a rapid charge arrangements to form a spatially separated radical ion-pair (Fc .+ -BDP-C60 .-). The latter exhibits a lifetime of about 400 ps which is much longer than those in the corresponding dyads (BDP-C60 and Fc-BDP), in which ion pairs recombined in about 40 ps. Thus, the strategy of appending one or two additional donor units on the isolated BDP-C60 dyad elongated the lifetime of the charge-separated pair by manifolds.

In conclusion, studying the photophysics of these elegant molecular systems not only adds up to our fundamental knowledge regarding the light-induced behaviour in these flexible D-A systems but also demonstrates some interesting potential applications. The panchromatically absorbing BDP-C60-DSBDP triad can be used in specific applications where a long lifetime of triplet state is desirable (photodynamic therapy, TTA-UC etc.). Further, charge separated state lifetime in BDP-C60 dyad could be enhanced by adding Fc unit, making these systems potentially important when immobilized on the surface to design photovoltaic systems. The same strategy of adding Fc can be applied to the BDP-C60-DSBDP triad to obtain a system that will entail a long charge separation while absorbing in the entire visible spectrum. The demand for clean, renewable energy alternatives that could replace traditional fossil has drawn even more research attention to the direct production of fuel by utilizing solar energy.

One of the most extensively researched field in artificial photosynthesis is solar-driven water splitting into its constituent parts, H2 and O2 1 . At the same time, using H2 and O2 in fuel cells or combustion will release energy and produce clean water as the only byproduct, resulting in a perfect sustainable energy system 2 . Solar-driven water splitting, which is a four-electron and four-proton process, is typically studied in two half-reactions: the hydrogen evolution reaction (HER) 3 and the oxygen evolution reaction (OER) 4 , where the reduction of protons to H2 and the oxidation of H2O to O2 take place. Water splitting process is multielectronic in nature which usually requires a positive Gibbs free energy (ΔG 0 = 237.13 kJ/mol) 5 , and thus, does not occur spontaneously leading to an overall low conversion efficiency in plants (11%) 6,7 . Therefore, there has been a significant amount of research effort to develop efficient catalysts for each half-reaction(s) or total water splitting 8,9,10 .

Classical noble metal catalysts such as those based on iridium and ruthenium and their corresponding oxides usually show high OER and HER catalytic activity 11,12,13,14,15 but these noble metal catalysts could significantly increase the cost of real-world application. To address this, there have been efforts to develop viable and cost-effective transition-metal-based catalysts including Ni, Fe, Mo, Co-based organometallic complexes and their corresponding metal oxides/hydrides/sulfides etc 16 ,17,18 . Broadly, there are two main families of OER and HER photocatalysts: heterogeneous (more particularly, insoluble) and homogeneous, both have their own significant advantages 19 . Insoluble water oxidation catalysts: WOCs (usually metal oxides) are stable and inexpensive to produce. The advantages of homogeneous or soluble WOCs (most typically, inorganic coordination compounds) include a well-defined structural active site that increases the selectivity and high turnover number (all the catalytic active sites can participate in the reaction process). Table 4.1 summarizes the pros and cons of the homogeneous and heterogeneous catalysts.

In this regard, the development of viable photocatalysts that combine the advantages of heterogeneous metal oxide catalysts (typically stable and cost-effective) and homogeneous catalysts (typically highly active, selective, and tunable) is interesting. As an emerging class of catalysts, polyoxometalates (POMs) exhibit special properties that combine the advantages of homogeneous molecular compounds and heterogeneous metal oxide catalysts. POMs which is often referred to as metal-oxo clusters correspond to a large group of mostly anionic molecular 'electron reservoirs" built from group VI transition-metal centers in their highest oxidation state (V IV, V , Mo VI , and W VI ) assembled with oxygen atoms within a wide variety of structures and compositions 20,21,22 . POMs have been known for their vast application realm as in electrocatalysis 23 , material science 24 , and photocatalysis 25 . The majority of these uses result from their ability to accept and release electrons, suggesting their redoxactive nature 26 . According to the geometry of the POMs, two major structural families have The concept of using POMs as potential water/proton reduction catalysts goes back to the 1960's, when Pope and Papaconstantinou published a series of systematic studies on the electronic, spectroscopic, and magnetic properties of reduced heteropolytungstates and heteropolymolybdates (also known as "heteropoly blues (HPB)") 28,29,30 . Later, researchers reported that the heteropolyblues in the presence of noble metal-based co-catalysts (usually Pt) could efficiently generate hydrogen, but most of these studies were conducted under UV-light irradiation 31,32 . The efforts to develop non-noble metal-containing POM-based water reduction catalysts able to operate under visible light remained a challenge due to the low absorption of POMs in the visible region. One of the first examples of photosensitizer-POM system found to be efficient for reduction of protons is that from the group of Izzet who reported an elegant HER-active POM hybrid compound whereby two iridium complexes, playing the role of a visible light photosensitizer, were covalently linked to a Dawson unit 33 . In the same year, Lv and colleagues published another interesting example where the catalytic sites of the manganese containing POMs were coupled with visible light absorbing photosensitizer (iridium complex)

to attain a high efficiency of HER in the presence of sacrificial electron donor 34 . In the subsequent years, the same group reported two more photocatalytic systems utilizing iridium complex as a photosensitizer and nickel containing POMs 35,36 . In 2019, Mialan and coworkers reported another nickel-substituted hybrid POM for which they investigated the hydrogen yield with the increasing concentration of the POM 37 . Recently, our group also reported a novel Thio-POM assembly, constructed by sandwiching two Mo3S4 cores (electrophilic units) in between two polyoxotungstates 25 . This POM assembly was photophysically investigated using [Ir(ppy)2(dtbbpy)] + as a photosensitizer and triethanolamine (TEOA) as a sacrificial donor, highlighting the very high rate constant for the electron transfer from the reduced photosensitizer to the Thio-POM catalyst in homogeneous conditions. The catalytic efficiency of this system for HER was observed to be the highest as compared to the above-mentioned Nicontaining POM systems. Further, we studied a set of three other Keggin-type POMs with the same anionic charge and similar reduction potentials in combination with the molybdenum halide cluster [Mo6I8Cl6] 2-in acetonitrile. The study will be presented as a second part of this Chapter (part 4.2), which expands on our preliminary findings that the rate of electron transfer from cluster to POM is a function of the charge density on the POM.

The above-mentioned investigations were carried out in organic solvent and we were further interested to explore PET in cluster-POM systems in aqueous environment. It is known that the tungsten and molybdenum halide complexes are hydrolytically unstable. After dissolution of the sodium salt of the cluster in water, it undergoes hydrolysis reaction leading to the replacement of the apical Cl ligand by water molecule, according to eq. 1 and 2 shown below 43 :

[{M6X8}Cl6] 2 -+ H2O → [{M6X8}Cl5(H2O)] - + Cl - eq.1 [{M6X8}Cl5(H2O)] - + H2O → [{M6X8}Cl4(H2O)2] ↓ + Cl - eq.2
This leads to the precipitation of the cluster in water. In a study conducted by Ivanov et al., using quantitative NMR spectroscopy to determine the Cl -released in the solution, it was revealed that the hydrolysis process corresponds to the release of about 2 Cl -per cluster;

however, the kinetics depend on the core of the cluster 43 . Therefore, the authors studied the rate of hydrolysis process in molybenum and tungsten halide cluster with varying inner ligands and it was found that the hydrolysis rate follows the following tendency: {W6I8} < {W6Br8} ≈ {Mo6I8} < {Mo6Br8}. It implies that the tungsten clusters are much more stable than their equivalent molybdenum analogues in water. For this purpose, we have used an octahedral tungsten halide cluster; instead of [Mo6I8Cl6] 2-used in our previous studies. It was demonstrated in the study by Ivanov et al. that even the {W6I8} cluster precipitated after standing for over a month. Therefore, in order to further increase the stability of Na2[W6I8Cl6] in aqueous solutions, cyclodextrins (CD) were employed to encapsulate the cluster. Actually, cyclodextrins are water soluble macrocycles (Fig. 4.1.3a) which feature crown-like hydrophillic portals on the outside and hydrophobic character in their cavity, making them attractive candidates to form robust inclusion complexes with hydrophobic molecules as ''guests'' 44 . Such inclusion complexes with metal clusters as well as with polyoxometalates have been reported previously 45,46,47 (4.1.3b and c). We report photophysical investigations demonstrating that the dynamic quenching of the electron-rich cluster-based complex [Mo6I8Cl6] 2-by polyoxometalates (POMs) involves a photoinduced electron transfer from the excited cluster to the POM units. This result paves the way for the rational design of all-inorganic photocatalytic systems using cluster-based complexes as robust noble metal-free photosensitizers and POMs as catalysts.

Demand for exploiting sustainable energy stimulates the research on homogeneous systems for catalytic solar fuel generation. In this context, one of the key challenges is to develop photocatalytic systems involving robust noble-metalfree components. 1 Polyoxometalates (POMs) are electron-poor molecular species that have been identified as promising earthabundant metal-based catalysts for light-driven hydrogen evolution reaction when they are associated with sunlight harvesting unit. [2][3][4][5][6][7][8][9][10] In most of these systems, the photosensitizer corresponds to noble-metal complexes such as [Ru(bpy)3] 2+ or [Ir(ppy)2(dtbbpy)] + , where bpy stands for 2,2'bipyridine, ppy for 2-phenylpyridine, and dtbbpy for 4,4′-di-tertbutyl 2,2′-bipyridine. Although the electron transfer between the excited state of such complexes and the POM is quite efficient, using cheap and abundant element-containing photosensitizers would be highly beneficial to reduce the environmental impact and cost of these photocatalytic devices. Covalent grafting of organic dyes (e.g., boron dipyrromethene -BODIPY, porphyrin, perylene) with POMs represents an attractive way to design photoactive POM-based systems. [11][12][13][14][15] Although this approach is elegant, such hybrid functionalized POMs exhibit drawbacks such as time-consuming synthetic procedures and in many cases, unavoidable photo-bleaching under prolonged operating irradiation. Molybdenum clusterbased halide complexes [Mo6X8Y6] 2-represent a class of electron-rich species that consists of six reduced molybdenum (formally Mo 2+ ) organized in an octahedral metal cluster linked to face-capping (X) and terminal halide ligands (Y). 16,17 Valence electrons are shared between metal atoms to form metal-metal bonds. These abundant metal based compounds are able to undergo reversible one-electron oxidation process. 16,18 Moreover upon irradiation in the UV-visible region, they exhibit intense luminescence with high quantum yields and considerably long-lived (µs order) triplet state. [19][20][21][22][23] These striking physical properties suggest that [Mo6X8Y6] 2-complexes should behave as photosensitizers in solution. Pioneered work by Gray and coworkers revealed that the luminescence of the molybdenum chloride di-anionic complex [Mo6Cl8Cl6] 2-is quenched in the presence of POMs in 2 M HCl. 24 Although the authors accepted that such emission quenching is due to a photo-induced electron transfer between the two inorganic entities, they were unable to prove the formation of the charge separated states.

In this communication, we demonstrate unambiguously by nanosecond transient absorption (TA) studies that [Mo6I8Cl6] 2- is able to transfer one electron to POMs upon visible-light irradiation. The mixed halide [Mo6I8Cl6] 2-cluster complex has been selected for this study (Fig. 1A) because it exhibits higher Please do not adjust margins Please do not adjust margins quantum yield than that of the [Mo6Cl8Cl6] 2-cluster-based complex previously studied by Gray 24 (47% vs 19%). 16 Moreover, key parameters of the quenching mechanism related to the cluster-POM interplay have been highlighted by using POMs with various charge densities, nuclearities, and redox potentials, such as the Keggin-type anions ([XW12O40] n-with X = P 5+ , Si 4+ , B 3+ , (2H) 2+ (Fig. 1B) and the Dawson-type anion [P2W18O62] 6-(Fig. 1C).

The electronic absorption spectra of the [Mo6I8Cl6] 2-complex and that of the POMs (as tetrabutylammonium salts in acetonitrile) in the UV-Visible range are shown in Fig. 1D. The cluster complex exhibits typical intense absorption in nearultraviolet with a broad low-energy absorption band located at 420 nm (ɛ = 3 300 M -1 .cm -1 ) that corresponds to metal to metal transitions, from bonding to antibonding character. 25 In contrast, the POMs exhibit no or weak absorption in the visible region. Indeed characteristic ligand-to-metal charge-transfer (OW) bands of tungsten-oxo frameworks are observed in UV region. 26,27 Further, excitation at 420 nm of [Mo6I8Cl6] 2-in deaerated acetonitrile gives rise to a broad and intense emission, centered at about 700 nm (Fig. S1, ESI), consistent with that previously reported by Kitamura and coworkers. 16 Time-resolved emission measurements (Fig. S2, ESI) show that the emission at around 700 nm decays with a time constant of 165 ± 10 μs. Measurements in aerated solution reveal a substantial decrease in the excited state lifetime down to 1.5 μs due to oxygen quenching (Fig. S3, ESI), supporting that the detected luminescence arises from the triplet state. 28 Adding the Keggin anion [PW12O40] 3-to argon-saturated solution of [Mo6I8Cl6] 2-(30 µM) provokes a strong emission quenching as evidenced by the evolution of the steady-state emission intensities (Fig. 2A) and of the lifetimes (Fig. 2B). For instance, adding about six equivalents of POM reduced the emission intensity of [Mo6I8Cl6] 2-by a factor of four. Plotting I0/I and τ0/τ as a function of [PW12O40] 3-concentration, yields a linear Stern-Volmer (SV) correlation having equivalent slopes (Fig. S4, ESI and Fig. 2C) with a quenching rate constant (kq) of 9 x 10 7 M -1 .s - 1 . The values of the quenching rate constants (kq) obtained from the SV plots of the emission lifetimes for the different cluster-POM pairs are given in Table 1. The respective values (k'q) obtained by the emission intensities are shown in ESI (Table S1).

A decrease in the emission intensities and lifetimes [Mo6I8Cl6] 2- upon adding [SiW12O40] 4-, [P2W18O62] 6-or [BW12O40] 5-anions was also observed, but with a much lower magnitude (Figs. S5, S6 and S7, ESI). The intensity and lifetime SV plots obtained for these three cluster/POM systems (Fig. 2C and S4, ESI) gave lower values of quenching rate constants given in Table 1 and Table S1. Such a linear behavior in both steady-state (I0/I) and time-resolved (τ0/τ) measurements shows that the quenching of the excited state is dynamic in nature and suggests that there is negligible ground state association between the photosensitizer and the quencher. 29 From the comparison of the four studied [Mo6l8Cl6] 2-/POM tandems, it appears that the quenching constants significantly decrease with the increase of the anionic charge density (q/W) of the POM unit (q/W = global charge/number of metal atoms, Table 1). The increase of the charge density gives rise to different contributions altering the transfer of electron and/or energy between the [Mo6I8Cl6] 2- donor and the POM acceptor. The energy transfer is ruled out as there is no overlap between the absorption of the POMs and the emission of the cluster. The main effect of the charge density may lie within the increase of the electrostatic repulsions between the two inorganic anionic partners of the tandem, therefore limiting in fine the approach of the photosensitizer in the close vicinity of the quenching unit.

Another contribution to the quenching mechanism is the first redox standard potential of the POM that decreases with its anionic charge density (see cyclic voltammograms in Fig. S8 and the one electron reduction values of POMs in Table 1), leading to the decrease of the thermodynamic driving force of the electron transfer from the cluster to the POM acceptor. To highlight the key role of the charge density of POMs on [Mo6I8Cl6] 2-/POM interplay and in particular the strong influence of coulombic repulsions within the tandem, we studied the influence of the highest charged Keggin anion of the series [H2W12O40] 6-on the emission lifetime of [Mo6I8Cl6] 2-. In similar experimental conditions, the quenching process was found negligible and even could not be determined.

It must be pointed out that that the quenching constants measured for the system [Mo6I8Cl6] 2-/[PW12O40] 3-in acetonitrile are slightly lower than those reported by Gray et al. (70 x 10 7 M - 1 .s -1 ) for the tandem [Mo6Cl8Cl6] 2-/[PW12O40] 3-in aqueous solution. 19,24 Although the nature of the molybdenum halide cluster should have an impact on the quenching constant, the nature of the solvent on the quenching efficiency is also crucial. Actually, transfer between negatively charged species depends strongly on their mutual coulombic repulsions that are stronger in organic solvent exhibiting low dielectric constant, such as acetonitrile, than in water. Again, this observation supports that one important contribution of the quenching process between This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3

Please do not adjust margins Please do not adjust margins molybdenum clusters and POMs lies in the electrostatic interaction, mostly directed by the respective charge density and its repartition of the involved ionic components. a The first standard potential of the POM (V vs Fc/Fc + ) has been determined by cyclic voltammetry studies in deaerated acetonitrile containing 0.1 M TBA.PF6.

To evidence the electron transfer process from the excited state of the octahedral cluster to the POMs, nanosecond TA measurements were performed in deaerated acetonitrile. After excitation of [Mo6I8Cl6] 2-at 420 nm, a positive transient absorption band at 520 nm accompanied by a less intense band at around 650 nm was observed, which has been ascribed to the triplet excited state of the cluster (Fig. 3A). 28 The transient absorption spectral features can be satisfactorily fitted with a monoexponential decay of 170 ± 10 µs time constant, which is close to the previously measured emission lifetime.

In the presence of the Keggin-type POM [PW12O40] 3-, the nanosecond TA spectra (λexc = 420 nm) of [Mo6I8Cl6] 2-revealed a different scenario (Fig. 3B). Until 0.5 µs, the TA spectra show the spectral features of the triplet excited state of [Mo6I8Cl6] 2-. Then, the signal of the excited state decays and transforms to a new spectroscopic feature absorbing in almost the entire visible range, extending to the near-infrared part (Fig. 3B). This new broad transient absorption band is consistent with the formation of a 1 e-reduced POM, which gives rise to intense intervalence charge transfer (IVCT) band spreading until the NIR domain. The formation of the reduced POM species results from an electron transfer from the cluster to the POM leading to the charge-separated species {oxidized cluster-reduced POM} observed at 200 µs. Beyond 200 µs, the transient absorption signal decays and no change in the spectral shape is observed, reflecting the charge recombination process. Globally, fitting the kinetic traces with a two-exponential model gives two time components of 23 ± 3 and 350 ± 30 μs. The resulting decayassociated difference spectra (DADS) are shown in Fig. S9, ESI.

The short time component of 23 µs is assigned to the decay of the excited [Mo6I8Cl6] 2-* concomitant with the rise of chargeseparated species, while the latter should correspond to the recombination of the charge-separated pair. Hypothesis of a photo-induced electron transfer has been further supported by comparison of the broad TA spectroscopic feature (retrieved from the 350 µs DADS) with the sum of the contributions arising from the oxidized mono-anionic [Mo6I8Cl6] -complex and the one electron-reduced POM. The UV-Vis signature of both species has been obtained from separated bulk electrolysis at controlled potential (see details in ESI). The oxidized [Mo6I8Cl6] -molybdenum halide cluster complex gives rise to two principal absorption bands located at about 600 and 800 nm, resulting in a color change from yellow to olive-green (Fig. S10, ESI). Besides, solutions of the oneelectron reduced derivative of the [PW12O40] 3-ion exhibit a characteristic deep blue color (𝜆 𝑎𝑏𝑠 𝑚𝑎𝑥 = 750 nm) which is assigned to the d-d transitions and IVCT bands (Fig. S11,ESI). 26,30 A good match with TA spectroscopic feature is overall obtained using a linear combination of the respective contributions of the spectro-electrochemical difference spectra of the oxidized cluster and the reduced POM (Fig. 4A). This is consistent with a photoinduced electron transfer from the triplet excited state of [Mo6I8Cl6] 2-* to the Keggin-type POM [PW12O40] 3-. The nanosecond TA experiments have been also carried out with higher charged Keggin-type POMs, [BW12O40] 5-and [H2W12O40] 6- (Fig. S12 and S13, ESI). In such systems, a small decrease in the lifetime of the excited state was observed. However, the spectroscopic feature and the dynamics of the chargeseparated state could not be detected due to the less efficient electron transfer. Interestingly, the photoinduced electron transfer was also measured between [Mo6I8Cl6] 2-and [P2W18O62] 6-, as revealed by the decay of the typical transient absorption signal of the [Mo6I8Cl6] 2-* triplet state, concomitant with the rise of a flat, featureless transient absorption signal in the entire visible range associated to the reduced POM (Fig. 3C). This signal is different from that of the system involving the Keggin anion [PW12O40] 3-and matches nicely with a combination involving spectra of the oxidized mono-anionic [Mo6I8Cl6] -complex and

POMs

Standard potential, V a Charge density (q/W) kq × 10 Please do not adjust margins Please do not adjust margins the one-electron reduced Dawson-type anion (Fig. 4B). Biexponential fitting of all the kinetic traces gives time constants of 42 ± 5 and 620 ± 50 μs corresponding to the rise and the decay of the charge-separated state, respectively (Fig. 3C and Fig. S14, ESI). Notably, the longer lifetime of the [Mo6I8Cl6] 2-* excited state observed when the tandem partner is a Dawsontype POM as compared that when the tandem partner is [PW12O40] 3-reflects the less efficient electron transfer consistent with the quenching constant value. 0.50). This work opens avenues toward the design of full-inorganic photocatalytic devices by using molybdenum octahedral clusters as noble-metal free inorganic photosensitizers associated to POM as specific catalytic unit efficient for targeted processes such as depolluting, biomass valorization or clean energy generation. 31 This work was supported by i) Labex Charmmmat (ANR-11-LABX-0039-grant, ii) CLUSPOM-H2 project (ANR-22-CE50-0007-01), iii) Paris-Saclay University, iv) UVSQ and v) CNRS.

Conflicts of interest

There are no conflicts to declare. NMR). [1][2][3][4][5][6][7] The protonation degree have been determined based on TGA analysis.

Notes and references

B

The TBA salts of POM have been prepared by metathesis process as described below.

Preparation of TBA3[PW12O40]. H3[PW12O40]

•14H2O (5 g, 1.6 mmol) was suspended in 40 mL of H2O.

In a separate beaker, tetrabutylammonium bromide (5 g, 15.5 mmol) was dissolved in 50 mL of distilled water. This resulting solution was added to the [PW12O40] 3-solution, leading to the precipitation of a white solid. After gentle stirring for 30 min at room temperature, the solid was collected by filtration through a fine frit, then washed with water (100 mL), ethanol (20 mL), and dried with diethyl ether (20 mL). Yield > 85 %. IR spectrum: 799 (s), 891 (s), 955 (m), 971 (m), 1008 (m), 1487 (s), 2360 (m), 2880 (s), 2950 (w), 2970 (s) cm -1 .

Preparation of TBA4

[SiW12O40]. Na0.6H3.4[SiW12O40]•16H2O (5 g, 1.
54 mmol) was suspended in 40 mL of H2O. In a separate beaker, tetrabutylammonium bromide (5 g, 15.5 mmol) was dissolved in 50 mL of distilled water. This resulting solution was added to the [SiW12O40] 4-solution, leading to the precipitation of a white solid. After gentle stirring for 30 min at room temperature, the solid was collected by filtration through a fine frit, then washed with water (100 mL), ethanol (20 mL), and dried with diethyl ether (20 mL). Yield > 85 %. IR spectrum: 784 (s), 875 (s), 920 (m), 974 (m), 1011 (m), 1487 (s), 2360 (m), 2880 (s), 2950 (w), 2970 (s) cm -1 .

Preparation of TBA5[BW12O40]

. K4.3Na0.7[BW12O40]•17H2O (5 g, 1.5 mmol) was suspended in 40 mL of H2O. The dodecatungstoboric acid has been prepared by passage of this aqueous solution through a column of Dowex 50WX8 (H + form). Then, the resulting colorless solution of H5[BW12O40] has been added to a 250 mL solution of tetrabutylammonium bromide (0.3 M). After stirring for 30 min at room temperature, the white solid was collected by filtration through a fine frit, then washed with water (100 mL), ethanol (20 mL), and dried with diethyl ether (20 mL). Yield > 85 %. IR spectrum: 800 (s), 892 (s), 946 (s), 989 (w), 1489 (s), 2360 (m), 2887 (s), 2950 (w), 2960 (s) cm -1 .

Preparation of TBA4.8H1.2[H2W12O40]. Rb6[H2W12O40]

•11H2O (5 g, 1.4 mmol) was suspended in 40 mL of H2O. The metatungstic acid has been prepared by passage of this solution through a column of Dowex 50WX8 (H + form). Then, the resulting colorless solution of H6[H2W12O40] has been added to a 250 mL solution of tetrabutylammonium bromide (0.3 M). After stirring for 30 min at room temperature, the white solid was collected by filtration through a fine frit, then washed with water (100 mL), ethanol (20 mL), and dried with diethyl ether (20 mL). Yield > 85 %. IR spectrum: 705 (w), 792 (s), 885 (s), 953 (s), 1480 (s), 2870 (s), 2950 (w), 2960 (s) cm -1 .

Preparation of TBA5.5H0.5[P2W18O62]. K6[P2W18O62]

•19H2O (5 g, 1 mmol) was suspended in 40 mL of H2O. Acid salt of [P2W18O62] 6-has been prepared by passage of this aqueous solution through a column of Dowex 50WX8 (H + form). Then, the resulting colorless solution of POMs has been added to a 75 mL solution of tetrabutylammonium bromide (0.3 M). After stirring for 30 min at room temperature, the white solid was collected by filtration through a fine frit, then washed with water (100 mL), ethanol (20 mL), and dried with diethyl ether (20 mL). Yield > 85 %. IR spectrum: 785 (w), 911 (s), 956 (s), 1090 (s), 1480 (s), 2870 (s), 2950 (w), 2960 (s) cm -1 .

b. Preparation of samples

Photophysical studies. To prevent the oxygen quenching of the cluster [Mo6I8Cl6] 2-, all the sample solutions were recorded under oxygen-free conditions. First, the solvent was purged with argon for 30 min and then the samples were prepared inside the glovebox with the argon-saturated solvent.

Measurement of the UV-vis spectrum of one-electron reduced POMs. Each one-electron reduced POM has been prepared under argon using bulk electrolysis in acetonitrile containing nBu4N.PF6 (0.1 M) as supporting electrolyte. To reduce the POM compounds, the chronoamperometry were performed at potential corresponding to about 120 mV below the half-wave potential of the first redox event. The reduction of the POMs led to the formation of blue solutions. To avoid the reoxidation by air (very fast process), we systematically handled the solutions under inert atmosphere.

Measurement of the UV-vis spectrum of one-electron oxidized cluster. The oxidation of the molybdenum halide cluster [Mo6I8Cl6] -by bulk electrolysis under argon in acetonitrile led to formation of insoluble precipitate at the working electrode interface. This phenomenon has been previously observed by Efremova and coworkers. 1 To tackle this problem, we performed the oxidation of the cluster Na2[Mo6I8Cl6] (2 mM) in 25 mM HClO4 aqueous solution containing γ-cyclodextrin (4 mM). The use of cyclodextrin is required to stabilize the cluster in water, the CD host avoids the hydrolysis of the terminal chloride ligands. 8 In such conditions, the metal atom cluster exhibits a reversible oxidation at +1 V vs SCE. 2 The controlled potential oxidation has been performed by applying the potential at +1.1 V vs SCE. During the oxidization of the cluster, the solution turned olive green (see Figure S13), and the process was monitored by steady-state voltammetry using rotating disk electrode to verify the completion of the one-electron oxidation.

c. Methods

a. Steady-state absorption and fluorescence spectroscopy

UV-vis ground state absorption spectroscopy was performed on a UV-2600 spectrophotometer from Shimadzu. The sample concentrations used for absorption and emission measurements were in the micro-molar range, and the spectra were recorded in standard quartz cuvettes with a 1 cm optical path. Emission spectra were measured with Scientific Fluoromax PLUS (Horiba Jobin-Yvon) at room temperature in deaerated acetonitrile solution. The presented emission spectra are corrected for the response of the emission monochromator and the detector. All samples were excited at 420 nm with an excitation and emission slit band pass of 2 nm, in both cases.

b. Time-resolved emission spectroscopy

The excitation light source for the time-resolved emission measurements is based on a Nd:YAG Laser (EKSPLA) yielding 4 ns pulses with a repetition rate of 10 Hz. After passing through the sample, the emission light was sent to the spectrograph (SPEX 270M, Jobin-Yvon) via a bundle of optical fibre for analysis. The dispersed light from the spectrograph is then detected by an intensified charge coupled device (ICCD) camera, PIMAX-4 (Princeton Instrument). The obtained spectra were not corrected by the sensitivity of the spectrometer. The emission kinetics were studied between 400 and 700 nm.

c. Transient absorption spectroscopy

To excite the samples for laser flash photolysis, an optical parametric oscillator (OPO) pumped by a pulsed nanosecond Nd:YAG laser (EKSPLA) was used. The pump laser operates at a frequency of 10 Hz. The resulting pulse width was 4 ns, with an energy of 1-2 mJ/pulse. For transient absorption measurements, a nanosecond white light super continuum laser (STM-2-UV LEUKOS, temporal width of < 1 ns) was used as a probe light. The probe laser was set to a repetition rate of 20 Hz. To monitor spectral energy distribution fluctuations, the probe beam was split by a beam splitter, which directed one part of the beam to the sample cell and the other to the reference. The sample's transmitted light enters the center of the optical fiber (round-to-linear) bundle, which is then routed to a spectrograph (SPEX 270M, Jobin-Yvon) for analysis. An intensified charge coupled device (ICCD) camera PIMAX-4 (Princeton Instrument), synchronized with the pump and probe lasers detects the dispersed white light from the spectrograph. The spectrograph records both the reference and sample spectra at the same time.

Using Equation ( 1), the difference absorption spectra, pump on and pump off can be recorded by varying the delay time between pump and probe:

ΔA = 𝑙𝑜𝑔 10 ( 𝑠 𝑟𝑒𝑓 𝑝 𝑠 𝑟𝑒𝑓 0 × 𝑠 𝑠𝑎𝑚𝑝𝑙𝑒 0 𝑠 𝑠𝑎𝑚𝑝𝑙𝑒 𝑝 ) (1)
where 𝑠 𝑟𝑒𝑓 𝑝 and 𝑠 𝑟𝑒𝑓 0 are reference spectra when the pump is on and off, respectively, while 𝑠 𝑠𝑎𝑚𝑝𝑙𝑒 𝑝 and 𝑠 𝑠𝑎𝑚𝑝𝑙𝑒 0 are sample spectra when the pump is on and off, respectively. All measurements were taken with a quartz cuvette of 10 mm x 10 mm dimension.

d. Electrochemical measurements:

Cyclic voltammetric (CV) experiments were performed with a compact PalmSens4 driven by a computer using PSTrace software. Measurements were performed at room temperature in a singlecompartment micro-cell. A glassy carbon (GC) electrode with a diameter of 2 mm was used as the working electrode. The auxiliary electrode was a Pt plate, and potentials are quoted against an Ag wire as reference electrode. All potentials are quoted relative to the ferrocene/ferrocenium internal standard. Electrochemical experiments were carried out in water-free, de-aerated acetonitrile with TBA.PF6 (0.1 M) serving as a supporting electrolyte. Prior to the experimental runs, all solutions were purged with argon for at least 10 min to eliminate O2, and they were maintained under a positive argon pressure throughout the experiment. The relevant potentials determined by the CVs were used to carry out chronoamperometry in the same three-electrode cell system described above (Preparation of samples).

Using controlled potential coulometry method, 9 the samples were quantitatively oxidized/reduced in an stepwise manner. Linear scan voltammetry (rotating dick electrode) measurements were recorded at each coulometric step (before and after reduction) to ensure 100% reduction / oxidation process. The solutions containing reduced POMs were then analyzed with the help of Perkin Elmer Lambda 750 UV-Vis-NIR spectrophotometer. Stern-Volmer (SV) plots were constructed to assess the Stern-Volmer constants (ksv), according to the well-noted relationship: τ0

/τ = 1+ ksv[Q] or I0/I = 1 + ksv[Q],
where τ0 and I0 are emission lifetime and intensity in the absence of quencher respectively and [Q] is quencher concentration. Finally, the quenching constants (kq) were determined using kq = ksv/τ0. It is to be noted that the three POMs have contribution in the cluster absorption region (see absorption spectra of the POMs and the cluster in Fig. 4.2.4) and therefore the emission spectra in all the cases have been corrected for the portion of the excitation photons absorbed by POMs using the following equation 51 :

𝐼 𝑓 𝑐𝑜𝑟𝑟 = 𝐼 𝑓 𝑜𝑏𝑠 𝐴 𝐶+𝑃 𝐴 𝐶 ( 1-10 -𝐴 𝐶 1-10 -𝐴 𝐶+𝑃 )
, where C and P denote cluster and POM, respectively. Wavelength (nm) ksv (M -1 ) k'sv (M -1 ) kq (M -1 s -1 ) k'q (M -1 s -1 )

[PW11VO40] 4- (2100 ±100) (3100 ±200) 1.5x10 7 2.0 x10 7

[SiW11MoO40] 4- (2700 ±100) (2100 ±70) 1.9x10 7 1.5x10 7

[SiMo12O40] 4- (3700 ±300) (2700 ±100) 2.6x10 7 1.9x10 7 Since the lifetime of the excited state remains unaffected by the loss of photons due to the absorption of the quencher, it is observed that the time-resolved quenching constants k'q obtained for the three cluster-POM pairs (POMs with the same charge) are close to each other.

Nonetheless, the kq and k'q values have the same order of magnitude in the three. This finding are in line with our observations in the part 1 of this chapter, which relates the rate of electron transfer from cluster to POM with the charge on the POM.

Comparison of TA signal with spectroelectrochemical spectra

Photoinduced electron transfer was confirmed by overlapping the TA spectra (at the delay time corresponding to the maximum of the charge separated state formation) and the linear sum of the one electron oxidized cluster and reduced POMs spectra. In all the three cases, a good overlap is found between the two, confirming the PET from cluster to POM (Fig. 4.2.11). In conclusion, we have investigated the dependence of the electron transfer efficiency in the [Mo6I8Cl6] 2-cluster and three Keggin-type polyoxometalate anions bearing the same charge.

Discrepencies in the values of the the steady-state and time-resolved quenching constants arise from the fact that the [PW11VO40] 4-and [SiMo12O40] 4-POMs strongly absorb in the absorption region of the cluster. Nevertheless, considering the more reliable quenching constants retrieved from time-resolved measurements (no correction factor involved), it is apparent that the quenching efficiency in the three POMs is quite similar, which is likely to be related to the charge on the POMs. Photoinduced electron transfer (PET) was confirmed by transient absorption (TA) and the complementary spectroelectrochemical measurements (SEC). The TA spectra corresponding to the charge separated species overlap with the linear combination of cluster oxidized and POMs reduced confirming PET from cluster to POMs. Wavelength (nm)

e x 10 3 (M -1 .cm -1
) 420 nm shows that the nature of the cluster is not affected by interaction with the CD. and the lifetime of the cluster in the absence of POM (τ0), a quenching constant of 25 x 10 7 M - 1 s -1 is obtained. It is to be noted that this quenching constant is about 15 times higher in magnitude than that obtained in [Mo6I8Cl6] 2--[P2W18O62] 6-study (see Article #3). While it may be related to the change in the solvent from organic to aqueous solution (coulombic repulsion is stronger in organic solvent than in water), we think that it is also related to the protecting effect of CD, which reduces the electronic repulsions between the two anionic entities thus enhancing the electron transfer efficiency. Further experiments should be performed with unprotected cluster (without CD) and POM to validate our hypothesis. Wavelength (nm)

Cluster 36 µM Cluster + 2.5 eq. POM Cluster + 3.5 eq. POM Cluster + 6 eq. POM Cluster + 11 eq. POM a. equivalents of POM in the aqueous solution of [W6I8Cl6] 2-@2CD, the emission intensity and the lifetime of the latter decreased by about 1.5 times (see spectra and decays i and ii). Then, 40 equivalents of CD were added to the above solution, leading to a small increase in the lifetime as well as emission intensity (see spectra and decays iii). When the CD concentration was increased further, a very weak gradual increase in intensity and lifetime was observed, though the intensity and lifetimes were not fully recovered even with 250 equivalents of CD.

Due to the very weak effect, it was complicated to study systematically its concentration dependence. Cluster 36 µM Cluster + 6 eq. POM Cluster + 6 eq. POM+ 40 eq. CD Cluster + 6 eq. POM+ 80 eq. CD Cluster + 6 eq. POM+ 200 eq. CD Cluster + 6 eq. POM+ 250 eq. CD i. iii.

ii. a.

t (µs)

Cluster 36 µM Cluster + 6 eq. POM Cluster + 6 eq. POM + 40 eq. CD Cluster + 6 eq. POM + 80 eq. CD Cluster + 6 eq. POM + 200 eq. CD Cluster + 6 eq. POM + 250 eq. CD i.

ii.

iii.

b. Control experiments were also performed by increasing the CD concentration in the ([W6I8Cl6] 2-@2CD) solution (see absorption spectra in Fig. 4.3.7a). No change in the emission intensity and lifetime was observed, suggesting an already optimized host-guest assembly of cluster and -CD, where one cluster unit is deeply embedded inside two units of CDs (Fig.

4.3.7b and c

). This means that further addition of CD does not lead to an enhanced shielding or protection of the cluster from the solvent environment. This optimized assembly ( 2 We have shown in the section 4.2.2 the role of CD to stabilize the cluster@CD-POM tandem, leading to an efficient quenching of the emission and the lifetime of the excited cluster in the presence of POM. Increasing cyclodextrin concentration in the cluster@CD-POM tandem solution results in a weak decrease in the emission quenching of the excited cluster. This effect is tentatively explained by additional interaction between CD and the POM, thus stabilizing the cluster@CD-POM tandem with less coulombic repulsions in a higher distance between the two anionic cluster and POM units. To further investigate the photoinduced processes, transient absorption measurements in this tandem will be described in the next section. Then, upon adding 40 equivalents of cyclodextrins into the 1:13 eq. solution of [W6I8Cl6] 2- @2CD cluster-POM, similar spectroscopic features were observed as that of cluster-POM mixture (Fig. 4.3.10a). Global fitting of the kinetics, on the other hand, produced slightly different time constants of about 3 µs and 110 µs (Fig. 4.3.10b). A small increase in the firsttime component corresponding to the electron transfer process (as compared to the cluster-POM solution without cyclodextrin) is suggestive of the hinderance in the electron transfer due to the higher shielding or isolation effect of cluster and POM by cyclodextrin. This effect agrees with our emission and lifetime quenching results. Moreover, a rise in the time-constant of the charge recombination can be again related to the increased isolation between cluster and POM that hinders the back electron transfer. To summarize, photoinduced electron transfer was investigated between a tungsten halide cluster and a Dawson-type POM pair. In order to solubilize the [W6I8Cl6] 2-cluster and to prevent the hydrolysis of the chlorine ligands, 1:2 inclusion complex of the cluster and -CD (two CDs per cluster) was used as photosensitizer/electron donor. Emission intensity and lifetime quenching measurements in the presence of POM reveal an efficient quenching of the cluster excited state with kq = 25x10 7 M -1 .s -1 . The high efficiency of the PET can be attributed to the decreased electronic repulsions between cluster and POM due to the shielding effect of the CD.

The effect of CD on the electron transfer efficiency was further studied by increasing the concentration of -CD in cluster-POM solutions.

The findings indicate that CD does, in fact, impede electron transfer by further separating the cluster and POM, however the effect is very weak. These observations were supported by performing TA studies that revealed a slightly longer charge-separation and recombination lifetimes. While longer charge-separation agreed with the quenching experiments, the longer recombination further reiterates that CD increases the separation between the cluster and POM units.

Conclusions and Prospects

The present chapter explores photoinduced electron transfer in the octahedral metal halide The third part of this chapter is focussed on studying the PET in hydrosoluble cluster-POM systems. Here, we used a tungsten halide cluster ([W6I8Cl6] 2-), which has shown to be more stable in water than the molybdenum counterpart. This cluster was further stabilized by encapsulating it inside two -CD units to form stable inclusion complex ([W6I8Cl6] 2-@2CD).

The effect of adding POM in the aqueous solution of the inclusion complex was studied using

Stern-Volmer analysis which revealed an efficient quenching of the cluster by POM. Moreover, the role of CD in the electron transfer efficiency from cluster to the POM was determined. The results show that while CD can slightly reduce electron transfer efficiency, it also stabilizes the charge separated state by keeping the cluster and the POM separated. The strategy of encapsulating the cluster and/or POMs inside the cavities of CDs is very promising to design a new class of multifunctonal materials, in particular light-driven devices for photocatalysis using a straightforward supramolecular self-assembly of cluster and POMs.

Overall, the work highlights the potential of metal halide cluster as a new class of inorganic photosensitizer/electron donor that can be coupled with polyoxometalates to realize allinorganic artificial photosynthetic systems for clean energy generation. Hybrid organic-inorganic materials serve as a natural interface between two branches of chemistry, each of which significantly contributing to the field of materials science 1 . Indeed, hybrid materials frequently involve a combination of components thoroughly studied in their respective fields but provide an additional dimension to their properties in becoming part of the hybrid compound. The design of a hybrid seeks to get the best out of two different chemical worlds, frequently with complementary strengths, to form new materials with new or improved properties. The concept of designing, focused on developing functional hybrid materials that harness the chemical activity of their organic-inorganic hybrid systems, is not new, and numerous examples in the literature were demonstrated with the intention of using them as multifunctional materials 2,3,4,5 . These include primarily two classes 6 : (I) innovative combinations where organic and inorganic partners were linked together through weak bonds (Van der Waals, ionic or hydrogen bonds), and (II) hybrid structures in which organic and inorganic components were linked together through covalent solid or iono-covalent chemical bonds.

Polyoxometalates (POMs) have emerged as ideal inorganic candidates for designing advanced organic-inorganic hybrids, owing to their relevance to analytical chemistry, catalysis, medicine, and materials science 7 . In fact, the most common route of incorporating POMs in advanced multifunctional materials is through organic-inorganic hybrids 8 . Most of these POM-based hybrids are "class I" materials, meaning that the POM and organic components are linked via non-covalent interactions 9 . Some examples in the literature also illustrate POM-based "class II" covalent hybrids 10,11 . Nonetheless, the covalent approach has several advantages because the covalent link improves hybrid stability and may improve the interaction between inorganic and organic components. As explained earlier, POMs stand as outstanding options for developing artificial photosynthetic devices due to their ability to store multiple electrons 12 . POMs have thus been frequently linked covalently to classical inorganic photosensitizers (ruthenium and iridium complexes) to study the charge accumulation on POMs and their subsequent use in photocatalysis 13,14,15 . All these inorganic covalent assemblies could perform photocatalytic hydrogen production with significantly better efficiency than the multi-component systems highlighting the importance of covalent linkage. However, such systems suffer from the use of noble metal photosensitizers. Moreover, they tend to show a low stability of the photosensitizers in the presence of acids, which is crucial to enhance the electron-accepting properties of POMs.

To circumvent these problems, covalently coupling organic residues with POMs offers an attractive way to design stable and inexpensive hybrid organic-inorganic photocatalytic architectures. Odobel et al. in 2009 published a series of studies on organic photosensitizers covalently linked to POM units (Fig. 5.1). In one of the reports, the authors first studied a covalently linked perylene-POM hybrid, where the two components were linked by means of a semi-rigid linker, thus favoring the existence of different families of conformations 16 . No spectral signature corresponding to the charge transfer state was observed, implying that the electron transfer occurred in only the folded conformer, in which the charge recombination is faster than intramolecular electron transfer. Then, they studied a hybrid system in which a flexible linker linked two perylenes to the POM unit. In this case, the flexible linker allows a faster electron transfer and more extensive fluorescence quenching of the photosensitizer suggesting that an orbital overlap is required between the units to realize an electron transfer.

But again, no spectroscopic signatures of the charge-separated state were observed because the rate of the reverse process exceeds that of the forward step. quenching, but the more flexible tether promotes this. In all cases, the excited singlet state on the porphyrin can transfer an electron to the POM. However, the rate of electron transfer is very slow and is in competition with the formation of the triplet state of the porphyrin. The presence of triethanolamine as a sacrificial electron donor leads to the formation of the porphyrin π-radical anion. This latter species can reduce POM, but again the rate of reaction is remarkably slow. Here, bimolecular electron transfer competes effectively with the intramolecular route.

Under these conditions, loading the POM with more than a single electron was impossible. Apart from these early reports on organic chromophore-POM hybrids, of late, the approach has been extended to covalently linked BODIPY-POM hybrids. The group of Izzet in 2018 illustrated for the first time the design and photophysics of the photoactive hybrids based on The group of Izzet in 2021 also reported the effect of acid on the photoinduced charge separation in BODIPY-POM hybrid systems 21 . The authors compared the electron transfer efficiency in two BODIPY-POM hybrids (K Mo Sn [BOD] and K W Sn [BOD]) (Fig. 5.6) with and without trifluoroacetic acid. On one hand, in K Mo Sn [BOD], the electron transfer was possible with and without acid; however, the rate of electron transfer was observed to be faster (91 ps), and the recombination was observed to be slower (1.3 µs) in the presence of acid (as against 2.2 ns and 16 ns, respectively in the absence of acid). On the other hand, in K W Sn [BOD], the electron transfer was only possible in the presence of the acid, although with a small efficiency.

Moreover, the charge recombination was more rapid as compared to the Mo analog. These observations were attributed to the difference in the driving forces of the charge separation and recombination (ΔGcs and ΔGcr, respectively). The higher the ΔGcs and ΔGcr values, the higher will be the rate of the charge separation and recombination, respectively. The presence of acid shifts the reduction potentials of the POMs to more positive values, thus increasing the driving force of the charge separation and, at the same time, lowering the driving force for recombination, i.e., stabilizing the charge-separated state. However, the authors suggested that the effect on the charge separation is not solely due to the reduction potentials. They proposed that the protonation of the POMs followed by an electron transfer is likely to affect the electron transfer rates. Further, for the recombination, since the reduced POMs are more basic than the oxidized counterparts, it is unlikely that deprotonation occurs first. Thus, a concerted protonelectron transfer or electron transfer followed by proton transfer can possibly explain the mechanism for charge recombination. Inspired by these studies and having explored the photophysical processes in all organic and all inorganic donor-acceptor systems, we were interested in studying organic-inorganic hybrid systems based on BODIPY-POM. So far, these hybrid platforms are mostly built with polyoxotungstates or polyoxomolybdates. The use of polyoxovanadates (POV) remains much less explored. POVs exhibit multiple accessible oxidation states (mainly based on the V V /V IV and, more rarely, V IV /V III redox couples), resulting in rich and intricate redox behavior 22,23 . In addition, they have higher redox potentials than polyoxotungstates and polyoxomolybdates 24 ,so their ability to accept electrons from photosensitizers should be more efficient. In this work, we have photophysically studied novel Lindqvist-type POV covalently linked with BODIPY units. These covalent hybrids were synthesized using Cu-catalyzed alkyl-azide click chemistry approach by the collaborative efforts of Dr. Clément Falaise and Dr. Emmanuel Allard at Institut Lavoisier de Versailles. The two arms of a POV were clicked with the BODIPY unit to form a hexadentate-BODIPY-polyoxovanadate hybrid (noted as BDP-POV-BDP) (Fig. 5.7a).

Similarly, more conjugated BODIPY (distyryl-BODIPY, noted as DSBDP) units were clicked with the POV to form DSBDP-POV-DSBDP hybrids (Fig. 5.7b). a.

(n-Bu4N + )2

(n-Bu4N + )2

and Table 5.1). Also, at negative potentials, the CV of the BDP-POV-BDP hybrid shows an irreversible reduction process of POV at -0.62 V and a quasi-reversible process at -1.13 V corresponding to the reduction of the BDP unit. In DSBDP-POV-DSBDP hybrid, again the redox properties are very close to that of the reference DSBDP unit showing a reversible reduction at -1.10 V and an irreversible reduction process of the POV unit at -0.68 V. The irreversible redox waves for the POM in the two hybrids can be explained by the deposition of the compounds on the surface of the electrode. The redox property of the POV was also investigated using 500 eq. of trifluoroacetic acid (TFA). The CVs recorded in the absence and in the presence of TFA show a positively-shifted (from -0.5 to -0.10 V) reversible reduction process as well as an increase in current intensity (Fig. 5.9). The increase in current intensity is certainly due to multielectron transfer caused by the presence of protons, as estimated by the ratio of the difference of the cathodic and anodic current intensities in the presence and absence of acid ([Ic-Ia]with acid/([Ic-Ia]without acid) , which was found to be approximately 4. The effect of protonation was also studied in the two hybrids, but the redox signals were not clearly defined. As observed in the CVs above (Fig. 5.8), the organic and inorganic counterparts show the reduction properties in the hybrids close to the individual entities, implying the electronic decoupling between the two units. Therefore, the potential of the protonated parent POV unit was considered to calculate the driving force of the electron transfer process in the presence of acid. The values for the electrochemical oxidation and reduction processes in the parent BODIPYs, parent POV, and the two hybrids are summarized in the Table 5.1.

b. Steady-state absorption and emission studies:

The steady-state absorption and emission spectra of the parent BODIPYs (BDP and DSBDP), and the polyoxovanadate unit were recorded in DCM. Due to the low solubility of the two BODIPY-POV hybrids (BDP-POV-BDP and DSBDP-POV-DSBDP) in DCM, their absorption and emission spectra were recorded in DCM with 1% DMF. The parent BDP shows a typical, intense S0 → S1 transition band at about 500 nm, characteristic of phenyl substituted BODIPYs 25,26 while the S0 → S2 transition is represented by a weaker and wider feature below 400 nm (Fig. 5.10a) 26 . In the case of DSBDP, the S0 → S1 absorption band is bathochromically shifted to 628 nm and gets broadened due to the extended conjugation of the BODIPY core.

The UV bands for DSBDP, with a maximum at 345 nm, can be attributed to the S0 → S2 transition. Besides, the POV unit weakly absorbs in the UV-Visible range (Fig. Upon exciting the parent BDP at 495 nm, an intense emission band centered at 525 nm was obtained with a quantum yield (ΦF) of about 73 % (scaled to 1 in Fig. 5.11a). This intense emission was greatly quenched by over 80 % (ΦF = 12 %) in the BDP-POV-BDP hybrid.

Further, excitation of the parent DSBDP at 620 nm gives a strong emission (ΦF = 68 %) with a maximum at about 650 nm and a less intense emission band at about 700 nm, which is significantly quenched by about 45 % in the DSBDP-POV-DSBDP hybrid (Fig. 5.11b). These quenchings are characteristic of the photoinduced electron and/or energy transfer reactions from the excited state of the BODIPY units to the POV unit. Indeed, due to the spectral overlap (although weak) between the BDP emission and POV absorption, the energy transfer from excited BDP to POV is possible (see the overlap of the POV absorption and BDP emission in Fig. 2 of Annexe B). However, in the case of DSBDP-POV hybrid, excitation energy transfer process can be ruled out due to the lack of spectral overlap between the DSBDP emission and POV absorption. The steady-state absorption and emission data are summarized in Table 5.2.

Moreover, upon adding 500 eq. of TFA to the hybrid solutions, the emission was further quenched by about 70 % in both the hybrids as compared to that without acid (see blue curves primarily suggests the contribution from solvation dynamics (solvent reorganization) 28 . Beyond 1 ps, the bands of the DSBDP excited-state decay with a simultaneous recovery of the GSB and emission bands. Kinetic fittings at individual probe wavelengths, for example, at 450 nm corresponding to the excited state absorption, and 640 and 700 nm corresponding to the GSB and stimulated emission converged with multiple time constants (Fig. 5.17 and Table 5.4).

Kinetic fit at 450 nm yields two time-constants of 76 ps and 4.3 ns. While the former is associated to the anisotropy of the molecule (see Fig. 5.18 comparing the GSB kinetics at magic angle and parallel polarization of the pump and probe), the latter is related to the relaxation of the excited state. For the negative TA bands (640 and 700 nm), kinetics could be fitted satisfactorily with at least three exponential functions giving time constants of about 0. 4.2, Fig. 5.12). The thermodynamic driving force of the electron transfer process (ΔGcs) from the excited state of BDP to POV was estimated to be -0.75 eV using Rehm-Weller equation 30 . Then, the DADS associated to 340 ps exhibits positive TA bands at 320 and 420 nm along with a negative GSB band, which are the typical spectroscopic features of BDP excited state. Again, this time-constant was also obtained in the individual fitting of the decay of the excited state of BDP and of the ground-state bleach recovery. Therefore, the 340-ps component is associated with the photoexcitation energy transfer process from 1 BDP* to POV. Finally, the DADS associated to the non-decaying component is the spectroscopic signature of the charge-separated state (BDP Electrochemical analysis of POMs performed by many research groups has revealed that the ability to accept electrons is directly related to the presence of protons 31,32 Finally, the BDP cation signature is retained at the end of the femtosecond measurement window. In both the hybrids, although an electron transfer was possible in the absence of the acid, the driving force was not enough to allow for an efficient charge-separation. In the presence of the acid, the driving force of charge-separation (ΔGcs) in both the hybrid compounds increases (see Table 5.7 for the summary of the ΔGcs values for the two hybrids, in the absence and presence of TFA), thus increasing the rate of electron transfer processes, consistent with the increased fluorescence quenching and the decreased lifetime of the excited state. The effect of H + on the lifetime of the excited state shows that protonation of the POM increases the thermodynamic favorability of the charge separation without significantly affecting the reorganization energy.

However, the process of nuclear motion, such as proton transfer is rarely evidenced experimentally. The protonation of the POV can be considered to occur either by concerted proton electron transfer (termed as CPET) and/or in a stepwise manner (electron transfer followed by proton transfer; ET-PT or proton transfer followed by electron transfer; PT-ET).

Given that the introduction of acid reduces the lifetime of the BODIPY excited states (from 74 to 4 ps), the mechanism of PT-ET or CPET is more convincing.

v. Nanosecond transient absorption

In order to ascertain the lifetimes of the charge-separated states which were obtained as a non decaying in femtosecond measurement in all the cases, TA measurements in nanosecond timescale were performed. Moreover, the nanosecond TA spectra were recorded in the presence and in the absence of acid to evaluate the effect of protons on the stabilization of the chargeseparated state.

BDP-POV-BDP and DSBDP-POV-DSBDP in the absence and presence of acid

Nanosecond TA spectra of the BDP-POV-BDP and DSBDP-POV-DSBDP hybrids were recorded upon respectively exciting the BDP and DSBDP units at 495 and 620 nm in DCM/DMF (99:1, v/v) mixture. In both the cases, a rise in the cation bands (400 nm for BDP .+ and 350-500 nm and 700 nm for DSBDP .+ ) is observed until 12 ns which is due to the IRF of the laser (Fig. 5.31a and b). From 12 to 300 ns, the TA bands are observed to decay to the ground state, marking the charge recombination step. Following the kinetics at 400 nm (BDP .+ ) gives a charge-separated state lifetime of about 30 ns (Fig. Surprisingly, the addition of TFA stabilizes the charge separated state in terms of energetics (see table 5.7) but not in terms of the lifetimes in the nanosecond range, which remain unaltered in case of both the hybrids (see the comparison of the nanosecond TA kinetics of the two hybrids with and without acid in Fig. 5.33), in the absence and in the presence of the acid. Further, the presence of a short component (several tens of ps, see Fig. POV-DSBDP as compared to BDP-POV-BDP. This finding was consistent with the more emission quenching and smaller emission mean-lifetime in the latter as compared to that in the former. Contrary to a similar non-conjugated reported system based on molybdate POM (Ered = -1.20 V) where no charge separated state could be observed, it could be well realized in our assemblies thanks to the higher reduction potential of POV.

We have also studied the effect of acid on the rate of electron transfer in the two hybrids. The reduction potential of the POM increases (becomes more positive) in the presence of protons, which resulted in a faster charge-separation due to an increased thermodynamic driving force of electron transfer (ΔGcs). These results were again in agreement with the increased emission quenching of the BODIPYs and the decreased lifetime of the excited state obtained by TA method in the two hybrids in the presence of acid. Based on the decreased excited state lifetime in the presence of acid, we proposed that the proton transfer followed by an electron transfer or CPET is a probable mechanism for faster charge-separation. For recombination, TA spectroscopy in nanosecond timescale showed that the charge-separated state lifetimes of the two hybrid systems remain unchanged even in the presence of acid. Therefore, electron transfer followed by proton transfer is proposed for the recombination step. The results in the presence of acid show that the electron transfer can be fastened due to protonation of POMs and an efficient electron transfer can be attained even in non-conjugated systems. These are preliminary results where we have used a high concentration of acid. Indeed, a more systematic study is required with gradually increasing the acid concentration in the system that can provide more complete kinetic and mechanistic insights into the electron transfer process in the presence of protons. The modest lifetimes of the charge-separated states (in the range of tens of nanoseconds), though, limit their use in photochemical devices. To address this, efforts are being made by our collaborators to design a photosensitizer-acceptor-catalyst system (P-A-C) based on BODIPY-C60-POM that would expectedly lead to a more stabilized charge-separated state by spatially distancing the ion-pairs.

Moreover, to use these assemblies in photocatalytic devices, accumulation of charges on the POM is necessary. Transient absorption with double-pump excitation measurements can be explored to probe the charge accumulation on the POM.

In all these assemblies, a charge-separated state was detected. More interestingly in the triad, a charge separation can be observed irrespective of the excitation wavelength. However, the lifetime of the charge-separated state was short. This is addressed in the third part of this chapter (presented as a sub-chapter; 2.3). The BDP-C60 dyad was functionalized with one or two additional donors, Fc, at β position of the BODIPY to design D-P-A triads (mono or bis Fc-BDP-C60). The aim was to study the multi-step electron or charge transfer pathway leading to the elongation of the charge-separated state lifetime due to the spatial positioning of the counter ions. The mono and bis Fc triads and the corresponding Fc-BDP dyads were investigated using femtosecond transient absorption and time-resolved emission methods. In the mono and bis-Fc-BDP dyads, exciting the BDP unit leads to an electron transfer process from Fc to BDP, resulting in the charge-separated state (Fc .+ -BDP .-) that relaxes to the ground state by populating 3 BDP* in 25 ps, which then populates 3 Fc*. In the triad, two photoinduced electron transfer processes were observed in two different conformersdue to the flexible linker between BDP and C60 units. In one of the conformers, an electron transfer from Fc to BDP was observed to form Fc .+ -BDP .--C60 which follows the same relaxation route as that of the isolated Fc-BDP

dyad. An ultrafast electron transfer from BDP to C60 was observed in a conformer with a folded geometry between BDP and C60 (electron transfer is preferred from BDP to C60 compared to Fc to BDP). The resulting charge-separated-state (Fc-BDP .+ -C60 .-) undergoes a rapid charge shift and/or electron transfer, eventually forming a spatially distanced ion-pair (Fc .+ -BDP-C60 .-

) that recombines in 400 ps. Clearly, the lifetime of the charge-separated state was elongated as compared to the isolated Fc-BDP and BDP-C60 dyads, thanks to sequential electron transfer or charge-shift mechanism.

In the second part of the thesis, intermolecular photoinduced processes were studied in allinorganic photosensitizer-acceptor systems, comprising octahedral metal halide cluster as photosensitizer/electron-donor unit and polyoxometalate as catalytic platform/electronacceptor. The first part of this chapter, presented as an Article manuscript (Article #3), explores the factors (charge density on POMs and reduction potential) affecting the rate of the electron transfer process from cluster ([Mo6I8Cl6] 2-) to POM. A series of five different Keggin polyoxotungstates (P, Si, B, H2) and a Dawson polyoxotungstate [P2W18O62] 6-were chosen for the study, which vary in the anionic charge density and the reduction potentials. Emission intensity and lifetime SV analysis were employed to retrieve the quenching rate constants. It was found that the quenching is dynamic in nature and is strongly related to the anionic charge density of POM. The magnitude of the charge density on the POM dictates the coulombic repulsions between the two anionic units (negatively charged cluster and negatively charged POMs), and it also affects the reduction potential of the POMs (POMs with the same metal ion). Therefore, the higher the charge density on the POM (lower the reduction potential), the higher will be the coulombic repulsions, and therefore, the rate of electron transfer from cluster to the POM will be lower. Using transient absorption spectroscopy, the kinetics of electron transfer and the signature of the charge-separated species were obtained. Ru, etc.). The fundamental insight into the photoinduced processes (mechanism of quenching, quenching efficiency, factors affecting the rate of quenching, etc.) within these systems is essential and serves as a stepping stone toward the development of the non-noble metal containing all-inorganic photocatalytic system.

The abovementioned work was carried out in an organic solvent, and we further explored the electron transfer properties of cluster-POM systems in water. However, the hydrolytic nature of clusters limits their investigation in aqueous solutions. To stabilize the clusters in water, cyclodextrin was employed to encapsulate the cluster in their hydrophobic cavities, which prevents the precipitation of the cluster. In this study, a tungsten halide cluster ([W6I8Cl6] 2-) was chosen as it is more stable than the corresponding Mo-based cluster. To stabilize it in water, two CDs were used to engulf one cluster ([W6I8Cl6] 2-@2CD). This inclusion compound was photophysically investigated with a Dawson POM ([P2W18O62] 6-) as a quencher. The dynamic quenching of the cluster was observed with the time constant of 25 x 10 7 M -1 .s -1 . Then, upon further increasing the CD concentration in the cluster@CD-POM solution, the emission intensity and the lifetime appeared to weakly increase. This effect is tentatively explained by additional interaction between CD and the POM, thus stabilizing the cluster@CD-POM tandem with less coulombic repulsions in a higher distance between the two anionic cluster and POM units. This results in a decrease of electron transfer efficiency and an increase in the charge separated lifetime. Our study highlights that the cluster with two CD units can be considered an optimized system that gives a considerably high quenching constant. Further studies should be performed with cluster alone (without CD) and POM to confirm whether the presence of two CDs around the cluster hinders the electron transfer process. It will help in establishing optimized conditions to obtain high efficiency of electron transfer while maintaining the stability of the clusters in aqueous solutions.

The last part of the thesis examines intramolecular photoinduced processes in organic-inorganic hybrids constructed using BODIPY and POM as the corresponding organic and inorganic equivalents. Two different BODIPY-POM-based hybrid covalent assemblies (BODIPY-POM-BODIPY and distyrylBODIPY-POM-distyrylBODIPY) were photophysically investigated, which combined the best components from the previous two projects, i.e., BODIPY as photosensitizer and POM as the catalyst. A Lindquist-type polyoxo-hexavanadate (POV) was used in the investigation, as POVs tend to show higher reduction potentials compared to the W and Mo-based POMs. However, they have remained unexplored in BODIPY-based hybrids.

The selective excitation of BODIPY units in the two hybrids resulted in photoinduced electron transfer, however, the efficiency is relatively low. Next, when protons were introduced to the solution, the efficiency of electron transfer was increased manifold and could be evidenced by the emission quenching in the steady-state emission and femtosecond TA measurements. These observations were supported by the positive shift of the reduction potential of the POV upon the addition of 500 eq. of H + , resulting in the increased driving force for the electron transfer process. These findings suggested that the protonation of POV facilitates the electron transfer process. The lifetime of the charge-separated states in both the hybrids was found to be in the order of tens of nanoseconds, and surprisingly it remained unaltered even in the presence of protons. The results indicate an important instance of photoinduced electron transfer in an entirely non-conjugated BODIPY-POM system, accredited to the high reduction potential of POV. Further, we have demonstrated that PET efficiency can be tuned by introducing protons into the system. These preliminary results pave the path for developing efficient organicinorganic hybrids based on BODIPY and POV.

Prospects

The long-lived triplet excited state localized on the triplet of DSBDP in the BDP-C60-DSBDP triad, regardless of excitation, opens a way to use this triad as an organic triplet photosensitizer.

A step toward stabilizing the charge-separated state was taken by designing the D-P-A system (Fc-BDP-C60). The long lifetime of the charge-separated state in Fc-BDP-C60 obtained by means of multi-step electron transfer is interesting in terms of its application in photovoltaic systems. This assembly can be taken to solid surfaces (gold, ITO, etc.) to realize photovoltaic devices, mimicking the antenna reaction center of natural photosynthesis. A similar approach of grafting an additional donor (e.g., Fc) on the BDP-C60-DSBDP triad should be applied to leverage the panchromatic ability of this triad while elongating the lifetime of the chargeseparated state. This will be taken into consideration for the design of the next generation of molecular assemblies in collaboration with Dr. Emmanuel Allard and his colleagues at Institut Lavoisier de Versailles.

The insights gained from our fundamental study, which investigated the potential and factors influencing PET efficiency from metal halide clusters to model POMs, are a first step toward using cluster-POM systems for clean energy applications. In the near future, thio-POMs, known for their high HER activity, will be studied with the cluster (inorganic noble-metal-free photosensitizer) to design novel visible-light photocatalytic systems for hydrogen production that operate not only in homogeneous solutions but also at photo-electrode interfaces (Fig. 6.1).

To realize this goal, the host-guest approach will be used, incorporating clusters and POMs within commercial CDs to design supramolecular, low-cost self-assemblies. The goal will be achieved in the frame of the 'ClusPOM-H2' project funded by ANR in close collaboration with different partner labs (ICSR, Rennes; ILV, Versailles; and ISMO). Overall, the fundamental studies on the photophysics of various donor-acceptor systems presented in this thesis add to our understanding of electron and energy transfer processes, paving the way to not only design more elegant systems in the future but also lead up to exploring their potential in artificial photosynthesis.

CHAPTER 3 .CHAPTER 4 .

 34 Acknowledgements ............................................................................................................................ Table of Contents ............................................................................................................................... List of abbreviations ........................................................................................................................... Résumé ............................................................................................................................................. Preface.............................................................................................................................................. CHAPTER 1. Introduction ........................................................................................................... 1.1 Natural and Artificial photosynthesis ......................................................................................... 1.2 Photophysical Concepts in Artificial Photosynthesis ................................................................. 1.2.1 Light-absorption and Excited-state deactivation ............................................................... 1.2.2 Excited state reaction mechanisms.................................................................................... a.Photoinduced Energy Transfer (EnT) ........................................................................................ b.Photoinduced Electron Transfer (PET) ...................................................................................... 1.3 Focus on photoinduced inter and intramolecular electron transfer ......................................... 1.3.1 Photoinduced intermolecular electron transfer in D-A systems ....................................... a.Static Quenching ...................................................................................................................... b.Dynamic quenching .................................................................................................................. 1.3.2 Photoinduced intramolecular electron transfer in D-A systems ....................................... 1.4 Light harvesting D-A systems: a puzzle of compounds .............................................................. 1.4.1 Photosensitizers ................................................................................................................. a.Organic Photosensitizers .......................................................................................................... b.Inorganic Photosensitizers ....................................................................................................... 1.5 Model D-A systems based on the previous photosensitizers .................................................. 1.6 Objective of the dissertation ...................................................................................................... References ........................................................................................................................................ CHAPTER 2. Experimental Techniques and Data analysis ............................................................. 2.1 Steady-state absorption and emission ....................................................................................... 2.2 Time-resolved spectroscopy ...................................................................................................... 2.2.1 Time correlated single photon counting ............................................................................ a.Nanosecond TA set-up: ............................................................................................................ b.Femtosecond TA set-up ............................................................................................................ 2.2.3 Data analysis of femtosecond TA ....................................................................................... a.Chirp correction ........................................................................................................................ b.Global analysis ......................................................................................................................... 2.3 Electrochemical measurements ................................................................................................. a.Cyclic voltammetry ................................................................................................................... b.Spectroelectrochemistry ........................................................................................................... References ........................................................................................................................................ Photoinduced processes in organic D-A assemblies ................................................... 3.1 Photoinduced electron and energy transfer processes in covalently linked BDP-C60 based molecular assemblies ....................................................................................................................... 3.1.1 Introduction and literature survey ..................................................................................... 3.1.2 Covalently linked BODIPY-fullerene based D-A systems: ................................................... 3.1.2 Background and motivation of the work ........................................................................... Article 1: Selective population of triplet excited state in heavy atom free BODIPY-C60 based molecular assemblies .................................................................................................................. Article 2: Ultrafast time-resolved spectroscopy elucidating photo-driven electron and energy transfer processes in a broadband light-absorbing BODIPY-C60-distyryl BODIPY triad .............. 3.2 Cascade photoinduced electron transfer processes in β-positioned ferrocene-BODIPY-C60 conjugates ...................................................................................................................................... 3.2.1 Introduction and literature survey ................................................................................... 3.2.2 Background and motivation of the work ......................................................................... 3.2.3 Results and Discussion: .................................................................................................... a.Steady-state absorption and emission ................................................................................... b.Time-resolved emission (TCSPC) ............................................................................................. c.Femtosecond transient absorption (TA) spectroscopy ........................................................... i.β-bis-Fc-BDP dyad ................................................................................................................... ii.β-bis-Fc-BDP-C60 triad ............................................................................................................. 3.2.4 Conclusion ........................................................................................................................ Conclusions and future perspective ............................................................................................... References ...................................................................................................................................... Photoinduced processes in all-inorganic molecular systems ..................................... 4.1 Photoinduced electron transfer in electron-rich octahedral metal halide cluster and polyoxometalate ............................................................................................................................ 4.1.1 Introduction and literature survey ................................................................................... 4.1.2 Background and motivation of the work ......................................................................... 4.1.3 Article Cluster-POM in acetonitrile .................................................................................. 4.2 Photoinduced electron transfer in octahedral metal halide cluster and same-charged polyoxometalate anions ................................................................................................................. 4.2.1 Redox properties of the POMs ......................................................................................... 4.2.2 Steady state and time resolved quenching of [Mo6I8Cl6] 2-by POMs ............................... 4.2.3 Transient absorption spectroscopy .................................................................................. 4.2.4 Comparison of TA signal with spectroelectrochemical spectra ....................................... 4.3 Photoinduced electron transfer between [W6I8Cl6] 2-@2CD encapsulated octahedral tungsten cluster in cyclodextrin and [P2W18O62] 6-polyoxometalate ............................................................. 4.3.1 Steady-state and time-resolved studies on [W6I8Cl6] 2-@2CD: ......................................... 4.3.4 Transient absorption spectroscopy of cluster in the presence of POM and CD .............. Conclusions and Prospects .............................................................................................................

Fig. 1 . 1

 11 Fig. 1.1 Schematic illustration of natural photosynthesis. The chlorophyll a in reaction center in photosystem II undergoes charge separation by transferring an electron to plastoquinone (Pq) which triggers a photochemical water oxidation reaction Photosystem I transfers excited electrons and protons to NADP reductase through the primary acceptor, ferredoxin (Fd) for the regeneration of NADPH. The NADPH regenerated by photosystem I is then consumed during the reduction of carbon dioxide to carbohydrates in the Calvin cycle.

Fig. 1 . 2

 12 Fig. 1.2 Simplified schematic representation of a typical artificial photosynthetic system. D and A denotes donor and acceptor, respectively. PS: photosensitizer, WOC: water oxidation catalyst, RC: reduction catalyst.

Fig. 1 . 3

 13 Fig. 1.3 Potential energy curves for the ground state and the excited state of a diatomic molecule. r0 and r0' represent the internuclear distance in the ground state and the excited state at the potential energy minima, ν represents vibrational levels. Adapted from reference 13 .

Fig. 1 . 4

 14 Fig. 1.4 Perrin-Jablonski diagram showing different photophysical processes. Adapted from reference 12 .

Fig. 1 . 5

 15 Fig. 1.5 Schematic representations of (a) Förster resonance energy transfer and (b) Dexter electron exchange energy transfer. Adapted and redrawn from reference 13 .

  (a) oxidative electron transfer processes: 1 D* + A → D .+ + A .- (b) reductive electron transfer processes: 1 A* + D → D .+ + A .- In the reductive quenching process, an electron from the D species moves into the vacancy that is left after the A species undergoes electron excitation (Fig.1.6a), finally resulting in an acceptor anion (A .-) and a donor cation (D .+ ). In oxidative quenching mechanism, light absorption by the donor molecule promotes an electron to its LUMO which is then transferred to the LUMO of the acceptor (Fig.1.6b), leading to the formation of a donor cation (D .+ ) and an acceptor anion (A .-). Although, oxidative and reductive properties are enhanced in the excited state as compared to that in the ground state, the fate of the electron transfer process (oxidative or reductive) is governed by the redox properties of D* and A, or D and A*, respectively.

Fig. 1 . 6

 16 Fig. 1.6 Schematic representation of reductive (a) and oxidative (b) photoinduced electron transfer (PET) processes at a typical donor (D) -acceptor (A) interface. Adapted and redrawn from the reference 9 .

Fig. 1 . 7

 17 Fig. 1.7 Marcus theory predictions for the variation of activation free energy (ΔG⧧) and rate of electron transfer (ln kET) with cell potential (ΔE0), solid green and dashed red curve, respectively. λ is the reorganization energy. Normal and inverted regions indicate less and more spontaneous redox reactions respectively, resulting in high (ΔG⧧) and thus small rate of electron transfer.

Fig. 1 . 8

 18 Fig. 1.8 Diagrammatic representation of static quenching processes between fluorophore (D) and quencher (Q) a) Sphere-of-action quenching. b) Quenching associated with ground-state non-fluorescent complex. Adapted from reference 9 .

Fig. 1 .

 1 Fig. 1.10 Absorption features and chemical structures of typical porphyrin (above) and phthalocyanin (below) dyes. Taken from reference 44 . Apart from the above-mentioned chromophores, during the last two decades 4,4-difluoro-4bora-3a,4a-diaza-s-indacenes, abbreviated as BODIPYs are frequently incorporated in D-A systems for photoinduced electron transfer reactions. In comparison to other classes of chromophore dyes, BDPs have piqued the interest of scientific community due to their outstanding photophysical, optoelectronic, and electrochemical properties 60,61 . Furthermore, in addition to the BDP platform's simple and straightforward synthesis, convenient and flexible tuning of its outstanding properties is possible through simple synthetic modifications on five different positions (Fig. 1.11). As a result, BODIPY derivatives have been successfully applied to a wide range of scientific and technological fields, including photodynamic therapy, photonics, and optoelectronics 62 .

Fig. 1 .

 1 Fig. 1.12 General illustration of chemical structures of [Ru(bpy)3] 2+ and [Ir(ppy)3] complexes.

  Fig. 1.13 Examples of molecular structures of organic D-A systems built with different photosensitizers and quinone or fullerene as acceptor units as reported in literature.

1

 1 photosensitizer(donor)-acceptor (D-A) assemblies.The chapter particularly deals with fast and ultrafast intramolecular PET in BODIPY covalently linked to C60 fullerene as an acceptor. A related study provided as a sub-chapter explores the aspect of cascade electron transfer in a three-component system (D-P-A), that uses ferrocene as an additional donor, which is linked to BODIPY-C60 dyad. Chapter 4 includes a study on all inorganic donor-acceptor systems. It presents photophysical investigations on intermolecular PET of a new class of inorganic photosensitizers (electron-rich octahedral metal halide clusters) using different polyoxometalate anions as electron acceptors and photocatalytic platforms. Chapter 5 deals with the study on covalently linked organic-inorganic hybrid systems. The study puts forth intramolecular photoinduced electron transfer in covalently linked BODIPY-POM hybrid systems using fast and ultrafast transient absorption methods. Finally, Chapter 6 offers an overall conclusion of this work and gives prospects for future investigations.

Fig. 2 . 1

 21 Fig. 2.1 Schematic representation of TCSPC setup at PPSM. Depending on the excitation wavelength, fluorescence decays were recorded at the PPSM laboratory, ENS Paris-Saclay, and in LASIR Lab in Université de Lille, using a homemade time-correlated single photon counting (TCSPC) setup.

  d.) Product absorption: Another positive signal might appear in TA spectra, which indicates the absorption of the transient product, triplet states, or charge-separated states, for example. e.) Radiative deactivation: Fluorescence or phosphorescence appears as negative signals in the ΔA profile.

Fig. 2 .

 2 Fig. 2.2 (a) A general representation of a TA set-up showing the pump pulse photoexciting the sample and the probe pulse monitoring the sample response at different delay times after photoexcitation and (b) contributions in a ΔA spectrum.

Fig. 2 . 3

 23 Fig. 2.3 Schematic representation of the nanosecond TA set-up built at ISMO. Letters I, L and M represent iris, lens and mirror, respectively. B.S. is beam splitter and O.C. is optical chopper and T.O. is transparent optics.

Fig. 2 . 4

 24 Fig. 2.4 Timing control and synchronization diagram of EKSPLA laser, CCD, and probe laser.

  mechanical delay line situated in the path of the probe pulse. The simplified figure of the transient absorption setup is shown in Figure 2.5 and a more detailed scheme of the TA setup is shown in Figure 2.6.

Fig. 2 . 5

 25 Fig. 2.5 Simplified scheme of a femtosecond transient absorption set-up.

Fig. 2 . 6

 26 Fig. 2.6 Detailed schematic diagram of the femtosecond transient absorption set-up housed in Quantum Electric Lab, Adam Mickiewizc University, Poznan. This figure is prepared with the help of Dr. Sabina Brazevic.

2. 2 . 3

 23 Fig 2.7

Fig. 3 . 1 . 2

 312 Fig. 3.1.2 General molecular structure of BODIPY compound and its typical absorption and emission features.

  Fig. 3.1.4 A few examples of the reported multichomophoric BDP-C60 based D-A systems 61,67,68 .

3 . 1 . 2

 312 Fig. 3.1.5 Two interesting examples of two-BODIPY-C60 systems reported in the literature 69,70 .

Fig. 3 . 1 . 6

 316 Fig. 3.1.6 Synthetic route of BDP-C60 dyad system under study. Using the same strategy, Emmanuel Allard et al. synthesized a new multichromophoric triad (BODIPY-C60-distyryl-BODIPY, referred to as BDP-C60-DSBDP) by clicking the alkyne group present on the other arm of the C60 platform with the more conjugated distyryl-BODIPY unit (Fig. 3.1.7). The presence of the alkyne groups on the two sides of the fullerene platform thus facilitated the design of a new covalently linked BODIPY-fullerene based D-A-D'system, a triad in which the fullerene is sandwiched between two differently functionalized BODIPYs

Fig. 3 . 1 . 7

 317 Fig. 3.1.7 Molecular structures of BDP-C60-DSBDP triad and DSBDP-C60 dyad systems under study.The BDP-C60-DSBDP triad along with the two reference dyads (BDP-C60 and DSBDP-C60) were photophysically characterized using steady-state absorption and emission, time-resolved emission and nanosecond and femtosecond transient absorption spectroscopy. The signatures of the oxidized and reduced species (charge separation upon photoexcitation) were obtained by using spectro-electrochemistry. The photophysical studies on these donor-acceptor systems have been published in two parts (Article #1 and Article #2).

Scheme 1

 1 Scheme 1 Structure of the BDP-C 60 -DSBDP triad, BDP-C 60 and DSBDP-C 60 dyads under study

  s 0 ref are reference spectra with and without pump, respectively while, s p sample and s 0 sample are sample spectra with and without pump, respectively.

  . SI-1 shows ΔA versus power (mW) plots for 3 C 60 * (Fig. SI-1a) and 3 DSBDP* (Fig. SI-1b) in the dyad and the triad, along with that of [Ru(bpy) 3 ] 2+ . For each excitation power, ΔA values have been noted at the maxima of the kinetic curves at 710 nm for 3 C 60 * and 455 nm for [Ru(bpy) 3 ] 2+ (Fig. SI-2), 710 nm for 3 DSBDP* and 455 nm for [Ru(bpy) 3 ] 2+ (Fig. SI-3). Putting in the values of slopes obtained by linear regression in the Eq. (

  60 and DSBDP-C 60 dyads is therefore the sum of the absorbance of fullerene and the respective BDPs (Fig.SI-5a). Similarly, the absorption spectrum of the triad almost overlaps with the sum of the corresponding reference compound spectra, signifying negligible ground-state interaction between BDPs and the fullerene moiety (Fig.SI-5b).

Fig. 1 a

 1 Fig. 1 a Absorption spectra of BDP-C 60 (pink), DSBDP-C 60 (green) and DSBDP-C 60 -BDP (cyan) in benzonitrile. b Emission spectra in benzonitrile of BDP (black, λ exc = 495 nm), DSBDP (red,

Fig. 2

 2 Fig. 2 Excitation spectra of a BDP-C 60 dyad (λ em = 560 nm; dashed green), b DSBDP-C 60 dyad (λ em = 670 nm; dashed blue), and c BDP-C 60 -DSBDP triad (λ em = 670 nm; dashed pink), overlapped with their respective absorption spectra; black solid curves

  Fig. SI-9), indicating the presence of only singlet excited states of the BDP and DSBDP. No signatures of long-lived 3 DSBDP* and 3 BDP* states with microsecond lifetimes were observed. Further, excitation of the fullerene platform at 420 nm (Fig. SI-10) reveals two positive transient absorption bands at 400

Fig. 3 Fig. 4 a

 34 Fig. 3 Fluorescence decay of parent BDP (black, λ exc = 495 nm, λ em = 535 nm), BDP-C 60 dyad (pink, λ exc = 495 nm, λ em = 535 nm), BDP-C 60 -DSBDP triad (cyan, λ exc = 495 nm, λ em = 535 nm; blue, λ exc = 495 nm, λ em = 650 nm) in benzonitrile

Fig. 5

 5 Fig. 5 Nanosecond transient absorption spectra of BDP-C 60 -DSBDP triad (6 µM, λ exc = 525 nm), in deaerated benzonitrile. Inset: Decay profile observed at 710 nm

Fig. 6 a

 6 Fig. 6 a Transient absorption spectra and b up-converted fluorescence spectra of BDP-C 60 dyad (10 μM) and perylene (400 μM) in deaerated toluene at 525 nm excitation and 40 mW laser power

  state and time-resolved emission, as well as nanosecond transient absorption. Excitation of BODIPY chromophores can lead to the formation of a triplet state centered on C 60 or the BODIPY moieties depending on the BODIPY structure. In the case of the BDP-C 60 dyad, nanosecond transient absorption studies confirm the formation of the triplet excited state of C 60 , whereas for the DSBDP-C 60 and the BDP-C 60 -DSBDP triad, the triplet excited state centered on the DSBDP moiety was detected. This phenomenon is supported by the lower positioning of the 3 DSBDP* energy level as compared to 3 C 60 *. The use of BDP-C 60 and BDP-C 60 -DSBDP as potential organic triplet photosensitizers for triplet-triplet annihilation upconversion was demonstrated. This work is useful for the development of a new wideband light-capturing, heavyatom-free organic triplet photosensitizer, with an appreciably long triplet lifetime (280 µs for 3 DSBDP * ) at room temperature.

Fig. 7 Fig. 8

 78 Fig. 7 Scheme of TTA up-conversion in the BDP-C 60 dyad and perylene. UC fluo: Up-converted fluorescence

Fig. SI- 1 .

 1 Fig. SI-1. Plots of ΔA vs. excitation power. a. [Ru(bpy)3] 2+ (black squares, 455 nm) and BDP-C60 (red dots, 710 nm), λexc = 470 nm, OD= 0.3 at 470 nm, b. Ru(bpy)3] 2+ (black squares, 455 nm), DSBDP-C60 (pink dots, 710 nm) and DSBDP-C60-BDP (blue triangles, 710 nm), λexc = 420 nm, OD= 0.3 at 420 nm. Optical path = 0.4 cm. Values under each curve represent the slope obtained by linear fitting.

Fig. SI- 2 .

 2 Fig. SI-2. Transient absorption decays recorded at varying powers of excitation pulse (λexc = 470 nm) a. [Ru(bpy)3] 2+ in water (recorded at 455 nm) and b. BDP-C60 dyad in benzonitrile. (recorded at 710 nm).

Fig. SI- 4 .Fig

 4 Fig. SI-4. Absorption spectra of BDP-N3 (black), DSBDP-N3 (red), and C60 platform (blue) in benzonitrile.

6 .Fig. SI- 7 .

 67 Fig. SI-6. Excitation spectra of a. BDP-N3; green dash (λem = 560 nm) and b. DSBDP-N3 (λem = 670 nm); red dash overlapped with their respective absorption spectra; black solid curves.

Fig. SI- 8 . 10 .

 810 Fig. SI-8. Fluorescence decays of parent DSBDP (red, λexc = 635 nm, λem = 650 nm), DSBDP-C60 dyad (green, λexc = 635 nm, λem = 650 nm) and BDP-C60-DSBDP triad (blue, λexc = 635 nm, λem = 650 nm) in benzonitrile.

Fig. SI- 11 .Fig. SI- 12 .Fig. SI- 14 .

 111214 Fig. SI-11. Nanosecond transient absorption spectra of a. BDP-C60 dyad (λexc = 525 nm), b. DSBDP-C60 dyad (λexc = 635 nm), c. BDP-C60-DSBDP triad (λexc = 635 nm) and d. BDP-C60-DSBDP triad (λexc = 525 nm) measured in oxygenated solutions. Concentration of 6 µM was used for all the measurements.

Fig. SI- 16 . 17 18 .

 161718 Fig. SI-15. Transient absorption decays of BDP-C60 dyad at 710 nm ( 3 C60*) and 485 nm ( 3 perylene*) fitted with bi-exponential function (λexc = 525 nm).

  Fig. SI-19 Time resolved fluorescence spectra of a. DSBDP-monomer (450 μM) and b. BDP-C60-DSBDP triad (10 μM) in deaerated toluene at 525 nm excitation and 40 mW laser power.

Fig. 1 Fig. 2 aFig. 3

 123 Fig.1 UV-Visible absorption and emission spectra of a BDP-C60 dyad (λexc = 495 nm), b DSBDP-C60 dyad (λexc = 605 nm) and c BDP-C60-DSBDP triad (λexc = 495 nm) recorded in benzonitrile

Fig. 4 Fig. 5 aFigFig. 7 b Fig. 8 a

 457b8 Fig. 4 Femtosecond transient absorption spectra of BDP-C60 at specified delay times: a -0.3 to 0.44 ps; b 0.51 to 100 ps; c 100 to 301 ps and d 301 to 2460 ps in deaerated benzonitrile. λexc = 525 nm. The grey and black curves in panel (a) represent the respective steady-state absorption (abs) and emission (em) spectra of pristine BDP

Fig. 9 Fig. 10 Fig. 11 aFig. 12

 9101112 Fig. 9 Energy level diagram presenting the photoinduced processes in DSBDP-C60 dyad in argon-saturated benzonitrile. Conformer 1 connotes a short D-A distance, undergoing rapid electron transfer. Conformers 2 and 3 with intermediate donor-acceptor distances undergo photoexcitation energy transfer from 1 DSBDP* to C60. The indicated lifetimes represent the kinetics of the dominant relaxation processes. For the sake of simplicity, an extended conformer with no electron/energy transfer process is not shown in the scheme

1

 1

  corresponding transient kinetics are shown in SI Fig. 5. Time constants obtained from the individual multiexponential fit are in agreement with the global multiexponential fit shown in Fig. 8a. Three exponential model satisfactorily fit the kinetic traces yielding the time constants of 4, 87 and 874 ps. The DADS (Fig. 8b) of the first component of 4 ps shows the negative bands at 687 and 1045 nm corresponding to the rise of DSBDP •+ and C 60 •-absorption, respectively. Similar 123 time constants are also observed in the individual fitting of the decay of 1 DSBDP* and the rising of chargeseparated state absorption. As in the BDP-C 60 dyad with the same flexible linker, different donor-acceptor distances are also possible in this dyad. The first time constant is therefore related to the formation of the charge-separated state in a short DSBDP-C 60 distance conformer (named Conformer 1, Fig.

3 . 4 . 5 .

 345 Nanosecond transient absorption spectra of a. BDP-C60 dyad (exc = 525 nm) and b. DSBDP-C60 dyad (red curve; exc = 635 nm) and the triad (green curve; exc = 525 nm) in deaerated benzonitrile at specified delay times. Femtosecond transient absorption spectra of DSBDP at specified delay times: a) -0.3 to 0.58 ps; b) 0.58 to 216 ps; and c) 216 to 2500 ps and the fitted kinetic traces at chosen probe wavelengths in deaerated benzonitrile. exc = 640 nm. Chemical oxidation of DSBDP-C60 dyad using FeCl3.6H2O. a. Absorbance of DSBDP-C60 dyad before and after adding the oxidant and b. the difference absorption spectrum of DSBDP .+ showing the positive band largely absorbing at 687 nm.

6 . 7 . 8 . 9 .

 6789 Time traces of transient absorptions of the DSBDP-C60 dyad at specific probe wavelengths in deaerated benzonitrile fitted with a three-exponential model except for 1045 nm which fits with a four exponential model, λexc = 640nm SI Table 2. Time constants obtained by fitting femtosecond transient absorption kinetics at specific probe wavelengths in the DSBDP-C60 dyad, λexc = 640 nm. (r): rising time, (d): decay time; a,d photoinduced charge transfer and charge recombination of DSBDP and C60* b energy transfer from 1 DSBDP* to C60, c second energy transfer from 1 DSBDP* to C60, e ISC process of 1 C60*. Decay traces of transient absorptions of BDP-C60-DSBDP dyad at specific wavelengths in deaerated benzonitrile fitted with a four-exponential model, λexc = 525 nm SI Table 3. Time constants obtained by fitting femtosecond transient absorption kinetics at specific probe wavelengths in the BDP-C60-DSBDP triad, λexc = 525 nm. (r): rising time, (d): decay time; a energy transfer from 1 BDP* to DSBDP, b,e photoinduced charge transfer and charge recombination of DSBDP and C60, c energy transfer from 1 DSBDP* to C60, d energy transfer from 1 BDP* to C60, f second energy transfer from 1 DSBDP* to C60, g decay of 1 C60* and formation of triplet excited states ( 3 C60* and 3 DSBDP*). # Note: time constant of <1 ps could not be retrieved with precision due to strong signal from benzonitrile. Femtosecond transient absorption spectra of BDP-C60-DSBDP at specified delay times: a. -0.32 to 0.44 ps; b. 0.44 to 20 ps; c. 20 to 300 ps and d. 300 to 2460 ps in solvent benzonitrile. exc = 580 nm. The dashed grey and black curves in panel (a) represent the respective steady-state absorption (SS abs) and emission (SS em) spectrum of pristine DSBDP Decay traces of transient absorptions of BDP-C60-DSBDP triad at specific wavelengths in deaerated benzonitrile fitted with a three-exponential model, λexc = 580 nm SI Table 4. Time constants obtained by fitting femtosecond transient absorption kinetics at specific probe wavelengths in BDP-C60-DSBDP triad, λexc = 580 nm. (r): rising time, (d): decay time; a,d photoinduced charge transfer and recombination from 1 DSBDP* to C60, b photoinduced energy transfer from 1 DSBDP* and C60, c second energy transfer from 1 DSBDP* to C60, e ISC process of 1 C60*.

10 . 1

 101 photosynthetic system requires not only fast charge separation but also slow charge recombination. Although many D-A systems have been reported to demonstrate efficient PET from D* to A, one notable limitation of these molecules is their inability to achieve temporal stabilization of the photo-produced charge-separated state 72 . The strategy of a single, shortrange photoinduced electron transfer process can minimize losing the input photon energy by a rapid electron transfer from the excited state. However, short spatial distancing between the ion pairs and, in some cases, strong coupling between donor-acceptor units can lead to a short-lived charge-separated pair. To circumvent this issue, an important strategy is to employ cascade electron transfer or multistep electron transfer in donor-acceptor multiad systems (D-P-A or D-A-A, Fig.3.2.1), which leads to a more stable charge-separated state owing to the spatially distant counter ion pair73 . The phenomenon was inspired by the natural photosynthetic system in which solar energy is converted into chemical energy by a cascade of photoinduced energy and electron transfers among donors and acceptors in the antenna complexes and reaction centers72,74 . The independent studies from the group ofMoore et al. and Sakata et al. are among the pioneering examples of multiple PET in D-A assemblies consisting of porphyrin-quinonebased triads. Sakata and coworkers reported a D-A-A system, wherein a porphyrin (P) was attached to two quinones (Q) molecules 75 . Upon photoexcitation of the porphyrin unit, a first electron transfer leads to the formation of P •+ -Q1 •--Q2, followed by an electron shift from Q1 to Q2, finally resulting in the spatially separated P •+ -Q1-Q2 •-ion pair. A few years later, Moore and co-workers published a carotenoporphyrin-quinone (C-P-A) triad, in which the photoexcited porphyrin (C-P * -A) transfers an electron to the quinone (C-P •+ -A •-), followed by the rapid electron transfer from carotenoid to the oxidized porphyrin resulting in a significantly longlived (lifetime in the range of µs) C •+ -P-A •-state as a final charge-separated state 76 . A simplified scheme of cascade electron transfer in a D-P-A system is presented in Fig.3.2.1.

(

  photosynthetic systems.

Fig. 3 . 2 . 1

 321 Fig. 3.2.1 Representation of stabilization of the charge separated state in a D-P-A system.Although a large number of the BODIPY-fullerene and Fc-fullerene D-A systems were published in the literature, we could find very few examples of Fc-BODIPY-C60 conjugated systems that were able to undergo cascade (multi-step) electron transfer leading to long-lived charge-separated states37,89,90 .Wijesinghe et al., for instance, reported a visible light-absorbing Fc-BODIPY-C60 triad (Fig.3.2.2), in which a cascade PET process occurred following photoexcitation of the BODIPY37 . The triad exhibited an electron transfer from Fc to

Fig. 3 . 2 . 2

 322 Fig. 3.2.2 Molecular structures of Fc-BODIPY-C60 triads reported in the literature37,89,90 .

Fig. 3 . 2 . 3

 323 Fig. 3.2.3 Molecular structures of the β positioned mono and bis Fc-BODIPY-C60 triads along with corresponding Fc-BODIPY dyads and the reference compounds. Firstly, the reference compounds β-mono-ethynylphenyl BODIPY (BDP-EtPh) and β-bisethynylphenyl BODIPY (EhPh-BDP-EtPh) were first obtained by ''Sonogashira cross coupling'' of ethynyl phenyl with β-mono-iodo-BODIPY and β-bis-iodo-BODIPY, following the strategy reported in the literature 91 .

3. 2 . 3

 23 Results and Discussion: a. Steady-state absorption and emission Absorption spectra of β-mono-Fc-BDP-C60 and β-bis-Fc-BDP-C60 triads recorded in benzonitrile along with their precursor dyads (β-mono-Fc-BDP and β-bis-Fc-BDP) are shown in Fig. 3.2.4. The ground state photophysical data are summarized in table 3.2.1. The absorption spectra of the reference compound BDP-EtPh show intense absorption band at 525 nm and a

Fig. 3 . 2 . 4

 324 Fig. 3.2.4 Steady state (a) absorption spectra of β-mono and bis-Fc-BDP-C60 conjugates along with the reference dyads and (b) emission spectra of β-mono-Fc-BDP dyad and the corresponding C60 triad recorded in benzonitrile (λexc=500 nm).

Fig. 3 . 2 . 5

 325 Fig. 3.2.5 Fluorescence decay of reference BDP-EtPh (λexc = 495 nm, λem = 578 nm), Fc-BDP dyad (λexc = 495 nm, λem = 555 nm), Fc-BDP-C60 triad (λexc = 495 nm, λem = 555 nm) in benzonitrile.

  spectra in UV-Vis-NIR region were recorded in argonsaturated benzonitrile and are shown in Fig. 3.2.6. For clarity, the spectra are presented in four different time windows. The first time window [-0.3 to 0.2 ps] depicts the rising of 1 BDP* bands at ca. 350 and 440 nm in the UV-visible part and a broad absorption band in NIR, accompanied with a negative absorption band centered at ca. 600 nm. The positive bands are attributed to the absorption of 1 BDP* while the negative TA band corresponds to the ground state depletion of BDP (inverted ground state absorption spectrum is added in the first time panel for comparison, grey curve in Fig. 3.2.6a). From 0.2 to 3 ps, the 1 BDP* bands in the visible as well as NIR regions decay at the same time as the rising of a new positive TA band at 550 nm and a broad absorption band centered at about 1100 nm in NIR. These spectral features can be attributed to electron transfer step from Fc to 1 BDP* to form a charge separated state Fc .+ -BDP .-. The BDP radical anion signature in UV-Visible part is confirmed by recording the spectral changes in the bis-Fc-BDP dyad at varying reduction potentials (-0.7 to -1.2 V) leading to one-electron reduction of the BDP unit. The difference spectroelectrochemical (SEC) spectra (Fig. 3.2.7) show a sharp band at 550 nm accompanied by a small absorption band at 440 nm. The SEC spectra provides a good overlap with the femtosecond TA spectra of the triad as well as the corresponding dyad (black curve in panel b. of Fig.3.2.6. and panel b of Fig. A2 in Annexe A).

Fig. 3 . 2 . 6

 326 Fig. 3.2.6 Femtosecond transient absorption spectra of bis Fc-BDP dyad (8 µM) at specified delay times: a -0.3 to 0.24 ps; b 0.24 to 3 ps; c 3 to 100 ps and d 100 to 2700 ps in solvent benzonitrile. λexc = 580 nm. The grey curve in panel (a) shows the inverted ground state absorption of BDP and the black curve in panel in benzonitrile(b) shows the spectroelectrochemical signature of one-electron reduced BDP unit in DCM.

Fig. 3 . 2 . 7

 327 Fig. 3.2.7 Spectroelectrochemical spectra of one electron oxidized BDP in bis Fc-BDP dyad (10 µM) relative to the neutral state with 0.1 M TBAPF6 in DCM solution showing the rise of BDP •+ , upon gradually increasing the oxidation potential. The negative bands correspond to the depletion of the electronic ground state.

Fig. 3 . 2 . 8

 328 Fig. 3.2.8 Multi-exponential fit of femtosecond transient absorption (λexc =580 nm) kinetic traces of the bis-Fc-BDP dyad at specific wavelengths.

Fig. 3 . 2 . 9

 329 Fig. 3.2.9 Energy level diagram presenting the photoinduced processes in the β-mono or bis-Fc-BDP dyads in argon-saturated benzonitrile. Note : CS and CR denote charge separation and charge recombination. Energies: 1 BDP* =2.29 eV (calculated from the mid-point of the absorption and emission specta), 3 Fc* 89 , 3 BDP* 96 .

Fig. 3 . 2 .

 32 Fig. 3.2.10 Femtosecond transient absorption spectra of bis-Fc-BDP-C60 triad (8 µM) at specified delay times : (a) -0.5 to 0.2 ps; (b) 0.2 to 3 ps; (c) 3 to 100 ps and (d) 100 to 2700 ps in deaerated benzonitrile. λexc = 580 nm. The pink curve in panel (a) represents the SEC signature of C60 anion in DCM.

Fig. 3 . 2 .

 32 Fig. 3.2.11 Multi-exponential fit of femtosecond transient absorption (λexc =580 nm) kinetic traces of the bis-Fc-BDP-C60 triad at specific wavelengths.Table 3.2.3 Time constants obatined by multi-exponential fitting of kinetic traces of bis-Fc-BDP-C60 triad at specific wavelengths. *Benzonitrile solvent has strong contributions within 2 ps time windows.

Fig. 3 .

 3 Fig. 3.2.12 Energy level diagram presenting the photoinduced processes in the mono or bis-Fc-BDP-C60 triads in argon-saturated benzonitrile. Conformer 1 connotes a shorter BDP-C60 distance that undergoes ultrafast electron transfer from BDP to C60. Conformer 2 with shorter Fc and BDP distance as compared to that of BDP and C60, undergoes rapid electron transfer from Fc to BDP. Energies: 1 BDP* =2.29 eV (calculated from the mid point of the absorption and emission specta), 3 Fc* 89 , 3 BDP* 96 .

  Fig. 4.1.1 Molecular structures of Keggin-type ([XM12O40] n-) and Dawson-type ([X2M18O62] n-), X = P, Si, Ge, and M= Mo, W, V. The pink pyramids represent the hetero atom (X), blue pyramids represent metal atom (M), and the red dots are the oxygen atoms through which M atoms are connected to each other.

2

 2 Background and motivation of the workThe above-mentioned reports suggest that earth abundant metal-containing POM architectures when coupled with a photosensitizer and a sacrificial electron donor show high catalytic efficiency for HER. However, the use of noble metal-based photosensitizer in these systems would not sustain a low-cost and wide-scale production of hydrogen evolving photocatalyst systems. Therefore, using photosensitizers built from less expensive elements is needed to expand the use of POMs in photocatalytic devices. To this end, octahedral metal halide clusters which represent a class of electron-rich species are one of the promising candidates. Metal halide clusters are known for their remarkable photophysical properties, including phosphorescent properties characterized by emission in the red/near-infrared region intense luminescence with high quantum yields in solutions and considerably long-lived (microsecond order) triplet state. Octahedral metal halide clusters such as M6X14 or M6X8Y6 types (M= Mo, W or Rh ; X, Y= Cl, Br, I) (bearing 2-charge) show several accessible oxidation levels.38 In the typical M6X14 cluster, the M6 cluster units are bonded to eight face-capping inner ligands apical ligands.39 In the hexanuclear metal cluster complexes of the type [{Mo6X8}Y6] 2-or [M6X14] 2-, six metal atoms (M) are organized in an octahedral metal core linked to eight face-capping (X) and six terminal ligands (X or Y) (Fig.4.1.2). These units are associated to counter cations (such as NH4 or BuN4.) and yield stable all-inorganic ions in solution38 .

Fig. 4 .

 4 Fig. 4.1.2 Molecular structural representation of a metal halide cluster of [M6X14] 2-or [M6X8Y6] 2-type. Black sphere: metal, light green sphere: capping ligand (X = Cl, Br or I), dark green sphere : apical ligand (Y = Cl, Br or I). These metal cluster (Mo6) units demonstrate remarkable optical properties (absorption from UV to Visible, emission ranging from visible to NIR and hence large Stoke's shift) suitable for developing a new family of photosensitizer for photovoltaics and photocatalytic systems 40 . Of all the cluster systems with different transition metal atom core (Mo, W or Rh), the molybdenum based octahedral clusters are more thoroughly investigated due to their exceptional optical properties 40 . Indeed, the incorporation of Mo6 units provides a broad emission window from deep red to NIR range along with a large absorption window from UV to visible range and a large Stokes shift, suitable for developing a new family of photosensitizers for photovoltaics and photocatalytic systems. 40,41 . After the first report by Maverick in 1981 on luminescence properties of [Mo6Cl14] 2-cluster 38 , maximum emission wavelength (λem), emission quantum yield (ϕem) and lifetime (τem) data have been published for various bromide and iodide analogues. All the clusters were shown to be luminescent at room temperature. Kitamura et al. have systematically studied redox, spectroscopic and photophysical properties of a series of [{Mo6X8}Y6] 2-(X, Y= Cl, Br or I) cluster in solution as well as in crystalline phase 41 . The work highlights that all the clusters show intense and long-lived emission. Further, the emission quantum yield and lifetime values can be tuned depending on the combinations of X and/or Y. Recently, Renaud and co-workers have investigated the potential application of weakly toxic molybdenum halide clusters as photosensitizers in all inorganic solar cells 40 . The authors pointed out that the change of ligands and/or metal can modify the absorptivity of the halide clusters and the electron transfer to TiO2 by tuning the HOMO/LUMO levels. The work

Fig. 4 . 1 . 3

 413 Fig. 4.1.3 Structural representations of (a) -cyclodextrin, (b) octahedral metal halide cluster and (c) Dawson POM encapsulated in -cyclodextrin (1:2 complex) 46 . Here, [W6I8Cl6] 2-with 2 equivalents of -CD (2 CD per cluster) was prepared by the team of Dr. Clément Falaise and Prof. Emmanuel Cadot at Institut Lavosier Versailles. The photosensitizing and electron donating ability of this inclusion complex ([W6I8Cl6@2CD] 2-is studied in combination with the Dawson-type polytungstate anion (K6[P2W18O62]). The effect of POM and CD on the emission quenching of the cluster is investigated in independent Stern-Volmer (SV) measurements. Further, the shielding effect of CD on the quenching of cluster by POM was studied by measuring the luminescence properties of the cluster-POM solution with increasing concentrations of CD. The results of this work are presented in the third part of this chapter (4.3).

Fig. 1 (

 1 Fig. 1 (A) Structural representation of the molybdenum halide cluster. Black sphere: Mo, yellow sphere: I, green sphere: Cl. (B) and (C) Structural representations of the Keggintype and Dawson-type POM. (D) Electronic absorption profiles of the cluster and the POMs (as tetrabutylammonium salts) in acetonitrile. Note: The molar extinction coefficients (ε) of POMs are scaled by dividing by a factor of 20.

  -CNRS UMR 8214, Université Paris-Saclay; Rue André Rivière 91400 Orsay (France). Email : rachel.meallet-renault@universite-paris-saclay.fr; karine.steenkeste@universite-paris-saclay.fr; minh-huong.ha-thi@universiteparis-saclay.fr b. ILV-CNRS UMR 8180, UVSQ, Université Paris-Saclay; 45 Avenue des Etats Unis, 78035 Versailles Cedex (France). Email : clement.falaise@uvsq.fr c. Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Univ Rennes; Avenue du General Leclerc, 35042 Rennes (France) † Footnotes relating to the title and/or authors should appear here. Electronic Supplementary Information (ESI) available: [Materials and methods, emission spectra, transient absorption spectra]. See DOI: 10.1039/x0xx00000x COMMUNICATION Journal Name 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

Fig. 2 .

 2 Fig. 2. (A) Steady-state emission spectra and (B) time-resolved emission decays of the [Mo6I8Cl6] 2-cluster-based complex (30 µM) in the presence of different concentrations of [PW12O40] 3-POM in deaerated acetonitrile and (C) Stern-Volmer plots showing the emission lifetime changes of the cluster in the presence of POMs.550 600 650 700 750 800

Fig. 4 .

 4 Fig. 4. Comparison of the second decay-associated difference spectrum (DADS) resulted from the global analysis on the TA measurements of the cluster and (A) [PW12O40] 3-and (B) [P2W18O62] 6-, with the linear combination (calculated) of the spectra of the oxidized cluster [Mo6I8Cl6] -and the one-electron reduced POMs.

  Chemicals and solvents for all synthesis and characterization procedures were purchased from chemPUR, Sigma-Aldrich, and TCI, and are used as it is. (TBA)2[Mo6I8Cl6], H3[PW12O40]•14H2O, Na0.6H3.4[SiW12O40]•16H2O, K4.3Na0.7[BW12O40]•17H2O, and Rb6[H2W12O40]•11H2O and K6[P2W18O62]•19H2O were prepared according to literature procedures and checked by routine analyses (IR, TGA, EDX, and

Figure S3 .

 S3 Figure S3. Time-resolved emission spectra detailed of [Mo6I8Cl6] 2-at different delays between pump (λexc = 420 nm, 20 mW) and probe in aerated acetonitrile. The inset shows the monoexponential fit of the kinetics at 700 nm.

Figure S4 .

 S4 Figure S4. Stern-Volmer plots comparing the quenching efficiencies of the four POMs under study obtained from emission intensity of TBA2[Mo6I8Cl6] cluster excited state in deaerated acetonitrile.

Figure S8 .Figure S9 .

 S8S9 Figure S8. Cyclic voltammograms of polyoxometalate anions in anhydrous and deaerated acetonitrile with 0.1 M TBA.PF6. Reference electrode: Ag wire; Working electrode: glassy carbon; and Auxiliary electrode: Carbon. The TBA salts of POMs ([H2W12O40] 6-[P2W18O62] 6-) showed distorted ill-defined waves in CH3CN due to the presence of protons. On the addition of about 1 eq. of TBA.OH per POM, the ill-defined waves disappeared completely, leaving a two-step redox wave.

Figure S10 .Figure S11 .

 S10S11 Figure S10. Spectro-electrochemical difference spectra of quantitatively oxidized molybdenum halide cluster.

Figure S12 .

 S12 Figure S12. (A) TA spectra along with the kinetic fit at 520 nm shown in the inset and (B) decay associated difference spectra (DADS) of cluster (60 µM)-[BW12O40] 5-(300 µM) POM pair obtained by global fitting of TA kinetics (λexc = 420 nm, 20 mW).

Figure S13 .Figure S14 . 4 . 2 .Fig. 4 . 2 . 1 Fig. 4 . 2 . 2 1 +

 S13S14424214221 Figure S13. (A) TA spectra along with the kinetic fit at 520 nm shown in the inset and (B) decay associated difference spectra (DADS) of cluster (60 µM)-[H2W12O40] 6-(300 µM) POM pair obtained by global fitting of TA kinetics (λexc = 420 nm, 20 mW).

Fig. 4 . 2 . 3

 423 Fig. 4.2.3 Steady state absorption spectra of (a) [PW11VO40] 4-, (b) [SiW11MoO40] 4-and (c) [SiMo12O40] 4-in the presence of increaing concentrations of POM in acetonitrile.

Fig. 4 . 2 . 4

 424 Fig. 4.2.4 Steady-state absorption spectra of the cluster and the three POMs in acetonitrile. As shown in Fig. 4.2.5a, the emission intensity of the cluster decreases with the increasing concentration of [PW11VO40] 4-. Further, the time-resolved emission quenching measurement under the same conditions reveals the lifetime of the excited cluster was decreased at varying concentration of the POM (Fig. 4.2.5b). Similarly, the emission intensity and the lifetimes of the cluster decreases gradually upon adding the [SiW11MoO40] 4-and [SiMo12O40] 4-POMs. The intensity and lifetime quenching for the two POMs are presented in Fig. 4.2.6 and 4.2.7, respectively. In all the cases, plotting I0/I and τ0/τ as a function of the POM concentration yields linear correlations (Fig. 4.2.8 a and b). The emission intensity and the lifetime Stern-Volmer plots give the slopes with the same order of magnitude (τ0/τ ≈ I0/I), however, there are considerable variation in the absolute values. This discrepency is more pronounced in the case of [PW11VO40] 4-and [SiMo12O40] 4-POMs, which can be related to the intense absorption of these two POMs at the excitation wavelength of the cluster resulting in an inaccurate correction of the emission intensities (see absorption spectra of the POMs in Fig.4.2.4). The values of the Stern-Volmer constants (ksv) and the quenching constants (kq) are summarized inTable 4.2.2.

Fig. 4 .

 4 Fig. 4.2.5 (a) Steady-state and (b) time-resolved emission quenching of the cluster by [PW11VO40] 4-in deaerated acetonitrile, λexc=420 nm.

Fig. 4 .Fig. 4 .

 44 Fig. 4.2.6 (a) Steady-state and (b) time-resolved emission quenching of the cluster by [SiW11MoO40]4-in deaerated acetonitrile, λexc=420 nm.

Fig. 4 .

 4 Fig. 4.2.8 (a) Emission intensity and (b) lifetime Stern-Volmer plots for the three cluster-POM pairs.

4. 2 . 3

 23 Transient absorption spectroscopy [Mo6I8Cl6] 2-cluster and POMs : Nanosecond transient absorption spectra (λexc = 420 nm) of cluster and [PW11VO40] 4-POM in deaerated acetonitrile show the signature of triplet excited state of cluster characterized by an intense absorption band at 500 nm accompanied with a less intense band at 620 nm 52 until 200 μs. Following 200 μs, a band absorbing in almost the entire visible range, extending to near-infrared part (Fig.4.2.9a) can be related to the evolution of charge separated species (Clus + -POM -). Globally fitting the kinetic traces with two-exponential model gives decay associated spectra with the time components of 32 μs and 312 μs (Fig.4.2.10a). While the 32 μs component is assigned to the rise of charge-separated species, the 312 μs time-constant is related to the Clus + /POM -recombination step. No clear rise for the charge separation is observed in kinetic traces, because the charge-separated species absorb at the same position as the excited state cluster.

Fig. 4 . 2 . 9

 429 Fig. 4.2.9 Transient absorption spectra of [Mo6I8Cl6] 2-, 60 µM and the three POM solutions (a) [PW11VO40] 4-, 600 µM; (b) [SiW11MoO40] 4-, 600 µM; and (c) [SiMo12O40] 4-, 600 µM; at indicated delay times between pump (λexc = 420 nm, 20 mW) and probe in deaerated acetonitrile. Insets show the biexponential fit of the kinetics giving the time-constants of the charge separation and recombination.

Fig. 4 . 2 .

 42 Fig. 4.2.10 Decay associated difference spectra of [Mo6I8Cl6] 2-cluster 60 µM with (a) [PW11VO40] 4-600 µM, (b) [SiW11MoO40] 4-600 µM, and (c) [SiMo12O40] 4-600 µM POMs in deaerated acetonitrile obtained by global fit of kinetic traces (λexc = 420 nm, 20 mW).

Fig. 4 . 2 .

 42 Fig. 4.2.11 Comparison of spectroelectro-chemical spectranoted as SEC (sum of oxidized cluster and reduced POM spectra) with nanosecond transient absorption (TA) spectra observed for the charge separated state of (a) [PW11VO40] 4-, (b) [SiW11MoO40] 4-, and (c) [SiMo12O40] 4-.

  -state and time-resolved studies on [W6I8Cl6] 2-@2CD:The recorded absorption and emission spectra of the cluster are shown in Fig.4.3.1. The absorption band of the tungsten cluster is blue shifted as compared to the molybdenum analogue (see Fig.1in Article manuscript #3) and the spectrum shows dominance of intense charge transfer transitions in near ultraviolet range, which is typical of hexanuclear tungsten halide clusters 53 . Upon excitation of [W6I8Cl6] 2-@2CD at 420 nm in deaerated water, an intense emission band is observed (centered at around 680 nm) in the red and NIR region, similar to TBA2[W6I8Cl6]that without CD in acetonitrile reported by Gray et al.54 .

Fig. 4 . 3 . 1

 431 Fig. 4.3.1 Steady-state absorption (red) and emission (blue) spectra of [W6I8Cl6] 2-@2CD in argon saturated water, λexc = 420 nm.In order to obtain the emission lifetime and the absorption profile of the excited state of the [W6I8Cl6] 2-@2CD cluster, time-resolved emission and transient absorption studies in nanosecond timescale were performed in deaerated water. Transient absorption spectra shown in Fig.4.3.2a exhibit the absorption features of the triplet-excited state of the cluster (λexc = 420 nm) in the entire visible range with the maximum at about 400 nm. The lifetime of the excited state is determined by monoexponential global fitting of the TA kinetic traces and is found to be about 10 µs, consistent with that reported by Gray et al.54 . This was further confirmed by recording the time-resolved emission (λexc = 420 nm) which shows the decay of the emission band yielding a time-constant of about 10 µs (Fig.4.3.2b), identical to the lifetime obtained with TA study. The consistency of the photophysical data with that of the reported literature

Fig. 4 .Fig. 4 . 3 . 3

 4433 Fig. 4.3.2 (a)Nanosecond transient absorption spectra and (b) time-resolved emission spectra at indicated delay times of the [W6I8Cl6] 2-@2CD in argon saturated water. Pump power = 20 mW at λexc = 420 nm. The interaction between the photosensitizer (i.e., [W6I8Cl6] 2-enveloped inside two CD units) and the electron acceptor/catalyst (Na6P2W18O62) was examined by measuring the emission quenching of the excited state of the cluster in argon saturated aqueous solutionsThe steady state emission intensities and lifetimes of the cluster were measured upon addition of different POM concentrations. The absorption spectra of the cluster with the different POM concentrations are shown in Fig. 4.3.3. It shows a small contribution of the POM absorption at the excitation wavelength of the cluster. Therefore, the emission spectra shown later are corrected for the portion of photons absorbed by the POM.

Fig. 4 .

 4 Fig. 4.3.4 (a) Steady-state, (b) time-resolved emission quenching of the [W6I8Cl6] 2-@2CD inclusion complex by [P2W18O62] 6-in argon saturated water, λexc=420 nm, and (c) steady-state and time-resolved Stern-Volmer plot 4.3.3 Role of cyclodextrin (CD) on electron transfer efficiency It is interesting to investigate the effect of increased concentration CDs on the quenching efficiency of the ([W6I8Cl6] 2-@2CD) by the POM. Two contrasting effects were expected: (a)increased quenching due to decreased coulombic repulsions or(b) decreased quenching due to the increased distance between cluster and POM. Therefore, to evaluate the above effects the emission intensity and the lifetime of the aqueous solution containing (i) ([W6I8Cl6] 2-@2CD), (ii) ([W6I8Cl6] 2-@2CD) and POM, and (iii) ([W6I8Cl6] 2-@2CD)-POM with increased CD

Fig. 4 . 3 . 5

 435 Fig. 4.3.5 Steady-state absorption spectra of [W6I8Cl6] 2-@2CD cluster-POM solution with increasing concentration of CD in deaerated water.

Fig. 4 .

 4 Fig. 4.3.6 (a) Steady-state,(b) time-resolved emission quenching of the [W6I8Cl6] 2-@2CD inclusion complex by [P2W18O62] 6-with the increased concentration of cyclodextrin CD in argon saturated water, λexc = 420 nm. Note :(i), (ii) and (iii) indicate emission data for [W6I8Cl6] 2-@2CD, [W6I8Cl6] 2-@2CD and [P2W18O62] 6-POM, and [W6I8Cl6] 2-@2CD-[P2W18O62] 6-POM with increasing equivalents of CD, respectively. τi=10µs, τiii= 3.5µs.

Fig. 4 .

 4 Fig. 4. 3.7 (a) Steady-state absorption spectra (b) steady-state emission spectra, and (c) time-resolved emission spectra of [W6I8Cl6] 2-@2CD cluster with increasing concentration of -CD in deaerated water.

4. 3 . 4

 34 Transient absorption spectroscopy of cluster in the presence of POM and CD Nanosecond transient absorption spectroscopy was employed to further elaborate on the charge transfer and recombination dynamics of the (i) [W6I8Cl6] 2-@2CD cluster in the presence of POM and (ii) cluster/POM with increased concentration of CD.Transient absorption spectra of the cluster solution in the presence of 13 equivalents of polyoxometalate were recorded in argon saturated aqueous solution (Fig.4.3.9a). When the [W6I8Cl6] 2-@2CD cluster was excited at 420 nm, the characteristic TA signal with the maximum absorption at about 400 nm, corresponding to the excited state of the cluster, was observed at 100 ns. Following 100 ns, this absorption feature begins to decay to concomitantly form a new broad transient band absorbing in the entire visible range with the absorption maximum at about 700 nm. Global fitting of the TA kinetic traces (Fig.4.3.9b) converged with two-time constants of about 2 µs and 70 µs corresponding to the formation and decay of the charge separated state, respectively.

Fig. 4 .Fig. 4 .

 44 Fig. 4.3.9 (a) Nanosecont transient absorption spectra of [W6I8Cl6] 2-@2CD (300 µM) in the presence of 13 equivalents of [P2W18O62] 6-POM in argon saturated water and (b) global bi-exponential kinetic fitting. Pump power = 20 mW at λexc = 420 nm.400

  clusters of the type [M6I8Cl6] 2-(M= Mo or W) and model Keggin and Dawson-type polyoxometalates (POMs) in organic and aqueous solvents. In the first part of this chapter, a complete photophysical investigation using steady-state absorption and emission, time-resolved emission and transient absorption spectroscopy was carried out in acetonitrile on [Mo6I8Cl6] 2-cluster and different POMs with varying charges. The second part of this chapter investigates the PET between the [Mo6I8Cl6] 2-cluster and three Kegging POMs having the same charge. Combining the results of the two studies, it appears that the quenching efficiency is influenced by the charge density of the POMs. Additionally, the spectroscopic signatures of the charge separated state obtained by transient absorption spectroscopy overlap with the sum of the spectroelectrochemical spectra of the oxidized cluster and the reduced POMs, thus approving the photo-driven electron transfer from excited cluster to POMs.

Fig. 5 . 1

 51 Fig.5.1 Molecular structures of perylene-POM organic-inorganic hybrids studied. Adapted from Odobel et al. 16 . In another report, the authors studied the efficiency of fluorescence quenching as a function of linker flexibility in a covalently linked porphyrin-Dawson system 17 (Fig.5.2). It was observed that the appended POM quenches fluorescence from the porphyrin excited state, with the rate being controlled by the flexibility of the linker. The rigid linker does not favor fluorescence

Fig. 5 . 2

 52 Fig.5.2 Molecular structures of porphyrin-POM organic-inorganic hybrids. Adapted from Odobel et al.17

  Yet another report highlights the energy and electron transfer processes in a multi-porphyrin-POM system (Fig.5.3) and subsequent charge accumulation on the POM in the presence of a sacrificial donor18 . Upon excitation of the peripheral Zn-porphyrin (ZnP) unit, an efficient energy transfer occurs to freebase porphyrin (FbP), which in turn transfers an electron to the POM. The subsequent hole transfer from FbP to the other ZnP helps to separate the charges, thus stabilizing the charge-separated state. Finally, in the presence of a sacrificial electron donor, the POM can store several electrons on a single cluster.

Fig. 5 . 3

 53 Fig.5.3 Molecular structures of ZnP-POM organic-inorganic hybrids. Adapted from Odobel et al.18

Fig. 5 . 5

 55 Fig.5.5 Molecular structures of BODIPY-POM organic-inorganic hybrids. Adapted from Streb et al. 20

Fig. 5 . 6

 56 Fig.5.6 Molecular structures of BODIPY-POM organic-inorganic hybrids. Adapted from Izzet et al. 21

Fig. 5 . 7

 57 Fig.5.7 Molecular structures of (a) (n-Bu4N + )2BDP-POV-BDP and (b) (n-Bu4N + )2DSBDP-POV-DSBDP used in the present study A complete photophysical investigation was carried out on the two hybrids in non-acidic dichloromethane using steady-state absorption and emission, time-resolved emission, and transient absorption spectroscopic methods (from femto to nanosecond time scales). Then, the effect of introducing protons (with trifluoroacetic acid, TFA) on the charge separation and recombination dynamics was studied, primarily with steady-state emission and transient absorption spectroscopies.

Fig. 5 . 8

 58 Fig.5.8 Cyclic voltammograms of (a) POV, parent BDP, BDP-POV-BDP, and (b) POV, parent DSBDP, and DSBDP-POV-DSBDP (from top to bottom) in deaerated DCM in the presence of 0.1M TBAPF6, Scan rate = 50 mVs -1 . Asterix (*) in the voltammograms depicts the reduction wave of POV.

Fig. 5 . 9

 59 Fig.5.9 Cyclic voltammograms of the parent POV unit in the absence and in the presence of TFA in deoxygenated DCM. Scan rate=50 mVs -1 .

  5.10a). The spectra of BDP-POV-BDP and DSBDP-POV-DSBDP are, in fact, the sum of the absorption spectra of the BODIPY and POV units, suggesting negligible electronic communication between the components (see the dotted and dashed curves in Fig.5.10a). No change in the spectral shape is observed upon addition of 500 eq. of trifluoroacetic acid (see Fig.B1for the absorption spectra in the absence and in the presence of the acid in Annexe B).

Fig. 5 .

 5 Fig.5.10 Steady-state (a) absorption spectra of parent BDP, parent DSBDP, POV unit, BDP-POV-BDP and DSBDP-POV-DSBDP hybrid and (b) emission spectra of parent BDP (λexc = 495 nm) and DSBDP and (λexc = 580 nm) their corresponding hybrids in DCM. The dotted and dashed curved represent the sum of the absorption of BODIPY and POV units.

Fig. 5 .

 5 Fig.5.11 Steady-state emission spectra of (a) parent BDP and BDP-POV-BDP hybrid (λexc = 495 nm) and (b) parent DSBDP and BDP-POV-BDP hybrid (λexc = 580 nm) in the absence and in the presence of 500 eq of TFA in DCM.

in Fig. 5 .Fig. 5 .

 55 Fig.5.12 Fluorescence decays of (a) the reference BDP and BDP-POV-BDP hybrid (λexc = 495 nm, λem = 540 nm) and (b) the reference DSBDP and DSBDP-POV-DSBDP hybrid (λexc = 350 nm, λem = 635 nm) in DCM.

Fig. 5 .

 5 Fig.5.13 Femtosecond transient absorption spectra of the parent BDP (10µM) detailed at specific delay times (a) [-0.1 to 0.4 ps] and (b) [0.4 to 2700 ps] in deaerated DCM (λexc= 495 nm).

Fig. 5 .

 5 Fig.5.14 Multiexponential fitting of the femtosecond TA kinetic traces of parent BDP in DCM (λexc = 495 nm) at indicated wavelengths.

Fig. 5 .

 5 Fig.5.15 Comparison of the fs TA kinetic traces of BDP at different angles between the polarizations of the pump and probe pulses to evidence the anisotropy effect. Similarly, the femtosecond TA spectra of DSBDP were recorded in DCM upon excitation at 620 nm. Until 370 fs, (see first time window from -0.3 to 1 ps, Fig. 5.16.a), the spectra show a rise in the typical DSBDP transient absorption bands at 450 and 640 nm concomitant with a rise in the negative TA bands related to the sum of GSB and stimulated emission at 640 nm and another small emission band at 700 nm (inverted ground-state absorption and emission bands are added in the first time window, Fig.5.16.a). Then from 370 fs to 1 ps, the principal excitedstate signal at 450 nm remains stagnant (no change in the position and the instensity) while the negative TA bands undergo a slight red shift. This dynamic red-shift in the emission band

Fig. 5 .Fig. 5 .

 55 Fig.5. 16 Femtosecond transient absorption spectra of the parent DSBDP (10µM) detailed at specific delay times (a) [-0.3 to 1 ps] and (b) [1 to 2700 ps] in DCM, (λexc = 620 nm). The black and grey curves represent ground state absorption and emission of DSBDP, respectively.

Fig. 5 .

 5 Fig.5.18 Comparison of the fs TA kinetic traces of DSBDP at different angles between the polarizations of the pump and probe pulses to evidence the anisotropy effect.

Fig. 5 .

 5 Fig.5.19 Femtosecond transient absorption spectra of the BDP-POV-BDP hybrid (10µM) detailed at specific delay times (a) [-0.3 to 1 ps], (b) [1 to 350 ps], and (c) [350 to 1700 ps] in deoxygenated DCM/DMF (99:1,v/v), (λexc = 495 nm).

Fig. 5 .

 5 Fig.5.20 Multiexponential individual fitting of femtosecond TA kinetic traces of the BDP-POV-BDP hybrid in deoxygenated DCM/DMF (99:1,v/v), (λexc = 495 nm) at indicated wavelengths.

Fig. 5 .

 5 Fig.5.21 (a) Multiexponential global fitting of femtosecond TA kinetic traces of the BDP-POV-BDP hybrid at indicated wavelengths and (b) associated DADS obtained in deoxygenated DCM/DMF (99:1,v/v), (λexc = 495 nm).

Fig. 5 .

 5 Fig.5.22 Comparison of the fs-TA kinetic traces of BDP-POV-BDP hybrid at different angles between the polarizations of the pump and probe pulses to evidence the anisotropy effect.

Fig. 5 .

 5 Fig.5.23 Femtosecond transient absorption spectra of the DSBDP-POV-DSBDP hybrid (10µM) detailed at specific delay times a. [-0.3 to 1 ps] and b. [1 to 2560 ps], in deoxygenated DCM/DMF (99:1,v/v), (λexc = 620 nm).

Fig. 5 .

 5 Fig.5.24 Comparison of the femtosecond transient absorption spectra of DSBDP-POV-DSBDP hybrid at 1 ps and 2.5 ns with the spectroelectrochemical (SEC) spectrum of one-electron oxidized DSBDP (DSBDP .+ ).

Fig. 5 .

 5 Fig.5.25 Multiexponential individual fitting of femtosecond TA kinetic traces of the DSBDP-POV-DSBDP hybrid in deoxygenated DCM/DMF (99:1,v/v), (λexc = 620 nm) at indicated wavelengths.

Fig. 5 .

 5 Fig.5.26 (a) Multiexponential global fitting of femtosecond TA kinetic traces of the DSBDP-POV-DSBDP hybrid and (b) DADS obtained in deoxygenated DCM/DMF (99:1,v/v), (λexc= 580 nm) at indicated wavelengths.

  recorded in the UV-visible range upon exciting the BDP and DSBDP at 495 and 620 nm, respectively.a. BDP-POV-BDPUpon adding 500 eq. of trifluoroacetic acid (TFA) to the solution of BDP-POV-BDP hybrid, the electron transfer was trigerred in a much shorter time-delay as compared to the one without acid. Indeed, as shown in the femtosecond transient absorption spectra in the presence of TFA in Fig.5.27, the panel a. reveals the formation of the singlet excited state of BDP until 0.5 ps, which further decays rapidly to simultaneously form a sharp TA band at 400 nm consistent with the formation of BDP radical cation, as previously observed with BDP-POV-BDP hybrid without acid(panel b. of Fig. 5.19). DADS obtained by the global fitting of the transient absorption data (Fig.5.28 a and b) exhibits three time components of 4 ps, 32 ps, and 200 ps and a non-decaying component associated to a constant. The DADS associated to the 4-ps component depicts the decay of singlet BDP bands at 350 and 420 nm along with a rise in the charge-separated band at 400 nm. Notably, the time-constant of the charge separation is enhanced by about 20 times as compared to that in the absence of the acid (τ = 74 ps). This enhancement in the charge-separation process is supported by the positively shifted reduction potential of the POV in the presence of acid (see Fig.5.9). The driving force of electron transfer is thermodynamically more favourable (ΔGcs= -1.10 eV) as compared to that without protons (ΔGcs= -0.75 eV). Then, the DADS corresponding to the 32-ps component shows the decay of a TA signal in the range of 320 to 450 nm, which appears to be a mixture of the singlet excited state of BDP and the charge-separated state. This means that an oxidized specie of BDP is undergoing recombination at the same time-frame of the energy transfer process from excited BDP to POV. The energy transfer process is indeed possible in another conformer with a relatively longer center-to-center distance between BDP and POV. The 200-ps DADS shows the decay of the spectral signal characteristic of the BDP excited state suggesting yet another energy transfer process from the excited BDP to the POV in a longer-distanced conformer.

Fig. 5 .

 5 Fig.5.27 Femtosecond transient absorption spectra of the BDP-POV-BDP hybrid (10µM) detailed at specific delay times a. [-0.3 to 0.5 ps] and b. [0.5 to 2700 ps], in deoxygenated DCM/DMF (99:1,v/v) with 500 eq. of TFA (λexc= 495 nm).

Fig. 5 .Fig. 5 .Fig. 5 .

 555 Fig.5.28 (a) Multiexponential global fitting of the femtosecond TA kinetic traces of the BDP-POV-BDP hybrid and (b) DADS obtained in deoxygenated DCM/DMF (99:1,v/v) with 500 eq. of TFA (λexc= 495 nm).

Fig. 5 .Fig. 5 .

 55 Fig.5.31 Nanosecond transient absorption spectra of (a) BDP-POV-BDP hybrid (λexc = 495 nm) and (b) DSBDP-POV-DSBDP hybrid (λexc = 620 nm) in deoxygenated DCM/DMF (99:1,v/v).

Fig. 5 .

 5 Fig.5.33 Nanosecond transient absorption kinetics traces of (a) BDP-POV-BDP hybrid (λexc = 495 nm) and (b) DSBDP-POV-DSBDP hybrid (λexc = 620 nm) with and without acid at 400 and 680 nm, respectively.

Fig. 5 .

 5 Fig.5.34 Summary of energetics of photoinduced electron transfer in the absence and presence of acid in (a) BDP-POV-BDP hybrid (λexc = 495 nm) and (b) DSBDP-POV-DSBDP hybrid (λexc = 620 nm).

Fig. 6 . 1

 61 Fig.6.1 Diagrammatic representation of (a) HER in solution and (b) photoelectrocatalysis using cluster-CD-POM supramolecular assemblies Further, the photophysical investigations on BODIPY-POV organic-inorganic hybrid systems provide an important instance of PET in covalently linked, non-conjugated BODIPY-POM systems. The next step would constitute the study of charge accumulation (pump-pump-probe experiments) on POV in the presence of a sacrificial electron donor (Fig.6.2). The lifetime of

Fig. 6 . 2

 62 Fig.6.2 Illustration of charge-accumulation in BODIPY-POM hybrid in the presence of sacrificial electron donor

  Fig.A1. Steady state (a) absorption spectra and (b) emission spectra of BDP-EtPh (λexc =534 nm) and EtPh-BDP-EtPh (λexc =578 nm) conjugates in benzonitrile

  Fig. A3. Multiexponential fitting of the kinetic traces of mono Fc-BDP dyad (λexc =580 nm) in benzonitrile at indicated wavelengths.

Fig

  Fig. A5. Femtosecond transient absorption spectra of mono-Fc-BDP-C60 triad (8 µM) at specified delay times: (a)-0.15 to 0.18 ps; (b) 0.11 to 2 ps; (c) 2 to 100 ps, and (d)100 to 2700 ps in deaerated benzonitrile. λexc = 580 nm. The pink curve in panel (a) represents the SEC signature of C60 anion

Fig. B1 .FiguresFig. 1 . 1

 B111 Fig. B1. Absorption spectra of parnt BODIPYs and the two hybrids in the presence and absence of acid

Fig. 2 . 1

 21 Fig. 2.1 Schematic representation of TCSPC setup at PPSM. .............................................................. Fig. 2.2 (a) A general representation of a TA set-up showing the pump pulse photoexciting the sample and the probe pulse monitoring the sample response at different delay times after photoexcitation and (b) contributions in a ΔA spectrum. ...................................................................................................... Fig. 2.3 Schematic representation of the nanosecond TA set-up built at ISMO. Letters I, L and M represent iris, lens and mirror, respectively. B.S. is beam splitter and O.C. is optical chopper and T.O. is transparent optics. .............................................................................................................................. Fig. 2.4 Timing control and synchronization diagram of EKSPLA laser, CCD, and probe laser. ........ Fig. 2.5 Simplified scheme of a femtosecond transient absorption set-up. ........................................... Fig. 2.6 Detailed schematic diagram of the femtosecond transient absorption set-up housed in Quantum Electric Lab, Adam Mickiewizc University, Poznan. This figure is prepared with the help of Dr. Sabina Brazevic. ..............................................................................................................................

Fig. 2 .Fig. 3 . 1 . 1 Fig. 3 . 2 . 1 Fig. 4 . 2 . 1

 2311321421 Fig. 2.7 (a) Uncorrected, (b) chirp correction and (c) chirp-corrected signals of pure DCM, λexc = 495 nm. .........................................................................................................................................................

Fig. 4 .

 4 Fig. 4. 3.7 (a) Steady-state absorption spectra (b) steady-state emission spectra, and (c) time-resolved emission spectra of [W6I8Cl6] 2-@2CD cluster with increasing concentration of -CD in deaerated water. ................................................................................................................................................... Fig. 4.3.8 Structural representation of the supramolecular POM@nCD showing the 1:1, 1:2, and 1:3 arrangement (from left to right) 46 . ....................................................................................................... Fig. 4.3.9 (a) Nanosecont transient absorption spectra of [W6I8Cl6] 2-@2CD (300 µM) in the presence of 13 equivalents of [P2W18O62] 6-POM in argon saturated water and (b) global bi-exponential kinetic fitting. Pump power = 20 mW at λexc = 420 nm. ............................................................................... Fig. 4.3.10 (a) Nanosecont transient absorption spectra of 1:13 eq. of [W6I8Cl6] 2-@2CD (300 µM)-[P2W18O62] 6-POM in the presence of 40 equivalents of -CD in argon saturated water and (b) global bi-exponential kinetic fitting. Pump power = 20 mW at λexc = 420 nm. ...........................................

Fig. 5 . 1

 51 Fig.5.1 Molecular structures of perylene-POM organic-inorganic hybrids studied. Adapted from Odobel et al. 16 . ..................................................................................................................................... Fig.5.2 Molecular structures of porphyrin-POM organic-inorganic hybrids. Adapted from Odobel et al. 17 ...................................................................................................................................................... Fig.5.3 Molecular structures of ZnP-POM organic-inorganic hybrids. Adapted from Odobel et al. 18 Fig.5.4 Molecular structures of BODIPY-POM organic-inorganic hybrids. Adapted from Izzert et al. 19 ............................................................................................................................................................. Fig.5.5 Molecular structures of BODIPY-POM organic-inorganic hybrids. Adapted from Streb et al. 20 ............................................................................................................................................................. Fig.5.6 Molecular structures of BODIPY-POM organic-inorganic hybrids. Adapted from Izzet et al. 21 ............................................................................................................................................................. Fig.5.7 Molecular structures of (a) (n-Bu4N + )2BDP-POV-BDP and (b) (n-Bu4N + )2DSBDP-POV-DSBDP used in the present study ........................................................................................................ Fig.5.8 Cyclic voltammograms of (a) POV, parent BDP, BDP-POV-BDP, and (b) POV, parent DSBDP, and DSBDP-POV-DSBDP (from top to bottom) in deaerated DCM in the presence of 0.1M TBAPF6, Scan rate = 50 mVs -1 . Asterix (*) in the voltammograms depicts the reduction wave of POV. ............................................................................................................................................................. Fig.5.9 Cyclic voltammograms of the parent POV unit in the absence and in the presence of TFA in deoxygenated DCM. Scan rate=50 mVs -1 . .......................................................................................... Fig.5.10 Steady-state (a) absorption spectra of parent BDP, parent DSBDP, POV unit, BDP-POV-BDP and DSBDP-POV-DSBDP hybrid and (b) emission spectra of parent BDP (λexc = 495 nm) and DSBDP and (λexc = 580 nm) their corresponding hybrids in DCM. The dotted and dashed curved represent the sum of the absorption of BODIPY and POV units. ....................................................... Fig.5.11 Steady-state emission spectra of (a) parent BDP and BDP-POV-BDP hybrid (λexc = 495 nm) and (b) parent DSBDP and BDP-POV-BDP hybrid (λexc = 580 nm) in the absence and in the presence of 500 eq of TFA in DCM. .................................................................................................................. Fig.5.12 Fluorescence decays of (a) the reference BDP and BDP-POV-BDP hybrid (λexc = 495 nm, λem = 540 nm) and (b) the reference DSBDP and DSBDP-POV-DSBDP hybrid (λexc = 350 nm, λem = 635 nm) in DCM. ....................................................................................................................................... Fig.5.13 Femtosecond transient absorption spectra of the parent BDP (10µM) detailed at specific delay times (a) [-0.1 to 0.4 ps] and (b) [0.4 to 2700 ps] in deaerated DCM (λexc= 495 nm). .............. Fig.5.14 Multiexponential fitting of the femtosecond TA kinetic traces of parent BDP in DCM (λexc = 495 nm) at indicated wavelengths. ...................................................................................................... Fig.5.15 Comparison of the fs TA kinetic traces of BDP at different angles between the polarizations of the pump and probe pulses to evidence the anisotropy effect. ........................................................

Fig. 5 .

 5 Fig.5. 16 Femtosecond transient absorption spectra of the parent DSBDP (10µM) detailed at specific delay times (a) [-0.3 to 1 ps] and (b) [1 to 2700 ps] in DCM, (λexc = 620 nm). The black and grey curves represent ground state absorption and emission of DSBDP, respectively. .............................. Fig.5.17 Multiexponential fitting of the femtosecond TA kinetic traces of parent DSBDP in DCM, (λexc = 620 nm) at indicated wavelengths. ........................................................................................... Fig.5.18 Comparison of the fs TA kinetic traces of DSBDP at different angles between the polarizations of the pump and probe pulses to evidence the anisotropy effect. .................................. Fig.5.19 Femtosecond transient absorption spectra of the BDP-POV-BDP hybrid (10µM) detailed at specific delay times (a) [-0.3 to 1 ps], (b) [1 to 350 ps], and (c) [350 to 1700 ps] in deoxygenated DCM/DMF (99:1,v/v), (λexc = 495 nm). .............................................................................................. Fig.5.20 Multiexponential individual fitting of femtosecond TA kinetic traces of the BDP-POV-BDP hybrid in deoxygenated DCM/DMF (99:1,v/v), (λexc = 495 nm) at indicated wavelengths. ............... Fig.5.21 (a) Multiexponential global fitting of femtosecond TA kinetic traces of the BDP-POV-BDP hybrid at indicated wavelengths and (b) associated DADS obtained in deoxygenated DCM/DMF (99:1,v/v), (λexc = 495 nm). .................................................................................................................. Fig.5.22 Comparison of the fs-TA kinetic traces of BDP-POV-BDP hybrid at different angles between the polarizations of the pump and probe pulses to evidence the anisotropy effect. ............................ Fig.5.23 Femtosecond transient absorption spectra of the DSBDP-POV-DSBDP hybrid (10µM) detailed at specific delay times a. [-0.3 to 1 ps] and b. [1 to 2560 ps], in deoxygenated DCM/DMF (99:1,v/v), (λexc = 620 nm). .................................................................................................................. Fig.5.24 Comparison of the femtosecond transient absorption spectra of DSBDP-POV-DSBDP hybrid at 1 ps and 2.5 ns with the spectroelectrochemical (SEC) spectrum of one-electron oxidized DSBDP (DSBDP .+ ). .......................................................................................................................................... Fig.5.25 Multiexponential individual fitting of femtosecond TA kinetic traces of the DSBDP-POV-DSBDP hybrid in deoxygenated DCM/DMF (99:1,v/v), (λexc = 620 nm) at indicated wavelengths. Fig.5.26 (a) Multiexponential global fitting of femtosecond TA kinetic traces of the DSBDP-POV-DSBDP hybrid and (b) DADS obtained in deoxygenated DCM/DMF (99:1,v/v), (λexc= 580 nm) at indicated wavelengths. ........................................................................................................................ Fig.5.27 Femtosecond transient absorption spectra of the BDP-POV-BDP hybrid (10µM) detailed at specific delay times a. [-0.3 to 0.5 ps] and b. [0.5 to 2700 ps], in deoxygenated DCM/DMF (99:1,v/v) with 500 eq. of TFA (λexc= 495 nm). ................................................................................................... Fig.5.28 (a) Multiexponential global fitting of the femtosecond TA kinetic traces of the BDP-POV-BDP hybrid and (b) DADS obtained in deoxygenated DCM/DMF (99:1,v/v) with 500 eq. of TFA (λexc= 495 nm). .................................................................................................................................... Fig.5.29 Femtosecond transient absorption spectra of the DSBDP-POV-DSBDP hybrid (10µM) detailed at specific delay times (a) [-0.3 to 0.3 ps] and (b) [0.2 to 2700 ps], in deoxygenated DCM/DMF (99:1,v/v) with 500 eq. of TFA (λexc = 620 nm). ............................................................. Fig.5.30 (a) Multiexponential global fitting of femtosecond TA kinetic traces of the DSBDP-POV-DSBDP hybrid and (b) DADS obtained in deoxygenated DCM/DMF (99:1,v/v), (λexc= 580 nm) with 500 eq. of TFA. ................................................................................................................................... Fig.5.31 Nanosecond transient absorption spectra of (a) BDP-POV-BDP hybrid (λexc = 495 nm) and (b) DSBDP-POV-DSBDP hybrid (λexc = 620 nm) in deoxygenated DCM/DMF (99:1,v/v). .............. Fig.5.32 Fitting of nanosecond transient absorption kinetics of (a) BDP-POV-BDP hybrid (λexc = 495 nm) and (b) DSBDP-POV-DSBDP hybrid (λexc = 620 nm) at indicated wavelengths. ..... Fig.5.33 Nanosecond transient absorption kinetics traces of (a) BDP-POV-BDP hybrid (λexc = 495 nm) and (b) DSBDP-POV-DSBDP hybrid (λexc = 620 nm) with and without acid at 400 and 680 nm, respectively. ......................................................................................................................................... Fig.5.34 Summary of energetics of photoinduced electron transfer in the absence and presence of acid in (a) BDP-POV-BDP hybrid (λexc = 495 nm) and (b) DSBDP-POV-DSBDP hybrid (λexc = 620 nm). .............................................................................................................................................................

Fig. 6 . 1

 61 Fig.6.1 Diagrammatic representation of (a) HER in solution and (b) photoelectrocatalysis using cluster-CD-POM supramolecular assemblies ...................................................................................... Fig.6.2 Illustration of charge-accumulation in BODIPY-POM hybrid in the presence of sacrificial electron donor ...................................................................................................................................... Fig.6.3 Illustration of charge-stabilization in BODIPY-C60-POM hybrid...........................................

Positive ΔA Negative ΔA

	2.) Probe laser (STM-2-UV LEUKOS) operated at a 2 kHz repetition rate, which is
	reduced by means of an optical chopper to 20 Hz. It is a supercontinuum laser (350-
	2400 nm) with an average output power of 24.2 mW.
	3.) ICCD (PI-MAX 4, Princeton Instrument) cameras gated at 20 Hz.
	4.) Photodiode (THORLABS DET 10 A/M)
	5.) Optical Chopper (THORLABS)
	6.) A quartz cell for the sample
	7.) System of lenses and mirrors for the laser beam alignment
	EKSPLA laser consists of a widely studied Nd: YAG (made up of a synthetic crystalline
	material Yttrium Aluminum Garnet (YAG) doped with neodymium (Nd)) as an active laser
	medium. The resultant pulse width from EKSPLA is 4 ns, and the peak optical power is tens of
	megawatts.
	a.	b.
		2. It
	consists of:
	1.) EKSPLA Laser (Nd: YAG laser head and OPO) that can be operated on a 10 Hz
	repetition rate, tunable from 410-2500 nm.

CHAPTER 3. Photoinduced processes in organic D-A assemblies 3.1 Photoinduced electron and energy transfer processes in covalently linked BDP-C60 based molecular assemblies

	b. Spectroelectrochemistry
	The solutions containing oxidized cluster and reduced POMs were analyzed with the help of
	Perkin Elmer Lambda 750 UV-Vis-NIR spectrophotometer. Spectroelectrochemical
	measurements of the BODIPYs were performed at PPSM laboratory, ENS Paris-Saclay, with
	the help of an optically transparent thin-layer electrochemical cell (OTTLE) connected with a
	PalmSens4 potentiostat. The cell employs a platinum mesh as a working electrode, a platinum
	wire as a counter electrode and a silver wire as a quasi-reference electrode. 0.1 M of TBAPF6
	was used as a supporting electrolyte. The spectra were recorded in Varian 5000 Cary 3.1.1 Introduction and literature survey
	spectrophotometer. For oxidation and reduction of inorganic molecules, i.e., cluster and POMs,
	SEC was carried out at ILV, in three-electrode cell system described above (see 2.3a). Using the
	controlled potential coulometry method, the samples were quantitatively oxidized/reduced in a
	stepwise manner. Linear scan voltammetry (rotating dick electrode) measurements were
	recorded at each coulometric step (before and after reduction) to ensure 100% reduction /
	oxidation process. The solutions containing reduced POMs or oxidized cluster were then
	analyzed with the help of Perkin-Elmer Lambda 750 UV-Vis-NIR spectrophotometer.
	Chronoamperometry was employed to oxidize or reduce the BODIPY, cluster and POM
	molecules. A UV-Vis spectroelectrochemical set-up was also made operational at ISMO.

Table 2

 2 Triplet formation quantum yields estimated using laser pulse actinometry for the two dyads and the triad under study

	Molecule	Triplet state Δε, M -1 cm -1 τ T, μs	ɸ T , %
	BDP-C 60	3 C 60 *	16 100	7 ± 0.8	40 ± 8
	DSBDP-C 60	3 DSBDP*	58 000	250 ± 20 37 ± 8
	DSBDP-C 60 -BDP 3 DSBDP*	58 000	280 ± 20 20 ± 4

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, Junge, H., Schulz, T.,

Marquet, N., Spannenberg, A., Gladiali, S., & Beller, M. (2011)

. Synthesis, characterisation and application of iridium(III) photosensitisers for catalytic water reduction. Chemistry -A European Journal,

17, 6998-7006. 3. You, Y., & Nam, W. (2014)

. Designing photoluminescent molecular probes for singlet oxygen, hydroxyl radical, and iron-oxygen species. Chemical Science, 5, 4123-4135. 4. Cakmak, Y., Kolemen, S., Duman, S., Dede, Y., Dolen, Y., Kilic, B., Kostereli, Z., Yildirim, L. T., Dogan, A. L., Guc, D., & Akkaya, E. U. (2011). Designing excited states: Theory-guided access to efficient photosensitizers for photodynamic action. 11. Ma, J., Yuan, X., Küçüköz, B., Li, S., Zhang, C., Majumdar, P., Karatay, A., Li, X., Yaglioglu, H. G., Elmali, A., Zhao, J., & Hayvali, M. (2014). Resonance energy transfer-enhanced rhodaminnestyryl bodipy dyad triplet photosensitizers. Journal of Materials Chemistry C, 2, 3900-3913. 12. N. O., Blesener, T. S., et al. (2019). Synthesis, characterization, and electron-transfer properties of ferrocene-BODIPY-fullerene NIR absorbing triads: Are Catecholopyrrolidine-Linked Fullerenes a good architecture to facilitate electron-transfer? Chemistry-A European Journal, 25, 8401-8414. 27. Shao, S., Thomas, M. B., Park, K. H., Mahaffey, Z., Kim, D., & D'Souza, F. (2018). Sequential energy transfer followed by electron transfer in a BODIPY-bisstyryl BODIPY bound to C60 triad 76 1 3 via a 'two-point' binding strategy. 34. Bensasson, R. V., Bienvenüe, E., Fabre, C., Janot, J. M., Land, E. J., Leach, S., Leboulaire, V., Rassat, A., Roux, S., & Seta, P. (1998). Photophysical properties of three methanofullerene derivatives. Chemistry-A European Journal, 4, 270-278. 35. Marfin, Y. S., Merkushev, D. A., Usoltsev, S. D., Shipalova, M. V., & Rumyantsev, E. V. (2015). Fluorescent properties of 8-substituted BODIPY dyes: Influence of solvent effects. Journal of Fluorescence, 25, 1517-1526. version: The primacy of kinetics. Journal of Physical Chemistry Letters, 5, 4062-4072.

  3 C60* bands appear at around 400 and 710 nm with the lifetime of 9 μs.

		0.02		0.03			t= 86ns±40ns		0.04	t= 90ns±40ns
		0.015		0.01 0.015 0.02 0.025 A	710nm	a			0.02	b
	A	0.01		0 0.005	0 0.2 0.4 0.6 0.8 1 t,ms			A	0	0.05
		0 0.005								0.75ms 0.85ms 1ms 1.1ms 1.2ms	-0.06 -0.04 -0.02	A	0.02 0.03 0.04 0.01 0 -0.01	710nm t,ms 0 0.2 0.4 0.6 0.8 1 1.2	0.5ms 0.75ms 0.95ms 1.1ms 1.35ms
			400		500	600	700	800			400	500	600	700	800
		0.06									0.08
		0.02 0.04				t= 0ns±30ns	c			0.02 0.04 0.06	t= 70ns±30ns d
	A	-0.04 -0.02 0	A	0.05 0.04 0.02 0.03		710nm				A 0.4ms	0 -0.02 -0.04	A	0.06 0.04	710nm	0.4ms
				0.01						0.5ms	-0.06	0.02	0.5ms
		-0.08 -0.06		0	0 0.2 0.4 0.6 0.8 1 t,ms			0.7ms 0.9ms 1ms	-0.1 -0.08	0	0 0.2 0.4 0.6 0.8 1 1.2 t,ms	0.7ms 1ms 1.2ms
			400		500	600	700	800		400	500	600	700	800
						Wavelength (nm)				Wavelength (nm)

Table 1

 1 Fluorescence lifetimes (contribution in %) in benzonitrile are expressed in nanoseconds with an error of ± 40 ps and ± 90 ps for 495 and 635 nm excitation, respectively, the longest lifetime was fixed with that of the parent BODIPYs, a λexc = 495 nm, λem = 535 nm, b λexc = 635 nm, λem = 650 nm, c λexc = 495 nm, λem = 650 nm

	Molecule	τ1 (amplitude, wt.%)	τ2 (amplitude, wt.%)	τ3 (amplitude, wt.%)	τ4 (amplitude, wt.%)
	BDP a	4.57	-	-	-
	BDP-C60 a	0.04 (39.19%)	0.23 (60.57%)	4.57 (0.25%)	-
	DSBDP b	4.46	-	-	-
	DSBDP-C60 b	0.15 (25.82%)	0.73 (73.11%)	4.46 (1.06%)	-
	DSBDP-C60-BDP a	0.02 (59.7%)	0.09 (37.77%)	0.225 (5.4%)	4.57 (0.40%)
	DSBDP-C60-BDP b	0.18 (28.18%)	0.79 (70.15%)	4.46 (1.34%)	-
	DSBDP-C60-BDP c	0.33 (28%)	0.88 (70%)	4.46 (0.80%)	-

Table 3 .

 3 2.1 Absorption and emission characteristics of Fc-BDP dyad and the triad along with the two reference compounds in benzonitrile.

	Compound	abs, nm	em, nm	F, %	τ, ns
		(ε, M -1 cm -1 )			(amplitude, wt.%)
	BDP-EtPh a	514 (80 000)	578	75	3.40
	PhEt-BDP-EtPh b	575 (85 000)	607	73	3.40
	mono Fc-BDP b	390 (16 500),	555	2.2	0.013 (96%), 0.330
		534 (71 700)			(3.04%),
					3.40 (0.6%)
	mono Fc-BDP-C60 b	328 (108 000),	555	1.4	0.009 (99.8%),
		534 (67 100)			0.184 (0.18%),
					3.40 (0.01%)
	bis Fc-BDP	420 (20 500),	--	--	--
		580 (79 500)
	bis Fc-BDP-C60	328 (115 000),	--	--	--
		580 (80 100)

F : Fluorescence quantum yield with 10 % error a exc = 495 nm, em = 578 nm, b exc = 495 nm, em= 555 nm, quantum yield measurement of the BODIPY moiety with Rhodamine 6G used as standard (exc = 495 nm,  = 95% in ethanol)

Table 3 .

 3 2.2 Time constants obatined by multi-exponential fitting of kinetic traces of bis-Fc-BDP dyad at specific wavelengths

	0.05
	0
	A				440 nm
					555 nm
					673 nm
	-0.05				1150 nm
	0.1	1	10	100	1000
			t (ps)

  Fc-BDP (Fig. 3.2.5 and table 3.2.1), which can be related to the short component of about 3 ps obtained by femtosecond TA measurements.

	Wavelength	IRF	A1	τ1 (ps)	A2	τ2 (ps)	A3	τ3 (ps)
	(nm)	(ps)
	440	0.7	0.03	2 ± 0.1	0.01	26 ± 4	0.005	420 ± 18
	( 1 BDP*, BDP .-		(d)	( 1 BDP*)	(d)	(BDP .-)	(d)	( 3 BDP*)
	& 3 BDP*)
	555	0.6	-0.04	2 ± 0.2	0.06	25 ± 4	-0.2	380 ± 26
	(GSB & BDP .-)		(r)	(BDP .-)	(d)	(BDP .-)	(r)	(GSB)
	673	0.3	0.01	1 ± 0.5	-0.005	26 ± 2	0.005	370 ± 7
	( 1 BDP* &		(d)	( 1 BDP*)	(r)	( 3 BDP*)	(d)	( 3 BDP*)
	3 BDP*)
	1150 nm	0.3 0.001	2 ± 0.01	0.001	25 ± 2
	( 1 BDP* & BDP .-)		(d)	( 1 BDP*)	(d)	(BDP .-)

of Annexe A. The TA kinetic traces of mono-Fc-BDP exhibit very similar dynamic features as that in bis-Fc-BDP analog. It is worth noting that a very short lifetime below the IRF (96%) was obtained in TCSPC decay of mono

  BDP dyads (see the fitting of 673 nm kinetic trace in Fig.3.2.8 and Table 3.2.2). Similarly, the multiexponential fit of the 1050 nm kinetic trace which majorly corresponds to C60 anion with small contributions from 1 BDP* and BDP anion transient species, yields three timeconstants. Again, the IRF considers the rise of C60 anion formed by an ultrafast electron transfer

	from 1 BDP* to C60. The time-constant of about 2 ps corresponds (appears as a plateau in the
	kinetic trace) to the rise of the charge separated state Fc	.+ -BDP	.--C60 as was previously observed
	in the Fc-BDP dyad. The time component of 23 ps can be attributed to the recombination of
	Fc .+ -BDP	.--C60. Finally, the longest time-constant of about 400 ps is attributed to the decay of
	Fc .+ -BDP-C60	.-.
				0.02
			A	0
						440 nm
						550 nm
				-0.02		673 nm 1050 nm
				0.1	1	10	100	1000
						t (ps)
	Further, fitting the TA kinetics at 673 nm ( 1 BDP* and 3 BDP*) yields three time constants: 3 ps,
	20 ps and 335 ps (Fig. 3.2.11 and table 3.2.3). The fit parameters did not yield the short
	component of <1 ps corresponding to the rise of 1 BDP* and C60 .-and can be considered within

3.2.11 and table 3.2.3)

. The 3 ps time-constant is attributed to the charge transfer from Fc to 1 BDP* to form Fc .+ -BDP .--C60, which further recombines to populate 3 BDP* in 27 ps. These time constants are in close agreement with those of isolated mono and bis Fc-BDP dyads (see the fitting of 550 nm kinetic trace in Fig.

3

.2.8 and Table

3

.2.2). The threeexponential fit of the 1050 nm kinetic trace also give similar three time-constants, but the longest time-constant of about 420 ps, which was not observed in the case of mono and bis Fc-BDP conjugates and is attributed to the decay of Fc .+ -BDP-C60 .-forming in the Conformer 1.

the IRF (long IRF due to benzonitrile). The 3 ps is attributed to the decay of 1 BDP* to form Fc .+ -BDP .--C60. The 20 ps component corresponds to the formation of 3 BDP* which decays to populate 3 Fc* in 335 ps. Again, these lifetimes are in good agreement with those obtained in Fc-

Table 4 .

 4 1.1 Pros and cons of homogeneous and heterogeneous catalysts

		Heterogenous catalyst	Homogenous catalyst
	Pros	Stable and inexpensive, ease of	High selectivity and high turnover
		separation and recycling, thermal	number, low mass transfer limitation
		stability
	Cons	Mass transfer resistance, small	Costly catalyst separation and recovery
		catalytic surface area

Table 4

 4

	.1.2 summarizes the reported data (turn over frequency

Table 4 .

 4 1.2 Summary of photosensitizer (PS)-POM systems reported in literature

	Structure of compounds	Type of POM	TOF, h -1	Photosensitizer	Quenching
				(PS)	mechanism
		Dawson-type	0.25	Iridium complex	Oxidative
		polyoxotungstate			quenching of excited
					PS by POM
	Izzet et al., 2013 33
		Mn-substituted	7.6	[Ru(bpy)3] 2+	Oxidative
		polyoxotungstate			quenching of excited
					PS by POM
	Lv et al., 2013 34
		Ni-substituted	116	[Ir(ppy)2(dtbbpy)] + Both
		polyoxotungstate			oxidative and
					reductive
					quenching
					mechanisms
					possible
	Lv et al., 2014 35				(reductive
					quenching
					of excited
					PS	by
					sacrificial
					donor	is
					dominant )
	Lv et al., 2015 36

Table . 1

 . Quenching rate constants (kq) determined from emission lifetime changes for each [Mo6I8Cl6] 2-/ -POM tandem.

Table 4 .

 4 2.2.

			Cluster 30mM
										Cluster	30	µM
		160
			Cluster+ 2 eq. POM
	Emission intensity x 10 3 (a.u.)	40 60 80 100 120 140	Cluster + 3 eq. POM Cluster + 5 eq. POM Cluster + 6 eq. POM Cluster + 8 eq. POM a.			Luminescence (a.u.) Intensity	0.5 1	b.		Cluster Cluster Cluster+ + + 5 2 3 eq. eq. eq. POM POM POM Cluster + 6 eq. POM Cluster + 8 eq. POM
		20
							0
		0
		600	650	700	750	800	0	200	400	600		800	1000
			Wavelength (nm)					t (ms)

Table 4 .

 4 2.2 Summary of steady-state and time-resolved Stern-Volmer (ksv and k'sv, respectively) and quenching (kq and k'q, respectively) constants.

  :1 CD:Cluster) is previously reported in literature for similar clusters 55 . in solid state 55 .

					Cluster 53 µM
									350
					Cluster + 56 eq. CD
	Absorbance (a.u.)	1 2	a.		Cluster + 110 eq. CD Cluster + 170 eq. CD Cluster + 226 eq. CD Cluster + 340 eq. CD	Emission x 10 3 (a.u.) intensity	50 100 250 300 150 200	b.	Cluster + 110 eq. CD Cluster + 170 eq. CD Cluster 53 µM Cluster + 56 eq. CD
											Cluster + 226 eq. CD
											Cluster + 340 eq. CD
									0
		0
			300	400	500	600				550	600	650	700	750	800
				Wavelength (nm)						Wavelength (nm)
						1200
					(a.u.)	1000		Cluster 53 µM Cluster + 56eq. CD
					Intensity	800	c.	Cluster+ 110 eq CD Cluster + 170 eq. CD Cluster+ 226 eq CD Cluster + 340 eq. CD
					Emission	400 600
						200
						0
						0	20	40	60	80
								t (µs)

Table 5 .

 5 1 Half-wave potentials (in V) of the parent BODIPYs, the POV unit, and two hybrids under study in deoxygenated DCM with 0.1 M TBAPF6. The potentials of irreversible processes are denoted with (irr). Note : the value of reduction potential of in the presence of acid is for multrielectron transfer process.

	Compound		[BODIPY] + /	[BODIPY] /	POV / POV +1e-
			[BODIPY]		[BODIPY] -
	POV								-0.40
	BDP		1.27		-1.16
	DSBDP		0.87		-1.10
	BDP-POV-BDP		1.24 (irr)		-1.13		-0.62 (irr)
	DSBDP-POV-BDP		0.82 (irr)		-1.10		-0.68 (irr)
	POV (500 eq. TFA)								-0.05
		0.000012		POV without acid
		0.000008		POV with 500 eq. acid
		0.000004
	Current (A)	-0.000004 0
		-0.000008
		-0.000012
		-0.000016
		-1	-0.8	-0.6	-0.4	-0.2	0	0.2	0.4
				Potential (V)

Table 5 .

 5 3 Time constants obtained by a multi-exponential fitting of the kinetic traces of parent BDP at specific wavelengths. Note: the value of τ2 was fixed at 2.3 ns.

	Wavelength	IRF (ps)	A1	τ1	A2	τ2
	(nm)
	350	0.271	-0.008	24 ps ± 3 ps	0.12	2.3 ns
	( 1 BDP*)
	422	0.094	-0.002	13 ps ± 3ps	0.04	2.3 ns
	( 1 BDP*)
	558	0.230	0.004	11 ps ± 3 ps	-0.06	2.3 ns
	(GSB)
	1154	0.170	-0.001	12 ps ± 7 ps	0.006	2.3 ns
	( 1 BDP*)

  8 ps, 100 ps and 4.3 ns. The presence of an additional short time-constant of about 0.8 ps indicates a solvent reorganization, which typically ranges from tens of femtoseconds to picoseconds, depending on solvent viscosity28 . The other two time-constants of 100 ps and 4.3 ns can be attributed to the anisotropy and excited state relaxation, respectively.

				DSBDP em
		0.02 0.04	1 DSBDP*	GSB and em DSBDP abs	em	a.	-0.3 0.02
	A	-0.02 0 -0.04					0.06 0.11 0.18 0.26
		-0.06 -0.08 -0.1					0.40 0.52 0.72 1 ps
		400	500	600	700		800
	A	-0.02 0.04 0.02 0 -0.04 -0.06 -0.08	Wavelength (nm)	b.	1 3 50 80 100 408 1010 1430 2060
		-0.1					2560 ps
		400	500	600	700		800
			Wavelength (nm)

Table 5 .

 5 4 Time constants (τ) and pre-exponential factors (A) obtained by multi-exponential fitting of kinetic traces of parent DSBDP at specific wavelengths. The value of τ3 was fixed at 4.3 ns.

	Wavelength (nm) IRF (ps)	A1	τ1 (ps)	A2	τ2 (ps)	A3	τ3
	450	0.160	--	--	-0.003	123 ± 10 0.05	4.3ns
	( 1 DSBDP*)
	640	0.118	-0.01	0.73 ± 0.1	-0.013	127 ± 9 -0.08	4.3ns
	(GSB+DSBDP
	em)
	665	0.111	0.004 0.64 ± 0.1	0.001	171 ± 30 0.006	4.3ns
	( 1 DSBDP*)
	700	0.121	0.001	0.86 ±	-0.002	138 ± 9 -0.01	4.3ns
	(DSBDP em)			0.18

Table 5 .

 5 5 Time constants (τ) and pre-exponential factors (A) obtained by a multi-exponential fitting of the kinetic traces of the BDP-POV-BDP hybrid at specific wavelengths. CS and EnT denote charge-separation and energy transfer, respectively

	Wavelength	IRF	A1	τ1 (ps)	A2	τ2 (ps)	A3	τ3 (ps)
	(nm)	(ps)
	400	0.158	0.004	7 ± 1	0.008	72 ± 5	0.008	370 ± 15
	( 1 BDP* &				(d)	( 1 BDP*, CS)	(d)	( 1 BDP*, EnT)
	BDP .+ )
	420 ( 1 BDP*) 0.110	0.005	3 ± 0.4	0.02	55 ± 2	0.012	325 ± 11
					(d)	( 1 BDP*, CS)	(d)	( 1 BDP*, EnT)
	475 (GSB)	0.089 -0.007	2 ± 0.3	-0.028	42 ± 3	-0.03	322 ± 9
					(r)	(CS)	(r)	(EnT)
	546 (GSB)	0.085 -0.009	4.5 ± 0.5	-0.025	53 ± 5	-0.03	343 ± 13
					(r)	(CS)	(r)	(EnT)
	1156	0.100	0.001	1.5 ± 0.6	0.0009	85 ± 10	0.002	553 ± 187
	( 1 BDP*)				(d)	( 1 BDP*, CS)	(d)	( 1 BDP*, EnT)

Table 5 .

 5 6 Time constants (τ) and pre-exponential factors (A) obtained by the multi-exponential fitting of individual kinetic traces of the DSBDP-POV-DSBDP hybrid at specific wavelengths. CS and EnT denote chargeseparation and energy transfer, respectively.

		0.02 0.03				a.	0.01		1 DSBDP*	b.	constant 0.5 ps 57 ps 1492 ps
		0.01					0.005
	A	0				A		DSBDP .+
							0
		-0.01	454 nm
		-0.02	700 nm 635 nm 660 nm				-0.005	1 DSBDP*
		-0.03								GSB DSBDP
		0.1	1	10	100	1000		400	500	600	700
				t (ps)					Wavelength (nm)

  . The anodic shift in the potential required for reduction observed upon acidification is typical of proton-coupled electron transfer because coupling protonation to electron transfer results in a decrease in the energy required for the charge transfer process to occur. Izzet et al. also demonstrated the enhanced electron transfer efficiency of BODIPY-POM hybrids in the presence of protons 21 . These prior reports motivated us to study the effect of protons on the electron transfer efficiency in our BODIPY-POV hybrid systems. The femtosecond transient absorption spectra were

	Wavelength	IRF (ps)	A1	τ1 (ps)	A2	τ2 (ps)	A3	τ3 (ns)
	(nm)
	450	0.166	0.001	0.45 ± 0.08 0.001	65 ± 14	0.02	0.0018 ±
	( 1 DSBDP*)							0.0002
	630	0.140	-0.006	0.56 ± 0.05 -0.006	58 ± 9	-0.032	1.7 ± 0.2
	(GSB & DSBDP
	em)
	650	0.116	0.001	0.72 ± 0.05 0.001	67 ± 11	0.002	1.6 ± 533
	( 1 DSBDP*)
	700	0.132	0.0002 0.68 ± 0.04 -0.001	34 ± 4	-0.004	1.3 ± 105
	(DSBDP em)

Table 5 .

 5 7 Summary of thermodynamic driving force of charge-separation and recombination in the two hybrids with and without acid.

	Compound

BDP-POV-BDP with 500 eq. acid -1.10 -1.32

DSBDP-POV-DSBDP with 500 eq. acid -1.02 -0.92

  The charge-separated state signatures were confirmed by overlapping the spectro-electrochemical spectra of one electron-oxidized cluster and one electron-reduced POM. Further study with the POMs ([PW11VO40] 4-, [SiMo12O40] 4-and [SiW11MoO40] 4-) bearing the same charge but varying the reduction potentials (due to the reduction of different metal centers; V or Mo) is presented as a second part of the chapter. The rate of electron transfer remains similar despite the reduction potentials differ complements our preliminary study that the charge density on POMs plays a major role in the rate of electron transfer. The use of noble-metal-free metal halide clusters as photosensitizers provides a potential alternative for classical, expensive photosensitizers (Ir,

POM archétypaux (à base de W) dans un solvant organique. Le travail a mis en évidence le processus de PET entre les deux entités et explore la dépendance de l'efficacité du transfert d'électrons en fonction de la densité de charge de l'atome métallique du POM. Ce même cluster [Mo6I8Cl6] 2-a éré étudié en présence de POM portant la même densité de charge mais ayant des potentiels de réduction différents : ces études ont révélé des constantes de vitesse d'extinction similaires, confirmant le rôle majeur de la densité de charge des POM sur l'efficacité du transfert d'électrons. Une étude supplémentaire sur les systèmes cluster-POM hydrosolubles, dans lesquels le cluster a été stabilisé en l'encapsulant à l'intérieur des cyclodextrines (CD), a montré que ces dernières stabilisent le tandem cluster-POM en séparant les deux entités. Ces études, bien que préliminaires, fournissent des informations essentielles sur les facteurs affectant le transfert d'électrons du cluster au POM dans les solutions organiques et aqueuses.Combinant les résultats obtenus sur les deux premières parties, la troisième partie de la thèse explore les processus photoinduits intramoléculaires dans des hybrides organiques-inorganiques construits sur des BODIPY et POM comme homologues organiques et inorganiques. En combinant le meilleur des entités des deux études précédentes, nous avons étudié les assemblages covalents hybrides BODIPY-POM (BODIPY-POM-BODIPY et distyrylBODIPY-POM-distyrylBODIPY). Le polyoxo-hexavanadate (POV) de type Lindquist a été utilisé dans l'étude comme accepteur d'électrons. Un transfert d'électrons a ainsi été observé dans les deux assemblages, mais avec une faible efficacité. L'ajout de protons au système a amélioré l'efficacité du PET du fait de la modification de propriétés redox du POM en présence de protons. Le PET observé dans notre étude, dont la cinétique peut être régulée avec l'introduction de protons, souligne le caractère prometteur de ces POM à base de V pour la construction de tels hybrides.En conclusion, ce travail de recherche contribue à notre compréhension fondamentale des processus photoinduits dans les systèmes donneurs-accepteurs complexes, et les résultats aideront à construire des systèmes plus efficaces pour la conversion de l'énergie solaire à l'avenir.

Photochemical & Photobiological Sciences (2022) 21:1573-1584

Univ. Lille, CNRS, UMR 8516, LASIRE, Laboratoire de Spectroscopie pour les Interactions, la Réactivité et l'Environnement, 59 000 Lille, France

Eur. Phys. J. Spec. Top.

Acknowledgements

Acknowledgements This work was supported by the CNRS (International Emerging Action, PICS project no 08198), the LabEx PALM ANR-10LABX-0039-PALM and CHARMMMAT ANR-11-LABX-0039, Région Ile-de-France DIM Nano-K. Anam Fatima is grateful for the MESRI grant (2019)(2020)(2021)(2022). Jad Rabah thanks the MESRI for a PhD fellowship (2017-2020).

List of abbreviations

BODIPY

Author contributions Conceptualization: AF, RMR, MH, TP; investigation: AF, JR, EA, HF, KW, GB, GC and MS; supervision: TP, RMR, KS and MH; writing-original draft preparation: AF, KS and MH; funding acquisition: MH, RMR and TP. All authors have read and agreed to the submitted version of the manuscript.

Declarations

Conflict of interest On behalf of all authors, the corresponding authors state that there is no conflict of interest.

Supplementary information

Selective population of triplet excited states in heavy atom free BODIPY-C60 based molecular assemblies

Anam Fatima, 1 Jad Rabah, 2 Emmanuel Allard, 2 * , Hélène Fensterbank, 2 Karen Wright, 2 Gotard Burdzinski,  Gilles Clavier,  Michel Sliwa,  Thomas Pino, 1 Rachel Méallet-Renault, 1 * Karine Steenkeste, 1 * Minh-Huong Ha-Thi 1 * Grant (2019-2022). Jad Rabah thanks the MESRI for a PhD fellowship (2017-2020).

Data availability statement This manuscript has data included as electronic supplementary material.

Declarations

Conflict of interest On behalf of all authors, the corresponding authors state that there is no conflict of interest.

123

Supplementary Information Wavelength (nm) SI Fig. 1. Excitation spectrum (dashed blue, λem = 670 nm) overlapped with the absorption spectrum (cyan) of the BDP-C60-DSBDP triad.

Conclusions and future perspective

In this Chapter, we have investigated the photoinduced processes events in two families of covalently linked BODIPY-fullerene based donor-acceptor systems.

In Section 3. and DSBDP-C60) depending on the structure of the BODIPY unit. In the BDP-C60 dyad, the triplet excited state is localized on C60 whereas in the DSBDP-C60 dyad and the triad (either exciting the BDP or DSBDP unit), the triplet excited state is finally populated on DSBDP. The investigations thus highlight that wherever DSBDP acts as a photoactive unit (either by direct or indirect excitation), the triplet-excited state is formed on DSBDP and not on C60. The 3 DSBDP* was found to be long-lived (250 µs) which opens a way to its use in several applications. As an example, we have demonstrated TTA-UC in these assemblies. The triad is particularly intriguing because it exhibits panchromatic light absorption and populates a longlived triplet state, suggesting that it could be used as a heavy-atom free, organic, and wide-band light capturing triplet photosensitizer.

Section 3.1 (B) further investigates the electron and energy transfer events that led to selective population of triplet-excited states in these covalent assemblies using femtosecond pump-probe time-resolved emission spectroscopies. Due to the presence of a flexible linker between the BODIPYs and C60 units, there can be several conformations possible as also demonstrated in a similar BDP-C60 dyad. Conformers with short distance between BDP units and C60 induce an electron transfer from BDP or DSBDP to the C60 unit leading to the formation of short-lived (several tens of ps) charge-separated ion pair. However, in intermediate and extended conformers, energy transfer events from BDP to C60 leads to the formation of 1 C60*, which further undergoes ISC to form 3 C60 in the BDP-C60 dyad. In DSBDP-C60 dyad and the triad, the

Table S1. Summary of emission quenching rate constants (k'q) obtained by the Stern-Volmer analysis of the emission intensities in the presence of different POMs. This study is an expansion of our results on the quenching of the excited state of cluster in the presence of POMs with different charges and redox petentials reported in Article #3. The similar quenching rate constants obtained in the case of POMs bearing the same charge confirms the effect of charge density of the POM anions on the rate of electron transfer to POM.

As we explained in Article # 3 that the charge density on the polyoxoanions affects the coulombic repulsions between the anionic cluster and POM as well as the reduction potential However, the signal for the singlet excited state of the BODIPY decayed slightly faster (1193 ps) than the pristine BODIPY (2120 ps). This observation was linked to an additional non radiative pathway to populate the BODIPY triplet state in the hybrid due to the heavy atom effect of the appended POM.

5.13.b).

Individial fitting of the kinetic traces (at 350, 422, and 1154 nm corresponding to the excitedstate absorption and 558 nm for the GSB) converged satisfactorily with time-components of about 15 ps and 2.4 ns (Fig. 5.14 and Table 5.3). While the 2.4 ns component is simply related to the relaxation of the singlet excited state of BDP, the shorter (about 15 ps) time-component is related to the anisotropy of the excited molecule, which was confirmed by performing measurements with different orientation of pump-probe polarization directions (i.e., θ = 0°, 90° and 57.4°). For a parallel polarization of the pump and the probe (θ = 0°), a more intense GSB signal is observed as compared to that in the perpendicular and the magic angle polarization conditions (Fig.5.15). This difference is due to the rotational diffusion of the molecule in the ground state. Notably, it is difficult to obtain a perfect magic angle condition (57.4°), therefore an anisotropy effect can still be observed but with a much lesser signal amplitude.

Chapter 6. Conclusions and Prospects

Conclusions

This research investigates the intramolecular and intermolecular electron and/or energy transfer processes using fast and ultrafast time-resolved spectroscopies, with the perspective of understanding the fundamental photoinduced processes within different donor-acceptor systems and exploring their potential application in clean energy generation.

The work can be broadly divided into two parts aiming to study (i) intra-and inter-molecular electron and energy transfer processes within photoactive covalent assemblages (all organic and organic-inorganic hybrids) and (ii) multicomponent systems (all-inorganic). The results are organized into three parts.

The first part is focused on photoinduced processes within BODIPY-C60-based molecular assemblies. The first sub-part of this chapter is presented in the form of two research articles that photophysically investigates a novel broadband light-absorbing triad (BDP-C60-DSBDP) along with its two reference dyads (BDP-C60 and DSBDP-C60) in different time-scales (femtosecond to nanosecond). The first article (Article #1) is dedicated to the study in nanosecond timescale in which triplet-excited states in the two reference dyads and the triad were characterized using nanosecond transient absorption spectroscopy and were quantified using the relative actinometry method. Upon selective excitation of BDP in the BDP-C60 dyad, the triplet excited state was localized on C60 with a quantum yield of 40 %, whereas in the DSBDP-C60 dyad, the triplet of DSBDP was obtained with 37 % yield. This observation is supported by the lower positioning of 3 DSBDP* (1.15 eV) than 3 C60* (1.56 eV). In the triad, the triplet was found to be localized on DSBDP regardless of the excitation wavelength (BDP or DSBDP). The lifetime of the triplet excited state of DSBDP was found to be much longer than triplet C60. The long-lived triplet excited state opens the avenues for its use in heavy-atomfree triplet organic photosensitizer where a long-lived triplet state is desirable, for example, PDT and TTA-UC. Further, we exemplified the use of these assemblies (dyads and triads) in TTA-UC to witness delayed fluorescence. For the BDP-C60 dyad, perylene was used as the triplet acceptor, which undergoes TTET from 3 C60* to form 3 perylene*, and then two triplet perylene undergo TTA to form 1 perylene*, which fluoresce with delayed emission lifetime 20 µs (prompt fluorescence lifetime of perylene is 3.8 ns). For the DSBDP-C60 triad, the DSBDP monomer was used as an annihilator, and yet again, a delayed fluorescence of DSBDP was observed (29 µs compared to the inherent fluorescence lifetime of 4.42 ns). In all, the work demonstrated the potential use of these covalent assemblies, particularly panchromatically absorbing triad, as heavy atom-free organic triplet photosensitizers.

Further, we investigated the photophysics of the triad and the two reference dyads (Article #2) to elucidate the ultrafast photoinduced processes (using pump-probe experiments in femtosecond time-scale and time-resolved emission), leading to the selective population of triplet excited state. The multiexponential emission decays and the fitting of fs TA kinetics suggested the presence of multiple relaxation pathways, which could be explained by the presence of flexible linkers leading to different spatial conformers depending on the relative distance between different units. Therefore, complex photophysics was observed in these systems with competitive electron and energy transfer processes. Upon selective excitation of the BDP in the BDP-C60 dyad, an ultrafast electron transfer (<1 ps) was observed in a folded conformer leading to the formation of a charge-separated state (BDP .+ -C60 .-) along with two successive energy transfer processes (forming 1 C60*) at different time scales in two other conformers with relatively relaxed geometries. Fitting of the kinetic traces (both individual and global fits) yielded the lifetime of the charge-separated state to be about 42 ps. In the other reference dyad (DSBDP-C60), the selective excitation of the DSBDP unit leads to an electron transfer process, however, with a longer time (4 ps) compared to that in the BDP-C60 dyad. We reasoned this by a smaller driving force of the electron transfer process in the DSBDP-C60 dyad (ΔGcs = -0.54 eV versus -0.70 eV in BDP-C60). The lifetime of the charge-separated state (DSBDP .+ -C60 .-) was found to be about 87 ps. In this dyad also, two energy transfer processes with different time constants were observed, suggesting multiple conformations with the flexible linker. The relaxation pathways was even more complex in the triad.. The triad was excited at two excitation wavelengths corresponding to the absorptions of BDP and DSBDP.

The excitation of BDP gives similar photoinduced processes as the isolated BDP-C60 dyad, along with an additional photoexcitation energy transfer from BDP to DSBDP. Then, once DSBDP is excited, it follows the same relaxation route as the DSBDP-C60 dyad (one electron and two energy transfer processes in different conformers). Furthermore, the direct excitation of DSBDP in the triad behaves identically to the isolated DSBDP-C60 dyad, indicating no additional photoinduced processes. The energy transfer processes in these assemblies lead to the formation of 1 C60*, which undergoes ISC to form the triplet of C60. In the DSBDP-C60 dyad and the triad, 3 C60* further populates 3 DSBDP* via ping-pong energy transfer. Table 3.2

List of Tables