Remerciements/Acknowledgments

Les remerciements sont sans aucun doute les pages les plus importantes de ce manuscrit de thèse. C'est en effet, l'opportunité pour moi de remercier officiellement toutes les personnes qui m'ont accompagnée durant ces trois ans. Toutefois, j'espère que personne ne découvrira rien de nouveau car j'espère vous avoir déjà fait savoir toute ma gratitude au quotidien.

Je tiens en premier lieu remercier le jury qui a accepté d'évaluer ce travail. J'adresse mes sincères remerciements aux professeurs Piétrick Hudhomme et José-Ramon Galan Mascaros, d'avoir accepté d'évaluer ce travail de thèse en tant que rapporteurs et ce, dans un intervalle de temps restreint. Je vous remercie vivement ainsi que les directeurs de recherche Valérie Marvaud et Gabor Molnar, pour l'ensemble de vos suggestions et cette riche discussion. Cela a été un réel honneur de pouvoir vous présenter ce travail.

A mes directeurs de thèse, David Kreher et Rodrigue Lescouëzec, un immense merci. Je pense que les collaborations sont des aventures scientifiques certes, mais surtout humaines. Je vous remercie donc d'avoir accepté de vivre celle-là avec moi. Merci pour toute la confiance que vous m'avez accordée pendant ces trois ans. Je ne pourrais jamais rendre justice à ces trois années passées ensemble en un simple paragraphe, c'est évident. Je pense que cette expérience est pour moi à l'image des matériaux, ce sont souvent derrière des défauts/imperfections que se cachent des propriétés incroyables.

Il est vrai que j'étais loin d'imaginer la difficulté associée à la nouveauté d'un projet. Le chemin a été parsêmé de nombreuses embûches, mais chacune d'entre elles m'a permis de faire de magnifiques rencontres… …Tout d'abord au sein de mes deux équipes de ERMMES et Polymères. Je tiens à remercier mes collègues mais surtout amies, Lydia Sosa-Vargas et Yanling Li… deux incroyables chercheuses. Merci pour tous les moments de partages, pour votre aide, votre soutien ainsi que pour votre générosité et vos conseils. Vous avez tellement enrichi ces 3 années.

Merci à tous étudiants rencontrés au sein de l'équipe ERMMES : Juan Ramon Jimenez Gallego (mi hermano scientifico, pour toute la convivialité espagnole que tu as parsemée au quotidien et dans la science), Yesica Flores (and her mexican drug cartel), Ang Li (my famous chinese actor), Yoan Prado, Rémi Plamont, Jana Glatz, Maxime Fusch, Coral Herranz ainsi que les étudiants M2. Je remercie également l'ensemble des permanents : Benoit Fleury, Laurent Lisnard, Yves Journaux, Mannan Seuleiman, Alexandrine Flambart ainsi que Christophe Cartier dit Moulin. Un grand merci à l'équipe dans sa globalité pour tous les agréables moments passés ensemble.

Je souhaite également remercier Michel Verdaguer avec qui j'ai eu le plaisir de discuter temps en temps.

Evidémment un grand merci aux membres de mon autre équipe (permanents et nonpermanents) : Fabrice Mathevet (qui a mis le feu au labo avec ses supers playlists), Morgan Auffray (et ses 'Tsamina mina heyhey'), Xiaolu Su (who plays badminton as no one !), Teng Teng (my fumehood neighbour), Romain Brisse, Dizheng Liu, Xiao Liu ainsi qu'aux derniers arrivants.

Je remercie l'équipe de Chimie des Polymères dans son ensemble (toutes les équipes rassemblées sous le titre). Je tiens tout d'abord à remercier Laurent Bouteiller de m'avoir permis de travailler au sein de cette équipe dans les meilleures conditions.

Un merci particulier à la nocturne du 4 ème étage, Gaëlle Pembouong, pour sa gentillesse et ses encouragements ainsi que Jutta Rieger. Merci également à Nicolas, Illy, Philliphe Guégan, Soum, Cécile Huin. Je voudrais remercier l'ensemble des personnes que j'ai rencontrées au sein des polymères. Je m'excuse de ne pouvoir toutes les citer, de part la taille l'équipe. Même si mes passages étaient moins fréquents sur la fin, je tiens vraiment à remercier tous les permanents et étudiants.

De part les collaborations developpées, j'ai eu la chance de pouvoir explorer différents laboratoires. Ce projet n'aurait évidemment pas la même forme sans la contribution des différents partenaires.

Je souhaite tout d'abord remercier Jessem Landoulsi du laboratoire de réactivité de surfaces (LRS). Ton implication dans les différentes pistes explorées (même les moins fructueuses) a donné une grande impulsion à ce projet notamment sur les films et SAMs. Je te remercie sincèrement pour ton aide, notamment dans mes moments de découragement, et de m'avoir poussée à essayer de valoriser au mieux l'ensemble du travail réalisé.

Merci également à Antoine Miche et Christophe Méthiviers pour les mesures XPS, ainsi que Pauline Cornette. Merci également aux diverses personnes rencontrées dont Laetitia Valentin et Vincent Humblot.

Merci aux électrochimistes du LISE, Laure Fillaud et Emmanuel Maisonhaute pour l'intérêt que vous avez porté à ce travail, notamment sur les films et études électrochimiques des cubes, mais également à ma formation. Un grand merci pour les riches et passionnantes discussions ainsi que manipes. Merci également à Thomas et Gabriel.

I would like to deeply thank the collaborators, Jan Dreiser and Niéli Daffé for their kind welcome at SLS (Paul Scherrer Institute, Switzerland). A huge thank also to Michal Studniarek (I now sign forms as -NH2), Kassymkhan Baisetov and Mehdi Heydari. I would have never imagined that beatime could be so thrilling and so …athletic. It was surely one of the best experience of my PhD. Thank you so much to all of you! Je souhaite également remercier Imad Arfaoui (MONARIS) pour les tests STM. Un grand merci pour ta jovialité, ta patience Imad et tout ce que tu m'as appris.

Je remercie également Pierre-Henri Aubert du LPPI à Cergy qui a eu la gentillesse de me former au profilomètre et mesure de conductivité. Merci également à Johan Biscaras de l'IMPMC (Sorbonne-Université) ainsi que Marco Faustini du LCMCP (sorbonne-Université) pour les tests faits sur les films composites.

Je souhaite aussi remercier l'équipe POMs. Je tiens à remercier le trio de folie ; Richard Villaneau, Geoffroy Guillemot et Ludovic Tortech ; pour avoir ensoleillé mes journées et pour tous le soutien que vous m'avez apportée. Un grand merci également à Sébastien Blanchard, Severine Renaudineau, Guillaume Izzet ainsi que tous les autres permanents. Merci également à Madeleine Piot, Ourania Makryagenni et Teng Zheng pour tous les moments partagés ensemble. A Maxime Laurans (le palais breton le plus difficile à satisfaire que je connaisse), un immense merci pour ton amitié.

Un grand merci également à l'équipe GOBs pour son hospitalité ! Je remercie très chaleureusement Berni Hasenknopf pour tous les encouragements (depuis la L2) ainsi que Guillaume Vives. Merci beaucoup à tous les étudiants et notamment à Lorien, Jiang, Hugo, Martin, Leonid, Jorge. Je voudrais surtout remercier Sawsen Cherraben … ma petite batata qui a embaumé mes journées tel un véritable parfumeur ;-). A Atika Bentayeb, khalti… merci pour tes rappels et ta sagesse. Merci pour tous les moments de complicité. De la même façon, je souhaite remercier Jamal Moussa de l'équipe ARC (IPCM) de m'avoir sans cesse poussée à donner le meilleur de moi-même et pour tout le soutien apporté depuis le master.

Enfin, à ma famille et tous mes proches, qui m'inculquent finalement les leçons les plus importantes que l'on puisse recevoir. A mes parents, les personnes les plus généreuses et sages que je connaisse. J'espère un jour réussir à égaler vos nombreuses qualités mais en attendant … Merci de me supporter (avec mes nombreux défauts). Merci à tous mes proches pour tout ce que vous m'apportez au quotidien. Vous avez contribué à votre façon à ce travail notamment avec votre indéfectible soutien… nul doute que personne ne pourra jamais mesurer toute la gratitude qui se cache derrière ses 5 lettres : merci.

Pour finir, merci à toute les personnes que j'ai pu rencontrer et qui m'ont offert ne serait-ce qu'un sourire pour égayer ma journée. Molecular switches are molecules that can adjust their (chemical, physical) properties in response to an external stimulus. A wide range of molecular switches has been described in literature whose switching properties can lie in a change of structure (isomerization), oxidation states (redox switches), electronic configuration (optical and magnetic switches). 1,2,3 The fascinating properties of molecular switches have drawn most attention in molecular electronics and more generally in advanced materials research.

Table of contents

In molecular electronics, the use of molecular switches has appeared as an attractive strategy to face limitations stated by Moore's law about 50 years ago. Indeed, electronic technologies are confronted to issues (e.g. increase of leakage current) originating from the shrinking of current silicon devices components. Molecular electronic is now regarded as a promising alternative technology for device miniaturization due to the high degree of control achieved on the properties and dimensionality through chemical syntheses.

Molecular switches are also of particular interest for the design of (multi)functional materials more specifically for the most evolved class of them, called "smart" materials. These latter are materials which can accommodate their (chemical, physical) properties under a modification of environmental conditions. Therefore, they are regarded as adjustable and scalable systems. Molecular switches had come out as promising precursors for the elaboration of responsive materials. The incorporation of molecular switches into processable materials that could be truly used in technological applications is a critical point in order to facilitate their technological transfer. Figure 1 illustrates possible approaches for integrating molecular switches into materials, as a first step of device prototypes design. This PhD project sets in this research topic of multifunctional hybrid (nano)materials.

The overall goal of this first collaboration between ERMMES and Polymères groups of the Institut Parisien de Chimie Moléculaire (IPCM) was to develop new routes to access hybrid responsive materials based on switchable molecular units. For this purpose, we mainly targeted two families of magnetic molecular switches to investigate their integration into organic polymers or their grafting onto surfaces: (i) iron(II) spin crossover complexes (SCO), (ii) a family of photomagnetic cyanide-bridged {Fe4Co4} cubes.

The first chapter provides a brief description of the two families of magnetic molecular switches selected in this work. It exposes the interests but also some issues raised by their integration into materials. Finally, examples of hybrid materials based on magnetic molecular switches are outlined.

In chapter 2, the synthetic work carried out for magnetic switches functionalization is presented. More specifically, the functionalization of the tris(pyrazolyl)borate "scorpionates" ligands that are employed in the preparation of SCO complexes and photoswitchable {Fe4Co4} cubes is described.

Prior to use the functionalized switches for material design, we studied the influence of the functional group on the switching properties for both the iron(II) SCO complexes and the {Fe4Co4} charge transfer complexes. Thus, the work performed on model {Fe4Co4} cubes to optimize their photomagnetic properties and to target the best candidates (for hybrid switchable materials fabrication) is presented here.

Once the functionalized building blocks were obtained, different strategies were explored to insert them in materials (figure 2).

The chapter 3 is dedicated to the preparation of organic polymers functionalized by switchable complexes, through chemical routes. First, we discuss a post-polymerization modification approach, established by Jäkle et al, to synthesize tris(1-pyrazolyl)borate functionalized polymers. Secondly, in order to enlarge the synthetic strategies of complexes-containing organic polymers, we designed new ligands functionalized with different polymerizable/functional groups. We describe here the new procedures developed for the functionalization of bis(pyrazolyl)pyridine (bpp) ligands family, well-known to induce SCO complexes.

In chapter 4, the incorporation of switchable complexes into organic polymers through electrochemical methods is described. This work was mainly carried out using {Fe4Co4} cubes because of their properties and robustness. Our goal here is to elaborate bistable thin-films of these {Fe4Co4} cubes directly into surfaces. Consequently, the {Fe4Co4} cages were functionalized at their periphery by an electroactive group (thiophene). Their ability to undergo electropolymerization process was then tested.

This electrochemical method was developed and optimized with a family of {Fe4Co4} cages with poor photomagnetic properties. The procedure was thus extended to a thiophene functionalized {Fe4Co4} cage with enhanced photomagnetic properties. We present the surface characterizations obtained on the resulting materials (of the different types of cubes used). Finally, we show some preliminary results of synchrotron measurements (XAS, XMCD) of the hybrid thin-films properties. Finally in chapter 5, we examine the ability of {Fe4Co4} cages functionalized by an anchoring group to self-assemble onto surfaces. Once again, thiophene group was employed but this time to take advantage of its affinity for gold substrates.

First, we detail conditions for the formation of self-assembled layers of thiophene functionalized {Fe4Co4} cages from solution. Through surface characterizations, we discuss their chemical composition, their morphology as well as their stability. In addition, the binding nature of the cages to the substrates is compared to the study of a model compound (3hexylthiophene).

CHAPTER 1

Context and state of art 14

I. Molecular Magnetic switches

Molecular magnetism is a fruitful source of functional complexes. Indeed, it has provided a broad spectrum of magnetic compounds exhibiting switchable properties. A nonexhaustive list, containing spin-crossover complexes, valence tautomerism complexes or even metal-to-metal charge transfer (MMCT) complexes (e.g.Prussian Blue Analogues), is illustrated figure 1. 4 This chapter will mainly focus on switchable compounds around which this work was built: Iron(II) spin crossover complexes and photomagnetic polymetallic complexes. The objective is to provide a general description of their properties, proving their ability to accommodate to a wide collection of functionalities. Then, we will discuss the processing methods developed to include these molecular switches into materials. Finally, we will outline the main scientific challenges encountered in this research area underway.

Figure 1. Illustrations of different magnetic molecular switches. Image extracted from reference [4].

Iron(II) Spin-crossover complexes

Phenomenon and techniques

The spin-crossover phenomenon was first discovered in an Fe(III) dithiocarbamate mononuclear complex by Cambi and coworkers 5 For strong ligand field (Δ > Π), the ground state is the one with the minimum spin multiplicity (S = 0) as all d electrons are paired in t2g orbitals (t2g) 6 . The Fe(II) complex is thus in a diamagnetic low-spin configuration. On the opposite, weak ligand fields (Δ < Π), stabilize the high-spin state of maximum multiplicity (S = 2).

For intermediate cases, when the ligand field and the mean pairing energy are of similar magnitude (Δ ≈ Π), the zero-point energy difference ΔE°HL can become close to thermal energy (ΔE°HL ≈ kBT). It is then possible to thermally induce the transition from one spin state to another. This molecular spin-state switching is called spin crossover (SCO). Spin-crossover complexes may also respond to other stimuli such as pressure, magnetic/electric fields or even light. The so-called Light-Induced Excited Spin-State Trapping (LIESST) effect will be discussed in more details section I. 1.3. Most of the reported SCO complexes are iron complexes, and the most prominent family being represented by the Fe(II) complexes in N6 donor environment. These complexes are particularly interesting because they exhibit drastic changes in their magnetic and optical signature, going from a diamagnetic LS state (S= 0) showing intense colour to a pale paramagnetic one (S= 2). The spin-state change of the compound is then easily detectable by SQUID magnetometry or optical measurements. These techniques are the most commonly used to measure the thermally-induced transition.

The spin transition results in other significant changes of the structural, physical and (less frequently) chemical properties of the complex. Any technique sensitive for instance to a change of electronic, vibrational, structural parameters during the transition can thus be relevant to follow the transition (e.g. IR, Raman, NMR, Mossbauer spectroscopies).

Single crystal X-ray Diffraction (or X-ray Powder Diffraction) is particularly used to probe the structural variations. Generally, Fe II -N bond lengths in LS state ( ̴ 1.8-2.0 Å) are about 10% shorter than in the HS state ( ̴ 2.0-2.2 Å), due to absence of electrons in antibonding eg orbitals. 6 The LS-HS transition is also easily revealed by significant distortions in the Fe(II) coordination sphere.

More rarely, Differential Scanning Calorimetry (DSC) is used to provide thermodynamic parameters (ΔH°; ΔS°) associated to the spin transition.

Spin crossover profiles

The general way to report a thermally induced spin transition is to plot the high spin fraction γHS as a function of temperature. The temperature for which LS and HS fractions are equal to 50% is called the transition temperatures, T1/2. Different nature of spin-state changes have been identified (fig. 3a-e). The spin state change can be gradual/continuous (fig. 3a) over a broad range of temperature. It can also be abrupt (fig. 3b-c) and arises within a narrow range of temperature (few Kelvin). In some more complex cases, the transition may show several steps (fig. 3d) or be incomplete (fig. 3e).

The curve shape is an indicator of the cooperativity. The cooperativity is due to elastic intermolecular interactions between the spin-crossover neighbouring molecules. A gradual transition accounts for the absence of cooperativity between SCO-centres. In this case, the transition can be described as a simple thermal equilibrium based on a Boltzmann distribution. This behaviour is typical of spin transition in solution or solid solutions.

On the opposite, sharp transitions are the signature of cooperative effects. In fact the structural changes occurring on one complex (i.e. the increase or decrease of the volume of the complex) can trigger the spin-state change on its neighbouring molecules. (fig. 3c) When cooperativity becomes strong enough, thermal hysteresis loop can be observed. The latter are thus the extreme illustration of cooperativity and are the most coveted spin transition profile.

Two transition temperatures border the loop width, T1/2↓ and T1/2↑. Within this loop, the compound is bistable, i.e. it can exist in two different spin states in for an identical value of the temperature. Hysteresis confers a memory effect as the compound retains a given spin state (memory) which depends of its entry pathway in the loop. The two spin states can be encoded with a binary code 0/1 (or ON/OFF) and the SCO material appears as a molecular memory component.

The design of SCO complexes with a complete, reproducible hysteresis around room temperature opens perspective for optical or magnetic data storage. This behaviour can be observed when intermolecular contacts (such as H-bonds, π-π interactions and so on) efficiently promote elastic interaction. A more rational way to achieve such cooperative behaviour is to design coordination polymers where SCO centres are linked by coordination bonds. 7,8 At the scale of an isolated molecule, it is very challenging to obtain bistability. A smart strategy was developed by Zarembovitch et al. in the nineties. It consisted in designing potential SCO complexes containing photo-isomerizable ligands. During isomerization, the ligand field is modified, triggering a spin-state change as shown figure 4. 9 The effect is called Ligand-Driven Light Induced Spin Change, LD-LISC. It has been evidenced in solution but it is difficult to realize in the solid state where dense molecular assembly can prevent the structural reorganisation accompanying the ligand isomerization.

In the present work, we mainly focus on molecular systems whose properties can be switched by a light stimulus. The next sections will thus provide a description of photoswitchable SCO and MMCT complexes.

Photomagnetism in spin crossover systems

As mentioned in section 1.1., in some cases spin-state switching can be triggered in SCO complexes by a light irradiation at low temperatures. This phenomenon is known as the Light-Induced Excited Spin-State Trapping (LIESST) effect. As for the thermal stimulus, the spinstate change is accompanied by strong changes in the magnetic properties (photomagnetic) and strong changes in the optical properties (photochromism). [9] This effect was first discovered in an Fe(II) SCO molecular system in solution by

McGarvey et al. 10,11 Then, Decurtins et al. 12 reported the light sensitive electronic changes of [Fe II (ptz)6](BF4) (ptz = 1-propyl-tetrazole), in solid state at low temperature (20 K). An explanation of the LIESST effect will be given through this example, (figure 5). 13 Upon irradiation at 514nm, the [Fe II (ptz)6](BF4) complex is excited from the initial singlet ground state, 1 A1 , to the excited singlet 1 T1. The excited singlet can relax back to the ground state 1 A1 or to a metastable quintuplet state 5 T2, through intersystem crossing.

At low temperatures, the system remains trapped in the paramagnetic metastable state, which can exhibit very long lifetime.

The activation energy barrier between the potential wells of the metastable spin state 5 T2 and the ground state 1 A1 can be overcome if the temperature is raised. Quantum relaxation can also occur at low temperatures. The limit temperature, above which the metastable magnetic information is lost, is called the TLIESST.

Graphically, it corresponds to the inflexion point of this relaxation curve and is determined by the minimum of the ∂χMT/∂T curve. 14 Interestingly, Hauser showed that it was also possible to convert the system back to LS state by irradiating the metastable state. 15 This effect was called reverse-LIESST. In principle, it is possible to obtain SCO complexes where one wavelength triggers the LS to HS conversion whereas another one triggers the HS to LS conversion. The described light induced bistability opens perspectives for using photomagnetic materials in technological applications. However, the main deficiency of LIESST effect lies in the low relaxation temperature (ca TLIESST < 150 K).

(Photo)magnetic polymetallic complexes

Photomagnetism in Prussian Blue Analogues (PBAs)

The photomagnetic effect can also be achieved in some mixed-valence polymetallic species. In these systems, a light irradiation induces an electron transfer from one metallic centre M to another M'. Interestingly, photomagnetism based on electron transfer has only been observed in cyanide-bridged compounds.

The first example was reported in a Prussian Blue Analogue (PBA), of formula K0.4Co1.3[Fe(CN)6]•5H2O, by Hashimoto and coworkers in 1996. 16 In this material, an increase of the magnetization and the magnetic ordering temperature Tc is observed upon red light irradiation at 5 K. This result was explained by the conversion of diamagnetic {Fe II LS-CN-Co III LS} pairs (Fe t2g 6 eg 0 , S = 0; Co t2g 6 eg 0 , S = 0) to paramagnetic {Fe III LS -CN-Co II HS} ones (Fe t2g 5 eg 0 , S = 1/2; Co t2g 5 eg 2 , S = 3/2) as illustrated figure 6.

This process is named Electron Transfer Coupled to a Spin Transition (ETCST) as a spin transition occurs on the Co ion. Similarly, to the LIESST effect, it is possible to thermally relax the photo-induced metastable state and extract a relaxation temperature, Trelax. In this family of compounds, relaxation temperatures can be usually higher than those of pure SCO systems but they remain under 200 K.

The discovery of Hashimoto et al. gave rise to many studies aiming at improving the photomagnetic properties. However, the task is challenging as Prussian Blue analogues are To deepen the knowledge of the ETCST phenomenon, a strategy has consisted in studying lower (soluble) dimensional models of PBAs. Indeed, the use of model compounds where the environment of the Fe and Co ions is well-controlled enable researchers to better control structural and electronic parameters influencing ETCST. Besides the use of soluble complexes allow the access of accurate electronic information through various solution techniques (electrochemistry, NMR etc) not accessible in PBAs due to their poor solubility.

From 3D PBAs networks to their 0D molecular models

In the past decade, molecular models scaling down to the smallest constitutive unit (M-CN-M' pair) of Fe/Co PBAs skeleton have been synthesized. Some examples exhibiting attractive ETCST are illustrated figure 7. 17,18,19,20 The conception of these well-defined soluble and photo responsive molecular models is based on an insightful choice of the building blocks. Octametallic cubic models are particularly interesting as they are the only ones for which the role of the alkali ion could be taken into account. We will focus the discussion on the two published examples (figure 7c-7d). Holmes and co-workers in 2008. 18 The {Fe4Co4} cube exhibits outstanding magnetic properties. Indeed, the thermally induced ETCST is abrupt and occurs closed to room temperature (T1/2 = 250 K). The conversion is complete as the maximum χMT value at high temperature is consistent with four isolated LS Fe(III) and four HS Co(II).

Concerning its photomagnetic properties, in addition to the remarkable efficiency of the photo-conversion, the measured Trelax is equal to 180 K. The measured lifetime of the metastable state at 180 K is around 10 years. It must be highlighted that commercial applications require to hold information more than 10 years at room temperature (stability ratio C -1 = ΔE/kBT > 50). 21 The achievement of such photo-responsive molecules is thus motivating for the design of molecular-based materials.

Another example going further in the mimicking of PBAs, were reported by D. Garnier 17-20) efficient increase of the magnetization is observed upon irradiation at 808 nm at 20 K. However, the relaxation temperatures are lower (Trelax = 80 K).

An innovation of this work can be ascribed to the detailed analysis of the molecular structure and electronic properties in crystal state and solution. Indeed, the solubility and the stability of the molecular cube containing potassium allowed its study by a complete set of techniques such as EPR, NMR spectroscopy and mostly by cyclic voltammetry. This enabled to highlight some remarkable electronic properties: a stability over a wide range of potential with 9 accessible redox states; their electro-chromism; their photo-magnetism; a slow magnetic relaxation ("field-induced SMM behaviour") at low temperature.

Following efforts in the group were devoted to the exploration of the influence of the inserted cation nature within a cube family of generic formula A  {Fe4Co4} where A = K + , Rb + , Cs + , Tl + , NH4 + and NH3CH3 + .

II. Implementing magnetic molecular switches: toward responsive molecular-based materials 1. Some challenges to face.

Given the multitude of fascinating properties of these magnetic molecular switches, research efforts have been driven by the desire to exploit them in devices for technological applications. 22,23 For instance, in 2008 Matsuda et al 24 developed a prototype of organic light emitting diode (OLED) containing a SCO complex, [Fe(dpp)2](BF4), to thermally tune the electroluminescence of a light emitting material.

Concerning SCO-complexes, it is worth mentioning the work of Bousseksou and coworkers that have massively demonstrated the great potential of these complexes-based materials. Indeed, they illustrated the interest of optical, mechanical and electrical SCO complexes properties for photonic devices, 25,26,27 actuators, 28,29,30 and electronic devices 31,32,33,34,35 respectively.

More recently, Clérac and al. 36 reported different SCO complexes or ETCST polymetallic complex-based microelectromechanical sensors (MEMs). Large resonance frequency shifts of the different hybrid devices were measured when the molecules switch under thermal or optical excitations.

However, the manufacturing of molecular switches-based devices necessarily implies the processing of molecular switches (e.g. deposition/patterning onto surfaces, structuration into layers etc). This step often constitutes the main challenge of the device fabrication and sometimes the limiting step.

First obstacles are intrinsic to the design of molecular-based materials, whatever the nature of functional molecules. Indeed, the transfer of these latter to materials can lead to a loss of the molecule structural integrity, as observed sometimes in the case of surfaces functionalization.

In particular, SCO and ETCST phenomena are highly sensitive as they are dependent of the molecule environment, the synthetic conditions and so on. As reported in solid state (crystals, bulk) studies, variations in crystallinity, solvent amount and polymorphism as well as aging of samples may all have a dramatic impact on the magnetic response. 37,38 For these reasons, the properties of hybrid (nano)materials based on (magnetic) molecular switches may be difficult to predict or rationalize. It can be even more difficult for nanomaterials due to some physical effects (e.g. size and quantum effects) appearing when working at the nanoscale.

Overall, the fabrication of molecular-based devices often entails the deposition of molecules onto surfaces as well-defined and homogeneous thin-films or layers. Over the last decades, high-vacuum sublimation techniques have been preferred to achieve these films on substrates. However, the scope of the molecules that could undergo these vacuum deposition processes is restricted. Indeed, molecules have to be sublimable, neutral and robust.

To extend the number of switchable molecules that could be integrated into devices, other efforts were recently invested in the design molecular based devices through solution approaches (e.g. such as spin coating, drop casting, layer-by-layer assembly). These alternatives techniques can be less demanding from a technological point of view.

In this project, we decided to follow this line and investigate routes to process the magnetic molecular switches through wet-chemistry approaches. On the one hand, we worked on the integration of molecular switches into organic polymers that are usually flexible materials with an ease to be processed. On the other hand, we have been interested in the deposition of magnetic molecular switches onto surfaces as hybrid (ultra)thin-films or selfassembled layers. This part was mainly investigated with the {Fe4Co4} cages that feature an efficient light induced-ETCST and remarkable solution properties (redox behaviour etc).

Moreover, to our knowledge photoswitchable low-dimensional polymetallic complexes have been poorly studied on surfaces.

In the following part, we will go through some examples of magnetic molecular switches on surfaces and switchable complexes-containing organic polymers. We will discuss the main strategies adopted and some issues encountered to access the two types of materials.

Magnetic molecular switches on surfaces

Molecular electronics and spintronics have highly stimulated the deposition of magnetic molecular switches as mono or submonolayer onto surfaces. However, so far, most of the research has been mainly focusing on an other class of magnetic molecular compoundes : Single Molecule Magnets (SMMs). These anisotropic complexes have attracted the interest of researchers because they can exhibit magnetic bistability, at the scale of one molecule, thus acting as molecular memories.

Many examples of SMMs on surfaces are based on lanthanide complexes. These systems can be monometallic and neutral and they thus allow the use of sublimation techniques to prepare monolayers or submonolayers. Moreover, some of these systems show outstanding properties with hysteresis observed up to 60 K. 39,40 The main difficulty in the field is to maintain the SMM properties onto surfaces and to avoid either (i) the decomposition of the polymetallic complexes during the deposition process, or (ii) the loss of the magnetic properties as the surface-molecule interaction can modify the electronic properties of the molecules. Different strategies have then been developped to preserve the molecules.

For instance, Serrano et al 41 prepared a bilayer of terbium double-decker (TbPc2) in which the first layer is used as a spacer with the substrate (see figure 8). Their scanning tunnelling microscopy (STM)

and spectroscopy experiments indicated that the use of a 'sacrificial' layer lowers the electronic coupling of the second-layer TbPc2 molecules with the gold surface.

Wet-chemistry routes were also investigated to graft SMMs onto surfaces but there are few examples of SMM adsorbates retaining their magnetic hysteresis. Indeed, the adsorption of the archetypal dodecamanganese ("Mn12") SMM on gold leads to a systematic reduction and disappearance of magnetic hysteresis. 42 First, an interesting example was reported by Rohlf et al. 44 in 2018 in which the authors investigated the switching properties of [Fe(pypyr(CF3)2)2(phen)] thin-films on 1T-TiTe2 substrate. This substrate was selected for its weak electron density at the Fermi level in order to avoid hybridization of the deposited SCO complex. More interestingly, XAS and SQUID experiments revealed a photoswiching of [Fe(pypyr(CF3)2)2(phen)] molecules when arranged in thin-films while it was not observed in the bulk (see figure 10).

Indeed, XAS spectra of the SCO thin-films recorded at the Fe L3 and L2 edges indicate an increase of the high-spin fraction at 28 K under irradiation (λ = 532 nm). On the opposite, under the same conditions the powder XAS spectra remain the same (corresponding to the complex in low spin state). In addition, the thermally-induced spin transition is shifted to lower temperatures when the SCO complex is deposited on surfaces (T1/2 is lowered by Extracted from reference [43].

approximatively 60 K relative to the powder T1/2). The authors attributed this difference to a decrease of the internal pressure in thin-films relative to the powder.

The second example selected their recent work, Baigari and al. 45 Finally, the description of discrete photomagnetic polymetallic complexes on surfaces is very limited. In addition, the deposition of these systems onto surfaces was mainly achieved through wet-chemistry approaches. For instance, Mardaud et al. 46 reported photomagnetic (ultra)thin-films of Mo/Cu cyanide-bridged complexes, whose deposition was done using Langmuir-Bludgett technique.

Figure 10. XA spectra at the Fe L3 and L2 edges for an Fe-pypyr powder (a) and thin film (b) on 1T-TiTe2. The blue spectra were acquired at 120 K. The red spectra were acquired at 28 K under illumination with a 532 nm laser. For comparison, a further XA spectrum acquired at 410 K is displayed in panel a. Temperature dependence of the HS fractions for Fe-pypyr powder and Fe-pypyr thin film (c). The dashed gray line is an extrapolation of the SQUID measurement. Extracted from reference [44].

Switchable complexes-containing organic polymers.

Numerous strategies have been considered to devise molecular-based switchable materials. In a non-exhaustive list, we will present some examples of the approaches related to our work and based on the use of organic polymers.

Schematically, synthetic approaches to embed switchable complexes in organic matrices can be divided into two main categories: (1) the "non-covalent approach", based on the dispersion of the switches in organic polymers. (2) The covalent approach, which consists in covalently grafting the switchable units to a polymeric backbone.

The design of these (nano)composites through a non-covalent approach can be carried out by straightforward routes, from a technical point of view (such as spin-coating, dipcoating, etc.). However, some restricting drawbacks are listed hereunder:

 the solvents allowing the dissolution of the switching units and the polymers are not always compatible  the resulting systems may not be thermodynamically stable and demixtion or inhomogeneity can occur.

 The dilution of switchable units into a polymeric matrix can lead to drastic change in their physical properties. In particular the cooperativity observed in bulk system can be lost when switchable molecules are dispersed in a matrix.

To overcome the last issue outlined, a common approach consists in using micro-or nano-particles of SCO-coordination polymers. For instance, Lapresta Fernadez et al. 47 dispersed the [Fe(NH2trz)3](BF4)2] polymeric SCO complex in several polymeric matrices. They were able to reproduce in the composite material the hysteretic behaviour of the bulk and use the material as thermal sensors (see figure 11).

Another emblematic example was given by Mascaros et al 48 [47].

synergic effect of the switching properties of the SCO compound with the electrical properties of polypyrrole. Indeed, the hybrid material exhibits a hysteretic behaviour in the conductivity measurement overlying with the hysteretic behaviour observed in the magnetic properties (figure 12).

For its part, the covalent approach may be more demanding from a chemical point of view. There are fewer examples of switchable hybrid materials obtained by this route due to the challenges presented by their synthesis and characterizations.

For instance, Ainscough et al 49 reported the preparation of spin-crossover grafted phosphazene polymers illustrated figure 13. They successfully synthesized cyclotriphosphazene ligands substituted terpyridine ligand as well as the corresponding polymers. However, the authors mentioned the difficulty to characterize the materials obtained after addition of iron(II) metal ions as they were insoluble.

On their side, Lemaire et al have mainly attempted to achieve the synthesis of switchable complexes based organic polymers by electropolymerization. They prepared for Figure 13. Schematic representation of SCO grafted phosphazene polymers. Extracted from ref. [49] example a bimetallic Fe(III) SCO complexes bearing thiophene entity 50 as presented figure 14. Unfortunately, the authors did not manage to electropolymerize thiophenefunctionalized complexes.

In the same spirit, Mitsumoto et al 51 reported in 2011 a Fe/Ni cyanidebridged square functionalized by an oligothiophene. Likewise, the authors could not reach the hybrid materials by electropolymerization as the material obtained was not stable. However, if this attempt was unfruitful, to our knowledge, there was no other investigations of (oligo)thiophenefunctionalized polymetallic complex for the design of switchable hybrid organic polymers (via electropolymerization process).

Even if the covalent strategy is more challenging, it can allow a better control of the material structure. Although, the integration of magnetic switches in organic polymers was found difficult to achieve, this approach has also yielded to some remarkable results using other type of switches. Indeed, we can mention the example obtained with a redox switch by

Choi et al 52 in 2007 with a great potential as non-volatile memory. Regarding the nature of X in the general structure scheme 1a), two families of poly(pyrazolyl) borates can be identified 4 : a. homoscorpionates : X is another pyrazolyl group (pz x ), identical to the two other. The resulting scorpionates act as tridentate ligand (in a fac manner), with C3v symmetry.

b. Heteroscorpionates : X can be a pyrazole pz y , different from the two others (pz x ) or anything else. Depending on the nature of R and R', bidentate or tridentate ligand can be obtained.

This work will restrain itself to the family of homoscorpionates tris(pyrazol-1-yl)

borates, with a particular interest for the third generation.

The archetype of trispyrazolyl borates family is the Tp ligand (Tp = tris(pyrazol-1-yl) borate) (scheme 1b) which belongs to the first generation of homoscorpionates. An extensive work on pyrazolyl-substituted scorpionates has led to the development of the second generation, noted Tp R (R being substituents at the 3, 4 or 5-position). An example is given scheme 1c) with Tp * . In contrast to the widespread number of modifications carried out on the pyrazole ring, there are only few examples of tris(pyrazol-1-yl)borates of the third generation. Indeed, the majority of trispyrazol-1-yl borates reported still contain an H as substituent R' (scheme 1a). Some investigations on the replacement of the {BH} bridgehead were conducted for example by M. Wagner's group. 5,6,7 H, Ph or Me3Si). The authors found a profound difference of the complexes magnetic behaviour regarding the nature of X. However, due to its distal position to the metallic core, they assumed that the magnetic properties were mainly governed by the complexes intrinsic distortions and crystalline supramolecular organization, more than the electronic effects of X.

Vitze et al 9 further investigated the influence of other phenyl ring substituents on the supramolecular arrangement such as the pentafluorophenyl fragment. They described the homoleptic iron (II) complex [Fe((C6F5B(pz)3)2] but did not report its magnetic properties.

In this contribution, we described the synthetic efforts made to enlarge the library of the third-generation scorpionates. More specifically, our interest has lied in devising tris(pyrazolyl) borates bearing a functional group for the design of advanced switchable complexes. In this regard, we conceived new thiophene-functionalized tris(pyrazol-1-yl) ligands (scheme 2c-d). Thiophene has been selected for its versatile character to act as an anchoring group but also as (electro)polymerizable group. Prior to use them for the design of switchable molecular based-nanomaterials (Chapter 4 and 5), we devoted some efforts to understand the influence of the functionalization on the properties of the iron(II) SCO complexes (this section ) and one those of {Fe4Co4} charge transfer complexes (section II).

In the first part of this chapter, four ligands illustrated scheme 2 were prepared. Ligand (4 A ) and (4 B ) were already described in literature 9,10 but no iron(II) complex was described with ligand (4 A ) and no magnetic measurements were reported for [Fe((C6F5B(pz)3)2]. As the phenyl ring substituents exhibit opposite electronic effects, ligands (4 A ) and (4 B ) appeared to be relevant for the study of model compounds. Thus, we decided to (re)investigate the iron(II) homoleptic complexes based on those two ligands. In parallel, we will detail the syntheses of ligands (4 C ) and (4 D ) and study thiophene influence on the electronic properties of the subsequent iron(II) complexes.

2. Syntheses of a third-generation tris(pyrazol-1-yl) borates serie. readily prepared bringing some modifications to the method outlined by Eaborn. 11 The compound was obtained in high yield (81%) from the reaction of the Grignard (4,tert-butylPhMgBr) on Me3SiCl in THF under reflux for 48 hours. Preparation of compounds 1 C and 1 D were based on Kumada and Suzuki coupling respectively. The precursor material (4-BrPhSiMe3) was prepared as reported elsewhere. 12 Trimethyl[4-(2-thienyl)phenyl]silane (1 C ) synthesis was carried out using the procedure (scheme 4) reported by Starikova et al. 13 It consists in the coupling of 2-thienylmagnesium bromide with 1-bromo-4-(trimethylsilyl)benzene (4-BrPhSiMe3) in 80 % isolated yield.

Compound 1 D was prepared through a two steps synthesis (scheme 5). First, the The chemical structure and purity of precursors and final compounds were proved by a combination of 1 H, 13 C NMR and elemental analysis (when possible). Detailed synthetic procedures and characterizations of compounds are joined in the experimental part. For the three compounds, the substituent on the one hand and the three pyrazoles on the other hand give rise to a set of characteristic signals in the aromatic region in 1 H as well as in 13 C NMR spectrum. Indeed, δ( ppm that can be assigned to the meta, para and ortho fluorine respectively. 19 Integration ratios also support the proposed pentafluorophenyl tris(pyrazol-1-yl) borate structure.

Syntheses of the homoleptic [Fe II (RB(pz)3)2] complexes

The series of homoleptic complexes of general formula [Fe(RB(pz)3)2] (where R = tbu Ph; . This observation seems to follow the electronic effect trend of the substituents. Each complex was characterized by elemental analysis, NMR, mass spectrometry, single-crystal XRD (when possible).

Characterizations

NMR

As stated above, the four complexes were expected to contain low-spin iron(II) at room temperature. This was shown by 1 H NMR solution characterizations. Indeed, at room temperature, complexes chemical shifts were observed within 0 to 15 ppm, the typical range of diamagnetic species. For [Fe(L)2] compounds (L = 4 A ; 4 C ; 4 D ); 1 H and 13 C NMR signals of pyrazole rings were slightly shifted regarding those of the free ligands but they are similar for the three complexes. NMR spectra of [Fe(C6F5Tp)2] complex are given as example below.

Optical Studies

UV-visible spectra were collected for all of the homoleptic complexes at room temperature in 10 -4 M dichloromethane solutions. The position of absorption maxima and the extinction coefficients are given table 1.

Compound ε/M -1 cm -1 λ (nm) Δ (cm Overall, the absorption spectra (figure 2) of the four complexes show a broad transition at ca. 520 nm, with ε < 400 M -1 cm -1 . Based on literature of the [Fe(Tp)2] pioneer complex 16 , we can assign this absorption band to the d-d transition 1 A1g ⟶ 1 T1g (in [Fe(Tp)2] 529 nm and ε = 90). Here the relative high value of the extinction coefficient may be due to the queue of the underlying ligand-centred transition.

The octahedral splitting parameter Δ was found to be within the range from 19 000 to 19 200 cm -1 for the four homoleptic iron(II) complexes. This family of third-generation of trispyrazolyl borates exert a ligand field slightly stronger than the archetypal Tp ligand, as a value of ̴ 18 700 cm -1 was reported by Jesson et al 16 for [Fe(Tp)2]. The [Fe( tbu PhTp)2] complex also seems to show a spin transition (figure 3 given table 3 and ) and close to values reported for tris(pyrazol-1yl)borate complexes in a low spin-state. 4 These structural observations are consistent with the magnetic measurements discussed above.

The supramolecular arrangement was also examined for all crystallized complexes. In [Fe( tbu PhTp)2], the unit cell contains three complexes with a conformation in which one of the pyrazole is practically orthogonal to the phenyl ring. There are CH-π interactions between the two molecules of the asymmetric unit. Indeed, the distance between the phenyl ring centroid and the pyrazole hydrogen Pz-H5 (the closest to the boron atom) is below the cut-off value of typical CH-π interactions, see appendix 1. 18,19 Moreover, the closest distance, d(PzH5-C(ph))= 2.63 Å (where C(ph) stands for the phenyl centroid). Given the SQUID measurement results, it is clear that those contacts have no impact on the magnetic properties of the complex as the spin transition shows no cooperative effect. The [Fe(2-TPhTp)2] complex structure comprised the same CH-π interactions described for [Fe( tbu PhTp)2] compound. The distance PzH5-C(ph) is however higher d(PzH5-C(ph)) = 2.78 Å.

Even though thiophene and phenyl rings are aligned parallel to each other along the a-axis, there are no π-π interactions between the sites. These structural data are coherent with a gradual spin transition.

In The [Fe(C6F5Tp)2] complex asymmetric unit contains two independent molecules. Both molecules are organized in sheets that are orthogonal to each other (appendix 2). Molecules belonging to the same sheet are parallels. The sheets arrangement allows an interesting interaction between metal ions sites. Indeed, the shortest intermolecular Fe-Fe distance corresponds to 6.801 Å.

Discussion

As briefly explained in the chapter 1, a steep spin transition is related to the existence of efficient intermolecular interactions that promote the cooperativity in the solid state. Here, the spin transition of the complexes are gradual.

In this study of [Fe(L)2] complexes (L = 4 A , 4 B , 4 C ), the substitution on the phenyl groups seems to permit the modulation of the spin transition temperature. However, UV-visible studies at 300 K did not enable us to highlight an influence of the phenyl ring substituent in the ligand serie (4 A , 4 B , 4 C , 4 D ). Consequently, the spin transition shift can hardly be attributed to substituent electronic effect or intermolecular interactions. It seems that factors governing the iron(II) spin state may arise from matrices effect.

Interestingly, in a more exhaustive study D.L. Reger and co-workers 20 have examined the structure-function correlations of several iron(II) complexes with tris(pyrazol-1-yl) substituted on the fourth position of boron RB(pz)3 (where R is different from H) and their methylated counterparts RB(3-Mepz)3. In the third-generation of ligand studied, the authors highlighted coordination sphere distortions (as discussed in X-ray crystals structures) as second factor influencing magnetic properties. Overall, it was showed that those intramolecular geometrical changes predominate intermolecular interactions in controlling the spin crossover.

The special case of the [Fe II (C6F5Tp)2] complex

From a gradual transition to an hysteresis loop

As the [Fe(C6F5Tp)2] complex shows almost a half conversion at the maximum available temperature (400 K, see figure 5), we further investigated the spin transition at higher temperature by using an oven. The temperature dependence measurement of the χmT product was performed using crystals obtained by slow evaporation of a DMF solution.

During the first heating, the χMT product exhibits a gradual transition from 0.06 cm 3 mol -1 K at 300 K to 3.10 cm 3 mol -1 K at 498 K. From 498 K to 500 K, the χMT brutally increases Nevertheless, the hysteresis profile of the [Fe(C6F5Tp)2] complex is atypical. Indeed, this thermo-induced spin transition appears as the superposition of a gradual conversion and an abrupt one. Thus, we assume that only a fraction of the material was converted at high temperature in a new phase participating to the hysteretic event, whereas the other one maintain a gradual behaviour. Regarding the χMT curve, this fraction is estimated at c.a. 33 %., considering the steep part of the hysteresis loop.

The SQUID measurement was repeated on a second sample (see appendix 3). The same behaviour was observed over the three thermal cycles. The difference lies in a lower low-spin to high-spin conversion and a sharper hysteresis loop (width = 49 K). As discussed earlier, gradual SCO is a molecular property while abrupt hysteretic spin transition expresses an effective communication between the SCO centres. To investigate this singular magnetic behaviour, different thermal analyses (Thermal Gravimetry Analysis and Differential Scanning Calorimetry), IR and NMR spectroscopy and single crystal X-rays diffraction studies were carried out.

Verification of the compound identity

First of all, after SQUID measurement [Fe(C6F5Tp)2] sample was recovered as pink crystals. A small amount of the compound appeared degraded, as it turned brown.

Differential Scanning Calorimetry (DSC) measurements

The thermal effects associated to the SCO process were monitored by differential scanning calorimetry on heating and cooling cycles. All the [Fe(C6F5Tp)2] samples were obtained as crystals from slow evaporation of a DMF solution.

The DSC thermograms of the complex upon three heating-cooling sequences, recorded with a rate of 1 °C/min, are given figure 7 (a-c). The first heating mode reveals a broad exothermic peak centred on 115°C (388 K), followed by a series of sharp endothermic peaks A summary of the hysteresis parameters is reported table 4. The enthalpy values (ΔH)

of the endothermic and exothermic peaks are 2.75 kJ/mol and 3.12 kJ/mol respectively.

Typical ΔH values for Fe(II) SCO complexes fall in the range 3-27 kJ/mol. The values obtained here are lower than expected for such hysteretic event. We may explain these results with the previous assumption of an incomplete (phase) conversion of the material. Regarding the SQUID measurement, these enthalpy values would be associated to the conversion of a 33% fraction of the material (or even less for DSC experiments). Thus, they should be considered as apparent enthalpies, the real ones would be roughly above 9 kJ/mol. The hysteresis observed by DSC showed a strong dependency to the experimental conditions. Indeed, increasing the scan rate to 2 °C/min resulted in a slight peaks broadening whereas the SCO phenomenon was not observed by operating at 10 °C/min (appendix 5). We assume with this temperature ramp (10°C/min) the material may not have time to convert (or rearrange). The sharpness of the transition was also significantly modified with variations of sample preparation. DSC scans of grinded crystals afford undefined signals, which extend over a range of ≈ 40 °C upon cooling (appendix 5). Similar enthalpy values as those of untreated crystals were extracted by integrating those spread peaks surface area.

The DSC study has provided evidence, akin to magnetic measurements, of the [Fe(C6F5Tp)2] complex hysteretic profile after annealing. To investigate the physical transition underlying the occurrence of a thermal bistability, temperature dependent single-crystal Xray diffraction were carried out.

Temperature dependence single-crystal X-ray diffraction

Crystal structures of fresh [Fe(C6F5Tp)2] single-crystals (obtained by slow evaporation of DMF solution) were resolved by XRD at 300 K and at 430 K (Table 5). The sample recorded at 430 K underwent a 2 K/min temperature ramp until 470 K and was cooled back to 430 K, to investigate crystallographic phase transition). No structures could have been obtained at higher temperature because of the limited quality of the XRD data. Tries at temperature above ΔHapp down (kJ.mol -1 ) -3.120 ca. 480 K showed that the white crystals (in high-spin state) shrink and melt (systematically felling off the sample holder).

Consequently, to get structural information of [Fe(C6F5Tp)2] after a thermal treatment, crystallographic datas were measured at 300 K from a sample analysed by DSC up to 500 K. 1 This would allow the comparison of LS state phases before and after annealing. i.e 2.10 Å. From 300 K to 430 K, the unit cell volume expansion is about 2.5 %. This variation is due to the increase of the metal-ligand distances together with thermal dilatation. Cell parameters also evolve with temperature as b and c slightly increase while a decreases. These variations were similar to those reported for [Fe(HB(pz)3] in the work of Salmon et al. 21 At 430 K, FeN-NB torsion angle is 6.2° for the complex in HS state which is almost the double that the value pointed out at room temperature (i.e. 3.6°). The spin state change is also exemplified by the decrease of the FeN-NC torsion angle, equal to ca. 160.8°. Besides those intramolecular differences no notable changes were detected in the intermolecular interactions. The crystal structures of the sample studied by DSC did not reveal any difference with the one obtained for fresh crystals (300 K). We assume that the selected crystals for XRD analysis, which maintain their crystallinity after the DSC thermal treatments, belong to the fraction which does not show the hysteretic behaviour. Temperature dependence powder XRD experiments are currently conducted to shed light upon the modifications generated by the annealing.

Conclusion

We have prepared new Fe(II) SCO complexes of the third-generation scorpionate

[RB(pz)3] -(where R = 4 A , 4 B , 4 C , 4 D ) . These systems show reversible and gradual spin-state conversion at high temperature with T1/2 temperature, which strongly depend on the ligand functionalization. As expected, the samples with electron-withdrawing R group on the phenyl ring show weaker ligand field and lower transition temperature.

An interesting magnetic behaviour is observed for the [Fe(C6F5Tp)2] derivative which show a conversion from a gradual spin transition to a hysteretic behaviour upon annealing.

The origin of these phenomenon still needs to be confirmed. At the moment, we assum that upon heating, the compounds melts and another crystal phase can be obtained with enhanced intermolecular interaction.

Overall, these mononuclear complexes are useful models to probe the influence of ligand functionalization on the magnetic behaviour. However, the sensitivity of the samples toward oxidation, their gradual SCO behaviour, their high transition temperature are a clear drawback for potential applications. For these reasons, we decided to investigate the influence of this third-generation tris(pyrazolyl)borate on more robust switchable complexes.

We selected the A  {Fe4Co4} cages reported in our group in 2016 and which show interesting photomagnetic effect and a remarkable thermal stability.

II. Functionalization of charge transfer cyanide-bridged {Fe 4 Co 4 } cubes

Note to the reader :

In the following discussions, independently of the inserted cation, the cubes containing 4x Fe II LS, 3x Co III LS and 1x Co II HS will be referred as "paramagnetic" while those consisting in pure 4x Fe II LS, 4x Co III LS will be referred as "diamagnetic".

Moreover, for simplicity reasons, the formula of paramagnetic

A  {[Fe II (L1)(CN)3]4[Co III (L2)]3[Co II (L2)]} cubes (
where A is a monocation; L1 and L2 stand for tris(pyrazol-1-yl)borates ligands was sometimes shorten to A  {Fe(L1)Co(L2)} (or sometimes A-FeL1CoL2). This abbreviation was also applied to diamagnetic cubes specifying the anion as followed: A  {Fe(L1)Co(L2)}X (where X is the anion). Any deviation to this notation will be specified.

Background study : Role of the inserted cations in a A  {Fe4Co4} cages family

The ETCST phenomenon identified in PBAs is highly dependent of the inserted cation.

For example, Bleuzen et al highlighted this point in a family of A2Co4[Fe(CN)6]3.3·11H2O (A = Na + , Rb + , Cs + ) PBAs in which the efficiency of the ETCST was increased when using the smaller cation. 22 On the one hand, the Na-containing PBA shows a thermally induced ETCST below 300 K whereas the Rb and Cs derivative remain diamagnetic in the all temperature range. On the other hand, photomagnetic effects are observed for all compounds but the relaxation temperature is higher for the Na-PBA. Both phenomenon can be explained by a better Moreover, it was also suggested (previously) that the filling of the PBA networks with large alkali ion could confers some rigidity to the network that can prevent the structural reorganisation due to the ETCST process.

This study was extended in the group ERMMES to lower dimensional PBAs models in order to probe the effect of the ion at the local scale, and to eliminate the potential influence of the solid-state interactions. A series of neutral photomagnetic A  {Fe4Co4} cages (where A = K + , Rb + , Cs + , Tl + , NH4 + and NH3CH3 + ) were investigated. 23,24 In fact, the octametallic anionic core proved its ability to host different monocations, which allowed the investigation of the cation effect on the structural and electronic properties of the A  {Fe4Co4} cages family.

The cubic framework consisted in 4x Fe II The major advantages of these systems are their good solubility and stability in most of common organic solvents. Indeed, an exhaustive array of characterizations in solution could have been obtained and compiled to those collected in solid state. Paramount results of this research that have provided a solid basis in our work of designing functionalized {Fe4Co4} cages and optimizing their properties will be summarized below.

Identification of structural disparities with the ion size

Single-crystal XRD analyses exposed significant differences when ranging the cation size. For instance, within the series of alkali cations (K + , Rb + , Cs + ) each of them adopt a singular position. The smaller the cation, the more off-centre its position (see figure 8). The cage bears a formal negative charge, on the {( pz Tp)Co II (NC)3} corner. Cations with a pronounced polarizing character, i.e. smaller cations, will tend to reside preferably close to the local negative charge. The larger the cation, the more stable cube

Besides the interaction between the Co II and the inserted cation, this latter builds contacts with the cage through interaction with the cyanide bridges (distances are lower than Van Der Walls radii). The number of interactions between the alkali ion and the cyanide πclouds increases with the cation size. Indeed, the Cs + interacts with the twelve cyanide bridges while the K + interacts only with three. The complete number of interactions of Cs + with cyanide bridges confers a larger stability in solution to the Cs  {Fe(Tp)Co( pz Tp)} comparing to the K  {Fe(Tp)Co( pz Tp)} cube (which is coherent with the high affinity of Prussian blue analogues for Cs + ions).

Additionally, the robustness of those polymetallic complexes in solution has enabled to emphasize the richness of their redox properties, by using cyclic voltammetry studies. First of all, the cubes were shown to be stable in a very large potential range (over 3V in CH3CN solution). Then, they show a remarkable redox versatility as nine reversibly accessible oxidation states are observed from the most reduced state, [Fe II 4Co II (HS)4] 4-, to the most oxidized one, [Fe III 4Co III (LS)4] 4-.

The smaller the cation, the better the photomagnetic properties

The photomagnetic behaviours of the A  {Fe4Co4} cages (A = K + , Rb + , Cs + ) were measured on crystalline materials, obtained either from the evaporation of an ACN solutions or from evaporation of DCM:DMF mixtures. Significant increases of magnetization were observed at 20 K upon irradiation with laser light at 405, 532, 635, 808 and 900 nm for all the cubes. However, different results were obtained depending on the crystal phases.

To discuss the cation influence on photomagnetic properties independently of intermolecular interactions, measurements were performed in frozen solutions (on either ACN or DCM) as shown in figure 9.

Similarly to the study of Bleuzen et al. on

PBAs, the efficiency of the photoconversion was improved when reducing the size of the inserted alkali cation as shown figure 2. The previous study (in ERMMES group) on lower dimensional PBAs led also to the conclusion that the guest cation can steer the magnetic properties of the {Fe4Co4} framework.

Additional effects of the hosted-cation on the cube properties

Single crystal XRD and EPR (Electron paramagnetic resonance) experiments jointly indicate a more anisotropic paramagnetic Co II HS when its proximity with the alkali cation rises.

This observation was reinforced by the retrieval of the Δ (axial distortion) parameter from the temperature dependence of the magnetic susceptibility measurements. Higher value of Δ parameter was obtained K  {Fe(Tp)Co( pz Tp)} cube (Δ = -994 cm -1 ) comparing to Cs  {Fe(Tp)Co( pz Tp)} cube (Δ = -710 cm -1 ). This reflects an increase of the distortion of the Co(II) coordination sphere when the alkali cation size is reduced.

The A  {Fe4Co4} cages show typical field-induced single-ion magnet (SIM) behaviour below 10 K. The analysis of the slow relaxation process showed that the higher energy barrier, Ueff, are directly correlated to the distortion of the Co(II) coordination sphere and to the magnetic anisotropy as seen by EPR measurements. For example, the stronger axial anisotropy of the Co(II) ion in the K  {Fe(Tp)Co( pz Tp)} cube leads to a higher energy barrier, Ueff = 90.7 cm -1 than the one measured in the Cs  {Fe(Tp)Co( pz Tp)}, where Ueff was found to be equal to 49.7 cm -1 . Overall the nature of the alkali ion clearly influenced through its influence on the Co(II) coordination sphere the slow relaxation of the magnetization. 23,24

Interests and objectives of this part

The foregoing appealing properties of these {Fe4Co4} cages spurred us to engineer their transfer into materials (Chapters 4 and 5). In this way, {Fe4Co4} cages were functionalized with a thiophene ligand (2-TPhTp or 3-TPhTp ligands). To target the best candidates, our work has included a preliminary study on the influence of the third-generation tris(pyrazol-1-yl)borates on the {Fe4Co4} (magnetic) properties.

For this purpose, we used tbu PhTp ligand as a model compound to elaborate a rational design of new {Fe4Co4} functionalized complexes with the improvement of their properties.

This choice was driven by the following reasons: (i) tert-butyle moiety have well-identified electronic effect (inductive donor) on the phenyl ring; (ii) the ligand (4 A ) is prepared in high yield by a more straightforward synthesis than the 2-TPhTp and 3-TPhTp ligands.

To consider the influence of the functionalized ligand on the electronic properties of both metal ions, and therefore on the photomagnetic effect, the following A  {Fe( tbu PhTp)Co(Tp)} and A  {Fe(Tp)Co( tbu PhTp)} (A stands for a monocation) cages series were prepared.

In the previous study of A  {Fe(Tp)Co( pz Tp)} cages (where A = K + , Rb + , Cs + , Tl + , NH4 + and NH3CH3 + ), the caesium was the cation conferring the greater structural stability to the {Fe(Tp)Co( pz Tp)} framework but with a moderate photoconversion. The best photoconversion was obtained with the potassium but this cube is less stable in solution. A compromise was reached with the Tl  {Fe(Tp)Co( pz Tp)} cage. Indeed, it combined a good stability and an efficient photomagnetic effect.

Therefore, two series of cubes bearing tbu PhTp (4 A ) (on either Fe or Co) were first investigated on the most stable compounds that is using caesium . The possibility to refine the {Fe4Co4} cube properties playing on the nature of the inserted cation was also explored by substituting the Cs + by K + and/or Tl + , in order to confirm our preliminary results.

The results will be discussed by taking the A  {Fe(Tp)Co( pz Tp)} cubes family as reference. The {B-H} signal integrating for one proton was expected but it was not observed.

Investigating the effect of tbu

For metal sites out of the 3-fold axis : the pyrazolyl rings are no longer equivalents.

{B-H} CH2Cl2

Protons signals assignments of Cs {Fe(Tp)Co( tbu PhTp)}ClO4

The 1 H NMR spectrum (figure 13) of the {Cs

 [Fe II (Tp)(CN)3]4[Co III ( tbu PhTp)]4]}ClO4
gives rise to a more straightforward interpretation than its paramagnetic counterpart. Indeed, as all cobalt ions are Co III LS, all the chemical shifts are observed in the diamagnetic region.

More importantly, accordingly to the Td symmetry of the diamagnetic cube, proton signals of the iron(II) Tp ligands are equivalent, along with those of the cobalt(III) tbu PhTp ligands.

Thus, ten different chemical shifts with a total integration of 128 protons are expected.

As at 300 K the {H-B} signal is usually broad and not detected at room temperature, the 1D 67 133 Cs NMR analyses of both paramagnetic and diamagnetic cubes reveal interesting information related to the interactions between the {Fe4Co4} cage and the inserted cation.

The caesium signal of in Cs  {Fe(Tp)Co( tbu PhTp)} compound is observed at -212.2 ppm (vs CsNO3, 0.1M, D2O) while the one of Cs  {Fe(Tp)Co( tbu PhTp)}ClO4 is located in the positive region at 47.5 ppm, in the 133 Cs chemical shift diamagnetic range (from +130 to -30 ppm).

These measurements give access to the 133 Cs paramagnetic chemical shift, which is defined as the difference between the experimental shift and the diamagnetic contribution, δ( para ) = δ exp -δ dia . The diamagnetic contribution is obtained by considering the chemical shift of an isostructural diamagnetic reference (here the diamagnetic cube).

In the present case δ( para ) = δ exp -δ dia = -249.7 ppm (at 300 K). The paramagnetic contribution has two components: the Fermi contact term (due to the presence of unpaired electron on the probed nucleus) and the dipolar contribution (a through space interaction). In the case of the paramagnetic cube, the dipolar contribution is to be positive because the caesium is located on the magnetic axis. Thus the observed negative shift necessarily arises from a Fermi contact term. The contact term can have two origins. It can be due to the delocalization of the unpaired electron from the paramagnetic source, the Co(II) ion, to the neighbouring atoms (the unpaired electron is localized in a molecular orbital that extend up to the probed nucleus) or, it can be due to the polarization of the valence orbitals on an atom by a neighbouring orbital of a neighbouring atom bearing spin density. In all the cases, the negative paramagnetic chemical shift measured on the caesium indicates that (i) this atom bears some negative spin density; (ii) an electronic communication through orbitals occurs between the cage and the intercalated caesium.

Strong shielded caesium signals corresponding to negative paramagnetic chemical shift have been reported in PBAs by Köhler and co-workers 25 . To explain the negative 133 Cs chemical shifts, the authors suggested an extension of the spin density from the d orbitals of the paramagnetic metallic ions to the caesium s orbital through a polarization mechanism of the close s and d orbitals. We believe that the spin density extension could also arise through the spin delocalization from the Co(II) ions to the cyanide ligand and through the interaction between the Cs orbitals and the π orbitals of the cyanide bridges . Those Cs-CN interactions are also highlighted by XRD analyses as discussed further, section II. 3.5.

Influence of the experimental conditions on paramagnetic: diamagnetic cubes ratio.

We examined the ratio of the paramagnetic and diamagnetic compounds under different synthetic conditions from the standard procedure described (entry 1, table 7). For reactions conducted according entry (1) and ( 2), the paramagnetic: diamagnetic ratio was estimated by 1 H NMR on the crude precipitate collected after work up (m = 114 mg for both conditions). In case of entry ( 3) and (4), no precipitate was observed at the end of the reaction.

The dilution of reaction up to 15 mL afforded a 1.5 : 1 mixture of paramagnetic and diamagnetic cubes (entry 2). This ratio was drastically affected when the reaction was conducted at longer times reaction. For 48 h in 3 mL (entry 3) or 36 h in 15 mL, only the diamagnetic cube Cs  {Fe(Tp)Co( tbu PhTp)}ClO4 was found. This is consistent with a progressive oxidation of the cube in solution. The paramagnetic cube becomes prevalent for short reaction times and concentrated solutions. As the time increases, the diamagnetic cube signal increases (at 47.9 ppm, as shown in the last spectrum). More importantly, an additional signal appears at -305.9 ppm after 2 days and persists until the analyses end. As this signal is more up-field shifted than the one of Cs  {Fe(Tp)Co( tbu PhTp)} cube, it might correspond to a species containing more paramagnetic metallic ions. It could look contradictory with the 1 H NMR analyses, which does not show new peaks however, it is not so unusual for a paramagnetic complex containing several paramagnetic ions to be NMR silent because of the fast 1 H nuclear relaxation. It seems that on cobalt, these third generation Tp stabilize the {Fe II 4Co III 4} diamagnetic state. These observations were supported by electrochemical studies presented further in this chapter. Indeed, it seems that the redox potential of the {Co( tbu PhTp)(NC)3} units are lower than that of the {Co( pz Tp)(NC)3} unit (see electrochemical studies further). As a consequence, the diamagnetic cube shows shorter Fe II -Co III edge distance, ca. 5.0 Å, than the paramagnetic in which the Fe II -CN-Co II distances reach ca. 5.2 Å. As all cobalt ions exhibit the same Co III LS electronic state, the Fe/Co framework is overall neutral, and the Cs + ion is well centred inside the cube. Thus, the Cs + -πCN distances are less scattered and are within a narrow range, from 3.52 to 3.61 Å. Finally, the perchlorate anion, which ensures the charge compensation sets at 7.74 Å from the inserted cation.

Magnetic properties

The Cs  {Fe(Tp)Co( tbu PhTp)}ClO4 cube did not show any increase of the magnetization upon irradiation (the χmT value remains equal to zero over the all light excitation duration).

This behaviour is reminiscent of that of the Cs  {Fe(Tp)Co( pz Tp)}ClO4 diamagnetic compound, which did not exhibit any photomagnetic properties either. Finally, it is also worth noticing that thermally-induced ETCST was not observed within the explored temperature range (2-400 K) for both diamagnetic and paramagnetic cubes. The magnetic susceptibility χM of Cs  {Fe(Tp)Co( tbu PhTp)} is plotted as a χM vs T graphic in figure 19b as example.

Conclusion

The third-generation tris(pyrazolyl) borate ligand tbu PhTp was used to synthesize a novel cubes family : A  {Fe(Tp)Co( tbu Tp)}. In contrast with the A  {Fe(Tp)Co( pz Tp)} family, the preparation of the paramagnetic compound is more difficult to achieve as the {Co( tbu Tp)(NC)3}

unit is prone to oxidation. However the paramagnetic cube can be precipited from concentrated solution and further purified to reach almost pure material. The diamagnetic cube, Cs  {Fe(Tp)Co( tbu PhTp)}ClO4, can be obtained by diluting the reaction media or increasing reaction times to promote the oxidation.

The observations made on tbu PhTp model cubes can be extended to the other thirdgeneration Tp ligand, described in the first part of this chapter, in particular to 2-TPhTp and 3-TPhTp. Within this series of ligands, the diamagnetic A  {Fe(Tp)Co( R Tp)}ClO4 ( R Tp = third generation Tp) is clearly the more stable form, even if we manage to isolate the paramagnetic derivative in solid-state. This behaviour could be explained by considering that the third generation ligands possess a higher ligand field that favour the oxidation state +III of the cobalt ion.

Unfortunately both of the diamagnetic and paramagnetic cubes do not show any thermally-induced or photo-induced ETCST. The stabilization of the diamagnetic state represents a drawback as the diamagnetic cubes are poorly photomagnetic (see below). They do not represent favourable candidates for the design of (photo)responsive hybrid materials.

In order to overcome this problem, we thus decided to explore the properties of cubes functionalized on the "iron corners".

Actually, we assume that the effect of the Tp functionalization should be less important on the electronic properties of the {Fe( R Tp)(CN)3} units (where R Tp stands for a third generation Tp ligand). Indeed, the strong ligand field of acceptor cyanides is expected to have a dominant influence on the iron electronic state and thus on its redox potential.

Investigations made on {Fe( tbu Tp)(CN)3} units and the corresponding cubes are presented in the following.

III. Refining the {Fe 4 Co 4 } cage ETCST through a multistep rational design 1. Influence of the molecular precursors electrochemical potential

In metal-metal charge transfer systems, it is well known that the nature of the peripheral ligands strongly influence the level of delocalization between the two metal ions and the overall electronic properties of the bimetallic entities. The influence of these ligands on the electronic properties can be followed by different experimental approaches. Indeed, UV-vis spectroscopy, electrochemistry and magnetometry (when unpaired electron are present) allow probing the level of electronic communication between the two metal sites.

In summary, the electronic delocalization will be enhanced when both entities have close redox potentials. On the opposite, when the redox potentials of both metal ions are well separated the redox state are well defined and the system are classified as class I mixed valence compounds.

Bernhardt et al. 28 showed in a series of [(L)Fe-CN-Co(L')] binuclear systems that they were able to adjust the MMCT transition energy and intensity by changing the nature of the blocking ligand and thus the redox potential of the metal ions. The control of the redox potential of the Fe and Co subunits was then proved to be efficient to predict the possible occurrence of ETCST phenomenon 29,30 .

For Previous results in ERMMES group agree with this statement. However intermolecular interactions can also play a crucial role in the occurrence of ETCST, eventually stabilizing one redox state in system where an ETCST phenomenon could be expected 31,32 .

To get a better understanding on the influence of the scorpionate ligands on the electronic properties of the Fe-CN-Co ion pairs in this work, we decided to investigate the redox properties of model complexes. The [Fe III/II ( R Tp)(CN)3] -/2-were used as model complex to probe the influence of the R Tp ligand. In the case of the cobalt units, homoleptic [Co III/II ( R Tp)2] +/0 complexes were used as models as it is not possible to prepare [Co III/II ( R Tp)(NC)3] -/2-. The cyclic voltammograms are presented figure 20 and 21. The redox potential of [Co III/II ( R Tp)2] +/0 complexes are spread in a larger redox range.

For example, the oxidation wave of [Co III/II (Tp)2] +/0 is located at 0.225 V, (vs SCE) well above that of [Co III/II ( tbu PhTp)2], which occurs at -0.132 V. This difference (ca. 0.357 V) accounts for a better stabilization of the Co III redox state when using third generation scorpionate ligand, tbu PhTp. This can explain the stabilization of {Fe II 4Co II 4} electronic state in the above-mentioned cubes. NMR Spectroscopy.

Design and study of the

The Bu4N[Fe III ( tbu PhTp)(CN)3] tricyanido iron(III) complex gives a paramagnetic 1 H spectrum (see figure 22) with all the expected signals. Indeed, at room temperature, chemical shifts were observed within -60 to 25 ppm.

The protons signals corresponding to the tert-butyl phenyl ring of tbu PhTp are observed at 22.13 and 13.55 ppm (Hphenyl ) and 4.26 ppm (Htbu) with the expected ratio 2:2:9. The set of pyrazolyl protons signals appeared at -4.88,-6.26 and -55.88 ppm.

The complex was also characterized by FT-IR spectroscopy, as the technique is particularly informative for cyanide compounds. Indeed, the complex spectrum recorded on crystals (see appendices) exhibits a sharp absorption band at 2121 cm -1 corresponding to the stretching of CN -, typical of ferricyanide compounds. 33 The alphatic C-H of tetrabutylammonium cation and the tert-butyl moiety appear within the range 2874 to 2961 cm -1 . The temperature dependence of the MT products (figure 25) between 2 and 300 K are ascribed to the presence of one paramagnetic Co(II) HS ion in each cube. The MT value of approx. 2.7 cm 3 mol -1 K near room temperature is in good agreement with the expected value for isolated high-spin Co(II) ions with a significant magnetic orbital contribution.

Single-crystal XRD

Above 300 K, the MT curves shows the onset of a thermally induce ETCST. This behaviour is reminiscent of that observed in the paramagnetic A{Fe(Tp)Co( pz Tp)} (A = K, Cs) model cubes. The ETCST is incomplete at 400 K, the maximum available temperature, and irreversible. The ETCST is irreversible in these experimental conditions. Indeed, upon cooling from 400 K to 2 K, the MT product slightly decreases in both cases but do not show any characteristic step. This behaviour was also previously observed in the model cubes, and it is very commonly observed for FeCo molecular switches. It is associated to the loss of crystallization molecules (the measurement is carried out under reduced He pressure) as the temperature increases. The desolvated compounds may exhibit different magnetic behaviour as the solvated crystal phases.

More importantly, both compounds show a photomagnetic effect at low temperature.

In fact, the MT curves (figure 26) measured at 20 K under a laser light irradiation at 808 nm (ca 5 mW/cm 2 ) show an increase which account for the conversion of Fe II LS(S=0)-CN-Co III LS(S=0) diamagnetic pairs into Fe III LS(S=1/2)-CN-Co II HS(S=3/2) paramagnetic ones.

The saturation is obtained within 20 min of irradiation for both Cs and Tl cubes.

Interestingly, the increase of the MT value is much larger (ca. 10 cm 3 mol -1 K) for the Tl cube than for the Cs one (ca. 1 cm 3 mol -1 K). This measurement thus confirms an anterior preliminar result on the model cubes, A  {Fe(Tp)Co( pz Tp)}, where the Tl derivative showed a much better photomagnetic effect than the Cs one.

IV. Cubes electrochemical studies in solution

Cyclic voltammetry is a powerful tool to probe the electronic properties of polymetallic species. reaction involving electron transfers. In the case of mixed-valence compounds this technique can provide, to a certain extent, qualitative information on the electronic delocalization. Indeed, the degree of electronic exchange bewteen metal ions associated by bridging ligand can confer them a peculiar electrochemical signature.

This point has been particularly evidence by seminal investigations on cyanide-bridged {Fe4Co4} cages, A . Their peculiar electrochemical behavior will be discussed here with the different model synthesized in this chapter.

Cyclic voltammetry studies were thus carried out on the polymetallic complexes :

Cs{Fe(Tp)Co( tbu PhTp)}, Cs{Fe( tbu PhTp)Co(Tp)} and Tl{Fe( tbu PhTp)Co(Tp)} cubes in acetonitrile, at room temperature.

The three cyclic voltammograms (CV) are presented figure 27 and display common features. First, the CV show eight oxidation and eight reduction redox waves, which are similar for all cubes and well reproduced upon cycling. This indicates the remarkable redox stability of these polymetallic species and their remarkable stability over a broad potential range.

For the three cubes, two distinct sets of four redox waves are observed: four quasireversible waves are observed at high potential whereas four (apparently) irreversible broad

waves are observed at lower potential. The quasi-reversible waves observed at high potential (between 0.7 and 1.5 V vs ECS) are ascribed to the Fe III/II redox systems corresponding to the four Fe vertices of the cube. Interestingly, although the Fe ions are chemically equivalent, the four waves are shifted from each other. This is ascribed to an efficient electronic communication between the Fe vertices, either from electrostatic origin, or due to a "secondfirst neighbour" interaction through the CN-M-NC link as previously described in cyanidobased polymetallic species. 34 The redox waves of the four Co III/II ions located in the other vertices are typical of slow We are currently carrying out further analyses in highly controlled conditions to be able to correlate the electronic properties probed by CV (as we did for the precursors) and the switchable properties observed in magnetism.

V. Conclusion

Four third-generation tris(pyrazolyl) borate were synthesized of which two were newly developed. These ligands were successively used to prepare and study two kinds of switchable complexes.

First, we have been able to synthesize and to characterize new [Fe( R Tp)2] neutral SCO complexes ( R Tp = 4 A , 4 B , 4 C , 4 D ) . These complexes show gradual transition at high temperature (ca. above 300 K). We observed that their transition temperature, T1/2, is not only controlled by the ligand substitution. As previously observed in SCO complexes, intermolecular interactions clearly play a critical role on the SCO behaviour.

Very interestingly, in the case of the [Fe(C5F6Tp)2] complex, we observed a drastic change in the SCO behaviour upon heating. Whereas the material exhibits a very gradual spinstate equilibrium in the P-1 crystal phase, SQUID and DSC results jointly show the changeover to a strongly cooperative transition with a hysteresis loop of ca 50 K after one thermal ramp.

XRD data did not allow us to probe any phase transition as crystals started to melt around 490 K. At present, we assume that upon heating the melting of the complex leads to a new crystal phase would be obtained upon melting. Current powder-XRD measurements are in progress and preliminary results tend to support our hypothesis.

To our knowledge, this behaviour is unprecedented and opens interesting perspective for the preparation of cooperative spin transition systems. In addition, this complex offers interesting perspectives for vacuum deposition on surfaces.

Second, we prepared a family of model {Fe4Co4} cubes with tbu PhTp ligand to study the effects of this new capping ligand on the Fe/Co framework. The "easiest" synthetic approach that consists in anchoring the R Tp ligand on the Co ions leads to diamagnetic cubes or to fragile paramagnetic ones (that are prone to oxidation). Unfortunately in both cases, the cubes are not photomagnetic.

With the assumption that the functionalization on the "{(L)Fe(CN)3} vertices" would have less effects on the electronic properties of the {Fe4Co4} cubes, we first prepared 'symmetric' system where the R TP ligand is located on the iron ions. The approach has proved to be efficient as photomagnetism was recovered going from the Cs  {Fe(Tp)Co( tbu PhTp)} to the Cs  {Fe( tbu PhTp)Co(Tp)} cube. As observed in previous study of the A  {Fe(Tp)Co( pz Tp)} family, the efficiency of the light-induced ETCST in the Cs-containing, cages is the lowest.

Consequently, we prepared the Tl  {Fe( tbu PhTp)Co(Tp)} cube that demonstrated a complete photoconversion upon irradiation with an improved relaxation temperature.

The capping ligand has versatile role in the design of the {Fe4Co4} cages as it first helps in the control of the dimensionality by blocking metal sites and (ii) its position allows to tune the cage magnetic properties.

Overall these studies have allowed:

(i) to establish reliable synthetic strategies for the preparation of functionalized FeCo cubes that can be inserted into hybrid material (chapter 4-5)

(ii) to improve our fundamental understanding on the parameters governing the electron transfer in these Fe/Co photomagnetic systems (in particular the role of the Fe/Co redox matching and the role of the alkali ion).

CHAPTER 3

Synthetic approaches for the design of new metallopolymers One of the major impetuses for the interest in spin crossover (SCO) complexes and photomagnetic complexes is the possibility of producing molecular materials for a wide range of applications: displays, sensors, memories and so on. 1,2 Despite their appealing magnetic properties, they are often crystalline materials difficult to process. It is one of the challenge that needs to be overcome for bringing magnetic molecular materials to commercial applicability.

To assist the processing of these molecular switches, one strategy consists in integrating them into organic polymers, either via a covalent approach or by dispersing them in a polymer matrix. Indeed, organic polymers present many advantages over small molecules in manufacturing procedures, in particular well-established techniques such as spin coating and ink-jet printing that are already used in industrial applications.

Thus, metal-containing organic polymers would combine the structural flexibility and processability of organic polymers with the interesting physical properties associated with the molecular switches.

To access these hybrid materials, several covalent-based synthetic approaches have been considered in this work :

a) The post-functionalization of an organic polymer. This approach relies on the chemical modification of the organic backbone to insert metal-binding sites.

(figure 1),

b) The (co)polymerization of a functionalized polymerizable ligand with an organic monomer (fig. 1). In both a) and b) case, the complex is formed in a second stage, by reaction of metal salts or with metallic complexes bearing labile ligands.

Considering those two strategies, the final metallopolymer will be obtained by reaction of a functionalized organic backbone with a metal ion.

Finally, it is also possible to envisage the metallopolymers through the (co)polymerization of a functionalized complex (bearing a polymerizable group). However, as this approach hasn't been explored in the frame of this work, it will not be discussed.

We first re-investigated the post-functionalization strategy reported by Y. Qin et al. 3 In their work, the authors grafted pyrazolyl groups on a preformed organic polymer bearing B(NMe2)2 functions. They thus obtained tris(pyrazolyl)borate (Tp) tridentate ligands covalently bound to an organic backbone. However, due to several experimental observations that we detail in the following, we had to move towards new types of ligands, in particular new 2,6-di-(pyrazol-1-yl)pyridine ligands (bpp). These latter were selected as they are known to provide (photomagnetic) iron(II) SCO complexes.

Thus, in this chapter, we first discuss our results regarding the tris(pyrazol-1-yl)borate group based polymers, which lead to a re-evaluation of the work of Qin et al. Then, we describe as well the synthesis and the functionalization of new bpp ligands. Finally we present some preliminary results regarding a non covalent approach. Although we did not have time to graft them to the main polymer chain, we believe they give an interesting potential access to the overall synthetic strategies of metallopolymers. Here, we chose to follow this post-polymerization approach to make use of Tpfunctionalized polymer as an organic matrix to host switchable magnetic complexes, either spin crossover complexes, or photomagnetic polymetallic complexes.

The synthetic route, illustrated scheme 1, is based on chemical modifications of a poly(4-trimethysilylstyrene). This later was obtained by atom transfer radical polymerization (ATRP) of 4-trimethylsilylstyrene. ATRP is controlled radical polymerization that enables the Scheme 1. Procedure route for the preparation of Tp-functionalized polymers preparation of well-defined polymers with predictable molecular weight and a narrow molecular weight distribution. 4 Then, the introduction of the tris(pyrazolyl)borate groups involved the same chemical steps as those described for tbu PhTp (4 A ). All the synthesis steps were conducted under inert atmosphere.

First, we prepared the para-trimethyl substituted styrene monomer from the reaction of a styrene Grignard reagent on Me3SiCl in THF under reflux overnight. The modified styrene was isolated and characterized by 1 H NMR, in agreement with literature.

The 4-trimethysilylstyrene was then polymerized in anisole (50%) in inert conditions following a typical protocol for atom transfer radical polymerization. 5 The reaction was initiated with 1-phenylethyl bromide (PEBr) and catalysed by

CuBr/pentamethyldiethylenetriamine (PMDETA). Experimental conditions were defined to target a molecular weight (Mn) of ca.20 x 10 3 . The molecular weight and the polydispersity index (PDI) of the resulting poly(4-trimethylsilylstyrene) were determined by gel permeation analysis (GPC), relative to polystyrene standards : they resulted in Mn = 21 500 g/mol and PDI = 1.1.

Furthermore, poly(4-trimethylsilylstyrene) was treated with 0.30 equivalent of BBr3

(relative to the trimethysilyl moieties) in DCM for 24h. Under these conditions, a statistical Me3Si substitution is expected. A subsequent in-situ treatment of copolymer 1 with small excess of Me3SiNMe2 for three hours led to the modified copolymer 2. The substituent exchange (Br-NMe2) was confirmed by 1 H NMR with the appearance of a new signal at δ = 2.63 ppm (consistent with the literature data).

The large 1 H NMR aromatic signals of the polymer backbone were detected between 6.20 -7.36 ppm while the aliphatic ones were identified between 1.23 -2.09 ppm. Finally, the broad signal due to the trimethylsilyl group sets at δ = 0.25 ppm.

The B(NMe2)2 loading was roughly estimated, similarly to Qin et al work by integration of B(NMe2)2 relative to SiMe3.. A degree of boron functionalization of 27% was found, which corresponds to an almost quantitative loading (see figure 2).

The last step of the polymer modification procedure was then achieved by reacting copolymer 2 with a mixture of pyrazole and sodium pyrazolide (2:1) in THF at reflux overnight under inert atmosphere. After the purification steps, a white precipitate was collected but its high insolubility prevented us from getting a reliable set of characterizations. At this point, it must be noted that Qin et al. characterized copolymer 3 by NMR in deuterated acetone, but they did not provide any spectrum. However they obtained and well characterized the corresponding Ru-metallopolymer, after reaction of copolymer 3 with a ruthenium salt.. We first assumed that our solubilisation issues were attributed to an evolution and/or degradation of our material in presence of water or alcohols. To have reliable evidences of the final copolymer reactivity, investigations were performed on a molecular model compound.

The tbu PhTp ligand (4 A ) was selected for this purpose as it can be considered as a model of the PhTp pattern present in the copolymer 3. In THF-d8, 1 H NMR spectrum (figure 3) contains all the set of aromatic signals corresponding to the tbu PhTp ligand. But in addition, even if relatively small at the beginning, two unexpected signals are also observed at 6.17 and 6.63 ppm at t = 0 min. More interesting, after 24h, this new set of signals (highlighted by stars on the spectrum) associated to the tbu PhTp evolution/degradation in solution appears and increases. We assigned this B-N cleavage to the presence of water and the 4tertbutylphenylboronic acid was obtained as the final product of tbu PhTp evolution. The decomposition product identity was ascertained by 1 H and 13 C NMR in dmso-d6 and by 11 B NMR in CD3CN (see figure 5). Indeed, the 11 B NMR chemical shift value (δ = 28.9 ppm) is typical of boronic acid species. Moreover, 1 H NMR and 13 C NMR were consistent with existing literature of the 4-tertbutylphenylboronic acid.

Conclusion

After several months of optimizations, we managed to obtain a Tp-functionalized polymer by following the procedure described by Y. Qin et al. even if the final expected product could not be characterized due to solubility issues.

Moreover, to understand this Tp-functionalized material behaviour, investigations were conducted on a model compound, tbu PhTp. The NMR solution study carried out in deuterated methanol revealed the high sensitivity of third-generation tris(pyrazolyl) borate moiety to hydrolytic cleavage.

Consequently, we assume that tris(pyrazolyl) borate functions in the modified polymer undergo the same process of decomposition in presence of water, resulting in the formation of boronic acid moieties. As an example and demonstrated in literature, 6,7 boronic acids can form six-membered rings (boroxine) by (thermal) condensation, see scheme 2. We thus believe, that boronic acid groups may act as cross-linking agent leading to the formation of insoluble polymers. Moreover, in basic media (as here due to the presence of sodium pyrazolide), boronate species can be obtained and may participate in the strengthening of the cross-linking phenomenon. At the present stage, we assume that Qin et al. were able to characterize the Ru-based copolymer 3 because they directly reacted a Ru preformed complex onto the organic copolymers. In fact, we suspect that the coordination of a metal to the trispyrazolyl borate moieties prevent their hydrolysis. This is somehow confirmed by our outcomes in chapter 2, as no degradation of our functionalizedtris(pyrazolyl) borates ligands was observed once complexes were formed.

In the light of these outcomes, after several other unsuccessfull attempts of synthesis even in glove box conditions, we decided to explore other synthetic routes for the design of switchable metallo-polymers through the functionalization of 2,6-di(pyrazol-1-yl)pyridine (bpp) ligands.

Scheme 2. Representation of boroxine obtained by condensation of the corresponding boronic acid

III. New functionalized 2,6-di(pyrazol-1-yl)pyridine ligands

Synthetic procedures are well-established for the derivatisation of almost every position of the (1-bpp) framework. This tridentate ligand and its derivatives have proved to be a fruitful source of iron(II) spin crossover complexes with abrupt thermal transition in an accessible temperature range (200 -300 K). 8 Moreover, these SCO complexes also display interesting photomagnetic properties.

In this part, we report purposeful ligand design for the future preparation of spin crossover complexes-based metallopolymers via different polymerization techniques.

With this aim in mind, we developed new routes for the functionalization of 2,6di(pyrazol-1-yl)pyridine ligand (bpp) on the 4-position of the pyridine. The substitution of this position has been well studied and do not significantly affect the metal centre. Thus, we developed synthetic routes for two new 2,6-di(pyrazol-1-yl)pyridine ligand derivatives, ligands (B) and (C) (see scheme 3), using the ligand (A) as starting material. The latter was already reported by M.A. Halcrow et coll 9 and also represents an interesting building block for the design of hybrid switchable polymers. Moreover, the relative homoleptic iron(II) complex [Fe(A)2] displays photo and thermal induced spin transition. 10 The new original ligands (B and C) were designed to pave the way to different metallopolymers synthetic strategies. Indeed, the alkyne function of ligand (B) could enable post-polymerization modification based on click chemistry while the ligand (C) features a methyl-methacrylate pattern suitable for (controlled) radical polymerization.

(B) (C) (A) Scheme 3. Representation of the two new 2,6-di(pyrazol-1-yl)pyridine (bpp derivatives developed in this work (B) and (C) and the precursor (A).

Synthesis of ligand (A)

The synthesis of ligand (A) was achieved in two steps following the procedure described in literature 7 (scheme 4). Bromination of citrazinic acid leads to the 2,6dibromoisonicotinic acid in good yield (86%). The final step consists in the nucleophilic coupling of pyrazoles with the brominated intermediate. The reaction results in the formation of the expected compound with high yield (75%).

The compound identity was confirmed by 1 H NMR and FT-IR spectroscopy and was subsequently used for the syntheses of ligands (B) and (C).

Synthesis of ligand (B) and spectroscopy

The preparation of the ligand (B) results from the formation of an amide bond between ligand (A) and propargylamine. The synthetic route development was thus inspired from peptid coupling procedures, testing two different coupling agents. Carbodiimide-mediated reactions are the most traditional strategy to form peptide bond. They are usually performed in mild conditions, i.e. CH2Cl2 (or DMF) between 4 and 25°C.

A first try was carried out in DCM using the carbonyl diimidazole (CDI) as coupling reagent. After 18 hours of stirring at room temperature, the mixture was analyzed by 1 H NMR and it only revealed the presence of starting materials. Thus, the chosen conditions were not sufficient to trigger the reaction.

The ligand (B) was then generated by using a one-pot method involving an alternative carbodiimide reagent and a N-hydroxy derivative as a coupling additive (to increase the efficiency of the reaction). More precisely, 2,6 di(pyazol-1-yl)isonicotinic acid, propargylamine were dissolved in DMF in presence of 1-Ethyl-3-(3dimethylaminopropyl)carbodiimide hypochloride (EDC.HCl) and 1-hydroxybenzotrialzole (HOBt). After the addition of Et3N, the 106 reaction was run at room temperature over 2 days. After work up and recrystallization in AcOEt, the required compound was obtained pure, in a moderate yield (45%).

Scheme 5. Synthetic route of ligand (B).

1 H and 13 C NMR analyses were obtained as long as FT-IR spectrum. The 1 H NMR spectrum is presented figure 6.

Synthesis of ligand (C) and spectroscopy

The established protocol for ligand (C), illustrated scheme 6 proceeds in two steps. Indeed, to promote the ester bond formation in the final step, the synthesis first requires the preparation of the 2,6di(pyrazol-1-yl)4-acid chloridpyridine as intermediate. Acyl chloride activates the carbonyl group and is more sensitive to nucleophilic attack than the carboxylic acid of ligand (A). The conversion of carboxylic acid into acyl chloride was carried out with SOCl2 under reflux. After 2 hours volatiles were removed.

The compound could not be isolated as pure but was used in the second step without further purification. Then, the second step consists in the addition of 2-hydroxyethylmethacrylate onto the acyl chloride-bpp derivative. The reaction was conduction in CH2Cl2 in presence of Et3N first at 0°C during 2 hours and then at room temperature overnight.

A recrystallization in toluene affords the expected bpp derivative in moderate yield (48%). The 1 H spectrum is presented figure 7.

IV. Conclusion

In this chapter, we essentially targeted covalent routes for the development of sidechains functionalized organic polymers.

First, we re-investigated the tris(pyrazolyl)borate functionalized polymers synthetic procedure reported by Jäkle et al. However, the desired Tp-functionalized polymer could not be isolated due to its instability related to the presence of water. Indeed, the 1 H NMR study of a model compound ( tbu PhTp) evidenced the decomposition of phenyl-substituted Tp.

Therefore, this Tp-functionalized polymer does not seem to be the best choice as for the design of robust hybrid switchable materials. This is the reason why in parallel, we dedicated efforts to the development of the functionalization of bpp skeleton. Those ligands have been intensively studied for the preparation of SCO complexes. A variety of functionalization routes have been consequently reported for the bpp derivatives on pyrazole rings as well as the pyridine group.

In this context, we identified new routes for the preparation of 4-position substituted bbp ligands, ligands (B) and (C). They both allow to design spin crossover complexes-based organic polymers through various approaches.

Current efforts are now devoted to the synthesis of the corresponding homoleptic iron(II) complexes to study first their magnetic properties. In a second time, the homo-and hetero-polymerization of the ligands (C) by radical polymerization and the postpolymerization modification strategy using ligand (B) will be investigated.

111 CHAPTER 4 Photoswitching in electropolymerized ultra-thin films of {Fe 4 Co 4 } cages

Hybrid thin films of poly(thiophene)-{Fe4Co4} cages: motivations and strategy

The work carried out on model {Fe4Co4} cages (chapter 1) has proved the value of these molecules as versatile building blocks for modern materials design. Indeed, we demonstrated the possibility to steer the morphology of their structures and simultaneously their physical properties. Moreover, these {Fe4Co4} cubes display 9 accessible oxidation states and a remarkable stability over a wide electrochemical window. Thus, they are promising electro and photo-switches for electrochromic devices and optical applications 1,2,3 , besides their potential as electronic components in molecular based-devices. 4,5 Our interest lies in implementing these polymetallic complexes into devices to exploit their properties. To achieve that, the first challenge to face consists in the device structuring that for many applications requires the fabrication of thin-films. 6 Thus, our aim was to develop a practical method to incorporate the {Fe4Co4} cages into homogeneous thin-films.

To complete this task, we decided to functionalize these polymetallic complexes by an organic group whose polymerization will result in their structuration into hybrid thin-films. In this case, we chose thiophene as functional group to benefit from its ease to be processed in addition to its physical/electrochemical properties. Indeed, thiophene polymers synthesis/ deposition onto surfaces can be easily achieved and controlled by different wet chemistry approaches. Among them, electropolymerization has been largely applied to prepare highquality films of conjugated polymers. This electrochemical method and more generally wetchemistry techniques are real alternatives to physical deposition techniques, which require stringent conditions.

The transition metal complexes functionalization with (oligo)thiophene and their successive electropolymerization has primarily been carried out on mononuclear complexes. 7 Moreover, the exercise has been rarely performed on polymetallic complexes. An example was reported by K. Mitsumoto et al 8 in 2011. The japanese team described the successful synthesis of a cyanide bridged Fe/Co square with a terthiophene ligand. However, the electropolymerization of their system was unfruitful as the films obtained were found unstable.

In our case the {Fe4Co4} cubes solublility and robustness in solution, as well as their notable electrochemical properties, prompted us to assess the electropolymerization as a method of processing them into thin-films.

Thus, we synthesized {Fe4Co4} building blocks using the 2-TPhTp and 3-TPhTp ligands (Figure 1). As each cube bears four ligands, hyperbranched structures were envisaged in both cases (figure 1). The polymerization of the 2-TPhTp functionalized cubes can give rise to a maximal sequence of two thiophenes, with the metallic core in the main chain. In contrast, the use of 3-TPhTp can lead to a more extended thiophene backbone as the cube will be in

Thiophene electropolymerization: general aspects and mechanism

Since A. Heeger, H. Shirakawa and A. MacDiarmid (Nobel Prize in 2000) demonstrated the metallic behaviour of oxidized (or reduced) polyacetylene in the late 1970s, π-conjugated polymers have been extensively studied. 9 The search for more stable conducting polymers than the cis-polyacetylene led to the considering of polyheterocycles (PHCs) (see scheme 1).

Polythiophene (PT) and its derivatives have become the most emblematic class of

PHCs. The effervescence of both academic and industrials researches on PT polymers and its derivatives is justified by their high stability, their structural versatility and their solution processability (e.g. spin coating and ink-jetting). 10,11 These materials are now used in several industrial applications such as antistatic and magnetic shielding coating, sensors, modified electrodes and molecular electronic devices. 12

Scheme 1. General formula of unsubstituted poly(heterocycles) (where X = N, S and R = H or O-C2H4-O …)

The synthesis of those materials can be achieved either by chemical or electrochemical methods. Electropolymerization is not the most convenient technique for large material production, however this process offers several advantages with respect to the chemical routes:

 Absence of catalyst and purification step.

 Direct grafting of the film onto the electrode surface, which is specific interest for modified electrodes or batteries.

 Access to different types of electrode materials (Pt, ITO, Glassy carbon, flexible supports etc)

 Control of the film thickness. The growth of the film can be tailored and followed in real time by monitoring the current response.

 A first in situ characterization of the generated film at the electrode surface is related to the technique itself. Indeed, information of the material composition is brought by its electrochemical study.

The polymer formation is a sequence of electrochemical and chemical steps. The anodic coupling mechanism broadly accepted is illustrated in figure 2 with thiophene. 13

Epa

The reaction is initiated by the formation of the thiophene radical cation, when the monomer solution is brought to its oxidation potential Epa. The α-α' coupling of two radical cations leads to a dihydro dimer dication followed by the loss of two protons resulting in the aromatic dimer. The retrieval of the aromaticity constitutes the driving force of this chemical step.

At the applied potential, the dimer exists in its oxidized form as its oxidation potential is lower than the monomer. Thus, the following step involves the coupling of the dimer radical cation with a monomer radical. In the ideal setting, the regioregular oligomers are obtained by α-α' coupling after the proton loss. In absence of substituent in β-position, undesired linkages (e.g. α-β') bring structure defects altering the chemical and physical properties (electrical conductivity etc).

Finally, the propagation process produces insoluble oligomers that precipitate onto the electrode, when its solubility limit is reached.

The procedure can be realized under potentiostatic or galvanostatic conditions or by cyclic potential sweeps. The multisweep voltammogram of the 2,2'-bithiophene, showed figure 2.b) provides evidence of the growing process.

Indeed, the first cycle only describes the irreversible oxidation of the 2,2'-bithiophene that occurs at Epa = 1.3 V vs SCE. Then, the redox signature of the polymer appear at lower potential and expand throughout repetitive potential sweepings. The current increase reflects the chain extension and its aromaticity.

Preliminary studies on mononuclear cobalt complexes.

Both the thiophene irreversible oxidation occurs at Epa thio = 1.62 V vs SCE. This value is close to the one observed for thiophene-substituted terpyridine complexes. Indeed, Beley et al 14 showed that the irreversible oxidation of thiophene moiety in a Ru(II) complex occurs at 1.55 V vs SCE.

In Related correlations of the effect of the thiophene substituent on its oxidation potential have been reported in literature. 15,16 Oxidation potential of thiophene monomers were found strongly dependent of the steric and electronic effect of the substituent.

For instance, 3-substitued thiophenes with electron withdrawing groups are generally difficult or impossible to electropolymerize. 6,17,18,19 The substituent position also exerts an influence on the one-electron redox wave reaction corresponding to the couple Co III /Co II . Indeed, the oxidation wave was detected at -0.174 V for [Co(2-TPhTp)2] and -0.118 V for [Co(3-TPhTp)2] (see Table 1).

Table 1. Electrochemical datas of the divalent cobalt complexes.

Surface modification : electropolymerization of [Co(2-TPhTp)2]

The polymer synthesis of [Co(2-TPhTp)2] was attempted with the previous experimental conditions (section III. 2.). Five successive scans between negative potentials and Epa thio result to a progressive attenuation of both thiophene and cobalt peaks (figure 4).

By increasing the number of cycles, we expect that the propagation will result in the polymer coating of the electrode when its solubility limit is reached.

Compound

(A). CV of the electrodeposited [Co(2-TPhTp)2] thin-film in ACN at different scan rates (B and C). Peak current as a function of the scan rate for baseline corrected cyclic voltammograms (D), obtained as for B and C figures. GC was used as working electrode, Pt as counter electrode, and a saturated calomel electrode as reference for all measurements.

of the electrodeposited film (figure 4 B-C) are characteristic of immobilized species. However, they differ from the ideal voltammetric profile of adsorbed species in the case of a fast reversible electron transfer, depicted figure 5. 20 The peak-to-peak separation ΔEp = (Epc -Epa) is relatively small ( < 59 mV ) as expected for a diffusionless electron transfer. However, the waves are broader than in the case of an ideal Nernstian behaviour with a peak half width of ca. 200 mV, which is higher than the expected value (90.6 mV). This deviation from ideality can be attributed to the occurrence of electrostatic interactions between the electroactive centres 21,22 or to the occurrence of different microenvironments that lead to a distribution of standard potentials.

Moreover, as show in the figure 4-D, the peak current is proportional to the scan rate as it is expected for thin films that are fully transformed within the time scale of the CV (on the order of RT/ FV). This contrasts with the ν 1/2 dependence of nernstian waves of diffusing species. In an ideal case, current law is expressed as follow by the equation (1):

𝑖 𝑝 = 𝑛 2 𝐹 2 𝜈𝐴 𝛤 4𝑅𝑇 (1)
here n is the number of electron involved, A the electrode surface area, Γ the surface coverage, ν the scan rate and F the Faraday constant. The surface coverage, 0.67 x 10 -6 mol/m², is then extracted from the peak currents plot. This surface concentration can also be obtained directly by charge integration according to the formula (2) :

𝛤 = 𝑄 𝑛𝐹𝐴 ( 2
)
where n is the number of electron involved, A the electrode surface area, F the Faraday constant and Q is the charge of the integrated peak area (subtracted from the background FWHM Q E /V I /A current). A value of 2.44 x 10 -6 mol/m² is obtained by this method, a value which is slightly higher than the one obtained from equation 1. The difference between the two values arises from the non-ideal behaviour of the voltammograms (figure 4 B-C).

Conclusion

In this work, we demonstrated the ability of the thiophene-functionalized Co(II) complexes to undergo electrochemical oxidation. The anodic potential of the thiophene group in [Co(2-TPhTp)2] lies in a similar electrochemical range than the one described for other thiophene-functionalized mononuclear complexes. The resulting electrodeposited film is proved to be robust as the intensity of the signal is unchanged after several potential cycles.

We also observed that multiple potential sweeps results in current decrease and the electrode coating. To ascertain this current reduction result from a partial passivation of the electrode without any desorption nor degradation, EQCM experiments will be carried out.

Electrochemistry in solution and surface modification

Cyclic voltammetry studies were performed in 0.1 mM solution in acetonitrile at room temperature, with Bu4NPF6 (0.1 M) as a supporting electrolyte. All experiments were carried out using a saturated calomel electrode as reference and a Pt wire as counter electrode.

Different electrode materials (Glassy carbon, Pt or ITO) were used as working electrode.

Electrochemical data obtained for the Cs  {Fe(Tp)Co(2-TPhTp)}PF6 and Cs  {Fe(Tp)Co(3-TPhTp)}PF6 cubes are collected table 2 and the corresponding cyclic voltammograms are presented figure 6 (A-C and D-F respectively).

In both compounds, the one-electron reversible redox waves corresponding to the Fe III /Fe II couples are within the range [0.9; 1.4] V vs SCE (fig. 6A &C). Their redox potential are found identical in both cubes (see table 2). The redox waves ascribed to Co III /Co II couples are less apparent and, when they can be observed they are within a wider electrochemical range, [-1; 0.5] V vs SCE. As discussed in chap. 1, section III. the electron transfer is accompanied by a change in the spin-state. The sluggish electrochemical behaviour results from the significant structural reorganization that is associated to the spin-state change. coupling can lead at maximum to a sequence of two thiophene units between two cubes. In contrast, 3-TPhTp α and α' positions are both free. The electropolymerization can then lead in this case to a longer thiophene chain than with 2-TPhTp ligand.

To study the electropolymerization of the two compounds, five repetitive sweepings were performed from -1 V to their corresponding thiophene anodic potential Epa thio (figure 6 C&F). For both cubes, the shape of voltammograms are similar to those obtained after 2 cycles. Nonetheless, during cycling the intensity of oxidation waves at 0.73 and 0.77 V vs SCE respectively slightly increase until the fourth cycles. In addition, the waves shift towards higher potential (10 mV shift for Cs  {Fe(Tp)Co(2-TPhTp)}PF6 and 5 mV for Cs  {Fe(Tp)Co(3-TPhTp)}PF6) indicating that the positive charge may be less stabilized as cubes polymerized.

The modified electrodes were washed with acetonitrile and cyclic voltammetry measurements were carried out in 0.1M Bu4NPF6 acetonitrile solution. Figures 7 show the variations of the cyclic voltammograms as function of the sweep rate. For both cubes, at low scan rates the redox waves of the iron entities are well-defined and exhibit the bell-shape typical of adsorbed reactants. They are preceeded by the irreversible anodic wave associated to oligo/polythiophene species.

In contrast, the cobalt redox waves are no longer distinguishable in the polymer film.

Multiple reasons might explain the broadening of the redox waves. The electron transfer coupled to a spin-state change may involve a larger structural reorganization due to steric constraints once the cubes are electropolymerized. The number of electropolymerized thiophene groups per cubes may not be homogeneous and can lead to the coexistence of different cobalt environments. Additionally, cobalt entities are more sensitive to electrostatic effects (distance between sites, dielectric constant of the medium) than the iron ones.

Although it is not possible to identify the cobalt ions signals, the four separated oneelectron processes of Fe III /Fe II couples remain a clear indicator of the cubes structural integrity.

C) D) B) A)

The dynamic of the electron transfer exchange within Cs  {Fe(Tp)Co(3-TPhTp)}PF6

and Cs  {Fe(Tp)Co(2-TPhTp)}PF6 films are highlighted by the plot of the (oligo)/polythiophene peak currents versus the scan rate (figure 8). It should be mentioned that due the difficulty to define a proper baseline on cubes electrodeposited CV, current peaks values of the oligomeric thiophene species were collected on CV (figure 7 A-D) that are uncorrected from blank.

The peak current of polythiophene remains proportional to v for both Cs  {Fe(Tp)Co(3-TPhTp)}PF6 and Cs  {Fe(Tp)Co(2-TPhTp)}PF6 thin-films, suggesting that the electrochemical process efficiently propagates through the entire film, even at 2.0 Vs -1 .

An approximate model has been suggested to describe the electron transfer through polymer films, and in an analogous way in dendrimers. 23,24 In the linear regime, all the electroactive centres in the electropolymerized film are involved in the electron transfer., and the centres that lies to far from the electrode surface can contribute through a multistep hopping mechanism :

𝐴 1 + 𝑒 (𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒) ⇌ 𝐵 1 𝐵 1 + 𝐴 2 ⇌ 𝐵 2 + 𝐴 1…
where Ai and Bi are the two possible states associated to one electroactive site.

In figure 8, we observe that at low scan rates the oxidation peak intensity of thiophene moities is similar to those of iron entities. Nevertheless, the intensities of the iron centres waves diminish comparatively to those of thiophene (when the scan rate becomes larger than 0.5 V s -1 ). We also notice that the background capacitive current increases with the potential.

It seems that electron-hopping rate becomes limiting at higher scan rates for the iron oxidation. This might be associated to a limitation of the PF6 -counterions diffusion into the film to compensate the charges created upon oxidation. In this model, the first PF6 - counterions corresponding to oligothiophene oxidation would be inserted with a lower timescale than the voltammetry one. Conversely In contrast, subsequent ions that are inserted upon iron centers oxidation would be more difficult to insert, becoming the limiting phenomenon at larger scan rates. Such mechanism would also explain that the film growth can be slowed after about 5 oxidation cycles as mentioned just Electropolymerization of the thiophene-functionalized polymetallic complexes affords robust films that are stable over a large electrochemical window. Indeed, signals intensity remains stable and no decomposition signs were noticed over multiple sweepings.

Further electrochemical studies have to be performed to obtain more informations of the electron transfer dynamic through the electropolymerized cubes films. Similarly to the thiophene-functionalized Co(II) complexes, EQCM experiments will be carried out to have a clearer understanding of the thin-film growth during the electropolymerization process. The chemical composition of the films and their morphology were studied by XPS and AFM measurements, and they are presented here under.

Electropolymerized thin-films chemical analyses

3.1. X-ray photoelectron spectroscopy (XPS): principle and specificity XPS is an analytical technique (1967) extensively used for the characterization solid surfaces. Its principle lies the photoelectric effect, whereby an incident light shining on a surface material can induce the emission of electrons, called photoelectrons. In XPS, the irradiation of an atom A by photons of a given energy (hν) leads to the formation of an ion in its excited state A +* and the ejection of an electron e from core levels. 25 Figure 9 summarizes the process.

The photoelectrons escape from the material with a kinetic energy (Ekin) which is the experimental quantity measured by the spectrometer. This kinetic energy (Ekin) can be estimated applying the principle of energy conservation:

Ekin = hν -EB -W-Ec
where hν is incident light energy, Ekin is the kinetic energy of the electrons, W is the spectrometer work function and Ec correspond to charge effects (which is equal to zero for conductive substrates).

Elemental analysis

The emitted electrons reaching the analyser without energy loss contribute to the characteristic peaks in the spectrum. While the kinetic energy is measured, spectra are usually displayed on binding energy scale to facilitate chemical identification. As core levels energies vary from element to element, the binding energy of an electron (EB) is a signature of the atom type.

Oxidation state identification

XPS is also a probe of the oxidation state of surface species. The positions of core level energy of an atom strongly depend on the species to which it is bonded. For instance, a decrease of electron density around this atom will yield peaks to higher binding energy.

Therefore, the distinction between oxidized and non-oxidized metal elements may be made via the binding energy position (e.g. Fe 2p, Cr 2p, Ni 2p, Mo 3d, Ti 2p, Si 2p …). 26 In some cases, the discrimination between non-oxidized and oxidized forms is not possible as their binding energies are similar.

This may be achieved using the modified Auger parameter, α' = EK (C'C''C''') + EB (C), a concept proposed by Wagner to probe changes in the local environment of the ionized atom, where EK (C'C''C''') is the kinetic energy of an Auger transition involving C', C'', and C''' core electron levels and EB (C) the binding energy of the corresponding XPS peak.

XPS depth profiling

This technique allows chemical information of material surfaces to be probed with a nanoscale resolution in the direction perpendicular to the surface. Indeed, an electron photoemitted at depth z in a flat and smooth solid in a direction making the angle  with the normal to the surface of the solid has a probability Q to leave the solid in this direction without energy loss:

Q = exp (-z /  cos)
where z / cos is the distance d travelled by the photoelectron in the solid and  is the inelastic mean free path (IMFP), i.e. the average distance between inelastic collisions :

Q d - d d  
The inelastic mean free path depends on the kinetic energy of the photoemitted electron and on the properties of the matrix, namely its density. The contribution to the intensity decreases exponentially with the depth: 63 % of the signal originates from a layer of thickness equal to λ cosθ, 86 % from a layer of thickness 2λ cosθ, 95 % from a layer of thickness 3λ cosθ (see figure 10). As the inelastic mean free path is commonly in the range of about 1.5 to 3.3 nm, XPS probe a surface layer with a thickness up to 3λ cosθ, i.e. about 5-10 nm.

Samples' preparation and XPS method

For XPS measurements, both Cs  {Fe(Tp)Co(2-TPhTp)}PF6 and Cs  {Fe(Tp)Co(3-TPhTp)}PF6 thin-films were prepared by multiplying potential cycles (5 times) on a Pt-coated mica substrate (0.6 cm x 0.6 cm), which was used as a working electrode. Voltammograms are presented figure 11. Experimental conditions were similar to those used for the electrochemistry study section II. The substrates were then washed with pure acetonitrile and dried with nitrogen gas flow. XPS analysis was performed on both cubes films as well as and the Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 pristine powder.

The binding energy scale was set by fixing C 1s component due to carbon only bound to carbon and hydrogen at 284.8 eV. This contribution mainly arises from adventitious organic contaminations 27 and from the possible presence of acetonitrile in the film. The following sequence of spectra was recorded: survey spectrum, C 1s (start), O 1s, N 1s, Fe 2p, Co 2p, Cs 3d, S2p, Cl 2p, C 1s (final). The C 1s peak was recorded in the beginning and at the end of the sequence to check the stability of sample charging and the absence of organic compounds degradation as a function of time. The data treatment was performed with the Casa XPS software (Casa Software Ltd., UK).

The chemical composition of the studied surfaces are given in mole fraction (%) with respect to the sum of all elements expect hydrogen. Finally, the record Co 2p signal was recorded at the beginning and at the end of the sequence measurement did not show any modification of the peak shape.

In contrast, the Fe 2p core spectra display a doublet peak whose signals are separated by 13 eV. The Fe 2p3/2 and Fe 2p1/2 contributions are located at 721 and 708 eV respectively with an intensity ratio of 2:1 as expected. These signals were associated to pure Fe(II) species in agreement with literature. A contribution of Fe(III) compounds was not detected as it would give a peak at binding energy higher than ̴ 710 eV. As Co entities, after electropolymerization Fe signals retain similar FWHM values to ones observed in the bulk.

The Cs 3d spectra include a doublet pair in an adjacent region to the Fe 2p region.

Characteristic 3d contributions appear at 724 eV (3d5/2) and 738 eV (3d3/2) with the expected intensity ratio of 3:2. Due to the proximity of Cs 3d and Fe 2p areas, the spectra involve an overlap of Cs 3d5/2 and Fe 2p1/2 peaks. Consequently, component quantification was made using only peaks which are well separated, i.e. Cs 3d3/2 and Fe 2p3/2. 3.

These peaks were decomposed with constraints insuring the consistency required to make quantitative comparison. In all spectra treatment, a distance constraint of 1.1 eV was applied between S 2p3/2 and S 2p1/2 components.

In the powder, the peak-fitting analysis of S 2p spectrum reveals a doublet whose peaks are at 164.01 eV for S 2p3/2 and 165.11 eV for S 2p1/2. In the electropolymerized thinfilm the decomposition of S 2p spectrum reveals the same doublet peaks. Indeed, S 2p3/2 and S 2p1/2 peaks were observed at 163.96 eV and 165.08 eV respectively. Peaks full width half maximum is also identical in both samples and is ca. 1.20 Finally, Cl 2p and F 1s spectra were recorded (see appendices) but Chlorine spectrum did not exhibit any signals. F 1s core level spectrum show a singulet at 686 eV which have been attributed to PF6 -anion (in accordance with literature values). This observation supports the anion exchange assumed earlier.

XPS quantification spectra

Molar concentration ratios involving the main constitutive elements of the cube are reported table 4. The other elements were detected and their molar concentrations are listed in appendix. The intensity ratio of Co III /Fe II is 1.12 in the electropolymerized thin-film that is close to the value obtained for the powder (1.07) and reasonably close to the theoretical one.

Similarly, a Cs + /Fe II ratio of 0.37 was observed for the powder and 0.35 in the electropolymerized thin-film, which is consistent with the expected value. However, none of these constitutional elements of the cube were detected. Indeed, XPS spectra did not display any peaks and quantitative results revealed concentration of 0% for these elements. The elements detected on this sample were carbon (64.39%) and oxygen (23.57%), originating from adventitious contamination.

AFM characterization of Cs  {Fe(Tp)Co(2-TPhTp)}PF6 thin-films of controlled thickness

We demonstrated the feasibility of depositing Cs  {Fe(Tp)Co(2-TPhTp)}PF6 onto a platinum substrate. Then, we aimed at evaluating the electropolymerization as a tool to prepare hybrid thin-films of controlled thickness and studying their growth on the substrate. The morphology of the electropolymerized films was probed by AFM in the dried state using peak force tapping mode. In this AFM method, the applied force is lower than in the classical tapping mode, thus allowing probing the interface with minimal disturbance.

AFM images recorded after different number of cyclic potential sweeps are presented figure 15. After 1 cycle, the AFM image (fig. 15a) revealed a relatively smooth surface (Rrms = 1.08 nm, computed from 2 µm x 2µm image). After 5 cycles, the roughness increased to a relatively limited degree (Rrms = 3.12 nm, Figure 15b) and the film remained homogenous with the presence of fairly regular particles. After 10 and 15 cycles, the roughness is of the same order of magnitude as 5 cycles samples (Rrms = 2.15nm and Rrms = 2.17nm, Figure 15c and 15d respectively). The average height of the particles was around 4nm for 1 cycle sample and about 8nm for the rest. However, the real particle height could not be evaluated reliably because the tip was too large to penetrate into the interstices between particles.

Interestingly, the increase of the number of cycles up to 10 and 15 did not influence the topography of the films. Films homogeneity was evaluated scanning different areas on each samples. No irregularities (i.e. grains or aggregates) were detected. These findings suggest that the film growth occurs in a homogenous way along the surface plan, independently on the number of cycles.

Thickness measurements : AFM scratching tests

The thickness of electropolymerized films, prepared with a different number of potential sweeps, was determined by AFM in the dried state using contact mode. For this purpose, each sample was scratched over a scan size applying 5V. To make reliable comparison between samples, we ensure in each case the material was completely removed and Pt substrate was reached. The topography of the scratched area was then obtained over a larger scan (15μmx 15μm or 10μmx 10μm).

The AFM image of the scratching test carried out on Cs  {Fe(Tp)Co(2-TPhTp)}PF6 electropolymerized thin-film prepared after 10 potential cycles is presented figure 16-A. The average thickness value is deducted from the height difference between points located in the scratched zone and un-scratched area (cross-section figure XA).

An average thickness of 14.8 ± 0.8 nm was measured for the sample prepared with 10 cycles. The thicknesses of other samples were measured similarly. Values ranging from 2.8 nm to 18.5 nm were noted and plotted as a function of the number of cycles (figure 16B).

Conclusion

The electropolymerization was used as a method to elaborate a nanostructured homogenous thin-films of Cs  {Fe(Tp)Co(2-TPhTp)}PF6 cube and to tune their thicknesses with the number of cyclic potential sweeps. However, as for the model compoundCs  complex was obtained as its counterpart Bu4N[Fe III ( tbu PhTp)(CN)3] in two steps but in lower yield (20%). In solution, [Fe III (2-TPhTp)(CN)3] compound was found to easily evolve to a brown precipitate. Moreover, as we suspected thiophene moiety to be sensitive to the employed oxidative conditions (H2O2). Thus, the minimum needed of hydrogen peroxide was used.

The Tl  {Fe(2-TPhTp)Co(Tp)} cube was then obtained following the standard protocol for paramagnetic cubes synthesis (additional details are afforded in the experimental part).

A set of characterization (FT-IR, HRMS) is joined in experimental part/appendices.

Electropolymerization and surface characterizations

Electropolymerization

The Tl  {Fe(2-PhTp)Co(Tp)} thin-films were prepared by multiplying potential cycles (5 times only) on a Pt-coated mica substrate (0.6 cm x 0.6 cm), which was used as a working electrode. The electropolymerization experimental conditions were similar to those used for the preparation of diamagnetic Cs  {Fe(Tp)Co(2-TPhTp)}PF6 thin-films (i.e. 0.1mM acetonitrile solution + 0.1M TBAPF6). The substrates were then washed with pure acetonitrile and dried with nitrogen gas flow. A cyclic voltammogram of Tl  {Fe(2-PhTp)Co(Tp)} electropolymerization and the corresponding electrochemical datas are presented figure 17 and table 5.

The electrochemical profile of the Tl  {Fe(2-PhTp)Co(Tp)} has several distinctive features. Indeed, as its paramagnetic {Fe4Co4} counterparts, the first redox process observed (here denoted by a pink circle at 0.48 V vs SCE) upon the first oxidation sweep is associated to the oxidation of the cobalt(II) to cobalt(III). Then, the four successive oxidation and reduction waves of iron entities appear within 0.90 and 1.60 V vs SCE (see table 5). It is somewhat difficult to fully dissociate the redox process of the fourth iron from the oxidation wave of thiophene that appears about 1.52 V vs SCE.

In Tl  {Fe(2-TPhTp)Co(Tp)}, the electrochemical oxidation of thiophene occurs at lower potential than in the diamagnetic cube Cs  {Fe(Tp)Co(2-TPhTp)}PF6. This may result from a greater stabilization of the radical cation when thiophene is linked on the iron entities.

The oxidation potential of the bithiophene is observed at 0.81 V vs SCE and it shifts by 15 mV towards higher potential during potential sweeping. This was also observed in the case of Cs  {Fe(Tp)Co(2-TPhTp)}PF6 although the shift was slightly less pronounced (8mV shift on Pt substrate). This could arise from an insufficient charge compensation during the electropolymerization processs mentioned above.

XPS analyses

Successively to its preparation, the Tl  {Fe(2-TPhTp)Co(Tp)} electropolymerized thinfilms was analyzed by XPS. An identical procedure was applied for the calibration and data treatments (peak-fitting, quantification etc) as the one described previously for Cs  The XPS spectrum of the inserted cation was also recorded. Indeed, Tl 4f core level spectrum displays two peaks at 119.9 eV and 124.4 eV corresponding to 4f7/2 and 4f5/2 respectively. The distance between these two signals is thus equal to 4.5 which is good agreement with XPS data tables. (Indeed, values observed here are in line with those reported for thallium salts) 28 .

S 2p

Tl +

The S 2p core level spectra core level spectrum of the Tl  {Fe(2-TPhTp)Co(Tp)} electropolymerized thin-film, is composed of two components, at 164.24 and 165.31 eV, that are assigned to the sulfur of the 2-TPhTp ligand (located on the iron ions). Finally, P 2p and F 1s core level spectra were recorded and exhibit signals attributed to PF6 -, which indicates the presence of supporting electrolyte in the electropolymerized thin-films. of the polymer. The thickness deducted from the section profile is about 3.6 nm. A similar result was obtained using a second cantilever (cantilever 2, k= 0.55 N/m) despite a voltage of 10 V (figure 20b). In this case, the height difference read on the cross-section corresponds to 2.30 nm.

Finally, the complete removal of the film was achieved using cantilever 3 with a constant spring of 6.9 N/m and applying 5 V. The AFM image of the scratched area is presented

Photomagnetic properties of Tl  {Fe(2-TPhTp)Co(Tp)} electropolymerized thin-films

The photomagnetic properties of Tl  {Fe(2-TPhTp)Co(Tp)} thin-films (5 cycles as for previous XPS and AFM studies) were investigated through synchrotron techniques at the X-Treme beamline at SLS (Paul Scherrer Institute Paul Scherer Institute (PSI).

X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) are surface sensitive techniques that allow probing electronic and magnetic properties (at low temperatures and under high magnetic fields). They are both element specific techniques. XAS can provide information about the local structure of the complex, the oxidation state of the metal ion etc while XMCD can provide local magnetic information, in particular the spin and orbital magnetic contribution on both metal ions. The XMCD is thus a highly valuable technique to probe at the local scale the redox state change and the spin state change occurring in a FeCo photomagnetic pair under light irradiation.

XAS and XMCD measurement were performed using a laser of 635 nm wavelength and moderate of intensity ~1 mW.mm² to photo-excite the molecules over 3 hours. Figure 21 shows XAS spectra at Fe and Co the L3 and L2 edges of the cube thin-film at 2K before and after laser irradiation.

Before irradiation, Co spectrum shows peaks at 779.

IV. Conclusion

One of the major aim of this project was to establish reliable and original synthetic routes to process switchable magnetic molecules , our final goal being to further integrate them into prototype of molecular devices. As many devices fabrication resort to thin-film manufacturing, we targeted a method to incorporate molecular switches into thin-films.

Due to the remarkable properties, we first selected {Fe4Co4} polymetallic complexes for the elaboration of bistable ultra-thin films. Thiophene has proved to be a suitable derivative to develop processable multifunctional building blocks. Here, we assessed the electropolymerization as a method to deposit thiophene-functionalized {Fe4Co4} cages directly onto surfaces as an hybrid thin-film. Electropolymerization is wet chemistry approach largely used for conductive polymers. An advantage relative to other deposition techniques as spin coating and ink jetting, is absence of the need for anchoring layer (between polymers and substrate).

A first work was carried out with Cs  {Fe(Tp)Co(2-TPhTp)}PF6 and Cs  {Fe(Tp)Co(3-TPhTp)}PF6 diamagnetic cubes to devise and to optimize the preparation of thin-films. These {Fe4Co4} cages successfully underwent electropolymerization to provide stable thin-films. In contrast, the electropolymerization of the only example of a polymetallic complex functionalized by an oligothiophene (Mitsumoto et al.) was unfruitful.

Electro-generated thin-films were successfully deposited on different types of substrates, as glassy carbon and Pt/mica and ITO (but this latter was not discussed in this chapter). Their characterization by XPS proved that the cube integrity was maintained after electropolymerization as well as the metal oxidation state.

The electropolymerization process also allowed us to control the thickness of the film by simply varying the number of potential sweepings. AFM experiments indicated that the films growth was homogeneous, independently of the number of cycles. Thus, we could prepare homogeneous thin-film, with a controlled thickness (in the range of nanometer) directly on electrodes.

As the Cs

Motivations and strategies

The functionalization of surfaces with magnetic molecules represents an active topic whose aim is to create molecular-based nanodevices for molecular spintronics. Most of the research in molecular magnetism has focused on single-molecule magnets adsorbed on surfaces. However since several years, there is a growing interest for the study of other switchable magnetic systems such as spin-crossover complexes In contrast, the investigation of (photomagnetic) charge transfer polymetallic complexes on surfaces has not been reported.

Despite the wide range of applications that could arise from the use of magnetic molecules in devices, there are still challenges to overcome. For example, integrity loss of coordination metal complexes, as well as their properties, could arise from their transfer from solution to surfaces. Indeed, many chemical or structural electronic changes can occur once molecules are adsorbed/anchored. Thus, designing nanoelectronic devices requires first to investigate the stability of the molecules and the integrity of the desired properties once adsorbed on surfaces.

In this chapter, we proposed to explore the functionalization of surfaces with the A  {Fe4Co4} cages studied in the ERMMES group, through soft-chemistry approaches. Our main objective consisted in preparing self-assembled monolayers of Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 and Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 on gold substrate. The use of these versatile polymetallic complexes was motivated by their robustness in solution and their remarkable properties as demonstrated in the previous chapters. To our knowledge the characterization of self-assembled monolayers (SAMs) of photomagnetic polymetallic complexes has never been reported in the literature. The thiophene functionalized A  {Fe4Co4} cages were selected to test thiophene ability to anchor such complexes on Au surfaces. (ii) Any evolution of oxidation state (that might arise from solution experiments) would be more easily noticeable in pure {Fe II 4Co III 4} cubes than in a {Fe II 4Co III 3Co II } mixed-valence one.

Secondly, the stability of cubes' layers was evaluated through desorption tests by means of X-rays photoelectron spectroscopy (XPS).

Foreword on thiophene self-assembled monolayers (SAMs)

Self-assembled monolayers are uniform assemblies of molecules that spontaneously adsorb and organize at interfaces. Organic self-assembled monolayers have been extensively studied as they provide a practical way to functionalize solid surfaces (metals, oxides, semiconductors) and to tailor the interfacial properties. SAMs can be prepared by ultra-high vacuum techniques or by immersion in a dilute solution. 1 Among the many organic SAMs, the ones derived from organosulfur compounds are the most reported in literature. Attention was mainly focused on alkane thiols due to their high structural order and their stability. On the opposite, the modification of surface by thiophene and its derivatives has been much less widespread. In fact, early studies and theoretical calculations 2,3 suggest that thiophene sulphur atom interact only weakly with gold.

However, few studies have also provided experimental evidences on the chemisorption of thiophene (and derivatives) on gold substrate by using XPS. 4,5,6 In practice, SAMs thiophene compounds were mostly prepared by dipping. Ethanol was widely used as it solvates a variety of thiophene compounds with varying degrees of polar character, it is inexpensive and has a low toxicity. However, the use of ethanol could not be applied to the A  {Fe4Co4} cages as these molecules decompose in protic solvents.

II. Testing conditions for thiophene functionalized {Fe 4 Co 4 } cages

SAMs formation from solution.

PM-IRRAS technique as a screening method

PM-IRRAS is a non-destructive physico-chemical analysis, which is convenient for the characterization of floating monolayers or thin films deposited on metal substrates. As any IR technique, PM-IRRAS provides information on the nature of chemical bonds. The experiment set up is depicted figure 1.

The metallic surface is irradiated with polarized light composed of a parallel polarized radiation p and a perpendicular polarized radiation s with respect to the incidence plan.

Following surface dipole selection rule, the dipole transition moment of the adsorbed molecules must have a component orientated along the surface normal to absorb the IR incident irradiation. Consequently, only the p-component will interact with the sample. 7,8 The technique can thus provide information on the molecules orientation adsorbed on surfaces. Although the technique is particularly useful for IR studies at the nanoscale, it is limited by the use of reflective substrates. absorption frequencies constitute a key marker for {Fe4Co4} cubes identification by IR spectroscopy. For these reasons, we have mainly worked with this marker for our study of SAMs formation using PM-IRRAS.

Effect of concentration

The figure 3 shows the PM-IRRAS spectrum of gold surface after 3 hours immersion in a solution of Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 in CHCl3 at 5.10 -4 M. The PM-IRRAS spectrum of the gold substrate, showed figure 3, presents characteristic bands of Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 compound, similarly to its FT-IR spectrum of (see appendix). This thus indicated that the adsorption of the diamagnetic cube on the substrate occurred.

Three absorption bands regions can be distinguished: (i) C-H stretch at 2928 cm -1 , (ii) cyanide characteristic vibrational mode at 2108 cm -1 and (iii) absorption bands (C=N,C=C stretching bands, etc) in the finger print region attributed to iron and cobalt capping ligands.

The {B-H} stretching originating from {FeTp(CN)3} units, expected at 2418 cm -1 is not visible probably to the low intensity of the band.

The cyanide band displays an identical pattern than the one presented in FT-IR spectrum (figure 2) suggesting that the cube integrity is maintained.

Influence of immersion time

We first tested the influence of immersion time on surface coverage. As the cyanide band shape remains unaffected/unchanged under different experimental conditions. Thus, the band was used for semi-quantification. Gold substrates coated on glass were dipped in a solution Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 in CHCl3 with different immersion time, i.e. {1h, 3h, 6h, 18h or 24h}.

The PM-IRRAS spectra of all samples show a ν(C-N) band located at 2108 cm -1 . Indeed, figure 4-B presents PM-IRRAS spectra of samples dipped 1h and 24h, centred on the cyanide band which are identical proving the shape doesn't evolve over dipping time. Cyanide band area of samples spectra was plotted as a function of their immersion time in solution, figure 4-A. The relationship between the peak area and the immersion time was found to be nearly constant. Even if deposition was already achieved after 1h in solution, the following experiments were mainly carried out choosing 3 hours of immersion time. As if the adsorption is fast, we assumed that organization of the adsorbates may require longer immersion time.

III. Testing the stability thiophene functionalized {Fe 4 Co 4 } cages

SAMs.

In the following investigations, X-rays photoelectron spectroscopy was used to evaluate (i)

the adsorption stability of the Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 and Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 layers on gold, subjected to increasing washing time. (ii) The binding nature of the two thiophene-functionalized adsorbates. To support our understanding on the cubes adsorption states, a comparative study was performed using the 3-hexylthiophene as model compound.

Sample preparation

The Au(111) surfaces used were of two kinds. Substrates of Au(111) (1.5cm x 1.5cm), deposited on glass coated with a Cr adhesion layer, were flame annealed and cleaned in ethanol in a sonicating bath for 10 min. Substrates were dried under a nitrogen flow and further treated by UV/O3 for 20 min. Substrates were then used immediately for adsorption tests. SAMs indicates the appearance of unbound S atoms peaks between 163 eV and 165 eV. Peaks at higher binding energy are associated to oxidized sulfur species. 9 These two S 2p dominant peaks were observed for both cubes independantly of washing time. Indeed, four hours of pure dichloromethane did not permit to remove the thiophene adsorbates.

Binding energy (eV) The measure of Sox/Stot ratio was found to increase with time for Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 as evidenced figure 9. However, figure 10 reveals that the amount of oxidized sulfur evolves differently for 3-hexylthiophene.

Indeed even after 24 hours in neat dichloromethane, the amount of oxidized sulfur seems relatively stable. We assume that the increase of oxidized sulfur amount for Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 arise from the oxidiation of the free thiophene of the cage. Indeed, it is worth noting that the thiophene-functionalized cages are expected to interact with gold substrate by two thiophene group (maximum). Thus, the two remaining thiophene groups may be prone to oxidation. In contrast, 3-hexylthiophene molecules are less sensitive to oxidation as they are well-adsorbed on gold surfaces. The deposition of the thiophene functionalized cubes on surfaces was also investigated using electrospray ionization, by our collaborators from the laboratoire de réactivité de surface (LRS), C. Méthiviers and J. Landoulsi. Indeed, they demonstrated the possibility to use electrospray ionization to deposit peptides onto different types of substrates. 13,14 The deposition is performed under ultra-high vacuum conditions (pressure 10 -10 Torr). However, in contrast to sublimation technique, electrospray ionization process allows the deposition of molecules in state which is closer to that existing in solution. indication that the cube framework is maintained after deposition.

Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 stoichiometry after deposition on gold substrates

To gain more refined information on the layer composition, elemental quantification of cage markers (i.e. Fe, Co and Cs) were evaluated on the basis of XPS measurements.

Quantitative analyses were performed on sets of Cs {Fe(Tp)Co(3-TPhTp)}ClO4 samples 3 and will be compared depending on the type substrate used (Au/mica or Au/glass), independently of the washing time applied.

Figure 12 presents the correlations between Fe concentrations versus the Co and Cs concentration for series of samples on Au/mica and Au/glass. Summarized datas are given table 2 and 3.

3 Data were collected on series of samples prepared by immersion in DCM solution during 3h using either Au/mica or Au/glass substrates. Taking together, the correlations between the molar concentration of Fe and Cs and table 3 reveals a relationship fairly close to the expected value 1:0.25 for Au/glass substrate.

For samples on Au/mica, the amount of Cs is slightly above the expected value (Cs/Fe = 0.472± 0.066). Correlations between Fe and Co show slight deviations from theoretical values (1:1).

Indeed, the Co/Fe ratio mean value was 1.71± 0.252 for Au/mica substrates and 0.691±0.105

for Au/glass. Even if this finding may raise some questions about the stoichiometry of the cages on surfaces, it cannot testify on its own of the occurrence of cage decomposition.

We do not exclude that some molecules degradation can occur during the deposition onto surfaces, however PM-IRRAS and XPS spectra did not point irregularities for Au/mica substrates. Some iron oxidation was observed on Au/glass that might originate from a difference of surface quality relative to Au/mica substrates. These questions clearly deserve further investigations.

IV. Conclusion

In this study, we aimed at investigating the ability of the thiophene-functionalized {Fe4Co4} cages to anchor gold substrates. For this purpose, we used Cs  {Fe(Tp)Co This PhD project was developed at the frontiers of ERMMES and Polymères research themes, in the framework of a first collaboration. Our major interest was to establish reliable synthetic routes for the design of hybrid (nano)materials based on magnetic molecular switches.

We mainly dedicated efforts on the integration of magnetic molecular switches into two main classes of materials: (i) surfaces and (ii) organic polymers. The switchable molecules of interest were primarily extracted from the library of thermo and photo-responsive molecules studied in the ERMMES group. Our attention was particularly focused on a family of photomagnetic cyanide-bridged {Fe4Co4} mixed-valence systems described previously in the group. These {Fe4Co4} cubes were first functionalized at their periphery, on the tris(pyrazolyl)borate capping ligand used for their synthesis.

We prepared a series of functionalized tris(pyrazol-1-yl)borate 'scorptionates' ligands, including two of them with a thiophene moiety. These ligands were first used to prepare different homoleptic iron(II) complexes which demonstrated an incomplete spin-transition at high temperature. However among them, the [Fe(C6F5Tp)2] revealed an outstanding magnetic properties change after thermal treatment.

The functionalized Tp ligands were then investigated on the cyanide-bridged {Fe4Co4} cages, using the tbu PhTp compound (4 A ) to build a series of model cages. The position of the ligand was found to play a crucial role on the cage properties. On cobalt, {Fe II 4Co III 4} diamagnetic cages appeared as the most stable compounds. Indeed, {Fe II 4Co III 3Co II } paramagnetic cages showed sensitivity to oxidation in solution and led to their diamagnetic equivalent. More importantly, positioning the ligand on cobalt led to extinction of the cages (photo)magnetic properties.

With the assumption that the functionalization on the "{(L)Fe(CN)3} vertices" would have less effect on the electronic properties of the {Fe4Co4} cubes, we prepared the symmetric system where the functionalized TP ligand was located on the iron ions. The approach was proven efficient as the photomagnetic properties of the cages were recovered. Finally, our study has also allowed confirming the critical role of the inserted alkali ion on the electronic properties of this cage the cubes properties. Indeed, the functionalized cages were first designed with caesium for stability reasons but these latter displayed low/moderate light-induced ETCST. The use of thallium permitted to refine the cages properties and to obtain a Tl + derivative showing a very efficient ETCST effect and an improved relaxation temperature.

This rational design of {Fe4Co4} cages with optimized properties enabled us to target the best candidates using next, thiophene derivatives for the design of responsive molecular based-materials.

The preparation of organic polymers functionalized with magnetic molecular switches was investigated by chemical or electrochemical routes. The first one explored aimed at synthesizing tris(pyrazolyl)borate functionalized polymers, through a post-polymerization modification approach. However, the instability of the final material led us to consider a different strategy. For this purpose, we then decided to start with the functionalization of coordinating ligands, known to induce iron(II) SCO complexes, to further (co)polymerize them.

To this end, new functionalized R-bpp derivatives were prepared and their insertion into organic skeleton will be tested via different approaches (post-polymerization modification strategy, radical (co)polymerization).

Electrochemical methods were mainly studied using the {Fe4Co4} clusters owing to their robustness in solution and other remarkable properties (solubility, electronic flexibility).

The {Fe4Co4} cages were functionalized with thiophene derivatives (2-TPhTp, 3-TPhTp) and successfully underwent electropolymerization process, maintaining the cube integrity. With 2-TPhTp derivatives, we demonstrated the possibility to deposit the {Fe4Co4} cages as homogenous ultra-thin films, directly onto Pt/mica (but also ITO) substrates. Their thickness could have been controlled by simple experimental conditions change without altering the homogeneity of the films.

The photo-switching properties of Tl + derivative thin-films were probed by synchrotron techniques (collab. J. Dreiser, N. Daffé, SLS-PSI, Switzerland). XAS and XMCD preliminary results indicated the photo-switching properties of the cages under light irradiation. Thus, we demonstrated that the photo-switching properties of the {Fe4Co4} cages are preserved in the thin-films. This responsive hybrid nanomaterial is, to our knowledge, unprecedented. It is worth noticing that an example of FeCo square complex functionalized by thiophene groups was already reported by K. Mitsumoto in 2011. Nonetheless, in this case the electropolymerization of the square was unfruitful and decomposition occurred. We believe that the remarkable stability of the {Fe4Co4} cubes was a key parameter for their electropolymerization.

Moreover, we demonstrated the possibility to efficiently process {Fe4Co4} cages onto different type of substrates (Pt, ITO etc) by electrochemical techniques. This straightforward approach also represents a concrete step towards {Fe4Co4} cubes-based device manufacturing. More interestingly, the potential of these new hybrid materials is multiple.

Indeed, they constitute modified electrodes that could be exploited for their photo-switches as well as redox-switches properties etc. Finally, the incorporation of {Fe4Co4} cages in conductive polymers clearly offers considerable perspectives for the development of multifunctional materials. Thus, it would be interesting to measure the electrical properties of these hybrid nanomaterials and test the influence of a light excitation.

Furthermore, we investigated the self-assembly of thiophene-functionalized {Fe4Co4} cages onto gold surfaces. ). The chemical composition, layer morphology of the functionalized surfaces were obtained through PM-IRRAS, XPS, AFM. We showed that the {Fe4Co4} cages with thiophene derivatives form homogenous and continuous layer. A comparative study with a model thiophene derivative indicated that the cages layers were anchored to the surface by physorption. However, the polymetallic complexes obtained relatively stable as demonstrated through desorption tests. In addition, the {Fe4Co4} cages adsorption on Au(111) was also performed by UHV electrospray ionization (collab. C. Méthiviers, J. Landoulsi, LRS) after which in-situ XPS and PM-IRRAS studied were carried out. Characterizations obtained from this original deposition technique supported those obtained from solution deposition approach.

This exploration of self-assembled monolayers of {Fe4Co4} cages is still in the making and need further investigations to fully understand the fate of molecules during/after deposition. This research concerns is of particular interest in the field of surfaces functionalization with magnetic molecules an active topic in spintronics and molecular-based nanodevices. To our knowledge, there is no example of light-induced ETCST self-assembled monolayers reported. Thus this study of those polymetallic complexes on surfaces can be of fundamental and technological interests for spintronic and molecular electronic.

Technical details

FT-IR spectra were collected in the 200-4000 cm -1 range. Measurements were carried out on a Vertex 70 Bruker instrument using the attenuated total reflection (ATR) technique on solid samples (with a 4 cm -1 resolution). The intensity of the absorption band can be indicated as vw (very weak), w (weak), m (medium), s (strong), vs (very strong), and br (broad).

NMR spectra were recorded on Bruker AC 300 and 400 MHz spectrometers using the signal of tetramethylsilane (TMS) for 1 H or carbon of deuterated solvents as internal standarts for 13 was by dual-space direct methods with SHELXT S3 with full-matrix least-squares refinement using SHELXL-2014/7. S4 All non-hydrogen atoms were refined anisotropically. H atoms were placed in calculated positions using a riding model.

[S2] O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H. Puschmann, J. Appl.

Cryst., 2009, 42, 339-341.

[S3] G. M. Sheldrick, Acta Cryst., 2015, A71, 3-8.

[S4] G. M. Sheldrick, Acta Cryst., 2015, C71, 3-8.

Photomagnetic measurements were performed on ca. 0.5 mg of sample. The fresh sample was introduced in the SQUID at 110 K under a helium flow and frozen before purging in order to avoid solvent loss. The irradiation was performed in the SQUID at 20 K ( or 15 K for solution measurements) in order to limit the effect of the heating due to the sample irradiation (H = 1 T). The relaxation of the photo-induced metastable state (χMT vs T curve) was measured at 0.4 K min -1 .

UV-Visible absorption spectra were recorded on a JASCO V670 spectrometer at 273 K in dichloromethane solutions (0.1 mM) using a 100 mm quarts cuvette.

Cyclic voltammetry measurements

GPC (SEC).

Size exclusion chromatography (SEC) was performed in tetrahydrofuran (THF).

Samples were analyzed at a concentration of 3 mg mL -1 after filtration through a 0.45 μm poresize membrane. Toluene was used as a flow rate marker. The flow rate was 1 mL min -1 and the separation was performed at 25 °C with two columns (PLgel Mixed-C, 7.5 mm × 300 mm; bead diameter: 5 μm; separation limits: 400 to 10 6 g mol -1 ). A differential refractive index detector (Viscotek VE 3580 RI Detector) was used, and molar masses (Mn, the number-average molar mass, Mw, the weight-average molar mass) and the dispersity (Ð = Mw/Mn) were derived from a calibration curve based on polystyrene (PS) from Polymer Standards Service. The OmniSEC 4.6 software was used for data acquisition and data analysis.

  Je tiens également à remercier les cristallographes Benoit Baptiste (IMPMC) et Lise-Marie Chamoreau (DRX, IPCM) pour toute leur aide et entrain engagés pour nous aider à résoudre le mystère du [Fe(C6F5Tp)2]. Merci également à Geoffray Gontard (DRX, IPCM) ainsi qu'Aurélie Bernard et Claire Troufflard de la plateforme RMN. Je souhaite aussi adresser mes sincères remerciements à Dalila Segouane (CSOB, IPCM) et Omar Khaled (MACO, IPCM) pour leur disponibilité et leur aide sur les mesures HRMS.

CHAPTER 1

 1 INTRODUCTION ............................................................... Context and state of art ..............................................

Figure 1 .

 1 Figure 1. Schematic representations of hybrid (nano)materials based on magnetic molecular switches: a) organic polymers side-chains functionalized by spin crossover complexesb) and thin-films of polymetallic complexes.

Figure 2 .

 2 Figure 2. Plan of the PhD thesis.

  in 1931. This observation was then extended to other octahedral transition metal complexes with d n (4 ≤ n ≤7) electronic configuration, mainly in the first row with d 4 Cr(II), d 5 Fe(III), d 6 Fe(II) and d 7 Co(II) ions. Spin transition was described in solid, liquid state as well as soft matter. Octahedral 3d complexes, with d 4 to d 7 electronic configuration, may exist either in high-spin state (HS) or low-spin state (LS). The example of an octahedral Fe(II) d 6 is given figure 2. Depending on the ligand field strength, Δ, relative to the mean pairing energy, Π, several cases arise.

Figure 2 .

 2 Figure 2. Electronic configuration for a d 6 iron(II) complex in high spin and low spin states.

Figure 3 .

 3 Figure 3. Schematic representation of the main possible profiles of a spin transition processes.

Figure 4 .

 4 Figure 4. Illustration of the LD-LISC effect extracted from reference[9]

Figure 5 .

 5 Figure 5. LIESST and reverse-LIESST mechanisms in a d6 spincrossover system. (scheme extracted from ref. 13).

Figure 6 .

 6 Figure 6. Schemes of a Fe/Co PBA 3D network (left) and representation of the ETCST phenomenon in a cyanide-bridged Fe/Co pair (right).

  The example 7c corresponds to the molecular cube of formula {[Co(Tpe)]4[( pz Tp)Fe(CN)3]4(ClO4)4 (where Tpe = 2,2,2,-tris(pyrazolyl)-ethanol) reported by

  et al. and J.R. Jimenez et al. in our group in 2016 and 2017 respectively. 20 The mixed-valence {Fe4Co4} cage of formula A {[Fe II (Tp)(CN)3]4[Co III ( pz Tp)]3[Co II ( pz Tp)]} contains an inserted alkali ion, A. They thus offer the possibility to probe the influence of A on the ETCST and to confront these results to those obtained in PBAs. These cubes show an incomplete gradual thermallyinduced ETCST at high temperature (T> 300 K). As the predecessor example of Clérac et al., an

Figure 7 .

 7 Figure 7. Selected examples of low-dimensional models of PBAs (from ref.17-20)

Figure 8 .

 8 Figure 8. STM image of TbPc2 on Au(111). inset: STM image of a single isolated TbPc2 molecule on Au(111). STM height profiles; left: profile across the film of 1st-and 2nd-layer TbPc2 molecules along the dashed line in (a); right: profile across a single TbPc2 molecule along the dashed line in the inset of (a). Extracted from reference[41].

  Figure 9. Schematic representation of [Fe4] cluster anchored on a gold surface through its thiolateterminated aliphatic chains.Extracted from reference[43].

  revealed the occurrence of an original magnetic phase (containing 1/3 of HS complexes ordered in a superlattice structure) inside an ordered monolayer. Interestingly, they also managed to probe the dynamics of a light-induced SCO of [Fe II ((3,5-(CH3)2Pz)3BH)2] sub-monolayers on Au(111) substrates (by STM and STS measurements).

  who dispersed different spintransition polymeric compounds in polypyrrole.

Figure 12

 12 Figure 12 presents a TEM image of composite made using [Fe(trz)(Htrz)2][BF4]. They observed a

Figure 11 .

 11 Figure 11. Illustration of SCO-based composites presented as thermochromic sensors. Image from ref[47].

Figure 12 .

 12 Figure 12. TEM image of [Fe(trz)(Htrz)2][BF4]/polypyrrole composite (left); Electrical characterization of the composite showing a thermal hysteresis in the conductivity (right). Images extracted from reference[48].

Figure 14 .spin crossover complexes based on scorpionate ligands 1 .

 141 Figure14. Structure of the iron(III) SCO complex bearing a thiophene group. XRD structure (left) was taken from reference[50].

2. 1 .

 1 Syntheses of ligands (4 A ), (4 C ) and (4 D ) Common steps of ligand (4 A ), (4 C ) and (4 D ) syntheses are illustrated in scheme 3. The synthesis of ligand (4 B ) will be discussed aside from the others. The starting material (1 A ) was

Scheme 2 .

 2 Scheme 2. Third-generation tris(pyrazol-1-yl)borates described in this work.

precursor 1 , 3 , 2 -

 132 dioxaborolane-4,4',5,5'-tetramethyl2-(3-thienyl) was obtained from the lithiation of 3-bromothiophene followed by treatment with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (IPTMDOB), in high yield (as reported elsewhere 14 ). The obtained compound was reacted with 1-bromo-4-(trimethylsilyl)benzene (4-BrPhSiMe3) under Suzuki conditions, in toluene for 24 hours, to yield to compound [4-(3-Thienyl)phenyl](trimethyl)silane] (1 D ) in moderate yield (50 %).

Scheme 4 .

 4 Scheme 4. Synthetic route to compound 1 C . (i) THF, reflux, 3h; (ii) THF, 0-25°C, 24h

4. 3 .

 3 Magnetic properties of the complexes The magnetic behaviours of the crystalline compounds [Fe(C6F5Tp)2], [Fe(2-TPhTp)2] and [Fe( tbu PhTp)2] are shown in figure 3 , which displays the temperature dependence of the χMT product (χM is the molar magnetic susceptibility) of the three complexes. The χMT curves of the complexes [Fe(C6F5Tp)2] (green curve) and [Fe(2-TPhTp)2] (orange) show a reversible, gradual spin transition, which is incomplete at 400 K. The transition starts roughly near 250 K for [Fe(C6F5Tp)2], near 300 K for [Fe(2-TPhTp)2]. The χMT values at 400 K, 1.34 and 0.55 cm 3 mol - 1 K for [Fe(C6F5Tp)2] and [Fe(2-TPhTp)2], respectively, are lower than the anticipated value for a complete spin transition (ca. 3.30 cm 3 mol -1 K ). Within this assumption, the relative population of HS:LS complexes is ca. 0.44 : 0.56 for [Fe(C6F5Tp)2] and 0.19 : 0.81 for [Fe(2-TPhTp)2].

Figure 3 .

 3 Figure 3. Temperature dependence of molar magnetic susceptibility of [Fe(C6F5Tp)2] (green curve);[Fe(2-TPhTp)2] (orange curve) and [Fe( tbu PhTp)2](blue curve) upon one thermal cycle. Inset : Beginning of [Fe( tbu PhTp)2] thermallyinduced spin-transition.

  reveal for the three compounds, an average Fe-N distance typical of low spin-state iron(II) ion (1.967 Å in [Fe( tbu PhTp)2]; 1.977 in [Fe(2-TPhTp)2]; 1.967 in [Fe(C6F5Tp)2]). Intrinsic bite angle of tris(pyrazol-1-yl) borates ligands accommodate well to Fe-N bond distance in the low spin-state (ca 2.0 Å). 17 In this electronic configuration, pyrazolyl ring twisting is minimized and then the FeN-NB distortion approaches 0° (while in the HS state it becomes significant). here the average FeN-NC torsion angles are similar in the three complexes (172.9 in [Fe( tbu PhTp)2]; 169.9 in [Fe(2-TPhTp)2]; 169.1 in [Fe(C6F5Tp)2]

Figure 4 .

 4 Figure 4. Molecular structures and atom labeling for compounds, [Fe(2-TPhTp)2] (top), [Fe(C6F5Tp)2]) (middle) and [Fe( tbu PhTp)2](bottom).

  opposite to [Fe( tbu PhTp)2] and [Fe(2-TPhTp)2] complexes, no PzCH-π interactions are observed in [Fe(C6F5Tp)2] structure. Due to its inverse electronic distribution comparing to non-fluorinated aromatic, the pentafluorophenyl ring cannot act as a hydrogen acceptor. The molecular arrangement obtained here is similar to the one obtained by Vitze et al when sample is crystallized in a THF/hexane mixture.)

from 3 .

 3 10 to 3.56 cm 3 mol -1 K at 500 K. The thermal treatment at high temperature clearly induces an irreversible change in the magnetic behaviour. In fact upon cooling, the transition becomes more abrupt and cooperative. During the two other heating-cooling cycles, the magnetic susceptibility displays a reversible hysteresis loop with T1/2↓ = 405 K and T1/2↑ = 479 K. The χMT values of 3.56 cm 3 K mol -1 at 500 K and 0.06 cm 3 mol -1 K and 300 K indicate the presence of pure high and low spin species respectively.

Figure 5 .

 5 Figure 5. Temperature dependence of molar magnetic suceptiblity of [Fe(C6F5Tp)2] crystals obtained on three consecutive cycles ( 1 st cycle in blue, 2 nd cycle in orange and 3 rd cycle in green). Data acquired in the sweep mode, scanning at 2 K/min. Inset : d(χmT)/dT vs. T dependence calculated from χmT = f(T) curve.

1 H, 19 F

 19 and11 B NMR in solution and IR spectroscopy analyses were carried out to determine if any chemical reaction could have occurred during the thermal cycles in the SQUID. NMR spectra (appendix 4) were analogous to the pristine compound. Similarly, the FT-IR spectra (figure6a) of the compound before and after SQUID measurements do not bring to light new peaks. The main difference consists in the presence of DMF (ν = 2932; 2842 and 1673 cm -1 ) in the pristine compound.The thermal behaviour of the sample analyzed by SQUID was investigated by TGA analysis under N2. The thermogram is presented figure6b. The TGA result shows that the sample undergoes an important weight loss beginning at 240 °C (513 K) with a total mass loss of 90%., which can be due to thermal degradation.

Figure 6 .

 6 Figure 6. TGA thermogram of [Fe(FTp)2] after SQUID analyses (experience showed figure5) under N2 with a 2°C/min ramp (a).FT-IR spectra of [Fe(FTp)2] complex before (orange) and after SQUID measurements (blue)(b)

  above 210 °C (483 K). The large endothermic signal might result from the oxidation of a small amount of [Fe(C6F5Tp)2] crystals. This partial oxidation reaction was noticed by a colour change from pink to orange-brown of few crystals. Thin endothermic signals produced at high temperature probably arise from some melting of the compound.Upon cooling, an exothermic peak is observed at 117 °C (390 K) corresponding to the conversion of high spin-state to low spin-state. Subsequent scanning resulted in a reproducible thermal behaviour, analogous to that observed by SQUID measurements. Upon heating, the endothermic peak associated to the conversion of LS to HS state is observed at 195 °C (468 K) while the back conversion (HS to LS state) is at 132 °C (405 K). These peaks match with the sharp transition of the thermal magnetic hysteresis.

Figure 7 .

 7 Figure 7. DSC thermograms of [Fe(FTp)2], first cycle (left); second cycle (middle) and third cycle (right)performed with 1°C/min ramp.( Exothermic up).

  stabilization of the Fe(III)-CN-Co(II) pair in the Na-containing PBA.The stabilization of the Fe(III)-CN-Co(II) pair was then explained by interaction of the alkali ions (Lewis acid) with the π system of the cyanide bridges (lewis base), which reduces the electronic density on the cyanide and decreases the ligand field on the cobalt. This leads to a stabilization of the Co(II) redox state and thus a stabilization of the Fe(III)-CN-Co(II) pairs in comparison to the Fe(II)-CN-Co(III) ones. The Fe(II)-CN-Co(III) remains the ground state in the Na-PBA but the metastable Fe(III)-CN-Co(II) is better stabilized and more easily accessible.

Figure 8 .

 8 Figure 8. X-ray structure of K-Fe(Tp)Co( pz Tp) (left), Rb-Fe(Tp)Co( pz Tp) (middle), Cs-Fe(Tp)Co( pz Tp) (right) recorded at 200 K. Orange balls represent Fe II LS; purple ones the Co III LS and the blue one the Co II HS

Figure 9 .

 9 Figure 9. XMT product versus time of A  {Fe(Tp)Co( pz Tp)} (A = K + , Rb + , Cs + ) in acetonitrile 5mM, at 15 K under irradiation at 808 nm (5mW/cm 2 ).[PhD.Juan, 2017]

H

  and 133 Cs NMR at room temperature, see figure 10. As observed for the A  {[Fe II (Tp)(CN)3]4[Co III ( pz Tp)]3[Co II ( pz Tp)]} series, the presence of one paramagnetic Co II HS ion in Cs  {[Fe(Tp)(CN)3]4[Co III ( tbu PhTp)]3[Co II ( tbu PhTp)]} induces :  Paramagnetic chemical shifts: at 300 K, values up to 90 ppm in both positive and negative frequency regions can be observed. The protons of the scorpionate ligand coordinated to the paramagnetic Co(II) centre are the most shifted ones. On the contrary, those on the opposite corner are supposed to be the less shifted ones. The relative intensity of the chemical shift help assigning the peaks: qualitatively the more shifted, the closer the protons are to the Co(II) ion.  A lowering of the symmetry from Td to C3v leading to a loss of the equivalence of the pyrazolyl rings of tris(pyrazolyl) borate ligands of Fe(II) and Co(III) located out of the 3fold rotation axis. A broadening of some NMR signals, due to nuclear-electron dipolar interactions. The protons, which undergo the strongest dipolar coupling with the paramagnetic source are expected to be the broadest (BH is a peculiar case as it is already broad in the diamagnetic complexes).

Figure 10 . 1 HFigure 11 .

 10111 Figure 10. 1 H NMR spectrum of Cs  {Fe(Tp)Co( tbu PhTp)} (1mM) in CD2Cl2 at 300 K

Figure 12 . 1 H

 121 Figure 12. 1 H NMR spectrum at 300 K of Cs  {Fe(Tp)Co( tbu PhTp)} in CD2Cl2. The coloured circles correspond to different sets of protons. The stars denote diamagnetic {Cs  {Fe(Tp)Co( tbu PhTp)}ClO4 compound traces.

Figure 13 . 1 H

 131 Figure 13. 1 H NMR spectrum at 300 K of Cs{ Fe(Tp)Co( tbu PhTp}ClO4 in CD2Cl2.

3. 3 .Figure 14 . 1 H

 3141 Figure 14. 1 H NMR spectra of Cs  {Fe(Tp)Co( tbu PhTp)} (1mM) in CD2Cl2 centred on [ -10, 20 ppm] range, recorded at 300 K at different intervals of time.

Figure 15 . 1 H

 151 Figure 15. 1 H NMR spectra of Cs  {Fe(Tp)Co( tbu PhTp)} (1mM) in presence of NaClO4 (100mM) in CD2Cl2 centred on the aromatic signals range, recorded at 300 K at different intervals of time.

Figure 16 . 4 .

 164 Figure 16. 133 Cs NMR spectra of Cs  {Fe(Tp)Co( tbu PhTp)} (1mM) in presence of NaClO4 (100 mM) in CD2Cl2 centred on [ -10, 20 ppm] range, recorded at 300 K at different intervals of time.

Finally

  Figure 17. FT-IR spectra of the Cs  {Fe(tp)Co( tbu PhTp)} (orange) and {Cs  Fe(tp)Co( tbu PhTp)}ClO4 (blue) cubes.

Figure 18 .

 18 Figure 18. Perspective view of the paramagnetic Cs  {Fe(Tp)Co( tbu PhTp)} (left) and the diamagnetic Cs  {Fe(Tp)Co( tbu PhTp)}ClO4 (right) structures.

Figure 19 .

 19 Figure 19. MT product versus time irradiation measured at 20 K and under a 808 nm light irradiation (5mW/cm 2 ) on the diamagnetic (black) and paramagnetic (blue) cubes (a). MT versus T curve of the Cs  {Fe(Tp)Co( bu PhTp)} cube (b) scan rate 2K/min.

  {(L)Fe-CN-Co(L')} units showing large splitting of the Fe III /Fe II and Co III /Co II redox potentials, stable redox states (either Fe III -CN-Co II or Fe II -CN-Co II ) are obtained. When moving closer the redox potential of both subunits, and thus the energy difference between the Fe III -CN-Co II or Fe II -CN-Co II states, ETCST can be observed. It is worth underlying that the matching of the redox potential have appeared in the literature as an important criteria to observe ETCST.

  Bu4N[Fe III/II ( pz Tp)(CN)3] (-373 mV vs SCE). The influence of the capping ligand on the redox potential seems relatively small as the cyanides mainly dominate the ligand field. It is worth underlying that both [Fe III/II ( pz Tp)(CN)3] -/2-and [Fe III/II (Tp)(CN)3] -/2-have led to similar Fe-Co system showing ETCST phenomenon, so the use of a species with an intermediate redox potential fullfill the condition to obtain FeCo photomagnetic molecules.

  paramagnetic A  {Fe( tbu PhTp)Co(Tp)} cubes 2.1. Bu4N[Fe( tbu PhTp)(CN)3] precursor synthesis and characterisationsSynthesis.The preparation of the [Fe( tbu PhTp)(CN)3] anionic complex follows a four-step synthesis and it is obtained with an high yield (80%). The first step involves the synthesis of the homoleptic [Fe( tbu PhTp)2] complex, following the procedure described part I. The cyanuration reaction was done by reacting [Fe II ( tbu PhTp)2] with 3.5 equivalents of KCN in isopropanol under light exclusion conditions to avoid side reactions. The mixture was heated to reflux until a colour change from pink to yellow was observed. The reaction resulted in the formation of K2[Fe II ( tbu PhTp)(CN)3] which was oxidized with a mild oxidant (H2O2) in water.Finally, a cation metathesis with one equivalent of Bu4NBr precipitates [Fe( tbu PhTp)(CN)3] -as a tetrabutyl ammonium salt. Crystals of the expected complex were obtained by slow evaporation of a MeOH solution.

Figure 22 .

 22 Figure 22.. 1 H NMR spectrum of the Bu4N[Fe( tbu PhTp)(CN)3] in CD2Cl2 at room temperature.

  figure 23. The Fe(III) ion adopts a slightly distorted octahedral coordination sphere. The average Fe-C bond lengths is 1.927 Å, which is typical for cyanide iron(III) complex. The Fe-N bonds are about the same magnitude (1.969 Å). The Fe-C-N angles are almost linear (ca. 177°-180°).The shortest Fe-Fe intermolecular distance is 8.08 Å, revealing that the complexes are well isolated from each other.

Figure 23 . 2 . 2 .

 2322 Figure 23. Perspective view of the structure of [Fe III ( tbu PhTp)(CN)3] -

Figure 24 .

 24 Figure 24. Perspective view of the paramagnetic Cs  {Fe( bu PhTp)Co(Tp)} structure.

Figure 25 .

 25 Figure 25. MT versus T curve of the Cs  {Fe( tbu PhTp)Co(Tp)} (left) and Tl  {Fe( tbu PhTp)Co(Tp)} (right) cubes

Figure 26 .

 26 Figure 26. Relaxation curves of the Cs  {Fe( tbu PhTp)Co(Tp)} (left) and Tl  {Fe( tbu PhTp)Co(Tp)} (right) cubes : variation of the MT products upon heating (0.4 K/min) after light irradiation at low temperature. The inflexion points (shown in the insets) give an estimate of the relaxation temperature.

Figure 27 .

 27 Figure 27. MT products versus irradiation time curves of the Cs  {Fe( tbu PhTp)Co(Tp)} (left) and Tl  {Fe( tbu PhTp)Co(Tp)} (right) cube. Laser light: 808 nm, ca 5 mW/cm 2 . The first jumps observed in the curves correspond to the switching ON of the laser light (photomagnetic effect. The second jumps observed after the saturation plateau correspond to the moment the light is switched off. The heating effect of the light disappears and the local temperature increases and the MT increases.

  photomagnetic properties. Indeed we observed that the position of the redox waves can vary with different parameters (temperature, water content, electrode surface state, etc.).

Figure 27 . 4 M

 274 Figure 27. CV Figure 20. Cyclic voltammograms of the Tl  {Fe( tbu PhTp)Co(Tp)} (violet); Cs  {Fe( tbu PhTp)Co(Tp)} (brown) and Cs  {Fe(Tp)Co( tbu PhTp)} (blue) in ACN (10 -- 4 M) with TBAPF6 (0.1M). Scan rate = 100 mV/s. GC was used as working electrode, Pt as counter electrode, and a saturated calomel electrode as reference.

Figure 28 .

 28 Figure 28. Multistep rational design for the {Fe4Co4} cages magnetic properties optimization.

Figure 1 .

 1 Figure 1. Schematic representation of synthetic strategies for the design of metallopolymers

Figure 2 . 1 H 2 .

 212 Figure 2. 1 H NMR spectra (300 K) of the starting Poly(4-trimethylstyrene) and the copolymer (2) recorded in CDCl3. Presence of benzene, Me3SiBr impurity (δ = 2.52 ppm), water and grease were identified and denoted by stars.

Figure 3 . 1 H

 31 Figure 3. 1 H NMR spectra centred on the aromatic signals range of tbu PhTp, recorded in deuterated THF at 300 K at different intervals of time.

FinallyFigure 4 . 1 H

 41 Figure 4. 1 H NMR spectra centred on the aromatic signals range of tbu PhTp, recorded in deuterated methanol at 300 K at different intervals of time.

Figure 5 .

 5 Figure 5. NMR characterizations of 4-tertbutylphenylboronic acid.1 H and13 C NMR spectra were recorded in dmso-d6 and in CD3CN for the11 B NMR spectrum. All measurements were performed at 300K.

Scheme 4 .

 4 Scheme 4. Synthetic route of ligand (A) used in this study.

Figure 6 . 1 H

 61 Figure 6. 1 H NMR spectrum of ligand (B) in dmso-d6 at room temperature

Scheme 5 .

 5 Scheme 5. Synthetic route of ligand (C).

Figure 7 . 1 H

 71 Figure 7. 1 H NMR spectrum of ligand (C)) in dmso-d6 at room temperature

  the side-chain. Therefore according to the ligand, the resulting hybrid polymer can exhibit different chemical/physical properties (e.g. electrical conductivity). . In the first part of this chapter, we investigated the ability of homoleptic cobalt complexes, [Co II (2-TPhTp)2] and [Co II (3-TPhTp)2], to form a thin film on the electrode surface. These two examples are used to present the electrochemical behaviour of adsorbed surface species. Then, the {Fe4Co4} cages electropolymerization was initiated and optimized with the most readily accessible thiophene-functionalized cubes, i.e. the Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 and Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 diamagnetic cubes. Although they are not photomagnetic, their electrochemical properties were expected to be similar to their paramagnetic counterparts.

Figure 1 .

 1 Figure 1. Representation of thiophene-functionalized {Fe4Co4} cages with 2-TPhTp (left) and 3-TPhTp (right).

Figure 2 .

 2 Figure 2. a) Proposed mechanism for polythiophene formation, involving electrochemical and chemical steps. b) Growth of a polymeric film on a Glassy carbon electrode from 1 mM bithiophene monomer solution in acetonitrile.

Full 1 H

 1 NMR, HRMS characterisations of both mononuclear complexes are joined in appendices.2.1. Electrochemistry in solutionThe electrochemical behaviours of both complexes were obtained by cyclic voltammetry at room temperature from 1mM DMF: ACN (1:9) solution containing 0.1 M Bu4NPF6. DMF was used as a co-solvent due to the low solubility of the complexes in ACN. A saturated calomel electrode was used as reference, Pt wire was the counter electrode and a glassy carbon electrode (Ф = 3 mm) was employed as working electrode. The divalent [Co(2-TPhTp)2]] and [Co(3-TPhTp)2] complexes cyclic voltammograms (CV) are presented in figure 3. The first interesting observation is the influence of the substitution position (α or β) on the thiophene ring, on the irreversible oxidation peak. For [Co(2-TPhTp)2]

  contrast, for [Co(3-TPhTp)2] the thiophene oxidation potential is anodically shifted compared to [Co(2-TPhTp)2], and it appears just before the solvent oxidation. We observed that [Co(3-TPhTp)2] thiophene group cannot be oxidized within the studied electrochemical window [-0.8; 2 V], which indicates a weaker stabilisation of the radical cation in [Co(3-TPhTp)2] than in [Co(2-TPhTp)2].

Figure 3 .Figure 4 .

 34 Figure 3. Cyclic voltammograms for 1 mM of the Co(2-TPhTp)2 (A) and Co(3-TPhTp)2 (B) in DMF:ACN (1:9) at v = 100 mV.s -1 . TBAPF6 (0.1 M) was used as supporting electrolyte, GC as working electrode, Pt as counter electrode, and a saturated calomel electrode as reference.

Figure 5 .

 5 Figure 5. Ideal cyclic voltammetric response for the reversible reaction of adsorbed species.

123II. 1 .

 1 Electropolymerization of thiophene-functionalized {Fe II 4 Co III 4 } diamagnetic building blocks. Syntheses of Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 and Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 precursors. Both compounds were prepared following the synthetic route described chapter 2 to obtain diamagnetic cubes (i.e. by oxidizing paramagnetic thiophene-functionalized cubes in presence of a perchlorate salt, NaClO4). After purification (detailed in appendix) the compounds were characterized by HRMS, NMR, IR and UV-visible spectroscopies. Note: The compounds were obtained as perchlorate salts. However, regarding the experimental conditions used for electrochemistry studies (i.e. the amount of supporting electrolyte) an anion exchange should occur in solution. For this reason, the compounds will be denoted Cs  {Fe(Tp)Co(2-TPhTp)}PF6 and Cs  {Fe(Tp)Co(3-TPhTp)}PF6 in the remaining part of this chapter.

Figure 6 .

 6 Figure 6. (Multi)sweep voltammograms of 0.1 mM of the Cs  {Fe(Tp)Co(2-TPhTp)}PF6 (A to C) and Cs  {Fe(Tp)Co(3-TPhTp)}PF6 (D to F) in ACN + TBAPF6 (0.1 M) at v = 100 mV.s -1 . GC was used as working electrode, Pt as counter electrode, and a saturated calomel electrode as reference for all measurements.

Figure 7 .

 7 Figure 7. CV of the electrodeposited Cs  {Fe(Tp)Co(2-TPhTp)}PF6 (A and C) thin-film and Cs  {Fe(Tp)Co(3-TPhTp)}PF6 (B and D) in ACN + 0.1M TBAPF6 at different scan rates. GC was used as working electrode, Pt as counter electrode, and a saturated calomel electrode as reference for all measurements.

  above and as observed by AFM scratching experiments (see also section II.2. part 2). A full quantitative analysis is impossible here because of the different oxidation waves entanglement and the impossibility to correct cyclic voltammograms. Consequently, surface coverage and thickness layer could not have been extracted by using the electrochemical data. To summarize, he electrochemical studies of the Cs  {Fe(Tp)Co(3-TPhTp)}PF6 and Cs  {Fe(Tp)Co(2-TPhTp)}PF6 cages have been performed. Both cubes retain typical features of the Fe/Co model cubes in their voltammograms. The electrochemical oxidation of the thiophene was successfully observed in both compounds. As in Co(II) homoleptic complexes, thiophene oxidation occurs at lower potential in 2-TPhTp functionalized cubes than in the one functionalized by 3-TPhTp. While the electrochemical oxidation of thiophene in [Co II (3-TPhTp)2] was barely visible and did not result in the formation of a thin-film, it was welldefined in Cs  {Fe(Tp)Co(3-TPhTp)}PF6 and resulted in the formation of a film. This difference is linked to a better stabilization of the radical cation in the polymetallic complex.

Figure 8 .

 8 Figure 8. Peak current as a function of the scan rate for cyclic voltammograms of Cs  {Fe(Tp)Co(2-TPhTp)}PF6 (blue squares) and Cs  {Fe(Tp)Co(3-TPhTp)}PF6 (red squares) obtained figure 7 (A-D). Oligo/polythiophene Ip values extracted on CV uncorrected from blank.

Figure 9 .

 9 Figure 9. Schematic representation showing photoionization of an atom by ejection of a 1s electron.

Figure 10 .

 10 Figure 10. Contribution (P) of the different sample regions as function of the depth.

Figure 12 .

 12 Figure 12. Representative Co 2p, Cs 3d and Fe 2p peaks recorded on Cs  {Fe(Tp)Co(2-TPhTp)}PF6 electropolymerized thin-film on Pt/mica (a) and on Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 powder (b).

Figure 13 .

 13 Figure 13. Representative S 2p peaks recorded on Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 powder (a) and the Cs  {Fe(Tp)Co(2-TPhTp)}PF6 electropolymerized thin-film on Pt/mica (b).

3. 4 .

 4 XPS analysis of Cs  {Fe(Tp)Co(3-TPhTp)}PF6 electropolymerized thin-film.

Figure 14

 14 Figure 14 presents the XPS spectra recorded for the Cs  {Fe(Tp)Co(3-TPhTp)}PF6 electropolymerized thin-film on Co 2p, Cs 3d, Fe2p and S 2p element regions.

For 4 . 1 .

 41 these reasons, five different samples of Cs  {Fe(Tp)Co(2-TPhTp)}PF6 thin-films were prepared by varying the electrochemical deposition conditions. Pt-coated mica substrates (0.6 cm x 0.6 cm) were used as working electrode films and films were obtained by performing either 1, 2, 5, 10 or 15 cyclic potential sweeps. Cyclic voltammograms are joined in appendices. Substrates were then washed with pure acetonitrile and dried with nitrogen gas flow. Films morphology and thickness were determined by AFM and are successively presented, as a function of the number of potential sweeps. Topography of Cs  {Fe(Tp)Co(2-TPhTp)}PF6 thin-films

Figure 15 .Figure 16 .

 1516 Figure 15. Representative AFM height images (2 µm x 2 µm; Peak Force Tapping; in air) recorded on Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 electropolymerized ultra-thin films on Pt/mica obtained after different number of cyclic potential sweeps: 1 cycle (a), 5 cycles (b), 10 cycles (c) and 15 cycles (d). Line scans were taken at the locations indicated by dashed lines.

  {Fe(Tp)Co( tbu PhTp)}ClO4 (chapter 2), the diamagnetic Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 cube does not exhibit any photomagnetic properties. To investigate the photomagnetic properties of {Fe4Co4}-electropolymerized thin-films, we thus decided to use the Tl  {Fe(2-TPhTp)Co(Tp)} cube. The latter was synthesized following the strategy developed in chapter 2 on a model cube containing the tbu PhTp ligand. It is worth noticing that the Tl  {Fe( tbu PhTp)Co(Tp)} was shown to display an enhanced photomagnetic properties, as observed on our thalliumcontaining model cube. Thus, electropolymerization strategy was applied to Tl  {Fe(2-TPhTp)Co(Tp)} whose resulting thin-films were first characterized by XPS and AFM and finally by synchrotron techniques (XAS and XMCD).

1 .

 1 Synthesis of Tl  {Fe(2-TPhTp)Co(Tp)} paramagnetic cube. The preparation of Tl  {Fe(2-TPhTp)Co(Tp) is based on Tl  {Fe( tbu PhTp)Co(Tp)} model one. We first had to prepare the Bu4N[Fe III (2-TPhTp)(CN)3] precursor. The tricyanido iron(III)

E

  {Fe(Tp)Co(2-TPhTp)}PF6 samples. The Co 2p, Fe 2p, Tl 4f and S 2p XPS core spectra of the Tl  {Fe(2-TPhTp)Co(Tp)} electropolymerized thin-films are presented figure 18. The Fe 2p peaks appeared at 721 and 708 eV corresponding respectively to Fe 2p3/2 and Fe 2p1/2 contributions. The Fe 2p signals

Figure 17 .

 17 Figure 17. Repeated potential scans (five consecutive scans) electropolymerization of Tl {Fe(2-TPhTp )Co(Tp)} onto a Pt/mica substrate in CH3CN + 1M NBuPF6 at 0.1 V s -1 . Table 5. Some electrochemical datas of Tl {Fe(2-TPhTp )Co(Tp)} electropolymerized thin-films on Pt/mica ( 5cycles) [A] and Cs  {Fe(Tp)Co(2-TPhTp)}PF6 electropolymerized thin-films on Pt/mica (5cycles) [B] as a comparison.

Figure 18 .

 18 Figure 18. Representative Co 2p, Fe 2p ,Tl 4f and S 2p peaks recorded on {Tl  Fe(2-TPhTp)Co(Tp)} electropolymerized thin-film on Pt/mica (b).

2. 3 . 10 nm 10 Figure 19 .

 3101019 figure19, presents a relatively smooth surface (Rrms = 2.56 nm, computed from 2 µm x 2µm image). Roughness value obtained here is of similar magnitude to the one noted for Cs  {Fe(Tp)Co(2-TPhTp)}PF6 thin-films (Rrms = 3.12 nm) prepared with identical number of cycles ( i.e. 5 cycles).

figure 20c .Figure 20 .

 20c20 figure 20c. The average value of the thickness was estimated to 19.64 ± 1.1 nm.

Figure 21 .

 21 figure26. Measurements performed evidence the photoconversion of the paramagnetic Tl 

  photomagnetic, we synthesized a paramagnetic Tl  {Fe(2-TPhTp)Co(Tp)} derivative exhibiting efficient photomagnetic properties. The electropolymerization was then extended to the Tl  {Fe(2-TPhTp)Co(Tp)} to prepare thin-films on Pt substrates. The thin-films composition and topography were also probed by XPS and AFM techniques respectively. Once again, we demonstrated that the integrity of the {Fe4Co4} cages were preserved after electropolymerization. The thin-films obtained were homogenous and were found to have a stronger adhesion of Pt substrate than the diamagnetic analogous thin-film of Cs  {Fe(Tp)Co(2-TPhTp)}PF6.

Figure 22 .

 22 Figure 22. Schematic representation of a electropolymerized thin-film of thiophene-functionalized {Fe4Co4} cages ( using 2-TPhTp).

First, we carried

  out efforts in defining experimental conditions for the adsorption of Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 and Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 cubes on gold from solution. Diamagnetic derivatives appeared more suitable to their paramagnetic counterparts to initiate adsorption tests, mainly for the following reasons: (i) Paramagnetic cubes functionalized with third-generation tris(pyrazolyl)borate are sensitive to oxidation in solution (as observed chapter 1).

As already seen (chapter 2 )Figure 1 .

 21 Figure 1.. Schematic illustration of PM-IRRAS experimental principle.

Figure 2 .

 2 Figure 2. FT-IR spectrum of the Cs  {Fe(Tp)Co(3-TPhTp)}ClO4.

Figure 3 .

 3 Figure 3.. PM-IRRAS spectrum of Au/glass substrate after 3 hours immersion in a 5.10 -4 M solution of Cs  {Fe(Tp)Co(2-TPhTp)}ClO4.

Figure 4 . 3 . 1 .

 431 Figure 4. A) Cyanide band area as a function of gold substrate immersion time in a solution of Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 in CHCl3. B) Comparison of PM-IRRAS spectra of gold subtrates after 1h or 24h immersion time.

Figure 7 .

 7 Figure 7. S 2p core level spectra of Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 on Au/glass (a); Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 on Au/glass (b) after 5 min, 1h or 4h of washing time in pure DCM. Schemes of 2-TPhTp and 3-TPhTp ligands are depicted next to XPS spectra.

Figure 8 .

 8 Figure 8. S 2p core level spectra of Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 on Au/mica (a); Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 on Au/glass (b) after 5 min or 24h of washing time in neat DCM. Schemes of the 3-TPhTp ligand and 3hexylthiophene are depicted next to the XPS spectra.

3. 3 .

 3 Electrospray deposition (ESI) of Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 on Au(111)

Figure 10 .Figure 9 .

 109 Figure 10. Evolution of Sox/Stot ratio with washing time of Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 and 3hexylthiophene deposited on Au/mica. (XPS analyses of these samples are showed figure 8).

Figure 11 .

 11 Figure 11. PM-IRRAS spectrum of Cs {Fe(Tp)Co(3-TPhTp)}ClO4 after electrospray ionization deposition.

Figure 10 .

 10 Figure 10. XPS spectra of Cs 3d and Fe 2p (a); Co 2p (b); S 2p (c) recorded on Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 after electrospray ionization deposition.

Figure 12 .

 12 Figure 12. Correlations between the indicated molar concentrations measured by XPS for Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 samples on Au/mica (orange) and Au/glass (blue). Dashed lines indicated theoretical relation between the two variables with their corresponding equations. Samples were prepared by dipping substrates 3h in cube solution and washed either 5min, 1h, 4h or 24h in pure DCM.

5 .

 5 figure 13a) to ca. 2 nm. Finally, the surface roughness (computed from 2 µm x 2µm image) was found to be to 3.15 nm indicating a relatively homogenous film.

Figure 13 .

 13 Figure 13. Representative AFM height images (2 µm x 2 µm; Peak Force Tapping; in air) recorded on Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 deposited on Au/mica from solution (10 -4 M in DCM). Image b (0.6 µm x 0.6 µm) correspons to a zoom taken at the center of the image a. The line scan (c) was taken at the location indicated by dashed line.

( 2 -

 2 TPhTp)}ClO4 and Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 diamagnetic cages to probe conditions to their deposition onto surfaces.The chemical composition of {Fe4Co4} cages layers were probed by XPS which revealed an influence of the type of substrates (i.e. oxidation of iron(II) on Au/glass). The binding nature of the thiophene functionalized cages was compared to a model thiophene derivative, the 3hexylthiophene. XPS investigations revealed sulfur bound atoms for 3-hexylthiophene molecules (in agreement with literature) whilel only physisorption was observed for Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 and Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 compounds. Still, the {Fe4Co4} cages layers have proved to be stable as 24 hours of washing in pure DCM did not enable to remove them.

Finally, as indicated

  by AFM characterizations, these polymetallic complexes deposited onto surfaces as homogenous smooth films. If our study need to be completed, this first study for the preparation of self-assembled {Fe4Co4} cages layers include some encouraging results.

  C. Chemical shifts, δ, are expressed in parts per millions (ppm) and are referenced using IUPAC recommendations. The measurements of the paramagnetic samples were carried out after thermal stabilization Mass spectrometric analyses (ESI mode) were performed on a quadrupole-Orbitrap Q Exactive mass spectrometer from Thermo Scientific. Elemental analyses (C, H, N) were carried out by combustion analysis using a vario MICRO cube apparatus from Elementar. TGA analysis: TGA measurements were performed on a Perkin Elmer STA-Q600 simultaneous thermal analyzer. DSC measurement was performed on an Instrument DSC Q2000 V24.4 Build 116, using the Module DSC Standard Cell RC. X-ray crystallography: Crystal data and details concerning data collection and refinement are given below. Data collection was performed on a STOE STADIVARI diffractometer using graphite-monochromated Mo Kα radiation with λ = 0.71073 Å. Using Olex2, S2 the structure was solved with the ShelXT [2] structure solution program using Direct Methods and refined with the ShelXL [3] refinement package using Least Squares minimisation. Structure solution for 1

  were carried out at room temperature in a standard cell equipped with a platinum wire counter electrode, Hg/Hg 2+ reference electrode using Metrohm PGSTAT 101 electrochemical analyzer. Different materials were used as working electrode (glassy carbon, Pt/mica). All measurements on {Fe4Co4} cages were performed in acetonitrile with 0.1 M [Bu4N][PF6] as the supporting electrolyte. XPS analyses were performed using an ESCA+ spectrometer (Omicron Nano-Technology) equipped with a monochromatized aluminum X-ray source (powered at 20 mA and 14 kV) and an MCD 128 channeltron detector. Charge stabilization was ensured using a CN10 device with an emission current of 5.0 μA and a beam energy of 1 eV. Analyses were performed in sweep mode; the resulting analyzed area was 1 mm in diameter. A pass energy of 20 eV was used for narrow scans. Under these conditions, the full width at half-maximum (fwhm) of the Ag3d5/2 peak of a clean silver reference sample was about 0.6 eV. The pressure in the analysis chamber was approximately 10 -10 Torr. The photoelectron collection angle, θ, between the normal to the sample surface and the analyzer axis was 45°. The following sequence of spectra was recorded: survey, C 1s, O 1s, N 1s, Al 2p, Cl 2p, P 2p, S2p, and C 1s again to check for charge stability as a function of time and the absence of sample degradation.

-1 ) [Fe( tbu PhTp)2] 158 520 19 200 [Fe(C6F5Tp)2] 145 526 19 000 [Fe(3-TPhTp)2] 119 524 19 100 [Fe(2-TPhTp)2] 145

	Figure 1.
	524	19 100

1

H,

11

B and

19

F NMR spectra of [Fe(C6F5Tp)2] recorded in CD2Cl2 at room temperature.

Figure 2. Visible spectroscopic characterization of [Fe(L)2] homoleptic complexes in DCM, 10 -4 M at 273K : [Fe(tbuPhTp)2] (blue curve); [Fe(C6F5Tp)2] (green curve); [Fe(3-TPhTp)2] (black curve) and [Fe(2-TPhTp)2] (orange curve). Table 1. Optical datas of [Fe(L)2] homoleptic complexes.

Table 2

 2

	. Crystallographic data for [Fe(C6F5Tp)2], [Fe(2-TPhTp)2] and [Fe( tbu PhTp)2] complexes.

tbu PhTp)2] and [Fe(C6F5Tp)2] complexes crystallized from DCM, 1,2-dichlorethane and DMF respectively and all belong to the triclinic P-1 space group. The molecular structure are presented figure 4. In each complexes, the iron(II) ion is coordinated by two tris(pyrazol-1-yl) borate ligands in an octahedral manner. Selected bond lengths are [Fe( tbu PhTp)2] [Fe(2-TPhTp)2] [Fe(C6F5Tp)2]

Table 3 .

 3 Selected bond distances, angles and torsion angles for [Fe(C6F5Tp)2], [Fe(2-TPhTp)2] and [Fe( tbu PhTp)2]

Table 4

 4

. (Thermodynamic) parameters of the [Fe(FTp)2] hysteresis loop

Table 5 .

 5

		[Fe(C6F5Tp)2] (1)	[Fe(C6F5Tp)2] (2)	[Fe(C6F5Tp)2] (3)	[Fe(C6F5Tp)2] (4)
	Formula	((refrefffrefTHF/Hex 5 C30H18B2FeF10N12	C30H18B2FeF10N12	C30H18B2FeF10N12	C30H18B2FeF10N12
	FW	814.03	814.03	814.03	814.03
	Color, shape	Violet, block	Pink, needles	Pink, needles	Pink, needles
	Solvent	Hexane/THF	Toluene	DMF	DMF
	Temperature	173(2)	200	300	430
	Crystal system Triclinic	Triclinic	Triclinic	Triclinic
	Space group	P -1	P -1	P -1	P -1
	a/ Å	11.0508(15)	11.0054(6)	11.0213(11)	11.0112(9)
	b/ Å	11.0935(13)	11.0441(6)	11.0866 (6)	11.3538 (12)
	c/ Å	13.6243(18)	13.6011(8)	13.8026 (14)	13.8099 (14)
	α/°	99.755(10)	99.8650(10)	96.162(6)	95.786 (7)
	β/°	96.682(11)	96.559(2)	100.160(8)	99.961(7)
	γ/°	91.510(10)	91.4250(10)	91.322(6)	91.781 (7)
	V/ Å 3	1633.1(4)	1616.37	1648.96	1689.74
	Z	2.0	2.0	2.0	2.0
	Dcalc. /g cm -3	1.655	1.673	1.637	1.637
	R-Factor (%)		3.86	6.21	7.61

Crystallographic data for [Fe(FTp)2] crystallized in different conditions : datas extracted from ref

[9]

(1); crystals obtained in Toluene (2) 2 at 300 K; in DMF at 300 K

(3)

; in DMF at 430 K

(4)

.

Structures collected at 300 K and 430 K in DMF belong to the same space group, P-1 and are isostructural. It is worth noticing that no solvent molecules were observed in the lattice whatever the solvent used for crystallization (DMF or Toluene). Selected bond lengths are presented table 6. The main difference arises from the fact that the two crystallographically independent complexes (Fe(1) and Fe(2)) are in different spin state at high temperature. Indeed, one complex displays Fe-N lengths, ca. 1.97 Å, corresponding to Fe II ions in LS state while for the second Fe-N lengths are in the range of those observed for HS state,

Table 6 .

 6

		Toluene(200K	DMF(300K)	Fe1 DMF(430K)	Fe2 DMF(430K)	Aft. DSC(300K)
		)	FeFe1 Bond distances (Å)	Fe2
	Fe(1)-N(1)	1.953	1.980	2.100	1.998	1.952
	Fe(1)-N(3)	1.969	2.001	2.080	1.967	1.972
	Fe(1)-(N5)	1.981	1.966	2.131	1.987	1.984
			Bond angles (°)
	N(1)-Fe(1)-N(3)	88.18	88.8	84.3	88.6	88.04
	N(3)-Fe(1)-N(5)	86.69	88.2	85.6	87.6	86.96
	N(5)-Fe(1)-N(1)	89.67	85.8	86.6	86.6	89.51
	N(1)-Fe(1)-N(1a)	180	180.0	180.0	180.0	180.0
	N(5)-Fe(1)-N(5a)	180	180.0	180.0	180.0	180.0
	N(3)-Fe(1)-N(3a)	180	180.0	180.0	180.0	180.0
	N(1)-Fe(1)-N(3a)	91.82	91.2	95.7	91.4	91.96
	N(3)-Fe(1)-N(5a)	93.31	91.8	94.4	92.4	93.04
	N(5)-Fe(1)-N(1a)	90.33	94.2	93.4	93.4	90.49
	N(2)-B(1)-N(4)	107.9	107.9	110.8	107.4	107.8
	N(4)-B(1)-N(6)	103.6	103.3	102.3	103.8	103.6
	N(6)-B(1)-N(2)	108.9	109.1	108.4	108.9	108.9
	FeN-NB average	3.75	1.45	6.2	3.6	2.16
	FeN-NC average	169.1	173.7	160.85	169.46	170.2

Selected bond lengths and angles of [Fe(FTp)2] complexes obtained in different conditions (in toluene at 200 K; in DMF at 300K; in DMF at 430 K showing values of the two non-equivalent complexes and after DSC treatment at 1°C/min)

  LS, 3x Co III LS and 1x Co II HS . The metallic ions occupy the corners alternatively and are connected through cyanide bridges. Two types of

metal sites can be distinguished, strong ligand field {(Tp)Fe(CN)3} and weaker ligand field {( pz Tp)Co(NC)3}.

PhTp ligand in a A  {Fe(Tp)Co( tbu PhTp)} cubes series

  H NMR analyses of the crude powder revealed the presence of the expected compound Cs  {[Fe II (Tp)(CN)3]4[Co III ( tbu PhTp)]3[Co II ( tbu PhTp)]} but also a minority (25 %) of Cs  {[Fe II (Tp)(CN)3]4[Co III ( tbu PhTp)]4}ClO4. Both compounds were isolated and characterized by 1

	The Cs  {Fe(Tp)Co( tbu PhTp)} cube was prepared by first reacting Bu4N[Fe(Tp)(CN)3]
	(117 mg, 0.2 mmol) with Co(ClO4)2 . 6H2O (73 mg, 0.2 mmol) in 3 mL of DMF. The solution
	initially red, turned to green after the addition of CsI (207 mg, 0.8 mmol) reflecting the change
	from {Fe III -CN-Co II } pairs to {Fe II -CN-Co III } ones. Then, the ligand Na[ tbu PhTp] (110 mg, 0.3
	mmol) was added as a solid and the mixture was stirred overnight. The suspension was filtered
	and the product was obtained as a blue powder after treatment (washing several times with

3.1. Synthesis and NMR characterization of Cs  {Fe(Tp)Co( tbu PhTp)} Et2O/DMF mixtures, 8/1, until the filtrate becomes colourless). Crystals of the compound were obtained by slow evaporation of a solution either in acetonitrile or in CH2Cl2/DMF (8:2).

1

Table 7 .

 7

The reactions were run with 0.2 mmol of Bu4N[Fe(Tp)(CN)3]; 0.2 mmol of Co(ClO4)2.6H2O; 0.3 mmol of Na[ tbu PhTp] and 0.8 mmol of CsI in DMF at room temperature.

Table 8 .

 8 Selected FT-IR (ATR) frequencies of {Fe(tp)Co( tbu PhTp)} paramagnetic and diamagnetic cubesThe ν(B-H) stretching also provides some information on the iron redox state. Indeed, the νB-H is found at ca. 2509 cm -1 in the Bu4N[Fe III (Tp)(CN)3] precursor whereas it is shifted to lower values, upon reduction. in both diamagnetic and paramagnetic cubes, ν(B-H) stretching is measured ca. 2483 cm -1 , indicating only pure Fe(II) ions.The metal-cyanide stretching vibration, ν(Fe-CN) was observed at 514 and 515 cm -1 in Cs  {Fe(Tp)Co( tbu PhTp)} and Cs  {Fe(Tp)Co( tbu PhTp)}ClO4 respectively. This observation is also in agreement with the presence of only Fe II ions in both cubes. In both cases, the methyl peak of the tert-butyl moiety appears at 2957 cm -1 .

		νB-H	νCN (cm -1 )	vClO4 (cm -1 )	νFe-CN (cm -1 )
	Cs  {Fe(tp)Co( tbu PhTp)}	2483	2120; 2103; 2090 -	514

Cs  {Fe(tp)Co( tbu PhTp)}ClO4

	2481	2125; 2106	1090;1063;1043 515

K 

{Fe(tp)Co( tbu PhTp)}ClO4

	2474	2125; 2105	1090;1064;1042 518

Table 9

 9

	. Electrochemical datas of Bu4N[Fe(L)(CN)3] complexes
	Compound	E°1/2 vs SCE /mV
	Fe( pz Tp)(CN) 3	-373
	Fe( tbu PhTp)(CN)3	-411
	Fe(Tp)(CN) 3	-426
	The redox process in Bu4N[Fe(

tbu PhTp)(CN)3] occurs at a half potential of -411 mV (vs SCE), slightly above that of Bu4N[Fe III/II (Tp)(CN)3] (-426 mV vs SCE) and below that of

  [Co(2-TPhTp)2] and [Co(3-TPhTp)2] complexes were obtained from a mixture of Co(BF4)2.6H2O cobalt salt and the corresponding ligand in acetonitrile. The yellow suspension was filtered after stirring for 15 minutes. The precipitate was washed successively with ACN

	and MeOH:H2O (1:2). After drying the complexes were obtained as pale yellow powders.
	[Co(3-TPhTp)2] % EA Calc : C, 56.95; H, 4.02; N, 20.97. % EA Found : C, 53,75; H, 3,76; N, 19,83.

Table 2 .

 2 Electrochemical datas of Cs  {Fe(Tp)Co(3-TPhTp)}PF6 and Cs  {Fe(Tp)Co(2-TPhTp)}PF6 cubes after 2 cycles.

	Epa thio (V)	Epa oligothio

  eV. This doublet was assigned to the sulfur of the 2-TPhTp ligand borne by cobalt ions

		Components	Position	FWHM
	Electropolymerized thin-films S1(2p3/2)	163.96	1.23
		S1(2p1/2)	165.08	1.23
	Pristine powder	S1(2p3/2)	164.01	1.21
		S1(2p1/2)	165.11	1.21

Table 3 .

 3 Characteristics and assignment of S 2p peaks components for Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 powder and the Cs  {Fe(Tp)Co(2-TPhTp)}PF6 electropolymerized thin-film on Pt/mica.

	S 2p

CasaXPS (This string can be edited in CasaXPS.DEF/PrintFootNote.txt)

Table 4 .

 4 Theoretical and experimental ratio of constitutive cubes elements in Cs  {Fe(Tp)Co(2-TPhTp)}ClO4 powder and the Cs  {Fe(Tp)Co(2-TPhTp)}PF6 electropolymerized thin-film.

Table 1 ,

 1 as well as those of the Cs 

						S 2p
		S 2p
		S 2p				S 2p
	175	170	165	160	175	170	165	160
	175	170	165	160	175	170	165	160
						Binding Energy (eV)

{Fe(Tp)Co(3-TPhTp)}ClO4 compound. For 3-hexylthiophene, the peak position of S1 and S2 are located at 161.2 eV and 162 and are the same as those observed for thiophene, bisthiophene and EDOT reported in CasaXPS (This string can be edited in CasaXPS.DEF/PrintFootNote.txt) Binding Energy (eV) CasaXPS (This string can be edited in CasaXPS.DEF/PrintFootNote.txt) Binding Energy (eV) 5min 24h CasaXPS (This string can be edited in CasaXPS.DEF/PrintFootNote.txt) Binding Energy (eV) CasaXPS (This string can be edited in CasaXPS.DEF/PrintFootNote.txt)

  table 1). Components located between 161 and 163 eV were assigned to bound S atoms on Au surface. As mentioned above, peaks at higher binding energy can be attributed to two other types of sulfur. Indeed, signals between 163 eV and 165 eV belong to unbound S atoms (S3 and S4) while those above 167 eV are attributed to oxidized sulfur (Sox).

Table 5 .

 5 Values of binding energy position of different thiophene derivatives.

	These results on model compound attest thus that only physisorbed thiophene species
	are observed in the case of the thiophene functionalized cages. Although the XPS spectra of
	the Cs  {Fe(Tp)Co(3-TPhTp)}ClO4 (and Cs  {Fe(Tp)Co(2-TPhTp)}ClO4) did not reveal
	chemisorbed thiophene, the layers obtained seem relatively stable. Indeed, even after 24h of
	washing in DCM, XPS S2p peaks are still observed (figure 8) as well as those of cage framework
	(see appendices).
	3.2. Evolution of oxidized sulfur amount in solution.
	An analysis of the evolution of the oxidized sulfur species of Cs  {Fe(Tp)Co(3-
	TPhTp)}ClO4 with washing time is presented figure 9 and compared to those of 3-

hexylthiophene figure

10

. Sox corresponds to the molar concentration of oxidized sulfur, i.e to species signal above 167 eV and Stot correspond to the sum of the molar concentration species detected in each sample.

Table 3 .

 3 Means of elements ratios computed from Au/glass samples. n stands for the number of samples studied.

	Au/mica	Experimen tal	Theoretical	n	Au/verre	ntal Experime	Theoretical	n
	Co/Fe	1.715 (± 0.252)	1	9	Co/Fe	0.105) 0.691 (±	1	6
	Cs/Fe	0.472 (± 0.066)	0.25	9	Cs/Fe	0.169) 0.368 (±	0.25	6

Table 2 .

 2 Means of elements ratios computed fromAu/mica samples. n stands for the number of samples studied.

DSC analysis conducted with identical experimental conditions as the one presented figure7(3 cycles from

°C to 230 °C using a 1 °C/min ramp).2 Structural datas of crystals from toluene solution were obtained by a post-doctorant (R. Plamont) of ERMMES team.

Binding energy (eV)Binding energy (eV) Binding energy (eV)

Appendix 6. 1 H-1 H COSY spectrum of Cs  {Fe(Tp)Co( tbu PhTp)}ClO4 in CD2Cl2 at 300K. Appendix 5. 1 H-1 H COSY spectrum of Cs  {Fe(Tp)Co( tbu PhTp)} in CD2Cl2 at 300K.

Abbreviations

ACN

Another remarkable features is the large splitting observed in these systems between the oxidation and the reduction waves. This is typical of slow redox systems, which undergo significant structural reorganization because the the reduction/oxidation is associated to a spin-state change. Here, once the Co II (HS) get oxidized it is immediately converted to a Co III (LS) species, and conversely, once the Co III (LS) species is reduced, they are converted to Co II (HS) species.

Further studies need to be carried out on the different cubes to compare the influence on the scorpionate ligands on the redox potential of the Fe and Co metal ions. At the present stage we were not able to draw clear trends because we observed that in the cubes the position of the Co redox wave are strongly dependent on the experimental conditions. 10. Electrochemical datas of the three cobalt complexes. components are observed at 782 and 797 eV, respectively with an intensity ratio of 2:1. These values as well as the distance between Co 2p3/2 and Co 2p1/2 peaks, 15 eV, are in good agreement with the literature regarding Co(III) species. 28 Moreover, peaks are well-defined with identical values of FWHM in the powder and in the electropolymerized thin-films. This indicates that the chemical environment of elements is preserved and after deposition.

XPS analysis of Cs

The presence of shake-up satellites, between the two doublet peaks, has been attributed to the occurrence of ligand-to-metal charge transfer during the photoemission process. 29 The small shoulders on Co 2p3/2 and Co 2p1/2 peaks basis (780 eV and 795 eV respectively) were assigned to Co(II). The unexpected observation of Co(II) might be associated to some ligand-to-metal charge transfer taking place under X-rays irradiation. (a) (b) Consequently, cyclic potential sweeps did not result in the electropolymerization of the Cs  {Fe(Tp)Co(3-TPhTp)}PF6 cube on Pt substrate. With platinium as a working electrode, potential are slightly shifted to higher potential. In this case, we assume that thiophene oxidation wave is no longer accessible which makes the electropolymerization process inefficient. This observation is also supported by the absence of a clear oxidation wave associated to an oligothiophene species in voltammogram figure 11b as observed in the voltammogram of Cs  {Fe(Tp)Co(2-TPhTp)}PF6 compound (figure 11a). Moreover, theoretical and experimental Co/Fe and Cs/Fe molar concentration ratios of the cube elements showed a good agreement (Table 4). This suggests that the Fe-Co

Cs 3d-Fe 2p Co 2p

Fe II cycles. Substrates were screwed on a nonmagnetic sample holder which was entered into the air-lock chamber and transfered into the SLS cryomagnet. For XAS the isotropic absorption cross-section is taken as σIso=(σ -+σ + )/2. By following the variation XMCD intensity as a function of the external magnetic field, one can measure element-specific magnetization curves.

Magnetization curves are measured by using the EMPHU-65 undulator which allows fast switching (5 Hz) of the X-ray circular helicity.

Compound syntheses

Safety remarks.

Some of the experiments involved hazardous products that should be handled with care.

If some hazard statements are provided just below, the reader is however strongly recommended to consult the full safety data sheet of the corresponding products. 4 As for all products, users should at least wear primary safety equipment (i.e. safety gloves, eye protection and lab coat).

Specific cautions: BBr3 is is toxic and highly corrosive ! n-Butyllithium is waterreactive and pyrophoric liquid. POBr3 is an unstable explosive and also reacts violently with water. Thallium salts are extremely toxic and are harmful if swallowed (through repeated exposure). Cyanide salts should be carefully neutralize as their contact with acids liberates very toxic gas.

All reactions and manipulations of air-sensitive compounds were carried out in dry, oxygene-free atmosphere using standard Schlenck techniques. The combined organic extracts were dried with anhydrous Na2SO4. The solvent was removed under vacuum and 2,6-dibromoisonicotinic acid was obtained as a slightly brown powder .

Synthesis

(Yield 7.0 g, 86%). 4 Safety documentation is available on www.sigmaaldrich.com

Synthesis of 1-Trimethylsilyl-4-tert-butylbenzene

1-Trimethylsilyl-4-tert-butylbenzene (according to ref with minor modifications). 5 A solution of 1-Bromo-4-tert-butylbenzene (10,65 g, 50 mmol) in dry THF (30 mL) was added dropwise, under argon atmosphere, on a suspension of Mg turnings in dry THF (20mL). A small amount of iodine (2 crystals) was added to initiate the reaction. The mixture was stirred at room temperature for 3 hours (control of the reaction can be performed by NMR). The solution was cooled to 0°C, using an ice-bath and trimethylsilylchlorid was added dropwise.

At the end of the addition, the ice-bath was removed and the reaction was stirred for 2 additional hours at room temperature. The mixture was then heated under reflux for 24 hours. After

Synthesis of 1-dibromoboryl-4-tert-butylbenzene

1-Dibromoboryl-4-tert-butylbenzene (according to litterature) 6 1-Trimethylsilyl-4-tert-butylbenzene (1.03 g, 5.03 mmol) was dissolved in DCM (5mL), under argon atmosphere, and BBr3 ( 5.48 mL, 1M in DCM) was added dropwise. The round-bottomed flask was covered with aluminium and stirred at room temperature for 2 hours.All the volatile materials were then removed under vacuum. The desired compound was obtained as a colourless liquid after distillation (106°C, 5.10-2 torr ).

(Yield 1,1g, 73%) A solution of bis(dimethylamino)-boryl-4-tert-butylbenzene in dry toluene (30 mL) was added dropwise on a suspension of pyrazole ( 0.871g, 12.66 mmol) and NaPz ( 0.570g, 6.33 mmol) in 70mL of dry toluene. The reaction mixture was heated to reflux for 16h. Then, the suspension was concentrated under reduced pressure and was filtered hot on a sintered glass filter. The precipitate was washed with toluene (3x 20 mL) and hexane (3x20 mL). The resulting solid was dried under vacuum at 50 °C for 30min. The desired compound was obtained as a white solid.

(Yield 1.7 g, 75%) A solution of K2[Fe II ( tbu PhTp)(CN)3] (230 mg, 0,41 mmol) was dissolved in water (5 mL) and tetra butylammonium bromide (132 mg, 0,41 mmol) was added. Hydrogen peroxyde (30%, 2mL) was added dropwise. Then the reaction was covered with aluminium foil and stirred overnight at room temperature. The orange precipitate was then filtered and washed with water (3x 10mL).

The solid was dissolved in MeOH. Crystals of the desired compound were produced by slow evaporation of the methanol solution.

(Yield 0.239g, 80%) (Yield 0.180g, 57%) of stirring, CsI (0.414 g, 1.6 mmol) was added and the reaction was stirred for 5 additional minutes. tbu PhTp (0.220 g, 0.6 mmol) was added and the reaction was stirred at room temperature for 1 day. Then, the resulting suspension was centrifugated, the supernatant was transfered and evaporated. The obtained blue solid was washed several times with a mixture of DMF/Et2O (2:8) until the solution was clear. The blue solid was dissolved in CH2Cl2 and filtered to remove a red precipitate. The filtrate was evaporated, the obtained solid was dissolved in a mixture of CH2Cl2/ Et2O (7:3) and the suspension was filtered (to remove salts).

This treatment was repeated 2 more times. Then the solvent was evaporated, the solid was dissolved in ACN (5 mL), filtered and the filtrate was evaporated. Suitable crystals of the compound for XRD were obtained by slow evaporation of a solution in ACN. Cs-FetpCo( tbu PhTp) was dissolved with an excess of NaClO4 in CH2Cl2. The mixture was stirred at room temperature for 24h. The solution was filtered to remove a yellow impuriy. The solvent was evaporated and th solid was recrystallized in ACN. If necessary, the solid was washed with a mixture of DMF/Et2O (2:8) several times before recrystallization.

Synthesis of Cs

[4-(2-Thienyl)phenyl](trimethyl)silane (according to ref with minor modifications)

A solution of 2-bromothiophene (4.89 g, 30 mmol) in dry THF (30 mL) was added dropwise to a suspension of Mg turnings (0.73g, 30mmol) under argon atmosphere at 0°C. After complete addition, the mixture was stirred at room temperature for 3 hours. The 2-thienylmagnesium bromide obtained was added dropwise to a solution of (4-bromophenyl)trimethylsilane (6.2g, 27 mmol) and Pd(dppf)Cl2 (0,11 g, 0,15 mmol) in anhydrous THF at 0°C. Then the ice-bath was removed and the reaction was stirred at room temperature for 24 hours.

At the end of the reaction, 75mL of water were added followed by 100 mL of THF. The layers were separated and the organic layer was washed with water (2x 50mL). The combined organic layers were dried over sodium sulfate and evaporated. The product was purified by column chromatography on silica gel (eluent : toluene) to afford the pure desired compound.

(Yield 5.02 g, 72%) Hz, 1H, Thio), 0.27 (s, 9H, SiMe3) ppm.

Synthesis of Na[2-TPhTp]

Na[2-TPhTp]: BBr3 (1M in DCM, 18 mL) was added dropwise to a stirred solution of ([4-(2-Thienyl)phenyl](trimethyl)silane (3.5 g, 15mmol) in dichloromethane (5 mL) and the solution was stirred at room temperature for 4h, under argon atmosphere. After removal of all the volatile materials a yellow solid was obtained. The resulting solid was dissolved in dichloromethane (10 mL) and Me3SiNMe2 (7 mL, 43.7 mmol) was added dropwise at 0°C. The mixture was warmed to room temperature over 1h and stirred for 4 additional hours.

C19H16BN6SNa MW: 394.11

All the volatiles were removed under reduced pressure and the yellow residue dried under vacuum for 3h. The resulting solid was dissolved in dry toluene (70 mL) and added on a suspension of pyrazole (2.2 g, 32 mmol) and Napz (1.4 g, 16 mmol) in toluene (50mL) at room temperature, under argon atmosphere. Then, the reaction was then heated to reflux for 16h.

Then the suspension was concentrated under reduced pressure and was filtered hot on a sintered glass filter. The precipitate was washed with toluene (3x20 mL) and hexane (3x20 mL). The resulting solid was dried under vacuum at 50 °C for 30min. The desired compound was obtained as a white solid.

(Yield 3,8 g, 64%)

Synthesis of [Fe(2-TPhTp)2]

[Fe II (2-TPhTp)2] : Fe(BF4)2,6H2O 180 mg, 0,53 mmol) was dissolved in MeOH (2 mL) with a small amount of ascorbic acid (2 mg) and added dropwise to a solution of 2-TPhTp (420 mg, 1.06 mmol) in ACN (20mL). The mixture was stirred for 15 minutes at room temperature. A pink solid was filtered and washed twice with ACN (2x 5mL). The solid was dissolved in DCM (5 mL), the solution was then filtered and evaporated.

Pink crystals of the desired compound were obtained by slow evaporation in 1,2dichloroethane.

(Yield 320 mg, 70%) (Yield 20mg, 35%) until the colour of the reaction changed from pink to yellow (it can take several days i.e. 4-5).

After cooling the solvent was evaporated under vacuum. The resulting solid was dissolved in acetonitrile and filtered. The compound was obtained as a yellow powder.

A solution of K2[Fe II ( tbu PhTp)(CN)3] (230 mg, 0,41 mmol) was dissolved in water (5 mL) and tetra butylammonium bromide (132 mg, 0,41 mmol) was added. Hydrogen peroxyde (30%, 2mL) was added dropwise. Then the reaction was covered with aluminium foil and stirred overnight at room temperature. The orange precipitate was then filtered and washed with water (3x 10mL).

The solid was dissolved in MeOH. Crystals of the desired compound were produced by slow evaporation of the methanol solution. days. Then, the resulting suspension was centrifugated, the supernatant was transferred and evaporated. The obtained blue solid was washed several times with a mixture of DMF/Et2O

(2:8) until the solution was clear. The blue solid was dissolved in CH2Cl2 and filtered to remove a red precipitate. The filtrate was evaporated, the obtained solid was dissolved in a mixture of CH2Cl2/ Et2O (7:3) and the suspension was filtered (to remove inorganic salts). This treatment was repeated 2 more times. Then the solvent was evaporated, the solid was dissolved in ACN (5 mL), filtered and the filtrate was evaporated. The compound was obtained as deep blue powder. the addition of solution of aq. Na2CO3 (20mL) , the mixture was degassed for 20min. Pd(PPh4) ( 0.231 g, 0.2 mmol) was then added and the reaction was stirred under reflux for 1 day. After cooling down to room temperature, the mixture was poured into 75 mL of water and 100mL of toluene. The organic phase was separated, washed with water (2x40 mL), dried over sodium sulfate and filtered. The solvent was evaporated and dried under vacuum for 2h. After purification by column chromatography on silica gel (eluent : toluene), the desired compound was obtained as white solid.

(Yield 1,17g, 50%) (Yield 3,72 g, 70%)

1 H NMR (300MHz, C6D6) : 2,39 (s, Me) ppm.