Dr Matthew Conley

Daniel Silverio

Dr Marc Renom Carrasco

Olivier Ouari

Le Pr

Paul

Dr David Gajan

Bénédicte

Gilles

Cécile G Torsten

Andrea Kévin

Marta Tobias

Jan

Elodie, Dorothéa Daniella Kristaps

Diane Guido

Ladislav, Sylvie, Houda, Manhal, ... Olivier

Eloïse Merci À Maëlle

Mathilde Arthur

Laure-Lise Benjamin

Antoine Nils

la vie du laboratoire. Merci donc à Au

Avant de rentrer dans le coeur de cette thèse, je tiens ici à remercier l'ensemble des personnes qui rendu ce travail possible, directement ou indirectement. Plus précisément, je remercie tout d'abord l'ensemble des membres de mon jury de soutenance : le Pr Reimer et le Dr Massiot qui ont accepté d'évaluer mon manuscrit en plein été ; le Dr Blackledge, la Dr Andraud, le Pr Mueller qui ont accepté de participer à mon jury de soutenance. Je remercie ma co-encadrante de thèse, la Dr Anne Lesage et mon directeur de thèse, le Pr Lyndon Emsley. Je les remercie pour leur confiance, pour la liberté qu'ils m'ont laissé dans mes travaux, pour le niveau d'équipement exceptionnel du CRMN à lequel ils m'ont permis d'avoir accès, et pour m'avoir proposé et guidé dans les projets intéressants qui sont présentés dans ce manuscript. Je vous remercie en particulier pour m'avoir appris pendant ces trois années, l'importance de la collaboration scientifique qui rend le travail humainement très intéressant et scientifique plus solide. Plus précisément je retiendrais qu'en Science, personne ne peut, ni ne sait tout faire mais qu'il est primordial de pouvoir identifier ce que l'on peux faire. Anne, je te remercie pour m'avoir accompagner quasi-quotidiennement au cours de cette thèse, pour la qualité de ton encadrement, et pour ta patience infinie à m'expliquer et me réexpliquer des expériences de RMN.

methods are available for surface science and give valuable information, they only lead to a partial understanding of surfaces at the molecular level. Moreover, these methods are not compatible with all kinds of materials and usually require the use of a model and pristine surface.

Solid-State NMR would be a method of choice to characterize surfaces. However, the approach suffers from its intrinsically low sensitivity and this is strongly emphasize in the case of surfaces where the atoms of interest are diluted in the matrix. Dynamic Nuclear Polarization (DNP) applied to surfaces (SENS) recently emerged as a very promising method to characterize surface sites. It offers a dramatic enhancement of NMR sensitivity and DNP applied to materials has led to many examples in the last ten years.

In the present thesis, I have shown that DNP SENS, in combination with EXAFS, allowed the detailed 3D structure determination of the silica-supported organometallic complex determined with a precision of 0.7 Å.

In parallel, some experimental aspect of DNP SENS have been explored. A spin diffusion has been developed to understand diffusion of hyperpolarization in porous media. A new aqueous DNP matrix, coined DNP Jelly, has been developed to characterize nanoparticles and thus expanding experimental range of DNP SENS. Finally, the first experiment of DNP NMR at fast magic angle spinning (up to 40 kHz) and high field are reported.

Résumé de la thèse

La capacité à déterminer les structures moléculaires en trois dimensions à partir de monocristaux par des méthodes de diffraction a transformé la chimie des matériaux. Le problème de la détermination de structure est en grande partie non résolu, en particulier si le système étudié est situé à une surface et n'a pas de périodicité, comme dans la plupart des matériaux fonctionnels actuels.

La Résonance Magnétique Nucléaire (RMN) à l'état solide serait une méthode de choix pour caractériser les surfaces mais la limite de détection de la RMN est beaucoup trop faible pour permettre à la RMN de caractériser les surfaces. L'introduction récente d'une nouvelle approche utilisant la Polarisation Dynamique Nucléaire (DNP) pour améliorer la sensibilité de la RMN des surfaces (DNP SENS) permet à présent de réaliser des expériences qui étaient totalement impossible il y a quelques années encore.

Plus particulièrement, grâce à la méthode DNP SENS, les présents travaux de thèse aboutissent à la première structure tridimensionnelle d'un complexe organométallique supporté sur silice, avec une précision de 0,7 Å.

De nombreux aspects de l'expérience DNP SENS ont été exploré. Le

Introduction

En chimie, la capacité à comprendre les propriétés des systèmes chimiques reposent sur leurs descriptions détaillées au niveau moléculaire. Au cours du dernier siècle, plusieurs méthodes comme la spectroscopie infrarouge (IR), la diffraction des rayons X (XRD)... ont été développées, permettant une compréhension de la structure de nombreux matériaux. Cependant, lorsque l'on a l'intention de décrire les espèces de surface au niveau atomique, le problème devient plus délicat et plusieurs questions clés Introduction récents et les dernières applications sont examinés. En particulier l'expérience dite DNP SENS est présenté, il s'agit d'une méthode très robuste pour la caractérisation des surfaces en RMN.

Après cette introduction, les résultats obtenus au cours des trois années de mes études doctorales sont présentés en deux parties séparées. La première partie est axée sur les aspects pratiques et instrumentaux de l'approche DNP SENS. L'objectif est de mieux comprendre le processus DNP lorsqu'il est exploité pour les surfaces et pour améliorer les conditions expérimentales et la formulation de l'échantillon, dans laquelle cette approche peut être appliquée. Cette première partie comprend trois chapitres. Tout d'abord, deux aspects fondamentaux de l'imprégnation de DNP SENS ont été étudiés: (i) la diffusion de l'agent polarisant dans le milieu poreux et (ii) le rapport entre le volume de solution utilisé pour l'imprégnation et la masse d'agent polarisant.

Cela a permis une meilleure compréhension de l'approche DNP SENS, en particulier, il semblerait que le processus de diffusion de spin joue un rôle clé dans DNP SENS. Un modèle de diffusion de spin de l'hyperpolarisation DNP adapté à une matrice poreuse est présenté.

Dans le deuxième chapitre, les premières expériences combinant la rotation à l'angle magique jusqu'à 40 kHz et les hauts champs magnétiques (800 MHz) en conditions DNP sont réalisées. L'effet de la rotation à l'angle magique sur le gain DNP est étudié. On observe que le gain DNP est maintenue jusqu'à 40 kHz. Cependant, une importante perte de signal a été observée à une fréquence de rotation élevée.

Enfin, dans le troisième chapitre, une nouvelle matrice DNP aqueuse, DNP Jelly, est proposée. Cette formulation est basée sur un gel de polyacrylamide. L'utilisation de DNP Jelly se révèle appropriée pour la caractérisation des nanoparticules CdTe présenté dans ce même chapitre.

Dans la deuxième partie, je montre que la détermination de la structure tridimensionnelle d'un complexe organométallique sur une surface de silice amorphe utilisant des mesures RMN à l'état solide est possible. La détermination de la structure est rendu accesible grâce à une approche combinant la spectroscopie RMN exaltée par polarisation nucléaire dynamique avec l'EXAFS. Le résultat est une structure détaillée du complexe

Introduction

In chemistry, the ability to understand the properties of chemical systems relies on their detailed description at the molecular level. Over the last century, several methods like infrared (IR) spectroscopy, mass balance analysis, X-ray diffraction (XRD)... were developed, allowing a structure-based understanding of many materials. However, when one intends to describe the surface species at the atomic level the problem becomes trickier, and several key questions often remain unanswered. Chemical processes that occur at a surface play a crucial role in modern chemistry. Many questions arise when exploring surfaces of modern materials, related to the description of adsorption-desorption processes, functionalization (structure and dynamics of surface functionalities), reactions at the surface, or diffusion of chemicals. In heterogeneous catalysis, the catalyst -often in the solid phase -is involved through its surface in the chemical process.

Currently, although an extensive range of surface-sensitive methods are available for surface science and give valuable information, they only lead to a partial understanding of surfaces at the molecular level. Moreover, these methods are not compatible with all kinds of materials and usually require the use of a model and pristine surface. Among the most recent development to provide an atomic-level description of surfaces, in 2015, de Oteyza et al. have used state-of-the-art non-contact Atomic Force Microscopy (nc-AFM) method to directly image covalent bond of adsorbates onto a Ag(100) surface, thus providing insights in the description of Bergman reaction of formation of cycle. [START_REF] De Oteyza | Direct Imaging of Covalent Bond Structure in Single-Molecule Chemical Reactions[END_REF] Adding solid-state nuclear magnetic resonance (SSNMR) to the chemist's toolbox for studying surfaces would improve future development in surface science as SSNMR is, in principle, a method of choice to assess such questions. [START_REF] Blanc | High resolution solid state NMR spectroscopy in surface organometallic chemistry: access to molecular understanding of active sites of well-defined heterogeneous catalysts[END_REF] Indeed, SSNMR has shown its capability to characterize many solid systems ranging from protein fibrils to polymers or pharmaceuticals and is commonly used for the study of materials. Moreover, SSNMR can be used to reveal critical dynamical information, making the approach not only suitable for the structure but also relevant for the study of complex chemical processes.

While SSNMR would be a method of choice for the characterization of surfaces, the approach suffers from its intrinsically low sensitivity. Moreover, the study of surface species where the atoms of interest are diluted in the matrix strongly emphasize this sensitivity issue. As a consequence SSNMR is often inefficient to characterize surfaces of many modern materials. Dynamic Nuclear Polarization (DNP) applied to surfaces recently emerged as a very promising method to characterize surface sites. It offers a Chapter 1 dramatic enhancement of NMR sensitivity by transferring polarization from unpaired electrons under cryogenic conditions. DNP has raised considerable attention for the last two decades. Application of DNP to materials has led to many examples in the last ten years.

In the present thesis, my interest is in characterizing surfaces from SSNMR enhanced with the DNP method. In particular, my aim is to demonstrate that the threedimensional structure of surface sites can be fully resolved.

In the first chapter of the present thesis, I have reviewed the characterization of single well-defined sites supported on surfaces using SSNMR. First, some important aspects of surface organometallic chemistry are reported, since this is an important route in chemistry to obtain single well-defined surface sites. I have then described the methods used in conventional SSNMR for the characterization of such materials. Finally, as the sensitivity issue is strongly exacerbated in the study of surface, the hyperpolarization approach by Dynamic Nuclear Polarization is analyzed. Basics concepts, recent developments, and the latest applications are reviewed, in particular the approach, coined DNP Surface Enhanced NMR Spectroscopy (DNP SENS), is presented, which is a very robust method for the characterization of surfaces.

After this introduction, the results obtained during the three years of my doctoral studies are presented in two separated parts. The first part is focused on practical and instrumental aspects of the DNP SENS approach, to better understand the DNP process itself when exploited for surfaces and to improve the experimental conditions/the formulation, in which this approach can be applied. This first part includes three chapters.

First, two fundamental aspects of impregnation in DNP SENS have been investigated: (i) the diffusion of the polarizing agent in the porous medium and (ii) the ratio between the volume of solution used for impregnation and the weight of the material. This has allowed for a better understanding of the DNP SENS approach, in particular, it appeared that spin diffusion process plays a key role in DNP SENS resulting in the development of a spin diffusion model of DNP hyperpolarization adapted to porous matrix.

In a second chapter, the first experiments combining fast magic angle spinning (up Introduction to 40 kHz) and DNP at high field (800 MHz) are reported. The effect of fast spinning on the DNP enhancement is investigated. DNP enhancement is maintained up to 40 kHz magic angle spinning (MAS) rate. However, an important loss of signal due an increase quenching has been observed at a high spinning rate.

Finally, in the third chapter, a new aqueous DNP matrix, coined DNP Jelly, is proposed. It is based on a polyacrylamide gel. The use of DNP Jelly is demonstrated to be suitable for the characterization of CdTe nanoparticles.

In the second part, I have shown that the determination of the three-dimensional structure of an organometallic complex on an amorphous silica surface using solid-state NMR measurements is possible. Structure determination is enabled through a dynamic nuclear polarization surface enhanced NMR spectroscopy approach. The result, in combination with EXAFS, is a detailed structure of the surface complex determined with a precision of 0.7 Å. A single well-defined conformation that is folded toward the surface in such a way as to include an interaction between the metal center and the surface atoms was observed.

Chapter 2

Introduction

In the present chapter, I review the recent SSNMR development for the study of surfaces. This chapter focuses on the ability of SSNMR to reveal an atomic level description of active sites on chemically functionalized surfaces with catalytic relevance.

The first part of this chapter is dedicated to Surface Organometallic Chemistry (SOMC) and Supported Homogeneous Catalysis (SHC). The idea of this first section is not to give an exhaustive description of the field but rather an overview of this important part of modern chemistry which has raised a critical attention in the chemists' community. In the second part, I report examples of the atomic description of surfaces using conventional SSNMR experiments. Note that Marchetti et al. very recently published an excellent review of studies conducted with SSNMR on surfaces and interfaces. [START_REF] Marchetti | Understanding Surface and Interfacial Chemistry in Functional Nanomaterials via Solid-State NMR[END_REF] As one of the pitfalls of characterizing surfaces from SSNMR is its intrinsically low sensitivity, in the third part, I concentrate on the description of state-of-the-art developments in hyperpolarization from Dynamic Nuclear Polarization (DNP) for SSNMR under Magic Angle Spinning (MAS). The review is stressed on the description of surfaces where DNP hyperpolarization is used to reveal surface structural and dynamical characteristics.

Well Defined Immobilized Heterogenous Catalysts

Generalities

In an industrial context, the use of heterogeneous catalysis is very relevant, as this class of catalysts usually simplifies purification steps. For instance, in 2015, Tsubogo et al. published [START_REF] Tsubogo | Multistep continuous-flow synthesis of (R)-and (S)-rolipram using heterogeneous catalysts[END_REF] the synthesis of (R)-Rolipram, an anti-inflammatory pharmaceutical, using a continuous flow system with multiple chemical reaction steps. The synthesis was achieved without the separation of any excess reactants, by-products, or catalysts. This was enabled by the use of a series of continuous flow reactors packed with different heterogeneous catalysts to ensure both chemo-and enantio-selectivities of the reaction, and a decent overall yield. This kind of process is economically relevant for the pharmaceutical industry, as it simplifies purifications steps, but also ecologically relevant as it should reduce the use of big amounts of organic solvents in purification steps. Here, the use of efficient and optimized heterogeneous catalysts is crucial in the success of Surface structure investigations in solid-state NMR such an approach as (i) they simplify the catalyst elimination step; (ii) avoid complex purification of the final products.

Thus there is an important interest in obtaining and optimizing heterogeneous catalysts. However, active sites characterization of heterogeneous catalysts is still challenging and never easy. SOMC and SHC are popular strategies to obtain supported single site catalysts on a surface where active sites are covalently immobilized onto a surface. These methods aim to "bridge the gap" [START_REF] Copéret | Homogeneous and Heterogeneous Catalysis: Bridging the Gap through Surface Organometallic Chemistry[END_REF] between surface science and molecular organometallic chemistry. Indeed, organometallic chemistry provides powerful tools to allow rational design of homogeneous organometallic catalysts from molecular description. [START_REF] Astruc | Organometallic Chemistry and Catalysis[END_REF] Having well-defined organometallic sites on a surface permits taking advantage of both tools of molecular organometallic chemistry and robustness of heterogeneous catalysts. In SOMC, well-defined surface catalytic sites are obtained by the reaction between an organometallic fragment and a surface. In SHC, a homogenous catalytic center is anchored to a surface via a pre-existing linker. [START_REF] Copéret | Strategies to Immobilize Well-Defined Olefin Metathesis Catalysts: Supported Homogeneous Catalysisvs[END_REF] For the last decades, various surface types have been used as supports for welldefined surface sites such as silica, [START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF] alumina, [START_REF] Trueba | γ-Alumina as a Support for Catalysts: A Review of Fundamental Aspects[END_REF] aluminosilicates, [START_REF] Machado | Aluminosilicate obtained by sol-gel process as support for an anionic iron porphyrin: Development of a selective and reusable catalyst for oxidation reactions[END_REF] etc. Silica is among the most popular support for single well-defined surface sites in heterogeneous catalysis. [START_REF] Copéret | Homogeneous and Heterogeneous Catalysis: Bridging the Gap through Surface Organometallic Chemistry[END_REF] Due to both the flexibility of the Si-O-Si angle and rigidity of O-Si-O angle, silica exists in a large number of different phases as represented in Figure 2.1. It can be non-porous, with at least fourteen known polymorphs, [START_REF]The Colloid Chemistry of Silica[END_REF][START_REF] Rimola | Silica surface features and their role in the adsorption of biomolecules: computational modeling and experiments[END_REF], or also gives dense non-periodic glass phase. It can also form porous materials, that can be periodic or non-periodic. However, in the context of catalysis, amorphous silica is usually the most widely used support in its porous or non porous forms. [START_REF] Rimola | Silica surface features and their role in the adsorption of biomolecules: computational modeling and experiments[END_REF] There is a considerable number of examples of well-defined catalytic centers supported on silica for various types of catalyzed reactions, such as metathesis, hydrogenation, Ziegler-Natta polymerization, hydrogenolysis, isomerization, epoxidation, to cite few examples; extensively reviewed in 2016 in [START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF].

While at the premises of SOMC, surfaces was intuited to act as classical ligand, it is now well established that surfaces adds some particularities. In particular, the surface is a rigid ligand and significantly reduces the mobility of organometallic species.

For example, silica surface allows for the stabilization of electron-deficient Ta(III) or Zr(IV) complexes, at oxidation number that has not be observed before in homogeneous Chapter 2 organometallic chemistry. [START_REF] Basset | On the Origins and Development of Surface Organometallic Chemistry in Mod[END_REF][START_REF] Quignard | Surface organometallic chemistry: synthesis and characterization of a tris(neopentyl)zirconium(IV) complex grafted to the surface of a partially dehydroxylated silica[END_REF][START_REF] Thieuleux | Methane activation by silica-supported Zr(IV) hydrides: the dihydride [(SiO)2ZrH2] is much faster than the monohydride [(SiO)3ZrH[END_REF] Figure 2.1: Different solid phases involving {SiO 4 } units. Reprinted with permission from [START_REF] Rimola | Silica surface features and their role in the adsorption of biomolecules: computational modeling and experiments[END_REF]. Copyright 2013 American Chemical Society.

Silica-based materials

As silica-based materials will be the focus of an important part of this thesis, some general features of this class of materials are therefore reported hereinafter. SOMC mainly uses amorphous silica in the form of non-porous particles or porous materials.

The most common way to synthesize silica is to start from a silica precursors such as tetraethylorthosilicate (TEOS) or tetramethylorthosilicate (TMOS), which condensate to afford silica materials via the so-called sol-gel approach. In the 90s, very efficient methods have been described to obtain mesoporous silicas, which exhibit a dense network of pores and therefore a large surface area. MCM-41, [START_REF] Beck | A new family of mesoporous molecular sieves prepared with liquid crystal templates[END_REF][START_REF] Kresge | Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism[END_REF] SBA-15, [START_REF] Zhao | Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores[END_REF] and SBA-16 are, [START_REF] Zhao | Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures[END_REF] for instance, popular mesoporous silica matrices. Mesoporous silica is obtained using the sol-gel approach in the presence of a structure directing agent (SDA).

The SDA first self-assembles in solution to form a liquid-crystal phase that works as a template for the formation of the porous network during the condensation of silica.

Control of silanol density

Under typical laboratory condition, silica surfaces exhibit two major functional groups: silanols (Si-OH), and siloxane bridges (Si-O-Si). Both can also be present in the bulk, but they are not accessible. On silica surface, silanols are found isolated or as geminal or vicinal silanols (see Figure 2.3). Si atoms bound to one OH are denoted as Q 3 (isolated and vicinal), Si atoms bound to two OH groups are named Q 2 (germinal). Q 4 denotes a silicon without OH group and involved in four Si-O-Si bridges. [START_REF] Rimola | Silica surface features and their role in the adsorption of biomolecules: computational modeling and experiments[END_REF] As such structure-templated sol-gel processes are performed under aqueous conditions, the existence of surface silanols cannot be avoided, however, their density can be controlled using de-hydroxylation by heating the silica under reduced pressure. [START_REF] Zhuravlev | The surface chemistry of amorphous silica. Zhuravlev model[END_REF] Strategies to functionalize silica surfaces

The first common strategy to functionalize silica surfaces is the post-synthetic functionalization route. The presence of silanols allows introducing a catalytic center, by a post-modification of the silica surface. This can be achieved for example by a reaction between surface silanols and organometallic precursors to afford the formation of a Chapter 2 surface organometallic fragment. [START_REF] Basset | Surface Organometallic Chemistry in Compr[END_REF][START_REF]Modern Surface Organometallic Chemistry[END_REF] Figure 2.4 lays out an example of a rhenium catalyst supported on silica SiO 2-(700) (silica partially at dehydroxylated at 700 • C) obtained through the reaction between surface silanols and an organometallic complex.

An alternative post-modification approach is to graft functionalities on the silica surface via a reaction with organosilanes (R 1 O) 3 SiR 2 , chlorosilanes ClSiR 3 , or silazanes HN(SiR 3 ) 3 . [START_REF] Hoffmann | Silica-Based Mesoporous Organic-Inorganic Hybrid Materials[END_REF] A second common strategy to incorporate functionalities to the silica surface is the use of co-condensation. In this approach, the silica is synthesized via condensation of tetraalkoxysilanes (e.g. TEOS or TMOS) and trialkoxyorganosilanes (e.g. (EtO) 3 SiR), thus the organic moiety R is directly incorporated on the surface. This strategy has the advantage of allowing a more homogenous distribution of the functionalities on the surface. [START_REF] Conley | Mesostructured Hybrid Organic-Silica Materials: Ideal Supports for Well-Defined Heterogeneous Organometallic Catalysts[END_REF] Tuning hydrophobicity of silica surfaces Surface silanols give silica a hydrophilic character because it enables the possibility of formation of hydrogen bonds between the surface and water molecules. Thus, it is possible to make the surface hydrophobic by capping silanols with hydrophobic organic moieties. That is usually achieved by reaction of surface silanols with chlorosilane, e.g. tetramethylsilyl chloride. Surface passivation also allows avoiding side-reactions with the surface silanols during the formation of the active center on the surface. For instance, Conley et al. explain that the passivation of surface silanols was required to use the reagents necessary to obtain an Ir metal carbene supported on the surface without sidereaction with surface silanols. [ The reference for chemical shift is tetramethylsilane. Data from [START_REF] Maciel | Silicon-29 Nuclear Magnetic Resonance Study of the Surface of Silica Gel By Cross Polarization and Mangic-Angle Spinning[END_REF] Cross-polarization (CP), was introduced by Hartmann and Hahn [START_REF] Hahn | Nuclear Double Resonance in the Rotating Frame[END_REF] in 1962 and was significantly improved by Lurie and Slichter two years later [START_REF] Lurie | Spin Temperature in Nuclear Double Resonance[END_REF] and then first successfully demonstrated for natural abundance 13 C NMR in 1972. [START_REF] Pines | Proton-Enhanced Nuclear Induction Spectroscopy. A Method for High Resolution NMR of Dilute Spins in Solids[END_REF][START_REF] Pines | Proton-enhanced NMR of dilute spins in solids[END_REF] The approach relies on the magnetization transfer from sensitive high gyromagnetic ratio nuclei to less sensitive low gyromagnetic nuclei. The method uses the dipolar coupling to perform the transfer.

As the dipolar coupling scales by r -3 , where r is the distance between the two nuclei, it provides a very efficient way to selectively polarize surfaces in organic-inorganic materials (such as functionalized silica) where the bulk does not contain any protons. In Lee-Goldburg (FSLG) decoupling scheme, they did not observe any resolution improvements and attributed the remaining signal broadening to inhomogeneous contributions and chemical exchange. [START_REF] Trébosc | Solid-state NMR study of MCM-41-type mesoporous silica nanoparticles[END_REF] Fast MAS also allowed them to implement a Carr-Purcell-Meiboom-Gill (CPMG) detection scheme to improve the sensitivity during the acquisition of 1 H- 29 Si HETCOR spectrum at natural abundance. [START_REF] Trebosc | Studies of Organically Functionalized Mesoporous Silicas Using Heteronuclear Solid-State Correlation NMR Spectroscopy under Fast Magic Angle Spinning[END_REF] In 2007, the same research group examined 1 H-detected schemes, based on the development of Ishii and Tycko, [START_REF] Ishii | Sensitivity Enhancement in Solid State 15N NMR by Indirect Detection with High-Speed Magic Angle Spinning[END_REF] for the acquisition of to be more sensitive because of the higher gyromagnetic ratio of the 1 H compared to 13 C nuclei. The authors reported a sensitivity gain of about 3.8, which is near to the theoretical value of 4. [START_REF] Wiench | Chemical shift correlation NMR spectroscopy with indirect detection in fast rotating solids: studies of organically functionalized mesoporous silicas[END_REF] The approach was also successfully applied on mesoporous silica nanoparticles functionalized with a rhodium phosphine complex. [START_REF] Rapp | A solid-state NMR investigation of the structure of mesoporous silica nanoparticle supported rhodium catalysts[END_REF] In contrast to these two examples using through-space correlation, a through-bond correlation experiment with 1 H detection has been reported on organically functionalized mesoporous silica nanoparticles. [START_REF] Mao | Indirectly detected through-bond chemical shift correlation NMR spectroscopy in solids under fast MAS: Studies of organic-inorganic hybrid materials[END_REF] This has been enabled by a sequence based on refocused INEPT HETCOR with 1 H detection, as represented on . [START_REF] Mao | Conformations of Silica-Bound (Pentafluorophenyl)propyl Groups Determined by Solid-State NMR Spectroscopy and Theoretical Calculations[END_REF] Later, the same group demonstrated that when PFP functionalized mesoporous silica nanoparticles were impregnated with heptane, only the PFP-p conformation was found. Thus, contact between hydrophilic surface silanols and heptane was avoided. [START_REF] Tsai | Rational Catalyst Design: A Multifunctional Mesoporous Silica Catalyst for Shifting the Reaction Equilibrium by Removal of Byproduct[END_REF] SSNMR was shown to be suited for studying the dynamics of grafted surface molecules.

In 2008, Blanc et al. studied the dynamics of a series of organometallic complexes supported on silica and found that the motions of the grafted molecules were not isotropic, likely due to the tethering of the molecules to the surface. They also discovered that the dynamics of the supported metal complex was dependent on the metal center. Reprinted with permission from [START_REF] Wiench | Chemical shift correlation NMR spectroscopy with indirect detection in fast rotating solids: studies of organically functionalized mesoporous silicas[END_REF]. Copyright 2007 American Chemical Society. Copyright 2009 American Chemical Society.

In all these studies, SSNMR has proven to be a very powerful method to access both structural and dynamical properties of surface sites. However, isotopic enrichment and/or long accumulation times are necessary, mainly due to the poor sensitivity of SSNMR exacerbated by the dilution of the investigated molecules in the sample. Various approaches have been developed to address this sensitivity issue: higher magnetic fields, CPMG acquisition, or 1 H detection. In parallel, the use of a molecular surface models to mimic the surface, such as polyhedral oligomeric silsesquioxanes (POSS) (as surface silanol models) have been developed [START_REF] Quadrelli | Molecular Insight for Silica-Supported Organometallic Chemistry through Transition Metal Silsesquioxanes in Mod[END_REF] and used for example in [START_REF] Chabanas | Perhydrocarbyl Re VII Complexes: Comparison of Molecular and Surface Complexes[END_REF][START_REF] Blanc | Surface versus Molecular Siloxy Ligands in Well-Defined Olefin Metathesis Catalysts: [(RO)3SiOMo(NAr)(CHtBu)(CH2tBu)[END_REF]. In two decades, computational modeling of surfaces has become an increasingly important tool to complement the experimental data. Despite these efforts, sensitivity remains a critical bottleneck.

In the following section, I explain how hyperpolarization by Dynamic Nuclear Polarization in NMR can address the sensitivity issue for surface NMR spectroscopy. Reprinted (adapted) with permission from [START_REF] Mao | Conformations of Silica-Bound (Pentafluorophenyl)propyl Groups Determined by Solid-State NMR Spectroscopy and Theoretical Calculations[END_REF]. Copyright 2010 American Chemical Society.

2.4 Hyperpolarizing surfaces in NMR using Dynamic Nuclear Polarization

Introduction

Dynamic Nuclear Polarization (DNP) takes advantage of the gyromagnetic ratio of the electron which is 658 times higher in magnitude than the gyromagnetic ratio of 1 H nuclei. The thermal polarization of electrons in a magnetic field is thus considerably greater than nuclear polarization. DNP allows transferring this high polarization to nuclei and thus to observe nuclear spin polarization at a much higher level than that achieved by the Boltzmann equilibrium (so-called hyperpolarization). DNP is not a new discovery in the field of NMR; in 1953, Overhauser proposed a theoretical model

showing that hyperpolarized metal nuclei can be obtained based on a cross-relaxation process between nuclei and saturated unpaired electrons. [START_REF] Overhauser | Polarization of Nuclei in Metals[END_REF] This was experimentally demonstrated by Carver and Slichter a few months later. [START_REF] Carver | Polarization of Nuclear Spins in Metals[END_REF] Conducting electrons of metallic lithium were saturated resulting in a 7 Li NMR signal enhancement of about 100. Although the first experiment was promising, efficient Overhauser Effect (OE) DNP demands short correlation times of the electrons spins and is thus mainly limited to mobile electrons in conductors or liquids. Solid Effect (SE) [START_REF] Abragam | Une nouvelle méthode de polarisation dynamique des noyaux atomiques dans les solides[END_REF] and Cross Effect (CE) [START_REF] Kessenikh | Proton Polarization in irradiated polyethylenes[END_REF] were discovered a few years later, but they unfavorably scaled with the magnetic field. As high-power and high-frequency microwaves sources were not available at that time, DNP NMR has not been considered as a relevant field of NMR development for several decades. [START_REF] Rossini | One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition[END_REF] The field reborn during the 90s with the pioneering work of Robert G. Griffin at the Massachusetts Institute of Technology (MIT) by combining newly available gyrotrons able to produce high-frequency and high-power microwaves to the design of DNP low temperature (LT) magic angle spinning (MAS)

probes. [START_REF] Becerra | A Spectrometer for Dynamic Nuclear Polarization and Electron Paramagnetic Resonance at High Frequencies[END_REF][START_REF] Gerfen | High frequency (140 GHz) dynamic nuclear polarization: Polarization transfer to a solute in frozen aqueous solution[END_REF][START_REF] Griffin | Clear signals from surfaces[END_REF] DNP NMR in the solid state is now a field in constant effervescence, and has been applied to a large number of systems ranging from small molecules in frozen solvents, [START_REF] Gerfen | High frequency (140 GHz) dynamic nuclear polarization: Polarization transfer to a solute in frozen aqueous solution[END_REF] proteins, [START_REF] Andreas | Dynamic Nuclear Polarization Study of Inhibitor Binding to the M2 18-60 Proton Transporter from Influenza A[END_REF][START_REF] Akbey | High-Temperature Dynamic Nuclear Polarization Enhanced Magic-Angle-Spinning NMR[END_REF][START_REF] Akbey | Structural biology applications of solid state MAS DNP NMR[END_REF][START_REF] Ni | Peptide and Protein Dynamics and Low-Temperature/DNP Magic Angle Spinning NMR[END_REF] crystals and organic powders, [START_REF] Van Der Wel | Dynamic Nuclear Polarization of Amyloidogenic Peptide Nanocrystals: GNNQQNY, a Core Segment of the Yeast Prion Protein Sup35p[END_REF][START_REF] Rossini | Dynamic Nuclear Polarization NMR Spectroscopy of Microcrystalline Solids[END_REF][START_REF] Mollica | Quantitative structural constraints for organic powders at natural isotopic abundance using dynamic nuclear polarization solid-state NMR spectroscopy[END_REF] functionalized surfaces, [START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF][START_REF] Lelli | Fast Characterization of Functionalized Silica Materials by Silicon-29 Surface-Enhanced NMR Spectroscopy Using Dynamic Nuclear Polarization[END_REF] nanoparticles, [START_REF] Hope | Surface-selective direct 17 O DNP NMR of CeO 2 nanoparticles[END_REF] microporous organic polymers, [START_REF] Blanc | Dynamic Nuclear Polarization NMR Spectroscopy Allows High-Throughput Characterization of Microporous Organic Polymers[END_REF] etc.

In particular, in the course of the thesis, MAS DNP NMR in the solid-state has been found to be particularly powerful to characterize surfaces. [START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF][START_REF] Rossini | Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF] Thus, historical and Surface structure investigations in solid-state NMR more recent concepts in MAS DNP are reported in the current section. One can note that DQ and ZQ transitions are dominated by the fluctuation of the electron-nuclear coupling (hyperfine and dipolar couplings). The sign of the enhancement is determined by the ratio of the two relaxation rates: W 2 < W 0 results in a positive nuclear enhancement and W 2 > W 0 in a negative enhancement. [START_REF] Maly | Dynamic nuclear polarization at high magnetic fields[END_REF][START_REF] Can | Mechanisms of dynamic nuclear polarization in insulating solids[END_REF] To be efficient, the OE requires the fluctuations of the hyperfine coupling at a frequency close to the Larmor frequency of the electron which was found possible in metals with free conducting electrons and in liquids. [START_REF] Carver | Polarization of Nuclear Spins in Metals[END_REF][START_REF] Griesinger | Dynamic nuclear polarization at high magnetic fields in liquids[END_REF] In insulating solids, OE was not expected to be very efficient. Surprisingly, in 2014, Can et al. were however able to observe sizeable OE DNP with the BDPA radical (see The Solid Effect. The Solid-Effect is a second DNP mechanism discovered in 1958. [START_REF] Abragam | Une nouvelle méthode de polarisation dynamique des noyaux atomiques dans les solides[END_REF] Similarly to the OE, it can be described by the same two-spin system involving a nucleus and an electron represented in Figure 2.12. However, instead of saturating the electron, continuous microwaves irradiation occurs at the forbidden DQ or ZQ transitions.

Chapter 2 Similarly to the previous effect, saturation of ZQ or DQ will lead to a positive or negative enhancement. The principle of SE is that non-secular nuclear-electron coupling is responsible for a partial mixing of the two-spin (unperturbed) system thus resulting in partially allowing the DQ/ZQ transitions and driving the polarization transfer from the electron to the nucleus. As shown in Figure 2.12(c), saturation of the ZQ transition results in negative enhancement whereas, as depicted in 2.12(d), the saturation of the DQ transition results in positive nuclear enhancement.

SE is observed with BDPA. Indeed the 1 H enhancement magnetic field dependance curves reported in Figure 2.13 shows positive and negative enhancements due to the SE. [START_REF] Can | Overhauser effects in insulating solids[END_REF] SE corresponds to the positive and negative satellites peak when microwaves irradiation saturate the ZQ and DQ transitions. SE is also observed on the organic radical trityl OX063 (see Figure 2.12(f ) for the structure). To be efficient, the EPR linewidth should not exceed the nuclear Larmor frequency, otherwise positive and negative SE can occur at the same time and hence, cancel each other. SE has not attracted considerable attention in high-field DNP NMR as the efficiency of the transfer drops dramatically with the magnetic field (ω 0 ): it scales with the permanent field as ω -2 0 . [START_REF] Corzilius | Solid effect in magic angle spinning dynamic nuclear polarization[END_REF][START_REF] Maly | Dynamic nuclear polarization at high magnetic fields[END_REF] The Cross Effect. The Cross-Effect is a DNP mechanism first evidenced in 1963. [START_REF] Kessenikh | Proton Polarization in irradiated polyethylenes[END_REF] In non-rotating sample and under high fields, the CE mechanism can be understood as followed: let us consider a three-spin system constituted of two electrons and one nucleus.

In (B) shows the effect of saturation of the EPR transition of e1 at ω 0,e1 and (C) gives the distribution after CE transitions occured. Adapted from [START_REF] Maly | Dynamic nuclear polarization at high magnetic fields[END_REF].

CE is usually performed with radicals exhibiting an inhomogeneously broadened EPR line, where the width of the EPR line is larger than the Larmor frequency of the nucleus.

Chapter 2

Thus the CE can occur when two radicals close in space with the correct g-tensor orientations give EPR resonances that match the CE condition ω 0,I = |ω 0,e1 -ω 0,e2 |. [START_REF] Maly | Dynamic nuclear polarization at high magnetic fields[END_REF][START_REF] Hu | High-frequency dynamic nuclear polarization using mixtures of TEMPO and trityl radicals[END_REF] Figure 2.17 shows the EPR line TEMPO radical compared to the DNP enhancement measured on the system as a function of the magnetic field. The DNP transfer is dominated by the CE mechanism, and the two maxima correspond to optimized negative and positive CE conditions. In this model, the CE mechanism scales with the inverse of the magnetic field as the width of the inhomogeneously broadened EPR line scales (in Hz) with the magnetic field leading to a decreasing spectral density and lower microwave saturation efficiency. [START_REF] Hu | High-frequency dynamic nuclear polarization using mixtures of TEMPO and trityl radicals[END_REF] The CE mechanism under MAS has been intensively discussed recently. In particular,

Thurber

Chapter 2

There is a fourth DNP mechanism coined Thermal Mixing. However, this mechanism is out of the scope of the present thesis as it is not significantly active in the conditions used for MAS DNP at low temperature (≈ 100 K).

Solvents in MAS DNP

As the materials studied by MAS DNP do not usually contain unpaired electrons, the sample needs to be doped with stable free radicals. These are typically stable organic radicals in aqueous media (e.g. water/glycerol) or organic solvents (e.g. 1,1,2,2tetrachloroethane). MAS DNP experiments are usually performed at low temperature (LT) around 100 K and solvents are thus frozen. LT experiments are used because they lead to longer electron relaxation times and thus facilitate the saturation of the electrons with continuous microwaves leading to higher enhancements. [START_REF] Gerfen | High frequency (140 GHz) dynamic nuclear polarization: Polarization transfer to a solute in frozen aqueous solution[END_REF] An important property of the solvent is to avoid the aggregation of radicals upon cooling the sample to approx. 100 K. Radical aggregation would result in the formation of electron-concentrated regions leading to shortening of the electron relaxation times and thus to reduced DNP enhancement. [START_REF] Gajan | Hybrid polarizing solids for pure hyperpolarized liquids through dissolution dynamic nuclear polarization[END_REF] Hence, glassy solvents are usually used in MAS DNP experiments such as water/DMSO, [START_REF] Hu | Dynamic Nuclear Polarization with Biradicals[END_REF] water/glycerol. [START_REF] Sauvée | Highly efficient, water-soluble polarizing agents for dynamic nuclear polarization at high frequency[END_REF] In 2012, Zagdoun et al.

compared different organic solvents for MAS DNP applications. They found that the best non-aqueous systems rely on halocarbon solvents such as 1,1,2,2-tetracholoroethane or 1,2-dichloroethane. [START_REF] Zagdoun | Non-aqueous solvents for DNP surface enhanced NMR spectroscopy[END_REF] Interestingly, the protonated frozen solvent provides a solid matrix, so-called DNP matrix, where 1 H-1 H spin diffusion, driven by 1 H homonuclear couplings, can efficiently distribute the hyperpolarization through the entire sample. Spin diffusion is particularly relevant as in most MAS DNP experiments direct DNP transfer occurs between the electron and 1 H and then 1 H hyperpolarization is transferred via CP to heteronuclei.

1 H-1 H spin diffusion numerical models of Van Der Wel et al. [START_REF] Van Der Wel | Dynamic Nuclear Polarization of Amyloidogenic Peptide Nanocrystals: GNNQQNY, a Core Segment of the Yeast Prion Protein Sup35p[END_REF] and later Rossini et al. [START_REF] Rossini | Dynamic Nuclear Polarization NMR Spectroscopy of Microcrystalline Solids[END_REF] have demonstrated the importance of this process in MAS DNP experiments. Some experimentations have been performed without the use of a solvent. For example, De Paëpe, Hediger, and co-workers have developed a matrix-free approach for MAS DNP where the polarizing agents are anchored to the sample of interest using a gluing agent. They demonstrated the applicability of the method for microcrystalline cellulose, lysozyme, and liposomes. [START_REF] Takahashi | Rapid Natural-Abundance 2D 13 C-13 C Correlation Spectroscopy Using Dynamic Nu-clear Polarization Enhanced Solid-State NMR and Matrix-Free Sample Preparation[END_REF][START_REF] Takahashi | Matrix-free dynamic nuclear polarization enables solid-state NMR 13C-13C correlation spectroscopy of proteins at natural isotopic abundance[END_REF][START_REF] Fernández-De Alba | Matrix-Free DNP-Enhanced NMR Spectroscopy of Liposomes Using a Lipid-Anchored Biradical[END_REF] Surface structure investigations in solid-state NMR Recently, the suppression of the solvent resonances in DNP MAS experiments has been proposed. As described in reference [START_REF] Yarava | Solvent suppression in DNP enhanced solid state NMR[END_REF], coherence lifetimes of frozen solvent (TCE or water/glycerol) are usually significantly shorter than those of the materials of interest (surfaces and microcrystalline powders in their studies), and thus solvent signals can be filtered out using spin echoes or spin-lock. In [START_REF] Lee | Solvent signal suppression for highresolution MAS-DNP[END_REF], the authors propose to use deuterated solvent and reintroduce 2 H-13 C dipolar couplings of the deuterated solvent to quickly dephase 13 C solvent signals.

Rational development of DNP polarizing agents

Development of optimized radicals has attracted wide attention from the NMR community in the last decade. The general objective here is to approach the maximum theoretical DNP enhancement of 658 for 1 H nuclei. The development of polarizing agents has mainly focused on CE DNP as the scaling of SE with the magnetic field is strongly unfavorable. For 1 H OE DNP in insulating solids, so far no other efficient radicals, except BDPA and its derivative SA-BDPA are known. The structure of the polarizing agent discuss in the current section are depicted in is slightly better than TEKPOL. [START_REF] Kubicki | Rational design of dinitroxide biradicals for efficient cross-effect dynamic nuclear polarization[END_REF] The careful analysis of this exhaustive series of biradicals lead to the conclusion that at a moderate magnetic field (400 MHz), the polarizing agent -by itself -does not prevent from reaching the maximum theoretical DNP enhancement and that other parameters should be identified to reach this maximum. For example, in reference [START_REF] Kubicki | Amplifying Dynamic Nuclear Polarization of Frozen Solutions by Incorporating Dielectric Particles[END_REF], the same authors observed that the DNP enhancement can be doubled by incorporating dielectrics particles (e.g. KBr) in the rotor, thus achieving a better distribution of the microwave field in the sample. shows that this expected CE in the mixture was actually observed. [START_REF] Hu | High-frequency dynamic nuclear polarization using mixtures of TEMPO and trityl radicals[END_REF] The same idea was used in reference [START_REF] Michaelis | High-field 13C dynamic nuclear polarization with a radical mixture[END_REF] where a mixture of trityl and BDPA was investigated. In this paper, the difference between the two narrow line EPR frequencies matches with ). [START_REF] Mathies | Efficient Dynamic Nuclear Polarization at 800 MHz/527 GHz with Trityl-Nitroxide Biradicals[END_REF] The influence of the tether has been considered. In particular they introduced TEMTriPol-1, represented in Figure 2.17(d), which generated a 1 H enhancement of 65 at 800 MHz, and more importantly they observed that this 1 H enhancement scaled favourably with the magnetic field. Although the electron-electron interaction in the TEMPO/trityl mixture is a dipolar interaction, the authors' calculations showed that there was a very high electron-electron exchange interaction in TEMTriPol-1. The authors correlated the important enhancement observed for TEMTriPol-1 and its field dependance to this unusual exchange interaction. Usually, in binitroxide radicals, the electron-electron dipolar coupling drove the CE transition, and the exchange interaction is negligible. The authors noted that this strong exchange interaction was only efficient at a higher field as they attributed CE DNP enhancement at low field (211 MHz) to intermolecular dipolar interactions between nitroxide units for TEMTriPol-1.

Finally, note that metal complexes are currently under investigation in the group of

Chapter 2

TEMTriPOL-1 Oschkinat and co-workers proposed to use high temperatures to improve the resolution of proteins studied by MAS DNP NMR, which is one of the major issues preventing a larger adoption of DNP for structural biology in solid-state NMR. [START_REF] Akbey | High-Temperature Dynamic Nuclear Polarization Enhanced Magic-Angle-Spinning NMR[END_REF] Using TOTAPOL in the glycerol/water matrix, they observed an important loss of DNP performance but also the concomitant improvement of NMR resolution.

(d)

NMR signal quenching in MAS DNP

For the moment, the quantification of the efficiency of MAS DNP experiments was limited to the sole evaluation of the DNP enhancements factor ε. However, it was shown that ε is not sufficient to reflect the quality of the MAS DNP experiments. This topic is discussed in the following section.

In 2012, Rossini et al. introduced the concept of overall sensitivity enhancement. [START_REF] Rossini | One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition[END_REF] In MAS DNP, the common way to measure the DNP performance is to evaluate ε as the ratio of the signal intensity with microwaves and the signal intensity without microwaves. In principle, this is not sufficient to evaluate the sensitivity gain between SSNMR experiment performed with a dry sample at room temperature. Indeed, paramagnetic species are introduced, thus inducing a partial loss of NMR signals due to paramagnetic quenching. Also, the effect of temperature and the recycling delay needs to be taken into account. In [START_REF] Rossini | One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition[END_REF] the overall sensitivity enhancement was then defined as:

Σ † = ε × θ × T 1 T DN P 298 K 100 K (2.2)
where ε is the DNP enhancement, T 1 the spin-lattice relaxation time measured on an undoped sample at room temperature, T DN P is the build-up time measured on a doped sample under MAS DNP conditions with microwaves. θ is the contribution factor, i.e. In reference [START_REF] Kobayashi | Analysis of sensitivity enhancement by dynamic nuclear polarization in solid-state NMR: a case study of functionalized mesoporous materials[END_REF] a similar expression was introduced to correctly quantify the sensitivity in the MAS DNP approach. While the previous equation 2.2 can reflect the overall sensitivity gain in MAS DNP experiments, it does not provide insights into the mechanism that leads to the reduction of the NMR signal upon radical addition. In particular, in 2014, the concept of depolarization when biradicals are used in combination with magic angle spinning has emerged. [START_REF] Mentink-Vigier | Nuclear depolarization and absolute sensitivity in magic-angle spinning cross effect dynamic nuclear polarization[END_REF][START_REF] Corzilius | Paramagnet induced signal quenching in MAS-DNP experiments in frozen homogeneous solutions[END_REF][START_REF] Thurber | Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves[END_REF] In [START_REF] Mentink-Vigier | Nuclear depolarization and absolute sensitivity in magic-angle spinning cross effect dynamic nuclear polarization[END_REF], a depolarization factor ε depo was defined as the ratio between the integrated NMR intensity of the sample under rotation at the magic angle and the static sample. (≈ 25 K). [START_REF] Thurber | Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves[END_REF] This effect has been interpreted as a loss of polarization compared to Boltzmann equilibrium due electron-electron dipolar rotor events when CE polarizing agents are present. [START_REF] Thurber | Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves[END_REF][START_REF] Mentink-Vigier | Nuclear depolarization and absolute sensitivity in magic-angle spinning cross effect dynamic nuclear polarization[END_REF] Finally, I finish this general section about MAS DNP NMR by mentioning the very most recent development in the elaboration of pulsed DNP schemes in the next section.

Pulsed Dynamic Nuclear Polarization

In the previous sections, all the results reviewed rely on continuous wave DNP where the sample is continuously irradiated with monochromatic microwave beam. The Griffin Group at the MIT is currently investigating the possibility to use pulsed microwaves and implementation of pulsed DNP schemes. The major motivation is that this approach could, in principle, circumvent the unfavorable field dependence of CW DNP used today.

However, this demands instrumentation development, as no commercial systems are available today to conduct these experiments.

The NOVEL experiment (Nuclear Spin Orientation via Electron Spin Locking) introduced in 1988; is in principle similar to Hartmann-Hahn Cross Polarization where two different spins are spin-locked in a double rotating frame. [START_REF] Henstra | Nuclear spin orientation via electron spin locking (NOVEL)[END_REF][START_REF] Henstra | The theory of nuclear orientation via electron spin locking (NOVEL)[END_REF] In the original NOVEL paper in 1988, the authors were able to obtain a nuclear enhancement of about 10 using a single crystal of silicon doped with boron acceptors in a 0.28 T magnet. [START_REF] Henstra | Nuclear spin orientation via electron spin locking (NOVEL)[END_REF] Can et al. recently re-visited the NOVEL experiment and applied it to current DNP systems. At 270 K and 0.35 T, an enhancement of 323 was observed with BDPA dissolved in partially deuterated polystyrene. [START_REF] Can | Time domain DNP with the NOVEL sequence[END_REF] Enhancements of 430 was obtained at 80 K and 0.28 T with the radical trityl OX063. [START_REF] Mathies | Pulsed Dynamic Nuclear Polarization with Trityl Radicals[END_REF] In 2017, the RA-NOVEL (Ramped-amplitude NOVEL) experiment has been introduced, where a ramp shape is used for the microwave locking pulse, leading to an improvement of NOVEL by a factor of about 1.6. [START_REF] Can | Rampedamplitude NOVEL[END_REF] The recent revisit of the Integrated Solid Effect (ISE) can also be mentioned here, originally introduced in 1988, [START_REF] Henstra | Enhanced dynamic nuclear polarization by the integrated solid effect[END_REF] modified to FS-ISE (Frequency Swept ISE).

The current section was very general and dedicated to introducing general concepts and recent developments of the MAS DNP method. In the next section, MAS DNP applied to surfaces is analyzed in more detail.

MAS DNP applied to Surfaces

The DNP SENS approach

In a pioneering experiment in 2010, MAS DNP was used to analyse a phenolfunctionalized SBA-15 silica, using a novel approach named DNP SENS (DNP Surface Enhanced NMR Spectroscopy). The incipient wetness impregnation technique was used to infuse the porous material with a polarizing agent solution (TOTAPOL dissolved in glycerol/water in that case). [START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF] As represented in Figure 2.20, the continuous irradiation of the sample with microwaves allows the transfer of electron polarization to adjacent 1 H, then

DNP SENS formulation

While DNP SENS were promising, it was still far from being optimized and thus providing the highest DNP sensitivity gains possible. Moreover, although the development of new polarizing agents for MAS DNP offered more and more efficient hyperpolarization, there were additional parameters that needed to be taken into account when MAS DNP was applied to materials and not only to bulk solutions (pure solution of polarizing agent) and it is reported in the next paragraphs.

Biradical concentration. The biradical concentration was the first and the most straightforward parameter to optimize. In reference [START_REF] Zagdoun | A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials[END_REF], to ensure the highest sensitivity gain for surface signal, the authors measured the overall sensitivity gain Σ † Si as function of the bTbK concentration. As shown in Silica surface passivation. As described in the section 2.2.2, as-synthetized silica exhibits surface silanols that can be post-modified to tune the hydrophobicity of the surface. In reference [START_REF] Zagdoun | Improved Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy through Controlled Incorporation of Deuterated Functional Groups[END_REF], the influence of different passivating groups has been investigated, they are shown in Figure 2.23. In particular, the presence of methyl groups has a substantial impact on the DNP performance: an enhancement ε Si,CP = 9 was measured when the surface was passivated with trimethylsilyl whereas ε Si,CP reached 74 using deuterated trimethylsilyl passivating groups. The increase of the enhancement has been interpreted as methyl groups acting as relaxation sinks that prevent high DNP enhancements.

DNP SENS on porous materials

As functionalized porous silica is a versatile material class, described in section 2.2 of this manuscript, the beginning of DNP SENS has focused on the description of porous silica. For example, DNP have allowed following chemical reactions on surfaces using The overall sensitivity enhancement Σ † Si takes the temperature into account, whereas Σ Si does not taking it. Reprinted with permission from [START_REF] Zagdoun | A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials[END_REF]. Copyright 2012 ACS. 13 C, 15 N CPMAS and 2D HETCOR spectra, as reported in Figure 2.22. In 2013, Samantaray et al. studied a ruthenium complex immobilized on SBA-15 silica. They were able to indirectly observe interactions between the metal and the silica surface when a flexible linker was used to anchor the organometallic complex to the surface. However, they found that a rigid linker prevents this interaction. These interactions were assumed to be able to increase the chemical lifetime of the supported Ru complex. [START_REF] Samantaray | Evidence for Metal-Surface Interactions and Their Role in Stabilizing Well-Defined Immobilized Ru-NHC Alkene Metathesis Catalysts[END_REF] In 2015, similar interactions were qualitatively observed on an Ir(I) catalyst immobilized on SBA-15. These stabilizing interactions were believed to rationalize the exceptional capability of the supported Ir(I) catalyst to realize the hydrogenation of trans-stilbene whereas its molecular analog was far less active. [START_REF] Romanenko | Iridium(I)/N-Heterocyclic Carbene Hybrid Materials: Surface Stabilization of Low-Valent Iridium Species for High Catalytic Hydrogenation Performance[END_REF] DNP SENS was also applied to Metal Organic Frameworks (MOFs). The latter are an important class of materials attracting considerable attention for their possible application in gas storage, gas separation, and heterogeneous catalysis. They consist of a crystalline 3D network where organic ligands are connected via metallic centers. [START_REF] D'alessandro | Carbon Dioxide Capture: Prospects for New Materials[END_REF] In [START_REF] Rossini | Dynamic nuclear polarization enhanced solid-state NMR spectroscopy of functionalized metal-organic frameworks[END_REF], DNP SENS enabled the use of { 1 H}-15 N CPMAS at natural 15 N isotopic abundance on N-functionalized MOFs. Interestingly, the authors noted the DNP enhancement ε evolved from 7 to 13 by simply allowing the non-functionalized MOFs -impregnated with a bCTbK solution in TCE -sample to rest at room temperature for 25 h. They did not observe a similar effect on the functionalized MOFs. The authors hypothesized that the polarizing agent (bCTbK) was blocked outside the pores due to its size and can slowly diffuse in the porous materials in the case of non-functionalized MOFs, however, this diffusion was hindered by organic moieties in N-functionalized MOFs. Moreover, as the radicals did not enter entirely into the material, the authors suggested that this Reprinted with permission from [START_REF] Kobayashi | DNP-Enhanced Ultrawideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal-Organic Frameworks[END_REF]. Copyright 2016 ACS.

Characterization of materials with low surface area and increasingly diluted species with DNP SENS

The exploration of materials has not been limited to porous materials with relatively high surface areas. Most recent developments in MAS DNP have revealed important structural aspects of materials exhibiting lower and lower surface areas, and on very diluted surface species where the sensitivity issue is strongly emphasized.

In reference [START_REF] Sangodkar | Influences of Dilute Organic Adsorbates on the Hydration of Low-Surface-Area Silicates[END_REF], DNP SENS allowed getting insights in the adsorption of sucrose on low surface area Ca 3 SiO 5 particles and revealed the consequences of this adsorption on the hydration properties of the particles.

Nanoparticles (NPs) also lead to low surface area but have attracted important attention due to their vast potential. DNP enhanced NMR would be a method of choice to study these insensitive systems. In [START_REF] Piveteau | Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF], it is shown that DNP SENS experiments can be carried out on NPs. The authors reported the analysis of oleate-capped 4 nm CdSe quantum dots (QDs). They observed that DNP SENS on these QDs cannot be directly performed on a solution containing the suspended QDs and the dissolved po-

MAS DNP for the observation of insensitive nuclei

In NMR on materials, there is a high interest to not be limited to spin-1/2 only, and relatively sensitive nuclei such as 1 H, 13 C, or 15 N because of the different wealth of elements in use in material science.

Oxygen, for instance, is found in many materials. for the characterization of a large variety of materials, in particular for the description of surfaces. However, these approaches can only be applied on materials that are compatible with the radicals. MAS DNP fails when the polarizing agent reacts with the surfaceactive sites. For example, in reference [START_REF] Samantaray | Evidence for Metal-Surface Interactions and Their Role in Stabilizing Well-Defined Immobilized Ru-NHC Alkene Metathesis Catalysts[END_REF], DNP SENS could not be applied successfully due to the reaction between the polarizing agent TOTAPOL and the ruthenium active center.

Currently, two strategies have been proposed to avoid the degradation of both the polarizing agent and the active sites. A first strategy is to study confined active sites in porous materials the pore of which are small enough to avoid the penetration of the radical, as demonstrated in 2017 by Pump et al. [START_REF] Pump | Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy[END_REF] An alternative strategy, maybe more versatile, consists in designing polarizing agents in which the free radical center is protected from the material by bulky organic groups. For example, in [START_REF] Liao | Dendritic polarizing agents for DNP SENS[END_REF], the use particles with a DNP enhancement of about 10, whereas no enhancement was obtained by impregnation of the material with a TEKPOL solution in TCE.

Conclusions

To optimize heterogeneous catalysts in an industrial chemical context, one needs to elucidate the link between structure and properties. However, this is never easy in heterogeneous catalysis as there is a lack of appropriate analytical methods to characterize active sites at an atomic level. Single well-defined catalytic centers immobilized on surfaces, from SOMC or SHC, is a chemical answer to this problematic. The idea is that one can benefit from the help of the -well-established -principles of organometallic chemistry to design heterogeneous catalysts. Among all the supports that can be used to Adapted from [START_REF] Liao | Dendritic polarizing agents for DNP SENS[END_REF] is not enough to address the problem of structure-properties relationship of the active center. Thus there is still a need for analytical methods to assess the structure of supported species. SSNMR is an excellent candidate to characterize such materials. Some examples where SSNMR has been successfully used to describe single well-defined surface sites have been reviewed. However, the use of isotopic labeling is almost inevitable due to the poor sensitivity of SSNMR. DNP SENS is a hyperpolarization approach that can circumvent this obstacle. The latest development in the DNP MAS field and recent examples where this approach has been used to characterize supported surface sites have been described. All in all, well-defined immobilized catalysts obtained from chemical approaches such as SOMC or SHC combined with DNP SENS can address the need of obtaining optimized heterogeneous catalysts.

Chapter 3

Introduction

The first example of Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) has been carried out in 2010 by Lesage et al. [START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF]. Since then the development of the approach has been focused on improving the sensitivity gain provided by DNP on NMR studies of materials, exploring more and more challenging systems, for example, applying the approach to water-sensitive materials, [START_REF] Zagdoun | Non-aqueous solvents for DNP surface enhanced NMR spectroscopy[END_REF] to radicalsensitive material, [START_REF] Pump | Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy[END_REF][START_REF] Liao | Dendritic polarizing agents for DNP SENS[END_REF] or to increasing low surface area materials [START_REF] Sangodkar | Influences of Dilute Organic Adsorbates on the Hydration of Low-Surface-Area Silicates[END_REF] as it has been reported in the chapter 2 of this manuscript.

During my Ph.D. work, I focus on the characterization of well-defined organometallic catalysts supported on the mesoporous silica SBA-15 by DNP SENS. The latter is a class of silica material consisting of amorphous silica with a dense hexagonal network of cylindrical pores whose diameter usually ranges from 5 to 30 nm [START_REF] Zhao | Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores[END_REF] as represented in Figure 3.1. Different kinds of functionalities can be introduced by covalent binding molecules on the silica surface. Surface functionalization is enabled for example via postmodification of the SBA-15 surface or via a sol-gel approach as described in chapter 2.

Nowadays advanced synthetic methods allow to introduce increasingly complex active centers, but also allow to ensure a homogenous distribution of the functionalities, to control the surface concentration of these functionalities, and to tune the hydrophobicity of the surface. As many of materials studied in this thesis are supported on silica, some fundamental experimental aspect of DNP SENS on mesoporous silica are investigated in a first Experimental aspects of impregnation in DNP SENS of porous silica materials step. Sample preparation in NMR enhanced with DNP is one of the most important factors to guarantee the success of the experience. Without being exhaustive, some important experimental aspects are described here. Experiments reported here are carried out on functionalized SBA-15 silica type material. The conclusions obtained here are mainly linked to general properties of the porous silica matrix rather than to the grafted functionality itself.

In a DNP enhanced NMR experiment, the diamagnetic sample must first be doped with a polarizing agent, which is commonly achieved by impregnating the sample with a radical solution leading to trivial questions. The design of the radical used for CE DNP has resulted in bigger and bulkier polarizing agents as described in the section 2.4.2. Thus when impregnating a porous material with a radical solution, there is a possibility that the pore size prevents the penetration of the radical inside the porous network. This is the case for small pores (pore sizes of approx. 1 nm such as for SBA-16 or MOFs) [START_REF] Rossini | Dynamic nuclear polarization enhanced solid-state NMR spectroscopy of functionalized metal-organic frameworks[END_REF][START_REF] Zhao | Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures[END_REF]. However, the answer is not straightforward for SBA-15 silica type where the pore diameter usually ranges from 5 to 30 nm [START_REF] Zhao | Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores[END_REF] and usually around 5 to 7 nm for the SBA-15 samples studied in the present work. I, therefore, investigate the diffusion into the pores of different polarizing agents from the bTbK family (bTbK, bCTbK, TEKPOL, and TEKPOL2) [START_REF] Zagdoun | Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K[END_REF][START_REF] Kubicki | Rational design of dinitroxide biradicals for efficient cross-effect dynamic nuclear polarization[END_REF]. To do so, a spin diffusion model in porous materials is developed. It reveals that spin diffusion plays a significant role in DNP SENS experiments applied to SBA-15 and that these models can be used to obtain insights on the location of polarizing agents in the material. These aspects have a substantial impact on the DNP SENS experiment. Indeed, the results show that biggest radicals such as TEKPOL or TEKPOL2 are not able to diffuse inside the porous network of SBA-15 silica.

When impregnating material, another fundamental question that can arise is the influence of the solvent volume for a given weight of porous silica. Hence, the dependence of ratio between the volume of radical solution and the weight of the porous SBA-15 materials on the DNP SENS experiment is investigated. The results reveal the porefilling mechanisms of the solvent inside the porous network. Regarding the impact on the DNP SENS experiment, it shows that the radical solution volume used to impregnate porous materials should be carefully monitored to ensure the highest possible DNP sensitivity gain.

Chapter 3

Diffusion of the polarizing agent into mesoporous materials

In all this section, the impregnation of an SBA-15 silica material functionalized with phenylimadazolium is investigated, the material is represented in ple around the magic angle. [START_REF] Thurber | Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves[END_REF][START_REF] Corzilius | Paramagnet induced signal quenching in MAS-DNP experiments in frozen homogeneous solutions[END_REF] In this section, the mechanism responsible for the loss of NMR signal upon introduction of paramagnetic species is not essential, however it is important to note that these effects are local and thus the loss of signal only occurs in the direct environnement of the radical.

We measure the contribution factor θ as defined in reference [START_REF] Rossini | One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition[END_REF]. The contribution factor indicates the fraction of the sample, impregnated with the radical solution, that contributes to the NMR signal compared to a sample where materials are impregnated with a pure solvent. It accounts for all the effects described previously, i.e. signal quenching and nuclear depolarization. To quantify the fraction of the surface that contributes, 29 Si Q 4,4 ′ signals can be used as a probe for surface contribution. θ is obtained by dividing the 29 Si Q 4,4 ′ signal area per unit of mass of the impregnated material with a radical solution by the area of the same peak with a material impregnated with pure solvent. The contribution factor for different radical solutions are reported in Table 3.1.

We observe that the contribution factor is significantly lower for small polarizing radicals, bCTbK, and bTbK whereas it reaches 83 % for TEKPOL2 and 80 % for TEKPOL.

These observations suggest that bigger radicals cannot enter into the silica pores and consequently less signal from the surface is quenched as radicals remain outside. With smaller radicals such as bCTbK, bTbK, on the contrary, a smaller contribution factor reveals higher concentration of radicals in the pore resulting in more quenching of the surface signals. Note that the quenching factor itself is not sufficient to get a formal proof that TEKPOL and TEKPOL2 do not enter in the SBA-15 pores whereas bTbK and bCTbK do enter. Indeed, as stated before, the aromatic rings decorating the radicals Chapter 3 may change significantly the interactions between the surface and the radicals. Thus, in the following section, localization of the polarizing agent in porous materials is confirmed by investigating the spin diffusion of the enhanced magnetization inside the pores.

Spin diffusion simulations

In DNP experiments in the solid-state,1 H-1 H spin diffusion is a very important aspect. This is a flip-flop process that efficiently diffuses the 1 H hyperpolarization locally created by DNP around each radical through the sample. Thus spin diffusion allows obtaining enhanced NMR signals from sample regions which are not directly polarized.

Actually, 1 H-1 H spin diffusion is essential in DNP experiments as the signals in the direct neighborhood of the radical are quenched and then not observed [START_REF] Rossini | One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition[END_REF][START_REF] Zagdoun | A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials[END_REF].

In 2007, Van Der Wel et al. have shown that it is possible to polarize amyloidogenic peptide nanocrystals in a TOTAPOL solution from the outside of the core of the crystal.

[68] In addition, they have observed that the DNP enhancement is not constant with the recycling time of the experiment and that the enhancement as a function of time can be modeled by using the Fick's law, which can be applied for processes of nuclear To probe the heterogeneity of the radical distribution in mesoporous silica, the 1 H en-Experimental aspects of impregnation in DNP SENS of porous silica materials observe for the two bulkier radicals, that the 1 H enhancement starts to increase at short polarization times, reaches a maximum and finally decreases to its steady-state value.

However, for bTbK and bCTbK, the enhancement quickly decreases and reaches its final value within approximately 10 seconds.

Spin diffusion model for relayed DNP in porous materials

To rationalize these behaviors, the spin diffusion model used in 2012 by Rossini et al. [START_REF] Rossini | One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition[END_REF] needs to be modified as the enhancements measured on mesoporous silica impregnated with TEKPOL or TEKPOL2 solutions in TCE have never been observed

or predicted with this model.

At the mesoscopic scale, spin diffusion can be described using the Fick's second law:

∂P ∂t = ∇(D∇P ) + σ( r, t) (3.1)
Experimental aspects of impregnation in DNP SENS of porous silica materials polarization in slices at x = constant), it can be reduced to a one dimensional problem: P ( r, t) = P (x, t). Finally, assuming the diffusion coefficient D can only depend on x, the diffusion equation can be written:

∂P ∂t = D ∂ 2 P ∂x 2 + P (x, t) -P 0 (x) T 1 (x) (3.2)
We first need to set initial and boundary conditions for the partial differential equation.

• Initial conditions. The enhancement as a function of time is recorded with a saturation-recovery experiment. At time t = 0 the nuclear polarization is equal to zero in the whole sample:

∀ x P (x, t = 0) = 0 (3.3)
• Boundary conditions. The system we considered is closed from the outside, i.e. no polarization can enter, nor leave the system through the border. Thus the polarization flow j P (x, t) at x = 0 and x = L can be set to zero at all times:

∀ t j P (x = 0, t) = j P (x = L, t) = 0 ⇔ ∀ t -D ∇P (x = 0, t) = -D ∇P (x = L, t) = 0 ⇔ ∀ t ∂P ∂x (x = 0, t) = ∂P ∂x (x = L, t) = 0 (3.4)
In the porous system, we now need to take into account the distribution of the polarizing agent into the system by properly defining the functions x → T 1 (x) and x → P 0 (x). The functions are represented in Figure 3.5. The cylindrical tube of total length L is divided in two: the source, where the DNP hyperpolarization is created and, the target where the DNP hyperpolarization is transported by spin diffusion. The source is located at both edge of the silica pore on a width P L , simulating partial diffusion of the polarizing agent into the pore. Due to direct paramagnetic relaxation enhancement (PRE) effect, [START_REF] Schlagnitweit | Nanostructure of Materials Determined by Relayed Paramagnetic Relaxation Enhancement[END_REF] we consider a shorter T source 1 in the region where the radical is located and a longer one T target 1 in the region where there is only pure TCE. A continuous T 1 function is created using the hyperbolic tangent (tanh):

T 1 (x) = T source 1 + T target 1 -T source 1 2 (tanh p(x -P L ) + tanh p(L -x -P L )) (3.5)
where p defines the slope of the hyperbolic tangent and consequently the thickness of the interface between solvent containing polarizing agent and pure TCE. P 0 is given by the Boltzmann distribution and set to 1 when microwaves on or off. In the absence of microwaves in the source region, the effect of depolarization induced by

Experimental aspects of impregnation in DNP SENS of porous silica materials binitroxides is taken into account. P 0 is set to ε depo = 0.5 where the value of ε depo is chosen to be in line with litterature. [START_REF] Mentink-Vigier | Nuclear depolarization and absolute sensitivity in magic-angle spinning cross effect dynamic nuclear polarization[END_REF][START_REF] Corzilius | Paramagnet induced signal quenching in MAS-DNP experiments in frozen homogeneous solutions[END_REF][START_REF] Thurber | Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves[END_REF] The continuous function P 0 is created again with the use of the hyperbolic tangent:

P 0 (x) =              ε 0 + 1-ε 0 2 (tanh p(x -P L ) + tanh p(L -x -P L ))
if microwaves are on

ε depo + 1-ε depo 2 (tanh p(x -P L ) + tanh p(L -x -P L ))
if microwaves are off

(3.6)
The partial differential equation is solved numerically in MatLab to get the polarization in the system as function of space and time (x, t) → P (x, t). Finally, the NMR signals intensity is obtained by integrating the polarization all over the sample and the enhancement by dividing simulated polarization build-up with and without microwaves:

S(t) = L 0 P (x, t)θ(x)dx and ε(t) = S on (t) S of f (t) (3.7)
where θ is a function that accounts for partial quenching of NMR signals due to presence of radicals. As in the systems considered in this section the distribution of radical is expected to be heteregenous, θ is not supposed to be constant along the pore and can varies between 0 (total quenching) and 1 (total contribution). To be consistent with the previous definition the contribution function θ is also defined with the hyperbolic tangent function:

θ(x) = θ min + θ max -θ min 2 (tanh p(x -P Q ) + tanh p(L -x -P Q )) (3.8)
Note that here defining such a contribution function assumes that there are regions in the sample where we can observe 1 H spin diffusion but the signal is not directly observed in such regions. This is possible if paramagnetic interactions between nitroxide radicals resulting in a gradient of concentration along the pore. As the radical is dispersed in a bigger volume, we do not expect a quenching effect on 1 H signals as strong as for TEKPOL2 as we interpreted this effect to a high local concentration of TEKPOL2.

In reference [START_REF] Zagdoun | A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials[END_REF], the quenching measured as a function of the radical concentration The enhancements as a function of time for mesoporous silica impregnated with a solution of TEKPOL or bTbK in TCE exhibit similar behaviors as TEKPOL2 and bCTbK respectively. In principle, spin diffusion simulations can be easily adapted to these cases using similar parameters for the simulations.

All in all, the results and simulations report here show that bulkier polarizing agents, TEKPOL and TEKPOL2, are not able to diffuse in the entire length of the pores of SBA-15. On the contrary, smaller radicals investigate in this section, bTbK and bCTbK can enter and diffuse into the SBA-15 porous network. Preventing the diffusion of the radical in the pores allows to obtain high contribution factor from the silica surface and thus, combined with high surface contribution factor θ, the sensitivity of the DNP SENS experiment for functionalized SBA-15 silica type material is significantly high with TEKPOL or TEKPOL2. Using bTbK or bCTbK an important part of the NMR signal is loss due to paramagnetic quenching effects.

Partial Impregnation Dynamic Nuclear Polarization

Another, basic but important, experimental aspects of DNP SENS on mesoporous materials is the volume of solution used to impregnate the sample. In this section, we analyze the DNP enhancement obtained on passivated mesoporous silica SBA-15

Experimental aspects of impregnation in DNP SENS of porous silica materials impregnated with different volumes of a 16 mM bCTbK solution in TCE. We choose to work with bCTbK as, according to section 3.2, it diffuses more easily into the porous material, thus simplifying the analysis of the effect of partial impregnation on DNP SENS. as a function of the solution volume. The latter is measured relative to the pore volume obtained by nitrogen physisorption measurement: 2.05 cm 3 .g -1 using the Barrett-Joyner-Halenda (BJH) approach [START_REF] Barrett | The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms[END_REF] and 1.99 cm 3 .g -1 using the Brunauer-Emmett-Teller (BET) approach. [START_REF] Brunauer | Adsorption of Gases in Multimolecular Layers[END_REF] We first observe an increase of both ε H and ε29 Si, CP enhancements up to approximately the complete pore filling of the mesoporous material Chapter 3

(relative pore filling of 1). After the complete filling of the pore, when adding more solution to impregnate the material, both ε H and ε29 Si, CP enhancements reach a plateau at their maximum value.

In practice, the visual aspect of the sample clearly indicates if the impregnation volume exceeds or not the porous volume of the sample. While a dry powder is obtained when the impregnation volume is smaller than the porous volume, a wet paste-like sample is obtained when it is larger.

These curves clearly show that partial impregnation of mesoporous material results in lower DNP performance whereas over-impregnation of the sample does not affect the DNP efficiency. However, we note that the absolute sensitivity of the DNP SENS experiments will decrease when overloading the sample, as adding too much solution will decrease the amount of material that can be packed in an NMR rotor.

It is particularly interesting to observe that the shape of the DNP curve evolution clearly change when the volume of the polarizing solution matches the porous volume measured using nitrogen physisorption. Indeed, if nitrogen can adsorb on nearly the entire available silica surface and fills all the pores (including micro-pores) of mesoporous silica SBA-15, this is not necessary the case for TCE. Indeed, the wettability of the silica surface may not be the same, and the ability to access the whole available porous volume may be different from nitrogen. Here the results suggest that the porous volume of the material revealed by DNP using TCE as solvent matches extremely well with the volume measured with nitrogen physisorption. Adsorption measurements with TCE or organic solvents, in general, are possible but not straightforward as this requires a special BET instrument.

Though the porous volume of the material measured with TCE matches with the volume measured by nitrogen adsorption, it does not explain why we observe an increase of the enhancement when the pore filling is incomplete. In 2014, Kubicki et al.

reported that the introduction of finely ground KBr particles in a 16 mM TEKPOL solution in TCE resulted in almost two times higher DNP enhancements measured in bulk solution. [START_REF] Kubicki | Amplifying Dynamic Nuclear Polarization of Frozen Solutions by Incorporating Dielectric Particles[END_REF] They hypothesized that the high dielectric properties of KBr particles allowed for a better distribution of the microwaves field inside the sample thus increasing

Experimental aspects of impregnation in DNP SENS of porous silica materials the average field experienced by the sample and thus resulting in higher DNP enhancement. This effect is related to the relative dielectric constant of the particles compared to TCE (ǫ r (TCE) = 2.5 and ǫ r (KBr) = 4.9). Here we believe that we observe a reverse dielectric effect. Porous materials are currently attracting attention in microelectronics for their properties of reaching low relative dielectric constants by increasing porosity.

The relative dielectric constant decreases when the porosity of the material is increased. secondly pore condensation at a high level of water content. [START_REF] Grünberg | Hydrogen Bonding of Water Confined in Mesoporous Silica MCM-41 and SBA-15 Studied by 1[END_REF] Later, in 2010, water filling of silica nanopores were analyzed in [START_REF] De La Llave | Water filling of hydrophilic nanopores[END_REF] along with dynamics of confined water. Considering the amount of TCE solvent used to impregnate samples and 1 H spectra usually obtained in DNP SENS, it seems reasonable to assume 1 H spectra (recorded without any solvent suppression techniques [START_REF] Yarava | Solvent suppression in DNP enhanced solid state NMR[END_REF][START_REF] Lee | Solvent signal suppression for highresolution MAS-DNP[END_REF]) will be dominated by the solvent resonances. If we analyze 1 H- 29 Si HETCOR usually obtained for mesoporous silica under DNP SENS conditions, see Figure ?? and also references [START_REF] Lelli | Fast Characterization of Functionalized Silica Materials by Silicon-29 Surface-Enhanced NMR Spectroscopy Using Dynamic Nuclear Polarization[END_REF][START_REF] Zagdoun | A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials[END_REF] for example, 29 Si nuclei are never directly polarized from the protons of the solvent. However, these 29 Si nuclei are always polarized from the covalently attached organic moieties used to functionalize the surface. This is interpreted in terms of 1 H spin diffusion, which relays the hyperpolarized magnetization through the solvent where it reaches the 1 H of the organic moieties and is transferred to neighboring 29 Si nuclei by CP. When the microwaves are off, the { 1 H}- 29 Si CP spectrum is that of the entire functionalized silica surface is observed. For an indirect DNP (hyperpolarized 1 H and CP to 29 Si), considering that there is no 1 H spin diffusion in the bulk silica when microwaves are on, only surface 29 Si in contact with the bCTbK solution can be hyperpolarized. Thus only these 29 Si nuclei are participating to the 1 H- (3) is a radial pore filling model.

non-enhanced regions in microwaves on spectra). Consequently, if the entire 1 H DNP enhancement is transferred to the 29 Si during CP, we expect ε29 Si, CP to match with ε1 H for regions where the solution wets the surface. Regions, where the surface is not wetted, will not be enhanced during microwaves on experiments and thus 29 Si will be dominated by enhanced regions; but they will still contribute as equally as wetted regions when microwaves are off.

For hypothesis (3), the whole surface is always entirely wetted with the polarizing solution whatever the volume of impregnation is. Thus the entire surface is enhanced and we expect ε1 H = ε29 Si, CP . On the contrary, for hypothesis (1) and ( 2), if the volume used to impregnate samples is lower than the porous volume, it will result in partial coverage of the surface. Let's write α the ratio between the wetted surface and the total surface of the sample (in Figure 3.10 α = 0.5 is represented). As { 1 H}- 29 Si CP spectra with microwaves ON are dominated by 29 Si signals from the wetted surface, and the Experimental aspects of impregnation in DNP SENS of porous silica materials loaded silica. However, the TCE volume needed to correctly impregnate samples can be simply estimated from the porous volume calculated with BET nitrogen adsorption measurements. We have also seen that impregnation of passivated mesoporous silica with TCE occurs with a radial-pore-filling mode: a TCE layer forms on the silica wall and progressively grows from the surface to the center of the pore.

Using both optimal volume of the polarizing solution and a bulky highly efficient radical like TEKPOL2 that partially diffuse inside the pores as demonstrated in this section, and thus lead to high contribution factors, extremely high overall enhancement can be obtained for mesoporous silica materials. This, in addition, yields long coherence lifetime for the surface nuclei, allowing for the application of efficient solvent suppression techniques as well as, more importantly, of echo-based experiments like INADEQUATE or REDOR-type experiments (as will be detailed in Chapter 6).

Part II Three-dimensional surface structure determination

Perspectives: exploring low coordinated Ir-NHC surface sites a similar 3D surface structure determination approach that the method designed for the platinum complex. Thus the materials, represented on

Chapter 8

The present manuscript comes to its end. It has been mainly focused on introducing three-dimensional structure determination of single surface sites using the DNP SENS approach. In the beginning of this work, the 3D structure of organometallic ligands anchored at a surface was never observed. It was not even known if such surface sites exhibited a well-defined structure or were completely disordered structures. Combining DNP SENS, with EXAFS, we have been able to demonstrate that a Pt organometallic complex immobilized onto SBA-15 silica has a well-defined structure and the organic moiety adopts a conformation so that surface-metal interactions are permitted. We note that we determined the structure in the presence of an organic solvent, which mimics the conditions of the catalyzed reaction (with the exception of the temperature). We believe that the ability to obtain 3D surface sites structure will facilitate the implementation of structure-activity relationships in many modern materials in particular for those relevant in heterogeneous catalysis. Thus, current investigations focus on the structure of a low-coordinated Ir(I) immobilized catalyst that is showing high catalytic activity in its immobilized version whereas its molecular analog is far less active.

In parallel, more fundamental aspects of DNP MAS have been investigated in order to expand the application range of Dynamic Nuclear Polarization NMR. First, by implementing fast Magic Angle Spinning conditions, we have observed that the MAS rate is not preventing high DNP enhancements with the gold standard AMUPOL in water/glycerol. However, we have reported that fast spinning has a dramatic effect on signal quenching. New polarizing agents should be able to circumvent this limitation.

For example, there is no depolarization for radicals performing OE DNP and thus the BDPA radical is a promising system to perform DNP MAS at fast spinning. Alternatively, CE DNP at fast MAS would lead to higher sensitivity using biradicals made of two narrow line radical anchored. In this case, lower depolarization effect can be expected. However, this type of radical is still complicated to obtained and the hybrid approach where a narrow line radical and a broad line radical are combined (e.g. for TEMTri-POL [START_REF] Mathies | Efficient Dynamic Nuclear Polarization at 800 MHz/527 GHz with Trityl-Nitroxide Biradicals[END_REF]) is a promising alternative.

Second, a new DNP matrix based on polyacrylamide gels has been introduced. It allows to characterize of CdTe COOH quantum dots and prevents the aggregation of the particles when cooling the sample.

Conclusions

Third, so as to have a better understanding of the DNP SENS experiment and in particular of incipient wetness impregnation, which is a key step to perform the experiment, we have analyzed some primary aspects of the approach on mesoporous materials. In particular, we found that spin diffusion is playing an important role and a numerical spin diffusion model adapted to mesoporous materials have allowed measuring pore lengths.

DNP MAS is currently a vibrant approach in SSNMR as it allows to implement experiments that were not possible before and to get unprecedented information at the atomic scale. However, during this thesis, going more deeply in fundamental aspects of DNP SENS, it is clear that the application of DNP MAS is strongly dependent on the nature of the sample of interest. There are still many samples where DNP MAS is not meeting up to the expectations, in particular in the field of structural biology where very poor NMR resolution are obtained due to important heterogeneous line broadenings. In material, investigation of radical-sensitive material is still challenging.

Recent introduction of dendritic binitroxides, or [START_REF] Liao | Dendritic polarizing agents for DNP SENS[END_REF] silica wall-embedded radical [START_REF] Besson | Silica materials with wall-embedded nitroxides provide efficient polarization matrices for dynamic nuclear polarization NMR[END_REF][START_REF] Silverio | Tailored Polarizing Hybrid Solids with Nitroxide Radicals Localized in Mesostructured Silica Walls[END_REF] are promising versatile strategies to study radical-sensitive material with DNP MAS.

  transport de de l'hyperpolarisation par diffusion de spin est primordial et un modèle numérique dans les matériaux mésoporeux a été développé. De plus, une nouvelle matrice aqueuse se basant sur des gels polyacrylamides a été mise au point et utilisée pour la caractérisation par RMN de nanoparticules permettant ainsi d'étendre les domaines d'application de DNP SENS. Enfin les premières expériences RMN DNP combinant hauts champs magnétiques et haute fréquence de rotation d'échantillon sont présentées.

  investigations in solid-state NMR 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Well Defined Immobilized Heterogenous Catalysts . . . . . . . . . . . . . 2.2.1 Generalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Silica-based materials . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Solid-State NMR Spectroscopy for single well-defined supported sites heterogeneous catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Hyperpolarizing surfaces in NMR using Dynamic Nuclear Polarization . . 2.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Basics of Magic Angle Spinning Dynamic Nuclear Polarization in NMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 MAS DNP applied to Surfaces . . . . . . . . . . . . . . . . . . . . 2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I Expanding the experimental range of Dynamic Nuclear Polarization for surfaces 3 Experimental aspects of impregnation in DNP SENS of porous silica materials 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Diffusion of the polarizing agent into mesoporous materials . . . . . . . . 3.2.1 Signal quenching induced on surface nuclei when introducing nitroxides into porous silica . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Spin diffusion simulations . . . . . . . . . . . . . . . . . . . . . . . 3.3 Partial Impregnation Dynamic Nuclear Polarization . . . . . . . . . . . . 3.3.1 Effect of partial pore filling on DNP SENS experiments . . . . . . 3.3.2 Pore filling mechanisms . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 1

Figure 2 . 2 ,

 22 Figure 2.2, reproduced from[START_REF] Hoffmann | Silica-Based Mesoporous Organic-Inorganic Hybrid Materials[END_REF], schematically lays out this synthetic process. After the condensation of silica, SDAs can be removed via calcination or extraction, yielding empty and accessible pores.[START_REF] Hoffmann | Silica-Based Mesoporous Organic-Inorganic Hybrid Materials[END_REF]

Figure 2 . 2 :

 22 Figure 2.2: Sol-gel approach for the synthesis of MCM-41 silica. Two routes are possible: a) true liquid-crystal template mechanism, and b) cooperative liquid-crystal template mechanism. Reprinted with permission from [20]. Copyright 2006 John Wiley and Sons.

  1 H-13 C HETCOR spectra on organically functionalized mesoporous species. Their pulse program is represented on Figure 2.7, based on CP correlation with (a) 13 C detection and (b) 1 H detection. The detection of the 1 H has the advantage

Figure 2 . 8 .

 28 In 2010, Mao et al. studied the conformation of a fluorocarbon moiety (PFP) grafted onto mesoporous silica nanoparticles (MSN). This material is interesting for its highly hydrophobic surface and hence its capacity to catalyze esterification and dehydration processes. Using 13 C-19 F HETCOR, and 19 F-29 Si HETCOR (through-bonds and throughspace versions of the experiments) in combination with computational chemistry, they were able to demonstrate the existence of two non-exchanging conformations. The organic moiety can fold to the surface (PFP-p) or stand upright (PFP-u) (see Figure2.9(a) for the structure).

Figure 2 .Figure 2 . 6 :

 226 Figure 2.6: 1 H MAS NMR on MCM-41 as function of MAS rate. Reprinted with permission from [43]. Copyright 2005 American Chemical Society.

Figure 2 .

 2 Figure 2.9(b-c)).[START_REF] Mao | Conformations of Silica-Bound (Pentafluorophenyl)propyl Groups Determined by Solid-State NMR Spectroscopy and Theoretical Calculations[END_REF] Later, the same group demonstrated that when PFP function-

2 Figure 2 . 7 :

 227 Figure 2.7: CP-based HETCOR with (a) 13 C detection and (b) 1 H detection.

Figure 2 . 8 :

 28 Figure 2.8: 1 H detected INEPT-based HETCOR. Reprinted with permission from [48].

Figure 2 . 9 :

 29 Figure 2.9: (a) Representation of the two surface conformations PFP-u and PFP-p on MSN-PFP. (b) 13 C-19 F INEPT HETCOR of MSN-PFP showing the two surface conformations PFP-p (red) and PFP-u (blue). (c) 2D 19 F-19 F DQMAS spectrum of PFP-MSN where DQ excitation and reconversion were achieved using the back-to-back (BABA) sequence, showing the two independent systems PFP-p (red) and PFP-u (blue).

Figure 2 .

 2 Figure 2.10 depicts the 7 Li NMR spectrum obtained with and without microwaves.

Chapter 2 Figure 2 . 10 :

 2210 Figure 2.10: Oscilloscope pictures nuclear resonance absorption vs static magnetic field. Top: 7 Li resonance, with microwaves; middle: 7 Li resonance enhanced by electron saturation; and bottom: 1 H resonance in glycerin sample. Reprinted with permission from [57]. Copyright 1953 American Physical Society.

2. 4 . 2

 42 Basics of Magic Angle Spinning Dynamic Nuclear Polarization in NMR Continuous waves (CW) MAS DNP instrumentation Before reviewing in more detail the recent developments in the field of MAS DNP, we will briefly describe current commercial MAS DNP instrumentation. As represented in Figure 2.11, the instrument is divided into two parts. The first one is a usual NMR magnet equipped with a LT-MAS probe, which can operate at temperatures around 100 K. The second magnet is equipped with a gyrotron tube that generates a continuous microwave irradiation. Commercial Bruker systems use 263 GHz, 395 GHz, and 527 GHz microwave frequencies for 400 MHz, 600 MHz, and 800 MHz NMR systems ( 1 HLarmor frequencies) respectively. The microwave beam is conducted to the NMR probe via a waveguide and used to irradiate the sample continuously.[START_REF] Rosay | Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results[END_REF][START_REF] Rosay | Instrumentation for solid-state dynamic nuclear polarization with magic angle spinning NMR[END_REF]

Figure 2 . 11 : 1 )Figure 2 . 12 :

 2111212 Figure 2.11: Commercial MAS DNP NMR system composed of a gyrotron, an NMR magnet, a wave guide, and a cryocabinet. Reprinted with permission from [77]. Copyright 2016 Elsevier Inc.

Figure 2 .

 2 Figure 2.12(b) and transition rates 2.12(b)(a). The OE is a cross-relaxation process between the two spins involving the zero-quantum (ZQ) and double-quantum (DQ) relaxation rates. In Figure 2.12(a) the ZQ transition occurs at a rate W 0 and the DQ transition at W 2 . If these two relaxation rates are different, then saturation of the electron at its Larmor frequency results in an enhancement of the nuclear polarization.

Figure 2 .

 2 12(e)) dispersed in polystyrene and with its derivative water-soluble version SA-BDPA (sulfonated BDPA) in a glycerol/water mixture. Moreover, they have observed that, with BDPA, OE DNP scales favorably with the magnetic field as one can see in Figure 2.13. In this figure the central transition corresponds to OE DNP and increases when going from 9.4 T to 18.8 T.[81] OE DNP with BDPA has been recently rationalized, with the help of ab initio molecular dynamics simulations, and attributed to fluctuating hyperfine couplings at about 650 GHz in BDPA. Thus, moving from 263 GHz for 9.4 T (400 MHz 1 H frequency), 395 GHz for 14.1 T (600 MHz 1 H frequency), and 527 GHz for 18.8 T (800MHz 1 H frequency), the fluctuation frequency of the hyperfine coupling gets closer to the electron Larmor frequency and hence the DNP enhancement increases.[START_REF] Pylaeva | Molecular Mechanism of Overhauser Dynamic Nuclear Polarization in Insulating Solids[END_REF]

Figure 2 .

 2 14, the system is represented by eight energy levels. When the CE condition ω 0,I = |ω 0,e1 -ω 0,e2 | (where ω 0,I is the nuclear Larmor frequency, ω 0,e1 the Larmor frequency of electron e1 and, ω 0,e2 the Larmor frequency of electron e2) is matched, |αβα and |βαβ are degenerate as represented on Figure 2.14(A). When one electron EPR transition is saturated with continuous microwave irradiation (e1 in Figure 2.14(B)), the CE transition leads to nuclear polarization enhancement (negative enhancement in

Figure 2 .Figure 2 . 14 :

 2214 Figure 2.14(C)). Saturation of electron e2 will lead to positive enhancement.

  et al. and Mentik-Vigier et al. have shown the important role of level anticrossings (LACs) occurring during the rotation of the sample at the magic angle.[85, 86] Corzilius recently schematically illustrated the CE MAS mechanism.[87] Figure 2.15(ab) gives the central energy levels (color code in Figure 2.15(d)) and electron frequencies as a function of the MAS rotor position and Figure 2.15(e) is a spectral representation. In Figure 2.15(e)-A, one starts with two separate electron spin resonances. Then due to the rotation of the sample, electron frequencies evolve and one electron matches with the continuous microwaves irradiation, which causes (partial) saturation of this electron, this corresponds to event (1) in Figure 2.15(a) and is spectrally represented in Figure 2.15(e)-B. Then, in Figure 2.15(e)-C, the CE condition is matched, leading to polarization transfer from the electrons to the nuclei during the LAC. It corresponds to event (2) in Figure 2.15(b), and one can observe the increased nuclear polarization during this event in the population representation Figure 2.15(c). Then, moving to Figure 2.15(e)-D, the frequencies of the two electrons cross and thus exchange their polarization in a second LAC event. This is denoted as event (3) in energy-level representation

Figure 2 .Figure 2 . 15 :

 2215 Figure 2.15(a-b) and the exchange can be clearly seen on the population in Figure 2.15(c). As rotation continues, a second CE LAC event occurs transferring more polarization to the nucleus. This second transfer is efficient because the two electrons just exchanged their polarization (as one can observe in panel Figure 2.15(b-c) when two CE events (2) happened successively, only the first one actually results in hyperpolarizing the nucleus). Then the second electron is saturated when crossing the continuous microwaves irradiation and the process continues. Note that in this model, LACs are

Figure 2 . 16 .

 216 In the 90s, TEMPO was the first radical investigated in DNP MAS. TEMPO is a stable nitroxide exhibiting a broad EPR spectrum in frozen solution and that can perform CE DNP. In 2004, Hu et al. proposed to use biradical to control the electron-electron dipolar coupling and reported polarizing agents constituted of two tethered TEMPO units (bis-TEMPO-n-ethyleneglycol: bTnE represented on Figure2.16). They observed an increase of the DNP enhancement by a factor of about 4.[START_REF] Hu | Dynamic Nuclear Polarization with Biradicals[END_REF] The water-soluble biradical TOTAPOL introduced in 2006 followed the same principle and the authors reported a DNP enhancement of 290 on 1 H. One year later, the concept of using rigid tether was introduced.[START_REF] Matsuki | Dynamic nuclear polarization with a rigid biradical[END_REF] As stated before, only the association of molecules with an orientation of the g-tensor that allows matching with the CE condition gives rise to effective CE. Thus, using rigid tethers allows imposing the appropriate relative orientation of the gtensor of the TEMPO units of the biradical. While authors reported a 1 H enhancement of 260 for bT2E, they obtained 325 under the same experimental conditions (≈ 80 K, 211 MHz NMR magnet under 160 GHz microwaves continuous irradiation), with the rigid biradical bTbK (see structure on Figure 2.16). Later, Zagdoun et al. introduced bulkier radicals in order to increase the electron relaxation times (T 1e and T 2e ) improv-Chapter 2 ing EPR saturation upon continuous microwaves irradiation. This lead to the design of TEKPOL, depicted in Figure 2.16, that can provide 1 H enhancements of 200, outperforming bTbK that gives 1 H enhancement of 50 under the same conditions (≈ 105K, 400 MHz NMR magnet, 263 GHz gyrotron).[99, 100] The same concept has been used for the design of AMUPOL (Figure 2.16), which is -to date -the best polarizing agent in aqueous solvents (water/glycerol).[91] In 2016, Kubicki et al. published a comparison of 37 CE biradicals in which they carefully compared the respective DNP performances and correlated them to the radical substituents. TEKPOL2 was introduced and with a DNP enhancement factor of 215 (≈ 105 K, 400 MHz NMR magnet, 263 GHz gyrotron)

Figure 2 . 16 :

 216 Figure 2.16: Structures of some landmark CE nitroxide-based polarizing agents.

Figure 2 .

 2 17(a-b) shows the field profiles of the radical trityl (SE) and TEMPO (CE). The superposition of the two EPR profiles shows that the difference between the narrow line EPR spectrum of trityl and the maximum (at the top of the field plot) of the integrated EPR line of TEMPO perfectly matched the Larmor frequency of the 1 H in their 5 T magnet. As the CE condition matches between the two radicals, the possibility to observe CE DNP between TEMPO and trityl in a TEMPO/trityl mixture was predicted.

Figure 2 .

 2 17(c)

the 13 C

 13 Larmor frequency under the given working conditions. Very recently, Mathies et al. went further and reported the investigation of mixed radicals made of a TEMPO unit tethered to a radical trityl, coined TEMTriPol (represented on Figure 2.17(d)

Figure 2 . 17 :

 217 Figure 2.17: (a) Integrated EPR profile of trityl and the 1 H enhancement as function magnetic field with microwave irradiation at 139.66 GHz. The positive and negative enhancement peaks correspond to SE DNP, when ZQ and DQ transition are irradiated. (b) The same representation for TEMPO, which exhibits a broad EPR profil resulting in CE DNP. (c) The same representation for a mixture of trityl and TEMPO. Credits: (ac) Reprinted with permission from [84]. Copyright 2007 American Institute of Physics (AIP). (d) Reprinted with permission from [104]. Copyright 2015 Wiley-VCH.

Figure 2 . 18 :

 218 Figure 2.18: (a) Phase transition of ortho-terphenyl (OTP). (b) Temperature dependance of 1 H enhancement for 32 mM BPDA in OTP or 16 mM TEKPOL in OTP-d 1 4 at 9.4 T and 18.8 T. T g shows the glass transition temperature of OTP. Credits: (a) Reprinted with permission from [106]. Copyright 2013 ACS. (b-c) Reprinted (adapted) with permission from [107]. Copyright 2015 ACS

  Surface structure investigations in solid-state NMR the fraction of nuclei that still contribute to the NMR signals after the introduction of the paramagnetic polarizing agent: θ = I doped sample,µwOF F /m doped sample I undoped sample,µwOF F /m undoped sample (2.3)

Figure 2 . 19 :

 219 Figure 2.19: Depolarization factor ε depo (ratio between the integrated NMR intensity of the sample under rotation at the magic angle and the static sample) for 4 mM AMUPOL in glycerol/water. Reprinted with permission from [109]. Copyright 2015 Phys. Chem. Chem. Phys. Owner Societies.

Figure 2 .

 2 [START_REF] Zhao | Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures[END_REF] depicts the evolution of ε depo as function of the MAS rate for a solution of 4 mM AMUPOL in glycerol/water at 400 and 800 MHz at approx. 100 K. One can observe that under rotation at the magic angle spinning the signal intensity of a sample doped with AMUPOL dramatically decreases compared to static conditions, demonstrating that MAS is here inducing an additional quenching effect. Even stronger depolarization effects have been observed at a lower temperature Chapter 2

Figure 2 .Figure 2 . 20 :

 2220 Figure 2.20: (a) Description of the DNP SENS experiment. (b) 13 C DNP SENS CPMAS spectrum, recorded on a phenol-functionalized SBA-15 silica impregnated with a 25 mM TOTAPOL solution in glycerol/water, at natural isotopic abundance (9.4 T, 105 K). Clear signals arise from the phenol functionality (atom labels are given in (a)), aliphatic signals were assigned to ethyl surface moieties remaining from the material synthesis. Reprinted with permission from [71]. Copyright 2010 ACS.

Figure 2 . 21 :

 221 Figure 2.21: (a) 1 H enhancement (ε H ) and 29 Si enhancement (via CP, ε Si,CP ) recorded on azide-functionalized SBA-15 silica with different concentrations of bTbK in TCE. (b) 29 Si quenching factor (θ Si ) as a function of the bCTbK concentration in TCE. (c)

Chapter 2 Figure 2 . 22 :

 2222 Figure 2.22: (A) Structure of functionalities immobilized on SBA-15 silica surface, Mat-PrN 3 , Mat-PrNH 2 , and Mat-PrIm (B) 13 C DNP SENS CPMAS spectra. (C) 15 N DNP SENS CPMAS spectra. (D) 1 H-13 C, 1 H-15 N, 1 H-29 Si DNP SENS HETCOR spectra of Mat-PrIm. The open circles indicate byproducts or incomplete conversion from the previous synthetic steps. Reprinted with permission from [75]. Copyright 2013 ACS.

Figure 2 . 23 : 2 Figure 2 .

 22322 Figure 2.23: (a) azide functionalized SBA-15 silica and the passivating groups studied.(b) 1 H enhancement (ε H ),29 Si enhancement (via CP, ε Si,CP ), and sensitivity enhancements Σ Si for the material with different passivating groups. Reprinted (adapted) with permission from[START_REF] Zagdoun | Improved Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy through Controlled Incorporation of Deuterated Functional Groups[END_REF]. Copyright 2013 ACS.

Chapter 2 withFigure 2 . 26 :

 2226 Figure 2.26: (a) Structure of tungsten metathesis catalyst immobilized on SiO 2-(700) particles and its metathesis reaction intermediates TBP and SP metallacyclobutane. (b) Refocused 13 C-13 C INADEQUATE obtained by reacting 2 with large excess of 13 C-ethylene yielding SP-2 and hypothesized metallacyclopentane. Reprinted with permission from [128]. Copyright 2016 Wiley-VCH.

  of dendrimer-derivatives of frequently used DNP polarizing agents (see the structure of PyPolB-D2[G3] on Figure2.28) was proposed, where the nitroxide is protected by bulky organosilicon moieties. With this approach, the authors recorded13 C CPMAS DNP SENS spectrum with an organo-tungsten complex immobilized onto SiO 2-(700)

Figure 2 . 28 :

 228 Figure 2.28: Structure of PyPolB-D2[G3]. Adapted from[START_REF] Liao | Dendritic polarizing agents for DNP SENS[END_REF]

2 Figure 3 . 1 :

 231 Figure 3.1: Functionalized mesoporous silica SBA-15.

Figure 3 . 1 .Figure 3 . 2 :Table 3 . 1 :

 313231 Figure 3.2: Radicals from the bTbK series[START_REF] Zagdoun | Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K[END_REF][START_REF] Kubicki | Rational design of dinitroxide biradicals for efficient cross-effect dynamic nuclear polarization[END_REF]

  polarization and spin diffusion. In 2013, Rossini et al. have polarized 1-100 µm glucose crystals and have applied a numerical spin diffusion model to obtain an estimation of the size of the glucose crystal in the powder by fitting the DNP enhancement as a function of time. [60] Figure 3.3 is a reproduction of the enhancement as function of the polarization time observed on glucose in [60]. They have observed that the enhancement starts at its maximum and slowly decreases to reach a steady-state value. The latter decreases with increasing size of particles. Here, I show how spin diffusion models allows to get insights into the location of the polarizing agent location in SBA-15 and also enables the measurement of pore lengths with DNP SENS. I first adapt the spin diffusion model developed by Rossini et al. to interpret the behavior of DNP enhancement in SBA-15. 1

Figure 3 . 3 :

 33 Figure 3.3: 1 H enhancement as a function of time simulated for a microcrystalline system. Reproduced with permission from [60]. Copyright 2012 American Chemical Society.

Figure 3 . 5 :

 35 Figure 3.5: Schematic representation of the system used for simulations with the T 1 and P 0 functions as defined in the main text. Radicals are located at the edges of the system (purple regions), the center of the pore only contains pure TCE (blue region) and the different regions are continuously connected.

and 1 HFigure 3 . 6 :Figure 3 . 5 . 1 = 0.5 s and T target 1 =Figure 3 . 6 .Figure 3 . 7 : 1 H

 136351136371 Figure 3.6: 1 H enhancement as a function of time recorded on functionnalized SBA-15 material impregnated with 16 mM TEKPOL2 in TCE: comparison between the experimental data and the curve obtained with spin diffusion model.

  has shown that increasing the radicals concentration has resulted in a decrease of the contribution factor. Using a 25 mM bCTbK solution, the authors have observed almost 80 % of the surface signals are quenched. Thus we simulate the enhancement as function of time, with the known pore length of L = 750 nm, a contribution function set to θ : x → 1 and P L = P Q = (1 -0.18) × L, for the 1 H enhancement curve measured with the material impregnated with bCTbK dissolved in TCE. Figure 3.7 shows the simulated curve is in reasonable agreement with the experimental data.

3. 3 . 1 Figure 3 . 8 :

 3138 Figure 3.8: 1 H and 29 Si (measured via CP from 1 H) enhancements as a function of the pore filling of the passivated functionalized SBA-15 material with a 16 mM bCTbK solution in TCE. Error bars on29 Si were estimated on the basis of the signal over noise ratio in microwaves on and off spectra. Error bars on 1 H enhancement are not represented as they were found to be lower than the size of the data points themselves.

Figure 3 .

 3 Figure 3.8 reports the DNP enhancement measured on 1 H and on 29 Si (through CP)

[ 141 , 142 , 143 ]

 141142143 In our case, we progressively reduce the apparent porosity of the material by filling it with TCE and thus the relative dielectric constant increases. Similarly to what was observed with the addition of high relative dielectric constant particles, filling the pores with TCE increases the apparent dielectric constant and thus yield to higher DNP enhancement. So far I only discussed the general trend observed for these enhancements as a function of the filling volume. In the following section I show that the results shown in Figure 3.8 also give insights into the pore filling mechanisms of TCE in the passivated SBA-15.

Figure 3 . 9 :

 39 Figure 3.9: Pore filling of liquid N 2 in MCM-41 porous material. A pore is schematically represented with different liquid N 2 volume. First, radial pore-filling occurs, then capillary condensation occurs in a second time.

Figure 3 .

 3 Figure 3.10 shows different simple models that we can imagine for the filling of pores in SBA-15 with the bCTbK solution in TCE. Hypothesis (1) is an axial pore filling model and implies that the filling of pores occurs linearly: for a relative pore filling degree of 0.5, half of each pores is filled with the polarizing agent solution. The second hypothesis (2) is that pores can be only entirely filled or completely empty: for a 0.5 pore filling one out of two pores is filled with solution one out of two pores is entirely empty. Hypothesis (3) is the radial pore filling model where the filling with the solution occurs progressively from the wall of the pores to the center.

Figure 3 . 10 :

 310 Figure 3.10: Pore filling with TCE of passivated SBA-15 hypotheses. (1) is an axial pore filling model ; (2) supposes pores can be only entirely filled or entirely emptied ;

Figure 7 . 2 ,

 72 have been formulated with15 N isotopic labelling to enable constraint measurements with29 Si-{ 15 N} and13 C-{ 15 N} DNP SENS REDOR experiments. Additionally, the complexes are13 C labeled at the carbene (C 6 ) position. This additional isotopic labeling will open new experimental possibilities regarding distance measurements and will allow one to orient the imidazole ring with respect to the silica support, which could not have been done by NMR in the previous study. In addition, we will be able to measure13 C-13 C DQ buildup curves under DNP conditions to determine the C 6 -C 2 distances and hence constrains the orientation of the imidazolium ring. In the previous study, this orientation has been set based on DFT structure optimization.

Figure 7 .Figure 7 . 2 :

 772 Figure7.2 shows the 13 C{ 1 H} and 15 N{ 1 H} DNP SENS CPMAS spectra on the three labelled materials (Figure7.2). The 13 C{ 1 H} CPMAS spectra clearly demonstrates the metallation from the NHC precatalyst to the silver materials has a partial yield. In contrast the transmetallation is complete as evidence by the 15 N{ 1 H} CPMAS spectra recorded on N 1 -labelled materials. Thus both the silver-and iridium-based materials still contains a non-negligible amount of the starting ligand, which will have to be considered in the structure determination. This issue can be solved by implementing selective recoupling NMR experiments that will exploit the fact that the carbene resonances in the organic and organometallic ligands have very distinct13 C chemical shifts. Selective REDOR pulse sequences have been developed for distance measurement in fully labeled solid proteins.[START_REF] Jaroniec | Frequency Selective Heteronuclear Dipolar Recoupling in Rotating Solids: Accurate 13 C-15 N Distance Measurements in Uniformly 13 C, 15 N-labeled Peptides[END_REF] This will lead to the determination of three unambiguous distance restraints between the (labeled) carbene and the surface silicon atoms in the Ir-materials. The 15 N chemical shifts of the N 1 atom in the organic and organometallic NHC ligands are too close to enable a selective recoupling of the29 Si-{ 15 N} dipolar interactions.These approaches are currently under investigations. We envision these methods can (i) demonstrate that 3D surface structure determination allows structure-property relationship on catalytically relevant materials and (ii) address surface characterization of a large number of immobilized catalyst where the conversion of surface sites is incomplete during the synthesis.

24]

	Chapter 2
	OH catalytic activity. In this section, I review examples where SSNMR has been success-
		Si
	O fully used to characterize functionalized surfaces, focusing on catalysts immobilized on O O
	surfaces.					t Bu
					pentane		t Bu
					20 o C	Re
	t BuCH 3				t Bu	H Bu t
			t Bu
					t Bu		Structural type δ Si (ppm)
		Si O	Re H	H	H t Bu		(HO) 2 Si(OSi) 2	-90.6
	O	O	O				(HO)Si(OSi) 3	-99.8
							Si(OSi) 4	-109.3
	Figure 2.4: Synthetic route to Rhenium complexes supported on silica. Reprinted with
	permission from [25]. Copyright 2009 John Wiley and Sons. Figure 2.5: 29 Si CPMAS spectrum of sil-
	ica gel obtained at 11.88 MHz (1014 scans). This approach has led to the synthesis of a large number of new heterogeneous cata-Reprinted with permission from [27]. Copy-lysts where the catalytic properties have been efficiently tuned from their homogeneous analogues. Supported nanoparticles also provide an efficient route to obtain well-defined right 1980 American Chemical Society.
	sites in heterogeneous catalysis and also take advantage from a rational design based
	on homogenous catalysts. However, supported nanoparticles cannot be considered as
	part of SOMC as this class of material do not systematically include an organic moi-
	ety to tether the NPs to a surface. In 2010, Fu et al. demonstrated the possibility to
	obtain a heterogeneous catalyst based on coordinately unsaturated ferrous (CUF) sites
	from Pt-Fe nanoparticles supported on silica particles. CUFs are attractive sites as they
	can stabilize the unusual Fe(IV) oxidation state. They demonstrated their catalyst to
	be efficient for carbon monoxide oxidation. Apart from that, CUF sites are particu-
	larly well-known catalytic sites in enzymes, and immobilizing such active sites provides
	attractive catalysts for oxidation processes in industry.[26]	H
	OH				OH		O	H
	Si O 2.3 Solid-State NMR Spectroscopy for single well-defined O O HO O Si Si O O O O O Si O O
							O
	supported sites heterogeneous catalysts
	silanol				vicinal silanol	geminal silanol
	Q 3				Q 2		Q 3
	Over the years, SSNMR has proven to be very efficient to characterize active cat-
	Figure 2.3: Isolated, vicinal, and geminal surface silanols alytic species located on a surface, providing valuable information for understanding the

Table 2 . 1

 21

: Chemical shift assignement.

  All the previous examples demonstrated that MAS DNP is a very promising method

	Chapter 2
	Radical sensitive materials
		17 O is the only NMR active
	isotope of oxygen, but it has a very low natural abundance (≈ 0.04 %) and complex
	spectra due to quadrupolar broadenings. Its low sensitivity makes it an ideal candidate
	for DNP enhanced NMR. The ability to perform DNP enhanced 1 H-17 O CPMAS NMR
	was demonstrated on H 2	17 O with indirect DNP (DNP transfer from electron to 1 H
	and then hyperpolarization is CP to 17 O),[129] and then at natural abundance with

  29 Si CP spectrum (we can neglect the contribution of Experimental aspects of impregnation in DNP SENS of porous silica materials

	Hyp (1)	Hyp (2)	Hyp (3)

restent souvent sans réponse. Les processus chimiques qui se produisent à une surface jouent un rôle crucial dans la chimie moderne. De nombreuses questions se posent lors de l'exploration de surfaces de matériaux modernes, liées à la description des processus d'adsorption-désorption, de la fonctionnalisation (structurale et dynamique des fonctionnalités de surface), des réactions à la surface ou de la diffusion de produits chimiques.En catalyse hétérogène, le catalyseur -souvent en phase solide -est impliqué par sa surface dans le processus chimique.Actuellement, bien qu'un vaste choix de méthodes sensibles à la surface soient disponibles, elles ne conduisent qu'à une compréhension partielle des surfaces au niveau moléculaire. En outre, ces méthodes ne sont pas compatibles avec toutes sortes de matériaux et exigent généralement l'utilisation d'un modèle et d'une surface vierge.Parmi les développements les plus récents pour fournir une description atomique des surfaces, en 2015, de Oteyza et al. ont utilisé la microscopie de force atomiqe sans contact (nc-AFM) pour observer l'image d'une liaison covalente d'adsorbats sur une surface de Ag[START_REF] Zagdoun | Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K[END_REF].[START_REF] De Oteyza | Direct Imaging of Covalent Bond Structure in Single-Molecule Chemical Reactions[END_REF]

A unified spin diffusion model for different set of data with different geometries (including porous silica) has been developed and submitted: Pinon, A ; Schlagnitweit, J ; Berruyer, P ; Rossini, A ; Lelli, M ; Socie, E ; Tang, M ; Pham, T ; Lesage, A ; Schantz, S ; Emsley, L. Measuring Nano to Micro Structures From Relayed DNP NMR.

In a frozen solution of binitroxide in TCE, we observe the enhancement is constant as a function of time. However, we believe spin diffusion also allow polarizing the entire frozen solution, but it happens almost instantaneously as the average distance between two polarizing agents is quite small (≈ 2.9 nm in a 16 mM binitroxide solution in TCE).

Remerciements

Chapter 1

Chapter 2 Surface structure investigations in solid-state NMR Surface structure investigations in solid-state NMR larizing agents, as freezing the sample at 100 K results in the aggregation of the QDs. Thus, the particles were dispersed in mesoporous SiO 2 to prevent aggregation. A 13 C- 111 Cd dipolar HMQC spectrum reported in Figure 2.25, allowed to directly observe the interactions between the oleate ligand and the QDs surface. Note that a combination of 13 C isotopic labeling and DNP MAS was required to obtain sufficient sensitivity for the 2D experiments. Pruski and co-workers also reported results on NPs using DNP SENS, [START_REF] Perras | Natural Abundance (17)O DNP Two-Dimensional and Surface-Enhanced NMR Spectroscopy[END_REF] This was enabled with PRESTO-QCPMG (phase-shifted recoupling effects a smooth transfer of orderquadrupolar CPMG), see Figure 2.27(a), and variation of τ 0 and τ 1 to get the transfer intensity curves (see Figure 2.27(b)) which was fitted to obtain distances. Using a similar approach, the same group also probed surface hydrogen bonding structural and dynamical features on mesoporous silica NPs and aluminosilicates. [START_REF] Perras | Probing Surface Hydrogen Bonding and Dynamics by Natural Abundance, Multidimensional[END_REF][START_REF] Perras | Natural Abundance 17O DNP NMR Provides Precise O-H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts[END_REF] In [START_REF] Hope | Surface-selective direct 17 O DNP NMR of CeO 2 nanoparticles[END_REF] the first surface-sensitive 17 where

is the local nuclear polarisation. At thermal equilibrium, P = P 0 = 1; at nuclear saturation, P = 0.

• σ( r, t) is the local creation term of Fick's law. In the spin diffusion process, polarization is created, after saturation, following spin-lattice relaxation process. Thus, this term can be written σ( r, t) = P ( r,t)-P 0 ( r)

. Note that if we assume no diffusion (D = 0) and homogenous spin-lattice relaxation time (T 1 ( r) = T 1 ), we are back to a simple exponential build-up of polarization after saturation. SBA-15 is a porous silica exhibiting a dense hexagonal network of pores. It is modeled with a single cylindrical tube aligned on the x axis, filled with the polarizing agent solution. As the system is invariant by all rotations around the x-axis and assuming that the polarization is independent of the distance r to the center of the pore (constant Chapter 3 whole surface contributes to when microwaves are OFF, we expect a mismatch between ε1 H and ε29 Si, CP : n tot spins contributed to µw OFF and αn tot to µw ON:

Figure 3.10 shows that ε29 Si, CP match with ε1 H for all relative pore fillings, confirming hypothesis (3) for the pore-filling mechanism of TCE in passivated mesoporous silica. Note that, contrary to the study of Grünberg et al. [START_REF] Grünberg | Hydrogen Bonding of Water Confined in Mesoporous Silica MCM-41 and SBA-15 Studied by 1[END_REF] of water filling in SBA-15 where they have observed water condensation of water after radial filling, our enhancement data do not allow us to reveal such precise description of the filling process of TCE inside SBA-15: radial pore filling model or a two-step pore-filling (radial pore filling followed by condensation like in MCM-41, Figure 3.9) will both give matching between ε1 H and ε29 Si, CP .

Conclusions

In this section, we analyze two different experimental aspects of impregnation in DNP SENS. The conclusions that we have obtained give us a more detailed picture of the impregnation process in porous materials and new insights for sample preparation which is maybe one of the most crucial factors to guarantee the success of a DNP SENS experiment.

The analysis of spin diffusion in functionalized mesoporous silica impregnated with different polarizing agent solutions from the bTbK series allows us to demonstrate that the impregnation leads to a heterogeneous distribution of the polarizing source inside the material. In particular, bulkier radicals (TEKPOL and TEKPOL2) diffuse a short way into the pores whereas smaller ones (bTbK and bCTbK) can diffuse on longer range.

The analysis of enhancement behaviors with spin diffusion model require to modify the model previously published for amyloid fibrils [START_REF] Van Der Wel | Dynamic Nuclear Polarization of Amyloidogenic Peptide Nanocrystals: GNNQQNY, a Core Segment of the Yeast Prion Protein Sup35p[END_REF] and microcrystals [START_REF] Rossini | One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition[END_REF]. In particular, the role of the quenching appears to be key to reproduce the enhancement curves.

The analysis of the influence of the volume of solvent used to impregnate the porous sample in DNS SENS shows that insufficient impregnation will lead to a significant loss of the DNP efficiency in mesoporous silica. We hypothesize the existence of a reverse dielectric effect that can explain the poor DNP enhancement obtained from partially Perspectives: exploring low coordinated Ir-NHC surface sites

Chapter 7

In the previous section, we have demonstrated, using the combination of DNP SENS and EXAFS, that the carbene platinum complex immobilized onto an SBA-15 silica surface exhibits a well-defined structure and we have been able to determine the threedimensional structure of this complex. While it was interesting as a proof-of-concept, it does not truly enable a structure-property relationship, as the Pt complex studied does not exhibit a particularly interesting catalytic activity! In this last chapter, we propose to investigate the structure of a low-coordinated Ir(I) complex immobilized onto SBA-15 silica, which is represented on material and its molecular analogue. Reprinted (adapted) with permission from [START_REF] Romanenko | Iridium(I)/N-Heterocyclic Carbene Hybrid Materials: Surface Stabilization of Low-Valent Iridium Species for High Catalytic Hydrogenation Performance[END_REF].

Copyright 2015 Wiley-VCH.

Here we propose to quantitatively investigate this Ir(I) catalyst by implementing

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