Dr Renaud Demadrille

Dr Marcelo Gennari

Duc, Christina Tania Kun

Nabil, Mariam, Youssef, Anthonin, Tim Jonathan Afridi

Petros, Miltos, Spiros, Despina Andreas Angeliki

Nikos

Manos Giota

Keywords: Dye-Sensitized Photo-Electrochemical Cell Hydrogen-Evolving Reaction OER, Oxygen-Evolving Reaction ITO, Indium-Tin Oxide FTO, Fluorine-doped Tin Oxide HOMO, Highest Occupied Molecular Orbital LUMO, Lowest Unoccupied Molecular Orbital Cyclic Voltammetry IEA, Irreversible Electron acceptor LSV, Linear Sweep Voltammetry E Elemental Analysis ESI-MS, Electron-Spray Ionization Mass Spectrometry HRMS, High-Resolution Mass Spectrometry NMR, Nuclear Magnetic Resonance ppm, parts per million NHE, Normal Hydrogen Electrode RHE, Reversible Hydrogen Electrode ALD, Atomic Layer Deposition DFT, Density Functional Theory CuAAC, Copper-catalyzed Azide-Alkyne Cycloaddition -ligand charge transfer MWCNT, Multi-Walled Carbon NanoTube PCET, Proton-Coupled Electron Transfer R.T Sacrificial Electron Donor SEM, Scanning Electron Microscopy XPS, X-ray Photoelectron Spectroscopy MALDI-ToF, Matrix-Assisted Laser Desorption/Ionization Time-of-Flight mass spectrometry Transparent Conducting Oxide

Pierre and Dr. D. Gasparutto for the MALDI-ToF measurements, Dr. J. Pécaut for the ESI-MS, Pr. D. Leonard for the ToF-SIMS analysis, C. Müller and Pr. B. Dietzek for the photophysical studies in solution

General Introduction

Energy is the most important tool of modern communities in order to facilitate their daily life with an annual global energy consumption of around 18 TW, which is predicted to reach 43 TW in 2100. 1 Modern energy economy is reliant on fossil fuels that cause serious environmental problems (climate change, melting of polar ice caps, air pollution, lower biodiversity) due to their combustion that produces carbon dioxide. This fact along with the depletion of fossil fuels because of their continuous use has forced the scientific community to find alternatives based on renewable and abundant energy sources, like solar, geothermal, wind power etc. (Figure 1). Solar energy is extremely advantageous, as the most abundant energy source. The sun delivers on earth in about one hour, the same amount of energy we are currently using in one year. Unfortunately, solar energy is intermittent and diffuse. Thus, there is an urgent need to find a way to store and convert this inexhaustible source of energy. Photovoltaics is a mature existing technology to capture sunlight and convert it into electricity.

Electricity can fulfill immediate energy needs. However, it cannot be stored. To achieve long-term storage of solar energy, different approaches have been proposed. Converting it into a chemical form, like a fuel, seems a promising way and it is crucially needed due to the current technological infrastructure. Fuels that can be produced directly or indirectly by solar energy through photo-, thermo-or electrochemical reactions are called solar fuels. Using solar energy for their production is a big challenge and scientists find inspiration from the most common natural process, photosynthesis.

In nature, there are many photosynthetic organisms, like algae, cyanobacteria and plants, called photoautotrophs, which are able to synthesize biomass directly from carbon dioxide and water using the energy from sunlight. They use the photogenerated electrons to reduce carbon dioxide and produce carbohydrates via the Calvin cycle, which are the energy reserves of the cell. Furthermore, some cyanobacteria and microalgae are capable of reducing protons to hydrogen, using enzymes called hydrogenases. 3 All these transformations are realized thanks to an amazing biologic machinery, presented schematically in Figure 2, consisting of two large protein complexes, the photosystem I (PS I) and the photosystem II (PS II), assisted by various redox cofactors and enzymes. The scientific community tries to mimic the basic mechanism of photosynthesis (artificial photosynthesis) [START_REF] Andreiadis | Artificial Photosynthesis: From Molecular Catalysts for Light-Driven Water Splitting to Photoelectrochemical Cells[END_REF][START_REF] Zhang | Artificial Photosynthesis : Opportunities and Challenges of Molecular Catalysts[END_REF] that consists of three elementary steps:

 initial light-harvesting thanks to molecular photosensitizers located at PSII and PSI  proton-coupled electron transfers between redox cofactors to accomplish long-lived charge separation  multielectronic redox catalysis involving specific enzymes and cofactors (OEC at PSII, RuBP, hydrogenases, etc)

Taking photosynthesis as a model, three main approaches exist to produce solar fuels: homogeneous photocatalytic systems using colloids, photovoltaics coupled with electrolysis and photoelectrochemical cells. 6 These systems present different degrees of technological maturity and manufacturing cost. A dye-sensitized-photoelectrochemical cell (DSPEC, Figure 2), that combines a photoanode in analogy to PSII, where water oxidation takes places to supply with electrons and protons the photocathode (similar to PSI), where proton (or carbon dioxide) reduction occurs, has gained an increased attention by molecular chemists. A first proof of concept was made in 1972, comprising a TiO2 photoanode and Pt as counter electrode. [START_REF] Fujishima | Electrochemical Photolysis of Water at a Semiconductor Electrode[END_REF] Since then, many efforts by different research groups have been devoted for the preparation of functional dye-sensitized photoanodes and photocathodes separately. 8,[START_REF] Walter | Solar Water Splitting Cells[END_REF][START_REF] Queyriaux | Molecular Cathode and Photocathode Materials for Hydrogen Evolution in Photoelectrochemical Devices[END_REF][START_REF] Xu | Water Splitting Dye-Sensitized Solar Cells[END_REF][START_REF] Gibson | Dye-Sensitized Photocathodes for H2 Evolution[END_REF][START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF][START_REF] Li | Device Fabrication for Water Oxidation, Hydrogen Generation, and CO2 Reduction via Molecular Engineering[END_REF] Understanding the principles that govern the activity and optimizing the performance of each component will pave the way for the preparation of competent DSPEC.

Until today, the bottleneck in a full DSPEC device in terms of performances is the photocathode part. In this sense, during my thesis, I have focused on the construction of dyesensitized photocathodes for hydrogen evolution. This work is a continuation of what has already been done in the "SolHyCat" group in the Laboratory of Chemistry and Biology of Metals. Based on the previous expertise of the lab, the main objectives of my PhD project were:

 To compare the efficiency of different light harvesters (P2) and catalytic units (HEC) for hydrogen-evolving photocathodes  To address the stability of the molecular-based photocathodes by performing postoperando analysis

In the first chapter, we will introduce the principles of dye-sensitized photocathodes for hydrogen production. Then, the contribution of the group to this field will be highlighted. We will show a cobalt catalyst that was used for the first covalent and noble metal-free dye-catalyst assembly and model photocathodes based on ruthenium complexes. Next, the already reported examples of functional photocathodes will be analyzed, including the work from the group.

Finally, figures of merit will be presented for all the different systems, as well as current limitations.

In the second chapter, the synthesis and characterization of a new dye-catalyst assembly consisting of a ruthenium photosensitizer functionalized with methyl phosphonate groups and a cobalt diimine-dioxime catalyst will be demonstrated.

The third chapter is dedicated to the assessment of the photoelectrochemical activity for hydrogen evolution of a NiO photocathode in aqueous media sensitized by the first dyad. Postoperando characterization to investigate the destiny of our molecular components will be carried out.

The fourth chapter will be concentrated on the preparation of a second dye-catalyst assembly with the same photosensitizer and a different cobalt tetraazomacrocyclic complex as a catalyst.

In the last chapter, the second dyad will be examined for proton reduction under the same experimental conditions with the first photocathode for direct comparison. In addition, preliminary attempts to use our novel dye-catalyst assembly for PEC carbon dioxide reduction will be presented.

Chapter I Hydrogen and dye-sensitized photocathodes for light-driven H 2 production

Hydrogen is a chemical element with just one proton and one electron and its name arises from the Greek words hydro for "water" and gene for "forming". It is the most abundant chemical substance in the universe and the element can be found in the stars and powers the universe through the proton-proton chain reaction and carbon-nitrogen cycle. Hydrogen also plays an important role in acid-base reactions, because most of them involve the exchange of protons between soluble molecules. Furthermore, it is essential for life, and present in nearly all molecules in living organisms. Preferably it is bonded to other elements, such as to oxygen to form the most important good of life, water (H2O).

Dihydrogen (named hydrogen in the following of this thesis), the simplest chemical compound, at standard temperature and pressure is generally nontoxic, colorless, tasteless and odorless gas. However, H2 is not available on earth under the form of H2. Nowadays hydrogen is almost exclusively used for industrial purposes in chemicals (ammonia, methanol) and refining. Glassmaking, production of plastics and printed circuit boards, hydrogenation of vegetables, animal oils and fats, metallurgy and weather balloons are some additional fields where hydrogen finds applications.

Hydrogen is a typical energy vector with a high energy density (120 MJ.kg -1 ). 1 kg of hydrogen generates three times more energy than 1 kg of gasoline (energy density of 44.4 MJ.kg -1 ). Furthermore, it is carbon-free, as its oxidation in a fuel cell, an already mature technology, produces electricity and only water as a by-product. However, the most important drawback is that its production is based mainly on fossil fuels. The most common raw materials of hydrogen are natural gas, hydrocarbon, coal and in lower scale electrolysis and biomass.

Until today, the most dominant procedure for H2 production is steam reforming from hydrocarbons, but the byproducts (CO, CO2 and other greenhouse gases) makes this process controversial. The scientific community tries to find environmentally friendly procedures for its production, like biomass and photoelectrolysis of water (Eqn. I.1) to store solar energy in a chemical form. The latter approach is the one our group is keen on and has invested scientific effort for years.

𝑯 𝟐 𝑶 ⇌ 𝑯 𝟐 + 𝟏 𝟐 𝑶 𝟐 ΔG = 237.2 kJ•mol -1 (298.15 K, 1 bar) Eqn. I.1

Splitting of water evolves hydrogen and oxygen and can be separated into two half reactions: water oxidation (Eqn. I.2) and proton reduction (Eqn. I.3). Throughout this work we will focus on the reductive half reaction (Eqn.I.3).

𝑯 𝟐 𝑶 ⇌ 𝟏 𝟐 𝑶 𝟐 + 𝟐 𝑯 + + 𝟐 𝒆 - E 0 = 1.23 V vs RHE Eqn. I.2

𝟐 𝑯 + + 𝟐 𝒆 -⇌ 𝑯 𝟐 E 0 = 0 V vs RHE Eqn. I. 3 Hydrogen evolution (Eqn. I.3) is a simple reaction, but as most of multielectronic processes, catalysts are crucial for speeding it up. Platinum and platinoids are the most widely used hydrogen-evolving catalysts (HEC). However, they are expensive and scarce (37 ppb in the Earth's crust), [START_REF] Gordon | Metal Stocks and Sustainability[END_REF] preventing the development of functional devices in a large and sustainable scale. As an alternative, bioinspired molecular catalysts have been developed based on the active site of hydrogenases, 3 which is a binuclear complex with two iron or a combination of nickel and iron atoms. Inspired by these enzymes, molecular chemists use first row transition metals, like Fe, Co, Ni for the preparation of noble metal-free HEC. [START_REF] Thoi | Complexes of Earth-Abundant Metals for Catalytic Electrochemical Hydrogen Generation under Aqueous Conditions[END_REF][START_REF] Tong | First-Row Transition Metal Polypyridine Complexes That Catalyze Proton to Hydrogen Reduction[END_REF] Homogeneous photocatalytic systems for the reductive [START_REF] Artero | Splitting Water with Cobalt[END_REF][START_REF] Dempsey | Hydrogen Evolution Catalyzed by Cobaloximes[END_REF][START_REF] Eckenhoff | Molecular Systems for Light Driven Hydrogen Production †[END_REF] part of water splitting emerged over the last 20 years. The three most important components of a photocatalytic system for hydrogen evolution are:

1. A photosensitizer to absorb visible light 2. A catalyst to reduce protons and generate hydrogen (HEC)

3. A sacrificial electron donor to supply electrons and regenerate the chromophore Photocatalytic systems based either on multicomponent or dye-catalyst assemblies have been developed. Supramolecular bimetallic structures, where intramolecular electron transfer and spatial charge separation can take place, seem particularly attractive. Ruthenium baseddyes coupled with noble metal catalysts (platinum, palladium) produced hydrogen in homogeneous systems. [START_REF] Kovacs | Supramolecular Bimetallic Assemblies for Photocatalytic Hydrogen Generation from Water[END_REF][START_REF] Braumüller | Synthesis and Characterization of an Immobilizable Photochemical Molecular Device for H2-Generation[END_REF] Our group prepared supramolecular dyads with noble metal-free catalysts (cobalt complexes) [START_REF] Fihri | Cobaloxime-Based Photocatalytic Devices for Hydrogen Production[END_REF][START_REF] Fihri | Efficient H2 -Producing Photocatalytic Systems Based on Cyclometalated Iridium-and Tricarbonylrhenium-Diimine Photosensitizers and Cobaloxime Catalysts[END_REF] and hydrogen was evolved under light irradiation.

Today, the challenge is to integrate these molecular components into a functional device.

The pioneering work from O'Regan and Grätzel on n-type dye-sensitized solar cells paved the way for intensive research of how to heterogenize light harvesters (ruthenium complexes) onto electrodes for light-driven processes. [START_REF] O'regan | A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films[END_REF]

I. Design and working principle of a dye-sensitized photocathode 1) Inspiration from DSSCs and basic principles

The technological development made in the field of DSSCs can act as a base to construct devices for conversion of solar energy in different chemical forms, like hydrogen. The preparation of efficient devices, called dye-sensitized photoelectrochemical cells (DSPECs,

Figure 2), where light-driven water-splitting can take place is far from reality. This field is still not as mature as DSSCs, even though decomposition of water with light-responsive electrodes was described for the first time in 1960s. [START_REF] Kallmann | Decomposition of Water by Light[END_REF] Here, we will focus on the photocathode side of a tandem device.

The first p-type DSSC (based on NiO semiconductor) was reported in 1999 [START_REF] He | Dye-Sensitized Nanostructured p-Type Nickel Oxide Film as a Photocathode for a Solar Cell[END_REF] and its basic principles [START_REF] Hagfeldt | Dye-Sensitized Solar Cells[END_REF][START_REF] Nikolaou | Recent Advances and Insights in Dye-Sensitized NiO Photocathodes for Photovoltaic Devices[END_REF][START_REF] Benazzi | Developing Photocathode Materials for P-Type Dye-Sensitized Solar Cells[END_REF] are depicted in Figure I. 1 (left). Light absorption by the photosensitizer generates a reactive excited state (PS * ), which has more oxidizing power than its ground state, and accepts an electron from the valence band of the p-type semiconductor. A charge separated state is generated with an electron on the dye (PS -) and a hole on the semiconductor. Then, two pathways are possible: Regeneration of the reduced dye by a redox mediator (RM), mostly iodide/triiodide in the solution (organic media) or by the semiconductor (recombination reaction). If electron transfer from the reduced dye to the redox mediator is thermodynamically feasible and kinetically faster than recombination, the reduced mediator (RM -) is formed and then oxidized at the counter electrode, producing an electric current in the external circuit.

As far as the basic principles of the dye-sensitized hydrogen-evolving photocathode is concerned, upon light irradiation, we have the formation of the excited state of the dye (PS * ).

An electron is transferred from the valence band of the semiconductor to form a charge separated state (PS -/SC(h + ). The holes (h + ) diffuse through NiO to a back contact, which in most cases is a glass coated with fluorine doped tin oxide (FTO). Then, an electron transfer from the reduced dye (regeneration of the dye) to the catalyst occurs. These steps will be repeated twice in order to accumulate two electrons on the catalyst for the reduction of protons to hydrogen.

In this case, two different recombination processes can take place (dashed arrows). One between the reduced dye and the semiconductor, like in DSSCs, and between the reduced catalyst and the semiconductor. Between DSSCs and p-type dye-sensitized photocathodes [START_REF] Walter | Solar Water Splitting Cells[END_REF][START_REF] Queyriaux | Molecular Cathode and Photocathode Materials for Hydrogen Evolution in Photoelectrochemical Devices[END_REF][START_REF] Xu | Water Splitting Dye-Sensitized Solar Cells[END_REF][START_REF] Gibson | Dye-Sensitized Photocathodes for H2 Evolution[END_REF][START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF] (Figure I.1) a lot of similarities can be observed, but there are also major differences. From a kinetic point of view, in a p-type DSSC a single electron transfer process occurs to produce electricity compared to multielectronic redox catalysis for the dye-sensitized hydrogen-evolving photocathode. For the latter, aqueous medium as a proton source is needed instead of an organic electrolyte for DSSCs.

In the following section, the main components of p-type dye-sensitized hydrogen-evolving photocathodes with basic requirements will be presented.

2) NiO as semiconductor

The semiconductor is responsible for efficient charge separation. Hole diffusion away from the dye to the charge-collecting substrate (mostly fluorine tin-doped oxide), enables the catalysis to occur. An ideal semiconductor should exhibit:

 Transparency in the visible region, thus enabling the dye to perform its role by minimizing the competition for absorption.

 Photostability and stability under aqueous conditions, as water is the solvent of interest.

 Accessibility and strong chemisorption with different anchoring groups for an efficient and robust grafting with the molecular compounds.

 Mesoporosity to offer high surface area in order to increase the grafting density of the molecular components. More light harvesters mean more efficient absorption of the incident light.

 High conductivity and fast hole diffusion to reduce unwanted recombination reactions.

 A valence band with a potential able to reduce the excited dye (hole injection) in order to start the photoinduced processes.

Nickel oxide (NiO) is the most common used wide band-gap p-type semiconductor. [START_REF] Odobel | Recent Advances in the Sensitization of Wide-Band-Gap Nanostructured p -Type Semiconductors. Photovoltaic and Photocatalytic Applications[END_REF] NiO is a low-cost and easy to process semiconductor with good transparency, as an indirect band gap of 3.6 eV implies that absorption in the visible region cannot occur. [START_REF] Boschloo | Spectroelectrochemistry of Nanostructured NiO[END_REF] Nickel vacancies (Ni 2+ , Ni 3+ ) and intercalated oxygen are responsible for its p-type semiconductivity. [START_REF] Dare-Edwards | Photoelectrochemistry of Nickel (II) Oxide[END_REF][START_REF] Nandy | Effect of Oxygen Partial Pressure on the Electrical and Optical Properties of Highly ( 200 ) Oriented p-Type Ni 1x O Films by DC Sputtering[END_REF] In addition, NiO is stable under mildly acidic aqueous conditions required for H2 evolution. Its pH-dependent flat valence band has a value of + 0.4 V vs NHE in neutral aqueous solution, [START_REF] Boschloo | Spectroelectrochemistry of Nanostructured NiO[END_REF] which lies between the frontier orbitals of typical photosensitizers. Thus, hole injection from the HOMO orbital of excited dyes can occur. [START_REF] Warnan | Multichromophoric Sensitizers Based on Squaraine for NiO Based Dye-Sensitized Solar Cells[END_REF] Different preparation methods exist [START_REF] Dini | The Influence of the Preparation Method of NiOx Photocathodes on the Efficiency of P-Type Dye-Sensitized Solar Cells[END_REF] (sol-gel deposition, [START_REF] Sumikura | Syntheses of NiO Nanoporous Films Using Nonionic Triblock Co-Polymer Templates and Their Application to Photo-Cathodes of p-Type Dye-Sensitized Solar Cells[END_REF][START_REF] Li | Double-Layered NiO Photocathodes for p-Type DSSCs with Record IPCE[END_REF] electrodeposition, [START_REF] Vera | Preparation and Characterization of Eosin B-and Erythrosin J-Sensitized Nanostructured NiO Thin Film Photocathodes[END_REF] hydrothermal synthesis [START_REF] Lepleux | Simple and Reproducible Procedure to Prepare Self-Nanostructured NiO Films for the Fabrication of P-Type Dye-Sensitized Solar Cells[END_REF] ) and nanostructured NiO films with different characteristics (porosity, thickness, conductivity) have been developed onto transparent conductive oxide substrates, like FTO. Recently, a comparison of dye-sensitized NiO photocathodes prepared by various techniques was realized. [START_REF] Wood | A Comprehensive Comparison of Dye-Sensitized NiO Photocathodes for Solar Energy Conversion †[END_REF] The same molecular dye and hydrogen-evolving catalyst were employed for a reliable correlation and some general trends were revealed. Thickness of around 1-2 μm and surface area of at least 40 m 2 g -1 were important for optimizing the performance of the photocathodes.

3) The anchoring groups Immobilization of the components onto the surface is a crucial step for the activity. Metal oxides are the most widely used SC. Their functionalization is based on the reactivity of surface bound -OH groups with anchoring groups of the molecular components to form monolayers. [START_REF] Pujari | Covalent Surface Modification of Oxide Surfaces Angewandte[END_REF][START_REF] Zhang | Anchoring Groups for Dye-Sensitized Solar Cells[END_REF] Chemical groups like carboxylic, phosphonic and cyanoacrylic acids are generally employed as anchoring groups, forming covalent inorganic ester linkages at the surface of the semiconductor for a strong coupling. Pyridines, catechols, amines and silanes can also be used.

Many parameters like pH, metal oxide, temperature, solvents affect the robustness of grafting.

Carboxylates are the most accustomed anchorages in photocathodes owing to their strong affinity to metal oxides, like NiO. [START_REF] Pellegrin | Ruthenium Polypyridine Complexes as Sensitizers in NiO Based P-Type Dye-Sensitized Solar Cells : Effects of the Anchoring Groups[END_REF] Different anchoring modes exist with monodentate or bidentate character due to the presence of two oxygen atoms. [START_REF] Galoppini | Linkers for Anchoring Sensitizers to Semiconductor Nanoparticles[END_REF] Bidentate modes show higher stability owing to superior contact with the semiconductors. Hydrogen bonding interactions may be also present. In a comparative study, the highest grafting densities on TiO2 were obtained with porphyrin dyes bearing carboxylates. [START_REF] Brennan | Comparison of Silatrane, Phosphonic Acid, and Carboxylic Acid Functional Groups for Attachment of Porphyrin Sensitizers to TiO2 in Photoelectrochemical Cells[END_REF] Phosphonate is another extensively studied anchoring group, as a result of their high affinity to metal substrates. [START_REF] Mutin | Hybrid Materials from Organophosphorus Coupling Molecules[END_REF][START_REF] Queffélec | Surface Modification Using Phosphonic Acids and Esters[END_REF][START_REF] Hotchkiss | The Modification of Indium Tin Oxide with Phosphonic Acids: Mechanism Tuning of Surface Properties , and Potential for Use in Organic Electronic Applications[END_REF] The three oxygen atoms allow mono-, bi-and tridentate anchoring modes, [START_REF] Brodard-Severac | High-Field 17-O MAS NMR Investigation of Phosphonic Acid Monolayers on Titania[END_REF] that may explain the superior anchorage stability compared to carboxylates in DSSCs. [START_REF] Hagfeldt | Dye-Sensitized Solar Cells[END_REF][START_REF] Brennan | Comparison of Silatrane, Phosphonic Acid, and Carboxylic Acid Functional Groups for Attachment of Porphyrin Sensitizers to TiO2 in Photoelectrochemical Cells[END_REF][START_REF] Gillaizeau-Gauthier | Phosphonate-Based Bipyridine Dyes for Stable Photovoltaic Devices[END_REF] Phosphonate groups were reported to offer more stable linkage under acidic aqueous conditions compared to carboxylate groups for ruthenium polypyridine complexes grafted onto TiO2. [START_REF] Hanson | Photostability of Phosphonate-Derivatized, Ru(II) Polypyridyl Complexes on Metal Oxide Surfaces[END_REF] A very recent study by the groups of F. Odobel and L. Hammarström with an immobilized on NiO organic dye bearing different anchoring groups showed that the phosphonic acid was the strongest one of the series, leading to the highest dye loading. [START_REF] Farré | A Comparative Investigation of the Role of the Anchoring Group on Perylene Monoimide Dyes in NiO-Based Dye-Sensitized Solar Cells[END_REF] The anchoring group may influence the electronic properties of the components and the charge transfer kinetics after light irradiation. Hence, choosing the right anchoring group is an important and demanding procedure.

4) The photosensitizer

The main requirements of a photosensitizer are:

 High extinction coefficient and a broad light absorption, mainly in the visible region.

 Its highest molecular orbital (HOMO) should be more positive than the valence band of the semiconductor so that hole injection [START_REF] Warnan | Multichromophoric Sensitizers Based on Squaraine for NiO Based Dye-Sensitized Solar Cells[END_REF] can occur upon excitation.

 The reduction potential of the dye ought to have enough reducing power. This means to be more negative than the catalytic reduction potential of the catalyst.

 High photo-and electro-chemical stability under aqueous conditions.

The molecular structure of the dye is a field of extensive research. Following hole injection, the reduced state of the dye is generated. At this point, it is crucial the electron density to be transferred to the catalyst. The molecular architecture is a key for the efficiency of this electron transfer. The best-performing photosensitizers in DSSCs are donor-π-acceptor moieties. [START_REF] Ooyama | Photophysical and Electrochemical Properties , and Molecular Structures of Organic Dyes for Dye-Sensitized Solar Cells[END_REF] The HOMO is positioned close to the anchoring group and the semiconductor for more efficient hole injection and the LUMO away from it, slowing down charge recombination and favoring electron transfer to the redox mediator. These properties were also exploited in the context of H2-evolving photocathodes. [START_REF] Walter | Solar Water Splitting Cells[END_REF][START_REF] Queyriaux | Molecular Cathode and Photocathode Materials for Hydrogen Evolution in Photoelectrochemical Devices[END_REF][START_REF] Xu | Water Splitting Dye-Sensitized Solar Cells[END_REF][START_REF] Gibson | Dye-Sensitized Photocathodes for H2 Evolution[END_REF][START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF] Similar directionality of the electron transfer can be also found in ruthenium dyes. [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] Examples of push-pull organic dyes and ruthenium photosensitizers integrated in dye-sensitized H2-evolving photocathodes will be presented later.

5) The catalyst

A catalyst for the reduction of protons to hydrogen is the final electron acceptor of the system. For catalysis to occur, its catalytic reduction potential must be less negative than the reduction potential of the dye. Stability under light irradiation, aqueous conditions and accumulation of two electrons (multielectronic processes for proton reduction) is decisive.

The development of new molecular catalysts based on non-noble metals (Fe, Co, Ni) is a crucial issue. [START_REF] Gordon | Metal Stocks and Sustainability[END_REF] An advantage of using molecular catalysts is that their electronic properties can be modified by changing the ligands substituents. In that context, bio-inspired complexes have emerged based on the catalytic center of hydrogenases. In the field of dye-sensitized hydrogenevolving photocathodes, cobalt and nickel complexes are the mostly used catalysts and more detailed analysis will follow. [START_REF] Walter | Solar Water Splitting Cells[END_REF][START_REF] Queyriaux | Molecular Cathode and Photocathode Materials for Hydrogen Evolution in Photoelectrochemical Devices[END_REF][START_REF] Xu | Water Splitting Dye-Sensitized Solar Cells[END_REF][START_REF] Gibson | Dye-Sensitized Photocathodes for H2 Evolution[END_REF][START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF]

II. Contribution from the group in the field

Our group is highly involved in the field of bio-inspired chemistry and artificial photosynthesis for the production of solar fuels. Over the last years, cobalt complexes proved to be efficient noble metal-free catalysts for proton reduction. [START_REF] Thoi | Complexes of Earth-Abundant Metals for Catalytic Electrochemical Hydrogen Generation under Aqueous Conditions[END_REF][START_REF] Artero | Splitting Water with Cobalt[END_REF][START_REF] Losse | Catalytic Hydrogen Production at Cobalt Centres[END_REF] In particular, a family of Co-based compounds called cobaloximes, with two glyoxime ligands (Chart I.1), [START_REF] Schrauzer | Organocobalt Chemistry of Vitamin B12 Model Compounds (Cobaloximes)[END_REF] were reported in 1980s by Ziessel for hydrogen evolution under homogeneous conditions. [START_REF] Hawecker | Efficient Homogeneous Photochemical Hydrogen Generation and Water Reduction Mediated by Cobaloxime or Macrocyclic Cobalt Complexes[END_REF] Increased research interest was revived after 2005 for electro-or photo-chemical hydrogen evolution and cobaloximes have been extensively used, including our group, due to their easy synthesis, high turnover numbers and relatively low overpotentials. [START_REF] Dempsey | Hydrogen Evolution Catalyzed by Cobaloximes[END_REF][START_REF] Eckenhoff | Cobalt Complexes as Artificial Hydrogenases for the Reductive Side of Water Splitting[END_REF][START_REF] Razavet | Proton Electroreduction Catalyzed by Cobaloximes : Functional Models for Hydrogenases[END_REF][START_REF] Baffert | Cobaloximes as Functional Models for Hydrogenases . 2 . Proton Electroreduction Catalyzed by Difluoroborylbis(Dimethylglyoximato)Cobalt (II) Complexes in Organic Media[END_REF][START_REF] Hu | Electrocatalytic Hydrogen Evolution by Cobalt Difluoroboryl-Diglyoximate Complexes[END_REF][START_REF] Hu | Electrocatalytic Hydrogen Evolution at Low Overpotentials by Cobalt Macrocyclic Glyoxime and Tetraimine Complexes[END_REF] More robust catalysts, like cobalt diimine-dioxime (Chart I.1), have attracted our interest and in the corresponding section, an overview of the work on this catalyst will be presented.

Chart I.1. Proton-bridged and difluoroboryl cobaloximes and cobalt dimine-dioxime complexes

1) The cobalt diimine-dioxime catalyst

One of the main limitations of cobaloximes is their low stability under acidic aqueous conditions. [START_REF] Mccormick | Impact of Ligand Exchange in Hydrogen Production from Cobaloxime-Containing Photocatalytic Systems[END_REF] Hydrolysis of the oxime bridge results in the displacement of the glyoxime ligands. This imperfection motivated our group to examine more robust cobalt catalysts for hydrogen evolution. Cobalt diimine-dioxime complexes (Chart I.1), promoted by Marzilli's group in late 1980s, with a single tetradentate ligand seemed a promising alternative. [START_REF] Seeber | Electrochemical Synthesis of "Costa-Type" Cobalt Complexes[END_REF] Synthesis is based on Schiff base condensation of butanedione-monoxime on diamine compounds. Metalation can readily occur by reaction with a metal-salt and oxidation to the more stable Co(III) derivative with two axial halide ligands in an octahedral configuration. [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine -Dioxime Complexes[END_REF] The C2 carbon of the propyl bridge can be functionalized enabling further grafting onto electrodes and covalent coupling with photosensitizers

i) Co(DO)(DOH)pnX2 as electrocatalyst for proton reduction

Our group reported for the first time in 2009 a series of tetradentate diimine-dioxime complexes with Co as the central metal for electrocatalytic hydrogen evolution. [START_REF] Jacques | Cobalt and Nickel Diimine-Dioxime Complexes as Molecular Electrocatalysts for Hydrogen Evolution with Low Overvoltages[END_REF] Their cyclic voltammograms (CV) display two reversible and monoelectronic cathodic processes attributed to two reductions, Co(III)/Co(II) and Co(II)/Co(I). In the presence of a proton source (organic acids), cobalt complexes showed catalytic activity for hydrogen evolution close to the second redox couple. Bulk electrolysis in organic media proved their ability to evolve hydrogen reaching faradaic efficiencies (F.E.) close to unity and tens of turnover numbers (TON). An increased stability at low pH compared to cobaloximes was observed and ascribed to the stable tetradentate nature of the ligand, which is resistant to hydrolysis.

When cobalt diimine-dioxime was grafted onto multiwall carbon nanotubes (Figure I.2) for the assessment of its catalytic activity under fully aqueous conditions, increased stability was observed. [START_REF] Andreiadis | Molecular Engineering of a Cobalt-Based Electrocatalytic Nanomaterial for H2 Evolution under Fully Aqueous Conditions[END_REF] The azido derivative complex was synthesized and coupled using Bertozzi's procedure (Cu-free azide-alkyne cycloaddition) on a functionalized cyclooctyne. [START_REF] Sletten | From Mechanism to Mouse: A Tale of Two Bioorthogonal Reactions[END_REF] Under bulk electrolysis at -0.59 V vs RHE in acetate buffer (pH 4.5), current densities of 1 mA.cm -2 were determined. Hydrogen was evolved reaching 55000 TONs after 7 hours of operation and 97% of faradaic efficiency (F.E.). Post-operando XPS analysis and CV excluded significant decomposition that could affect hydrogen evolution. [START_REF] Cobo | A Janus Cobalt-Based Catalytic Material for Electro-Splitting of Water[END_REF] Finally, this molecular cathode was tested under the presence of oxygen, which did not affect significantly the activity of the catalyst. [START_REF] Kaeffer | Oxygen Tolerance of a Molecular Engineered Cathode for Hydrogen Evolution Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] Oxygen tolerance could be beneficial as traces of oxygen can be present in DSPEC devices.

ii) Photocatalytic hydrogen evolution with Co(DO)(DOH)pnX2

Co-diimine-dioxime catalyst and its derivatives have been operated in homogeneous photocatalytic systems using different photosensitizers and sacrificial electron donors. [START_REF] Probst | Photocatalytic H2 Production from Water with Rhenium and Cobalt Complexes[END_REF][START_REF] Guttentag | Photocatalytic H2 Production with a Rhenium / Cobalt System in Water under Acidic Conditions[END_REF][START_REF] Zhang | Phosphine Coordination to a Cobalt Diimine -Dioxime Catalyst Increases Stability during Light-Driven H2 Production[END_REF] Our group employed [Co(DO)(DOH)pnBr2] in combination with an iridium photosensitizer. [START_REF] Zhang | Phosphine Coordination to a Cobalt Diimine -Dioxime Catalyst Increases Stability during Light-Driven H2 Production[END_REF] Under optimal conditions in a mixture of organic/aqueous solvents and after 10 hours of illumination, the photocatalytic system reached 700 TON. Long-term stability was improved by addition of PPh3, known for stabilizing low oxidation states of cobalt complexes. Thus, the catalytic activity was enhanced by stabilizing the Co(I) state, the starting point of the mechanism. The stability of the catalyst further increased the overall activity of the system for 10 hours, as the presence of a stable electron acceptor (catalyst) throughout photocatalysis, decreased the possibility of accumulating reduced dye species that can be decomposed over time. The importance of the ratio of dye/catalyst (optimal 2/1), the SED (TEA more efficient than TEOA), the pH (optimal pH 10), the amount of water in the mixture (1/1 water/acetonitrile) and the choice of photosensitizer (ruthenium complex was less active) were also examined.

2) Model dye-sensitized photocathodes

Before the preparation of dye-sensitized hydrogen-evolving photocathodes, our group developed model dye-sensitized photocathodes, where only the light harvester was immobilized in order to learn about grafting and to study the photoelectrochemical activity under pertinent conditions. Their performance was evaluated in aqueous media with the presence of an irreversible electron acceptor as a simplified replica of the catalyst.

Ruthenium polypyridine complexes were grafted onto homemade nanostructured mesoporous ITO films, prepared by template sol-gel technique. [START_REF] Hamd | Dye-Sensitized Nanostructured Crystalline Mesoporous Tin-Doped Indium Oxide Films with Tunable Thickness for Photoelectrochemical Applications[END_REF] We selected phosphonate as anchoring groups based on the studies presented in the corresponding section. Complexes with two phosphonates (RuP2 OH -bpy) were immobilized on the films by soaking overnight in a methanolic solution. CV of the electrodes showed a typical oxidation peak for Ru(II)/Ru(III).

Its intensity increased linearly with increasing scan rates, an indication of efficient grafting.

Dye loading of some nmol.cm -2 was estimated by integrating the anodic peak and by UV-Vis spectroscopy. Photoelectrochemical measurements were carried out with the presence of a cobalt complex as an irreversible electron acceptor (IEA) in aqueous solution. Photocurrent densities of -50 μΑ.cm -2 were obtained under visible light irradiation, ascribed to an efficient electron transfer from the dye to the electron acceptor.

Continuing the work on ruthenium dyes, we compared the spectroscopic properties and the photoelectrochemical activities of NiO (prepared by screen-printing) photocathodes sensitized by different ruthenium tris diimine complexes, bearing either two or four methyl phosphonate anchoring groups and a bipyridine or a dipyridophenazine ancillary ligand (dppz) (Chart I.2). [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] The methylene spacers were used as they can reduce the electronic coupling between NiO and the light harvester, slowing down the unwanted recombination processes. [START_REF] Pellegrin | Ruthenium Polypyridine Complexes as Sensitizers in NiO Based P-Type Dye-Sensitized Solar Cells : Effects of the Anchoring Groups[END_REF][START_REF] Gennari | Long-Lived Charge Separated State in NiO-Based P-Type Dye-Sensitized Solar Cells with Simple Cyclometalated Iridium Complexes[END_REF] PEC activity was assessed in aqueous electrolyte with [Co(NH3)5Cl]Cl2 as IEA at 0 V vs Ag/AgCl and the most active complex was RuP4 OH -dppz. The corresponding photocathode displayed photocurrents up to -190 μA.cm -2 . After light excitation, the formation of a long-lived charged separated state with a hole on NiO and one electron on the complex (PS -) was observed by nanosecond transient absorption spectroscopy (TAS). The more extended π-ligand stabilizes the electron density away from the NiO surface, leading to increased efficiency. Two recombination pathways were proposed for all complexes. A faster one (some hundreds of ps), when the electron density is localized on the phosphonate bpy ligands closer to NiO and a slower one (2 ns) when it is localized on the ancillary ligand (bpy or dppz).

Chart I.2. Ruthenium polypyridine complexes bearing methyl phosphonate anchoring groups immobilized on ITO and NiO films.

Based on the results, the anchoring groups did not affect the electronic properties of the dyes, in agreement with the non-conjugated nature of the linker. Increasing their number from two to four, the dye loading was two times higher. Furthermore, a third ligand with an electronwithdrawing nature is important for the delocalization of the electron on the bridging ligand in a dye-catalyst assembly, thus away from the semiconductor. In the literature, higher efficiencies for DSSCs were also observed for ruthenium polypyridine complexes onto NiO electrodes, when they were functionalized with an electron accepting group on the bipyridine ligands away from the anchoring groups. [START_REF] Han | Interfacial Electron Transfer Yields in Dye-Sensitized NiO Photocathodes Correlated to Excited-State Dipole Orientation of Ruthenium Chromophores[END_REF] It should be highlighted that these observations were the basis for designing the systems I worked on during this thesis.

In parallel and in collaboration with Pr.

III. State-of-the-art on dye-sensitized hydrogen-evolving photocathodes

One important characteristic of a dye-sensitized photocathode is how the components are assembled together in the architecture. "layer-by-layer" approach.

1) Catalyst in solution

The pioneering work of L. Sun in 2012 was the first example of a functional dye-sensitized hydrogen-evolving photocathode. [START_REF] Li | Visible Light Driven Hydrogen Production from a Photo-Active Cathode Based on a Molecular Catalyst and Organic Dye-Sensitized p-Type Nanostructured NiO[END_REF] A molecular H2-evolving catalyst, more specifically a cobaloxime (HER 1), and an organic dye (PS 1) were employed. The photosensitizer was grafted onto nano-structured NiO films through carboxylate anchoring groups and the catalyst was physisorbed onto the electrode. In phosphate buffer at pH 7, a photocurrent density of around -10 μΑ.cm -2 was observed at -0.4 V vs Ag/AgCl. Applying this potential, hydrogen evolution was confirmed by a Clark-type electrode in the solution, with up to 90 nmol after 30 minutes of continuous visible light irradiation. Photocurrent decay due to catalyst decomposition or leaching from the surface could be solved with re-addition of fresh catalyst.

Following this study, the field started to evolve and research groups around the world were keen on optimizing the activity of the photocathode counterpart of a tandem cell.

Aqueous stability is one of the main limitations for the activity of dye-sensitized photocathodes. A work published by Y. Wu in 2016 tried to address this challenge by taking advantage of the role of membranes in natural cells. [START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF] They constructed an organic donoracceptor dye (PS 2) capable of resisting to dye desorption and semiconductor degradation at acidic aqueous solution. The hydrophobic hexyl groups created a protective layer in which water and protons cannot penetrate, whereas the two perylenemonoimide groups can act as the hydrophilic environment for interaction with the protic electrolyte. Many molecular catalysts, including cobaloximes, are unstable in acidic conditions. Hence, a cubane molybdenum sulfide cluster (HER 2) was chosen as a HER catalyst. [START_REF] Hinnemann | Biomimetic Hydrogen Evolution : MoS2 Nanoparticles as Catalyst for Hydrogen Evolution[END_REF] The activity was assessed in pH 0 at -0.3 V vs NHE and the evolution of hydrogen was confirmed by gas chromatography, reaching an average faradaic efficiency of 50%. Higher photocurrents were observed at lower pH with maximum at pH 0 (-300 μΑ.cm -2 at -0.3 V vs NHE applied potential) sustained for almost 16

hours. Such a stability is attributed to the shielding that the hydrophobic hexyl groups offer to the system. Thus, water cannot easily reach the semiconductor surface to detach the dye.

An alternative approach to stabilize the dye against desorption relies on the deposition of a thin layer of Al2O3 by atomic layer deposition (ALD), as reported by the group of M. R.

Wasielewski's. [START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3-Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF] The understanding of the electron transfer dynamics at the NiO interface and more specifically between the semiconductor and the photosensitizer and between the latter and the catalyst is quite tricky. Our group in collaboration with B. Dietzek and A. B. Muñoz-García, recently addressed the first process using quantum chemistry and transient absorption measurements and they correlated the results with a dye-sensitized photocathode able to evolve hydrogen. [START_REF] Massin | Investigating Light-Driven Hole Injection and Hydrogen Evolution Catalysis at Dye-Sensitized NiO Photocathodes : A Combined Experimental-Theoretical Study[END_REF] An organic push-pull dye (PS 4) was grafted onto mesoporous NiO films and was examined for PEC hydrogen evolution with a cobaloxime (HER 1) in solution.

Chronoamperometric measurements were carried out at -0.4 V vs Ag/AgCl for 6 hours under continuous irradiation in aqueous electrolyte with a faradaic efficiency of 3% and 12 nmol.cm - 2 of evolved hydrogen. The key feature of this study is that a long-lived charge separated state is needed for the activity, especially when the catalyst is dissolved in the solution and diffusion governs the electron transfer.

Chart I.4. Hydrogen-evolving catalysts (HER 1-8) used for dye-sensitized hydrogen-evolving photocathodes.

2) Covalent or supramolecular dye-catalyst assemblies

The first example of a functional dye-sensitized hydrogen-evolving photocathode based on a supramolecular assembly was reported in 2013 by the group of Y. Wu. [START_REF] Ji | Photostable P-Type Dye-Sensitized Photoelectrochemical Cells for Water Reduction[END_REF] A NiO photocathode was soaked in a solution of a cyclometalated ruthenium sensitizer which was grafted onto NiO through carboxylate groups. Then, the sensitized film was placed in an aqueous solution of a cobaloxime for 5 minutes and axial coordination of the pyridyl group of the dye with the cobalt metal center resulted in Dyad 1. Chronoamperometric measurements at -0.2 V vs NHE under continuous light irradiation for 2.5 hours in aqueous phosphate buffer at pH 7 were carried out and an amount of 290 nmol of hydrogen was produced, corresponding to 45% F.E. An increase of F.E. to 68%, when the photoelectrocatalytic experiments were conducted at less negative potentials (-0.1 V vs NHE) was observed. ALD coating before sensitization increased the photocurrent density probably due to suppression of the hole-electron recombination processes.

Chart I.6. Dye-catalyst assemblies (Dyad 1-6) used for dye-sensitized hydrogen-evolving photocathodes.

In 2016, our group designed the first noble metal-free covalent dye-catalyst assembly based on an organic push-pull dye and the cobalt diimine-dioxime catalyst (Dyad 2a). [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] The pushpull character appears attractive by enabling the unidirectional electron transfer from the NiO surface toward the catalyst. [START_REF] Ooyama | Photophysical and Electrochemical Properties , and Molecular Structures of Organic Dyes for Dye-Sensitized Solar Cells[END_REF] The two components were attached using click chemistry, the copper-catalyzed azide-alkyne cycloaddition (CuAAC). [START_REF] Kolb | Click Chemistry : Diverse Chemical Function from a Few Good Reactions[END_REF] Long term electrolysis at -0.14 V vs RHE in a pH 5.5 MES buffer was carried out. Chenodeoxycholic acid (CDCA) was used as a coadsorbent to avoid aggregation of the dyad on the NiO surface and hydrogen was confirmed by gas chromatography with F.E. of 8-10%. Post-operando XPS analysis revealed that one possible reason for the low faradaic efficiency can be the competitive reduction of the bulk NiO electrode, although reductive decomposition of the molecular assembly could not be excluded. [START_REF] Kaeffer | The Dark Side of Molecular Catalysis: Diimine -Dioxime Cobalt Complexes Are Not the Actual Hydrogen Evolution Electrocatalyst in Acidic Aqueous Solutions[END_REF] Re-evaluation using a microsensor Clack-type electrode for quantifying hydrogen in the solution showed 66% increase of the F.E. [START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF] Our group highlighted the importance to quantify H2 in the solution and in the headspace in order to have more accurate results, as a fraction of H2 remains diluted in the electrolyte even after some hours of experiments.

Moreover, the insertion of a second thiophene unit (Dyad 2a) between the donor and the acceptor part of the dye was shown to slightly improve the efficiency due to a better charge separation or a slower charge recombination.

Another dyad covalently linking an organic dye derived from PS1 with the cobalt diiminedioxime complex on mesoporous NiO films was prepared using a copper-free click reaction (Dyad 3). [START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye -Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF] Time-resolved fluorescence and transient absorption spectroscopy showed that upon irradiation of the photocathode, an ultrafast hole injection from the excited dye into NiO occurs, followed by an electron transfer from the reduced dye to the catalyst. The second electron transfer to reduce to Co(I) could not be identified, but hydrogen evolution was detected during photoelectrochemical experiments. In acetate buffer at pH 4.8 at 0 V vs Ag/AgCl, the system produced an average photocurrent of -10 μA.cm -2 but only 2.5 nmol of hydrogen and 0.05 TON.

In 2019, dye-catalyst assemblies based on ruthenium photosensitizers and noble metal-based molecular catalysts (Pt, Pd) were grafted onto NiO films (Dyad 4, 5). [START_REF] Põldme | Photoelectrocatalytic H2 Evolution from Integrated Photocatalysts Adsorbed on NiO[END_REF] The corresponding photocathodes exhibited cathodic and stable photocurrents at pH 3. Continuous irradiation (AM 1.5 filter, 1 sun) for 1 hour with an applied potential confirmed the reduction of protons to hydrogen with faradaic efficiencies varying from 30 to 90%. At -0.2 V vs Ag/AgCl, 46 TON were determined for the Ru-Pd photocatalyst. On one hand, post-operando analysis showed that the photosensitizer and the Pd catalyst remained stable after 1 hour of PEC tests. On the other hand, decomposition of the Pt complex and formation of nanoparticles could explain the higher performance of the system with Dyad 4.

Recently, the group of C. Olivier in collaboration with our group developed an original supramolecular dyad based on a pyridyl-functionalized ruthenium-diacetylide photosensitizer [START_REF] Massin | Design and Synthesis of Novel Organometallic Dyes for NiO Sensitization and Photo-Electrochemical Applications[END_REF] coupled via axial coordination to a cobaloxime (Dyad 6). [START_REF] Lyu | H2 -Evolving Dye-Sensitized Photocathode Based on a Ruthenium Diacetylide / Cobaloxime Supramolecular Assembly[END_REF] Theoretical calculations showed that in the LUMO, electron density is localized on the cobalt atom of the dyad, in contrast to what observed for the HOMO. Thus, electron transfer from the excited state of the dye to the catalyst should occur. The photoelectrocatalytic activity of the NiO photocathode was assessed in acetate buffer at pH 4.5 at + 0.07 V vs RHE under visible-light irradiation for 4.5 hours. The photocurrent density was -3.2 μΑ.cm -2 with gradual decrease during the long-term photoelectrolysis, probably due to leaching of the dyad from the surface. Gas chromatography proved the PEC hydrogen evolution with F.E. of 27%. Control experiments without the coordinated cobaloxime showed only traces of hydrogen produced.

3) Co-grafting

The group of L. Sun was the first to prepare dye-sensitized hydrogen-evolving photocathodes by the co-grafting strategy. [START_REF] Fan | Pt-Free Tandem Molecular Photoelectrochemical Cells for Water Splitting Driven by Visible Light[END_REF] A molecular ruthenium complex (PS 5) bearing phosphonate anchoring groups and a cobaloxime with a pyridine-2,6-dicarboxylic acid were co-anchored onto NiO films (HER 5). The corresponding photocathode afforded cathodic photocurrents of -13 μΑ.cm -2 at -0.4 V vs Ag/AgCl at pH 7. A tandem cell with a photoanode based on the same dye and a molecular WOC was examined at + 0.4 V vs Ag/AgCl with an average photocurrent density of -40 μΑ.cm -2 . Although the amount of H2 produced was not reported, this study was an inspiration for the construction of full DSPECs. Moving one step forward, they prepared the first functional tandem cell for water splitting. [START_REF] Li | Organic Dye-Sensitized Tandem Photoelectrochemical Cell for Light Driven Total Water Splitting[END_REF] On the photocathode, an organic dye (PS 1) and a cobaloxime (HER 6) bearing carboxylate and phosphonate anchoring groups respectively were co-grafted onto NiO films. At -0.2 V vs Ag/AgCl in a pH 7 phosphate buffer solution, the photocathode produced 290 nmol of hydrogen with a faradaic efficiency of 68% after 90 minutes of continuous irradiation. The co-grafting is clearly beneficial for the stability and activity of the photocathode compared to the previous reported system with the physisorbed catalyst. [START_REF] Li | Visible Light Driven Hydrogen Production from a Photo-Active Cathode Based on a Molecular Catalyst and Organic Dye-Sensitized p-Type Nanostructured NiO[END_REF] The tandem cell with a TiO2 photoanode in aqueous electrolyte at pH 7 without any applied bias evolved 330 nmol of hydrogen with a 55% faradaic efficiency.

A biomimetic Fe-Fe catalyst (HER 7a) was co-adsorbed with a coumarin dye (PS 6) on mesoporous NiO films by the group of L. Hammarström. [START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye -NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF] The photocathode produced 13 nmol of hydrogen (3 TON) with a 50% F.E. after 1100 s of irradiation in acetate buffer (pH 4.5) with an applied potential of -0.3 V vs Ag/AgCl. The loss of activity was caused by the catalyst decomposition and detachment, as evidenced by FTIR spectrum. Transient absorption measurements revealed the surface electron transfer process between the molecular components in the coadsorbed system, following the ultrafast hole injection from the excited dye. It was observed that the catalyst reduction favorably competes with the relatively slow charge recombination between the reduced dye and the semiconductor. Finally, the reduced catalyst is sufficiently long-lived for catalysis to occur. This was ascribed to the distance between the iron center and the NiO surface due to the long linker. [START_REF] Gardner | Light-Driven Electron Transfer between a Photosensitizer and a Proton-Reducing Catalyst Co-Adsorbed to NiO[END_REF] This was further supported by the fact that a similar photocathode based on HER 7b catalyst with a shorter linker did not produce any hydrogen.

Our group contributed to the co-grafting strategy with the preparation of a NiO photocathode based on an organic dye (PS 7) and a cobalt-diimine-dioxime catalyst (HER 8). [START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2-Evolving Dye-Sensitized Photocathode[END_REF] Higher dye loading and stability against detachment are the main reasons for this improvement.

The best performing photocathode was a NiO co-sensitized with a perylene monoimide dye (PS 9) and HER 4. 215 nmol.cm -2 of hydrogen, 48 TON and 41% FE after 2 hours of continuous irradiation at 0 V vs RHE in aqueous electrolyte (pH 3) were determined. One of the main reasons for this higher activity is the higher light harvesting efficiency in the visible region of PS9 compared to PS8.

In 2019, our group in collaboration with O. Ishitani and R. Abe reported a functional tandem photoelectrochemical cell. 102 An organic push-pull dye (PS 7) and a cobaloxime (HER 5) were grafted onto NiO electrodes. P-type semiconductors like CuGaO2, which are more conductive and with a more positive flat band potential compared to NiO, were also tested. 103 Under the best conditions, the CuGaO2 photocathode was more efficient reaching 555 nmol of H2 with a 75% F.E. and TON ranging from 82 to 640 over 2 hours of continuous irradiation at 0 V vs RHE. With this photocathode in hand, which is the most efficient in the literature in terms of TON, a tandem cell with a photoanode was prepared. The TaON/CoOx photoanode and the photocathode split water into hydrogen and oxygen with comparatively activities of the separate electrodes. After performing ICP-OES measurements, leaching of the cobalt complex from the surface was evidenced probably due to weak interaction between the pyridyl group and the metal center.

4) "Layer-by-layer" assembly "Layer-by-layer" assembly can contribute to spatial separation similarly to the covalent approach but without involving demanding synthetic steps for a covalent coupling. The first example of a dye-sensitized photocathode based on this architecture was reported in 2016 by the group of E. Reisner. 104 NiO films were first sensitized by a Ru-PS (PS 10), then the electrode was dipped in a solution of ZrOCl2 for the bounding of the phosphonic acid groups of the Ru-complex to Zr(IV). Finally, the nickel catalyst (HER 4) was attached to Zr(IV) and the assembly was assessed in aqueous electrolyte (pH 3) at + 0.3 V vs RHE under continuous irradiation. Moderate amounts of hydrogen (6 nmol in the headspace) and F.E. (around 9%)

were obtained. Slightly higher activity was observed with two layers of the Ni-catalyst after two immobilization cycles, showing that the ratio of dye/catalyst influences the performance.

The low activity of this assembly was attributed to inefficient charge transfer dynamics at the interfaces, as well as a low loading of the catalyst (around 5 nmol.cm -2 ). Direct comparison with a co-grafting approach, which exhibited negligible photocurrents, indicated its superior activity. A possible explanation is the proximity of the catalyst with the semiconductor for the co-grafting method, leading to fast recombination processes. Furthermore, the "layer-by-layer" assembly had a positive impact on the stability of the system, as 50% of the light harvester remained grafted on the surface after 2 hours of PEC test compared to some traces when the two molecular components were immobilized on the surface.

The same year, T. J. Meyer's group prepared a tri-layer photocathode based on a donor-dyecatalyst assembly to further increase the distance of the catalyst (HER 4) from the surface. 105 The insertion of dianiline as electron donor in between NiO and the light harvester (PS 11) allows ITO to be used as SC instead of p-type NiO. Transient-absorption measurements revealed relative slow recombination processes between the reduced dye and the hole in NiO.

Hence, electron transfer to the catalyst can favorably compete and this is the basis of the efficiency of this system. PEC tests on macro-mesoporous IO-ITO offered high surface loading and higher activity to previously reported photocathodes with 53% F.E. in MES buffer (pH 5.1) at + 0.05 V vs RHE. Initial photocurrent density of -56 μA.cm -2 dropped by 50% within 4 h of operation. Post-operando analysis showed that desorption or degradation of the catalyst are the two main problems for the decay of the photocurrent. In contrast, negligible H2 evolution was observed using meso-ITO films, demonstrating that macromesopores ensure sufficient assembly coverage.

Continuing their work, the preparation of multilayered assemblies with atomic layer deposition was accomplished. 12) decreased the efficiency of the photocathodes mainly due to their highly conjugated nature that accelerates back electron transfer.

5) Narrow band gap photoactive semiconductors

Fabrication of hydrogen-evolving photocathodes with narrow band gap semiconductors, which can behave as light harvesters and achieve charge separation, is an alternative to p-type semiconductors. Although the scope of this thesis is different, some recent breakthroughs on this field and particularly when combined with molecular photosensitizers will be presented.

P-type silicon (second most abundant element in the earth's crust) materials are attractive photocathodes due to their low-cost, visible light absorption and has been already used for hydrogen evolution. 108 A nanostructured p-Si photocathode in water with the Ni bis(diphosphine)-based catalyst (HER 4) anchored on the surface has been prepared by the group of E. Reisner. 109 In a more elegant approach by T. J. Meyer, a p-Si photocathode modified by a "layer-by-layer" assembly of an organic dye (derivative of PS3) and the same nickelcatalyst (HER 4) showed significant activity for hydrogen evolution with photocurrent density of -1 mA.cm -2 at 0 V vs NHE. 110 The additional dye (heterojunction) was used as an electron transfer mediator between the photo responsive p-Si electrode and the catalyst. A tandem cell with a SnO2 photoanode with two Ru-complexes as PS and OEC achieved water splitting and the efficiency seems to be hindered by the performance of the photoanode. Thus, in this case the limiting factor for an efficient tandem cell is the photoanode and not the photocathode. This work is an inspiration for utilization of both a photoactive semiconductor and a molecular photosensitizer in order to afford more efficient light absorption and higher performances.

Mesoporous perovskite LaFeO3 is a stable material under irradiation and in aqueous solutions and has a proper band gap energy for visible light absorption. 111 Hence, potential candidate as a p-type visible-light-absorbing semiconductor for light-driven H2 generation.

Successful modification with a molecular dye and HER 4 produced hydrogen with photocurrent densities of -20 μΑ.cm 2 and 48% F.E. at + 0.6 V vs RHE by the group of L. Sun. 112 Inferior activity was obtained for the photocathode without the grafted dye. ICPE spectra indicated that both the perovskite and the molecular dye absorb light and contribute to the photocurrent generation.

IV. Figures of merit and current limitations

1) Performance of the state-of-the-art photocathodes

The development of artificial photosynthetic systems, capable of harvesting light energy and converting it into an electrochemical gradient of potential to perform catalytic reactions of interest (oxidation of water, reduction of protons, carbon dioxide, etc.) is a field of intensive research. Over the last years, dye-sensitized photoanodes for water oxidation have reached milestones with high catalytic efficiencies and photocurrent densities of 1 mA.cm -2 . 113 For the construction of an operating tandem DSPEC, the two counterparts should exhibit similar activity.

The reported dye-sensitized photocathodes are based on four different architectures, as already presented. The first architecture follows the concept of p-DSSCs but with a H2-evolving catalyst in the solution instead of a redox mediator. This approach is not promising, as the electron transfer is diffusion-limited and recombination reactions can be favored. Systems that bring the molecular components in closer contact are needed. The second arrangement based on a covalent or supramolecular dye-catalyst assembly can enhance the directionality of electron transfer and the charge separation, because the catalyst is placed away from the semiconductor. In addition, the dyad enables to perform analytical studies in solution prior to grafting. It is also a logical continuation of the work already done with dyads or triads in the field of artificial photosynthesis. 114,115,116 For these reasons and based on the previous expertise of the group, we decided to adopt this architecture in this work, even though it is synthetically challenging. Co-grafting of the catalyst and the photosensitizer on the surface is a more straightforward procedure and it gives the opportunity to deconvolute the effects of the photosensitizer and the catalyst on the performance. Although the electron transfer between the dye and the catalyst is efficient due to the attachment of both on the electrodes, the proximity of the catalyst to the semiconductor can enhance unwanted recombination processes. In this approach, the ratio of adsorbed dye/catalyst is also difficult to be determined. Furthermore, the surface coverage of the film with the photosensitive unit is limited, as there is a competition for attachment with the catalyst. A way to overcome the complexity of synthesis and keep the directionality of the electron transfer is the "layer-by-layer" assembly of the molecular components. Nonetheless, the ratio of the two subunits is doubtful, arising questions about the influence of the dye and the catalyst on the activity.

In Table I.1, the figures of merit (the photocurrent densities (j), the amount of produced hydrogen, the turnover numbers and the faradaic efficiencies) together with the different experimental conditions of the hydrogen evolving photocathodes reported up to date are presented.

-Photocurrent density: Overall, the different photocathodes exhibit low photocurrent densities, in the order of some tens of μA.cm -2 , far away from the goal of 1 mA.cm -2 . Only two examples with values higher than 100 μA.cm -2 exist. [START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF][START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3-Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF] -Amount of H2 produced: Gas chromatography was mainly used for the quantification of the amount of hydrogen produced. High-sensitive micro Clark-type electrode was also employed in some cases for the determination of diluted hydrogen in aqueous solution. Recently, our group highlighted the importance to combine both techniques in order to have more accurate results especially when low photocurrents are observed, as a fraction of H2 remains in the electrolyte even after some hours of experiments. [START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF] In many cases, the surface of the electrode is not indicated in order to perform a comparison taking into account the size of the photocathodes. Thus, comparing the different systems with the amount of H2 produced is not so accurate.

-TON: In order to correlate the overall performance of the system with the intrinsic activity of the molecular components, the loading of the molecular components needs to be taken into consideration. Turn-over number (TON) is a reliable factor for direct comparison between the different photocathodes, at it excludes material-and device-based parameters. It indicates how many times the catalyst has participated in the catalytic cycle for the reduction of a substrate (proton, carbon dioxide, etc). Nevertheless, a lot of studies do not provide TON values, as surface characterization techniques are scarce and the amount of the loaded catalyst is not determined. In particular for co-grafting and "layer-by-layer" approaches, the surface organization that can affect the activity is questionable.

-Faradaic efficiency: The faradaic efficiency shows the percentage of charge converted to hydrogen (2 electrons per molecule of H2). We could say that it is a selectivity index for proton 2. Calculated based on the numerical values provided in the studies.

2) Kinetic issues

The few reported NiO-photocathodes still exhibit low activity in terms of photocurrent The origin of these unwanted recombination pathways was enlightened for NiO-based DSSCs. 117 One of the sources behind these detrimental procedures is the hole extinction coefficient 10 -8 -10 -7 cm 2 s -1 , which was estimated two times lower than for TiO2 photoanodes. 118,119 In addition, Ni 3+ defect species can act as hole (Ni 4+ ) and electron traps. 117,120,121 Atomic layer deposition (ALD) of thin layers of insulating Al2O3 onto NiO electrodes before sensitization was employed to slow down the recombination processes, 122,123 but undesirable slower electron injection from the semiconductor to the photosensitizer can occur with increasing number of ALD cycles. 124 Hence, a compromise between slower recombination and slower electron injection should be considered for optimization of the systems.

Nevertheless, ALD-treatment was used for hydrogen-evolving photocathodes, showing a beneficial impact. [START_REF] Ji | Photostable P-Type Dye-Sensitized Photoelectrochemical Cells for Water Reduction[END_REF]105 Efforts to replace NiO with other materials to increase the efficiency of H2-evolving photocathodes are ongoing with some interesting preliminary results. Delafossites (such as CuGaO2, CuCrO2) [START_REF] Yu | Cu(I)-Based Delafossite Compounds as Photocathodes in p-Type Dye-Sensitized Solar Cells[END_REF]100,101,102,103 is another family of metal oxides with interesting properties for both p-type DSSCs and DSPECs. Materials like CuO 125 and conductive ITO 105,126 with fast hole mobility could also offer alternative solutions.

It should be noted that most photophysical studies performed on NiO-photocathodes to address the kinetics of electron transfers were not recorded under operando conditions and especially under the potential applied for the PEC tests. However, recent studies by the group of L. Hammarström and T. J. Meyer revealed that charge recombination kinetics are strongly affected by the potential applied to the NiO film. 117,121 In particular, transient absorption measurements recorded on films sensitized with the ruthenium phosphonate complex [(Ru(bpy)2 (bpyPO3H2)] 2+ established that the lifetime of the charge separated state (hole on NiO and reduced dye) is drastically increased when potentials more negative than OCP are applied to the electrode. 121 This has also prevented to study the following processes that are the thermal electron transfer from the reduced dye to the catalyst and the charge recombination between the reduced catalyst and the NiO. In particular, the formation of the reduced catalyst has never been observed for dye-sensitized hydrogen-evolving photocathodes based on cobalt catalysts.

3) Stability issues

Long-term stability is another limiting factor for the efficiency of the dye-sensitized photocathodes. However, only a few studies addressed this issue by performing post-operando analysis to find out the destiny of the molecular components. [START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2-Evolving Dye-Sensitized Photocathode[END_REF] Most of the systems exhibit low stability due to desorption of the molecular components from the surface. Thus, more robust anchoring groups or stabilizing methods should be found. Atomic layer deposition of thin layers of Al2O3 can also contribute to decrease desorption. [START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3-Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF] Meyer's group proposed a what he called "mummy" strategy. 127 After grafting of a Ru-complex onto TiO2, Al2O3 ALD used for protection and for consequent grafting of a WOC. In a final step, Al2O3 ALD was employed for further catalyst stabilization and the photoanode successfully oxidized water.

For the different systems, the same catalysts are repeated and notably complexes based on cobalt and nickel. As far as cobalt catalysts concern, their decomposition under experimental conditions is speculated to be a limiting factor. More stable and robust catalysts should be employed to face this detrimental phenomenon, like polypyridyl cobalt complexes that showed resistance under reductive conditions. 128,129,130 Analyzing the state-of-the-art photocathodes, ideas of the path that we should follow can be extracted and more specifically:

 The various reported experimental parameters make it difficult to rationalize the activity of the different systems. Thus, we will assess the performance of the new photocathodes under strictly identical conditions to rationally improve the molecular design.

 The kinetics of the electron transfer under the potential applied for the photoelectrochemical experiments are unexplored. In this sense, we will undertake photophysical studies under quasi-operando conditions in order to understand the bottleneck of the electron transfer.

 Post-operando analysis is insufficient for a clear understanding of the fate of the molecular components. Undertaking in-depth characterization could help selecting more robust components and optimizing the molecular assemblies.

Chapter II Synthesis and characterization in solution of a novel dye-catalyst assembly based on the cobalt diiminedioxime complex

In this chapter, the synthesis and characterization of a dye-catalyst assembly, RuP4 OEt -Co, relying on a ruthenium tris-diimine photosensitizer functionalized with four methyl phosphonate groups for a covalent anchoring onto semiconducting films and covalently linked to the cobalt diimine-dioxime catalyst is presented (Scheme II.1). The choice of the components is not random, as we will rely on existing knowledge from the group and this part of my thesis project directly follows-up on Nicolas Queyriaux PhD thesis (defended in March 2016). In our effort to obtain a rational study for dye-sensitized photocathodes, we decided to synthesize a molecular architecture similar to the first reported dyad (Dyad 2) [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] with a different light harvesting unit, keeping the catalyst and the triazole assembly identical.

The structure of the light harvester, bearing a functionalized imidazo [4,5-f] [1,10]phenanthroline ligand, stems from supramolecular ruthenium-based assemblies studied for photocatalytic proton reduction. [START_REF] Fihri | Cobaloxime-Based Photocatalytic Devices for Hydrogen Production[END_REF][START_REF] Fihri | Efficient H2 -Producing Photocatalytic Systems Based on Cyclometalated Iridium-and Tricarbonylrhenium-Diimine Photosensitizers and Cobaloxime Catalysts[END_REF] Furthermore, the decision to functionalize the dye with four methyl phosphonate groups is inspired by our model NiO photocathodes sensitized by ruthenium complexes. [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] Finally, an optimized protocol using copper-catalyzed azide-alkyne cycloaddition for coupling the two subunits will be employed for the construction of the final assembly. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF]131 The redox and spectroscopic properties of the final dyad will be also evaluated.

Thermodynamic considerations supported by transient absorption spectroscopy and spectrophotochemical measurements will be presented to analyze the feasibility of the working mechanism onto NiO electrodes. We should note that the anchoring groups were protected until the end of this chapter for solution studies.

Chart II.1. Structure of RuP4 OEt -Co.

I. Synthetic strategy

In Scheme II.1, two proposed synthetic routes are depicted for the preparation of RuP4 OEt -Co, which will be analyzed in the following part.

Scheme II.1. Two proposed synthetic routes for the preparation of RuP4 OEt -Co.

1) Copper-catalyzed Azide-Alkyne Cycloaddition (CuAAC) coupling

Efficient and modular synthetic strategies to combine in a single molecule a photosensitizer and an electroactive catalytic unit appear particularly interesting, even though the preparation can be synthetically demanding. We selected Copper-catalyzed Azide-Alkyne Cycloaddition (CuAAC), established by Sharpless in 2002, 132 one of the most famous examples of "click" chemistry. [START_REF] Kolb | Click Chemistry : Diverse Chemical Function from a Few Good Reactions[END_REF] Regioselectivity to give 1,4-disubstituted 1,2,3-triazoles, mild experimental conditions, high yields and compatibility with most organic and metallo-organic functions are the main advantages of this synthetic tool. Examples of ruthenium tris-diimine assemblies coupled with a molecular moiety using CuAAC have been already reported. 133,134 Route 1, where Co-N3 can be coupled through CuAAC with RuP4 OEt -EPIP, is the most obvious one to build the dyad. However, we observed that under standard copper-catalyzed conditions for the azide-alkyne cycloaddition reaction, partial substitution of the cobalt center of the catalyst by copper ions can occur. [START_REF] Andreiadis | Molecular Engineering of a Cobalt-Based Electrocatalytic Nanomaterial for H2 Evolution under Fully Aqueous Conditions[END_REF] In parallel, a second route was developed by our group based on the use of the copper diimine-dioxime complex Cu-N3, to perform the CuAAC coupling either with a push-pull organic dye [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] or a ruthenium tris-diimine complex functionalized with an alkyne group, which was the first example of a paramagnetic binuclear Ru(II)-Cu(II) complex (Ru-Cu) efficiently produced using click chemistry (Scheme II.2). 131 In the case of the push-pull organic dye, the targeted dyad (Dyad 2) was isolated after performing transmetallation from copper to cobalt. Thus, the second route will be employed in order to prepare our final dye-catalyst assembly (RuP4 OEt -Co).

Scheme II.2. CuAAC to obtain the paramagnetic Ru-Cu dyad.

2) Functionalization with methyl phosphonate groups

Grafting of the molecular components onto semiconducting materials is crucial for the overall activity. Ruthenium-polypyridine photosensitizers are widely used for both photoanode and photocathode applications due to their long-lived photoexcited state, strong visible absorption and chemical stability. 135 Various synthetic modifications, consisting of two main steps, are well-described to tune their redox and spectroscopic properties. Initially, ligands are synthesized and purified and coordination on ruthenium follows. Despite its dominant role for the synthesis of ruthenium polypyridine complexes, 'chemistry on the complex' is an alternative with limited usage. 136 Different coordination routes have been developed starting by various ruthenium precursors, like [Ru(DMSO)4Cl2], [Ru(arene)Cl2]2 where arene = cymene, benzene, etc. or RuCl3 hydrate. 137,138 Based on the model photocathodes sensitized by ruthenium complexes from the group, four methyl phosphonate anchoring groups increased the dye loading onto NiO films, without modifying the electronic properties of the complex. [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] Two main synthetic routes to prepare ruthenium dyes with four methyl phosphonate anchoring groups (Scheme II.3) have been reported. The first one, called traditional route 22,139 ,140,141,142 relies on the preparation of the [cis-Ru(4,4′-(CH2PO3Et2)2-bpy)2Cl2] intermediate, while the second approach developed in 2010 exploits the properties of organometallic ruthenium complexes. 138,143,144 With the traditional route, the anchoring groups are introduced first followed by the substituted bpy ligand, while the latter route proceeds in the opposite order.

Scheme II.3. Schematic representation of the two synthetic routes to prepare ruthenium complexes with four methyl phosphonate groups.

In a recent publication, we evaluated the efficiency of both methods for the preparation of ruthenium complexes with substituted bpy or phen ligands. 145 The synthetic limitations encountered with the organometallic route forced us to re-examine the traditional approach and in particular the procedure reported by the group of S. Rau in order to prepare RuP4 OEt -phendione, starting from Ru(DMSO)4Cl2. [START_REF] Braumüller | Synthesis and Characterization of an Immobilizable Photochemical Molecular Device for H2-Generation[END_REF] The intermediate product after the introduction of the methyl phosphonate groups ([cis-Ru(4,4′-(CH2PO3Et2)2-bpy)2Cl2x(DMSO)x]Clx) was not purified and used immediately to the next reaction to obtain the final complex in high yields (86% overall crude yield). We reproduced this synthesis with 78% yield, and we were able to isolate RuP4 OEt -dppz (55% yield). RuP4 OEt -TMS-EPIP and RuP4 OEt -EPIP were also synthesized with lower yields (20% and 30% accordingly) and protection of the terminal alkyne is not required anymore with this procedure. Hence, the traditional route is a more efficient and straightforward way to prepare different ruthenium derivatives bearing four methyl phosphonate groups. The synthesis of the last two complexes will be presented in more details in the following part of this chapter.

II. Synthesis of RuP4

OEt -Co 1) Preparation of RuP 4 OEt -EPIP

We synthesized RuP4 OEt -EPIP through two different synthetic routes (Scheme II.4, 5):

i) TMS-protected route

Starting from the commercially available compound, Ru(dmso)4Cl2, 148 we were able to obtain RuP4 OEt -EPIP (Scheme II.4) in three steps. 145 The intermediate [cis-Ru(4,4′-(CH2PO3Et2)2-bpy)2Cl2-x(DMSO)x]Clx of the first reaction was not purified and could be stored under argon for days. We proceeded to the next reaction using an average molecular weight (1160 g.mol -1 ) and the TMS-EPIP ligand, 146 which was already available in the lab, in a mixture of ethanol and water (3/1) under argon.

Scheme II.4. Synthetic procedure for the preparation of RuP4 OEt -EPIP through the TMS-protected ligand.

A disappointing 10% yield was obtained during our first attempts. By increasing the amount of ligand to 1.5 eq, the yield was improved to 20%. Purification was achieved by column chromatography on silica gel with gradient elution (MeCN/aqueous 0.4 M KNO3, from 98:2 to 90:10). Addition of saturated aqueous solution of KPF6, did not precipitate the desired

ii) Direct route

We succeeded to prepare RuP4 OEt -EPIP in 30% yield by directly reacting EPIP ligand with

[cis-Ru(4,4′-(CH2PO3Et2)2-bpy)2Cl2-x(DMSO)x]
Clx (Scheme II.5). Maximum yield was 30%, after increasing the temperature and using 1.5 eq of the ligand. This procedure is more straightforward, as it contains one less reaction and purification step. Thus, we adopted this procedure for the synthesis of RuP4 OEt -EPIP. The final compound was characterized by NMR (more details in the corresponding part) and mass spectrometry. The expected peak in ESI-MS at m/z: 667.3 for [M-2PF6] 2+ is a proof for the isolation of the right complex. During the preparation of RuP4 OEt -EPIP, we observed by-products with partly hydrolyzed phosphonate groups, which are retained on silica gel column and could give a possible explanation for the low final yields. 148

EPIP ligand was prepared by Debus-Radziszewski condensation between the 4ethynylbenzaldehyde and the phendione (Scheme II.5). 146 After neutralization with aqueous NH4OH, the precipitate was filtered and we obtained the desired ligand with 90% yield (m/z:

321.3 for [M+H] + ).
3) CuAAC coupling The final dyad was isolated as a hexafluorophosphate salt after extraction with DCM with 55% yield. Due to its paramagnetic nature, NMR was not used for the characterization. The final structure was confirmed by ESI-MS with a main peak at m/z: 559.3 for [M-H2O-3PF6] 3+ with a characteristic isotopic pattern.

Scheme II.7. CuAAC to obtain the paramagnetic RuP4 OEt -Cu.

4) Transmetallation from RuP 4 OEt -Cu to RuP 4 OEt -Co

A key step for the synthesis of the final dyad is the exchange from copper to cobalt (Scheme II.8). [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF][START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2-Evolving Dye-Sensitized Photocathode[END_REF] Initially, we performed this reaction using Ru-Cu, after resynthesizing the complex (Scheme II.2). 131 We decided to work first with Ru-Cu, as the absence of phosphonate groups could be beneficial for the efficient transmetallation and isolation of the dyad. In addition, the final Ru-Co dyad could also serve as a reference system for photophysical studies. Metal exchange occurs by reacting the copper-derivative complex (Ru-Cu) with a Co(II)-salt (in our case CoBr2.hydrate) in acetone under air bubbling for the oxidation of Co(II) to Co(III) species, a more stable derivative. In contrast to previous dyads with organic dyes, ruthenium compounds exhibit poor solubility in acetone, affected by its charged nature, and addition of methanol was needed. After purification with column chromatography on silica gel using a mixture of acetone/H2O/aqueous KBr 90/9/1, we obtained Ru-Co in 60% yield.

Scheme II.8. Transmetallation procedure to obtain the final ruthenium-cobalt assemblies.

RuP4 OEt -Co was isolated (as a hexafluorophosphate salt) in 60% yield with the same synthetic and purification protocol. The main peak in HRMS and ESI-MS with a characteristic isotopic pattern at m/z: 916.6 for [M-2PF6] 2+ is a good proof for the final dye-catalyst assembly.

In addition, the diamagnetic dyad was confirmed by 1 H-NMR (see corresponding part below). On the cathodic part, two peaks were assigned to the reduction of the Co(III) center to Co(II) at -0.60 V vs Fc +/0 and then reduction of Co(II) to Co(I) at -0.98 V vs Fc +/0 , by analogy with the Co-N3. Three quasi-reversible electrochemical processes are monitored for RuP4 OEt -Co at -1.73 V, -1.93 V and -2.45 V vs Fc +/0 . These processes are ascribed to the reduction of the three polypyridine ligands and are in good agreement with the reduction waves of RuP4 OEt -EPIP.

5) 1 H NMR characterization

The electrochemical properties of the two separate components are present and quite unmodified in the final dye-catalyst assembly. This observation indicates that there is no electronic coupling between the two metallic centers at the ground state of the dyad.

ii) Spectro-electrochemical measurements

Following the analysis of the redox couples of the dyad, electrolysis of RuP4 OEt -Co and UV-

2) UV-Visible spectroscopy i) Determination of molar extinction coefficients

The UV-Vis spectra of RuP4 OEt -EPIP, RuP4 OEt -Co and Co-N3 were recorded in acetonitrile

(Figure II.5).
The visible part of the spectra is dominated by the classical MLCT transition (double humped band) 135 with maxima for RuP4 OEt -Co and RuP4 OEt -EPIP at 462 nm (ɛ = 19700 M -1 cm -1 and 19100 M -1 cm -1 accordingly) and with a shoulder at 430 nm for both complexes. A small bathochromic shift on this region (5 to 7 nm), was previously observed for methyl phosphonate-functionalized complexes due to the weak electron donating character of the phosphonate groups, which can stabilize the bpy-based π * orbital. [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF]145 In the UV part, an intense band displayed at 291 nm (ɛ = 115000 M -1 cm -1 ) for the dyad and at 290 nm (ɛ = 110000 M -1 cm -1 ) for the ruthenium dye, is attributed to a π-π* transition based on the polypyridine ligands. For both ruthenium-based complexes, the shoulder at around 350 nm and the tail at 500 nm can be attributed to transitions involving the imidazole ligand. 150 Moreover, a contribution from the Co-catalytic center in the near UV region is ascribed to ligand-centered transitions. The absorption spectra of the dyad correlates well with the sum of the absorption of the two separate components in Figure II.5 (black dotted line). The fact that the spectroscopic properties remain almost unmodified between the precursor complexes and the dyad is again an indication of an electron decoupling between the dye and the catalyst at the ground state of the dyad. An important feature, implying no significant modification of the properties of the two subunits in the final assembly.

ii) Protonation state of the imidazole bridge

The imidazole ring is part of the bridge connecting our chromophore with the cobalt complex. From the literature, it is known that the protonation state of this unit can modify the photophysical properties of similar ruthenium-polypyridine complexes. 150,153,154,155 In order to elucidate the protonation state of RuP4 OEt -Co, titration measurements were performed using aliquots of TFA and TEA solutions ( Absorbance Wavelength (nm)

Without addition of TFA Addition of 0,2 eq TFA Addition of 0,4 eq TFA Addition of 0,6 eq TFA Addition of 0,8 eq TFA Addition of 1 eq TFA Addition of 2 eq TFA Absorbance Wavelength (nm)

Without addition of TEA Addition of 0,2 eq TEA Addition of 0,4 eq TEA Addition of 0,6 eq TEA Addition of 0,8 eq TEA Addition of 1 eq TEA The protonic state of the imidazole ring affects part of the photophysical properties of the compounds. Thus, some photophysical studies will be also carried out with the presence of 1 eq of base for efficient deprotonation of the imidazole unit. In addition, molar extinction coefficient was determined after addition of 1 eq of TEA and a lower value of 17700 M -1 cm -1 was obtained for the MLCT transition at 463 nm for RuP4 OEt -Co, in agreement with our titration experiments that the protonated complex exhibit higher MLCT band. In the next chapter, the PEC tests will be carried out at pH 5.5 with NiO as electron source and the basic principles of a NiO photocathode have been already presented in the first chapter.

IV) Thermodynamic considerations

The NiO valence band edge potential at pH 5.5 was previously estimated to be + 0.46 V vs NHE [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] from the + 0.37 V vs NHE value determined at pH 7 by Wu and coworkers. 156 The energy of the excited state (E00) for RuP4 OEt -Co was estimated 2.01 eV from the maximum emission wavelength at 618 nm at room temperature in acetonitrile (recoded in Germany). From this value and the data in Table II.2, we could estimate a redox potential of + 0.81 V vs NHE for Ru * /Ru -with the following equation:

E(Ru * /Ru -) =E(Ru II /Ru II -L -) + E00 Eq. II.1 where E(Ru II /Ru II -L -) is Ered3 = -1.20 V vs NHE for RuP4 OEt -Co (Table II.2).
Hole injection from the excited dye to the valence band of NiO is thermodynamically possible with ΔGinj = -0.35 eV using Eq. II.2. Generation of Co(II) and Co(I) from the reduced dye can occur with driving forces of ΔG1 = -1.13 eV and ΔG2 = -0.75 eV respectively (Eq. where E(Co III/II ) and E(Co II/I ) are Ered1 = -0.07 V and Ered2 = -0.45 V vs NHE from Table II.2. From the estimated driving forces, hole injection from the excited dye to NiO and thermal intramolecular electron transfer from the reduced dye to the catalyst can thermodynamically take place. As far as the first reduction of the cobalt center concerns, spectroelectrochemical measurements implemented by the group on NiO electrodes with the cobalt diimine-dioxime complex, showed that the catalyst is at its Co(II) state without irradiation and further electron transfer is needed to be activated to Co(I). [START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2-Evolving Dye-Sensitized Photocathode[END_REF] 2) Photolysis experiment

II.3 and 4).

Light irradiation and monitoring of the evolution of the UV-Visible spectra may give us experimental proof for the ability of our dyad to generate under homogeneous conditions the Co(I) reduced state of the catalyst, which is the entry point of the mechanism. [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine -Dioxime Complexes[END_REF] Photolysis experiments of RuP4 OEt -Co (4.5 × 10 -5 M) in degassed acetonitrile using TEOA as SED (10% v/v) with a Xe-lamp under visible light irradiation were performed, following already reported procedures from the group. [START_REF] Zhang | Phosphine Coordination to a Cobalt Diimine -Dioxime Catalyst Increases Stability during Light-Driven H2 Production[END_REF][START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] The time-dependent differential spectra of RuP4 OEt -Co are Afterwards and within some minutes a double band with maxima at 600 and 709 nm is generated and increases in intensity for 30 minutes, when a plateau is reached. Based on previous reports, [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] this band is typical for the formation of Co(I). The absence of broad and more pronounced peaks for Ru(I) around 500 nm (Figure II.4), hints that accumulation of reduced species does not occur. This could be a result of regeneration of the reduced dye by the catalyst and formation of Co(I). Since the SED is in high excess compared to the catalyst, reductive quenching is more possible to occur under homogeneous conditions and the clear evolution of Co(I) implies that our dyad is able to accumulate Co(I).

A direct comparison (Figure II.8) under strictly identical experimental conditions for Dyad 2 with the same catalyst and an organic dye [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] showed that Co(I) is formed after 60 minutes of irradiation and with lower intensity. Faster and more efficient electron transfer from ruthenium to the cobalt complex is indicated for RuP4 OEt -Co. However, this is not conclusive as the efficiency of the reductive quenching of the dye from the SED should be also taken into consideration for both cases. Nevertheless, the build-up of Co(I) species for our dyad under homogenous conditions is a first promising sign for the photoelectrochemical tests that will follow.

V. Conclusions and perspectives

A new dye-catalyst assembly was synthesized and characterized for further use in a NiO photocathode for proton reduction. The light harvester is a ruthenium polypyridine complex and the hydrogen-evolving catalyst a cobalt diimine-dioxime. The traditional route proved a straightforward procedure for the preparation of ruthenium polypyridine complexes with four methyl phosphonate groups, even though the yields were moderate. The choice of the two components (PS, HEC) is crucial, along with the nature of the bridging ligand, as its structural and electronic properties affect the kinetics and the catalytic activity of the dyad. 158 Coppercatalyzed azide-alkyne cycloaddition was employed to couple the two moieties under mild conditions and with satisfying yields. This procedure can be a useful tool for the preparation of more sophisticated architectures. Transmetalation from copper to cobalt yielded the final dyecatalyst assembly. Full characterization was made for the dyad, which was isolated under the protonated form of the imidazole ring. Electronic decoupling at the ground state of the architecture between the two components is an important feature. This means that pairing the two subunits does not affect their initial redox and spectroscopic properties. Controlled electrochemical reduction of RuP4 OEt -Co revealed the characteristic signatures for the reduced species of ruthenium and cobalt center.

All the thermodynamic requirements are met to have an active NiO photocathode (at pH 5.5) through a reductive mechanism. Previous TAS measurements and some initial results for our system excluded the oxidative quenching. Photolysis experiments under the presence of a SED with UV-Visible monitoring indicated accumulation of Co(I) under homogeneous conditions.

Thus, it can be assumed that reduction of the catalytic moiety should occur through two successive thermal electron transfers from the reduced dye. Direct comparison with the first reported dyad of the group (Dyad 2) showed more efficient and faster evolution of Co(I), when ruthenium was employed as PS. Hence, assessment of the photoelectrocatalytic activity for hydrogen production of our novel dye-catalyst assembly seems promising.

Characterization and H 2 -evolving activity assessment of a novel RuP 4 OH -Co-based NiO photocathode

State-of-the-art photocathodes for proton reduction started to develop within the last 10 years and represent a continuously growing field with intensive research interest. In this chapter, we will prepare RuP4 OH -Co sensitized NiO electrodes and assess their photoelectrocatalytic activity for proton reduction in fully aqueous medium. For comparison purposes, we will work under experimental conditions identical to the previously reported ones from the group for organic dye-based photocathodes. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] This will enable us to correlate the performance with the nature of the photosensitizer, as the other parameters will remain unmodified. The influence of the light irradiation will be also emphasized. Post-operando characterization of the photocathodes using XPS, MALDI-ToF, ToF-SIMS and UV-Vis spectroscopy will be performed in order to shed light on the fate of our molecular dye-catalyst assembly after the activity tests.

I. Preparation of the sensitized NiO films

In this part, preparation of NiO films will be presented, as well as SEM characterization.

Then, the synthetic protocol for the deprotection of the phosphonate ester groups will be demonstrated to obtain the corresponding acid anchoring groups. Finally, the procedure to sensitize the homemade NiO films will be explained.

1) F108-templated homemade NiO films i) Preparation

NiO is the main p-type SC for the construction of p-type DSSCs [START_REF] Hagfeldt | Dye-Sensitized Solar Cells[END_REF][START_REF] Nikolaou | Recent Advances and Insights in Dye-Sensitized NiO Photocathodes for Photovoltaic Devices[END_REF][START_REF] Benazzi | Developing Photocathode Materials for P-Type Dye-Sensitized Solar Cells[END_REF] and DSPECs. [START_REF] Walter | Solar Water Splitting Cells[END_REF][START_REF] Queyriaux | Molecular Cathode and Photocathode Materials for Hydrogen Evolution in Photoelectrochemical Devices[END_REF][START_REF] Xu | Water Splitting Dye-Sensitized Solar Cells[END_REF][START_REF] Gibson | Dye-Sensitized Photocathodes for H2 Evolution[END_REF][START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF] Their low-cost synthesis and good transparency make this material attractive for photoelectrocatalytic applications and a variety of protocols [START_REF] Wood | A Comprehensive Comparison of Dye-Sensitized NiO Photocathodes for Solar Energy Conversion †[END_REF] to prepare mesoporous films are available. In this work, homemade nanostructured mesoporous NiO films were obtained according to the procedure previously reported by E. Suzuki 37 and later by our group. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] Anhydrous NiCl2 and F108 triblock co-polymer were dissolved into a mixture of water and ethanol. After centrifugation, the supernatant solution was deposited on 12 FTO glass substrates by spin-coating. The group of E. Suzuki used a doctor blade method. However, we could obtain more reproducible films with spin-coating. Finally, the films were annealed in air at 450 ⁰C for 30 minutes to remove organic impurities and sinter the particles together. The procedure was repeated four times to obtain four-layered films.

ii) SEM characterization

A small piece of 1 cm 2 of NiO films was cut and sent for scanning electron microscopy.

Adina Morozan, a research engineer in our group, performed these measurements and some 2

) Immobilization on NiO films i) Hydrolysis of the phosphonate ester groups

In order to graft the dyad onto NiO films, hydrolysis of the phosphonates esters to the corresponding acids is needed. Two synthetic procedures exist. Strong acids like HCl or H2SO4

under reflux conditions are able to perform this deprotection. [START_REF] Gillaizeau-Gauthier | Phosphonate-Based Bipyridine Dyes for Stable Photovoltaic Devices[END_REF]139 However, this harsh environment could be detrimental for our dye-catalyst assembly and most specifically for the cobalt complex. Alternatively, the McKenna reaction based on the use of bromo-trimethylsilane (TMSBr) is a more convenient way for our dyad, as milder conditions are employed. [START_REF] Gillaizeau-Gauthier | Phosphonate-Based Bipyridine Dyes for Stable Photovoltaic Devices[END_REF]144 RuP4 OEt -Co was solubilized in dry DCM, TMSBr was added dropwise under an argon atmosphere and the reaction mixture was stirred at room temperature for 3 days. During this first step, silyl dealkylation takes place. After hydrolysis of the silyl ester in methanol, the deprotected compound was obtained (Chart III.1). RuP4 OEt -EPIP was also deprotected (RuP4 OH -EPIP) following the same synthetic protocol in order to use it as a reference compound for control tests. ii) Film sensitization

Sensitization of the films was achieved by soaking them in 0.1 mM deprotected complexes (RuP4 OH -Co, RuP4 OH -EPIP) solutions in MeOH for 24h. Finally, the films were placed in pure methanol to remove physisorbed compounds from the surface and dried under air flow. The sensitized films were kept protected from light. This procedure has been previously optimized in the group. [START_REF] Massin | Dye-Sensitized PS-b-P2VP-Templated Nickel Oxide Films for Photoelectrochemical Applications[END_REF][START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] It was observed that soaking for 24 hours is sufficient to achieve maximum grafting of the molecular components onto NiO films.

II. Characterization of RuP4 OH -Co sensitized NiO films

Here, we will first determine the amount of dyad immobilized on the films. Subsequently, the sensitized NiO films will be characterized by various techniques in order to prove the integrity of the molecular assembly after grafting and before starting the photoelectrocatalytic tests for proton reduction.

1) Dyad loading determination

The dyad loading was estimated by UV-Vis spectroscopy after the desorption of the dyad from the surface of the film. This was achieved by dipping the sensitized NiO films in methanolic solution of 1 M phenyl phosphonic acid. [START_REF] Lyu | H2 -Evolving Dye-Sensitized Photocathode Based on a Ruthenium Diacetylide / Cobaloxime Supramolecular Assembly[END_REF]162 The phenyl phosphonic acid, an ideal The estimation of the immobilized compounds was previously made by measuring the UV-Vis absorption spectra of the sensitized NiO films. [START_REF] Massin | Dye-Sensitized PS-b-P2VP-Templated Nickel Oxide Films for Photoelectrochemical Applications[END_REF][START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] However, scattering could complicate the interpretation of the spectra depending on the NiO film thickness. Thus, the methodology employed here is more accurate.

12 different samples gave us an average dyad loading of 5.2 ± 0.8 nmol.cm -2 (Table III The obtained dyad loading value is slightly lower than for previous dye-catalyst assemblies with the same NiO preparation (6-8 nmol.cm -2 ). [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF][START_REF] Lyu | H2 -Evolving Dye-Sensitized Photocathode Based on a Ruthenium Diacetylide / Cobaloxime Supramolecular Assembly[END_REF] Lower solubility of the dyad and its bulky structure could be a possible explanation. Higher grafting efficiencies (> 13 nmol.cm -2 ) were determined previously in the group for ruthenium complexes bearing methyl phosphonate groups immobilized on NiO films. [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] Thicker films due to different NiO preparation and the less bulky nature of the complexes can justify the higher dye loadings.

2) MALDI-ToF analysis

The solutions containing RuP4 OH -Co in 1 M phenyl phosphonic acid after detaching from the surface were sent for MALDI-ToF analysis. Identical fragmentation is observed (Figure accordingly. Ruthenium-containing ions were identified thanks to its specific isotopic pattern.

III
Ions corresponding to the type CaHbNcOdPeRu + are present with methyl phosphonate groups.

The peak at m/z: 616 (c), corresponding to [Ru(bpy)(CH2 ˙)(bpy)(CH2-PO3H2)2] + with the break of the C-P bond supports a chemisorption rather than physisorption process of grafting. 160 In the range of m/z: 300-400 (d), a fraction at 362 is ascribed to the third diimine ligand with the triazole ring. In addition, peaks corresponding to the loss of nitrogen or carbon atoms of the main fragment at m/z: 362 are observed. ToF-SIMS analysis gave us signatures of ruthenium-cobalt containing ions. However, the intact molecular peak was not detected probably due to the harsh nature of the technique, which facilitates fragmentation. Nevertheless, several secondary ions for the different parts of RuP4 OH -Co gave reliable evidence that the whole dyad is grafted onto NiO films.

III. Photoelectrocatalytic activity of RuP4 OH -Co sensitized films

The PEC activity for proton reduction of the dyad will be evaluated in a 3-electrode set-up configuration with the sensitized NiO film as the working electrode.

1) Experimental set-up

In order to accurately compare all the dye-sensitized photocathodes prepared in the group, a standardized procedure was defined in collaboration with S. Bold (PhD student), based on the previous studies from the group. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF][START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF] 1) Every NiO film was cut in half after sensitization. One half was used to determine the grafting density and the other half was used for the assessment of the PEC activity and for postoperando analysis. For the preparation of the working electrode, a silver wire was deposited on the FTO substrate of the film using silver conducting paste to increase the contact and it was isolated with a tape.

2) The experiments were carried out in a three-electrode cell with quartz windows (Figure M NaCl buffer at pH 5.5. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] The counter electrode was separated with a frit to avoid the influence of the oxidation reaction taking place. Then, the electrolyte in the two-compartment cell was degassed with nitrogen for 30 minutes. The volume of the solution was around 5 ml and the headspace approximately 2 ml.

3) Irradiation was carried out with a Xe lamp operated at 280 W. A water-filled filter for elimination of IR irradiation (λ < 800 nm) and a UV cut-off filter (λ ˃ 400 nm) or an AM 1.5

G filter, as a mimicking for natural sunlight, were employed. The irradiation was calibrated at the equivalent of 1 sun. In the following part and in Chapter V, these two types of irradiation will be referred as visible light irradiation (400-800 nm), when the UV cut-off filter will be used and AM 1.5G irradiation when the AM 1.5G filter will be employed. The cell was placed at a fixed distance from the lamp and the light power was calibrated prior to every experiment.

4) Prior to the long-term chronoamperometric experiments, linear sweep voltammograms were recorded for each sensitized NiO film under dark conditions (lamp off), continuous irradiation (lamp on) and then chopped light irradiation (lamp on-off), screening from + 0.5 V to -0.5 V vs Ag/AgCl with a scan rate of 10 mV.s -1 . For all the experiments, the potential is reported vs Ag/AgCl and vs RHE. The potential against a reversible hydrogen electrode (RHE)

was calculated using the equation below (Eq.III.1):

E (V) vs RHE = E (V) vs Ag/AgCl + 0.199 + 0.059 × pH Eq.III.1

5) The amounts of evolved hydrogen were determined after chronoamperometry runs by gas chromatography in the headspace (50 μl sampling) and in the electrolyte, using a sensitive micro clark-type electrode (polarized at + 1000 mV), as an important fraction of hydrogen might remain in the solution for hours. [START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF] Calibration curves enabled us to correlate the response of the GC and of the electrode to the amount of produced hydrogen. Highest photocurrent densities were observed at increasingly negative potentials. This is a result of slowing down the recombination processes, as proposed by L. Hammarström and coworkers for DSSCs. 117 Recently, Papanikolas and his group also proved that at more negative potentials, the intragap states of NiO acting as hole traps are populated with electrons and the unwanted recombination processes can be decreased. 121 Under visible light irradiation, photocurrent densities of -58 ± 11 μA.cm -2 (3 independent samples) after subtraction of the dark current at -0.4 V vs Ag/AgCl were determined. A 4-fold increase (-15 μA.cm -2 ) in comparison with the previous dyad with the same cobalt catalyst (Dyad 2) is estimated. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] If we take into consideration the dyad loading of our system, electron transfer efficiency of 11 ± 3 μA.nmoldyad -1 can be determined. For Dyad 2, this efficiency was low, 2 μA.nmoldyad -1 . In agreement with the photolysis experiment (Figure II.8), more efficient electron transfer from the reduced ruthenium dye to the cobalt catalyst, could explain these differences.

ii) Under simulated AM 1.5G solar irradiation

Our dyad exhibits a broad absorption band at 300 nm due to π-π * transitions based on the diimine ligands (Figure II.5). It was previously observed that photocatalytic systems with multicomponent or supramolecular ruthenium-cobalt assemblies displayed lower activity when a UV cut-off filter was employed. [START_REF] Fihri | Cobaloxime-Based Photocatalytic Devices for Hydrogen Production[END_REF][START_REF] Hawecker | Efficient Homogeneous Photochemical Hydrogen Generation and Water Reduction Mediated by Cobaloxime or Macrocyclic Cobalt Complexes[END_REF] Using an AM 1.5 G filter could enable us to take advantage of this strong absorption of ruthenium polypyridine complexes in the near-UV region. In addition, the air mass filter is a mimic of natural sunlight and it is used as a standardized filter for DSSCs.

LSV were recorded using the AM 1.5 G filter instead of the cut-off filter (> 400nm) and the IR filter (calibrated at 70 mW.cm -2 ), leading to the generation of higher cathodic photocurrents Compared to RuP4 OH -Co-sensitized films, pristine NiO and RuP4 OH -EPIP-sensitized films, display significantly lower photocurrent densities. These results clearly confirmed the important role played by the cobalt complex for the enhancement of the photocurrents. We therefore ascribe this performance to electron transfer from the p-type NiO to the cobalt diimine-dioxime catalyst through the excitation of the dye, suggesting that catalysis is at the origin of the photocurrents. We can also notice that after an initial spike due to the accumulation of charge carriers at the electrode/electrolyte interface, [START_REF] Lyu | H2 -Evolving Dye-Sensitized Photocathode Based on a Ruthenium Diacetylide / Cobaloxime Supramolecular Assembly[END_REF] the cathodic photocurrent of the film sensitized by RuP4 OH -Co, gradually decreases with time. The reasons behind this decay will be analyzed in the post-operando characterization section.

Preliminary tests with lower (0.5 sun) or higher (1.5 sun) light intensities were carried out with no significant differences in the photocurrent densities. Thus, subsequent longer-term irradiation experiments were performed under 1 sun illumination.

3) Assessment of the H 2 -evolving activity Chronoamperometric measurements were carried out under continuous irradiation for 2 hours at -0.4 V vs. Ag/AgCl (+ 0.13 V vs RHE) for direct comparison with the previous dyesensitized photocathode from the group. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] These tests will allow us to correlate the cathodic photocurrents with catalytic hydrogen production. In At the end of the two hours experiments, we measured for hydrogen in the headspace with gas chromatography and in the electrolyte with a micro clark-type electrode probe, following a reported protocol from the group. [START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF] In Table III 6 times less hydrogen was produced with NiO films sensitized by RuP4 OH -EPIP (< 10 nmol.cm -2 ) under irradiation with the two different light sources. The production of low amounts of hydrogen without the presence of a catalyst was previously observed in the literature for p-type dye-sensitized photocathodes [START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3-Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF][START_REF] Lyu | H2 -Evolving Dye-Sensitized Photocathode Based on a Ruthenium Diacetylide / Cobaloxime Supramolecular Assembly[END_REF]167 and was attributed to the formation of Ni(0) nanoparticles, which are active for proton reduction. 168 Decomposition of ruthenium, as evidenced in homogeneous photocatalytic systems, cannot be excluded for the evolution of H2. 169,170,171 Traces of hydrogen was produced when pristine NiO films were illuminated with visible light (less than 1 nmol.cm -2 ). The production of higher amounts of H2 for pristine NiO films when AM 1.5G filter was employed (5 nmol.cm -2 ) can be ascribed to the excitation of the semiconductor, as it can absorb in the UV region (band gap of 3.6 eV). Nevertheless, the increased photocurrents and hydrogen evolution for RuP4 OH -Co-sensitized films could be ascribed to the electron transfer from the valence band of NiO to the cobalt diimine-dioxime catalyst through the excited state of the photosensitizer, which can then efficiently drive proton reduction.

An average faradaic efficiency of 24 ± 5 % and 26 ± 7 % was determined under visible light and AM 1.5 G irradiation accordingly, implying that the electrons are consumed for other reactions and not just for proton reduction. Although these values are still low, they are more than twice higher than all the other reported dye-sensitized photocathodes based on the cobalt diimine-dioxime catalyst (around 10%). [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF][START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF][START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2-Evolving Dye-Sensitized Photocathode[END_REF] Main reasons for the modest F.E. could be the reduction of trapped oxygen, as well as partial reduction of NiO, [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] recently confirmed by Simonov and his group. 168 Based on the fast decay of the photocurrents after some minutes of irradiation (Figure

III.
12), we decided to carry out CA experiments for 30 minutes instead of 2 hours. The amount of evolved H2 and the TON were in the same range than the previous one (Table III.2 c).

However, a lower charge passed during the course of this experiment leading to a faradaic efficiency of 72%. This experiment is important because it shows that, during the first 30 minutes of activity, our system displays a high selectivity for hydrogen production. Restarting the measurement for 2 hours, the amount of hydrogen produced was not further increased.

Hence, it seems that our system is deactivated during the course of the experiment. Postoperando analysis may shed light on the origin of this deactivation and will be analyzed in detail in the corresponding section. assembly. This technique will be employed here to characterize the films with the potential applied during the PEC tests. Thus, the influence of this potential on the charge-separated state lifetime can be identified, as it has already been done for ruthenium-sensitized NiO photocathodes. 117,121 The experiments below were performed by S. Bold in the group of Pr. B.

Dietzek in Germany and some key information will be presented.

Using femtosecond laser pulse, TA spectra were recorded without applied potential (opencircuit potential, OCP), at -0.74 V Fc +/0 , equivalent to the -0.4 V vs. Ag/AgCl applied in the photoelectrochemical tests, and -1.15 V vs. Fc +/0 . At -0.74 V vs. Fc +/0 , the cobalt catalyst is under its Co(II) state, [START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2-Evolving Dye-Sensitized Photocathode[END_REF] while at -1.15 V vs. Fc +/0 it is reduced to the Co(I) state. Thus, the measurement at -1.15 V vs. Fc +/0 serves as a reference measurement where any electron transfer to the cobalt catalyst is not thermodynamically possible.

TA and more pronounced at more negative applied potentials. For RuP4 OH -Co, the faster hole injection process is more efficient than for RuP4 OH -EPIP, which could be explained by different electron density distributions in their excited states. In the case of the dyad, the electron density may be localized on the extended ligand due to the modification caused by the triazole unit and the hole injection can be more efficient. 172 Hole injection is followed by charge recombination, which is drastically affected by the applied potential, increasing from a time constant of tens of picoseconds at OCP to > 7 ns (τ3) at negative applied potentials, leading to a long-lived charge-separated state. In parallel, remaining 3 MLCT decays directly back to the ground state in ≈ 1 ns (τ2). state in the case of RuP4 OH -Co, which is not present in our spectra. From the femto-second TA-SEC measurements, the most important observation is a long-lived separated state.

Since thermal electron transfer in triazole-bridged dyads has been shown to occur on the nsµs time scale, 173,174,175,176,177 TA experiments were performed up to 15 μs. However, the TA spectra in this time range are spectrally identical to the last spectra and decay without any spectral changes. In particular, the signature of the Co(Ι) state did not appear on the TA spectra of RuP4 OH -Co -sensitized NiO films at -0.74 V vs. Fc +/0 , and the decay kinetics of the reduced Ru complex were not accelerated in comparison to RuP4 OH -ΕPIP-sensitized films.

Overall, by applying a reductive potential, the lifetime of the charge-separated state was increased from hundreds of picoseconds to up to 15 µs, an increase of up to 5 orders of magnitude compared to OCP measurements. However, the formation of Co(I) species could not be detected. Therefore, the electron transfer from the reduced dye to Co(II) appears to be quite inefficient. On the basis of the extinction coefficients of the reduced dye and Co(I) from the UV-Vis SEC experiment and the experimental noise of the TA-SEC measurement, we could estimate that less than 12% of the reduced dyes can undergo electron transfer to the catalyst.

Thus, this inefficient thermal electron transfer to the catalyst is in agreement with the low activity of our photocathode.

V. Post-operando analysis of RuP4 OH -Co-sensitized NiO films

The PEC activity of our photocathode in fully aqueous conditions was promising. However, after 30 minutes of continuous irradiation at -0,4 V vs Ag/AgCl, most of the initial photocurrent is lost and the hydrogen production reaches a plateau. Post-operando analysis is of significant importance for finding out what is the destiny of the molecular dyad and may provide answers for the photocurrent decay. After completing the PEC tests, the surface of the NiO films were characterized by XPS and TOF-SIMS or they were placed in 1 M methanolic solution of phenyl phosphonic acid and the corresponding desorption solutions were analyzed by UV-Vis and MALDI-ToF.

1) XPS analysis

Post-operando XPS analysis of the sensitized NiO films was carried out, providing important information. In Figure III.15, the XPS spectra for Co 2p after catalysis (right) can be fitted with the same components as bare NiO films (left). Thus, partial or complete loss of Co is indicated. Previous high resolution HAXPES measurements, showed decreased amount of cobalt after 1000 s of photoelectrolysis, using a dye-catalyst assembly based on the same cobalt diimine-dioxime complex. [START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye -Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF] In addition, in our case, desorption of more than 40% was estimated for N 1s, which was higher than for ruthenium or phosphorus. This can further support the degradation of the catalyst, containing nitrogen atoms. Finally, no significant changes were observed for Ni 2p concerning the shape of the main peaks from 852 to 868 eV (Figure III.17 left) after catalysis (blue line). However, a small shift of the peak at 853 eV, indicating evolution of Ni(0) cannot be excluded. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] This is in agreement with the production of traces of H2 ascribed to Ni nanoparticles, obtained during our PEC tests.

2) TOF-SIMS analysis

Post-operando TOF-SIMS analysis for the sensitized NiO films was carried out in positive and negative mode.

i) Positive mode

In the positive mode (Figure III.18), the most important differences between the spectra before and after the PEC tests is that Co-containing fragments are limited and with lower intensity, in particular those at m/z: 738 and 659. In addition, peaks containing ruthenium corresponding to a type of CaHbNcOdPeRu + similar to what observed before catalysis are still present, but anew in lower intensity. These observations imply that desorption and/or decomposition pathways based on the catalyst is a limiting factor for the efficiency of the photocathode. Peaks that can be ascribed to the triazole unit with the imidazole group of the third diimine ligand are also observed in the region between m/z: 321 and 392 (d), identical with the ones before catalysis. This is an indication that the triazole bridge is stable during a photoelectrocatalytic test. Triazole has been reported to be resistant to hydrolysis, oxidation and reduction. [START_REF] Kolb | Click Chemistry : Diverse Chemical Function from a Few Good Reactions[END_REF] However, the group after performing post-operando TOF-SIMS measurements has found fragments due to reductive hydrogenation of the triazole unit for the co-grafted cobalt catalyst onto NiO films. [START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2-Evolving Dye-Sensitized Photocathode[END_REF] The loss of active immobilized Co complexes was ascribed to the instability of the triazole unit. Under our experimental conditions, similar decomposition pathways cannot be identified. The covalent grafting of the dye and the HER catalyst may control the electron transfer to the catalyst through the triazole bridge in a way that reductive bleach cannot occur.

ii) Negative mode

In Desorption was estimated to be 76 ± 8 % (3 independent samples) for films irradiated with visible light and 80 ± 7 % (4 independent samples) when AM 1.5 G illumination was employed.

Hence, the light source does not affect the desorption of the molecular components from the surface. The fact that most of the initial dyad is detached from the NiO surface after 2 hours is one limiting point for long-term activity. Hence, under our experimental conditions, desorption is a main issue to consider.

Desorption with similar ruthenium complexes have been already reported in the literature.

Ruthenium-polypyridine dyes with two methyl phosphonic acid groups [(Ru(bpy)2(4,4'-(PO3H2)2bpy)] 2+ immobilized onto TiO2 showed increased detachment in aqueous media after light irradiation in a range of pH from 1 to 7, reaching bare films after 15 hours. [START_REF] Hanson | Photostability of Phosphonate-Derivatized, Ru(II) Polypyridyl Complexes on Metal Oxide Surfaces[END_REF] In another study, the same complex grafted onto TiO2 completely desorbed after 3 hours of continuous irradiation at pH 7.8 under open circuit conditions. 178 Furthermore, 20% desorption was previously observed from the group with ruthenium complexes bearing 4 methyl phosphonate groups after 10 minutes of photoelectrochemical tests in acetate buffer at pH 4.5. [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] Detachment of the molecular components from the surface is often acknowledged in the field of photoelectrocatalysis driving to low performances of p-type dye-sensitized photocathodes. [START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye -Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF][START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye -NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2-Evolving Dye-Sensitized Photocathode[END_REF]104 About 40% desorption of ruthenium from the film was determined after performing XPS analysis, half from the estimation based on UV-Vis spectra. Decomposition of ruthenium could be the answer to this difference, even though XPS spectra do not support this assumption.

[Ru(bpy)3] 2+ is known to undergo bipyridine substitution during photogeneration of hydrogen in acidic water 128,179 with appearance of visible bands assigned to new coordination species with solvent molecules or anions. Furthermore, partial photochemical decomposition pathways under irradiation of ruthenium polypyridine complexes grafted onto metal oxides were reported in methanol, but not in water, with new bands at 500 nm for ligand substitution. [START_REF] Hanson | Photostability of Phosphonate-Derivatized, Ru(II) Polypyridyl Complexes on Metal Oxide Surfaces[END_REF] Another issue, is that for the XPS analysis we used a small piece of the lower part of a NiO film that could be irradiated less and decrease the desorption rate. Finally, a last issue might be linked to a lack of solubility of the dyad (or its derivatives) in the phenyl phosphonic acid methanolic solution employed for the desorption. Indeed, substitution of the halide ligands on the cobalt center occurs in aqueous medium leading to highly charged (5+) dyads.

4) MALDI-ToF analysis

The desorption solutions of the NiO films after 2 hours of PEC tests were sent for MALDI-TOF analysis. The peak at m/z: 1448, characteristic of the dyad, was absent from the spectrum In some cases, a peak at m/z: 1152 ascribed to the complete loss of the catalyst due to detachment from the triazole unit was also present. Supplementary post-operando techniques are needed to further elucidate the destiny of the dyad, as MALDI-ToF favors fragmentation, arising doubts for the speculated decomposition products. Here again, the fact that this analysis only provides information on the compounds soluble in the desorption solution has to be taken into consideration. Nevertheless, the fact that the signature of the intact dyad was not found after catalysis, is a good explanation for the deactivation of our photocathode due to the loss of the catalytic center.

VI. Conclusions and perspectives

The novel ruthenium-cobalt dyad was grafted onto nanostructured mesoporous NiO films, with dyad loading of 5.2 ± 0.8 nmol.cm -2. Even though this value is lower compared to the previous grafting efficiencies from the group, similar dyad loadings (4-5 nmol.cm -2 ) have been reported for ruthenium based dyads on NiO electrodes. 180 Efficient sensitization and the integrity of the immobilized assembly were confirmed by XPS, ToF-SIMS and MALDI-ToF.

Photoelectrochemical characterization was carried out in fully aqueous conditions (pH 5.5) at -0.4 V vs Ag/AgCl (+ 0.13 RHE) under standardized conditions. When visible light was employed, cathodic photocurrents were developed, reaching -58 ± 11 μΑ.cm -2 . Long-term photoelectrolysis was performed and 28 ± 2 nmol.cm -2 of H2 was evolved with 5 ± 1 TON. This new photocathode displayed superior activity compared to the previous system of the group based on a dyad with an organic dye [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] or with a porphyrin (see Annexes), highlighting the role of ruthenium, as all the other parameters were identical (setup, catalyst, triazole unit). More efficient and faster electron transfer from the dye to the catalyst, in agreement with our photolysis experiments (Figure II.8) could be a possible explanation.

Higher photocurrents were obtained when the activity was implemented with AM 1.5G illumination. Two times higher amount of H2 and TON were estimated, reaching 60 ± 9 nmol.cm -2 and 13 ± 2 TON accordingly. More efficient light harvesting due to the strong absorption at 300 nm of ruthenium polypyridine complexes could be the main reason for this improvement. However, the performance is still low compared to homogeneous multicomponent photocatalytic systems employing the same cobalt catalyst (300 TON after 2 hours of continuous irradiation), where the electron transfer is controlled by diffusion. [START_REF] Zhang | Phosphine Coordination to a Cobalt Diimine -Dioxime Catalyst Increases Stability during Light-Driven H2 Production[END_REF] In addition, it is even lower if we compare it with electrocatalytic systems (55000 TON), [START_REF] Andreiadis | Molecular Engineering of a Cobalt-Based Electrocatalytic Nanomaterial for H2 Evolution under Fully Aqueous Conditions[END_REF] where the electron supply is sufficient. Thus, in our case the electron supply to the catalyst is limited, leading to moderate activity.

In order to investigate the electron transfer dynamics in our system, TA-SEC measurements were carried out. We could observe a fast hole injection, a long-lived primary charge separated state (one-electron reduced dye and oxidized Ni species) and charge recombination processes for our NiO/dyad system at quasi-operando conditions. The main kinetic issue does not seem to be the charge recombination, as it has been mainly proposed in the literature, but rather the efficiency of the electron transfer to the catalyst. No spectral feature could be identified for the formation of Co(I), the entry point of the mechanism for proton reduction, [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine -Dioxime Complexes[END_REF] suggesting that this electron transfer is not efficient and below the detection limit of the TA spectroscopy. This can explain the low activity of our photocathode. Different molecular design should be envisaged to enhance this thermal intramolecular electron transfer to the catalyst.

Although higher faradaic efficiencies were obtained compared to the previous dye-sensitized photocathodes of the group, the values are still low. After 2 hours of PEC experiments the estimated F.E. did not exceed 25%. However, almost 70% F.E. were determined during the first 30 minutes. This means that the catalyst works properly at the beginning of the tests. The lower F.E. after 2 hours raise questions about the stability of the system. Post-operando characterization was crucial to explain this behavior and to understand what happens at the end of the catalytic experiments.

Almost 80% desorption from the surface occurs during the course of the PEC tests. The group has addressed the instability of the cobalt diimine-dioxime catalyst under photochemical conditions [START_REF] Zhang | Phosphine Coordination to a Cobalt Diimine -Dioxime Catalyst Increases Stability during Light-Driven H2 Production[END_REF] and Alberto and his coworkers assumed its decomposition under light irradiation. [START_REF] Guttentag | Photocatalytic H2 Production with a Rhenium / Cobalt System in Water under Acidic Conditions[END_REF] Our group proposed that its immobilization onto carbon nanotubes increased the stability under electrochemical conditions. [START_REF] Andreiadis | Molecular Engineering of a Cobalt-Based Electrocatalytic Nanomaterial for H2 Evolution under Fully Aqueous Conditions[END_REF] However, in the work presented here XPS, MALDI-ToF and ToF-SIMS showed partial or complete disappearance of cobalt after catalysis. Decomposition pathways based on the catalytic part were identified.

To conclude, the main factors limiting the H2-evolving performances of the RuP4 OH -Cobased photocathode are:

-an inefficient electron supply to the catalyst, -a high percentage of desorption in the electrolyte -a hydrolysis of the catalytic center.

Synthesis and characterization in solution of a novel dyad based on a cobalt tetraazomacrocyclic catalyst

In this chapter, we will present the synthesis and characterization of a new dye-catalyst assembly relying on the same ruthenium polypyridine photosensitizer functionalized with four methyl phosphonate groups (RuP4 OEt -EPIP) and a cobalt tetraazomacrocyclic complex, referred below as Cat1 (Scheme IV.1). Its redox and spectroscopic properties will be evaluated along with some thermodynamic considerations for the feasibility of the electron transfer from the NiO electrode to the catalyst.

Chart IV.1. Structure of Cat1 and the final dye-catalyst assembly, RuP4 OEt -Cat1.

The motivation to introduce a novel catalyst arises by the increased activity and stability of polypyridinic and aminopyridinic cobalt complexes under reductive aqueous conditions reported over the last 10 years. 128,129,181 In addition, a comparative study by our group testing different molecular HER catalysts and employing a Clark-type microsensor, proved the superior activity of Cat1 for proton reduction under homogeneous conditions compared to the cobalt diimine-dioxime catalyst. 182 In particular, at pH 4.5 with [Ru(bpy)3 2+ ] as light absorber, Cat1 exhibited a 5-fold increase in TON (102 vs 20.8).

I. Cat1 for H2 evolution under homogeneous conditions

Cat1 is a cobalt(III) complex based on a tetradentate macrocyclic ligand, which is a 14membered core (2,12-dimethyl-3,7,11,17 -tetraazabicyclo[11.3.1 ]heptadeca-1 (17),2,11,13,15-pentaene). 183,184 Cyclic voltammogram in 0.1 M N n Bu4PF6 in acetonitrile at a glassy carbon electrode revealed three reversible reduction potentials, ascribed to two metal reduction processes (Co(III)/Co(II), Co(II)/Co(I)) and one ligand based reduction, already reported for similar nickel complexes. 185 X-ray crystallography determined a six-coordinate complex with a distorted octahedral geometry and two trans to each other chloro ligands, under its Co(III) state. 186 Cat1 was first reported as a catalyst for the reduction of CO2. 187 A quick literature screening of Cat1 for photo-and electro-catalytic proton reduction will be presented in the following part along with some recent results, by our group and others, to further assemble it with a light-harvesting unit.

1) Electrocatalytic hydrogen evolution

Electrocatalytic hydrogen evolution was first studied in acetonitrile by the group of T.

Lau. 186

2) Photocatalytic hydrogen evolution

Photo-driven hydrogen evolution using Cat1 was first examined in an acetonitrile/water mixture (3/2) with Ir-PS, TEOA as SED and acetic acid as proton source, attaining 180 TON after 7 hours of continuous irradiation. 186 Maximal TON were achieved at pH 7.1. Cat1 exhibited higher activity with [Ru(bpy)3 2+ ] as light harvester. Ascorbate and ascorbic acid were used as SED and proton source respectively in fully aqueous conditions. 169 The highest TONcat were 1000 and obtained with 10 times higher concentration of dye compared to the catalyst at pH 4. 12 ml of hydrogen was produced and the photocatalytic system was active for more than 20 hours. In a recent study, Cat1 under similar conditions at pH 4.1, reached 680 TON after 4 hours of continuous illumination. 188

Cat1 seems to be stable over time. One of the limiting factors for long-term operation was suggested to be the photosensitizer due to decomposition. Higher TONcat (more than 4000) were obtained after substitution of ruthenium photosensitizer with an organic dye at pH 4.5 after 24 hours of irradiation. 170 Decreasing the amount of catalyst, increased the observed TONcat to 8952. 75% of the initial concentration of the dye remains after 22 hours of irradiation. Thus, increased stability of the light harvester under reduced conditions can explain the increased TON compared to the previous systems. Comparing with other cobalt catalysts, among them Co-diimine-dioxime and noble metal complexes, Cat1 showed superior activity, probably due to the macrocyclic nature of the ligand. 169 Resistance to hydrolysis and stabilization of low oxidation states of cobalt are the possible reasons leading to enhanced activity.

Trying to shed light on the mechanism for proton reduction, 190 electrochemical generation of Co(I) revealed its absorption spectrum with two intense peaks at 325 and 432 nm and a broad absorption above 500 nm. 169 Co(I) species were stable under anaerobic conditions in acetonitrile and mechanistic studies with addition of acids speculated a similar catalytic pathway with Codiimine-dioxime. [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine -Dioxime Complexes[END_REF] Protonation of Co(I) species leads to a Co(III)-hydride. Further reduction to Co(II)-hydride and an additional protonation are needed for hydrogen evolution. However, reduction to Co(0), and then further protonation to obtain a Co(II)-hydride species, seemed to be also feasible. 188 It is clear that the working mechanism for proton reduction with Cat1 is not as well-described as in the case of cobalt diimine-dioxime and investigation is ongoing.

3) Functionalization of Cat1 for covalent assembly with lightharvesting units the pyridine moiety of the ligand. To prove the beneficial influence of grafting, a direct comparison with the unfunctionalized catalyst was realized. Under the same experimental conditions and light illumination, the non-bonded catalyst evolved two times less hydrogen.

Long-term experiments showed that the immobilized catalyst was still active after 32 hours of irradiation, compared to 16 hours for the catalyst in solution. Photoluminescence and TAS revealed that for the hybrid system, electron transfer to the catalyst is faster and back electron is slower, in agreement with its enhanced photocatalytic activity. Diffusion-limited processes is the principal interpretation for the inferior activity of the non-grafted catalyst.

Chart IV.2. A covalently bonded hybrid assembly of Cat1 on QDs.

ii) Covalent dye-catalyst assemblies prepared in our group

Before starting this thesis, our group synthesized the first covalent dye-catalyst assemblies with Cat1 (manuscript submitted for publication). 192 Using our previously-reported CuAAC procedure, 87 J. Massin (post-doc) and Q. Vacher (master student) succeeded to couple Cat1 with two different organic dyes, obtaining the two following dyads (Chart IV.3).

Chart IV.3. Structure of the two dyads prepared in our group.

Sebastian Bold, a PhD student in our team, assessed the photoelectrocatalytic activity of T1-Cat1 and T2R-Cat1 for hydrogen evolution with a promising outcome. Notably for T2R-Cat1, faradaic efficiencies higher than 65% and a 26-fold increase in TON were achieved compared to similar photocathodes based on the cobalt diimine-dioxime catalyst, examined under strictly identical conditions (4 hours of continuous irradiation at -0.4 V vs Ag/AgCl at pH 5.5). [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] During my thesis, I participated to this project and optimized the synthesis and purification of T1-Cat1. Guided by these attractive results, I also tried to functionalize T2R-Cat1 with methyl phosphonic acid anchoring groups instead of carboxylic acid ones in an attempt to examine in more details the role played by the nature of the anchoring groups on the grafting stability of the photocathode (T2RP-Cat1, more information in Annex). Following the superior hydrogen-evolving performance of these organic dyads with Cat1, coupling of this catalyst with RuP4 OEt -EPIP was envisaged and will be presented in the following part.

II. Synthesis of RuP4 OEt -Cat1

1) Synthetic strategies to couple RuP 4 OEt -EPIP and Cat1

The construction of the catalytic center and the coupling method can be challenging and here we will demonstrate our synthetic approaches to obtain the final dyad. Two synthetic routes (Scheme IV.1) were proposed to obtain RuP4 OEt -Cat1 (Chart IV.1). Source of inspiration of Route 1 is the way we already isolated RuP4 OEt -Co. It is based on a CuAAC coupling between RuP4 OEt -EPIP and an azido functionalized derivative of Cat1 (N3-Cat1). The latter will be prepared using a cobalt-templated synthesis from a diacetylpyridine precursor functionalized by an azido group on the para-position of the pyridine moiety (N3-DAP). The fact that exchanging from cobalt to copper occurs under normal Cu-driven cycloaddition [START_REF] Andreiadis | Molecular Engineering of a Cobalt-Based Electrocatalytic Nanomaterial for H2 Evolution under Fully Aqueous Conditions[END_REF] highlights the need to establish appropriate conditions to avoid this detrimental effect, [START_REF] Kolb | Click Chemistry : Diverse Chemical Function from a Few Good Reactions[END_REF]193 even though the more robust nature of Cat1 might prevent this phenomenon by stabilizing the cobalt(III) center.

Following the efficient synthesis of the organic based-dyads with Cat1 (Chart IV.3), Route 2 is based on the CuAAC of RuP4 OEt -EPIP with the catalyst precursor bearing an azido functional group (N3-DPA). Then, cobalt-templated macrocyclization will take place for the ruthenium-based complex (RuP4 OEt -DAP) to obtain the final dyad.

Scheme IV.1. Two retro-synthetic pathways to obtain RuP4 OEt -Cat1.

Both routes use the same precursors. RuP4 OEt -EPIP, whose synthesis is reported in chapter II and N3-DAP, whose synthesis will be presented later. We should note that the anchoring groups were protected until the end of this chapter (like RuP4 OEt -Co) and characterization in solution was carried out under the protected form due to low solubility of phosphonic acids in most common solvents.

2) Cobalt-templated synthesis of Cat1

Before testing the 2 routes, synthesis of a batch of Cat1 was carried out in order to test the procedure of the cobalt-templated synthesis before applying it to a more complex system.

Furthermore, a reference batch of Cat1 is useful for comparison purposes. Synthesis of Cat1 is well-described by different research groups and it is based on a metal-templated synthesis (Scheme IV.2). 169,184,186 2,6 diacetylpyridine,3, salt in 1/1/1 ratio are heated at 75°C in a mixture of ethanol and water for 5 hours. Water is needed for avoiding the precipitation of the intermediate complex of 2,6 diacetylpyridine and Co(II). Some drops of acetic acid are added for buffering the amine solution and catalyzing the Schiff-base condensation. Addition of HCl (to provide an additional Cl anion) and oxidation to the more stable Co(III)-species with air-bubbling overnight yielded our final Cat1 in 30% yield, after precipitation with cold ethanol. Further crystallization as a perchlorate salt can also be undertaken. 169

Scheme IV.2. General procedure for the preparation of Cat1.

3) Preparation of N 3 -DAP

Starting from the commercially available chelidamic acid (Scheme IV.3), an aromatic nucleophilic substitution from hydroxyl to chloro groups takes place, as well as activation of the acid groups from thionyl chloride. 194 Subsequently, the chlorinated intermediate reacts with morpholine under inert atmosphere and a di-morpholino compound is obtained (1) with initial yields below 20%. Increase to 65% was achieved after addition of some drops of DMF, acting catalytically. NMR characterization and ESI-MS were used to identify the desired compound (more information in Experimental part). Then, a ketone-derivative ( 2) 195 was synthesized by replacing the two morpholine amides using a Grignard reagent (MeMgCl). 196 Purification was accomplished on silica gel column with DCM as eluent and ESI-MS and NMR proved its efficient synthesis. N3-DAP was isolated by substitution of para-chloro group on pyridine using an excess of NaN3 in a moderate 30% yield. 197 The same reaction was carried out with a commercially available bromo-derivative of ( 2 -Leaving the reaction under stirring at 75°C for 6 hours, like the previous reports for the construction of the catalytic center for Cat1 or overnight did not significantly affected the final yields. However, increasing the reaction time to 48 hours was detrimental for the synthetic procedure.

-Using acetonitrile instead of dichloromethane as co-solvent did not have any important impact.

-Two different cobalt salts were used (CoCl2.6H2O and CoBr2.hydrate). After adding

CoBr2.hydrate, we could not obtain the expected dyad. Its highly hygroscopic nature influences the accurate amount that we weighted, and this should affect the efficiency of the templatedsynthesis. Our highest yield did not exceed 17%, which is lower than the one obtained for Cat1 (30%).

Co(II) salt

In addition, the yields for the dyads based on organic dyes were significantly higher, reaching 70% for T1-Cat1 (Annexes). The presence of some starting material (RuP4 OEt -DAP) was identified by ESI-MS analysis of the crude mixture, suggesting that a lack of efficiency of the cyclization might be an issue. In addition, the fact that the phosphonated derivative of T2R-Cat1 (T2RP-Cat1, see Annexes) could not be isolated, suggests that the presence of phosphonate groups in T2RP-Cat1 and RuP4 OEt -Cat1, but not in T1-Cat1 and T2R-Cat1 could be an issue. Partial hydrolysis of the phosphonate esters might occur under the acidic conditions employed in the template synthesis procedure (acetic acid, HCl), rendering the complex more difficult to purify and isolate (different solubility, stronger adsorption onto silica gel).

7) ESI-MS analysis

ESI-MS was an important tool for us to confirm that we synthesized the desired dye-catalyst assembly. Before purifying the dyad, the crude mixture was checked by ESI-MS to confirm that the template synthesis was successful, and we could often detect mono-hydrolyzed derivatives, explaining the low final yields after column chromatography.

After purification, spectra acquired in the positive mode ( The methyl phosphonate groups are observed unmodified with 8 protons for the methylene spacer (multiplet at 3.37 ppm) and 16 (multiplet at 3.99 ppm) and 24 (multiplet at 1.14 ppm)

protons for the -CH2 and -CH3 groups of the phosphonate esters.

ii) RuP4 OEt -Cat1

The On the anodic scan of the voltammogram, at + 0.90 V vs Fc +/0 the reversible redox couple is associated to the metal-centered oxidation of Ru(II) to Ru(III). [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] On the cathodic part, two peaks for the redox couples of the cobalt center Co(III)/(II) at -0.41 V vs Fc +/0 and Co(II)/Co(I) at -0.87 V vs Fc +/0 are recognized, in comparison with the corresponding values of -0.51 V and -0.98 V Fc +/0 for Cat1. The third ligand-based reduction of Cat1 is not detectable in the dyad due to overlap with the ligand-based reductions of ruthenium. Three quasi-reversible electrochemical processes are monitored for RuP4 OEt -Cat1 at -1.74 V, -1.95 V and -2.50 V vs Fc +/0 , attributed to the reduction of the three polypyridine ligands. As previously observed for RuP4 OEt -Co, the individual redox properties of the photosensitizer (RuP4 OEt -EPIP) and the catalyst (Cat1) are not modified after their efficient coupling, in agreement with their electronic decoupling at the ground state of the dyad RuP4 OEt -Cat1.

2) UV-Visible spectroscopy i) Protonation state of the imidazole bridge

The absorption spectrum of RuP4 OEt -Cat1 displays the dominant MLCT transition in the visible region and the π-π * ligand-based one at 300nm. We previously showed by UV-vis titration that for the isolated RuP4 OEt -Co, the imidazole ring of the bridging ligand is protonated. In this sense, similar titration measurements were performed for RuP4 OEt -Cat1 We should mention that the reaction yields and the ε values have been calculated taking into consideration the protonated imidazole derivatives. taken into account here, we will focus on the reductive quenching one, considering that hole injection is occurring very fast (some ps) for ruthenium polypyridine dyes grafted onto NiO. [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] Fast hole injection was also proved for RuP4 OEt -Co after performing TA-SEC measurements.

using
In addition, for T2R-Cat1 generation of the reduced dye occurred before electron transfer to Cat1, showing that reductive quenching will most likely take place. 192 We will determine if electron transfer to the catalytic center to generate Co(I) species is possible. Emission spectra for RuP4 OEt -Cat1 have not been measured yet by our collaborators in Germany, but we can use the estimated value of 2.01 eV for RuP4 OEt -Co, as this number does not remarkably vary among different ruthenium complexes. From the estimated energy of the excited state (2.01 eV) and using Eq.II.1 and Ered3 in Table IV a. In V vs NHE. The redox potentials were converted from Fc +/0 (Table IV. From Table IV.3 it is clear that generation of Co(I) is feasible from a thermodynamic point of view. Thus, all criteria are met to prepare functional photocathodes with RuP4 OEt -Cat1. We should note that the redox potentials of the dyad were determined in organic media. Even though for Cat1 chloro axial ligands are substituted by water in aqueous conditions and the redox couples can be slightly shifted, [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine -Dioxime Complexes[END_REF]188 we can have a good estimation of what happens in aqueous media.

2) Photolysis experiment

To further study the ability of the dye-catalyst assembly to generate and accumulate a Co(I) state under irradiation, a photolysis experiment was performed in degassed acetonitrile using TEOA as SED (10 % v/v) The clear formation of Co(I) implies that RuP4 OEt -Cat1 can accumulate the reduced species of cobalt, the entry point of the proposed catalytic cycle. 169 Since TEOA is in high excess, reductive quenching is more likely to occur under these photochemical conditions, followed by thermal electron transfer from the reduced dye to the catalyst. Here, Co(I) is generated within the first minute of irradiation and reaches a plateau after 3 minutes. These features seem promising especially if we compare them with the photolysis experiment of RuP4 OEt -Co, where accumulation of the Co(I)-species took more than 30 minutes (Figure II.8). This suggests that the overall process is faster in the case of RuP4 OEt -Cat1.

V. Conclusions and perspectives

A second novel dye-catalyst assembly with a cobalt tetraazomacrocyclic complex was synthesized and characterized for further integration in a NiO hydrogen-evolving photocathode.

Synthesis of the dyad was accomplished in two steps from RuP4 OEt -EPIP: First, CuAAC was performed to couple the light harvester with a catalyst precursor bearing an azido group, followed by the cobalt-templated synthesis of the catalytic center. Full characterization of the final dye-catalyst assembly was performed and established that : i) the dyad was isolated under its protonated imidazole form at the end of the synthesis; ii) the redox and spectroscopic properties of the subunits remaining unmodified after the covalent coupling and iii) all the thermodynamic considerations for electron transfer to the catalyst can be met for an efficient NiO photocathode at pH 5.5 based on RuP4 OEt -Cat1. Interestingly, a photolysis experiment revealed that the Co(I) state is formed within the first minutes of irradiation, suggesting that the electron transfer processes are faster for RuP4 OEt -Cat1 than RuP4 OEt -Co.

This dyad thus seems very promising for PEC applications. However, its synthesis and purification were really low yielding (< 20%) and demanding. Comparison with the other Cat1based dyads prepared in the group suggests that hydrolysis of the phosphonate groups during the course of the reaction could be an issue. A solution could be to fully hydrolyze the phosphonate esters to the corresponding acids before purification of the final dyad assembly by size-exclusion chromatography. 144

In the retrosynthetic scheme below (Scheme IV.7), an alternative procedure is envisaged.

The idea is to perform first the click coupling between N3-DAP and the EPIP ligand. Then, cobalt-templated synthesis will follow to obtain the catalyst with the third diimine ligand (EPIP-Cat1) and finally coordination of this ligand to ruthenium for isolating the desiring RuP4 OEt -Cat1. The main advantage is that possible hydrolysis of the phosphonate groups due to the presence of HCl and acetic acid could not anymore happen. Synthetic difficulties for this route might be the unwanted coordination of cobalt from both sides of the ligand (EPIP-DAP).

Photoelectrochemical activity for proton and carbon dioxide reduction of a novel RuP 4 OH -Cat1 based NiO photocathode

In the last chapter, the novel RuP4 OH -Cat1-sensitized NiO photocathode will be assessed for proton reduction under strictly identical conditions with RuP4 OH -Co. This will give us the opportunity to expand our research and correlate the photoelectrocatalytic activity with the choice of the catalyst. In addition, we hope that we will be able to minimize the desorption rate from the surface, which was observed for our previous photocathode.

Cat1 has found few applications in the field of carbon dioxide reduction under photo-or electro-chemical conditions. In this frame, some preliminary results utilizing our dyad on NiO electrodes for PEC carbon dioxide reduction will be displayed in the second half of this chapter.

A literature screening on p-type photocathodes for CO2 reduction inspired us to work in fully aqueous media. Finally, comparison with the already existing molecular dye-sensitized photocathodes for proton and carbon dioxide reduction will be made in order to understand the future potentials of our photocathodes.

I. Preparation and characterization of the sensitized NiO films

Following the synthesis of RuP4 OEt -Cat1, grafting onto homemade mesoporous nanostructured NiO electrodes was undertaken and the resulting sensitized films were characterized before assessing their photoelectrocatalytic activity.

1) Film sensitization and dyad loading determination The first sensitized film had a dyad loading of 4.2 nmol.cm -2 (F1 in Table V.1), thus in the range of the 5.2 ± 0.8 nmol.cm -2 previously obtained with RuP4 OH -Co. However, this result could not be reproduced (F2-F11 in Table V ii) Alternative sensitization procedure using RuP4 OTMS -Cat1

The low grafting densities obtained with RuP4 OH -Cat1 compared to our previous system, motivated us to search for ways to face this issue. A recent paper from S. Rau and T. Jacob described the immobilization of ruthenium polypyridine complexes onto NiO using trimethylsilylated (TMS) substituted phosphonate anchoring groups instead of the acidic form of the latter. 199 Sensitization was realized in the glovebox, as TMS-groups are prone to hydrolysis under non-inert atmosphere, leading to higher initial kinetic adsorption rates and slightly higher grafting densities compared to classical sensitization with phosphonic acid groups. They ascribed this behavior to the presence of a good leaving group, like TMS, which can accelerate grafting during the hetero condensation reaction with the reactive hydroxyl groups, taking place on the surface of the NiO film. In addition, lower desorption and better performances for DSSCs in organic media compared to the deprotected complexes were obtained due to a higher binding strength to the surface.

Driven by this study, we performed the McKenna reaction in the glovebox using dry acetonitrile and an excess of TMSBr, to obtain the corresponding protected dyad RuP4 OTMS -Cat1 (Scheme V.2).

Scheme V.2. Synthetic procedure to obtain RuP4 OTMS -Cat1.

Grafting onto NiO films was also undertaken in the glovebox, using dry acetonitrile as solvent, and after rinsing and drying, the sensitized films were kept protected from light. Using different batches of NiO, an average grafting density of 1.9 ± 0.5 nmol.cm -2 was determined.

F1 F2 F3 F4 F5 1.8 2.4 1.1 2.2 2.0 Table V.2.
Surface concentration (nmol.cm -2 ) of RuP4 OTMS -Cat1 determined by the desorption procedure for 5 freshly grafted films (F).

Although the dye loading was not enhanced, this procedure seemed to result in more reproducible values. Electrodes sensitized by the two different experimental procedures were assessed for photoelectrochemical H2 production, for comparison purposes.

2) MALDI-ToF analysis

The desorption solutions were analyzed by MALDI-ToF with two different methods in order to prove that the integral dye-catalyst assembly (RuP4 OH -Cat1) has been grafted onto NiO films. Similarly to RuP4 OH -Co, all peaks are detected under their single charged form.

i) Linear positive mode

In linear positive mode, with which the resolution for the isotopic pattern is not possible, we could observe a signal at 1465.6 corresponding to our dyad with the loss of the two Cl -axial ligands and the PF6 -counter ions (Figure V.2). The peak at 1439.7 is ascribed to the loss of two nitrogen atoms due to fragmentation and the one at 1313.8 to some hydrolyzed starting material (RuP4 OH -DAP). This can be linked to the desorption conditions or fragmentation during the experiment. Cat1 was tentatively analyzed to better understand the fragmentation patterns, but its low molecular weight did not enable us to observe its spectroscopic signature in MALDI-ToF.

ii) Reflecting positive mode

The spectra were also recorded using a reflecting positive mode to provide the isotopic pattern of the peaks (Figure V.3). The signal at m/z 1465.9 is still present with a characteristic isotopic pattern for ruthenium, but in lower intensity, as this method favors fragmentation.

Thus, the main peak is at 1438, corresponding to our dye-catalyst assembly with the loss of two nitrogen atoms.

II. Photoelectrocatalytic activity for H2 evolution

The photocathode in The photocathode develops cathodic photocurrents after continuous light irradiation (red line)

with onset potential at + 0.41 V vs Ag/AgCl (+ 0.94 V vs RHE). Furthermore, chopped light irradiation (blue line) implies that the generated current is photo-induced.

Higher photocurrent densities were recorded at increasingly negative potentials. 117,121 This is in agreement with the TA-SEC measurements for the RuP4 OH -Co based photocathode (presented in Chapter III), where at more negative applied potentials a long-lived charged separated state is present along with increasing time constants for charge recombination, leading to enhanced activity. At -0.4 V vs Ag/AgCl we reached an average of 67 ± 36 μA.cm - Photoelectrochemical tests were also carried out using the alternative sensitization procedure with RuP4 OTMS -Cat1 under strictly identical conditions. From the LSV (Figure V.5 bottom), the photocurrents are higher (134 ± 57 μA.cm -2 determined for 3 different samples) compared to the ones obtained with the classical immobilization procedure. This mainly arises from the higher grafting densities, as the electron transfer efficiency is only slightly enhanced (75 ± 8 compared to 59 ± 22 μA.nmoldyad -1 ). Nevertheless, investigating the photoelectrocatalytic activity under continuous irradiation will indicate if this grafting procedure could have a positive impact in a long-term scale.

-0,6 -0,4 -0,2 0,0 0,2 0,4 For the dye-sensitized photocathodes using cobalt diimine-dioxime as catalyst, pH 5.5 was initially chosen as the optimal pH. Preliminary results obtained with organic dyes coupled with Cat1 (Chart IV.3) indicated that the chemical nature of the buffer solution does not affect the activity, as similar values were obtained using MES or Britton-Robinson (B.R.) buffer at pH 5.5. In this work, we started to examine more acidic pH values, as Cat1 was mostly employed at lower pH in homogeneous photocatalytic systems. Hence, we tested B.R. buffer, a wide range pH buffer system, 200 at pH 3 (Figure V.6), as a representative value.

- After recording LSVs under dark and light irradiation at pH 3, the cathodic photocurrents were significantly decreased compared to the photocathodes at pH 5.5 (Figure V.5) as well as the electron transfer efficiency (43 ± 8 μA.nmol -1 dyad for 2 different samples). This could be a result of different kinetics of the electron transfer at lower pH. Furthermore, at this value, our dye-catalyst assembly might be protonated on the imidazole ring (Chart II.4) 150 affecting the possible intramolecular electron transfer to the catalyst through the bridging ligand.

Nevertheless, a more detailed study is needed to conclude about the influence of the pH on the performance.

2) H 2 -evolving activity assessment

Long-term chronoamperometric measurements were performed under continuous irradiation and coupled to hydrogen detection in order to correlate the cathodic photocurrents observed in the previous LSV experiments with photoelectrocatalytic hydrogen evolution. For a direct comparison with our previous photocathode, strictly identical conditions were employed, i.e.

the same applied potential (-0.4 V vs Ag/AgCl) with AM 1.5 G irradiation for 2 hours. The initial photocurrent density of 110 μΑ.cm -2 (Figure V.7) is in good accordance with the values obtained during the LSV (Figure . V.5) for the NiO films sensitized by RuP4 OH -Cat1. A decay of the photocurrent is observed over the 2 hours experiment, with only 3.5 μΑ.cm -2 remaining when the lamp was switched off at the end of the experiment. Gas chromatographic analysis in the headspace of the cell and using the micro-clark electrode in the solution confirmed the evolution of hydrogen. The overall faradaic yield was determined to be 50 ± 3 % and hydrogen is produced with an impressive average TON value of 237 ± 14 (Table V.3), the highest values reported until today for NiO dye-sensitized photocathodes (Table I Measuring hydrogen after 2 hours of PEC tests for the photocathodes sensitized by RuP4 OTMS -Cat1, F.E. of 64 ± 2% were estimated, indicating that the selectivity for proton reduction is a little bit increased. TON of 371 ± 128 TON were determined, showing anew an enhancement. The data tend to indicate that the grafting procedure influences the photoelectrocatalytic activity with slightly better F.E and TON in the case of RuP4 OTMS -Cat1sensitized films. More experiments are needed in order to confirm the superior performance of the photocathodes prepared with the protected silylated derivatives of the dyad, as different batches of NiO films may also be responsible for the activity. The increase in performance for the RuP4 OH -Cat1-based photocathode compared to RuP4 OH -Co is undeniable with two times higher F.E. and 18-fold increase in TON. The more efficient and faster electron uptake (catalysis) by the catalyst, as the same harvesting unit and bridging ligand are used, and its different nature (robust macrocyclic complex) could be hidden behind this behavior. It should be noted that the first example to integrate Cat1 into a photocathode was made in 2015. 201 In this work, a ruthenium complex grafted onto mesoporous NiO films, produced negligible photocurrents in organic media, probably due to diffusionlimited electron-transfer to the free in solution catalyst. Thus, coupling Cat1 with the dye and immobilizing on the electrode could face the problem of diffusion and directionality of the electron transfer and enhance the activity.

3) Preliminary post-operando analysis

The PEC activity of RuP4 OH -Cat1 and RuP4 OTMS -Cat1 after 2 hours in fully aqueous conditions is clearly superior to the one previously recorded with RuP4 OH -Co. Nevertheless, 92 to 97% drop of the initial current density is observed under continuous light irradiation, implying stability problems. Post-operando analysis is crucial for understanding the source of this photocurrent decay. After the 2 hours PEC tests, the NiO films were dipped in the methanolic solution of 1 M phenylphosphonic acid and the solutions were characterized by UV-Vis spectroscopy and MALDI ToF.

i) UV-Vis characterization

Analyzing the desorption solutions for the fresh half of sensitized NiO films and the other half after 2 hours of PEC test gave us evidence for significant detachment of the dyad from the NiO during the experiment (Figure V.9). We could estimate desorption of 69 ± 1% ( 3independent experiments) for the normal grafting and 74 ± 6% (2 independent experiments) for the alternative grafting using the TMS-protected derivative. Thus, a stronger binding onto NiO when the TMS-protected dyad is employed could not be confirmed in contrast to the study from the group of S. Rau. 199 In addition, these values are similar to the one obtained for RuP4 OH -Co dyad. 50% desorption of the initial dyad is also determined after carrying out PEC tests at pH 3. T.

J. Meyer's group highlighted that desorption of [Ru(bpy)2(4,4′-(PO3H2)2bpy)] 2+ from metal oxides (TiO2 and ZrO2) is pH-dependent in aqueous conditions. [START_REF] Hanson | Photostability of Phosphonate-Derivatized, Ru(II) Polypyridyl Complexes on Metal Oxide Surfaces[END_REF] Notably, at pH higher than 5, desorption took place even without light irradiation. Based on this work and our study here, phosphonates could offer more stable linkage at more acidic pH, but this stabilization is not enough, as inferior performance for hydrogen evolution was obtained.

ii) MALDI-ToF analysis

The These results tend to indicate that the catalyst is completely lost after 2 hours of PEC activity.

However, it should be noted that this peak is also present for the fresh NiO films (Figure V.2 and 3). Thus, additional measurements are needed to accurately identify possible decomposition pathways. Furthermore, quantifying cobalt by ICP-AES may help to clarify if some cobalt is still present or not at the end of the PEC experiments.

III. Photoelectrochemical activity for CO2 reduction 1) General introduction

The worldwide consumption of fossil fuels increased the levels of carbon dioxide in the atmosphere with negative impacts on the environment. 202 A solution to decrease the accumulation of CO2 lies in the development of selective and efficient methods to reduce it into carbon-based fuels using solar or other renewable energy sources. 203 Plants are able to harness the solar energy in order to convert CO2 into carbohydrates, the fuels of the cells (photosynthesis) and this is the inspiration for molecular chemists to develop sunlight-driven devices for CO2 reduction (artificial photosynthetic systems).

Carbon dioxide can be reduced to a mixture of products (Eq.V.1) due to close thermodynamics such as CO, HCOOH, CH3OH, CH4, CH2CH2, C2H5OH, CH3COOH and oxalate. CO2 is an inert substrate and reduction to a variety of products at reasonable rates demands the use of potentials more negative than those corresponding to the thermodynamic equilibrium. Thus, efficient, stable and selective catalysts are needed to decrease the reaction barriers due to multiple electron-proton reduction processes and to target specific compounds.

CO2 + e - CO2 .- E 0 = -1.9 V CO2 + 2H Electro-and/or photochemical reduction of carbon dioxide has gained scientific interest from 1980s. 204,205 The following years, various molecular complexes based on noble metals (Re and Ru mostly) but also on first-row transition metals like Cu, Co, Ni, Mn and Fe have been reported

as carbon dioxide reduction catalysts. [START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF]206,207,208,209,210,211 As photosensitizers, organic dyes, ruthenium, iridium polypyridine complexes and porphyrins have been employed. In addition, dyads with ruthenium and rhenium play a central role in the field. 212,213 Although a fine tuning of the catalyst structure can lead to good selectivity, few of these systems can work in fully aqueous conditions, where competition with proton reduction can take place, and even less can reduce CO2 beyond the two-electron reduction products, CO and HCOOH (Eqn. V.1).

Photochemical systems need sacrificial electron donors to function. Immobilization onto surfaces are important for construction of functional devices that can achieve carbon dioxide reduction with water as reductant. A few examples of molecular photocathodes for CO2 reduction were reported up to date and will be presented below, along with the use of Cat1 for photo-or electro-chemical CO2 reduction.

2) Dye-sensitized photocathodes for CO 2 reduction

The field of dye-sensitized photocathodes for CO2 reduction was emerged the last years [START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF]214 with the first functional systems appearing in 2014. 215,216 In this field, two main approaches exist for the construction of the photocathodes, the covalent dye-catalyst assemblies and the "layer-by-layer" approach and the main breakthroughs will be presented in the following section. Alternatively, examples of molecular photocathodes based on narrow band-gap semiconductors combined with molecular catalysts are presented in a third part.

i) Covalent dye-catalyst assemblies

The first example of a p-type dye-sensitized photocathode with a molecular catalyst adopted the covalent dye-catalyst assembly architecture. 216 A dyad composed of a zinc porphyrin as photosensitizer and a rhenium bipyridyl complex (Dyad a) as catalyst proved to be active for In these studies, the amount of grafted dyad was relatively low (not more than 4 nmol.cm -2 )

and more importantly the detachment from the surface limits the efficiency of the photocathodes. 217,180 In order to improve both the surface concentration in active compound and the stability of the grafting, a loading method using reductive electropolymerization was employed by Ishitani's group. 218 First, a ruthenium complex with methyl phosphonic acid groups and two 4-methyl-4′-vinyl-2,2′-bipyridine ligands (Dye a) were grafted onto NiO films.

Then, the RuRe dyad with two 4-methyl-4′-vinyl-2,2′-bipyridine ligands (Dyad c) was electrodeposited in acetonitrile and three times more immobilized dyad compared to normal grafting was estimated from CV and ICP measurements. Desorption significantly decreased in aqueous NaHCO3 solution, because of the hydrophobic nature of the vinyl polymer groups.

Irradiation of the photocathode showed high cathodic photocurrents and long-term photoelelectrolysis at -0.7 V vs Ag/AgCl produced 500 nmol of CO as well as H2 and HCOOH with a total of 85% F.E. Hydrogen and formate were products of the decomposition of ruthenium as evidenced by control tests. The higher amount of CO and F.E. compared to the non-polymerized dyad 180 can be explained by the higher loading and the lower desorption of the dyad as evidenced by XPS and UV-Vis at the end of the experiment. However, only 16

TONCO were achieved, which represents a twice lower activity, suggesting that the electron transfer from the dye to the catalyst might be less efficient in this polymerized architecture.

Chart V.2. Ruthenium and rhenium complexes used to build CO2-reduction photocathodes by the electropolymerisation technique.

In a similar approach, NiO films were silanized with a vinyltrimethoxysilane bridge and ruthenium (Dye b) and rhenium complexes (Chart V.2) were stepwise reductively electropolymerized, forming carbon-carbon bonds, with 1/1 ratio. 219 Photocurrent densities of -20 μA.cm -2 were recorded under light irradiation at -0.7 V vs Ag/AgCl in neutral aqueous conditions. CO and H2 were the two products during the course of 10 hours photoelectrolysis with 65% F.E. and 58 TON for CO. Time-resolved spectroscopic measurements confirmed that fast hole injection occurs after dye excitation; In addition, relatively slow charge recombination enables electron transfer to the catalyst. Photocurrent decay was limited due to the stability of the assembly onto the surface, but rhenium decomposition or detachment was observed with post-operando XPS analysis.

GuGaO2 semiconductors offer an alternative to NiO, because of its higher conductivity and more positive flat band potential. 103 O. Ishitani's group grafted Dyad b onto GuGaO2 in order to compare the efficiency with NiO under similar experimental conditions. 220 The photocathode exhibited cathodic photocurrents with 0.4 V more positive potential than NiO in aqueous electrolyte (pH 6.6). The observed current is a result of photo-induced hole injection from the excited photosensitizer to the semiconductor. Under continuous light irradiation at -0.3 V vs Ag/AgCl for 15 hours, the amount of CO was estimated 1 μmol with 122 TON and a total F.E.

of 81% (including hydrogen). This is a clear improvement compared to the NiO-based photocathode. 180 The most significant achievement of this work is the preparation of the first bias-free tandem cell active for CO2 reduction under visible light illumination and using water as a reductant. The photocathode evolved CO (22 TON) and H2 with a total F.E. of 72% and the CoOX/TaON photoanode O2 with 70% F.E.

ii) "Layer-by-layer" assembly

A "layer-by-layer" approach based on Zr(IV)-phosphonate bridges was used for the construction of dye-sensitized photocathodes for CO2 reduction in 2019 by the group of T. J.

iii) Molecular photocathodes based on narrow band-gap semiconductors

Most of the constructed photocathodes for CO2 fixation are based on noble metal catalysts, notably ruthenium and rhenium. Last year, a functional photocathode using a p-Si photoelectrode and an immobilized phosphonated cobalt bis(terpyridine) catalyst was prepared. 222 High grafting density (45 nmol.cm -2 ) was determined due to a mesoporous TiO2 scaffold on the electrode. The photocathode reduced CO2 to mostly CO, formate and hydrogen in a mixture of organic/aqueous media, but also in fully aqueous conditions. In 6/4 MeCN/H2O at -1.0 V vs Fc + /Fc, a total F.E. of 77% and 330 TON were estimated. Control experiments with a cobalt salt proved that the molecular catalyst is crucial for the activity, even though some H2 and minimal amounts of CO were measured due to a heterogenous Co-deposit. Stability for more than 24 hours is an important characteristic of this system. In addition, the performance of the catalyst outperforms its previous activity in solution or grafted onto dark electrodes. 208,223 Most importantly, the corresponding photocathode is the most efficient reported for CO2 reduction, highlighting the fact that precious metal free catalysts can compete the noble metalbased one.

The groups of M. Robert and F. Odobel reported a photocathode based on Cu(In,Ga)Se2

(CIGS) as narrow band-gap SCs and a cobalt quaterpyridine. 224 The molecular catalyst was functionalized with phosphonic acid anchoring groups for the immobilization on a TiO2 protective layer of the architecture. LSVs in fully aqueous media (pH 6.8) under intermittent irradiation showed impressive photocurrent densities higher than 3 mA.cm -2 at potentials more negative than 0 V vs RHE. 2 hours PEC tests were carried out at -0.06 V vs RHE and high selectivity for CO (97%, 3% H2), activity (8000 TON) and stability were obtained.

Recently, T. J. Meyer's group reported a combination of molecular and semiconductor p-n junctions for efficient photoelectrocatalytic CO2 reduction (Figure V.13). 225 For the semiconductor, one-dimensional n-GaN nanowire arrays were grown on n + -p-p + silicon wafers. A NiO overlayer was deposited with ALD for binding the first monolayer of the molecular assembly, an electron relay (NPhN) with carboxylic acid anchoring groups and different structural variations. Then, a ruthenium dye and the common used rhenium catalyst with carboxyl groups were subsequently immobilized on the electrode by Zr(IV)-bridging units in a "layer-by-layer" assembly. Finally, a protective NiO layer was added by ALD for shielding the anchorage on the electrodes and the binding between the different layers. Complementary light absorption occurs due to the presence of silicon wafers and the molecular dye. Thus, the system takes advantage of the whole visible spectrum. Solar illumination (AM 1.5G) of the photocathode in aqueous electrolyte (pH 6.8) generates cathodic photocurrents from 0 V to -0.4V vs RHE. At -0.25 V vs RHE, reduction of CO2 to formate was accomplished with F.E.

from 35 to 65% with quite stable photocurrent densities (-1.1 mA.cm -2 ) for 20 hours. 3) Cat1 as catalyst for CO 2 reduction

Cat1 has been first used as catalyst for CO2 reduction in the eighties. 187 CO and H2 were generated under irradiation of CO2-saturated aqueous (pH 4) photocatalytic system containing Cat1, [Ru(bpy)3] 2+ and ascorbate as SED. The selectivity for CO was low, as hydrogen was produced in higher yields. In addition, electrochemical reduction of CO2 was carried out in fully aqueous media or in a mixture with DMF. From CVs, catalytic wave was observed at more negative potentials than the second reduction wave (Co(II)/Co(I)) in the presence of CO2. CO and H2 were produced, even though some oxalate and formate were also present after electrolysis (no exact quantification). Higher selectivity for CO was found in a mixture of H2O/DMF (5/95) and the process is catalytic with TON values above unity (7). F.E. of 70% was estimated at -1.3 V vs SCE.

Many years later, Peters and his team reexamined the electrocatalytic CO2 reduction with Cat1 in a mixture of organic/aqueous conditions. 226 Catalytic activity was observed close to the third reduction potential (ligand-based redox couple), increasing with the addition of water.

Controlled potential electrolysis with a glassy carbon at -2.0 V vs Ag/AgNO3 for 40 minutes proved the reduction of CO2 to CO. Higher performance (45% F.E. and 4 TON) and selectivity (CO/H2 3/2) for CO evolution were obtained in an electrolyte containing 10 M H2O. Cat1 was also active for photocatalytic CO2 in the same media with Ir(ppy)3 light harvester and TEA as electron donor. 227,228 CO was detected in the solution and in the gas headspace with FT-IR and some intermediate species of the catalytic cycle could be characterized.

i) Experimental set-up and conditions

The set-up for the photoelectrochemical CO2 reduction experiments is similar to the one previously employed for H2 evolution. The photocathode performances were assessed in a three-electrode cell, using a 50 mM NaHCO3 aqueous electrolyte 180,220 and two different applied potentials for comparison purposes. Irradiation was carried out with a 300W ozone-free

Xe lamp operated at 280 W, equipped with a water-filled filter for elimination of IR irradiation and AM 1.5G filter. Pure CO2 was bubbled for at least 40 minutes in the electrolyte in order to saturate the solution. CO and H2 were quantified in the headspace by gas chromatography with FID and TCD detectors respectively. HCOOH in the solution was quantified by ion-exchanged chromatography at the end of the experiment. Prior to every chronoamperometric test, linear sweep voltammograms were recorded screening from + 0.6 V to -0.8 V vs Ag/AgCl.

ii) Photoelectrochemical characterization

PEC measurements with the NiO/RuP4 OH -Cat1 photocathode were conducted to examine its ability for reducing CO2 in water. Ag/AgCl, whereas the alternative sensitization with TMS-protected dyads was used for the experiments at -0.7 V vs Ag/AgCl). By contrast, when pristine NiO films were used, very low photocurrents were recorded, corresponding to the production of only traces of CO and H2. The NiO photocathode sensitized by RuP4 OH -EPIP produced slightly higher amount of H2 but also traces of CO. The former can derive from some ruthenium-based activity through decomposition processes as previously reported under similar experimental conditions. 180,220 This first series of experiments thus clearly prove that the presence of Cat1 is required for substantial CO2 reduction activity. Of note, low amounts of formate are also produced in the electrolyte solution, but roughly the same amount is quantified whatever the system tested (with our without Cat1), including for pristine NiO films.

In addition, PEC tests were performed under N2 atmosphere at -0.7 V vs Ag/AgCl. In that case, only small quantities of CO were produced (9 nmol.cm -2 ) and the main activity is the reduction of protons from the aqueous electrolyte. Interestingly, our photocathode is still active for PEC H2 evolution under these quasi neutral conditions (pH 6.6) with an amount of evolved hydrogen (234 nmol.cm -2 ) in the range of the ones previously obtained at pH 5.5 (Table V.3).

Finally, 2 experiments were run over a longer time (19 hours) and these preliminary results indicate that our photocathode is still active after 3 hours at both potentials: up to 291 TON for CO were indeed achieved under these conditions (-0.4 V vs Ag/AgCl). However, additional experiments and post-operando analysis are required in order to fully understand this behavior.

IV. Conclusions and perspectives

In the first part of this chapter, a novel dye-catalyst assembly was grafted onto NiO films.

Photoelectrocatalytic tests were carried out under strictly identical conditions for direct comparison with the previous H2-evolving photocathode. Under fully aqueous conditions at pH 5.5 at an applied potential of -0.4 V vs Ag/AgCl (+ 0.13 RHE) with AM 1.5 G solar irradiation, higher photocurrents and electron transfer efficiency were obtained, implying more efficient electron uptake (catalysis) by the cobalt tetraazomacrocyclic catalyst. The produced H2 and F.E.

were also enhanced, reaching F.E. higher than 50% compared to 25% for the previous system.

Higher than 200 TON were determined, showing an 18-fold increase. The attractive activity of the new dyad is ascribed to the cobalt complex as the photosensitizer, the anchorage and the bridging ligand are the same. To the best of our knowledge, these TON values are the highest ever reported for a dye-sensitized NiO photocathode.

Low grafting densities were estimated due to the moderate solubility of the dyad and the different batches of NiO films. Slightly increase was accomplished after grafting the dyecatalyst assembly under its TMS-protected form and almost 400 TON were achieved after 2 hours of CA experiments. However, the results were not always reproducible, and more measurements are essential to identify a clear trend. Nevertheless, improving the grafting procedure either by electropolymerization or by introducing new anchoring groups could further improve the performance of the photocathode.

Post-operando analysis showed desorption after 2 hours of PEC test, in agreement with RuP4 OH -Co, regardless the grafting method. ALD could be used in order to face this detrimental phenomenon. Preliminary MALDI-ToF spectra suggested that loss of the catalyst can occur during the long-term tests. Obviously, more post-mortem techniques like ToF-SIMS, XPS, ICP-AES are needed to obtain a more distinct view of the possible decomposition pathways and the destiny of the molecular dye-catalyst assembly. This will enable us to correlate the activity of the photocathode with the nature of the catalyst and in a following step to optimize the molecular architecture to further boost the long-term operation.

In the last part of this thesis, we showed that our new dyad is also promising for photoelectrocatalytic CO2 reduction. To the best of our knowledge, this is the first example of functional dye-sensitized photocathode based on a cobalt catalyst, which produces CO with high efficiency and selectivity in fully CO2-saturated aqueous media. The evolution of hydrogen due to the competitive proton reduction can produce the industrially useful syngas. 229 The TON values are higher than the previous NiO-photocathodes with the RuRe dyad in aqueous media 180 and outperforms previous work with Cat1 under electro or photo-chemical conditions. In addition, these preliminary results exhibit the highest TON with a dye-sensitized photocathode for CO2 reduction to date. Further experiments, post-operando characterization and mechanistic investigations are under way in order to shed light on the activity and stability of the photocathode.

General conclusion and perspectives

This thesis falls within the field of bio-inspired chemistry and artificial photosynthesis for solar fuel production. Production of hydrogen or carbon-based fuels in a tandem DSPEC, where a photoanode for water oxidation is coupled to a photocathode performing the reduction processes of interest (proton, carbon dioxide) represents the ultimate goal. Unfortunately, up to now the performances of the p-type photocathode are a limiting factor to develop competent full devices and the construction of efficient dye-sensitized photocathodes is currently the subject of intensive research interest.

Here, we presented the synthesis and characterization of two covalent dye-catalyst assemblies functionalized with methyl phosphonate anchoring groups for grafting onto NiO.

For both architectures, the same ruthenium tris-diimine complex as photosensitizer was employed coupled with two different cobalt complexes.

Our first dyad with a cobalt diimine-dioxime catalyst was active for hydrogen evolution in fully aqueous medium. Strictly identical experimental conditions were used for the photoelectrocatalytic tests in order to have a direct comparison with the previous photocathode from the group based on a push-pull organic dye-catalyst assembly. The superior performance of our photocathode could be ascribed to the ruthenium photosensitizer. We could further increase the activity when standardized AM 1.5G filter was used due to more efficient light harvesting by the ruthenium complex. The obtained activity lies in the range of the previously reported best performing dye-sensitized hydrogen-evolving photocathodes with photocurrents of some tens of μA and TONs (Table I.1).

Transient absorption spectroscopy and post operando experiments allowed us to identify the main factors limiting the performance of the photocathode. Firstly, the thermal electron transfer from the reduced dye to the catalyst is one of the kinetic bottlenecks. Secondly, desorption of the dyad from the surface of the electrode and degradation of the cobalt diimine-dioxime catalyst are two issues that should be taken into consideration for the overall efficiency and long-term stability of our system. We should mention that post-operando analysis in order to elucidate the fate of the molecular components in the literature is scarce.

We designed a second dye-catalyst assembly with a more robust cobalt tetraazomacrocyclic complex. Its preparation proved to be a big challenge as synthesis and purification are highly demanding and optimization is ongoing in the lab. Nevertheless, the corresponding photocathode exhibits the highest TON reported until today for p-type dye-sensitized NiO photocathodes for proton reduction. The enhanced activity is clearly ascribed to the catalytic unit, as all the other parameters remained identical. Supplementary post-operando analysis is needed in order to understand the reasons behind this increase in performance. Our novel dyad was also used for CO2 reduction and the preliminary results showed high TON and F.E. for CO.

Overall, this work now provides some guidance to build more performant dye-sensitized photocathodes:

 Cat1 opens the way to integrate more stable and robust aminopyridyl and polypyridyl cobalt complexes in these devices.

 Covalent dye-catalyst architectures achieving more efficient electron transfer need to be designed.

 To improve the stability of the molecular anchoring, different approaches can be followed. Deposition of a thin oxide layer by ALD, typically alumina, at the top of the sensitized film is well-known to decrease desorption from the surface of metal oxides. Formation of a hydrophobic monolayer and (electro)polymerized assemblies are other solutions in order to diminish the detachment of the molecular components.

Finally, a detailed screening of the anchoring groups is crucial in order to obtain the most stable anchorage (not sensitive to pH).

 Improvements of the semiconductor is also necessary for future applications. New materials like CuGaO2 or CuCrO2 have been employed in the field and outperformed analogous NiO-based photocathodes. With these materials, the construction of more efficient systems could be accomplished.

Finally, the performance of the RuP4 OH -Cat1-based photocathode and in particular its activity under neutral pH starts to pave the way for its integration in a tandem dye-sensitized photoelectrochemical cell.

I. Chemical products and solvents

II. Characterization methods and equipment 1) Column chromatography

Two techniques were chosen for carrying out separative chromatography. In a first case, these were performed manually on 60 M silica gel with a grain size of 0.04-0.063 nm (Macherey-Nagel). In a second case, the separative chromatography is assisted by a Puriflash 450 device (Interchim), with preconditioned silica cartridges (Interchim, Puriflash Silica HP 15 μm), after liquid deposition of the product.

2) UV-Vis absorption spectroscopy UV-vis absorption spectra were recorded either on a Shimadzu UV-1800 spectrometer or on an Agilent Cary 60 UV-vis spectrometer in 1 cm optical light path quartz cuvettes.

3) Nuclear magnetic resonance spectroscopy (NMR)

1 H and 13 C NMR spectra were recorded at 298 K in 5 mm o.d. tubes either on a Bruker AC 300 spectrometer equipped with a QNP probehead operating at 300.0 MHz for 1 H and 75.5

MHz for [START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF] C or on a Bruker Avance III spectrometer equipped with a Prodigy cryoprobe and operating at 500.0 MHz for 1 H and 126 MHz for [START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF] C. The deuterated solvents are indicated in all cases and the chemical sifts are given in ppm and referenced to the solvent residual peak.

The obtained spectra were treated using the software TOPSPIN (BRUKER). The multiplicity of the peaks is as follows: s (singlet), d (doublet), t (triplet), m (multiplet).

4) Electrospray ionization mass spectrometry (ESI-MS)

Electrospray ionization mass spectrometric measurements were carried out on a Thermoquest Finnigan LCQ spectrometer in positive and negative mode by Colette Lebrun and Jacques Pécaut at IRIG/SyMMES/CIBEST/CEA Grenoble.

5) High resolution mass spectrometry (HRMS)

Accurate mass measurements (HRMS) were performed on a Bruker maXis mass spectrometer by the "Fédération de Recherche" ICOA/CBM (FR 2708) platform.

6) Elemental analysis

Elemental analysis was carried out on a Thermofisher Scientific "Flash 2000" by the "Plateforme d'analyse pour la chimie" (GDS 3648, Strasbourg) for the analysis of C, H and N.

11) Electrochemistry in solution

Electrochemical analysis was performed using a BioLogic SP300 potentiostat controlled via EC-Lab V10 software. Electrochemical experiments were recorded in a three-electrode cell combining a glassy carbon as working electrode, a platinum wire counter electrode, and a custom-made Ag/AgCl reference electrode (separated from the solution by a Vycor frit).

Typical measurements were carried out in argon-purged acetonitrile solution (0.1 M n-Bu4NBF4 as supporting electrolyte) of 1 mM of each compound. Ferrocene was added at the end of every measurement as an internal reference.

12) Spectro-electrochemistry UV-Vis-SEC measurements were performed in a three-electrode setup in a H2O/O2-free glovebox, using a patterned "Honeycomb" Spectroelectrochemical Cell Kit Chelidamic acid monohydrate (2.0 g, 10. 93 mmol) was dissolved in SOCl2 (20 ml). Dry DMF (0.3 ml) was added at 0°C and the paleyellow reaction mixture was refluxed overnight under argon. The solvent was removed under vacuum with a liquid nitrogen trap and a white solid was obtained.

The crude product was dissolved in dry CHCl3 (20 ml) and a morpholine (3.8 ml, 43.72 mmol) solution in dry chloroform (3.8 ml) was added dropwise at 0°C. The mixture was stirred at room temperature for 3 hours. The solvent was removed under vacuum, aqueous saturated NaHCO3 solution (10 ml) was added and extractions with ethyl acetate were realized, followed by extractions with brine solution and water. The organic layer was dried with Na2SO4, filtered, and the solvent was removed in vacuum to obtain a white solid. Yield: 65% (2.3 g). 1 H NMR (300 MHz, CDCl3): δ (ppm) 7.74 (s, 2H), 3.80 (br, 8H), 3.68-3.64 (m, 4H), 3.60-3.56 (m, 4H).

13

C NMR (300 MHz, CDCl3): δ (ppm) 165. 3, 153.4, 146.6, 125.4, 66.8, 47.6 MeMgCl 3M solution in THF (7 ml, 17 mmol) was added dropwise at 0°C. The reaction mixture was stirred for 30 min under argon at 0°C, followed by quenching with aqueous saturated NH4Cl solution (50 ml) and extraction with CH2Cl2. The organic layer was dried on Na2SO4, filtered and evaporated under vacuum. The crude product was purified by column chromatography on silica gel (CH2Cl2) to obtain white crystals. Yield: 64% (0.85 g). 1 H NMR (300 MHz, CDCl3): δ (ppm) 8.18 (s, 2H), 2.78 (s, 6H). [START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF] C NMR (300 MHz, CDCl3): δ (ppm) 198. 1, 153.9, 146.9, 124.9, 25.6. ESI-MS: m/z 197.9 [M+H] + , 219.9 [M+Na]

+ , 235.9 [M+K] + .

4-azido-2,6-diacetylpyridine (N3-DPA)

2 (600 mg, 3.04 mmol) and NaN3 (3.9 g, 60 mmol) were mixed in DMF (10 ml) and heated to 55°C. After 24 hours, DMF was removed by distillation under reduced pressure. The crude product was purified on silica gel column chromatography using 50% pentane in CH2Cl2 as eluent to give a white solid. Yield: 30% (200 mg). 1 H NMR (300 MHz, CDCl3): δ (ppm) 7.83 (s, 2H), 2.78 (s, 6H). [START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF] C NMR (300 MHz, CDCl3): δ (ppm) 198. 6, 154.5, 151.5, 114.7, 25.6 during the course of the two-hours PEC experiment. In addition, a clear evolution of four Q bands is observed, suggesting a demetallation of the porphyrin core. As demetallation of porphyrins is reported to take place under acidic conditions, the photoelectrocatalytic activity of the films was also assessed at pH 7. Although the ZnP-Co-sensitized film displays improved stability at this less acidic pH (less desorption and demetallation), the activity was not enhanced and the same amount of hydrogen was produced ( The overall performances of this novel dyad are slightly lower than the ones previously reported for a NiO-based photocathode based on the same cobalt diimine-dioxime complex covalently coupled with a push-pull organic dye. This might be due to a lack of directionality in the structure. Indeed, the electron-withdrawing ability of the carboxylate anchoring group can stabilize the electron on the opposite side of the catalytic center, preventing the generation of the catalytically active species.

using dichloromethane/ethanol (98/2) as eluent to give yellow oil (B). From the 1 H NMR (CDCl3, 300 MHz), the peak at 9.81 ppm confirmed the presence of the aldehyde derivative.

Although many efforts were made (increasing temperature, reaction time, different mixture of solvents for column chromatography), we always observed the starting material. Thus, the crude product was used for the next reaction.

Propargyl cyanoacetamide

To a solution of 1-cyanoacetyl-3,5-dimethylpyrazole (5.1 g, 31.2 mmol) in toluene ( 40 Following the synthetic protocol for Dyad 2, we synthetized T1P-Co as follows:

T1P-Cu (C) (110 mg, 0.14 mmol) and Cu-N3 (60 mg, 0.14 mmol) were dissolved in 7.3 ml of degassed CH2Cl2. Sodium ascorbate (26 mg, 0.13 mmol) and CuSO4.5H2O (10 mg, 0.04 mmol) were dissolved in 2.5 ml of degassed H2O and then added to the reaction mixture. 8 ml of degassed

MeOH was added until a single phase was obtained. The reaction mixture was stirred at room temperature for 24 hours under argon and dark conditions. After removal of the organic solvents, aqueous saturated KPF6 solution was added and the mixture was extracted with CH2Cl2. The crude product was purified by column chromatography on silica gel using as eluent a mixture of CH3CN/aqueous KNO3 solution at 10% of the saturating concentration. The product was obtained after evaporation of the organic solvents, addition of KPF6-saturated aqueous solution and extraction with CH2Cl2. The final compound was dried under vacuum to give a brown solid, Yield: 65% (115 mg H, 4.70; N, 10.37; found: C, 44.47; H, 4.74; N, 9.83.

Résumé en français des travaux

Abstract

Production of solar fuels in water splitting dye-sensitized photoelectrochemical cells is a highly promising approach in order to fulfill future energy demands and to face the environmental problems arising from the combustion of fossil fuels. Our group previously constructed NiObased dye-sensitized photocathodes for H2 evolution, based on the first covalent organic dyecatalyst assemblies. Here, we report the synthesis and characterization of two novel dyads, both relying on a ruthenium tris-diimine photosensitizer covalently linked via copper-catalyzed azide-alkyne cycloaddition (CuAAC) to two different H2-evolving cobalt catalysts. The H2evolving photoelectrochemical activities of the resulting photocathodes were assessed under standardized conditions. This allowed us to establish that i) the dyads based on a Ru photosensitizer outperform the previous ones relying on push-pull organic dyes; ii) replacing the cobalt diimine-dioxime catalyst previously employed by the group by a more robust cobalt tetraazamacrocyclic catalyst was crucial to improve the activity, with the highest TON so far reported for hydrogen-evolving dye-sensitized photocathodes. Transient absorption spectroelectrochemical measurements revealed that the performances are limited by the low efficiency of the electron transfer from the reduced dye to the catalytic unit. In addition, longterm stability is strongly affected by the desorption of the dyads from the surface of the electrode during the course of the photoelectrochemical tests. Finally, promising preliminary results were also obtained for photoelectrochemical CO2 reduction to CO in aqueous medium, thus opening interesting perspectives in the field.

Résumé

La production de carburants solaires dans des cellules photo-électrochimiques à colorant réalisant la photodissociation de l'eau est une approche extrêmement prometteuse pour répondre aux futures demandes énergétiques et faire face aux problèmes environnementaux liés à l'utilisation de combustibles fossiles. Au laboratoire, de premières photocathodes de production d'hydrogène à colorant ont été construites, basées sur le premier assemblage colorant-catalyseur covalent sans métal noble. Dans ces travaux, la synthèse et la caractérisation de deux nouveaux assemblages colorant-catalyseur est décrite, tous deux reposant sur le même photosensibilisateur de ruthénium lié de façon covalente via une cycloaddition azoture-alcyne catalysée au cuivre (CuAAC) à deux catalyseurs de cobalt différents. Les activités photoélectrochimiques de production d'hydrogène des photocathodes correspondantes ont été déterminées dans des conditions standardisées. Ceci nous a permis d'établir que i) les dyades à base de ruthénium surpassent celles précédemment décrites à base de colorants organiques ; ii) remplacer le catalyseur diimine dioxime de cobalt précédemment utilisé dans le groupe par un catalyseur tétraazamacrocyclique de cobalt s'est révélé crucial pour améliorer l'activité, et a permis d'obtenir les TONs les plus élevés à ce jour pour des photocathodes de production d'hydrogène à colorant. Des caractérisations spectroscopiques avancées ont révélé que les performances sont limitées par le manque d'efficacité du transfert d'électrons du colorant réduit vers le catalyseur. Par ailleurs, la stabilité est fortement impactée par la désorption de la dyade de la surface de l'électrode au cours des tests photo-électrochimiques. Enfin, de premiers résultats préliminaires sur la réduction photo-électrochimique du CO2 en CO en milieu aqueux ont été obtenus et ouvrent des perspectives intéressantes dans le domaine.

2

 2

Figure 1 .

 1 Figure 1. Total energy reserves (renewable energy sources and fossil fuels) taken from reference 2.

Figure 2 .

 2 Figure 2. Schematic representation of the natural photosynthetic process (top) 4 and water splitting photoelectrochemical cell, DSPEC (bottom). Cubes correspond to light harvesters (P1, P2) and spheres to oxygen or hydrogen-evolving catalysts (OEC, HEC) accordingly.

Figure I. 1 .

 1 Figure I.1. Working principles of a p-type DSSC (left) and a dye-sensitized photocathode in a DS-PEC for H2 production (right). Orange cubes correspond to light harvesters (PS) and green spheres either to redox mediators (RM) or to hydrogen-evolving catalysts (HEC).

Figure I. 2 .

 2 Figure I.2. Cobalt diimine-dioxime grafted onto multiwall carbon nanotubes.

Figure I. 3 .

 3 Figure I.3. Left, simplified mechanism for H2 evolution by Co(DO)(DOH)pnX2, right, possible pathways for hydrogen evolution, involving the oxime bridge with DFT optimized structures, taken from reference 66 .

Figure I. 4 .

 4 Figure I.4. Different architectures for the construction of p-type dye-sensitized hydrogen-evolving photocathodes: a) catalyst in the solution, b) covalent or supramolecular assembly c) co-grafting, d)

3 .

 3 Nanostructured NiO films sensitized with PS3 were protected by 30 cycles of Al2O3 ALD and the photoelectrocatalytic activity was assessed using two different molecular catalysts, a cobaloxime (HER 1) and a nickel Dubois-type catalyst (HER 3) in aqueous solution. The best performing electrode with the nickel catalyst evolved 1.82 μmol of hydrogen at -0.4 V vs Ag/AgCl under illumination for 2 hours. The F.E. was close to unity and the TON 1.5, an indication that catalytic hydrogen evolution occurs. NiO films without ALD treatment produced less amount of hydrogen and complete loss of the dye observed by UV-Photosensitizers (PS 1-12) used for dye-sensitized hydrogen-evolving photocathodes.

  densities and TON compared to TiO2 photoanodes. In Figure I.5, the photoinduced processes occurring in dye-sensitized photocathode based on a covalently linked dye-catalyst assembly are depicted. Upon light excitation, fast hole injection occurs resulting in a charge separated state with a hole on NiO and an electron on the photosensitizer. Competition between charge recombination and catalysis, after thermal electron transfer from the reduced dye to the catalyst, is believed to be the main kinetic issue (Figure I.5).

Figure I. 5 .

 5 Figure I.5.Photoinduced processes in a dye-sensitized photocathode based on a covalent dye-catalyst assembly.

  compound and further extraction with DCM was needed to isolate RuP4 OEt -TMS-EPIP. The final compound has been characterized by 1 H and 13 C NMR, ESI-MS, HRMS and elemental analysis (see experimental part). The main peak from ESI-MS at m/z: 703.4 with the characteristic isotopic pattern for ruthenium is attributed to [M-2PF6] 2+ . Deprotection of the triple bond of RuP4 OEt -TMS-EPIP in basic conditions (K2CO3) resulted the desired compound (RuP4 OEt -EPIP) with hexafluorophosphate as counter ions after addition of KPF6 and with the same purification protocol already presented.

RuP4

  OEt -EPIP and Cu-N3 are ready to be coupled. The optimized synthetic procedure to obtain Ru-Cu 131 was successfully applied to yield the paramagnetic RuP4 OEt -Cu dyad (Scheme II.7). CuSO4 is reduced in situ to catalytic Cu(I) species by an excess of sodium ascorbate under inert atmosphere. A mixture of three solvents (DCM/MeOH/H2O) is essential for a homogeneous solution. The reaction mixture is left under stirring for 2 days at room temperature and protected from light. Purification was accomplished on silica gel column chromatography with a gradient elution of MeCN/aqueous 0.4 M KNO3 (from 95/5 to 80/20).

1 H

 1 NMR spectra of the compounds were recorded in CD3CN. The presence of 4 methyl phosphonate groups is undoubtedly with 8 protons for the methylene spacer, 16 and 24 protons for the ethyl part of the phosphonate groups (Figure II.1).

Figure II. 1 . 1 H

 11 Figure II.1. 1 H NMR spectra of RuP4 OEt -Co (top) and RuP4 OEt -EPIP (bottom) recorded in CD3CN.

Figure II. 2 .Figure II. 3 .

 23 Figure II.2. Selected regions of 1 H NMR spectra of RuP4 OEt -Co (top) and RuP4 OEt -EPIP (bottom) recorded in CD3CN.

  Vis monitoring will enable us to identify the spectroscopic signature of Co(II), Co(I) and Ru(I) species and subsequently perform transient absorption measurements. UV-Visible spectroelectrochemistry measurements were carried out in the glovebox in acetonitrile with a patterned "Honeycomb" Spectroelectrochemical Cell Kit (AKSTCKIT3) purchased by Pine Instrument Company (Figure II.4). A silver wire was used as the reference electrode and a gold as the working electrode. Prior to every measurement, a cyclic voltammogram was recorded in order to find the correct potential to apply for reduction to Co(II), Co(I) and Ru(I). At -0.15 V applied potential, reduction to Co(II) state takes place and a positive absorption band with a maximum at around 500 nm arises (blue line).151 Further reduction to Co(I) state at more negative potentials (-0.55 V), a double band with maxima at 600 and 720 nm appears (orange line). These results are in accordance with the previously reported Co(I) signature, obtained during the course of an electrolysis experiment with Co-diimine-dioxime complex.[START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2-Evolving Dye-Sensitized Photocathode[END_REF] Upon further reduction, we could observe the reduced species of the dye with a broad absorption at 369 and 534 nm, typical for Ru(I) species (green line).152

Figure II. 4 .

 4 Figure II.4. Differential steady-state UV-Vis-SEC spectra of a 0.1 mM solution of RuP4 OEt -Co in acetonitrile using 0.1 M TBABF4 as electrolyte at different oxidation states of the catalyst and dye. All potentials were measured vs. Ag wire.

Figure II. 5 .

 5 Figure II.5. UV-Visible absorption spectra of RuP4 OEt -Co (blue line), RuP4 OEt -EPIP (orange line), Co-N3 (green line) and sum of the absorption Co-N3 and RuP4 OEt -EPIP (dashed line) recorded in acetonitrile.

Figure II. 6 )

 6 in acetonitrile (1.1 × 10 -5 M). No spectral changes were observed even when 2 eq. of TFA were added (Figure II.6a).

Figure

  Figure II.6a. UV-Vis titration at the ground state of RuP4 OEt -Co in acetonitrile (1.1 × 10 -5 M) using aliquots of TFA solutions.

Figure

  Figure II.6b. UV-Vis titration at the ground state of RuP4 OEt -Co in acetonitrile (1.1 × 10 -5 M) using aliquots of TEA solutions.

1 )

 1 Feasibility of working mechanism on NiO electrodesAfter synthesis and characterization of the final dye-catalyst assembly and before proceeding to the photoelectrochemical tests, it is crucial to check if thermodynamically our dyad is able to perform proton reduction. To be catalytically active, the Co(I) state (Figure I.3) 66 needs to be generated under light-driven conditions, in the presence of a source of electrons (a sacrificial electron donor in the solution or a semiconductor like NiO in the case of the photocathode). Two mechanisms can account for this Co(I) formation, either an oxidative or a reductive one (Figure II.7). In the latter, the excited PS can be quenched reductively by the supplier of electrons (NiO or SED), generating PS -. Then, thermal intramolecular electron transfer from PS -to the hydrogen evolution catalyst (HEC) will reduce Co(III) to Co(II). One more electron transfer to the cobalt center is needed to result a Co(I) species. The other path is a photoinduced intramolecular electron transfer from the excited dye to the catalyst (oxidative quenching) to obtain Co(II) with formation of PS + . Regeneration of the oxidized PS occurs by the SED or NiO. Subsequently, a second photo-driven intramolecular electron transfer will reduce the catalyst to Co(I). The relative viability of each of these two mechanisms is determined by the rates of the respective electron transfers.

Figure II. 7 .

 7 Figure II.7. Two possible mechanisms for the formation of Co(I) in a dye-catalyst assembly.

  ΔGinj = e[EVB(NiO) -E(Ru * /Ru -)] Eq. II.2 ΔG1 = e[E(Ru II /Ru II -L -) -E(Co III/II )] Eq. II.3 ΔG2 = e[E(Ru II /Ru II -L -) -E(Co II/I )] Eq. II.4

Figure II. 8 .

 8 Figure II.8. Differential spectra of UV-Vis monitored photolysis experiment following the formation of Co(I) species for RuP4 OEt -Co (top) and Dyad 2 (bottom) (4.5 × 10 -5 M) in CH3CN/TEOA (90:10 v/v) under visible light illumination.

  SEM images are depicted below. Porosity of 15 to 20 nm were calculated (Figure III.1), indicating that our films are mesoporous based on IUPAC nomenclature (2-50 nm). Thickness of up to 1 μm (Figure III.1) were measured for the nanostructured mesoporous four-layered NiO films.[START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF] This value is in the optimal range of 1-2 μm, based on a benchmarking analysis trying to correlate the preparation methods of NiO electrodes with the activity in DSSCs and DSPECs applications.[START_REF] Wood | A Comprehensive Comparison of Dye-Sensitized NiO Photocathodes for Solar Energy Conversion †[END_REF]

Figure III. 1 .

 1 Figure III.1. Scanning electron microscopy (SEM) images of a mesoporous NiO film with porous size of 18 nm (left) and the four spin-coated layers onto FTO glass substrate with thickness of 790 nm (right).

Chart III. 1 .

 1 Figure III.2. MALDI-ToF spectra of RuP4 OH -Co recorded in methanol (top). Left bottom is zoom in the region m/z: 1424-1460 of the spectrum where the main peak at 1448 is displayed and right bottom is depicted the theoretical isotopic pattern for RuP4 OH -Co without the two bromo axial ligands.

Figure III. 3 .

 3 Figure III.3. UV-vis absorption spectra (epsilon curve) of RuP4 OEt -Co (black line) and RuP4 OEt -EPIP (red line) in a methanolic solution of 1 M phenylphosphonic acid.

. 4 )

 4 with the non-grafted complex (Figure III.2), implying that integrity of the molecular assembly is retained after immobilization on NiO films. Hence, the higher in intensity peak at m/z: 1448 corresponds to the molecular weight of the dye-catalyst assembly after loss of the two bromo axial ligands.

Figure III. 4 . 68 Figure III. 6 .

 4686 Figure III.4. MALDI-ToF spectrum of RuP4 OH -Co dyad recorded in a methanolic solution of 1 M phenylphosphonic acid after detached from NiO films. Zoom in m/z: 1438-1460, where the isotopic pattern of the main peak at 1448.16 is presented.

Figure III. 7 . 55 Figure III. 8 .

 7558 Figure III.7. Positive mode ToF-SIMS spectra of NiO films freshly sensitized by RuP4 OH -Co in the m/z range a) 700-800, b) 650-700, c) 600-650, d) 300-400. Some proposed fragments are also depicted.

III. 9 )

 9 , using the NiO-sensitized film as the working electrode (3 to 3.4 cm 2 ), Ag/AgCl (KCl 3M) as the reference electrode and a titanium wire as the counter electrode. As supporting electrolyte we used a non-coordinating 0.1 M 2-(N-morpholino)ethanesulfonic acid (MES)/0.1

Figure III. 9 .Figure III. 10 .

 910 Figure III.9. Typical cell used for the PEC experiments (left) and NiO photocathode assessed for PEC proton reduction sensitized by RuP4 OH -Co (right) at pH 5.5.

(Figure III. 11 .

 11 Figure III.11. Steady-state photocurrents recorded in a pH 5.5 MES 0.1 M/NaCl 0.1 M supporting electrolyte under AM 1.5 G irradiation (70 mW.cm -2 ), at an applied potential of -0.4 V vs Ag/AgCl, for NiO electrodes of pristine (black line), RuP4 OH -EPIP (red line) and RuP4 OH -Co (blue line).

Figure III. 12 ,

 12 photocurrents with catalytic hydrogen production. In Figure III.12, the photocurrent densities and charges are depicted for light illumination using the AM 1.5G filter for pristine NiO films and sensitized by RuP4 OH -EPIP and by RuP4 OH -Co. For RuP4 OH -Co, fast photocurrent decay is observed, stabilizing after around 30 minutes. After 120 minutes, the lamp was switched off and the presence of low cathodic photocurrent was confirmed (< -2 μA.cm -2 ) with almost 98% drop of the initial photocurrent density. A second series of experiments were undertaken under visible light irradiation (data not presented), leading to similar observations. Comparing these photocurrents and charges with the ones recorded for pristine NiO films and sensitized by RuP4 OH -EPIP, clearly establish that the photo response is significantly enhanced by the presence of the catalyst in accordance with the steady-state photocurrents measurements(Figure III.11).

. 2 ,Figure III. 12 .

 212 Figure III.12. Representative photocurrent densities (top) and corresponding charges (bottom) recorded during the course of two-hour PEC measurements under continuous light irradiation (AM 1.5G filter) at an applied potential of -0.4 V vs Ag/AgCl for NiO electrodes of pristine (black line), RuP4 OH -EPIP (red line) and RuP4 OH -Co dyad (blue line).

2 .

 2 Photoelectrochemical experiments assessing hydrogen production activity on NiO electrodes. Conditions are as follows: MES buffer at pH = 5,5, -0.4 V vs. Ag/AgCl applied potential, 2 h runtime. a-Measured by gas chromatography. b-Measured with a micro-Clark electrode. c-After half an hour instead of two hours. d-Fifth measurement (half-an-hour instead of two hours) not included IV. Transient absorption-spectroelectrochemistry (TA-SEC) on the sensitized films To investigate the light-induced processes occurring in the photocathode under quasioperando conditions, transient absorption spectroelectrochemistry (TA-SEC) on NiO films was carried out at different applied potentials. This technique was previously developed by Sebastian Bold in collaboration with our group to study Dyad 2 in solution, combining the advantages of electrochemistry with transient absorption spectroscopy. 151 TA-SEC enabled to study the excited state dynamics and the intramolecular electron transfer in the dye-catalyst

  spectra of NiO films sensitized by RuP4 OH -Co(Figure III.13) show the following general shape: at early times, a spectrum with ground state bleach (GSB) at 480 nm and excited stimulated emission (ESA) at around 590 nm, corresponding to the 3 MLCT state. At OCP, this spectrum decays almost completely after 7 ns. However, at more negative potentials it develops into a long-lived spectrum with positive bands at 540 and 480 nm with a complete loss of the GSB. The band at 540 nm is characteristic for the one-electron reduced dye (Figure II.4), while the band at 480 nm is tentatively ascribed to oxidized Ni species.[START_REF] Boschloo | Spectroelectrochemistry of Nanostructured NiO[END_REF] This observation thus confirms the formation of a charge-separated state, produced by hole injection from the excited dye into the NiO film. Similar TA spectra with RuP4 OH -EPIP-sensitized NiO films were obtained.

Figure III. 13 .

 13 Figure III.13. TA-SEC spectra of RuP4 OH -Co-sensitized NiO films at early delays up to 7 ns at (A) OCP, (B) -0.74 V and (C) -1.15 V vs. Fc +/0 applied potential.

Figure III. 14 .

 14 Figure III.14. Jablonski diagram summarizing the light-induced processes occurring for RuP4 OH -Cosensitized NiO films at -0.74 V vs. Fc +/0 (equivalent to -0.4 V vs Ag/AgCl used in the photoelectrochemical tests).

Figure III. 15 .

 15 Figure III.15. Left XPS core-level spectrum of pristine NiO films and right XPS core-level spectrum after catalysis in the region where cobalt was expected to appear.

Figure III. 16 .

 16 Figure III.16. Left, XPS core-level spectrum of P 2p of pristine NiO films (red line), NiO films sensitized by RuP4 OH -Co before catalysis (blue line) and after 2 hours of PEC test (green line). Right XPS core-level spectrum of C 1s of pristine NiO films (red line) and Ru 3d of NiO films sensitized by RuP4 OH -Co before (blue line) and after (green line) PEC test. Overlap between ruthenium and carbon is present in this region.

Figure III. 17 .

 17 Figure III.17.XPS core-level spectrum of pristine NiO films (red line), NiO films sensitized by RuP4 OH -Co before catalysis (green line) and after 2 hours of PEC test. Left spectrum depicts Ni 2p and right O 1s.

Figure III. 18 .

 18 Figure III.18. Positive mode ToF-SIMS spectra of NiO electrodes sensitized by RuP4 OH -Co after 2 hours of PEC tests in the m/z range a) 700-800, b) 650-700, c) 600-650, d) 300-400. Inset show proposed fragments.

  the negative mode(Figure III.19), peaks containing Co have been disappeared at m/z: 817 and 738. Same NiaPbOcHd -fragments were detected in the range of m/z: 150-200 with the spectra before catalysis, proving that the phosphonate linkage is present even after two hours of catalysis (c).

Figure III. 19 .

 19 Figure III.19. Negative mode ToF-SIMS spectra of NiO electrodes sensitized by RuP4 OH -Co after 2 hours of PEC tests in the m/z range a) 800-1000, b) 700-800, c) 150-200.

Figure III. 20 .

 20 Figure III.20. UV-Visible spectra for RuP4 OH -Co in methanolic solution of 1 M phenylphosphonic acid, where half of a fresh NiO film was dipped (red line) and the other half was dipped after 2 hours of PEC experiment (black line) with visible irradiation (left) or with AM 1.5 G illumination (right).

(

  Figure III.21). One peak is observed at m/z: 1224, tentatively attributed to the complete decoordination of the cobalt center. Hydrolytic cleavage of the C-N bonds of the diiminedioxime ligand seems the most probable explanation. The experimental isotopic pattern for the proposed molecular structure, ascribed to the degraded product, correlates well with the theoretical one.

Figure III. 21 .

 21 Figure III.21. Top, MALDI-TOF spectrum recorded in a methanolic solution of 1 M phenylphosphonic acid after 2 hours of PEC test, middle, the experimental isotopic pattern of the main peak at 1224 and bottom, the theoretical isotopic pattern are depicted.

  Scheme IV.3. Synthetic procedure to obtain N3-DAP.

Figure IV. 1 )

 1 exhibit different peaks attributed to the final dyad thanks to the specific isotopic pattern of ruthenium. The dominant peak detected at m/z: 588.1 is the signature of RuP4 OEt -Cat1 with a Co(III)Cl2 center [M-3PF6] 3+ and the peak at m/z: 954.4 corresponds to the same complex with a loss of 2PF6 [M-2PF6] 2+ . An additional peak at m/z: 881.5 is ascribed to RuP4 OEt -Cat1 with the loss of one proton [M-H-2PF6] 2+ .

Figure. IV. 1 .

 1 Figure. IV.1. Top, ESI-MS spectrum of purified RuP4 OEt -Cat1, middle its experimental isotopic pattern and bottom its theoretical one.

Figure IV. 2 . 1 H

 21 Figure IV.2. 1 H NMR spectra of RuP4 OEt -EPIP (top) and RuP4 OEt -DAP (bottom) recorded in CD3CN at 300MHz.

  1 H-NMR of RuP4 OEt -Cat1 recorded at 500MHz is depicted in Figure IV.3. We faced some problems with the interpretation of the spectrum and especially close to the catalytic center. Comparison with the spectrum of RuP4 OEt -DAP (Figure IV.2) reveals that new peaks arise in the region of 6 ppm to 1.5 ppm; further comparison with the spectrum of T2R-Cat1allow to attribute them to the protons of the catalytic moiety. Interestingly, the peak at 5.95 ppm is the proton of Co-NH (one of the footprints of the complex) based on the detailed NMR analysis of T2R-Cat1. The interpretation of the latter proved also to be difficult due to some conformational restrictions in the dyad; various solvent mixtures were tested before being able to obtain a 500 MHz spectrum of correct quality. Efforts to improve the quality of 1 H NMR of RuP4 OEt -Cat1 are thus ongoing using different solvents, varying the experimental parameters and working with a 500MHz NMR spectrometer.

Figure IV. 3 . 1 HIIIFigure IV. 4 .

 314 Figure IV.3. 1 H NMR spectra of RuP4 OEt -Cat1 (top) and selected regions (bottom) recorded in CH3CN at 500MHz. Middle, selected region of 1 H NMR spectra of T2R-Cat1 recorded in CH3CN at 500MHz

Figure IV. 5 . 150 Chart IV. 4 .

 51504 Figure IV.5. UV-Vis titration at the ground state of RuP4 OEt -Ca1 in acetonitrile using aliquots of TFA (top) or TEA (bottom) solutions.

Figure IV. 6 .IV. Thermodynamic considerations 1 )

 61 Figure IV.6. UV-Visible absorption spectra of RuP4 OEt -Cat1 (red line), RuP4 OEt -EPIP (black line) and Cat1 (blue line) recorded in acetonitrile.

. 3 ,

 3 we can determine oxidizing power of + 0.80 V vs NHE for Ru * /Ru -. Hole injection from the excited dye to the valence band of NiO (+ 0.46 V vs NHE valence band edge potential) is thermodynamically feasible with ΔGinj = -0.34 eV using Eq. II.2. Generation of Co(II) and Co(I) from the reduced light harvester can take place with driving forces of ΔGred1 = -1.33 eV and ΔGred2 = -0.87 eV respectively (Eq. II.3 and 4). Thus, hole injection and thermal intramolecular electron transfer from the reduced dye to the catalyst can thermodynamically occur.

1 )

 1 to NHE, considering E°(Fc +/0 ) = + 0.53 V vs. NHE in acetonitrile. 157 b. In eV.

Figure IV. 7 .

 7 Figure IV.7. Differential spectra of UV-Vis monitored photolysis experiment of RuP4 OEt -Cat1 (3.6 × 10 -5 M) in CH3CN/TEOA (90:10 v/v) under visible light illumination.

1 .Figure V. 1 .

 11 Scheme V.1. Hydrolysis of the phosphonate ester groups to obtain RuP4 OH -Cat1.

Figure V. 2 .

 2 Figure V.2. MALDI-TOF spectrum of RuP4 OH -Cat1 dyad recorded with linear positive mode in a methanolic solution of 1 M phenylphosphonic acid.

Figure V. 3 .

 3 Figure V.3. MALDI-TOF spectrum of RuP4 OH -Cat1 dyad recorded with reflecting positive mode in a methanolic solution of 1 M phenylphosphonic acid.

Figure V. 4

 4 was assessed for photoelectrocatalytic hydrogen evolution in fully aqueous media at pH 5.5 in MES buffer using AM 1.5G solar irradiation in order to provide a direct comparison with the RuP4 OH -Co-based photocathode. NiO films sensitized by the two grafting procedures were tested. In addition, some preliminary experiments were carried out at more acidic pH to test the stability of the anchorage.

Figure V. 4 .

 4 Figure V.4. Photocathode assessed for PEC H2 evolution based on RuP4 OH -Cat1.

2 ( 3

 3 photoelectrochemical characterization is in agreement with the photolysis experiments (Figure IV.7) where the evolution of Co(I) for RuP4 OEt -Cat1 was suggested to be faster than for RuP4 OEt -Co (Figure II.8).

Figure V. 5 .

 5 Figure V.5. Linear sweep voltammograms of NiO electrodes sensitized by RuP4 OH -Cat1 (top) and RuP4 OTMS -Cat1 (bottom) in pH 5.5 using 0.1 M MES 0.1 M/NaCl as supporting electrolyte under AM 1.5 G solar irradiation (70 mW.cm -2 , red line), dark conditions (black line) and chopped light irradiation (blue line).

Figure V. 6 .

 6 Figure V.6. Linear sweep voltammograms of NiO electrodes sensitized by RuP4 OH -Cat1 dyad in pH 3 using Britton-Robinson as supporting electrolyte under AM 1.5 G solar irradiation (70 mW.cm -2 , red line), dark conditions (black line) and chopped light irradiation (blue line).

Figure V. 7 .Figure V. 8 .

 78 Figure V.7. 2 hours chronoamperometry measurements of pristine NiO films (blue line), NiO films sensitized by RuP4 OH -Cat1 dyad (black line) and RuP4 OH -EPIP (red line) in pH 5.5 using 0.1 M MES 0.1 M/NaCl as supporting electrolyte under AM 1.5G light irradiation (70 mW.cm -2 ).

Figure V. 9 .

 9 Figure V.9. UV-Visible spectra for RuP4 OH -Cat1 using normal grafting procedure (left) or grafting using RuP4 OTMS -Cat1 (right) in methanolic solution of 1 M phenylphosphonic acid, where half of a fresh NiO film was dipped (black line) and the other half was dipped after 2 hours of PEC experiment (red line).

  desorption solutions were sent for MALDI-ToF analysis. The peak at m/z 1465 characteristic for RuP4 OH -Cat1 (Figure V.2 and 3) is completely absent from the spectra regardless the grafting procedure (Figure V.10). One main peak is observed at m/z: 1152, tentatively attributed to the cleavage of C-N bond between the triazole unit and the cobalt complex with a characteristic isotopic pattern for ruthenium.

Figure V. 10 .

 10 Figure V.10. MALDI-TOF spectra recorded in a methanolic solution of 1 M phenylphosphonic acid after 2 hours of PEC test with normal grafting procedure (top) and with grafting using TMS-protected phosphonate groups (middle).

  photocurrents of around -20 μA.cm -2 were obtained and CO was evolved with 6% F.E. and 10 TON. Co-absorption of the dyad and the dye increased the TON to 122, as the light harvester plays the role of the electron reservoir to the system.

Figure V. 11 .

 11 Figure V.11. A schematic representation of the hybrid photoelectrochemical cell, taken from reference 180 .

  Meyer (Figure V.12).221 Ruthenium and catalytic rhenium complexes bearing two methyl phosphonic acid groups were used and a ratio of roughly 1/1 was determined by XPS. The initial part of the assembly is an electron donating dianiline coupled onto mesoporous NiO films. The corresponding photocathode was illuminated with visible light (1 sun) in a CO2saturated acetonitrile solution and photocurrent densities of -65 μA.cm -2 were obtained at -0.54 V vs NHE. 8 times lower responses were measured for the assembly without the dianiline, which acts as an electron donor and promotes the intramolecular electron transfer to the excited dye. After 20 minutes of continuous irradiation, CO was produced with 85% F.E. and 22 TON.Transient absorption measurements showed that hole injection takes place in sub-nanoseconds and that direct electron transfer to the catalyst is in competition with charge recombination on the microsecond scale.

Figure V. 12 .

 12 Figure V.12. Dye-sensitized photocathode for CO2 reduction based on "layer-by-layer" assembly. The figure was taken by reference 221 .

Figure V. 13 .

 13 Figure V.13. "Layer-by-layer" molecular p-n-junction assembly formed by Zr(IV)-phosphonate bridging. The figure was taken by reference 225 .

4 )Figure V. 14 .

 414 photoelectrocatalytic CO2 reduction (Figure V.14). In the following part, some preliminary results with RuP4 OH -Cat1 grafted onto NiO electrodes in fully aqueous media will be presented. For the PEC tests, we decided to use the conditions from the study by O. Ishitani in collaboration with our group for the NiO photocathode based on Dyad b (aq. 50 mM NaHCO3 pH 6.6). 180

Figure V. 15 Figure V. 15 .Figure V. 16 .

 151516 photocathode under dark conditions and illumination. Photocathodic current responses can be clearly observed starting at approximately + 0.4 V vs Ag/AgCl (+ 1.0 V vs RHE) and increasing at more negative potentials, reaching photocurrent densities of -100 μA.cm -2 . Of note, the photocurrent density recorded with our films is significantly higher than the ones previouslyreported for NiO-based photocathodes.217,180

  (AKSTCKIT3) purchased by Pine Instrument Company. The cell kit includes 1 mm quartz cuvette, a platinum honeycomb auxiliary electrode, a gold honeycomb working electrode and a Ag wire as reference electrode. A BioLogic VSP-300 potentiostat connected to the glovebox was used. The spectra were recorded with an optical fiber external-sampler connected to the UV-Vis 15) Desorption and dyad and dye loading quantification Sensitized NiO films were soaked in 1 M methanolic solution of phenylphosphonic acid (5 ml) for 4 hours to desorb immobilized compounds. Afterwards, UV-Vis spectra of the desorption solutions were measured. From the maximum absorption at the MLCT band, the concentration of the dye or the dyad were determined using the molar extinction coefficient at the MLCT band (recorded in 1 M methanolic solution of phenylphosphonic acid) through the Beer-Lambert law (A = ε × d × C, where d = 1 cm). Then, the moles of the compounds in each solution were calculated. Finally, using the surface area of each film, the dye and dyad loadings were estimated. 16) Quantification of Η 2 and CO The amount of H2 and CO were measured in the headspace by sampling aliquots (50 μl) in a Perkin Elmer Clarus 580 gas chromatograph equipped with a molecular sieve 5 Ǻ column (30 m -0.53 mm) and a TCD and FID detector respectively. Additionally, the amount of hydrogen dissolved in solution was determined by a Unisense H2 microsensor. The zero value for the microsensor was measured in the assembled cell prior to irradiation. Calibration curves with known amounts of H2 and CO enabled us to convert the signal of the GC and the probe in amount of produced H2 and CO. 17) Quantification of HCOOH A 883 Basic IC plus enabling chemical suppression was used for the determination of formate in the solution. The ion chromatograph is equipped with the Metrohm Suppressor Module Rotor A. The instrument contains a high-pressure pump, an inline filter, a pulsation absorber, an injection valve, a peristatic pump, a conductivity detector and a separation column. For the elution of anions, NaHCO3 and Na2CO3 were used and for the suppression H2SO4. The instrument is operated with the MagIC Net Basic software. For the quantification of formate, we diluted 500 μl of the electrolyte in 5 ml milli-Q water before every injection. A calibration curve allowed us to convert the area of the peak to a concentration of formate. -EPIP (160 mg, 0.15 mmol) was dissolved in 5 ml THF. K2CO3 (20 mg, 0.15) in 5 ml MeOH was added dropwise and the reaction mixture was stirred at room temperature for 4 hours. The crude product was purified by column chromatography on silica gel (gradient elution of MeCN/0.4 M aqueous KNO3, from 98:2 to 90:10). The fractions were concentrated and the product was precipitated by dropwise addition of a saturated KPF6 aqueous solution. The orange precipitate was filtered, washed with water and dried under vacuum. Yield: 87% (130 mg). 1 H-NMR (300 MHz, CD3CN): δ (ppm) 8.94 (d, J = 8.3 Hz, 2H), 8.50 -8.44 (m, 4H), 8.32 (d, J = 8.1 Hz, 2H), 8.05 (t, J = 8.3 Hz, 2H), 7.90 (t, J = 7.9 Hz, 2H), 7.81 (d, J = 7.7 Hz, 2H), 7.78 (d, J = 4.5 Hz, 2H), 7.60 -7.55 (m, 4H), 7.48 (d, J = 8.2 Hz, 2H), 7.40 (t, J = 6.7 Hz, 2H), 7.18 (t, J = 6.3 Hz, 2H), 3.60 (s, 1H). ESI-MS: m/z 367.0 [M-2PF6] 2+ , 879.0 [M-PF6] + . No titrations measurements were performed for the identification of the protonation state of the imidazole unit. [Ru(4,4´-(CH2PO3Et2)2-bpy)2Clx(DMSO)2-x]Cl2-x (1 equivalent, using an average molecular weight of 1164) and TMS-EPIP (75 mg, 0.33 mmol) in a water/ethanol (1:3) mixture was refluxed overnight. After cooling down to room temperature, 5 ml of a saturated NH4PF6 aqueous solution was added to the reaction mixture. Extraction with dichloromethane and removal of the organic solvent yield a crude reddish solid. After purification by column chromatography on silica gel with gradient elution MeCN/0.4 M aqueous KNO3 (from 98:2 to 90:10) and removal of volatiles, dropwise addition of a saturated NH4PF6 aqueous solution and extraction with dichloromethane, gave an orange powder. Yield: 20% (75 mg). 1 H NMR (300 MHz, CD3CN): δ (ppm) 9.04 -8.91 (m, 2H), 8.43 (d, J = 14.7 Hz, 4H), 8.26 (d, J = 8.1 Hz,

  photocathodes based on the same cobalt diimine-dioxime catalyst. At the end of the experiments, the amount of hydrogen produced was measured in the headspace and in the electrolyte solution with 8 ± 1 % F.E. and 7 ± 1 nmol.cm -2 . By comparison, control experiments performed with ZnP-ref-sensitized (just the photosensitizer) NiO films under identical conditions produced only 3 nmol.cm -2 .Post-operando characterization of the dyad was performed after desorption from the film in a 1 M phenylphosphonic acid solution in THF. The UV-vis spectrum of the solution displays an overall lower intensity of the signals by comparison with the spectrum of the freshly-grafted film attributed to some desorption of ZnP-Co from the NiO surface (estimated to≈ 40 %)

  spectra of ZnP-Co in a THF solution of 1 M phenylphosphonic acid, where half of a fresh NiO film was dipped (black line) and the other half was dipped after 2 hours of PEC experiment (red line). Zoom in the region of the Q bands.

  ml), propargylamine(2 ml, 31.2 mmol) was added. The mixture was refluxed for 2 min and then stirred at room temperature. The precipitate was filtered, washed with pentane and dried to obtain a yellow-brownish solid. Yield: 83% (3.3 g).1 H NMR (Acetone-d6, 300 MHz): δ (ppm) 5.75 (s, 1H), 4.02 (br, 2H), 3.61 (s, 2H), 2.69 (br, 1H). ESI-MS: m/z 123.1 [M+H] + , 145.1 [M+Na] + , 161.0 [M+K] + .TPA-alkyne derivative (C)(B) (200 mg, 0.31 mmol) and propargyl cyanoacetamide(45 mg, 0.37 mmol) were solubilized in 15 ml dry CHCl3 and piperidine (0.2 ml, 1.4 mmol) was added dropwise. The reaction mixture was stirred overnight under argon and protected from light. 1 M aqueous HCl was added and the mixture was extracted and washed with water. After evaporation of the solvent, the crude product was purified by column chromatography on silica gel using dichloromethane/ethanol (98/2) as eluent and the orange solid (C) was dried under vacuum.ESI-MS: m/z 759.1 [M+H] + . From the 1 H NMR (CDCl3, 300 MHz), the new peaks at 8.36 (s, 1H) and 2.30 (s, 1H) ppm confirmed the presence of the alkyne derivative. Although many efforts were made, we always observed the starting material with a peak at 9.85 ppm. The crude product was used for the next reaction.Reference dye (D)(B)(25 mg, 0.038 mmol) and ethyl cyanoacetate (0.1 ml, 1.33 mmol) were solubilized in 5 ml dry CHCl3 and piperidine (0.1 ml, 0.95 mmol) was added dropwise. The reaction mixture was stirred overnight under argon and protected from light. 1 M aqueous HCl was added and the mixture was extracted and washed with water. After evaporation of the solvent, the crude product was purified by column chromatography on silica gel using dichloromethane/ethanol (98/2) as eluent. The excess ethyl cyanoacetate was removed by precipitation of the compound in a mixture of ethanol/water and filtration with celite. The red solid (D) was dried under vacuum. Yield: 60% with some starting material (17 mg).

L

  'énergie est l'outil le plus important des communautés modernes afin de faciliter leur vie quotidienne avec une consommation d'énergie mondiale annuelle d'environ 18 TW, qui devrait atteindre 43 TW en 2100. L'économie énergétique moderne repose sur les combustibles fossiles qui causent de graves problèmes environnementaux (changement climatique, fonte des calottes polaires, pollution de l'air, baisse de la biodiversité) en raison de leur combustion qui produit du dioxyde de carbone. Ce fait, associé à l'épuisement des combustibles fossiles en raison de leur utilisation continue, a contraint la communauté scientifique à trouver des alternatives basées sur des sources d'énergie renouvelables et abondantes, comme le solaire, la géothermie, l'énergie éolienne, etc. (Figure 1).

Figure 1 .

 1 Figure 1. Réserves énergétiques totales (sources d'énergie renouvelables et combustibles fossiles).

Figure 2 .

 2 Figure 2. Représentation schématique du processus photosynthétique naturel (en haut) et cellule photoélectrochimique de division de l'eau, DSPEC (en bas). Les cubes correspondent aux collecteurs de lumière (P1, P2) et les sphères à l'oxygène ou aux catalyseurs dégageant de l'hydrogène (OEC, HEC) en conséquence.

  Covalent dye-catalyst assemblies ............................................................................ ii) "Layer-by-layer" assembly ....................................................................................... iii) Molecular photocathodes based on narrow band-gap semiconductors ................. 3) Cat1 as catalyst for CO2 reduction ........................................................................................ 4) Assessment of the photoelectrocatalytic activity for CO2 reduction of RuP4 OH -Cat1 sensitized NiO films under fully aqueous conditions ............................................................................. i) Experimental set-up and conditions .......................................................................... ii) Photoelectrochemical characterization ..................................................................... iii) Assessment of the photoelectrocatalytic activity ......................................................

	TMSBr: trimethylsilyl bromide
	ToF-SIMS: Time-of-Flight Secondary Ionization Mass Spectroscopy
	TON: turnover number
	F.E.: Faradaic Efficiency
	UV-Vis: UltraViolet Visible
	IC: Ion chromatography
	TAS: Transient Absorption Spectroscopy
	TA-SEC: Transient Absorption-Spectroeletrochemistry

i)

  B. Dietzek and Dr. F.H. Schacher an organic pushpull dye bearing carboxylate groups was immobilized onto porous nanostructured tri-layered NiO films prepared by co-polymer template technique and deposited onto FTO.

[START_REF] Massin | Dye-Sensitized PS-b-P2VP-Templated Nickel Oxide Films for Photoelectrochemical Applications[END_REF]

Photocurrent densities of -300 μA.cm -2 were measured during PEC experiments in water (pH 4.5) with [Co(NH3)5Cl]Cl2 as IEA at + 0.2 V vs NHE. Following this work, metallo-organic dyes, synthesized by our collaborator Dr. C. Olivier, were immobilized on NiO substrates fabricated by Dyenamo.

Electrode architecture Components Electrode material pH 1 Applied potential (V) j (μA.cm -2 ) H 2 (nmol) (nmol.cm -2 ) TON F.E. (%) Reference a

	c	PS6/HER7a	NiO	4,5	-0,3 (Ag/AgCl)	-10	12,4 (12,4 2 )	≤ 3	50	L.Hammarström et al. 2016 96
	c	PS7/HER8	NiO	5,5	-0,4 (Ag/AgCl)	-8	n.r.	n.r.	9,3 ± 1.5	V. Artero et al. 2018 98
	a c a c	HER1/PS 1 PS2/HER2 PS8/HER4 PS3/HER1 PS3/HER3 PS7/HER5	NiO NiO CuCrO2 NiO NiO CuGaO2 NiO NiO	7 0 3 n.r. 7 7 5.5	-0,4 (Ag/AgCl) -0,3 (NHE) 0 (NHE) 0 (RHE) -0,4 (Ag/AgCl) 0,41 (RHE) 0,14 (RHE) 0,14 (RHE)	-10 -15,1 -5,8 -300 183 ± 36 -200 -100 -45 -12 -12	90 (90 2 ) 94 ± 10 (376 ± 40) 2 35 ± 2 n.r. 130 ± 90 2 (342) 1820 ± 20 (4789) 555 540 ± 80 347 ± 29	˂1 2 126 ± 13 n.r. ˂1 2 1.5 82-640 14-84 n.r.	n.r. ± 8 ± 8 49 ± 11 60 ± 10 98 ± 10 74 ± 8 ± 2	L. Sun et al. E. Reisner et al. 2018 100 Y. Wu et. al. 2016 82 R. Wasielewski et al. 2017 84 R. Abe, O. Ishitani, V.Artero et al. 2019 102
	a c	PS4/HER1 PS9/HER4	NiO IO-CuCrO2	4,5 3	-0,4 (Ag/AgCl) 0 (RHE)	-6 -25	̴ 40 (12) 54 ± 3 (215 ± 10)	˂1 2 48 ± 2	3 ± 8	M. Chavarot-Kerldou et al. 2019 85 E. Reisner et al. 2019 101
	b c	Dyad 1 PS8/HER4	NiO IO-CuCrO2	7 3	-0,2 (NHE) 0 (RHE)	-15 -18	290 (290 1 ) 40 ± 6 (160 ± 24)	n.r. 36 ± 5	45 40 ± 14	Y. Wu et al. E. Reisner et al. 2019 101
	b d	Dyad 2a PS10/HER4	NiO NiO	5,5 3	-0,4 (Ag/AgCl) +0,3 RHE	-15 -9	37 ± 6 (26 ± 10) 7 ± 1	˂1 2 n.r.	9 ± 1 ± 2	V.Artero et al. 2016 87 E. Reisner et al. 2016 104
	b d	Dyad 2b PS11/HER4	NiO io-ITO	5,5 5,1	-0,4 (Ag/AgCl) +0,05 RHE	-11 -56	16 ± 5 >1000	n.r. >15 2	13 ± 1 ± 5	V.Artero et al. 2018 90 T. J. Meyer et al. 2016 105
	b b d	Dyad 3 Dyad 4 PS11/HER4 PS12/HER4	NiO NiO Doped NiO/ioITO	4,8 3 5	0 (Ag/AgCl) -0,2 (Ag/AgCl) -0,25 NHE	-10 -33 ± 7 -40 ≥ -15	2,5 <300 (<380) 700 ± 80 (700 ± 80) 2 n.r.	0,05 47 20 2 n.r.	n.r. 96 ± 3 n.r.	H. Tian et al. 2017 91 E. A. Gibson et al. 2019 92 T. J. Meyer et al. 2017 107
	b	Dyad 5	NiO	3	-0,2 (Ag/AgCl)	-36 ± 2	500 (400)	46	98	E. A. Gibson et al. 2019 92
	b	Dyad 6	NiO	4,5	-0,4 (Ag/AgCl)	-3,2	109 (33 2 )	2,5	27	C. Olivier et al. 2019 93
	c	PS5/HER5	NiO	7	-0,4 (Ag/AgCl)	-13	n.r.	n.r.	n.r.	L. Sun et al.
	c	PS1/HER6	NiO	7	-0,2 (Ag/AgCl)	-35	290 (290 2 )	n.r.	68	L. Sun et al.
	reduction. F.E. close to unity are favorable, indicating that all the photo-driven electrons are
	used for H2 evolution. The reported values drastically vary from < 10% to almost 100%, from
	one system to the other.
	Until today, comparing the four different figures of merit, no direct proof can be extracted
	of which architecture or molecular components (photosensitizers and hydrogen-evolving
	catalysts) are the most appropriate for a functional photocathode. It is important to highlight
	that from one study to the other, too many parameters vary (different pH, electrolytes, applied
	potentials, film preparation and light intensity) for an accurate comparison to be made.

Table I .1. Figures of merit of

 I the reported dye-sensitized hydrogen-evolving photocathodes together with the different experimental conditions employed.1. The nature of the electrolyte is not taken into account in this table.

Table II .

 II

1. Electrochemical properties of complexes RuP4 OEt -Co, RuP4 OEt -EPIP and Co-N3 (in V vs Fc +/0 ).

Table II .2. Redox properties of Co-N3, RuP4 OEt -EPIP and RuP4 OEt -Co.

 II

a-In V vs NHE. The redox potentials were converted from Fc +/0 (Table

II.

1) to NHE, considering E°(Fc +/0 ) = + 0.53 V vs. NHE in acetonitrile. 157 b-In eV.

Table III .

 III

	Irradiation	Film	Surface Area (cm 2 )	Dye and dyad loading (nmol.cm -2 )	H2 in the headspace a (nmol)	H2 in solution b (nmol)	Total H2 (nmol)	Total H2 (nmol.cm -2 )	Charge passed (mC.cm -2 )	F.E. (%)	TON	Ref. exp.
		NiO	3.3 3.0	--	0 0	3 1	3 1	1 < 1	2 3	7 3	--	SB117 SB120
		average						1 ± 1	3 ± 1	5 ± 2
		NiO|RuP4 OH -	3.2	3.7	2	13	15	5	9	10	-	EG323
	400-800 nm	EPIP average	3.3	1.9	0	11	11	3 4 ± 1	8 9 ± 1	8 9 ± 1	-	EG324
		NiO|RuP4 OH -Co	3.3 3 3.1	6.4 5.6 4.7	70 57 30	33 21 56	102 78 85	31 26 28	32 22 18	19 23 31	5 4 6	EG273 EG304 EG314
		average						28 ± 2	24 ± 6	24 ± 5	5 ± 1
		NiO	2.8 2.6	--	6 4	6 9	13 13	5 5	7 8	13 12	--	EG266 EG311
		average						5 ± 0	8 ± 1	13 ± 1
		NiO| RuP4 OH -	2.6	4.2	6	30	36	14	13	20	-	EG264
	AM 1.5 irradiation (IR filtered)	EPIP average NiO| RuP4 OH -Co	3.2 3.2 2.9 3.2 2.5	2.1 5.5 4.2 5.3 3.3	3 78 115 102 74.1	16 154 70 60 53	19 232 185 162 127	6 10 ± 4 72 64 51 51	6 10 ± 4 38 46 54 42	23 22 ± 2 37 27 18 23	-13 15 10 15	EG325 EG257 EG270 EG272 EG342
			3.0	5.2 c	25 c	153 c	178 c	59	16 c	72 c	11 c	EG418
		average d						60 ± 9	45 ± 6	26 ± 7 13 ± 2

  Mei Wang's group in China lately reported the first example of a functionalized Cat1 derivative for covalent grafting onto CuInS2/ZnS QDs (Chart IV.2).191 A Pd-catalyzed crosscoupling reaction was used for the introduction of a pyridine dicarboxylate anchoring group on

	i)	Covalent grafting onto semiconductors QDs
	Although Cat1 has been employed for the photo-or electrocatalytic proton and carbon
	dioxide reduction the last 30 years, mainly multicomponent systems were studied and there are
	almost no examples of substituted Cat1 derivatives. Only a single example of Cat1

functionalized on the pyridyl moiety for grafting onto quantum dots (QDs) was reported.

191

Recently, in our group two dye-catalyst assemblies based on organic dyes (submitted for publication) were prepared.

192

Table IV . 2 .

 IV2 Electrochemical properties of complexes RuP4 OEt -Cat1, RuP4 OEt -EPIP and Cat1 vs Fc +/0 .

Table IV .3. Redox properties of Cat1, RuP4 OEt -EPIP and RuP4 OEt -Cat1.

 IV

Table V . 1 .

 V1 Surface concentration (nmol.cm -2 ) of RuP4 OH -Cat1 determined by the desorption procedure for 11 freshly grafted films (F).

	F1	F2	F3	F4	F5	F6	F7	F8	F9	F10	F11
	4.3	2.0	1.0	0.9	0.5	1.3	2.3	1.3	1.4	0.5	1.1

.1). Using different batches of NiO films, in addition to the low solubility of the complex, resulted in lower grafting densities, varying from 0.5 to 2.3 nmol.cm -2 .

60 ± 13 Table V.3. Photoelectrochemical experiments assessing hydrogen production activity of the RuP4 OH - Cat1 or RuP4 OTMS -Cat1 on

  NiO electrodes. Conditions are as follows: MES buffer at pH = 5,5 or B.R. buffer at pH 3, AM 1.5 G solar irradiation, -0.4 V vs. Ag/AgCl applied potential, 2 h runtime.

	pH	Film	Surface Area (cm 2 )	Surface concentration (nmol.cm -2 )	Total H2 (nmol.cm -2 )	Charge passed (mC.cm -2 )	F.E. (%)	TON	Ref. exp.
		NiO	2.8 2.6	--	5 5	7 8	13 12	--	EG266 EG311
		average			5 ± 0	8 ± 1	13 ± 1
		NiO|RuP4 OH -	2.6	4.2	14	13	20	-	EG264
		EPIP	3.3	2.1	6	6	23	-	EG325
		average			10 ± 4	10 ± 4	22 ± 2
			3.0	1.0	218	79	53	227	EG328
		NiO|RuP4 OH -	2.9	0.9	214	86	49	227	EG330
	5.5	Cat1	3.1 3.1	1.3 4.3	302 1098	120 407	49 52	235 258	EG425 EG261
		average					50 ± 3 237 ± 14
		NiO|Ru P4 OTMS -Cat1	3.1 3.0 3.2	1.8 2.4 1.1	767 1078 263	230 318 83	64 65 61	431 459 224	EG414 EG426 EG447
		average					64 ± 2	371 ± 128
			3.2	0.5	36	20	36	69	EG331
	3.0	NiO|RuP4 OH -Cat1	3.0	2.3	131	74	24	51	EG437
		average					30 ± 8

Table V .

 V 4 gathers the figure of merits obtained for the different systems and experimental conditions tested. Under CO2-saturated conditions, the RuP4-Cat1 sensitized photocathode is active for CO2 reduction to CO. Hydrogen was also produced, as expected due to the presence of water and the competitive proton reduction, but interestingly, the selectivity for CO is superior over H2, with 230 ± 27 TON for CO (52 % F.E.) produced at -0.4 V vs Ag/AgCl compared to 61 ± 18 TON for H2 (13 % F.E.). Surprisingly, we observed that the activity is

	lower at -0.7 V vs Ag/AgCl with 121 ± 11 TON (44 % F.E.) achieved to produce CO and 31
	± 2 TON for H2 (11 % F.E.). Experiments under strictly identical conditions are now needed to
	understand if these differences really arise from the applied potential or if the grafting method
	plays a role here (classical sensitization employed for the set of experiments at -0.4 V vs

Table V . 4 .

 V4 Photoelectrochemical experiments assessing CO2 reduction activity of the RuP4 OH -Cat1 dyad on NiO electrodes (surface area 3.0 ± 0.2 cm 2 ).

		Potential	Surface	Charge	n(CO) /	n(H2) /	n(HCOOH) /
	sample	(V vs Ag/AgCl)	conc.	density	nmol.cm -²	nmol.cm -²	nmol.cm -²	FECO/ %	FEH2/ % FEHCOOH/	Ref. exp.
			(nmol.cm -2 )	(mC.cm -2 )	(TON)	(TON)	(TON)			%
	NiO	-0.4	-	11	1	1	22	2	2	38	EG400
		-0.7	-	10	6	4	-a		8	-	EG450
	RuP4 OH -EPIP	-0.4	5.1	68	5 (1)	120 (22)	-a	1	34	-	EG453
		-0.7	5.4	54	9 (2)	91 (16)	46	3	32	16	EG444
		-0.4	1.4	104	298 (211)	60 (43)	-a		11	-	EG397
	RuP4 OH -Cat1			219 b	410 (291)	86 (61)	42 (30)		8	4
		-0.4	0.5	56	135 (249)	43 (79)	14 (26)		15	5	EG398
		-0.7	2.2	134	329 (129)	86 (34)	-a		12	-	EG441
	RuP4 OTMS -			282 c	633 (249)	149 (59)	62 (24)		10	4
	Cat1	-0.7	2.0	116	240 (113)	61 (28)	28 (13)		10	5	EG446
	RuP4 OH -Cat1	-0.7	1.1	91	11 (10)	234 (206)	-a	3	50	-	EG454
	(N2 saturated)			244 c	34 (30)	361 (317)	-a	3	28	-
	Conditions are as follows: CO2-saturated 50 mM aqueous NaHCO3 at pH = 6.6, AM 1.5 G solar irradiation, 3 h runtime. a) HCOOH was not quantified, b) 21
	h runtime, c) 19 h runtime.

  All reagents were purchased from Sigma Aldrich or Strem and used as obtained unless otherwise stated. Solvents were dried from appropriate drying agents (sodium wire and

	benzophenone for THF, calcium hydride for CH2Cl2) and freshly distilled under argon before
	use. Reagent-grade solvents were used without further purification. The 4,4´-
	bis(diethylphosphonomethyl)-2,2´-bipyridine (4,4´-(CH2PO3Et2)2-bpy) and N2,N2'-2-
	azidopropanediylbis(2,3-butadione-2-imine-3-oxime) ligands were custom-synthesized by the
	companies Oribase Pharma and Provence Technologies, respectively.

  . ESI-MS: m/z 340.1 [M+H] + , 362.1 [M+Na] + .

4-chloro-2,6-diacetylpyridine (2)

1 (2.3 g, 6.78 mmol) was dissolved under argon in 60 ml of freshly distilled THF.

  Table above).H2 measurements at the end of the photoelectrolysis experiments (continuous irradiation for 2 hours; MES buffer, pH 5.5 or B.R. buffer pH 7; applied potential: +0.14 V vs RHE). a) Measured by gas chromatography and micro-clark type electrode.

	Film	pH	Surface Area (cm 2 )	Dye loading (nmol.cm -2 )	Total H2 (nmol) a	Total H2 (nmol.cm -2 )	Charge passed (mC.cm -2 )	F.E. (%)	TON	% desorp.
	ZnP-	5.5	3.3	7.4	25	8	17	9	1	30
	Co		3.1	6.1	17	6	19	6	1	48
	average					7 ± 1	18 ± 1	8 ± 1	1	39 ± 9
	ZnP-	7	3.2	7.1	23	7	13	11	1	19
	Co		3.2	5.1	15	5	21	5	1	31
	average					6 ± 1	17 ± 4	8 ± 3	1	25 ± 6
	ZnP-	5.5	3.0	3.6	10	3	4	17	-	35
	ref		3.2	3.1	8	3	10	8	-	67
	average					3	7 ± 3	13 ± 4		51 ± 16

  1 H NMR (CDCl3, 300 MHz): δ (ppm)8.11 (s, 1H), 7.57 (br, 1H), 7.38 (d, J = 8.1 Hz, 2H), 7.19 (br, 1H), 7.08 (d, J = 8.1 Hz, 4H), 6.91-6.84 (m, 6H), 3.90 (quint., J = 7.1 Hz, 10H), 3.01 (s, 2H), 2.94 (s, 2H), 1.17-1.09 (m, 15H). HR-MS (ESI+): m/z calcd for C38H45N2O8P2S 751.2366; found: 751.2357 [M+H] + .

  ). HR-MS (ESI+): m/z calcd for C50H62N10O9CuP2S 551.6591; found 551.6584 [M-H2O-PF6] 2+ and m/z calcd for C50H61N10O9CuP2S 1102.3109; found 1102.3102 [M-H2O-PF6] + . E.A.: calcd for C50H63N10O10CuP3SF6 + 1.2 KPF6: C, 44.46;

cm -1 and for RuP4 OEt -EPIP 18400 M -1 cm -1 .

Acknowledgments

The list of people who deserve my thanks for allowing me to carry out this thesis is long and I am afraid and sorry that I may forget some of them. This PhD thesis is the output of the effort

Scheme II.5. Synthetic procedure for the preparation of RuP4 OEt -EPIP through the direct route.

2) Preparation of Cu- N 3 The N2,N2'-2-azidopropanediylbis (2,3-butadione-2-imine-3-oxime) ligand was customsynthesized by the company "Provence Technologies" following a reported procedure. [START_REF] Andreiadis | Molecular Engineering of a Cobalt-Based Electrocatalytic Nanomaterial for H2 Evolution under Fully Aqueous Conditions[END_REF] A concentrated solution of CuClO4.6H2O in acetone was added in a hot ligand solution (acetone) under stirring (Scheme II.6). [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine -Dioxime Catalyst[END_REF]131,149 The reaction mixture was left standing at room temperature overnight and the precipitate was filtrated, yielding the paramagnetic Cu-N3.

Scheme II.6. Synthesis of Cu-N3.

Chapter III

The inefficient supply of electrons to the catalyst implies that a new molecular design, favoring thermal electron transfer from the reduced dye, is needed.

Stronger attachment of the molecular architecture on the electrodes is crucial for long-term operation. This can be achieved either by functionalization of the dye with different anchoring groups or by protecting the interface of the anchorage from water (ALD).

More robust and noble metal-free catalysts should be used to enhance the performance of the photocathodes and this will be the subject of the next chapter. 129 Chapter IV 4) Route 1: attempts to prepare N 3 -Cat1 We tried to synthesize N3-Cat1 by performing cobalt-templated synthesis on N3-DAP using the previously presented procedure for the preparation of the non-substituted Cat1.

CoCl2•6H2O, N3-DAP and 3,3'-diaminodipropylamine in equimolar amounts were heated at 75°C in a homogeneous mixture of EtOH/DCM/H2O (3/1.5/1) for 5 hours (Scheme IV.4). Some drops of acetic acid were added for buffering the amine solution and catalyzing the Schiffbase condensation. After oxidation to Co(III) and precipitation with KCl, a green solid was collected. However, ESI-MS demonstrated that the final compound was amino-functionalized with three peaks at m/z: 402. 1, 366.1 and 330.2 corresponding to [M-Cl] + , [M-2Cl-H] + and [M-3Cl-2H] + accordingly. A previous attempt to isolate the same complex in the group was also unsuccessful. Hence, the reduction through hydrogenation from azido to amino due to the presence of water 198 is favorable under these conditions and in our hands subsequent CuAAC to obtain RuP4 OEt -Cat1 through Route 1 was not possible. The construction of the catalytic center on RuP4 OEt -DAP was realized by a cobalt-templated synthesis based on the procedure employed to prepare Cat1 (Scheme IV.2). CoCl2•6H2O,

RuP4

OEt -DAP and 3,3'-diaminodipropylamine were heated at 75°C in a homogeneous mixture of EtOH/DCM/H2O (3/1.5/1) overnight (Scheme IV.6). DCM was added for solubility reasons and the reaction was left overnight in order to try to increase the reaction yield (more details in Table IV.1). Some drops of acetic acid were also added.

Scheme IV.6. Cobalt-templated synthesis to obtain RuP4 OEt -Cat1.

After air bubbling overnight to oxidize to the more stable diamagnetic Co(III) species in the presence of HCl, column chromatography on silica was carried out. RuP4 OEt -Cat1 is highly charged and the purification was challenging. We found out that the best combination of solvents for good separation is to start with acetone/aq. 0.4 M KNO3 (85/15) and some few mgs of starting material (RuP4 OEt -DAP) could be removed. Then, we changed to acetone/H2O/aq. saturated KCl (90/9/1) and the desired compound was desorbed from silica. After addition of KPF6 and filtration, RuP4 OEt -Cat1 was obtained under its hexafluorophosphate salt, albeit with a yield not exceeding 20%.

Many efforts were devoted for synthesizing RuP4 OEt -Cat1 and to try to optimize the reaction conditions and the purification (Table IV Finally, the issue of the protonation state of the imidazole ring that can affect the solubility and the photophysical properties can be avoided by preparing an oxazole derivative. Attempts to optimize the synthesis and the purification are ongoing by Dr. Tania Straistari (post-doc in our group).

Chapter V

2 hours chronoamperometric measurements at pH 3, showed again photocurrent decay and the total amount of hydrogen produced was low (EG 331 and EG437). In addition, F.E. and TON were estimated 30 ± 8% and 60 ± 13 accordingly. Hence, the moderate photoelectrochemical behavior is followed by a moderate photoelectrocatalytic activity.

The photoelectrocatalytic activity of the sensitized NiO-films with RuP4 OTMS -Cat1 were also tested for hydrogen evolution.

Experimental part

spectrometer. 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) in dry acetonitrile was used as electrolyte solution. Initially, a cyclovoltammetry (CV) was carried out to determine the redox potentials. Then, potentials were applied in a chronoamperometry (CA) and UV-Vis spectra were recorded, until equilibrium was reached.

13)

NiO film preparation FTO glass substrates (Solems, TEC 7, 7Ω resistance) were used as TCO with dimensions of 40 × 20 × 3.1 mm. A cleaning protocol for the substrates including Gigapur, deionized water and ethanol was adopted. After drying with compressed-air, the glasses were treated with UV-Vis ozone cleaning to remove organic impurities. NiCl2 (1.5 g) and F108 triblock co-polymer (1.5 g) were mixed in milli-Q water (4.5 ml) and ethanol (11.2 ml). The resulting viscous solution was sonicated overnight. The following day, the mixture was centrifuged for 30 minutes at 5000 rpm and filtrated. The supernatant solution was spin-coated for 60 seconds at 5000 rpm onto the conductive part of 12 FTO glasses. Sintering in the air in a flat oven was the last step (temperature increase within 30 minutes to 450°C, where stabilizes for 30 minutes).

To increase the thickness, the procedure was repeated four times and light brown NiO films with 4 layers were prepared.

14) Photoelectrochemical measurements

The photoelectrochemical measurements were carried out in a home-made two compartement three-electrode cell with a sensitized NiO film as working electrode, a titanium wire as auxiliary electrode and a Ag/AgCl reference electrode. The reference electrode was made of a Ag wire dipped into a 3 M aqueous KCl solution and separated from the electrolyte by a Vycor frit. The auxiliary electrode was also separated from the main compartment of the cell by a Vycor frit. Irradiation was performed using a Newport 300 W ozone-free Xe lamp at 280 W equipped with a water-filled Spectra-Physics 6123NS liquid filter to eliminate infrared radiation (λ > 800 nm) and either a 400 nm longpass filter (Spectra-Physics 59472) or AM 1.5G filter (2 × 2 IN 2 81094 oriel instruments). The power density was measured at a specific distance from the lamp, using a Newport PM1918-R power-meter. When the 400 nm longpass filter was used, the irradiation power was adjusted to 65 mW.cm -2 (equivalent to one sun irradiation) using appropriate neutral density filters. When the AM 1.5G filter was used, the irradiation power was adjusted to 1 sun without having fixed the water-filled liquid filter (this corresponds to 70 mW.cm -2 when the water-filled liquid filter is present).

III. Synthesis 2-(4-ethynylphenyl)-1H-imidazo[4,5-f][1,10]phénanthroline (EPIP)

A solution of phendione (1.0 g, 4. 91, 143.71, 130.47, 122.45, 121.64, 117.96, 147.82, 132.34, 129.62, 126.27, 123.25, 83.22, 82.31 157.7, 157.5, 152.6, 152.4, 151.1, 146.8, 145.9, 133.4, 131.5, 129.6, 127.6, 127.0, 126.1, 125.7, 105.1, 97.6, 63.6, 34.3, 32.5, 16.6, 1.4 No titrations measurements were performed for the identification of the protonation state of the imidazole unit.

RuP4 OEt -Cu

In an argon-purged round bottom flask, ). The organic solvents were evaporated and the product was extracted with dichloromethane after the addition of 5ml of saturated KPF6 aqueous solution.

The organic layer was evaporated and dried under vacuum to give an orange powder. Yield: 60% (85 mg). 1 H NMR (300 MHz, CD3CN): δ (ppm) 9.07 (s, 1H), 9.00 (t, J = 8.9 Hz, 2H), NMR (300 MHz, CDCl3): δ 9.84 (s, 2H), 7.72 (d, J = 8.7 Hz, 4H), 7.43 (d, J = 8.8 Hz, 2H), 7.11 (d, J = 8.6 Hz, 4H), 6.98 (d, J = 8.8 Hz, 2H).

N,N-bis(4-hydroxymethylphenyl)-4-bromoaniline

N,N-bis(4-carboxyphenyl)-4-bromoaniline (1.7 g, 4.47 mmol) was dissolved in 60 ml CH2Cl2.

25 ml of absolute ethanol was added followed by sodium borohydride (600 mg, 15.86 mmol).

The mixture was stirred under argon, at room temperature, for 2 hours with a color change from yellow to almost colorless. The reaction was quenched by the addition of 50 ml water. After extractions with brine, the organic layer was dried over Na2SO4, filtered and the solvent was evaporated. Yield: 90% (1.5 g).

TPA-phosphonate derivative (A)

In a oven dried schlenk N,N-bis(4-hydroxymethylphenyl)-4-bromoaniline (1.1 gr, 2.86 mmol), ZnBr2 (1.5 gr, 6.70 mmol) and P(OCH2CH3)3 (10 ml, 58 mmol) were added under argon. The solution was purged with argon and 20 ml dry CHCl3 was added. The reaction mixture was stirred overnight, at room temperature, using a balloon filled with argon and protected from night. After the addition of 300 ml of hexane, the mixture was stirred vigorously for at least 5 minutes until the appearance of a waxy solid. The supernatant was removed and the precipitate was extracted with dichloromethane and washed with water. The organic layer was dried over Na2SO4, filtered and the solvent was evaporated. The crude product was purified by column chromatography on silica gel using dichloromethane/ethanol (98/2) as eluent to give a yellowish oil. The product was placed in a refrigerator overnight until a white solid (A) was obtained.

Yield: 50% (900 mg). 146.04, 132.17, 130.78, 126.32, 125.13, 124.31, 114.92, 62.09, 33.09, 16.39.

TPA-aldehyde derivative (B)

(A) (300 mg, 0.48 mmol), 5-formyl-2-thiophene boronic acid (118 mg, 0.76 mmol), Pd2dba3 (9.3 mg, 0.01 mmol), HP(tBu)3BF4 (9.3 mg, 0.03 mmol), and K3PO4 (0.75 mL, 1 M) were added in 6 ml DME. The mixture was stirred at room temperature under argon for 24 hours, protected from light. After addition of H2O (20 ml), the mixture was extracted with CH2Cl2 and dried over Na2SO4. The crude product was purified by column chromatography on silica gel As a perspective, an alternative approach could be followed. As already explained in Chapter IV, the hydrolysis of the phosphonate groups in T2RP-Cat1 and RuP4 OH -Cat1, could be the main issue for the isolation of the final dyads. Thus, the template synthesis of the cobalt catalyst could be performed using first a precursor without the phosphonate anchors. The last reaction would be the Knoevenagel condensation to couple the dye with the catalyst, as presented below.

IV. Publications related to the