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Pour tous les manuscrits utilisés dans cette thèse cumulative, les autorisations de réimpression nécessaires pour la publication secondaire ont été obtenues auprès des éditeurs. Les co-auteurs des manuscrits utilisés dans cette thèse cumulative ont été informés de leur utilisation, des contributions des autres doctorants en tant que coauteurs des publications ainsi que des autorisations de réimpression pour une thèse cumulative.

Sebastian Bold

Jena,

List of Abbreviations

Introduction

As fossil fuel reserves dwindle and global warming increases, caused by the intensive use of fossil fuels as primary energy source, the worldwide energy demand keeps rising due to an ever-increasing human population. This situation forces humanity to use alternative, carbon emission-free and renewable energy sources such as solar, wind, tidal and geothermal energy. Solar energy in particular has an immense potential in terms of available energy to be gained. 1 So far, the use of solar energy is tantamount to generation of electricity by photovoltaic cells to be fed into the electrical grid or stored in batteries. However, fuels will still be needed in the future for specific purposes such as mobility where high energy densities are required, as in the case of freight trucks and air travel. Fuels are also an attractive option to move stored energy from areas with high solar influx, where the use of solar energy is more productive, to those without strong solar irradiation; or to compensate the intermittent nature of solar energy. Fuels that are produced with solar energy receive the name "solar fuels".

Currently, the most technologically mature solution to produce such fuels is to combine photovoltaics with electrolysers, so that the electricity generated by the photovoltaics is used on site to drive the redox reaction that produces the desired fuel, e.g. to reduce protons in water to produce molecular hydrogen. However, this system requires several distinct components and on large scale, the balance-of-system costs can be substantial. 2 An alternative is to directly convert the energy of the sunlight into chemical energy by photocatalytic processes. The inspiration for this approach comes from the photosynthetic system found in a number of organisms in nature, which uses sunlight to drive various redox reactions, most prominently reductive reactions on carbon dioxide to produce sugars. 3 Certain cyanobacteria are even able to reduce protons to molecular hydrogen. 4 The photosynthetic system faces the difficult task of generating a charge-separated state (CSS) that is long-lived enough to drive the reaction, and has therefore evolved to a complex machinery comprised of two photosystems (PSI and PSII) with various charge shuttles and enzymes to catalyse the redox reactions. This inspired the field of artificial photosynthesis, which mimics the concept of natural photosynthesis, but tries to reduce the complexity such that the individual parts of the system are ideally easily and cheaply synthesized: the photosystems are replaced by molecular dyes for efficient light absorption, the enzymes by bio-inspired molecular catalysts, and the redox shuttles by a variety of approaches to shuttle electrons. In the centre, the studied systems are presented. In chapter 3, the photophysics of the organic dyes and some of their dye-catalyst dyads are investigated in solution using TA spectroscopy and spectroelectrochemistry. In chapter 4, NiO photocathodes sensitized with the dyads are investigated in terms of their activity for hydrogen production, stability under operando conditions and the light-induced processes as a function of applied potential. The latter was also performed for a Ru-Co dyad.

One is to immobilize the components on semiconductor (SC) particles and suspend those in solution, which is a very low-cost, but also less mature technology. 5 Furthermore, both half reactions, in the case of water splitting proton reduction and water oxidation, have to be carried out on the same particle, which can lead to difficulties. A more sophisticated approach is to separate the two reactions into two separate half cells with a photocathode and a photoanode, consisting of SC films sensitized with a dye and catalyst. The resulting system is called a dye-sensitized photoelectrochemical cell (DSPEC). 3 In a tandem cell with both photoanode and photocathode, the current of the worse performing half-cell is the limiting factor for the overall performance. To date, the photoanodes greatly outperform the photocathodes, generating the need for an improvement in dye-sensitized photocathodes (DSPC). [6][7][START_REF] Li | Organic Dye-Sensitized Tandem Photoelectrochemical Cell for Light Driven Total Water Splitting[END_REF] To do this, it is important to obtain insights into the bottleneck processes in the photocathode to be able to address them. Spectroscopic techniques, especially time-resolved techniques, can help to give a mechanistic understanding of the light-induced processes in the DSPCs. [START_REF] Ponseca | Ultrafast Electron Dynamics in Solar Energy Conversion[END_REF][START_REF] Hammarström | Accumulative Charge Separation for Solar Fuels Production: Coupling Light-Induced Single Electron Transfer to Multielectron Catalysis[END_REF] Arguably the most important technique is transient absorption (TA) spectroscopy, which has the advantage of giving both kinetic as well as spectral information, allowing to identify intermediates and the kinetics of their formation and decay. However, to date the number of studies on DSPCs for proton reduction is limited, and usually not performed under conditions that are close to the operando conditions of the The goal of these studies was to identify the best-performing system and its bottlenecks to then rationally improve upon the molecular design for a higher performance of the photocathodes.

In the second chapter, I will present the state of the art of DSPCs in terms of activity and stability. The third chapter will deal with the photophysics in solution of the organic dyes and dye-catalyst assemblies synthesized in our lab, with one dyad studied by TA-SEC. The fourth chapter presents the results on the NiO photocathodes sensitized with the organic dye-catalyst assemblies, including their activity, post-and in-operando characterization and TA-SEC measurements. In addition, TA-SEC measurements of NiO photocathodes sensitized with a ruthenium-cobalt covalent assembly are presented.

2 Dye-sensitized H 2 -evolving Photocathodes: State of the Art

General considerations and design

Using sunlight to drive water splitting is attractive due to the abundance of sunlight and the relatively low energy necessary. Thermodynamically, 1.23 V is the standard potential for water splitting, [START_REF] Nocera | The Artificial Leaf[END_REF] and 1.23 eV, the energy needed to be inserted into a chemical bond, [START_REF] Lewis | Powering the Planet: Chemical Challenges in Solar Energy Utilization[END_REF] corresponds to a photon with a wavelength of ca. 1000 nm. Most of the sunlight spectrum consists of photons of higher energy which -based on thermodynamic considerations -should make for an easy water splitting with sunlight. However, much of the initial excitation energy is lost in relaxation processes in the photosensitizer (PS) as well as in the charge transfer to the catalyst, on top of the overpotential needed by the catalyst to drive the redox reaction. At the same time, a certain energy difference of the CSSs is needed to prevent recombination and ensure a fast downhill charge transfer cascade to the catalyst. In consequence, two PS are needed to provide sufficient driving force for water splitting through their combined excitation energy, analogous to the natural photosynthetic system which contains two photosystems. Therefore, the molecular components of molecular devices for watersplitting must be optimized to reduce these energy losses.

While the goal is full water splitting, the two half reactions are often realized and studied separately. In the following, only systems for proton reduction will be discussed. Inspired by natural photosynthesis, researchers have tried to reduce the complexity of the natural photosynthetic apparatus by identifying its key components and then synthesizing in the laboratory molecules that perform their functions: In general, in all photocatalytic systems there is a PS which absorbs light and then transfers an electron to a catalyst. Once the catalyst is fully charged, it reduces protons from the surrounding solution to molecular hydrogen. In addition, there must be an electron donor to regenerate the oxidized PS. While in full water splitting, these electrons come ultimately from the oxidation of water, when studying the proton reduction separately a different source is required. The arguably easiest way to achieve photocatalytic proton reduction is in fully homogeneous systems, where PS and catalyst are dissolved in solution. [START_REF] Hogue | A Smorgasbord of 17 Cobalt Complexes Active for Photocatalytic Hydrogen Evolution[END_REF][START_REF] Gueret | Cobalt(II) Pentaaza-Macrocyclic Schiff Base Complex as Catalyst for Light-Driven Hydrogen Evolution in Water: Electrochemical Generation and Theoretical Investigation of the One-Electron Reduced Species[END_REF][START_REF] Gueret | Challenging the [Ru(Bpy)3]2+ Photosensitizer with a Triazatriangulenium Robust Organic Dye for Visible-Light-Driven Hydrogen Production in Water[END_REF][START_REF] Panagiotopoulos | Photochemical Hydrogen Production and Cobaloximes: The Influence of the Cobalt Axial N-Ligand on the System Stability[END_REF][START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine-Dioxime Complexes[END_REF] In addition, it is necessary to add a sacrificial electron donor (SED) which is ultimately the source of the electron needed for the reduction. While homogeneous photocatalysis is easy to achieve and often gives high turnover numbers (TON), the use of the SED, usually an organic molecule such as triethylamine or ascorbic acid, is prohibitive for commercial application. It can however serve to quickly identify promising candidates for application in other techniques.

Full water splitting can be achieved by dye-sensitized SC nanoparticles that perform both proton reduction and water oxidation. While this is a cheap method of achieving full water splitting, so far the achieved conversion efficiencies remain low. Their use is also hampered by the fact that hydrogen and oxygen evolve in the same solution and must afterwards be separated, which is a technical challenge.

To overcome these drawbacks, DSPEC have been proposed, based on the design of dye-sensitized solar cells (DSSC). Here, the two redox reactions (proton reduction and water oxidation) are separated into two half cells, each of which performs only one of the two reactions. Each half cell consists of a SC film on a conductive substrate which is connected to the electrical circuit. The SC film is sensitized with a PS that absorbs light and injects charge into the SC from its excited state. The contrary charge is then transferred to a catalyst, which may be immobilized on the film or the PS and which then performs the redox reaction (Figure 2.1). By connecting two such photoelectrodes, a photocathode for proton reduction and a photoanode for water oxidation, via a diaphragm and an external circuit, a tandem DSPEC can be built for full water splitting. In the ideal case, this is achieved without applying a potential bias across the two half cells, but applying an additional bias may be necessary to drive water splitting for ineffective systems. This way, the production of hydrogen and oxygen is spatially separated and both electrodes can be optimized for the respective reaction. Some key properties of the DSPEC are essential for high currents:

1) Efficient light absorption by the PS. Preferably, the whole visible light spectrum should be used by the PS to maximize sunlight absorption. In addition, high extinction coefficients are beneficial to increase light absorption. In a tandem cell where light passes through two photoelectrodes, the absorption spectra of the two PS should ideally be complementary so each uses a different part of the solar spectrum. [START_REF] Li | Organic Dye-Sensitized Tandem Photoelectrochemical Cell for Light Driven Total Water Splitting[END_REF][START_REF] Yu | Recent Advances in Dye-Sensitized Photoelectrochemical Cells for Solar Hydrogen Production Based on Molecular Components[END_REF] 2) Fast and long-lived charge separation. Charge separation must occur faster than the deactivation of the PS excited state. More importantly, the created CSSs must be longlived, since charge transfer to the catalyst may be slow and catalysis operates on very long timescales compared to typical excited state processes. In addition to this, the redox reactions in question are multi-electron reactions that require multiple charges to be transferred to the catalyst before the reaction can occur. During this time, the charges already transferred must remain on the catalyst.

3) Fast catalysis at low overpotential. To reduce the CSS lifetime requirements and increase currents, the reaction taking place on the catalyst should be fast. A low overpotential ensures that also low-energy photons can be used to drive the reaction.

Today, hydrogen-producing DSPEC have been published and bias-free water splitting has been achieved. However, the first systems based on first-row transition metal catalysts were published less than ten years ago and thus DSPEC are still an immature technology that suffers from low activity and are far from the currents needed for commercialization. This is especially true for the photocathodes which lag behind the photoanodes, with photocurrent densities typically lower than 100 µA•cm -2 (Table 2.1) compared to typically hundreds of µA•cm -2 for photoanodes. [START_REF] Xu | Water Splitting Dye-Sensitized Solar Cells[END_REF] Therefore, work still must be done to understand and address the issues facing the development of more performant DSPCs.

The major problems so far seem to be the often-slow ET kinetics to the catalyst, which in combination with generally fast charge recombination (CR) lead to a low amount of long-lived, catalytically active catalysts. In addition, long-term stability under operando conditions is still an issue, as the photocurrent generated by most photocathodes quickly diminishes. These issues will be discussed in more detail in the respective sections: section 2.2 deals with the different components of DSPC such as the PS (section 2.2.1), catalysts (section 2.2.2) and PS-catalyst dyads (section 2.2.3) as well as the SC materials (section 2.2.4); section 2.4 presents the stability issues affecting DSPEC performance; and section 2.5 discusses the light-induced processes in DSPC. Furthermore, PS used in DSPCs should possess certain properties to maximize activity: First, the extinction coefficients should be high in the visible region to maximize light absorption by the photocathode, with typical extinction coefficients of > 10,000 M -1 cm -1 . [START_REF] Decavoli | Molecular Organic Sensitizers for Photoelectrochemical Water Splitting[END_REF] Related to this is the spectral distribution of absorption:

Panchromatic dyes that absorb across the whole solar spectrum are desired to use the maximum possible amount of incoming sunlight. [START_REF] Hagfeldt | Dye-Sensitized Solar Cells[END_REF][START_REF] Odobel | New Photovoltaic Devices Based on the Sensitization of P-Type Semiconductors: Challenges and Opportunities[END_REF] The challenge lies in pushing the dye absorption to the red part of the spectrum while maintaining high extinction coefficients as well as the needed energy levels for catalysis. This is especially important for tandem DSPCs with two photoelectrodes, where the absorption of the two PS must be tuned to absorb different parts of the sunlight if a single light source is to be used. 23 Push-pull dyes are a popular choice since the frontier orbitals can be designed to optimize the light-induced processes: the HOMO is localized close to the anchoring groups to ensure fast HI, and the LUMO is localized far away from the anchoring groups to overcome fast CR in dye-sensitized SC, [START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF][START_REF] Qin | Synthesis and Mechanistic Studies of Organic Chromophores with Different Energy Levels for P-Type Dye-Sensitized Solar Cells[END_REF] especially the widely used NiO.

Additionally, in dye-catalyst assemblies this can be useful to localize the LUMO close to the catalyst unit to ensure fast ET. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] An important parameter for long-term performance is the stability of the dye under the relatively harsh -aqueous and reductive -operando conditions used in photoelectrochemical experiments. Related to this is the stability of the anchorage to the SC film, as a common problem in DSPCs is the hydrolysis of the anchoring group in aqueous media which leads to desorption of the PS from the SC surface. 3,[START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF][START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2 -Evolving Dye-Sensitized Photocathode[END_REF][START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF][START_REF] Sahara | Photoelectrochemical Reduction of CO2 Coupled to Water Oxidation Using a Photocathode with a Ru(II)-Re(I) Complex Photocatalyst and a CoOx /TaON Photoanode[END_REF] The choice of an adequate anchoring group for the PS in a given DSPC system can increase its stability markedly. 3 Typical anchoring groups are carboxylic acid or phosphonic acid groups that form esters on metal oxide surfaces. 3,[START_REF] Guerrero | Phosphonate Coupling Molecules for the Control of Surface/Interface Properties and the Synthesis of Nanomaterials[END_REF][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] Farré | A Comparative Investigation of the Role of the Anchoring Group on Perylene Monoimide Dyes in NiO-Based Dye-Sensitized Solar Cells[END_REF] These esters can be hydrolysed in the aqueous electrolyte, resulting in desorption of the anchored PS. The hydrolysis reaction kinetics depend on the pH of the electrolyte, with phosphonic acids being reported to be more stable in acidic aqueous conditions. [START_REF] Hanson | Photostability of Phosphonate-Derivatized, Ru II Polypyridyl Complexes on Metal Oxide Surfaces[END_REF] There have been efforts to apply different anchoring groups to overcome this issue. [START_REF] Materna | Using Surface Amide Couplings to Assemble Photocathodes for Solar Fuel Production Applications[END_REF] Lastly, if the DSPC systems are to be transferred to commercial application, the cost of the PS and the rarity of the used materials will play an important role, as expensive molecular components can be prohibitive for large-scale applications. [START_REF] Zhang | Artificial Photosynthesis: Opportunities and Challenges of Molecular Catalysts[END_REF] The two most common types of PS are metal complexes and organic dyes. The most widely used metal complexes are Ruthenium polypyridyl complexes due to their relatively high extinction coefficients in the visible region, well-understood photophysics and long excited state lifetimes as well as good chemical stability. [START_REF] Bräutigam | Hole Injection Dynamics from Two Structurally Related Ru-Bipyridine Complexes into NiOx Is Determined by the Substitution Pattern of the Ligands[END_REF] The simplest and by far most used in DSPC is tris(bipyridine)ruthenium (Ru(bpy)3), where one or more of the bipyridine (bpy) ligands are functionalized with anchoring groups ( Figure 2.2). 7,[START_REF] Youngblood | Photoassisted Overall Water Splitting in a Visible Light-Absorbing Dye-Sensitized Photoelectrochemical Cell[END_REF][START_REF] Shan | Photogeneration of Hydrogen from Water by a Robust Dye-Sensitized Photocathode[END_REF] Ru polypyridyl complexes with π-extended ligands replacing usually one of the bpy have been described in studies on photocatalytic hydrogen evolution in homogeneous solution, [START_REF] Kowacs | Supramolecular Bimetallic Assemblies for Photocatalytic Hydrogen Generation from Water[END_REF][START_REF] Tschierlei | Photochemical Fate: The First Step Determines Efficiency of H2 Formation with a Supramolecular Photocatalyst[END_REF][START_REF] Rau | A Supramolecular Photocatalyst for the Production of Hydrogen and the Selective Hydrogenation of Tolane[END_REF][START_REF] Zedler | Unraveling the Light-Activated Reaction Mechanism in a Catalytically Competent Key Intermediate of a Multifunctional Molecular Catalyst for Artificial Photosynthesis[END_REF][START_REF] Pfeffer | Tuning of Photocatalytic Activity by Creating a Tridentate Coordination Sphere for Palladium[END_REF][START_REF] Karnahl | Tuning of Photocatalytic Hydrogen Production and Photoinduced Intramolecular Electron Transfer Rates by Regioselective Bridging Ligand Substitution[END_REF] but so far have not been used in DSPC. In these systems, the LUMO is usually localized on the extended ligand, providing a higher distance between the charges in the CSS after HI, which is hoped to reduce deleterious CR. [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] Extended ligands have also been used to introduce a catalytic moiety by coordinating metal ions at the far end to create photocatalytically active dyads. [START_REF] Kowacs | Supramolecular Bimetallic Assemblies for Photocatalytic Hydrogen Generation from Water[END_REF][START_REF] Tschierlei | Photochemical Fate: The First Step Determines Efficiency of H2 Formation with a Supramolecular Photocatalyst[END_REF][START_REF] Rau | A Supramolecular Photocatalyst for the Production of Hydrogen and the Selective Hydrogenation of Tolane[END_REF][START_REF] Zedler | Unraveling the Light-Activated Reaction Mechanism in a Catalytically Competent Key Intermediate of a Multifunctional Molecular Catalyst for Artificial Photosynthesis[END_REF][START_REF] Pfeffer | Tuning of Photocatalytic Activity by Creating a Tridentate Coordination Sphere for Palladium[END_REF][START_REF] Karnahl | Tuning of Photocatalytic Hydrogen Production and Photoinduced Intramolecular Electron Transfer Rates by Regioselective Bridging Ligand Substitution[END_REF][START_REF] Põldme | Photoelectrocatalytic H2 Evolution from Integrated Photocatalysts Adsorbed on NiO[END_REF][START_REF] Li | Photochemical Hydrogen Production Catalyzed by Polypyridyl Ruthenium -Cobaloxime Heterobinuclear Complexes with Different Bridges[END_REF][START_REF] Kohler | Photoinduced Electron Transfer Kinetics of Linked Ru-Co Photocatalyst Dyads[END_REF][START_REF] Fihri | Cobaloxime-Based Photocatalytic Devices for Hydrogen Production[END_REF] Despite their attractive features as PS, Ru complexes have a major drawback in the rarity and therefore price of Ru, which can hinder the potential commercialization of Ru-based systems. Additionally, their extinction coefficients are typically not as high as those of the best organic PS. Nevertheless, they remain attractive in order to provide a fundamental understanding of the light-induced processes occurring at the electrodeelectrolyte interface.

The use of organic PS can partly eliminate these issues, as they are typically cheaper to synthesize and can offer very good and tuneable photophysical properties, especially high extinction coefficients. A wide variety of organic PS has been integrated in functional DSPC, including triarylamine-based push-pull dyes (D-π-A), 6,[START_REF] Li | Organic Dye-Sensitized Tandem Photoelectrochemical Cell for Light Driven Total Water Splitting[END_REF][START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF][START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2 -Evolving Dye-Sensitized Photocathode[END_REF][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][START_REF] Massin | Investigating Light-Driven Hole Injection and Hydrogen Evolution Catalysis at Dye-Sensitized NiO Photocathodes: A Combined Experimental-Theoretical Study[END_REF] perylene mono-and diimides (PMI/PDI), [START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF][START_REF] Creissen | Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst[END_REF] coumarin 343 (C343) [START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF] and isoindigo dyes (DPP-P, Figure 2.2). [START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF][START_REF] Creissen | Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst[END_REF] Push-pull dyes consist of an electron-rich donor and an electron-poor acceptor part which are typically linked with by a conjugated linker. These systems offer several advantages: As discussed earlier, the deliberate localization of the HOMO and LUMO can easily be achieved to optimize the light-induced processes. For this purpose, the anchoring groups are placed at the electron-rich part of the molecule where the HOMO is located; in the push-pull dyes used in DSPC, in most cases this is the triarylamine moiety. Localization of the HOMO close to the anchoring groups, and therefore to the SC surface, ensures fast HI after excitation of the PS. Conversely, the LUMO is preferably located on the distant end of the PS molecule to slow down CR once HI has occurred. The LUMO is located at the electron-acceptor group, which is either a cyanoacrylate or a PDI/ naphthalene diimide (NDI) group. The conjugated linker that connects the two groups is usually a (oligo-)thiophene. The choice of the three components that make up the push-pull affects the photophysical properties, most importantly the extinction coefficient and absorption maxima position, and can therefore be used to tune the dye to the specific needs of the DSPC. [START_REF] Decavoli | Molecular Organic Sensitizers for Photoelectrochemical Water Splitting[END_REF] Further elements can be added to the dye structure to fulfil additional tasks, as is the case of BH4, based on a tetrathiophene bridge and PMI acceptor: Here, long alkyl chains were introduced on the bridge to form a hydrophobic layer on the film surface in order to protect the anchoring group from hydrolysis and thus to increase the stability of the photocathode. [START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF]

Hydrogen evolution reaction (HER) catalysts

Catalysts are needed to reduce the activation energy of the proton reduction and thus drive the reaction as closely as possible to the thermodynamic potential. The discussion here will focus on those HER catalysts that have been integrated into DSPC to limit the scope of this section. However, the majority of the studies concerning the catalytic activity and mechanism were carried out in solution. Ideally, the overpotential for a redox reaction at a catalyst is zero, and the reaction is completely reversible without hysteresis. Desirable properties of HER catalysts are a high turnover frequency (TOF), produced by fast reaction kinetics, and a high stability under operando conditions, e.g. in aqueous conditions with applied potential and under irradiation. The combination of these two properties leads to high TON. From a mechanistic point of view, the catalyst must be able to consecutively accept and store two electrons since proton reduction is a two-electron process. The reduction potential of both the intermediate and final, catalytically active species must lie between the reduction potential of the reduced dye and that of protons to ensure a downhill ET cascade. Additionally, the catalyst must be stable in all of the oxidation states involved in the catalytic cycle. With a future application of the DSPEC technology in mind, rare and expensive metals such as platinum [START_REF] Gordon | Metal Stocks and Sustainability[END_REF] and palladium must be avoided to reduce costs. Therefore, research has focused on using Earth-abundant elements as the catalytic centres in HER catalysts such as the first-row transition metals Co, Ni and Fe.

In nature, evolution has produced highly performing HER catalysts in the hydrogenase enzymes, most notably the [NiFe]-and [FeFe]-hydrogenases (Figure 2.3).

These contain a bimetallic catalytic centre with CN -and CO ligands, while ironsulphur clusters ensure ET to the catalytic site, and nearby amino acids work as proton relays. [START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF] Hydrogenases display very high TOF at mild reaction conditions, reaching rates of up to 10,000 s -1 , which make them an attractive choice of HER catalyst. [START_REF] Jeuken | Biophotoelectrochemistry: From Bioelectrochemistry to Biophotovoltaics[END_REF] However, they are also very labile and are especially sensitive to oxygen; they therefore need to be protected or kept in ideal working conditions. [START_REF] Oughli | A Redox Hydrogel Protects the O2-Sensitive [FeFe]-Hydrogenase from Chlamydomonas Reinhardtii from Oxidative Damage[END_REF][START_REF] Plumeré | A Redox Hydrogel Protects Hydrogenase from High-Potential Deactivation and Oxygen Damage[END_REF] This, in combination with their bulkiness which prevents high surface coverage of the enzyme, [START_REF] Jeuken | Biophotoelectrochemistry: From Bioelectrochemistry to Biophotovoltaics[END_REF] complicate their use as catalysts in DSPC, and at the moment there is no example of a hydrogenase-based DSPC in the literature. Rather, so-called bioinspired HER catalysts have been developed that mimic essential features of hydrogenases to attain high TOF, but are less bulky, have higher long-term stability and can be synthesized in the laboratory. Some of those catalysts directly use the active site of hydrogenases such as catalyst [Fe-Fe] (Figure 2.3). [START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Gatty | Direct Evidence of Catalyst Reduction on Dye and Catalyst Co-Sensitized NiO Photocathodes by Mid-Infrared Transient Absorption Spectroscopy[END_REF][START_REF] Brown | Ultrafast Electron Transfer between Dye and Catalyst on a Mesoporous NiO Surface[END_REF] Most mimic only the functions of certain groups in the enzymes while using completely different molecular structures. A good example of this approach is the molecular Ni catalyst developed by DuBois in which amine groups on the ligands serve as proton relays, and which achieved a TOF of > 100,000 s -1 . [START_REF] Helm | A Synthetic Nickel Electrocatalyst with a Turnover Frequency Above 100,000 S-1 for H2 Production[END_REF] In consequence, this catalyst has been a popular choice to develop functional DSPC. [START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF][START_REF] Shan | Photogeneration of Hydrogen from Water by a Robust Dye-Sensitized Photocathode[END_REF][START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF][START_REF] Creissen | Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst[END_REF][START_REF] Gross | Photoelectrochemical Hydrogen Production in Water Using a Layer-by-Layer Assembly of a Ru Dye and Ni Catalyst on NiO[END_REF][START_REF] Shan | Modulating Hole Transport in Multilayered Photocathodes with Derivatized P-Type Nickel Oxide and Molecular Assemblies for Solar-Driven Water Splitting[END_REF] The second popular class of molecular HER catalysts in DSPC are Co catalysts. The most widely used ones are "cobaloximes", bis-(dimethylglyoximato)cobalt, that were first reported for proton reduction in 1983 [START_REF] Hawecker | Efficient Homogeneous Photochemical Hydrogen Generation and Water Reduction Mediated by Cobaloxime or Macrocyclic Cobalt Complexes[END_REF] and rediscovered in the 2000s for electroand photoelectrochemical proton reduction due to low synthetic requirements, high TON and low overpotentials of < 100 mV. [START_REF] Dempsey | Hydrogen Evolution Catalyzed by Cobaloximes[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] Eckenhoff | Cobalt Complexes as Artificial Hydrogenases for the Reductive Side of Water Splitting[END_REF][START_REF] Hu | Electrocatalytic Hydrogen Evolution at Low Overpotentials by Cobalt Macrocyclic Glyoxime and Tetraimine Complexes[END_REF][START_REF] Hu | Electrocatalytic Hydrogen Evolution by Cobalt Difluoroboryl-Diglyoximate Complexes[END_REF][START_REF] Razavet | Proton Electroreduction Catalyzed by Cobaloximes: Functional Models for Hydrogenases[END_REF] They consist of a cobalt metal centre coordinated to two dimethylglyoxime ligands in a planar geometry, and, depending on the oxidation state, up to two axial ligands. Protons are coordinated at the oxygen atoms of the oxime bridge, which can therefore possibly function as a proton relay. [START_REF] Bhattacharjee | A Computational Study of the Mechanism of Hydrogen Evolution by Cobalt(Diimine-Dioxime) Catalysts[END_REF] Although cobaloximes show high performance in organic media with TON of > 50,000 (during 7 hours of electrolysis for a TOF of ≈ 2 s -1 ), [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine-Dioxime Complexes[END_REF] they are limited in their usefulness for water splitting by their lability in aqueous acidic conditions due to demetallation.

To mitigate this, the two dimethylglyoxime ligands were connected via an alkyl chain to give a single ligand that is less easily displaced due to the chelate effect, improving stability considerably. [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine-Dioxime Complexes[END_REF][START_REF] Artero | Splitting Water with Cobalt[END_REF][START_REF] Seeber | Electrochemical Synthesis of Costa-Type Cobalt Complexes[END_REF] The resulting complexes are called Co diimine-dioximes and will be called Co throughout this thesis. In addition, the alkyl chain offers an opportunity to functionalize the catalyst, for example to tether it to an electrode [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine-Dioxime Complexes[END_REF][START_REF] Rosser | Understanding Immobilized Molecular Catalysts for Fuel-Forming Reactions through UV/Vis Spectroelectrochemistry[END_REF][START_REF] Andreiadis | Molecular Engineering of a Cobalt-Based Electrocatalytic Nanomaterial for H2 Evolution under Fully Aqueous Conditions[END_REF] or a PS. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF][START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF] On cobaloximes, such functionalization is usually carried out by axial coordination of pyridines. [START_REF] Li | Photochemical Hydrogen Production Catalyzed by Polypyridyl Ruthenium -Cobaloxime Heterobinuclear Complexes with Different Bridges[END_REF][START_REF] Reynal | Distance Dependent Charge Separation and Recombination in Semiconductor/Molecular Catalyst Systems for Water Splitting[END_REF][START_REF] Lyu | H2 -Evolving Dye-Sensitized Photocathode Based on a Ruthenium-Diacetylide/Cobaloxime Supramolecular Assembly[END_REF][START_REF] Ji | Photostable P-Type Dye-Sensitized Photoelectrochemical Cells for Water Reduction[END_REF] While this provides an easy way for functionalization, it is also very labile, especially since cobaloximes undergo ligand exchange during the catalytic cycle. [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine-Dioxime Complexes[END_REF][START_REF] Mccormick | Impact of Ligand Exchange in Hydrogen Production from Cobaloxime-Containing Photocatalytic Systems[END_REF] In the catalytic mechanism (Figure 2.4), the Co III precatalyst is reduced to the Co II state (Co II L) by a first reduction step to enter the catalytic cycle. From this state, the Co II hydride (Co II HLH) is formed through two proton-coupled ET steps via a Co I state with a protonated oxime bridge (Co I LH). Co II HLH then produces hydrogen either through external protonation or through an intramolecular mechanism to end up in the Co II L state ready to initialise catalysis again. [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine-Dioxime Complexes[END_REF] The protonation of the oxime bridge allows to lower the overpotential required for catalysis to occur and is proposed to facilitate hydrogen formation through an intramolecular protonation step. During catalysis, the axial halide ligands are displaced by solvent molecules, typically water or ACN. While the Co II state still possesses two axial ligands, in the Co I state one ligand is lost to form a pentacoordinated complex on which the hydride can form. Despite the increased stability achieved with the tetradentate ligand, cobalt diimine dioximes still suffer from decomposition in aqueous reductive conditions, creating a particulate state that is also active for proton reduction. [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] Therefore, research should focus on more robust catalysts for hydrogen evolution. One such class are cobalt complexes using polypyridyl and aminopyridyl ligands which have been reported to be active in fully aqueous conditions. [START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF] In particular, the complex [Co III (CR)X2] + (Cat1) 2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1 (17),2,11,13,15-pentaene, X = Cl, Br) based on a tetraaza-macrocyclic ligand has shown very good HER performance in homogeneous photocatalytic conditions. [START_REF] Hogue | A Smorgasbord of 17 Cobalt Complexes Active for Photocatalytic Hydrogen Evolution[END_REF][START_REF] Gueret | Cobalt(II) Pentaaza-Macrocyclic Schiff Base Complex as Catalyst for Light-Driven Hydrogen Evolution in Water: Electrochemical Generation and Theoretical Investigation of the One-Electron Reduced Species[END_REF][START_REF] Gueret | Challenging the [Ru(Bpy)3]2+ Photosensitizer with a Triazatriangulenium Robust Organic Dye for Visible-Light-Driven Hydrogen Production in Water[END_REF][START_REF] Varma | Efficient Photocatalytic Hydrogen Production in Water Using a Cobalt(III) Tetraaza-Macrocyclic Catalyst: Electrochemical Generation of the Low-Valent Co(I) Species and Its Reactivity toward Proton Reduction[END_REF] It has achieved TON of > 1000 in homogeneous photocatalytic conditions, outperforming Co by a factor of 70. [START_REF] Varma | Efficient Photocatalytic Hydrogen Production in Water Using a Cobalt(III) Tetraaza-Macrocyclic Catalyst: Electrochemical Generation of the Low-Valent Co(I) Species and Its Reactivity toward Proton Reduction[END_REF] This has been ascribed to its greater stability granted by the macrocyclic ligand which prevents hydrolysis and decomplexation. The catalytic cycle of this catalyst has not been fully cleared up until now, partly due to the difficulty imposed by the non-redox-innocence of the ligand: While the Co III/II reduction clearly occurs on the metal centre, the formal Co II/I and Co I/0 couples are partly or fully ligand-based reductions. [START_REF] Gueret | Cobalt(III) Tetraaza-Macrocyclic Complexes as Efficient Catalyst for Photoinduced Hydrogen Production in Water: Theoretical Investigation of the Electronic Structure of the Reduced Species and Mechanistic Insight[END_REF] Electrocatalytic hydrogen evolution in aqueous solution was observed at potentials slightly negative of the Co II/I couple, suggesting the formal Co I state initiates catalysis. [START_REF] Grau | Electrochemically and Photochemically Induced Hydrogen Evolution Catalysis with Cobalt Tetraazamacrocycles Occurs Through Different Pathways[END_REF] In spite of its activity in homogeneous photocatalysis, hydrogen evolution with Cat1 in DSPC has so far not been reported in the literature, and it has never been immobilized on electrodes.

(CR =

Dye-catalyst assemblies

In an effort to improve upon the results obtained with co-grafted DSPC, PS have been coupled to catalysts to yield supramolecular dye-catalyst assemblies anchored on the SC film via anchoring groups on the PS unit (Figure 2.5), which could have several advantages. Firstly, the PS-catalyst distance can be precisely controlled by the choice of linker and is possibly much closer than in co-grafted systems where the distribution on the SC surface is random. Furthermore, in addition to the PS-catalyst distance, the choice of the linker group can also influence the ET kinetics in the assembly, possibly granting faster forward ET and slower CR. Slower CR can also be achieved by the fact that the catalyst is located farther away from the SC surface instead of directly anchored on it. Lastly, the surface coverage of active molecules could be increased since there is a single molecule grafted on the surface which contains both PS and catalyst, as opposed to co-grafted systems where both components occupy space on the SC surface.

In many of the assemblies, the catalyst is grafted to the PS via complexation of the PS as a ligand to the metal centre of the catalyst. This approach was used to create dyads with a Ru polypyridyl PS and a Pt or Pd catalyst, where one ligand is coordinated to both the Ru and the Pt/Pd metal centre (Ru-Pd and Ru-Pt). [START_REF] Põldme | Photoelectrocatalytic H2 Evolution from Integrated Photocatalysts Adsorbed on NiO[END_REF] In these assemblies, the excited electron relaxes to a state where it is localized on the extended ligand, from where it can more easily transfer to the catalytically active unit. 86 Complexation was also popular for coupling cobaloxime HER catalysts to PS containing a pyridine group that displaced one of the axial ligands on the Co complex, as in the case of O22-cobaloxime [START_REF] Ji | Photostable P-Type Dye-Sensitized Photoelectrochemical Cells for Water Reduction[END_REF] and Ru-acetylide-cobaloxime. [START_REF] Lyu | H2 -Evolving Dye-Sensitized Photocathode Based on a Ruthenium-Diacetylide/Cobaloxime Supramolecular Assembly[END_REF] However, these assemblies suffer from stability issues since under operation, ligand exchange takes place that removes the catalyst from the PS unit. [START_REF] Li | Photochemical Hydrogen Production Catalyzed by Polypyridyl Ruthenium -Cobaloxime Heterobinuclear Complexes with Different Bridges[END_REF] To mitigate this, replacing the cobaloxime catalyst by the Co diimine-dioxime one opened up the possibility of covalent linkage at the alkyl chain, which was exploited to create dyads with organic push-pull dyads via triazole linkers created by click chemistry (T1-Co, 26 T2-Co [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] and PB-2 [START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF] ).

Finally, in a variation of the complexation approach, a layer-by-layer strategy was employed to construct dye-catalyst (PS-Zr-NiP) [START_REF] Gross | Photoelectrochemical Hydrogen Production in Water Using a Layer-by-Layer Assembly of a Ru Dye and Ni Catalyst on NiO[END_REF][START_REF] Shan | Modulating Hole Transport in Multilayered Photocathodes with Derivatized P-Type Nickel Oxide and Molecular Assemblies for Solar-Driven Water Splitting[END_REF] and donor-dye-catalyst (D-Zr-PS-Zr-NiP) [START_REF] Shan | Photogeneration of Hydrogen from Water by a Robust Dye-Sensitized Photocathode[END_REF] assemblies (or monolayers) using Zr IV -phosphonate bridges to connect the different units. The catalyst of choice in these assemblies was NiP. This approach has the advantage of a very easy assembly by sequential immersion of the SC film into solutions of the different units. [START_REF] Gross | Photoelectrochemical Hydrogen Production in Water Using a Layer-by-Layer Assembly of a Ru Dye and Ni Catalyst on NiO[END_REF] In general, the assemblies created by complexation are much easier in terms of synthesis and offer high modularity for optimization of the systems, while covalent linkage typically yields a more stable assembly.

Semiconductors

SC films play an important role in the design of the photocathode as they provide the support of the PS and HER catalyst, are the immediate electron donor upon excitation of the PS and provide the contact to the external circuit. As such, there are demanding requirements placed on the SC material to ensure high activity of the overall DSPC.

A necessary requirement for a functional DSPC is the position of the VB potential of the SC material. It must be more negative than the reduction potential of the excited dye for HI to occur (∆Ginj < 0). The precise match between the SC VB potential and the dye excited state reduction potential can be more easily fine-tuned by structural modifications on the dye. However, the reduction potential of the reduced dye must remain negative enough that ET to the catalyst is possible to drive proton reduction.

SC materials for DSPC must also possess a high charge carrier mobility for injected charges to quickly move away from the surface and reach the back contact instead of recombining. Unfortunately, hole mobility in p-type SC is generally much slower than electron mobility in n-type SCs: in the case of NiO, the hole diffusion coefficient can be several orders of magnitude lower than the electron diffusion coefficient in TiO2. [START_REF] Watson | Electron Injection At Dye-Sensitized Semiconductor Electrodes[END_REF][START_REF] Mori | Charge-Transfer Processes in Dye-Sensitized NiO Solar Cells[END_REF][START_REF] Li | Double-Layered NiO Photocathodes for p-Type DSSCs with Record IPCE[END_REF] This is caused by the localization of holes in oxygen 2p orbitals, which raises the energy barrier that needs to be overcome for migration through the material. [START_REF] Wrede | Towards Sustainable and Efficient P-Type Metal Oxide Semiconductor Materials in Dye-Sensitised Photocathodes for Solar Energy Conversion[END_REF][START_REF] Banerjee | Recent Developments in the Emerging Field of Crystalline P-Type Transparent Conducting Oxide Thin Films[END_REF][START_REF] Kawazoe | Transparent p -Type Conducting Oxides: Design and Fabrication of p-n Heterojunctions[END_REF][START_REF] Yim | Computational Discovery of P-Type Transparent Oxide Semiconductors Using Hydrogen Descriptor[END_REF] The low hole mobility prevents injected holes from quickly moving away from the film surface, making it more likely for them to recombine. In the future, if very active molecular components are developed, the conductivity of p-type SC could become a hard limit for the achievable photocurrent. Furthermore, the SC surface must be reactive toward the anchoring groups of the molecular components of the DSPC. In metal oxides, typically surface OH groups react with either carboxylic acid or phosphonic acid groups to form the respective ester and thus covalently graft the molecules to the film surface. To increase the area of the electrode-electrolyte interface and the loading of the active molecular species and therefore increase the photocurrent flowing through the photocathode, the SC films must possess a large surface area. This is achieved by preparing porous films via a range of techniques: from sol-gel processes, [START_REF] Zhu | Photoelectrochemistry of Mesoporous NiO Electrodes in Iodide/Triiodide Electrolytes[END_REF][START_REF] Qin | Design of an Organic Chromophore for P-Type Dye-Sensitized Solar Cells[END_REF][START_REF] Qin | High Incident Photon-to-Current Conversion Efficiency of p-Type Dye-Sensitized Solar Cells Based on NiO and Organic Chromophores[END_REF] often using polymer-templated precursors, [START_REF] Mori | Charge-Transfer Processes in Dye-Sensitized NiO Solar Cells[END_REF][START_REF] Li | Double-Layered NiO Photocathodes for p-Type DSSCs with Record IPCE[END_REF][START_REF] Bräutigam | Porous NiOx Nanostructures Templated by Polystyrene-Block-Poly(2-Vinylpyridine) Diblock Copolymer Micelles[END_REF][START_REF] Nakasa | A High Voltage Dye-Sensitized Solar Cell Using a Nanoporous NiO Photocathode[END_REF][100][101] in which the SC is formed in an annealing process directly on the conducting substrate, to the use of a suspension of preformed SC nanoparticles that is deposited on the substrate by spraying, screen printing or doctor-blading. 102,103 Especially polymer-templated films give high porosity due to the mesoporous structure. Other preparation techniques include chemical vapour deposition, 104 electrodeposition 105 and hydrothermal synthesis. 106 However, a high porosity can also hinder charge transport to the back contact and facilitate undesired recombination of injected charges. For future commercial application, the cost and ease of large-scale fabrication is also an issue. Finally, the SC must be stable under operando conditions and be resistant to photocorrosion, because of which wide bandgap SC are chosen (NiO: 3.6-4.0 eV). [START_REF] Odobel | New Photovoltaic Devices Based on the Sensitization of P-Type Semiconductors: Challenges and Opportunities[END_REF] This also ensures high transparency in the visible region to optimize absorption by the PS.

The currently most widely used p-type SC material for DSPC is NiO due to its ease of fabrication, good thermal and chemical stability, [START_REF] Odobel | New Photovoltaic Devices Based on the Sensitization of P-Type Semiconductors: Challenges and Opportunities[END_REF] and good transparency in the visible region. [START_REF] Zhu | Photoelectrochemistry of Mesoporous NiO Electrodes in Iodide/Triiodide Electrolytes[END_REF] However, it also has several severe drawbacks that has prompted research into alternative materials, so far with mixed success: In addition to the low hole mobility already discussed, a severe issue is the abundance of trap states in the bandgap, mostly produced by defects on the film surface. 107,108 These trap states are the cause of very fast CR after HI, which severely limits the activity of the DSPC since most of the energy of absorbed photons is lost (see section 4.1.2 ). While this can be mitigated by applying a negative potential, the goal of DSPC research is bias-free water splitting, where the potential generated by the photoanode is likely not negative enough to fully fill the trap states with electrons, especially at non-equilibrium with high photocurrents.

Alternative SC materials that have been used in DSPC have been the delafossites CuCrO2 [START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF]53 and CuGaO2,6 CuFe2O4, 109 LaFeO3, 110 and indium-doped tin oxide (ITO). [START_REF] Shan | Photogeneration of Hydrogen from Water by a Robust Dye-Sensitized Photocathode[END_REF][START_REF] Shan | Modulating Hole Transport in Multilayered Photocathodes with Derivatized P-Type Nickel Oxide and Molecular Assemblies for Solar-Driven Water Splitting[END_REF] ITO is an attractive material due to its properties, however, indium is scarce and must be avoided for large-scale application. [START_REF] Wrede | Towards Sustainable and Efficient P-Type Metal Oxide Semiconductor Materials in Dye-Sensitised Photocathodes for Solar Energy Conversion[END_REF] CuGaO2 showed higher photocurrent and more positive photocurrent onset potential. 6 CuCrO2 also showed higher photocurrents than NiO. [START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF][START_REF] Creissen | Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst[END_REF] CuFe2O4 and LaFeO3 both are small-bandgap SC that absorb visible light and drive proton reduction under irradiation when functionalized with a catalyst; 111 the photocurrent for both was also enhanced when additionally sensitized with a PS. 109,110 In spite of these efforts, so far no highly better p-type SC material than NiO has been developed.

Photocathode architecture and performance

DSPCs have been realized in essentially three different architectures of increasing complexity. While the PS is always grafted to the SC film surface to ensure fast HI, the catalyst is introduced in the system in different ways (

Catalysts in solution

The synthetically easiest way to prepare photocathodes for proton reduction is to simply dissolve the catalyst in the electrolyte solution, which is directly inspired by the working principle of p-DSSCs, in which the dissolved redox mediator oxidises the anchored, reduced PS. This approach avoids having to introduce anchoring groups on the catalyst and makes it possible to quickly screen several catalysts for optimal performance with a given dye-sensitized SC film. 112 Indeed, the simplicity of this approach caused it to be used in the first ever example of a DSPC for hydrogen evolution in 2012. [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] In this study, the organic dye P1 was anchored on NiO and a cobaloxime catalyst dissolved in solution, which under irradiation produced 320 nmol•cm -2 H2. [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] However, in this approach the catalyst must diffuse to the SC surface for ET to occur from the reduced state of the dye to the catalyst. The downside of this approach is thus that ET is inherently diffusion limited, meaning that if the lifetime of the reduced state of the dye is not sufficiently long-lived, the ET yield decreases.

Furthermore, since proton reduction is a multi-electron process, singly-reduced (nonactive) catalysts can diffuse away from the SC film surface before being reduced again, spending time in solution in this state, which can potentially lead to unwanted side reactions. In spite of this, the "catalyst in solution" approach has been successfully used showing high photocurrents with good production of hydrogen (up to 1150 nmol H2•cm -2 ), [START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF] where the system using NiP 52 outperformed cobaloximes (Table 2.1). [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][START_REF] Massin | Investigating Light-Driven Hole Injection and Hydrogen Evolution Catalysis at Dye-Sensitized NiO Photocathodes: A Combined Experimental-Theoretical Study[END_REF][START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF] Due to the high amount of catalyst necessary in solution, TON per catalyst are small (< 2)

for all systems built with this approach.

Co-grafting

In order to eliminate the need for catalyst diffusion and to increase spatial proximity between the PS and the catalyst, researchers started to co-graft the catalyst on the film surface together with the PS. This was hoped to accelerate ET to the catalyst to therefore achieve higher photocurrents and hydrogen production. However, it is necessary to functionalize the catalysts with anchoring groups: NiP, for example, was functionalized with phosphonate groups, while cobaloximes were coordinated to pyridines anchored via phosphonic acid 8 or carboxylic acid groups. 6,7 Furthermore, the grafting of catalyst on the surface necessarily reduces the maximum amount of PS that can be grafted on the film surface, and degraded catalysts cannot be replaced by intact ones from the solution. Co-grafting the catalyst also does not allow precise control of the PS-catalyst distance, as their distribution on the film surface is random.

For this approach, NiP [START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF][START_REF] Creissen | Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst[END_REF] and cobaloximes [6][7][START_REF] Li | Organic Dye-Sensitized Tandem Photoelectrochemical Cell for Light Driven Total Water Splitting[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] remained the most popular catalysts and a system using NiP and DPP-P on CuCrO2 showed the highest performance with 376 nmol H2•cm -2 and a TON of 126 (Table 2.1). [START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF] In general, total amounts of hydrogen produced with this approach were lower compared to dissolving the catalyst in solution, but TON were higher due to the much lower amount of catalyst used, ranging from 3 to 126, where reported. 6,[START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF][START_REF] Creissen | Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst[END_REF][START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF]

Dye-catalyst assemblies

In order to precisely control the PS-catalyst distance and to achieve fast ET, PS have been coupled to catalysts to yield supramolecular dye-catalyst assemblies either by complexation or by covalent linkage, as described in section 2.2.3. The increased control over the final system comes at the cost of much higher synthetical difficulty in the preparation of these assemblies. In terms of performance, these systems so far are comparable to the co-grafted systems: the best-performing system RuP2-Zr-NiP achieved 700 nmol H2•cm -2 and TON of 20, 64 while especially those systems based on organic sensitizers with cobalt catalysts showed very little activity (TON < 1, Table 2.1). [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF][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] 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 terms of amount of hydrogen produced, the layer-by-layer strategy with Ru(bpy)3 and NiP yielded the best results; [START_REF] Shan | Photogeneration of Hydrogen from Water by a Robust Dye-Sensitized Photocathode[END_REF][START_REF] Shan | Modulating Hole Transport in Multilayered Photocathodes with Derivatized P-Type Nickel Oxide and Molecular Assemblies for Solar-Driven Water Splitting[END_REF] however, these were achieved in ITO as opposed to the usually used NiO which might influence the performance. In fact, the RuP3-Zr-NiP system anchored on NiO performed poorly. [START_REF] Gross | Photoelectrochemical Hydrogen Production in Water Using a Layer-by-Layer Assembly of a Ru Dye and Ni Catalyst on NiO[END_REF] TON-wise, Ru-Pd and Ru-Pt performed best, [START_REF] Põldme | Photoelectrocatalytic H2 Evolution from Integrated Photocatalysts Adsorbed on NiO[END_REF] but these consist of two rare metal centers that would economically be unviable in large-scale application. In summary, none of the photocathode architectures published thus far clearly outperforms the others.

Therefore, they are all still viable routes of investigation. In theory, supramolecular assemblies have the highest promise since the final architecture can be precisely controlled; however, it is not clear if the possibly thus gained increase in activity makes up for the increased difficulty in their preparation. In commercial systems, this would depend on the cost of synthesis compared to the amount of hydrogen produced and other factors that involve costs, such as the area needed for the mounted systems. In addition, the wide parameter space in terms of pH, SC material, applied potential etc. makes it very difficult to compare the results achieved by different systems and identify critical parameters.

Stability issues

During operation, most DSPCs reported so far lose their activity over the course of minutes to hours. This can be traced to several processes caused by the operando conditions, including the aqueous electrolyte at usually acidic pH, the applied reductive potential and the irradiation with visible light.

The issue most often described is desorption of grafted molecules from the SC film surface. [START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2 -Evolving Dye-Sensitized Photocathode[END_REF][START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF][START_REF] Sahara | Photoelectrochemical Reduction of CO2 Coupled to Water Oxidation Using a Photocathode with a Ru(II)-Re(I) Complex Photocatalyst and a CoOx /TaON Photoanode[END_REF] This is a consequence of the hydrolysis of the anchoring groups, which are usually surface carboxylic or phosphonic esters formed between the respective acid on the PS/catalyst and OH groups on the film surface. In operation, the aqueous electrolyte solution can hydrolyse the esters and thus cause leaching of the formerly grafted molecules into the solution. To eliminate this degradation channel, the use of different anchoring groups that are more resistant to hydrolysis such as silatrane and siloxane groups have been proposed. 3,[START_REF] Materna | Using Surface Amide Couplings to Assemble Photocathodes for Solar Fuel Production Applications[END_REF] The pH of the electrolyte solution must also be taken into account, as different esters are most stable at different pH values.

However, so far no anchoring group has been discovered that withstands the acidic pH required for high catalytic turnover frequencies and thus for high currents. A promising approach are the use of protecting layers that cover the anchoring groups.

One type are thin metal oxide layers that are deposited after sensitization, typically prepared by atomic layer deposition since it is a low-temperature technique that does not destroy the grafted molecules. 3 The metal oxide used for this so far has been Al2O3, however, in the only study on DSPC photocathodes it was used in, it did not lead to an obvious improvement in stability of the systems. [START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF] A different approach was used by Wu et al., who functionalized their organic push-pull PS (BH4) with long alkyl chains on the bridge to form a hydrophobic protection layer which blocked the aqueous electrolyte from reaching the anchoring groups and thus prevented hydrolysis. This strategy had remarkable success, as their photocathode showed stable photocurrents for 16 hours in acidic electrolyte. [START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF] This enabled them to even use a pH 0 electrolyte, which gave ≈6 times higher photocurrents than using pH 5, which is in a range more typically used in DSPC.

The second issue is the degradation or modification of the molecular components during operation. The photoreductive conditions during photoelectrochemical experiments can lead to damage to the molecular structures such as bond cleavage.

This issue has been somewhat overlooked so far and has only been studied in detail for one photocathode using a PDI PS and a cobalt diimine dioxime catalyst. [START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2 -Evolving Dye-Sensitized Photocathode[END_REF] Related to this is pure photodamage, for example structural rearrangements such as isomerizations at double bonds. 113 Typically, the PS used in DSPC come from the more mature field of DSSC, but photodegradation is expected to be more severe in DSPC than in DSSC as the catalysis is much slower than the one-ET to the electrolyte taking place in DSSC, which leads to an accumulation of negative charges on the film surface.

This accumulation can lead to faster damage to photoactive systems. 114 To eliminate this issue, molecular structures must be devised that are stable under photoreductive conditions, and especially stable in their reduced state. To protect the PS, fast ET to the catalyst is desired to reduce the time the PS spends in its reduced state. So far, there

have not been any efforts directed especially at making more stable PS for DSPC. A stability issue that concerns the catalysts used in DSPC is their hydrolysis and demetallation. For example, cobaloximes are very stable in electrocatalysis in organic media, but have been found to quickly hydrolyse and demetalate in aqueous conditions. [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine-Dioxime Complexes[END_REF] This might partially be caused by the absorbance of visible light most catalysts show and which can lead to undesired side reactions, especially in lower oxidation states. A solution to this issue is to use macrocyclic, chelating ligands that do not decoordinate as easily, making the catalyst more resistant to hydrolysis.

Alternatively, polypyridinic Co complexes, in which low oxidation states are better stabilized, can be used. 115 If possible, the catalyst itself should also show low absorbance of visible light.

In summary, there are several stability issues plaguing DSPC and that prevent their long-term activity. However, so far little effort has been undertaken to study the degradation processes in detail, which could be achieved using a variety of in-and post-operando spectroscopic techniques. This information could then be used to rationally design more stable systems.

Kinetic issues

As sunlight is the energy source used for water splitting in DSPEC, light-induced processes and their kinetics are an essential part of DSPEC research. Figure 2.8 shows the light-induced processes that take place in a photocathode upon excitation with light. Initially, a photon is absorbed by the PS to raise it to the excited state. From there, the excited dye can inject a hole into the VB of the SC (kHI,red) to yield the primary CSS, with the negative charge localized on the PS. From this state, thermally activated ET can then occur to the catalyst (kET,red) to yield the secondary CSS. The hole injected into the SC can diffuse away from the film surface to eventually end up at the back contact (kdiff). Since the excited state is quenched by an ET to the excited PS, this pathway occurs via reductive quenching. There is also the possibility of an oxidative quenching pathway, in which the excited state is quenched by ET from the PS to the catalyst (kET,ox)

to give the primary CSS with an electron on the catalyst and a hole on the PS. The oxidised PS is then regenerated by HI into the SC (kHI,ox) to end up in the same secondary CSS as in the reductive quenching pathway.

In parallel to these desired "forward" processes, detrimental processes occur in which the excitation energy is lost. In general, these processes become a problem if the forward process from the respective state is not significantly faster than the detrimental process. First, the excited PS can return to the ground state before charge separation can occur (kD). This is only an issue if the PS has a very short excited state lifetime on the picosecond to a few nanosecond timescale while HI or ET is slow. The more important issue is CR from the several CSS involved in the light-induced processes (krec) since CR often occurs on timescales that are similar or faster than the forward reactions. This especially affects DSPEC since the targeted redox reactions are multi-electron processes and the CSS therefore must be long-lived enough to enable multiple successive charge transfers. The study of the intramolecular and interfacial processes and their kinetics is therefore of utmost importance in DSPEC research. Ideally, this should be done under operando conditions since the light-induced processes could depend on the surrounding electrolyte solution and especially the applied potential. To perform these measurements, in this thesis the relatively new technique of TA-SEC, i.e. TA spectroscopy at an applied potential, was used to study NiO films sensitized with dyecatalyst assemblies. To this end, a dedicated spectroelectrochemical cell was designed and built. Complementary to these experiments, traditional TA spectroscopy was also measured of the molecular dyads in solution.

Spectroscopy in solution

Parts of this chapter have been published in the following articles: The easiest way to study the excited state behaviour of a given compound is in solution, and this is usually the starting point for every investigation. In this chapter, characterization of the photophysical properties of a series of organic push-pull dyes and dye-catalyst assemblies in solution will be discussed. The synthesis of the compounds was performed by Quentin Vacher, Julien Massin and Emmanouil Giannoudis. Since in DSPEC, absorption of a photon by the PS is the first step in a cascade of processes that ultimately ends with catalysis, it is of paramount importance to investigate the excited state processes taking place in a dye or dye-catalyst assembly.

In particular, ET processes are fundamental, which is why a study of their kinetics and the detection of the formed states is critical. Measurements in solution provide the easiest and fastest way to study these processes.

Section 3.2 deals with the excited state photophysics of a series of push-pull dyes in solution. In section 3.3, the dye-catalyst dyads based on those dyes are investigated.

The focus will be on the deactivation processes from the thermalized singlet excited state, including internal conversion, intersystem crossing (ISC), energy transfer and ET.

Spectroscopic techniques

To gain a full understanding of the performance of DSPEC systems, it is indispensable to study the light-induced processes taking place, if possible at operando conditions.

To achieve this, a variety of spectroscopic techniques are available, from basic steadystate such as absorption and emission spectroscopy to more advanced time-resolved techniques. The technique that will be crucial in this thesis is TA spectroscopy, a pump-probe technique that yields time-resolved spectral information. This allows to identify the transient species involved in the light-induced processes as well as the kinetics of the processes that lead to their formation and decay. In TA spectroscopy, a laser pump pulse excites the sample and after a time delay, a probe pulse is used to measure the absorbance of the sample (Figure 3. 1). By varying the pump-probe delay, the temporal evolution of the spectral response and therefore of the excited state can be recorded. The pump pulse is usually intense, spectrally narrow and tuned to a defined wavelength to excite a selected transition in the sample. The probe pulse is typically weak and, if possible, spectrally broad to enable studying a wide spectral window. The spectra that are measured are differential spectra where the reference absorbance without excitation is subtracted from the absorbance at time t after excitation. A complementary technique are time-resolved emission measurements using a streak camera or time-correlated single photon counting. These techniques allow to measure the temporal, and in the case of the streak camera spectral, evolution of emission after excitation by a pump pulse. This technique can be used to substantiate results from TA measurements and yield information on the contributions from emissive states. to which the organic dyes studied in this thesis belong. In general, these dyes show two main absorption bands: one band in the UV caused by a transition localized on the TPA moiety and an intramolecular charge transfer (ICT) band in the visible part of the spectrum (ε = 29,000 to 58,000 M -1 cm -1 ). [START_REF] Qin | Synthesis and Mechanistic Studies of Organic Chromophores with Different Energy Levels for P-Type Dye-Sensitized Solar Cells[END_REF][116][117][118][119][120][121] Upon excitation of the latter, electron density is shifted from the TPA part, where the HOMO is localized, to the cyanovinyl acceptor part, where the LUMO is localized. The absorption, but especially the emission from the ICT state shows a strong solvatochromic effect due to the stabilization of the charge-transfer state in polar solvents. 116,122,123 While the excited state lifetimes observed are in the range of 450 ps to 3.4 ns in non-polar solvents (hexane, toluene and tetrahydrofuran), 116,120,[123][124][125] in polar solvents such as acetonitrile (ACN) they are much lower, in the range of 3 ps to 1.2 ns. [START_REF] Qin | Synthesis and Mechanistic Studies of Organic Chromophores with Different Energy Levels for P-Type Dye-Sensitized Solar Cells[END_REF]116,117,119,124 The general excited-state processes published in literature are a cascade of relaxation processes, including solvent rearrangement and molecular motions such as flattening, that lead from the initially excited Franck-Condon state to a thermalized ICT state (sometimes called the S1), from which return to the ground state occurs. 116,117,119,122,123 These results are at first sight not promising for photocatalytic applications due to the short excited state lifetime in polar solvents, which creates the need for fast excited state quenching processes in the final assembly.

Organic push-pull (D-π-A) dyes in this thesis

The organic dyes investigated in this thesis are push-pull organic dyes consisting of a TPA donor and a cyanoacrylate acceptor part (Figure 3.3), connected by a bridge varying in length and rigidity from a single thiophene (T1) over a dithiophene (T2) to the rigid cyclopentadithiophene (CPDT, T2R). In T2R, the linker additionally possesses alkyl chains that were introduced with the intent of reducing dye aggregation on the sensitized film surface and of creating a hydrophobic blocking layer that protects the anchoring groups from hydrolysis under operando conditions. [START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF] Furthermore, the acceptor group can be prepared as a cyanoacrylamide group with an alkyne group to further attach the catalyst via copper-catalyzed azide-alkyne cycloaddition. In addition, upon excitation, these push-pull dyes should exhibit ICT that localizes the hole (the HOMO) on the TPA, close to the anchoring groups, to accelerate HI and the electron (the LUMO) on the cyanoacrylate group close to the catalyst for fast ET to the latter.

In terms of their photophysics, the three dyes show very similar behaviour. In UV-Vis absorption spectra (Figure 3.3), they all show two main bands: localized at 355 nm, the π-π* transition localized on the TPA, and whose position and intensity are independent of the linker. At longer wavelengths, there is a band produced by the ICT from the TPA donor to the cyanoacrylate acceptor group, which is the HOMO-LUMO transition. The spectral position and intensity of this band depends on the linker, experiencing a bathochromic and hypochromic shift upon changing the single thiophene linker in T1 for bithiophene linkers, with the effect much more pronounced for the rigid CPDT linker in T2R. The bathochromic shift is a result of a smaller HOMO-LUMO gap, suggesting that the linker takes part in one or both of the two frontier orbitals. DFT calculations in literature performed on an analogue of T2R show that it is indeed the HOMO which is more strongly localized on the bridge than in an analogue of T1. 126 motions such as rotation and flattening, as previously observed in the literature. 116,117,119,122,123 While the spectral response to this process is identical, the kinetics vary between dyes and especially depending on the solvent. In ACN, these relaxation processes are very fast, occurring within the first few ps. T1 and T2 show a biphasic process with time constants of t1 = 0.2 ps and t2 = 0.9 ps (T1), 1.7 ps (T2), while T2R shows a monophasic process with t1 = 1 ps. The higher time constant for the slow process in T2 might be due to the involvement of a rotation around the bond connecting the two thiophenes in this cooling process, a motion only possible in T2.

The influence of the solvent is much more pronounced: for T1, switching the solvent to dimethyl formamide (DMF) increased the time constants for cooling to t1 = 0.7 ps and t2 = 2.6 ps, while using methanol (MeOH) lead to a biphasic process with t1 = 1.3 ps and t2 = 11 ps for T2R. This can be explained by their higher viscosity compared to ACN (0.37 cp, DMF: 0.92 cp, MeOH: 0.55 cp), retarding relaxation processes that involve major molecular motion. In the case of MeOH, an additional factor could be the formation of hydrogen bonds. After the relaxation cascade is complete, the dyes are in the thermalized singlet excited ICT state, from which they decay back to the ground state in either monophasic (T2R)

or biphasic fashion (T1, T2) on a sub-ns to ns timescale. This fast excited state decay has been suggested to occur due to a twisted geometry with increased charge separation and localization in the S1 state, which was proposed to approximate the S1 and S0 potential energy surfaces, leading to a fast internal conversion and thus to a fast excited state decay and low emission quantum yield. 116 This was supported by an observation on a TPA-cyanoacrylate dye, in which the S0 and S1 state were essentially degenerate at 90° and 270° rotation around the double bond, providing a low-barrier mechanism for internal conversion to the ground state. 113 In T1 and T2, this might be the reason for the biphasic decays which could result from different rotational conformers. The faster excited state decay for T2 might be due to the lower HOMO-LUMO gap or to the additional (rotational) degrees of freedom introduced by the longer linker.

Photophysics of dye-catalyst assemblies in solution

In the photophysics of dye-catalyst dyads, the most interesting question is the kinetics of ET and CR. They are important to determine the yield of CSS and its lifetime. This ET can occur either via an oxidative quenching, i.e. from the excited dye to the catalyst, or via a thermal ET, i.e. from the reduced dye (generated by a reductive quenching of the excited PS by an external electron donor) to the catalyst. Depending on the kinetics of the processes involved, dyads can undergo one of the two or possibly a mixture of both mechanisms.

In this thesis, covalent dye-catalyst assemblies based on the dyes T1, T2 and T2R covalently linked to the cobalt HER catalysts Co and Cat1 via copper-catalysed azidealkyne cycloaddition ("click chemistry") were investigated (Figure 3.3). The idea behind these dye-catalyst assemblies is to provide a directionality of the ET away from the electrode surface when grafted, and to fix the two units in a spatially close conformation to ensure fast and efficient ET from the excited or reduced dye to the catalyst. To investigate whether this goal was achieved, the resulting dyads were studied by TA spectroscopy and spectroelectrochemistry.

Electrochemical measurements showed that there is no ground state interaction between the dye and catalyst unit in any of the dyads, which is corroborated by their UV-Vis spectra. Three reduction processes were observed for Cat1 of which two are positive of the dye reduction potentials and are therefore accessible by ET from the reduced dye. The first two reductions, to the Co II and Co I state, are localized on the metal-organic centre while the third process is ligand-based. [START_REF] Varma | Efficient Photocatalytic Hydrogen Production in Water Using a Cobalt(III) Tetraaza-Macrocyclic Catalyst: Electrochemical Generation of the Low-Valent Co(I) Species and Its Reactivity toward Proton Reduction[END_REF][START_REF] Grau | Electrochemically and Photochemically Induced Hydrogen Evolution Catalysis with Cobalt Tetraazamacrocycles Occurs Through Different Pathways[END_REF]128,129 Concerning the dyes, the reduction potential is unaffected by the introduction of the CPDT linker, but the oxidation potential decreases by ca. 220 mV, equivalent to a HOMO shift to higher energy, due to the electron-rich dithiophene unit. Steady-state spectroscopy and electrochemical measurements showed that i) HI from the excited state of the dyes to NiO and ii) ET from either the excited or reduced dye to the catalyst up to its Co I state is thermodynamically possible.

Emission spectroscopy indicated an interaction in the excited state in the form of emission quenching in the T1 and T2R dyads which reduced the emission quantum yield by 68 to 92 %. Thus, the covalent assemblies with the cobalt catalysts seem to enable non-radiative decay pathways for the emissive state. The nature and kinetics of these non-radiative pathways were the focus of the TA spectroscopic experiments performed on the dyads in solution.

Upon excitation, the initial behaviour of all dyads studied is identical to that of the respective dyes up until the formation of the thermalized singlet excited ICT state, irrespective of the catalyst unit. In short, the initially excited Franck-Condon state relaxes on the sub-ps to ps timescale in a relaxation cascade to the thermalized singlet excited state via solvent rearrangement and molecular motions such as rotation and flattening. The spectral shape of the TA spectra and their evolution during the relaxation process is identical to that of the respective dyes. After the initial relaxation, the deactivation pathways taking place from the thermalized singlet excited state are however quite distinct between the different dyads and in comparison to the respective dyes.

T1-Co

The original dyad T1-Co shows a spectrally identical, but faster excited state deactivation in the TA experiments compared to the dye T1. This suggests that the same deactivation process takes place in the dyad but is accelerated. Since the dyads were constructed to enable fast ET from the dye to the catalyst, we would like to know whether the excited state deactivation involves such an ET process. The setup, which was designed by Dr. Linda Zedler, consists of a three-electrode setup in a 1 mm cuvette (scheme in Figure 3.5). The working electrode was a glassy carbon electrode with 0.4 mm thickness and a drilled hole with 1 mm diameter through which the pump and probe beam were focused colinearly. This electrode design enables a high local concentration of reduced species at the focal point of the pump and probe beams, taking advantage of the highly resistant glassy carbon material while achieving optimal light transmission despite its opacity. The counter electrode was a Pt wire and the pseudo-reference electrode a Ag wire.

During the experiment, the sample was reduced in a chronoamperometric measurement before starting the TA measurement. During the measurement, the potential was continously applied to ensure a constant high concentration of reduced species in the sample space. In the experiment, it was observed that the oxidation state of the catalyst had little to no influence on the excited state kinetics (Figure 3.6). Importantly, there was no spectral or kinetic indication which would point to ET to the cobalt catalyst: i) the Co I spectral signature was not observed when exciting the sample in the Co II state, the spectral shape was in fact identical at all applied potentials; and ii) the excited state decay kinetics were not faster for the measurements with thermodynamically possible ET in comparison to the reference measurement at Co I , where it was not. In fact, a counter-intuitive faster excited state decay was observed at more negative potentials, which might be due to a Coulombic repulsion between electron density at the cobalt catalyst and the electron localized on the dye acceptor group, leading to a destabilization of the ICT state and therefore to its faster decay. Thus, it could be concluded that ET does not take place in the excited state of T1-Co and that the faster excited state decay must stem from a different pathway. An alternative explanation for the increase in the non-radiative rate could be an increased density of vibrational modes upon addition of the cobalt moiety. Dexter energy transfer from the excited donor chromophore to short-lived cobalt-centred dd-states might also play a role.

Therefore, it must be deduced that the ET that enables hydrogen production in photoelectrochemical experiments with T1-Co takes place on a much slower timescale than its rather short excited state lifetime. Therefore, it seems likely that instead of an oxidative quenching pathway, which we failed to observe in our TA-SEC experiment, a reductive quenching pathway is indeed active. Here, the excited state is first quenched by HI into NiO, which is typically ultrafast (see section 4.1.2.3), and subsequently ET from the reduced dye to the catalyst takes place.

This hypothesis is supported by observations made by Aukauloo and coworkers who studied a series of dye-catalyst dyads with Ru polypyridyl complexes connected to various acceptor moieties by triazole bridges prepared by "click" chemistry. In their studies, the ruthenium excited state was usually quenched very quickly by an external SED or acceptor, followed by a thermally activated intramolecular ET to the acceptor unit. [132][133][134][135][136] The ET was observed on the ns-µs timescale, with the fastest occurring in < 20 ns in a Ru(bpy)3-NDI assembly 134 and the slowest with a characteristic time constant of 31 µs in a Ru(bpy)3-Ni(cyclam) assembly. 133 These results suggest that indeed the CSS generated by HI on the NiO surface might have to be stable for up to tens of µs for ET to the catalytic center to occur, and thus much longer than the 1.7 ns and 350 ps lifetimes of the excited state of T1 and T1-Co, respectively.

It also highlights that the triazole bridge might be synthetically appealing, but not ideal in terms of the excited state photophysics and ultimately the performance of these dyads. Further evidence for this comes from studies with triazole-bridged donoracceptor assemblies without SED, e.g. looking at excited state ET. ET in triazolebridged dyads was reported to occur on a wide range of timescales, ranging from ultrafast ET on the timescale of ps [137][138][139][140][141][142] to slower ET in ns-ms [139][140][141]143,144 to dyads where ET is not observed, even though it is thermodynamically possible. [145][146][147] Similarly, the CR rates drastically differed between systems, from the ps time scale 137,141,142 to hundreds of ns. 138,143,148 An interesting behaviour was observed in a Zn porphyrin-NDI dyad, where ET efficiency depended on whether the donor or acceptor part of the molecule was excited. Excitation of the acceptor lead to a high yield of CSS through fast ET (<10 ps), while excitation of the donor Zn porphyrin lead to a low yield because of slower ET (tET = 9 ns) combined with the deactivating ISC to the triplet state on the Zn porphyrin. 141 One study also found an influence of the connectivity of the donor and acceptor moieties on the triazole, e.g. which part was connected to a carbon or a nitrogen atom on the triazole ring. 148 In general, it seems that ET trough the triazole is rather slow in most assemblies; many of the fast ET rates can be ascribed to throughspace ET made possible by flexible linkers, e.g. in studies in polymers functionalized with Ru dyes. 149-152

T2R dyads

The study of the dyads based on the T2R revealed yet another influence of the cobalt moieties on the excited state behaviour of the dye unit. In this case, both dyads, based on Co and Cat1, were studied by fs-TA spectroscopy in MeOH. Interestingly, both dyads show identical excited state behaviour, except for a somewhat more pronounced emission quenching in T2R-Cat1. There is no SE band at any point in time.

Due to the solvent, the initial relaxation process is slow and finished only after some tens of ps. Following the thermalization and unlike all the other compounds, a very long-lived signal forms in about 120 ps. Spectrally, it consists of GSB at 350 and around 490 nm and a broad ESA band above 510 nm with two maxima at 590 and 725 nm. This spectrum is ascribed to the triplet state of the T2R dye likely localized on the CPDT bridge, formed by ISC from the thermalized singlet ICT state (Figure 3.8). The extinction coefficients of oligothiophene triplet states observed in literature are in the same order of magnitude as that of T2R, in line with the observed similar intensity of the long-lived band compared to the GSB. 153 Although the triplet absorption of pure oligothiophenes is found at higher energies than those observed in our dyads for the triplet state band, the bithiophene structure in T2R is part of an extended conjugated charge-transfer system which can have a strong effect on the excited state energies. ET to the Co catalysts is discarded on the basis of i) the identical spectral shape for both Co and Cat1 dyads, where the two catalysts would give a different spectral response upon reduction by ET, and ii) the similar intensity of the double band to the other dyebased bands, while extinction coefficients of the optical transitions on the cobalt complexes are much lower than those of the dye (factor > 10).

The fast ISC is explained by the rigid CPDT linker containing two thiophene units. Biand oligothiophenes are known to undergo fast ISC on the same time scale as observed here and could potentially show similar behaviour when used as a linker in T2R. [153][154][155] The extremely fast ISC rates for bithiophene are explained in the literature by the high spin-orbit coupling factor of sulfur 156 and a very small energy difference of the S1 and T2 state. 154 Since the fast ISC only takes place in the dyads and not in the T2R dye, an interaction of the CPDT linker with the cobalt complexes is likely responsible for the fast ISC. Literature on ISC induced by cobalt in binary systems is scarce and the observed effect attributed to the paramagnetic nature of Co II ions which cannot be the explanation for our diamagnetic Co III complex. 157,158 Instead, alternative pathways must be considered involving higher energy states on the cobalt complexes. In addition, the rigidity of the CPDT linker likely plays a role in the ISC since for T2-Co, only a very small yield of triplet state was detected.

In the literature, there are some other examples of heavy-atom free triplet sensitizers that operate by radical pair ISC, which is much slower than that observed in our systems, [159][160][161][162] or through the use of spin convertors, most prominently fullerenes. [163][164][165] In our case, the Co complex could act as a spin convertor after ET since ISC has been observed to occur on a ps time scale in the ground state of Co complexes 166 and < 1 ps in the excited state, 167 and thus could be much faster than the ISC rates observed here.

If the ISC and back CT to the dye are much faster than the CT to the complex, the latter would be the gating process and there would be no accumulation of the CT state with the electron localized on the Co complex, explaining why this species was not observed in the experiment.

T2-Co

The T2-Co dyad shows quite a different excited state behaviour than T1-Co. Where T1-Co showed a much faster decay of the thermalized singlet ICT state than T1, this process is barely accelerated in T2-Co compared to its dye T2 to occur with an average time constant of 500 ps. The spectral shape is identical to that of T2 with a GSB centred at 455 nm and ESA at 620 nm, while the SE could not be observed in either T2 or T2-Co since it is bathochromically shifted out of the spectral window of our setup. As in the case of the T1 systems, the excited state decay processes are of the same nature but are slightly accelerated in the dyad. It is not clear why there seems to be a lesser acceleration of excited state decay by interaction with the cobalt catalyst than in T1-Co.

From DFT calculations on similar dyes, the LUMO does not seem to be more strongly located on the linker, e.g. away from the cobalt catalyst, than in T1. 127 A possible explanation can be offered when inspecting the spectrum at long delay times. In T2-Co, after 9 ns the signal has not completely decayed to zero, instead, there still is some GSB and ESA. The spectral shape of the ESA is broader than the initial one, extending farther into the red. In comparison to T2, this contribution can already be observed at earlier delay times. In analogy to the results for the T2R dyads (see section 3.3.2), this signal is attributed to the triplet state of the T2 dye formed by cobaltinduced ISC. Since the triplet state is more long-lived than the singlet state, the convoluted signal of the two states can result in a net slower signal decay than in T1-Co. and 625 nm for T2-Co).

Conclusion and Outlook

Comparing the data from the different experiments on the excited state behaviour of the organic dyes and dyads in solution, several insights can be gained.

First, all dyes and dyads show identical initial ultrafast cooling processes to a thermalized singlet excited state whose kinetics are affected by solvent viscosity.

Deactivation from this state is similar in the different dyes with a direct decay to the ground state on the sub-ns to ns timescale, but occurs in very different fashion for the different dyads:

1. T1-Co shows faster deactivation than the T1 dye due to interactions with Co dd states or additional vibrational modes.

2. The T2R dyads undergo fast ISC to yield a long-lived triplet state, likely through interaction of the rigid CPDT linker and higher Co states.

3. T2-Co shows only slightly faster decay than T2 and forms the excited triplet state in low yield.

The choice of linker in the design of the push-pull dye thus determined the nature and kinetics of the decay process from the singlet ICT state. ET from the excited state to the catalytic centre (Co or Cat1) was not observed in any of the dyads, despite its thermodynamic feasibility. This is likely due to a slow ET process competing with the fast excited state deactivation observed for these organic dyes and dyads. Thus, it seems that in DSPC, the oxidative quenching pathway is not active in these dyads, but rather a reductive quenching pathway where the electron is transferred from the reduced dye to the catalyst.

This work is one of few examples of TA-SEC measurements in solution on donoracceptor dyads and the only one so far using an organic dye as photoactive unit and electron donor. Furthermore, it was the first time these measurements were performed on a dyad containing a cobalt catalyst active for proton reduction as acceptor. This enabled us to probe the second ET step that leads to the catalytically active state of the catalyst without using a SED or a chemical reductant which could interfere in the photophysical measurements. In general, this technique opens up the possibility of probing light-induced processes at different oxidation states which is important in the context of systems designed for photo-induced multi-electron catalysis such as proton reduction and water oxidation.

The results obtained for T1-Co in the TA-SEC experiments show clearly that emission quenching alone is not sufficient evidence for photoinduced ET. It is necessary to additionally observe the spectral signature of the charge-transfer state or an influence of the oxidation state on the decay kinetics. The latter observation is now made possible by TA-SEC measurements without the need for a chemical reductant.

In general, the results show that the interaction of the donor and acceptor units, in this case the Co complexes, in molecular dyads can lead to unexpected results: Firstly, to a fast ground state decay that outcompetes ET. More interestingly, a fast ISC on the sub-ns timescale occurs in the T2R dyads. Such a fast, cobalt-induced ISC has so far not been observed in the literature, as it is likely a product of the interaction of the Co cobalt complex with the rigid CPDT linker containing two thiophene rings, where the triplet state is localized and which are known themselves to undergo ISC to the triplet state. 153,154 This shows that all the possible excited-state interactions, in addition to the desired ones, must be taken into account when designing donor-acceptor dyads.

However, the unexpectedly obtained fast ISC shows a possible new architecture for organic triplet sensitizers by using tethered Co complexes instead of heavy atoms or other spin convertors.

Furthermore, our results challenge the use of triazole bridges as linkers for donoracceptor dyads designed for light-induced charge-transfer, since ET cannot compete with the other deactivation processes. Therefore, a different linker design should be considered to achieve fast ET. The data was analyzed using a customized data analysis software (Pascher Instruments AB, Sweden). First, the data was corrected for the chirp and subsequently, a sum of exponential functions was fitted to the data. The pulse overlap region of ±200 fs was removed from the data to exclude the coherent artefacts present in this temporal region.

Experimental Section

The TA measurements were carried out in ACN in a standard 1 mm cuvette (OD400nm = 0.3).

Dye-sensitized photocathodes

Parts of this chapter have been published in the following articles:

Activity and stability

The results in this paragraph were obtained by Emmanouil Giannoudis at CEA Grenoble and are taken from his doctoral thesis.

Ru and RuCo show grafting densities of 5.2 ± 0.8 nmol/cm 2 . At -0.4 V vs. Ag/AgCl applied potential and under 1 sun equivalent irradiation in the visible region (400-800 nm), RuCo showed photocurrent densities of 58 ± 11 µA•cm -2 , while the use of an AM The obtained TON values for this assembly are very low compared to those obtained with the same catalyst under homogeneous photocatalytic conditions (300 TON) 170 or in electrocatalytic conditions when anchored on carbon nanotubes (7000 TON). [START_REF] Andreiadis | Molecular Engineering of a Cobalt-Based Electrocatalytic Nanomaterial for H2 Evolution under Fully Aqueous Conditions[END_REF] This suggests that the assembly suffers from performance-limiting issues which, in addition to the stability issues, might be kinetic issues, i.e. the catalyst does not receive electrons at a rate necessary for fast catalysis. To investigate this, TA-SEC experiments were performed.

Time-resolved spectroscopy on dye-sensitized NiO films

When studying PS for solar energy applications, the photophysics in solution are only the first step to obtain a general idea of the excited state processes in the molecule. For application in DSPEC, the second, very important step is to investigate the photophysics of the complete dye-sensitized film as this is the active system. When immobilizing PS on p-type SCs such as NiO, additional pathways are opened up in the excited state such as HI into the VB of the SC and the following CR as well as intermolecular interactions on the surface that can lead to excimer formation and fast excited state decay. [START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF]171 Typically kinetics of interfacial processes on metal oxide films are non-exponential, often stretching across timescales of several orders of magnitude.

This applies to HI and even more so to CR (see below). In practice, these processes can be fitted either with multiexponential fits, where it must be made clear that the individual time constants obtained do not reflect separate processes, or by using a stretched exponential fit. This stretching of the timescales observed for interfacial processes can also lead to an overlapping of different processes, complicating data analysis. This can lead to uncertain assignment of HI kinetics as they often occur at the same time as other ultrafast processes such as thermal relaxation of excited states.

Furthermore, sometimes the analysis is even more challenging to similar spectral signatures of excited and reduced/oxidised dyes.

It must be pointed out that a large part of the literature on excited state processes in dye-sensitized NiO films comes from the field of DSSC due to its longer history and higher maturity. Consequently, there are few studies on films with a grafted catalyst, as catalysts are not needed in DSSC due to the more easily achievable one-electron redox reaction taking place between the PS and the redox mediator. Therefore, only the reduced PS needs to be produced, and efforts were directed only to produce a very long-lived reduced dye state after excitation. To this end, some dyads were introduced containing dedicated electron acceptor units, that did however not pose the additional challenge of having to be a catalyst.

TA-SEC setup

The principal technique to gain insights on the light-induced processes in dyadsensitized photocathodes used in this work was TA-SEC. For this purpose, a spectroelectrochemical cell was designed that fulfilled the requirements of these challenging experiments:

First, the precise application of a potential to the photocathode had to be ensured to be able to accurately reproduce the applied potential during the photoelectrochemical experiments. An implicit consequence of the applied potential is the investigation of the light-induced processes at different oxidation states of the catalyst unit. In addition, we needed to investigate the ET kinetics in the absence of catalysis to accumulate the Co I state in order to detect it. Therefore, the measurements must be carried out in the absence of water, using dry organic solvent. To perform these experiments, the cell had to be able to house a three-electrode setup with a reference electrode and an auxiliary electrode. In addition, the cell must allow to measure under inert conditions for the duration of the TA experiment, e.g. several hours, principally to prevent water and oxygen from entering the cell, which could react with the reduced state of the catalyst, thus preventing its detection. Also, the dye-sensitized NiO film must be fixed in plane to avoid a loss of focus, while being able to move it in plane to avoid photodamage from the pump beam which was identified as a major issue especially for the organic dye-catalyst assemblies. This was exacerbated by the low concentration of sample molecule on the film surface combined with light scattering on the NiO film, which together demanded long exposure time to obtain a good signalto-noise ratio. In addition, the film must be electrically contacted to be connected to the potentiostat. The final challenge was to limit the path length of the laser beams through the cell to a few millimetres which was important for the detection of ultrafast kinetics such as HI in the fs-TA-SEC experiment. A long path length leads to a loss of temporal evolution due to the chirp caused by dispersion in the electrolyte solution.

The setup used in the TA-SEC experiments in solution (section 3.3.1) did not fulfil all of these criteria: the use of the leakless Ag/AgCl reference electrode, as opposed to a pseudo-reference Ag wire used in the measurements in solution, demanded a wider cell due to its higher diameter (2 mm). In addition, in the setup for solution the film could not be fixed in plane which prohibited movement to avoid photodamage.

Therefore, a spectroelectrochemical cell was designed, largely based on the design of a photoelectrochemical cell by Zahner Elektrik 172 but adapted to fs-TA measurements. ) and sintering at 450°C, followed by sensitization with the molecular components. This film is contacted via a conducting foam to a wire connected to the potentiostat. The reference and auxiliary electrodes are passed through rubber stoppers and a channel into the cuvette created by the cell body and the glass windows. The final path length through the assembled cell is 5-6 mm. For the cell body, a polymer was chosen since metal could not be used as it is a conductor, and the design would have been more difficult to achieve with a cell made of glass. The polymer had to be resistant to the electrolyte used and hard enough to resist the pressure of the flanges. Polytetrafluoroethylene is ideal in terms of chemical inertness, however it is much too soft and was deformed during use. The polymer finally chosen was polypropylene which is more resistant to pressure but limits the choice of solvent.

It is however stable in contact with ACN, which was used as the electrolyte solvent.

In a typical experiment, the cell was assembled in a glovebox under inert conditions.

Once closed, it was tight for the duration of a TA experiment. Oxygen and water levels in the cell were not measured to prove this, but no influence of passed time on the lifetime of the CSSs was observed. Then, TA spectra were recorded at different applied potentials, i.e. during a chronoamperometric measurement, while moving the film in plane to avoid photodamage. This was achieved by mounting the cell on a yz-stage.

The potential of the reference electrode was calibrated versus the ferrocene +/0 couple.

TA-SEC results on RuCo-sensitized NiO films

Prior to the measurements, the sensitized films were washed with water to adjust the protonation state of the imidazole group. Then, TA spectra were recorded of Ru-and RuCo-sensitized NiO films without any applied potential (open circuit potential (OCP)), at -0.74 V and -1.15 V vs. Fc +/0 . -0.74 V vs. Fc +/0 corresponds to -0.4 V vs.

Ag/AgCl, the potential employed in the photoelectrochemical experiments (e.g., the operando potential), and is sufficiently reductive to reduce the Co centre to its Co II state. -1.15 V vs. Fc +/0 served as a control measurement since the Co centre is at its Co I state at this potential, making ET from the reduced or excited dye thermodynamically impossible. The kinetics and the spectral shape were very similar for Ru and RuCo, except for some spectral broadening for the latter, and therefore the results for the two compounds will be discussed together. At the end of the discussion, a focus will be laid on the ET process in the dyad.

TA spectra of NiO films sensitized with Ru and RuCo at early times show a spectrum with GSB at 480 nm and ESA around 590 nm, corresponding to the triplet metal-toligand charge transfer ( 3 MLCT) state (Figure 4.3). At OCP, this spectrum decays almost completely within 7 ns ns except for a weak positive band centred at 520 nm.

At negative applied potentials, however, it develops into a long-lived spectrum with positive bands at 540 and 480 nm, completely losing the GSB. The band at 540 nm is characteristic for the one-electron reduced Ru dye (dash-dot line in Figure 4.3), while the band at 480 nm is tentatively assigned to oxidized Ni centres produced by HI, 173,174 leading to the assignment of this spectrum to the primary CSS produced by HI from the excited dye into the NiO film (formal NiO + |Ru I ). From a three-component exponential fit to the experimental data, the time constant for HI is determined to be Since at more negative applied potentials the initial signal at 480 nm (GSB) is weaker while the signal at 540 nm (reduced Ru dye) is stronger, we postulate an additional ultrafast (< 2 ps) HI process that rises in amplitude at more negative applied potentials.

For RuCo, the ultrafast HI process has higher amplitude than for Ru, reflected in a higher initial ratio of signal at 540 nm to 480 nm. HI is followed by CR, which is strongly affected by the applied potential, increasing from a time constant of tens of picoseconds at OCP to > 7 ns at negative applied potentials, leading to a long-lived CSS. At OCP, the fast CR following the fast HI leads to the overall much faster decay of 3 MLCT signal than in solution. In parallel, there is a process with a characteristic time constant of τ2 = 950 to 1470 ps which shows the spectral signature of a decay of 3 MLCT to the ground state (concerted loss of GSB and ESA). This decay might be produced by an accelerated direct decay of excited molecules that have not undergone HI and thus remain in the 3 MLCT state, due to self-quenching of tightly packed compounds on the NiO surface. 175 Alternatively, a combination of HI and CR can lead to the same spectral result. The lower amplitude and slower rate for this component at more negative applied potentials supports this assignment: HI is faster, decreasing the amount of long-lived 3 MLCT states, and CR is slowed down by the lower number of holes on the NiO surface. Most importantly, the TA spectra of RuCo in the time window up to 7 ns do not show any spectral indication of charge transfer to the cobalt catalyst unit. Such a charge transfer would be observable as an increasing absorption band between 550 and 750 nm, produced by the Co I state in the case of RuCo at -0.74

V vs. Fc +/0 , which is not present in the TA spectra.

Since thermally activated ET in triazole-bridged dyads has been shown to occur on the ns-µs time scale, 132,133,135,136,176 we performed TA experiments under identical conditions in the ns-µs time scale. The TA spectra in this time range are spectrally identical to the last spectra recorded in the fs-TASEC experiment and decay without any spectral changes, with 15% of the initial signal still left after 45 µs (Figure 4.3). The decay occurs in a non-exponential fashion as typical for CR reactions in molecularly functionalized NiO photoelectrodes [START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF]107,108,177 and could be approximated with a three-component exponential fit which yields similar lifetimes under all experimental conditions (τ1 ≈ 86 ns, τ2 ≈ 1.4 µs, τ3 ≈ 14 µs, Table 4.1), with an average lifetime of τave = 3.6 to 5.2 µs. In these experiments, an additional band was observed at 390 nm which in the fs-TA-SEC experiment was blocked by the pump scatter, and which was also observed in UV-Vis SEC measurements upon one-electron reduction of Ru. Since this band is produced by the one-electron reduced bpy ligand, it seems that the excess electron is partly localized on the anchoring bpy ligands, possibly through thermally activated interligand ET from the extended ligand. This partial localization on the anchoring ligand could lead to faster CR due to the spatial proximity to the NiO surface (Figure 4.4), as previously discussed for related ruthenium complexes. 45 Overall, by applying a reductive potential, the lifetime of the CSS was increased from hundreds of picoseconds to up to 15 µs, an increase of up to 5 orders of magnitude.

However, in spite of the long-lived CSS, no direct evidence for ET to the cobalt catalyst could be observed: within the experimental signal to noise, the spectral signature of the Co I state did not appear in the TA spectra of RuCo-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 Ru-sensitized films. On the basis of the ratio of the extinction coefficients of the reduced dye and the Co I state from the UV-Vis SEC experiment, and an experimental noise of 0.52 mOD, the limit of detection at a signal-to-noise ratio of 2:1 is at 12% efficiency for the ET in the ns-TASEC experiment, meaning that less than 12% of the reduced state of the Ru dye undergoes ET.

Influence of the applied potential on the interfacial processeshole injection

From the TA-SEC data it becomes apparent that the applied potential influences the kinetics of the interfacial charge-transfer processes, e.g. HI and CR, as observed before. 107,108 HI in Ru(Co)-sensitized NiO films occurs in a biphasic fashion with an ultrafast (τ0 < 2 ps) and a slower component that takes place in tens of picoseconds (τ1 = 30 to 70 ps). This is in line with the published literature, where HI in dye-sensitized NiO films was observed in some cases in < 1 ps to some ps, [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF][START_REF] Põldme | Photoelectrocatalytic H2 Evolution from Integrated Photocatalysts Adsorbed on NiO[END_REF][START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF][START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Brown | Ultrafast Electron Transfer between Dye and Catalyst on a Mesoporous NiO Surface[END_REF][START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF]107,125,[178][179][180][181][182][183][184][185][186][187][188][189][190][191][192] while filling the trap states with electrons, thereby raising the Fermi level and producing an electron-filled surface. This leads to a higher driving force for HI, resulting in faster kinetics and especially higher amplitude ultrafast HI. It was also observed that for

RuCo, especially at more negative potentials, the ratio of the amplitude of the ultrafast to the slow HI component is higher compared to Ru. This could be caused by an influence of the triazole linker on the ligand energy levels, if the presence of the triazole leads to a faster localization of the excited electron on the extended ligand.

Influence of the applied potential on the interfacial processescharge recombination

The second interfacial process influenced by the applied potential is CR of This behaviour was also observed in the literature on dye-sensitized NiO films: as for HI, CR occurs in a highly non-exponential fashion, stretching over timescales of several orders of magnitude. In most of the studied dye-sensitized NiO films, CR occurs on a sub-ns timescale with time constants of tens to hundreds of ps. [START_REF] Bräutigam | Hole Injection Dynamics from Two Structurally Related Ru-Bipyridine Complexes into NiOx Is Determined by the Substitution Pattern of the Ligands[END_REF][START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF][START_REF] Põldme | Photoelectrocatalytic H2 Evolution from Integrated Photocatalysts Adsorbed on NiO[END_REF][START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF][START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF]101,107,125,[178][179][180][181][182][183][184][185][186][187][188][189][190][191][192][193] Indeed, CR can be so fast that no signal of the reduced PS is observed which can also complicate the assignment of the HI process. However, the generally faster excited state decay than that observed for the PS in solution, due to this fast HI and CR, can be a hint for the existence of both, even if the reduced PS is not observed. Despite this fast recombination and due to the highly non-exponential behaviour of CR on NiO films, there is often some reduced dye still observed on the ns-µs timescale. [START_REF] Bräutigam | Hole Injection Dynamics from Two Structurally Related Ru-Bipyridine Complexes into NiOx Is Determined by the Substitution Pattern of the Ligands[END_REF][START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF][START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF][START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Brown | Ultrafast Electron Transfer between Dye and Catalyst on a Mesoporous NiO Surface[END_REF]101,177,[180][181][182][183][184]186,188,190,191,193,194,[196][197][198][199] However, this is usually only a small amount (< 10 %) of initially present CSS, but in some cases more efficient retardation of CR has been achieved. The most successful strategy to extend the lifetime of the CSS was installing a secondary electron acceptor such as NDI or PMI on the dye to which ET occurs after HI. 101,181,190 A certain degree of success was also achieved by introducing electron-withdrawing trifluoromethyl groups on the ligands of a Ru(bpy) 3 complex 193 or longer linkers between the Ru centre and the anchoring groups. 188 Some studies only present data recorded on the ns-µs time scale, which does not allow to determine the yield of CSS since the fast CR on the sub-ns timescale cannot be observed. Due to the higher pump power used in ns-TA setups, even small amounts of CSS can still be observed, and so the slow CR part of a multiexponential (fast) decay could mistakenly be assumed to be the only CR process taking place in a system, leading to an overestimation of CSS average lifetime.

For RuCo-sensitized NiO films at more negative applied potentials, the fast CR channel is increasingly deactivated, leading to a build-up of CSS that lives for up to 15 µs compared to hundreds of picoseconds at OCP, an increase of up to five orders of magnitude. To explain this strong increase in lifetime, the reason for the fast CR at OCP must be determined and the mechanism by which the application of a negative potential eliminates it. A first hint as to the reason for the fast CR came from a study using a series of PMI dyes with increasing driving force for recombination. Instead of a Marcus inverted behaviour as expected for the large driving forces calculated for CR with holes in the NiO VB, a Marcus normal behaviour was observed. 186 This led to the idea that CR actually takes place with intragap trap states, for which the driving force for CR should be lower and lie in the Marcus normal region. Further insight comes from the few studies that employed TA-SEC. The first study to use this technique did not investigate the exact role of NiO, but did observe potential-dependent kinetics of CR for a PMI-oligothiophene-TPA push-pull dye, with lifetimes increasing from ca.

2 µs at -0.15 V vs. Fc +/0 to a biphasic decay with 50-100 µs and ms lifetime at -0.9 V vs Fc +/0 . 199 A similar long-lived CSS was observed for a polymer consisting of a Ru(bpy)3

PS and a Re catalyst for CO2 reduction, immobilized on NiO and at an applied potential of -0.5 V vs. NHE: the reduced Ru(bpy)3 polymer alone showed a lifetime of 18 µs (from a Kohlrausch-Williams-Watts stretched exponential fit), while with the Re catalyst this was reduced to 6 µs due to ET to the catalyst, the reduced state of which recombined in 1.7 ms. 200 Another study looked more closely at the origin of this dependence in NiO films sensitized with Ru-NMI in a potential range of -0.54 to 0.36 V vs Fc +/0 : D'Amario and coworkers observed a biexponential decay of the reduced dye species, with a fast decay with a potential-independent lifetime of ca. 15 ns whose amplitude increased at positive potential and a slow decay whose rate increased at more positive potential (lifetime decreased from 500 s to 700 µs), with a high dispersion of rates. 108 Based on these observations, the authors proposed that CR takes place via two pathways: the fast decay would be due to CR with the injected hole in the valence, while the slow decay results from CR with the injected hole after its relaxation into the intragap trap states. A somewhat different explanation was proposed by Dillon and coworkers in what is to date the only TA-SEC study on dye-sensitized NiO films that investigated the ultrafast processes in a fs-TA setup. 107 They observed faster HI at more negative applied potential, until the multiexponential HI kinetics became biexponential at very negative applied potential with 2.8 and 39 ps time constants, and the yield of reduced dye increased from 0% at 0.4 V to 91% at -0.4 V vs. Ag/AgCl. This was ascribed to a retardation of CR. To obtain a good fit for their data, the authors proposed the model presented in section 4.1.2.3, consisting of predominantly electron-filled and predominantly electron-depleted surface sites. Applying a negative potential to the NiO film reduces these Ni sites to Ni 2+ , thus filling the intragap states and producing an electron-filled surface (Figure 4.5). This, in turn, leads to slower CR: from their model, the authors obtained a ratio between the fast and slow CR times on the sub-ns data, which increased from 1:1 at 0.4 V to 1:13,000 at -0.4 V vs. Ag/AgCl. In a ns-TA SEC experiment, it was reflected in a rise of the characteristic time constant for CR from 430 ns at -0.2 V to 750 ns at -0.6 V vs. Ag/AgCl. Note that these experiments were performed with Ru(bpy)3 as sensitizer, which in another study at OCP was found to not inject holes from its excited state due to the localization of the LUMO at the anchoring ligand, [START_REF] Bräutigam | Hole Injection Dynamics from Two Structurally Related Ru-Bipyridine Complexes into NiOx Is Determined by the Substitution Pattern of the Ligands[END_REF] which should also accelerate CR from the reduced dye.

The lifetime obtained for the CSS in Ru(Co)-sensitized NiO films is thus longer than that observed for Ru(bpy)3 at applied potential, 107 in which the electron is located unfavourably close to the NiO surface, but much shorter than that of the Ru(bpy)2bpy-NDI complex, which possessed the additional electron acceptor NDI. 108 On this timescale, the influence of the increasingly negative potential when going from -0.74

V to -1.15 V vs. Fc +/0 seems to be weak as it only leads to a 10-40 % increase in lifetime.

In the fs-TA-SEC experiment, however, the signal of the CSS increased two-fold when going to -1.15 V vs. Fc +/0 , suggesting that the overall yield of long-lived CSS is still significantly enhanced at more negative potential.

Electron transfer

Arguably the most important process for the use of the dyad in light-driven proton reduction is the ET process from the excited or reduced dye to the catalyst unit. In RuCo-sensitized NiO films, no evidence for such a process could be observed in the TA-SEC experiments. Firstly, there was no direct spectral evidence of the formation of the Co II species (in the OCP measurement) or the Co I species (in the measurement at -0.74 V vs. Fc +/0 ) that would form upon such an ET process. Secondly, the decay kinetics of the CSS on the Ru dye are not accelerated in RuCo in comparison to Ru, which would be a sign for a competing process such as ET. However, from photoelectrochemical experiments it is known that to some degree, ET must occur since hydrogen is produced. Since the TON after 30 minutes is only 11, it stands to reason that the ET to the catalyst might be very inefficient. Thus, the amount of Co I formed might simply lie below the limit of detection of the TA setup. Considering a signal-to-noise ratio of 2:1 as detectable, the ET efficiency was estimated to be below 12 % on the ns-µs timescale. In light of the low TON achieved, a much lower efficiency seems plausible.

Similar observations have been made in the literature: in a dyad especially relevant to the Ru complexes studied in this thesis, i.e. based on a Ru(bpy)2ip PS and a copper diimine-dioxime complex as acceptor linked by a triazole group, no ET could be unequivocally observed: 145 In the dyad, the excited state lifetime of the PS was reduced compared to the PS alone (380 ns vs. 1.1 µs), but the CSS could not be detected. In addition to the issues known for triazole linkers in such donor-acceptor dyads (see section 3.3.1), a study by Mulfort and co-workers suggests that indeed the combination of a Ru complex donor with a Co complex acceptor might be the issue: they could not observe ET in either of two such assemblies, whether they were linked by a methylene spacer or the Co was coordinated to an extended tetrapyridophenazine ligand of the Ru complex. [START_REF] Kohler | Photoinduced Electron Transfer Kinetics of Linked Ru-Co Photocatalyst Dyads[END_REF] In contrast, Ru-Pt and Ru-Pd assemblies also prepared by coordinating the catalytic metal centre to extended ligands on the Ru PS have shown fast ET to the catalytic centre in the sub-ns region. [START_REF] Kowacs | Supramolecular Bimetallic Assemblies for Photocatalytic Hydrogen Generation from Water[END_REF][START_REF] Tschierlei | Photophysics of an Intramolecular Hydrogen-Evolving Ru-Pd Photocatalyst[END_REF]201,202 This could suggest that the Co complexes employed are poor acceptors for photoinduced intramolecular ET.

Due to the relative novelty of the DSPEC field, there are few studies containing timeresolved spectroscopy on dye-sensitized NiO films that contain a co-grafted catalyst [START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Brown | Ultrafast Electron Transfer between Dye and Catalyst on a Mesoporous NiO Surface[END_REF]177,182 or dye-catalyst dyads. [START_REF] Põldme | Photoelectrocatalytic H2 Evolution from Integrated Photocatalysts Adsorbed on NiO[END_REF][START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF]198,200 This might be partly due to the fact that the catalysts employed typically do not have strong optical transitions, which complicates the analysis by UV-Vis TA spectroscopy. However, visible-pump midinfrared-probe TA spectroscopy has been employed to study the CO vibrational bands of FeFe catalysts. 182 In combination with C343 as PS, following ultrafast HI to yield the reduced C343, fast ET to the catalyst took place in some ps. [START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Brown | Ultrafast Electron Transfer between Dye and Catalyst on a Mesoporous NiO Surface[END_REF] The lifetime of the reduced catalyst depended on the anchoring group employed: a carboxylic acid group directly on the coordinating phenyl group led to fast CR in 2 and 30 µs, [START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Brown | Ultrafast Electron Transfer between Dye and Catalyst on a Mesoporous NiO Surface[END_REF] while the use of a longer linker extended the lifetime to up to 20 ms. [START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF] When this catalyst was cografted with the push-pull dye E2, ET took place from the excited state of the dye instead and this led to fast CR between dye and catalyst. 182 As for dye-catalyst dyads studied on NiO, the number is also very limited. [START_REF] Põldme | Photoelectrocatalytic H2 Evolution from Integrated Photocatalysts Adsorbed on NiO[END_REF][START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF]198,200 The first study was on a porphyrin-Re dyad, in which HI was observed in < 30 ps and CR from the porphyrin dye in 180 ns, however only ns-TA data was recorded. ET to the catalyst was not directly observed but assumed to occur due to previous results in solution. 198 In Ru-Pd and Ru-Pt dyads, HI was observed within the pump pulse and CR in 30 ps, with some residual 3 MLCT population which decayed in 1 ns. The analysis was difficult due to overlapping signals for the CSS and residual excited state in both dyads, as well as the multiexponential decay kinetics as mentioned before. [START_REF] Põldme | Photoelectrocatalytic H2 Evolution from Integrated Photocatalysts Adsorbed on NiO[END_REF] The CSS with the reduced catalyst was not unequivocally observed. In PB-2, an organic dyad comprised of a push-pull dye clicked to a cobalt diimine-dioxime catalyst, fast HI in 1 and 30 ps and fast CR in 350 ps was observed, as expected for the organic dye. [START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF] The authors proposed ET to the catalyst in 330 ps based on the faster decay kinetics of the reduced dye, but did not obtain spectral evidence of the reduced catalyst and therefore could not determine the lifetime of the reduced catalyst. In the only example where this was possible, a negative potential of -0.5 V vs. NHE was applied to the film to retard CR (see section 4.1.2.4), but only ns-TA data was recorded. 200 The molecular component in this case was a polymer structure containing Ru(bpy)3 as PS and a Re catalyst. While the Ru-polymer alone showed CR with an average lifetime of 18 µs (from a stretched exponential fit), in the dye-catalyst polymer this was decreased to 6.4 µs due to ET to the catalyst, from which CR occurred in 1.7 ms. In summary, for a mixture of several reasons -low extinction coefficents of catalyst optical transitions, fast CR on NiO films without applied potential, leading to overlapping of different processes -the observation of reduced catalyst on dye-sensitized NiO films and the determination of its lifetime remains rare in the literature.

In our case, since the formation of the reduced catalyst state could not be observed, its lifetime could not be assessed, but it is likely higher than that of the reduced Ru complex due to its longer distance from the NiO surface and lower driving force for recombination, as observed for the Ru complex with a NDI acceptor group which showed several orders of magnitude higher lifetime than the Ru(bpy)3 alone. 107,108

Summary and Outlook

In summary, a long-lived CSS on the Ru dye was achieved at applied potential, but ET to the cobalt catalyst was not observed which seems to be the main impediment to high activity of the dyad (Figure 4.8). This low activity, in addition to the low stability of the assembly, leads to the low TON observed in photoelectrochemical experiments.

Future designs of dye-catalyst dyads should therefore aim to improve the ET kinetics (see section 4.3).

A sophisticated spectroelectrochemical cell was built that enabled TA measurements on films at applied potential under inert conditions that were stable for hours. The cell thus permitted long measurement times, important to obtain a good signal to noise ratio in dye-sensitized films which typically have low signal intensities and high noise due to scattering. It was used to perform the first-ever TA measurements at operando potential on NiO films sensitized with a dye-catalyst dyad (RuCo) that were functional for photoelectrochemical H2 evolution. These measurements showed that the low activity of the photocathodes was due to a slow and inefficient ET to the catalyst from the reduced Ru complex. 4.12; N, 9.23; found: C, 39.01; H, 4.53; N, 8.74.

Experimental section

Synthesis of

Synthesis of RuP4

OEt -Co. RuP4 OEt -Cu (110 mg, 0.05 mmol) and CoBr2 hydrate (100 mg, 0.45 mmol, 9 eq) were solubilized in a mixture of acetone/methanol 9 / 1 (15 ml) and stirred under air bubbling overnight. The organic solvents were evaporated and the crude product was purified by column chromatography on siliga gel using a 90:9:1 acetone/H2O/KBr (sat. aq. solution) eluent mixture. The organic solvents were evaporated and the product was extracted with dichloromethane after the dropwise addition of a saturated KPF6 aqueous solution. After evaporation of dichloromethane and drying under vacuum, RuP4 OEt -Co was isolated as an orange powder. Yield: 60 % (70 mg). H, 4.04; N, 9.26; found: C, 39.21; H, 4.44; N, 8.93. At the end of the synthesis, we observed that RuP4 OEt -Co was isolated under a protonated imidazole (ImH2 + ) form (see acido-basic titration Figure S2). A similar observation was made for the Ru-imidazole precursor RuP4 OEt -EPIP. For a related Ruimidazole complex, Aukauloo and coworkers determined pKa values of 3.1 for the first protonation (ImH2 + ↔ ImH) and of 8.7 for the second protonation (ImH ↔ Im -) of the imidazole group in aqueous solution 168 . This supports that RuP4 OEt -Co is under its neutral Im form under our operating photoelectrochemical conditions (pH 5.5). However, as the imidazole-protonation state might affect intramolecular electron transfer efficiencies (either photoinduced or thermal electron transfer processes), spectroscopic data were recorded in acetonitrile solution both in the absence and in the presence of 1 equivalent of base (triethylamine) and transient absorption -spectroelectrochemical (TA-SEC) measurements were recorded on sensitized films that have been washed

with distillated water beforehand.

Synthesis of RuP4-Co. RuP4

OEt -Co (16 mg, 0.007 mmol) was added in an oven-dried schlenk under argon and was solubilized in 2 ml of dry dichloromethane. Under argon, TMSBr (21 µl, 0.17 mmol) was added and the reaction mixture was stirred at room temperature, under argon for 3 days. After evaporation of the solvent, 5 ml of methanol was added and the solution was stirred for 3 hours. The organic solvent was evaporated and the orange powder was dried under vacuum. Yield: 90 % (13 mg). The deprotected dyad was then immediately employed for the NiO film sensitization.

The same procedure was used to prepare RuP4-EPIP from RuP4 OEt -EPIP.

fs-TA spectroscopy. A custom-built setup was utilized to acquire fs-TA data. The TA setup is described in detail elsewhere 203 . A white-light supercontinuum probe pulse at The pump pulses used were of 400 nm wavelength and a pulse duration of approximately 100 fs. The repetition rate of the pump pulses was reduced to 0.5 kHz by a mechanical chopper and the polarization adjusted to the magic angle of 54.7° by a Berek compensator and a polarizer. The data was analyzed using a customized data analysis software written by Jens Uhlig (Lund University). First, the data was corrected for the chirp and subsequently, a sum of exponential functions was fitted to the data.

The artifact region of ±1 ps was removed from the data during the fitting process to exclude the coherent artefacts present in this temporal region. Preparation of NiO films for spectroelectrochemistry. NiO films were prepared using a F108

precursor solution prepared by mixing 2 g of NiCl2, 2 g of F108 triblock copolymer in a mixture of ethanol (15 mL) and Milli-Q water (6 mL), according to our previously reported procedures [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] . The mixture was sonicated for 6 h, centrifuged and passed through a 45 µm pore size polyethersulfone syringe filter. Round ITO-coated glass substrates (Solems ITO SOL 30, 1 mm thickness) were cleaned by sonication in isopropanol, distilled water and ethanol for 10 min each. The F108 precursor solution was then spin-coated on the substrates (5000 rpm, 60 s) followed by sintering in an oven in air (30 min ramp to 450 °C, 30 min at 450 °C). Spin-coating and sintering was repeated for a total of four layers. The NiO films were then soaked in methanolic solutions (0.2 mM) of RuP4-EPIP and RuP4-Co at room temperature for 24 h. After sensitization, films were rinsed with methanol and dried with pressured air. To deprotonate the initially protonated species that results from the synthesis, the films were then soaked in distilled water for 5 minutes and rinsed with dry acetonitrile.

TA-SEC measurements. TA-SEC experiments on films were performed using a home- The results confirm recent studies and thus consolidate the picture of the influence of the applied potential on the interfacial process kinetics in NiO films sensitized with Ru complexes, affirming the need to apply a potential to the NiO film to enable a longlived charge separation due to the high amount of intragap trap states at the NiO film surface. This is prohibitive to the use of NiO as p-type SC material in DSPEC for biasfree water splitting as fast CR prevents catalytic turnover and thus decreases the DSPC activity. To overcome this, methods will have to be developed that eliminate the intragap trap states at the NiO film surface and can therefore eliminate the fast CR pathway. Instead, alternative p-type SC could also be developed that do not present such a high density of trap state. So far, the search for better-performing p-type SC materials has been difficult.

The TA-SEC results also suggest the existence of distinct slow and fast HI and CR pathways, depending on whether the excess electron density is localized on the anchoring or the extended ligand. This insight might help in the ligand design of future molecular systems by trying to localize the excess electron density even more strongly on the extended, non-anchoring ligand. This should furthermore lead to faster ET transfer to the acceptor unit.

On the issue of ET from the reduced dye to the cobalt catalyst, the results challenge the use of the triazole linker even further: despite the conjugated linker and long lifetime of the reduced donor unit, i.e. the Ru complex, no ET could be observed. This suggests that the triazole group is a poor conductor of charge, at least under the conditions present here. Therefore, future synthetic efforts should foremost be concerned with introducing a different linker structure that enables fast ET to the catalyst.

Organic covalent dye-catalyst assemblies

The main part of the work on DSPCs performed in this thesis was done on a series of dye-catalyst assemblies based on the organic push-pull dyes (T1, T2R) linked to cobalt catalysts (Co, Cat1) (Figure 4.9). All four combinations of the two different dyes with the two different cobalt catalysts were studied to investigate the respective influence of both building blocks on performance, under otherwise identical experimental conditions. The dyad T1-Co had been previously published, [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] but was re-examined under identical conditions to the other dyads to allow comparison to the newly synthesized dyads.

As mentioned in section 3.2, introducing the rigid CPDT linker led to a bathochromic and hypochromic shift in the absorption spectrum (λmax = 493 nm, εmax = 57,400 M -1 •cm -1 in MeOH) for T2R in comparison to T1, making T2R the better light-absorbing unit, as observed before in the literature. 126,127 The alkyl side chains on the linker were intended to prevent desorption of the compounds from the NiO surface by protecting the anchoring groups from the aqueous electrolyte. [START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF]205 The catalyst Cat1 has shown good activity and especially stability for photocatalytic proton reduction in aqueous solution. [START_REF] Gueret | Challenging the [Ru(Bpy)3]2+ Photosensitizer with a Triazatriangulenium Robust Organic Dye for Visible-Light-Driven Hydrogen Production in Water[END_REF][START_REF] Varma | Efficient Photocatalytic Hydrogen Production in Water Using a Cobalt(III) Tetraaza-Macrocyclic Catalyst: Electrochemical Generation of the Low-Valent Co(I) Species and Its Reactivity toward Proton Reduction[END_REF][START_REF] Grau | Electrochemically and Photochemically Induced Hydrogen Evolution Catalysis with Cobalt Tetraazamacrocycles Occurs Through Different Pathways[END_REF]128,[206][207][208] The stability is conferred to the complex by its macrocyclic ligand that resists decomplexation and hydrolysis, while cobaloximes-based catalyst such as Co are known to decompose under aqueous photocatalytic conditions via demetallation, 209 hydrolysis and/or reduction of the oxime-based ligands, [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine-Dioxime Complexes[END_REF][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]210,211 as already observed for RuCo.

For these dyads, and especially for the best-performing T2R-Cat1, the wide range of analytical and spectroscopic techniques mentioned in the introductory paragraph were used to determine their activity, stability, degradation processes and investigate the light-induced processes. Where possible, time-resolved measurements were conducted at the operando potential to reflect the system's behaviour in operando.

The NiO photocathodes were prepared by spin-coating and sintering a F108templated NiCl2 sol-gel [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF][START_REF] Li | Double-Layered NiO Photocathodes for p-Type DSSCs with Record IPCE[END_REF]100 to give 4-layered films with a thickness of 880 ± 170 nm as determined by scanning electron miscroscopy. The films were sensitized by soaking for 24 h in methanolic solutions of the deprotected dyes and dyads. The grafting densities, determined by desorption of the molecules from the surface followed by measuring UV-Vis absorption spectra of the resulting solution, were in the range of 6 to 14 nmol•cm -2 , similar to those observed in literature on other dye-sensitized NiO films. [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][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]121 As the grafting density varies between films and especially between different batches of prepared NiO, the grafting density was determined for each

(Photo-)electrochemistry and hydrogen production activity

In photoelectrochemical experiments in 2-(N-morpholino)ethanesulfonic acid buffer at pH 5.5, all dyads showed a photocurrent produced by irradiation with visible light.

In linear scan voltammograms, photocurrent is observed below an applied potential of ca. 0.95 V vs. relative hydrogen electrode (RHE) and increases substantially below 0.5 V vs. RHE (-0.06 V vs. Ag/AgCl). All chronoamperometric measurements were carried out at -0.4 V vs. Ag/AgCl applied potential (0.14 V vs. RHE). In chronoamperometric experiments under chopped light irradiation, cathodic and anodic current spikes were detected upon turning the light on and off, respectively.

This was ascribed to charge accumulation at the electrode-electrolyte interface and will be explained in the section on TA-SEC measurements. NiO films sensitized with just the dyes T1 and T2R show very low photocurrents, which is enhanced ten-fold by the introduction of the Cat1 catalyst in the dyads with up to -6.3 µA•cm -2 for T2R-Cat1sensitized films after 20 minutes (Figure 4.10). Interestingly, the T1-Co dyad initially showed by far the highest photocurrent which decayed rapidly. While it is not entirely clear why this dyad shows this behaviour, it might be a product of a fast degradation of the catalyst unit, as the hydrogen production -the activity of the system -does not correlate with the high photocurrent. To compare the intrinsic activity of the dyads, the photocurrent was normalized to the grafting density of the photocathodes: the Cat1 and T2R dyads showed slower decay in photocurrent and higher overall photocurrent than their Co and T1 counterparts, respectively and cumulatively: on average, the Cat1 dyads produced 3.2 times higher current than the Co dyads and the T2R dyads 1.7 times as much as the T1 dyads, for a total 5.4-fold increase of T2R-Cat1 over T1-Co, making T2R-Cat1 the most active dyad.

This was also true when looking at the hydrogen produced, where T2R-Cat1 performed best both in terms of activity -the rate of hydrogen production -and stability -the duration of hydrogen production. After the first four hours of irradiation, the Cat1 dyads achieved TON values for hydrogen production of 13 ± 1 (T1-Cat1) and 22 ± 7 (T2R-Cat1) and Faradaic efficiencies around 65 %. The activity of the Co dyads was much lower, with TON values below 2 and Faradaic efficiencies of ≈ 10 %, as observed before for dyads based on the same diimine dioxime catalysts. [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][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] Continuing irradiation for a total of 22 hours almost doubled the TON values for the Cat1 systems, while the Co dyads showed no increase in produced hydrogen, meaning they had lost their activity during the first 4 hours, as also observed for RuCo. Thus, after 22 hours, the Cat1 dyads produced 26 times more hydrogen than their Co counterparts. This is largely thanks to the high increase in Faradaic efficiency and long-term stability of Cat1 in aqueous medium, as already observed in solution photocatalytic studies; [START_REF] Varma | Efficient Photocatalytic Hydrogen Production in Water Using a Cobalt(III) Tetraaza-Macrocyclic Catalyst: Electrochemical Generation of the Low-Valent Co(I) Species and Its Reactivity toward Proton Reduction[END_REF] and which is more generally observed for polypyridinic and aminopyridinic cobalt catalysts. [START_REF] Varma | Efficient Photocatalytic Hydrogen Production in Water Using a Cobalt(III) Tetraaza-Macrocyclic Catalyst: Electrochemical Generation of the Low-Valent Co(I) Species and Its Reactivity toward Proton Reduction[END_REF]115,[212][213][214] The low Faradaic efficiency of Co, on the other hand, could be a result of the various degradation processes that plague cobaloximes-based catalysts. [START_REF] Kaeffer | Hydrogen Evolution Catalyzed by Cobalt Diimine-Dioxime Complexes[END_REF][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][209][210][211] In addition, the T2R dyads showed 1.7-fold higher hydrogen production than their T1 analogues, correlating with the higher photocurrent. The individual influence of both the catalyst and dye units could thus be quantified. It becomes clear that the nature of the catalytic centre was key to the photocathode performance, much more than the exact dye structure.

Operando and post-operando characterization

To evaluate the nature and the kinetics of the deactivation processes of the photocathodes, they were studied post-operando by ICP-MS and UV-Vis spectroscopy as well as in-operando by time-resolved UV-Vis spectroelectrochemistry. Postoperando UV-Vis spectra of solutions obtained by desorption from the used films revealed that the π-π* band on the TPA was only slightly reduced in intensity, more so for T1 than for the T2R systems. As a reduction in band intensity can be attributed to desorption from the film surface during operation, it seems that desorption was no major issue affecting the stability of the systems. The increased resistance of the T2R systems against desorption confirmed the design choice of using hydrophobic alkyl chains to protect the anchoring groups from hydrolysis by the aqueous electrolyte. [START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF] The low solubility of the organic compounds in water can be another factor improving the stability of the anchorage.

At the same time, the ICT band shows a sharp decrease in intensity while a new band appears at higher energies. Time-resolved UV-Vis SEC experiments under operando conditions (Figure 4.11) showed that i) the spectral changes take place only in photoelectrochemical conditions, i.e. with both applied potential and irradiation, and ii) that they do so with characteristic time constants of 1 and 90 min for T2R-Cat1, and 0.75 and 21 min for T2R. Irradiation alone led to a slight spectral change attributed to photoisomerization on the cyanoacrylate double bond, 113 while the applied potential by itself did not cause any changes. UV-Vis SEC of the T2R and T1 dye in dry ACN

shows an identical irreversible spectral change upon reduction of the dyes. In conclusion, the dye structure degrades in its reduced state irrespective of the surrounding solvent. The exact structure of the degradation product could not be determined, although the observations that i) the π-π* absorption of the triarylamine group is still present, and ii) a similar alteration is observed for T1 and T2R under reductive conditions suggest that the degradation occurs on the acceptor group of the push-pull structures, i.e. the cyanoacrylate/cyanoacrylamide groups. Since T2R

showed faster degradation kinetics than T2R-Cat1, it seems that there is an additional process competing with degradation in the dyad, which TA-SEC measurements showed was ET to the catalyst (see section 4.2.3). In addition, post-operando ICP-MS measurements performed for T2R-Cat1 showed that a high amount of cobalt is lost during operation (84 % after 4 hours, 93 % after 22 h), indicating a loss of the catalytic unit or demetallation. The clear correlation found between the photocurrent decrease during the 4-hour photoelectrochemical measurement, the loss of absorbance, and the loss of Co from the surface, strongly suggests that i) the dye degradation is the main deactivation process in our system and ii) it involves a bond breaking leading to the detachment of the catalytic centre from the assembly. The dyads remaining intact on the surface (16 % after 4 h) account for the H2-evolving activity still detected after 4 hours under irradiation. Assuming the correlation between the photocurrent decrease and the loss of catalytic unit holds true, the average surface concentration of intact dyads present during the first four hours of activity was estimated to be 2.0 ± 0.4 nmol•cm -2 , leading to a more accurate TON value of 94 ± 16 per active dyad, corresponding to a TOF of 6.5•10 -3 s -1 (0.4 min -1 ). By contrast to other studies, where dye/catalyst desorption from the SC film is the main reported deactivation process, [START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF][START_REF] Sahara | Photoelectrochemical Reduction of CO2 Coupled to Water Oxidation Using a Photocathode with a Ru(II)-Re(I) Complex Photocatalyst and a CoOx /TaON Photoanode[END_REF][START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF] only a low amount of dyads is released into solution.

TA-SEC

As for RuCo, TA-SEC measurements on dye-sensitized NiO films were also carried out with the best-performing system T2R-Cat1 and the T2R dye as a reference. Again, TA spectra were recorded at OCP, the operando potential (-0.74 V vs. Fc+/0 ≙ -0.4 V vs Ag/AgCl) where the cobalt catalyst is reduced to the Co II state and a more reductive potential (-1.15 V vs. Fc +/0 ) where it is reduced to the Co I state. The latter serves as a reference measurement since ET is not thermodynamically possible from the reduced dye to the Co I state of the catalyst.

As the spectral shape and initial kinetics of T2R-Cat1 and T2R-sensitized films are identical, they will be discussed together. The TA spectra initially show GSB at 490 nm and ESA at > 550 nm (Figure 4.12). The ESA evolves in some ps to form a band at 620 nm. At OCP, this signal decays completely within 1.6 ns together with the GSB. At negative applied potentials, it is increasingly long-lived and reaches the ns-ms scale.

In addition, T2R-Cat1-sensitized films showed a signal at 430 nm on the ns-µs timescale which did not decay completely in 1 ms and which was was absent at all other conditions (T2R at all potentials and T2R-Cat1 at -1.15 V vs. Fc +/0 ).

Kinetic and spectral analysis of the data showed that the initially populated excited state undergoes ultrafast HI in < 1 ps to form the primary CSS with an electron localized on the organic push-pull dye. On this timescale the TA spectra change from that characteristic of the vibrationally hot, excited state of the dye (GSB at around 500 nm and ESA maximum at 680 nm) to that of the reduced dye, which produces the band observed at 620 nm. This was supported by UV-Vis spectroelectrochemical measurements. Contrary to the case of Ru(Co), no influence of the applied potential on the principal HI kinetics was observed since the kinetics are already ultrafast even at OCP. This can be explained by the closer spatial proximity of the HOMO to the NiO surface and the absence of retarding factors such as electron density on the anchoring ligands, as in the case of Ru(Co). In general, such ultrafast HI is often observed for organic dyes on NiO. [START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF][START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Brown | Ultrafast Electron Transfer between Dye and Catalyst on a Mesoporous NiO Surface[END_REF][START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF]101,125,[178][179][180][181][182][183][184]186,[190][191][192] A second process taking place in some ps spectrally shows a further decay of residual excited state of the dye, which probably occurs through a slower HI component and fast CR, since amplitude and rate decrease at more negative applied potential. The third process, and the most affected by the applied potential, is CR: the characteristic time constant for CR increases from ≈ 150 ps at OCP to >> 2 ns at applied potential. At -0.74 V, the average lifetime of the primary CSS, obtained by a multiexponential fit of the non-exponential decay, was 28 µs for T2R and 7 µs for T2R-Cat1, which at -1.15 V vs Fc +/0 increased to 131 µs (67% relative amplitude) and < 1 ms (33 % rel. ampl.) for T2R and similar values for T2R-Cat1. It

shows the same non-exponential behaviour as observed for RuCo and in literature. 107,108 The explanation for the influence of the applied potential on the CR kinetics is the same as that presented for Ru(Co) (section 4.1.2.4).

Most importantly, the signal observed at 430 nm is caused by the Co I state of the catalyst unit, as confirmed by UV-Vis SEC measurements. The presence of this signal confirms ET from the reduced state of the dye to the catalyst to form the catalytically active state. The major part of this ET takes place in 1.6-20 ns and is thus much slower than HI. Therefore, in T2R-Cat1-sensitized NiO films, charging of the catalyst occurs through a reductive quenching mechanism, in which the reduced dye is formed first, and ET is thermally activated. The sub-20 ns ET time constant is quite fast compared

to other examples of thermally activated ET in triazole-bridged donor-acceptor dyads in solution, which takes place on the ns-µs timescale. [132][133][134][135][136] ET from a Ru dye to a Ni catalyst in a layer-by-layer assembly was determined to occur before 50 ns, either from the excited or reduced state of the dye. [START_REF] Shan | Photogeneration of Hydrogen from Water by a Robust Dye-Sensitized Photocathode[END_REF] A recent study on a Ru-Re system anchored on NiO by electropolymerization also showed ET on the ns-µs scale (< 10 µs), which was attributed to electron tunnelling between the components due to the presence of an electronically insulating alkyl linker, 200 as is also the case in T2R-Cat1. This casts doubts on the design of the T2R-Cat1 dyad, since its linker contains both an insulating methylene group and a triazole group, the use of which as a linker in donor-acceptor dyads is disputed in literature (see section 4.1.2.5). This translates into a slow ET process in comparison to some examples of co-grafted systems where ET from the dye (excited or reduced state) to the catalyst was observed on the ps timescale. [START_REF] Brown | Ultrafast Electron Transfer between Dye and Catalyst on a Mesoporous NiO Surface[END_REF]182 Assuming the faster decay of the primary CSS in T2R-Cat1 compared to T2R is due to a slower ET process, this process would take place in a non-exponential fashion with an average time constant of 9 µs. However, the ET observed is not quantitative since a significant amount of signal produced by the reduced dye remains in parallel to the signal of reduced catalyst.

The secondary CSS with the electron localized on the cobalt catalyst decays in a nonexponential fashion with a lifetime of 30 µs (84 % rel. ampl.) and < 1 ms (16% rel.

ampl.). The CR from this state is therefore slower than that from the reduced dye at this potential, which can be explained with the lower driving force for recombination and the higher distance between the catalyst and the NiO surface.

Summary and Outlook

In summary, the use of TON as a metric for hydrogen evolution while using standardized experimental conditions to assess the performances of the dyadsensitized photocathodes, together with in-depth post-and in-operando characterization including especially TA-SEC measurements, allowed a detailed comparison of their activity, stability and the underlying photophysical processes. A comparison of the activity metrics such as photocurrent, the amount of hydrogen produced and TON with the state of the art in the literature on DSPCs for hydrogen production is difficult as these parameters vary considerably from one study to another, preventing the establishment of a sound structure-activity relationship, which is necessary for a rational improvement of the molecular systems. By using TON as a measure of the intrinsic activity of the dye-catalyst molecules and systematically varying both the dye and catalyst unit, we were able to quantify the respective contribution of each unit to the overall hydrogen production.

In spite of the difficulty of comparison with literature values, it can be stated that the T2R-Cat1-sensitized photocathode favourably competes with the best-performing dye-sensitized NiO photocathodes for hydrogen production in the literature in terms of Faradaic efficiency, stability and hydrogen produced. [START_REF] Xu | Water Splitting Dye-Sensitized Solar Cells[END_REF][START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF][START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF] This is certainly the case when comparing to other dye-catalyst assemblies based on Earth-abundant materials, none of which have achieved TON or amounts of produced hydrogen close to that of T2R-Cat1, with typically more than ten times less. The only superior assemblies used Pd and Pt catalysts and/or Ru complexes as PS. [START_REF] Põldme | Photoelectrocatalytic H2 Evolution from Integrated Photocatalysts Adsorbed on NiO[END_REF][START_REF] Ji | Photostable P-Type Dye-Sensitized Photoelectrochemical Cells for Water Reduction[END_REF] The TA-SEC results on sensitized NiO films obtained in this thesis present a novelty as it is the first time that i) an organic dye and ii) a functional hydrogen-evolving NiO photocathode, especially using a dye-catalyst dyad, were studied using this technique.

In addition, it was the first time a long-lived Co I state of a cobalt HER catalyst was observed on a dye-sensitized NiO photocathode and its lifetime could be determined.

Crucially, the observed lifetime of the Co I state of up to > 1 ms is enough to reach the timescale on which catalysis occurs, giving further support for the observed role for activity of the cobalt catalyst.

From the post-and operando characterization, two main impediments for the performance were identified:

The degradation of the dye structure in the reduced state which leads to a loss of absorption and a loss of the catalyst unit and therefore strongly limits the stability of the assembly. This contrasts with the existing literature, where desorption from the film surface is given as the main deactivation process. [START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF][START_REF] Sahara | Photoelectrochemical Reduction of CO2 Coupled to Water Oxidation Using a Photocathode with a Ru(II)-Re(I) Complex Photocatalyst and a CoOx /TaON Photoanode[END_REF][START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF] To overcome this, a more robust acceptor group than the cyanoacrylate group should be introduced in the assembly that does not easily degrade, while still fulfilling the energetic requirements for ET to the catalyst.

The low efficiency of the ET from the reduced dye to the catalyst. This non-quantitative ET leads to an accumulation of charges on the dye that are not consumed by catalysis and thus produce the current spikes observed in the chopped light chronoamperometric experiments by charging and discharging of the electrode-electrolyte interface. This accumulation in turn leads to degradation of the dye due to its instability. Similar observations were recently made for organic SC photocathodes with a PDI acceptor polymer and an upper limit for charge accumulation was determined for stable longterm operation, above which the PDI underwent irreversible change. 114 In our case, the accumulated reduced state of the dye is not stable and yields over the course of 2 to 4 hours a modified assembly without the catalyst unit and with a very low light harvesting efficiency in the visible region. This process is clearly accelerated in the absence of the catalyst (T2R vs T2R-Cat1 photocathode) which highlights that ET to the catalyst competes with the reduced dye alteration. A fast ET to the catalyst reduces the lifetime of the reduced dye and thereby protects it from degradation. Therefore, changing the dyad structure in a way that enables fast and quantitative ET to the catalyst will improve both the activity and the stability of the system at the same time which could lead to significantly increased hydrogen evolution.

These results also highlight the value of the TA-SEC measurements at operando potential which allow to determine the limiting processes and identify the weak points of the assembly. This is especially important since, as demonstrated here, a slow forward process can not only hinder activity of the assembly, but also compromise its long-term stability.

Comparing RuCo to T2R-Cat1

In the TA-SEC experiments, both Ru(Co) and T2R(-Cat1) show a potential dependence of HI and CR. In Ru(Co), the HI kinetics show a stronger dependence, probably due the generally slower HI and the different localization of the excited electron on the ligands, whereas in T2R HI is ultrafast at all potentials. As the time constant for HI is much lower than the excited state lifetime in all cases, it becomes obvious that the excited state lifetime of the dye does not have an impact on the system performance, but rather the lifetime of the CSS formed by HI. Here, it is interesting to observe that the organic dye T2R shows longer lifetimes of the CSS than Ru. This is probably related to the localization of the excess electron in the reduced state: while in T2R it is very localized on the cyanoacrylate acceptor group, in Ru it seems to be partially localized on the anchoring bpy ligands, whose closer proximity to the NiO surface could lead to faster CR. Therefore, the CSS lifetime of Ru might be improved by increasing the energy difference between the extended ligand and the bpy ligands or increasing the spatial distance, both of which could be achieved through an additional acceptor unit appended to the extended ligand. This strategy yielded very long CSS lifetimes in a Ru(bpy)2bpy-NDI complex on NiO. 108 Alternatively, if very fast ET to the catalyst unit can be promoted by a different linker architecture, this would serve the same purpose as the additional acceptor group.

A puzzling observation is that the CSS lifetime of T2R, in addition to being higher, is also more affected by the applied potential than Ru -that is,

( ) ( )
is greater for T2R than for Ru: when increasing the negative bias from -0.74 V to -1.15 V vs Fc +/0 , the lifetime of the CSS of Ru less than doubled, while it increased more than 15-fold for T2R. The energy levels of the reduced dyes cannot play a role since they differ only by ca. 100 mV and are still about 500 mV negative of the most negative applied bias. Thus, despite the much shorter excited state lifetime of T2R in solution, the light-induced processes are more favourable than for Ru, including faster HI and slower CR, when immobilized on NiO.

Since ET from the reduced dye to the catalyst is a crucial process, a comparison of ET efficiency of the two dyads studied by TA-SEC is insightful. However, since both the dye and catalyst unit as well as the exact linker structure is different in the two dyads, the validity of this comparison and the assignment of the kinetic differences to certain structural motives is limited. However, since ET could not be observed spectroscopically for RuCo while for T2R-Cat1 it was observed in < 20 ns, even though incomplete, there are obviously structural factors in T2R-Cat1 that promote ET. Since the linker is shorter and fully conjugated in RuCo, which should more easily promote ET in comparison to the longer and electronically isolating linker in T2R-Cat1, the exact linker structure does not seem to be the decisive factor here. One issue might be the localization of the excess charge: in T2R-Cat1 it is localized on the acceptor group very close to the linker, while in RuCo part of the charge seems to be localized on the anchoring bpy groups far away from the linker. Another factor could be the structure of the cobalt complex and especially its ligands: while the diimine-dioxime ligand of Co is bound to the linker via a propyl chain, Cat1 is connected at the pyridine unit of the macrocyclic ligand. This electron-poor unit with a conjugated π-system should be a much better acceptor unit than the alkyl chain of Co. In addition, in Co the reductions are exclusively metal-based, while for Cat1 a ligand-based reduction is proposed for the formal Co I state in some of the reported studies. [START_REF] Gueret | Cobalt(III) Tetraaza-Macrocyclic Complexes as Efficient Catalyst for Photoinduced Hydrogen Production in Water: Theoretical Investigation of the Electronic Structure of the Reduced Species and Mechanistic Insight[END_REF] Therefore, the electron-accepting molecular orbital could be much bigger in Cat1 leading to more efficient ET. To fully understand these influences, TA-SEC measurements on T2R-Co and Ru-Cat1 would help to understand the exact role played by the nature of the catalyst unit in the ET process.

Another advantage of the organic systems is the higher grafting density achieved in comparison to the Ru complexes, which can be attributed to the higher steric demand of the Ru core compared to the planar T2R. The major drawback for the organic dye is its instability in the reduced state and therefore under photoelectrochemical conditions which leads to its fast degradation. However, the Ru complexes are desorbed much more easily from the NiO film surface, possibly because they are cationic which could lead to a higher solubility in water than the organic dyes which are insoluble.

In conclusion, the more favourable kinetics of the light-induced processes for T2R, in addition to the better light absorption properties and the use of Earth-abundant materials for its synthesis, makes T2R the more attractive light-harvesting unit, if the issues with the lack of stability of its reduced state can be resolved. Cat1 is clearly a better catalyst unit in comparison to Co due to its higher stability in aqueous conditions, but also possibly by being a better electron acceptor for ET from the reduced dye. In the future, the dye structures should be optimized for faster and quantitative ET to the catalyst unit, which is the main impediment for high activity in the current dyads. This would further serve to protect the dyes from degradation in their reduced state.

Summary

Dye-sensitized photoelectrochemical cells represent a highly promising technology for the production of solar fuels via light-driven water splitting as an integral part of a carbon-neutral economy. In that context, the design of more performant dye-catalyst assemblies is highly demanded as they should allow to precisely control over the excited state processes, especially ET. However, detailed investigations on the lightinduced processes of dyad-sensitized photocathodes under operando conditions are scarce in the literature, and few studies have addressed in depth the degradation processes in those photocathodes.

Therefore, this work aimed at providing a comprehensive understanding of the performances of hydrogen-evolving DSPC based on a series of molecular dye-catalyst assemblies varying by the nature of the dye and the catalyst, including the elucidation of the systems' weak points and bottlenecks for hydrogen production. To this end, a full characterization of the photoelectrochemistry and activity of the photocathodes was combined with the study of the excited state processes both in solution and on films to gain a full understanding of the systems and their performance-limiting factors. Especially important was the determination of the lifetime of the CSS and the kinetics of the ET to the catalyst unit. The mainly studied compounds were a series of four noble-metal free dyads which varied in the nature of the push-pull organic dye (T1 or T2R) and in that of the catalyst for proton reduction (Co or Cat1). Assessing the activity of all four combinations enabled the determination of each part's contribution to the overall performance and therefore to link the molecular structure to the activity of the system. The elucidation of such a structure-acitivity relationship is important to be able to rationally improve the molecular structure of the dyads. The bestperforming system T2R-Cat1 was studied in detail, notably including the excited state processes at applied potential by TA-SEC and post-and in-operando measurements to determine the deactivation pathways and kinetics. In addition, NiO photocathodes sensitized with a Ruthenium-cobalt dyad were studied by TA-SEC for comparison. To access all oxidation states of the catalyst in the dyad, the measurements were carried out at applied potential (TA-SEC). Despite the emission quenching and reduced excited state lifetime in the dyad compared to the dye, ET in the excited state could not be observed, neither directly by a spectral contribution of the reduced cobalt complex nor indirectly by a potential dependence of the kinetics. The faster excited state decay in the dyad can be explained by an increased density of vibrational modes upon addition of the cobalt moiety or Dexter energy transfer from the excited donor chromophore to short-lived cobalt-centered dd-states. The study showed that in photocathodes sensitized with T1-Co, the oxidative quenching pathway is not active, but rather a reductive quenching pathway where the electron is transferred from the reduced dye, produced by HI, to the catalyst. Furthermore, the TA-SEC technique enables the investigation of the light-induced processes at different oxidation states which is important in the context of systems designed for photo-induced multielectron catalysis such as proton reduction and water oxidation.

Additionally, the T2R dye and the dyads based on it were also studied in solution by TA spectroscopy. While the initial relaxation processes were identical to the T1 systems, the dyads showed a markedly different behaviour from T1-Co, undergoing fast ISC to the long-lived excited triplet state of the dye, which did not occur in the dye alone. The fast ISC was explained by an interplay of both the rigid CPDT bridge and the cobalt catalyst, possibly involving higher energy states of the cobalt complex that lie between the singlet and triplet state energy of the T2R dye. This study showed how the interaction of different structures in a dye-catalyst assembly can lead to unexpected excited state behaviour that must be considered when designing a dye-catalyst dyad. examples in the literature where this technique was applied, and this work was the first time it has been applied to NiO films sensitized with a dye-catalyst dyad that were functional for photoelectrochemical H2 evolution. To this end, a dedicated spectroelectrochemistry cell was designed that enabled femtosecond-TA experiments on films with a precise applied potential under inert conditions that were stable for several hours and enabled us to measure TA-SEC spectra with good signal-to-noise ratio.

The TA-SEC data showed that HI occurs in a biphasic fashion with a sub-ps and a slower time constant with tens of picoseconds. The amplitude depended on the applied potential, favouring fast HI at more negative applied potential. Likewise, CR from the reduced Ru complex was affected by the applied potential, slowing down severely at negative applied potential: while at OCP a major contribution takes place on the same timescale as HI, preventing a build-up of charge-separated species, at negative applied potentials it occurs on the ns-µs timescale with average lifetimes of 3.6-5.2 µs. All the interfacial processes were found to be non-exponential. The influence of the applied potential could be explained by the reduction of Ni 3+ defect states to Ni 2+ , corresponding to the filling of intragap trap states with electrons. This transformation of an electron-depleted to an electron-rich NiO surface leads to both accelerated HI and retarded CR. Most importantly, no ET from the reduced Ru complex to the catalyst was observed, putting the efficiency of the formation of the catalytically active state to below 12%, determined by the limit of detection of the setup, in line with the low activity for hydrogen evolution. These results highlight the importance of the applied potential on the light-induced processes on sensitized NiO photocathodes and revealed the inefficient ET as a weak point of the assembly. This is in contrast to the literature where a fast recombination process from the primary CSS is invoked to explain the lack of efficiency of the photocathodes since the photophysical measurements are carried out on dry films without an applied potential. It could not be established which factor prevented ET, whether the donor or acceptor units or the triazole linker. The TA-SEC results also suggest the existence of distinct slow and fast HI and CR pathways, depending on whether the excess electron density is localized on the anchoring or the extended ligand. This insight might help in the ligand design of future molecular systems by trying to localize the excess electron density even more strongly on the extended, non-anchoring ligand. This should furthermore lead to faster ET to the acceptor unit.

The main part of the work on NiO photocathodes was performed on organic dyecatalyst dyads. Cyclic voltammetry experiments showed that there is no interaction in the ground state between the dye and catalyst unit and that the oxidation potential of T2R is shifted negatively in comparison to T1 due to the electron-rich CPDT linker. UV-Vis absorption spectra showed that this linker also caused a bathochromic and hyperchromic shift in the dye which made it the better light-absorbing unit. Then, the activity of the compounds when immobilized on NiO was studied in photoelectrochemical experiments in terms of photocurrent and hydrogen produced.

The photocurrent increased in the order T1-Co < T2R-Co < T1-Cat1 < T2R-Cat1 with a total of 5-fold increase in current from T1-Co to T2R-Cat1. The amount of hydrogen produced follows the same trend with the Cat1 dyads being significantly more productive than the Co dyads due to a much higher Faradaic efficiency. The individual contribution of the dye and catalyst units could thus be established to lead to a 26-fold increase for Cat1 in comparison to Co and a 1.7-fold increase for T2R in comparison to T1, highlighting the decisive role of the catalyst. A crucial factor was the stability of the assemblies, with the Cat1 dyads showing photocurrent and hydrogen evolution after more than four hours, when the Co dyads are already inactive. This confirms the previously observed higher activity and especially stability for Cat1 in homogeneous photocatalysis, but is in contrast to the literature on DSPC where desorption from the electrode surface is usually given as the main deactivation process.

The stability was further investigated by post-and in-operando measurements. The post-operando measurements revealed that the dyad loses both its main absorption band and the cobalt during operation. Time-resolved in-operando UV-Vis spectroelectrochemical experiments under operando conditions on the bestperforming T2R-Cat1 system, in combination with reductive UV-Vis SEC in organic solvent, showed that the T2R dye is unstable in its reduced state and degrades over the timescale of minutes to hours under operando conditions. From the available evidence, it seems that there is a bond breaking at the acceptor cyanoacrylate group that leads to the loss of the main charge-transfer band and the cobalt catalyst. On a positive note, the dyads did not easily desorb from the NiO surface due to hydrolysis, especially those based on T2R, ascribed to the protective effect of the hydrophobic alkyl side chains on the CPDT linker.

The light-induced processes in T2R-Cat1-sensitized NiO films were investigated by TA-SEC. Fast HI on the sub-ps timescale was observed that was unaffected by the applied potential, producing the first CSS. The CR kinetics from this state depended on the applied potential as observed also for RuCo, increasing from ca. 150 ps without applied potential to the µs-ms scale at applied potentials. Most importantly, ET from the reduced dye to the catalyst was observed within 20 ns after excitation and possibly a slower component on the µs timescale to form the catalytically active Co I state. The Co I state decayed on the µs scale (τ = 30 µs), with 16 % left after 1 ms. This is an important finding as it puts the lifetime of the catalytically state of the catalyst in the range of the timescale of catalysis. The ET to the catalyst was however not quantitative, leading to charge accumulation on the reduced dye and therefore ultimately to its degradation. Therefore, an improved molecular design of the dyad that assures fast and quantitative ET to the catalyst should solve both the issue of low activity and reductive degradation of the dye unit. These results also highlight the value of the TA-SEC measurements at operando potential which allow to determine the limiting processes and identify the weak points of the assembly. This is especially important since, as demonstrated here, a slow forward process can not only hinder activity of the assembly, but also compromise its long-term stability.

This dissertation thus provides key insights on the light-induced processes in NiO photocathodes sensitized with dye-catalyst dyads and their dependence on the applied bias, together with an assessment of the photocathode performance and the deactivation processes and their kinetics. The obtained findings of the systems' weak points, especially that of very inefficient ET from the dye to the catalyst and the reductive degradation of the organic dye units, will help to rationally design dyads with improved activity and stability.

Zusammenfassung

Farbstoffsensibilisierte Reproduced from Bold, S. et al., 2020, Catalysts, 10(11) Very big thanks go to Dr. Ruri Wahyuono for the supervision and great co-working during my Master's thesis and beyond, and for introducing me to the field of dyesensitized stuff. Thank you for your great patience, your unparalleled willingness to help, not to mention the delicious food. I wish you all the best for your start in Indonesia!

Equally great thanks go to Dr. Emmanouil "Manos" Giannoudis for being a great colleague, for always echarme una mano, for lots and lots of synthesis that I probably would have spectacularly failed at, and most of all for being a great friend. I am very happy our paths have crossed. ("Amazing!")

A special thanks goes also to Mr. Alpaca, Dr. Mathias Micheel, for his help with various laser setups, for the great friendship and the grand Jena sports tour.

Thanks also to Dr. Nick Randell for good beer, skiing and great scientific and nonscientific discussion during coffee (did I mention beer?).

I want to thank the whole group in Jena for their help with many issues and the nice atmosphere in the group. Special thanks go to Dr. Maria Wächtler for a lot of organizational help and Dirk Müller and Dr. Denis Akimov for their help with the laser setups at IPHT. I want to thank Dr. Linda Zedler and Dr. Ying Zhang for getting me started with the spectroelectrochemical measurements that would so define my thesis. Last but not least, I want to thank soon-to-be Dr. Kilian Schneider and Dr. Yusen Luo for very successful sharing of the fs-TA lab.

I also want to thank my group in Grenoble for the very warm welcome and the great and motivating atmosphere in the lab. A special thanks goes to Dr. Matthieu Koepf for getting me started in the lab and showing me all the tricks for the photoelectrochemical measurements, and for all the help he's given me in the past years which was way too much. Overall thanks for being the nicest person ever, and for the accordion sessions. My thanks go also to Jennifer Fize and Dr. Adina Morozan for their continuous help in the lab.

I also want to thank the workshop at IPHT for their great help in building and improving the cell for the TA-SEC measurements based on my somewhat crude first design.

I would also like to acknowledge the financial support of the Franco-German University for my cotutelle thesis.

My thanks go also to the graduate academy in Jena and the école doctorale in Grenoble for helping me with the cotutelle agreement.

Finally, I am deeply grateful to my family which has always supported me in all of my choices, my travels, my moving to Jena and my studies.

Mis últimas y más grandes gracias van para Johanna Solano. Gracias por aguantar mis a veces largas horas de laboratorio y trabajo en casa. Gracias por soportar no verme la mitad del año mientras estaba en Francia. Gracias por apoyarme siempre y darme confianza en mí mismo. Gracias por hacerme reír y relajarme cuando estaba bajo estrés. Gracias por todo. Te liebo.

Appendix 2

Updated publications

Publication 3 -Spectroscopic investigations provide a rational for the hydrogenevolving activity of dye-sensitized photocathodes based on a cobalt tetraazamacrocyclic catalyst.

Reproduced with permission from ACS Catalysis, 2021, 11, 6, 3662-3678

Figure 2 . 8 :Figure 3 . 2 :Figure 3 . 3 :Figure 3 . 4 :Figure 3 . 8 :

 2832333438 Figure 1.1 Schematic outline of this thesis' structure. .....................................................Figure 2.1: Schematic representation of a tandem DSPEC for full water splitting .....Figure 2.2: Core structures of PS used in DS-photocathodes .........................................Figure 2.3: Catalysts for HER ..............................................................................................Figure 2.4: General catalytic mechanism for hydrogen evolution by cobalt diimine dioxime complexes ...............................................................................................................Figure 2.5: Dye-catalyst assemblies reported in the literature for proton reduction in DSPC. ......................................................................................................................................Figure 2.6: Schematic representation of energy levels in a DSPC for hydrogen evolution .................................................................................................................................Figure 2.7: Different photocathode architectures used in DSPCs for proton reduction. ..................................................................................................................................................Figure 2.8: Schematic representation of the light-induced processes in DSPEC photocathodes .......................................................................................................................Figure 3.1: Left: Schematic depiction of a TA experiment .............................................. Figure 3.2: Chemical structures of organic push-pull dyes, containing TPA donor and cyanovinyl-based acceptor units, investigated by TA spectroscopy in the literature. ................................................................................................................................ Figure 3.3: Compounds whose photophysics were studied in solution and their UV-Vis absorption spectra .......................................................................................................... Figure 3.4: Results of TA spectroscopy of organic push-pull dyes in solution. ........... Figure 3.5: Scheme of the electrochemical and principal light-induced processes taking place during the TA-SEC measurements on T1-Co ............................................. Figure 3.6: fs-TA-SEC data of T1-Co in DMF at various applied potentials ................ Figure 3.7:Results of TA spectroscopy of T2 and T2-Co in solution ............................. Figure 3.8: Deactivation pathways of the singlet excited ICT state of the dye-catalyst dyads .......................................................................................................................................Figure 4.1: Chemical structures of the dye Ru and the dyad RuCo and their UV-Vis absorption spectra in ACN ..................................................................................................Figure 4.2: Cell for TA-SEC measurements on films in the assembled and disassembled state. ...............................................................................................................Figure 4.3: fs-TA-SEC (top left) and ns-TASEC (top right) data of NiO films sensitized with Ru at OCP and two applied potentials (0.74 and -1.15 V vs. Fc +/0 ).....Figure 4.4: Schematic representation of different HI and CR kinetics caused by differing localization of the excited electron (NiO|Ru*) and the excess electron in the CSS (NiO + |Ru -) ......................................................................................................................

Figure 4 . 1 :

 41 Figure 1.1 Schematic outline of this thesis' structure. .....................................................Figure 2.1: Schematic representation of a tandem DSPEC for full water splitting .....Figure 2.2: Core structures of PS used in DS-photocathodes .........................................Figure 2.3: Catalysts for HER ..............................................................................................Figure 2.4: General catalytic mechanism for hydrogen evolution by cobalt diimine dioxime complexes ...............................................................................................................Figure 2.5: Dye-catalyst assemblies reported in the literature for proton reduction in DSPC. ......................................................................................................................................Figure 2.6: Schematic representation of energy levels in a DSPC for hydrogen evolution .................................................................................................................................Figure 2.7: Different photocathode architectures used in DSPCs for proton reduction. ..................................................................................................................................................Figure 2.8: Schematic representation of the light-induced processes in DSPEC photocathodes .......................................................................................................................Figure 3.1: Left: Schematic depiction of a TA experiment .............................................. Figure 3.2: Chemical structures of organic push-pull dyes, containing TPA donor and cyanovinyl-based acceptor units, investigated by TA spectroscopy in the literature. ................................................................................................................................ Figure 3.3: Compounds whose photophysics were studied in solution and their UV-Vis absorption spectra .......................................................................................................... Figure 3.4: Results of TA spectroscopy of organic push-pull dyes in solution. ........... Figure 3.5: Scheme of the electrochemical and principal light-induced processes taking place during the TA-SEC measurements on T1-Co ............................................. Figure 3.6: fs-TA-SEC data of T1-Co in DMF at various applied potentials ................ Figure 3.7:Results of TA spectroscopy of T2 and T2-Co in solution ............................. Figure 3.8: Deactivation pathways of the singlet excited ICT state of the dye-catalyst dyads .......................................................................................................................................Figure 4.1: Chemical structures of the dye Ru and the dyad RuCo and their UV-Vis absorption spectra in ACN ..................................................................................................Figure 4.2: Cell for TA-SEC measurements on films in the assembled and disassembled state. ...............................................................................................................Figure 4.3: fs-TA-SEC (top left) and ns-TASEC (top right) data of NiO films sensitized with Ru at OCP and two applied potentials (0.74 and -1.15 V vs. Fc +/0 ).....Figure 4.4: Schematic representation of different HI and CR kinetics caused by differing localization of the excited electron (NiO|Ru*) and the excess electron in the CSS (NiO + |Ru -) ......................................................................................................................

Figure 4 . 2 :

 42 Figure 1.1 Schematic outline of this thesis' structure. .....................................................Figure 2.1: Schematic representation of a tandem DSPEC for full water splitting .....Figure 2.2: Core structures of PS used in DS-photocathodes .........................................Figure 2.3: Catalysts for HER ..............................................................................................Figure 2.4: General catalytic mechanism for hydrogen evolution by cobalt diimine dioxime complexes ...............................................................................................................Figure 2.5: Dye-catalyst assemblies reported in the literature for proton reduction in DSPC. ......................................................................................................................................Figure 2.6: Schematic representation of energy levels in a DSPC for hydrogen evolution .................................................................................................................................Figure 2.7: Different photocathode architectures used in DSPCs for proton reduction. ..................................................................................................................................................Figure 2.8: Schematic representation of the light-induced processes in DSPEC photocathodes .......................................................................................................................Figure 3.1: Left: Schematic depiction of a TA experiment .............................................. Figure 3.2: Chemical structures of organic push-pull dyes, containing TPA donor and cyanovinyl-based acceptor units, investigated by TA spectroscopy in the literature. ................................................................................................................................ Figure 3.3: Compounds whose photophysics were studied in solution and their UV-Vis absorption spectra .......................................................................................................... Figure 3.4: Results of TA spectroscopy of organic push-pull dyes in solution. ........... Figure 3.5: Scheme of the electrochemical and principal light-induced processes taking place during the TA-SEC measurements on T1-Co ............................................. Figure 3.6: fs-TA-SEC data of T1-Co in DMF at various applied potentials ................ Figure 3.7:Results of TA spectroscopy of T2 and T2-Co in solution ............................. Figure 3.8: Deactivation pathways of the singlet excited ICT state of the dye-catalyst dyads .......................................................................................................................................Figure 4.1: Chemical structures of the dye Ru and the dyad RuCo and their UV-Vis absorption spectra in ACN ..................................................................................................Figure 4.2: Cell for TA-SEC measurements on films in the assembled and disassembled state. ...............................................................................................................Figure 4.3: fs-TA-SEC (top left) and ns-TASEC (top right) data of NiO films sensitized with Ru at OCP and two applied potentials (0.74 and -1.15 V vs. Fc +/0 ).....Figure 4.4: Schematic representation of different HI and CR kinetics caused by differing localization of the excited electron (NiO|Ru*) and the excess electron in the CSS (NiO + |Ru -) ......................................................................................................................

Figure 4 . 3 :

 43 Figure 1.1 Schematic outline of this thesis' structure. .....................................................Figure 2.1: Schematic representation of a tandem DSPEC for full water splitting .....Figure 2.2: Core structures of PS used in DS-photocathodes .........................................Figure 2.3: Catalysts for HER ..............................................................................................Figure 2.4: General catalytic mechanism for hydrogen evolution by cobalt diimine dioxime complexes ...............................................................................................................Figure 2.5: Dye-catalyst assemblies reported in the literature for proton reduction in DSPC. ......................................................................................................................................Figure 2.6: Schematic representation of energy levels in a DSPC for hydrogen evolution .................................................................................................................................Figure 2.7: Different photocathode architectures used in DSPCs for proton reduction. ..................................................................................................................................................Figure 2.8: Schematic representation of the light-induced processes in DSPEC photocathodes .......................................................................................................................Figure 3.1: Left: Schematic depiction of a TA experiment .............................................. Figure 3.2: Chemical structures of organic push-pull dyes, containing TPA donor and cyanovinyl-based acceptor units, investigated by TA spectroscopy in the literature. ................................................................................................................................ Figure 3.3: Compounds whose photophysics were studied in solution and their UV-Vis absorption spectra .......................................................................................................... Figure 3.4: Results of TA spectroscopy of organic push-pull dyes in solution. ........... Figure 3.5: Scheme of the electrochemical and principal light-induced processes taking place during the TA-SEC measurements on T1-Co ............................................. Figure 3.6: fs-TA-SEC data of T1-Co in DMF at various applied potentials ................ Figure 3.7:Results of TA spectroscopy of T2 and T2-Co in solution ............................. Figure 3.8: Deactivation pathways of the singlet excited ICT state of the dye-catalyst dyads .......................................................................................................................................Figure 4.1: Chemical structures of the dye Ru and the dyad RuCo and their UV-Vis absorption spectra in ACN ..................................................................................................Figure 4.2: Cell for TA-SEC measurements on films in the assembled and disassembled state. ...............................................................................................................Figure 4.3: fs-TA-SEC (top left) and ns-TASEC (top right) data of NiO films sensitized with Ru at OCP and two applied potentials (0.74 and -1.15 V vs. Fc +/0 ).....Figure 4.4: Schematic representation of different HI and CR kinetics caused by differing localization of the excited electron (NiO|Ru*) and the excess electron in the CSS (NiO + |Ru -) ......................................................................................................................

Figure 4 . 5 :Figure 4 . 7 -

 4547 Figure 4.5: Potential dependence of the NiO band electronics and its influence on HI and CR kinetics. ..................................................................................................................... Figure 4.6: Decay-associated spectra obtained by a three-exponential fit of the TA-SEC data of NiO films sensitized with RuCo ...................................................................Figure 4.7 -Synthesis of Ru and RuCo ..............................................................................Figure 4.8: Scheme showing the light-induced processes in a NiO films sensitized with RuCo at an applied potential of -0.74 V vs. Fc +/0 .....................................................Figure 4.9: Chemical structures of the organic dye-catalyst dyads studied in this work and their UV-Vis absorption spectra in DMF (T1 dyads) and MeOH (T2R dyads). ....................................................................................................................................Figure 4.10: Results of the photoelectrochemical measurements of the organic dyecatalyst dyads ........................................................................................................................Figure 4.11: In-operando characterization of NiO films sensitized with T2R-Cat1 ....Figure 4.12: Results of TA-SEC measurements of NiO films sensitized with T2R-Cat1 and T2R .................................................................................................................................. Figure 5.1. Schematic representation of the principal characterization techniques used in this work on the DSPCs. .......................................................................................

Figure 4 . 8 :

 48 Figure 4.5: Potential dependence of the NiO band electronics and its influence on HI and CR kinetics. ..................................................................................................................... Figure 4.6: Decay-associated spectra obtained by a three-exponential fit of the TA-SEC data of NiO films sensitized with RuCo ...................................................................Figure 4.7 -Synthesis of Ru and RuCo ..............................................................................Figure 4.8: Scheme showing the light-induced processes in a NiO films sensitized with RuCo at an applied potential of -0.74 V vs. Fc +/0 .....................................................Figure 4.9: Chemical structures of the organic dye-catalyst dyads studied in this work and their UV-Vis absorption spectra in DMF (T1 dyads) and MeOH (T2R dyads). ....................................................................................................................................Figure 4.10: Results of the photoelectrochemical measurements of the organic dyecatalyst dyads ........................................................................................................................Figure 4.11: In-operando characterization of NiO films sensitized with T2R-Cat1 ....Figure 4.12: Results of TA-SEC measurements of NiO films sensitized with T2R-Cat1 and T2R .................................................................................................................................. Figure 5.1. Schematic representation of the principal characterization techniques used in this work on the DSPCs. .......................................................................................

Figure 4 . 9 :

 49 Figure 4.5: Potential dependence of the NiO band electronics and its influence on HI and CR kinetics. ..................................................................................................................... Figure 4.6: Decay-associated spectra obtained by a three-exponential fit of the TA-SEC data of NiO films sensitized with RuCo ...................................................................Figure 4.7 -Synthesis of Ru and RuCo ..............................................................................Figure 4.8: Scheme showing the light-induced processes in a NiO films sensitized with RuCo at an applied potential of -0.74 V vs. Fc +/0 .....................................................Figure 4.9: Chemical structures of the organic dye-catalyst dyads studied in this work and their UV-Vis absorption spectra in DMF (T1 dyads) and MeOH (T2R dyads). ....................................................................................................................................Figure 4.10: Results of the photoelectrochemical measurements of the organic dyecatalyst dyads ........................................................................................................................Figure 4.11: In-operando characterization of NiO films sensitized with T2R-Cat1 ....Figure 4.12: Results of TA-SEC measurements of NiO films sensitized with T2R-Cat1 and T2R .................................................................................................................................. Figure 5.1. Schematic representation of the principal characterization techniques used in this work on the DSPCs. .......................................................................................

Figure 4 .

 4 Figure 4.5: Potential dependence of the NiO band electronics and its influence on HI and CR kinetics. ..................................................................................................................... Figure 4.6: Decay-associated spectra obtained by a three-exponential fit of the TA-SEC data of NiO films sensitized with RuCo ...................................................................Figure 4.7 -Synthesis of Ru and RuCo ..............................................................................Figure 4.8: Scheme showing the light-induced processes in a NiO films sensitized with RuCo at an applied potential of -0.74 V vs. Fc +/0 .....................................................Figure 4.9: Chemical structures of the organic dye-catalyst dyads studied in this work and their UV-Vis absorption spectra in DMF (T1 dyads) and MeOH (T2R dyads). ....................................................................................................................................Figure 4.10: Results of the photoelectrochemical measurements of the organic dyecatalyst dyads ........................................................................................................................Figure 4.11: In-operando characterization of NiO films sensitized with T2R-Cat1 ....Figure 4.12: Results of TA-SEC measurements of NiO films sensitized with T2R-Cat1 and T2R .................................................................................................................................. Figure 5.1. Schematic representation of the principal characterization techniques used in this work on the DSPCs. .......................................................................................

Figure 4 . 12 :

 412 Figure 4.5: Potential dependence of the NiO band electronics and its influence on HI and CR kinetics. ..................................................................................................................... Figure 4.6: Decay-associated spectra obtained by a three-exponential fit of the TA-SEC data of NiO films sensitized with RuCo ...................................................................Figure 4.7 -Synthesis of Ru and RuCo ..............................................................................Figure 4.8: Scheme showing the light-induced processes in a NiO films sensitized with RuCo at an applied potential of -0.74 V vs. Fc +/0 .....................................................Figure 4.9: Chemical structures of the organic dye-catalyst dyads studied in this work and their UV-Vis absorption spectra in DMF (T1 dyads) and MeOH (T2R dyads). ....................................................................................................................................Figure 4.10: Results of the photoelectrochemical measurements of the organic dyecatalyst dyads ........................................................................................................................Figure 4.11: In-operando characterization of NiO films sensitized with T2R-Cat1 ....Figure 4.12: Results of TA-SEC measurements of NiO films sensitized with T2R-Cat1 and T2R .................................................................................................................................. Figure 5.1. Schematic representation of the principal characterization techniques used in this work on the DSPCs. .......................................................................................

Figure 1 . 1

 11 Figure 1.1 Schematic outline of this thesis' structure.In the centre, the studied systems are presented. In chapter 3, the photophysics of the organic dyes and some of their dye-catalyst dyads are investigated in solution using TA spectroscopy and spectroelectrochemistry. In chapter 4, NiO photocathodes sensitized with the dyads are investigated in terms of their activity for hydrogen production, stability under operando conditions and the light-induced processes as a function of applied potential. The latter was also performed for a Ru-Co dyad.

  photoelectrochemical cell. The work in this thesis is situated in the context of the work performed in the "SolHyCat" group at the Laboratory of Chemistry and Biology of Metals, CEA Grenoble, on the construction of DSPCs for proton reduction on a molecular basis, notably covalent dye-catalyst assemblies; and the work in the group of Molecular Photonics at the Friedrich-Schiller University Jena, on the investigation of the light-induced processes in such molecular dyes and dyads. The aim of this thesis, as outlined in Figure 1.1, was to unite the expertise of both groups to a) construct dye-sensitized NiO photocathodes with a series of newly synthesized covalent dye-catalyst assemblies featuring an organic dye and a cobalt catalyst, and assess their activity in photoelectrochemical proton reduction b) to determine possible degradation mechanisms that reduce the activity and lifetime of the photocathodes c) to investigate the light-induced processes in the assemblies in solution and on NiO, especially the electron transfer (ET) from the dye to the catalyst unit. Especially important, the established TA spectroscopy should be combined with electrochemistry to perform transient absorption spectroelectrochemistry (TA-SEC) experiments in solution and on film, allowing to measure the light-

Figure 2 . 1 :

 21 Figure 2.1: Schematic representation of a tandem DSPEC for full water splitting, in this case with dye-catalyst supramolecular assemblies.

2. 2

 2 Components of dye-sensitized photocathodes for proton reduction2.2.1 PhotosensitizersPS are essential in DSPEC as they perform the absorption of incoming photons and are the site where the first CSS is formed. They are the only component that is always anchored to the SC film via anchoring groups. The necessary thermodynamic requirement a PS must fulfil to ensure a functional photocathode for solar fuel (or hydrogen) production is the position of the redox energy levels (e.g., its HOMO and LUMO): The reduction potential of the HOMO must be positive of that of the SC valence band (VB) to ensure fast hole injection (HI) after excitation of the PS, and the reduction potential of the LUMO must be negative of that of the catalyst to enable ET to the catalyst to form the catalytically active state.

Figure 2 . 2 :

 22 Figure 2.2: Core structures of PS used in DS-photocathodes : Tris(bipyridine)ruthenium (Ru(bpy)3), push-pull organic dyes (D-π-A), porphyrins, perylene monoimides (PMI) and perylene diimides (PDI), as well as coumarin C343 and the isoindigo dye DPP-P. Rests (R) are varied, with at least one being an anchoring group.

Figure 2 . 3 :

 23 Figure 2.3: Catalysts for HER: the active sites of [FeFe]-and [NiFe]-hydrogenases (top) and HER catalysts used in DSPC until now (bottom).

Figure 2 . 4 :

 24 Figure 2.4: General catalytic mechanism for hydrogen evolution by cobalt diimine dioxime complexes starting from the Co III L precatalyst on the left side. The structure in brackets is the transition state for the intramolecular elimination of molecular hydrogen, in which a proton bound to an oxime oxygen atom comes near the Co center.

Figure 2 . 5 :

 25 Figure 2.5: Dye-catalyst assemblies reported in the literature for proton reduction in DSPC.

Figure 2 . 6 :

 26 Figure 2.6: Schematic representation of energy levels in a DSPC for hydrogen evolution, including the VB and conduction band of the SC, the ground state and excited state redox potential for dye reduction, the redox potential of the catalyst to reduce it to its active state and the redox potential for proton reduction.

Figure 2 . 7 )

 27 . It is either simply dissolved in the electrolyte solution, co-grafted on the SC film surface alongside the PS or attached to the PS in the supramolecular assemblies shown in section 2.2.3. The different architectures have advantages and disadvantages that will be presented, together with their photoelectrochemical performances.

Figure 2 . 7 :

 27 Figure 2.7: Different photocathode architectures used in DSPCs for proton reduction.

Figure 2 . 8 :

 28 Figure 2.8: Schematic representation of the light-induced processes in DSPEC photocathodes, here shown with a dye-catalyst supramolecular assembly.

Publication 1 -

 1 Bold, S., Zedler, L., Zhang, Y., Massin, J., Artero, V., Chavarot-Kerlidou, M., & Dietzek, B. Electron transfer in a covalent dye-cobalt catalyst assembly -a transient absorption spectroelectrochemistry perspective. Chemical Communications, 2018, 54(75), 10594-10597. Publication 2 -Bold, S., Straistari, T., Muñoz-García, A. B., Pavone, M., Artero, V., Chavarot-Kerlidou, M., & Dietzek, B. Investigating Light-Induced Processes in Covalent Dye-Catalyst Assemblies for Hydrogen Production. Catalysts, 2020, 10(11), 1340.

Figure 3 . 1 :

 31 Figure 3.1: Left: Schematic depiction of a TA experiment. The sample is excited by the pump pulse and its absorbance is measured by the probe pulse after the time delay Δt. Differential spectra are obtained by subtracting the absorption spectrum in the ground state from the absorption spectrum at Δt. The time delay is produced by a mechanical delay stage for ultrafast measurements (< 10 ns) and by an electronic delay for longer timescales (ns-ms). Right: Schematic depiction of the processes taking place in a TA experiment.

3. 2

 2 Photophysics of push-pull organic dyes 3.2.1.1 Organic push-pull (D-π-A) dyes in the literature Due to the large number of published D-π-A compounds, I will focus on those based on a triphenylamine (TPA) donor and a cyanovinyl-based acceptor group (Figure 3.2),

Figure 3 . 2 :

 32 Figure 3.2: Chemical structures of organic push-pull dyes, containing TPA donor and cyanovinyl-based acceptor units, investigated by TA spectroscopy in the literature.

Figure 3 . 3 :

 33 Figure 3.3: Compounds whose photophysics were studied in solution and their UV-Vis absorption spectra (solid lines) and emission maxima (triangles) in various solvents of increasing polarity (Toluene < Chloroform < Dichloromethane < Dimethylformamide < ACN).This is further corroborated by electrochemical measurements, which show a strong negative shift of the redox potential of the dye oxidation upon introducing the CPDT linker (ca. 230 mV negative shift for T2R compared to T1) while the dye reduction potential is much less affected by this change (ca. 50 mV negative shift). Since the dye oxidation is equivalent to removal of an electron from the HOMO and dye reduction to the addition of an electron to the LUMO, these measurements also suggest that the principal change is in the HOMO position which is raised by the electron-rich CPDT linker. This leads to more negative oxidation potentials and a smaller HOMO-LUMO gap. The T2 dye represents an intermediate case.127

Figure 3 . 4 :

 34 Figure 3.4: Results of TA spectroscopy of organic push-pull dyes in solution. Top: TA spectra of T1 in ACN and kinetic traces of the ESA (λ = 535 nm in ACN, 550 nm in DMF) and SE (λ = 705 nm) at early delay times in ACN and DMF that shows the influence of solvent viscosity on the relaxation kinetics. Bottom: Jablonski scheme showing the excited state processes taking place in the organic push-pull dyes T1, T2 and T2R.

  Figure 3.5 shows the principal possible processes during the TA-SEC measurement of T1-Co. Of special interest were the measurements at the Co II state, which should provide the possibility of observing the Co I state after ET, and at the Co I state which served as reference since ET is not thermodynamically possible in this state.

Figure 3 . 5 :

 35 Figure 3.5: Scheme of the electrochemical and principal light-induced processes taking place during the TA-SEC measurements on T1-Co and the differential absorption spectra of the T1-Co II and T1-Co I species obtained by UV-Vis spectroelectrochemistry. k are rate constants for the different processes, with kD = decay to the ground state, kR = radiative decay, kNR = nonradiative decay, kET = ET, kBET = back ET. Bottom right: Schematic representation of the spectroelectrochemical cell used in this experiment.

Figure 3 .

 3 Figure 3.6: fs-TA-SEC data of T1-Co in DMF at various applied potentials: OCP (left), Co(II) (middle) and Co(I) (right) reductive potential. The upper graphs show the spectra at early delay times while the middle ones show late delay times. The bottom graphs show the decay-associated spectra (DAS) obtained by a four-component parallel exponential fit.

Figure 3 . 7 :

 37 Figure 3.7:Results of TA spectroscopy of T2 and T2-Co in solution . a) TA data of T2 and T2-Co in ACN after the initial relaxation cascade, normalized to the GSB after 50 ps. b) Kinetic traces of T1, T1-Co, T2 and T2-Co in ACN from 50 ps to 9 ns with the average lifetimes calculated from the biphasic decay at the maxima of the ESA (λ = 430 nm for T1 and T1-Co, 455 nm for T2 and T2-Co) and GSB (λ = 535 nm for T1, 550 nm for T1-Co, 620 nm for T2

Figure 3 . 8 :

 38 Figure 3.8: Deactivation pathways of the singlet excited ICT state of the dye-catalyst dyads via direct decay and ISC to the triplet ICT state. The width of the arrows indicates the amplitude of the processes. The spectra depicted are the TA spectra of the thermalized singlet ICT state (recorded of T2R in MeOH) and of the triplet ICT state (T2R-Cat1 in MeOH).

3. 4 . 1

 41 Synthesis of T2 and T2-CoT2-Co. The synthesis of T2-Co has been published and can be found inWindle, C. D., Massin, J., Chavarot-Kerlidou, M., Artero, V. Dalton Transactions, 2018, 10509-10516. T2. The reference dye T2 was synthesized according to the same protocol as T2-Co, using ethyl 2-cyanoacetate instead of propargyl cyanoacetamide during the Knoevenagel reaction.

3. 4 . 2

 42 Transient absorption spectroscopyTransient absorption measurements of T1, T2, T1-Co and T2-Co. A custom-built setup was utilized to acquire fs-TA data. The TA setup is described in detail in R.Siebert, D. Akimov, M. Schmitt, A. Winter, U. S. Schubert, B. Dietzek, J. Popp, ChemPhysChem, 2009, 10, 910-919. A white-light supercontinuum probe pulse at 1 kHz repetition rate was used to analyze the excited state dynamics. The supercontinuum was generated by focusing a minor part of the output of the Ti:Sapphire amplifier into a CaF2 plate mounted on a rotating stage. After generation of the white light continuum, it is split into a probe and reference beam. Using a concave mirror with 500 nm focal length, the probe pulse was focused onto the sample by a concave mirror of 500 mm focal length.The spectra of probe and reference were detected by a Czerny-Turner spectrograph of 150 mm focal length (SP2150, Princeton Instruments) equipped with a diode array detector (Pascher Instruments AB, Sweden). The pump pulses used were of 403 nm wavelength and a pulse duration of approximately 100 fs. The repetition rate of the pump pulses was reduced to 0.5 kHz by a mechanical chopper and the polarization adjusted to the magic angle of 54.7° by a Berek compensator and a polarizer. The time delay is generated by reflecting the probe beam on a pair of mirrors mounted on a movable delay line and therefore adjusting the path length of the probe beam.

Publication 3 -

 3 Photocathode Based on a Ruthenium Dye-Cobalt Diimine Dioxime Catalyst Assembly: Insights from Advanced Spectroscopy and Post-operando Characterization. ACS AppliedMaterials & Interfaces, 2021, 13, 42, 49802-49815.

Figure 4 . 1 :

 41 Figure 4.1: Chemical structures of the dye Ru and the dyad RuCo and their UV-Vis absorption spectra in ACN. The inset shows a close-up of the metal-to-ligand charge transfer (MLCT) band.

1. 5

 5 filter increased this value to 84 ± 7 µA•cm -2 due to the strong absorption band in the UV of the Ru complex. 98 % of the initial photocurrent was lost during the first two hours of photoelectrochemical measurements due to desorption from the NiO surface and degradation of the cobalt catalyst. After two hours under photoelectrochemical conditions with visible light (-0.4 V vs. Ag/AgCl applied potential, 2-(Nmorpholino)ethanesulfonic acid buffer solution with pH = 5.5), hydrogen was produced with TON of 5 ± 1 (28 ± 2 nmol•cm -2 H2) with a low Faradaic efficiency of 25%. While the amount of produced hydrogen doubled with the use of the AM 1.5 filter (TON = 13 ± 2, 60 ± 9 nmol•cm -2 H2), the Faradaic efficiency stayed identical. The low value of the Faradaic efficiency was ascribed to side reaction on the cobalt diiminedioxime catalyst. Short measurements of 30 minutes duration showed a much higher F.E. of 72% and almost identical TON as after two hours, meaning most of the hydrogen is produced in the initial minutes. This means that the intrinsic Faradaic efficiency of the assembly is high, but decreases quickly due to side reactions and degradation of the catalyst. MALDI-ToF MS measurements showed that demetallation due to the cleavage of the C-N bonds of the diimine-dioxime ligand was the primary degradation process. In addition, 80% of the initially present dyads were lost from the NiO surface due to desorption during the first two hours under photoelectrochemical conditions.

Figure 4 . 2 :

 42 Figure 4.2: Cell for TA-SEC measurements on films in the assembled and disassembled state. The general design consisted of a cell body with two transparent windows pressed to it with flanges and sealed with rubber O-rings (Figure 4.2). One of the windows is a glass substrate with a transparent conducting oxide coating (fluorine-doped tin oxide (FTO) or ITO) on which the sample film is deposited prior to the experiment. The films were prepared by spin-coating a precursor solution of NiCl2 and the F108 block copolymer in ethanol/water on ITO-coated glass slides (Solems, FTO coating for the photoelectrochemical experiments) and sintering at 450°C, followed by sensitization

Figure 4 . 3 :

 43 Figure 4.3: fs-TA-SEC (top left) and ns-TASEC (top right) data of NiO films sensitized with Ru at OCP and two applied potentials (0.74 and -1.15 V vs. Fc +/0 ). The inverted absorption spectrum of RuCo in ACN (dashed), the TA spectrum of RuCo in ACN (dot) and the scaled differential absorption spectrum upon one-electron reduction of the Ru complex in RuCo (dashdot) are given for comparison. Bottom: Kinetic traces at 530 nm for Ru-and RuCo-sensitized NiO films at the two applied potentials, normalized for each potential at 20 ns time delay.

Figure 4 . 5 :

 45 Figure 4.5: Potential dependence of the NiO band electronics and its influence on HI and CR kinetics.It might also be caused by different local environments on the NiO and predominantly electron-or hole-filled surfaces as suggested by Dillon and co-workers for a Ru(bpy)3 complex grafted on NiO, 107 and the time constants observed here fit well with those obtained in their study.(2.8 and 39 ps). To obtain a good fit for their TA-SEC data, the authors proposed a model consisting of predominantly electron filled and predominantly electron depleted surface sites. This, in a band picture, corresponds to empty and filled intragap states between the NiO valence and conduction bands. In a chemical picture, these sites/trap states are essentially Ni 2+ atoms that can be oxidized to Ni 3+ . Applying a negative potential to the NiO film reduces these Ni sites to Ni 2+ , thus filling the intragap states and producing an electron-filled surface. This model explains the influence of the applied potential on the HI kinetics: the NiO film as prepared -equivalent to the OCP measurements -presents a primarily electrondepleted surface, corresponding to unfilled trap states or Ni 3+ atoms at defect sites, respectively(Figure 4.5). As the surface electrons are those that will transfer to the excited Ru complex during HI, a low concentration of surface electrons leads to slow HI; in a band/energy picture the electrons in the NiO have low driving force for HI resulting in slow HI. Applying a reductive potential reduces the Ni atoms to Ni 2+ ,

  NiO + |Ru I (Co). From the fit of the fs-TA-SEC data, the amplitude of the CR component increases at more reductive potential. Since the time constant of this component is > 7 ns, this increase in amplitude means that more CSS is present at the end of the 7 ns time window of the experiment as a result of slower CR kinetics, in line with the higher signal of the reduced dye. The lower signal at OCP means that the formed CSS already decays in under 7 ns, and the failure of a build-up of signal of the reduced dye at 540 nm shows that CR takes place on the same time scale as HI. This also shows in the decay-associated spectra of the slow HI component which is positive at > 500 nm at OCP, while at reductive potentials it is negative. This difference is caused by the convolution of HI and CR spectral features, as the differential DAS show: subtracting the HI DAS at more reductive potentials which show much slower CR reveals the spectral signature of CR in the first DAS at OCP(Figure 4.6). This means that at OCP, CR occurs on the same time scale as HI, namely tens of picoseconds.

Figure 4 . 6 :

 46 Figure 4.6: Decay-associated spectra obtained by a three-exponential fit of the TA-SEC data of NiO films sensitized with RuCo. Left: DAS of τ1 at different applied potentials (solid), associated with HI and fast CR. The dotted lines show the differential spectra of the DAS at negative applied potentials subtracted from OCP which reveals the contribution of fast CR at OCP. Right: DAS of τ3, associated with CR.

  Ru and RuCo. All reagents were purchased from Sigma Aldrich or Strem and used as obtained unless otherwise stated. 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-3oxime) ligands were custom-synthesized by the companies Oribase Pharma and Provence Technologies, respectively. The 2-(4-ethynylphenyl)-1H-imidazo[4DO)(DOH)N3pnBr2] (CoN3) complexes were prepared according to previously reported procedures. 1 H and 13 C NMR spectra were recorded at 298 K on a Bruker Avance 300 MHz spectrometer and the resulting spectra are referenced to the residual solvent peak and reported in relative to tetramethylsilane reference (δ = 0 ppm). UVvis absorption spectra were recorded either on a Shimadzu UV-1800 spectrometer or on an Agilent Cary 60 UV-vis spectrometer.

Figure 4 . 7 -

 47 Figure 4.7 -Synthesis of Ru and RuCo

1

  kHz repetition rate was used to analyze the excited state dynamics. The supercontinuum was generated by focusing a minor part of the output of the Ti:Sapphire amplifier onto a CaF2 plate mounted on a rotating stage and after generation, it was split into a probe and reference beam. Using a concave mirror, the probe pulse was focused onto the sample. The spectra of probe and reference were detected by a Czerny-Turner spectrograph of 150 mm focal length (SP2150, Princeton Instruments) equipped with a diode array detector (Pascher Instruments AB, Sweden).

  ns-TA spectroscopy. The setup for ns transient absorption measurements was previously described 204 . The pump pulse was generated by a Continuum Surelite OPO Plus (pumped by a Nd:YAG laser, ≈20 ns pulse duration, 10 Hz repetition rate). The probe light was generated by a 75 W Xenon arc lamp and focussed through the sample and detected by a Hamamatsu R928 photomultiplier. The signal was amplified and processed by a commercially available detection system (Pascher Instruments AB, Sweden). The measurements were performed upon 410~nm (2.0 W) at sample (RuP4OEt-EPIP and RuP4-Co) concentrations of 15 µM (OD at λexc ≈ 0.25) in acetonitrile and upon addition of 1 molar equivalent of triethylamine (TEA).

  built spectroelectrochemical cell, designed to work in a three-electrode setup under inert conditions and to optimize time resolution in the fs measurements, with a minimized path length (ca. 4 mm). The sensitized NiO film was used as the working electrode, a miniature leakless Ag/AgCl electrode (EDAQ) as reference electrode and a Pt wire as auxiliary electrode. The cell was assembled in a glove box and filled with 0.1 M TBABF4 in ACN. Transient absorption measurements were performed at open circuit potential and different applied potentials on the same film, moving the film in the plane periodically to prevent photodegradation. Prior to each experiment, the reference electrode was calibrated using ferrocene. In the ns-TASEC experiments, the pump pulse (λ = 485 nm) power was set to 0.2 mJ . To remove pump scatter, a 475 nm short pass and a 500 nm long pass edge filters were used for short and long wavelengths, respectively.

Figure 4 . 8 :

 48 Figure 4.8: Scheme showing the light-induced processes in a NiO films sensitized with RuCo at an applied potential of -0.74 V vs. Fc +/0 . Blue colouring indicates the localization of negative charge, e.g. an electron, while red colouring indicates the localization of positive charge, e.g. a hole.

Figure 4 . 10 :

 410 Figure 4.10: Results of the photoelectrochemical measurements of the organic dye-catalyst dyads. Top: Chronoamperometry during the first four hours of measurement, normalized to the amount of grafted dyad. Bottom left: Chronoamperometry under chopped light irradiation, showing the current spikes due to charge accumulation on the dye. Bottom right: TON of produced hydrogen over 22 hours.

Figure 4 .

 4 Figure 4.11: In-operando characterization of NiO films sensitized with T2R-Cat1: UV-Vis spectra recorded during four hours under operando conditions (blue spectra) and the average current density of four different T2R-Cat1-sensitized NiO films plotted against 1-10 -A , where A is the integrated absorbance in the visible range recorded during the operando UV-Vis measurements after subtracting the NiO background. 1-10 -A is proportional to the number of absorbed photons and was scaled to the current density at 600 s to account for the initial current spike.

Figure 4 . 12 :

 412 Figure 4.12: Results of TA-SEC measurements of NiO films sensitized with T2R-Cat1 and T2R. Left: fs-TA-SEC spectra at OCP and -0.74 V vs. Fc +/0 applied potential as well as DAS at -0.74 V vs. Fc +/0 applied potential. Right: ns-TA-SEC spectra at -0.74 V and 1-.15 V vs. Fc +/0 with reference spectra of the Co I state of Cat1 (dashed) and of the one-electron reduced dye (dotted). Bottom: Schematic representation of the light-induced processes taking place in a NiO|T2RCat1 photocathode at -0.74 V vs. Fc +/0 applied potential.

Figure 5 . 1 .

 51 Figure 5.1. Schematic representation of the principal characterization techniques used in this work on the DSPCs.The first important part of the work was the investigation of the photophysics of the organic dyes and dyads in solution. The dyad T1-Co, which had already shown promising performance in a functional H2-evolving NiO photocathode, was studied by TA spectroscopy to identify the light-induced processes and the possible ET to the catalyst. The T1 dye underwent ultrafast relaxation to the thermally relaxed excited singlet state, from which it decayed to the ground state in the sub-ns to ns timescale.

  The fast ISC induced by the Co complex observed here might open up new possibilities in the area of organic triplet sensitizers.As part of the work on DSPCs, NiO films sensitized with RuCo were studied by TA-SEC to investigate the light-induced processes in the dyad and the potential dependence of the interfacial processes such as HI and CR. There are very few

7 RésuméPublication 1 -Publication 2 -

 712 Photokathoden mit der Untersuchung der Prozesse im angeregten Zustand sowohl in Lösung als auch auf Filmen kombiniert, um ein vollständiges Verständnis der Systeme und ihrer leistungsbegrenzenden Faktoren zu erlangen. Besonders wichtig war die Bestimmung der Lebensdauer des ladungsgetrennten Zustands und der Kinetik des Elektronentransfers zur Katalysatoreinheit. Bei den hauptsächlich untersuchten Verbindungen handelte es sich um eine Serie von vier edelmetallfreien Dyaden, die sich in der Art des organischen Push-Pull-Farbstoffs (T1 oder T2R) und in der des Katalysators für die Protonenreduktion (Co oder Cat1) unterschieden. Die Bewertung der Aktivität aller vier Kombinationen ermöglichte die Bestimmung des Beitrags jedes Teils zur Gesamtleistung und damit die Verknüpfung der molekularen Struktur mit der Aktivität des Systems. Die Aufklärung einer solchen Struktur-Aktivitäts-Beziehung ist wichtig, um die molekulare Struktur der Dyaden verbessern zu können. Das am besten funktionierende System T2R-Cat1 wurde im Detail untersucht, insbesondere einschließlich der Prozesse im angeregten Zustand bei angelegtem Potential durch transiente Absorptions-Spektroelektrochemie und post-und inoperando Messungen zur Bestimmung der Deaktivierungswege und -kinetik. Darüber hinaus wurden NiO-Photokathoden, die mit einer Ruthenium-Cobalt-Dyade

  Figure 1.1 Schematic outline of this thesis' structure. ..................................................... Figure 2.1: Schematic representation of a tandem DSPEC for full water splitting .....Figure 2.2: Core structures of PS used in DS-photocathodes .........................................Figure 2.3: Catalysts for HER ..............................................................................................Figure 2.4: General catalytic mechanism for hydrogen evolution by cobalt diimine dioxime complexes ............................................................................................................... Figure 2.5: Dye-catalyst assemblies reported in the literature for proton reduction in DSPC. ......................................................................................................................................Figure 2.6: Schematic representation of energy levels in a DSPC for hydrogen evolution .

		VIII
	SC SE List of Figures	semiconductor stimulated emission
	ACN SEC	acetonitrile spectroelectrochemistry
	bpy SED	2,2'-bipyridine sacrificial electron donor
	CPDT TA	cyclopentadithiophene transient absorption
	CR TOF	charge recombination turnover frequency
	CSS TON	charge-separated state turnover number
	DMF TPA	dimethyl formamide triphenylamine
	DSPC VB	dye-sensitized photocathode valence band
	DSPEC	dye-sensitized photoelectrochemical cell
	DSSC	dye-sensitized solar cell
	ESA	excited state absorption
	ET	electron transfer
	FTO	fluorine-doped tin oxide
	GSB	ground state bleach
	HER	hydrogen evolution reaction
	HI	hole injection
	ICT	intramolecular charge transfer
	ISC	intersystem crossing
	ITO	indium-doped tin oxide
	MeOH	methanol
	MLCT	metal-to-ligand charge transfer
	NDI	naphthalene diimide
	OCP	open-circuit potential
	PMI / PDI	perylene monoimide / perylene diimide
	PS	photosensitizer
	RHE	relative hydrogen electrode

................................................................................................................................

Figure 2.7: Different photocathode architectures used in DSPCs for proton reduction. ..................................................................................................................................................

Table 2 .

 2 1: Key figures of DSPCs for proton reduction published in the literature.

	2 DYE-SENSITIZED H2-EVOLVING PHOTOCATHODES: STATE OF THE ART

[Rh(dmbpy)2Cl2)] + 32 [Rh(dtBubpy)2Cl2)] + 80 D-π-A (

		D-π-A (T1)	Cobaloxime	4.5 7	NiO NiO	-0.4 0.14 (RHE) -12 vs. -6 +0.18 vs.	6 2	̴ 40 540 ±	3	± 8	˂1 1 14	M.	Chavarot-
							Ag/AgCl RHE			(12) 80		(after 1	Kerlidou et al.
	Co-grafting	Ru(bpy)3 D-π-A (P1) DPP-P Ru-Pd PMI (PMI-P) Ru-acetylide-cobaloxime	cobaloxime cobaloxime NiP NiP	7 7 3 5.5 3 3 4.5	NiO NiO IO-NiO NiO IO-CuCrO2 NiO	+ 0.05 vs. RHE -0.4 vs. Ag/AgCl +0.2 vs. RHE -0.2 vs. Ag/AgCl RHE 0 vs. RHE +0.41 vs. -0.4 vs. -0.2 vs. Ag/AgCl Ag/AgCl 0.14 +0.18 vs. vs RHE RHE 0 vs. RHE RHE +0.07 vs. -0.4 vs. Ag/AgCl	-13 -18 -35 -12 -36 ± 2 -25 -3.2	2 1.5 2 1 2 4.5	n.r. 40 ± 6 290 (290 a ) 300 324 (410) 54 ± 3 10) (215 ± 109 (33 a )	n.r. ± 14 36 ± 5 n.r. 68 n.r. h) 46 n.r. ± 8 2.5 48 ± 2	L. Sun et al. 2019 53 L. Sun et al. E. Reisner et al. C. Olivier et al. 2019 79
		C343		[Fe-Fe]	4.5	NiO CuCrO2	-0.3	vs.	< -10	0.3	12.4 (160 ±	50	≤ 3	L.Hammarström
							Ag/AgCl			(12.4 1 ) 24)		et al. 2016 54
	Supramolecular	O22-			7	NiO	+0.17 vs. -0.2 vs.	-15	2.5	290		n.r.	Y. Wu et al.
	assembly	cobaloxime				RHE NHE				(290 a )		2013 80
							+ 0.2 vs.
		D-π-A (RBG-	Co	5.5	NiO	-0.4 RHE	vs.	-8	2	n.r.	9.3 ±	n.r.	V. Artero et al.
		174) T1-Co			5.5	NiO	Ag/AgCl -0.4 vs.	-15	2	37 ± 6	1.5 ± 1	˂1 a	V.Artero et al.
							+0.14 vs. Ag/AgCl			(26 ±		2016 26
							RHE + 0.14 vs.			10)
		DPP-P		NiP	3	CuCrO2	0 (RHE) RHE		-15.1	2	94 ±	34 ± 8	126 ±
		T2-Co	BH4)	Mo3S4	0 5.5	NiO NiO	0 (NHE) -0.4 vs.	183 ± 36 16.6 -11 2	n.r. 10 16 ± 5 ± 1 49 ± n.r. 13 n.r.	Y. Wu et. al. V.Artero et al.
		PMI PB-2 PMI D-π-A (RBG-174) Ru-Pt	Cobaloxime NiP cobaloxime	1 4.8 1 7 3	NiO NiO NiO NiO CuGaO2 NiO	-0.4 Ag/AgCl vs. Ag/AgCl + 0.14 vs. RHE 0 vs. RHE 0 vs. -0.4 vs. Ag/AgCl Ag/AgCl 0.41 (RHE) -45 -225 -5.8 -10 -140 + 0.48 vs. RHE -0.2 vs. -33 ± 7	2 2 0.3 2 2 1	124 a (376 ± 40) 1 (326) 35 ± 2 31 ± 8 60 ± ˂1 1 n.r. 2.5 n.r. 0.05 437 a (1150) 98 ± 1.5 555 74 82 198 47	2016 24 2018 87 E. Reisner et al. H. Tian et al. M. Wasielewski et al. 2017 52 2017 77 V. Artero et al. E. A. Gibson et
							Ag/AgCl			(250)		al. 2019 46

PS-Zr-NiP (RuP3-Zr-NiP)

	3	NiO	+0.3	vs.	-7	2	6 ± 1	± 2	≈1 a	E. Reisner et al.
			RHE							2016 63
	D

-Zr-PS-Zr- NiP (Dianiline-Zr- RuP2-Zr-NiP)

	5.1	io-ITO	-0.25	vs.	-56	4	>1000 ± 5	>15 a	T. J. Meyer et al.
			NHE						2016 38

PS-Zr-NiP (RuP2-Zr-NiP)

	5	Doped	-0,25 NHE -40	2	700 ±	± 3	20 a	T. J. Meyer et al.
		NiO/ioITO			80
					(700 ±
					80) a

  Observation of the spectral signature of the reduced catalyst together with the signature of the oxidized dye unit would enable an unequivocal assignment of the deactivation to ET. Steady-therefore be observable in TA spectroscopy if ET takes place upon excitation of a dyad already in the Co II oxidation state.Thus, to study whether the faster excited state decay of T1-Co is due to ET, TA spectroscopy was performed in a spectroelectrochemical cell at various applied potentials: at the open-circuit potential (OCP, e.g., without an applied potential, where the Co catalyst is in its initial Co III state), and at potentials sufficiently negative to reduce the cobalt catalyst to its Co II and Co I state. It was hypothesized that under these conditions, the existence of ET could be unequivocally confirmed or ruled out, either by the appearance of the Co I state after excitation of the Co II state of the dyad, or by a marked difference in excited state decay kinetics, especially in comparison to the reference Co I state.

	state UV-Vis spectroelectrochemical experiments were carried out to reveal the
	spectral features of the respective units in their oxidised and reduced state: in T1-Co,
	upon reduction of the initially present Co III state of the catalyst to the Co II state, the
	catalyst shows only a low-intensity absorption at 512 nm (Figure 3.5). At the same

wavelength, the oxidised dye shows a strong bleach of its ICT absorption, which would hide the Co II band in case of ET. However, the Co I state shows a much stronger absorption at 600-800 nm, which does not overlap with the bleach of the oxidised dye and should

  More in general, this setup allows to study excited state processes and especially ET kinetics at a variety of oxidation states of the donor and/or acceptor in a dyad, or indeed a multicomponent system. This is very valuable in the field of multielectronic redox catalysis where multiple ET steps are required: While the first ET step, which is the only one accessible by traditional TA spectroscopy, might occur rapidly due to a high driving force or favourable orientation/geometry, this is not necessarily the case for subsequent ET steps. These might be severely slowed down or inactivated by lower driving force, structural rearrangements taking place upon the preceding ET step or other influences. TA-SEC makes it possible to study each ET step separately, arguably most importantly the final step that leads to the formation of the catalytically active species, in this case the Co I state. To date, this has been used to study a reduced copper PS130 and two dye-catalyst assemblies, a Ru-Rh 42 and a Ir-polyoxometalate dyad.131

Table 4 .

 4 1: Obtained time constants by fitting the TA-SEC data of Ru-and RuCo-sensitized NiO films at OCP and different applied potentials with multiexponential fits.

				fs-TASEC					ns-TASEC
		Applied potential/ V vs. Fc +/0	τ0	τ1 / ps	τ2 / ps	τ3	τ1 / µs	τ2 / µs	τ3 / µs	Normalized integral DAS	τave / µs
		OCP		67 1070
	Ru	-0.74		52 735		0.1 1.6 15 1 0.97 0.48	3.6
		-1.15	< 2	31 655	> 7	0.1 1.5 16 1 1.18 0.93	5.2
		OCP	ps	67 950	ns
	RuCo	-0.74		69 1360		0.1 1.2 13 1 0.77 0.66	3.9
		-1.15		45 1470		0.1 1.2 13 1 0.85 0.77	4.3

  , 1340. Published by the Multidisciplinary Digital Publishing Institute (MDPI).Publication 3 -Spectroscopic investigations provide a rational for the hydrogenevolving activity of dye-sensitized photocathodes based on a cobalt tetraazamacrocyclic catalyst.Reproduced with permission from ACS Catalysis, submitted for publication on17.11.2020. Unpublished work ©2020 American Chemical Society.A doctoral thesis does not write itself -normal! During my journey, I have had the great luck to be able to count on the great help and support of many people, without whom this thesis would not have been possible.First and foremost, I want to thank my supervisors Dr. Benjamin Dietzek and Dr. Murielle Chavarot-Kerlidou for the chance to work in their respective groups and the great collaboration. Murielle, thank you for always having an open ear, for the long (sometimes very long) scientific discussions and your support in all kinds of matters, not least French bureaucracy! Benjamin, thank you for the challenging tasks, the scientific discussions and the trust you placed in me. My thanks go also to Dr. Vincent Artero for giving me the opportunity to work in his group at CEA which was an amazing place to do science.

	Authors	SB	TS	ABMG MP	VA	MCK	BD
	Synthesis Authors	SB	x JM	EG	MK	VA	BD	MCK
	Characterization Synthesis	x	x x	x
	Time-resolved Characterization spectroscopy (Photo-)electrochemistry Data analysis and processing Time-resolved spectroscopy	x x x x x	x	x	x			x
	DFT calculations Data analysis and processing	x		x	x		x	x
	Data interpretation Data interpretation	x x		x	x	x	x x	x x
	Concept development Concept development Preparation of the manuscript Revision of the manuscript Preparation of the manuscript Revision of the manuscript Proposed publication equivalents Proposed publication equivalents	x x x x x 1.0 1.0		x	x	x x x	x x x x x x	x x x x x x

  Publication 4 -Hydrogen Production at a NiO Photocathode Based on a Ruthenium Dye-Cobalt Diimine Dioxime Catalyst Assembly: Insights from Advanced Spectroscopy and Post-operando Characterization Reproduced with permission from ACS Appl. Mater. Interfaces 2021, 13, 42, 49802-49815. ©2021 American Chemical Society Authors SB EG CM AS JB NQ CG DL CSP DG DA SK VA BD MCK

	Synthesis		x			x
												. ©2021
	American Chemical Society Characterization x x	x
	(Photo-	x	x
	Authors )electrochemistry Time-resolved x		x	SB	JM	EG	MK		VA		BD	MCK
	Synthesis spectroscopy				x	x
	Time-resolved	x
	Characterization spectroelectrochemistry			x	x	x
	Mass spectrometry								x	x	x
	(Photo-)electrochemistry		x				x
	Time-resolved spectroscopy Calculations	x x	x						x
	Data analysis and processing XPS	x		x	x					x	x
	Data interpretation processing Data analysis and x x	x	x	x	x		x		x	x	x	x
	Concept development Data interpretation x x	x	x			x		x	x x	x	x	x	x	x	x
	Preparation of the manuscript Concept development x x		x	x							x	x	x	x	x
	Revision of the manuscript manuscript Preparation of the x x	x						x	x	x	x	x	x
	Proposed publication Revision of the x x manuscript equivalents Proposed publication 1.0 1.0		1.0							x	x	x	x
	equivalents

Acknowledgements

some PS have slower HI that takes place in some tens of ps. [START_REF] Bräutigam | Hole Injection Dynamics from Two Structurally Related Ru-Bipyridine Complexes into NiOx Is Determined by the Substitution Pattern of the Ligands[END_REF][START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF][START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF]107,183,193 These slower HI kinetics are usually observed for Ru complexes, while organic dyes almost exclusively show ultrafast HI. There are very few examples where HI was very slow (250 ns), 194 did not occur at all 175 or only from the oxidised PS after oxidative quenching with an electron acceptor. 195 A good example for multiexponential HI is that of Ru(bpy)3 by Dillon and coworkers, who used a four-component multiexponential fit for HI with time constants of 2.8, 39, 280 ps and 10 ns.

The amplitude of the ultrafast HI component increases at more negative potentials which translates to a lower GSB signal and a higher signal of the reduced Ru at 540 nm at the beginning of the experiment. The slower component increases both in rate and in amplitude at more negative potentials. The biphasic HI kinetics might be caused by a different localization of the excited electron: if the electron is primarily located on the anchoring bpy ligand, the excess charge density slows down HI into NiO, while localization on the extended ligand presents no impediment and thus leads to fast HI, as observed before for Ru(bpy)3. [START_REF] Bräutigam | Hole Injection Dynamics from Two Structurally Related Ru-Bipyridine Complexes into NiOx Is Determined by the Substitution Pattern of the Ligands[END_REF] In this case, the slower component might be an interligand ET from the bpy to the imidazophenanthroline ligand that gatekeeps the fast HI.

Declaration of Originality

I certify that the work presented here is, to the best of my knowledge and belief, original and the result of my own investigations, except as acknowledged, and has not been submitted, either in part or whole, for a degree at this or any other university.

Selbständigkeitserklärung

Ich erkläre, dass ich die vorliegende Arbeit selbständig und nur unter Verwendung der angegebenen Hilfsmittel und Quellen angefertigt habe und dass ich nicht die gleiche oder eine in wesentlichen Teilen ähnliche Arbeit bei dieser oder einer anderen Hochschule als Dissertation eingereicht habe.

Déclaration d'originalité

Je certifie que le travail présenté ici est, au meilleur de ma connaissance et de ma conviction, original et le résultat de mes propres enquêtes, sauf comme reconnu, et n'a pas été soumis, en partie ou en totalité, pour un diplôme à cette ou une autre université.

Sebastian Bold

Jena, XX.XX.2020

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