Sebastian Bold

Julien Massin

Emmanouil Giannoudis

Matthieu Koepf

Vincent Artero

Benjamin Dietzek

Murielle Chavarot-Kerlidou

Spectroscopic investigations provide a rationale for the hydrogen-evolving activity of dye-sensitized photocathodes based on a cobalt tetraazamacrocyclic catalyst

Keywords: Cobalt catalyst, dye-sensitized, solar fuels, photoelectrocatalysis, spectroscopy

Dye-sensitized photoelectrochemical cells (DSPECs) are a promising approach to produce solar fuels, e.g. by reduction of protons to molecular hydrogen. Here, we present functional NiO photocathodes sensitized with covalent organic dye-catalyst assemblies integrating a robust cobalt tetraazamacrocyclic complex. This catalyst proved to be decisive to improve the stability of these systems, hydrogen being produced with a 26-fold increase in turnover numbers compared to similar photocathodes based on a cobaloxime catalyst, all other conditions being strictly identical otherwise. Transient absorption spectroelectrochemical (TA-SEC) measurements observed the catalytically competent Co I state in a functional dye-sensitized photocathode, with a lifetime of up to > 1 ms, comparable to the timescale of catalysis. They also unveiled the lack of efficiency of the thermally activated electron transfer from the reduced dye to the catalyst, which firstly limits the photocurrent density for hydrogen production. A second consequence is the accumulation of photogenerated charges on the acceptor side of the dye, ultimately leading to its degradation, as observed in operando and post-operando characterization of the system. This study thus provides tracks to improve the performances of hydrogen-evolving dye-sensitized photocathodes toward their integration into functional DSPECs.

Introduction

Storing solar energy, which by nature is intermittent, in chemical bonds, i.e. a solar fuel, is highly appealing to move from fossil fuels to renewable energies. [START_REF] Nocera | Solar Fuels and Solar Chemicals Industry[END_REF] One promising approach to producing solar fuels relies on artificial photosynthesis, which mimics the natural photosynthetic process with synthetic components tailored to perform the desired redox reaction. [START_REF] Zhang | Artificial Photosynthesis: Opportunities and Challenges of Molecular Catalysts[END_REF][START_REF] House | Artificial Photosynthesis: Where Are We Now? Where Can We Go?[END_REF][START_REF] Berardi | Molecular Artificial Photosynthesis[END_REF] In that context, the production of hydrogen (H2) in water splitting dye-sensitized photoelectrochemical cells (DSPECs) is the subject of intensive research efforts. [START_REF] Brennaman | Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells[END_REF][START_REF] Yu | Recent Advances in Dye-Sensitized Photoelectrochemical Cells for Solar Hydrogen Production Based on Molecular Components[END_REF] Tandem DSPECs combine two photoelectrodes, a photoanode driving water oxidation to oxygen and a photocathode where protons and electrons are recombined to produce H2, in that way mimicking the Z-scheme of natural photosynthesis. [START_REF] Windle | Earth-Abundant Molecular Z-Scheme Photoelectrochemical Cell for Overall Water-Splitting[END_REF][START_REF] Fan | Pt-Free Tandem Molecular Photoelectrochemical Cells for Water Splitting Driven by Visible Light[END_REF][START_REF] Li | Organic Dye-Sensitized Tandem Photoelectrochemical Cell for Light Driven Total Water Splitting[END_REF] However, the dye-sensitized photocathodes reported to date lag well behind their photoanode counterparts in terms of photocurrent density and stability and are the limiting factor in tandem DSPEC, calling for investigations to understand their limitations and to develop guidelines to improve their performances. [START_REF] Windle | Earth-Abundant Molecular Z-Scheme Photoelectrochemical Cell for Overall Water-Splitting[END_REF][START_REF] Fan | Pt-Free Tandem Molecular Photoelectrochemical Cells for Water Splitting Driven by Visible Light[END_REF][START_REF] Li | Organic Dye-Sensitized Tandem Photoelectrochemical Cell for Light Driven Total Water Splitting[END_REF] Recently, molecular hydrogen-evolving catalysts based on first-row transition metals (Fe, Co, Ni) have been successfully integrated in dye-sensitized photocathodes. In these studies, the synthetic tunability of the catalyst most of the time governs the electrode construction strategy:

(i) immobilisation of supramolecular assemblies built by axial coordination of the dye on the Co center of a cobaloxime catalyst, [START_REF] Ji | Photostable P-Type Dye-Sensitized Photoelectrochemical Cells for Water Reduction[END_REF][START_REF] Lyu | H2 -Evolving Dye-Sensitized Photocathode Based on a Ruthenium-Diacetylide/Cobaloxime Supramolecular Assembly[END_REF] (ii) fully covalent dye-catalyst assemblies prepared thanks to a copper-catalyzed azide-alkyne cycloaddition (CuAAC) strategy using an azidosubstituted cobalt diimine-dioxime complex, [START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF][START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF][START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] (iii) layer-by-layer deposition of the dye and of a nickel bis(diphosphine) catalyst (Dubois type complex) functionalized with four phosphonate groups, [START_REF] Wang | Layer-by-Layer Molecular Assemblies for Dye-Sensitized Photoelectrosynthesis Cells Prepared by Atomic Layer Deposition[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] (iv) alternatively, co-immobilisation of dyes and catalysts at the semiconductor surface, [START_REF] Windle | Earth-Abundant Molecular Z-Scheme Photoelectrochemical Cell for Overall Water-Splitting[END_REF][START_REF] Fan | Pt-Free Tandem Molecular Photoelectrochemical Cells for Water Splitting Driven by Visible Light[END_REF][START_REF] Li | Organic Dye-Sensitized Tandem Photoelectrochemical Cell for Light Driven Total Water Splitting[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 | Dye-Sensitized LaFeO3 Photocathode for Solar-Driven H2 Generation[END_REF][START_REF] Creissen | Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst[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] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF] simplifying the synthesis and allowing to vary the dye/catalyst ratio. In addition to this diversity in the electrode architectures, which in turn strongly affects the electron transfer kinetics to the catalytic center, H2 evolution activity was assessed under a wide range of conditions (nature of the electrolyte, pH, applied potential…).

Altogether, this prevents direct comparison of the performances from one system to another to identify the best-performing components and to establish structure-activity relationships necessary for a rational improvement of the photocathode design.

In this study, we present a new noble metal-free covalent dye-catalyst assembly based on pushpull organic dyes covalently linked via CuAAC "click" chemistry to the tetraazamacrocyclic cobalt complex Cat1 as catalytic unit. The superior activity and stability of the latter, compared to the cobalt diimine-dioxime catalyst, was previously reported for photocatalytic proton reduction under fully aqueous conditions. [START_REF] Nie | Charge Transfer Dynamics and Catalytic Performance of a Covalently Linked Hybrid Assembly Comprising a Functionalized Cobalt Tetraazamacrocyclic Catalyst and CuInS2/ZnS Quantum Dots for Photochemical Hydrogen Production[END_REF][START_REF] Grau | Electrochemically and Photochemically Induced Hydrogen Evolution Catalysis with Cobalt Tetraazamacrocycles Occurs Through Different Pathways[END_REF][START_REF] Leung | Electro-and Photocatalytic Hydrogen Generation in Acetonitrile and Aqueous Solutions by a Cobalt Macrocyclic Schiff-Base Complex[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] Sandroni | Cadmium-Free CuInS2/ZnS Quantum Dots as Efficient and Robust Photosensitizers in Combination with a Molecular Catalyst for Visible Light-Driven H2 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] Gimbert-Suriñach | Efficient and Limiting Reactions in Aqueous Light-Induced Hydrogen Evolution Systems Using Molecular Catalysts and Quantum Dots[END_REF][START_REF] Roy | A Systematic Comparative Study of Hydrogen-Evolving Molecular Catalysts in Aqueous Solutions[END_REF] In parallel, the dye structure was modified from the previously published T1-Co [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] with a cyclopenta[1,2-b:5,4-b']dithiophene (CPDT) bridge to bathochromically shift the absorption and increase the extinction coefficient in the visible region. [START_REF] Yen | Arylamine-Based Dyes for p-Type Dye-Sensitized Solar Cells[END_REF][START_REF] Li | Synchronously Reduced Surface States, Charge Recombination, and Light Absorption Length for High-Performance Organic Dye-Sensitized Solar Cells[END_REF] The alkyl chains introduced on the linker are intended (i) to limit dyad-dyad interactions and (ii) to prevent dyad desorption by forming a hydrophobic layer at the surface of the NiO film. [START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF][START_REF] Yu | Decoupling PH Dependence of Flat Band Potential in Aqueous Dye-Sensitized Electrodes[END_REF] Photoelectrochemical activity was assessed under strictly identical conditions to our previously-reported system, [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] thus providing a comprehensive comparison with T1-Co [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] and highlighting the beneficial role played by the T2R and Cat1 moieties on the H2 production performances. In addition, in-depth operando and post-operando characterizations of the T2R-Cat1-sensitized photocathode allowed to identify relevant degradation processes and to determine their kinetics. Finally, the light-driven formation of the different intermediates at the surface of the NiO film, including the catalytically competent Co I state of Cat1, was investigated by transient absorption spectroelectrochemical (TA-SEC) [START_REF] Bold | Electron Transfer in a Covalent Dye-Cobalt Catalyst Assembly -a Transient Absorption Spectroelectrochemistry Perspective[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] Zhang | Excited-State Dynamics of Heteroleptic Copper(I) Photosensitizers and Their Electrochemically Reduced Forms Containing a Dipyridophenazine Moiety -a Spectroelectrochemical Transient Absorption Study[END_REF][START_REF] Dillon | Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes[END_REF][START_REF] D'amario | Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells[END_REF] measurements and provided a mechanistic rational to improve the design of these dye-sensitized photocathodes.

Figure 1.

Structures of the H2-evolving catalyst Cat1, the dyes T1 and T2R, and the dyads T1-Cat1 and T2R-Cat1.

Results

Synthesis of the novel Cat1-based dyads. Functionalization of Cat1 is a challenging task, as reflected by the scarcity of procedures in the literature to prepare Cat1 derivatives. Up to now, its catalytic performances were assessed either under electroassisted conditions [START_REF] Grau | Electrochemically and Photochemically Induced Hydrogen Evolution Catalysis with Cobalt Tetraazamacrocycles Occurs Through Different Pathways[END_REF][START_REF] Leung | Electro-and Photocatalytic Hydrogen Generation in Acetonitrile and Aqueous Solutions by a Cobalt Macrocyclic Schiff-Base Complex[END_REF][START_REF] Mccrory | Electrocatalytic Hydrogen Evolution in Acidic Water with Molecular Cobalt Tetraazamacrocycles[END_REF] or under photocatalytic conditions in multicomponent homogenous systems. [START_REF] Grau | Electrochemically and Photochemically Induced Hydrogen Evolution Catalysis with Cobalt Tetraazamacrocycles Occurs Through Different Pathways[END_REF][START_REF] Leung | Electro-and Photocatalytic Hydrogen Generation in Acetonitrile and Aqueous Solutions by a Cobalt Macrocyclic Schiff-Base Complex[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] Sandroni | Cadmium-Free CuInS2/ZnS Quantum Dots as Efficient and Robust Photosensitizers in Combination with a Molecular Catalyst for Visible Light-Driven H2 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] Gimbert-Suriñach | Efficient and Limiting Reactions in Aqueous Light-Induced Hydrogen Evolution Systems Using Molecular Catalysts and Quantum Dots[END_REF][START_REF] Roy | A Systematic Comparative Study of Hydrogen-Evolving Molecular Catalysts in Aqueous Solutions[END_REF] The sole exception reported to date relies on a palladium-catalyzed cross-coupling introduction of a pyridine dicarboxylate anchoring group on the pyridine moiety of the ligand, [START_REF] Nie | Charge Transfer Dynamics and Catalytic Performance of a Covalently Linked Hybrid Assembly Comprising a Functionalized Cobalt Tetraazamacrocyclic Catalyst and CuInS2/ZnS Quantum Dots for Photochemical Hydrogen Production[END_REF] successfully allowing to assemble Cat1 with quantum dots [START_REF] Nie | Charge Transfer Dynamics and Catalytic Performance of a Covalently Linked Hybrid Assembly Comprising a Functionalized Cobalt Tetraazamacrocyclic Catalyst and CuInS2/ZnS Quantum Dots for Photochemical Hydrogen Production[END_REF] or a TiO2-protected silicon photoelectrode. [START_REF] Nie | Boosting the Performance of a Silicon Photocathode for Photoelectrochemical Hydrogen Production by Immobilization of a Cobalt Tetraazamacrocyclic Catalyst[END_REF] On the other hand, CuAAC is a highly versatile, efficient and modular coupling reaction. It was previously employed by some of us to assemble the push-pull organic dye T1 (Figure 1) with the H2evolving cobalt diimine-dioxime catalyst Co(N 2 ,N 2 '-propanediylbis(2,3-butandione-2-imine-3oxime))Br2 (Co), leading to the first functional noble metal-free NiO photocathode based on a covalent assembly. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] We first decided to use the same strategy to covalently assemble Cat1 with T1 and with the novel T2R dye (Figure 1); however, all our attempts to prepare the cobalt complex Cat1 substituted by an azido group in para position of the pyridine ring proved unsuccessful. An alternative route (Scheme 1) was thus developed: CuAAC coupling was first carried out between the alkyne-substituted dyes ( tBu T1-alkyne and tBu T2R-alkyne, see SI) and the 4-azido-2,6-diacetylpyridine precursor (Scheme 1), followed by the templated synthesis of the tetraazamacrocyclic cobalt complex. This way, the targeted dyads were isolated in two steps under their tert-butyl ester protected forms ( tBu T1-Cat1 and tBu T2R-Cat1, Figure 1) in 50-60 % overall yield. Full synthetic details and characterizations are provided in the Supporting Information. Scheme 1. Synthetic strategy for the preparation of tBu T1-Cat1 and tBu T2R-Cat1.

Spectroscopic and electrochemical characterization in solution.

All compounds were characterized by cyclic voltammetry (Table 1, Figures 2 &S1). The cyclic voltammogram of tBu T2R-Cat1 displays five redox events (Figure 2). On the anodic scan, two waves at +0.43 V and +0.58 V vs. Fc +/0 are assigned to oxidation processes occurring on the dye moiety, by comparison with the cyclic voltammogram of tBu T2R (Figure 2). The latter is characterized by two quasi-reversible waves at +0.48 V and +0.66 V vs. Fc +/0 , attributed to the oxidation of the triphenylamine (TPA) moiety and of the electron-rich CPDT bridge, respectively, as reported for related CPDT-based push-pull dyes. [START_REF] Yen | Arylamine-Based Dyes for p-Type Dye-Sensitized Solar Cells[END_REF][START_REF] Cheng | Organic Dyes Incorporating the Cyclopentadithiophene Moiety for Efficient Dye-Sensitized Solar Cells[END_REF] On the cathodic scan of tBu T2R-Cat1, the two reversible processes observed at -0.54 V and -0.97 V vs. Fc +/0 are in full agreement with the two successive reversible reductions observed on the Co III catalyst Cat1 (Figure 2) yielding the formal Co II and Co I states, respectively. [START_REF] Leung | Electro-and Photocatalytic Hydrogen Generation in Acetonitrile and Aqueous Solutions by a Cobalt Macrocyclic Schiff-Base Complex[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] The reduction of the acceptor part of the dye moiety is observed at -1.62 V vs. Fc +/0 and shows a loss of reversibility compared to tBu T2R. The cyclic voltammogram of tBu T1-Cat1 (Figure 2) displays very similar reduction processes for the catalyst (-0.53 V and -1.03 V vs. Fc +/0 ) and dye (-1.65 V vs. Fc +/0 ) units; the single anodic event at +0.64 V vs. Fc +/0 is assigned to the TPA-centered oxidation process, by comparison with the cyclic voltammogram of T1 (Figure S1a). [START_REF] Massin | Dye-Sensitized PS-b-P2VP-Templated Nickel Oxide Films for Photoelectrochemical Applications[END_REF][START_REF] Ji | Linker Effect in Organic Donor-Acceptor Dyes for p-Type NiO Dye Sensitized Solar Cells[END_REF] The UV-Vis absorption spectra (Figure 3, Table S1) show two main absorption bands between 300 and 600 nm. The band at 350 nm is assigned to a π-π* transition on the TPA moiety [START_REF] Ishow | Comprehensive Investigation of the Excited-State Dynamics of Push-Pull Triphenylamine Dyes as Models for Photonic Applications[END_REF] and its position remains unchanged for all dyads in comparison with the parent dye. At longer wavelengths, all compounds show the intramolecular charge transfer (ICT) transition from the TPA donor to the cyanoacrylate acceptor group, as typical for this class of push-pull dyes. [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF][START_REF] Yen | Arylamine-Based Dyes for p-Type Dye-Sensitized Solar Cells[END_REF][START_REF] Massin | Dye-Sensitized PS-b-P2VP-Templated Nickel Oxide Films for Photoelectrochemical Applications[END_REF] For the tBu T2R series, this absorption is red-shifted in comparison to the tBu T1 series by ca. 0.33 eV (in ACN, Figure S2), which is attributed to the more extended conjugated system due to the CPDT linker. Furthermore, extinction coefficients for tBu T2R (57,400 M -1 cm -1 , max = 499 nm) exceed those of tBu T1 (27,700 M -1 cm -1 , max = 448 nm).

Overall, in the covalent assemblies, the electrochemical and spectroscopic properties of the catalytic and dye units are retained, reflecting their electronic decoupling within the dyads, as previously established for T1-Co [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] and more generally frequently encountered for triazolebased dyads assembled by CuAAC coupling. NiO film sensitization and characterization. NiO films were prepared by spin-coating and sintering a F108 templated hydro-alcoholic 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][START_REF] Sumikura | Syntheses of NiO Nanoporous Films Using Nonionic Triblock Co-Polymer Templates and Their Application to Photo-Cathodes of p-Type Dye-Sensitized Solar Cells[END_REF] The 4-layered films have a thickness of 880 ± 170 nm as determined by SEM (Figure S3, Table S2). They were sensitized by soaking for 24 h in methanolic solutions of the deprotected dyads (or dyes).

The amount of dyad (or dye) loading was quantified by dyad (or dye) desorption in a methanolic 1M phenylphosphonic acid solution followed by UV-vis absorption measurement of the resulting solution, [START_REF] Lyu | H2 -Evolving Dye-Sensitized Photocathode Based on a Ruthenium-Diacetylide/Cobaloxime Supramolecular Assembly[END_REF][START_REF] Ameline | Isoindigo Derivatives for Application in P-Type Dye Sensitized Solar Cells[END_REF] using the molar extinction coefficient of the TPA-centered band at 350 nm (Figure S4, Table S3). Grafting densities are in the range of 6-14 nmol•cm -2 (Table S4), in good agreement with the literature on other dye-sensitized NiO films. [START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF][START_REF] Kaeffer | 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] Massin | Dye-Sensitized PS-b-P2VP-Templated Nickel Oxide Films for Photoelectrochemical Applications[END_REF] We observed that the grafting density substantially varies from one batch of NiO films to another, preventing any straightforward analysis of the loading based on the molecular structures of the dyads. This supports the need to verify the grafting efficiency for each individual sample. For this, we used half of each freshly-sensitized NiO film to determine the dyad (or dye) loading while the other half was used for the assessment of photoelectrochemical hydrogen production. Hence, an accurate quantification of the amount of dyad grafted on the considered film was obtained and used to calculate the turnover number (TON) values for hydrogen production activity (see below).

Photoelectrochemical activity. The photoelectrochemical activity of the dye-sensitized NiO photocathodes was assessed in a three-electrode configuration setup under conditions identical to our previous studies [START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF][START_REF] Kaeffer | 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] Lyu | H2 -Evolving Dye-Sensitized Photocathode Based on a Ruthenium-Diacetylide/Cobaloxime Supramolecular Assembly[END_REF] using an aqueous (N-morpholino)ethanesulfonic acid (MES) buffer solution at pH 5.5 as electrolyte; samples were irradiated with visible light at the equivalent of 1 sun intensity, using a 400 nm longpass filter and an IR filter. Linear sweep voltammograms (LSV) were recorded from +1.00 V to 0.00 V vs. RHE (Figure 4a). The large cathodic signal observed between +1.00 and ≈+0.6 V vs. RHE is attributed to capacitive charging of the NiO films. [START_REF] Boschloo | Spectroelectrochemistry of Nanostructured NiO[END_REF] Under irradiation, the dyad-sensitized NiO films start exhibiting cathodic photocurrents at ≈ +0.95 V vs. RHE, which are stable down to ≈ +0.5 V vs. RHE before increasing at more cathodic potentials. For the film sensitized with T2R-Cat1, the photocurrent reaches 13.5 A•cm -2 at +0.14 V vs. RHE (dark current subtracted). Chronoamperometric measurements under chopped light irradiation yielded insights into the magnitude and stability of the photocurrents measured at + 0.14 V vs. RHE, the potential applied for hydrogen production (Figure 4b). When light is turned on, the systems show an initial cathodic photocurrent spike; in addition, upon switching off the irradiation, a reverse anodic spike of lower intensity is observed, suggesting that some photogenerated electrons are accumulated on the dyad (see below) and not fully consumed by catalysis. [START_REF] Ji | Photostable P-Type Dye-Sensitized Photoelectrochemical Cells for Water Reduction[END_REF][START_REF] Lyu | H2 -Evolving Dye-Sensitized Photocathode Based on a Ruthenium-Diacetylide/Cobaloxime Supramolecular Assembly[END_REF][START_REF] Kirner | Visible-Light-Assisted Photoelectrochemical Water Oxidation by Thin Films of a Phosphonate-Functionalized Perylene Diimide Plus CoO x Cocatalyst[END_REF] Importantly, almost no photocurrent is generated in the absence of the catalytic unit (-0.4 A•cm -2 and -0.6 A•cm -2 after 20 minutes for films sensitized with T1 and T2R, respectively), while the presence of Cat1 leads to a ten-fold increase of the photocurrent density (-3.7 and -6.3 µA•cm - 2 for T1-Cat1 and T2R-Cat1, respectively). In longer-term photoelectrochemical experiments under continuous light irradiation, all photocathodes show steady-state photocurrents (Figure 5, Figure S8), albeit with marked differences in photocurrent magnitude and stability over time.

To investigate the role of the novel catalytic unit, the photoelectrochemical activity of the previously-reported dyad containing the cobalt diimine-dioxime catalyst Co(DO)(DOH)pnBr2 (T1-Co) was reassessed, together with the activity of a novel dyad T2R-Co combining T2R

with Co(DO)(DOH)pnBr2 (see SI for synthesis and characterisations in Table S1, Figures S1b, S4-7). After normalisation of the photocurrent density to the amount of grafted dyad to compare the intrinsic activity of the dyads (Figure 5b), the Cat1 and T2R dyads showed slower decay in photocurrent and higher overall photocurrent than their Co and T1 counterparts, respectively.

On average, the Cat1 dyads produced 3.2 times higher accumulated 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. Hydrogen-evolving activity assessment. To confirm that the observed photocurrent originates from the reduction of protons into hydrogen, the H2 evolution activity of the sensitized NiO photocathodes was assessed during long-term chronoamperometric experiments at an applied potential of +0.14 V vs. RHE and under continuous visible light irradiation. The amount of hydrogen produced was measured after 4 and 22 hours both in the headspace of the cell by gas chromatography and in the electrolyte using a micro Clark-type electrode probe following an established protocol. [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] Turnover numbers (TONCo) were calculated, using the dyad loading quantified for the freshly sensitized film. (T1-Cat1) and 22 ± 7 (T2R-Cat1) and Faradaic efficiencies around 65 % (Table 2). The activity of the Co dyads was much lower, with TON values below 2 and Faradaic efficiencies of ≈ 10 %, in agreement with previous reports. [START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF][START_REF] Kaeffer | 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] Comparing the influence of the lightharvesting units, the T2R systems produced around twice as much hydrogen as their T1 counterparts. 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. For all systems, the Faradaic efficiency decreased between 4 and 22 hours of irradiation (to ≈ 35 % for Cat1 and ≈ 3 % for Co).

Post-operando characterization

Molecular dyads were desorbed post-operando from the NiO film in a phenylphosphonic acid methanolic solution, with the aim to quantify the amount of dyad desorbed from the NiO surface during the course of the photoelectrochemical tests as well as to identify potential structural modifications. The absorbance of the desorption solutions was measured to quantify the remaining amount of dyad still present on the surface after a 22h PEC experiment. The resulting spectra are shown in Figure 6 and Figure S9, together with the spectra recorded for the corresponding dyads desorbed from a fresh film for comparison. a lesser degree of desorption which we ascribe to the protection against hydrolysis granted by the hydrophobic alkyl chains on the CPDT linker. [START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF] The strong decrease in absorbance at the ICT band, accompanied by the formation of a new band at 400 nm for T2R-Cat1, is likely caused by structural modification of the dye during the photoelectrochemical experiments, which expose the dye to relatively harsh conditions, i.e. applied reductive potential, light irradiation and aqueous buffer. We therefore decided to investigate which of these parameters is likely at the origin of the changes in dye absorption.

For this purpose, UV-Vis absorption spectra were recorded over time in a home-made cell on NiO films sensitized with T2R-Cat1 while varying the experimental parameters, including full operando conditions. First, spectra were recorded over one hour on a T2R-Cat1-sensitized NiO film with the operando potential applied in the absence of irradiation (Figure S10a); under these conditions, no spectral change was observed, apart from an initial jump in absorbance caused by the electrochromic nature of NiO. [START_REF] Sonavane | Efficient Electrochromic Nickel Oxide Thin Films by Electrodeposition[END_REF][START_REF] Xia | Electrochromic Properties of Porous NiO Thin Films Prepared by a Chemical Bath Deposition[END_REF] Second, when the film was submitted to visible light irradiation in the absence of any applied potential (Figure S10b), small amplitude spectral changes were observed that might be explained by isomerization of the cyanoacrylate double bond. [START_REF] Zietz | Photoisomerization of the Cyanoacrylic Acid Acceptor Group -a Potential Problem for Organic Dyes in Solar Cells[END_REF] By contrast, when the film was exposed both to visible light irradiation and to the applied potential, i.e. operando conditions, fast spectral changes were observed (Figure 7a), with features similar to those previously identified for the desorbed dyad in the post-operando measurements (Figure 5). A kinetic analysis revealed a biexponential decay with characteristic lifetimes of 6 and 90 min (Figure S11). In comparison, a T2R-sensitized NiO film displayed faster degradation kinetics, with ≈1 and 21 min characteristic lifetimes (Figure S12-13). Thus, it appears that both the applied potential and the visible light irradiation are necessary to cause the dye modification observed post-operando; in addition, this process seems to be accelerated when the catalyst is not present at the surface of the film.

To further study this process, UV-Vis spectroelectrochemical experiments were recorded on tBu T2R in dry 0.1 M nBu4NBF4 acetonitrile (ACN) solution. Under these conditions, the same irreversible spectral changes were observed upon exhaustive reduction of the dye (Figure S14), thus excluding any contribution from the aqueous electrolyte in the former experiments. tBu T1

showed an identical spectral response upon reduction, suggesting the same degradation process also takes place for this dye (Figure S14c). 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.

To ascertain whether the modification suffered by the dye leads to the loss of the catalytic center, e.g. by bond cleavage, the amount of Co present at the surface of the film was determined pre-and post-operando by ICP-MS (Table S5). After respectively 4 and 22 h of PEC activity, 84 and 93 % of the initial amount of Co was lost, which correlates very well to the loss of absorbance (84 % after 4 h). Furthermore, the photocurrent recorded on T2R-Cat1-sensitized NiO films correlates with the number of absorbed photons (Figure 7b), which is also tied to the absorbance. These findings suggest that the same underlying process causes both the loss of Co and of the dye absorbance, as separate, independent processes would lead to a stronger loss of photocurrent compared to either the loss of absorbance or of Co loading. Assuming therefore that 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

(Table S6).

Time-resolved spectroscopy and spectroelectrochemistry

Since the light-induced processes in the dye-sensitized photocathodes are crucial to charge the catalyst and thus enable catalytic proton reduction, they were investigated by time-resolved spectroscopy. In a first step, time-resolved emission was measured for all compounds grafted onto NiO and ZrO2 films, the latter serving as a non-quenching reference (see the Supporting Information for a detailed analysis, and Figure S15, Table S7). All systems showed a complete emission quenching within the duration of the instrumental response function (FWHM = 430 ps) on NiO, which was not observed on ZrO2. This quenching is attributed to hole injection from the excited dye into the valence band of the NiO film. [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF][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] Han | Interfacial Electron Transfer Yields in Dye-Sensitized NiO Photocathodes Correlated to Excited-State Dipole Orientation of Ruthenium Chromophores[END_REF][START_REF] Kou | Visible Light-Induced Reduction of Carbon Dioxide Sensitized by a Porphyrin-Rhenium Dyad Metal Complex on p-Type Semiconducting NiO as the Reduction Terminal End of an Artificial Photosynthetic System[END_REF] To further investigate the sensitized NiO films, we turned to transient absorption (TA) spectroscopy. We aimed to investigate the electron transfer dynamics under conditions that are as close as possible to the operando ones, i.e. with the operando potential applied to the film, albeit in the absence of catalysis in order to accumulate and detect the reduced Co species, which was achieved by using dry acetonitrile as solvent. Compared to unbiased dye-sensitized NiO films, applying an external negative bias was recently shown to sharply slow down the recombination rate of reduced dye molecules with holes in the NiO film. [START_REF] Dillon | Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes[END_REF][START_REF] D'amario | Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells[END_REF][START_REF] Daeneke | Dominating Energy Losses in NiO P-Type Dye-Sensitized Solar Cells[END_REF] This phenomenon is attributed to the suppression of a detrimental nongeminate recombination pathway by populating the NiO intragap states upon applying a negative potential. [START_REF] Dillon | Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes[END_REF] To study the influence of applied potential on the photoinduced kinetics, we performed TA-SEC experiments on NiO films sensitized either with the dyad T2R-Cat1 or the dye T2R as reference. In these measurements, data was recorded at open-circuit potential (OCP), i.e. in the electrolyte but without any applied bias, as well as during application of two cathodic potentials to the sensitized film: first, the potential employed in the photoelectrochemical experiments (-0.74 V vs. Fc +/0 ≙ -0.4 V vs. Ag/AgCl), sufficient to reduce the catalyst to its Co II state; [START_REF] Kaeffer | Insights into the Mechanism and Aging of a Noble-Metal Free H2 -Evolving Dye-Sensitized Photocathode[END_REF] then, a more cathodic one (-1.15 V vs. Fc +/0 ) where the catalyst is reduced to the Co I state. The latter measurement served as a reference since no electron transfer is thermodynamically possible to the catalyst in the Co I state (the third reduction process on the catalyst occurs at more negative potential than the dye reduction -see Figure 2). A detailed spectroscopic analysis of the fs-and ns-TA data is provided in the Supporting Information and the characteristic time constants are summarized in Table S8. Since both T2R-and T2R-Cat1-sensitized NiO films showed the same behaviour in the fs-TA-SEC experiment, they will be discussed together.

After laser pulse excitation and regardless of the potential applied to the electrode, the fs-TA spectra of T2R-and T2R-Cat1-sensitized NiO films initially show ground-state bleach (GSB) at 500 nm and excited state absorption (ESA) at > 550 nm, attributed to the vibrationally hot excited state of the dye (Figure 8, Figure S16-17). [START_REF] Bold | Investigating Light-Induced Processes in Covalent Dye-Catalyst Assemblies for Hydrogen Production[END_REF] This is followed by a fast recovery of GSB accompanied by a blue-shift of ESA to form a band with a maximum at 620 nm. Since the 620nm band is characteristic of the one-electron reduced dye (Figure S18), this first process is assigned to hole injection, leading to the formation of the primary charge-separated state NiO + |T2R -(-Cat1). Applying a three-component exponential fit to the data, a characteristic time constant of τ1 < 1 ps is obtained for this process, in line with the ultrafast hole injection observed in literature for other organic dyes grafted on NiO. [START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF][START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Massin | Investigating Light-Driven Hole Injection and Hydrogen Evolution Catalysis at Dye-Sensitized NiO Photocathodes: A Combined Experimental-Theoretical Study[END_REF][START_REF] Black | Investigating Interfacial Electron Transfer in Dye-Sensitized NiO Using Vibrational Spectroscopy[END_REF][START_REF] Morandeira | Coumarin 343-NiO Films as Nanostructured Photocathodes in Dye-Sensitized Solar Cells: Ultrafast Electron Transfer, Effect of the I3/I -Redox Couple and Mechanism of Photocurrent Generation[END_REF][START_REF] Morandeira | Photoinduced Ultrafast Dynamics of Coumarin 343 Sensitized P-Type-Nanostructured NiO Films[END_REF][START_REF] Sheibani | A Study of Oligothiophene-Acceptor Dyes in p-Type Dye-Sensitized Solar Cells[END_REF][START_REF] Smeigh | Ultrafast Recombination for NiO Sensitized with a Series of Perylene Imide Sensitizers Exhibiting Marcus Normal Behaviour[END_REF][START_REF] Gibson | A P-Type NiO-Based Dye-Sensitized Solar Cell with an Open-Circuit Voltage of 0.35 V[END_REF][START_REF] Le Pleux | Photophysical and Photovoltaic Investigations of Acceptor-Functionalized Perylene Monoimide Dyes for Nickel Oxide p-Type Dye-Sensitized Solar Cells[END_REF][START_REF] Yu | Electron Transfer Kinetics of a Series of Bilayer Triphenylamine-Oligothiophene-Perylenemonoimide Sensitizers for Dye-Sensitized NiO[END_REF] As this class of push-pull organic dyes are known to undergo a fast relaxation cascade to the relaxed ICT state on the same subps time scale (Figure S17), [START_REF] Bold | Electron Transfer in a Covalent Dye-Cobalt Catalyst Assembly -a Transient Absorption Spectroelectrochemistry Perspective[END_REF][START_REF] Ishow | Comprehensive Investigation of the Excited-State Dynamics of Push-Pull Triphenylamine Dyes as Models for Photonic Applications[END_REF][START_REF] Bold | Investigating Light-Induced Processes in Covalent Dye-Catalyst Assemblies for Hydrogen Production[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] Flender | Excited-State Relaxation of the Solar Cell Dye D49 in Organic Solvents and on Mesoporous Al 2 O 3 and TiO 2 Thin Films[END_REF][START_REF] Zhu | Intramolecular Charge Transfer and Solvation of Photoactive Molecules with Conjugated Push-Pull Structures[END_REF][START_REF] Ishow | Photoinduced Intramolecular Charge-Transfer Dynamics of a Red-Emitting Dicyanovinyl-Based Triarylamine Dye in Solution[END_REF] it seems likely that hole injection and the relaxation cascade occur simultaneously and that τ1 therefore reflects a convolution of the two. A second process, with a characteristic time constant 2 ≈ 8 ps, can be assigned to the decay of residual excited state to the ground state since the spectral change it causes is a loss of the typical features of the T2R dye in the excited state. This excited state likely decays through hole injection coupled to fast charge recombination, as the latter has been previously observed on similar timescales for NiO films sensitized with organic dyes. [START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Morandeira | Photoinduced Ultrafast Dynamics of Coumarin 343 Sensitized P-Type-Nanostructured NiO Films[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] The final spectral change is a decay of the characteristic feature of the reduced dye at 620 nm and of the remaining GSB and is thus assigned to charge recombination. This process shows a strong dependence on the applied potential: at OCP, the signal of the primary charge-separated state completely decays within the time window of 1.6 ns, while it is very long-lived when a cathodic potential is applied to the film. This is also reflected in the time constant obtained from the fit, which increases from τ3 = 190 ps for T2R and 110 ps for T2R-Cat1 at OCP, similar to values found in literature, [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] Black | Investigating Interfacial Electron Transfer in Dye-Sensitized NiO Using Vibrational Spectroscopy[END_REF][START_REF] Morandeira | Coumarin 343-NiO Films as Nanostructured Photocathodes in Dye-Sensitized Solar Cells: Ultrafast Electron Transfer, Effect of the I3/I -Redox Couple and Mechanism of Photocurrent Generation[END_REF][START_REF] Morandeira | Photoinduced Ultrafast Dynamics of Coumarin 343 Sensitized P-Type-Nanostructured NiO Films[END_REF][START_REF] Sheibani | A Study of Oligothiophene-Acceptor Dyes in p-Type Dye-Sensitized Solar Cells[END_REF][START_REF] Smeigh | Ultrafast Recombination for NiO Sensitized with a Series of Perylene Imide Sensitizers Exhibiting Marcus Normal Behaviour[END_REF][START_REF] Gibson | A P-Type NiO-Based Dye-Sensitized Solar Cell with an Open-Circuit Voltage of 0.35 V[END_REF][START_REF] Yu | Electron Transfer Kinetics of a Series of Bilayer Triphenylamine-Oligothiophene-Perylenemonoimide Sensitizers for Dye-Sensitized NiO[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] Morandeira | Improved Photon-to-Current Conversion Efficiency with a Nanoporous p-Type NiO Electrode by the Use of a Sensitizer-Acceptor Dyad[END_REF] to > 1 ns at -0.74 V and -1.15 V vs. Fc +/0 . Such behaviour of dye-sensitized NiO films has previously been associated with filling of intragap trap states. [START_REF] Dillon | Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes[END_REF][START_REF] D'amario | Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells[END_REF] These trap states are caused by Ni 3+ defects on the NiO surface and provide a pathway for fast non-geminate charge recombination for electrons localized on the reduced dye, leading to characteristic lifetimes in the picosecond range for charge recombination. [START_REF] Dillon | Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes[END_REF][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] Han | Interfacial Electron Transfer Yields in Dye-Sensitized NiO Photocathodes Correlated to Excited-State Dipole Orientation of Ruthenium Chromophores[END_REF][START_REF] Black | Investigating Interfacial Electron Transfer in Dye-Sensitized NiO Using Vibrational Spectroscopy[END_REF][START_REF] Smeigh | Ultrafast Recombination for NiO Sensitized with a Series of Perylene Imide Sensitizers Exhibiting Marcus Normal Behaviour[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][START_REF] Põldme | Photoelectrocatalytic H2 Evolution from Integrated Photocatalysts Adsorbed on NiO[END_REF] Indeed, charge recombination may be so fast that no signal of the reduced dye is observed. [START_REF] Dillon | Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes[END_REF][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] Application of a cathodic potential fills these trap states, thus deactivating the charge recombination channels and extending the charge-separated state lifetime. Most importantly regarding the activity for hydrogen evolution, on the timescale of the femtosecond transient absorption measurements (< 1.6 ns) there is no spectral evidence for electron transfer from the reduced dye to the catalyst in the T2R-Cat1-sensitized films. To accurately determine the lifetime of the primary charge-separated state and to investigate if electron transfer to the catalytic center occurs on a longer time scale, we turned to nanosecond TA-SEC experiments (which cover the time range from 20 ns to 1 ms). Kinetic traces measured at 620 and 505 nm represent the primary charge-separated state NiO + |T2R -(-Cat1) with the electron localized on the dye unit (Figure 9, S19). The long-lived signal decaying in a nonexponential fashion on a microsecond timescale indicates charge recombination, as observed in dye-sensitized NiO films. [START_REF] Kamire | Photodriven Hydrogen Evolution by Molecular Catalysts Using Al2O3 -Protected Perylene-3,4-Dicarboximide on NiO Electrodes[END_REF][START_REF] Dillon | Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes[END_REF][START_REF] D'amario | Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells[END_REF][START_REF] Gardner | Light-Driven Electron Transfer between a Photosensitizer and a Proton-Reducing Catalyst Co-Adsorbed to NiO[END_REF] For NiO|T2R-Cat1, the average lifetime of the primary charge-separated state extracted from the fit at -0.74 V vs. Fc +/0 was 7 µs, while at -1.15 V vs.

Fc +/0 it increased by more than one order of magnitude to 131 µs (27% relative decay amplitude)

and > 1 ms (73% relative decay amplitude). NiO|T2R showed a similar trend with somewhat longer lifetimes, especially at -0.74 V vs. Fc +/0 (τave = 28 µs, -1.15 V vs. Fc +/0 : 184 µs (67%)

and > 1 ms (33 %), Table S8). [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] Furthermore, this positive signal is absent when the catalytic unit is not present at the surface of the film (T2R at -0.74 V vs. Fc +/0 ), or when it is already reduced to the Co I state prior to excitation (T2R-Cat1 at -1.15 V vs. Fc +/0 ), i.e. two conditions where no electron transfer is possible (Figure 9). This allowed us to unambiguously assign this spectral signature to the formation of the Co I state of Cat1 by thermally activated electron transfer from the reduced dye species. 33 Regarding the kinetics of the electron transfer from the reduced dye to the cobalt catalyst, its absence in the fs-TA experiments combined with the maximum signal at 430 nm being present directly after the pump pulse in the ns-TA experiment puts the characteristic time constant for this process in a time window comprised between 1.6 and 20 ns. In addition, the faster decay of the signal of the reduced dye in the dyad compared to T2R is likely caused by an additional, slower electron transfer process to the catalyst in the dyad. Assuming the decrease in lifetime of the reduced dye in T2R-Cat1 in comparison to T2R is only due to this further process, its characteristic time constant is estimated to be 9 µs. Once formed, the Co I state undergoes much slower charge recombination than the reduced dye, with average lifetimes of 30 µs (84 % relative decay amplitude) and > 1 ms (16 % relative decay amplitude).

In summary, the TA-SEC experiments revealed the following scheme (summarized in Figure 11 and S22):

(i) after light excitation, an ultrafast hole injection process leads to the generation of the primary charge-separated state NiO + |T2R --Cat1, (ii) the subsequent thermal electron transfer to the cobalt catalyst predominantly takes place within 20 ns, and the lifetime of the catalytically competent Co I state ranges from the s to the ms timescale, (iii) the yield of this thermal electron transfer process is however far from unity, with a substantial amount of reduced dye still detectable at the surface of the film with an average lifetime of 7 µs.

Figure 11. Scheme summarizing the light-induced processes in a T2R-Cat1-sensitized NiO film at -0.74 V vs. Fc +/0 (corresponding to the +0.14 V vs. RHE potential applied during the photoelectrochemical measurements). The times indicated are inverse first order rates. Since the charge recombination and electron transfer processes are non-exponential, the times given are averages of the inverse first order rates of a multiexponential fit (see Table S8).

Discussion

The photocathodes introduced in this study contain the tetraazamacrocyclic cobalt catalyst Cat1 and clearly display improved performances compared to their counterparts relying on the cobalt diimine-dioxime catalyst Co. Although quite similar photocurrent densities were recorded (≈10 µA•cm -2 @ +0.14 V vs. RHE), significantly higher amounts of hydrogen were produced The superior hydrogen production activity observed with Cat1 cannot be simply rationalized thermodynamically. Indeed, both Cat1 and Co dyads display a significant driving force for hole injection from the dye excited state into NiO (0.6 to 0.9 eV; see Table S1) to form the reduced dye. The reduced dye itself can subsequently reduce the catalytic center to the Co I state via thermally assisted electron transfer, with very close driving forces for the two catalysts (Table S1). The clear-cut difference in the photocathode performances is rather assigned to the increased stability of Cat1 in fully aqueous medium, which previously resulted in a 70-fold higher amount of H2 produced under homogeneous photocatalytic conditions, compared to Co. [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] More generally, the robustness of polypyridinic and aminopyridinic cobalt catalysts has been recognized over the past few years [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] Queyriaux | Electrocatalytic Hydrogen Evolution with a Cobalt Complex Bearing Pendant Proton Relays: Acid Strength and Applied Potential Govern Mechanism and Stability[END_REF][START_REF] Queyriaux | Recent Developments in Hydrogen Evolving Molecular Cobalt(II)-Polypyridyl Catalysts[END_REF][START_REF] Queyriaux | A Noble Metal-Free Photocatalytic System Based on a Novel Cobalt Tetrapyridyl Catalyst for Hydrogen Production in Fully Aqueous Medium[END_REF][START_REF] Zee | Metal-Polypyridyl Catalysts for Electro-and Photochemical Reduction of Water to Hydrogen[END_REF] whereas cobaloximes are more prone to decompose under aqueous conditions of activity, via demetalation, [START_REF] Materna | Understanding the Performance of NiO Photocathodes with Alkyl-Derivatized Cobalt Catalysts and a Push-Pull Dye[END_REF] 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][START_REF] Anxolabéhère-Mallart | Boron-Capped Tris(Glyoximato) Cobalt Clathrochelate as a Precursor for the Electrodeposition of Nanoparticles Catalyzing H2 Evolution in Water[END_REF][START_REF] Cobo | A Janus Cobalt-Based Catalytic Material for Electro-Splitting of Water[END_REF] if they are not constantly and rapidly supplied with reductive equivalents. [START_REF] Andreiadis | Molecular Engineering of a Cobalt-Based Electrocatalytic Nanomaterial for H2 Evolution under Fully Aqueous Conditions[END_REF] The latter process might account for the low F.E. for hydrogen production of the Co photocathodes reported here and in previous studies from our group, [START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF][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] together with other side-reactions such as the reduction of oxygen that might diffuse in the cell during the course of the experiment. [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] Despite having replaced Co by the more robust H2-evolving catalyst Cat1, the overall performances of our photocathodes still need to be increased to reach a level of activity comparable to the current photoanode state of the art, with photocurrents in the mA•cm -2 range. [START_REF] Lee | Progress on Ternary Oxide-Based Photoanodes for Use in Photoelectrochemical Cells for Solar Water Splitting[END_REF] Detailed investigations on the most efficient T2R-Cat1-based photocathode helped us to identify two parameters currently limiting its performances:

1) Operando and post-operando characterizations revealed that the T2R dye structure is modified under reductive conditions, with the loss of the main part of its absorption in the visible region as a consequence. We failed to identify the exact structure of this novel derivative, 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, indicate that this modification occurs on the acceptor group of the push-pull structures, i.e. the cyanoacrylate moiety in the case of T2R and the cyanoacrylamide one for T2R-Cat1. In addition, post-operando ICP-MS measurements indicated that an important amount of Co catalyst is lost during the first 4 hours of activity. The clear correlation found between the photocurrent decrease, 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, rather than decomposition of the catalyst, and (ii) it involves a bond breaking, leading to the detachment of the whole catalytic center from the assembly. This is further supported by the recent observation that Cat1 grafted on TiO2 was stable for 10 hours under photoelectrochemical conditions similar to ours (pH 4.5 acetate buffer, 1 sun visible light irradiation), [START_REF] Nie | Boosting the Performance of a Silicon Photocathode for Photoelectrochemical Hydrogen Production by Immobilization of a Cobalt Tetraazamacrocyclic Catalyst[END_REF] rendering a catalyst-centered degradation process very unlikely. After 4 hours of activity, the dyads still intact on the surface (retained Co loading: 16 %) account for the H2-evolving activity still detected between 4 and 22 hours. By contrast to other reported studies, where dye/catalyst desorption from the semiconductor film is the main deactivation process, [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] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF][START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-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] only a low amount of dyads is released in solution. The presence of the lateral alkyl chains on the CPDT bridge likely stabilizes the dyads on the NiO surface by forming a hydrophobic layer able to protect the carboxylate ester anchors from nucleophilic attack from water, as previously reported for another push-pull organic dye-based photocathode. [START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF] The low solubility of these dyads in aqueous medium, compared for instance to cationic ruthenium photosensitizers, might be another factor improving the stability of the sensitized NiO surface.

2) TA-SEC measurements recorded under quasi-operando conditions (same potential applied to the film as for the chronoamperometric measurements but in anhydrous acetonitrile instead of aqueous electrolyte) revealed the kinetics of the light-induced processes and provided information about the different intermediates formed at the surface of the film. First, the formation of a long-lived (> 1 ms) reduced organic dye moiety in the T2R-Cat1 dyad was observed due to hole injection to the NiO. The charge recombination kinetics strongly depended on the applied potential, leading to a six order of magnitude increase in the lifetime of the primary charge separated state at -0.74 V vs. Fc +/0 with respect to OCP. While a similar behaviour was recently reported for NiO films sensitized with a push-pull organic dye [START_REF] Daeneke | Dominating Energy Losses in NiO P-Type Dye-Sensitized Solar Cells[END_REF] or ruthenium tris-diimine photosensitizers, [START_REF] Dillon | Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes[END_REF][START_REF] D'amario | Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells[END_REF]100 it is, to the best of our knowledge, unprecedented for a hydrogen-evolving photocathode based on a covalent dye-catalyst assembly. It must be stated that, due to similar time constants, the excited state relaxation of the dye as well as the hole injection and charge recombination processes overlapped temporally and could not be deconvoluted with the sum of exponentials analysis performed here. In the future, more advanced tools such as the recently reported stochastic kinetics simulations 101 could be applied to provide a better description of these early events. Second, thermally activated electron transfer from the reduced dye to the Co II center yielding the reduced (Co I ) catalyst predominantly occurred on a time window of 1.6 to 20 ns, with only a small contribution taking place on longer timescales (average time constant of 9 s). This appears to be comparable to other triazole-based dyads studied in solution, where this process has been observed on the ns-µs time scale. [START_REF] Sheth | Light-Induced Tryptophan Radical Generation in a Click Modular Assembly of a Sensitiser-Tryptophan Residue[END_REF][102][103][104][105] A recent study on a Ru-Re system anchored on NiO by electropolymerization also showed electron transfer 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. 100 Electron transfer 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] For co-immobilised systems, charge transfer from the dye (excited or reduced state) to the catalyst has been observed in as fast as a few ps. [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] Materna | Understanding the Performance of NiO Photocathodes with Alkyl-Derivatized Cobalt Catalysts and a Push-Pull Dye[END_REF]106 Direct spectroscopic observation of the reduced catalyst signature at the surface of a dye-sensitized NiO film and determination of its lifetime are however quite scarce in the literature. [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]100,106 The group of L. Hammarström previously reported a diiron catalyst reduced state with different lifetimes depending on the nature of its anchor to NiO: when grafted with a carboxylate anchoring group, this reduced state was stable on the 100 s timescale and no H2 was produced.

Employing a phosphonate anchor, the lifetime reached the ms timescale allowing for hydrogen production. [START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF] In our dyad, in the absence of protons, the reduced Co I state also has a lifetime reaching the ms timescale, which can be explained by a lower driving force for charge recombination together with an increased donor-acceptor distance (e.g. Co I to NiO + ) compared to the primary charge separated state, in which the dye is reduced. The formation of this catalytically competent species appears however to be quite inefficient; an accurate quantification of the electron transfer yield was not possible from the ns-TA-SEC experiment since the signal of the Co I species was already built up at the beginning of the experiment, yet it was far from unity as an important amount of reduced dye remained present at the surface of the film on the µs time scale. This observation can be put in line with the presence of anodic discharging spikes when light is switched off in the chopped-light chronoamperometric measurements (Figure 4b), corresponding to the reoxidation of this accumulated reduced dye.

Taken altogether, these data highlight that neither a low light absorption efficiency nor a sluggish multielectronic catalytic process limit the dye-sensitized photocathode performances.

Rather, the accumulation of photogenerated charges at the surface of the NiO film must be considered as the performance-limiting factor. Similar observations were recently made for organic semiconductor photocathodes with a perylene diimide (PDI) acceptor polymer: an upper limit for charge accumulation was determined for stable long-term operation, above which the PDI underwent irreversible change. In our case, the reduced state of the dye is not stable, which causes the loss of the catalytic unit, accompanied by a strong decrease of the light harvesting efficiency in the visible region. This process is clearly accelerated in the absence of the catalyst (T2R vs. T2R-Cat1 photocathode) highlighting that electron transfer to the catalyst can compete with the reduced dye alteration. A fast electron transfer to the catalyst indeed reduces the lifetime of the reduced dye and thereby protects it from degradation. This charge extraction process is unfortunately not very efficient for the T2R-Cat1 photocathode, suggesting that only a small fraction of dyads is fully operative at the surface of the film, possibly for conformational or topological reasons. This raises questions regarding the covalent design of our dye-catalyst assemblies; indeed, the presence of a flexible methylene spacer between the acceptor side of the dye and the triazole-catalyst moiety might prevent the thermal electron transfer step, thus occurring only in very specific conformations of the assembly.

Future work should clearly focus on the design of novel photocathode architectures, integrating dyes that are more robust under reductive conditions, and bridging units allowing a faster and more efficient electron supply to the catalyst.

Conclusion

In conclusion, we successfully synthesized two novel noble metal-free covalent dye-catalyst assemblies based on the robust H2-evolving tetraazamacrocyclic cobalt catalyst Cat1. The performances of the resulting photocathodes were assessed under conditions strictly identical to our previously reported dyad based on the cobalt diimine-dioxime catalyst, [START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF][START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] enabling to establish structure-activity relationships. Switching to Cat1 was instrumental to increase both the activity and the stability of the systems, which could be further improved by the introduction of an alkyl-substituted CPDT bridge in the push-pull structure of the dye (T2R). The resulting T2R-Cat1-sensitized photocathode effectively produced hydrogen during 4 hours with 70 % Faradaic efficiency and still displays some activity after 22 hours, which ranks this construction within the best-performing H2-evolving dye-sensitized photocathodes reported up to now [START_REF] Creissen | Solar H2 Generation in Water with a CuCrO2 Photocathode Modified with an Organic Dye and Molecular Ni Catalyst[END_REF]107,108 and opens perspectives toward an integration into a tandem photoelectrochemical cell. In order to provide meaningful insights into the factors affecting the performances of the T2R-Cat1-sensitized photocathode over time, we relied on a combination of operando and post-operando characterization techniques. Hence, we established that the dye structure is altered under reductive conditions. This non-exponential process, occurring on a minute to hour timescale, is correlated to the photocurrent decrease and to the loss of Co observed during the first four hours of photoelectrochemical activity. Thus, despite significant improvements, stability is still hampered by the lack of robustness of the organic dye. In parallel, the electron transfer dynamics at the NiO interface were investigated thanks to transient absorption measurements recorded with the operando potential applied to the film. We detected the formation of the catalytically competent Co I state with a lifetime of > 1 ms, sufficient to initiate catalysis.

From these results, one major bottleneck hindering photoelectrochemical performances is the low yield of the thermal electron transfer from the reduced dye to the catalyst; in addition to limiting the photocurrent density, this leads to the accumulation of charges on the acceptor part of the dye and ultimately to its degradation. This study thus provides a clear rational toward the optimization of the performances of dye-sensitized photocathodes for hydrogen production and we will focus our efforts on designing novel dyad assemblies and/or photocathode architectures achieving faster electron transfer to the catalyst. In that context, a DFT-driven approach exploiting Marcus theory to determine structures warranting higher electron transfer rates could be very valuable. Replacing NiO by other p-type semiconductors represents another track for improvement, as recently highlighted in studies by our group and others. [START_REF] Windle | Earth-Abundant Molecular Z-Scheme Photoelectrochemical Cell for Overall Water-Splitting[END_REF][START_REF] Li | Dye-Sensitized LaFeO3 Photocathode for Solar-Driven H2 Generation[END_REF][START_REF] Creissen | Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst[END_REF] Finally, this work paves the way for the integration of robust cobalt polypyridinyl catalysts into functional photoelectrochemical devices for solar fuels production. [START_REF] Queyriaux | Electrocatalytic Hydrogen Evolution with a Cobalt Complex Bearing Pendant Proton Relays: Acid Strength and Applied Potential Govern Mechanism and Stability[END_REF][START_REF] Queyriaux | Recent Developments in Hydrogen Evolving Molecular Cobalt(II)-Polypyridyl Catalysts[END_REF]109 Experimental Section.

Synthesis. All reagents were purchased from Sigma Aldrich or Strem and used as obtained unless otherwise stated. Reagent-grade solvents were used without further purification.

1 H NMR experiments were carried out on Bruker Avance 300 or 500 MHz spectrometers and the resulting spectra are referenced to the residual solvent peak and reported in relative to tetramethylsilane reference (δ = 0 ppm). Electrospray ionization mass spectrometric (ESI-MS) measurements were carried out on a Thermoquest Finnigan LCQ spectrometer. Accurate mass measurements (HRMS) were recorded on a Bruker maXis mass spectrometer by the "Fédération de Recherche" ICOA/CBM (FR 2708) platform. Elemental analysis was performed on a Thermofisher Scientific "Flash 2000" by the "Plateforme d'analyse pour la chimie" (GDS 3648, Strasbourg). tBu T1, [START_REF] Massin | Dye-Sensitized PS-b-P2VP-Templated Nickel Oxide Films for Photoelectrochemical Applications[END_REF] its alkyne derivative tBu T1-alkyne, 16 Cat1 33 and tBu T1-Co [START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] were synthesized according to previously reported procedures. Detailed syntheses for the dye precursors, the building block, tBu its alkyne derivative tBu T2R-alkyne and T2R-Co are described in the Supporting Information.

General procedure for the CuAAC click coupling. [START_REF] Windle | A Protocol for Quantifying Hydrogen Evolution by Dye-Sensitized Molecular Photocathodes and Its Implementation for Evaluating a New Covalent Architecture Based on an Optimized Dye-Catalyst Dyad[END_REF][START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] The alkyne-substituted dye (0.5 mmol) and 4-azido-2,6-diacetylpyridine (1 equiv.) were dissolved in 20 mL of degassed CH2Cl2.

Sodium ascorbate (0.5 mmol) and CuSO4•5H2O (0.2 mmol) were dissolved in 10 mL of degassed H2O and added to the reaction mixture. Degassed MeOH was added dropwise until a single phase was obtained. The reaction mixture was stirred at room temperature for 24 hours.

After removal of the volatile solvents, the mixture was extracted with CH2Cl2, washed with brine solution, dried over Na2SO4 and filtered. The crude product was purified on silica gel column to yield the diacetylpyridine-functionalized dye.

General procedure for the template synthesis of Cat1. The diacetylpyridine-functionalized dye (0.3 mmol) was dissolved in EtOH (2 mL) and CH2Cl2 (1 mL), and kept at 40 °C under argon.

CoCl2•6H2O (1 equiv.) and water (0.4 mL) were added and the solution was stirred at 55 °C to dissolve the salt. The reaction mixture was warmed to 75°C before addition of 3,3'diaminodipropylamine (1 equiv.) and glacial acetic acid (0.03 mL) and was then stirred at 75 °C for 5 hours under argon. After cooling down to room temperature, 0.1 mL of concentrated HCl was added and the reaction mixture was stirred at room temperature for 16 hours, then kept for 2 hours at -15 °C and centrifuged. The precipitate was washed with H2O and EtOH before drying under vacuum to yield the dyad as a red solid.

General procedure for the tert-butyl ester groups hydrolysis prior to film sensibilisation.

Hydrolysis of the tert-butyl ester groups was carried out 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][START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF][START_REF] Massin | Dye-Sensitized PS-b-P2VP-Templated Nickel Oxide Films for Photoelectrochemical Applications[END_REF] by reacting the tert-butyl ester protected dyads (or dyes) with trifluoroacetic acid (TFA) in CH2Cl2 for 5 hours at room temperature.

NiO film preparation and sensitization. NiO films were prepared using a F108 precursor solution prepared by mixing 1 g of NiCl2, 1 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][START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF] The mixture was sonicated for 6 h, centrifuged and passed through a 45 µm pore size polyethersulfone syringe filter. FTO-coated glass substrates (Solems TEC 7, sheet resistance 7Ω.□) for the hydrogen production experiments or round ITO-coated glass substrates (Solems ITO SOL 30, 1 mm thickness) for the operando UV-Vis experiments and transient absorption measurements were cleaned by sonication in isopropanol, distilled water and ethanol for 10 min each, followed by ozone cleaning for 15 min. 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 film thickness was measured by scanning electron microscopy (SEM) with a scanning electron microscope Zeiss Ultra 55 operating at 4 kV.

NiO electrodes were soaked in methanolic solutions (0.2 mM) of the different dyes and dyads (acidic form) at room temperature for 24 h under orbital stirring. For the TCSPC measurements, sensitizing solutions with 10 mM chenodeoxycholic acid (CDCA) added were used for sensitization. After sensitization, films were then rinsed with methanol and dried with pressured air. After sensitization, NiO films were cut in half. One half of the sample was used to determine grafting densities pre-operando while the other half was used for photoelectrochemical experiments and afterwards desorbed to determine grafting densities and possible degradation post-operando.

Dyad loading quantification. Sensitized NiO films were soaked in 1M methanolic phenylphosphonic acid solution for 3 hours to desorb grafted compounds. Afterwards, UV-Vis spectra of the desorption solutions were then measured and dyad (or dye) loading was determined using the molar extinction coefficient at the  band (Figure S3) and the film surface areas. Cobalt quantification was performed by inductively coupled plasma mass spectrometry (ICP-MS) using an iCAP RQ quadrupole mass instrument (Thermo Fisher Scientific GmbH, Germany). The samples were prepared by soaking each film in 1 mL of aq.

HNO3 (65% w/v) for 1 min, rinsing with 1 mL Milli-Q water, hydrolyzing again with 1 mL of HNO3, rinsing with 1 mL H2O, then 2.5 mL H2O. The resulting solution (6.5 mL at 20% w/v HNO3) was diluted 20 times to yield a solution at 1% w/v HNO3, which was used for the measurement.

Steady-state spectroscopy. Steady-state absorption spectra were recorded on an Agilent Cary 60 or a Jasco V780 spectrophotometer in 1 cm cuvettes. All solvents used for spectroscopy were of spectroscopic grade. Transient absorption spectroelectrochemistry (TA-SEC) measurements. The setup for femtosecond transient absorption measurements has been previously described. 111 An amplified Ti:Sapphire laser (Legend, Coherent Inc) produced the 800 nm fundamental beam which was split into two beams. One of the beams was used to pump an optical-parametric amplifier (TOPAS-C), whose output was spectrally centered at 480 nm and used as pump pulses for the pump-probe experiments. The pump pulses were typically set to 0.4 µJ per pulse, which was lowered to 0.2 µJ per pulse for measurements at -1.15 V vs. Fc +/0 to avoid photodegradation.

Time

To achieve the pump-probe delay of 2 ns, the pump beam was directed over a delay line. The supercontinuum used as probe pulse was generated by passing the second beam of the fundamental 800 nm through a CaF2 window, with probe intensities falling into the range of hundred nJ. The probe light was split into two beams, one of which was focussed through the sample, while the other served as reference. A 488 nm notch filter was placed between sample and detector to remove scattered pump light. The probe and reference beams were detected by a double-stripe diode array and converted into differential absorption signals using a commercially available detection system (Pascher Instruments AB, Sweden). The recorded data was corrected for the chirp and globally fitted using a sum of three exponential. During the fitting, the pulse overlap region of ± 200 fs was excluded to avoid contributions from coherent artifacts. 112 The setup for ns transient absorption measurements was previously described: 113 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 pump pulse energy was set to 0.13 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.

TA-SEC experiments were performed using a home-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

Figure 2 .

 2 Figure 2. Cyclic voltammograms of Cat1 (grey), tBu T2R (red), tBu T1-Cat1 (green) and tBu T2R-Cat1 (black). CVs were recorded at 50 mV/s scan rate in 0.1 M TBABF4 in dry DMF except for the reductive scan of T1-Cat1, in which TBACl was used as supporting electrolyte.

Figure 3 .

 3 Figure 3. UV-Vis absorption spectra of tBu T1 and tBu T1-Cat1 in DMF and of tBu T2R, tBu T2R-Cat1 and Cat1 in MeOH.

Figure 4 .

 4 Figure 4. a) Linear scan voltammograms (LSV) of NiO films sensitized with the Cat1 dyads recorded in the dark and under continuous visible light irradiation. b) Chopped-light chronoamperometric measurements recorded in aqueous 0.1 M pH 5.5 MES buffer at -0.4 V vs. Ag/AgCl (+0.14 V vs. RHE) on NiO films sensitized with the dyes and the Cat1 dyads.

Figure 5 .

 5 Figure 5. Photocurrents recorded on NiO films sensitized with the Cat1-and Co-based dyads during the first 4 hours of photoelectrochemical experiments (applied potential: +0.14 V vs. RHE). a) Absolute current density and b) current normalized to the amount of dyad grafted on the film, allowing to quantify the intrinsic activity of the dyads.

Figure 6 .

 6 Figure 6. UV-Vis spectra of the Cat1 dyads desorbed from fresh (dashed lines) and postoperando (after 22h of photoelectrochemical activity assessment, solid lines) NiO films in a 1 M phenylphosphonic acid solution in methanol, normalized to the surface area of the films.

Figure 7 .

 7 Figure 7. a) UV-Vis absorption monitoring of T2R-Cat1-sensitized NiO films under operando photoelectrochemical conditions: UV-Vis spectra (solid) and differential spectra (dashed) obtained by subtracting the spectrum obtained at applied potential before irradiation (0 min). b) 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 8

 8 Figure 8Left: fs-TA-SEC spectra at different pump-probe delay times after excitation at 480 nm of T2R-Cat1-sensitized NiO films at OCP, -0.74 V (corresponding to +0.14 V vs. RHE employed for the PEC tests) and -1.15 V vs. Fc +/0 applied potential, using 0.1 M nBu4NBF4 electrolyte in ACN. Right: Decay-associated spectra obtained with a three-component exponential fit of the respective fs-TA-SEC data. τ1 corresponds to hole injection with associated relaxation, τ2 to a decay of residual excited state and τ3 to charge recombination from the charge-separated state.

Figure 9 .

 9 Figure 9. Kinetic traces at characteristic wavelengths obtained in ns-TASEC experiments recorded on NiO films sensitized with T2R and T2R-Cat1 at applied potentials of -0.74 and -1.15 V vs. Fc +/0 , using 0.1 M nBu4NBF4 electrolyte in ACN. The signals at 620 and 505 nm are characteristic of the reduced state of the T2R dye, while the signal at 430 nm is ascribed to the Co I state of the catalyst.

Figure 10 .

 10 Figure 10. ns-TASEC spectra recorded on a NiO film sensitized with T2R-Cat1 at -0.74 V and -1.15 V vs. Fc +/0 applied potentials at different delay times together with reference spectra obtained via UV-vis SEC of the Co I state (dashed) and of the one-electron reduced T2R in solution (dotted).

  photocathodes still display some activity over 22 hours whereas the Co systems are fully

  photoelectrochemical experiments were recorded using either a BioLogic VSP 300,

Table 1 .

 1 Electrochemical potentials (E1/2 in V vs. Fc +/0 ) of investigated compounds determined by cyclic voltammetry.

	Sample	Edye +/0	Edye 2+/+	ECo III/II	ECo II/I	ECo I/0	Edye 0/-b)
	Cat1	---	---	-0.58	-1.08	-1.84	---
	tBu T1	0.71	---	---	---	---	-1.52
	tBu T2R	0.48	0.66	---	---	---	-1.57
	tBu T1-Cat1	0.64 a)	---	-0.53	-1.03	---	-1.65
	tBu T2R-Cat1	0.43 a)	0.58 a)	-0.54	-0.97	---	-1.65

a) Peak potentials in the anodic scan. b) Peak potentials in the cathodic scan.

  [START_REF] Zieschang | Photoinduced Electron Transfer Dynamics in Triarylamine-Naphthalene Diimide Cascades[END_REF][START_REF] De Miguel | Triazole Bridges as Versatile Linkers in Electron Donor-Acceptor Conjugates[END_REF][START_REF] Sheth | Light-Induced Tryptophan Radical Generation in a Click Modular Assembly of a Sensitiser-Tryptophan Residue[END_REF][START_REF] Baron | Click Chemistry on a Ruthenium Polypyridine Complex. An Efficient and Versatile Synthetic Route for the Synthesis of Photoactive Modular Assemblies[END_REF][START_REF] Baron | Efficient Electron Transfer through a Triazole Link in Ruthenium(II) Polypyridine Type Complexes[END_REF][START_REF] Queyriaux | CuAAC-Based Assembly and Characterization of a Ruthenium-Copper Dyad Containing a Diimine-Dioxime Ligand Framework[END_REF]

Table 2 .

 2 FiguresAfter 4 hours of irradiation, the Cat1 dyads produced hydrogen with TON values of 13 ± 1

		After 4 hours		After 22 hours
	Sample	H2 / (nmol•cm -2 )	FE (%)	TON	H2 / (nmol•cm -2 )	FE (%)	TON
	T1-Cat1	96 ± 14	65 ± 17	13 ± 1	162 ± 20	31 ± 4	23 ± 1
	T2R-Cat1	187 ± 25	66 ± 7	21 ± 6	365 ± 122	39 ± 8	39 ± 8
	T1-Co	11 ± 1	9 ± 1	0.7 ± 0.1	13 ± 4	3 ± 0	0.9 ± 0.3
	T2R-Co	10 ± 2	11 ± 2	1.5 ± 0	10 ± 3	3 ± 2	1.5 ± 0.2

of merit of the different dyad-sensitized photocathodes determined from chronoamperometric measurements under continuous visible-light irradiation, in aqueous MES buffer electrolyte at pH 5.5, at an applied potential of +0.14 V vs. RHE (full set of experimental data in Table

S4

).

  Table of content (TOC) figure:

Funding Sources

This work was supported by the European Commission's Seventh Framework Program (FP7/2007-2013) under grant agreement n° 306398 (FP7-IDEAS-ERC, Project PhotocatH2ode), the French National Research Agency in the framework of the "Investissements d'avenir" program (ANR-15-IDEX-02, Labex ARCANE and CBH-EURGS, ANR-17-EURE-0003) and the Franco-German University.

ASSOCIATED CONTENT

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.

AUTHOR INFORMATION

Corresponding Author

* Benjamin Dietzek; E-mail: benjamin.dietzek@leibniz-ipht.de * Murielle Chavarot-Kerlidou; E-mail: murielle.chavarot-kerlidou@cea.fr Present Addresses †Present address: Aix Marseille Univ, CNRS, Centrale Marseille, iSm2, Marseille, France.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Supporting Information. The following files are available free of charge.

Detailed synthetic procedures and additional spectroscopic and photoelectrochemical data (PDF).

ACKNOWLEDGMENT