Sebastian Bold
email: sebastian.bold@uni-jena.de

Tatiana Straistari
email: tatiana.straistari@cea.fr

Ana B Muñoz-García
email: anabelen.munozgarcia@unina.it

Michele Pavone
email: michele.pavone@gmail.com

Vincent Artero
email: vincent.artero@cea.fr

Murielle Chavarot-Kerlidou
email: murielle.chavarot-kerlidou@cea.fr

Benjamin Dietzek
email: benjamin.dietzek@leibniz-ipht.de

Investigating Light-Induced Processes in Covalent Dye-Catalyst Assemblies for Hydrogen Production

Keywords: solar energy conversion, push-pull dye, intersystem crossing

The light-induced processes occurring in two dye-catalyst assemblies for light-driven hydrogen production were investigated by ultrafast transient absorption spectroscopy. These dyads consist of a push-pull organic dye based on a cyclopenta[1,2-b:5,4-b']dithiophene (CPDT) bridge, covalently linked to two different H 2 -evolving cobalt catalysts. Whatever the nature of the latter, photoinduced intramolecular electron transfer from the excited state of the dye to the catalytic center was never observed. Instead, and in sharp contrast to the reference dye, a fast intersystem crossing (ISC) populates a long-lived triplet excited state, which in turn non-radiatively decays to the ground state. This study thus shows how the interplay of different structures in a dye-catalyst assembly can lead to unexpected excited state behavior and might open up new possibilities in the area of organic triplet sensitizers. More importantly, a reductive quenching mechanism with an external electron donor must be considered to drive hydrogen production with these dye-catalyst assemblies.

Introduction

The conversion of solar energy into a storable fuel, such as hydrogen (H 2 ) through sunlight-driven water splitting, is currently the subject of intensive research efforts [START_REF] Walter | Solar Water Splitting Cells[END_REF][START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF]. In that context, photosynthesis has been a great source of inspiration for molecular chemists, leading to the development of a wide variety of H 2 -evolving molecular dyads [START_REF] Pfeffer | Palladium versus Platinum: The Metal in the Catalytic Center of a Molecular Photocatalyst Determines the Mechanism of the Hydrogen Production with Visible Light[END_REF][START_REF] Dempsey | Hydrogen Evolution Catalyzed by Cobaloximes[END_REF][START_REF] Shan | Photogeneration of hydrogen from water by a robust dye-sensitized photocathode[END_REF][START_REF] Mengele | Generation of a stable supramolecular hydrogen evolving photocatalyst by alteration of the catalytic center[END_REF][START_REF] Luo | Tethered sensitizer-catalyst noble-metal-free molecular devices for solar-driven hydrogen generation[END_REF][START_REF] Braumüller | Synthesis and characterization of an immobilizable photochemical molecular device for H2-generation[END_REF][START_REF] Põldme | Photoelectrocatalytic H 2 evolution from integrated photocatalysts adsorbed on NiO[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] Shan | Controlling Vertical and Lateral Electron Migration Using a Bifunctional Chromophore Assembly in Dye-Sensitized Photoelectrosynthesis Cells[END_REF][START_REF] Windle | A protocol for quantifying hydrogen evolution by dye-sensitized molecular photocathodes and its implementation for evaluating a new covalent architecture based on an optimized dye-catalyst dyad[END_REF][START_REF] Kaeffer | Covalent Design for Dye-Sensitized H2 -Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF], inspired by the Photosystem I-Hydrogenase couple found in some hydrogen producing photosynthetic micro-organisms [START_REF] Mctavish | Hydrogen Evolution by Direct Electron Transfer from Photosystem I to Hydrogenases[END_REF]. Recently, the first functional H 2 -evolving photocathodes integrating such molecular dyads were reported in the literature [START_REF] Dalle | Electro-and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes[END_REF][START_REF] Shan | Photogeneration of hydrogen from water by a robust dye-sensitized photocathode[END_REF][START_REF] Põldme | Photoelectrocatalytic H 2 evolution from integrated photocatalysts adsorbed on NiO[END_REF][START_REF] Shan | Controlling Vertical and Lateral Electron Migration Using a Bifunctional Chromophore Assembly in Dye-Sensitized Photoelectrosynthesis Cells[END_REF][START_REF] Gibson | Dye-sensitized photocathodes for H2 evolution[END_REF], paving the way for solar fuels production in dye-sensitized photoelectrochemical cells. Our group contributed to the field with the design of covalent dye-catalyst assemblies based on H 2 -evolving cobalt catalysts [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].

Understanding the excited state dynamics and electron transfer mechanisms in these systems is key to rationally improve their design and optimize their H 2 evolution performances. We recently addressed the photoinduced processes at work with our first photoelectrocatalytically active dyad, thanks to a combination of classical spectroscopic techniques and transient absorption spectroelectrochemistry [START_REF] Bold | Electron transfer in a covalent dye-cobalt catalyst assembly-a transient absorption spectroelectrochemistry perspective[END_REF]. In the present study, we investigate the ultrafast excited state dynamics of two novel dye-catalyst assemblies specifically designed for improved photoelectrocatalytic hydrogen production in dye-sensitized photocathodes [START_REF] Bold | Spectroscopic investigations provide a rational for the hydrogen-evolving activity of dye-sensitized photocathodes based on a cobalt tetraazamacrocyclic catalyst[END_REF]. They comprise a push-pull organic dye (T2R) covalently assembled with two different H2-evolving cobalt catalysts: either the cobalt diimine-dioxime complex Cat2 [START_REF] Kaeffer | Hydrogen evolution catalyzed by cobalt diimine-dioxime complexes[END_REF] or the cobalt tetraazamacrocyclic catalyst Cat1 [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][START_REF] Roy | A Systematic Comparative Study of Hydrogen-Evolving Molecular Catalysts in Aqueous Solutions[END_REF] (Figure 1). The metal-free dye structure relies on a triphenylamine (TPA) donor group functionalized by two protected carboxylic acid groups for its future anchoring onto semiconducting oxide-based electrodes and a cyanoacrylate/cyanoacrylamide acceptor group, separated by the electron-rich cyclopenta[1,2b:5,4-b']dithiophene (CPDT) bridge. The latter is classically employed as a building block in organic photovoltaics or for dye-sensitized solar cell applications, thanks to its increased light absorption properties in the visible region of the spectrum [START_REF] Yen | Arylamine-Based Dyes for p-Type Dye-Sensitized Solar Cells[END_REF][START_REF] Cai | Achieving organic solar cells with efficiency over 14% based on a non-fullerene acceptor incorporating a cyclopentathiophene unit fused backbone[END_REF].

Results

Steady-State Spectroscopy and Density Functional Theory (DFT) Calculations

First insights into the light-induced processes in T2R, T2R-Cat1, and T2R-Cat2 are obtained from steady-state spectroscopy. In acetonitrile (ACN), the absorption spectra for all compounds show two main bands at 355 and around 495 nm (Figure 2). The band at 355 nm is assigned to the triphenylamine-centered transition, while the additional small band at 298 nm might be due to a localized transition on the bithiophene linker [START_REF] Fujitsuka | Laser flash photolysis study on the photoinduced reactions of 3,3 -bridged bithiophenes[END_REF][START_REF] Paa | Intersystem crossing in oligothiophenes studied by fs time-resolved spectroscopy[END_REF]. Below 320 nm, the dyads additionally show contribution from the respective cobalt catalysts. In the visible region, the strong band at 495 nm results from the HOMO-LUMO transition with intramolecular charge-transfer (ICT) character from the TPA donor to the cyanoacrylate acceptor. Compared to the two dyads, the ICT band of T2R is slightly red-shifted (∆E ≈ 200 cm -1 ), which might stem from the stronger electron-withdrawing character of the ester terminal group in comparison with the amide, slightly increasing the push-pull character of the dye. As previously reported [START_REF] Yen | Arylamine-Based Dyes for p-Type Dye-Sensitized Solar Cells[END_REF], the presence of the rigid CPDT unit strongly enhances the light-harvesting efficiency in the visible region (ε of 60,300 M -1 •cm -1 ) compared to our previously-studied dyads relying on mono-or bis-thiophene units (ε of 31,800 and 38,600 M -1 •cm -1 , respectively) [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] Bold | Electron transfer in a covalent dye-cobalt catalyst assembly-a transient absorption spectroelectrochemistry perspective[END_REF][START_REF] Massin | Dye-sensitized PS-b-P2VP-templated nickel oxide films for photoelectrochemical applications[END_REF].

The computed time-dependent (TD)-DFT vertical transition energies confirm the experimental assignments of the optical absorption spectra. The first bright transition (i.e., with a high oscillator strength value) at 472 nm for the T2R isolated dye corresponds to the HOMO-LUMO transition (ICT).

The second transition at 331 nm corresponds to the HOMO-LUMO+1 transition localized on the TPA

Results

Steady-State Spectroscopy and Density Functional Theory (DFT) Calculations

First insights into the light-induced processes in T2R, T2R-Cat1, and T2R-Cat2 are obtained from steady-state spectroscopy. In acetonitrile (ACN), the absorption spectra for all compounds show two main bands at 355 and around 495 nm (Figure 2). The band at 355 nm is assigned to the triphenylamine-centered transition, while the additional small band at 298 nm might be due to a localized transition on the bithiophene linker [START_REF] Fujitsuka | Laser flash photolysis study on the photoinduced reactions of 3,3 -bridged bithiophenes[END_REF][START_REF] Paa | Intersystem crossing in oligothiophenes studied by fs time-resolved spectroscopy[END_REF]. Below 320 nm, the dyads additionally show contribution from the respective cobalt catalysts. In the visible region, the strong band at 495 nm results from the HOMO-LUMO transition with intramolecular charge-transfer (ICT) character from the TPA donor to the cyanoacrylate acceptor. Compared to the two dyads, the ICT band of T2R is slightly red-shifted (∆E ≈ 200 cm -1 ), which might stem from the stronger electron-withdrawing character of the ester terminal group in comparison with the amide, slightly increasing the push-pull character of the dye. As previously reported [START_REF] Yen | Arylamine-Based Dyes for p-Type Dye-Sensitized Solar Cells[END_REF], the presence of the rigid CPDT unit strongly enhances the light-harvesting efficiency in the visible region (ε of 60,300 M -1 •cm -1 ) compared to our previously-studied dyads relying on mono-or bis-thiophene units (ε of 31,800 and 38,600 M -1 •cm -1 , respectively) [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] Bold | Electron transfer in a covalent dye-cobalt catalyst assembly-a transient absorption spectroelectrochemistry perspective[END_REF][START_REF] Massin | Dye-sensitized PS-b-P2VP-templated nickel oxide films for photoelectrochemical applications[END_REF]. moiety (Supplementary Figure S1). These results are in full agreement with TD-DFT calculations previously reported for a structural analog of T2R, varying only by the nature of the acceptor group (dicyanovinyl unit instead of cyanoacrylate unit) [START_REF] Yen | Arylamine-Based Dyes for p-Type Dye-Sensitized Solar Cells[END_REF]. For the T2R-Cat1 complex, the absorption transition energies are very similar to T2R, with the first ICT transition at 476 nm (HOMO -> LUMO+4) and the second (TPA) one at 335 nm (HOMO -> LUMO+6) (Supplementary Figure S2). The same electronic structure pattern is found for the T2R-Cat2 complex. Considering their targeted application for hydrogen production under aqueous conditions, the electronic properties of the dyads were further studied in methanol (MeOH), which is a protic solvent of higher polarity (ET N = 0.76) and viscosity (η = 0.59 mPa•s -1 ) than ACN (ET N = 0.46 and η = 0.35 mPa•s -1 ) [START_REF] Reichardt | Properties, Purification, and Use of Organic Solvents[END_REF][START_REF] Montalti | 9-Solvent Properties[END_REF]. The UV-Vis absorption spectra in MeOH (Supplementary Figures S2B andS3) are similar to those recorded in ACN. In particular, no significant shift of the ICT band is observed. The computed time-dependent (TD)-DFT vertical transition energies confirm the experimental assignments of the optical absorption spectra. The first bright transition (i.e., with a high oscillator strength value) at 472 nm for the T2R isolated dye corresponds to the HOMO-LUMO transition (ICT). The second transition at 331 nm corresponds to the HOMO-LUMO+1 transition localized on the TPA moiety (Supplementary Figure S1). These results are in full agreement with TD-DFT calculations previously reported for a structural analog of T2R, varying only by the nature of the acceptor group (dicyanovinyl unit instead of cyanoacrylate unit) [START_REF] Yen | Arylamine-Based Dyes for p-Type Dye-Sensitized Solar Cells[END_REF]. For the T2R-Cat1 complex, the absorption transition energies are very similar to T2R, with the first ICT transition at 476 nm (HOMO→LUMO+4) and the second (TPA) one at 335 nm (HOMO→LUMO+6) (Supplementary Figure S2). The same electronic structure pattern is found for the T2R-Cat2 complex.

Considering their targeted application for hydrogen production under aqueous conditions, the electronic properties of the dyads were further studied in methanol (MeOH), which is a protic solvent of higher polarity (E T N = 0.76) and viscosity (η = 0.59 mPa•s -1 ) than ACN (E T N = 0.46 and η = 0.35 mPa•s -1 ) [START_REF] Reichardt | Properties, Purification, and Use of Organic Solvents[END_REF][START_REF] Montalti | 9-Solvent Properties[END_REF]. The UV-Vis absorption spectra in MeOH (Supplementary Figures S2B andS3) are similar to those recorded in ACN. In particular, no significant shift of the ICT band is observed. Upon excitation at 520 nm, the compounds show emission, which is centered at 674 nm for T2R and 662 nm for the dyads (Figure 2B). As in the absorption spectra, the dyads show identical spectra while that of T2R is red-shifted (∆E ≈ 281 cm -1 ), which again is ascribed to the different nature of the acceptor group. All compounds show a large Stokes shift of 5100-5200 cm -1 , caused by the stabilization of the charge-transfer state in the polar solvent MeOH. While the emission quantum yield of T2R is 0.25, the dyads show much lower quantum yields, i.e., 0.02 (T2R-Cat1) and 0.08 (T2R-Cat2), corresponding to an emission quenching of 92% (T2R-Cat1) and 68% (T2R-Cat2), respectively. The covalent assembly with the cobalt catalysts seemingly opens up non-radiative decay pathways for the emissive state, leading to this pronounced emission quenching.

The emission excitation spectra show the same bands as the absorption spectra at 355 and 395 nm, indicating that excitation in both the TPA and ICT bands leads to identical emission. For the dyads, there is a discrepancy between absorption and excitation spectra below 320 nm that is caused by the absorption band of the cobalt complexes, whose excitation does not lead to emission.

Transient Absorption (TA) Spectroscopy of T2R

To obtain further insights into the nature of the excited states and of the non-radiative decay induced by the covalent attachment to the catalytic center, we performed time-resolved transient absorption spectroscopy in MeOH with excitation at 480 nm. The transient absorption spectra recorded for T2R at short delay times (Figure 3A) display a ground state bleach (GSB) centered at 490 nm and at 350 nm, while an excited-state absorption (ESA) is observed above 615 nm and peaks around 690 nm. Within 50 ps, the ESA undergoes a blue-shift via a transient double band structure with maxima at 575 and 656 nm to form a broad single ESA band with a maximum at 605 nm. Simultaneously, a negative differential absorption band develops above 690 nm (Figure 3A). Once this spectral evolution is concluded, no further spectral shifts take place and an overall signal decay is observed. This decay, however, is not finished within the experimentally accessible time window of 1.6 ns. The spectral evolution observed for T2R can be rationalized with a model consisting of three consecutive processes starting from the initially excited ICT state, yielding the bleach at 480 nm. The additional bleach at 350 nm is attributed to the GSB of the TPA absorption, supporting the calculations, which show this band to be a HOMO-LUMO+1 transition [START_REF] Yen | Arylamine-Based Dyes for p-Type Dye-Sensitized Solar Cells[END_REF]. The negative differential absorption at >690 nm is due to stimulated emission (SE). The red-shift compared to the steady-state emission measurements, where the maximum was located at 674 nm, can be explained by an overlap of the negative SE band with the positive ESA band. A sequential three-component exponential fit Table 1. Characteristic time constants of light-induced processes upon excitation at 480 nm. The corresponding rates are given in Table S2. Based on the spectral evolution associated with the first two processes, i.e., a blue-shift of ESA with no concurrent GSB decay, they can be assigned to relaxation of the initially populated hot state ICT FC via a partly relaxed ICT ‡ state. The observed red-shift of the SE, from the initial dip at 600 nm to the negative band at >690 nm, further supports this assignment. The exact nature of these relaxation processes is not clear in this class of push-pull dyes [START_REF] Ishow | Photoinduced intramolecular charge-transfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution[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] 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] Oum | Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO 2 , ZrO 2 and in acetonitrile[END_REF]. Quite generally, it is ascribed to a solvation process in response to the photoinduced intramolecular shift of electron density [START_REF] Oum | Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO 2 , ZrO 2 and in acetonitrile[END_REF], coupled to a relaxation cascade through molecular motion such as flattening and rotation around bonds [START_REF] Ishow | Photoinduced intramolecular charge-transfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution[END_REF]. In addition, the coexistence of various conformers in solution might further complicate the assignment of the process to molecular degrees of freedom. The spectral evolution observed for T2R can be rationalized with a model consisting of three consecutive processes starting from the initially excited ICT state, yielding the bleach at 480 nm. The additional bleach at 350 nm is attributed to the GSB of the TPA absorption, supporting the calculations, which show this band to be a HOMO-LUMO+1 transition [START_REF] Yen | Arylamine-Based Dyes for p-Type Dye-Sensitized Solar Cells[END_REF]. The negative differential absorption at >690 nm is due to stimulated emission (SE). The red-shift compared to the steady-state emission measurements, where the maximum was located at 674 nm, can be explained by an overlap of the negative SE band with the positive ESA band. A sequential three-component exponential fit satisfyingly fits the data and gives characteristic time constants associated with the processes of τ 1 = 1.3, τ 2 = 11 and τ 3 = 1320 ps (Table 1, species-associated spectra in Figure 5).

1 τ n = 1/k n.
Based on the spectral evolution associated with the first two processes, i.e., a blue-shift of ESA with no concurrent GSB decay, they can be assigned to relaxation of the initially populated hot state ICT FC via a partly relaxed ICT ‡ state. The observed red-shift of the SE, from the initial dip at 600 nm to the negative band at >690 nm, further supports this assignment. The exact nature of these relaxation processes is not clear in this class of push-pull dyes [START_REF] Ishow | Photoinduced intramolecular charge-transfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution[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] 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] Oum | Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO 2 , ZrO 2 and in acetonitrile[END_REF]. Quite generally, it is ascribed to a solvation process in response to the photoinduced intramolecular shift of electron density [START_REF] Oum | Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO 2 , ZrO 2 and in acetonitrile[END_REF], coupled to a relaxation cascade through molecular motion such as flattening and rotation around bonds [START_REF] Ishow | Photoinduced intramolecular charge-transfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution[END_REF].

In addition, the coexistence of various conformers in solution might further complicate the assignment of the process to molecular degrees of freedom. To this end, we computed the DFT minimum energy structures for four conformations for the T2R dye, considering different relative positions of the CPDT system, the CN, and the CO moieties (Figure 6, Table 2). These four conformations were found to be very close in energy with energy differences in the ground state below 0.1 eV (<2.3 kcal/mol), meaning all conformers are likely present in solution. The TD-DFT vertical electronic transitions computed were scarcely affected by the different dye conformations with differences below 2 nm (Table 2). However, the molecular dynamics and the interconversion between the different conformers, not explored here, could be responsible for the complex relaxation processes in the excited state. In MeOH, the cooling processes are slowed down in comparison with measurements performed in ACN (Supplementary Figure S4), where the cooling process could be fitted with a single component with τ = 1 ps. This can be explained by the higher viscosity of MeOH, which slows down the relaxation processes involving major molecular motion, as observed before for similar push-pull dyes [START_REF] Bold | Electron transfer in a covalent dye-cobalt catalyst assembly-a transient absorption spectroelectrochemistry perspective[END_REF][START_REF] Ishow | Photoinduced intramolecular charge-transfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution[END_REF]. This effect could be further increased To this end, we computed the DFT minimum energy structures for four conformations for the T2R dye, considering different relative positions of the CPDT system, the CN, and the CO moieties (Figure 6, Table 2). These four conformations were found to be very close in energy with energy differences in the ground state below 0.1 eV (<2.3 kcal/mol), meaning all conformers are likely present in solution. The TD-DFT vertical electronic transitions computed were scarcely affected by the different dye conformations with differences below 2 nm (Table 2). However, the molecular dynamics and the interconversion between the different conformers, not explored here, could be responsible for the complex relaxation processes in the excited state. In MeOH, the cooling processes are slowed down in comparison with measurements performed in ACN (Supplementary Figure S4), where the cooling process could be fitted with a single component with τ = 1 ps. This can be explained by the higher viscosity of MeOH, which slows down the relaxation processes involving major molecular motion, as observed before for similar push-pull dyes [START_REF] Bold | Electron transfer in a covalent dye-cobalt catalyst assembly-a transient absorption spectroelectrochemistry perspective[END_REF][START_REF] Ishow | Photoinduced intramolecular charge-transfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution[END_REF]. This effect could be further increased by hydrogen bonds formed by MeOH, which ACN does not. The third process is the return to the ground state of the thermalized excited state 1 ICT, reflected in a concerted decay of GSB, ESA, and SE. This excited state behavior is in line with that observed previously for a related compound with a single thiophene linker [START_REF] Bold | Electron transfer in a covalent dye-cobalt catalyst assembly-a transient absorption spectroelectrochemistry perspective[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].

Catalysts 2020, 10, x FOR PEER REVIEW 8 of 15 by hydrogen bonds formed by MeOH, which ACN does not. The third process is the return to the ground state of the thermalized excited state 1 ICT, reflected in a concerted decay of GSB, ESA, and SE. This excited state behavior is in line with that observed previously for a related compound with a single thiophene linker [START_REF] Bold | Electron transfer in a covalent dye-cobalt catalyst assembly-a transient absorption spectroelectrochemistry perspective[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]. Table 2. Relative energies (∆E) at the B3LYP level of theory, including zero-point vibrational energies within the harmonic approximation, and vertical transition energies at the CAM-B3LYP level of theory, including the acetonitrile solvent medium via the PCM, with the oscillator strengths given in parenthesis. For the labelling, refer to Figure 6.

T2R-A T2R-B T2R-C T2R-D

Transient Absorption Spectroscopy of the Dyads

In the dyads (Figure 3B,C), the initial TA spectra are identical to those of T2R (Supplementary Figure S5a) and the processes taking place within the first 20 ps after photoexcitation are similar in timescale and spectral changes to T2R. They are therefore attributed to the relaxation of the initially excited hot singlet state of the dye moiety to the thermalized singlet excited state 1 ICT. This state shows a more pronounced double band structure in the ESA than T2R, with maxima at 589 nm and 640 nm, while the absence of the negative SE band above 695 nm is in line with the reduced emission quantum yield of the dyads when compared to T2R. In contrast to T2R, an additional spectral evolution is observed within ca. 300 ps, with a decrease in intensity of the ESA and the formation of a long-lived species, characterized by a double band absorption at 590 and 725 nm. This new band persists up to the longest experimentally accessible delay times (1.6 ns), while a partial decay of the ESA band around 600 nm continues. The bleach at 500 nm shows less decay for the dyads compared to T2R (33% remaining signal for the dyads and 20% for the dye). The characteristic absorption observed at the end of the accessible time window (1.6 ns) is identical for both dyads (Supplementary Figure S5b), irrespective of the nature of the catalytic center. Furthermore, it does not match the spectra of the Co II states of the catalysts, which show maxima at 595 and 512 nm for Cat1 and Cat2, respectively (Supplementary Figure S6) [START_REF] Bold | Electron transfer in a covalent dye-cobalt catalyst assembly-a transient absorption spectroelectrochemistry perspective[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] Muresan | Immobilization of a Molecular Cobaloxime Catalyst for Hydrogen Evolution on a Mesoporous Metal Oxide Electrode[END_REF]. Additionally, the extinction coefficients of the optical transitions on the cobalt complex are an order of magnitude lower than those of the dye, while the band observed in the TA spectrum shows a similar intensity to the other dye-based bands. These observations lead us to discard a photoinduced electron transfer process to the catalyst and formation Table 2. Relative energies (∆E) at the B3LYP level of theory, including zero-point vibrational energies within the harmonic approximation, and vertical transition energies at the CAM-B3LYP level of theory, including the acetonitrile solvent medium via the PCM, with the oscillator strengths given in parenthesis. For the labelling, refer to Figure 6.

T2R-A T2R-B T2R-C T2R-D

Transient Absorption Spectroscopy of the Dyads

In the dyads (Figure 3B,C), the initial TA spectra are identical to those of T2R (Supplementary Figure S5a) and the processes taking place within the first 20 ps after photoexcitation are similar in timescale and spectral changes to T2R. They are therefore attributed to the relaxation of the initially excited hot singlet state of the dye moiety to the thermalized singlet excited state 1 ICT. This state shows a more pronounced double band structure in the ESA than T2R, with maxima at 589 nm and 640 nm, while the absence of the negative SE band above 695 nm is in line with the reduced emission quantum yield of the dyads when compared to T2R. In contrast to T2R, an additional spectral evolution is observed within ca. 300 ps, with a decrease in intensity of the ESA and the formation of a long-lived species, characterized by a double band absorption at 590 and 725 nm. This new band persists up to the longest experimentally accessible delay times (1.6 ns), while a partial decay of the ESA band around 600 nm continues. The bleach at 500 nm shows less decay for the dyads compared to T2R (33% remaining signal for the dyads and 20% for the dye). The characteristic absorption observed at the end of the accessible time window (1.6 ns) is identical for both dyads (Supplementary Figure S5b), irrespective of the nature of the catalytic center. Furthermore, it does not match the spectra of the Co II states of the catalysts, which show maxima at 595 and 512 nm for Cat1 and Cat2, respectively (Supplementary Figure S6) [START_REF] Bold | Electron transfer in a covalent dye-cobalt catalyst assembly-a transient absorption spectroelectrochemistry perspective[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] Muresan | Immobilization of a Molecular Cobaloxime Catalyst for Hydrogen Evolution on a Mesoporous Metal Oxide Electrode[END_REF]. Additionally, the extinction coefficients of the optical transitions on the cobalt complex are an order of magnitude lower than those of the dye, while the band observed in the TA spectrum shows a similar intensity to the other dye-based bands. These observations lead us to discard a photoinduced electron transfer process to the catalyst and formation of a charge-transfer species with the electron localized on the cobalt complex (Co II state), in spite of its thermodynamic feasibility (see Supplementary Table S1). Instead, we propose that a triplet state forms by 1 ICT→ 3 ICT intersystem crossing. Bi-and oligothiophenes are known to undergo fast ISC on the timescale of 50 ps to 1 ns [START_REF] Rentsch | Size dependence of triplet and singlet states of α-oligothiophenes[END_REF][START_REF] Grebner | Size-Dependent Properties of Oligothiophenes by Picosecond Time-Resolved Spectroscopy[END_REF], reminiscent of the behavior observed here for the CPDT bridge of our push-pull dye system. The extinction coefficients of the oligothiophene triplet states are also in the same order of magnitude as that of T2R (≈38,000 M -1 cm -1 for the oligothiophenes, ≈55,000 M -1 cm -1 for T2R and T2R-based dyads) [START_REF] Grebner | Size-Dependent Properties of Oligothiophenes by Picosecond Time-Resolved Spectroscopy[END_REF], in line with the observed similar intensity of the ESA band at 725 nm compared to the GSB. The triplet absorption of pure oligothiophenes is observed at higher energies (2.7 eV corresponding to 460 nm for terthiophene) [START_REF] Grebner | Size-Dependent Properties of Oligothiophenes by Picosecond Time-Resolved Spectroscopy[END_REF] than those observed here. However, the CPDT structure is part of an extended conjugated charge-transfer system which can have a strong effect on the excited state energies. In addition, the long lifetime observed for this species is in full agreement with a triplet state, which generally show longer lifetimes than singlet states due to the spin-forbidden nature of the decay to the ground state. Finally, the decay of ESA signal between 540 and 690 nm observed after 300 ps is attributed to direct decay of 1 ICT to the ground state, analogous to the situation in T2R.

Thus, we assume the model shown in Figure 4: following the initial relaxation processes (k 1 and k 2 ), the resulting singlet ICT excited state can decay directly to the ground state (k 3 ) as well as, in the case of the dyads, undergo ISC to form the triplet state (k 4 ), which in turn decays back to the ground state (k 5 ). When fitting the TA data with this model, the direct decay (k 3 ) was not fitted, probably due to the low spectral amplitude and low number of recorded points. However, from the spectral evolution after 300 ps, which shows further signal decay at 500 nm and around 600 nm, i.e., at the position of GSB and ESA of the 1 ICT state, the presence of this direct decay must be deduced. We therefore eliminated the contribution of the 3 ICT state from the TA spectra by subtracting the spectrum recorded at 300 ps (ε = ε(λ, ∆t)ε(λ, 300 ps)). The spectra thus obtained showed the inverted TA spectrum of the 1 ICT state, validating the assignment of direct 1 ICT decay to this spectral evolution (Figures S7 andS8). The decay at the maximum at 600 nm was fitted with a monoexponential decay to obtain the values for τ 3 (370 ps for T2R-Cat1 and 660 ps for T2R-Cat2), which were then fixed in the global fit to the presented model to yield the species-associated spectra and rates of the different processes (Figure 5B,C, Table 1). The obtained rate for the direct decay of the singlet excited state to the ground state is double that observed for T2R. We previously observed a similar trend for a related assembly with an organic dye and cobalt complex, where the excited state lifetime decreased from 1840 to 470 ps [START_REF] Bold | Electron transfer in a covalent dye-cobalt catalyst assembly-a transient absorption spectroelectrochemistry perspective[END_REF]. We proposed Dexter energy transfer to short-lived cobalt-centered states as a possible cause for the reduction in lifetime, an explanation that can equally hold true for the compounds studied here. The rate of ISC was determined to be k 4 = 6.5 × 10 9 s -1 (τ 4 = 150 ps) and k 4 = 5.8 × 10 9 s -1 (τ 4 = 170 ps) for T2R-Cat1 and T2R-Cat2, respectively, while the lifetime of the triplet state was τ 5 >> 2 ns, reflecting the long-lived signal observed.

Discussion

The ISC rate observed in our systems is comparable to literature values for bi-and terthiophene (50-135 ps) [START_REF] Grebner | Size-Dependent Properties of Oligothiophenes by Picosecond Time-Resolved Spectroscopy[END_REF][START_REF] Paa | Femtosecond time-resolved measurements of terthiophene: Fast singlet-triplet intersystem crossing[END_REF]. The extremely fast ISC rates for bithiophene are explained in the literature by a very small energy difference of the S 1 and T 2 state [START_REF] Rentsch | Size dependence of triplet and singlet states of α-oligothiophenes[END_REF]. In general, the sulfur atom plays a role in the fast ISC processes in thiophenes due to its high spin-orbit coupling factor [START_REF] Janssen | Triplet-state photoexcitations of oligothiophene films and solutions[END_REF]. However, as ISC is absent in T2R despite the identical dye structure as in the dyads, the bithiophene structure cannot be the sole source of the ISC and the explanations offered in the literature likely do not fully account for the fast ISC in our systems. In particular, those invoking the population of higher vibrational states in the singlet excited state do not apply in this case, as all systems show identical initial kinetics involving ultrafast relaxation to a thermalized singlet excited state that is finished before ISC is observable. The obvious difference in the dyads in comparison to the T2R is, however, the presence of the cobalt complex. In the literature, descriptions of ISC induced by cobalt in binary systems are limited (to the best of our knowledge) to the influence of the paramagnetic nature of Co II ions [START_REF] Volchkov | Induced intersystem crossing at the fluorescence quenching of laser dye 7-amino-1,3-naphthalenedisulfonic acid by paramagnetic metal ions[END_REF][START_REF] Takashima | Metal ion-dependent fluorescent dynamics of photoexcited zinc-porphyrin and zinc-myoglobin modified with ethylenediaminetetraacetic acid[END_REF]. However, the cobalt complexes in this study are diamagnetic Co III complexes. Instead, alternative pathways must be considered, most probably involving higher energy states on the cobalt complexes. In fact, there are several such states between the HOMO and LUMO energy levels of the dye, which could play a role in the observed ISC (Supplementary Figure S2). The latter was tentatively investigated by density functional calculations, unfortunately without success.

In the literature on heavy-atom free triplet sensitizers, there are examples of radical pair intersystem crossing (RP-ISC), where after charge transfer an electron changes its spin to go from the 1 CT state to the 3 CT state via hyperfine coupling, which then relaxes to a localized triplet state [START_REF] Ji | Color-tunable delayed fluorescence and efficient spin-orbit charge transfer intersystem crossing in compact carbazole-anthracene-BODIPY triads employing the sequential electron transfer approach[END_REF]. In our case, it would be thermodynamically feasible for the excited electron to transfer to a higher state on the cobalt and back to the dye, accompanied by a spin change. However, RP-ISC alone occurs on time scales of nanoseconds and is therefore much slower than the ISC observed in the T2R dyads [START_REF] Weiss | Direct measurement of singlet-triplet splitting within rodlike photogenerated radical ion pairs using magnetic field effects: Estimation of the electronic coupling for charge recombination[END_REF][START_REF] Dance | Time-resolved EPR studies of photogenerated radical ion pairs separated by p-phenylene oligomers and of triplet states resulting from charge recombination[END_REF][START_REF] Colvin | Intersystem crossing involving strongly spin exchange-coupled radical ion pairs in donor-bridge-acceptor molecules[END_REF]. A different approach is the use of spin convertors-most prominently fullerenes-that, after energy transfer from the chromophore to the spin convertor, undergo ISC to yield the triplet state on the spin convertor, which then by a second energy transfer step relaxes to yield the triplet state localized on the chromophore [START_REF] Wu | Light-harvesting fullerene dyads as organic triplet photosensitizers for triplet-triplet annihilation upconversions[END_REF][START_REF] Huang | Energy-funneling-based broadband visible-light-absorbing Bodipy-C 60 triads and tetrads as dual functional heavy-atom-free organic triplet photosensitizers for photocatalytic organic reactions[END_REF][START_REF] Huang | Visible-light-harvesting triphenylamine ethynyl C60-BODIPY dyads as heavy-atom-free organic triplet photosensitizers for triplet-triplet annihilation Upconversion[END_REF]. In our case, the Co complex could act as a spin convertor after electron transfer since ISC has been observed to occur on a ps time scale in the ground state of Co complexes [START_REF] Marroux | Spin Changes Accompany Ultrafast Structural Interconversion in the Ground State of a Cobalt Nitrosyl Complex[END_REF] and <1 ps in the excited state [START_REF] Mccusker | Picosecond Excited-State Dynamics in Octahedral Cobalt(III) Complexes: Intersystem Crossing versus Internal Conversion[END_REF], 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.

Materials and Methods

Cat1 [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 Cat2 [START_REF] Jacques | Cobalt and nickel diimine-dioxime complexes as molecular electrocatalysts for hydrogen evolution with low overvoltages[END_REF] were prepared according to previously reported procedures. Detailed synthetic procedures for T2R, T2R-Cat1, and T2R-Cat2 are reported elsewhere [START_REF] Bold | Spectroscopic investigations provide a rational for the hydrogen-evolving activity of dye-sensitized photocathodes based on a cobalt tetraazamacrocyclic catalyst[END_REF]. For T2R-Cat2, the copper-catalyzed azide-alkyne cycloaddition (CuAAC) coupling we previously reported for related dyads based on push-pull dyes [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] was employed. The synthesis of T2R-Cat1 relies on a CuAAC coupling between 4-azido-2,6-diacetylpyridine and the alkyne derivative of T2R, followed by the template-directed synthesis of the macrocyclic catalytic center. Spectroscopic-grade solvents (Sigma Aldrich, UV-Vis grade or better) were used for all the spectroscopic characterizations.

Steady-state UV-vis absorption spectroscopy: Steady-state absorption spectra were recorded on a Jasco V780 spectrophotometer or a Specord S600 (Analytik Jena) in 1 cm cuvettes.

Steady-state emission spectroscopy: Emission spectra were measured on an Edinburgh FLS980 emission spectrofluorometer in a 1 cm cuvette at 90 • angle. The solutions were prepared to have an absorbance of 0.05 at the excitation wavelength. Quantum yields were determined on the same instrument equipped with an integrating sphere, with solution at 0.1 absorbance at the excitation wavelength.

Transient absorption spectroscopy: The setup for femtosecond transient absorption measurements has been previously described [START_REF] Karnahl | Tuning of Photocatalytic Hydrogen Production and Photoinduced Intramolecular Electron Transfer Rates by Regioselective Bridging Ligand Substitution[END_REF]. 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 0.5 µJ per pulse. 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 CaF 2 window, with probe intensities falling into the range of hundred nJ. The probe light was split into two beams, one of which was focused through the sample, while the other served as reference. 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, Lund Sweden). The recorded data were 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 artefacts [START_REF] Dietzek | Appearance of coherent artifact signals in femtosecond transient absorption spectroscopy in dependence on detector design[END_REF]. The measurements were performed in 1 mm cuvettes on solutions prepared to have an absorbance of 0.3 at the excitation wavelength (480 nm).

Computational details: All the calculations were carried out with the Gaussian16 suite of quantum chemistry programs [START_REF] Frisch | Revision A.02[END_REF]. The electronic ground state and the vertical excitation energies of T2R, T2R-Cat1, and T2R-Cat2 complexes have been characterized with DFT [START_REF] Hohenberg | Inhomogeneous Electron Gas[END_REF] and TD-DFT [START_REF] Runge | Density-Functional Theory for Time-Dependent Systems[END_REF] approaches, respectively. Structural optimizations were carried out with the B3LYP hybrid density functional [START_REF] Becke | Density-functional thermochemistry. III. The role of exact exchange[END_REF] and the Def2TZVP [START_REF] Weigend | Accurate Coulomb-fitting basis sets for H to Rn[END_REF] basis set for C, H, O, N, and S atoms, and the SDD ECP and basis for the Co atom [START_REF] Dolg | Energy-adjusted ab initio pseudopotentials for the first row transition elements[END_REF]. Because the excitation energies have a strong character of intramolecular charge transfer, we recall to the CAM-B3LYP level of theory with the same basis set for all the TD-DFT calculations [START_REF] Yanai | A new hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP)[END_REF]. In all the calculations, we have included the Acetonitrile solvent medium via the polarizable continuum model (PCM) [START_REF] Tomasi | Quantum Mechanical Continuum Solvation Models[END_REF].

Conclusions

The photoinduced processes at work in two novel dye-catalyst assemblies designed for hydrogen production in dye-sensitized photocathodes have been investigated by steady-state and ultrafast spectroscopic techniques. Comprehensive studies on the excited state dynamics of push-pull organic dyes are not common in the literature [START_REF] Ishow | Photoinduced intramolecular charge-transfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution[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] 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] Oum | Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO 2 , ZrO 2 and in acetonitrile[END_REF][START_REF] Li | Influence of Thiophene Moiety on the Excited State Properties of Push-Pull Chromophores[END_REF][START_REF] Labrunie | Triphenylamine-Based Push-Pull σ-C 60 Dyad As Photoactive Molecular Material for Single-Component Organic Solar Cells: Synthesis, Characterizations, and Photophysical Properties[END_REF][START_REF] Oum | Electron and hole transfer dynamics of a triarylamine-based dye with peripheral hole acceptors on TiO 2 in the absence and presence of solvent[END_REF]. Compared to metallo-organic photosensitizers such as Ru(bpy) 3 2+ , various conformers indeed coexist in solution for these structures, increasing the number of potential relaxation pathways, thus rendering the photophysical analysis much more complex. The results presented here nevertheless highlight that appending a cobalt-based catalytic unit to a push-pull organic dye strongly affects its photophysical properties. In particular, the quenching of the dye emission within the dyad is not the signature of a photoinduced intramolecular electron transfer process to the catalytic center. Instead, a new non-radiative relaxation pathway is opened, involving intersystem crossing to a long-lived triplet excited state. Although the mechanism behind that is still unclear, this opens new perspectives of application for this family of dyads as organic triplet sensitizers. This study also provides key information regarding light-driven hydrogen evolution with these dye-catalyst assemblies, i.e., an initial reductive quenching mechanism with the electron donor is required for catalysis to occur.

Supplementary Materials:

The following are available online at http://www.mdpi.com/2073-4344/10/11/1340/s1. S1: Photophysical and redox properties of Cat1, Cat2, T2R, T2R-Cat1, and T2R-Cat2, Table S2: Rates and characteristic time constants of excited-state processes.

Figure 1 .

 1 Figure 1. Structures of the dye T2R, catalysts Cat1 and Cat2, and dyads T2R-Cat1 and T2R-Cat2.

Figure 1 .

 1 Figure 1. Structures of the dye T2R, catalysts Cat1 and Cat2, and dyads T2R-Cat1 and T2R-Cat2.

Figure 2 .

 2 Figure 2. (A) UV-Vis absorption spectra of T2R, T2R-Cat1, T2R-Cat2, Cat1 and Cat2 recorded in acetonitrile. (B) Normalized absorption, emission and excitation spectra of T2R, T2R-Cat1 and T2R-Cat2 recorded in MeOH. The emission quantum yield was 0.25 (T2R), 0.02 (T2R-Cat1) and 0.08 (T2R-Cat2).

Figure 2 .

 2 Figure 2. (A) UV-Vis absorption spectra of T2R, T2R-Cat1, T2R-Cat2, Cat1 and Cat2 recorded in acetonitrile. (B) Normalized absorption, emission and excitation spectra of T2R, T2R-Cat1 and T2R-Cat2 recorded in MeOH. The emission quantum yield was 0.25 (T2R), 0.02 (T2R-Cat1) and 0.08 (T2R-Cat2).

15 Figure 3 .

 153 Figure 3. TA spectra at different pump-probe delays (A,C,E) and kinetic traces (B,D,F) of T2R (A,B), T2R-Cat1 (C,D) and T2R-Cat2 (E,F) in MeOH after excitation at 480 nm. The solid lines in the kinetic traces are fits of the experimental data to the models presented in Figure 4.

Figure 3 .

 3 Figure 3. TA spectra at different pump-probe delays (A,C,E) and kinetic traces (B,D,F) of T2R (A,B), T2R-Cat1 (C,D) and T2R-Cat2 (E,F) in MeOH after excitation at 480 nm. The solid lines in the kinetic traces are fits of the experimental data to the models presented in Figure 4.

Figure 4 .

 4 Figure 4. Photophysical model for the light-induced processes upon excitation at 480 nm. The pathway to the triplet state by ISC (τ4) is only open for the dyads. Dashed arrows indicate radiative transitions.

  2 ns T2R-Cat2 0.9 7 660 170 >> 2 ns 1 τn = 1/kn.

Figure 4 .

 4 Figure 4. Photophysical model for the light-induced processes upon excitation at 480 nm. The pathway to the triplet state by ISC (τ 4 ) is only open for the dyads. Dashed arrows indicate radiative transitions.

Catalysts 2020 , 15 Figure 5 .

 2020155 Figure 5. Species-associated spectra obtained by fitting the TA data of (A) T2R, (B) T2R-Cat1 and (C) T2R-Cat2 obtained at 480 nm excitation with the model shown in Figure 4. The dashed and dotted lines in the first panel show the inverted and scaled steady-state absorption and emission spectra.

Figure 5 .

 5 Figure 5. Species-associated spectra obtained by fitting the TA data of (A) T2R, (B) T2R-Cat1 and (C) T2R-Cat2 obtained at 480 nm excitation with the model shown in Figure 4. The dashed and dotted lines in the first panel show the inverted and scaled steady-state absorption and emission spectra.

Figure 6 .

 6 Figure 6. Minimum energy structures of four conformations for the T2R dye obtained by DFT calculations.

Figure 6 .

 6 Figure 6. Minimum energy structures of four conformations for the T2R dye obtained by DFT calculations.

Figure S1 :

 S1 Frontier molecular orbitals of T2R, Figure S2: Frontier molecular orbitals of T2R-Cat1, Figure S3: UV-Vis absorption spectra of T2R, T2R-Cat1, and T2R-Cat2 in MeOH, Figure S4: TA spectra and species-associated spectra of T2R in ACN, Figure S5: Transient absorption spectra at 0.4 ps and 1600 ps time delay in MeOH, Figure S6: UV-Vis spectroelectrochemical spectra of T2R-Cat1, Figure S7: Differential TA spectra of T2R-Cat1 in MeOH of the direct decay and monoexponential fit, Figure S8: Differential TA spectra of T2R-Cat2 in MeOH of the direct decay and monoexponential fit, Table

Table 1 .

 1 Characteristic time constants of light-induced processes upon excitation at 480 nm. The corresponding rates are given in TableS2.

	-		Time Constants 1 /ps
		τ 1	τ 2	τ 3	τ 4	τ 5
	T2R	1.3 11 1320
	T2R-Cat1	0.8	7	370	150 >> 2 ns
	T2R-Cat2	0.9	7	660	170 >> 2 ns

Acknowledgments:

We thank Yves Carstensen and Maria Sittig for their help in measuring the emission quantum yields. We thank Jens Uhlig for providing us with his data analysis software used for fitting the TA data.

Conflicts of Interest:

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Funding: This research was supported by the European Commission's Seventh Framework Program (FP7/2007-2013) under grant agreement n • 306398 (FP7-IDEAS-ERC, Project PhotocatH 2 ode), 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.


Julien Massin

Maximilian Bräutigam

Sebastian Bold

Maria Wächtler

Michèle Pavone

Ana B Muñoz-García

Benjamin Dietzek

Vincent Artero

Murielle Chavarot-Kerlidou

Michele Pavone

Investigating Light-Driven Hole Injection and Hydrogen Evolution Catalysis at Dye-Sensitized NiO Photocathodes: A Combined Experimental-Theoretical Study

Keywords: Solar fuels, dye-sensitized photoelectrodes, DSPEC, NiO, push-pull dye, cobaloxime, transient absorption measurements

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Introduction

The production of fuels through light-driven processes is a promising solution for the durable storage of solar energy. [START_REF] Faunce | Artificial photosynthesis as a frontier technology for energy sustainability[END_REF][START_REF] Thapper | Artificial Photosynthesis for Solar Fuels -an Evolving Research Field within AMPEA, a Joint Programme of the European Energy Research Alliance[END_REF] Among other technologies, photoelectrochemical cells (PECs) couple the harvesting of light with fuel-forming catalysis in a single device. [START_REF] Walter | Solar water splitting cells[END_REF] Dye-sensitized PECs (DS-PECs) form a particular class of such devices based on the grafting of active photocatalytic components at the surface of transparent conductive electrode substrates, extending the tandem dye-sensitized solar cells (DSSCs) technology. [START_REF] Odobel | Recent Advances in the Sensitization of Wide-Band-Gap Nanostructured p-Type Semiconductors. Photovoltaic and Photocatalytic Applications[END_REF][START_REF] Brennaman | Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells[END_REF][START_REF] Xu | Water splitting dye-sensitized solar cells[END_REF] Significant achievements in this direction have been reported recently regarding the preparation of molecular-based photocathodes active for H2 production or CO2 reduction, including their implementation in operating DS-PECs. [START_REF] Gibson | Dye-sensitized photocathodes for H2 evolution[END_REF][START_REF] Queyriaux | Molecular cathode and photocathode materials for hydrogen evolution in photoelectrochemical devices[END_REF][START_REF] Kumagai | Hybrid photocathode consisting of a CuGaO2 p-type semiconductor and a Ru(II)-Re(I) supramolecular photocatalyst: non-biased visible-light-driven CO2 reduction with water oxidation[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] Wang | A donor-chromophorecatalyst assembly for solar CO2 reduction[END_REF] Almost all of them rely on nickel oxide (NiO), which is a low-cost and easy-to-process p-type semiconductor, suitable for hole injection from an excited molecular photosensitizer to its valence band. [START_REF] Wang | A donor-chromophorecatalyst assembly for solar CO2 reduction[END_REF][START_REF] Li | Visible light driven hydrogen production from a photo-active cathode based on a molecular catalyst and organic dyesensitized p-type nanostructured NiO[END_REF][START_REF] Ji | Photostable p-Type Dye-Sensitized Photoelectrochemical Cells for Water Reduction[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 | Covalent Design for Dye-Sensitized H2-Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst[END_REF][START_REF] Click | Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production[END_REF][START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-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] Kamire | Photodriven hydrogen evolution by molecular catalysts using Al2O3protected 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][START_REF] Wang | Layer-by-Layer Molecular Assemblies for Dye-Sensitized Photoelectrosynthesis Cells Prepared by Atomic Layer Deposition[END_REF][START_REF] Shan | Direct photoactivation of a nickel-based, water-reduction photocathode by a highly conjugated supramolecular chromophore[END_REF][START_REF] Shan | Controlling Vertical and Lateral Electron Migration Using a Bifunctional Chromophore Assembly in Dye-Sensitized Photoelectrosynthesis Cells[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 | Insights into 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][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] Various nanostructured NiO film preparation methods have been reported over the years [START_REF] Dini | The influence of the preparation method of NiOx photocathodes on the efficiency of p-type dye-sensitized solar cells[END_REF] and recently evaluated with respect to their performance as photocathodes in a comprehensive benchmarking study. [START_REF] Wood | A comprehensive comparison of dye-sensitized NiO photocathodes for solar energy conversion[END_REF] Irrespective of the molecular design of the photocathode, i.e. the specific nature/structure of dyes and catalysts that are employed and how they are assembled (catalyst dissolved in solution vs. co-grafting of dye and catalyst onto NiO vs. covalent dye-catalyst assembly), two key kinetic processes for the photoelectrocatalytic activity can be identified: i) light-driven charge separation and hole injection into NiO and ii) generation of a long-lived charge-separated intermediate to cope with the low timescale of catalysis compared to the light-driven processes. [START_REF] Ponseca | Ultrafast Electron Dynamics in Solar Energy Conversion[END_REF] In order to meet these key requirements, organic push-pull dyes have advanced into versatile sensitizer systems, [START_REF] Odobel | Recent Advances in the Sensitization of Wide-Band-Gap Nanostructured p-Type Semiconductors. Photovoltaic and Photocatalytic Applications[END_REF][START_REF] Nikolaou | Recent advances and insights in dye-sensitized NiO photocathodes for photovoltaic devices[END_REF] even though the injection kinetics observed for Ru(II)-bipyridyl-derived complexes have also been discussed in the context of intramolecular push-pull properties. [START_REF] Brautigam | 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] Han | Interfacial electron transfer yields in dye-sensitized NiO photocathodes correlated to excited-state dipole orientation of ruthenium chromophores[END_REF] Generally, such chromophores are designed to have their HOMO localized near the NiO surface and the LUMO distant to it. Thus, their excited states are ideally suited to promote electron injection from occupied NiO levels, more specifically Ni d and ONiO p states, into the HOMO of the electronically excited photosensitizer. Coumarin C343-sensitized NiO films were among the first studied systems for elucidating electron transfer dynamics by ultrafast spectroscopic techniques, [START_REF] Morandeira | Photoinduced Ultrafast Dynamics of Coumarin 343 Sensitized p-Type-Nanostructured NiO Films[END_REF][START_REF] Morandeira | Coumarin 343-NiO Films as Nanostructured Photocathodes in Dye-Sensitized Solar Cells: Ultrafast Electron Transfer, Effect of the I 3-/I -Redox Couple and Mechanism of Photocurrent Generation[END_REF][START_REF] Mori | Charge-Transfer Processes in Dye-Sensitized NiO Solar Cells[END_REF] including light-driven electron transfer to a co-grafted H2-evolving catalyst. 18, 39- 41 This motivated two of us to investigate the structure, energetics and electronic properties of the NiO(100)/C343 interface by an original ab initio study which highlighted how the local molecular structure at the interface can affect relevant parameters for performance, such as the dye binding strength or the dye-electrode hole injection driving force. [START_REF] Munoz-Garcia | Structure and energy level alignment at the dyeelectrode interface in p-type DSSCs: new hints on the role of anchoring modes from ab initio calculations[END_REF] We now extend this study to NiO electrodes functionalized by a push-pull organic dye. Grafted onto NiO, the T1 dye containing a triarylamine electron-donor part and an ethyl cyanoacetate electron-acceptor part separated by a thiophene unit (Fig. 1) was proven efficient to generate photocurrents in the presence of irreversible electron acceptors in aqueous electrolytes. [START_REF] Massin | Dye-Sensitized PS-b-P2VP-templated Nickel Oxide Films for Photoelectrochemical Applications[END_REF] To characterize the initial light-driven processes occurring at NiO electrodes sensitized with T1, we use the ab initio approach to get insights into the thermodynamics of the hole injection process, we characterize this process kinetically using ultrafast transient absorption spectroscopy and finally demonstrate that light-driven charge separation at this dye-sensitized NiO electrode can be coupled to H2 evolution catalysis under fully aqueous conditions.

Results and discussion

DFT Calculations

HOMO and LUMO densities of the isolated T1 dye (Fig. S1) are typical for a push-pull dye, with a significant part of the HOMO and LUMO density localized on the anchoring and acceptor group, respectively. Since T1 features two carboxylic acid anchoring groups, we have considered two different binding geometries corresponding to one and two carboxylate groups attached to a NiO (100) surface, which we call mono-branch (M) and bi-branch (B) throughout the text (Fig. 2). In both cases, we have considered the bidentate anchoring mode for the carboxylate groups, which exhibits ~ 0.12 eV more negative binding energies than the monodentate one. For both the M and B configurations we found energy minima, as confirmed by harmonic molecular frequencies calculations. Of course, other local minima can be found because of the dye's flexible tail, but an extensive sampling of the dye conformational space is beyond the scope of this work, which is focused on the T1-NiO interface rather than on the dye molecular dynamics. Moreover, we checked the effects of dispersion on the dye binding energy by including the D3 correction scheme by Grimme, [START_REF] Grimme | A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu[END_REF] but we find no qualitative differences with pristine DFT-PBE+U, the B case always slightly favored over the M geometry with and without the inclusion of dispersion forces. On these structural minima, we have evaluated the thermodynamic driving force for hole injection (E in Fig. 1) from the corresponding atomprojected density of states (PDOS) of M and B systems (Fig. 3). Resulting values for E are collected in Table 1. In all three environments considered (vacuum, acetonitrile and water), hybridized Ni dand O p-states cross the Fermi level and are above the dye HOMO energies, resulting in a favorable E for hole injection. Still, we find that the solvent strongly affects the relative position of the NiOVB-T1HOMO peaks, bringing a very weak driving force for hole injection in vacuum (0.05 eV) to a very convenient driving force of 0.26 eV < E < 0.35 eV in acetonitrile and water. This effect is related to the solvent offsetting the interfacial electrostatic dipole generated by the surface OH groups upon bidentate binding of the carboxylate groups. 1.

Table 1. Calculated driving force for hole injection (E), calculated as the energy difference between the highest occupied PDOS peak of the dye and the Fermi level (p-NiO VB edge, the highest energy occupied state in the VB).

E (eV)=EF-EHOMO M B Vacuum -0.05 -0.05 CH3CN -0.30 -0.18 H2O -0.35 -0.26
Besides presenting a good thermodynamic driving force for hole injection, the recombination of the excited electron of the dye with the holes at the semiconductor valence band should be minimized in efficient p-DSSCs. We have evaluated this effect by obtaining the band-decomposed charge densities of M and B systems for the lowest-energy unoccupied band as an approximation of the LUMO of the dye when attached to NiO. These charge densities provide qualitative insights into where the electron will reside after photo-excitation of the dye. As shown by Fig. 4, in both M and B cases, the pseudo-LUMO densities are localized on the T1 dye electron-acceptor group, as the LUMO of the isolated dye (Fig. S1). On the contrary, for the nonpush-pull benchmark C343 dye (Fig. 4, right) the pseudo-LUMO density is not only localized on the dye, but involves a significant amount of unoccupied states from the electrode surface slab.

From electron transfer theory applied to p-DSSCs, [START_REF] Ma | Predicting Energy Conversion Efficiency of Dye Solar Cells from First Principles[END_REF] the probability of charge recombination is inversely proportional to the distance between the dye photo-excited electron and the hole at the electrode surface. While for the C343-NiO system this distance is practically zero, the T1 pseudo-LUMO density stays well localized on the electron-acceptor group and at a reasonable distance from the NiO surface (> 10 Å). So, it is expected that recombination processes are minimized for T1 adsorbed on NiO. We must note that these results can only provide qualitative insights. On one hand, the quite large size of the system, including the T1 dye, the NiO slab and the implicit solvent, prevents an ab initio characterization beyond state-of-the-art DFT-PBE+U (as for example GW approaches): we must note, for example, that for a correct description of the NiO conduction band an hybrid HF-DFT approach with ~35% of exact exchange would be better suited than DFT-PBE+U [ref]. On the other hand, the p-NiO/T1/solvent interface is still a very simplified model of a much more complex real electrode, where several other species are involved in a complex surface equilibrium. Still, our computational results highlight the different contributions of the adsorption mode (M vs B) and of the solvent polarity on (i) the hole injection driving force and (ii) the probability of charge recombination, setting the conceptual framework for the analysis of the spectrocopic and the electrochemical results (see below).

Transient absorption spectroscopy

Time-resolved transient absorption measurements were performed for T1 dissolved in acetonitrile and grafted onto mesoporous NiO films, both in the presence and in the absence of chenodeoxycholic acid (CDCA). CDCA is a well-established co-adsorbent in dye-sensitized solar cells to avoid aggregation of sensitizer on the surface of TiO2 and hence to reduce dye-dye interactions, which are prone to quench excited states and hence diminish the efficiency of the solar cells. [START_REF] Ito | High-conversion-efficiency organic dye-sensitized solar cells with a novel indoline dye[END_REF][START_REF] Favereau | Diketopyrrolopyrrole derivatives for efficient NiO-based dye-sensitized solar cells[END_REF] In solution, the transient spectra immediately after excitation are dominated by positive excited state absorption (ESA) for wavelengths shorter than 600 nm and longer than 650 nm (Fig. 5A).

The negative feature between 600 and 650 nm results from dominating contributions from stimulated emission (SE), which is supported by comparison with the absorption and emission spectrum of the dye in solution. Contributions originating from ground state bleach (GSB) can be ruled out because the ground state is not absorbing in this spectral region. Within the first 4 ps the negative SE feature between 600 and 650 nm disappears while in parallel negative signal is formed for wavelengths longer than 650 nm. These changes probably result from a change in the ESA band superimposed on the SE contributions to the overall transient spectrum, but also an additional redshift of the SE signal on this time scale can contribute to the observed spectral changes. After 4 ps an overall decay of the signal intensity without significant changes in the spectral shape is observed (Fig. 5B). These observations can be explained in line with the established model for the relaxation in the excited state manifold in structurally related push-pull systems upon intra-molecular charge-transfer ( 1 ICT) excitation. [START_REF] Ishow | Photoinduced intramolecular chargetransfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution[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] Zhu | Intramolecular Charge Transfer and Solvation of Photoactive Molecules with Conjugated Push-Pull Structures[END_REF] A relaxation cascade leads from the initially populated ICTFC state to a relaxed 1 ICT states. In T1 a relaxed emissive ICT' state is formed with a time constant of τ1 = 0.9±0.1 ps leading to the strong spectral changes observed in the transient spectra occurring up to 4 ps. This ICT' state is depopulated via emission and/or ISC. The decay of the overall signal can be fitted with two exponentials, τ2 = 118±10 ps and τ3 = 1.9±0.4 ns. The value of τ3 = 1.9 ns agrees with the lifetime of 1.7 ns determined by emission lifetime measurements (Fig. S5). The observed spectral signatures and photoinduced processes are not impacted by the presence of CDCA (Fig. S6). 2 and Fig. S6). To experimentally address the question whether the packing of T1 on the NiO surface impacts the dynamics, T1 and CDCA were co-grafted onto the NiO electrodes. CDCA was indeed previously employed as anti-aggregation agent to improve the photovoltaic efficiency of DSSCs. [START_REF] Ito | High-conversion-efficiency organic dye-sensitized solar cells with a novel indoline dye[END_REF][START_REF] Favereau | Diketopyrrolopyrrole derivatives for efficient NiO-based dye-sensitized solar cells[END_REF] As shown in the supporting information (Fig. S3, S4 and S6), the addition of CDCA does not impact the photoinduced processes, i.e. characteristic time-constants of 1 = 300 fs, 2 = 1.7 ps, 3 = 17 ps and 4 = 266 ps plus a long lived low amplitude component result from the fit (Table 2 and Fig. S6). Hence, interactions between individual T1 molecules can be excluded to account for the observed photoinduced dynamics in the T1-sensitized NiO electrodes.

Table 2. Time constants ( ) derived from a global multiexponential fit, for amplitude spectra (decay associated spectra -DAS) see Fig. S6, and relative decay amplitude (A) for the bleach formed on NiO in the spectral range between 500 nm and 510 nm determined by integration of the signal in this range.

 T1 (CH3CN) T1+CDCA (CH3CN) T1@NiO T1+CDCA@NiO 1 / ps (A1) 0.9 (n.a.) 0.8 (n.a.) 0.4 (0.17 latter is particularly apparent when using transition metal complexes as photosensitizers, in which the photoactive core consists of an asymmetrically coordinated redox active metal ion, e.g.

Ru(II). [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] Associated with the overall decay of the photoinduced absorption signal of T1@NiO is a shift of the zero-crossing of the OD-signal from 515 nm (at early delay times) to 530 nm (at long delay times). This indicates that recombination of holes in the NiO with the reduced photosensitizer occurs fast -as typically observed for NiO photocathodes. [START_REF] Odobel | Recent advances and future directions to optimize the performances of p-type dye-sensitized solar cells[END_REF] The sub-ns timescale of charge recombination, processes associated with the characteristic time constant 2=280 ps, is kinetically very much like the data reported by Hammarström et al., who reported charge recombination times of 335 ps in a structurally very related system. [START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF] Interestingly, charge recombination for T1@NiO is one order of magnitude slower than the one previously reported for coumarin C343@NiO (characteristic time constant of 23 ps), [START_REF] Morandeira | Photoinduced Ultrafast Dynamics of Coumarin 343 Sensitized p-Type-Nanostructured NiO Films[END_REF] as predicted by our DFT calculations (see above).

Nonetheless, the decay of the photoinduced signal is not complete within the experimentally accessible delay times of 1.8 ns. [START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF] We cannot quantify the actual timescale for the slow charge recombination, which is outside the range of experimentally accessible delay times. Meyer and coworkers resorting to a cathode architecture, in which the photosensitizer and the NiO are separated by an extended supramolecular spacer, reported charge separation lifetimes extending µs to ms. [START_REF] Shan | Generation of Long-Lived Redox Equivalents in Self-Assembled Bilayer Structures on Metal Oxide Electrodes[END_REF] To estimate the yield of this long-lived species, we compared the amplitudes of the ground-state bleach signal measured at the very blue edge of the experimentally accessible spectral window (to avoid contributions from the overlaying broad and red-shifted photoinduced absorption band). At a delay time of 400 fs the ground-state bleach has an amplitude of about 2.5 mOD, which decays to 1 mOD at 1.8 ns delay time. From the relative amplitude of the long lived component determined by integration over the bleach region in the fit (Table 2) a yield of 35% of the long lived species can be estimated, which is very reasonable. [START_REF] Queyriaux | Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine Sensitized NiO Photocathodes[END_REF] It should be noted that this value presents an upper limit of the actual yield of the long-lived species as, at early times, comparably strong photoinduced absorption might overlap with the ground-state bleach. On the contrary, at long delay times, the OD spectrum resembles the inverted ground state spectrum of T1 with only minor contributions from a broad unstructured photoinduced absorption. The absence of any noticeable contribution from photoinduced absorption at long delay times reflects the difficulty of the spectroscopic detection of holes in NiO, which has already been pointed out by Hagfeldt and by Hammarström. [START_REF] D'amario | Unveiling hole trapping and surface dynamics of NiO nanoparticles[END_REF][START_REF] Boschloo | Spectroelectrochemistry of Nanostructured NiO[END_REF] It should also be mentioned here that recent studies by the groups of Hammarström [START_REF] D'amario | Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells[END_REF] and Meyer 56 highlighted the dependence of the charge separated state lifetime on the external bias applied to the electrode. Vacant electronic states extending well above the valence band are indeed present in NiO films; filling these states by applying a negative potential to the film in a classical three-electrode setup as the one employed here for the photoelectrochemical measurements (see below) drastically increases the lifetime of the reduced dye on the surface.

Photoelectrochemical study

The existence of a long-lived charge-separated state is a key feature for the observation of catalytic photocurrent in functional photocathodes reported in literature. [START_REF] Antila | Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes with a Biomimetic FeFe-Catalyst[END_REF][START_REF] Kamire | Photodriven hydrogen evolution by molecular catalysts using Al2O3protected 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][START_REF] Shan | Generation of Long-Lived Redox Equivalents in Self-Assembled Bilayer Structures on Metal Oxide Electrodes[END_REF] This is particularly crucial when the catalyst is dissolved in solution: electron transfer from the reduced dye to the catalyst is, in this specific case, controlled by diffusion of the latter in the electrolyte; it thus occurs -at the fastest -on the nanosecond timescale.

To verify whether the long-lived species detected above for the T1-sensitized NiO films could initiate catalysis, the photoelectrochemical properties of these electrodes were investigated in aqueous sodium acetate buffer (0.1 M, pH=4.5) in the absence and in the presence of the cobaloxime H2-evolving catalyst [Co(dmgBF2)2(H2O)2] (Fig. 1), previously employed for similar purposes. 12, 20 30 The redox properties of the latter (E 0 (Co II/I ) = -0.40V vs NHE) are suitable for Co(I) generation by electron transfer from the reduced dye to the Co(II). The latter process is exergonic with a Gibbs free energy (GET = E(PS/PS -) -E(Co II /Co I )) of -0.64 eV, essential to enter the H2-evolving catalytic cycle. [START_REF] Artero | Splitting water with cobalt[END_REF] Linear sweep voltammograms (Fig. 6) were recorded first in the dark and then under visible light irradiation (400-800 nm; 65 mW•cm -2 ~1 sun) on a range of potentials preventing direct Co(II) to Taking these observations into consideration, chronoamperometric measurements were carried out at +0.05 V vs. RHE (-0.4 V vs. Ag/AgCl), where high photocurrent density is observed. A photocurrent density of 6 A•cm -2 was measured under chopped light irradiation at T1-sensitized NiO photocathodes in the presence of [Co(dmgBF2)2(H2O)2] in solution (Fig. 6, Table S1). As the co-grafting of T1 with CDCA does not impact the photoinduced dynamics of the photoelectrode, the slightly lower photocurrent value recorded in the presence of CDCA (Fig. S7 and Table S1) is attributed to the lower grafting density of the T1 dye when co-adsorbed with CDCA (Experimental part and Fig. S2).

Finally, photoelectrochemical H2 generation was confirmed during prolonged chronoamperometric measurements (6 hours at +0.05 V vs. RHE, Fig. S8), coupled to gas chromatography analysis of the headspace of the cell. Hydrogen was successfully detected with a faradaic efficiency of 3% (12 nmolH2•cm -2 ), compared to 1% (3 nmolH2•cm -2 ) in the absence of catalyst in solution (Fig. S9, Table S1). As expected from the photocurrent measurements, (B) (A) T1/CDCA co-grafted electrodes are slightly lower active (Fig. S8, Table S1). Production of low amounts of hydrogen at dye-sensitized NiO photocathodes in the absence of a molecular catalyst was previously observed in the literature and recently attributed to the light-driven generation of Ni nanoparticles at defects of the NiO film. [START_REF] Hoogeveen | Origin of Photoelectrochemical Generation of Dihydrogen by a Dye-Sensitized Photocathode without an Intentionally Introduced Catalyst[END_REF] The observation of a four-time increase in the amount of hydrogen produced per square centimeter when the cobalt catalyst is present in solution (Table S1) supports a cobalt-center molecular process. Although the results reported here are not intended to be of any practical application, the observed catalytic activity can be directly put in line with the generation of a charge-separated state long-lived enough to allow intermolecular electron transfer to the molecular catalyst in solution.

Conclusion

The field of dye-sensitized photoelectrodes for the generation of solar fuels expands rapidly both regarding the preparation of novel systems and spectroscopic characterization of their mechanism. However, first-principles studies that include both the molecular constituents and the metal oxide electrode surface remain scarce. We demonstrate here for the first time how DFT studies can determine the structural and electronic features of the interfaces between a push-pull dye and the p-NiO (100) surface. Although theoretical calculations cannot reproduce the complex structure of mesoporous NiO electrodes used experimentally in prototypical dyesensitized photocathode system, theory provides a rationale for mechanistic studies and activity measurements regarding the binding modes of the dye molecules and their effect, together with solvent polarity, on the driving force for hole injection. Further work will aim at the modelling of photocathode architectures incorporating catalytic units. 16 , 26 ASSOCIATED CONTENT Supporting Information.

Figure 1 .

 1 Figure 1. Energy level diagram at pH 4.5 of a p-type NiO photocathode sensitized with T1, together with the chemical structure and Co II /Co I redox potential (vs. NHE) of the H2-evolving cobaloxime catalyst [Co(dmgBF2)2(H2O)]2].The NiO valence band edge potential at this pH value was estimated from the 0.37 V vs. NHE value determined at pH 7.[START_REF] Natu | Valence band-edge engineering of nickel oxide nanoparticles via cobalt doping for application in p-type dye-sensitized solar cells[END_REF]

42

 42

Figure 2 .

 2 Figure 2. Left: Lateral views of the optimized structures of T1 dye upon adsorption on NiO (100) surface in the mono-branch (M) and bi-branch (B) adsorption modes. The limits of the unit cell used are shown in dashed grey. Right: Top view of 3x3 unit cells of M and B systems. Color legend: Ni (silver), O (red), C (green), N (blue), S (yellow), H (light pink).

  While qualitatively similar, M systems present ~0.1 eV of extra driving force with respect to B ones in both acetonitrile and water.

Figure 3 .

 3 Figure 3. Projected density of states (PDOS) at the DFT-PBE+U level of theory for Ni d states (black line), ONiO p states (red line) and T1 dye (green line) in the M and B adsorption mode calculated in vacuum (top panels), acetonitrile (central panels) and water (bottom panels). The Fermi energy (EF) is set to zero. Corresponding E values are reported in Table1.

Figure 4 .

 4 Figure 4. Isosurfaces of band-decomposed charge density of the lowest unoccupied bands of adsorbed T1 in both M (left) and B (middle) adsorption modes on NiO (100) in comparison with that of coumarine C343 dye (right) computed at the same level of theory (PBE+U, H2O -implicit solvent). Isosurface value: 0.001 e -bohr -3 .

Figure 5 .

 5 Figure 5. Transient absorption spectra of T1 in acetonitrile solution (A, B) and of a NiO film sensitized with T1 (C) at chosen delay times. The steady state absorption and emission (ex = 400 nm) spectra of T1 are given for comparison in the relevant spectral range (D).

33 )

 33 The thermodynamic driving force calculated to 0.3 eV (see above) is reflected in the rather rapid electron injection visible in the transient absorption data shown in Fig.5C. From the data, a characteristic injection time of τ1= 0.3±0.1 ps can be deduced. Probably even faster components contribute to the charge injection, which are hidden by the coherent artifact.[START_REF] Kovalenko | Femtosecond hole-burning spectroscopy of the dye DCM in solution: the transition from the locally excited to a charge-transfer state[END_REF][START_REF] Dietzek | Appearance of coherent artifact signals in femtosecond transient absorption spectroscopy in dependence on detector design[END_REF] This sub-ps timescale for hole injection is consistent with literature, e.g. a recent study by Hammarström and coworkers, who use a push-pull chromophore structurally related to P1, for covalent attachment of a Co-based molecular catalyst.[START_REF] Pati | Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices[END_REF] However, these authors observed in addition a second injection time constant in the ps-range, which is absent in our experiments. Instead, on the ps to ns timescale we observe an overall decay of the transient absorption signal, which is characterized by ground-state bleach below roughly 525 nm and a rather broad photoinduced absorption band peaking at 555 nm and occurs multiexponentially. Such multiexponential decay, reflecting the (partial) charge recombination, is typical for dye-sensitized semiconductors and can be associated with (a) charge-recombination from holes of different natures, i.e. localized in the VB or trapped in surface and/or in intra-band states,[START_REF] D'amario | Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells[END_REF][START_REF] Shan | Generation of Long-Lived Redox Equivalents in Self-Assembled Bilayer Structures on Metal Oxide 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 | Unveiling hole trapping and surface dynamics of NiO nanoparticles[END_REF] (b) different molecular conformations of the reduced photosensitizer (c) different localizations of the radical anion. The

  Co(I) reduction at the electrode. Photocurrent generation was observed with an onset potential of +0.4 V vs NHE.

Figure 6 .

 6 Figure 6. (A) Linear sweep voltammograms of a T1-sensitized NiO electrode recorded in acetate buffer (0.1 M, pH 4.5) in the presence of [Co(dmgBF2)2(H2O)2] (1 mM), in the dark (dotted line) or under visible-light irradiation (solid line). (B) Chopped-light chronoamperometric measurements recorded in acetate buffer (0.1 M, pH 4.5) in the presence of [Co(dmgBF2)2(H2O)2] (1 mM) at an applied potential of 0.05 V vs RHE for NiO electrodes sensitized by T1 alone (blue line) or in the presence of CDCA as co-absorbent (green line).

ACKNOWLEDGMENT

This work was supported by the French National Research Agency in the framework of the "Investissements d'avenir" program (Labex program ARCANE -ANR-11-LABX-0003-01, Graduate school of Chemistry, Biology and Health of Univ. Grenoble Alpes-CBH-EUR-GS, ANR-17-EURE-0003 and ANR-15-IDEX-02), the European Research Council under the European Union's Seventh Framework Program FP/2007-2013 (ERC Grant Agreement n.306398 and COST Action CM1202 PERSPECT-H2O), Studienstiftung des Deutschen Volkes (M.B.). The project these results are based upon was supported by the Free State of Thuringia under the number 2015 FGI 0008 and co-financed by European Union funds under the European Regional Development Fund (EFRE).

AUTHOR INFORMATION

Corresponding Authors

* Ana B. Mun͂ oz-Garcia; E-mail: anabelen.munozgarcia@unina.it * Benjamin Dietzek; E-mail: benjamin.dietzek@leibniz-ipht.de * Vincent Artero; E-mail: vincent.artero@cea.fr

Author Contributions

Notes

The authors declare no competing financial interest.