Murielle Chavarot-Kerlidou

Light‐driven electron transfer and subsequent multielectron storage is among the key aspects of photochemical reactions in artificial photosynthesis and molecular electronics. Following our previously introduced design and characterization of Ru(II)‐based photosensitizers, four new Ru complexes with π‐extended ligands featuring a flavin‐inspired subunit were investigated via density functional theory in order to evaluate their electrochemical properties ahead of a time and resource‐demanding synthesis. Two complexes, Ru‐Me ₂ allox ^B and Ru‐Me ₂ deazaallox ^B , with a bent ligand architecture, were identified as promising candidates for application in light‐driven charge accumulation and subsequently synthesized. The electrochemical characterization of Ru‐Me ₂ allox ^B confirmed the theoretical predictions and its photophysical properties were investigated using UV/Vis absorption, resonance Raman, time‐resolved emission, and time‐resolved absorption spectroscopy in combination with quantum chemical simulations. Furthermore, first insights into the electronic distribution in the singly reduced complex were modelled computationally and obtained by EPR and UV/Vis absorption spectroscopy and spectroelectrochemistry. These results underline the promising multielectron storage capacity of the newly designed π‐extended alloxazine ligand.

This study reports the synthesis, electrochemical characterization, and photophysical properties of two novel photoactive polyoxometalate (POM)–Ir photosensitizer hybrid assemblies. Under visible light irradiation in the presence of triethylamine and trifluoroacetic acid, the polyoxomolybdate-based dyad photoaccumulates the electrons via an efficient intramolecular photoinduced electron transfer process.

Molecular charge accumulating systems that act as both, photosensitizer and electron storage unit, are of interest in the context of multielectron redox processes, e. g. in solar fuel production. To this end, the photophysical properties of RuL1, a ruthenium tris‐diimine complex with an alloxazine‐based ligand as bioinspired structural motif, were investigated. The study includes absorption, emission, resonance Raman and transient absorption spectroscopy in combination with quantum chemical simulations to determine the light‐driven reactivity of the complex. Moreover, spectroelectrochemistry was employed for an in‐depth characterization of the optical properties of the reduced complex. Finally, a photolysis experiment using triethanolamine as electron source, in conjunction with redox titrations, demonstrated that visible light irradiation triggers the formation of the doubly‐reduced singly‐protonated derivative of RuL1, where both redox equivalents are stored on the alloxazine‐based ligand.

Systems integrating quantum dots with molecular catalysts are attracting ever more attention, primarily owing to their tunability and notable photocatalytic activity in the context of the hydrogen evolution reaction (HER) and CO₂ reduction reaction (CO₂RR). CuInS₂ (CIS) quantum dots (QDs) are effective photoreductants, having relatively high-energy conduction bands, but their electronic structure and defect states often lead to poor performance, prompting many researchers to employ them with a core-shell structure. Molecular cobalt HER catalysts, on the other hand, often suffer from poor stability. Here, we have combined CIS QDs, surface-passivated with l-cysteine and iodide from a water-based synthesis, with two tetraazamacrocyclic cobalt complexes to realize systems which demonstrate high turnover numbers for the HER (up to >8000 per catalyst), using ascorbate as the sacrificial electron donor at pH = 4.5. Photoluminescence intensity and lifetime quenching data indicated a large degree of binding of the catalysts to the QDs, even with only ca. 1 μM each of QDs and catalysts, linked to an entirely static quenching mechanism. The data was fitted with a Poissonian distribution of catalyst molecules over the QDs, from which the concentration of QDs could be evaluated. No important difference in either quenching or photocatalysis was observed between catalysts with and without the carboxylate as a potential anchoring group. Femtosecond transient absorption spectroscopy confirmed ultrafast interfacial electron transfer from the QDs and the formation of the singly reduced catalyst (Co^II state) for both complexes, with an average electron transfer rate constant of <k_ET> ≈ (10 ps)^-1. These favorable results confirm that the core tetraazamacrocyclic cobalt complex is remarkably stable under photocatalytic conditions and that CIS QDs without inorganic shell structures for passivation can act as effective photosensitizers, while their smaller size makes them suitable for application in the sensitization of, inter alia, mesoporous electrodes.

Herein, the synthesis and optoelectronic properties of a novel push–pull organic dye, pRK1, specifically designed for use in dye‐sensitized photocathodes for hydrogen generation, are reported. The chemical structure of this dye, which incorporates a benzothiadiazole moiety, is inspired by RK1, a dye previously reported as a photosensitizer in n‐type dye‐sensitized solar cells (DSSCs) with power conversion efficiencies above 10% and high stability. The photoelectrochemical activity for hydrogen evolution of pRK1 after grafting onto NiO photocathodes in combination with a cobalt tetrapyridyl catalyst [Co(bapbpy)(OH₂)₂](BF₄)₂ in aqueous solution is evaluated and compared with two reference dyes from the literature, RuP₂‐bpy and P1. It is shown that among the three photocathodes studied in this work, NiO|pRK1 is the most efficient, producing up to 1.9 μmol cm⁻² of hydrogen with a faradaic efficiency of 66%, under visible light irradiation in aqueous electrolyte at pH 4.5. pRK1 shows a turnover number (TON_dye) of up to 145 during the 6 h chronoamperometric test, almost twice that of P1. This study demonstrates that the chemical structure of high performance dyes commonly used in DSSCs can be successfully modified to meet the requirements for light‐driven water splitting in dye‐sensitized photoelectrochemical cells.

A molecular dyad featuring a benzothiadiazole-based dye and a cobalt diimine–dioxime catalyst is developed for hydrogen production from water.

The design of photoactive systems capable of storing and relaying multiple electrons is highly demanded in the field of artificial photosynthesis, where transformations of interest rely on multielectronic redox processes. The photophysical properties of the ruthenium photosensitizer [(bpy)₂Ru(oxim‐dppqp)]²⁺ (Ru), storing two electrons coupled to two protons on the π‐extended oxim‐dppqp ligand under light‐driven conditions, are investigated by means of excitation wavelength‐dependent resonance Raman and transient absorption spectroscopies, in combination with time‐dependent density functional theory; the results are discussed in comparison to the parent [(bpy)₂Ru(dppz)]²⁺ and [(bpy)₂Ru(oxo‐dppqp)]²⁺ complexes. In addition, this study provides in‐depth insights on the impact of protonation or of accumulation of multiple reducing equivalents on the reactive excited states.

The mechanism of H₂ evolution catalyzed by the [Co(N₄H)Cl₂]⁺ complex is elucidated with quantitative determination of the rate-constants for the various protonation steps, and the identification of the aliphatic amine to act as a proton relay.