Exploiting the disulfide/dithiol switch in photoinduced proton coupled electron transfer reactions

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The transformation of small molecules relevant for sustainable energy scenarios, specifically the generation of solar fuels, usually requires the transfer of multiple electrons and protons. Catalysts mediating these challenging transformations of mostly inert molecules thus have to orchestrate the complex multielectron, multiproton transfer to/from the substrate, viz. the proton-coupled electron transfer (PCET) events that are also widespread in biological systems. These processes may proceed sequentially via proton transfer followed by electron transfer (PT-ET), or vice versa (ET-PT), or the electron and proton may be transferred in a concerted fashion (CPET) to avoid any charged intermediates of potentially high energy. Because of their inherent nonequilibrium nature when occurring from photoexcited states, PCET scenarios driven by light are particularly complex and not well understood. However, understanding and controlling the dynamics of multiple light-triggered charge transfer processes will ultimately be crucial for improved sunlight to fuel conversion and for any desired energy storage in solar fuels. This fundamental research project targets the elucidation of the sequence of events during photoinduced PCET processes using novel photoexcitable metal complexes that are decorated with a disulfide/dithiol switch in the ligand periphery. The proposed chemistry capitalizes on the 2e−/2H+ nature of the disulfide/dithiol interconversion and on the potential inversion that often occurs after the first reduction of a disulfide moiety. Building upon the progress achieved during the first funding period, key questions of this project concern the choreography of the individual chemical steps following photoexcitation of well-designed molecular and supramolecular systems, including ET, PT and the rupture of the S-S bond. These questions are addressed by an interdisciplinary project team and in close collaborations with several research groups of the priority program, using a suite of experimental and computational methods. The conceptual guideline, and the ultimate goal, is the triggering of a second ET event, potentially coupled to PT, following the initial excited state PCET process. This will open a promising entry towards multielectron/multiproton chemistry initiated by single photoexcitation event.