Intramolecular Electron Transfer Governs Photoinduced Hydrogen Evolution by Nickel-Substituted Rubredoxin: Resolving Elementary Steps in Solar Fuel Generation

The field of solar fuels is a rapidly growing area of research, though low overall efficiencies continue to preclude large-scale implementation. To resolve the elementary processes involved in light-driven energy storage and identify key factors contributing to efficiency losses, systematic investigation and optimization are necessary. In this work, a ruthenium chromophore is directly attached to a model hydrogenase enzyme, nickel-substituted rubredoxin, to construct a molecular system capable of photoinduced hydrogen evolution. Time-resolved absorption and emission spectroscopy reveal direct, rapid intramolecular electron transfer (ET) between the two metal centers to generate a charge-separated state that persists for ∼1 μs, though this species is not productive for hydrogen evolution. Investigation of the photochemical behavior under catalytic conditions in conjunction with thermochemical analyses suggests that ET to the catalytic nickel site from the reductively quenched ruthenium center is the rate-d...