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-determining step. By eliminating the need for three components to diffuse together, direct mechanistic information about catalysis is obtained in a time-resolved manner. This approach is generalizable to study the activity and intramolecular charge transfer properties of a wide range of photosensitizers and catalysts, with applicability toward diverse energy conversion reactions.