Synergistic Two-Electron Transfer Enables Minute-Scale Redox Accumulation under Low-Light Conditions

The conversion of sunlight into chemical fuels offers a promising route for renewable energy storage. A key challenge is the accumulation of multiple redox equivalents under low solar irradiance, as fuel-forming reactions require more than one redox equivalent. From a mechanistic viewpoint, this poses two major challenges: (1) the first redox equivalent must persist long enough for a second photon to be absorbed and (2) the second photon must not induce charge recombination. We address both challenges using a new molecular design concept with a covalently linked triad comprising a ruthenium-based photosensitizer covalently linked to a two-electron acceptor and a terminal electron relay. Upon light excitation in the presence of ascorbate, the photosensitizer donates an electron to the terminal relay, storing it on the millisecond time scale. Re-excitation of the photosensitizer followed by its reduction by ascorbate enables an overall two-electron transfer from the reduced photosensitizer and the reduced terminal relay to the central acceptor. This acceptor allows reversible accumulation of two reduction equivalents via disulfide bond cleavage and protonation on a minute time scale. Unlike conventional designs with linear redox gradients, this system uses a peripheral relay to suppress recombination and support the accumulation of redox equivalents under sunlight-level irradiances. These findings offer a new strategy for solar-driven multielectron chemistry and advance molecular approaches to artificial photosynthesis.