Photochemical and Redox‐Controlled ON–OFF Switching in Proton‐Coupled Electron Transfer

We report a reversible ON–OFF proton‐coupled electron transfer (PCET) switch based on ruthenium(II) polypyridyl complexes, triggered by both light and redox stimuli. Upon photoirradiation, a carboxylate‐ligated Ru(II) complex undergoes photosubstitution to generate a metastable, PCET‐active aqua complex. This metastable complex exhibits redox‐induced conformational change, regenerating the original carboxylate complex. The switching behavior enables external control of PCET activity, directly modulating electrocatalytic water oxidation. Electrochemical and spectroscopic analyses reveal that the mechanism is governed by a pH‐dependent interplay between intramolecular conformational change and bimolecular electron exchange. Importantly, the rate‐determining step shifts from unimolecular conformational change near neutral pH to bimolecular electron exchange under acidic conditions. These findings establish a strategy for designing tunable molecular switches and redox‐responsive functional materials.