Working the other way around: photocatalytic water oxidation triggered by reductive quenching of the photoexcited chromophore

A detailed photophysical investigation of the photocatalytic water oxidation system based on the tetraruthenium polyoxometalate [Ru₄(μ-O)₄(μ−OH)₂(H₂O)₄(γ-SiW₁₀O₃₆)₂]¹⁰⁻ (1) as the catalyst, the tetranuclear ruthenium dendrimer [Ru{(μ-2,3-dpp)Ru(bpy)₂}₃]⁸⁺ (2) as the light-harvesting photosensitizer, and persulfate (S₂O₈²⁻) as the sacrificial electron acceptor has shown that the water oxidation mechanism proceeds through an unusual, “anti-biomimetic” pathway: The first photochemical event is indeed a reductive quenching of the excited photosensitizer by the catalyst, followed by electron scavenging by the sacrificial electron acceptor, both occurring on the picosecond time scale within ion-paired species. As an unprecedented photoreaction mechanism for molecular water oxidation systems, these results suggest a new way to combine photosensitizers and catalysts, taking advantage of suitable chemical interactions and alternative photoinduced processes for the construction of efficient water-splitting devices.