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 [Ru-4(mu-O)(4)(mu-OH)(2)(H2O)(4)(gamma-SiW10O36)(2)](10-) (1) as the catalyst, the tetranuclear ruthenium dendrimer [Ru{(mu-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.