High-Redox-Potential Chromophores for Visible-Light-Driven Water Oxidation at Low pH

Photochemical water oxidation at low pH requires chromophores with adequate photophysical properties and high redox potentials capable of oxidizing the catalysts. We report here a series of homoleptic Ru(II) polypyridyl complexes and their performances in photochemical water oxidation at pH 1 using persulfate as the sacrificial electron acceptor. These chromophores incorporate CF3 or PO3H2 groups to increase the Ru-III/II standard potentials to 1.3-1.6 V versus NHE, while their homoleptic nature retains photophysical properties (absorption spectra, lifetimes, emission energies, emission quantum yields) comparable to [Ru(bpy)(3)](2+). With [(bda)Ru(isq)(2)] as the catalyst, the chromophores promote visible-light-driven water oxidation with high activity at low pH. The chromophores with phosphonate groups as substituents showed higher activity than expected based on driving force arguments. In addition, kinetic isotope effects as high as similar to 3 were measured, suggesting the involvement of concerted proton-coupled transfer pathways between the catalyst and the chromophores with the phosphonate group acting as the proton acceptor. These chromophores therefore carry out the light absorption, charge migration, and proton transfer roles of P-680, Tyr161, and His190 in Photosystem II on a single molecule and are good candidates for photoelectrochemical water oxidation at low pH in dye-sensitized photoelectrochemical cells.