Photoisomerization and proton-coupled electron transfer (PCET) promoted water oxidation by mononuclear cyclometalated ruthenium catalysts.

Photoisomeric transformations in ruthenium polypyridyl complexes have been rarely reported. Herein we report the geometrical transformation of cyclometalated trans-[Ru(tpy)(PAD)(OH(2))](+) ([1](+)) to the cis-[Ru(tpy)(PAD)(OH(2))](+) ([1a](+)) (tpy = 2,2';6',2"-terpyridine, PAD = 2-(pyrid-2'-yl)acridine) isomer upon irradiation of visible light (λ ≥420 nm). Due to a proton-induced tautomeric equilibrium between the Ru-C bond and Ru═C coordination, the π* energy levels of PADH are lower than those of tpy by 12.61 and 12.24 kcal mol(-1), respectively, in [1](+) and [1a](+). Isomers [1](+) and [1a](+) both act as catalytic oxygen-evolving complexes (OECs) chemically as well as electrochemically.