Photoisomeric transformations in ruthenium polypyridyl complexes have been rarely reported. Herein we report the geometrical transformation of cyclometalated trans-[Ru(tpy)(PAD)(OH2)](+) ([1](+)) to the cis-[Ru(tpy)(PAD)(OH2)](+) ([1a](+)) (tpy = 2,2';6',2 ''-terpyridine, PAD = 2-(pyrid-2'-yl)acridine) isomer upon irradiation of visible light (lambda >= 420 nm). Due to a proton-induced tautomeric equilibrium between the Ru-C bond and Ru=C coordination, the pi* 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.