Substituent effect and wavelength dependence of the photoinduced Ru-O homolysis in the [Ru(bpy)2(py-SO3)]+-type complexes

[Ru(bpy)(2)(py-SO3)](+) (3, bpy = 2,2'-bipyridine, py-SO3 = pyridine-2-sulfonate) was recently found to undergo py-SO3 ligand dissociation and py-SO3+ radical generation under hypoxic conditions upon irradiation (Chem. Commun., 2015, 51, 428). To explore the substituent effect on the Ru-O homolysis by which the py-SO3+ radical may be produced, [Ru(4,4'-(R)(2)-bpy)(py-SO3)](+), where R = OCH3 (1), CH3 (2), COOCH3 (4), were synthesized and their photochemical properties were investigated. The py-SO3+ radical generation efficiencies followed the order of 4 > 3 > 2 > 1, and the radical generation efficiencies are wavelength dependent. As a result, 3 and 4 may lead to DNA covalent binding and DNA cleavage upon 355 nm irradiation, but merely DNA covalent binding upon 470 nm irradiation. In contrast, 1 and 2 can serve as DNA photo-binding agents only due to their less efficient Ru-O homolysis. The Ru-O homolysis via the (3)sigma(Ru-O)pi(star)(R-bpy) state is proposed to rationalize the substituent effect and the wavelength dependence, which is supported by TD-DFT calculations. This work gave insights into the mechanism of the Ru-O homolysis and provided guidelines for developing new [Ru(bpy)(2)(py-SO3)](+)-type complexes with higher Ru-O homolysis efficiency. Such complexes have dual activities of photoactivated chemotherapy (PACT) and photodynamic therapy (PDT) under hypoxic conditions and are therefore promising as a new class of antitumor drugs.