[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 in 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 3σ(Ru-O)p*(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) in hypoxic conditions and are therefore promising as a new class of antitumor drugs. 

Dalton Trans., 2016