Ruthenium complexes capable of light-triggered cytotoxicity are appealing potential prodrugs for photodynamic therapy (PDT) and photoactivated chemotherapy (PACT). Two groups of Ru(II) polypyridyl complexes with 2-(2-pyridyl)-benzazole ligands were synthesized and investigated for their photochemical properties and anticancer activity to compare strained and unstrained systems that are likely to have different biological mechanisms of action. The structure-activity relationship was focused on the benzazole core bioisosterism and replacement of coligands in Ru(II) complexes. Strained compounds rapidly ejected the 2-(2-pyridyl)-benzazole ligand after light irradiation, and possessed strong toxicity in the HL-60 cell line both under dark and light conditions. In contrast, unstrained Ru(II) complexes were non-toxic in the absence of light, induced cytotoxicity at nanomolar concentrations after light irradiation, and are capable of light-induced DNA damage. The 90-220-fold difference in light and dark IC values provides a large potential therapeutic window to allow for selective targeting of cells by exposure to light.