This study investigates the correlation between photocytotoxicity and the prolonged excited-state lifetimes exhibited by certain Ru(II) polypyridyl photosensitizers comprised of pi-expansive ligands. The eight metal complexes selected for this study differ markedly in their triplet state configurations and lifetimes. Human melanoma SKMEL28 and human leukemia HL60 cells were used as in vitro models to test photocytotoxicity induced by the compounds when activated by either broadband visible or monochromatic red light. The photocytotoxicities of the metal complexes investigated varied over 2 orders of magnitude and were positively correlated with their excited-state lifetimes. The complexes with the longest excited-state lifetimes, contributed by low-lying (IL)-I-3 states, were the most phototoxic toward cancer cells under all conditions.