Five ruthenium(II) complexes, [Ru(II)(tpy)(dppz)(py-R)](2+) (tpy = 2,2':6',2 ''-terpyridine; dppz = dipyrido[3,2-a:2',3'-c]phenazine; py-R = 4-substituied pyridine; R = N(CH3)(2), NH2, OCH3, H, NO2), were synthesized; and the substituent effects oil the photophysical property, electrochemical property, DNA binding, and DNA photocleavage of the complexes were examined carefully. Increasing the electron-donating ability of the substituent R from NO to N(CH3)(2) leads to a cathodic shift of Ru-based oxidation potential, a red shift of the (MLCT)-M-1 absorption at room temperature and the (MLCT)-M-3 emission at 77 K, and enhancement of the DNA photocleavage ability. DNA photocleavage control experiments and the EPR spin-trapping technique confirm that the photocleavage abilities of the complexes originate from O-1(2) production. Time-resolved absorption Spectra Suggest that the (MLCT)-M-3 lifetime plays ail important role in the photosensitized O-1(2) generation of these complexes, which in turn depends strongly on the electron-donating ability of the substituent R. By changing the substituent of pyridine from the electron-withdrawing to the electron-donating group, the photocleavage abilities of the complexes varied from inactive to active, providing a new strategy for the development of DNA photocleavers of tpy-based Ru(II) complexes.