Ruthenium(II) Terpyridyl Complexes Exhibiting DNA Photocleavage: The Role of the Substituent on Monodentate Ligand

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-substituted pyridine; R = N(CH3)2, NH2, OCH3, H, NO2), were synthesized; and the substituent effects on 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 NO2 to N(CH3)2 leads to a cathodic shift of Ru-based oxidation potential, a red shift of the 1MLCT absorption at room temperature and the 3MLCT 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 1O2 production. Time-resolved absorption spectra suggest that the 3MLCT lifetime plays an important role in the photosensitized 1O2 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.