Marked Differences in Light-Switch Behavior of Ru(II)Complexes Possessing a Tridentate DNA Intercalating Ligand

摘要

The tridentate ligand 3-(pyrid-2'-yl)dipyrido[3,2-a:2',3'-c]phenazine(pydppz)has been prepared in two steps by elaboration of 2-(pyrid-2'-yl)-1,10-phenanthroline.Both homoleptic [Ru(pydppz)2]~(2+)and heteroleptic [Ru(tpy)(pydppz)]~(2+)(tpy = 2,2';6',2"-terpyridine)complexes have been prepared and characterized by ~1H NMR.The absorption and emission spectra are consistent with low-lying MLCT excited states,which are typical of Ru(II)complexes.Femtosecond transient absorption measurements show that that the ~3MLCT excited state of the heteroleptic complex [Ru(tpy)(pydppz)]~(2+)(tau ≈ 5 ns)is longer-lived than that of the homoleptic complex [Ru(pydppz)2]~(2+)(r = 2.4 ns)and that these lifetimes are significantly longer than that of the ~3MLCT state of the parent complex [Ru(tpy)2]~(2+)(r = 120 ps).These differences are explained by the lower-energy ~3MLCT excited state present in [Ru(tpy)(pydppz)]~(2+)and [Ru(pydppz)2]~(2+)compared to [Ru(tpy)2]~(2+),resulting in less deactivation of the former through the ligand-field state(s).DFT and TDDFT calculations are consistent with this explanation.[Ru(tpy)(pydppz)]~(2+)and [Ru(pydppz)2]~(2+)bind to DNA through the intercalation of the pydppz ligand;however,only the heteroleptic complex exhibits luminescence enhancement in the presence of DNA.The difference in the photophysical behavior of the complexes is explained by the inability of [Ru(pydppz)2]~(2+)to intercalate both pydppz ligands,such that one pydppz always remains exposed to the solvent.DNA photocleavage is observed for [Ru(tpy)(pydppz)]~(2+)in air,but not for [Ru-(pydppz)2]~(2+).The DNA damage likely proceeds through the production of small amounts of ~1O_2 by the longer-lived complex.Although both complexes possess the intercalating pydppz ligand,they exhibit different photophysical properties in the presence of DNA.