Marked differences in light-switch behavior of Ru(II) complexes possessing a tridentate DNA intercalating ligand.

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) and heteroleptic Ru(tpy)(pydppz) (tpy = 2,2';6',2' '-terpyridine) complexes have been prepared and characterized by (1)H 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 (3)MLCT excited state of the heteroleptic complex Ru(tpy)(pydppz) (tau approximately 5 ns) is longer-lived than that of the homoleptic complex Ru(pydppz)(2) (tau = 2.4 ns) and that these lifetimes are significantly longer than that of the (3)MLCT state of the parent complex Ru(tpy)(2) (tau = 120 ps). These differences are explained by the lower-energy (3)MLCT excited state present in Ru(tpy)(pydppz) and Ru(pydppz)(2) compared to Ru(tpy)(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) and Ru(pydppz)(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) to intercalate both pydppz ligands, such that one pydppz always remains exposed to the solvent. DNA photocleavage is observed for Ru(tpy)(pydppz) in air, but not for Ru(pydppz)(2). The DNA damage likely proceeds through the production of small amounts of (1)O(2) by the longer-lived complex. Although both complexes possess the intercalating pydppz ligand, they exhibit different photophysical properties in the presence of DNA.