PHOTOPHYSICS OF POLYAZAAROMATIC RUTHENIUM(II) COMPLEXES INTERACTING WITH DNA

The effects of the interaction of polyazaaromatic Ru(II) complexes with DNA on their photophysics are examined by analyzing the dependence of their luminescence lifetimes with temperature. Five complexes whose excited-state lifetimes in the absence of DNA are controlled by deactivation rate constants of different relative magnitudes have been chosen: (i) Ru(bpy/phen)(2)(hat)(2+)(bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, hat = 1,4,5,8,9,12-hexaazatriphenylene) classified in this paper as first category complexes whose (3)MLCT (metal-to-ligand charge transfer) excited-state lifetimes are mainly controlled by nonradiative deactivations to the ground state (k(nr)), (ii) Ru(tap)(3)(2+) (tap = 1,4,5,8-tetraazaphenanthrene) and Ru(tap)(2)(hat)(2), complexes of the third category, according to this classification, whose luminescence decays are mainly controlled by a thermally activated crossing from the (3)MLCT state to the (3)MC (metal centered) state, and (iii) Ru(phen)(3)(2)+, a complex of the so-called second category, which exhibits an excited-state behavior intermediate between the two other cases and has been chosen because of its numerous studies with DNA. The data presented in this paper provide evidence that DNA affects both k(nr) and the crossing to the (3)MC. This is unambiguously shown with those complexes of the first category for which DNA decreases k(nr) as well as with the complexes of the third category for which the polynucleotide diminishes clearly the crossing to the (3)MC. Ru(phen)(3)(2+) illustrates a case where DNA influences both deactivation pathways with, however, less clear-cut effects as compared to those observed with the complexes of the two other categories. Moreover, the increase of luminescence lifetime of Ru(phen)(3)(2)+ upon binding to DNA is accentuated by a reduced oxygen quenching on the double helix. This is not the case for the tap and hat complexes, the luminescence of which is practically O-2 insensitive.