PHOTOPHYSICS OF POLYAZAAROMATIC RUTHENIUM(II) COMPLEXES INTERACTING WITH DNA

The effects of the interaction of polyazaaromatic Ru(II) complexes wit h DNA on their photophysics are examined by analyzing the dependence o f their luminescence lifetimes with temperature. Five complexes whose excited-state lifetimes in the absence of DNA are controlled by deacti vation rate constants of different relative magnitudes have been chose n: (i) Ru(bpy/phen)(2)(hat)(2+)(bpy = 2,2'-bipyridine, phen = 1,10-phe nanthroline, hat = 1,4,5,8,9,12-hexaazatriphenylene) classified in thi s paper as first category complexes whose (3)MLCT (metal-to-ligand cha rge transfer) excited-state lifetimes are mainly controlled by nonradi ative 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 lumines cence decays are mainly controlled by a thermally activated crossing f rom the (3)MLCT state to the (3)MC (metal centered) state, and (iii) R u(phen)(3)(2)+, a complex of the so-called second category, which exhi bits an excited-state behavior intermediate between the two other case s and has been chosen because of its numerous studies with DNA. The da ta presented in this paper provide evidence that DNA affects both k(nr ) and the crossing to the (3)MC. This is unambiguously shown with thos e complexes of the first category for which DNA decreases k(nr) as wel l as with the complexes of the third category for which the polynucleo tide diminishes clearly the crossing to the (3)MC. Ru(phen)(3)(2+) ill ustrates 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 lumi nescence lifetime of Ru(phen)(3)(2)+ upon binding to DNA is accentuate d by a reduced oxygen quenching on the double helix. This is not the c ase for the tap and hat complexes, the luminescence of which is practi cally O-2 insensitive.