Résumé : Nanosecond transient resonance Raman and picosecond transient absorption spectroscopic investigations of the two structurally analogous Ru-polypyridyl complexes, [Ru(phen)2dppz]2+ (2),are presented (phen = 1,10-phenanthroline, dppz = dipyrido [3,2-a:2′,3′-c] phenazine; tap = 1,4,5,8 tetraazaphenanthrene). The findings offer insight into the differing nature of the lowest excited states of the two complexes, and describe the role of these states within the very distinct photophysical behaviour of each, both in relation to solvent response and their interaction with DNA (facilitated in each case through the intercalating dppz ligand). The active, solvent-sensitive, dppz-based 3MLCT states involved in the 'light-switch' behaviour of (1) are probed, alongside evidence of a progression through a precursor transient state when the complex is in non-aqueous environment. Evidence has been provided of a photophysical pathway for (2), involving formation of a tap-based lowest 3MLCT state. When (2) is bound to DNA through the dppz ligand, a photo-driven electron transfer process ensues between the guanine base of DNA and the lowest 3MLCT state. © 2001 Elsevier Science B.V. All rights reserved.