Résumé : The excited state of the 1,4,5,8-tetra-azaphenanthrene complex, [Ru(TAP)3]2+, which unlike [Ru(phen)3] 2+ (phen = 1,10-phenanthroline) or [Ru(bipy)3] 2+ (bipy = 2,2′-bipyridyl) is strongly quenched upon binding to poly[d(G-C)], is found to be much more effective than either [Ru(phen) 3]2+ or [Ru(bipy)3]2+ in causing cleavage of the DNA backbone.