Rapid energy transfer in bichromophoric tris-bipyridyl/cyclometallated ruthenium(ii) complexes

The synthesis, characterization, and photophysical properties of the N ₆-N₅C bichromophoric [(bpy) ₂Ru(I)Ru(ttpy)][PF₆]₃ (bpy is 2,2′-bipyridine and ttpy is 4′-p-tolyl-2,2′:6′,2″-terpyridine) and [(bpy) ₂Ru(II)Ru(ttpy)][PF₆]₃ (I and II are bpy-dipyridylbenzene ditopic ligands bridged by an ethynyl and phenyl unit, respectively) complexes are reported together with the model mononuclear complexes [(bpy)₂Ru(I)][PF₆]₂, [(bpy) ₂Ru(II)][PF₆]₂, [Ru(VI)(ttpy)][PF₆] (VI is 3,5-di(2-pyridyl)-biphenyl) and [Ru(dpb)(ttpy)][PF₆] (Hdpb is 1,3-di(2-pyridyl)-benzene). The electrochemical data show that there is little ground state electronic communication between the metal centers in the bimetallic complexes. Selective excitation of the N₅C unit in the bichromophoric systems leads to luminescence typical for a bis-tridentate cyclometallated ruthenium complex and is similar to the [Ru(VI)(ttpy)][PF ₆] model complex. In contrast, the luminescence from the tris-bidentate N₆ unit is efficiently quenched by energy transfer to the N₅C unit. The energy transfer rate has been determined by femtosecond pump-probe measurements to 0.7 ps in the ethynyl-linked [(bpy) ₂Ru(I)Ru(ttpy)][PF₆]₃ and to 1.5 ps in the phenyl-linked [(bpy)₂Ru(II)Ru(ttpy)][PF₆]₃ (in acetonitrile solution at 298 K), and is inferred to occur via a Dexter mechanism.