Time-resolved spectroscopic studies of the influence of the electronic environment on the charge-transfer excited states of mono- and di-nuclear Ru(II) complexes

Resonance Raman (RR) spectroscopic studies, supplemented by excited state absorption techniques, were used to probe the effect of modifications to the electronic environment on the excited states of mono- and di-nuclear Ru(II) polypyridyl complexes containing triazole-based bridging ligands. Ground state RR spectroscopy showed that coordination of the sigma-donating bridging ligands to a second Ru(bpy)(2) moiety resulted in stabilization of the metal d(pi)-orbitals, whereas excited state RR spectroscopy revealed a concomitant stabilization of the pi*-level of the bridging ligand. With 3,5-bis(pyrazin-2-yl)-1,2,4-triazole) as bridging ligand (BL), the latter effect was sufficient to cause a switching from a bpy-based to a BL-based (MLCT)-M-3 state, with evidence suggesting charge polarisation towards a pyrazine fragment. Further, for pyrazine-containing bridging ligands, protonation of the triazole fragment resulted in a switching of the lowest excited state to a BL-based excited state. (C) 1998 Elsevier Science S.A.