Resonance Raman and spectroelectrochemical investigation of the location of the lowest excited state in mono- and dinuclear ruthenium(II) complexes containing pyrazine moieties

The excited-state properties of mononuclear and dinuclear Ru(bpy)(2) complexes with bpzt(-) bridging ligand (bpy = 2,2'-bipyridine, and bpzt(-) = 3,5-bis(pyrazin-2-yl)-1,2,4-triazole) have been probed by resonance Raman (RR), excited-state absorption (ESA), and spectroelectrochemical techniques. Excited-state RR spectroscopy provides evidence of a lowering of the pi*-level of the bridging ligand upon addition of a second Ru(bpy)(2) unit to the mononuclear complex, resulting in switching from a bpy-based to a bpzt-based lowest excited state, within which the data suggest there is charge polarization toward the more readily reduced pyrazine moiety. Further illustration of the sensitivity of these states to the electronic environment has also been observed in the excited-state RR spectra of the protonated mononuclear complex, where the pi*-accepting-level of bpzt is again lowered below that of bpy.