Metal-to-ligand charge-transfer excited-states in binuclear copper(I) complexes. Tuning MLCT excited-states through structural modification of bridging ligands. A resonance Raman study

The resonance Raman spectra of the ground- and lowest excited-states for a series of binuclear copper(I) complexes with bridging ligands based on 2,3- di-(2-pyridyl)quinoxaline have been measured. Analyses of the ground-state resonance Raman spectra show strong enhancement of an inter-ring stretching mode and quinoxaline-based modes; consistent with the Frank-Condon state h aving bonding changes about the copper(I) centre and on the quinoxaline rin g system. The electronic absorption data also suggest the initially formed MLCT-state is localised towards the quinoxaline ring system. The excited-st ate resonance Raman spectroscopy shows similar spectral features for comple xes which have substituent changes at the pyridyl rings of the bridging lig and. They show spectra associated with the radical anion of the bridging li gand. This is consistent with the formation of an excited-state which is me tal-to-ligand charge-transfer in nature and where the radical anion is loca lised on the quinoxaline ring system of the bridging ligand.