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

Abstract

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 having bonding changes about the copper(I) centre and on the quinoxaline ring system. The electronic absorption data also suggest the initially formed MLCT-state is localised towards the quinoxaline ring system. The excited-state resonance Raman spectroscopy shows similar spectral features for complexes which have substituent changes at the pyridyl rings of the bridging ligand. They show spectra associated with the radical anion of the bridging ligand. This is consistent with the formation of an excited-state which is metal-to-ligand charge-transfer in nature and where the radical anion is localised on the quinoxaline ring system of the bridging ligand.