Role of MLCT States in the Franck-Condon Region of Neutral, Heteroleptic Cu(I)-4H-imidazolate Complexes: A Spectroscopic and Theoretical Study.

The impact of the electronic structure of a series of 4H-imidazolate ligands in neutral, heteroleptic Cu(I) complexes is investigated. Remarkable broad and strong ligand-dependent absorption in the visible range of the electromagnetic spectrum renders the studied complexes promising photosensitizers for photocatalytic applications. The electronic structure of the Cu(I) complexes and the localization of photoexcited states in the Franck-Condon region are unraveled by means of UV-vis absorption and resonance Raman (rR) spectroscopy supported by time-dependent density functional theory (TD-DFT) calculations. The visible absorption bands stem from a superposition of bright metal-to-ligand charge-transfer (MLCT) and π-π* as well as weakly absorbing MLCT states. Additionally, the analysis of involved molecular orbitals and rR spectra upon excitation of MLCT and π-π* states highlights the impact of the electronic structure of the 4H-imidazolate ligands on the properties of the corresponding Cu(I) complexes to avail a toolbox for predictive studies and efficient complex design.