Luminescent Cyclometalated N-heterocyclic Carbene-containing Organogold(III) Complexes: Synthesis, Characterization, Electrochemistry, and Photophysical Studies

A new class of luminescent mononuclear and dinuclear N-heterocyclic carbene-containing gold(III) complexes has been synthesized and characterized. The X-ray crystal structures of most of the complexes have also been determined. Electrochemical studies reveal a ligand-centered reduction originated from the RC--N--CR moieties with no oxidation waves. Interestingly, one of the dinuclear complexes exhibited two distinct reduction couples instead of one with the first reduction occurring at less cathodic potential, probably related to the splitting of the pi* orbital-based LUMO resulting from intramolecular pi-pi interaction. The electronic absorption and luminescence behaviors of the complexes have also been investigated. In dichloromethane solution at room temperature, the low-energy absorption bands are assigned as the intraligand pi-pi* transition, with mixing of a charge transfer character from the aryl ring to the pyridine moiety of the cyclometalating RC--N--CR ligand. The low-energy emission bands are ascribed to origins mainly derived from the intraligand pi-pi* states with an aryl to pyridine charge transfer character of the cyclometalating RC--N--CR ligand.