Optical properties of transition metal complexes with N-heterocyclic carbenes as ligands. 1,3-di-t-Butylimidazol-2-ylidene as charge transfer donor and acceptor

The N-heterocyclic carbene 1,3-di-t-butylimidazol-2-ylidene (dbiy) can function as charge transfer (CT) donor and acceptor if it is coordinated to oxidizing and reducing metals, respectively. The complex TiIVCl4(dbiy)2 shows a ligand-to-metal CT absorption at 450 nm whereas Ni0(CO)2(dbiy)2 is characterized by a metal-to-ligand charge transfer absorption at λmax=365 nm. While TiIVCl4(dbiy)2 is not luminescent, Ni0(CO)2(dbiy)2 displays a room temperature emission at λmax=510 nm which is assumed to originate from the lowest-energy MLCT triplet.