Luminescence processes of hexanuclear methylpyridinethiolato-copper (I) crystals

Three kinds of luminescent crystals, copper(I) cluster compounds with different ligand configurations (hexanuclear methylpyridinethiolato-copper(I): [Cu6(nL-mpyt)6]; nL being the ligand configuration), have been investigated by observing their photoluminescence spectra. In the three samples, two luminescence bands appear predominantly: one (CC-band), appearing in the lower-energy region around 1.8 eV, has been assigned to the transition between intra-molecular orbitals (MO) of a Cu cluster center(CC), and the other, (the CT-band), appearing at higher energy than the CC-band, to a charge transfer (CT) transition from the CC–MO to a ligand MO. The CC-band has common characteristics in the three samples. On the other hand, the difference in the ligand structures gives the differences in the CT-band peak energy and bandwidth, and of the Stokes shift. Such differences reflect the influence of the bonding nature between the CC and the ligands, and of the mass of the ligands, on the CT luminescence processes. These properties arising from dynamical processes in these materials have been discussed in terms of a configuration-coordinate model.