Theoretical study on electronic excitation spectra of Mo and Re cluster complexes: [(Mo6Cl8)Cl6]2− and [(Re6S8)Cl6]4−

Abstract

We report here on the electronic structures of [(Mo6Cl8)Cl6]2− and [(Re6S8)Cl6]4− obtained by the Tamm–Dankoff configuration interaction approximation, where Darwin and mass-velocity terms are taken into account through the model core potentials, and the spin–orbit interaction term is included in a semiempirical manner. These complexes have absorption spectra ranging from the visible to the near-ultraviolet energy region and have the long-lived luminescence. The calculated results of the transition moments roughly reproduce the experimental absorption and emission spectra. Especially for the absorption spectra, the theoretical strong intensity for the region higher than 4.5 eV in the two complexes is in good agreement with the experimental results. The obtained intense peak at 3.9 eV may correspond to the experimental broad peak at approximately 3.6 eV for the Mo complex. For the Re complex, the calculated intensity distribution gives one broad peak at around 3.1 eV. This peak may correspond to the experimental peak at 2.9 eV. In the experimental emission spectra, the maxima are located at 1.62 eV for the Mo complex and approximately 1.8 eV for the Re complex. Theoretically, 12 and eight states are found in the energy range between 1.5 and 2.0 eV for the Mo and Re complexes, respectively. At least one, if not more, of these states would be responsible for each emission maximum. To determine the character of the above transitions, the absorption and emission transitions were experimentally shown to be ligand-to-metal charge-transfer transitions and metal-localized transitions, respectively. Theoretically, absorption transitions were characterized as a mixture of metal-localized transitions and ligand-to-metal charge-transfer transitions, and emission transitions a mixture of metal-localized transitions and metal-to-ligand charge-transfer transitions.