Electronic Properties of [Core plus exo]-type Gold Clusters: Factors Affecting the Unique Optical Transitions

Unusual visible absorption properties of [core+exo]-type Au-6 (1), Au-8 (2), and Au-11 (3) clusters were studied from experimental and theoretical aspects, based on previously determined crystal structures. Unlike conventional core-only clusters having no exo gold atoms, these nonspherical clusters all showed an isolated visible absorption band in solution. Density functional theory (DFT) studies on corresponding nonphenyl models (1'-3') revealed that they had similar electronic structures with discrete highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) bands. The theoretical spectra generated by time-dependent DFT (TD-DFT) calculations agreed well with the experimentally measured properties of 1-3, allowing assignment of the characteristic visible bands to HOMO-LUMO transitions. The calculated HOMO-LUMO transition energies increased in the order Au-11 < Au-6 < Au-g, as was found experimentally. Frontier orbital analyses indicated that the HOMO and LUMO were both found in proximity to the terminal Au-3 triangles containing the exo gold atom, with the HOMO -> LUMO transition occurring in the core -> exo direction. The HOMO/LUMO distribution patterns of 1' and 3' were similar to each other but were markedly different from that of 2', which has longer core-to-exo distances. These findings showed that not only nuclearity (size) but also geometric structures have profound effects on electronic properties and optical transitions of the [core+exo]-type clusters.