Computational analyses of singlet–singlet and singlet–triplet transitions in mononuclear gold‐capped carbon‐rich conjugated complexes

Density functional theory and CASSCF calculations have been used to determine equilibrium geometries and vibrational frequencies of metal‐capped one‐dimensional π‐conjugated complexes (H3P)Au(CC)n(Ph) (n = 1–6), (H3P)Au(CCC6H4)(CCPh), and H3PAu(CCC6H4)CCAuPH3 in their ground states and selected low‐lying ππ* excited states. Vertical excitation energies for spin‐allowed singlet–singlet and spin‐forbidden singlet–triplet transitions determined by the time‐dependent density functional theory show good agreement with available experimental observations. Calculations indicate that the lowest energy 3(ππ*) excited state is unlikely populated by the direct electronic excitation, while the low‐lying singlet and triplet states, slightly higher in energy than the lowest triplet state, are easily accessible by the excitation light used in experiments. A series of radiationless transitions among related excited states yield the lowest 3(ππ*) state, which has enough long lifetimes to exhibit its photochemical reactivities. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1214–1221, 2005