Effects of chain length and Au spin-orbit coupling on 3(ππ*) emission from bridging Cn2- units:: Theoretical characterization of spin-forbidden radiative transitions in metal-capped one-dimensional carbon chains [H3PAu(CC)nAuPH3]

Density functional theory and CASSCF calculations have been used to optimize the geometries of binuclear gold(I) complexes [H3PAu(CdropC)(n)AuPH3] (n=1-6) in their ground states and selected lowest energy (3)(pipi*) excited states. Vertical excitation energies obtained by time-dependent density functional calculations for the spin-forbidden singlet-triplet transitions have exponential-decay size dependence. The predicted singlet-triplet splitting limit of [H3PAu(CdropC)(proportional to) AuPH3] is about 8317 cm(-1). Calculated singlet-triplet transition energies are in reasonable agreement with available experimental observations. The effect of the heavy atom Au spin-orbit coupling on the (3)(pipi*) emission of these metal-capped one-dimensional carbon allotropes has been investigated by MRCI calculations. The contribution of the spin- and dipole-allowed singlet excited state to the spin-orbit-coupling wave function of the (3)(pipi*) excited state makes the low-lying acetylenic triplet excited states become sufficiently allowed so as to appear in both electronic absorption and emission.