WHY IS 1,6,6A-LAMBDA(4)-TRITHIAPENTALENE COLORED

Low-energy excited electronic states of the title compound are studied by quantum chemical configuration interaction (CI) calculations, using the semiempirical AM1 model and ab initio Hartree-Fock theory. Strong CI is predicted between excited electronic configurations of pi-pi* and sigma-sigma* character, which explains the failure of pi-electron calculations to reproduce the observed absorption spectrum. The computed transitions are compared with the measured linear dichroism (LD) absorption spectrum of TTP in stretched polyethylene. The long-axis polarized transition observed at 20 900 cm(-1), giving rise to the orange color of the compound, can be explained as the result of strong interaction between the HOMO-LUMO (pi-pi*) and the SHOMO-SLUMO (sigma-sigma*) configurations. The sigma and sigma* orbitals involved in the transition are associated with the unique, electron-rich three-center bonding of the sulfur atoms.