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, usin g the semiempirical AM1 model and ab initio Hartree-Fock theory. Stron g 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 comput ed transitions are compared with the measured linear dichroism (LD) ab sorption spectrum of TTP in stretched polyethylene. The long-axis pola rized transition observed at 20 900 cm(-1), giving rise to the orange color of the compound, can be explained as the result of strong intera ction between the HOMO-LUMO (pi-pi) and the SHOMO-SLUMO (sigma-sigma* ) configurations. The sigma and sigma orbitals involved in the transi tion are associated with the unique, electron-rich three-center bondin g of the sulfur atoms.