The lowest energy singlet state of tetrathiophene, an oligomer of polythiophene

Fluorescence and fluorescence excitation spectra have been measured for solid solutions of tetrathiophene in tetradecane at 12 and 4.2 K. At 4.2 K, the spectra exhibit full vibrational resolution {inhomogeneous origin [full width at half-maximum, (FWHM)] approximately 5 cm−1 }. Narrow band excitation and detection establish that there are four independent, but nearly identical excitation/emission pairs with dipole allowed origins at 22 248, 22 241, 22 214, and 22 187 cm−1. The data are consistent with the idea that these multiple spectra come from a single tetrathiophene confomer which can occupy four different sites in the tetradecane lattice. The vibrational frequencies of the modes in the ground (excited) state that couple strongly to the electronic excitation are 162, 327, 703, 1478, and 1531 cm−1 (161, 333, 688, 1235, and 1551 cm−1 ). The overlap of the excitation and emission origins and vibrational development of the spectra establish that the S0 to S1 transition is symmetry allowed (1 1Ag to 1 1Bu for C2h all-trans tetrathiophene) with the excited 1 1Bu state described reasonably as being derived from the ground state by the promotion of one electron from the highest energy occupied molecular orbital to the lowest energy unoccupied molecular orbital (HOMO to LUMO).