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 lcub;inhomogeneous origin lsqb;full width at halfhyphen;maximum, (FWHM)rsqb; approximately 5 cmminus;1rcub;. Narrow band excitation and detection establish that there are four independent, but nearly identical excitation/emission pairs with dipole allowed origins at 22thinsp;248, 22thinsp;241, 22thinsp;214, and 22thinsp;187 cmminus;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 cmminus;1(161, 333, 688, 1235, and 1551 cmminus;1). The overlap of the excitation and emission origins and vibrational development of the spectra establish that theS0toS1transition is symmetry allowed (1thinsp;1Agto 1thinsp;1BuforC2hallhyphen;transtetrathiophene) with the excited 1thinsp;1Bustate 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).
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