Lowest energy excited singlet state of 2,2′:5′,2′′-terthiophene, an oligomer of polythiophene

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Abstract
Fluorescence excitation and emission spectra have been measured for solid solutions of 2,2′:5′,2″-terthiophene in n-decane at 77, 10, and 4.2 K. At 4.2 K the spectra exhibit full vibrational resolution (origin inhomogeneous FWHM approximately 5 cm−1). At 4.2 K narrow band excitation and detection establish that there are four independent but nearly identical excitation/emission pairs with origins at 24 806, 24 827, 24 835, and 24 879 cm−1. Whether this multiplicity is caused by the presence of different isomers or comes from a single isomer that can occupy in the n-decane lattice in four different ways is not known with certainty. However, the similarity of the vibronic development and energetic considerations suggest that the spectra arise from a single isomer present in four rather similar n-decane sites or from four slightly different conformations of a single isomer. The overlap of excitation and emission origins and the vibrational development of the spectra establish that the S0 to S1 transition is symmetry allowed (probably reasonably described as the 1 1A1 to 1 1B2 transition where the excited state is derived from the ground state by the promotion of one electron from the HOMO to LUMO).