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Polarization dependence of the absorption spectra (ABS) and the luminescence spectra in a-quaterthiophene (4T) and a-quinquethiophene (5T) has been measured directly with near normal incident light on the ab, bc, and ca crystal planes of their thick single crystals (SCs). Vibronic excitons (Frenkel excitons) have been observed on each crystal plane of 4T and 5T. The polarizations of the ABS are consistent with the prediction obtained from the MOPAC calculations of the transition dipole moment directions of the molecules in the SCs. Excited electron relaxes to the lowest excited state which is composed of the LUMO states.

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