Low Temperature Absorption and Luminescence of Oligothiophene Single Crystals

The optical properties of high temperature phase alpha-quaterthiophene and alpha-hexathiophene single crystals were investigated by low temperature photoacoustic and photoluminescence spectroscopy. The absorption and fluorescence spectra of both materials showed similar well-resolved lines, which are explained by vibronic progression of the 0-0 transition. Five vibronic modes were identified and ascribed to out of phase translation of adjacent monomer units, ring deformation, and ring breathing. The energy of the out of phase translation and the antisymmetric ring deformation was slightly reduced for the hexamer compared to the tetramer, whereas the other modes were independent of the conjugation length. Only a small stokes shift (18 cm sup (-1)) for alpha-quaterthiophene and 10 cm sup (-1) for alpha-hexathiophene) is observed in these high quality crystals and is ascribed to a shallow X-trap bellow the bottom of the excitonic band.