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 photoacoustic and photoluminescence spectroscopy at low temperatures. The absorption and fluo rescence spectra of both materials consist of a series of similar well-reso lved lines, which are explained by vibronic progression of the 0-0 transiti on. Five vibronic modes are identified and ascribed to the out of phase tra nslation of adjacent monomer units, ring deformation, and ring breathing mo des. The energies of the out of phase translation and the antisymmetric rin g deformation are slightly smaller for the hexamer than for the tetramer, w hereas the other modes were independent of the conjugation length. Only a s mall Stokes shift is observed in these high quality crystals (18 cm(-1) for alpha-quaterthiophene and 10 cm(-1) for alpha-hexathiophene) and is ascrib ed to a shallow defect state below the bottom of the excitonic band. (C) 19 99 American Institute of Physics. [S0021-8979(99)00205-4].