A new type of surface exciton has been observed on large grown single-crystal surfaces of the bc plane in bithiophene, quarterthiophene, and quinquethiophene and of the ab plane in terthiophene, by reflection measurements. These surfaces are well-grown flat surfaces. The surface exciton absorption intensities decrease with time and are faster in the order of bithiophene, terthiophene, quarterthiophene, and quinquethiophene. This is the same order as their sublimation rates. The present new type of surface exciton is not confined in a surface layer potential like other ones reported before. The energy is lowered by twisting the molecular form between cis and trans forms. The surface exciton absorption band was not observed on the other side walls of the thick crystals, which are the ab and the ac planes in bithiophene, quarterthiophene, and quinquethiophene single crystals (SCs) and the bc and ac planes in a terthiophene SC. Vibronic exciton absorption bands were observed directly by the reflection measurements on these side walls.

Polarization dependence of the absorption spectra and the luminescence spectra in a-bithiophene and a -terthiophene has been measured directly with near normal incident light on the ab, bc, and ca crystal planes of their thick single crystals. The absorption spectra were obtained from the reflection spectra using the Kramers-Kronig transformation. Vibronic excitons (Frenkel excitons) have been observed on each crystal plane of a-bithiophene and a-terthiophene. The polarizations of the absorption spectra are consistent with the prediction obtained from the MOPAC calculations of the transition dipole moment directions of the molecules in the single crystals.