X-ray diffraction and ultvaviolet-visible absorption studies on electrochemically synthesized poly(thiophene-3-methanol) films

To investigate the effect of the CH2OH substituent on the energy gap of polythiophene, and to study the possible long-range order in this polymer, ex situ optical absorption and X-ray diffraction measurements were performed on electrosynthesized polythiophene-3-methanol (PTOH) films. In addition, the evolution of the optical absorption of PTOH as a function of applied potential was followed in situ. The diffraction pattern of PTOH is similar to that of liquid thiophene and bithiophene with only one broad peak at Q=1.4 Angstrom(-1), indicating that despite the two-dimensional deposition process of PTOH, its structure is exceptionally disordered. The optical absorption spectrum of PTOH shows maximum absorption at 3.05 eV, which corresponds to a conjugation length of only 5-6 monomer units. Nevertheless, the evolution of the optical spectrum upon oxidation is characterized by similar features as observed for polythiophenes of higher degree of conjugation, namely by a decrease in the intensity of the absorption due to valence to conduction band transition and a simultaneous appearance of a new broad absorption band at 1.65 eV. However, only low doping levels are obtained for PTOH. These results are discussed on the basis of the C-O-C coupling in PTOH.