Raman spectroscopic studies on the structural changes of electrosynthesized polythiophene films during the heating and cooling processes

Abstract

Polythiophene (PT), poly (3-chlorothiophene) and poly (3-methylthiophene) have been electrochemically synthesized by direct oxidative polymerization of corresponding monomers at stainless steel electrode and in boron trifluoride diethyl etherate solution. The as-grown PT films have been peeled off from the electrode surfaces and characterized by resonance Raman spectroscopy in the temperature scale of 78–573 K. During the heating process, the Raman bands related to the oxidized species decreased gradually, mainly due to the effects of oxygen and moisture in air. Finally, the polymers changed into the neutral states and then decomposed. During the cooling process, the polymer chain segments of the neutral species changed from a disordered (coil-like) to an ordered (rod-like) structure and caused the elongation of conjugated chain length. This led to the dramatic decrease of the Raman intensities of the bands related to the neutral species. On the other hand, the Raman intensities of the bands associated with the oxidized species are nearly independent of temperature. This is mainly because the oxidized species have quinoidic structures and their conformation does not change during the cooling process.