Polythiophene puzzle - a plausible solution

Previous in situ spectroelectrochemical studies on the redox transformation of poly(3-methylthiophene) have been completed by further electrochemical, EQCM and in situ conductance measurements. In the case of thicker films, the generally observed single, wide anodic voltammetric wave could be separated into two oxidation steps, which are not consecutive, but complementary processes. The mass vs. potential curve from the EQCM measurements also reflects the double pattern of the electronic process. The virtual stoichiometric factor per electron of the anion during the first oxidation is lower than 0.5, and it is close to 1 in the second oxidation region. The experimentally obtained smaller values are explained by the parallel removal of solvent. The conductance vs. logarithm of charge curves at small sweep rate show no hysteresis. However, during the first oxidation step the film is nonconducting; its transformation to conducting film is coupled with the second oxidation step. On the basis of the experimental observations, a detailed mechanism of the redox transformation is suggested in which the roles of the solvated and nonsolvated parts of the neutral film are distinguished. The achievement of the quasi-metallic state is connected to a capacitive process in which an intermediate of uncompensated charge is involved. (C) 1998 Elsevier Science S.A.