Experimental and Theoretical Studies on the Redox Properties of Conducting Polymers

Voltammetric measurements on selected polypyrrole (PPy) films and defined soluble oligomers belonging to the oligo‐p‐phenylene and the oligo‐p‐phenylenevinylene series provide new insights into the charging/discharging properties of conducting polymers. It turns out that the current response in the whole accessible potential range of conventional conducting polymers is mainly caused by faradaic charge transfer reactions rather than capacitive effects. This is supported by digital simulations which are based on a model of n‐fold electron transfers within a finite layer. In addition, from the data of defined oligomers it becomes clear that the number and energies of accessible redox states in such materials depend on the extension of conjugation in the monomeric unit and on the chain length of the polymer. In general, in systems such as PPy up to a charging level of approximately 20 – 25 mol% all redox states are degenerated. Then, the successive redox states are gradually shifted to higher energies in accordance with increasing Coulomb repulsion between the charges. The results show that the bipolaron model must be amended. Nevertheless, experimental data reveal that the charging steps in conducting polymers are coupled with structural relaxation processes; but they occur, in contrast to the theoretical predictions, already on the polaron state.