At the beginning of potential cycling the polymer is essentially made up of the hexainerfZg1 and the corresponding behavior is displayed, i.e. conductivity attains a maximum at the second redox potential of the material (polaron-bipolaron mixed-valence conduction). Further polymerization, promoted by potential cycling, produces longer oligomeric segments with a random distribution resulting in a relationship of conductivity vs. potential similar to that of the octamer.
Capacitive currents in polyconjugated conducting polymers have been attributed, on the basis of the redox behavior of transition metal oxides"' as well as of oligomers,f4, ' 1 to sequences of several electron transfers. Since a metal-like capacitive behavior is also displayed by well-defined lowweight oligomers involved in single two-electron oxidations, this suggestion cannot be invoked as the primary explanation of the phenomenon. In any case, in our picture of oligomers in the solid state as molecular conductors with a band of closely spaced states, the latter may be viewed as redox states and the hypothesis of multiple redox states may be thus reconciled with the picture of the conducting polymer chain as a molecular capacitor.[']
Experimen tal
Experiments were performed ai 25 'C undev nitrogen in acetonitrile containing the supporting electrolyte tetraethylammoniumperchlorate (TEAP) in 0.1 M concentration. The counter electrode was platmum; the reference clectrode was silver/O 1 M silver perchlorate in acetonitrile (0.34 V vs. SCE); the working electrode for cyclic voltammetry and chronopotentiometry was a platinum minidisc electrode (3 x lo-' cm'). Electrochemistry was performed with a PAR 273 potenliostat provided with M270 analytical software.
The apparatus mid procedures used in the in situ conductivity experiments were described previously in detail [15]. The relevant working electrode was a two-band platinum electrode (0.3 cm x 0.03 cm for each band) with interband spacing of 6 bm operating as a two-probe device with a bipotcntiostat.