Mixed-valence conduction in redox-substituted polythiophenes. Enhancement of redox electron-exchange rate by polyconjugated chains

The in situ conductivity of some poly(cyclopentadithiophenes) modified at the 4 position with reducible (p-nitrophenyl and p-N-methylpyridyl) or oxidizable (ferrocene) redox groups was investigated in acetonitrile. In the nitrophenyl- and pyridyl-functionalized polymers the reduction, which is confined to the pendant redox group, makes the polymer mixed-valence conductive. The conductivity ((3–6) × 10−3 S cm−1), which is the highest reported for a redox polymer, remains within the limits expected for a direct intersite electron hopping. In the ferrocene-functionalized polymers the oxidation involves both the redox group and the backbone. Redox conduction at the ferrocene group is enhanced by hopping through the conductive polymer backbone. The hopping rate is increased by a decrease of the ferrocene-backbone distance and by conjugation of ferrocene with the backbone itself making conductivity increase up to 1 S cm−1.