Electrochemical, magnetic and electrical properties of a,w-capped sexithiophene films. Part 1.Neutral-polaron and polaron-bipolaron conductivities.

Thin solid films of fourteen sexithiophenes variously substituted at the terminal a,w-positions were electrochemically deposited on electrodes by anodic coupling of the corresponding half-capped terthiophenes. Sexithiophene films were investigated by cyclic voltammetry, EQCM, in-situ ESR and in-situ conductivity. Reversible oxidation is composed by two separate one-electron steps, the first of which is split into two 0.5-electron steps for most 3,4-ethylenedioxy-substituted terthiophenes. ESR indicates strong magnetic dimerization for the one electron oxidized species. Conductivity (in the range 10-4-10-1 S cm-1) is redox type and develops at the neutral-cation (neutral-polaron) and cation-dication (polaron-bipolaron) levels with values 5-10 times higher for the latter. The influence of steric, electron donor-acceptor and conjugative effects on conductivity was evaluated by a suitable choice of end-capping substituents.