Electrochemical, magnetic and electrical properties of a,w-capped sexithiophene films. Part 2. Conduction in Sexithiophenes with a,w-Aryl-Extended Conjugation

Thin films of tetrahexyl-sexithiophenes, fluorenonyl- and fluorenyl-protected at the terminal a,w-positions, were investigated by cyclic voltammetry, EQCM, in-situ ESR and in-situ conductivity. Reversible oxidation is composed by three separate steps, two one-electron and a further multilelectron processes. ESR indicates strong magnetic dimerization for the one-electron oxidized species. Conductivity is redox type at the cation-dication (polaron-bipolaron) state and metal-like at doping levels higher than the bipolaron with a 20-fold increase at full oxidation. Hexyl-substitutedƒn a,w-capped octathiophene, decathiophene and dodecathiophene films have been similarly investigated to compare the effects of fluorenonyl- and fluorenyl-extension of sexithiophene with that given by additional thiophene rings. The conductivity, which increases progressively (by 3 orders of magnitude) with the oligothiophene chain length, is metal-like. In dodecathiophene the doping charge increases markedly approaching four electrons per molecule and conductivity, maximized at the two-electron level, decreases linearly approaching zero at the four-electron level. A bipolaron model of conductivity accounts for conductivity in oligothiophenes.