Electrochemical, Magnetic, and Electrical Properties of ?,?-Capped Sexithiophene Films. 2. Conduction in Sexithiophenes with ?,?-Aryl-Extended Conjugation

Thin films of tetrahexylsexithiophenes, fluorenonyl- and fluorenyl-protected at the terminal R,ö- positions, were investigated by cyclic voltammetry, electrochemical quartz crystal microbalance analysis, in situ ESR, and in situ conductivity. Reversible oxidation is composed of three separate steps, two one-electron processes and a further multielectron process. 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-substituted R,ö-capped octathiophene, decathiophene, and dodecathiophene films have been similarly investigated to compare the effects of fluorenonyl and fluorenyl extension of sexithiophene with those 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 the 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.