Electrochemical, magnetic, and electrical properties of a-w-capped sexithiophene film. Part 3. Conduction in poly(bis-terthienyl-B) (B = ethane, disulfide, diacetylene, ethylene)

Electrochemical oxidation of bis-terthienyl-B (B = ethane, disulfide, diacetylene, acetylene, ethylene) has been investigated. Monomers without and with 3,3''-dialkylsubstitution are regularly coupled to polymers alternating sexithiophene and B moieties. The corresponding terthienyl homopolymers have also been produced for comparison. The polymers obtained are characterized by cyclic voltammetry, EQCM, UV-vis and FTIR spectroscopy, in-situ ESR and in-situ conductivity. The conductivities of p-doped polymers with ethane or disulfide bridges fall in a narrow range (1-5x10-2 S cm-1) and are practically the same of a,w-dimethylsexithiophene (1x10-2 S cm-1) suggesting that conduction proceeds without the help of the links via a direct redox hopping between sexithiophene blocks of adjacent polymer chains. In the cases of diacetylene and acetylene conduction changes from redox to bipolaron-type but only in the case of ethylene the conductivity jumps to high values (1-5 S cm-1) corresponding to those of the polymers without the bridging moiety B, i.e. with shorted thiophene rings. DFT calculations of the bridge energy levels account for this result.