ELECTROCHEMICAL PREPARATION OF CONDUCTING POLY(TRANS-1,2-DI(2-THIENYL)ETHYLENE) AND POLY(1,4-DI(2-THIENYL)-1,3-BUTADIENE) AND THEIR PROPERTIES - COMPARISON WITH POLYTHIOPHENE

The electrochemical and optical properties in poly(trans-1,2-di(2-thienyl)ethylene), (trans-PTE), and poly(1,4-di(2-thienyl)-1,3-butadiene) (PTB), in comparison with those of polythiophene (PT), during electrochemical p-type doping have been investigated by cyclic voltammetry and in situ optical-absorption-spectrum measurements. The evolution of localized states is evidenced by the spectral change associated with doping. These results are discussed in terms of polaron and/or bipolaron models. The band gap of trans-PTE and PTB was evaluated to be both about 1.9 eV and smaller than that of PT by about 0.2 eV. This result can be interpreted in terms of a conformation change in the molecular structure, such as a change in the torsion angle between neighbouring thiophene rings. The polaron states in trans-PTE appear in the band gap, about 0.65 eV above the valence band but about 0.6 eV below the conduction band. The polaron states in PTB appear at about 0.7 and 0.6 eV below and above the conduction and valence bands respectively. The top of the valence band of both trans-PTE and PTB is located at a higher-energy state than that of PT by about 0.2 eV. This result satisfactorily explains the stability of p-type dopants in trans-PTE and PTB compared with PT. The bottom of the conduction band of both trans-PTE and PTB is in almost the same position as that of PT and successful n-type doping in trans-PTE and PTB is expected.