The electrochemical properties of poly(2,5-thienylene vinylene) synthesized via water-soluble precursor polymer

Abstract

The electrochemical, optical, and magnetic properties in poly(2,5-thienylene vinylene), PTV, in comparison with those of polythiophene, PT, during electrochemical p-type doping have been investigated by cyclic voltammetry, optical absorption spectrum, and electron spin resonance (ESR) measurements. The evolution of localized states is evidenced by the spectral change associating with doping. In the lightly doped state only two absorption peaks originating from the transitions between two gap states and the valence band appear within the gap region at about 0.6 and 1.2 eV. The spin susceptibility increased by about 1 order of magnitude from 3.7×10−7 to 2.6×10−6 emu/mol upon doping up to a dopant concentration of about 2.8 mol %. With further increasing dopants concentration, the spin susceptibility decreases slightly. The spin density evaluated from the susceptibility was inconsistent with the polaron density assumed to be formed by dopants of 2.8 mol %. Namely, even at lightly doped state, the newly developed species turn into p-type spinless bipolarons. The band gap of PTV was evaluated to be about 1.8 eV and smaller than that of PT by about 0.3 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 neighboring thiophene rings. The electronic band structure of PTV was determined. The bipolaron states in PTV appears in the band gap: about 0.6 eV below and above the conduction and the valence band, respectively. The bottom of the conduction band of PTV is in almost the same position as that of PT and successful n-type doping in PTV is expected.