Poly(3-arylthiophenes):: Syntheses of monomers and spectroscopic and electrochemical characterization of the corresponding polymers

Several arylthiophene derivatives were synthesized using the Kumada cross-coupling reaction and electropolymerized at a platinum electrode with the aim of investigating the effect of the number of thiophene unit and substitution pattern of the monomers on the spectroscopic and charge storage properties of the resulting polymers. The electrochemical oxidation of these monomers showed a decrease of the anodic peak potential as the number of thiophene units is increased: 1.2-1.4, 1.0, and 0.75 V for the mono-, bi-, and terthiophene, respectively. The polymers having one phenyl ring per thiophene unit displayed a larger band gap with the exception ofpoly-3-phenyl-thiophene, which showed a much smaller band gap. The lower band gap for the later can be explained by the higher reactivity of the cation radical(as evidenced bg;a high oxidation potential) which led to a highly conjugated polymer. The cyclic voltammograms of the same polythiophene derivatives is characterized by a significant charge unbalance between the doping and undoping charge and most showed a limited cycle lifetime. On the other hand, poly-3-phenyl thiophene, poly-3 '-(1-naphthyl)-2,2 ': 5 ' ,2 -terthiophene are the more stable and also show a relatively good balance between doping and undoping charge for both the p- and n-doping redox processes. The highest voltammetric charge density evaluated by considering the weight of monomer unit was recorded for poly-3-phenyl thiophene despite the fact that some poly terthiophene derivatives have a lower phenyl ring/thiophene unit ratio.