Voltammetric properties of poly(4,4′-dialkyl-2,2′-bithiophenes) and poly(3-alkylthiophenes)

The voltammetric behaviour of two types of soluble poly(alkylthiophenes), namely poly(4,4′-dialkyl-2,2′-bithiophenes) and the corresponding poly(3-alkylthiophenes), has been studied. Three types of experiments were carried out. (1) Classical cyclic voltammetry, i.e. cyclic voltammetry performed in electrolytes containing a large excess of electroactive component (LiClO₄). (2) Cyclic voltammetry carried out in electrolytes containing an electrochemically inactive component ((Bu₄N)₄SiW₁₂O₄₀), which provided ionic conductivity, and a small admixture of electroactive component (LiClO₄). (3) "Memory effect" studies in which the cyclic voltammetry run was preceded by a waiting time τ. It has been demonstrated that the voltammetric behaviour of the oxidation peak in poly(4,4′-dialkyl-2,2′-bithiophenes) is very complex and cannot be approximated by either the diffusion limit or the charge transfer limit. The behaviour of the reduction part of the cycle is qualitatively similar to the behaviour of poly(3-alkylthiophenes) and is characteristic of systems in which the overall electrochemical process is limited by charge transfer processes. Studies carried out with low concentrations of electroactive component in the electrolyte showed that the doping of poly(4,4′-dialkyl-2,2′-bithiophenes) consists of two parts: "quasi-irreversible doping", which occurs preferentially and is independent of the concentration of LiClO₄, and a reversible part which increases with the concentration. The memory effect, i.e. the shift in oxidation peak position induced by the so-called waiting time τ (the time during which the polymer is kept at the potential of the reduced state, 0 V), is much more pronounced in poly(4,4′-dialkyl-2,2′-bithiophenes) than in poly(3-alkylthiophenes). The existence of the memory effect is associated with the difficulty of de-doping (slow release of the residual charge) and the slow relaxation process of the reduced flexible polymer matrix. An increase in τ allows for more complete recovery of the residual charge, which in turn leads to higher charge being introduced during the oxidative doping.