Electrochemical properties of polyindole and poly(5-cyanoindole) in LiClO4—acetonitrile and in HCl and HClO4 solutions

Electropolymerization of indole and 5-cyanoindole leads to the formation of electroactive polymer films. Their analysis by FT-IR spectroscopy, as a function of the oxidation potential in aprotic medium (LiClO4—acetonitrile) shows that they exhibit the same type of redox reactions. The coupling between the monomer units occurs in both cases between the C2 and C3 positions. On the contrary, when polyindole and poly(5-cyanoindole) are cycled in HCl or HClO4 solutions, differences appear in their electrochemical behaviour. Two types of redox reversible processes are observed in both polymers. For polyindole, these transformations are pH independent. For poly(5-cyanoindole), the reaction occurring at the most positive potential is pH dependent and is related to both protonic and electronic exchanges between the polymer and the electrolytic solution. The other redox transformation, at less positive potentials, is pH independent and can be described as an electronic exchange only. Mechanisms describing these two redox reversible reactions occurring when poly(5-cyanoindole) is cycled in acid solutions are proposed.