Electrooxidation of 5-substituted indoles

Abstract

The electrochemical oxidation of a wide variety of 5-substituted indole monomers at a platinum electrode results in the formation of a redox active film. Electrochemical and spectroscopic evidence is consistent with the redox species in the film being a cyclic trimer. In contrast, the electropolymerisation of 5-aminoindole and 5-hydroxyindole on a platinum electrode does not result in redox active film formation. This is attributed to the adsorption of the monomer onto the metal electrode via the substituent, which inhibits this reaction. However, electropolymerisation of these monomers onto a predeposited film of 5-cyanoindole or 5-nitroindole results in the formation of the cyclic trimer. Electrochemical studies using a rotating-ring disc electrode (RRDE) have confirmed the stoichiometry of the trimerisation reaction and also that the redox active cyclic trimer species shows reversible one electron redox activity. The half-wave potential for the reduction of each of these trimers shows a linear dependence with the Hammett substituent constant, σ⁺ or σ^-, as appropriate, determined for a para-substituted aromatic indicating that the 5-substituent is conjugated into the π-electron system of the trimer. This indicates that judicious choice of substituents allows control of the trimer redox potential.