Electrochemical copolymerisation of 5-substituted indoles

Abstract

The electrochemical oxidation of mixtures of indole-5-carboxylic acid (ICA) and 5-cyanoindole (CI) produces a film containing free cyclic trimers and polymeric species consisting of linked cyclic trimer units. Trimers containing 3 ICA molecules and 3 CI molecules are present, together with cotrimers which contain 2 ICA and 1 CI molecule and 1 ICA and 2 CI molecules. A mechanism is proposed which explains the observed rate of trimer formation and the relative amounts of trimer and cotrimer formed. In this mechanism, trimer formation occurs via adsorbed ICA and CI radical cations, with the proportion of the trimers in the layer being governed by the coupling of a statistical distribution of these radical cations; the overall rate of formation of each trimer is then given by the combined coupling rate of the ICA and CI radical cations involved. As for the monosubstituted indoles, further linking of trimers and cotrimers occurs when forming the polymer species. The oxidation potentials of the cotrimers lie between those of the (3 ICA) trimer and the (3 CI) trimer, consistent with the proposal that the oxidation potential correlates with the combined electron-withdrawing/donating power of all three substituents in the trimer. Copolymerisation on an electrode surface by the electrochemical oxidation and coupling of preformed ICA and CI trimers is also possible.