The redox reaction and induced structural changes of 5-substituted indole films

Abstract

The electrochemical behaviour of two types of electrodeposited redox active indole trimer films, 5-cyanoindole (CI) and indole-5-carboxylic acid (ICA), have been studied in acetonitrile electrolyte systems. Chronoamperometry, cyclic voltammetry and transmission line analysis of ac impedance data have been used to monitor the kinetics and mechanism of the electron transfer process with prolonged redox cycling. As-deposited films of CI and ICA each show high electronic conduction, consistent with the films behaving as a porous metal. CI films show a relatively large, potential dependent barrier to ion insertion, consistent with a compact, poorly solvated structure. In contrast, ICA films display a higher film capacitance and a lower barrier to ion insertion, indicating a more open and solvated film. On prolonged slow redox cycling over several days, CI shows little change in coat structure, whereas ICA shows a marked change in its redox reaction, consistent with a change in the mechanism of electron transfer to redox hopping, and in the mechanism of ion transfer to cation insertion. This can be explained by the irreversible deprotonation of a carboxylic acid substituent on the trimer centre during oxidation, which induces a change in redox mechanism and film structure. Transmission line analysis of small amplitude ac impedance data is shown to be an excellent method for monitoring this and other such changes in modified electrode systems.