The redox mechanism of polyaniline studied by simultaneous ESR-UV-vis spectroelectrochemistry

The electrochemical redox process to charge and discharge polyaniline layers is studied by simultaneous Electron Spin Resonance (ESR) and UV-vis spectroscopic measurements. For this purpose, optically transparent electrodes were applied under potential-controlled conditions. The use of a calibrated manganese ESR standard ensures the calculation of the absolute numbers of free spins in the polymer layer during electrochemical doping. A general procedure is given to analyse the simultaneously recorded electrochemical and spectroscopic data. The procedure proposed following the time dependence of charge, ESR absorption and the absorbance demonstrates that the electrochemical charging/discharging mechanism of polyaniline can be described on the base of three oxidation states. In this way, the separated UV-vis spectra and the time/potential dependence of each redox state can be shown. Only little changes are observed in the shape of the separated spectra using the polaron/bipolaron model in comparison to the polaron/sigma-dimer model. A square ladder scheme of the redox mechanism is discussed, considering neutral, polaronic and bipolaronic states and the protonation equilibria. Furthermore, experimental evidence for the Faradaic nature of the "capacitive" current plateau is presented. (C) 1999 Elsevier Science S.A. All rights reserved.