POTENTIAL-MODULATED ULTRAVIOLET VISIBLE AND RAMAN-SPECTRA OF POLYPYRROLE THIN-FILMS IN AQUEOUS-ELECTROLYTES - COMBINATION WITH VOLTAMMETRIC SCANNING AND THE INFLUENCE OF DIOXYGEN ON THE STABILITY OF RADICAL CATIONS AND DICATIONS OF THE CONDUCTING POLYMER

For non-degenerate ground-state polymers such as polypyrrole, it has been shown by other authors that radical cations (polarons) and dications (bipolarons) are the species responsible for their electronic conduction. The evolution of these species with electrode potential and doping level has also been considered in previous studies. We now show, via a combination of open-circuit relaxation and UV-VIS spectroelectrochemical measurements, that the carrier evolution pathway critically depends on the O2 content of the ambient in which neutral polypyrrole is electrochemically or chemically conditioned. Specifically a three-state spectral transition characterizes the oxidation of the polymer in an N2 atmosphere. On the other hand, in an O2-saturated environment, the presence of an isosbestic point at ca. 480 nm implies clean interconversion between two forms. We present evidence for our proposal that the polaron state is unstable in polypyrrole in O2, and the polymer undergoes a direct PP0 --> PP2+ oxidation in this case. On the other hand, oxidation in N2 involves the usual PP0 --> PP.+ --> pp2+ sequence. The poor resolution typical of cyclic voltammetric data on polymers such as polypyrrole is underlined when UV-VIS spectroelectrochemical data are analysed in the form of dA/dt vs. potential plots (A = absorbance at a characteristic wavelength). The similar utility of Raman scattering data is also illustrated via the use of dl/dt vs. potential plots for polypyrrole electrodes redox-cycled in KCl (l = intensity of scattered light for a specific band undergoing potential modulation).