Changes in the population of neutral species and charge carriers during electrochemical oxidation of polypyrrole

The absorbance changes due to neutral species and charge carriers on anodic oxidation of polypyrrole were studied. The deconvolution of the spectra obtained for varying oxidation states and in the presence of KF, KCI, KBr and KI yields four optical transitions: two are due to the aromatic and quinoid forms of the neutral segments of polypyrrole, and two are associated with charge carriers. During anodic oxidation, the population decay of both neutral conformations follows first-order laws. The oxidation rates of the quinoid segments are higher than those of the aromatic form, independent of the nature of the halide in solution and, at high potentials, are affected by the recombination of polarons to yield bipolarons. It was also observed that the redox transition takes place at lower rates, but to higher oxidation states, in the presence of F- compared with the other halides, possibly due to a parallel, overoxidation reaction.