An IR spectroscopic study of the electrochemical reduction of polypyrrole doped with dodecylsulfate anion

In situ transmission IR spectra (4000—450 cm−1) of the oxidized and reduced forms of electrochemically grown polypyrrole films have been recorded. The changes that occurred in the IR spectra of oxidized polypyrrole doped with dodecylsulfate were evaluated as it was progressively reduced electrochemically at constant current. The spectral data were interpreted in terms of changes in bond order and found to be consistent with a quinoid-to-benzoid transition on reduction. Spectral subtractions were used to isolate the spectra of the oxidized and reduced species. The data suggest that the conducting species in dodecylsulfate-doped polypyrrole is the same as that obtained with p-toluenesulfonate and we conclude that the lower conductivity of the former is caused by a lower level of active species, rather than the presence of different charge carriers in the former polymer. The rate of loss of active species on reduction was found to be double the expected rate, consistent with results obtained by other workers. In terms of the polaron/bipolaron concept, we suggest this is consistent with disruption of bipolarons by removal of one positive charge from each as the first step in the reduction. We also examined the spectra of reduced films that had been exposed to air. The data indicate a shift in bond order towards a quinoid structure but a key part of the spectrum assigned to the α-carbon atoms did not change in proportion to the rest of the spectrum; this suggests that delocalization along the chain through a quinoid structure was not achieved to a significant extent by oxygen doping.