X-ray photoelectron spectroscopic studies of polypyrrole synthesized with oxidative Fe(III) salts

Abstract

Conductive polypyrrole (PPY) complexes obtained from chemical polymerization and oxidation of pyrrole by Fe(III) oxidants, such as FeCl3 and Fe(ClO4)3, have been studied by X-ray photoelectron spectroscopy (XPS). The C12p core-level spectrum of the PPY/chloride complexes reveals the presence of the 'dopant' in three distinct chemical states. Thus, any reaction stoichiometry derived from the total chlorine balance of the complex alone can be misleading. As in the case of the nitrogens in polyaniline, proton modifications of the pyrrolylium nitrogens give rise to a number of intrinsic oxidation states, ranging from that consisting of about 25% oxidized imine-like (=N-) structure, observed in the present study, to the fully reduced PPY (all-NH-structure). The former is obtained by deprotonation of the oxidized and positively charged pyrrolylium nitrogens and is susceptible to reprotonation by protonic acids, while the latter is obtained from the reduction of the deprotonated structure by phenylhydrazine and is susceptible to reoxidation by electron acceptors. From the actual chloride anion content of a complex and the corresponding intrinsic oxidation state of the polymer, a reaction stoichiometry that accounts for 25% oxidized pyrrole units is favored. Similar intrinsic structures are observed in PPY complexes synthesized with Fe(ClO4)3. In this case, although the dopant exists predominantly as perchlorate species, a small amount of the dopant also exists as the chloride species.