XPS STUDIES ON CONDUCTING POLYMERS - POLYPYRROLE FILMS DOPED WITH PERCHLORATE AND POLYMERIC ANIONS

Conducting polypyrrole (PPy) films, doped with perchlorate and polymeric anions, were prepared by electropolymerization and analyzed by XPS in order to clarify the interaction between polymer redox sites and various dopants. The X-ray photoemission data indicate that the electrostatic interaction of ClO4- anions with PPy occurs at specific sites. The electric field produced by the anion affects the entire pyrrole ring in its proximity and makes all atoms, carbons and nitrogen, to look more positive. If the electrostatic bonding were with the nitrogen heteroatom alone, this would yield a considerably large shift than is observed in the N 1s core level spectra. The geometric arrangement of the ring-anion pair consistent with the observed interaction behavior requires that the tetrahedral ClO4- anion overlays the planar ring with its triangular oxygen plane parallel to the planar ring. The negative charge of the counterion is localized, and the polymer responds to it by local modification of the chain geometry and the appearance of defined bipolaron entities. However, if the PPy is doped with large anions, uniform electronic charge extraction from the carbon backbone prevails. In this case, a majority of the negative charge carriers are presumably not placed close enough, nor at a favorable disposition with respect to the planar pyrrole ring, to accomplish interaction at specific sites.