Could the acid doping of polyaniline represent the charge transfer interaction?

Abstract

It is proposed to describe the acid doping of the emeraldine base (EB) of polyaniline (PANI) as a donor-acceptor interaction, followed by a charge separation (charge transfer) inside the complex formed according to the scheme: D + H-Acc= (D→H-Acc) D +-H +Acc −, where D is the imine center of EB and H-Acc the acceptor possessing the mobile H atom. The hydrogen mediator supplied from the acceptor molecule is joined to the polymer chain in the second stage of transformation. The electrochemical system, PANI electrode/hydroquinone (4F-hydroquinone) solution, has been investigated as a model of this D-H-Acc interaction. It has been found that the redox reactions of both hydroquinone systems take place on PANI practically without overpotential and, probably, the two-electron charge transfer processes are realized in such systems. Reaction currents depend on PANI quantities deposited on the Pt electrode. On the PANI/electrolyte interface the stoichiometric complex is formed; it consists of nearly four units of aniline and one molecule of quinone (hydroquinone). There exists a great electronic exchange into this surface complex involving the acid centers of PANI as hydrogen mediators in the charge transfer on an organic acceptor. D-H-Acc interaction between EB and hydroquinone/quinone under the ‘wet’ electrochemical and ‘dry’ solid-state condition has been compared. We have concluded that the electron exchange between the EB chain and surface adsorbed molecules is the necessary condition for the doping occurring in both cases. The similarity has been revealed between the reactive center in the PANI-quinone/hydroquinone redox system and the donor-acceptor interaction in porphyrin-quinone photochemical reactions and other biological systems. It is proposed to involve the quinone and other acceptor molecules in the optical processes occurring with the π-electron system of conducting polymers.