Suppression of the emeraldine base-to-emeraldine salt transition in ultra-thin films of polyaniline

Abstract

Electronic absorption spectra of thin films of polyaniline (emeraldine base (EB) and emeraldine salt (ES)) prepared by adsorption at the solid/liquid interface and by casting from dimethylformamid and dimethyl sulfoxide solutions were investigated. Prepared by the same technique and using the same solution, the films however demonstrated divergence in their absorption spectra, depending on the film thickness. The benzenoid-to-quinoid excitonic transition in the EB was blue shifted as the film thickness was decreased. In films exposed to equal levels of acid, thicker films showed more pronounced spectral features of the ES. Spontaneous suppression of the protonation features was found in spectra of adsorbed films of ES as compared to those of a solution from which the film was adsorbed. Also, the protonation features in cast films of ES were weakened as the film thickness was decreased. Possible mechanisms responsible for this phenomenon are discussed.