Spectroscopic studies of polyaniline in solution and in spin-cast films

Abstract

Absorption spectra are presented for polyaniline in dilute solution in concentrated sulfuric acid and in the form of spin-cast films (spun from sulfuric acid solution and subsequently washed with water). The results from samples prepared with different molecular weights and with different degrees of protonation (in the case of the films) are compared. For the emeraldine salt, we observed three absorption features, at 1.0 eV (with a tail extending deep into the infrared), 1.5 eV and 3.0 eV. The relative intensities of the 1.0 and 1.5 eV absorptions are strongly dependent on both the molecular weight and the protonation level, with important differences for the solid film and the solution spectra. The results imply that the 1.0 eV absorption is the principal signature of the intrachain (free carrier) excitations, and that the 1.5 eV absorption arises from interchain excitation. A method to prepare stable fully oxidized polyaniline in concentrated sulfuric acid is presented; the spectrum of the charged bipolaron lattice of fully oxidized polyaniline exhibits a strong absorption band centered at 2.2 eV and a weaker absorption at 4 eV. The available data provide the basis for an understanding of the electronic structure of the four principal forms of polyaniline: the fully reduced leucoemeraldine, the emeraldine base, the oxidized and fully protonated emeraldine salt, and the fully oxidized bipolaron lattice.