Effect of pH on the structure of absorption spectra of highly protonated polyaniline analyzed by the Alentsev–Fock method

This paper presents the results of the investigation of the effect of pH of highly acidic medium (pH 0 and 1) on the structure of the electronic spectra of polyaniline films in the spectral range 300–900 nm. The analysis was carried out using the Alentsev–Fock method, which requires no preliminarily notion about the possible shape and/or number of the individual absorption bands and extensive statistical calculations. Along with the six absorption maximums described earlier by other authors three new individual bands were separated. On the basis of the comparison of our results with the literature data, we are inclined to assign the new band near 530–570 nm (depending on the pH) to the donor–acceptor interaction between the quinoid fragments and the counter anions and the new band at λmax>900 nm to the ‘free carriers’ absorption. We also suppose that the new band near 810–830 nm may be associated with the second absorption band of the cation-radical dimers. The effect of an increase in pH on the structure of polyaniline absorption spectra manifests itself in the increase of the portions of the bands corresponding to different electron transitions in the quinoid fragments, as well as in the decrease of the portions of the bands related to the conductivity of the film (i.e. the bands of the cation-radical dimers (or polarons) and free charge carriers).