Semi-empirical study of chain conformation and absorption spectra of polyanilines: size, solvent and disorder effects

In this paper, we present electronic structure calculations and conformational studies of polyaniline oligomers to investigate the influence of oligomer size, oxidation state and solvent effects in the UV-Vis absorption spectrum. Torsional barrier and solvent effect calculations are carried out within AM1 semi-empirical technique. Absorption spectra are simulated through ZINDO/S-CI calculations. Based on the analysis of the torsion potential curves, an interpretation is given for the diverse thermochromic behavior observed in insulating polyanilines as a function of oxidation state. Pernigraniline base, the fully oxidized form of polyaniline, has been reported to present an unusual redshift in its spectrum with increasing temperature. The potential energy associated to ring twisting calculated for pernigraniline exhibits a double well shape, which is different from the potentials obtained for the other forms. We concluded that the chain flexibility depends strongly on the oxidation state and is the source of the diverse chromic behavior. (C) 2000 Published by Elsevier Science B.V.