Evolution of structure and electronic properties in oxidized polyaniline as a function of the torsion angle between adjacent rings

We present semi-empirical Hartree-Fock and valence effective Hamiltonian (VEH) calculations on polyaniline in its oxidized (so-called 2A) form. In particular, we study the evolution of geometrical structure and electronic properties as a function of the torsion angle between adjacent rings. The geometry is optimized at each fixed value of the torsion angle using the modified neglect of differential overlap (MNDO) method. A minimum in the total energy is obtained for a torsion angle of 87°. No drastic changes in the geometrical structure are found for torsion angles between 90° and 30°. The geometry within the six-membered carbon rings stays almost constant while, for example, the bond angle at the nitrogen increases from 125.1° for a 90° torsion angle to 131.5° for a 30° torsion angle. Very interestingly, the electronic properties are found to be rather insensitive to the changes in torsion angle, for example the band gap between HOMO and LUMO is 1.93 eV and 1.60 eV for torsion angles of 90° and 30°, respectively. However, the width of the highest occupied band is almost zero for a 90° torsion angle and increases up to 1.0 eV for a 30° torsion angle. Comparisons are made between polyaniline and other similar polymers.