Calculations of Band Gaps in Polyaniline from Theoretical Studies of Oligomers

Geometries and band gaps of polyaniline oligomers up to decamer have been systematically calculated and analyzed using various computational techniques such as molecular mechanics, semiempirical, and ab initio methods. On the basis of fully optimized geometries of neutral and charged forms of polyaniline oligomers, excitation energies are calculated at the semiempirical ZINDO (INDO/S) level and extrapolated to the band gap value of the infinite chain. Band gaps are also approximated by extrapolating the HOMO/LUMO difference calculated at the density functional level (B3LYP/6-31G*). The ZINDO//AM1 band gaps in the reduced and oxidized form of polyaniline (4.3 and 2.7 eV) are in good agreement with experimental values (3.8 ± 2 and 1.8 ± 3 eV, respectively). The doped form of polyaniline (two positive charges per four aniline units) has been computed with a spin-unrestricted method (UAM1) and the band gap approximated from an extrapolation of the tetramer and octamer. The calculated band gap of 1.3 eV (UZINDO//UAM1) is in good agreement with experiment (1.5 eV). The influence of ring torsional angle and interchain interaction on the band gap of the polyaniline system are also discussed.