Car-Parrinello molecular dynamics study of electronic and structural properties of neutral polyanilines

We present an ab initio quantum-mechanical study, based on Car-Parrinello molecular dynamics, of electronic, optical, and structural properties of the three base forms of polyaniline polymers. To avoid finite-size effects encountered in the study of oligomers, we have represented each polymer by a periodic isolated infinite chain. We have provided detailed information on the morphology of each chain in terms of bond lengths, bond angles, torsion angles, twisting and bending of the chain backbone, analyzing also temperature effects. From the ground-state geometry optimization we have observed a definite nonplanarity of the nitrogen atoms which constitute the chain backbone in the emeraldine base polymer. Moreover, by monitoring the time evolution of the geometry of the pernigraniline base polymer, we have evidenced an oscillating quinoid-phenilene character of each chain ring. Electronic band-structure and density of states have been calculated for emeraldine, pernigraniline, and leucoemeraldine base polymers within the Kohn-Sham density-functional theory. We have analyzed the global electronic charge density and the nature of the highest occupied molecular orbital and lowest unoccupied molecular orbital levels and reproduced existing experimental absorption and ultraviolet photoelectron spectra.