FROM NEUTRAL OLIGOANILINES TO POLYANILINES - A THEORETICAL INVESTIGATION OF THE CHAIN-LENGTH DEPENDENCE OF THE ELECTRONIC AND OPTICAL-PROPERTIES

We investigate theoretically the chain-length dependence of the electr onic and optical properties of molecules representative of the three b ase forms of polyaniline (PAni), i.e., leucoemeraldine, emeraldine, an d pernigraniline. The results establish that the PAni oligomers show a weak evolution of their electronic and optical properties as the chai n grows, in contrast to the trends observed with ether conjugated syst ems. Such behavior is related to a lack of significant pi delocalizati on of the molecular orbitals, due to the presence along the backbone o f nitrogen atoms that interconnect adjacent phenylene rings, and to hi gh-torsion angles induced by steric hindrance between the rings. We al so emphasize the need for a correlated approach to properly describe t he so-called ''excitonic feature'' observed around 2 eV in the optical spectrum of the emeraldine base form.