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 electronic and optical properties of molecules representative of the three base forms of polyaniline (PAni), i.e., leucoemeraldine, emeraldine, and pernigraniline. The results establish that the PAni oligomers show a weak evolution of their electronic and optical properties as the chain grows, in contrast to the trends observed with ether conjugated systems. Such behavior is related to a lack of significant pi delocalization of the molecular orbitals, due to the presence along the backbone of nitrogen atoms that interconnect adjacent phenylene rings, and to high-torsion angles induced by steric hindrance between the rings. We also emphasize the need for a correlated approach to properly describe the so-called ''excitonic feature'' observed around 2 eV in the optical spectrum of the emeraldine base form.