Open-chain Boundary Condition For Polyaniline And Its Long-range Exciton

We studied the self-consistent-variation ground state for an open chain with finite length of a PNB polymer, in the framework of the Baranowski-Büttner-Voit model. Assuming suitable end values in the end-fixed condition, or adding a revision term to the Hamiltonian to substitute the environment about the ends in free-end condition, the numerical solutions show, both geometrical and electronic structures, are similar to those in the Born-Karman condition, with no end collapses, and no defective levels in the gap. On the basis, self-trapping exciton state was obtained for free-end condition. We also studied its responses to electrostatic fields, and tried to understand the observed electrochromism of the polymer by the blue shift. The results showed: an applying field resulted in a blue shift of the exciton absorption. As the field was stronger than 925 kV/cm, the exciton was dissociated into a soliton and an anti-soliton, while zero-field absorption of the exciton was splitted into two peaks at 1.1 eV and 1.4 eV, which was consistent with the observed two short-lived peaks (1.0 eV and 1.3 eV ) of the photoinduced absorption in the polymer. It is the first time that both of the two short-lived peaks were obtained in one chain.Our calculations can reproduce a lot of observed data in polyaniline. So our solution may stimulate further work on polyaniline and can be popularized to phenyel-based polymer. It will play an important part in polymer design.