First-principles studies of the structural and optical properties of crystalline poly(para-phenylene)

We present first-principles local-density band structure calculations for one-dimensional and three-dimensional crystalline poly (para-phenylene) (PPP) using the full-potential linearized augmented-plane-wave and the pseudopotential methods. Optimized structural parameters for PPP chains and for orthorhombic crystalline phases with space groups Pbam and Pnnm are determined. A torsion angle of 27° is predicted in PPP chains and 17° in the crystals. The dielectric tensor and the absorption coefficient are calculated. We find very good agreement with experimental data, indicating that the excitations are extended band states. The interchain coupling leads to energy band splittings of the order of 0.5 eV. It is shown that the energy gap can be varied over a wide energy range by relatively small structural modifications.