Influence of symmetry of the unit cell on electronic properties of conjugated heteropolyarylenes

Model calculations of the band structures of polyparaphenylene (PPP) and polypyridine (PP) have been carried out with different symmetries for the unit cell. It has been shown that the inclusion of a heteroatom in PPP partially reduces the band gap and the dispersion of the valence band formed by the π levels of the pyridine rings (in the case of angularly coordinated 2,2′-PP, the valence band is populated by the lone pairs of the nitrogen atoms). The steric influence of substituents on the electronic properties of polymers has been evaluated. Drastic narrowing of the forbidden gap has been discovered upon the transition of PP to its quinoidal structure, and it may attest to the possibility of the realization of a bipolar mechanism of conduction at high degrees of doping of such polymers.