Theoretical study of the electronic properties and crystal structure of poly(perinaphthalene): On the origin of high observed conductivities

Combining the quantum chemical valence effective Hamiltonian (VEH) technique and crystal packing methods, we have investigated the electronic properties and crystal structure of poly(perinaphthalene), PPN. The goal is to understand the origin of high conductivities observed for a pyrolysis product which contains PPN chains. VEH band structure calculations for an isolated PPN chain predict a band gap which is small, 0.44 eV. The highest occupied and lowest unoccupied (HOMO and LUMO) bands have a combined width of about 9 eV. Analogous calculations for poly(perianthracene), PPA, yield a band gap (2.26 eV) which is much larger than that calculated for PPN. The symmetries of the HOMO and LUMO bands provide an explanation for the band gap difference between PPN and PPA. Crystal packing analysis predicts the existence of two phases for PPN, a phase in which PPN molecules are arranged as overlapping dimer pairs and a nondimeric phase in which there is little intermolecular overlap. The observed diffraction patterns for PPN suggests the packing of overlapping molecules in a third type of structure, possibly stabilized by a degree of irregular reaction during synthesis. Quantum chemical calculations indicate that interchain overlap decreases the band gap of PPN, from 0.44 eV for the isolated chain to 0.29 eV.