Conjugated heterocyclic copolymers: Correlation between electronic structures and conducting properties

The theoretical electronic characteristics of two conjugated regular copolymers, poly(1,4-phenylene-2′,5′-thienylene) (PT 2-5) and poly(1,4-phenylene-2′,6′-pyridinediyl) (PP 2-6), were determined using the valence effective Hamiltonian (VEH) method. The VEH method allows one to calculate ionization potential, bandgap, bandwidth and electron affinity in relation to the molecular composition and geometry of the polymeric repeating unit. The crystalline organization of the two copolymers was studied by X-ray diffraction analysis and compared to that of the well-ordered corresponding homopolymer, polyparaphenylene. The d.c. conductivity properties of the products chemically doped with iodine or arsenic pentafluoride were then correlated with the electronic and crystalline structural data. Good overall agreement was found between the conductivity levels upon doping and the electronic properties from VEH calculations. In addition, the conductivity seemed to be partly governed by the crystallinity ratio of the polymeric system when the less oxidizing iodine dopant was used. In this case, the lower the crystallinity ratio, the higher the concentration of iodine required to produce a conductivity enhancement.