MODIFIED EXTENDED HUCKEL BAND CALCULATIONS ON CONJUGATED POLYMERS

In order to more accurately predict band gaps, corresponding to pi-pi* transitions of one-dimensional conducting polymers, the formula for the off-diagonal elements, H(ij)alpha-beta in the extended Huckel (EH) band calculation method was modified according to the form H(ij)alpha-beta = K1 (H(ii)alpha-alpha + H(jj)beta-beta) exp (-K2R(alpha-beta))S(ij)alpha-beta. Parametrizations for the constants K1 and K2 were performed so as to yield reasonable band gaps for the pure hydrocarbon polymers transpolyacetylene, poly(para-phenylene), and poly(phenylene vinylene). Since there is a large difference in bond alternations along polymeric chains between ab initio and modified neglect of diatomic overlap optimized geometries, especially for heterocyclic polymers, the valence orbital exponents of oxygen, nitrogen, and sulfur were separately adjusted, depending on the chosen geometry, to reproduce the band gaps of polyfuran, polypyrrole, and polythiophene. It is found that geometrical relaxations in the presence of heteroatoms strongly affect the C1-C4 interactions as well as bond alternations, which in turn affect the band gap. Modified EH band calculations were performed for various polymers. The predicted band gaps had average errors of ca. 10% (less than 0.3 eV) compared to the experimental values, and the method produced band structures consistent with electron-energy-loss spectroscopic observations.