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 t he 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 perfo rmed so as to yield reasonable band gaps for the pure hydrocarbon poly mers transpolyacetylene, poly(para-phenylene), and poly(phenylene viny lene). Since there is a large difference in bond alternations along po lymeric chains between ab initio and modified neglect of diatomic over lap optimized geometries, especially for heterocyclic polymers, the va lence orbital exponents of oxygen, nitrogen, and sulfur were separatel y adjusted, depending on the chosen geometry, to reproduce the band ga ps of polyfuran, polypyrrole, and polythiophene. It is found that geom etrical 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 variou s polymers. The predicted band gaps had average errors of ca. 10% (les s than 0.3 eV) compared to the experimental values, and the method pro duced band structures consistent with electron-energy-loss spectroscop ic observations.