Segment model for polyacetylene

Abstract

Electronic excitations of finite polyenes require explicit inclusion of electron-electron correlations in Pari-ser-Parr-Pople (PPP) models. Molecular parameters are retained for long (N≥100) polyene segments arising from cross links or defects in polyacetylene, (CH)_X. Noninteracting even and odd segments and their ions have different absorptions and stabilities, as shown for cell models with electron-hole symmetry. Odd segments (polyene radicals) are related to the paramagnetism and act as localized traps for charges in undoped or lightly-doped (CH)_X. Finite segments have long been considered for (CH)_X, for example in connection with Raman studies. We have emphasized here the differences between even and odd segments that persist for large N and have considered fixed distributions of noninteracting segments. Although complete e-e and e-ph analyses are still impractical, symmetry properties of cell models and molecular extrapolations delineate the main features. Transport via charges trapped on odd segments, optical excitations of even and odd segments and their ions, and magnetic properties all differ from previous infinite-strand models. We are pursuing more detailed applications of the segment model.