Optical excitations of even and odd polyenes with molecular PPP correlations

Abstract

Exact low-lying states of Pariser-Parr-Pople (PPP) and Hubbard models with N≤12 sites are obtained by diagrammatic valence-bond (VB) methods. Comparison of finite chains and rings for regular Hubbard models with U=4|t| demonstrates the accuracy of N→∞ extrapolations. Standard molecular PPP parameters for alternating C---C bonds give an N→∞ absorption maximum at E_g=2.8±0.2 eV for gas-phase polyacetylene. Molecular PPP parameters in regular polyene radicals lead to positive and negative spin densities, with |ρ₋/ρ₊|~0.42 at N=50. slightly above the (CH)_x value. Correlations in interacting Heisenberg antiferromagnetic chains are shown to shift the midgap state and to produce long-range interactions among spin solitons. The qualitatively different electronic structure of even and odd polyenes is used to associate (CH)_x gap state with excitations of a static distribution of finite segments with molecular PPP correlations.