Excitation shifts of parallel conjugated polymers due to π-electron dispersion forces

Abstract

Molecular exciton methods are extended to obtain excited-state shifts of parallel conjugated strands arising from interchain Coulomb interactions. A method is developed to evaluate second-order corrections involving π-electron dispersion forces, with exact Pariser–Parr–Pople states of isolated segments as zeroth-order functions. Crystal shifts are estimated for polyacetylene (PA) at interchain separation d=3.9 Å; the two-photon excitation to the covalent 2 1Ag state is hardly shifted, but the one-photon excitation to 1 1Bu is red shifted by ∼0.30 eV and specific higher n 1Ag states are strongly red shifted by ∼1 eV. Their stabilization correlates with charge separation and ionicity in the excited state. Excitation shifts due to π-electron dispersion forces do not parallel changes in the static polarizability and show unusual long-range behavior of d−3 in the interchain separation.