Interchain dispersion and second hyperpolarizability of conjugated polymers

Abstract

π-electron dispersion forces between parallel conjugated strands are shown to alter χ(3) coefficients, even in the limit of nonoverlapping strands, through dipole processes involving virtual excitation and deexcitation of adjacent strands. The position and relative intensity of two- and three-photon resonances in the third-harmonic-generation (THG) spectrum depend both on the strength of interchain dispersion forces and on the lattice coordination z. The admixture of even-parity states above the one-photon gap Eg with biexcitons on adjacent strands shifts the two-photon resonance to lower energy and reduces its amplitude, while the amplitude of the three-photon resonance at Eg/3 increases with z due to new biexcitonic pathways. THG spectra based on exact solutions to Pariser–Parr–Pople (PPP) models of polyenes are reproduced by a dimer model for each strand. Interchain dispersion in the lattice of dimers leads to an exciton problem whose solution explicitly gives the dependence on interchain dispersion, intrachain correlations, and lattice coordination. Interchain dispersion in polyacetylene, with z=6 neighboring strands in van der Waals contact, accounts qualitatively for its THG spectrum, while two-photon processes in conjugated polymers whose backbones are separated by bulky side groups are in accord with single-strand PPP results.