Nonlinear optical processes in conjugated polymers: configuration interaction description of linear polyenes and VEH/SOS evaluation of polyarylene vinylenes

We have used a Multi-Reference Determinant (MRD) Configuration Interaction (CI) approach, including double excitations within a Pariser-Parr-Pople Hamiltonian, to investigate linear polyenes containing from four to sixteen carbons. We have calculated the low-lying excited states, the two-photon absorption spectrum, and the Third Harmonic Generation response. We have analyzed the length dependence of the static third-order susceptibility, χ⁽³⁾: an indication of saturation behavior has been found. Focusing our attention on the two-photon resonance peak present in the free-electron laser THG measurements on polyacetylene, we conclude that the experimental data can be explained within a strongly correlated electron model. The Valence Effective Hamiltonian (VEH) method has been extended to calculate nonlinear optical coefficients and has been applied to investigate the static and dynamic third-order polarizabilities in poly(p-phenylene vinylene) and poly(thienylene vinylene). The relation between aromaticity and hyperpolarizability is discussed.