Linear Polyenes: The Interplay between Electronic Structure, Geometric Structure, and Nonlinear Optical Properties

Abstract

This contribution deals with theoretical investigations of the electronic and optical properties of two series of all-trans linear polyenes. In the first case, the focus is on unsubstituted polyenes and the geometry relaxation processes occurring in the first singlet, one-photon optically allowed excited state. Calculations are performed at various levels of sophistication, from a simple Su-Schrieffer-Heeger (Huckel-like) Hamiltonian up to Pariser-Parr-Pople single CI and Restricted Hartree-Fock ab initio Hamiltonians. It is found that explicit consideration of the electron-lattice coupling is essential in order to obtain a coherent evolution of the 1Bu state geometry relaxation, in going from short polyenes to long polyenes and polyacetylene. The relevance of our results in terms of the nonlinear optical properties of these compounds is pointed out. In the second case, the electronic structure and second-order polarizability, β, are calculated at the ab initio level for two series of novel push-pull polyene molecules, benzodithiapolyenals and dithiolylidenepolyenals. The benzodithiapolyenal molecules have recently been reported to present among the largest µ.β values ever measured. The theoretical results allow for an in-depth understanding of the properties of these systems.