Theoretical aspects and design of conjugated polymers for nonlinear optics

Organic conjugated polymers present strong electron-phonon coupling; i.e., there is a close interplay between the geometric structure and the electronic structure of the material. An electronic process leads to important local modifications of the chain geometry, which in turn induce the appearance of localized electronic states in the gap. As a result, nonlinear charged excitations such as solitons, polarons, or bipolarons play a major role in the nonlinear optical processes. A comparison has been made between the rigid band models used to describe the hyperpolarizabilities of small polyenes and the models (such as the instanton concept) proposed for polyacetylene chains. The evolution of the soliton characteristics as a function of chain length has been calculated by combining ab initio techniques and semiempirical techniques including configuration interaction. Novel macromolecular architectures that could lead to high third-order nonlinear optical effects have been investigated.