Physical origin of the third-order nonlinear optical properties of polyenes

Abstract

The results of an experimental study of the third-order nonlinearity and second-order hyperpolarizabilities for several π-conjugated polyenes are given. In this paper we present also a theoretical analysis of the second-order hyperpolarizabilities

of polyenes modified. Theoretical calculations of the

were carried out at the restricted Hartree–Fock level using the semiempirical Austin model 1 and parametric method 3 (PM3) quantum-chemical self consistent field–molecular-orbital approaches within the HyperChem 7.0 program package. Calculations based on the PM3-type semiempirical molecular-orbital approach have been performed to investigate the energy and structural stabilities of polyenes. We have calculated electronic charge-density distributions of these molecules. Calculations have unambiguously shown an essential influence of modifications of the double bonds and distribution of electronic charge density. Moreover, we have observed a substantial change in the charge-density gradients under the influence of aromatic ring modifications. Changes in the charge-density gradients leads to a strong non-uniformity of the space electronic charge distribution. In the performed calculation we predict that the third-order nonlinearity and second-order hyperpolarizabilities increase with increasing elongation of polyene chain. We found that the theoretical calculations give a true tendency for changes in the corresponding susceptibilities; however, the values are different because of the underestimated absolute energy gap values obtained within the semiempirical methods.

Acknowledgement

This paper was partly supported by the Komitet Badan Naukowych (project 2PO3B 101 24).