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

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. [ABSTRACT FROM AUTHOR]