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 pi- conjugated polyenes are given. In this paper we present also a theoretical analysis of the second- order hyperpolarizabilities gamma(t(3))(ijkl) of polyenes modified. Theoretical calculations of the gamma(t(3))(ijkl) 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 modi. cations 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.