Abstract

We have described how the first hyperpolarizability, β, varies as a function of ground-state polarization and bond length alternation, BLA.¹ Previously, our predictions of this relationship were derived by examining an prototypical push-pull polyene under the presence of static homogenous electric field.¹ In order to develop a more realistic understanding of how real molecules behave in solution, we have now examined both p-nitroaniline and the push-pull polyene 1,1-dicyano-6-dibutylamino-hexatriene at the Hartree Fock ab initio level on the basis of an expanded Self Consistent Reaction-Field approach to simulate solvents with dielectric constants up to 37.5. The calculations were carried out using both spherical and elliptical cavities and taking into account multipole interactions up to 2⁶ poles. Predictions were compared to Hyper-Rayleigh Scattering (HRS) results determined by Persoons et al. As had been shown earlier,² the second-order NLO response of 1,1-dicyano-6-dibutylamino-hexatriene exhibits a peak as a function of solvent polarity, which was confirmed by the HRS measurements. Interestingly, we found that this peaked behavior of β as a function of solvent dielectric constant was predicted only when an elliptical cavity and the up to l = 6 pole contributions were taken into account, but was not observed when the calculations employed either an elliptical cavity and a dipole (l = 1) approximation, or a spherical cavity with up to l = 6 pole contributions. These results suggest that it is possible to model the geometric and electronic structure of polarizable molecules using a Self Consistent Reaction-Field approach, but it is necessary to build into the calculations mechanisms to account for the subtleties of the molecular charge distributions, not taken into account within a simple dipole approximation.

© 1995 Optical Society of America