THEORETICAL AND EXPERIMENTAL INVESTIGATIONS OF THE NONLINEAR-OPTICAL PROPERTIES OF VANILLIN, POLYENOVANILLIN, AND BISVANILLIN DERIVATIVES

The potential interest of vanillin, isovanillin, thiovanillin, and isothiovanillin as starting units for the design of hyperpolarizable molecules such as polyenovanillin (Ar-(CP double bond CH)(n)-CH double bond O) and bisvanillin derivatives was investigated by a CNDO/S method using a specific program that allowed the calculation of all the components of the first-order molecular hyperpolarizability tenser beta in nonpolar solvents. The reliability of the calculations was assessed by comparison with EFISH measurements on model systems. Among the vanillin derivatives, thiovanillin, where the sulfur group is in para to the aldehyde group, showed the largest hyperpolarizability, beta. In polyenovanillins, the magnitude of beta was predicted to increase as m(2) (m being the number of unsaturated bonds between the donor substituents of the aromatic ring and the aldehyde group), without saturation effect on the hyperpolarizability density p = beta/V at least up to m = 9. The calculations indicated that the thiovanillyl group (and its S-alkylated derivatives) was equivalent to the p-(dimethylamino)phenyl group in its ability to enhance the hyperpolarizability of polyenic systems, a result which may have important practical consequences. In bisvanillin, for torsion angles between the benzene rings greater than 25 degrees, the magnitude of beta was calculated to be the vector addition of the contributions of the monomer units, while for smaller angles, approaching planarity (in the cisoid conformation), a significant enhancement due to the conjugation of the two benzene rings was evidenced. It was concluded from this analysis that polyenovanillin and bisvanillin derivatives represent an interesting new family of hyperpolarizable molecules for applications in nonlinear optics.