HARMONIC RAYLEIGH-SCATTERING FROM NONLINEAR OCTUPOLAR MOLECULAR MEDIA - THE CASE OF CRYSTAL VIOLET

Experimental evidence is provided to supplement earlier molecular engineering propositions as to the high nonlinearities of octupolar origin. The absence of dipole moment in such structures precludes the utilization of electric-field-induced second-harmonic generation (EFISH) in solution: harmonic light scattering (HLS), which does not require dipolar electric field poling, is shown to provide quantitative access to the quadratic nonlinearity of octupolar nonlinear molecules. Crystal violet, a trigonal conjugated cationic dye with electronic charge transfer from peripheral dimethylamino donor groups to an electron deficient sp2 hybridized central carbon atom, is chosen to illustrate the relevance of HLS in this context as well as provide an experimental estimate of the beta magnitude of a prototypical octupolar system. A resonantly enhanced beta value of 580 x 10(-30) esu is found at 1.064 mum, in acetone solution (lambda(max) = 590 nm). Due care is taken of solvent calibration while a detailed account of symmetry considerations and tensorial averaging in solution are provided for various molecular point groups of relevance (D3, D3h, C2v and T(d)). It is shown that besides the exploration of the new family of octupolar nonlinear systems, the HLS experiment should be also used to revisit the more traditional dipolar molecular systems: it complements EFISH experiments in providing a measure of the anisotropy of molecular quadratic hyperpolarizabilities.