Design strategies versus limiting theory for engineering large second-order nonlinear optical polarizabilities in charged organic molecules

The combination of two design strategies for engineering large second-order nonlinear optical polarizabilities is critically analyzed and contrasted with a recent limiting theory for nonlinear molecular polarizabilities. Elongation of the conjugation path length and N-arylation in stilbazolium chromophores both are established engineering guidelines to enhance molecular optical nonlinearities. These two strategies have now been combined, resulting in extended and N-aryl substituted "stilbazolium" chromophores. The second-order nonlinear molecular polarizabilities, or first hyperpolarizabilities β, of these ionic compounds were measured by hyper-Rayleigh scattering with suppression of the multiphoton fluorescence contributions. Static first hyperpolarizabilities β0 were estimated by using the two-level model. The resulting large β0 values are compared with theoretical limiting values obtained from recently derived generally applicable sum-rules and with experimental values for neutral dipolar chromophores. It is concluded that the two design strategies are independently valid, and that their combination will therefore provide the maximum attainable β0 in an ionic compound. Crystals containing optimized ionic chromophores can hence be expected to possess electrooptic coefficients far greater than those obtained so far from either ionic crystals of stilbazolium dyes or from neutral dipolar guest chromophores in polymer matrixes.