| Organic electro-optic (EO) materials have received a great deal of interest as alternatives to traditional inorganic crystalline materials. Among the most successful of organic EO systems are dipolar charge-transfer chromophore guest-host systems. The molecular figure of merit for optical nonlinearity of EO chromophores is the first hyperpolarizability (beta). Hyper-Rayleigh scattering (HRS) is considered to be the most direct technique for measuring beta. In this work, HRS is used to examine how beta varies with molecular architecture in order to define "structure-property" relationships. The development of in-house femtosecond and nanosecond HRS experimental apparatuses is described, and fluorescence discrimination efforts are detailed, including the novel technique of shifted-excitation hyper-Rayleigh scattering. With careful choice of excitation wavelength to minimize resonant enhancement, HRS results are reported for a variety of chromophore architectures, including ferrocenyl-based chromophores, multiple-chromophore bundles, and zwitterionic/neutral-ground-state chromophore dimers. Phenomenological accounts are given of the dispersion of beta with excitation wavelength, demonstrating resonant enhancement of beta. Hyperpolarizability studies demonstrating the challenge of characterization within the context of resonant enhancement are detailed for chromophores with novel conjugated-bridge architectures, and a preliminary dispersion study of a tricyanopyrroline compound is reported. |
