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Synthesis and Characterization of Polyfunctional Polyhedral Silsesquioxane Cages.

By Santy Sulaiman


Recent studies on octameric polyhedral silsesquioxanes, (RSiO1.5)8, indicate that the silsesquioxane cage is not just a passive component but appears to be involved in electron delocalization with conjugated organic tethers in the excited state. This dissertation presents the synthesis and characterization of (RSiO1.5)8 molecules with unique photo-physical properties that provide support for the existence of conjugation that involves the (RSiO1.5)8 cage. The dissertation first discusses the elaboration of octavinylsilsesquioxane via cross-metathesis to form styrenyl-functionalized octasilsesquioxane molecules. Subsequent Heck coupling reactions of p-bromostyrenyl derivative provides vinylstilbene-functionalized octasilsesquioxane. The amino derivative, NH2VinylStilbeneOS, show highly red-shifted emission spectrum (100 nm from the simple organic analog p-vinylstilbene) and high two-photon absorption (TPA) cross-section value (100 GM/moiety), indicating charge-transfer processes involving the silsesquioxane cage as the electron acceptor. The unique photophysical properties of polyfunctional luminescent cubic silsesquiox-anes synthesized from ortho-8-, (2,5)-16-, and 24-brominated octaphenylsilsesquioxane (OPS) via Heck coupling show how the steric interactions of the organic tethers at the silsesquioxane cage corner affect conjugation with the silsesquioxane cage. Furthermore, the high TPA cross-section (10 GM/moiety) and photoluminescence quantum yield (20%) of OPS functionalized with 24 acetoxystyrenyl groups suggest that the existence excited states in these molecules with similar energies and decay rates: normal radiative p-p* transition and charge transfer involving the silsesquioxane cage. The fluoride ion-catalyzed rearrangement reactions of cage and polymeric silses-quioxanes provide a convenient route to a mixture of deca- and dodecameric silsesquioxane molecules in high yields, giving us the opportunity to investigate the effect of silsesquioxane cage geometry on their photophysical properties. The ability to recycle polymeric silsesquioxane resins, byproducts from cubic silsesquioxane syntheses, into useful cage silsesquioxane molecules adds another advantage. Lastly, we present the synthesis of octa(aminophenyl)silsesquioxane-based epoxy resins with coefficient of thermal expansion (CTE) as low as 25oC/ppm without ceramic fillers. The CTEs of these resins can be tailored over an order of magnitude by choosing epoxy crosslinking agents having different flexibilities

Topics: Silsesquioxane, Hybrid Materials
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