Novel Syntheses, Functionalization, and Applications of Octa-, Deca- and Dodecasilsesquioxanes.

Abstract

The construction of materials nanometer-by-nanometer in principle leads to the controlled design of a variety of materials with well-defined nanometer-sized architectures and novel yet predictable behaviors. Polyhedral silsesquioxanes of the formula (RSiO1.5)n, where n = 8, 10, or 12 and R is an organic functionality, represent “ideal” nanometer-sized building blocks that allow for subsequent and selective chemical modification to provide a wide variety of derivatives. This permits the specific assembly of these molecular components into larger, well-defined structures with tailorable properties. This dissertation is dedicated to the syntheses, functionalization, and applications of octa-, deca-, and dodecasilsesquioxanes. The objectives of this work were to develop simple, effective routes to nanoscale composite precursors based on silsesquioxanes with tunable properties for use in a variety of applications. These properties were readily achieved by direct chemical modification of the organic periphery. Our investigations demonstrate that octasilsesquioxane-based nanocomposites can be tailored to exhibit barrier properties with very low permeability to oxygen or employed as high temperature, thermal cross-linking agents and/or potential platforms to supramolecular structures. The use of incompletely condensed, cyclic silsesquioxane tetramers as possible precursors to fully condensed two-faced “Janus” octamers was also explored. Finally, we report the novel fluoride-mediated synthesis of functionalized deca- and dodecameric silsesquioxane cages from random-structured and generally “useless” polymeric silsesquioxane precursors. Statistical control of the numbers and types of moieties on the cages is achieved simply by altering the ratio of starting materials. The utility of these types of reactions is demonstrated in the modification of vinylxPh10-x T10 and vinylxPh12-x T12 cages (x~2) with 4-bromostyrene using simple metathesis chemistry. Subsequent Heck coupling with other vinyl cages leads to “string of beads” silsesquioxane oligomers joined by conjugated organic tethers. These materials exhibit uncharacteristically pronounced red-shifted emissions (≈120 nm) suggestive of extensively conjugated compounds. Such behavior may be a direct result of electronic interaction of the organic groups in the periphery with a predicted, spherical LUMO located within the cage cores. If this is indeed the case, functionalized silsesquioxanes could represent an exciting new class of 3-D semiconducting materials.