|About the Book|
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 vinylxPh 12-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 (&ap-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.