Why do the [PhSiO(1.5)](8,10,12) cages self-brominate primarily in the ortho position? Modeling reveals a strong cage influence on the mechanism.

(PhSiO1.5)8,10,12 cages are bulky, electron withdrawing like CF3; yet self-brominate (60 °C), favoring ortho substitution: PhT8 (≈85%), PhT10 (≈75%) and PhT12 (60%). First-principles calculations suggest bromination initiates when Br2 is "trapped" via H-bonding to ortho-H's, followed by polarization via strong interactions with cage faces, possibly cage LUMOs.

Nano building blocks via iodination of [PhSiO1.5]n, forming [p-I-C6H4SiO1.5]n (n = 8, 10, 12), and a new route to high-surface-area, thermally stable, microporous materials via thermal elimination of I2.

We describe the synthesis and characterization of the homologous p-iodophenylsilsesquioxanes (SQs) [p-I-C(6)H(4)SiO(1.5)](n) (n = 8, 10, 12) via ICl-promoted iodination (-40 to -60 degrees C) with overall yields of 80-90% and > 95% para selectivity following recrystallization. Characterization by NMR, FTIR, TGA, and single-crystal X-ray diffraction are reported and compared to data previously published for I(8)OPS. Coincidentally, we report a new synthesis of the elusive pentagonal decaphenyl SQ (dPS) [C(6)H(4)SiO(1.5)](10) and its characterization by NMR and single-crystal X-ray studies. These unique macromolecules possess equivalent chemical functionality but varying symmetries (cubic, pentagonal, and D(2d) dodecahedral), offering the potential to develop homologous series of functionalized star and dendrimer compounds with quite different core geometries and thereby providing the potential to greatly vary structure-property relationships in derivative compounds and nanocomposites made therefrom. We find that all three compounds decompose on heating to approximately 400 degrees C/N(2) with loss of I(2) to form robust, microporous materials with BET surface areas of 500-700 m(2)/g, pore volumes of 0.25-0.31 cm(3)/g, average pore widths of 8 A, and oxidative stabilities > or = 500 degrees C and with solid-phase morphologies varying from crystalline to mostly amorphous, as indicated by powder XRD and SEM studies. These latter findings point to important symmetry effects relating directly to packing in the crystalline phase prior to thermolysis.

Synthesis and photophysical properties of stilbeneoctasilsesquioxanes. Emission behavior coupled with theoretical modeling studies suggest a 3-D excited state involving the silica core.

A set of stilbene-substituted octasilicates [p-RStil(x)Ph(8-x)SiO(1.5)](8) (R = H, Me, MeO, Cl, NMe(2) and x = 5.3-8) and [o-MeStilSiO(1.5)](8) were prepared. Model compounds were also prepared including the corner and half cages: [p-MeStilSi(OEt)(3)], [p-Me(2)NStilSi(OSiMe(3))(3)], and [p-Me(2)NStilSi(O)(OSiMe)](4). These compounds were characterized by MALDI-TOF, TGA, FTIR, and (1)H NMR techniques. Their photophysical properties were characterized by UV-vis, two-photon absorption, and cathodoluminescence spectroscopy (on solid powders), including studies on the effects of solvent polarity and changes in concentration. These molecules are typically soluble, easily purified, and robust, showing T(d(5%)) > 400 degrees C in air. The full and partial cages all show UV-vis absorption spectra (in THF) identical to the spectrum of trans-stilbene, except for [o-MeStilSiO(1.5)](8), which exhibits an absorption spectrum blue-shifted from trans-stilbene. However, the partial cages show emissions that are red-shifted by approximately 20 nm, as found for stilbene-siloxane macrocycles, suggesting some interaction of the silicon center(s) with the stilbene pi* orbital in both the corner and half cages. In contrast, the emission spectra of the full cages show red-shifts of 60-100 nm. These large red-shifts are supported by density functional theoretical calculations and proposed to result from interactions of the stilbene pi* orbitals with a LUMO centered within the cage that has 4A(1) symmetry and involves contributions from all Si and oxygen atoms and the organic substituents. Given that this LUMO has 3-D symmetry, it appears that all of the stilbene units interact in the excited state, consistent with theoretical results, which show an increased red-shift with an increase in the functionalization of a single corner to functionalization of all eight corners with stilbene. In the case of the Me(2)N- derivatives, this interaction is primarily a charge-transfer interaction, as witnessed by the influence of solvent polarity on the emission behavior. More importantly, the two-photon absorption behavior is 2-3 times greater on a per p-Me(2)Nstilbene basis for the full cage than for the corner or half cages. Similar observations were made for p-NH(2)stilbenevinyl(8)OS cages, where the greater conjugation lengths led to even greater red-shifts (120 nm) and two-photon absorption cross sections. Cathodoluminescence studies done on [p-MeStilSiO(1.5)](8) or [p-MeStilOS](8) powders exhibit essentially the same emissions as seen in solution at high dilution. Given that only the emissions are greatly red-shifted in these molecules, whereas the ground-state UV-vis absorptions are not changed from trans-stilbene, except for the ortho derivative, which is blue-shifted 10 nm. It appears that the interactions are only in the excited state. Theoretical results show that the HOMO and LUMO states are always the pi and pi* states on the stilbene, which show very weak shifts with increasing degrees of functionalization, consistent with the small changes in the UV-vis spectra. The band gap between the lowest unoccupied 4a1 symmetry core state localized inside the silsesquioxane cage and the highest occupied state (pi state on stilbene), however, is markedly decreased as the number of stilbene functional groups is increased. This is consistent with the significant red-shifts in the emission spectra. The results suggest that the emission occurs from the 4a1 state localized on the cage. Moreover, for the compounds [p-RStil(6-7)Ph(2-1)OS](8), the emissions are blue-shifted compared to those of the fully substituted compounds, suggesting the molecular symmetry is reduced (from cubic), thereby reducing the potential for 3-D delocalization and raising the energy of the LUMO. The implications are that these octafunctional molecules exhibit some form of 3-D interaction in the excited state that might permit their use as molecular transistors as well as for energy collection and dispersion as molecular antennas, for example, and for nonlinear optical applications.

Fluoride rearrangement reactions of polyphenyl- and polyvinylsilsesquioxanes as a facile route to mixed functional phenyl, vinyl T10 and T12 silsesquioxanes.

Polyphenylsilsesquioxane [PhSiO(1.5)](n) (PPS) and polyvinylsilsesquioxane [vinylSiO(1.5)](n) (PVS) are polymeric byproducts of the syntheses of the related T(8) octamers [PhSiO(1.5)](8) and [vinylSiO(1.5)](8). Here we demonstrate that random-structured PPS and PVS rearrange in the presence of catalytic amounts of Bu(4)N(+)F(-) in THF to form mixed-functionality polyhedral T(10) and T(12) silsesquioxane (SQ) cages in 80-90% yields. Through control of the initial ratio of starting materials, we can statistically tailor the average values for x for the vinyl(x)Ph(10-x)T(10) and vinyl(x)Ph(12-x)T(12) products. Metathetical coupling of x approximately = 2 vinyl cages with 4-bromostyrene produces SQs with an average of two 4-bromostyrenyl substituents. These products can be reacted via Heck coupling with vinylSi(OEt)(3) to produce SQs with vinylSi(OEt)(3) end-caps. Alternately, Heck coupling with the originally produced x approximately = 2 vinyl SQs leads to "beads on a chain" SQ oligomers joined by conjugated organic tethers. The functionalized T(10) and T(12) cages, metathesis, and Heck compounds were characterized by standard analytical methods (MALDI-TOF MS, (1)H and (13)C NMR spectroscopy, TGA, and GPC). MALDI confirms the elaboration of the cages after each synthetic step, and GPC verifies the presence of higher molecular weight SQ oligomers. TGA shows that all of these compounds are thermally stable in air (>300 degrees C). The UV-vis absorption and emission behavior of the Heck oligomers reveals exceptional red-shifts (> or = 60 nm) compared to the vinylSi(OEt)(3) end-capped model compounds, suggesting electronic interactions through the SQ silica cores. Such phenomena may imply 3-D conjugation through the cores themselves.

High-throughput screening of nanoparticle catalysts made by flame spray pyrolysis as hydrocarbon/NO oxidation catalysts.

We describe here the use of liquid-feed flame spray pyrolysis (LF-FSP) to produce high surface area, nonporous, mixed-metal oxide nanopowders that were subsequently subjected to high-throughput screening to assess a set of materials for deNO(x) catalysis and hydrocarbon combustion. We were able to easily screen some 40 LF-FSP produced materials. LF-FSP produces nanopowders that very often consist of kinetic rather than thermodynamic phases. Such materials are difficult to access or are completely inaccessible via traditional catalyst preparation methods. Indeed, our studies identified a set of Ce(1-x)Zr(x)O(2) and Al(2)O(3)-Ce(1-x)Zr(x)O(2) nanopowders that offer surprisingly good activities for both NO(x) reduction and propane/propene oxidation both in high-throughput screening and in continuous flow catalytic studies. All of these catalysts offer activities comparable to traditional Pt/Al(2)O(3) catalysts but without Pt. Thus, although Pt-free, they are quite active for several extremely important emission control reactions, especially considering that these are only first generation materials. Indeed, efforts to dope the active catalysts with Pt actually led to lower catalytic activities. Thus the potential exists to completely change the materials used in emission control devices, especially for high-temperature reactions as these materials have already been exposed to 1500 degrees C; however, much research must be done before this potential is verified.

Nano-alpha-Al2O3 by liquid-feed flame spray pyrolysis.

Nanometre-sized particles of transition (t)-aluminas are important for the fabrication of high-quality alumina ceramics. Multiple tons are produced each year using a variety of gas-phase processes. The nanoparticles produced by these methods consist mainly of the undesired delta phase with some gamma- and theta-Al(2)O(3). Nano-t-aluminas should provide access to dense nano/submicrometre-grained alpha-Al(2)O(3) shapes offering significant advantages over micrometre-grained shapes. Unfortunately, polymorphism coupled with the high activation energy for nucleating alpha-Al(2)O(3) greatly impedes efforts to process dense alpha-Al(2)O(3) with controlled grain sizes, especially for submicrometre materials. Typically alpha-Al(2)O(3) nucleation within t-aluminas is sporadic rather than uniform, leading to exaggerated grain growth and vermicular microstructures without full densification (5). Thus, production of quantities of nano-alpha-Al(2)O(3) from multiple nano-t-aluminas for seeding or direct processing of alpha-Al(2)O(3) monoliths could greatly change how alpha-Al(2)O(3) components are processed. We report here that liquid-feed flame spray pyrolysis of nano-t-aluminas converts them to dispersible 30-80 nm alpha-Al(2)O(3) powders (50-85% phase transformed). Surprisingly, the powder surfaces are fully dehydrated. These powders pressureless sinter to more than 99.5% dense alpha-Al(2)O(3) with final grain sizes < or =500 nm without sintering aids.

Continuous-wave ultraviolet laser action in strongly scattering Nd-doped alumina.

We report electrically pumped, cw laser action near 405 nm from Nd(3+) -doped delta -alumina nanopowders. To our knowledge, this is the first report of stimulated emission from the high-lying F(2) -excited states, achieved through feedback from strong elastic scattering of light over transport path lengths shorter than half a wavelength.

Organic/inorganic hybrid composites from cubic silsesquioxanes.

A new class of epoxy nanocomposites with completely defined organic/inorganic phases was prepared by reacting octakis(glycidyldimethylsiloxy)octasilsesquioxane [(glydicylMe(2)SiOSiO(1.5))(8)] (OG) with diaminodiphenylmethane (DDM) at various compositional ratios. The effects of reaction curing conditions on nanostructural organization and mechanical properties were explored. A commercial epoxy resin based on the diglycidyl ether of bisphenol A (DGEBA) was used as a reference material throughout these studies. FTIR was used to follow the curing process and to demonstrate that the silsesquioxane structure is preserved during processing. OG/DDM composites possess comparable tensile moduli (E) and fracture toughness (K(IC)) to, and better thermal stabilities than, DGEBA/DDM cured under similar conditions. Dynamic mechanical analysis and model reaction studies suggest that the maximum cross-link density is obtained at N = 0.5 (NH(2):epoxy groups = 0.5) whereas the mechanical properties are maximized at N = 1.0. Digestion of the inorganic core with HF followed by GPC analysis of the resulting organic tether fragments when combined with the model reaction studies confirms that, at N = 0.5, each organic tether connects four cubes, while, at N = 1.0, linear tethers connecting two cubes dominate the network structure. Thus, well-defined nanocomposites with controlled variation of the organic tether architecture can be made and their properties assessed.