The objectives of this work are to demonstrate facile routes to 3-D star materials with octa-and hexadecafunctionality to provide new nanoconstruction tools for the synthesis of new types of stars, dendrimers, and hyperbranched molecules or for the assembly of novel nanocomposites. A further objective is to identify novel properties inherent in the resulting new compounds. Octavinylsilsesquioxane (OVS, [VinylSiO 1.5 ] 8 ) with perfect 3-D or cubic symmetry is elaborated through metathesis with substituted styrenes to produce a series of RStyrenylOS compounds. The p-BrStyrenylOS compound is then further reacted with other sets of p-substituted styrenes via Heck coupling to produce a set of R′VinylStilbeneOS compounds. The R′ ) NH 2 compound is then reacted with 3,5-dibromo or dinitrobenzoyl chloride to produce hexadecafunctional 3-D stars. These synthetic methods provide perfect single core and then core-shell 3-D stars including in the third generation branch points such that these molecules can be used for the synthesis of new dendrimers or hyperbranched molecules. Further, the first sets of materials are fully conjugated. Investigation of the UV-vis absorption, photoluminescence, and two-photon absorption properties of the R′VinylStilbeneOS compounds, especially where R′ ) NH 2 , reveals exceptional red-shifts (120 nm), charge-transfer behavior, and excellent two-photon absorption properties that may suggest that the silica core serves the role of electron acceptor in the system and interacts equally with all eight organic moieties. This observation may imply 3-D conjugation through the 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 chan...

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The availability of pure samples of o-Br 8 OPS, 2,5-Br 16 OPS, and Br 24 OPS provides a rare opportunity to synthesize sets of corresponding stilbene derivatives: o-RStyr 8 OPS, RStyr 16 OPS, and RStyr 24 OPS where R ¼ 4-methyl (Me), Boc-protected 4-amino (NBoc), or 4-acetoxy (Ace). These derivatives show unique UV-Vis absorption and photoluminescent behavior that points to interesting interactions between the organic tethers and the silsesquioxane cage. o-RStyr 8 OPS shows blue-shifts in the absorption spectra compared to p-MeStyr 8 OPS, suggesting that the stilbene groups sit over and interact with the face of the electrophilic silsesquioxane cage as is the case with the parent molecule, o-Br 8 OPS. The emission spectra of o-RStyr 8 OPS are similar to p-MeStyr 8 OPS indicating similar excited states involving the core LUMO. RStyr 16 OPS exhibits absorption and emission spectra as well as F PL similar to 1,4-distyrylbenzene, pointing to disruption in conjugation with the silsesquioxane cage because of steric interactions. RStyr 24 OPS offers absorption maxima that are blue-shifted and emission maxima that are red-shifted relative to RStyr 16 OPS. We speculate that RStyr 24 OPS is so sterically hindered that interactions with the cage face must occur. NBocStyr 24 OPS and AceStyr 24 OPS show moderate F PL and high two photon cross-section values, leading us to conclude that there are two excited states of nearly equivalent energy in these molecules with similar decay rates: a normal radiative p-p* transition and charge transfer involving the silsesquioxane cage. These same functional groups can be anticipated to offer much greater two photon absorption if different methods can be found for protecting the free amine from oxidation or replacing the acetoxy group (e.g. perhaps using alkyl or aryl groups).

Introduction. Cubic silsesquioxanes (see Figure 1) are unique molecules that combine three-dimensional cubic symmetry with single nanometer diameters and a core that is the smallest single crystal of silica. Symmetry places a functional group on each vertex in a different octant in Cartesian space providing the opportunity to form covalent bonds accordingly, such that the potential exists to construct materials in 1-, 2-, or 3-dimensions nanometer by nanometer. In principle, this permits manipulation of global properties by tailoring structures at nanometer length scales, allowing the finest control possible. It also provides access to materials with highly reproducible properties and the potential to predict and design them for specific applications. [1][2][3][4][5][6][7][8][9][10] Results and Discussion. We recently began exploring the chemistries and properties of epoxy resins and polyimides made with octaaminophenylsilsesquioxane, [NH 2 PhSiO 1.5 ] 8 , OAPS. [11][12][13][14] In early studies we demonstrated that global silsesquioxane nanocomposite properties can be tailored by controlling the structure of the organic tether linking cube vertices, at nanometer length scales. [15][16][17][18][19] We report here efforts to develop single-phase materials that offer control of the coefficients of thermal expansion (CTE) of silsesquioxane epoxy resins over an order of magnitude. Control of CTE is of considerable importance in multiple materials applications (e.g., coatings that offer resistance to abrasion, corrosion, photooxidation, hydrophobicity, staining, etc.) where the polymer coating is applied to glass, ceramic, or metal substrates with quite dissimilar CTEs. In such instances, thermal cycling often leads to loss of adhesion followed by coating failure via chemical and/or mechanical mechanisms. 20 CTE mismatches are also quite problematic in electronic applications, for example, in interlayer dielectrics and flip-chip underfills. 21 In the latter case, the underfill epoxy must match the CTEs of silicon-based ICs (CTEs of 2-3 µm/°C) with substrates (CTEs of 20-40 µm/°C) to ensure good thermal management. Current epoxy materials require silica fillers to adjust CTEs to g20 µm/°C. Such CTEs are intermediate between substrates and silicon to minimize fatigue at solder joints. These fillers raise resin viscosities to levels near 50 000 mPa‚s, making processing very difficult. Likewise, corrosionresistant epoxy resin coatings on Al alloys for aircraft bodies must minimize environmental corrosion and offer good abrasion resistance and curing at temperatures <50 °C but also have CTEs close to those of the alloys, typically 22-24 µm/°C. Such values were heretofore unknown for simple epoxy systems and especially for primer coats on aircraft fuselages that are typically DGEBA/DDM materials (60-70 µm/°C). 22 Epoxy resin thermosets studied here were produced from a series of epoxys (see Table 1 and Figure 2) formulated using OAPS as the curing agent. The formulations chosen were made according to our original model...

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New aminophenylsilsesquioxanes, dodecaaminophenylsilsesquioxane (NH2PhSiO1.5)12 (DAPS), and hexadecaaminooctaphenylsilsesquioxane ((NH2)2PhSiO1.5)8 (HDAPS), were prepared from dodecaphenylsilsesquioxane (PhSiO1.5)12 (DPS), and octaphenylsilsesquioxane (PhSiO1.5)8 (OPS), by nitration in fuming HNO3, followed by reduction with HCO2H/Et3N/Pd/C. The effects of the cage structure and numbers of NH2 groups on the chemical and physical properties of these compounds were compared with octaaminophenylsilsesquioxane (NH2PhSiO1.5)8 (OAPS), and analogous epoxy resin nanocomposites. The cage structure does not affect nitration patterns (primarily meta and para), but affects coefficients of thermal expansion (CTEs), likely because of overall crosslink densities and because of the epoxy tether structures joining cube vertices. CTEs as low as 25 ± 2 ppm°C–1 were found, perhaps the lowest reported for an unfilled epoxy resin. The thermal stabilities of synthesized HDAPS epoxy nanocomposites were 20–30°C lower than those made with OAPS. This result may be a consequence of less-complete curing due to gelation before stoichiometric curing, caused by the much higher crosslink densities in OAPS.