Precise synthesis of nanobuilding blocks with accurately positioned functional groups presents a daunting challenge. Herein, a practical synthesis and thorough characterization of a series of T8 -polyhedral oligomeric silsesquioxane (POSS) di- and triadducts is reported. Upon addition of triflic acid across the double bonds in octavinylPOSS (V8 T8 ) followed by hydrolysis, the cubic symmetry of the T8 -POSS cage (Oh ) is broken into C2v (ortho-), C2v (meta-), and D3d (para-) for diadducts and further to Cs (oom-), Cs (omp-), and C3v (mmm-) for triadducts in a stochastic fashion. Their structures and regioconfigurations have been unambiguously demonstrated by (1) H, (13) C, and (29) Si NMR spectroscopy, as well as MALDI-TOF mass spectrometry. The assignment of the diadducts was further corroborated by converting each individual diadduct into triadduct(s), the structure of which is controlled by the symmetry of the precursor. Except for the C3v triadduct, they can all be prepared in synthetically useful quantities. The presence of two types of highly reactive and mutually orthogonal functional groups facilitates further modification into complex nanostructures and composite materials. These unique regioisomers provide a versatile platform for constructing giant molecules and Janus silsesquioxanes.
A series of regioisomeric Janus-type polyhedral oligomeric silsesquioxanes (POSS) with multiple, mixed surface functional groups has been successfully synthesized based on the cubic T8 -POSS framework in two consecutive thiol-ene reactions. The first thiol-ene addition of β-mercaptoethanol leads to a statistical mixture of multi-adducts where the regioisomers of bis-adducts (ortho, meta, and para isomers) can be isolated at synthetically useful quantities by flash column chromatography. Then, the second thiol-ene reaction readily installs a variety of functional groups onto the remaining vinyl groups of each regioisomer, providing an easy access to precisely defined, hetero-bifunctional Janus POSS nano-building blocks. The configurations and structures of the products have been unambiguously proven by using (1) H, (13) C, and (29) Si NMR spectroscopy as well as MALDI-TOF mass spectrometry.
Long and rigid objects formed by self‐assembly in water are useful as templates or for their rheological or biological properties. They are usually obtained by combining hydrogen bonding and strong hydrophobic interactions brought by an alkyl or alkylene chain. A simple access to well‐defined rod‐like assemblies in water is reported based on a penta‐urea sticker directly connected to poly(ethylene oxide) side chains. These assemblies are characterized by an average length of several hundreds of nanometers and a monodisperse radius (4.5 nm) resulting from a reduced lateral aggregation of the stickers.
We report the symmetry-dictated mesophase formation and phase diagram of a series of T 8 -polyhedral oligomeric silsesquioxane (POSS) derivatives bearing perfluoroalkyl chains and hydroxyl alkyl groups. The phase structures and phase behaviors of these molecular Janus particles were studied by DSC, POM, FT-IR, and WAXD experiments. It was found that introducing one hydroxyl alkyl group leads to a decreased crystal melting point, and incorporating two hydroxyl alkyl groups at different POSS positions causes the formation of lamellar liquid crystal mesophases with feature sizes and transition temperatures that depend on the symmetry of these regioisomers. Interestingly, installing only one substituent with two hydroxyl groups leads to monotropic phase transitions, where the mesophase appears only upon cooling from the isotropic melt within the narrow supercooling window. Phase diagrams were systematically constructed for these compounds and understood based on the fine influence of symmetry on their possible molecular packing scheme and their distinct hydrogen bonding patterns, as evidenced by their infrared spectra at different temperatures. These results are not only fundamental to understanding the effects of regioisomerism on giant molecule self-assembly but also important for the development of hybrid materials with tailored nanostructures.
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