Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 9, Issue 7, Pages 1594-1599Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.200390183
Keywords
bridged silsequioxanes; hybrid silica; nanstructures; self-assembly; sol-gel
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A new approach for the morphological control of bridged silsesquioxanes has been achieved by the hydrolysis of silylated organic molecules bearing urea groups. The urea groups are responsible for the auto-association of the molecules through intermolecular hydrogen-bonding interactions. The self-assembly leads to supramolecular architectures that have the ability to direct the organization of hybrid silicas under controlled hydrolysis. The hydrolysis of the chiral diureido derivatives of trans- (1,2) -diaminocyclohexane 1 under basic conditions has been examined. The solid-state NMR spectra (Si-29 and C-13) showed the hybrid nature of these materials with wholly preserved Si-C covalent bonds throughout the silicate network. Hybrid silicas with hollow tubular morphologies were obtained by the hydrolysis of the enantiomerically pure compounds, (R,R)-1 or (S,S)-1, whereas the corresponding racemic mixture, rac-1, led to a hybrid with ball-like structures. The tubular shape is likely to result from a combination of two phenomena: the auto-association abilities and a self-templating structuration of the hybrid materials by the organic crystalline precursor. Electronic microscopy techniques (SEM and TEM) gave evidence for the self-templating pathway. The formation of the ball-like structures occurs through a usual nucleation growth phenomenon owing to a higher solubility of the corresponding crystals in the same medium.
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