Novel Silica Structures Which Are Prepared by Transcription of Various Superstructures Formed in Organogels

Abstract

Three cholesterol-based gelators were synthesized which have a monobenzo-18-crown-6 (2), monoaza-18-crown-6 (3), and 1,10-diaza-18-crown-6 (4), respectively. These gelators could gelatinize 8 of 14 organic solvents tested herein, indicating that they possess a versatile gelation ability. Scanning electron microscopy observations of these xerogels showed that in cyclohexane 2, 3, and 4 assemble into a fibrous network structure, a curved lamellar structure, and a cylindrical tubular structure, respectively. Sol−gel polymerization of tetraethoxysilane was carried out in these gel systems. The silica obtained from 2 in the absence of metal salt had a conventional granular structure whereas that in the presence of KClO4 had a hollow fiber structure featuring the rough surface and the thick tube wall. This structure is created by the adsorption of anion-charged silica particles onto the cation-charged organogel fibers. On the other hand, the silica obtained from 3 and 4 had a hollow fiber structure featuring the smooth surface and the thin tube wall both in the absence and the presence of metal salt. In the absence of metal salt, the cationic charge generated by protonation of azacrown ethers plays a crucial role in the creation of such a hollow fiber structure. In the presence of KClO4 or CsClO4, sol−gel polymerization resulted in the tubular silica with a multilayer structure like a roll of paper. The findings suggest that sol−gel polymerization proceeds along the surface of the curved lamellar surface of 3 or 4 and the silica eventually grows up as a tubular structure. These results indicate that various novel silica structures can be prepared by transcription using various superstructures in organogels as a template.