New diazacrown-appended cholesterol gelators 1 and 2 were synthesized, and their gelation ability was evaluated in organic solvents. Very surprisingly, 1 + acetic acid gel results in spherical vesicles with two distinct sizes 200 and 2500 nm in diameter. In particular, the smaller vesicles are linked linearly, which is considered to serve as a driving-force for gelation. In contrast, 2 has a multilayered tubular structure. To characterize their aggregation modes in the gel phase, the organogels were observed by CD spectroscopy. The CD spectrum of 1 + acetic acid gel exhibits a negative sign for the first Cotton effect, indicating that the dipole moments in the gelator aggregate orient into an anticlockwise direction. On the other hand, 2 exhibits a positive sign for the first Cotton effect, indicating that they orient into a clockwise direction. The results indicate that the aggregate of 2 is stabilized by intermolecular cholesterol-cholesterol and azobenzene-azobenzene interactions, whereas the CD sign from the aggregate of 1 is indicative of an intramolecular azobenzene-azobenzene interaction. The spherical vesicle structures of organogel 1 were successfully transcribed into silica structures by the sol-gel polymerization of tetraethoxysilane (TEOS) in the gel phase. The TEM observation established that the wall of the spherical silica obtained in the acidic conditions consists of the multilayered vesicle structure. On the other hand, addition of Pd(NO3)2 changed the silica structure into fluffy globules with ~6000 nm in diameter. The EPMA observation established that Pd(II) ions are densely deposited on the surface of this globular silica. Hence, this process is useful as a new method to create metal catalytic sites on the silica support. These results indicate that the spherical multilayered structure of the organogel can be precisely transcribed into the silica structure. We thus believe that the sol-gel polymerization using molecular assembly templates strongly built in the organogel phase is a new strategy to create superstructured silica materials.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry