Received July 10, 2007

In Final Form: September 6, 2007

Abstract:

Guest-free -cyclodextrin self-assembly (-CD-SA) was successfully obtained through a simple treatment such as sonication of -CD in a specific solvent. From wide-angle X-ray diffraction (WAXD), it was found that the crystalline structure of -CD changed upon increasing the treatment time, resulting in -CD-SA in which the -CDs were closely packed in the vertical direction and hexagonally aligned in the horizontal direction (what is called as "channel structure"). In particular, these structures were developed only in tetrahydrofuran (THF) as a specific solvent. In addition, it was found by inclusion experiment and field-emission scanning electron microscopy (FE-SEM) that propionic acid was able to be included into the channel of -CD-SA and that -CD-SA had -CD bundles with a fibril-like shape, respectively. These results demonstrate that the -CD-SA consists of nanofibril-like -CD bundles with cylindrical nanopores open at least at one end, resulting from the dispersion of -CD molecules by sonication in THF and the subsequent re-formation of strong hydrogen bonding between the -CDs with the aid of THF (so-called "slow recrystallization"). Interestingly, it was observed from FE-SEM and nitrogen adsorption-desorption measurement that the -CD-SA had a wormhole-like mesopore with inkbottle shape (average desorption pore size = ca. 25 nm). This mesoporous structure was considered to be attributed to the formation of a mesoporous framework by the disordered aggregation of the nanofibril-like -CD bundles.

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