Characterization and mechanism study of micrometer-sized secondary assembly of beta-cyclodextrin

We herein report a beta-cyclodextrin-based secondary assembly (beta-CD SA) obtained from an aqueous solution. It was found that the addition of a very small amount of organic molecule 2-phenyl-5-(4-diphenylyl) 1,3,4-oxadiazole (PBD) into an aqueous solution of 10 mM beta-CD led to the formation of a micrometer-sized rodlike SA, which made the mixture turbid immediately. After careful characterization, the structure and the formation mechanism of the beta-CD SA were suggested. PBD first induces beta-CDs to form rigid nanotubes through head-to-head or tail-to-tail routes. Using the solid nanotubes as recrystallization centers, other beta-CDs assembled to channel in the c axis direction and hexagonally aligned in the b axis direction, leading to the formation of a beta-CD SA. In the beta-CD SA, most of the beta-CDs were not occupied by PBD. In the course of formation, intermolecular hydrogen-bonding plays a prominent role. The results reported herein would be helpful in constructing cyclodextrin-based architectures in water.