Hydrogels Composed of Organic Amphiphiles and alpha-Cyclodextrin: Supramolecular Networks of Their Pseudorotaxanes in Aqueous Media

Mixtures of N-alkyl pyridinium compounds [py-N-(CH2)(n)OC6H3-3,5-(OMe)(2)](+)(X-) (1bCl: n=10, X=Cl; 1cBr: n=12, X=Br) and alpha-cyclodextrin (alpha-CD) form supramolecular hydrogels in aqueous media. The concentrations of the two components influences the sot gel transition temperature, which ranges from 7 to 67 degrees C. Washing the hydrogel with acetone or evaporation of water left the xerogel, and C-13 CP/MAS NMR measurements, powder X-ray diffraction (XRD), and scanning electron microscopy (SEM) revealed that the xerogel of 1bCl (or 1cBr) and alpha-CD was composed of pseudorotaxanes with high crystallinity. C-13{H-1} and H-1 NMR spectra of the gel revealed the detailed composition of the components. The gel from 1bCl and a-CD contains the corresponding [2]- and [3]pseudorotaxanes, [1b center dot(alpha-CD)]Br and [1b center dot.(alpha-CD)2]Br, while that from 1cBr and a-CD consists mainly of [3]pseudorotaxane [1c center dot(alpha-CD)(2))Br. 2D ROESY H-1 NMR measurements suggested intermolecular contact of 3,5-dimethoxyphenyl and pyridyl end groups of the axle component. The presence of the [3]pseudorotaxane is indispensable for gel formation. Thus, intermolecular interaction between the end groups of the axle component and that between alpha-CDs of the [3]pseudorotaxane contribute to formation of the network. The supramolecular gels were transformed into sols by adding denaturing agents such as urea, C6H3-1,3,5(OH)(3), and [py-N-nBu](+)(Cl-).