Structural factors of amphiphilic calix[6]biscrowns affecting their vesicle–nanotube transitions in self-assembly

Morphological transitions of self-assemblies of organic amphiphiles responsive to environmental stimuli have drawn much attention. However, little has been investigated regarding the effects of structural factors of calixarenes on their self-assembly behavior. In this paper, we studied the self-assembly of several amphiphilic calix[6]biscrowns with emphasis on morphological transitions of the assemblies with changes in medium polarity and the key structural factors of the calix[6]biscrowns affecting the transition. It was found that when the medium polarity increased, the calix[6]biscrowns with amide linkages between the backbone and the alkyl tails performed vesicle–nanotube transitions, while the calix[6]biscrowns without such linkages showed a decrease in size of the vesicles only. Further studies, including X-ray crystallography, FT-IR and Langmuir–Blodgett film, indicated that the hydrogen bonding between the amide linkages is responsible for this vesicle-to-nanotube transition. The effect of the nature of the end groups of the tails has also been studied.