Sulfonato and Phosphonatocalix[n]arenes in Self-Assembly

Crystallization of p-sulfonatocalixarene and p-phosphonatocalixarene with a diversity of components (metal ions and/or organic molecules) can afford self-assembled arrays that contain unique packing motifs, albeit with limited degrees of predictability on the nature of the structures. The typical self-assembly of the common truncated cone-shaped sodium p-sulfonatocalixarenes is the bilayer arrangement with or without molecules embedded within their cavities. Despite p-sulfonatocalixarene molecules assembling into bilayers, perturbation of such arrangement is possible under specific conditions, including formation of curved structures as in spheroids and helical tubules. Attaching alkyl chains to the other rim of p-sulfonatocalixarenes renders them amphiphilic, and they can self-assemble into high-curvature aggregates such as vesicles, micelles, and other spheroidal entities. In comparison, the self-assembly of p-phosphonatocalixarene is limited owing to the challenge in high-yielding syntheses, and the complexities of having an additional ionizable proton on each upper-rim functional group when dealing with the conjugate acids. p-Phosphonatocalixarene forms nanorafts in water, ultimately crystallizing in a bilayer arrangement. Variation of the substituted alkyl chain on the p-phosphonatocalixarene lower rim affords different calixarene conformations and packing motif.