Architecture-controlled "SMART" Calix[6]arene self-assemblies in aqueous solution

Self-assemblies of a calix[6]arene (1) functionalized at the small rim by three imidazolyl arms and at the large rim by three hydrophilic sulfonato groups have been studied in water. Transmission electron microscopy, atomic force microscopy, and in situ dynamic light scattering showed that 1 forms multilamellar vesicles at a concentration equal to or higher than 10(-4) M. At pH 7.8 and 10(-4) M, the multilamellar vesicles present a relatively large polydispersity (50-250 nm in diameter). However, after sonication unilamellar vesicles of much lower polydispersity and smaller size are obtained. The impact of the pH and the presence of Ag+ ions have also been investigated. Whereas increasing the pH led to the formation of giant vesicles (450 nm), monodisperse vesicules of 50 nm were obtained at a pH (6.5) that is only slightly higher than the pKa of the tris(imidazole) core of 1. Most interestingly, in the presence of silver ions, micelles (2.5 nm large) were obtained instead of vesicles. These observations are attributable to the imidazole core in 1 that is not only sensitive to the presence of protons but also can bind a silver cation. The resulting geometrical change in the monomeric units triggers the collapse of the vesicles into micelles. This shows that the implementation of an acid-base functionality such as an imidazole group in the hydrophobic core of the amphiphilic calix[6]arene makes the aggregation architecture responsive to the pH and to metal ions.