Cucurbituril-modulated supramolecular assemblies: from cyclic oligomers to linear polymers.

Employing bis(p-sulfonatocalix[4]arenes) (bisSC4A) and N',N''hexamethylenebis(1-methyl-4,4'-bipyridinium) (HBV(4+)) as monomer building blocks, the assembly morphologies can be modulated by cucurbit[n]uril (CB[n]) (n = 7, 8), achieving the interesting topological conversion from cyclic oligomers to linear polymers. The binary supramolecular assembly fabricated by HBV(4+) and bisSC4A units, forms an oligomeric structure, which was characterized by NMR spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), dynamic light scattering (DLS), isothermal titration calorimetry (ITC), and gel permeation chromatography (GPC) experiments. The ternary supramolecular polymer participated by CB[8] is constructed on the basis of host-guest interactions by bisSC4A and the [2]pseudorotaxane HBV(4+)@CB[8], which is characterized by means of AFM, DLS, NMR spectroscopy, thermogravimetric analysis (TGA), UV/Vis spectroscopy, and elemental analysis. CB[n] plays vital roles in rigidifying the conformation of HBV(4+), and reinforcing the host-guest inclusion of bisSC4A with HBV(4+), which prompts the formation of a linear polymer. Moreover, the CB[8]-participated ternary assembly could disassemble into the molecular loop HBV(2+)@CB[8] and free bisSC4A after reduction of HBV(4+) to HBV(2+), whereas the CB[7]-based assembly remained unchanged after the reduction. CB[8] not only controlled the topological conversion of the supramolecular assemblies, but also improved the redox-responsive assembly/disassembly property practically.