Supramolecular Self-Assemblies of β-Cyclodextrins with Aromatic Tethers: Factors Governing the Helical Columnar versus Linear Channel Superstructures

A series of 6-O-(p-substituted phenyl)-modified β-cyclodextrin derivatives, i.e., 6-O-(4-bromophenyl)-β-CD (1), 6-O-(4-nitrophenyl)-β -CD (2), 6-O-(4-formylphenyl)-β-CD (3), 6-phenylselenyl-6-deoxy-β-CD (4), and 6-O-(4-hydroxybenzoyl)-β-CD (5), were synthesized, and their inclusion complexation behavior in aqueous solution and self-assembling behavior in the solid state were comparatively studied by NMR spectroscopy, microcalorimetry, crystallography, and scanning tunneling microscopy. Interestingly, (seleno)ethers 1-4 and ester 5 displayed distinctly different self-assembling behavior in the solid state, affording a successively threading head-to-tail polymeric helical structure for the (seleno)ethers or a mutually penetrating tail-to-tail dimeric columnar channel structure for the ester. Combining the present and previous structures reported for the relevant β-CD derivatives, we further deduce that the pivot heteroatom, through which the aromatic substituent is tethered to β-CD, plays a critical role in determining the helix structure, endowing the 2-fold and 4-fold axes to the N/O- and S/Se-pivoted β-CD aggregates, respectively. This means that one can control the self-assembling orientation, alignment, and helicity in the solid state by finely tuning the pivot atom and the tether length. Further NMR and calorimetric studies on the self-assembling behavior in aqueous solution revealed that the dimerization step is the key to the formation of linear polymeric supramolecular architecture, which is driven by favorable entropic contributions.