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

Abstract

A series of 6-O-(p-substituted phenyl)-modified beta-cyclodextrin derivatives, i.e., 6-O-(4-bromophenyl)-beta-CD (1), 6-O-(4-nitrophenyl)-beta-CD (2), 6-O-(4-formylphenyl)-beta-CD (3), 6-phenylselenyl-6-deoxy-beta-CD (4), and 6-O-(4-hydroxybenzoyl)-beta-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 beta-CD derivatives, we further deduce that the pivot heteroatom, through which the aromatic substituent is tethered to beta-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 beta-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.

Self-assembly behavior of phenyl modified β-cyclodextrins

“…In contrast, modified β-CDs 2 [11] and 3 [10] behave somewhat differently, forming the self-inclusion complexes in solution, the aromatic moiety of which is shallowly embedded in own β-CD cavity. Therefore, the difference of conformation in aqueous solution may be attributed to the diversity of pivot heteroatoms in 1-4.…”

“…We have recently reported systematical studies on the binding behaviors of mono-modified β-CDs both in solution and the solid state. The obtained results indicated that the assembly modes of polymeric supramolecules might be gov erned through the precise control of structures and chemical properties of substituents [10] . In the present study, we wish to report the self-assembly behavior of mono(6-phenolic-6-deoxy)-β-cyclodextrin (1) both in solution and the solid state by means of X-ray crystallography and 1 H NMR.…”

“…The dihedral angles between the plane of aromatic ring and the heptagon composed of seven glycosidic oxygen atoms in crystals 1 and 2 are about 2-fold as compared with these of crystals 3 and 4. The contrasting 2-and 4-fold helical structures may be accounted for in terms of the larger atomic radius and lower electronegativity of S and Se atoms compared to those of N and O atoms [10] . Comparison of the present results with the previous reports reveals that the packing mode of 1 may be analogous to that of the relevant complexes reported previously, but the depth of penetration is appreciably different.…”

“…Using the present and previous structures reported for the relevant β-CD derivatives (Fig. 1), i.e., mono(6-anilino-6-deoxy)-β-CD (2) [11] , mono(6-phenylselenyl-6-deoxy)-β-CD (3) [10] , and mono(6-phenylthio-6-deoxy)-β-CD (4) [16] , the factors governing the formations of supramolecular assemblies are further discussed. It is of our particular interest to investigate how the slight difference of pivot heteroatoms, through which the aromatic substituent is tethered to β-CD, affects the binding behavior of modified β-CDs with phenyl substituents both in solution and the solid state.…”

“…The conformations of many modified CDs have been carefully studied both in solution and the solid state, and most studies show that the formation of selfinclusion complexes or supramolecular assemblies mainly depend on the spatial size-fit and/or geometric complementarity between the substituents and CD cavities [5][6][7][8][9][10][11][12][13][14][15][16] . We have recently reported systematical studies on the binding behaviors of mono-modified β-CDs both in solution and the solid state.…”