Possessing two β-cyclodextrin cavities in close vicinity and a functional linker with good structural variety in a single molecule, bridged bis(β-cyclodextrin)s can significantly enhance the original binding ability and molecular selectivity of native β-cyclodextrin and thus be successfully utilized in drug carriers, solubilizers, catalysis, photochemical materials, etc. This Account describes recent developments in the intramolecular cooperative binding and multiple recognition of bridged bis(β-cyclodextrin)s with functional linkers in solution, as well as their molecular assembly behaviors through the intermolecular cooperative binding. It also gives a description of unique properties and wide applications of bis(β-cyclodextrin)s and their assemblies.

A water-soluble 8-aminoquinolino-b-cyclodextrin/1-adamantaneacetic acid (1/ADA) system is prepared in situ and exhibits a unique switch-on fluorescence response to Zn2+ over other common metal ions. Spectrophotometric studies demonstrate that this system can strongly coordinate Zn2+ through a cyclodextrin/substrate/metal triple recognition mode, and the resulting 1/ADA/Zn2+ ternary complex emits the blue-green fluorescence (λ= 490 nm) that can be easily distinguished by eyes in aqueous solution. Significantly, the switch-on fluorescence response of 1/ADA to Zn2+ is barely affected by various metal ions except Cu2+. As a result, this system can behave as an efficient supramolecular fluorescence sensor for Zn2+ in water.

,6'-bis(β-cyclodextrin) s linked by 2,2'-bipyridine-4,4'-dicarboxy tethers; that is, 2,2'-bipyridine-4,4'-dicarboxy-bridged bis(6-O-β-cyclodextrin) (2) and N,N'-bis(2-aminoethyl)-2,2'-bipyridine-4,4'-dicarboxamide-bridged (3), N,N'-bis(5-amino-3-azapentyl)-2,2'-bipyridine-4,4'-dicarboxamide-bridged (4) and N,N'-bis(8-amino-3,6-diazaoctyl)-2,2'-bipyridine-4,4'-dicarboxamide-bridged bis(6-amino-β-deoxy-b-cyclodextrin) (5), has been synthesized as cooperative multipoint-recognition receptor models. The inclusion complexation behavior of 2-5 with organic dyes; that is, ammonium 8-anilino-1-naphthalenesulfonate, Brilliant Green, Methyl Orange, Acridine Red, and Rhodamine B, has been investigated in aqueous phosphate buffer solutions (pH 7.20) at 25 ℃ by means of ultraviolet, fluorescence, and circular dichroism spectrometry as well as by fluorescence lifetime measurements. The spectral titrations gave the complex stability constants (KS) and Gibbs’ free energy changes (△G°) for the inclusion complexation of 2-5 with the organic dyes and other thermodynamic parameters (△H°and △S°) for the inclusion complexation of 2-4 with the fluorescent dyes Acridine Red and Rhodamine B. Bis(β-cyclodextrin)s 2-5 displayed higher binding abilities toward most of the examined dye molecules than native β-cyclodextrin 1; this is discussed from the viewpoints of the size/shape-fit concept, the induced-fit interaction, and cooperative, multipoint recognition by the bridging chain and the dual hydrophobic cavities. Thermodynamically, the inclusion complexation of 2-4 with Acridine Red is totally enthalpy driven with a negative or minor positive entropic contribution, but the inclusion complexation with Rhodamine B is mainly entropy-driven with a mostly positive, but occasionally negative, enthalpic contribution; in some cases this determines the complex stability.

A series of cyclodextrin-based polypseudorotaxanes (PPRs) were constructed by threading native β-cyclodextrin or L-tryptophan-modified β-cyclodextrin onto the amino-terminated PPG chains of different lengths. Subsequently, these PPRs were further assembled to form netlike supramolecular aggregates through the linkage of gold nanoparticles, and the resulting water-soluble Au-PPR aggregates were comprehensively characterized by FT-IR, UV, NMR, fluorescence spectroscopy, powder X-ray diffraction patterning, TG-DTA, and transmission electron microscopy. The results showed that the size and sedimentation rate of the Au-PPR aggregates were mainly dependent on the lengths of the PPG chains. Significantly, the Au-PPR aggregate 8, involving many L-tryptophan residues, showed not only a satisfactory water solubility but also a good capturing ability for fullerenes in aqueous solution. The 8-fullerene conjugate thus formed exhibited a good DNA cleavage ability under light irradiation.