The molecular host cucurbit [7]uril forms an extremely stable inclusion complex with the dicationic ferrocene derivative bis(trimethylammoniomethyl)ferrocene in aqueous solution. The equilibrium association constant for this host-guest pair is 3 ؋ 10 15 M ؊1 (Kd ‫؍‬ 3 ؋ 10 ؊16 M), equivalent to that exhibited by the avidinbiotin pair. Although purely synthetic systems with larger association constants have been reported, the present one is unique because it does not rely on polyvalency. Instead, it achieves its extreme affinity by overcoming the compensatory enthalpyentropy relationship usually observed in supramolecular complexes. Its disproportionately low entropic cost is traced to extensive host desolvation and to the rigidity of both the host and the guest.cucurbituril ͉ entropy control ͉ ferrocene derivatives ͉ host-guest complexation ͉ thermodynamics

Cucurbit[n]uril (CB[n], n = 5-10), a new family of molecular hosts comprising n glycoluril units, have gained much attention in the new millennium for their exceptional molecular recognition ability. The CB homologues have brought dynamism to CB chemistry, as witnessed by the heightened interest in the field for the last several years. Compared to the chemistry of cyclodextrins and calixarenes, however, that of CB[n] has developed slowly until recently, which may be attributed mainly to their poor solubility in common solvents, and inability to functionalize these molecules. The direct functionalization method of CB[n] propelled CB chemistry to a new height as this new method not only solved the solubility problem but also opened up the gateway to the generation of tailor-made CB[n] derivatives. The functionalization of CB[n] led us to investigate numerous applications including artificial ion channels, vesicles, stationary phases in chromatography, ISEs, polymers, nanomaterials, and many others. This tutorial review describes the recent advances and challenges in the functionalization of CBs along with the applications of functionalized CBs.

A dicationic ferrocene derivative has previously been shown to bind cucurbit[7]uril (CB[7]) in water with ultra-high affinity (ΔGo= −21 kcal/mol). Here, we describe new compounds that bind aqueous CB[7] equally well, validating our prior suggestion that they, too, would be ultra-high affinity CB[7] guests. The present guests, which are based upon either a bicyclo[2.2.2]octane or adamantane core, have no metal atoms, so these results also confirm that the remarkably high affinities of the ferrocene-based guest need not be attributed to metal-specific interactions. Because we used the M2 method to compute the affinities of several of the new host-guest systems prior to synthesizing them, the present results also provide for the first blinded evaluation of this computational method. The blinded calculations agree reasonably well with experiment and successfully reproduce the observation that the new adamantane-based guests achieve extremely high affinities, despite the fact that they position a cationic substituent at only one electronegative portal of the CB[7] host. However, there are also significant deviations from experiment, and these lead to the correction of a procedural error and an instructive evaluation of the sensitivity of the calculations to physically reasonable variations in molecular energy parameters. The new experimental and computational results presented here bear on the physical mechanisms of molecular recognition, the accuracy of the M2 method, and the usefulness of host-guest systems as test-beds for computational methods.

The formation of inclusion complexes between cucurbit[7]uril (CB[7]) and ferrocene and its derivatives has been investigated. The X-ray crystal structure of the 1:1 inclusion complex between ferrocene and CB[7] revealed that the guest molecule resides in the host cavity with two different orientations. Inclusion of a set of five water-soluble ferrocene derivatives in CB[7] was investigated by 1H NMR spectroscopy and calorimetric and voltammetric techniques. Our data indicate that all neutral and cationic guests form highly stable inclusion complexes with CB[7], with binding constants in the 10(9)-10(10) M(-)(1) and 10(12)-10(13) M(-1) ranges, respectively. However, the anionic ferrocenecarboxylate, the only negatively charged guest among those surveyed, was not bound by CB[7] at all. These results are in sharp contrast to the known binding behavior of the same guests to beta-cyclodextrin (beta-CD), since all the guests form stable inclusion complexes with beta-CD, with binding constants in the range 10(3)-10(4) M(-1). The electrostatic surface potentials of CB[6], CB[7], and CB[8] and their size-equivalent CDs were calculated and compared. The CD portals and cavities exhibit low surface potential values, whereas the regions around the carbonyl oxygens in CBs are significantly negative, which explains the strong affinity of CBs for positively charged guests and also provides a rationalization for the rejection of anionic guests. Taken together, our data suggest that cucurbiturils may form very stable complexes. However, the host-guest interactions are very sensitive to some structural features, such as a negatively charged carboxylate group attached to the ferrocene residue, which may completely disrupt the stability of the complexes.

Host-stabilized charge-transfer (CT) interactions and supramolecular assemblies built with these interactions are described. A variety of supramolecular assemblies including polyrotaxanes, molecular necklaces, and rotaxane dendrimers were synthesized through the intramolecular or intermolecular host-stabilized CT complex formation using cucurbit[8]uril (CB[8]) and D-A molecules having both electron-donor and electron-acceptor units connected by various types of linkers. Applications, including the design and synthesis of redox-driven molecular machines such as molecular loop locks, development of redox-controllable vesicles and detection of biologically important molecules, are also described.