A series of macrocyclic Eu, Gd, and Tb complexes has been prepared in which the intramolecular ligation of a beta-arylsulfonamide nitrogen is rendered pH-dependent, giving rise to changes in the hydration state, q, at the lanthanide center. In complexes based on DO3A, variation of the p-substituent in the arylsulfonamide moiety determines the apparent protonation constant log K(MLH) with values of 5.7, 6.4, and 6.7 for the -CF(3), -Me, and -OMe substituents, respectively. Introduction of three beta-carboxyalkyl substituents, alpha to three ring nitrogens, inhibits displacement of the bound water by added protein and also suppresses intermolecular binding by endogenous anions (lactate, HCO(3)(-)). Measurements of the pH dependence of the form and intensity of the Eu complexes revealed that intramolecular carboxylate coordination occurred competitively. This was reduced either by enhancing the electron density at the sulfonamide nitrogen or by enlarging the chelate ring from 7--8. Amplification of the relaxivity changes in the pH range 8--5 occurred on protein binding, and over the pH range 7.4--6.8 a 48% change in relaxivity was defined for [Gd.3a] (298 K, 65.6 MHz) in 50% human serum solution.

Semithiobambusurils, which represent a new family of macrocyclic host molecules, have been prepared by a convenient, scalable synthesis. These new cavitands are double functional: they strongly bind a broad variety of anions in their interiors and metal ions at their sulfur-edged portals. The solid-state structure of semithiobambus[4]uril with HgCl2 demonstrates the ability of these compounds to form linear chains of coordination polymers with thiophillic metal ions. The crystal structure of semithiobambus[6]uril with tetraphenylphosphonium bromide exhibits the unique anion-binding properties of the host cavity and the characteristics of the binding site.

This account highlights three significant fields of applications for cucurbiturils (CBs). The first relates to cucurbituril‐based devices and sensors involving spectral effects associated with the formation and dissociation of inclusion complexes. The second refers to the use of CBs as catalysts that control the rate and selectivity of chemical reactions. The third deals with the potential use of CBs for the assembly of mechanical devices and molecular motors. An example of the sensor application is based on the discovery that 4‐aminobipyridine derivatives form strong inclusion complexes with CB6, exhibiting remarkably large enhancements of fluorescence intensity and quantum yields. This reversible binding phenomenon can be used for the design of switchable beacons that can be incorporated into cascades of binding networks for various applications. The second application is exemplified by selective, enhanced photoaddition reactions as well as [3+2]cycloaddition and other reactions mediated by the CB cavity. The third application is based on the idea that low‐barrier molecular rotary motors having rotaxane architecture can be constructed using a cucurbituril host and a polyyne guest serving as stator and rotator, respectively. The “low friction” between these components is supported by molecular mechanics calculations with model systems and experimentally verified by X‐ray crystallography with several synthetic host–guest complexes, all suggesting that the diyne rod floats at the center of the macrocyclic host with no apparent van der Waals contacts between them.