2. Switchable Host-Guest Interactions of Supramolecular Rings and Cages

摘要

Noncovalent interactions between covalently structured subunits form the basis of the vastly growing field of supramolecular chemistry [1]. Dispersion forces, hydrogen bonds, dipole-dipole/ion-dipole/ion-ion attractions, π —π-interactions, and metal coordination can be regarded as the main types of such weak interactions. Moreover, hydrophobic interactions are considered to be the most important driving force for the assembly of many supramolecular systems in water. Many more variants of these main classes have been described in recent years such as cation-π [2] and even anion-π interactions [3, 4]. Some effects such as the "fluorous-fluorous interactions" have even led to the development of an entirely new discipline [5]. Supramolecular interactions span a wide range on the energy scale ranging from fractions of a kilojoule per mole for weak van der Waals attractions to hundreds of kilojoules per mole for strong ligand-metal bonds in coordination compounds [1]. This latter fact already shows that the borders between what is regarded as "covalent compounds" and "supramolecular compounds" is somewhat fuzzy and the strengths of binding of particular building blocks within a supramolecular assembly should not be the only criterion by which the system is described. In addition, the chosen solvent may have a dramatic effect on the strength of supramolecular interactions. A further principle found in many supramolecular structures is self-assembly [6]. Although cases of clean formation of a single supramolecular structure under kinetic control are known, the end product of a self-assembly process is often the thermodynamic product showing the lowest energy on the potential energy surface that spans all possible products (isomers) of the respective supramolecular reaction. For the system to reach the thermodynamic minimum, all substeps of assembly and disassembly are required to occur dynamically in a manner of coupled equilibrium processes under the given reaction conditions (temperature, solvent, concentration, and composition).