The inclusion of guest molecules by pillar[n]arene hosts in nonaqueous solution

Pillar[n]arenes are a relatively new family of macrocyclic molecular hosts, consisting of methylene-linked phenol-based monomers attached in a para-substituted configuration. The macrocyclic nature of these molecules provides a well-defined internal cavity, into which smaller guest molecules can be included to form supramolecular host-guest inclusion complexes. The para-oriented attachment yields relatively rigid cylindrical or pillar-like molecular and cavity shapes, hence the name. In addition to the unique shape of these cavities (compared to the related meta-substituted calixarenes, for example), the aromatic nature of the cavity walls also contributes to distinctive host properties of pillar[n]arenes and guest affinities. Neutral pillar[n]arenes, such as those with alkyl substituents, are generally insoluble in water, and thus their host properties are studied in nonaqueous media. This review article will provide a comprehensive summary of the reported host binding studies of pillar[n]arenes in nonaqueous solution. A wide range of specific pillar[n]arenes have been studied in this way, as have a wide variety of sizes and types of guests. The emphasis will be on studies in which the value of the binding association constant K_a was measured and reported, to consider the effect of guest size, shape, and nature and the host size and substituents on the binding ability of pillar[n]arenes. The main objective of the paper is to provide a summary and overview of the host binding abilities of this fascinating family of molecular hosts in nonaqueous solution.