Stimuli-Responsive Supramolecular Assemblies Constructed from Pillar[ n]arenes.

Abstract

Supramolecular assemblies are constructed from at least two molecules through various noncovalent bonding modes such as hydrogen bonding, cationic-anionic electrostatic interactions, aromatic interactions, metal-ligand bonding, hydrophobic-hydrophilic interactions, and charge-transfer interactions. Owing to the dynamic and reversible nature of these noncovalent bonds, the assembly and disassembly of these molecules are dynamic and reversible. Molecules self-assemble to form the most conformationally and thermally stable structures through these noncovalent interactions. The formation of these noncovalent interactions is affected by the properties of the environment such as its polarity, temperature, and pressure; thus, the structure of the assembled compounds is determined by the environment. The sizes and shapes of the supramolecular assemblies play an important role in determining their functions. Therefore, controlling their size and shape is important. Introducing stimuli-responsive groups into supramolecular assemblies is a useful way to control their size and shape. Controlling supramolecular structures and motions with external stimuli, i.e., periodic and rotational motions on the molecular scale, structures, and molecular weights at the nano- and micrometer scales, visible shrinking/expansion, and adhesive behavior at a macroscopic scale, is very useful. Macrocyclic host molecules are useful building blocks for the construction of stimuli-responsive supramolecular assemblies because their host ability can be tuned by changing the shape and electron density of the cavity. The size-dependent hosting ability of the cavity is similar to the lock-and-key model in biological systems. Stimuli-responsive supramolecular assemblies have been developed by using macrocyclic compounds such as cyclodextrins, cucurbit[ n]urils, calix[ n]arenes, crown ethers, and related macrocycles. We successfully developed new pillar-shaped macrocyclic hosts in 2008, which were coined pillar[ n]arenes. The unique structural features of pillar[ n]arenes allowed new properties. This year, 2018, marks one decade of research into pillar[ n]arene chemistry, and in that time the properties of pillar[ n]arenes have been widely investigated by various scientists. Thanks to their efforts, the characteristic properties of pillar[ n]arenes that result from their pillar-shaped structures have been elucidated. Their host ability, the chirality of their pillar-shaped structure, and their versatile functionality are unique features of pillar[ n]arenes not seen in other well-known hosts, and these properties are very useful for the creation of new stimuli-responsive supramolecular assemblies. In this Account, we describe photo-, pH- and redox-responsive supramolecular assemblies based on pillar[ n]arenes. First, we discuss molecular-scale stimuli-responsive supramolecular assemblies, i.e., pseudorotaxanes, pseudocatenanes, and supramolecular polymers. We also highlight subnanometer- and micrometer-scale stimuli-responsive supramolecular assembles such as particles and vesicles. Finally, we discuss the macroscopic stimuli-responsive structural changes of surfaces and gels. This Account will provide useful information for researchers working on not only pillar[ n]arene chemistry but also the chemistry of other macrocyclic hosts, and it will inspire new discoveries in the field of supramolecular assemblies and systems containing macrocyclic hosts.