Smart Materials Based on Synthetic Host Molecules: The Role of Host-Guest Chemistry in the Fabrication and Application.

Stimuli-responsive or smart materials have recently shown a significant impact on the frontier of material science and engineering. The exponential development of synthetic host molecules (SHMs) over the last decades and their corresponding host-guest chemistry, have empowered researchers with new opportunities to design and construct tailored or guest-specific smart materials. In this Minireview, we present the recent advancements in synthetic host based smart materials, ranging from the fabrication strategies to the state-of-art applications including adsorption, separation, luminescence, self-healing and actuation. The role that the host-guest chemistry plays in these systems is highlighted throughout to give a better prospective of the available possibilities for emerging materials of future economies.

Symmetrically Tetra-functionalized Pillar[6]arenes Prepared by Fragment Coupling.

Due to the highly symmetrical structures generated from one-pot syntheses, the partial functionalization of macrocycles is usually beset with low yields and onerous purifications of the target multifunctional macrocycles. To improve this situation, taking pillar[6]arenes as an example, a two-step fragment coupling method has been developed for synthesizing symmetrically tetra-functionalized pillar[6]arenes, namely X-pillar[6]arenes. This method is simple and versatile, which makes hetero-fragment coupling and pre-functionalization available. Nine new macrocycles and a pillar[6]arene-based cage have been prepared. In addition, one of the newly synthesized macrocycles, COOEtEtXP[6], exhibits a strong cyan luminescence in the solid state under irradiation by 365 nm UV light. This emission originates from intramolecular through-space conjugation. Meanwhile, formation of a supramolecular polymer by multiple non-covalent intra/intermolecular interactions helps rigidify the structure and make COOEtEtXP[6] an efficient solid-state emitter. It is believed that this fragment coupling can also be used to realize the multi-functionalization of other macrocycles.

Multifunctional Pillar[n]arene-Based Smart Nanomaterials.

The construction of smart nanomaterials from host macrocycles that are responsive to specific stimuli has gained significant attention in recent years. The application of pillar[n]arenes has been of particular interest given their ease of functionalization and tunability of the intrinsic cavity electronic properties that allows them to encapsulate a great variety of guests and complex with metal ions with high selectivity via noncovalent interactions, endowing them with captivating properties and functions. Herein, we present the most recent advances in the design and functionalization of pillar[n]arene-based smart nanomaterials, and their applications for sensing, catalysis, drug delivery, and artificial transmembrane channels.

Selective Separation of Phenanthrene from Aromatic Isomer Mixtures by a Water-Soluble Azobenzene-Based Macrocycle.

Selective separation of phenanthrene (PHE) from aromatic isomer mixtures is a big challenge in industry. In this work, a light-responsive water-soluble azobenzene-based macrocycle 1 is synthesized and an aqueous solution of E,E-1 is employed to separate PHE from anthracene via a solid-liquid extraction method under ambient conditions. After five extraction cycles, the average purity for PHE is about 91.1% and macrocycle 1 can be reused at least five times without obvious reduction of separation performance for PHE. This work not only comprises a new and clean way to separate PHE by taking advantage of a macrocyclic host but also promotes the application of host-guest chemistry.

Molecular Cages Self-Assembled by Imine Condensation in Water.

Self-assembly by imine condensation in aqueous media is a formidable task because of the labile nature of imines in the presence of water. Here, by taking advantage of multivalence and ligand preorganization, basket-shaped triscationic cage molecules are self-assembled in high yields in both water and organic solvent, by condensing a hexaformyl and bisamine. These cages, especially the chiral ones, are stable or inert in aqueous solution, that is, no decomposition was observed upon dilution, precipitation, or exposure to competitive amines or aldehydes. Such water-compatibility allows the hosts to take advantage of the hydrophobic effect to accommodate hydrophobic guests. The chiral cage S-23+ selectively binds and distinguishes one of two enantiomers, opening up opportunities for applications such as chiral compound separation. Chiral narcissistic self-sorting and sergeants-and-soldiers effects occur during cage formation when two amino precursors are involved in self-assembly.

Azobenzene-Based Macrocyclic Arenes: Synthesis, Crystal Structures, and Light-Controlled Molecular Encapsulation and Release.

Azobenzene (azo)-based macrocycles are highly fascinating in supramolecular chemistry because of their light-responsiveness. In this work, a series of azo-based macrocyclic arenes 1, 2, 3, and 4, distinguished by the substituted positions of azo groups, is rationally designed and synthesized via a fragment-cyclization method. From the crystal and computed structures of 1, 2, and 3, we observe that the cavity size of these azo-macrocycles decreases gradually upon E→Z photoisomerization. Moreover, light-controlled host-guest complexations between azo-macrocycle 1 and guest molecules (7,7,8,8-tetracyanoquinodimethane, terephthalonitrile) are successfully achieved. This work provides a simple and effective method to prepare azo-macrocycles, and the light-responsive molecular-encapsulation systems in this work may further advance the design and applications of novel photo-responsive host-guest systems.

π-Metalated [15]Paracyclophanes: Synthesis and Binding to Oxo-Anions via Anion-π Interactions.

The host-guest properties of [15]paracyclophane ([15]PCPs) are engendered by π-metalation, which exhibits fantastic regioselectivity toward a macrocyclic molecule. The synthesis and characterization of mono-, di-, and trimetalated [15]PCPs are discussed in this article, and the anion binding behavior of 3Ru-[15]PCP-II6+, one of the trimetalated [15]PCPs, driven by anion-π interactions, is comprehensively demonstrated in both solution and the solid state. The anion binding properties of 3Ru-[15]PCP-II6+ in solution are investigated by 1H NMR titrations, showing selectivity toward ReO4- in both organic and aqueous solutions. The binding mode is unexpected; the anionic guest stacks over the host rather than threads it. This selectivity for ReO4- is also supported by water-nitromethane extraction experiments, which demonstrate that its partition from water into the organic phase by 3Ru-[15]PCP-II·6OTf is maintained to some extent in the presence of excess Cl-, SO42-, H2PO4-, NO3-, and ClO4-.

Formation of Linear Side-Chain Polypseudorotaxane with Supramolecular Polymer Backbone through Neutral Halogen Bonds and Pillar[5]arene-Based Host-Guest Interactions.

Linear side-chain polypseudorotaxane with supramolecular polymer backbone was assembled by neutral halogen bonds (XB) and pillar[5]arene-based host-guest interactions in solution and in the solid state. The formation of the halogen-bonded supramolecular polymer backbone and side-chain polypseudorotaxane in solution was characterized by 1 H NMR spectroscopy, diffusion ordered NMR spectroscopy and scanning electron microscopy experiments. Furthermore, the solid-state structures of these two highly organized supramolecular architectures were provided by single-crystal X-ray analysis.