Guest-Induced Folding and Self-Assembly of Conformationally Adaptive Macrocycles into Nanosheets and Nanotubes

Macrocyclic host molecules with aromatic building blocks: the state of the art and progress

Macrocyclic host molecules play the central role in host-guest chemistry and supramolecular chemistry. The highly structural symmetry of macrocyclic host molecules can meet people's pursuit of aesthetics in molecular design, and generally means a balance of design, synthesis, properties and applications. For macrocyclic host molecules with highly symmetrical structures, building blocks, which could be described as repeat units as well, are the most fundamental elements for molecular design. The structural features and recognition ability of macrocyclic host molecules are determined by the building blocks and their connection patterns. Using different building blocks, different macrocyclic host molecules could be designed and synthesized. With decades of developments of host-guest chemistry and supramolecular chemistry, diverse macrocyclic host molecules with different building blocks have been designed and synthesized. Aromatic building blocks are a big family among the various building blocks used in constructing macrocyclic host molecules. In this feature article, the recent developments of macrocyclic host molecules with aromatic building blocks were summarized and discussed.

Conformationally adaptive macrocycles with flipping aromatic sidewalls

Conformationally adaptive macrocycles possess multiple well-defined conformations through quickly flipping their aromatic sidewalls. The macrocycles combine the binding power of all the conformations. Upon binding a guest, one or a combination of conformations are selected to achieve the maximized binding affinity. In addition, the complex conformational network is responsive to changes in temperature or solvent. It has been demonstrated that these macrocycles have unique properties in chirality sensing, stimuli-responsive self-assembly, and molecular switches. In this tutorial review, we summarize recent advances on conformationally adaptive macrocycles with an emphasis on our own research. We believe that this class of macrocycles will have a bright future in supramolecular chemistry and beyond.

A 2,3-dialkoxynaphthalene-based naphthocage

A 2,3-dialkoxynaphthalene-based naphthocage was synthesized. It shows similarly strong binding affinity to organic cations as the 2,6-dialkoxynaphthalene-based naphthocage but different guest preference and conformational response.

2,3-Dibutoxynaphthalene-based tetralactam macrocycles for recognizing precious metal chloride complexes

Two new tetralactam macrocycles with 2,3-dibutoxynaphthalene groups as sidewalls have been synthesized and characterized. The macrocycle containing isophthalamide bridges can bind square-planar chloride coordination complexes of gold(III), platinum(II), and palladium(II) in CDCl₃, while the macrocycle with 2,6-pyridine dicarboxamide bridging units cannot. This may be due to the shrunken cavity caused by intramolecular hydrogen bonds in the latter tetralactam macrocycle. The binding of the isophthalamide-based macrocycle is mainly driven by hydrogen bonds and electrostatic interactions. This naphthalene-based macrocycle has similar binding affinities to all the three abovementioned precious metal chloride complexes. This is in contrast to the fact that the tetralactam macrocycle with anthracene as the sidewalls only show good binding affinities to AuCl₄⁻. The superior binding to all three complexes may be due to the conformational diversity of the naphthalene-based macrocycle, which make it conformationally adaptive to maximize the binding affinities. In addition, the macrocycle shows fluorescent quenching when adding the chloride metal complexes in its solution and may be used as a fluorescent sensor for the detection of these coordination complexes.

Gemini-Type Supramolecular Amphiphile Based on a Water-Soluble Pillar[5]arene and an Azastilbene Guest and Its Application in Stimuli-Responsive Self-Assemblies

Supramolecular amphiphiles are a type of intriguing building blocks to fabricate self-assembled nanostructures that can be applied in diverse fields. Gemini-type supramolecular amphiphiles, containing two hydrophobic tails and two hydrophilic head groups linked by a spacer, are good candidates to fabricate many advanced materials that are able to apply in surface modification, drug/gene delivery, and solubilization. Pillararenes, the fifth generation of macrocyclic host molecules, have been used to fabricate many supramolecular amphiphiles that played important roles in biomedical fields and materials science. However, compared with single-chain and bola-type supramolecular amphiphiles, the studies of gemini-type supramolecular amphiphiles based on pillararenes are very rare. Herein, a new strategy to prepare gemini-type supramolecular amphiphiles was reported. A new acid-responsive host?guest recognition motif in water on the basis of a 4,4?-azastilbene derivative (G1) and a water-soluble pillar[5]arene (WP5) was fabricated. The gemini-type supramolecular amphiphile was constructed by an azastilbene amphiphilic guest (G2) and WP5. Then its application in stimuli-responsive self-assemblies was investigated. G2 self-assembled into nanoribbons in water. Upon addition of WP5, the gemini-type supramolecular amphiphile formed, leading to the formation of disklike micelles. After further addition of hydrochloric acid, the morphology changed into nanosheets.

Molecular recognition using tetralactam macrocycles with parallel aromatic sidewalls

This review summarizes the supramolecular properties of tetralactam macrocycles that have parallel aromatic sidewalls and four NH residues directed into the macrocyclic cavity. These macrocycles are versatile hosts for a large number of different guest structures in water and organic solvents, and they are well-suited for a range of supramolecular applications. The macrocyclic cavity contains a mixture of polar functional groups and non-polar surfaces which is reminiscent of the amphiphilic binding pockets within many proteins. In water, the aromatic surfaces in the tetralactam cavity drive high affinity due the hydrophobic effect and the NH groups provide secondary interactions that induce binding selectivity. In organic solvents, the supramolecular factors are reversed; the polar NH groups drive high affinity and the aromatic surfaces provide the secondary interactions. In addition to an amphiphilic cavity, macrocyclic tetralactams exhibit conformational flexibility, and the combination of properties enables them to be effective hosts for a wide range of guest molecules including organic biscarbonyl derivatives, near-infrared dyes, acenes, precious metal halide complexes, trimethylammonium ion-pairs, and saccharides.

Probing the guest-binding preference of three structurally similar and conformationally adaptive macrocycles

A hybrid macrocycle was synthesized by combining the features of oxatub[4]arene and zorb[4]arene. Three macrocycles show different guest-binding preferences, even in a complex mixture.

A conformationally adaptive macrocycle: conformational complexity and host-guest chemistry of zorb[4]arene

Large amplitude conformational change is one of the features of biomolecular recognition and is also the basis for allosteric effects and signal transduction in functional biological systems. However, synthetic receptors with controllable conformational changes are rare. In this article, we present a thorough study on the host–guest chemistry of a conformationally adaptive macrocycle, namely per-O-ethoxyzorb[4]arene (ZB4). Similar to per-O-ethoxyoxatub[4]arene, ZB4 is capable of accommodating a wide range of organic cations. However, ZB4 does not show large amplitude conformational responses to the electronic substituents on the guests. Instead of a linear free-energy relationship, ZB4 follows a parabolic free-energy relationship. This is explained by invoking the influence of secondary C–H···O hydrogen bonds on the primary cation···π interactions based on the information obtained from four representative crystal structures. In addition, heat capacity changes (ΔC_p) and enthalpy–entropy compensation phenomena both indicate that solvent reorganization is also involved during the binding. This research further deepens our understanding on the binding behavior of ZB4 and lays the basis for the construction of stimuli-responsive materials with ZB4 as a major component.

Spectral study on conformation switchable cationic calix[4]carbazole serving as curcumin container, stabilizer and sustained-delivery carrier

A fluorescent 2,7-dimethoxy-substituted calix[4]carbazole (1) is facilely synthesized. The spectral behaviors of both the guest-induced switchable conformation of 1 and its abilities serving as the stabilizer and molecular carrier of curcumin are investigated. UV-vis, fluorescence and NMR spectral results show that upon binding to curcumin, the 1,3-alternate conformation of 1 is converted to be the cone one. The relative high association constant (6.4×10⁶M^-1) of 1 binding to curcumin enables it to stabilize the curcumin, to suppress its degradation, and to sustainably deliver it into the EYPC vesicles within 20h. Moreover, the cytotoxicity assay shows that 1 does not interfere the antiproliferative activities of curcumin. All these properties endow 1 the potential capability of serving as the molecular drug carrier. Our current result may pave the way looking for more efficient fluorescent calixcarbazoles and thereof spectral utilities.

Molecular Recognition, Conformational Behavior, and Spectral Characteristics of Oxatub[4]arene Macrocycle

In the present work, we analyze molecular recognition behavior of synthetic hydroxylated oxatub[4]arene (TA4) receptor toward the methyl viologen in different redox states. The supramolecular binding of methyl viologen guest toward TA4 macrocyclic scaffold has been studied employing the dispersion corrected ωB97X-D based density functional theory. The methyl viologen in dicationic and neutral forms revealed distinct features in electronic, ¹H nuclear magnetic resonance, and infrared spectra. Quantum theory of atoms in molecules in conjunction with the noncovalent interaction reduced density gradient in real space have been used as tools to characterize the underlying host-guest binding.

Regioselective Synthesis of Methylene-Bridged Naphthalene Oligomers and Their Host-Guest Chemistry

In this research, we report the regioselective synthesis of methylene-bridged naphthalene oligomers from 2,6-dialkoxyl naphthanene and paraformaldehyde by using p-TsOH as the catalyst and CH₂Cl₂ as the solvent. The structures were characterized by NMR spectroscopy and X-ray crystallography. Their host-guest chemistry with organic cations was studied, and optimal naphthalene numbers in the oligomers were revealed for different guests. In addition, the reason for the unsuccessful synthesis of methylene-bridged naphthalene macrocycles was discussed.

Effects of side chains of oxatub[4]arene on its conformational interconversion, molecular recognition and macroscopic self-assembly

The side-chain length of oxatub[4]arenes was shown to affect its conformational interconversion, molecular recognition and macroscopic self-assembly behavior.