Naphthotubes: Macrocyclic Hosts with a Biomimetic Cavity Feature

Macrocyclic hosts are the principal tools used in supramolecular chemistry because they can recognize other small molecules through non-covalent interactions. However, in terms of recognition ability, known macrocyclic hosts are often not comparable to bioreceptors. This may be due to the lack of functional groups inside the deep cavity, which is a common feature of bioreceptors. Most of the known macrocyclic hosts contain either a hydrophobic cavity or polar binding sites only. Macrocyclic hosts with functional groups inside a hydrophobic cavity are rare. In 2004, Glass and co-workers reported a pair of water-soluble naphthalene-based molecular tubes with amide protons in the well-defined deep cavity. The cavity feature is very similar to that of bioreceptors. However, the amide protons were not used in molecular recognition and were replaced in 2012 with allyl groups in order to improve the hydrophobic effect. We started our work on the basis of the Glass molecular tubes but paid close attention to the functional groups in the deep cavity. In this Account, we summarize our results on these biomimetic receptors, which we call naphthotubes. The inward-directed functional groups endow the corresponding naphthotubes with unique recognition abilities. Naphthotubes with hydrogen-bond acceptors (ether, ester, and imine) prefer to bind organic cations; naphthotubes with hydrogen-bond donors (urea, thiourea, and amide) can bind neutral molecules; amine naphthotubes are stimuli-responsive to acid/base. In particular, the water-soluble amide naphthotubes are able to selectively recognize highly hydrophilic molecules in water-a generally accepted challenge in supramolecular chemistry. The unique recognition ability of these naphthotubes provides the basis for their applications in sensing, self-assembly, and molecular machines. Fluorescent sensing of environmental contaminants in water, chiroptical sensing of small chiral molecules, allosteric cooperative self-assembly, dissipative self-assembly, and directional molecular shuttles have been demonstrated with these naphthotubes. Overall, we hope to convey the message that these naphthotubes have unique recognition properties and promising applications in diverse fields. We believe that further exploration of this class of macrocycles may lead to practical applications in, for example, biomedical science, environmental science, and other related fields.

Co-Inclusion of cyclic ethers and chloroform by a macrocycle with benzophenone-3,3′,4,4′-tetracarboxylic diimide units

Crystallization of a diimide-based macrocycle having adamantane parts and several cyclic ethers in chloroform provided inclusion crystals, where both guests were cooperatively accommodated within inner spaces between the macrocycles.

Fluorescent monitoring of the reaction kinetics of nonfluorescent molecules enabled by a fluorescent receptor

Fluorescent monitoring of the reaction kinetics of nonfluorescent molecules was achieved in water by using a fluorescent receptor.

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.

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.

Achieving Strong Positive Cooperativity through Activating Weak Non-Covalent Interactions

Positive cooperativity achieved through activating weak non-covalent interactions is common in biological assemblies but is rarely observed in synthetic complexes. Two new molecular tubes have been synthesized and the syn isomer binds DABCO-based organic cations with high orientational selectivity. Surprisingly, the ternary complex with two hosts and one guest shows a high cooperativity factor (α=580), which is the highest reported for synthetic systems without involving ion-pairing interactions. The X-ray single-crystal structure revealed that the strong positive cooperativity likely originates from eight C-H⋅⋅⋅O hydrogen bonds between the two head-to-head-arranged syn tube molecules. These relatively weak hydrogen bonds were not observed in the free hosts and only emerged in the complex. Furthermore, this complex was used as a basic motif to construct a robust [2+2] cyclic assembly, thus demonstrating its potential in molecular self-assembly.

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.

Oxatub[5,6]arene: synthesis, conformational analysis, and the recognition of C60 and C70

Large oxatub[n]arenes (n = 5, 6) have been synthesized and characterized. Their conformational complexity was analyzed and host-guest chemistry was studied. In particular, the rather flexible oxatub[6]arene is able to accommodate C60 and C70 with moderate binding affinities. Computational results suggest that there are different binding affinities to C60 or C70 for each conformer of oxatub[6]arene. This endows oxatub[6]arene with conformational adaptability according to the needs of its guests.

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

A conformationally adaptive macrocycle is presented, namely zorb[4]arene, which exists in multiple conformations in the uncomplexed state. The binding cavity of zorb[4]arene is concealed, either due to a collapsed conformation or by self-inclusion. The zorb[4]arene with long alkyl chains manifests itself with surprisingly low melting point and thus exist as an oil at room temperature. Binding of a guest molecule induces the folding and conformational rigidity of zorb[4]arene and leads to well-defined three-dimensional structures, which can further self-assemble into nanosheets or nanotubes upon solvent evaporation, depending on guest molecules and the conformations they can induce.

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.

Oxatub[4]arene: a molecular "transformer" capable of hosting a wide range of organic cations

The molecular "transformer", oxatub[4]arene, was found to be able to host a wide range of organic cations. The strong binding ability is believed to originate from its four interconvertible and deep-cavity conformers. The binding behavior of such adaptable receptors may provide implications for molecular recognition in nature.

endo-Functionalized molecular tubes: selective encapsulation of neutral molecules in non-polar media

Four endo-functionalized molecular tubes show high binding affinity and selectivity to neutral molecules.

Oxatub[4]arene: a smart macrocyclic receptor with multiple interconvertible cavities

We report a smart macrocycle with four interconvertible cavities, which can select the best-fit cavity for a given guest.

There are a large number of synthetic macrocyclic receptors in the literature. No particular one is suitable for all guests or purposes. For a broader guest binding scope or multiple purposes, a macrocycle with multiple interconvertible cavities will be advantageous. Here, we report a naphthalene-based macrocyclic receptor with an adaptable cavity, namely oxatub[4]arene. It has four representative conformations resulting from the flipping of naphthalene rings, each with a deep and well-defined cavity. Different guests select one host conformer or a combination of conformers. All the four conformers have been detected and characterized based on 2D NMR spectra and X-ray single crystal structures. Thermodynamically, these conformers constitute a reservoir, that responds to the structural changes of guests, and thus maximizes the association free energies. This smart macrocycle may provide a new platform for the construction of molecular machines and devices or stimuli-responsive materials.