Ether Naphthotube Host-Guest Complexes and [2]Rotaxanes with Dications.

The interactions between ether naphthotube and a series of dication guests in organic solution were investigated. It was found that ether naphthotube formed stable host-guest complexes selectively with these guests in a 1 : 1 stoichiometric ratio with association constants ranging from 102 to 106 M-1, which were confirmed by 1H-NMR spectra and ITC experiments. The host-guest interactions are driven by enthalpy change as the entropic factors are unfavorable. Positive correlations between ΔH and ΔS have been observed in the host-guest complexes. Furthermore, the para-substitution of the guests can significantly affect the binding affinities through a combination of field/inductive and resonance effects by following a linear free energy relationship. Based on the host-guest complexes composed of ether naphthotube and organic cations, two interlocked [2]rotaxanes were prepared by cationization reaction and Huisgen cycloaddition between the cations and the stopper components. The ether naphthotube-based host-guest complexes are useful for creating sophisticated interlocked molecules.

Acid/base responsive pseudo[3]rotaxanes from amine naphthotubes and bis-pyridinium/isoquinolinium guests.

A novel cooperative pseudo[3]rotaxane system was successfully constructed by the inclusion complexation of two identical amine naphthotubes with a bis-pyridinium/isoquinolinium guest. Single crystal structure analysis revealed that weak Csp3-H⋯O hydrogen bonds between the two hosts are responsible for the positive cooperativity during the formation of pseudo[3]rotaxanes. Moreover, intermolecular charge-transfer interactions between the electron-rich host and the electron-poor guests were observed. The pseudo[3]rotaxanes showed pH-controllable association/dissociation processes with naked-eye color changes in solution.

Mechanical Training Enabled Reinforcement of Polyrotaxane-Containing Hydrogel.

Many strategies have been developed for constructing anisotropic hydrogels, however, it remains a challenge to fabricate hydrogels with anisotropic nanocrystalline domains from intrinsically soft networks. Here, we report a naphthotube-based polyrotaxane-containing hydrogel that can be reinforced via mechanical training. During the training process, the hydrogel can adopt reorientation of polymer chains to form anisotropic structures driven by external uniaxial force. Due to the multiple hydrogen bonding sites and movable feature of naphthotube, the sliding of naphthotube on PEG chains simultaneously inducing the zipping of adjacent polymer chains to form densely anisotropic nanocrystalline domains through hydrogen bonded networks. Thus, the trained hydrogel exhibits an enhanced tension stress of ≈110 kPa, which realize a remarkable enhancement of ≈10 times compare to initial state. This study provides a new tactic for improving the mechanical performance of soft materials.

Precise Recognition in Water by an Endo-Functionalized Cavity: Tuning the Complementarity of Binding Sites.

Precise binding towards structurally similar substrates is a common feature of biomolecular recognition. However, achieving such selectivity-especially in distinguishing subtle differences in substrates-with synthetic hosts can be quite challenging. Herein, we report a novel design strategy involving the combination of different rigid skeletons to adjust the distance between recognition sites within the cavity, which allows for the highly selective recognition of hydrogen-bonding complementary substrates, such as 4-chromanone. X-ray single-crystal structures and density functional theory calculations confirmed that the distance of endo-functionalized groups within the rigid cavity is crucial for achieving high binding selectivity through hydrogen bonding. The thermodynamic data and molecular dynamics simulations revealed a significant influence of the hydrophobic cavity on the binding affinity. The new receptor possesses both high selectivity and high affinity, which provide valuable insights for the design of customized receptors.

Synthesis, Configurational Analysis, Molecular Recognition and Chirality Sensing of Methylene-Bridged Naphthotubes.

Macrocyclic hosts with a well-defined cavity are particularly appealing for supramolecular chemistry, but they are still rare. In this research, we reported a new class of macrocyclic hosts, namely methylene-bridged naphthotubes, with well-defined cavities. They were synthesized through TFA-catalyzed Friedel-Crafts reactions between alkoxy-substituted bisnaphthalenes and paraformaldehyde. A configurational selection was observed. Three-membered macrocycles possess a single cavity, while four-membered macrocycles adopt a double-cavity conformation or a self-filling conformation depending on the alkoxy length. The small homologue shows quite strong binding affinities (up to 108  M-1 ) to organic cations, which is better than structurally similar but flexible macrocycles. This demonstrates the advantage of a well-defined cavity in molecular recognition. Moreover, these naphthotubes can be used as chirality sensors for chiral organic cations through different chirality transfer mechanisms.

Conformational Effects on the Threading Kinetics of Dumbbell-Shaped Guests into the Cavity of Oxatub[4]arene.

Unprecedented threading kinetics were revealed between viologen-based guests and conformationally adaptive oxatub[4]arene. Three representative conformations of oxatub[4]arene are involved in the kinetic and thermodynamic products which follow the opposite orders in their rankings. Consequently, error correction was involved and a complex kinetic process was observed in a simple two-component system. Moreover, it was found that some viologen-based guests have much faster threading kinetics than those of DABCO-based with the same stoppers. This was enabled by an unprecedented threading mechanism in which a tilted conformation of the guests is adopted by involving one linear alkyl group on the 3,5-dialkoxybenzyl stoppers, the viologen core, and the methylene spacers in the transition states. This new mechanism even allows the viologen-based guests with the 3,5-dicetyloxybenzyl stoppers to form a pseudorotaxane with oxatub[4]arene.

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.

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.

Effective and Rapid Removal of Polar Organic Micropollutants from Water by Amide Naphthotube-Crosslinked Polymers.

It is challenging to remove polar organic micropollutants from water through adsorption-mediated processes. Macrocycle-crosslinked polymers were recently shown to be effective adsorbents for nonpolar or charged organic micropollutants through specific host-guest binding, but are rarely used for the treatment of neutral and polar organic micropollutants. This is due to the challenge of recognizing polar molecules in water by macrocyclic hosts. In this research, we report two amide naphthotube-crosslinked polymers which can effectively and rapidly adsorb a wide scope of polar organic micropollutants from water through biomimetic molecular recognition. Amide naphthotubes possess hydrogen bonding sites in their deep hydrophobic cavities and can effectively bind polar organic micropollutants in water through the hydrophobic effects and shielded hydrogen bonds. The cross-linked polymers containing amide naphthotubes are even able to remove a complex mixture of organic micropollutants from water and the used materials can be easily regenerated through washing with MeOH or EtOH. This research provides a solution for the treatment of polar organic micropollutants by using biomimetic molecular recognition in water.

Biomimetic Recognition of Quinones in Water by an Endo-Functionalized Cavity with Anthracene Sidewalls.

Selective molecular recognition in water is the foundation of numerous biological functions but is a challenge for most synthetic hosts. We employ the concept of endo-functionalized cavity and the strategy of simultaneous construction to address this issue. The concept and the strategy were demonstrated in the construction of a biomimetic host for selectively recognizing quinones in water. The host was synthesized by joining two pieces of bent anthracene dimer through amide bond formation, affording a deep hydrophobic cavity and inward-directing hydrogen bonding sites. The host can recognize quinones over their close analogues in water, and its association affinity to p-benzoquinone is the highest among all the known hosts and is even comparable to that of the bioreceptor. The binding with an anthraquinone reaches nanomolar affinity. Shielded hydrogen bonding, C-H⋅⋅⋅π, and charge transfer interactions, and the hydrophobic effect are responsible for the high binding affinity and selectivity.

Redox-Responsive Host-Guest Chemistry of a Flexible Cage with Naphthalene Walls.

"Naphthocage", a naphthalene-based organic cage, reveals very strong binding (up to 1010 M-1) to aromatic (di)cationic guests, i.e., the tetrathiafulvalene mono- and dication and methyl viologen. Intercalation of the guests between two naphthalene walls is mediated by C-H···O, C-H···π, and cation···π interactions. The guests can be switched into and out of the cage by redox processes with high binding selectivity. Oxidation of the flexible cage itself in the absence of a guest leads to a stable radical cation with the oxidized naphthalene intercalated between and stabilized by the other two. Encapsulated guest cations are released from the cavity upon cage oxidation, paving the way to future applications in redox-controlled guest release or novel stimuli-responsive materials.

Stabilization of the Closed-Ring Isomer of Spiropyran by Amide Naphthotube in Water and Its Application in Naked-Eye Detection of Toxic Paraoxon.

The thermodynamically unstable, colourless closed-ring isomer of spiropyran can be stabilized in water by the anti-configurational isomer of amide naphthotube. The influence of the binding on the thermodynamics and kinetics of spiropyran have been studied. The complex was further used to prepare a test paper that allows naked-eye detection of toxic paraoxon.

Mono-functionalized derivatives and revised configurational assignment of amide naphthotubes.

Amide naphthotubes with four carboxylate sidechains have been reported by us for selectively recognizing hydrophilic molecules in water and they have found applications in sensing and self-assembly. Modification of these macrocycles on the sidechains would further expand their applicability. Herein, we report the synthesis of mono-functionalized amide naphthotubes with one alkyne and three carboxylate groups. These naphthotubes show rather different binding affinities from that of the amide naphthotubes with four carboxylate sidechains. The partial self-inclusion of the alkyne group in the cavity was invoked to explain these differences. In addition, the syn- and anti-configurational isomers of the naphthotubes with four carboxylate groups were found to be assigned incorrectly in our earlier publication. Further evidence is provided here for the new assignment. The implications of this new structural assignment for the conclusions drawn in the earlier works are discussed as well.

Multiple Stimuli-Responsive Conformational Exchanges of Biphen[3]arene Macrocycle.

Conformational exchanges of synthetic macrocyclic acceptors are rather fast, which is rarely studied in the absence of guests. Here, we report multiple stimuli-responsive conformational exchanges between two preexisting conformations of 2,2',4,4'-tetramethoxyl biphen[3]arene (MeBP3) macrocycle. Structures of these two conformations are both observed in solid state, and characterized by 1H NMR, 13C NMR and 2D NMR in solution. In particular, conformational exchanges can respond to solvents, temperatures, guest binding and acid/base addition. The current system may have a role to play in the construction of molecular switches and other stimuli-responsive systems.

Prismarene: An Emerging Naphthol-Based Macrocyclic Arene.

Phenol-based macrocyclic arenes have been widely used in supramolecular chemistry, significantly enriching the toolbox of the field. In contrast, naphthol-based macrocyclic arenes are rather underdeveloped. Very recently, Gaeta and co-workers successfully synthesized such macrocycles (referred to as prism[n]arenes) with good guest-binding ability by reacting 2,6-dimethoxynaphthalene with paraformaldehyde under optimized conditions. In view of the simple synthesis and good host-guest chemistry, we anticipate that this macrocycle will find similar success and wide applications as the phenol-based macrocyclic arenes.

Adsorptive Separation of Benzene, Cyclohexene, and Cyclohexane by Amorphous Nonporous Amide Naphthotube Solids.

Benzene hydrogenation is an important industrial process. The reaction is incomplete, resulting in a mixture of benzene, cyclohexane, and/or cyclohexene that have to be separated before any further reactions. The currently used extractive and azeotropic distillations are operationally complex and energy intensive. Adsorptive separation provides an alternative energy-efficient method. However, the separation of the ternary mixture by adsorptive separation has not yet been reported. In the present research, we report two macrocyclic hosts with hydrogen-bonding sites in their cavities that are able to separate the ternary mixture of benzene, cyclohexene, and cyclohexane. N-H⋅⋅⋅π interactions were found to play a key role in the selective separation. In addition, fast adsorption, high loading ratios, and easy recycling are achieved with the present system, which is promising for practical applications.

Naphthocage: A Flexible yet Extremely Strong Binder for Singly Charged Organic Cations.

We report a quite flexible naphthol-based cage (so-called "naphthocage") which adopts a self-inclusion conformation in its free state and is able to bind singly charged organic cations extremely strongly ( Ka > 107 M-1). Ion-selective electrodes prepared with this naphthocage show a super-Nernstian response to acetylcholine. In addition, the highly stable complex (1010 M-1) between ferrocenium and the naphthocage can be switched electrochemically, which lays a basis for its application in stimuli-responsive materials.

Molecular Recognition of Hydrophilic Molecules in Water by Combining the Hydrophobic Effect with Hydrogen Bonding.

During the last half a century, great achievements have been made in molecular recognition in parallel with the invention of numerous synthetic receptors. However, the selective recognition of hydrophilic molecules in water remains a generally accepted challenge in supramolecular chemistry but is commonplace in nature. In an earlier Communication [ Huang et al. J. Am. Chem. Soc. 2016 , 138 , 14550 ], we reported a pair of endo-functionalized molecular tubes that surprisingly prefer highly hydrophilic molecules over hydrophobic molecules of a similar size and shape. The hydrophobic effect and hydrogen bonding were proposed to be responsible, but their exact roles were not fully elucidated. In this Article, we present a thorough study on the binding behavior of these molecular tubes toward 44 hydrophilic molecules in water. Principal component analysis reveals that the binding strength is weakly correlated to the hydrophobicity, volume, surface area, and dipole moment of guests. Furthermore, molecular dynamics simulations show the hydrophobic effect through releasing the poorly hydrogen-bonded cavity water contributes to the binding of all the hydrophilic molecules, while hydrogen bonding differentiates these molecules and is thus the key to achieve a high selectivity toward certain hydrophilic molecules over other molecules with a similar size and shape. Therefore, a good guest for these molecular tubes should meet the following criteria: the hydrogen-bonding sites should be complementary, and the molecular volume should be large enough to expel all the cavity water but not too large to cause steric hindrance. This rule of thumb may also be used to design a selective receptor for certain hydrophilic molecules. Following these guidelines, a "best-fit" guest was found for the syn-configured molecular tube with a binding constant as high as 106 M-1.

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 (ΔCp) 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.

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.