Pillar-Shaped Macrocyclic Hosts Pillar[n]arenes: New Key Players for Supramolecular Chemistry

Separation performances of extended pillar[6]arenes, a new stationary phase for gas chromatography

Pillar[n]arenes (P[n]As, n = 5-10), a new generation of macrocyclic hosts, display interesting properties such as 3D π-electron cavity, host-guest interactions, good stability, and easy functionalization, which make these polymers very promising for separation applications. In particular, extended P[n]As (EP[n]As) show large-sized cavities, structural flexibility, cavity adaptability, and synthetic accessibility. These characteristics have been exploited in this work, which reports for the first time the investigation of two EP[n]As (LP6A-C10 and BpP6A-C10) as stationary phases for gas chromatography (GC). LP6A-C10 and BpP6A-C10 columns exhibited moderate polarity (average polarity 117 and 118) and high column efficiencies (3000 plates/m and 3235 plates/m). These columns achieved complete challenging separation of halogenated benzene, benzaldehyde, phenol, and aniline isomers, which are difficult to resolve due to their high resemblance in structures and properties. Experimental results demonstrate the high selectivity and inertness of the prepared columns and their distinct advantages if compared with commercial HP-5 and HP-35 columns. EP[n]As columns displayed good separation repeatability with RSD values of 0.01%-0.02 % for run-to-run, 0.01%-0.22 % for day-to-day, and 1.36-3.56 % for column-to-column. This work demonstrates the promising future of EP[n]A stationary phases for chromatographic separations.

Chalcogen Bond-Driven Alkylations: Selenoxide-Pillar[5]arene as a Recyclable Catalyst for Displacement Reactions in Water

A novel strategy to catalyze alkylation reactions through chalcogen bond interaction using a supramolecular structure is presented herein. Utilizing just 1.0 mol % of selenoxide-pillar[5]arene (P[5]SeO) as the catalyst we achieved efficient catalysis in the cyanation of benzyl bromide in water. Our approach demonstrated high efficiency and effectiveness, with the results supported by designed control experiments and theoretical models, highlighting the catalytic effect of the pillar[5]arene through noncovalent interactions. Quantum-chemical calculations (ωB97X-D/def2-TZVP@SMD) pointed out that the catalyzed cyanation reaction followed an SN2-like mechanism, with energy barriers (ΔH≠) ranging from 16.7 to 18.2 kcal mol-1, exhibiting dissociative character depending on the para-substituent. 1H NMR analysis revealed that P[5]SeO acted as a catalyst through inclusion complex formation, facilitating the transfer of the electrophilic substrate to the aqueous solution for nucleophilic displacement. Our reaction protocol proved applicable to various substrates, including aromatic and alpha-carbonyl derivatives. The use of sodium azide as the nucleophile was also feasible. Importantly, our method allowed scalability, and the catalyst P[5]SeO could be recovered and reused effectively for multiple reaction cycles, showcasing sustainability.

Recent progress using novel tetraphenylethylene-based macrocyclic hosts in water

Macrocyclic structures are popular in supramolecular chemistry and have enjoyed considerable success as platforms for elaboration to container compounds and new materials. Host/guest studies in organic media have relied heavily on structures derived from crown ethers, calixarenes, cucurbiturils, resorcinarenes and pillararenes over the past decades. More recently, their water-soluble versions have been developed for potential applications in biology. Inspired by nature and the need for large-sized containers, Cao and co-workers have designed and synthesized a series of novel macrocyclic hosts based on the tetraphenylethylene (TPE) platform. These compounds have cationic frameworks with well-defined hydrophobic cavities for recognition of biomolecules (e.g. amino acids, nucleosides, peptides, proteins, coenzyme factors) in water. They offer multiple adaptive responses as sensors through fluorescence, circular dichroism and circularly polarized luminescence. These TPE-based hosts also show promising applications as stimuli-responsive fluorescent materials, in drug delivery and as artificial photofunctional systems. Herein, this review highlights this work as it establishes a new class of biomimetic, water-soluble supramolecular macrocyclic hosts.

A Crystalline 3D Supramolecular Polymer Constructed by Clamparene-Based Controllable Self-Assembly and Its Application in Photothermal Conversion

The development of well-defined three-dimensional supramolecular polymers presents significant challenges, particularly in achieving crystalline state structures. This study addresses this challenge by presenting the construction of a crystalline three-dimensional supramolecular polymer through the self-assembly of clamparene (CLP) and a naphthalene diimide derivative (NDIOH) in the solid state. The hierarchical self-assembly progresses from one-dimensional linear supramolecular polymers to two-dimensional supramolecular polymers and ultimately to a crystalline three-dimensional supramolecular polymer. Moreover, the prepared crystalline three-dimensional supramolecular polymer demonstrates effective photothermal conversion. This work advances the understanding and design of functional three-dimensional supramolecular polymers in the crystalline state.

Catalytic Enantioselective Synthesis of Planar Chiral Pillar[5]arenes via Asymmetric Sonogashira Coupling

As a novel type of macrocycles with attractive planar chirality, pillar[5]arenes have gained increasing research interest over the past decades, enabling their widespread applications in diverse fields such as porous materials, molecular machines, and chiral luminescence materials. However, the catalytic methodology towards the enantioselective synthesis of planar chiral pillar[5]arenes remains elusive. Here we report a novel method for the enantioselective synthesis of planar chiral pillar[5]arenes via asymmetric Sonogashira coupling, giving access to a wide range of highly functionalized planar chiral pillar[5]arenes, including both homo- and hetero-rimmed ones, with excellent enantioselectivities. Attractively, the resultant planar chiral pillar[5]arenes show great potential for widespread use in many areas such as chiral luminescent materials. This work not only enables the successful synthesis of planar chiral pillar[5]arenes with abundant structural and functional diversity as key building blocks for practical applications but also enriches the asymmetric cross-coupling methodologies in organic synthetic chemistry.

Enantiopure Macrocycles Based on Tröger's Base and Diphenyl Maleimide Exhibiting Strong Chiral Emission and Host-Guest Properties

While a plenty of macrocyclic hosts have been developed in supramolecular chemistry, those that combine chiral luminescent properties and host-guest recognition abilities are still uncommon. Herein, two pairs of enantiomeric macrocycles were synthesized via Suzuki-Miyaura [2+2] cyclization reactions using Tröger's base and diphenyl maleimide as the building blocks. The diphenyl maleimide units impart these macrocycles with highly strong fluorescence, achieving quantum yields up to 100 % in apolar solvents. Furthermore, the chiral, V-shaped Tröger's base units provide the macrocycles with circularly polarized luminescence (|glum|=1.68×10-3) and well-define cavity for hosting electron-deficient or positively charged guests with Ka up to 1.7×106 M-1.

Chiral Membrane Containing Subnanometer Channels for Enantioselective Transport Amino Acids

The research of chiral separation technology is of great significance for understanding the origin of life and promoting the application of chiral molecules. Herein, anionic chiral pillar[6]arene and cationic pillar[6]arene were designed and synthesized, and a chiral pillar[6]arene membrane was constructed by layer-by-layer assembly through electrostatic interactions. The transport rates of l-Ala and d-Ala in this channel were 14.33 and 1.86 μM cm-2 h-1, respectively, and the transport rate of l-Ala was 7.7 times that of d-Ala. In the transport experiment of Ala racemate, the ee value of l-Ala in the permeate after a single separation was 62%, indicating that this channel has a certain chiral separation ability. The effects of the assembly method, the number of chiral layers, and the driving force on the separation effect were further studied, and the selective transport mechanism was explored through the host-guest interaction at the molecular level and theoretical simulation. This strategy provides a new template for expanding the application of chiral pillar[n]arenes in the field of membrane separation.

Binding of Bioactive Ammonium Ions in Water with a Cavity-Based Selectivity: Water Solubilization versus Micellar Incorporation

Many bioactive molecules contain primary ammonium groups, generating significant interest in developing selective receptors for ammonium ions. A promising strategy involves the use of polyaromatic cavitands to achieve size and shape selectivity through their cavity. However, designing effective receptors for ammonium ions in aqueous media is challenging due to the competitive nature of water. Calix[5]arenes are known to selectively bind primary ammonium ions over secondary, tertiary, and quaternary ammonium ions in organic solvents. Here, we report on the binding properties of a calix[5]arene, which bears carboxyl groups on its small rim, in organic solvents and aqueous media. This receptor was transferred in water either through deprotonation of its carboxyl groups or by incorporation into dodecylphosphocholine micelles. 1H Nuclear Magnetic Resonance data confirmed the endo complexation of various primary ammonium ions in not only organic solvents but also both aqueous media. Cavity-based selectivity was also observed, validating the cavitand strategy for the selective binding of ammonium ions in water. Unique binding properties, driven by the calix[5]arene's intrinsic recognition ability and the hydrophobic effect, were observed in water. Notably, binding affinities for dopamine and lysine derivatives with log Ka values of >3.9 were determined. The direct solubilization of the receptor outperformed micellar incorporation due to the hydrophilic nature of the primary ammonium ions, which hinders their uptake into micelles. These results offer promising perspectives for the development of efficient chemosensors for the characterization of bioactive ammonium ions in water.

A pseudo-cubic metal-organic cage with conformationally switchable faces for dynamically adaptive guest encapsulation

The creation of hosts capable of accommodating different guest molecules may enable these hosts to play useful roles in chemical purifications, among other applications. Metal-organic cages are excellent hosts for various guests, but they generally incorporate rigid structural units that hinder dynamic adaptation to specific guests. Here we report a conformationally adaptable pseudo-cubic cage that can dynamically increase its cavity volume to fit guests with differing sizes. This pseudo-cube incorporates a tetramine subcomponent with 2,6-naphthalene arms that cooperatively adopt a non-planar conformation, enabling the cage faces to switch between endo and exo states. A wide range of guest molecules were observed to bind within the cavity of this cage, spanning a range of sizes from 46% to 154% of the cavity volume of the empty cage. Experimental and computational evidence characterizes the flipping of cage faces from endo to exo, expanding the cavity upon binding of larger guests.

Transmembrane Ion Channels: From Natural to Artificial Systems

Natural channel proteins allow the selective permeation of ions, water or other nutritious entities across bilayer membranes, facilitating various essential physiological functions in living systems. Inspired by nature, chemists endeavor to simulate the structural features and transport behaviors of channel proteins through biomimetic strategies. In this review, we start from introducing the inherent traits of channel proteins such as their crystal structures, functions and mechanisms. Subsequently, different kind of synthetic ion channels including their design principles, dynamic regulations and therapeutic applications were carefully reviewed. Finally, the potential challenges and opportunities in this research field were also carefully discussed. It is anticipated that this review could provide some inspiring ideas and future directions towards the construction of novel bionic ion channels with higher-level structures, properties, functions and practical applications.

Generation of Triplet States by Host-Stabilized Through-Space Conjugation for the Construction of Efficient Supramolecular Photocatalysts

Promoting the generation of triplet states is essential for developing efficient photocatalytic systems. This research presents a novel approach of host-stabilized through-space conjugation via the combination of covalent and non-covalent methods. The designed building block, 4,4'-(1,4(1,4)-dibenzene cyclohexaphane-1,4-diyl)bis(1-phenylpyridinium) chloride, features inherently stable through-space conjugation. When this block forms a 1 : 1 host-guest complex with cucurbit[8]uril, the through-space conjugation is further stabilized within the confined cavity. Both the generation and lifetime of triplet state are significantly increased, resulting from the host-stabilized through-space conjugation. Additionally, the ultrahigh binding constant of 6.58×1014 M-1 ensures the persistence of host-stabilization effect. As a result, the host-guest complex acts as a highly efficient catalyst in the photocatalytic oxidation of thioether and aromatic alcohol. In the photodegradation of lignin, a complex natural product, the host-guest complex also exhibits high efficiency, demonstrating its robustness. This line of research is anticipated to enrich the toolbox of supramolecular photochemistry and provide a strategy for fabricating efficient supramolecular photocatalysts.

Recent Applications of Pillararene-Inspired Water-Soluble Hosts

Pillararenes and their derivatives have emerged in supramolecular chemistry as unique macrocycles for applications in host-guest chemistry, materials science and biomimetics. Many variations have been conceived and synthesized in recent years and in this review, we relate progress in water-soluble versions: leaning towerarenes, extended-pillararenes, biphenarenes, helicarenes and octopusarenes. These are applied in targeted drug delivery, selective uptake and release of aromatic guests, fabrication of gold/silver and mesoporous silica nanoparticles, cell imaging, pollutant separation, biomedicine (e. g. biofilm disruptors, taste masking containers, neuromuscular blockers, antidotes for macromolecular biotoxin) and enantioselective recognition. It is intended that this review will be helpful for research in synthetic macrocyclic chemistry and supramolecular functional systems, leading to practical applications in various research areas.

Lighting Up Bispyrene-Functionalized Chiral Molecular Muscles with Switchable Circularly Polarized Excimer Emissions

Aiming at the further extension of the application scope of traditional molecular muscles, a novel bispyrene-functionalized chiral molecular [c2]daisy chain was designed and synthesized. Taking advantage of the unique dimeric interlocked structure of molecular [c2]daisy chain, the resultant chiral molecular muscle emits strong circularly polarized luminescence (CPL) attributed to the pyrene excimer with a high dissymmetry factor (glum) value of 0.010. More importantly, along with the solvent- or anion- induced motions of the chiral molecular muscle, the precise regulation of the pyrene stacking within its skeleton results in the switching towards either "inversed" state with sign inversion and larger glum values or "down" state with maintained handedness and smaller glum values, making it a novel multistate CPL switch. As the first example of chiral molecular muscle-based CPL switch, this proof-of-concept study not only successfully widens the application scopes of molecular muscles, but also provides a promising platform for the construction of novel smart chiral luminescent materials for practical applications.

Temperature-Dependent Left- and Right-Twisted Conformational Changes in 1 : 1 Host-Guest Systems: Theoretical Modeling and Chiroptical Simulations

An efficient strategy for high-performance chiral materials is to design and synthesize host molecules with left- and right- (M- and P-)twisted conformations and to control their twisted conformations. For this, a quantitative analysis is required to describe the chiroptical inversion, chiral transfer, and chiral recognition in the host-guest systems, which is generally performed using circular dichroism (CD) and/or proton nuclear magnetic resonance (1H NMR) spectroscopies. However, the mass-balance model that considers the M- and P-twisted conformations has not yet been established. In this study, we derived the novel equations based on the mass-balance model for the 1 : 1 host-guest systems. Then, we further applied them to analyze the 1 : 1 host-guest systems for the achiral calixarene-based capsule molecule, achiral dimeric zinc porphyrin tweezer molecule, and chiral pillar[5]arene with the chiral and/or achiral guest molecules by using the data obtained from the CD titration, variable temperature CD (VT-CD), and 1H NMR experiments. The thermodynamic parameters (ΔH and ΔS), equilibrium constants (K), and molar CD (Δϵ) in the 1 : 1 host-guest systems could be successfully determined by the theoretical analyses using the derived equations.

Mechanically Interlocked Macrocycles on Covalent Networks for Energy and Environmental Applications

Macrocycles' unique properties of interacting with guest molecules have been an intriguing scientific endeavor for many decades. They are potentially practically useful for engineering applications, especially in energy and environmental applications. These applications are usually demanding, involving a high temperature, pH, voltage, etc., thus, finding suitable substrates that can endure working environments and sustain macrocycles' properties is highly desirable. In that sense, covalent networks are ideal as they are chemically/electrochemically/thermally stable and can be porous by design. Emerging porous materials, especially covalent organic frameworks (COFs), could be suitable as their porous spaces allow macrocycles to interact with guest species. In the past seven years, we have seen the rise of mechanically interlocked macrocycles on covalent networks (MIMc-CNs) that translate macrocycles' properties into macroscale materials. In this conceptual review, we first describe the idea of integrating MIMcs into COFs or conventional amorphous polymers. Next, we review the reported representative MIMc-CNs used in energy and environmental applications. We also provide a brief outlook for the future directions for the MIMc-CNs research.

Photoswitchable Topological Regulation of Covalent Macrocycles, Molecular Recognition, and Interlocked Structures

Macrocycles represent one important class of functional molecules, and dynamic macrocycles with the potential of cleavability, adaptability, and topological conversion are challenging. Herein we report photoswitchable allosteric and topological control of dynamic covalent macrocycles and further the use in guest binding and mechanically interlocked molecules. The manipulation of competing ring-chain equilibria and bond formation/scission within reaction systems enabled light-induced structural regulation over dithioacetal and thioacetal dynamic bonds, accordingly realizing bidirectional switching between crown ether-like covalent macrocycles and their linear counterparts. The on-demand photoswitchable topological transformation of macrocycles further allowed guest recognition/release exhibiting controllable binding affinity and selectivity. To showcase the capability light-triggered assembly/disassembly of diverse mechanically interlocked structures, such as rotaxanes and catenanes, was achieved. The realization of photoswitchable topological conversion of covalent macrocycles, which has been rarely reported before, demonstrates the potential of light-triggered reactivity control and structural reconfiguration for enhanced complexity and sophisticated function. The strategies and results should be appealing to endeavors in molecular recognition, dynamic assemblies, molecular machines, and intelligent materials.

Fluorinated Conjugated Microporous Polymers Based on Pillar[n]arenes for Removal of Water Pollutants and Their Cation Selective Adsorption

Organic dyes are widely used in many applications. However, the leakage of organic dyes into the natural environment has become a severe and worldwide problem owing to their high toxicity and nonbiodegradability. Therefore, the development of effective removal technologies for organic dyes is required. In this article, we report the synthesis and adsorption properties of highly fluorinated conjugated microporous polymers based on pillar[n]arenes. The polymers exhibited large Brunauer-Emmett-Teller surface areas of up to 1063 m2 g-1 and selective adsorptive removal of cationic organic dyes from aqueous solutions. Comparison with the nonfluorinated polymers indicated that the adsorption mechanism mainly relies on the fluorine-cation electrostatic interaction. The maximum adsorption capacity reached 313 mg g-1 for crystal violet, which is higher than those of conventional adsorbents. Additionally, the fluorinated polymers could function as proton channels when they were embedded into lipid membranes.

Glyoxylic acid monohydrate promoted reductive addition of sodium sulfinates to pillar[4]arene[1]quinone

An efficient synthesis of sulfonate esters through reductive addition of sodium sulfinates to pillar[4]arene[1]quinone has been established (15 examples). Compared to the arylsulfonylation of p-quinone with sodium arylsulfinates under other acidic conditions, this work affords the hydroquinone-type 4-O-sulfonyl derivatives by using glyoxylic acid monohydrate as a promoter. The protocol features mild reaction conditions and high selectivity and is an alternative protocol for the O-sulfonylation of pillar[4]arene[1]hydroquinone.

A Pillar[5]arene-Based π-Conjugated Organic Small Molecule Emitter: Synthesis, Self-Assembly, and Selective Sensing of Cr2O7 2- Anion

A triphenylamine-containing π-conjugated pillar[5]arene luminescent small organic molecule has been synthesized via Suzuki-coupling reaction. This molecule can self-assemble to form linear supramolecular polymers in both solution and solid state. The molecule shows enhanced emission compared with parent pillar[5]arene in dilute solution. Based on the bright luminescent behavior, its sensing ability for Cr2O7 2- anion was studied.

Advances in Chiral Macrocycles: Molecular Design and Applications

Chiral macrocycles have recently emerged as promising materials for enantioselective recognition, asymmetric catalysis, and circularly polarized luminescence (CPL) due to their terminal-free structure, preorganized chiral cavities, and unique host-guest and self-assembly properties. This review summarizes recent advances in the design and synthesis of chiral macrocycles with central, axial, helical, and planar chirality, each imparting distinct structural and chiroptical characteristics. We highlight key strategies for constructing these macrocycles and their applications in optoelectronic and catalytic systems. Emphasis is placed on the balance between rigidity and flexibility in macrocycle design, essential for effective molecular recognition, adaptable catalysis, and CPL. We conclude with perspectives on future opportunities, anticipating ongoing developments in chiral macrocycle research.

Diastereoselective Polypseudorotaxane Formation with Planar Chiral Pillar[5]arenes via Co-crystallization Processes

As the number of chiral ring molecules in chiral polyrotaxane increases, the number of possible stereoisomers exponentially increases. Consequently, the selective synthesis of a specific stereoisomer becomes much more challenging. To address this problem, we co-crystallized poly(ethylene glycol) and a diastereomeric ring molecule, pillar[5]arene, in the solid state. The co-crystallization formed polypseudorotaxanes with a high diastereomeric excess (ca. 88 % de), meaning that polypseudorotaxanes containing (S, pS) stereoisomer pillar[5]arene rings were synthesized selectively. By contrast, in solution and evaporation systems, the selectivity remained low (ca. 10 % de). The results suggested that the packing effect by the co-crystallization contributed to the denser assembly of ring molecules on the polymeric chain, resulting in the diastereoselective formation. High diastereoselectivity was also observed even in higher-molecular-weight poly(ethylene glycol)s. These selectivities arose from the cooperative effects of the ring molecules on the polymeric chain, which were supported by calculating the stabilization energy.

Boosting the circularly polarized luminescence of pyrene-tiaraed pillararenes through mechanically locking

Attributed to their unique dynamic planar chirality, pillar[n]arenes, particularly pillar[5]arenes, have evolved as promising platforms for diverse applications such as circularly polarized luminescence (CPL) emitters. However, due to the unit flipping and swing, the achievement of excellent CPL performances of pillar[5]arenes in solution state remains a formidable challenge. To deal with this key issue, a mechanically locking approach has been successfully developed, leading to boosted dissymmetry factor (glum) values of pyrene-tiaraed pillar[5]arenes up to 0.015 through the formation of corresponding [2]rotaxanes. More importantly, taking advantage of the stably locked co-conformers, these resultant [2]rotaxanes maintain excellent CPL performances in diverse solvents and wide range of concentrations, making them promising candidates for practical applications. According to this proof-of-concept study, we have not only successfully developed a powerful strategy for the rational design of chiral luminescent materials with desired CPL performances but also contributed a promising platform for the construction of smart chiral materials.

Molecular-Squeeze Triggers Guest Desorption from Sponge-Like Macrocycle Crystals

Desorption in conventional porous sorbents often employ external forces including inert gas blowing, heating, vacuum treatment to trigger guest release. We here report an unprecedented molecular-squeeze triggered guest release behavior from sponge-like macrocycle crystals. The crystals function as typical sponge to include guest molecules within their microscopic voids that are adaptively formed, thus acting as adsorbents for toluene/pyridine separations. Intriguingly, vaporized ethyl acetate (EA) molecules trigger the guest release from the crystals without entering the pores or voids of the crystals to replace the guests. Instead, they work as external forces applied directly onto the crystals themselves, ''squeezing" the materials to close the voids through supramolecular interactions between EA and macrocycles on the crystal surface and release the guest molecules. Various experimental techniques as well as molecular dynamics simulations reveal the mechanism of the molecular-squeeze induced guest release procedure. The EA-regenerated crystals can be recycled multiple times without the loss of separation performance. Compared with conventional guest release procedure, this method is manipulated in a mild condition, showing the potential in saving cost and energy.

Preparation of a 1,1'-Binaphthyl-based Chiral Polyimine Macrocycle Bonded Chiral Stationary Phase by Thiol-ene Click Reaction and Its Enantioseparation Performance in High-Performance Liquid Chromatography

Chiral macrocycles have emerged as attractive media for chromatographic enantioseparation due to their excellent host-guest recognition properties. In this study, a new chiral stationary phase (CSP) based on 1,1'-binaphthyl chiral polyimine macrocycle (CPM) was reported. The CPM was synthesized by one-step aldehyde-amine condensation of (S)-2,2'-dihydroxy-[1,1'-binaphthalene]-3,3'-dicarboxaldehyde with 1,2-phenylenediamine and bonded on thiolated silica via the thiol-ene click reaction to afford the CSP. The enantioseparation performance of the CSP was evaluated by separating different types of racemates including alcohols, esters, ketones, amides, organic acids, and ethers in both normal-phase (NP) and reversed-phase (RP) elution modes. As a result, enantioseparations of 10 and 15 racemates were achieved in the two elution modes, respectively. Meanwhile, the effects of chromatographic conditions on separation, such as mobile phase composition and injection mass, were studied in detail. Moreover, a comparison of the proposed CSP for the separation of the tested racemates with commercial Chiralcel OD-H and Chiralpak AD-H columns was also conducted, and results revealed that the proposed CSP can achieve some enantioseparations that cannot be achieved by the two commercial columns. This study indicates that the chiral macrocycle is a promising chiral selector for high-performance liquid chromatography.

Facile synthesis of a new chiral polyimine macrocycle and its application for enantioseparation in high-performance liquid chromatography

Macrocyclic compounds such as crown ethers and cyclodextrins play an important role in the field of chromatography and show excellent separation performance. The design of simple and convenient methods for the efficient synthesis of novel chiral macrocycles for chromatographic separation is of great significance. In this work, a novel chiral polyimine macrocycle (PIMC) was designed and synthesized by the simply one-step reaction of 2,6-diformyl-4-tert-butylphenol with (S)-(-)-1,2-propanediamine. Then, it was bonded onto silica by the thiol-ene click reaction to construct a new chiral stationary phase (CSP) for high-performance liquid chromatography (HPLC). The chiral separation performance of the proposed CSP was examined by separating various racemates in the normal-phase (NP) and reversed-phase (RP) HPLC. In total, twelve and nine racemates, including ethers, esters, amines, alcohols, organic acids, ketones, and epoxides, were separated to varying degrees via NP-HPLC and RP-HPLC, respectively, Moreover, the CSP offered good chiral separation complementarity to Chiralcel OD-H and Chiralpak AD-H columns for resolution of these test racemates, and it can separate several racemic compounds that either cannot be separated or cannot be separated well be separated by the two commercially available columns. After the column was used for hundreds of injections, the relative standard deviations of the retention time and resolution were below 0.56 % and 0.45 %, respectively, showing the good reproducibility and stability of the CSP. This study provides a simple and convenient approach to synthesize a novel chiral macrocycle and CSP and also indicates the broad application prospects of such chiral PIMCs in HPLC chiral separation.

Partially Sulfated Pillar[5]Arenes: Synthesis and Molecular Recognition Properties

We report the synthesis and characterization of sulfated pillar[5]arene hosts (P5S2-P5S10) that differ in the number of sulfate substituents. All five P5Sn hosts display high solubility in water (73-131 mM) and do not undergo significant self-association according to 1H NMR dilution experiments. The x-ray crystal structures of P5S6, P5S6 ⋅ Me6HDA, P5S8 ⋅ Me6HDA, and P5S10 ⋅ Me6HDA reveal one intracavity molecule of Me6HDA and several external molecules of Me6HDA which form a network of close methonium ⋅ ⋅ ⋅ sulfate interactions. The thermodynamic parameters of complexation between P5Sn and the panel of guests was measured by direct or competitive isothermal titration calorimetry. We find that the binding free energy toward a guest becomes more negative as the number of sulfate substituents increase. Conversely, the binding free energy of a specific sulfated pillar[5]arene toward a homologous series of guests becomes more negative as the number of NMe groups increases. The ability to tune the host ⋅ guest affinity by changing the number of sulfate substituents will be valuable in supramolecular polymers, separation materials, and latching applications.

Reversible encapsulation and release of fullerenes using calix[n]phenoxazines

This investigation presents the synthesis of butyl-decorated calix[n]phenoxazines of varying sizes by kinetic control and the ring-expansion of calix[3]phenoxazine, which uniquely exhibits distinct binding affinities for fullerenes C60 and C70. Calix[3]phenoxazine demonstrates a higher binding affinity for cationic ammonium, which can be reversibly deprotonated and protonated, enabling the reversible release and reloading of fullerenes. This system holds potential for applications in fullerene extraction and separation.

Template and Solid-State-Assisted Assembly of an M9L6 Expanded Coordination Cage for Medium-Sized Molecule Encapsulation

The M6L4 cage, self-assembling from six Pd(II) or Pt(II) 90-degree blocks and four triazine-cored triangular ligands, has an effective hydrophobic cavity of about 450 Å3 capable of encapsulating one or more small molecules. Here, from the same components, we successfully constructed an M9L6 cage with an internal volume expanded to 1540 Å3 via the self-assembly of an M8L6 precursor using pillar[5]arene as a template. This cage retains the high molecular recognition ability of the M6L4 cage while recognizing medium-sized guest molecules with molecular weights of up to ∼1600.

Helical-Sense Matching Facilitates Supramolecular Copolymerization of Helical-Chiral Pillar[5]arenes

Supramolecular polymerization using two-dimensional π-conjugated chiral monomers has been mainly demonstrated because the supramolecular polymerization can be controlled by stereocommunication through π-π stacking between the two-dimensional chiral monomers. We herein report supramolecular copolymerization utilizing three-dimensional pentahedrons with twisted helical chirality through different combinations of helical-chiral acidic and basic pillar[5]arenes as comonomers. In this case, helical-sense matching is key to facilitating the supramolecular copolymerization. Based on the unique helical chirality of the three-dimensional pillared structure of the pillar[5]arenes and alternate ion-pairing interactions between acidic and basic groups on their bilateral rims, the homochiral helical-sense matching system forms kinetically stable nanowire-shaped supramolecular copolymers, generating the supramolecular gel in high concentrations. At elevated temperatures, the nanowire structure undergoes a transformation into thermodynamically stable nanoparticles, resulting in a gel-to-sol transition. This process can be hindered by introducing linear guest molecules, which prohibit the unit swing of pillar[5]arenes and stabilize the nanowires and supramolecular gel. By tailoring the enantiomeric ratio (e.r.) values of the chiral combinations, the helical-sense-dependent gel-to-sol transition was realized, specifically by decreasing the e.r. values. Because of helical-sense mismatching, the heterochiral system generates short, branched nanowires and presents as a turbid solution. These distinct differences reveal that the helical-sense matching between three-dimensional chiral pillar[5]arene comonomers is important for supramolecular copolymerization.

Recent advances in novel chiral macrocyclic arenes

Chiral macrocyclic arenes possess confined three-dimensional asymmetric cavities, electron-rich structures, chiral luminescence properties and excellent enantioselective recognition properties and have become a frontier and hotspot of macrocyclic chemistry and supramolecular chemistry. In recent years, there has been growing interest in the development of novel chiral macrocyclic arenes, which have found applications in various research areas. In this review, the construction, properties and functional applications of novel chiral macrocyclic arenes in enantioselective recognition, chiral sorting and construction of chiral luminescent materials according to their chiral types, including central, axial, planar, and inherent chiralities, are summarized. It is expected that this review will be helpful for research on supramolecular chemistry and for promoting the development of synthetic chemistry, materials chemistry and biochemistry.

Host-Guest Assemblies of Polyoxovanadate Clusters as Supramolecular Catalysts

Supramolecular functional materials can be used to overcome some of the most challenging tasks in materials science, where the dynamic nature of supramolecular interactions can be leveraged to fine-tune the properties of the material for a given task. The Lindqvist hexavanadate family of polyoxometalates (POMs) have emerged as particularly interesting candidates to be used in supramolecular materials due to their redox and Lewis acid properties that enable their application in the fields of energy conversion/storage or catalysis. Despite their promising potential, hexavanadate clusters are underrepresented in the field of supramolecular materials, mainly due to the synthetic challenges related to their inherent reactivity. In this work, pillar[5]arene was successfully grafted onto a Lindqvist hexavanadate and the resulting structure was confirmed by single crystal X-ray diffraction (SC-XRD), presenting the first example of a crystal structure of a POMcovalently functionalized with a pillar[5]arene. By introducing a ditopic guest molecule that could interlink pillar[5]arene moieties, host-guest interactions were leveraged as the driving force for the formation of supramolecular assemblies incorporating hexavanadate clusters in a controlled manner. The enhanced catalytic performance of the resulting aggregates confirmed their potential application as functional catalytic materials. This novel approach for developing hexavanadate-based catalysts reported here showcases the potential of using host-guest interactions as a means to introduce catalytically active metal-oxo clusters into supramolecular frameworks.

Structural insights into 1,4-bis-(neopent-yloxy)pillar[5]arene and the pyridine host-guest system

The crystal structure of 1,4-bis-(neopent-yloxy)pillar[5]arene, C95H140N2O10 (TbuP), featuring two encapsulated pyridine mol-ecules, reveals significant host-guest inter-actions. Inter-estingly, the pyridine guests are positioned near the neopent-yloxy substituents instead of the electron-rich aromatic core of the pillar[5]arene. This spatial arrangement suggests a preference for the pyridine mol-ecules to engage with the aliphatic regions of the host. Detailed analysis of the structural characteristics of this host-guest system (TbuP·2Py), as well as its packing pattern within the crystal network, is presented and discussed.

A chiral supramolecular liquid crystal based on pillararene and its application in information encryption

A chiral supramolecular liquid crystal based on a pillararene mesogen was constructed. The regulation of liquid crystal behavior was achieved through the host-guest interactions between the pillararene-based mesogen and a tetraphenylethylene-containing guest. In addition, this supramolecular liquid crystal system, showing pH-responsive fluorescence emission character, was applied as an information encryption material capable of storing multiple levels of distinct information, thereby enriching the application of liquid crystal materials in the field of information security.

Phenyl-Extended Resorcin[4]arenes: Synthesis and Highly Efficient Iodine Adsorption

The continuous exploration of new analogs of calixarenes and pillararenes unlocks infinite opportunities in supramolecular chemistry and materials. In this work, we introduce a new class of macrocycle, phenyl-extended resorcin[4]arenes (ExR4), a unique and innovative design that incorporates unsubstituted phenylene moieties into the resorcin[4]arene scaffold. Single-crystal analysis reveals a chair-like conformation for per-methylated ExR4 (Me-ExR4) and a twisted "Figure-of-eight" shaped conformation for per-hydroxylated ExR4 (OH-ExR4). Notably, OH-ExR4 demonstrates exceptional adsorption capability toward I3 - ions in an aqueous solution, with a rapid kinetic rate of 1.18×10-2 g ⋅ mg-1 ⋅ min-1. Furthermore, OH-ExR4 shows excellent recyclability and potential as a stationary phase in column setups. The discovery of ExR4 opens up new avenues for constructing new macrocycles and inspires further research in functional adsorption materials for water pollutant removal.

Supramolecular Exchange Reaction in the Cavity of Naphthalene Diimides Extended-Pillar[6]arene

A new naphthalene diimides extended-pillar[6]arene 1 with a large cavity and rich host-guest complexation properties was synthesized in high yield. It can not only form 1 : 2 complexes with large size polycyclic aromatic hydrocarbons but also form 1 : 1 : 1 ternary complex with perylene and 2,7-diazapyrenium. Moreover, the supramolecular exchange reaction from a 1 : 2 host-guest complex 1 ⋅ (G3)2 formed by 1 and perylene to a 1 : 1 : 1 ternary complex 1 ⋅ G3 ⋅ G5 formed by 1 with perylene and 2,7-diazapyrenium salt was also investigated by 1H NMR experiments as well as theoretically calculations.

Halogen bonding BODIPY-appended pillar[5]arene for the optical sensing of dicarboxylates and a chemical warfare agent simulant

A pillar[5]arene host, functionalised with halogen bonding (XB) recognition sites and BODIPY fluorophores, demonstrates strong binding and optical sensing of environmentally relevant dicarboxylates and a chemical warfare agent simulant, in organic and competitive aqueous-organic media - enabled by the unprecedented combination of fluorophore-conjugated XB interactions with the hydrophobic pillar[5]arene host cavity.

Structural Deformations in Cucurbit[n]urils: Analysis, Host-Guest Dependence, and Automated Ellipticity Measurements Using ElliptiCB[n]

Cucurbit[n]urils (CB[n]s) are cyclic macrocycles with rich host-guest chemistry. In many cases, guest binding in CB[n]s results in host structural deformations. Unfortunately, measuring such deformations remains a major challenge, with only a handful of manual estimations reported in the literature. To address this challenge, we have developed the public program ElliptiCB[n], which is available on GitHub, that provides a robust and automated method for measuring the elliptical deformations in CB[n] hosts. We outline the development and validation of this approach, apply ElliptiCB[n] to measure the ellipticity of the 1113 available CB[n] structures from the Cambridge Structural Database (CSD), and directly investigate the structural deformations of CB[5], CB[6], CB[7], CB[8], and CB[10] hosts. We also report the general landscape of accessible CB[n] elliptical deformations and compare ellipticity distributions across CB[n] hosts and host-guest complexes. We found that in almost all cases guest binding significantly impacts the distribution of host ellipticity distributions and that these distributions are dissimilar across host-guest complexes of differently sized CB[n]s. We anticipate that this work will provide a useful approach for understanding of the flexibility of CB[n] hosts and will also enable future measurement and standardization of ellipticity measurements of CB[n]s.

Quantum Chemical Approaches for Manipulation and Evaluation of Intracage Microelectric Field Strength of Molecular Containers in Y@C64 and Y@C64X4 (X = Cl, F, and H, Y = NH4Cl, H3O-Cl, and 2H2O)

The effect of an oriented external electric field (EEF) on materials has led to the ongoing development, which stimulates us to consider whether intracage microelectric fields (IMEFs) can be used to substitute for the EEF. Focusing on the manipulation and evaluation of the IMEF of asymmetric molecular containers, the host-guest compounds of interesting pineapple-shaped Y@C64X4 (X = vacant, Cl, F, and H; Y = NH4Cl, H3O-Cl, and 2H2O) are theoretically constructed and the strength of the IMEF was evaluated by the intrapotential energy surface analysis by using the point charge (q = +1) scanning method. Interestingly, the left and right halves of each cage are like two IMEFs connected in reverse series. Both the addition of four X atoms and the orientation of the guest can sensitively influence the IMEF's strengths and directions of both half cages and further determine the entire cage's IMEF. Subsequently, the IMEF can sensitively change the binding characteristics and properties of the guest species. Therefore, the manipulation and evaluation of the IMEF can be achievable. This work may provide support for an asymmetric molecular container with an IMEF to manipulate the novel structural and chemical bond characteristics of the guest species.

HFIP as a versatile solvent in resorcin[n]arene synthesis

Herein, we present 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as an efficient solvent for synthesizing resorcin[n]arenes in the presence of catalytic amounts of HCl at ambient temperature and within minutes. Remarkably, resorcinols with electron-withdrawing groups and halogens, which are reported in the literature as the most challenging precursors in this cyclization, are tolerated. This method leads to a variety of 2-substituted resorcin[n]arenes in a single synthetic step with isolated yields up to 98%.

Rotaxane Formation from Borate Ion-Containing Crown Ether and Ammonium Ion: Enhancement of Their Association through Ion-Pairing

We synthesized [2]rotaxanes featuring a catechol borate ion-containing crown ether and secondary ammonium ions. These rotaxane components show both ion-ion interactions and hydrogen bonds. X-ray crystallography and NMR spectroscopy allowed elucidation of the rotaxane structure. Moreover, 1H NMR spectroscopy revealed the rotaxane synthesis can be thermodynamically controlled. The binding affinity between the borate-containing crown ether and ammonium ions is enhanced by ion-pairing.

HI-6-Loaded Vehicle of Liposomes Mediated by an Amphiphilic Pillar[5]arene against Paraoxon Poisoning

Organophosphate (OP) intoxication has become a severe common health matter all over the world. For the treatment of acute OP poisoning, the effective intracerebral delivery of acetylcholinesterase reactivators is crucial. Here, an amphiphilic hydrazide-pillar[5]arene (HP5A-6C), which could be readily integrated into liposomal bilayers' zwitterionic disaturated phosphatidylcholine (DSPC), was synthesized. A T7 peptide-containing guest (G) was attached on the surface via a noncovalent interaction to make mixed liposomes a particularly appealing candidate for brain-targeting delivery. Such coassembly could remain stable at room temperature for up to 6 weeks, and safety evaluations initially verified its fine biological compatibility. The hydrophilic interiors of T7/HP5A-6C@DSPC could further load HI-6 with 89.70% encapsulation efficiency. Support for brain-targeting potency came from imaging results. Notably, intravenous injection of HI-6-loaded vesicles exhibited a remarkable therapeutic effect on paraoxon (POX)-poisoned mice, effectively alleviating seizures and brain damage and significantly increasing the improving survival rate to 60% over the course of 7 days.

Guest-Induced Conformational Transformations in Tiara[5]arene Crystals: A Pathway for Molecular Sieving

In pursuit of environmental sustainability and energy efficiency, assorted macrocyclic compounds have recently emerged as crystalline adsorbents for the efficient molecular sieving of various chemical commodities. Herein, we delve into the conformational characteristics and solid-state packing modes of tiara[5]arenes (T[5]), a rim-differentiated pillar[5]arene derivative. By meticulously exploring the conformational space, we have successfully identified a multitude of distinct T[5] conformers within a relatively narrow energy range of 22 kJ/mol. This finding underscores the inherent conformational flexibility of this macrocyclic scaffold, enabling T[5] to adapt diverse packing arrangements in the solid state. While solvent-free T[5] crystals do not exhibit permanent porosity, they undergo solvomorphic interconversions when exposed to various guest compounds. Our study demonstrates that T[5]-based crystalline materials exhibit a notable preference for selectively capturing aromatic and olefinic solvents, such as benzene, toluene, chlorobenzene, and cyclohexene, over their aliphatic hydrocarbon counterparts from equivalent volume liquid mixtures, achieving up to 10:1 selectivity between benzene and cyclohexane.

Telluride-Based Pillar[5]arene: A Recyclable Catalyst for Alkylation Reactions in Aqueous Solution

The syntheses of previously unknown sulfide- and telluride-pillar[n]arenes are reported here. These macrocycles, among others, were tested as catalysts for alkylation reactions in aqueous solutions. Telluride-pillar[5]arene (P[5]-TePh) showed the best performance, emulating the behavior of the methyltransferase enzyme cofactor S-adenosyl-l-methionine. Using 1.0 mol % of P[5]-TePh, benzyl bromides reacted with NaCN/NaN3 in water, yielding organic nitriles/azides. The catalyst was recycled and efficiently reused for up to six cycles. 1H NMR experiments indicate a possible interaction between the substrate and P[5]-TePh's cavity.

Six-Cyclic Crown Ether-Type Pillar[5]Arene: Enhanced Binding Ability to Bispyridinium Derivatives

A six-cyclic crown ether-type pillar[5]arene was synthesized, and the five ethylene oxide loops were located outside the cavity and not affected by temperature changes which was confirmed by variable-temperature NMR experiment in DMSO-d6 and CDCl3 and 2D 1H-1H NOESY experiment in CDCl3. The six-cyclic pillar[5]-crown also showed greater binding ability of host-guest with bis(pyridinium) derivatives than conventional alkoxy pillar[5]arenes that illustrated through 1H NMR titration spectroscopic experiment in acetone-d6/CDCl3 (1 : 1) and UV-vis titration experiments in CHCl3 at room temperature. The five benzocrown ethers at the periphery were able to bind metal cations by 1H NMR titration spectroscopic experiment in CD2Cl2/methanol-d4(9 : 1).

Preparation and evaluation of a 1,1'-bi-2-naphthol-based chiral macrocycle bonded silica chiral stationary phase for high performance liquid chromatography

Macrocycles play vital roles in supramolecular chemistry and chromatography. 1,1'-Bi-2-naphthol (BINOL)-based chiral polyimine macrocycles are an emerging class of chiral macrocycles that can be constructed by one-step aldehyde-amine condensation of BINOL derivatives with other building blocks. These macrocycles exhibit good characteristics, such as facile preparation, rigid cyclic structures, multiple chiral centers, and defined molecular cavities, that make them good candidates as new chiral recognition materials for chromatographic enantioseparations. In this study, a BINOL-based [2+2] chiral polyimine macrocycle was synthesized by one-step condensation of enantiopure (S)-2,2'-dihydroxy-[1,1'-binaphthalene]-3,3'-dicarboxaldehyde with (1R,2R)-1,2-diaminocyclohexane. The product was modified with 5-bromo-1-pentene and then attached to thiolated silica using click chemistry to construct a new chiral stationary phase (CSP). The enantioselectivity of the new CSP was explored by separating various racemates under normal phase (NP) and reversed phase (RP) high performance liquid chromatography (HPLC). Thirteen racemates and eight racemates were enantioseparated under the two separation modes, respectively, including chiral alcohols, phenols, esters, ketones, amines, and organic acids. Among them, nine racemates achieved baseline separation under NP-HPLC and seven racemates achieved baseline separation under RP-HPLC. High resolution separation was observed with benzoin (Rs = 5.10), epinephrine (Rs = 4.98), 3-benzyloxy-1,2-propanediol (Rs = 4.42), and 4,4'-dimethylbenzoin (Rs = 4.52) in NP-HPLC, and with 4-methylbenzhydrol (Rs = 4.72), benzoin ethyl ether (Rs = 3.79), 1-phenyl-1-pentanol (Rs = 3.68), and 1-(3-bromophenyl)ethanol (Rs = 3.60) in RP-HPLC. Interestingly, the CSP complemented Chiralcel OD-H, Chiralpak AD-H, and CYCLOBOND I 2000 RSP columns for resolution of these test racemates, separating several racemic compounds that could not be well separated by the three commercially available columns. The influences of injected sample amount on separation were also evaluated. It was found that the column exhibited excellent stability and reproducibility after hundreds of injections, and the relative standard deviations (n = 5) of the retention time and resolution were less than 0.49% and 0.69%, respectively. This study indicates that the BINOL-based chiral macrocycle has great potential for HPLC enantioseparation.

Bispillar[5]arene-Based Slide-Ring Polyrotaxanation Enables Enhanced Toughness, Recyclability, Impact, and Puncture Resistance of Polyisoprene Elastomers

A series of slide-ring polyrotaxanes (SRPs) have been constructed by the solvent-free blending of a ditopic pillar[5]arene (DP5A) and polyisoprene (PIP) after thermal annealing. Solid-state 13C NMR experiments supported the fact that the pillar[5]arene rings of DP5A were threaded by PIP chains to afford physically interlocked networks. Tensile tests revealed that 1% of DP5A can improve the elongation at break from 50 to 239%, the tensile modulus from 2.1 to 3.9 MPa, and the toughness from 0.35 to 4.5 MJ/m3. Impact and puncture resistance experiments show that the DP5A-doped materials exhibit remarkable enhancement of protective and impalement-resistant performance. The samples can be also recycled repeatedly due to their physical crosslinking nature. The important stress delocalization effects have been attributed to the pulley effect of DP5A in the SRP materials, which represents a supramolecular approach for improving the performance of PIP elastomers.

A Supramolecular Biosensor for Rapid and High-Throughput Quantification of a Disease-Associated Niacin Metabolite

Metabolic abnormalities play a pivotal role in various pathological conditions, necessitating the quantification of specific metabolites for diagnosis. While mass spectrometry remains the primary method for metabolite measurement, its limited throughput underscores the need for biosensors capable of rapid detection. Previously, we reported that pillar[6]arene with 12 carboxylate groups (P6AC) forms host-guest complexes with 1-methylnicotinamide (1-MNA), which is produced in vivo by nicotinamide N-methyltransferase (NNMT). P6AC acts as a biosensor by measuring the fluorescence quenching caused by photoinduced electron transfer upon 1-MNA binding. However, the low sensitivity of P6AC makes it impractical for detecting 1-MNA in unpurified biological samples. In this study, we found that P6A with 12 sulfonate groups (P6AS) is a specific and potent supramolecular host for 1-MNA interactions even in biological samples. The 1-MNA binding affinity of P6AS in water was found to be (5.68 ± 1.02) × 106 M-1, which is approximately 700-fold higher than that of P6AC. Moreover, the 1-MNA detection limit of P6AS was determined to be 2.84 × 10-7 M, which is substantially lower than that of P6AC. Direct addition of P6AS to culture medium was sufficient to quantify 1-MNA produced by cancer cells. Furthermore, this sensor was able to specifically detect 1-MNA even in unpurified human urine. P6AS therefore enables rapid and high-throughput quantification of 1-MNA, and further improvement of our strategy will contribute to the establishment of high-throughput screening of NNMT inhibitors, diagnosis of liver diseases, and imaging of human cancer cells in vivo.

Crystal structure and supra-molecular features of a host-guest inclusion complex based on A1/A2-hetero-difunctionalized pillar[5]arene

A host-guest supra-molecular inclusion complex was obtained from the co-crystallization of A1/A2-bromo-but-oxy-hy-droxy difunctionalized pillar[5]arene (PilButBrOH) with adipo-nitrile (ADN), C47H53.18Br0.82O10·C6H8N2. The adipo-nitrile guest is stabilized within the electron-rich cavity of the pillar[5]arene host via multiple C-H⋯O and C-H⋯π inter-actions. Both functional groups on the macrocyclic rim are engaged in supra-molecular inter-actions with an adjacent inclusion complex via hydrogen-bonding (O-H⋯N or C-H⋯Br) inter-actions, resulting in the formation of a supra-molecular dimer in the crystal structure.

Kinetic-thermodynamic correlation of conformational changes in ammonium complexes of a flexible naphthocage

Conformational changes in non-covalent complexes are of fundamental importance to many chemical and biological processes. Yet, these low-energy structural changes are usually fast and difficult to monitor, which poses challenges in their detailed kinetic understanding. The correlation between kinetics and thermodynamics of the conformational change of a model supramolecular system featuring a flexible naphthocage and quaternary ammonium guests is described in this work. Guest binding initially locks the host in two major conformations, which then equilibrates over time to the more stable conformer. The overall rate of the system to attain conformational equilibrium is found to inversely correlate with the thermodynamic stability of the host-guest complexes, and hence not only can the kinetic parameters of the conformational exchange be predicted from the easily obtainable thermodynamic data, but the kinetic profile can also be rationalized by using the structural properties of the different guests.