A1/A2-Diamino-Substituted Pillar[5]arene-Based Acid–Base-Responsive Host–Guest System

Rational Design of Supramolecular Receptors for Consistent Binding Affinities under High-Salinity Conditions

The development of water-soluble multicharged macrocycles has opened promising pathways in biomedical applications, enabling selective molecular recognition for therapeutic and diagnostic uses. Yet, traditional polyanionic and polycationic receptors often face performance limitations under realistic operating conditions. A major drawback is the natural tendency of these polycharged hosts to experience increasing screening effects as concentration rises due to self-ion pairing phenomena, which can reduce binding efficiency by several orders of magnitude. These issues are further intensified when polyionic receptors are used in high-salinity environments, typically used to replicate physiological settings, where the abundance of ions introduces additional screening effects that diminish the supramolecular affinity for a wide range of guests. This study presents a new approach that leverages zwitterionic synthetic receptors with rationally engineered architectures to overcome these challenges. By incorporation of specific structural features, self-ion pairing is eliminated, effectively making host concentration no longer a controlling factor in the thermodynamics of the complexation process. Additionally, these dual-charged hosts achieve self-contained stabilization, naturally shielding recognition sites from external ion interference under high-salinity conditions. Furthermore, the ability of these supramolecular hosts to encapsulate zwitterionic guests, a challenging task due to the strong solvation of these molecules in aqueous solution, adds significant value to the functional versatility of these macrocycles. Altogether, these findings represent a significant advancement in the design of stable and adaptable receptor systems for complex environments.

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.

Ortho-Functionalization of Pillar[4]arene[1]benzoquinone Monoxime via Selective 1,4-Addition of Grignard Reagents

Ortho-functionalization of pillar[n]arenes has been a formidable challenge, partially due to the fragility of their macrocyclic skeletons. In this concise report, we describe a facile synthetic method for monoarylation/alkylation at the position ortho to the oxime functionality in pillar[4]arene[1]benzoquinone monoxime (1) via addition of Grignard reagents. The described method enables the creation of various mono-ortho-alkyl/aryl-substituted pillar[5]arene derivatives that were previously inaccessible.

Pillar[5]arenes decorated with six-membered-ring aromatics at all the substitution positions

Macrocyclic molecules have characteristic properties different from linear ones, such as high symmetry and guest-inclusion ability. To bring drastic changes to these properties, direct introduction of many substituents is a challenging but effective tool. Herein, we attain direct installation of ten six-membered-ring aromatic π-units into both rims of a pillar[5]arene. In contrast to previous pillar[n]arenes with less hindered five-membered-ring units, which showed conformational complexity and crushed crystal structures, the per-phenyl-substituted pillar[5]arene has a cylinder-shaped crystal structure with a dichloromethane inside the cavity and is obtained as a single pair of D 5-symmetric enantiomers. The average dihedral angles between the core and peripheral benzene rings sharply increase from 38° to 66°. These differences indicate the importance of local steric repulsion on both rims for determining the structures and properties of macrocycles.

Polydopamine modified by pillar[5]arene in situ for targeted chemo-photothermal cancer therapy

A pillar[5]arene-modified polydopamine (PDA-P[5]OH) displaying pH/NIR dual-responsive properties was constructed successfully in situ for targeted chemo-photothermal cancer therapy.

Per-Arylation of Pillar[n]arenes: An Effective Tool to Modify the Properties of Macrocycles

Installation of various substituents is a reliable and versatile way to alter the properties of macrocyclic molecules, but high-yield and controlled methods are not always available especially for multifold reactions. Herein, we report 10- and 12-fold introduction of aryl substituents onto both rims of cylinder-shaped pillar[n]arenes, which usually have alkoxy substituents slanting to the cylinder axes. Although alkoxy pillar[5]arenes exist as D₅-symmetric enantiomeric pairs, arylated pillar[5]arenes provide crushed single-crystal structures and stereoisomerism including C₂-symmetric conformations depending on the aryl groups. Pillar[n]arenes with 2-benzofuranyl groups display bright fluorescence with quantum yields of 88-90% and no host-guest complexation with electron-deficient molecules in solution due to large deviation from alkoxy compounds. A benzofuran-appended pillar[6]arene instead captures small gaseous molecules in the solid state, probably owing to outside spaces surrounded by aromatic rings.

Counteranions at Peripheral Sites Tune Guest Affinity for a Protonated Hemicryptophane

The affinity of small molecules for biomolecular cavities is tuned through a combination of primary and secondary interactions. It has been challenging to mimic these features in organic synthetic host molecules, however, where the cavities tend to be highly symmetric and nonpolar, and less amenable to chemical manipulation. Here, a host molecule composed of a TREN ligand and cyclotriveratrylene moiety was investigated. Size-matched polar guests were encapsulated within the cavity via triple protonation of the TREN moiety with various sulfonic acids. X-ray crystallography confirmed guest encapsulation and identified three methanesulfonates, p-toluenesulfonates, or 2-naphthalenesulfonates hydrogen-bonded with H3TREN at the periphery of the cavity. These structurally diverse counteranions were shown by 1H NMR spectroscopy to differentially regulate guest access at the three portals, and to undergo competitive displacement in solution. This work reveals "counteranion tuning" to be a simple and powerful strategy for modulating host-guest affinity, as applied here in a TREN-hemicryptophane.

Pillar[4]arene[1]thioarene: Synthesis and Host-Guest Binding Properties

The synthesis and characterisation of novel class of pillar[4]arene[1]thioarenes (P[4]A[1]SMe) are reported. Synthesised via an oxidation-thionation strategy, the replacement of a single dialkoxybenzene panel from parent pillar[5]arene (P[5]A) is achieved. 1H-NMR spectroscopic titration experiments, supported by DFT computational studies, revealed that P[4]A[1]SMe show starkly modulated host-guest binding properties for electron deficient aliphatic guests.

ortho-Functionalization of Pillar[5]arene: An Approach to Mono-ortho-Alkyl/Aryl-Substituted A1/A2-Dihydroxypillar[5]arene

Despite the fact that the rim and lateral functionalizations of pillar[n]arenes have been well explored, ortho-functionalization has rarely been realized. In this work, we report a facile method of introducing a single functionality ortho to the hydroxyl group in A1/A2-dihydroxypillar[5]arene via a Grignard addition to pillar[4]arene[1]quinone followed by a dienone-phenol rearrangement. The described ortho-alkylation/arylation method allowed formation of various mono ortho-alkyl/aryl-substituted A1/A2-dihydroxypillar[5]arenes previously difficult to obtain.

Rim-differentiated Co-pillar[4+1]arenes

A series of rim-differentiated Co-pillar[4+1]arenes featuring penta-substituted "upper" rims and mono-functionalisable "lower" rims was successfully synthesised and fully characterised. These novel pillar[5]arene-based scaffolds with clickable moieties and extra synthetic handles are versatile platforms for self-assembled molecular architectures and biological applications.

Supramolecular-Macrocycle-Based Crystalline Organic Materials

Supramolecular macrocycles are well known as guest receptors in supramolecular chemistry, especially host-guest chemistry. In addition to their wide applications in host-guest chemistry and related areas, macrocycles have also been employed to construct crystalline organic materials (COMs) owing to their particular structures that combine both rigidity and adaptivity. There are two main types of supramolecular-macrocycle-based COMs: those constructed from macrocycles themselves and those prepared from macrocycles with other organic linkers. This review summarizes recent developments in supramolecular-macrocycle-based COMs, which are categorized by various types of macrocycles, including cyclodextrins, calixarenes, resorcinarenes, pyrogalloarenes, cucurbiturils, pillararenes, and others. Effort is made to focus on the structures of supramolecular-macrocycle-based COMs and their structure-function relationships. In addition, the application of supramolecular-macrocycle-based COMs in gas storage or separation, molecular separation, solid-state electrolytes, proton conduction, iodine capture, water or environmental treatment, etc., are also presented. Finally, perspectives and future challenges in the field of supramolecular-macrocycle-based COMs are discussed.

per-Alkoxy-pillar[5]arenes as Electron Donors: Electrochemical Properties of Dimethoxy-Pillar[5]arene and Its Corresponding Rotaxane

1,4-dimethoxypillar[5]arene undergoes reversible multielectron oxidations forming stable radical cations, a property retained when incorporated in [2]rotaxanes, suggesting that pillar[5]arenes can be employed as viable, yet unreported, electron donors.

Vapochromic crystals: understanding vapochromism from the perspective of crystal engineering

Vapochromic materials, which undergo colour and/or emission changes upon exposure to certain vapours or gases, have received increasing attention recently because of their wide range of applications in, e.g., chemical sensors, light-emitting diodes, and environmental monitors. Vapochromic crystals, as a specific kind of vapochromic materials, can be investigated from the perspective of crystal engineering to understand the mechanism of vapochromism. Moreover, understanding the vapochromism mechanism will be beneficial to design and prepare task-specific vapochromic crystals as one kind of low-cost 'electronic nose' to detect toxic gases or volatile organic compounds. This review provides important information in a broad scientific context to develop new vapochromic materials, which covers organometallic or coordination complexes and organic crystals, as well as the different mechanisms of the related vapochromic behaviour. In addition, recent examples of supramolecular vapochromic crystals and metal-organic-framework (MOFs) vapochromic crystals are introduced.

Controlled binding of organic guests by stimuli-responsive macrocycles

Synthetic supramolecular chemistry pursues not only the construction of new matter, but also control over its inherently dynamic behaviour.

Resolution and Racemization of a Planar-Chiral A1/A2-Disubstituted Pillar[5]arene

Butoxycarbonyl (Boc)-protected pillar[4]arene[1]-diaminobenzene (BP) was synthesized by introducing the Boc protection onto the A1/A2 positions of BP. The oxygen-through-annulus rotation was partially inhibited because of the presence of the middle-sized Boc substituents. We succeeded in isolating the enantiopure RP (RP, RP, RP, RP, and RP)- and SP (SP, SP, SP, SP, and SP)-BP, and studied their circular dichroism (CD) spectral properties. As the Boc substituent is not large enough to completely prevent the flip of the benzene units, enantiopure BP-f1 underwent racemization in solution. It is found that the racemization kinetics is a function of the solvent and temperature employed. The chirality of the BP-f1 could be maintained in n-hexane and CH2Cl2 for a long period at room temperature, whereas increasing the temperature or using solvents that cannot enter into the cavity of BP-f1 accelerated the racemization of BP-f1. The racemization kinetics and the thermodynamic parameters of racemization were studied in several different organic solvents.

Constitutional isomers of brominated-functionalized copillar[5]arenes: synthesis, characterization, and crystal structures

We herein report the preparation of constitutional isomers of brominated-functionalized pillar[5]arenes via co-condensation of 1,4-bis(2-bromoethoxy)benzene and 1,4-dimethoxybenzene. The structures of the obtained isomers were then established using single crystal X-ray diffraction. We also found that the isomeric yield distribution of the different constitutional isomers was independent of the monomer's mole feed ratio, as revealed by HPLC analysis of the crude mixture. Finally, further characterization of the separated constitutional isomers indicated that they possess different melting points, NMR spectra, crystal structures, binding constants and stacking patterns in the solid state.

Constitutional isomers of brominated-functionalized pillar[5]arenes were synthesized using a co-cyclization strategy.

Synthesis, functionalization, and isolation of planar-chiral pillar[5]arenes with bulky substituents using a chiral derivatization agent

Bulky perneopentyloxy-pillar[5]arene (Pillar-1) was synthesized and its conformational mobility was investigated using variable-temperature ¹H NMR spectroscopy. The host–guest interactions between Pillar-1 and n-octyltrimethylammonium hexafluorophosphate (OMA) were investigated, and the formation of a 1 : 1 complex was revealed via¹H NMR. Planar-chiral isomers were synthesized via the reaction of a hydroxy-functionalized pillar[5]arene with chiral derivatization agent (S)-(+)-MTPA-Cl. The (Sp, R)-and (Rp, R)-forms of the pillar[5]arene diastereomers were isolated by HPLC, and their structures were analyzed by ¹⁹F NMR. HPLC measurements indicated that racemization did not take place at 40 °C for 72 h.

Bulky perneopentyloxy-pillar[5]arene was synthesized. Complexation behavior and conformational mobility were investigated using ¹H NMR spectroscopy. Isolation of planar-chiral pillar[5]arenes using a chiral derivatization agent were carried out.

Unidirectional complexation of pillar[4]arene[1]benzoquinoneoxime with alkyl alcohols

Unidirectional binding between a pillar[4]arene[1]benzoquinoneoxime host and n-alkyl alcoholic guests was realized with the hydroxy heads of the guests in direct contact with the oxime group of the macrocyclic host.

Chiral discrimination of 2-heptlyaminium salt by planar-chiral monohydroxy-functionalized pillar[5]arenes

A chiral receptor was synthesized based on monohydroxy-functionalized pillar[5]arenes and its ability to discriminate alkyl aminium salts is demonstrated.

Systematic rim cyano-functionalization of pillar[5]arene and corresponding host–guest property varieties

Synthetic investigation of the systematic replacement of alkoxy groups in pillar[5]arene by cyano ones was carried out. Host–guest property studies of nCN-pillar[5]arenes and 1,4-dicyanobutane showed the functionization number related host–guest interaction varieties.

Restricting shuttling in bis(imidazolium)…pillar[5]arene rotaxanes using metal coordination

Metal coordination to a series of bis (imidazolium)…pillar[5]arene [2]rotaxanes through the formation of metal-carbene bonds facilitates a new strategy to restrict the shuttling motion in [2]rotaxanes. Whereas the pillar[5]arene macrocycle rapidly shuttles along the full length of the bis (imidazolium) rod for the parent [2]rotaxane, Ag(i) coordination to the imidazolium groups through the formation of N-heterocyclic carbenes leads to restricted motion, effectively confining the shuttling motion of the [2]rotaxane. The Ag(i) coordinated [2]rotaxanes can be reacted further, either removing the Ag-carbene species to recreate the parent [2]rotaxane, or reaction with more bulky Pd(ii) species to further restrict the shuttling motion through steric inhibition.

A dual redox-responsive supramolecular amphiphile fabricated by selenium-containing pillar[6]arene-based molecular recognition

A dual redox-responsive pillar[6]arene-based supramolecular amphiphile was fabricated in water. The self-assembly behavior of this supramolecular amphiphile in response to dual redox stimuli was investigated.

Encapsulated di-chloro-ethane-mediated inter-locked supra-molecular polymeric assembly of A1/A2-dihydroxy-oct-yloxy pillar[5]arene 1,2-di-chloro-ethane monosolvate

Crystals of 1-(1,4-dihy-droxy)-2,3,4,5-(1,4-dioct-yloxy)-pillar[5]arene, C99H158O10·C2H4Cl2, were grown from a 1,2-dicholoro-ethane/n-hexane solvent system. In the crystal, the encapsulated 1,2-di-chloro-ethane solvent is stabilized by C-H⋯π inter-actions and mediates the formation of an inter-locked supra-molecular polymer via C-H⋯Cl inter-actions.

Single-crystal X-ray diffraction study of a host-guest system comprising monofunctionalized-hydroxy pillar[5]arene and 1-octa-namine

Co-crystallization of a monofunctionalized hy-droxy pillar[5]arene with 1-octa-namine resulted in the formation of an inclusion complex where the alkyl chain is threaded in the macrocycle cavity, namely 1,2,3,4-(1,4-dimeth-oxy)-5-(1-hy-droxy-4-meth-oxy)-pillar[5]arene-1-octa-namine-water (1/1/1), C44H48O10·C8H19N·H2O. The guest compound is stabilized inside the cavity by hydrogen-bonding and C-H⋯π inter-actions. The water mol-ecule in the asymmetric unit mediates the formation of a supra-molecular dimer by hydrogen-bonding inter-actions. These functionalized-pillararene hosts expand the possibility of exploring more supra-molecular inter-actions with various guest species.

A new approach for the synthesis of mono- and A1/A2-dihydroxy-substituted pillar[5]arenes and their complexation with alkyl alcohols in solution and in the solid state

A new method for synthesizing hydroxy-functionalized pillar[5]arenes by catalytic hydrogenation to remove the benzyl protecting groups of a pillararene precursor has been employed.

Guest-regulated chirality switching of planar chiral pseudo[1]catenanes

Pillar[5]arene-derived pseudo[1]catenanes in dichloromethane exist in an equilibrium between a self-included conformational state and a de-threading one.

Engineering a pillar[5]arene-based supramolecular organic framework by a co-crystallization method

A perhydroxyl-pillar[5]arene and 4,4-bipyridine based supramolecular organic framework (SOF) was prepared by using a co-crystallization approach.

Bis- and mono(m-benzoic acid)-functionalized pillar[5]arenes

Installation of m-benzoic acid functionalities on pillar[5]arene rims resulted in bis- and mono(m-benzoic acid)-functionalized pillar[5]arenes 1 and 2. Bis(m-benzoic acid)-functionalized pillar[5]arene 1 was able to self-assemble to form one-dimensional channels with DMF molecules residing in pillar[5]arene cavities. Esterification of two carboxylic acids in 1 with decane-1,10-diol did not afford a [1]catenane, but a bicyclic compound. Although 1-decanol esterification of mono(m-benzoic acid)-functionalized pillar[5]arene 2 did not form a self-included [1]pseudorotaxane-like structure, a mono(decyl m-benzoate)-functionalized pillar[5]arene bearing an ethyl acetate chain was found to form a self-included complex with the ethyl acetate moiety residing inside the pillar[5]arene cavity.

A redox-responsive supramolecular amphiphile fabricated by selenium-containing pillar[5]arene-based host–guest recognition

A novel redox-responsive molecular recognition motif was built between a neutral selenium-containing pillar[5]arene and a pyridinium bromide salt in water. It was further used to construct the first pillararene-based selenium-containing supramolecular amphiphile with application in controlled release.

Synthesis of novel chiral imidazolium stationary phases and their enantioseparation evaluation by high-performance liquid chromatography

Two novel chiral stationary phases (CSPs) were prepared by bonding chiral imidazoliums on the surface of silica gel. The chiral imidazoles were derivatized from chiral amines, 1-phenylethylamine and 1-(1-naphthyl)ethylamine. The obtained CSPs were characterized by Fourier Transform Infrared (FT-IR) spectroscopy and elemental analysis (EA), demonstrating the bonding densities of CSP 1 and CSP 2 were 0.43 mmol g^-1 and 0.40 mmol g^-1, respectively. These two CSPs could be used to availably separate 8 pharmaceuticals, 7 mandelic acid/its derivatives, 2 1-phenylethylamine derivatives, 1 1,1'-bi-2-naphthol, and 1 camphorsulfonic acid in high-performance liquid chromatography (HPLC). It is found that CSP 1 could effectively enantioseparate most chiral analytes, especially the acidic components, while CSP 2 could enantiorecognize all chiral analytes, although a number of components did not achieve baseline separation. Additionally, the effects of mobile phase composition, mobile phase pH and salt content, chiral selector structures, and analyte structures on the enantiorecognitions of the two CSPs were investigated. It is found that high acetonitrile content in mobile phases was conducive to enantiorecognition. Mobile phase pH and salt content could alter the retention behaviors of different enantiomers of the same chiral compound, resulting in better enantioresolution. Moreover, both chiral selector structures and substituted groups of analytes played a significant role in the separation of chiral solutes.