Synthesis of a Water-Soluble Carboxylatobiphen[4]arene and Its Selective Complexation toward Acetylcholine

Rapid Detection of Zeranol Contamination in Cereals Using a Quaternary Ammonium-Functionalized Terphen[3]arene-Based Optical Sensor

To ensure food safety and quality, sensitive and accurate methods for rapidly detecting mycotoxins have become imperative. Zeranols (ZERs) are a class of mycotoxins commonly found in cereals, posing serious health risks, including hormonal disruption and carcinogenic potential. In response to this pressing concern, we have developed a simple yet highly sensitive and high-throughput supramolecular sensing approach based on novel macrocycles known as extended biphen[n]arenes for monitoring ZERs in cereal matrices. The proposed approach utilizes the indicator displacement assay (IDA) and employs quaternary ammonium-functionalized terphen[3]arene (CTP3) as the host molecule for capturing ZERs. This method achieves a highly sensitive response, owing to the robust synergistic interactions between CTP3 and ZERs, ensuring the reliable detection of these harmful compounds. Significantly, not only does the established approach provide an alternative strategy for inspecting high-risk grains contaminated by ZERs, but it also demonstrates novel applications of biphen[n]arenes for mycotoxin detection.

Water soluble macrocyclic host for recognition of N-methylquinolinium salts in water

In this paper, we reported the synthesis of water soluble macrocyclic arenes 1 containing anionic carboxylate groups. It was found that host 1 could form a 1 : 1 complex with N-methylquinolinium salts in water. Moreover, the complexation and decomplexation of the complexes between host and the guests could be achieved by changing the pH of the solution, and the process could also be observed by naked eye.

Recent advances in the synthesis and applications of macrocyclic arenes

Macrocyclic arenes including calixarenes, resorcinarenes, cyclotriveratrylene, pillararenes and so on have emerged as highly attractive synthetic macrocyclic hosts due to their unique structures, facile functionalization, and broad range of applications. In recent years, there has been growing interest in the development of novel macrocyclic arenes composed of various aromatic building blocks bridged by methylene groups, which have found applications in various research areas. Consequently, the development of novel macrocyclic arenes has become a frontier and hot topic in supramolecular and macrocyclic chemistry. In this review, we feature the recent advances in the synthesis and applications of novel macrocyclic arenes that have emerged in the last decade. The general synthetic strategies employed for these macrocyclic arenes are systematically summarized, and their wide applications in molecular recognition and assemblies, molecular machines, biomedical science and functional materials are highlighted.

Biphen[n]arenes: Modular Synthesis, Customizable Cavity Sizes, and Diverse Skeletons

Macrocyclic compounds are fundamental tools in supramolecular chemistry and have been widely used in molecular recognition, biomedicine, and materials science. The construction of new macrocycles with distinctive structures and properties would unleash new opportunities for supramolecular chemistry. Traditionally popular macrocycles, e.g., cyclodextrins, calixarenes, cucurbiturils, and pillararenes, possess specific cavities that are usually less than 10 Å in diameter; they are normally suitable for accommodating small- or medium-sized guests but cannot engulf giant molecules or structures. Furthermore, the skeletons of traditional macrocycles are impoverished and incapable of being changed; functional substituents can be introduced only on their portals.Thus, it is very challenging to construct macrocycles with customizable cavity sizes and/or diverse backbones. We have developed a versatile and modular strategy for synthesizing macrocycles, namely, biphen[n]arenes (n = 3-8), based on the structure- or function-oriented replacement of reaction modules, functional modules, and linking modules. First, two reaction modules and one functional module are connected by Suzuki-Miyaura coupling to obtain a monomer having two reaction sites. Then Friedel-Crafts alkylation between the monomer and an aldehyde (linking module) serves to afford diversely functionalized macrocycles. Moreover, large macrocycles can be achieved by using long and rigid oligo(para-phenylene) monomers. Because of the modular synthesis and plentiful molecular supplies, the biphen[n]arenes showed interesting recognition properties for both small molecules and large polypeptides. Customizable functional backbones and binding sites endowed this new family of macrocycles with peculiar self-assembly properties and potential applications in gas chromatography, pollutant capture, and physisorptive separation. Biphen[n]arenes would be a promising family of workhorses in supramolecular chemistry.In this Account, we summarize our recent work on the chemistry of biphen[n]arenes. We introduce their design and modular synthesis, including systematic exploration for reaction modules, customizable cavity sizes, skeleton functionalization, pre- and postmodification, and molecular cages. Thereafter, we discuss their host-guest properties, involving the binding for small guests by cationic/anionic/neutral biphen[n]arenes, as well as the complexation of polypeptides by large quaterphen[n]arenes. In addition, we outline the self-assembly and potential applications of this new family of macrocycles. Finally, we forecast their further development. The chemistry of biphen[n]arenes is still in its infancy. Continued exploration will not only further expand the supramolecular toolbox but also open new avenues for the use of biphen[n]arenes in the fields of biology, pharmaceutical science, and materials science.

Construction of pillar[4]arene[1]quinone-1,10-dibromodecane pseudorotaxanes in solution and in the solid state

We report novel pseudorotaxanes based on the complexation between pillar[4]arene[1]quinone and 1,10-dibromodecane. The complexation is found to have a 1:1 host–guest complexation stoichiometry in chloroform but a 2:1 host–guest complexation stoichiometry in the solid state. From single crystal X-ray diffraction, the linear guest molecules thread into cyclic pillar[4]arene[1]quinone host molecules in the solid state, stabilized by CH∙∙∙π interactions and hydrogen bonds. The bromine atoms at the periphery of the guest molecule provide convenience for the further capping of the pseudorotaxanes to construct rotaxanes.

Coordination-Driven Poly[2]Pseudorotaxanes in Highly Polar Organic Solvent

Self-assembly of polypseudorotaxanes in high-polar organic solvents is difficult due to remarkably weak interactions between macrocycles and axles. Reported here is a novel metal-coordinated poly[2]pseudorotaxane constructed by pillar[5]arene, 1,4-bis(4-pyridyl pyridinium)butane, and [PdCl₂(PhCN)₂] in highly polar organic solvent of dimethyl sulfoxide (DMSO). Utilizing a combination of ¹H NMR, NOESY, DOSY, DLS, SEM, and viscosity measurements, the formation of polypseudorotaxane was shown to be dependent on the concentration of [2]pseudorotaxanes/[PdCl₂(PhCN)₂] and temperature. Furthermore, a temperature-responsive supramolecular gel with reversibly gel-sol transformation was obtained via spontaneous assembly of the polypseudorotaxanes at high concentrations.

Hemicryptophanes with Improved Fluorescent Properties for the Selective Recognition of Acetylcholine over Choline

The synthesis of two new fluorescent hemicryptophanes is reported. They were found to be efficient and selective receptors for acetylcholine over choline. When compared to other hemicryptophane hosts previously reported for the selective recognition of acetylcholine, they display improved fluorescent properties: their maximum emission wavelengths are red-shifted and the quantum yields are higher. NMR titration experiments and density functional theory (DFT) calculations support the results obtained from fluorescence spectroscopy and give insights into the interactions involved in the host/guest complexes and into the selectivity for acetylcholine over choline.

A Modular Synthetic Strategy for Functional Macrocycles

Reported here is a molecule-Lego synthetic strategy for macrocycles with functional skeletons, involving one-pot and high-yielding condensation between bis(2,4-dimethoxyphenyl)arene monomers and paraformaldehyde. By changing the blocks, variously functional units (naphthalene, pyrene, anthraquinone, porphyrin, etc.) can be conveniently introduced into the backbone of macrocycles. Interestingly, the macrocyclization can be tuned by the geometrical configuration of monomeric blocks. Linear (180°) monomer yield cyclic trimers and pentamers, while V-shaped (120°, 90° and 60°) monomers tend to form dimers. More significantly, even heterogeneous macrocycles are obtained in moderate yield by co-oligomerization of different monomers. This series of macrocycles have the potential to be prosperous in the near future.

Computational Study of Encapsulation of Polyaromatic Hydrocarbons by Endo-Functionalized Receptors in Nonpolar Medium

Polycyclic aromatic hydrocarbons (PAHs) constitute a large group of organic pollutants produced from either natural or artificial sources during the incomplete combustion of fossil fuels or derived from various industrial processes (such as refinery processes of crude petroleum). They are seriously hazardous to human health, and removing them is of major importance. The complexation likeliness with and selective recognition of PAH guests by endo-functionalized molecular tube hosts (host-abu and host-abtu) in a nonpolar medium are investigated using classical molecular dynamics simulation and quantum calculation to probe the factors and the molecular mechanism involved in complexation processes. We examine the role of different guest molecules in the structural changes of hosts, a prelude to van der Waals interactions and binding free energy in the complexation process. These types of host-guest interactions depend on various factors. We find that (i) both the host molecules (host-abtu and host-abu) interact with the guest π-electron cloud almost equally and (ii) these interactions also depend on the molecular size of PAHs. The larger the nonpolar surface area of PAHs, the greater the interactions with the host, and the more extensive the π-electron cloud of the guest, the stronger the interactions. The linear PAHs interact more strongly than isomeric branched/curved PAHs, and the presence of heteroatoms on PAHs decreases the interactions with the host by creating repulsion between the lone pairs of heteroatoms and the π-electron cloud of the host. Noncovalent van der Waals interactions and N-H···π interactions dominate the high affinities of PAHs toward host-abu and host-abtu. The potential of mean force and molecular mechanics Poisson-Boltzmann surface area calculations reveal that all host-guest complexes are energetically stable.

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.

Specific Encapsulation of Acetylcholine Chloride by a Self-Assembled Molecular Capsule with Sulfonamido Moiety

New molecular capsule 1₂, which encapsulates only acetylcholine chloride in the ion-pair form, has been developed. Cavitand 1 with four sulfonamido moieties on the upper rim of tetraimino-cavitand self-assembled to form a stable molecular capsule in the presence of an acetylcholine chloride guest through eight intermolecular -NH···O═S hydrogen bonds, two from each of the four paired sulfonamide units.

Efficient Separation of cis- and trans-1,2-Dichloroethene Isomers by Adaptive Biphen[3]arene Crystals

Reported here is the highly efficient separation of industrially important cis- and trans-1,2-dichloroethene (cis-DCE and trans-DCE) isomers by activated crystalline 2,2',4,4'-tetramethoxyl biphen[3]arene (MeBP3) materials, MeBP3α. MeBP3 can be synthesized in excellent yield (99 %), and a cyclic pentamer is also obtained when using 1,2-dichloroethane as the solvent. The structure of MeBP3 in the CH₃ CN@MeBP3 crystal displays a triangle-shape topology, forming 1D channels through window-to-window packing. Desolvated crystalline MeBP3 materials, MeBP3α, preferentially adsorb cis-DCE vapors over its trans isomer. MeBP3α is able to separate cis-DCE from a 50:50 (v/v) cis/trans-isomer mixture, yielding cis-DCE with a purity of 96.4 % in a single adsorption cycle. Single-crystal structures and powder X-ray diffraction patterns indicate that the uptake of cis-DCE triggers a solid-state structural transformation of MeBP3, suggesting the adaptivity of MeBP3α materials during the sorption process. Moreover, the separation can be performed over multiple cycles without loss of separation selectivity and capacity.

Selective recognition of acetylcholine over choline by a fluorescent cage

A fluorescent hemicryptophane has been synthesized and can be used as a turn on receptor of acetylcholine. A binding constant of 2.4 × 104 M-1 was measured for this neurotransmitter, and its selective and sensitive detection over choline and choline phosphate was achieved. NMR and DFT calculations provide insight into the interactions involved in this selective recognition process.

New opportunities in synthetic macrocyclic arenes

This feature article summarizes the latest research progress in the design and development of new synthetic macrocyclic arenes.

Carboxylato-pillar[6]arene-based fluorescent indicator displacement assays for the recognition of monoamine neurotransmitters

The complexation of three cationic fluorescent dye guests with the anionic host carboxylato-pillar[6]arene (WP6) was investigated by optical and NMR spectroscopy. Among the selected indicators – a stilbazolium dye (i1) and two naphthalimide derivatives with positively charged ‘anchor’ groups (i2 and i3) – i1 gave a large turn-on, i2 and i3 a large turn-off fluorescence response to the complexation. The size selectivity of the complex formation of pillararenes was demonstrated by comparing the binding constants of the complexes of the three indicators with WP6 and its smaller homologue, WP5. The systems WP6·i1 and WP6·i2 were tested as indicator displacement assays for the sensing of monoamine neurotransmitters. The WP6·i1 system functioned as a turn-off, the WP6·i2 system as a turn-on sensor for neurotransmitters, and both assays showed a good selectivity to histamine over the other neurotransmitter analytes.

The complexation of three cationic fluorescent dye guests with the anionic host carboxylato-pillar[6]arene (WP6) was investigated by optical and NMR spectroscopy.

Synthesis of a water-soluble 2,2'-biphen[4]arene and its efficient complexation and sensitive fluorescence enhancement towards palmatine and berberine

A water-soluble 2,2′-biphen[4]arene (2,2’-CBP4) containing eight carboxylato moieties was synthesized and characterized. Its complexation behavior towards two alkaloids, palmatine (P) and berberine (B), was investigated by means of fluorescence and ¹H NMR spectroscopy in aqueous phosphate buffer solution (pH 7.4). In the presence of 2,2’-CBP4, ¹H NMR signals of P and B displayed very large upfield shifts, indicating the formation of inclusion complexes with strong binding affinities. Fluorescence titration experiments showed that P and B exhibited dramatic fluorescence enhancement of more than 600 times upon complexation with 2,2’-CBP4. Particularly, the fluorescence intensity is strong enough to be readily distinguished by the naked eye. Although the two guests have similar structures, the association constant of B with 2,2’-CBP4 (K_a = (2.29 ± 0.27) × 10⁶ M⁻¹) is 3.9 times larger than that of P (K_a = (5.87 ± 0.24) × 10⁵ M⁻¹).

Synthesis of diamido-bridged bis-pillar[5]arenes and tris-pillar[5]arenes for construction of unique [1]rotaxanes and bis-[1]rotaxanes

The pillar[5]arene mono- and di(oxyalkoxy)benzoic acids were successfully prepared in high yields by sequential alkylation of ω-bromoalkoxy-substituted pillar[5]arenes with methyl or ethyl p-hydroxybenzoate followed by a hydrolytic reaction under basic conditions. Under catalysis of HOBt/EDCl, the amidation reaction of pillar[5]arene mono(oxybutoxy)benzoic acid with monoamido-functionalized pillar[5]arenes afforded diamido-bridged bis-pillar[5]arenes. 1H NMR and 2D NOESY spectra clearly indicated that [1]rotaxanes were formed by insertion of longer diaminoalkylene unit into the cavity of one pillar[5]arene with another pillar[5]arene acting as a stopper. The similar catalysed amidation reaction of pillar[5]arene di(oxybutoxy)benzoic acid with monoamido-functionalized pillar[5]arenes resulted in the diamido-bridged tris-pillar[5]arenes, which successfully form the unique bis-[1]rotaxanes bearing longer than diaminopropylene diamido bridges.

Unusual binding selectivity with non-selective homoditopic pillar[5]arene oxime: serendipitous discovery of a unique approach to heterobinuclear metalation in solution

A heterobinuclear complexation strategy on homoditopic pillar[5]arene was developed by using a pillar[5]arene with two rims decorated with benzaldehyde oximes. The unique selective recognition process was found to result from vesicular formation based on a controllable self-assembly, leading to binding of thorium(iv) only onto one rim, with the other rim being unoccupied for subsequent complexation of a second different metal ion.

Ultrasensitive and specific fluorescence detection of a cancer biomarker via nanomolar binding to a guanidinium-modified calixarene

We designed a water-soluble guanidinium-modified calix[5]arene to target lysophosphatidic acid (LPA), an ideal biomarker for early diagnosis of ovarian and other gynecologic cancers, achieving binding on the nanomolar level. An indicator displacement assay, coupled with differential sensing, enabled ultrasensitive and specific detection of LPA. Moreover, we show that using a calibration line, the LPA concentration in untreated serum can be quantified in the biologically relevant low μM range with a detection limit of 1.7 μM. The reported approach is feasible for diagnosing ovarian and other gynecologic cancers, particularly at their early stages.

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

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

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.

Pillar[5]arene-Diketopyrrolopyrrole Fluorescent Copolymer: A Promising Recognition and Adsorption Material for Adiponitrile by Selective Formation of a Conjugated Polypseudorotaxane

Conjugated pillar[5]arene-diketopyrrolopyrrole copolymer (P1) is synthesized by the copolymerization of a difunctionalized pillar[5]arene and a diketopyrrolopyrrole-based monomer, which shows large extinction coefficients (1.1 × 10⁴ m^-1 cm^-1 ) at 519 nm and strong emission at 587 nm. P1 exhibits very strong host-guest binding affinity towards adiponitrile but low binding affinity towards 1,4-dihalobutane and 1,4-bis(imidazol-1-yl)butane. Such an enhanced selectivity is first found in the polypseudorotaxane between pillararene and neutral guests in organic solution and is successfully used for the recognition and adsorption of adiponitrile by the formation of a P1-adiponitrile polypseudorotaxane.

Noncovalent Interactions Accompanying Encapsulation of Resorcinol within Azacalix[4]pyridine Macrocycle

Electronic structure and noncovalent interactions within the inclusion complexes of resorcinol and calix[4]pyridine (CXP[4]) or azacalix[4]pyridine (N-CXP[4]) macrocycles have been analyzed by employing hybrid M06-2X density functional theory. It has been demonstrated that substitution of a heteroatom (-NH-) at the bridging position of the CXP[4] alters the shape of the cavity from a "box-shaped" to funnel-like one. Penetration of resorcinol guest within the CXP[4] cavity renders a "butterfly-like" structure to the complex, whereas the N-CXP[4] complex reveals distorted geometry with the guest being nearer to one of the pyridine rings at the upper rim of the host. Underlying hydrogen bonding, π···π, and other weak interactions are characterized using the Quantum Theory of Atoms in Molecules (QTAIM) and Noncovalent Interactions Reduced Density Gradient (NCI-RDG) methods. The coexistence of multiple intermolecular interactions is envisaged through the frequency shifts of the characteristic -NH and -OH vibrations in their calculated vibrational spectra. The guest protons confined to the host cavity exhibit shielding, while those facilitating hydrogen bonding engender the downfield signals in their calculated ¹H NMR spectra.

Formation of charge-transfer complexes based on a tropylium cation and 2,6-helic[6]arenes: a visible redox stimulus-responsive process

Charge-transfer complexes between 2,6-helic[6]arenes and a tropylium cation formed, and they could undergo visible redox stimulus-responsive and switchable complexation processes.

Synthesis of a water-soluble 2,6-helic[6]arene derivative and its strong binding abilities towards quaternary phosphonium salts: an acid/base controlled switchable complexation process

A water-soluble 2,6-helic[6]arene derivative was synthesized, and it showed strong binding ability and acid/base stimulus-responsive complexation towards quaternary phosphonium salts.

2,2′-Biphen[n]arenes (n = 4–8): one-step, high-yield synthesis, and host–guest properties

A new family of supramolecular macrocycles, 2,2′-biphen[n]arenes (n = 4–8), has been synthesized through a single-step reaction with a yield of 51%.

The first water-soluble pillar[5]arene dimer: synthesis and construction of a reversible fluorescent supramolecular polymer network in water

The first water-soluble pillar[5]arene dimer was successfully designed and synthesized. It can complex with carboxylate anion functionalized tetraphenyl ethylene to form a reversible fluorescent supramolecular polymer network in water.

Pillar[n]arene-based supramolecular organic frameworks with high hydrocarbon storage and selectivity

The high hydrocarbon storage capacity and adsorption selectivity of two low-density, solution-processable pillar[n]arene-based SOFs have been investigated for the first time.

Highly efficient complexation of sanguinarine alkaloid by carboxylatopillar[6]arene: pKa shift, increased solubility and enhanced antibacterial activity

Efficient complexation of sanguinarine by carboxylatopillar[6]arene. The pK_a shift, increased solubility, and enhanced antibacterial activity are described.

Water-soluble pillar[5]arene induced the morphology transformation of self-assembled nanostructures and had further application in paraquat detection

Hex-4ClPBI can self-assemble into nanotubes in water, and the tubular structures can be transformed into nanoribbons and further vesicles by addition of H⁺ and further WP5.