Cucurbit[6]uril-Based Supramolecular Assemblies: Possible Application in Radioactive Cesium Cation Capture

A cucurbit[6]uril-based fluorescence supramolecular assembly for information encryption and visualization detection of nitro compounds and antibiotics

A novel CB[6]-based supramolecular assembly [K(ANS)(CB[6])2(DMF)2(H2O)0.5] (1) (CB[6] = cucurbit[6]uril, ANS- = 8-amino-1-naphthalene sulfonic acid ion) was successfully synthesized under solvothermal condition. Performance studies have shown that 1 exhibited excellent chemical stability and recycling performance. Meanwhile, 1 exhibited remarkable potential as a fluorescence sensor for the detection of 2,4,6-trinitrophenol (TNP), 4-nitrophenol (4-NP), and rifampicin (RFP) in both aqueous environments and practical samples. This sensing capability is achieved through fluorescence quenching, which offers fast response times and exceptional sensitivity, with detection limits of 0.19 μM for both TNP and 4-NP, and 0.21 μM for RFP. Even more remarkably, an anti-counterfeiting ink based on 1 and a portable test hydrogel were devised for encrypting information and visually detecting using a smartphone application. This work has the potential to expand the utilization of CB[6]-based materials in optical applications.

Recent update on the electroactive oligopyrrolic macrocyclic hosts with a Bucky-ball heart

Supramolecular chemistry is a multidisciplinary research area mostly associated with the investigation of host-guest interactions within intricate three-dimensional (3D) molecular architectures held together reversibly by various non-covalent interactions. Continuous efforts to develop such kinds of complex host-guest systems with designer oligopyrrolic macrocyclic receptors are a rapidly growing research domain, which is deeply involved in applied supramolecular chemistry research. These host-guest supramolecular complexes can be constructed by combining suitable electron-rich oligopyrrolic donors (as a host) with complementary electron-poor guests (as acceptors), held together by the ionic force of attraction triggered by intermolecular charge/electron transfer (CT/ET) transitions. Some of these resulting CT/ET ensembles are potential candidates for the construction of efficient optoelectronic materials, optical sensors, molecular switches, etc. In this Feature Article we aim to focus on these supramolecular ensembles composed by size and shape complementary electroactive oligopyrrolic molecular containers, which are suitable for spherical guest (e.g., buckminsterfullerene) complexation. We also provide a "state-of-the-art" overview on plausible applications of these particular host-guest systems. Our aim is to cover only specific electron-rich tetrathiafulvalene (TTF)-based oligopyrrolic receptors, e.g., TTF-calix[4]pyrroles, TTF-cryptands, TTF-porphyrins and exTTF-porphyrin-based molecular motifs reported to date, along with a brief outlining of their "functional behaviour" in materials chemistry research.

A multifunctional cucurbit[6]uril-based supramolecular assembly for fluorescence sensing of TNP and Ba2+ and information encryption

Both nitroaromatic compounds (NACs) and heavy metal ions are accumulative high-risk environmental pollutants, so high-sensitivity detection of these environment pollutants is necessary. In this work, a cucurbit[6]uril (CB[6]) based luminescent supramolecular assembly [Na₂K₂(CB[6])₂(DMF)₂(ANS)(H₂O)₄](1) (CB[6] = cucurbit[6]uril, ANS^2- = 8-Aminonaphthalene-1,3,6-trisulfonic acid ion) has been synthesized under solvothermal conditions, using ANS^2- as a structural inducer. Performance studies have shown that 1 exhibits excellent chemical stability and easy regeneration ability. It can highly selective sensing of 2,4,6-trinitrophenol (TNP) through fluorescence quenching with a strong quenching constant (K_sv = 2.58 × 10⁴ M^-1). Additionally, the fluorescence emission of 1 can be effectively enhanced with Ba²⁺ in aqueous solution (K_sv = 5.57 × 10³ M^-1). More impressively, Ba²⁺@1 was successfully used as anti-counterfeiting fluorescent ink functional material with strong information encryption function. This work illustrates the application prospects of luminescent CB[6]-based supramolecular assembly in environmental pollutants detection and information anti-counterfeiting for the first time, which extends the multifunctional application scope of CB[6]-based supramolecular assembly.

Investigation of the Interaction of Benzo(a)Pyrene and Fluoranthene with Cucurbit[n]urils (n = 6-8): Experimental and Molecular Dynamic Study

The inclusion complexes of cucurbit[n]uril, CB[n] (n = 6-8), with poly aromatic hydrocarbon (PAH) Benzo(a)Pyrene (BaP), and fluoranthene (FLT) were investigated carefully in aqueous media. Fluorescence and ¹H NMR spectroscopy were used to characterize and investigate the inclusion complexes that were prepared in the aqueous media. The most predominant complexes of both guests with hosts were the 1:1 guest: host complexes. Stability constants of 2322 ± 547 M^-1, 7281 ± 689 M^-1, 3566 ± 473 M^-1 were obtained for the complexes of BaP with CB[6], CB[7], and CB[8], respectively. On the other hand, stability constants of 5900.270 ± 326 M^-1, 726.87 ± 78 M^-1, 3327.059 ± 153 M^-1 were obtained for the complexes of FLT with CB[6], CB[7], and CB[8], respectively. Molecular dynamic (MD) simulations were used to study the mode and mechanism of the inclusion process and to monitor the stability of these complexes in aqueous media at an atomistic level. Analysis of MD trajectories has shown that both BaP and FLT form stable inclusion complexes with CB[7] and CB[8] in aqueous media throughout the simulation time, subsequently corroborating the experimental results. Nevertheless, the small size of CB[6] prohibited the encapsulation of the two PAHs inside the cavity, but stable exclusion complex was observed between them. The main driving forces for the stability of these complexes are the hydrophobic forces, van der Waals interactions, electrostatic effect, the π····π and C-H···π interaction. These results suggest that BaP and FLT can form stable complexes with CB[n] (n = 6-8) in solution.

In Search of Preferential Macrocyclic Hosts for Sulfur Mustard Sensing and Recognition: A Computational Investigation through the New Composite Method r2SCAN-3c of the Key Factors Influencing the Host-Guest Interactions

Sulfur mustard (SM) is a harmful warfare agent that poses a serious threat to human health and the environment. Thus, the design of porous materials capable of sensing and/or capturing SM is of utmost importance. In this paper, the interactions of SM and its derivatives with ethylpillar[5]arene (EtP[5]) and the interactions between SM and a variety of host macrocycles were investigated through molecular docking calculations and non-covalent interaction (NCI) analysis. The electronic quantum parameters were computed to assess the chemical sensing properties of the studied hosts toward SM. It was found that dispersion interactions contributed significantly to the overall complexation energy, leading to the stabilization of the investigated systems. DFT energy computations showed that SM was more efficiently complexed with DCMP[5] than the other hosts studied here. Furthermore, the studied macrocyclic containers could be used as host-based chemical sensors or receptors for SM. These findings could motivate experimenters to design efficient sensing and capturing materials for the detection of SM and its derivatives.

Double-Walled Metal-Organic Framework with Regulable Pore Environments for Efficient Removal of Radioactive Cesium Cations

An anion double-walled metal-organic framework [Co₂Li₄(BTC)₃(DMF)(H₂O)·(CH₃)₂N]_n (1) based on heterobimetallic Li⁺ and Co²⁺ ions was successfully constructed. Utilizing selective destruction and formation of Co-O/Co-N bonds in the metal chains, [Co₂Li₄(BTC)₃(py)(H₂O)·(CH₃)₂N]_n (2) and [Co₂Li₄(BTC)₃(pi)(H₂O)·(CH₃)₂N]_n (3) with the same skeleton but distinct pore structures can be surprisingly obtained. Additionally, compounds 2 and 3 can be transformed into [Co₂Li₄(BTC)₃(H₂O)₂·(CH₃)₂N]_n (4) by soaking them in an ethanol solution. This kind of single-crystal-to-single-crystal transformation successfully regulates the pore structure of MOFs and enriches the diversity of the pore wall on the premise of retaining the original framework. Most impressively, compound 1 shows high adsorption capacity for Cs⁺ cations and is a good candidate to selectively accommodate Cs⁺ among the common alkali metal ions, which is future identified by single-crystal X-ray diffraction and inductively coupled plasma mass spectrometry (ICP-MS) test. Meanwhile, compound 1 can selectively adsorb methylene blue (MB) and crystal violet (CV) molecules over Rhodamine B (RMB).

Selective Separation of Hexachloroplatinate(IV) Dianions Based on Exo-Binding with Cucurbit[6]uril

The recognition and separation of anions attracts attention from chemists, materials scientists, and engineers. Employing exo-binding of artificial macrocycles to selectively recognize anions remains a challenge in supramolecular chemistry. We report the instantaneous co-crystallization and concomitant co-precipitation between [PtCl₆ ]^2- dianions and cucurbit[6]uril, which relies on the selective recognition of these dianions through noncovalent bonding interactions on the outer surface of cucurbit[6]uril. The selective [PtCl₆ ]^2- dianion recognition is driven by weak [Pt-Cl⋅⋅⋅H-C] hydrogen bonding and [Pt-Cl⋅⋅⋅C=O] ion-dipole interactions. The synthetic protocol is highly selective. Recognition is not observed in combinations between cucurbit[6]uril and six other Pt- and Pd- or Rh-based chloride anions. We also demonstrated that cucurbit[6]uril is able to separate selectively [PtCl₆ ]^2- dianions from a mixture of [PtCl₆ ]^2- , [PdCl₄ ]^2- , and [RhCl₆ ]^3- anions. This protocol could be exploited to recover platinum from spent vehicular three-way catalytic converters and other platinum-bearing metal waste.

Cucurbituril-Encapsulating Metal-Organic Framework via Mechanochemistry: Adsorbents with Enhanced Performance

The first examples of monolithic crystalline host-guest hybrid materials are described. The reaction of 1,3,5-benzenetricarboxylic acid (H3 BTC) and Fe(NO3 )3 ⋅9 H2 O in the presence of decamethylcucurbit[5]uril ammonium chloride (MC5⋅2 NH4 Cl⋅4 H2 O) directly affords MC5@MIL-100(Fe) hybrid monoliths featuring hierarchical micro-, meso- and macropores. Particularly, this "bottle-around-ship" synthesis and one-pot shaping are facilitated by a newly discovered Fe-MC5 flowing gel formed by mechanochemistry. The designed MC5@MIL-100(Fe) hybrid material with MC5 as active domains shows enhanced CH4 and lead(II) uptake performance, and selective capture of lead(II) cations at low concentrations. This shows that host-guest hybrid materials can exhibit synergic properties that out-perform materials based on individual components.

Potential Applications of Cucurbit[n]urils and Their Derivatives in the Capture of Hazardous Chemicals

Cucurbit[n]urils (Q[n]s) are a relatively young family of macrocycles, consisting of glycoluril units bridged by methylene groups, and their unique structures have attracted extensive attention from chemists in recent decades. Due to the presence of a rigid hydrophobic inner cavity and two polar outer portals lined with carbonyl groups, Q[n]s not only encapsulate guest species into the cavity, but also coordinate with metal ions/clusters. Considerable achievements have been obtained in the fields of Q[n]s-based host-guest chemistry, coordination chemistry, as well as the combination of host-guest and coordination chemistry. Furthermore, the outer surface of Q[n]s has been demonstrated to be capable of interacting with definite species to generate supramolecular architectures in recent years. With more in-depth research into Q[n]s, their application studies have also emerged as a hot topic. This Minireview focuses on recent advances in the potential applications of solid-state materials based on Q[n]s and their derivatives for the capture and adsorption of hazardous chemicals from a solution or a gas mixture.

Cucurbit[n]uril-Based Supramolecular Frameworks Assembled through Outer-Surface Interactions

Porous materials, especially metal-organic frameworks, covalent organic frameworks, and supramolecular organic frameworks, are widely used in heterogeneous catalysis, adsorption, and ion exchange. Cucurbit[n]urils (Q[n]s) suitable building units for porous materials because they possess cavities with neutral electrostatic potential, portal carbonyls with negative electrostatic potential, and outer surfaces with positive electrostatic potential, which may result in the formation of Q[n]-based supramolecular frameworks (QSFs) assembled through the interaction of guests within Q[n]s, the coordination of Q[n]s with metal ions, and outer-surface interaction of Q[n]s (OSIQ). This review summarizes the various QSFs assembled via OSIQs. The QSFs can be classified as being assembled by 1) self-induced OSIQ, 2) anion-induced OSIQ, and 3) aromatic-induced OSIQ. The design and construction of QSFs with novel structures and specific functional properties may establish a new research direction in Q[n] chemistry.

Macrocyclic Pyridone Pentamers for Highly Selective High-Capacity Removal of Caesium Ions from Radioactive High-Salinity Waste

We report here that macrocyclic H-bonded pyridone pentamers, containing five properly and convergently spaced electron-rich O-atoms that decorate a rigid cavity of 2.85 Å across, exhibit an extraordinarily high yet pH-independent capacity and selectivity in Cs⁺ removal. In particular, with [host]=240 μM and [Cs⁺ ]=15 μM, a single extraction efficiently removes more than 91% of Cs⁺ ions from artificial sea water, containing various competitive metal ions at a total concentration of 0.68 M ([total Mⁿ⁺ ]/[Cs⁺ ]=4.5×10⁴ ]). To our best knowledge, these pyridone pentamers represent the best small organic molecule-based extractants that target Cs⁺ ions.

Prussian blue nanocubes decorated on nitrogen-doped hierarchically porous carbon network for efficient sorption of radioactive cesium

Eliminating the radioactive ¹³⁷Cs from nuclear waste is critical to the human health and environment. Prussian blue (PB)-based materials are considered as promising adsorbents for the removal of cesium. Herein, we demonstrate a facile strategy to achieve controllable synthesis of PB nanocrystals decorated on nitrogen-doped hierarchically porous carbon (NHPC) derived from cattle bone as adsorbent to remove cesium. The PB nanocrystals with a nanocube morphology are well distributed on NHPC, which is beneficial to increase the reachable surface area during adsorption. The resulting adsorbent exhibits a remarkable adsorption performance with a capacity of 125.31 mg g^-1, a superior recyclability with 87 % of initial capacity retained after 5 cycles, and an outstanding adsorption selectivity for cesium. X-ray diffraction, X-ray photoelectron spectroscopy combined with ⁵⁷Fe Mössbauer spectroscopy results reveal that cesium ions are inserted into the crystal channels of PB to generate a new phase (CsFe₂(CN)₆·3H₂O) after adsorption. Moreover, the adsorption process is spontaneous and endothermic which can be described by the Langmuir isotherm and pseudo-second-order kinetic models. This strategy for synthesis of PB/carbon adsorbents offers efficient candidate for removal of ¹³⁷Cs from wastewater.

Cucurbit[7]uril-Based Metal-Organic Rotaxane Framework for Dual-Capture of Molecular Iodine and Cationic Potassium Ion

Metal-organic rotaxane frameworks (MORFs) attracted much attention in the past years for construction of intelligent functional materials. Herein, a one-pot synthesis is reported of a three-dimensional (3D) cucurbit[7]uril (Q[7])-based MORF under hydrothermal conditions, namely Q[7]-MORF-1, formed by encapsulating the anionic benzoate moieties of the tricarboxylate ligand into the cavity of Q[7]. Furthermore, Q[7]-MORF-1 shows dual-capture capacity for iodine and K⁺ selectively among the alkali metal ions. The captured molecular iodine is included in the cavity of Q[7] through halogen-bonding interactions and the K⁺ cations are positioned at the carbonyl port of the Q[7] through K-O coordination interactions.

Separation Performance of Capillary Gas Chromatography Based on Monohydroxycucurbit[7]Uril Incorporated Into Sol-Gels as the Stationary Phase

A novel monohydroxycucurbit[7]uril-based stationary phase for capillary gas chromatography (GC) has been produced. For this, a capillary column coating was made using a sol–gel technique, incorporating synthesized monohydroxycucurbit[7]uril [(OH)Q[7]] and hydroxy-terminated poly(dimethylsiloxane) (OH-PDMS) into the sol–gel network through hydrolysis and polycondensation and chemical sol–gel grafting to the inner wall of a fused-silica tube. The preparation method may produce a coating with greater integrity, which gives the prepared column a higher separation efficiency and better selectivity toward analytes than a reported stationary phase based on neat cucurbit[n]urils (Q[n]s). The prepared (OH)Q[7]/PDMS column had 3,225 theoretical plates per meter determined using naphthalene at 120°C and exhibited a weakly polar nature. The (OH)Q[7]/PDMS column has high resolution over a broad spectrum of analytes with symmetrical peak shapes and exhibited better separation performance than commercial capillary columns and reported columns based on neat Q[n]s that failed to resolve some critical analytes. Moreover, the column also showed good thermal stability up to 300°C and separation repeatability with relative standard deviation values in the range of 0.01–0.11% for intraday, 0.11–0.32% for interday and 0.29–0.58% for column-to-column. In addition, the energy effect on the retention of analytes on the (OH)Q[7]/PDMS stationary phase was investigated. The results indicated that retention on the column was determined mainly by the enthalpy change. As demonstrated, the proposed coating method can address some disadvantages that exist with the reported Q[n]s columns and combine the full advantages of (OH)Q[7] with the sol–gel coating method while achieving outstanding GC separation performance.

Noncomplexed Cucurbituril-Mediated Structural Evolution of Layered Uranyl Terephthalate Compounds

Template synthesis is one of the most feasible ways to explore new uranyl compounds with intriguing structures and properties. Here we demonstrate the preparation of six novel "sandwichlike" uranyl coordination polymers (UCPs) based on two-dimensional uranyl-terephthalate acid (H₂TP) networks using CBn (n = 5, 6, 8) as template ligands in the presence of different cations (Na⁺, K⁺, Cs⁺, or H₂N(CH₃)₂⁺). Compound 1 ([UO₂(TP)₂][Na₂(CB5)(H₂O)](H₂O)₅) is composed of layered uranyl-TP networks with the complex of CB5 and sodium cations as template ligands. In compound 2 ([(UO₂)₂(TP)₃]₂(CB6)(H₂O)₁₀), CB6 located between uranyl-TP networks contacts them by π-π interactions and hydrogen bonds. Compound 3 ([(UO₂)₂(TP)₃]₂[Na₂(H₂O)₁₀(CB6)]) is the same as compound 2 except for sodium cations bonding with CB6. Similarly in compound 4 ([(UO₂)₂(TP)₃][Cs(H₂O)₃(CB6)]), CB6 is a capsulelike structure capped with two cesium cations and interacts with uranyl-TP networks through π-π and C-H···π interactions. Compound 5 ([(UO₂)₂(TP)₃(HCOO)₂][K(H₂O)₂(CB5)]₂[H₂N(CH₃)₂]₂(CB6)(H₂O)₆) consists of both templates of CB6 and CB5 in which each CB5 is capped with one potassium cation while the H₂N(CH₃)₂⁺ cation is held at CB6 portals. In compound 6 ([(UO₂)₂(TP)₃]₂[UO₂(TP)₂(H₂O)₂][Cs(CB8)₃(H₂O)₄](H₂O)₁₆), CB8 ligands are connected by cesium cations to form a triangle motif and are further located between the uranyl-TP networks as template agents. All of the 2D layered structures with free CBn or cation-anchored CBn intercalate into the laminates of uranyl-terephthalate and shows a cucurbituril-mediated structural evolution. The regulating role of CBn as structure-directing template agents for the construction of layered UCPs through outer-surface interactions with layers of uranyl terephthalate is demonstrated, especially for the case of CB6 with contractive interlayer spacing.

Evaluation of the supramolecular interaction of Congo red with cucurbiturils using mass spectrometry and spectroscopic methods

Cucurbit[n]urils decolourise aqueous solutions of Congo red by forming outer-surface adducts, which are also detected in gas-phase ESI-MS studies.

A cucurbit[6]uril-based supramolecular assembly test strip for immediate detection of nitrofuran antibiotics in water

A CB[6]-based supramolecular assembly has been designed and prepared into test strips, which can detect nitrofuran antibiotics by the naked eye.

A cucurbit[6]uril-based supramolecular assembly as a highly sensitive and quickly responsive luminescent sensor for the detection of fluoroquinolone antibiotics in simulated wastewater

A highly sensitive and quickly responsive CB[6]-based supramolecular assembly was synthesized and used to detect antibiotics through luminescence quenching.

An N-linked disalicylaldehyde together with its caesium ion and dichloromethane sensing performances: ‘dual key & lock’ LMCT-enhanced fluorescence strategy

The disalicylaldehyde-Cs⁺sensing system, a novel approach for quick and reusable detection of Cs⁺together with convenient CH₂Cl₂monitoring.

Architecture transition of supramolecular polymers through hierarchical self-assembly: from supramolecular polymers to fluorescent materials

A new supramolecular crosslinked polymer and a gel with excellent self-repairing capability were prepared by double host–guest interactions.

Specific K+ Binding Sites as CO2 Traps in a Porous MOF for Enhanced CO2 Selective Sorption

Metal-organic frameworks (MOFs) can be fine-tuned to boost sorbent-sorbate interactions in order to improve gas sorption and separation performance, but the design of MOFs with ideal structural features for gas separation applications remains a challenge. Herein it is reported that unsaturated alkali metal sites can be immobilized in MOFs through a tetrazole based motif and that gas affinity can thereby be boosted. In the prototypal MOF of this type-NKU-521 (NKU denotes Nankai University), K⁺ cations are effectively embedded in a trinuclear Co²⁺ -tetrazole coordination motif. The embedded K⁺ sites are exposed to the pores of NKU-521 through water removal, and the isosteric heat (Q_st ) for CO₂ is boosted to 41 kJ mol^-1 . The nature of the binding site is validated by molecular simulations and structural characterization. The K⁺ cations in effect serve as gas traps and boost the CO₂ -framework affinity, as measured by the Q_st , by 24%. In addition, the impact of unsaturated alkali metal sites upon the separation of hydrocarbons is evaluated for the first time in MOFs using ideal adsorbed solution theory (IAST) calculations and column breakthrough experiments. The results reveal that the presence of exposed K⁺ sites benefits gas sorption and hydrocarbon separation performances of this MOF.

Anion-adaptive crystalline cationic material for 99TcO4- trapping

Efficient anion recognition is of great significance for radioactive 99TcO4- decontamination, but it remains a challenge for traditional sorbents. Herein, we put forward a tactic using soft crystalline cationic material with anion-adaptive dynamics for 99TcO4- sequestration. A cucurbit[8]uril-based supramolecular metal-organic material is produced through a multi-component assembly strategy and used as a sorbent for effective trapping of TcO4-. Excellent separation of TcO4-/ReO4- is demonstrated by fast removal kinetics, good sorption capacity and high distribution coefficient. Remarkably, the most superior selectivity among metal-organic materials reported so far, together with good hydrolytic stability, indicates potential for efficient TcO4- removal. The structure incorporating ReO4- reveals that the supramolecular framework undergoes adaptive reconstruction facilitating the effective accommodation of TcO4-/ReO4-. The results highlight opportunities for development of soft anion-adaptive sorbents for highly selective anion decontamination.

Triangular Regulation of Cucurbit[8]uril 1:1 Complexes

Triangular shapes have inspired scientists over time and are common in nature, such as the flower petals of oxalis triangularis, the triangular faces of tetrahedrite crystals, and the icosahedron faces of virus capsids. Supramolecular chemistry has enabled the construction of triangular assemblies, many of which possess functional features. Among these structures, cucurbiturils have been used to build supramolecular triangles, and we recently reported paramagnetic cucurbit[8]uril (CB[8]) triangles, but the reasons for their formation remain unclear. Several parameters have now been identified to explain their formation. At first sight, the radical nature of the guest was of prime importance in obtaining the triangles, and we focused on extending this concept to biradicals to get supramolecular hexaradicals. Two sodium ions were systematically observed by ESI-MS in trimer structures, and the presence of Na⁺ triggered or strengthened the triangulation of CB[8]/guest 1:1 complexes in solution. X-ray crystallography and molecular modeling have allowed the proposal of two plausible sites of residence for the two sodium cations. We then found that a diamagnetic guest with an H-bond acceptor function is equally good at forming CB[8] triangles. Hence, a guest molecule containing a ketone function has been precisely triangulated thanks to CB[8] and sodium cations as determined by DOSY-NMR and DLS. A binding constant for the triangulation of 1:1 to 3:3 complexes is proposed. This concept has finally been extended to the triangulation of ditopic guests toward network formation by the reticulation of CB[8] triangles using dinitroxide biradicals.

Pillararene-based supramolecular polymers

Pillararenes, as a new type of macrocyclic hosts, possess columnar structures and electron-rich cavities. Pillararenes not only recognize suitable cations, but also bind many neutral molecules. Due to the easy modification of pillararenes, various functional groups can be conveniently attached to the rim of pillararenes to provide suitable interaction sites, and the modified pillararenes even bind anionic guests. Thus, pillararenes and their derivatives have presented intriguing and unique host-guest recognition nature in the past few years, which make them ideal building blocks for the preparation of supramolecular polymers. Pillararene-based supramolecular polymers (PSPs) not only possess many merits of traditional covalent polymers but also have many specific properties, such as self-reparability, degradability, and self-adaptation. This feature paper gives an overview of the preparation of PSPs and covers recent research advance and future trends of pillararene-based host-guest pairs, assembly methods, topological architectures, stimuli-responsiveness, and functional features. We expect that the review will be helpful to researchers working in the fields of supramolecular chemistry and polymer science.

Porous supramolecular assemblies and functional properties of perhydroxylated cucurbit[6]uril and polyoxometallates

(HO)₁₂Q[6] and POMs were used as basic building blocks, and interactions yielded two novel types of supramolecular assemblies at different HCl concentrations.

Outer surface interactions to drive cucurbit[8]uril-based supramolecular frameworks: possible application in gold recovery

Q[8]-based honeycomb-like frameworks could be obtained in [AuCl₄]⁻ free aqueous HNO3 solution, aqueous HCl and HNO3 solutions that contain [AuCl₄]⁻. These frameworks exhibit a high selectivity for imprisoning [AuCl₄]⁻.

4-Sulfocalix[4]arene/Cucurbit[7]uril-Based Supramolecular Assemblies through the Outer Surface Interactions of Cucurbit[n]uril

Upon mixing of aqueous solutions of the freely soluble building blocks cucurbit[7]uril (Q[7]) and 4-sulfocalix[4]arene (SC[4]A), white microcrystals instantly separate in near-quantitative yield. The driving force for this assembly is suggested to be the outer-surface interaction of the Q[n]. Dynamic light scattering, scanning electron microscopy, and NMR (diffusion-ordered NMR spectroscopy) analyses have confirmed the supramolecular aggregation of Q[7] and SC[4]A. Titration ¹H NMR spectroscopy and isothermal titration calorimetry have shown that the interaction ratio of Q[7] and SC[4]A is close to 3:1. Moreover, the Q[7]/SC[4]A-based supramolecular assembly can accommodate molecules of some volatile compounds or luminescent dyes. Thus, this work offers a simple and highly efficient means of preparing adsorbent or solid fluorescent materials.

Sequence-Specific Self-Assembly of Positive and Negative Monomers with Cucurbit[8]uril Linkers

The self-assembly into dynamic oligomers of Cucurbit[8]uril (CB[8]), a positive ditopic Ir(III) bis-terpyridine complex, and a negative ditopic Fe(II) bis-terpyridine complex flanked by four butyrate side chains was assessed to answer a seemingly straightforward question: does CB[8] adopt a social self-sorting pattern by encapsulating both positive and negative units into a heteroternary complex? We showed that this is indeed the case, with CB[8] linking a positive Ir unit to a neighboring negative Fe unit whenever possible. Furthermore, the solubility of the dynamic oligomers was significantly affected by their sequence; upon addition of 0.6-1.2 equiv of positive Ir oligomer to its negative Fe counterpart, the predominant assembly present in solution was a mixed oligomer with a (Fe-Ir-Ir-) _n sequence. Weak interactions between the negative butyrate side chains and the partially positive outer wall of CB[7] were also identified by two-dimensional nuclear magnetic resonance techniques, and resulted in a negative p K_a shift (0.10 p K_a unit) for the terminal carboxylic groups.

Coordination supramolecular assemblies of a monohydroxycucurbit[7]uril and their potential applications in gas sorption

Coordination supramolecular assemblies of monohydroxycucurbit[7]uril ((HO)Q[7]) with alkaline earth metal ions (AE²⁺) have been formed in aqueous HCl solution in the presence of tetrachloride cadmium anions ([CdCl₄]^2-) as a structure directing agent. The driving force for the assembly could be attributed to the interaction of the positive electro-potential outer-surface of (HO)Q[7] molecules with [CdCl₄]^2- anions and ionic dipole interaction of the hydroxyl of (HO)Q[7] molecules with [CdCl₄]^2- anions. Moreover, the porous structure of the (HO)Q[7]/AE²⁺-based coordination supramolecular assemblies could result in potential applications in the selective sorption of polar volatile organic molecules, which may be useful in molecular sieves, sensors, absorption and separation.

Cucurbit[6]uril-based multifunctional supramolecular assemblies: synthesis, removal of Ba(ii) and fluorescence sensing of Fe(iii)

Supramolecular assemblies formed of cucurbit[6]uril and flexible aromatic compounds were achieved via outer-surface interactions, and show selective adsorption and sensing properties.

Highly selective absorption of polychloromethanes in perhydroxylated cucurbit[6]uril-based supramolecular assemblies

Two perhydroxylated cucurbit[6]uril-based porous assemblies have been obtained by the crystallization of perhydroxylated cucurbit[6]uril {(HO)₁₂Q[6]} from aqueous HCl solutions of different concentrations, which exhibit highly selective absorption of polychloromethanes.

Multiple Efficient Fluorescence Emission from Cucurbit[10]uril-[Cd4Cl16]8--Based Pillared Diamond Porous Supramolecular Frameworks

Cucurbit[10]uril (Q[10] or CB[10]), with the largest rigid cavity (ca. 1.0 nm) yet characterized in the cucurbiturils family, and indeed among all artificial macrocyclic receptors to date, has been successfully exploited to construct a novel Q[10]-[Cd₄Cl₁₆]^8--based pillared diamond porous supramolecular framework. Single-crystal X-ray diffraction analysis revealed that the three-dimensional open-nanotube-type porous framework is constructed from free Q[10] molecules and [Cd₄Cl₁₆]^8- cluster anions through the outer surface interactions of Q[10]. Notably, the Q[10]-based porous framework host can accommodate guest dyes, such as rhodamine B (G1), pyrenemethanamine hydrochloride (G2), and bathocuproine hydrochloride (G3), to form solid materials with efficient red-green-blue (RGB) fluorescence. This work not only exemplifies a facile approach for the construction of macrocycle-based porous frameworks but also offers a simple, lower cost, yet still highly efficient means of preparing multi-emitting, including white-light-emitting, solid luminescent materials.

Cyclodextrin/Paclitaxel Dimer Assembling Vesicles: Reversible Morphology Transition and Cargo Delivery

Here, we developed stable supramolecular binary vesicles on the basis of the host-guest interaction between β-cyclodextrins (β-CDs) and paclitaxel (PTX) dimer. The inclusion complexation between PTX dimer and β-CDs in water was studied by proton nuclear magnetic resonance spectroscopy and two-dimensional rotating-frame Overhauser effect spectroscopy. The resulting inclusion complex was amphiphilic and could self-assemble into vesicles with average diameter of 230 nm. The vesicles could evolve to nanoparticles (NPs) by adding competitive binding guest amantadine hydrochloride or by digesting β-CDs through α-amylase. Moreover, this process was reversible, and the NPs could also transform to vesicles by adding enough β-CDs again. The obtained hollow supramolecular vesicles were further explored to load hydrophilic dye indocyanine green molecule or hydrophobic anticancer drug doxorobicin for their controlled release under external stimulus. This work provides a new strategy for the design of supramolecular systems by using prodrug as building blocks.