Cucurbit[6]uril-based supramolecular frameworks assembled via the outer surface interaction of cucurbit[n]urils

Curcurbit[n]uril(Q[n])-based supramolecular frameworks(QSFs) constructed from the outer surface interaction of Q[n]s(OSIQ) have the characteristic of simplicity,diversity and modulability.Their simplicity is reflected in their simple composition and preparation methods used for QSFs.The diversity of supramolecular organic frameworks(SOFs) is reflected in the synthesis methods and structural characteristics of the as-obtained QSFs,as well as the variety of structural directing agents and basic building blocks used to prepare QSFs.The modulability is reflected by the controllable channel size in the QSFs,which can be adjusted using different sizes of Q[n]s.In this work,the first re ported cucurbituril Q[6]was selected as the basic building block and three Q[6]-based su p ramolecular frameworks were obtained from aqueous HCl solutions in the presence of [CdCl₄]^2-respectively.The OSIQs are the main driving forces for the formation of these frameworks.This study shows the diversity of the QSFs.     

A novel pillar[5]arene-cucurbit[10]uril based host-guest complex: Synthesis,characterization and detection of paraquat

The macrocyclic family comprising pillar[n]arenes and cucurbit[n]urils have received much attention recently. However, studies on the construction of supramolecular complexes formed directly with derivatized pillar[n]arenes and cucurbit[n]urils are scant. Given the interest in such systems, herein we have synthesized a new type of naphthalene-derivatized pillar[n]arene NTP5 and selected Q[10] as the host molecule. The 4-[2-(1-naphthalenyl)ethenyl]pyridine of NTP5 is encapsulated by Q[10] and formed a host-guest complex in water-acetic acid (1:1) solution accompanied by enhanced fluorescence, which changed the morphology of NTP5 from a sphere to a porous form. In addition, the fluorescence of Q[10]-NTP5 can be quenched by the addition of the highly toxic pesticide paraquat (PQ), and the mechanism was shown to be the formation of a new charge transfer ternary system of Q[10]-NTP5-PQ. This work provides new ideas for the contribution of supramolecular assemblies based on derivatized pillar[n]arenes and their combination with cucurbit[n]urils and reveals their potential applications.     

Cucurbit[n]urils-induced room temperature phosphorescence of quinoline derivatives

The cucurbit[7,8]urils (Q[7] and Q[8])-induced room temperature phosphorescence (RTP) of quinoline and its derivatives were firstly found in the cucurbit[n]urils chemistry. The luminophores (quinolines) and their RTP are affected by the concentration of different Q[n]s, heavy metal ions and amounts, and pH. The RTP lifetime of the luminophore has been investigated. In presence of Na₂SO₃, the cation Tl⁺ led to stronger Q[n]-induced RTP, while the RTP lifetimes of luminophore/Q[7 or 8]/KI were generally longer than that of luminophore/Q[7 or 8]/TlNO₃, the RTP lifetimes of these systems were between 0.18 and 47.4 ms. Contrary to the stable 1:2 Q[8]:guest ternary inclusion complexes at lower pHs, as suggested by ¹H NMR, electronic absorption and fluorescence spectroscopy, low Q[8]-induced room temperature phosphorescence was observed. However, at higher pHs, high intensity of cucurbit[n]urils-induced room temperature phosphorescence of these quinoline derivatives were observed, and a 1:1 Q[8]:guest inclusion complex was formed. Investigations of dependence of RTP intensity on concentration of Q[n] revealed that the highest intensity of the Q[n]-induced RTP was observed at a low mole ratio of host:guest, which is closed to 1:1. It was presumably resulted from the strong interaction of Q[n] and these guests due to the combined hydrophobic cavity interaction and the hydrophilic portal interaction of the cucurbit[n]urils with the nitrogen heterocycles guest.     

Controllable fabrication of a supramolecular polymer incorporating twisted cucurbit[14]uril and cucurbit[8]uril via self-sorting

A linear supramolecular polymer with controllable features based on twisted cucurbit[14]uril (tQ[14]) and cucurbit[8]uril (Q[8]) was firstly fabricated via an effective self-sorting strategy. Herein we designed a monomer, 1?butyl?1′-(naphthalen- 2-ylmethyl)-4,4′-bipyridinium bromide (BNB), that contains bipyridyl, aliphatic butyl and aromatic naphthyl groups, simultaneously. Two host molecules, tQ[14] and Q[8] were employed to develop an effective strategy for constructing a linear supramolecular polymer with controllable features. The alkyl groups on both sides of BNB could insert into the two cavities of tQ[14], the naphthyl part of BNB via π-π stacking in Q[8] cavity, serving as the driving force for supramolecular polymerization. Through self-sorting of the monomer, tQ[14] and Q[8], led to the formation of the linear supramolecular polymer. Depolymerization could be achieved by addition of adamantane hydrochloride (AH) which driven two BNB guest molecules out of the Q[8] cavity. This self-sorting strategy has great potential, not only for designing supramolecular polymer materials with different controllable structures through introduction of multiple functional groups, but also for broadening the application of twisted cucurbit[14]uril in supramolecular chemistry.     

Host-guest interaction tailored cucurbit[6]uril-based supramolecular organic frameworks (SOFs) for drug delivery

An approach for the construction of crystalline porous supramolecular organic frameworks (SOFs) via outer-surface interactions of cucurbit[6]uril (Q[6]) with high yield is presented. This approach enables the noncovalent integration of guest molecules into ordered topologies and creates new host–guest-complex-based SOFs; i.e., the topology can be predesigned and constructed by using [ZnCl₄]^2? anions to induce the formation of solid Q[6]-SOFs, and the pore wall surface can be easily modified by the Q[6]-encapsulated guest molecules. In addition, one of prepared solid Q[6]-SOFs showed a high drug-loading capacity and smart potential release control for drug-delivery applications     

Cucurbit[10]uril-based chemistry

With the biggest cavity in the cucurbit[n]urils (CB[n]s) family, CB[10] has shown its unique molecular recognition properties. This review gives a brief summary of the research progresses in the CB[10]-based chemistry, involving its purification and applications in fields such as molecular recognition and molecular assembly.     

Direct syntheses of a series of cucurbit[n]uril-anchored polyacrylamides

Using the one-pot, direct strategy reported by Su and co-workers, we have synthesised a series of cucurbit[n]urils (Q[n], n = 5–8) and alkyl-substituted cucurbit[6]urils (SQ[6]s) anchored on polymers. Acrylamide, as a typical monomer, was used to synthesise a series of Q[n]s (n = 5–8) and SQ[6]-anchored polyacrylamides (PAMs) using a persulfate salt as initiator and oxidant. The Q[n]s (n = 5–8) and SQ[6]-anchored PAM samples have been characterised by ¹H NMR, ¹H NMR titrations of probe guests, Fourier-transform infrared and thermogravimetric analyser. The results confirmed that PAM chains had been successfully grafted on the back of the Q[n]s (n = 5–8) and SQ[6]s through an in situ radical polymerisation approach. It was further confirmed that the hydrophobic cavities of the Q[n]s on the polymers were still freely accessible. This synthetic approach may be extended to a variety of Q[n]s that are difficult to functionalise.     

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.     

Preparation and characterization of inclusion complexes of antitumor camptothecin with cucurbit[n = 7, 8]urils

The slightly water-soluble anticancer drug camptothecin (CPT) and its inclusion complexes with cucurbit[n = 7, 8]uril (Q[n] (n = 7, 8)) were investigated. The formation of 1:2 complexes with Q[n] (n = 7, 8) in aqueous solution was confirmed by fluorescence spectroscopy and the apparent stability constants were determined to be higher than 3.01 × 10¹² L²/mol². The solid inclusion complexes of CPT and Q[n] (n = 7, 8) were also prepared by the co-evaporation method and characterized by Fourier transformation-infrared spectroscopy, differential scanning calorimetry and powder X-ray diffraction. Aqueous solubility and dissolution studies indicate that the complexes exhibited significantly increased dissolution rates compared with the pure drug and physical mixtures. The potential of Q[7] or Q[8] for stabilizing lactone modality of CPT was investigated by the High Performance Liquid Chromatography (HPLC) method. The results reveal more than 63% CPT lactone form (active form) in CPT-Q[7] or Q[8] complexes compared to only 36% CPT lactone form in the absence of Q[7] or Q[8] after being incubated in the phosphate buffer solution (pH 7.4 at 37°C) for 5 h.     

Structural deformation and host–guest properties of doubly-reduced cycloparaphenylenes, [n]CPPs2− (n = 6, 8, 10, and 12)

Chemical reduction of several cycloparaphenylenes (CPPs) ranging in size from [8]CPP to [12]CPP has been investigated with potassium metal in THF. The X-ray diffraction characterization of the resulting doubly-reduced [n]CPPs provided a unique series of carbon nanohoops with increasing dimensions and core flexibility for the first comprehensive structural analysis. The consequences of electron acquisition by a [n]CPP core have been analyzed in comparison with the neutral parents. The addition of two electrons to the cyclic carbon framework of [n]CPPs leads to the characteristic elliptic core distortion and facilitates the internal encapsulation of sizable cationic guests. Molecular and solid-state structure changes, alkali metal binding and unique size-dependent host abilities of the [n]CPP²⁻ series with n = 6–12 are discussed. This in-depth analysis opens new perspectives in supramolecular chemistry of [n]CPPs and promotes their applications in size-selective guest encapsulation and chemical separation.

The series of doubly-reduced cycloparaphenylenes (CPPs) with increasing dimensions and flexibility shows the size-dependent structural changes and enhanced host abilities.     

Coordination of Alkaline Earth Metal Ions in the Inverted Cucurbit[7]uril Supramolecular Assemblies Formed in the Presence of [ZnCl4]2− and [CdCl4]2−

A convenient method to isolate inverted cucurbit[7]uril (iQ[7]) from a mixture of water‐soluble Q[n]s was established by eluting the soluble mixture of Q[n]s on a Dowex (H⁺ form) column so that iQ[7] could be selected as a ligand for coordination and supramolecular assembly with alkaline earth cations (AE²⁺) in aqueous HCl solutions in the presence of [ZnCl₄]^2? and [CdCl₄]^2? anions as structure‐directing agents. Single‐crystal X‐ray diffraction analysis revealed that both iQ[7]–AE²⁺–[ZnCl₄]^2?–HCl and iQ[7]–AE²⁺–[CdCl₄]^2?–HCl interaction systems yielded supramolecular assemblies, in which the [ZnCl₄]^2? and [CdCl₄]^2? anions presented a honeycomb effect, and this resulted in the formation of linear iQ[7]/AE²⁺ coordination polymers through outer‐surface interactions of Q[n]s.     

Substituted cucurbit[n]uril rings, catenanes and channels

The developments into the synthesis of networked rings, and channels generated in crystalline structures of substituted cucurbit[n]uril with metal ions and organic cations are reviewed. Pertinent chemical features and characteristics are discussed in the context of structural design and supramolecular architecture when utilizing substituted cucurbit[n]uril as building blocks.     

六、七、八元瓜环与苯胺系列衍生物的相互作用

利用紫外吸收光谱、荧光光谱以及¹H NMR方法详细考察了六、七、八元瓜环(Q[6], Q[7], Q[8])与苯胺系列衍生物客体的相互作用和体系pH对其作用的影响. 实验结果表明, 3种瓜环与苯胺系列衍生物客体的相互作用强弱、作用比例以及作用模式与体系的酸度密切相关: 在“高”或“低”pH条件下, 未观察到瓜环与这些客体的明显作用; 在介于“高”与“较高”或“低”与“较低”的pH范围, 瓜环与这些客体发生相互作用, 形成1∶1的包结配合物; 而在介于“较高”与“较低”的pH范围, 瓜环与这些客体发生相互作用, 可形成1∶2的包结配合物. 对于不同的瓜环-客体作用体系, 相应的pH范围各不相同. 本文利用简便的实验方法, 测试了这些pH值及其范围. 根据测定的结果, 结合瓜环以及客体的结构特征, 对体系主客体在不同的酸度区域表现出的不同作用模式进行了探讨.     

Isolated supramolecular [Ru(bpy)3]-viologen-[Ru(bpy)3] complexes with trapped CB[7,8] and photoinduced electron-transfer study in nonaqueous solution

The synthesis of two supramolecular diruthenium complexes, 1 ?CB[7] and 1 ?CB[8] (CB[n]=cucurbit[n]uril), which contain the respective host CB[7] and CB[8], were synthesized and isolated. In the case of host CB[8], the desired supramolecular complex was obtained by utilizing dihydroxynapthalene as a template during the synthesis. The ¹H NMR spectra, electrochemistry, and photochemistry of these supramolecular complexes were performed in nonaqueous solution. The results show that both CB[7,8] hosts mainly bind to the linker part in solution in acetonitrile. This binding also lowers the oxidation potential of the ruthenium metal center and hinders the quenching effect by the viologen moiety. It has also been shown that external methylviologen can be included into 1 ?CB[8]. Analysis with NMR spectroscopy, electrochemistry, and photochemistry clearly shows a viologen radical dimer formation between the bound viologen and free methylviologen, thereby showing that the unique abilities of the CB[8] host can be utilized even in nonaqueous solution.     

Interaction of Cucurbit[n = 6∼8]urils and Benzimidazole Derivatives

The structures and optical properties of host–guest complexes produced from cucurbit[n = 6–8]urils and some benzimidazole derivatives have been investigated by ¹H NMR spectroscopy, electronic absorption spectroscopy and fluorescence spectroscopy. The experimental results reveal that calculations of A∼N_Q[n]/N_guest and I_f∼N_Q[n]/N_guest for the same association complex both support a good fit to an identical binding model. In particular, the A∼N_Q[n]/N_guest, I_f∼N_Q[n]/N_guest calculations and the ¹H NMR determinations for three Q[6]–ge(1∼3) complexes and three Q[8]–ge(1∼3) complexes all support a binding model of 1:1 and 1:2 respectively.     

A hemicyanine and cucurbit[n]uril inclusion complex: competitive guest binding of cucurbit[7]uril and cucurbit[8]uril

The interaction between the hemicyanine indole derivative H and the cucubit[n]urils Q[7] and Q[8] has been studied using ¹H NMR and UV spectroscopy as well as by fluorescence experiments. Competitive studies on the inclusion of H by Q[7] and Q[8] have also been conducted, and reveal that on changing the size of the Q[n] cavity, the binding behaviour can be very different.     

[n]Cycloparaphenylene-Pillar[5]arene Bismacrocycles: Their Circularly Polarized Luminescence and Multiple Guest Recognition Properties

Both pillar[n]arenes (P[n]As) and [n]cycloparaphenylenes ([n]CPPs) play an important role in supramolecular chemistry. Herein, we report the precise synthesis of two multifunctional bismacrocycles [n]CPP-P[5]A by integrating P[5]A into the [n]CPP backbone. The photoluminescence quantum yield (Φ_F) of the bismacrocycles was found to show a dramatic increase relative to the corresponding [n]CPPs. The chiral enantiomers (pR)/(pS)-[8]CPP-P[5]A were successfully isolated by chiral HPLC, and showed promising properties of circularly polarized luminescence (g_lum≈0.02). In addition, [n]CPP-P[5]A bismacrocycles are capable of binding pyridinium salts and fullerene derivatives with high affinity and specificity within the two distinct cavities. Transient absorption studies showed that photo-induced electron transfer occurs in [10]CPP-P[5]A⊃C₆₀ complex. Our results suggest that [n]CPP-P[5]A are potentially useful in CPL-active materials, multiple guest recognition and supramolecular polymer preparation.     

Synthesis and Cation-Binding Properties of Novel p-Isopropenylcalix[n]arenes and Their Acetylated Derivatives

A novel two-step method was developed to prepare individual p-isopropenylcalix[n]arenes [n=4, 6, 8]. In the first step, linear phenolic oligomers were prepared in a basic medium from p-isopropenylphenol and paraformaldehyde. The second step, cyclization of the linear oligomers was carried out at higher temperatures. Ethylene glycol was used as medium, and sodium tetraborate-decahydrate as the catalyst. O-Acetylated derivatives were obtained by acetylation of the phenolic hydroxyl groups of p-isopropenylcalix[n]arenes [n=4, 6, 8]. The p-isopropenyl-calix[n]arenes and their acetylated derivatives were characterized by IR, ¹H NMR and UV spectroscopy, and elemental analysis. Their ability to extract metal cations from aqueous solutions was evaluated via metal picrate extraction experiments. A parallel investigation of the cation-binding ability of the p-tert-butylcalix[n]arenes [n=4, 6, 8] and their acetylated derivatives was also performed. The p-isopropenylcalix[6]arene hexaacetate is the most effective extracting agent for metal picrates, and shows strong affinity towards Rb cation.     

Supramolecular assemblies of cucurbit[n]urils and 4-aminopyridine controlled by cucurbit[n]uril size (n = 5, 6, 7 and 8)

The binding interactions between 4-aminopyridine (4-AP) and a series of cucurbit[n]urils (Q[5], Q[6], TMeQ[6], Q[7], Q[8]) have been studied using ¹H NMR spectroscopy, UV–vis absorption spectroscopy, isothermal titration calorimetry (ITC) and X-ray crystallography. The data indicates that the Q[5]@4-AP complex exhibits exo binding, which is not observed in the other four host-guest complexes. Furthermore, X-ray crystallography clearly reveals how the Q[n]s bind with 4-AP to form complexes, for example Q[5] forms an outer-surface complex, whilst Q[6], TMeQ[6] and Q[7] formed 1:1 host and guest type complexes, and Q[8] formed a stable 1:2 ternary complex due to its large cavity, which can accommodate two 4-AP molecules.     

Cucurbit[n]urils for Supramolecular Catalysis

The control over chemical reactivity and selectivity are always pursued. Using non-covalent interactions to achieve efficient and selective catalysis is an essential goal of supramolecular catalysis. Supramolecular catalysis based on cucurbit[n]urils (CB[n]s) possesses distinct characteristics for the unique structure of CB[n]s. CB[n]s are a family of pumpkin-shaped host molecules with various molecular sizes, rigid structures, electronegative portals and wealthy host-guest chemistry. Herein, we summarize the three major mechanisms of CB[n]s based supramolecular catalysis. Owing to the structural properties of CB[n]s, CB[n]s can serve as nanoreactors and steric hindrance to modulate the reactivity of substrates. They can also catalyze the reactions by modulating the reactivity of ionized intermediates. Recent progresses on the CB[n]s based supramolecular catalysis are introduced in this Minireview and the future development in this field is discussed. It is anticipated that this review provides insights into the mechanism of CB[n]s based supramolecular catalysis and may help scientists find new opportunities in this field.