Assembly and Applications of Macrocyclic-Confinement-Derived Supramolecular Organic Luminescent Emissions from Cucurbiturils.

Cucurbit[n]urils (Q[n]s or CB[n]s), as a classical of artificial organic macrocyclic hosts, were found to have excellent advantages in the fabricating of tunable and smart organic luminescent materials in aqueous media and the solid state with high emitting efficiency under the rigid pumpkin-shaped structure-derived macrocyclic-confinement effect in recent years. This review aims to give a systematically up-to-date overview of the Q[n]-based supramolecular organic luminescent emissions from the confined spaces triggered host-guest complexes, including the assembly fashions and the mechanisms of the macrocycle-based luminescent complexes, as well as their applications. Finally, challenges and outlook are provided. Since this class of Q[n]-based supramolecular organic luminescent emissions, which have essentially derived from the cavity-dependent confinement effect and the resulting assembly fashions, emerged only a few years ago, we hope this review will provide valuable information for the further development of macrocycle-based light-emitting materials and other related research fields.

Cucurbit[7]uril-Encapsulation-Controlled Supramolecular Photoproduct and Radical Fluorescence Emission.

Herein, a host-guest interaction-controlled photoproduct created by using cucurbit[7]uril (Q[7])-based pseudorotaxane structures is reported. The assembly exhibited controlled behavior towards the reduction of the ethylene (C=C) bond in the tetrakis(pyridin-4-yl)ethylene (TPyE) guest molecule under UV light irradiation. This can be attributed to the Q[7] encapsulation masking the four pyridinium arms of the guest, which inhibits planarization of the TPyE core to form the cyclization product. In particular, the strong affinity of Q[7] for the butyl-substituted guest (TPyE-4C) led to an unusual radical fluorescence emission of the photoirradiation-triggered intermediate of the guest molecule being observed in aqueous solution. This work provides a valuable paradigm and new insight for macrocycle-based host-guest interactions in supramolecular catalysis and luminescent radical materials.

A New Cationic Fluorescent Probe for HSO3- Based on Bisulfite Induced Aggregation Self-Assembly.

In comparison with the numerous studies that have centered on developing molecular frameworks for the functionalization of fluorescent materials, less research has addressed the influence of the side chains, despite such appendages contributing significantly to the properties and applications of fluorescent materials. In this work, a new series of cationic fluorescent probes with AIE characteristics have been developed, which exhibit unique sensitivity for charge-diffusion anions, namely HSO3-, via the interactions of ions and the cooperation of the controllable hydrophobicity. The impact of the alkyl chain length attached at the cationic probes suggested that the fluorescent intensity and sensitivity of the probes could be partially enhanced by adjusting their aggregation tendency through the action of the hydrophobic effect under aqueous conditions. DLS and SEM images indicated that different particle sizes and new morphologies of the probes were formed in the anion-recognition-triggered self-assembly process, which could be attributed to the composite effect of electrostatic actions, Van der Waals forces and π-π stacking.

Hierarchical Supramolecular Self-Assembly: Fabrication and Visualization of Multiblock Microstructures.

Due to the fast dynamics and re-equilibration of supramolecular self-assembly, bottom-up molecular strategies to fabricate well-defined and controllable multiblock structures are rare. Herein, we propose a new concept for fabrication of fluorescent multiblock microcolumns containing 1 to 7 blocks via hierarchical supramolecular self-assembly based on cucurbit[8]uril (CB[8]), NaBr and an AIEgen guest. Through the complexation between CB[8] and different numbers of AIEgen guests (2, 1, 0), the competitive displacement caused by the binding of the sodium cation to the CB[8] portal, and the reversible assembly of positively charged guests in salt solutions, one-pot hierarchical supramolecular self-assembly is realized. The molecular structure of each block is analyzed by single-crystal X-ray diffraction. The AIEgen enables the self-assembly of multiblocks to be visualized, understood, and regulated.

An Air-Stable Organic Radical from a Controllable Photoinduced Domino Reaction of a Hexa-aryl Substituted Anthracene.

Air-stable organic radicals and radical ions have attracted great attention for their far-reaching application ranging from bioimaging to organic electronics. However, because of the highly reactive nature of organic radicals, the design and synthesis of air-stable organic radicals still remains a challenge. Herein, an air-stable organic radical from a controllable photoinduced domino reaction of a hexa-aryl substituted anthracene is described. The domino reaction involves a photoinduced [4 + 2] cycloaddition reaction, rearrangement, photolysis, and an elimination reaction; 1H/13C NMR spectroscopy, high resolution mass spectrometry, single-crystal X-ray diffraction, and EPR spectroscopy were exploited for characterization. Furthermore, a photoinduced domino reaction mechanism is proposed according to the experimental and theoretical studies. In addition, the effects of employing push and pull electronic groups on the controllable photoinduced domino reaction were investigated. This article not only offers a new blue emitter and novel air-stable organic radical compound for potential application in organic semiconductor applications, but also provides a perspective for understanding the fundamentals of the reaction mechanism on going from anthracene to semiquinone in such anthracene systems.

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.

Host-guest interactions in nor-seco-cucurbit[10]uril: novel guest-dependent molecular recognition and stereoisomerism.

The unique monomer and excimer fluorescence emissions of pyrene were first exploited as distinctly photophysical signals to identify the possible diastereomers of guests within nor-seco-cucurbit[10]uril (NS-CB[10]) cavities. Further experiments revealed that balancing the hydrophilic and hydrophobic effects of the guest in aqueous solution can improve the molecular recognition and binding ability of NS-CB[10].

Host-Guest Interaction of Cucurbit[8]uril with N-(3-Aminopropyl)cyclohexylamine: Cyclohexyl Encapsulation Triggered Ternary Complex.

The host-guest interaction of a series of cyclohexyl-appended guests with cucurbit[8]uril (Q[8]) was studied by ¹H NMR spectroscopy, isothermal titration calorimetry (ITC), and X-ray crystallography. The X-ray structure revealed that two cycloalkane moieties can be simultaneously encapsulated in the hydrophobic cavity of the Q[8] host to form a ternary complex for the first time.

Outer Surface Interactions of Cucurbit[6]uril That Trigger the Assembly of Supramolecular Three-Dimensional Polycatenanes.

A supramolecular polycatenane stabilized by outer surface interactions between cucurbit[6]uril (Q[6] or CB[6]) cations and [CdCl4 ]2- is reported. The arrangement of the poly-interlocked supramolecular framework is evidently constructed by a favorable combination of weak non-covalent interactions such as hydrophobic effects, hydrogen bonding, ion-dipole interactions, and C-H⋅⋅⋅Cl interactions in the solid state. Further study suggests that such 3D poly-interlocked species could be rapidly precipitated from the mixture solution in high yield owing to electrostatic interaction of the [CdCl4 ]2- anion with the electropositive outer surface of Q[6] host. This work thus demonstrates a simple, low-cost, and efficient non-covalent approach for the construction of a infinite 3D interlocked structure in water besides through coordination or covalent bonds.

Facile Cucurbit[8]uril-Based Supramolecular Approach To Fabricate Tunable Luminescent Materials in Aqueous Solution.

Light-emitting materials with tunable properties may offer fascinating applications in optoelectronic devices, fluorescent sensors, and imaging agents. Herein, a new supramolecular approach based on host-guest interactions that greatly decreases the number of required synthetic steps and produces a system with tunable and dynamical photophysical properties was developed. Because of the novel electronic distributions of the chromophore guest within the rigid hydrophobic cavity of the cucurbit[8]uril host in this system, color tuning of emissions such as cyan, yellow, green, and white light with efficiency increased fluorescence lifetime, and quantum yield was easily achieved by simple addition of the host in aqueous solution. Stimulus-responsive tuning of color has long been an important area of research into light emissions. The current study distinguishes itself by its combination of simple steps using a single synthetic receptor and a single organic fluorophore guest in a single solution. Our results may provide a promising advancement of the fabrication of smart and tunable luminescent materials.

Anion Recognition Triggered Nanoribbon-Like Self-Assembly: A Fluorescent Chemosensor for Nitrate in Acidic Aqueous Solution and Living Cells.

A water-soluble π-conjugated bispyridinium phenylenevinylene-based fluorogenic probe has been developed as a novel fluorescent chemosensor for highly selective, sensitive, and rapid detection of NO3(-) anion in acidic aqueous media. This system self-assembles to a nanoribbon as a result of ionic interaction. The positively charged chemosensor generates a nearly instantaneous significant fluorescence signal (475 vs 605 nm) in response to NO3(-) in the green/yellow spectral region, with a large Stokes shift (130 nm). The fluorescence changes can be attributed to the self-aggregation of the sensor triggered by ionic interaction, which occurs as a consequence of the subtle cooperation of electrostatic ionic bonding, van der Waals forces, and π-stacking of the π-conjugated aromatic moieties. Importantly, this chemosensor has been employed for the first time for the fluorescence detection of intracellular NO3(-) anion in cultured cells.

Self-assemblies based on the "outer-surface interactions" of cucurbit[n]urils: new opportunities for supramolecular architectures and materials.

Supramolecular architectures and materials have attracted immense attention during the last decades because they not only open the possibility of obtaining a large variety of aesthetically interesting structures but also have applications in gas storage, sensors, separation, catalysis, and so on. On the other hand, cucurbit[n]urils (Q[n]s), a relatively new class of macrocyclic hosts with a rigid hydrophobic cavity and two identical carbonyl fringed portals, have attracted much attention in supramolecular chemistry. Because of the strong charge-dipole and hydrogen bonding interactions, as well as hydrophobic and hydrophilic effect derived from the negative portals and rigid cavities of Q[n]s, nearly all research in Q[n]s has been focused on utilizing the portals and cavities to construct supramolecular assemblies similar to other macrocyclic receptors such as cyclodextrin and calixarenes. Interestingly, a recent study revealed that other weak noncovalent interactions such as hydrogen bonding and π···π stacking, as well as C-H···π and ion-dipole interactions, could also be defined as "outer-surface interactions", which are derived from the electrostatically positive outer surface of Q[n]s. These interactions could be the driving forces in the formation of various novel Q[n]-based supramolecular architectures and functional materials. In this Account, we provide a comprehensive overview of supramolecular self-assemblies based on the outer-surface interactions of Q[n]s. These outer-surface interactions include those between Q[n]s, Q[n]s and aromatic molecules, Q[n]s and calixarenes, Q[n]s and inorganic complex ions, and Q[n]s and polyoxometalates. Pioneering work has shown that such weak noncovalent interactions play very important roles in the formation of various Q[n]-based functional materials and supramolecular architectures. For example, hydrogen bonds in outer-surface interactions between Q[n] molecules not only function as the sole driving force in the formation of one-dimensional Q[n] porous channels but also assist the bonding forces of the channels in capturing and accommodating acetylene molecules and carbon dioxide in the channel cavities. Moreover, upon introduction of a third species such as an aromatic molecule or inorganic anion into the Q[n]/metal system, "outer-surface interactions" could lead to Q[n]/metal-based self-assemblies from simple finite supramolecular coordination complexes to infinite polydimensional supramolecular architectures and other structures. Overall, this Account focuses on the novel self-assembly driving force derived from Q[n]s including (i) concepts of the outer-surface interactions of Q[n]s, (ii) providing plausible explanations of the mechanisms of the outer-surface interactions of Q[n]s, and (iii) introduction of an overview of the developments and practical applications of outer-surface interactions of Q[n]s in supramolecular chemistry. It is hoped that this study based on the outer-surface interactions of Q[n]s can enrich the field of molecular engineering of functional supramolecular systems and provide new opportunities for the construction of functional materials and architectures.

The first study about the relationship between the extractability of thiacalix[4]arene derivatives and the position of the coordination binding sites.

Three organic ionophores (2-4) based on the p-tert-butylthiacalix[4]arene backbone, blocked in the 1,3-alternate conformation, bearing two pyridyl coordinating moieties (ortho for 2, meta for 3 and para for 4), have been synthesized and characterized in the solid state. The solvent extraction experiments with the metal ions showed that the ability of these derivatives to complex with Ag(+) appeared to be largely dependent on the position of the nitrogen atoms of the pyridyl ring. Two different complexation modes have been confirmed by 1H NMR titration. Ionophore 2 armed with two pyridyl moieties, complexed with Ag(+) cation through N···Ag(+)···S interactions; however, ionophore 3 and ionophore 4 complexed with Ag(+) through metal-nitrogen (N···Ag(+)) interactions. The DFT computational studies were consistent with the experimental findings. These findings will provide us with an important rule to design an appropriate thiacalix[4]arene ionophore in the future. Another study on the possibility for application of ionophores 2-4 for the treatment of waste water containing Cr(VI) and Cr(III), showed that ionophore 3 was useful in the application of the solvent extraction method in selective treatment of waste water containing Cr(VI) and Cr(III) prior to discharge.

Tetrachloridometallate dianion-induced cucurbit[8]uril supramolecular assemblies with large channels and their potential applications for extraction coating on solid-phase microextraction fibers.

Q[8]-based porous materials were synthesized in the presence of [Md-blockCl4](2-) anions as structure inducers. The driving forces of the structure-directing effect of the [Md-blockCl4](2-) anions may be due to the ion-dipole interaction and hydrogen bonding between the [Md-blockCl4](2-) anions and ≡CH or ═CH2 groups on the backs of Q[8] molecules. Moreover, the tests of potential applications show that these porous materials can not only capture organic molecules through the cavity of Q[8] moieties but also adsorb larger organic molecules with different selectivities.

A study of allosteric binding behaviour of a 1,3-alternate thiacalix[4]arene-based receptor using fluorescence signal.

A novel heteroditopic thiacalix[4]arene receptor L possessing 1,3-alternate conformation, which contains two pyrene moieties attached to the lower rim via urea linkages together with a crown ether moiety appended at the opposite side of the thiacalix[4]arene cavity, has been synthesized. The complexation behaviour of receptor L was studied by means of fluorescence spectra and (1)H NMR titration experiments in the presence of K(+) ions and a variety of other anions. The results suggested that receptor L can complex efficiently via the urea cavity or the crown ether moiety, and a positive/negative allosteric effect operating in receptor L was observed.

Cucurbit[n]uril-based coordination chemistry: from simple coordination complexes to novel poly-dimensional coordination polymers.

Cucurbit[n]urils are a family of molecular container hosts bearing a rigid hydrophobic cavity and two identical carbonyl fringed portals. They have attracted much attention in supramolecular chemistry because of their superior molecular recognition properties in aqueous media. This review highlights the recent advances and challenges in the field of cucurbit[n]uril-based coordination chemistry. It not only presents progress in the knowledge of such macrocyclic compounds, which range from simple to complicated architectures, but also presents new routes of synthesis and their advantages in hybrid porous solids. The concept of structure "inducer" for their structural design to achieve predictable structures and controlled pores is described. The large pore sizes and hydrophobic cavities of these compounds that lead to unprecedented properties and potential applications are also discussed.

Construction of cucurbit[7]uril based tubular nanochannels incorporating associated [CdCl4]2- and lanthanide ions.

There is intensive interest in the design of tubular channels because of their novel structures and various applications in a variety of research fields. Herein, we present a series of coordination-driven Q[7]-derived organic nanochannels using an anion-induced strategy under different acid concentrations. An advantage of this approach is that the tubular channels not only retain the original character of the parent macrocyclic receptors but also provide deep hydrophobic cavities possessing guest binding sites. Importantly, this study also emphasizes the efficiency of the macrocyclic receptors in providing a tubular hydrophobic cavity by directly stacking on top of one another with the anion-fixed and by acid control. The resulting combination of hydrogen bonding, C-H···Cl, and ion-dipole interactions helps to stabilize these supramolecular architectures. Such systems are both tunable and versatile and allow for interconvertibility in the construction of nanochannels based on these macrocyclic receptors.

Tri-substituted hexahomotrioxacalix[3]arene derivatives bearing imidazole units: synthesis and extraction properties for cations and chromate anions.

The article describes the synthesis and extraction properties of the new hexahomotrioxacalix[3]arenebased derivatives cone- and partial-cone-2 bearing 1-methyl-1H-imidazole moieties at the lower rim. It has been demonstrated that O-alkylation of the flexible macrocycle 1 with 2-(chloromethyl)-1-methyl-1Himidazole hydrochloride affords tri-O-alkylated products with a cone or partial-cone conformation. Alkali metal salts such as NaH and Cs2CO3 can play an important role in the conformer distribution via a template effect. The conformation of receptors 2 has been confirmed by X-ray crystallographic analysis. Furthermore, the complexation properties of receptors 2 toward selected alkali/transition metal cations and alkylammonium ions are reported. The new 1-methyl-1H-imidazole-substituted hexahomotrioxacalix[3] arene frameworks are also efficient extractors of HCr2O7(-)/Cr2O7(2-) anions at low pH.

Highly emissive hand-shaped π-conjugated alkynylpyrenes: synthesis, structures, and photophysical properties.

Three alkynyl-functionalised, hand-shaped, highly fluorescent and stable emitters, namely, 2-tert-butyl-4,5,7,9,10-pentakis(p-R-phenylethynyl)pyrenes have been successfully synthesized via a Pd/Cu-catalysed Sonogashira cross-coupling reaction. The chemical structures of the alkynylpyrenes were fully characterized by their (1)H/(13)C NMR spectra, mass spectroscopy and elemental analysis. Synchrotron single-crystal X-ray analysis revealed that there is a 1-D, slipped, face-to-face motif with off-set, head-to-tail stacked columns, which are clearly influenced by the single, bulky, tert-butyl group in the pyrene ring at the 2-position. Detailed studies on the photophysical properties in both solutions and thin films strongly indicate that they might be promising candidates for optoelectronic applications, such as organic light-emitting devices (OLEDs) or as models for investigating the fluorescent structure-property relationship of the alkynyl-functionalised pyrene derivatives.

Hexahomotrioxacalix[3]arene derivatives as ionophores for molecular recognition of dopamine, serotonin and phenylethylamine.

The lower rim functionalized hexahomotrioxacalix[3]arene derivatives cone-3 and cone-5 bearing three benzyl and three N,N-diethyl-2-aminoethoxy groups, respectively, were synthesized from triol 1. Their complexation with 2-(3,4-dihydroxyphenyl)ethylamine (dopamine), 5-hydroxytryptamine (serotonin), and 2-phenylethylamine (phenethylamine), which have biologically important activities, has been studied by (1)H-NMR spectroscopy. The chemical shifts of the aromatic protons of the host and guest molecules and the up-field shifts of the ethyl protons of the guest molecules strongly suggest the formation of inclusion complexes in solution. The formation of the host-guest complexes is assisted by a hydrogen bond and/or an electrostatic interaction between the host and ammonium ion (RNH(3)(+)) of the guest. The structures of receptors cone-3 and cone-5 have been determined by X-ray crystallography.