Supramolecular organic frameworks: engineering periodicity in water through host-guest chemistry

Emerging Two-Dimensional Crystallization of Cucurbit[8]uril Complexes: From Supramolecular Polymers to Nanofibers

The binding of imidazolium salts to cucurbit[8]uril, CB[8], triggers a stepwise self-assembly process with semiflexible polymer chains and crystalline nanostructures as early- and late-stage species, respectively. In such a process, which involves the crystallization of the host-guest complexes, the guest plays a critical role in directing self-assembly toward desirable morphologies. These include platelet-like aggregates and two-dimensional (2D) fibers, which, moreover, exhibit viscoelastic and lyotropic properties. Our observations provide a deeper understanding of the self-assembly of CB[8] complexes, with fundamental implications in the design of functional 2D systems and crystalline materials.

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].

In Situ Loading and Delivery of Short Single- and Double-Stranded DNA by Supramolecular Organic Frameworks

Short DNA represents an important class of biomacromolecules that are widely applied in gene therapy, editing, and modulation. However, the development of simple and reliable methods for their intracellular delivery remains a challenge. Herein, we describe that seven water-soluble, homogeneous supramolecular organic frameworks (SOFs) with a well-defined pore size and high stability in water that can accomplish in situ inclusion of single-stranded (ss) and double-stranded (ds) DNA (21, 23, and 58 nt) and effective intracellular delivery (including two noncancerous and six cancerous cell lines). Fluorescence quenching experiments for single and double end-labeled ss- and ds-DNA support that the DNA sequences can be completely enveloped by the SOFs. Confocal laser scanning microscopy and flow cytometry reveal that five of the SOFs exhibit excellent delivery efficiencies that, in most of the studied cases, outperform the commercial standard Lipo2000, even at low SOF–nucleic acid ratios. In addition to high delivery efficiencies, the water-soluble, self-assembled SOF carriers have a variety of advantages, including convenient preparation, high stability, and in situ DNA inclusion, which are all critical for practical applications in nucleic acid delivery.

Facile Construction of a Supramolecular Organic Framework Using Naphthyl Viologen Guests and CB[8] Host via Charge-Transfer Complexation

Herein, we report the synthesis of guest-host systems comprising naphthyl-viologen-naphthyl (Np-Vio-Np) and viologen-naphthyl-viologen (Vio-Np-Vio) guest molecules and their subsequent supramolecular polymerization in the presence of a CB[8] host in water. In addition, the guest complexation of ethyl-terminated trimeric viologen (ETV) with Np-Vio-Np and CB[8] was investigated. As a result of supramolecular interactions, 2D supramolecular organic frameworks with high internal periodicity were constructed. 1H NMR studies clearly demonstrated the formation of a host-stabilized charge-transfer complex via folding back (Np-Vio-Np and Vio-Np-Vio) in the presence of CB[8]. In the case of ETV + Np-Vio-Np + CB[8], a large polymeric network was formed as indicated by the NMR titrations. UV-vis and fluorescence studies clearly confirm the formation of an inter/intra molecular CT complex upon complexation with cucurbit[8]uril. The size obtained using the dynamic light scattering (DLS) method pinpoints the formation of larger supramolecular aggregates in the order of μm through host-guest assembly, which is further complemented by FESEM and TEM. SAXS measurements indicate the formation of a 2D supramolecular polymer/polymer aggregate with long-range order.

Multifunctional porous hydrogen-bonded organic framework materials

Hydrogen-bonded organic frameworks (HOFs) represent an interesting type of polymeric porous materials that can be self-assembled through H-bonding between organic linkers. To realize permanent porosity in HOFs, stable and robust open frameworks can be constructed by judicious selection of rigid molecular building blocks and hydrogen-bonded units with strong H-bonding interactions, in which the framework stability might be further enhanced through framework interpenetration and other types of weak intermolecular interactions such as ππ interactions. Owing to the reversible and flexible nature of H-bonding connections, HOFs show high crystallinity, solution processability, easy healing and purification. These unique advantages enable HOFs to be used as a highly versatile platform for exploring multifunctional porous materials. Here, the bright potential of HOF materials as multifunctional materials is highlighted in some of the most important applications for gas storage and separation, molecular recognition, electric and optical materials, chemical sensing, catalysis, and biomedicine.

A pore-expanded supramolecular organic framework and its enrichment of photosensitizers and catalysts for visible-light-induced hydrogen production

A 3.6 nm-pore SOF is constructed, which adsorbs both photosensitizers and polyoxometallates for visible light-induced proton reduction to produce H₂.

Unveiling the formation 1 : 2 supramolecular complexes between cucurbit[7]uril and a cationic calix[4]arene derivative

The formation of host–guest complexes between cucurbit[7]uril (CB7) and a tetracationic calix[4]arene derivative in the so-called cone conformation was investigated by ¹H NMR, DOSY NMR, isothermal titration calorimetry and ESI-MS.

Cucurbit[8]uril-Based Polymers and Polymer Materials

Cucurbit[8]uril (CB[8]) is unique and notable in the cucurbit[n]uril family, since it has a relatively large cavity and thus is able to simultaneously accommodate two guest molecules. Typically, an electron-deficient first guest and an electron-rich second guest can be bound by CB[8] to form a stable 1:1:1 heteroternary supramolecular complex. Additionally, two homo guests can also be strongly dimerized inside the cavity of CB[8] to form a 2:1 homoternary supramolecular complex. During the past decade, by combining polymer science and CB[8] host-guest chemistry, a variety of systems have been established to construct supramolecular polymers with polymer chains typically at the nanoscale/sub-microscale, and CB[8]-based micro/nanostructured polymer materials in the form of polymer networks and hydrogels, microcapsules, micelles, vesicles, and colloidal particles, normally in solution and occasionally on surfaces. This Review summarizes the noncovalent interactions and strategies used for the preparation of CB[8]-based polymers and polymer materials with a focus on the representative and latest developments, followed by a brief discussion of their characterization, properties, and applications.

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.

Towards the Prediction of Global Solution State Properties for Hydrogen Bonded, Self-Associating Amphiphiles

Through this extensive structure-property study we show that critical micelle concentration correlates with self-associative hydrogen bond complex formation constant, when combined with outputs from low level, widely accessible, computational models. Herein, we bring together a series of 39 structurally related molecules related by stepwise variation of a hydrogen bond donor-acceptor amphiphilic salt. The self-associative and corresponding global properties for this family of compounds have been studied in the gas, solid and solution states. Within the solution state, we have shown the type of self-associated structure present to be solvent dependent. In DMSO, this class of compound show a preference for hydrogen bonded dimer formation, however moving into aqueous solutions the same compounds are found to form larger self-associated aggregates. This observation has allowed us the unique opportunity to investigate and begin to predict self-association events at both the molecular and extended aggregate level.

2:2 Complexes from Diphenylpyridiniums and Cucurbit[8]uril: Encapsulation-Promoted Dimerization of Electrostatically Repulsing Pyridiniums

Rigid linear compounds G1 and G2, which contained two 4-phenylpyridinium (PhPy⁺ ) units, have been prepared to investigate their binding with cucurbit[8]uril (CB[8]). X-ray crystallographic structures revealed that in the solid state both compounds were included by CB[8], through antiparallel stacking, to form 2:2 quaternary complexes (G1)₂ @(CB[8])₂ and (G2)₂ @(CB[8])₂ . For the former complex, CB[8] entrapped G1 by holding two heterodimers of its Py⁺ and benzyl units, which were at opposite ends of the backbone. In contrast, for the first time, the second complex disclosed parallel stacking of two cationic Py⁺ units of G2 in the cavity of CB[8] in the solid state, despite the generation of important electrostatic repulsion. Isothermal titrations in water afforded high apparent association constants of 4.36×10⁶ and 6.43×10⁶  m^-1 for 1:1 complexes G1@CB[8] and G2@CB[8], respectively, and ¹ H NMR spectroscopy experiments in D₂ O confirmed a similar stacking pattern to that observed in the solid state. A previous study and crystal structures of the 2:1 complexes formed between three new controls, G3-5, and CB[8] did not display such unusual stacking of the cationic Py⁺ unit; this may be attributed to the multivalency of the two CB[8] encapsulation interactions.

Host-guest chemistry in two-dimensional supramolecular networks

Nanoporous supramolecular networks physisorbed on solid surfaces have been extensively used to immobilize a variety of guest molecules. Host-guest chemistry in such two-dimensional (2D) porous networks is a rapidly expanding field due to potential applications in separation technology, catalysis and nanoscale patterning. Diverse structural topologies with high crystallinity have been obtained to capture molecular guests of different sizes and shapes. A range of non-covalent forces such as hydrogen bonds, van der Waals interactions, coordinate bonds have been employed to assemble the host networks. Recent years have witnessed a surge in the activity in this field with the implementation of rational design strategies for realizing controlled and selective guest capture. In this feature article, we review the development in the field of surface-supported host-guest chemistry as studied by scanning tunneling microscopy (STM). Typical host-guest architectures studied on solid surfaces, both under ambient conditions at the solution-solid interface as well as those formed at the ultrahigh vacuum (UHV)-solid interface, are described. We focus on isoreticular host networks, hosts functionalized pores and dynamic host-guest systems that respond to external stimuli.

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

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

Towards quantifying the role of hydrogen bonding within amphiphile self-association and resultant aggregate formation

The role of hydrogen bonding within aggregate formation and CMC: can these properties be predicted by low level computational modelling?

Herein, we present a series of five tetrabutylammonium (TBA) sulfonate–urea amphiphilic salts. In solution these amphiphilic salts have been shown to form a variety of self-associated species. The proportion and type of which are both solvent and concentration dependent. In DMSO-d₆ a variety of NMR experiments provide evidence towards the formation of mainly dimeric over larger aggregate species. Increasing the percentage of water was shown to increase the concentration of the larger aggregates over dimers in solution. A correlation was established between critical micelle concentration (CMC) values obtained in a 1 : 19 EtOH : H₂O mixture, dimeric self-association constants obtained in a DMSO-d₆ – 0.5% H₂O and the results of simple semi-empirical PM6 computational modelling methods. This approach begins to quantify the role of hydrogen bonding in amphiphile self-association and the effects it imparts on surfactant properties. This consequently provides preliminary evidence that these properties maybe predicted by simple low level computational modelling techniques.

Two-Dimensional Materials as Prospective Scaffolds for Mixed-Matrix Membrane-Based CO2 Separation

Membrane-based CO₂ separation technology plays a significant role in environmental remediation and clean energy. Two-dimensional (2D) materials with atomically precise structures have emerged as prospective scaffolds to develop mixed-matrix membranes (MMMs) for gas separation. Summarized in this perspective review are the latest breakthrough studies in the synthesis of 2D-material-based MMMs to separate CO₂ from gas mixtures. 2D materials including graphene oxide (GO), metal-organic framework (MOF)-derived nanosheets, covalent organic frameworks (COFs), and transition metal dichalcogenides (TMDs), as fascinating building blocks, have been comprehensively summarized, together with a focus on synthetic processes and gas separation properties. Challenges and the latest advances in the manufacture of novel synthetic 2D materials are briefly discussed to foresee emerging opportunities for the development of new generations of 2D-material-based MMMs.

Supramolecular organic frameworks (SOFs): homogeneous regular 2D and 3D pores in water

Studies on periodic porosity and related properties and functions have been limited to insoluble solid-state materials. Self-assembly provides a straightforward and efficient strategy for the construction of soluble periodic porous supramolecular organic frameworks (SOFs) in water from rationally designed molecular building blocks. From rigid tri- and tetra-armed building blocks and cucurbitu[8]ril (CB[8]), a number of two-dimensional (2D) honeycomb, square and rhombic SOFs can be generated, which is driven by CB[8]-encapsulation-enhanced dimerization of two aromatic units on the periphery of the multi-armed molecules. By utilizing the same three-component host−guest motif as the driving force, three-dimensional (3D) diamondoid and cubic SOFs can be obtained from tetrahedral and [Ru(bipy)3]2+-derived octahedral monomers and CB[8]. All of the 2D and 3D periodic frameworks are soluble in water, and are able to maintain the periodicity as well as the pore sizes in the solid state. 3D SOFs are highly efficient homogeneous polycationic frameworks for reversible adsorption of anionic species including organic dyes, peptides, nucleic acids, drugs, dendrimers and Wells-Dawson-typed polyoxametallates (WD-POMs). WD-POM molecules adsorbed in the [Ru(bipy)3]2+-based SOF can catalyse the reduction of proton to H2 upon visible-light sensitization of [Ru(bipy)3]2+, which allows multiple electron transfer from [Ru(bipy)3]2+ to WD-POM. This review summarizes the design, formation and characterization of this new family of self-assembled frameworks, highlights their applications as homogeneous porous materials and finally outlines some future research directions.

Supramolecular anion recognition in water: synthesis of hydrogen-bonded supramolecular frameworks

The interaction of tetratopic amidinium-containing receptors with terephthalate anions leads to porous framework materials assembled through charge-assisted hydrogen bonds. The frameworks form in good yield within minutes in water at room temperature, but no framework material is obtained if other anions (Cl^-, Br^-, NO₃^-, SO₄^2- or isophthalate^2-) are used in place of terephthalate. Two forms of the framework can be prepared: one with a connected pore network, and a more dense phase with discrete voids. We demonstrate that these are the kinetic and thermodynamic products, respectively. Either framework can be prepared independently and can be converted to the other form in response to stimuli. Furthermore, the frameworks can be controllably disassembled and reassembled in response to acid/base triggers suggesting that this new class of materials may have applications in the selective encapsulation and release of guests.

Crystalline nanotubular framework constructed by cucurbit[8]uril for selective CO2 adsorption

A crystalline nanotubular framework (1) constructed by cucurbit[8]uril exhibits highly selective adsorption for CO₂ over N₂, CH₄, and H₂.

Postmodification of a supramolecular organic framework: visible-light-induced recyclable heterogeneous photocatalysis for the reduction of azides to amines

Postmodification has been utilized to introduce [Ru(BPY)₃]²⁺ to 3D SOF to prepare a catalyst for visible light-induced recyclable heterogeneous photoreduction of aromatic azides into amines.

The controlled synthesis of pillar[6]arene-based hexagonal cylindrical structures on an electrode surface via electrochemical oxidation

Electrochemical oxidation of pillar[6]arene containing six hydroquinones resulted in the formation of hexagonal cylindrical structures on an electrode surface driven by charge transfer interaction.

Metal-Organic Polyhedron Capped with Cucurbit[8]uril Delivers Doxorubicin to Cancer Cells

Self-assembly of ligand 1 and Pd(NO3)2 delivers Fujita-type metal-organic polyhedron (MOP) 3 which bears 24 covalently attached methyl viologen units on its external surface, as evidenced by 1H NMR, diffusion-ordered spectroscopy NMR, electrospray mass spectrometry, transmission electron microscopy, and atomic force microscopy measurements. MOP 3 undergoes noncovalent complexation with cucurbit[n]urils to yield MOPs 4-6 with diameter ≈5-6 nm. MOP 5 can be fully loaded with doxorubicin (DOX) prodrug 2 via hetero-ternary complex formation to yield 7. The MOPs exhibit excellent stability toward neutral to slightly acidic pH in 10 mM sodium phosphate buffer, mitigating the concern of disassembly during circulation. The results of MTS assays show that MOP 7 is 10-fold more cytotoxic toward HeLa cells than equimolar quantities of DOX prodrug 2. The enhanced cytotoxicity can be traced to a combination of enhanced cellular uptake of 7 and DOX release as demonstrated by flow cytometry and confocal fluorescence microscopy. The confluence of properties imparted by the polycationic MOP architecture and plug-and-play CB[n] complexation provides a potent new platform for drug delivery application.

Synthesis of Water-Soluble Deep-Cavity Cavitands

An efficient, four-step synthesis of a range of water-soluble, deep-cavity cavitands is presented. Key to this approach are octahalide derivatives (4, X = Cl or Br) that allow a range of water-solubilizing groups to be added to the outer surface of the core host structure. In many cases, the conversion of the starting dodecol (1) resorcinarene to the different cavitands avoids any chromatographic procedures.

Fluorescence enhancement through the formation of a single-layer two-dimensional supramolecular organic framework and its application in highly selective recognition of picric acid

A novel two-dimensional (2D) supramolecular organic framework (SOF) has been assembled, which was further used as a fluorescent chemosensor to detect picric acid with high selectivity and sensitivity.