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

Cucurbit[8]uril-Mediated Supramolecular Heterodimerisation and Photoinduced [2+2] Heterocycloaddition to Generate Unexpected [2]Rotaxanes

In contrast to the self-assembly of homosupramolecules, the self-assembly of heterosupramolecules is more challenging and significant in various fields. Herein, we design and investigate a cucurbit[8]uril-mediated heterodimerisation based on an arene-fluoroarene strategy. Furthermore, the heteroternary complex is found to be able to undergo a photoinduced [2+2] heterocycloaddition, resulting in the formation of an unexpected [2]rotaxane. This work demonstrates a novel supramolecular heterodimerisation system that not only contributes to the development of photoisomerisation systems, but also enriches synthetic methods for mechanically interlocked molecules.

Double-cavity cucurbiturils: synthesis, structures, properties, and applications

Double-cavity Q[n]s are relatively new members of the Q[n] family and have garnered significant interest due to their distinctive structures and novel properties. While they incorporate n glycoluril units, akin to their single-cavity counterparts, their geometry can best be described as resembling a figure-of-eight or a handcuff, distinguishing them from single-cavity Q[n]s. Despite retaining the core molecular recognition traits of single-cavity Q[n]s, these double-cavity variants introduce fascinating new attributes rooted in their distinct configurations. This overview delves into the synthesis, structural attributes, properties, and intriguing applications of double-cavity Q[n]s. Some of the applications explored include their role in supramolecular polymers, molecular machinery, supra-amphiphiles, sensors, artificial light-harvesting systems, and adsorptive separation materials. Upon concluding this review, we discuss potential challenges and avenues for future development and offer valuable insights for other scholars working in this area with the aim of stimulating further exploration and interest.

Cucurbituril-based supramolecular host-guest complexes: single-crystal structures and dual-state fluorescence enhancement

Two supramolecular complexes were prepared using cucurbiturils [CBs] as mediators and a four-armed p-xylene derivative (M1) as a guest molecule. The single crystals of these two complexes were obtained and successfully analyzed by single-crystal X-ray diffraction (SCXRD). An unexpected and intriguing 1 : 2 self-assembly arrangement between M1 and CB[8] was notably uncovered, marking its first observation. These host-guest complexes exhibit distinctive photophysical properties, especially emission behaviors. Invaluable insights can be derived from these single-crystal structures. The precious single-crystal structures provide both precise structural information regarding the supramolecular complexes and a deeper understanding of the intricate mechanisms governing their photophysical properties.

p-Sulfonato-Calix[4]arene Micelles Stabilize a Povidone Iodine Solution: Supramolecular Interactions, Iodine Retention, and Bactericidal Activity

Povidone iodine (PVPI) is an antiseptic widely used against a broad spectrum of pathogens. However, undesired side-effects are still associated with PVPI treatment due to the irritant effect of iodine. Reducing the concentration of a PVPI formulation could provide safer and more friendly formulations, for routine use and applications in very delicate organs such as the eye. However, managing the storage of a low-concentration solution of PVPI is challenging due to the high iodine volatility. In this study, we demonstrated that an amphiphilic p-sulfonato-calix[4]arene derivative forming micelles (SC4OC6) improves the stability of a 0.1% PVPI aqueous buffered solution. UV-vis and NMR spectra as well as dynamic and electrophoretic light scattering measurements showed that SC4OC6 establishes non-covalent supramolecular interactions with PVPI, resulting in the formation of nanoaggregates with a negatively charged surface. Isothermal titration calorimetry provided the aggregation parameters and evidenced that the formation of the supramolecular assembly is an enthalpically favored process. The interaction of SC4OC6 with PVPI enhances the iodine retention and stability of the solution without affecting the rapid and effective bactericidal activity of PVPI, as demonstrated by a time-killing assay with Staphylococcus epidermidis.

Small Molecule NIR-II Dyes for Switchable Photoluminescence via Host -Guest Complexation and Supramolecular Assembly with Carbon Dots

Small molecular NIR-II dyes are highly desirable for various biomedical applications. However, NIR-II probes are still limited due to the complex synthetic processes and inadequate availability of fluorescent core. Herein, the design and synthesis of three small molecular NIR-II dyes are reported. These dyes can be excited at 850-915 nm and emitted at 1280-1290 nm with a large stokes shift (≈375 nm). Experimental and computational results indicate a 2:1 preferable host-guest assembly between the cucurbit[8]uril (CB) and dye molecules. Interestingly, the dyes when self-assembled in presence of CB leads to the formation of nanocubes (≈200 nm) and exhibits marked enhancement in fluorescence emission intensity (Switch-On). However, the addition of red carbon dots (rCDots, ≈10 nm) quenches the fluorescence of these host-guest complexes (Switch-Off) providing flexibility in the user-defined tuning of photoluminescence. The turn-ON complex found to have comparable quantum yield to the commercially available near-infrared fluorophore, IR-26. The aqueous dispersibility, cellular and blood compatibility, and NIR-II bioimaging capability of the inclusion complexes is also explored. Thus, a switchable fluorescence behavior, driven by host-guest complexation and supramolecular self-assembly, is demonstrated here for three new NIR-II dyes.

Bottom-up supramolecular assembly in two dimensions

The self-assembly of molecules in two dimensions (2D) is gathering attention from all disciplines across the chemical sciences. Attracted by the interesting properties of two-dimensional inorganic analogues, monomers of different chemical natures are being explored for the assembly of dynamic 2D systems. Although many important discoveries have been already achieved, great challenges are still to be addressed in this field. Hierarchical multicomponent assembly, directional non-covalent growth and internal structural control are a just a few of the examples that will be discussed in this perspective about the exciting present and the bright future of two-dimensional supramolecular assemblies.

The self-assembly of molecules in two dimensions (2D) is gathering attention from all disciplines across the chemical sciences. This perspective discusses the main strategies to direct the supramolecular self-assembly of organic monomers in 2D.

Crystallization and near-infrared emission from host–guest based supramolecular polymers

Crystallization process of a NIR emissive supramolecular polymer formed by host–guest complexation of a distyrylanthracene derivative and cucurbiturils is described.

Controlled synthesis of organic two-dimensional nanostructures via reaction-driven, cooperative supramolecular polymerization

The bottom-up approach of supramolecular polymerization is an effective synthetic method for functional organic nanostructures. However, the uncontrolled growth and polydisperse structural outcome often lead to low functional efficiency. Thus, precise control over the structural characteristics of supramolecular polymers is the current scientific hurdle. Research so far has tended to focus on systems with inherent kinetic control by the presence of metastable state monomers either through conformational molecular design or by exploring pathway complexity. The need of the hour is to create generic strategies for dormant states of monomers that can be extended to different molecules and various structural organizations and dimensions. Here we venture to demonstrate chemical reaction-driven cooperative supramolecular polymerization as an alternative strategy for the controlled synthesis of organic two-dimensional nanostructures. In our approach, the dynamic imine bond is exploited to convert a non-assembling dormant monomer to an activated amphiphilic structure in a kinetically controlled manner. The chemical reaction governed retarded nucleation-elongation growth provides control over dispersity and size.

A Woven Supramolecular Metal-Organic Framework Comprising a Ruthenium Bis(terpyridine) Complex and Cucurbit[8]uril: Enhanced Catalytic Activity toward Alcohol Oxidation

The self-assembly of a diamondoid woven supramolecular metal-organic framework wSMOF-1 has been achieved from intertwined [Ru(tpy)₂ ]²⁺ (tpy=2,2',6',2''-terpyridine) complex M1 and cucurbit[8]uril (CB[8]) in water, where the intermolecular dimers formed by the appended aromatic arms of M1 are encapsulated in CB[8]. wSMOF-1 exhibits ordered pore periodicity in both water and the solid state, as confirmed by a combination of ¹ H NMR spectroscopy, UV-vis absorption, isothermal titration calorimetry, dynamic light scattering, small angle X-ray scattering and selected area electron diffraction experiments. The woven framework has a pore aperture of 2.1 nm, which allows for the free access of both secondary and primary alcohols and tert-butyl hydroperoxide (TBHP). Compared with the control molecule [Ru(tpy)₂ ]Cl₂ , the [Ru(tpy)₂ ]²⁺ unit of wSMOF-1 exhibits a remarkably higher heterogeneous catalysis activity for the oxidation of alcohols by TBHP in n-hexane. For the oxidation of 1-phenylethan-1-ol, the yield of acetophenone was increased from 10 % to 95 %.

Coumarin-Containing Pillar[5]arenes as Multifunctional Liquid Crystal Macrocycles

Liquid crystal macrocycles (LCMs) combine the unique properties of liquid crystals with those associated with macrocyclic compounds-shape persistence and the capability of hosting small molecules. Herein, we investigate the grafting of coumarin-containing promesogenic moieties to pillar[5]arene as a strategy to obtain multifunctional LCMs. Pillar[5]arenes containing 10 and 30 coumarin units are glassy materials with nematic mesomorphism. Moreover, the coumarin moieties afford the pillar[5]arene derivatives with enhanced film-forming and photoresponsive properties. Photodimerization of the coumarin moieties results in cross-linked polymer networks, which can be used as alignment layers. Therefore, liquid-crystal coumarin-containing pillar[5]arenes represent a significant addition to the family of LCMs and may become useful for the development of engineered, hierarchical structures and materials.

Multifunctional Supramolecular Assemblies with Aggregation-Induced Emission (AIE) for Cell Line Identification, Cell Contamination Evaluation, and Cancer Cell Discrimination

It is undoubted the important role of cells in biology and medicine, but worldwide misidentified and cross-contaminated cell lines have caused much trouble in related fields. Herein, three kinds of supramolecular AIE (aggregation-induced emission) nanoassemblies were constructed by the host-guest interaction between tetraphenylethene (TPE) derivatives and cucurbit[8]uril (CB[8]). Based on the recognized mechanism of AIE, the TPE derivatives could achieve stronger fluorescence emission and higher fluorescence quantum yield after assembling with CB[8]. Moreover, the constructed supramolecular AIE complexes obtained well-confirmed nanostructures and exhibited different sizes and shapes. Correspondingly, they generated characteristic biological properties and fluorescence enhancement of cells. Inspired by the concept of Big Data Analysis, these fluorescence signals were further transformed into a unique fingerprint of cells via linear discriminant analysis. Immediately, we realized the veracious identification between a normal cell line, two cancer cell lines, and two metastasized cancer cell lines in a qualitative analysis. More importantly, it was well used to monitor the evaluation of cross-contaminated cells and the discrimination of cancer cells. As a proper bioapplication of ideal supramolecular nanomaterials, this system was easy to learn and apply, and the whole procedure was kept to 20 min, without cell disruption, centrifugation, or washing steps.

Correlating Solution- and Solid-State Structures of Conformationally Flexible Resorcinarenes: Significance of a Sulfonyl Group in Intramolecular Self-Inclusion

The synthesis of tetramethoxyresorcinarene podands bearing p-toluene arms connected by -SO3 - (1) and -CH2 O- (2) linkers is presented herein. In the solid state, the resorcinarene podand 1 forms an intramolecular self-inclusion complex with the pendant p-toluene group of a podand arm, whereas the resorcinarene podand 2 does not show self-inclusion. The conformations of the flexible resorcinarene podands in solution were investigated by variable-temperature experiments using 1D and 2D NMR spectroscopic techniques as well as by computational methods, including a conformational search and subsequent DFT optimisation of representative structures. The 1 H NMR spectra of 1 and 2 at room temperature show a single set of proton signals that are in agreement with C4v symmetry. At low temperatures, the molecules exist as a mixture of boat conformations featuring slow exchange on the NMR timescale. Energy barriers (ΔG≠ 298 ) of 55.5 and 52.0 kJ mol-1 were calculated for the boat-to-boat exchange of 1 and 2, respectively. The results of the ROESY experiments performed at 193 K and computational modelling suggest that in solution the resorcinarene podand 1 adopts a similar conformation to that present in its crystal structure, whereas podand 2 populates a more versatile range of conformations in solution.

Control over the Aspect Ratio of Supramolecular Nanosheets by Molecular Design

Recent developments in kinetically controlled supramolecular polymerization permit control of the size (i.e., length and area) of self-assembled nanostructures. However, control of molecular self-assembly at a level comparable with organic synthetic chemistry and the achievement of structural complexity at a hierarchy larger than the molecular level remain challenging. This study focuses on controlling the aspect ratio of supramolecular nanosheets. A systematic understanding of the relationship between the monomer structure and the self-assembly energy landscape has derived a new monomer capable of forming supramolecular nanosheets. With this monomer in hand, the aspect ratio of a supramolecular nanosheet is demonstrated that it can be controlled by modulating intermolecular interactions in two dimensions.

Dynamic host-guest interaction enables autonomous single molecule blinking and super-resolution imaging

Synthetic host-guest complexes are inherently dynamic as they employ weak and reversible noncovalent interactions for their recognition processes. We strategically exploited dynamic supramolecular recognition between fluorescently labeled guest molecules to complementary cucurbit[7]uril hosts to obtain stochastic switching between fluorescence ON- and OFF-states, enabling PAINT-based nanoscopic imaging in cells and tissues.