Pillar[5]arene as a Co-Factor in Templating Rotaxane Formation

Preparation of Pillar[5]arene-Based [2]Rotaxanes by a Stopper-Exchange Strategy

A pillar[5]arene-containing rotaxane building block bearing exchangeable stoppers has been prepared in multigram scale quantities with high yields from the reaction of 2,4-dinitrophenol (DNP) with the inclusion complex resulting from the association of dodecanedioyl chloride with 1,4-diethoxypillar[5]arene. Stopper exchange reactions have been achieved by treatment of the resulting DNP diester with various amines through an addition-elimination mechanism preventing the unthreading of the axle component during the reaction and thus preserving the [2]rotaxane structures. The resulting diamide [2]rotaxane derivatives have thus been obtained in good to excellent yields. Importantly, [2]rotaxanes difficult or impossible to prepare by direct introduction of the two stoppers in a single synthetic step are now easily available.

Supramolecular catalyst functions in catalytic amount: cucurbit[8]uril accelerates the photodimerization of Brooker's merocyanine

Supramolecular catalysis aims to modulate chemical reactions on both selectivity and rate by taking advantage of supramolecular chemistry. However, due to the effect of product inhibition, supramolecular catalysts are usually added in stoichiometric amounts. Herein, we report a supramolecular catalysis system in which 1% of the supramolecular catalyst, cucurbit[8]uril, is able to significantly accelerate the photodimerization of Brooker's merocyanine. This catalytic process is realized in a cyclic manner because the photodimerized product can be spontaneously replaced by monomeric reactants via competitive host-guest complexation. Thus, a catalytic amount of cucurbit[8]uril is sufficient to accomplish photodimerization within 10 min. This line of research will enrich the field of supramolecular catalysis and allow the development of more efficient catalytic systems.

Improved Solubility and Bioactivity of Camptothecin Family Antitumor Drugs with Supramolecular Encapsulation by Water-Soluble Pillar[6]arene

Water-soluble pillar[6]arene (WP6) was used to solubilize camptothecin family antitumor drugs. In the presence of WP6, the solubility of camptothecin (CPT) and 10-hydroxycamptothecin (HCPT) was enhanced by 380 and 40 times, respectively. The solubility enhancement is proved to be the result of the host-guest encapsulation by WP6. WP6 has a low cytotoxicity against normal MC 3T3-E1 cells, whereas the bioactivity of CPT and HCPT is substantially improved as a result of the solubility enhancement.

One-pot synthesis of hetero[6]rotaxane bearing three different kinds of macrocycle through a self-sorting process

A hetero[6]rotaxane bearing three different kinds of macrocycle is designed and successfully synthesized through a one-pot “click” reaction by employing a facile and efficient integrative self-sorting principle.

In this article, a six-component self-sorting process that involves three types of crown ether macrocycle and three types of cation guest molecule was carefully and thoroughly investigated. The six components include three kinds of crown ether, namely bis(p-phenylene-34-crown-10) (BPP34C10), dibenzo-24-crown-8 (DB24C8) and benzo-21-crown-7 (B21C7), and their corresponding cation guest molecules, namely a 4,4′-bipyridine dication (BPY²⁺) and dibenzylammonium (DBA) and benzylalkylammonium (BAA) ions, respectively. Based on this well-established highly selective six-component self-sorting process, a hetero[6]rotaxane bearing three different kinds of crown ether macrocycle was designed and successfully synthesized through a facile and efficient one-pot “click” stoppering strategy. Such work is proposed to be a significant advance in the construction of mechanically interlocked molecules with high structural complexity, as well as a good supplement in the areas of multi-component self-sorting and noncovalent self-assembly.

2,2′-Biphen[n]arenes (n = 4–8): one-step, high-yield synthesis, and host–guest properties

A new family of supramolecular macrocycles, 2,2′-biphen[n]arenes (n = 4–8), has been synthesized through a single-step reaction with a yield of 51%.

Water-soluble pillar[5]arene induced the morphology transformation of self-assembled nanostructures and had further application in paraquat detection

Hex-4ClPBI can self-assemble into nanotubes in water, and the tubular structures can be transformed into nanoribbons and further vesicles by addition of H⁺ and further WP5.

Pillar[5]arene-based [1]rotaxane: high-yield synthesis, characterization and application in Knoevenagel reaction

Quantitative synthesis of a pillararene-based [1]rotaxane has been achieved via a “self-threading-stoppering” approach, followed by its first application in organic catalysis.

A pillar[5]arene based gel from a low-molecular-weight gelator for sustained dye release in water

A soft gel based on pillar[5]arene was successfully prepared using a carbazone reaction.

Photoresponsive AA/BB supramolecular polymers comprising stiff-stilbene based guests and bispillar[5]arenes

We report a novel photoresponsive AA/BB supramolecular polymer comprising stiff-stilbene bridged guests and disulfide-bridged bispillar[5]arenes.

Macrocyclic shape-persistency of cyclo[6]aramide results in enhanced multipoint recognition for the highly efficient template-directed synthesis of rotaxanes

Examples of using two-dimensional shape-persistent macrocycles, i.e. those having noncollapsible and geometrically well-defined skeletons, for constructing mechanically interlocked molecules are scarce, which contrasts the many applications of these macrocycles in molecular recognition and functional self-assembly. Herein, we report the crucial role played by macrocyclic shape-persistency in enhancing multipoint recognition for the highly efficient template-directed synthesis of rotaxanes. Cyclo[6]aramides, with a near-planar conformation, are found to act as powerful hosts that bind bipyridinium salts with high affinities. This unique recognition module, composed of two macrocyclic molecules with one bipyridinium ion thread through the cavity, is observed both in the solid state and in solution, with unusually high binding constants ranging from ∼10¹³ M^-2 to ∼10¹⁵ M^-2 in acetone. The high efficacy of this recognition motif is embodied by the formation of compact [3]rotaxanes in excellent yields based on either a "click-capping" (91%) or "facile one-pot" (85%) approach, underscoring the great advantage of using H-bonded aromatic amide macrocycles for the highly efficient template-directed synthesis of mechanically interlocked structures. Furthermore, three cyclo[6]aramides bearing different peripheral chains 1-3 demonstrate high specificity in the synthesis of a [3]rotaxane from 1 and 2, and a [2]rotaxane from 3via a "facile one-pot" approach, in each case as the only isolated product. Analysis of the crystal structure of the [3]rotaxane reveals a highly compact binding mode that would be difficult to access using other macrocycles with a flexible backbone. Leveraging this unique recognition motif, resulting from the shape-persistency of these oligoamide macrocycles, in the template-directed synthesis of compact rotaxanes may open up new opportunities for the development of higher order interlocked molecules and artificial molecular machines.

A Kinetic Self-Sorting Approach to Heterocircuit [3]Rotaxanes

In this proof-of-concept study, an active-template coupling is used to demonstrate a novel kinetic self-sorting process. This process iteratively increases the yield of the target heterocircuit [3]rotaxane product at the expense of other threaded species.

Twisted Cucurbit[n]urils

Two new twisted cucurbiturils, cucurbit[13]uril (tQ[13]) and cucurbit[15]uril (tQ[15]), have been synthesized and separated, and their structures have been confirmed by NMR spectroscopy and MALDI-TOF mass spectrometry together with the X-ray structures of two new complexes, {Dy(H2O)4Cd(H2O)4tQ[13]}·2.5[CdCl4]·65H2O and {Cd0.5(H2O)2tQ[15]}·[CdCl4]·47H2O. tQ[15] is the largest cucurbit[n]uril (Q[n]) in the Q[n] family reported to date. The X-ray diffraction studies of both complexes indicated that these large tQ[n]s effectively exhibit two different cavities-a central cavity and two side cavities. Preliminary host-guest behavior by each of the new systems was investigated by NMR studies.

Pillar-Shaped Macrocyclic Hosts Pillar[n]arenes: New Key Players for Supramolecular Chemistry

In 2008, we reported a new class of pillar-shaped macrocyclic hosts, known as "pillar[n]arenes". Today, pillar[n]arenes are recognized as key players in supramolecular chemistry because of their facile synthesis, unique pillar shape, versatile functionality, interesting host-guest properties, and original supramolecular assembly characteristics, which have resulted in numerous electrochemical and biomedical material applications. In this Review, we have provided historical background to macrocyclic chemistry, followed by a detailed discussion of the fundamental properties of pillar[n]arenes, including their synthesis, structure, and host-guest properties. Furthermore, we have discussed the applications of pillar[n]arenes to materials science, as well as their applications in supramolecular chemistry, in terms of their fundamental properties. Finally, we have described the future perspectives of pillar[n]arene chemistry. We hope that this Review will provide a useful reference for researchers working in the field and inspire discoveries concerning pillar[n]arene chemistry.

Cooperative capture synthesis: yet another playground for copper-free click chemistry

Click chemistry describes a family of modular, efficient, versatile and reliable reactions which have acquired a pivotal role as one of the most useful synthetic tools with a potentially broad range of applications. While copper(i)-catalysed alkyne-azide cycloaddition is the most widely adopted click reaction in the family, the fact that it is cytotoxic restricts its practice in certain situations, e.g., bioconjugation. Consequently, researchers have been exploring the development of copper-free click reactions, the most popular example so far being strain-promoted alkyne-azide cycloadditions. An early example of copper-free click reactions that is rarely mentioned in the literature is the cucurbit[6]uril (CB6) catalysed alkyne-azide cycloaddition (CB-AAC). Despite the unique ability of CB-AAC to generate mechanically interlocked molecules (MIMs) - in particular, rotaxanes - its slow reaction rate and narrow substrate acceptance limit its scope. In this Tutorial Review, we describe our efforts of late in developing the fundamental principles and practical applications of a new copper-free click reaction - namely, cooperative capture synthesis, whereby introducing a cyclodextrin (CD) as an accelerator in CB-AAC, hydrogen bonding networks are formed between the rims of CD and CB6 in a manner that is positively cooperative, giving rise to a high level of pre-organisation during efficient and quick rotaxane formation. For example, [4]rotaxanes can be prepared nearly quantitatively within a minute in water. Furthermore, we have demonstrated that CB-AAC can accommodate a wider substrate tolerance by introducing pillararenes as promoters. To date, we have put cooperative capture synthesis into practice by (i) preparing polyrotaxanes containing up to 200 rings in nearly quantitative yields, (ii) trapping conformational isomers of polymacrocycles as rings in rotaxanes, (iii) demonstrating solid-state fluorescence and Förster resonance energy transfer (FRET) processes by fixing the fluorophores in a rotaxane and (iv) establishing the principle of supramolecular encryption in the preparation of dynamically and reversibly tunable fluorescent security inks.

Efficient complexation between pillar[5]arenes and neutral guests: from host–guest chemistry to functional materials

This feature article covers the molecular recognition of pillar[5]arenes and neutral guests, and its application in making supramolecular structures, polymers and functional materials.

Pillar[5]arene derivatives containing two dinitrophenyl rings: syntheses, conformations and the tubular self assembly in the solid state

Pillar[5]arene derivatives with two dinitrophenyl rings were synthesized in 90% and 99% yields, respectively. The di(2,4-dinitro-5-fluoro-phenyl)-modified pillar[5]arene self assembled into column in the solid state.

Biologically Active Heteroglycoclusters Constructed on a Pillar[5]arene-Containing [2]Rotaxane Scaffold

A synthetic approach combining recent concepts for the preparation of multifunctional nanomolecules (click chemistry on multifunctional scaffolds) with supramolecular chemistry (self‐assembly to prepare rotaxanes) gave easy access to a large variety of sophisticated [2]rotaxane heteroglycoclusters. Specifically, compounds combining galactose and fucose have been prepared to target the two bacterial lectins (LecA and LecB) from the opportunistic pathogen Pseudomonas aeruginosa.

Formation of a hetero[3]rotaxane by a dynamic component-swapping strategy

Acid-catalysed scrambling of the mechanically interlocked components between two different homo[3]rotaxanes, constituted of dumbbells containing two secondary dialkylammonium ion recognition sites encircled by two [24]crown-8 rings, each containing a couple of imine bonds, affords a statistical mixture of a hetero[3]rotaxane along with the two homo[3]rotaxanes, indicating that neither selectivity nor cooperativity is operating during the assembly process.

Enantiopure pillar[5]arene active domains within a homochiral metal-organic framework

Enantiopure struts containing pillar[5]arenes incorporating planar chirality have been linked together with Zn4O clusters in order to create metal-organic frameworks that include homochiral active domains and so have the potential to act as a solid support in chiral chromatography.

Well-defined nano-sunflowers formed by self-assembly of a rod-coil amphiphile in water and their morphology transformation based on a water-soluble pillar[5]arene

Well-defined nano-sunflowers were constructed by self-assembling a rod-coil amphiphile in water, and their morphologies transformed by addition of a water soluble pillar[5]arene and Ag2O were also investigated.

Two Stepwise Synthetic Routes toward a Hetero[4]rotaxane

Heterorotaxanes have been emerging as an important class of mechanically interlocked molecules and have attracted much attention in recent years. Driven by the distinguishable host-guest interactions between crown ether macrocycles and ammonium with different sizes, a novel hetero[4]rotaxane was successfully prepared by employing the combination of copper-catalyzed "click" reaction and P(n-Bu)3-catalyzed esterification reaction as stoppering reactions. The hetero[4]rotaxane contains an interlocked species in which a dibenzo[24]crown-8 ring threaded by a dibenzylammonium-containing component with two benzo[21]crown-7 macrocycles at both ends to act as stoppers, and each of the two benzo[21]crown-7 rings is also threaded with a benzylalkylammonium unit to form the second interlocked species. The hetero[4]rotaxane was prepared through two different stepwise synthetic routes, and the complicated chemical structure of the hetero[4]rotaxane was well-characterized by (1)H NMR spectroscopy and high-resolution electrospray ionization (HR-ESI) mass spectrometry. The investigation shows that the construction of complicated topological heterorotaxane can be achieved via distinct approaches with high efficiencies, which may provide a foundation for the construction of more sophisticated heterorotaxane systems or functional supermolecules.

Tunable solid-state fluorescent materials for supramolecular encryption

Tunable solid-state fluorescent materials are ideal for applications in security printing technologies. A document possesses a high level of security if its encrypted information can be authenticated without being decoded, while also being resistant to counterfeiting. Herein, we describe a heterorotaxane with tunable solid-state fluorescent emissions enabled through reversible manipulation of its aggregation by supramolecular encapsulation. The dynamic nature of this fluorescent material is based on a complex set of equilibria, whose fluorescence output depends non-linearly on the chemical inputs and the composition of the paper. By applying this system in fluorescent security inks, the information encoded in polychromic images can be protected in such a way that it is close to impossible to reverse engineer, as well as being easy to verify. This system constitutes a unique application of responsive complex equilibria in the form of a cryptographic algorithm that protects valuable information printed using tunable solid-state fluorescent materials.

Solid-state fluorescent materials show promise for potential applications in security and anti-counterfeiting technologies. Here, the authors report a heterorotaxane which has found application in security inks with highly tunable solid-state fluorescence through supramolecular encapsulation.

A [2]rota[2]catenane, constructed from a pillar[5]arene-crown ether fused double-cavity macrocycle: synthesis and structural characterization

A [2]rota[2]catenane, based on a pillar[5]arene-crown ether fused bicycle, was constructed and its structure was determined crystallographically.

Integrative self-sorting: a versatile strategy for the construction of complex supramolecular architecture

Integrative self-sorting expands the concepts of self-assembly and allows the programming of supramolecular architecture on a higher level of complexity.

Formation of ring-in-ring complexes between crown ethers and rigid TVBox8+

An octacationic tetraviologen-based cyclophane—so called TVBox⁸⁺—can form a ring-in-ring complex with bis-1,5-dinaphtho[50]crown-14, which represents a key intermediate for constructing molecular Borromean rings in a stepwise manner.

A novel dynamic pseudo[1]rotaxane based on a mono-biotin-functionalized pillar[5]arene

A stable pillar[5]arene-based pseudo[1]rotaxane P1′ was synthesized by the click reaction, which exhibited a dynamic slow disassembly process upon adding a strong-polar solvent or competitive guest. Moreover, this dynamic behavior might be used as a switch to turn on or off the bioactivity of the biotin moiety in aqueous solution.

A novel pentiptycene bis(crown ether)-based [2](2)rotaxane whose two DB24C8 rings act as flapping wings of a butterfly

A novel [2](2)rotaxane based on pentiptycene-derived bis(crown ether) can be efficiently synthesized via a "click chemistry" method and the subsequent N-methylation. Due to the different affinities of DB24C8 with the ammonium and triazolium stations, the wing-flapping movement of the DB24C8 "wings" in the [2](2)rotaxane can be easily achieved by acid/base stimulus.

Pillar[5]arene-based nonionic polyrotaxanes and a topological gel prepared from cyclic host liquids

We synthesize nonionic polymer-based polyrotaxanes by click reactions in pillar[5]arene-based cyclic host liquids and a topological gel by a metathesis reaction.

A pillar[5]arene and crown ether fused bicyclic host: synthesis, guest discrimination and simultaneous binding of two guests with different shapes, sizes and electronic constitutions

A pillar[5]arene- and crown ether-based bicyclic host molecule binds two guest species selectively by its two independent cyclic host subunits.

Pillar[5]- and pillar[6]arene-based supramolecular assemblies built by using their cavity-size-dependent host–guest interactions

We discuss various pillar[5]- and pillar[6]arene-based supramolecular assemblies built using their host–guest interactions.