Pre-regulation of the planar chirality of pillar[5]arenes for preparing discrete chiral nanotubes

Histidine-modified pillar[5]arene-functionalized mesoporous silica materials for highly selective enantioseparation

Chiral enantiomers, especially the enantiomers of chiral drugs often exhibit different pharmacological activity, metabolism and toxicity, thus it is of great research significance to scientifically and reasonably develop single chiral drugs with low toxicity and high efficiency. Among them, high performance liquid chromatographic techniques based on chiral stationary phases (CSPs) has become one of the most attractive methods used to evaluate the enantiomeric purity of single-enantiomers compound of pharmacological relevance. In this work, pillar[5]arene functionalized with L- and D-histidine, respectively, were modified on the surface of mesoporous silica as novel chiral stationary phases called L/DHis-BP5-Sil. Notably, L/D-histidine had the characteristics of low steric hindrance and easy derivatization. Although the π-π interaction of imidazole group was weaker than that of benzene ring, the benzene ring bonding imidazole-conjugated ring in the structure produced better enantioseparation effect. The results showed that L/DHis-BP5-Sil can separate a variety of complex structural enantiomers with excellent reproducibility, thermal stability and separation performance. Hence, the unique advantage of the highly selective separation of L/DHis-BP5-Sil provides new insights into the enantioseparation field.

Applications of Supramolecular Polymers Generated from Pillar[n]arene-Based Molecules

Supramolecular chemistry enables the manipulation of functional components on a molecular scale, facilitating a "bottom-up" approach to govern the sizes and structures of supramolecular materials. Using dynamic non-covalent interactions, supramolecular polymers can create materials with reversible and degradable characteristics and the abilities to self-heal and respond to external stimuli. Pillar[n]arene represents a novel class of macrocyclic hosts, emerging after cyclodextrins, crown ethers, calixarenes, and cucurbiturils. Its significance lies in its distinctive structure, comparing an electron-rich cavity and two finely adjustable rims, which has sparked considerable interest. Furthermore, the straightforward synthesis, uncomplicated functionalization, and remarkable properties of pillar[n]arene based on supramolecular interactions make it an excellent candidate for material construction, particularly in generating interpenetrating supramolecular polymers. Polymers resulting from supramolecular interactions involving pillar[n]arene find potential in various applications, including fluorescence sensors, substance adsorption and separation, catalysis, light-harvesting systems, artificial nanochannels, and drug delivery. In this context, we provide an overview of these recent frontier research fields in the use of pillar[n]arene-based supramolecular polymers, which serves as a source of inspiration for the creation of innovative functional polymer materials derived from pillar[n]arene derivatives.

Alignment and Dynamic Inversion of Planar Chirality in Pillar[n]arenes

Pillar[n]arenes are symmetrical macrocyclic compounds composed of benzene panels with para-methylene linkages. Each panel usually exhibits planar chirality and prefers chirality-aligned states. Because of this feature, pillar[n]arenes are attractive scaffolds for chiroptical materials that are easy to prepare and optically resolve and show intense circular dichroism (CD) signals. In addition, rotation of the panels endows the chirality of pillar[n]arenes with a dynamic nature. The chirality in tubular oligomers and supramolecular assemblies sometimes show time- and procedure-dependent alignment phenomena. Furthermore, the CD signals of some pillar[n]arenes respond to the addition of chiral guests when their dynamic chirality is coupled with host-guest properties. By using diastereomeric pillar[n]arenes with additional chiral structures, the response can also be caused by achiral guests and changes of the environment, providing molecular sensors.

Cation controlled rotation in anionic pillar[5]arenes and its application for fluorescence switch

Controlling molecular motion is one of hot topics in the field of chemistry. Molecular rotors have wide applications in building nanomachines and functional materials, due to their controllable rotations. Hence, the development of novel rotor systems, controlled by external stimuli, is desirable. Pillar[n]arenes, a class of macrocycles, have a unique planar chirality, in which two stable conformational isomers pR and pS would interconvert by oxygen-through-the-annulus rotations of their hydroquinone rings. We observe the differential kinetic traits of planar chirality transformation in sodium carboxylate pillar[5]arene (WP5-Na) and ammonium carboxylate pillar[5]arene (WP5-NH₄), which inspire us to construct a promising rotary platform in anionic pillar[5]arenes (WP5) skeletons. Herein, we demonstrate the non-negligible effect of counter cations on rotational barriers of hydroquinone rings in WP5, which enables a cation grease/brake rotor system. Applications of this tunable rotor system as fluorescence switch and anti-counterfeiting ink are further explored.

Guest-Induced Planar-Chiral Pillar[5]arene Surface for Selectively Adsorbing Protein Based on Host-Guest Chemistry

In living systems, the adsorption of a protein on biointerfaces is a universal phenomenon, such as the specific binding of an antibody and antigen, which plays an important role in body growth and life maintenance. The exploration of a protein-selective adsorption on the biointerface is of great significance for understanding the life process and treatment in vitro. Herein, on the basis of biomimetic strategies, we fabricated a planar-chiral NH₂-pillar[5]arene modified silicon surface (pR-/pS-NP5 surfaces) for a highly enantioselective adsorption of protein by taking advantage of the guest-induced planar chirality of pillar[5]arenes. Results from practical experiments and theoretical calculations show that the pR-NP5 surface possesses a high adsorption capacity and chiral selectivity for bovine serum albumin (BSA). Moreover, it was identified that the guest-induced chiral effect the generation and amplification of planar chirality, which was much beneficial for enhancing the interaction between planar-chiral pillar[5]arene host and BSA. The binding capacity of pR-NP5 and BSA is stronger than that of pS-NP5, thus promoting the chiral selective adsorption of BSA. This work affords a deeper understanding of the chiral influence of protein adsorption on biointerfaces and meanwhile provides a new perspective for chiral-sensing applications.

Real-time chirality transfer monitoring from statistically random to discrete homochiral nanotubes

Real time monitoring of chirality transfer processes is necessary to better understand their kinetic properties. Herein, we monitor an ideal chirality transfer process from a statistically random distribution to a diastereomerically pure assembly in real time. The chirality transfer is based on discrete trimeric tubular assemblies of planar chiral pillar[5]arenes, achieving the construction of diastereomerically pure trimers of pillar[5]arenes through synergistic effect of ion pairing between a racemic rim-differentiated pillar[5]arene pentaacid bearing five benzoic acids on one rim and five alkyl chains on the other, and an optically resolved pillar[5]arene decaamine bearing ten amines. When the decaamine is mixed with the pentaacid, the decaamine is sandwiched by two pentaacids through ten ion pairs, initially producing a statistically random mixture of a homochiral trimer and two heterochiral trimers. The heterochiral trimers gradually dissociate and reassemble into the homochiral trimers after unit flipping of the pentaacid, leading to chirality transfer from the decaamine and producing diastereomerically pure trimers.

"Rim-Differentiated" Pillar[6]arenes

A "rim-differentiated" pillar[6]arene (RD-P[6]) was obtained successfully, with the assistance of a dimeric silver trifluoroacetate template, among eight different constitutional isomers in a direct and regioselective manner. The solid-state conformation of this macrocycle could switch from the 1,3,5-alternate to a truly rim-differentiated one upon guest inclusion. This highly symmetric RD-P[6] not only hosts metal-containing molecules inside its cavity, but also can form a pillar[6]arene-C₆₀ adduct through co-crystallization on account of donor-acceptor interactions. The development of synthetic strategies to desymmetrize pillararenes offers new opportunities for engineering complex molecular architectures and organic electronic materials.

Dynamic combinatorial libraries of a dimercapto-pillar[5]arene

We report the synthesis and dynamic covalent chemistry (DCvC) of an A1/A2-dimercapto-functionalized pillar[5]arene (Di-SH-P5). The introduction of thiol moieties into this macrocyclic host makes it an effective building block for making a dynamic combinatorial library (DCL), giving rise to a set of cyclic trimers with intriguing host-guest properties as the dominant constituents.

Chiral pillar[5]arene-functionalized silica microspheres: synthesis, characterization and enantiomer separation

Chiral pillar[5]arene-functionalized silica microspheres were prepared and characterized for the first time, which can be used as a new kind of chiral stationary phases for effective enantioseparation under reversed-phase and...

CPL on/off control of an assembled system by water soluble macrocyclic chiral sources with planar chirality

Herein, we report the synthesis and planar chiral properties of a pair of water-soluble cationic pillar[5]arenes with stereogenic carbons. Interestingly, although units of the molecules were rotatable, only one planar chiral diastereomer existed in water in both cases. As a new type of chiral source, these molecules transmitted chiral information from the planar chiral cavities to the assembly of a water-soluble extended π-conjugated compound, affording circularly polarized luminescence (CPL). The chirality transfer process and resulting CPL were extremely sensitive to the feed ratio of the chiral pillar[5]arenes owing to the combined action of their planar chirality, bulkiness, and strong binding properties. When a limited amount of chiral source was added, further assembly of the extended π-conjugated compound into helical fibers with CPL was triggered. Unexpectedly, larger amounts of chiral source destroyed the helical fiber assemblies, resulting in elimination of the chirality and CPL properties from the assembled structures.

Readily obtained pillar[5]arenes with pure planar chirality enabled CPL on/off control of an assembled system by varying the feed ratio.

Planar Chirality: A mine for catalysis and structure discovery

Planar chirality is one of the most fascinating expressions of chirality, exploited by Nature to lock three-dimensional chiral conformations and, more recently, by chemists to create new chiral reagents, catalysts and functional organic materials. Nevertheless, the shortage of protocols able to induce and secure asymmetry during the generation of these unique chiral entities has dissuaded chemists to exploit their structural properties. This Minireview intends to illustrate the limited but remarkable catalytic methodologies reported for the production of planar chirality in strained molecules and serve as source of inspiration for the development of new unconventional protocols that are expected to come in the near future.

Reversible "On/Off" Chiral Amplification of Pillar[5]arene Assemblies by Dual External Stimuli

We report dual-stimuli, thermo- and photostimuli, responsive chiral assemblies, of planar-chiral pillar[5]arenes with azobenzene groups on their rims. The azobenzene-substituted planar-chiral pillar[5]arenes were synthesized by copper(I)-catalyzed alkyne-azide cycloaddition "click" reaction of azide-substituted planar-chiral pillar[5]arenes containing S or R stereogenic carbon atoms with an alkyne-substituted azobenzene. These decaazides with stereogenic carbons could act as starting points for a large library of planar-chiral pillar[5]arenes. Homeotropic alignment of azobenzenes, caused by the mesogenic property of the azobenzene groups, was induced by annealing a film of the azobenzene-substituted planar-chiral pillar[5]arenes. The alignment resulted in chiral propagation from the planar-chiral pillar[5]arene cores to the azobenzene area and caused significant chiral amplification consequently. These aligned chiral assemblies were collapsed by trans to cis photoisomerization of the azobenzene groups, resulting in chiral amplification off, and reconstructed by cis to trans thermo-isomerization, again turning on the chiral amplification.