Pillararene Host–Guest Complexation Induced Chirality Amplification: A New Way to Detect Cryptochiral Compounds

The contradiction between the rising demands of optical chirality sensing and the failure in chiral detection of cryptochiral compounds encourages researchers to find new methods for chirality amplification. Inspired by planar chirality and the host–guest recognition of pillararenes, we establish a new concept for amplifying CD signals of cryptochiral molecules by pillararene host–guest complexation induced chirality amplification. The planar chirality of pillararenes is induced and stabilized in the presence of the chiral guest, which makes the cryptochiral molecule detectable by CD spectroscopy. Several chiral guests are selected in these experiments and the mechanism of chiral amplification is studied with a non-rotatable pillararene derivative and density functional theory calculations. We believe this work affords deeper understanding of chirality and provides a new perspective for chiral sensing.     

Chiroptical Sensing of Amino Acid Derivatives by Host–Guest Complexation with Cyclo[6]aramide

A hydrogen-bonded (H-bonded) amide macrocycle was found to serve as an effective component in the host–guest assembly for a supramolecular chirality transfer process. Circular dichroism (CD) spectroscopy studies showed that the near-planar macrocycle could produce a CD response when combined with three of the twelve L-α-amino acid esters (all cryptochiral molecules) tested as possible guests. The host–guest complexation between the macrocycle and cationic guests was explored using NMR, revealing the presence of a strong affinity involving the multi-point recognition of guests. This was further corroborated by density functional theory (DFT) calculations. The present work proposes a new strategy for amplifying the CD signals of cryptochiral molecules by means of H-bonded macrocycle-based host–guest association, and is expected to be useful in designing supramolecular chiroptical sensing materials.     

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.     

Selective sensing and transport in bionic nanochannel based on macrocyclic host-guest chemistry

Imitating the signal transduction and transmembrane transport co ntrolled by biological channels in the cell membra ne,artificial nanochannels with a similar capability of sensing and transport are constructed as bionic nanochannels.To accomplish selective sensing and transport of biological analyte(as "guest"),the bionic nanochannels are modified with the artificial receptor(as "host"),Based on selective recognition between host and guest,bionic nanochannels translate the stimulus of the guest to electrochemical signal as sensors,and further regulate the transmission of guest as transporters.Howeve r,throughout all kinds of guests,the selective sensing and transpo rt of ions and chiral molecules is a challenging problem.And throughout all hosts of ions and chiral molecules,the macrocyclic hosts with multisite of recognition show better selectivity,such as crown ethers,cyclodextrins,calixarenes,and pillararenes.In this article,we highlight recent advances in the macrocyclic host-based nanochannels for the selective sensing and transport of ionic and chiral guests,summarize the similarities and differences of different kinds of macrocyclic host-based nanochannels,and expect the research direction and application prospect.     

Chiral Recognition and Separation by Chirality‐Enriched Metal–Organic Frameworks

Endowed with chiral channels and pores, chiral metal–organic frameworks (MOFs) are highly useful; however, their synthesis remains a challenge given that most chiral building blocks are expensive. Although MOFs with induced chirality have been reported to avoid this shortcoming, no study providing evidence for the ee value of such MOFs has yet been reported. We herein describe the first study on the efficiency of chiral induction in MOFs using inexpensive achiral building blocks and fully recoverable chiral dopants to control the handedness of racemic MOFs. This method yielded chirality‐enriched MOFs with accessible pores. The ability of the materials to form host–guest complexes was probed with enantiomers of varying size and coordination and in solvents with varying polarity. Furthermore, mixed‐matrix membranes (MMMs) composed of chirality‐enriched MOF particles dispersed in a polymer matrix demonstrated a new route for chiral separation.     

Highly Efficient Chirality Transfer from Diamines Encapsulated within a Self‐Assembled Calixarene–Salen Host

A calix[4]arene host equipped with two bis‐[Zn(salphen)] complexes self‐assembles into a capsular complex in the presence of a chiral diamine guest with an unexpected 2:1 ratio between the host and the guest. Effective chirality transfer from the diamine to the calix–salen hybrid host is observed by circular dichroism (CD) spectroscopy, and a high stability constant K_2,1 of 1.59×10¹¹ M ^?2 for the assembled host–guest ensemble has been determined with a substantial cooperativity factor α of 6.4. Density functional calculations are used to investigate the origin of the stability of the host–guest system and the experimental CD spectrum compared with those calculated for both possible diastereoisomers showing that the M,M isomer is the one that is preferentially formed. The current system holds promise for the chirality determination of diamines, as evidenced by the investigated substrate scope and the linear relationship between the ee of the diamine and the amplitude of the observed Cotton effects.     

Emerging Cubic Chirality in γCD‐MOF for Fabricating Circularly Polarized Luminescent Crystalline Materials and the Size Effect

The chiral feature of γCD‐MOF, and especially the emergent cubic void, was not unveiled so far. Now, through the host–guest interaction between γCD‐MOF and achiral luminophores with different charges and sizes, the unique cubic chirality of the emerging void in γCD‐MOF as well as a size effect on CPL induction are revealed for the first time. Numerous achiral luminophores could be integrated into γCD‐MOF and emitted significantly boosted circularly polarized luminescence. While the small sized luminophores preferred to be loaded into the intrinsic void of γCD, large ones were selectively encapsulated into the cubic void. Interestingly, when the size of the guest luminophores was close to the cube size, it showed strong negative CPL. Otherwise, either positive or negative CPL was induced.     

Planar Chiral Organoboranes with Thermoresponsive Emission and Circularly Polarized Luminescence: Integration of Pillar[5]arenes with Boron Chemistry

Enantiopure molecules based on macrocyclic architecture are unique for applications in enantioselective host‐guest recognition, chiral sensing and asymmetric catalysis. Taking advantage of the chiral transfer from the intrinsically planar chirality of pillar[5]arenes, we herein present an efficient and straightforward approach to achieve early examples of highly luminescent chiral systems ( P5NN and P5BN ). The optical resolution of their enantiomers has been carried out via preparative chiral HPLC, which was ascribed to the molecular functionalization of pillar[5]arenes with π‐conjugated, sterically bulky triarylamine (Ar₃N) as an electron donor and triarylborane (Ar₃B) as an acceptor. This crucial design enabled investigations of the chiroptical properties, including circular dichroism (CD) and circularly polarized luminescence (CPL) in the solid state. The intramolecular charge transfer (ICT) nature in P5BN afforded an interesting thermochromic shift of the emission over a wide temperature range.     

Enantiomer discrimination using terahertz spectroscopy via formation of a diastereomer salt

This paper presents a new recognition method using terahertz (THz) spectroscopy. By forming a guest–host diastereomer with an optically active compound (host) as a chiral recogniser, the chirality of the enantiomer (guest) can be clearly discriminated according to its absorption spectra. In this proof-of-concept study, we examined a pair of host–guest (h–g) complexes made of (R)- or (S)-α-methylbenzylamine (guest) and cholic acid (host).     

Detection and amplification of chirality by helical polymers

A unique feature of synthetic helical polymers for the detection and amplification of chirality is briefly described in this article. In sharp contrast to host-guest and supramolecular systems that use small synthetic receptor molecules, chirality can be significantly amplified in a helical polymer, such as poly(phenylacetylene)s with functional pendants, which enable the detection of a tiny imbalance in biologically important chiral molecules through a noncovalent bonding interaction with high cooperativity. The rational design of polymeric receptors can be possible by using chromophoric helical polymers combined with functional groups as the pendants, which target particular chiral guest molecules for developing a highly efficient chirality-sensing system. The chirality sensing of other small molecular and supramolecular systems is also briefly described for comparison.     

Pillararene‐Based Assemblies: Design Principle,Preparation and Applications

In this review, we highlight recent advancements on pillararene‐based assemblies. The driving forces for the formation of the pillararene‐based assemblies are discussed first. The host–guest interactions are deemed as not only general strategy for constructing assemblies but also essential components for preventing the assemblies from the dissociation. Solvent effect is also important in the assembling process, since it could influence the host–guest interactions and provide solvophobic effect on pillararenes for the assembly. Then, several pillararene‐based assembly architectures are introduced, including pillararene‐based interlocked structures, such as (poly)pseudorotaxanes, (poly)rotaxanes, and daisy chains, classified by their topological structures and synthetic strategy. The morphologies of the supramolecular assemblies are divided into several types, for example, nanospheres, nanotubes and supramolecular polymers. Furthermore, the functions and potential applications are summarized accompanied with related assembly structures. The review not only provides fundamental findings, but also foresights future research directions in the research area of pillararene‐based assemblies.     

Emerging Cubic Chirality in γCD-MOF for Fabricating Circularly Polarized Luminescent Crystalline Materials and the Size Effect

The chiral feature of γCD-MOF, and especially the emergent cubic void, was not unveiled so far. Now, through the host–guest interaction between γCD-MOF and achiral luminophores with different charges and sizes, the unique cubic chirality of the emerging void in γCD-MOF as well as a size effect on CPL induction are revealed for the first time. Numerous achiral luminophores could be integrated into γCD-MOF and emitted significantly boosted circularly polarized luminescence. While the small sized luminophores preferred to be loaded into the intrinsic void of γCD, large ones were selectively encapsulated into the cubic void. Interestingly, when the size of the guest luminophores was close to the cube size, it showed strong negative CPL. Otherwise, either positive or negative CPL was induced.     

Planar-chiral pillar[5]arene: chiral switches induced by multiexternal stimulus of temperature, solvents, and addition of achiral guest molecule

We synthesized chiral-substituents modified pillar[5]arene for the first time. The chiral-substituents modified pillar[5]arene showed planar chirality and interconversion between (pS) and (pR) forms took place quickly. The planar chirality was switched by temperature, solvents, and addition of achiral guest. As the measurement temperature increased, the diastereomeric excess was decreased. The diastereomeric excesses were high in low-permittivity solvents, while a low diastereomeric excess was observed in high-permittivity solvents. Addition of achiral guest induced an increase of negative CD intensities.     

Quantified chirality, molecular similarity, and helical twisting power in lyotropic chiral nematic guest/host systems

Lyotropic liquid crystals can exhibit phase chirality. The mechanism behind the transfer of chirality between a chiral dopant and a liquid crystalline host phase is still under discussion. Our own recent results and proposals are the following. Lyotropic phase chirality can exist even at very low concentrations of chiral dopants, with less than 1 chiral dopant per 50 micelles. There is evidence for an intramicellar double twist which could be due to the induction of chiral conformations in the achiral surfactant chains. The chirality of arbitrary molecules can be quantified by means of the 'Hausdorff distance'. Increasing chirality of a dopant does not necessarily imply increasing helical twisting power, and molecular similarity between chiral guest and achiral host is essential for effective chirality transfer.     

Chirality induction in a cation-driven assembly using a crowned metalloporphyrin

15-Crown-5-appended metalloporphyrin causes a K(+)-driven self-organization to bind a bifunctional guest ditopically, thereby allowing the circular dichroism (CD) detection of chirality induced in the ensemble when chiral amines are employed as the guest; the chiroptical properties are discussed.     

Simultaneous enhancement of phosphorescence and chirality by host–guest recognition of molecular tweezers

A novel type of host–guest recognition systems have been developed on the basis of a Au(III) molecular tweezer receptor and chiral Pt(II) guests. The complementary host–guest motifs display high non-covalent binding affinity (K_a: ～10⁴ L/mol) due to the participation of two-fold intermolecular π–π stacking interactions. Both phosphorescence and chirality signals of the Pt(II) guests strengthen in the resulting host–guest complexes, because of the cooperative rigidifying and shielding effects rendered by the tweezer receptor. Their intensities can be reversibly switched toward pH changes, by taking advantage of the electronic repulsion effect between the protonated form of tweezer receptor and the positive-charged guests in acidic environments. Overall, the current study demonstrates the feasibility to enhance and modulate phosphorescence and chirality signals simultaneously via molecular tweezer-based host–guest recognition.     

Selective Encapsulation and Chiral Induction of C60 and C70 Fullerenes by Axially Chiral Porous Aromatic Cages

Chiral induction has been an important topic in chemistry, not only for its relevance in understanding the mysterious phenomenon of spontaneous symmetry breaking in nature but also due to its critical implications in medicine and the chiral industry. The induced chirality of fullerenes by host–guest interactions has been rarely reported, mainly attributed to their chiral resistance from high symmetry and challenges in their accessibility. Herein, we report two new pairs of chiral porous aromatic cages (PAC), R- PAC-2 , S- PAC-2 (with Br substituents) and R- PAC-3 , S- PAC-3 (with CH₃ substituents) enantiomers. PAC-2 , rather than PAC-3 , achieves fullerene encapsulation and selective binding of C₇₀ over C₆₀ in fullerene carbon soot. More significantly, the occurrence of chiral induction between R- PAC-2 , S- PAC-2 and fullerenes is confirmed by single-crystal X-ray diffraction and the intense CD signal within the absorption region of fullerenes. DFT calculations reveal the contribution of electrostatic effects originating from face-to-face arene-fullerene interactions dominate C₇₀ selectivity and elucidate the substituent effect on fullerene encapsulation. The disturbance from the differential interactions between fullerene and surrounding chiral cages on the intrinsic highly symmetric electronic structure of fullerene could be the primary reason accounting for the induced chirality of fullerene.     

Supramolecular chirogenesis in zinc porphyrins: interaction with bidentate ligands,formation of tweezer structures,and the origin of enhanced optical activity

The complexation behavior, binding properties, and spectral parameters of supramolecular chirality induction in the achiral host molecule, syn (face-to-face conformation) ethane-bridged bis(zinc porphyrin), upon interaction with chiral bidentate guests (diamines and amino alcohols) have been studied by means of UV-vis, CD, fluorescence, (1)H NMR, and ESI MS techniques. It was found that the guest structure plays a decisive role in the chirogenesis pathway. The majority of bidentate ligands (except those geometrically unsuitable) exhibit two major equilibria steps: the first guest ligation leading to formation of the 1:1 host-guest tweezer structure (K(1)) and the second guest molecule ligation (K(2)) forming the anti bis-ligated species (1:2). The second ligation is much weaker (K(1) > K(2)) due to the optimal geometry and stability of the 1:1 tweezer complex. The enhanced conformational stability of the tweezer complex ensures an efficient chirality transfer from the chiral guest to the achiral host, consequently inducing a remarkably high optical activity in the bis-porphyrin.     

Chirality transfer from chiral solvents and its memory in an azobenzene derivative exhibiting photo-switchable racemization

The transfer and dynamic fixation of chirality in cyclic azobenzenes using R-(+)-1-phenylethylalcohol (R-PEA) and S-(-)-1-phenylethylalcohol (S-PEA) as solvents or additives are investigated. The cyclic azobenzenes used in this study carry a 1,5-dioxynaphthalene moiety as rotating unit, connected to the photoisomerizing (E-Z) azobenzene unit with spacers of varying lengths. With suitable lengths of the spacers the molecules exhibit stable enantiomers originated from the element of planar chirality in the E form due to the stopped rotation of the rotor, while in the Z form the allowed rotation results in racemization. The CD spectra of racemic compounds in the E form in chiral solvents were inert or almost negligible before irradiation, while 366 nm irradiation causing E-Z photoisomerization resulted in induction of clear CD bands. The thermal or photochemical reverse Z-E isomerization causes a change in the CD spectra to new ones which are reasonably matching with the spectra of the pure enantiomers recorded in non-chiral solvents. The obtained new CD spectra are maintained even in a racemic solvent system attained by the dilution with an equal amount of chiral solvent of opposite stereostructure. These results indicate that the chirality is transferred from the chiral solvents or additives to the racemizing Z form of cyclic azobenzene and it is fixed in the non-racemizing E form. The molecule without racemization in both E and Z forms did not show any significant induced CD bands irrespective of E-Z isomerizations. The molecule showing racemization in E and Z forms just shows the non-fixed induced CD. The property of photo-switchable racemization is necessary for the effective transfer and temporal fixation of the chirality in this type of chirality sensors.     

Calixarene/pillararene-based supramolecular selective binding and molecular assembly

This review mainly summarized the recent researches on the supramolecular selective binding and molecular assembly based on calixarene and pillararene. Several representative examples were provided to expound the progress in the area of recognition and sensing, multi-functional assembly and crosslinked multi-dimensional materials.