Thermal Chiroptical Switch Based on an Ultrahigh Temperature-Initiated Macrocycle Gel Platform

Responsive chiral optical materials have gained considerable interests from the fields of sensing, display, and optical devices. Materials that are capable of changing chiral optics under harsh conditions such as strong basic/acidic or ultrahigh temperature provides thoughts for the design of materials working at special environments, which however, are still underdeveloped. Here, a proof-of-concept design of organogel is reported that acts as matrices for thermal chiroptical switch with critical working temperature above 100 °C. The reversible solution-to-gel transition of the specific β-cyclodextrin/dimethyl formide/LiCl system is initialized at about 130 °C, when the luminophores with aggregation-induced-emission property shall be lighted up with transferred chirality from inherent chiral β-cyclodextrin. It allows for the controlled emergence of circularly polarized luminescence. This delicate design enables successful fabrication of ultrahigh temperature thermal chiroptical switch.

Crystal structure and supra-molecular features of a host-guest inclusion complex based on A1/A2-hetero-difunctionalized pillar[5]arene

A host-guest supra-molecular inclusion complex was obtained from the co-crystallization of A1/A2-bromo-but-oxy-hy-droxy difunctionalized pillar[5]arene (PilButBrOH) with adipo-nitrile (ADN), C47H53.18Br0.82O10·C6H8N2. The adipo-nitrile guest is stabilized within the electron-rich cavity of the pillar[5]arene host via multiple C-H⋯O and C-H⋯π inter-actions. Both functional groups on the macrocyclic rim are engaged in supra-molecular inter-actions with an adjacent inclusion complex via hydrogen-bonding (O-H⋯N or C-H⋯Br) inter-actions, resulting in the formation of a supra-molecular dimer in the crystal structure.

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.

Chiroptical regulation of macrocyclic arenes with flipping-induced inversion of planar chirality

Studies on various macrocyclic arenes have received increasing attention due to their straightforward syntheses, convenient derivatization, and unique complexation properties. Represented by pillar[n]arenes, several distinctive macrocyclic arenes have recently emerged with the following characteristics: they possess a pair of enantiomeric planar chiral conformations, and interconversion between these enantiomeric conformations can be achieved through the flipping of ring units. Complexation of a chiral guest with these macrocyclic arenes will lead to a shift of the equilibrium between the Rp and Sp conformers, leading to intriguing possibilities for chiral induction and sensing. By the introduction of bulky substituents on the rims, employing rotaxanation or pseudocatenation, planar chirality could be locked, enabling the enantiomeric separation of the chiral structures. The induced or separated chiral conformers/compounds exhibit significant chiroptical properties. These macrocyclic arenes, with flipping-induced inversion of planar chirality, demonstrated intriguing chiral induction dynamics and kinetics. In this featured review, we systematically summarize the progress in chiroptical induction/regulation of these macrocyclic arenes, particularly in the fields of chiral sensing, molecular machines, molecular recognition, and assembly.

Noncovalently bound and mechanically interlocked systems using pillar[n]arenes

Pillar[n]arenes are pillar-shaped macrocyclic compounds owing to the methylene bridges linking the para-positions of the units. Owing to their unique pillar-shaped structures, these compounds exhibit various excellent properties compared with other cyclic host molecules, such as versatile functionality using various organic synthesis techniques, substituent-dependent solubility, cavity-size-dependent host-guest properties in organic media, and unit rotation along with planar chiral inversion. These advantages have enabled the high-yield synthesis and rational design of pillar[n]arene-based mechanically interlocked molecules (MIMs). In particular, new types of pillar[n]arene-based MIMs that can dynamically convert between interlocked and unlocked states through unit rotation have been produced. The highly symmetrical pillar-shaped structures of pillar[n]arenes result in simple NMR spectra, which are useful for studying the motion of pillar[n]arene wheels in MIMs and creating sophisticated MIMs with higher-order structures. The creation and application of polymeric MIMs based on pillar[n]arenes is also discussed.

External-stimulus-triggered conformational inversion of mechanically self-locked pseudo[1]catenane and gemini-catenanes based on A1/A2-alkyne-azide-difunctionalized pillar[5]arenes

Herein, we report a methodology for constructing mechanically self-locked molecules (MSMs) through the efficient intramolecular copper(i)-catalyzed alkyne–azide cycloaddition (CuAAC) of self-threaded A1/A2-azido-propargyl-difunctionalized pillar[5]arenes. The obtained monomeric “pseudo[1]catenane” and dimeric “gemini-catenane” were isolated and fully characterized using mass spectrometry, nuclear magnetic resonance (NMR) spectroscopy, and X-ray crystallography. Upon investigation by ¹H NMR spectroscopy in chloroform, the observed motion for the threaded ring in the pseudo[1]catenane was reversibly controlled by the temperature, as demonstrated by variable-temperature ¹H NMR studies. Two gemini-catenane stereoisomers were also isolated in which the two pillar[5]arene moieties threaded by two decyl chains were aligned in different topologies. Furthermore, the conformational inversion of pseudo[1]catenane and the gemini-catenanes triggered by solvents and guests was investigated and probed using ¹H NMR spectroscopy, isothermal titration calorimetry, and single-crystal X-ray analysis.

Mechanically self-locked molecules (MSMs) through the efficient intramolecular copper(i)-catalyzed alkyne–azide cycloaddition (CuAAC) of self-threaded A1/A2-azido-propargyl-difunctionalized pillar[5]arenes.

Pillararenes: fascinating planar chiral macrocyclic arenes

Chiral macrocycles possess significant value in chiral science and supramolecular chemistry. Pillararenes, as a class of relatively young supramolecular macrocyclic hosts, have been widely used for host-guest recognition and self-assembly. Since the position of substituents on the benzene rings breaks the molecular symmetry (symmetric plane and symmetric center), pillararenes possess planar chirality. However, it is a great challenge to synthesize stable and resolvable enantiomers because of the easy rotation of the phenylene group. In this review, we summarize the construction methods of resolvable chiral pillararenes. We also focus on their applications in enantioselective recognition, chiral switches, chirality sensing, asymmetric catalysis, circularly polarized luminescence, metal-organic frameworks, and highly permeable membranes. Finally, we discuss the future research perspectives in this field of pillararene-based planar chiral materials. We hope that this review will encourage more researchers to work in this exciting field.

Pillar[5]arene-Derived endo-Functionalized Molecular Tube for Mimicking Protein-Ligand Interactions

Artificial tubular molecular pockets bearing polar functionalities on their inner surface are useful model systems for understanding the mechanisms of protein-ligand interactions in living systems. We herein report a pillar[5]arene-derived molecular tube, [P4-(OH)BPO], whose endo conformational isomer endo-[P4-(OH)BPO] possesses an inwardly pointing hydrogen-bond (H-bond) donor (OH) in its deep cavity and a strong H-bond acceptor (C═O) on its predominantly hydrophobic inner surface, rendering it a perfect protein binding pocket mimetic. A fragment-based drug design model was established using endo-[P4-(OH)BPO] and a library of various shape-complementary fragment ligands (1-38). On the basis of the binding affinity data for "fragment-pocket" complexes G⊂endo-[P4-(OH)BPO] (G = 1-38), two rationally designed "lead molecules" (39 and 40) were identified as being able to enhance binding affinity significantly by forming H-bonds with both the donor and acceptor of endo-[P4-(OH)BPO]. The described work opens new avenues for developing pillar[n]arene-derived protein binding pocket-mimetic systems for studies of protein-ligand interactions and mechanisms of enzymatic reactions.

Pressure-driven, solvation-directed planar chirality switching of cyclophano-pillar[5]arenes (molecular universal joints)

Planar chiral cyclophanopillar[5]arenes with a fused oligo(oxyethylene) or polymethylene subring (MUJs), existing as an equilibrium mixture of subring-included (in) and -excluded (out) conformers, respond to hydrostatic pressure to exhibit dynamic chiroptical property changes, leading to an unprecedented pressure-driven chirality inversion and the largest ever-reported leap of anisotropy (g) factor for the MUJ with a dodecamethylene subring. The pressure susceptivity of MUJs, assessed by the change in g per unit pressure, is a critical function of the size and nature of the subring incorporated and the solvent employed. Mechanistic elucidations reveal that the in-out equilibrium, as the origin of the MUJ's chiroptical property changes, is on a delicate balance of the competitive inclusion of subrings versus solvent molecules as well as the solvation of the excluded subring. The present results further encourage our use of pressure as a unique tool for dynamically manipulating various supramolecular devices/machines.

Stereochemical Inversion of Rim-Differentiated Pillar[5]arene Molecular Swings

To investigate the dynamic stereochemical inversion behavior of pillar[5]arenes (P[5]s) in more detail, we synthesized a series of novel rim-differentiated P[5]s with various substituents and examined their rapid rotations by variable-temperature NMR (203–298 K). These studies revealed for the first time the barrier of “methyl-through-the-annulus” rotation (ΔG^‡ = 47.4 kJ·mol^–1 in acetone) and indicated that for rim-differentiated P[5]s with two types of alkyl substituents, the smaller rim typically determines the rate of rotation. However, substituents with terminal C=C or C≡C bonds give rise to lower inversion barriers, presumably as a result of attractive π–π interactions in the transition state. Finally, data on a rim-differentiated penta-methyl-penta-propargyl P[5] exhibited the complexity of the overall inversion dynamics.

Redox-Triggered Chirality Switching and Guest-Capture/Release with a Pillar[6]arene-Based Molecular Universal Joint

A chiral electrochemically responsive molecular universal joint (EMUJ) was synthesized by fusing a macrocyclic pillar[6]arene (P[6]) to a ferrocene-based side ring. A single crystal of an enantiopure EMUJ was successfully obtained, which allowed, for the first time, the definitive correlation between the absolute configuration and the circular dichroism spectrum of a P[6] derivative to be determined. The self-inclusion and self-exclusion conformational change of the EMUJ led to a chiroptical inversion of the P[6] moiety, which could be manipulated by both solvents and changes in temperature. The EMUJ also displayed a unique redox-triggered reversible in/out conformational switching, corresponding to an occupation/voidance switching of the P[6] cavity, respectively. This phenomenon is an unprecedented electrochemical manipulation of the capture and release of guest molecules by supramolecular hosts.

Competitive Selection of Conformation Chirality of Water-Soluble Pillar[5]arene Induced by Amino Acid Derivatives

The competitive conformation chirality of dynamically racemic water-soluble pillar[5]arene WP5 can be induced by 19 different l-amino acid ethyl ester hydrochlorides. Among them, l-Arg-OEt and 18 other l-amino acid ethyl ester hydrochlorides can induce the opposite-handedness conformation of WP5. This was ascribed to the different binding models with a side-chain moiety or ethyl ester moiety of amino acids toward the cavity of WP5.

Resolution and Racemization of a Planar-Chiral A1/A2-Disubstituted Pillar[5]arene

Butoxycarbonyl (Boc)-protected pillar[4]arene[1]-diaminobenzene (BP) was synthesized by introducing the Boc protection onto the A1/A2 positions of BP. The oxygen-through-annulus rotation was partially inhibited because of the presence of the middle-sized Boc substituents. We succeeded in isolating the enantiopure RP (RP, RP, RP, RP, and RP)- and SP (SP, SP, SP, SP, and SP)-BP, and studied their circular dichroism (CD) spectral properties. As the Boc substituent is not large enough to completely prevent the flip of the benzene units, enantiopure BP-f1 underwent racemization in solution. It is found that the racemization kinetics is a function of the solvent and temperature employed. The chirality of the BP-f1 could be maintained in n-hexane and CH2Cl2 for a long period at room temperature, whereas increasing the temperature or using solvents that cannot enter into the cavity of BP-f1 accelerated the racemization of BP-f1. The racemization kinetics and the thermodynamic parameters of racemization were studied in several different organic solvents.

Critical Role of Achiral Guest Molecules in Planar Chirality Inversion of Alanine-Appended Pillar[5]arenes

Planar chirality inversion of pillar[5]arenes bearing d- or l-alanine substituents on both rims was investigated upon addition of guest molecules having pyridinium or imidazole moieties and long alkyl chains. The d- and l-alanine-substituted pillar[5]arenes exhibited pS and pR planar chirality, respectively. However, this planar chirality was inverted upon inclusion of certain achiral molecules, comprising pyridinium or imidazole moieties and long alkyl chains with terminal hydroxyl or methyl groups.

Pillar[5]-bis-thiacrown: An Adaptive Tricyclic Host Selectively Recognizing an Organic Guest by Dimetalation

Some biological receptors change their shapes and rigidity by metalation to recognize substrates precisely via adaptive guest binding process. Herein we present a semi-flexible tricyclic host molecule whose conformation is rigidified by dimetalation to uptake organic guests selectively. Considering two metal binding sites and an empty space between them, pillar[5]-bis-thiacrown (L) was synthesized. The tricyclic host L forms a disilver(I) complex [Ag₂ L(NO₃ )₂ ], with an Ag⋅⋅⋅Ag separation of 9.976 Å. Binding studies based on ¹ H NMR including 2D NOESY and DOSY experiments towards α,ω-dicyanoalkanes [CN(CH₂ )_n CN, n=2-6, shortly C2-C6] demonstrated that the dimetalated L, Ag₂ L preferentially recognizes C2 over other guests than that of free L. Furthermore, the dimetalated the host only uptakes C2 in the presence of other guests. Crystal structures support the idea that the space between two silver(I) centers plays a decisive role on the selective guest binding forming an Ag-C2-Ag@L arrangement via the length-selective recognition. This work demonstrates the chemical example of the adaptive guest binding and presents a new perspective on the metallosupramolecules of pillararenes.