Enantiodifferentiation in the Photoisomerization of (Z,Z)-1,3-Cyclooctadiene in the Cavity of γ-Cyclodextrin–Curcubit[6]uril-Wheeled [4]Rotaxanes with an Encapsulated Photosensitizer

para-Phenylenediamine Dimer as a Redox-Active Guest for Supramolecular Systems

Redox-active components are highly valuable in the construction of molecular devices. We combined two p-phenylenediamines (p-PDA) with a biphenyl (BiPhe) unit to prepare a supramolecular guest 4 consisting of three binding sites for cucurbit[7/8]uril (CBn) and/or cyclodextrins (CD). Supramolecular properties of 4 were investigated using NMR, UV-vis, mass spectrometry and isothermal titration calorimetry. Our analysis revealed that 4 forms higher-order host-guest complexes, wherein a CD unit occupies the central BiPhe site, secured by two CBn units at the terminal p-PDA sites. Additionally, 1 : 1 complexes with α-CD and β-CD, a 1 : 2 complex with γ-CD and 2 : 1 complexes with CB7 and CB8 were identified. Through UV-vis and cyclic voltammetry, redox processes leading to the formation of a stable, deep blue dication diradical of 4 are elucidated. Furthermore, it is demonstrated that CB7 selectively protects oxidised 4 from reduction in the presence of a reducing agent. The supramolecular and redox properties of the structural motif represented by 4 render it an interesting candidate for the construction of supramolecular devices.

Two Squares in a Barrel: An Axially Disubstituted Conformationally Rigid Aliphatic Binding Motif for Cucurbit[6]uril

Novel binding motifs suitable for the construction of multitopic guest-based molecular devices (e.g., switches, sensors, data storage, and catalysts) are needed in supramolecular chemistry. No rigid, aliphatic binding motif that allows for axial disubstitution has been described for cucurbit[6]uril (CB6) so far. We prepared three model guests combining spiro[3.3]heptane and bicyclo[1.1.1]pentane centerpieces with imidazolium and ammonium termini. We described their binding properties toward CB6/7 and α-/β-CD using NMR, titration calorimetry, mass spectrometry, and single-crystal X-ray diffraction. We found that a bisimidazolio spiro[3.3]heptane guest forms inclusion complexes with CB6, CB7, and β-CD with respective association constants of 4.0 × 104, 1.2 × 1012, and 1.4 × 102. Due to less hindering terminal groups, the diammonio analogue forms more stable complexes with CB6 (K = 1.4 × 106) and CB7 (K = 3.8 × 1012). The bisimidazolio bicyclo[1.1.1]pentane guest forms a highly stable complex only with CB7 with a K value of 1.1 × 1011. The high selectivity of the new binding motifs implies promising potential in the construction of multitopic supramolecular components.

Catenane and Rotaxane Synthesis from Cucurbit[6]uril-Mediated Azide-Alkyne Cycloaddition

The chemistry of mechanically interlocked molecules (MIMs) such as catenane and rotaxane is full of new opportunities for the presence of a mechanical bond, and the efficient synthesis of these molecules is therefore of fundamental importance in realizing their unique properties and functions. While many different types of preorganizing interactions and covalent bond formation strategies have been exploited in MIMs synthesis, the use of cucurbit[6]uril (CB[6]) in simultaneously templating macrocycle interlocking and catalyzing the covalent formation of the interlocked components is particularly advantageous in accessing high-order catenanes and rotaxanes. In this review, catenane and rotaxane obtained from CB[6]-catalyzed azide-alkyne cycloaddition will be discussed, with special emphasis on the synthetic strategies employed for obtaining complex [n]rotaxanes and [n]catenanes, as well as their properties and functions.

Development of fluorophoric [2]pseudorotaxanes and [2]rotaxane: selective sensing of Zn(II)

Fluorophoric [2]pseudorotaxanes {NiPR1(ClO4)2-NiPR3(ClO4)2} are synthesized by utilizing newly designed fluorophoric bidentate ligands (L1-L3) and a heteroditopic naphthalene containing macrocycle (NaphMC) with high yields via Ni(II) templation and π-π stacking interactions. Subsequently, a fluorophoric [2]rotaxane (NAPRTX) is established through a Cu(I) catalysed click reaction between an azide terminated pseudorotaxane, {NiPR4(ClO4)2}, which contains the newly designed fluorophoric ligand L4, and alkyne terminated bulky stopper units. All these fluorophoric [2]pseudorotaxanes and the [2]rotaxane were characterized using numerous techniques such as mass spectrometry, NMR, UV/Vis, PL, and elemental analysis, wherever applicable. Furthermore, to investigate the effect of the fluorophoric moieties, the coordinating ability of chelating units, and size and shape of the three dimensional cavity generated by the mechanical bond in the interlocked [2]rotaxane (NAPRTX), we have performed a sensing study of various metal ions. Thus, the interlocked [2]rotaxane is found to have potential as a selective fluorescent sensor for Zn(II) metal ions over other transition, alkali and alkaline earth metal ions, where the 2,2'-bipyridyl arylvinylene moiety of the axle acts as a fluorescence signalling unit.

γ-Cyclodextrin-based [2]rotaxane stoppered with gold(I)-ethynyl complexation: phosphorescent sensing for nitroaromatics

A [2]rotaxane is assembled by γ-cyclodextrin (γ-CD) with threaded 1,4-diethynylbiphenyl and bulky Au(I)-phosphine stoppers. The phosphorescence of the [2]rotaxane has been observed in aerated aqueous solution and found to be quenched by nitroaromatics due to γ-CD-based co-binding, providing a sensitive approach towards explosive-sensing.

Catalytic Chiral Photochemistry Sensitized by Chiral Hosts-Grafted Upconverted Nanoparticles

Singlet chiral photocatalysis is highly challenging. Herein, we report fluorescence resonance energy transfer (FRET)-based chiral photocatalysis with γ-cyclodextrin (CD)-grafted lanthanide-doped upconverted nanoparticles (UCNP). The CD-modified UCNP strongly emits in the UV wavelength region upon excitation with a 980 nm laser, which selectively sensitizes the photosubstrates complexed by CD on the surface of UCNP through FRET. Therefore, enantiodifferentiating photocyclodimerization of anthracene or naphthalene derivatives sensitized by the CD-modified UCNP gives photoproducts in good enantioselectivity even in the presence of a catalytic amount of CD-modified UCNP. Moreover, the photocatalysts are readily separated and could be reused for at least six cycles without decreasing the enantioselectivity.

Chiral Photocatalyst Structures in Asymmetric Photochemical Synthesis

Asymmetric catalysis is a major theme of research in contemporary synthetic organic chemistry. The discovery of general strategies for highly enantioselective photochemical reactions, however, has been a relatively recent development, and the variety of photoreactions that can be conducted in a stereocontrolled manner is consequently somewhat limited. Asymmetric photocatalysis is complicated by the short lifetimes and high reactivities characteristic of photogenerated reactive intermediates; the design of catalyst architectures that can provide effective enantiodifferentiating environments for these intermediates while minimizing the participation of uncontrolled racemic background processes has proven to be a key challenge for progress in this field. This review provides a summary of the chiral catalyst structures that have been studied for solution-phase asymmetric photochemistry, including chiral organic sensitizers, inorganic chromophores, and soluble macromolecules. While some of these photocatalysts are derived from privileged catalyst structures that are effective for both ground-state and photochemical transformations, others are structural designs unique to photocatalysis and offer insight into the logic required for highly effective stereocontrolled photocatalysis.

Optimizing Photochirogenic Performance by Solvent-Driven Conformational Fixation in Enantiodifferentiating Photoisomerization of (Z)-Cyclooctene Mediated by Sensitizing β-Cyclodextrin Hosts

Catalytic enantiodifferentiating photoisomerization of cyclooctene (1Z) included and sensitized by regioisomeric 6-O-(o-, m-, and p-methoxybenzoyl)-β-cyclodextrins (CDs) was performed under a variety of solvent conditions for higher enantioselectivities. The enantiomeric excess (ee) of chiral (E)-isomer (1E) produced was a critical function of all the internal and external factors examined, in particular, the sensitizer structure and the solvent conditions, to afford (R)-1E in record-high ee's of up to 67% upon sensitization with the meta-substituted β-CD host in water and salt solutions but neither in 50% aqueous ethanol nor with the ortho- and para-substituted hosts. The mechanistic origin of the sudden ee enhancement achieved under the specific conditions is discussed on the basis of the circular dichroism spectral behaviors upon substrate inclusion and the compensatory enthalpy-entropy relationship of the activation parameters for the enantiodifferentiating photoisomerization.

Supramolecular Enantiodifferentiating Photocyclodimerization of 2-Anthracenecarboxylic Acid Mediated by Bridged β-Cyclodextrins: Critical Effects of the Host Structure, pH and Co-Solvents

Several sulfoxide- and sulfone-bridged β-cyclodextrin (CD) dimers were synthesized for mediating the enantiodifferentiating [4+4] photocyclodimerization of 2-anthracenecarboxylic acid (AC). The complexation behavior of these chiral hosts with AC was investigated by UV-vis, circular dichroism, fluorescence, and NMR spectroscopies and certified the formation of 1 : 1 and 1 : 2 host-guest complexes. The product distribution and enantioselectivity of the photoreaction turned out to be a critical function of the chemical structure of bridged CDs. Comparing to the sulfur-bridged 2A_X -3G_X β-CD dimer 7, the conversion of the photolyzes with sulfoxide-bridged was significantly improved, and the ee of cyclodimer 2 was remarkably increased from -82.8% with 7 to -96.7% with the sulfoxide-bridged 2A_X -3G_X β-CD dimer 8. The relative yields and ee values of the slipped cyclodimers 5 and 6 were greatly enhanced in the presence of 6 M CsCl. The reaction selectivity is susceptible to the pH variation of the aqueous buffer solution, demonstrating that the supramolecular photochirogenesis is controlled by multidimensional factors, including the chemical structure of the chiral host, solvent, and pH conditions.

Advances in Chirality Sensing with Macrocyclic Molecules

The construction of chemical sensors that can distinguish molecular chirality has attracted increasing attention in recent years due to the significance of chiral organic molecules and the importance of detecting their absolute configuration and chiroptical purity. The supramolecular chirality sensing strategy has shown promising potential due to its advantages of high throughput, sensitivity, and fast chirality detection. This review focuses on chirality sensors based on macrocyclic compounds. Macrocyclic chirality sensors usually have inherent complexing ability towards certain chiral guests, which combined with the signal output components, could offer many unique advantages/properties compared to traditional chiral sensors. Chirality sensing based on macrocyclic sensors has shown rapid progress in recent years. This review summarizes recent advances in chirality sensing based on both achiral and chiral macrocyclic compounds, especially newly emerged macrocyclic molecules.

Modes of Micromolar Host-Guest Binding of β-Cyclodextrin Complexes Revealed by NMR Spectroscopy in Salt Water

Multitopic supramolecular guests with finely tuned affinities toward widely explored cucurbit[n]urils (CBs) and cyclodextrins (CDs) have been recently designed and tested as functional components of advanced supramolecular systems. We employed various spacers between the adamantane cage and a cationic moiety as a tool for tuning the binding strength toward CB7 to prepare a set of model guests with K_CB7 and K_β-CD values of (0.6-5.0) × 10¹⁰ M^-1 and (0.6-2.6) × 10⁶ M^-1, respectively. These accessible adamantylphenyl-based binding motifs open a way toward supramolecular components with an outstanding affinity toward β-cyclodextrin. ¹H NMR experiments performed in 30% CaCl₂/D₂O at 273 K along with molecular dynamics simulations allowed us to identify two arrangements of the guest@β-CD complexes. The approach, joining experimental and theoretical methods, provided a better understanding of the structure of cyclodextrin complexes and related molecular recognition, which is highly important for the rational design of drug delivery systems, molecular sensors and switches.

A Supramolecular Strategy for Enhancing Photochirogenic Performance through Host/Guest Modification: Dicationic γ-Cyclodextrin-Mediated Photocyclodimerization of 2,6-Anthracenedicarboxylate

Possessing an extra anionic handle for chiral supramolecular interactions, 2,6-anthracenedicarboxylate exhibited greater photochirogenic performance than 2-anthracenecarboxylate to afford the anti-cyclodimer in up to 94% yield and -72% enantiomeric excess upon photoirradiation with dicationic γ-cyclodextrins.

Locking the Dynamic Axial Chirality of Biphenyl Crown Ethers through Threading

This paper describes the syntheses of [2]rotaxanes comprising 23- and 26-membered biphenyl crown ethers as the macrocyclic components and secondary ammonium ions as the dumbbell-shaped components, and the locking of the dynamic axial chirality of the biphenyl moieties in these structures. Chiral high-performance liquid chromatography (HPLC) revealed that our [2]rotaxane featuring the 26-membered crown ether racemized at room temperature, but the racemization of the [2]rotaxane featuring the 23-membered crown ether did not proceed at room temperature over a period of three days. After separation of the enantiomers of the [2]rotaxane incorporating the 23-membered crown ether through chiral HPLC, we studied its racemization at elevated temperature. The rate of stereoinversion in dimethylsulfoxide (a polar solvent) was faster than that in o-dichlorobenzene (a nonpolar solvent), and herein we discuss these kinetic parameters.

A Photochemical/Thermal Switch Based on 4,4'-Bis(benzimidazolio)stilbene: Synthesis and Supramolecular Properties

Stilbene derivatives are well-recognised substructures of molecular switches based on photochemically and/or thermally induced (E)/(Z) isomerisation. We combined a stilbene motif with two benzimidazolium arms to prepare new sorts of supramolecular building blocks and examined their binding properties towards cucurbit[n]urils (n=7, 8) and cyclodextrins (β-CD, γ-CD) in water. Based on the ¹ H NMR data and molecular dynamics simulations, we found that two distinct complexes with different stoichiometry, i. e., guest@β-CD and guest@β-CD₂ , coexist in equilibrium in a water solution of the (Z)-stilbene-based guests. We also demonstrated that the bis(benzimidazolio)stilbene guests can be transformed from the (E) into the (Z) form via UV irradiation and back via thermal treatment in DMSO.

pH-Controlled Chirality Inversion in Enantiodifferentiating Photocyclodimerization of 2-Antharacenecarboxylic Acid Mediated by γ-Cyclodextrin Derivatives

Several γ-cyclodextrin (γ-CDx) derivatives were used as chiral hosts for the photocyclodimerization of 2-anthracenecarboxylic acid (AC). The effect of pH on photoreactivity and stereochemical outcome of photoproducts was investigated. Upon changing the solution pH, the stereochemical outcome of HH cyclodimer 3 was inverted from 25.2% to -64.4% and 41.2% to -76.2%, respectively, in the photocyclodimerization of AC mediated by bis-quinoline-modified γ-CDx 7 and its N-methylated derivative 8.

Binding study on 1-adamantylalkyl(benz)imidazolium salts to cyclodextrins and cucurbit[n]urils

Adamantane-based imidazolium salts with flexible linkers between the adamantane cage and imidazolium core were prepared. Their supramolecular behaviour towards the natural cyclodextrins α-CD, β-CD and γ-CD and cucurbit[n]urils (n = 7, 8) was studied.

Synthesis of a [6]rotaxane with singly threaded γ-cyclodextrins as a single stereoisomer

A series of hetero [4]-, [5]- and [6]rotaxanes containing both cucurbit[6]uril (CB[6]) and γ-cyclodextrin (γ-CD) as the macrocyclic components have been synthesized via a threading-followed-by-stoppering approach. Due to the orthogonal binding of CB[6] to ammonium and γ-CD to biphenylene/tetra(ethylene glycol), the [n]rotaxanes display a specific sequence of the interlocked macrocycles. In addition, despite of the asymmetry of γ-CD with respect to the orthogonal plane of the axle, only one stereoisomer of the [6]rotaxane was obtained.

Precise Manipulation of Temperature-Driven Chirality Switching of Molecular Universal Joints through Solvent Mixing

Three chiral bicyclic pillar[5]arene derivatives termed as molecular universal joints (MUJs), were synthesized and separated enantiomerically. These MUJs showed temperature-driven chirality switching in certain solvents. Herein, it is demonstrated that temperature-driven chirality switching could also be realized by mixing two miscible organic solvents, in each of which chirality inversion is not accomplishable. Additionally, solvent mixing drastically varied the inversion temperature of the MUJs, for example, from far below zero to room temperature. Moreover, the temperature-driven S_p /R_p to R_p /S_p chirality switching direction could be reversed by the solvent mixing and it was critically controlled by the mixing ratios of the two solvents. These observations allowed precise manipulation of the chirality switching behavior of the MUJs. Such a chirality switching was ascribed to the influences of solvent and temperature on the in-out equilibrium of the side rings, which is delicately controlled by several processes, including the solvation/desolvation and the inclusion/exclusion of the side rings and solvent molecules. Crucially, the solvent mixing introduced new supramolecular processes, in particular the desolvation of solvent molecules from the mixed solvent system and the solvation of the side ring by the mixed solvent, which significantly disturbed the original in-out equilibrium of MUJs and drastically switched the entropy and enthalpy changes of conformational interconversion.

Synthesis, enantioseparation and photophysical properties of planar-chiral pillar[5]arene derivatives bearing fluorophore fragments

Planar chiral pillar[5]arene derivatives (P5A-DPA and P5A-Py) bearing bulky fluorophores were obtained in high yield by click reaction. The photophysical properties of both compounds were investigated in detail. P5A-DPA with two 9,10-diphenylanthracene (DPA) pigments grafted on the pillar[5]arene showed a high fluorescence quantum yield of 89.5%. This is comparable to the monomer DPA-6, while P5A-Py with two perylene (Py) pigments grafted on the pillar[5]arene showed a significantly reduced quantum yield of 46.4% vs 78.2% for the monomer Py-6. The oxygen-through-annulus rotation of the phenolic units was inhibited for both compounds due to the bulky chromophore introduced, and the resolution of the enantiomers was achieved due to the bulky size of the fluorophores. The absolute configuration of the enantiomers was determined by circular dichroism (CD) spectra. The solvent-induced aggregation behavior was investigated with the enantiopure P5A-DPA and P5A-Py. It was found that the CD signals were enhanced by aggregation. P5A-DPA showed aggregation-induced emission enhancement, while P5A-Py showed aggregation-induced emission quenching, accompanied by excimer emission when aggregating in water and THF mixed solution.

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.

An Ultimate Stereocontrol in Supramolecular Photochirogenesis: Photocyclodimerization of 2-Anthracenecarboxylate Mediated by Sulfur-Linked β-Cyclodextrin Dimers

Stereoisomeric β-cyclodextrin (CD) dimers linked with a sulfur atom or an arene spacer were designed to create a tethered dual CD capsule for precisely manipulating the regio- and enantioselectivities of the photocyclodimerization of 2-anthracenecarboxylate (AC) to four stereoisomeric classical 9,10:9',10'-cyclodimers and two nonclassical 5,8:9',10'-cyclodimers. Among the dimeric CD hosts prepared, exo-3-thia-β-CD dimer formed 1:1 and 1:2 host-guest complexes with AC in aqueous solutions, the former of which hindered but the latter facilitated the AC photocyclodimerization with regio- and enantioselectivities much higher than those obtained with native β-CD or the rest of the β-CD dimers. The stereochemical outcomes turned out to be highly sensitive to and hence critically manipulable by the linking position and configuration of the connected saccharide units and the linker length, as well as the external variants, such as temperature, pH, and added salt. Eventually, the photocyclodimerization of AC mediated by the dimeric β-CD host gave enantiopure syn-head-to-tail-9,10:9',10'-cyclodimer in 97-98% yield in a pH 5.1 buffer solution at 0.5 °C and also in an aqueous CsCl solution at -20 °C.

Dynamic Control of the Self-Assembling Properties of Cyclodextrins by the Interplay of Aromatic and Host-Guest Interactions

The presence of a doubly-linked naphthylene clip at the O-2^I and O-3^II positions in the secondary ring of β-cyclodextrin (βCD) derivatives promoted their self-assembly into head-to-head supramolecular dimers in which the aromatic modules act either as cavity extension walls (if the naphthalene moiety is 1,8-disubstituted) or as folding screens that separate the individual βCD units (if 2,3-disubstituted). Dimer architecture is governed by the conformational properties of the monomer constituents, as determined by NMR, fluorescence, circular dichroism, and computational techniques. In a second supramolecular organization level, the topology of the assembly directs host-guest interactions and, reciprocally, guest inclusion impacts the stability of the supramolecular edifice. Thus, inclusion of adamantane carboxylate, a well-known βCD cavity-fitting guest, was found to either preserve the dimeric arrangement, leading to multicomponent species, or elicit dimer disruption. The ensemble of results highlights the potential of the approach to program self-organization and external stimuli responsiveness of CD devices in a controlled manner while keeping full diastereomeric purity.

Construction and heterogeneous photooxidization reactivity of a cyclodextrin/porphyrin polyrotaxane network

A supramolecular polyrotaxane network, based on a CD-based inclusion complex and porphyrin derivative, was synthesized by imine condensation reactions. Then it could be applied as a heterogeneous catalyst for generating singlet oxygen under photoirradiation, which showed its good ability to photooxidize anthracene derivatives.

Room-temperature phosphorescent γ-cyclodextrin-cucurbit[6]uril-cowheeled [4]rotaxanes for specific sensing of tryptophan

The room temperature phosphorescence of iodine-substituted-γ-CD-CB[6]-cowheeled[4] rotaxanes was quenched specifically by tryptophan among plasma amino acids.

Heterorotaxanes

Heterorotaxanes, in which at least two types of macrocycles were introduced as the wheel components in rotaxanes, have attracted more and more attention during the past few decades owing to their unique structural features and intriguing properties. The coexistence of varied macrocycles endows the resultant heterorotaxanes not only versatile shuttling and switching behaviors but also great potential for the construction of functional rotaxane systems for applications. In this feature article, a survey of the successful synthesis of heterorotaxanes will be provided based on the various strategies towards the synthesis of heterorotaxanes, i.e. orthogonal binding approach, self-sorting approach, cooperative capture approach, active metal template approach, etc.

Stereoselective Synthesis of Mechanically Planar Chiral Rotaxanes

Chiral interlocked molecules in which the mechanical bond provides the sole stereogenic unit are typically produced with no control over the mechanical stereochemistry. Here we report a stereoselective approach to mechanically planar chiral rotaxanes in up to 98:2 d.r. using a readily available α-amino acid-derived azide. Symmetrization of the covalent stereocenter yields a rotaxane in which the mechanical bond provides the only stereogenic element.

Induced chirality sensing through formation and aggregation of the chiral imines double winged with pyrenes or perylenes

Reaction of chiral amines with benzaldehydes 3,5-disubstituted by two pyrenes or perylenes afforded corresponding double winged chiral imines, which aggregated to show significantly enhanced circular dichroism spectra at the transition bands of the chromophores in the mixture solutions of DMF and H2O.

Chirality in rotaxanes and catenanes

Although chiral mechanically interlocked molecules (MIMs) have been synthesised and studied, enantiopure examples are relatively under-represented in the pantheon of reported catenanes and rotaxanes and the underlying chirality of the system is often even overlooked. This is changing with the advent of new applications of MIMs in catalysis, sensing and materials and the appearance of new methods to access unusual stereogenic units unique to the mechanical bond. Here we discuss the different stereogenic units that have been investigated in catenanes and rotaxanes, examples of their application, methods for assigning absolute stereochemistry and provide a perspective on future developments.

Enhanced chiral recognition by γ-cyclodextrin-cucurbit[6]uril-cowheeled [4]pseudorotaxanes

Mixing γ-cyclodextrin (γ-CD), cucurbit[6]uril (CB[6]) and tetraammonium-bearing axles together led to a spontaneous formation of γ-CD-CB[6]-cowheeled [4]pseudorotaxanes. The well-defined unsymmetrical cavities thus formed enhance the binding affinity towards chiral amines by factors of several hundreds and show remarkably improved chiral discrimination.

Supramolecular Photochirogenesis Driven by Higher-Order Complexation: Enantiodifferentiating Photocyclodimerization of 2-Anthracenecarboxylate to Slipped Cyclodimers via a 2:2 Complex with β-Cyclodextrin

Chiral slipped 5,8:9',10'-cyclodimers were preferentially produced over classical 9,10:9',10'-cyclodimers upon supramolecular photocyclodimerization of 2-anthracenecarboxylate (AC) mediated by β-cyclodextrin (β-CD). This photochirogenic route to the slipped cyclodimers, exclusively head-to-tail (HT) and highly enantioselective, has long been overlooked in foregoing studies but is dominant in reality and is absolutely supramolecularly activated by 2:2 complexation of AC with β-CD. The intricate structural and photophysical aspects of this higher-order complexation-triggered process have been comprehensively elucidated, while the absolute configurations of the slipped cyclodimers have been unambiguously assigned by comparing the experimental and theoretical circular dichroism spectra. In the 2:2 complex, two ACs packed in a dual β-CD capsule are not fully overlapped with each other but are only partially stacked in a slipped anti- or syn-HT manner. Hence, they do not spontaneously cyclodimerize upon photoexcitation but instead emit long-lived excimer fluorescence at wavelengths slightly longer than the monomer fluorescence, indicating that the slipped excimer is neither extremely reactive nor completely relaxed in conformation and energy. Because of the slipped conformation of the AC pair in the soft capsule, the subsequent photocyclodimerization becomes manipulable by various internal or external factors, such as temperature, pressure, added salt, and host modification, enabling us to exclusively obtain the slipped cyclodimers with high regio- and enantioselectivities. In this supramolecularly driven photochirogenesis, the dual β-CD capsule functions as a chiral organophotocatalyst to trigger and accelerate the nonclassical photochirogenic route to slipped cyclodimers by preorganizing the conformation of the encapsulated AC pair, formally mimicking a catalytic antibody.