Exploiting Anion-π Interactions for Efficient and Selective Catalysis with Chiral Molecular Cages

Anion-π+ AIEgens for Fluorescence Imaging and Photodynamic Therapy

Fluorescence imaging-guided photodynamic therapy (PDT) has attracted extensive attention due to its potential of real-time monitoring the lesion locations and visualizing the treatment process with high sensitivity and resolution. Aggregation-induced emission luminogens (AIEgens) show enhanced fluorescence and reactive oxygen species (ROS) generation after cellular uptake, giving them significant advantages in bioimaging and PDT applications. However, most AIEgens are unfavorable for the application in organisms due to their severe hydrophobicity. Anion-π+ type AIEgens carry intrinsic charges that can effectively alleviate their hydrophobicity and improve their binding capability to cells, which is expected to enhance the bioimaging quality and PDT performance. This concept summarizes the applications of anion-π+ type AIEgens in fluorescence imaging, fluorescence imaging-guided photodynamic anticancer and antimicrobial therapy in recent years, hoping to provide some new ideas for the construction of robust photosensitizers. Finally, the current problems and future challenges of anion-π+ AIEgens are discussed.

Supramolecularly modulated carbon-centered radicals: toward selective oxidation from benzyl alcohol to aldehyde

The reactivity of ketyl radicals and benzoyl radicals, two key intermediates of photo-induced oxidation of benzyl alcohol, can be stabilized by the host-guest interaction of the radicals with cucurbit[7]uril. As a result, the selectivity of photo-induced oxidation from benzyl alcohol to aldehyde is significantly improved by diminishing side reactions and inhibiting the generation of carboxylic acid products. This work presents a new route to modulate the reactivity of radical intermediates, enriching the chemistry of supramolecular intermediates and the toolbox of supramolecular catalysis.

Post-Assembly Polymerization of Discrete Anion-Coordinated Triple Helicate

Hierarchical self-assembly has been recently employed in the construction of anion-coordination-driven gel materials. However, the post-assembly modification strategy, which may be a highly efficient strategy to realize the functionalization of discrete 'aniono' supramolecular architectures, has not been employed yet. Herein we report the first example of anion-coordination-driven gel material cross-linked by well-defined 'aniono' triple helicate through post-assembly polymerization. The obtained gel shows self-healing property and excellent compatibility with various surfaces, including glass, rubber, leaf, PP, and metal. The viscoelastic gel constructed through the post-assembly modification strategy enriches the method to construct the anion-coordination-driven smart materials.

Chiral Covalent Organic Cages: Structural Isomerism and Enantioselective Catalysis

Covalent organic cages are a prominent class of discrete porous architectures; however, their structural isomerism remains relatively unexplored. Here, we demonstrate the structural isomerism of chiral covalent organic cages that renders distinct enantioselective catalytic properties. Imine condensations of tetra-topic 5,10-di(3,5-diformylphenyl)-5,10-dihydrophenazine and ditopic 1,2-cyclohexanediamine produce two chiral [4 + 8] organic cage isomers with totally different topologies and geometries that depend on the orientations of four tetraaldehyde units with respect to each other. One isomer (PN-1) has an unprecedented Johnson-type J26 structure, whereas another (PN-2) adopts a tetragonal prismatic structure. After the reduction of the imine linkages, the cages are transformed into two amine bond-linked isomers PN-1R and PN-2R. After binding to Ni(II) ions, both can serve as efficient catalysts for asymmetric Michael additions, whereas PN-2R affords obviously higher enantioselectivity and reactivity than PN-1R presumably because of its large cavity and open windows that can concentrate reactants for the reactions. Density-functional theory (DFT) calculations further confirm that the enantioselective catalytic performance varies depending on the isomer.

Cage Match: Comparing the Anion Binding Ability of Isostructural Versus Isofunctional Pairs of Metal-Organic Nanocages

Affinities of six anions (mesylate, acetate, trifluoroacetate, p-toluenecarboxylate, p-toluenesulfonate, and perfluorooctanoate) for three related Pt2+ -linked porphyrin nanocages were measured to probe the influence of different noncovalent recognition motifs (e. g., hydrogen bonding, electrostatics, π bonding) on anion binding. Two new hosts of M6 L3 12+ (1b) and M4 L2 8+ (2) composition (M=(en)Pt2+ , L=(3-py)4 porphyrin) were prepared in a one-pot synthesis and allowed comparison of hosts that differ in structure while maintaining similar N-H hydrogen-bond donor ability. Comparisons of isostructural hosts that differ in hydrogen-bonding ability were made between 1b and a related M6 L3 12+ nanoprism (1a, M=(tmeda)Pt2+ ) that lacks N-H groups. Considerable variation in association constants (K1 =1.6×103  M-1 to 1.3×108  M-1 ) and binding mode (exo vs. endo) were found for different host-guest combinations. Strongest binding was seen between p-toluenecarboxylate and 1b, but surprisingly, association of this guest with 1a was only slightly weaker despite the absence of NH⋅⋅⋅O interactions. The high affinity between p-toluenecarboxylate and 1a could be turned off by protonation, and this behavior was used to toggle between the binding of this guest and the environmental pollutant perfluorooctanoate, which otherwise has a lower affinity for the host.

Helical Cage Rotors Switched on by Brake Molecule with Variable Fluorescence and Circularly Polarized Luminescence

While chiral molecular rotors have unique frames and cavities to possibly generate switchable chiroptical functions, it still remains a formidable challenge to precisely restrict their rotations to activate certain functions such as fluorescence as well as circularly polarized luminescence (CPL), which are strongly related to the local molecular rotations. Herein, we design a pair of enantiopure helical cage rotors, which emit light neither at the molecular state nor in the crystal or aggregation states, although they contain luminophore groups. However, upon mounting with fluoroaromatic borane (TFPB) as a molecular brake, the phenyl rotation of the helical cage can be effectively hindered and fluorescence and CPL activities of the molecular cage are switched on. Crystal structure analysis reveals that the rotation is restricted through synergistic B-O-H-N bonding and a fluoroaromatic-aromatic (ArF-Ar) dipole interaction. Moreover, the helical cages are switched on stepwise with color-variable fluorescence and CPL by the inner brake in the molecular state and the outer brake in the supramolecular assemblies, respectively. This work not only provides the design idea of chiroptical molecular rotors but also unveils how fluorescence and CPL could be generated in cage rotor systems.

Anion-π catalysis on carbon allotropes

Anion-π catalysis, introduced in 2013, stands for the stabilization of anionic transition states on π-acidic aromatic surfaces. Anion-π catalysis on carbon allotropes is particularly attractive because high polarizability promises access to really strong anion-π interactions. With these expectations, anion-π catalysis on fullerenes has been introduced in 2017, followed by carbon nanotubes in 2019. Consistent with expectations from theory, anion-π catalysis on carbon allotropes generally increases with polarizability. Realized examples reach from enolate addition chemistry to asymmetric Diels-Alder reactions and autocatalytic ether cyclizations. Currently, anion-π catalysis on carbon allotropes gains momentum because the combination with electric-field-assisted catalysis promises transformative impact on organic synthesis.

Host-Guest-Interaction Enhanced Nitric Oxide Photo-Generation within a Pillar[5]arene Cavity for Antibacterial Gas Therapy

Supramolecular macrocycles with intrinsic cavities have been widely explored as containers to fabricate versatile functional materials via specific host-guest recognitions. However, relatively few studies have focused on the modulation of guest reactivity within a macrocyclic cavity. Here, we demonstrate the confinement effect of pillar[5]arene with an electron-rich and precise cavity that can dramatically enhance guest photoactivity and nitric oxide (NO) generation upon visible light irradiation. Mechanism studies reveal that it is achieved through increasing the ground state nitro-aromatic torsion angle, suppressing the intersystem crossing relaxation path of the S1 state, and accelerating the isomerization reaction path of guest molecules. This NO-generating system displays broad-spectrum antibacterial, biofilm inhibition, and dispersal activities. Moreover, it can accelerate the healing of methicillin-resistant Staphylococcus aureus (MRSA)-infected wounds in vivo.

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.

Recent applications of organic cages in sensing and separation processes in solution

Organic cages are three-dimensional polycyclic compounds of great interest in the scientific community due to their unique features, which generally include simple synthesis based on the dynamic covalent chemistry strategies, structural tunability and high selectivity. In this feature article, we present the advances over the last ten years in the application of organic cages as chemosensors or components in chemosensing devices for the determination of analytes (pollutants, analytes of biological interest) in complex aqueous media including wine, fruit juice, urine. Details on the recent applications of organic cages as selective (back-)extractants or masking agents for potential applications in relevant separation processes, such as the plutonium and uranium recovery by extraction, are also provided. Over the last ten years, organic cages with permanent porosity in the liquid and solid states have been highly appreciated as porous materials able to discriminate molecules of different sizes. These features, combined with good solvent processability and film-forming tendency, have proved useful in the fabrication of membranes for gas separation, solvent nanofiltration and water remediation processes. An overview of the recent applications of organic cages in membrane separation technologies is given.

Electric field-assisted anion-π catalysis on carbon nanotubes in electrochemical microfluidic devices

The vision to control the charges migrating during reactions with external electric fields is attractive because of the promise of general catalysis, emergent properties, and programmable devices. Here, we explore this idea with anion-π catalysis, that is the stabilization of anionic transition states on aromatic surfaces. Catalyst activation by polarization of the aromatic system is most effective. This polarization is induced by electric fields. The use of electrochemical microfluidic reactors to polarize multiwalled carbon nanotubes as anion-π catalysts emerges as essential. These reactors provide access to high fields at low enough voltage to prevent electron transfer, afford meaningful effective catalyst/substrate ratios, and avoid interference from additional electrolytes. Under these conditions, the rate of pyrene-interfaced epoxide-opening ether cyclizations is linearly voltage-dependent at positive voltages and negligible at negative voltages. While electromicrofluidics have been conceived for redox chemistry, our results indicate that their use for supramolecular organocatalysis has the potential to noncovalently electrify organic synthesis in the broadest sense.

Anion-(π)n -π Catalytic Micelles

Anion-π catalysis operates by stabilizing anionic transition states on π-acidic aromatic surfaces. In anion-(π)n -π catalysis, π stacks add polarizability to strengthen interactions. In search of synthetic methods to extend π stacks beyond the limits of foldamers, the self-assembly of micelles from amphiphilic naphthalenediimides (NDIs) is introduced. To interface substrates and catalysts, charge-transfer complexes with dialkoxynaphthalenes (DANs), a classic in supramolecular chemistry, are installed. In π-stacked micelles, the rates of bioinspired ether cyclizations exceed rates on monomers in organic solvents by far. This is particularly impressive considering that anion-π catalysis in water has been elusive so far. Increasing rates with increasing π acidity of the micelles evince operational anion-(π)n -π catalysis. At maximal π acidity, autocatalytic behavior emerges. Dependence on position and order in confined micellar space promises access to emergent properties. Anion-(π)n -π catalytic micelles in water thus expand supramolecular systems catalysis accessible with anion-π interactions with an inspiring topic of general interest and great perspectives.

Chiral Adaptive Induction of an Achiral Cucurbit[8]uril-Based Supramolecular Organic Framework by Dipeptides in Water

Chiral induction by natural biomolecules can reveal the indispensable role of chiral structures in life and can be used to develop the chirality-sensing biomolecular recognition. Here, we present the synthesis and characterization of an achiral supramolecular organic framework (SOF-1) constructed from cucurbit[8]uril (CB[8]) and hexaphenylbenzene (HPB) derivative (1) in water. Due to the propeller-like rotational chiral conformation of HPB units and the specific recognition properties of CB[8], SOF-1 demonstrates chiral adaptive induction in water when interacting with the N-terminal Trp-/Phe-containing dipeptides including L-TrpX and L-PheX (X is an amino acid residue), respectively, exhibiting contrasting circular dichroism (CD) and circularly polarized luminescence (CPL) spectra. Consequently, SOF-1 has been developed as a supramolecular host and chiroptical sensor capable of recognizing and distinguishing the sequence-opposite Trp-/Phe-containing dipeptide pairs including L-TrpX/L-XTrp and L-PheX/L-XPhe based on the sequence-selective CD responses.

A Highly Water-Soluble Aggregation-Induced Emission Luminogen with Anion-π+ Interactions for Targeted NIR Imaging of Cancer Cells and Type I Photodynamic Therapy

The low oxygen dependence of type I photosensitizers (PSs) has made them a popular choice for treating solid tumors. However, the drawbacks of poor water solubility, short emission wavelength, poor stability, and inability to distinguish cancer cells from normal cells limit the application of most type I PSs in clinical therapy. Thereby, developing novel type I PSs for overcoming these problems is an urgent but challenging task. Herein, by utilizing the distinctive structural characteristics of anion-π+ interactions, a highly water-soluble type I PS (DPBC-Br) with aggregation-induced emission (AIE) characteristic and near-infrared (NIR) emission is fabricated for the first time. DPBC-Br displays remarkable water solubility (7.3 mM) and outstanding photobleaching resistance, enabling efficient and precise differentiation between tumor cells and normal cells in a wash-free and long-term tracking manner via NIR-I imaging. Additionally, the superior type I reactive oxygen species (ROS) produced by DPBC-Br provide both specific killing of cancer cells in vitro and inhibition of tumor growth in vivo, with negligible systemic toxicity. This study rationally constructs a highly water-soluble type I PS, which has higher reliability and controllability compared with conventional nanoparticle formulating procedures, offering great potential for clinical cancer treatment.

A Benzothiadiazole-Based Self-Assembled Cage for Cadmium Detection

A turn-on fluorescent probe, cage 1, was efficiently self-assembled by condensing 4,4'-(benzothiadiazole-4,7-diyl)dibenzaldehyde and TREN in chloroform. The formation of cage 1 was characterized and confirmed by NMR spectroscopy, mass spectrometry, and theoretical calculations. The yield of cage 1 could be controlled by tuning the reaction conditions, such as the precursor concentration. Interestingly, the addition of 10 equiv of Cd²⁺ relative to cage 1 could increase the fluorescence almost seven-fold. ¹H NMR and fluorescence experiments indicating fluorescence enhancement may be caused by the decomposition of cage 1. Such a high selectivity toward Cd²⁺ implies that the cage could potentially be employed in cadmium detection.

Controlled Construction of Heteroleptic [Pd2 (LA )2 (LB )(LC )]4+ Cages: A Facile Approach for Site-Selective endo-Functionalization of Supramolecular Cavities

Construction of supramolecular structures with internal functionalities is a promising approach to build enzyme-like cavities. The endo-functionalized [Pd₁₂ L₂₄ ] and [Pd₂ L₄ ] coordination cages represent the most successful systems in this regard. However, these systems mainly contain one type of endo-moiety. We herein provide a solution for the controlled endo-functionalization of [Pd₂ L₄ ] cages. Site-selective introduction of the endo-functional group was achieved through the formation of heteroleptic [Pd₂ (L^A )₂ (L^B )(L^C )] cages. Using two orthogonal steric control elements is the key for the selective formation of the hetero-assemblies. We demonstrated the construction of two hetero-cages with a single internal functional group as well as a hetero-cage with two distinct endohedral functionalities. The endo-functionalized hetero-cages bound sulfonate guests with fast-exchange dynamics. This strategy provides a new solution for the controlled endo-functionalization of supramolecular cavities.

Asymmetric Dihydroxylation-Based Kinetic Resolution of Allylic Amides Enabled by Noncovalent π-Interactions

While Sharpless asymmetric dihydroxylation is widely utilized to convert various alkenes into diols with excellent enantioselectivies, kinetic resolution by means of this fundamental catalysis has generally proven to be ineffective. Here we report that, by relying on noncovalent π-interactions that purposely include the substrate's stereocenter in the corresponding catalyst-substrate interaction framework, AD-based kinetic resolution of allylic amides is realized. This method enables such versatile chiral building blocks to be easily accessed with excellent enantiomeric excesses (ee's).

[2+3] Amide Cages by Oxidation of [2+3] Imine Cages – Revisiting Molecular Hosts for Highly Efficient Nitrate Binding

The pollution of groundwater with nitrate is a serious issue because nitrate can cause several diseases such as methemoglobinemia or cancer. Therefore, selective removal of nitrate by efficient binding to supramolecular hosts is highly desired. Here we describe how to make [2+3] amide cages in very high to quantitative yields by applying an optimized Pinnick oxidation protocol for the conversion of corresponding imine cages. By NMR titration experiments of the eight different [2+3] amide cages with nitrate, chloride and hydrogen sulfate we identified one cage with an unprecedented high selectivity towards nitrate binding vs. chloride (S=705) or hydrogensulfate (S>13500) in CD₂Cl₂/CD₃CN (1 : 3). NMR experiments as well as single‐crystal structure comparison of host‐guest complexes give insight into structure‐property‐relationships.

Inherently Chiral Cages via Hierarchical Desymmetrization

A new type of cage inherent chirality was accessed by hierarchical desymmetrization of a D_3h-symmetric prismlike cage motif. The dissymmetric C_3v cage precursor C1 bearing two different phloroglucinol caps was first synthesized. The subsequent progressive substitutions on the three triazine arms by different nucleophiles furnished the desired C₁-symmetric inherently chiral cages C3 and C4 with rich structural diversity. Resolution of the racemic cages was achieved by chiral chromatography, and the enantiopure cages were readily obtained on the gram scale. Convenient post-synthetic transformations of the chiral cages with retention of enantiomeric purity were also realized. The absolute configuration was determined by X-ray crystallography, and a chirality descriptor was provided to define the cage chirality. With the inherently chiral array of the electron-deficient triazine surfaces constituting three individual chiral V-shaped π cavities, regio- and enantioselective anion-π binding was probed for the first time with minimum interference of other interactions. As exemplified with chiral phosphate anions (CPAs), it was found that cage (-)-C3a preferably binds (S)-CPA^- in the most electron-deficient cavity through synergistic anion-π interactions with considerable chiral selectivity.

Activation Modes in Asymmetric Anion-Binding Catalysis

Over the past two decades, enantioselective anion-binding catalysis has emerged as a powerful strategy for the induction of chirality in organic transformations. The stereoselectivity is achieved in a range of different reactions by using non-covalent interactions between a chiral catalyst and an ionic substrate or intermediate, and subsequent formation of a chiral contact ion-pair upon anion-binding. This strategy offers vast possibilities in catalysis and the constant development of new reactions has led to various substrate activation approaches. This review provides an overview on the different activation modes in asymmetric anion-binding catalysis by looking at representative examples and recent advances made in this field.

Synthesis of a Highly Electron-Deficient, Water-Stable, Large Ionic Box: Multielectron Accumulation and Proton Conductivity

π-acidic boxes exhibiting electron reservoir and proton conduction are unprecedented because of their instability in water. We present the synthesis of one of the strongest electron-deficient ionic boxes showing e^- uptake as well as proton conductivity. Two large anions fit in the box to form anion-π interactions and form infinite anion-solvent wires. The box with NO₃^-···water wires confers high proton conductivity and presents the first example that manifests redox and ionic functionality in an organic electron-deficient macrocycle.

A Se···O bonding catalysis approach to the synthesis of calix[4]pyrroles

Described herein is a chalcogen bonding catalysis approach to the synthesis of calix[4]pyrrole derivatives. The Se···O bonding interactions between selenide catalysts and ketones gave rise to the catalytic activity in the condensation reactions between pyrrole and ketones, leading to the generation of calix[4]pyrrole derivatives in moderate to high yields. This chalcogen bonding catalysis approach was efficient since only 5 mol % catalyst loading was used to promote the consecutive condensation processes while the reactions could be carried out at room temperature, thus highlighting the potential of this type of nonclassical interactions in catalyzing relative complex transformations.

Anion-π Catalysis Enabled by the Mechanical Bond

We report a series of rotaxane‐based anion–π catalysts in which the mechanical bond between a bipyridine macrocycle and an axle containing an NDI unit is intrinsic to the activity observed, including a [3]rotaxane that catalyses an otherwise disfavoured Michael addition in >60 fold selectivity over a competing decarboxylation pathway that dominates under Brønsted base conditions. The results are rationalized by detailed experimental investigations, electrochemical and computational analysis.

Triazine- and Binaphthol-Based Chiral Macrocycles and Cages: Synthesis, Structure, and Solid-State Assembly

A series of triazine- and binaphthol-based homochiral and heterochiral macrocycles and cages were easily synthesized. Either fragment coupling or a one-pot approach afforded the desired products in 52-91% yields on a multigram scale as enantiopure forms. As disclosed by the crystal structures, these macrocycles and cages possess intriguing chiral cavities and assembly properties. In particular, (S,S,S)-Cage features a D₃-symmetric double-faced propeller-like structure with three chiral pockets at the side. It forms a highly ordered hexagonal column-like assembly and multiple distinct helical channels in its crystal.

Decoded fingerprints of hyperresponsive, expanding product space: polyether cascade cyclizations as tools to elucidate supramolecular catalysis

Simple enough to be understood and complex enough to be revealing, cascade cyclizations of diepoxides are introduced as new tools to characterize supramolecular catalysis. Decoded product fingerprints are provided for a consistent set of substrate stereoisomers, and shown to report on chemo-, diastereo- and enantioselectivity, mechanism and even autocatalysis. Application of the new tool to representative supramolecular systems reveals, for instance, that pnictogen-bonding catalysis is not only best in breaking the Baldwin rules but also converts substrate diastereomers into completely different products. Within supramolecular capsules, new cyclic hemiacetals from House–Meinwald rearrangements are identified, and autocatalysis on anion–π catalysts is found to be independent of substrate stereochemistry. Decoded product fingerprints further support that the involved epoxide-opening polyether cascade cyclizations are directional, racemization-free, and interconnected, at least partially. The discovery of unique characteristics for all catalysts tested would not have been possible without decoded cascade cyclization fingerprints, thus validating the existence and significance of privileged platforms to elucidate supramolecular catalysis. Once decoded, cascade cyclization fingerprints are easily and broadly applicable, ready for use in the community.

Hyperresponsive XL product space identifies polyether cascade fingerprinting as an attractive tool to elucidate supramolecular catalysis, including pnictogen-bonding, capsule and anion–π catalysts.

Modulation of π character upon complexation captured by molecular rotation spectra

Two configurations of the furan–CF3Cl complex have been observed by high-resolution rotational spectroscopy. One is characterized by a dominant Cl lone pairs∙∙∙π*aromatic interaction and the other is stabilized by a...

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.

Chiral macrocycle-induced circularly polarized luminescence of a twisted intramolecular charge transfer dye

The host-guest binding between a chiral macrocycle and an achiral dye could suppress the twisted intramolecular charge transfer (TICT) process, leading to enhanced emission and bright circularly polarized luminescence (CPL) from the originally achiral TICT-dye.

Putting Anion-π Interactions at Work for Catalysis

Since its discovery two decades ago, anion-π interaction has been increasingly recognized as an important driving force. Extensive theoretical and experimental efforts on the ground-state anion-π binding and recognition have laid the bases for exploring its relevance in catalysis. Accordingly, the concept of "anion-π catalysis" that employing an electron-deficient π surface (π-acidic surface) for anionic reaction intermediate and transition state stabilization has emerged. This article shortly reviews the emergence and development of this concept, aiming to provide an emphasis on the general concept and key progress in this exciting area. To highlight the essential contribution of anion-π interactions, the contents are organized according to their role engaged in catalytic process, for example from both ground-state and transition-state stabilization to solely transition-state stabilization, mainly by a single π-face, and to cooperative π-face activation. A concluding remark and outlook on future development of this field is also given.