Enantioselective Recognition for Many Different Kinds of Chiral Guests by One Chiral Receptor Based on Tetraphenylethylene Cyclohexylbisurea

Recognition of Amino Acid Salts by Temperature-Dependent Allosteric Binding with Stereodynamic Urea Receptors

Positive homotropic artificial allosteric systems are important for the regulation of cooperativity, selectivity and nonlinear amplification. Stereodynamic homotropic allosteric receptors can transmit and amplify induced chirality by the first ligand binding to axial chirality between two chromophores. We herein report stereodynamic allosteric urea receptors consisting of a rotational shaft as the axial chirality unit, terphenyl units as structural transmission sites and four urea units as binding sites. NMR titration experiments revealed that the receptor can bind two carboxylate guests in a positive homotropic allosteric manner attributed to the inactivation by intramolecular hydrogen-bonding between urea units within the receptor. In addition, the VT-CD spectra observed upon binding of the urea receptor with l- or D-amino acid salts in MeCN showed interesting temperature-dependent Cotton effects, based on the differences of the receptor shaft unit and the guest structure. The successful discrimination of hydrocarbon-based side chains of amino acid salts indicated that the input of chiral and steric information for the guest was amplified as outputs of the Cotton effect and the temperature-dependence of VT-CD spectra through cooperativity of positive allosteric binding.

Rapid room-temperature phosphorescence chiral recognition of natural amino acids

Chiral recognition of amino acids is very important in both chemical and life sciences. Although chiral recognition with luminescence has many advantages such as being inexpensive, it is usually slow and lacks generality as the recognition module relies on structural complementarity. Here, we show that one single molecular-solid sensor, L-phenylalanine derived benzamide, can manifest the structural difference between the natural, left-handed amino acid and its right-handed counterpart via the difference of room-temperature phosphorescence (RTP) irrespective of the specific chemical structure. To realize rapid and reliable sensing, the doped samples are obtained as nanocrystals from evaporation of the tetrahydrofuran solutions, which allows for efficient triplet-triplet energy transfer to the chiral analytes generated in situ from chiral amino acids. The results show that L-analytes induce strong RTP, whereas the unnatural D-analytes produce barely any afterglow. The method expands the scope of luminescence chiral sensing by lessening the requirement for specific molecular structures.

Amino Acid-Based Molecular and Membranous Chiral Tools for Enantiomeric Recognition

Given the need, both academic and industrial, for new approaches and technologies for chiral discrimination of enantiomers, the present work demonstrates the development through rational design and integration of two new chiral platforms (molecular and membranous) for enantioselective recognition through visual as well as microscopic observation. The molecular platform (TPT) is based on the tryptophan derivative developed through the condensation of two tryptophan units with terepthaloyl chloride. While TPT based on l-tryptophan recognizes R-mandelic acid over the S-isomer, the host with reverse chirality (TPDT) recognizes S-mandelic acid over R-isomer. The role of chemical functionality in this sensitive recognition process was established experimentally by developing an analogue of TPT and by judiciously using different chiral analytes. Importantly, a detailed theoretical study at the molecular level revealed the U-shaped conformation of TPT, creating a cavity for accommodating a chiral guest with selective functional interaction resulting in the discrimination of enantiomers. Finally, a chiral polymeric mat of poly(methyl methacrylate) (PMMA)/polyacrylonitrile (PAN) (2:3) impregnated with TPT was developed via electrospinning. The resulting fibrous mat was successfully utilized for chiral recognition through microscopic and architectural observation. Hence, the present work reports simple chiral tools for enantiomeric recognition.

Enantioselective Fluorescence Recognition of Free α-Amino Acids by Ion-Type Ammonium Salt-Based Sensors

Optically pure amino acids have extensive applications in pharmaceuticals, pesticides, food, materials, and other fields. Enantiomers recognition of chiral amino acids using optical methods with synthetic chiral sensors has attracted extensive attention. Most reported sensors typically identify guests by covalent or hydrogen bonding or hydrophobic interaction with amino acids and their derivatives. In this paper, a series of ion-type quaternary ammonium salt-based enantioselective fluorescent sensors were synthesized for chiral recognition of free α-amino acids via electrostatic interaction. The fluorescence intensity ratios ID/IL (ID, IL, fluorescence intensity of sensor when treated with D- or L-amino acid) were up to 2.1 and enantioselective fluorescence enhancement ratios ef (ef=[IL-I0]/[ID-I0] or [ID-I0]/[IL-I0]. (I0, fluorescence intensity of the sensor)) were up to 5.0. Among them, sensor 3 showed best enantioselective recognition performance toward tryptophan (Trp), and L-Trp significantly quenched the fluorescence of sensor 3, but D-Trp greatly enhanced the fluorescence of sensor 3, its ID/IL was 2.11 and ef was 1.8. The mechanistic investigation by NMR spectrum revealed that a tight three-point interaction, including electrostatic interaction, hydrogen bond, and π-π stacking, between sensor 3 and D-Trp was formed.

Effective enantioselective recognition by steady-state fluorescence spectroscopy: Towards a paradigm shift to optical sensors with unusual chemical architecture

A series of amino acid-derived 1,2,3-triazoles presenting the amino acid residue and the benzazole fluorophore connected by a triazole-4-carboxylate spacer was studied for enantioselective recognition using only steady-state fluorescence spectroscopy in solution. In this investigation, the optical sensing was performed with D-(-) and L-(+)-Arabinose and (R)-(-) and (S)-(+)-Mandelic acid as chiral analytes. The optical sensors showed specific interactions with each pair of enantiomers, allowing photophysical responses, which were used for their enantioselective recognition. DFT calculations confirm the specific interaction between the fluorophores and the analytes corroborating the observed high enantioselectivity of these compounds with the studied enantiomers. Finally, this study investigated nontrivial sensors for chiral molecules by a mechanism different than turn-on fluorescence and has the potential to broad chiral compounds with fluorophoric units as optical sensors for enantioselective sensing.

Aggregation-induced emission of a bis(imino)acenaphthene zinc complex with tetraphenylethene units

Using bis(imino)acenaphthene (BIAN) zinc(II) and palladium(II) complexes with tetraphenylethene (TPE) units as bulky aryl groups, Zn-2 and Pd-2 have been designed and developed, and their photophysical properties in solution and in the solid state have been investigated. Both in solution and in the solid state Zn-2 and Pd-2 show two photoabsorption bands in the ranges of 300 nm to 350 nm and 450 nm to 600 nm, which are assigned to the π-π* transition originating from both the TPE units and naphthalene units and the intraligand charge transfer (ILCT) between the TPE units and the BIAN unit, respectively. Density functional theory (DFT) calculations demonstrated that for Zn-2 the highest occupied molecular orbitals (HOMO) are localized on the TPE units, while the lowest unoccupied molecular orbitals (LUMO) are localized on the BIAN unit, leading to the appearance of a photoabsorption band on the ILCT. The emission from Zn-2 was quenched in solution, but appeared as phosphorescence at around 600 nm by photoexcitation at the ILCT band in the solid state as well as in the aggregated state, which was formed by the addition of n-hexane as a poor solvent to the dichloromethane (DCM) solution. The aggregate formation of Zn-2 in the DCM/n-hexane (10 wt%/90 wt%) solution was confirmed by the Tyndall scattering and scanning electron microscopy (SEM) measurements, demonstrating the aggregation-induced emission (AIE) characteristics of Zn-2. On the other hand, Pd-2 was non-emissive in the solid state and in the aggregated state as well as in solution. Moreover, the DCM-inclusion complexes of Zn-2 and Pd-2 were obtained and their photophysical properties were investigated. It was found that the photoluminescence quantum yield (Φ_PL-solid) values of Zn-2 and Zn-2-DCM in the solid state are less than 1%. Single-crystal X-ray structural analysis of Zn-2-DCM revealed the absence of intermolecular π-π interactions. Consequently, it was suggested that the low Φ_PL-solid value of Zn-2 is mainly due to the radiationless relaxation of the excitons by dynamic rotation of the phenyl groups of the TPE units, even in the solid state and in the aggregation state.

Challenges and opportunities for chiral covalent organic frameworks

As highly versatile crystalline porous materials, covalent organic frameworks (COFs) have emerged as an ideal platform for developing novel functional materials, attributed to their precise tunability of structure and functionality. Introducing chiral functional units into frameworks produces chiral COFs (CCOFs) with chiral superiorities through chirality conservation and conversion processes. This review summarises recent research progress in CCOFs, including synthetic methods, chiroptical characterisations, and their applications in asymmetric catalysis, chiral separation, and enantioselective recognition and sensing. Challenges and limitations are discussed to uncover future opportunities in CCOF research.

Self-Reporting Activated Ester-Amine Reaction for Enantioselective Multi-Channel Visual Detection of Chiral Amines

Chiral recognition is of importance not only in living systems but also in estimating the optical purity of enantiomeric drugs and fabricating advanced materials. Herein we report a novel self-reporting activated ester-amine reaction that can provide multi-channel visual detection of organic amines. It relies on the reaction extent dependent cis-transoid to cis-cisoid helical transition of the polyphenylacetylene backbone and the thus triggered fluorescence. Owing to the high selectivity, this visual process can recognize structurally diverse achiral amines and quantitatively check the impurity content. It also shows an outstanding enantioselectivity towards various chiral amines and can be applied to determine enantiomeric composition. The multiple responses in absorption, circular dichroism, photoluminescence, and circularly polarized luminescence make the helical transition of the polymer backbone a potential detection mode for high-throughput screening of chiral chemicals.

ESIPT-Active 8-Hydroxyquinoline-Based Fluorescence Sensor for Zn(II) Detection and Aggregation-Induced Emission of the Zn(II) Complex

A D-π-A type quinoline derivative, 2-(((4-(1, 2, 2-triphenylvinyl)phenyl)imino)methyl)quinolin-8-ol (HL), was synthesized and structurally characterized. The five-membered ring formed by the O-H···N hydrogen bond in HL contributed to the excited-state intramolecular proton transfer (ESIPT) behavior of HL, which was further verified by theoretical computations. Upon coordination with Zn²⁺, the hydroxyl proton in HL was removed, resulting in the inhibition of ESIPT. In the meanwhile, the formed Zn ₂ L ₄ complex displayed aggregation-induced emission (AIE) character in THF/H₂O mixtures, which is conducive to the fluorescence enhancement in aqueous media. Structure analysis suggested that the origin of the AIE characteristic was attributed to restriction of intramolecular rotations along with the formation of J-aggregates. Based on ESIPT coupled with AIE, HL could recognize Zn(II) in aqueous media via an orange fluorescence turn-on mode. Benefitting from the AIE property, chemosensor HL was successfully applied to fabricate test strips for rapid sensing of Zn(II) ions.

Visualization of Enantiorecognition and Resolution by Chiral AIEgens

Enantioselective recognition and separation have attracted much attention in pharmaceutical analysis, food chemistry, and life science. Herein, we propose an efficient strategy to achieve such purposes using optically active luminogens with aggregation-induced emission (AIE) characteristics. These AIE luminogens (AIEgens) show strong enantiomeric discrimination for 12 kinds of chiral acids and unprotected amino acids. In particular, an exceptionally high enantioselectivity for d/l-Boc-glutamic acid was observed, as demonstrated by the large difference between the formed AIEgen/acid complexes. Due to the AIE effect, enantioselective separation was achieved by aggregation of the AIEgens with one enantiomer in the mixed acid solution. Through analysis of the fluorescence standard curve, the aggregates of AIEgen/chiral acid possessed 90% d-analyte, from which the enantiomeric excess (ee) value was assessed to be 80% ee. Such a result is in good agreement with that (91% d-analyte and 82% ee) by chiral HPLC analysis. Thus, this simple one-step aggregation method can serve as a preliminary screening tool for high-throughput analysis or separation of chiral chemicals.

Chiral gelators for visual enantiomeric recognition

Introduction of chirality in supramolecular gels has allowed the effective translation and amplification of molecular chirality. Upon integrating the stimuli-responsive nature of these gels with supramolecular chirality, a new platform for the discrimination of the enantiomeric guests through naked eye can be developed. Over the past decade, several groups have reported the development of chiral supramolecular gels for enantioselective recognition through gel formation or collapse. However, to the best of our knowledge, we are yet to come across a review highlighting the utilization of chiral supramolecular gels for macroscopic discrimination of enantiomers. In this article, we have articulated the chiral gelators developed to date for the recognition of different enantiomeric analytes focusing on their mode of recognition with an in-depth analysis of the mechanism of interactions assisting the recognition process.

Enantioselective determination of chiral acids and amino acids by chiral receptors with aggregation-induced emissions

The chiral AIEgens showed satisfying enantiomer discrimination not only for amino acids but also for chiral acids.

Enantioselective recognition of chiral acids by supramolecular interactions with chiral AIEgens

Novel chiral AIEgens bearing optically pure amino groups were synthesized and showed excellent discrimination for a series of chiral acidic compounds and amino acids. Interestingly, after supramolecular assembly with 4-sulfocalix[4]arene, the obtained complexes showed enhanced enantioselectivity for chiral acids.

A chiral spirobifluorene-based bis(salen) zinc(ii) receptor towards highly enantioselective binding of chiral carboxylates

We have designed a new chiral receptor based on two salen zinc(ii) complex units connected with a spirobifluorene framework. The chiral receptor is proven to enantioselectively bind chiral carboxylate guests and the differences between the binding constants of enantiomeric guests were up to more than one order of magnitude.

Reversible Chromatic Change of Supramolecular Gels for Visual and Selective Chiral Recognition of Histidine

In the present work, a chemical reaction has been performed for supramolecular gels to achieve multiple-stimuli-responsive smart soft materials. Simple chiral binaphthalene-based receptors can condense with histidine (His) to yield a Schiff base, which would help to achieve visual chiral recognition of unprotected l/d-His through gel formation along with specific selectivity toward 20 amino acids. Through intermolecular hydrogen bonds, the resultant Schiff base molecules assemble with excess His molecules to form three-dimensional (3D) networks of metastable cross-linked nanospheres and stable nanofibers in EtOH/water and MeOH/water, respectively. Significantly, this condensation reaction exhibits unique reversible and chromatic phenomena between sol-gel phase transitions in EtOH/water, which provide a way to design chemical reaction-based multivisual-change supramolecular gels for sensing and switching applications.

Enantioselective Fluorescent Recognition of Free Amino Acids: Challenges and Opportunities

Fluorescent probes that can discriminate enantiomers of amino acids in organic media or aqueous solution are discussed. This Minireview focuses on recent progress in the studies of three classes of probes including those made of cyclodextrins, 1,1'-binaphthyl compounds, and nanomaterials, and uses them to illustrate the design strategies, applications, and limitations in this area. These probes are potentially useful for rapid analysis of asymmetric reactions for amino acid synthesis as well as the real-time imaging of amino acids in biological systems. The challenges in these applications are analyzed. Working in this field of enantioselective fluorescent recognition of amino acids offers great opportunities to make new scientific discoveries and to develop important practical applications.

Chiral recognition and enantiomer excess determination based on emission wavelength change of AIEgen rotor

Chiral recognition, such as enantioselective interactions of enzyme with chiral agents, is one of the most important issues in the natural world. But artificial chiral receptors are much less efficient than natural ones. For tackling the chiral recognition and enantiomer excess (ee) analysis, up until now all the fluorescent receptors have been developed based on fluorescence intensity changes. Here we report that the chiral recognition of a large number of chiral carboxylic acids, including chiral agrochemicals 2,4-D, is carried out based on fluorescent colour changes rather than intensity changes of AIEgen rotors. Moreover, the fluorescence wavelength of the AIEgen rotor linearly changes with ee of the carboxylic acid, enabling the ee to be accurately measured with average absolute errors (AAE) of less than 2.8%. Theoretical calculation demonstrates that the wavelength change is ascribed to the rotation of the AIEgen rotor upon interaction with different enantiomers.

Artificial receptors for chiral recognition are important in enantiomer excess analysis but current artificial detectors are based on fluorescence intensity changes only. Here the authors propose a different detection mechanism based on change of the fluorescence emission wavelength of an AIEgen rotor.

Spontaneous chiral self-assembly of achiral AIEgens into AIEgen-silica hybrid nanotubes

Antipodal chiral AIEgen-silica hybrid mesostructures were fabricated by spontaneous chiral self-assembly of the achiral amphiphilic AIEgen 2-[4-(1,2,2-triphenylethenyl)phenoxy]-acetic acid via a co-structure directing route in the absence of any symmetry-breaking agent. The produced AIEgen-silica hybrid mesostructured nanotubes showed chiral features with optical activity originating from J-aggregation with an excess of one stereochemical hand of the helical conformation of the AIEgen.

Synthesis of novel chiral fluorescent sensors and their application in enantioselective discrimination of chiral carboxylic acids

Novel chiral fluorescent sensors are synthesized from a dibromide containing a tetraphenylethylene moiety and enantiomerically pure amino alcohols and an amine. The sensors are applied for the chiral recognition of a wide range of chiral carboxylic acids and related derivatives.

Chiral Receptors for Lysine Based on Covalently Linked Bis- and Tris-binaphthylphosphoric Acids

The synthesis and application of three chiral receptors based on the covalent linkage of 1,1'-binaphthylphosphoric acids is reported. The binding of the lysine enantiomers to the chiral receptors was investigated by DOSY-NMR and NMR titrations, revealing that the bisphosphoric acid 1d acts as a highly stereoselective receptor for binding of d-lysine.

Cross-Coupling Approach to an Array of Macrocyclic Receptors Functioning as Chiral Solvating Agents

Chiral macrocyclic receptors 1 with multiple hydrogen-bonding sites in the cavity were synthesized and used as NMR chiral solvating agents (CSAs). The Suzuki-Miyaura cross-coupling reaction gave rapid access to a series of variants 1b-p of unsubstituted parent compound 1a. Among them, 1d with the 4-cyanophenyl group at the 3,3'-positions of the binaphthyl moiety was the most excellent CSA for a benchmark analyte compound, 2-chloropropionic acid (CPA); both of the quartet and doublet signals of CPA were split most completely in CDCl₃. Binding constants ( K_a) determined in CDCl₃ by NMR titrations indicated that ( R)-1d was the most enantioselective ( K_a( S)/ K_a( R) = 5.4). Interestingly, the K_a value of ( R)-1d for ( S)-CPA (5900) was greater than that of ( R)-1a for ( S)-CPA (3080), which strongly suggests an attractive interaction between the 4-cyanophenyl group of ( R)-1d and ( S)-CPA. The X-ray crystal structure of 1d indicates that one of the two H atoms meta to the cyano group is directed toward the cavity. DFT calculations suggested that this H atom of the 4-cyanophenyl group of ( R)-1d forms a weak hydrogen bond with the Cl atom of ( S)-CPA (C-H···Cl-C hydrogen bond).

Visual and Colorimetric High-Throughput Analysis of Chiral Carboxylic Acids Based on Enantioselective Charge Shielding of Gold Nanoparticles

Because chiral carboxylic acids (CCAs) are a class of important biological molecules and common functional moieties found in pharmaceutical molecules, the chiral analysis of CCAs has received much attention. Herein, we developed a simple, rapid, and cost-effective method for visual and colorimetric high-throughput analysis of CCAs using chiral di-imine structure-modified gold nanoparticles (C-AuNPs) as the probe. The C-AuNPs are positively charged in the presence of zinc ion, and they can be enantioselectively shielded by the negatively charged CCA enantiomers. Therefore, upon the addition of different concentrations and enantiomeric excess (ee) of CCAs, the C-AuNP-based sensor shows the different levels of aggregation along with the visual changes in solution color, which can achieve simultaneous analysis of the concentration and ee of CCAs. The chiral recognition mechanism based on C-AuNPs was investigated by the determination of binding constants ( K) and molecular simulation methods. Our approach is expected to have the wide-ranging applications in the developing region for enantio-sensing of various chiral drugs and biomolecules.

A new aggregation-induced emission fluorescent probe for rapid detection of nitroreductase and its application in living cells

The biological activity of nitroreductase (NTR) is closely related to biological hypoxia status in organisms. The development of effective methods for monitoring the activity of NTR is of great significance for medical diagnosis and tumor research. Toward this goal, we have developed a new aggregation-induced emission (AIE) fluorescence NTR probe TPE-HY used the tetraphenylethene as the fluorophore, and used the nitro group as the NTR recognition site. The probe TPE-HY has many excellent properties, including rapid response, AIE characteristics, high sensitivity and selectivity, and low cytotoxicity. Importantly, the probe TPE-HY is successfully applied to monitor endogenous NTR in living HeLa cells.

A photo-stable fluorescent chiral thiourea probe for enantioselective discrimination of chiral guests

Herein, a chiral thiourea Schiff base derived from (1R,2R)-1,2-cyclohexanediamine and tetraphenylethylene (TPE) was applied as a highly effective chiral sensor for the enantioselective discrimination of various acids and aminesviaion-pair and hydrogen-bond interaction.

A fluorescent cross-linked supramolecular network formed by orthogonal metal-coordination and host–guest interactions for multiple ratiometric sensing

A supramolecular network can be used for the ratiometric sensing of pH, cyclen and Cl⁻ due to the incorporation of two fluorophores and two non-covalent interactions.

Enantioselective recognition of carboxylic acids by novel fluorescent triazine-based thiazoles

Hydrogen bonding and π-π interactions take special part in the enantioselectivity task. In this regard, because of having both hydrogen acceptor and hydrogen donor groups, melamine derivatives become more of an issue for enantioselectivity. In the light of such information, triazine-based chiral, fluorescence active novel thiazole derivatives L1 and L2 were designed and synthesized from (S)-(-)-2-amino-1-butanol and (1S,2R)-(+)-2-amino-1,2-diphenylethanol. The structural establishment of these compounds was made by spectroscopic methods such as FTIR, ¹ H, and ¹³ C NMR. While the solution of these compounds in DMSO did not show any fluorescence emission, it was observed that the emission increased 44-fold for L1 and 55-fold for L2 in 95% water, similar to the aggregation-induced emission (AIE) characterized compounds. In this regard, enantioselective capabilities of these compounds against carboxylic acids were tested, and in experiments carried out at a ratio of 40/60 DMSO/H₂ O, it was determined that R-2ClMA increased the fluorescence emission of L1 chiral receptor by 2.59 times compared to S-isomer.

Sui Generis Helicene-Based Supramolecular Chirogenic System: Enantioselective Sensing, Solvent Control, and Application in Chiral Group Transfer Reaction

A novel dioxa[6]helicene-based supramolecular chirogenic system (1) as a specific chiral recognition host for enantiopure trans-1,2-cyclohexanediamine (2) is reported. Host 1 with an inherent free phenolic group and a (1S)-camphanate chiral handle on the opposite terminal rings of the helicene chromophore acted as an efficient turn on fluorescent sensor for S,S-2 with an excellent enantioselective factor, α = K SS /K RR = 6.3 in benzene. This specific host-guest interaction phenomenon is found to be solvent-dependent, which leads to an enantioselective chiral (camphanate) group transfer to the diamine guest molecule. In the case of R,R-2, the de value is up to 68% even at room temperature. Intriguingly, the induced helicity in dioxa[6]helicene diol 6, upon supramolecular hydrogen-bonding interactions, is of opposite sense with positive helicity for S,S-2 and negative helicity for R,R-2, as shown by circular dichroism spectroscopy and in combination with theoretical calculations. This chiral supramolecular system is found to be an excellent host-guest pair for enantiomeric recognition of 2, based on their electronic and steric factors.

Fluorescent chirality recognition by simple boronate ensembles with aggregation-induced emission capability

Chiral boronate ensembles showed enantioselective aggregation behaviors for chiral diamines and cinchona alkaloids, enabling the fluorescent recognition of their chirality.

Chiral and non-conjugated fluorescent salen ligands: AIE, anion probes, chiral recognition of unprotected amino acids, and cell imaging applications

Chiral and non-conjugated fluorescent salen ligands with a cyclohexane/1,2-diphenylethane bridge have small π-conjugated systems but exhibit strong aggregation/anion/amino acid-induced emission.

Reversal aggregation-induced circular dichroism from axial chirality transfer via self-assembled helical nanowires

Two chiral binaphthyl-based enantiomers, (R/S)-7, can produce gradual reversal AICD signals from solution to aggregation, which can be attributed to axial chirality transfer to self-assembled helical nanowires in aggregation state.