Conjugated polymer-enhanced enantioselectivity in fluorescent sensing

Regioselective Substitution of BINOL

1,1'-Bi-2-naphthol (BINOL) has been extensively used as the chirality source in the fields of molecular recognition, asymmetric synthesis, and materials science. The direct electrophilic substitution at the aromatic rings of the optically active BINOL has been developed as one of the most convenient strategies to structurally modify BINOL for diverse applications. High regioselectivity has been achieved for the reaction of BINOL with electrophiles. Depending upon the reaction conditions and substitution patterns, various functional groups can be introduced to the specific positions, such as the 6-, 5-, 4-, and 3-positions, of BINOL. Ortho-lithiation at the 3-position directed by the functional groups at the 2-position of BINOL have been extensively used to prepare the 3- and 3,3'-substituted BINOLs. The use of transition metal-catalyzed C-H activation has also been explored to functionalize BINOL at the 3-, 4-, 5-, 6-, and 7-positions. These regioselective substitutions of BINOL have allowed the construction of tremendous amount of BINOL derivatives with fascinating structures and properties as reviewed in this article. Examples for the applications of the optically active BINOLs with varying substitutions in asymmetric catalysis, molecular recognition, chiral sensing and materials are also provided.

Enantioselective and Chemoselective Optical Detection of Chiral Organic Compounds without Resorting to Chromatography

Enantiorecognition and resolution are of essential importance in many diverse areas of science. Whenever there arises a need to analyze/investigate enantiomers in different situations chromatography stands up in our minds immediately. Nevertheless, chemoselective and enantioselective recognition/discrimination (without going for separation) constitutes a different perception and requirement. The techniques using chiroptical sensing cause detection based on molecular interactions induced in different manners. Enantioselective sensing of monosaccharides in γ-cyclodextrin assembly and by diboronic acid based fluorescent sensors, application of bi-naphthol and H8 BINOL based sensors and dendrimers, metal-to-ligand charge transfer transitions in CD, exciton-coupled circular dichroism, surface enhanced Raman spectroscopy, and enantioselective indicator displacement sensor arrays for enantioselective recognition/detection of chiral organic compounds, such as amines, amino acids/alcohols, and hydroxycarboxylic acids have been discussed in progressive manner with mechanistic explanations, wherever available. Besides, the chiroptical vs LC approach has been discussed. The present paper is focused on certain different non-chromatographic optical techniques and aims to extend an understanding and a view to consider such techniques which have been successful in selective detection, and determination of absolute configuration and enantiomeric excess, (without resorting to separation vis-à-vis LC) and that have potential use in high-throughput chiral assay and combinatorial search for asymmetric catalysts and reagents.

Chemoselective and Enantioselective Fluorescent Identification of Specific Amino Acid Enantiomers

The enantiomers of chiral amino acids play versatile roles in biological systems including humans. They are also very useful in the asymmetric synthesis of diverse chiral organic compounds. Therefore, identification...

Fluorescent and Colorimetric Dual-signal Enantiomers Recognition via Enzyme Catalysis: The Case of Glucose Enantiomers Using Nitrogen-doped Silicon Quantum Dots/Silver Probe Coupled with β-D-Glucose Oxidase

Chiral enantiomer recognition is important but facing tough challenges in the direct quantitative determination for complex samples. In this work, via chosing nitrogen-doped silicon quantum dots (N-SiQD) as optical nanoprobe and constructing N-SiQD/silver (N-SiQD/Ag NPs) complex, β-D-GOx as model enzyme and glucose enantiomers as analytes, a fluorescent and colorimetric dual-signal chiral sensing strategy was proposed herein for chiral recognition based on specific enzyme-catalyzed reaction. N-SiQD can exhibit intense fluorescence, while it can be quenched by Ag NPs owing to the formation of N-SiQD/Ag NPs. In the presence of glucose isomer, D-glucose is catalytically hydrolyzed by β-D-GOx to form H₂O₂ owing to the specific enzyme catalyzed reaction between D-glucose and β-D-GOx, and H₂O₂ can etch Ag NPs from the N-SiQD/Ag NPs probe to change the solution color from brown to colorless and restore the N-SiQD fluorescence; while these phenomena cannot be caused by L-glucose, a dual-signal sensing method was thus constructed for recognizing glucose enantiomers. It is believed that the chiral enantiomers recognition strategy via enzyme catalysis has great application for selective and quantificational detection of enantiomers in the complex sample system.

Crystalline C-C and C═C Bond-Linked Chiral Covalent Organic Frameworks

While crystalline covalent organic frameworks (COFs) linked by C-C bonds are highly desired in synthetic chemistry, it remains a formidable challenge to synthesize. Efforts to generate C-C single bonds in COFs via de novo synthesis usually afford amorphous structures rather than crystalline phases. We demonstrate here that C-C single bond-based COFs can be prepared by direct reduction of C═C bond-linked frameworks via crystal-to-crystal transformation. By Knoevenagel polycondensation of chiral tetrabenzaldehyde of dibinaphthyl-22-crown-6 with 1,4-phenylenediacetonitrile or 4,4'-biphenyldiacetonitrile, two olefin-linked chiral COFs with 2D layered tetragonal structure are prepared. Reduction of olefin linkages of the as-prepared CCOFs produces two C-C single bond linked frameworks, which retains high crystallinity and porosity as well as high chemical stability in both strong acids and bases. The quantitative reduction is confirmed by Fourier transform infrared and cross-polarization magic angle spinning ¹³C NMR spectroscopy. Compared to the pristine structures, the reduced CCOFs display blue-shifted emission with enhanced quantum yields and fluorescence lifetimes, while the parent CCOFs exhibit higher enantioselectivity than the reduced analogs when be used as fluorescent sensors to detect chiral amino alcohols via supramolecular interactions with the built-in crown ether moieties. This work provides an attractive strategy for making chemically stable functionalized COFs with new linkages that are otherwise hard to produce.

In situ self-assembly of conjugated polyelectrolytes for cancer targeted imaging and photodynamic therapy

The construction of intelligent self-assembly systems with cancer targeting photodynamic therapy abilities is highly required for increasing the precise therapeutic efficiency in clinical treatment. Herein, a cationic water soluble conjugated polymer (PFT-SH) functionalized with thiol groups was designed and synthesized via a palladium-catalyzed Suzuki coupling reaction. Firstly, PFT-SH can enter cells and form loose aggregations by hydrophobic and π-π stacking interactions. Secondly, a high level of H₂O₂ in cancer cells oxidizes sulfhydryl groups to disulfide bonds and then forms more and larger aggregations. Finally, PFT-SH showed remarkable ROS producing ability under white light irradiation with 78% quantum yields (Φ_Δ). Due to this unique self-aggregation property, PFT-SH was successfully used to achieve in situ self-assembly specifically inside cancer cells for targeted imaging. Both the specific aggregation of PFT-SH in cancer cells and its ROS producing ability led to its use in the targeted killing of cancer cells through efficient photodynamic therapy.

Accurate chiral pattern recognition for amines from just a single chemosensor

The current work proposes a novel method for accurate pattern recognition of (mono- and di-) amines and determination of enantiomeric excess (ee) using molecular self-assembly.

Creation of Centimeter-Sized 2 D Crystalline Film by Crystallization of Homopolymer in Solution

The 2 D assembly of polymers to form free-standing and large crystalline films is quite appealing but very challenging. Although there have been some works using interface templates, reports of in situ assembly in solution are still rare. Herein, a simple strategy is developed for the creation of a free-standing and centimeter-sized 2 D crystalline polymer film through crystallization of an amphiphilic brush polydiacetylene (PDA) in solution. The film exhibits good shape memory, a low dielectric constant, and good carrier mobility. This strategy may be applied extensively to produce a variety of other macroscopic 2 D crystalline polymer films for applications in electronics, catalysis, and so on.

Optical Analysis of Reaction Yield and Enantiomeric Excess: A New Paradigm Ready for Prime Time

This Perspective highlights the advances of optical methods for asymmetric reaction discovery. Optical analysis allows for the determination of absolute configuration, enantiomeric excess and reaction yield that is amenable to high-throughput experimentation. Thus, the synthetic organic community is encouraged to incorporate the methods discussed to expedite the development of high-yielding, enantioselective transformations.

Synthesis, chiroptical properties, and self-assembled nanoparticles of chiral conjugated polymers based on optically stable helical aromatic esters

By Suzuki coupling reaction, three pairs of chiral conjugated polymers with optically stable helical aromatic ester subunits as the main-chain were designed and synthesized. Polymers (+)-P-P1 and (-)-M-P1, (+)-P-P2 and (-)-M-P2 showed strong fluorescence emission, strong mirror image CD and circularly polarized luminescence (CPL) signals in THF. For polymers (+)-P-P3 and (-)-M-P3, containing the tetraphenylethene (TPE) moiety, they not only showed obvious aggregation induced enhancement emission (AIEE), but also exhibited mirror image CD signals and aggregation-induced enhancement CPL signals in THF-water mixtures. Moreover, (+)-P-P3 and (-)-M-P3 could also form chiral nanoparticles by solvent evaporation induced self-assembly. Interestingly, it was further found that the size of the nanoparticles could be controlled by the changing of THF/water ratio, and their CPL properties were also shown.

Microscopic chiral pockets in a tris(chelated) iridium(iii) complex as sites for dynamic enantioselective quenching

A microscopic pocket surrounded by bulky ligands in Ir(iii) acted as a site discriminating the chirality of an approaching molecule.

Polymer Amplified Enantioselectivity in the Fluorescent Recognition of Prolinol

A 1,1'-bi-2-naphthol (BINOL)-aldehyde-based polymer has been synthesized that exhibits enantioselective fluorescent enhancement toward prolinol. It is found that the polymer shows greatly amplified enantioselectivity over the parent small-molecule sensor under the same conditions. This is attributed to the photoinduced electron transfer processes between the BINOL units in the polymer chain as well as the different steric environment provided by the polymer.

Simultaneous Determination of Concentration and Enantiomeric Composition in Fluorescent Sensing

Fluorescent sensors have found broad applications in determining the concentrations of diverse analytes by measuring specific sensor-analyte fluorescent responses. For a chiral substrate containing varying composition of two enantiomers, both the concentration and enantiomeric composition should greatly influence the fluorescent response of an enantioselective fluorescent sensor. Thus, multiple independent measurements are normally needed to determine both the concentration and enantiomeric composition of a chiral compound. In order to facilitate the application of the enantioselective fluorescent sensors, our laboratory has developed four strategies to simultaneously determine the concentration and enantiomeric composition of various chiral substrates by a single fluorescence measurement. A mixture of a chiral BINOL-based dialdehyde and an achiral compound salicylaldehyde in the presence of Zn²⁺ is used to interact with chiral diamines, amino alcohols, and amino acids. The fluorescence enhancement at λ₁ = 447 nm due to the achiral sensor is mostly determined by the concentration of the substrates, and the fluorescence enhancement at λ₂ > 500 nm due to the chiral sensor is highly enantioselective. A 3D graph combining the fluorescence intensities at λ₁ and λ₂ can be used to determine the enantiomeric composition. A chiral conjugated polymer containing the BINOL-dialdehyde units is shown to amplify the enantioselectivity of the small molecule sensor under the same conditions. Combination of the chiral polymer with salicylaldehyde allows simultaneous concentration and enantiomeric composition determination. In a pseudoenantiomeric sensor pair of the BINOL-based amino alcohols, one sensor shows greater fluorescence enhancement with one enantiomer of chiral α-hydroxy carboxylic acid at λ₁ = 374 nm and another sensor shows greater fluorescence enhancement with another enantiomer at λ₂ = 330 nm. Using a mixture of this sensor pair allows the determination of both concentration and enantiomeric composition with one fluorescence measurement. A BINOL-based trifluoromethyl ketone is found to exhibit dual emission responses toward a chiral diamine at λ₁ = 370 nm and λ₂ = 438 nm. The fluorescence enhancement at λ₁ is mostly determined by the substrate concentration and that at λ₂ is highly enantioselective. Thus, using one sensor with one measurement gives both parameters. A BINOL-naphthyl imine compound is designed to show two different fluorescent responses toward functional chiral amines in the presence of Zn²⁺. When the naphthylamine unit is displaced off the sensor by a chiral amine substrate via imine metathesis, the emission of naphthylamine is restored at λ₁ = 427 nm, which allows determination of the substrate concentration. The fluorescence enhancement at λ₂ > 500 nm due to the formation of the new chiral imine products is highly enantioselective. The work discussed here has provided convenient methods to obtain the two important parameters of a chiral molecule by a single fluorescence measurement. They should contribute to the development of analytical tools for the rapid assay of chiral compounds.

Fluorescence recognition of chiral amino alcohols by using a novel ionic liquid sensor

A novel task-specific ionic liquid derived from l-phenylalaninol was prepared as an enantioselective fluorescent sensor for the first time.

Photophysical Diversity of Water-Soluble Fluorescent Conjugated Polymers Induced by Surfactant Stabilizers for Rapid and Highly Selective Determination of 2,4,6-Trinitrotoluene Traces

The increasing application of fluorescence spectroscopy in development of reliable sensing platforms has triggered a lot of research interest for the synthesis of advanced fluorescent materials. Herein, we report a simple, low-cost strategy for the synthesis of a series of water-soluble conjugated polymer nanoparticles with diverse emission range using cationic (hexadecyltrimethylammonium bromide, CTAB), anionic (sodium dodecylbenzenesulfonate, SDBS), and nonionic (TX114) surfactants as the stabilizing agents. The role of surfactant type on the photophisical and sensing properties of resultant polymers has been investigated using dynamic light scattering (DLS), FT-IR, UV-vis, fluorescence, and energy dispersive X-ray (EDS) spectroscopies. The results show that the surface polarity, size, and spectroscopic and sensing properties of conjugated polymers could be well controlled by the proper selection of the stabilizer type. The fluorescent conjugated polymers exhibited fluorescence quenching toward nitroaromatic compounds. Further studies on the fluorescence properties of conjugated polymers revealed that the emission of the SDBS stabilized polymer, N-methylpolypyrrole-SDBS (NMPPY-SDBS), is strongly quenched by 2,4,6-trinitrotoluene molecule with a large Stern -Volmer constant of 59 526 M(-1) and an excellent detection limit of 100 nM. UV-vis and cyclic voltammetry measurements unveiled that fluorescence quenching occurs through a charge transfer mechanism between electron rich NMPPY-SDBS and electron deficient 2,4,6-trinitrotoluene molecules. Finally, the as-prepared conjugated polymer and approach were successfully applied to the determination of 2,4,6-trinitrotoluene in real water samples.