INTRODUCTION
Enantioselective recognition is essential in asymmetric synthesis, pharmaceutical, agrochemical, and food industries.[] To date, some classical methods have been employed to analyze chiral compounds, such as nuclear magnetic resonance (NMR),[] UV-Vis spectra,[] high-performance liquid chromatography (HPLC),[] circular dichroism (CD),[] fluorescence spectroscopy.[] Among them, fluorescence technique has drawn large attention in enantiorecognition thanks to its high sensitivity, low-cost, and easy operation.[] Until now, many chiral fluorescent probes have been developed to distinguish enantiomers, including covalent organic frameworks,[] metal−organic frameworks (MOFs),[] small organic molecules,[] and chiral polymers.[] While, the most existed sensors are often applied to chiral detection but rarely used for enantioseparation.[]
Over the past decades, tremendous efforts have been made to develop optically pure molecules mainly through asymmetric synthesis and enantioselective separation. Although asymmetric synthesis has made a significant progress,[] most of the commercially chiral compounds are still obtained by enantioseparation due to its advantages of easy operation, inexpensive, and high reliability for mass production. Now, several enantioseparation tactics were developed, for example, stereoselective crystallization,[] enzyme controlled separation,[] and enantiomer-selective-magnetization,[] selective aggregation,[] chromatography.[] Undoubtedly, selective crystallization is one of the most inexpensive and convenient technique to realize mass production.[] However, the separation efficiency always needs to be analyzed by HPLC. Here, if we use the fluorescence technique to visualize the resolution process by selective complexation with one enantiomer in the racemate, it will greatly simplify the operation process, reduce cost and increase the separation efficiency. Unfortunately, traditional fluorophores with planar structures often emitted strong fluorescence in the molecularly dissolved state, but faint emission in the condensed phase,[] which dramatically inhibited their real world application.
In 2001, Tang and coworkers found a kind of twisted fluorophore that is faintly emissive in the solution but fluoresces intensely in the aggregated state, this intriguing phenomenon was coined as aggregation-induced emission (AIE).[] Their photoluminescence (PL) behaviors can be explained by restriction of intramolecular motion.[] Tetraphenylethylene (TPE), as the most investigated AIEgen, was often utilized to build versatile materials for bioimaging, chemosensing, optoelectronic devices, stimuli-responsive materials, and chiral receptors.[] For example, Zheng and coworkers reported several chiral probes based on TPE that can recognize enantiomers with high enantioselectivity.[] Up to now, chiral AIEgens with excellent recognition performance and separation ability are rarely reported.
Here, we reported two TPE-based chiral probes bearing optically pure (1R, 2R)- or (1S, 2S)- diaminocyclohexane named as (1R, 2R)-TM or (1S, 2S)-TM, which presented excellent enantiomeric discrimination for 5 pairs of chiral acids. Especially, (1R, 2R)-TM can enantioselectively aggregate with dibenzoyl-l-tartaric acid from a pair of enantiomers to emit bright fluorescence accompanied by a PL intensity ratio (Il/Id) up to 281. Such big differences in PL and morphologies can be used to visualize the chiral separation process. Chiral HPLC analysis demonstrated that the precipitates were composed of 82% dibenzoyl-l-tartaric acid. Moreover, their sensing mechanism was investigated by NMR titration and 2D NOESY spectrum. Therefore, it is a promising strategy to enantioselectively separate optically pure chemicals by chiral AIEgens.
RESULTS AND DISCUSSION
Generally, chiral fluorescent probes were composed of fluorophore and chiral source. TPE was wildly utilized to build chemo/bio-sensors.[] It is anticipated that AIEgens decorated with (1R, 2R)/(1S, 2S)-(±)-cyclohexanediamine will endow the probes with the ability of enantioselective recognition of chiral acids through acid-base interaction. Then, chiral fluorescent probes (1R, 2R)/(1S, 2S)-TM were prepared by the reaction route as shown in Schemes . (1R, 2R)/(1S, 2S)-(±)-cyclohexanediamine and 4-bromobenzaldehyde were reacted to afford Schiff base 3 in THF. Then, the intermediate (1R, 2R)/(1S, 2S)-3 were reduced by sodium borohydride to give (1R, 2R)/(1S, 2S)-4 with good yields. Finally, the target molecules (1R, 2R)/(1S, 2S)-TM were obtained through Suzuki coupling reaction of (1R, 2R)/(1S, 2S)-4 interacted and 4,4,5,5-tetramethyl-2-(1,2,2-triphenylvinyl)-1,3,2-dioxaborolane 7. Their molecular structural characterizations were verified by NMR spectra and high resolution mass spectrometry (HRMS).
Photophysical properties
The absorption spectra of (1R, 2R)/(1S, 2S)-TM were measured in tetrahydrofuran (THF). The as-prepared chiral AIEgens showed a main absorption peak at 311 and 241 nm (Figure ), which come from intramolecular charge transfer and π-π transition, respectively. CD spectra were also recorded in THF. As Figure indicated, (1S, 2S)-TM presented three positive CD peaks at ∼317, ∼284, and ∼231 nm, while (1R, 2R)-TM exhibited negative ones with identical CD intensity. Their AIE behaviors were validated in the THF/water solution by PL spectroscopy. (1R, 2R)-TM showed weak fluorescence in THF. When adding water to THF, the fluorescence intensity had no obvious change from 0 to 80% water fraction (Figure ). Further increasing the water fraction, a sky-blue fluorescence was emerged at 477 nm. When raising the water fraction to 95%, the PL intensity was enhanced by 211-fold in comparison with the THF solution. (1S, 2S)-TM also showed similar behaviors with (1R, 2R)-TM in THF/water mixtures (Figure ). The fluorescence quantum yield (QY) of (1R, 2R)-TM was recorded in 0 and 95% water fraction is about 0% and 86%, respectively. These results indicate that chiral (1R, 2R)/(1S, 2S)-TM are typical AIEgens.
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Enantioselective recognition
Enantioselective recognition of chiral acids by (1R, 2R)-TM proceeded in the mixture of THF and water. As shown in Figure , Figure , and Table , (1R, 2R)-TM can selectively precipitate with one of enantiomers to induce different PL intensities. For example, (1R, 2R)-TM can enantioselectively interact with (−)-Dibenzoyl-l-tartaric acid to afford sediments with bright PL emission. While, under the same conditions, the complex of (1R, 2R)-TM and (+)-Dibenzoyl-d-tartaric acid gave a clear solution with very weak fluorescence. The PL intensity ratio of Il/Id was up to 281, declaring a good recognition for chiral Dibenzoyltartaric acid. As the Job's plot illustrated, (1R, 2R)-TM complexed in a 1:1 ratio with (−)-Dibenzoyl-l-tartaric acid (Figure ). Interestingly, with prolonging the standing time, the turbid solution of (1R, 2R)-TM and (−)-Dibenzoyl-l-tartaric acid formed flocculent precipitate with bright fluorescence. While, the mixture of (1R, 2R)-TM and (+)-Dibenzoyl-d-tartaric acid was still in the clear solution (Figure ). When the concentration of (1R, 2R)-TM and (−)-Dibenzoyl-l-tartaric acid were increased, an organic gel was easily generated. However, the mixture of (1R, 2R)-TM and (+)-Dibenzoyl-d-tartaric acid did not show significant change (Figure ). These results implied that (1R, 2R)-TM may be served as chiral selectors for enantioselective separation just through a simple filtration.
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As a control experiment, Di-p-toluoyl tartaric acid and tartaric acid were chosen as chiral analytes to study their enantioselective recognition behaviors by (1R, 2R)-TM. As illustrated in Figure , Figure , and Table , (1R, 2R)-TM exhibited good discrimination for Di-p-toluoyl tartaric acid. After complexation with l-Di-p-toluoyl tartaric acid, tiny aggregates were formed with brightly sky-blue fluorescence. In comparison, the mixture of (1R, 2R)-TM and d-Di-p-toluoyl tartaric acid only exhibited weak fluorescence in the solution. The fluorescence intensity ratio (Il/Id) was up to 23. While, for tartaric acid, there is no obvious fluorescence change upon interaction with (1R, 2R)-TM neither in THF solution nor in the mixed solvents, indicating that phenyl rings in Dibenzoyltartaric acid and Di-p-toluoyl tartaric acid played vital role in enantioselective recognition. Additionally, (1R, 2R)-TM can also respond to other chiral acids with good enantioselectivity, such as mandelic acid (Ir/Is = 2.7), 2-chloromandelic acid (Ir/Is = 2.1), malic acid (Il/Id = 2.0) (Figure and Table ). Here, it is worth noting that the fluorescence of (1R, 2R)-TM was significantly enhanced in the presence of (−)-Dibenzoyl-l-tartaric acid or l-Di-p-toluoyl tartaric acid. However, for mandelic acid, chloromandelic acid, malic acid, and tartaric acid, the fluorescence intensity obviously quenched compared to the free (1R, 2R)-TM after interaction. It is inferred that (1R, 2R)-TM could closely combine with (−)-Dibenzoyl-l-tartaric acid or l-Di-p-toluoyl tartaric acid with suitable configuration and molecular size to form compact complex, which will restrict the free motion of AIEgens to activate the radiative transition, thus emitting bright fluorescence. For other chiral acids in Figure , we think the small stereospatial structure cannot well match with (1R, 2R)-TM, thus forming a loose complex with weak emission. Moreover, for those acids, there may exist charge transfer between the AIEgen and acids to result in decreased emission. From our experience, the solvents also played important role in enantioselective recognition. Because different chiral analytes have various polarity and solubility, to achieve optimal enantioselectivity for analytes, it should be measured case by case to study the solvent proportions and concentration. Of course, the recognition could be tested in same solvent condition, but the fluorescence intensity ratio can't always reach the maximum in a single solvent system for all chiral analytes. Therefore, the solvent proportions and concentrations are different for corresponding analytes.
CD spectroscopy was then employed to investigate the interaction of chiral AIEgens and chiral acids. (±)-Dibenzoyl-d/l-tartaric acid, (±)-d/l-Di-p-toluoyl tartaric acid, R/S-(±)-Mandelic acid, R/S-(±)-2-Chloromandelic acid and (±)-d/l-Malic acid were tested in THF. As presented in Figure and Figure , the CD intensity changed remarkably after complexation with enantiomers in contrast to the CD spectra of (1R, 2R)-TM or (1S, 2S)-TM. Among them, chiral acids (±)-Dibenzoyl-d/l-tartaric acid, (±)-d/l-Di-p-toluoyl tartaric acid, R/S-(±)-Mandelic acid, R/S-(±)-2-Chloromandelic acid showed an enhanced CD intensity upon interaction with (1R, 2R)/(1S, 2S)-TM. For (±)-d/l-Malic acid, their complexes just enhanced slightly. The above results implied that (1R, 2R)/(1S, 2S)-TM can distinguish chiral acids by CD spectroscopy.
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Study of enantioselective recognition mechanism
To reveal the possible sensing mechanism, scanning electron microscopy (SEM), fluorescence microscope, and confocal microscope were used to study their morphologies change (Figure and Figure ). SEM images of (1R, 2R)-TM and the mixture of (1R, 2R)-TM and (+)-Dibenzoyl-d-tartaric acid were first collected in THF/water mixture. As shown in Figure , both (1R, 2R)-TM and the complex of (1R, 2R)-TM and (+)-Dibenzoyl-d-tartaric acid formed amorphous nanoparticles in the THF/water mixture (fw = 65.8%). While, upon complexation with (−)-Dibenzoyl-l-tartaric acid, the aggregates gave micron, even millimeter grade floccule suspension (Figure ). Inspired by the morphology difference, the concentration and standing time were further increased, a gel was facilely acquired with blue fluorescence (Figure ). From SEM images we can see that abundant blocky structures of various sizes were formed (Figure ). Based on above results, it is hypothesized that (−)-Dibenzoyl-l-tartaric acid can well match with (1R, 2R)-TM to form ordered assembles to further emit strong fluorescence due to the suitable spatial configuration and AIE effect.
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Then, 1H NMR titrations were carried out to disclose the chiral sensing mechanism in THF-d8. As illustrated in Figure , the resonance of protons of (1R, 2R)-TM and (−)-Dibenzoyl-l-tartaric acid were changed obviously. After addition of 0.2−1.8 equivalents of (−)-Dibenzoyl-l-tartaric acid to the solution of (1R, 2R)-TM (10 mM), the methyl proton (Ha), phenyl ring protons (Hb, and Hc) of (−)-Dibenzoyl-l-tartaric acid and proton Hd, Hf, and Hg of (1R, 2R)-TM presented an upfield shift of δ −0.071, −0.075, −0.074, −0.144, −0.080, and −0.170, respectively (Table and Figure ). While, the proton He, Hh, Hj, and Hk of (1R, 2R)-TM showed an evident downfield shift of δ 0.131, 0.285, 0.665, 0.052, and 0.099, respectively. Chemical shifts of (1R, 2R)-TM and (+)-Dibenzoyl-d-tartaric acid had a similar tendency as the solution of (1R, 2R)-TM and (−)-Dibenzoyl-l-tartaric acid (Table and Figure ). According to the NMR titration results, it was inferred that the amino groups of (1R, 2R)-TM can easily interact with carboxyl groups of (−)-Dibenzoyl-l-tartaric acid to generate corresponding ionic compounds through acid-base interaction. The obtained −NH2+ group can efficiently decrease the electron density of neighboring protons to cause an obvious downfield shift in NMR spectra. Inversely, an upfield shift of NMR was recorded because of the enhanced shielding effect of electron-rich acid ions (−COO−). Besides, after interaction of (1R, 2R)-TM and (−)-Dibenzoyl-l-tartaric acid, the proton Hh and Hk exhibited much more upfield shift than that of (+)-Dibenzoyl-d-tartaric acid. It is implied that hydrogen bonds were formed between the benzoyl groups of (−)-Dibenzoyl-l-tartaric acid, and Hh of (1R, 2R)-TM. 1H NMR titration also suggested that (1R, 2R)-TM complexed with (−)-Dibenzoyl-l-tartaric acid in a 1:1 molar ratio as well (Figure ), this was consistent with the PL titration (Figure ). Moreover, 2D NOESY NMR spectrum of the mixture of (1R, 2R)-TM and (−)-Dibenzoyl-l-tartaric acid was collected in THF-d8 (Figures and ). On account of the suitable spatial configuration and acid–base interaction between amino and carboxyl groups, intermolecular NOE signals of Ha−Hi, Ha−Hk between (1R, 2R)-TM and (−)-Dibenzoyl-l-tartaric acid were observed, which indicated that carboxylic groups are close to amino groups of (1R, 2R)-TM. While, for the mixture of (1R, 2R)-TM and (+)-Dibenzoyl-d-tartaric acid, intermolecular NOE signals cannot be detected, which implied a weak interaction between (1R, 2R)-TM and (+)-Dibenzoyl-d-tartaric acid (Figures and ).
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On the basis of the 1H NMR titrations and 2D NOESY NMR spectra of (1R, 2R)-TM and (−)-Dibenzoyl-l-tartaric acid, it is found that the acid–base interaction of amino and carboxyl groups played vital role in the recognition process. To evaluate the effect of pH on chiral recognition, fluorescence spectra were collected with different equivalents of hydrochloric acid (0.5 to 1000 equiv.) under the same test conditions. As illustrated in Figure , (1R, 2R)-TM emitted weak fluorescence in THF/water mixture (fw = 65.8%). When adding the hydrochloric acid to the solution for 0.5 h or overnight, the fluorescence intensity showed a minor change in contrast to Figure . Thus, we think the pH has a minor effect on the emission of (1R, 2R)-TM.
Therefore, we speculated the mechanism for enantioselective recognition is that amino groups of (1R, 2R)/(1S, 2S)-TM can afford efficiently charge-aided hydrogen bonding interaction with the carboxyl group of chiral acid. Due to the opposite configuration of enantiomers, (1R, 2R)-TM could closely combine with one enantiomer with suitable configuration to form compact complex, which will restrict the free motion of AIEgens to activate the radiative transition, thus resulting in bright emission. For the other acidic enantiomers, it possessed oppositely stereospatial structure and showed low spatial matching to (1R, 2R)-TM, and forming loose complex with weak or no emission. To better understand their interaction mode, a simulation of the interaction of (1R, 2R)-TM and (−)-Dibenzoyl-l-tartaric acid was conducted (Figure ).
Enantioselective separation
Enantioselective separation based on fluorescence technique has increased large interest owing to its high selectivity and sensitivity for chiral chemicals. Considering the conspicuous enantioselectivity for l- and d-dibenzoyl tartaric acid, (1R, 2R)-TM was used to capture (−)-Dibenzoyl-l-tartaric acid in the racemic solution to realize enantioselective resolution. Generally, (1R, 2R)-TM as a chiral sensor can selectively recognize (−)-Dibenzoyl-l-tartaric acid, similarly, (1S, 2S)-TM is able to detect (+)-Dibenzoyl-d-tartaric acid. Therefore, the enantiomer excess of chiral acids can be quantitatively analyzed by (1R, 2R)-TM and (1S, 2S)-TM. Taking Dibenzoyl-l/d-tartaric acid as an example, the PL intensity of (1R, 2R)-TM gradually enhanced along with increasing molar percent of (−)-Dibenzoyl-l-tartaric acid in the mixture of dibenzoyl tartaric acid (Figure ). Meanwhile, the (1S, 2S)-TM showed similar results to (+)-Dibenzoyl-d-tartaric acid. Inspired by the experiment data, we assumed that (1R, 2R)-TM might be used for enantioselective resolution. Thus, the dibenzoyl tartaric acid was taken as an example to verify the hypothesis. (1R, 2R)-TM (1.6 mg, 0.002 mmol) was dissolved in THF (3 mL). Then, the mixture of (−)-Dibenzoyl-l-tartaric acid and (+)-Dibenzoyl-d-tartaric acid was added to the solution. Subsequently, deionized water was dropped into the solution and incubated for another 6 h. Then a large number of floccules appeared with intense fluorescence (Figure ). The obtained floccules were attained by simple filtration. The residue and filtrate were gathered and dried. The optical purities of sediments and filtrate were verified through chiral HPLC. As illustrated in Figure , (−)-Dibenzoyl-l-tartaric acid, (+)-Dibenzoyl-d-tartaric acid, and the mixture of d/l-Dibenzoyltartaric acid and (1R, 2R)-TM were recorded on the chiral phase column of CHIRALPAK IF-3 (Column size: 0.46 cm I.D. × 25 cm L × 3 μm, Mobile phase: n-hexane/ethanol/trifluoroacetic acid = 90/10/0.1). Under this condition, these compounds clearly split and the retention time of (−)-Dibenzoyl-d-tartaric acid, (+)-Dibenzoyl-l-tartaric acid, and (1R, 2R)-TM was 7.78, 6.91 and 5.65 min, respectively. The chiral HPLC results revealed that the residue was comprised of 82% (−)-Dibenzoyl-l-tartaric acid and 18% (+)-Dibenzoyl-d-tartaric acid (Figure ). The solution was mainly consisting of (+)-Dibenzoyl-d-tartaric acid with an enantiomer content of 85% (Figure ). Although the separation efficiency cannot compare with the chiral HPLC, this promising strategy showed great potential in separation of chiral drugs and reagents.
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CONCLUSION
In this work, two optically active AIEgens were facilely prepared, namely (1R, 2R)-TM and (1R, 2R)-TM, which can discriminate 5 kinds of chiral acids. Especially, the fluorescence intensity ratio of two complexes was up to 281 for (−)-Dibenzoyl-l-tartaric acid and (+)-Dibenzoyl-d-tartaric acid. Besides, these chiral acids are able to induce obvious CD signal change upon interaction with (1R, 2R)-TM. The results of 1H NMR and 2D NOESY NMR suggested that the acid–base interaction of amino and carboxyl groups and suitable stereoselectivity were responsible for the enantioselective recognition. Then, the enantioselective resolution of d/l-dibenzoyl tartaric acid was realized by using the chiral AIEgens. The HPLC results revealed that the enantiomer separation efficiency was achieved to be 82% in the sediments (enantiomeric excess value was assessed to be 64% ee). Compared with the chiral HPLC separation, it is anticipated that this strategy will afford a simple and convenient method for enantioselective recognition and separation.
EXPERIMENTAL SECTION
Materials
All reagents and solvents were chemically pure (CP) grade or analytical reagent (AR) grade and were used as received unless otherwise indicated.
Synthesis of (1R, 2R)/(1S, 2S)-3 and (1R, 2R)/(1S, 2S)-4
(1R, 2R)/(1S, 2S)-3 and (1R, 2R)/(1S, 2S)-4 are known molecules and were synthesized according to previous literature.[] The detailed synthetic procedures are as below.
(1R, 2R)-3
To a two-neck 50 mL flask, (1R, 2R)-1 (1.00 g, 8.75 mmol, 1.00 equiv.) and 4-Bromobenzaldehyde (2, 3.23 g, 17.5 mmol, 2.00 equiv.) were dissolved in tetrahydrofuran (30 mL) and refluxed for 12 h. Then, the reaction mixture was cooled to room temperature. The precipitate was filtrated and washed with ethanol. After dried by vacuum oven at 45°C for another 8 h, white solid was obtained with an excellent yield (3.78 g, 96%). 1H NMR (400 MHz, CDCl3, δ) 8.10 (s, 2H), 7.44 (s, 8H), 3.40–3.33 (m, 2H), 1.84 (t, J = 25.4, 15.1, 6.6 Hz, 6H), 1.48 (t, J = 8.9 Hz, 2H). 13C NMR (100 MHz, CDCl3, δ) 159.63, 135.14, 131.62, 129.26, 124.62, 73.72, 32.77, 24.38.
(1S, 2S)-3
The synthetic procedure was similar to that described for (1R, 2R)-3. Compound (1S, 2S)-3 (3.70 g, 94%) was obtained as a white solid. 1H NMR (400 MHz, CDCl3, δ) 8.10 (s, 2H), 7.43 (s, 8H), 3.36 (d, J = 4.2 Hz, 2H), 1.81 (t, J = 23.2, 10.4 Hz, 6H), 1.48 (t, J = 10.1 Hz, 2H). 13C NMR (100 MHz, CDCl3, δ) 159.63, 135.14, 131.61, 129.26, 124.63, 73.72, 32.78, 24.38.
(1R, 2R)-4
In a one-neck 100 mL flask, compound (1R, 2R)-3 (1.00 g, 2.23 mmol, 1.00 equiv.) was dissolved in the mixed solvent of ethanol and tetrahydrofuran (1:1, 40 mL). Then sodium borohydride (186 mg, 4.46 mmol, 2.2 equiv.) was added under the ice bath, the reaction mixture was carried out at room temperature for 12 h. Then, quenched with water (30 mL) and extracted with CH2Cl2. The organic phase was collected and dried with anhydrous MgSO4. The organic layer was collected and evaporated under reduced pressure. After drying by vacuum oven at 45°C overnight, white solid was obtained with an excellent yield (0.98 g, 98%). 1H NMR (400 MHz, CDCl3, δ) 7.44–7.39 (m, 4H), 7.19–7.15 (m, 4H), 3.83 (d, J = 13.4 Hz, 2H), 3.60 (d, J = 13.4 Hz, 2H), 2.22 (dd, J = 5.5, 3.6 Hz, 2H), 2.13 (dd, J = 10.8, 2.9 Hz, 2H), 1.90 (s, 2H), 1.75–1.70 (m, 2H), 1.25–1.18 (m, 2H), 1.07–0.98 (m, 2H). 13C NMR (100 MHz, CDCl3, δ) 140.05, 131.36, 129.69, 120.48, 60.85, 50.20, 31.51, 24.94.
(1S, 2S)-4
The synthetic procedure was similar to that described for (1R, 2R)-4. Compound (1S, 2S)-4 (0.99 g, 99%) was obtained as a white solid. 1H NMR (400 MHz, CDCl3, δ) 7.41-7.37 (m, 4H), 7.15 (d, J = 8.4 Hz, 4H), 3.81 (d, J = 13.4 Hz, 2H), 3.57 (d, J = 13.4 Hz, 2H), 2.20 (dd, J = 5.4, 3.7 Hz, 2H), 2.10 (dd, J = 10.9, 2.7 Hz, 2H), 1.91 (s, 2H), 1.72–1.68 (m, 2H), 1.20 (t, J = 9.6 Hz, 2H), 1.05–0.95 (m, 2H). 13C NMR (100 MHz, CDCl3, δ) 140.06, 131.36, 129.70, 120.48, 60.85, 50.19, 31.50, 24.96.
Synthesis of 7
To a solution of 4-(1-bromo-2,2-diphenylvinyl)biphenyl (5) (2.00 g, 5.99 mmol, 1 equiv.) in toluene (60 mL) was added PdCl2(PPh3)2 (210.00 mg, 0.299 mmol, 0.05 equiv.), bis(pinacolato)diboron (2.28 g, 8.98 mmol, 1.5 equiv.), PhONa (1.04 g, 8.98 mmol, 1.5 equiv.), and cyclopentyl methyl ether (20 mL) under nitrogen atmosphere. The mixture was stirred at 105°C for 18 h. The reaction mixture was treated with water (10 mL) at room temperature, extracted with dichloromethane (20 mL), washed with brine, and dried over anhydrous MgSO4. The product was purified by column chromatography (silica gel, ethyl acetate/petroleum ether 1/20) to give 7 (2.00 g, 87%) as faint yellow solid. 1H NMR (400 MHz, CDCl3, δ) 7.36–7.26 (m, 5H), 7.15–7.01 (m, 8H), 6.96 (dd, J = 7.8, 5.5 Hz, 2H), 1.11 (s, 12H).13C NMR (100 MHz, CDCl3, δ) 151.32, 144.63, 141.74, 130.86, 129.68, 129.37, 127.89, 127.47, 126.70, 125.78, 83.63, 24.50.
Synthesis of (1R, 2R)-TM
(1R, 2R)-4 (1.00 g, 2.10 mmol, 1.00 equiv.), boronic ester 7 (1.72 g, 4.50 mmol, 2.20 equiv.), and cesium carbonate (2.74 g, 8.4 mmol, 4.00 equiv.), and Pd(PPh3)4 (121.30 mg, 0.10 mmol, 0.05 equiv.) were added in the mixture solution of toluene, EtOH, and water (4:1:1, 60 mL) under N2 atmosphere at 90°C for 12 h. After cooling to room temperature, the solvent was removed under vacuum, and the residue was purified by silica gel column chromatography to give the desired product (1R, 2R)-TM (1.43 g, 85%) as a white solid. 1H NMR (400 MHz, CDCl3, δ) 7.06 (t, J = 16.4, 9.2, 6.4 Hz, 17H), 6.95 (d, J = 8.2 Hz, 2H), 3.78 (d, J = 13.2 Hz, 1H), 3.56 (d, J = 13.2 Hz, 1H), 2.20 (d, J = 9.0 Hz, 1H), 2.10 (d, J = 12.9 Hz, 1H), 1.89 (s, 1H), 1.72 (d, J = 8.3 Hz, 1H), 1.22 (t, J = 10.1 Hz, 1H), 1.07–0.98 (m, 1H). 13C NMR (100 MHz, CDCl3, δ) 143.89,143.65, 142.11, 140.77, 139.14, 131.40, 131.15, 127.57, 127.25, 126.31, 60.81, 50.56, 31.60, 25.05. HRMS (ESI+) m/z: [M + H]+ calcd for C60H54N2, 803.4360, found 803.4440.
(1S, 2S)-TM
The synthetic procedure was similar to that described for (1R, 2R)-TM. Compound (1S, 2S)-TM (1.35 g, 80%) was obtained as a white solid. 1H NMR (400 MHz, CDCl3, δ) 7.05 (t, J = 12.4, 6.6 Hz, 17H), 6.95 (d, J = 8.1 Hz, 2H), 3.79 (d, J = 13.2 Hz, 1H), 3.57 (d, J = 13.3 Hz, 1H), 2.21 (d, J = 9.0 Hz, 1H), 2.10 (d, J = 12.8 Hz, 1H), 2.02 (s, 1H), 1.73 (d, J = 8.3 Hz, 1H), 1.22 (t, J = 10.2 Hz, 1H), 1.08–0.99 (m, 1H). 13C NMR (100 MHz, CDCl3, δ) 143.89, 143.64, 142.15, 140.78, 139.04, 131.29, 127.58, 127.26, 126.32, 60.76, 50.51, 31.54, 25.04. HRMS (ESI+) m/z: [M + H]+ calcd for C60H54N2, 803.4360; found 803.4450.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (52173152 and 21805002), the Guangdong Basic and Applied Basic Research Foundation (2020A1515110476), the Fund of the Rising Stars of Shaanxi Province (2021KJXX-48), the Natural Science Basic Research Plan in Shaanxi Province of China (2019JQ-302, 2021JQ-801), the Research Foundation of Education Department of Shaanxi Province (20JS005), the Young Talent fund of University Association for Science and Technology in Shaanxi, China (20190610 and 20210606), and the Scientific and Technological Innovation Team of Shaanxi Province (2022TD-36).
CONFLICT OF INTEREST
The authors declare no conflict of interest.
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Abstract
Enantioselective recognition and separation are the most important issues in the fields of chemistry, pharmacy, agrochemical, and food science. Here, we developed two optically active diamines showing aggregation‐induced emission (AIE) that can discriminate 5 kinds of chiral acids with high enantioselectivity. Especially, a very high fluorescence intensity ratio (I
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Neither ProQuest nor its licensors make any representations or warranties with respect to the translations. The translations are automatically generated "AS IS" and "AS AVAILABLE" and are not retained in our systems. PROQUEST AND ITS LICENSORS SPECIFICALLY DISCLAIM ANY AND ALL EXPRESS OR IMPLIED WARRANTIES, INCLUDING WITHOUT LIMITATION, ANY WARRANTIES FOR AVAILABILITY, ACCURACY, TIMELINESS, COMPLETENESS, NON-INFRINGMENT, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Your use of the translations is subject to all use restrictions contained in your Electronic Products License Agreement and by using the translation functionality you agree to forgo any and all claims against ProQuest or its licensors for your use of the translation functionality and any output derived there from. Hide full disclaimer
Details
1 AIE Research Center, Shaanxi Key Laboratory of Phytochemistry, College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji, China
2 Shenzhen Institute of Molecular Aggregate Science and Engineering, School of Science and Engineering, The Chinese University of Hong Kong‐Shenzhen, Shenzhen, China