Chiral Poly(ionic liquid) with Nonconjugated Backbone as a

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Chiral Poly(ionic liquid) with Non-conjugated Backbone as Fluorescent Enantioselective Sensor for Phenylalaninol and Tryptophan Datong Wu, Yin Yu, Jie Zhang, Lili Guo, and Yong Kong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04869 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Applied Materials & Interfaces

Chiral Poly(ionic liquid) with Non-conjugated Backbone as Fluorescent Enantioselective Sensor for Phenylalaninol and Tryptophan Datong Wu, Yin Yu, Jie Zhang, Lili Guo and Yong Kong*

Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China *

Corresponding author. E-mail address: [email protected]

Keywords: Poly(ionic liquid); Non-conjugated backbone; Fluorescence; Chiral recognition; Sensor

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ABSTRACT Here a novel fluorescent chiral poly(ionic liquid) (S)-PCIL-4 with non-conjugated backbone is designed and synthesized in the control of micelle through free radical polymerization, whose fluorescence emission maximum is at λem,max = 430 nm. It is observed that polymers with spatially proximate units (phenyl group and pyridinium cation) have photoluminescence through spatial π-π and ion-π interaction. Then (S)-PCIL-4 can be served as a fluorescent turn-off/on sensor for chiral recognition of phenylalaninol and tryptophan in the presence of Cu(II). For example, when (S)-PCIL-4-Cu(II) is treated with (R/S)-phenylalaninol, it will exhibit different fluorescence responses. Values of the enantiomeric fluorescence difference ratio for phenylalaninol and tryptophan are 1.10 and 1.08, respectively. In brief, we believe that the approach opens up a possible pathway to prepare a variety of fluorescent polymers with nonconjugated backbone and proves to be desirable in further application.

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INTRODUCTION So far, diverse techniques, like, NMR,1,2 HPLC,3,4 mass spectra,5,6 optical sensors,7,8 and electrochemical analysis,9,10 have been widely carried out for chiral recognition. Among them, fluorescent chiral sensors with straightforward signals such as enantioselective fluorescence enhancement or quenching attract growing interest in enantiomeric composition determination.11-14 The approach has the advantages of high selectivity and real-time analysis. There is no doubt that the ingenious structural design plays a key role in effective recognition. A mountain of fluorescent sensors mainly derived from 1,1’-bi-2-naphthol (BINOL) or naphthalene have been reported.15-24 However, a common problem encountered with the synthesis of chiral sensors is lacking of enough pure natural fluorescent chiral molecules. To enrich novel species as enantioselective receptors, it is necessary to prepare the artificial sensors with fire-new backbones. Chart 1. Architecture of TSDE-based fluorescent PCIL.

Poly(ionic liquid)s (PILs) including polymeric backbone inherit certain special natures of ionic liquids (ILs) such as low flammability, excellent electrical conductivity, and designable structures for practical use. They are emerging as a novel class of materials in diverse fields.25-30 Monomers (for example, N-vinylimidazolium or vinylpyridium-based ILs) can be applied for the synthesis of functional PILs directly. However, there is difficulty in the 3



preparation of fluorescent PILs unless monomer itself is comprised of fluorophore. Up to date, fluorescent chiral PILs are rarely reported.31 Considering the great difficult in preparation of fluorescent PILs, it is desirable to build a brand-new approach. Through-space delocalization effect (TSDE) for luminescence has been observed in special molecules composing of a donor and an accepter with spatially proximate units.32-35 Their chemical structures are not limited to conjugated bonds, but still have photoluminescence with spatial π-π electronic interaction.32 As shown in Chart 1, realizing the great significance of TSDE effect between physically separated units, we hypothesize that the pyridinium cation and phenyl group comprised of not only π-π interaction but also ion pair-π interaction are suitable for the synthesis of fluorescent PILs. Moreover, it can provide adjustable functionality for the true application.

Figure 1. Schematic illustration of the synthesis of (S)-PCIL-4 and chemical structures of pre-analysis racemates: (i) 3-chloropropylisocyanate, dichloromethane, 0 °C, 4 h; (ii) 1% mol AIBN, 5% mol CTAB, acetonitrile, 70 °C, 10 h; (iii) (S)-2, acetonitrile, 85 °C, 48 h.

For the above reasons, we propose a novel pathway to construct the target fluorescent polymeric chiral ionic liquid (S)-PCIL-4 through free radical polymerization. The chemical structure of (S)-PCIL-4 is shown in Figure 1. The whole synthesis process requires two steps. Firstly, polyethylene (PSVP-3) is chosen as the backbone by using 2-azoisobutyronitrile (AIBN) as initiator and cetyltrimethyl ammonium bromide (CTAB). CTAB can form micelle to control homogeneity in ethanol and avoid the open-ended growth of polymers. Among 4


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them, phenyl and pyridine groups are physically separated, but meanwhile allow TSDE effect to construct fluorescence polymers. Moreover, such a geometry can govern the enantioselectivity by communicating any binding groups to the fluorophore. Secondly, the chiral molecule (a phenylalanine derivative, (S)-2) with alkyl chloride is linked to pyridyl nitrogen group to form pyridinium cation. Moreover, in the present work, the novel fluorescent chiral poly(ionic liquid) is used as a switchable sensor with fluorescent turn-off/on responses for the simultaneous concentration calculation of Cu(II) and chiral compounds including phenylalaninol and tryptophan. Of note, when different configurations of racemates are treated with the sensor, it will exhibit different fluorescence intensity, contributing to a rapid enantioselective recognition.

EXPERIMENTAL SECTION Reagents and Materials. All chemicals involved in this experiment were analytical reagent grade at least. Raw chemicals including styrene, 4-vinylpyridine, vinylimidazole, 2,2’-azobis(2-methylpropionitrile), cetyltrimethyl ammonium bromide,

L-phenylalanine

methyl ester, 2-chloroethyl isocyanate purchased from Aladdin BioChem Technology Co., Ltd. (Shanghai, China) were used to prepare the target functional polymers without further puriﬁcation. L-Phenylalaninol, D-phenylalaninol, L-tryptophan, D-tryptophan, and metal salts (CuCl2, CoCl2, AlCl3, FeCl3, NiCl2, ZnCl2) were bought from Sinopharm Group Co. Ltd. (Shanghai, China). Instrumentation. UV-vis absorption spectra were conducted on a UV-1700 UV-vis spectrophotometer (Shimadzu, Japan) equipped with a 1 cm quartz cell. Fluorescent emission spectra were obtained on a LS-55 ﬂuorescence spectrometer (PerkinElmer, USA). 1H and 13C NMR were recorded on Avance 400 MHz NMR Spectrometer (Bruker, Switzerland) using TMS as internal reference. Fourier-transformed infrared (FTIR) spectra were recorded on

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Nicolet NEXUS-470 FTIR spectrometer (Thermo Nicolet, USA). The weight-average relative molecular mass (Mw) and number-average relative molecular mass (Mn) were determined by using 1515GPC-Waters (Waters, USA) with the parameters including injection volume (50.0 µL) and run time (40.0 min). Preparation of (S)-2. As shown in Figure 1, (S)-1 (6.0 mmol, 1.074 g) dissolved in anhydrous CH2Cl2 (50 mL) was stirred at 0 oC.36 3-Chloropropylisocyanate (6.0 mmol, 0.63 g) dissolved in 15 mL CH2Cl2 was added dropwise for 30 min, and stirring was continued for 4 h at room temperature. The product was then concentrated under reduced pressure without further purification for further synthesis. 1H NMR: (400 MHz, DMSO-d6) δ ppm 7.31-7.16 (m, 5H), 6.48-6.46 (d, J = 8.0 Hz, 1H), 6.41-3.38 (t, J = 6.0 Hz, 1H), 4.42-4.36 (m, 1H), 3.60 (s, 3H), 3.55-3.52 (t, J = 6.0 Hz, 2H), 3.30-3.25 (m, 2H), 2.99-2.85 (m, 2H). 13C NMR (100 MHz, DMSO-d6): δ 172.96, 157.11, 137.02, 129.13, 128.24, 126.53, 54.03, 51.68, 44.48, 41.27, 37.46. Preparation of PSVP-3. Free radical polymerization of the monomers, styrene and 4-vinylpyridine with different molar ratios (1:1, 2:1, 3:1, 1:2, and 1:3), was used to synthesize the target fluorescent polymers. As a typical procedure, freshly purified styrene (0.52 g, 0.5 mmol), 4-vinylpyridine (0.525 g, 0.5 mmol), AIBN (5.2 mg, 1.0 wt %), and CTAB (26.0 mg, 5.0 wt %) in 50 mL CH3CH2OH/CH3CN (v/v = 5/95) were gently mixed and stirred in a flask. Then the flask contents were placed under N2(g) and heated to 70 oC. The whole polymerization was carried out for 12 h and concentrated under reduced pressure. The product PSVP-3 was thoroughly washed with ethyl acetate (3 × 30 mL) and dried at room temperature with the yield of 68%. Preparation of (S)-PCIL-4. To the solution of PSVP-3 (0.3 g) and (S)-2 (0.426 g, 1.5 mmol) in 30 mL acetonitrile was added under N2(g). The reaction mixture was stirred at 85 oC for 48 h, followed by the removal of solvent under vacuum to give the product. Then the mixture was washed with ethyl acetate (3 × 30 mL) to remove unreacted (S)-2 and dried at

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room temperature with the yield of 79%. Fluorescence quenching for Cu(II). A stock solution of a sensor (2.0 mg mL-1) in ethanol was freshly prepared for each measurement. For the fluorescence quenching study, a sensor solution was mixed with varying concentrations (2.5-100 µM) of Cu(II) stock solution at room temperature in a 1 cm quartz cell. The mixture solution for the fluorescence spectra collected in the range of 400-600 nm with fluorescence excitation at 385 nm was taken within 10 min. Typical procedure for chiral recognition. In order to achieve the best recognition efficiency, different concentration of Cu(II) ranging from 25 µM to 100 µM were discussed in detail. After comparison of the results, the ethanol stock solutions of (S)-PCIL-4 (2.0 mg mL-1) and Cu(II) (50 µM) with weak fluorescent intensity were applied for each measurement. Moreover, stock solutions of phenylalaninol enantiomers with different concentrations (20-300 µM) were prepared in ethanol. For the fluorescence enhancement study, a sensor solution was mixed with the phenylalaninol solution at room temperature in a 1 cm quartz cell. Fluorescence excitation at 385 nm was carried out for all of the fluorescence measurements unless otherwise indicated.

RESULTS AND DISCUSSION Optimization of reaction conditons. In this study, we find that CTAB can affect the fluorescent emission spectra of PCILs. Different weights of CTAB (0.0, 1.0, 3.0, and 5.0 wt %) were added to the solution including freshly purified styrene (0.52 g, 0.5 mmol), 4-vinylpyridine (0.525 g, 0.5 mmol), and AIBN (5.2 mg, 1.0 wt %). The molecular weights of PCILs would increase with the decrease of CTAB. Meantime, the fluorescent intensity decreases accordingly (Figure S3). Of note, when CTAB is not added in the reaction, it could not produce the fluorescent PCIL. Further, the polymers arising from different molar ratios of monomers have different emission spectra. To be specific, the emission spectra (λex = 385 nm) would shift in the positive direction with the increase of pyridinium cation (Figure S4 and S5). 7



And the fluorescent intensity decreases obviously when the molar ratio of styrene and 4-vinylpyridine is 1/3. In short, the results proves the simultaneous interaction including π-π effect and ion-pair effect in PCILs.

Figure 2. (A) FTIR spectra of (a) (S)-2; (b) PSVP-3; (c) (S)-PCIL-4; (B) Partial 1H NMR spectra of (S)-PCIL-4 in methanol-d4; (C) Emission spectra of PSVP-3 (2.0 mg mL-1, λex,max = 350 nm, λem,max = 410 nm) and (S)-PCIL-4 (2.0 mg mL-1, λex,max = 385 nm, λem,max = 430 nm) in ethanol; (D) Emission and excitation spectra of (S)-PCIL-4 (2.0 mg mL-1). Characterization of (S)-PCIL-4. It is necessary to verify the successful synthesis of the target molecule. Textural properties (such as chemical structures, the ratio of phenyl and IL moiety, and molecular weights) and photoluminescent property of the employed polymers are validated by 1H NMR, GPC, FTIR, and fluorescent emission spectra. FTIR spectra of (S)-2, PSVP-3, and (S)-PCIL-4 are displayed in Figure 2A. In curve a, the bending vibration of N-H belonging to the carbamido group is recorded at 3412 cm-1. The bands at 2953 and 2864 cm-1 are assigned to the asymmetric stretching vibration and symmetric stretching vibration of C-H. The signal at 1747 cm-1 is for the stretching vibration

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of C=O bond. In curve b, characteristic peaks at 1487 and 1474 cm-1 are belonged to pyridium moiety. Compared with curve a and b, most characteristic peaks are same in curve c, proving the linkage between the copolymer framework and (S)-2. A partial 1H NMR spectrum of (S)-PCIL-4 dissolved in CD3OD-d4 is shown in Figure 2B. It displays certain peaks related to characteristic groups of PSVP-3 and (S)-2. Namely, proton signals of aromatic rings (phenyl and pyridinium cation groups) and –CH2– and –CH– groups adjacent to the carbamido group occurred in 9.0-6.4 ppm and 4.8-4.1 ppm, respectively. According to the proton ratio of 5.5 between these characteristic groups, we confirm the x value (the molar ratio of styrene to (S)-CIL) is 1.3 by calculation. The weight-average relative molecular mass (Mw) and number-average relative molecular mass (Mn) of (S)-PCIL-4 are determined by gel permeation chromatography (GPC) and the values are 3753 and 3534, respectively (Figure S1). Further, the polydispersity index (PDI) is 1.06, indicating a narrow distribution of polymer weights due to the addition of CTAB. Figure 2C shows PL spectra of PSVP-3 and (S)-PCIL-4 in ethanol at the concentration of 2.0 mg L-1. Specifically, PSVP-3 presents a broad emission band with the maxima emission (λem,max) located at 410 nm in the maximal exciting light (λex,max) of 350 nm. On the other hand, the simultaneous interaction, π-π interaction and ion pair-π interaction, can improve the luminous efficiency. λem,max of the fluorescent PCIL at 430 nm with higher fluorescent intensity is obtained, meanwhile λex,max is at 385 nm (Figure 2D). In addition, solvents have significant influence on emission spectra of PCILs due to their solvent effect (Figure S6). (S)-PCIL-4 dissolved in ethanol has a narrow peak and relatively high fluorescence intensity.

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Figure 3. (A) Emission spectra of (S)-PCIL-4 (2.0 mg mL-1, λex,max = 385 nm) upon addition of increasing concentration of Cu(II) (2.5-100 µM) in ethanol (Inset: linear relationship of fluorescence intensity versus the concentration of Cu(II). (B) Emission spectra of (S)-PCIL-4 (2.0 mg mL-1, λex,max = 385 nm) with or without of Cu(II) (50 µM), (R)- and (S)-5 (300 µM). Enantioselective recognition. The as-prepared fluorescent (S)-PCIL-4 is sensitive to metal ions. Once copper salt is added to the (S)-PCIL-4 solution, it would generate yellow precipitate with the formation of the coordination complex (S)-PCIL-4-Cu(II), resulting in fluorescence quenching (λex,max = 385 nm, λem,max = 430 nm,) as a fluorescence “turn off” mode. Figure 3A shows that the fluorescence of (S)-PCIL-4 (2 mg mL-1) is almost quenched with Cu(II) concentration increased to 0.1 mM. Further, as shown in Figure 3B, the as-obtained (S)-PCIL-4-Cu(II) complex can regain fluorescent emission peaks in the presence of (rac)-amino alcohols (or amino acids), meanwhile the cloudy solution becomes clear (Figure S7). Namely, a tunable mode is established between “turn off” and “turn on”. And it displays a competitive binding formation among (S)-PCIL-4, Cu(II), and amino alcohols, proving that amino alcohols is easier to form the coordination bond with Cu(II). Significantly, the mixture solution with different enantiomers of amino alcohols presents disparate fluorescent intensity, giving the positive information for chiral recognition.37,38

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Figure 4. Emission spectra of (S)-PCIL-4 + Cu(II) (2.0 mg mL-1 + 50 µM, λex,max = 385 nm) upon addition of increasing amount of (A) (S)-5 and (B) (R)-5 in the range of 0-300 µM (Inset: linear relationship of (I-I0)/I0 versus the concentration of (rac)-5). (C) Linear relationship of (I-I0)/I0 versus different ee of (rac)-5. (D) The Benesi-Hildebrand plot.

Next, we probe the chiral recognition performance of (S)-PCIL-4-Cu(II) in detail. (R)- and (S)-5 are tested as a typical example. First of all, the examined mixture solution containing (S)-PCIL-4 (2.0 mg mL-1) and Cu(II) (50 µM) is prepared in situ. When (S)-PCIL-4-Cu(II) is treated with (R)- or (S)-5 in different concentrations, the yellow precipitate disappears and distinct fluorescence enhancement can be observed with the increased concentration of (R)- or (S)-5. As shown in Figure 4A, in ethanol, the fluorescence intensity of the mixture solution increases 1.48-fold on addition of (S)-5 (0.3 mM). In contrast, the fluorescence intensity for (R)-5 (0.3 mM) increases 1.34-fold (Figure 4B). The enantiomeric fluorescence difference ratio is 1.10. According to the results, coordinative bonds formed between (S)-5 and Cu(II) have higher stability constant compared to (R)-5. On the other hand, the difference of fluorescence enhancement can be viewed as an analytical tool for chiral recognition of (rac)-5. 11



Further, the varied fluorescence intensity with the addition of (R) or (S)-5 exhibits a good linear relationship in the concentration range of 0.02-0.3 mM. At the same time, the linear relationship between the enantiomeric fluorescence difference ratio [(I − I0)/I0] and ee values of (rac)-5 (0.3 mM) is determined (Figure 4C). In brief, the novel fluorescent sensor provides high advantages in the simultaneous determination of concentration and enantiomeric composition of enantiomers by fluorescence measurement.

Figure 5. Emission spectra of (S)-PCIL-4 + Cu(II) (2.0 mg mL-1 + 50 µM, λex,max = 385 nm) upon addition of increasing amount of (A) (S)-6 and (B) (R)-6. Inset: linear relationship of (I-I0)/I0 versus the concentration of (rac)-6. Chiral recognition of amino acid. We also investigate the generality of amino acid (tryptophan) by this approach. As shown in Figure 5, fluorescence responses toward the mixture solution composed of enantiomers, Cu(II), and (S)-PCIL-4 are similar to those of phenylalaninol. The (S)-enantiomer gives better capability of the fluorescence recovery. We also conclude the linear relationship between the concentration and fluorescence for tryptophan. Besides that, the enantiomeric fluorescence difference ratio is 1.08. Table 1. The apparent association constants (KBH, M-1) obtained from fluorescence titration in ethanol.

Entry

Guest

Ka / M-1

KBH(S−S)/KBH(S−R)

1

(S)-5

4831

3.37

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2

(R)-5

1434

3

(S)-6

6038

4

(R)-6

3979

1.52

Mechanism investigation. Inspired by the results, a better-defined chiral environment for chiral recognition requires two criteria.36,39 Firstly, a competitive process for the coordinative bond formation occurs among (S)-PCIL-4, Cu(II), and (rac)-5. Particularly, the formation rates of two diastereomeric host-guest complexes toward (R)- and (S)-configuration are different. Secondly, the whole reactions are reversible and less covalent bond architecture is necessary, allowing tunable bond selectivity. Figure 4D shows the Benesi-Hildebrand plots for (S)-PCIL-4-Cu(II) toward (S)- and (R)-5 in ethanol. The association constants KBH are found to be 4831 M−1 with (S)-5 and 1434 M−1 with (R)-5, giving an enantioselectivity factor KBH(S−S)/KBH(S−R) of 3.37. Further, Table 1 presents values of KBH and KBH(S−S)/KBH(S−R) for the enantiomers of (rac)-5 and (rac)-6.

Figure 6. Possible binding modes between (S)-PCIL-4-Cu(II) and (rac)-5 for explaining enantioselectivity. As mentioned above, the complex formation is in the control of a competitive process between the receptor and acceptor. The molar ratio between receptor and acceptor is 1:1 according to the Benesi-Hildebrand plot with good linear relationship (Figure 4D). Referred to reported work,40-43 the enantioselective behavior of the receptor can be explained from the 13



point of the complex stability. As shown in Figure 6, we postulate the possible binding modes between (S)-PCIL-Cu(II) and (rac)-5, including coordinative binding motifs present. Namely, carbamido group of (S)-PCIL-4 and –OH and –NH2 belonging to amino alcohols conjugate with Cu(II). Space steric hindrance between phenyl and methyl ester groups is lower than that of phenyl and phenyl groups, which provides higher stability for (S)-PCIL-4-Cu(II)-(S)-5.

CONCLUSIONS In summary, we have presented here an innovative analytical approach for enantioselectvie recognition with the new fluorescent sensor. The artificial sensor built with the fire-new backbone can afford photoluminescence in the presence of spatial π-π and ion-π interaction. This setup offers the possibility for the preparation of fluorescent PILs without certain fluorophore. Further, the functional PILs are sensitive to Cu(II) and amino alcohol (amino acid) via tunable fluorescent enhancement and quenching. Significantly, (S)-PCIL-4-Cu(II) toward (R)- and (S)-enantiomers has different fluorescence intensity because of different association constants. The concentration and enantiomeric composition of enantiomers can be simultaneously determined in one step. The potential application of this setup opens up a possible pathway to synthesize a large variety of fluorescent PILs under their designability and work here broadens their scope in diverse fields.

ASSOCIATED CONTENT Supporting Information GPC curve of (S)-PCIL-4; 1H NMR of (S)-PCIL-4; emission spectra of prepared materials; emission spectra of prepared materials with the different molar ratios of styrene and 4-vinylpyridine; emission spectra of (S)-PCIL-4 in different solvents, and photographs of prepared materials (PDF)

AUTHOR INFORMATION Corresponding Author 14


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*E-mail: [email protected]

Notes There are no conflicts to declare.

ACKNOWLEDGEMENTS We gratefully acknowledge financial support from the Natural Science Foundation of Jiangsu Province (BK20171194).

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Chiral Poly(ionic liquid) with Nonconjugated Backbone as a

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