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Highly enantioselective recognition of alaninol via the chiral BINAM-based fluorescence polymer sensor
Polymer 101 (2016) 93e97
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Highly enantioselective recognition of alaninol via the chiral BINAMbased fluorescence polymer sensor Wenjie Zhang a, Guo Wei b, Ziyu Wang a, Jing Ma a, Chengjian Zhu a, Yixiang Cheng a, * a
Key Lab of Mesoscopic Chemistry of MOE, Collaborative Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, China b School of Chemistry and Materials Science, Nanjing Normal University, Nanjing, 210097, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2016 Received in revised form 13 August 2016 Accepted 18 August 2016 Available online 20 August 2016
A novel chiral polymer P1 was designed and synthesized by the polymerization of (S)-2, 20 -binaphthyladiamine (BINAM) derivative monomer (S-M-1) with 2, 5-diiodo-1, 4-dioctyloxybenzene (M-2) via Pd(II)-catalyzed Sonogashira coupling reaction, and P2 could be obtained by the reduction reaction of P1 with NaBH4. Interestingly, the resulting chiral (S)-BINAM-based polymer P1 sensor can act as a “turn on” fluorescence enhancement sensor towards (D)-alaninol, and the value of enantiomeric fluorescence difference ratio (ef) can reach as high as 14.46. On the contrary, no fluorescence response behavior toward alaninol enantiomers could be observed for the polymer P2. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Chiral polymer Enantioselective recognition Sensor
1. Introduction The chiral enantioselective recognition based on fluorescence chemosensor is attracting particular interest because of the potential applications for screening high-throughput chiral catalysts, designing the highly active asymmetric catalysis, understanding the interactions between biological molecules, developing the useful chiral resolution processes, and so on [1]. Much attention has been paid on the design of the novel chiral fluorescence sensors which can afford a simple, rapid, and real-time analytical methodology for the detection of chiral enantiomer composition [2]. Recently, the enantioselective recognition system has been focused on the chiral polymer-based fluorescence sensors by introducing different functional groups in the conjugated polymer main chain backbones [3]. Meanwhile the delocalizable p-electronic conjugated “molecular wire” polymers can greatly enhance the fluorescence response signal when they can form the corresponding hostguest complexes with the chosen enantiomers [4]. The optically active 2, 20 -binaphthyladiamine (BINAM) has been regarded as one of the most important C2-symmetric chiral compounds due to the confined rotation of the two naphthalene [5]. These BINAM-based derivatives can not only show the stable chiral
* Corresponding author. E-mail addresses: [email protected] (C. Zhu), [email protected] (Y. Cheng). http://dx.doi.org/10.1016/j.polymer.2016.08.061 0032-3861/© 2016 Elsevier Ltd. All rights reserved.
configuration as well as high chiral induction, but also chiral discrimination response behavior in molecule recognition and asymmetric catalysis [6]. In our previous work, we first found an in situ generated 1:1 Zn(II)-containing BINAM-based polymer complex which can exhibit the direct and visual enantioselective recognition response of the N-Boc-protected alanine [7]. Recently our group also found a novel chiral (S)-BINAM-based polymer which can act as the pronounced fluorescence enhancement sensor only on Hþ and trivalent metal cations (Fe3þ, Cr3þand Al3þ) [2g]. In this paper, we continued to design and synthesize a novel (S)BINAM-based polymer P1 by modifying the microenviroment of the chiral binding moiety, which can show highly enantioselective recognition response of (D)-alaninol. 2. Results and discussions The synthetic procedures of the chiral polymers P1 and P2 sensors are illustrated in Scheme 1. The monomer (S-M-1), (S)-2, 20 binaphthyladiamine (BINAM) derivative was synthesized by the reaction of (S)-2, 20 -BINAM (BINAM ¼ 1, 10 -binaphthyl-2, 20 diamine) with 5-ethynyl-2-hydroxybenzaldehyde [8,9a] via nucleophilic addition-elimination reaction. The linker monomer M-2, 2, 5-diiodo-1, 4-dioctyloxybenzene was prepared according to the reported literature [7,9]. The corresponding chiral (S)-BINAMbased polymer P1 could be obtained by Pd(II)-catalyzed Sonogashira coupling reaction of the chiral monomer S-M-1 with the
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Scheme 1. Synthesis procedures of compounds M-1, M-2 and the chiral polymer P1, P2.
linker monomer M-2 and collected as a dark yellow powder in 71.7% yield. The chiral polymer P2 could be obtained with NaBH4 by the reduction reaction of P1 in 89.7% yield. The chiral polymer P1 has one well-resolved peak situated at d ¼ 8.61 ppm (the Supporting Information, Figs. SI 5 and 6, which can be assigned to the imine group (C (H) ¼ N). As we can see from the 1H NMR spectra of the polymer P2, this peak disappeared indicating that the imine group in P1 has been almost reduced for P2. The values of Mw, Mn, and PDI of the chiral polymer P1 are 16540, 11650 and 1.42, respectively. Both P1 and P2 are air-stable solid and have good solubility in common organic solvents including toluene, THF, CHCl3, CH2Cl2, and CH3CN, which can be due to the nonplanarity of the twisted chain framework and the flexible n-octyl substituents. In addition, the ethynyl linker can reduce steric hindrance between phenyl groups and also produce a beneficial influence on the stability of the resulting chiral polymer. According to the TGA results of the polymers P1 and P2 have highly thermal stability and no loss weight before 300 C, and tend to only 50% decompose at 700 C (see supporting information, Fig. SI 8. Therefore, the designed chiral polymers can act as a stable fluorescence chemosensor material. The fluorescence spectra of the chiral polymer P1 and P2 was measured in THF solution (10 mM corresponding to 1, 10 -binaphthyl moiety). The chiral polymer P1 shows very weak fluorescence emission situated at 419 nm (FPL ¼ 0.02) due to the strong and ordered intramolecular hydrogen bonding between imine and
hydroxyl of phenol, which leads to non-radiative transition. On the contrary, P2 can emit strong blue fluorescence situated at 402 nm (FPL ¼ 0.26), which may be attributed to the weak and disordered intramolecular hydrogen bonding between hydroxyl of phenol and amine (Fig. SI 9) [10]. Compared with our previous works, the first polymer sensor could response to both Hþ and trivalent metal cations, but the second chiral polymer sensor could exhibit “turn-on” fluorescence response only to Zn2þ(Scheme 2.) [2g] On the contrary, the chiral polymer P1 in this paper can exhibit the excellent fluorescence response on the enantioselective recognition of the alaninol enantiomers. As is evident from Scheme 2, although three kinds of (S)BINAM-based polymers have the similar polymer conjugated backbone structure, they can show quite different fluorescence response behaviors, which can be attributed to the obvious difference from the coordination binding space blocks [11]. In this paper, we further carried out the fluorescence responses of the chiral polymer P1 towards various chiral guest molecules in THF solution (10 mM). The enantioselective recognition response behavior of chiral guest isomers is related to the enantiomeric fluorescence difference ratio, ef, according to ef ¼ (ID I0)/(IL I0), in which I0 is the fluorescence intensity in the absence of the chiral substrate, herein, ID and IL represent the fluorescence intensities in the presence of (D)- and (L)-chiral enantiomers, respectively. Interestingly, we found that (L)-alaninol has a little effect on the
Scheme 2. Difference from chiral polymer linkers and chiral binding building block.
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fluorescence intensity of the chiral polymer P1 (Fig. 1d), but (D)alaninol can lead to an obvious fluorescence enhancement (Fig. 1d). In comparison to an only 3.69-fold increase upon the addition of (L)-alaninol, the fluorescence intensity of the chiral polymer P1 showed a pronounced gradual enhancement as high as 21.39-fold with the molar ratio increases of (D)-alaninol from 1:10 to 1:80 (Fig. 1d). The ef value for the chiral polymer P1 can reach as high as 14.46, demonstrating that the resulting (S)-BINAM-based polymer sensor P1 can exhibit excellent enantioselective recognition ability towards (D)-alaninol. The fluorescent enhancement response of the (S)-BINAM-based chiral polymer P1 for enantiomers of alaninol as a guest molecule can be regarded as the suppressed photoinduced electron transfer (PET) effect [12] from interactions of the protons of alaninol with the nitrogen atoms of the imine moieties through intramolecular hydrogen-bonding [13]. Upon the formation of hosteguest complexes, the lone pair of electrons on the nitrogen atom (imine group) is no longer available for PET, which can lead to the enhancement of fluorescence about the (S)-BINAM-based polymer sensor. Herein, the (S)-binaphthalene-based chiral polymer P1 has a rigid and stable main chain backbone, and “turn on” fluorescence-enhancement response behavior towards (D)-alaninol can be attributed to the inherent chiral interactions in the microenvironments in the building blocks and the steric repulsion between chiral receptors and chiral guest molecules. The building block of the (S)-binaphthalene receptor, which is composed of imine and hydroxy groups that are controlled by the dihedral angle of binaphthalene, can orient a well-defined spatial arrangement in the regular polymer backbone and well accommodate the formation of a more-stable S-D complex compared to the diastereomeric S-L complex (see supporting information, Fig. SI 17. Most importantly, the (S)-binaphthalene-based chiral polymer P1 emits only weak blue fluorescence in THF solution, but the solution color of P1 can turn on bright blue upon the addition of (D)-alaninol, which can be clearly observed by the naked eye (Fig. 1d, inset) under a commercially available UV lamp (lex ¼ 353 nm). On the contrary, the color of the (S)-binaphthalene-based chiral polymer sensor P1 solution still keeps weak blue upon the addition of (L)-alaninol indicating that the resulting (S)-binaphthalene-based chiral polymer P1 can act as a highly sensitive and selective fluorescence sensor for the direct visual discrimination of the alaninol enantiomers at a low concentration. In order to ascertain the enantioselectivity of the resulting (S)binaphthalene-based chiral polymer P1 towards other chiral enantiomers including phenylalaninol, phenylglycinol, valinol and phenylethylamine as shown in Fig. 2, we further performed a set of comparable experiments. Almost no enantioselective recognition response could be detected, and no fluorescence responses can be observed (supporting information Figs. SI 11e14. What's more, we also studied the fluorescence response behaviors of the chiral polymer P2 (10 mM in THF solution) towards various chiral guest molecules (Fig. 3). We found that almost no fluorescence change of P2 could be observed upon the addition of these guest molecules except phenylethylamine enantiomers (Fig. 3). But both R-/Sphenylethylamine enantiomers can lead to the similar fluorescence enhancement effect, no enantioselective recognition response can be observed. It may be attributed to the flexible and twisted
Fig. 1. Fluorescence spectra of (a) the chiral polymer P1 (10 mM in THF) and guest molecules L-/D-alaninol (50equiv. in THF) (b) polymer P1 (10 mM in THF) in the presence of D-alaninol (0, 10, 20, 30, 40, 50, 60, 70, 80 equiv.) (c) polymer P1 (10 mM in THF) in the presence of L-alaninol (0, 10, 20, 30, 40, 50, 60, 70, 80 equiv) and (d) Fluorescence enhancement of the chiral polymer P1 (10 mM in THF) with (L)- and (D)alaninol (lem ¼ 425 nm, lex ¼ 353 nm; slit: 10 nm, 8 nm) (insert: the photo of the fluorescence color changes of the polymer sensor P1 (1) when it interacts with Lalaninol (2) and D-alaninol (3)).
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Fig. 2. Chiral guests used for the enantioselective recognition.
polymers is carried out on a Shimadzu 10 Å with THF as the eluent and polystyrene as the standard. All reagents and solvents are commercially available A.R. grade. Et3N and THF are purified by distillation from sodium in the presence of benzophenone. The concentrations of the stock solution of amine alcohols are 1 104 mM in THF. Initially, each experiment was started with a 3.0 mL polymer solution with a known concentration (10 mM). The adding amount of each experiment example of amine alcohols has been noted in the graphical interpretation and the figure.
Fig. 3. Fluorescence spectra of the chiral polymer P2 (10 mM in THF, lex ¼ 353 nm, lem ¼ 425 nm, slit: 5 nm, 4 nm) and various guest molecules (50equiv. in THF).
backbone of the chiral polymer P2 in the disordered state and the irregular chain backbone in comparison with the rigid chain backbone structure of P1. 3. Conclusion In summary, we developed a novel (S)-BINAM-based chiral polymer sensor P1 which can act as a fluorescence sensor for enantioselective recognition response towards (D)-alaninol, and the color change can be clearly observed by the naked eyes under a commercially available UV lamp for direct and visual discrimination of alaninol enantiomers at low concentration. Furthermore, no response behavior toward chiral guest enantiomers could be observed for the corresponding chiral polymer P2 due to the flexible and twisted polymer chain backbone. 4. Experimental section 4.1. Instrumentation and materials NMR spectra were obtained using a 400-Bruker spectrometer 400 MHz for 1H NMR and 100 MHz for 13C NMR and reported as parts per million (ppm) from the internal standard TMS. Electrospray ionization mass spectra (ESI-MS) were measured on a Thermo Finnigan LCQ Fleet system. UVevis spectra were taken on a Perkine Elmer Lambda 25 spectrometer. Fluorescent spectra are collected with a Perkin-Elmer LS 55 spectrometer. C, H, O, N of elemental analyses are measured on an Elemental Vario MICRO analyzer. Specific rotation was measured with a Ruololph Research Analytical Autopol IV-T/V. Thermogravimetric analyses (TGA) are conducted on a Perkin-Elmer Pyris-1 instrument under N2 atmosphere. The gel permeation chromatography (GPC) analysis of the
4.1.1. Synthesis of (S) e 2, 20 -binaphthyladiamine (BINAM) derivative monomer (S-M-1) (S)-2, 20 -BINAM (568.70 mg, 2.0 mmol) and 5-ethynyl-2hydroxybenzaldehyde [2a,9b] (584.60 mg, 4.0 mmol) were dissolved in 40 mL dry ethanol. The mixed solution was first stirred at 40 Cfor 12 h under an Ar atmosphere, and then the reaction mixture was cooled to room temperature before the solvent was removed by a rotary evaporator, and the mixed solvents of ethyl acetate and petroleum ether was added to the result mixture for recrystallization to afford the yellow solid product (0.91 g, yield 84.0%). 1H NMR (400 MHz, CDCl3) d: 12.35 (s, 2H), 8.64 (s, 2H (HC] N)), 8.12 (d, J ¼ 8.8 Hz, 2H (ArH)), 7.98 (d, J ¼ 8.1 Hz, 2H (ArH)), 7.65 (d, J ¼ 8.8 Hz, 4H (ArH)), 7.47 (s, 4H (ArH)), 7.39 (d, J ¼ 2.1 Hz, 2H (ArH)), 7.26 (s, 2H (ArH)), 6.64 (d, J ¼ 8.6 Hz, 2H (ArH)), 2.94 (s, 2H (HC^C)). 13C NMR (100 MHz, CDCl3) d: 161.29, 160.74, 143.04, 136.02, 133.16, 132.69, 130.23, 129.89, 128.38, 127.22, 126.44, 126.25, 119.09, 117.52, 116.53, 112.40, 82.92, 75.85. ESI-MS: [Mþ1]þ: 541.15. Anal. Calcd for C38H24N2O2: C, 84.42; H, 4.47; N, 5.18. Found: C, 83.86; H, 5.13; N, 4.62.
4.1.2. Synthesis of the chiral polymer sensor P1 S-M-1 (108.20 mg, 0.20 mmol) and 2, 5-diiodo-1, 4-dioctyloxybenzene (M-2) [8] (117.20 mg, 0.20 mmol), CuI (1.0 mg, 0.005 mmol) and Pd (PPh3)2Cl2 (3.5 mg, 0.005 mmol) are dissolved in 6 mL Et3N and 6 mL THF. The solution was first stirred for 48 h at 80 Cunder an Ar atmosphere, and then the mixture was cooled to room temperature and filtered. The residue was dissolved in a small quantity of THF after the solvent was removed under reduced pressure, and then 80 mL of methanol was added to precipitate the polymer. A dark yellow polymer solid was filtered off and washed by using methanol several times. Further purification was carried out by dissolving the polymer in THF to precipitate in methanol solution again. The polymer was dried under vacuum for 24 h at room temperature. The final yield is 71.7% (75.50 mg). GPC results: Mw ¼ 16540, Mn ¼ 11650, PDI ¼ 1.42. [a] 25 D ¼ þ296.0 (c 0.20, THF). 1 H NMR (400 MHz, CDCl3) d: 12.34 (m, OH), 8.61 (s, HC]N), 8.23e7.60 (m, ArH), 7.40e7.10 (m, ArH), 6.91e6.56 (m, ArH), 3.95e3.41 (m, CH2O), 1.74e1.20 (m, CH2), 0.82e0.75 (m, CH3). Anal. Calcd for C60H60N2O4: C, 82.54; H, 6.93; N, 3.21. Found: C, 81.96; H, 6.33; N, 2.40.
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4.1.3. Synthesis of the chiral polymer sensor P2 P1 (0.089 g) was dissolved in 10 mL THF and 10 mL MeOH mixed solvents, and then NaBH4 (450 mg) was added to the above solution in batches. The reaction mixture was first stirred until the yellow color turn light at room temperature. 10 mL water was added to stop the reduction reaction, and then the solution continued to stir for another 30 min. The mixture was extracted by CH2Cl2 (3 20 mL) solvent. The organic layers were dried with anhydrous Na2SO4 and evaporated under reduced pressure to collect the polymer P2 as a dark yellow solid (0.079 g, 89.7%). 1H NMR (400 MHz, CDCl3) d: 7.32e7.26 (m, ArH), 7.00e6.98 (m, ArH), 6.82e6.81 (m, ArH), 3.99e3.75 (m, CH2O), 1.87e1.18 (m, CH2), 0.96e0.78 (m, CH3). Acknowledgment This work was supported by the National Natural Science Foundation of China (21372114 and 21474048). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2016.08.061. References [1] (a) C. Wang, E. Wu, X.D. Wu, X.C. Xu, G.Q. Zhang, L. Pu, J. Am. Chem. Soc. 137 (2015) 3747e3750; (b) Y. Zhou, J. Yoon, Chem. Soc. Rev. 41 (2012) 52e67; (c) U. Filip, J. Janusz, J. Org. Chem. 80 (2015) 4235e4243; (d) G.H. Xie, W. Tian, L.P. Wen, K. Xiao, Z. Zhang, Q. Liu, G.L. Hou, P. Li, Y. Tian, L. Jiang, Chem. Commun. 51 (2015) 3135e3138; (e) L. Zhang, Q.X. Jin, K. Lv, L. Qin, M.H. Liu, Chem. Commun. 51 (2015) 4234e4236; (f) F.Y. Song, N. Fei, F. Li, S.W. Zhang, Y.X. Cheng, C.J. Zhu, Chem. Commun. 49 (2013) 2891e2893; (g) H.J. Huang, W.T. Yang, J.P. Deng, RSC Adv. 5 (2015) 26236e26245; (h) J.M. Jiao, F. Li, S.W. Zhang, Y.W. Quan, W.H. Zheng, Y.X. Cheng, C.J. Zhu, Macromol. Rapid Commun. 35 (2014) 1443e1449; (i) S.S. Yu, L. Pu, Tetrahedron 71 (2015) 745e772; (j) G. Wei, F.D. Meng, Y.X. Wang, Y.X. Cheng, C.J. Zhu, Macromol. Rapid Commun. 35 (2014) 2077e2081. [2] (a) K.W. Bentley, C. Wolf, J. Am. Chem. Soc. 135 (2013) 12200e12203; (b) S.S. Yu, W. Plunkett, M. Kim, L. Pu, J. Am. Chem. Soc. 134 (2012) 20282e20285; (c) K.L. Wen, S.S. Yu, Z. Huang, L.M. Chen, M. Xiao, X.Q. Yu, L. Pu, J. Am. Chem. Soc. 137 (2015) 4517e4524; (d) F. Wang, R. Nandhakumar, Y. Hu, D. Kim, K.M. Kim, J. Yoon, J. Org. Chem. 78 (2013) 11571e11576; (e) K.W. Bentley, C. Wolf, J. Org. Chem. 79 (2014) 6517e6531; (f) A. Bencini, C. Coluccini, A. Garau, C. Giorgi, V. Lippolis, L. Messori, D. Pasini, S. Puccionia, Chem. Commun. 48 (2012) 10428e10430; (g) L. Wang, F. Li, X.H. Liu, G. Wei, Y.X. Cheng, C.J. Zhu, J. Polym. Sci. Part A Polym. Chem. 51 (2013) 4070e4075; (h) Q.T. Li, H.M. Guo, Y.B. Wu, X. Zhang, Y.F. Liu, J.Z. Zhao, J. Fluoresc. 21 (2011) 2077e2084. [3] (a) Q. Li, C.J. Wang, H.L. Tan, G. Tang, J. Gao, C.H. Chen, RSC Adv. 6 (2016) 17811e17817; (b) D. Giri, S.K. Patra, RSC Adv. 5 (2015) 79011e79021; (c) B. Wu, L. Xu, S.F. Wang, Y. Wang, W.A. Zhang, Polym. Chem. 6 (2015) 4279e4289; (d) Y.X. Lei, H. Li, W.X. Gao, M.C. Liu, J.X. Chen, J.C. Ding, X.B. Huang, H.Y. Wu, J. Mater. Chem. C 2 (2014) 7402e7410; (e) J. Qiao, C.F. Chen, L. Qi, M.R. Liu, P. Dong, Q. Jiang, X.Z. Yang, X.Y. Mu, L.Q. Mao, J. Mater. Chem. B 2 (2014) 7544e7550. [4] (a) X.Z. Zhang, C.F. Chamberlayne, A. Kurimoto, N.L. Frankb, Harbron E. J. Chem. Commun. 52 (2016) 4144e4147; € hler, R. Hildner, J. Phys. Chem. A 120 (b) S. Baderschneider, U. Scherf, J. Ko (2016) 233e240; (c) K. Nakano, M. Nakano, B. Xiao, E.J. Zhou, K. Suzuki, I. Osaka, K. Takimiya, K. Tajima, Macromolecules 49 (2016) 1752e1760; (d) Q.Q. Shi, W.Q. Chen, J.F. Xiang, X.M. Duan, X.W. Zhan, Macromolecules 44 (2011) 3759e3765; (e) J. Wen, D. Luo, L. Cheng, K. Zhao, H.B. Ma, Macromolecules 49 (2016) 1305e1312.
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Highly enantioselective recognition of alaninol via the chiral BINAMbased fluorescence polymer sensor Wenjie Zhang a, Guo Wei b, Ziyu Wang a, Jing Ma a, Chengjian Zhu a, Yixiang Cheng a, * a
Key Lab of Mesoscopic Chemistry of MOE, Collaborative Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, China b School of Chemistry and Materials Science, Nanjing Normal University, Nanjing, 210097, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2016 Received in revised form 13 August 2016 Accepted 18 August 2016 Available online 20 August 2016
A novel chiral polymer P1 was designed and synthesized by the polymerization of (S)-2, 20 -binaphthyladiamine (BINAM) derivative monomer (S-M-1) with 2, 5-diiodo-1, 4-dioctyloxybenzene (M-2) via Pd(II)-catalyzed Sonogashira coupling reaction, and P2 could be obtained by the reduction reaction of P1 with NaBH4. Interestingly, the resulting chiral (S)-BINAM-based polymer P1 sensor can act as a “turn on” fluorescence enhancement sensor towards (D)-alaninol, and the value of enantiomeric fluorescence difference ratio (ef) can reach as high as 14.46. On the contrary, no fluorescence response behavior toward alaninol enantiomers could be observed for the polymer P2. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Chiral polymer Enantioselective recognition Sensor
1. Introduction The chiral enantioselective recognition based on fluorescence chemosensor is attracting particular interest because of the potential applications for screening high-throughput chiral catalysts, designing the highly active asymmetric catalysis, understanding the interactions between biological molecules, developing the useful chiral resolution processes, and so on [1]. Much attention has been paid on the design of the novel chiral fluorescence sensors which can afford a simple, rapid, and real-time analytical methodology for the detection of chiral enantiomer composition [2]. Recently, the enantioselective recognition system has been focused on the chiral polymer-based fluorescence sensors by introducing different functional groups in the conjugated polymer main chain backbones [3]. Meanwhile the delocalizable p-electronic conjugated “molecular wire” polymers can greatly enhance the fluorescence response signal when they can form the corresponding hostguest complexes with the chosen enantiomers [4]. The optically active 2, 20 -binaphthyladiamine (BINAM) has been regarded as one of the most important C2-symmetric chiral compounds due to the confined rotation of the two naphthalene [5]. These BINAM-based derivatives can not only show the stable chiral
* Corresponding author. E-mail addresses: [email protected] (C. Zhu), [email protected] (Y. Cheng). http://dx.doi.org/10.1016/j.polymer.2016.08.061 0032-3861/© 2016 Elsevier Ltd. All rights reserved.
configuration as well as high chiral induction, but also chiral discrimination response behavior in molecule recognition and asymmetric catalysis [6]. In our previous work, we first found an in situ generated 1:1 Zn(II)-containing BINAM-based polymer complex which can exhibit the direct and visual enantioselective recognition response of the N-Boc-protected alanine [7]. Recently our group also found a novel chiral (S)-BINAM-based polymer which can act as the pronounced fluorescence enhancement sensor only on Hþ and trivalent metal cations (Fe3þ, Cr3þand Al3þ) [2g]. In this paper, we continued to design and synthesize a novel (S)BINAM-based polymer P1 by modifying the microenviroment of the chiral binding moiety, which can show highly enantioselective recognition response of (D)-alaninol. 2. Results and discussions The synthetic procedures of the chiral polymers P1 and P2 sensors are illustrated in Scheme 1. The monomer (S-M-1), (S)-2, 20 binaphthyladiamine (BINAM) derivative was synthesized by the reaction of (S)-2, 20 -BINAM (BINAM ¼ 1, 10 -binaphthyl-2, 20 diamine) with 5-ethynyl-2-hydroxybenzaldehyde [8,9a] via nucleophilic addition-elimination reaction. The linker monomer M-2, 2, 5-diiodo-1, 4-dioctyloxybenzene was prepared according to the reported literature [7,9]. The corresponding chiral (S)-BINAMbased polymer P1 could be obtained by Pd(II)-catalyzed Sonogashira coupling reaction of the chiral monomer S-M-1 with the
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Scheme 1. Synthesis procedures of compounds M-1, M-2 and the chiral polymer P1, P2.
linker monomer M-2 and collected as a dark yellow powder in 71.7% yield. The chiral polymer P2 could be obtained with NaBH4 by the reduction reaction of P1 in 89.7% yield. The chiral polymer P1 has one well-resolved peak situated at d ¼ 8.61 ppm (the Supporting Information, Figs. SI 5 and 6, which can be assigned to the imine group (C (H) ¼ N). As we can see from the 1H NMR spectra of the polymer P2, this peak disappeared indicating that the imine group in P1 has been almost reduced for P2. The values of Mw, Mn, and PDI of the chiral polymer P1 are 16540, 11650 and 1.42, respectively. Both P1 and P2 are air-stable solid and have good solubility in common organic solvents including toluene, THF, CHCl3, CH2Cl2, and CH3CN, which can be due to the nonplanarity of the twisted chain framework and the flexible n-octyl substituents. In addition, the ethynyl linker can reduce steric hindrance between phenyl groups and also produce a beneficial influence on the stability of the resulting chiral polymer. According to the TGA results of the polymers P1 and P2 have highly thermal stability and no loss weight before 300 C, and tend to only 50% decompose at 700 C (see supporting information, Fig. SI 8. Therefore, the designed chiral polymers can act as a stable fluorescence chemosensor material. The fluorescence spectra of the chiral polymer P1 and P2 was measured in THF solution (10 mM corresponding to 1, 10 -binaphthyl moiety). The chiral polymer P1 shows very weak fluorescence emission situated at 419 nm (FPL ¼ 0.02) due to the strong and ordered intramolecular hydrogen bonding between imine and
hydroxyl of phenol, which leads to non-radiative transition. On the contrary, P2 can emit strong blue fluorescence situated at 402 nm (FPL ¼ 0.26), which may be attributed to the weak and disordered intramolecular hydrogen bonding between hydroxyl of phenol and amine (Fig. SI 9) [10]. Compared with our previous works, the first polymer sensor could response to both Hþ and trivalent metal cations, but the second chiral polymer sensor could exhibit “turn-on” fluorescence response only to Zn2þ(Scheme 2.) [2g] On the contrary, the chiral polymer P1 in this paper can exhibit the excellent fluorescence response on the enantioselective recognition of the alaninol enantiomers. As is evident from Scheme 2, although three kinds of (S)BINAM-based polymers have the similar polymer conjugated backbone structure, they can show quite different fluorescence response behaviors, which can be attributed to the obvious difference from the coordination binding space blocks [11]. In this paper, we further carried out the fluorescence responses of the chiral polymer P1 towards various chiral guest molecules in THF solution (10 mM). The enantioselective recognition response behavior of chiral guest isomers is related to the enantiomeric fluorescence difference ratio, ef, according to ef ¼ (ID I0)/(IL I0), in which I0 is the fluorescence intensity in the absence of the chiral substrate, herein, ID and IL represent the fluorescence intensities in the presence of (D)- and (L)-chiral enantiomers, respectively. Interestingly, we found that (L)-alaninol has a little effect on the
Scheme 2. Difference from chiral polymer linkers and chiral binding building block.
W. Zhang et al. / Polymer 101 (2016) 93e97
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fluorescence intensity of the chiral polymer P1 (Fig. 1d), but (D)alaninol can lead to an obvious fluorescence enhancement (Fig. 1d). In comparison to an only 3.69-fold increase upon the addition of (L)-alaninol, the fluorescence intensity of the chiral polymer P1 showed a pronounced gradual enhancement as high as 21.39-fold with the molar ratio increases of (D)-alaninol from 1:10 to 1:80 (Fig. 1d). The ef value for the chiral polymer P1 can reach as high as 14.46, demonstrating that the resulting (S)-BINAM-based polymer sensor P1 can exhibit excellent enantioselective recognition ability towards (D)-alaninol. The fluorescent enhancement response of the (S)-BINAM-based chiral polymer P1 for enantiomers of alaninol as a guest molecule can be regarded as the suppressed photoinduced electron transfer (PET) effect [12] from interactions of the protons of alaninol with the nitrogen atoms of the imine moieties through intramolecular hydrogen-bonding [13]. Upon the formation of hosteguest complexes, the lone pair of electrons on the nitrogen atom (imine group) is no longer available for PET, which can lead to the enhancement of fluorescence about the (S)-BINAM-based polymer sensor. Herein, the (S)-binaphthalene-based chiral polymer P1 has a rigid and stable main chain backbone, and “turn on” fluorescence-enhancement response behavior towards (D)-alaninol can be attributed to the inherent chiral interactions in the microenvironments in the building blocks and the steric repulsion between chiral receptors and chiral guest molecules. The building block of the (S)-binaphthalene receptor, which is composed of imine and hydroxy groups that are controlled by the dihedral angle of binaphthalene, can orient a well-defined spatial arrangement in the regular polymer backbone and well accommodate the formation of a more-stable S-D complex compared to the diastereomeric S-L complex (see supporting information, Fig. SI 17. Most importantly, the (S)-binaphthalene-based chiral polymer P1 emits only weak blue fluorescence in THF solution, but the solution color of P1 can turn on bright blue upon the addition of (D)-alaninol, which can be clearly observed by the naked eye (Fig. 1d, inset) under a commercially available UV lamp (lex ¼ 353 nm). On the contrary, the color of the (S)-binaphthalene-based chiral polymer sensor P1 solution still keeps weak blue upon the addition of (L)-alaninol indicating that the resulting (S)-binaphthalene-based chiral polymer P1 can act as a highly sensitive and selective fluorescence sensor for the direct visual discrimination of the alaninol enantiomers at a low concentration. In order to ascertain the enantioselectivity of the resulting (S)binaphthalene-based chiral polymer P1 towards other chiral enantiomers including phenylalaninol, phenylglycinol, valinol and phenylethylamine as shown in Fig. 2, we further performed a set of comparable experiments. Almost no enantioselective recognition response could be detected, and no fluorescence responses can be observed (supporting information Figs. SI 11e14. What's more, we also studied the fluorescence response behaviors of the chiral polymer P2 (10 mM in THF solution) towards various chiral guest molecules (Fig. 3). We found that almost no fluorescence change of P2 could be observed upon the addition of these guest molecules except phenylethylamine enantiomers (Fig. 3). But both R-/Sphenylethylamine enantiomers can lead to the similar fluorescence enhancement effect, no enantioselective recognition response can be observed. It may be attributed to the flexible and twisted
Fig. 1. Fluorescence spectra of (a) the chiral polymer P1 (10 mM in THF) and guest molecules L-/D-alaninol (50equiv. in THF) (b) polymer P1 (10 mM in THF) in the presence of D-alaninol (0, 10, 20, 30, 40, 50, 60, 70, 80 equiv.) (c) polymer P1 (10 mM in THF) in the presence of L-alaninol (0, 10, 20, 30, 40, 50, 60, 70, 80 equiv) and (d) Fluorescence enhancement of the chiral polymer P1 (10 mM in THF) with (L)- and (D)alaninol (lem ¼ 425 nm, lex ¼ 353 nm; slit: 10 nm, 8 nm) (insert: the photo of the fluorescence color changes of the polymer sensor P1 (1) when it interacts with Lalaninol (2) and D-alaninol (3)).
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Fig. 2. Chiral guests used for the enantioselective recognition.
polymers is carried out on a Shimadzu 10 Å with THF as the eluent and polystyrene as the standard. All reagents and solvents are commercially available A.R. grade. Et3N and THF are purified by distillation from sodium in the presence of benzophenone. The concentrations of the stock solution of amine alcohols are 1 104 mM in THF. Initially, each experiment was started with a 3.0 mL polymer solution with a known concentration (10 mM). The adding amount of each experiment example of amine alcohols has been noted in the graphical interpretation and the figure.
Fig. 3. Fluorescence spectra of the chiral polymer P2 (10 mM in THF, lex ¼ 353 nm, lem ¼ 425 nm, slit: 5 nm, 4 nm) and various guest molecules (50equiv. in THF).
backbone of the chiral polymer P2 in the disordered state and the irregular chain backbone in comparison with the rigid chain backbone structure of P1. 3. Conclusion In summary, we developed a novel (S)-BINAM-based chiral polymer sensor P1 which can act as a fluorescence sensor for enantioselective recognition response towards (D)-alaninol, and the color change can be clearly observed by the naked eyes under a commercially available UV lamp for direct and visual discrimination of alaninol enantiomers at low concentration. Furthermore, no response behavior toward chiral guest enantiomers could be observed for the corresponding chiral polymer P2 due to the flexible and twisted polymer chain backbone. 4. Experimental section 4.1. Instrumentation and materials NMR spectra were obtained using a 400-Bruker spectrometer 400 MHz for 1H NMR and 100 MHz for 13C NMR and reported as parts per million (ppm) from the internal standard TMS. Electrospray ionization mass spectra (ESI-MS) were measured on a Thermo Finnigan LCQ Fleet system. UVevis spectra were taken on a Perkine Elmer Lambda 25 spectrometer. Fluorescent spectra are collected with a Perkin-Elmer LS 55 spectrometer. C, H, O, N of elemental analyses are measured on an Elemental Vario MICRO analyzer. Specific rotation was measured with a Ruololph Research Analytical Autopol IV-T/V. Thermogravimetric analyses (TGA) are conducted on a Perkin-Elmer Pyris-1 instrument under N2 atmosphere. The gel permeation chromatography (GPC) analysis of the
4.1.1. Synthesis of (S) e 2, 20 -binaphthyladiamine (BINAM) derivative monomer (S-M-1) (S)-2, 20 -BINAM (568.70 mg, 2.0 mmol) and 5-ethynyl-2hydroxybenzaldehyde [2a,9b] (584.60 mg, 4.0 mmol) were dissolved in 40 mL dry ethanol. The mixed solution was first stirred at 40 Cfor 12 h under an Ar atmosphere, and then the reaction mixture was cooled to room temperature before the solvent was removed by a rotary evaporator, and the mixed solvents of ethyl acetate and petroleum ether was added to the result mixture for recrystallization to afford the yellow solid product (0.91 g, yield 84.0%). 1H NMR (400 MHz, CDCl3) d: 12.35 (s, 2H), 8.64 (s, 2H (HC] N)), 8.12 (d, J ¼ 8.8 Hz, 2H (ArH)), 7.98 (d, J ¼ 8.1 Hz, 2H (ArH)), 7.65 (d, J ¼ 8.8 Hz, 4H (ArH)), 7.47 (s, 4H (ArH)), 7.39 (d, J ¼ 2.1 Hz, 2H (ArH)), 7.26 (s, 2H (ArH)), 6.64 (d, J ¼ 8.6 Hz, 2H (ArH)), 2.94 (s, 2H (HC^C)). 13C NMR (100 MHz, CDCl3) d: 161.29, 160.74, 143.04, 136.02, 133.16, 132.69, 130.23, 129.89, 128.38, 127.22, 126.44, 126.25, 119.09, 117.52, 116.53, 112.40, 82.92, 75.85. ESI-MS: [Mþ1]þ: 541.15. Anal. Calcd for C38H24N2O2: C, 84.42; H, 4.47; N, 5.18. Found: C, 83.86; H, 5.13; N, 4.62.
4.1.2. Synthesis of the chiral polymer sensor P1 S-M-1 (108.20 mg, 0.20 mmol) and 2, 5-diiodo-1, 4-dioctyloxybenzene (M-2) [8] (117.20 mg, 0.20 mmol), CuI (1.0 mg, 0.005 mmol) and Pd (PPh3)2Cl2 (3.5 mg, 0.005 mmol) are dissolved in 6 mL Et3N and 6 mL THF. The solution was first stirred for 48 h at 80 Cunder an Ar atmosphere, and then the mixture was cooled to room temperature and filtered. The residue was dissolved in a small quantity of THF after the solvent was removed under reduced pressure, and then 80 mL of methanol was added to precipitate the polymer. A dark yellow polymer solid was filtered off and washed by using methanol several times. Further purification was carried out by dissolving the polymer in THF to precipitate in methanol solution again. The polymer was dried under vacuum for 24 h at room temperature. The final yield is 71.7% (75.50 mg). GPC results: Mw ¼ 16540, Mn ¼ 11650, PDI ¼ 1.42. [a] 25 D ¼ þ296.0 (c 0.20, THF). 1 H NMR (400 MHz, CDCl3) d: 12.34 (m, OH), 8.61 (s, HC]N), 8.23e7.60 (m, ArH), 7.40e7.10 (m, ArH), 6.91e6.56 (m, ArH), 3.95e3.41 (m, CH2O), 1.74e1.20 (m, CH2), 0.82e0.75 (m, CH3). Anal. Calcd for C60H60N2O4: C, 82.54; H, 6.93; N, 3.21. Found: C, 81.96; H, 6.33; N, 2.40.
W. Zhang et al. / Polymer 101 (2016) 93e97
4.1.3. Synthesis of the chiral polymer sensor P2 P1 (0.089 g) was dissolved in 10 mL THF and 10 mL MeOH mixed solvents, and then NaBH4 (450 mg) was added to the above solution in batches. The reaction mixture was first stirred until the yellow color turn light at room temperature. 10 mL water was added to stop the reduction reaction, and then the solution continued to stir for another 30 min. The mixture was extracted by CH2Cl2 (3 20 mL) solvent. The organic layers were dried with anhydrous Na2SO4 and evaporated under reduced pressure to collect the polymer P2 as a dark yellow solid (0.079 g, 89.7%). 1H NMR (400 MHz, CDCl3) d: 7.32e7.26 (m, ArH), 7.00e6.98 (m, ArH), 6.82e6.81 (m, ArH), 3.99e3.75 (m, CH2O), 1.87e1.18 (m, CH2), 0.96e0.78 (m, CH3). Acknowledgment This work was supported by the National Natural Science Foundation of China (21372114 and 21474048). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2016.08.061. References [1] (a) C. Wang, E. Wu, X.D. Wu, X.C. Xu, G.Q. Zhang, L. Pu, J. Am. Chem. Soc. 137 (2015) 3747e3750; (b) Y. Zhou, J. Yoon, Chem. Soc. Rev. 41 (2012) 52e67; (c) U. Filip, J. Janusz, J. Org. Chem. 80 (2015) 4235e4243; (d) G.H. Xie, W. Tian, L.P. Wen, K. Xiao, Z. Zhang, Q. Liu, G.L. Hou, P. Li, Y. Tian, L. Jiang, Chem. Commun. 51 (2015) 3135e3138; (e) L. Zhang, Q.X. Jin, K. Lv, L. Qin, M.H. Liu, Chem. Commun. 51 (2015) 4234e4236; (f) F.Y. Song, N. Fei, F. Li, S.W. Zhang, Y.X. Cheng, C.J. Zhu, Chem. Commun. 49 (2013) 2891e2893; (g) H.J. Huang, W.T. Yang, J.P. Deng, RSC Adv. 5 (2015) 26236e26245; (h) J.M. Jiao, F. Li, S.W. Zhang, Y.W. Quan, W.H. Zheng, Y.X. Cheng, C.J. Zhu, Macromol. Rapid Commun. 35 (2014) 1443e1449; (i) S.S. Yu, L. Pu, Tetrahedron 71 (2015) 745e772; (j) G. Wei, F.D. Meng, Y.X. Wang, Y.X. Cheng, C.J. Zhu, Macromol. Rapid Commun. 35 (2014) 2077e2081. [2] (a) K.W. Bentley, C. Wolf, J. Am. Chem. Soc. 135 (2013) 12200e12203; (b) S.S. Yu, W. Plunkett, M. Kim, L. Pu, J. Am. Chem. Soc. 134 (2012) 20282e20285; (c) K.L. Wen, S.S. Yu, Z. Huang, L.M. Chen, M. Xiao, X.Q. Yu, L. Pu, J. Am. Chem. Soc. 137 (2015) 4517e4524; (d) F. Wang, R. Nandhakumar, Y. Hu, D. Kim, K.M. Kim, J. Yoon, J. Org. Chem. 78 (2013) 11571e11576; (e) K.W. Bentley, C. Wolf, J. Org. Chem. 79 (2014) 6517e6531; (f) A. Bencini, C. Coluccini, A. Garau, C. Giorgi, V. Lippolis, L. Messori, D. Pasini, S. Puccionia, Chem. Commun. 48 (2012) 10428e10430; (g) L. Wang, F. Li, X.H. Liu, G. Wei, Y.X. Cheng, C.J. Zhu, J. Polym. Sci. Part A Polym. Chem. 51 (2013) 4070e4075; (h) Q.T. Li, H.M. Guo, Y.B. Wu, X. Zhang, Y.F. Liu, J.Z. Zhao, J. Fluoresc. 21 (2011) 2077e2084. [3] (a) Q. Li, C.J. Wang, H.L. Tan, G. Tang, J. Gao, C.H. Chen, RSC Adv. 6 (2016) 17811e17817; (b) D. Giri, S.K. Patra, RSC Adv. 5 (2015) 79011e79021; (c) B. Wu, L. Xu, S.F. Wang, Y. Wang, W.A. Zhang, Polym. Chem. 6 (2015) 4279e4289; (d) Y.X. Lei, H. Li, W.X. Gao, M.C. Liu, J.X. Chen, J.C. Ding, X.B. Huang, H.Y. Wu, J. Mater. Chem. C 2 (2014) 7402e7410; (e) J. Qiao, C.F. Chen, L. Qi, M.R. Liu, P. Dong, Q. Jiang, X.Z. Yang, X.Y. Mu, L.Q. Mao, J. Mater. Chem. B 2 (2014) 7544e7550. [4] (a) X.Z. Zhang, C.F. Chamberlayne, A. Kurimoto, N.L. Frankb, Harbron E. J. Chem. Commun. 52 (2016) 4144e4147; € hler, R. Hildner, J. Phys. Chem. A 120 (b) S. Baderschneider, U. Scherf, J. Ko (2016) 233e240; (c) K. Nakano, M. Nakano, B. Xiao, E.J. Zhou, K. Suzuki, I. Osaka, K. Takimiya, K. Tajima, Macromolecules 49 (2016) 1752e1760; (d) Q.Q. Shi, W.Q. Chen, J.F. Xiang, X.M. Duan, X.W. Zhan, Macromolecules 44 (2011) 3759e3765; (e) J. Wen, D. Luo, L. Cheng, K. Zhao, H.B. Ma, Macromolecules 49 (2016) 1305e1312.
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