Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 150 (2015) 814–820
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Substituent effects on anion sensing of salicylidene Schiff base derivatives: Tuning sensitivity and selectivity Libin Zang a,b, Shimei Jiang a,⇑ a b
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun 130012, PR China College of GeoExploration Science and Technology, Jilin University, Changchun, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Synthesis of four novel colorimetric
anion sensors based on salicylidene Schiff base. Sensitivity and selectivity are interrelated with the substituent of chromophores. F sensing through color changes based on deprotonation. Sensor 1 can be applied in qualitative detection of F in aqueous medium.
a r t i c l e
i n f o
Article history: Received 5 February 2015 Received in revised form 18 May 2015 Accepted 4 June 2015 Available online 17 June 2015 Keywords: Salicylidene Schiff base Anion sensing Substituent effects Sensitivity Selectivity
a b s t r a c t A series of colorimetric anion sensors using the salicylidene Schiff bases with different substituents, including electron donating group (tert-butyl, in sensor 2), conjugated group (naphthyl, in sensor 3) and electron withdrawing group (chlorine, in sensor 4), respectively, have been developed. The substituents can not only impact chromogenic signal output, but also tune the sensitivity and selectivity of the anion sensing by their specific electron push–pull features. In particular, both 1 (without substituent) and 2 show high selectivity for F over Cl , Br , I , AcO and H2PO4 , but the sensitivity of 2 is poorer than 1 due to the effect of electron donating groups. Sensor 3 exhibits higher sensitivity for F than 1, but it is disturbed by the weak response to AcO and H2PO4 . Sensor 4 has the highest sensitivity for F , but shows the significant response to AcO and H2PO4 , which also decreases the selectivity for F . Finally, analytical applications of 1 for the detection of F in aqueous medium and toothpaste have been studied. Ó 2015 Published by Elsevier B.V.
Introduction Anion binding and sensing is now a major field within supramolecular chemistry with potential applications in pollutant sequestration, biomedical and environmental monitoring, anion exchange, and anion transport [1,2]. Considerable attention has been focused on the design of colorimetric hosts that can ⇑ Tel.: +86 431 85168474; fax: +86 431 85193421. E-mail addresses: [email protected] (L. Zang), [email protected] (S. Jiang). http://dx.doi.org/10.1016/j.saa.2015.06.028 1386-1425/Ó 2015 Published by Elsevier B.V.
selectively recognize anion species through visible color changes with the advantages that the responses can be conveniently detected by the naked eye [3,4]. A variety of colorimetric sensors containing a number of different types of anion binding group have been designed and tested for anion recognition and sensing over the past years [5–11]. Most of these sensors are developed based on hydrogen bonding interaction [12–14], deprotonation [15–17] or chemical reaction [18–20]. However, many anion sensors are not able to differentiate F , AcO and H2PO4 because of their similar basicity and surface charge density [21,22]. Among various
L. Zang, S. Jiang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 150 (2015) 814–820
anions, F is attracting a great deal of interest because of the importance of this anion in dental health and its possible toxicity when administered in high doses [23,24]. Thus, there is considerable current interest in sensitive and selective recognition of F . Salicylidene Schiff base derivatives, as an excellent class of anion receptors, have been integrated into a variety of anion sensors to detect F or other specific anionic guests [25,26]. However, it is not sufficient to engage in the act of solving the problem of differentiating F , AcO , and H2PO4 [27]. In the literature, several studies have shown that the substituents have a great impact on the selectivity and sensitivity of anion sensors [12,28]. Therefore, changing substituting group of sensors should be an effective way to solve this problem. However, to the best of our knowledge, the substituent effects on anion sensing of salicylidene Schiff base derivatives have not been reported so far. Herein, four anion chemosensors based on salicylidene Schiff bases bearing pyrene moiety have been designed and synthesized. As depicted in Scheme 1, they are sensor 1 (without substituent), electron donating groups (tert-butyl) substituted sensor 2, conjugated group (naphthyl) substituted sensor 3, and electron withdrawing groups (chlorine) substituted sensor 4. These representative substituent groups will help us understand the substituent effects on anion sensing process, including color change, the sensitivity and the selectivity. Meanwhile, the introduction of pyrene group induces an increase in conjugate level of sensors, and this will provide a good chromogenic signal output in the visible region. All the four sensors exhibit excellent responsive properties but various effects in molecular recognition of anions. The mechanisms controlling the maximum of the absorption shift, as
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well as the sensitivity and selectivity of anion recognition have been investigated in detail (see below). Experimental Materials and methods All the materials for synthesis and spectra were purchased from commercial suppliers and used without further purification. All solvents and reagents used in the spectroscopic studies were analytical grade. All anions were in the form of tetrabutylammonium (TBA) salts. 1H NMR (TMS) was recorded on a Bruker UltraShield 500 MHz spectrometer. UV–vis absorption spectra were taken on a Shimadzu 3100 UV–VIS–NIR recording spectrophotometer using a 2 nm slit width in 1 cm quartz cells. The binding abilities of 1, 2, 3 and 4 with anions were investigated by UV–vis absorption spectroscopy in CH3CN solution using a constant host concentration (10 lM) and increasing concentrations of anions. The equilibrium constants of the sensors and anions were calculated by the method which had been reported previously [29]. Synthesis Sensors 1, 2, 3 and 4 were prepared from the condensation of pyren-1-ylmethylene-hydrazine [30] with Salicylaldehyde, 3,5-ditert-butyl-2-hydroxy-benzaldehyde, 2-hydroxy-naphthale ne-1-carbaldehyde and 3,5-dichloro-2-hydroxy-benzaldehyde, respectively, as depicted in Scheme 1 and characterized by 1H NMR (Figs. S1–S5).
Scheme 1. The synthetic routes of 1, 2, 3 and 4.
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Pyren-1-ylmethylene-hydrazine A 0.2000 g (0.87 mmol) sample of l-pyrenecarboxaldehyde was suspended in 10 mL of ethanol, to which was added 0.17 g (2.61 mmol) of hydrazine hydrate, and the mixture was stirred for 4 h at room temperature. During the reaction, a yellow solid precipitate appeared. After the reaction, the precipitate was then filtered and was washed with 15 mL of cold ethanol, affording the product as a yellow powder (0.0919 g, 43%). 1H NMR (500 MHz, DMSO): 7.193 (s, 2H), 8.065 (t, J = 7.5 Hz, 1H), 8.149 (s, 2H), 8.269 (m, 4H), 8.394 (d, J = 8.0 Hz, 1H), 8.710 (d, J = 9.5 Hz, 1H), 8.773 (s, 1H). 2-(Pyren-1-ylmethylene-hydrazonomethyl)-phenol (1) A 0.0919 g (0.38 mmol) sample of pyren-1-ylmethylenehydrazine was suspended in 10 mL of ethanol, to which was added 35 lL of Salicylaldehyde, and the mixture was stirred for 15 h at room temperature. During the reaction, a yellow solid precipitate appeared. After the reaction, the precipitate was then filtered and was washed with 15 mL of cold ethanol, affording the product 1 as a yellow powder (0.1129 g, 80%). 1H NMR (500 MHz, DMSO): 7.034 (m, 2H), 7.446 (t, J = 7.0 Hz, 1H), 7.770 (d, J = 6.0 Hz, 1H), 8.173 (t, J = 7.5 Hz, 1H), 8.284 (d, J = 9.0 Hz, 1H), 8.352 (d, J = 9.0 Hz, 1H), 8.424 (m, 4H), 8.757 (d, J = 8.5 Hz, 1H), 9.152 (s, 1H), 9.181 (d, J = 9.5 Hz, 1H), 9.901 (s, 1H), 11.473 (s, 1H). 2,4-Di-tert-butyl-6-(pyren-1-ylmethylene-hydrazonomethyl)-phenol (2) A 0.2443 g (1.0 mmol) sample of pyren-1-ylmethylenehydrazine was suspended in 30 mL of ethanol, to which was added 0.2348 g (1.0 mmol) of 3,5-di-tert-butyl-2-hydroxy-benzaldehyde, and the mixture the mixture was refluxed for 5 h. During the reaction, a yellow solid precipitate appeared. After the reaction, the precipitate was then filtered and was washed with 15 mL of cold ethanol, affording the product 2 as a yellow powder (0.3505 g, 76%). 1H NMR (500 MHz, CDCl3): 1.391 (s, 9H), 1.556 (s, 9H), 7.278 (d, J = 2.5 Hz, 1H), 7.515 (d, J = 2.5 Hz, 1H), 8.087 (t, J = 7.5 Hz, 1H), 8.131 (d, J = 9.0 Hz, 1H), 8.196 (d, J = 9.0 Hz, 1H), 8.275 (m, 4H), 8.723 (d, J = 8.0 Hz, 1H), 8.930 (d, J = 9.5 Hz, 1H), 9.005 (s, 1H), 9.690 (s, 1H), 12.429 (s, 1H). 1-(Pyren-1-ylmethylene-hydrazonomethyl)-naphthalen-2-ol (3) A 0.2440 g (1.0 mmol) sample of pyren-1-ylmethylenehydrazine was suspended in 30 mL of ethanol, to which was added 0.1720 g (1.0 mmol) of 2-hydroxy-naphthalene-1-carbaldehyde, and the mixture was refluxed for 5 h. During the reaction, a yellow solid precipitate appeared. After the reaction, the precipitate was then filtered and was washed with 15 mL of cold ethanol, affording the product 3 as a yellow powder (0.2917 g, 73%). 1H NMR (500 MHz, DMSO): 7.310 (d, J = 9.0 Hz, 1H), 7.429 (t, J = 7.5 Hz, 1H), 7.616 (t, J = 7.5 Hz, 1H), 7.825 (d, J = 8.0 Hz, 1H), 7.897 (d, J = 8.5 Hz, 1H), 8.073 (t, J = 7.5 Hz, 1H), 8.114 (d, J = 8.5 Hz, 1H), 8.179 (d, J = 9.0 Hz, 1H), 8.267 (m, 5H), 8.762 (d, J = 8.0 Hz, 1H), 8.905 (d, J = 9.5 Hz, 1H), 9.670 (s,1H), 9.900 (s, 1H). 2,4-Dichloro-6-(pyren-1-ylmethylene-hydrazonomethyl)-phenol (4) A 0.2440 g (1.0 mmol) sample of pyren-1-ylmethylenehydrazine was suspended in 30 mL of ethanol, to which was added 0.1910 g (1.0 mmol) of 3,5-dichloro-2-hydroxy-benzaldehyde, and the mixture was refluxed for 20 h at room temperature. During the reaction, a yellow solid precipitate appeared. After the reaction, the precipitate was then filtered and was washed with 15 mL of cold ethanol, affording the product 4 as a yellow powder (0.3505 g, 84%). 1H NMR (500 MHz, DMSO): 7.776 (d, J = 2.5 Hz, 1H), 7.844 (d, J = 2.5 Hz, 1H), 8.183 (t, J = 7.5 Hz, 1H), 8.294 (d, J = 9.0 Hz, 1H), 8.371 (d, J = 9.0 Hz, 1H), 8.438 (m, 4H), 8.771 (d, J = 8.0 Hz,
1H), 9.146 (s, 1H), 9.154 (d, J = 8.0 Hz, 1H), 10.066 (s, 1H), 12.556 (s, 1H). Results and discussion Fluoride sensing The fluoride binding properties of 1, 2, 3 and 4 are firstly studied by UV–visible spectroscopy. In detail, the fluoride anions in the form of tetrabutylammonium (TBA) salts are added gradually to CH3CN solution of 1, 2, 3 and 4 (10 lM). As shown in Fig. 1, the max absorption peaks at 388 nm (sensor 1, e = 38,100 cm 1 M 1), 391 nm (sensor 2, e = 38,600 cm 1 M 1), 416 nm (sensor 3, e = 39,300 cm 1 M 1) and 387 nm (sensor 4, e = 34,700 cm 1 M 1) decrease gradually upon the addition of F , and new bands develop simultaneously at 494 nm (sensor 1, e = 25,900 cm 1 M 1), 532 nm (sensor 2, e = 24,700 cm 1 M 1), 516 nm (sensor 3, e = 28,900 cm 1 M 1) and 486 nm (sensor 4, e = 28,200 cm 1 M 1), respectively. There are isosbestic points in the four UV–vis titration spectra, confirming the changes between two clearly defined structures. The absorption spectra of the four sensors show large red shifts (99–141 nm, Table 1) after adding F . It is noteworthy that the red shift of 2 is much larger than that of other three sensors. These phenomena are the results of intramolecular charge transfer (ICT) [27]. As can be expected from the UV–vis spectra, color changes occur after adding F to the four sensors. As shown in Fig. 2, the color of solution of 1 turns red, solution of 2 turns purple, solution of 3 turns pink, and solution of 4 turns orange red, respectively. These phenomena mirror the red-shift observed in the absorption spectra. These results not only show that all the four sensors have good responsive properties to F , but also indicate the introduction of different substituents can effectively modulate the color change, the molar absorptivity and red shift of the sensors. As mentioned above, anion sensors based on salicylidene Schiff base always form hydrogen bonding or deprotonation on the OH group during the sensing process. In our case, deprotonation of the sensors is elucidated in the 1H NMR titration spectra. The protons of 1 give the clearest signal and the 1H NMR titration spectra of 1 ultimately are used in determining the structural changes that were occurring. As depicted in Fig. 3, the signal of OH proton at 11.473 ppm disappears after the addition of 0.5 equiv of F . Upon the gradual addition of F , the signal of Ha, Hb, Hc, Hd and He shift upfield gradually, and it indicates the increase of the electron density on the benzene ring owing to the deprotonation of the OH group. A new 1:2:1 triplet signal at 16.2 ppm appears after the addition of 5 equiv of fluoride, which is attributed to the FHF dimer (Fig. 3). The existence of this new species confirms the deprotonation of the OH group [29]. Substituent effects on sensitivity and selectivity The benzene ring of the sensor 1 has no substituent. In this case, the absorption spectra would reach saturation after adding 70 equiv of F (0.7 mM). However, sensor 2 has two tert-butyl substituents, which are electron donating. Hence, compared to 1, there will be a slightly larger negative charge in the conjugated system. The bond between the proton and oxygen of the hydroxyl group will be stronger and thus deprotonation will be more difficult. This is reflected experimentally with the observation that more F (about 100 equiv, 1 mM) is needed to allow for deprotonation. In other words, electron donating groups will decrease the sensitivity for F . Conjugated group has exactly the opposite effect. The naphthyl group in sensor 3 has a conjugation effect, which will decrease the negative charge in the conjugated system. Thus, the deprotonation
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Fig. 1. Absorption spectra changes of 1 (a), 2 (b), 3 (c), 4 (d) in CH3CN (10 lM) upon the addition of F respectively. Inset: absorbance of new peaks as a function of F respectively.
Table 1 Spectroscopic properties of 1, 2, 3 and 4 in the absence and presence of F in CH3CN. Compound
kmax (nm)
e (cm
1 1+F 2 2+F 3 3+F 4 4+F
388 494 391 532 416 516 387 486
38,100 25,900 38,600 24,700 39,300 28,900 34,700 28,200
1
M
1
)
Dkred
shift
(nm)
106 141 100 99
of the hydroxyl group will be easier to take place. And the saturation value of F was 34 equiv (0.34 mM) accordingly. In addition, the naphthyl group in 3 gives rises to a much higher kmax compared to that of 1. The chlorine substituents in sensor 4 have a high electronegativity and thus have an electron withdrawing effect. Hence, compared to 1, the bond between the proton and oxygen of the hydroxyl group will be weaker and thus deprotonation will be easier. Experimentally, sensor 4 only needs 3 equiv F (30 lM) to get its limiting value. The results suggest that electron withdrawing groups increased the sensitivity of fluoride response significantly. Furthermore, the equilibrium constants of sensors 1, 2, 3 and 4 with fluoride ions are calculated according to UV–vis titration spectra [29]. As shown in Table 2, the order of fluoride affinities in four sensors is 2 < 1 < 3 < 4, which is consistent with above conclusions. Meanwhile, the Benesi–Hildebrand plots indicate that the stoichiometry of sensors and fluoride is 1:2 (Fig. S8). Generally, the
electron donating groups would decrease the sensitivity of the sensors, but the conjugation effect and electron withdrawing groups would increase the sensitivity to fluoride. Actually, these substituent groups influence the acidity of OH proton, which, in turn, change the availability of OH moiety for deprotonation and affinity of sensors toward anions. Though all the four sensors display good sensing ability to fluoride, the selectivity of fluoride over other anions, such as AcO and H2PO4 , is also important. Up to now, most of the related research show that the performance of fluoride sensors suffered from deleterious interference of these anions. As shown in Fig. 2, both 1 and 2 respond independently to F , while other anions cannot cause change neither in absorption spectra nor in color, even when adding enough anions. On the contrary, 3 and 4 exhibit significant responses not only to the most basic F but also to moderate basic AcO and H2PO4 . Sensor 3 has weaker binding ability to AcO and H2PO4 than F , and the absorption spectra would reach saturation after adding 1200 equiv (12 mM) of AcO and 1500 equiv (15 mM) of H2PO4 (Fig. S7). The equilibrium constants of 3 with AcO and H2PO4 are 6.62 104 M 2 and 3.34 104 M 2, respectively. Interestingly, after adding AcO and H2PO4 the saturation peak intensities of absorption spectra are also lower than F . By contrast, sensor 4 displays the strong interaction with F , AcO and H2PO4 . The saturation value of AcO and H2PO4 are 9 equiv (90 lM) and 17 equiv (170 lM), respectively (Fig. S8). The equilibrium constants of 4 with AcO and H2PO4 are 3.81 109 M 2 and 8.43 108 M 2, even higher than other sensors with the most electronegative fluoride. This is presumably due to the strong electron withdrawing group increasing the acidity of the OH group of sensors. To sum up, as depicted in Fig. 4, either no substitute or with
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L. Zang, S. Jiang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 150 (2015) 814–820
Fig. 2. UV–visible spectra changes and color changes observed upon the addition of various anions to the solutions of 1, 2, 3 and 4 in CH3CN (10 lM).
electron donating groups, sensor 1 and 2 only responded to F with high selectivity. However, the conjugation effect and electron withdrawing group will decrease the selectivity of recognition to F .
Analytical application In view of the high selectivity and sensitivity of sensor 1 in the pure organic solvent, we also designed the experiments in DMSO–
L. Zang, S. Jiang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 150 (2015) 814–820
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Fig. 3. Partial 1H NMR spectra of 1 (2 mM, DMSO-d6) with the addition of 0 equiv, 0.5 equiv, 1.0 equiv, 2.0 equiv, 5.0 equiv, 10 equiv, 20 equiv of TBAF respectively.
Table 2 Equilibrium constants of 1, 2, 3 and 4 with F , AcO , H2PO4 in CH3CN as determined from UV–vis absorption titrations. Anion
Sensor 1 (Ks in M 2)
Sensor 2 (Ks in M 2)
Sensor 3 (Ks in M 2)
Sensor 4 (Ks in M 2)
F AcO H2PO4
4.65 106 – –
1.01 106 – –
4.58 107 6.62 104 3.34 104
6.54 109 3.81 109 8.43 108
H2O (95:5, v/v) solution to further investigate its performance in the presence of water. It is well-known that upon adding the protic solvents such as water will competitively form hydrogen-bonding with the binding site of sensors for anions. Meanwhile, anions are strongly solvated in aqueous medium due to their high hydration free energy which makes the detection of anions in water medium much more difficult [31]. Fortunately, in case of sensor 1, the adding of F showed detectable color and spectral responses in mixed solvent DMSO–H2O (95:5, v/v) (Fig. 5). These results clearly
Fig. 4. Proposed sensing mechanisms and properties of 1, 2, 3 and 4 for the detection of anions.
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substituents of chromophores constituting the anion binding site. The sensitivities follow the trend 4 > 3 > 1 > 2, which is in consistence with the push–pull feature of the substituents. Additionally, for the selectivity of F recognition, both 1 and 2 are much higher. However, 3 and 4 are not able to differentiate F , AcO and H2PO4 . This work confirms the importance of electron push–pull character for the substituent effects. Furthermore, we also demonstrate that the conjugation effect and electron withdrawing groups can also induce a decrease in the selectivity for F . Finally, the real sample analysis of sensor 1 is successfully achieved through the qualitative estimation of F in water and the commercially available toothpaste by simple and easy colorimetric method. This study benefits the future potential application of the influence rule of the substituent on the anion recognition of salicylidene Schiff base anion chemosensors, as well as their substituent structures, for their possible use in the designing and synthesizing of more effective anion sensors. Fig. 5. UV–visible spectra changes and of 1 (50 lM) in DMSO–H2O (95:5, v/v) upon the addition of (0–150 equiv). Inset showing the color change observed after adding F .
Acknowledgements This work was supported by the National Basic Research Program of China (2012CB933800) and the National Natural Science Foundation of China (21374036). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.06.028. References
Fig. 6. (a) Color change of test strips of 1 with F in water; (b) real sample analysis of 1 (1 10 3 M, DMSO) for the qualitative detection of F in toothpaste.
indicated the potential application of 1 for the selective detection of F in aqueous medium. To evaluate the practical applicability of sensor 1, paper-made test strips were prepared by immersing filter papers into a CH2Cl2 solution of 1 (4 103 M) and then dried. Then, the test strips were immersed in an aqueous solution of F (0.08 M), and the strips were subsequently dried. As shown in Fig. 6a, obvious color change was observed in the presence of F . This result indicated that the paper-made test strips could detect F in water qualitatively. Finally, the analytical application of the sensor 1 for the analysis of real samples was investigated. For real sample analysis, we carried out the qualitative detection of F ions in commercially available toothpaste. The test solutions for this experiment were prepared by adopting recently reported method [32]. The toothpaste sample solution was prepared as 100 mg/mL in water. Upon the addition of toothpaste solution, a distinct color change of 1 was observed for the qualitative detection of F (Fig. 6b).
Conclusions In this study, a series of novel colorimetric sensors with various substituents for the optical measurement of anions has been developed based on the salicylidene Schiff base system. It is found that all of these anion sensors display colorimetric responses for F but with different anion sensitivity and response selectivity for different electron push–pull feature of the substituents. Considering the sensitivity and selectivity of the anion sensors, we conclude that the deprotonation induced by anions interrelate with the
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Substituent effects on anion sensing of salicylidene Schiff base derivatives: Tuning sensitivity and selectivity Libin Zang a,b, Shimei Jiang a,⇑ a b
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun 130012, PR China College of GeoExploration Science and Technology, Jilin University, Changchun, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Synthesis of four novel colorimetric
anion sensors based on salicylidene Schiff base. Sensitivity and selectivity are interrelated with the substituent of chromophores. F sensing through color changes based on deprotonation. Sensor 1 can be applied in qualitative detection of F in aqueous medium.
a r t i c l e
i n f o
Article history: Received 5 February 2015 Received in revised form 18 May 2015 Accepted 4 June 2015 Available online 17 June 2015 Keywords: Salicylidene Schiff base Anion sensing Substituent effects Sensitivity Selectivity
a b s t r a c t A series of colorimetric anion sensors using the salicylidene Schiff bases with different substituents, including electron donating group (tert-butyl, in sensor 2), conjugated group (naphthyl, in sensor 3) and electron withdrawing group (chlorine, in sensor 4), respectively, have been developed. The substituents can not only impact chromogenic signal output, but also tune the sensitivity and selectivity of the anion sensing by their specific electron push–pull features. In particular, both 1 (without substituent) and 2 show high selectivity for F over Cl , Br , I , AcO and H2PO4 , but the sensitivity of 2 is poorer than 1 due to the effect of electron donating groups. Sensor 3 exhibits higher sensitivity for F than 1, but it is disturbed by the weak response to AcO and H2PO4 . Sensor 4 has the highest sensitivity for F , but shows the significant response to AcO and H2PO4 , which also decreases the selectivity for F . Finally, analytical applications of 1 for the detection of F in aqueous medium and toothpaste have been studied. Ó 2015 Published by Elsevier B.V.
Introduction Anion binding and sensing is now a major field within supramolecular chemistry with potential applications in pollutant sequestration, biomedical and environmental monitoring, anion exchange, and anion transport [1,2]. Considerable attention has been focused on the design of colorimetric hosts that can ⇑ Tel.: +86 431 85168474; fax: +86 431 85193421. E-mail addresses: [email protected] (L. Zang), [email protected] (S. Jiang). http://dx.doi.org/10.1016/j.saa.2015.06.028 1386-1425/Ó 2015 Published by Elsevier B.V.
selectively recognize anion species through visible color changes with the advantages that the responses can be conveniently detected by the naked eye [3,4]. A variety of colorimetric sensors containing a number of different types of anion binding group have been designed and tested for anion recognition and sensing over the past years [5–11]. Most of these sensors are developed based on hydrogen bonding interaction [12–14], deprotonation [15–17] or chemical reaction [18–20]. However, many anion sensors are not able to differentiate F , AcO and H2PO4 because of their similar basicity and surface charge density [21,22]. Among various
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anions, F is attracting a great deal of interest because of the importance of this anion in dental health and its possible toxicity when administered in high doses [23,24]. Thus, there is considerable current interest in sensitive and selective recognition of F . Salicylidene Schiff base derivatives, as an excellent class of anion receptors, have been integrated into a variety of anion sensors to detect F or other specific anionic guests [25,26]. However, it is not sufficient to engage in the act of solving the problem of differentiating F , AcO , and H2PO4 [27]. In the literature, several studies have shown that the substituents have a great impact on the selectivity and sensitivity of anion sensors [12,28]. Therefore, changing substituting group of sensors should be an effective way to solve this problem. However, to the best of our knowledge, the substituent effects on anion sensing of salicylidene Schiff base derivatives have not been reported so far. Herein, four anion chemosensors based on salicylidene Schiff bases bearing pyrene moiety have been designed and synthesized. As depicted in Scheme 1, they are sensor 1 (without substituent), electron donating groups (tert-butyl) substituted sensor 2, conjugated group (naphthyl) substituted sensor 3, and electron withdrawing groups (chlorine) substituted sensor 4. These representative substituent groups will help us understand the substituent effects on anion sensing process, including color change, the sensitivity and the selectivity. Meanwhile, the introduction of pyrene group induces an increase in conjugate level of sensors, and this will provide a good chromogenic signal output in the visible region. All the four sensors exhibit excellent responsive properties but various effects in molecular recognition of anions. The mechanisms controlling the maximum of the absorption shift, as
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well as the sensitivity and selectivity of anion recognition have been investigated in detail (see below). Experimental Materials and methods All the materials for synthesis and spectra were purchased from commercial suppliers and used without further purification. All solvents and reagents used in the spectroscopic studies were analytical grade. All anions were in the form of tetrabutylammonium (TBA) salts. 1H NMR (TMS) was recorded on a Bruker UltraShield 500 MHz spectrometer. UV–vis absorption spectra were taken on a Shimadzu 3100 UV–VIS–NIR recording spectrophotometer using a 2 nm slit width in 1 cm quartz cells. The binding abilities of 1, 2, 3 and 4 with anions were investigated by UV–vis absorption spectroscopy in CH3CN solution using a constant host concentration (10 lM) and increasing concentrations of anions. The equilibrium constants of the sensors and anions were calculated by the method which had been reported previously [29]. Synthesis Sensors 1, 2, 3 and 4 were prepared from the condensation of pyren-1-ylmethylene-hydrazine [30] with Salicylaldehyde, 3,5-ditert-butyl-2-hydroxy-benzaldehyde, 2-hydroxy-naphthale ne-1-carbaldehyde and 3,5-dichloro-2-hydroxy-benzaldehyde, respectively, as depicted in Scheme 1 and characterized by 1H NMR (Figs. S1–S5).
Scheme 1. The synthetic routes of 1, 2, 3 and 4.
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Pyren-1-ylmethylene-hydrazine A 0.2000 g (0.87 mmol) sample of l-pyrenecarboxaldehyde was suspended in 10 mL of ethanol, to which was added 0.17 g (2.61 mmol) of hydrazine hydrate, and the mixture was stirred for 4 h at room temperature. During the reaction, a yellow solid precipitate appeared. After the reaction, the precipitate was then filtered and was washed with 15 mL of cold ethanol, affording the product as a yellow powder (0.0919 g, 43%). 1H NMR (500 MHz, DMSO): 7.193 (s, 2H), 8.065 (t, J = 7.5 Hz, 1H), 8.149 (s, 2H), 8.269 (m, 4H), 8.394 (d, J = 8.0 Hz, 1H), 8.710 (d, J = 9.5 Hz, 1H), 8.773 (s, 1H). 2-(Pyren-1-ylmethylene-hydrazonomethyl)-phenol (1) A 0.0919 g (0.38 mmol) sample of pyren-1-ylmethylenehydrazine was suspended in 10 mL of ethanol, to which was added 35 lL of Salicylaldehyde, and the mixture was stirred for 15 h at room temperature. During the reaction, a yellow solid precipitate appeared. After the reaction, the precipitate was then filtered and was washed with 15 mL of cold ethanol, affording the product 1 as a yellow powder (0.1129 g, 80%). 1H NMR (500 MHz, DMSO): 7.034 (m, 2H), 7.446 (t, J = 7.0 Hz, 1H), 7.770 (d, J = 6.0 Hz, 1H), 8.173 (t, J = 7.5 Hz, 1H), 8.284 (d, J = 9.0 Hz, 1H), 8.352 (d, J = 9.0 Hz, 1H), 8.424 (m, 4H), 8.757 (d, J = 8.5 Hz, 1H), 9.152 (s, 1H), 9.181 (d, J = 9.5 Hz, 1H), 9.901 (s, 1H), 11.473 (s, 1H). 2,4-Di-tert-butyl-6-(pyren-1-ylmethylene-hydrazonomethyl)-phenol (2) A 0.2443 g (1.0 mmol) sample of pyren-1-ylmethylenehydrazine was suspended in 30 mL of ethanol, to which was added 0.2348 g (1.0 mmol) of 3,5-di-tert-butyl-2-hydroxy-benzaldehyde, and the mixture the mixture was refluxed for 5 h. During the reaction, a yellow solid precipitate appeared. After the reaction, the precipitate was then filtered and was washed with 15 mL of cold ethanol, affording the product 2 as a yellow powder (0.3505 g, 76%). 1H NMR (500 MHz, CDCl3): 1.391 (s, 9H), 1.556 (s, 9H), 7.278 (d, J = 2.5 Hz, 1H), 7.515 (d, J = 2.5 Hz, 1H), 8.087 (t, J = 7.5 Hz, 1H), 8.131 (d, J = 9.0 Hz, 1H), 8.196 (d, J = 9.0 Hz, 1H), 8.275 (m, 4H), 8.723 (d, J = 8.0 Hz, 1H), 8.930 (d, J = 9.5 Hz, 1H), 9.005 (s, 1H), 9.690 (s, 1H), 12.429 (s, 1H). 1-(Pyren-1-ylmethylene-hydrazonomethyl)-naphthalen-2-ol (3) A 0.2440 g (1.0 mmol) sample of pyren-1-ylmethylenehydrazine was suspended in 30 mL of ethanol, to which was added 0.1720 g (1.0 mmol) of 2-hydroxy-naphthalene-1-carbaldehyde, and the mixture was refluxed for 5 h. During the reaction, a yellow solid precipitate appeared. After the reaction, the precipitate was then filtered and was washed with 15 mL of cold ethanol, affording the product 3 as a yellow powder (0.2917 g, 73%). 1H NMR (500 MHz, DMSO): 7.310 (d, J = 9.0 Hz, 1H), 7.429 (t, J = 7.5 Hz, 1H), 7.616 (t, J = 7.5 Hz, 1H), 7.825 (d, J = 8.0 Hz, 1H), 7.897 (d, J = 8.5 Hz, 1H), 8.073 (t, J = 7.5 Hz, 1H), 8.114 (d, J = 8.5 Hz, 1H), 8.179 (d, J = 9.0 Hz, 1H), 8.267 (m, 5H), 8.762 (d, J = 8.0 Hz, 1H), 8.905 (d, J = 9.5 Hz, 1H), 9.670 (s,1H), 9.900 (s, 1H). 2,4-Dichloro-6-(pyren-1-ylmethylene-hydrazonomethyl)-phenol (4) A 0.2440 g (1.0 mmol) sample of pyren-1-ylmethylenehydrazine was suspended in 30 mL of ethanol, to which was added 0.1910 g (1.0 mmol) of 3,5-dichloro-2-hydroxy-benzaldehyde, and the mixture was refluxed for 20 h at room temperature. During the reaction, a yellow solid precipitate appeared. After the reaction, the precipitate was then filtered and was washed with 15 mL of cold ethanol, affording the product 4 as a yellow powder (0.3505 g, 84%). 1H NMR (500 MHz, DMSO): 7.776 (d, J = 2.5 Hz, 1H), 7.844 (d, J = 2.5 Hz, 1H), 8.183 (t, J = 7.5 Hz, 1H), 8.294 (d, J = 9.0 Hz, 1H), 8.371 (d, J = 9.0 Hz, 1H), 8.438 (m, 4H), 8.771 (d, J = 8.0 Hz,
1H), 9.146 (s, 1H), 9.154 (d, J = 8.0 Hz, 1H), 10.066 (s, 1H), 12.556 (s, 1H). Results and discussion Fluoride sensing The fluoride binding properties of 1, 2, 3 and 4 are firstly studied by UV–visible spectroscopy. In detail, the fluoride anions in the form of tetrabutylammonium (TBA) salts are added gradually to CH3CN solution of 1, 2, 3 and 4 (10 lM). As shown in Fig. 1, the max absorption peaks at 388 nm (sensor 1, e = 38,100 cm 1 M 1), 391 nm (sensor 2, e = 38,600 cm 1 M 1), 416 nm (sensor 3, e = 39,300 cm 1 M 1) and 387 nm (sensor 4, e = 34,700 cm 1 M 1) decrease gradually upon the addition of F , and new bands develop simultaneously at 494 nm (sensor 1, e = 25,900 cm 1 M 1), 532 nm (sensor 2, e = 24,700 cm 1 M 1), 516 nm (sensor 3, e = 28,900 cm 1 M 1) and 486 nm (sensor 4, e = 28,200 cm 1 M 1), respectively. There are isosbestic points in the four UV–vis titration spectra, confirming the changes between two clearly defined structures. The absorption spectra of the four sensors show large red shifts (99–141 nm, Table 1) after adding F . It is noteworthy that the red shift of 2 is much larger than that of other three sensors. These phenomena are the results of intramolecular charge transfer (ICT) [27]. As can be expected from the UV–vis spectra, color changes occur after adding F to the four sensors. As shown in Fig. 2, the color of solution of 1 turns red, solution of 2 turns purple, solution of 3 turns pink, and solution of 4 turns orange red, respectively. These phenomena mirror the red-shift observed in the absorption spectra. These results not only show that all the four sensors have good responsive properties to F , but also indicate the introduction of different substituents can effectively modulate the color change, the molar absorptivity and red shift of the sensors. As mentioned above, anion sensors based on salicylidene Schiff base always form hydrogen bonding or deprotonation on the OH group during the sensing process. In our case, deprotonation of the sensors is elucidated in the 1H NMR titration spectra. The protons of 1 give the clearest signal and the 1H NMR titration spectra of 1 ultimately are used in determining the structural changes that were occurring. As depicted in Fig. 3, the signal of OH proton at 11.473 ppm disappears after the addition of 0.5 equiv of F . Upon the gradual addition of F , the signal of Ha, Hb, Hc, Hd and He shift upfield gradually, and it indicates the increase of the electron density on the benzene ring owing to the deprotonation of the OH group. A new 1:2:1 triplet signal at 16.2 ppm appears after the addition of 5 equiv of fluoride, which is attributed to the FHF dimer (Fig. 3). The existence of this new species confirms the deprotonation of the OH group [29]. Substituent effects on sensitivity and selectivity The benzene ring of the sensor 1 has no substituent. In this case, the absorption spectra would reach saturation after adding 70 equiv of F (0.7 mM). However, sensor 2 has two tert-butyl substituents, which are electron donating. Hence, compared to 1, there will be a slightly larger negative charge in the conjugated system. The bond between the proton and oxygen of the hydroxyl group will be stronger and thus deprotonation will be more difficult. This is reflected experimentally with the observation that more F (about 100 equiv, 1 mM) is needed to allow for deprotonation. In other words, electron donating groups will decrease the sensitivity for F . Conjugated group has exactly the opposite effect. The naphthyl group in sensor 3 has a conjugation effect, which will decrease the negative charge in the conjugated system. Thus, the deprotonation
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Fig. 1. Absorption spectra changes of 1 (a), 2 (b), 3 (c), 4 (d) in CH3CN (10 lM) upon the addition of F respectively. Inset: absorbance of new peaks as a function of F respectively.
Table 1 Spectroscopic properties of 1, 2, 3 and 4 in the absence and presence of F in CH3CN. Compound
kmax (nm)
e (cm
1 1+F 2 2+F 3 3+F 4 4+F
388 494 391 532 416 516 387 486
38,100 25,900 38,600 24,700 39,300 28,900 34,700 28,200
1
M
1
)
Dkred
shift
(nm)
106 141 100 99
of the hydroxyl group will be easier to take place. And the saturation value of F was 34 equiv (0.34 mM) accordingly. In addition, the naphthyl group in 3 gives rises to a much higher kmax compared to that of 1. The chlorine substituents in sensor 4 have a high electronegativity and thus have an electron withdrawing effect. Hence, compared to 1, the bond between the proton and oxygen of the hydroxyl group will be weaker and thus deprotonation will be easier. Experimentally, sensor 4 only needs 3 equiv F (30 lM) to get its limiting value. The results suggest that electron withdrawing groups increased the sensitivity of fluoride response significantly. Furthermore, the equilibrium constants of sensors 1, 2, 3 and 4 with fluoride ions are calculated according to UV–vis titration spectra [29]. As shown in Table 2, the order of fluoride affinities in four sensors is 2 < 1 < 3 < 4, which is consistent with above conclusions. Meanwhile, the Benesi–Hildebrand plots indicate that the stoichiometry of sensors and fluoride is 1:2 (Fig. S8). Generally, the
electron donating groups would decrease the sensitivity of the sensors, but the conjugation effect and electron withdrawing groups would increase the sensitivity to fluoride. Actually, these substituent groups influence the acidity of OH proton, which, in turn, change the availability of OH moiety for deprotonation and affinity of sensors toward anions. Though all the four sensors display good sensing ability to fluoride, the selectivity of fluoride over other anions, such as AcO and H2PO4 , is also important. Up to now, most of the related research show that the performance of fluoride sensors suffered from deleterious interference of these anions. As shown in Fig. 2, both 1 and 2 respond independently to F , while other anions cannot cause change neither in absorption spectra nor in color, even when adding enough anions. On the contrary, 3 and 4 exhibit significant responses not only to the most basic F but also to moderate basic AcO and H2PO4 . Sensor 3 has weaker binding ability to AcO and H2PO4 than F , and the absorption spectra would reach saturation after adding 1200 equiv (12 mM) of AcO and 1500 equiv (15 mM) of H2PO4 (Fig. S7). The equilibrium constants of 3 with AcO and H2PO4 are 6.62 104 M 2 and 3.34 104 M 2, respectively. Interestingly, after adding AcO and H2PO4 the saturation peak intensities of absorption spectra are also lower than F . By contrast, sensor 4 displays the strong interaction with F , AcO and H2PO4 . The saturation value of AcO and H2PO4 are 9 equiv (90 lM) and 17 equiv (170 lM), respectively (Fig. S8). The equilibrium constants of 4 with AcO and H2PO4 are 3.81 109 M 2 and 8.43 108 M 2, even higher than other sensors with the most electronegative fluoride. This is presumably due to the strong electron withdrawing group increasing the acidity of the OH group of sensors. To sum up, as depicted in Fig. 4, either no substitute or with
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Fig. 2. UV–visible spectra changes and color changes observed upon the addition of various anions to the solutions of 1, 2, 3 and 4 in CH3CN (10 lM).
electron donating groups, sensor 1 and 2 only responded to F with high selectivity. However, the conjugation effect and electron withdrawing group will decrease the selectivity of recognition to F .
Analytical application In view of the high selectivity and sensitivity of sensor 1 in the pure organic solvent, we also designed the experiments in DMSO–
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Fig. 3. Partial 1H NMR spectra of 1 (2 mM, DMSO-d6) with the addition of 0 equiv, 0.5 equiv, 1.0 equiv, 2.0 equiv, 5.0 equiv, 10 equiv, 20 equiv of TBAF respectively.
Table 2 Equilibrium constants of 1, 2, 3 and 4 with F , AcO , H2PO4 in CH3CN as determined from UV–vis absorption titrations. Anion
Sensor 1 (Ks in M 2)
Sensor 2 (Ks in M 2)
Sensor 3 (Ks in M 2)
Sensor 4 (Ks in M 2)
F AcO H2PO4
4.65 106 – –
1.01 106 – –
4.58 107 6.62 104 3.34 104
6.54 109 3.81 109 8.43 108
H2O (95:5, v/v) solution to further investigate its performance in the presence of water. It is well-known that upon adding the protic solvents such as water will competitively form hydrogen-bonding with the binding site of sensors for anions. Meanwhile, anions are strongly solvated in aqueous medium due to their high hydration free energy which makes the detection of anions in water medium much more difficult [31]. Fortunately, in case of sensor 1, the adding of F showed detectable color and spectral responses in mixed solvent DMSO–H2O (95:5, v/v) (Fig. 5). These results clearly
Fig. 4. Proposed sensing mechanisms and properties of 1, 2, 3 and 4 for the detection of anions.
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substituents of chromophores constituting the anion binding site. The sensitivities follow the trend 4 > 3 > 1 > 2, which is in consistence with the push–pull feature of the substituents. Additionally, for the selectivity of F recognition, both 1 and 2 are much higher. However, 3 and 4 are not able to differentiate F , AcO and H2PO4 . This work confirms the importance of electron push–pull character for the substituent effects. Furthermore, we also demonstrate that the conjugation effect and electron withdrawing groups can also induce a decrease in the selectivity for F . Finally, the real sample analysis of sensor 1 is successfully achieved through the qualitative estimation of F in water and the commercially available toothpaste by simple and easy colorimetric method. This study benefits the future potential application of the influence rule of the substituent on the anion recognition of salicylidene Schiff base anion chemosensors, as well as their substituent structures, for their possible use in the designing and synthesizing of more effective anion sensors. Fig. 5. UV–visible spectra changes and of 1 (50 lM) in DMSO–H2O (95:5, v/v) upon the addition of (0–150 equiv). Inset showing the color change observed after adding F .
Acknowledgements This work was supported by the National Basic Research Program of China (2012CB933800) and the National Natural Science Foundation of China (21374036). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.06.028. References
Fig. 6. (a) Color change of test strips of 1 with F in water; (b) real sample analysis of 1 (1 10 3 M, DMSO) for the qualitative detection of F in toothpaste.
indicated the potential application of 1 for the selective detection of F in aqueous medium. To evaluate the practical applicability of sensor 1, paper-made test strips were prepared by immersing filter papers into a CH2Cl2 solution of 1 (4 103 M) and then dried. Then, the test strips were immersed in an aqueous solution of F (0.08 M), and the strips were subsequently dried. As shown in Fig. 6a, obvious color change was observed in the presence of F . This result indicated that the paper-made test strips could detect F in water qualitatively. Finally, the analytical application of the sensor 1 for the analysis of real samples was investigated. For real sample analysis, we carried out the qualitative detection of F ions in commercially available toothpaste. The test solutions for this experiment were prepared by adopting recently reported method [32]. The toothpaste sample solution was prepared as 100 mg/mL in water. Upon the addition of toothpaste solution, a distinct color change of 1 was observed for the qualitative detection of F (Fig. 6b).
Conclusions In this study, a series of novel colorimetric sensors with various substituents for the optical measurement of anions has been developed based on the salicylidene Schiff base system. It is found that all of these anion sensors display colorimetric responses for F but with different anion sensitivity and response selectivity for different electron push–pull feature of the substituents. Considering the sensitivity and selectivity of the anion sensors, we conclude that the deprotonation induced by anions interrelate with the
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