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Novel chemosensor for multiple target anions: The detection of F− and CN− ion via different approach
Journal of Fluorine Chemistry 175 (2015) 180–184
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor
Novel chemosensor for multiple target anions: The detection of F and CN ion via different approach Duraisamy Udhayakumari a, Sivan Velmathi a,*, Maria Susai Boobalan b a b
Department of Chemistry, National Institute of Technology, Organic and Polymer Synthesis Laboratory, Tiruchirappalli 620 015, India Department of Chemistry, St. Joseph’s College (Autonomous), Tiruchirappalli 620002, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 February 2015 Received in revised form 30 March 2015 Accepted 18 April 2015 Available online 12 May 2015
A novel chemosensor based on azo linked Schiff base (receptor 1) was synthesized by simple condensation method. The sensor studies of receptor 1 disclose the selectivity toward fluoride and cyanide ion with a color change among the other common anions in aqueous medium. The binding affinity of receptor 1 was also enlightened by 1H NMR titration and DFT calculation. ß 2015 Elsevier B.V. All rights reserved.
Keywords: Azo dye Nucleophilic addition Cyanide detection Aqueous solution Colorimetric
1. Introduction Selective sensing of anions has received considerable attention over recent years due to their important role in the biological process, medicinal chemistry, environmental chemistry and catalysis. In addition, colorimetric chemosensors have many advantages like low cost, suitability for biological concern, easy detection and high selectivity [1–5]. During the last few years, a number of colorimetric anion sensors have been developed using urea, thiourea, amide and pyrrole based binding units. Most of the receptors sense different anions such as fluoride, acetate, nitrate, dihydrogen phosphate and hydroxyl group [6–12]. Cyanide is one of the most toxic anion, harmful to the environment. It plays an important role in various industrial processes, for example gold mining, synthetic fibers and resins [13–17]. On the other hand, fluoride is a biologically important anion due to its wide role in dental care and treatment of osteoporosis [18–20]. Novel colorimetric cyanide ion sensors, which allow naked eye detection of the color change without resorting to the use of expensive instruments is most attractive approach. However, finding a simple receptor selectively sensing cyanide anion in aqueous medium is still a challenge. To the best of our knowledge, a simple
* Corresponding author. Tel.: +91 431 2503640/9486067404; fax: +91 431 2500133. E-mail address: [email protected] (S. Velmathi). http://dx.doi.org/10.1016/j.jfluchem.2015.04.014 0022-1139/ß 2015 Elsevier B.V. All rights reserved.
sensor that can sense differentially F and CN ion has rarely been reported [21–23]. Recently we reported azo Schiff base [24,25] to act as a colorimetric, fluorescent chemosensors for F /AcO ions in organic and semi-aqueous medium. In continuation of our research interest in the chemosensors, we herein report the application of receptor 1 toward sensing of CN in pure aqueous medium. In this paper, we report a simple receptor which can selectively detect cyanide in aqueous medium via colorimetric and UV–vis spectroscopic method. With appropriate analysis, it is possible to detect fluoride and cyanide ions separately in the same sample by tuning the solvent medium. The absorption spectra indicated the formation of complex between receptor and anions in 1:1 stoichiometric ratio with micromolar detection limit. The interaction mechanism between the receptor 1 with fluoride and cyanide was investigated using 1H NMR titration experiment. 2. Materials and methods All reagents used were obtained commercially and used without further purification. In the titration experiments, all the anions were added in the form of tetrabutylammonium (TBA) salts and analytical grade solvents such as acetonitrile (CH3CN) and ethanol (EtOH) were purchased commercially. Shimadzu UV-2600 UV-vis spectrophotometer was used to record UV–visible spectra using quartz cell with 1 cm path length. 5 10 5 M solution of receptor 1 in CH3CN and 1.5 10 3 M solutions of the anions in
D. Udhayakumari et al. / Journal of Fluorine Chemistry 175 (2015) 180–184
CH3CN, H2O were prepared. 0.2 equiv. (20 mL)–2 equiv. (200 mL) of guest solution was added to 3 mL of R1 taken in the UV cuvette. 3. Results and discussion The synthesis of chemosensor (receptor 1) is outlined in Scheme 1. 1 mmol of 4-amino azobenzene (0.200 g) and 1 mmol of 2-hydroxy-5-(phenyldiazenyl) benzaldehyde (0.229 g) in ethanol (10 mL) was reflux for 2–3 h at 85 8C. After cooling, the precipitate formed was filtered, washed, recrystallized from EtOH and dried in vacuum oven. Yield: 85%, mp: 160 8C. 1H NMR (300 MHz, DMSO-d6 d ppm): 9.25 (s, 1H, OH), 8.37 (s, 1H, CH5 5N), 8.02–8.07 (d, 3H, Ar, J = 15 Hz), 7.87–7.94 (m, 4H, Ar, J = 5.7 Hz), 7.55–7.71 (m, 9H, Ar, J = 8.4 Hz), 7.18–7.21(d, 1H, Ar, J = 9 Hz). HRMS: Calcd: 406.1600, Found: 406.1660 (Figs. S1 and S2). The sensing behavior of receptor 1 with anions was observed by ‘‘naked-eye’’ and detection was further confirmed by UV–vis spectrophotometer. In the presence of different counter anions, receptor 1 showed selective sensing of CN in aqueous environment. To investigate the anion recognition property of receptor 1, four different types of colorimetric and UV–vis titrations were carried out. The binding affinity of receptor 1 (5 10 5 M in CH3CN) in the presence of various anions such as F , Cl , Br , AcO , H2PO4 , HSO4 , HO and CN (1.5 10 3 M in CH3CN) was carried out. Fig. 1a and b shows the color changes and the absorption spectrum of receptor 1 with different anions in CH3CN. The solution of receptor 1 resulted in an immediate color change from colorless to orange for F , yellow color for AcO and green color for CN ions. The other ions like Cl , Br , H2PO4 , HSO4 and HO did not induce any color changes and absorption changes even in large excess. The incremental addition of F and CN shows a band at 450 nm increased, at the same time the band at 350 nm decreased with the clear and single isosbestic point at 385 nm (Figs. S3 and S4). In second titration, the effect of water on the binding affinity for fluoride was carried out in different water–acetonitrile mixtures
181
(0%, 20%, 40%, 60%, 80% and 90% water, respectively) with receptor 1 (CH3CN). 20% aq solution of F resulted in a marginal color change, but did not show any optical changes (Fig. S5). So the outcome of the second titration indicates, receptor can sense fluoride, only in organic medium and not in aqueous medium. In the third titration, in order to gain systematic knowledge on the effect of water on the binding affinity and selectivity for cyanide (100% H2O), we took receptor 1 in water–acetonitrile mixtures (0%, 20%, 40%, 60%, 80% and 90% water) and CN ion in 100% H2O. When receptor 1 was taken in 20:80 water–acetonitrile mixtures as solvent, CN showed obvious interaction with the receptor 1. The color changed to green upon addition of CN , correspondingly in optical spectrum the band at 350 nm shifted (red shift) to 390 nm. When increase the percentage of water from 40% to 90%, same result was observed in color change and UV–vis change (Fig. 2a and b). From these results, it is confirmed that cyanide can be selectively sensed in 90% aqueous medium. In general, hydration will terminate the anion sensing behavior, water the strong competitive solvent in the hydrogen bonding and lead to the leaching of anions and regenerate the receptor molecule. We chose receptor 1 in 90% H2O (5 10 5 M) and anions in 100% H2O (1.5 10 3 M), only CN ion showed obvious interaction with the receptor 1 (Fig. 3a). The color changed to green and the band shifted to 350–390 nm (40 nm) only for CN ion (Fig. 3b). Increasing the amount of water content is avoiding completely the interference of F and ACO ions. The anions such as F , AcO , H2PO4 and HSO4 should interact with water through hydrogen-bonding leading to a large decrease in their nucleophilicity. While cyanide has weaker hydrogen-bonding ability and stronger nucleophilicity toward the carbonyl group. Cyanide has much weaker hydrogen bonding ability in comparison with other anions like F and AcO and stronger nucleophilicity toward the imine group [26–28], which results in the addition of CN ions to the carbon atom of an electron deficient imine group and, subsequently, fast proton transfer of
Scheme 1. Synthetic route of receptor 1.
Fig. 1. (a) The corresponding color changes and (b) absorption spectra of receptor 1 when treated with 2 equiv. of various anions (F , Cl , Br , AcO , H2PO4 , HSO4 , HO and CN ) in CH3CN.
182
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Fig. 2. (a) The corresponding color changes and (b) absorption spectra of receptor 1 in water–acetonitrile mixtures (0%, 20%, 40%, 60%, 80% and 90% water, respectively) when treated with 2 equiv. of CN ion in 100% H2O.
Fig. 3. (a) The corresponding color changes and (b) absorption spectra of receptor 1 in water (90%)–acetonitrile (10%) mixtures when treated with 2 equiv. of various anions (F , Cl , Br , AcO , H2PO4 , HSO4 , HO and CN ) in 100% H2O.
the phenol hydrogen to the neighboring nitrogen anion through an intramolecular hydrogen bond. The detection of anions by receptor 1 is further supported by 1H NMR titration. 1H NMR titrations were carried out in DMSO-d6. Tetra butyl ammonium salt of fluoride and cyanide were used as anions sources. Without the addition of anions, receptor 1 showed signals at d 8.37 ppm due to HC5 5N proton and 9.25 ppm corresponds to OH proton. After the addition of 2 equiv. of F ions, the resonance signal of –OH proton becomes disappeared (Fig. 4). This might be due to the formation of hydrogen bonds between the anions and the –OH group of receptor 1. This indicates that deprotonation of the receptor 1 can possibly occur. In addition, the signals correspond to imine and aromatic protons were shifted to upfield with the addition of F ions. The 1H NMR titration studies sustain the proposed sensing mechanism of receptor 1 with F ions (Scheme 2). But in the case of CN ions to receptor 1, the –OH signal of receptor 1 disappeared and a new peak at 6.36 ppm started appearing. All the aromatic protons were shifted to upfield. This result strongly supports the nucleophilic attack of CN ions toward the carbon atom of an electron deficient imine group of receptor 1 (Scheme 3).
Binding constant (Ka) for all anion complexes of the receptor 1 were calculated using Benesi–Hildebrand plot and stoichiometry of the complex was calculated using Job’s plot. The detection limit (LOD) of F and CN ions by receptor 1 was calculated using the formula, LOD = 3s/m. Where ‘‘s’’ the standard deviation and ‘‘m’’ is the slope obtained in the plot of absorbance vs. concentration of anions. The detection limit value implies that the receptor 1 can easily detect F and CN at micro molar level in both organic and aqueous medium. Binding constants and detection limit are calculated with repeated titrations and the values are given in Table 1. The receptor 1 was compared with some reported chemosensors for CN (Table 2). While each chemosensor showed some advantages such as easy synthesis, high sensitivity, no interference and naked eye detection. Our receptor 1 exhibited quite appealing analytical features such as easy synthesis, high selectivity and naked-eye sensing (R15 5CN ) [29–34]. Density Functional Theory (DFT) calculations have been used to understand the behavior of receptor 1 with the F and CN ions. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for receptor 1 and their complexes have been generated from the optimized structures of the receptor
Fig. 4. 1H NMR spectrum of receptor 1 recorded in DMSO-d6 in the absence and presence of F and CN ions.
D. Udhayakumari et al. / Journal of Fluorine Chemistry 175 (2015) 180–184
183
Scheme 2. The proposed sensing mechanism for receptor 1 with fluoride.
Scheme 3. The proposed sensing mechanism for receptor 1 with cyanide.
1 and their respective anion complexes (Fig. 5). The structural optimization and the computational calculations were carried out using Gaussian09 quantum chemistry package and results were viewed with GaussView5 GUI. The appropriate choice of model chemistry for the computation of electronic structure property is indispensable. In the present scenario, Density Functional Theory (DFT) supported B3LYP/6-31G (d, p) model chemistry was implemented particularly for Schiff base ligands under gas phase condition. DFT calculations were carried out to investigate the change of the absorption spectra upon the
addition of F and CN . From the energy level diagram (Fig. 6), it can be seen that the addition of the anions lead to the stabilization of the HOMO and LUMO of the sensor molecule. From the calculated results, the energy levels of HOMO and LUMO of receptor 1 increased after the addition of F and CN ions when compared to that of the free receptor 1 (Table 3). For F and CN complexes of receptor 1, the absorption bands predicted by the theoretical methods is red shifted compared with those of the independent probes, which is in good agreement with the experimental observations.
Table 1 Binding constant, stoichiometry, detection limit value of the complex formed for receptor 1 with CN and F ion. S. no 1 2 3
Receptor + ions R1 + F R1 + CN R1 + CN
Solvent system
Binding constant (M
1
)
4
100% CH3CN 100% CH3CN 90% H2O (R1) 100% H2O (CN )
4.6 10 3.3 105 5.5 104
Stoichiometry
LOD (M
1
1:1 1:1 1:1
1.5 10 1.9 10 5.2 10
6
)
6 6
Table 2 Comparison of some of the reported chemosensors for CN with the present work. Sensors
Solvent system
Sensing ions
LOD
Binding constant (M 1)
Linear range (R2)
Reference
Thiazole based sensor
DMSO/bis-tris buffer (8:2)
– 0.990 0.990 0.990
[30] [31] [32] [33]
Imidazo-anthraquinones based sensor
CH3CN:H2O (97:3)
–
[34]
Azo linked Schiff base sensor
CH3CN CH3CN:H2O (1:9, v/v)
3.2 102 1.0 102 – 2.3 103 – 5.89 104 (R1) 3.98 108 (R2) 6.80–8.96 6.31–8.50 4.6 104 5.5 104
[29]
H2O CH3CN:DMSO:HEPES (93:1:6) EtOH:H2O (3:7) CH3CN:DMSO (99:1)
60 mM 140 mM 0.01–0.07 mM 0–1.9 mM 1.5 mM 1.79 mM (R1) 2.43 mM (R2) 3.31 mM 4.97 mM 1.5 mM 1.9–5.2 mM
0.994
Azo based sensor Azo coupled tripodal based sensor Azo coupled NBD unit based sensor Bis-pyrene derivative based sensor
CN AcO CN CN CN CN
0.993
This work
CN F F CN
Table 3 Energies of HOMO and LUMO of the R1 and complexes. R1 and complexes R1 R1 + F R1 + CN
HOMO (eV) 5.774 3.434 3.282
LUMO (eV) 2.361 0.807 0.633
DE (eV)
Theoretical wavelength (nm)
Observed wavelength (nm)
3.413 2.627 2.649
363 471 350
354 450 385
D. Udhayakumari et al. / Journal of Fluorine Chemistry 175 (2015) 180–184
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Fig. 5. Optimized structures of (a) receptor 1, (b) receptor 1 + F and (c) receptor 1 + CN .
Fig. 6. Energy levels of various HOMO and LUMO’s of receptor 1 in the absence and presence of F and CN .
4. Conclusion In summary, we have developed a novel and highly sensitive receptor 1 that can detect both F and CN ion selectively with the distinct color changes, where F ion shows colorless to orange and CN ion shows colorless to green. Further, receptor 1 showed selective detection of cyanide ion in 90% aqueous medium based on a nucleophilic addition followed by a intra molecular proton exchange process. Receptor 1 was found to provide sensitive and good selective recognition of CN confirmed through its absorption spectrum. Acknowledgment Authors express their thanks to DRDO (ERIP/ER/1006004/M/01/ 1333 dated 23-05-2011) for financial assistance in the form of a major sponsored project. 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.jfluchem.2015.04.014. References [1] P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. 40 (2001) 486–516. [2] S.K. Kim, J. Yoon, Chem. Commun. 7 (2002) 770–771. [3] A. Bianchi, K.B. James, E.G. Espana, Supramolecular Chemistry of Anions, WileyVCH, New York, USA, 1997. [4] T. Gunnlaugsson, M. Glynn, G.M. Tocci, P.E. Kruger, F.M. Pfeffer, Coord. Chem. Rev. 250 (2006) 3094–3117. [5] R. Martinez-Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419–4476.
[6] S.J. Coles, J.G. Frey, P.A. Gale, M.B. Hursthouse, M.E. Light, K. Navakhun, G.L. Thomas, Chem. Commun. (2003) 568–569. [7] L.S. Evans, P.A. Gale, Chem. Commun. (2004) 1286–1287. [8] S.J. Loeb, J. Am. Chem. Soc. 126 (2004) 5030–5031. [9] P.A. Gale, M.E. Light, B. McNally, K. Navakhun, K.E. Sliwinski, B.D. Smith, Chem. Commun. (2005) 3773–3775. [10] T. Gunnlaugsson, A.P. Davis, J.E. O’Brien, M. Glynn, Org. Biomol. Chem. 3 (2005) 48–56. [11] Y. Zhou, J.F. Zhang, J. Yoon, Chem. Rev. 114 (2014) 5511–5571. [12] F. Wang, L. Wang, X. Chen, J. Yoon, Chem. Soc. Rev. 43 (2014) 4312–4324. [13] J.N. Babu, V. Bhalla, M. Kumar, H. Singh, Org. Chem. 3 (2006) 787–793. [14] H. Deng, Y.C. Qiu, C. Daiguebonne, Inorg. Chem. 47 (2008) 5866–5872. [15] J.L. Liu, Y. Liu, Q. Liu, C.Y. Li, J. Am. Chem. Soc. 133 (2011) 15276–15279. [16] L. Deng, Chen, M. Zhou, S.J. Guo, Anal. Chem. 82 (2010) 4283–4287. [17] K.W. Kulig, Cyanide Toxicity, U. S. Department of Health and Human Services, Atlanta, GA, 1991. [18] W. Huang, X. Yu, H. Lin, H. Lin, J. Incl. Phenom. Macrocycl. Chem. 69 (2011) 69–73. [19] Y. Li, J. Li, H. Lin, J. Shao, Z.S. Cai, H. Lin, J. Lumin. 130 (2010) 466–472. [20] H.S. Jung, H.J. Kim, J. Vicens, J.S. Kim, Tetrahedron Lett. 50 (2009) 983–987. [21] J. Isaad, Perwuelz Tetrahedron Lett. 51 (2010) 5810–5814. [22] Y.Y. Guo, X.L. Tang, F.P. Hou, J. Wu, W. Dou, W.W. Qin, J.X. Ru, G.L. Zhang, W.S. Liu, X.J. Yao, Sens. Actuators B 181 (2013) 202–208. [23] S.K. Kwon, S. Kou, H.N. Kim, X. Chen, H. Hwang, S.-W. Nam, S.H. Kim, K.M.K. Swamy, S. Park, J. Yoon, Tetrahedron Lett. 49 (2008) 4102–4105. [24] H.T. Niu, X. Jiang, J. He, J.P. Cheng, Tetrahedron Lett. 49 (2008) 6521–6524. [25] S. Suganya, S. Velmathi, J. Mol. Recognit. 26 (2013) 259–267. [26] V. Reena, S. Suganya, S. Velmathi, J. Fluor. Chem. 153 (2013) 89–95. [27] J.Y. Noh, I.H. Hwang, H. Kim, E.J. Song, K.B. Kim, C. Kim, Bull. Korean Chem. Soc. 34 (2013) 1985–1990. [28] J. Kang, E.J. Song, H. Kim, Y.H. Kim, Y.M. Kim, S.J. Kim, C. Kim, Tetrahedron Lett. 54 (2013) 1015–1019. [29] G.R. You, G.J. Park, S.A. Lee, Y.W. Choi, Y.S. Kim, Sens. Actuators B 202 (2014) 645– 655. [30] J. Isaad, A.E. Achari, Tetrahedron 67 (2011) 4939–4947. [31] D.Y. Lee, N. Singh, A. Satyender, D.O. Jang, Tetrahedron Lett. 52 (2011) 6919–6922. [32] Y.K. Tsui, S. Devaraj, Y.P. Yen, Sens. Actuators B 161 (2012) 510–519. [33] M.L. Jong, H. Moon, K.M.K. Swamy, Y. Jeong, G. Kim, J. Choi, J.Y. Lee, J. Yoon, Sens. Actuators B 199 (2014) 369–376. [34] R.M.F. Batist, E. Oliveira, P.G. Costa S, C. Lodeiro, M.M.M. Raposo, Supramol. Chem. 26 (2014) 71–80.
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor
Novel chemosensor for multiple target anions: The detection of F and CN ion via different approach Duraisamy Udhayakumari a, Sivan Velmathi a,*, Maria Susai Boobalan b a b
Department of Chemistry, National Institute of Technology, Organic and Polymer Synthesis Laboratory, Tiruchirappalli 620 015, India Department of Chemistry, St. Joseph’s College (Autonomous), Tiruchirappalli 620002, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 February 2015 Received in revised form 30 March 2015 Accepted 18 April 2015 Available online 12 May 2015
A novel chemosensor based on azo linked Schiff base (receptor 1) was synthesized by simple condensation method. The sensor studies of receptor 1 disclose the selectivity toward fluoride and cyanide ion with a color change among the other common anions in aqueous medium. The binding affinity of receptor 1 was also enlightened by 1H NMR titration and DFT calculation. ß 2015 Elsevier B.V. All rights reserved.
Keywords: Azo dye Nucleophilic addition Cyanide detection Aqueous solution Colorimetric
1. Introduction Selective sensing of anions has received considerable attention over recent years due to their important role in the biological process, medicinal chemistry, environmental chemistry and catalysis. In addition, colorimetric chemosensors have many advantages like low cost, suitability for biological concern, easy detection and high selectivity [1–5]. During the last few years, a number of colorimetric anion sensors have been developed using urea, thiourea, amide and pyrrole based binding units. Most of the receptors sense different anions such as fluoride, acetate, nitrate, dihydrogen phosphate and hydroxyl group [6–12]. Cyanide is one of the most toxic anion, harmful to the environment. It plays an important role in various industrial processes, for example gold mining, synthetic fibers and resins [13–17]. On the other hand, fluoride is a biologically important anion due to its wide role in dental care and treatment of osteoporosis [18–20]. Novel colorimetric cyanide ion sensors, which allow naked eye detection of the color change without resorting to the use of expensive instruments is most attractive approach. However, finding a simple receptor selectively sensing cyanide anion in aqueous medium is still a challenge. To the best of our knowledge, a simple
* Corresponding author. Tel.: +91 431 2503640/9486067404; fax: +91 431 2500133. E-mail address: [email protected] (S. Velmathi). http://dx.doi.org/10.1016/j.jfluchem.2015.04.014 0022-1139/ß 2015 Elsevier B.V. All rights reserved.
sensor that can sense differentially F and CN ion has rarely been reported [21–23]. Recently we reported azo Schiff base [24,25] to act as a colorimetric, fluorescent chemosensors for F /AcO ions in organic and semi-aqueous medium. In continuation of our research interest in the chemosensors, we herein report the application of receptor 1 toward sensing of CN in pure aqueous medium. In this paper, we report a simple receptor which can selectively detect cyanide in aqueous medium via colorimetric and UV–vis spectroscopic method. With appropriate analysis, it is possible to detect fluoride and cyanide ions separately in the same sample by tuning the solvent medium. The absorption spectra indicated the formation of complex between receptor and anions in 1:1 stoichiometric ratio with micromolar detection limit. The interaction mechanism between the receptor 1 with fluoride and cyanide was investigated using 1H NMR titration experiment. 2. Materials and methods All reagents used were obtained commercially and used without further purification. In the titration experiments, all the anions were added in the form of tetrabutylammonium (TBA) salts and analytical grade solvents such as acetonitrile (CH3CN) and ethanol (EtOH) were purchased commercially. Shimadzu UV-2600 UV-vis spectrophotometer was used to record UV–visible spectra using quartz cell with 1 cm path length. 5 10 5 M solution of receptor 1 in CH3CN and 1.5 10 3 M solutions of the anions in
D. Udhayakumari et al. / Journal of Fluorine Chemistry 175 (2015) 180–184
CH3CN, H2O were prepared. 0.2 equiv. (20 mL)–2 equiv. (200 mL) of guest solution was added to 3 mL of R1 taken in the UV cuvette. 3. Results and discussion The synthesis of chemosensor (receptor 1) is outlined in Scheme 1. 1 mmol of 4-amino azobenzene (0.200 g) and 1 mmol of 2-hydroxy-5-(phenyldiazenyl) benzaldehyde (0.229 g) in ethanol (10 mL) was reflux for 2–3 h at 85 8C. After cooling, the precipitate formed was filtered, washed, recrystallized from EtOH and dried in vacuum oven. Yield: 85%, mp: 160 8C. 1H NMR (300 MHz, DMSO-d6 d ppm): 9.25 (s, 1H, OH), 8.37 (s, 1H, CH5 5N), 8.02–8.07 (d, 3H, Ar, J = 15 Hz), 7.87–7.94 (m, 4H, Ar, J = 5.7 Hz), 7.55–7.71 (m, 9H, Ar, J = 8.4 Hz), 7.18–7.21(d, 1H, Ar, J = 9 Hz). HRMS: Calcd: 406.1600, Found: 406.1660 (Figs. S1 and S2). The sensing behavior of receptor 1 with anions was observed by ‘‘naked-eye’’ and detection was further confirmed by UV–vis spectrophotometer. In the presence of different counter anions, receptor 1 showed selective sensing of CN in aqueous environment. To investigate the anion recognition property of receptor 1, four different types of colorimetric and UV–vis titrations were carried out. The binding affinity of receptor 1 (5 10 5 M in CH3CN) in the presence of various anions such as F , Cl , Br , AcO , H2PO4 , HSO4 , HO and CN (1.5 10 3 M in CH3CN) was carried out. Fig. 1a and b shows the color changes and the absorption spectrum of receptor 1 with different anions in CH3CN. The solution of receptor 1 resulted in an immediate color change from colorless to orange for F , yellow color for AcO and green color for CN ions. The other ions like Cl , Br , H2PO4 , HSO4 and HO did not induce any color changes and absorption changes even in large excess. The incremental addition of F and CN shows a band at 450 nm increased, at the same time the band at 350 nm decreased with the clear and single isosbestic point at 385 nm (Figs. S3 and S4). In second titration, the effect of water on the binding affinity for fluoride was carried out in different water–acetonitrile mixtures
181
(0%, 20%, 40%, 60%, 80% and 90% water, respectively) with receptor 1 (CH3CN). 20% aq solution of F resulted in a marginal color change, but did not show any optical changes (Fig. S5). So the outcome of the second titration indicates, receptor can sense fluoride, only in organic medium and not in aqueous medium. In the third titration, in order to gain systematic knowledge on the effect of water on the binding affinity and selectivity for cyanide (100% H2O), we took receptor 1 in water–acetonitrile mixtures (0%, 20%, 40%, 60%, 80% and 90% water) and CN ion in 100% H2O. When receptor 1 was taken in 20:80 water–acetonitrile mixtures as solvent, CN showed obvious interaction with the receptor 1. The color changed to green upon addition of CN , correspondingly in optical spectrum the band at 350 nm shifted (red shift) to 390 nm. When increase the percentage of water from 40% to 90%, same result was observed in color change and UV–vis change (Fig. 2a and b). From these results, it is confirmed that cyanide can be selectively sensed in 90% aqueous medium. In general, hydration will terminate the anion sensing behavior, water the strong competitive solvent in the hydrogen bonding and lead to the leaching of anions and regenerate the receptor molecule. We chose receptor 1 in 90% H2O (5 10 5 M) and anions in 100% H2O (1.5 10 3 M), only CN ion showed obvious interaction with the receptor 1 (Fig. 3a). The color changed to green and the band shifted to 350–390 nm (40 nm) only for CN ion (Fig. 3b). Increasing the amount of water content is avoiding completely the interference of F and ACO ions. The anions such as F , AcO , H2PO4 and HSO4 should interact with water through hydrogen-bonding leading to a large decrease in their nucleophilicity. While cyanide has weaker hydrogen-bonding ability and stronger nucleophilicity toward the carbonyl group. Cyanide has much weaker hydrogen bonding ability in comparison with other anions like F and AcO and stronger nucleophilicity toward the imine group [26–28], which results in the addition of CN ions to the carbon atom of an electron deficient imine group and, subsequently, fast proton transfer of
Scheme 1. Synthetic route of receptor 1.
Fig. 1. (a) The corresponding color changes and (b) absorption spectra of receptor 1 when treated with 2 equiv. of various anions (F , Cl , Br , AcO , H2PO4 , HSO4 , HO and CN ) in CH3CN.
182
D. Udhayakumari et al. / Journal of Fluorine Chemistry 175 (2015) 180–184
Fig. 2. (a) The corresponding color changes and (b) absorption spectra of receptor 1 in water–acetonitrile mixtures (0%, 20%, 40%, 60%, 80% and 90% water, respectively) when treated with 2 equiv. of CN ion in 100% H2O.
Fig. 3. (a) The corresponding color changes and (b) absorption spectra of receptor 1 in water (90%)–acetonitrile (10%) mixtures when treated with 2 equiv. of various anions (F , Cl , Br , AcO , H2PO4 , HSO4 , HO and CN ) in 100% H2O.
the phenol hydrogen to the neighboring nitrogen anion through an intramolecular hydrogen bond. The detection of anions by receptor 1 is further supported by 1H NMR titration. 1H NMR titrations were carried out in DMSO-d6. Tetra butyl ammonium salt of fluoride and cyanide were used as anions sources. Without the addition of anions, receptor 1 showed signals at d 8.37 ppm due to HC5 5N proton and 9.25 ppm corresponds to OH proton. After the addition of 2 equiv. of F ions, the resonance signal of –OH proton becomes disappeared (Fig. 4). This might be due to the formation of hydrogen bonds between the anions and the –OH group of receptor 1. This indicates that deprotonation of the receptor 1 can possibly occur. In addition, the signals correspond to imine and aromatic protons were shifted to upfield with the addition of F ions. The 1H NMR titration studies sustain the proposed sensing mechanism of receptor 1 with F ions (Scheme 2). But in the case of CN ions to receptor 1, the –OH signal of receptor 1 disappeared and a new peak at 6.36 ppm started appearing. All the aromatic protons were shifted to upfield. This result strongly supports the nucleophilic attack of CN ions toward the carbon atom of an electron deficient imine group of receptor 1 (Scheme 3).
Binding constant (Ka) for all anion complexes of the receptor 1 were calculated using Benesi–Hildebrand plot and stoichiometry of the complex was calculated using Job’s plot. The detection limit (LOD) of F and CN ions by receptor 1 was calculated using the formula, LOD = 3s/m. Where ‘‘s’’ the standard deviation and ‘‘m’’ is the slope obtained in the plot of absorbance vs. concentration of anions. The detection limit value implies that the receptor 1 can easily detect F and CN at micro molar level in both organic and aqueous medium. Binding constants and detection limit are calculated with repeated titrations and the values are given in Table 1. The receptor 1 was compared with some reported chemosensors for CN (Table 2). While each chemosensor showed some advantages such as easy synthesis, high sensitivity, no interference and naked eye detection. Our receptor 1 exhibited quite appealing analytical features such as easy synthesis, high selectivity and naked-eye sensing (R15 5CN ) [29–34]. Density Functional Theory (DFT) calculations have been used to understand the behavior of receptor 1 with the F and CN ions. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for receptor 1 and their complexes have been generated from the optimized structures of the receptor
Fig. 4. 1H NMR spectrum of receptor 1 recorded in DMSO-d6 in the absence and presence of F and CN ions.
D. Udhayakumari et al. / Journal of Fluorine Chemistry 175 (2015) 180–184
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Scheme 2. The proposed sensing mechanism for receptor 1 with fluoride.
Scheme 3. The proposed sensing mechanism for receptor 1 with cyanide.
1 and their respective anion complexes (Fig. 5). The structural optimization and the computational calculations were carried out using Gaussian09 quantum chemistry package and results were viewed with GaussView5 GUI. The appropriate choice of model chemistry for the computation of electronic structure property is indispensable. In the present scenario, Density Functional Theory (DFT) supported B3LYP/6-31G (d, p) model chemistry was implemented particularly for Schiff base ligands under gas phase condition. DFT calculations were carried out to investigate the change of the absorption spectra upon the
addition of F and CN . From the energy level diagram (Fig. 6), it can be seen that the addition of the anions lead to the stabilization of the HOMO and LUMO of the sensor molecule. From the calculated results, the energy levels of HOMO and LUMO of receptor 1 increased after the addition of F and CN ions when compared to that of the free receptor 1 (Table 3). For F and CN complexes of receptor 1, the absorption bands predicted by the theoretical methods is red shifted compared with those of the independent probes, which is in good agreement with the experimental observations.
Table 1 Binding constant, stoichiometry, detection limit value of the complex formed for receptor 1 with CN and F ion. S. no 1 2 3
Receptor + ions R1 + F R1 + CN R1 + CN
Solvent system
Binding constant (M
1
)
4
100% CH3CN 100% CH3CN 90% H2O (R1) 100% H2O (CN )
4.6 10 3.3 105 5.5 104
Stoichiometry
LOD (M
1
1:1 1:1 1:1
1.5 10 1.9 10 5.2 10
6
)
6 6
Table 2 Comparison of some of the reported chemosensors for CN with the present work. Sensors
Solvent system
Sensing ions
LOD
Binding constant (M 1)
Linear range (R2)
Reference
Thiazole based sensor
DMSO/bis-tris buffer (8:2)
– 0.990 0.990 0.990
[30] [31] [32] [33]
Imidazo-anthraquinones based sensor
CH3CN:H2O (97:3)
–
[34]
Azo linked Schiff base sensor
CH3CN CH3CN:H2O (1:9, v/v)
3.2 102 1.0 102 – 2.3 103 – 5.89 104 (R1) 3.98 108 (R2) 6.80–8.96 6.31–8.50 4.6 104 5.5 104
[29]
H2O CH3CN:DMSO:HEPES (93:1:6) EtOH:H2O (3:7) CH3CN:DMSO (99:1)
60 mM 140 mM 0.01–0.07 mM 0–1.9 mM 1.5 mM 1.79 mM (R1) 2.43 mM (R2) 3.31 mM 4.97 mM 1.5 mM 1.9–5.2 mM
0.994
Azo based sensor Azo coupled tripodal based sensor Azo coupled NBD unit based sensor Bis-pyrene derivative based sensor
CN AcO CN CN CN CN
0.993
This work
CN F F CN
Table 3 Energies of HOMO and LUMO of the R1 and complexes. R1 and complexes R1 R1 + F R1 + CN
HOMO (eV) 5.774 3.434 3.282
LUMO (eV) 2.361 0.807 0.633
DE (eV)
Theoretical wavelength (nm)
Observed wavelength (nm)
3.413 2.627 2.649
363 471 350
354 450 385
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Fig. 5. Optimized structures of (a) receptor 1, (b) receptor 1 + F and (c) receptor 1 + CN .
Fig. 6. Energy levels of various HOMO and LUMO’s of receptor 1 in the absence and presence of F and CN .
4. Conclusion In summary, we have developed a novel and highly sensitive receptor 1 that can detect both F and CN ion selectively with the distinct color changes, where F ion shows colorless to orange and CN ion shows colorless to green. Further, receptor 1 showed selective detection of cyanide ion in 90% aqueous medium based on a nucleophilic addition followed by a intra molecular proton exchange process. Receptor 1 was found to provide sensitive and good selective recognition of CN confirmed through its absorption spectrum. Acknowledgment Authors express their thanks to DRDO (ERIP/ER/1006004/M/01/ 1333 dated 23-05-2011) for financial assistance in the form of a major sponsored project. 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.jfluchem.2015.04.014. References [1] P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. 40 (2001) 486–516. [2] S.K. Kim, J. Yoon, Chem. Commun. 7 (2002) 770–771. [3] A. Bianchi, K.B. James, E.G. Espana, Supramolecular Chemistry of Anions, WileyVCH, New York, USA, 1997. [4] T. Gunnlaugsson, M. Glynn, G.M. Tocci, P.E. Kruger, F.M. Pfeffer, Coord. Chem. Rev. 250 (2006) 3094–3117. [5] R. Martinez-Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419–4476.
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