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A highly selective colorimetric and turn-on fluorescent probe for cyanide anion
Tetrahedron 68 (2012) 2523e2526
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A highly selective colorimetric and turn-on fluorescent probe for cyanide anion Yan-Duo Lin a, Yung-Shu Peng b, Weiting Su c, Chin-Hsin Tu b, Chia-Hsing Sun b, Tahsin J. Chow a, c, * a
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan Department of Chemistry, Soochow University, Taipei 111, Taiwan c Department of Chemistry, National Taiwan University, Taipei 106, Taiwan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2011 Received in revised form 3 January 2012 Accepted 13 January 2012 Available online 20 January 2012
A new colorimetric and turn-on fluorescence chemosensor was developed for cyanide anion with high selectivity in the presence of other anions in an aqueous THF solution. The sensing mechanism is attributed to the interruption of p-conjugation by a nucleophilic addition of cyanide to the b-position of a dicyanoethylene unit. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Anion sensing Chemosensor Cyanide sensing Fluorescence Chemodosimeter
1. Introduction The recognition and detection of the cyanide ion are of growing interest because it binds efficiently to and deactivates the cytochrome c and inhibits the electron-transport chain in mitochondria and thus causes lethal poisons to living animals.1 Nonetheless, cyanides are widespread used in industrial processes, including gold mining, electroplating, metallurgy, and the synthesis of fibers and resins.2 Consequently, cyanide raises the potential contamination in the environment. Taking into account the adverse effects, a great deal of attention has been devoted to the discovery of improved analytical methods for the detection of cyanide. Considerable efforts for cyanide anion detection have been reported previously.3,4 However, most analytical approaches rely on hydrogen bonding interactions and, as a consequence, generally exhibited modest selectivities with respect to other anions. An alternate approach, reaction-based receptor for cyanide anions were adopted by many researches including the formation of cyanide complexes with metal ion, boron derivatives, and CdSe quantum dots.5 Most recently, receptors based on squaraine,6 acridium salts,7 oxazines,8 trifluoro-acetophenone derivatives,9 and benzyl derivatives10 have been developed on the basis of the nucleophilic
* Corresponding author. Tel.: þ886 2 27898552; fax: þ886 2 27884179; e-mail address: [email protected] (T.J. Chow). 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2012.01.026
reactions of cyanide anion to give rise to the observable signals. Among them only limited turn-on fluorescent probes for cyanide anion work effectively in aqueous or partially aqueous environments.11 Searching for effective chemosensors for detection of cyanide anion in aqueous media is still a challenging task. By virtue of the strong nucleophilic character of cyanide, reaction-based receptor for cyanide anion has been developed in order to avoid the complication induced hydrogen bonding. In this report we describe a novel colorimetric and fluorescent cyanideselective sensor 1, which can work effectively in a partially aqueous medium. Compound 1 can detect cyanide ion via naked eye discernible color change and exhibits a unique ‘turn-on’ fluorescence of high selectivity in a water/THF solution. The structure of compound 1 is composed of a trans-4-(N,N-diphenylamino)stilbene (2) as a signaling unit,12 and a dicyanoethylene group as a sensing unit (Scheme 1). The electrophilic nature of dicyanoethylene group can be modulated by cyanide anion, which interrupts the p-conjugation.13 The role of dicyanoethylene in the sensing mechanism can be clarified by a comparison with the spectrum of compound 2. 2. Result and discussion The synthesis of 1 was achieved readily through a condensation of the precursor aldehyde 3 with malononitrile in the presence of NEt3.14 Its structure was confirmed by 1H NMR, 13C NMR, and HRMS spectroscopic data (ESI).
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Y.-D. Lin et al. / Tetrahedron 68 (2012) 2523e2526
N
R
CN
CN α NC Hc
β
Hd
2) R = H 3) R = CHO
Ha Hc Hd
CN-
Hb
NC Hc
CN Hc
Hd
Hd
Fig. 2. Color changes observed for 1 (3.00105 M) upon the addition of F, Cl, Br, I, ClO 4 , H2PO4 , HSO4 , NO3 , CH3CO2 , and CN (20 equiv, respectively) in water/THF (40:60, v/v) solution.
Compound 1 shows two distinct absorption bands at w320 and 465 nm. The former is attributed to the pep* transition and the latter an intramolecular charge transfer (ICT) transition. Spectral response of 1 was examined with the tetrabutylammonium (TBA) salt of a series of anions including F, Cl, Br, I, ClO 4 , H2PO4 , HSO4 , NO3 , CH3CO2 , OH , and CN in 40% aqueous THF solutions (Fig. 1). Solutions were allowed to equilibrate for 15 min before taking the measurement. The sensing reaction was virtually completed within 12 min as indicated in the inset of Fig. 1 (also S6 and S7 in SD). Upon the addition 20 equiv of each anion, the absorption spectra of 1 did not exhibit significant change except cyanide ion. In the presence of CN ion, both absorption bands diminished, while two new bands at 296 and 377 nm showed up. The bleaching of the color can be observed clearly by naked eyes (Fig. 2). A gradual spectral change are shown in the inset of Fig. 1, in which three clear isosbestic points were observed at 411, 358, and 302 nm, indicating its clean transformation to a new species. An apparent fluorescence enhancement was also observed upon the addition of 20 equiv of cyanide (Fig. 3). The cyanide sensing fluorescence of 1 was monitored by titration in a mixed solution of 40:60 (v/v) water/THF upon excitation at 358 nm (i.e., isosbestic
point). With the addition of cyanide ion, the fluorescence intensity of the solution at 475 nm was gradually increased as shown in the inset of Fig. 3. The selectivity of fluorescence test was very high because the intensity enhancement was not observed in a solution containing all other anions before the addition of cyanide. The mechanism of a sensory response of compound 1 to cyanide anion is depicted in Scheme 1. The cyanide ion attacks the b-position of the dicyanoethylene moiety of 1,13 so that interrupts the pconjugation of the ICT transition. The resulted chromophore comprises the main part of the p-aminostilbene moiety, yet without the dicyanoethylene acceptor site. Its emission spectrum looks quite similar to that of trans-4-(N,N-diphenylamino)stilbene moiety (2) (S9 in SD). The yellow color fades away due to the diminishing of the ICT absorption. At the same time the intensity of fluorescence increases due to a higher quantum yield of the new chromophore.12 The sensing property of 1 toward cyanide ion is thus of two folds: both a visual color change and a fluorescence enhancement. A critical test on anion competition experiment was conducted by mixing 1 with cyanide and with all other types of anion in an aqueous THF solution. The absorption band stayed virtually the same upon the addition of 20 equiv each of all other anions to a solution of 1 (30 mM) (Fig. 1). As soon as a solution of CN ion was added to the anions mixture, the absorption intensity at 465 nm faded away, along with a concomitant appearance of new absorption band at 377 nm. The spectral pattern is the same as that of 1 upon adding CN ion alone. For emission spectra, the fluorescence intensity of 1 varied only slightly in the presence of other anions without CN (Fig. 3). When CN was added into a solution containing all other anions, a significant fluorescence enhancement was immediately observed. It is clear that compound 1 possesses an excellent selectivity for cyanide anion in the presence of other anions, which make it very useful in practical applications.
Fig. 1. Absorption spectra of 1 (3105 M in water/THF 40:60, v/v) solution with and without anions F, Cl, Br, I, ClO 4 , H2PO4 , HSO4 , NO3 , CH3CO2 , OH , and CN (20 equiv each). The inset shows the time-dependent change in the absorption spectra of 1 (3105 M) upon reaction with CN (3 equiv).
Fig. 3. Fluorescence spectra of 1 upon an excitation at 358 nm (3.0105 M) in water/ THF (40:60, v/v) solution in the presence of anions (20 equiv each). The inset shows the continuous change of spectra upon the addition of CN (0e3 equiv at 0.2 equiv interval).
N
N
1
4-
weak fluorescence
strong fluorescence
Scheme 1. Chemical structures of 1e3, and the reaction of 1 with cyanide ion for the formation of 4 (deprotonated form of 4).
Y.-D. Lin et al. / Tetrahedron 68 (2012) 2523e2526
Structural details in the interaction of 1 with cyanide anion were revealed by monitoring the changes in 1H NMR experiments. In Fig. 4 it shows the 1H NMR spectra of 1 upon the addition of tetrabutylammonium cyanide in DMSO-d6 solution. The addition of cyanide resulted in a slow reduction of the vinylic proton signal (Ha) at 8.46 ppm and finally disappeared with addition of 1 equiv of cyanide, while a new signal appeared at 4.45 ppm, which corresponds to the b-proton (Hb). Therefore, the results are consistent with the proposed mechanism that cyanide adds to the b-position of dicyanoethylene group. Meanwhile, the upfield shift of the two set of aromatic proton (Hc and Hd) are observed at the ortho and meta position relative to the ethylene group. However, no signal was observed corresponding to the malononitrile moiety in the 1H NMR spectra, probably due to its higher acidity, thus exchanged rapidly with the solvent.13a The formation of cyanide adduct 4 was further confirmed by high resolution FABMS where a peak at m/z 451.1931 corresponding to [1-CNþH]þ (Fig. S5).
Fig. 4. 1H NMR spectral changes of 1 in DMSO-d6 upon the addition of CN anion.
3. Conclusion In summary, we have successfully designed and synthesized a new highly selective chemosensor 1 for cyanide in aqueous THF solution. Sensor 1 exhibits a unique colorimetric and fluorescence enhancement only with cyanide ion even in the excess amounts of other anions, demonstrating its excellent selectivity over other anions. The color change could be observed directly by naked eyes. The turn-on fluorescence result from interruption of the pushepull chromophore through a nucleophilic attack by a cyanide anion. The resulted trans-4-(N,N-diphenylamino)stilbene chromophore exhibits substantially higher fluorescence quantum efficiency. The detection limit (signal-to-background, S/B)15 of compound 1 was estimated to be 6.6105 M. The sensitivity is close to the maximum permissive level in drinking water, which is 1.9 mM according to the World Health Organization (WHO).16 It can be used as a convenient and effective method to detect cyanide in aqueous media.
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of freshly prepared anion solution (0.03 M) at the prescribed increments. Solutions were allowed to equilibrate for 15 min before taking each measurement. Experiments with longer equilibration times did not produce noticeable differences. Solvents of reagent grade were used for syntheses, and those of spectroscopy grade for spectra measurements. Compounds purchased from commercial sources were used as received. The syntheses of compounds 314 have been reported previously. 4.1.1. 2-{4-[2-(4-Diphenylaminophenyl)vinyl]benzylidene}malononitrile (1). A mixture of p-diphenylamino-p0 -formylstilbene 3 (1 g, 2.7 mmol), malononitrile (0.2 g, 3.2 mmol), and NEt3 (2 mL) in dry CH2Cl2 (27 mL) was placed in a three-necked flask under a nitrogen atmosphere. It was stirred at room temperature for 18 h. Afterward the solvent was removed under reduced pressure, and the residue was purified by column chromatograph eluted with ethyl acetate/ hexane (1:10) to afford 1 as dark-red solids (0.70 g, 58% yield). Mp 167e168 C; 1H NMR (400 MHz, DMSO-d6): d 8.46 (s, 1H), 7.96 (d, J¼8.4 Hz, 2H), 7.80 (d, J¼8.4 Hz, 2H), 7.56 (d, J¼8.6 Hz, 2H), 7.48 (d, J¼16.4 Hz, 1H), 7.37e7.33 ppm (m, 4H), 7.23 (d, J¼16.4 Hz, 1H), 7.13e7.06 (m, 6H), 6.96 (d, J¼8.6 Hz, 2H); 13C NMR (100 MHz, DMSO-d6): d 160.36, 147.75, 146.69, 143.76, 132.51, 131.30, 130.13, 129.82, 129.68, 129.60, 128.35, 126.93, 125.10, 124.62, 123.72, 122.13, 113.70 ppm; FAB-HRMS calcd for C30H21N3 (MþHþ) 424.1814, found 424.1827. 4.1.2. 2-Cyano-3-{4-[2-(4-diphenylaminophenyl)vinyl]phenyl}succino-nitrile (4). A mixture of 1 (0.30 g, 0.71 mmol) and tetrabutylammonium cyanide (0.38 g, 14.2 mmol) in DMSO (10 mL) was placed in a three-necked flask under nitrogen atmosphere. It was stirred by a magnetic bar at room temperature for 3 h. Precipitates were formed upon the addition of distilled water, and the solids of 4 were filtered off and washed with water. (0.22 g, 71% yield). 1H NMR (400 MHz, CDCl3): d 7.61 (d, J¼8.2 Hz, 2H), 7.47 (d, J¼8.2 Hz, 2H), 7.39 (d, J¼8.4 Hz, 2H), 7.30e7.26 (m, 4H), 7.15e7.12 (m, 5H), 7.08e7.04 (m, 4H), 6.98 (d, J¼16.3 Hz, 1H), 4.46 (d, J¼5.9 Hz, 1H), 4.26 (d, J¼5.9 Hz, 1H); 13C NMR (100 MHz, DMSO-d6): d 148.10, 147.36, 140.59, 130.92, 130.36, 129.34, 128.43, 127.69, 127.53, 126.19, 125.76, 123.34, 123.08, 114.79, 109.26, 38.70, 29.58 ppm; FAB-HRMS calcd for C31H23N4 (MþHþ) 451.1923, found 451.1931. Acknowledgements Financial supports were provided by the Institute of Chemistry, Academia Sinica, and the National Science Council of Taiwan. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tet.2012.01.026. References and notes
4. Experimental section 4.1. General 1 H and 13C NMR spectra were recorded on a Bruker 400 MHz spectrometer. Fast atom bombardment (FAB) mass spectra were recorded on a Jeol JMS 700 double-focusing spectrometer. UV spectra were measured on a Jasco V-530 double beam spectrophotometer. Fluorescence spectra were recorded on a Hitachi F4500 fluorescence spectrophotometer. Absorption and fluorescence sensing measurements were performed in 3105 M water/ THF (40:60, v/v) solutions in all cases. To an optical cell containing compound 1 (2 mL) and a magnetic stir bar was added the aliquots
1. (a) Kulig, K. W. Cyanide Toxicity; U.S. Department of Health and Human Services: Atlanta, GA, 1991; (b) Baskin, S. I.; Brewer, T. G. In Medical Aspects of Chemical and Biological Warfare; Sidell, F., Takafuji, E. T., Franz, D. R., Eds.; TMM Publication: Washington, DC, 1997; Chapter 10, pp 271e286. 2. (a) Young, C.; Tidwell, L.; Anderson, C. Cyanide: Social, Industrial and Economic Aspects; Minerals, Metals, and Materials Society: Warrendale, 2001; (b) Miller, G. C., Pritsos, C. A. Cyanide: Soc., Ind. Econ. Aspects, Proc. Symp. Annu. Meet. TMS 2001, 73e81. 3. (a) Xu, Z.; Chen, X.; Kim, H. A.; Yoon, J. Chem. Soc. Rev. 2010, 39, 127; (b) An, K.; Castellano, F. N. J. Am. Chem. Soc. zenbacher, P., Jr.; Tayson, D. S.; Jursíkova 2002, 124, 6232; (c) Lin, Z.; Chen, H. C.; Sun, S.-S.; Hsu, C.-P.; Chow, T. J. Tetrahedron 2009, 65, 5216; (d) Kim, H. N.; Guo, Z.; Zhu, W.; Yoon, J.; Tian, H. Chem. Soc. Rev. 2011, 40, 79; (e) Ren, J.; Zhu, W.; Tian, H. Talanta 2008, 75, 760. 4. (a) Sun, S.-S.; Lees, A. J. Chem. Commun. 2000, 1687; (b) Gimeno, N.; Li, X.; Durrant, J. R.; Vilar, R. Chem.dEur. J. 2008, 14, 3006; (c) Kim, Y.-H.; Hong, J.-I. Chem. Commun. 2002, 512; (d) Liu, H.; Shao, X.-B.; Jia, M.-X.; Jiang, X.-K.; Lia, Z.-
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5.
6. 7. 8. 9.
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T.; Chen, G.-J. Tetrahedron 2005, 61, 8095; (f) Kim, S.-H.; Hong, S.-J.; Yoo, J.; Kim, S.-K.; Sessler, J. L.; Lee, C.-H. Org. Lett. 2009, 11, 3626. (a) Ganesh, V.; Sanz, M. P. C.; Mareque-Rivas, J. C. Chem. Commun. 2007, 5010; (b) Zeng, Q.; Cai, P.; Li, Z.; Qina, J.; Tang, B. Z. Chem. Commun. 2008, 1094; (c) Lou, X.; Zhang, L.; Qin, J.; Li, Z. Chem. Commun. 2008, 5848; (d) Ros-Lis, J. V.; ~ ez, R.; Soto, J. Chem. Commun. 2005, 5260; (e) Hudnall, T. W.; Martínez-M an Gabbaï, F. P. J. Am. Chem. Soc. 2007, 129, 11978; (f) Touceda-Varela, A.; Stevenn, J. A.; Dryden, D. T. F.; Mareque-Rivas, J. C. Chem. Comson, E. I.; Galve-Gasio mun. 2008, 1998; (g) Chung, S.-Y.; Nam, S.-W.; Lim, J.; Park, S.; Yoon, J. Chem. Commun. 2009, 2866. ~ ez, R.; Soto, J. Chem. Commun. 2002, 2248. Ros-Lis, J. V.; Martínez-M an Yang, Y.-K.; Tae, J. Org. Lett. 2006, 8, 5721. (a) Tomasulo, M.; Raymo, F. M. Org. Lett. 2005, 7, 4633; (b) Tomasulo, M.; Sortino, S.; White, A. J. P.; Raymo, F. M. J. Org. Chem. 2006, 71, 744. Chung, Y. M.; Raman, B.; Kim, D. S.; Ahn, K. H. Chem. Commun. 2006, 186.
10. Cho, D.-G.; Kim, J. H.; Sessler, J. L. J. Am. Chem. Soc. 2008, 130, 12163. 11. (a) Chen, C.-L.; Chen, Y.-H.; Chen, C.-Y.; Sun, S.-S. Org. Lett. 2006, 8, 5053; (b) Sabir, H. M.; Rupesh, B.; Mukesh, C.; Carolina, E.; Antonio, F. New J. Chem. 2011, 35, 57; (c) Sun, Y.; Liu, Y.; Chen, M.; Guo, W. Talanta 2009, 80, 996; (d) Lee, K.-S.; Kim, H.-J.; Kim, G.-H. ,; Shin, I.; Hong, J.-I. Org. Lett. 2008, 10, 49; (e) Jo, J.; Lee, D. J. Am. Chem. Soc. 2009, 131, 16283. 12. Yang, J.-S.; Chiou, S.-Y.; Liau, K.-L. J. Am. Chem. Soc. 2002, 124, 2518. 13. (a) Hong, S.-J.; Yoo, J.; Kim, S.-H.; Kim, J. S.; Yoon, J.; Lee, C.-H. Chem. Commun. vez, P.; García, F. C.; Serna, F.; de la Pen ~ a, J. L.; 2009, 189; (b) Vallejos, S.; Este García, J. M. Chem. Commun. 2010, 7951. 14. Hwang, S.; Lee, J. H.; Park, C.; Lee, H.; Kim, C.; Park, C.; Lee, M.-Y.; Lee, W.; Park, J.; Kim, K.; Park, N.-G.; Kim, C. Chem. Commun. 2007, 4887. 15. Garner, A. L.; Koide, K. J. Am. Chem. Soc. 2008, 130, 16472. 16. Guidelines for Drinking-Water Quality; World Health Organization: Geneva, 1996.
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Tetrahedron journal homepage: www.elsevier.com/locate/tet
A highly selective colorimetric and turn-on fluorescent probe for cyanide anion Yan-Duo Lin a, Yung-Shu Peng b, Weiting Su c, Chin-Hsin Tu b, Chia-Hsing Sun b, Tahsin J. Chow a, c, * a
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan Department of Chemistry, Soochow University, Taipei 111, Taiwan c Department of Chemistry, National Taiwan University, Taipei 106, Taiwan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2011 Received in revised form 3 January 2012 Accepted 13 January 2012 Available online 20 January 2012
A new colorimetric and turn-on fluorescence chemosensor was developed for cyanide anion with high selectivity in the presence of other anions in an aqueous THF solution. The sensing mechanism is attributed to the interruption of p-conjugation by a nucleophilic addition of cyanide to the b-position of a dicyanoethylene unit. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Anion sensing Chemosensor Cyanide sensing Fluorescence Chemodosimeter
1. Introduction The recognition and detection of the cyanide ion are of growing interest because it binds efficiently to and deactivates the cytochrome c and inhibits the electron-transport chain in mitochondria and thus causes lethal poisons to living animals.1 Nonetheless, cyanides are widespread used in industrial processes, including gold mining, electroplating, metallurgy, and the synthesis of fibers and resins.2 Consequently, cyanide raises the potential contamination in the environment. Taking into account the adverse effects, a great deal of attention has been devoted to the discovery of improved analytical methods for the detection of cyanide. Considerable efforts for cyanide anion detection have been reported previously.3,4 However, most analytical approaches rely on hydrogen bonding interactions and, as a consequence, generally exhibited modest selectivities with respect to other anions. An alternate approach, reaction-based receptor for cyanide anions were adopted by many researches including the formation of cyanide complexes with metal ion, boron derivatives, and CdSe quantum dots.5 Most recently, receptors based on squaraine,6 acridium salts,7 oxazines,8 trifluoro-acetophenone derivatives,9 and benzyl derivatives10 have been developed on the basis of the nucleophilic
* Corresponding author. Tel.: þ886 2 27898552; fax: þ886 2 27884179; e-mail address: [email protected] (T.J. Chow). 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2012.01.026
reactions of cyanide anion to give rise to the observable signals. Among them only limited turn-on fluorescent probes for cyanide anion work effectively in aqueous or partially aqueous environments.11 Searching for effective chemosensors for detection of cyanide anion in aqueous media is still a challenging task. By virtue of the strong nucleophilic character of cyanide, reaction-based receptor for cyanide anion has been developed in order to avoid the complication induced hydrogen bonding. In this report we describe a novel colorimetric and fluorescent cyanideselective sensor 1, which can work effectively in a partially aqueous medium. Compound 1 can detect cyanide ion via naked eye discernible color change and exhibits a unique ‘turn-on’ fluorescence of high selectivity in a water/THF solution. The structure of compound 1 is composed of a trans-4-(N,N-diphenylamino)stilbene (2) as a signaling unit,12 and a dicyanoethylene group as a sensing unit (Scheme 1). The electrophilic nature of dicyanoethylene group can be modulated by cyanide anion, which interrupts the p-conjugation.13 The role of dicyanoethylene in the sensing mechanism can be clarified by a comparison with the spectrum of compound 2. 2. Result and discussion The synthesis of 1 was achieved readily through a condensation of the precursor aldehyde 3 with malononitrile in the presence of NEt3.14 Its structure was confirmed by 1H NMR, 13C NMR, and HRMS spectroscopic data (ESI).
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Y.-D. Lin et al. / Tetrahedron 68 (2012) 2523e2526
N
R
CN
CN α NC Hc
β
Hd
2) R = H 3) R = CHO
Ha Hc Hd
CN-
Hb
NC Hc
CN Hc
Hd
Hd
Fig. 2. Color changes observed for 1 (3.00105 M) upon the addition of F, Cl, Br, I, ClO 4 , H2PO4 , HSO4 , NO3 , CH3CO2 , and CN (20 equiv, respectively) in water/THF (40:60, v/v) solution.
Compound 1 shows two distinct absorption bands at w320 and 465 nm. The former is attributed to the pep* transition and the latter an intramolecular charge transfer (ICT) transition. Spectral response of 1 was examined with the tetrabutylammonium (TBA) salt of a series of anions including F, Cl, Br, I, ClO 4 , H2PO4 , HSO4 , NO3 , CH3CO2 , OH , and CN in 40% aqueous THF solutions (Fig. 1). Solutions were allowed to equilibrate for 15 min before taking the measurement. The sensing reaction was virtually completed within 12 min as indicated in the inset of Fig. 1 (also S6 and S7 in SD). Upon the addition 20 equiv of each anion, the absorption spectra of 1 did not exhibit significant change except cyanide ion. In the presence of CN ion, both absorption bands diminished, while two new bands at 296 and 377 nm showed up. The bleaching of the color can be observed clearly by naked eyes (Fig. 2). A gradual spectral change are shown in the inset of Fig. 1, in which three clear isosbestic points were observed at 411, 358, and 302 nm, indicating its clean transformation to a new species. An apparent fluorescence enhancement was also observed upon the addition of 20 equiv of cyanide (Fig. 3). The cyanide sensing fluorescence of 1 was monitored by titration in a mixed solution of 40:60 (v/v) water/THF upon excitation at 358 nm (i.e., isosbestic
point). With the addition of cyanide ion, the fluorescence intensity of the solution at 475 nm was gradually increased as shown in the inset of Fig. 3. The selectivity of fluorescence test was very high because the intensity enhancement was not observed in a solution containing all other anions before the addition of cyanide. The mechanism of a sensory response of compound 1 to cyanide anion is depicted in Scheme 1. The cyanide ion attacks the b-position of the dicyanoethylene moiety of 1,13 so that interrupts the pconjugation of the ICT transition. The resulted chromophore comprises the main part of the p-aminostilbene moiety, yet without the dicyanoethylene acceptor site. Its emission spectrum looks quite similar to that of trans-4-(N,N-diphenylamino)stilbene moiety (2) (S9 in SD). The yellow color fades away due to the diminishing of the ICT absorption. At the same time the intensity of fluorescence increases due to a higher quantum yield of the new chromophore.12 The sensing property of 1 toward cyanide ion is thus of two folds: both a visual color change and a fluorescence enhancement. A critical test on anion competition experiment was conducted by mixing 1 with cyanide and with all other types of anion in an aqueous THF solution. The absorption band stayed virtually the same upon the addition of 20 equiv each of all other anions to a solution of 1 (30 mM) (Fig. 1). As soon as a solution of CN ion was added to the anions mixture, the absorption intensity at 465 nm faded away, along with a concomitant appearance of new absorption band at 377 nm. The spectral pattern is the same as that of 1 upon adding CN ion alone. For emission spectra, the fluorescence intensity of 1 varied only slightly in the presence of other anions without CN (Fig. 3). When CN was added into a solution containing all other anions, a significant fluorescence enhancement was immediately observed. It is clear that compound 1 possesses an excellent selectivity for cyanide anion in the presence of other anions, which make it very useful in practical applications.
Fig. 1. Absorption spectra of 1 (3105 M in water/THF 40:60, v/v) solution with and without anions F, Cl, Br, I, ClO 4 , H2PO4 , HSO4 , NO3 , CH3CO2 , OH , and CN (20 equiv each). The inset shows the time-dependent change in the absorption spectra of 1 (3105 M) upon reaction with CN (3 equiv).
Fig. 3. Fluorescence spectra of 1 upon an excitation at 358 nm (3.0105 M) in water/ THF (40:60, v/v) solution in the presence of anions (20 equiv each). The inset shows the continuous change of spectra upon the addition of CN (0e3 equiv at 0.2 equiv interval).
N
N
1
4-
weak fluorescence
strong fluorescence
Scheme 1. Chemical structures of 1e3, and the reaction of 1 with cyanide ion for the formation of 4 (deprotonated form of 4).
Y.-D. Lin et al. / Tetrahedron 68 (2012) 2523e2526
Structural details in the interaction of 1 with cyanide anion were revealed by monitoring the changes in 1H NMR experiments. In Fig. 4 it shows the 1H NMR spectra of 1 upon the addition of tetrabutylammonium cyanide in DMSO-d6 solution. The addition of cyanide resulted in a slow reduction of the vinylic proton signal (Ha) at 8.46 ppm and finally disappeared with addition of 1 equiv of cyanide, while a new signal appeared at 4.45 ppm, which corresponds to the b-proton (Hb). Therefore, the results are consistent with the proposed mechanism that cyanide adds to the b-position of dicyanoethylene group. Meanwhile, the upfield shift of the two set of aromatic proton (Hc and Hd) are observed at the ortho and meta position relative to the ethylene group. However, no signal was observed corresponding to the malononitrile moiety in the 1H NMR spectra, probably due to its higher acidity, thus exchanged rapidly with the solvent.13a The formation of cyanide adduct 4 was further confirmed by high resolution FABMS where a peak at m/z 451.1931 corresponding to [1-CNþH]þ (Fig. S5).
Fig. 4. 1H NMR spectral changes of 1 in DMSO-d6 upon the addition of CN anion.
3. Conclusion In summary, we have successfully designed and synthesized a new highly selective chemosensor 1 for cyanide in aqueous THF solution. Sensor 1 exhibits a unique colorimetric and fluorescence enhancement only with cyanide ion even in the excess amounts of other anions, demonstrating its excellent selectivity over other anions. The color change could be observed directly by naked eyes. The turn-on fluorescence result from interruption of the pushepull chromophore through a nucleophilic attack by a cyanide anion. The resulted trans-4-(N,N-diphenylamino)stilbene chromophore exhibits substantially higher fluorescence quantum efficiency. The detection limit (signal-to-background, S/B)15 of compound 1 was estimated to be 6.6105 M. The sensitivity is close to the maximum permissive level in drinking water, which is 1.9 mM according to the World Health Organization (WHO).16 It can be used as a convenient and effective method to detect cyanide in aqueous media.
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of freshly prepared anion solution (0.03 M) at the prescribed increments. Solutions were allowed to equilibrate for 15 min before taking each measurement. Experiments with longer equilibration times did not produce noticeable differences. Solvents of reagent grade were used for syntheses, and those of spectroscopy grade for spectra measurements. Compounds purchased from commercial sources were used as received. The syntheses of compounds 314 have been reported previously. 4.1.1. 2-{4-[2-(4-Diphenylaminophenyl)vinyl]benzylidene}malononitrile (1). A mixture of p-diphenylamino-p0 -formylstilbene 3 (1 g, 2.7 mmol), malononitrile (0.2 g, 3.2 mmol), and NEt3 (2 mL) in dry CH2Cl2 (27 mL) was placed in a three-necked flask under a nitrogen atmosphere. It was stirred at room temperature for 18 h. Afterward the solvent was removed under reduced pressure, and the residue was purified by column chromatograph eluted with ethyl acetate/ hexane (1:10) to afford 1 as dark-red solids (0.70 g, 58% yield). Mp 167e168 C; 1H NMR (400 MHz, DMSO-d6): d 8.46 (s, 1H), 7.96 (d, J¼8.4 Hz, 2H), 7.80 (d, J¼8.4 Hz, 2H), 7.56 (d, J¼8.6 Hz, 2H), 7.48 (d, J¼16.4 Hz, 1H), 7.37e7.33 ppm (m, 4H), 7.23 (d, J¼16.4 Hz, 1H), 7.13e7.06 (m, 6H), 6.96 (d, J¼8.6 Hz, 2H); 13C NMR (100 MHz, DMSO-d6): d 160.36, 147.75, 146.69, 143.76, 132.51, 131.30, 130.13, 129.82, 129.68, 129.60, 128.35, 126.93, 125.10, 124.62, 123.72, 122.13, 113.70 ppm; FAB-HRMS calcd for C30H21N3 (MþHþ) 424.1814, found 424.1827. 4.1.2. 2-Cyano-3-{4-[2-(4-diphenylaminophenyl)vinyl]phenyl}succino-nitrile (4). A mixture of 1 (0.30 g, 0.71 mmol) and tetrabutylammonium cyanide (0.38 g, 14.2 mmol) in DMSO (10 mL) was placed in a three-necked flask under nitrogen atmosphere. It was stirred by a magnetic bar at room temperature for 3 h. Precipitates were formed upon the addition of distilled water, and the solids of 4 were filtered off and washed with water. (0.22 g, 71% yield). 1H NMR (400 MHz, CDCl3): d 7.61 (d, J¼8.2 Hz, 2H), 7.47 (d, J¼8.2 Hz, 2H), 7.39 (d, J¼8.4 Hz, 2H), 7.30e7.26 (m, 4H), 7.15e7.12 (m, 5H), 7.08e7.04 (m, 4H), 6.98 (d, J¼16.3 Hz, 1H), 4.46 (d, J¼5.9 Hz, 1H), 4.26 (d, J¼5.9 Hz, 1H); 13C NMR (100 MHz, DMSO-d6): d 148.10, 147.36, 140.59, 130.92, 130.36, 129.34, 128.43, 127.69, 127.53, 126.19, 125.76, 123.34, 123.08, 114.79, 109.26, 38.70, 29.58 ppm; FAB-HRMS calcd for C31H23N4 (MþHþ) 451.1923, found 451.1931. Acknowledgements Financial supports were provided by the Institute of Chemistry, Academia Sinica, and the National Science Council of Taiwan. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tet.2012.01.026. References and notes
4. Experimental section 4.1. General 1 H and 13C NMR spectra were recorded on a Bruker 400 MHz spectrometer. Fast atom bombardment (FAB) mass spectra were recorded on a Jeol JMS 700 double-focusing spectrometer. UV spectra were measured on a Jasco V-530 double beam spectrophotometer. Fluorescence spectra were recorded on a Hitachi F4500 fluorescence spectrophotometer. Absorption and fluorescence sensing measurements were performed in 3105 M water/ THF (40:60, v/v) solutions in all cases. To an optical cell containing compound 1 (2 mL) and a magnetic stir bar was added the aliquots
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