- Email: [email protected]
A pyrenyl-appended triazole-based ribose as a fluorescent sensor for Hg2+ ion
Carbohydrate Research 345 (2010) 2557–2561
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Note
A pyrenyl-appended triazole-based ribose as a fluorescent sensor for Hg2+ ion Kuan-Hao Chen, Cheng-Yi Lu, Hsiu-Jung Cheng, Shau-Jiun Chen, Ching-Han Hu, An-Tai Wu ⇑ Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan
a r t i c l e
i n f o
Article history: Received 7 March 2010 Received in revised form 6 September 2010 Accepted 9 September 2010 Available online 17 September 2010 Keywords: Chemosensor Ribose Fluorescence Pyrene
a b s t r a c t For the efficient detection of toxic trace metal ions, two pyrenyl-appended triazole-based D-ribose fluorescent chemosensors 6 and 7 were prepared and their fluoroionophoric properties toward transition metal ions were investigated. Chemosensors 6 and 7 exhibit highly selective recognition toward Hg2+ ion among a series of tested metal ions in CH2Cl2/MeOH solution. The association constants of 6 and 7 are calculated to be 1.73 105 M 1 and 4.44 105 M 1, respectively. Both 6 and 7 formed complexes with the Hg2+ ion at a 1:1 ligand-to-metal ratio with a detection limit of 10–15 lM Hg2+. Computational analysis demonstrated that the Hg2+ ion occupied the coordination center of 6 with N2 and N3 atoms in two triazole groups, thus separating and distorting the two parallel pyrenes away from each other. Ó 2010 Elsevier Ltd. All rights reserved.
The design and synthesis of new chemosensors for the efficient detection of trace metal ions are among the most important research topics in environmental chemistry and biology.1 In particular, mercury ion is a significant environmental pollutant that accumulates in plants, soil, and water. In the marine environment, mercury ion is converted by bacteria into methylmercury, a highly potent neurotoxin. Methylmercury is passed up the food chain and bioaccumulates in humans.2–4 Accordingly, it is imperative to develop analytical methods for the sensitive and selective detection of trace amounts of mercury ion.5,6 Carbohydrate-based chemosensors are chiral entities with hydroxyl groups and oxygen atoms that form quite suitable cation binding sites. Thus, in the design of chemosensors, the incorporation of sugar molecules is a good strategy for capturing cations.7–9 Currently, there are very few sugar-based fluorescent chemosensors for selective detection of mercury ion described in the literature.10 A good chemosensor must also possess a selective and sensitive signaling mechanism. Recently, pyrene has emerged as one of the most effective functional groups for fluorescence signaling.11–15 In this study, we combined these two entities in a novel chemosensor by connection with triazole groups for metal ion screening studies because the triazole groups are lately recognized as potential metal ion binding site.15,16 The designed ribose-based pyrenyl-appended chemosensors 6 and 7 exhibit high selectivity and sensitivity for Hg2+ ion compared to other transition and heavy metal ions. The synthesis of fluorescent sensors is outlined in Scheme 1. The 5-O-Ms-C-riboside 1 can be easily prepared from ribose in five steps.17 The ester groups of 1 were hydrolyzed with NaOH to ⇑ Corresponding author. Tel.: +886 4 7232105 3542; fax: +886 4 7211 190. E-mail address: [email protected] (A.-T. Wu). 0008-6215/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carres.2010.09.010
produce the corresponding acid 2 in 70% yield. We used Steglich esterification to couple compound 2 with two different lengths of polyethylene glycol in the presence of DMAP and DCC to obtain 3 or 4 in 68% and 63% yields, respectively. Under microwave conditions, 3 or 4 was treated with 1-(propagyloxymethyl)pyrene 5 and PPh3CuBr as the catalyst in toluene at reflux to obtain compound 6 or 7 in 63% and 57% yields, respectively. We first studied the optical properties of both 6 and 7 in CH2Cl2/ MeOH solution. On excitation at 343 nm, the maximum absorption wavelength of the pyrene in 6 and 7 displayed a strong excimer emission at 478 nm and a weak monomer emission at 395 nm (Fig. 1a and b), respectively. The relative ratio of excimer to monomer (Iexcimer/Imonomer) bands for 6 and 7 is about 5–5.5 indicating that 6 and 7 have similar conformational rigidity. The chemosensing behavior of 6 (10 lM) and 7 (15 lM) was investigated by comparing their fluorescence intensities before and after addition of 10 equiv of the following 11 metal ions (as perchlorate salts): Li+, Na+, K+, Ca2+, Mg2+, Hg2+, Co2+, Ni2+, Cu2+, Pb2+, and Zn2+. The results indicated that the monomer and excimer emissions of 6 were strongly quenched only in the presence of the Hg2+ ion (Fig. 1a). This selective quenching by the Hg2+ ion was also observed for compound 7 (Fig. 1b). The quenching efficiency (I I0/I0 100%) for 6 and 7 observed at 478 nm was nearly 100% (Fig. 2); in contrast, the other metal ions caused comparatively small changes in fluorescence intensity. In addition, Cd2+ is an important cation because, like Hg2+ it is also toxic. Therefore, we also performed the selectivity study on Cd2+. The results showed almost no effect on the fluorescence changes of 6 and 7 upon addition of Cd2+ (Fig. S14). Moreover, we tested whether the addition of excess metal ions would change the UV–vis spectra of both 6 and 7. We found that the spectra of
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O
O N3
N3
OCH3
O
O
NaOH O
O
O n = 2, 3 ( )n HO O OH
O
O
dioxane
DCC, DMAP
O
O
O
O O O
O
O O 3n=2 4n=3
N3
2
1
)n
( O
OH
(
O O
O
) n
N3
O O
O
O O
O
OH propargyl bromide
O 3 or 4
O N N
PPh3¡PCuBr
NaH
O
N
M.W
N
O
N N
O
5
6n=2 7n=3 Scheme 1. Synthesis of chemosensors 6 and 7.
(a)
Metal ions Na, Zn, Ca, K, Li, Pd, Mg, Ni, Co
478 nm Free 6
500
Cu Na Zn
400 300 200
377 nm 396 nm
Hg
100 0 400
450
500
550
Li
Hg Pb Mg
Ni
Co
-20 -40
6 7
-60 -80
600
Wavelength (nm)
Fluorescence Intensity (a.u.)
K
-100 350
(b)
Ca
0
Cu
[( I-I0) /I0] x 100 %
Fluorescence Intensity (a.u.)
600
700
Na, Zn, Ca, K, Li, Pd, Mg, Ni, Co
478 nm
Free 7
600
Figure 2. Fluorescence intensity changes ((I I0)/I0 100%) of 6 (10 lM, dark bars) and 7 (15 lM, light bars) in the presence of various metal ions in CH2Cl2/MeOH (80:20, v/v); kex = 344 nm.
500
Cu
400 300 200
377 nm
100
396 nm
Hg
0 350
400
450
500
550
600
Wavelength (nm) Figure 1. Fluorescence spectra of (a) 6 (10 lM) and (b) 7 (15 lM) in the presence of 10 equiv of various metal ions in CH2Cl2/MeOH (80:20, v/v); kex = 344 nm.
both 6 (Fig. 3a) and 7 (Fig. 3b) were altered only in the presence of the Hg2+ ion. The presence of Hg2+ ion caused a small red shift that
occurred at three kmax absorption wavelengths (312, 328, and 344 nm). These red shifts are attributable to the deformation of electrical environments owing to Hg2+ complex formation. Additionally, the chain lengths of the ethylene ethers on 6 and 7 had no influence on the quenching of the excimer and monomer emissions upon the addition of the Hg2+ ion. The declining excimer and monomer emissions that we found in both 6 and 7 were similar to the findings of Kim and coworkers.13 They showed that once Pb2+, Hg2+, and Cu2+ions were added into a pyrenyl-appended triazole-based calix[4]arene as a fluorescent sensor, a quenched fluorescence in both monomer and excimer emissions was observed. Recently, Chung and coworkers18 described a bis-triazoles-chained pyrenes fluorescent chemosensor that had dual-mode recognition for Pb2+ and Cd2+ ions. In comparison, our results showed both 6 and 7 were unimode Hg2+-selective fluorescent sensors. The differences could be attributed to the addition of the ribose-based moiety to the bis-triazoles-chained pyrenes.
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344 nm
Free 6, Na, Zn, Ca, K, Li, Pd, Mg, Ni, Co, Cu
0
(a)
-100
481 nm
1.0
346 nm
Absorbance
328 nm
0.5
0.0 300
Fluorescence Intensity (a.u.)
600
Hg 312 nm
320
340
360
380
0
-300 -400
500
-500 -600
400
0
5
10
15
20
2+
[ Hg ] / [ 6 ]
30 e.q 300 200 100 0 350
Wavelength (nm)
-200
I - I0
(a) 1.5
400
450
500
550
600
% (?X)
(b) 1.0
344 nm
Free 7, Na, Zn, Ca, K, Li, Pd, Mg, Ni, Co, Cu
0 -100
346 nm
600
0
481 nm
328 nm
0.4
Hg 312 nm
0.2
0.0 300
320
-200 -300 -400
0.6
340
360
380
Wavelength (nm) Figure 3. UV–vis spectra of (a) 6 (10 lM) and (b) 7 (15 lM) in the presence of 10 equiv of various metal ions in CH2Cl2/MeOH (80:20, v/v); kex = 344 nm.
Fluorescence Intensity (a.u.)
Absorbance
0.8
I - I0
(b)
500
-500 -600
400
0
5
10
15
20
2+
[ Hg ] / [ 7 ]
10 eq 300 200 100 0 350
400
450
500
550
600
Wavelength (nm) Figure 4. Fluorescence spectra of (a) 6 (10 lM) and (b) 7 (15 lM) in CH2Cl2/MeOH (80:20, v/v) upon addition of increasing concentrations of Hg(ClO4)2; kex = 344 nm.
Previous reports13,14 showed that host molecules with plural pyrenyl groups exhibit intramolecular excimer emission by two different mechanisms. One results19 from p–p stacking of the pyrene rings in the free state, which results in a characteristic decrease of excimer emission intensity and a concomitant increase of monomer emission upon metal ion complexation. The other mechanism is ascribed to the interaction of an excited pyrene (Py*) unit with a ground state pyrene (Py) unit.14 The remarkable excimer emission in this study is probably attributed to an intramolecular interaction between Py and Py* because the chemosensor properties of 6 and 7 do not conform to the former mechanism upon addition of the Hg2+ ion. However, the monomer fluorescence quenching cannot be easily explained because it may be difficult to distinguish between the reverse PET mechanism and intra-complex quenching since Hg2+ ion is known as effective quenching metals (e.g., open-shell, paramagnetic, large or easily reducible cation).20,21 Taken together the data above indicate that both monomer and excimer fluorescences diminished dramatically upon addition of Hg2+ probably because of the quenching induced by electron transfer. To further investigate the chemosensor properties of 6 and 7, we performed fluorescence titrations of 6 (10 lM) and 7 (15 lM) in the presence of different concentrations of Hg2+ ion in CH2Cl2/ MeOH. Figure 4 shows the gradual reductions in fluorescence intensity for 6 and 7 upon addition of increasing concentrations of Hg2+ ion. From the fluorescence titration profiles, the association constant for 6*Hg2+ and 7*Hg2+ in CH2Cl2/MeOH solution was calculated to be 1.73 105 M 1 and 4.43 105 M 1, respectively, by a Stern-Volmer plot (Figs. S15 and S17). From the titration result the detection limits for the Hg2+ ion were determined to be 10 and
15 lM, for 6 and 7, respectively. The detection limits were sufficiently low to detect the submillimolar concentrations of Hg2+ ion found in many chemical and biological systems. In the Job plot (Figs. S16 and S18), a maximum fluorescence change was observed when the molar fraction of the ionophore 6 or 7 versus Hg2+ was 0.5, which indicated that only a 1:1 complex was formed. Selectivity for the Hg2+ ion was further ascertained with competition experiments. We found that the fluorescence intensity of either 6 or 7 in the presence of 10 equiv of Hg2+ ion was almost unaffected by the addition of 10 equiv of competing metal ions (Li+, Na+, K+, Ca2+, Mg2+, Co2+, Ni2+, Cu2+, Pb2+, and Zn2+) (Figs. S19 and S20). To better understand the coordination behavior of 6 with the Hg2+ ion, 1H NMR experiments were carried out in CDCl3 solution. The spectral differences are depicted in Figure 5. The complexation of 6 with Hg2+ is expected to reduce the electron density of the coordination sites and induce a downfield shift of the nearby proton signals; however, upfield shift was observed for most of the protons. In the presence of 1.0 equiv of the Hg2+ ion, the protons of Ha-Hc were upfield shifted by 0.18, 0.25, and 0.10 ppm, respectively. Two methylene protons in the triazole-CH2–O (Hg) and O–CH2-pyrene (Hh) were upfield shifted by 0.22 ppm and 0.18 ppm, respectively. In addition, the protons in ribose-CH2triazole (He) and ribose ring (Hd) were upfield shifted by 0.18–0.25 ppm and it formed a sharper peak. The protons in the isopropyl groups of the ribose were also upfield shifted by 0.16 ppm while the protons in pyrene were upfield shifted by 0.15–0.2 ppm. Based on the results of metal ion-induced chemical
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Figure 5. 1H NMR spectra of 6 (5 mM) in the (a) absence and (b) presence of 1.0 equiv of Hg(ClO4)2 in CDCl3.
shift changes in the 1H NMR spectrum, we proposed that the upfield shifts may be due to the perchlorate counter anion that could not be well separated from Hg2+ in the organic solvent. To investigate further the Hg2+ ion accurate coordinating mode of the 6–Hg2+ complex, computations for 6–Hg2+ were conducted at the B3LYP/6-31G(d) level using the GAUSSIAN 03 suite of programs. On the basis of these calculations, we found three stationary points, which have been verified as genuine minima via vibrational frequency analyses. The optimized structure for the most stable isomer of 6–Hg2+ is illustrated in Figure 6. It is clear that the Hg2+ ion occupied the coordination center of 6 with N2 and N3 atoms (i.e., the average bond length of Hg–N2 and Hg–N3 was estimated at 4.09 and 4.17 Å, respectively) in two triazole groups, thus separating and distorting two pyrenes each other.
In summary, two novel ribosyl-based-Hg2+-selective fluorescent sensors 6 and 7 were designed and synthesized by coupling ethylene ether, ribosyl, and pyrenetriazolemethyl moieties. The chain lengths of the ethylene ether groups did not affect their selectivity for the Hg2+ ion. Both 6 and 7 show high selectivity for Hg 2+ ion over other metal ions. The detection limits of both 6 (10 lM) and 7 (15 lM) are sufficiently low to detect the trace amounts of Hg2+ ion found in many chemical and biological systems. Computational calculations demonstrated that a triazolecoordinated Hg2+ ion center was formed in 6–Hg2+, thus quenching both monomer and excimer of 6. Although there is no concrete evidence to support the breakage of the excimer complex, the most characteristic in this study is that both monomer and excimer fluorescences diminished dramatically upon addition of Hg2+ ion probably because of the quenching induced by electron transfers.
K.-H. Chen et al. / Carbohydrate Research 345 (2010) 2557–2561
Figure 6. Conformation of 6–Hg2+ optimized by density functional theory.
Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carres.2010.09.010. References 1. de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515–1566.
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2. McKeown-Eyssen, G. E.; Ruedy, J.; Neims, A. Am. J. Epidemol. 1983, 118, 470– 479. 3. Davidson, P. W.; Myers, G. J.; Cox, C.; Shamlaye, C. F.; Marsh, D. O.; Tanner, M. A.; Berlin, M.; Sloane-Reeves, J.; Cernichiari, E.; Choisy, O.; Choi, A.; Clarkson, T. W. Neurotoxicology 1995, 16, 677–688. 4. Grandjean, P.; Weihe, P.; White, R. F.; Debes, F. Environ. Res. 1998, 77, 165– 172. 5. Yuan, M.; Li, Y.; Li, J.; Li, C.; Liu, X.; Lv, J.; Xu, J.; Liu, H.; Wang, S.; Zhu, D. Org. Lett. 2007, 9, 2313–2316. 6. Zhu, M.; Yuan, M.; Liu, X.; Xu, J.; Lv, J.; Huang, C.; Liu, H.; Li, Y.; Wang, S.; Zhu, D. Org. Lett. 2008, 10, 1481–1484. 7. Yuasa, H.; Miyagawa, N.; Izumi, T.; Nakatani, M.; Izumi, M.; Hashimoto, H. Org. Lett. 2004, 6, 1489–1492. 8. Singhal, N. K.; Ramanujam, B.; Mariappanadar, V.; Rao, C. P. Org. Lett. 2006, 8, 3525–3528. 9. Xie, J.; Ménand, M.; Maisonneuve, S.; Métivier, R. J. Org. Chem. 2007, 72, 5980– 5985. 10. Ou, S.; Lin, Z.; Duan, C.; Zhang, H.; Bai, Z. Chem. Commun. (Camb) 2006, 42, 4392–4394. 11. Moon, S.-Y.; Youn, N. J.; Park, S. M.; Chang, S.-K. J. Org. Chem. 2005, 70, 2394– 2397. 12. Park, S. M.; Kim, M. H.; Choe, J.-I.; No, K. T.; Chang, S.-K. J. Org. Chem. 2007, 72, 3550–3553. 13. Park, S. Y.; Yoon, J. H.; Hong, C. S.; Souane, R.; Kim, J. S.; Matthews, S. E.; Vicens, J. J. Org. Chem. 2008, 73, 8212–8218. 14. Kim, S. K.; Lee, S. H.; Lee, J. Y.; Lee, J. Y.; Bartsch, R. A.; Kim, J. S. J. Am. Chem. Soc. 2004, 126, 16499–16506. 15. Zhou, W.; Li, Y.; Li, Y.; Liu, H.; Wang, S.; Li, C.; Yuan, M.; Liu, X.; Zhu, D. Chem. Asian J. 2006, 1–2, 224–230. 16. Chang, K.-C.; Su, I.-H.; Senthilvelan, A.; Chung, W. S. Org. Lett. 2007, 9, 3363– 3366. 17. Wu, A. T.; Wu, P. J.; Zou, W.; Chir, J. L.; Chang, Y. C.; Tsai, S. Y.; Guo, C. Q.; Chang, W. S.; Hsieh, Y. C. Carbohydr. Res. 2008, 343, 2887–2893. 18. Hung, H. C.; Cheng, C. W.; Ho, I. T.; Chung, W. S. Tetrahedron Lett. 2009, 50, 302– 305. 19. Nakahara, Y.; Kida, T.; Nakatsuji, Y.; Akashi, M. J. Org. Chem. 2004, 69, 4403– 4411. 20. Choi, M.; Kim, M.; Lee, K. D.; Han, K.-N.; Yoon, I.-A.; Chung, H.-J.; Yoon, J. Org. Lett. 2001, 3, 3455–3457. 21. Akkaya, E. U.; Huston, M. E.; Czarnik, A. W. J. Am. Chem. Soc. 1990, 112, 3590– 3593.
Contents lists available at ScienceDirect
Carbohydrate Research journal homepage: www.elsevier.com/locate/carres
Note
A pyrenyl-appended triazole-based ribose as a fluorescent sensor for Hg2+ ion Kuan-Hao Chen, Cheng-Yi Lu, Hsiu-Jung Cheng, Shau-Jiun Chen, Ching-Han Hu, An-Tai Wu ⇑ Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan
a r t i c l e
i n f o
Article history: Received 7 March 2010 Received in revised form 6 September 2010 Accepted 9 September 2010 Available online 17 September 2010 Keywords: Chemosensor Ribose Fluorescence Pyrene
a b s t r a c t For the efficient detection of toxic trace metal ions, two pyrenyl-appended triazole-based D-ribose fluorescent chemosensors 6 and 7 were prepared and their fluoroionophoric properties toward transition metal ions were investigated. Chemosensors 6 and 7 exhibit highly selective recognition toward Hg2+ ion among a series of tested metal ions in CH2Cl2/MeOH solution. The association constants of 6 and 7 are calculated to be 1.73 105 M 1 and 4.44 105 M 1, respectively. Both 6 and 7 formed complexes with the Hg2+ ion at a 1:1 ligand-to-metal ratio with a detection limit of 10–15 lM Hg2+. Computational analysis demonstrated that the Hg2+ ion occupied the coordination center of 6 with N2 and N3 atoms in two triazole groups, thus separating and distorting the two parallel pyrenes away from each other. Ó 2010 Elsevier Ltd. All rights reserved.
The design and synthesis of new chemosensors for the efficient detection of trace metal ions are among the most important research topics in environmental chemistry and biology.1 In particular, mercury ion is a significant environmental pollutant that accumulates in plants, soil, and water. In the marine environment, mercury ion is converted by bacteria into methylmercury, a highly potent neurotoxin. Methylmercury is passed up the food chain and bioaccumulates in humans.2–4 Accordingly, it is imperative to develop analytical methods for the sensitive and selective detection of trace amounts of mercury ion.5,6 Carbohydrate-based chemosensors are chiral entities with hydroxyl groups and oxygen atoms that form quite suitable cation binding sites. Thus, in the design of chemosensors, the incorporation of sugar molecules is a good strategy for capturing cations.7–9 Currently, there are very few sugar-based fluorescent chemosensors for selective detection of mercury ion described in the literature.10 A good chemosensor must also possess a selective and sensitive signaling mechanism. Recently, pyrene has emerged as one of the most effective functional groups for fluorescence signaling.11–15 In this study, we combined these two entities in a novel chemosensor by connection with triazole groups for metal ion screening studies because the triazole groups are lately recognized as potential metal ion binding site.15,16 The designed ribose-based pyrenyl-appended chemosensors 6 and 7 exhibit high selectivity and sensitivity for Hg2+ ion compared to other transition and heavy metal ions. The synthesis of fluorescent sensors is outlined in Scheme 1. The 5-O-Ms-C-riboside 1 can be easily prepared from ribose in five steps.17 The ester groups of 1 were hydrolyzed with NaOH to ⇑ Corresponding author. Tel.: +886 4 7232105 3542; fax: +886 4 7211 190. E-mail address: [email protected] (A.-T. Wu). 0008-6215/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carres.2010.09.010
produce the corresponding acid 2 in 70% yield. We used Steglich esterification to couple compound 2 with two different lengths of polyethylene glycol in the presence of DMAP and DCC to obtain 3 or 4 in 68% and 63% yields, respectively. Under microwave conditions, 3 or 4 was treated with 1-(propagyloxymethyl)pyrene 5 and PPh3CuBr as the catalyst in toluene at reflux to obtain compound 6 or 7 in 63% and 57% yields, respectively. We first studied the optical properties of both 6 and 7 in CH2Cl2/ MeOH solution. On excitation at 343 nm, the maximum absorption wavelength of the pyrene in 6 and 7 displayed a strong excimer emission at 478 nm and a weak monomer emission at 395 nm (Fig. 1a and b), respectively. The relative ratio of excimer to monomer (Iexcimer/Imonomer) bands for 6 and 7 is about 5–5.5 indicating that 6 and 7 have similar conformational rigidity. The chemosensing behavior of 6 (10 lM) and 7 (15 lM) was investigated by comparing their fluorescence intensities before and after addition of 10 equiv of the following 11 metal ions (as perchlorate salts): Li+, Na+, K+, Ca2+, Mg2+, Hg2+, Co2+, Ni2+, Cu2+, Pb2+, and Zn2+. The results indicated that the monomer and excimer emissions of 6 were strongly quenched only in the presence of the Hg2+ ion (Fig. 1a). This selective quenching by the Hg2+ ion was also observed for compound 7 (Fig. 1b). The quenching efficiency (I I0/I0 100%) for 6 and 7 observed at 478 nm was nearly 100% (Fig. 2); in contrast, the other metal ions caused comparatively small changes in fluorescence intensity. In addition, Cd2+ is an important cation because, like Hg2+ it is also toxic. Therefore, we also performed the selectivity study on Cd2+. The results showed almost no effect on the fluorescence changes of 6 and 7 upon addition of Cd2+ (Fig. S14). Moreover, we tested whether the addition of excess metal ions would change the UV–vis spectra of both 6 and 7. We found that the spectra of
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O
O N3
N3
OCH3
O
O
NaOH O
O
O n = 2, 3 ( )n HO O OH
O
O
dioxane
DCC, DMAP
O
O
O
O O O
O
O O 3n=2 4n=3
N3
2
1
)n
( O
OH
(
O O
O
) n
N3
O O
O
O O
O
OH propargyl bromide
O 3 or 4
O N N
PPh3¡PCuBr
NaH
O
N
M.W
N
O
N N
O
5
6n=2 7n=3 Scheme 1. Synthesis of chemosensors 6 and 7.
(a)
Metal ions Na, Zn, Ca, K, Li, Pd, Mg, Ni, Co
478 nm Free 6
500
Cu Na Zn
400 300 200
377 nm 396 nm
Hg
100 0 400
450
500
550
Li
Hg Pb Mg
Ni
Co
-20 -40
6 7
-60 -80
600
Wavelength (nm)
Fluorescence Intensity (a.u.)
K
-100 350
(b)
Ca
0
Cu
[( I-I0) /I0] x 100 %
Fluorescence Intensity (a.u.)
600
700
Na, Zn, Ca, K, Li, Pd, Mg, Ni, Co
478 nm
Free 7
600
Figure 2. Fluorescence intensity changes ((I I0)/I0 100%) of 6 (10 lM, dark bars) and 7 (15 lM, light bars) in the presence of various metal ions in CH2Cl2/MeOH (80:20, v/v); kex = 344 nm.
500
Cu
400 300 200
377 nm
100
396 nm
Hg
0 350
400
450
500
550
600
Wavelength (nm) Figure 1. Fluorescence spectra of (a) 6 (10 lM) and (b) 7 (15 lM) in the presence of 10 equiv of various metal ions in CH2Cl2/MeOH (80:20, v/v); kex = 344 nm.
both 6 (Fig. 3a) and 7 (Fig. 3b) were altered only in the presence of the Hg2+ ion. The presence of Hg2+ ion caused a small red shift that
occurred at three kmax absorption wavelengths (312, 328, and 344 nm). These red shifts are attributable to the deformation of electrical environments owing to Hg2+ complex formation. Additionally, the chain lengths of the ethylene ethers on 6 and 7 had no influence on the quenching of the excimer and monomer emissions upon the addition of the Hg2+ ion. The declining excimer and monomer emissions that we found in both 6 and 7 were similar to the findings of Kim and coworkers.13 They showed that once Pb2+, Hg2+, and Cu2+ions were added into a pyrenyl-appended triazole-based calix[4]arene as a fluorescent sensor, a quenched fluorescence in both monomer and excimer emissions was observed. Recently, Chung and coworkers18 described a bis-triazoles-chained pyrenes fluorescent chemosensor that had dual-mode recognition for Pb2+ and Cd2+ ions. In comparison, our results showed both 6 and 7 were unimode Hg2+-selective fluorescent sensors. The differences could be attributed to the addition of the ribose-based moiety to the bis-triazoles-chained pyrenes.
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K.-H. Chen et al. / Carbohydrate Research 345 (2010) 2557–2561
344 nm
Free 6, Na, Zn, Ca, K, Li, Pd, Mg, Ni, Co, Cu
0
(a)
-100
481 nm
1.0
346 nm
Absorbance
328 nm
0.5
0.0 300
Fluorescence Intensity (a.u.)
600
Hg 312 nm
320
340
360
380
0
-300 -400
500
-500 -600
400
0
5
10
15
20
2+
[ Hg ] / [ 6 ]
30 e.q 300 200 100 0 350
Wavelength (nm)
-200
I - I0
(a) 1.5
400
450
500
550
600
% (?X)
(b) 1.0
344 nm
Free 7, Na, Zn, Ca, K, Li, Pd, Mg, Ni, Co, Cu
0 -100
346 nm
600
0
481 nm
328 nm
0.4
Hg 312 nm
0.2
0.0 300
320
-200 -300 -400
0.6
340
360
380
Wavelength (nm) Figure 3. UV–vis spectra of (a) 6 (10 lM) and (b) 7 (15 lM) in the presence of 10 equiv of various metal ions in CH2Cl2/MeOH (80:20, v/v); kex = 344 nm.
Fluorescence Intensity (a.u.)
Absorbance
0.8
I - I0
(b)
500
-500 -600
400
0
5
10
15
20
2+
[ Hg ] / [ 7 ]
10 eq 300 200 100 0 350
400
450
500
550
600
Wavelength (nm) Figure 4. Fluorescence spectra of (a) 6 (10 lM) and (b) 7 (15 lM) in CH2Cl2/MeOH (80:20, v/v) upon addition of increasing concentrations of Hg(ClO4)2; kex = 344 nm.
Previous reports13,14 showed that host molecules with plural pyrenyl groups exhibit intramolecular excimer emission by two different mechanisms. One results19 from p–p stacking of the pyrene rings in the free state, which results in a characteristic decrease of excimer emission intensity and a concomitant increase of monomer emission upon metal ion complexation. The other mechanism is ascribed to the interaction of an excited pyrene (Py*) unit with a ground state pyrene (Py) unit.14 The remarkable excimer emission in this study is probably attributed to an intramolecular interaction between Py and Py* because the chemosensor properties of 6 and 7 do not conform to the former mechanism upon addition of the Hg2+ ion. However, the monomer fluorescence quenching cannot be easily explained because it may be difficult to distinguish between the reverse PET mechanism and intra-complex quenching since Hg2+ ion is known as effective quenching metals (e.g., open-shell, paramagnetic, large or easily reducible cation).20,21 Taken together the data above indicate that both monomer and excimer fluorescences diminished dramatically upon addition of Hg2+ probably because of the quenching induced by electron transfer. To further investigate the chemosensor properties of 6 and 7, we performed fluorescence titrations of 6 (10 lM) and 7 (15 lM) in the presence of different concentrations of Hg2+ ion in CH2Cl2/ MeOH. Figure 4 shows the gradual reductions in fluorescence intensity for 6 and 7 upon addition of increasing concentrations of Hg2+ ion. From the fluorescence titration profiles, the association constant for 6*Hg2+ and 7*Hg2+ in CH2Cl2/MeOH solution was calculated to be 1.73 105 M 1 and 4.43 105 M 1, respectively, by a Stern-Volmer plot (Figs. S15 and S17). From the titration result the detection limits for the Hg2+ ion were determined to be 10 and
15 lM, for 6 and 7, respectively. The detection limits were sufficiently low to detect the submillimolar concentrations of Hg2+ ion found in many chemical and biological systems. In the Job plot (Figs. S16 and S18), a maximum fluorescence change was observed when the molar fraction of the ionophore 6 or 7 versus Hg2+ was 0.5, which indicated that only a 1:1 complex was formed. Selectivity for the Hg2+ ion was further ascertained with competition experiments. We found that the fluorescence intensity of either 6 or 7 in the presence of 10 equiv of Hg2+ ion was almost unaffected by the addition of 10 equiv of competing metal ions (Li+, Na+, K+, Ca2+, Mg2+, Co2+, Ni2+, Cu2+, Pb2+, and Zn2+) (Figs. S19 and S20). To better understand the coordination behavior of 6 with the Hg2+ ion, 1H NMR experiments were carried out in CDCl3 solution. The spectral differences are depicted in Figure 5. The complexation of 6 with Hg2+ is expected to reduce the electron density of the coordination sites and induce a downfield shift of the nearby proton signals; however, upfield shift was observed for most of the protons. In the presence of 1.0 equiv of the Hg2+ ion, the protons of Ha-Hc were upfield shifted by 0.18, 0.25, and 0.10 ppm, respectively. Two methylene protons in the triazole-CH2–O (Hg) and O–CH2-pyrene (Hh) were upfield shifted by 0.22 ppm and 0.18 ppm, respectively. In addition, the protons in ribose-CH2triazole (He) and ribose ring (Hd) were upfield shifted by 0.18–0.25 ppm and it formed a sharper peak. The protons in the isopropyl groups of the ribose were also upfield shifted by 0.16 ppm while the protons in pyrene were upfield shifted by 0.15–0.2 ppm. Based on the results of metal ion-induced chemical
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Figure 5. 1H NMR spectra of 6 (5 mM) in the (a) absence and (b) presence of 1.0 equiv of Hg(ClO4)2 in CDCl3.
shift changes in the 1H NMR spectrum, we proposed that the upfield shifts may be due to the perchlorate counter anion that could not be well separated from Hg2+ in the organic solvent. To investigate further the Hg2+ ion accurate coordinating mode of the 6–Hg2+ complex, computations for 6–Hg2+ were conducted at the B3LYP/6-31G(d) level using the GAUSSIAN 03 suite of programs. On the basis of these calculations, we found three stationary points, which have been verified as genuine minima via vibrational frequency analyses. The optimized structure for the most stable isomer of 6–Hg2+ is illustrated in Figure 6. It is clear that the Hg2+ ion occupied the coordination center of 6 with N2 and N3 atoms (i.e., the average bond length of Hg–N2 and Hg–N3 was estimated at 4.09 and 4.17 Å, respectively) in two triazole groups, thus separating and distorting two pyrenes each other.
In summary, two novel ribosyl-based-Hg2+-selective fluorescent sensors 6 and 7 were designed and synthesized by coupling ethylene ether, ribosyl, and pyrenetriazolemethyl moieties. The chain lengths of the ethylene ether groups did not affect their selectivity for the Hg2+ ion. Both 6 and 7 show high selectivity for Hg 2+ ion over other metal ions. The detection limits of both 6 (10 lM) and 7 (15 lM) are sufficiently low to detect the trace amounts of Hg2+ ion found in many chemical and biological systems. Computational calculations demonstrated that a triazolecoordinated Hg2+ ion center was formed in 6–Hg2+, thus quenching both monomer and excimer of 6. Although there is no concrete evidence to support the breakage of the excimer complex, the most characteristic in this study is that both monomer and excimer fluorescences diminished dramatically upon addition of Hg2+ ion probably because of the quenching induced by electron transfers.
K.-H. Chen et al. / Carbohydrate Research 345 (2010) 2557–2561
Figure 6. Conformation of 6–Hg2+ optimized by density functional theory.
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