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A new selective fluorescent probe for lead ions
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Chinese Chemical Letters 20 (2009) 326–329 www.elsevier.com/locate/cclet
A new selective fluorescent probe for lead ions Mao Zhong Tian *, Feng Feng, Shuang Ming Meng, Yue Hua Yuan School of Chemistry and Chemical Engineering, Shanxi Datong University, Datong 037009, China Received 16 June 2008
Abstract A new fluorescent probe (BPb1) for Pb2+ has been synthesized, where diethanolamine (receptor) is linked with 4,4-difluoro-4bora-3a, 4a-diaza-s-indacene (BODIPY) (fluorophore) via a methylene group (spacer). The absorption (496 nm) and emission (505 nm) wavelengths are in visible range. The fluorescence quantum yields of the lead-free and lead-bound states of BPb1 in acetonitrile are 0.013 and 0.693, respectively. The large chelation enhanced fluorescence effect (CHEF) with Pb2+ can be explained by the blocking of the photoinduced electron transfer (PET) process. # 2008 Mao Zhong Tian. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Probe; Pb2+; Selectivity; Photo-induced electron transfer; Fluorescence
At present, the development of fluorescence probes for sensing of metal ions is an important goal in chemistry and biology due to their relatively high sensitivity, selectivity and short response time, local observation, etc [1]. Even though fluorescent probes for various metal ions have been developed over the last decades, there have been relatively few reports on Pb2+ selective fluorescent probes [2–20]. BODIPY dyes have excellent photophysics and photochemistry properties [21], so a few BODIPY-based fluorescence probes have been reported recently [22–27]. Herein, we reported a new BODIPY incorporated Pb2+ fluorescence probe based on photoinduced electron transfer mechanism. Synthesis of probe BPb1 is shown in Scheme 1.
1. Experimental All reactions were carried out under a nitrogen atmosphere with dry, freshly distilled solvents under anhydrous conditions. All the materials were obtained from commercial suppliers and were used without further purification. 1 H NMR and 13C NMR spectra were obtained on a Varian INVOA 400 MHz spectrometer. High resolution mass spectra were obtained using Q-TOF mass spectrometry (Micromass, England). UV absorption spectra were obtained on a Perkin Elmer Lambda 35 spectrophotometer. Fluorescence emission spectra were recorded with a PTI-700 fluorimeter. Melting point was determined by an X-6 micro-melting point apparatus and was uncorrected. Chloroacetyl chloride (1.94 mmol) and 1,4-dimethylpyrrole (3.9 mmol) were dissolved in 150 mL of absolute CH2Cl2 under a nitrogen atmosphere, the absolute CH2Cl2 was bubbled with N2 for 0.5 h prior to use. Then the mixture * Corresponding author. E-mail address: [email protected] (M.Z. Tian). 1001-8417/$ – see front matter # 2008 Mao Zhong Tian. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2008.11.013
M.Z. Tian et al. / Chinese Chemical Letters 20 (2009) 326–329
327
Scheme 1. Synthesis of probe BPb1.
was stirred for 5 h at room temperature. The black solution was added 4 mL of triethylamine, followed by 8 mL of BF3OEt2. The mixture was stirred under a N2 atmosphere for another 4 h at room temperature. Finally, the reaction mixture was filtered, and evaporated to dryness. The crude compound was purified by flash column chromatography on silica gel (16:1 hexanes–EtOAc) to afford a red solid 1 in ca. 43% yield. A suspension of compound 1 (0.34 mmol), 2-(2-hydroxyethylamino)-ethanol (0.34 mmol), potassium iodide (0.10 mmol) and potassium carbonate (0.34 mmol) in 30 mL THF was stirred for 8 h at room temperature under N2 atmosphere. After filtration, THF was removed by evaporation. The crude compound was purified by flash column chromatography (200:1 CH2Cl2–MeOH) to afford a yellow solid compound BPb1 in ca. 53% yield. mp: 142–143 8C; 1H NMR (400 MHz, CDCl3, d ppm): 6.16 (s, 1H, CH), 6.15 (s, 1H, CH), 3.73 (t, 4H, J = 4.6 Hz, OCH2), 2.96 (t, 4H, J = 4.6 Hz, CH2N), 2.80 (s, 2H, NCH2C), 2.63 (s, 3H, CH3), 2.53 (s, 3H, CH3), 2.45 (s, 3H, CH3), 2.44 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3, d ppm): 149.14, 126.37, 125.99, 125.03, 122.24, 120.92, 117.99, 60.2, 45.98, 31.13, 21.41, 14.73, 13.65. HRMS: calcd. for C18H26BF2N3O2 [M + H]+, 366.2164; found, 366.2152. 2. Results and discussion The absorption maximum wavelength of BPb1 was 496 nm. The maintaining of the maximum absorption wavelength upon the binding of Pb2+ indicates a PET mechanism [1]. The fuorescence spectra of BPb1 were obtained by excitation at 496 nm (Fig. 1.). BPb1 showed a large CHEF effect with Pb2+. The fluorescence quantum yield was improved from wF = 0.013 to wF = 0.693 when the Pb2+ was added [28]. During the complexation, the position of the fluorescence maximum emission wavelength did not change, which indicating that the large CHEF effects with Pb2+ can be explained by the blocking of the PET process. PET fluorescent probes are of great interest and promise because of their various applications. The electron transfer between the fluorophore (as signaling unit) and the receptor results in ‘‘switching off’’ of the fluorescence intensity. The presence of guests (metal ions or proton) capable of binding with the lone pair electrons of the receptor causes the PET
Fig. 1. Emission spectra of 1 mmol/L BPb1 in the presence of 0–300 eq. of Pb2+. These spectra were measured in acetonitrile (excitation at 496 nm). The slit width was 4 nm for both excitation and emission.
328
M.Z. Tian et al. / Chinese Chemical Letters 20 (2009) 326–329
Fig. 2. The fluorescence response of BPb1 to various cations and its selectivity for Pb2+. The colorless bars represent the integrated emission of BPb1 (1 mmol/L) in the presence of 50 equiv. of the cations of interest; the dark gray bars represent the changes in integrated emission that occur upon subsequent addition of 50 equiv of Pb2+ to solutions containing BPb1 (1 mmol/L) and 50 equiv. of the cations of interest. The responses were normalized with respect to the integrated emission intensity of free dye (F0); excitation was provided at 496 nm and emission was integrated from 500 to 650 nm.
interaction to be cut off and the fluorescence of the system is ‘‘switched on’’. In the present case, the receptor (nitrogen and oxygen atoms in BPb1), efficiently participates in complexation with Pb2+, which suppresses the process of PET from the amine nitrogen to the fluorophore and prevents PET quenching. The fluorescence response fits to a Hill coefficient of BPb1 suggested that the stoichiometry for the BPb1–Pb2+ complex was 1:1. The apparent dissociation constant of 1:1 BPb1–Pb2+ complex can be got according to the following equation [29]: F F min ½M ¼ F max F min ½M þ K d F min and F max are the integrated emission intensities in the absence of and with an excess of Pb2+, respectively. Kd is the apparent dissociation constant. [M] is the concentration of Pb2+. By plotting the normalized integrated emission intensity changes with p[Pb2+], a sigmoidal curve is obtained and the apparent dissociation constant, Kd, is deduced to be 68.8 mmol/L. As for BPb1, the concentration range that is appropriate for optimal measurements is such that 6.88 mmol/L < [Pb2+] < 688 mmol/L [30]. The fluorescence signaling ability of BPb1 has been examined for several alkali, alkaline earth, transition, and heavy metal ions (Fig. 2). The changes in the fluorescence emission observed upon metal cation addition depend on the nature of the added metal cations. The selectivity of BPb1 for Pb2+ over Zn2+, Cd2+, and Hg2+ is particularly important because Zn2+, Cd2+, and Hg2+ are metal ions that frequently interfere with Pb2+ analysis. The highest effect has been observed in the presence of Pb2+ ions. There were relatively small fluorescence enhancement effects with Cr3+, Cd2+, Ag+, and Hg2+. The competition-based fluorescence effect profiles for these metal ions are also shown in Fig. 2 (dark gray bars). The measurements were done with 1 mmol/L of ligand. No significant interference was observed upon complexation with competing cations under these conditions except for Fe3+ and Co2+. At the same time, we also investigated the effect of some common anions (Cl, NO3, AcO, H2PO4, PO43 and SO42) on the fluorescence spectra of BPb1 in the absence and the presence of Pb2+. The results suggest that these coexistent anions did not interfere obviously the detection of Pb2+ either. 3. Conclusions In summary, we have developed a new simple and selective fluorescent probe BPb1 for Pb2+, which displayed a fluorescence enhancement response toward Pb2+ via 1:1 binding mode. BPb1 showed good selectivity for Pb2+ over the other metal ions and anions examined. Variations in the ligands on the BODIPY may provide various fluorescent probes, which are selective for different metal ions.
M.Z. Tian et al. / Chinese Chemical Letters 20 (2009) 326–329
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Acknowledgment Financial support by Shanxi Scholarship Council of China (No. 200310) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515. S. Deo, H.A. Godwin, J. Am. Chem. Soc. 122 (2000) 174. P. Chen, B. Greenberg, S. Taghavi, C. Romano, D.v.d. Lelie, C. He, Angew. Chem., Int. Ed. 44 (2005) 2715. J. Li, Y. Lu, J. Am. Chem. Soc. 122 (2000) 10466. J. Liu, Y. Lu, J. Am. Chem. Soc. 125 (2003) 6642. J. Liu, Y. Lu, J. Am. Chem. Soc. 126 (2004) 12298. I. Chang, J.J. Tulock, J. Liu, W. Kim, D.M. Cannon, J.Y. Lu, P.W. Bohn, J.V. Sweedler, D.M. Cropek, Environ. Sci. Technol. 39 (2005) 3756. I. Kim, A. Dunkhorst, J. Gilbert, U.H.F. Bunz, Macromolecules 38 (2005) 4560. M.Y. Chae, J. Yoon, A.W. Czarnik, J. Mol. Recognit. 9 (1996) 297. W.S. Xia, R.H. Schmehl, C.J. Li, J.T. Mague, C.P. Luo, D.M. Guldi, J. Phys. Chem. B 106 (2002) 833. C.T. Chen, W.P. Huang, J. Am. Chem. Soc. 124 (2002) 6246. R. Metivier, I. Leray, B. Valeur, Chem. Commun. (2003) 996. X. Qi, E.J. Jun, L. Xu, S.J. Kim, J.S.J. Hong, Y.J. Yoon, J. Yoon, J. Org. Chem. 71 (2006 2881). T. Hayashita, D. Qing, M. Minagawa, J.C. Lee, C.H. Ku, N. Teramae, Chem. Commun. (2003) 2160. M. Sun, D. Shangguan, H. Ma, L. Nie, X. Li, S. Xiong, G. Liu, Biopolymers 72 (2003) 413. S.K. Kim, S.H. Lee, J.Y. Lee, R.A. Bartsch, J.S. Kim, J. Am. Chem. Soc. 126 (2004) 16499. J.Y. Kwon, Y.J. Jang, Y.J. Lee, K.M. Kim, M.S. Seo, W. Nam, J. Yoon, J. Am. Chem. Soc. 127 (2005) 10107. Y. Zhang, W.C. Xiang, R.H. Yang, F. Liu, K.A. Li., J. Photochem. Photobiol. A 173 (2005) 264. K. Kavallieratos, J.M. Rosenberg, W.Z. Chen, T. Ren, J. Am. Chem. Soc. 127 (2005) 6514. Q. He, E.W. Miller, A.P. Wong, C.J. Chang, J. Am. Chem. Soc. (2006) 9316. R.P. Haugland, Molecular Probes, Inc., Eugene, Orgon, 2002. M. Baruah, W. Qin, N. Basaric´, W.M. De Borggraeve, N. Boens, J. Org. Chem. 70 (2005) 4152. X. Peng, J. Du, J. Fan, J. Wang, Y. Wu, J. Zhao, S. Sun, T. Xu, J. Am. Chem. Soc. 129 (2007) 1500. X. Qi, E.J. Jun, L. Xu, S.J. Kim, J.S.J. Hong, Y.J. Yoon, J. Yoon, J. Org. Chem. 71 (2006) 2881. Y. Wu, X. Peng, B. Guo, J. Fan, Z. Zhang, J. Wang, A. Cui, Y. Gao, Org. Biomol. Chem. 3 (2005) 1387. K. Huang, W. Xie, H. Wang, H. Zhang, Talanta 73 (2007) 62. K. Huang, C. Xu, W. Xie, H. Zhang, H. Wang, Spectrochim. Acta Part A 69 (2008) 437. T. Hirano, K. Kikuchi, Y. Urano, T. Nagano, J. Am. Chem. Soc. 124 (2002) 6555. B. Valeur, Wiley-VCH, Weinheim, 2002, p. 287. J.R. Lakowicz. Kluwer Academic/Plenum, vol. 4, 1994, p. 24.
Chinese Chemical Letters 20 (2009) 326–329 www.elsevier.com/locate/cclet
A new selective fluorescent probe for lead ions Mao Zhong Tian *, Feng Feng, Shuang Ming Meng, Yue Hua Yuan School of Chemistry and Chemical Engineering, Shanxi Datong University, Datong 037009, China Received 16 June 2008
Abstract A new fluorescent probe (BPb1) for Pb2+ has been synthesized, where diethanolamine (receptor) is linked with 4,4-difluoro-4bora-3a, 4a-diaza-s-indacene (BODIPY) (fluorophore) via a methylene group (spacer). The absorption (496 nm) and emission (505 nm) wavelengths are in visible range. The fluorescence quantum yields of the lead-free and lead-bound states of BPb1 in acetonitrile are 0.013 and 0.693, respectively. The large chelation enhanced fluorescence effect (CHEF) with Pb2+ can be explained by the blocking of the photoinduced electron transfer (PET) process. # 2008 Mao Zhong Tian. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Probe; Pb2+; Selectivity; Photo-induced electron transfer; Fluorescence
At present, the development of fluorescence probes for sensing of metal ions is an important goal in chemistry and biology due to their relatively high sensitivity, selectivity and short response time, local observation, etc [1]. Even though fluorescent probes for various metal ions have been developed over the last decades, there have been relatively few reports on Pb2+ selective fluorescent probes [2–20]. BODIPY dyes have excellent photophysics and photochemistry properties [21], so a few BODIPY-based fluorescence probes have been reported recently [22–27]. Herein, we reported a new BODIPY incorporated Pb2+ fluorescence probe based on photoinduced electron transfer mechanism. Synthesis of probe BPb1 is shown in Scheme 1.
1. Experimental All reactions were carried out under a nitrogen atmosphere with dry, freshly distilled solvents under anhydrous conditions. All the materials were obtained from commercial suppliers and were used without further purification. 1 H NMR and 13C NMR spectra were obtained on a Varian INVOA 400 MHz spectrometer. High resolution mass spectra were obtained using Q-TOF mass spectrometry (Micromass, England). UV absorption spectra were obtained on a Perkin Elmer Lambda 35 spectrophotometer. Fluorescence emission spectra were recorded with a PTI-700 fluorimeter. Melting point was determined by an X-6 micro-melting point apparatus and was uncorrected. Chloroacetyl chloride (1.94 mmol) and 1,4-dimethylpyrrole (3.9 mmol) were dissolved in 150 mL of absolute CH2Cl2 under a nitrogen atmosphere, the absolute CH2Cl2 was bubbled with N2 for 0.5 h prior to use. Then the mixture * Corresponding author. E-mail address: [email protected] (M.Z. Tian). 1001-8417/$ – see front matter # 2008 Mao Zhong Tian. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2008.11.013
M.Z. Tian et al. / Chinese Chemical Letters 20 (2009) 326–329
327
Scheme 1. Synthesis of probe BPb1.
was stirred for 5 h at room temperature. The black solution was added 4 mL of triethylamine, followed by 8 mL of BF3OEt2. The mixture was stirred under a N2 atmosphere for another 4 h at room temperature. Finally, the reaction mixture was filtered, and evaporated to dryness. The crude compound was purified by flash column chromatography on silica gel (16:1 hexanes–EtOAc) to afford a red solid 1 in ca. 43% yield. A suspension of compound 1 (0.34 mmol), 2-(2-hydroxyethylamino)-ethanol (0.34 mmol), potassium iodide (0.10 mmol) and potassium carbonate (0.34 mmol) in 30 mL THF was stirred for 8 h at room temperature under N2 atmosphere. After filtration, THF was removed by evaporation. The crude compound was purified by flash column chromatography (200:1 CH2Cl2–MeOH) to afford a yellow solid compound BPb1 in ca. 53% yield. mp: 142–143 8C; 1H NMR (400 MHz, CDCl3, d ppm): 6.16 (s, 1H, CH), 6.15 (s, 1H, CH), 3.73 (t, 4H, J = 4.6 Hz, OCH2), 2.96 (t, 4H, J = 4.6 Hz, CH2N), 2.80 (s, 2H, NCH2C), 2.63 (s, 3H, CH3), 2.53 (s, 3H, CH3), 2.45 (s, 3H, CH3), 2.44 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3, d ppm): 149.14, 126.37, 125.99, 125.03, 122.24, 120.92, 117.99, 60.2, 45.98, 31.13, 21.41, 14.73, 13.65. HRMS: calcd. for C18H26BF2N3O2 [M + H]+, 366.2164; found, 366.2152. 2. Results and discussion The absorption maximum wavelength of BPb1 was 496 nm. The maintaining of the maximum absorption wavelength upon the binding of Pb2+ indicates a PET mechanism [1]. The fuorescence spectra of BPb1 were obtained by excitation at 496 nm (Fig. 1.). BPb1 showed a large CHEF effect with Pb2+. The fluorescence quantum yield was improved from wF = 0.013 to wF = 0.693 when the Pb2+ was added [28]. During the complexation, the position of the fluorescence maximum emission wavelength did not change, which indicating that the large CHEF effects with Pb2+ can be explained by the blocking of the PET process. PET fluorescent probes are of great interest and promise because of their various applications. The electron transfer between the fluorophore (as signaling unit) and the receptor results in ‘‘switching off’’ of the fluorescence intensity. The presence of guests (metal ions or proton) capable of binding with the lone pair electrons of the receptor causes the PET
Fig. 1. Emission spectra of 1 mmol/L BPb1 in the presence of 0–300 eq. of Pb2+. These spectra were measured in acetonitrile (excitation at 496 nm). The slit width was 4 nm for both excitation and emission.
328
M.Z. Tian et al. / Chinese Chemical Letters 20 (2009) 326–329
Fig. 2. The fluorescence response of BPb1 to various cations and its selectivity for Pb2+. The colorless bars represent the integrated emission of BPb1 (1 mmol/L) in the presence of 50 equiv. of the cations of interest; the dark gray bars represent the changes in integrated emission that occur upon subsequent addition of 50 equiv of Pb2+ to solutions containing BPb1 (1 mmol/L) and 50 equiv. of the cations of interest. The responses were normalized with respect to the integrated emission intensity of free dye (F0); excitation was provided at 496 nm and emission was integrated from 500 to 650 nm.
interaction to be cut off and the fluorescence of the system is ‘‘switched on’’. In the present case, the receptor (nitrogen and oxygen atoms in BPb1), efficiently participates in complexation with Pb2+, which suppresses the process of PET from the amine nitrogen to the fluorophore and prevents PET quenching. The fluorescence response fits to a Hill coefficient of BPb1 suggested that the stoichiometry for the BPb1–Pb2+ complex was 1:1. The apparent dissociation constant of 1:1 BPb1–Pb2+ complex can be got according to the following equation [29]: F F min ½M ¼ F max F min ½M þ K d F min and F max are the integrated emission intensities in the absence of and with an excess of Pb2+, respectively. Kd is the apparent dissociation constant. [M] is the concentration of Pb2+. By plotting the normalized integrated emission intensity changes with p[Pb2+], a sigmoidal curve is obtained and the apparent dissociation constant, Kd, is deduced to be 68.8 mmol/L. As for BPb1, the concentration range that is appropriate for optimal measurements is such that 6.88 mmol/L < [Pb2+] < 688 mmol/L [30]. The fluorescence signaling ability of BPb1 has been examined for several alkali, alkaline earth, transition, and heavy metal ions (Fig. 2). The changes in the fluorescence emission observed upon metal cation addition depend on the nature of the added metal cations. The selectivity of BPb1 for Pb2+ over Zn2+, Cd2+, and Hg2+ is particularly important because Zn2+, Cd2+, and Hg2+ are metal ions that frequently interfere with Pb2+ analysis. The highest effect has been observed in the presence of Pb2+ ions. There were relatively small fluorescence enhancement effects with Cr3+, Cd2+, Ag+, and Hg2+. The competition-based fluorescence effect profiles for these metal ions are also shown in Fig. 2 (dark gray bars). The measurements were done with 1 mmol/L of ligand. No significant interference was observed upon complexation with competing cations under these conditions except for Fe3+ and Co2+. At the same time, we also investigated the effect of some common anions (Cl, NO3, AcO, H2PO4, PO43 and SO42) on the fluorescence spectra of BPb1 in the absence and the presence of Pb2+. The results suggest that these coexistent anions did not interfere obviously the detection of Pb2+ either. 3. Conclusions In summary, we have developed a new simple and selective fluorescent probe BPb1 for Pb2+, which displayed a fluorescence enhancement response toward Pb2+ via 1:1 binding mode. BPb1 showed good selectivity for Pb2+ over the other metal ions and anions examined. Variations in the ligands on the BODIPY may provide various fluorescent probes, which are selective for different metal ions.
M.Z. Tian et al. / Chinese Chemical Letters 20 (2009) 326–329
329
Acknowledgment Financial support by Shanxi Scholarship Council of China (No. 200310) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515. S. Deo, H.A. Godwin, J. Am. Chem. Soc. 122 (2000) 174. P. Chen, B. Greenberg, S. Taghavi, C. Romano, D.v.d. Lelie, C. He, Angew. Chem., Int. Ed. 44 (2005) 2715. J. Li, Y. Lu, J. Am. Chem. Soc. 122 (2000) 10466. J. Liu, Y. Lu, J. Am. Chem. Soc. 125 (2003) 6642. J. Liu, Y. Lu, J. Am. Chem. Soc. 126 (2004) 12298. I. Chang, J.J. Tulock, J. Liu, W. Kim, D.M. Cannon, J.Y. Lu, P.W. Bohn, J.V. Sweedler, D.M. Cropek, Environ. Sci. Technol. 39 (2005) 3756. I. Kim, A. Dunkhorst, J. Gilbert, U.H.F. Bunz, Macromolecules 38 (2005) 4560. M.Y. Chae, J. Yoon, A.W. Czarnik, J. Mol. Recognit. 9 (1996) 297. W.S. Xia, R.H. Schmehl, C.J. Li, J.T. Mague, C.P. Luo, D.M. Guldi, J. Phys. Chem. B 106 (2002) 833. C.T. Chen, W.P. Huang, J. Am. Chem. Soc. 124 (2002) 6246. R. Metivier, I. Leray, B. Valeur, Chem. Commun. (2003) 996. X. Qi, E.J. Jun, L. Xu, S.J. Kim, J.S.J. Hong, Y.J. Yoon, J. Yoon, J. Org. Chem. 71 (2006 2881). T. Hayashita, D. Qing, M. Minagawa, J.C. Lee, C.H. Ku, N. Teramae, Chem. Commun. (2003) 2160. M. Sun, D. Shangguan, H. Ma, L. Nie, X. Li, S. Xiong, G. Liu, Biopolymers 72 (2003) 413. S.K. Kim, S.H. Lee, J.Y. Lee, R.A. Bartsch, J.S. Kim, J. Am. Chem. Soc. 126 (2004) 16499. J.Y. Kwon, Y.J. Jang, Y.J. Lee, K.M. Kim, M.S. Seo, W. Nam, J. Yoon, J. Am. Chem. Soc. 127 (2005) 10107. Y. Zhang, W.C. Xiang, R.H. Yang, F. Liu, K.A. Li., J. Photochem. Photobiol. A 173 (2005) 264. K. Kavallieratos, J.M. Rosenberg, W.Z. Chen, T. Ren, J. Am. Chem. Soc. 127 (2005) 6514. Q. He, E.W. Miller, A.P. Wong, C.J. Chang, J. Am. Chem. Soc. (2006) 9316. R.P. Haugland, Molecular Probes, Inc., Eugene, Orgon, 2002. M. Baruah, W. Qin, N. Basaric´, W.M. De Borggraeve, N. Boens, J. Org. Chem. 70 (2005) 4152. X. Peng, J. Du, J. Fan, J. Wang, Y. Wu, J. Zhao, S. Sun, T. Xu, J. Am. Chem. Soc. 129 (2007) 1500. X. Qi, E.J. Jun, L. Xu, S.J. Kim, J.S.J. Hong, Y.J. Yoon, J. Yoon, J. Org. Chem. 71 (2006) 2881. Y. Wu, X. Peng, B. Guo, J. Fan, Z. Zhang, J. Wang, A. Cui, Y. Gao, Org. Biomol. Chem. 3 (2005) 1387. K. Huang, W. Xie, H. Wang, H. Zhang, Talanta 73 (2007) 62. K. Huang, C. Xu, W. Xie, H. Zhang, H. Wang, Spectrochim. Acta Part A 69 (2008) 437. T. Hirano, K. Kikuchi, Y. Urano, T. Nagano, J. Am. Chem. Soc. 124 (2002) 6555. B. Valeur, Wiley-VCH, Weinheim, 2002, p. 287. J.R. Lakowicz. Kluwer Academic/Plenum, vol. 4, 1994, p. 24.