A new colorimetric chemodosimeter for mercury ion via specific thioacetal deprotection in aqueous solution and living cells

Tetrahedron Letters 53 (2012) 7031–7035

Contents lists available at SciVerse ScienceDirect

Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet

A new colorimetric chemodosimeter for mercury ion via specific thioacetal deprotection in aqueous solution and living cells Ajit Kumar Mahapatra a,⇑, Rajkishor Maji a, Prithidipa Sahoo a, Prasanta Kumar Nandi a, Subhra Kanti Mukhopadhyay b, Avishek Banik b a b

Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711103, India Department of Microbiology, The University of Burdwan, Burdwan, West Bengal, India

a r t i c l e

i n f o

Article history: Received 22 May 2012 Revised 4 October 2012 Accepted 6 October 2012 Available online 13 October 2012 Keywords: Chemodosimeter Near-infrared Mercury Dithiane Colorimetric Living cell

a b s t r a c t A sensitive and selective new chemodosimeteric chemosensor (DIBT) for mercury ions was successfully devised and characterized. Upon addition of Hg2+ to DIBT, a red-edge absorption band at 972 nm was observed. An important feature for the new chemodosimeter is its high selectivity toward mercury ions over the other competitive species due to Hg2+-triggered specific C–S cleavage, making the ‘naked-eye’ detection of mercury ions possible in aqueous solution and living cells such as Candida albicans. Ó 2012 Elsevier Ltd. All rights reserved.

The design and construction of artificial receptors and the development of new methods for the selective detection and visualization of Hg2+ ions in chemical and biological systems are critically important, considering the ion’s toxic impact on our environment.1 Hg2+ is one of the most hazardous components in the environment.2 The Environmental Protection Agency (EPA) standard for the maximum allowable level of inorganic Hg2+ in drinking water is 2 ppb.3 There are many kinds of chemical and physical sensors developed for the detection of Hg2+, among which the fluorescence based sensors represent a simple, but sensitive technique providing detection limits as low as ppb.4 However, most of the reported Hg2+ sensors have shortcomings, particularly in terms of sensitivity, selectivity, interference from other metal ions, the turn-off response, and low solubility in aqueous solution.5 So far, the development of practical fluorescent chemosensors for many heavy transition metal (HTM) ions is still a challenge. There are currently two strategies to design ‘turn-on’ sensors for Hg2+ are (i) Hg2+ chelation enhanced fluorescence (CHEF) due to the blockage of photoinduced electron transfer (PET) and (ii) Hg2+-triggered reaction to form fluorescent compound.6 Since the quenching nature of Hg2+ makes the CHEF mechanism not always work, the latter for chemodosimeters is drawing also much attention. 7 Compared to fluorometric sensors, colorimetric sensors have ⇑ Corresponding author. Tel.: +91 33 2668 4561; fax: +91 33 2668 4564. E-mail address: [email protected] (A.K. Mahapatra). 0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2012.10.026

attracted much attention for allowing the so-called ‘naked-eye’ detection in a straightforward and inexpensive manner, offering qualitative and quantitative information without using expensive equipment. Colorimetric chemosensors of cations can be generated based on the right combination of receptor and chromophore. One such combination would be a deprotection of 1,3-dithiane to carbaldehyde induced specifically by Hg2+, which is an efficient well known umpolung reaction in organic synthesis.8 It is known that dithiane can be selectively deprotected to aldehyde by Hg2+ in aqueous solution. Based on this reaction, we designed a simple indole-3dithiane derivative, namely DIBT (Dithianindolylbenzothiazole), by generating benzothiazole as the middle unit at the C-2 position of an indole conjugated system. We anticipate that the deprotection of the dithiane unit by Hg2+ would induce an intramolecular charge transfer (ICT) from the end indole ring nitrogen units to the electron-withdrawing ability of recovery carbaldehyde, which could in turn lead to colorimetric and fluorescent changes. Furthermore, dithianindole moieties are amenable to structural modification by introduction of the benzothiazole group at the C-2 position, so that the new absorption band appeared at the red-edge or near the infrared (NIR) spectral range. Chemosensors with near infrared (NIR, 700–1000 nm) optical response are useful probes for in vitro and in vivo biological sensing and imaging of metal ions.9 The reactive probes have great advantages in selectivity and sensitivity over other sensing strategies by utilization of a specific

7032

A. K. Mahapatra et al. / Tetrahedron Letters 53 (2012) 7031–7035

chemical reaction (usually irreversible) between guest molecules and target species.10 These chemical reactions are driven by receptor and analyte (usually a metal ion), leading to making or breaking of covalent bonds rather than the formation of supramolecular complexes that lead to the formation of a fluorescent or colored product displaying a unique spectroscopic change. Indeed, several selective chemodosimeters for Hg2+ ions in water or organic solvents have been exploited on the basis of irreversible mercury-promoted desulfurization reactions, including hydrolysis, cyclization, and elimination reactions that stem from the strong thiophilic affinity of Hg2+ ions.11 To the best of our knowledge, this represents the first example of a sensor (except cyanine based) with a significant red-edge absorption band in the NIR region above 900 nm in the presence of the Hg2+-induced chemodosimeter reaction. With this in mind, herein we report a new fluorescent ‘turn-on’ Hg2+ chemodosimeter DIBT, which can act as an efficient colorimetric sensor that can selectively detect Hg2+ by visible color changes with a characteristic NIR absorbance band at 972 nm in aqueous medium. The synthesis of DIBT was readily accomplished by implementing Scheme 1. Benzothiazole derivative 1 was prepared by the reaction of indole-2-carboxylic acid with 2-aminothiophenol (PCl3, toluene, 80%).12 Reaction of the benzothiazole derivative 1 with POCl3-DMF afforded 2 (65%). Subsequently, 3 could be prepared from the reaction of 2 with 1,3-ethanedithiol in the presence of Et2O_BF3 in diethyl ether as a pale yellow solid in good yield (88%). These compounds were characterized by 1H and 13C NMR spectroscopy, ESI-MS, and elemental analysis (See Supplementary data Figs. S1–S8). Specifically, we incorporated a 1,3-dithiane unit in 3-position of indole moiety, thus extending the conjugation system with long wavelength through Hg2+-triggered carbaldehyde recovery and that can perform ‘‘naked-eye’’ detection of Hg2+ ion in the NIR region. To obtain an optimum response toward Hg2+, the benzothiazole moiety serves as the chromophore core with better photo stability. The ionophoric properties of DIBT were investigated by UV–vis and fluorescent measurements. UV/Vis absorption was firstly examined with different amounts of Hg2+ ion to test the sensing ability of DIBT in the HEPES buffer (50 mM HEPES, 100 mM KNO3, pH 7.40, containing 20% acetonitrile) solution at room temperature. Free chemodosimeter DIBT displays one major absorption band centered around 346 nm (Fig. 1a), which is responsible for the very light yellow color of the solution. When adding excess Hg2+ (2 mM) to DIBT (5 lM) in the same HEPES buffer, the color of the solution turned from slight yellow

to orange-yellow (Fig. 1a Inset), which was clearly recognizable to the naked eye. It should be noted that the sensing process is very fast, within 60 s. At a lower Hg2+ concentration (<30 lM), the absorption band at 279 nm was gradually enhanced, while the intensity of the main absorption band at 346 nm was decreased correspondingly. When the Hg2+concentration was further increased (>50 lM), the main absorption band at 346 nm progressively decreased in intensity and shifts to 436 nm, while a new NIR band at 972 nm increased continuously in intensity. Such a red-edge absorption band implies that Hg2+ is able to enhance the ICT effect by specific Hg2+ promoted selective dethioacetalization liberating 2, and the presence of Hg2+ becomes observable to the naked eye. This new red-edge absorption band at 972 nm is apparently attributed to ICT, as the generated aldehyde group coordinates with Hg2+ ions and acts as an electron-withdrawing group in the ICT process. The presence of two well defined isosbestic points at k = 312 and 369 nm is consistent with the presence of an equilibrium process corresponding to DIBT and mercury complex DIBT + Hg2+ (Fig. 1a). More importantly, the absorption spectra of 2 and DIBT + Hg2+ are nearly identical, indicating that desulfurization of DIBT by Hg2+ ions affords 2 (Fig. 1b). Upon interaction with various metal ions such as Zn2+, Mn2+, Ni2+, Fe2+, Cu2+, Cd2+, Pb2+ and Ag+, no significant spectral changes were observed upon the addition of 50 equiv of each ions (Fig. 2). The only exception was Ag+, which gave rise to a little absorbance in the NIR region. Judging from the titrations, the stoichiometries of the complexes were ascertained from the break (a maximum at 0.5 mol fraction) of the UV–vis titration curves on the basis of Job’s plot method,13 and it was found to be 1:1 (Fig. S9). After selective dethioacetalization, the generated aldehyde derivative 2, which further chelates a second Hg2+ ion is responsible for intramolecular charge transfer. Note that a fluorometric detection of Hg2+ is also possible for DIBT. In the fluorescence spectra (Fig. 3a), free chemodosimeter DIBT exhibits weak emission when excited at 346 nm and the fluorescence titration of DIBT in the presence of different Hg2+ concentrations was then performed in the same HEPES buffer solution. The introduction of Hg2+ (2.1 equiv) elicited a significant turn-on fluorescence response at 412 nm within 1 minute, while the fluorescence change induced by 50 equiv other transition metal ions were negligible even after one hour of incubation under ambient condition. The fluorescence response of DIBT with various cations and its selectivity for Hg2+ are shown in Figure 3b in the form of bar graphs. Obviously, the release of recovery aldehyde 2 via a Hg2+-induced C–S bond cleavage can induce a turn-on fluorescence response which can be explained in terms of the excited-state ICT

H2N PCl3 / Toluene

+ N H

COOH

Heat

HS

N

N H

1

S

CHO

POCl3 / DMF

S

Propane-1,3-dithiol

Heat

N

N H

2

S

BF3-Et2O

S

N H

3 : DIBT Scheme 1. Synthesis of DIBT.

N S

A. K. Mahapatra et al. / Tetrahedron Letters 53 (2012) 7031–7035

7033

Figure 1. (a) Absorption spectra of DIBT (20.0 lM, 4  10 6 M) in the presence of different concentrations of Hg2+ in the HEPES buffer (50 mM HEPES, 100 mM KNO3, pH 7.40, containing 20% acetonitrile). Inset: color changes of DIBT upon additions of Hg2+ (2 mM). (b) Normalized absorption spectra of DIBT (black), DIBT + Hg2+ (red), 2(green), and 2 + Hg2+ (blue).

Figure 2. Absorption spectra of DIBT (4  10 50 equiv of various cations.

6

M) in the absence and presence of

process from the donor to the acceptor. Furthermore, the fluorescence intensities at 412 nm have a linear relationship within the Hg2+concentrations from 0.01 to 2 mM (Fig. 3a, inset), and the detection limit was evaluated to be 4.0  10 7 M. The sensor DIBT reported here should have high sensitivity and selectivity toward Hg2+, operating through the high thiophilic

affinity between Hg2+and sulfur. As expected, the selectivity of DIBT toward Hg2+ over other relevant metal ions is remarkably high. The photograph in Figure 4 shows the solution color after addition of excess amount of various cations in the HEPES buffer (50 mM HEPES, 100 mM KNO3, pH 7.40, containing 20% acetonitrile) solution at room temperature. The unique red-edge absorption of DIBT was observed only with the addition of Hg2+. DIBT exhibited a distinct color change from light yellow to orangeyellow upon addition of Hg2+. The color persisted even in the presence of various other metal ions, confirming the preferential binding of DIBT to Hg2+. Almost no change in absorption spectra was found with other metal ions such as Ag+, Cd2+, Cu2+, Mn2+, Ni2+, Pb2+, Zn2+, and Fe2+. Consequently, chemodosimeter DIBT possesses high selectivity toward Hg2+ over other competitive metal ions. Thus, DIBT can colorimetrically discriminate Hg2+ from all other metal ions with a characteristic NIR signature in aqueous medium. From the results observed, we proposed a reaction mechanism for Hg2+-promoted desulfurization mechanism. The coordination of Hg(ClO4)2 or HgCl2 with the sulfur atom, due to the thiophilicity of the mercury ion, activates the carbon atom of the thioacetal that is then hydrolyzed by water to afford the corresponding aldehyde unit (Scheme 2). To verify our proposed interaction of Hg2+ ions with DIBT, we performed theoretical calculations of DIBT and compound 2 at the DFT level by using the Gaussian 03 package14 with B3LYP/ 6-31+G⁄⁄. The calculation results reveal that the highest occupied molecular orbital (HOMO) is mainly located on the whole

Figure 3. (a) Fluorescence spectra of DIBT (4.0  10 7 M) in the presence of different concentrations of Hg2+ when excited at 346 nm. Inset: fluorescence at 412 nm as a function of [Hg2+]. (b) Bar diagram showing fluorescence intensity of DIBT (4  10 7 M) at 412 nm in the absence and presence of 50 equiv of various cations in the HEPES buffer (50 mM HEPES, 100 mM KNO3, pH 7.40, containing 20% acetonitrile).

7034

A. K. Mahapatra et al. / Tetrahedron Letters 53 (2012) 7031–7035

DIBT Ag +

Cd +2

Cu +2

Mn +2

Hg+2

Ni+2

Pb+2

Zn +2

Fe +2

Figure 4. Colorimetric photographs of DIBT upon the addition of 50 equiv of various cations.

H

H2O S

O S

N H DIBT : Hg 2+

N H

Hg2+

N

Hg2+

N S

2 : Hg 2+

S

Scheme 2. Reaction of DIBT with Hg2+ releases 2.

p-conjugated indole framework and dithiane sulfurs while the lowest unoccupied molecular orbitals (LUMO) of DIBT and 2 are mainly distributed over the benzothiazole moieties including thiazole sulfur and the intervening bond between them (Fig. 5). Hence, the interaction of the nitrogen atom with Hg2+ can change the orbital energy level, realizing the optical detection. The UV–vis and fluorescence spectra of DIBT system upon interaction with excess equiv of Hg2+ are attributable to the releasing of 2 (Scheme 2). To gain insights into the sensing mechanism, we decided to study the sensing process by 1H NMR and mass spectra in the presence of Hg2+ ions. Upon gradual addition of Hg2+ ion solution into DIBT in a 1:1 CDCl3:D2O, the 1,3-dithiane methane proton at d 6.33 ppm disappeared with a concomitant appearance of a new peak at d 10.82 ppm, assignable to the corresponding aldehyde proton of DIBT (Fig. 6). The product of DIBT + Hg2+ was isolated by a silica gel column and was then subjected again to 1 H NMR analysis. The 1H NMR and mass spectra of the resulting product are essentially identical to that of free 2. In ESI Mass spectra (Fig. S2), DIBT gives a m/z peak of [M+1]+ at 369.00. Likewise, DIBT + Hg2+ displays (Fig. S10) a m/z peak of [M+1]+ at 279.0586 corresponding to that of 2. Thus, the studies of NMR, mass spectrometry, absorption spectrometry, theoretical calculation, and fluorescence spectrometry demonstrated that the

(a)

Figure 6. Partial 1H NMR (500 MHz) spectra of (a) probe DIBT, (b) DIBT and 0.5 equiv of Hg2+, (c) DIBT and 1.0 equiv of Hg2+, and (d) DIBT and 2.0 equiv of Hg2+ (identical with standard compound 2) in 50:50 CDCl3:D2O.

sensor of DIBT is likely functioned by the chemodosimetric mechanism (Scheme 2). The excellent ‘‘turn-on’’ sensing behaviour of DIBT for Hg2+ in aqueous media displays its potential to monitor Hg2+ in living organisms,15 therefore the Candida albicans cells (IMTECH No. 3018) were preloaded with 10 lM of compound DIBT for 30 min at 37 °C, and it showed negligible intracellular fluorescence imaged (Fig. 7a). In contrast, cells treated with mercury and the compound DIBT showed greatly enhanced intracellular fluorescence (Fig. 7b– c). Thus, the results indicate that DIBT is cell membrane permeable and able to response to Hg2+ in the living cells.

(b)

HOMO

DIBT (c)

LUMO

(d)

2

HOMO

LUMO

Figure 5. Calculated structures of (a) DIBT and (b) the HOMO and LUMO distributions of DIBT. Calculated structures of (c) molecule 2, and (d) the HOMO and LUMO distributions of 2 using (B3LYP/6-31+G⁄⁄).

A. K. Mahapatra et al. / Tetrahedron Letters 53 (2012) 7031–7035

(a)

(b) 2.

3. 4.

(c)

(d)

5.

Figure 7. (a) Fluorescence microscope images of Candida albicans cells only, (b) images of cells + DIBT (c) images of cells + DIBT + Hg2+ (5 lM), (d) images of cells + DIBT + Hg2+ (25 lM).

In conclusion, we have developed a sensitive and selective ICTbased new fluorescent chemodosimeter DIBT for Hg2+ in aqueous media, which exhibits several advantages: (i) a red-edge absorption maximum at 972 nm of DIBT was observed upon titration with Hg2+ ions, (ii) exhibiting high Hg2+ selectivity over various competitive cations, ascribing to the strong affinity of Hg2+ toward sulfur, (iii) bringing a color change from very light yellow to orange to the naked eye, realizing a quantitative determination of Hg2+ with a colorimetric method. This assay offers a method for the detection of Hg2+ that is easier and more convenient for application due to suitable sensitivity and desirable response time. The living cell image experiments further demonstrate their value in the practical applications of biological systems. Further evaluations of this method with additional real samples are underway in our laboratory.

6.

7.

8. 9.

Acknowledgments

10.

We thank CSIR [Project No. 01(2460)/11/EMR-II] for the financial support. R.K.M. thanks the DST (WB) for fellowship. We also acknowledge UGC-SAP and MHRD for funding instrumental facilities in our department.

11.

12.

References and notes 1. (a) Guzzi, G.; La Porta, C. A. M. Toxicology 2008, 244, 1–12; (b) Basu, N.; Scheuhammer, A.; Grochowina, N.; Klenavic, K.; Evans, D.; O Brien, M.; Chan, M. Environ. Sci. Technol. 2005, 39, 3585–3591; (c) Desvergne, J.P.; Czarnik, A.W. Chemosensors of Ion and Molecule Recognition, Kluwer: Dordrecht, Springer, 1997; (d) Fluorescent Chemosensors for Ion and Molecule Recognition; Czarnik, A.

13. 14. 15.

7035

W., Ed.; American Chemical Society: Washington, DC, 1992; (e) Amendola, V.; Fabbrizzi, L.; Forti, F.; Licchelli, M.; Mangano, C.; Pallavicini, P.; Poggi, A.; Sacchi, D.; Taglieti, A. Coord. Chem. Rev. 2006, 250, 273–299; (f) Ojida, A.; Nonaka, H.; Miyahara, Y.; Tamaur, S.; Sada, K.; Hamachi, I. Angew. Chem., Int. Ed. 2006, 45, 5518–5521. (a) Clarkson, T. W.; Magos, L.; Myers, G. J. N. Engl. J. Med. 2003, 349, 1731–1739; (b) Carvalho, C. M. L.; Chew, E.-H.; Hashemy, S. I.; Lu, J.; Holmgren, A. J. Biol. Chem. 2008, 283, 11913–11923; (c) Tchounwou, P. B.; Ayensu, W. K.; Ninashvili, N.; Sutton, D. Environ. Toxicol. 2003, 18, 149–175. Mercury Update: Impact of Fish Advisories. EPA Fact Sheet EPA-823-F-01-011; EPA, Office of Water: Washington, DC, 2001. (a) Nolan, E. M.; Lippard, S. J. Chem. Rev. 2008, 108, 3343–3480; (b) Yoon, S.; Albers, A. E.; Wong, A. P.; Chang, C. J. J. Am. Chem. Soc. 2005, 127, 16030–16031; (c) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2006, 128, 14474–14475; (d) Wegner, S. V.; Okesli, A.; Chen, P.; He, C. J. Am. Chem. Soc. 2007, 129, 3474– 3475; Angew. Chem. Int. Ed. 2007, 46, 5549–5553 (e) Guo, Z. Q.; Zhu, W. H.; Shen, L. J.; Tian, H. Angew. Chem. 2007, 119, 5645–5649; Angew. Chem. Int. Ed. 2008, 47, 193–197 (f) Wu, D.; Descalzo, A. B.; Weik, F.; Emmerling, F.; Shen, Z.; You, X. Z.; Rurack, K. Angew. Chem. 2008, 120, 199–203; (g) Liu, L.; Zhang, D. Q.; Zheng, X. P.; Wang, Z.; Zhu, D. B. J. Nanosci. Nanotechnol. 2009, 9, 3975–3981; (h) Mor-Piperberg, G.; Tel-Vered, R.; Elbaz, J.; Willner, I. J. Am. Chem. Soc. 2010, 132, 6878–6879. (a) Qian, F.; Zhang, C.; Zhang, Y.; He, W.; Gao, X.; Hu, P.; Guo, Z. J. Am. Chem. Soc. 2009, 131, 1460–1468; (b) Xu, Z.; Baek, K.-H.; Kim, H. N.; Cui, J.; Qian, X.; Spring, D. R.; Shin, I.; Yoon, J. J. Am. Chem. Soc. 2010, 132, 601–610; (c) Santra, M.; Ryu, D.; Chatterjee, A.; Ko, S. K.; Shin, I.; Ahn, K. H. Chem. Commun. 2009, 2115–2117; (d) Yang, Y. K.; Ko, S. K.; Shin, I.; Tae, J. Org. Biomol. Chem. 2009, 7, 4590–4593; (e) Strasdeit, H. Angew. Chem. 2008, 120, 840–842. Angew. Chem. Int. Ed. 2008, 47, 828–830; (f) Chen, X.; Baek, K. H.; Kim, Y.; Kim, S. J.; Shin, I.; Yoon, J. Tetrahedron 2010, 66, 4016–4021. (a) Yoon, S.; Miller, E. W.; He, Q.; Do, P. H.; Chang, C. J. Angew. Chem., Int. Ed. 2007, 46, 6658; (b) Xie, Z.; Wang, K.; Zhang, C.; Yang, Z.; Chen, Y.; Guo, Z.; Lu, G.; He, W. . New J. Chem. 2011, 35, 607; (c) Lim, C. S.; Kang, D. W.; Tian, Y. S.; Han, J. H.; Hwang, H. L.; Cho, B. R. Chem. Commun. 2010, 46, 2388; (d) Ho, M. L.; Chen, K. Y.; Lee, G. H.; Chen, Y. C.; Wang, C. C.; Lee, J. F.; Chung, W. C.; Chou, P. T. Inorg. Chem. 2009, 48, 10304; (e) Tian, M.; Ihmels, H. Chem. Commun. 2009, 45, 3175; (f) Tian, M.; Ihmels, H.; Benner, K. Chem. Commun. 2010, 46, 5719. (a) Mahapatra, A. K.; Roy, J.; Sahoo, P. Tetrahedron Lett. 2011, 52, 2965–2968; (b) Tang, B.; Ding, B.; Xu, K.; Tong, L. Chem. Eur. J. 2009, 15, 3147; (c) Zhang, X.; Xiao, Y.; Qian, X. Angew. Chem., Int. Ed. 2008, 47, 8025; (d) Choi, M. G.; Kim, Y. H.; Namgoong, J. E.; Chang, S. K. Chem. Commun. 2009, 45, 3560; (e) Wu, J. S.; Hwang, I. C.; Kim, K. S.; Kim, J. S. Org. Lett. 2007, 9, 907; (f) Ros-Lis, J. V.; Marcos, M. D.; Martinez-Manez, R.; Rurack, K.; Soto, J. Angew. Chem., Int. Ed. 2005, 44, 4405; (g) Zhang, G.; Zhang, D.; Yin, S.; Yang, X.; Shuai, Z.; Zhu, D. Chem. Commun. 2005, 41, 2161; (h) Liu, B.; Tian, H. Chem. Commun. 2005, 41, 3156; (i) Jiang, W.; Wang, W. Chem. Commun. 2009, 45, 3913; (j) Lin, W.; Cao, X.; Ding, Y.; Yuan, L.; Long, L. Chem. Commun. 2010, 46, 3529; (k) Ko, S. K.; Yang, Y. K.; Tae, J.; Shin, I. J. Am. Chem. Soc. 2006, 128, 14150. Green, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis; WileyInterscience: New York, 1999. (a) Patonay, G.; Antoine, M. D. Anal. Chem. 1991, 63, 321A–327A; (b) Thompson, R. B. In Topics in Fluorescence Spectroscopy 4; Lakowicz, J. R., Ed., 1st ed.; Plenum: New York, 1994; pp 151–181; (c) Stoyanov, S. Pract. Spectrosc. 2001, 25, 35–93; (d) Frangioni, J. V. Curr. Opin. Chem. Biol. 2003, 7, 626–634; (e) Fabian, J.; Nakazumi, H.; Matsuoka, M. Chem. Rev. 1992, 92, 1197–1226. (a) Dujols, V.; Ford, F.; Czarnik, A. W. J. Am. Chem. Soc. 1997, 119, 7386–7387; (b) Kovacs, J.; Rçdler, T.; Mokhir, A. Angew. Chem. 2006, 118, 7979–7981. Angew. Chem. Int. Ed. 2006, 45, 7815–7817; (c) Mart nez-M_Çez, R.; Sancenn, F. Coord. Chem. Rev. 2006, 250, 3081–3093; (d) Song, F.; Watanabe, S.; Floreancig, P. E.; Koide, K. J. Am. Chem. Soc. 2008, 130, 16460–16461. (a) Mahapatra, A. K.; Roy, J.; Sahoo, P.; Mukhopadhyay, S. K.; Chattopadhyay, A. Org. Biomol. Chem. 2012, 10, 2231–2236; (b) Quan, D. T.; Kim, J. S. Chem. Rev. 2010, 110, 6280–6301. Youk, J.-S.; Kim, Y. H.; Kim, E.-J.; Youn, N. J.; Chang, S.-K. Bull. Korean Chem. Soc. 2004, 25, 869. Connors, K. A. Binding Constants, The Measurement of Molecular Complex Stability; Wiley, John & Sons: New York, 1987. (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648; (b) Dill, J. D.; Pople, J. A. J. Chem. Phys. 1975, 62, 2921. Peng, X.; Du, J.; Fan, J.; Wang, J.; Wu, Y.; Zhao, J.; Sun, S.; Xu, T. J. Am. Chem. Soc. 2007, 129, 1500.