Recent progress on fluorescent chemosensors for metal ions

Inorganica Chimica Acta 381 (2012) 2–14

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Review

Recent progress on fluorescent chemosensors for metal ions Yongsuk Jeong a, Juyoung Yoon a,b,⇑ a b

Department of Bioinspired Science (WCU), Ewha Womans University, Seoul 120-750, Republic of Korea Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea

a r t i c l e

i n f o

Article history: Available online 17 September 2011 Fluorescence Spectroscopy: from Single Chemosensors to Nanoparticles Science – Special Issue Keywords: Fluorescent chemosensors Cu2+ sensor Hg2+ sensor Zn2+ sensor Pb2+ sensor Cd2+ sensor

a b s t r a c t The recognition and sensing of the biologically and environmentally important metal ions has emerged as a significant goal in the field of chemical sensors in recent years. Among the various analytical methods, fluorescence has been a powerful tool due to its simplicity, high detection limit and application to bioimaging. This review highlights the fluorescent chemosensors for metal ions, which have been recently developed from our laboratory. This review was categorized by target metal ions, such as Cu2+, Hg2+, Zn2+, Pb2+, Cd2+, Vanadate, Ag+ and Au3+. Selectivity and sensitivity for these metal ions were achieved by introducing various ligands to core fluorophores, such as, rhodamine, fluorescein, pyrene, anthracene, naphthalimide, coumarin, and BODIPY. Ó 2011 Elsevier B.V. All rights reserved.

Yongsuk Jeong was born in Anyang, Korea in 1985. She received B.S. degree from Department of Chemistry of Ewha Womans University. She is on a Master course in Prof. Juyoung Yoon’s laboratory in Ewha Womans University.

Juyoung Yoon was born in Pusan, Korea in 1964. He received his Ph.D. (1994) from The Ohio State University. After completing postdoctoral research at UCLA and at Scripps Research Institute, he joined the faculty at Silla University in 1998. In 2002, he moved to Ewha Womans University, where he is currently a professor of Department of Chemistry and Nano Science and Department of Bioinspired Science. His research interests include investigations of fluorescent chemosensors, molecular recognition and organo EL materials.

⇑ Corresponding author at: Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea. Tel.: +82 2 3277 2400; fax: +82 2 3277 2384. E-mail address: [email protected] (J. Yoon). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.09.011

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Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Fluorescent chemosensors for metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Chemosensors for Cu2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Chemosensors for Hg2+/CH3Hg+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3. Chemosensors for Zn2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.4. Chemosensors for Pb2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.5. Chemosensors for Cd2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.6. Chemosensor for vanadate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.7. Chemosensors for noble metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Conclusions and future perspectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1. Introduction The recognition and sensing of the biologically and environmentally important metal ions has emerged as a significant goal in the field of chemical sensors in recent years [1,2]. Several methods, such as high performance liquid chromatography, mass spectrometry, and atomic absorption spectroscopy, have been developed to analyze the concerned targets. However, these methods suffer either from extensive, time consuming procedures or the use of sophisticated instrumentation. Fluorescence has been a powerful tool due to its simplicity, high detection limit and application to bioimaging [3]. More specifically, the fluorogenic methods in conjunction with suitable probes are preferable approaches to measure these analytes since fluorimetry is rapidly performed, nondestructive, highly sensitive, suitable for high-throughput screening applications, and most importantly, can afford real information on the localization and quantify of the targets of interest. In this current review, we focus on the fluorescent chemosensors for metal ions, which have been recently developed from our laboratory. This review was categorized by target metal ions, such as Cu2+, Hg2+, Zn2+, Pb2+, Cd2+, Vanadate and precious metal ions (Ag+ and Au3+). Selectivity and sensitivity for these metal ions were achieved by introducing various ligands to core fluorophores, such as, rhodamine, fluorescein, pyrene, anthracene, naphthalimide, coumarin, and BODIPY. 2. Fluorescent chemosensors for metal ions 2.1. Chemosensors for Cu2+ Among various metal ions, copper ion plays a critical role as a catalytic cofactor for a variety of metalloenzymes, including superoxide dismutase, cytochrome c oxidase and tyrosinase. However, under overloading conditions, copper can cause neurodegenerative diseases (e.g., Alzheimer’s and Wilson’s diseases) probably due to its involvement in the production of reactive oxygen species [4,5]. Owing to its biological importance, fluorescent chemosensors, which can monitor Cu2+ in living cells, have attracted much attention in recent years [6]. Our group also contributed for Cu2+ selective fluorescent chemosensors by utilizing a new binaphthyl derivative, which bears two pyrene groups and crown other unit (1) [7]. In the absence of Cu2+, this probe showed strong excimer emission (kmax = 477 nm) in CH3CN. On the contrary a unique blue shift was observed upon the addition of Cu2+. When Cu2+ was added to the CH3CN solution, probe 1 displayed a large fluorescent enhancement with about 40 nm blue shift, which was attributed to the formation of a static pyrene excimer (Fig. 1). From the fluorescence titration, the association constant of 1 with Cu2+ was observed to be 6.56  104 M 1.

O O

O O 2+

Cu

O

O O O O O

O

Cu2+ O

O

O O

O

Dynamic Excimer λem = 477 nm

Static Excimer λem = 447 nm

1 Fig. 1. Proposed binding mode of compound 1 with Cu2+.

New rhodamine derivatives 2 and 3 bearing binaphthyl group were synthesized as selective fluorescent and colorimetric sensors for Cu2+ (Fig. 2) [8]. Highly selective ‘‘Off–On’’ type fluorescent changes were observed upon the addition of Cu2+ among the various metal ions in CH3CN–HEPES buffer. Probe 2 displayed a 380fold increase in its emission upon the addition 8.0 equiv. of Cu2+ and the value of log K for the binding of 2 and Cu2+ was calculated as 4.93. For probe 2, a carbonyl oxygen as well as crown ether oxygens can provide a nice binding pocket for Cu2+. On the other hand, the values of log K1:1 log K1:2 for the binding of 3 and Cu2+ were determined as 4.19 and 4.83, respectively. A chemo-sensing of 3 with Cu2+ was successfully applied to the microfluidic system, too. We recently reported naphthalimide derivative 4 which is linked by a piperazine ring as a selective fluorescent chemosensors for Cu2+ in aqueous solutions [9]. Free 4 in polar solvents displayed a dynamic excimer emission (Fig. 3), which resulted from a naphthalimide dimer formed in the excited state; whereas, the 4/Cu2+ (1:1) complex display a static excimer emission arising from a naphthalimide dimer in the ground state. The method of continuous variations was used to explain the final stoichiometry of the 4-Cu2+ complex which indicated the formation of a 4/Cu2+ (1:2) complex showing naphthalimide monomer emission. Significantly, the fluorescence responses of 4 to Cu2+ in aqueous solutions (CH3CN:HEPES = 1:1, v/v) induced a selective increase in monomer emission whereas other metal ions produced a negligible change. The dissociation constant (Kd) of 4 with Cu2+ was determined to be 3.4  10 4 M.

4

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N N

O

N

N N

O

N

O

N

N

O OHHO

N

O

N

O N

O

O

O

O

O

O

N

3

2 Fig. 2. Structures of 2 and 3.

Cu2+ (1 equiv.) HN

O

O

N

N

NH

NH

HN O

CH3CN N

dynamic excimer 4

bance at 424 nm decreased sharply, while the ones at 356 nm and 557 nm increased significantly, which induced a color change from primrose yellow to pink. The nonlinear fitting of the titration curve and the data of Job’s plot from absorption spectra assumed a 1:1 stoichiometry for the 5-Cu2+ complex with an association constant of 2.5  104 M 1. The first example of boronic acid-linked fluorescent and colorimetric chemosensor for copper ions was reported recently [11]. The monoboronic acid-conjugated rhodamine probe 6 (Fig. 4) displayed a highly selective fluorescent enhancement with Cu2+ among the various metal ions in 20 mM HEPES (0.5% CH3CN) at pH 7.4. Upon addition of Cu2+ to this solution, a pink color (kmax = 556 nm) was developed and the resulting species exhibited strong orange fluorescence (kmax = 572 nm). These absorption and emission changes were attributed to the Cu2+-induced spirolactam ring opening process as shown in Fig. 4. The association constant of 6 for Cu2+ was determined to be 2.8  103 M 1. Furthermore, this monoboronic acid-conjugated rhodamine probe was applied to detect copper ions in mammalian cells and zebra fish. New BODIPY derivatives (7 and 8) were synthesized as ‘‘Off–On’’ fluorescent chemosensor and fluorescent chemodosimeter for Cu2+ (Fig. 5) [12]. Compound 8 showed a highly selective CHEF (chelation enhanced fluorescence) effect only with Cu2+ among the metal ions examined. The fluorescence emission intensity of 8 reached its maximum when 5 equiv. of Cu2+ was added, and a gradual decrease in its emission intensity as well as a red-shift (9 nm) were observed as the concentration of Cu2+ increased. A similar red shift and an absorbance decrease of 8 were also observed in its UV spectra. A small amount of water comes from copper perchlorate-hy-

O Cu2+ N

static excimer Cu2+ (1 equiv.) CH3CN O N

O Cu2+

HN

N

O

N Cu2+

O

NH

monomer Fig. 3. Proposed mechanism of stepwise binding mode of 4 with Cu2+.

A rhodamine-pyrene derivative 5 has been synthesized as a ratiometric and ‘‘off-on’’ sensor for the detection of Cu2+ in CH3CN–HEPES buffer (0.02 M, pH 7.4) (4:6, v/v) [10]. When Cu2+ was added to the solution, a significant decrease of the fluorescence intensity of 424 nm and a new fluorescence emission band centered at 575 nm, which was attributed to the Cu2+ induced ring opening of the spirolactam moiety (Fig. 4). In addition, the absorO

HO

N N

N

O

O

O

Cu2+

N

N

N N

O

N

5

O

(HO)2B

N

O

O

Cu2+

N N

N

H O B O H

N

N

O

6 Fig. 4. Proposed binding modes of 5 and 6 with Cu2+.

N

N

5

Y. Jeong, J. Yoon / Inorganica Chimica Acta 381 (2012) 2–14

O

O O

O O O

O N

H O

O

H O N

N

Cu2+

fast

N

N

B

N

F F

N

N B F F

N B F F 7

8

Fig. 5. Proposed mechanism of recognition/reactivity of compound 7and 8 towards Cu2+.

drate as well as tightly bound Cu2+ in the binding site promoted hydrolysis of acetyl groups in 8 resulting in 7-Cu2+. These results demonstrated that this compound can be utilized as a selective fluorescent chemodosimeter for Cu2+ (see Fig. 6). We reported a small library of fluorophore-triazine tripod fluorescent system, which can accommodate a combination of three different functional groups, such as fluorophore (BODIPY), ligand (or ligands) and auxiliary group [13]. The binding properties of these tripod fluorescent systems were determined using Ag+, Ca2+, Cd2+, Co2+, Cu2+, Cs+, Hg2+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+ and Zn2+ ions (2 eq.) to evaluate the metal ion binding properties of these compounds in acetonitrile. For different ligands, compound 9 (Fig. 5) bearing one 2-methylpyridine binding unit displayed a highly selective fluorescent quenching effect with only Cu2+ among the metal ions examined. Compound 10 bearing one di-(2-picolyl)amine (DPA) unit also displayed a large and selective

CHEQ (chelation enhanced fluorescence quenching) effect with Cu2+, even though there were relatively small CHEQ effects with Hg2+, Pb2+ and Zn2+. Compound 13 bearing two signal BODIPY units and one DPA unit displayed a large CHEQ effect with Cu2+, and relatively small CHEQ effects with Hg2+, Pb2+ and Zn2+. Compound 11 bearing two 2-methylpyridine binding units displayed a highly selective fluorescent quenching effect with only Cu2+, and compound 12 bearing two DPA binding units displayed large CHEQ effects with Co2+, Cu2+ and Ni2+. Two 4,5-disubstituted-1,8-naphthalimide derivatives 14 and 15 were synthesized as ratiometric fluorescent sensor and colorimetric sensors for Cu2+ [14]. In 100% aqueous solutions of 14, Cu2+ induced a fluorescent enhancement centered at 478 nm at the expense of the fluorescent emission of 14 centered at 534 nm. 15 senses Cu2+ by means of a colorimetric (primrose yellow to pink) method with a thorough quench in emission attributed to the deprotonation of

N Cl N

H N

H N

N N

N

Cl N

N

O

N

B

H N

N

N N

N

N

N

N

N

O

O

N

N

N

F F 9

B

N

F F 11

F F 10

N

B

N

N N N

N N

N

N

N

N

N N

N

O

N N

O

O N F B F N N

B

N

N 13

F F 12 Fig. 6. Structures of compounds 9–13.

N B F F

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ments. Later, we also utilized Cu2+ complex of this fluorescein derivative for the detection of cyanide in aqueous solution [18]. Sulfur containing anthracene derivatives (Fig. 9) were synthesized as fluorescent chemosensors for Cu2+ [19]. Compound 19 displayed a highly selective CHEQ effect only with Cu2+ among the metal ions examined whereas compound 20, 1,8-isomer, showed quite different emission patterns, a large CHEF effect along with a red-shift ( 40 nm) upon the addition of Cu2+. Conjugated polymer based sensors have been extensively studied because their absorption, emission, and redox characteristics are sensitive to environmental perturbations [20]. Among them, polydiacetylenes (PDAs) have attracted significant attention for their unique chromatic properties [21]. We recently synthesized new azide- and alkyne- functionalized polydiacetylene (PDA-aa) vesicles (Fig. 10) and applied them as a new method for visual detection of Cu2+ [22]. In the presence of ascorbic acid, Cu2+ can be reduced to Cu+, which catalyzed the click reaction between the two functional groups. After incubation with Cu2+ and ascorbic acid, PDAaa solution changed its color from blue to red, which was attributed to conformational transition of the conjugated backbone. Other metal ions were explored but only induced negligible color changes.

the secondary amine conjugated to the naphthalimide fluorophore. 14-Cu2+ and 15-Cu2+ were further applied to sense cyanide in ratiometric way via colorimetric and fluorescent changes. A new naphthalimide-calix [4] arene was synthesized as a two-faced and highly selective fluorescent chemosensor for Cu2+ or F [15]. This chemosensor displayed a selective fluorescence quenching effect only with Cu2+ among the various metal ions (Fig. 8). From the fluorescent titrations, the association constant of 16 with Cu2+ in acetonitrile was calculated to be 6.1  104 M 1. Similar high selectivity for Cu2+ in the presence of 10% aqueous system (CH3CN:water = 9:1, v/v) was also observed for the fluorescent study. The weak and red-shift emission was recorded for 16 with Cu2+, this unique change can be attributed to the deprotonation of naphthalimide NH in the presence of Cu2+. A new cavitand derivative 17 bearing four coumarin groups (Fig. 7) was also synthesized and studied as a Cu2+ selective fluorescent chemosensor [16]. Compound 17 in acetonitrile-chloroform (4:1, v/v) showed unique large CHEQ effect only with Cu2+ among the metal ions examined. The job plot using the fluorescence changes indicated a 1:4 binding for compound 17 with Cu2+. From the fluorescence titration experiments, the Kd value with Cu2+ was observed to be 3 lM. The binding of this complex with dicarboxylates was further demonstrated via the fluorescent changes. A new fluorescent chemosensor based on the fluorescein derivative which effectively recognized Cu2+ in nanomolar range at pH 7.4 [17]. Compound 18 (Fig. 9) displayed a large CHEQ effect with Cu2+ and the dissociation constants of complex 18 with Cu2+ was calculated to be 26 nM. Furthermore, the usefulness of the title fluorescent chemosensor 18 as a sensor was demonstrated by monitoring Cu2+ ion uptake by copper binding proteins such as transferrin and amyloid precursor protein, respectively. This highly sensitive Cu2+-selective chemosensor can be suitable for many other biological applications possibly including in vivo experi-

O

N

O

O

N

Mercury is one of the most prevalent toxic metals in the environment, and gains access to the body orally or dermally. The US EPA (Environmental Protection Agency) standard for the maximum allowable level of inorganic Hg in drinking water is 2 ppb [23]. Due to the high toxicity of mercury, considerable attention has been devoted to the development of new fluorescent chemosensors for the detection of mercury and mercuric salts [24]. We recently reported two rhodamine hydrazone derivatives bearing thiol and carboxylic acid groups, respectively, as selective fluorescent and colorimetric chemosensors for Hg2+ [25]. The ring-opening process of spirolactam induced large fluorescent enhancement and colorimetric change upon the addition of Hg2+. In CH3CN-H2O (1:99, v/v) solution, about 10-fold and 50-fold enhancements in fluorescent intensities of 21 and 22 were observed upon the addition of 100 equiv. Hg2+ (Fig. 11). By monitoring the fluorescence of microchannel containing 21/22 with Hg2+, a linear response was observed in the range of 1 nM–1 lM with the detection limits of 1 nM for 21 and 4.2 nM for 22, respectively. The Job’s plots indicated the 2:1 stoichiometry for the binding of 21 and Hg2+ and the 1:1 stoichiometry for the binding of 22 and Hg2+. Both chemosensors were also successfully applied to visualize Hg2+ accumulated in the nematode C. elegans, which was previously exposed to nanomolar concentrations of Hg2+.

O

HN 6HN

HN 6 N

N

N

O H 14

2.2. Chemosensors for Hg2+/CH3Hg+

N

O H 15

Fig. 7. Structures of compounds 14 and 15.

O

O

O

O

O

O

O

O

O

OH OH

O

Cu2+ O OO O

NH HN

O

N 16

N N

O

Cu O

O

O

OO

H

H R

N Cu2+

2+

O

H R

N

Cu2+

R

17, R = CH2CH2Ph Fig. 8. Strructures of 16 and 17–4Cu2+.

O

H R

7

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HO2C HO2C

CO2H N

HO

N O

Cl

S

CO2H

S

S

O Cl COOH

S

20

19

18

Fig. 9. Structures of 18–20.

H N 7

O H N

7

N

N

3

N

N

3

O

N3 H N 7

N

N

3

O

PDA-aa

Cu2+

ascorbic acid

H N 7

O H N

7

H N O

N N N

N

N

N

3

O

7

N 3

N

N

3

Fig. 10. PDA-aa vesicle functionalized with azide and alkyne groups and the detection of Cu2+ using click chemistry.

A rhodamine-based sensor 23 (Fig. 11) bearing histidine group was also reported for the detection of Hg2+ [26]. Two carbonyl oxygens as well as imidazole nitrogen in probe 23 provided a nice binding pocket for Hg2+. In 0.02 M pH 7.4 HEPES:EtOH (1:9, v/v), the addition of 100 equiv. Hg2+ induced over 100-fold increase in fluorescence due to the spiro-lactam ring opening. In contrast, no responses were observed when other metal ions were added. From the fluorescence titrations, the association constant of 23 with Hg2+ was calculated to be 2.0  103 M 1. Probe 23 was also further applied to detect Hg2+ in the cell. Rhodamine derivatives 24 and 25 (Fig. 12) bearing mono and bis-boronic acid groups displayed selective fluorescent and colorimetric changes for Hg2+ in CH3CN–HEPES buffer (pH 7.4, 10 mM) (9:1, v/v) [27]. Two boronic acid derivatives displayed selective and large fluorescent enhancements and distinct color changes

with Hg2+. The association constants of 24 and 25 with Hg2+ were calculated as 3.3  103 M 1 and 2.1  104 M 1, respectively. Bisboronic probe 25 displayed about 9-fold tighter binding with Hg2+ compared to mono-boronic probe 24, which can be attributed to an additional boronic acid moiety of 25. Two new rhodamine derivatives (26 and 27) bearing urea groups were synthesized as Hg2+ selective fluorescent and colorimetric chemosensors (Fig. 13) [28]. The dimeric system 27 showed a highly selective fluorescent enhancement and colorimetric changes upon the addition of Hg2+ in acetonitrile, on the contrary, compound 26 showed a poorer selectivity toward Hg2+. The association constants of 26 and 27 with Hg2+ were calculated as 2.9  104 M 1 and 3.2  105 M 1, respectively. A new rhodamine 6G derivative bearing spirothiolactone ring 28 (Fig. 14) showed a very high selectivity towards the Hg2+ ion

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NH O O

O

SH

N

O

NH

N

N

O

N

N

O

22

21

NH2

N

N N

N N

N

O

COOH

N

23

Fig. 11. Structures of 21–23.

(HO)2B

cies were attributed to deselenation reaction. The fluorescence intensity of 29 was linearly proportional to the Hg2+ concentration of 0–30 nM. Job’s plot indicated the binding mode of 1:1 stoichiometry between 29 and Hg2+. This sensor was successfully applied to detect inorganic mercury/methylmercury species in cells and zebrafish. As other approach, we synthesized an anthracene derivative, which bears azathiacrown ligand on the 1,8-positions of anthracene framework [32]. The fluorescent chemosensor 30 (Fig. 15) diplayed extreme selectivity for Hg2+ in aqueous solution at physiological pH and it showed a selective large CHEQ effect with Hg2+ (Ka = 1.95  105 M 1). The anthracene moiety in host 30 acts not only as a fluorescence source but also as a template for introducing the binding selectivity. These results suggest that the rigid capping of azacrown ligand onto a fluorophore framework may be employed successfully in the creation of selective chemosensors. We further extended this similar strategy to acridine derivatives, which bear immobilized azacrown or azathiacrown ligands

(HO)2B

O

O NH

N

N

N (HO)2B

N

O 24

N

N

N

O 25

Fig. 12. Structures of 24 and 25.

over other metal ions in the CH3CN–HEPES buffer (0.01 M, pH 7.4) (1:99, v/v) [29]. An enhancement of up to 200-fold ‘‘Off–On’’ type fluorescence for 28 was observed after the addition of the Hg2+. A unique 2:1 binding mode (28:Hg2+) was confirmed by electrospray-ionization mass spectroscopy (ESIMS), the Job’s plot, and X-ray crystal structure data. The spirothiolactone ring-opened structure, as well as the coordination of the two sulfur atoms to Hg2+, was clearly confirmed. Probe 28 was also successfully applied to visualize Hg2+ accumulated in the nematode C. elegans. Methylmercury species, which can readily pass through biological membranes, are much more toxic than inorganic mercury species [30]. Based on the high affinity between mercury and selenium, we designed a fluorescent chemodosimeter 29 (Fig. 14) based on rhodamine B selenolactone for inorganic mercury and methylmercury species [31]. The fluorescence enhancement and UV-vis spectral change induced by mercury/methylmercury spe-

O O Se S N N H

O 28

29

Fig. 14. Structures of 28 and 29.

O O

O NH

O

HN

NH

HN

N

N

O

N H

N

27

O

N

N

O

N

Hg2+

NO2 NH

HN O

HN

NH

O NH

N

N

N

O

N N

O

O

O

O

NH N N O

O

26 N

N

Fig. 13. Structures of rhodamine B urea derivatives (26 and 27) and a proposed binding mode of 27 with Hg2+.

N

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(Fig. 15) [33]. Compound 31 and 32 displayed large CHEF effects with Hg2+ and Cd2+ among the metal ions examined at pH 7.4. The association constants of compound 31 with Hg2+ and Cd2+ were calculated to be 1.18  105 and 4.48  103 M 1, on the other hand, the association constants of compound 32 with Hg2+ and Cd2+ were calculated to be >108 and 3.28  104 M 1, respectively. These results explain that cooperative binding from an immobilized ligand and nitrogen on acridine can provide such selectivity. The practical use of these probes was demonstrated by their applications to the detection of Hg2+ and Cd2+ ions in mammalian cells. Two new selenium moieties were also introduced to the the 1,8-positions of anthracene framework [19]. Compound 33 and 34 (Fig. 15) displayed highly selective CHEF (chelation enhanced fluorescence) effects only with Hg2+ among the metal ions examined in acetonitrile chloroform (4:1, v/v). The association constants were calculated as 3.6  104 and 4.4  104 M 1, respectively. The job plots indicated 1:1 binding between host 33/34 and Hg2+. Even though pyridine moiety in 34 contains additional nitrogen for the binding with Hg2+, the association constants turned out to be very similar. Two binaphthyl-azacrown-anthracene fluorophores (35 and 36, Fig. 15) were synthesized as selective fluorescent chemosensors for Hg2+ in CH3CN–0.01 M HEPES (pH 7.4) (4:1, v/v) [34]. Both of these compounds showed large fluorescence enhancements with Hg2+ even though there were moderate fluorescent enhancements with Zn2+ and Cu2+. Furthermore, fluorescent emissions from the binaphthyl and anthracene groups using different excitation wavelengths showed different patterns with these metal ions. All three metal ions, such as Hg2+, Zn2+ and Cu2+, showed CHEF effects when either 336 nm or 390 nm were used as the excitation wavelength. On the other hand, Zn2+ induced CHEF effects and Cu2+ induced large CHEQ effects when 290 nm was used as the excitation wavelength. The different CHEF and CHEQ effects in the dual emission changes from a binaphthyl group and a second fluorophore can provide more precise detections of their metal ions. We synthesized other tripod fluorescent systems 33 and 34 (Fig. 16), which bear a triazine core for combining three different functional groups, such as fluorophore (BODIPY), ligand and auxiliary group [35]. To confirm about the auxiliary subunit effect, the

S N

S

S

S

N

N

X

X

X

X

N

N

31: X= O 32: X= S

30

N Se

Se

Se

fluorescent emission changes in compounds 33 and 34 with Hg2+ (10 eq.) were productively compared (Fig. 16); a quenching effect with compound 33, and an enhancement and a slight red shift with compound 34. The large chelation enhanced fluorescence (CHEF) effect of compound 34 upon the addition of Hg2+ can be explained by the blocking of the PET mechanism. Another BODIPY derivative bearing piperazine group 39 (Fig. 16) was also synthesized and characterized by X-ray crystallography [36]. Among the various metal ions, 39 showed a selective CHEQ effect with Hg2+ in CH3CN-water (95:5, v/v). From the fluorescence titration experiments, the association constant of 1 with Hg2+ was observed to be 2800 M 1, respectively [11]. The job plots using the fluorescence changes indicated 1:1 binding for 1 with Hg2+. 2.3. Chemosensors for Zn2+ Many of the enzymes available in the human body as well as in sea organisms contain zinc as a very essential element and many pathological processes such as cerebral ischemia, Alzheimer’s disease, infantile diarrhea involve intracellular zinc detection [37,38]. Accordingly, its cellular imaging with high sensitivity and selectivity over biologically abundant cations has been actively studied [39]. We recently reported a simple and effective fluorescent sensor 40 (Fig. 17) based on the hyrazone-pyrene [40]. This probe displayed a highly selective fluorescent enhancement with Zn2+, and application of this probe to detect the intrinsic Zn2+ ions present in pancreatic-cells was successfully demonstrated. The absence of any significant change in absorption spectra upon the addition of Zn2+ indicated that large fluorescence enhancement with Zn2+ can be attributed to the blocking of the PET process from nitrogen in the hydrazone moiety to pyrene. An NBD based TRPEN chemosensor 41 was also synthesized recently [41]. Compound 41 displayed a red-to-yellow color change and a selective fluorescence enhancement in the presence of Zn2+ with a slight wavelength change. The addition of Zn2+ to 41 in 100% aqueous solution (0.1 M HEPES, pH 7.2) caused a large CHEF, which was explained by photoinduced electron transfer (PET) and internal charge transfer (ICT) mechanisms. The dissociation constant (Kd) of 41 with Zn2+was calculated as 1.3 ± 0.13 lM. The practical use of this probe was demonstrated by its application to the biologically relevant detection of Zn2+ ions in pancreatic b-cells. Even though DPA-based receptors have higher affinities for Zn2+ over alkali and alkaline-earth metal ions, they show similar affinities for most of transition and heavy metal (HTM) ions. Recently, we reported a new strategy called ‘receptor transformer’. We synthesized an amide- containing DPA receptor for Zn2+, combined with a naphthalimide fluorophore (42, Fig. 18) [42]. Compound 42 binds Zn2+ in an imidic acid tautomeric form of the amide-

Se

N

A N

34

33

N

N

N N

37 : A = HN

Cl

38 : A = HN

N

N

N

O O O

O N O

N R

(R)-35: R = H (R)-36: R = CN Fig. 15. Structures of 30–36.

N

N

B F F

N B F F 39

Fig. 16. Structures of compounds 37–39.

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HC

Cd2+, however, the fluorescent changes for other metal ions were relatively reduced. The association constants were calculated as 4.2  105 M 1 for Zn2+, and 1.3  104 M 1 for Cd2+, respectively. Two carboxylate oxygens vs phenolate could be the reason for the larger association constants of probe 44 compared to those of probe 43. Probe 45, in which DPA ligand was introduced at the 4-position, showed large fluorescence enhancements with Zn2+ and Cd2+ and fluorescence quenching effects with Cu2+ and Hg2+. From the fluorescence titrations, the association constants were calculated as 1.4  105 M 1 for Zn2+, and 1.2  104 M 1 for Cd2+, respectively.

NO2

N NH2

N O

OH

N N

N

N N

N

41

40

Fig. 17. Structure of compounds 40 and 41.

2.4. Chemosensors for Pb2+ DPA receptor in aqueous solutions with the highest affinity (Kd = 5.7 nM), while most other HTM ions are bound to the chemosensor in an amide tautomeric form (Fig. 18). Due to this differential binding mode, 42 showed excellent selectivity for Zn2+ over most competitive HTM ions, and an enhanced fluorescence (22-fold) as well as a red-shift in emission from 483 nm to 514 nm was observed with Zn2+. Interestingly, the 42/Cd2+ complex (Kd = 48.5 nM) showed an enhanced (21-fold) blue-shift in emission from 483 nm to 446 nm. We demonstrated that 42 could discriminate in vitro and in vivo Zn2+ and Cd2+ with green and blue fluorescence, respectively. Finally, 42 was successfully applied to detect zinc ions during the development of living zebrafish embryos. New 4- or 8-substituted-7-hydroxycoumarin derivatives 43–45 (Fig. 19) were recently reported as fluorescent sensors for metal ions in HEPES buffer solutions (20 mM, pH 7.4) containing 1% DMSO [43]. Probe 43, which has an iminodiacetic acid diethyl ester ligand at the 8-position, displayed a selective and large fluorescence enhancement with Zn2+ even though there was a small fluorescence enhancement with Cd2+ and fluorescence quenching effects with Cu2+ and Ni2+. The association constant of probe 43 with Zn2+ was calculated as 1.7  104 M 1. Among the metal ions examined, probe 44, which has an iminodiacetic acid group at the 4-position, showed large fluorescence enhancements only with Zn2+ and Cd2+. As compared to probe 43, probe 44 displayed less selectivity for Zn2+ over

O

N

O

Lead is a poisonous metal and its poisoning mostly comes from ingestion of contaminated food or water. Long-term exposure to lead or its salts can damage nervous connections (especially in young children) and cause blood and brain disorders [44]. In 2005, we reported a rhodamine-B derivative 46 as a fluorescent chemosensor for Pb2+ (Fig. 20) [45]. A single crystal of compound 46 was characterized using X-ray crystallography, which for the first time represented the unique spirolactam-ring formation. Upon the addition of Pb2+ to a colorless solution of 46 in acetonitrile, both a pink color and the fluorescence characteristics of rhodamine B appeared. Because both changes disappeared upon the addition of excess cyclen or ethylenediamine, it is believed that the complexation of 46 with Pb2+ is reversible. Recently, we have developed a new PDA-based chemosensor system for the detection of Pb2+ in aqueous solution [46]. UV irradiation of the mixtures of both DA monomers (47:PCDA = 1:9) induced the formation of stable and blue-colored PDA molecules 48 (Fig. 21). 48 displayed a selective and clear blue-to-red transition only with Pb2+ in HEPES (10 mM, pH 7.4) among various metal ions. The blue-to-red transition of the PDAs was also accompanied by the enhancement of fluorescence. The fluorescence spectra of the PDAs 48 showed a gradual increase in the presence of 0– 9 lM Pb2+ with the detection limit of 0.8 ppm.

O

N

O

O

M2+ HN

O

Zn2+

aqueous solution

O

N

N

O N

N N

aqueous solution

N

H

N

M2+ N

N

N

42

42-M2+ amide tautomer

Zn2+ N

N

42-Zn2+ imidic acid tautomer

Fig. 18. Different binding modes of 42 with Zn2+ over other metals in aqueous solution.

OH O

O

O

OH O

N O

O

OH

N

HO

N

O O

O 43

N O

N OH

O 44

Fig. 19. Structures of compound 43–45.

O 45

O

H

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N N N

O

N

O N

N

N

O

N

2+

Pb

N

N

N

O

N

46 Fig. 20. Proposed binding mechanism for the fluorescence enhancement of 46 upon addition of Pb2+.

2.5. Chemosensors for Cd2+

yellow fluorescence emission. Therefore, the two step recognition processes of vanadate with 51 were successfully indicated by the colorimetric and fluorescent changes (Fig. 23).

A new anthracene derivative as a reverse PET chemosensor for metal ions was synthesized in 2001 [47]. An anthryl tetra acid 49 (Fig. 23) showed large fluorescence quenching effects in 100% aqueous solution (pH 7.0) with metal ions via photoinduced electron transfer. Unlike other metal ions, the addition of Cd2+ induced an additional broad, red-shifted band yielding the composite spectrum with kmax 435 nm. Based on the NMR experiments, unique chelatoselective fluorescence perturbation in the presence of Cd2+ was attributed to the electrophilic aromatic cadmination at the 9-position of anthracene. On the contrary, chemosensor 50 (Fig. 22) displayed a selective CHEF effect with Cd2+ among the metal ions examined at pH 10 [48].

2.7. Chemosensors for noble metal ions Noble metals, such as gold, silver, platinum and palladium, are widely used to prepare dental materials, catalysts, fuel cells, jewelry, and anticancer drugs. However, their frequent use can result in a high level of residual noble metal ions, which may result in the contamination of water systems and soil and therefore cause a health hazard. In this regards, selective sensing methods would be very useful for real-time monitoring of these metal ions in environmental and biological samples. In 2002, we reported two fluorescence sensors 52 and 53 with an anthracene-functionalized pyrazole receptor for Ag+ (Fig. 24) [50]. Compound 52 displayed fluorescence quenching effects with Ag+ and Cu2+ ions in CHCl3–ethanol (7:3, v/v). The association constants for Ag+ and Cu2+ were calculated to be 1.25  105 and 1.34  105 M 1, respectively. Compound 53 displayed a selective fluorescent quenching effect only with Ag+ ion, and its association constant was calculated as 2.44  103 M 1, which means that 52 binds with Ag+ about 100-fold more strongly than 53. The overall fluorescent emission change of 53 was ca. 20-fold and that of the 52 was 0.3-fold, which can be attributed to the additional Ag+–p interaction in the case of 53 (Fig. 24). We also reported two fluorescein derivatives 54 and 55 bearing morpholine and thiomorpholine substituents, respectively for the detection of Ag+ (Fig. 26) [51]. Chemosensors 54 and 55 showed

2.6. Chemosensor for vanadate We recently reported tris(2-((ethylimino)methyl)pyren-1-ol)amine 51 as a first optical sensor of tetrameric vanadate and unique binding process was proposed by both distinct colorimetric and fluorescent changes (Fig. 23) [49]. During the titration, 51 was supposed to bind V1 (monomeric, VO43 ), V4 (tetrameric, V4O124 ), and V5 (pentameric, V5O155 ) simultaneously, and the color of the solution 51 changed from pink to yellow, with a strong yellow-green fluorescence. Interestingly, the equilibrium of oligomeric vanadates shifted towards V4 slowly, since 51 preferentially binds V4 over V1 and V5. and the situation reached the maximum point after 8 h of the addition of monovanadate. The color of solution 51 changed from yellow to orange, with a strong

N

O

N

N

7 N H 47 11

N

N

N N

HO

N

11

HO

HO

HN O

7

7

11

11

7

11

HN

HO

O

O

O

7

N

10

7

7

10

48 Fig. 21. Self-assemble and polymerization of monomer 47 and 48.

O

O

O

UV 7

HO

HO

O

10

7

10

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N N

CO2H HO2C HO2C

CO2H N

N

N

CO2H

N

CO2H

N N

N N

Ag+

N N

Ag+

CO2H

52 50

49

CO2H

N N

+

Ag

N N

Ag+

N N

Fig. 22. Structure of compounds 49 and 50.

N N +

high binding selectivity towards Ag ions and showed completely different fluorescent and colorimetric changes upon the addition of Ag+. Chemosensor 54 showed a selective CHEF effect with Ag+ at pH 7.4 (0.01 M HEPES:DMSO = 95:5, v/v). On the other hand, the fluorescence of chemosensor 55 showed a selective CHEQ effect and a light yellow to pink color change took place upon the addition of Ag+. Two different binding modes are explained in Fig. 25. The data from the fluorescence titration experiments gave association constants of 3.5  103 M 1 and 3.2  109 M 2 for 54 and 55, respectively, with Ag+. Two naphthalimide derivatives 56 and 57 were reported recently (Fig. 26) [52]. Probe 56 can detect Ag+ with a selective fluorescence enhancement (14-fold) and high association constant (Ka = 1.24  105 M 1) in CH3CN–H2O (50:50, v/v; 0.5 M HEPES buffer at pH 7.4) solution. Furthermore, Ag+ could be detected at least down to 1.0  10 8 M. On the other hand, the reference compound 57 without the carbonyl group did not show a strong binding with

53 Fig. 24. Proposed binding modes of 52 and 53 with Ag+.

Ag+, which suggests that the carbonyl group between the 1,8naphthalimide and [15] aneNO2S2 plays an important role for the selective fluorescence enhancement. In 2011, we synthesized a bis-pyrene derivative 58 bearing two pyrenes and reported as a ratiometric fluorescent chemosensor for silver ions at physiological pH (Fig. 27) [53]. Compound 58 showed a selective fluorescence change only with Ag+ in DMSO-HEPES (pH 7.4, 1:1, v/v), although there was a relatively smaller quenching effect with Hg2+. In the absence of metal ions, a strong excimer emission was observed at 463 nm, along with a monomer emission at 399 nm. upon the addition of Ag+, the excimer peak was significantly reduced with the enhancement of the monomeric peak.

O O O V O O

V1

O V OO O V O O

O O V O O

O

O

O

V

N

N

O N

N

V V O O O O

V5 HO

O O O O V V O O O

N

HO

HO

N

N

V2

O O O V O HO V O O O O V V O O O

V4

O O

OO V N

N

51

V O NO OO O V O V N O

O

N

O O OO V O O O N V O V O O N O O O

O

V

N O

V

Fig. 23. Propose binding mechanism of compound 51 with different vanadate species.

N

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O

O N

N

H O

O

Cl

O O H O

Ag+

N Ag+

N

O

HO

O

Cl CO2

Cl

Cl CO2

54 Fluorescent Yellow Solution

High Fluorescent Yellow Solution

S

S H O

N

N O

Cl

PET N

H O

N

S Ag+

Cl

S O

Ag+

O

O

Cl

CO2

Ag+

Cl CO2

55 Fluorescent Yellow Solution

Less Fluorescent Pink Solution

Fig. 25. Proposed binding modes of 54 and 55 with Ag+.

O

N

O

NH

O

N

O

The product was isolated and characterized as the oxazolecarbaldehyde of 55. There was also a large enhancement (6-fold) in the UV absorption (kmax = 562 nm) of probe 59 upon the addition of Au3+. The rate constant for the conversion of 59 (5 lM) to 60 was measured in the presence of Au3+ (10 equiv.), and estimated to be Kobs = 4.5 (±0.20)  10 4 s 1. The detection limit was estimated to be 63 ppb in EtOH–water (1:1, v/v). Finally, probe 59 was successfully applied to the cell imaging of Au3+.

O

NH S

S O

O

N

N S

O

S

56

O

57

Fig. 26. Structures of compound 56 and 57.

The 1:1 stoichiometry of the 53-Ag+ complex was confirmed and the association constant of 58 with Ag+ was calculated as 3.2  105 M 1. A rhodamine-alkyne derivative 59 as the first fluorescent and colorimetric chemodosimeter for Au3+ was reported recently (Fig. 28) [54]. Probe 59 displayed a selective fluorescence enhancement (over 100-fold) and colorimetric change (from colorless to pink) with Au3+ in EtOH–HEPES buffer (0.01 M, pH 7.4) (1:1, v/v).

N

3. Conclusions and future perspectives Due to the biologically and environmentally importance, detection of metal ions has emerged as a significant goal in the field of chemical sensors in recent years. Certainly fluorescence has been proven to be the most powerful tool due to its simplicity, high detection limit and application to bioimaging. In this review, we focused our recent contributions to this field. This review was categorized by target metal ions, such as Cu2+, Hg2+, Zn2+, Pb2+, Cd2+, Vanadate, Ag+ and Au3+. Selectivity and sensitivity for these metal ions were achieved by introducing various ligands to core fluorophores, such as, rhodamine, fluorescein, pyrene, anthracene, naphthalimide, coumarin, and BODIPY. Generally, there are three different approaches to design fluorescent chemosensors. The most popular way involves the use of sensors in which the binding sites and signaling subunits are

N N

N

Ag+ N

N

Ag+ N

58 Fig. 27. Proposed binding mode of 58 with Ag+.

N

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Et2N

O

NEt2

Et2N

O

NEt2 [18] [19]

Au3+ N

N

O 59

O

CHO

[20] [21]

60

Fig. 28. Au3+-induced transformation from 59 to 60.

linked covalently. A coordination complex-based displacement approach has also been used. A third method is known as a chemodosimeter approach. These types of sensors rely on the occurrence of specific, most often irreversible chemical reactions. In this review, these three approaches are adopted to design various chemosensors. Especially, second approach was used for the detection of cyanide from Cu2+ complex of chemosensors [14,18,55]. In addition, metal complexes can be further used as fluorescent chemosensors for biologically important anionic species, such as pyrophosphate [55], ATP [57], UTP [56]. We believe fluorescent chemosensors will be developed intelligently with aid of molecular recognition, supramolecular chemistry and ligand engineering. Nowadays, specific and sensitive fluorescent chemodosimeters are actively reported utilizing conventional organic reactions. Application to biology and environmental science is another driving force for researchers of this field in the future. Acknowledgment This work was supported by National Research Foundation (NRF) Grant (2011-0020450), WCU (R31-2008-000-10010-0) and by the Converging Research Center Program through the Ministry of Education, Science and Technology (2011K000720). J.Y. also deeply thanks to previous and present group members. References [1] H.N. Kim, M.H. Lee, H.J. Kim, J.S. Kim, J. Yoon, Chem. Soc. Rev. 37 (2008) 1465. [2] (a) D.T. Quang, J.S. Kim, Chem. Rev. 110 (2010) 6280; (b) 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; (c) J.S. Kim, S.Y. Lee, J. Yoon, J. Vicens, Chem. Commun. (2009) 4791; (d) J.F. Zhang, Y. Zhou, J. Yoon, J.S. Kim, Chem. Soc. Rev. 40 (2011) 3416. [3] (a) X. Chen, Y. Zhou, X. Peng, J. Yoon, Chem. Soc. Rev. 39 (2010) 2120; (b) R. Martinez-Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419; (c) Z. Xu, X. Chen, H.N. Kim, J. Yoon, Chem. Soc. Rev. 39 (2010) 127; (d) Z. Xu, S.K. Kim, J. Yoon, Chem. Soc. Rev. 39 (2010) 1457; (e) S.-K. Ko, X. Chen, J. Yoon, I. Shin, Chem. Soc. Rev. 40 (2011) 2120; (f) S.K. Kim, H.N. Kim, Z. Xiaoru, H.N. Lee, H.N. Lee, J.H. Soh, K.M.K. Swamy, J. Yoon, Supramol. Chem. 19 (2007) 221; (g) X. Chen, X. Tian, I. Shin, J. Yoon, Chem. Soc. Rev. 40 (2011) 4783; (h) Y. Zhou, Z. Xu, J. Yoon, Chem. Soc. Rev. 40 (2011) 2222. [4] G. Multhaup, A. Schlicksupp, L. Hesse, D. Beher, T. Ruppert, C.L. Masters, K. Beyreuther, Science 271 (1996) 1406. [5] R.A. Lovstad, BioMetals 17 (2004) 111. [6] E.L. Que, D.W. Domaille, C.J. Chang, Chem. Rev. 108 (2008) 1517. [7] E.J. Jun, H.N. Won, J.S. Kim, K.H. Lee, J. Yoon, Tetrahedron Lett. 47 (2006) 4577. [8] X. Chen, M.J. Jou, H. Lee, S. Kou, J. Lim, S.-W. Nam, S. Park, K.-M. Kim, J. Yoon, Sens. Actuators B 137 (2009) 597. [9] Z. Xu, J. Yoon, D.R. Spring, Chem. Commun. 46 (2010) 2563. [10] Y. Zhou, W. Fang, Y. Kim, S. Kim, J. Yoon, Org. Lett. 11 (2009) 4442. [11] K.M.K. Swamy, S.-K. Ko, S.K. Kwon, H.N. Lee, C. Mao, J.-M. Kim, K.-H. Lee, J. Kim, I. Shin, J. Yoon, Chem. Commun. (2008) 5915. [12] 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. [13] X. Qi, S.K. Kim, S.J. Han, L. Xu, A.Y. Jee, H.N. Kim, C. Lee, Y. Kim, M. Lee, S.-J. Kim, J. Yoon, Supramol. Chem. 21 (2009) 455. [14] Z. Xu, J. Pan, D.R. Spring, J. Cui, J. Yoon, Tetrahedron 66 (2010) 1678. [15] Z. Xu, S. Kim, H.N. Kim, S.J. Han, C. Lee, J.S. Kim, X. Qian, J. Yoon, Tetrahedron Lett. 48 (2007) 9151. [16] Y.J. Jang, B.-S. Moon, J.H. Park, J.Y. Kwon, Y.J. Yoon, K.D. Lee, J. Yoon, Tetrahedron Lett. 47 (2006) 2707. [17] (a) E.J. Jun, J.-A. Kim, K.M.K. Swamy, S. Park, J. Yoon, Tetrahedron Lett. 47 (2006) 1051;

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