Surfactant-sensitized ratiometric fluorescent chemodosimeter for the highly selective detection of mercury(II) ions based on vinyl ether oxymercuration

Abstract

The ratiometric fluorescence chemodosimeter N-butyl-4-(prop-1-en-1-yloxy)-1,8-naphthalic anhydride (NT-VE) was designed for Hg²⁺ recognition by oxymercuration at ambient temperature with high selectivity and no interference from other metal cations such as Cu²⁺, Ag⁺, Au³⁺, Fe³⁺, etc. The NT-VE could be incorporated into sodium dodecyl-benzenesulfonate (SDBS) micelles, which was confirmed by the clear emission enhancement observed in 0.1 mM SDBS. The hydrophobic core of the SDBS enhanced the solubility of NT-VE, enabling the detection of Hg²⁺ in aqueous solution, and the negatively charged micelle surface increased the local concentration of Hg²⁺ for amplified sensitivity. This work demonstrated the excellent performance of the NT-VE chemodosimeter for ratiometric detection of Hg²⁺ in aqueous solution. The NT-VE/SDBS system could detect Hg²⁺ over a linear range of 0.05–10 μM and a detection limit of 9 ppb (45 nM) in water.

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Introduction

Mercury contamination has become a serious problem worldwide, because of geological events and the widespread use of mercury species in modern human activities. For example, mercury mining and zinc smelting areas are considered as the hot spots in mercury pollution in China, while waste water from dental offices is the major pollution source in US.^1,2 Upon entering freshwater and marine ecosystems, bacteria convert inorganic mercury ions to extremely toxic methylmercury, which eventually reaches the top of the food chain and bioaccumulates in large predatory fish, such as tuna and swordfish, consumed by humans.^3–5 Being lipophilic, methylmercury is readily absorbed through the GI tract and damages the central nervous system.⁶ Therefore, the detection of Hg²⁺ ions in water is very important.

Various analytical methods, such as cold vapour atomic absorption spectroscopy, inductively coupled plasma mass spectroscopy, etc., have been developed for detection of Hg²⁺ ions. Among these techniques, the fluorescence method is desirable by the virtue of its high sensitivity and simplicity.⁷ Therefore, numerous fluorescent chemodosimeters have been developed and applied in the selective detection of Hg²⁺ ions. Typically, these probes contain sulfur or nitrogen atoms that can coordinate the Hg²⁺ as part of off-on fluorescent switches.⁸ However, there are two aspects which potentially influence the accuracy and selectivity of these Hg²⁺ probes as well as their potential applications in real world “dirty” samples and in biological settings with low concentration of Hg²⁺. First, there is a storage issue because oxygen in air or other oxidizing agents may oxidize and decompose the sulfur or nitrogen moiety. Second, these Hg²⁺ probes lack selectivity toward Hg²⁺ ions in the presence of other metals, because the sulfur or nitrogen moieties can interact with other heavy metals such as Ag⁺, Cu²⁺, or Au³⁺ ions.

Recently, Kazunori Koide and co-worker developed Hg²⁺ probes based on the oxymercuration of vinyl ethers.⁹ Although, their probes could detect mercury species at ambient temperature in the presence of various organic, inorganic, and anionic contaminants, their probes were based on the specificity to Hg²⁺ depending on single emission intensity changes which were affected by a variety of factors, such as instrumental efficiency, probe molecule concentration, its stability under photoillumination, and microenvironment around probe.^10,11 In addition, these probes also suffered from low water solubility, requiring use of an organic solvent in the sensing system, which may be problematic in potential applications. Therefore, to overcome these limitations mentioned above, it is necessary to develop ratiometric probes with high accuracy, selectivity and sensitivity for the detection of Hg²⁺ in water.

Surfactants have attrackted interest because they can modulate the photophysical properties and the sensing behavior of fluorescent probes by self-assembly into supramolecular structures called micelles. Moreover, surfactants can enhance the solubility of the hydrophobic sensors in aqueous solution as they provide both a hydrophobic interior environment and a polar or ionic surface, thereby enabling the analysis to be carried out in water.^12,13 Although several groups have utilized fluorescent probe/surfactant assemblies for detection of metal ions or anions, development of novel fluorescent probe/surfactant assemblies for the detection of metal ions, such as Hg²⁺ is still needed.

Herein, we address this need and present the synthesis and characterization of a ratiometric fluorescent probe, N-butyl-4-(prop-1-en-1-yloxy)-1,8-naphthalic anhydride (NT-VE), for Hg²⁺ ions sensing. The chemodosimeter is based on 1,8-naphthalimide dye, which is the characteristic of an ICT fluorophore.¹⁴ Given the high π electrophilicity of Hg²⁺ towards the oxymercuration of vinyl ether, we chose to incorporate vinyl ether as the reaction unit.⁹ The anionic surfactant, SDBS, is believed to tune the photophysical properties and sense behaviour of NT-VE. The optimized NT-VE/SDBS assemblies were found to be highly sensitive and selective to Hg²⁺ in aqueous solution. Because of these designed features, NT-VE/SDBS assemblies have a number of advantages that include: (1) turn-on and ratiometric Hg²⁺ detection; (2) Hg²⁺-specific fluorescence response; (3) a detection limit for Hg²⁺ in the nanomolar range; (4) capability to perform assays in aqueous solution. In addition, NT-VE/SDBS assemblies exhibit good selectivity for Hg²⁺ over many competing species.

Experimental section

Apparatus

The ¹H NMR and ¹³C NMR spectra were recorded on a Bruker AMX400 spectrometer (¹H, 400 MHz; ¹³C, 100.6 MHz). Spectra were referenced internally by using the residual solvent (¹H: δ_CDCl3 = 7.26 ppm or δ_d6-DMSO = 2.54 ppm) relative to SiMe₄. Electrospray ionization (ESI) mass spectra were recorded on a Bruker Apex IV FTMS mass spectrometer.

The pH measurements were carried out with a pH acidometer (Mettler Toledo FE-30). Absorption spectra were recorded on a Purkinje TU-1901 spectrophotometer. Fluorescence measurements were taken on a Hitachi F-7000 fluorescence spectrometer with a 10 mm quartz cuvette.

Procedures

The spectral properties of the reaction between Hg²⁺ and NT-VE were measured in a solution of sodium dodecyl-benzene sulfonate (SDBS) and potassium biphthalate (KHP 20 mM, pH = 4). All fluorescence spectra were recorded in a quartz cuvette at room temperature in the range 400–700 nm with excitation wavelength at 380 nm before or after reaction for 30 min. The excitation and emission bandwidths were both set to 2.5 nm.

Synthesis

NT-VE. The chemodosimeter NT-VE was synthesized according to Scheme 1b.^15,16 1.726 g (6 mmol) N-buty-4-chloro-1,8-naphthalic anhydride (NT-1), 1.800 g (30 mmol) allyl alcohol, 60.0 mg (0.2 mmol) 18-crown-6 as a phase transfer catalyst and 2.6 g (20.0 mmol) K₂CO₃ as a base were dissolved in 20 mL DMF. The reaction mixture was stirred for 12 hours at 80 °C. After being cooled to room temperature, the mixture was added to 200 mL H₂O, extracted with CHCl₃ and concentrated to obtain the crude product NT-2, which was purified by column chromatography (using CHCl₃–petroleum = 1 : 10 as eluent). The product NT-2 (463.5 mg, 1.5 mmol) was refluxed with 7.6 mg (0.02 mmol) dichlorobis(benzonitrile)palladium(II) as a catalyst in 2 mL toluene for 8 hours to obtain NT-VE. The crude product was purified by column chromatography (using CH₂Cl₂–petroleum = 2 : 1 as eluent), and a pale yellow solid was obtained. ¹H NMR for the NT-VE (400 MHz, CDCl₃): δ = 0.91–0.98 (t, J = 7.4 Hz, 3H; CH₃), 1.36–1.45 (m, 2H; CH₂), 1.67–1.70 (m, 2H; CH₂), 1.71–1.8 (d, 2H; CH₂), 4.13–4.21 (t, J = 7.6 Hz, 3H; CH₃), 5.2–5.7 (m, 1H; CH=CH), 6.69 (d, 1H; CH=CH), 7.25 (d, J = 8.2 Hz, 1H; ArH), 7.71 (m, J = 7.8 Hz, 1H; ArH), 8.54–8.61 (m, 3H; ArH). MS (ESI positive) calcd for C₁₉H₁₉NO₃: 310.12 [M + H]⁺; found: 310.1.

Scheme 1 (a) Design rationale for Hg²⁺ chemodosimeter. (b) Synthesis of chemodosimeter NT-VE.

NT-OH. 159.8 mg (4 mmol) Hg(ClO₄)₂·3H₂O was dissolved in 1 mL H₂O (pH = 2) and added to 4 mL THF which contained 463.5 mg (1.5 mmol) NT-VE. The reaction mixture was stirred for 30 min at room temperature. After extraction with ethyl acetate and concentrated, the mixture was purified by column chromatography (using CHCl₃–petroleum ether = 1 : 10 as eluent) to give the compound NT-OH. ¹H NMR for the NT-OH (400 MHz, DMSO-d₆): δ = 0.85 (t, J = 6.8 Hz, 3H; CH₃), 1.23–1.39 (m, 2H; CH₂), 1.56–1.64 (m, 2H; CH₂), 4.03 (t, J = 7.4 Hz, 2H; CH₂), 7.16 (d, J = 8.0 Hz, 1H; ArH), 7.75–7.79 (m, 1H; ArH), 8.35 (d, J = 1.2 Hz, 1H; ArH), 8.38–8.48 (d, J = 7.6 Hz, 1H; ArH), 8.54 (d, J = 8.4 Hz, 1H; ArH), 11.85 (s, 1H; OH). MS (ESI positive) calcd for C₁₆H₁₅NO₃ 270.11 [M + H]⁺; found: 270.1.

Results and discussion

Design and synthesis of NT-VE

Several considerations influence the design of the chemodosimeter NT-VE. The first objective was to employ a fluorophore with a ratiometric fluorescent response. The Hg²⁺ ion sensor based on the 1,8-naphthalimide, NT-1 (Scheme 1b) offered an excellent ICT structure, a typical feature for construction of a ratiometric fluorescent chemodosimeter.¹⁴ The second objective was to obtain highly selective recognition of Hg²⁺ ion compared to other metal ions. The vinyl ether was previously employed for the synthesis of Hg²⁺ probes due to the unique ability of Hg²⁺ to convert an alkyne to a ketone.⁹ Therefore, in the design of the Hg²⁺ probe, we chose 1,8-naphthalimide as the fluorophore and the vinyl ether as the specific reaction site for Hg²⁺. We expected that the reaction of NT-VE with Hg²⁺ would trigger the cleavage of the O–CH=CH bond, leading to the longer wavelength fluorescence of compound NT-OH.

Spectroscopic properties to NT-VE

We determined the spectroscopic characteristics of Hg²⁺ (10 μM) interactions with NT-VE (5 μM). The absorption spectra of the chemodosimeter NT-VE in the absence and presence of the Hg²⁺ were obtained. NT-VE displayed one absorption band centered at 380 nm. Upon the addition of Hg²⁺, there was no significant change in the absorption band (Fig. S1†), but the fluorescence spectra changed remarkably (Fig. S1† and 1a).

Fig. 1 (a and b) Fluorescence response curve (λ_ex, 380 nm) of NT-VE (5 μM) in the presence of different concentration of Hg²⁺ ions (final concentration: 0, 0.2, 0.4, 0.8, 2, 4, 6, 8, 10 μM) in pH = 4 buffer, (a) without SDBS added, (b) with 0.1 mM SDBS added.

In Scheme 1a and Fig. 1a, NT-VE showed blue-violet fluorescence at 460 nm. Upon the addition of Hg²⁺ ions, the vinyl ether on the NT-VE is cleaved, and NT-OH, which exhibits a bright yellow emission at 560 nm owing to the stronger electron-donor ability of hydroxyl group, is simultaneously released. Therefore, it is expected that the distinct fluorescence change from blue-violet to yellow upon the addition of Hg²⁺ ions reflects the realization of the oxymercuration reaction of NT-VE. The recognition mechanism of the NT-VE toward Hg²⁺ shown in Scheme 1a, was further investigated by ¹H NMR, ¹³C NMR, and MS (Fig. S2 to S6†). To disclose the sensing mechanism of the NT-VE toward the Hg²⁺ ion, the reaction of NT-VE with Hg²⁺ was conducted under the same conditions as described above. The reaction products were subjected to electrospray ionization mass spectral analysis. The peak at m/z 270.11 [M − H]⁺ corresponding to compound NT-OH was observed (as shown in Fig. S4 to S6†). Thus, recognition of Hg²⁺ led to the oxymercuration of the vinyl ether chemodosimeter (Scheme 1).

SDBS effect on fluorescent spectra of NT-VE

Surfactants can modulate the sensitivity of the probe because of their amphiphilic characteristics. The dispersibility of NT-VE in water with different types of surfactants, including cetyltrimethylammonium bromide, sodium dodecylbenzene sulfonate (SDBS), sodium dodecylsulfonate and sodium lauryl sulphate were investigated. As shown in Fig. S7,† the NT-VE in water showed an obvious difference in the presence of SDBS due to the good compatibility between the hydrophobic probe and the dodecyl groups in the micelle interior. In addition, the negatively charged micelle surface increased the local concentration of Hg²⁺ in the near vicinity of NT-VE. Micelle incorporation of the hydrophobic probe can drag the receptor into the counter ionic cloud, in which the local concentration of cationic analyte is boosted by electrostatic interaction. Thus, the combination of micelle and the probe made the reaction between NT-VE and Hg²⁺ ions facile. The fluorescent response in the presence of Hg²⁺ further confirmed this interaction.

We next examined the SDBS-induced spectral changes of chemodosimeter NT-VE in aqueous solution (Fig. 1b). When 0.1 mM SDBS was present, fluorescence intensity of VT-NE at 460 nm was enhanced 3.22-fold. The result indicated that the hydrophobic fluorophore was successfully incorporated into the SDBS micelle.

To obtain a sensor system with better sensing properties, and to understand the role of SDBS assemblies in adjusting the sensor's sensitivity, a series of NT-VE/SDBS solutions with a constant amount of NT-VE (5 μM) but different concentrations of SDBS were prepared in pure water, and their responses to Hg²⁺, were measured (Fig. S9†). Clearly, the fluorescence intensity ratio (I_{560 nm}/I_{460 nm}) was enhanced as the SDBS concentration increased up to 0.1 mM. However, the fluorescence intensity ratio (I_{560 nm}/I_{460 nm}) became much poorer when concentrations of SDBS were more than 0.1 mM. In other words, the NT-VE/SDBS assemblies showed higher sensitivity at a concentration of 0.1 mM SDBS, at which the NT-VE exhibited the largest ratiometric response (I_{560 nm}/I_{460 nm}) to Hg²⁺. As a result, we chose the system containing 0.1 mM SDBS to detect Hg²⁺.

Fluorescence response during Hg²⁺ ion titration

Titration of NT-VE (5 μM) with Hg²⁺ gradually changed the fluorescence spectra of NT-VE. The addition of 2 equiv. of Hg²⁺ in the absence of SDBS, resulted in a 4.08-fold increase in the emission intensity at 560 nm and a 1/1.16-fold decrease of in the emission intensity at 460 nm, leading to an overall 4.73-fold ratiometric response (Fig. 1a). However in the presence of 0.1 mM SDBS, addition of 2 equiv. of Hg²⁺ resulted in a 3.44-fold increase in the emission intensity at 560 nm and a 1/3.98-fold decrease in the emission intensity at 460 nm, which resulted in an overall 13.69-fold ratiometric response (Fig. 1b). Stronger fluorescence of free sensor of NT-VE and a higher ratiometric response in the presence of SDBS facilitated the Hg²⁺ detection at low concentration.

We then optimized the conditions for Hg²⁺-promoted hydrolysis of the vinyl ether of NT-VE to form NT-OH in various buffer solutions in the presence of 0.1 mM SDBS at 25 °C. The pH titration of Hg²⁺ and NT-VE led to a strong fluorescence change in the pH range of 2–4, while there was no change in the pH range of 6–12 (Fig. 2a), which suggested that the Hg²⁺-promoted hydrolysis occurs in acidic solution. Considering that pH 4 can be neutralized more easily than pH 2 or 3, pH 4.0 was chosen for the remaining part of study.

Fig. 2 (a) Hg²⁺ (10 μM) reaction with NT-VE (5 μM) in various buffers (0.1 mM SDBS, phosphate buffer solution 20 mM pH = 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12) adjusted by KOH or HNO₃. (b) Fluorescence response curves (λ_ex, 380 nm) of NT-VE (5 μM) in the presence of 20 μM Hg²⁺ (10 μM) every 2 min for 1 hour.

We further investigated the influence of the reaction time on the conversion of NT-VE to NT-OH in response to Hg²⁺ (20 μM) by adding SDBS (0.1 mM) at pH 4.0. The conversion of NT-VE to NT-OH in the presence of Hg²⁺ and SDBS showed 75% completion in 20 min, although the reaction still proceeded (Fig. 2b). A 30 min duration, which was half the time needed in the previous report,⁹ was considered as an optimal balance between sensitivity and convenience.

The determination of Hg²⁺ with chemidosimeter NT-VE was investigated in an aqueous solution containing 0.1 mM SDBS and potassium biphthalate (KHP 20 mM, pH = 4) buffer solution. In the fluorescence spectra, NT-VE displayed one major fluorescence emission centered at 460 nm. While adding different concentration of Hg²⁺, the maximum emission band underwent a red shift to 560 nm with an isoemission point at 516 nm (Fig. 1b). Additionally, the fluorescence intensity ratio (I_{560 nm}/I_{460 nm}) showed a linear dependence on the Hg²⁺ concentration in the range of 0.05 to 10 μM with a detection limit of 0.045 μM (Fig. 3).

We also evaluated the selectivity of the NT-VE probe toward Hg²⁺ under the same conditions. As expected, almost no fluorescence change were observed in the presence of Na⁺, K⁺, Mg²⁺, Ca²⁺, Cu²⁺, Ni³⁺, Al³⁺, Pb²⁺, Cd²⁺, Cr³⁺, Zn²⁺, Ba²⁺, Mn²⁺, Co²⁺, Fe³⁺ and Ag⁺ (Fig. 4b). Also, the effects of interference of the above-mentioned analytes on monitoring Hg²⁺ were investigated (Fig. 4a). These results demonstrated that the probe NT-VE possesses high selectivity toward Hg²⁺.

Fig. 3 Fluorescence response curve (λ_ex, 380 nm) of NT-VE (5 μM) in the presence of different concentration of Hg²⁺ (final concentration: 0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 μM) in pH = 4 buffer with 0.1 mM SDBS. Concentration of Hg²⁺ in the inset figure are 0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.

Fig. 4 (a) Fluorescence response of NT-VE (5 μM) to Hg²⁺ (10 μM) with interference with metal ions (100 μM) in pH = 4 buffer: Na⁺, K⁺, Mg²⁺, Ca²⁺, Cu²⁺, Ni³⁺, Al³⁺, Pb²⁺, Cd²⁺, Cr³⁺, Zn²⁺, Ba²⁺, Mn²⁺, Co²⁺, Fe³⁺ and Ag⁺. (b) Metal ion selectivity of NT-VE (5 μM), 1-probe only, 2-Hg²⁺, 3–18 100 μM Na⁺, K⁺, Mg²⁺, Ca²⁺, Cu²⁺, Ni³⁺, Al³⁺, Pb²⁺, Cd²⁺, Cr³⁺, Zn²⁺, Ba²⁺, Co²⁺, Fe³⁺ and Ag⁺.

Conclusions

This work described the design and synthesis of the Hg²⁺ sensor NT-VE. It is believed that the vinyl ether is a specific reaction site for Hg²⁺ and helps to enhance the selectivity to Hg²⁺. Further, optical spectroscopy measurements revealed that the sensitivity of the sensor could be tuned by the commercially available anionic surfactant, SDBS. The selected NT-VE/SDBS sensor platform exhibited high sensitivity toward Hg²⁺ in aqueous solution, with a linear range 0.05–10 μM and a low detection limit of 9 ppb. Moreover, this work represented a simple way to obtain a ratiometric, receptor-free, inexpensive and sensitivity-tunable Hg²⁺ fluorescence sensor by using the combination of SDBS (modulated the sensing behaviour) and ratiometric Hg²⁺-ion sensor NT-VE.

Acknowledgements

We gratefully acknowledge financial support from the National Nature Science Foundation of China (no. 21275018, 21203008, and 20975012) and the 111 Project (B07012) for financial support.

Notes and references

1. (a) Y. Lin, T. Larssen, R. D. Vogt and X. Feng, Appl. Geochem., 2012, 25, 60; (b) X. Feng and G. N. Bigham, Appl. Geochem., 2011, 26, 153.
2. (a) H. Zhang, X. Feng, T. Larssen, L. Shang and P. Li, Environ. Sci. Technol., 2010, 44, 4499; (b) X. Feng and G. Qiu, Sci. Total Environ., 2008, 400, 227.
3. M. Nendza, T. Herbst, C. Kussatz and A. Gies, Chemosphere, 1997, 35, 1875–1885.
4. H. H. Harris, I. J. Pickering and G. N. George, Science, 2003, 301, 1203.
5. S. Hardy and P. Jones, J. Chromatogr. A, 1997, 791, 333–338.
6. (a) T. W. Clarkson, L. Magos and G. J. Myers, N. Engl. J. Med., 2003, 349, 1731–1737; (b) P. M. Bolger and B. A. Schwetz, N. Engl. J. Med., 2002, 347, 1735–1736.
7. (a) E. M. Nolan and S. J. Lippard, J. Am. Chem. Soc., 2003, 125, 14270–14271; (b) X. F. Guo, X. H. Qian and L. H. Jia, J. Am. Chem. Soc., 2004, 126, 2272–2273; (c) Y. K. Yang, K. J. Yook and J. Tae, J. Am. Chem. Soc., 2005, 127, 16760–16761; (d) C. Díez-Gil, A. Caballero, I. Ratera, A. Tárraga, P. Molina and J. Veciana, Sensors, 2007, 7, 3481–3488; (e) F. L. Song, S. Watanabe, P. E. Floreancig and K. Koide, J. Am. Chem. Soc., 2008, 130, 16460–16461; (f) Y. Zhang, Q. Yuan, T. Chen, X. Zhang, Y. Chen and W. Tan, Anal. Chem., 2012, 84, 1956–1962; (g) B. Yin, M. You, W. Tan and B. Ye, Chem.–Eur. J., 2012, 18, 1286–1289.
8. (a) S. K. Ko, Y. K. Yang, J. Tae and I. Shin, J. Am. Chem. Soc., 2006, 128, 14150–14155; (b) B. Liu and H. Tian, Chem. Commun., 2005, 3156; (c) S. Yoon, E. W. Miller, Q. He, P. H. Do and C. J. Chang, Angew. Chem., Int. Ed., 2007, 46, 6658–6661; (d) G. Q. Shang, X. Gao, M. X. Chen, H. Zheng and J. G. Xu, J. Fluoresc., 2008, 18, 1187; (e) B. Tang, L. J. Cui and K. H. Xu, ChemBioChem, 2008, 9, 1159–1164; (f) E. M. Nolan and S. J. Lippard, Chem. Rev., 2008, 108, 3443; (g) W. Huang, P. Zhou, W. Yan, C. He, L. Xiong, F. Li and C. Duan, J. Environ. Monit., 2009, 11, 330–335; (h) M. G. Choi, Y. H. Kim, J. E. Namgoong and S. K. Chang, Chem. Commun., 2009, 3560–3562; (i) M. Santra, D. Ryu, A. Chatterjee, S. K. Ko, I. Shin and K. H. Ahn, Chem. Commun., 2009, 2115–2117; (j) R. Huang, X. Zheng, C. Wang, S. Yan, J. Yuan, X. Weng and X. Zhou, Chem.–Asian J., 2012, 7, 915–918; (k) Y. Gong, X. Zhang, Z. Chen, Y. Yuan, Z. Jin, L. Mei, J. Zhang, W. Tan, G. Shen and R. Yu, Analyst, 2012, 137, 932–938; (l) W.-J. Shi, J.-Y. Liu and D. K. P. Ng, Chem.–Asian J., 2012, 7, 196–200.
9. A. Shin and K. Kazunori, J. Am. Chem. Soc., 2011, 133, 2556–2566.
10. (a) J. V. Mello and N. S. Finney, Angew. Chem., Int. Ed., 2001, 40, 8; (b) H. Fu, B. H. Loo, D. Xiao, R. Xie, X. Ji, J. Yao, B. Zhang and L. Zhang, Angew. Chem., Int. Ed., 2002, 41, 6; (c) Y. Kubo, M. Yamamoto, M. Ikeda, M. Takeuchi, S. Shinkai, S. Yamaguchi and K. Tamao, Angew. Chem., Int. Ed., 2003, 42, 2036–2040.
11. (a) X. Zhang, Y. Xiao and X. Qian, Angew. Chem., 2008, 120, 8145; (b) A. Coskun, M. D. Yilmaz and E. U. Akkaya, Org. Lett., 2007, 9(4), 607; (c) E. M. Nolan and S. J. Lippard, J. Mater. Chem., 2005, 15, 2778; (d) M. Tian and H. Ihmels, Chem. Commun., 2009, 3175–3177; (e) E. M. Nolan and S. J. Lippard, J. Am. Chem. Soc., 2007, 129, 5910–5918; (f) A. Coskun and E. U. Akkaya, J. Am. Chem. Soc., 2006, 128, 14474; (g) S. V. Wegner, A. Okesli, P. Chen and C. He, J. Am. Chem. Soc., 2007, 129, 3474; (h) C. Li, X. Zhang, L. Qiao, Y. Zhao, C. He, S. Huan, L. Lu, L. Jian, G. Shen and R. Yu, Anal. Chem., 2009, 81, 9993; (i) W. Xuan, C. Chen, Y. Cao, W. He, W. Jiang, K. Liu and W. Wang, Chem. Commun., 2012, 48, 7292; (j) Z. Han, H. Luo, X. Zhang, R. Kong, G. Shen and R. Yu, Spectrochim. Acta, Part A, 2009, 72, 1084–1088; (k) J. Jiang, W. Liu, J. Cheng, L. Yang, H. Jiang, D. Bai and W. Liu, Chem. Commun., 2012, 48, 8371–8373; (l) K. Mahapatraa, G. Hazraa, N. K. Dasa, P. Sahooa, S. Goswami and H.-K. Funb, J. Photochem. Photobiol., A, 2011, 222, 47–51.
12. (a) L. Ding, S. Wang, Y. Liu, J. Cao and Y. Fang, J. Mater. Chem. A, 2013, 1, 8866–8875; (b) A. Mallick, M. C. Mandal, B. Haldar, A. Chakrabarty, P. Das and N. Chattopadhyay, J. Am. Chem. Soc., 2006, 128, 3126–3127.
13. J. Wang, X. Qian, J. Qian and Y. Xu, Chem.–Eur. J., 2007, 13, 7543–7552.
14. R. M. Duke, B. E. Veale, F. M. Preffer, P. E. Kruger and T. Gunnlaugsson, Chem. Soc. Rev., 2010, 39, 3936–3953.
15. V. B. Bojinova, I. P. Panovaa, D. B. Simeonovb and N. I. Georgieva, J. Photochem. Photobiol., A., 2010, 210, 89–99.
16. P. Golborn and F. Scheinmann, J. Chem. Soc., Perkin Trans. 1, 1973, 2870–2875.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra46693k

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