ESIPT based Hg²⁺ and fluoride chemosensor for sensitive and selective ‘turn on’ red signal and cell imaging

Abstract

An excited state intramolecular proton transfer (ESIPT) enabled fluorescent sensor phenanthroline diimino phenol (PDP) for Hg²⁺ has been designed and synthesized. PDP acts as a dual sensor and selectively detects only Hg²⁺ in mixed aqueous medium and fluoride in acetonitrile medium over other competing metal ions and anions. The binding of PDP with Hg²⁺ is supported by DFT. The ESIPT phenomenon in PDP is favored in the presence of Hg²⁺, which is rarely reported, along with an intense red fluorescence suppressing other competing metal ions. Among different anionic analytes, only fluoride shows a visually detectable exciting color change from pale yellow to pink with almost similar emission characteristics. PDP also demonstrates its importance in the fluorescent imaging of Hg²⁺ ions in human cancer cells.

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Mercury is well-documented as one of the most toxic metals, but came to attention only after the manifestation of the Minamata disaster in Japan. Mercury and compounds with its different states have high chemical activity. The most toxic species are the organomercuric compounds, particularly CH₃Hg⁺, which are soluble in fats, the lipid fraction of cell membranes and brain tissues. Because the Hg–C bond is covalent in nature alkyl mercury is retained in cells for a prolonged period of time, and can move through the placental wall and enter foetal tissues causing damage to unborn babies. It can cause adverse damage to the central nervous system and transmission of nerve impulses as well as various cognitive disorders in the human body.^1,2 Mercury occurs as a trace component in air, water or soil, and is generated by many sources such as gold production, coal plants, thermometers, barometers and mercury lamps.³ Hg²⁺, present in water and soil, is known to bio-accumulate in the human body and in other higher organisms through propagation in the food chain. The major intake occurs through our daily diet, such as through contaminated fish.^4,5 The high toxicity and serious effects on human health and the environment resulting from mercury contamination mean that it is necessary to develop a sensitive chemosensor for Hg²⁺ ions that will be capable of selectively detecting this ion in biological and environmental samples in the presence of other interfering cations.

The development of fluorescent chemosensors capable of selective interactions with simple anions has also been a dynamic area of research in supramolecular chemistry to investigate a variety functions like sensing and catalysis.^6–8 Among inorganic anions, fluoride, the smallest anion with high charge density, is an attractive target for sensor design owing to its role in chemical, environmental and biological processes like dental care⁹ and osteoporosis.¹⁰ Excess fluoride released from different sources has significant potential for ecological damage.¹¹ Thus the development of sensitive and selective sensors for fluoride is also an important area of continuing interest. To date most of the reported sensors for fluoride, which is a very strong hydrogen bond acceptor, are based upon either hydrogen bonding interactions with suitably pre-organized N–H, C–H or O–H groups, Lewis acid–base interactions, anion–π interactions or anion induced chemical reactions.^12–16

Presently atomic absorption spectrometry¹⁷ and inductively coupled plasma mass spectrometry¹⁸ are the most common methods for the detection of heavy metal ions. However these instrumentally intensive methods only measure the total metal ion content, and often require extensive sample preparation. Thus, a simple and inexpensive method that not only detects but also quantifies heavy metal ions is desirable for real-time monitoring of environmental, biological, and industrial samples.¹⁹ Among the various detection techniques, fluorescence chemosensors are the most convenient methods and convert the binding event into a detectable feedback signal. These involve the use of either a fluorescent/colorimetric group as a reporter functionality that responds through changes in its emission/optical spectral pattern.^20–23 In this regard, metal-selective fluorescent chemosensors serve as useful tools for the detection of metal ions and are being widely exploited to detect biologically or environmentally relevant metal ions.^24–30

For the optical detection of various analytes, the signaling could be UV-visible absorption and/or fluorescence spectral changes upon modulations in the photo induced electron transfer (PET), intramolecular charge transfer (ICT), metal to ligand charge transfer (MLCT), twisted intramolecular charge transfer (TICT), fluorescence resonance energy transfer (FRET), and excimer/exciplex formation. Recently among these sensing phenomena, excited state intramolecular proton transfer (ESIPT) has attracted special interest for designing molecular probes to sense biologically important species, due to its extensive applications for laser dyes,³¹ UV-photostabilizers,³² scintillators,³³ membrane,³⁴ and protein probes,³⁵ and as a potential component for photoswitches and organic LEDs.^36–38

A variety of molecules bearing an H-bond donor group, associated with a basic site (O, N) in the ground state having intramolecular H-bonding interactions, usually undergo an ESIPT process in which a rapid photoinduced proton transfer results. In the ESIPT system, protons in the excited state leave or join a molecule at a different rate to that in the ground state. The ESIPT process is easy to recognize in steady-state spectra, the absorbance is generally similar to that of the parent chromophore but the fluorescence is significantly different. In ESIPT processes large Stokes shifts are observed, a desirable optical response from any fluorescent probe as they become strategically more advantageous over normal probes by minimizing the error arising from physical or chemical fluctuations in the sample and experimental conditions.^39–41 While several anion selective probes have been developed, metal cation as well as anion detection based on modulation of ESIPT has not been much explored.^42–44

In this context, we report here the synthesis and fluoroionophoric properties of an ESIPT enabled phenanthroline diimino phenol (PDP) sensor which is aimed towards the selective recognition of Hg²⁺ ions in mixed aqueous solution along with a fluorescence bio-imaging capability, computational study and its sensitivity towards fluoride ions in acetonitrile medium. The PDP sensor was synthesized in one step by the conventional Schiff’s base condensation reaction of 2-hydroxy-5-methylbenzene-1,3-dicarbaldehyde (2)⁴⁵ with 1,10-phenanthroline-5-amine (3) in ethanol as a yellowish precipitate with good yield (Scheme 1). The yellow solid product was recrystallized in absolute ethanol, giving the PDP sensor in 62% yield. Its molecular structure and purity were established from spectroscopic studies like ¹H NMR, and MS spectra (Fig. S7 and S8, ESI†).

Scheme 1 Synthetic scheme for sensor 1.

The chemosensing abilities of PDP, within acetonitrile/aqueous (1 : 1) HEPES buffered (pH 7.4) solution (C = 2 × 10⁻⁵ M), towards various metal ions (C = 2 × 10⁻⁴ M) were studied by means of UV-vis as well as fluorescence spectroscopy. The UV-vis spectrum of the PDP shows a characteristic absorption band with a peak at 380 nm (Fig. 1). This absorption band, on gradual addition of Hg²⁺ as chloride salt solution (C = 2 × 10⁻⁴ M), slowly diminished accompanied with an isosbestic point at 418 nm, which indicates the formation of a new complex between the PDP and Hg²⁺ ions. But a more interesting outcome was observed when this PDP was excited at 380 nm during fluorescence titration.

Fig. 1 UV-vis absorption spectra of PDP (20 μM) in acetonitrile/aqueous (1 : 1) HEPES buffered (pH 7.4) solution upon addition of 0 to 2 equivalents of HgCl₂ (C = 2 × 10⁻⁴ M).

The PDP sensor contains an acidic –OH group at the 2-position with respect to the diimine moiety. The location of the acidic –OH group and =N– groups is such that intramolecular hydrogen bonding appears in a six membered cyclic manner which may encourage keto–enol tautomerization (Scheme 2) at its ground state. Absorption of radiation causes excitation of the ground state enol (E) to the excited state enol (E*). This E* form either emits radiation and returns to the ground state enol form (E) or undergoes ESIPT to form the excited keto (K*), then returns to keto (K) through emission of radiation at a higher wavelength.⁴⁶

Scheme 2 Probable mode of intramolecular proton transfer.

To elucidate the quantitative binding affinity of the PDP sensor, fluorescence titrations of PDP with Hg²⁺ were performed. Upon excitation at 380 nm in CH₃CN/aqueous (1 : 1, v/v) HEPES buffered solution (pH 7.4) (C = 2 × 10⁻⁵ M), the fluorescence spectrum of the PDP sensor exhibits dual emission peaks (Fig. 2). One weak band at 466 nm is supposed to be originated from excited enol (E*) and another emission at 617 nm is due to the excited state keto (K*) originating from the excited state enol (E*). Upon the gradual addition of up to 3 equivalents of Hg²⁺ into the solution of PDP, it was found that the emission at 466 nm remains almost unaffected whereas that at 617 nm is slightly red shifted to 635 nm with about six fold fluorescence enhancement. The red emission at 635 nm which is intensified on addition of Hg²⁺ indicates that the complexation between PDP and Hg²⁺ drives the equilibrium (Scheme 2) towards the right side, i.e. facilitating the formation and hence increasing population of the keto tautomer (K) of the PDP. Moreover, the Job’s plot, using the fluorescence titration method which exhibited a maximum at 0.5 mole fraction of Hg²⁺, indicates 1 : 1 stoichiometry between the Hg²⁺ and the PDP sensor (Fig. S1†). Thus, on the basis of the 1 : 1 stoichiometry and the fluorescence titration data, the association constant of sensor 1 for Hg²⁺ was calculated on the basis of the Benesi–Hildebrand plot⁴⁷ (Fig. S3†) and was found to be 7.5 × 10³ M⁻¹ with a detection limit of 4.9 μM using 20 μM of aqueous-PDP solution (Fig. S5†).

Fig. 2 (a) Fluorescence titration spectra (λ_ex = 380 nm) of the PDP sensor (20 μM) in CH₃CN/aqueous HEPES buffer (1 mM, pH 7.4; 1 : 1 v/v) upon incremental addition of 0 to 3 equiv. of Hg²⁺ (C = 2 × 10⁻⁴ M). (b) Fluorescence of PDP in the absence & presence of Hg²⁺).

One of the challenges for Hg²⁺ probes is to obtain systems that are selective over a wide range of potentially competing ions, such as Mn²⁺, Cr³⁺, Pb²⁺, Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺ and Cu²⁺ as their perchlorate, nitrate or chloride salts. The PDP showed almost negligible effect on the fluorescence behavior in the presence of these competing ions (up to 3 equivalents), which did not suppress its selectiveness towards the Hg²⁺ ion. Only Cr³⁺ and Cd²⁺ tried to interact with PDP, but could not succeed over Hg²⁺ to produce any effective interference (Fig. 3).

Fig. 3 Fluorescence titration spectra (λ_ex = 380 nm) of PDP (20 μM) upon addition of 3 equiv. of different metal ions, in CH₃CN/aqueous HEPES buffer (1 mM, pH 7.4; 1 : 1 v/v).

To further explore the utility of PDP as a fluorescence sensor, we have studied its anion sensing capability in pure acetonitrile through UV-vis and fluorescence titration methods. The UV-vis spectrum of PDP (C = 2 × 10⁻⁵ M in CH₃CN) is characterized by an intense band centered at 380 nm which is probably responsible for the light yellow color of the solution. It diminished gradually with increasing new absorption at 525 nm in a ratiometric manner, having one isosbestic point at 433 nm, on gradual addition of tetra-butyl ammonium fluoride (TBAF) solution (C = 1 × 10⁻³ M) from 0 to 1.5 equivalents after which it gets saturated. This ratiometric change of the absorption band from 380 to 525 nm is associated with an exciting visually detectable color change from light yellow to pink (Fig. 4).

Fig. 4 UV-vis absorption spectra of PDP (20 μM) in acetonitrile upon addition of 0 to 1.5 equiv. TBAF (C = 1 × 10⁻³ M). (Inset: visual color change of PDP in the absence & presence of Hg²⁺.)

The fluoroionophoric behavior of PDP towards fluoride ions was also investigated under a similar set of conditions as for the UV-vis titration. When excited at 380 nm PDP showed a weak dual emission peak centered at 480 and 615 nm due to its inherent ESIPT tendency. On gradual addition of the fluoride ion the emission band at 480 nm remained almost unaltered whereas the emission at 615 nm was red shifted to 653 nm with a remarkable intensity enhancement of about 20 fold after the addition of 0 to 1.5 equivalents of TBAF (Fig. 6).

Fig. 5 UV-vis spectra of PDP (20 μM) upon addition of 1.5 equiv. of different anions in CH₃CN.

Fig. 6 Fluorescence titration spectra (λ_ex = 380 nm) of the PDP sensor (20 μM) in CH₃CN upon incremental addition of 0 to 1.5 equiv. of TBAF.

The addition of other interfering anions (C = 1 × 10⁻³ M) like Br⁻, Cl⁻, and I⁻ as their tetrabutyl ammonium salts and PO₄³⁻, H₂PO₄⁻, NO₃⁻, NO₂⁻ and SO₃²⁻ as their sodium salts showed no significant change in the UV-visible or fluorescence spectra, only CH₃COO⁻ showed slight interference (Fig. 5 and 7). This affirms the selectiveness of PDP towards fluoride ions. Job plot (Fig. S2†) analysis shows 1 : 1 stoichiometry for complexation between the PDP and fluoride ion with an association constant of 5.7 × 10³ M⁻¹ (Fig. S4†). It was also found that a minimum of 12.7 μM of fluoride can be detected by using 20 μM of PDP solution fluorometrically (Fig. S6†).

Fig. 7 Fluorescence spectra of PDP (20 μM) upon addition of 1.5 equiv. of different anions in CH₃CN.

To study the interaction between F⁻ and the PDP sensor further, ¹H NMR titration experiments were carried out in CDCl₃. Fig. 8 shows the partial ¹H NMR spectra of PDP upon addition of TBAF. After addition of 1.5 equivalents of TBAF the –OH signal (H_a) at 13.56 ppm completely disappeared and a slight up field chemical shift of aldimine–CH=N protons and aromatic ring protons were found, owing to through bond delocalization, followed by a new signal appearing at 7.99 ppm (H_a′) which affirms the fluoride assisted proton transfer (–C=C–NH_a′) through six member cyclic intra-molecular hydrogen bonding to facilitate the equilibrium as shown in Scheme 2 towards the keto (K) form rather than the enol (E) form.

Fig. 8 Partial ¹H NMR spectra of PDP alone and in the presence of TBAF (1.5 equiv.).

The experimental results we have obtained so far reveal that only Hg²⁺ and F⁻ among different competing metal ions and anions, respectively, drive the equilibrium, as shown in Scheme 2, towards the keto tautomer. This may be due to the fact that the presence of fluoride ions makes the –OH proton (H_a) more facile towards ESIPT through H-bonding interaction.

To achieve some theoretical support, DFT calculations were performed using the Gaussian 03 (Revision B.04)⁴⁸ package while Gauss View was used for visualization of molecular orbitals. The keto and enol forms of PDP were optimized with the B3LYP⁴⁹ functional and 6-311+G(d,p) basis set with no symmetry constraints. For Hg²⁺ bound species, LANL2DZ for Hg and 6-31g*+ for the rest of the atoms were used for optimization. Molecular orbitals were analysed using the AOMix program.⁵⁰

The Time Dependent DFT (TD-DFT) calculations were performed in the same basis set using the CPCM model with a water and acetonitrile mixed solvent at a 1 : 1 ratio and it was found that the HOMO to LUMO electronic transition (49.28%) with f = 0.5641 at 371.18 nm is due to the enol form and that at 404.64 nm (66.36%) with f = 0.5796 is due to the keto form, the average of both these transitions is close to the experimentally observed transition centered at 380 nm.

In Hg²⁺ bound species the TD-DFT calculation showed that the transition at 455.10 nm is due to HOMO to LUMO (66.26%) with f = 0.6163 and the HOMO−1 to LUMO+1 (63.12%) with f = 0.2566 transition at 378.37 nm, respectively, and these match with the experimental finding of the gradual decrease in the absorption at 380 nm concomitant with the formation of newer absorption peaks. The enol form of PDP is more stable (19.0 kcal mol⁻¹) compared with its keto form due to the loss of conjugation in the associated benzene ring. The HOMO–LUMO orbitals of the enol and keto forms along with the optimized structure of PDP are shown in Fig. 9.

Fig. 9 HOMO–LUMO along with the optimized structure of PDP.

Thus in the presence of Hg²⁺, it is supposed that the size of Hg²⁺ mostly fits with the cavity available in the keto form (K) rather than the enol (E) form of PDP to form an energetically more stable PDP–Hg²⁺ complex and hence shifts the equilibrium towards the keto tautomer, which is also supported by the DFT calculation. Hence the population of the keto form is favored, enhancing the intensity of the emission at 635 nm in the fluorescence spectra in the presence of Hg²⁺. The phenanthroline nitrogens do not involve in direct coordination with the Hg²⁺ here, but probably favor the acidity of the phenolic hydrogen towards ESIPT. The probable role of phenanthroline nitrogen is to increase the acidity of the phenolic hydrogen by the electron withdrawing resonance of one nitrogen (which is conjugated with the phenolic OH) of one pyridine ring of each phenanthroline ring in PDP.

Cell imaging

A human lung cancer cell line (NCI-H460) was used for testing fluorescence of PDP with HgCl₂. NCI-H460 cells were grown in RPMI medium supplemented with heat-inactivated FCS (10%; v/v), streptomycin (100 U ml⁻¹) and penicillin (0.1 mg ml⁻¹), in a humidified atmosphere of 5% CO₂ at 37 °C. Cells were trypsinized and seeded in 6 well plates at a density of 1 × 10⁵ cells per cm³. After 24 h of cell seeding, cells were treated with different concentrations of PDP (2 × 10⁻⁶ M) and HgCl₂ (0.2 × 10⁻³ M, 0.5 × 10⁻³ M, 1 × 10⁻² M, 1.5 × 10⁻² M, 2 × 10⁻² M). After 3 to 4 h cells were washed with phosphate buffer saline and fresh medium was added to each well, then pictures were taken using a fluorescence inverted microscope (DeWinter Victory-FL using a UV filter).

The cell imaging study showed (Fig. 10) that cells treated with PDP alone showed no fluorescence. But cells treated with PDP and different concentrations of HgCl₂ exhibit red fluorescence.

Fig. 10 Red fluorescence images of NCI-H460 cells in the presence of both PDP (2 × 10⁻³ M) and HgCl₂ (1 × 10⁻² M) (b). Corresponding bright field image (a) and merge image (c) of the cells. The photographs were taken with a DeWinter Victory-FL fluorescence inverted microscope using DeWinter Biowizard software v 4.4 at 40×.

In conclusion, the easily synthesizable chemosensor PDP displayed effectiveness towards selective and sensitive sensing not only for Hg(II) in mixed aqueous medium, but also visually detects fluoride in acetonitrile medium with exciting ‘turn on’ red fluorescence based upon excited state intramolecular proton transfer (ESIPT), which is supported by DFT study and ¹H NMR titration. To the best of our knowledge, anions are common for promoting the keto–enol tautomerism through ESIPT but here Hg²⁺ is a special case which directly controls the extent of tautomerism, favoring the keto form through ESIPT. In view of the high selectivity, sensitivity and quick synthetic accessibility, PDP is potentially useful as a Hg²⁺ sensor in aqueous medium and a fluoride sensor in non-aqueous medium. Moreover, the capability of fluorescence imaging for Hg²⁺ in living cells shows PDP has encouraging chemical and spectroscopic properties that meet the criteria for further biological applications.

Acknowledgements

Authors thank the DST and CSIR (Govt. of India) for financial support.

Notes and references

1. C. M. L. Carvalho, E.-H. Chew, S. I. Hashemy, J. Lu and A. Holmgren, J. Biol. Chem., 2008, 283, 11913.
2. T. W. Clarkson, L. Magos and G. J. Myers, N. Engl. J. Med., 2003, 349, 1731.
3. F. Di Natale, A. Lancia, A. Molino, M. Di Natale, D. Karatza and D. Musmarra, J. Hazard. Mater., 2006, 132, 220.
4. W. F. Fitzgerald, C. H. Lamborg and C. R. Hammerschmidt, Chem. Rev., 2007, 107, 641.
5. M. Nendza, T. Herbst, C. Kussatz and A. Gies, Chemosphere, 1997, 35, 1875.
6. J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspective, VCH, Weinheim, 1995.
7. J. W. Steed and J. L. Atwood, Supramolecular Chemistry: A Concise Introduction, Wiley, Chichester, 2000.
8. L. E. Santos-Figueroa, M. E. Moragues, E. Climent, A. Agostini, R. Martinez-Manez and F. Sancenon, Chem. Soc. Rev., 2013, 42, 3489.
9. M. Kleerekoper, Endocrinol. Metab. Clin. North Am., 1998, 27, 441.
10. E. Gazzano, L. Bergandi, C. Riganti, E. Aldieri, S. Doublier, C. Costamagna, A. Bosia and D. Ghigo, Curr. Med. Chem., 2010, 17, 2431.
11. H. Lu, Q. Wang, Z. Li, G. Lai, J. Jiang and Z. Shen, Org. Biomol. Chem., 2011, 9, 4558.
12. C. R. Wade, A. E. J. Broomsgrove, S. Aldridgeand and F. P. Gabbaï, Chem. Rev., 2010, 110, 3958.
13. A. N. Swinburne, M. J. Paterson, A. Beeby and J. W. Steed, Chem.–Eur. J., 2010, 16, 2714.
14. E. B. Veale, G. M. Tocci, F. M. Pfeffer, P. E. Krugera and T. Gunnlaugsson, Org. Biomol. Chem., 2009, 7, 3447.
15. K. M. Mahoney, P. P. Goswami and A. H. J. Winter, Organomet. Chem., 2013, 78, 702.
16. L. Fu, F.-L. Jiang, D. Fortin, P. D. Harvey and Y. Liu, Chem. Commun., 2011, 47, 5503.
17. V. Iyengar and J. Wolttlez, Clin. Chem., 1988, 34, 474.
18. A. T. Townsend, K. A. Miller, S. Mc and S. Aldous, J. Anal. At. Spectrom., 1998, 13, 1213.
19. H. N. Kim, W. X. Ren, J. S. Kim and J. Yoon, Chem. Soc. Rev., 2012, 41, 3210.
20. Y. Zhou, Z. Xu and J. Yoon, Chem. Soc. Rev., 2011, 40, 2222.
21. D. T. Quang and J. S. Kim, Chem. Rev., 2010, 110, 6280.
22. X. Chen, Y. Zhou, X. Peng and J. Yoon, Chem. Soc. Rev., 2010, 39, 2120.
23. J. F. Zhang, Y. Zhou, J. Yoon and J. S. Kim, Chem. Soc. Rev., 2011, 40, 3416.
24. (a) S. Goswami, A. K. Das and S. Maity, Dalton Trans., 2013, 42, 16259–16263; (b) A. P. Singh, D. P. Murale, Y. Ha, H. Liew, K. M. Lee, A. Segev, Y.-H. Suh and D. G. Churchill, Dalton Trans., 2013, 42, 3285; (c) A. Pariyar, S. Bose, S. Singh Chhetri, A. N. Biswas and P. Bandyopadhyay, Dalton Trans., 2012, 41, 3826.
25. L. Neupane, J. Y. Park, J. H. Park and K.-H. Lee, Org. Lett., 2013, 15, 254.
26. S. Khatua and M. Schmittel, Org. Lett., 2013, 15, 4422.
27. Z. Yang, l. Hao, B. Yin, M. She, M. Obst, A. Kappler and J. Li, Org. Lett., 2013, 15, 4334.
28. X. Zhang, H. Zhao, X. Cao, N. Feng, D. Tian and H. Li, Org. Biomol. Chem., 2013, 11, 8262.
29. X. Zhang, X.-J. Huang and Z.-J. Zhu, RSC Adv., 2013, 3, 24891.
30. V. Bhalla, Roopa, M. Kumar, P. Raj Sharma and T. Kaur, Dalton Trans., 2013, 42, 15063.
31. K. Sakai, T. Tsuzuki, Y. Itoh, M. Ichikawa and Y. Taniguchi, Appl. Phys. Lett., 2005, 86, 081103.
32. J. Catalan, F. Fabero, R. M. Claramunt, M. D. S. Maria, M. D. C. Focesfoces, F. H. Cano, M. Martinezripoll, J. Elguero and R. Sastre, J. Am. Chem. Soc., 1992, 114, 5039.
33. J. M. Kauffman, Radiat. Phys. Chem., 1993, 41, 365.
34. V. V. Shynkar, A. S. Klymchenko, C. Kunzelmann, G. Duportail, C. D. Muller, A. P. Demchenko, J. M. Freyssinet and Y. Mely, J. Am. Chem. Soc., 2007, 129, 2187.
35. M. B. Cardoso, D. Samios, N. P. Silveira, F. S. Rodembusch and V. Stefani, Photochem. Photobiol. Sci., 2007, 6, 99.
36. T. Mutai, H. Tomoda, T. Ohkawa, Y. Yabe and K. Araki, Angew. Chem., 2008, 120, 9664.
37. S. Park, J. Seo, S. H. Kim and S. Y. Park, Adv. Funct. Mater., 2008, 18, 726.
38. S.-J. Lim, J. Seo and S. Y. Park, J. Am. Chem. Soc., 2006, 128, 14542.
39. M. Taki, J. L. Wolford and T. V. O’Halloran, J. Am. Chem. Soc., 2004, 126, 712.
40. J. Seo, S. Kim and S. Y. Park, J. Am. Chem. Soc., 2004, 126, 11154.
41. W. Jiasheng, L. Weimin, G. Jiechao, Z. Hongyan and W. Pengfei, Chem. Soc. Rev., 2011, 40, 3483.
42. S. Goswami, S. Maity, A. K. Das, A. C. Maity, T. K. Mandal and S. Samanta, Tetrahedron Lett., 2013, 54, 5232.
43. Z. Xu, L. Xu, J. Zhou, Y. Xu, W. Zhu and X. Qian, Chem. Commun., 2012, 48, 10871.
44. R. Hu, J. Feng, D. H. Hu, S. Q. Wang, S. Y. Li, Y. Li and G. Q. Yang, Angew. Chem., Int. Ed., 2010, 49, 4915.
45. R. R. Gagne, L. C. Spiro, J. T. Smith, A. C. Hamann, R. W. Thies and K. A. Shiemke, J. Am. Chem. Soc., 1981, 103, 4073.
46. K. Dlng, J. S. Courtney, J. A. Strandjord, S. Flom, D. Frledrich and F. P. Barbara, J. Phys. Chem., 1983, 87, 1184.
47. H. Benesi and H. Hildebrand, J. Am. Chem. Soc., 1949, 71, 2703.
48. M. J. Frisch, et al., Gaussian, Inc., Pittsburgh, PA, 2003.
49. (a) A. D. J. Becke, Chem. Phys., 1993, 98, 5648; (b) C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter Mater. Phys., 1988, 37, 785.
50. (a) S. I. Gorelsky, AOMix: Program for Molecular Orbital Analysis, University of Ottawa, Ottawa, 2007, http://www.sg-chem.net/; (b) S. I. Gorelsky and A. B. P. J. Lever, Organomet. Chem., 2001, 635, 187.

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Footnote

† Electronic supplementary information (ESI) available: Details of synthetic procedure and spectral data. See DOI: 10.1039/c4ra07838a

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