(S)-BINOL-based boronic ester fluorescence sensors for enantioselective recognition of α-phenylethylamine and phenylglycinol

Abstract

Four chiral fluorescence sensors (S)-L1–4 incorporating boronic ester and (S)-1,1′-bi-2-naphthol (BINOL) moieties were synthesized and developed for the enantioselective recognition of α-phenylethylamine and phenylglycinol enantiomers. The sensor (S)-L1 shows an obvious fluorescence quenching “turn-off” response towards enantiomers of both α-phenylethylamine and phenylglycinol. Interestingly, the sensor (S)-L2 can exhibit remarkable fluorescent enhancement “turn-on” response behavior towards (S)-α-phenylethylamine, but shows no response towards phenylglycinol enantiomers. The Stern–Volmer constant (K_sv) values of (S)-L1 are 0.63 × 10³ L mol⁻¹ and 4.48 × 10³ L mol⁻¹ for (L)- and (D)-phenylglycinol, respectively, and the value of the enantiomeric fluorescence difference ratio (ef) of (S)-L2 is 4.6 for α-phenylethylamine, demonstrating that (S)-L2 could be used as a fluorescence sensor for simple and direct visual discrimination of organic molecule enantiomers. On the contrary, no fluorescence enantioselective recognition response could be observed when using the (S)-BINOL-based boronic ester sensors (S)-L3 and (S)-L4 incorporating bigger naphthyl or 8-methoxyquinolinyl substituent groups.

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Introduction

The enantioselective recognition of chiral molecules is one of the most important fundamental features of various systems in nature and of biological processes.¹ Significant attention has been paid to fluorescence sensors for enantioselective recognition due to their high selectivity and their potential applications within biological, analytical and clinical biochemistry fields.² These fluorescence sensors can effectively provide a real-time analytical tool for the direct and simple detection of chiral enantiomers.³ Recently, many fluorescence sensors based on various chiral centers and fluorophores have been developed for the enantioselective recognition of chiral organic molecules, such as amines, amino alcohols, carboxylic acids and amino acids.⁴ However, thus far there have been no reports on using (S)-BINOL-based boronic ester molecules as fluorescence sensors for direct visual discrimination of chiral enantiomers at low concentrations.

Chiral amines and amino alcohols are important building blocks for the preparation of a variety of biologically active molecules and pharmaceutical compounds. In fact, the pharmaceutical activity of chiral molecules often resides in only one enantiomer, and the problem of undesirable effects being elicited by inactive enantiomers is now widely recognized. Much work has been devoted to developing rapid and highly selective methods for fluorescent enantioselective recognition of amines and amino alcohols.⁵ Recently, Pu’s group reported an (S)-BINOL-based fluorescence sensor for the simultaneous determination of both concentration and enantiomeric composition.⁶ Although considerable progress has been made with respect to the enantioselective recognition of amines and amino alcohols, the search for highly sensitive and effective sensors still remains of great interest.

In past years, the development of enantioselective fluorescence sensors has been mainly focused on a variety of optically active 1,1′-bi-2-naphthol derivatives, chiral macrocycles, dendrimers and oligomers,⁷ and the design of a novel fluorescence sensor is urgently required. A chiral fluorescence boronic acid sensor has attracted much attention for several decades.⁸ Recently, Zhao's group developed a series of carbazole-based bisboronic acids as enantioselective fluorescence sensors for mandelic acid and tartaric acid.⁹ On the other hand, although there have been some reports on boronic moiety-based enantioselective sensors incorporating BINOL, anthracene and carbazole systems,¹⁰ most of these were focused on the recognition of hydroxyl acids and saccharides, and there have been no examples on the recognition of amines or amino alcohols. In this paper, we describe four (S)-BINOL-based boronic ester ligands, (S)-L1–4, as fluorescence sensors for the enantioselective recognition of α-phenylethylamine and phenylglycinol. More importantly, (S)-L2 can exhibit a bright blue fluorescence color change in the presence of α-phenylethylamine under a commercially available UV lamp, which can be clearly observed by the naked eye.

Results and discussion

The synthetic procedures for the four (S)-BINOL-based boronic ester ligands (S)-L1–4 are shown in Scheme 1. (S)-2,2′-Bis(methoxymethoxy)-1,1′-binaphthol was synthesized according to the literature.¹¹ Compound 2 was firstly treated with n-BuLi, followed by substitution of B(OMe)₃ to give the MOM-protected intermediate, and was then treated with HCl (1 mol L⁻¹) to afford compound 3 as a white powder. 3 was dissolved in THF and treated with H₂O₂ to afford pure compound 4 as a purple powder after purification by chromatography.¹² (S)-L1 could be obtained by the reaction of 3 and 1,2-dihydroxybenzene under reflux using toluene as a solvent, in a total yield of 36.5%, while (S)-L2 could be synthesized by reflux of 4 and phenylboronic acid overnight using toluene as a solvent, in a total yield of 30.5%. (S)-L3 could be obtained by reflux of 3 and 2,3-dihydroxynaphthalene, in a total yield of 36.1%. (S)-L4 could be obtained by reflux of 4 and 8-methoxyquinolin-5-boronic acid, in a total yield of 30.7%.

Scheme 1 Synthetic procedure for the chiral ligands (S)-L1–4.

The enantioselective fluorescence behavior of the chiral sensor (S)-L1 towards R/S-α-phenylethylamine and L/D-phenylglycinol was investigated. As is shown in Fig. 1a, the fluorescence intensity of (S)-L1 (1.0 × 10⁻⁵ mol L⁻¹ in toluene) decreased to about 44% of its initial intensity upon the addition of (R)-α-phenylethylamine (1.0 × 10⁻² mol L⁻¹ in THF), and to about 30% of its initial intensity upon the addition of (S)-α-phenylethylamine (1.0 × 10⁻² mol L⁻¹ in THF). The fluorescence quenching efficiency is related to the Stern–Volmer constant (K_sv), which is determined by monitoring changes in fluorescence intensity and applying measurable values to the Stern–Volmer equation: I₀/I = 1 + K_sv[Q]. Here, I₀ and I represent the fluorescence intensity of (S)-L1 in the absence and in the presence of the guest molecule, and [Q] is the concentration of the guest molecule.¹³ As shown in Fig. 2a, at a guest molecule concentration range of 1 × 10⁻⁴ to 3 × 10⁻⁴ mol L⁻¹, the Stern–Volmer constant (K_sv) is 2.71 × 10³ L mol⁻¹ for (R)-α-phenylethylamine, and 3.96 × 10³ L mol⁻¹ for (S)-α-phenylethylamine, indicating that (S)-L1 shows nearly equal fluorescence quenching behavior towards the enantiomers of α-phenylethylamine (K^S_sv/K^R_sv = 1.46). It was also found that the fluorescence intensity of (S)-L1 was slightly quenched upon the addition of (L)-phenylglycinol, but (D)-phenylglycinol had a much greater fluorescence quenching effect on (S)-L1 under the same conditions (Fig. 1b). At a guest molecule concentration range of 1 × 10⁻⁴ to 3 × 10⁻⁴ mol L⁻¹, the K_sv is 0.63 × 10³ L mol⁻¹ for (L)-phenylglycinol, and 4.48 × 10³ L mol⁻¹ for (D)-phenylglycinol (Fig. 2b). This indicates that (S)-L1 exhibits remarkable fluorescence enantioselectivity towards (D)-phenylglycinol (K^D_sv/K^L_sv = 7.11).

Fig. 1 Fluorescence emission spectra of (S)-L1 (1.0 × 10⁻⁵ mol L⁻¹ in toluene) towards: (a) (R)- and (S)-α-phenylethylamine (1.0 × 10⁻² mol L⁻¹ in THF) at a 1 : 80 molar ratio; (b) (L)- and (D)-phenylglycinol (1.0 × 10⁻² mol L⁻¹ in THF) at a 1 : 80 molar ratio. λ_ex = 342 nm, λ_em = 398 nm.

Fig. 2 Stern–Volmer plots of (S)-L1 (1.0 × 10⁻⁵ mol L⁻¹ in toluene) in the presence of (a) (R)- and (S)-α-phenylethylamine, and (b) (L)- and (D)-phenylglycinol.

The fluorescence response behavior of (S)-L2 with (R)/(S)-α-phenylethylamine was also investigated by fluorescence spectroscopy. Fig. 3a shows the fluorescence spectra of (S)-L2 upon the addition of (R)/(S)-α-phenylethylamine at 1 : 70 molar ratios. Interestingly, we found that the guest molecule (R)-α-phenylethylamine had less of a fluorescence enhancement effect on (S)-L2, while (S)-α-phenylethylamine exerted a much greater enhancement effect under the same determined conditions. The enantioselective effect of the fluorescence sensor could be measured by the value of the enantiomeric fluorescence difference ratio, ef [ef = (I_S − I₀)/(I_R − I₀)]. Here, I_R and I_S represent the fluorescence intensity in the presence of the (R)- or (S)- guest molecules, respectively.¹⁴ According to the fluorescence spectra of (S)-L2 towards (R)- and (S)-α-phenylethylamine, the ef value is 4.6, which indicates that the chiral ligand (S)-L2 exhibits excellent enantioselective fluorescence behavior towards (S)-α-phenylethylamine. This could be attributed to an inherent chiral recognition process based on steric repulsion between the (S)-BINOL moiety and (S)-phenylethylamine. The chirality of (S)-BINOL is derived from the restricted rotation of the two naphthalene rings. The rigid structure and C₂ symmetry of the chiral binaphthyl molecules play an important role in the inherently chiral induction. The dihedral angle between the two naphthalene rings, which is controlled by the substituted groups at the minor-groove 3,3′-position of BINOL, ranges from 60 to 120°.¹⁵ However, in our case, the dihedral angle is controlled by the boronic ester and benzene moieties at the 2,2′,3,3′-positions, and thus, the building block of the (S)-L2 receptor is suited to the formation of a more stable S–S complex compared to an S–R diastereomeric complex.

Fig. 3 (a) Fluorescence emission spectra of (S)-L2 (1.0 × 10⁻⁵ mol L⁻¹ in toluene) towards (R)- and (S)-α-phenylethylamine (1.0 × 10⁻² mol L⁻¹ in THF) at 1 : 70 molar ratios. λ_ex = 328 nm, λ_em = 393 nm. (b) Fluorescence enhancement of (S)-L2 (I/I₀) vs. increasing molar ratios of R- or S-α-phenylethylamine at λ_ex = 328 nm. Inset: photograph of (1) (S)-L2, (2) (S)-L2 with (R)-α-phenylethylamine, and (3) (S)-L2 with (S)-α-phenylethylamine at 1 : 70 molar ratios (1.0 × 10⁻⁴ mol L⁻¹ in toluene).

The fluorescence enhancement behavior of α-phenylethylamine on (S)-L2 was further investigated at a much broader concentration range. As shown in Fig. 3b, when (S)-L2 was treated with excess amounts of (R)- or (S)-α-phenylethylamine, the fluorescence intensity of (S)-L2 was gradually enhanced upon addition of the guest molecule in the concentration range of 1 : 10 to 1 : 70 molar ratios. Maximum fluorescence enhancement was 3.6 fold for (R)-α-phenylethylamine, and 10.8 fold for (S)-α-phenylethylamine. We also investigated the fluorescence behavior of (S)-L1 and (S)-L2 towards other enantiomeric guest molecules, including tartaric acid, phenylalaninol, valinol, and 1,2-diamino hexane, but no enantioselective recognition effect could be detected (see the ESI, Fig. S11–S19†).

In order to further investigate the mechanism of the enantioselective recognition process, ¹H NMR titration of (S)-L1 and (S)-L2 was carried out, as shown in the ESI, Fig. S33 and S34.† The H signal of the amino group of the guest compound was shifted upfield about 0.08–0.1 ppm upon addition of the host molecule, which can be attributed to the B atom of the host molecule acting as an electron acceptor (or Lewis acid), and the amino group of the guest compound acting as an electron donor (or Lewis base) to form a host–guest complex in an enhancement enantioselective recognition process. Herein, we further investigated the sensors (S)-L3 and (S)-L4 as fluorescence sensors for the enantioselective recognition of enantiomeric guest molecules under the same conditions. As is evident from Fig. 4, no enantioselective recognition effect could be observed for either (S)-L3 or (S)-L4. As demonstrated previously, a rigid structure is important for the chiral recognition process. In the case of (S)-L3 and (S)-L4, the dihedral angel is controlled by naphthyl and quinolinyl moieties respectively, which is different to (S)-L1 and (S)-L2. Consequently, the building blocks of the chiral receptor are no longer conformationally suited to the formation of an S-L or S-D diastereomeric complex, and no enantioselectivity could be observed.

Fig. 4 Fluorescence emission spectra of (a) (S)-L3 (1.0 × 10⁻⁵ mol L⁻¹ in toluene, λ_ex = 342 nm, λ_em = 398 nm), and (b) (S)-L4 (1.0 × 10⁻⁵ mol L⁻¹ in toluene, λ_ex = 424 nm, λ_em = 497 nm) towards (L)- and (D)-phenylglycinol (1.0 × 10⁻² mol L⁻¹ in THF) at 1 : 100 molar ratios.

The stoichiometries and association constants for (S)-L2 (the host) and α-phenylethylamine were also determined and calculated using the Benesi–Hildebrand method.¹⁶ Assuming that the guest molecule forms an n:1 inclusion complex with the host, the stoichiometries and association constants can be calculated by using nonlinear fitting (eqn (1)). Here, K is the association constant, I represents the fluorescence intensity and C_G is the concentration of the guest molecule.

In the case where a 1 : 1 stoichiometry is determined, eqn (2) is applicable and is more accurate for the calculation of the association constant than eqn (1). Here, C_H is the concentration of the host, and other symbols have the same meaning as for eqn (1).

Stoichiometric, association constant and response selectivity (rs) data are listed in Table 1. The host compound (S)-L2 formed a 1 : 1 stoichiometric complex with (R)/(S)-α-phenylethylamine in toluene, as shown in the ESI, Fig. S6 and S9.† The value of rs could reach a maximum of 130.18, demonstrating that (S)-L2 shows excellent enantioselectivity towards (S)-α-phenylethylamine.

Table 1 Association constants and enantioselectivity of (S)-L2 towards α-phenylethylamine

Host	Guest	Conformation	n	K (L mol^−1)	KS/KR	rs^a
a rs = response selectivity; rs = (KSFS)/(KRFR), where FR or S refers to the maximum fluorescence enhancement by the (R)- or (S)-enantiomer.
(S)-L2	Phenylethylamine	R	1.01 ± 0.05	(1.02 ± 0.02) × 10^3	28.33	130.18
		S	1.00 ± 0.03	(2.89 ± 0.02) × 10^4

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Conclusions

In summary, (S)-BINOL-based boronic ester ligands were developed for enantioselective fluorescence sensors. Most importantly, (S)-L2 can exhibit a highly selective and sensitive enantioselective “turn-on” fluorescence response towards (S)-α-phenylethylamine for simple and direct visual discrimination at low concentrations.

Experimental section

General methods

NMR spectra were recorded on a Bruker-300 spectrometer, 300 MHz for ¹H NMR and 75 MHz for ¹³C NMR, and are reported as parts per million (ppm) from the internal standard TMS. Fluorescence spectra were obtained from an RF-5310 PC spectrometer. C, H, and N elemental analyses were performed on an Elementar Vario MICRO analyzer. Specific rotation was determined using a Rudolph Research Analytical Autopol I. All solvents and reagents were commercially available A. R. grade. All solvents were dried using standard procedures immediately before use.

Synthesis

2,2′-Dihydroxy-1,1′-binaphthyl-3,3′-diboronic acid (3). 2,2′-Bis(methoxymethoxy)-1,1′-binaphthol (2) (4.25 g, 11.3 mmol) was dissolved in anhydrous THF, and n-BuLi (15.0 mL, 2.5 mol L⁻¹ in hexane, 37.5 mmol) was added by syringe injection at −78 °C under a N₂ atmosphere. After the mixture was stirred at room temperature for 1 h, trimethoxyborane (7.6 mL, 67.8 mmol) was added by syringe injection at −78 °C under a N₂ atmosphere. The reaction mixture was gradually warmed to room temperature and stirred overnight. The reaction was quenched by water at 0 °C, and extraction was carried out using ethyl acetate. The combined organic layer was washed with brine and dried over anhydrous Na₂SO₄. After removal of the solvent under reduced pressure, the crude product was purified by chromatography (hexane–ethyl acetate = 10/1) to afford the intermediate. This intermediate was dissolved in HCl (1 mol L⁻¹) and stirred overnight to afford 3 as a white precipitate, in a 36.8% yield (1.56 g). [α]²⁵_D = +124.0° (c = 0.2, THF). ¹H NMR (300 MHz, d₆-acetone): δ 8.53 (s, 1H), 7.91 (d, 1H, J = 12 Hz), 7.26 (m, 2H), 7.04 ppm (q, 1H, J = 12, 3 Hz). ¹³C NMR (75 MHz, d₆-DMSO): δ 97.13, 107.96, 109.36, 110.98, 114.72, 132.01, 151.73, 152.99, 161.05 ppm. Elemental analysis calcd (%) for C₂₀H₁₆B₂O₆: C 64.24, H 4.31; found: C 64.19, H 4.88.

3,3′-Dihydroxy-2,2′-binaphthol (4). Compound 3 (0.5 g, 1.3 mmol) was dissolved in THF (50 mL), and 30% aq. H₂O₂ (1 mL) was slowly added under an ice-water bath, and the mixture was refluxed overnight. After the mixture was allowed to come to room temperature, the reaction was quenched by Na₂SO₃ (5 mL, aq.) and extraction was carried out using ethyl acetate. The combined organic layer was washed with brine and dried over anhydrous Na₂SO₄. The solvent was removed under reduced pressure, and the residue was purified by chromatography on silica gel (CHCl₃–MeOH = 10/1) to give 4 as a pale purple powder, in an 83% yield (0.35 g). ¹H NMR (300 MHz, d₆-DMSO): δ 10.05 (s, 1H), 8.39 (s, 1H), 7.65 (d, 1H, J = 9 Hz), 7.24 (s, 1H), 7.16 (dt, 1H, J = 9, 1 Hz), 6.96 (dt, 1H, J = 9, 1 Hz), 6.81 ppm (d, 1H, J = 9 Hz).

Synthesis of (S)-L1. Compound 3 (100 mg, 0.27 mmol) and 1,2-dihydroxybenzene (54.1 mg, 0.54 mmol) were dissolved in toluene (30 mL) at 50 °C. Anhydrous Na₂SO₄ (ca. 3.0 g) was added, and the mixture was refluxed overnight. After cooling to room temperature, the mixture was filtered, the solvent was evaporated under reduced pressure, and (S)-L1 was obtained as a white solid in a nearly 100% yield (138.1 mg). [α]²⁵_D = +179.2° (c = 0.2, THF). ¹H NMR (300 MHz, d₆-acetone): δ 8.53 (s, 1H), 7.91 (d, 1H, J = 9 Hz), 7.27 (m, 2H), 7.04 (d, 1H, J = 9 Hz), 6.82 (q, 2H, J = 3 Hz), 6.67 ppm (q, 2H, J = 3 Hz). ¹³C NMR (75 MHz, d₆-DMSO): δ 107.96, 114.72, 124.50, 126.14, 126.34, 126.47, 127.40, 128.39, 128.70, 132.01, 151.73, 154.09, 156.03 ppm. Elemental analysis calcd (%) for C₃₂H₂₀B₂O₆: C 73.61, H 3.86; found: C 73.63, H 3.90. ESI-MS ([C₃₂H₂₀B₂O₆ − H]⁻) calcd 521.14; found 521.10.

Synthesis of (S)-L2. Compound 4 (100 mg, 0.31 mmol) and phenylboronic acid (75.6 mg, 0.62 mmol) were dissolved in toluene (30 mL) at 50 °C. Anhydrous Na₂SO₄ (ca. 3.0 g) was added, and the mixture was refluxed overnight. After cooling to room temperature, the mixture was filtered, the solvent was evaporated under reduced pressure, and (S)-L2 was obtained as a white solid in a nearly 100% yield (150.1 mg). [α]²⁵_D = +69.4° (c = 0.2, THF). ¹H NMR (300 MHz, d₆-acetone): δ 7.87 (d, 2H, J = 12 Hz), 7.69 (d, 1H, J = 12 Hz), 7.36 (m, 4H), 7.22 (t, 1H, J = 9 Hz), 7.02 ppm (m, 2H). ¹³C NMR (75 MHz, d₆-DMSO): δ 98.87, 107.82, 109.17, 111.53, 114.21, 118.86, 121.22, 124.67, 127.79, 128.55, 132.10, 153.35, 160.16, 166.60 ppm. Elemental analysis calcd (%) for C₃₀H₂₀B₂O₄: C 78.42, H 4.11; found: C 78.39, H 4.10. ESI-MS ([C₃₂H₂₀B₂O₄ − H]⁻) calcd 489.15; found 489.10.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21074054, 21172106, 51173078) and the National Basic Research Program of China (2010CB923303).

Notes and references

1. (a) S. Garten, P. U. Biedermann, S. Topiol and I. Agranat, Chirality, 2010, 22, 662; (b) L. Pu, Acc. Chem. Res., 2012, 45, 150; (c) J. I. Yeh, B. Shivachev, S. Rapireddy, M. J. Crawfore, R. R. Gil, S. C. Du, M. Madrid and D. H. Ly, J. Am. Chem. Soc., 2010, 132, 10717; (d) D. K. Bwambok, S. K. Challa, M. Lowry and I. M. Warner, Anal. Chem., 2010, 82, 5028; (e) K. Micoine, B. Hasenknopf, S. Thorimbert, E. Lacote and M. Malacria, Angew. Chem., Int. Ed., 2009, 48, 3466; (f) A. M. Kelly, Y. Perez-Fuertes, J. S. Fossey, S. L. Yeste, S. D. Bull and T. D. James, Nat. Protoc., 2008, 3, 215.
2. (a) G. A. Hembury, V. V. Borovkov and Y. Inoue, Chem. Rev., 2008, 108, 1; (b) L. Pu, Chem. Rev., 2004, 104, 1687; (c) X. Mei and C. Wolf, Chem. Commun., 2004, 2078; (d) R. Al, F. Tfibel, F. Merola and P. Pernot, J. Chem. Soc., Perkin Trans. 1, 1999, 2, 341; (e) Y. Xu and M. E. McCarroll, J. Phys. Chem. B, 2005, 109, 8144.
3. (a) Z. B. Li, J. Lin, Y. C. Qin and L. Pu, Org. Lett., 2005, 7, 3441; (b) S. S. Yu and L. Pu, J. Am. Chem. Soc., 2010, 132, 17698.
4. (a) S. S. Yu, A. M. Deberardinis, M. Turlington and L. Pu, J. Org. Chem., 2011, 76, 2814; (b) Z. Li, J. Lin, M. Sabat, M. Hyacinth and L. Pu, J. Org. Chem., 2007, 72, 4905; (c) S. Shirakawa, A. Moriyama and S. Shimizu, Org. Lett., 2007, 9, 3117; (d) H. L. Liu, H. P. Zhu, X. L. Hou and L. Pu, Org. Lett., 2010, 12, 4172.
5. (a) G. Qing, T. Sun, Y. He, F. Wang and Z. Chen, Tetrahedron: Asymmetry, 2009, 20, 575; (b) K. M. Kim, H. Park, H. J. Kim, J. Chin and W. Nam, Org. Lett., 2005, 7, 3525; (c) X. He, Q. Zhang, X. H. Liu, L. L. Lin and X. M. Feng, Chem. Commun., 2011, 47, 11641; (d) J. M. Jiao, X. H. Liu, X. R. Mao, J. F. Li, X. Y. Cheng and C. J. Zhu, New J. Chem., 2013, 37, 317.
6. S. S. Yu, W. Plunkett, M. Kim and L. Pu, J. Am. Chem. Soc., 2012, 134(50), 20282.
7. (a) R. Corradini, C. Paganuzzi, R. Marchelli, S. Pagliari, S. Sforze, A. Dossena, G. Galavernaa and A. Duchateau, J. Mater. Chem., 2005, 15, 2741; (b) S. P. Upadhyay, R. R. S. Pissurlenkar, E. C. Coutinho and A. V. Karnik, J. Org. Chem., 2007, 72, 5709; (c) Y. K. Kim, H. N. Lee, N. J. Singh, H. J. Choi, J. Y. Xue, K. S. Kim, J. Yoon and M. H. Hyun, J. Org. Chem., 2008, 73, 301; (d) H. L. Liu, Q. Peng, Y. D. Wu, D. Chen, X. L. Hou and L. Pu, Angew. Chem., Int. Ed., 2010, 49, 602.
8. (a) S. D. Bull, M. G. Davidson, J. M. H. Van Den Elsen, J. S. Fossey, A. Toby, A. Jenkins, Y. B. Jiang, Y. J. Kubo, F. Marken, K. Sakurai, J. Z. Zhao and T. D. James, Acc. Chem. Res., 2013, 46(2), 312; (b) Z. Q. Guo, I. Shin and J. Yoon, Chem. Commun., 2012, 48, 5956.
9. (a) F. Han, L. Chi, X. F. Liang, S. M. Ji, S. S. Liu, F. Zhou, Y. B. Wu, K. L. Han, J. Z. Zhao and T. D. James, J. Org. Chem., 2009, 74, 1333; (b) X. Zhang, L. N. Chi, S. M. Ji, Y. B. Wu, P. Song, K. L. Han, H. M. Guo, T. D. James and J. Z. Zhao, J. Am. Chem. Soc., 2009, 131, 17452.
10. (a) X. Zhang, Y. B. Wu, S. M. Ji, H. M. Guo, P. Song, K. L. Han, W. T. Wu, W. H. Wu, T. D. James and J. Z. Zhao, J. Org. Chem., 2010, 75, 2578; (b) Y. B. Wu, H. M. Guo, T. D. James and J. Z. Zhao, J. Org. Chem., 2011, 76, 5685; (c) Y. B. Wu, H. M. Guo, X. Zhang, T. D. James and J. Z. Zhao, Chem.–Eur. J., 2011, 17, 7632.
11. (a) H. Ishitani, M. Ueno and S. Kobayashi, J. Am. Chem. Soc., 2000, 122, 8180; (b) X. B. Huang, Y. Xu, Q. Miao, L. L. Zong, H. W. Hu and Y. X. Cheng, Polymer, 2009, 50, 2793.
12. H. Danjo, K. Hirata, S. Yoshigai, I. Azumaya and K. Yamaguchi, J. Am. Chem. Soc., 2009, 131, 1638.
13. (a) Y. Z. Wu, Y. Dong, J. F. Li, X. B. Huang, Y. X. Cheng and C. J. Zhu, Chem.–Asian J., 2011, 6, 2725; (b) L. F. Zheng, X. B. Huang, Y. G. Shen and Y. X. Cheng, Synlett, 2010, 453; (c) Q. Wang, X. Chen, L. Tao, L. Wang, D. Xiao, X. Q. Yu and L. Pu, J. Org. Chem., 2007, 72, 97.
14. (a) J. Lin, Q. Hu, M. Xu and L. Pu, J. Am. Chem. Soc., 2002, 124, 2088; (b) Y. Xu, L. F. Zheng, X. B. Huang, Y. X. Cheng and C. J. Zhu, Polymer, 2010, 51, 994.
15. L. Pu, Chem. Rev., 1998, 98, 2405.
16. (a) H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1949, 71, 2703; (b) I. D. Kuntz, F. P. Gasparro, M. D. Johnston and R. P. Taylor, J. Am. Chem. Soc., 1968, 90, 4778; (c) B. Valeur, J. Pouget and J. Bourson, J. Phys. Chem., 1992, 96, 6545; (d) J. B. Birks, Photophysics of Aromatic Molecules, Wiley, New York, 1970, p. 313; (e) F. Y. Wu, Z. Li, Z. C. Wen, N. Zhou, Y. F. Zhao and Y. B. Jiang, Org. Lett., 2002, 4, 3203.

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Footnote

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c3ra45682j

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