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Novel ratio fluorescence probes for selectively detecting zinc ion based on Y-type quinoxaline framework
Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Novel ratio fluorescence probes for selectively detecting zinc ion based on Y-type quinoxaline framework Hui Han a,n, Ying Ren Liu a, Chuan Dong a, Xiang-en Han b,n a b
Institute of Environmental Science, Shanxi University, Taiyuan 030006, Shanxi, China School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 May 2016 Received in revised form 13 November 2016 Accepted 21 November 2016
Several simple Y-type 2, 3-di(2′-pyridyl or quinoyl)quinoxaline derivatives had been synthesized, which is firstly utilized for detecting zinc ion as the receptor and fluorophore. They showed significant fluorescent turn-on and ratiometric detection of Zn2 þ with red shift and remarkable fluorescence ratio enhancement. In addition, these probes displayed high selectivity for zinc ion over other relevant ions. Theoretical calculations were carried out to investigate the recognition mechanism of the intramolecular charge transfer. & 2016 Elsevier B.V. All rights reserved.
Keywords: Zinc ion fluorescence probe Quinoxaline Selectivity Ratio Intramolecular charge transfer
1. Introduction As the second abundant nutritionally essential elements in human body, zinc ion plays indispensable roles in various biological processes such as the redox state, maintaining brain function, regulating gene expression, enzymatic function, keeping mammalian reproduction and cellular signaling [1–4]. Disturbance of intracellular free Zn2 þ homeostasis in living organisms has been considered to be associated with a series of diseases, such as alzheimer's disease, parkinson's disease, diabetes, prostate cancer and immune dysfunction [5–7]. Therefore, monitoring the distribution and concentration of free Zn2 þ in environmental or biologic samples has attracted increasing interests. Among numerous methods, fluorescent probe is considered as a powerful tool in detection of zinc ion due to the simplicity, high selectivity, high sensitivity, and low damage to biological samples [8–10]. The past decades a number of Zn2 þ fluorescent probes have been exploited through changing diverse fluorophores and Zn2 þ receptors [11–17]. However, a majority of these probes suffer from diverse drawbacks, such as selectivity, sensitivity, water solubility as well as cell membrane permeability, which keep them from further application in living cells. Furthermore, only a few of them that could sense Zn2 þ ratiometrically. It is generally believed n
Corresponding authors. E-mail addresses: [email protected] (H. Han), [email protected] (X.-e. Han).
that fluorescent probes of ratiometric type exhibit advantages of better resistance to variation of sensor concentration, higher sensitivity, and signal of color change [18]. Although there has been a great effort to develop ratiometric Zn2 þ -selective probes in the past few years, the ratiometric and targetable ones are really inadequate. On the other hand, selecting efficient fluorophores or the recognition group is the vital importance to the design and synthesis of fluorescent probes. Recent years, dipyridylmethylamine used to selectively detect zin ion as the recognition group is the classical structure. Quinoline as an efficient fluorophore has been widely used in various ion probes. Quinoxaline derivatives have attracted extensive interest due to their promising perspectives, particularly in the area of efficient luminescent materials [19], biologically active molecules [20,21], photon polymerization [22] or the sensitized dye applied in dye sensitized solar cell [23,24]. However, there were still no relevant reports upon the application of the combination of quinoxaline and pyridine (or quinoline) as the zinc ion probe. Herein in this work, we reported for the first time that the probe based on the Y-type 2,3-di(2′-pyridyl or quinoyl)quinoxaline skeleton (Scheme 1) could be used to selectively detect Zn2 þ . Results showed that all the probes exhibited significant fluorescent turn-on and ratiometric detection of Zn2 þ with red shift and remarkable fluorescence ratio enhancement. Moreover, they displayed 2:1 metal-ligand ratio when their Zn2 þ complexes were formed.
http://dx.doi.org/10.1016/j.jlumin.2016.11.065 0022-2313/& 2016 Elsevier B.V. All rights reserved.
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H. Han et al. / Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Scheme 1. The synthetic routes of the probe (QP1-QP5).
2. Experimental 2.1. Materials and apparatus All solvents and reactants were commercially available and used without further purification unless for special needs. Melting points were recorded on electrothermal digital melting point apparatus. NMR spectra were recorded at 295 K on a Bruker Advance DPX-400 MHz spectrometer using CDCl3 as solvent and TMS as internal standard. Absorption measurements were conducted on a Shimadzu UV-2501PC spectrophotometer. Fluorescence spectra were recorded on a Hitachi FL-4500 spectrofluorometer. Elemental analysis data was determined 5E-CHN2000 Elemental analysis instrument. 2.2. Fluorescence measurements A stock solution (1 10-3 mol L-1 (QP1, QP2) and 1 10-4 mol L-1 (QP3-QP5)) was prepared in DMF solution. A working solution of metal ion (1 10-5 mol L-1 and 1 10-6 mol L-1) was prepared. Whereas metal ions include K þ , Ca2 þ , Na þ , Mg2 þ , Al3 þ , Mn2 þ , Sn4 þ , Ba2 þ , Cd2 þ , Fe3 þ , Ni2 þ , Zn2 þ , Cu2 þ and Co2 þ . The fluorescence characteristics of all probes in DMF solution were recorded. The fluorescence titrations were carried out by addition of small aliquots of zinc ion working solution to samples solution in a 1 cm quartz cell. After well mixed, the solution was allowed to stand in ultrasonic cleaning machine for 30 s, and absorption or fluorescence spectra were recorded. Excitation and emission slit widths were 2 nm/1 nm, respectively. 2.3. Synthesis of probes 2.3.1. Synthesis of compound 2 To a mixture of thiamine chloride (1.5 g, 1 mmol) and trimethylamine (3 ml) in ethanol (30 ml) was added 2-pyridyllaldehyde (50 mmol), and the mixture was stirred at room temperature for 12 h. The solution was filtered to afford a pure product as a white solid (compound 1). A mixture of the solid and concentrated nitric acid (15 ml) was stirred at 40 °C for 1.5 h. Then the solution was cooled to room temperature, and was carefully neutralized through the addition of a solution of 10% sodium carbonate. The resulting mixture was filtered, and the solid collected was washed with water. After drying under reduced pressure, the title compound was obtained. Compound 2, white solid, Yield:82%; m.p. 154–156 °C; 1H NMR (400 MHz, CDCl3) δ 8.55-8.42 (m, 2 H), 8.21-8.08 (m, 2 H), 7.89 (d, J ¼ 7.8 Hz, 2 H), 7.41-7.34 (m, 2 H). Elem anal. Calcd. for C12H8N2O2: C, 67.92; H 3.80; N 15.08; Found: C, 67.88; H 3.82; N 15.11.
2.3.2. Synthesis of compound 3 Selenium dioxide (5.5 g, 0.05 mol) was dissolved in dioxane (48 ml) and water (2 ml). To this solution was added in portions with stirring during fifteen minutes, a solution of quinaldine (5.7 g, 0.04 mol) in dioxane (10 ml). During the addition the flask was heated to reflux and then maintained at this temperature for one hour. The selenium precipitate was filtered from the hot reaction mixture. The dioxane and water were distilled from the filtrate under reduced pressure, leaving a semisolid residue, which was purified by flash chromatography (silica gel, EtOAc: hexane / 1:3) to provide the product as a faint yellow solid, Yield: 91%; m.p. 179– 181°C; 1H NMR (400 MHz, CDCl3) δ 8.70 (d, J ¼ 8.4 Hz, 1 H), 8.38 (d, J ¼ 8.3 Hz, 1 H), 8.22 (d, J ¼ 10.1 Hz, 1 H), 7.90 (d, J ¼ 8.2 Hz, 1 H), 7.80 (t, J ¼7.4 Hz, 1 H), 7.64 (t, J ¼ 7.4 Hz, 1 H). Elem anal. Calcd. for C20H12N2O2: C, 76.91; H 3.87; N 8.97; Found: C, 76.89; H 3.89; N 8.95. 2.3.3. Synthesis of compound QP1-QP5 A mixture of o-dione (1 mmol) and 4-methyloxy-1,2-diaminobenzene (1 mmol) in ethanol (20 mL) was added glacial acetic acid (0.1 ml), and the mixture was refluxed for 12 h. The concentration of the mixture under reduced pressure afforded a crude product, which was purified by flash chromatography (silica gel, EtOAc: hexane/1:3) to provide the pure product. Compound QP1, Yield: 92%; m.p. 195–196 °C; 1H NMR (400 MHz, CDCl3) δ 8.44 (dd, J ¼ 11.3, 4.5 Hz, 2H), 8.14 (d, J ¼ 9.1 Hz, 1H), 7.94– 7.86 (m, 2H), 7.82 (t, J ¼ 7.7 Hz, 2H), 7.56–7.46 (m, 2H), 7.26 (dd, J ¼ 12.8, 6.9 Hz, 3H), 4.03 (s, 3H); 13C NMR(100 MHz, CDCl3) δ 161.36, 157.51, 152.35, 149.92, 148.69, 142.89, 137.40, 136.62, 130.36, 124.23, 122.88, 106.69, 55.91. HRMS: calcd: 314.1258, found: 414.1271. Elem anal. Calcd. for C19H14N4O: C, 72.59; H 4.49; N 17.82; Found: C, 72.57; H 4.52; N 17.79. Compound QP2, Yellow solid; Yield: 87%; m.p. 93–95 °C; 1H NMR (400 MHz,CDCl3) δ 8.67 (d, J ¼ 8.7 Hz, 2H), 8.21 (d, J ¼ 8.8 Hz, 2H), 7.89 (d, J ¼ 8.0 Hz, 1H), 7.86-7.77 (m, 4H), 7.60 (m, J ¼ 19.4, 15.6, 10.5 Hz, 4H), 7.33 (t, J ¼ 8.1 Hz, 1H), 7.18 (s, 1H), 3.90 (s, 3H); Elem anal. Calcd. for C27H18N4O: C, 78.24; H 4.38; N 13.52; Found: C, 78.25; H 4.40; N 17.51. Compound QP3, Yeild: 88%, m.p.: 95–98 °C, 1H NMR(400 MHz, CDCl3) 8.39 (d, J ¼ 8.3 Hz, 1H), 8.28 (d, J ¼ 8.4 Hz, 1H), 8.15 (d, J ¼ 8.2 Hz, 1H), 7.94 (d, J ¼ 8.21 Hz, 1H), 7.86 (t, J ¼ 7.6 Hz, 1H), 7.75 (t, J ¼ 7.5 Hz, 1H). Elem anal. Calcd. for C26H16N4: C, 81.23; H 4.20; N:14.57; Found: C, 81.25; H 4.19; N 14.56. Compound QP4, Yeild: 86%, m.p.: 99–101 °C, 1H NMR(400 MHz, CDCl3)δ 8.64 (d, J ¼ 8.7 Hz, 2H), 8.57 (d, J ¼ 8.5 Hz, 2H), 8.33–8.28 (m, 3H), 8.19 (t, J ¼ 7.8 Hz, 2H), 8.11 (d, J ¼ 8.5 Hz, 2H), 7.99 (dd, J ¼ 8.6, 2.8 Hz, 2H), 7.88 (t, J ¼ 9.8 Hz, 2H), 7.36 (d, J ¼ 8.3 Hz, 1H), 2.44 (s, 3H). C27H18N4: C, 81.39; H 4.55; N 14.06; Found: C, 81.42; H
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4.54; 14.04;. Compound QP5, Yellow solid; Yield: 87%; m.p. 93–95 °C; 1H NMR (400 MHz,CDCl3) δ 8.67 (d, J ¼ 8.7 Hz, 2H), 8.21 (d, J ¼ 8.8 Hz, 2H), 7.89 (d, J ¼ 8.0 Hz,1H), 7.86–7.77 (m, 4H), 7.60 (m, J ¼ 19.4, 15.6, 10.5 Hz, 4H), 7.33 (t, J ¼ 8.1 Hz,1H), 7.18 (s,1H), 3.90 (s 3H); 13C NMR (100 MHz, CDCl3) δ 158.96, 158.15, 156.95 ,148.09, 147.58, 147.15, 146.68, 137.28, 136.77, 131.90, 130.20, 129.6, 128.96, 127.91,126.56, 121.56, 119.22, 118.36, 114.80, 111.44, 101.78, 55.85. HRMS: calcd: 414.1526, found 414,1511. Elem anal. Calcd. for C27H18N4O: C, 78.24; H 4.38; N 13.52; Found: C, 78.25; H 4.40; N 17.51.
promoted by the binding of Zn2 þ and the recognition group of the probe. These compounds in Table 2 possess a high HOMO energy level (-5.788 to -6.624 eV), which could lead to their better holetransport properties. The low LUMO energy of these compounds (-1.809 to -2.305 eV) is supposed to facilitate the acceptance of electrons from the cathode. In addition, the results of theoretical calculation showed that the Eg level of the probe based 2,3-di(2′pyridyl)quinoxaline was bigger than the probe based on 2, 3-di(2′quinoyl) quinoxaline. The bigger conjugated system of quinoline reduce the Eg level. Moreover, the HOMO level of QP2 was lower than that of other probes.
3. Results and discussion
3.2. Responses behaviors of the probe to metal ions
3.1. Theoretical calculation
The fluorescence response behaviors of QP1-QP5 with various metal ions in DMF solution were investigated. The results exhibit that the enhancement of fluorescent intensity could be found only in the presence of zinc ions (Fig. 1 and Figure S7-Figure S9), which indicate the high selectivity of probes for detecting zinc ion. In emission spectra, Fe3 þ , Co2 þ , Cu2 þ , Ni2 þ have similar changes, they cause a fluorescence quenching (Fig. 1 and Fig. 2).
To gain insight into the optimized structure and electronic state of the probes, DFT calculations were performed on the dyes using the Gaussian 09 package in gas phase. Electronic calculations were performed using B3LYP as an exchange correlation functional and 6–31 G (d, p) basis set. The optimized structure of probes was shown in Table 1. The molecular orbital energy surface of the HOMO and LOMO was collected in Table 1. According to the surface distribution of the electron cloud, the electron transfer could be found from the HOMO to the LOMO. The binding with Zn2 þ of these probes is bond to lead to great changes for the intramolecular electric cloud and subsequent changes of the fluorescent spectra, which is the basis of fluorescence probes for detection. Moreover, the intramolecular charge translation of the probe molecular was
Table 2 The calculated molecular orbit data of probes. Compound
QP1
QP2
QP3
QP4
QP5
ELOMO(eV) EHOMO(eV) Eg(ELOMO-EHOMO)
-1.809 -5.895 4.086
-2.305 -6.624 4.319
-1.938 -5.837 3.899
-1.863 -5.788 3.760
-1.846 -5.855 3.667
Table 1 Calculated spatial distributions of the HOMO and LUMO levels of partial compounds.
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Fig. 1. Metal ion selectivity of probes QP1 and QP5 (probe-10mM (QP1) and 1 mM (QP5), metal ion-30mM (QP1) and 3 mM (QP5)). Red bars: fluorescence responses of the probe with different metal ions followed by addition of Zn2 þ . Excitation was performed at 384 nm (QP1) and 352 nm (QP5).
Table 3 Titration curve data of probes QP1-QP5. Compound Excited
QP1 QP2 QP3 QP4 QP5
The maximum emission wavelength/ nm
wavelength/nm Probe
Probeþ Zn2 þ
384 360 352 341 352
480 467 433 423 435
416 460 405 401 396
n
17.11 2.05 7.13 3.46 28.7
n: The proportion of fluorescence intensity for the probesolution after the addition of zinc ion.
Fig. 2. Fluorescence spectra of QP1 and some metal ions (wherein the metal ion is twice the molar amount of probe, excited wavelength: 384 nm).
This may be due to that paramagnetic ions Fe3 þ , Co2 þ , Cu2 þ , Ni2 þ occupying open shell d-orbitals can induce the electron or energy transfer between the metal ions and probes, and then provide a very fast and efficient non-radiative decay of the excited states.
Zn2 þ with closed shell d-orbitals, however, cannot introduce such processes [25–27]. Therefore, the probe can be applied to ratiometric detection of Zn2 þ without the potential influence from other biological metal ions. In addition, we could find that only paramagnetic ions Fe3 þ , 2þ Co , Ni2 þ , Cu2 þ , Zn2 þ can show a fluorescence response with probes. The reason may be that their ion's radius just can form coordination with the nitrogen atom in the quinoxaline and pyridine rings.
Fig. 3. Fluorescent spectra of QP1 and QP5 in DMF solution with changing of zinc ion content (the concentration of probe: 1 10-5 mol/L(QP1), 1 10-6 mol/L(QP5); titrant zinc ion concentration: 1 10-3 mol/L(QP1), 1 10-4 mol/L(QP5); each time dropping to 2uL) The inset shows boost of fluorescence intensity with increasing [Zn2 þ ], λex ¼ 384 nm(QP1), 352 nm(QP5). I/I0 is for the data of fluorescence intensity in 480 nm(QP1) and 435 nm(QP2).
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Fig. 4. The change of color for the probe solution after adding zinc ion (the inset picture was obtained under the irradiation of ultraviolet light at 365 nm).
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can find that the fluorescence intensity of the probe solution exhibited no obvious changes when the concentration of zinc ion is twice the concentration of the probe, so we can surmised that probes might form a 1: 2 complex with zinc ion. The relevant data of fluorescent spectrum for probes was shown in Table 3. Upon the addition of zinc ion, the fluorescent intensity of the probe QP5 show the biggest change about 28.7 times (Fig. 4). As shown in Fig. 3, the fluorescence spectra of QP1 and QP5 exhibited a linear response to Zn2 þ concentration (0 to 2 equiv.), from which the limit of detection for Zn2 þ was experimentally determined to be nM (Figure S14, Figure S15 and Table S1). The result suggested that the probe have a low detecting limit to Zn2 þ and could be suitable for monitoring the low-level intracellular free Zn2 þ . Generally, the intracellular free Zn2 þ is maintained in extremely low concentration (pM to nM range) by the zinc metalloproteins, whereas the free Zn2 þ concentration could reach as high as μM level after Zn2 þ release in response to certain stimulations, such as inflammation or oxidative stress. Thus, these probes are expected to be applied in intracellular zinc detection. Fig. 4 showed the solution emission photos of compounds QP1-QP5, when illuminated with a 365 nm UV lamp. It is clearly that all of the compounds have a good fluorescence, whiles the aggregate emits bright green or yellow fluorescence when illuminated with a 365 nm UV lamp (right).
Fig. 5. The reaction procedure of probe QP5 with Zn2 þ .
3.4. Mechanism of the probe
Fig. 6. Infrared spectrum of QP5 upon the addition of Zn2 þ .
3.3. Responses behaviors of the probe to zinc ion Emission spectra of Zn2 þ titration for probes were determined. The results show that the fluorescent spectra of all probes display an obvious red-shift and a remarkable enhancement of fluorescence intensity upon addition of Zn2 þ , which indicate a typical ratiometric fluorescent character (Fig. 3 and Figure S10- Figure S13). As shown in the inset curve of Fig. 3, a titration experiment of zinc ion was carried out. Generally, with the increase of Zn2 þ concentration, the complex formed by the probe and Zn2 þ gradually increase and the fluorescence intensity also gradually increase. When the fluorescence intensity did not change, the probe and zinc ion no longer formed complex, then the formed ratio of the complex is the concentration ratio of the probe and ion. We
Fig. 5 shows the recognition mechanism of the probe (QP5 as the example). The promoted intramolecular charge transition and enhanced molecular rigidity upon the addition of zinc ion are seen as the cause of the fluorescent properties change. Experimental and theoretical investigations have indicated that the intramolecular rotations in the fluorescent molecular would deactivate the corresponding excited states, thus making them nonemissive in the respective solutions. The intramolecular steric interactions are blocked in their complexes and herein their emissions are enhanced. In our work, nitrogen atom of pyridine and nitrogen atom of quinoxaline in the probe molecular was designed to bond with zinc ion. The rotation of single bond between pyridine and quinoxaline is restricted when these probes bind to zinc ion, so it gives a higher fluorescent emission. On the other hand, the structure of probes QP5 are a typical Dπ-A configuration, when the complex was formed, the transfer of electrons from the methoxy group to the pyridine ring is promoted by the electron-withdrawing property of metal ions. So we think intramolecular charge transfer (ICT) is looked as the detected mechanism of probes. The infrared spectra of QP2 upon addition of Zn2 þ are shown in Fig. 6. Notably, the characteristic absorption peaks at about 1500 cm-1 show a severe broadness and strength upon the addition of Zn2 þ , and the absorption peaks of aryl ring becomes less in the region of 1250–750cm-1. These data support that strong coordination bonding interactions between the probe and Zn2 þ .
4. Conclusions In conclusion, a series of novel fluorescent probes (QP1-QP5) for Zn2 þ based on quinoxline skeleton was designed and synthesized. Moreover, these probes possess the advantage of good sensitivity and selectivity toward Zn2 þ over other metal ions and showed 2: 1 metal-to-ligand ratio, respectively, when their Zn2 þ complex formed. Therefore, the 2, 3-di(2′-pyridyl or quinoyl)quinoxaline segment exhibits a good application prospect as the receptor in Zn2 þ sensing. We can always predict that this class of new recognition groups can be modified to other light emitting groups to
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come to a class of zinc ion probes with excellent properties.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2016.11.065.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Novel ratio fluorescence probes for selectively detecting zinc ion based on Y-type quinoxaline framework Hui Han a,n, Ying Ren Liu a, Chuan Dong a, Xiang-en Han b,n a b
Institute of Environmental Science, Shanxi University, Taiyuan 030006, Shanxi, China School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 May 2016 Received in revised form 13 November 2016 Accepted 21 November 2016
Several simple Y-type 2, 3-di(2′-pyridyl or quinoyl)quinoxaline derivatives had been synthesized, which is firstly utilized for detecting zinc ion as the receptor and fluorophore. They showed significant fluorescent turn-on and ratiometric detection of Zn2 þ with red shift and remarkable fluorescence ratio enhancement. In addition, these probes displayed high selectivity for zinc ion over other relevant ions. Theoretical calculations were carried out to investigate the recognition mechanism of the intramolecular charge transfer. & 2016 Elsevier B.V. All rights reserved.
Keywords: Zinc ion fluorescence probe Quinoxaline Selectivity Ratio Intramolecular charge transfer
1. Introduction As the second abundant nutritionally essential elements in human body, zinc ion plays indispensable roles in various biological processes such as the redox state, maintaining brain function, regulating gene expression, enzymatic function, keeping mammalian reproduction and cellular signaling [1–4]. Disturbance of intracellular free Zn2 þ homeostasis in living organisms has been considered to be associated with a series of diseases, such as alzheimer's disease, parkinson's disease, diabetes, prostate cancer and immune dysfunction [5–7]. Therefore, monitoring the distribution and concentration of free Zn2 þ in environmental or biologic samples has attracted increasing interests. Among numerous methods, fluorescent probe is considered as a powerful tool in detection of zinc ion due to the simplicity, high selectivity, high sensitivity, and low damage to biological samples [8–10]. The past decades a number of Zn2 þ fluorescent probes have been exploited through changing diverse fluorophores and Zn2 þ receptors [11–17]. However, a majority of these probes suffer from diverse drawbacks, such as selectivity, sensitivity, water solubility as well as cell membrane permeability, which keep them from further application in living cells. Furthermore, only a few of them that could sense Zn2 þ ratiometrically. It is generally believed n
Corresponding authors. E-mail addresses: [email protected] (H. Han), [email protected] (X.-e. Han).
that fluorescent probes of ratiometric type exhibit advantages of better resistance to variation of sensor concentration, higher sensitivity, and signal of color change [18]. Although there has been a great effort to develop ratiometric Zn2 þ -selective probes in the past few years, the ratiometric and targetable ones are really inadequate. On the other hand, selecting efficient fluorophores or the recognition group is the vital importance to the design and synthesis of fluorescent probes. Recent years, dipyridylmethylamine used to selectively detect zin ion as the recognition group is the classical structure. Quinoline as an efficient fluorophore has been widely used in various ion probes. Quinoxaline derivatives have attracted extensive interest due to their promising perspectives, particularly in the area of efficient luminescent materials [19], biologically active molecules [20,21], photon polymerization [22] or the sensitized dye applied in dye sensitized solar cell [23,24]. However, there were still no relevant reports upon the application of the combination of quinoxaline and pyridine (or quinoline) as the zinc ion probe. Herein in this work, we reported for the first time that the probe based on the Y-type 2,3-di(2′-pyridyl or quinoyl)quinoxaline skeleton (Scheme 1) could be used to selectively detect Zn2 þ . Results showed that all the probes exhibited significant fluorescent turn-on and ratiometric detection of Zn2 þ with red shift and remarkable fluorescence ratio enhancement. Moreover, they displayed 2:1 metal-ligand ratio when their Zn2 þ complexes were formed.
http://dx.doi.org/10.1016/j.jlumin.2016.11.065 0022-2313/& 2016 Elsevier B.V. All rights reserved.
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Scheme 1. The synthetic routes of the probe (QP1-QP5).
2. Experimental 2.1. Materials and apparatus All solvents and reactants were commercially available and used without further purification unless for special needs. Melting points were recorded on electrothermal digital melting point apparatus. NMR spectra were recorded at 295 K on a Bruker Advance DPX-400 MHz spectrometer using CDCl3 as solvent and TMS as internal standard. Absorption measurements were conducted on a Shimadzu UV-2501PC spectrophotometer. Fluorescence spectra were recorded on a Hitachi FL-4500 spectrofluorometer. Elemental analysis data was determined 5E-CHN2000 Elemental analysis instrument. 2.2. Fluorescence measurements A stock solution (1 10-3 mol L-1 (QP1, QP2) and 1 10-4 mol L-1 (QP3-QP5)) was prepared in DMF solution. A working solution of metal ion (1 10-5 mol L-1 and 1 10-6 mol L-1) was prepared. Whereas metal ions include K þ , Ca2 þ , Na þ , Mg2 þ , Al3 þ , Mn2 þ , Sn4 þ , Ba2 þ , Cd2 þ , Fe3 þ , Ni2 þ , Zn2 þ , Cu2 þ and Co2 þ . The fluorescence characteristics of all probes in DMF solution were recorded. The fluorescence titrations were carried out by addition of small aliquots of zinc ion working solution to samples solution in a 1 cm quartz cell. After well mixed, the solution was allowed to stand in ultrasonic cleaning machine for 30 s, and absorption or fluorescence spectra were recorded. Excitation and emission slit widths were 2 nm/1 nm, respectively. 2.3. Synthesis of probes 2.3.1. Synthesis of compound 2 To a mixture of thiamine chloride (1.5 g, 1 mmol) and trimethylamine (3 ml) in ethanol (30 ml) was added 2-pyridyllaldehyde (50 mmol), and the mixture was stirred at room temperature for 12 h. The solution was filtered to afford a pure product as a white solid (compound 1). A mixture of the solid and concentrated nitric acid (15 ml) was stirred at 40 °C for 1.5 h. Then the solution was cooled to room temperature, and was carefully neutralized through the addition of a solution of 10% sodium carbonate. The resulting mixture was filtered, and the solid collected was washed with water. After drying under reduced pressure, the title compound was obtained. Compound 2, white solid, Yield:82%; m.p. 154–156 °C; 1H NMR (400 MHz, CDCl3) δ 8.55-8.42 (m, 2 H), 8.21-8.08 (m, 2 H), 7.89 (d, J ¼ 7.8 Hz, 2 H), 7.41-7.34 (m, 2 H). Elem anal. Calcd. for C12H8N2O2: C, 67.92; H 3.80; N 15.08; Found: C, 67.88; H 3.82; N 15.11.
2.3.2. Synthesis of compound 3 Selenium dioxide (5.5 g, 0.05 mol) was dissolved in dioxane (48 ml) and water (2 ml). To this solution was added in portions with stirring during fifteen minutes, a solution of quinaldine (5.7 g, 0.04 mol) in dioxane (10 ml). During the addition the flask was heated to reflux and then maintained at this temperature for one hour. The selenium precipitate was filtered from the hot reaction mixture. The dioxane and water were distilled from the filtrate under reduced pressure, leaving a semisolid residue, which was purified by flash chromatography (silica gel, EtOAc: hexane / 1:3) to provide the product as a faint yellow solid, Yield: 91%; m.p. 179– 181°C; 1H NMR (400 MHz, CDCl3) δ 8.70 (d, J ¼ 8.4 Hz, 1 H), 8.38 (d, J ¼ 8.3 Hz, 1 H), 8.22 (d, J ¼ 10.1 Hz, 1 H), 7.90 (d, J ¼ 8.2 Hz, 1 H), 7.80 (t, J ¼7.4 Hz, 1 H), 7.64 (t, J ¼ 7.4 Hz, 1 H). Elem anal. Calcd. for C20H12N2O2: C, 76.91; H 3.87; N 8.97; Found: C, 76.89; H 3.89; N 8.95. 2.3.3. Synthesis of compound QP1-QP5 A mixture of o-dione (1 mmol) and 4-methyloxy-1,2-diaminobenzene (1 mmol) in ethanol (20 mL) was added glacial acetic acid (0.1 ml), and the mixture was refluxed for 12 h. The concentration of the mixture under reduced pressure afforded a crude product, which was purified by flash chromatography (silica gel, EtOAc: hexane/1:3) to provide the pure product. Compound QP1, Yield: 92%; m.p. 195–196 °C; 1H NMR (400 MHz, CDCl3) δ 8.44 (dd, J ¼ 11.3, 4.5 Hz, 2H), 8.14 (d, J ¼ 9.1 Hz, 1H), 7.94– 7.86 (m, 2H), 7.82 (t, J ¼ 7.7 Hz, 2H), 7.56–7.46 (m, 2H), 7.26 (dd, J ¼ 12.8, 6.9 Hz, 3H), 4.03 (s, 3H); 13C NMR(100 MHz, CDCl3) δ 161.36, 157.51, 152.35, 149.92, 148.69, 142.89, 137.40, 136.62, 130.36, 124.23, 122.88, 106.69, 55.91. HRMS: calcd: 314.1258, found: 414.1271. Elem anal. Calcd. for C19H14N4O: C, 72.59; H 4.49; N 17.82; Found: C, 72.57; H 4.52; N 17.79. Compound QP2, Yellow solid; Yield: 87%; m.p. 93–95 °C; 1H NMR (400 MHz,CDCl3) δ 8.67 (d, J ¼ 8.7 Hz, 2H), 8.21 (d, J ¼ 8.8 Hz, 2H), 7.89 (d, J ¼ 8.0 Hz, 1H), 7.86-7.77 (m, 4H), 7.60 (m, J ¼ 19.4, 15.6, 10.5 Hz, 4H), 7.33 (t, J ¼ 8.1 Hz, 1H), 7.18 (s, 1H), 3.90 (s, 3H); Elem anal. Calcd. for C27H18N4O: C, 78.24; H 4.38; N 13.52; Found: C, 78.25; H 4.40; N 17.51. Compound QP3, Yeild: 88%, m.p.: 95–98 °C, 1H NMR(400 MHz, CDCl3) 8.39 (d, J ¼ 8.3 Hz, 1H), 8.28 (d, J ¼ 8.4 Hz, 1H), 8.15 (d, J ¼ 8.2 Hz, 1H), 7.94 (d, J ¼ 8.21 Hz, 1H), 7.86 (t, J ¼ 7.6 Hz, 1H), 7.75 (t, J ¼ 7.5 Hz, 1H). Elem anal. Calcd. for C26H16N4: C, 81.23; H 4.20; N:14.57; Found: C, 81.25; H 4.19; N 14.56. Compound QP4, Yeild: 86%, m.p.: 99–101 °C, 1H NMR(400 MHz, CDCl3)δ 8.64 (d, J ¼ 8.7 Hz, 2H), 8.57 (d, J ¼ 8.5 Hz, 2H), 8.33–8.28 (m, 3H), 8.19 (t, J ¼ 7.8 Hz, 2H), 8.11 (d, J ¼ 8.5 Hz, 2H), 7.99 (dd, J ¼ 8.6, 2.8 Hz, 2H), 7.88 (t, J ¼ 9.8 Hz, 2H), 7.36 (d, J ¼ 8.3 Hz, 1H), 2.44 (s, 3H). C27H18N4: C, 81.39; H 4.55; N 14.06; Found: C, 81.42; H
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4.54; 14.04;. Compound QP5, Yellow solid; Yield: 87%; m.p. 93–95 °C; 1H NMR (400 MHz,CDCl3) δ 8.67 (d, J ¼ 8.7 Hz, 2H), 8.21 (d, J ¼ 8.8 Hz, 2H), 7.89 (d, J ¼ 8.0 Hz,1H), 7.86–7.77 (m, 4H), 7.60 (m, J ¼ 19.4, 15.6, 10.5 Hz, 4H), 7.33 (t, J ¼ 8.1 Hz,1H), 7.18 (s,1H), 3.90 (s 3H); 13C NMR (100 MHz, CDCl3) δ 158.96, 158.15, 156.95 ,148.09, 147.58, 147.15, 146.68, 137.28, 136.77, 131.90, 130.20, 129.6, 128.96, 127.91,126.56, 121.56, 119.22, 118.36, 114.80, 111.44, 101.78, 55.85. HRMS: calcd: 414.1526, found 414,1511. Elem anal. Calcd. for C27H18N4O: C, 78.24; H 4.38; N 13.52; Found: C, 78.25; H 4.40; N 17.51.
promoted by the binding of Zn2 þ and the recognition group of the probe. These compounds in Table 2 possess a high HOMO energy level (-5.788 to -6.624 eV), which could lead to their better holetransport properties. The low LUMO energy of these compounds (-1.809 to -2.305 eV) is supposed to facilitate the acceptance of electrons from the cathode. In addition, the results of theoretical calculation showed that the Eg level of the probe based 2,3-di(2′pyridyl)quinoxaline was bigger than the probe based on 2, 3-di(2′quinoyl) quinoxaline. The bigger conjugated system of quinoline reduce the Eg level. Moreover, the HOMO level of QP2 was lower than that of other probes.
3. Results and discussion
3.2. Responses behaviors of the probe to metal ions
3.1. Theoretical calculation
The fluorescence response behaviors of QP1-QP5 with various metal ions in DMF solution were investigated. The results exhibit that the enhancement of fluorescent intensity could be found only in the presence of zinc ions (Fig. 1 and Figure S7-Figure S9), which indicate the high selectivity of probes for detecting zinc ion. In emission spectra, Fe3 þ , Co2 þ , Cu2 þ , Ni2 þ have similar changes, they cause a fluorescence quenching (Fig. 1 and Fig. 2).
To gain insight into the optimized structure and electronic state of the probes, DFT calculations were performed on the dyes using the Gaussian 09 package in gas phase. Electronic calculations were performed using B3LYP as an exchange correlation functional and 6–31 G (d, p) basis set. The optimized structure of probes was shown in Table 1. The molecular orbital energy surface of the HOMO and LOMO was collected in Table 1. According to the surface distribution of the electron cloud, the electron transfer could be found from the HOMO to the LOMO. The binding with Zn2 þ of these probes is bond to lead to great changes for the intramolecular electric cloud and subsequent changes of the fluorescent spectra, which is the basis of fluorescence probes for detection. Moreover, the intramolecular charge translation of the probe molecular was
Table 2 The calculated molecular orbit data of probes. Compound
QP1
QP2
QP3
QP4
QP5
ELOMO(eV) EHOMO(eV) Eg(ELOMO-EHOMO)
-1.809 -5.895 4.086
-2.305 -6.624 4.319
-1.938 -5.837 3.899
-1.863 -5.788 3.760
-1.846 -5.855 3.667
Table 1 Calculated spatial distributions of the HOMO and LUMO levels of partial compounds.
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Fig. 1. Metal ion selectivity of probes QP1 and QP5 (probe-10mM (QP1) and 1 mM (QP5), metal ion-30mM (QP1) and 3 mM (QP5)). Red bars: fluorescence responses of the probe with different metal ions followed by addition of Zn2 þ . Excitation was performed at 384 nm (QP1) and 352 nm (QP5).
Table 3 Titration curve data of probes QP1-QP5. Compound Excited
QP1 QP2 QP3 QP4 QP5
The maximum emission wavelength/ nm
wavelength/nm Probe
Probeþ Zn2 þ
384 360 352 341 352
480 467 433 423 435
416 460 405 401 396
n
17.11 2.05 7.13 3.46 28.7
n: The proportion of fluorescence intensity for the probesolution after the addition of zinc ion.
Fig. 2. Fluorescence spectra of QP1 and some metal ions (wherein the metal ion is twice the molar amount of probe, excited wavelength: 384 nm).
This may be due to that paramagnetic ions Fe3 þ , Co2 þ , Cu2 þ , Ni2 þ occupying open shell d-orbitals can induce the electron or energy transfer between the metal ions and probes, and then provide a very fast and efficient non-radiative decay of the excited states.
Zn2 þ with closed shell d-orbitals, however, cannot introduce such processes [25–27]. Therefore, the probe can be applied to ratiometric detection of Zn2 þ without the potential influence from other biological metal ions. In addition, we could find that only paramagnetic ions Fe3 þ , 2þ Co , Ni2 þ , Cu2 þ , Zn2 þ can show a fluorescence response with probes. The reason may be that their ion's radius just can form coordination with the nitrogen atom in the quinoxaline and pyridine rings.
Fig. 3. Fluorescent spectra of QP1 and QP5 in DMF solution with changing of zinc ion content (the concentration of probe: 1 10-5 mol/L(QP1), 1 10-6 mol/L(QP5); titrant zinc ion concentration: 1 10-3 mol/L(QP1), 1 10-4 mol/L(QP5); each time dropping to 2uL) The inset shows boost of fluorescence intensity with increasing [Zn2 þ ], λex ¼ 384 nm(QP1), 352 nm(QP5). I/I0 is for the data of fluorescence intensity in 480 nm(QP1) and 435 nm(QP2).
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Fig. 4. The change of color for the probe solution after adding zinc ion (the inset picture was obtained under the irradiation of ultraviolet light at 365 nm).
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can find that the fluorescence intensity of the probe solution exhibited no obvious changes when the concentration of zinc ion is twice the concentration of the probe, so we can surmised that probes might form a 1: 2 complex with zinc ion. The relevant data of fluorescent spectrum for probes was shown in Table 3. Upon the addition of zinc ion, the fluorescent intensity of the probe QP5 show the biggest change about 28.7 times (Fig. 4). As shown in Fig. 3, the fluorescence spectra of QP1 and QP5 exhibited a linear response to Zn2 þ concentration (0 to 2 equiv.), from which the limit of detection for Zn2 þ was experimentally determined to be nM (Figure S14, Figure S15 and Table S1). The result suggested that the probe have a low detecting limit to Zn2 þ and could be suitable for monitoring the low-level intracellular free Zn2 þ . Generally, the intracellular free Zn2 þ is maintained in extremely low concentration (pM to nM range) by the zinc metalloproteins, whereas the free Zn2 þ concentration could reach as high as μM level after Zn2 þ release in response to certain stimulations, such as inflammation or oxidative stress. Thus, these probes are expected to be applied in intracellular zinc detection. Fig. 4 showed the solution emission photos of compounds QP1-QP5, when illuminated with a 365 nm UV lamp. It is clearly that all of the compounds have a good fluorescence, whiles the aggregate emits bright green or yellow fluorescence when illuminated with a 365 nm UV lamp (right).
Fig. 5. The reaction procedure of probe QP5 with Zn2 þ .
3.4. Mechanism of the probe
Fig. 6. Infrared spectrum of QP5 upon the addition of Zn2 þ .
3.3. Responses behaviors of the probe to zinc ion Emission spectra of Zn2 þ titration for probes were determined. The results show that the fluorescent spectra of all probes display an obvious red-shift and a remarkable enhancement of fluorescence intensity upon addition of Zn2 þ , which indicate a typical ratiometric fluorescent character (Fig. 3 and Figure S10- Figure S13). As shown in the inset curve of Fig. 3, a titration experiment of zinc ion was carried out. Generally, with the increase of Zn2 þ concentration, the complex formed by the probe and Zn2 þ gradually increase and the fluorescence intensity also gradually increase. When the fluorescence intensity did not change, the probe and zinc ion no longer formed complex, then the formed ratio of the complex is the concentration ratio of the probe and ion. We
Fig. 5 shows the recognition mechanism of the probe (QP5 as the example). The promoted intramolecular charge transition and enhanced molecular rigidity upon the addition of zinc ion are seen as the cause of the fluorescent properties change. Experimental and theoretical investigations have indicated that the intramolecular rotations in the fluorescent molecular would deactivate the corresponding excited states, thus making them nonemissive in the respective solutions. The intramolecular steric interactions are blocked in their complexes and herein their emissions are enhanced. In our work, nitrogen atom of pyridine and nitrogen atom of quinoxaline in the probe molecular was designed to bond with zinc ion. The rotation of single bond between pyridine and quinoxaline is restricted when these probes bind to zinc ion, so it gives a higher fluorescent emission. On the other hand, the structure of probes QP5 are a typical Dπ-A configuration, when the complex was formed, the transfer of electrons from the methoxy group to the pyridine ring is promoted by the electron-withdrawing property of metal ions. So we think intramolecular charge transfer (ICT) is looked as the detected mechanism of probes. The infrared spectra of QP2 upon addition of Zn2 þ are shown in Fig. 6. Notably, the characteristic absorption peaks at about 1500 cm-1 show a severe broadness and strength upon the addition of Zn2 þ , and the absorption peaks of aryl ring becomes less in the region of 1250–750cm-1. These data support that strong coordination bonding interactions between the probe and Zn2 þ .
4. Conclusions In conclusion, a series of novel fluorescent probes (QP1-QP5) for Zn2 þ based on quinoxline skeleton was designed and synthesized. Moreover, these probes possess the advantage of good sensitivity and selectivity toward Zn2 þ over other metal ions and showed 2: 1 metal-to-ligand ratio, respectively, when their Zn2 þ complex formed. Therefore, the 2, 3-di(2′-pyridyl or quinoyl)quinoxaline segment exhibits a good application prospect as the receptor in Zn2 þ sensing. We can always predict that this class of new recognition groups can be modified to other light emitting groups to
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come to a class of zinc ion probes with excellent properties.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2016.11.065.
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