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A molecular dual fluorescence-ON probe for Mg2+ and Zn2+: Higher selectivity towards Mg2+ over Zn2+ in a mixture
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A molecular dual fluorescence-ON probe for Mg2 þ and Zn2 þ : Higher selectivity Towards Mg2 þ over Zn2 þ in A mixture Shubhra Bikash Maity, Parimal K. Bharadwaj
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S0022-2313(14)00354-8 http://dx.doi.org/10.1016/j.jlumin.2014.06.020 LUMIN12747
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Journal of Luminescence
Received date: 24 March 2014 Revised date: 28 May 2014 Accepted date: 10 June 2014 Cite this article as: Shubhra Bikash Maity, Parimal K. Bharadwaj, A molecular dual fluorescence-ON probe for Mg2 þ and Zn2 þ : Higher selectivity Towards Mg2 þ over Zn2 þ in A mixture, Journal of Luminescence, http://dx.doi.org/10.1016/j. jlumin.2014.06.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Revised Manuscript Number: LUMIN-D-14-00378
A Molecular Dual Fluorescence-ON Probe for Mg2+ and Zn2+: Higher Selectivity Towards Mg2+ over Zn2+ in a Mixture
Shubhra Bikash Maity and Parimal K. Bharadwaj Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India Email:[email protected]
ABSTRACT A Schiff base incorporating a coumarin fluorophore has been synthesized from easily available materials and is characterized by X-ray crystallography and other techniques. The probe serves as a dual analyte sensor and quantifies Mg2+ and Zn2+ ions by emission enhancement at different wavelengths without interference from a host of biologically relevant alkali/alkaline earth and transition metal ions. In presence of Mg2+ the light yellow color of the probe in methanol changes to yellow-orange while in presence of Zn2+ ion it changes to orange and hence can be detected through naked eye. The probe selectively gives emission of Mg2+ when Zn2+ ion is also present. Keywords: Schiff base, Chemosensor, Magnesium, Zinc, Selectivity. 1. Introduction Magnesium ion is one of the most abundant divalent metal ions in bio-systems. This ion plays significant roles both in cell proliferation as well as in cell death and is also involved [1-10] in the regulation of hundreds of enzymes and molecules having different structures and functions. Alteration in total or free magnesium can have significant consequences for cell metabolism and its functions. Selective detection of Mg2+ in the presence of other biologically relevant Ca2+, Na+ and K+ or transition-metal ions is, therefore, of particular importance. A number of fluorescent chemosensors for Mg2+ have been reported with receptors that included diaza-18-crown-6 [11], benzo-15-crown-5 [12], charged β
diketone [13-16], calix[4]arene [17, 18], porphyrin related macrocycles [19, 20] and iminelinked compounds [21-24]. Most of them require large number of synthetic steps or interference from other metal ions. Therefore, development of chemosensors that selectively and sensitively detect and quantify Mg2+ ion particularly in the presence of alkali, alkaline earth and biologically relevant transition metal ions continues to be an important goal. In the present study, we report a sensor L that can detect Mg2+ ion. It can also detect Zn2+ via emission at a different wavelength although Mg2+ exhibits greater selectivity over Zn2+ when both are present. We have chosen coumarin as the flurophore because it exhibits large Stokes shift and visible excitation and emission wavelengths [25-27]. 2. Experimental 2.1 Materials Reagent grade 4-diethylaminosalicylaldehyde, ethylnitroacetate and all metal perchlorate salts were bought from Aldrich Chemicals (USA), while 4-methylphenol, nbutanol, piperidine and acetic acid were bought from S. D. Fine Chemicals (INDIA). All chemicals were used as received without further purification. All solvents were bought from S. D. Fine Chemicals (INDIA) and were dried prior to use following the standard literature procedure. All reactions were carried out under dinitrogen protection unless stated. 2.2 Instrumentation 1
H NMR (500 MHz) and
13
C NMR spectra (125 MHz) of the compounds were
recorded on a JEOL DELTA2 spectrometer in CDCl3 with Si(CH3)4 as the internal standard. The ESI-Mass data were obtained in methanol from a WATERS-Q-Tof Premier Mass Spectrometer. UV-vis spectra were recorded on a Shimadzu 2450 UV-Vis spectrophotometer in methanol medium at 298 K. Melting points were determined with an electrical melting point apparatus by PERFIT, India and were uncorrected. Steady-state fluorescence spectra were obtained using a Perkin-Elmer LS 50B Luminescence Spectrometer at 293 K with
excitation and emission band-pass of 2.5 and 2.5 nm or 3.5 nm. The excitation wavelength was 450 nm and the spectra were recorded in the range 490-700nm. Fluorescence quantum yield [28, 29] were determined by comparing the corrected spectra with that of fluorescein in ethanol taking the total area under the fluorescence spectra using the following equation ΦS = ΦR (FSAR/FRAS) × (ηS/ηR) 2 ……………… (i) Where, Φ stands for quantum yield, F stands for area under the fluorescence spectra, A stands for absorbance value and η stands for the refractive index value. The subscript ‘R’ indicates the value of the parameter for reference (i.e. fluorescein) and ‘S’ subscript indicates the value of the parameter for the sample. 2.3 Synthesis 2.3.1 Synthesis of A Compound A was synthesized according to the previously reported method [30]. A 250 mL round bottom flask equipped with spinning bar was charged with 4-methylphenol (5.17 g, 47.81 mmol) and hexamine (13.40 g, 95.62 mmol). To this trifluoroaceticacid (40 mL, 0.52 mol) was added and the yellow solution became hot. A reflux condenser was fitted and the solution was heated to 145 °C for 24 h, during which it turned dark brown. The hot solution was poured into ~ 100 mL 4N HCl solution and stirred for 2 hours, during which it becomes solidified. The solid was filtered and washed several times with ice cold water (2 × 100 mL) to make it acid free. The white solid obtained was dried over P2O5 yields 2.95 g of the desired product. Yield: 37.6 %; M. Pt. 130 °C; 1H NMR (500 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 2.37 (s, 3H), 7.75 (s, 2H), 10.19 (s, 2H), 11.43 (s, 1H) (Figure S1); 13C NMR (125 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 20.17, 122.98, 129.61, 138.08, 161.85, 192.30 (Figure S2).
2.3.2 Synthesis of B and C Compound B and C were synthesized according to the literature method reported earlier [23]. Synthesis of B: To a solution of 4-diethylaminosalicylaldehyde (1.4 g, 7.24 mmol) and ethylnitroacetate (0.80 mL, 7.96 mmol) in 25 mL of n-butanol was added catalytic amount of piperidine (0.1 mL) and glacial acetic acid (0.2 mL) and the reaction mixture was refluxed for 12 h. The bright red solid formed during the course of reaction was filtered under hot condition, washed 2 to 3 times with n-BuOH (2 × 20 mL) and finally dried under vacuum to give 1.7 gm of nitro derivative. Yield: 90 %; M. Pt. 170 °C; 1H NMR (500 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 1.26 (t, 6H, J = 6.9 Hz), 3.48 (q, 4H, J = 6.9 Hz), 6.46 (d, 1H, J = 2.3Hz), 6.70 (dd, 1H, J = 2.3 & 9.1 Hz), 7.41 (d, 1H, J = 9.1 Hz), 8.69 (s, 1H) (Figure S3); 13C NMR (125 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 12.50, 45.61, 96.88, 106.28, 111.23, 126.91, 132.65, 143.42, 153.55, 154.66, 158.85 (Figure S4). Synthesis of C: In a 100 mL RB flask stannous chloride (4.3 g, 19.06 mmol) and 20 mL 15% HCl solution were taken. To this compound B (1 g, 3.82 mmol) was added portion wise over a period of 15 minutes and the solution was stirred at room temperature for 6 hours. Then NaOH solution (5M) was added under ice cold condition and the pH of the solution was adjusted to ~10. The aqueous phase was extracted with ethyl acetate (3 × 30 mL). The organic layer was dried over anhydrous Na2SO4 and evaporated to dryness yielded the amine component as yellowish-orange solid. Yield: 90 %; M. Pt. 85 °C; 1H NMR (500 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 1.16 (t, 6H, J = 7.2 Hz), 3.36 (q, 4H, J = 7.2 Hz), 3.85 (s, br, 2H), 6.51 (s, 1H ), 6.57 (d, 1H, J = 8.4 Hz), 6.69 (s, 1H), 7.10 (d, 1H, J = 8.6 Hz) (Figure S5); 13C NMR (125 MHz, 25 °C, CDCl3, Si(CH3)4) δ: 12.57, 44.75, 98.05, 109.41, 109.72, 114.67, 126.08, 127.57, 147.54, 151.71, 160.46 (Figure S6).
2.3.3 Synthesis of L To a solution of compound A (0.164 g, 1mmol) in 20 mL of absolute ethanol was added compound C (0.232 g, 1 mmol) and the solution was reflux for half an hour under dinitrogen atmosphere. A light orange colored precipitate was formed and it was filtered, washed with hot absolute ethanol (2 × 20 mL) and then diethyl ether to afford compound L as light orange colored solid. Recrystallization from acetone produced orange red colored crystal suitable for X-ray diffraction. Yield: 70 %; M. Pt. 200 °C; 1H NMR (500 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 1.24 (t, 6H, J = 7.3 Hz), 2.32 (s, 3H), 3.44 (q, 4H, J = 7.3 Hz), 6.51 (d, 1H, J = 1.8 Hz), 6.37 (dd, 1H, J = 2.7, 9.1 Hz), 7.34 (d, 1H, J = 9.1 Hz), 7.54 (s, 1H), 7.68 (d, 2H, J =6.4 Hz), 9.63 (s, 1H), 10.51 (s, 1H), 14.28 (s, 1H) (Figure S7);
13
C NMR
(125 MHz, 25 °C, CDCl3, Si(CH3)4) δ: 12.55, 20.28, 45.06, 97.04, 103.61, 109.80, 123.89, 128.29, 129.59, 131.77, 138.32, 139.13, 151.24, 155.28, 158.58, 162.22, 189.69 (Figure S8); ESI Mass: (m/z): Calcd for 379.1659 [M + H]+ Found 379.1635 (Figure S9). Anal. Calcd. for C22H22N2O4: C, 69.83; H, 5.86; N, 7.40% Found C, 69.78; H, 5.85; N, 7.45%.
3. Results and discussion 3. 1 Synthesis The synthesis of L is shown in Scheme 1. The probe L can be synthesized by adding 2,6-diformyl-4-methylphenol into a hot ethanolic solution of 3-amino-7-(diethylamino)-2Hchromen-2-one in a 1:1 molar ratio. After 30 min, the solution was cooled to room temperature whereupon the product separated as a yellow solid. It was collected by filtration, washed with cold ethanol and air-dried. This compound was characterized satisfactorily by NMR spectroscopy, ESI-Mass spectrometry and elemental analysis as shown in the experimental section.
Scheme 1. Synthetic route to fluorescent sensor L Single crystals of L can be grown by slow evaporation of an acetone solution at room temperature. A perspective view of L is shown in Figure 1 while the crystal data and refinement parameters, bond distances and bond angles are collected in Tables S1 and S2 respectively (supporting information).
Fig. 1. ORTEP diagram of L showing 50% probability thermal ellipsoids on all nonhydrogen atoms. Oxygen atoms are red, carbons are grey, nitrogens are blue and hydrogens are white. 3.2 Spectral characteristics Sensor L is moderately soluble in almost all common organic solvents. The absorption and emission spectra of the dye were recorded in methanolic solution containing 1% DMF as cosolvent. We have tried other solvent systems as well. The emission responses remain unaltered in ethanol as well as in THF. To test the sensing ability of L in organic-aqueous
medium, L was treated with various metal ions in MeOH-H2O (or buffered) system. However, addition of water decreased the emission intensity and completely vanishes in 5 % water (v/v) within 10 minutes. The solvent dependency of the emission response due to the complexation of metal ions by ligands in water is difficult due to the strong solvation of water molecules with metal ions. Also, in MeOH-H2O (or buffered) system the selectivity of the dye L towards metal ions gets lost. The photophysical properties of L were investigated by monitoring the absorption and emission spectral changes that occur upon addition of the perchlorate salt of a cation including, Li+, Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+. Metal-free L shows an absorption maximum at 381 nm with a shoulder at higher wavelength that is redshifted in the presence of Mg2+, Ca2+ and Zn2+ and remain more or less the same in presence of the remaining metal ions (Fig. 2a). A clear isosbestic point is observed at 470 nm when the spectra are recorded with varying concentrations of Mg2+ and Zn2+ (Fig. S10a and S10b in the SI) indicating the existence of a single equilibrium between L and either Mg2+ or Zn2+ ion. The metal-free L exhibits only a weak emission upon excitation at 450 nm due to predominant non-radiative deactivation from C=N bond isomerisation [31] as well as excited state intramolecular proton transfer (ESIPT) process [32-34]. While addition of Li+, Na+, K+, Ca2+ has no effect on the emission, paramagnetic metal ions Mn2+, Fe2+, Co2+, Ni2+ and Cu2+ causes quenching. In contrast, addition of Mg2+ and Zn2+ result in strong enhancement of the emission intensity with a peak at 550 nm for Mg2+ and a peak 558 nm for Zn2+ respectively along with a change in color from light yellow to yellow-orange upon binding with Mg2+ while in presence of Zn2+ ion it changes to orange and hence can be detected through naked eye (Fig. 2b). It is assumed that the photoluminescence band and the extent of fluorescence intensity enhancement depend on the degree of rigidification, and relative orientation of the
hydroxyl group attached to the phenyl moiety. There is about 10 nm shift of the band with the two metal ions that accounts for subtle color differences.
Fig. 2. (a) Absorption spectral nature of the dye L towards various metal ions; (b) Emission spectra of L in the presence of 10 equiv of various metal ions in methanol (contain 1% DMF as co-solvent), excitation at 450 nm, slit: 2.5 nm/3.5 nm [L] = 10 µM. Inset: Color changes upon addition of Mg2+ and Zn2+ ion to the methanolic solution of the dye L. The significant enhancement in the two cases is due to blocking of the C=N bond isomerization resulting in inhibition of the ESIPT process. The quantum yield value further corroborates this result. The quantum yield value (Φ) of the free dye L increase significantly from 0.015 to 0.23 upon binding to Mg2+ where as it changes to 0.14 upon Zn2+ binding.
3.3 Binding stoichiometry of L and Mg2+ or Zn2+ The binding stoichiometry for both the Mg2+ and Zn2+ ions with L were evaluated from the fluorescence titration measurements. The emission titration results shows both Mg2+ and Zn2+ forms 1: 1 complex as there is no change in the emission intensity and reached a plateau upon addition of more than 1 equivalent metal ions (Fig.3a and 3b).
Fig. 3. (a) Fluorescence titration spectra of L (10 µM) in the presence of different concentration of Mg(ClO4)2. Inset: the fluorescence intensity at 550 nm as a function of Mg2+ concentration; (b) Fluorescence titration spectra of L (10 µM) in the presence of different concentration of Zn(ClO4)2. Inset: the fluorescence intensity at 558 nm as a function of Zn2+ concentration. λexc = 450 nm. Slit: 2.5 nm/ 2.5 nm. The binding stoichiometry was further supported from the Job’s plot experiments [35] (Figs. S11 and S12 in the SI). Both Mg2+ and Zn2+ shows maximum emission intensity when the equimolar amounts of L and Mg2+/Zn2+ are present in the solution. The ESI-Mass spectra of both the complexes were recorded. The peaks at 401.1484 (70%) ([(L - H+) + Mg2+]+) for Mg2+ and 524.1762 (25%) ([(L - H+) + Zn2+ + 2CH3CN]+) for Zn2+ respectively clearly provide the additional evidence indicating formation of 1:1 complexes with Mg2+ or Zn2+ ions (Figs. S13 and S14 in the SI). The association constants [36-38] value were obtained from the emission titration results were found to be 2.39 × 104 M-1 for Mg2+ and 2.11 × 104 M-1 for Zn2+ respectively (Figs. S15 and S16 in the SI).
3.4 Selectivity of L to Mg2+ over other metal ions The ion selectivity of L was examined in the presence of Na+, K+ and Ca2+, which are physiologically important. Recently available fluorescent probes for Mg2+, usually suffers from the interference of Ca2+ ion in the quantitative measurement in the cell. In case of L no interference from these ions could be observed. Biologically relevant d-block metal ions
including Mn2+ , Fe2+/Fe3+ , Co2+ , Ni2+ and Cu2+ ions induced a quenching of the emission with a clear exception of Zn2+ ion (Fig. 4a).
Fig. 4. (a) The selectivity of L towards Mg2+ and other metal ions in CH3OH solution. I0 is the fluorescence emission intensity of the sensor L in absence of metal ions at 550 nm and I is the emission intensity at 550 nm in presence of various metal ions. λexc = 450 nm; (b) Selectivity of the dye L for Mg2+ in methanol (containing 1% DMF as co-solvent). Gray bars indicate the fluorometric response of the dye L with 50 equiv of the metal ion of interest (except for Mg2+, Mg2+ ion used = 10 equiv) and green bars represent the final integrated fluorescence response (If) after addition of 10 equiv of Mg2+ to each solution containing other metal ions over the initial integrated emission (I0). The fluorescence enhancement with Zn2+ is relatively lower compared to the value with Mg2+ ion. The strong complexation of ligand L with Mg2+ allows replacement of Zn2+ from the complex by Mg2+ ion. It is found that 10 equivalent excess of Mg2+ion able to replace the Zn2+ ion from the L.Zn2+ complex (Fig. S17 in the SI) however, excess (~ 30 equiv) Zn2+ ion was not able to replace the Mg2+ ion from the L.Mg2+ complex. Furthermore, fluorescence studies indicate that even 50 equivalents physiologically relevant cations Na+, K+ or Ca2+ do not interfere with the L.Mg2+ complex formation (Fig. 4b).
3.5 The irreversible behaviours of the binding of Mg2+ and Zn2+ by L For probing the reversible nature, both the Mg2+ and Zn2+ complexes were treated with sodium salts of various anions such as F–, Br–, I–, AcO–, NO2–, NO3–, HCO3– , BF4– and PF6–. Emission spectroscopy was used to ascertain whether these ions have any effect on the emission characteristics. As can be seen from the Fig. 5 and Fig. 6 that even using excess of
any of these salts had no effect or little effect on the emission quantum yields.. This indicated that the sensing process of L towards Mg2+ or Zn2+ is irreversible in nature.
Fig. 5. Emission responses of the magnesium complex of ligand L (5 µM) towards various anions (200 µM) in methanol (containing 1% DMF as co-solvent). Excitation at 450 nm, slit: 2.5 nm/2.5 nm.
Fig. 6. Emission responses of the zinc complex of ligand L (10 µM) towards various anions (200 µM) in methanol (containing 1% DMF as co-solvent). Excitation at 450 nm, slit: 2.5 nm/3.5 nm.
3.6 Proposed binding mode of L towards Mg2+ or Zn2+ A metal ion binds L at the site shown in Scheme 2. The IR spectra (Figs. S18-S20 in the SI) of free L and its Mg2+ and Zn2+ complexes are consistent with this mode of binding.
Scheme 2. Proposed binding mode of L with metal ion. In the IR spectra, the stretching vibration of coumarin carbonyl at 1709 cm-1 and that of C=N bond stretching vibration at 1594 cm-1 decreased by 16 cm-1 and 7 cm-1 respectively upon Mg2+ binding whereas the changes was 16 cm-1 and 9 cm-1 upon Zn2+ binding. The intense broad band at 1100 cm-1 in the metal complexes is an indicative [39] of the presence of ionic perchlorate meaning the fourth coordination site of the metal ion may be occupied by a solvent molecule. The 1H NMR titration experiments also support this proposed mechanism (Figs. S21 and S22 in the SI). The intramolecular hydrogen bond leads to chemical shift of the OH group appearing at a very low field, 14.3 ppm. There is a downfield shift of the hydroxyl proton upon gradual addition of both Mg2+ and Zn2+ to the solution of L in CDCl3:CD3CN (1:1,v/v) and became broad, which can be attributed to the breaking of the intramolecular hydrogen bond by Mg2+//Zn2+····O–H interactions. Meanwhile, the signals of the hydrogen atoms in aromatic rings and CH=N showed a significant up-field shift, indicating interaction of the aromatic groups with Mg2+//Zn2+ ions. These results also suggested that the carbonyl oxygen of coumarin ring, imine nitrogen as well as phonolic oxygen may participate in binding with Mg2+/Zn2+.
4. Conclusions In conclusion, we have designed a fluorescent probe L based on C=N bond isomerisation mechanism that gives very strong fluorescence upon addition of Mg2+ ion as well as Zn2+ ion. The binding of Mg2+ is highly selective, allowing its detection in the presence of alkali, alkaline earth metal ions as well as host of first-row transition-metal ions. We are presently working on other acyclic receptors with selectivity for particular metal ion in aqueous environment.
Acknowledgments We gratefully acknowledge the financial support received from the Department of Science and Technology, New Delhi, India (to PKB) and SRF from the CSIR to SBM.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/xxx/j.jlumin.2014.xx.xxx. Crystallographic data for the structure reported in this paper have also been deposited with the CCDC as deposition no. CCDC 988879 (available free of charge, on application to the CCDC, 12 Union Rd., Cambridge CB2 1EZ, U.K.; e-mail [email protected]). References [1] H. Rubin, Arch. Biochem. Biophys. 458 (2007) 16. [2] H. Rubin, Bioessays 27 (2005) 311. [3] J. R. Moll, A. Acharya, J. Gal, A. A. Mir, C. Vinson, Nucleic Acids Res. 30 (2002) 1240. [4] T. Shoda, K. Kikuchi, H. Kojima, Y. Urano, H. Komatsu, K. Suzuki, T. Nagano, Analyst. 128 (2003) 719. [5] F. I. Wolf, A. Torsello, A. Fasanella, A. Cittadini, Mol. Aspects Med. 24 (2003) 11.
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Highlight of the manuscript ¾ A Schiff base incorporating a coumarin fluorophore has been synthesized. ¾ It acts as a dual analyte sensor and quantifies Mg2+ and Zn2+ ions by emission enhancement at different wavelengths. ¾ It shows excellent selectivity for Mg2+ ion in presence of alkali, alkaline earth metals as well as first row transition metals.
A molecular dual fluorescence-ON probe for Mg2 þ and Zn2 þ : Higher selectivity Towards Mg2 þ over Zn2 þ in A mixture Shubhra Bikash Maity, Parimal K. Bharadwaj
www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(14)00354-8 http://dx.doi.org/10.1016/j.jlumin.2014.06.020 LUMIN12747
To appear in:
Journal of Luminescence
Received date: 24 March 2014 Revised date: 28 May 2014 Accepted date: 10 June 2014 Cite this article as: Shubhra Bikash Maity, Parimal K. Bharadwaj, A molecular dual fluorescence-ON probe for Mg2 þ and Zn2 þ : Higher selectivity Towards Mg2 þ over Zn2 þ in A mixture, Journal of Luminescence, http://dx.doi.org/10.1016/j. jlumin.2014.06.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Revised Manuscript Number: LUMIN-D-14-00378
A Molecular Dual Fluorescence-ON Probe for Mg2+ and Zn2+: Higher Selectivity Towards Mg2+ over Zn2+ in a Mixture
Shubhra Bikash Maity and Parimal K. Bharadwaj Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India Email:[email protected]
ABSTRACT A Schiff base incorporating a coumarin fluorophore has been synthesized from easily available materials and is characterized by X-ray crystallography and other techniques. The probe serves as a dual analyte sensor and quantifies Mg2+ and Zn2+ ions by emission enhancement at different wavelengths without interference from a host of biologically relevant alkali/alkaline earth and transition metal ions. In presence of Mg2+ the light yellow color of the probe in methanol changes to yellow-orange while in presence of Zn2+ ion it changes to orange and hence can be detected through naked eye. The probe selectively gives emission of Mg2+ when Zn2+ ion is also present. Keywords: Schiff base, Chemosensor, Magnesium, Zinc, Selectivity. 1. Introduction Magnesium ion is one of the most abundant divalent metal ions in bio-systems. This ion plays significant roles both in cell proliferation as well as in cell death and is also involved [1-10] in the regulation of hundreds of enzymes and molecules having different structures and functions. Alteration in total or free magnesium can have significant consequences for cell metabolism and its functions. Selective detection of Mg2+ in the presence of other biologically relevant Ca2+, Na+ and K+ or transition-metal ions is, therefore, of particular importance. A number of fluorescent chemosensors for Mg2+ have been reported with receptors that included diaza-18-crown-6 [11], benzo-15-crown-5 [12], charged β
diketone [13-16], calix[4]arene [17, 18], porphyrin related macrocycles [19, 20] and iminelinked compounds [21-24]. Most of them require large number of synthetic steps or interference from other metal ions. Therefore, development of chemosensors that selectively and sensitively detect and quantify Mg2+ ion particularly in the presence of alkali, alkaline earth and biologically relevant transition metal ions continues to be an important goal. In the present study, we report a sensor L that can detect Mg2+ ion. It can also detect Zn2+ via emission at a different wavelength although Mg2+ exhibits greater selectivity over Zn2+ when both are present. We have chosen coumarin as the flurophore because it exhibits large Stokes shift and visible excitation and emission wavelengths [25-27]. 2. Experimental 2.1 Materials Reagent grade 4-diethylaminosalicylaldehyde, ethylnitroacetate and all metal perchlorate salts were bought from Aldrich Chemicals (USA), while 4-methylphenol, nbutanol, piperidine and acetic acid were bought from S. D. Fine Chemicals (INDIA). All chemicals were used as received without further purification. All solvents were bought from S. D. Fine Chemicals (INDIA) and were dried prior to use following the standard literature procedure. All reactions were carried out under dinitrogen protection unless stated. 2.2 Instrumentation 1
H NMR (500 MHz) and
13
C NMR spectra (125 MHz) of the compounds were
recorded on a JEOL DELTA2 spectrometer in CDCl3 with Si(CH3)4 as the internal standard. The ESI-Mass data were obtained in methanol from a WATERS-Q-Tof Premier Mass Spectrometer. UV-vis spectra were recorded on a Shimadzu 2450 UV-Vis spectrophotometer in methanol medium at 298 K. Melting points were determined with an electrical melting point apparatus by PERFIT, India and were uncorrected. Steady-state fluorescence spectra were obtained using a Perkin-Elmer LS 50B Luminescence Spectrometer at 293 K with
excitation and emission band-pass of 2.5 and 2.5 nm or 3.5 nm. The excitation wavelength was 450 nm and the spectra were recorded in the range 490-700nm. Fluorescence quantum yield [28, 29] were determined by comparing the corrected spectra with that of fluorescein in ethanol taking the total area under the fluorescence spectra using the following equation ΦS = ΦR (FSAR/FRAS) × (ηS/ηR) 2 ……………… (i) Where, Φ stands for quantum yield, F stands for area under the fluorescence spectra, A stands for absorbance value and η stands for the refractive index value. The subscript ‘R’ indicates the value of the parameter for reference (i.e. fluorescein) and ‘S’ subscript indicates the value of the parameter for the sample. 2.3 Synthesis 2.3.1 Synthesis of A Compound A was synthesized according to the previously reported method [30]. A 250 mL round bottom flask equipped with spinning bar was charged with 4-methylphenol (5.17 g, 47.81 mmol) and hexamine (13.40 g, 95.62 mmol). To this trifluoroaceticacid (40 mL, 0.52 mol) was added and the yellow solution became hot. A reflux condenser was fitted and the solution was heated to 145 °C for 24 h, during which it turned dark brown. The hot solution was poured into ~ 100 mL 4N HCl solution and stirred for 2 hours, during which it becomes solidified. The solid was filtered and washed several times with ice cold water (2 × 100 mL) to make it acid free. The white solid obtained was dried over P2O5 yields 2.95 g of the desired product. Yield: 37.6 %; M. Pt. 130 °C; 1H NMR (500 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 2.37 (s, 3H), 7.75 (s, 2H), 10.19 (s, 2H), 11.43 (s, 1H) (Figure S1); 13C NMR (125 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 20.17, 122.98, 129.61, 138.08, 161.85, 192.30 (Figure S2).
2.3.2 Synthesis of B and C Compound B and C were synthesized according to the literature method reported earlier [23]. Synthesis of B: To a solution of 4-diethylaminosalicylaldehyde (1.4 g, 7.24 mmol) and ethylnitroacetate (0.80 mL, 7.96 mmol) in 25 mL of n-butanol was added catalytic amount of piperidine (0.1 mL) and glacial acetic acid (0.2 mL) and the reaction mixture was refluxed for 12 h. The bright red solid formed during the course of reaction was filtered under hot condition, washed 2 to 3 times with n-BuOH (2 × 20 mL) and finally dried under vacuum to give 1.7 gm of nitro derivative. Yield: 90 %; M. Pt. 170 °C; 1H NMR (500 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 1.26 (t, 6H, J = 6.9 Hz), 3.48 (q, 4H, J = 6.9 Hz), 6.46 (d, 1H, J = 2.3Hz), 6.70 (dd, 1H, J = 2.3 & 9.1 Hz), 7.41 (d, 1H, J = 9.1 Hz), 8.69 (s, 1H) (Figure S3); 13C NMR (125 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 12.50, 45.61, 96.88, 106.28, 111.23, 126.91, 132.65, 143.42, 153.55, 154.66, 158.85 (Figure S4). Synthesis of C: In a 100 mL RB flask stannous chloride (4.3 g, 19.06 mmol) and 20 mL 15% HCl solution were taken. To this compound B (1 g, 3.82 mmol) was added portion wise over a period of 15 minutes and the solution was stirred at room temperature for 6 hours. Then NaOH solution (5M) was added under ice cold condition and the pH of the solution was adjusted to ~10. The aqueous phase was extracted with ethyl acetate (3 × 30 mL). The organic layer was dried over anhydrous Na2SO4 and evaporated to dryness yielded the amine component as yellowish-orange solid. Yield: 90 %; M. Pt. 85 °C; 1H NMR (500 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 1.16 (t, 6H, J = 7.2 Hz), 3.36 (q, 4H, J = 7.2 Hz), 3.85 (s, br, 2H), 6.51 (s, 1H ), 6.57 (d, 1H, J = 8.4 Hz), 6.69 (s, 1H), 7.10 (d, 1H, J = 8.6 Hz) (Figure S5); 13C NMR (125 MHz, 25 °C, CDCl3, Si(CH3)4) δ: 12.57, 44.75, 98.05, 109.41, 109.72, 114.67, 126.08, 127.57, 147.54, 151.71, 160.46 (Figure S6).
2.3.3 Synthesis of L To a solution of compound A (0.164 g, 1mmol) in 20 mL of absolute ethanol was added compound C (0.232 g, 1 mmol) and the solution was reflux for half an hour under dinitrogen atmosphere. A light orange colored precipitate was formed and it was filtered, washed with hot absolute ethanol (2 × 20 mL) and then diethyl ether to afford compound L as light orange colored solid. Recrystallization from acetone produced orange red colored crystal suitable for X-ray diffraction. Yield: 70 %; M. Pt. 200 °C; 1H NMR (500 MHz, CDCl3, 25 °C, Si(CH3)4) δ: 1.24 (t, 6H, J = 7.3 Hz), 2.32 (s, 3H), 3.44 (q, 4H, J = 7.3 Hz), 6.51 (d, 1H, J = 1.8 Hz), 6.37 (dd, 1H, J = 2.7, 9.1 Hz), 7.34 (d, 1H, J = 9.1 Hz), 7.54 (s, 1H), 7.68 (d, 2H, J =6.4 Hz), 9.63 (s, 1H), 10.51 (s, 1H), 14.28 (s, 1H) (Figure S7);
13
C NMR
(125 MHz, 25 °C, CDCl3, Si(CH3)4) δ: 12.55, 20.28, 45.06, 97.04, 103.61, 109.80, 123.89, 128.29, 129.59, 131.77, 138.32, 139.13, 151.24, 155.28, 158.58, 162.22, 189.69 (Figure S8); ESI Mass: (m/z): Calcd for 379.1659 [M + H]+ Found 379.1635 (Figure S9). Anal. Calcd. for C22H22N2O4: C, 69.83; H, 5.86; N, 7.40% Found C, 69.78; H, 5.85; N, 7.45%.
3. Results and discussion 3. 1 Synthesis The synthesis of L is shown in Scheme 1. The probe L can be synthesized by adding 2,6-diformyl-4-methylphenol into a hot ethanolic solution of 3-amino-7-(diethylamino)-2Hchromen-2-one in a 1:1 molar ratio. After 30 min, the solution was cooled to room temperature whereupon the product separated as a yellow solid. It was collected by filtration, washed with cold ethanol and air-dried. This compound was characterized satisfactorily by NMR spectroscopy, ESI-Mass spectrometry and elemental analysis as shown in the experimental section.
Scheme 1. Synthetic route to fluorescent sensor L Single crystals of L can be grown by slow evaporation of an acetone solution at room temperature. A perspective view of L is shown in Figure 1 while the crystal data and refinement parameters, bond distances and bond angles are collected in Tables S1 and S2 respectively (supporting information).
Fig. 1. ORTEP diagram of L showing 50% probability thermal ellipsoids on all nonhydrogen atoms. Oxygen atoms are red, carbons are grey, nitrogens are blue and hydrogens are white. 3.2 Spectral characteristics Sensor L is moderately soluble in almost all common organic solvents. The absorption and emission spectra of the dye were recorded in methanolic solution containing 1% DMF as cosolvent. We have tried other solvent systems as well. The emission responses remain unaltered in ethanol as well as in THF. To test the sensing ability of L in organic-aqueous
medium, L was treated with various metal ions in MeOH-H2O (or buffered) system. However, addition of water decreased the emission intensity and completely vanishes in 5 % water (v/v) within 10 minutes. The solvent dependency of the emission response due to the complexation of metal ions by ligands in water is difficult due to the strong solvation of water molecules with metal ions. Also, in MeOH-H2O (or buffered) system the selectivity of the dye L towards metal ions gets lost. The photophysical properties of L were investigated by monitoring the absorption and emission spectral changes that occur upon addition of the perchlorate salt of a cation including, Li+, Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+. Metal-free L shows an absorption maximum at 381 nm with a shoulder at higher wavelength that is redshifted in the presence of Mg2+, Ca2+ and Zn2+ and remain more or less the same in presence of the remaining metal ions (Fig. 2a). A clear isosbestic point is observed at 470 nm when the spectra are recorded with varying concentrations of Mg2+ and Zn2+ (Fig. S10a and S10b in the SI) indicating the existence of a single equilibrium between L and either Mg2+ or Zn2+ ion. The metal-free L exhibits only a weak emission upon excitation at 450 nm due to predominant non-radiative deactivation from C=N bond isomerisation [31] as well as excited state intramolecular proton transfer (ESIPT) process [32-34]. While addition of Li+, Na+, K+, Ca2+ has no effect on the emission, paramagnetic metal ions Mn2+, Fe2+, Co2+, Ni2+ and Cu2+ causes quenching. In contrast, addition of Mg2+ and Zn2+ result in strong enhancement of the emission intensity with a peak at 550 nm for Mg2+ and a peak 558 nm for Zn2+ respectively along with a change in color from light yellow to yellow-orange upon binding with Mg2+ while in presence of Zn2+ ion it changes to orange and hence can be detected through naked eye (Fig. 2b). It is assumed that the photoluminescence band and the extent of fluorescence intensity enhancement depend on the degree of rigidification, and relative orientation of the
hydroxyl group attached to the phenyl moiety. There is about 10 nm shift of the band with the two metal ions that accounts for subtle color differences.
Fig. 2. (a) Absorption spectral nature of the dye L towards various metal ions; (b) Emission spectra of L in the presence of 10 equiv of various metal ions in methanol (contain 1% DMF as co-solvent), excitation at 450 nm, slit: 2.5 nm/3.5 nm [L] = 10 µM. Inset: Color changes upon addition of Mg2+ and Zn2+ ion to the methanolic solution of the dye L. The significant enhancement in the two cases is due to blocking of the C=N bond isomerization resulting in inhibition of the ESIPT process. The quantum yield value further corroborates this result. The quantum yield value (Φ) of the free dye L increase significantly from 0.015 to 0.23 upon binding to Mg2+ where as it changes to 0.14 upon Zn2+ binding.
3.3 Binding stoichiometry of L and Mg2+ or Zn2+ The binding stoichiometry for both the Mg2+ and Zn2+ ions with L were evaluated from the fluorescence titration measurements. The emission titration results shows both Mg2+ and Zn2+ forms 1: 1 complex as there is no change in the emission intensity and reached a plateau upon addition of more than 1 equivalent metal ions (Fig.3a and 3b).
Fig. 3. (a) Fluorescence titration spectra of L (10 µM) in the presence of different concentration of Mg(ClO4)2. Inset: the fluorescence intensity at 550 nm as a function of Mg2+ concentration; (b) Fluorescence titration spectra of L (10 µM) in the presence of different concentration of Zn(ClO4)2. Inset: the fluorescence intensity at 558 nm as a function of Zn2+ concentration. λexc = 450 nm. Slit: 2.5 nm/ 2.5 nm. The binding stoichiometry was further supported from the Job’s plot experiments [35] (Figs. S11 and S12 in the SI). Both Mg2+ and Zn2+ shows maximum emission intensity when the equimolar amounts of L and Mg2+/Zn2+ are present in the solution. The ESI-Mass spectra of both the complexes were recorded. The peaks at 401.1484 (70%) ([(L - H+) + Mg2+]+) for Mg2+ and 524.1762 (25%) ([(L - H+) + Zn2+ + 2CH3CN]+) for Zn2+ respectively clearly provide the additional evidence indicating formation of 1:1 complexes with Mg2+ or Zn2+ ions (Figs. S13 and S14 in the SI). The association constants [36-38] value were obtained from the emission titration results were found to be 2.39 × 104 M-1 for Mg2+ and 2.11 × 104 M-1 for Zn2+ respectively (Figs. S15 and S16 in the SI).
3.4 Selectivity of L to Mg2+ over other metal ions The ion selectivity of L was examined in the presence of Na+, K+ and Ca2+, which are physiologically important. Recently available fluorescent probes for Mg2+, usually suffers from the interference of Ca2+ ion in the quantitative measurement in the cell. In case of L no interference from these ions could be observed. Biologically relevant d-block metal ions
including Mn2+ , Fe2+/Fe3+ , Co2+ , Ni2+ and Cu2+ ions induced a quenching of the emission with a clear exception of Zn2+ ion (Fig. 4a).
Fig. 4. (a) The selectivity of L towards Mg2+ and other metal ions in CH3OH solution. I0 is the fluorescence emission intensity of the sensor L in absence of metal ions at 550 nm and I is the emission intensity at 550 nm in presence of various metal ions. λexc = 450 nm; (b) Selectivity of the dye L for Mg2+ in methanol (containing 1% DMF as co-solvent). Gray bars indicate the fluorometric response of the dye L with 50 equiv of the metal ion of interest (except for Mg2+, Mg2+ ion used = 10 equiv) and green bars represent the final integrated fluorescence response (If) after addition of 10 equiv of Mg2+ to each solution containing other metal ions over the initial integrated emission (I0). The fluorescence enhancement with Zn2+ is relatively lower compared to the value with Mg2+ ion. The strong complexation of ligand L with Mg2+ allows replacement of Zn2+ from the complex by Mg2+ ion. It is found that 10 equivalent excess of Mg2+ion able to replace the Zn2+ ion from the L.Zn2+ complex (Fig. S17 in the SI) however, excess (~ 30 equiv) Zn2+ ion was not able to replace the Mg2+ ion from the L.Mg2+ complex. Furthermore, fluorescence studies indicate that even 50 equivalents physiologically relevant cations Na+, K+ or Ca2+ do not interfere with the L.Mg2+ complex formation (Fig. 4b).
3.5 The irreversible behaviours of the binding of Mg2+ and Zn2+ by L For probing the reversible nature, both the Mg2+ and Zn2+ complexes were treated with sodium salts of various anions such as F–, Br–, I–, AcO–, NO2–, NO3–, HCO3– , BF4– and PF6–. Emission spectroscopy was used to ascertain whether these ions have any effect on the emission characteristics. As can be seen from the Fig. 5 and Fig. 6 that even using excess of
any of these salts had no effect or little effect on the emission quantum yields.. This indicated that the sensing process of L towards Mg2+ or Zn2+ is irreversible in nature.
Fig. 5. Emission responses of the magnesium complex of ligand L (5 µM) towards various anions (200 µM) in methanol (containing 1% DMF as co-solvent). Excitation at 450 nm, slit: 2.5 nm/2.5 nm.
Fig. 6. Emission responses of the zinc complex of ligand L (10 µM) towards various anions (200 µM) in methanol (containing 1% DMF as co-solvent). Excitation at 450 nm, slit: 2.5 nm/3.5 nm.
3.6 Proposed binding mode of L towards Mg2+ or Zn2+ A metal ion binds L at the site shown in Scheme 2. The IR spectra (Figs. S18-S20 in the SI) of free L and its Mg2+ and Zn2+ complexes are consistent with this mode of binding.
Scheme 2. Proposed binding mode of L with metal ion. In the IR spectra, the stretching vibration of coumarin carbonyl at 1709 cm-1 and that of C=N bond stretching vibration at 1594 cm-1 decreased by 16 cm-1 and 7 cm-1 respectively upon Mg2+ binding whereas the changes was 16 cm-1 and 9 cm-1 upon Zn2+ binding. The intense broad band at 1100 cm-1 in the metal complexes is an indicative [39] of the presence of ionic perchlorate meaning the fourth coordination site of the metal ion may be occupied by a solvent molecule. The 1H NMR titration experiments also support this proposed mechanism (Figs. S21 and S22 in the SI). The intramolecular hydrogen bond leads to chemical shift of the OH group appearing at a very low field, 14.3 ppm. There is a downfield shift of the hydroxyl proton upon gradual addition of both Mg2+ and Zn2+ to the solution of L in CDCl3:CD3CN (1:1,v/v) and became broad, which can be attributed to the breaking of the intramolecular hydrogen bond by Mg2+//Zn2+····O–H interactions. Meanwhile, the signals of the hydrogen atoms in aromatic rings and CH=N showed a significant up-field shift, indicating interaction of the aromatic groups with Mg2+//Zn2+ ions. These results also suggested that the carbonyl oxygen of coumarin ring, imine nitrogen as well as phonolic oxygen may participate in binding with Mg2+/Zn2+.
4. Conclusions In conclusion, we have designed a fluorescent probe L based on C=N bond isomerisation mechanism that gives very strong fluorescence upon addition of Mg2+ ion as well as Zn2+ ion. The binding of Mg2+ is highly selective, allowing its detection in the presence of alkali, alkaline earth metal ions as well as host of first-row transition-metal ions. We are presently working on other acyclic receptors with selectivity for particular metal ion in aqueous environment.
Acknowledgments We gratefully acknowledge the financial support received from the Department of Science and Technology, New Delhi, India (to PKB) and SRF from the CSIR to SBM.
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Highlight of the manuscript ¾ A Schiff base incorporating a coumarin fluorophore has been synthesized. ¾ It acts as a dual analyte sensor and quantifies Mg2+ and Zn2+ ions by emission enhancement at different wavelengths. ¾ It shows excellent selectivity for Mg2+ ion in presence of alkali, alkaline earth metals as well as first row transition metals.