Bis(rhodamine)-based polyether type of turn-on fluorescent sensors: selectively sensing Fe(III)

Accepted Manuscript Bis(rhodamine)-based polyether type of turn-on fluorescent sensors: selectively sensing Fe(III) Yang-Lin Lin, Robert Sung, Kuangsen Sung PII:

S0040-4020(16)30748-7

DOI:

10.1016/j.tet.2016.07.077

Reference:

TET 27973

To appear in:

Tetrahedron

Received Date: 4 March 2016 Revised Date:

17 July 2016

Accepted Date: 30 July 2016

Please cite this article as: Lin Y-L, Sung R, Sung K, Bis(rhodamine)-based polyether type of turn-on fluorescent sensors: selectively sensing Fe(III), Tetrahedron (2016), doi: 10.1016/j.tet.2016.07.077. 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 proof before it is published in its final 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.

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Graphical Abstract

Bis(rhodamine)-based polyether type of turn-on fluorescent sensors: selectively sensing Fe(III) Yang-Lin Lin, Robert Sung and Kuangsen Sung

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Department of Chemistry, National Cheng Kung University, Tainan, Taiwan, ROC

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Tetrahedron journal homepage: www.elsevier.com

Yang-Lin Lin, a Robert Sung a,† and Kuangsen Sung a, ∗ a

Department of Chemistry, National Cheng Kung University, Tainan, Taiwan, ROC

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Bis(rhodamine)-based polyether type of turn-on fluorescent sensors: selectively sensing Fe(III)

Due to the paramagnetic nature of Fe(III), receptor ligand becomes one of the key points in developing fluorescent Fe(III) sensors. We report that a new type of receptor ligands that consist of a polyether capped with two amide groups of N-(rhodamine-6G)lactam, selectively sense Fe(III). Its detection limit for Fe(III) is as low as 10 µM, and its linear range for sensing Fe(III) is 14-69 µM.

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Article history: Received Received in revised form Accepted Available online

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2009 Elsevier Ltd. All rights reserved.

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Keywords: fluorescent sensors rhodamine Fe(III)

——— ∗ Corresponding author; E-mail: [email protected] †

Dr. Robert Sung’s current address: Faculty of Family Medicine, Northern Ontario School of Medicine, Ontario, Canada

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2. Results and discussion

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The bis(rhodamine)-based polyether type of sensor 1 or 2 was prepared by treating tetraethylene glycol or hexaethylene glycol with tosyl chloride, then with NaN3, and then with PPh3, followed by 2 equivalent of rhodamine 6G in methanol in the presence of NEt3. (Scheme 1) HO

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1. TsCl, NaOH, THF/H2O 2. NaN3, DMF 3. PPh3, THF 4. 2 eq. rhodamine 6G

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Figure 1. 2/Fe(III), 2/Cr(III) and 2/other metal-ions complexes in 1:1 CH3CN/H2O, where other metal-ions are Na(I), K(I), Ca(II), Mg(II), Pb(II), Co(II), Cu(II), Ni(II), Zn(II) and Hg(II) The Fe(III)-sensing mechanism of 1 and 2 is proposed in Scheme 2. Before sensing Fe(III) or Cr(III), the N-(rhodamine6G)lactam group in 1 and 2 stays in the lactam form, which is colorless because the xanthene chromophore does not stay in fully conjugated resonance. A new absorption peak at 530 nm and a new emission at 550 nm appear when 1 and 2 sense Fe(III) or Cr(III), indicating that Fe(III) or Cr(III) induces the ringopening of the lactam form of rhodamine7 in 1 and 2 to form the highly conjugated form of rhodamine, which makes the energy gap between the HOMO and the LUMO smaller and absorbs visible light.

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The Fe(III) is one of the most essential trace elements in biological systems, and plays important roles in cellular metabolism.1a,b Metabolism disorders involving Fe(III) may cause anemia, liver and kidney damage, diabetes, and heart failure.1c,d In addition, Fe(III) also involves biological electron transfer that is mediated by a heterogeneous group of redox cofactors in the oxidative phosphorylation.2 The orderly flow of electrons through chains of the redox cofactors is fundamental to the generation of metabolic energy.2 Hence, development of Fe(III) sensors is important. So far, few fluorescent Fe(III) sensors3 have been developed because the paramagnetic nature of Fe(III) makes development of fluorescent Fe(III) sensors become a challenge. The Fe(III) has 5 electrons in 3d orbital. Weak-field ligands in spectrochemical series make Fe(III) high-spin, causing Fe(III) highly paramagnetic,4 and that significantly quenches fluorescence according to the photophysical principle.5 On the other hand, strong-field ligands make Fe(III) low-spin,6 and that will not quench fluorescence much. As a result, one of the key points in developing fluorescent Fe(III) sensors is what type of receptor ligand one adopts. The receptor ligands for the known fluorescent Fe(III) sensors include amide, amine, azacrown ether, hydroxylamine, hydroxamate, and triazole.3 We wonder if a polyether type of ligands are qualified to become a receptor ligand for sensing Fe(III). Hence, we prepared 1 and 2, both of which consist of a polyether type of receptor ligands capped with two amide end groups of N-(rhodamine-6G)lactam.

2 emit fluorescence at 550 nm. We also did competition experiment of Fe(III) with other metal-ions, fluorescence intensity for the solutions of 1/ Fe(III) and 2/ Fe(III) was not reduced.

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2 1. Introduction

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Scheme 1. Synthesis of the polyether type of bis(rhodamine)based sensors 1 and 2 Both 1 and 2 in 1:1 CH3CN/H2O are non-fluorescent and colorless. For the metal-ion sensing test, 1 or 2 at the concentration of 18 µM in 1:1 CH3CN/H2O was treated with 1 equivalent of various metal-ions such as Na(I), K(I), Ca(II), Mg(II), Pb(II), Co(II), Cu(II), Ni(II), Fe(III), Cr(III), Zn(II) and Hg(II). What we found was that Fe(III) made the solutions of 1 and 2 turn pink significantly, and a new absorption peak at 530 nm appeared. (Fig. 1) Similarly, Cr(III) made the solutions of 1 and 2 turn very pale pink, and a new and very small absorption peak at 530 nm appeared. But, other metal-ions did not change the color of the solutions of 1 and 2. Similarly, Fe(III) made the solutions of 1 and 2 emit fluorescence at 550 nm significantly, Cr(III) made the solutions of 1 and 2 emit fluorescence at 550 nm mildly, and other metal-ions did not make the solutions of 1 and

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Scheme 2. Proposed Fe(III)-sensing mechanism for 1 and 2 In order to have insights into the stoichiometry for the 1/Fe(III), 2/Fe(III), 1/Cr(III), and 2/Cr(III) complexes, we carried out a Job plot for each complex, where the total molar concentration of the metal-ion [Fe(III) or Cr (III)] and the ligand (1 or 2) was held constant, but their mole fractions were varied. Absorbance (A) of the solution at 530 nm is plotted against the mole fraction of the ligand. The maxima in these plots occur at the mole fraction of 0.5, indicating that these complexes are 1:1 complex. One of the Job plots is shown in Fig. 2, where the stoichiometry of 2/Fe(III) complex was investigated.

Figure 2. Job’s plot for the 2/Fe(III) complex in 1:1 CH3CN/H2O

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normalized fluorescence intensity of 1, 2 and 3 was plotted against log of changing concentrations of Fe(III) and Cr(III). Linear regression analysis was applied to these curves in order to find out the linear range8 of 1, 2 and 3 for sensing Fe(III) and Cr(III). The point at which the linear regression curve crosses the X-ordinate axis is taken as the detection limit.8

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Figure 3. The 1H NMR spectrum of 2 in CD3CN (top 1) and its enlarged view (top 3). The 1H NMR spectrum of 2 in CD3CN with addition of 0.3 eq of Fe(NO3)3 in D2O (top 2) and its enlarged view (bottom).

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We also used 1H NMR spectra to confirm that 2/Fe(III) forms a 1:1 complex and the polyether part of 2 functions as a multidentate ligand for Fe(III) in the complex. Because Fe(III) is paramagnetic, it would make resonance peaks of 1H NMR spectra broad and poor resolution. As shown in Fig. 3, after addition of 0.3 eq of Fe(NO3)3 into 2 in CD3CN/D2O, the integration areas and chemical shifts of the resonance peaks around 3.5-3.2 ppm (-OCH2CH2O-) do change, and based on the resonance peaks around 8.0-6.0 ppm (the bottom spectrum in Fig. 3), only two chemicals [2 and 1:1 2/Fe(III) complex] can be found in the solution. Once Fe(III) is bound with 2, the lactam form of rhodamine group, which does not have chelationenhanced fluorescence (CHEF), will be converted into the highly conjugated form, which shows CHEF, and three types of protons on the phenyl group are shifted to higher field. (the bottom spectrum in Fig. 3) If one Fe(III) were bound with only one of two rhodamine groups in 2, the solution of 2 with 0.3 eq of Fe(NO3)3 would have a chance to form 1:2 2/Fe(III) complex with two Fe(III) binding with two rhodamine groups of 2. Then, we should see at least three chemicals in the solution, such as 2, 1:1 2/Fe(III) complex with one Fe(III) chelated with only one rhodamines, and 1:2 2/Fe(III) complex with two Fe(III) chelated with two rhodamines. However, only two chemicals [2 and 1:1 2/Fe(III) complex] can be found in the solution, indicating that the 1:1 2/Fe(III) complex has one Fe(III) chelated with two rhodamines and polyether at the same time. This is also supported by the Job plot (Fig. 2) we did with UV spectrometer. In addition, this result is consistent with another experimental result that Fe(III) is chelated with the azacrown ether and carbonyl moiety in the rhodamine-azacrown ether conjugate.3g However, addition of more than 10 eq of Fe(III) makes fluorescence emission of the rhodamine-azacrown ether conjugate shift from 575 nm to 525 nm, indicating that several Fe(III) and the rhodamine-azacrown ether conjugates aggregate together to form complicated 1:3.3 complexes. This phenomenon does not occur in the titration of 2 with Fe(III), where 1:1 2/Fe(III) complex was found. It is likely because 2 and the rhodamine-azacrown ether conjugate have completely different structures even though they both have rhodamine and polyether moiety.

To find out if the polyether part of 1 or 2 functions as a multidentate ligand that cooperates with the amide group of N(rhodamine-6G)lactam in selectively sensing Fe(III), we prepared 311 as a control compound, where only amide and hydroxyl groups may bind with Fe(III). Job plots have also been done for 3/Fe3+ and 3/Cr3+ complexes, and they were confirmed to be 1:1 complexes. One of the Job plots is shown in Fig. 4, where the stoichiometry of 3/Fe(III) complex was investigated. In order to get the detection limit and the linear range of 1, 2 and 3 for sensing Fe(III) and Cr(III), we did fluorescence titration of 1, 2 and 3 with Fe(III) and Cr(III), and these fluorescence titration figures are shown in Fig. 5, 6 and 7. The increase in fluorescence intensity produced by increasing the concentration of Fe(III) and Cr(III) from 0 to the specified concentration is clearly resolved and has a good signal-to-noise ratio. The fluorescence intensity data were normalized between the minimum fluorescence intensity and the maximum fluorescence intensity according to the equation of [(Imin - I)/(Imin - Imax)]. The

Figure 4. Job’s plot for the 3/Fe(III) complex in 1:1 DMSO/H2O

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Figure 5. Fluorescence titration of 1 (19 µM) with Fe3+ (0, 17, 35, 52, 69, 87, 104, 139 and 173 µM) (top) and with Cr3+ (0, 19, 39, 58, 77, 96, 116, 154, 193 and 289 µM) in 1:1 CH3CN/H2O (bottom)

Figure 6. Fluorescence titration of 2 (14 µM) with Fe3+ (0, 14, 28, 42, 55, 69, and 139 µM) (top) and with Cr3+ (0, 15, 31, 46, 62, 77, 92, 123, 154, 231, 308, 462 and 616 µM) in 1:1 CH3CN/H2O (bottom)

Figure 7. Fluorescence titration of 3 (19 µM) with Fe3+ (0, 19, 38, 57, 76, 95, 114, 152 and 191 µM) (top) and with Cr3+ (0, 21, 42, 64, 85, 106, 127, 169, 212, 318, 424 and 635 µM) in 1:1 DMSO/H2O (bottom)

The detection limit and the linear range of 1 for sensing Fe(III) are 33 µM and 35-104 µM, respectively. The detection limit and the linear range of 1 for sensing Cr(III) are 38 µM and 39-154 µM, respectively. For 2, its detection limit and linear range for sensing Fe(III) are 10 µM and 14-69 µM, respectively, and its detection limit and linear range for sensing Cr(III) are 30 µM and 46-154 µM, respectively. The detection limit and the linear range of 3 for sensing Fe(III) are 22 µM and 38-114 µM, respectively. The detection limit and the linear range of 3 for sensing Cr(III) are 43 µM and 85-212 µM, respectively. Some fluorescent Fe(III) sensors have the detection limit as low as 0.2 µM, but they all are turn-off fluorescent carbon or quantum dots9. To my knowledge, 2 hits a record-low detection limit for the turn-on fluorescent Fe(III)-sensing. [CM] = (KD)[(F-Fmin)/(Fmax-F)]=(1/KSTAB)[(F-Fmin)/(Fmax-F)]

(1)

In order to obtain the stability constants (KSTAB) for the complexes of 1/Fe(III), 1/Cr(III), 2/Fe(III), 2/Cr(III), 3/Fe(III) and 3/Cr(III), fluorescence titration of 1, 2 and 3 with Fe(III) and Cr(III) (Fig. 4, 5 and 6) and the well-known Eq. 110 were used, where [CM] is metal ion concentration, KD is dissociation constant, KSTAB is stability constant, Fmin is the fluorescence intensity when the ligand is in the free form, Fmax is the fluorescence intensity when the ligand is totally complexed, F is the fluorescence intensity when the ligand is partially complexed by the metal ion. As shown in Table 1, the stability constants of the 1/Fe(III), 2/Fe(III) and 3/Fe(III) complexes are much higher than those of the corresponding 1/Cr(III), 2/Cr(III) and 3/Cr(III) complexes, indicating that 1, 2 and 3 sense Fe(III) much better than Cr(III). The stability constant of the 1/Fe(III) complex bearing 3 ether oxygen atoms is a little higher than that of the

ACCEPTED MANUSCRIPT 3/Fe(III) complex, while the stability constant of the 2/Fe(III) complex bearing 5 ether oxygen atoms is much higher than that of the 3/Fe(III) complex, indicating that the polyether part of 2 bearing 5 ether oxygen atoms significantly binds with Fe(III).

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Figure 8. The suggested binding pattern for the 2/Fe(III) complex

Table 1. The stability constants (KSTAB) for the complexes of 1/Fe(III), 1/Cr(III), 2/Fe(III), 2/Cr(III), 3/Fe(III) and 3/Cr(III) Complex

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3. Conclusion

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It was reported that linear polyethers as multidentate ligands wrap a metal cation preferentially in an equatorial zone around the metal cation.11 It is also known that Fe(III) can form stable seven-coordinate12 or eight-coordinate13 complexes. Hence, we suggest that the compound 2 might bind with Fe(III) like the complex shown in Fig. 8, where the linear polyether sevendentate ligand of 2 wrap Fe(III) in an equatorial zone around the Fe(III) cation.

dried over MgSO4, and concentrated under vacuum. To the residue was added NaN3 (1.1 g, 15.5 mmol) in 25 mL of DMF. The reaction was stirred at 60 °C for 24 hrs, cooled to room temperature, and extracted with ethyl acetate. The combined organic layers were dried over MgSO4, and concentrated under vacuum. To the residue was added triphenylphosphine (4 g, 15.3 mmol) in 20 mL of THF. The reaction was stirred at 25 °C for 8 hrs, and extracted with toluene. The combined aqueous layers were concentrated under vacuum. To the residue in 6 mL of MeOH and 1 mL of NEt3 was added Rhodamine 6G (3.45 g, 7.2 mmol). The reaction was stirred under reflux for 8 hrs, and cooled to room temperature. The precipitate was collected by filtration and purified by column chromatography to give 1 (21%) or 2 (14%) as dark red solid. 1: 1HNMR (400 MHz, CDCl3) δ 7.90 (m, 2H), 7.41(t, J=3.6 Hz, 4H), 7.01 (m, 2H), 6.32 (s, 4H), 6.22 (s, 4H), 3.51 (s, 4H), 3.27-3.26(m, 12H), 3.21 (q, J=7.2 Hz, 8H), 3.08 (t, J=7.2 Hz, 4H), 1.85(s,12H) 1.31 (t, J=7.2 Hz, 12H); 13C NMR (100 MHz, CDCl3) δ 167.8, 153.4, 151.1, 146.9, 131.9, 130.4, 127.9, 127.4, 123.2, 122.2, 117.3, 105.4, 96.1, 69.8, 69.3, 67.2, 64.4, 38.7, 37.8, 16.2, 14.2; HRMS (FAB, M+) m/z calcd for C60H68N6O7 984.5149, found 984.5158. 2: 1 HNMR(400MHz, CDCl3) δ 7.90 (t, J=3.6 Hz, 2H), 7.41(t, J=3.6 Hz, 4H), 7.01 (m, 2H), 6.32 (s, 4H), 6.22 (s, 4H), 3.28-3.51(m, 20H), 3.21 (q, J=7.2 Hz, 8H), 3.10 (t, J=7.2 Hz, 4H), 1.85(s, 12H) 1.31 (t, J=7.2 Hz, 12H); 13C NMR (100MHz, CDCl3) δ 169.3, 154.8, 152.6, 146.9, 131.9, 130.4, 127.9, 127.4, 123.2, 122.2, 117.3, 106.9, 97.5, 71.4, 70.9, 68.7, 65.9, 40.2, 39.3, 17.6, 15.7; HRMS (FAB, M+) m/z calcd for C64H76N6O9 1072.5673, found 1072.5665.

We thank the National Science Council of Taiwan for financial support (NSC101-2113-M-006-001-MY3).

Supplementary Material

Supplementary data associated with this article can be found in the online version. References and notes 1.

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According to the 1H NMR spectrum of the 2/Fe(III) complex and the stability constants for the complexes of 1/Fe(III), 1/Cr(III), 2/Fe(III), 2/Cr(III), 3/Fe(III) and 3/Cr(III), the polyether part of 2 functions as a multidentate ligand that cooperates with the amide group of N-(rhodamine-6G)lactam in selectively sensing Fe(III).

Acknowledgments

4. Experimental section 4.1. General

Na(I), K(I), Ca(II), Mg(II), Pb(II), Co(II), Cu(II), Ni(II), Fe(III), Cr(III), Zn(II) and Hg(II) of nitrate salts and all the reagents were purchased from Aldrich Co., Ltd. The compound 3 was prepared according to the literature.14 4.2. General method to prepare 1 and 2 To a solution of tetraethylene glycol (or hexaethylene glycol) (5.2 mmol) and NaOH (1.7 g, 42.5 mmol) in 45 mL of THF and 5 mL of water was added tosyl chloride (3 g, 15.7 mmol). The reaction was stirred at 0 °C for 1 h and 25 °C for 4 hrs, and extracted with ethyl acetate. The combined organic layers were

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