Easily accessible ferric ion chemosensor based on rhodamine derivative and its reversible OFF–ON fluorescence response

Tetrahedron 70 (2014) 7527e7533

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Easily accessible ferric ion chemosensor based on rhodamine derivative and its reversible OFFeON fluorescence response Haiyang Liu a, b, Xuejuan Wan a, b, *, Liqiang Gu b, Tianqi Liu b, Youwei Yao b, * a Key Laboratory of Special Functional Materials of Shenzhen, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, PR China b Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 June 2014 Received in revised form 23 July 2014 Accepted 5 August 2014 Available online 10 August 2014

An easily accessible ferric ion (Fe3þ) fluorescence chemosensor was designed and synthesized via microwave reaction based on rhodamine derivative. This sensor displayed a favorable selectivity for Fe3þ ion over a range of other common metal cations in acetonitrile/Tris buffer (4:1, v/v, pH 7.0) with a 1:1 stoichiometry, leading to prominent fluorescence OFFeON switching of the rhodamine fluorophore. The association constant between the sensor and Fe3þ was determined to be 1.51105, and the corresponding detection limit was calculated to be 3.2107 M according to fluorescence titration analysis. The recognizing behavior of the chemosensor toward Fe3þ has been investigated in detail by FTIR, NMR, and HRMS measurements. Additionally, the recognition process of the chemosensor for Fe3þ is chemically reversible on the addition of phosphate anion ðPO3 4 Þ. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Fluorescent chemosensor Easy-to-get Ferric ion OFFeON Phosphate anion Reversibility

1. Introduction As one of the most essential trace elements in biological systems, ferric ion (Fe3þ) plays an important role in many biochemical processes at the cellular level.1 Abnormal concentrations of Fe3þ ion within the body can induce serious biological disorders,2 such as liver and kidney damage, heart failure, and diabetes. Therefore, rapid and efficient detecting of Fe3þ ion is essential for real-time monitoring of drinking water and biological sample. In the past decades, numerous conventional methods have been intensively utilized for the detection of trace ferric ions,3 such as atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectrometry (ICP-AES), inductively coupled plasma mass spectroscopy (ICP-MS), and anodic stripping voltammetry. However, the above-mentioned expensive and time-consuming detection methods are inappropriate to meet the demand for realtime monitoring. Due to their high sensitivity, visual simplicity, instantaneous response, and real-time detection, fluorescent techniques are the most convenient methods and are extremely desired by downstream users,4 and consequently numerous

* Corresponding authors. Tel.: þ86 (0)755 26534059; fax: þ86 (0)755 26534457 (X.W.); tel./fax: þ86 (0)755 26036796 (Y.Y.); e-mail addresses: [email protected] (X. Wan), [email protected] (Y. Yao). http://dx.doi.org/10.1016/j.tet.2014.08.008 0040-4020/Ó 2014 Elsevier Ltd. All rights reserved.

fluorescent chemosensors for Fe3þ were reported in recent years.5 Considering the application potential in the near future, the exploration of concise synthesis procedure for Fe3þ fluorescence chemosensor with most inexpensive and easy-to-get starting material still possesses significant research potential. Rhodamine fluorophore modified with selective ligand has consistently demonstrated its potential to construct fluorescent sensors, ascribing to their excellent optical performance (high absorption coefficient and fluorescent quantum yield).2f,6 These rhodamine derivatives (spirolactam form) are usually non-fluorescent, whereas the ion-recognition induced ring-opening of corresponding spirolactam can give rise to strong fluorescence emission.7 Based on the above equilibrium, diverse fluorescence OFFeON chemosensors for transition metal ions have been reported,8 including Cu2þ, Hg2þ, Pb2þ, Zn2þ, Cr3þ, and Fe3þ. Yang et al. synthesized a novel rhodamine-azacrown derivative and reported that the obtained molecule can selectively detect Fe3þ and Al3þ cations in different solvents.8c Quite recently, our group developed a watersoluble Cr3þ chemosensor with rhodamine-labeled nanogel, which possesses tunable detection sensitivity.9 In view of recent literature, heteroatoms in proper chemical environment could act as chelating sites toward metal ions, such as the carbonyl (eCOe) oxygen,10 amide (eCONHe) nitrogen,11 amino (eNH2 and eNHe) nitrogen,5b,12 imide (eN]CRe) nitrogen,5a,13 and hydroxyl (eOH) oxygen14 et al. Given this, many fluorescent

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chemosensors for Fe3þ detection containing appropriate heteroatoms based on chromophores were developed.15,16 Considering the complexity of the synthetic route, hard-to-get starting material and not-rise yield of the target probes, we try to design some facile fluorescent sensors based on simple chromophores. Here, we design and synthesize a new and simple rhodamine B-based chemosensor (RhBEP, Scheme 1) for Fe3þ detection by facile synthesis via microwave reaction (only 15 min). The obtained chemosensor displayed favorable detection selectivity for Fe3þ over other common metal ions in acetonitrile/Tris buffer (4:1, v/v), accompanied with macroscopic fluorescence signal ‘turn-on’. The carbonyl oxygen atoms of the rhodamine spirocyclic and o-phthalic anhydride are supposed to act as chelating sites for Fe3þ. The sensing mechanism toward the reversible recognition of Fe3þ was studied in detail via FTIR, 1H NMR, and HRMS.

153.65, 153.45, 148.73, 133.41, 132.45, 132.30, 131.19, 129.00, 127.95, 123.87, 123.34, 122.88, 108.14, 105.60, 97.89, 65.27, 44.30, 39.16, 36.96, 12.64. HRMS calcd for C38H39N4O4 [RhBEPþH]þ: 615.2966, found: 615.2956. Anal. Calcd for C38H38N4O4: C, 74.24; H, 6.23%; N, 9.11. Found: C, 74.11; H, 6.41; N, 9.07%. 2.3. Testing and calculating methods 2.3.1. General spectroscopic procedures. Stock solutions (0.001 M) of chlorinated salts (Naþ, Kþ, Mg2þ, Ca2þ, Co2þ, Ni2þ, Fe2þ, Al3þ, Cr3þ, Cu2þ, Zn2þ, Fe3þ) and perchlorate salts (Hg2þ, Pb2þ, Cd2þ) were first prepared with 10 mM TriseHCl buffer solution (pH 7.0). RhBEP stock solution was also prepared in acetonitrile (CH3CN). Test solutions were prepared by placing calculation amount of RhBEP stock solution into a test tube, adding an appropriate aliquot

Scheme 1. Schematic illustration for the synthesis of RhBEP.

2. Experimental section 2.1. Materials and general methods Rhodamine B (98%, Aladdin), ethylenediamine (99%, Aladdin), and o-phthalic anhydride (99%, Aladdin) were used as received. Other reagents were purchased from Sinopharm Chemical Reagent Co. and used as received. Water was deionized with a Milli-QSP reagent water system (Millipore) to a specific resistivity of 18.4 MU cm. All nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Ultrashield TM 400 PLUS spectrometer. UVevis spectroscopy measurements were acquired on an SCINCO S-4100 UV/Vis spectrophotometer. Fluorescence spectra were recorded using a Jobin Yvon FluoroLog-3-TCSPC spectrofluorometer. Mass spectrum (MS) was obtained with a Waters Q-TOF premier Mass Spectrometer. IR spectra were taken in KBr disks on a Bruker VERTEX 70 spectrometer. Microwave reactions were conducted by CEM DISCOVER microwave reactor made in USA. 2.2. Synthesis of RhBEP General approach employed for the synthesis of RhBEP is shown in Scheme 1. Rhodamine B ethylenediamine (RhB-EN) was synthesized according to literature procedures.17 Into a special microwave reaction tube (CEM configuration of 85 mL) equipped with a magnetic stirring bar, RhB-EN (0.242 g, 0.5 mmol), o-phthalic anhydride (0.074 g, 0.5 mmol), and 35 mL anhydrous toluene were added sequentially. The reaction mixture was stirred at room temperature for 5 min and then heated under microwave irradiation for 15 min at 120  C. After cooling to room temperature in a high speed air flow bath, the solvent was removed under reduced pressure and the crude product was purified by column chromatography on SiO2 using EtOAc/n-hexane (1:2, v/v) as the eluant to afford RhBEP as a white powder (0.252 g, 82% yield). 1H NMR (400 MHz, CDCl3) d 7.88e7.85 (m, 1H), 7.73 (dd, J¼5.4, 3.0 Hz, 2H), 7.63 (dd, J¼5.4, 3.0 Hz, 2H), 7.41 (dd, J¼5.5, 3.1 Hz, 2H), 7.05e7.02 (m, 1H), 6.47 (d, J¼8.8 Hz, 2H), 6.36 (s, 2H), 6.15 (d, J¼8.8 Hz, 2H), 3.65 (t, J¼5.7 Hz, 2H), 3.42 (t, J¼5.7 Hz, 2H), 3.26 (q, J¼7.0 Hz, 8H), 1.14 (t, J¼7.0 Hz, 12H). 13C NMR (100 MHz, CDCl3) d 168.69, 168.12,

of each metal or anions stock, and then diluting the solution to 3.0 mL with acetonitrile and buffer solution. RhBEPeFe3þ solution for anions detection was prepared via adding 10 equiv of Fe3þ and then appropriate amount of anions to RhBEP solution. All fluorescence spectra were recorded 2 h after the addition and mixing at 25  C with the excitation wavelength set at 520 nm (excitation/ emission slit widths: 5 nm). 2.3.2. Fluorescent quantum yield. The fluorescence quantum yield (Funk) was determined relative to Rhodamine 6G (FR¼0.94 in ethanol) as standard according to the previous literature18 and Funk was calculated using the following equation (Eq. 1):



Funk ¼ Fstd $

    2 h Iunk Astd $ $ unk hstd Istd Aunk

(1)

where I and A were the integrated fluorescence intensities and the absorption, respectively, and h was the indices of refraction of the solutions, and unk and std represented the sample and standard compound solutions, respectively. 2.3.3. Calculation of the association constant. The association constant of RhBEPeFe3þ was obtained from nonlinear curve fitting of the fluorescence titration data according to BenesieHildebrand equation (Eq. 2),19 where F0, F, and Fmax are the fluorescence intensity of RhBEP in the absence of Fe3þ, at a certain concentration of Fe3þ ion, and a complete-interaction concentration of Fe3þ, [M] is the metal ion concentration, n is the binding stoichiometry, and Ka is the association constant.

  Fmax  F ¼ n log½M þ log Ka log F  F0

(2)

3. Results and discussion Taking advantages of the equilibrium between spirolactam (non-fluorescent) to ring-open amide (fluorescent) form of rhodamine chromophore, RhBEP was utilized as effective chemosensor for Fe3þ in CH3CN/Tris buffer (4:1, v/v, pH 7.0). This fluorescence probe was facilely synthesized by RhB-EN and o-phthalic anhydride

via microwave reaction in toluene as shown in Scheme 1, and its structure was then confirmed via 1H NMR (Fig. S1), 13C NMR (Fig. S2), MS measurements (Fig. S3), and FTIR measurements (Fig. S4). 3.1. Fluorescence OFFeON sensing of Fe3D cation The detection selectivity of RhBEP (10 mM) toward Fe3þ ions over 10 equiv other common metal cations such as Naþ, Kþ, Mg2þ, Ca2þ, Co2þ, Ni2þ, Fe2þ, Al3þ, Cr3þ, Cu2þ, Zn2þ, Hg2þ, Pb2þ, and Cd2þ in CH3CN/Tris buffer (4:1, v/v, pH 7.0) was first investigated and the results were shown in Fig. 1. RhBEP shows scarcely fluorescence emission among the range of 540e700 nm in the mixed solution, indicating that the chemosensor exists as a spirocycle-closed form initially. In CH3CN/Tris buffer media, the addition of Fe3þ can lead to a most remarkable fluorescence enhancement over other cations except Cr3þ (Cr3þ only enhanced the emission intensity of RhBEP to some extent), along with a clear color change from colorless to pink, since there is an Fe3þ-induced ring-opening under this condition (Fig. 1). Fluorescence quantum yield for RhBEP/Fe3þ was calculated as F¼0.48 (RhBEP with 20 equiv Fe3þ) (while F was calculated only 0.11 for RhBEP/Cr3þ under the same conditions), using rhodamine 6G (in ethanol) as a reference.18 In a competitive study, the solution of RhBEPeFe3þ showed almost no changes in the presence of other common metal ions (Fig. S5). Distinctive colorimetric transition from colorless to pink and a fluorometric transition from non-fluorescent to strong bright pink fluorescent emission can also be clearly discerned by naked eye (Fig. 1b), indicating that RhBEP could serve as a selective OFFeON fluorescence and colorimetric sensor for Fe3þ in CH3CN/Tris buffer (4:1, v/v, pH 7.0).

Fluorescence Intensity

(a)

3x10

6

2x10

6

3+

Fe

Fluorescence Intensity

H. Liu et al. / Tetrahedron 70 (2014) 7527e7533

6.0x10

6

4.0x10

6

2.0x10

6

0.0

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(a) Only RhBEP (b) RhBEP+ Fe

2

4

6

8

10

3+

12

pH Fig. 2. Fluorescence intensity recorded for RhBEP (10 mM, CH3CN/Tris buffer (4:1, v/v)) at various pH values in the (a) absence and (b) presence of 20 equiv Fe3þ (lex¼520 nm, lem¼582 nm). The pH value was modulated by adding 2.0 M HCl and 2.0 M NaOH solutions.

(for RhBEP) located at 582 nm decreased with increasing pH value (at acidic conditions, pH<5.0), because rhodamine derivative was in the open-ring state with proton; at pH>5.0, the fluorescence intensity almost did not vary with different pH value change owing to ring-closed spirolactam form transition of the rhodamine moiety. The addition of Fe3þ can lead to the fluorescence enhancement over a comparatively mild pH range (5.0e8.0), which is similar with most of the rhodamine-based fluorescence probes in literature reports.20e24 Acidic environment was disadvantageous for the detection of Fe3þ, as the pH-induced similar spirocycle-opening reaction for rhodamine unit will also arouse strong fluorescence emission, which is not suitable for cation sensing. Considering the chemosensor possesses most remarkable fluorometric intensity changes in Fe3þ-sensing under physiological pH window, all the fluorescence measurements were conducted in CH3CN/Tris buffer (pH 7.0).

3.3. Titration analysis +

1x10

6

0 540

+

2+

3+

2+

2+

Blank,Na ,K ,Ca ,Al ,Co ,Mg , 2+ 2+ 2+ 2+ 2+ 2+ 2+ Ni ,Cu ,Zn ,Fe ,Pb ,Cd ,Hg 3+

Cr

570

600 630 660 Wavelength (nm)

690

Fe3þ-sensing capability of the obtained chemosensor was further investigated in detail with spectrophotometric titration analysis. Typical UVevis absorbance spectra recorded for RhBEP upon gradual addition of Fe3þ ions were shown in Fig. 3. Addition of increased amounts of Fe3þ to the solution of RhBEP could lead to the appearance and enhancement of a new absorption band at

(b)

0.5 0.4

Absorption

0.5

0.3

Fig. 1. (a) Fluorescence emission spectra, and (b) optical photographs (visible light and UV 365 nm) obtained for RhBEP (10 mM for a, and 20 mM for b) in CH3CN/Tris buffer (4:1, v/v, pH 7.0) after the addition of 10.0 equiv of Kþ, Naþ, Mg2þ, Cr3þ, Ca2þ, Al3þ, Ni2þ, Fe3þ, Fe2þ, Cu2þ, Zn2þ, Pb2þ, Cd2þ, Hg2þ, and Co2þ, respectively (lex¼520 nm).

3.2. pH effect to the sensing system The effect of pH on fluorescence response of the chemosensor RhBEP to Fe3þ was also investigated. The variation of fluorescence intensity for RhBEP probe against different pH values in the absence and presence of Fe3þ (20 equiv) was recorded and the results were shown in Fig. 2. It could be observed that the fluorescence intensity

Absorption

0.2

0.4

0.1 0.0 0

0.3

5

10

15

20

[Fe3+]/[RhBEP]

25

0.2 0.1 0.0 450

500

550

600

650

Wavelength (nm) Fig. 3. UVevis spectra and (inset) absorbance changes (560 nm) recorded for RhBEP (10 mM) upon gradual addition of Fe3þ (0e25 equiv) in CH3CN/Tris buffer (4:1, v/v, pH 7.0).

H. Liu et al. / Tetrahedron 70 (2014) 7527e7533

6.0x10

4.0x10

F lu o re s c e n c e In te n s ity

Fluorescence Intensity

560 nm, accompanied with a w110-fold enhancement for absorbance intensity, which can be assigned to the Fe3þ ion-triggered ring-opening reaction of the RhBEP. The excitation wavelength for Fe3þ-sensing was located at 520 nm, considering the integrity of the fluorescence emission spectra. The variations of the fluorescence intensity versus Fe3þ concentration were also recorded in the fluorescence titration measurement. Accordingly, upon gradual addition of Fe3þ ions (0e25 equiv, relative to [RhBEP]), fluorescence emission band centered at 582 nm dramatically increased w150fold, indicating that the opened-ring form of RhBEP turned into the main species upon Fe3þ binding, which could be clearly discerned in Fig. 4. With addition of Fe(III), the fluorescence intensity of the testing solution increased gradually and saturated after addition of about 20 equiv of Fe (III) (Fig. 4, inset). This trend kept similar to ultraviolet absorption titration analysis (Fig. 3, inset), which was also consistent with Jadeja’s and Li’s research.15 Plotting of the fluorescence intensity versus Fe3þ concentration (0e1.0 equiv) afforded a good linear relationship (R2¼0.9888) (Fig. S6). The detection limit (DL) was then determined to 3.2107 M from the equation DL¼k*Sb1/S,25 suggesting that RhBEP has high specificity toward Fe3þ under current working conditions (The United States Environmental Protection Agency, China’s Ministry of Health and Health Canada have set the maximum contaminant level for iron in drinking water at 0.30 mg/L, 5.4106 M). To evaluate the application of this chemosensor RhBEP in monitoring Fe3þ ions in drinking water, RhBEP was applied to the determination of Fe(III) in tap water/acetonitrile (1:4, v/v). Fe3þ ion was deliberately introduced to simulate contaminated tap water. As shown in Fig. S7, the fluorescence intensity increased gradually upon the addition of Fe3þ (0e20 mM). The result indicated the suitability of this chemosensor for the determination of Fe3þ ions in a natural water sample. Compared with the reported rhodamine-based sensors,15,16 this system may possess good potentiality for daily real-time monitoring of ferric ions in drinking water because of lower detection limit and the concise synthetic route of the chemosensor.

6

Fe

3+

6

6.0x10

6

4.0x10

6

2.0x10

6

0.0 0

2.0x10

5

10

3+

15

20

[Fe ]/ [RhBEP]

25

6

0.0 540

570

600

630

660

from pink to colorless. These findings demonstrated that phosphate anion could react with ferric ion to form a low-solubility product (FePO4, Ksp¼1.31022) and clenched ferric ion replacing RhBEP in the mixed solution (acetonitrile/Tris buffer, 4:1). Then, PO3 4 was chosen as a quenching unit to study the reversibility of the coordination process. After addition of 10 equiv Na3PO4 to the 10 mM RhBEP with Fe3þ (10 equiv), the fluorescence intensity at 582 nm was quenched (a2, line B in Fig. 5) due to the competitive binding of 3þ Fe3þ from RhBEP by PO3 4 , and further addition of 10 equiv Fe could recover the strong fluorescence again (a3, line C in Fig. 5). This reversible process was also monitored by 1H NMR (Fig. S9). 3þ When PO3 solution, the chem4 was introduced into RhBEPþFe ical shifts in 1H NMR returned to the original state (the rhodamine unit with ring-closed), which was similar to previous reports.26,27 These indicated that the recognition process of compound RhBEP with Fe3þ is chemically reversible.

Fluorescence Intensity

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3x10

6

2x10

6

1x10

6

0 540

C

A

a2

a1

a3

B

570

600 630 Wavelength (nm)

660

690

Fig. 5. Fluorescence intensity (at 582 nm) of RhBEP (10 mM) to Fe3þ in CH3CN/Tris buffer solution (4:1, v/v, pH 7.0). (A) Line A: RhBEP with 10 equiv Fe3þ; (B) line B: RhBEP with 10 equiv Fe3þ and then addition of 10 equiv PO3 4 ; (C) line C: RhBEP with 3þ 10 equiv Fe3þ and PO3 (inset under UV: a1, 4 , then addition of 10 equiv Fe 3þ 3þ 3 RhBEPþFe ; a2, RhBEPþFe , addition of PO4 ; a3, RhBEPþFe3þ, addition of PO3 4 , 3þ then addition of Fe ).

To determine the stoichiometry of the ferriceligand complex, Job’s method for absorbance measurement was carried out. Keeping the sum of the initial concentration of Fe3þ and RhBEP at 50 mM, the molar ratio of Fe3þ was varied from 0 to 1. The maximum absorption band was reached at a molar fraction of 0.5 (Fig. 6). According to n¼x/(1x), in which x is the molar fraction of Fe3þ corresponding to the maximal absorbance change and n is the binding stoichiometry, 0.5 indicates the 1:1 stoichiometry between Fe3þ and RhBEP.

690

Wavelength (nm)

3.4. Sensing mechanism The reversibility is an important aspect of any chemical sensor for detection of specific metal ions. In order to investigate the reversibility in colorimetric and fluorogenic signaling of RhBEPeFe3þ complex, a series of sodium salts of different anions such as ClO 4,   3 Br, SO2 (AcO), HPO2 were, re4 , NO3 , CH3COO 4 , and PO4 spectively, added into the solution containing RhBEP and Fe3þ, and their fluorescence intensity changes were monitored (Fig. S8). Among all these anions, the enhanced fluorescence intensity due to cation coordination to RhBEP immediately reduced upon addition of PO3 4 , accompanied with subsequent colorimetric transition

Absorption

Fig. 4. Fluorescence emission spectra and (inset) variation of fluorescence intensity recorded for RhBEP (10 mM) upon gradual addition of Fe3þ (0e25 equiv) in CH3CN/Tris buffer (4:1, v/v, pH 7.0) (lex¼520 nm).

0.3

0.2

0.1 0.5

0.0 0.0

0.2

0.4 3+

0.6

0.8

1.0

3+

X={[Fe ]/[Fe +RhBEP]} Fig. 6. Job’s plot obtained for RhBEP and Fe3þ ion in CH3CN/Tris buffer (4:1, v/v, pH 7.0). The total concentration of RhBEP and Fe3þ was fixed at 50 mM. The absorbance was measured at 560 nm.

H. Liu et al. / Tetrahedron 70 (2014) 7527e7533

Another direct evidence to study the bonding mechanism between cation and the chemosensor was obtained by comparing the ESI mass spectra of RhBEPeFe3þ complex with the original sensor. As shown in Fig. 7, the cluster peak at m/z¼740.1609 (calcd¼740.1614) corresponding to [RhBEPþFe3þþ2Cl]þ and m/ z¼776.1378 (calcd¼776.1381) corresponding to [RhBEPþFe3þþ3ClþHþ]þ can be clearly observed when 2 equiv of FeCl3 was added to RhBEP, suggesting a 1:1 metal-probe binding stoichiometry. Based on this 1:1 model, the association constant Ka between RhBEP and Fe3þ was calculated to be 1.51105 from nonlinear curve fitting of the fluorescence titration data (Figs. 4 and S6), according to BenesieHildebrand equation.19

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change. The characteristic amide carbonyl of rhodamine unit absorption peak at 1689 cm1 shifted to 1619 cm1 and another characteristic amide carbonyl from o-phthalic anhydride unit absorption peak at 1719 cm1 shifted to 1704 cm1 in the presence of Fe3þ ion, indicating that these two amide carbonyl oxygens were actually involved in the recognition of Fe3þ, which was consistent with Yang’s and Zeng’s research.8c,30 1H NMR spectra of RhBEP in the absence and presence of Fe3þ ion were also compared as shown in Fig. 8. The paramagnetic nature of Fe3þ ion broadened the 1H NMR spectra and almost all the chemical shift of protons had changed upon addition of Fe3þ, especially the protons of rhodamine chromophore (such as He, Hf, Hg, and Hh, the Dd was up to 0.88, 1.12,

Fig. 7. ESI mass spectra of RhBEP in the presence of FeCl3 (2 equiv), indicating the formation of a 1:1 RhBEP/Fe3þ complex.

In order to have more insight into the cation binding properties of the RhBEP, FTIR spectra (Fig. S10) and 1H NMR spectra (Fig. 8) of RhBEP and RhBEPeFe3þ complex in CD3OD were measured. In the peculiar molecular structure of RhBEP, there exist three amide carbonyls in which the heteroatoms O and N are considered with the ability to coordinate metal ion according to the previous literature.28,29 The shift of amide carbonyl absorption band in IR spectrum was proved to be an effective method to confirm the structure

d

o,l

g f

e CD

3

e

(a) H2O

h

,n b,c +m a

8

o,l g

d,f e

7

jk

(b)

6

h

k

j

OD

d

4. Conclusions gf

3

o,l

i

CD

a

Enlarged figure

b,c

+m ,n

OD

a b,c+m,n

1.20, and 0.40 ppm, respectively) and the two methylenes (eCH2CH2e) between rhodamine unit and phthalimide (the Dd of Hj and Hk was 0.15 and 0.13 ppm, respectively). This indicated that the opening of the spirolactam ring in RhBEP and the significant changes of chemical circumstance of the two methylenes occurred in the presence of Fe3þ, which was also consistent with pervious reports.25c,31 According to these results, a possible interaction mode for RhBEP with Fe3þ was proposed in Scheme 2. Once the Fe3þ ion encountered the chemosensor RhBEP, the heteroatoms O of the rhodamine spirocyclic and o-phthalic anhydride unit could act as chelating sites for Fe3þ, and the spirolactam ring turned open, leading to fluorescence turn-on. When PO3 4 was introduced into the above system, Fe3þ ion was abstracted by the anion, and the spirolactam ring restored to its original state (ring-closed), leading to the oneoff switching fluorescence.

5

4

3

δ / ppm Fig. 8. 1H NMR spectra of RhBEP in CD3OD in absence (a) and presence (b) of Fe3þ.

In conclusion, an easily accessible fluorescent chemosensor (RhBEP) for Fe3þ was synthesized and reported. Compared with other Fe3þ chemosensors, RhBEP with concise skeleton (by reacting RhB-EN with o-phthalic anhydride) can be facilely synthesized with two inexpensive and easily available starting materials by microwave reaction, and showed excellent selectivity and sensitivity to Fe3þ in CH3CN/Tris (4:1, v/v) with dramatic enhanced fluorescence intensity and color change (from colorless to red). In this paper, the fluorescence OFFeON mechanism of Fe3þ-sensing was adequately supported by reversible recognition measurement, Job’s plot

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Scheme 2. Proposed mechanism for the recognition of Fe3þ.

evaluation, HRMS, FTIR, and 1H NMR study. The detection limit of the sensor for Fe3þ is low to 3.2107 M according to fluorescence titration analysis.

8.

Acknowledgements The financial support from National Natural Science Foundation of China (NSFC) Project (21204042), Fundamental Research Project of Shenzhen (JCYJ20120830152316443) and Technology Innovation Program of Shenzhen (CXZZ20130322101824104) is gratefully acknowledged.

9. 10.

Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.tet.2014.08.008.

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