Highly selective fluorescent chemosensors for the detection of Hg2+ based on photochromic diarylethenes with a terminal terpyridine unit

Accepted Manuscript 2+ Highly selective fluorescent chemosensors for the detection of Hg based on photochromic diarylethenes with a terminal terpyridine unit Shouzhi Pu, Hongjing Jia, Congbin Fan, Gang Liu, Yinglong Fu, Shuhong Jing PII:

S0040-4020(15)00393-2

DOI:

10.1016/j.tet.2015.03.069

Reference:

TET 26551

To appear in:

Tetrahedron

Received Date: 26 January 2015 Revised Date:

9 March 2015

Accepted Date: 20 March 2015

Please cite this article as: Pu S, Jia H, Fan C, Liu G, Fu Y, Jing S, Highly selective fluorescent 2+ chemosensors for the detection of Hg based on photochromic diarylethenes with a terminal terpyridine unit, Tetrahedron (2015), doi: 10.1016/j.tet.2015.03.069. 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.

ACCEPTED MANUSCRIPT

Graphical Abstract

Highly selective fluorescent chemosensors for the detection of

terpyridine unit

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Hg2+ based on photochromic diarylethenes with a terminal

Shouzhi Pu*, Hongjing Jia, Congbin Fan, Gang Liu, Yinglong Fu, Shuhong Jing

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Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, PR China author:

E-mail

[email protected]

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+86-791-83831996.

address:

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*Corresponding

1

(S.

Pu);

Tel./Fax:

ACCEPTED MANUSCRIPT

Highly selective fluorescent chemosensors for the detection of Hg2+ based on photochromic diarylethenes with a terminal terpyridine unit

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Shouzhi Pu*, Hongjing Jia, Congbin Fan, Gang Liu, Yinglong Fu, Shuhong Jing Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, PR China

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Abstract: Two new diarylethenes with a terpyridine unit were synthesized and their photochromic and

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fluorescent properties were investigated. The diarylethenes exhibited favorable photochromism by UV/Vis irradiation. Compared to the one with a vinyl-linked terpyridine unit, the one with a simple benzene linker had a smaller molar absorption coefficient and absorption maximum, and larger cyclization/cycloreversion quantum yields. Hg2+ coordination with the diarylethenes resulted in

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remarkable photochromism and fluorescence changes. The fluorescence intensity of the diarylethene with a benzene linker was enhanced significantly by 2.5-fold and its emission peak exhibited a notable redshift from 420 to 463 nm with an evident color change from dark blue to bright cyan. In contrast, the one with

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a vinyl linker showed a dramatic increment of fluorescence intensity by 35-fold with a color change from

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darkness to bright green. Both of the diarylethenes were highly selective towards the recognition of Hg2+ without interference from other metal ions. Keywords: Photochromism; Diarylethene; Terpyridine; Hg2+ Chemosensor; Fluorescence switch.

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ACCEPTED MANUSCRIPT 1. Introduction To date, numerous efforts have been made in the development of effective fluorescent probes for the recognition of metal ions on the basis of photochromic diarylethenes with functionalized groups due to

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their instantaneous response, visual simplicity, and high sensitivity.1-3 For instance, Tian and co-workers reported a new multistate 1,8-naphthalimidepiperazine-tethered dithienylethene molecule with fluorescence tunable by Cu2+, proton, and light.4 They also reported several new chemsensors based on

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photochromic perhydrogencyclopentene in response to ions such as Hg2+, F-, and Al3+.5,6 Bozec and

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co-workers synthesized a dithienylethene containing 2,20-bipyridine and demonstrated its fluorescence change induced by Zn2+, Re+, and Ru2+.7 Previously, our group revealed that the diarylethenes with a rhodamine or salicylidene Schiff base unit could be selective chemosensors towards Al3+ with significant color and fluorescence changes.8-10 The obtained results have contributed to a broader understanding of

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the specific recognition characteristics of photochromic diarylethenes toward metal ions. Mercury is one of the most prevalent toxic pollutants and its inorganic ion Hg2+ can be converted into neurotoxic methylmercury induced by marine bacteria. The toxin accumulates in seafood, thereby

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entering food chain.11-15 Once ingested, methylmercury triggers several nervous disorders such as

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acrodynia, Hunter–Russell syndrome, and Minamata disease.16 Therefore, sensitive and on-site detection of Hg2+ is of great concern in environmental and food chemistry. Among various methods for the detection of Hg2+, fluorescent chemosensors have been developed as a general method by their distinct advantages of high sensitivity, good selectivity, quick response, and simple instrumentation. Although some fluorescent sensors for the detection of Hg2+ have been reported,17-19 there are still only a few sensors with ideal sensitivity and selectivity for Hg2+ recognition due to its fluorescence quenching nature via enhanced spin-orbit coupling associated with the heavy atom effect.

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ACCEPTED MANUSCRIPT Terpyridine is one of the most prominent N-heterocyclic ligands for transition metal ions, and its derivatives can serve as versatile templates in the field of supramolecular and coordination chemistry, as well as material sciences.23-25 The strong binding affinities make them highly remarkable building blocks

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for ion recognitions. So far, there are several reports concerning the ion-recognition sensors of terpyridine and its derivatives based on their strong binding affinities,26-31 and several successful attempts have been made to develop terpyridine-containing diarylethenes for selective detection of metal ions.30-33 But their

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sensitivity and selectivity still needs to be further enhanced. In this work, we report the synthesis of two

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novel diarylethenes 1O and 2O with photochromic perfluorocyclopentene and terpyridine units (Scheme 1) and their responses to light and metal ions. The two diarylethenes could serve as fluorescence probes

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for Hg2+ ion with high selectivity.

Scheme 1. Photochromism of diarylethenes 1 and 2 by photoirradiation.

2. Experimental

2.1. General methods All solvents used were of analytical grade and purified by distillation before use. NMR spectra were recorded on a Bruker AV400 (400 MHz) spectrometer using CDCl3 as the solvent and tetramethylsilane as an internal standard. Infrared spectra (IR) spectra were recorded on a Bruker Vertex-70 spectrometer.

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ACCEPTED MANUSCRIPT Elemental analysis was carried out with a PE CHN 2400 analyzer. Mass spectra were obtained on an Agilent 1100 ion trap MSD spectrometer. Melting point was measured using a WRS-1B melting point apparatus. Absorption spectra were measured using an Agilent 8453 UV/Vis spectrophotometer.

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Photo-irradiation was carried out using an SHG-200 UV lamp, CX-21 ultraviolet fluorescence analysis cabinet, and a BMH-250 visible lamp. Lights of appropriate wavelengths were isolated using different light filters. Fluorescence spectra were measured on a Hitachi F4600 fluorescence spectrophotometer.

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Except for Mn(II), K(I), and Ba(II) (all of their counter ions were chloride ions), other tested metal ions

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were obtained by the dissolution of their respective metal nitrates (0.10 mmol) in distilled water (10 mL). The PMMA films were prepared by dissolving 10 mg of diarylethene sample and 100 mg of poly-methylmethacrylate (PMMA) in chloroform (1 mL) with the aid of ultrasound, and the homogeneous solution was spin-coated on a quartz substrate (20 × 10 × 1mm3) at 1500 rpm.

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Synthesis

The synthetic route of diarylethenes 1O and 2O is shown in Scheme 2. The precursors 3 and 5 were

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synthesized by the reported method.

Scheme 2. Synthetic route to diarylethenes 1O and 2O.

2.2. Synthesis of 1-[2-methyl-5-(4''-pheny-2, 2 ′:6 ′, 2''-terpyridine)vinyl-3-thienyl]-2-(2-methyl-3benzothiophenyl) perfluorocylopentene (1O) A mixture of 2-acetylpyridine (0.73 g, 6.00 mmol) and compound 3 (1.56 g, 3.00 mmol) in C2H5OH (50

5

ACCEPTED MANUSCRIPT mL) was stirred vigorously with 15% aq. KOH (15 mL) and conc. NH4OH (10 mL) for 24h at room temperature. Then the reaction refluxed for 2 days under nitrogen atmosphere. The precipitate was collected by vacuum filtration, washed with water then with cold methanol, and dissolved in ethyl acetate.

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The organic fraction was washed with 1% NaHCO3 and dried with MgSO4. The crude compound was purified directly by column chromatography on silicagel using ethyl acetate as the eluent to give compound 1O (0.24 g) in 11% yield. 1H NMR (CDCl3, 400 MHz, TMS): δ (ppm): 2.00 (s, 3H), 2.33 (s,

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3H), 7.14 (s, 1H), 7.23 (m, 2H,), 7.32 (m, 4H), 7.42 (d, 2H, J = 8.0 Hz), 7.48 (d, 2H, J = 8.0 Hz), 7.60 (m,

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4H), 7.75 (t, 2H), 7.88 (m, 2H,), 8.71 (m, 6H). 13C NMR (CDCl3, 100MHz): δ (ppm): 14.3, 15.0, 118.0, 120.9, 121.1, 121.6, 122.6, 123.0, 123.4, 124.1, 124.5, 124.8, 125.1, 125.4, 125.9, 127.3, 133.4, 136.4, 137.24, 137.8, 140.6, 141.7, 142.0, 148.6, 155.5, 155.7. IR (KBr, ν, cm-1): 500, 540, 570, 621, 660, 742, 792, 828, 892, 966, 992, 1049，1112, 1139, 1190, 1273, 1338, 1390, 1438, 1467, 1515, 1566, 1583, 1603,

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1635, 1906, 2856, 2922, 3056, 3128, 3358. Anal. calcd for C40H25F6N3S2(%): C, 66.20; H, 3.47; N, 5.79, found: C, 66.32; H, 3.09; N, 6.05. LRMS (ESI–): m/z calcd. for C40H25F6N3S2 [M-H]– 725.1; found 724.0. Synthesis

of

1-{2-methyl-5-[2-(4''-pheny-2,

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2.3.

2

′:6

′,

2''-terpyridine)]

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vinyl-3-thienyl}-2-(2-methyl-3- benzothiophenyl) perfluorocylopentene (2O) Compounds 5 (1.06 g, 2.38 mmol) and 6 (3.10 g, 4.76 mmol) mixed in 100 mL THF and a small amount of tris(o-tolyl)phosphine and palladium acetate added in the mixture and then stirred in a flask at 363K for 48h under an argon atmosphere. Finally, the reaction mixture was filtered, extracted, and dried with anhydrous MgSO4. The crude compound was purified directly by column chromatography on silicagel using ethyl acetate as the eluent to give compound 2O (0.45 g) in 25% yield. M.p. 453–454K; 1H NMR (CDCl3, 400 MHz, TMS): δ (ppm): 1.90 (s, 3H), 2.30 (s, 3H), 6.79 (d, 1H, J = 8 Hz), 7.01 (s, 1H), 7.13,

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ACCEPTED MANUSCRIPT 7.17 (d, 1H, J = 16 Hz), 7.26 (t, 2H), 7.35 (m, 4H), 7.55 (m, 2H), 7.75 (m, 1H), 7.88 (m, 2H), 7.90 (m, 2H), 8.68 (d, 2H, J = 8 Hz), 8.75 (s, 4H). 13C NMR(CDCl3, 100MHz): δ (ppm): 14.9, 15.0, 118.5, 120.3, 121.4, 121.7, 122.1, 123.9, 124.5, 124.9, 125.0, 126.0, 126.9, 127.7, 127.9, 128.0, 128.2, 136.9, 137.3,

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137.7, 138.2, 138.2, 140.4, 141.7, 142.6, 149.1, 149.5, 156.0, 156.1. IR (KBr, ν, cm-1): 659, 755, 792, 754, 833, 894, 991, 1046, 1111, 1191, 1274, 1339, 1388, 1439, 1467, 1566, 1584, 2921, 3340, 3443, 3707. Anal. calcd for C42H27F6N3S2(%): C, 67.10; H, 3.62; N, 5.59, found: C, 67.22; H, 3.59; N, 5.50.



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3. Results and discussion

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LRMS (ESI–): m/z calcd. for C42H27F6N3S2 [M-H]– 751.2; found 750.0.

3.1. Photochromism

Fig. 1 shows the absorption spectral and color changes of diarylethenes 1 and 2 induced by

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photoirradiation in both dichloromethane and PMMA films. The absorption maximum of 1O was observed at 322 nm (ε = 2.5 × 104 mol-1 L cm-1) in dichloromethane, which resulted from π-π* transition.34 Upon irradiation with 297 nm UV light, the colorless solution of 1O turned pink and a new

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absorption band centered at 550 nm (ε = 8.7 × 103 mol-1 L cm-1) emerged due to the formation of the

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ring-closed isomer 1C. Reversely, the pink solution became colorless upon irradiation with visible light (λ > 500 nm), indicating that 1C returned to its initial open-ring isomer 1O. The absorption spectral change of 2O was similar to that of 1O as shown in Fig. 1B. The absorption maximum of 2O was observed at 352 nm (ε = 8.1 × 104 mol-1 L cm-1) and that of 2C was observed at 565 nm (ε = 3.6 × 104 mol-1 L cm-1) in dichloromethane. The photoisomerization process could be seen with naked eyes with a color change from colorless to purple. In photostationary state, the isosbestic points were observed at 355 nm for 1 and 380 nm for 2, respectively. Compared to those of diarylethene 1O and 1C, the absorption

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ACCEPTED MANUSCRIPT maximum of 2O is red-shifted by 28 nm and that of 2C is red-shifted by 13 nm, which arised from the

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higher extent of the π-conjugation in 2.

(B)

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(A)

(C)

(D)

Fig. 1 Absorption spectral and color changes of diarylethenes 1 and 2 by photoirradiation in dichloromethane (2.0 × 10-5

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mol L-1) and PMMA films (10%, w/w): (A) 1 in dichloromethane, (B) 2 in dichloromethane, (C) 1 in a PMMA film, (D) 2 in a PMMA film.

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In PMMA films, the absorption maxima observed of 1C and 2C in photostationary state were observed at 555 nm and 573 nm, respectively. As shown in Fig. 1C and Fig. 1D, the colorless 1O/PMMA film turned brown and 2O/PMMA turned purple when irradiated with 297 nm light. Compared to those in dichloromethane, the absorption maxima of their closed-ring isomers in PMMA films are red-shifted by 5 nm for 1C and 8 nm for 2C, which may be ascribed to the polar effect of the polymer matrix and stabilization of molecular arrangement in the solid medium.35, 36 The absorption spectral features of 1 and 2 in both dichloromethane and PMMA films are summarized in Table 1. Compared with those of 1, the

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ACCEPTED MANUSCRIPT molar absorption coefficients of 2 with a C=C increased notably for its open-ring and closed-ring isomers, but its cyclization quantum yield (Φo-c, 2 = 0.32) and cycloreversion quantum yield (Φc-o, 2 = 0.08) decreased notably. In addition, the photoconversion ratios of 1 and 2 were measured by HPLC analysis in

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photostationary state with the value of 74% for 1 and 81% for 2 in methanol. Table 1. Absorption characteristics and photochromic reactivity of diarylethenes 1 and 2 in dichloromethane (2.0 × 10-5 mol L-1) and PMMA films (10%, w/w) λo,max/nma (ε/L mol-1 cm-1)

λc,max/nmb (ε/L mol-1 cm-1)

PMMA film

dichloromethane

1

322 (2.5×104)

323

550 (8.7×103)

2

352 (8.1×104)

353

565 (3.6×104)

PR/%d

PMMA film

Φo-c

Φc-o

555

0.41

0.11

74

573

0.32

0.08

81

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dichloromethane

Φc

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Compd

a

Absorption maxima of open-ring isomers.

b

Absorption maxima of closed-ring isomers.

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Quantum yields of open-ring (Φo-c) and closed-ring isomers (Φc-o), respectively.

d

Photoconversion ratios in the photostationary state.

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3.2. Fluorescence properties

The fluorescence properties of diarylethenes 1 and 2 were measured in dichloromethane (2.0 × 10-5 mol

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L-1) at room temperature. The emission peak of 1O emerged at 425 nm with excitation at 340 nm, and that of 2O appeared at 498 nm with excitation at 380 nm. Therefore, both of the two diarylethenes

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showed a larger Stokes shift and their fluorescence spectral edges exhibited a notable redshift in comparison with their absorption edges, which may be attributed to the twisted intermolecular charge transfer (TICT) excited states in their molecular structures.37 By using anthracene as a reference, the fluorescence quantum yields of 1O and 2O were determined to be 0.032 and 0.026, respectively. As observed for most of the reported diarylethenes,38-40 the two diarylethenes functioned as a notable fluorescence switch upon alternating irradiation with UV and visible light in dichloromethane (Fig. 2). Upon irradiation with 297 nm UV light, the fluorescence emission intensity of 1O and 2O decreased 9

ACCEPTED MANUSCRIPT dramatically due to the formation of the closed-ring isomers of 1C and 2C. The back irradiation with appropriate visible light recovered their original emission spectra due to the regeneration of the open-ring isomers 1O and 2O. In photostationary state, the emission intensities of 1O and 2O were quenched to ca.

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500

400 Vis UV

300

200

100

0

350

400

450

500

550

600

200

150

Vis UV

100

50

0

Wavelength(nm)

400

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Emission Intensity (a.u.)

Emission Intensity (a.u.)

12% and 11%, respectively.

450

500

550

600

650

(B)

2800

2100 Vis UV

1400

700

0 480

495

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Emission Intensity (a.u.)

3500

510

525

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Wavelength (nm)

(C)

Emission Intensity (a.u.)

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Wavelength(nm)

1200

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Vis UV

600

300

530

540

550

560

570

580

Wavelength (nm)

(D)

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Fig. 2 Emission intensity changes of 1 and 2 by photoirradiation in dichloromethane (2.0 × 10-5 mol L-1) and PMMA films (10%, w/w): (A) 1 in dichloromethane, (B) 2 in dichloromethane, (C) 1 in a PMMA film, (D) 2 in a PMMA film.

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As observed in solution, diarylethenes 1O and 2O showed favorable fluorescence switching properties in PMMA amorphous films (10%, w/w), and the results are illustrated in Fig. 2C and Fig. 2D. In PMMA films, the emission peaks of 1O and 2O were observed at 498 and 544 nm, respectively. In the photostationary state, the fluorescent modulation efficiencies of diarylethenes 1 and 2 in PMMA films were determined to be 94% and 77%, respectively. Compared to the reported dithienylethene derivatives,41-43 the fluorescent modulation efficiencies of diarylethenes 1 and 2 were enhanced significantly in both solution and solid medium, which is ideal for practical applications in optoelectronic 10

ACCEPTED MANUSCRIPT devices, such as optical recording media and fluorescent photoswitches.44-47 3.3. Absorption and fluorescence response to Hg2+ The changes in the absorption of diarylethenes 1 and 2 induced by Hg2+ were investigated in

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dichloromethane at room temperature. As shown in Fig. 3A, addition of Hg2+ to the solution of 1O produced complex 1O' (Hg2+-bound state of 1O) with a notable absorption shift from 322 nm to 337 nm. This may be attributed to the coordination between Hg2+ and the terpyridine component of 1O.48 Upon

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irradiation with 297 nm light, the colorless solution of 1O' turned purple and a new absorption band

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centered at 553 nm emerged due to the formation of the closed-ring isomer complex 1C'. Compared to 1C, the absorption maximum of 1C' showed almost no change. Reversely, the purple solution turned colorless and the absorption spectrum reverted to that of the original open-ring isomer 1O' upon irradiation with visible light (λ > 500 nm). Similarly, a 22 nm red-shift for the absorption maximum of

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2O' (Hg2+-bound state of 2O) in dichloromethane was observed, as compared to that of 2O (Fig. 3A). 2O' also underwent photoisomerization upon photoirradiation with concomitant color change from colorless to purple. In photostationary state, the absorption maximum of 2C' was observed at 572 nm (ε =

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4.8 × 104 mol-1 L cm-1).

2.0

Absorbance

Absorbance

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0.5 0.4 0.3

1O'

1C'

0.2 0.1

1.6 2C'

1.2 2O

0.8 0.4

1O

2O'

0.0

0.0

300

400

500

600

300

700

400

500

600

700

Wavelength (nm)

Wavelength (nm)

(A)

(B)

Fig. 3 Absorption spectral changes of 1O and 2O by the stimulation of Hg2+ and light in dichloromethane (2.0 × 10-5 mol L-1): (A) 1O, (B) 2O. 11

ACCEPTED MANUSCRIPT Diarylethenes 1 and 2 were identified as an efficient fluorescent chemosensors for the detection of Hg2+. Fig. 4 shows the fluorescence changes of 1O and 2O induced by Hg2+ and EDTA in dichloromethane (2.0 × 10-5 mol L-1). In a fluorometric titration, addition of Hg2+ to the solution of 1O resulted in a

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gradual decrease of the emission intensity centered at 425 nm and a progressive increase of the emission intensity centered at 465 nm (Fig. 4A). At the same time, a concomitant color change from dark blue to bright cyan was observed by naked-eyes due to the formation of 1O'. The changes in the emission 49

Hg2+ causes the largest spectral shift and

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characteristics reflected the changes in the EET efficiency.

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results in the most efficient energy transfer for the bound state. Compared to 1O, the emission peak of 2O' was only enhanced by 35-fold without any red-shift (Fig. 4B). Using anthracene as a reference, the fluorescence quantum yields of 1O' and 2O' are 0.16 and 0.32, respectively. In this case, the 'chelation enhanced fluorescence' effect is more significant than the quenching effect of the proximate 'heavy

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atom'.50,51 It should be noted that the fluorescence of 1O and 2O were restored after adding 25.0 equiv of EDTA. The reversible processes were ascribed to the complexation/dissociation reaction between Hg2+

1O'

Emission Intensity (a.u.)

1O

1O'

1500

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Emission Intensity (a.u.)

2000

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and EDTA.

1000

500

0 350

Hg2+ EDTA

1O

400

450

550

2O'

2500 2000 1500

Hg2+

EDTA

1000 500 2O

0

500

2O'

2O

3000

600

Wavelength (nm)

400

450

500

550

600

650

Wavelength (nm)

(A)

(B)

Fig. 4 Fluorescence and color changes of 1O and 2O by the stimulation of Hg2+/EDTA in dichloromethane (2.0 × 10-5 mol L-1): (A) 1O, (B) 2O.

By plotting the fluorescence intensity ratio [F(465nm)-F0] / F0 as a function of Hg2+ concentration, the

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ACCEPTED MANUSCRIPT relationship curve of 1O were obtained as shown in Fig. 5A. The intensity ratio of 1O' increased gradually with the addition of Hg2+ and reached maximum after adding 1.2 µmol of Hg2+ (12 µL, 1.0 × 10-1 mol L-1), indicating that 1O could be potentially used as a ratiometric fluorescent probe for detecting

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Hg2+. In contrast, the addition of 2.0 µmol of Hg2+ (20 µL, 1.0 × 10-1 mol L-1) to the solution of 2O resulted in the maximum emission intensity without any spectral red-shift (Fig. 5B).

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1.5

1.0

0.5

30

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[F(489nm)-F0] /F0

2.0

25 20 15 10

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[F(465nm)-F0] / F0

2.5

5

0

1

2

3

4

5

Hg2+ (equiv.)

(A)

6

7

0

1

2

3

4

5

6

7

Hg2+ (equiv.)

(B)

Fig. 5 Emission intensity ratio changes of 1O (λex, 340 nm) and 2O (λex, 380 nm) by the stimulation of Hg2+ in

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dichloromethane (2.0 × 10-5 mol L-1): (A) 1, (B) 2.

Fig. 6 shows the fluorescent changes of 1O' and 2O' upon alternating irradiation with UV/Vis light in

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dichloromethane (2.0 × 10-5 mol L-1) at room temperature. As observed for most of the reported diarylethenes,38-40 both 1O' and 2O' exhibited notable fluorescence switching properties by

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photoirradiation. As shown in Fig. 6, the emission intensity of 1O' and 2O' decreased evidently upon irradiation with 297 nm light due to the formation of the non-fluorescence closed-ring isomer of 1C' and 2C'. When arrived at photostationary states, the emission intensity was quenched to ca. 23% for 1O' and 10% for 2O', respectively. The back irradiation with appropriate visible light regenerated the open-ring isomers and recovered their original emission intensity. The dual-controlled photoswitching behaviors of 1 and 2 are shown in Scheme 3 and Scheme 4, indicating that the two diarylethenes are one of the promising candidates for fluorescent switchable devices induced by either Hg2+/EDTA or UV/Vis light.

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ACCEPTED MANUSCRIPT 1C'

1O'

1500

1000

Vis UV

500 1C'

450

500

550

2O'

2000 1500

Vis UV

1000 500 400

600

2C'

2500

0

0

400

2O'

3000

2C'

450

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1O'

Emission Intensity (a.u.)

Emission Intensity (a.u.)

2000

500

550

600

650

Wavelength (nm)

Wavelength (nm)

(A)

(B)

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mol L-1): (A) 1O', (B) 2O'.

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Fig. 6 Emission intensity and fluorescence color changes of 1O' and 2O' by photoirradiation in dichloromethane (2.0 × 10-5

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Scheme 3. Multi-state fluorescence switching behaviors of 1O induced by the stimulation of Hg2+/EDTA and UV/Vis irradiation.

In order to calculate the stoichiometry of the Hg2+ and 1O or 2O, Job’s plots were performed by the reported method.52 By keeping the sum of the initial concentration of Hg2+ and 1O/2O at 2.0 × 10-5 mol L-1, the Hg2+ molar ratio changed from 0 to 1, and the Job’s plot curves of 1O and 2O were obtained (Fig. 7). The results showed that the maximum values were achieved when the molar fractions of [Hg2+]/([1O/2O]+[Hg2+]) were about 0.3. Therefore, the complex ratios of 1O-Hg2+ and 2O-Hg2+ were 2:1 in dichloromethane. In addition, the binding constants of the two compounds were determined to be

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ACCEPTED MANUSCRIPT

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3.0 × 107 mol L-1for 1O and 1.8 × 104 mol L-1 for 2O based on the Hildebrand-Benesi equation.53

(B)

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(A)

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Fig. 7 Job’s plot showing the 2:1 complexes Hg2+/1O and Hg2+/2O: (A) 1O, (B) 2O.

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Scheme 4. Multi-state fluorescence switching behaviors of 2O induced by the stimulation of Hg2+/EDTA and UV/Vis irradiation.

In order to confirm the selectivity of 1O and 2O as chemosensors for Hg2+, the changes in the fluorescence of 1O and 2O induced by other metal ions including K+, Ca2+, Zn2+, Sn2+, Pb2+, Ba2+, Mg2+, Mn2+, Ni2+, Co3+, Sr2+, Pb2+, Al3+, Cu2+, Fe3+, and Cr3+ were studied under the same experimental conditions. The fluorescence and color changes of 1O and 2O induced by these metal ions in dichloromethane are shown in Fig. 8. When the above ions were individually added into the solutions containing 1O and 2O, no obvious changes in the fluorescence and color were observed. The results 15

ACCEPTED MANUSCRIPT indicated that the fluorescence and color greatly changed only when Hg2+ was added the solutions of 1O and 2O. Furthermore, no obvious interference in fluorescence was observed when Hg2+ (5 equiv) was added with other metal ions (5 equiv). No significant variation in emission intensity was found in

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comparison with that containing Hg2+ alone. Therefore, 1O and 2O could serve as selective fluorescent chemosensors for the recognition of Hg2+ even competing with other ions in dichloromethane.

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1.5 1.0 0.5

25 20 15 10

5

2+

2+

2+

3+

2+

2+

Ni Cr M n Cu Cd

3+

2+

2+

2+

+

3+

Al Zn Sn Pb Fe Co

K+ M g 2+ Ca 2 + Ba 2

2+

2+

2+

3+

2+

3+

3+

N i C r M n C u Cd

2+

+

2+

Sr Pb Fe Co

K+ M g 2+ Ca 2 + Ba 2 + Al 3 + Zn 2

（B）

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EP

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(A)

No ne Hg 2+

0

0.0 N on e H g 2+

30

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[F(489nm)-F0] / F0

2.0

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[F(465nm)-F0] / F0

2.5

(C)

(D)

Fig. 8 Fluorescence responses of 1O and 2O to various metal ions (5 equiv) in dichloromethane (2.0 × 10-5 mol L-1): (A) 1O, (B) 2O, (C) photos of the fluorescence color changes for 1O, (D) photos of the fluorescence color changes for 2O.

4. Conclusions In

summary,

two

new

fluorescent

chemosensors

16

were

successfully

obtained

by

using

ACCEPTED MANUSCRIPT perfluorodiarylethene as a photochromic trigger and 2,2':6',2''-terpyridine unit as a chromophore. The two compounds have multi-addressable responses to both light and metal ion inputs, which result in different fluorescence behaviors under alternating UV/Vis irradiation and complexation/dissociation with

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Hg2+/EDTA. With the stimulation of Hg2+, the fluorescence intensity of the two derivatives increased dramatically with notable color changes, indicating that they could be utilized as naked-eyes fluorescence chemosensors for recognition of Hg2+ with high selectivity in dichloromethane. This work provided a

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new strategy for the design and synthesis of photochromic diarylethenes with favorable multi-state

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fluorescence switching behaviors and the recognition of specific metal ions. Acknowledgments

This work was supported by the National Natural Science Foundation of China (21262015, 51373072, 21363009), the Project of Jiangxi Advantage Sci-Tech Innovative Team (20142BCB24012), the Science

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Funds of Natural Science Foundation of Jiangxi Province (20132BAB203005, 20142BAB203005), and the Project of the Science Funds of Jiangxi Education Office (KJLD12035, GJJ12587, GJJ13577). References

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