A highly selective fluorescence probe for Al3+ based on a new diarylethene with a 6-(hydroxymethyl)picolinohydrazide unit

Tetrahedron 72 (2016) 8449e8455

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A highly selective fluorescence probe for Al3þ based on a new diarylethene with a 6-(hydroxymethyl)picolinohydrazide unit Xiaoxia Zhang a, Renjie Wang a, b, **, Gang Liu a, Congbin Fan a, Shouzhi Pu a, * a b

Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China College of Chemistry, Nanchang University, Nanchang, Jiangxi 330031, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 September 2016 Received in revised form 26 October 2016 Accepted 2 November 2016 Available online 3 November 2016

A new asymmetrical photochromic diarylethene with a 6-(hydroxymethyl)picolinohydrazide unit has been synthesized. Its multichromism behaviors induced by base/acid and light were investigated. The compound exhibited high selectivity and sensitivity toward Al3þ, and the fluorescence intensity of the diarylethene was remarkably enhanced by 95-fold and the emission peak was blue-shifted from 585 nm to 542 nm with a concomitant color change from dark to bright green triggered by Al3þ. The complexation-decomplexation reaction between the diarylethene and Al3þ was reversible with a binding constant of 1.22  104 L mol1, and the detection limit was determined to be 2.88  108 mol L1. Our experimental results demonstrated that the diarylethene could be served as a selective fluorescent sensor for the recognition of Al3þ with high sensitivity and selectivity in methanol. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Diarylethene Photochromism Fluorescence switch Al3þ recognition Fluorescent sensor

1. Introduction Photochromic compounds can reversibly interconvert between open-ring and closed-ring isomers, which produce different optical, electronic and geometrical properties upon light irradiation.1 Among various photochromic materials,2 photochromic diarylethene derivatives have received considerable attention due to their excellent thermal irreversible properties, outstanding fatigue resistance, high sensitivity and fluorescence switchable characteristics.3 During the past decades, numerous diarylethene molecules with both photochromic and multi-regulative fluorescence molecular switch properties have been reported,4 and the most widely used synthetic strategy for designing these molecules is to connect a diarylethene backbone with a functional receptor, which can selectively identify specific external stimuli such as metal ions, anions and small molecules.5 In these compounds, the tunable host-guest recognition can efficiently modulate by the diarylethene photocontrol unit. Aluminium is a kind of widespread metal element in daily life

* Corresponding author. ** Corresponding author. Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China. E-mail addresses: [email protected] (R. Wang), [email protected] (S. Pu). http://dx.doi.org/10.1016/j.tet.2016.11.007 0040-4020/© 2016 Elsevier Ltd. All rights reserved.

because it is the third most prevalent metal in the earth's crust, and it is found in its ionic form Al3þ in most animals, plant tissues and nature waters. Recently, Al3þ has been increasingly proved to have considerable toxicity in biological systems.6 There is evidence that excessive intake of aluminium can lead to severe health problems of human such as impairment of memory, rickets, anaemia, headache and Alzheimer's disease.7 Moreover, high concentration of Al3þ in the ecosystem can affect the growth of plant roots, freshwater fishes and cause product reduction. Therefore, exploring Al3þ sensors with high sensitivity and excellent selectivity signaling mechanisms has attracted widespread attention.8 For example, Misra et al. reported a new fluorescent probe based on a pyrene moiety, which exhibited a “turn-on” fluorescence response toward Al3þ at nano-molar range via chelation enhanced fluorescence.9 Recently, Pang et al. developed a series of bishydrazide derivatives with high selective detection of Al3þ, and the fluorescence of these derivatives could be remarkably enhanced through inhibiting the non-radiative PET and ESIPT process.10 However, the detection of Al3þ is much more difficult than other metal cations because of its poor coordination ability, strong hydration ability and the lack of spectroscopic characteristics.11 To obtain a highly sensitive and selective sensor toward Al3þ, the majority of Al3þ receptors contain functional structures such as Nsalicylidenehydrazide ligand, in which the nitrogen atom of the hydrazide, the oxygen atom of amide group and the oxygen atom of

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the phenolic hydroxy group can compose an efficient chelating site for Al3þ.12 The 6-(hydroxymethyl)picolinohydrazide unit is also a fascinating group because its structure can be modified through the Schiff base reaction with salicylaldehyde and form the Al3þ recognition group, i.e., N-(2-hydroxybenzylidene)-6-(hydroxymethyl) picolinohydrazide unit, in which the phenolic eOH and the nitrogen atom of pyridine ring not only provide the chelation site for Al3þ but also act as proton donors/acceptors. Therefore, it can be predicted that introducing a 6-(hydroxymethyl)picolinohydrazide unit into a diarylethene skeleton will produce a multi-responsive molecule. To the best of our knowledge, acid/base controlled multichromism and fluorescence chemosensor molecular switch based on a photochromic diarylethene with a 6-(hydroxymethyl)picolinohydrazide unit has not hitherto been reported. On the basis of the mentioned facts, a new photochromic diarylethene fluorescence chemosensor bearing a 6-(hydroxymethyl) picolinohydrazide unit (1O) with high selectivity for Al3þ was constructed, and its multichromism behavior and dual-responsive properties induced by acid/base, light and metal ions were systematically investigated. The diarylethene could undertake a fluorescent chemosensor for recognition of Al3þ with high selectivity in methanol solution. The schematic illustration of photochromism is shown in Scheme 1. 2. Experimental 2.1. General methods All solvents were purified by distillation before use. 1H NMR and C NMR spectra were recorded on a Bruker AV400 (400 MHz) spectrometer with THF-d8 as the solvent and tetramethylsilane as an internal standard. Infrared spectra (IR) were recorded on a Bruker Vertex-70 spectrometer. Melting point was measured on a WRS-1B melting point apparatus. Absorption spectra were measured using an Agilent 8453 UV/vis spectrophotometer. Photoirradiation was carried out using an SHG-200 UV lamp, a CX-21 ultraviolet fluorescence analysis cabinet, and a BMH-250 visible lamp. The light intensity was 20 mW/cm2 for 297 nm light, 75 mW/ cm2 for 254 nm light and 14 mW/cm2 for visible light, respectively. Fluorescence spectra were measured using a Hitachi F-4600 spectrophotometer, and the breadths of excitation and emission slit were both selected 5 nm. Fluorescence quantum yield was measured with an Absolute PL Quantum Yield Spectrometer QY C11347-11. Mass spectra were obtained on a Bruker AmaZon SL Ion Trap Mass spectrometer. Elemental analysis was measured with a PECHN 2400 analyzer. Expect for Mn2þ, Hg2þ, Kþ, and Ba2þ (their counterions were chloride ions), other metal ions were obtained by dissolving their respective metal nitrates (0.10 mmol) in distilled water (10.0 mL). 13

2.2. Synthesis of 1-(3,5-dimethyl-4-isoxazolyl)-2-{2-methyl-5-[(4hydroxyl-3- (2-hydroxybenzylidene)-6-(hydroxymethyl) picolinohydrazide)-phenyl]-3-thienyl}perfluorocyclopentene (1O) The synthetic route for diarylethene 1O is shown in Scheme 2. First, the diarylethene salicylaldehyde derivative 2 was prepared by

the reported method.13 In a 100 mL flask, compound 2 (0.20 g, 0.41 mmol) and 6-(hydroxymethyl)picolinohydrazide (0.07 g, 0.41 mmol) were dissolved in anhydrous ethanol (50.0 mL). After refluxing for 24 h, the mixture was cooled to room temperature and concentrated under vacuum. The crude product was purified by recrystallization with ethanol to give the diarylethene 1O (0.15 g, 0.24 mmol) as a yellow solid in 59% yield. M.p. 487e488 K; 1H NMR (400 MHz, THF-d8, TMS), d 1.97 (s, 3H), 2.02 (s, 3H), 2.20 (s, 3H), 4.64 (t, 1H, J ¼ 4.0 Hz), 4.70 (d, 2H, J ¼ 8.0 Hz), 5.94 (d, 1H, J ¼ 8.0 Hz), 7.25 (s, 1H), 7.50 (d, 2H, J ¼ 8.0 Hz), 7.67 (d, 1H, J ¼ 8.0 Hz), 7.93 (t, 1H, J ¼ 8.0 Hz), 8.05 (d, 1H, J ¼ 4.0 Hz), 8.71 (s, 1H), 11.57 (s, 1H), 11.61 (s, 1H). 13C NMR (100 MHz, THF-d8, TMS), d 7.73, 9.10, 11.55, 102.53, 115.50, 116.82, 118.90, 119.22, 121.31, 122.53, 122.90, 125.62, 126.64, 135.93, 138.58, 140.73, 146.54, 148.21, 155.94, 157.85, 159.31, 167.85; IR (KBr, n, cm1):538, 593, 681, 753, 800, 891, 986, 1058, 1129, 1189, 1272, 1339, 1441, 1537, 1601, 1660, 1739, 2852, 2930, 3197, 3450; HRMS-ESI (m/z): [M þ Na]þ Calcd. For (C29H22F6N4O4SNa)þ, 659.1164, found: 659.1181. 3. Results and discussion 3.1. Photochromism of 1O The absorption spectral changes of 1O were measured in methanol (C ¼ 2.0  105 mol L1) as shown in Fig. 1. Upon irradiation at 297 nm light, the absorption band of 1O at 301 nm (ε, 5.28  104 L mol1 cm1) decreased with an increased new absorption band at 568 nm (ε, 6.49  103 L mol1 cm1) due to the formation of the closed-ring isomer 1C. This change could be seen with naked eyes as the colorless solution turned to purple. Alternatively, the purple solution of 1C could be bleached completely when illuminated with visible light (l > 500 nm). In the photostationary state (PSS), a clear isosbestic point was observed at 322 nm, indicating the reversible two-component photochromic reaction scheme.4f In addition, the cyclization/cycloreversion quantum yields of 1 in methanol were calculated to be 0.47 and 0.02 according to the reported method.5e Compared with the precursor compound 2,13 the cyclization quantum yield of 1 was notably increased. The result indicated that introducing the 6(hydroxymethyl)picolinohydrazide unit to diarylethene could promote the photocyclization reaction of 1. In addition, it was worth noting that the absorption band of 1 at 568 nm exhibited gradual blue shift with extended exposure time upon irradiation with 297 nm light (Fig. S1, Supporting information (SI)), indicating some new reaction happened during this process. In our previous work, we reported an asymmetrical photochromic diarylethene with a 2-(20 -hydroxyphenyl)benzothiazole (HBT) unit,5e which could be induced by intramolecular proton transfer reaction upon irradiation with higher energy UV light. Therefore, we predicted that diarylethene 1 might undertake similar change when irradiated with higher UV light. Upon illumination with 254 nm light, the absorption peak of the open-ring isomer at 301 nm gradually decreased, and the absorption band of 1C dramatically hypochromatic shifted to 485 nm accompanied with the molar absorption coefficient increased from ε (1C, 6.49  103 L mol1 cm1) to ε0 (1C′, 8.33  103 L mol1 cm1)

Scheme 1. Photochromism of diarylethene 1O.

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Scheme 2. Synthetic route of diarylethene 1O.

Absorbance

1.00

UV

Vis

0.75

0.50

0.25

Vis

UV

120 s 80 50 20 0

0.00 300

400

500

600

700

Wavelength (nm) Fig. 1. Absorption spectral changes of 1 by photoirradiation in methanol (C ¼ 2.0  105 mol L1) at room temperature.

11.57 ppm (Phenolic eOH). However, the proton peak of the phenolic hydroxyl group disappeared when 1O was illuminated by 254 nm light for 20 min. Meanwhile, a strong and wide peak was observed at 11.61 ppm, whose integral area was increased by two folds. The results indicated that the proton of phenolic hydroxy was transferred to the nitrogen atom and formed a secondary amine. Fig. 3 shows the fluorescence spectral change of 1O in methanol upon photoirradiation. Upon excitation at 350 nm light, a weak fluorescence peak at 585 nm was observed and the fluorescence quantum yield was determined to be 0.002. The emission intensity of 1O was decreased when irradiated with 297 nm light. When arriving at the PSS, the fluorescence intensity of 1O was quenched to ca. 51% due to the formation of the non-fluorescent closed-ring isomer 1C. The residual fluorescence in the photostationary state may be attributed to the incomplete cyclization and the existence of isomers with parallel conformation.14 Back irradiation of the visible light (l > 500 nm) could regenerate the open-ring isomer 1O and recovere the original emission spectra.

3.2. Multichromism of 1 with stimulation of base/acid (Fig. 2A). The photo-induced intramolecular proton transfer effect might be responsible for this phenomenon, in which the proton of phenolic hydroxy group transferred to the nitrogen atom of Schiff base, and the structure of 1C changed from the enol form to the ketone form (1C′, Scheme 3). After illumination with 254 nm light for 20 min, 1C reached to the PSS and the color changed from purple to red due to the formation of 1C′. Upon irradiation with visible light for 60 min, the absorption band of 1C′ at 485 nm declined and the red color solution could not completely bleach (Figs. S2 and SI). The results were well consistent with the diaryethene with a HBT group, in which the ketone form closed-ring isomer was stable and exhibited less photoactive to visible light.5e In order to determine the structure of ketone form (1C′), the changed process of 1O to 1C′ were tracked by the 1H NMR method (Fig. 2B). Before photoirradiation, two signal peaks appeared at 11.61 ppm (NeH) and

Upon stimulation with base/acid, diarylethene 1 showed multichromism as similar as the precursor diarylethene.15 Adding NaOH (2.7 mL, C ¼ 1.0 mol L1) to the solution containing 1O produced the deprotonated compound 1O, and the structure of 1O was confirmed by 1H NMR titration studies. Only the phenolic hydroxyl proton peak was vanished during the NaOH solution titration process (Figs. S3 and SI). The absorption maximum of 1O at 301 nm gradually red shifted to 317 nm during the dropwise addition of NaOH solution from 0 to 45.0 equiv, and a new visible absorption band appeared at 405 nm (ε, 1.60  104 L mol1 cm1) with the color changed from colorless to yellow due to the formation of deprotonated diarylethene 1O (Fig. 4A). Neutralization with hydrochloric acid (HCl, 2.7 mL, C ¼ 1.0 mol L1) could recover the absorption spectrum of 1O. Notably, the deprotonated 1O

Fig. 2. Absorption spectral and 1H NMR spectrum changes of 1 by photoirradiation in methanol (C ¼ 2.0  105 mol L1): (A) absorption spectral changes of 1C upon irradiation with 254 nm light; (B) the 1H NMR spectrum changes of 1 in the PSS in THF-d8.

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Emission intensity (a.u.)

Scheme 3. Structural and color changes among different forms of 1 with different UV light in methanol (C ¼ 2.0  105 mol L1).

60

UV

Vis

45

30

15

420

480

540

600

660

Wavelength (nm)

Fig. 3. Changes in the fluorescence of 1O upon alternating irradiation with UV and visible light in methanol (C ¼ 2.0  105 mol L1), excited at 350 nm.

exhibited very low photo reaction activity. The absorption maximum of 1O at 405 nm exhibited a minor blue shift and no obvious absorption and color change were found after irradiation with 297 nm light for 30 min (Figs. S4 and SI). The results were different from the similar diarylethenes with a phenolic eOH group,4a,16 where deprotonated compounds showed good photochromism and exhibited large red-shift during stimuli with UV light. The increased electron-withdrawing ability of deprotonated 6-(hydroxymethyl)picolinohydrazide unit of 1O may be responsible for this phenomenon. The strong pull electron effect decreased the electron cloud density of activated carbon atom and suppressed the photocyclization reaction to produce the closedring isomer 1C. Interestingly, the closed-ring isomer 1C could be transformed to deprotonated 1C via the stimulation of NaOH solution. Upon addition of NaOH (0 / 5.0 equiv), the purple solution of 1C changed to green due to the formation of deprotonated 1C, whose absorption was remarkably redshifted from 568 nm to 592 nm (Fig. 4B). The absorption spectrum and color of 1C could be converted back to 1C via neutralization with HCl (5.0 equiv). The color and structure changes between neutral and deprotonated states induced by light and acid/base are described in Scheme S1 (SI).

0.9

Absorbance

Absorbance

1.00

0.75

0.50 -

1O OH

+

H

1O

0.6

0.3

0.25

1C

1O

0.00 300

360

420

Wavelength (nm)

(A)

480

1C¯

0.0 300

450

600

Wavelength (nm)

750

(B)

Fig. 4. Absorption spectral and color changes of 1 by NaOH/HCl stimuli in methanol (C ¼ 2.0  105 mol L1) at room temperature: (A) absorption spectral changes of 1O upon addition of NaOH/HCl, (B) absorption spectral changes of 1C upon addition of NaOH/HCl.

5000 1O-Al

EDTA Al3+

1O

Emission intensity (a.u.)

Emission intensity (a.u.)

X. Zhang et al. / Tetrahedron 72 (2016) 8449e8455

5000 1O-Al 4000 Vis

(A)

UV

3000 2000 1000 1C-Al 0 450

Wavelength (nm)

8453

500

550

600

Wavelength (nm)

650

700

(B)

Fig. 5. Emission spectral changes and photos of 1O induced by Al3þ/EDTA and light stimuli in methanol (C ¼ 2.0  105 mol L1) at rt, excited at 350 nm: (A) 1O induced by Al3þ/ EDTA; (B) emission intensity changes of 1OeAl3þ by alternating irradiation with UV/vis light.

3.3. Fluorescence response to metal ions The metal binding ability of 1O was evaluated by fluorescence spectral analysis in methanol (C ¼ 2.0  105 mol L1). As shown in Fig. 5A, the emission intensity of 1 was drastically enhanced when the Al3þ concentration increased from 0 to 2.0 equiv due to the formation of metal complex 1OeAl3þ. Compared with 1O (lem ¼ 585 nm), the emission peak of 1OeAl3þ (lem ¼ 542 nm) was blue shifted by 43 nm and its emission intensity remarkably increased by 95-fold, accompanied with a distinct fluorescence color change from dark to bright green (Fig. 5A, inset). The subsequent addition of an excess amount of EDTA solution could recover

its original fluorescence spectrum, indicating that the complexation/dissociation reaction between Al3þand 1O was reversible. When the amount of Al3þ was added to 3.0 equiv, the fluorescence enhancement effect was arrived at a plateau, and the fluorescent quantum yield of 1OeAl3þ was determined as 0.15. A very good linear relationship was observed between the ratio of the fluorescence intensities (I542 nm/I585 nm) and the concentration of Al3þ added (0 / 2.0 equiv) (Figs. S5 and SI). The results indicated that 1O could be potentially used as a ratiometric fluorescent probe for detecting Al3þ quantitatively. In addition, the complex 1OeAl3þ functioned as a notable fluorescence switch upon irradiation with UV/vis light. The emission intensity was quenched to ca. 23% due to

Fig. 6. Emission intensity and photos in fluorescence changes of 1O induced by the addition of various metal ions (2.0 equiv) in methanol (C ¼ 2.0  105 mol L1): (A) emission intensity changes; (B) photos demonstrating changes in its fluorescence.

Emission intensity (a.u.)

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1200

900

600

300

0

0.2

0.4 3+

0.6

0.8

1.0

3+

χ = [Al ]/([Al ]+[1O]) Fig. 7. Job's plot showing the 1:1 complex between 1O and Al3þ.

the formation of the 1CeAl3þ complex (Fig. 5B). The binding ability of 1O with various metal cations including Al3þ, Fe3þ, Cr3þ, Cu2þ, Mn2þ,Ca2þ, Mg2þ, Co2þ, Ni2þ, Ba2þ, Cd2þ, Hg2þ, Sr2þ, Zn2þ, Pb2þ and Kþ were further investigated to explore its selectivity by fluorescence spectroscopy. Fig. 6 shows the emission spectra and fluorescence color changes of 1O in methanol (C ¼ 2.0  105 mol L1) induced by the addition of various metal ions (2.0 equiv). It can be seen that the fluorescence of 1O was remarkably enhanced only when Al3þ was added (Fig. 6A). The addition of other cations, such as Fe3þ, Cr3þ, Cu2þ, Mn2þ,Ca2þ, Mg2þ, Co2þ, Ni2þ, Ba2þ, Cd2þ, Hg2þ, Sr2þ, Zn2þ, Pb2þ and Kþ resulted in no obvious change in the fluorescence of 1O. The results indicated that diarylethene 1 has specific recognition toward Al3þ. The fluorescence enhancement of 1O induced by Al3þ may be attributed to the excited state intramolecular proton transfer and chelation enhanced fluorescence mechanism.13 According to the HSAB concept, Al3þ is a hard acid, which is prone to bind with hard centers such as the oxygen atom of the phenolic hydroxy group, the nitrogen atom of the pyridyl moiety and the hydrazide nitrogen atom. The complexation of 1O with Al3þ was confirmed through 1H NMR titration studies (Figs. S6 and SI). It is clearly seen that the proton of phenolic hydroxy (d ¼ 11.54 ppm) in 1OeAl3þ gradually vanished after the addition of 4.0 equiv Al3þ, suggesting that the phenolic proton may take part in complexation with Al3þ.

Additionally, the absence of a peak at 4.71 ppm demonstrated the hydroxymethyl proton of the pyridine ring also participated in coordination. Subsequently, the competitive experiments of 1O with various metal ions including Zn2þ, Mg2þ, Ca2þ, Kþ, Co2þ, Cd2þ, Ba2þ, Hg2þ, Sr2þ, Pb2þ, Mn2þ, Ni2þ, Cu2þ and Fe3þ were performed in methanol (Figs. S7 and SI). It can be easily seen that there was no distinct interference when Al3þ was added in the presence of Zn2þ, Mg2þ, Ca2þ, Kþ, Co2þ, Cd2þ, Ba2þ, Hg2þ, Sr2þ, Pb2þ, Mn2þ and Ni2þ, while the fluorescence intensity could be completely inhibited by Cu2þ and Fe3þ. However, it is easy to eliminate the paramagnetic effect of Cu2þ and Fe3þ on quenching the fluorescence of 1OeAl3þ complex.17 The results demonstrated that the interference of the two metal ions (Cu2þ and Fe3þ) was negligible during Al3þ detection. As a result, the diarylethene could be served as a selective sensor for recognition of Al3þ even competing with other related species in methanol. In order to further confirm the stoichiometry relationship between 1O and Al3þ, Job's plot was performed by keeping the sum of the initial concentration of Al3þ and 1O at 2.0  105 mol L1 and the changes of Al3þ molar ratio from 0 to 1, and the results were depicted in Fig. 7. It can be easily seen that the concentration of complex 1OeAl3þ approached the maximum value when the molar fraction of [1O]/([1O] þ [Al3þ]) was 0.5, indicating that the complex ratio of compounds between 1O and Al3þ is 1:1 in methanol. Additionally, the LRMS spectra of 1OeAl3þ exhibited a peak at þ þ 724.1, which may be attributed to [MþAl3þþNO 3 eH ] (Figs. S8 and SI). Based on the above mentioned findings, the probable complex structure of 1OeAl3þ was presented in Scheme 4. Meanwhile, the binding constant (Ka) of 1O with Al3þ was determined to be 1.22  104 L mol1 with good linear relationship (R ¼ 0.989, Figs. S9 and SI), as obtained by fitting the data to the Benesildebrand expression.18 The detection limit of Al3þ for 1O was determined from the fluorescence spectral changes, and it was found to be 2.88  108 mol L1 (Figs. S10 and SI) by the reported method.19 Therefore, diarylethene 1 could be used as a good selective and high sensitive fluorescence sensor for detection of Al3þ in methanol. 4. Conclusions In summary, a novel asymmetrical photochromic diarylethene with a 6-(hydroxymethyl)picolinehydrazide unit was synthesized, and its multichromism behaviors stimulated by light and acid/base were systematically investigated. The results demonstrated the

Scheme 4. Schematics of molecular structures and fluorescence changes of 1O induced by Al3þ/EDTA and light stimuli.

X. Zhang et al. / Tetrahedron 72 (2016) 8449e8455

deprotonation effect could suppress photocyclization of the openring isomer, while the closed-ring isomer showed remarkable bathochromic shift with the color change from purple to green. Moreover, the diarylethene can be used as a fluorescent sensor for detection of Al3þ with high sensitivity and selectivity. The fluorescence emission peak displayed distinct blueshift and 95-fold fluorescence enhancement after the addition of Al3þ. This work provides a useful design strategy for constructing a fluorescent chemosensor for high selectivity recognition of Al3þ based on photochromic diarylethenes with a certain functionalized group.

4.

5.

Acknowledgements 6.

The authors are grateful for the financial support from the National Natural Science Foundation of China (21362013, 51373072), the Science Funds of Natural Science Foundation of Jiangxi Province (20142BAB203005, 20132BAB203005), the Project of the Science Funds of Jiangxi Education Office (KJLD12035), and the Masters' Innovative Foundation of Jiangxi Province (YC2014-S432, YC2014S433).

8.

Appendix A. Supplementary data

9.

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.tet.2016.11.007.

7.

10. 11.

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