English

• 1.   Introduction

  Cations and anions have series important meanings in biological processes.^[1] Among those ions, Zn²⁺ is one of the most abundant transitional elements, in the terms of biological processes, such as enzyme regulation, brain function, protein synthesis, DNA (deoxyribonucleic acid) repair and neurotransmission.^[2] Those evidences exhibited that Zn²⁺ is related to a various of neurological diseases, such as Alzheimer's diseases, amyotrophic lateral sclerosis, hypoxia-ischemia and epilepsy.^[3] According to the WHO (World Health Organization) data displayed, more than 40% children in Africa and Asia were caused harmful problems because of the limited uptake of Zn²⁺.^[4] Meanwhile, a large amount of Zn²⁺ can lead to phytotoxic effects on soil microbes.^[5] Therefore, it is highly desirable to develop sensitive and selective sensor to detect Zn²⁺ in the environment, living cells, and tissues. Similarly, phosphate and cyanide anions are required to avoid excessive presence in biological processes generated by stoichiometric imbalance in living systems. Cyanide is highly toxic and it has significant lethality when it is exposed to the organism and the environment, such as causing vomiting, convulsion, thus causing dyspnea and even death.^[6] However, this does not hinder its application in metallurgy, plastic processing, petrochemical, photographic, resins and herbicides.^[7] ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^ - $ plays an important role in metabolic process, energy storage and construction of the backbone of DNA and RNA (ribonucleic acid).^[8] Some common diseases, including hyperthyroidism, kidney failure, bone disorders, and hypertension are caused by excessive phosphates.^[9] As a result, highly sensitive and selective sensors for Zn²⁺, CN^-, and ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^ - $ detection are becoming very urgent. Although, numerous chemosensors had been reported, the probe can identify the CN^- and Zn²⁺ at the same time with different detection ways are rarely developed. In addition, as one of good photoswitch unit, diarylethene derivatives have been widely used in the design and synthesis of photochemical sensors due to their high sensitivity, real-time analysis and simple operation.^[10] During the past decades, many diarylethene chemosensors were reported to recognize Zn²⁺ or CN^-.^[11] However, photocontrolled diarylethene chemosensors can detect the ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^ - $, CN^- and Zn²⁺ at the same time and they are rarely.

  Taking into account the above mentioned facts, a new diarylethene chemosensor (1O) with a naphthol unit was synthesized and characterized, in which 2-hydroxybenzyli- dene-aminonaphthol was attached to a photochromic diarylethene as pendant ligand (Eq. 1). The compound exhibited "naked-eye" recognition of CN^-, and remarkable "Turn-on" fluorescence response for detecting of Zn²⁺ with high selectivity. The schematic illustration of photochromism is shown in Figure 1.

  (1)

  Figure 1

  

  Figure 1.  Photochromism of diarylethene 1O

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  2.   Results and discussion

  2.1   Photochromic properties of diarylethene 1O

  The photochromic behaviors of diarylethene 1O were tested by photoirradiation in acetonitrile (c=2.0×10^-5 mol·L^-1) at 297 K. As shown in Figure 2, owing to the π→π^* transition, a sharp opened-ring isomer (1O) absorption peak was observed at 274 nm (ε_max=5.05×10⁴ L·mol^-1·cm^-1). The peak of the open-ring isomer at 274 nm was declined slowly with irradiation of 297 nm UV light, meanwhile, a new broad absorption peak at 516 nm was gradually appeared (ε_max=3.68×10³ L·mol^-1·cm^-1) due to the π electron delocalization, indicating the formation of the closed-ring form 1C.^[12] After arrived at the photostationary state (PSS), the color of 1O solution changed from colorless to light pink, and a clear isosbestic point was observed at 409 nm, which supported the reversible two component photochromic reaction. Conversely, upon irradiation with visible light of wavelength longer than 500 nm, the closed-ring isomer 1C was reversed back to 1O accompanying with the solution color changed from pink to colorless. The cyclization and cycloreversion quantum yields of 1O were determined by using the 1, 2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene as reference, and it values were determined to be 0.037 and 0.20, which was smaller than 2O (Ф₂, _O-C=0.21). The results demonstrate that the amino-naphthol group could inhibit the photocyclization of 1O, the reason may be due to its strong electron-withdrawing property.

  Figure 2

  

  Figure 2.  Absorption spectral and color changes of diarylethene 1O by photoirradiation in acetonitrile (c=2.0×10^-5 mol·L^-1)

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  2.2   Colorimetric detection for CN^-

  The photophysical properties of colorimetric sensing ability of diarylethene 1O were investigated by monitoring the absorption spectral behavior upon the addition of various anions, such as CN^-, F^-, Cl^-, Br^-, SCN^-, AcO^-, ${\rm{CO}}_{\rm{3}}^{2 - }$, ${\rm{SO}}_{\rm{3}}^{2-}$, ${\rm{SO}}_{\rm{4}}^{2-}$, ${\rm{PO}}_{\rm{4}}^{3-}$, ${{\rm{S}}_2}{\rm{O}}_3^{2-}$, ${\rm{HCO}}_{\rm{3}}^{-}$, ${\rm{HSO}}_3^{-}$, ${\rm{HPO}}_{\rm{4}}^{2-}$, ${\rm{HSO}}_{\rm{4}}^{2-}$ and ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$ in acetonitrile (0.2 μmol· L^-1). Figure 3A shows variations of the absorption spectra of 1O in acetonitrile in the presence of different anions. It is clear that 1O shows an absorption band centered at 490 nm upon the addition of CN^- and F^-, while other anions have no effect on its absorption spectra. The results indicated that 1O could efficient detection CN^- and F^- with good sensitivity over other anions in acetonitrile. Figure 3B exhibited that with increasing concentration of CN^-, the absorbance of 1O at 490 nm gradually enhanced. After addition of 24.0 μmol·L^-1 of CN^-, the absorption intensity of 1O arrived at a plateau accompanied with a notable color change from colorless to orange due to the formation of 1O+CN^-. Figure 3B inset displayed a good linear calibration curve between absorbance at 490 nm and concentration of CN^-, indicating that 1O could be potentially used as a ratiometric colorimetric sensor for quantitatively detecting CN^- at the range of 0→30.0 μmol·L^-1. Similar response to F^- was observed under the same conditions for 1O (Figure 4). An obvious absorbance was gradually appeared at 490 nm and the color solution of 1O changed from colorless to orange with the stimulation of F^- (0→80.0 μmol·L^-1). Figure 4 inset also exhibited a good linearity between the concentration of F^- and the absorption in the range of 0→48.0 μmol·L^-1, demonstrating that 1O could quantitatively detect F^- at the relevant concentration.

  Figure 3

  

  Figure 3.  (A) Various anions induced the absorption of 1O, and (B) changes in the absorption of 1O in the presence of different amount of CN^- in acetonitrile

  Inset: (A) the absorption intensity at 490 nm with the addition of different anions), and (B) the solution color and absorbance changes at 488 nm with the addition of CN^-

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  Figure 4

  

  Figure 4.  Absorption spectral changes of 1O in the presence of different amount of F^- in acetonitrile

  Inset was the solution color and absorbance changes at 488 nm with the addition of F^-

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  The binding mode and stoichiometry of 1O to CN^-/F^- were evaluated by Job's plot experiments, ^[13] the absorbance at 490 nm was plotted as a function of the molar fraction of 1O under a constant total concentration at 2.0× 10^-5 mol·L^-1, and the changes of CN^-/F^- molar fraction from 0 to 1 (Figure 5). The results showed that the molar fraction of [1O]/[1O+CN^-/F^-] was 0.5 when the concentration of the complex 1O+CN^-/F^- arrived the maximum value, demonstrating 1:1 binding stoichiometry ration for 1O+CN^-/F^-. The results were further confirmed by the ¹H NMR titration experiments. It was found that two peaks at 8.73 (H^a, phenol-H) and 13.18 (H^b, naphthol-H) were present in the ¹H NMR spectrum of 1O in CD₃CN without CN^-/F^-. However, the signal peaks intensity at 8.73 and 13.18 gradually vanished accompany with the addition of CN^-/F^- to 1O, suggesting the formation of 1O+CN^-/F^- compound. The binding constants (K_a) of 1O with CN^-/F^- were determined to be 1.20×10⁴ and 1.86×10⁴ L·mol^-1 according to the Hildebrand-Benesi experiments.^[14] The limit of detection (LOD) of 1O toward CN^-/F^- was calculated as 1.03×10^-6 and 3.43×10^-6 mol·L^-1, respectively. Moreover, the competitive experiments of 1O+ CN^-/F^- with various anions were carried out in order to estimate the specificity. As shown in Figure 6, there were no obvious changes in the absorbance of 1O+CN^-/F^- when other anions were added to the solution containing 1O+CN^-/F^- except for ${\rm{HSO}}_{\rm{4}}^{-}$ and CH₃COO^-. Particularly, the absorbance and color of 1O+F^- could notably decrease and nearly bleach by stimulating with HSO₄^- and CH₃COO^-. These results indicated that 1O could be served as a high selectivity and sensitivity colorimetric probe to recognize CN^-/F^-.

  Figure 5

  

  Figure 5.  (A) Job's plot showing the 1:1 complex of (A) 1O with CN^-, and (B) 1O with F^-

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  Figure 6

  

  Figure 6.  Competitive tests for the fluorescence responses of 1O induced by CN^- (A) and F^- (B) in the presence of other anions

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  Subsequently, the photo-isomerization of 1O+CN^- compound was studied by UV-Vis spectral absorption. As shown in Figure 7, upon irradiation with 313 nm light, the absorbance peaks at 338 and 490 nm were gradually decreased, and a new obvious visible absorption band appeared at 644 nm (ε_max=4.74×10³ L·mol^-1·cm^-1) with the orange solution turned to green. The reason may be ascribed to the formation of the closed-ring isomer 1C+CN^-, which could be completely recovered to the original color and absorption spectrum of 1O+CN^- when irradiated with visible light (λ > 500 nm).

  Figure 7

  

  Figure 7.  Absorption spectral changes of 1O+CN^- by photo- irradiation with UV/Vis light in acetonitrile

  Inset: the solution color changes

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  2.3   Fluorescence studies of 1O toward Zn²⁺

  The interactions of 1O with various metal ions were also studied by fluorescence spectrum in acetonitrile (c=2.0×10^-5 mol·L^-1) (Figure 8A). It could be seen that 1O showed poorly fluorescent at 603 nm. Upon the addition of 0.2 mmol·L^-1 Al³⁺, Zn²⁺, Mg²⁺, Cd²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Cr³⁺, Ca²⁺, Ba²⁺, Pb²⁺, Sr²⁺, Sn²⁺, Hg²⁺, Mn²⁺, K⁺and Ag⁺ in acetonitrile, the fluorescence emission intensity of 1O exhibited no obvious changes. However, when stimulated by Zn²⁺, a strong emission peak was observed at 578 nm and the fluorescence color changed from dark to bright yellow (Figure 8B), indicating the formation of 1O-Zn²⁺ complex, which emission wavelength was distinct blue-shifted 25 nm comparison with 1O. The complexation between 1O and Zn²⁺ induced that fluorescence enhancement features maybe attributed to the blocked C=N isomerization and chelated enhanced fluorescence (CHEF) effects.^[15]

  Figure 8

  

  Figure 8.  Changes in the fluorescence of 1O induced by the addition of various metal ions (0.5 mmol·L^-1) in acetonitrile (c=2.0×10^-5 mol·L^-1)

  (A) Emission spectra; (B) photographs of fluorescence

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  Further, in order to study the binding properties of 1O to Zn²⁺, the fluorescence titration experiments were performed. As shown in Figure 9A, the original fluorescence emission peak at 578 nm was gradually increased with increasing the concentration of Zn²⁺ (0→1.4 mmol·L^-1), and then the spectrum followed by a plateau with further titration. Compared to 1O, the emission intensity of 1O-Zn²⁺ was enhanced 32-fold, and the fluorescence quantum yield was determined to be 0.15. The corresponding binding constant was estimated to be 3.03×10³ L·mol^-1, and the limit of detection (LOD) was calculated to be 2.98×10^-8 mol·L^-1. Notably, the fluorescence intensity of 1O-Zn²⁺ could completely restore when added excessive EDTA aqueous solution, demonstrating that EDTA binding with Zn²⁺ was stronger than 1O in acetonitrile. Furthermore, the strong fluorescence emission band of 1O-Zn²⁺ at 578 nm could switch off by irradiating with 313 nm light because the formation of the non-fluo- rescence closed-ring isomer 1C-Zn²⁺ (Figure 9B). The fluorescence of 1O-Zn²⁺ was quenched to 91% and the bright yellow solution became dark when the photoisomerization reaction arrived at the PSS. It is worth noting that the fluorescence of 1C-Zn²⁺ can not be restored to the initial state under the exposure with visible light. The results indicated that 1O could be used as an "off-on-off" fluorescence switch under the dual regulation of Zn²⁺ and 313 nm light.

  Figure 9

  

  Figure 9.  (A) Changes in fluorescence of 1O induced by Zn²⁺/ EDTA, and (B) changes in emission intensity of 1O-Zn²⁺ by photo irradiation

  Inset: its emission intensity curve at 578 nm with the addition of different concentration of Zn²⁺

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  Subsequently, Job's plot curve and ¹H NMR spectra experiments were carried out to determine the binding mode between 1O and Zn²⁺. As shown in Figure 10A, the complex concentration got the maximum when the molecular fraction of 1O reached 0.7, suggesting 2:1 stoichiometric complexation between 1O and Zn²⁺. These data were further supported by ¹H NMR titration analysis of 1O-Zn²⁺complex formation (Figure 10B). It was clearly seen that the peak intensity of OH^a (naphthol-H^a) at 13.77 was gradually disappeared with increasing Zn²⁺ from 0 to 0.4 mmol·L^-1. While the chemical shifts of OH^b (phenol-H^b) at 9.48 and CH^c (CH^c=N) at 8.98 undergo an upfield shift to 9.22 and 8.80, respectively. Meanwhile, two new signals were observed at 10.66 and 9.81, corresponding to the OH^b' (phenol-H^b') and CH^c' (CH^c'=N). These results further confirmed that the stoichiometric ratio between 1O and Zn²⁺ were 2:1. Based on these a plausible binding scheme was presented in Scheme 1, where the oxygen atoms of phenol/naphthol and the nitrogen atoms of C=N double bond are the most chelating atoms and participated in the complexation.

  Figure 10

  

  Figure 10.  (A) Job's plot showing the 2:1 complex of 1O with Zn²⁺, and (B) ¹H NMR spectra of 1O in the presence of Zn²⁺(0→0.4 mmol·L^-1) in acetonitrile

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  Scheme 1

  

  Scheme 1.  Photochromism and fluorescence changes of 1O induced by Zn²⁺, EDTA and lights

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  Figure 11A shows the selectivity of the fluorescence response of 1O-Zn²⁺ to various anions in acetonitrile (c=2.0×10^-5 mol·L^-1). The emission spectral of 1O-Zn²⁺induced by anions (0.1 mmol·L^-1) such as CN^-, F^-, Cl^-, Br^-, SCN^-, AcO^-, ${\rm{CO}}_{\rm{3}}^{2-}$, ${\rm{SO}}_{\rm{3}}^{2-}$, ${\rm{SO}}_{\rm{4}}^{2-}$, ${\rm{PO}}_{\rm{4}}^{3-}$, ${{\rm{S}}_2}{\rm{O}}_3^{2-}$, ${\rm{HCO}}_{\rm{3}}^{-}$, ${\rm{HSO}}_3^{-}$, ${\rm{HSO}}_{\rm{4}}^{-}$, ${\rm{HPO}}_{\rm{4}}^{2-}$ and ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$. It could be seen that no obvious effects on the fluorescence intensity of 1O-Zn²⁺when Cl^-, Br^-, SCN^-, ${\rm{SO}}_{\rm{3}}^{2-}$, ${\rm{SO}}_{\rm{4}}^{2-}$, ${{\rm{S}}_2}{\rm{O}}_3^{2-}$, ${\rm{HSO}}_3^{-}$, and ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$ were added to 1O-Zn²⁺ solution (0.5 mmol·L^-1). While CN^-, F^-, AcO^-, ${\rm{CO}}_{\rm{3}}^{2-}$, ${\rm{PO}}_{\rm{4}}^{3-}$ and ${\rm{HCO}}_{\rm{3}}^{-}$ can induce its fluorescence intensity decrease to some extent. Notably, when ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$ and ${\rm{HSO}}_{\rm{4}}^{-}$ were added to the acetonitrile solution containing 1O-Zn²⁺, the fluorescence intensity of 1O-Zn²⁺ was quenched to 97% and 94%, respectively, with an obvious color change from bright yellow to dark (Figure 11B). The results demonstrated that 1O-Zn²⁺ complex could be served as a good probe to detect ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$ and ${\rm{HSO}}_{\rm{4}}^{-}$. In a fluorometric titration, addition of ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$ to the solution of 1O-Zn²⁺ resulted in a gradually decrease of the emission intensity centered at 578 nm. By plotting the fluorescence intensity at 578 nm as a function of ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$ concentration (0→20 μmol·L^-1), a good relationship curve was obtained (Figure 11B). Similarly, ${\rm{HSO}}_{\rm{4}}^{-}$ can also turn off the fluorescence emission intensity of 1O-Zn²⁺. The results indicated that 1O-Zn²⁺could be potentially used as ratiometric fluorescent sensor for recognizing ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$/ ${\rm{HSO}}_{\rm{4}}^{-}$. Furthermore, the association constants were calculated to be 8.29×10⁴ L·mol^-1 (R=0.993) for ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$ and 6.86×10⁴ L·mol^-1 (R=0.997) for ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$ according to a Benesi-Hildebrand plot, and the LOD of 1O-Zn²⁺ for detecting ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$/${\rm{HSO}}_{\rm{4}}^{-}$ were estimated to be 6.55× 10^-7 and 1.27×10^-7 mol·L^-1, respectively.

  Figure 11

  

  Figure 11.  (A) Changes in fluorescence of 1O-Zn²⁺ induced by different anions, and (B) changes in emission intensity of 1O-Zn²⁺ upon the addition of H₂PO₄^-

  Inset: its emission intensity curve at 578 nm with the addition of different concentration of H₂PO₄^-

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  Subsequently, in order to comparison with reported chemosensors, some recently published related studies were summarized in Table 1, and some important parameters such as the LOD and binding constants, and stokes shift values. It could be seen that 1O had lower LOD value for detecting Zn²⁺, and high sensitivity in the range of 10^-7 for recognizing ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$. The binding constant of 1O was of the order 10³ for Zn²⁺, which was the small level compared to other probes. However, diarylethene 1O could effectively recognize both CN^- and ${{\rm{H}}_{\rm{2}}}{\rm{PO}}_{\rm{4}}^{-}$ with the normal level binding constants (10⁴), the multi sensing characteristics of 1O had much more advantages than other probes. The results provided a useful design strategy for constructing a multi-functional probe for the selective recognition based on unimolecule photochromic diaryl- ethene with a certain functionalized group.

  Table 1

  

  Table 1.  Comparative study of the analytical performance of 1O with other reported sensors

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  Ref.	Sensor	Binding constant Ka/(L·mol-1)	LOD/(L·mol-1)	Stokes shift/nm
  [16]		3.07×103 (Zn2+)	1.22×10-6 (Zn2+)	30 nm
  [17]		1.90×105 (Zn2+)	6.85×10-8 (Zn2+)	28 nm
  [18]		2.50×106 (Zn2+)	4.90×10-8 (Zn2+)	64 nm
  [19]		1.17×104 (Zn2+)	4.90×10-8 (Zn2+)	23 nm
  This work		3.03×103 (Zn2+) 1.20×104 (CN-) 8.29×104 (${{\rm{H}}_{\rm{2}}}{\rm{PO}}_{\rm{4}}^{-}$)	2.98×10-8 (Zn2+) 1.03×10-6 (CN-) 6.55×10-7 (${{\rm{H}}_{\rm{2}}}{\rm{PO}}_{\rm{4}}^{-}$)	25 nm

  3.   Conclusions

  In conclusion, a new colorimetric and fluorometric diarylethene sensor containing a 2-hydroxybenzylidene-ami- nonaphthol group had been developed. The compound exhibited "naked-eye" detection for CN^- with high selectivity and lower LOD in acetonitrile. Meanwhile, it also showed highly sensitively for detecting Zn²⁺ with a 2:1 binding stoichiometry, and the LOD for Zn²⁺ reached to 0.3 nmol·L^-1. Additionally, the 1O-Zn²⁺ complex could efficiently recognize ${{\rm{H}}_{\rm{2}}}{\rm{PO}}_{\rm{4}}^{-}$/${\rm{HSO}}_{\rm{4}}^{-}$ with the LOD of 6.55×10^-7 and 1.27×10^-7 mol·L^-1, respectively. These results suggested that 1O could be used as an efficient multifunctional sensor for detecting CN^-, ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$, and Zn²⁺.

  4.   Experimental section

  4.1   General methods

  All solvents were of analytical grade and used without further purification. Other reagents were used as-received. ¹H NMR and ¹³C NMR spectra were measured with a Bruker AVANCE NEO 500 MHz FT-NMR spectrometer (tetramethylsilane as the internal standard). Mass spectra were collected using an Agilent 1100 ion trap LC/MS MSD system. Metal ions, such as Al³⁺, Zn²⁺, Mg²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Cr³⁺, Ca²⁺, Pb²⁺, Sr²⁺, Mn²⁺and Ag⁺were prepared by dissolving the corresponding metal nitrates (0.1 mmol) in distilled water (2.0 mL), other metal ions like Ba²⁺, Cd²⁺, Hg²⁺, Sn²⁺, and K⁺ were prepared with their chlorides. Anions, except ${\rm{CO}}_{\rm{3}}^{2-}$, ${\rm{SO}}_{\rm{3}}^{2-}$, ${\rm{SO}}_{\rm{4}}^{2-}$, ${\rm{PO}}_{\rm{4}}^{3-}$, ${{\rm{S}}_2}{\rm{O}}_3^{2-}$, ${\rm{HCO}}_{\rm{3}}^{-}$, ${\rm{HSO}}_3^{-}$, ${\rm{HSO}}_{\rm{4}}^{-}$, ${\rm{HPO}}_{\rm{4}}^{{\rm{2-}}}$ and ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$ that were prepared with their sodium salts, remaining anions (CN^-, F^-, Cl^-, Br^-, SCN^- and AcO^-) were obtained by their tetrabutylammonium salts (0.1 mmol) in distilled water (2.0 mL). EDTA solution was gained with ethylenediaminetetraacetic acid disodium salt (Na₂EDTA) (1.0 mmol) in distilled water (10.0 mL). Hitachi F-4500 fluorescence spectrophotometer and Agilent 8454 were used in performing fluorescence spectra and UV/Vis photoirradiation with an MUL-165 UV lamp and MVL-210 visible lamp. Melting point was determined using a WRS-1B melting point apparatus. Elemental analysis was studied by a PE CHN 2400 analyzer. Photoirradiation experiments were performed on a setup consisting of an SHG-200 UV lamp, CX-21 ultraviolet fluorescence analysis cabinet, and BMH-250 visible lamp.

  4.2   Synthesis of 1-(3, 5-dimethylisoxazole-4-yl)-2-(2- methyl-5-[(3-aminonaphthol-2-yl)phenol-yl]-thiophe- ne-3-yl)perfluorocyclopentene (1O)

  1-(3, 5-dimethyl-4-isoxazol)-2-(2-methyl-5-[4-hydroxy-3-aldehyde]-thiophene-3-yl)perfluorocyclopentene (2) was synthesized following a series of previously reported methods.^[20]

  In a 100 mL flask, a mixture of diarylethene (2) (0.24 g, 0.5 mmol), 2-aminonaphthol (3) (0.08 g, 0.5 mmol) in anhydrous ethanol (5.0 mL) was stirred and refluxed for 6 h. After cooling the mixture solution to the room temperature, the crude product was washed several times with ethanol and diethyl ether, and dried under vacuum to afford 0.20 g (64% yield) 1O as a red solid. m.p. 215~216 ℃; ¹H NMR (500 MHz, DMSO-d₆) δ: 13.82 (s, 1H), 10.14 (s, 1H), 9.14 (s, 1H), 7.99 (s, 1H), 7.84 (s, 1H), 7.78 (d, J=5.0 Hz, 1H), 7.69~7.66 (m, 2H), 7.42 (s, 1H), 7.36 (t, J=15.0 Hz, 1H), 7.28 (t, J=15.0 Hz, 2H), 7.01 (d, J=5.0 Hz, 1H), 2.25 (s, 3H), 2.02 (s, 3H), 1.97 (s, 3H); ¹³C NMR (126 MHz, DMSO-d₆) δ: 10.72, 12.27, 14.58, 104.45, 110.44, 117.62, 118.32, 120.16, 121.70, 123.94, 124.66, 126.25, 126.57, 128.40, 129.53, 130.87, 133.86, 140.97, 142.22, 150.49, 158.57, 161.64, 162.99, 170.76; ESI-MS m/z: 629.09 [M+H]⁺. Anal. calcd for C₃₂H₂₂F₆N₂O₃S: C 61.14, H 3.53, N 4.46; found C 61.12, H 3.55, N 4.44.

  Supporting Information   ¹H NMR spectra of 1O in DMSO, ¹³C NMR spectra of 1O in DMSO and ESI-MS spectra of 1O in CH₃CN. ¹H NMR titration of 1O in the presence of CN^- and F^-, the binding constant between 1O+CN^-/F^-, the limit of detection (LOD) of CN^- and F^-, the binding constant for 1O-Zn²⁺and the limit of detection (LOD), changes in emission intensity of 1O-Zn²⁺upon the addition of different amount of ${\rm{HSO}}_{\rm{4}}^{-}$, the binding constant for 1O-Zn²⁺with ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$ and ${\rm{HSO}}_{\rm{4}}^{-}$, the limit of detection (LOD) of ${{\rm{H}}_2}{\rm{PO}}_{\rm{4}}^{-}$ and HSO₄^-. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/.

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  Figure 1  Photochromism of diarylethene 1O

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  Figure 2  Absorption spectral and color changes of diarylethene 1O by photoirradiation in acetonitrile (c=2.0×10^-5 mol·L^-1)

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  Figure 3  (A) Various anions induced the absorption of 1O, and (B) changes in the absorption of 1O in the presence of different amount of CN^- in acetonitrile

  Inset: (A) the absorption intensity at 490 nm with the addition of different anions), and (B) the solution color and absorbance changes at 488 nm with the addition of CN^-

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  Figure 4  Absorption spectral changes of 1O in the presence of different amount of F^- in acetonitrile

  Inset was the solution color and absorbance changes at 488 nm with the addition of F^-

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  Figure 5  (A) Job's plot showing the 1:1 complex of (A) 1O with CN^-, and (B) 1O with F^-

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  Figure 6  Competitive tests for the fluorescence responses of 1O induced by CN^- (A) and F^- (B) in the presence of other anions

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  Figure 7  Absorption spectral changes of 1O+CN^- by photo- irradiation with UV/Vis light in acetonitrile

  Inset: the solution color changes

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  Figure 8  Changes in the fluorescence of 1O induced by the addition of various metal ions (0.5 mmol·L^-1) in acetonitrile (c=2.0×10^-5 mol·L^-1)

  (A) Emission spectra; (B) photographs of fluorescence

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  Figure 9  (A) Changes in fluorescence of 1O induced by Zn²⁺/ EDTA, and (B) changes in emission intensity of 1O-Zn²⁺ by photo irradiation

  Inset: its emission intensity curve at 578 nm with the addition of different concentration of Zn²⁺

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  Figure 10  (A) Job's plot showing the 2:1 complex of 1O with Zn²⁺, and (B) ¹H NMR spectra of 1O in the presence of Zn²⁺(0→0.4 mmol·L^-1) in acetonitrile

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  Scheme 1  Photochromism and fluorescence changes of 1O induced by Zn²⁺, EDTA and lights

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  Figure 11  (A) Changes in fluorescence of 1O-Zn²⁺ induced by different anions, and (B) changes in emission intensity of 1O-Zn²⁺ upon the addition of H₂PO₄^-

  Inset: its emission intensity curve at 578 nm with the addition of different concentration of H₂PO₄^-

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  Table 1.  Comparative study of the analytical performance of 1O with other reported sensors

  Ref.	Sensor	Binding constant Ka/(L·mol-1)	LOD/(L·mol-1)	Stokes shift/nm
  [16]		3.07×103 (Zn2+)	1.22×10-6 (Zn2+)	30 nm
  [17]		1.90×105 (Zn2+)	6.85×10-8 (Zn2+)	28 nm
  [18]		2.50×106 (Zn2+)	4.90×10-8 (Zn2+)	64 nm
  [19]		1.17×104 (Zn2+)	4.90×10-8 (Zn2+)	23 nm
  This work		3.03×103 (Zn2+) 1.20×104 (CN-) 8.29×104 (${{\rm{H}}_{\rm{2}}}{\rm{PO}}_{\rm{4}}^{-}$)	2.98×10-8 (Zn2+) 1.03×10-6 (CN-) 6.55×10-7 (${{\rm{H}}_{\rm{2}}}{\rm{PO}}_{\rm{4}}^{-}$)	25 nm

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