English

• 0.   Introduction

  Iron is one of the most abundant elements in the earth′s crust, and it is not only found in large quantities in the natural environment, but also plays an important role in the circulatory system of living organisms^[1]. In addition, Fe ion is also one of the major industrial pollutants that can pollute the ecological environment. Some analytical means, including atomic absorption spectrometer, inductively coupled plasma spectrometry (ICP), etc. have been used to detect Fe³⁺. Although these methods have high detection accuracy, they also have many disadvantages, such as that the instrumentation is too large and not easy to carry, and the detection cost is high. Therefore, it is important to develop an inexpensive and portable method for the detection of iron ion^[2].

  Luminescent coordination polymers (LCPs) are luminescent materials formed by metal ions/cluster nodes (second building units, SBUs) and organic linkers^[3-5]. When LCPs come into contact with the guest substance, the resulted host - guest interactions may change the luminescence properties of LCPs, which can be used for detection of the guest substance^[6-7].

  Herein, we selected a ligand H₂L (H₂L=2 - (1, 3 - dioxo - 1H - benzo[de]isoquinolin - 2(3H) - yl)terephthalic acid) to react with zinc ion to prepare a two-dimension- al (2D) LCP with (4, 4) - net topology, namely [Zn₂(L)₂ (DMSO)₂(DMF)] (1, DMSO=dimethyl sulfoxide, DMF= dimethylformamide). The ability of 1 as a luminescence probe for the detection of Fe³⁺ was investigated. The results showed that 1 had a high sensitivity for Fe³⁺ and the limit of detection (LOD) was calculated to be 2.8 μmol·L^-1. The probe can be recycled after DMF washing, indicating a good potential for Fe³⁺ detection.

  1.   Experimental

  1.1   Materials and methods

  All the reagents and solvents were commercially purchased, and used as received without further purification. H₂L was synthesized according to reported method^[8]. The IR spectra (650~4 000 cm^-1) were recorded from a Nicolet - 20DXB spectrometer using KBr pellets. Thermogravimetric analyses (TGA) were carried out on a TA - Q50 thermogravimetric analyzer under N₂ atmosphere with the heating rate of 10 ℃ · min^-1. Elemental analyses (C, H and N) were performed on a Vario EL Ⅲ elemental analyzer. Powder X - ray diffraction (PXRD) patterns were collected on D/MAX-2400 X - ray diffractometer with Cu Kα radiation (λ = 0.154 060 nm) at a scan rate of 10 (°)·min^-1 (Voltage: 40 kV, Current: 25 mA, Scan range: 5°~50°). The lumi- nescence spectra were collected on Hitachi F-7000 FL Spectrophotometer.

  1.2   Synthesis of [Zn₂(L)₂(DMSO)₂(DMF)] (1)

  A mixture containing 0.02 mmol H₂L and 0.02 mmol ZnSO₄·H₂O in DMF/H₂O/DMSO (3 mL/0.1 mL/0.5 mL) in a 20 mL scintillation vial was heated at 115 ℃ for 1 d and then cooled to room temperature. The colorless crystals were collected, washed with DMF and dried in air (Yield: 53.4%). Element analysis Calcd. for C₄₇H₃₇N₃O₁₅S₂Zn₂(%): C, 52.34; H, 3.46; N, 3.90. Found(%): C, 51.89; H, 3.06; N, 3.77. IR (cm^-1): 3 003(w), 1 701(m), 1 659(s), 1 588(s), 1 492(w), 1 411 (m), 1 374(s), 1 298(w), 1 242(s), 1 199(w), 1 031(m), 961(w), 892(w), 773(s), 704(w), 657(w).

  1.3   Structure determination

  The intensity data from single crystals of 1 were collected at 150 K on a Bruker SMART APEX Ⅱ CCD area detector system with graphite-monochromated Mo Kα (λ =0.071 073 nm) radiation. Data reduction and unit cell refinement were performed with Smart - CCD software. The structure was solved by direct meth - ods using SHELXS - 2014 and were refined by full - matrix least squares methods using SHELXS - 2014^[9]. All non - hydrogen atoms were refined anisotropically. The hydrogen atoms related to C and N atoms were gen- erated geometrically. A summary of crystal structure refinement data is given in Table 1. Selected bond lengths and angles of 1 are given in Table 2.

  Table 1

  

  Table 1.  Crystal data collection and structure refinement parameters for 1

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  Formula	C47H37N3O15S2Zn2	Dc/(g·cm-3)	1.594
  Formula weight	1 078.65	μ/mm-1	1.237
  Crystal system	Monoclinic	F(000)	2 208
  Space group	P21/c	θ range/(°)	2.192~24.999
  a/nm	1.578 75(9)	Reflection collected, unique, observed	100 290, 7 900, 6 821
  b/nm	1.858 13(11)	Rint	0.053 9
  c/nm	1.536 32(10)	GOF on F2	1.055
  β/(°)	94.370(2)	R1, wR2[I > 2σ(I)]	0.041 9, 0.115 4
  V/nm3	4.493 7(5)	R1, wR2 (all)	0.049 3, 0.118 9
  Z	4

  Table 2

  

  Table 2.  Selected bond lengths (nm) and bond angles (°) for 1

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  Zn1—O1	0.203 3(2)	Zn1—O9	0.209 8(2)	Zn2—O8A	0.192 5(2)
  Zn1—O5	0.238 5(3)	Zn1—O10	0.210 5(3)	Zn2—O11	0.196 2(3)
  Zn1—O6	0.207 9(2)	Zn2—O2	0.194 2(2)
  Zn1—O7	0.203 7(2)	Zn2—O3B	0.195 2(2)
  O1—Zn1—O6	151.99(10)	O7A—Zn1—O6	97.59(10)	O2—Zn2—O11	101.76(10)
  O1—Zn1—O7	110.42(9)	O7—Zn1—O9	93.79(9)	O3B—Zn2—O11	112.88(12)
  O1—Zn1—O9	86.72(9)	O9—Zn1—O10	173.73(11)	O8A—Zn2—O2	119.26(10)
  O6—Zn1—O5	58.38(9)	O10—Zn1—O5	87.84(10)	O8A—Zn2—O3B	117.83(11)
  O7—Zn1—O5	155.78(9)	O2—Zn2—O3B	96.42(10)	O8A—Zn2—O11	107.54(11)
  Symmetry codes: A: -x, 1/2+y, 1/2-z; B: 1-x, 1/2+y, 1/2-z.

  1.4   Luminescence sensing of metal ions in disper-sions

  Each luminescence detection experiment was re- peated at least three times, and the results were essen- tially the same each time. The excitation slit width and emission slit width of the instrument were all set to 10 nm for each detection experiment. Each titration assay: 3 mg of fine powdered 1 was dispersed in 6 mL of DMF solution and sonicated for 1 h to form a homogeneous suspension. Titration assay procedure: DMF solution dissolved with metal ions was added drop by drop to the suspension, shaken well, and its luminescence intensity was measured immediately.

  2.   Results and discussion

  2.1   Crystal structure of 1

  Single - crystal X - ray diffraction analysis shows 1 bears a 2D framework based on dinuclear {Zn ₂(COO)₄} SBUs and L^2- ligands (Fig. 1). Fig. 1a shows the different coordination environments of the two Zn²⁺ ions. Zn1 has a distorted octahedral {O₆} coordination polyhedron, and is coordinated by four carboxylate oxygen atoms form a DMF and a DMSO terminal molecules. In contrast, Zn2 bears a tetrahedral {O₄} coordination polyhedron and is coordinated by three carboxylate oxygen atoms and a terminal DMF molecule. The Zn—O bond distances are in a range of 0.192 5(2)~0.238 5(3) nm, which are similar to the reported Zn complex^[10]. Two neighboring Zn²⁺ ions are bridged by two carboxylate groups with Zn…Zn separation of 0.41 nm and each metal ion is further chelated by a carboxylate group, leading to a {Zn₂(COO)₄} SBU. The ligands (L^2-) adopting two coordination modes, μ₃-κ_O¹∶κ_O¹∶κ_O² and μ₃ - κ_O1∶κ_O1∶κ_O1, respectively, are employed to coordinate to two metal ions that belong to two {Zn₂(COO)₄} SBUs. Thus, the ligand L^2- can be treated as a 2 - connected linker.

  Figure 1

  

  Figure 1.  Coordination environment of metal ions in 1 (a); Two coordination modes of ligand L^2- in 1 (b); Framework topology of 2D net of 1 (c); Packing of structure of 1 viewed along b (d) and c (e) axes, respectively

  Symmetry codes: A: -x, y+1/2, -z+1/2; B: 1-x, y+1/2, -z+1/2; The π-π interactions are represented as blue dotted lines; The H, C, N and S atoms of DMF and DMSO molecules and the hydrogen atoms of L^2- have been omitted for clarity in Fig.d and e

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  The combination of the 4-connected SBUs and 2- connected linkers leads to a 2D neutral network with (4, 4)-net type topology. The 2D neutral network is observed along b axis, extending in a wave shape. In the quadrilateral unit of the layered structure, the two ligands in opposite positions are packed so tightly that strong offset face-to-face π-π interaction occurs between their naphthalene groups with ring-ring separation (center to center) of 0.36 nm. These π-π interactions connect the layers into a 3D supramolecular structure.

  2.2   Characterization of the compound

  The purity and crystallinity of the bulk samples of 1 were confirmed by PXRD analysis (Fig. 2a). TGA of 1 (Fig. 2b) revealed that it was stable before 183 ℃. The first weight loss of 7.01% corresponds to the loss of one DMF molecule (Calcd. 6.77%). Then there was a short plateau in a range of 238 ~ 311 ℃. When the temperature continued to rise, there was a significant drop in the weight of 1, which means that the compound decomposes completely.

  Figure 2

  

  Figure 2.  (a) PXRD patterns of 1; (b) TGA curve of 1

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  The solid-state photoluminescence spectra of H₂L and 1 were tested, as shown in Fig. 3a. At the excitation wavelength of 365 nm, H₂L and 1 had emission peaks at 469 and 457 nm, respectively. 1 and H₂L had similar emission peaks with similar positions and peak shapes, so the blue emission of 1 can be attributed to the ligand - related emission. The CIE coordinates of 1 were (0.16, 0.18), which were very close to (0.14, 0.08) of the ideal blue light source. The luminescence spectrum of DMF suspension of 1 is shown in Fig. 3b, which was similar with the solid-state spectrum.

  Figure 3

  

  Figure 3.  Luminescence spectra of H₂L and 1 in solid-state (a) and DMF suspension of 1 (b)

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  2.3   Luminescence sensing of Fe³⁺

  We prepared a suspension of 1 with DMF and added different metal ion solutions (1.0 mmol·L^-1) to the suspension. As shown in Fig. 4, the luminescence intensities of the suspensions showed different degrees of changes after the addition of different metal ions. For example, a slight enhancement of the emission intensity of the suspension occurred after the addition of Al³⁺ solution, while a certain degree of weakening of the intensity occurred after the addition of Mn²⁺, Cr³⁺ and Cu²⁺, respectively. And only Fe³⁺ had a significant quenching effect on the luminescence of the suspension, which could be clearly observed by the naked eye. The addition of other metal ions did not affect the emission intensity of the suspensions.

  Figure 4

  

  Figure 4.  (a) Luminescence intensities of DMF dispersion of 1 after addition of different metal cations (1.0 mmol·L^-1); (b) Anti-interference experiments of other metal ions on Fe³⁺ detection

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  To test the ability of compound 1 to detect iron ion, we added Fe³⁺ solution dropwisely to the DMF suspension of 1 and measured the emission spectra of the whole process. As shown in Fig. 5a, the intensity of the emission decreased when the amount of Fe³⁺ increased. The relationship between the Fe³⁺ concentration and luminescent intensity is illustrated in Fig. 5b and can be fitted well with a linear equation. The correlation coefficient (R²) was calculated to be 0.996 33, indicating it can be a good probe for the quantitative detection of Fe³⁺ in a concentration range of 0~90 μmol·L^-1. Based on the signal-noise ratio of being three, its LOD was calculated to be 2.8 μmol·L^-1, which was compara - ble to other MOF - based probes for iron ion^[11]. Anti - interference experiments of other metal ions on the detection of Fe³⁺ were also performed (Fig. 4b). The results show that the sensing behavior of 1 toward Fe³⁺ is not affected by other ions.

  Figure 5

  

  Figure 5.  (a) Luminescence intensity of DMF dispersion of 1 after incremental addition of Fe³⁺ solution with 365 nm excitation wavelength; (b) Variation in luminescence intensity of DMF suspension of 1 as a function of concentration of Fe³⁺ solution

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  Regeneration is an important parameter to measure whether a probe has application. The used suspension was centrifuged to separate the solids. The initial luminescence can be restored by washing the resulted solid with DMF for several times. The entire detection-recovery cycle can be repeated at least three times. Meanwhile, PXRD results showed that the structure of 1 remained unchanged after three cycles of detection - recovery tests. Such results indicate that 1 has a good application prospect as a probe for detecting Fe^{3+ [4-8]}.

  According to the literature reports^[12], we believe that the reason for the quenching effect of Fe³⁺ on the luminescence of suspension of 1 is that the Fe³⁺ solution has a significant absorption between 310~425 nm. The excitation light source used in the detection was 365 nm, so there was an energy competition between Fe³⁺ and 1, which quenched the emission of 1.

  3.   Conclusions

  In summary, a two - dimensional LCP was synthesized by hydrothermal synthesis using H₂L ligands and zinc ions. The compound has a 2D (4, 4)-network topology. The results from the titration analysis showed that 1 exhibited a sensitive and selective quenching response to Fe³⁺ with LOD of 2.8 μmol·L^-1, and thus could be an Fe³⁺ probe with practical applications.

  
  Acknowledgements: This research is supported by National Natural Science Foundation of China (Grant No.21871038).
• 1. [1]

     徐中轩, 李立凤, 徐仕菲, 陈雪婷. 无机化学学报, 2020, 36(11): 2087-2092 doi: 10.11862/CJIC.2020.230XU Z X, LI L F, XU S F, CHEN X T. Chinese J. Inorg. Chem. , 2020, 36(11): 2087-2092 doi: 10.11862/CJIC.2020.230

  2. [2]

     高玲玲, 李梦雪, 杜意恩, 陈勇强. 无机化学学报, 2020, 36(12): 2359-2366 doi: 10.11862/CJIC.2020.249GAO L L, LI M X, DU Y E, CHEN Y Q. Chinese J. Inorg. Chem. , 2020, 36(12): 2359-2366 doi: 10.11862/CJIC.2020.249

  3. [3]

     Feng X, Zhao J S, Liu B, Wang L Y, Ng S, Zhang G, Wang J G, Shi X G, Liu Y Y. Cryst. Growth Des. , 2010, 10(3): 1399-1408 doi: 10.1021/cg901391y

  4. [4]

     Liu R, Huang M M, Yao X X, Li H H, Yang F L, Li X L. Inorg. Chim. Acta, 2015, 434: 172-180 doi: 10.1016/j.ica.2015.05.019

  5. [5]

     Niu Y W, Liu X, Zhao L, Guo Y M, Li W X, Ma M M, Li X L. Polyhedron, 2019, 157: 241-248 doi: 10.1016/j.poly.2018.10.016

  6. [6]

     Li B, Fang W J, Liu S Q, Zhao H, Zhang J J. Chin. J. Struct. Chem. , 2020, 39(1): 110-117

  7. [7]

     Zhao H, Ni J, Zhang J J, Liu S Q, Sun Y J, Zhou H J, Li Y Q, Duan C Y. Chem. Sci. , 2018, 9(11): 2918-2926 doi: 10.1039/C8SC00021B

  8. [8]

     Singh D, Bhattacharyya P K, Baruah J B. Cryst. Growth Des. , 2010, 10(1): 348-356 doi: 10.1021/cg900938x

  9. [9]

     Sheldrick G M. Acta Crystallogr. Sect. C, 2015, C71: 3-8

  10. [10]

      Feng X, Feng Y Q, Liu L, Wang L Y, Song H L, Ng S W. Dalton Trans. , 2013, 42(21): 7741-7754 doi: 10.1039/c3dt33002h

  11. [11]

      Xu H, Pan Z R. Chin. J. Struct. Chem. , 2019, 38(12): 2121-2128

  12. [12]

      Zhan D Y, Saeed A, Li Z X, Wang C M, Yu Z W, Wang J F, Zhao N J, Xu W H, Liu J H. Dalton Trans. , 2020, 49(48): 17737-17744 doi: 10.1039/D0DT03781H

• 

  Figure 1  Coordination environment of metal ions in 1 (a); Two coordination modes of ligand L^2- in 1 (b); Framework topology of 2D net of 1 (c); Packing of structure of 1 viewed along b (d) and c (e) axes, respectively

  Symmetry codes: A: -x, y+1/2, -z+1/2; B: 1-x, y+1/2, -z+1/2; The π-π interactions are represented as blue dotted lines; The H, C, N and S atoms of DMF and DMSO molecules and the hydrogen atoms of L^2- have been omitted for clarity in Fig.d and e

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  Figure 2  (a) PXRD patterns of 1; (b) TGA curve of 1

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  Figure 3  Luminescence spectra of H₂L and 1 in solid-state (a) and DMF suspension of 1 (b)

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  Figure 4  (a) Luminescence intensities of DMF dispersion of 1 after addition of different metal cations (1.0 mmol·L^-1); (b) Anti-interference experiments of other metal ions on Fe³⁺ detection

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  Figure 5  (a) Luminescence intensity of DMF dispersion of 1 after incremental addition of Fe³⁺ solution with 365 nm excitation wavelength; (b) Variation in luminescence intensity of DMF suspension of 1 as a function of concentration of Fe³⁺ solution

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  Table 1.  Crystal data collection and structure refinement parameters for 1

  Formula	C47H37N3O15S2Zn2	Dc/(g·cm-3)	1.594
  Formula weight	1 078.65	μ/mm-1	1.237
  Crystal system	Monoclinic	F(000)	2 208
  Space group	P21/c	θ range/(°)	2.192~24.999
  a/nm	1.578 75(9)	Reflection collected, unique, observed	100 290, 7 900, 6 821
  b/nm	1.858 13(11)	Rint	0.053 9
  c/nm	1.536 32(10)	GOF on F2	1.055
  β/(°)	94.370(2)	R1, wR2[I > 2σ(I)]	0.041 9, 0.115 4
  V/nm3	4.493 7(5)	R1, wR2 (all)	0.049 3, 0.118 9
  Z	4

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  Table 2.  Selected bond lengths (nm) and bond angles (°) for 1

  Zn1—O1	0.203 3(2)	Zn1—O9	0.209 8(2)	Zn2—O8A	0.192 5(2)
  Zn1—O5	0.238 5(3)	Zn1—O10	0.210 5(3)	Zn2—O11	0.196 2(3)
  Zn1—O6	0.207 9(2)	Zn2—O2	0.194 2(2)
  Zn1—O7	0.203 7(2)	Zn2—O3B	0.195 2(2)
  O1—Zn1—O6	151.99(10)	O7A—Zn1—O6	97.59(10)	O2—Zn2—O11	101.76(10)
  O1—Zn1—O7	110.42(9)	O7—Zn1—O9	93.79(9)	O3B—Zn2—O11	112.88(12)
  O1—Zn1—O9	86.72(9)	O9—Zn1—O10	173.73(11)	O8A—Zn2—O2	119.26(10)
  O6—Zn1—O5	58.38(9)	O10—Zn1—O5	87.84(10)	O8A—Zn2—O3B	117.83(11)
  O7—Zn1—O5	155.78(9)	O2—Zn2—O3B	96.42(10)	O8A—Zn2—O11	107.54(11)
  Symmetry codes: A: -x, 1/2+y, 1/2-z; B: 1-x, 1/2+y, 1/2-z.

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•