English

• Recently, environmental pollution problem has drawn more and more attentions. Metal ions, particu-larly copper and iron ions, are common pollutants. Although copper and iron perform outstanding functions in many industrial applications^[1-4], it is easily to cause massive copper and iron ions entering human body through foods and drinking water, if industrial waste is not discharged as required. An adequate concentration of Cu²⁺ ions is needed in bone develop-ment, cellular metabolism, and plays an important role in significant catalytic cofactor for iron^[5-6]. Much like copper, iron plays a critical role in human metabolism. Most importantly, iron has significant function in oxygen transport and metabolism of mitochondrial^[7]. However, overloading of copper and iron ions can cause a series of diseases, such as anemia, brain-dysfunction, liver cirrhosis, renal insufficiency hepatic cirrhosis, anemia, hereditary hemochromatosis and even death^[8-13]. Thus, it is extremely urgent to develop an efficient method to detect Cu²⁺ and Fe³⁺ ions in the areas of environmental and medical sciences and the nuclear industry.

  Luminescence sensing has been demonstrated to be an effective approach to detect Cu²⁺ and Fe³⁺ ions for their obvious advantages, including high sensitivity and selectivity, low cost, rapid response time, simple manipulation, as well as the possibility to be applied to either solid phase or solution^[14-16]. Metal-organic frameworks (MOFs), as an outstanding class of micro-porous and mesoporous materials, have superior functional properties and applications in the fields of gas storage and separation^[17-19], heterogeneous cataly-sis^[20-22], photoluminescence^[23-24], magnetic materials^[25-26], electrocatalysis^[27-28], molecular recognition^[29] and drug delivery^[30]. Ln MOFs-based luminescence sensors have successfully attracted enormous attention, due to their unique luminescent properties of visible pure colors, large Stokes′ shift values and relatively long lumines-cence lifetimes, which attributed to f-f transitions via an "antenna effect"^[31-35]. However, rare Ln MOFs could simultaneously probe Cu²⁺ and Fe³⁺ ions.

  In this work, a new Ln-MOFs [Tb(L)(H₂O)₂]_n (1) (H₃L=4, 4′, 4″-((1, 3, 5-triazine-2, 4, 6-triyl)tris(sulfaned-iyl))tribenzoic acid) had been obtained. All L^3- ligands bridge metal ions to form a novel 3D framework with 1D open channels (1.550 73(6) nm×0.926 97(4) nm). In addition, luminescent studies revealed that complex 1 exhibited a unique turn-off luminescent response to Cu²⁺ and Fe³⁺ in CH₃CN solution.

  1.   Experimental

  1.1   Materials and measurements

  All other commercially available chemical reagents and solvents were of reagent grade and used without further purification. Powder X-ray diffraction (PXRD) was studied on a Rigaku Ultima Ⅳ diffracto-meter (Cu Kα radiation, λ=0.154 06 nm, 40 kV, 40 mA, 2θ=5°~45°) at room temperature. The IR spectra were recorded on a FTIR-8700 spectrometer using KBr pellets in a range of 4 000~400 cm^-1. Elemental analysis(EA) was carried out on a Perkin-Elmer 2400(Ⅱ) element analysis instrument. Thermogravimetric analyses(TGA) were taken on a Mettler-Toledo TGA/SDTA 851e in a range of 25~700 ℃ (the heating rate was 10 ℃·min^-1) under air atmosphere. The fluorescent spectra were performed on a HITACHI F-7000 fluore-scence spectrometer at room temperature. The UV-Vis spectra were recorded by using a UV-visible spectrophotometer (Evolution 500LC).

  1.2   Preparation of complex 1

  A mixture of Tb(NO₃)₃·2H₂O (0.190 5 g, 0.5 mmol) and H₃L (0.267 3 g, 0.5 mmol) was dissolved in 2 mL deionized water and 2 mL CH₃CN. And the whole solution was transferred in a Teflon-lined stainless steel container and heated to 100 ℃ for 3 days, and then it was slowly cooled to room temperature over 3 days. After filtered and washed with CH₃CN, colorless and block single crystals were obtained in 45% yield based on H₃L. Elemental Anal. Calcd. for complex 1(%): C, 39.48; H, 2.19; N, 5.76. Found(%): C, 39.62; H, 2.28; N, 5.25. IR (KBr, cm^-1): 3 190 (m, br), 1 687 (s), 1 631 (m), 1 594 (m), 1 557 (s), 1 478 (vs), 1 424 (s), 1 397 (s), 1 254 (vs), 1 172 (w), 1 075 (w), 1 000 (w), 906 (w), 844 (s), 761 (s), 678 (m), 662 (w), 575 (m).

  1.3   X-ray crystallography

  The single-crystal X-ray diffraction data colle-ction was performed on a Bruker SMART APEX-Ⅱ CCD detector diffractometer equipped with Mo Kα radiation (λ=0.071 073 nm) at room temperature. The absorption correction was performed by using the SADABS program^[36]. Data intensity was corrected by Lorentz-polarization factors and empirical absorption. The structure of the complex was measured by direct methods and expanded with Fourier techniques. Anisotropic displacement parameters based on F ² were applied to all non-hydrogen atoms in full-matrix least-squares refinements^[37]. The hydrogen atoms were assi-gned with isotropic displacement factors and included in the final refinement cycles by the use of geometrical restrains. All calculations were performed with SHELX-2014 package^[38].The details of all pertinent crystallographic data and structure refine-ment for complex 1 are summarized in Table 1. The selected bond lengths and angles are listed in Table 2.

  Table 1

  

  Table 1.  Crystal data and structure refinement for complex 1

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  Empirical formula	C24H16N3O8S3Tb		μ/mm-1	2.592
  Formula weight	729.5		F(000)	2 864
  Crystal system	Monoclinic		θ range / (°)	4.792~55.174
  Space group	C2/c		Reflection collected	73 495
  a / nm	2.228 23(10)		Unique reflection (Rint)	6 968 (0.028 0)
  b / nm	0.926 97(4)		Data with I > 2σ(I)	6 657
  c / nm	3.066 06(13)		Parameter refined	358
  β / (°)	107.207 0(10)		R1, wR2 [I > 2σ(I)]	0.049 0, 0.151 9
  V / nm3	6.049 5(5)		R1, wR2 (all data)	0.051 2, 0.153 0
  Z	8		Goodness-of-fit (on F2)	1.186
  Dc / (g·cm-3)	1.602

  Table 2

  

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

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  Tb(1)-O(1)	0.226 4(5)	Tb(1)-O(4)#2	0.242 4(5)	Tb(1)-O(6)#1	0.235 2(5)
  Tb(1)-O(2W)	0.244 5(5)	Tb(1)-O(2)#2	0.236 5(5)	Tb(1)-O(3)#2	0.259 3(5)
  Tb(1)-O(5)#3	0.239 0(5)	Tb(1)-O(1W)	0.241 7(5)
  O(1)-Tb(1)-O(6)#1	84.43(17)	O(1W)-Tb(1)-O(4)#2	132.00(16)	O(1)-Tb(1)-O(2)#2	101.65(17)
  O(1)-Tb(1)-O(2W)	73.45(18)	O(6)#1-Tb(1)-O(2)#2	146.44(17)	O(6)#1-Tb(1)-O(2W)	138.68(18)
  O(1)-Tb(1)-O(5)#3	153.95(18)	O(2)#2-Tb(1)-O(2W)	73.53(19)	O(6)#1-Tb(1)-O(5)#3	111.12(16)
  O(5)#3-Tb(1)-O(2W)	81.45(17)	O(2)#2-Tb(1)-O(5)#3	77.26(16)	O(1W)-Tb(1)-O(2W)	70.42(18)
  O(1)-Tb(1)-O(1W)	86.34(18)	O(4)#2-Tb(1)-O(2W)	144.45(18)	O(6)#1-Tb(1)-O(1W)	73.80(17)
  O(1)-Tb(1)-O(3)#2	76.82(16)	O(2)#2-Tb(1)-O(1W)	138.98(18)	O(6)#1-Tb(1)-O(3)#2	79.96(15)
  O(5)#3-Tb(1)-O(1W)	78.65(17)	O(2)#2-Tb(1)-O(3)#2	69.66(16)	O(1)-Tb(1)-O(4)#2	127.01(17)
  O(5)#3-Tb(1)-O(3)#2	125.22(16)	O(6)#1-Tb(1)-O(4)#2	76.37(16)	O(1W)-Tb(1)-O(3)#2	150.03(17)
  O(2)#2-Tb(1)-O(4)#2	73.75(17)	O(4)#2-Tb(1)-O(3)#2	51.60(15)	O(5)#3-Tb(1)-O(4)#2	78.16(17)
  O(2W)-Tb(1)-O(3)#2	125.96(17)
  Symmetry codes: #1: -x+1, y, -z+1/2; #2: -x+3/2, -y+1/2, -z+1; #3: x+1/2, -y+3/2, z+1/2; #4: x-1/2, -y+3/2, z-1/2.

  CCDC: 1936537.

  2.   Results and discussion

  2.1   Crystal structure of the complex

  Single crystal X-ray diffraction result of complex 1 reveals that it crystallizes in the monoclinic system with C2/c space group. The coordination environment of the metal center is shown in Fig. 1a. The asymmetric unit is composed of one metal center, one fully deprotonated L^3- anion, and two H₂O molecules. Each Tb³⁺ center is eight-coordinated by six O atoms from six different L^3- ligands (Tb-O 0.226 4(5)~0.259 3(5) nm) and two O atoms from two H₂O molecules (Tb-O 0.241 7(5)~0.244 5(5) nm), resulting in a dodecahedron geometry. The bond angles around Tb(Ⅲ) in complex 1 are from 51.60(15)° to 153.95(18)° (Table 2).

  Figure 1

  

  Figure 1.  (a) Coordination environments of Tb(Ⅲ) ions in complex 1 showing 50% probability displacement ellipsoids; (b) 1D helical chains in complex 1; (c) 2D layer of complex 1 viewed along b axis; (d) 3D network in complex 1 with green spheres representing pores

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  As to L^3- ligand, each ligand coordinates to four Tb(Ⅲ) ions by two different carboxylate groups adopting μ₁-η¹:η¹ and μ₂-η¹:η¹ coordination modes (Fig. 2). Car-boxylate groups from different L^3- ligands can be viewed as μ₂-bridging ligands, which bridge conti-guously two metal atoms to form an infinite 1D chains (Fig. 1b). Furthermore, L^3- ligands link two infinite 1D chains to make up a 2D layer parallel to ac plane (Fig. 1c). In the complex, the layers are further extended by the L^3- ligands, resulting in an open 3D network structure with 1D channels of 1.550 73(6)nm×0.926 97(4) nm along the c-axis (Fig. 1d) (the size is calculated by the distance between the pair of the symmetric Tb1 atoms and C19 atoms in the pore). Note that, PLATON analysis reveals a moderate void of 1.337 5 nm³, which retains 22.1% of the volume of unit cell (6.049 5 nm³).

  Figure 2

  

  Figure 2.  Binding mode of the linker of complex 1

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  2.2   Powder X-ray diffraction (PXRD) and thermogravimetric analysis

  To check the framework stability of complex 1, PXRD determination and thermogravimetric analysis (TGA) of complex 1 were performed (Fig. 3). The PXRD pattern of 1 shows that complex 1 is almost similar to the simulated one. The result confirms the phase purity and good stability of complex 1. In TGA, complex 1 was heated to 700 ℃ in a rate of 10 ℃·min^-1 under air atmosphere. The TGA curve displayed the first weight loss of 4.17% before 125 ℃, which may correspond to the loss of H₂O molecules (Calcd. 4.93%). Subsequently, the ligand began to decompose and the framework of complex 1 was destroyed at 525 ℃.

  Figure 3

  

  Figure 3.  (a) TGA curve of complex 1; (b) PXRD patterns of complex 1 and complex 1 after immersing in Cu(NO₃)₂ or Fe(NO₃)₃

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  2.3   Luminescence property

  The solid-state fluorescent spectra of the free ligand (H₃L) and complex 1 were measured at room temperature, respectively (Fig. 4). Complex 1 exhibited four characteristic luminescent emission peaks at 490, 545, 585 and 622 nm, upon excitation at 330 nm, which are contributed to the ⁵D₄→⁷F₆, ⁵D₄→⁷F₅, ⁵D₄→⁷F₄, and ⁵D₄→⁷F₃ f-f transitions, respectively^[39].

  Figure 4

  

  Figure 4.  (a) Excitation (black line) and emission (red line) spectra of complex 1; (b) Emission spectrum of free ligand H₃L (λ_ex=320 nm)

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  To evaluate the influence of different kinds of metal cations on the luminescence sensing ability of complex 1, the powder of complex 1 (5.00 mg) was dispersed and immersed in different CH₃CN solution (5 mL) containing 10 mmol·L^-1 of M(NO₃)_x (M=Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, Zn²⁺, Mn²⁺, Cd²⁺, Ni²⁺, Co²⁺, Fe³⁺ and Cu²⁺) to form stable suspension by ultrasound. The corresponding luminescence curves still showed four characteristic emission peaks of Tb³⁺ ions, and only the strongest emission bands at 545 nm were monitored under the adding of various cations (Fig. 5). The results of fluorescent sensing measurements show that Zn²⁺ and Pb²⁺ could improve the emission intensity of complex 1 with small enhancement effects. As for Mn²⁺, Sr²⁺, K⁺, Mg²⁺, Ca²⁺, Ni²⁺, Cd²⁺, Ba²⁺, Na⁺ and Co²⁺, the emission intensities exhibited no differences from each other. However, Cu²⁺ and Fe³⁺ exhibited a relatively high quenching effect on the luminescence of complex 1, which indicates that complex 1 can be considered as a promising fluorescent probe for Cu²⁺ or Fe³⁺.

  Figure 5

  

  Figure 5.  (a) Luminescence intensities of complex 1 in the presence of different metal ions in CH₃CN solution when excited at 330 nm; (b) Bar diagram of luminescence intensities at 545 nm of complex 1 in the presence of different metal ions in CH₃CN solution

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  Moreover, to further explore the relationship between Cu²⁺ concentration and quenching effect, a series of suspensions of Cu²⁺-1 in CH₃CN solutions with different concentrations of Cu²⁺ was made. A plot of the luminescence intensities of Cu²⁺-1 clearly shows the decrease of the luminescence intensity with incre-asing concentration of Cu²⁺ (Fig. 6a). The quenching effect was calculated by the Stern-Volmer equation: I₀/I=1+K_svc_M, where I₀ and I are the luminescence intensity of 1 in the absence and presence of metal ion, respectively; c_M is the concentration of metal ion (0~1 mmol·L^-1); K_sv is the quenching effect coefficient of metal ion (Fig. 7a). Same as Cu²⁺, the corresponding luminescence intensity with different concentrations of Fe(NO₃)₃ and Stern-Volmer plots for Fe³⁺ are shown in Fig. 6b and Fig. 7b, respectively. They cannot be well fitted by the Stern-Volmer equation, indicating that the dynamic and static quenching processes coexist^[40].

  Figure 6

  

  Figure 6.  Variation of luminescence intensity of complex 1 suspension with different concentrations of Cu(NO₃)₂ (a) and Fe(NO₃)₃(b)

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  Figure 7

  

  Figure 7.  Corresponding Stern-Volmer plots for complex 1 suspension with different concentrations of Cu(NO₃)₂ (a) and Fe(NO₃)₃ (b)

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  Interfering experiments of mixed other metal cations (10 mmol·L^-1) with Cu²⁺ or Fe³⁺ (10 mmol·L^-1) on the emission of complex 1 were studied. Significant quenching is also seen in Fig. 8. The results show that the quenching effect of Cu²⁺ or Fe³⁺ on 1 is not influenced by other metal cations, suggesting that complex 1 exhibits high selectivity for Cu²⁺ and Fe³⁺ sensing.

  Figure 8

  

  Figure 8.  Bar diagram of relative luminescence intensity of complex 1 with mixed metal ions

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  To explain the probing mechanism of complex 1 toward Cu²⁺ and Fe³⁺, the PXRD of Cu²⁺-1 and Fe³⁺-1 and UV-Vis spectra of complex 1, Fe(NO₃)₃ and Cu(NO₃)₂ were studied. The discussions about probable mechanism are given as follows: (1) the result of PXRD shows good agreement and proves that the basic 3D framework of complex 1 still remains (Fig. 3b); (2) it can be seen that the UV-Vis absorption band of a CH₃CN solution of Cu²⁺ (260~390 nm) or Fe³⁺ (240~440 nm) covers the excitation spectrum of complex 1 (330 nm) (Fig. 9), which decreases the essential energy requirement for the "antenna effect". Therefore, the existence of competitive absorption between metal ions (Cu²⁺ and Fe³⁺) and complex 1 might be responsible for the luminescence quenching behavior^[41-43].

  Figure 9

  

  Figure 9.  UV-Vis spectra of complex 1, Fe(NO₃)₃ and Cu(NO₃)₂

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  3.   Conclusions

  In summary, we demonstrates an unusual metal-organic framework [Tb(L)(H₂O)₂]_n upon solvothermal conditions. Each L^3- ligand bridge metal atoms to form a 3D framework with 1D open channels (1.550 73(6) nm ×0.926 97(4) nm). The emission spectra of Cu²⁺-1 and Fe³⁺-1 show almost complete fluorescence quenching. Clearly, complex 1 can be considered as a promising fluorescent probe for Cu²⁺ and Fe³⁺.

  
  
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• 

  Figure 1  (a) Coordination environments of Tb(Ⅲ) ions in complex 1 showing 50% probability displacement ellipsoids; (b) 1D helical chains in complex 1; (c) 2D layer of complex 1 viewed along b axis; (d) 3D network in complex 1 with green spheres representing pores

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  Figure 2  Binding mode of the linker of complex 1

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  Figure 3  (a) TGA curve of complex 1; (b) PXRD patterns of complex 1 and complex 1 after immersing in Cu(NO₃)₂ or Fe(NO₃)₃

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  Figure 4  (a) Excitation (black line) and emission (red line) spectra of complex 1; (b) Emission spectrum of free ligand H₃L (λ_ex=320 nm)

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  Figure 5  (a) Luminescence intensities of complex 1 in the presence of different metal ions in CH₃CN solution when excited at 330 nm; (b) Bar diagram of luminescence intensities at 545 nm of complex 1 in the presence of different metal ions in CH₃CN solution

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  Figure 6  Variation of luminescence intensity of complex 1 suspension with different concentrations of Cu(NO₃)₂ (a) and Fe(NO₃)₃(b)

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  Figure 7  Corresponding Stern-Volmer plots for complex 1 suspension with different concentrations of Cu(NO₃)₂ (a) and Fe(NO₃)₃ (b)

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  Figure 8  Bar diagram of relative luminescence intensity of complex 1 with mixed metal ions

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  Figure 9  UV-Vis spectra of complex 1, Fe(NO₃)₃ and Cu(NO₃)₂

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  Table 1.  Crystal data and structure refinement for complex 1

  Empirical formula	C24H16N3O8S3Tb		μ/mm-1	2.592
  Formula weight	729.5		F(000)	2 864
  Crystal system	Monoclinic		θ range / (°)	4.792~55.174
  Space group	C2/c		Reflection collected	73 495
  a / nm	2.228 23(10)		Unique reflection (Rint)	6 968 (0.028 0)
  b / nm	0.926 97(4)		Data with I > 2σ(I)	6 657
  c / nm	3.066 06(13)		Parameter refined	358
  β / (°)	107.207 0(10)		R1, wR2 [I > 2σ(I)]	0.049 0, 0.151 9
  V / nm3	6.049 5(5)		R1, wR2 (all data)	0.051 2, 0.153 0
  Z	8		Goodness-of-fit (on F2)	1.186
  Dc / (g·cm-3)	1.602

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

  Tb(1)-O(1)	0.226 4(5)	Tb(1)-O(4)#2	0.242 4(5)	Tb(1)-O(6)#1	0.235 2(5)
  Tb(1)-O(2W)	0.244 5(5)	Tb(1)-O(2)#2	0.236 5(5)	Tb(1)-O(3)#2	0.259 3(5)
  Tb(1)-O(5)#3	0.239 0(5)	Tb(1)-O(1W)	0.241 7(5)
  O(1)-Tb(1)-O(6)#1	84.43(17)	O(1W)-Tb(1)-O(4)#2	132.00(16)	O(1)-Tb(1)-O(2)#2	101.65(17)
  O(1)-Tb(1)-O(2W)	73.45(18)	O(6)#1-Tb(1)-O(2)#2	146.44(17)	O(6)#1-Tb(1)-O(2W)	138.68(18)
  O(1)-Tb(1)-O(5)#3	153.95(18)	O(2)#2-Tb(1)-O(2W)	73.53(19)	O(6)#1-Tb(1)-O(5)#3	111.12(16)
  O(5)#3-Tb(1)-O(2W)	81.45(17)	O(2)#2-Tb(1)-O(5)#3	77.26(16)	O(1W)-Tb(1)-O(2W)	70.42(18)
  O(1)-Tb(1)-O(1W)	86.34(18)	O(4)#2-Tb(1)-O(2W)	144.45(18)	O(6)#1-Tb(1)-O(1W)	73.80(17)
  O(1)-Tb(1)-O(3)#2	76.82(16)	O(2)#2-Tb(1)-O(1W)	138.98(18)	O(6)#1-Tb(1)-O(3)#2	79.96(15)
  O(5)#3-Tb(1)-O(1W)	78.65(17)	O(2)#2-Tb(1)-O(3)#2	69.66(16)	O(1)-Tb(1)-O(4)#2	127.01(17)
  O(5)#3-Tb(1)-O(3)#2	125.22(16)	O(6)#1-Tb(1)-O(4)#2	76.37(16)	O(1W)-Tb(1)-O(3)#2	150.03(17)
  O(2)#2-Tb(1)-O(4)#2	73.75(17)	O(4)#2-Tb(1)-O(3)#2	51.60(15)	O(5)#3-Tb(1)-O(4)#2	78.16(17)
  O(2W)-Tb(1)-O(3)#2	125.96(17)
  Symmetry codes: #1: -x+1, y, -z+1/2; #2: -x+3/2, -y+1/2, -z+1; #3: x+1/2, -y+3/2, z+1/2; #4: x-1/2, -y+3/2, z-1/2.

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