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• With growing concerns over the major ecosystem and health risk, the detection of harmful volatile organic compounds(VOCs) has attracted tremendous attention over the decades. Acting as a typically toxic molecule of VOCs, carbon disulfide(CS₂) can trigger severe multiorgan diseases when people are continuously exposed to it even in extremely low concentrations^[1-2]. Currently, well-trained canines, sophisticated analytical instruments, chemiluminescence-based methods and nanoprobes are used for CS₂ sensing^[3-6]. However, the above detection methods have different degrees of weakness, such as low sensitivity, high costs and high complexity. Hence, it is a significant but challenging task to explore new sensing materials for rapid and selective detection of the CS₂.

  Fluorescent metal-organic-frameworks(MOFs) have received much attention due to their potential application in sensitive and selective detection of hazardous substances^[7-11]. As a new type of chemosensor, the fluorescent detection based on MOFs can be conveniently monitored by using changes in fluorescent properties caused by host-guest interactions as interpretable signals^[12]. Magnesium as a candidate metal ion for the construction of MOFs has caused the climax of researches over the past few years because of its low-cost, nontoxicity and especially the unique 3d⁰ electron configuration favoring ligand-centered emission^[13-15]. Some Mg-MOFs have been explored as fluorescence sensors for typical VOCs and water, such as [Mg₂(BINDI)₂(DMF)₂] H₂O(H₄BINDI=N, N′-bis(5-isophthalic acid) naphthalenediimide)^[14], [NH₂(CH₃)₂][Mg₃(NDC)_2.5(HCO₂)₂(DMF)_0.75(H₂O)_0.25]·1.25DMF·0.75H₂O(H₂NDC=1, 4-naphthalene dicarboxylic acid)^[16], Mg₅(OH)₂(BTEC)₂(H₂O)₄·11H₂O(H₄BTEC=1, 2, 4, 5-benzenetetracarboxylic acid)^[17] and [Mg(H₂dhtp)(H₂O)₂]·DMA(H₄dhtp=2, 5-dihydroxy-terepthalic acid)^[18]. Herein, we present the solvothermal synthesis, crystal structure and characterizations of a new fluorescent Mg-MOF named as [Mg₄(1, 4-NDC)₄(DMA)₂(CH₃OH)₂(H₂O)₂]·DMA·CH₃OH(1). Fluorescence measurements demonstrated that the title compound displayed a purple light emission(λ_em=385 nm) and further fluorescent study indicated that compound 1 exhibited a selective and sensitive sensing property for CS₂ with a low concentration.

  1   Experimental

  1.1   Reagent and Instrument

  All reagents and chemicals were purchased from commercial sources and used without further purification. (Mg(NO₃)₂·6H₂O(≥99%, Tianjin BoDi Chemical Co., Ltd.); 1, 4-H₂NDC(≥95%, Beijing HWRK Chem Co., Ltd.); DMA(≥99%, Shanghai Titan Chemical Co., Ltd.); methanol anhydrous(≥99.5%, Shanghai Titan Chemical Co., Ltd.). Powder X-ray diffraction(PXRD) patterns were recorded on a Rigaku Miniflex Ⅱ diffractometer using CuKα radiation(λ=0.154178 nm). Elemental analyses(EA) of C, N, H were performed on a German Elementary Vario Ⅲ instrument. Thermogravimetric analysis(TGA) was carried out with a NETZACH STA 449F3 unit at a heating rate of 10 ℃/min under a nitrogen atmosphere. Emission and excitation spectra of compound 1 in the solid state and at a suspension system were recorded on a Perkin-Elmer LS55 luminescence spectrometer at room temperature.

  1.2   Synthesis of compound 1

  A mixture of 1 mmol Mg(NO₃)₂·6H₂O(0.256 g) and 1 mmol 1, 4-H₂NDC(0.216 g) in 4 mL DMA(N, N-dimethylacetamide) and 1 mL anhydrous methanol was sealed into a stainless steel reactor with a 20 mL teflon-lined bomb. The mixture was heated at 130 ℃ for 3 days and then was slowly cooled to room temperature. The colorless block-shaped crystals were obtained after being filtered and air-dried. Yield, 84.3%(0.284 g) based on Mg. Anal.(calc.) for compound 1:C 55.90%(56.16%), H 4.82%(5.01%), N 3.08%(3.12%).

  1.3   Determination of crystal structure

  A suitable single crystal of compound 1 was carefully selected under an optical microscope and glued to a thin glass fiber. The intensity data were collected on a SuperNova CCD diffractometer with MoKα radiation(λ=0.071073 nm) at 100(2) K. The structure was solved by direct methods and refined by full-matrix least-squares on F² using the SHELX-2016 program package^[19]. All the non-hydrogen atoms were refined anisotropically, and the hydrogen atoms bonded to carbon were located by geometrical calculations, while those for O atoms were located from difference-Fourier maps and refined with restrained O-H distances. The empirical formula of compound 1 was further confirmed by the EA and TGA results. The crystallographic data and details of structural refinements for compound 1 are listed in Table 1.

  Table 1.  Crystallographic data and structural refinement details for compound 1

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  Empirical formula	C63H67Mg4N3O24
  Formula mass	1 347.431 00(2)
  T/K	100(2)
  Crystal system	Monoclinic
  Space group	P21/c
  a/nm	2.060 90(12)
  b/nm	2.210 14(13)
  c/nm	1.503 85(10)
  α/(°)	90
  β/(°)	111.399(3)
  γ/(°)	90
  V/nm3	6.377 6(7)
  Z	4
  Dcalc./(g·cm-3)	1.403
  λ/nm	0.071 073
  μ/mm-1	0.142
  F(000)	2 824
  Reflections measured	28 773
  Independent reflections	12 199
  No. of parameters	878
  GOF on F2	1.033
  R1[I > 2σ(I)]a	0.059 6
  wR2[I > 2σ(I)]a	0.122 5
  R1[all data]	0.089 9
  wR2[all data]	0.146 1
  CCDC	1552691
  a.R1=Σ‖Fo|-|Fc‖/Σ|Fo|, wR2={Σw[(Fo)2-(Fc)2]2/Σw[(Fo)2]2}1/2.

  1.4   Fluorescence detection measurements

  The fluorescence properties of compound 1 and the 1, 4-H₂NDC ligand were investigated in the solid state at room temperature and the fluorescent spectra were recorded. Then 2 mg of compound 1(the as-made crystalline sample of compound 1 was loaded into an agate mortar and was manually ground with the pestle to afford a fine powder) was dispersed in 2 mL of CS₂, CH₃OH, acetone, DMA and acetonitrile, respectively. After ultrasonic treatment for a few seconds, the suspension was placed in a quartz cell of 1 cm width for fluorescence detection. Detailed detections were carried out by gradually adding the CS₂ as quenchers in an incremental fashion with a pipette. The corresponding fluorescent spectra were recorded at 298 K. For all the measurements, the dispersed suspensions of compound 1 were excited at λ_ex=345 nm(λ_em=385 nm) and the corresponding emission wavelengths were monitored from 360 nm to 650 nm.

  2   Results and discussion

  2.1   Crystal structure descriptions

  Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the monoclinic space group P2₁/c. The crystallographic asymmetric unit contains one formula unit. All the Mg²⁺ ions are six-coordinated except that there are some differences in the coordinated modes. The Mg(1) and Mg(3) atoms have similar coordination modes that are coordinated by five carboxylic O atoms from four 1, 4-NDC^2- ligands(one carboxylic group adopts a chelating coordination mode) and one O atom from a water molecule(Fig. 1A); while the Mg(2) and Mg(4) are both coordinated by four carboxylic O atoms from four 1, 4-NDC^2- ligands in a monodentate way and one O atom from a methanol molecule and one O atom from a DMA molecule(Fig. 1A). The Mg—O bond lengths range from 0.1991(3) nm to 0.2206(3) nm, which are comparable to those in the reported magnesium-carboxylate compounds^[20-23]. As shown in Fig. 1B, the 1, 4-NDC ligands(L1, L2, L3, L4) adopt different coordination modes which can be depicted as (k¹-k¹-μ₂)-(k¹-k¹-μ₂)-μ₄(L1 and L3) and (k¹-k¹-μ₂)-(k¹-k²-μ₂)-μ₄(L2 and L4), respectively.

  图1 The coordination environments of Mg atoms(A), coordination modes of the four crystallographically independent 1, 4-NDC^2- ligands(B), and the 1D chains in compound 1 extended along the c axis(C, D) Figure1. The coordination environments of Mg atoms(A), coordination modes of the four crystallographically independent 1, 4-NDC^2- ligands(B), and the 1D chains in compound 1 extended along the c axis(C, D)

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  As shown in Fig. 1C and Fig. 1D, there exist corner-shared coordination polyhedra of dinuclear [Mg1Mg2] and [Mg3Mg4] units in the structure which can be viewed as the secondary building units(SBU) for compound 1. Then, the adjacent [Mg1Mg2] and [Mg3Mg4] units are, respectively, interconnected by the COO^- groups of ligands L1 and L3 to form one-dimensional(1D) infinite chains of [—Mg1—Mg2—Mg1—Mg2—] and [—Mg3—Mg4—Mg3—Mg4—] along the c-axis. Further, each [—Mg1—Mg2—Mg1—Mg2—] chain connects to four adjacent [—Mg3—Mg4—Mg3—Mg4—] chains by the ligands L3 and L4 to form a 3D skeleton. The coordinated and free DMA, methanol, water molecules settle in the cages of the 3D framework(Fig. 2A). The solvent accessible volume is 43.6% if all the solvent molecules are removed according to the calculation performed by PLATON analysis. Topologically, when regarding the 1, 4-NDC^2- ligands and each dinuclear secondary building unit as 3-connected and 5-connected nodes respectively, the structure of compound 1 could be simplified as a typical fsc-3, 5-C2/c topology, Fig. 2B.

  图2 View of the 3D framework of compound 1 along the c axis showing the cages in which the DMA and methanol molecules are filled(A) and the topology of compound 1(B) Figure2. View of the 3D framework of compound 1 along the c axis showing the cages in which the DMA and methanol molecules are filled(A) and the topology of compound 1(B)

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  2.2   Thermal stability

  The phase purity of compound 1 was confirmed by PXRD(see Fig.S1 in Supporting Information) carried out with the polycrystalline sample of compound 1. Thermogravimetric analysis of compound 1 was performed under a N₂ atmosphere from 25 to 800 ℃ with a heating rate of 10 ℃ /min on pure powdered sample. The thermogravimetric curve for compound 1 is shown in Fig. 3. The 9.01% mass loss from room temperature(RT) to ~180 ℃ should be attributed to the loss of the free guests(calcd. 8.84%); the 19.72% mass loss of compound 1 from 180 to 500 ℃ corresponds to the departure of the coordinated water, CH₃OH and DMA molecules in the structure(calcd. 20.36%). The mass of the samples remained nearly constant from 600 to 800 ℃ and the characterization of PXRD suggested that the residual powder was MgO(see Fig.S2, in Supporting Information).

  图3 TG curve for compound 1 Figure3. TG curve for compound 1

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  2.3   Fluorescence detection properties

  As shown in Fig. 4, the fluorescent spectra of compound 1 in the solid state exhibited a purple emission band with an intense peak maximum at 385 nm upon excitation at 345 nm at room temperature. Compared with the emission band of the free 1, 4-H₂NDC ligand(λ_em=485 nm monitored at λ_ex=390 nm), compound 1 showed blue-shift emission. The title compound should exhibit a ligand-centered emission due to the unique electron configuration of Mg²⁺. Further fluorescence measurement for compound 1 has been carried out to identify whether compound 1 has a luminescent response to volatile organic molecules. As shown in Fig. 5A, compound 1 was dispersed in five kinds of typically used solvents and it was interesting to see that the fluorescence intensities of compound 1 were heavily dependent on the identity of the organic solvent molecules.

  图4 Solid state fluorescence spectra of compound 1 and 1, 4-H₂NDC Figure4. Solid state fluorescence spectra of compound 1 and 1, 4-H₂NDC

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  图5 Emission spectra of compound 1 dispersed in different solvents(A) and emission spectra of compound 1 dispersed in the acetone with various contents of CS₂(inset is the SV plot for the quenching of compound 1 by CS₂)(B) Figure5. Emission spectra of compound 1 dispersed in different solvents(A) and emission spectra of compound 1 dispersed in the acetone with various contents of CS₂(inset is the SV plot for the quenching of compound 1 by CS₂)(B)

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  Notably, we found that compound 1 exhibited a significant quenching of fluorescence when dispersed in CS₂. Since compound 1 demonstrated the strongest fluorescence emission in acetone, the acetone was chosen as the dispersed solvent to find the potential fluorescence detection of compound 1 for CS₂. The sensing sensitivity towards CS₂ was examined in detail through gradually increasing CS₂ contents into the emulsions of compound 1 dispersed in acetone to monitor the emissive response. As depicted in Fig. 5B, the fluorescent intensity of compound 1 was almost completely quenched when only 80 μL CS₂(0.4%(volume fraction)) was added, indicating that compound 1 was a benign candidate for selective sensing of CS₂. Stern-Volmer equation(SV plot):I₀/I=1+K_sv×[M] was applied to judge the quenching effect(I₀ and I are the suspension luminescence intensity of compound 1 without and with addition of quencher, and [M] is the molarity of quencher and K_sv is the quenching constant)^[24]. As shown in Fig. 5B, the SV plot displays a good linear behavior and the K_sv constant calculated from the experimental data is 1.45×10³ L/mol. Compared to the former reports detecting CS₂ based on the Mg-MOF luminescence intensity, the quenching concentration of CS₂ for the title compound is slightly higher^[17]. This could be attributed to the larger channels in the former reported Mg-MOFs, which facilitate the interactions between CS₂ and MOFs^[16-17]. Further investigation is still required for exploring the mechanism of fluorescence quenching in compound 1.

  3   Conclusions

  In summary, a novel 3D Mg-MOF, namely [Mg₄(1, 4-NDC)₄(DMA)₂(CH₃OH)₂(H₂O)₂]·DMA·CH₃OH has been synthesized under solvothermal conditions and characterized. Fluorescence study indicates that the title compound shows a highly sensitive fluorescent response for CS₂ with a low concentration. Future work will continue to study the construction of fluorescent Mg-MOFs, explore their sensing properties towards the harmful volatile organic compounds and aim at a deep understanding of the relationship of structure and property.

  Supporting Information [PXRD patterns] is available free of charge on the website of Chinese Journal of Applied Chemistry(http://yyhx.ciac.jl.cn/).

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• 

  Figure 1  The coordination environments of Mg atoms(A), coordination modes of the four crystallographically independent 1, 4-NDC^2- ligands(B), and the 1D chains in compound 1 extended along the c axis(C, D)

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  Figure 2  View of the 3D framework of compound 1 along the c axis showing the cages in which the DMA and methanol molecules are filled(A) and the topology of compound 1(B)

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  Figure 3  TG curve for compound 1

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  Figure 4  Solid state fluorescence spectra of compound 1 and 1, 4-H₂NDC

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  Figure 5  Emission spectra of compound 1 dispersed in different solvents(A) and emission spectra of compound 1 dispersed in the acetone with various contents of CS₂(inset is the SV plot for the quenching of compound 1 by CS₂)(B)

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  Table 1.  Crystallographic data and structural refinement details for compound 1

  Empirical formula	C63H67Mg4N3O24
  Formula mass	1 347.431 00(2)
  T/K	100(2)
  Crystal system	Monoclinic
  Space group	P21/c
  a/nm	2.060 90(12)
  b/nm	2.210 14(13)
  c/nm	1.503 85(10)
  α/(°)	90
  β/(°)	111.399(3)
  γ/(°)	90
  V/nm3	6.377 6(7)
  Z	4
  Dcalc./(g·cm-3)	1.403
  λ/nm	0.071 073
  μ/mm-1	0.142
  F(000)	2 824
  Reflections measured	28 773
  Independent reflections	12 199
  No. of parameters	878
  GOF on F2	1.033
  R1[I > 2σ(I)]a	0.059 6
  wR2[I > 2σ(I)]a	0.122 5
  R1[all data]	0.089 9
  wR2[all data]	0.146 1
  CCDC	1552691
  a.R1=Σ‖Fo|-|Fc‖/Σ|Fo|, wR2={Σw[(Fo)2-(Fc)2]2/Σw[(Fo)2]2}1/2.

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