English

• 1. INTRODUCTION

  In the past few years, much attention has been given to the design and synthesis of three-dimensional (3D) luminescent metal-organic frameworks (LMOFs) not only due to their fascinating structural characteristics such as high porosity, large surface area, regular porous channel and diverse topological structures^[1-4] but also to their great potential applications in the field of gas storage/sorption^[5], molecular recognition^[6], heterogeneous catalysis^{[7, 8]}, nonlinear optics^[9] and luminescent sensing^[10]. LMOFs sensors have exhibited outstanding characteristics like high selectivity, low detection limits and real-time response in the detection of explosives^[11-13], hazards metal ions^[13-15] and volatile organic compounds^[16-18]. Although great progress has been made, new LMOFs with novel topology and special function are still desirable.

  Ammonia is one of the most important building blocks widely used in the synthesis of chemical fertilizer, pharma-ceutical, et al. However, the heavy use of ammonia would cause severe environmental pollution and great damage to human health due to its volatile and toxic properties^[19]. Thus, the development of efficient method for the sensing of ammonia vapors has attracted considerable interests^{[20, 21]}. Meanwhile, 1-[bis(4-carboxylphenyl)methyl]-1, 3-diazole (H₂bcd) as a typical "Y" shape luminescent ligand offers three divergent coordination sites. From the crystal engineering point of view, H₂bcd could be a promising ligand for MOFs construction. However, MOFs based on H₂bcd and its analogue (4, 4-(1H-1, 2, 4-triazol-1-yl)methylene-bis(benzonic acid)) have been scarcely reported thus far^[22].

  Herein, a three-dimensional (3D) luminescent zinc metal-organic framework (MOF), Zn₄(µ₄-O)(bcd)₃ (complex 1) has been revealed. Complex 1 was synthesized by using H₂bcd and Zn(NO₃)₂·6H₂O under hydrothermal conditions. Crystal structure analysis reveals that complex 1 was crystallized in trigonal system, R$ \stackrel{-}{3} $ space group. Characteristic fluorescent emission was observed for complex 1 in solid-state and remarkable fluorescent enhancement was recorded after exposure to ammonia vapor.

  2. EXPERIMENTAL

  2.1 Reagents and instruments

  The ligand (H₂bcd) and all the reagents were commercially available and used without further purification. Elemental analyses for C, H and N were carried out with a Vario EL elemental analyzer. IR spectra were recorded on a Shimadzu IR Affinity-1S FTIR spectrophotometer using the KBr pellet. The thermogravimetric analyses (TGA) were performed on a Netzsch TG-209 thermogravimetry analyzer in N₂ atmosphere. The powder X-ray diffraction (PXRD) patterns were measured with a Bruker D8 ADVANCE X-ray diffractometer. Photoluminescence spectra were performed on an Agilent Cary Eclipse fluorescence spectrophotometer at room temperature.

  2 2 Structure determination

  The single-crystal data of 1 were collected on a Xcalibur Eos Gemini diffractometer with MoKα radiation (λ = 0.71073 Å) at 173 K. Using Olex2^[23], the structure was solved with the ShelXS structure solution program by direct methods, and refined with the ShelXL refinement package with full-matrix least-squares minimization^[24]. All empirical absorption corrections were applied using the SCALE3 ABSPACK program^[25]. All the Zn(II) atoms were first located. Then carbon, nitrogen and hydrogen atoms of the organic framework were subsequently found. All of the non-hydrogen atoms were located from the initial solution and refined anisotropically. The selected bond lengths and bond angles are summarized in Table 1. The crystal data and structure refinements for complex 1 are listed in Table 1S.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for Complex 1

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(2)	1.932(3)		Zn(1)–O(5)	1.949(5)		Zn(2)–O(3)	2.090(3)
  Zn(2)–O(1)	2.252(3)		Zn(2)–N(1)	2.052(4)		Zn(2)–O(4)	2.041(3)
  Zn(2)–O(5)	1.9597(11)		Zn(1)–O(2)#2	1.932(3)		Zn(1)–O(2)#1	1.932(3)
  Zn(2)#1–O(5)	1.997(11)		Zn(2)#2–O(5)	1.997(11)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(2)–Zn(1)–O(5)	112.23(9)		O(2)#2–Zn(1)–O(2)	106.58(10)		O(5)–Zn(2)–O(3)	101.53(10)
  O(5)–Zn(2)–N(1)	147.68(16)		O(5)–Zn(2)–O(4)	112.23(14)		O(5)–Zn(2)–O(1)	88.07(9)
  O(3)–Zn(2)–O(1)	169.95(12)		N(1)–Zn(2)–O(3)	88.11(13)		N(1)–Zn(2)–O(1)	82.12(13)
  O(4)–Zn(2)–O(3)	91.61(12)		O(4)–Zn(2)–N(1)	98.08(13)		O(4)–Zn(2)–O(1)	87.45(12)
  O(2)#1–Zn(1)–O(5)	112.23(9)		O(2)#2–Zn(1)–O(5)	112.23(9)		O(2)#1–Zn(1)–O(2)#1	106.58(10)
  Zn(1)–O(5)–Zn(2)#2	102.71(13)		O(2)#1–Zn(1)–O(2)	106.58(10)
  Symmetry codes for 1: #1: –y, x – y, z; #2: y – x, –x, z; #3: –1/3 – x, –2/3 – y, 4/3 – z; #4: –2/3 – x, –1/3 – y, 5/3 – z

  2 3 Synthesis of Zn₄(µ₄-O)(bcd)₃ (1)

  Zn(NO₃)₂·6H₂O (0.018 g, 0.06 mmol), H₂bcd (0.006 g, 0.02 mmol), H₂O (10 mL) and NaOH solution (0.5 mol/L, 5 drops) were added to a 25 mL Teflon-lined stainless-steel vessel. The mixture was reacted at 160 ℃ for 5 days to yield colourless block crystals (complex 1, 72% yield based on Zn). Anal. Calcd (%) for C₅₄H₃₆N₆O₁₃Zn₄ (1): C, 52.37; H, 2.93; N, 6.78. Found (%): C, 52.01; H, 3.32; N, 6.54. IR (KBr, cm⁻¹): 3425(w, O-H), 1673(s, C=O), 1607(s, C=N), 1560(s, C=C), 1545(m, C=C), 1520(C=C), 1504(w), 1417(m), 1399(m), 1336(w), 1281(w), 1125(w), 1016(w), 989(w), 882(w, C-H), 837(w, C-H), 821(w, C-H), 770(w), 753(w), 715(w), 670(w), 648(w).

  3. RESULTS AND DISCUSSION

  3 1 Crystal structure description of complex 1

  Single-crystal X-ray analysis revealed that complex 1 crystallizes in trigonal space group R$ \stackrel{-}{3} $. The asymmetric unit contains four Zn(II) ions, three bcd^2- ligands and one μ₄-O atom (Fig. 1a). As shown in Fig. 1a, the Zn(1) center shows a tetrahedral coordination with three oxygen atoms from three COO^- groups of three independent ligands and one μ₄-O(5) atom, and Zn(2) center shows a distorted octahedral coordination sphere with four O atoms from three COO^- groups of three independent ligands together with one μ₄-O(5) and one N atoms of the imidazole moiety of another ligand. μ₄-O(5) bridged four Zn (Zn(1), Zn(2), Zn(2#1), Zn(2#2)) to form a cluster. The cluster is edge-bridged by six carboxylate groups from six bcd^2- ligands, three N atoms from three bcd^2- ligands and one O atom to provide the Zn₄(O)(COO)₆N₃ secondary building unit (SBU) (Fig. 1b). The distance between Zn(1)···Zn(2) and Zn(2)···Zn(2#1) ions were found to be 3.053 and 3.311 Å, respectively. In the framework, each secondary building unit is coordinated with nine bcd^2- ligands (Fig. 1c), and each bcd^2- ligand coordinated with three Zn₄(O)(COO)₆N₃ units (Fig. 1d). The SBU structure in complex 1 is similar to that in MOF-5 with the formula [Zn₄O(R-CO₂)₆] where each Zn atom is tetrahedrally surrounded by six carboxylates^[26]. As a result, the coor-dination model of Zn(II) ion and ligand gave rise to a 3D porous framework (Fig. 1e). To better understand the nature of the framework, a topological approach has been applied. Nodes and connection nets were simply represented by the reduction of multidimensional structures. Zn₄(O)(COO)₆N₃ SBU can be regarded as 9-connected nodes, and all crystallographically independent bcd^2- ligands act as 3-connected linkers (Fig. 1f). Therefore, the whole structure can be represented as a 3, 9-connected network with {4²; 6}³-{4⁶; 6²¹; 8⁹} topology. The dimensionality and topology of the network presented in this work are entirely different from the reported two-fold interpenetrated structures (Zn(HIMB)₂ and [Zn(IMB)]·1.5H₂O) due to the change of the linking modes of H₂bcd and the formation of Zn₄(O)-(COO)₆N₃ units^[22a].

  Figure 1

  

  Figure 1.  (a) Coordination environments of Zn(II) ion in complex 1 (Hydrogen atoms are omitted for clarity; Symmetry codes 1: #1: –y, x – y, z; #2: y – x, –x, z; #3: –1/3 – x, –2/3 – y, 4/3 – z; #4: –2/3 – x, –1/3 – y, 5/3 – z); (b) Zn₄(O)(COO)₆N₃ unit; (c) Zn₄(O)(COO)₆N₃ unit coordinated with nine bcd^2- ligands; (d) bcd^2- ligand coordinated with three Zn₄(O)(COO)₆N₃ units; (e) View of the 3D structure constructed from bcd^2- and Zn²⁺ along the ab plane; (f) Topological views showing the 3D framework of 1 (The pink spheres represent Zn nodes and green spheres and lines show the bcd^2- nodes)

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  3 2 PXRD and TG analyses

  3   2. 1 PXRD analyses

  The purity of the bulky crystalline samples of complex 1 was confirmed by PXRD at room temperature. As shown in Fig. 2, the PXRD pattern of the as-synthesized complex 1 matched well with the simulated one based on the single-crystal diffraction data, which confirms the good purity of 1.

  Figure 2

  

  Figure 2.  PXRD pattern of complex 1

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  3   2. 2 TG analysis

  The stability of 1 was evaluated by the thermogravimetric analysis. The experiment was performed on bulky crystalline samples of complex 1 under N₂ atmosphere at a heating rate of 10 ℃/min. As depicted in Fig. 3, the framework is stable up to 411 ℃ and starts to collapse above this temperature. The sharp decrease in weight upon heating occurs between 411 and 500 ℃, which could be attributed to the loss of part coordinated bcd^2- ligands. A slow weight loss can be observed from 500 to 800 ℃. The weight of the final residue (32.83%) is assumed to be a small amount of bcd^2- ligand (6.75%) and ZnO (26.08%). The thermogravimetric analysis result shows that complex 1 has good thermal stability, which is the foundation for further applications^[27].

  Figure 3

  

  Figure 3.  TG curve of complex 1

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  3 3 Solid-state photoluminescent spectra

  The solid-state fluorescence spectra of complex 1 and the free ligand H₂bcd were recorded on an Agilent Cary Eclipse fluorescence spectrophotometer excited at 300 nm (The excitation wavelength was chosen according to the excita-tion spectra of complex 1) at room temperature (slits: 5 nm/10 nm). As shown in Fig. 4, H₂bcd displays a broad band emission range from 320 to 510 nm, which could be assigned to the intra-ligand n-π* or π-π* transitions^[28]. However, compared with the emission band of the free ligand, a remarkable red-shift was observed for complex 1 (from 360 to 425 nm), which could be attributed to the coordination effect^[29]. And the remarkable fluorescent emission of complex 1 could be ascribed to the ligand-to-ligand charge transfer. In addition, the fluorescence decay assay for complex 1 was carried out on a HORIBA Fluorolog®-3 fluorescence spectrophotometer and the average life time of complex 1 was found to be 3.82 ns.

  Figure 4

  

  Figure 4.  (a) Solid-state photoluminescent spectra of complex 1 and H₂bcd at room temperature (λ_ex = 300 nm); (b) Fluorescence decay assay of complex 1 (λ_ex = 290 nm)

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  3 4 Fluorescence enhanced sensing of ammonia vapor

  In order to reveal the potential application of complex 1 in the detection of organic vapors, test strips were prepared by simply deposited complex 1 on filter papers. The as-prepared paper-based sensor was covered on the bottle of benzene, m-xylene, ethylbenzene, ethanol (EtOH), acetonitrile, ammonia (NH₃, 20 wt%), ethyl acetate (EtOAc), petrol (PE), tetrahy-drofuran (THF), acetone, toluene, boron trifluoride etherate (BF₃·Et₂O), diethyl ether (Et₂O) and acetic acid. The fluorescent intensities of the sensor before and after exposure to various organic vapors were recorded and displayed in Fig. 4.

  As shown in Fig. 5, no obvious intensity change was observed for the paper-based sensor after exposure to neutral organic vapor such as benzene, m-xylene, ethylbenzene, ethanol (EtOH), acetonitrile, ethyl acetate (EtOAc), petrol (PE), tetrahydrofuran (THF), acetone, toluene and die-thyl ether (Et₂O). However, remarkable fluorescent enhan-cement was observed after exposure to ammonia vapor (F/F₀ = 1.66, F₀ and F are the fluorescent intensities of complex 1 coated test strips before and after exposure to organic vapor, respectively). Interestingly, slightly fluorescent decrease was observed for this paper-based sensor after exposure to acidic vapors such as acetic acid and BF₃·Et₂O.

  Figure 5

  

  Figure 5.  Fluorescent intensity changes of complex 1 coated test strips before and after exposure to various vapors in air (F₀ and F are the fluorescent intensities of complex 1 coated test strips in the absence and presence of organic vapors, respectively)

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  4. CONCLUSION

  A novel 3D LMOF Zn₄(µ₄-O)(bcd)₃ (complex 1) was successfully designed and synthesized by using 1-[bis(4-carboxylphenyl)methyl]-1, 3-diazole (H₂bcd) and Zn(NO₃)₂· 6H₂O under hydrothermal conditions. Structural analyses reveal that complex 1 crystallizes in trigonal system, R$ \stackrel{-}{3} $ space group. Every asymmetric unit of complex 1 contains four Zn(II) ions, three bcd^2- ligands and one μ₄-O atom. The whole structure can be represented as a 3, 9-connected network with {4²; 6}³{4⁶; 6²¹; 8⁹} topology. Remarkable fluorescent emission was observed for complex 1 centered at 425 nm upon excitation at 300 nm, which could be ascribed to the ligand-to-ligand charge transfer. In addition, obvious fluorescent enhancement was observed for complex 1 after exposure to ammonia vapor, which might be used in the selective and qualitative sensing of ammonia vapor in air.

  
  
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• 

  Figure 1  (a) Coordination environments of Zn(II) ion in complex 1 (Hydrogen atoms are omitted for clarity; Symmetry codes 1: #1: –y, x – y, z; #2: y – x, –x, z; #3: –1/3 – x, –2/3 – y, 4/3 – z; #4: –2/3 – x, –1/3 – y, 5/3 – z); (b) Zn₄(O)(COO)₆N₃ unit; (c) Zn₄(O)(COO)₆N₃ unit coordinated with nine bcd^2- ligands; (d) bcd^2- ligand coordinated with three Zn₄(O)(COO)₆N₃ units; (e) View of the 3D structure constructed from bcd^2- and Zn²⁺ along the ab plane; (f) Topological views showing the 3D framework of 1 (The pink spheres represent Zn nodes and green spheres and lines show the bcd^2- nodes)

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  Figure 2  PXRD pattern of complex 1

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  Figure 3  TG curve of complex 1

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  Figure 4  (a) Solid-state photoluminescent spectra of complex 1 and H₂bcd at room temperature (λ_ex = 300 nm); (b) Fluorescence decay assay of complex 1 (λ_ex = 290 nm)

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  Figure 5  Fluorescent intensity changes of complex 1 coated test strips before and after exposure to various vapors in air (F₀ and F are the fluorescent intensities of complex 1 coated test strips in the absence and presence of organic vapors, respectively)

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for Complex 1

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(2)	1.932(3)		Zn(1)–O(5)	1.949(5)		Zn(2)–O(3)	2.090(3)
  Zn(2)–O(1)	2.252(3)		Zn(2)–N(1)	2.052(4)		Zn(2)–O(4)	2.041(3)
  Zn(2)–O(5)	1.9597(11)		Zn(1)–O(2)#2	1.932(3)		Zn(1)–O(2)#1	1.932(3)
  Zn(2)#1–O(5)	1.997(11)		Zn(2)#2–O(5)	1.997(11)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(2)–Zn(1)–O(5)	112.23(9)		O(2)#2–Zn(1)–O(2)	106.58(10)		O(5)–Zn(2)–O(3)	101.53(10)
  O(5)–Zn(2)–N(1)	147.68(16)		O(5)–Zn(2)–O(4)	112.23(14)		O(5)–Zn(2)–O(1)	88.07(9)
  O(3)–Zn(2)–O(1)	169.95(12)		N(1)–Zn(2)–O(3)	88.11(13)		N(1)–Zn(2)–O(1)	82.12(13)
  O(4)–Zn(2)–O(3)	91.61(12)		O(4)–Zn(2)–N(1)	98.08(13)		O(4)–Zn(2)–O(1)	87.45(12)
  O(2)#1–Zn(1)–O(5)	112.23(9)		O(2)#2–Zn(1)–O(5)	112.23(9)		O(2)#1–Zn(1)–O(2)#1	106.58(10)
  Zn(1)–O(5)–Zn(2)#2	102.71(13)		O(2)#1–Zn(1)–O(2)	106.58(10)
  Symmetry codes for 1: #1: –y, x – y, z; #2: y – x, –x, z; #3: –1/3 – x, –2/3 – y, 4/3 – z; #4: –2/3 – x, –1/3 – y, 5/3 – z

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