English

• 0.   Introduction

  Over the last couple of decades, metal-organic frameworks (MOFs) have drawn attention due to their fascinating structures, intriguing topologies, and potential application in gas capture and separation, magnetism, molecular recognition, catalysis, drug delivery, and luminescence^[1-8]. The applicability of MOFs owes to the high surface area, excellent thermal stability, abundant luminescent sites, high selectivity, and so on. Nowadays, due to the quick development of science and technology, the effective detection of pollutants from wastewater has drawn great interest owing to its importance in homeland security and human health^[9-10]. Nitroaromatic compounds (NACs) are widely used as explosives materials and chemical intermediates. Among the NACs commonly used, 2, 4, 6-trinitrophenol (TNP) has the highest explosive power and was identified as a toxic pollutant. Its discharge into the environment leads to the contamination of soil and water, resulting in health problems for humans and wildlife^[11-12]. Moreover, Fe³⁺ ions play key roles in various biochemical processes for humans and other living organisms. Its overload or deficiency can result in insomnia, skin ailments, and even cancer^[13]. In recent years, MOF-based probes have been synthesized to detect heavy metal ions, small organic molecules, and nitro explosives in wastewater owing to their advantages, such as quick response, efficient selectivity, high sensitivity, and real-time monitoring^[14]. Zinc-based MOFs (Zn-MOFs) are one of the most important components in luminescent MOFs^[15-16].

  Herein, three new Zn-MOFs, namely {[Zn₂(BIDPS)₂ (OBA)₂]·DMA}_n (1), {[Zn(BIDPT)(PA)]·DMF}_n (2), and {[Zn(BIDPS) (PA) (H₂O) ₂] ·2H₂O}_n (3) (BIDPS=4, 4'-bis (imidazol-l-yl)-phenyl sulphone, H₂OBA=4, 4'-oxybisbenzoic acid, H₂PA=pamoic acid, BIDPT=4, 4'-bis (imidazol-l-yl)diphenyl thioether), have been successfully obtained under solvothermal condition. Herein, we described their synthesis, structures, and chemical stabilities. It was concluded that compounds 1-3 had fluorescence sensing ability for TNP and Fe³⁺ in an aqueous solution.

  1.   Experimental

  1.1   Materials and measurement

  All the chemicals except the BIDPS and BIDPT ligands were commercially available and used directly without further purification. The ligands BIDPS and BIDPT were synthesized according to the literature method^[17]. FT-IR (KBr pellet) absorption spectra were recorded on a Nicolet (Impact 410) spectrometer in a range of 400-4 000 cm^-1. Elemental analyses (C, H, N) were performed with a Perkin-Elmer model 240C elemental analyzer. Thermogravimetric analyses (TGA) were performed by using a Perkin-Elmer thermal analyzer. Powder X-ray diffraction (PXRD) patterns were conducted on a Bruker D8 Advance X-ray instrument with Cu Kα radiation (λ=0.154 06 nm, U=40 kV, I=40 mA, 2θ =5°-50°) at room temperature. Fluorescence spectra were collected on a SHIMAZU VF-320 fluorescence spectrophotometer.

  1.2   Synthesis of compound 1

  A mixture of Zn(NO₃)₂·6H₂O (1 mmol), H₂OBA (0.1 mmol), and BIDPS (0.1 mmol) was dissolved in 4 mL of DMA/CH₃OH/H₂O (1∶1∶2, V/V). The final mixture was sealed in a Teflon-lined stainless autoclave (15 mL) and heated at 95 ℃ for 72 h. Then the autoclave was cooled to room temperature at a descending rate of 5 ℃·h^-1. Some colorless crystals were obtained and washed with ethanol, then dried in air (Yield: 36% based on Zn). Anal. Calcd. for C₆₈H₅₃Zn₂N₉O₁₅S₂(%): C, 57.07; H, 3.73; N, 8.81. Found(%): C, 57.10; H, 3.68; N, 8.78. FT-IR (KBr, cm^-1): 3 422(m), 3 169(s), 1 646 (m), 1 592(m), 1 407(s), 1 288(s), 1 218(w), 1 179(w), 890(m), 856(w), 687(w), 554(w) (Fig. S1, Supporting information).

  1.3   Synthesis of compound 2

  A mixture of Zn(NO₃)₂·6H₂O (1 mmol), H₂PA (0.1 mmol), and BIDPT (0.1 mmol) was dissolved in 6 mL of DMF/CH₃CN/H₂O (2∶2∶2, V/V). The final mixture was sealed in a Parr Teflon-lined stainless autoclave (15 mL) and heated at 120 ℃ for 72 h. Then the autoclave was cooled to room temperature at a descending rate of 5 ℃·h^-1. Some brown block crystals were obtained and washed with ethanol, then dried in air (Yield: 42% based on Zn). Anal. Calcd. for C₄₄H₃₅ZnN₅O₇S(%): C, 62.67; H, 4.18; N, 8.31. Found(%): C, 62.75; H, 4.15; N, 8.28. FT-IR (KBr, cm^-1): 3 410(m), 3 178(m), 1 647 (m), 1 596(s), 1 409(s), 1 286(s), 1 234(w), 1 182(m), 860(m), 816(m), 752(m), 589(w), 558(w) (Fig.S2).

  1.4   Synthesis of compound 3

  A mixture of Zn(NO₃)₂·6H₂O (1 mmol), H₂PA (0.1 mmol), and BIDPS (0.1 mmol) was dissolved in 7 mL of DMF/CH₃CN/H₂O (2∶3∶2, V/V). The final mixture was sealed in a Parr Teflon-lined stainless autoclave (15 mL) and heated at 85 ℃ for 72 h. Then the autoclave was cooled to room temperature at a descending rate of 5 ℃·h^-1. Some brown block crystals were obtained and washed with ethanol, then dried in air (Yield: 48% based on Zn). Anal. Calcd. for C₄₁H₃₆ZnN₄O₁₂S(%): C, 56.33; H, 4.15; N, 6.41. Found(%): C, 56.31; H, 4.12; N, 6.39. FT-IR (KBr, cm^-1): 3 434(vs), 1 590(s), 1 508(s), 1 480(s), 1 412(s), 1 362(s), 1 186(m), 1 108(w), 832 (m), 815(m), 762(m), 722(m), 582(w), 536(w) (Fig.S3).

  1.5   Crystal structure determination

  The diffraction data for compounds 1-3 were collected on a Bruker SMART CCD diffractometer using Mo Kα radiation (λ=0.071 073 nm). The structure solutions and refinements of 1-3 were achieved by direct methods and refined by full-matrix least-square techniques on F² using the SHELXL-2014 program^[18-19]. All non-hydrogen atoms were refined with anisotropic thermal parameters and all the hydrogen atoms were placed at their ideal positions. Crystallographic data and structure refinements for 1-3 are summarized in Table 1, and selected bond distances and bond angles are listed in Table 2. The relevant data for hydrogen bonds are given in Table 3.

  Table 1

  

  Table 1.  Crystal data and structure refinement parameters for compounds 1-3

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  Compound	1	2	3
  Formula	C68H53Zn2N9O15S2	C44H35ZnN5O7S	C41H36ZnN4O12S
  Formula weight	1 431.05	843.20	874.17
  T/K	100.0	293(2)	165.0
  Crystal system	Monoclinic	Orthorhombic	Monoclinic
  Space group	C2/c	P212121	P21/n
  a/nm	2.399 0(5)	1.119 8(5)	0.777 8(3)
  b/nm	1.756 6(4)	1.314 3(7)	2.699 3(9)
  c/nm	1.400 9(3)	2.493 1(9)	1.828 0(6)
  β/(°)	90.237(1)		99.551(1)
  V/nm3	5.903 6(2)	3.669 2(3)	3.784 7(2)
  Z	4	4	4
  Dc/(g·cm-3)	1.610	1.394	1.534
  F(000)	2 944	1 584	1 808
  Reflection, independent	35 808, 5 838	19 882, 6 503	35 495, 6 953
  Observed reflection [I > 2σ(I)]	5 812	4 258	6 761
  2θ for data collection/(°)	5.474-52.068	7.092-134.468	5.416-50.74
  GOF on F2	1.052	1.140	1.032
  Rint	0.060 2	0.085 1	0.069 6
  R1, wR2* [I > 2σ(I)]	0.039 5, 0.084 8	0.089 7, 0.184 1	0.038 9, 0.073 6
  R1, wR2 (all data)	0.053 0, 0.091 8	0.149 0, 0.220 8	0.062 4, 0.084 1
  * R1=∑||Fo|-|Fc||/|Fo|, wR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

  Table 2

  

  Table 2.  Selected bond distances (nm) and bond angles (°) of compounds 1-3

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  1
  Zn1—O1	0.195 9(2)	Zn1—O5#1	0.197 1(2)	Zn1—N1	0.202 3(2)
  Zn1—N4#2	0.204 3(2)
  O1—Zn1—O5#1	109.52(8)	O1—Zn1—N1	122.58(8)	O1—Zn1—N4#2	106.78(9)
  O5#1—Zn1—N1	97.15(8)	O5#1—Zn1—N4#2	120.72(8)	N1—Zn1—N4#2	100.82(9)
  2
  Zn1—N2	0.200 9(1)	Zn1—N4#1	0.205 6(1)	Zn1—O1	0.237 7(1)
  Zn1—O2	0.205 0(1)	Zn1—O6#2	0.194 1(9)
  N2—Zn1—N4#1	101.5(5)	N2—Zn1—O1	108.7(5)	N2—Zn1—O2	93.5(5)
  N4#1—Zn1—O1	147.2(5)	O2—Zn1—N4#1	109.1(5)	O1—Zn1—O2	57.7(4)
  O6#2—Zn1—N2	113.4(5)	O6#2—Zn1—N4#1	92.7(4)	O6#2—Zn1—O1	86.7(4)
  O6#2—Zn1—O2	141.5(5)
  3
  Zn1—O1	0.200 0(2)	Zn1—O5#1	0.222 8(2)	Zn1—O6#1	0.223 2(2)
  Zn1—O9	0.209 7(2)	Zn1—O10	0.206 9(2)	Zn1—N1	0.206 5(2)
  O1—Zn1—O5#1	108.36(8)	O1—Zn1—O6#1	167.21(8)	O1—Zn1—O9	94.16(8)
  O1—Zn1—O10	87.55(8)	O1—Zn1—N1	96.41(9)	O5#1—Zn1—O6#1	58.87(7)
  O9—Zn1—O5#1	88.88(8)	O9—Zn1—O6#1	86.49(8)	O10—Zn1—O5#1	82.01(8)
  O10—Zn1—O6#1	89.86(8)	O10—Zn1—O9	170.80(9)	N1—Zn1—O5#1	154.75(8)
  N1—Zn1—O6#1	96.27(8)	N1—Zn1—O9	94.30(9)	N1—Zn1—O10	94.49(9)
  Symmetry codes: #1: -1/2+x, 1/2+y, z; #2: -1/2+x, 1/2-y, 1/2+z for 1; #1: 1/2-x, -y, -1/2+z; #2: -1/2-x, 1-y, -1/2+z for 2; #1: 1/2-x, -1/2+y, 1/2-z for 3.

  Table 3

  

  Table 3.  Hydrogen bond parameters of compound 3

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  D—H…A	d(D—H)/nm	d(H…A)/nm	d(D…A)/nm	∠DHA/(°)
  O3—H3…O2	0.084	0.179	0.254 1(3)	148.6
  O4—H4…O5	0.084	0.184	0.259 1(3)	147.1
  O9—H9A…O6#1	0.087	0.193	0.278 9(3)	168.9
  O9—H9B…O11#2	0.087	0.201	0.281 7(3)	153.5
  O10—H10A…O11	0.087	0.181	0.268 3(3)	176.2
  O10—H10B…O6#3	0.087	0.196	0.278 2(3)	157.2
  O11—H10A…N4#4	0.087	0.191	0.276 6(3)	165.7
  O11—H11B…O7#5	0.087	0.208	0.290 0(3)	156.3
  O12—H12C…O2	0.087	0.196	0.282 6(3)	174.7
  O12—H12D…O5#4	0.087	0.214	0.289 2(3)	144.2
  Symmetry codes: #1: 1/2-x, -1/2+y, 1/2-z; #2: 1+x, y, z; #3: -1/2+x, 3/2-y, 1/2+z; #4: 1/2-x, 1/2+y, 1/2-z; #5: -1/2+x, 3/2-y, -1/2+z for 3.

  CCDC: 2129934, 1; 2129682, 2; 2129683, 3.

  2.   Results and discussion

  2.1   Crystal structure

  2.1.1   Crystal structure of compound 1

  Single X-ray crystallography shows that compound 1 crystallizes in the monoclinic system with the C2/c space group. The asymmetric unit of 1 contains one independent Zn(Ⅱ) cation, one OBA^2- anion, one BIDPS molecule, and half of the lattice DMA molecule, which is disordered over two general positions with the site occupancy of 0.5. Zn(Ⅱ) is four-coordinated and exhibits a slightly distorted tetrahedral geometry surrounded by two oxygen atoms (O1 and O5) from two OBA^2- anions, two nitrogen atoms (N1 and N4) from two BIDPS molecules (Fig. 1a). Zn(Ⅱ) centers are linked by OBA^2- to form an infinite 1D wave-like chain with Zn…Zn separation of 1.486 7 nm. Zn(Ⅱ) centers are connected by BIDPS ligands via Zn—N coordination bonds, affording extraordinary meso-helical with a pitch of 2.773 1 nm. The meso-helical contains both left- and right-handed helical loops and displays a "∞" shape (Fig. 1b). The meso-helices can be regarded as a 3D representation of a lemniscate. Furthermore, the meso-helicals connect 1D wave-like chains constructed by Zn(Ⅱ) centers and OBA^2-, resulting in a 3D structure (Fig. 1c). A better insight into the structure, and a topological approach can be adopted. The Zn(Ⅱ) can be seen as four-connected nodes, while OBA^2- anions and BIDPS molecules are linkers. Based on the simplification principle, this 3D structure can be simplified into a four-connected cds topology with the point symbol of {6⁵·8}. The overall structure has two equivalent nets, adopting 2-fold interpenetration to minimize the large voids in the independent net (Fig. 1d). The compound formulated [Zn(BIDPT) (OBA)]_n has a similar 3D layer structure^[20].

  Figure 1

  

  Figure 1.  Crystal structure of 1: (a) coordination environment of the Zn(Ⅱ); (b) perspective view of the meso-helical chain formed by Zn(Ⅱ) and BIDPS; (c) 3D structure; (d) topological representation the two-fold interpenetrating cds topology

  Symmetry codes: #1:-1/2+x, 1/2+y, z; #2:-1/2+x, 1/2-y, 1/2+z

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  2.1.2   Crystal structure of compound 2

  The single-crystal X-ray diffraction reveals that compound 2 crystallizes in the orthorhombic crystal system with the P2₁2₁2₁ space group. The asymmetric unit of 2 consists of one Zn(Ⅱ) metal center, one BIDPT molecule, one PA^2- anion, and one uncoordinated DMF molecule. The Zn(Ⅱ) ion is five-coordinated by three oxygen atoms from two different PA^2- anions (Zn—O: 0.194 1(9)-0.237 7(1) nm) and two nitrogen atoms from two BIDPT molecules (Zn—N: 0.200 9(1) and 0.205 6(1) nm) to form a distorted square-pyramid coordination geometry (Fig. 2a). The bond angles and bond lengths around each Zn(Ⅱ) ion are typical and comparable with similar Zn(Ⅱ) complexes in the literature^[21-22].

  The hydroxyl and carboxyl groups of fully deprotonated PA^2- ion adopt a trans conformation to minimize the steric hindrance. The PA^2- anion adopts chelated and mono-dentated coordination modes with Zn(Ⅱ) ions, forming a 1D infinite left-handed helical chain of [Zn(PA)] _n with a pitch of 2.493 nm. It is noteworthy that the V-shaped BIDPT ligand links neighboring Zn ions to form another left-helical chain [Zn(BIDPT)]_n²ⁿ⁺ (Fig. 2b). The two left-helical chains are connected and form a 2D sheet (Fig. 2c). The 2D sheet has a dimension of 1.474 nm×1.627 nm, as determined by the Zn…Zn distance. The Zn(Ⅱ) ions can be regarded as 4-connected nodes and the two ligands as linkers. So, 2 can be represented as a sql network with the point symbol of {4⁴·6²}. Two sets of layers are oriented in different directions, and the two sets of layers catenate to each other in a parallel-parallel arrangement to form a 2D + 2D → 3D inclined polycatenation structure (Fig. 2d). The rhombic grid coordination polymer {[Cd(TPPA)(pht)(H₂O)](H₂O)₂}_n also formed a 2D → 3D inclined polycatenation structure^[23].

  Figure 2

  

  Figure 2.  Crystal structure of 2: (a) coordination environment of the Zn(Ⅱ); (b) left-handed helical chains constructed by [Zn(PA)]_n and [Zn(BIDPT)]_n²ⁿ⁺, respectively; (c) 2D network; (d) 2D + 2D → 3D inclined polycatenation framework

  Symmetry codes: #1: 1/2-x, -y, -1/2+z; #2:-1/2-x, 1-y, -1/2+z

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  2.1.3   Crystal structure of compound 3

  Compound 3 crystallizes in the monoclinic crystal system with the space group of P2₁/n. Its asymmetric unit consists of one Zn(Ⅱ) cation, one BIDPS molecule, one protonated PA^2- anion, two coordinated water molecules, and two lattice water molecules. As shown in Fig. 3a, each Zn(Ⅱ) ion is six-coordinated by one nitrogen atom from one BIDPS molecule (Zn—N: 0.206 5(2) nm), five oxygen atoms from two coordinated water mol- ecules and two carboxylate groups (Zn—O: 0.200 0(2)- 0.223 2(2) nm) in a slightly distorted octahedral coordination geometry.

  Figure 3

  

  Figure 3.  Crystal structure of 3: (a) coordination environment of the Zn(Ⅱ); (b) 3D network assembled by O—H…O and O—H…N hydrogen bonds; (c) 1D tube formed by Zn and O—H…O hydrogen bonds

  Symmetry code: #1: 1/2-x, -1/2+y, 1/2-z

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  In the network of 3, each fully protonated pamoic acid adopts chelated and mono-dentated coordination modes with Zn(Ⅱ) centers. In such a way, the Zn(Ⅱ) ions are connected by PA^2- anions to afford a right-handed helical chain along the b-axis direction with a pitch of 2.699 nm (Fig.S4). The BIDPS molecules coordinate to the Zn(Ⅱ) cations on both sides of the helical chain alternately, with the N1 atom coordinated to the Zn(Ⅱ) centers. Two adjacent 1D chains are joined together through intermolecular hydrogen bonds (O9—H9A…O6) to form a wavy 2D network, and the intramolecular hydrogen bonds among O4— H4…O5 and O3—H3… O2 make the molecule more stable (Fig. S5). Furthermore, the 2D networks are held together by other hydrogen bonds to form a 3D architecture (Fig. 3b). Remarkably, Zn(Ⅱ) ions are connected by O—H…O hydrogen bonds between coordinated and lattice water molecules, forming a 1D tube with Zn…Zn shortest distances of 0.550 4 nm (Fig. 3c).

  2.2   Chemical stabilities and TGA

  The phase purity of 1-3 has been confirmed by PXRD. The peak positions in the experimental patterns are consistent with the simulated ones, indicating the pure samples of 1-3. They are stable in the aqueous solutions in a pH range from 4 to 10, which demonstrates that 1-3 possesses good chemical stabilities (Fig.S6-S8).

  The thermal stability of compounds 1-3 was also estimated (Fig.S9-S11). Compound 1 showed the weight loss of uncoordinated DMA molecule from 151 to 167 ℃ (Calcd. 6.09%, Obsd. 6.15%). The framework began to decompose from 285 ℃, and the ZnO residue of 13.41% (Calcd. 11.38%) was observed. For 2, the first weight loss of 8.77% in a range of 158-190 ℃ is related to the loss of uncoordinated DMF molecule (Calcd. 8.67%). Then the decomposition happened from 190 to 700 ℃. The main residue should be ZnO (Calcd. 9.65%, Obsd. 12.47%). Compound 3 was stable before 132 ℃. The coordinated and lattice water molecules were lost from 132 to 173 ℃ (Calcd. 8.24%, Obsd. 8.31%). The overall framework began to decompose from 293 ℃, and the ZnO residue of 9.30% (Calcd. 9.36%) was observed at 700 ℃.

  2.3   Detection of TNP

  In view of the excellent luminescence properties of d¹⁰ MOFs, the solid-state fluorescence spectra of ligands (BIDPS and BIDPT) and compounds 1-3 were studied at room temperature (Fig.S12). Their maximum emission peaks were 436, 421, 467, 463, and 471 nm for BIDPS, BIDPT, and 1-3, respectively. The red shift compared to the luminescence of the ligands (BIDPS and BIDPT) reflected the interaction between the Zn²⁺ and the organic linkers^[24].

  Considering chemical stability in aqueous solution and strong luminescent properties of 1-3, we investigated their sensing ability for nitro compounds, including 4-nitrophenol (4-NP), 2, 4-dinitrotoluene (2, 4-DNT), TNP, 2, 4-dinitrophenol (2, 4-DNP), nitrobenzene (NB), 3-nitrophenol (3-NP), 4-nitrotoluene (NT), 1, 3-dinitrobenzene (1, 3-DNB), 1, 4-dinitrobenzene (1, 4-DNB). The luminescence response was measured by gradually dropping of fresh nitro compounds in an aqueous solution (5 mmol·L^-1) to the aqueous suspension of 1. The emission intensities of 1 gradually reduced with the increasing amount of TNP solution and decreased to 7.7% of the original intensity with the addition of 90 µL TNP solution (Fig. 4a). Upon addition of 90 µL of 3-NP and 4-NP, the emission intensity was quenched by 16% and 21%, while slight quenching effects were observed in the luminescence intensities of 1 after adding the same amount of the remaining nitro compounds. The results show that compound 1 can selectively detect TNP. Quantitatively, the quenching efficiency of TNP was analyzed using the Stern-Volmer (SV) equation: I₀/I=1+K_SVc, where I₀ and I are the luminescence intensity before and after the addition of analytes, respectively, K_SV is quenching constant (L·mol^-1) and c is the concentration of the analytes. The SV plots for TNP were near linear at the low concentration (R²=0.992). However, with the increase in concentration, the SV plot deviates from the linearity because of self-absorption or energy transfer^[25-26]. As shown in Fig. 4b, the K_SV for 1 was calculated to be 9.60×10³ L·mol^-1 based on the linear part. The detection limit was calculated based on the equation reported in the literature^[27]. It was 1.5 µmol·L^-1, which was compared to other MOF-based sensors for TNP^[28].

  Figure 4

  

  Figure 4.  (a) Photoluminescence spectra of 1 upon the addition of TNP; (b) SV plot for 1 with the gradual addition of TNP in water

  Inset: linear fit for SV plot

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  Compounds 2 and 3 were employed to sense nitro compounds with the same procedure. Like 1, 2 and 3 could also selectively detect TNP. With the increasing amount of TNP, the luminescent intensities of 2 and 3 decreased gradually. The emission intensities were quenched by 92.0% and 91.6% for 2 and 3 after the addition of 90 µL TNP solution. Under the same concentration, quenching efficiencies of 3-NP and 4-NP were 18% and 20% for 2, 13% and 15% for 3, respectively, and slight quenching effects were observed in the case of other nitro compounds. As shown in Fig. 5, K_SV and detection limit for TNP were calculated to be 7.03×10³ L·mol^-1 and 2.8 µmol·L^-1 for 2, 8.58×10³ L· mol^-1 and 2.3 µmol·L^-1 for 3, respectively. The K_SV and detection limit values for 2 and 3 were comparable to those of reported MOFs for sensing TNP in aqueous system^[29-30]. The large K_SV and small detection limit values indicate 1-3 may act as excellent sensors for TNP detection in the aqueous solution.

  Figure 5

  

  Figure 5.  (a, c) Photoluminescence spectra of 2 and 3 upon the addition of TNP, respectively; (b, d) SV plots for 2 and 3 with the gradual addition of TNP in water, respectively

  Inset: linear fit for SV plot

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  Furthermore, the selective detection of TNP in the presence of other competitive nitro compounds was investigated. The fluorescence intensities of 1-3 suspension in 1 mmol·L^-1 aqueous solution with individual nitro compounds, excluding TNP, were measured first. After that, fluorescence measurements were done again after the addition of 1 mmol·L^-1 TNP. Fluorescence intensity decreased to a large degree upon the addition of TNP (Fig.S13). This result demonstrates the quenching response of TNP on compounds 1-3 dispersions are not affected by any other individual interfering nitro compounds.

  2.4   Detection of metal ions

  Given the sensing abilities of nitro compounds by compounds 1-3, their abilities to sense metal ions were also examined. The same procedure was conducted to investigate the luminescence sensing of metal ions. Different aqueous solutions (5 mmol·L^-1) containing M(NO₃)_x (M^x+=K⁺, Na⁺, Cd²⁺, Zn²⁺, Ca²⁺, Ni²⁺, Ba²⁺, Cu²⁺, Co²⁺, and Fe³⁺) were studied as analytes. The photoluminescence response was measured after the gradual addition of different metal ions to the suspension of 1-3 in an aqueous solution. However, only Fe³⁺ exhibited the most significant quenching effect on the luminescence intensities of 1-3. As shown in Fig. 6, as the Fe³⁺ concentration increased, the luminescence intensities corresponding to 1-3 gradually decreased, and the efficiencies was up to 93.1% (1), 92.0% (2), and 91.3% (3), when the concentration of Fe³⁺ were up to 225 µmol·L^-1. At the low concentration, the K_SV and detection limit were calculated to be 1.30×10⁴ L·mol^-1, 1.4 µmol·L^-1 for 1, 7.95×10³ L·mol^-1, 2.1 µmol·L^-1 for 2, 7.56×10³ L·mol^-1, 2.3 µmol·L^-1 for 3, respectively.

  Figure 6

  

  Figure 6.  (a, c, e) Photoluminescence spectra of 1-3 upon the addition of Fe³⁺, respectively; (b, d, f) SV plots for 1-3 with the gradual addition of Fe³⁺ in water, respectively

  Inset: linear fit for SV plot

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  In addition, interference experiments of other cations on Fe³⁺ sensing were also studied. In the absence of Fe³⁺, the fluorescence intensities of 1-3 dispersed with other cations (1 mmol·L^-1) in water decreased slightly, but after the addition of Fe³⁺ (1 mmol·L^-1) to the above solution, the fluorescence intensities of 1-3 decreased sharply and almost completely extinguished (Fig. S14). The results show that the sensing behaviors of 1-3 towards Fe³⁺ are not affected by other metal ions.

  2.5   Recyclability after sensing experiments

  To further estimate the reutilization of 1-3 for TNP/Fe³⁺ sensing detection, the used suspension was centrifuged to separate the solids and washed with water, then dried. The recovered samples of 1-3 were soaked again in the aqueous solution (0.01 mol·L^-1) of TNP/Fe³⁺ for 2 h, respectively. The initial luminescence can be regained by washing the resulting solid with water. The PXRD confirmed that the structural integrity of 1-3 could be well-retained after quenching and recovery (Fig.S15-S17). The results show that 1-3 can be used as stable and recyclable chemical sensors for detecting TNP and Fe³⁺.

  3.   Conclusions

  In conclusion, we synthesized three new luminescent Zn-MOFs based on 4, 4'-bis(imidazol-l-yl)-phenyl sulphone, 4, 4'-bis(imidazol-l-yl)diphenyl thioether, pamoic acid, and 4, 4'-oxybisbenzoic acid. The crystal structure analysis results show that 1-3 are 3D networks. 1-3 possessed strong luminescence with excellent selectivity and sensitivity for TNP and Fe³⁺ in an aqueous medium. The structures of 1-3 remained intact after quenching and recovery, indicating they are efficient and recyclable sensing materials.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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• 

  Figure 1  Crystal structure of 1: (a) coordination environment of the Zn(Ⅱ); (b) perspective view of the meso-helical chain formed by Zn(Ⅱ) and BIDPS; (c) 3D structure; (d) topological representation the two-fold interpenetrating cds topology

  Symmetry codes: #1:-1/2+x, 1/2+y, z; #2:-1/2+x, 1/2-y, 1/2+z

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Crystal structure of 2: (a) coordination environment of the Zn(Ⅱ); (b) left-handed helical chains constructed by [Zn(PA)]_n and [Zn(BIDPT)]_n²ⁿ⁺, respectively; (c) 2D network; (d) 2D + 2D → 3D inclined polycatenation framework

  Symmetry codes: #1: 1/2-x, -y, -1/2+z; #2:-1/2-x, 1-y, -1/2+z

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Crystal structure of 3: (a) coordination environment of the Zn(Ⅱ); (b) 3D network assembled by O—H…O and O—H…N hydrogen bonds; (c) 1D tube formed by Zn and O—H…O hydrogen bonds

  Symmetry code: #1: 1/2-x, -1/2+y, 1/2-z

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) Photoluminescence spectra of 1 upon the addition of TNP; (b) SV plot for 1 with the gradual addition of TNP in water

  Inset: linear fit for SV plot

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  Figure 5  (a, c) Photoluminescence spectra of 2 and 3 upon the addition of TNP, respectively; (b, d) SV plots for 2 and 3 with the gradual addition of TNP in water, respectively

  Inset: linear fit for SV plot

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  Figure 6  (a, c, e) Photoluminescence spectra of 1-3 upon the addition of Fe³⁺, respectively; (b, d, f) SV plots for 1-3 with the gradual addition of Fe³⁺ in water, respectively

  Inset: linear fit for SV plot

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  Table 1.  Crystal data and structure refinement parameters for compounds 1-3

  Compound	1	2	3
  Formula	C68H53Zn2N9O15S2	C44H35ZnN5O7S	C41H36ZnN4O12S
  Formula weight	1 431.05	843.20	874.17
  T/K	100.0	293(2)	165.0
  Crystal system	Monoclinic	Orthorhombic	Monoclinic
  Space group	C2/c	P212121	P21/n
  a/nm	2.399 0(5)	1.119 8(5)	0.777 8(3)
  b/nm	1.756 6(4)	1.314 3(7)	2.699 3(9)
  c/nm	1.400 9(3)	2.493 1(9)	1.828 0(6)
  β/(°)	90.237(1)		99.551(1)
  V/nm3	5.903 6(2)	3.669 2(3)	3.784 7(2)
  Z	4	4	4
  Dc/(g·cm-3)	1.610	1.394	1.534
  F(000)	2 944	1 584	1 808
  Reflection, independent	35 808, 5 838	19 882, 6 503	35 495, 6 953
  Observed reflection [I > 2σ(I)]	5 812	4 258	6 761
  2θ for data collection/(°)	5.474-52.068	7.092-134.468	5.416-50.74
  GOF on F2	1.052	1.140	1.032
  Rint	0.060 2	0.085 1	0.069 6
  R1, wR2* [I > 2σ(I)]	0.039 5, 0.084 8	0.089 7, 0.184 1	0.038 9, 0.073 6
  R1, wR2 (all data)	0.053 0, 0.091 8	0.149 0, 0.220 8	0.062 4, 0.084 1
  * R1=∑||Fo|-|Fc||/|Fo|, wR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

  下载: 导出CSV

  Table 2.  Selected bond distances (nm) and bond angles (°) of compounds 1-3

  1
  Zn1—O1	0.195 9(2)	Zn1—O5#1	0.197 1(2)	Zn1—N1	0.202 3(2)
  Zn1—N4#2	0.204 3(2)
  O1—Zn1—O5#1	109.52(8)	O1—Zn1—N1	122.58(8)	O1—Zn1—N4#2	106.78(9)
  O5#1—Zn1—N1	97.15(8)	O5#1—Zn1—N4#2	120.72(8)	N1—Zn1—N4#2	100.82(9)
  2
  Zn1—N2	0.200 9(1)	Zn1—N4#1	0.205 6(1)	Zn1—O1	0.237 7(1)
  Zn1—O2	0.205 0(1)	Zn1—O6#2	0.194 1(9)
  N2—Zn1—N4#1	101.5(5)	N2—Zn1—O1	108.7(5)	N2—Zn1—O2	93.5(5)
  N4#1—Zn1—O1	147.2(5)	O2—Zn1—N4#1	109.1(5)	O1—Zn1—O2	57.7(4)
  O6#2—Zn1—N2	113.4(5)	O6#2—Zn1—N4#1	92.7(4)	O6#2—Zn1—O1	86.7(4)
  O6#2—Zn1—O2	141.5(5)
  3
  Zn1—O1	0.200 0(2)	Zn1—O5#1	0.222 8(2)	Zn1—O6#1	0.223 2(2)
  Zn1—O9	0.209 7(2)	Zn1—O10	0.206 9(2)	Zn1—N1	0.206 5(2)
  O1—Zn1—O5#1	108.36(8)	O1—Zn1—O6#1	167.21(8)	O1—Zn1—O9	94.16(8)
  O1—Zn1—O10	87.55(8)	O1—Zn1—N1	96.41(9)	O5#1—Zn1—O6#1	58.87(7)
  O9—Zn1—O5#1	88.88(8)	O9—Zn1—O6#1	86.49(8)	O10—Zn1—O5#1	82.01(8)
  O10—Zn1—O6#1	89.86(8)	O10—Zn1—O9	170.80(9)	N1—Zn1—O5#1	154.75(8)
  N1—Zn1—O6#1	96.27(8)	N1—Zn1—O9	94.30(9)	N1—Zn1—O10	94.49(9)
  Symmetry codes: #1: -1/2+x, 1/2+y, z; #2: -1/2+x, 1/2-y, 1/2+z for 1; #1: 1/2-x, -y, -1/2+z; #2: -1/2-x, 1-y, -1/2+z for 2; #1: 1/2-x, -1/2+y, 1/2-z for 3.

  下载: 导出CSV

  Table 3.  Hydrogen bond parameters of compound 3

  D—H…A	d(D—H)/nm	d(H…A)/nm	d(D…A)/nm	∠DHA/(°)
  O3—H3…O2	0.084	0.179	0.254 1(3)	148.6
  O4—H4…O5	0.084	0.184	0.259 1(3)	147.1
  O9—H9A…O6#1	0.087	0.193	0.278 9(3)	168.9
  O9—H9B…O11#2	0.087	0.201	0.281 7(3)	153.5
  O10—H10A…O11	0.087	0.181	0.268 3(3)	176.2
  O10—H10B…O6#3	0.087	0.196	0.278 2(3)	157.2
  O11—H10A…N4#4	0.087	0.191	0.276 6(3)	165.7
  O11—H11B…O7#5	0.087	0.208	0.290 0(3)	156.3
  O12—H12C…O2	0.087	0.196	0.282 6(3)	174.7
  O12—H12D…O5#4	0.087	0.214	0.289 2(3)	144.2
  Symmetry codes: #1: 1/2-x, -1/2+y, 1/2-z; #2: 1+x, y, z; #3: -1/2+x, 3/2-y, 1/2+z; #4: 1/2-x, 1/2+y, 1/2-z; #5: -1/2+x, 3/2-y, -1/2+z for 3.

  下载: 导出CSV
•