Luminescent MetalâOrganic Frameworks with Anthracene

Article pubs.acs.org/crystal

Luminescent Metal−Organic Frameworks with Anthracene Chromophores: Small-Molecule Sensing and Highly Selective Sensing for Nitro Explosives Xiao Li, Liu Yang, Liang Zhao,* Xin-Long Wang, Kui-Zhan Shao, and Zhong-Min Su* Institute of Functional Material Chemistry, Local United Engineering Lab for Power Battery, Northeast Normal University, Changchun, 130024 Jilin, People’s Republic of China S Supporting Information *

ABSTRACT: Five new luminescent metal−organic frameworks (MOFs), [Cd(bdc)(dia)0.5(MeOH)](DMA)0.5 (1), [Cd2O(1,4ndc)2(dia)2(H2O)]·DMA (2), [Cd(1,4-ndc)(dia)]·MeOH (3), [Cd(bpdc)(dia)0.5]·DMF (4), [Cd(bpdc) (dia)]·DEF (5) (dia = 9,10bis(1H-imidazol-1-yl)anthracene), were obtained from solvothermal conditions and characterized by PXRD, IR, TG, and PL spectra. Single-crystal X-ray diffraction analyses showed that all compounds 1−5 display three-dimensional architectures with diverse topologies. Compound 1 was a 6-con pcu network based on dinuclear Cd cluster. Compound 2 features an eight-connected network constructed from a binuclear Cd cluster. Compound 3 presents a 3-fold interpenetrated four-connected dia topology. Compounds 4 and 5 exhibit a 3-fold interpenetrated six-connected pcu framework and a 3-fold interpenetrated four-connected dia net, respectively. Because compounds 3−5 have one-dimensional (1D) channels, the luminescence properties of 3−5 in different solvents were investigated systematically. Furthermore, compounds 3−5 display highly sensitive, selective, and well-recyclable properties in the detection of nitrobenzene (NB) and 2,4,6-trintrophenol (TNP) as fluorescent sensors.

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INTRODUCTION Rapid and selective detection of explosive and explosive-like substances plays a paramount role in homeland safety and environmental security.1 Among these explosives, nitro-organic compounds, for example nitrobenzene (NB), 2,4,6-trinitnphenol (TNP), and 2,4-dinitrotolunene (2,4-DNT), etc., are the major molecules. Therefore, it is very significant for fast and highly selective detection of nitro-organic compounds.2 However, current detection methods,3 either employing trained canines or technique-based instruments, are not always available. This is because these instruments need frequently careful calibrations. Compared with traditional detection methods, optical sensing has proven to be a promising method to detect explosives rapidly. It is cost-effective, fast, and easily portable based on fluorescence techniques.4 Thus, there is an enormous demand to synthesize novel luminescent materials for detecting explosives. Metal−organic frameworks (MOFs), possessing the merits of richness topology structures and permanent porosities, have shown a series of promising applications in many fields, such as drug delivery,5 heterogeneous catalysis,6 gas storage, separations,7 and chemical sensing,8 etc. Among these, luminescent MOFs can be used as sensory materials with several particular advantages. First, in luminescent MOFs, both the organic ligands containing aromatic or conjugated π moieties and the metal components can raise optical emission or photoluminescence. Second, on the other hand, the permanent © XXXX American Chemical Society

porosity in luminescent MOFs can capture guest molecules, which can increases the opportunities of interactions between guest and host molecules. This can result in the alternation of the physicochemical properties such as the shift of the emission spectrum and the color change of compounds. Third, nitroaromatics, as exemplary explosive or explosive-like molecules, have high electron affinity, which make them known as strong quenchers.9 Therefore, it is of much significance to synthesize luminescent MOFs used for detecting explosives. As is well-known, MOFs are constructed by rational selfassembly of metal cations (or metal clusters) and fuctional organic molecules to build zero-, one-, two-, or threedimensional crystalline solids. It should be mentioned that the ligands play a vital role in the design and construction of diverse MOFs. Generally, we fabricate aromatic carboxylate and nitrogen donor ligands to construct MOFs with intriguing networks. Among the versatile ligands, π-conjugated molecules show superior potentials in the formation of luminescent materials.10 As a crucial class of π-conjugated molecules, the anthracene derivates possess unique properties in building luminescent materials.11 It should be an excellent strategy to synthesize luminescent materials with outstanding sensing Received: March 29, 2016 Revised: May 30, 2016

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DOI: 10.1021/acs.cgd.6b00482 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. Crystallographic Data for 1−5

a

compound

1

2

3

4

5

chem formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) temp (K) Z Dcalcd (g·cm−3) GOF on F2 no. of unique data no. of params refined R1 [I > 2σ(I)]a wR2 [I > 2σ(I)]b R1a (all data) wR2b (all data) Rint

C21H14CdN2.5O5.5 501.75 triclinic P1̅ 9.8155(16) 10.1884(15) 12.871(2) 69.002(3) 69.676(3) 63.281(3) 1046.3(3) 293(2) 2 1.593 1.078 3675 277 0.0637 0.1679 0.1092 0.2015 0.0365

C68H51Cd2N9O11 1394.98 orthorhombic Fdd2 18.9871(8) 43.590(3) 14.5389 90 90 90 12033.1(11) 293(2) 8 1.540 1.049 5276 376 0.0287 0.0712 0.0306 0.0720 0.0302

C33H24CdN4O5 668.96 monoclinic C2/c 16.2323(11) 9.9006(11) 18.3224(14) 90 103.015(2) 90 2868.9(4) 293(2) 4 1.549 1.073 2532 202 0.0487 0.1345 0.574 0.1418 0.0273

C27H21CdN3O5 580.87 monoclinic P21/n 7.6268(4) 18.5423(9) 17.2482(8) 90 100.0690(10) 90 2401.6(2) 293(2) 4 1.607 1.060 4213 281 0.0283 0.0630 0.0384 0.0655 0.0330

C39H33CdN5O5 764.09 monoclinic C2/m 26.054(4) 22.388(4) 9.2545(14) 90 92.154(4) 90 5394.5(15) 293(2) 4 0.941 0.967 6916 204 0.0621 0.1459 0.1276 0.1586 0.0849

R1 = ∑∥F0| − |Fc∥/∑|F0|. bwR2 = [∑w(F02 − Fc2)2/∑w(F02)2]1/2. spectrophotometer which is equipped with a xenon lamp and a quartz carrier at ambient temperature. Syntheses of Compounds 1−5. [Cd(bdc)(dia)0.5(MeOH)](DMA)0.5 (1). A mixture containing Cd(NO)3·4H2O (30 mg, 0.1 mmol), dia (30 mg, 0.1 mmol), and bdc (16 mg, 0.1 mmol) was dissolved in 8 mL of DMA-MeOH (1:1, v/v), and then the solutions was stirred for 30 min at ambient temperature. Then the mixture was placed in a 23 mL Teflon-lined autoclave under autogenous pressure and heated at 100 °C for 3 days. After cool to ambient temperature, quite a few light yellow blocks of crystals were obtained, washed with methanol, and dried under ambient conditions. Yield: 51 mg (51% yield based on dia). Anal. Calcd for C21H14CdN2.5O5.5: C, 50.22; H, 2.79; N, 6.97. Found: C, 50.08; H, 2.78; N, 6.99. IR (KBr, cm−1): 3453m, 3126m, 1671m, 1497m, 1402w, 1150m, 1022m, 920m, 762m, 650m, 588m, 433m. [Cd2(1,4-ndc)2(dia)2(H2O)]·DMA (2). A mixture of Cd(NO)3·4H2O (30 mg, 0.1 mmol), dia (30 mg, 0.1 mmol), and 1,4-ndc (11 mg, 0.05 mmol) was dissolved in 8 mL of DMA-H2O (4:4, v/v), and then the solution was stirred for 30 min at ambient temperature. Then the mixture was placed in a 23 mL Teflon-lined autoclave under autogenous pressure and heated at 100 °C for 3 days. After cooling to ambient temperature, many light yellow blocks of crystals were gained, washed with distilled water, and then dried under ambient conditions. Yield: 45 mg (66% yield based on dia). Anal. Calcd for C68H51CdN9O11: C, 58.50; H, 3.65; N, 9.03. Found: C, 58.65; H, 3.64; N, 9.05. IR (KBr, cm−1): 3741m, 3388m, 1624m, 1502m, 1411m, 794m. [Cd(1,4-ndc)(dia)]·MeOH (3). The same synthetic procedure as that for 2 was used other than that the solvent was changed to H2O− MeOH−DMA (1:1:2, v/v/v) which gave yellow blocks of crystals in 62% yield. Anal. Calcd for C33H24CdN4O5: C, 59.19; H, 3.59; N, 8.37. Found: C, 59.32; H, 3.57; N, 8.34. IR (KBr, cm−1): 1617w, 1592w, 1206m, 1157m, 1030m, 867m, 750m, 654m, 611w. [Cd(bpdc)(dia)0.5]·DMF (4). A mixture of Cd(NO)3·4H2O (30 mg, 0.1 mmol), bpdc (12 mg, 0.05 mmol), and dia (30 mg, 0.1 mmol) was dissolved in 8 mL of DMF−MeOH−H2O (2:1:1, v/v/v) and then stirred for 30 min at room temperature. Then the solution was placed in a 23 mL Teflon-lined autoclave under autogenous pressure and heated at 100 °C for 3 days. After the sample cooling to room temperature, we obtained yellow blocks of crystals in a 58% yield.

properties though the incorporation of anthracene moieties into networks. Meanwhile, the five-membered heterocycles, for example imidazole, triazole, and tetrazoles and so on, are promising candidates in the design and fabrication of luminescent MOFs.12 Keeping the aforementioned reasons in mind, we select the ligand 9,10-bis(1H-imidazol-1-yl)anthracene (dia), in which the anthracene spacers are directly bonded to the imidazole as bulky bis(imidazole) ligand to obtain microporous luminescent materials and investigate their sensing abilities. In this work, we demonstrate five luminescent MOFs, compounds 1−5, using dia ligand and three different rigid carboxylic acids. Their crystal structures have been solved by single-crystal X-ray diffraction and subsequently characterized by IR spectra, elemental analysis, powder X-ray diffraction (PXRD), and thermalgravimetric analyses (TGA). The luminescent properties of 3−5 have been investigated systematically, which indicates that it should be a hopeful strategy to synthesize luminescent materials possessing excellent sensing properties by incorporating anthracene moieties into MOFs. Meanwhile, compounds 3−5 exhibit high sensitivity for nitrobenzene (NB) and 2,4,6-trintrophenol (TNP) through luminescence quenching experiments.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All of the commercially available chemicals in the reactions were used as received without further purification. The ligand of dia was synthesized according to a revised procedure from the reported literature.13 IR spectra were carried out by an Alpha Centaurt FT/IR spectrophotometer using KBr pellets in the range 400−4000 cm−1. Thermogravimetric analysis (TGA) was performed on a PerkinElmer TG-7 analyzer heated from room temperature to 800 °C under nitrogen atmosphere at a rate of 10 °C min−1. Elemental analyses (C, H, and N) were measured on a PerkinElmer 240C elemental analyzer. PXRD patterns were accomplished on a Siemens D5005 diffractometer with Cu Kα (λ = 1.5418 Å) radiation in the range of 3−50° at a rate of 5 min−1 at 293 K. Fluorescence spectra were obtained on an F-4600 FL B



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Anal. Calcd for C27H22CdN3O5: C, 55.78; H, 3.79; N, 7.23. Found: C, 55.85; H, 3.80; N, 7.25. IR (KBr, cm−1): 3104m, 1673m, 1604m, 1521w, 1255m, 1141m, 1031m, 911s, 854m, 799s, 703m, 660m. [Cd(bpdc)(dia)]·DEF (5). The synthetic procedure was the same as that for 4, except that the solvent was changed to 8 mL of DEF. Finally, we obtained yellow blocks of crystals in 78% yield. Anal. Calcd for C39H33CdN5O5: C, 61.25; H, 4.32; N, 9.16. Found: C, 61.22; H, 4.31; N, 9.18. IR (KBr, cm−1): 3114m, 2927m, 1672m, 1536W, 1253W, 1174m, 1027m, 932s, 852m, 772s, 706m, 658m. Photoluminescence Experiments. The finely ground sample (3 mg) was immersed in 2 mL corresponding solution, treated by ultrasonication for 30 min, and subsequently aged to make the suspension stable enough for measurement. X-ray Crystallographic Analysis. X-ray single-crystal data collection of 1−5 was obtained on a Bruker SMART APEX II CCD diffractometer equipped with a graphite monochromator using Mo Kα radiation (λ = 0.71073 Å) at 293 K. A multiscan technique was used to perform adsorption corrections. All of the structures were solved using direct methods and refined using the full matrix least-squares method on F2 with anisotropic thermal parameters for all non-hydrogen atoms using the SHELXL-97 program.14 All hydrogen atoms were located in calculated positions and refined isotropically. In 1, the disordered atoms O5A and O5B, and C19A and C19B of the methanol molecule were split over two sites. Meanwhile, C20, C21, C23, C24, O6, and N3 of the DMA molecule were disordered over two sites with occupancy of 0.5 and were anisotropically refined. In 3, four carbon atoms (C1, C2, C6, and C7) and four oxygen atoms (O1A, O1B, O2A, and O2B) of 1,4-ndc were disordered over two sites with occupancy of 0.5 and were anisotropically refined. In 2, 4, and 5, free solvent molecules were highly disordered, and we failed to locate and refine the solvent peaks. Then, we used the SQUEEZE routine of PLATON to remove the diffused electron densities resulting from the residual solvent molecules and made further refinements using the data generated.15 The final formulas of 2−5 were determined by the combination of elemental analysis, TGA data, and the SQUEEZE results. The crystallographic data for 1−5 are listed in Table 1. Moreover, the selected bonds distances and bond angles are summarized in Supporting Information Table S1. In addition, crystallographic data of 1−5 have been deposited in the Cambridge Crystallographic Data Center as supplementary publication with CCDC nos. 1471006− 1471010.

Figure 1. (a) Asymmetric unit of 1. (b, c) Three-dimensional architecture of 1. (d) Diagrammatic drawing of the six-connected pcu net for 1.

bonded by four bdc ligands and two dia ligands, further connecting six {Cd2} units, thus resulting in a six-connected pcu network with the point symbol of {412·63} (Figure. 1d). Structure of Compound 2. X-ray determination has revealed that 2 crystallizes in an orthorhombic system with Fdd2 space group. It exists as two Cd(II) ions, two dia ligands, two 1,4-ndc ligands, and one water molecule in the asymmetric unit, as illustrated in Figure. 2a. Cd1 is hexacoordinated in a distorted octahedral fashion, which is surrounded by three oxygen atoms from three 1,4-ndc ligands, two nitrogen atoms from two dia ligands, and one coordinated water molecule. The average Cd−O and Cd−N distances are from 2.295(3) to 2.395(3) Å and from 2.294(3) to 2.231(4) Å, respectively. The neighboring Cd(II) ions bonded by two 1,4-ndc ligands form a dinuclear {Cd2} unit (Scheme 2b). As shown in Scheme 1b, the two carboxylate groups of 1,4-ndc ligands present two different coordination modes: one carboxylate group employs bidentate bridging coordination fashion μ2-, while the other carboxylic group is coordinated to one Cd(II) ion using a single oxygen atom. Then the dinuclear {Cd2} units further connected via 1,4-ndc ligands to generate a 2D structure (Figure. 2b), which is further assembled into 3D self-penetrating networks by the connection of dia ligands, as depicted in Figure. 2c. The topological analysis revealed that the topology of 2 is a uninodal eight-connected net by TOPOS software as shown in Figure. 2d, with the point symbol {420·54·64} based on defining {Cd2} units as eight-connected nodes. Structure of Compound 3. X-ray determination has exhibited that 3 reveals a 3D framework, crystallizing in the monoclinic space group C2/c. The asymmetric unit contains one Cd(II) ion with half-occupancy, half of a 1,4-ndc ligand, and half of a dia ligand, as displayed in Figure. 3a. Cd1 exhibits a distorted octahedral coordination configuration, connected by four oxygen atoms from two 1,4-ndc ligands (Cd−O, 2.303(8)−2.342(8) Å) and two nitrogen atoms from two dia ligands (Cd−N, 2.245(7) Å). Finally, the adjacent Cd(II) ions are connected by 1,4-ndc ligands and dia ligands to assembly into a 3D framework with 1D channels along the c axis (Figure. 3b,c). Topologically, 3 is a 3-fold interpenetrated dia network with the Schäfli symbol of {66} as depicted in Figure. 3d. Structure of Compound 4. Signal-crystal structure analysis shows that 4 crystallizes in monoclinic space group P21/n. The asymmetric unit consists of one Cd(II) ion, one bpdc ligand, and half of a dia ligand as shown in Figure. 4a. The

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RESULTS AND DISCUSSION Structure of Compound 1. X-ray determination has revealed that 1 crystallized in the triclinic system with P1̅ space group. The asymmetric unit includes one Cd(II) ion, one coordinated bdc ligand, and a half dia ligand are shown in Figure 1a. Cd1 adpots a distorted pentagonal bipyramid geometry by connecting to five carboxyl oxygen atoms from three bdc ligands, one oxygen atom from methanol molecule, and one nitrogen atom from one dia ligand. The Cd−N bond distance is 2.234(6) Å, and the average Cd−O bond distance ranges from 2.291(5) to 2.489(2) Å. It is notable that there are two types of bdc ligands in 1: (i)-bdc and (ii)-bdc (Scheme 1a). The two carboxylate groups of (i)-bdc display bridging tridentate coordination mode μ2-η2-; meanwhile (ii)-bdc exhibits chelating bidentate coordination fashion η2-. In Scheme 2a, the neighboring Cd(II) ions are further linked together through four carboxylate groups of bdc ligands of which two carboxylate groups feature bridging tridentate coordination mode, and the other two possess chelating bidentate coordination fashion to form a dinuclear {Cd2} unit. Panels b and c of Figure 1 show that the {Cd2} units are linked by four bdc ligands to form a two-dimensional layer, then the neighboring layers are further connected to construct a threedimensional pillared-layer structure by using the dia ligands as pillars. The topological analysis revealed that each {Cd2} unit is C



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Scheme 1. Different Coordination Modes of Carboxylic Acid in 1−5a

a

Panel a: left, (i)-bdc; right, (ii)-bdc.

Scheme 2. Dinuclear Cd-Cluster Substructure of 1, 2, and 4

Figure 2. (a) Coordination environment of dinuclear Cd(II) in 2. (b) 2D layer in 2. (c) 3D sturcture of 2. (d) View of the eight-connected topology of 2. Figure 3. (a) Coordination environment of Cd(II) in 3. (b) Channels of 3 running along the c axis. (c) 3D porous structure in 3. (d) 3-fold interpenetrated net for 3.

Cd1 ion possesses a distorted octahedral fashion, coordinated by one nitrogen atom from one dia ligand and five oxygen atoms from three bpdc ligands. The Cd−N bond length is 2.237(3) Å, and the Cd−O distance ranges from 2.227(3) to 2.384(3) Å. The adjacent Cd(II) ions are bridged by four carboxylate groups from four bpdc ligands to form a {Cd2} unit (Scheme 2c). The {Cd2} units are connected by bpdc and dia ligands along the special direction to construct a 3D metal− organic framework with one-dimensional channel along the b axis and the c axis, as displayed in Figure. 4b,c. Topologically, the structure of 4 is a 3-fold interpenetrated pcu network when the dinuclear {Cd2} units defined six-connected nodes. Structure of Compound 5. Single-crystal X-ray diffraction analysis shows that 5 crystallizes in monoclinic space group C2/ m. As depicted in Figure 5a, in the asymmetric unit of 5, there are one Cd(II) ion, one bpdc ligand, and one dia ligand. The Cd1 ion possesses a distorted octahedral fashion, which consists of two nitrogen atoms from two dia ligands (Cd−N, 2.231(4) Å) and four oxygen atoms from two bpdc ligands (Cd−O, 2.245(5)−2.548(5) Å). The three-dimensional framework exhibited in Figure 5c is generated from the extension of

Figure 4. (a) Coordination environment of Cd(II) in 4. (b) Channels of 4 along the b axis. (c) Channels of 4 along the c axis. (d) 3-fold interpenetrated net for 4.

D



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collapsed and decomposed with the residue CdO being 17.79% compared to its calculated 16.75%. Luminescence Properties. The solid-state photoluminescence (PL) spectra of 3−5 and the free ligands (1,4-ndc, bpdc, and dia) were measured at room temperature. As exhibited in Figure S3, the free dia ligand exhibits emission at 474 nm upon excitation at 370 nm; meanwhile, the free 1,4-ndc and bpdc ligands show emission at 481 and 400 nm upon excitation at 322 and 338 nm, respectively. Compounds 3−5 all present similar emission peaks at 428 nm (λex = 380 nm), 434 nm (λex = 370 nm), and 425 nm (λex = 368 nm) (Figure S6), respectively, which all can be assigned to dia ligands.16 We investigated the luminescent properties of finely ground samples 3−5 in different organic solvents, including N,Ndimethylformamide (DMF), N,N-dimethylacetamide (DMA), methanol, ethanol, acetone, acetonitrile, dichloromethane, chloroform, ethyl acetate, toulene, and nitrobenzene (NB). Figure 6 shows the most distinct peculiarity is that PL

Figure 5. (a) Coordination environment of Cd(II) in 5. (b) Channels of 5 running along the b axis. (c) 3D structure of 5. (d) 3-fold interpenetrated net for 5.

the adjacent Cd ions by the linkage of bpdc ligands and dia ligands with 1D channels along the b axis (Figure. 5b). To simplify this network, the Cd1 ions can be defined as four-connected nodes, so the structure of 5 is a 3-fold interpenetrated dia network (Figure 5d). Overall, 1−5 are all three-dimensional networks constructed by Cd ions, dia ligands, and dicarboxylate acids. In addition, the dicarboxylate acids in 1−5 are different and exhibit different coordination modes. Furthermore, the Cd1 ion in 1 adopts a distorted pentagonal bipyramid geometry, whereas the Cd1 ions in 2−5 are all hexacoordinated in disordered octahedral coordination configurations. Meanwhile, the secondary building units are dinuclear Cd clusters in 1, 2, and 4 and the substructures in 3 and 5 are based on single Cd ions. 3−5 have 1D channels along a special axis. Lastly, from a topological point of view, all of the five compounds display uninodal topologies and 3−5 exhibit 3-fold interpenetrated networks. Thermogravimetric Analyses. The thermogravimetric analyses of 1−5 were measured under N2 atmosphere from 25 to 800 °C with a heating rate of 10 °C min−1. The corresponding results are shown in Figure S2. Compound 1 shows a slow weight loss of 8.46% before 355 °C, which corresponds to the loss of DMA molecules (calculated, 8.68%). Then, with the increase of temperature, the framework collapsed and decomposed. The residue is CdO (experimental, 25.51%; calculated, 25.28%). Compound 2 exhibits two separate weight loss steps, which are the loss of DMA molecules and the collapse and decomposition of the framework. The first weight loss is 6.32% (calculated, 6.24%). Lastly, the weight of CdO residue is 18.79% (calculated, 18.35%). Compound 3 reveals two weight loss steps, which correspond to the loss of methanol molecules (experimental, 4.63%; calculated, 4.78%) and the decomposition of the framework (experimental, 20.17%; calculated, 19.13%). Compound 4 shows two weight loss steps. The first loss step (25− 350 °C) corresponds to the loss of guest DMF molecules (experimenta,: 13.11%; calculated, 12.58%). The second weight loss step corresponds to the decomposition of the framework (experimental, 22.03%; calculated, 22.18%). Compound 5 displays a slow weight loss of 14.53% before 300 °C, which is attributed to the loss of DEF molecules (calculated, 13.24%). Then with the increase of temperature, the framework

Figure 6. (a) Emission spectra of 3 in different organic solvents. (b, c) Comparison of luminescence intensities of 4 and 5 in different organic solvents.

intensities are largely dependent on the solvents, particularly for NB which exhibits complete quenching effects. Because NB is toxic and detrimental to the environments, it is necessary to examine the sensing sensitivity of fluorescence quenching by NB in more detail. The PL spectra of 3−5 in DMF with gradually increasing NB concentrations were analyzed (Figure 7). The quenching efficiency is defined by (I0 − I)/I0 × 100%, where I0 and I are the luminescent intensities of the compound before and after the addition of the analyte, respectively. As a result, the quenching efficiencies of 3−5 were estimate to be E



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Figure 8. (a) Quenching efficiency of 3−5 in different NB concentrations. (b) Stern−Volmer plots of 3−5 with I0/I − 1 versus the NB concentration in DMF.

Figure 7. (a−c) Emission spectra of 3−5 in different concentrations of NB in DMF.

nitroaromatic explosives, for instance 1,4-dinitrobenzene (1,4DNB), 3-nitrophenol (3-NP), 2,4-dinitrotolunene (2,4-DNT), 2-nitrotoluene (2-NT), 3-nitrotoluene (3-NT), 1,3-dinitrobenzene (1,3-DNB), and 2,4,6-trinitrophenol (TNP). We analyzed the PL spectra of 3−5 emulsions in different nitro-explosives with identical amounts. As depicted in Figure 9, all of 3−5 exhibit different degrees of fluorescence quenching effects for the eight nitro-compounds; especially when in TNP solution, the luminescence signals almost completely disappear with the spectra red-shifted by 38, 33, and 40 nm for 3−5, respectively.19 Although all the nitro-explosives can weaken the emission intensities of 3−5, the order of quenching efficiency is obviously different (Figure 9). For 3, the order of quenching efficiency is TNP > 3-NP > 1,4-DNB > 2-NT > 2,4-DNT > 3NT > NB > 1,3-DNB. By contrast, the quenching efficiency of 4 is in the following order: TNP > 3-NT > 1,3-DNB > 3-NP > 2,4-DNT > NB > 2-NT > 1,4-DNB. Lastly, the quenching efficiency of 5 follows the order: TNP > 2-NT > 2,4-DNT> 3NT > 1,3-DNB > 3-NP > 1,4-DNB > NB. The high quenching efficiency for TNP prompted us to further investigate the sensibility of 3−5 for TNP. The fluorescent intensities of 3 decrease clearly upon incremental addition of TNP (Figure 10), and the intensity versus TNP concentration plots of 3−5 all are bent downward as shown in Figure 11. The quenching efficiency versus TNP concentration plots bend upward and reach nearly 100% at 50 ppm, which means complete quenching (Figure 12). We can see the Stern−Volmer plots of 3−5 are all nearly linear at low concentrations and the plots subsquently become bent upward at high concentrations from Figure S7. Such high sensitivity and selectivity may be attributed to photoinduced electron-transfer and energy-transfer processes. TNP has the lowest unoccupied

75.2%, 58.9%, and 56.9% at 600 ppm, of which 3 exhibits the best fluorescence quenching effect (Figure 8a). On the other hand, the different quenching efficiencies can be quantitatively explained by the Stern−Volmer equation: (I0/I) = KSV[A] + 1, where KSV is the quenching constant [M−1] and [A] is the molar concentration of the analyte.17 From Figure 8b, the Stern−Volmer plots of 3−5 indicate that the quenching efficiencies are gradually increased versus the NB concentration. The detection abilities of 3−5 can be restored by washing the sample with DMF several times. The PXRD patterns after five cycles of quenching−recovery measurements are unchanged and the PL intensities are hardly decreased, revealing that 3−5 are stable and recycleable in detecting NB, as shown in Figure S8. We have made the cyclic votammetry measurements that confirm the reduction potentials of 3−5 are −1.37, −1.49, and −1.57 V, respectively. Li and co-workers reported the reduction potential of NB is −1.15 V,18a which is more positive than those of 3−5, indicating that 3−5 act as electron donor in the case of NB. The possible quenching mechanism of NB could be assumed to be two factors.18 First, the samples of 3−5 are welldispersed in solutions with NB, which can make NB molecules be closely adhered to the surface of the material particles and possibly enhance the host−guest interactions. Second, the nitro groups of NB are electrowithdrawing, which results in electron transfer from electron-donating framework to the highly electron-deficient NB molecules. This process occurs on excitation, which makes the fluorescence quenching. The fluorescence quenching of NB urges us to investigate other nitro explosives. Furthermore, we select a variety of F



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Figure 11. (a−c) Luminescence intensities of 3−5 in different concentrations of TNP in DMF. Figure 9. (a) Emission spectra of 3 in DMF with different explosives. (b, c) Fluorescence intensity of 4 and 5 in DMF with different explosives.

Figure 12. Quenching efficiency of 3−5 in different TNP concentrations.

unchanged after five cycles and the PXRD patterns of the recovered sample after five cycles were sustained unchanged indicating high sensitivity, recyclability, and stability for the detection of TNP (Figure S9).

Figure 10. Emission spectra of 3 in different concentrations of TNP in DMF.

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molecular orbital (LUMO) energy of all the analytes studied,20 so it is the strongest electron acceptor in the excited state. When TNP is on excitation, electrons are transferred from the conduction band (CB) to the LUMO. On the other hand, the spectral overlap between the optical emission spectrum of the TNP is much greater than the overlap from the other nitro compounds. Finally, the short-ranged electron-transfer and long-ranged energy-transfer processes result in high quenching effect.21 In addition, the fluorescence intensity was almost

CONCLUSION In summary, we have successfully prepared five luminescent MOFs based on ligand 9,10-bis(1H-imidazol-1-yl)anthracene with a flat anthracene chromophore and three different rigid carboxylic acid ligands under different slovothermal conditions. Compounds 1−5 exhibit three-dimensional frameworks with various architectures. The luminescent properties of 3−5 are investigated systematically. Compounds 3−5 all show distinctly G



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solvent-dependent emission intensities. Furthermore, in the detection of explosives, 3−5 can be used as fluorescent sensors for detecting NB and TNP with high stability and recyclability, which indicates that 3−5 are outstanding candidates in the detection of explosives.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00482. Selected bond lengths and angles, PXRD, TGA, and emission spectra of ligands, 1,4-ndc, bpdc, and compounds 3−5, Stern−Volmer plots of 3−5 for TNP and figure of cycle times for NB and TNP (PDF) Accession Codes

CCDC 1471006−1471010 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*(Z.-M.S.) E-mail: [email protected]. *(L.Z.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSFC of China (Grant Nos. 21471027 and 21131001), the National Key Basic Research Program of China (Grant No. 2013CB834802), the Fundamental Research Funds for the Central Universities (Grant No. 2412015BJ001), and Changbai Mountain Scholars of Jilin Province.

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REFERENCES

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