Luminescent Anionic Metal–Organic Framework with Potential

Jun 9, 2014 - Highly Selective Bifunctional Luminescent Sensor toward Nitrobenzene ..... based on 1-(imidazo-1-ly)-4-(1,2,4-triazol-1-ylmethyl)benzene...
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Luminescent Anionic Metal−Organic Framework with Potential Nitrobenzene Sensing Yuan-Chun He,† Hong-Mei Zhang,† Ying-Ying Liu,*,† Qiu-Yi Zhai,† Qiu-Tong Shen,† Shu-Yan Song,*,‡ and Jian-Fang Ma*,† †

Key Lab of Polyoxometalate Science, Department of Chemistry, Northeast Normal University, Changchun 130024, P. R. China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China

‡

S Supporting Information *

ABSTRACT: Solvothermal reaction of multidentate organic ligand, 5,5′,5″,5‴,5],5]′-[1,2,3,4,5,6-phenylhexamethoxyl]hexaisophthalic acid (H12L), with Cd(NO3)·4H2O produced an anionic 3D metal−organic framework [(CH3)2NH2]4[Cd3(H2L)] (1), which features a novel cageto-cage connection. Interestingly, the 1D channel, which is stacked by cages, is divided into uniform segments by molecular-scale bricks. The luminescent properties of compound 1 have been explored, which shows that 1 is a potential luminescent sensory material for nitrobenzene.

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Scheme 1. View of the H12L Ligand in Compound 1

etal−organic frameworks (MOFs) as a kind of multifunctional material have attracted much interest because of potential applications such as gas storage/separation, catalyst, magnetism, and sensing.1 Among various applications, studies on luminescent MOFs for sensing small molecules have developed significantly because of its quick response, high sensitivity, and reversibility.2 As one of the important chemical intermediates, nitrobenzene has been widely used for dye, pharmaceuticals, and pesticide.3 It is also a well-known explosive and highly toxic contaminant with broad range of detriment.4 Usually, canines and sophisticated instrumental methods are used to detect nitrobenzene, but these methods are inconvenient and not always available.5 Therefore, the sensitive and selective detection of nitrobenzene is very necessary. Up to now, some luminescent MOFs for sensing nitrobenzene have been reported.6 For example, Chang et al. reported a high sensitivity luminescent MOF [NH2(CH3)2]2[Cd17(L)12(μ3-H2O)4(DMF)2(H2O)2]·solvent (H3L = 2,4,6-tri[1-(3-carboxylphenoxy)ylmethyl]mesitylene and DMF = N,N-dimethylformamide), which quenched at 100 ppm with a high quenching efficiency of 92.5%. To the best of our knowledge, it is the most sensitive for nitrobenzene detection at present.6d Here, we report a novel anionic-MOF, [(CH3)2NH2]4[Cd3(H2L)] (1) (H12L = 5,5′,5″,5‴,5],5]′[1,2,3,4,5,6-phenylhexamethoxyl]hexaisophthalic acid; Scheme 1). H12L contains six flexible “−H2C−O−” bonds, so the six phenylene groups of the outer edge can twist freely in the reaction. In addition, 12 carboxyl groups can offer enough oxygen atoms to meet the coordination needs of metal ions. As a d10 metal, Cd(II) ion is difficult to oxidize or reduce because the d → d electron transition cannot occur. Nevertheless, © XXXX American Chemical Society

luminescence properties of quite a lot of Cd(II) complexes induced by organic ligands have been detected.7 The luminescence property of 1 in the solid state and its further investigation of nitrobenzene emulsion are demonstrated. The quenching mechanism for nitrobenzene detection is discussed. Compound 1 was successfully synthesized with H12L and Cd(NO3)·4H2O in DMF at 95 °C for 3 days. The purity of the Received: March 28, 2014 Revised: May 16, 2014

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Figure 1. (a) Coordination environments of the Cd(II) ions in 1. (b) View of the octahedral cage. (c) 3D framework built by octahedral cages. (d) Augmented form of framework, where the red octahedron is the 6-c [Cd2(COO)6] unit, magenta tetrahedron is the 4-c [Cd(COO)4] unit, the 6-c blue hexagon is the central benzene of H2L, and the 3-c green triangle is the branch benzene of H2L.

Single crystal X-ray analysis reveals that 1 crystallizes in the tetragonal space group P42/ncm. As shown in Figure 1a, the asymmetric unit of 1 contains three-quarters Cd(II) ion and a quarter organic H2L anion, and the negative charge of the framework is balanced by one (CH3)2NH2+ cation, which was generated from the decomposed DMF. Cd1 is eightcoordinated by carboxylate oxygen atoms from four H2L anions to form a 4-connected [Cd(COO)4] unit (Supporting Information Figures S3a and S4). Cd2 is six coordinated by carboxylate oxygen atoms from five H2L anions. Two symmetry related Cd2 ions are bridged by carboxylate groups, forming a 6-connected [Cd2(COO)6] paddlewheel unit (Supporting Information Figures S3b and S5). Each partly deprotonated H2L anion coordinates to 14 Cd(II) ions, with six methoxylisophthalate arms adopting two kinds of coordination modes: μ3-η1:η1:η1:η1 and μ1-η1:η1:η0:η0 (Supporting Information Scheme S1). In this way, two [Cd(COO)4] units and two [Cd2(COO)6] paddlewheel units are linked by two H2L anions to form an octahedral cage (Figures 1b and S6 in Supporting Information). By sharing these two kinds of Cd(II) units as vertexes, the cages are linked through H2L anions to generate a fascinating 3D porous framework with two mutually connected open channels (Figures 1c and S7 in Supporting Information). The smaller triangle channels (ca. 5.1 × 5.1 × 7.9 Å3) can be observed along the a or b axis (Supporting Information Figure S8). The large channel (ca. 10.8 × 10.8 Å2 by ignoring the suspending groups) can be discovered along the c axis, which is protected by protonated carboxyl groups (Figure S9). If considering the large channel as an idealized tube, the

Figure 2. Solid-state emission spectra of H12L and 1 at room temperature.

samples was proven by the powder X-ray diffraction pattern, which matches well with the simulated one (Supporting Information Figure S1). Thermogravimetric (TG) measurement was carried out from room temperature to 600 °C under nitrogen flow (Supporting Information Figure S2). The molecular formula of 1 was calculated and confirmed by the elemental analysis data and thermogravimetric analyses, because the SQUEEZE function in PLATON was applied.8 B

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Figure 3. (a) Emission spectra of 1 in different solvents. (b) LUMO and HOMO of nitrobenzene and ligand.

protonated carboxyl groups can be regarded as molecular-scale bricks. These bricks divide large channel into uniform segments. The famous HKUST-1 also displays a 3D porous framework based on octahedral cages.9 In HKUST-1, the six vertices of the octahedral cage are the same units, while compound 1 contains three kinds of vertices, which are [Cd(COO)4] units, [Cd2(COO)6] paddlewheel units, and the central benzene rings of H2L anions. Another main difference lies in the channels. Due to the existence of the molecular-scale bricks, mutually connected open channels are formed in 1 instead of the continuous channels as that in HKUST-1. To better understand the structure of 1, topology analysis is used to simplify the 3D framework.10 Because of the large structure of the H2L anion, its central and branch benzenes are classified as 6- and 3-connected nodes, respectively (Figure 4d).10b On the basis of this simplification, the 3D framework can be seen as a (3-c)4(4-c)(6-c)2 net with a point symbol of (52·6)4(54·62)(54·66·83·92)2.10a Cd(II)-containing MOFs have attracted intense interest for their potential prospects in chemical sensors, photochemistry, and electroluminescent display.11 The photoluminescence spectra of 1 as well as ligand H12L in the solid state were measured at room temperature (Figure 2). Due to seven conjugative groups, H12L exhibits strong emission peak at 440 nm (λex = 330 nm). This may be assigned to the π* → n or π* → π transition.12 Compound 1 shows emission peak at 446 nm (λex = 348 nm). The emission peak of 1 is similar to that of H12L ligand, so its emission may originate from the intraligand fluorescent emission of the H2L anion.13

Figure 4. (a) Emission spectra of 1 at different nitrobenzene concentrations. (b) Emission intensity at different nitrobenzene concentrations.

Figure 5. Emission spectra of 1 in toluene, ethylbenzene, isopropylbenzene, and nitrobenzene.

To examine the potential of compound 1 toward sensing of small molecules, its luminescence properties were investigated in different solvent emulsions. Two milligrams of 1 was ground down, and then immersed in different organic solvents. After treatment by ultrasonication for 30 min, the samples were suspended in solvents and so made into emulsions. The organic solvents used are acetonitrile, DMF, methanol, ethanol, C

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acetone, tetrahydrofuran (THF), dichloromethane (CH2Cl2), and nitrobenzene. Compared with 1, the emission positions of the emulsions do not change very much (Figure 3a). The emission intensities decrease in different degrees when the former seven solvents were used. In contrast, the nitrobenzene emulsion displays the most obvious quenching effect. The luminescence quenching may be due to the photoinduced electron-transfer mechanism. The electron-transfer progress can be interpreted by inductive effect. The nitrobenzene with electron-deficient property can obtain an electron from excited ligand,14,6d which has been confirmed by molecular orbital theory.6f The frontier molecular orbitals of nitrobenzene and the ligand are calculated by density functional theory at the level of B3LYP/6-31G*.6f The LUMO of nitrobenzene is a lowlying π*-type orbital stabilized by the −NO2 through conjugation, so it is lower than LUMO of ligand (Figure 3b). Therefore, the excited state electrons can transfer from MOF to nitrobenzene, which leads to luminescence quenching. The sensing properties of 1 for nitrobenzene were further investigated by monitoring a series of emulsions of 1 in DMF with gradually increased nitrobenzene concentration. As shown in Figure 4a, the luminescence intensity gradually decreases with increasing nitrobenzene concentration. At a concentration of 1000 ppm, the luminescence intensity is nearly completely quenched with a high quenching efficiency of 92% (Figure 4b). The result suggests that compound 1 has good sensitivity in detecting small amounts of nitrobenzene in solution compared with most reported structures. 14b,6f For all we know, [NH2(CH3)2]2[Cd17(L)12(μ3-H2O)4(DMF)2(H2O)2]·solvent, which has a quenching efficiency of 92.5%, is the most sensitive for nitrobenzene detection.6d There is a certain gap between compound 1 and [NH2(CH3)2]2[Cd17(L)12(μ3H2O)4(DMF)2(H2O)2]·solvent. To further verify the effect of the nitro group on sensing of nitrobenzene, emission spectra of 1 in toluene, ethylbenzene, and isopropylbenzene were measured (Figure 5). Obviously, the luminescence intensity of the nitrobenzene emulsion is far below those of others in the benzene series. The result reveals that the nitro group is the main cause of the quenching effect. In addition, the emission peaks of 1 in toluene, ethylbenzene, and isopropylbenzene show blue-shift with increasing nonpolarity of the solvent.15 In summary, a novel anionic-MOF 1 was synthesized using multidentate organic carboxylic ligand H12L and Cd(NO3)· 4H2O under solvothermal condition. Compound 1 features a novel cage-to-cage connected 3D framework with 1D channels. The channel partitions are achieved through molecular-scale bricks. The luminescent property of 1 displays selectivity and sensitivity to nitrobenzene. The result shows that 1 may be used for nitrobenzene sensing application.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21071028, 21001023, 21277022, 21371030, 21301026) and the Fundamental Research Funds for the Central Universities of China. Thanks for the referees’ instructive suggestion. And we thank Dr. ShiZheng Wen and Wei-Qiu Kan (HuaiYin Normal University) for calculating the frontier molecular orbitals of nitrobenzene and ligand.

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Experimental process, CIF files, PXRD patterns, TGA, figures, and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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