A Luminescent Zinc(II) Metal–Organic Framework (MOF) with

Article pubs.acs.org/IC

A Luminescent Zinc(II) Metal−Organic Framework (MOF) with Conjugated π‑Electron Ligand for High Iodine Capture and NitroExplosive Detection Ru-Xin Yao,† Xin Cui,† Xiao-Xia Jia,† Fu-Qiang Zhang,† and Xian-Ming Zhang*,†,‡ †

School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, People’s Republic of China Institute of Crystalline Materials, Shanxi University, Taiyuan 030006, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: A porous luminescent zinc(II) metal−organic framework (MOF) with a NbO net [Zn2(tptc)(apy)2−x(H2O)x]·H2O (1) (where x ≈ 1, apy = aminopyridine, H4tptc = terphenyl-3,3″,5,5″-tetracarboxylic acid), constructed using paddlewheel [Zn2(COO)4] clusters and πelectron-rich terphenyl-tetracarboxylic acid, has been solvothermally synthesized and characterized. Interestingly, the material displays efficient, reversible adsorption of radioactive I2 in vapor and in solution (up to 216 wt %). The strong affinity for I2 is mainly due to it having large porosity, a conjugated π-electron aromatic system, halogen bonds, and electron-donating aminos. Furthermore, luminescent study indicated that 1 exhibits high sensitivity to electron-deficient nitrobenzene explosives via fluorescence quenching.

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INTRODUCTION Research on clean, safe nuclear energy has been growing rapidly, in order to reduce greenhouse gas emissions and meet increased worldwide energy demands.1 One urgent issue is the disposal of nuclear waste products. As one important radioisotope in nuclear waste, 129I has attracted particular attention, because it has a long radioactive half-life (1.57 × 107 years), is difficult to deal with, and threatens human health.2 Therefore, the effective capture of radioactive iodine in nuclear effluents remains a challenge. In this respect, several adsorbents for iodine have been studied in the last few decades, including chalcogenide aerogels,3 functionalized clays, and silver-based porous zeolitic materials, with the loading amount varying from 8 wt % to 175 wt %.4 However, their application has been restricted by expensive costs, limited adsorption ability, or environmental issues. Thus, materials with steady high-iodine trapping ability are actively being sought. Porous metal−organic frameworks (MOFs), as an excellent platform for host−guest chemistry, have been utilized to controllable capture and release of various analytes including drugs, explosives, and radioactive wastes,5,6 mainly because of their adjustable pore structures, functionalities, and large surface areas. However, MOFs materials with high iodine uptake capacity remain very limited. It is noted that the adsorption capacity for iodine is related to not only the pore size, but also the effective sorption sites, the interaction of I2 and framework, and the effect of I2 and I2 (may form polyiodide anions). One effective strategy to create MOFs with high iodine affinity is the introduction of conjugated π-electron organic ligands or electron-pair donors pyridine, ROH, R2O, RNH2 and © XXXX American Chemical Society

so on, which could produce stable charge-transfer (CT) complexes or halogen bonds with iodine, thus increasing the adsorption amount for iodine.7−9 For instance, a double-walled MOF with a rigid organic ligand has been reported by Zeng’s group, who presented the iodine loading and release dynamics.7 The exceptional affinity for I2 (100 wt %) may be attributed to the regular π-electron walls of pybz. Recently, conjugated microporous polymer nanotubes were reported as absorbents with high I2 adsorption capacity (208 wt %).8b Zhu’s group reported a series of novel charged PAF materials with three efficient adsorption sites, which exhibit the highest adsorption values (271, 276, and 260 wt %) for iodine among all porous materials.1 Initiated by Chen and Schröder, an porous threedimensional (3D) framework with a NbO netMOF-505is of particularly interest; this is constructed from paddlewheel dinuclear copper clusters and tetracarboxylates ligand with mbenzenedicarboxylate moieties.10,11 Because of the intriguing interactions with unsaturated metal sites and objective adsorbates, as well as the adjustable cage sizes and surface areas, these materials exhibit high adsorption capacities for small molecule gases (H2, CO2, CH4, or C2H2). However, radioactive iodine capture and explosives detection are a few of the applications that have been explored using these aromatic MOFs. We previously reported a pyridyl-decorated MOF-505 material with hierarchical porosity, exhibiting large CO2 uptake capacity and high CO2/N2 selectivity.12 Inspired by the work, Received: May 31, 2016

A

DOI: 10.1021/acs.inorgchem.6b01312 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry combining luminescent zinc paddlewheel SBUs with πelectron-rich terphenyl-tetracarboxylic acid ligand, we successfully synthesized a novel multifunctional MOFs [Zn2(tptc)(apy)2−x(H2O)x]·H2O (1) (where x ≈ 1, H4tptc = terphenyl3,3″,5,5″-tetracarboxylic acid, and apy = aminopyridine) with a NbO net under solvothermal conditions. Three phenyl rings, strong electron-donator aminos, unsaturated ZnII sites, and luminescent properties make the material exhibit exceptional performance for iodine and nitrobenzene explosives.

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EXPERIMENTAL SECTION

Synthesis of [Zn2(tptc)(apy)2−x(H2O)x]·H2O (1). Zn(Ac)2·6H2O (0.100 g, 0.30 mmol), H4tptc (0.040 g, 0.10 mmol), apy (0.010 g, 0.10 mmol), DMA (5 mL), and H2O (1 mL) was stirred and added dropwise (10 drops) into a HNO3 solution (6 mol/L). White block crystals of 1 were obtained after 5 days at 100 °C (75.6% yield). Anal. Calcd (%) for C25.7H14.8N2Zn2O8.5 (1): C, 49.92; H, 2.41; N, 4.53. Found: C, 49.95; H, 2.38; N, 4.55. IR (KBr, cm−1; see Figure S1 in the Supporting Information): 3437(b), 2925(w), 1628(s), 1522(w), 1455(w), 1402(m), 1355(m), 1267(w), 1214(w), 1115(w), 1057(w), 1031(m), 780(w), 717(w), 597(w). Detailed structure parameters and crystallographic data are summarized in Table 1, and the corresponding bond lengths and bond angles are shown in Table S2 in the Supporting Information.

Table 1. Refinement Parameters and Crystal Data of 1

a

parameter

value

formula formula weight, Fw crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalc (g cm−3) μ (mm−1) F(000) size (mm) No. of reflections Tmax/Tmin Rint No. of data/parameters S R1,a wR2b[I > 2σ (I)] R1,wR2 (all data) Δρmax/Δρmin (e Å−3)

C25.7H13.8N2Zn2O8 617.74 trigonal R3̅m 18.8317(10) 18.8317(10) 38.5698(13) 90 90 120 11845.6(13) 9 0.779 0.938 2792.0 0.28 × 0.26 × 0.22 2961 0.745/1.000 0.0507 2961/125 1.037 0.0582, 0.1623 0.0714, 0.1712 0.93/−0.49

Figure 1. Depictions of (a) the asymmetric unit of 1, (b) a small pore (9.9 Å × 17.0 Å), (c) a larger fusiform pore (18.8 Å × 24.7 Å), (d) the three-dimensional (3D) framework of 1, and (e) the NbO topology network (for the sake of simplicity, apy and the partially occupied H2O molecule are represented by a terminal N atom).

carboxylates ligands, and each ligand is linked by four paddlewheel clusters, thus forming an NbO-type framework with a short Schläfli symbol of (6482) (Figure 1e). There exist two types of pores with D3 symmetry (Figure 1d), which are alternately stacked to form a 3D structure. The larger fusiform pore (18.8 Å × 24.7 Å) is surrounded with 12 [Zn2(COO)4] SBUs, while small pores (9.9 Å × 17.0 Å) are encapsulated by six [Zn2(COO)4] SBUs (Figures 1b and 1c). PLATON calculation shows that solvent accessible volume of the channel is 58.1% of the total volume (6880.0 Å3 out of the 11845.6 Å3 per unit cell volume). Both thermogravimetric analysis (TGA) and X-ray diffraction (XRD) show good stability. As shown in TGA (Figure S3 in the Supporting Information), we can see that the structure remains stable below 263 °C, which is in agreement with variable-temperature PXRD data (Figure S4 in the Supporting Information). Capture and Release of Radioactive Iodine. Complex 1 has excellent porous character and remarkable stability, which prompted us to investigate absorption for I2 molecules, because of environmental issues in the nuclear energy.13,14 Highly efficient I2 enrichment of 1 was successfully performed (the loaded material is denoted as 1@I2, as indicated in the energydispersive spectroscopy (EDS) plot in Figure S5 in the Supporting Information) by the approach of I2 vapor at ordinary pressure 75 °C (typical conditions of fuel reprocessing). Activated crystals were placed in a preweighed vial, and then they were sealed at 75 °C under ambient pressure. The crystals color progressively transformed to black (Figure 2a). As shown in Figure 2b, the adsorption ability of 1 was very quick originally, and then no further change was observed after 50 h,

R1 = ∑||F0| − |Fc||/∑|F0|. bwR2 = [∑w(F02 − Fc2)2/∑w(F02)2]1/2.

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RESULTS AND DISCUSSION Complex 1 crystallizes in a trigonal R3̅m space group. The Zn2+ is coordinated by four oxygen atoms and one nitrogen atom (or one terminal H2O molecule) forming a square pyramidal shape (Figure 1a). Two Zn2+ ions are linked by four carboxylate oxygen atoms, generating paddlewheel dinuclear [Zn2(COO)4] secondary building units (SBUs), with one terminal apy or H2O molecule at the paddlewheel axis (Figure S2 in the Supporting Information). Each SBU is bridged by four terphenylB

DOI: 10.1021/acs.inorgchem.6b01312 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) Change in the adsorbed amount of I2, relative to time; (b) photographs of crystals before and after I2 adsorption.

neglected halogen bonds.15b As stated by Jin et al, halogen bonds are directional noncovalent interactions between the σhole or π-hole and the negative or electron-rich sites, which have not been taken into consideration in GCMC molecular force field simulations. Performing precise stimulation of I2 adsorption in 1 with a large crystal cell and more than 600 atoms using a quantum ab initio method requires massive calculation, and this is not available to us at present. In addition, iodine uptake of 1 was further investigated in solution. After activated crystals (30 mg) of 1 were soaked in a cyclohexane solution of I2, the dark purple iodine solution faded to pale red and the crystal color deepened gradually (see Figures 4a and 4b) over time. The adsorption kinetics of I2 was monitored using UV-vis spectroscopy (Figure 4c). Initially, the adsorption of I2 increased rapidly, then slowly increased until saturation was reached, which can be fitted by Lagergren pseudo-second-order kinetic models (see Figures S7 and S8 in the Supporting Information). A removal efficiency of 80.6% was achieved for iodine solution, which far surpassed that of reported MIL-53-NH2 (60%).17 In comparison, the uptake of iodine on 1 remarkably exceeds that of commercial activated carbon and zeolite 13X in a cyclohexane solution (see Figure S9 in the Supporting Information). Poor X-ray diffraction (XRD) results of the crystal were obtained; although the original features were retained upon capturing iodine, we cannot successfully obtain the crystal data of loaded I2 (see Figure S10 in the Supporting Information). Most of the peak positions in the powder XRD analysis of 1@I2 are similar to that of the compound, indicating that the host framework is preserved, but intensity and width of peaks are different (see Figure S11 in the Supporting Information). Note that the I2 sorption process for 1 is invertible. By immersing 1@I2 in fresh ethanol, the adsorbed I2 can be readily removed from the framework. The crystal color changed gradually from black, to yellow, to colorless, being opposite to the progress observed for captured I2, while that of the ethanol solution deepened little by little (see Figure S12 in the Supporting Information), indicating that 1 is recyclable in the practical application of I2 capture. UV-vis spectra were further investigated the delivery of I2 from 1@I2 with time. It increases linearly before 80 min, as zero-order equation empirically adjusted, which is managed by the host−guest effect (see Figure 5). Photoluminescence and Sensing. As shown in Figure S13 in the Supporting Information, the luminescent spectra of free ligand H4tptc and 1 have been investigated in the solid state. Compared with H4tptc, 1 exhibits enhanced blue emission peaks at 400 and 464 nm with 319 nm excitation, which can be mainly ascribed to ligand-centered electronic transitions perturbed by metal ions,18 because similar emissions

which was indicatative that the system had reached equilibrium. For 1, the amount of I2 loading was 216 wt %, which is close to the value indicated by TGA on 1@I2 (Figure S3 in the Supporting Information). Furthermore, the adsorption value is similar to the estimated value of 227%, according to the density of I2 (4.93 g cm−3) and the pore volume of 1 (0.46 cm3 g−1), which is one of the maximum values in all reported porous adsorbents (zeolites, MOFs, and POFs; see Table S1).15,16 The high affinity of 1 can be ascribed to plentiful phenyl rings, large porosity, and strong electron-donor aminos, which can be interpreted as follows. First, iodine molecules can be adsorbed in the inner of pore using I−I···N and I−I···π halogen bonds.15a Furthermore, the iodine molecules can form intermolecular I− I···I−I halogen bonds, so as to connect firmly. The cooperation effect of three types of halogen bonds largely improves the adsorption ability of I2. The adsorbed I2 species remains neutral, which is confirmed by XPS of 1@I2 (Figure S6 in the Supporting Information). Grand Canonical Monte Carlo (GCMC) simulations were adopted13 to study the interactions of I2 and the host framework 1. A model with Lennard-Jones potential and electrostatic was simulated in the system. Preferential arrangements of I2 within the cages are shown in Figure 3, because of

Figure 3. Uptake of 40 molecules in 1@I2; each structure cell has been optimized using Grand Canonical Monte Carlo (GCMC) simulations.

the space steric hindrance of apy, but strong electron-donating aminos may enhance the interaction of I2 and pyridine ring. The strong interactions occur between I2 and the carbonyl oxygen atoms of the Zn paddlewheel cluster, I2, and phenyl ring of the π-electron-rich terphenyl-tetracarboxylic acid ligand. Calculations revealed that the maximal uptake is 40 I2 molecules per unit cell, which is lower than the experimental value (47 I2 molecules). This slight difference between the experimental and GCMC simulated numbers may be due to C

DOI: 10.1021/acs.inorgchem.6b01312 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) Series of photomicrographs showing the change in crystal color over time; (b) series of photomicrographs showing the color change of cyclohexane solution of I2 over time; and (c) iodine adsorption kinetics plot of 1.

Figure 5. UV-vis spectra of I2 released over time in ethanol. Inset shows a plot describing the controlled delivery of I2 ([I2] = Kt) in the first 1.5 h.

Figure 6. Fluorescence spectra of 1 with different concentrations of NB. Inset shows photographs of the liquid suspension (a) in the absence of NB and (b) the presence of NB (20 ppm).

where KSV is the Stern−Volmer constant (KSV = 4860 M−1), F0 and F are the respective luminescence intensities before and after the addition of NB, and C is the molar concentration of NB. These observations indicate that 1 may efficiently sense trace amounts of NB explosive and has promising potential for application as an explosives detector. Thermal Properties. As for 1 (Figure S3), the first weight loss of 3.2% within 90 °C can be assigned to the loss of terminal apy or H2O molecules. The second weight loss of 31.41% (calcd: 28.92%) is attributed to the removal of tptc4− ligand below 270 °C, following the decomposition of the framework. The ultimate residue for 1 (23.84%) is close to the percentage of zinc oxide (calcd 26.18%). 1@I2 displayed an obvious larger mass loss than 1 from RT to 500 °C, corresponding to the release of I2 and the collapse of the framework. The final residue of 8.60% for 1@I2 is in agreement with the percentage of zinc oxide (calcd 8.28%).

are detected at 415 and 464 nm (λex = 327 nm) for free ligand H4tptc. The fluorescent lifetime of 1 (1.46 μs) was increased, compared to that of free ligand (1.03 μs); in ethanol solution, the quantum yield (ϕPL) is 23.50%. The π-electron-rich feature stimulated us to investigate the sensing behavior of 1 toward nitro-aromatic explosives.5,19 Compared with various spectroscopic and analytical techniques, fluorescence is a sensitive, simple, and rapid detection method.20 Protonic and aromatic solvents were selected, namely, pyridine, 2-propanol, water, acetonitrile, DMF, 1,4dioxane, nitrobenzene (NB), and 1,2,4-trichlorobenzene. Notably, the fluorescence spectra enhance or quench (Figure S14 in the Supporting Information) with the polarity of solvent modified, because of the donor−acceptor electron transfer mechanism.21a,22 Fluorescence is remarkably different, by a factor of at least 100, in 2-propanol and NB. Fluorescence titration experiments were performed to study the sensitivity exhibited by a liquid suspension of 1 dispersed in 2-propanol via a gradual addition of 0.01 mmol/L NB stock solution (Figure 6). Interestingly, 1 exhibits an extremely high quenching efficiency (QE) of 96% for 20 ppm of NB in 2propanol solution. QE is defined as QE (%) = 1 −

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CONCLUSIONS In summary, a novel porous luminescent metal-organic framework (MOF) with a NbO net has been successfully synthesized, constructed using π-electron-rich terphenyl3,3″,5,5″-tetracarboxylic acid and paddlewheel [Zn2(COO)4] clusters, which displays excellent I2 adsorption capacity in vapor and in solution, which is due to large porosity, effective sorption sites, halogen bonds, and charge-transfer interactions. Furthermore, it exhibited extremely high detection sensitivity to nitrobenzene explosives (down to the ppm level). The present work suggests that the dual functional material may be a promising high-capacity absorbent for radioactive iodine and a

F F0

with F = KSVC + 1 F0 D

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sensitive luminescence sensor of nitro-explosives, with regard to environmental and security concerns.

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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.inorgchem.6b01312. UV-vis spectra in cyclohexane solution and ethanol solution, TGA plots, and powder XRD data (PDF) Crystallographic data (CCDC No. 1439878) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the 973 Program (No. 2012CB821701), the National Natural Science Foundation (Nos. 20925101, 21271125 & 21401119), a Plan for 10 000 Talents in China, Ministry of Education of China (No. IRT1156) and is greatly appreciated.

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REFERENCES

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b01312 Inorg. Chem. XXXX, XXX, XXX−XXX