Article pubs.acs.org/IC
Two 3D Isostructural Ln(III)-MOFs: Displaying the Slow Magnetic Relaxation and Luminescence Properties in Detection of Nitrobenzene and Cr2O72− Rui-Cheng Gao, Fu-Sheng Guo, Nan-Nan Bai, Yun-Long Wu, Fan Yang, Ji-Ye Liang, Zhen-Jing Li, and Yao-Yu Wang* Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710127, P. R. China S Supporting Information *
ABSTRACT: Two new three-dimensional isostructural lanthanide metal− organic frameworks (Ln(III)-MOFs), [LnL(H2O)3]·3H2O·0.75DMF (1Ln; Ln = Dy(III) and Eu(III) ions, H3L = biphenyl-3′-nitro-3,4′,5tricarboxylic acid, DMF = N,N′-dimethylformamide), were synthesized and characterized. The appearance of temperature-dependent out-of-phase (χ″M) signal reveals that complex 1-Dy displays slow magnetic relaxation behavior with the energy barrier (ΔUeff) of 57 K and a pre-exponential factor (τ0) of 3.89 × 10−8 s at 1200 Oe direct current field. The luminescence explorations demonstrated that 1-Eu exhibits high quenching efficiency and low detection limit for sensing nitrobenzene and Cr2O72−. Meanwhile, the fluorescence intensity of the quenched 1-Eu samples will be resumed after washing with DMF or water, indicating that 1-Eu may be used as a highly selective and recyclable luminescence sensing material for sensing nitrobenzene and Cr2O72− anion.
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INTRODUCTION Lanthanide metal−organic frameworks (Ln(III)-MOFs), selfassembled from lanthanide metal ions with organic linkers, are a very promising class of functional materials due to significant magnetic anisotropy or excellent luminescence from the unique 4f orbit of Ln(III) ions.1 In magnetic researches, Dy(III) ion, which has a big magnetic moment and a significant magnetic anisotropy, is always considered as a good candidate to design and obtain a single-molecule magnet (SMM).2 SMMs are of interest because of the potential application in high-density magnetic memories and quantum computing,3 which commonly connects inevitably to their unique physical properties, such as freezing of the magnetization below the so-called “blocking temperature” (TB), quantum tunnelling of the magnetization (QTM), and quantum phase interference.4 Although there are a great deal of SMMs and single-ion magnets in literature reports synthesized by Dy(III) ion, it remains a big challenge for the design and assembly of Dy(III)MOFs displaying SMM behaviors.5 The two main obstacles are complex coordination environment and the diverse geometry of Dy(III) ion in the MOFs.6 Meanwhile, as we all know, most Ln(III)-MOFs may display narrow and typical fluorescent emissions due to the internal 4f−4f electron transitions of Ln(III) ions, such as Eu(III) and Tb(III). The unique property prompts researchers to explore it as a fluorescent sensor.7 Ln(III)-MOFs sensors with its excellent optical properties through an “antenna effect”, for instance, large Stokes shifts, © XXXX American Chemical Society
and high color purity, take a great advantage of other MOFsupported sensors.8 In previous studies, our group performed many investigations on Ln(III)-MOFs supported sensors, and all of them displayed high sensitivity and excellent selectivity.9 However, compared with other environment factors such as solvent, temperature, pH value, etc, the central metal ions and ligands are considered as two main factors that influence the structures and properties of coordination complexes.10 To construct desired high-dimensional structural MOFs, rigid ligands have been extensively utilized and explored.11 In previous researches, rigid poly(carboxylic acid) ligands are usually used to primarily construct rigid metal-carboxylate chains. Then, the chains as rod-shaped secondary building units (SBUs) are very favorable to produce high-dimensional frameworks, which can keep their robust frameworks after the removal of guest molecules.10−12 These characteristics are all helpful on building functional materials with possible applications in catalysis, gas separation, gas storage, ion exchange, and magnetism. On the basis of these discussed aspects above, herein, the biphenyl-3′-nitro-3,4′,5-tricarboxylic acid (H3L) was chosen as the organic linker with the lanthanide ions (Dy(III) and Eu(III)) to construct the Ln(III)-MOFs with magnificent architectures and fascinating functions. Fortunately, two new Received: August 5, 2016
A
DOI: 10.1021/acs.inorgchem.6b01899 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry three-dimensional (3D) isostructural Ln(III)-MOFs: [LnL(H2O)3]·3H2O·0.75DMF (1-Ln) were successfully synthesized via solvothermal reaction. The magnetic investigation showed that 1-Dy exhibits slow magnetic relaxation behavior with an energy barrier (ΔUeff) of 57 K. And luminescence studies indicated that 1-Eu displays the quenching effect on nitrobenzene and Cr2O72−, which may be used as a chemical sensor in detecting these substances.
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EXPERIMENTAL SECTION
Materials and General Methods. Commercially available starting materials and reagents of analytical grade were used as received. And the H3L ligand was bought from Jinan Camolai Trading Company. Infrared spectra were recorded in the range of 400−4000 cm−1 on EQUINOX55 FT-IR spectro-photometer. Elemental analyses (C, H, and N) were performed on a PerkinElmer 2400C elemental analyzer. Thermogravimetric analyses (TGA) were performed under the protection of N2 using a Netzsch TG209F3 instrument at a heating rate of 10 °C min−1. The powder X-ray diffraction (PXRD) was collected by using a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, λ = 1.5418 Å) with 2θ (5−50°). A luminescent spectrum was measured on an Edinburgh FLS55 luminescence spectrometer. Magnetic measurements were measured by using a Quantum Design MPMSXL-7 SQUID magnetometer on polycrystalline samples. [LnL(H2O)3]·3H2O·0.75DMF (1-Ln). A mixture of Ln(NO3)3·6H2O (0.15 mmol), H3L (0.0166 g, 0.05 mmol), H2O (2 mL), and DMF (2 mL) was sealed in a 25 mL Teflon-lined stainless steel container. After that the vessel cooled to room temperature with a rate of 5 °C h−1, and finally the block crystals of 1-Ln were obtained. [DyL(H2O)3]·3H2O·0.75DMF (1-Dy). Yield. 64.70% Calcd for C17.25H23.25N1.75O14.75Dy: C, 31.70; H, 3.58; N, 3.75. Found: C, 31.45; H, 3.43; N, 3.91. IR (KBr, cm−1; Figure S2a): 3754w, 3602m, 2417m, 2921m, 2856w, 1670s, 1615s, 1552s, 1455s, 1392s, 1249m, 1105m, 790m, 724m, 583w. 470w. [EuL(H2O)3]·3H2O·0.75DMF (1-Eu). Yield. 57.83%. Calcd for C17.25H23.25N1.75O14.75Eu: C, 32.21; H, 3.64; N, 3.81. Found: C, 31.98; H, 3.40; N, 4.09. IR (KBr, cm−1; Figure S2b): 3418s, 2928m, 1663s, 1614s, 1551s, 1454s, 1393s, 1106m, 930w, 782m, 722m, 580m, 469w. X-ray Data Collection and Structure Determination. The single-crystal X-ray diffraction measurements of complexes 1-Dy and 1-Eu were experimented on a Bruker Smart CCD to get the crystal data at 296 K using ω rotation scans with widths of 0.3° and Mo Kα radiation (λ = 0.710 73 Å). The structures were solved by the direct methods and refined by full-matrix least-squares refinements based on F2 with the SHELXTL program.13 All non-hydrogen atoms were refined anisotropically. Anisotropic thermal parameters were applied to non-hydrogen atoms, and all hydrogen atoms from the organic ligands were calculated and added at idealized positions. Other details of relevant crystallographic data are given in Table S1, and the selected bond lengths and angles were listed in Tables S2 and S3.
Figure 1. Coordination environment of Dy(III) ion in 1-Dy. Symmetry codes: No. 1: −0.5 + x, 0.5 + y, 1.5 − z; No. 2: x, 1 − y, 0.5 + z; No. 3: −x, y, 1.5 − z.
displaying a trigonal dodecahedron geometry (D2d). The bond lengths [Dy−O = 2.286−2.446 Å] and angles around Dy(III) ion [O−Dy−O = 53.7(3)−151.5(4)°] are falling within the normal ranges generally observed in the literature. In 1-Dy, each ligand employs quadridentate coordination mode to link four Dy(III) centers by utilizing three deprotonated carboxylate groups of the ligand, adopting μ1η1:η1 chelating mode, μ2-η1:η1 bridging mode, and μ1-η0:η1 bridging mode (Figure 2a). Every two adjacent Dy(III) ions,
Figure 2. (a) Coordination models of the L3− in 1-Dy. (b) Two adjacent trigonal dodecahedron geometry of Dy(III) ion viewed as an SBU. (c) View of the 3D network for 1-Dy along the c axis. (d) View of the topology net for 1-Dy.
connecting each other through carboxylate groups (μ2-η1:η1) of two different ligands, viewed as an SBU (Figure 2b), and the adjacent SBUs are further linked by L3− ligands to form a 3D framework eventually. As shown in Figure 2c, there is one type of one-dimensional porous channel along c-axis in 1-Dy, where the nitro groups of ligands are located. Topologically, all the Dy(III) ions and L3− ligands can be viewed as four connected nodes (Figure S4), therefore, the network of 1-Dy belongs to a uninodal 4-c net topology with the Schlafli symbols as {42·6·83} (Figure 2d). Magnetic Properties. The direct-current (dc) magnetic susceptibilities of 1-Dy were measured in the temperature range from 1.8 to 300 K under 500 Oe applied magnetic field (Figure 3). The value of χMT of 1-Dy is 14.08 cm3 K mol−1 at room temperature, which is in good agreement with the expected value of 14.17 cm3 K mol−1 for one isolated Dy(III) ion (S = 5/
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RESULTS AND DISCUSSION Structure Description. Single-crystal X-ray diffraction study reveals that both 1-Dy and 1-Eu crystallize in the orthorhombic system with Pbcn space group, showing the isotypic structure, which can also be proved by the PXRD patterns (Figure S1), FT-IR (Figure S2), and TGA (Figure S3) experiments. Thus, the structure of 1-Dy is only discussed briefly as an example. As Figure 1 shows, the asymmetric unit consists of one crystallographically independent Dy(III) ion, one L3− ligand, and three coordinated water molecules. The central Dy(III) ion is coordinated by eight oxygen atoms from one chelating carboxylate group (O5 and O6), three bis (monodentate) bridging carboxylate groups (O1, O7, and O8), and three coordinated water molecules (O9, O10, and O11), B
DOI: 10.1021/acs.inorgchem.6b01899 Inorg. Chem. XXXX, XXX, XXX−XXX
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and χ″ do not go to zero below the maxima but increase at very low temperature, which reveals the onset of quantum tunneling and is commonly observed in lanthanide SMMs.4b To suppress the QTM, the ac susceptibilities under an optimized dc field of 1200 Oe were further performed (Figure 5). The frequencies
Figure 3. Temperature dependence of the χMT product for 1-Dy at 500 Oe.
2, L = 5, 6H15/2, g = 4/3) in the free-ion approximation.14 As the temperature is lowered, the χMT value for 1-Dy starts to decrease tardily from 300 to ∼50 K. Then, the curves degrade sharply and achieve minimum values of 10.75 cm3 K mol−1 at 1.8 K for 1-Dy. Thermal depopulation of the Dy(III) Stark sublevels and the weak anti-ferromagnetic interactions between the metal centers are responsible for the decrease.15 The field dependence of the magnetization of 1-Dy was measured at 2.0, 3.0, and 5.0 K (Figure 4 and Figure S5). At 2.0
Figure 5. Temperature dependence of the in-phase (a) and out-ofphase (b) ac susceptibility for 1-Dy under 1200 Oe dc field.
selected were 20, 100, 200, 300, 400, 500, 600, 700, 800, 900, and 1000 Hz in 1-Dy. However, the results indicate that both in-phase (χ′) and out of-phase (χ″) susceptibilities do not come down in the low-temperature range. The relaxation probability via the quantum pathway still exists. The relaxation time τ data of complex 1-Dy derived from the χ″ peaks follow the Arrhenius law [τ = τ0exp(ΔUeff/T)] (τ = relaxation time; τ0 = pre-exponential factor; ΔUeff = effective barrier). The preexponential factor (τ0) and the effective barrier (ΔUeff) are 3.89 × 10−8 s and 57 K for 1-Dy (Figure 6). The data plotted as Cole−Cole plots can be fitted to the generalized Debye model with α parameters below 0.33 (Figure S7 and Table S4), indicating the presence of a moderate relaxation process.15a,17 Besides, the shift of the peak temperature (TP) of χ′M is calculated by a parameter φ = (ΔTP/TP)/Δ(log f) = 0.23 for 1Dy, which falls in the range of a normal value for a superparamagnet but not a spin glass (0.01 < φ < 0.08). All the discussions above strongly suggest that 1-Dy exhibits slow magnetic relaxation behavior.14d,18 Luminescent Properties. The outstanding optical properties of Ln(III)-MOFs inspire us to carefully explore their
Figure 4. M vs H plots at 2.0, 3.0, and 5.0 K for 1-Dy.
K, the magnetization measurements of 1-Dy increases with a relatively rapid speed at low fields. Then the magnetization increases slowly at high fields, up to 5.5 Nβ without a clear saturation, which is similar to that observed in other dysprosium complexes.15e In addition, the curves at different temperatures without overlapping further proves the significant magnetic anisotropy and/or low-lying excited states in the complex.15c,16 The dynamics of the magnetization was investigated by measuring alternating-current (ac) susceptibility in zero dc fields. At a relatively high temperature range, the in-phase (χ′) and out-of-phase (χ″) susceptibilities in 1-Dy display apparent temperature-dependent peaks (Figure S6). The signal clearly indicates the slow relaxation of magnetization. When cooled, χ′ C
DOI: 10.1021/acs.inorgchem.6b01899 Inorg. Chem. XXXX, XXX, XXX−XXX
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Furthermore, the luminescence intensities are measured by the addition of nitrobenzene into the suspension of 1-Eu (dispersing 3 mg of 1-Eu sample into 3 mL of DMF) to research the quenching effect of nitrobenzene to 1-Eu. As shown in Figure 8a, with the nitrobenzene concentration increasing from 0 to 0.528 mM of nitrobenzene concentration, the luminescence intensity of 1-Eu@nitrobenzene suspension decreases distinctly. The relationship between the luminescence intensity and the concentration of nitrobenzene follows the first-order exponential decay formula of I = 0.973 exp(−C/ 0.065) + 0.012 (Figure 8b), showing that luminescence quenching is diffusion-controlled (I = luminescence intensity of 1-Eu@nitrobenzene suspensions/luminescence intensity of 1-Eu@DMF suspension; C = nitrobenzene concentration (mM); R2 = 0.998), which proves 1-Eu has the ability to measure quantitatively. Additionally, the recyclable performance of 1-Eu was also investigated. The samples of 1-Eu were added into nitrobenzene to completely form 1-Eu@nitrobenzene, and then 1Eu@nitrobenzene was simply washed with DMF several times. The results show that the fluorescence intensity of compound 1-Eu can be partially resumed (Figure S12a). In previous studies, the reasons for luminescence quenching caused by nitro compounds were basically attributed to two reasons: photoinduced electron transfer and resonance energy transfer.19 The Stern−Volmer relationship between the relative fluorescence intensity of 1-Eu and nitrobenzene concentration was fitted as an exponential function, which is speculated that the presence of simultaneous dynamic and static quenching.20a,b The electrostatic interactions between the phenyl rings of L3− ligands of 1-Eu and nitrobenzene is the possible reason for static quenching, while the dynamic one may be due to the collisional encounters between the fluorescent moieties of 1-Eu and nitrobenzene.20 Detection of Anions. The finely ground sample of 1-Eu (3 mg) was immersed in 3 mL of different kalium salt solutions of the same concentration (0.1 M) of C2O42−, I−, IO3−, BrO3−, Cl−, CO32−, SO42−, Br−, HSO4−, and Cr2O72− in neutral conditions. As demonstrated in Figure 9, the intensity at 614 nm is obviously enhanced by the addition of C2O42−, I−, IO3−, and BrO3−, whereas other anions (Cl−, CO32−, SO42−, Br−, HSO4−) tested just caused negligible change to the intensity of
Figure 6. Relaxation time, ln(τ), versus T−1 plot for 1-Dy under 1200 Oe dc field. The solid line is fitted with Arrhenius law.
potential applications. The solid-state luminescent properties of 1-Eu were explored first at room temperature. 1-Eu displays bright red luminescence under UV irradiation. The solid-state photoluminescence spectra, which excited at 395 nm, shows the strong emission peaks at 590, 614, 648, and 694 nm for 1-Eu, originate from 5D0−7F1,5D0−7F2, 5D0−7F3, and 5D0−7F4 f−f transitions of Eu(III) ions (Figure S8). The decay lifetime curve reveals the exponential decays with the lifetime of 0.31 ms for 1-Eu (Figure S9). Detection of Small Organic Molecules. First, the remarkable bright-red luminescence of 1-Eu prompts us to investigate its potential for sensing common organic solvent molecules. Before the fluorescence study, the crystalline materials of 1-Eu were ground into powder samples and added in 3 mg samples into 3 mL of different organic solvents for 24 h, then treated by ultrasonic agitation for 30 min to form stable suspensions. The solvents used in the texts are H2O, methanol, ethanol, tetrahydrofuran (THF), DMF, acetone, N,N-dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), N-methyl pyrrolidone (NMP), acetonitrile, 1,4-dioxane, dichloromethane (CH2Cl2), toluene, benzene (C6H6), chlorobenzene (C6H5Cl), cyclohexane, and nitrobenzene. As shown in Figure 7, the luminescence intensities are greatly dependent on solvents, particularly for nitrobenzene, which appears a significant quenching effect. PXRD patterns collected for each 1-Eu@solvent are similar to that of 1-Eu, showing that the framework of 1-Eu is intact in all the solvents (Figure S10) .
Figure 7. (a) Luminescence spectra and (b) the luminescent intensity at 614 nm intensities of 1-Eu in various organic pure solvents when excited at 395 nm. D
DOI: 10.1021/acs.inorgchem.6b01899 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 8. (a) Luminescence spectra of 1-Eu@nitrobenzene@DMF suspensions with nitrobenzene concentration varying from 0 to 0.528 mM (excited at 395 nm). (b) The plot of relative intensity vs nitrobenzene concentration.
Figure 9. (a) Luminescence spectra and (b) the luminescence intensity at 614 nm intensities of 1-Eu in various anions or pure solvents when excited at 395 nm.
Figure 10. (a) Luminescence spectra of 1-Eu@Cr2O72@H2O suspensions with Cr2O72− concentration varying from 0 to 4.14 mM (excited at 395 nm). (b) The plot of relative intensity vs Cr2O72− concentration.
1-Eu at 614 nm. Only Cr2O72− anion gave a significant fluorescence quenching effect. PXRD (Figure S11) was measured, and the patterns showed that the basic framework
of 1-Eu still remained unchangeable. The result indicates that 1-Eu has highly selective detection and specific recognition of Cr2O72− anion in aqueous solutions. To further explore 1-Eu as E
DOI: 10.1021/acs.inorgchem.6b01899 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry a luminescent probe for Cr2O72−, as shown in Figure 10, a series of titration experiments were performed, in which the luminescence intensities of the sample are measured by the addition of Cr2O72− aqueous into the suspension of 1-Eu sample. The luminescence intensities decreases as the concentration of Cr2O72− increased, and those of 1-Eu are almost completely quenched at the concentration of 4.14 mM. And the relationship between the relative luminescence intensity and the concentration of Cr2O72− also can be fitted as the following formula: I = 1.007 exp(−C/0.463) + 0.003 (I = luminescence intensity of 1-Eu@Cr2O72− suspensions/luminescence intensity of 1-Eu@H2O suspension; C = Cr2O72− concentration (mM); R2 = 0.996). Meanwhile, multiple cycles of Cr2O72− sensing experiments were performed and showed that the material could greatly regain its intensity after washing by water several times (Figure S12b). The result reveals that 1-Eu could be employed as a fluorescence sensor for detecting Cr2O72− with high sensitivity and recyclability. The mechanism of luminescence quenching is usually attributed to two aspects: (1) the Cr2O72− anions are absorbed on the surface of the target complexes, (2) the competition for the excitation energy between the Cr2O72− anions themselves and 1-Eu framework due to their weak interactions. Both of the two factors possibly lead to the change of the luminescence change and quenching.9b,21
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CONCLUSION
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ASSOCIATED CONTENT
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ACKNOWLEDGMENTS
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REFERENCES
We are thankful for the financial support from the NSFC (Grant Nos. 21531007, 21201139, 21501142, and 21371142), NSF of Shaanxi Province (Grant Nos. 2013JQ2016, 2016JQ2003), Postdoctoral Science Foundation of China (Grant No. 2015M572590), and the Open Foundation of Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education (Grant No. 338080049).
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In this study, we report the magnetic and luminescent properties of two Ln(III)-MOFs [LnL(H 2 O) 3 ]·3H 2 O· 0.75DMF (1-Ln; Ln = Dy(III) and Eu(III)) prepared from a H3L ligand. 1-Dy exhibits slow magnetic relaxation behavior with an energy barrier (ΔUeff) of 57 K and a pre-exponential factor (τ0) of 3.89 × 10−8 s. Luminescence studies of 1-Eu show the quenching effect on nitrobenzene and Cr2O72−. More importantly, the solid products can keep its original network and be recycled after several times detecting experiments, which may be used as a chemical sensor in sensing nitrobenzene and Cr2O72−. The successful synthesis of the compounds may provide new methods to construct new Ln(III)-MOFs functional materials.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01899.
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Additional figures, selected bond and length table, PXRD, TG, IR spectra and crystallographic data of 1Dy and 1-Eu. (PDF) Crystallographic information files for compounds 1-Dy and 1-Eu (CCDC 1497230 and 1497231, respectively). (CIF)
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The authors declare no competing financial interest. F
DOI: 10.1021/acs.inorgchem.6b01899 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b01899 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b01899 Inorg. Chem. XXXX, XXX, XXX−XXX
