Anionic Lanthanide Metal–Organic Frameworks: Selective Separation

37 mins ago - Synopsis. Four new isostructural anionic lanthanide metal−organic frameworks [(CH3)2NH2]1.5[Ln1.5(TATAT)(H2O)4.5]·x(solvent) (Ln-MOFs...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Anionic Lanthanide Metal−Organic Frameworks: Selective Separation of Cationic Dyes, Solvatochromic Behavior, and Luminescent Sensing of Co(II) Ion Zheng Cui, Xiaoying Zhang,* Shuang Liu, Lei Zhou, Wenliang Li,* and Jingping Zhang* Advanced Energy Materials Research Center, Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China

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ABSTRACT: Four new microporous isostructural anionic lanthanide metal− organic frameworks (Ln-MOFs), [(CH3)2NH2]1.5[Ln1.5(TATAT)(H2O)4.5]· x(solvent) {Ln = Tb, Eu, Dy, and Gd; H6TATAT = 5,5′,5″-(1,3,5-triazine2,4,6-triyl)tris(azanediyl)triisophthalate}, were successfully constructed. The Ln-MOFs are three-dimensional (3D) anionic frameworks and have two sizes of square channels (8.9 × 8.9 Å and 4.3 × 4.3 Å) with a Lewis basic nitrogendecorated pore environment. The 3D frameworks of Ln-MOFs can be simplified as (4,6)-connected she networks. Because of the anionic framework properties, Ln-MOFs can efficiently select and separate cationic dyes in the presence of anionic or neutral dyes of similar sizes. The adsorption amounts of methylene blue for Tb-MOF, Eu-MOF, Dy-MOF, and Gd-MOF are 147, 141, 133, and 143 mg g−1, respectively. Moreover, Tb-MOF and Eu-MOF allow easy detection and identification of ethanol, acetonitrile, and diethyl ether through solvatochromism. Diethyl ether vapor also rapidly changes the colors of Tb-MOF and Eu-MOF. The photoluminescence experiments show that the absolute quantum yields of Tb-MOF (upon excitation at 341 nm), Eu-MOF (upon excitation at 396 nm), Dy-MOF (upon excitation at 341 nm), and GdMOF (upon excitation at 370 nm) are 32.5%, 11.0%, 2.1%, and 7.1%, respectively. In addition, Tb-MOF can detect Co2+ ion with high selectivity and quenching efficiency of 87%.

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INTRODUCTION Metal−organic frameworks (MOFs, also known as porous coordination polymers or PCPs) have received great attention in applications such as gas storage,1−5 separation,6−11 catalysis,5,9,12−15 drug delivery,16−18 proton conduction,19−22 magnetism,23,24 luminescence,25−27 and sensing28−31 due to their controllable pore sizes and functionalized pore structures.32−35 Although there are some applications of MOFs that can be achieved based on the frameworks of MOFs26,36−38 (independent of guest molecules) or the interactions between MOFs’ surfaces and target molecules,39,40 most applications are based on the interactions between the frameworks and guest molecules in MOFs’ channels. Therefore, it is important to construct suitable channels for MOFs by reasonably selecting ligands and metals to achieve proper interactions between the frameworks and guest molecules. A variety of interactions (dispersion interactions, hydrogenbonding interactions, π−π stacking interactions, coordination interactions, etc.) between MOFs and guest molecules may occur by selecting highly conjugated ligands containing multiple Lewis basic nitrogen atoms and exposing them to the pore surfaces of the MOFs.28,41−46 In addition, choosing organic ligands containing multicarboxyl coordination sites and metal ions (excluding metal clusters) can often construct anion frameworks,47−53 which can selectively adsorb and separate molecules of different charges by electrostatic interactions.49,52,54−58 Lanthanide (Ln) ions have high coordination © XXXX American Chemical Society

numbers and rich coordination geometries and thus can form structurally diverse MOFs,59,60 and they also possess unique luminescent properties such as narrow emission bands, large Stokes shifts, and long luminescence lifetimes.61,62 Organic dyes have been widely used in many industries, including paper, printing, plastics, textiles, and more. However, the large emissions of organic dye wastes cause environmental pollution and threaten human health, because most of them are toxic and even carcinogenic.52,54−57,63,64 So far, many chemical, physical, and biological methods have been proposed to remove organic dye contaminants, and adsorption in these methods is more effective because of its high efficiency and economy.65−68 Compared with traditional adsorbents (activated carbons, zeolites, and polymeric materials), MOFs are more suitable for the selective adsorption and separation of organic dye molecules due to their tunable pore sizes and diverse structures.32−35,69 The mechanisms of MOFs as adsorbents mainly include size-based exclusion and/or the differences in the interactions between different molecules and MOFs. In these interactions (van der Waals forces, electrostatic interactions, coordination interactions, hydrogen-bonding interactions, hydrophobic interactions, π−π stacking interactions, etc.), strong electrostatic interactions allow Received: May 15, 2018

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DOI: 10.1021/acs.inorgchem.8b01319 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

presence of the sample, respectively, and λ, h, and c are the wavelength, the Planck constant, and the speed of light, respectively. The luminescence lifetimes (τ) were measured with an FLSP-920 steady-state and time-resolved fluorescence spectrometer at room temperature. X-ray photoelectron spectroscopy (XPS) spectra were recorded with Al Kα radiation and energy step size of 0.1 eV. UV−Vis absorption spectra were performed on a Varian Cary 50 UV−vis spectrophotometer. UV−Vis diffuse reflectance spectra were measured on an Agilent Cary 7000 universal measurement spectrophotometer. IR spectra were obtained as KBr pellets on a Thermo Scientific Nicolet 6700 FT-IR spectrometer. Geometry optimization of all dye molecules was conducted by the density functional theory on the B3LYP/6-31G(d, p) level.82−85 Dye Adsorption and Separation. The dyes used in the experiment were cationic methylene blue (MB), anionic methyl orange (MO), anionic orange II (OrII), neutral methyl red (MR), and cationic rhodamine 6G (Rh6G). Tb-MOF (Eu-MOF, Dy-MOF, or Gd-MOF, 10 mg) was soaked in 3 mL of DMF solutions of MB&MO (4/8 ppm), MB&OrII (4/8 ppm), MB&MR (4/4 ppm), MB&Rh6G (4/4 ppm), MB (4 ppm), MO (8 ppm), OrII (8 ppm), MR (4 ppm), and Rh6G (4 ppm) in cuvettes. The dyes adsorption processes were recorded by measuring changes in UV−vis absorption spectra over time. The values of the absorbance of OrII and MO were somewhat low at a concentration of 4 ppm, and therefore a concentration of 8 ppm was used. The maximum adsorption capacity of MB was measured by immersing 20 mg of as-synthesized Tb-MOF, Eu-MOF, Dy-MOF, and Gd-MOF in 10 mL of DMF solution of MB (1 g L−1) for 20 h at room temperature. Dye Release. Tb-MOF (Eu-MOF, Dy-MOF, or Gd-MOF, 10 mg) was soaked in 3 mL of DMF solution of MB (4 ppm) for 4 h, then filtered and washed with DMF to obtain MB@Tb-MOF (MB@ Eu-MOF, MB@Dy-MOF, or MB@Gd-MOF). Dye-releasing experiments were then performed by immersing MB@Tb-MOF (MB@EuMOF, MB@Dy-MOF, or MB@Gd-MOF, 10 mg) in pure DMF (3 mL) and a NaCl-saturated DMF solution (3 mL), respectively. UV− Vis absorption spectra were measured to monitor the dye-releasing process. Reusability for MB Adsorption. Tb-MOF (Eu-MOF, Dy-MOF, or Gd-MOF, 30 mg) was immersed in 9 mL of DMF solution of MB (4 ppm) for 4 h, and MB in the solution was detected by measuring UV−vis absorption spectra. The filtered MB@Tb-MOF (MB@EuMOF, MB@Dy-MOF, or MB@Gd-MOF) was then soaked in 9 mL of NaCl-saturated DMF solution for 6 h, and the MB in the solution was recorded by UV−vis absorption spectra. The crystals after the dye release experiment were then soaked in an excess of NaCl-saturated DMF solution overnight to exchange as much of the MB in the MB@ Ln-MOFs as possible. The crystals were further filtered out, and the above adsorption−desorption cycle was repeated. The crystals were washed at least three times with DMF during all filtration. Luminescence Experiments. Tb-MOF (Eu-MOF, Dy-MOF, or Gd-MOF, 30 mg) was soaked in 10 mL of DMF solutions of M(NO3)x (M = Zn2+, Ni2+, Cd2+, Na+, Cu2+, Cr3+, Co2+, Mg2+, Mn2+, Pb2+, or Fe3+; metal ion concentration: 1 mmol L−1) for 12 h, then filtered and washed with DMF to obtain the crystals of Mx+@TbMOF (Mx+@Eu-MOF, Mx+@Dy-MOF, or Mx+@Gd-MOF). The antijamming experiments were performed by soaking 30 mg of TbMOF in 10 mL of DMF solutions mixed with M(NO3)x (M = Zn2+, Ni2+, Cd2+, Na+, Cu2+, Cr3+, Mg2+, Mn2+, Pb2+, or Fe3+; metal ion concentration: 1 mmol L−1) and Co(NO3)2 (1 mmol L−1) for 12 h. Reusability for Co2+ Sensing. Tb-MOF (30 mg) was soaked in 10 mL of DMF solutions of Co(NO3)2 (1 mmol L−1) for 12 h and then filtered to obtain the crystals of Co2+@Tb-MOF. Activation of Co2+@Tb-MOF was achieved by soaking it in 20 mL of DMF solution of dimethylamine hydrochloride (10 mmol L−1) for 12 h. The above sensing-activation cycle was repeated for the activated crystals. The crystals were washed at least three times with DMF in all filtration operations, and then the crystals were air-dried to test the luminescence properties.

charged MOFs to efficiently and sensitively separate dyes with different charges.70 MOFs can also be used as excellent candidates for chemosensing, because they could adsorb guest molecules to influence properties (optical, electrical, and magnetic)29,71 of MOFs by analyte-probe interactions.28 Among these properties, luminescence-based sensing is highly desirable due to its ease of operation, simplicity of technique, and nondestructiveness.28,72 Cobalt, as an essential trace element in human body, plays a crucial role in the synthesis of vitamin B12 and hemoglobin. Because cobalt deficiency and excessive intake will seriously affect human health, the detection of cobalt is very important.73−77 Although a Tb-doped Zn-based MOF78 and a nanocomposite79 formed by Eu-based MOF and vitamin B12 have been reported for the detection of Co2+ ion, the single-component lanthanide MOF for selective luminescence sensing of Co2+ ion has not been reported so far. According to the above considerations, we selected highly conjugated chromophore 5,5′,5″-(1,3,5-triazine-2,4,6-triyl)tris(azanediyl)triisophthalate (H6TATAT) with multiple Lewis basic sites and carboxyl groups as a linker. By solvothermal reactions, TATAT6− ligands connect lanthanide metals to form four new isostructural anionic Ln-MOFs: [(CH3)2NH2]1.5[Ln1.5(TATAT)(H2O)4.5]·x(solvent) {Ln = Tb, x(solvent) = 4DMF; Ln = Eu, x(solvent) = 6DMF·H2O; Ln = Dy, x(solvent) = 5DMF·4.5H2O; Ln = Gd, x(solvent) = 5.5DMF·0.5H2O; DMF = dimethylformamide}. Ln-MOFs have two kinds of cages with different window sizes of 8.9 × 8.9 Å and 4.3 × 4.3 Å (considering the van der Waals radius of the atoms, the same below), and the Lewis basic nitrogen atoms of TATAT6− ligands are exposed on the surfaces of cages. Ln-MOFs show high selectivity for adsorption and separation of cationic dyes by cationic exchange in the presence of anionic or neutral dyes of similar sizes. Interestingly, Tb-MOF and Eu-MOF exhibit obvious solvatochromic behaviors when soaked in ethanol, acetonitrile, and diethyl ether, which can be used to visually detect and identify them. In addition, Tb-MOF is an excellent sensor for the detection of Co2+ ion.

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EXPERIMENTAL SECTION

Materials and Methods. All the chemicals were purchased from the commercial companies and used as obtained, except for H6TATAT, which was synthesized according to the previously published procedure.80 Elemental analyses (C, H, N) were performed on a EuroVector EA3000 elemental analyzer. Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku SmartLab Xray diffractometer using Cu Kα radiation at room temperature. Thermogravimetric analyses (TGA) were performed on a PerkinElmer TG-7 analyzer in a nitrogen atmosphere at a heating rate of 10 °C min−1. Luminescence spectra were recorded on a Hitachi F-7000 luminescence spectrometer at room temperature. The absolute quantum yields (Φ) were measured with a Hamamatsu absolute PL quantum yield spectrometer C9920−02 and were calculated by the following expression:81 Φ=

∫ PN(Em) = PN(Abs) ∫

λ sample reference [I (λ) − Iem (λ)]dλ hc em λ reference sample [I (λ) − Iex (λ)]dλ hc ex

where PN(Em) is the number of photons emitted from a sample, PN(Abs) is the number of photons absorbed by the sample, Ireference ex and Ireference are the intensities of the excitation light and photoem sample and Isample luminescence when there is no sample respectively, Iex em are the intensities of the excitation light and photoluminescence in the B

DOI: 10.1021/acs.inorgchem.8b01319 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Syntheses of Ln-MOFs. A mixture of Tb(NO3)3·6H2O (45 mg, 0.1 mmol), H6TATAT (62 mg, 0.1 mmol), and benzoic acid (220 mg, 1.8 mmol) in DMF/H2O (6/3 mL) was sealed in a Teflon-lined stainless steel container (15 mL). Then, the mixture was heated at 100 °C for 72 h, and finally the colorless cubic crystals of Tb-MOF were obtained. Eu-MOF, Dy-MOF, and Gd-MOF were synthesized by following the same procedure of Tb-MOF, except that Tb(NO3)3· 6H2O was replaced by Eu(NO3)3·6H2O (45 mg, 0.1 mmol), Dy(NO3)3·5H2O (44 mg, 0.1 mmol), and Gd(NO3)3·6H2O (45 mg, 0.1 mmol), respectively. Yield: 94% based on Tb(NO3)3·6H2O, 88% based on Eu(NO3)3·6H2O, 86% based on Dy(NO3)3·5H2O, and 89% based on Gd(NO3)3·6H2O for Tb-MOF, Eu-MOF, Dy-MOF, and Gd-MOF, respectively. Anal. Calcd for C42H61N11.5O20.5Tb1.5 (Tb-MOF, 1293.40): C 39.00, H 4.75, N 12.45; found: C 38.84, H 5.83, N 13.13%. Anal. Calcd for C48H77N13.5O23.5Eu1.5 (Eu-MOF, 1447.16): C 39.84, H 5.36, N 13.07; found: C 39.78, H 6.10, N 13.35%. Anal. Calcd for C45H77N12.5O26Dy1.5 (Dy-MOF, 1452.92): C 37.20, H 5.34, N 12.05; found: C 37.07, H 5.51, N 12.14%. Anal. Calcd for C46.5H72.5N13O22.5Gd1.5 (Gd-MOF, 1409.54): C 39.62, H 5.18, N 12.92; found: C 39.53, H 6.20, N 13.45%. X-ray Crystallography. Single-crystal X-ray diffraction data of Tb-MOF, Eu-MOF, Dy-MOF, and Gd-MOF were collected on a Bruker APEXII CCD diffractometer using the graphite monochromated Mo Kα radiation (λ = 0.710 73 Å). Absorption corrections were applied using multiscan technique. All the structures were solved by Direct Method of SHELXS-9786 and refined by full-matrix leastsquares techniques using the SHELXL-201487 program within WinGX.88 The disordered guest molecules were removed by the SQUEEZE program of PLATON.89 Then the structures were refined further using the data generated.

Figure 1. A-cage (a) and B-cage (b) constructed from six-connected TATAT6− ligands and four-connected Tb3+ ions in Tb-MOF. C gray, N blue, O red, and Tb turquoise.

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RESULTS AND DISCUSSION Structure of [(CH3)2NH2]1.5[Ln1.5(TATAT)(H2O)4.5]·x(solvent). Both single-crystal X-ray diffraction determinations and PXRD (Figure S1) measurements demonstrate that the four compounds [(CH3)2NH2]1.5[Ln1.5(TATAT)(H2O)4.5]· x(solvent) are isostructural and crystallize in cubic space group Pm3̅m (Tables S1 and S2). Herein, we just describe the structure of Tb-MOF in detail. Tb-MOF is a three-dimensional (3D) anionic framework in which protonated [(CH3)2NH2]+ cations generated by DMF thermal decomposition52,56,58,90 are present as counterions. There is one TATAT6−, one and a half Tb3+, and four and a half coordinated H2O in an asymmetric unit of Tb-MOF (Figure S2). Uncoordinated solvent molecules were determined by elemental analyses and taking thermogravimetric analyses (TGA) into account (Figure S3). Each Tb3+ is ninecoordinated by six carboxyl oxygen atoms from four TATAT6− ligands and three oxygen atoms from three coordinated water molecules. The six carboxylate groups of TATAT6− ligand adopt two coordination fashions μ1-η0:η1 and μ1-η1:η1 connecting Tb3+. Each TATAT6− ligand connects six Tb3+ ions acting as a hexagonal molecular building block. Since the hexagonal molecular building block has three short sides and three long sides, eight TATAT6− ligands bridge 24 Tb3+ to form two sizes of truncated octahedron cages (A-cage and Bcage; Figure 1). The diameters of A-cage and B-cage are 19.8 and 21.8 Å, respectively. Each A-cage connects eight adjacent B-cages via sharing the hexagonal TATAT6− ligands and connects six adjacent A-cages via sharing the square windows and vice versa (Figure 2a,b), thus forming a 3D porous structure. Sharing the square windows between the same cages causes Tb-MOF to form two channels with sizes of 8.9 × 8.9 Å and 4.3 × 4.3 Å along the crystal axes (Figure S4). From the topological point of view, the Tb3+ ions and TATAT6− ligands act as four-connected and six-connected nodes, respectively. As

Figure 2. (a) Arrangement of A-cages (yellow) and B-cages (bright green) in Tb-MOF. (b) The connection between A-cages in TbMOF. (c) The net she of Tb-MOF. (d) Natural tiling of the net she for Tb-MOF. Color codes: C gray, N blue, O red, and Tb turquoise.

a consequence, Tb-MOF adopts a (4,6)-connected network with the Schläfli symbol {44.62}3{46.66.83}2, which corresponds to she topology (Figure 2c,d).91 The potential solventaccessible volume of Tb-MOF, calculations using the PLATON software,89 is 15793.3 Å3 corresponding to 74.4% of the 21 225.2 Å3 unit cell volume. There are two kinds of guest molecules in the Ln-MOFs, namely, neutral molecules (DMF and H2O) and cationic [(CH3)2NH2]+. For specific applications, DMF (or H2O) and cationic [(CH3)2NH2]+ can C

DOI: 10.1021/acs.inorgchem.8b01319 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. UV−Vis spectral changes of DMF solutions of MB (a), MB&MO (b), MB&OrII (c), and MB&MR (d) in the presence of Tb-MOF.

molecules in MB@Ln-MOFs can be released in DMF solution of NaCl and hardly in pure DMF (Figures 4 and S13−S16).

be exchanged by neutral solvent molecules (ethanol, acetonitrile, diethyl ether, etc.) and cationic dyes, respectively. At 77 K, Ln-MOFs exhibit linear sorption isotherms with low gas uptakes for N2 (Figure S5), presumably because LnMOFs lose porosity during outgassing.56,92 PXRD analyses indicate that the frameworks of Ln-MOFs after the N2 adsorption experiments are collapsed (Figure S6). Dye Adsorption and Separation Properties. The large channel windows (8.9 × 8.9 Å and 4.3 × 4.3 Å) allow LnMOFs (Ln = Tb, Eu, Dy, and Gd) to adsorb large guest molecules. Furthermore, the anionic framework features of LnMOFs make them selective for the adsorption of guest molecules with different charges. Therefore, it is possible for Ln-MOFs to selectively adsorb cationic dyes in the presence of anionic or neutral dyes. We chose four organic dyes, cationic methylene blue (MB), anionic methyl orange (MO), anionic orange II (OrII), and neutral methyl red (MR) with similar sizes as the adsorption targets for Ln-MOFs (Figure S7). The four Ln-MOFs can selectively capture cationic MB rapidly. While the concentrations of coexisting anionic MO, anionic OrII, and neutral MR remain essentially unchanged (Figures 3b−d, S8b−d, S9b−d, and S10b−d). The adsorption experiments of individual dye also show that Tb-MOF, Eu-MOF, Dy-MOF, and Gd-MOF capture only MB and have almost no absorption of MO, OrII, and MR (Figures 3a, S8a, S9a, S10a, S11, and S12). As explained above, this phenomenon of selective adsorption of cationic dye (MB) is mainly due to the anionic framework properties of Ln-MOFs; that is, only the cationic dye can be rapidly adsorbed into the channels of the MOFs by ion exchange with [(CH3)2NH2]+. This mechanism was also confirmed by dye release experiments. The dye

Figure 4. Dye-releasing process of MB@Tb-MOF in a NaClsaturated DMF solution.

This means that the cationic MB in the MB@Ln-MOFs cannot be exchanged by neutral DMF molecules but can be exchanged by Na+ ions from NaCl. The role of NaCl in dye release is to provide Na+ ions to exchange cationic MB in MB@Ln-MOFs. Considering charge conservation, we can speculate that the cationic MB is indeed introduced into the Ln-MOFs via ion exchange with [(CH3)2NH2]+ in the original MOFs. Comparing the cationic MB molecular size (4.0 × 7.8 × 16.4 Å, Figure S7) with the Ln-MOFs’ pore sizes (8.9 × 8.9 Å and 4.3 × 4.3 Å), it can be concluded that the MB molecules can be adsorbed into the pores of the Ln-MOFs and that they D

DOI: 10.1021/acs.inorgchem.8b01319 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry are mainly adsorbed in the larger channel (8.9 × 8.9 Å). We smashed the crystals of MB@Tb-MOF and MB@Eu-MOF and found that the inside of the crystals was still blue (Figure S17), which confirmed that the MB molecules are adsorbed inside the crystals. PXRD analyses show that the frameworks of Ln-MOFs after the dye release experiments are stable (Figures S18−S21). The reusability experiments of the Ln-MOFs for MB adsorption show that the Ln-MOFs still exhibit high adsorption capacity for MB after five cycles (Figure S22). The PXRD patterns indicate that the frameworks of Ln-MOFs are basically unchanged after five adsorption−desorption cycles for MB (Figures S18−21). In addition, Ln-MOFs can selectively adsorb MB in the presence of rhodamine 6G (Rh6G) with the same charge and larger size (Figures 5, S7, and S23−S26), which is mainly due to the size exclusion effect; that is, the size of the dye Rh6G is too large to enter the channels of Ln-MOFs.

Figure 6. Solvatochromic behaviors of Tb-MOF for ethanol (a), acetonitrile (b), and diethyl ether (c).

MOF (or Eu-MOF) soaked in DMF, DMA, acetone, dichloromethane, trichloromethane, methanol, THF, and ethanediol maintained colorless. Furthermore, the vapor of diethyl ether markedly changed the colors of Tb-MOF and EuMOF within 20 min at room temperature (∼12 °C; Figure 7).

Figure 7. Response of Tb-MOF (a) and Eu-MOF (b) to the vapor of ethyl ether.

The UV−vis diffuse reflectance spectra of Tb-MOF and EuMOF show an obvious absorption in the visible region after adsorption of ethanol, acetonitrile, and diethyl ether (Figures S28 and S29), which is consistent with the observed changes in the color of the crystals. The decrease of the CO stretching vibration peak (1659 cm−1) of DMF in the infrared spectroscopy (IR) indicates that DMF in the channels of Tb-MOF and Eu-MOF is exchanged by ethanol, acetonitrile, and diethyl ether (Figures S30 and S31).42 Such solvatochromic phenomenon demonstrates that Tb-MOF and Eu-MOF can be used as naked eye detectors for ethanol, acetonitrile, and diethyl ether. Although the color change of crystals caused by water has been observed in the reported lanthanide MOFs,107,108 the solvatochromic behavior of lanthanide MOFs for organic solvent has not been reported so far. PXRD analyses show that the frameworks of Tb-MOF and Eu-MOF are collapsed after immersion in acetonitrile, while the structures of Tb-MOF and Eu-MOF are relatively intact after immersion in ethanol and diethyl ether (Figures S32 and S33). Because other solvents can also cause the collapse of framework but no solvatochromism, structural collapse is not the primary cause of solvatochromism. We put ethanol@TbMOF, acetonitrile@Tb-MOF, diethyl ether@Tb-MOF, ethanol@Eu-MOF, acetonitrile@Eu-MOF, and diethyl ether@EuMOF in DMF, acetone, or methanol for 5 d and found that the color of the crystals does not fade. And even though they are dried at 100 °C for 15 h, the color of the crystals is unchanged. These results indicate that the ethanol, acetonitrile, and diethyl ether molecules have strong interactions with the frameworks of Tb-MOF and Eu-MOF, such as dispersion interactions and hydrogen-bonding

Figure 5. UV−Vis spectral changes of DMF solutions of MB&Rh6G in the presence of Tb-MOF.

Furthermore, compared with some other adsorbents,58,90 Tb-MOF, Eu-MOF, Dy-MOF, and Gd-MOF have a rapid uptake rate of MB, which can absorb 92.6%, 90.8%, 93.8%, and 93.3% of dye molecules, respectively, in 200 min. The high adsorption rates are mainly due to the anionic framework as well as large window (8.9 × 8.9 Å) and cage dimensions (diameter: 21.8 Å) of the MOFs. In NaCl-saturated DMF solution, the MB release efficiencies are 40.3% for Tb-MOF, 33.5% for Eu-MOF, 34.9% for Dy-MOF, and 37.0% for GdMOF in 360 min. The adsorption amounts of MB for TbMOF, Eu-MOF, Dy-MOF, and Gd-MOF are 147, 141, 133, and 143 mg g−1, respectively, which are higher than some other adsorbents.58,93−96 Compared with the previously reported lanthanide MOFs,62,97−103 Tb-MOF, Eu-MOF, Dy-MOF, and Gd-MOF exhibit high adsorption rates and good adsorption selectivity for MB (Table S3). Solvatochromism. One of the easiest ways of detection is to change MOFs’ colors by analytes. This phenomenon is referred to as solvatochromism, with the detected molecules being solvent.42,44,104−106 Inspired by the solvatochromic behavior of tetrazine-based and triazine-based compounds previously reported in the literatures,42,44,58,105 we explored the solvatochromism of Tb-MOF and Eu-MOF. Interestingly, we found that the color of Tb-MOF (or Eu-MOF) soaked in ethanol, acetonitrile, and diethyl ether gradually changed from colorless to golden, dark orange, and dark red as the immersion time increased (Figures 6 and S27), respectively, while the TbE

DOI: 10.1021/acs.inorgchem.8b01319 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. Excitation and emission spectra of Tb-MOF (a), Eu-MOF (b), Dy-MOF (c), and Gd-MOF (d) at room temperature.

characteristic bands of Dy3+, which are assigned to the f−f electronic transitions 4F9/2 → 6H15/2 (486 nm) and 4F9/2 → 6 H13/2 (578 nm) (Figure 8c). Tb-MOF, Eu-MOF (excitation below 370 nm), and Dy-MOF exhibit strong characteristic emission peaks of Ln3+, indicating that TATAT6− ligand is an excellent antenna chromophore and sensitizes Ln3+ emission through the antenna effect (energy transfer from chromophore to Ln3+).61,109,110 The energy transfer from ligand to Eu3+ in Eu-MOF is incomplete at less than 370 nm excitation, which leads to the appearance of ligand-based emission. The characteristic emission bands of Eu3+ in Eu-MOF are mainly derived from the energy harvesting of Eu3+ when excited at 385 and 396 nm, while the energy absorption of TATAT6− ligand is mainly used for ligand-based emission.109,111,112 Gd-MOF displays a ligand-centered broad emission band with a peak at 462 nm (Figure 8d),30,113 similar to the 465 nm emission peak of the free H6TATAT ligand (Figure S36). The absence of the characteristic 4f−4f emission of Gd3+ may be due to the high energy level of Gd3+ that hinders energy transfer from ligand.114−116 The absolute quantum yields of Tb-MOF (upon excitation at 341 nm), Eu-MOF (upon excitation at 396 nm), Dy-MOF (upon excitation at 341 nm), and GdMOF (upon excitation at 370 nm) are 32.5%, 11.0%, 2.1%, and 7.1%, respectively. The four Ln-MOFs are anionic frameworks and have excellent luminescent properties. Furthermore, the Lewis basic triazine and imino N atoms of the TATAT6− ligand on the surface of the pores are potential binding sites for guest molecules and thus for the luminescent detection of metal ions.

interactions. The strong absorption bands of H6TATAT, TbMOF, and Eu-MOF in the UV region can be attributed to the n → π* and π → π* transitions of the ligand (Figures S28 and S29). The new absorption band of Tb-MOF (or Eu-MOF) in the visible region after adsorbing ethanol, acetonitrile, or diethyl ether may arise from solvent-to-ligand charge transfer104 or metal-to-ligand charge transfer affected by solventMOF interactions.42 Photoluminescent Properties and Luminescent Sensing. The solid photoluminescence excitation and emission spectra of compounds Tb-MOF, Eu-MOF, Dy-MOF, and GdMOF were investigated at room temperature. Tb-MOF exhibits four emission peaks at 492, 548, 585, and 625 nm, attributed to the 5D4 → 7FJ (J = 6−3) transitions of the Tb3+ (Figure 8a), respectively. Eu-MOF displays the characteristic emission bands of Eu3+ at 582, 595, 619, 654, and 707 nm, which correspond to the 5D0 → 7FJ (J = 0−4) transitions (Figure 8b), respectively. The excitation spectra of Eu-MOF show broad bands of TATAT6− ligand and sharp characteristic excitation bands of Eu3+ when monitored at 619 nm. Therefore, we measured the emission spectra of Eu-MOF at different excitation wavelengths. The results show that the emission spectra of Eu-MOF contain ligand-centered emission and metal-centered emission, and the relative intensities of the two kinds of emissions vary with the excitation wavelength (Figure S34). The excitation-wavelength-dependent photoluminescence allows Eu-MOF to have color-tunable property by adjusting the wavelength of the excitation light (Figure S35). Under excitation at 341 nm, Dy-MOF emits two F

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the coordination between the Lewis basic N atoms of the TATAT6− ligands and Co2+ ions.117−120 Also, Co2+ can easily enter the pores of the Tb-MOF by exchange with counter [(CH3)2NH2]+ cations (Figure S50). The luminescence decay curves of 5D4 → 7F5 (548 nm) of Tb-MOF and Co2+@TbMOF were measured at room temperature under excitation of 341 nm. The decay behavior of Tb-MOF can be well-fitted by a single exponential function (I = A exp(−t/τ)) with a lifetime of τ = 1137.72 μs, while the decay behavior of Co2+@Tb-MOF can be well-fitted by a double exponential function (I = A1 exp(−t/τ1) + A2 exp(−t/τ2)) with lifetimes of τ1 = 20.53 μs and τ2 = 279.00 μs (Figure S51). In addition, the solid UV−vis absorption spectra of Co2+@Tb-MOF overlap with the emission spectra of Tb-MOF to some extent (Figures S52 and S53). As mentioned above, the reasons why the Co2+@ Tb-MOF has a stronger luminescence quenching effect can be summarized as follows: (1) the cationic exchange and strong interactions between Lewis basic N atoms in TATAT6− ligands and Co2+ can weaken the energy transfer from ligands to Tb3+ and thus reduce the luminescence intensity of Tb-MOF; (2) the luminescence lifetime of 5D4 → 7F5 of Tb3+ is reduced from 1137.72 to 20.53 μs and 279.00 μs in the presence of Co2+, maybe because the interactions between Co2+ and the framework increase the nonradiative transition of the excited state Tb3+. This will reduce the radiative transition probability of the excited state Tb3+; (3) the Co2+ ions in the channels of Co2+@Tb-MOF absorb part of the emission of Tb-MOF (radiative energy transfer), so that the apparent luminescence intensity is reduced. Therefore, the luminescence quenching of Co2+@Tb-MOF may be caused by a combination of factors, including cationic exchange, strong framework−Co2+ interactions, and radiative energy transfer. The reusability experiments of Tb-MOF for Co2+ ion sensing can be achieved by using dimethylamine hydrochloride to activate the quenched Tb-MOF. The framework of the activated Tb-MOF is still stable (Figure S54). Although the luminescence intensity of Tb-MOF after activation is lower than that of the original Tb-MOF, it can still be used to detect Co2+ ion (Figures S55 and S56).

Therefore, we soaked 30 mg of Tb-MOF (Eu-MOF, Dy-MOF, or Gd-MOF) in 10 mL of DMF solutions of M(NO3)x (M = Zn2+, Ni2+, Cd2+, Na+, Cu2+, Cr3+, Co2+, Mg2+, Mn2+, Pb2+, or Fe3+; metal ion concentration: 1 mmol L−1) for 12 h; then, the crystals were filtered and washed with DMF to obtain the Mx+@Tb-MOF (Mx+@Eu-MOF, Mx+@Dy-MOF, or Mx+@ Gd-MOF). The photoluminescence spectra of Mx+@Tb-MOF, Mx+@Eu-MOF, Mx+@Dy-MOF, and Mx+@Gd-MOF were measured upon excitation at 341, 396, 341, and 370 nm, respectively. The results reveal that Cu2+@Eu-MOF, Fe3+@ Dy-MOF, and Mn 2+ @Gd-MOF exhibit the strongest luminescence quenching in Mx+@Eu-MOF, Mx+@Dy-MOF, and Mx+@Gd-MOF (Figures S37−S42), respectively. The quenching efficiencies of Cu2+@Eu-MOF, Fe3+@Dy-MOF, and Mn2+@Gd-MOF are 50%, 66%, and 64%, respectively, while Co2+@Tb-MOF displays the most distinct luminescence quenching among Mx+@Tb-MOF, and its quenching efficiency is 87% (Figures 9 and S43). It is worth noting that the Tb-

Figure 9. Luminescent intensity at 548 nm of Tb-MOF treated with 1 mmol L−1 various metal cations in DMF.

MOF is the first single-component lanthanide MOF for the selective luminescence detection of Co2+ ion. The sensitivity and selectivity of Eu-MOF, Dy-MOF, and Gd-MOF for the luminescent detection of Cu2+, Fe3+, and Mn2+, respectively, are normal, so there is no further study on the metal ion sensing properties of Eu-MOF, Dy-MOF, and Gd-MOF. The PXRD patterns show that the frameworks of Mx+@Eu-MOF, Mx+@Dy-MOF, and Mx+@Gd-MOF remain unchanged (Figures S44−S46). We investigated the effect of Tb-MOF on the luminescent detection of Co2+ in the presence of other metal ions. The antijamming experiments indicate that adding 1 equiv of Co2+ ions to DMF solution containing Ni2+, Na+, Cr3+, Mg2+, Mn2+, or Fe3+ can reduce the luminescence intensity of Mx+@Tb-MOF by more than 40% (Figure S47). This means that the presence of other metal ions (Zn2+, Ni2+, Cd2+, Na+, Cu2+, Cr3+, Mg2+, Mn2+, Pb2+, or Fe3+) reduces the selective detection of Co2+ ions by Tb-MOF. To explore the possible luminescence quenching mechanism of Co2+@Tb-MOF, we measured the PXRD patterns of Mx+@ Tb-MOF, and the results show that the structure of Tb-MOF holds integrity after adsorption of metal ions (Figure S48). The N 1s XPS spectra of Co2+@Tb-MOF and Tb-MOF exhibit that Co2+@Tb-MOF has a higher binding energy (399.5 eV) for the N 1s peak compared to Tb-MOF (399.1 eV; Figure S49). The increase of binding energy indicates a decrease in the electron density of the N atoms, which can be attributed to

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CONCLUSIONS

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ASSOCIATED CONTENT

In summary, we have synthesized four new isostructural MOFs (Tb-MOF, Eu-MOF, Dy-MOF, and Gd-MOF) with porous anionic frameworks by solvothermal method using ligands containing multiple Lewis basic sites. The four Ln-MOFs are capable of highly selective adsorption and separation of cationic dyes (methylene blue) based on cationic exchange. After soaking with ethanol, acetonitrile, and diethyl ether, the color of Tb-MOF (or Eu-MOF) changed significantly from colorless to golden, dark orange, and dark red, respectively. This allows Tb-MOF and Eu-MOF to be used to sense ethanol, acetonitrile, and diethyl ether by the naked eye. In addition, Tb-MOF exhibits excellent detection ability toward Co2+ ion.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01319. G

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Inorganic Chemistry Additional structure figures, TGA, PXRD, IR, XPS, UV− vis absorption spectra, UV−vis diffuse reflectance spectra, and luminescence plots (PDF)

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Accession Codes

CCDC 1815628−1815631 contain 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

*E-mail: [email protected]. (J.Z.) *E-mail: [email protected]. (X.Z.) *E-mail: [email protected]. (W.L.) ORCID

Jingping Zhang: 0000-0001-8004-3673 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21573036 and 21274017), Foundation of the Education Department of Jilin Province (111099108), and Jilin Provincial Research Center of Advanced Energy Materials (Northeast Normal Univ.).

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DOI: 10.1021/acs.inorgchem.8b01319 Inorg. Chem. XXXX, XXX, XXX−XXX