Improving Water-Stability and Porosity of Lanthanide Metal–Organic

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Improving Water-Stability and Porosity of Lanthanide Metal− Organic Frameworks by Stepwise Synthesis for Sensing and Removal of Heavy Metal Ions Sheng-Quan Lu, Yong-Yao Liu, Zhi-Ming Duan, Zhao-Xi Wang, Ming-Xing Li, and Xiang He* Department of Chemistry, College of Sciences, Shanghai University, Shanghai 200444, China

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S Supporting Information *

ABSTRACT: Employing polynuclear cluster-based nodes can make metal−organic frameworks have higher porosity and good stability. In this work, by using compound [(Me2NH2)Eu(L)(DMA)(H2O)]n (1) as reactant, polynuclear cluster-based [(Me2NH2)Eu5(L)4(DMA)4(H2O)6]n (2) (H4L = 1,3-bis-[3,5-bis(carboxy)phenoxy]propane) can be synthesized successfully. Although two compounds are all three-dimensional microporous anionic frameworks, the water-stability and porosity of compound 2 is higher than that of compound 1. Compound 2 is assembled by two kinds of multinuclear metal units in which one Eu(III) ion center is hexa-coordinated. The luminescence studies exhibited that compound 2 performed a highly selective photoluminescence quenching toward nitrobenzene and Fe(III) ion. So, compound 2 can be possibly developed as a dual functional chemosensor for detecting Fe(III) ion and nitrobenzene. Furthermore, compound 2 also has the potential ability to absorb and remove toxic metal ions from aqueous solution.

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lifetime,29,30 high quantum yields31−33 and visible color with the naked eye,34−36 which derived from f−f transitions via socalled “antenna effect”.37,38 However, it is still difficult for chemists to design and prepare Ln-MOFs that can respond quickly and sensitively to metal ions or small solvent molecules. Because high coordination numbers and sensitivity to external conditions are all factors that influence the synthesis results.39−41 However, rational design or precise control syntheses is a very important way to obtain functional MOFs for their application. Based on the above considerations, increasing the stability and porosity of MOFs is an imperative issue to be solved. As references demonstrated, utilizing multinuclear clusters results in MOFs with higher stability and larger porosity.19,42,43 In order to design MOFs with ultrastability and high porosity, stepwise pre- and postsynthetic modification methods were used as effectively.44−47 In this work, by stepwise synthesis, polynuclear cluster-based [(Me2NH2)-

INTRODUCTION With development of industries, chemical pollutants are gradually entering into our living environment. For example, toxic organic matters and heavy metal ions are all seriously affect people’s daily life. As an important industrial raw material, nitrobenzene (NB) can be used in the preparation of plastic and pesticides,1−4 but extensive use of it can bring severe environmental and health issues. However, the pollution caused by heavy metal ions is also a problem. Although iron element is very important in living biological systems, changing Fe3+ concentration will cause damage to the human body.5−7 As a result, searching of new chemosensors for rapid detection of NB and metal ions in solvent systems and even to find materials to capture and remove metal ions is quite desirable.8,9 Lanthanide metal−organic frameworks (Ln-MOFs) have gained much attention on detecting metal cations,10−13 anions,14−16 and small organic molecules17−20 attributed to their rapid response, low cost, high sensitivity, easy recyclability, and efficiency.21−24 As fluorescent probes, compared to transition metal MOFs, Ln-MOFs have superior photoluminescent properties such as extremely sharp characteristic emission,25,26 large Stocks shifts,27,28 long luminescent © XXXX American Chemical Society

Received: April 17, 2018 Revised: June 19, 2018

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DOI: 10.1021/acs.cgd.8b00575 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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atmosphere. The photoluminescent properties were performed with on a Shimadzu RF-5301PC fluorescence spectrophotometer in 200− 800 nm region. Synthesis of [(Me2NH2)Eu(L)(DMA)(H2O)]n (1). Eu(NO3)3·6H2O (45 mg, 0.10 mmol) and H4L (12 mg, 0.03 mmol) were added with 7.5 mL of DMA and 1.5 mL of H2O with the addition of 8 drops of 2 mol/L HNO3. The mixture transferred into a 15 mL Teflon-lined stainless steel autoclave, sealed tightly, heated at 140 °C for 72 h and then cooled to room temperature at a rate of 10 °C/h. Colorless block-like crystals of 1 were collected. Elemental analysis (%): Calcd for C25H31N2EuO12, C, 42.68; H, 4.44; N, 3.98. Found, C, 43.03; H, 4.66; N, 3.95. IR (KBr pellet, cm−1): 3242 w, 2959 w, 2918 w, 2776 w, 1659 s, 1609 s, 1550 s, 1448 m, 1374 s, 1261 s, 1132 m, 1070 w, 1020 w, 927 w, 786 m, 712 m. Synthesis of [(Me2NH2)Eu5(L)4(DMA)4(H2O)6]n (2). Freshly prepared 1 (20 mg) and 9 mg of Eu(NO3)3·6H2O were soaked in DMA aqueous solution (v/v = 1:1, 9 mL) with the addition 1 mL of 2 mol/ L HNO3 aqueous solution in a 20 mL Teflon-lined stainless steel vessel and heated at 100 °C for 72 h. Colorless block-like crystals of 2 were isolated by washing with DMA. Elemental analysis (%): Calcd for C94H104N5Eu5O50, C, 39.43; H, 3.66; N, 2.45. Found, C, 39.91; H, 3.99; N, 2.95. IR (KBr pellet, cm−1): 3845 w, 3741 w, 2918 w, 1610 s, 1552 s, 1453 s, 1381s, 1319 m, 1262 s, 1134 w, 1070 w, 1021 w, 924 w, 779 m, 714 m. X-ray Crystallography. The data of 1 and 2 were collected on a Bruker Smart Apex-II CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å) at room temperature. Empirical absorption corrections were applied using SADABS program. The structures were solved by direct method and refined by full-matrix least-squares on F2 with anisotropic displacement.50 Non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms on carbon were calculated in ideal positions with isotropic displacement parameters set to 1.2 × Ueq of the attached atom. The unit cell of 1−2 contains a large region of highly disordered solvent molecules, which could not be modeled as satisfactory discrete atomic sites, and therefore, PLATON/SQUEEZE was employed to remove these

Eu 5(L) 4 (DMA) 4 (H2O) 6 ]n (2) (H4L = 1,3-bis-[3,5-bis(carboxy)phenoxy]propane; Scheme 1) can be synthesized Scheme 1. Schematic Diagram Structures of H4L

successfully by using [(Me2NH2)Eu(L)(DMA)(H2O)]n (1) and europium nitrate as reactants. The topology of two compounds can all be represented as the same 4,8-connected net. Because the water-stability and porosity of compound 2 is higher than those of compound 1, compound 2 is selected to study the luminescent properties and shows high sensitivity for nitrobenzene molecules and Fe3+ ions. Furthermore, compound 2 also showed potential for toxic metal ion capture and removal from aqueous solution. The detection sensitivity can reach ppm level for Fe3+ or Pb2+.

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EXPERIMENTAL SECTION

Materials and Methods. The ligand 1,3-bis-[3,5-bis(carboxy)phenoxy]propane were prepared according to the literature method.48,49 All other reagents were received from commercial suppliers and used as received without further purification. The powder X-ray diffraction patterns (PXRD) were obtained from a Rigaku D/Max2550 V/PC. Elemental analyses of C, H, and N were performed with a Vario EL III analyzer. The Fourier transform infrared (FT-IR) spectra using KBr pellets were recorded with a Nicolet A370 FT-IR spectrometer in the range 4000−400 cm−1. Thermogravimetric analyses (TGA) were completed on a Netzsch STA 449C thermal analyzer at a heating rate of 10 °C·min−1 under N2

Figure 1. (a) View of the coordination environment of Eu3+ center in 1. (b) View of the dinuclear Eu3+ cluster in 1. (c) Three-dimensional structure along the c-axis of 1. (d) Three-dimensional topology along the c-axis of 1. B

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Figure 2. (a) View of the coordination environment of Eu3+ center in 2. (b) View of the dinuclear and trinuclear Eu3+ cluster in 2. (c) View of the 3D MOF along the a-axis of 2. (d) View of the 3D topology along the a-axis of 2.

network with rectangular channels of 10.83 Å × 8.99 Å (Figure 1c, considering van der Waals radii). As estimated by PLATON program, the total effective free volume of 1 with removal of solvent molecules is 13.6%, and cations and solvent molecules as guests are located in the voids.52 As binuclear Eu3+ units can be treated as 8-connected nodes and the L4− ligands as 4-connected nodes, compound 1 can be regarded as a 4,8-connected net with a topological point symbol of {412· 610·86}{46}2 (Figure 1d). Structure of [(Me2NH2)Eu5(L)4(DMA)4(H2O)6]n (2). The crystal structure of 2 recrystallizes in the space group P1̅ and features a three-dimensional framework structure. The asymmetric unit of 2 contains two and a half europium ions, two H4L ligands, one DMA molecule , and three water molecules as shown in Figure 2a The coordination environment of Eu1 is the same as that in compound 1, and two Eu3+ ions are bridged by carboxylate groups as a binuclear unit with a Eu···Eu separation of 4.4185 Å (Figure 2b). Eu2 is located in the center of inversion, coordinated by six carboxylic oxygens from H4L ligands. As we know, hexa-coordinated Eu(III) ion is rarely reported.53−55 The hexa-coordinated Eu3+ ion is formed due to the strong hindrance. The hexa-coordinated Eu3+ ion exists between the two eight-coordinated Eu3+ ions with the distance of 4.5405 Å. The coordination environments of the two eight-coordinated Eu3+ ions made the space of the central six-coordinated Eu3+ ion so crowded. Also, there are six isophthalic acidic groups of L ligands around the six-

electron densities.51 As two Eu-MOFs are all anionic frameworks, (Me2NH2)+ should exist in the channel. A summary of the crystallographic data for 1 and 2 is listed in Table S1. Selected bond lengths and angles for 1 and 2 are collected in Table S2.

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RESULTS AND DISCUSSION Structure of [(Me2NH2)Eu(L)(DMA)(H2O)]n (1). Compound 1 crystallizes in monoclinic space group C2/c and displays a 3D architecture. The asymmetric unit contains one crystallographically independent Eu3+ ion, one L4− ligand, one DMA, and one lattice water molecule (Figure 1a). Each Eu3+ ion is eight coordinated by one monodentate carboxylic oxygen atom (O10), two chelate carboxylic oxygen atoms (O3, O4), four bridging carboxylic oxygen atoms from different H4L ligands (O1, O2, O7, O8), and one oxygen atom from a DMA molecule (O23), displaying a distorted square antiprism coordination geometry (Figure S1a). The average Eu−O bond length is 2.413 Å within the range of 2.327−2.546 Å. The bond angles for O−Eu−O are in the range 51.39−160.70°. All O−Eu−O bond angles and Eu−O bond lengths are within the expected ranges. The two neighboring Eu3+ ions are bridged by four μ2-η1:η1-carboxylate groups to generate the [Eu2(COO)4] group as a binuclear unit with a Eu···Eu separation of 4.3901 Å (Figure 1b). These units are further linked through isophthalic acid moieties of the totally deprotonated L4− ligands to generate a 1D chain along the b axis (Figure S1b). Each binuclear unit is linked by eight L4− ligands to form a 3D C

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coordinated Eu3+ ion, making other coordinated groups hardly link to Eu3+ ions. All these make strong steric hindrance to the hexa-coordinated Eu3+ ions. Eu3 is coordinated by one chelating carboxylic oxygen, three μ2-bridging carboxylic oxygens, and three water molecules to form a distorted octahedral environment. The neighboring two Eu3’s are connected through one Eu2, forming a trinuclear unit of [Eu3(CO)6] (Figure 2b). Two kinds of polynuclear units are connected through isophthalic acid moieties of the totally deprotonated H4L ligands (Figure S2). Both binuclear and trinuclear are linked by eight L4− ligands to form a 3D network, which has channels of 15.68 Å × 8.28 Å along the a-axis (Figure 2c, considering van der Waals radii). As estimated by PLATON program, the total effective free volume of 2 with removal of solvent molecules is 24.9%. On considering binuclear and trinuclear cores as eight-connected node and L4− ligand as four-connected node, the framework of 2 can also be represented as a (4,8)-connected 3D cationic framework (Figure 2d), which has the same topological point symbol as compound 1. Coordination Modes of the Tetracarboxylate Ligand. From the structure descriptions above, we can see that the L4− anion can adopt different kinds of coordination modes, despite all of the H4L ligands in 1 and 2 deprotonated and characterized as a V-shape conformation. In 1, the L4− anions connect six metal ions and adopt the μ7-η1:η1:η1:η1:η1:η1:η1:η0 mode (Scheme S1), while the dihedral angle between two aromatic rings of ligand is 86.789° and the angle of the central −CH2−CH2−CH2− group is 115.492(17)° (Figure S3a). In 2, two types of L4− anions exhibit two coordination modes (La and Lb). La is connected to seven Eu(III) ions with all the carboxylate groups in μ2-η1:η1 modes (Scheme S1), while Lb is the same as mode I. The dihedral angle between two aromatic rings of La and Lb is 89.474° and 83.374° (Figure S3b), and the angles of the central −CH 2 −CH 2 −CH 2 − group are 114.740(86)° and 114.528(77)°, respectively. The dihedral and torsion angles of the H4L ligand in these two compounds are summarized in Table S3. Synthesis. As the ligand has flexibility, dynamic structural transformations might happen. So, stepwise synthesis was tried. By using crystals of compound 1 (20 mg) and 9 mg of Eu(NO3)3·6H2O soaked in 9 mL of DMA aqueous solution (v/v = 1:1) with the addition 1 mL of 2 mol/L HNO3 aqueous, which heated at 100 °C for 3 days, compound 2 was obtained. However, this conversion is irreversible (Figure 3a). Compound 1 cannot be obtained when H4L ligand is added under the same conditions (Figure S4). The conversion relationship between compounds 1 and 2 is shown in Figure 3b. In comparing the structure of 1 and 2, we find both compounds have similar structure. First, these two compounds have the same topology. Second, the deprotonated H4L ligand has similar coordination modes. Finally, the deprotonated H4L ligand also has the similar conformation. These similarities made this transformation successful. This transformation is through postsynthetic uptake of free Eu(III) ions in solution and results in an expansion of some units from dinuclear to trinuclear. This process not only provides us a method to construct polynuclear-based MOFs with presynthisized polynuclear units but also represents a new type of transformation. Step-wise synthesis can give us an effective route to regulate the structures and their chemical properties of MOFs.

Figure 3. (a) XRD patterns for compounds 1 and 2 and simulated 1 and 2. (b) Illustration of the detailed structure conversion of compounds 1 and 2.

Infrared Spectra. As shown in Figure S5, the broad band around 3400 cm−1 can be assigned to the characteristic O−H vibration. The absence of absorption peaks at 1720−1680 cm−1 in both compounds 1 and 2 indicates that the H4L ligands were completely deprotonated. The band at 1610 cm−1 for 1 and 2 can be attributed to the CO vibration of DMA. The peaks between 1555 and 1440 cm−1 are rising from the C−C stretching vibrations of aromatic rings. The symmetric COO− vibrations are located in the 1400−1300 cm−1 region. PXRD Pattern and Thermal Analysis. Powder X-ray diffractions for 1 and 2 are shown in Figure S6. According to the results, all the diffraction peaks that we synthesized were coinciding to the simulated patterns, showing good crystalline phase purity. To confirm the stability of compounds 1−2 in water, PXRD (Figure S7) measurements were conducted for 1 and 2 after a week of immersion in pure water. According to the PXRD data, the water-stability of two compounds was different. PXRD pattern of compound 2 are still consistent with that of as-prepared 2, indicating its water-stability is better than compound 1, which is essential to its practical application. To estimate the thermal stability of 1 and 2, thermogravimetric analyses were carried out in 20−900 °C range under nitrogen atmosphere (Figure S8). For 1, a weight loss of 2.15% from beginning to 134 °C is consistent with the removal of one lattice water (calcd 2.55%), and then a weight reduction is 17.74% in 134−400 °C range, showing great resemblance to the loss of one DMA molecule and one dimethylamine cation ion (calcd 18.93%). Compound 1 begins to collapse from 400 °C. Compound 2 lost 6.00% of its weight from the room temperature to 187 °C, which is attributed to the loss of two DMA molecules (calcd 6.08%), and then a weight loss of 11.81% in the temperature of 187−400 °C is in agreement with the weight of six coordinated water molecules, two DMA coordinated molecules, and the encapsulated [Me2NH2]+ cations (calcd 11.36%). Above 400 °C, the frameworks decompose rapidly. D

DOI: 10.1021/acs.cgd.8b00575 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 4. (a) Emission spectra of compound 2 immersed into different solvents. (b) Luminescent intensity of 5D0−7F2 transition of compound 2 immersed into different solvents.

Figure 5. (a) Emission spectra of compound 2 immersed into DMA of different metal ions. (b) Luminescent intensity of 5D0−7F2 transition of compound 2 immersed into DMA of different metal ions.

Figure 6. (a) Emission intensity of 5D0−7F2 transition of compound 2 immersed into different concentrations of Fe(NO3)3 DMA solutions. (b) Plots of the relative luminescent intensity (I0/I − 1) versus the concentration of Fe3+.

yields and luminescence lifetimes are similar to other Eu3+ compounds.56,57 Detection of Organic Molecules. Because the porosity of 2 is higher than 1, we select 2 to further explore its luminescence for sensing organic molecules. The luminescence characteristic emissions of 2 were affected by different solvents (DMF, DMA, diethyl ether, ethanol, methanol, chloro-

Luminescent Property. The luminescent spectra of compounds 1 and 2 show that both of them exhibited characteristic red light emission of the Eu3+ ion at 592, 617, 653, and 702 nm, which can be ascribed to the 5D0−7F1, 5 D0−7F2, 5D0−7F3, and 5D0−7F4 transitions of Eu(III) ions. The luminescence quantum yield (ΦQY) and lifetime of 2 was 12% and 1.006 ms, respectively (Figure S10). The quantum E

DOI: 10.1021/acs.cgd.8b00575 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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fluorescence, but as indicated for the PXRD of compound 2 (Figure S14), the framework retained its structural integrity after adding into Fe(III) ion. Hence, the reason for luminescence quenching is probably that the interaction between Fe3+ and oxygen atoms changes the electron energy level of ligand, resulting in the inefficient energy transfer between ligand and Eu(III) ions.57 So far, numbers of MOFs have been successfully used to detect small molecules and ions. Such as, Zang5 synthesized one water-stable Tb-DSOA compound and used it to detect Fe(III) ions in the range of 10−6−10−1 mol/L. The Ksv value of Tb-DSOA for Fe3+ ions is 3543 M−1. Liu63 used Tb-MOFs to sense Fe3+ ions, and the value of Ksv was 2063 M−1. When the concentration is 1 × 10−2 mol/L, the quenching efficiency could almost reach 100%. Comparably, 2 exhibits similar sensitivity and selectivity with others’ reports. Heavy Metal Ions Removal. Because of the anionic framework and water stability of 2, we study its ability to absorb and remove metal ions from wastewater. In order to check the ability to capture in low concentration, 20 mg of compound 2 was soaked in 20 mL of 1 ppm metal ion solution (Cu2+, Pb2+, Fe3+, Cr3+) for cation exchange. Using inductively coupled plasma optical emission spectroscopy (ICP-OES), the initial and residual metal ion concentration was analyzed. The results show that the solution was almost completely cleared within 24 h of metal ions addition (Table 1), especially for

methane, acetonitrile, acetone, water, hexane, benzene, and nitrobenzene). As shown in Figure 4, the predominant feature is that its fluorescence intensity could be selectively quenched by NB. Other solvents have negligible effect. It indicated 2 can selectively detect NB in solution. Furthermore, the sensitivity of 2 toward NB was investigated by increasing the NB concentration in 3 mL of DMA suspension containing 3 mg of finely dispersed 2. In order to explore the luminescence quenching degree, the quantified value of the quenching effect of nitrobenzene was obtained using the Stern−Volmer equation I0/I = 1+Ksv [M], in which I0 and I are the luminescence intensities of the DMA solvent of compound 2 before and after the addition of nitrobenzene, and [M] is the concentration of nitrobenzene.58 As shown in Figure S11, the Stern−Volmer plots for nitrobenzene exhibited good linear correlations, and the value of Ksv was estimated as 155.7 M−1. The results show that 2 are highly sensitive to nitrobenzene molecular, which indicates its potential application as sensor materials for explosives. The reason for luminescence quenching was basically attributed to the charge transfer from the electron-donating framework of 2 to the electronwithdrawing −NO2 group in NB.59,60 The stability of 2 treated with NB was proved by PXRD in Figure S12, which showed that the framework retained its structural integrity. Metal Ion Sensing. The [Me2NH2]+ cation inserted in the channels is beneficial for 2 to detect metal ions. The crystalline materials of 2 were ground into powder samples and added in 3 mg samples into 3 mL of 0.01 M DMA solvents containing metal ions Mn+ (M = Na+, K+, Ag+, Ca2+, Cu2+, Mn2+, Co2+, Ni2+, Mg2+, Cd2+, Pb2+, Zn2+, Fe3+, Cr3+,) at room temperature for 1 day, then treated by ultrasonic agitation for over 30 min to form stable suspensions. As shown in Figure 5, the fluorescence responses of 2 are strongly dependent on the metal ions. Obviously, Fe(III) exhibits a drastic quenching effect on the emission of 2. Other cations have an insignificant effect on the decrease of the fluorescence intensity, suggesting that 2 has a high selectivity for sensing Fe(III) ion. To understand the response of the fluorescence of 2 to Fe(III) ion, the fluorescence titration upon the addition of a solution of Fe(NO3)3 dissolved in DMA to 2 was further performed. As shown in Figure 6, the increasing of Fe(III) ion concentration from 1 × 10−4 to 2.5 × 10−3 mol L−1 causes the monotonic and drastic decrease of the fluorescence intensity of 2. The highest quenching efficiency is achieved by a concentration of Fe(III) ion at 2.5 × 10−3 M. In order to investigate the luminescence quenching degree, the quenching coefficient was calculated by using the Stern−Volmer equation. The SV plot exhibited good linear correlation, and the value of Ksv was estimated as 3.5 × 103 M−1. Thus, 2 can be considered as potential material for selective sensing of Fe(III) ion. In general, there are lots of metal ions coexisting in practical biological and environment systems;61,62 thus, to further investigate other metal ions that affect the selectivity for Fe(III) ions, the Fe3+ and same equiv of other metal ions (Na+, K+, Ag+, Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Cr3+) were added to the stable DMA suspension of 2. As shown in Figure S13, it is very interesting to note that the luminescence intensities of DMA suspension of mixed metal ions were dramatically quenching, which clearly reveals the high selectivity of compound 2 toward Fe3+ ions, even including other metal ions. Next, the possible mechanism is discussed. Although framework collapse is always a common way to quench the

Table 1. Results for the Heavy Metal Ion Exchange from 1 ppm Aqueous Solution heavy metal

initial concentration of ions (ppm)

concentration of ions after exchange (ppm)

removal efficiency (%)

Cu2+ Pb2+ Fe3+ Cr3+

1 1 1 1

0.061 0.005 0.008 0.254

93.9 99.5 99.2 74.6

Pb2+ ions (99.5%) and Fe3+ ions (99.2%). Thus, 2 can be used for toxic metal ion capture and removal from aqueous solution. The detection sensitivity can reach ppm level. Its high stability can be proved by the PXRD after immersion in the aqueous solution of Fe(III) ions (Figure S15).

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CONCLUSION In summary, two 3D anionic Eu-based MOFs with the same topology have been synthesized by using flexible carboxylic ligand H4L. Compound 1 presents a 3D structure with dinuclear metal unit, while compound 2 is assembled not only by dinuclear unit but also trinuclear unit in which one Eu(III) ion center is rarely reported hexa-coordinated. By stepwise synthesis, compound 2 can be irreversibly obtained from compound 1. This method can efficiently tune structures of MOFs and their chemical properties. Due to that the porosity of 2 is higher than 1, compound 2 is selected to further study the luminescent property and show high sensitivity for nitrobenzene molecules and Fe3+ ions. The quenching constants of nitrobenzene and Fe3+ are 155 and 3500 M−1, respectively, which is comparable with the previously reported MOF-based fluorescent probes for the corresponding analytes. Furthermore, compound 2 also shows potential for capture and removal of toxic metal ions from aqueous solution with a detection limit in the ppm level. F

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b00575. Details about crystallographic data, additional structure graphics, PXRD, IR, TGA, and photoluminescence spectra (PDF) Accession Codes

CCDC 1527356 and 1838243 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 Author

*E-mail: [email protected]. Tel: +86-21-66132670. ORCID

Zhi-Ming Duan: 0000-0002-8332-6131 Zhao-Xi Wang: 0000-0002-2689-7034 Ming-Xing Li: 0000-0003-0000-9876 Xiang He: 0000-0002-2417-9063 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 21203117, 21201118) and Natural Science Foundation of Shanghai (No. 17ZR1410600) for financial support.

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REFERENCES

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DOI: 10.1021/acs.cgd.8b00575 Cryst. Growth Des. XXXX, XXX, XXX−XXX