Lanthanide–Potassium Biphenyl-3,3′-disulfonyl-4,4′-dicarboxylate

Article pubs.acs.org/IC

Lanthanide−Potassium Biphenyl-3,3′-disulfonyl-4,4′-dicarboxylate Frameworks: Gas Sorption, Proton Conductivity, and Luminescent Sensing of Metal Ions Li-Juan Zhou,†,§ Wei-Hua Deng,‡,§ Yu-Ling Wang,*,† Gang Xu,‡ Shun-Gao Yin,† and Qing-Yan Liu*,† †

College of Chemistry and Chemical Engineering, Key Laboratory of Small Functional Organic Molecule of Ministry of Education, Jiangxi Normal University, Nanchang, Jiangxi 330022, P. R. China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China S Supporting Information *

ABSTRACT: A novel sulfonate−carboxylate ligand of biphenyl-3,3′-disulfonyl4,4′-dicarboxylic acid (H4−BPDSDC) and its lanthanide−organic frameworks {[LnK(BPDSDC)(DMF)(H2O)]·x(solvent)}n (JXNU-2, where JXNU denotes Jiangxi Normal University, DMF indicates dimethylformamide, and Ln = Sm3+, Eu3+, and Pr3+) were synthesized and structurally characterized. The three isomorphous lanthanide compounds feature three-dimensional frameworks constructed from one-dimensional (1D) rod-shaped heterometallic Ln−K secondary building units and are an illustration of a Kagome-like lattice with large 1D hexagonal channels and small 1D trigonal channels. The porous material of the representive JXNU-2(Sm) has an affinity to quadrupolar molecules such as CO2 and C2H2. In addition, the JXNU-2(Sm) compound exhibits humidity- and temperature-dependent proton conductivity with a large value of 1.11 × 10−3 S cm−1 at 80 °C and 98% relative humidity. The hydrophilic sulfonate group on the surface of channels facilitates enrichment of the solvate water molecules in the channels, which enhances the proton conductivity of this material. Moreover, the JXNU-2(Eu) material with the characteristic bright red color shows the potential for recognition of K+ and Fe3+ ions. The enhancing Eu3+ luminescence with the K+ ion and quenching Eu3+ luminescence with the Fe3+ ion can be associated with the functional groups of the organic ligand.

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INTRODUCTION The lanthanide−organic framework (LnOF) solids offer great promise for the development of materials with potential applications in photoluminescence,1 molecular magnets,2 adsorption,3 proton conduction,4 and sensing.5 Their aesthetically appealing structural topologies are associated with the high coordination number and flexible coordination geometry of the lantahanide ions.6 In contrast to the transition metalbased metal−organic frameworks (MOFs), the LnOFs are highly sought after because they can display distinct intrinsic optical and magnetic properties endowed by the lanthanide ions with 4f configuration.7 However, the selection of the organic ligand is very important for the preparation of the functional MOFs.8 For oxygen-containing organic ligand, the carboxylate ligands provide the most successful functional ligands due to the carboxylate group, which can exhibit various coordination modes when coordinated to the metal centers.9 In comparison with carboxylate group, the coordination chemistry of sulfonate has been less well-investigated due to the perception that sulfonate is a poor ligand.10 However, the weaker ligation nature of the sulfonate group facilitates formation of crystalline © XXXX American Chemical Society

products and may enable solid-state dynamics for the resulting frameworks.11 In this contribution, a novel sulfonate− carboxylate ligand, biphenyl-3,3′-disulfonyl-4,4′-dicarboxylic acid (H4−BPDSDC), was synthesized. The H4−BPDSDC ligand with a biphenyl backbone and two carboxyl and two sulfonyl functional groups can be considered as a remarkable organic precursor for preparation of the functional MOFs. Moreover, the H4−BPDSDC ligand with strong coordination ability of carboxylate groups and weak coordination ability of sulfonate groups can coordinate to the metal ions synergistically, which will give some interesting physical properties. Herein, three LnOFs {[LnK(BPDSDC)(DMF)(H2O)]·x(solvent)}n (JXNU-2, where JXNU denotes Jiangxi Normal University, DMF indicates dimethylformamide, and Ln = Sm3+, Eu3+, and Pr3+) were synthesized and structurally characterized. The sulfonate groups attached to the π-rich biphenyl backbones in these MOFs are on the surface of the one-dimensional (1D) channels, which endows the present Received: April 14, 2016

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DOI: 10.1021/acs.inorgchem.6b00928 Inorg. Chem. XXXX, XXX, XXX−XXX

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yl. 1H NMR (DMSO-d6, ppm): 7.86 (s, 4H), 8.12 (s, 2H); IR spectrum (cm−1, KBr pellet): 3521 (s), 2924 (m), 1715 (s), 1597 (m), 1547 (w), 1505 (m), 1400 (m), 1384 (m), 1372 (m), 1287 (m), 1265 (m), 1213 (s), 1183 (w), 1135 (m), 1046 (s), 1021 (s), 900 (m), 825 (s), 809 (m), 732 (m), 648 (s), 626 (s). Synthesis of {[LnK(BPDSDC) (DMF)(H2O)]·x(solvent)}n (JXNU2) (Ln = Sm3+, Eu3+ and Pr3+). The same process was employed to prepare these compounds, as follows. A mixture of lanthanide salt (SmCl3·6H2O (36.4 mg, 0.1 mmol), or Eu(NO3)3·6H2O (44.6 mg, 0.1 mmol), or Pr(NO3)3·6H2O (41.7 mg, 0.1 mmol)) and biphenyl-3,3′disulfonyl-4,4′-dicarboxylic acid (40.2 mg, 0.1 mmol) in 8 mL of dimethylformamide (DMF) and 2 mL of H2O was introduced into a 25 mL Parr Teflon-lined stainless steel vessel and heated at 85 °C for 4 d. Then the mixture was cooled naturally to form crystals. Crystalline product was filtered, washed with DMF, and dried at ambient temperature. Yield: 31.5 mg for JXNU-2(Sm); 30.2 mg for JXNU2(Eu); 32.3 mg for JXNU-2(Pr). IR spectrum (cm−1, KBr pellet): for JXNU-2(Sm), 3429 (s), 2935 (w), 1644 (s), 1601 (w), 1566 (w), 1534 (w), 1499 (w), 1480 (w), 1406 (s), 1253 (s), 1182 (m), 1088 (s), 1023 (s), 903 (w), 866 (m), 793 (m), 755 (m), 726 (w), 672 (m), 626 (m), 585 (w), 432 (m); for JXNU-2(Eu), 3415 (s), 2936 (w), 1655 (s), 1536 (w), 1499 (w), 1479 (w), 1394 (s), 1249 (m), 1179 (m), 1087 (s), 1022 (s), 903 (w), 887 (w), 853 (m), 793 (m), 753 (m), 726 (w), 626 (m), 585 (w), 530 (m), 430 (m); for JXNU-2(Pr), 3423 (s), 2937 (w), 1644 (s), 1600 (w), 1566 (m), 1534 (w), 1480 (w), 1398 (s), 1250 (m), 1179 (m), 1087 (s), 1021 (s), 902 (w), 888 (w), 861(s), 793 (m), 755 (m), 698 (w), 626 (m), 550 (w), 432 (m). X-ray Crystallography. X-ray diffraction data of compounds JXNU-2 were collected on a Bruker Apex II CCD diffractometer equipped with a graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). Data reduction was performed using SAINT and corrected for Lorentz and polarization effects. Adsorption corrections were applied using the SADABS routine.12 The structures were solved by the direct methods and successive Fourier difference syntheses and refined by the full-matrix least-squares method on F2 (SHELXTL Version 5.1).13 All non-hydrogen atoms were refined with anisotropic thermal parameters except for the oxygen, carbon, and nitrogen atoms of the coordination solvent molecules, which were kept isotropic thermal parameters due to poor thermal behavior. Aromatic hydrogen atoms were assigned to calculated positions. Hydrogen atoms of the solvent molecules were not located. For compounds JXNU-2(Sm) and JXNU-2(Eu), the coordinated water and DMF molecules, except for the oxygen atom of the DMF, could not be located in the difference Fourier map due to their disorder, but they are included in the formulas. The unit cell includes a large region of disordered guest solvent molecules, which could not be modeled and were treated by the SQUEEZE routine.14 The structures were then refined again using the data generated. Note that the formulas do not include the guest solvent molecules. The R1 values are defined as R1 = ∑||Fo| − |Fc||/∑| Fo| and wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2. The details of the crystal parameters, data collection, and refinement are summarized in Table 1, and the selected bond lengths are listed in Table S1.

MOFs with some interesting physical properties such as gas adsorption, proton conduction, and luminescent sensing.

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EXPERIMENTAL SECTION

Physical Measurements. IR (KBr pellets) spectra were recorded in the 400−4000 cm−1 range using a PerkinElmer Spectrum One FTIR spectrometer. Elemental analyses were performed on Elementar PerkinElmer 2400CHN microanalyzer. Thermogravimetric analyses were performed on a PE Diamond TG/DTA unit at a heating rate of 10 °C/min under a nitrogen atmosphere. Powder X-ray diffraction patterns were performed on a Rigaku Miniflex Π powder diffractometer using Cu Kα radiation (λ = 1.5418 Å). Gas sorption isotherms were measured using a Micromeritics ASAP2020 gas adsorption instrument up to 1 atm of gas pressure. The highly pure N2 (99.999%) and CO2 (99.999%) gases were used in the sorption experiments. The proton conductivity data were interrogated by the conventional quasi-four-probe method using gold paste and gold wires (50 μm diameter) with an Impedance/Gain-Phase Analyzer (Solartron SI 1260) under different environmental conditions. The electrochemical impedance spectroscopy (EIS) was taken by applying 100 mV of alternating current amplitude voltage over the frequency from 1 to 1 × 106 Hz in 10 steps on a logarithmic scale. The impedance results were analyzed in ZView2 (Scribner Associates), which was used to generate Nyquist plots with the real parts (Z′) as the abscissa and the imaginary part (Z″) as the ordinate. A pellet of JXNU-2(Sm) (2.5 mm in diameter and 1.27 mm in thickness) was compressed under a pressure of ∼500 MPa and exposed to controlled humidity and temperature environments, which was performed using an XKCTS80Z incubator (Shenzhen selenium control testing equipment corp). Fluorescence spectra were measured at room temperature with a single-grating Edinburgh EI980 fluorescence spectrometer. Chemicals. All chemicals were of reagent grade and used as commercially obtained. The biphenyl-3,3′-disulfonyl-4,4′-dicarboxylic acid (H4−BPDSDC) ligand was prepared as follows (Scheme 1).

Scheme 1. Preparation of H4−BPDSDC Ligand

Synthesis of H4−BPDSDC Ligand. (1) Synthesis of 3,3′Disulfonyl-4, 4′-dimethylbiphenyl. Sulfuric acid (30 mL; 98%) was placed in a 100 mL three-necked flask. The flask was immersed in an ice bath for 0.5 h, and then 10 g of 4,4′-dimethylbiphenyl was slowly added into the flask. The mixture was stirred at 60 °C for 1 h, 80 °C for 3 h, and 90 °C for 3 h. After it cooled to room temperature, the white powder product was obtained by adding lots of acetonitrile and methylene chloride. Yield: ∼86% based on 4,4′-dimethylbiphenyl. 1H NMR (deuterated dimethyl sulfoxide (DMSO-d6), ppm): 2.50 (s, 6H), 7.23 (d, 2H), 7.46 (d, 2H), 8.00 (s, 2H); IR spectrum (cm−1, KBr pellet): 3430 (s), 2244 (w), 1722 (m), 1476 (s), 1373 (m), 1184 (m), 1093 (m), 1071 (s) (OSO of −SO3H group), 815 (s), 720 (s), 629 (s) (C−S), 588 (w), 535 (m), 514 (m). (2) Synthesis of Biphenyl-3,3′-disulfonyl-4,4′-dicarboxylic Acid. 3,3′-disulfonyl-4,4′-dimethylbiphenyl (7.5 g), NaOH (0.5 g), and deionized water (75 mL) were placed in a three-necked flask. The mixture was stirred at 70 °C. Then 4.5 g of NaOH was slowly added into the mixture (pH = 12). The resulting mixture became clear, and after it stayed 1 h, finely powdered KMnO4 (12.3 g) was added into the flask in small portions. After it stirred at 90 °C for 9 h, the mixture was cooled to room temperature and filtrated. The filtrate was acidified with 12 M HCl. White solid was obtained from the resulting mixture after 10 h. Yield: ∼67% based on 3,3′-disulfonyl-4,4′-dimethylbiphen-

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RESULTS AND DISCUSSION Crystal Structure. Single-crystal X-ray diffraction analyses revealed that compounds JXNU-2(Sm), JXNU-2(Eu), and JXNU-2(Pr) are isomorphous and crystallize in a rhombohedral space group R3c (Table 1). There are one Ln3+ ion, one K+ ion, and one tetraanionic BPDSDC4− ligand in the asymmetric unit. Note that these compounds contain the K+ ion, which derives from the H4−BPDSDC ligand containing a small amount of K+ impurity. As shown in Figure 1, Ln atom is ninecoordinated by six carboxylate O atoms and two sulfonate O atoms from four BPDSDC4− ligands and one O atom of a DMF molecule. The coordination gemometry of the Ln atom can be described as a singly capped square antiprism with O7F at the capped position (Figure S1 in Supporting Information). The Ln−O bond distances vary from 2.380(4) to 2.674(4) Å (Table B

DOI: 10.1021/acs.inorgchem.6b00928 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data for JXNU-2 formula fw temp (K) cryst syst space group Z a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Dcalcd (g·cm−3) μ (mm−1) no. of reflns collected independent reflns obsd reflns.(I > 2σ(I)) F(000) R[int] R1a (I > 2σ(I)) wR2a (all data) CCDC no. a

Sm

Eu

Pr

C17H15NO12S2KSm 678.87 296(2) trigonal R3c 18 48.832(2) 48.832(2) 8.4834(6) 90 90 120 17 519(2) 1.158 1.760 42 323 8874 8435 5994 0.0434 0.0341 0.0974 1471126

C17H15NO12S2KEu 680.48 296(2) trigonal R3c 18 48.7999(3) 48.7999(3) 8.45290(10) 90 90 120 17 433.1(3) 1.167 1.872 32 991 8884 8342 6012 0.0302 0.0290 0.0808 1471127

C17H15NO12S2KPr 669.43 296(2) trigonal R3c 18 48.884(2) 48.884(2) 8.5632(6) 90 90 120 17 721(2) 1.129 1.486 35 014 8607 8116 5940 0.0400 0.0337 0.0937 1471128

R1 = ∑||Fo| − |Fc||/∑|Fo| and wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2.

Figure 1. Coordination environments of Ln3+ and K+ ions.

Figure 2. 3D framework showing the trigonal and hexagonal channels highlighting the heterometallic rod-shaped SBU.

S1). The K+ ion is six-coordinated by four sulfonate O atoms from three BPDSDC4− ligands, one carboxylate O of the fourth BPDSDC4− ligand, and one coordinated water molecule. The K−O bond distances range from 2.564(5) to 2.896(5) Å. As schemed in Figure S2, the BPDSDC4− ligand bridges four Ln3+ and four K+ ions through its four η2-O atoms and five unidentate O atoms. The Ln3+ and K+ ions are interconnected by the BPDSDC4− ligands to give a three-dimensional (3D) framework constructed from the 1D rod-shaped secondary building units (SBUs) running alone the c axis (Figure 2). The 1D rod-shaped SBU is constructed by the heterometallic Ln3+ and K+ ions bridged by the carboxylate and sulfonate groups, which is distinct from the common observed rod-shaped SBUs based on the homometallic carboxylate rods.15 Each rod-shaped heterometallic SBU is connected with four adjacent SBUs through the biphenyl linkers, resulting in the final 3D framework with two kinds of 1D channels propagating along

the c axis: small trigonal channels and large hexagonal channels (Figure 2). This 3D structure is an illustration of a Kagome-like lattice16 and reminiscent of vanadium-based terephthalate compound of MIL-68 (V(OH) (terephthalate)) based on the 1D rod-shaped vanadium(III)-hydroxyl−carboxylate SBUs.17 The trigonal channel has an approximate dimensionality of 13.5 Å, and the diameter of the hexagonal channel is ∼28.2 Å (based on the Ln ions). As depicted in Figure 2, the free sulfonate O10 atoms protrude into the channels in the small trigonal channels, while the sulfonate atoms of O4 and O5 atoms point into the hexagonal channels. These channels are occupied by large amounts of highly disordered solvated water and DMF molecules, which could not be located from the sing-crystal X-ray diffraction data (XRD). Gas Adsorption Properties. The experimental powder XRD patterns match well with the simulated ones, indicative of the phase purity of these compounds (Figure S3). Since the C

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Figure 3. (a) The N2 sorption isotherm at 77 K and (b) CO2 sorption isotherms at 273 and 293 K (solid symbols: adsorption, open symbols: desorption) for JXNU-2(Sm). (inset) The enthalpy of adsorption (Qst) as a function of CO2 uptake.

three compounds are isomorphous, the JXNU-2(Sm) is described here representatively to illustrate their gas adsorption and proton conductivity. The total solvent-accessible volume for JXNU-2(Sm) is estimated to be 58% using the PLATON program.14 The resulting thermogravimetric analyses revealed that the removal of solvent molecules occurs between 40 and 220 °C, and no further weight loss is observed below 380 °C (Figure S4). The activated sample of JXNU-2(Sm) was prepared by evacuation at 30 °C for 10 h and then at 150 °C for 10 h. The stability of the guest-free material is confirmed by the PXRD patterns (Figure S3a). The permanent porosity of JXNU-2(Sm) is verifieded by its N2 sorption isotherms at 77 K, which exhibits a typical reversible type I behavior and a very sharp uptake at P/P0 < 0.05 (Figure 3a), corroborating the microporous nature of JXNU-2(Sm). JXNU-2(Sm) takes up 230 cm3 (STP) g−1 of N2 at 1 atm with a Langmuir and BET surface areas of 985.6 and 738.6m2 g−1, respectively. As shown in Figure 3b, CO2 uptakes of JXNU-2(Sm) at 273 and 293 K (1 atm) are 66.6 and 41.2 cm3 g−1, respectively. Furthermore, the CO2 isotherms measured at 273 and 293 K were fitted using the virial equation (Figure S5), which gives the CO2 isosteric heat of adsorption (Qst) of 27 kJ mol−1 at zero loading and 26.4 kJ mol−1 at the maximum measured loading (Figure 3b inset). The Qst value for CO2 does not change appreciably with loading, indicating uniformity of the binding sites. Additionally, the Qst value for the present compound is moderately high, which is larger than those of MOFs with no open metal sites or polar functional groups (with Qst of 16−20 kJ mol−1),18 but it is comparable to those of MAF-25 (26.3(5) kJ mol−1)19 and InOF-1(29 kJ mol−1).20 The sorption performance of JXNU2(Sm) for CO2 prompted us to study the sorption of several fuel molecules such as CH4, C2H6, C3H8, and C2H2. At 273 K, JXNU-2(Sm) takes up less CH4 but adsorbs some amounts of C2H6, C3H8, and C2H2. Its uptake capacity for C2H6, C3H8, and C2H2 is 38.3, 60.7, and 79.1 cm3 g−1 at 1 atm, respectively (Figure 4). A hysteresis profile is observed in the isotherms of C2H2, which indicates a strong interaction between the adsorbed C2H2 molecules and the sulfonate-containing pore surface.21 The micropore surface is decorated with the sulfonate groups, which has a polarized environment and has a high affinity to C2H2. Proton Conduction. It is well-known that the organic polymers with sulfonate group, like Nafion, are an important kind of solid-state proton-conductive materials.22 We were inspired by the structural features that the sulfonate oxygen atoms lined in the surfaces of the channels of these compounds. The sulfonate groups along with the coordinated water

Figure 4. CH4, C2H6, C3H8, and C2H2 sorption isotherms at 273 K (solid symbols: adsorption, open symbols: desorption) for JXNU2(Sm).

molecules will provide the proton-conducting pathways for the proton transfer, indicating that these compounds with the hydrophilic channels can potentially be a proton-conducting material. The proton conductivity of JXNU-2(Sm) was therefore evaluated by the AC impedance method using a compacted pellet of the powdered sample. The Nyquist plots of the pellet samples were obtained at 30 °C under different relative humidity (RH). As shown in Figure S6, the Nyquist plots displayed a semicircle in the high-frequency component and an inclined tail in the low-frequency component, which is a fingerprint of proton conductor and shows two distinct charge transport regimes. The semicircle corresponds to the bulk resistance, which is attributed to the grain interior contribution, and the tail corresponds to the pile-up of protons at the electrodes.23 A noticeable decrease of the size of the semicircle with increasing humidity was observed in the Nyquist plots (Figure S6), which indicates that the conductivity of JXNU2(Sm) is highly humidity-dependent and increases by raising humidity. As shown in Figure 5, the proton conductivity of JXNU-2(Sm) is 6.64 × 10−7 S cm−1 at 30 °C and 50% RH, which is significantly less than that obtained at 98% RH by 2 orders of magnitude. The conductivity of 4.69 × 10−5 S cm−1 at 98% RH and 30 °C for JXNU-2(Sm) is comparable to those of other LnOFs such as Gd(III)-compound with hydrophilic channels formed by carboxylate oxygen atoms and coordinated water molecules (6.3 × 10−5 S cm−1 at 25 °C and 97% RH)24 and layered La3+ compound (4.24 × 10−5 S cm−1 at 25 °C and 95% RH).25 The abundant hydrophilic sulfonate groups are arranged at surface of channels, which would facilitate enrichment of the solvate water molecules for this high-proton D

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be 0.628 eV from the linear fitting of the Arrhenius plot of ln(σT) versus T−1 (Figure 6). The value is too large to assume that the conduction mechanism is only of the Grotthuss fashion.27 Instead, a vehicle mechanism could operate for the proton conduction of JXNU-2(Sm).27 The hydrophilic sites present in the channels would facilitate enrichment of the solvate water molecules in the channels, which movement as a solvated H+. Thus, the solvated water uptake induces a vehicletype proton transfer. Luminescent Properties. The Eu-containing compounds can show characteristic luminescent properties in the visible region. Thus, the luminescent spectra of compound JXNU2(Eu) suspended in DMF solution were measured. JXNU2(Eu) emits the characteristic bright red color of Eu3+ upon excitation at 316 nm (Figure S8). The emission peaks at 579, 593, 618, 652, and 696 nm can be attributed to 5D0 → 7Fn (n = 0, 1, 2, 3, 4) transitions of Eu3+ ions, respectively.28 The 5D0 emission decay curve for JXNU-2(Eu) in the solid state was monitored within the 5D0 → 7F2 transition under the excitation wavelength (Figure S9). The lifetime for 5D0 → 7F2 of Eu3+ ion was calculated to be 0.864 ms. The absolute emission quantum yield for the Eu3+ compund is 5.49%. The sulfonate groups on the surface of the 1D channels may capture the additional metal ions and serve as the fluorescent sensing sites for the metal ions. The as-prepared crytalline sample of JXNU-2(Eu) was ground and dispersed in DMF solution to form a suspension by ultrasound method. Then the interesting metal ions were added into the above resulting mixture. As shown in Figure S8, the well-resolved Eu3+-luminescence was observed in the emission spectra of the metal-incorported JXNU-2(Eu) in DMF suspension (metal ion concentration: 1 mM) upon excitation at 316 nm. The luminescence intensities of the dominant emission peaks (618 nm) are shown in Figure 7. The Li+, Na+,

Figure 5. Plots of log(σ) vs RH of JXNU-2(Sm) at 30 °C with three cycles.

conductivity. As shown in Figure 5, the humidity-dependent proton conductivity of JXNU-2(Sm) can be measured repeatedly by decreasing and increasing the humidity in the range of 50−98% without distinct change in their curves shapes. This observation suggests JXNU-2(Sm) is stable under impedance measurements and can reach equilibrium quickly, a property that is essential for sensor applications. Thus, JXNU2(Sm) could be a good candidate as a promising humidity sensor. To study the temperature effect and ensure meaningful correlation to proton conduction measurements, we further interrogated them with impedance as a function of temperature. Temperature dependence of the conductivity of JXNU-2(Sm) was measured from 40 to 80 °C at 98% RH for pellet sample (Figure S7). The Nyquist plots showed the decreased size of the semicircle when switching temperature from 40 to 80 °C, which displayed a noticeable increase in the conductivity (σ) when increasing temperature. The sample at 80 °C and 98% RH shows the highest proton conductivity with the value of 1.11 × 10−3 S cm−1 (Figure 6), which is comparable to that of the highest conductive hydrated metal−organic framework (MOF) material.26 The activation energy Ea was estimated to

Figure 7. Luminescence intensity of the transition (618 nm) of JXNU2(Eu) with addition of different metal ions (1 mM). I and I0 denote the fluorescence intensity of JXNU-2(Eu) with and without metal ions of interest, respectively.

Mg2+, Ba2+, and Co2+ ions have a negligible effect on the luminescence intensity. The addition of Fe3+ and Cu2+ ions to the suspension of JXNU-2(Eu) can weaken the emission intensity of JXNU-2(Eu). Remarkably, the emission intensity of JXNU-2(Eu) was enhanced significantly after the addition of K+. The luminescence intensity increases as the concentration of K+ increases, while the emission intensity decreases with increasing the concentration of Fe3+ (Figure 8). The PXRD patterns of the K+ and Fe3+-ion-incorporated JXNU-2(Eu) both are in good agreement with the parent JXNU-2(Eu) (Figure

Figure 6. Arrhenius-type plots of ln(σT) vs 1000/T for JXNU-2(Sm) at a RH of 98%. The black solid lines represent the best fit of the data. The measurements were performed at 10 °C intervals over a temperature range from 40 to 80 °C, with each data point corresponding to three independent tests. (inset) Nyquist plot for JXNU-2(Sm) at 80 °C and 98% RH. E

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Figure 8. Emission spectra of JXNU-2(Eu) with different concentrations of K+ (a) and Fe3+ (b) in DMF solution.

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S3b), suggesting that the framework of JXNU-2(Eu) remains intact when metal ions are added in. A few reports have been documented that some transition metal ions such as Cu2+ and Fe3+ can quench the lanthanide luminescence of the MOFs.29 For the present case, it is likely that the Cu2+ and Fe3+ entered into the channels and were captured by the sulfonate groups on the channels. The electronic structure of the ligand was perturbed by the introduction of metal-ion guests into the MOF channels, which reduces the energy transfer from ligand to the Eu3+ center, resulting in luminescent quenching. However, for K+ ion, it is not the case. The K+ ion has a large ionic radius and may form a cation−π interaction with the phenyl of the ligand when entering into the channel, which is similar to the Ag+ ion.30 It has been reported that the cation−π interaction can enhance the intersystem crossing (ISC) and increase the population of the excited triplet state of a fluorophore,31 which subsequently increases the population of the 5D0 excited state of the Eu3+ ion. Thus, the addition of K+ ion into the compound JXNU-2(Eu) increases the Eu3+ emission. To our knowledge, the luminescence sensitivity to K+ ion with MOF has not been reported previously. Concerning material of JXNU-2(Eu), it serves as a turn-on emission, which is preferred for detecting an analyte, highlighting the potential for recognition of K+ ion.

Corresponding Authors

*E-mail: [email protected]. Fax: +86-791-88336372. (Y.L.W.) *E-mail: [email protected]. (Q.-Y.L.) Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NNSF of China (Grant Nos. 21361011, 21101081, 51402293, and 21561015), the NSF of Jiangxi (Grant No. 20151BAB203002), NSF of Fujian for Distinguished Young Scholar (Grant No. 2016J06006), and the Young Scientist Training Project of Jiangxi Province (Grant No. 20153BCB23017).

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REFERENCES

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CONCLUSIONS In conclusion, the lanthanide-organic frameworks JXNU-2 based on the novel biphenyl-3,3′-disulfonyl-4,4′-dicarboxylate ligand have been synthesized and characterized. The 3D frameworks feature 1D hexagonal channels and 1D trigonal channels and exhibit affinity to quadrupolar molecules such as C2H2 and CO2, revealing potential for application in gas separation. Moreover, the JXNU-2 framework shows highly humidity-dependent proton conductivity associated with the hydrophilic channels, which has a potential application for an impedance humidity sensor. Finally, the JXNU-2(Eu) material acts as a turn-on emission for K+ ion, highlighting the potential for K+ ion sensor.

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AUTHOR INFORMATION

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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.inorgchem.6b00928. Table about bond lengths; PXRD patterns; TG curves; luminescence plots; and proton condutivity data (PDF) X-ray structure data (CIF) F

DOI: 10.1021/acs.inorgchem.6b00928 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.6b00928 Inorg. Chem. XXXX, XXX, XXX−XXX