Multifunctional Metal–Organic Frameworks with Fluorescent Sensing

Nov 10, 2016 - Metal–organic frameworks (MOFs) have attracted great attention in the ... Chang-Peng Men , Yu-Qing Luo , Shao-Jie Mo , Wen-Huan Huang...
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Multifunctional Metal−Organic Frameworks with Fluorescent Sensing and Selective Adsorption Properties Yu-Ling Li,†,‡ Yue Zhao,† Peng Wang,† Yan-Shang Kang,† Qing Liu,† Xiu-Du Zhang,† and Wei-Yin Sun*,† †

Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, China ‡ School of Chemistry & Pharmaceutical Engineering, Nanyang Normal University, Nanyang, Henan 473061, China S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) have attracted great attention in the past years due to their diverse structures as well as interesting properties. However, MOFs with multifunctionality are still challenging. Under solvothermal conditions, reactions of 1,3,5-tris(1-imidazolyl)benzene (tib) and 4,4′,4″-benzene-1,3,5-triyl-tribenzoic acid (H3BTB) with Cd(II) salt give rise to t w o n o v e l M O F s [ C d 3 ( t i b ) 2 (BTB) 2 ]· 3 D E F· 4 . 5 H 2 O (1 ) an d [Cd3(tib)2(BTB)2(DMA)2(H2O)2]·2DMA·8H2O (2) (DEF = N,N-diethylformamide, DMA = N,N-dimethylacetamide) with three-dimensional framework structures. It is fascinating that 1 and 2 not only show unique selectivity for detection of acetone through fluorescence quenching mechanism but also exhibit selective adsorption of gas (CO2 over N2 at 298 K) and dye (methyl orange) molecules.

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INTRODUCTION Porous metal-organic frameworks (MOFs) with effective voids and/or inner cavities have attracted remarkable attention in recent years because of their intriguing structures, interesting properties, and potential applications including gas storage/ separation, catalysis, and luminescent sensing materials. More importantly, it is desired that such multiple applications can be combined together into individual frameworks to give multifunctional MOFs.1−6 Despite remarkable progress having been achieved in the MOFs area, it remains a significant challenge to introduce more functional MOFs in a rational and systematic way. Among the potential applications, fluorescent sensing based on luminescent MOFs shows remarkable promise owing to its short response time, high sensitivity, simplicity, as well as low cost.7,8 This method has been used to detect small molecules, such as acetone and 2,4-dinitrotoluene (DNT), with high toxicity harming our wellbeing and environment.9,10 Up to now, there are MOFs reported for sensing of acetone.9 For example, Chen et al. reported an acetone-responsive MOF [Eu(BTC)(H2O)]·1.5H2O (BTC3− = 1,3,5-benzenetricarboxylate).9a However, most of the reported sensors are focused on lanthanide-organic frameworks, and only several MOFs with transition-metal centers have been reported. Chang and his coworkers presented two Cd(II) MOFs with unique selectivity for detection of acetone molecule.9f In addition, the reported lanthanide MOFs must be activated by removing solvent molecules before using them as sensors, since the solvent molecules may hinder the acetone to interact with metal centers. In contrast, the preactivation process is not essential for © XXXX American Chemical Society

the ligand-based luminescence sensors, which is of benefit to the practical application. Bearing the above-mentioned statement in mind and considering the advantage of sensing properties of MOFs, in this work, mixed ligands of 1,3,5-tris(1-imidazolyl)benzene (tib) and 4,4′,4″-benzene-1,3,5-triyl-tribenzoic acid (H3BTB; Scheme 1) were chosen for the following considerations: (i) Scheme 1. Schematic Drawing of Ligands tib and H3BTB

they are rigid organic ligands with high symmetry that generally facilitate crystallization in high-symmetry space groups and result in formation of stable frameworks; (ii) the conjugated πelectron skeletons in tib and H3BTB are favorable for construction of luminescent materials.10 As continuing of our development of MOF-based fluorescence sensors,9h we constructed two luminescent MOFs, namely, [Cd3(tib)2(BTB)2]·3DEF·4.5H2O (1) and Received: August 4, 2016

A

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Figure 1. (a) Coordination environment of Cd(II) in 1 with ellipsoids drawn at the 30% probability level. Hydrogen atoms and free solvent molecules are omitted for clarity. (b) Cd-tib cage unit in 1. (c) Fourfold interpenetrated Cd-BTB framework (different color shows different CdBTB ring, red: Cd-tib cage). (d) 3D structure of 1. (e) Topology of 1 (pink: Cd, green: tib and BTB3− ligands). 1:1) was sealed in an 18 mL glass vial, heated at 90 °C for 72 h, and then slowly cooled to room temperature; colorless block shaped crystals of 1 were obtained in 85% yield (based on tib). Anal. Calcd for 1 (C99H96N15O19.5Cd3): C, 55.43; H, 4.51; N, 9.79%. Found: C, 55.38; H, 4.59; N, 9.72%. IR (KBr pellet, cm−1): 3407 (m), 3238 (m), 3147 (m), 1622 (m), 1587 (s), 1511 (s), 1388 (s), 1310 (m), 1249 (s), 1112 (m), 1077 (s), 1017 (s), 936 (m), 867 (s), 816 (m), 781 (s), 708 (m), 649 (m), 470 (m). Synthesis of [Cd3(tib)2(BTB)2(DMA)2(H2O)2]·2DMA·8H2O (2). Complex 2 was synthesized by the same procedure used for preparation of 1, except that DMA/H2O (8 mL, v/v, 1:1) was used instead of DEF/ H2O. After the reaction mixture cooled to room temperature, colorless block-shaped crystals of 2 were obtained in 80% yield (based on tib). Anal. Calcd for 2 (C100H110N16O26Cd3): C, 52.46; H, 4.84; N, 9.80%. Found: C, 52.42; H, 4.92; N, 9.72%. IR (KBr pellet, cm−1): 3407 (m), 3220 (m), 3128 (m), 1622 (s), 1588 (s), 1536 (m), 1510 (s), 1398 (s), 1380 (s), 1243 (m), 1180 (m), 1074 (s), 1017 (s), 939 (m), 857 (m), 785 (s), 654 (m), 476 (m). X-ray Crystallography. Diffraction data collections for 1 and 2 were finished on a Bruker Smart Apex II CCD area-detector diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). The integration of the diffraction data as, well as the intensity corrections for the Lorentz and polarization effects, was performed using the SAINT program.12 Semiempirical absorption correction was performed using SADABS program.13 The structures of 1 and 2 were solved by direct methods using SHELXS-2014 and all the non-hydrogen atoms were refined anisotropically on F2 by the fullmatrix least-squares technique with SHELXL-2014.14 The hydrogen atoms except for those of water molecules were generated geometrically and refined isotropically using the riding model. Because the guest solvent molecules in 1 are highly disordered and impossible to refine using conventional discrete-atom models, the SQUEEZE subroutine of the PLATON software suite was used to remove the

[Cd3(tib)2(BTB)2(DMA)2(H2O)2]·2DMA·8H2O (2) (DEF = N,N-diethylformamide, DMA = N,N-dimethylacetamide). The results show that the as-synthesized MOFs can be used as acetone sensors without preactivation. In addition, the gas and dye sorption properties of the frameworks were investigated. To the best of our knowledge, this is the first example of Cdtib-carboxylate MOFs showing attractive combined properties.

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EXPERIMENTAL SECTION

Materials and Measurements. All commercially available chemicals and solvents are of reagent grade and were used as received. Ligand tib was synthesized according to the procedures reported previously.11 Elemental analyses for C, H, and N were performed on an Elementar Vario MICRO Elemental analyzer. FT-IR spectral measurements were performed on a Bruker Vector 22 FT-IR spectrophotometer using KBr pellets. Thermogravimetric analyses (TGA) were taken on a Mettler-Toledo (TGA/DSC1) thermal analyzer under nitrogen with a heating rate of 10 °C min−1. Powder X-ray diffraction (PXRD) analyses were performed on a Bruker D8 Advance X-ray diffractometer with Cu Kα (λ = 1.5418 Å) radiation. Sorption data were obtained on a Belsorp-max volumetric gas sorption instrument. The morphology was measured on a FEI Quanta 200 environmental scanning electron microscope (ESEM). The luminescence spectra were recorded on a Perkin-Elmer LS 55 spectro-fluorometer with a xenon arc lamp as the light source. The pass width of 10 nm was used in the measurements of emission and excitation spectra, and all the measurements were performed under the same experimental conditions. UV−vis spectra were collected on a UVProbe 2.33 spectro-photometer. Synthesis of [Cd3(tib)2(BTB)2]·3DEF·4.5H2O (1). A mixture of tib (6.9 mg, 0.025 mmol), H3BTB (11.0 mg, 0.025 mmol), and Cd(NO3)2·4H2O (30.8 mg, 0.1 mmol) in DEF/H2O (5 mL, v/v, B

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Figure 2. (a) Coordination environment of Cd(II) in 2 with ellipsoids drawn at the 30% probability level. Hydrogen atoms and free solvent molecules are omitted for clarity. (b) 3D structure of 2. (c) Topology of 2 (rose: Cd, green: tib and BTB3− ligands). scattering from the highly disordered solvent molecules.15 The formula of 1 was obtained based on volume/count_electron analysis, TGA, and elemental analysis. The reported refinements are of the guest-free structures obtained by the SQUEEZE routine, and the results are attached to the CIF file. The details of the crystal data for 1 and 2 are summarized in Table S1, and selected bond lengths and angles are listed in Table S2. Fluorescence Titrations. The fluorescence spectra of 1 and 2 were measured with 3 mL of their suspensions in CH3CN (1 mg/mL) at 298 K. All titrations were performed by the gradual addition of acetone in an incremental fashion. Each titration was repeated several times to get concordant value. No shape change but only intensity decrease was observed in the emission spectra during the titration process. The fluorescence quenching efficiency (%) was calculated with (1 − I/I0) × 100, where I0 and I are the fluorescence intensities before and after the addition of acetone, respectively. Selective Dye Removal. An aqueous stock solution of methyl orange (MO), methylene blue (MB), or rhodamine B (RhB; 1000 ppm) was prepared by dissolving MO (C14H14N3NaO3S, MW: 327.34, Sigma-Aldrich), MB (C16H18ClN3S, MW: 373.9, Sigma-Aldrich), or RhB (C28H31ClN2O3, MW: 479.01, Sigma-Aldrich) in deionized water. Aqueous solutions of 10 ppm MO, MB, or RhB were obtained by dilution of the stock solution with water. The MO, MB, or RhB concentrations were determined by using absorbance (at 464, 665, and 554 nm, respectively) of the solutions. Before adsorption, the adsorbents were desolvated and kept in a desiccator. Then, an exact amount of the adsorbents (10 mg) was put in the aqueous dye solutions (20 mL, 10 ppm). The dye solutions containing the adsorbents were mixed well under magnetic stirring and maintained for a fixed time (12 to 24 h) at 25 ◦C in dark. After adsorption for a predetermined time, the solution was separated from the adsorbents, and the dye concentration was calculated according to the absorbance.

around Cd1 are in the range of 53.36(17)°−164.9(3)° (Table S2). It is interesting that each tib ligand connects three Cd atoms in 1, and 12 Cd atoms and 8 tib ligands form a cagelike unit structure with a diameter of 8 Å (Figure 1b). To the best of our knowledge, this kind of Cd-tib cagelike structure has not been observed in the reported complexes. Furthermore, six BTB3− ligands and six Cd atoms form a large ring with a diameter of ∼25 Å (Figure S2). The vacancy of the ring is large enough to allow other ones to embed in it; thus, four such cyclic structures interpenetrate each other to form a fourfold interpenetrated Cd-BTB framework (Figure 1c). Such special structure further packages tib to form the final three-dimensional (3D) framework structure of 1 (Figure 1d). Three types of channels can be identified in the framework of 1 as shown in Figure 1d; one is large cuboctahedral M12L8 (diameter ≈ 8 Å) cage, and the other two small ones are octahedral M6L4 (diameter ≈ 4 Å) and M2L4 (diameter ≈ 2 Å) cages. The large openings allow for ready access, passage, and exchange of guest species as subsequently observed. The high solvent-accessible volume is 4309.6 Å3 of the 10 938 Å3 unit cell volume (39.4% of the total crystal volume) calculated by PLATON.16 To better understand the framework structure of 1, topological analysis was performed. Cd1, tib, and BTB3− ligands can be considered as four-, three-, and three-connectors, respectively, as illustrated in Figure S3. According to the simplification principle,17 the resulting framework of 1 is an unusual (3, 3, 4)-connected three-nodal 3D framework with point symbol of {83}4{86}3 as shown in Figure 1e. When the reaction medium was changed from DEF to DMA, complex 2 was isolated. The results of crystallographic analysis revealed that 2 crystallizes in triclinic space group of P1̅ (Table S1). The asymmetric unit of 2 contains two Cd2+ ions, one of which is sitting on an inversion center, one tib ligand, one BTB3− ligand, and coordinated DMA and water molecules (Figure 2a). Both Cd1 and Cd2 are six-coordinated with distorted octahedral geometry but different coordination environments. In addition to two nitrogen atoms from two different tib and three (for Cd1) or two (for Cd2) oxygen ones from two BTB3− ligands, there are one oxygen from DMA and two coordinated water molecules for Cd1 and Cd2, respectively. The Cd−N bond distances range from 2.242(3) to 2.310(3) Å, and the Cd−O ones are in the range of 2.216(2)−2.567(3) Å. In addition, the coordination angles around Cd are in the scope of 53.49(9)°−180° (Table S2). The tib ligands and Cd atoms interconnect to form a onedimensional (1D) chain (Figure S4a), while the BTB3− ligands

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RESULTS AND DISCUSSION Description of Crystal Structures. The results of crystallographic analysis revealed that 1 crystallizes in cubic space group of P432 (Table S1). The asymmetric unit of 1 is composed of one-sixth molecule of [Cd3(tib)2(BTB)2]. The complete deprotonation of H3BTB to form BTB3− was confirmed by IR spectral data, since no vibrational band originated from −COOH was observed between 1680 and 1760 cm−1 (Figure S1). The coordination environment of Cd(II) atom in 1 is shown in Figure 1a. Cd1 is six-coordinated by two nitrogen atoms from two different tib ligands, four carboxylate oxygen ones from two BTB3− ligands to give a seriously distorted octahedral coordination geometry. The Cd− O bond lengths in 1 are 2.221(4) and 2.616(5) Å, and the Cd− N one is 2.245(5) Å. In addition, the coordination angles C

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Inorganic Chemistry also link the Cd to give a 1D chain (Figure S4b). The 1D chains are further connected together to form the 3D structure of 2 (Figure 2b). The high solvent-accessible volume is 592.3 Å3 of the 2573.9 Å3 unit cell volume (23.0% of the total crystal volume) calculated by PLATON.16 For the topological analysis of 2, the Cd1, Cd2, tib, and BTB3− ligands can be considered as four-, four-, three-, and three-connectors, respectively (Figure S5). According to the simplification principle,17 the framework of 2 is an unusual (3, 3, 4, 4)-connected 4-nodal 3D net with point symbol of {4·82}2{4·85}2{83}2{85·12}, as illustrated in Figure 2c. Effect of Reaction Solvent on the Structures of the Complexes. The different framework structures of 1 and 2 imply the existence of effect of reaction medium,18 since 1 and 2 were obtained under the same reaction conditions except for the different reaction solvents of aqueous DEF for 1 and aqueous DMA for 2. The different basicity of (CH3CH2)2NH and (CH3)2NH generated in situ through the decomposition of solvent DEF and DMA, respectively, may lead to the formation of 1 and 2 with different structures. In addition, different coordination ability and/or steric hindrance of DEF and DMA makes no coordination of DEF in 1, while DMA serves as both coordinated and free solvent molecules in 2. The coordination of DMA in 2 may prevent the interpenetration of the framework 2. The results imply that the reaction solvent plays important role in determining the framework structures of 1 and 2. Thermogravimetric Analysis and Powder X-ray Diffraction. Complexes 1 and 2 are air-stable, and their thermal stability was studied by TG measurements in the temperature range of 30−800 °C (Figure S6). The TG curve of 1 shows weight loss of 16.0% from 30 to 350 °C, due to the loss of free water and DEF molecules (calcd 17.9 wt %). The TG curve of the activated sample of 1 confirms the complete removal of free solvent molecules in the framework. The TG curve of 2 shows weight loss of 23.4% from 30 to 320 °C, corresponding to the loss of free and coordinated water and DMA molecules (calcd 23.1 wt %). The TG curve of the activated sample 2 shows a weight loss of 10.2% from 30 to 320 °C, corresponding to the loss of coordinated water and DMA molecules (calcd 10.7 wt %), which confirms the complete removal of free solvent molecules in the framework of 2. The phase purity of the bulky samples was proved by PXRD data, since the PXRD patterns of the as-synthesized samples are consistent with the simulated ones (Figure S7). Furthermore, the PXRD patterns of 1 under varied temperatures indicate that the framework is stable up to 280 °C, while 2 after solvent exchange and vacuum activation can only be stable up to ∼40 °C. Figure S8 shows the PXRD patterns of 1 and 2 in different solvent, confirming that the crystalline structure of 1 and 2 can be preserved. Fluorescence Sensing Property. MOFs with d10 metal ions and π-conjugated skeleton ligands are considered to be potential fluorescence materials.19 The luminescence properties of 1 and 2 were studied in the solid state at room temperature. It was found that 1 and 2 show apparent fluorescence enhancement, and intense emission bands were observed at λem = 398 nm (λex = 290 nm) for 1 and λem = 384 nm (λex = 290 nm) for 2 as exhibited in Figure 3, which may be originated from the tib ligand emission, since the free tib ligand exhibits an emission at 409 nm (λex = 290 nm).20 It has been reported that the formation of architectures can increase the rigidity of the aromatic backbone of the ligands and enhance the intra/ intermolecular interactions among the organic ligands, which

Figure 3. Solid-state photoluminescent spectra of free tib ligand, 1, and 2 at room temperature, λex = 290 nm.

are favorable for energy transfer.21 The observed much stronger emission intensities of 1 and 2 than those of free tib imply that the formation of MOFs enhances the fluorescence of the ligand. This is known as aggregation-induced emission (AIE), which is considered to be caused by the coordination of organic ligands to metal ions that restricts the deformation of the ligand and induces the nonradiative relaxation.22 We also examined the luminescent properties of 1 and 2 in suspension solutions. The morphologies of well-ground crystalline powder samples of 1 and 2 without activation were investigated by SEM, and the results are shown in Figure S9. The stable suspension solutions used for fluorescent measurements were obtained by immersing the powder samples of 1 and 2 without activation (3 mg) in definite solvent (3 mL), ultrasonicating (1 h), and aging (3 d). Emission spectra of 1 and 2 dispersed in different solvents clearly show that 1 and 2 have the strongest emissions at 393 and 380 nm (λex = 290 nm), respectively, in CH3CN (Figure S10). Furthermore, the emission intensities of 1 and 2 in CH3CN dispersions are stronger than those of free tib in CH3CN under the same concentration (Figure S11). Therefore, CH3CN was employed as dispersion solvent in the sensing studies. The fluorescence-sensing experiments were performed for 1 and 2 dispersed in CH3CN by addition of other organic molecules including methanol, ethanol, isopropanol, isobutanol, DMF, DMA, THF, CHCl3, CH2Cl2, benzene, and acetone.23 As illustrated in Figure 4, only addition of acetone can quench the fluorescence emissions of 1 and 2 efficiently. Almost complete quenching (98% and 97% decrease in emission intensities of 1 and 2, respectively) was observed by addition of 2.0 vol % acetone, while with the addition of 2.0 vol % other organic molecules no remarkable decrease of emission intensities of 1 and 2 was detected (Insets of Figure 4). Furthermore, it was found that the quenching efficiency of 1 and 2 in addition of acetone was not affected by the existence of other organic molecules (Figure 5). The results demonstrate that 1 and 2 can selectively sense acetone molecule and have comparably minimal detection amount of acetone with the previously reported MOF-based acetone sensors (Table S3). More importantly, 1 and 2 without activation, namely, the asobtained samples, were used for acetone sensing, which is different from the reported lanthanide-based MOFs.9a To examine the sensing sensitivities of 1 and 2 for acetone in detail, the quenching efficiencies of 1 and 2 dispersed in CH3CN suspensions with amounts of acetone were investigated. As exhibited in Figure 6, the fluorescent quenching efficiency increased steeply within low concentration of acetone D

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Figure 4. Quenching efficiency variation of 1 (a) and 2 (b) dispersed in CH3CN via addition of 2.0 vol % different organic molecules. (insets) Fluorescence spectra of 1 (a) and 2 (b) dispersed in CH3CN with addition of different organic molecules.

Figure 5. Quenching efficiency of 1 (a) and 2 (b) upon the addition of different organics (green) and subsequent addition of acetone (pale), λex = 290 nm.

dispersed in CH3CN indicate that a 50% decrease of the luminescence intensity was reached at an acetone content of 0.33 vol %, and almost complete quenching was reached at a concentration of 2.0 vol % (Figure 7). Thus, these two complexes reported in this work can be considered as potential candidates for selective sensing of acetone molecules. The observed fluorescence quenching effect may be attributed to the interactions between the framework and small organic molecules. Upon excitation, an energy transfer from the organic ligands to the acetone molecules occurred and resulted in fluorescence quenching.9 It is known that the repeatability and recyclability of the sensing performance of the MOFs is important for their practical application. For this purpose, the recycling sensing performance experiments of 1 and 2 were performed by repeatedly conducting sensing tests with acetone in acetonitrile. As shown in Figure S13, no obvious decrease of the fluorescent quenching efficiency of 1 and 2 was observed after three cycles of sensing acetone, indicating a high repeatability in sensing performance. The maintenance of the framework structures of 1 and 2 after recycling sensing experiments was confirmed by XRD measurements (Figure S14). The results show the good

Figure 6. Plots of the quenching efficiencies with amounts of acetone.

(0−0.17 vol %). Linear correlations (R2 = 0.985 for 1 and 0.993 for 2) between the fluorescence response and the acetone content were obtained (Figure S12), indicating the diffusioncontrolled fluorescence quenching of 1 and 2 by acetone molecules.8 The results of fluorescence titration of 1 and 2 E

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Figure 7. Fluorescence titration of compounds 1 (a) and 2 (b) dispersed in CH3CN (1 mg/mL) with gradual addition of acetone, λex = 290 nm.

Figure 8. (a) N2 sorption isotherms at 77 K for 1′ and 2′ (filled shape, adsorption; open shape, desorption). (b) N2 and CO2 sorption isotherms at 298 K for 1′and 2′ (filled shape, adsorption; open shape, desorption).

and 1.0 atm, which are comparable with those of reported MOFs.28,29 In addition, the observed hysteresis upon desorption in the CO2 isotherms of 1′ and 2′ suggests the presence of strong interactions between CO2 and the framework that may hinder the desorption of CO2 from the framework and/or the diffusion through the narrow pore apertures.30 Furthermore, the CO2/N2 adsorption selectivity calculated by initial slope method at 298 K and 1.0 atm are 64 for 1′ and 18 for 2′. The results show that 1′ has better CO2 over N2 selectivity than 2′, and further comparison of the CO2/ N2 selectivity with various materials is summarized in Table S4. The results indicate that 1 and 2 might have potential application for separation of CO2 and N2.31 To further understand the interactions between CO2 and the frameworks, the adsorption enthalpies (Qst) for CO2 were calculated by using the data of CO2 isotherms at 273 and 298 K (Figure S15). On the one hand, The Qst values for 1′ and 2′ calculated using the virial method are 31.36 and 28.60 kJ mol−1, respectively (Figure S16), which are comparable with those of reported MOFs such as [Cu(bpy-n)2(SiF6)] (27 kJ mol−1),32 MIL-53(Al) (20.1 kJ mol−1),33 and HKUST-1 (30 kJ mol−1).34 On the other hand, the porous structures of 1 and 2 encourage us to investigate their dye adsorption property. Methyl orange (MO), methylene blue (MB), and rhodamine B (RhB; Scheme S1) are the most common dyes. To examine the

repeatability and recyclability of the sensing performance of 1 and 2. Selective Adsorption Property. To get information on surface area and porous property of the complexes, the performance of N2 and CO2 adsorption was investigated for the activated samples 1′ and 2′. PXRD data suggest that the activated samples 1′ and 2′ maintained their crystallinity (Figure S7). The N2 adsorption isotherms of 1′ and 2′ were measured at 77 K, and the results are given in Figure 8a. The N2 adsorption isotherms are typical type-I isotherms and show a steep rise at the low-pressure region, which is a characteristic of microporous materials.24 According to the Brunauer− Emmett−Teller (BET) equation, the BET surface area and total pore volume are 517 m2 g−1 and 0.25 m3 g−1 for 1′, 358 m2 g−1 and 0.23 m3 g−1 for 2′, which are higher than those of Zn4(pydc)4(DMF)2·3DMF (319 m2 g−1 and 0.12 m3 g−1),25 MA-MOF 235 (148 m2 g−1 and 0.075 m3 g−1),26 and ST-MOF 235 (135 m2 g−1 and 0.067 m3 g−1).27 It is interesting that 1′ and 2′ show the ability to selectively do sorption of CO2 over N2 at 1.0 atm. As shown in Figure 8b, the adsorption isotherms of N2 measured at 298 K for 1′ (1.12 cm3 g−1 at 1 atm) and 2′ (3.42 cm3 g−1 at 1 atm) indicated that almost no N2 adsorptions were observed. However, the CO2 adsorption isotherms showed that the values of CO2 adsorption are up to 24.60 cm3 g−1 for 1′ and 14.65 cm3 g−1 for 2′ at 298 K F

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Figure 9. Adsorption capability of 1′ and 2′ toward MO (a, b), MB (c, d), and RhB (e, f). Experiment conditions: C0 (MB): 10 mg L−1, C0 (MO): 10 mg L−1, adsorbent dose: 10 mg/20 mL).

solution, and the RhB concentrations varied from 10 to 8.73 ppm for 1′ and 9.04 ppm for 2′ in 24 h (Figure 9e,f and Tables S9 and S10). Therefore, the adsorption ability of 1′ and 2′ is less for MB compared with MO and scarcely for RhB. The selective adsorption of MO over MB and RhB in water for 1′ and 2′ may be caused by the different molecular size, because MO has smaller size of 3.1 × 4.3 × 14.5 Å than that of MB (4.2 × 5.0 × 13.4 Å) and RhB (5.4 × 9.8 × 14.1 Å; Scheme S1).35 In addition, there may be electrostatic interactions between the anionic MO and cationic metal center (Cd2+) as well as the π−π* interactions between aromatic moieties of the frameworks and dyes. The results show that 1 and 2 have highly selective adsorption ability for removal of MO dye from polluted water.

adsorption ability of compounds 1 and 2 for these dyes, the activated samples of 1′ and 2′ (10 mg) were dipped into an aqueous solution of MO, MB, and RhB (10 ppm, 20 mL) at ambient temperature in dark. For 1′, the yellow MO solution becomes nearly colorless, and the MO concentration decreases to almost zero, implying the complete removal of MO from aqueous solution during 24 h (Figure 9a and Table S5). Similar phenomenon was also observed for 2′; the color change from yellow to almost colorless together with the decrease of the MO concentration from 10 to 0.77 ppm indicates the effective removal of MO in 24 h (Figure 9b and Table S6). However, in contrast to the nearly complete removal of MO by 1′ in 12 h (Figure 9a and Table S5), the yellow solution color and 5.7 ppm MO concentration indicate incomplete removal of MO by 2′ after 12 h (Figure 9b and Table S6). It means that 1′ has stronger adsorption ability for MO than that of 2′. In the case of MB, the blue solution color changes are limited, and the MB concentrations decreased from 10 to 3.08 ppm for 1′ and to 4.43 ppm for 2′ after 24 h (Figure 9c,d and Tables S7 and S8). In addition, there are hardly any color changes for the rose RhB

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CONCLUSION In summary, we have successfully constructed two mutifunctional Cd(II) frameworks [Cd3(tib)2(BTB)2]·3DEF·4.5H2O (1) and [Cd3(tib)2(BTB)2(DMA)2(H2O)2]·2DMA·8H2O (2). It is fascinating that 1 and 2 were found to not only show G

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Inorganic Chemistry unique selectivity for detection of acetone via fluorescence quenching mechanism but also show selective adsorption of gas (CO2 over N2 at 298 K) and dye (methyl orange) molecules in aqueous solution. This study provides new physical insights into the rational design of MOF-based multifunctional materials.

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ASSOCIATED CONTENT

* Supporting Information S

These materials are available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01869. PXRD patterns, TG, SEM, and additional figures (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 25 89683485. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the National Natural Science Foundation of China (Grant Nos. 21331002 and 21573106) for financial support of this work. This work was also supported by a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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REFERENCES

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