2-Fold Interpenetrating Bifunctional Cd-Metal–Organic Frameworks

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2‑Fold Interpenetrating Bifunctional Cd-Metal−Organic Frameworks: Highly Selective Adsorption for CO2 and Sensitive Luminescent Sensing of Nitro Aromatic 2,4,6-Trinitrophenol Xu-Jia Hong,† Qin Wei,† Yue-Peng Cai,*,† Sheng-Run Zheng,† Ying Yu,† Yan-Zhong Fan,‡ Xian-Yan Xu,§ and Li-Ping Si*,†

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†

School of Chemistry and Environment, Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, Guangdong Provincial Engineering Technology Research Center for Materials for Energy Conversion and Storage, South China Normal University, Guanghzou, 510006, P. R. China ‡ School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guanghzou, 510275, P. R. China § College of Chemistry and Environmental Engineering, Shaoguan University, Shaoguan, 512005, P. R. China S Supporting Information *

ABSTRACT: A robust primitive diamond-type topology 3-D metal− organic framework (MOF) of {[Cd4(hbhdpy)2(bdc)3(DMA)2]·(H2O)4}n (1, DMA = N,N-dimethylacetamide) was constructed from the planar secondary building units of the dinuclear cadmium clusters, Cd2(μ2-O)2, and two linear organic linkers of the new multidentate Schiff base of 4-(2hydroxy-3-methoxy-benzyli-denehydrazino-carbonyl)-N-pyridin-4-yl-benzamide (Hhbhdpy) through the solvothermal reaction. 1 presents a 2-fold interpenetrating network along with confined narrow channels and rich acylamide groups as well as potential metal open sites for excellent selective CO2 uptake over CH4/N2 and high luminescent response for 2,4,6trinitrophenol (TNP) in DMA solution under ambient conditions. With 2amino-1,4-dicarboxy-benzene (H2bdc-NH2) replacing H2bdc, an aminefunctionalized MOF of {[Cd4(hbhdpy)2(bdc-NH2)3 (DMA)2]·(H2O)4}n (1-NH2) as an isomorphism of 1, was synthesized under the same reaction conditions. Compared with 1, the corresponding bifunctional features of 1-NH2 is more obvious. To the best of our knowledge, it is the first reported interpenetrating Cd-MOFs with highly sensitive luminescence response for TNP molecules combined with excellent selectivity for CO2/N2 and CO2/CH4. KEYWORDS: metal−organic frameworks, 2-fold interpenetrating Cd-MOFs, bifunctional features, selective adsorption, luminescent sensing, 2,4,6-trinitrophenol

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INTRODUCTION

and hydrothermal stabilities of these MOFs for large scale industrial applications must be satisfied.6−8 On the other hand, the combination of porosity and luminescence have made MOFs become the potential chemical sensors and devices for detecting metal ions (such as Cu2+, Ni2+, Ba2+, etc.),16−18 inorganic anions (for instance, NO3−, Cl−, I−, etc.), or other organic molecules (for example, 2,4,6trinitrophenol, 2,4,6-trinitro-toluene, etc.).19−22 Among them, especially noteworthy nitro aromatics, such as nitrobenzene (NB), 1,4-dinitrobenzene (DNB), and 2,4,6-trinitrophenol (TNP) are of high toxicity and have considerable harm to microorganisms and the human body deriving from accidents in the production and storage processes and illegal wastewater emissions in the environment. Accordingly, it is crucial for

Metal−organic frameworks (MOFs) have emerged as good sorbents for CO2 storage and capture on account of their tunable pore size and high surface area.1−5 Until now, there are only a few MOFs which show highly selective adsorption of CO2 over other gases such as CH4 or N2. Clearly it is still a challenge to construct viable CO2-capture MOFs which exhibit high CO2 selectivity.6−8 Many strategies have been explored in MOFs construction in order to enhance the CO2 adsorption selectivity and capacity, including decorating the pores with the polarizing groups such as nitrogen bases or other polarizing groups, narrowing the pore size by interpenetration from the increase of organic ligand flexibility, and changing the ligands’ size and length as well as coordination numbers of metal ions in those MOFs with isoreticular structures.9−15 Because some strategies may decline the surface area and CO2 uptake capacity, it is obviously necessary to balance the high selectivity and large storage capacity. At the same time, the high chemical © 2017 American Chemical Society

Received: November 2, 2016 Accepted: January 17, 2017 Published: January 17, 2017 4701

DOI: 10.1021/acsami.6b14051 ACS Appl. Mater. Interfaces 2017, 9, 4701−4708

ACS Applied Materials & Interfaces

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homeland security, security screening, and environmental monitoring to rapidly detect these nitro aromatics. Because of its rapidity, sensitivity, and convenience, the luminescence quenching method has become to be a very effective strategy for the sensing of these nitro aromatics.23−27 Bearing all of that in mind, a new class of lengthy linear multichelate Schiff base ligand involving acylhydrazone and aminocarbonyl groups, namely, 4-(2-hydroxy-3-methoxy-benzyli-dene-hydrazinocarbonyl)-N-pyridin-4-yl-benzamide (Hhbhdpy, Scheme 1), was chosen to construct the 3-D MOFs

Research Article

EXPERIMENTAL SECTION

Materials and Physical Measurements. The ligand 4-(2hydroxy-3-methoxy-benzylidene-hydrazi-nocarbonyl)-N-pyridin-4-ylbenzamide (Hhbhdpy) was prepared according to the literature,28,29 and the other materials were purchased and used without further purification. The PerkinElmer 240C analytical instrument was used to analyses elemental for C, H, N. IR spectra were recorded on a Nicolet FT-IR-170SX spectrophotometer in KBr pellets. Thermogravimetric analyses were performed on PerkinElmer TGA7 analyzer in flowing air atmosphere at a heating rate of 10 °C/min. Hitachi F-2500 and Edinburgh-FLS-920 were used to record the solid state luminescent spectra were at room temperature with a light source of xenon arc lamp and the pass width of 5.0 nm. Synthesis of {[Cd4(hbhdpy)2(bdc)3(DMA)2]·(H2O)4}n (1). A mixture of Hhbhpy (0.0780 g, 0.2 mmol), Cd(OAc)2 (0.4 mmol), H2bdc (0.0498 g, 0.3 mmol), DMA (5 mL), and water (0.5 mL) was sealed in a 15 mL Teflon-lined stainless steel vessel and heated at 100 °C for 3 days. After cooling to room temperature at a rate of 10 °C/h, yellow block crystals suitable for X-ray diffraction analysis were obtained. The yield was 75% for 1 (based on Hhbhpy). IR (KBr, cm−1): 3413 (br), 1572(m), 1413(w), 1328(w), 1211(s), 966 (m), 902 (w), 753 (s); elemental analysis calcd (%) for 1 (C74H72N10O26Cd4): C, 45.14; H, 3.66; N, 7.12. Found: C, 45.21; H, 3.63; N, 7.15. Synthesis of {[Cd4(hbhdpy)2(bdc-NH2)3(DMA)2]·(H2O)4}n (1NH2). The 1-NH2 was prepared by the same condition except that the replacement of H2bdc by auxiliary ligand H2bdc-NH2. After cooling to room temperature at a rate of 10 °C/h, yellow block crystals suitable for X-ray diffraction analysis were obtained. The yield was 72% for 1NH2 (based on Hhbhpy). IR (KBr, cm−1): 3426 (br), 1572(m), 1415(w), 1320(w), 1257(w), 1211(s), 969 (m), 904 (w), 750 (s); elemental analysis calcd (%) for 1-NH2 (C74H73N13O26Cd4): C: 44.21; H:3.63; N: 9.06; found: C 44.18, H 3.65, N 9.08. X-ray Data Collection and Structure Refinement. Data collections were performed at 298 K on a Bruker Smart Apex II diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) for compounds 1 and 1-NH2. The multiscan program SADABS are used for the absorption corrections.30 The SHELXS-9731 and SHELXL-9732 program packages were used for structural solutions and full-matrix least-squares refinements based on F2, respectively. All the non-hydrogen atoms were refined anisotropically. The nitrogen atom N5 and four carbon atoms C27, C28 and C32, C33 of the coordinated (bdc-NH2)− in the asymmetric unit of 1-NH2 were disordered into two sites with 0.5 occupancy of per position, respectively. For 1-NH2, the SQUEEZE option in PLATON33 was used to remove the disordered solvent water molecules, and the actual water molecules in the unit cell are determined by elemental and thermogravimetric analyses (EA and TGA). The hydrogen atoms of coordinated water were located from difference maps and refined with isotropic temperature factors and that of organic motives were placed at calculated position. The uncoordinated solvent water hydrogen atoms have been not added. The data collections, crystal parameters and refinements for complexes 1 and 1-NH2 are present in Table S1. Table S2 shows the selected angles and bond lengths. The Supporting Information presents more details. CCDC numbers of 1495775 and 1495776 are for compounds 1 and 1-NH2, respectively. Gas Adsorption Experiments. The N2, CH4, and CO2 sorption measurements were performed on automatic volumetric adsorption equipment (Belsorp-max). The MOFs were immersed in MeOH for 48 h to remove the nonvolatile solvates (DMA) before gas adsorption measurements. After the removal of methanol by decanting, the samples were activated by heating at 70 °C under vacuum conditions.

Scheme 1. Structures of Ligands for Hhbhdpy, H2bdc, and H2bdc-NH2

with the following advantages: (i) the selection of lengthy linear multichelate ligand, Hhbhdpy, is helpful to the assembly of the interpenetrating networks with high stability. (ii) The NO3 set from one end of Hhbhdpy may be designed to chelatingcoordinate to many metal centers for constructing the polynuclear metal-cluster nodes; meanwhile, the existence of the pyridyl N atoms from another end of Hhbhdpy is beneficial to the formation of high dimensional luminescent frameworks. (iii) The free acylamide as the polarity group modifying the pores can enhance the interaction between framework and carbon dioxide/nitroaromatic molecules. (iv) Reacting with the cadmium ion with the variable coordination number as the metal center, the resulting Cd-MOFs may provide potential open metal sites. (v) Co-coordination of the auxiliary ligands, H2bdc/H2bdc-NH2, can further modify MOF structure to improve its performance. Under guidance of this strategy, a luminescent microporous MOF, {[Cd4(hbhdpy)2(bdc)3(DMA)2]·(H2O)4}n (MOF-1) and its amine-functionalized isomorphic form, {[Cd4(hbhdpy)2(bdcNH2)3-(DMA)2]·(H2O)4}n (MOF-1-NH2) (Hhbhdpy = 4-(2hydroxy-3-methoxy-benzylidenehydrazin-ocarbonyl)-N-pyridin4-yl-benzami-de, and H2bdc =1,4-dicarboxybenzene, H2bdcNH2 = 2-amino-1,4-dicarboxybenzene, DMA = N,N-dimethylacetamide) were solvothermally constructed. Moreover, the lengthy linear Hhbhdpy ligand together with H2bdc/H2bdcNH2 afford the 2-fold interpenetrating primitive diamond-like 66 topology networks with reduced void spaces and confined narrow channels. MOF-1 thereby presents segmented pores, rich acylamide groups, and potential metal open sites for selective CO2 uptake over CH4/N2 and highly selective sensing of the nitro explosive 2,4,6-trinitrophenol (TNP). Compared with 1, the 1-NH2 showed better bifunctional properties. Obviously, our studies also prove that interpenetration can be an effective means to boost the stability of MOF structures as well as to render proper pore sizes and rich acylamide groups as well as potential metal open sites for guest separation and fluorescent sensing. To the best of our knowledge, this is the first report of interpenetrating MOFs with such high sensitivity luminescence response to TNP molecules with excellent selectivity for CO2 in CO2/N2 and CO2/CH4 to date.

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RESULTS AND DISCUSSION Synthesis and Structure Characterization. Yellow block crystals of 1/1-NH2 were obtained from the solvothermal reaction of Cd(OAc)2 with Hhbhdpy and H2bdc/H2bdc-NH2 in N,N-dimethylacetamide (DMA) at 100 °C for 3 days. Compounds 1 and 1-NH2 are isomorphous and crystallize in 4702

DOI: 10.1021/acsami.6b14051 ACS Appl. Mater. Interfaces 2017, 9, 4701−4708

Research Article

ACS Applied Materials & Interfaces the monoclinic crystal system, space group P21/c. Compound 1 is employed as a representative to be described in detail. There are two Cd2+ ions, one deprotonated hbhdpy− anion, one and a half bdc2− anions, one coordinated DMA molecule, and two lattice water molecules in the asymmetric unit of 1 (Figure 1a).

metallocyclic channels accommodated by the lattice water molecules with about 18.6 × 9.5 Å2 size (just considering a single noninterpenetrating 3-D framework in 1) viewed along the b axis (Figure S1). Meanwhile, the resulting 3-D network may be also viewed as the fusion of two 1-D wave-like chains of [(bdc)3(Cd2O2)2]n and [(hbhdpy) (Cd2O2)]n hinged by dinuclear Cd2O2 units as depicted in Figures S1 and S2. Because of the lengthy multidentate Hhbhdpy ligand, a 2fold interpenetrating network with 50.1% solvent accessible voids (calculated using the PLATON software after elimination of guest and coordinated solvent molecules) and 1-D channels of about 9.9 × 9.5 Å2 (excluding van der Waals radii of the atoms) along the [010] direction, is thus generated further stabilizing the resulting 3-D framework (illustrated in Figure 2).

Figure 1. In complex 1, (a) the coordination environment of Cd(II) ions. Symmetry codes: (i) 1 − x, −0.5 + y, 0.5 − z; (ii) −x, 1 − y, 1 − z; (iii) −x, 1 − y, −z. (b) The dinuclear Cd2(μ2-O)2 unit. (c) The coordination modes I−III of Hhbhdpy and H2bdc ligands.

Cd1 is seven-coordinated to two oxygen atoms (O2, O3) and one nitrogen atom (N1) from one deprotonated ligand hbhdpy−, two carboxyl oxygen atoms (O5, O6) from one deprotonated ligand bdc2−, one pyridyl nitrogen atom (N4i) from the other deprotonated ligand hbhdpy−, and one oxygen atom (O11) from one coordinated solvent of DMA, showing a distorted pentagonal bipyramidal coordination geometry. Cd2 is bonded with two oxygen atoms (O1, O2) from the deprotonated ligand hbhdpy− and five carboxyl oxygen atoms (O6, O7ii, O8ii, O9, O10) from three deproto-nated ligand bdc2− with the distorted pentagonal bipyramidal coordination geometry. The Cd−O bond distances are in the range of 2.237(3)−2.587(2) Å, and the Cd−N bond distances fall in the range of 2.380(2)−2.399(3) Å. Two adjacent Cd(II) centers are linked by one carboxylate oxygen atom (O6) from one bdc2− ligand and another phenolate oxygen atom (O2) from one deprotonated ligand hbhdpy− with the Cd1···Cd2 separation of 3.620(2) Å, forming the dinuclear Cd2(η2-O)2 unit (Figure 1b). In the present case, the Hhbhdpy in compound 1 was partially deprotonated and only one coordination mode was observed (Figure 1c), in which one end of ligand hbhdpy− chelating-coordinated to two Cd(II) ions in the same dinuclear Cd2O2 unit by one methoxyl oxygen, one hydroxyl oxygen, one acyl oxygen atom, and one acylhydrazone nitrogen atom with the NO3 set; the other end of hbhdpy− ligand was bridging-coordinated to one Cd(II) ion in the adjacent dinuclear Cd2O2 unit via one pyridyl nitrogen atom, showing mode I: μ3-η1:η2:η1:η1:η1. However, ligand 1,4-dicarboxy-benzene (H2bdc) was completely deprotonated and presented two different coordination modes (II) and (III) (Figure 1c), namely, each bdc2− ligand via μ2η1:η1:η1:η1 and μ3-η1:η1:η2:η1 mannars linked two and three Cd(II) centers in mode II and III, respectively. On the basis of the coordination modes of Cd(II) ions and two ligands of hbhdpy− and bdc2−, each dinuclear Cd2O2 unit can be connected to the four same adjacent ones to form one 3-D 4-c diamond-like 66 topological network with 1-D square

Figure 2. 2-fold interpenetrated 3-D network with 1-D channel along the b axis in complex 1: (a) ball-and-stick model and (b) topological configuration containing two same 66 frameworks.

Whereas a reduced void space deriving from interpenetration may create appropriate confined narrow pores/channels for adsorption of guest molecules. More importantly, the main ligands of hbhdpy− have free acylamide groups located in the channel and the planar dinuclear [Cd2(hbhdpy) (DMA)]3+ units formed in reaction (Figure 1) hold potential unsaturated metal centers (UMCs) that chemically interact with CO2 or nitroaromatics and render its potential for highly selective CO2 uptake and sensitive luminescence response for TNP molecules. Compound 1-NH2 is isomorphous with 1 (see Figures S3− S5 in the Supporting Information), and its related information as well as selected distances and angles are listed in Tables S1 and S2. It is worth mentioning that with H2bdc-NH2 replacing H2bdc, the 1-D channel surrounding the final product 1-NH2 is composed of the polar groups, namely, the amine and acylamide, though the pore size of 8.72 × 5.92 Å2 and solvent 4703

DOI: 10.1021/acsami.6b14051 ACS Appl. Mater. Interfaces 2017, 9, 4701−4708

Research Article

ACS Applied Materials & Interfaces accessible void of 42% in 1-NH2 compared with 1 have decreased. Powder X-ray Diffraction and Thermogravimetric Analyses. Numerous single crystals of 1 and 1-NH2 were collected for the powder X-ray diffraction (PXRD) measurement at room temperature. The PXRD patterns of 1 and 1NH2 match quite well with the simulated ones of 1 and 1-NH2 from the single-crystal-structure analyses, showing not only the similarity in structures of 1 and 1-NH2 but also the phase purity of as synthesized samples (Figure 3). The thermal stabilities of

Figure 4. (a) Adsorption isotherms of 1 for CO2, CH4, and N2 at 273 and 298 K, (b) adsorption isotherms of 1-NH2 for CO2, CH4, and N2 at 273 and 298 K (adsorption and desorption lines are presented with closed and open symbols, respectively). (c) The CO2/CH4 or CO2/N2 selectivity for 1 and 1-NH2 at 298 K calculated by the IAST method in CO2/CH4 (50/50) or CO2/N2 (15/85) binary mixtures. (d) The Qst of 1 and 1-NH2 for CO2. Figure 3. Related powder X-ray diffraction patterns in 1 and 1-NH2.

channels forming strong quadrupole−quadrupole interaction between 1 and CO2. Moreover, in order to further improve the sorption selectivity for CO2, the pores in 1 were modified with amine group to give 1-NH2. The result showed that 1-NH2 displayed a much higher uptake of CO2 at 273 K, 1 bar (49.3 cm3 g−1, 2.2 mmol g−1) with a less uptake of CH4 and N2 (4.0 cm3 g−1 and 2.0 cm3 g−1, respectively). Also at 298 K, 1 bar, the 1-NH2 showed better adsorption of CO2 than 1 (Figure 4b). To further investigate the CO2 selectivity, the separation selectivities of CO2 versus CH4 (50/50 mol ratios) and N2 (15/85 mol ratios) at room temperature were calculated up to 100 kPa using the ideal adsorption solution theory (IAST) based method (Figure 4c and Figure S12 in the Supporting Information). For 1, the selectivity of CO2/N2(CO2/CH4) displayed slight decreasing trend from 84.3 to 60.8 (9.7 to 8.0) in the range of 0−100 kPa. However, the corresponding value was almost improved two times from 157.6 to 104 (17.7 to 12.8) for 1-NH2. These results suggest that the 3-D interpenetrating 1/1-NH2 networks have a strong affinity toward CO2 over N2 and CH4, in which the performance of 1-NH2 from further amino functionalization of 1 is more effective. As we have learned, this performance is much better than some porous organic frameworks and zeolite imidazolate frameworks (Table S4).34,35 Moreover, the dynamic column breakthrough experiments were also performed to test the performance of the materials in the actual adsorption-based separation and purification processes. The mixture of CO2/N2 (15/85, v/v) was flowed over a packed bed of 1 and 1-NH2 solid at 298 K. As shown in Figure 5a,b, 1 can successfully separate the CO2 from the CO2/ N2 (15/85, v/v) mixture while the NH2 functional material, 1NH2, performs more efficiently. The dynamic CO2 adsorption capacity (0.83 mmol g−1) in 1-NH2 at room temperature is higher than that of 1 (0.41 mmol g−1). The result is similar to the known IRMOF-74-III-CH2NH2 (0.8 mmol g−1)36 and better than the HKUST-1 (0.45 mmol g−1).37 When it comes to the mixture of CO2/CH4 (50/50, v/v), the breakthrough occurs at a shorter dimensionless time due to the relatively

1 and 1-NH2 were also investigated under N2 atmosphere from room temperature to 800 °C with a heating rate of 10 °C/min (Figure S6). As showed in Figure S6, the 1 and 1-NH2 have the similar thermal stability. A weight loss from 147 to 211 °C was observed, which can be attributed to the loss of the DMA solvent and lattice water molecules. The TGA results further indicate that, after removal of guest molecules, 1 and 1-NH2 exhibit relatively high thermal stability up to 300 °C with certain structural robustness. Gas Adsorption and Separation Performance. Considering that 1 and 1-NH2 possess vacant pores and open metal sites as well as polarity group in the channels, we attempted to study its application in CO2 capture and separation. In order to remove the solvents in the MOFs with integrity of the frameworks, the as-synthesized 1 and 1-NH2 were immersed in MeOH for 2 days to exchange the high-boiling point solvent, DMA, followed by heating at 70 °C under vacuum conditions to afford the activated samples of 1 and 1-NH2. The TGA and XRD results showed that the DMA molecules were completely removed in the activated 1 and 1-NH2 while the integrity of the frameworks was still kept (Figures S6−S8). The N2 adsorption isotherms at 77 K indicated that both the activated 1 and 1NH2 displayed the fully reversible type-I isotherm with the Langmuir area of 1154.3 m2 g−1 and 1032.1 m2 g−1, respectively (Figure S9). The CO2, CH4 and N2 adsorption isotherms were measured at 273 and 298 K, showing the completely reversible without hysteresis (Figure 4a,b). It was shown that at 273 K (1 bar), 1 possesses good CO2 capture with an uptake of 33.6 cm3 g−1(1.5 mmol g−1) while the CH4 and N2 uptakes at 273 K (1 bar) were only 3.9 cm3/g and 1.9 cm3/g, respectively. At 298 K (1 bar), 1 still held the amount of 17.7 cm3 g−1 of CO2 capture while that of CH4 and N2 is quite minimal (2.1 cm3 g−1 and 1.6 cm3 g−1). It is obvious that 1 displayed the better sorption selectivity for CO2 over N2 and CH4, which could be attributed to the open metal sites and the acylamide groups in the 4704

DOI: 10.1021/acsami.6b14051 ACS Appl. Mater. Interfaces 2017, 9, 4701−4708

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ACS Applied Materials & Interfaces

Luminescent Sensing of Small Organic Molecules. As the aromatic Schiff base compounds generally possess a strong luminescent property, the solid-state emission spectra of the samples 1 and 1-NH2 as well as the free ligands were measured at room temperature (Figure 7). When excited at the

Figure 5. At 298 K and 1 bar, column breakthrough experiment for gas mixture carried out on (i) CO2/N2 = 15/85 (v/v) in 1 (a) and 1-NH2 (b), (ii) and CO2/CH4 = 50/50 (v/v) in 1 (c) and 1-NH2 (d).

Figure 7. Room-temperature emission spectra for free ligands and complexes 1 as well as 1-NH2 (Ex = 368 nm).

small separation ratio of CO2/CH4 compared to the CO2/N2 (15/85, v/v) binary mixture. To better understand these results, the adsorption enthalpies (Qst) of CO2 are counted from the adsorption data at 273 and 298 K by using the Clausius−Clapeyron equation to quantitatively assess the binding strengths between CO2 and the frameworks (see the Supporting Information). As depicted in Figure 4d, the Qst values at zero coverage, respectively, reach 26.0 and 29.5 kJ mol−1 for 1 and 1-NH2 showing the good interaction of CO2-framework in 1 and 1-NH2. The high Qst value of 1-NH2 is consistent with the fact that the amine group in the pores of MOFs could enhance the interaction of CO2framework. What’s more, the DFT calculations were also provided to further confirm the probable position of CO2 in the pores. A specific spatial possibility distribution can be exhibited in Figure S9 and Figure 6, which illustrate the tendency of

wavelength of 368 nm, the compounds 1 and 1-NH2 showed the resemblant emission peak with the ligand Hhbhdpy at the wavelength of about 492 nm, indicating that the luminescence of 1 and 1-NH2 are based on the ligand Hhbhdpy rather than H2bdc or H2bdc-NH2. However, it is worth noting that compared with 1, the NH2 functionalized MOF, 1-NH2, had the higher fluorescence intensity which may be attributed to the greater quantity of electrons transferred from the H2bdc-NH2 to the ligand Hhbhdpy. The above porous and luminescent properties of 1 and 1NH2 prompted us to explore their potential sensing for small organic solvent molecules. In order to test the stability of 1 and 1-NH2, their grinding powder samples were immersed in different organic solvents (such as DMA, DMF, methanol, ethanol, acetonitrile, benzene, 1,4-dioxane, and dichloromethane, etc.) for 12 h and then treated by ultrasonication for 30 min to form the suspension, finally confirmed by the powder X-ray diffraction. The results show that the integrity of the frameworks 1 and 1-NH2 is well maintained (Figure S7) and the common solvents have slight influence to the luminescence of 1 or 1-NH2 (Figure S15). In addition to the common organic solvents, the detection of the aromatic compounds seems more important because they are widespread in refinery operations, plastic processing, and fuel operations but potentially carcinogenic and neurotoxic. It is found that through the host−guest interactions, the luminescent MOFs (LMOFs) can be used as the sensors to quickly and easily detect the aromatic compounds, e.g., benzene, toluene, aniline, and nitrobenzene.38−40 As shown in Figure 8, among the aromatic compounds mentioned above, the nitrobenzene exhibits the most significant quenching effect. Moreover, we also investigate the possibility of sensing other nitrocompounds, for instance, 4-nitrophenol (NP), 2,4,6-trinitrophenol (TNP), 4-nitroaniline (NA), 4-nitrotoluene (NT), nitrobenzene (NB), and 1,4-dinitrobenzene (DNB), some of which are associated with highly explosive materials. When the concentration is 0.15 mM in DMA, the aforementioned six nitro-compounds can obviously weaken the photoluminescent intensity of both 1 and 1-NH2 emission (Figure 9a). Among the six nitro compounds, TNP has the highest fluorescent quenching efficiency for 1/1-NH2 up to 81.7%/89.1%, and the

Figure 6. Spatial possibility distribution of 1 (left) and 1-NH2 (right) on the direction of the b axis.

particles after equilibrium. On the direction of b and c, there was an obvious difference between two MOFs. For 1, the position of CO2 was mainly influenced by the acylamino and metal-cluster jointly. After amination, the molecules of CO2 begin to adsorb parallel with the plane of acylaminos for the existence of these two groups on both sides. Besides, the specific spatial possibility distribution increased near the amino. In one pore, because of the symmetry of both acylaminos and amino, CO2 will have a conformation that was completely parallel with their plane. This phenomenon provided an evidence that the amino and acylamide group in 1-NH2 provide the interaction sites with CO2. 4705

DOI: 10.1021/acsami.6b14051 ACS Appl. Mater. Interfaces 2017, 9, 4701−4708

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ACS Applied Materials & Interfaces

spectra (Figure S19), the absorption efficiencies at 368 nm follow the order TNP > (DNP > NP) > (NA > NT ∼ NB), conforming to the quenching trend but not complete. In a word, the quenching mechanism may be the combination of the framework−guest interactions, absorption efficiencies, and the different bandgaps between the framework and guest.41 To further reveal the quenching efficiency of TNP, 1 mM TNP solution was gradually added to a dispersed solution of 1 in DMA and the fluorescence intensity showed decrease gradually (Figures S21 and S22). Besides, the Stern−Volmer equation, (I0/I) = Ksv[A] + 1, was used to calculated the quenching constant (Ksv).41,42 Figure 9b shows that the Stern− Volmer plot for TNP is the typically linear at low concentrations, and the values of Ksv for TNP of 1 and 1NH2 are estimated to be 2.5 × 104 and 4.8 × 104 M−1, respectively. Likewise, the difference of Ksv values also reflects that free NH2 group in channels of MOFs may improve the fluorescence sensitivity of sensing TNP, and its Ksv value from Table S6 is higher than most of the reported interpenetrating MOFs. For the practical applications, the detection of TNP was also performed in the medium of water. The same quenching detection efficiency could also be observed and the Ksv value of 1 and 1-NH2 both present a slightly decrease which might due to the effect of water polarity on charge transfer (Figures S23− S26). What’s more, the detection limits (DL) was calculated based on the equation DL = 3Sb/Ksv in which the Sb is the standard deviations for 10 repeated fluorescent measurement of blank solutions and it came out that the DL of 1 and 1-NH2 were calculated to be 1.3 and 0.29 ppm, respectively (Table S7).

Figure 8. Fluorescence intensities of 1 (a) and 1-NH2 (b) samples that were dispersed in DMA upon addition of various organic solvents (1 mM).

Figure 9. (a) Degree of fluorescence quenches upon addition of the nitrobenzene derivatives (0.15 mM). Inset: The quenching efficiency (red bar graph for 1 and blue bar graph for 1-NH2). (b) The Stern− Volmer plot of I0/I versus the TNP concentration of 1 and 1-NH2.

quenching trend is TNP > NP > DNB > NT > NA > NB for the two MOFs, indicating a high selectivity of 1 and 1-NH2 for TNP. This result is possibly due to the presence of OH− group in TNP. It is reported that the highly acidic OH− group can interact strongly with the fluorophore via electrostatic interactions and consequently quenching luminescence from the energy transfer mechanism.41,42 For 1, the OH− group of the TNP can interact with the free Lewis-base site from the acylamino of ligand Hhbhdpy. It was found that the order of the quenching efficiency in DMA was TNP > NP, which is in agreement with the order of acidity of these two aromatic nitro compounds. Compared to 1, the presence of another free amide group from the auxiliary ligand H2bdc-NH2 in the pores of 1-NH2 may enhance the interaction of acidic phenolic hydroxyl groups with it, and all six nitro-compounds show higher quenching efficiency with quenching trend similar to 1. Obviously, this phenomenon further supports the fluorescence notion of the aforementioned quenching mechanism. In order to confirm the quenching mechanism, the HOMO and LUMO energy levels of MOFs and nitro aromatic were measured through the electrochemical cyclic voltammetry and the UV−vis absorption spectroscopy. As shown in Figures S16 and S17, the oxidation potential of MOFs and nitro aromatic were measured through CV and the HOMO of which were calculated by the equipiton EHOMO = −(Eox + 4.80) eV. Then their LUMO energy were calculated by the addition of Eg which were obtained by the UV−vis absorption spectroscopy (Figures S18 and S19). As displayed in Table S5, the ELUMO of both 1 and 1-NH2 were higher than the nitro aromatic which indicated the energy transfer between the MOFs and nitro aromatic. Besides, except the ELUMO of NT and NA, the ELUMO of the nitro aromatic compounds were increased with the trend of TNP < NP < DNB < (NT > NA) < NB, matching with the quenching trend in general and further confirm the quenching mechanism (Figure S20). Moreover, as showed in the UV−vis absorption

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CONCLUSIONS In summary, two isomorphous microporous Cd-MOFs (namely, 1 and 1-NH2) with 2-fold interpenetration, exhibiting high selective CO2 adsorption over CH4 and N2 gases and excellent selective sensing of the nitro explosive 2,4,6trinitrophenol (TNP), were systematically studied. Compared with 1, the corresponding bifunctional features of 1-NH2 is more evident. Obviously, it can be further anticipated that interpenetration can be considered as an effective means to not only enhance the stability of MOF structures but also render appropriate pore sizes and open metal action sites for small molecular separation and nitro explosive detection. Meanwhile, this work also further highlights that the interpenetrating microporous MOFs may be rationally designed by elaborately selecting multifunctional ligand to serve as CO2 captor for commercial utilization, climate control and energy development, and practical fluorescence-responsive sensor for pollutant monitoring.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14051. Additional structural figures, selected bond lengths, bond angles, crystal data, PXRD patterns, TGA curves, fitting for sorption isotherms, photoluminescent spectra, and tables (PDF) Report of checkcif (CIF) CIF file for 1 and 1-NH2 (CIF) 4706

DOI: 10.1021/acsami.6b14051 ACS Appl. Mater. Interfaces 2017, 9, 4701−4708

Research Article

ACS Applied Materials & Interfaces

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yue-Peng Cai: 0000-0003-4028-9358 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks for financial aid from the National Natural Science Foundation of P. R. China (Grant Nos. 21471061, 21671071, and 21575043), Science and Technology Planning Project of Guangdong Province (Grant Nos. 2013B010403024 and 2015B010135009), Science and Technology Program of Guangzhou (Grant No. 2014J4100051), and the N.S.F. of Guangdong Province (Grant Nos. 2014A030311001 and C86186).

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