2-Fold Interpenetrating Bifunctional Cd-MetalâOrganic Frameworks

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The 2-fold Interpenetrating Bifunctional Cd-MOFs: High Selective Adsorption for CO and Sensitive Luminescent Sensing of Nitro Aromatic 2,4,6-trinitrophenol 2

Xu-Jia Hong, Qin Wei, Yuepeng Cai, Sheng-Run Zheng, Ying Yu, Yan-Zhong Fan, Xianyan Xu, and Liping Si ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 17, 2017

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ACS Applied Materials & Interfaces

The 2-fold Interpenetrating Bifunctional Cd-MOFs: High Selective Adsorption for CO2 and Sensitive Luminescent Sensing of Nitro Aromatic 2,4,6-trinitrophenol Xu-Jia Honga,Qin Weia,Yue-Peng Caia,*Sheng-Run Zhenga,Ying Yua,Yan-Zhong Fanb,Xian-Yan Xuc,Li-Ping Sia* a

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 b Guanghzou, 510006, P. R. China; School of Chemistry and Chemical Engineering Sun Yat-Sen University Guanghzou, 510275, P. R. China; c College of Chemistry and Environmental Engineering Shaoguan University Shaoguan, 512005, P. R. China. Corresponding author E-mail: [email protected] and [email protected]

ABSTRACT: A robust primitive diamond-type topology 3-D MOF of {[Cd4(hbhdpy)2(bdc)3(DMA)2]·(H2O)4}n (1, DMA = N,Ndimethylacetamide) was constructed from the planar secondary building units of the dinuclear cadmium clusters, Cd2(µ2O)2, and two linear organic linkers of the new multidentate Schiff base of 4-(2-hydroxy-3-methoxy-benzyli-denehydrazinocarbonyl)-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 2,4,6-trinitrophenol (TNP) in DMA solution under ambient conditions. With 2-Amino-1,4-dicarboxy-benzene (H2bdc-NH2) replacing H2bdc, an amine-functionalized 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. Key Words: metal-organic frameworks, 2-fold interpenetrating Cd-MOFs, bifunctional features, selective adsorption, luminescent sensing, 2,4,6-trinitrophenol 19-22

■INTRODUCTION Metal−organic frameworks (MOFs) have emerged as good sorbents for CO2 storage and capture on account of their 1-5 tunable pore size and high surface area. Till now, there are only a few MOFs which show high 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 6-8 exhibit high CO2 selectivity. 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 9-15 isoreticular structures. 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 and hydrothermal stabilities of these MOFs for large scale 6-8 industrial applications must be satisfied. On the other hand, the combination of porosity and luminescence have made MOFs become the potential chemical sensors and devices for detecting metal ions 2+ 2+ 2+ 16-18 (such as Cu , Ni , Ba , etc.), inorganic anions (for - instance, NO3 , Cl , I , etc.) or other organic molecules (for example, 2,4,6-trinitrophenol, 2,4,6-trinitro-toluene,

etc.). 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, illegal wastewater emissions in the environment. Accordingly, it is crucial for homeland security, security screening and environmental monitoring to rapidly detect these nitro aromatics. Due to its rapidity, sensitivity and convenience, the luminescence quenching method has become to be a very effective strategy for the 23-27 sensing of these nitro aromatics. N

O

H N

OH HO

N O

R O

OH O

O

N H

Hhbhdpy

R = H for H2bdc; NH2 for H2bdc-NH2

Scheme 1. Structures of ligands for Hhbhdpy, H 2 bdc and H 2 bdc-NH 2 .

Bearing all of that in mind, a new class of lengthy linear multi-chelate Schiff base ligand involving acylhydrazone and aminocarbonyl groups, namely 4-(2-hydroxy-3methoxy-benzyli-dene-hydrazinocarbonyl)-N-pyridin-4-ylbenzamide (Hhbhdpy, Scheme 1), was chosen to construct the 3-D MOFs with the following advantages: (i) the selection of lengthy linear multi-chelate ligand, Hhbhdpy, is helpful to the assembly of the interpenetrating networks

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with high stability. (ii) The NO3 set from one end of Hhbhdpy may be designed to chelating-coordinate 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 CdMOFs may provide potential open metal sites. (v) Cocoordination 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(bdc-NH2)3-(DMA)2]·(H2O)4}n (MOF-1-NH2) (Hhbhdpy = 4-(2-Hydroxy-3-methoxy-benzylidenehydrazinocarbonyl)-N-pyridin-4-yl-benzami-de, and H2bdc = 1,4dicarboxybenzene, H2bdc-NH2 = 2-Amino-1,4dicarboxybenzene, DMA = N,N-Dimethylacetamide) were solvothermally constructed. Moreover, the lengthy linear Hhbhdpy ligand together with H2bdc/H2bdc-NH2 afford the 6 2-fold interpenetrating primitive diamond-like 6 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. ■EXPERIMENTAL SECTION Materials and physical measurements The ligand 4-(2-Hydroxy-3-methoxy-benzylidene-hydrazinocarbonyl)-N-pyridin-4-yl-benzamide (Hhbhdpy) was 28-29 prepared according to the literature, and the other materials were purchased and used without further purification. The Perkin-Elmer 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. Thermogravi-metric analyses were performed on Perkin-Elmer 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)

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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 −1 (KBr, cm ): 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 1-NH2 (based on −1 Hhbhpy). IR (KBr, cm ): 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 multi-scan program SADABS are used for 30 31 the absorption corrections. The SHELXS-97 and SHELXL32 97 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 (bdcNH2) in the asymmetric unit of 1-NH2 were disordered into two sites with 0.5 occupancy of per position, respectively. 33 For 1-NH2, the SQUEEZE option in PLATON was used to remove the disordered solvent water molecules, and the actual water molecules in the unit cell are determined by elemental and thermo-gravimetric 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. 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 non-volatile solvates (DMA) before gas


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adsorption measurements. After the removal of methanol by decanting, the samples were activated by heating at 70°C under vacuum conditions. ■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 the monoclinic crystal system, space group P21/c. Compound 1 is employed as a representative to be described in detail. There are two 2+ Cd ions, one deprotonated hbhdpy anion, one and a half 2bdc anions, one coordinated DMA molecule and two lattice water molecules in the asymmetric unit of 1 (Figure 1a). 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 2one deprotonated ligand bdc , 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 deproto2nated ligand bdc 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 2carboxylate oxygen atom (O6) from one bdc ligand and another phenolate oxygen atom (O2) from one deprotonated ligand hbhdpy with the Cd1···Cd2 separation 2 of 3.620(2) Å, forming the dinuclear Cd2(η -O)2 unit (Figure

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.

1b). In 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 atoms and one

acylhydrazone nitrogen atom with 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 1 2 1 1 1 nitrogen atom, showing mode I: µ3-η :η :η :η :η . However, ligand 1,4-dicarboxy-benzene (H2bdc) was completely deprotonated and presented two different coordination 2modes (II) and (III) (Figure 1c), namely, each bdc ligand 1 1 1 1 1 1 2 1 via µ2-η :η :η :η and µ3-η :η :η :η mannars linked two and three Cd(II) centres in mode II and III, respectively. Based on the coordination modes of Cd(II) ions and two 2− ligands of hbhdpy and bdc , each dinuclear Cd2O2 unit can be connected to the four same adjacent ones to form 6 one 3-D 4-c diamond-like 6 topological network with 1-D square metallocyclic channels accommodated by the 2 lattice water molecules with about 18.6 × 9.5 Å size (just considering a single non-interpenetrating 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 S1-2. Because of the lengthy multidentate Hhbhdpy ligand, a 2-fold interpenetrating network with 50.1% solvent accessible voids (calculated using the PLATON software after elimination of guest and coordinated solvent 2 molecules) and 1-D channels of about 9.9×9.5 Å (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). 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 3+ [Cd2(hbhdpy)(DMA)] units formed in reaction (Figure 1) hold potential unsaturated metal centres (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 Figure S3-5 in Supporting Information), and its related information as well as selected distances and angles are listed in Table S12. It is worth mentioning that with H2bdc-NH2 replacing H2bdc, the 1-D channel surrounding the final product 1NH2 is composed of the polar groups, namely the amine 2 and acylamide, though the pore size of 8.72 × 5.92 Å and solvent 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 1-NH2 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 1 and 1-NH2 were also investigated under N2 atmosphere from room temperature to 800°C with a


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(a)

(b)

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

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.

Figure 3. The related powder X-ray diffraction patterns in 1 and 1-NH2.

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

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that the DMA molecules were completely removed in the activated 1 and 1-NH2 while the integrity of the frameworks was still kept (Figures S6-8). The N2 adsorption isotherms at 77K indicated that both the activated 1 and 1-NH2 displayed the fully reversible type-I isotherm with 2 -1 2 -1 the Langmuir area of 1154.3 m g and 1032.1 m g , respectively (Figure S9). The CO2, CH4 and N2 adsorption isotherms were measured at 273K and 298K, showing the completely reversible without hysteresis (Figures 4a-b). It was shown that at 273K (1 bar), 1 possesses good CO2 capture with 3 -1 -1 uptake of 33.6 cm g (1.5 mmol g ) while the CH4 and N2 3 3 uptakes at 273K (1bar) were only 3.9 cm /g and 1.9 cm /g, respectively. At 298 K (1bar), 1 still held the amount of 3 -1 17.7 cm g of CO2 capture while that of CH4 and N2 is quite 3 -1 3 -1 minimal (2.1 cm g and 1.6 cm g ). 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 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 273K, 1 bar 3 -1 -1 (49.3cm g , 2.2 mmolg ) with a less uptake of CH4 and N2 3 -1 3 -1 (4.0 cm g and 2.0 cm g respectively). And at 298K, 1bar, 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)

Figure 4 (a) Adsorption isotherms of 1 for CO2, CH4 and N2 at 273 K and 298 K, (b) Adsorption isotherms of 1-NH2 for CO2, CH4 and N2 at 273 K 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.

and N2 (15:85 mol ratios) at room temperature were calculated up to 100KPa using the ideal adsorption solution theory (IAST) based method (Figures 4c and S12 in ESI). 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 0-100KPa. 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 towards CO2 over N2 and CH4, in which the performance of


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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 34-35 imidazolate frameworks (Table S4). 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 Figures 5a-b, 1 can successfully separate the CO2 from the CO2/N2 (15:85, v/v) mixture while the NH2 functional material, 1-NH2, performs more efficiently. The dynamic CO2 adsorption capacity (0.83 -1 mmolg ) in 1-NH2 at room temperature is higher than that -1 of 1 (0.41 mmolg ). The result is similar to the known -1 36 IRMOF-74-III-CH2NH2 (0.8 mmolg ) and better than the -1 37 HKUST-1 (0.45 mmolg ). 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 small separation ratio of CO2/CH4 compared to the CO2/N2 (15:85, v/v) binary mixture.

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).

To better understand these results, the adsorption enthalpies (Qst) of CO2 are counted from the adsorption data at 273 and 298 K by using Clausius-Clapeyron equation to quantitatively assess the binding strengths between CO2 and the frameworks (see ESI). As depicted in Figure 4d, the Qst values at zero coverage respectively -1 reach 26.0 and 29.5 KJ·mol 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 CO2-framework. What’s more, the DFT calculations was 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 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 metalcluster 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.

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

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 wavelength of 368 nm, the compounds 1 and 1-NH2 showed the resemblant emission peak with the ligand Hhbhdpy at the wavelength of

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

about 492 nm, indicating that the luminescence of 1 and 1NH2 are based on the ligand Hhbhdpy rather than H2bdc or H2bdc-NH2. However, it is worthnoting 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 H2bdcNH2 to the ligand Hhbhdpy. The above porous and luminescent properties of 1 and 1-NH2 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, 1,4dioxane and dichloro-methane, etc.) for 12h 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).


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

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, 38-40 toluene, aniline, and nirtrobenzene. As shown in Figure 8, among the aromatic compounds mentioned above, the nirtrobenzene exhibits the most significant quenching effect. Moreover, we also investigate the possibility of sensing other nitro-compounds, for instance, 4nitrophenol (NP), 2,4,6-trinitrophenol (TNP), 4-nitroaniline (NA), 4-nitrotoluene (NT), nitrobenzene (NB) and 1,4dinitrobenzene (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 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 41-42 from the energy transfer mechanism. 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 Figure S16-17, the oxidation potential of MOFs and nitro aromatic were measured through CV and the HOMO of which were caculated by the equipiton EHOMO = -

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(Eox + 4.80) eV. Then their LUMO energy were calculated by the addition of Eg which were obtained by the Uv-vis absorption spectroscopy(Figure S18-19). 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 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 41 framework and guest . 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 S21-22). Besides, the SternVolmer equation, (I0/I) = Ksv[A] + 1 , was used to calculated 41-42 the quenching constant (Ksv). 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 14 4 -1 NH2 are estimated to be 2.5 × 10 and 4.8 × 10 M , 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 S23-26). What’s more, the detection limits (D.L.) was calculated based on the equation D.L.=3Sb/Ksv in which the Sb is the standard deviations for ten repeated fluorescent measurement of blank solutions and it came out that the D.L. of 1 and 1NH2 were calculated to be 1.3 ppm and 0.29 ppm respectively (Table S7).

Figure 9 (a)The 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.

Conclusions


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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 2,4,6-trinitrophenol (TNP), were systematically studied. Compared with 1, the corresponding bifunctional features of 1-NH2 is more evident. Obviuosly, 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 multi-functional 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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Acknowledgements. Thanks for financial aid from the National Natural Science Foundation of P. R. China (Grant No.21471061, 21671071 and 21575043), Science and Technology Planning Project of Guangdong Province (Grant No. 2013B010403024 and 2015B010135009), Doctoral Program Foundation of Institutions of Higher Education of China (Grant No. 201244407110007) and the N.S.F. Of Guangdong Province (Grant No. 2014A030311001 and C86186).

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Supporting Information Available: Additional structural figures, selected bond lengths, bond angles, crystal data, PXRD patterns, TGA curves, fitting for sorption isotherms, photoluminescent spectra, and tables (PDF) The report of checkcif (PDF) CIF file for 1 and 1-NH2 (CIF) References (1)

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2-Fold Interpenetrating Bifunctional Cd-MetalâOrganic Frameworks

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