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Solvent-dependent zinc(II) coordination polymers with mixed ligands: selective sorption and fluorescence sensing† Ji-Ai Hua, Yue Zhao, Yan-Shang Kang, Yi Lu and Wei-Yin Sun* Starting from the same metal salts and mixed organic ligands of 1,3,5-tris(1-imidazolyl)benzene (tib) and 2-bromo-1,4-benzenedicarboxylic
acid
(H2BDC-Br),
two
novel
zinc(II)
coordination
polymers
[Zn2(tib)2(BDC-Br)]2·2SO4·17H2O (1) and [Zn4(tib)2(BDC-Br)3(H2O)4SO4]·7.5H2O·2.5DMF (2) (DMF = N,Ndimethylformamide) were obtained by using different solvent systems of DMF/H2O and DMF/EtOH/H2O, Received 12th April 2015, Accepted 13th May 2015
respectively. 1 is an unusual (3,4)-connected 3D net with a Point symbol of {4·8·104}{4·8·10}, while 2 is a
DOI: 10.1039/c5dt01386k
complicated 1D chain, which is further connected to form a 3D supramolecular architecture by hydrogen bonding interactions. In particular, 1 and 2 exhibit selective adsorption of CO2 over N2 and show good
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selectivity for detection of acetone via fluorescence quenching.
Introduction Porous metal–organic frameworks (MOFs) with diverse architectures, tunable pore size, surface area and functionality are promising materials for gas storage/separation, drug delivery, catalysis and luminescent sensors.1–4 Particularly, luminescent sensing MOFs are of significant interest due to their distinct advantages of short response time and high sensitivity,5,6 which can be used to detect small molecules, especially sensing of volatile organic solvent molecules. To achieve useful luminescent MOF sensors, an effective method is to use the ligand-based strategy, arising from π-conjugated rigid organic ligands with Lewis basic sites,7–9 in which the π-conjugated skeleton provides luminescence and rigid backbones while Lewis basic sites provide the binding site. To date, a variety of luminescent MOFs have been reported for sensing small organic molecules or metal ions.10 However, most of them were focused on the lanthanide MOFs,11 and only several transition metal MOFs have been reported for molecular sensing.8,12–15 For example, Sun et al. presented two Cd(II) MOFs demonstrating unique selectivity
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 210093, China. E-mail:
[email protected]; Fax: +86 25 83314502 † Electronic supplementary information (ESI) available: Hydrogen bonding data of 2 (Table S1), space-filling view of 1 (Fig. S1), 3D supramolecular structure of 2 (Fig. S2), PXRD (Fig. S3), TGA (Fig. S4) and photographs under ultraviolet light (Fig. S5). CCDC 1056413 and 1056414. For the ESI and crystallographic data in CIF or other electronic formats see DOI: 10.1039/c5dt01386k
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for the detection of acetone.12 Wen and co-workers reported the first Zn(II) MOF [Zn(pbdc)(bimb)·(H2O)] [H2pbdc = 5-(4-pyridyl)isophthalic acid, bimb = 4,4′-bis(1-imidazolyl)biphenyl] for detecting both pesticides and solvent molecules simultaneously.13 Bearing the aforementioned ideas in mind and taking advantage of the sensing properties of MOFs, a mixed ligand of 1,3,5-tris(1-imidazolyl)benzene (tib) and 2-bromo-1,4-benzenedicarboxylic acid (H2BDC-Br) was chosen in this work for the following reasons: (a) they are rigid building blocks containing imidazole and carboxylate groups that tend to form robust MOFs with metal centers; (b) both contain conjugated π-electron skeletons, which are favorable for luminescent materials; (c) the non-coordinated bromo-functional group may supply additional interaction sites, which can serve as intermolecular interacting sites for specific interaction performance.16 Fortunately, solvothermal reactions of the mixed ligand with ZnSO4·7H2O in a different mixed solvent system at 85 °C produced two new MOFs [Zn2(tib)2(BDC-Br)]2·2SO4·17H2O (1) and [Zn4(tib)2(BDC-Br)3(H2O)4SO4]·7.5H2O·2.5DMF (2), which were structurally characterized by single crystal and powder X-ray diffraction analyses, IR spectroscopy, elemental and thermogravimetric analyses (TGA). Furthermore, sensitive fluorescence quenching phenomena were observed in both the MOFs when exposed to acetone. In addition, the gas sorption properties of the two complexes were also investigated.
Experimental Materials and methods All commercially available chemicals and solvents of reagent grade were used as received without further purification. The
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ligand tib was synthesized according to the method reported previously.17 Elemental analyses for C, H and N were performed using a Perkin-Elmer 240C Elemental Analyzer at the analysis center of Nanjing University. FT-IR spectra were recorded in the range of 400–4000 cm−1 on a Bruker Vector 22 FT-IR spectrophotometer using KBr pellets. Powder X-ray diffraction (PXRD) measurements were carried out by using a Bruker D8 Advance X-ray diffractometer using Cu-Kα radiation (λ = 1.5418 Å), in which the X-ray tube was operated at 40 kV and 40 mA. TGA were performed using a Mettler-Toledo (TGA/ DSC1) thermal analyzer under a N2 atmosphere with a heating rate of 10 °C min−1. Sorption experiments were carried out with a Belsorp-max volumetric gas sorption instrument. The luminescence spectra for the powdered solid samples were recorded on an Aminco Bowman Series 2 spectrofluorometer with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra the pass width is 5 nm, and all the measurements were carried out under the same experimental conditions. Preparation of [Zn 2 (tib) 2 (BDC-Br)] 2 ·2SO 4 ·17H 2 O (1). A mixture of tib (8.2 mg, 0.03 mmol), H2BDC-Br (7.4 mg, 0.03 mmol) and ZnSO4·7H2O (14.4 mg, 0.05 mmol) was dissolved in a DMF–H2O mixed solvent (8 mL; v/v, 1/3). The final mixture was sealed in a Teflon-lined stainless steel container and heated at 85 °C for 3 days. After being cooled to room temperature, yellow block crystals of 1 were obtained in 73% yield. Anal. Calcd for C76H88Br2N24O33S2Zn4: C 38.82, H 3.77, N 14.30%. Found: C 38.87, H 3.73, N 14.36%. IR (KBr pellet, cm−1): 3421 (m), 3102 (w), 1617 (s), 1520 (s), 1508 (m), 1369 (s), 1272 (m), 1116 (w), 1073 (s), 1013 (w), 952 (w), 856 (m), 766(s), 657 (m), 602 (w), 536 (w). Preparation of [Zn4(tib)2(BDC-Br)3(H2O)4SO4]·7.5H2O·2.5DMF (2). Complex 2 was synthesized by the same procedure used for preparation of 1 except for that a DMF–EtOH–H2O mixed solvent (8 mL; v/v/v, 1/1/2) was used instead of DMF–H2O. Colorless block crystals of 2 were obtained in 63% yield. Anal. Calcd for C61.50H73.50Br3N14.50O30SZn4: C 36.40, H 3.65, N 10.01%. Found: C 36.27, H 3.59, N 10.07%. IR (KBr pellet, cm−1): 3421 (m), 3126 (w), 1653 (w), 1617 (s), 1514 (s), 1357 (s), 1248 (w), 1116 (w), 1073 (m), 1031 (w), 1013 (w), 947 (m), 832 (m), 766 (m), 681 (w), 651 (m), 591 (w), 536 (w). Crystallography Crystallographic data collections for 1 and 2 were carried out on a Bruker Smart Apex II CCD area-detector diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 293(2) K using the ω-scan technique. The diffraction data were integrated by using the SAINT program,18 which was also used for the intensity corrections for the Lorentz and polarization effects. Semi-empirical absorption corrections were applied using the SADABS program.19 The structures were solved by direct methods and all the non-hydrogen atoms were refined anisotropically on F2 by the full-matrix least-squares technique using the SHELXL-97 crystallographic software package.20 The hydrogen atoms except for those of water molecules were generated geometrically and refined isotropically
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using the riding model. The hydrogen atoms of coordinated water molecules were found from the Fourier map directly, while those of free water molecules were not found. Because the free water and DMF molecules in 2 were highly disordered and could not be modeled suitably, the SQUEEZE routine of
Table 1
Crystal data and structure refinements for complexes 1 and 2
Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z ρcalcd (g cm−3) μ (mm−1) F(000) Rint Data collected Independent data Goodness-of-fit R1a (I > 2σ(I)) wR2b (I > 2σ(I))
1
2
C76H88Br2N24O33S2Zn4 2351.12 Tetragonal P42/nbc 18.198(14) 18.198(14) 28.872(5) 90.00 90.00 90.00 9561.5(18) 4 1.633 1.965 4792 0.0397 50 178 4230 1.078 0.0625 0.1866
C61.50H73.50Br3N14.50O30SZn4 2029.12 Triclinic ˉ P1 12.906(16) 18.564(2) 20.441(3) 96.646(2) 108.014(2) 100.107(2) 4508.8(10) 2 1.495 2.483 2050 0.0528 28 627 18 608 1.011 0.0560 0.1007
a R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = |∑w(|Fo|2 − |Fc|2)|/∑|w(Fo)2|1/2, where w = 1/[σ2(Fo2) + (aP)2 + bP]. P = (Fo2 + 2Fc2)/3.
Table 2
Selected bond lengths (Å) and angles (°) for complexes 1 and 2a
1 Zn(1)–O(1) Zn(1)–N(1) O(1)–Zn(1)–N(1) O(1)–Zn(1)–N(5)#1 N(1)–Zn(1)–N(5)#1
1.999(3) 2.011(3) 106.75(13) 119.51(13) 111.30(13)
Zn(1)–N(5)#1 Zn(1)–N(3)#2 O(1)–Zn(1)–N(3)#2 N(1)–Zn(1)–N(3)#2 N(5)#1–Zn(1)–N(3)#2
2.014(3) 2.023(3) 101.68(13) 114.14(13) 103.33(13)
2 Zn(1)–O(1) Zn(1)–O(5) Zn(1)–N(1) Zn(1)–N(9)#1 Zn(2)–O(4)#2 Zn(2)–O(2W) Zn(2)–N(3) Zn(2)–O(1W) O(1)–Zn(1)–O(5) O(1)–Zn(1)–N(1) O(5)–Zn(1)–N(1) O(1)–Zn(1)–N(9)#1 N(1)–Zn(1)–N(9)#1 O(4)#2–Zn(2)–O(2W) O(4)#2–Zn(2)–N(3) O(2W)–Zn(2)–N(3) O(4)#2–Zn(2)–O(1W) O(2W)–Zn(2)–O(1W) O(9)–Zn(3)–O(7)#3 O(7)#3–Zn(3)–N(5)
1.933(3) 1.955(3) 1.990(4) 1.999(4) 1.930(3) 1.972(4) 1.990(5) 2.056(4) 96.64(16) 113.81(18) 114.91(17) 112.90(17) 109.43(17) 105.97(18) 132.07(19) 109.58(18) 103.35(15) 100.15(16) 96.24(14) 118.46(16)
Zn(3)–O(9) Zn(3)–O(7)#3 Zn(3)–N(5) Zn(3)–N(11)#1 Zn(4)–O(11) Zn(4)–N(7) Zn(4)–O(4W) Zn(4)–O(13) O(9)–Zn(3)–N(11)#1 O(7)#3–Zn(3)–N(11)#1 N(5)–Zn(3)–N(11)#1 O(11)–Zn(4)–N(7) O(11)–Zn(4)–O(4W) N(7)–Zn(4)–O(4W) O(11)–Zn(4)–O(13) N(7)–Zn(4)–O(13) O(4W)–Zn(4)–O(13) N(7)–Zn(4)–O(3W) O(4W)–Zn(4)–O(3W) O(13)–Zn(4)–O(3W)
1.954(3) 1.965(3) 1.995(4) 1.986(4) 1.958(3) 1.987(4) 2.010(4) 2.110(4) 114.08(17) 105.29(15) 108.60(16) 138.88(18) 107.48(17) 111.43(16) 94.54(14) 96.80(17) 92.09(15) 86.72(16) 81.87(15) 173.81(15)
a Symmetry transformations used to generate equivalent atoms: #1 y + 1/2, −x, −z + 1/2; #2 −y + 1/2, −x + 1/2, −z + 1/2 for 1; #1 x + 1, y + 1, z; #2 x − 1, y − 1, z; #3 −x + 2, −y + 1, −z + 1 for 2.
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PLATON21 was applied to remove their contributions to the scattering. SQUEEZE removed 7.5H2O and 2.5DMF molecules per formula unit. This value is calculated based on volume/ count_electron analysis and the TG data. The reported refinements are of the guest-free structures obtained by the SQUEEZE routine, and the results are attached to the CIF file. The atoms O4, O5, O6, O11 and O12 in 1 and O15, O16 and S1 in 2 are disordered. The details of the crystal parameters, data collection, and refinements for the complexes are summarized in Table 1 and the selected bond lengths and angles are listed in Table 2 and the hydrogen bonding data for 2 are given in Table S1 (ESI†).
Results and discussion Crystal structure of [Zn2(tib)2(BDC-Br)]2·2SO4·17H2O (1) Crystallographic analysis reveals that 1 crystallizes in a tetragonal space group P42/nbc, and the asymmetric unit consists of one fourth of [Zn2(tib)2(BDC-Br)]2·2SO4·17H2O. As illustrated in Fig. 1a, Zn1 is four-coordinated by three imidazole nitrogen atoms (N1, N5A, N3B) from three different tib ligands and one carboxylate oxygen atom (O1) from BDC-Br, generating a distorted tetrahedral coordination geometry, with Zn1–N bond distances in the range of 2.011(3)–2.023(3) Å and the Zn1–O distance of 1.999(3) Å (Table 2). In 1, each BDC-Br acts as a μ2bridging linker with two carboxylate groups adopting (κ1)–(κ1)μ2-BDC-Br coordination mode to connect two Zn(II) atoms. Meanwhile, each tib connects three Zn(II) atoms to form a brick-wall-like two-dimensional (2D) network (Fig. 1b), which can be simplified to a 3-connected uninodal 2D net with a {4·8·10} topology (Fig. 1c). The 2D networks are further connected together by BDC-Br to give rise to the final three-dimensional (3D) framework of 1 (Fig. 1d) with one-dimensional (1D) channels along the c axis (Fig. S1, ESI†). The high-solventaccessible volume is 2817.2 Å3 out of the 9562.0 Å3 unit cell volume (29.5% of the total crystal volume) calculated by using PLATON.21b TOPOS analysis reveals that this framework can be simplified as a (3,4)-connected 2-nodal 3D net with the Point (Schläfli) symbol of {4·8·104}{4·8·10} (Fig. 1e),22 in which the Zn(II) atom, tib and BDC-Br ligands are considered as four-, three- and two-connectors, respectively. Crystal structure of [Zn4(tib)2(BDC-Br)3(H2O)4SO4]· 7.5H2O·2.5DMF (2) When DMF–EtOH–H2O, instead of DMF–H2O, was used as reaction medium with the other reaction conditions unchanged, 2 was obtained. X-ray diffraction analysis reveals that the asymmetric unit of 2 consists of [Zn4(tib)2(BDC-Br)3(H2O)4 SO4]·7.5H2O·2.5DMF. As shown in Fig. 2a, both Zn1 and Zn3 are coordinated by two carboxylate oxygen atoms (O1, O5 for Zn1; O7C, O9 for Zn3) and two imidazole nitrogen atoms (N1, N9A for Zn1; N5, N11A for Zn3) to give distorted tetrahedral coordination geometries, Zn2 is also four-coordinated in a tetrahedral coordination geometry, but with the four positions occupied by one imidazole nitrogen
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atom (N3) of tib, one carboxylate oxygen atom (O4B) of BDC-Br and two coordinated water molecules (O1W, O2W). However, Zn4 is situated in a distorted trigonal bipyramidal environment, with the equatorial positions occupied by two oxygen atoms (O11, O4W) and one nitrogen atom (N7), and the axial positions coordinated by two oxygen atoms (O13, O3W). The Zn(1)N2O2, Zn(2)NO3 and Zn(3)N2O2 tetrahedra connect together via a tib ligand, and the Zn(3)N2O2 tetrahedron and Zn(4)NO4 trigonal bipyramid are linked together by one BDC-Br. In 2, all the BDC-Br ligands act as μ2-linkers with carboxylate groups adopting (κ1)–(κ1)-μ2-BDC-Br coordination modes. Each tib also acts as a tridentate linker to connect three Zn(II) atoms, and finally the two kinds of ligands link the Zn(II) atoms to form a 1D double ladder chain (Fig. 2b). In addition, the adjacent 1D chains are further linked together by hydrogen bonds to give a 2D structure (Fig. 2c), and finally to generate the 3D supramolecular structure of 2 (Fig. S2, ESI†). Effects of the reaction solvent on the structures It is known that the formation and/or crystallization of MOFs can be influenced by the reaction solvent, temperature, pH value and so on. In this regard, as one of the essential external factors, the solvent is a crucial one, such as its size, polarity and solubility of organic linkers in it have great influence on regulating the structures.23 The reaction of the same initial reactants in different solvents can generate different structures, which may provide direct evidence of the structural influence from these external factors. In our case, the solvent is a crucial factor. 1 and 2 were obtained under the same external reaction conditions except for the change of the solvents, and the structural differences should be mainly ascribed to the addition of EtOH, which further influence the solubility of the ligands and metal salts in the different solvents. The results indicate that the solvents play a key role in influencing the coordination environments of the Zn(II) ions and the linking modes of tib and BDC-Br ligands, further influencing the final structures of the complexes. PXRD and thermal stability The phase purity of the bulk samples was checked by PXRD in the solid state. For complexes 1 and 2, the as-synthesized samples are consistent with the simulated ones, indicating the phase purity of the bulk samples (Fig. S3, ESI†). To identify the thermal stabilities of the complexes, TG measurements were carried out and the results are shown in Fig. S4a (ESI†). Complex 1 shows a weight loss of 12.84% in the temperature range of 30–155 °C, which is attributed to the complete departure of the free water molecules (calcd 13.02%), and the residue is stable up to about 330 °C and then begins to decompose. For 2, the TG curve displays two step weight losses. In the first step, a weight loss of 10.28% from room temperature to 125 °C was observed, suggesting the loss of the free and coordinated water molecules (calcd 10.20%). In the second step, a weight loss of 8.52% was observed in the temperature range of 125–320 °C, corresponding to the removal of free DMF molecules (calcd 9.01%).24
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Fig. 1 (a) Coordination environment of Zn(II) in 1 with the ellipsoids drawn at the 30% probability level. Hydrogen atoms and free water molecules are omitted for clarity. (b) 2D network of Zn(II)-tib. (c) Topology of the Zn(II)-tib network. (d) 3D framework of 1. (e) Topology of 1: bright green, Zn atoms; red, tib ligands.
The desolvated samples 1′ and 2′ were obtained by heating the as-synthesized samples at 200 °C for 10 h under vacuum, respectively. The framework integrity was then examined by PXRD. As shown in Fig. S3,† 1′ and 2′ remain highly crystal-
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lized and retain the main framework features but with loss or shift of several peaks and occurrence of a few new peaks in 1′. This is probably due to the distortion or shrinking of the crystal lattice to some degree in response to heating and
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Fig. 3 Gas adsorption isotherms of CO2 (195 K) for 1 (a) and 2 (b), and N2 (77 K) for 1 (c) and 2 (d): filled shape, adsorption; open shape, desorption.
Fig. 2 (a) Coordination environment of Zn(II) in 2 with the ellipsoids drawn at the 30% probability level. Hydrogen atoms and free water molecules are omitted for clarity. (b) 1D chain of 2. (c) 2D network of 2 linked by hydrogen bonds.
removal of guest molecules, which is commonly observed in the MOF structures.25 In addition, as shown in Fig. S4b,† the TGA result of 1′ confirms that the free water molecules have been removed, while 2′ still shows a weight loss of 4.62%, which can be ascribed to the loss of the four coordinated water molecules (calcd 4.21%),26 as indicated by strong hydrogen bonds as listed in Table S1.† Gas adsorption The porosity and stability of the frameworks 1 and 2 prompted us to examine their gas uptake behavior. The activated
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samples 1′ and 2′ were prepared by heating the complexes at 200 °C under vacuum for 10 h, respectively. It is interesting that 1′ and 2′ show the ability to selectively adsorb CO2 over N2. As shown in Fig. 3a, the CO2 adsorption isotherm at 195 K for 1′ showed a type-I adsorption isotherm behavior, given the surface area of 177.69 m2 g−1 as calculated by the Brunauer– Emmett–Teller (BET) method. The CO2 uptake value is 66.484 cm3 g−1 (130.59 mg g−1) at 0.99 atm corresponding to 6.0 CO2 molecules per formula unit, which is moderate compared with the porous materials reported.27 For comparison, the adsorption capacity of 2′ (36.084 cm3 g−1 at 1 atm for CO2) is significantly less than 1′ (Fig. 3b), and the BET surface area is 94.03 m2 g−1. This phenomenon may be attributed to the influence of the different structures of 1 and 2. In addition, both the CO2 isotherms of 1 and 2 exhibited hysteresis upon desorption, which may arise from the strong interactions between CO2 and the framework hindering escape from the framework during the adsorption–desorption process as well as diffusion through narrow pore apertures, and similar sorption results have been observed in the recent reports.28 The adsorption isotherms of N2 measured at 77 K for 1′ and 2′ indicated that almost no N2 adsorptions were observed (1.825 cm3 g−1 at 1 atm for 1′ and 1.429 cm3 g−1 at 1 atm for 2′). The selective CO2 uptake over N2 observed for 1 and 2 may be partially attributed to the molecular size, given the smaller kinetic diameter of CO2 (3.30 Å) compared to that of N2 (3.64 Å),29 indicating that channels in the frameworks are small to allow the incorporation of N2. The results indicate that 1 and 2 might be promising materials for selective adsorptive separation of CO2 from industrial flue gas or the removal of CO2 from natural gas. Photoluminescence properties MOFs constructed from d10-metal ions and conjugated organic ligands are promising candidates for potential luminescent materials.30 Accordingly, the solid state luminescence properties of 1 and 2 were investigated at room temperature. As
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Fig. 4 Solid-state emission spectra of 1 and 2 as well as tib at room temperature.
shown in Fig. 4, 1 and 2 display apparent fluorescence enhancement and the intense emission bands were observed at λem = 439 nm (λex = 329 nm) for 1, λem = 443 nm (λex = 330 nm) for 2 compared with the free tib ligand (λem = 430 nm, λex = 376 nm), which may be attributed to the intraligand transition of tib due to their close resemblance of the emission bands.31 Obviously, the overall architectures of 1 and 2 endowed the rigidity of the aromatic backbones and maximized the intramolecular/intermolecular interactions among the organic moieties for energy transfer, and hence effectively reduced the intraligand HOMO–LUMO energy gap.13 In addition, since the emission bands of the carboxylate ligands are usually weak compared to that of tib, it was considered that the carboxylate ligands have no significant contribution to the fluorescence emission of 1 and 2 in the presence of the N-donor ligand.32 Compared with the free tib ligands, the red-shift of the emissions of 1 and 2 is considered to be caused by the coordination of the ligand to the metal centers.33 It is known that some fluorescent MOF materials are sensitive to the presence or absence of guest molecules. To examine the potential sensing of small solvent molecules, the fluo-
Fig. 5
rescence properties of 1 and 2 were investigated by immersing fresh samples in different solvents. For 1, the most interesting feature is that its photoluminescence (PL) intensity is largely dependent on the solvent molecules, particularly in the case of MeOH and acetone, which exhibit the most significant enhancing and quenching effects, respectively (Fig. 5a). This behavior usually exists in the examples of lanthanide-MOFs, while it is scarcely observed in Zn-MOFs. The intensity of the PL of 1 depends on the identity of the solvent molecule, with the sequence of MeOH > 2-PA > EtOH > CHCl3 > THF > NMP > 1,2ethanediol > CH3CN > DMA > DMF > CH2Cl2 > acetone, while for 2, the enhancing behavior effect is observed in 2-PA (2-propanol) and the quenching effect in acetone with the order of 2-PA > DMA > NMP > 1,2-ethanediol > CH3CN > CHCl3 > EtOH > DMF > MeOH > THF > CH2Cl2 > acetone (Fig. 5b). It can be clearly seen from the photographs of the samples of 1 and 2 showing the fluorescence under ultraviolet light (Fig. S5, ESI†). This phenomenon may be mainly ascribed to different interactions between the framework architecture and distinct solvent molecules.8 In this regard, such solvent-dependent luminescence properties are of interest for the sensing of solvent molecules. In addition, quenching effects of the two complexes are also verified by the following facts: after the assynthesized samples of 1 and 2 are soaked in acetone for 24 h, the luminescence intensities are completely quenched as verified by using their solid luminescence spectra (Fig. 4). The PXRD patterns of 1 and 2 soaked in acetone for 24 h are identical to the as-synthesized samples, which suggest that the crystal lattices remain robust after treatment. The PXRD patterns further confirm the high framework stabilities of 1 and 2 (Fig. S3, ESI†). These exciting results indicate that 1 and 2 could be served as promising luminescent probes for detecting small acetone molecules. To examine the sensing sensitivity toward acetone in detail, a batch of emulsions of 1 and 2 dispersed in MeOH and 2-PA solution, respectively, with gradually increasing acetone contents were prepared to monitor the emissive response. A gradual decrease of the fluorescence intensity was observed upon the addition of acetone to the MeOH emulsion of 1 and 2-PA emulsion of 2, respectively (Fig. 6a and b). The fluorescence decrease was nearly proportional to the acetone con-
PL intensities of 1 (a) and 2 (b) introduced into various pure solvents when excited at 329 nm for 1 and 330 for 2.
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Fig. 6 PL spectra of the dispersed 1 in MeOH (a) and 2 in 2-PA (b) in the presence of various contents of the acetone solvent (excited at 329 nm and 330 nm, respectively).
Conclusion
Fig. 7 PL intensities of 1 in MeOH (a) and 2 in 2-PA (b) as a function of acetone content.
In summary, we have successfully synthesized two zinc(II) coordination polymers with different structures by using different reaction solvents. The results indicate that the solvent plays a critical role in determining the structures of MOFs. Interestingly, 1 and 2 show the ability to selectively adsorb CO2 over N2, indicating their potential application in gas separation. More importantly, the two complexes show good sensing and detection of acetone by the fluorescence quenching method. It is the first report that Zn(II)-MOFs based on tib and dicarboxylate exhibit excellent luminescence sensing ability for acetone molecules. It can be seen that the mixed ligand strategy is effective in constructing new MOFs with different pore sizes, which make MOFs good candidates for separating and sensing small molecules. It is expected that more luminescent sensing MOFs will emerge in the future.
Acknowledgements centration. The decreasing trends of the fluorescence intensity at 439 nm for 1 and 443 nm for 2 versus the volume ratio of acetone could be fitted with a first-order exponential decay, indicating that fluorescence quenching of 1 and 2 using acetone is diffusion-controlled (Fig. 7a and b).34 Therefore, 1 and 2 can be regarded as candidates for the selective sensing of acetone. The physical interaction of the solute and solvent plays an important role in such fluorescence enhancing and quenching effects of small solvent molecules. Such a quenching mechanism is due to a competition of adsorption of the light source energy between the excited MOFs and acetone molecules adsorbed in the pores and on the surface of the MOF particles. Upon excitation, there is a competition of absorption of the light source energy between the solvent molecules and organic ligands. The energy absorbed by the organic ligands is transferred to acetone molecules, resulting in a decrease in the luminescence intensity.34
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This work was financially supported by the National Natural Science Foundation of China (grant no. 21331002 and 21401099). This work was also supported by a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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