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Four uncommon nanocage-based Ln-MOFs: highly selective luminescent sensing for Cu2+ ions and selective CO2 capture†
Received 24th April 2014, Accepted 22nd May 2014
Bo Liu,‡ Wei-Ping Wu,‡ Lei Hou* and Yao-Yu Wang
DOI: 10.1039/c4cc03049d www.rsc.org/chemcomm
Four uncommon isostructural nanocage-based 3D Ln-MOFs, 1-Ln (Ln = Eu, Tb, Gd and Dy), were constructed using a new diisophthalate ligand with active pyridyl sites. 1-Eu exhibits highly efficient luminescent sensing for Cu2+ ions and selective CO2 capture.
Metal–organic frameworks (MOFs) as fluorescent probes for pollutant detection, such as poisonous gases, organic solvents, and heavy metals, have recently received increasing attention because of the advantages of a fast response and high sensitivity to specific guests, and noninvasive operation.1 Accordingly, many MOFs display excellent luminescent sensing functions towards guests such as Cu2+, Fe3+, Al3+ and Zn2+ metal ions, and DMF, acetone and explosive organic nitro compounds.2 Compared to transition metals, lanthanide MOFs (Ln-MOFs) as fluorescent sensors have been rarely investigated, however, the intense luminescent signal and bright luminescent colours of Ln3+ ions, especially for Eu3+ and Tb3+, make Ln-MOFs the most promising sensor materials.3 Notably, these materials usually exhibit luminescent sensing for two or more metal ions simultaneously, and achieving a detectable signal only for a specific molecule or ion is a vital task. In addition, the cage-based porous MOFs and metal–organic polyhedra (MOPs) have recently attracted the increasing attention of chemists because of their fascinating structural topologies and unique applications for selective guest inclusions, gas storage, and as nanoscale reaction vessels.4 While numerous investigations focused on the transition metal cage-based systems, very few were performed on the cage-based Ln-MOFs due to the restrictions of flexible coordination geometries and high coordination numbers of Ln3+ ions.4b Thus it is
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Synthesis, crystallographic details, TGA, PXRD, sorption patterns, and additional figures. CCDC 998805 and 998812. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc03049d ‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2014
an urgent and appealing project to construct new cage-based Ln-MOFs and discover more excellent properties. An undeveloped 2,6-di(30 ,50 -dicarboxylphenyl)pyridine (H4L) linker is herein employed to build cage-based Ln-MOFs, based on the following advantages: (i) the isophthalate unit easily forms discrete cages with metal ions, so the two elongated isophthalates in H4L can link adjacent cages to generate high dimensional cage-based frameworks;5 (ii) lanthanide ions prefer binding carboxylate O atoms rather than the N atom of the central pyridine in H4L, so this N atom as a functional site is predicted to recognize small Lewis acidic molecules and metal ions. In this contribution we used H4L to construct four new isostructural Ln-MOFs, [H2N(Me)2][Ln3(L)2(HCOO)2(DMF)2(H2O)] (1-Ln) (Ln = Eu, Tb, Gd and Dy), showing unusual nanocage-based 3D frameworks with trinuclear Ln3 clusters. 1-Eu reveals excellent luminescent sensing for Cu2+ ions and selective CO2 capture over N2 and H2. The structure of 1-Eu is described representatively. 1-Eu crystallizes in the cubic space group Im3% and exhibits a cage-based 3D framework with a trinuclear [Eu3(HCOO)2(R-COO)8] cluster. The asymmetric unit contains two independent Eu3+ ions, two fullydeprotonated L4 linkers, two formate anions, one DMF ligand and one aqua ligand. The formate anions are derived from the decomposition of DMF molecules. Eu1 is nona-coordinated by one aqua ligand and eight O atoms of two formate groups and four carboxylate groups from four L4 linkers, displaying a monocapped square antiprism geometry (Fig. 1). Eu2 adopts a bicapped trigonal prism geometry defined by six O atoms of four carboxylate groups from four L4 linkers, one O atom of the formate group, and one DMF ligand. Two formate anions show bidentate and chelatebridging m3-Z2:Z2 coordination fashions, respectively. Two L4 linkers have the same mirror symmetries at the central pyridine N atoms, as well as similar coordination modes (Fig. S1†). One Eu1 and two Eu2 atoms are ligated by one formate anion and four carboxylate groups of four L4 linkers to form a trinuclear [Eu3(HCOO)2(R-COO)8] (Eu3) cluster with a mirror plane through Eu1 and the C24-carboxylate group. The most interesting feature is that twelve Eu3 clusters and twelve N1pyridine-containing L4 linkers interlink to form a nanocage (cage A) with a diameter of 2.4 nm (Fig. 2a and Fig. S2a†), which
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Fig. 1 Coordination environments of Eu3+ ions in 1-Eu. Symmetry codes: #1 x, 2 y, z; #2 0.5 z, 0.5 + x, y 0.5; #3 0.5 z, 1.5 x, y 0.5; #4 1 y, 1 + z, x; #5 1 y, 1 z, x.
represents one of the largest sizes in present metal–organic cages,4,6 featuring a tro-type truncated octahedron topology with face symbol 4668 (6 tetragonal and 8 hexagonal faces), differing from the commonly reported tetrahedral and octahedral cages.4 Excluding DMF and aqua ligands, each cage has eight hexagonal windows (4.4 4.1 Å2, excluding van der Waals radii of the atoms) ringed by three Eu3 clusters and three isophthalate units of three L4 linkers. Although the metal–organic cages or polyhedra were well investigated, the lanthanide-based cages have been documented in only two Tb-MOFs.7 The neighbouring cages in 1-Eu are joined together by pyridyl spacers of L4 into a 3D framework. Notably, in the framework twelve Eu3 clusters, eight N2pyridine-containing and four N1pyridine-containing L4 linkers interconnect to form another trotype nanocage (cage B) (Fig. 2b and Fig. S2b†), which has a relatively smaller diameter of 1.1 nm compared to cage A due to the inward direction of the central pyridines of the N1-containing L4 linkers. Cage B also has a hexagonal window (5.5 4.5 Å2), surrounded by three Eu3 clusters and three isophthalate units of three L4 linkers. The cages A and B arrange along the cubic cell axes; as a result, another octahedral nanocage (cage C) with a diameter of 1.1 nm is formed among them by six Eu3 clusters and twelve isophthalate units of twelve L4 linkers (Fig. 2c and Fig. S2c†). The cage C interlinks the cages A and B by window sharing to offer a 3D porous system (Fig. 2d), possessing 60.1% void space after excluding H2N(Me)2 cations, DMF and H2O ligands. The three cages indicate different
Fig. 2 Different structural views of 1-Ln: (a) cage A; (b) cage B; (c) cage C; (d) interlinked 3D pore; (e) three kinds of cages shown as a face-transitive tiling; (f) (3,6)-connected topology.
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face-transitive tilings, as shown in Fig. 2e. Topologically, each Eu3 cluster links six L4 units, and each L4 connects three clusters, so they are simplified as 6- and 3-connected nodes, respectively. Thus, the framework of 1-Eu is designated as a binodal (3,6)-connected net with point symbol (462)2(426786) (Fig. 2f), which is unprecedented in references despite 55 known (3,6)-connected nets in the RCSR database.8 1-Eu and 1-Tb display bright red and green luminescence under UV irradiation, respectively (Fig. S5†). The solid-state photoluminescence spectra show the strong emission peaks at 580, 592, 615 and 698 nm for 1-Eu, and at 489, 543, 582 and 621 nm for 1-Tb, corresponding to 5D0–7F0, 5D0–7F1, 5D0–7F2 and 5D0–7F4 f–f transitions of Eu3+ ions, and 5D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3 f–f transitions of Tb3+ ions, respectively (Fig. S6†). The decay lifetime curves reveal the double-exponential decays with the lifetimes of 0.29 and 0.46 ms for 1-Eu, and 0.34 and 1.41 ms for 1-Tb, respectively (Fig. S7†). The most significant structural feature of 1-Ln is the presence of free Lewis basic pyridyl sites on the cage surface, highlighting the potential for sensing functions and recognizing metal ions. The luminescence spectra of 1-Eu dispersed in DMF solution containing the same concentrations of M(NO3)x (M = Na+, K+, Mg2+, Ca2+, Zn2+, Cd2+, Mn2+, Co2+, Cu2+, Ni2+, In3+ and Tb3+) were studied (Fig. 3a and Fig. S8†). Interestingly, the Cu2+ ion shows a significant quenching effect on the luminescence intensity of 1-Eu; in contrast, other metal ions enhance luminescence intensity to a degree, indicating highly selective sensing of 1-Eu for Cu2+ ions. The enhancement of luminescence in 1-Eu by metal ions other than Cu2+ presumably results from the binding of metal ions in cages, which results in exclusion of other solvent molecules from the space close to metal ions, inhibiting nonradiative deactivation by solvents. This phenomenon in 1-Eu reveals two main differences from the most reported findings:1a,2 one is that, except Cu2+, all metal ions increase the luminescence intensity of 1-Eu, which commonly cause small or even negligible effects on the luminescence intensity of reported complexes; the other is that, resembling the Cu2+ situation, Mn2+, Ni2+ and Co2+ ions with different electron configurations also imposed a certain degree of luminescent quenching in previous reports, however, they do not produce any quenching effects in 1-Eu. This unique quenching effect in 1-Eu possibly results from the stronger affinity of pyridine N atoms in L4 toward Cu2+, as well as the smaller ionic radius of Cu2+, which renders Cu2+ ions more readily able to make contact with the pyridine N atom than Mn2+, Ni2+ and Co2+ ions and as a result, decreases the efficiency of energy transfer from L4 linkers to f–f transitions of Eu3+ centers.9 The sensitization of luminescent quenching of 1-Eu toward Cu2+ ions was examined by varying the concentration of Cu2+ ions. As illustrated in Fig. 3b, the luminescence intensity of 1-Eu is almost completely quenched at a Cu(NO3)2 concentration of 102 M. According to the Stern–Volmer equation: I0/I = 1 + Ksv[M],2a the quenching coefficient Ksv of 2350 40 M1 is obtained from the luminescent data (Table S1†), which is the highest value in documented MOFs for Cu2+ sensing in DMF.2a,9 The highly selective and sensitive sensing of 1-Eu for Cu2+ ions inspired us to further examine the influences of other metal ions on Cu2+ sensing functions. It is very striking that the luminescence intensity of 1-Eu shows no significant change after adding the different mixed metal ions Na+/Mg2+/Ca2+, Mg2+/Ca2+, Zn2+/Cd2+, Co2+/Ni2+, Zn2+/Cd2+/Ni2+, and Zn2+/Cd2+/Co2+
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Fig. 3 (a) Luminescence intensity at 615 nm of 1-Eu treated with different metal ions (102 M) in DMF solutions. (b) Luminescence spectra of 1-Eu in DMF solutions with Cu(NO3)2 at different concentrations, and color changes of 1-Eu in DMF after adding Cu2+ (102 M) under UV light. (c) Luminescence intensity at 615 nm of 1-Eu dispersed in DMF with addition of different mixed ions (101 M) (m1: blank; m2: Na+/Mg2+/Ca2+; m3: Mg2+/Ca2+; m4: Zn2+/Cd2+; m5: Co2+/Ni2+; m6: Zn2+/Cd2+/Ni2+; m7: Zn2+/Cd2+/Co2+) and Cu2+-incorporated systems (102 M). (d) Luminescence spectra of 1-Eu in DMF in the presence of different amounts of Cu2+ (inset: the plot of intensity versus Cu2+ concentration).
(each ion at a concentration of 101 M) (Fig. 3c and Fig. S10†), while the luminescence intensity is close to complete quenching once the Cu2+ ion (102 M) is incorporated. The Ksv of Cu2+ ions in mixed systems slightly exceeds the values of pure 1-Eu in DMF (Table S2†). To the best of our knowledge, this selective Cu2+ luminescence quenching phenomenon is observed for the first time in MOFs, highlighting the potential of 1-Eu for sensing Cu2+ ions. Furthermore, the titration experiments of Cu2+ ions in 1-Eu and the mixed ion systems Na+/Mg2+/Ca2+, Co2+/Ni2+ and Zn2+/Co2+/Ni2+ were also studied (Fig. 3d and Fig. S11†). The luminescence intensity at 615 nm versus the volume ratio of Cu2+ ions can be well fitted by a singleexponential function, establishing a diffusion-controlled process for the Cu2+ ion luminescent quenching behaviour. In addition, gas sorption measurements confirmed the permanent porosity of 1-Eu, which reveals no adsorption of N2 and H2 at 77 K, but selectively adsorbs CO2 at 195 K with an uptake of 166.3 cm3 g1 at 1 atm (Fig. 4a). To further evaluate sorption properties of 1-Eu, the isotherms of CO2 at 273 and 293 K were measured, showing uptakes of 51.7 and 33.2 cm3 g1, respectively.
Fig. 4 (a) Sorption isotherms of 1-Eu: N2 77 K, H2 77 K, and CO2 195 K (inset: 273 and 293 K). (b) Six cycles of CO2 uptake of 1-Eu at 293 K.
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At low and ambient temperature, all CO2 sorption isotherms display small hystereses, which possibly arise from the hindered escape of CO2 due to the relatively narrow windows of cages, as well as the interactions between the pyridine N atoms of L4 and CO2 molecules. The isosteric heat of adsorption (Qst) for 1-Eu was calculated using the adsorption data collected at 273 and 293 K (Fig. S14 and S15†). At zero coverage, the Qst is 23.1 kJ mol1, comparable with some MOFs containing acylamide or amine groups.10 Moreover, six cycles of CO2 sorption isotherms at 293 K show almost equal capacities (Fig. 4b), revealing sorption recurrence and complete regeneration of the material. Although the sorption capacity of CO2 in 1-Eu is not as high as those of other MOFs,11 the sorption selectivities and stability of 1-Eu suggests that the present solid can be potentially applied in gas separation processes. In summary, four unusual Ln-MOFs with unprecedented nanocages have been constructed. 1-Eu reveals high luminescent selectivity and sensitivity for Cu2+ ions as a result of weak contacts between pyridyl sites in L4 and Cu2+, as well as showing selective capture for CO2. The present results could provide a facile route to design and synthesize novel cage-based Ln-MOFs as promising functional materials. This work is supported by NSFC (21371142, 20931005, 91022004 and 21001088), and NSF of Shaanxi province (2013KJXX-26, 13JS114 and 2014JQ2049).
Notes and references 1 (a) Y. Cui, Y. Yue, G. Qian and B. Chen, Chem. Rev., 2012, 112, 1126; (b) L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105; (c) H.-L. Jiang, Y. Tatsu, Z.-H. Lu and Q. Xu, J. Am. Chem. Soc., 2010, 132, 5586; (d) J.-H. Wang, M. Li and D. Li, Chem. Sci., 2013, 4, 1793; (e) J. Tian, L. V. Saraf, B. Schwenzer, S. M. Taylor, E. K. Brechin, J. Liu, S. J. Dalgarno and P. K. Thallapally, J. Am. Chem. Soc., 2012, 134, 9581; ( f ) Z. Hu, B. J. Deibert and J. Li, Chem. Soc. Rev., 2014, DOI: 10.1039/c4cs00010b. 2 (a) B. Chen, L. Wang, Y. Xiao, F. R. Fronczek, M. Xue, Y. Cui and G. Qian, Angew. Chem., Int. Ed., 2009, 48, 500; (b) Y.-Q. Chen, G.-R. Li, Z. Chang, Y.-K. Qu, Y.-H. Zhang and X.-H. Bu, Chem. Sci., 2013, 4, 3678; (c) Z. Chen, Y. Sun, L. Zhang, D. Sun, F. Liu, Q. Meng, R. Wang and D. Sun, Chem. Commun., 2013, 49, 11557; (d) H. Yang, F. Wang, Y.-X. Tan, Y. Kang, T.-H. Li and J. Zhang, Chem. – Asian J., 2012, 7, 1069; (e) Q. Tang, S. Liu, Y. Liu, J. Miao, S. Li, L. Zhang, Z. Shi and Z. Zheng, Inorg. Chem., 2013, 52, 2799. 3 (a) Y. Li, S. Zhang and D. Song, Angew. Chem., Int. Ed., 2013, 52, 710; (b) B. Chen, Y. Yang, F. Zapata, G. Lin, G. Qian and E. B. Lobkovsky, Adv. Mater., 2007, 19, 1693. 4 (a) M. O’Keeffe and O. M. Yaghi, Chem. Rev., 2012, 112, 675; (b) J. J. Perry IV, J. A. Perman and M. J. Zaworotko, Chem. Soc. Rev., 2009, 38, 1400; (c) D. J. Tranchemontagne, Z. Ni, M. O’Keeffe and O. M. Yaghi, Angew. Chem., Int. Ed., 2008, 47, 5136; (d) J.-R. Li, D. J. Timmons and H.-C. Zhou, J. Am. Chem. Soc., 2009, 131, 6368. 5 Y. He, B. Li, M. O’Keeffe and B. Chen, Chem. Soc. Rev., 2014, DOI: 10.1039/c4cs00041b. ¨der, Acc. Chem. Res., 6 (a) Y. Yan, S. Yang, A. J. Blake and M. Schro 2014, 47, 296; (b) R. Yun, Z. Lu, Y. Pan, X. You and J. Bai, Angew. Chem., Int. Ed., 2013, 52, 1; (c) Q.-R. Fang, D.-Q. Yuan, J. Sculley, W.-G. Lu and H.-C. Zhou, Chem. Commun., 2012, 48, 254; (d) K. Koh, A. G. Wong-Foy and A. J. Matzger, J. Am. Chem. Soc., 2009, 131, 4184. 7 (a) Y. K. Park, S. B. Choi, H. Kim, K. Kim, B.-H. Won, K. Choi, J.-S. Choi, W.-S. Ahn, N. Won, S. Kim, D. H. Jung, S.-H. Choi, G.-H. Kim, S.-S. Cha, Y. H. Jhon, J. K. Yang and J. Kim, Angew. Chem., Int. Ed., 2007, 46, 8230; (b) Y. He, H. Furukawa, C. Wu, M. O’Keeffe and B. Chen, CrystEngComm, 2013, 15, 9328. 8 Please see the Web site of the O’Keeffe group at Arizona State University, http://rcsr.anu.edu.au. 9 (a) Y. Xiao, Y. Cui, Q. Zheng, S. Xiang, G. Qian and B. Chen, Chem. Commun., 2010, 46, 5503; (b) Z. Hao, X. Song, M. Zhu, X. Meng,
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11 (a) A. R. Millward and O. M. Yaghi, J. Am. Chem. Soc., 2005, 127, 17998; (b) S. R. Caskey, A. G. Wong-Foy and A. J. Matzger, J. Am. Chem. Soc., 2008, 130, 10870; (c) D.-D. Zhou, C.-T. He, P.-Q. Liao, W. Xue, W.-X. Zhang, H.-L. Zhou, J.-P. Zhang and X.-M. Chen, Chem. Commun., 2013, 49, 11728; (d) S. C. Xiang, Y. B. He, Z. J. Zhang, H. Wu, W. Zhou, R. Krishna and B. L. Chen, Nat. Commun., 2012, 3, 954.
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S. Zhao, S. Su, W. Yang, S. Song and H. Zhang, J. Mater. Chem. A, 2013, 1, 11043; (c) J.-M. Zhou, W. Shi, H.-M. Li, H. Li and P. Cheng, J. Phys. Chem. C, 2014, 118, 416. 10 (a) B. Zheng, J. Bai, J. Duan, L. Wojtas and M. J. Zaworotko, J. Am. Chem. Soc., 2011, 133, 748; (b) F. Wang, Y.-X. Tan, H. Yang, Y. Kang and J. Zhang, Chem. Commun., 2012, 48, 4842.
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Four uncommon nanocage-based Ln-MOFs: highly selective luminescent sensing for Cu2+ ions and selective CO2 capture†
Received 24th April 2014, Accepted 22nd May 2014
Bo Liu,‡ Wei-Ping Wu,‡ Lei Hou* and Yao-Yu Wang
DOI: 10.1039/c4cc03049d www.rsc.org/chemcomm
Four uncommon isostructural nanocage-based 3D Ln-MOFs, 1-Ln (Ln = Eu, Tb, Gd and Dy), were constructed using a new diisophthalate ligand with active pyridyl sites. 1-Eu exhibits highly efficient luminescent sensing for Cu2+ ions and selective CO2 capture.
Metal–organic frameworks (MOFs) as fluorescent probes for pollutant detection, such as poisonous gases, organic solvents, and heavy metals, have recently received increasing attention because of the advantages of a fast response and high sensitivity to specific guests, and noninvasive operation.1 Accordingly, many MOFs display excellent luminescent sensing functions towards guests such as Cu2+, Fe3+, Al3+ and Zn2+ metal ions, and DMF, acetone and explosive organic nitro compounds.2 Compared to transition metals, lanthanide MOFs (Ln-MOFs) as fluorescent sensors have been rarely investigated, however, the intense luminescent signal and bright luminescent colours of Ln3+ ions, especially for Eu3+ and Tb3+, make Ln-MOFs the most promising sensor materials.3 Notably, these materials usually exhibit luminescent sensing for two or more metal ions simultaneously, and achieving a detectable signal only for a specific molecule or ion is a vital task. In addition, the cage-based porous MOFs and metal–organic polyhedra (MOPs) have recently attracted the increasing attention of chemists because of their fascinating structural topologies and unique applications for selective guest inclusions, gas storage, and as nanoscale reaction vessels.4 While numerous investigations focused on the transition metal cage-based systems, very few were performed on the cage-based Ln-MOFs due to the restrictions of flexible coordination geometries and high coordination numbers of Ln3+ ions.4b Thus it is
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Synthesis, crystallographic details, TGA, PXRD, sorption patterns, and additional figures. CCDC 998805 and 998812. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc03049d ‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2014
an urgent and appealing project to construct new cage-based Ln-MOFs and discover more excellent properties. An undeveloped 2,6-di(30 ,50 -dicarboxylphenyl)pyridine (H4L) linker is herein employed to build cage-based Ln-MOFs, based on the following advantages: (i) the isophthalate unit easily forms discrete cages with metal ions, so the two elongated isophthalates in H4L can link adjacent cages to generate high dimensional cage-based frameworks;5 (ii) lanthanide ions prefer binding carboxylate O atoms rather than the N atom of the central pyridine in H4L, so this N atom as a functional site is predicted to recognize small Lewis acidic molecules and metal ions. In this contribution we used H4L to construct four new isostructural Ln-MOFs, [H2N(Me)2][Ln3(L)2(HCOO)2(DMF)2(H2O)] (1-Ln) (Ln = Eu, Tb, Gd and Dy), showing unusual nanocage-based 3D frameworks with trinuclear Ln3 clusters. 1-Eu reveals excellent luminescent sensing for Cu2+ ions and selective CO2 capture over N2 and H2. The structure of 1-Eu is described representatively. 1-Eu crystallizes in the cubic space group Im3% and exhibits a cage-based 3D framework with a trinuclear [Eu3(HCOO)2(R-COO)8] cluster. The asymmetric unit contains two independent Eu3+ ions, two fullydeprotonated L4 linkers, two formate anions, one DMF ligand and one aqua ligand. The formate anions are derived from the decomposition of DMF molecules. Eu1 is nona-coordinated by one aqua ligand and eight O atoms of two formate groups and four carboxylate groups from four L4 linkers, displaying a monocapped square antiprism geometry (Fig. 1). Eu2 adopts a bicapped trigonal prism geometry defined by six O atoms of four carboxylate groups from four L4 linkers, one O atom of the formate group, and one DMF ligand. Two formate anions show bidentate and chelatebridging m3-Z2:Z2 coordination fashions, respectively. Two L4 linkers have the same mirror symmetries at the central pyridine N atoms, as well as similar coordination modes (Fig. S1†). One Eu1 and two Eu2 atoms are ligated by one formate anion and four carboxylate groups of four L4 linkers to form a trinuclear [Eu3(HCOO)2(R-COO)8] (Eu3) cluster with a mirror plane through Eu1 and the C24-carboxylate group. The most interesting feature is that twelve Eu3 clusters and twelve N1pyridine-containing L4 linkers interlink to form a nanocage (cage A) with a diameter of 2.4 nm (Fig. 2a and Fig. S2a†), which
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Fig. 1 Coordination environments of Eu3+ ions in 1-Eu. Symmetry codes: #1 x, 2 y, z; #2 0.5 z, 0.5 + x, y 0.5; #3 0.5 z, 1.5 x, y 0.5; #4 1 y, 1 + z, x; #5 1 y, 1 z, x.
represents one of the largest sizes in present metal–organic cages,4,6 featuring a tro-type truncated octahedron topology with face symbol 4668 (6 tetragonal and 8 hexagonal faces), differing from the commonly reported tetrahedral and octahedral cages.4 Excluding DMF and aqua ligands, each cage has eight hexagonal windows (4.4 4.1 Å2, excluding van der Waals radii of the atoms) ringed by three Eu3 clusters and three isophthalate units of three L4 linkers. Although the metal–organic cages or polyhedra were well investigated, the lanthanide-based cages have been documented in only two Tb-MOFs.7 The neighbouring cages in 1-Eu are joined together by pyridyl spacers of L4 into a 3D framework. Notably, in the framework twelve Eu3 clusters, eight N2pyridine-containing and four N1pyridine-containing L4 linkers interconnect to form another trotype nanocage (cage B) (Fig. 2b and Fig. S2b†), which has a relatively smaller diameter of 1.1 nm compared to cage A due to the inward direction of the central pyridines of the N1-containing L4 linkers. Cage B also has a hexagonal window (5.5 4.5 Å2), surrounded by three Eu3 clusters and three isophthalate units of three L4 linkers. The cages A and B arrange along the cubic cell axes; as a result, another octahedral nanocage (cage C) with a diameter of 1.1 nm is formed among them by six Eu3 clusters and twelve isophthalate units of twelve L4 linkers (Fig. 2c and Fig. S2c†). The cage C interlinks the cages A and B by window sharing to offer a 3D porous system (Fig. 2d), possessing 60.1% void space after excluding H2N(Me)2 cations, DMF and H2O ligands. The three cages indicate different
Fig. 2 Different structural views of 1-Ln: (a) cage A; (b) cage B; (c) cage C; (d) interlinked 3D pore; (e) three kinds of cages shown as a face-transitive tiling; (f) (3,6)-connected topology.
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face-transitive tilings, as shown in Fig. 2e. Topologically, each Eu3 cluster links six L4 units, and each L4 connects three clusters, so they are simplified as 6- and 3-connected nodes, respectively. Thus, the framework of 1-Eu is designated as a binodal (3,6)-connected net with point symbol (462)2(426786) (Fig. 2f), which is unprecedented in references despite 55 known (3,6)-connected nets in the RCSR database.8 1-Eu and 1-Tb display bright red and green luminescence under UV irradiation, respectively (Fig. S5†). The solid-state photoluminescence spectra show the strong emission peaks at 580, 592, 615 and 698 nm for 1-Eu, and at 489, 543, 582 and 621 nm for 1-Tb, corresponding to 5D0–7F0, 5D0–7F1, 5D0–7F2 and 5D0–7F4 f–f transitions of Eu3+ ions, and 5D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3 f–f transitions of Tb3+ ions, respectively (Fig. S6†). The decay lifetime curves reveal the double-exponential decays with the lifetimes of 0.29 and 0.46 ms for 1-Eu, and 0.34 and 1.41 ms for 1-Tb, respectively (Fig. S7†). The most significant structural feature of 1-Ln is the presence of free Lewis basic pyridyl sites on the cage surface, highlighting the potential for sensing functions and recognizing metal ions. The luminescence spectra of 1-Eu dispersed in DMF solution containing the same concentrations of M(NO3)x (M = Na+, K+, Mg2+, Ca2+, Zn2+, Cd2+, Mn2+, Co2+, Cu2+, Ni2+, In3+ and Tb3+) were studied (Fig. 3a and Fig. S8†). Interestingly, the Cu2+ ion shows a significant quenching effect on the luminescence intensity of 1-Eu; in contrast, other metal ions enhance luminescence intensity to a degree, indicating highly selective sensing of 1-Eu for Cu2+ ions. The enhancement of luminescence in 1-Eu by metal ions other than Cu2+ presumably results from the binding of metal ions in cages, which results in exclusion of other solvent molecules from the space close to metal ions, inhibiting nonradiative deactivation by solvents. This phenomenon in 1-Eu reveals two main differences from the most reported findings:1a,2 one is that, except Cu2+, all metal ions increase the luminescence intensity of 1-Eu, which commonly cause small or even negligible effects on the luminescence intensity of reported complexes; the other is that, resembling the Cu2+ situation, Mn2+, Ni2+ and Co2+ ions with different electron configurations also imposed a certain degree of luminescent quenching in previous reports, however, they do not produce any quenching effects in 1-Eu. This unique quenching effect in 1-Eu possibly results from the stronger affinity of pyridine N atoms in L4 toward Cu2+, as well as the smaller ionic radius of Cu2+, which renders Cu2+ ions more readily able to make contact with the pyridine N atom than Mn2+, Ni2+ and Co2+ ions and as a result, decreases the efficiency of energy transfer from L4 linkers to f–f transitions of Eu3+ centers.9 The sensitization of luminescent quenching of 1-Eu toward Cu2+ ions was examined by varying the concentration of Cu2+ ions. As illustrated in Fig. 3b, the luminescence intensity of 1-Eu is almost completely quenched at a Cu(NO3)2 concentration of 102 M. According to the Stern–Volmer equation: I0/I = 1 + Ksv[M],2a the quenching coefficient Ksv of 2350 40 M1 is obtained from the luminescent data (Table S1†), which is the highest value in documented MOFs for Cu2+ sensing in DMF.2a,9 The highly selective and sensitive sensing of 1-Eu for Cu2+ ions inspired us to further examine the influences of other metal ions on Cu2+ sensing functions. It is very striking that the luminescence intensity of 1-Eu shows no significant change after adding the different mixed metal ions Na+/Mg2+/Ca2+, Mg2+/Ca2+, Zn2+/Cd2+, Co2+/Ni2+, Zn2+/Cd2+/Ni2+, and Zn2+/Cd2+/Co2+
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Fig. 3 (a) Luminescence intensity at 615 nm of 1-Eu treated with different metal ions (102 M) in DMF solutions. (b) Luminescence spectra of 1-Eu in DMF solutions with Cu(NO3)2 at different concentrations, and color changes of 1-Eu in DMF after adding Cu2+ (102 M) under UV light. (c) Luminescence intensity at 615 nm of 1-Eu dispersed in DMF with addition of different mixed ions (101 M) (m1: blank; m2: Na+/Mg2+/Ca2+; m3: Mg2+/Ca2+; m4: Zn2+/Cd2+; m5: Co2+/Ni2+; m6: Zn2+/Cd2+/Ni2+; m7: Zn2+/Cd2+/Co2+) and Cu2+-incorporated systems (102 M). (d) Luminescence spectra of 1-Eu in DMF in the presence of different amounts of Cu2+ (inset: the plot of intensity versus Cu2+ concentration).
(each ion at a concentration of 101 M) (Fig. 3c and Fig. S10†), while the luminescence intensity is close to complete quenching once the Cu2+ ion (102 M) is incorporated. The Ksv of Cu2+ ions in mixed systems slightly exceeds the values of pure 1-Eu in DMF (Table S2†). To the best of our knowledge, this selective Cu2+ luminescence quenching phenomenon is observed for the first time in MOFs, highlighting the potential of 1-Eu for sensing Cu2+ ions. Furthermore, the titration experiments of Cu2+ ions in 1-Eu and the mixed ion systems Na+/Mg2+/Ca2+, Co2+/Ni2+ and Zn2+/Co2+/Ni2+ were also studied (Fig. 3d and Fig. S11†). The luminescence intensity at 615 nm versus the volume ratio of Cu2+ ions can be well fitted by a singleexponential function, establishing a diffusion-controlled process for the Cu2+ ion luminescent quenching behaviour. In addition, gas sorption measurements confirmed the permanent porosity of 1-Eu, which reveals no adsorption of N2 and H2 at 77 K, but selectively adsorbs CO2 at 195 K with an uptake of 166.3 cm3 g1 at 1 atm (Fig. 4a). To further evaluate sorption properties of 1-Eu, the isotherms of CO2 at 273 and 293 K were measured, showing uptakes of 51.7 and 33.2 cm3 g1, respectively.
Fig. 4 (a) Sorption isotherms of 1-Eu: N2 77 K, H2 77 K, and CO2 195 K (inset: 273 and 293 K). (b) Six cycles of CO2 uptake of 1-Eu at 293 K.
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At low and ambient temperature, all CO2 sorption isotherms display small hystereses, which possibly arise from the hindered escape of CO2 due to the relatively narrow windows of cages, as well as the interactions between the pyridine N atoms of L4 and CO2 molecules. The isosteric heat of adsorption (Qst) for 1-Eu was calculated using the adsorption data collected at 273 and 293 K (Fig. S14 and S15†). At zero coverage, the Qst is 23.1 kJ mol1, comparable with some MOFs containing acylamide or amine groups.10 Moreover, six cycles of CO2 sorption isotherms at 293 K show almost equal capacities (Fig. 4b), revealing sorption recurrence and complete regeneration of the material. Although the sorption capacity of CO2 in 1-Eu is not as high as those of other MOFs,11 the sorption selectivities and stability of 1-Eu suggests that the present solid can be potentially applied in gas separation processes. In summary, four unusual Ln-MOFs with unprecedented nanocages have been constructed. 1-Eu reveals high luminescent selectivity and sensitivity for Cu2+ ions as a result of weak contacts between pyridyl sites in L4 and Cu2+, as well as showing selective capture for CO2. The present results could provide a facile route to design and synthesize novel cage-based Ln-MOFs as promising functional materials. This work is supported by NSFC (21371142, 20931005, 91022004 and 21001088), and NSF of Shaanxi province (2013KJXX-26, 13JS114 and 2014JQ2049).
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8734 | Chem. Commun., 2014, 50, 8731--8734
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