Luminescent Response of One Anionic Metal–Organic Framework

Jan 14, 2014 - Synopsis. A novel anionic metal–organic framework based on octa-nuclear zinc-cluster had been synthesized utilizing the flexible hexa...
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Luminescent Response of One Anionic Metal−Organic Framework Based on Novel Octa-nuclear Zinc Cluster to Exchanged Cations Bao Li,*,† Xi Chen,† Fan Yu,§ Wenjing Yu,† Tianle Zhang,*,† and Di Sun‡ †

School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China ‡ School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, P. R. China § Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, Jianghan University, Wuhan 430056, P. R. China S Supporting Information *

ABSTRACT: Utilizing the flexible hexa-carboxylate ligand derived from cyclotriphosphazene, a novel anionic metal− organic framework based on an octa-nuclear zinc cluster has been synthesized, showing the variable luminescent properties controlled by different guest cations.

he current interest in the field of MOFs has been focused on charged MOF materials, including anionic or cationic ones, because of their potential applications in molecular selection, ion exchange, gas sorption, and catalysis.1−3 Under the direction of crystal engineering, the topological structure of charged MOFs can be controlled to some degree by the careful selection of metal ions with specific coordination preference, spacer ligands with different geometrical characters, and the reaction conditions, etc.4 To a certain extent, utilizing the multicarboxylate ligands reacted with transitional metal ions is very facile to assemble the charged architectures under one-pot synthesis procession.5 Until now, great progress in tunable applications with dynamic structures had been achieved on cationic framework, but the anionic ones were rarely reported.3,6 In addition, luminescent response to guest counterions in cationic/anionic framework attracted less attention compared to the corresponding investigation in guest-molecular separation and other applications.7 Recently, cluster-based MOFs have also been of the current interest since the discovery of MOF-5.8 The versatile metalclusters [named as second-building-units (SBUs)] could replace the single metal-ion nodes and serve as the highly connected ones, and then reduce the interpenetration and increase the pore size and thermal stability.9 Despite such superiorities of cluster-based MOFs, to rationally synthesize MOFs with expected cluster nodes is still unconsummated.10 Among the various SBUs, metal-carboxylate clusters are of particular importance due to the flexible coordination modes and tailorable features of carboxylate ligands, in which zinccarboxylate ones including neutral dinuclear-zinc paddle-wheel and octahedral Zn4O(COO)6 ones are widely reported.11

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© 2014 American Chemical Society

However, higher-nuclear neutral zinc-based SBUs, especially charged ones, are very rare.12 To the best of our knowledge, the highest nuclear of zinc-SBUs in MOFs was the cuboid {Zn8O} cluster reported recently.13 With all these aforementioned considerations in mind, to synthesize novel charged cluster-based MOFs, hexakis(4formylphenoxy) cyclotriphosphazene (H6L1, Scheme S1 of the Supporting Information) was utilized to react with the zinc ion based on several reasons: (1) To assemble the functional materials, Zn(II) ions were selected because of the interesting luminescent properties originated from the connection of Zn(II) ions and carboxylate ligands; (2) investigating the possibilities of constructing novel zinc-based SBUs due to the facile tendency of zinc ion to assemble the clusters and variable configurations of flexible H6L;14 (3) the flexible derivated ligand tends to result in soft framework, which would be sensitive to the chemical environment referred to the exchanged ions of different shape, size, or coordinating nature, and subsequently lead to structural variations and tunable luminescent behaviors. Herein, we report the synthesis and structural characterization of one novel anionic-MOF, {[Zn11O(C42H24O18P3N3)4 (H2O)6]·xGuest}n (1), assembled from the connection of peculiar octa-nuclear zinc clusters and L1 linkers, as well as the variable luminescent properties of cation-exchanged samples. Crystalline sample of 1 was obtained from the solvothermal reaction in acidic DMF solvent containing the Zn(NO3)2·6H2O Received: November 24, 2013 Revised: January 7, 2014 Published: January 14, 2014 410

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negative, which causes the negative framework and needs the positive ions to balance the whole charge of the structure. In the crystal structure of 1, two types of L1 ligands adopt the slightly distorted tetrahedral configuration and connect four different metal nodes. The first type of L1 ligands connects two octa-nuclear SBUs and two monozinc nodes, while the other one links three mononuclear nodes and one octa-nuclear SBU (Figure 2). In this way, the whole charged 3D framework has

and H6L1. The intensity of the common X-ray diffraction equipment in our laboratory was too weak to give the satisfied data, except for the approximate large cell parameters. Therefore, the comparably qualified single-crystal analysis of 1 was finally obtained by the utilization of Synchrotron Radiation Facility at 173 K after repeated attempts.15 Crystal data and selected parameters of 1 were gathered in Table S1 of the Supporting Information. Compound 1 crystallizes in the trigonal space group R3c̅ , and the asymmetric unit contains three complete (Zn1, Zn3, and Zn5) and two one-third occupied (Zn2 and Zn4) zinc ions, one complete and one one-third-occupied hexa-carboxylate ligands, one one-third-occupied oxygen ion, and two coordinated aqua molecules (Figure S1 of the Supporting Information). All of the zinc ions are the tetra-coordinated mode, and the coordination spheres could be seen as slightly distorted tetrahedron, except for the octahedral coordination environment of the Zn(4) atom. Zn(1) and Zn(3) atoms with three syn−syn μ2b-bridging carboxyls of different L1 ligands form the noncentrosymmetric [Zn2(COO)3] unit with the Zn···Zn distance of 3.368 Å, whose axial vertices are occupied by one monodentate carboxyl of another L1 ligand and one four-connected oxygen ion (O25). In contrast, the similar dinuclear unit with the Zn···Zn distance of 3.669 Å is constructed by Zn(2) and Zn(4) atoms that the axial vertices are occupied by O25 and three-coordinated aqua molecules on Zn(4) atom. Furthermore, the central oxygen ion O25 locates on the C3-axial position and connects four dinuclear units to assemble the peculiar octanuclear-zinc cluster-based SBU that displays the slightly distorted tetrahedral configuration (Figure 1). In terms of the tetrahedron of the

Figure 2. Perspective view of the four-connection mode of the two kinds of hexa-carboxylate ligands.

been assembled by the connection of different zinc nodes and distinct L1 linkers. Very large pores are left in the structure, especially the 1D rhombic channel reserved along the a axis, which are occupied by solvent molecules and counter cations that could not be characterized due to the very weak diffraction intensity of 1 (Figure 3). Calculated using the PLATON

Figure 1. Perspective view of the (a) structure and (b) sevenconnection mode of octa-nuclear zinc-based SBU. Asymmetric code: A, −x + y − 3, −x + 2, z; B, −y + 2, x − y − 1, z; C, x − y + 1/3, x − 4/3, −z + 1/3; D, y + 1/3, −x + y + 4/3, −z + 1/3. Numbering of oxygen atoms in carboxylates was omitted excepted for the coordination water molecules O26, O26C, and O26D.

Figure 3. View of the 3D structure along the b axial direction (left) and the simplified ball-and-stick model (right) of complex 1 along the a axis: the red, green, and blue balls represent the 7-connected octanuclear SBUs, 3-connected mononuclear nodes, and 4-connected hexa-carboxylate ligands.

octa-nuclear SBU, three dinuclear vertices in one plane are interlinked by three benzoate substituents belonging to one L1 ligand located on the same side (Figure 1b). In addition, three vertices of Zn(1)−Zn(3) SBUs were capped by one monodentate carboxylate ligand of other L1 ligands. As such, each octa-nuclear SBU is further extended by seven distinct ligands in the two-connection mode. Besides the octa-nuclear zinc-nodes, one independent monozinc node with three monodentate carboxyls of different ligands and one coordinated aqua molecule is also presented in the crystallographical structure (Figure S2 of the Supporting Information). All of the Zn−O distances range from 1.848(8) to 2.159(10) Å, corresponding to the normal ZnII−O values. Furthermore, it must be noted that the charge of the two kinds of SBUs is

routine, the solvent accessible volume in the dehydrated structure of 1 is about 73.1%. With consideration of the monozinc ions, octa-nuclear SBUs, and two types of ligands as three-, seven-, and four-connecting nodes, respectively, the overall structure of 1 topologically possesses a novel 4-nodal 3,4,4,7-connected net with the point (Schläfli) symbol {42· 6}3{43·62·8}3{43·63}{46·66·86·103} calculated with TOPOS (Figure 3b).16 The thermal behavior of 1 was examined by thermogravimetric analyses (TGA) (Figure S3 of the Supporting Information). The TGA curve of 1 shows that it undergoes rapid loss of solvent or countercation below 200 °C. Then the stability of the dehydrated sample could stay up to 400 °C. The 411

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Ca2+ could be detected compared to the original zinc-MOF, showing the broad emission from 360 to 440 nm along with one peak value of 397 nm (λex = 330 nm) (Figure S5 of the Supporting Information). Similar emission peaks were also given by the Zn-MOF@Na and Zn-MOF@K samples. The variable luminescent properties of the original and cation-exchanged Zn-MOFs might be ascribed to the modifiable coordination environment caused by the different interactions originated from importing cations into the zinc framework, especially the small-for-size Li+ and Mg2+ ions compared to the other alkali or alkali-earth metal ions with weak interactions to the Zn-MOF. The immersed Li+ and Mg2+ ions possibly tend to bind onto the original framework via the remnant coordination sites as oxygen atoms of carboxyls on L1 ligands or coordinated water molecules, thus resulting in the variable stable framework, exhibited the changeable luminescent properties (Figure S6 of the Supporting Information).17 The Na+, K+, and other cations could also be exchanged into Zn-MOF, but the emission spectra of these samples do not change significantly because these cations could interact weakly with the Zn-MOF skeleton. In conclusion, an unprecedented octa-nuclear zinc-based cluster acted as the new SBU in MOFs has been presented. The anionic-framework of 1 exhibits the fantastic topological structure and modifiable luminescent properties along with the different cation-exchange treatments. The modifiable luminescent-response properties allow 1 possibly to act as the luminescent sensor to specific cations. The subsequent work will be focused on probing novel anionic framework and consequent properties of a series of zeolite-like MOFs constructed by the other metallic nodes/SBUs and distinct hexa-carboxylate ligands derived from cyclotriphosphazene.

much more stable framework of 1 might be ascribed to the high connection mode between the metallic nodes and L1 ligands. In addition, the photoluminescences of H6L1 and 1 in the solid state were also investigated at room temperature. A strongest emission peak at 339 nm (λex = 290 nm) was exhibited for the free ligand, which could presumably be attributed to the π−π* transitions, shown in Figure S4 of the Supporting Information. The emission spectrum of 1 shows the broad peaks located in the range from 340 to 440 nm along with two peak values of 374 and 392 nm (λex = 340 nm), which may be caused by σdonation from the coordination environments of the Zn(II) centers and could be assigned as ligand-to-metal charge-transfer (LMCT) (Figure 4). With the consideration that photo-

Figure 4. The luminescent spectra of 1, Zn-MOF@Li and Zn-MOF@ Eu. The zinc-MOF doped with Eu3+ ions exhibits the distinct emission spectra, showing two parts of luminescent properties: the broad peak in the range from 360 to 450 nm with three peak values of 370, 382, and 402 nm assigned to the LMCT of zinc-MOF skeleton and the other part from 580 to 710 nm shows the typical f−f transition of immersed Eu(III) ion upon excitation at 340 nm as 5D0 → 7F1 transition at 592 nm, 5D0 → 7F2 transition at 612 nm, 5D0 → 7F3 transition at 652 nm, and 5D0 → 7F4 transition at 702 nm.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental process, crystal data, TGA, XRF, and XRD results. X-ray crystallographic file in cif format for 1. This material is available free of charge via the Internet at http:// pubs.acs.org.

luminescent behavior of MOFs was closely associated with the metal ions and the ligands coordinated to metal centers, the near two emission peaks are in consistent with the two different coordination environments of metallic nodes in 1. Because 1 has the anionic open-framework along with the obvious channels occupied by organic cations, 1 was performed to carry out the cation-exchange experiment to investigate the modified properties based on the matrix of the anionic open zinc-MOF. Small-for-size cations as Li+ and Mg2+ with strong binding interactions were selected first. The emission spectrum of zinc-MOF doped with Li+ ions exhibited the broad emission from 345 to 440 nm but along with one relatively sharp peak at 370 nm and one gentle peak emerged in the previous one (λex = 320 nm) (Figure 4). Strikingly, the distinct emission peak of the zinc-MOF after cation-exchange with Mg2+ was given. The peak configuration incorporated two peaks and was similar to Zn-MOF@Li but with the different range from 460 to 580 nm and peak value of 500 nm (Figure S5 of the Supporting Information). Then mass cation Eu3+ with typical peaks was also applied to cation exchange. The luminescent spectra of the resulting sample clearly showed two parts of luminescent peaks, originated from the parent skeleton and immersed Eu3+ ion, which clearly manifested that Eu3+ ions could be exchanged into the zinc-MOF (Figure 4). At last, some cations with weak binding abilities as Na+, K+, Ca2+, and Ba2+ ions were selected. No obvious change of emission peak of the sample immersed in

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the National Natural Science Foundation of China for financial support (Grants 21101066 and 20871049), and the Analytical and Testing Center, Huazhong University of Science and Technology for spectral measurements.

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REFERENCES

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