Chinese Chemical Letters  2019, Vol. 30 Issue (2): 499-501   PDF    
A zinc(Ⅱ) MOF based on secondary building units of infinite wavy-shaped chain exhibiting obvious luminescent sense effects
Kai Lia, Kunhuan Heb, Quanwen Lib, Bin Xiab, Qinglun Wangb,*, Yinghui Zhanga,*     
a School of Materials Science and Engineering, TKL of Metal and Molecule-Based Material Chemistry, Nankai University, Tianjin 300350, China;
b College of Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China
Abstract: A new metal-organic framework (MOF), {[Zn7(BPS)4(OH)6(H2O)2]·5H2O]n} (1), (H2BPS = 4, 4'-bibenzoic acid-2, 2'-sulfone), based on a wavy and infinite chain-shaped secondary building units, has been synthesized under solvothermal conditions and characterized by single crystal X-ray diffraction and further confirmed by PXRD, TGA and IR spectrum. The solid-state emission spectra reveal that compound 1 presents strong luminescence emission bands at room temperature. The fluorescent properties of compound 1 in diverse organic solvents indicated that 1 has palpable luminescent sense effects for DMF and DMAC.
Keywords: Metal-organic frameworks     Secondary building unit     Solid-state emission spectra     Fluorescent properties     Luminescent sense effects    

The design and construction of metal-organic frameworks (MOFs) have stimulated tremendous attention of chemists for their intriguing structures and novel properties with wide potential applications in many useful areas, such as catalysis, gas storage/ separation, drug delivery, magnetic and luminescent materials [1-6]. Thus, the field of MOFs has undergone a blowout development during the past decade. To achieve controllable preparation desired MOFs with targeted structures and properties, much effort has been devoted to judicious selection or design organic linkers with specific functional groups (such as: -CH3, -Br, -NH2 and -NO2), secondary building units (SBUs) and metal center [7-9]. However, controllable preparation of MOFs is still a challenge in many cases because of the crystallization process related with many subtle factors, including metal-ligand ratio, pH, solvent, temperature, the counterion and so on [10, 11].

It is remarkable that SBUs has been identified as an effective tool in the construction of MOFs. There is a major class of MOFs in which the metal-containing secondary building units (SBUs) are infinite in one dimension [12, 13]. Such SBUs generally can be viewed as rods or chains. Until now, a large number type of rodbased MOFs with a series of interesting structures and topology have been constructed and characterized [14-16], such as MOFs with flat ribbon-, sinusoidal ribbon-, solid columnar- and aperiodic helical-shaped SBUs. Rod-based MOFs have attracted considerable attention since they can further enhance the stability of MOFs, which is great significant because of the high thermal stability is especially required for MOFs materials in practical application, and their resistance to interpenetration and potential for isoreticular expansion [17, 18]. Thereby, rod-based MOFs commonly have permanent porosity and rigid architectures and shows excellent potential for gas storage, luminescent sense, separations and catalysis [19].

Multi-carboxylates bridging ligands were often used to construct rod-based coordination polymers [20]. In this article, as an extension of investigation on rod-based MOFs, we have used bi-carboxylates organic linker H2BPS (H2BPS = 4, 4'-bibenzoic acid-2, 2'-sulfone), which has been widely employed for MOF construction because its nonlinear modes of coordination [21-24], to successfully synthesize a new MOF, namely, {[Zn7(BPS)4 (OH)6(H2O)2]·5H2O]n} (1).

X-ray crystal structural analyses reveal that 1 crystallizes in the triclinic system with P-1 space group. Its asymmetric unit contains three and a half crystallography independent zinc(Ⅱ) atoms, three hydroxy ions, two BPS2- ligand anions, one coordinated H2O molecule and two point five lattice H2O molecules, as shown in Fig. S1a in Supporting information. The Zn1 atom (occupancy is 50%) is coordinated with four carboxylic oxygen atoms and two hydroxyl oxygen atoms (Zn-O distances range from 2.059 Å to 2.173 Å) that resides in a six-coordinated octahedral coordination geometry. The Zn2 is completed by two carboxylic oxygen atoms from BPS2- and three hydroxy oxygen atoms (Zn-O distances range from 2.018 Å to 2.169 Å) that belong to triangulate dipyramidal configuration. The Zn3 is surrounded by two hydroxyl oxygen atoms, one μ3-OH- and one μ2-OH- with Zn-O distances ranging from 1.926 Å to 2.015 Å, which belongs to tetrahedral configuration. Zn4 exhibits triangulate dipyramidal configuration and is coordinated with two carboxylic oxygen atoms from two different BPS2-, one coordinated H2O molecule, one μ2-OH- and one μ3- OH- [Zn-O distances range from 2.017 Å to 2.121 Å] (Fig. S1b in Supporting information). Furthermore, Zn(Ⅱ) ions are associated together by μ3-OH- and μ2-OH-, forming a wavy and infinite chain-shaped secondary building units (SBUs). In order to better observe the framework, we extract heptanuclear Zn7 as an independent units, as shown green square in Fig. 1a. The Zn7 units are parallel arrayed to generate an infinite wavy-shaped chain. The Zn7 units are connected with two similar units by four pairs of 'double-bridge', forming a 3D metal-organic framework with 1D regular parallelogram channels along a axis (Figs. 1b and c). Similar rod-shaped or cluster secondary building units can be found in the literatures [25, 26].

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Fig. 1. Structure of compound 1: (a) infinite wavy-shaped chain metal-carboxylate building unit, green square represent Zn7 units; (b) projection view of the framework along a axis; green and purple balls represent 1D channels. (c) projection view of the framework along c axis (light gray: C; red: O; yellow: S; turquoise: Zn).

To confirm the phase purity of compound 1, powder XRD patterns were carried out. As shown in Fig. S2 in Supporting information, all major peaks of the experimental PXRD patterns for compound 1 match quite well with those of simulated PXRDs, indicating its excellent crystalline phase purity. TGA performed on the as-synthesized samples revealed that compound 1 has high thermal stability (Fig. S3 in Supporting information). The TGA curve of 1 shows a gradual weight-loss step of 6.25% (70–350 ℃), corresponding to escape of all lattice H2O and coordinated H2O molecules in the pores (calcd. 6.27%), followed by a sharp weight loss when exceed 400 ℃, which may be attributed to the decomposition of the coordinated BPS2- ligands before the framework decomposes completely giving a 50.9% residue, which belongs to the formation of ZnO (calcd. 51.15%).

The photoluminescence properties of 1 and free ligands were investigated in the solid state at room temperature. As depicted in Fig. 2, it can be observed that 1 and free ligands all present intense emission bands upon excitation at 300 nm. For free ligands, the maximum emission peaks is at 423 nm, which is attributable to the π*-π or π*-n charge transitions [27, 28]. For 1, the maximum is at 435 nm. In comparison with the emission peaks of free ligands, the maximum emission wavelength of compound 1 exhibits a slight red-shift from 423 nm to 435 nm, we believe that these procedures are generally neither ligand-to-ligand charge transfer (LLCT) nor metal-to-ligand charge transfer (MLCT) [29], because the Zn(Ⅱ) of d10 configuration is relative difficult to oxidize or to reduce. Therefore, the emission of 1 may be ascribed to intraligand fluorescent emission (π*-π or π*-n charge transitions) [30].

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Fig. 2. Solid-state emission spectra of H2BPS and 1.

To examine the effect of different solvent molecules on the luminescent properties of compound 1, crystal samples of 1 have been heated 10 h at 453 K under vacuum to remove guest H2O molecules. Then, crystal samples of 1 were soaked into diverse solvents of MeOH, EtOH, DMF, DMAC (dimethylacetamide), MeCN and acetone for two days before measuring luminescent properties. As shown in Fig. 3, the intensities and maximum emission wavelength of 1 strongly depend on the organic solvents. It is noteworthy that the maximum emission wavelength of 1 presents clearly red-shift for EtOH and acetone. The luminescent intensities of 1 exhibit obvious increase for MeCN. On the contrary, 1 shows the most critical quenching effect to DMF and DMAC, which suggests 1 has palpable luminescent sense effects for DMF and DMAC. The rest of solvents have relatively weak effects on the emission of 1. The luminescent change mechanism may be ascribed to the different solvent molecules in channels of 1, which may significant influence the intraligand electron transfer process. Especially the electron-withdrawing property of amide group in DMF and DMAC, which is likely to finally result in obvious quenching effects [31, 32].

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Fig. 3. Emission spectra (a) and intensities (b) for 1 in different organic solvents.

In summary, a new Zn(Ⅱ) metal-organic frameworks (MOFs) has been successfully obtained. X-ray crystal structural analyses reveal that 1 is based on SBUs of infinite zinc oxide chain. These 1D zigzag chain-shaped SBUs are interconnected through the carboxylic groups of BPS2- to generate a 3D metal-organic framework with 1D parallelogram channels. The maximum emission wavelength of 1 exhibits a relatively obvious red-shift upon excitation at 300 nm compared with free ligand, which may be ascribed to intraligand emission states. The fluorescent properties of 1 in diverse solvents indicate that 1 has palpable quenching effect to DMF and DMAC, which suggest 1 has potential application in the sensing and recognition of DMF and DMAC.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21771111, 21601092 and 21371104) and the Tianjin Natural Science Foundation (Nos. 15JCZDJC38800 and 16JCZDJC36900).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2018.05.001.

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