A Water-Stable Metal–Organic Framework with a Double-Helical

Communication pubs.acs.org/IC

A Water-Stable Metal−Organic Framework with a Double-Helical Structure for Fluorescent Sensing Xiao-Jing Liu,† Ying-Hui Zhang,† Ze Chang,† Ai-Lin Li,† Dan Tian,‡ Zhao-Quan Yao,† Yan-Yuan Jia,‡ and Xian-He Bu*,†,‡ †

School of Materials Science and Engineering, TKL of Metal- and Molecule-Based Material Chemistry and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300350, China ‡ Department of Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

stabilize the framework. However, the enhanced stability of MOFs raised by interpenetration often comes at the expense of reduced porosity. On the other hand, multihelical structures experience dense stacking of the subunit similar to that in interpenetrations and thus could also benefit the stabilization of MOFs. The helical structure as a highly fascinating and ubiquitous structure in nature has been observed in MOFs with attractive functions.11 More importantly, MOFs with helical structure usually show retained porosity and thus are available as promising functional materials, such as biological detection, adsorption and separation, and asymmetric catalysis.12 Based on aforementioned considerations, our attention is to introduce the helical structure into fluorescent MOFs, aiming at the development of highly stable MOF-based fluorescent sensors and exploit their practical application in an aqueous environment. In this work, we report a new Cd-based MOF, namely [Cd2(tib)2(bda)2]·(solvent)n (1) [tib = 1,3,5-tris(1-imidazolyl) benzene and H2bda = 2,2′-biphenyl dicarboxylic acid], which is characterized for its fascinating double helical structure formed through the dense stacking of bda2− and tib ligands surrounding Cd ions. Benefiting from the stabilization effect of double helical structure, 1 reveals improved solvent resistance, especially toward aqueous solution. Moreover, 1 exhibits a selective quenching response to ketones, and this attractive property makes 1 an ideal candidate for the detection of acetone in aqueous solution. The crystallographic analysis reveals that complex 1 crystallizes in the tetragonal I41 space group. The asymmetric unit of 1 consisted two crystallographically independent Cd2+ ions, two bda2− ligands, and two tib ligands (Figure S1). A fascinating double-helical structure is formed through the intertwinement of two couples of helical chains based on bda2− and tib ligands, respectively, by sharing bridging Cd ions (Figure S2a, b). For the bda2−-based couple, the Cd1-based helix is R-helical while the Cd2-based one is L-helical (Figure S2c, d). Meanwhile, an additional linkage between two neighboring Cd2+ ions by the tib ligand produces the second couple of helical chains (Figure S2e,f), in which the Cd1-based helix is L-helical while the Cd2-based one is R-helical. Notably, the two helixes of each couple are not strictly mirror symmetric,

ABSTRACT: Water instability is a crucial limiting factor in the practical application of most fluorescent metal− organic frameworks (MOFs). Here, by introducing a fascinating double-helical structure generated through dense stacking of organic ligands, a water-stable fluorescence MOF has been synthesized, namely, [Cd2(tib)2(bda)2]·(solvent)n (1) [tib =1,3,5-tris(1-imidazolyl) benzene; H2bda = 2,2′-biphenyl dicarboxylic acid]. This helical structure helps to enhance the stability of 1 against common organic solvents and water, even acid/ base aqueous solutions with a pH value ranging from 3 to 11. Furthermore, this material can be a potential fluorescent sensor for ketones.

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n recent years, metal−organic frameworks (MOFs) have received much attention in broad fields,1 due to their tailorable structure and intrinsic functions, realized through a rational choice of metal nodes and organic ligands.2 For example, the collocation of chromophoric organic ligands and metal ions has produced diverse fluorescent MOFs with emission wavelengths covering a wide region, which have been utilized as fluorescent sensors3 for the detection of volatile organic compounds, ionic species, and energetic materials.4 The MOF-based sensors exhibit some notable advantages such as high sensitivity, short response time, and operability.5 Nevertheless, the application of most MOF-based fluorescent sensors is significantly limited due to the sensors’ unsatisfying solvent resistance, especially against an aqueous environment.6 Even now, improving water stability is still a major challenge for the application of MOF-based sensors.7 Relevant analysis reveals that the displacement of organic ligand by water molecule as well as hydrolysis account for the degradation of MOFs in water.8 Farrusseng and co-workers have illustrated the relationship between the stability of MOFs and the metal−ligand bond,9 and the vital effect of steric structure on the stability of MOFs is also emphasized. That is, not only rational design of the electronic interaction between ligand and metal ions but also logical construction of crystal structure is essential to the fabrication of solvent-resistant MOFs. As proposed in many reports, the introduction of framework interpenetration is one efficient way to enhance the stability of MOFs,10 owing to the additional weak interactions derived from close entanglement of organic ligand that could © XXXX American Chemical Society

Received: April 14, 2016

A

DOI: 10.1021/acs.inorgchem.6b00935 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 1. Helical chain framework assembled by coordination interaction of Cd2+ ions with tib (a) and bda2− (b) ligands, respectively. (c) The double-helical structure formed through dense stacking of two helical chains framework. (d) The topology of the 3D framework.

emission intensities of the suspensions depend greatly on the solvents used, and particularly, were significantly quenched under conditions with ketones as solvents (Figure 2).

and every helix is surrounded by four helixes of different rotation direction (Figure 1a,b). Furthermore, the R-helix of one couple intertwines with the L-helix of another couple by sharing the metal ions, generating two kinds of double-helical structure extending along the c axis (Figure 1c). The doublehelical structures are linked with each other through the coordination interaction between the third imidazole ring of tib and one Cd2+ ion from nearby double-helices, forming a threedimensional (3D) framework. The framework possess channels running along the a and b directions (accessible size 6.0 × 2.4 Å2, considering the van der Waals radius of atoms; Figure S3). After removal of uncoordinated solvent molecules from the pore space of 1, the accessible volume is about 32% predicted by the SOLV function of PLATON.13 Considering the tib ligands and Cd2+ metal ions as three and five coordinated nodes, respectively, the 3D framework of 1 can be simplified into a network with 3,5-connected topology (Schläfli symbol of {3·6·7}{32·62·73·82·9}; Figure 1d). The formation of a double-helical structure depends greatly on the coordination characters of the ligands. The H2bda ligand shows twisted configuration to form the first helical chain, and the imidazole ligand form the second helical chain by adopting the obtuse angle arrangement. The imidazole and carboxylate moieties arranged densely around the metal centers, which effectively protects the central metal ions from the interference of the solvent molecule, and therefore, this feature is advantageous to the solvent resistance of 1. To confirm the phase purity and stability of complex 1 toward different solvents, PXRD (Figure S4) measurements were conducted for a sample of 1 after 12 h of immersion in different solvents, including common organic solvents, pure water, hydrochloric acid (pH = 3), and sodium hydroxide (pH = 11) aqueous solutions. All the PXRD patterns of treated 1 are in good agreement with that of as-prepared 1, indicating the high stability of the framework even under acid and base conditions, which is essential to its practical application. Recently, the investigation of MOFs-based chemosensors is one hotspot because of their intriguing photophysical properties.14 The solid 1 has strong absorbance less than 310 nm (Figure S5a) and a strong emission at about 325 nm (λex = 275 nm; Figure S5b). Considering its remarkable stability, we further investigate the luminescent properties of 1 in water and common organic solvents, including N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), ethanol, methanol, npropanol, tetrahydrofuran (THF), acetonitrile, dioxane, dichloromethane (CH2Cl2), acetone, methyl isobutyl ketone, and cyclohexanone. Spectra measurement indicates that the

Figure 2. The quenching efficiencies of emission (325 nm) of 1 along with gradual addition of ketones, λex = 275 nm. The inset represents fluorescent emission of 1 dispersed in different solvents.

Ketone is one kind of important chemical raw material which has been widely used in industrial production. However, their toxicity is a substantial threat to human health.15 To evaluate the fluorescent sensing behavior of 1 toward ketones, we investigated the fluorescent variation of 1 upon the addition of acetone, isobutyl ketone, and cyclohexanone in ethanol (Figure S6). The fluorescent variations of 1 in ethanol were collected in Figure 2, and a great decreasing of emission intensity of the suspension was observed upon the addition of ketones. When the concentration of ketones was increased to 0.7 vol %, the emission intensity of suspension was quenched by >80%. The detection limits (Figure S7) of the complex 1 to acetone, methyl isobutyl ketone, and cyclohexanone are about 0.055, 0.057, and 0.052 vol‰, respectively. This result manifests the high-efficiency sensing of 1 to ketones in ethanol. The detection of ketones in aqueous solution is much more meaningful than in ethanol in the viewpoint of application. Considering acetone is soluble in water, fluorescent spectra variation of acetone in water was observed. After increasing acetone content to 0.7 vol %, the fluorescence emission of suspension was quenched in an almost completely similar manner to that in ethanol (Figure S8). Meanwhile, the B

DOI: 10.1021/acs.inorgchem.6b00935 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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interference of other organic solvents was also exploited in aqueous solution. No obvious change of the quenching efficiency was observed upon the addition of 3.5 vol % (5fold amount of acetone content) of other organic solvents (Figure S9), which indicates that the detection sensitivity of 1 to ketones is hardly disturbed by other organic solvents with distinct functional groups. Additionally, the regeneration ability of luminescent sensors is very important. After being centrifuged, rinsed with water three times, and dried in the air, the fluorescence response of 1 is highly reversible in five sensing-recovery cycles (Figure S10). All the results mentioned above make 1 a promising fluorescent probe for the detection of acetone in aqueous solution. To clarify the sensing mechanism of 1 toward ketones, the UV−vis spectra of organic solvents were further investigated. The ketones has absorption between 250 and 330 nm, and other organic solvents exhibit absorbance only in a region less than 260 nm (Figure S11). So there should be a competition in the absorption of excitation energy between the ketones and 1. The emission of complex 1 suspension is around 325 nm, which also overlaps the absorption band of ketones. Such spectra overlap might results in energy transfer (ET) between the MOF and ketones and quench the emission of 1.16 Therefore, the selective fluorescence quenching of 1 under ketone conditions might be attributed to the combination of competitive absorption and an ET mechanism. In summary, a novel cadmium MOF (1) based on a doublehelical structure has been synthesized and investigated with its fluorescent property. This double helical structure may effectively protect central metal ions from the interference of solvent molecules and thus provide high resistance of 1 against common organic solvents and aqueous solutions with a pH value ranging from 3 to 11. Moreover, 1 exhibits a selectively fluorescent quenching response to ketones and thus is a potential fluorescent sensor for detecting acetone in aqueous solution.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00935. Experimental section, supporting tables and figures, and additional characterizations (PDF) Crystallographic detail (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-22-23502809. Fax: +86-22-23502458. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program of China (2014CB845600), the NSF of China (21531005, 21290171, and 21421001), and the MOE Innovation Team of China (IRT13022).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00935 Inorg. Chem. XXXX, XXX, XXX−XXX