Multifunctional 3-Fold Interpenetrated Porous Metal–Organic

Communication pubs.acs.org/crystal

Multifunctional 3-Fold Interpenetrated Porous Metal−Organic Frameworks Composed of Unprecedented Self-Catenated Networks Feng Luo,*,† Zi-Zun Yuan,† Xue-Feng Feng,† Stuart R. Batten,‡ Jian-Qiang Li,† Ming-Biao Luo,† Shu-Juan Liu,† Wen-Yuan Xu,† Gong-Ming Sun,† Yu-Mei Song,† Hai-Xiao Huang,† and Xian-Zhao Tian† †

College of Biology, Chemistry and Material Science, East China Institute of Technology, Fuzhou, Jiangxi, China School of Chemistry, Monash University, Victoria 3800, Australia

‡

S Supporting Information *

ABSTRACT: A 3-fold interpenetrating porous metal−organic framework, namely, Zn(bdc)(L)·solvents (1, L = N4,N4′di(pyridin-4-yl)biphenyl-4,4′-dicarboxamide, H2bdc = terephthalic acid), composed of self-catenated nets with unprecedented hxg-d-4-Fddd topology is reported. Upon removal of guest molecules, the crystal lattice is unaffected, and the resulting permanent porosity can be applied for gas chromatography separation of various organic molecules. Furthermore, remarkable tunable photoluminescence and direct white emission dependent on excitation wavelength, as well as visible light photocatalytic activity, are also observed, creating a new multifunctional material antetype. the first time to generate a metal−organic compound, acts as a linear linker with a bridging length of ca. 2.3 nm (see Supporting Information, Figure S1). As shown in Figure 1, along the a direction, an interweaving double-helical structure is observed, where L ligands bridge Zn(II) ions together to create one-dimensional (1D) helical chains with a repeat distance of ca. 34.4 Å. Bdc2− ligands then bridge these interweaving double-helices together, resulting in a three-dimensional (3D) net. Topological analysis via the TOPOS program gives a four-connecting 658 hxg-d-4-Fddd topology.4 In the literature, there are already 13 nets with point symbol (65.8) including cds, usf, dmp, mok, ict, uni, unl, unm, uns, 4/6/t7, sqc2077, 4/6/h14, 4/6/h6, and the present one should present a new 65.8 topology matrix.5 Note that as described in Figure 1, each six-membered ring, composed of six Zn(II) ions, two bdc2−, and four L ligands, interpenetrates with two identical six-membered rings, indicating self-penetration. Furthermore, in 1 three such hxg-d-4-Fddd nets interpenetrate each other, resulting overall in a structure showing 3-fold interpenetrating (class Ia) composed of self-catenated hxg-d-4Fddd nets (Figure S2, Supporting Information). The overall stability of the crystal is supported by such H-bonds, and it follows that the network that hold the structure is not the 3-fold interpenetrated 4-connected net, but a single net that arise considering the H-bonding, giving a single binodal (3, 6)-connected new net with (4·6·8)2(42·6·76·85·9) point symbol4 (Figure S3, Supporting Information), The potential

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he exploitation of the functions of metal−organic frameworks (MOFs) is currently a topic of great interest. Reports focused on, but not limited to, luminescence and porosity are increasing in number significantly.1 For the next generation, from the viewpoint of energy-saving and sustainable development, the design and synthesis of multifunctional MOF materials are expected. In the pursuit of such materials, the incorporation of magnetism, luminescence, or optoelectronic effects into porous materials has been developed.2 In addition, several exceptional functions such as tunable luminescence, direct white light emission, gas-chromatographic (GC) separation, or photocatalytic activity have been developed recently.3 The interest in this theme stems from their broad applications in daily living, manufacturing, industry, and so on. From the viewpoint of multifunctionality, combining these properties into a single MOF may lead to novel classes of materials; this represents a very interesting but large challenge. To this end, our group has successfully prepared a new 3-fold interpenetrating MOF, namely, Zn(bdc)(L)·solvents (1), which displays such multifunctionality  tunable luminescence, direct white light emission, GC separation for many kinds of organic molecules, as well as visible light photocatalytic activity. Polymer 1 is synthesized solvo(hydro)thermally from the selfassembly of Zn(NO3)2, H2bdc, and the acylamide ligand L. The included solvent in 1 is estimated from EA and thermogravimetric (TG) analysis to be one DMF and 2.5 H2O per Zn atom. The phase purity of the bulk sample was confirmed by EA and X-ray diffraction (XRD) analysis. The asymmetric unit of 1 contains one crystallographyindependent tetrahedral Zn(II) ion that is coordinated by two bdc2− oxygens and two L nitrogens. The bdc2− ligand shows the bis-monodentate coordination mode and the L ligand, used for © 2012 American Chemical Society

Received: April 30, 2012 Revised: May 31, 2012 Published: June 7, 2012 3392

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Figure 1. (a) View of the 3D net of 1 composed of double-helices linked by 1D Zn-bdc2− chains, (b) a single self-catenated hxg-d-4-Fddd net (2-c nodes represent the bent ligand, so each 6-ring shows 12 sides), and (c) the catenation of three six-membered rings marked in red, green, and blue (here the 6-sided rings show crossings that do not exist in the real structure because of the bending of the long ligand).

solvent-accessible volume is ca. 5452.8 Å3, equal to 35.6% of the cell volume.6 The thermostability of 1 was estimated by TG and XRD analysis (Figures S4 and S5). The loss of guest molecules is between ca. 30−290 °C (calc. 15.9%, exp. 16.0%). The guest-free net can then be maintained until 350 °C. After this temperature, chemical decomposition is observed. Furthermore, the XRD pattern from the crystal samples calcined at 350 °C for 24 h is consistent with the XRD pattern from single crystal data, implying permanent porosity of 1 and that the framework integrity remains even after the removal of guest molecules. This is unusual as the removal of guest molecules from interpenetrating structures usually causes the gliding or shrinkage of frameworks, resulting in a decrease or even loss of porosity.7 The exceptional high thermostability for the interpenetrating nets here is likely due to the extensive N−H···O hydrogen bonding between the interpenetrating nets. In recent years, porosity has often been associated with gas absorption and related to applications such as CO2 and H2 storage.8 Very recently, in light of the regular pore shape and functionalized pore walls in MOFs, these materials have been applied to high-resolution gas or liquid chromatographic separation, and several remarkable achievements in separation of n-alkanes, substituted aromatics, persistent organic pollutants, as well as chiral compounds have been reported.9 Nevertheless, reports of GC separation of other organic molecules such as alcohols, ethers, or esters have been noticeably absent. Herein, in view of the regular pore shape and functionalized pore walls provided by the acylamide unit, we carried out the application of

1 to GC separation for organic molecules, especially for alcohols, ethers, and esters. A 1-coated capillary column was fabricated via a dynamic coating method.9 It was first examined for separation of xylene isomers and alkanes that are important raw chemicals in industry and petroleum refining. As shown in Figure 2, this MOF column can clearly separate xylene isomers and alkanes within 3 min without the need for temperature programming. Our MOF column also affords high resolution and selectivity for the separation of alcohols, ketones, esters, ethers, aldehydes, olefin, and halohydrocarbon. Their elution sequence is consistent with their boiling points. All these organic molecules cover a broad range of boiling points and are baseline separated without temperature-programmed control. Unexpectedly, the retention time of chloroform (bp. 61.2°) is longer than that for 1,1,1-trichlroethane (bp. 74°), possibly due to the pore-filling effect and guest−host interactions.9 The present MOF column also shows excellent selectivity for positional isomers of alcohols. The separation factors (R) defined by the ratio of adjusted retention time (R = t2′/t1′) are 1.32 for propyl alcohol/isopropanol, 1.21 for butyl alcohol/ isobutyl alcohol, and 1.21 for 1-pentanol/isoamyl alcohol, clearly showing that the alcohol isomers can be well resolved. As observed in other MOF columns, the selective GC separation of these organic molecules on the 1-coated column is due mainly to their differing van der Waals interactions with the microporous walls. In 1 long and narrow, elliptical pore shapes, with a length of ca. 10.5 Å and largest width of ca. 4.5 Å, are observed. Thus, the retention of these organic molecules on the column depends mostly on the length of the linear part of the 3393

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Figure 2. GC chromatograms on the 1-coated capillary column (15 m long ×0.02 mm i.d.): (a) separation of xylene isomers at 120 °C under a N2 linear velocity of 55.65 mL/min, (b) separation of alkanes at 90 °C under a N2 linear velocity of 17.5 mL/min, (c) separation of alcohols at 180 °C under a N2 linear velocity of 40 mL/min, (d) separation of ketones at 140 °C under a N2 linear velocity of 17.5 mL/min, (e) separation of esters at 90 °C under a N2 linear velocity of 40 mL/min, (f) separation of ethers at 180 °C under a N2 linear velocity of 55.65 mL/min, (g) separation of aldehydes at 180 °C under a N2 linear velocity of 55.65 mL/min, (h) separation of olefin at 130 °C under a N2 linear velocity of 40 mL/min, (i) separation of halohydrocarbon at 120 °C under a N2 linear velocity of 55.65 mL/min, (j) separation of propyl alcohol/isopropanol at 120 °C under a N2 linear velocity of 55.65 mL/min, (k) separation of butyl alcohol/isobutyl alcohol at 120 °C under a N2 linear velocity of 55.65 mL/min, (l) separation of 1-pentanol/isoamyl alcohol at 120 °C under a N2 linear velocity of 55.65 mL/min. S = thermal conductivity detector response.

Figure 3. The emission spectrum of 1 under different excitation wavelengths (left), and the CIE chromaticity coordinates of the emissions spectrum excited at 320−410 nm (right).

elute faster than its linear isomers. Other factors such as the match between organic molecules and pore channels, dispersion, dipole− dipole, and hydrogen bond forces should also be taken into account.9 The solid state photoluminescence properties of 1 were studied. Figure 3 depicts the exceptional character of tunable

organic molecules, for example, (i) the GC separation of alcohols where shorter linear chains have weaker van der Waals interactions and elute faster than longer linear chains, (ii) the GC separation of isomers of alcohols where shorter linear chains of one carbon atom have weaker van der Waals interactions and 3394

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photoluminescence dependent on excitation wavelength and direct white emission. If excited at 320 nm, a strong emission bond is located at 385 nm, belonging to blue emission, whereas when excited at 410 nm, yellow emission around 550 nm is observed. By contrast, employing an excitation wavelength of 380 nm will produce equal blue and yellow emission, resulting overall in direct white emission. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of the emissions spectrum excited at 320−410 nm also suggest that the photoluminescence of 1 is tunable from yellow to white and then to blue by means of the variation of excitation light. The acylamide ligand shows photoluminescence at 404 nm, confirming that the emission around 380 nm is derived from the π−π* transition of organic ligands (Figure S6, Supporting Information). The 550 nm emission may be due to metal-toligand charge transfer (MLCT).10 Moreover, the UV−vis absorption spectra at room temperature in the solid shows two broad absorption bands at 200−400 nm, 400−550 nm, where it is obvious that the absorption in the visible region is weaker that that in the UV region, which is consistent with the photoluminescence intensity of blue and yellow emission, and the tunable photoluminescence properties (Figure S7, Supporting Information). To our best of knowledge, the instances of MOFs featuring white light emission is still highly rare, and similar tunable photoluminescence dependent on excitation wavelength and direct white light emission was only observed in two cases,11 but for Ag(I) two-dimensional (2D) coordination polymer and In(III) 3D coordination polymer rather than the Zn(II) 3D porous MOF reported here. Photocatalytic degradation of a commonly used dye, rhodamine B (RhB), was investigated. The photocatalytic process was traced from the variation of the color in the reaction system by monitoring the characteristic absorbance at 554 nm. Control experiments without catalyst, visible light simulated by xenon lamp light, and both were also carried out. With 1 acting as a catalyst under a xenon lamp light the color of the RhB solution completely disappeared, with a degradation rate of up to 90% after 780 min. Furthermore, 1 also acted as a catalyst in ambient light, with the depigmentation of the RhB solution clearly observed, with a degradation rate of up to 50% after 770 min. The results imply absorption or daylight photocatalytic activity. The absorption of RhB solution is confirmed by the color change of 1 (from colorless to lilac) after it is immerged in RhB solution (Figure 4). In order to quantify the photocatalytic reactions, the kinetics can be interpreted by Langmuir−Hinshelwood (L−H) kinetics defined as 1/r0 = 5(1/k0C0) + (K0/k0), where r0, C0, k0, and K0 present the initial rate, the initial concentration of RhB solution, the kinetic rate constant, and the equivalent adsorption equilibrium coefficient, respectively. For 1, the parameters for the photocatalytic degradation of RhB, k0 and K0, are 0.0034 min−1 and 0.0058 ppm−1, whereas for 1 in ambient light, the corresponding parameters are 0.00064 min−1 and 0.0161 ppm−1 (Figure S8, Supporting Information). The low value of K0 indicates negligible adsorption; however, for 1 in ambient light the value suggests considerable adsorption but dominating photocatalytic degradation. To test the photocatalytic reaction mechanism of 1, 2 mL of tertiary butyl alcohol was added in RhB solution, and no degradation could be observed, suggesting a free radical photocatalytic reaction mechanism.12 Furthermore, for 1 the photocatalytic activity was also investigated under UV; however, under this condition the photocatalytic degradation of RhB solution is not observed, implying no photocatalytic activity

Figure 4. The photocatalytic degradation of RhB solution for 1. The black and red curves present 0.006 g of 1 under xenonlamp light and ambient light, respectively. The concentration of rhodmine B solution is 5.0625 mg/L.

of 1 under UV. Thereby, it is evident that for the two types of electronic states in 1, the π−π* transition of organic ligands in the UV region has no photocatalytic activity, while the MLCT effect in the visible light region probably is responsible for the photocatalytic activity. In summary, we present here a rare porous 3-fold interpenetrating net composed of self-catenated net and high thermostability. The extensive typical N−H···O hydrogen bonding between the acylamide and bdc2− ligands is likely responsible for the high thermostability observed in the interpenetrating structure, even after the removal of guest molecules. Most importantly, the results presented here attest to this type of material having significant potential in the GC separation of organic molecules, tunable luminescence, direct white light emission, and visible light photocatalytic activity.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

The synthesis, structure, additional figures, and cif files are available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Fax: +86-794-8258320. Tel: +86-794-8258320. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the topology analysis from the reviewers. This work was supported by the Doctoral Start-up Fund of East China Institute of Technology, the Natural Science Foundation of Jiangxi Province of China (No. 2010GQH0005), the China Postdoctoral Science Foundation (No. 20100480725), and the Foundation of Key Laboratory of Radioactive Geology and Exploration Technology Fundamental Science for National Defense (2010RGET07).

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REFERENCES

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