COMMUNICATION DOI: 10.1002/asia.201402785

A Multifunctional 3D Chiral Porous Ferroelectric Metal–Organic Framework for Sensing Small Organic Molecules and Dye Uptake Li-Hui Cao,[a] Yong-Li Wei,[a] Can Ji,[a] Ming-Li Ma,[a] Shuang-Quan Zang,*[a] and Thomas C. W. Mak[a, b]

Abstract: A flexible aromatic multicarboxylate ligand and CdII ions assemble into a chiral multihelical porous metal– organic framework with second-order nonlinear optical and ferroelectric properties. The obtained guest-free form highly selectively senses small organic molecules and adsorbs large dye molecules.

trolled through integrating two or more properties into a single substance.[10] Recently, several investigators have also begun exploring the potential of MOFs as optical material.[11] The tunable porous structures and properties of MOFs provide an important advantage for chemosensory and adsorptive materials.[12] Designability and high crystallinity of MOFs provide a unique opportunity to regulate the properties and analyze the structure–activity relationships. The organic linkers often contain conjugated p moieties that can give rise to optical emission upon irradiation. Hence, to incorporate both ferroelectric and optical properties into MOFs, we employ a flexible aromatic multicarboxylate ligand [H5L = 3,5-bis(1-methoxy-3,5-benzene dicarboxylic acid) benzoic acid], which has a variety of flexible coordination forms to introduce helical segments into the framework. Herein, we present the synthesis, structure and properties of a 3D chiral porous metal–organic framework, {[Cd5(L)2 ACHTUNGRE(H2O)4ACHTUNGRE(DMAc)3]·10H2O·7DMAc}n (1) (Cd-MOF, DMAc = N,N’-dimethylacetamide), with a decorated 4,5,6-c net topology based on two different cadmium secondary building units (SBUs) and the flexible ligand H5L. The obtained CdMOF exhibits ferroelectric and second-order nonlinear optical properties, and at the same time, its potential for sensing small molecules and dye adsorption are studied. X-ray crystallography reveals that 1 crystallizes in the chiral space group P21. The fundamental building unit of 1 contains one binuclear Cd secondary building unit (SBU), one trinuclear Cd SBU, and two independent L5 ligands named model I and model II, respectively (Figures S1–S3, Supporting Information). The flexibility of the L5 ligand allows for multiple conformations. In model I, the L5 ligand takes an anti-conformation with five carboxylate groups coordinating to ten CdII cations, while in model II, the L5 ligand adopts a syn-conformation and connects to nine cadmium cations. Complex 1 possesses a 3D chiral framework based on two different layers (A and B), which were constructed by the binuclear and trinuclear Cd SBUs, respectively (Figure 1 and Figure S2). The two sheets give an ABAB packing style and are connected together by the L5 ligand. As shown in Figure 2, taking the Cd5 ion not into account, layer A is composed of the Cd4 ion and two different isophthalic acid groups from two different L5 ligands and exhibits a (4,4) lattice, which is common in the structure of the reported co-

The rational design and synthesis of metal–organic frameworks (MOFs) are of great interest due to their potential applications in gas storage and separation, heterogeneous catalysis, ferroelectric, nonlinear optics and sensors.[1, 2] Among the reported MOFs, helical structures have been attracting attention owing to their fascinating architectures[3] and potential applications in many fields.[4] Helical structural motifs of these polymers are related to the geometry and the number of coordination sites provided by organic ligands.[5] With the aim of understanding the structural behaviors of the natural helical biopolymers and developing potential applications in enantioselective catalysis and separation, chemists have synthesized a large number of artificial helical coordination polymers with single- or multiplestranded helical structures.[6] Although plenty of helical structures have been reported, multihelices coexisting in one MOF are rare until now.[7] Functional materials, especially crystallized materials, with second-order nonlinear-optical (NLO) or ferroelectric properties are currently of immense interest for their promising applications in a variety of new technologies,[8, 9] such as electric devices, optical communication, signal processing, information storage, and nonlinear optical devices. At the same time, multifunctional materials are attractive for technological applications because their properties can be con[a] L.-H. Cao, Dr. Y.-L. Wei, C. Ji, M.-L. Ma, Prof. S.-Q. Zang, Prof. T. C. W. Mak College of Chemistry and Molecular Engineering Zhengzhou University Zhengzhou, China Fax: (+ 86)-371-6778-0136 E-mail: [email protected] [b] Prof. T. C. W. Mak Department of Chemistry and Center of Novel Functional Molecules The Chinese University of Hong Kong Shatin, New Territories, Hong Kong SAR (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402785.

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Figure 1. X-ray crystal structure of 1 viewed along the a-axis, exhibiting two different layers (A and B).

Figure 3. Double left-handed helices in layer B; views of the inner helix and outer helix, respectively.

portion of the L5 ligand, these two kinds of helical chains can be distinguished as inner and outer helices. The inner helix is made up of isophthalic acid from model I and two Cd ions (Cd1 and Cd2), while the outer helix employs the 1and 5-carboxylate groups in model II that are connected through Cd2 and Cd3 ions. Viewed along the b-axis, we can see clearly that the inner helix is contained by the outer helix. It should be noted that although layer B, which was constructed by trinuclear Cd units, is mesomeric, the neighboring layer A built from binuclear Cd units has different helices in different directions, resulting in the layer A to be chiral; the complicated framework is a 3D chiral network in general. It is noteworthy that the connection of layers in 1 also results in the formation of two types of cavities, which were filled with disordered solvent molecules. The total solvent-accessible volume calculated using PLATON14 is 62.9 % of the unit cell volume, and the moderate pore sizes indicate that a variety of small organic molecules can be accessible to such two different cavities in 1. Complex 1 crystallizes in polar space groups P21, belonging to 10 polar point groups (1, m, 2, mm2, 3, 3m, 4, 4mm, 6, and 6mm),[15] which are associated with a second-harmonic generation (SHG) response and ferroelectric behavior. Our preliminary measurements on a powdered sample of 1 suggest that, at ambient temperature, 1 is approximately 0.5 times as SHG-active as urea, which is about 200 times higher than that of a-quartz and compares favorably with the values of reported frameworks.[16] To detect ferroelectricity, the electrical hysteresis loop of 1 was recorded at

Figure 2. The homochiral layer A constructed by binuclear Cd-SBUs (Cd5 was omitted for clarity).

ordination polymers13 based on isophthalic acid and its derivatives. Additional investigations of this structure indicate that three different types of helices coexist in layer A. Along the a-axis, one Cd4 ion and two isophthalic acids from two independent ligands connect together to form a left-handed helical chain. At the same time, the Cd4 ion links two isophthalic acid molecules to generate two different types of right-handed helical chains along the b-axis (Figure S4, Supporting Information). As far as we know, example of three different types of helical chains coexisting in the same layer has not been reported until now. All the helical chains in layer A bear the same handedness, leading to the homochiral layer. A very unique feature of this framework is that the alternate left-handed (L) and right-handed (R) unequal double-helical chains (helix-in-helix) are further interconnected through a trinuclear Cd unit to generate the 2D mesomeric layer B (Figure S5, Supporting Information). As shown in Figure S6 in the Supporting Information, the left-handed (L) and right-handed (R) helices are enantiomers, so only the structure of left-handed helix will be discussed herein. Figure 3 clearly shows that each double-helical chain is constructed from two types of coaxial helical chains interlinked through Cd ions. On the basis of the length of the

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compared with 1 at 472 nm, possibly due to the disappearance of the interactions between the framework and solvent molecules. A quenching efficiency of 71.1 % for 1 was estimated using the formula (I I0)/I  100 %, where I and I0 are the maximum fluorescence intensity of 1 and 1 a, respectively. The luminescence decay curves of 1 and 1 a were also obtained at room temperature (Figure S13, Supporting Information). The corresponding lifetime (t) for 1 is about t1 = 0.035 (99.74 %) ms and t2 = 0.790 (0.26 %) ms, and that for 1 a is about t1 = 0.060 (98.70 %) ms and t2 = 1.225 (1.30 %) ms. The UV/Vis spectra of ligand H5L and complexes 1 and 1 a are presented in Figure S14 in the Supporting Information. The spectra have a distinct low-energy absorption band at 220–360 nm, which can be attributed to the intraACHTUNGREliACHTUNGREgand p–p* transitions corresponding to the fluorescence emission. As mentioned above, the two different cavities of 1 are filled with a large number of disordered solvent molecules. Considering the intrinsic structure, it is anticipated that the solvent molecules could be replaced by other different common organic solvents (Figures S9 and S10, Supporting Information). In this regard, to examine the potential sensing of small organic solvent molecules, the fluorescence properties of 1 a in different solvents (denoted as 1 a solvents) were investigated. Samples of 1 a were immersed in various pure solvents for 3 days and were then removed by filtration. For 1 a, as shown in Figure 5, the PL intensity is largely dependent on the solvent molecules, particularly in

Figure 4. Electric hysteresis loops obtained at ambient temperature with pellets of powdered 1 in applied fields of  6,  8,  10,  12,  14,  16,  18, and  20 kV cm 1 for the successive loops.

room temperature using powdered samples in pellets (Figure 4). The experimental results indicate the presence of electric hysteresis loops with a remnant electric polarization (Pr) of 0.013 mC cm 2 and an electric coercive field (Ec) of 74.76 kV cm 1; the spontaneous saturation polarization (Ps) of complex 1 is ca. 0.016 mC cm 2, which is smaller than that for a typical ferroelectric compound (e.g., NaKC4H4O6·4H2O, Rochelle salt; usually Ps = 0.25 mC cm 2). MOFs with transition-metal ions without unpaired electrons, especially those having d10 configurations, display regular photoluminescent properties. Those MOFs are of great interest because of their potential applications in the areas of chemical sensors and photochemistry. The solid-state photoluminescent spectra of 1, 1 a (the desolvated of 1), and the free ligand (H5L) were studied at room temperature (Figure S12, Supporting Information). Excitation of the solid samples at 360 nm produces a luminescence peak with a maximum at 424 nm for 1. The fluorescence emission band for 1 might be attributed to the intraACHTUNGREliACHTUNGREgand emission from L5 because the free H5L exhibits a luminescence at 419 nm with the excitation at 360 nm. Interestingly, the solid-state PL spectrum studied on 1 a revealed unique fluorescence quenching behav- Figure 5. PL intensities of 1 a when introduced into various pure solvents (a, b). PL spectra of solid-state 1 a ior and presented a red shift water in the presence of various amounts of acetone (c, d).

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the case of acetone. When 1 a was dispersed in H2O, N,N’-dimethylacetamide (DMAc), or DMF, its fluorescence intensity basically remained the same. A slight increase occurred when methanol, ethanol, or benzene were introduced into 1 a. In the case of 1 a acetone, the PL intensity exhibited the most significant enhancing effect, which is about seven times higher than 1 a. To further study the effect of acetone on the luminescent intensity, 1 a was first dispersed in distilled water and then the acetone solvent content was gradually increased to monitor the emissive response. As shown in Figure 5 c and 5 d, the fluorescence intensity of 1 a gradually increased with the addition of acetone. These results suggest that 1 a could be a promising luminescent probe for detecting acetone. Dyes are widely employed in many industries, including paper, printing, plastics, textiles, cosmetics, pharmaceuticals and so on.[17] However, many dyes are considered to be toxic, and most dyestuffs are stable to light and oxidants, which make them difficult to degrade.[18] The result of sensing small organic molecules indicates that 1a is a waterstable MOF at room temperature. To prove the permanent porosity and accessibility of 1 a to larger molecules, three different dye molecules (Figure S15, Supporting Information) were adsorbed in solution. The result shows that 1 a is able to adsorb toluidine blue (TB), methylene blue (MB), and rhodamine 6G (R6G) from aqueous solution. The strongest emission peaks of dye@1 a are at 704 nm, 715 nm, and 617 nm, respectively, for TB, MB, and R6G at room temperature, which are similar to the previous results obtained on dye-doped materials (Figure S16, Supporting Information). All the dye@1 a materials exhibit a regular stability as judged from the powder X-ray diffraction (PXRD) data of dye@1 a after loading (Figure S11, Supporting Information). In conclusion, we have designed an unprecedented chiral metal–organic framework (Cd-MOF) with second-order nonlinear optical and ferroelectric properties. The porous luminescent Cd-MOF holds potential for sensing small molecules and for adsorbing large dye molecules. It is believed that these results will further facilitate the incorporation of preconceived multifunction into porous MOFs. It is expected that more multifunctional MOFs will emerge in the future.

3420(s), 2926(w), 1606(s), 1576(s), 1451(w), 1403(m), 1370(s), 1316(w), 1261(m), 1039(s), 779(s), 724(m), 596(m).

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20901070, 21371153) and the Program for Science&Technology Innovation Talents in Universities of Henan Province (13HASTIT008), the Key Scientific and Technological Project of Henan Province (132102210411) and Zhengzhou University.

Keywords: ferroelectricity · helical structures · metal– organic frameworks · multifunctional materials · secondharmonic generation

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Experimental Section Synthesis of {[Cd5(L)2ACHTUNGRE(H2O)4ACHTUNGRE(DMAc)3]·10H2O·7DMAc }n (1) CdACHTUNGRE(NO3)2·4 H2O (30.8 mg, 0.1 mmol), H5L (10.2 mg, 0.02 mmol), DMAc/ H2O (1.5/1 mL) (DMAc = N,N’-dimethylacetamide) were mixed in a Teflon-lined stainless steel vessel (23 mL). The mixture was heated under autogenous pressure at 90 8C for 72 h, and then cooled to RT. Colorless platelet crystals that were suitable for single-crystal X-ray analysis were obtained by filtration. The resulting colorless block crystals were washed with DMAc (yield: 69 % based on cadmium) and has a formula of {[Cd5(L)2ACHTUNGRE(H2O)4ACHTUNGRE(DMAc)3]·10 H2O·7 DMAc}n, which was derived from crystallographic data, elemental analysis (calcd: C, 40.09; H, 5.38; N, 5.19; found: C, 39.78; H, 4.99; N, 5.42 %.), and thermogravimetric analysis (TGA, Figure S8, Supporting Information). IR (KBr pellet, cm 1): n˜ =

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