Two New Nonlinear Optical and Ferroelectric Three-Dimensional

Dec 1, 2009 - The two complexes are isostructural and exhibit an acentric sqp-net, in which ... such as nonlinear optical (NLO) second harmonic genera...
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DOI: 10.1021/cg901066v

Two New Nonlinear Optical and Ferroelectric Three-Dimensional Metal-Organic Frameworks with an sqp-net

2010, Vol. 10 25–28

Yong-Tao Wang,*,† Gui-Mei Tang,† Yong-Qin Wei,‡ Ting-Xiao Qin,† Tian-Duo Li,† Chao He,‡ Ji-Bei Ling,‡ Xi-Fa Long,*,‡ and Seik Weng Ng§ †

Department of Chemical Engineering, Shandong Institute of Light Industry, Jinan 250353, P. R. China, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou, Fujian 350002, P. R. China, and §Department of Chemistry, Univeristy of Malaya, Kuala Lumpur 50603, Malaysia

‡

Received September 2, 2009; Revised Manuscript Received November 19, 2009

ABSTRACT: Two novel three-dimensional metal-coordination polymers [Cd2(L)4(μ2-X)(X)(H2O)]n [X=Cl (1) and Br (2)] have been solvothermally synthesized by the self-assembly of an unsymmetric bridging ligand, [L = 2-(1H-imidazol-1-ylmethyl)-1H-benzoimidazole], and cadmium salts with different anions, respectively. The two complexes are isostructural and exhibit an acentric sqp-net, in which the cadmium ions act as unusual five-connected nodes. The induction of the flexible and unsymmetric ligand leads them to crystallize in a noncentrosymmetric polar packing arrangement, which displays a second harmonic generation response and ferroelectric behaviors. The rational synthesis and design of noncentrosymmetric or polar compounds have attracted considerable attention in functional materials with specific chemical and physical properties such as nonlinear optical (NLO) second harmonic generation (SHG), piezoelectricity, pyroelectricity, and ferroelectricity.1-5 Of particular importance are nonlinear SHG and ferroelectricity, because the former plays an important role in telecommunications, optical storage, and information processing, and the latter may be useful in some scientific fields such as switchable NLO devices, ferroelectric random access memories, and light modulators.6 Up to now, there are some documents of SHG response and ferroelectricity in pure organic or inorganic compounds;7 however, investigation of metal-organic frameworks (MOFs) possessing NLO-active and ferroelecitric behaviors remains relatively scarce.4d,e,8 The hydrothermal or solvothermal technique has been widely used to develop an intriguing variety of novel MOFs.9-14 Indeed, works in our and other laboratories show that NLO-active MOFs can be efficiently generated through the introduction of an unsymmetric bridging ligand under the solvothermal reactions.9b,14 Motivated by these works, we envisioned that an unsymmetric bridging ligand can provide convenient access to MOF crystals with SHG response and ferroelectricity under solvothermal conditions. Therefore, we synthesized one new unsymmetric bridging ligand containing unsymmetric a bis-imiazole functional group, 2-(1H-imidazol-1-ylmethyl)-1H-benzoimidazole (L, Scheme 1). Unsymmetric bridging ligand L has two sites (bis-monodentate) for metal ligation and may extend into polymeric structures with bridging geometries. Herein, we wish to report the syntheses, solid state structures, photoluminescence, SHG efficiencies, and ferroelectric properties of [Cd2(L)4(μ2-X)(X)(H2O)]n [X = Cl (1) and Br (2)] with a 5-connected topological network, which were obtained by two cadmium salts with an unsymmetric bridging ligand L in the presence of methanol and water under solvothermal conditions (Scheme 1). The free ligand L has been synthesized and characterized by IR, NMR, and elemental analysis (EA) (see Supporting Information (SI)). At the outset of this study, we examined the reactions of CdCl2 with L under solvothermaly conditions. The solvothermal reaction of CdCl2 with L at 160 °C afforded the pale-yellow block

Scheme 1. Self-assemblies of the Unsymmetric Bis-imidazole Bridging Ligand L and Two Cd(II) Salts

*Author to whom correspondence should be addressed. E-mail: ceswyt@ sohu.com (Y.-T.W.); [email protected] (X.-F.L.). Fax: int code þ86 0531 8900 0551. Telephone: þ86 0531 8900 0551.

crystals compound of 1 (see SI). Under similar reaction conditions, when CdBr2 took the place of CdCl2, pale-yellow block crystals of 2 were obtained. IR spectra and EA results have confirmed the formulation of compounds 1 and 2. Subsequent structural determinations of 1 and 2 are unambiguously supported by X-ray crystallographic analysis. Compounds 1 and 2 are very stable in air at room temperature and are insoluble in water and common organic solvents. Single-crystal X-ray diffraction analysis15 reveals that compounds 1 and 2 are isomorphous and are three-dimensional polymers which crystallize in the form of the acentric space group P4nc. As shown in Figure 1a, there are two crystallographical independent Cd centers, one L molecule, two chloro anions, and one water molecule in which all Cd centers display a proximately octahedral coordination geometry. However, it is interesting to note that the compositions of the two Cd centers are different: the Cd1 metal center is bonded to four nitrogen atoms from four different imidazole rings, an oxygen atom of a water molecule, and a chloride atom; Cd2 connects to four nitrogen atoms from four different benzoimidazole groups and two chloride anions. In the surroundings of the Cd1 metal center, the four bonded N atoms of only the four same imidazole rings are located in planes, while the water molecule and chloride anion occupy the axial positions. However, in the Cd2 center, the equatorial plane is formed by four N atoms from only the four same benzoimidazole groups, and the axial positions are occupied by two chloride atoms. It is first observed that metal centers are coordinated by only one functional group of an unsymmetric bridging ligand. The Cd-N bond distances range from 2.3084(17) to 2.4286(17) A˚ and from 2.309(2) to 2.438(2) A˚ for 1 (X = Cl) and 2 (X = Br), respectively.2a The Cd-X bond distances fall in the ranges 2.622(3)-2.672(3) A˚ and 2.7749(16)-2.7919(9) A˚ for 1 and 2,

r 2009 American Chemical Society

Published on Web 12/01/2009

pubs.acs.org/crystal

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Wang et al.

Figure 2. Electric hysteresis loops for a pellet obtained from powdered samples of 1 (a) and 2 (b), respectively.

Figure 1. (a) ORTEP drawings of 1 (X = Cl) and 2 (X = Br) with 30% thermal ellipsoids. Hydrogen atoms are omitted for clarity. (b) View of a 2-D (4,4) layer with a [Cd2(L)4(Cl)(H2O)]n motif in 1 along the c axis. (c) View of a 3-D structure along the a axis in 1. The sign μ denotes the direction of the crystal packing polarity. (d) Schematic representation of a 5-connected 3-D sqp-net in 1, which is constructed by chloro anions (green) pillaring (4, 4) sheets.

respectively, which is in agreement with those found in the previously published literatures.3c,12c The L ligand adopts a μ2bridging mode with two N atoms ligating two Cd atoms separately; consequently, each ligand acts as a bis-monodentate spacer bridging two Cd centers. Each Cd center is bridged by four L ligands to four adjacent metal centers, which results in an infinite two-dimensional (4,4) layer along the ab plane (Figure 1b

and Figure S1). The adjacent layers are linked in parallel to each other in an ABAB fashion. Notably, these layers further extend to the 3-D structure through interlamellar Cl2 anions as pillars, which is the first time 3-D structures constructed by haloride anions are reported as pillars.12c Thus, the cadmium ions act as five-connected nodes and are linked by the L and chloride linkers to form the 3-D network (Figure 1c). Topological analysis shows that the structure of 1 can be described as a nonuniform 5connected binodal (44 3 66) topological sqp-network predicted by Wells,16 as shown in Figure 1d. To the best of our knowledge, no five-connected MOFs with noncentrosymmetric packing arrangements have been reported to date. In this work, in view of the fact that both compounds crystallize in a noncentrosymmetric space group (P4nc), we investigate the second-harmonic generation (SHG) measurements17 on the powdered samples of 1 and 2. The preliminary experimental results indicate that they are NLO-active and exhibit strong SHG efficiencies of 3 and 2 times that of KDP (KH2PO4), respectively, which is assigned mainly to a good donor-acceptor system (Figure S2) and the orderly arrangement of the organic moieties. The section of Cl-Cd-Cl-H2O shows a dipole moment along the c axis (Figures S3 and S4). The whole 3-D crystal structure exhibits a dipole moment along the c direction (Figures 1c and S5), owing to all the segments of Cl-Cd-Cl-H2O lying parallel to each other. Thus, it strengthens the high nonlinearity and hyperpolarizability of 1 to result in a good SHG response. The SHG efficiency of 1 is higher than that of 2, presumably because the withdrawing electron of chloride is higher than that of bromide. It is the first time that compounds 1 and 2 illustrate a NLO property among metal complexes constructed by unsymmetric bis-imidazole ligands. As mentioned above, samples 1 and 2 crystallize in the acentric space group P4nc, which is associated with the point group C4v,

Communication

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Supporting Information Available: Synthesis of L, preparation of crystals 1 and 2, additional figures, the TGA cures, and CIF format files of 1 and 2, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 3. Fluorescent emission spectra of L, 1, and 2 in the solid state at room temperature, respectively.

one of the ten polar point groups (C1, Cs, C2, C2v, C4, C4v, C3, C3v, C6, C6v) required for the ferroelectric property. Our primarily experimental results reveal that there is an electric hysteresis loop in compound 1 (a typical ferroelectric feature) with a remnant polarization (Pr) of 0.17 μC cm-2, a coercive field (Ec) of 2082 V cm-1, and a saturation spontaneous polarization (Ps) of 0.54 μC cm-2 (Figure 2a). The Ps value of 1 is significantly higher than that of the typical ferroelectric compound Rochelle salt (NaKC4H4O6 3 4H2O, Ps = 0.25 μC cm-2) and equal to that of C13H14ClN5O2Cd (Ps =0.50 μC cm-2).8g Similarly, as illustrated in Figure 2b, the Pr, Ec, and Ps of compound 2 can be evaluated to be 0.14 μC cm-2, 2370 V cm-1, and 0.39 μC cm-2, respectively, being similar to those found in the ferroelectric polymer C14H12CuN4O4.4e The thermogravimetric analysis (TGA) curve of the crystalline samples showed weight loss in the temperature ranges 85-132 °C and 80-127 °C for 1 and 2, respectively, ascribed to one coordinated molecule (found 1.86 and 1.99%; calculated 1.63 and 1.51% for 1 and 2, respectively) (Figures S6 and S7). The dehydrated materials are stable up to 273 and 305 °C for 1 and 2, respectively. However, the decomposition was still not complete at 800 °C, which could be confirmed by the black color of the residual and the slope of the TGA curves. The fluorescent properties of compounds 1 and 2 at room temperature have been investigated. As shown in Figure 3, complexes 1 and 2 display an extensive blue luminescence emission peak at 457 and 455 nm upon excitation at a wavelength of 280 nm, respectively. Compared with the luminescent spectra of 1 and 2, that of free L ligand is investigated in the solid state at room temperature. The free L exhibits a fluorescence emission band centered at 451 nm (λex =280 nm) (Figure 3). The profiles of 1 and 2 are similar to that of L, which is tentatively assigned to the intraligand fluorescent emissions.18 In summary, two noncentrosymmetric three-dimensional coordination polymers with an sqp-net have been successfully achieved with an unsymmetric bis-imidazole bridging ligand through self-assembly under solvothermal conditions. It is the first time that the ferroelectric and NLO properties of MOFs gernerated by the unsymmetric bis-imidazole system are investigated, which provides a new impetus to explore NLO and ferroelectric materials through MOFs. Further work is in progress to study other new materials with fascinating physical properties. Acknowledgment. This work was financially supported by the Project of Shandong Province Higher Educational Science and Technology Program (J09LB03), the Starting Funding of Shandong Institute of Light Industry (Y.-T.W.), and the Project of Fujian Province (2007F3115). Y.-T.W. thanks Prof. Chun-Lei Wang and Dr. Ming-Lei Zhao for helpful assisstance in discussing the dielectric properties. We would like to thank the reviewers for excellent comments.

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