New Ferroelectric and Nonlinear Optical Porous Coordination Polymer Constructed from a Rare (CuBr)∞ Castellated Chain Yi-Ming Xie,†,‡ Jiu-Hui Liu,† Xiao-Yuan Wu,† Zhen-Guo Zhao,† Qi-Sheng Zhang,† Fei Wang,† Shan-Ci Chen,† and Can-Zhong Lu*,†
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 3914–3916
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 35002, China, and College of Materials Science and Engineering, Huaqiao UniVersity, the Key Laboratory for Functional Materials of Fujian Higher Education, Quanzhou, Fujian 362021, China ReceiVed June 13, 2008; ReVised Manuscript ReceiVed August 29, 2008
ABSTRACT: A novel three-dimensional porous coordination polymer {[CuLBr] · 0.5H2O}n, solvothermally synthesized by the self-assembly of 4,4′-H2bpz (L ) 4,4′-H2bpz ) 3,3′,5,5′-tetramethyl-4,4′-bipyrazole) and CuBr, has a non-centrosymmetric polar packing arrangement, resulting in a strong second harmonic generation response and ferroelectric property. Developing new multifunctional materials that display the coexistence or synergism of more than two physical properties in one molecule, such as homochiral and absorbent materials,1 magnetism and ferroelectricity materials,2 nonlinear optical (NLO) and ferroelectric materials,3 is of immense current interest due to their tremendous potential applications in many fields. Ferroelectric and NLO behaviors are very useful and important physical properties in areas such as ferroelectric random access memories, switchable NLO devices, optical communication, signal processing and light modulators.4 Non-centrosymmetry is essential and the allimportant term for these materials. Although a great deal of progress has been made in the syntheses of acentric structures, challenges still remain in the preparation of acentric polar packing materials. Pyrazole and its derivatives have been widely employed to design porous coordination polymers (PCPs) due to their importance in medicine, biology and industry.5 Although many studies have been carried out based on 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (4,4′H2bpz) ligand, their focuses have been mainly on the structure development for novel topological architectures6 and physical properties such as absorbency, photoluminescence, magnetism and framework flexibility7 and so far very rarely concern the development of ferroelectric and NLO properties. During our research on the assembly of PCPs of pyrazole and its derivatives, the attractive and structurally simple bifunctional tectonic 4,4′-H2bpz ligand was widely used, which has an angle of rotation around 50-90° and, very often, will result in noncollinear orientation of two N-M vectors. Recently, a porous coordination polymer {[Cu(4,4′H2bpz)Br] · 0.5H2O}n (1) [4,4′-H2bpz ) 3,3′,5,5′-tetramethyl-4,4′bipyrazole] with non-centrosymmetrica polar packing arrangement was obtained. Herein, we report the synthesis, crystal structure, ferroelectric and dielectric, NLO and photoluminescence properties of the porous coordination polymer 1. The ligand 4,4′-H2bpz was prepared according to the literature8 and satisfactorily characterized by elemental analysis and IR. Colorless block-like crystals of compound 1 were synthesized by the reaction of CuBr, 4,4′-H2bpz, CH3CH2OH and H2O under hydrothermal conditions, which was formulated as {[Cu(4,4′H2bpz)Br] · 0.5H2O}n on the basis of elemental analysis. This compound is very stable in air at ambient temperature and is almost insoluble in most common solvents such as water, alcohol, acetonitrile, chloroform, acetone, toluene and so on. The structure of the title compound was also identified by satisfactory elemental analysis, IR and X-ray diffraction. Powder X-ray diffraction studies * To whom correspondence should be addressed. Fax: (+86)-591-83714946; tel: (+86)-591-83705794; e-mail: czlu@ fjirsm.ac.cn. † Chinese Academy of Sciences. ‡ Huaqiao University.
Figure 1. ORTEP drawing of 1 with 50% thermal ellipsoids. Hydrogen atoms and disordered O atoms are omitted for clarity (symmetry codes: A:3655, -x + 1, -y, z; B:7655, 3/2 - x, y, z; C:4654, 3/2 - x, 1/2 y, -1/2 + z).
indicate that the product is in pure phase (Figure S1, Supporting Information). Single crystal X-ray crystallographic analysis9 reveals that the title compound has a non-interpenetrating, three-dimensional porous framework constructed from copper ions and µ2-4,4′-H2bpz ligands. The asymmetric unit of 1 contains half a 4,4′-H2bpz ligand, half a copper ion and half a bromine ion (Figure 1). The Cu(1) atom is coordinated by two Br(1) and two N(1) atoms from two different 4,4′-H2bpz ligands (Cu-Br, 2.5037(13)-2.7813(13) Å; Cu-N, 1.988(3)Å;N-Cu-N,130.6(2)°;N-Cu-Br,93.90(11)-111.65(11)°; Br-Cu-Br, 108.53(3)°) to form a distorted tetrahedral geometry, which can be considered as a four-connected node to form a diamond-net structure (Figure S2, Supporting Information). It is notable that the castellated (CuBr)∞ chains in compound 1 are unusual (Figure 2a). Much less common than split-stair chains, castellated chains have been observed previously only in [Cu2Cl2(tmtttf)]∞ (tmttf ) tetrakis(methylsulfanyl)tetrathiafulvalene), which form two-dimensional sheets.10 The framework is based on inorganic columns of the castellated (CuBr)∞ chains. Each castellated chain is linked to four neighboring castellated chains by 4,4′H2bpz molecules to form a 3-D porous framework (Figure 2b). The arrangement of inorganic chains and organic linkers leads to a structure with a hexagonal array of channels. As shown in Figure S3, Supporting Information, the framework exhibits one type of channel with dimensions of approximately 4.82 × 7.83 Å along the c direction (considering the van der Waals radii).11 It is worth noting that the framework is neutral, obviating the need for counterions in their cavities. Calculations using PLATON12 show that total potential solvent area volume is about 411.9 Å3 per unit cell, comprising 26.3% of the crystal volume of 1 (disorder solvent H2O is omitted).
10.1021/cg800624z CCC: $40.75 2008 American Chemical Society Published on Web 10/10/2008
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Figure 2. (a) The castellated (CuBr)∞ chains in compound 1; (b) A 3-D porous framework of compound 1 along the c-axis (disorder solvent H2O is omitted).
Figure 3. Electric hysteresis loop of compound 1, observed for a powdered sample in the form of a pellet on a ferroelectric tester at room temperature.
Compound 1 crystallizes in the noncentric space group Ima2, which is associated with the point group C2V, one of the 10 polar point groups (C1, C2, Cs, C2V, C3, C3V, C4, C4V, C6, C6V) required for ferroelectricity. Experimental results indicate that there is an electric hysteresis loop that is a typical ferroelectric feature with a remnant polarization (Pr) of ca. 0.027 µC · cm-2 and coercive field (Ec) of ca. 1.88 kV · cm-1. The saturation of the spontaneous polarization (Ps) of 1 is ca. 0.0858 µC · cm-2 (Figure 3), which is similar to those found in homochiral trinuclear discrete Ni(II) and [Cd(trtr)2]n (trtr ) 3-(1,2,4-triazole-4-yl)-1,2,4-trizoole)) complexes.13 To the best of our knowledge, compound 1 represents the first example of metal pyrazole complexes with ferroelectric property. The behavior of permittivity (ε) ) ε1(ω) - iε2(ω) has been also studied, where ε1(ω) and iε2(ω) are the real (dielectric constant) and imaginary (dielectric loss) parts, respectively. The frequency dependence of the dielectric constant ε1 at 473 K indicates that ε1 rapidly decreases with the increase of frequency, but from 10000 Hz to higher frequencies, ε1 remains nearly unchanged (Figure S4, Supporting Information). This discrepancy suggests the presence of a dipole relaxation at low frequencies, similar to that found in the [Eu(Lig)2(H2O)2][ClO4] (Lig ) lactate).14 The noncentrosymmetric structure of 1 prompted us to study its NLO property. SHG measurements on the powder samples reveal
Figure 4. (a) Diffuse reflectance spectrum of compound 1; (b) emission spectra of 4,4′-H2bpz and 1 in the solid state at room temperature.
that compound 1 exhibits SHG efficiency similar to that of KDP (KH2PO4). Further absorption spectrum measurements indicate that compound 1 is transparent in the range of 580-2000 nm. From 580 to 240 nm, the absorption increases with decreasing wavelength, and the largest absorption rate is about 28% at 240 nm (Figure S5, Supporting Information). Hence compound 1 might be used in UV and near-IR regions as an NLO material. Optical diffuse reflectance study of compound 1 reveals an optical band gap of 3.82 eV (Figure 4a), which is at least similar to the band gap of LiNbO3 (3.5 eV). It is known that the band gap is consistent with the laser damage threshold. The larger the band gap is, the higher the laser damage threshold will be. Therefore, compound 1 will be expected to exhibit excellent laser damage threshold properties. To the best of our
3916 Crystal Growth & Design, Vol. 8, No. 11, 2008 knowledge, compound 1 represents the first example of NLO property among metal pyrazole complexes. The photoluminescence spectrum study shows that compound 1 displays an extensive blue luminescence emission in the solid state at room temperature with a maximum at ca. 440 nm (Figure 4b). Since similar emission with λmax ) 460 nm was also deserved for 4,4′-H2bpz in solid state. The fluorescent emission of 1 may be tentatively assigned to the intraligand transition of the free ligand fluorescent emission.15 The thermal stability of compound 1 has been studied through thermogravimetric analysis (TGA), which was performed from 30 to 1200 °C. The TGA of compound 1 (Figure S6, Supporting Information) under a N2 atmosphere demonstrates that the material is stable up to ca. 250 °C, whereupon the loss of ligands were observed from 250 to 480 °C (57.9% weight loss observed, 58.1% theoretical). In conclusion, a 3-D porous CuBr pyrazole coordination polymer with unusual castellated (CuBr)∞ chains, strong ferroelectric and NLO properties was prepared. The high thermal stability, insolubility in common solvents, as well as wide transparency range of 1 make it an excellent candidate for ferroelectric and NLO materials. The ferroelectric and NLO properties are investigated simultaneously in the pyrazole system for the first time, which opens a new avenue to ferroelectric and NLO materials through PCPs. The present work indicates that the pyrazole complex has many potential properties in the application for absorbent, ferroelectric and NLO materials. Our future efforts will be devoted to explore other similar systems to find other new multifunctional PCPs materials.
Acknowledgment. We gratefully acknowledge the financial support of the 973 key program of the MOST (2006CB932904, 2007CB815304, 2007CB815300), the National Natural Science Foundation of China (20425313, 20521101 and 50772113), the Chinese Academy of Sciences (KJCX2-YW-MO5) and the Natural Science Foundation of Fujian Province (2006L2005, 2006F3135, 2006F3141).
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Supporting Information Available: X-ray crystallographic files in CIF format for 1; syntheses details, physical properties measurements details, XRD pattern, frequency dependence of permittivity, UV-visible absorption spectra, TG diagram. This material is available free of charge via the Internet at http://pubs.acs.org.
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