A New Acentric Heterometallic Inorganic-organic Hybrid Framework [ZnK2(m-BDC)2(H2O)2]n: Fluorescent, NLO and Ferroelectric Properties①

XUE Li-Ping LIU Yue-Cheng HAN Yun-Hu LI Qi-Peng TIAN Chong-Bin LIN Ping DU Sho-Wu②

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A New Acentric Heterometallic Inorganic-organic Hybrid Framework [ZnK2(m-BDC)2(H2O)2]n: Fluorescent, NLO and Ferroelectric Properties①

XUE Li-Pinga, bLIU Yue-ChengaHAN Yun-Hua, bLI Qi-Penga, bTIAN Chong-BinaLIN PingaDU Shao-Wua②

a(350002)b(100049)

Solvothermal reactions of Zn(NO3)2·6H2O with isophthalic acid in the presence of potassium nitrate lead to an acentric three-dimensional (3D) hetero-metallic inorganic-organic hybrid framework[ZnK2(m-BDC)2(H2O)2]n(1, m-H2BDC = benzene-1,3-dicarboxylic acid). The crystal was characterized by elemental analysis, IR spectroscopy, TGA and single-crystal X-ray diffraction analysis. Complex 1 is of orthogonal crystal system, acentric space group21with= 19.764(6),= 19.948(6),= 12.039(4) ?,= 4746(3) ?3,= 4, C32H20O19K4Zn2,M= 995.66,D= 1.393 g/cm3,(000) = 2000 and= 1.426 mm-1. The final= 0.0761 and= 0.1841. The crystal exhibits remarkable blue luminescence emissions with high quantum yield of 56.02% and displaysmodest powder SHG efficiency of about 0.7 times that produced by a potassium dihydrogen phosphate (KDP) powder. In addition, it also exhibits potential ferroelectric property.

heterometallic, solvothermal synthesis, photoluminescence, ferroelectric

1 INTRODUCTION

The design and construction of novel functional metal-organic frameworks (MOFs) with second- order nonlinear optical (NLO), ferroelectric and photoluminescence are currently considered as one of the most important research areas[1, 2]. MOFs with ?uorescent properties are of great interest for their potential applications as light-emitting diodes (LEDs). NLO and ferroelectric behaviors are very useful and important physical properties in areas including ferroelectric random access memories, switchable NLO devices, optical communication, signal processing, and light modulators[3, 4]. However, noncentrosymmetry is an essential requirement for NLO materials, while ferroelectricity strictly re- quires that the compounds must crystallize in non- centrosymmetric space groups belonging to one of the 10 polar point groups (1,2,s,2v,4,4V,3,3v,6,6v)[5]. Therefore, the rational design and preparation of acentric or chiral networks are still a great challenge for synthetic chemists because most complexes are inclined to crystallize in a centric space group[6].

Herein, we report the synthesis of a new 3D Zn/K noncentrosymmetric framework [ZnK2(m- BDC)2(H2O)2]n(1).Compound1 was characterized by single-crystal X-ray structure analyses, powderXRD, thermogravimetric and IR analyses.Besides, theluminescent and second-order NLO properties as well as the ferroelectricity of 1were also investi- gated.

2 EXPERIMENTAL

2. 1 Materials and Instruments

All the chemicals were purchased commercially and used as received. Thermogravimetric analysis- mass spectrometry analysis (TGA-MS) experiments were performed using a TGA/NETZSCH STA449C instrument heated from 30～800℃ (heating rate of 10 ℃/min, nitrogen stream). The powder X-ray diffraction (XRD) patterns were recorded on crushed single crystals in the 2range of 5～50o using Cu-radiation. The XRD were measured on a PANalytical X’pert PRO X-ray diffractometer. IR spectra using the KBr pellet technique were recorded on a Spectrum-One FT-IR spectrophotometer in the range of 4000～450 cm?1. Elemental analyses (C, H, and N) were measured with an Elemental Vairo EL III Analyzer. Fluorescence spectra for the solid samples were performed on an Edinburgh Analytical instrument FLS920. The second-order nonlinear optical intensity was approximately estimated on a pulsed Q-switched Nd:YAG laser at a wavelength of 1064 nm with an input pulse of 350 mV.

2. 2 Synthesis

[ZnK2(m-BDC)2(H2O)2]n(1) A mixture of Zn(NO3)2·6H2O (0.0892 g, 0.3 mmol), KNO3(0.0505 g, 0.5 mmol) and-H2BDC (0.1661 g, 1.0 mmol) in CH3CH2OH (10 mL) was stirred at room temperature for a few minutes. The resulting slurry was transferred into a 20 mL Teflon-lined stainless steel vessel, which was heated at 130 ℃and maintained at this temperature for 24 h. The system was cooled to room temperature at a rate of 1.25 ℃/h.Yellow crystalswere obtained, washed with CH3CH2OH and dried in air (yield 65% based on Zn). Elemental Anal. Calcd. for C32H20O19K4Zn2(995.66): C, 38.60; H, 2. Found: C, 38.72; H, 1.89. IR (KBr, cm?1): 3452 m, 3219 s, 3068 w, 1629 s, 1591 s, 1494 w, 1402 m, 1094 w, 948 w, 770 w, 705 w, 581 s.

2. 3 Structure determination

Single-crystal X-ray diffraction data were collec- ted on a Rigaku diffractometer with a Mercury CCD area detector (Mo,= 0.71073 ?) at room tem- perature. Empirical absorption corrections were applied to the data using the Crystal Clear program. The structure was solved by direct methods using SHELXS-97 and refined on2by full-matrix least- squares with the SHELXL-97 program package[7, 8]. Metal atoms in each compound were located from the-maps and other non-hydrogen atoms were located in successive difference Fourier syntheses. All non-hydrogen atoms were refined anisotropically. Crystal data for 1: crystal sizes 1.00mm × 0.40mm × 0.35mm,(000) = 2000,= 293(2) K, 3.23<< 27.5°. The final refinement converged at= 0.0761 and= 0.1841 (= 1/[2(2) + (0.0694)2+ 17.8472], where= (F2+ 2F2)/3) for 3416 observed reflections (> 2()),= 1.08, (Δ/)max= 0.000, (Δ)max= 0.885 and (Δ)min= –0.598 e/?3. The selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for 1

Symmetry codes: (i) ?+1/2,+1/2,; (ii), ?,+1/2; (iii) ?+1/2, ??1/2,+1/2; (vii), ?,?1/2; (viii) ?+1/2, ??1/2,?1/2

3 RESULTS AND DISCUSSION

3. 1 Structural description

[ZnK2(m-BDC)2(H2O)2]nThe single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in orthorhombic acentric space group21. The asymmetry unit of 1 consists of one crystallographically independent Zn(II) center, two K(I) ions, two-BDC2–and two H2O. As depicted in Fig. 1a, the Zn(II) center is six-coordinated by six oxygen atoms (O(1), O(2), O(3), O(5), O(7i) and O(8i)) from four-BDC2–ligands and can be described as a distorted octahedron {Zn1O6}. The K1 residues in a four-coordinated geometrycom- posed of two carboxylate oxygen atoms (O(1), O(6vii)) from two distinct m-BDC2–ligands and two oxygen atoms (O(9w), O(10w)) from water, furnishing a slightly distorted tetrahedral coor- dination geometry {K1O4}. The K(2) is embraced by four carboxylate oxygen atoms (O(4), O(5), O(3ii) and O(8iii)) from four different m-BDC2–ligands.The Zn–O distances are in the range of 1.976(6)～2.579(11) ? and the K–O distances vary from 2.654(6) to 3.020(8) ?. In 1, there exist m-BDC2?dianions that display (k1-μ2)-(k1-μ2)-μ5, (k1-μ2)(k1- μ2)-μ6and (k1-μ2)-(k1-μ2)-μ4three coordination modes, respectively[9](Fig. 1b). As shown in Fig. 2a, helices consisted of two Zn(II) ions and two m-BDC2-dianions as a cycle (left- handed helix). By sharing Zn(II) ions, the helices are linked together to form a 2D layered structure. And the K(I) ions connect the 2D layered structures, forming the whole 3D structure (Fig. 2b). The pore volume ratios are calculated to be 23.2% using theprogram[10].The numbers of solvent molecules were disordered.

Fig. 1. (a) View of the coordination environment of metal ions in 1. The labels for carbon atoms are not shown for clarity.Symmetry codes: (i) ?+1/2,+1/2,; (ii), ?,+1/2; (iii) ?+1/2, ??1/2,+1/2; (vii), ?,?1/2; (viii) ?+1/2, ??1/2,?1/2. (b) Coordination modes of the m-BDC2–ligands

Fig. 2. (a) View of a left-handed helix, (b)View of the 3D framework from helix

The m-BDC ligands adopt (k1-μ2)-(k1-μ2)-μ5, (k1-μ2)-(k1-μ2)-μ6and (k1-μ2)-(k1-μ2)-μ4coordination modes, linking Zn(II) and K(I) ions into a 3D architecture with channels when viewed along the c axis(Fig. 3a). From a topological point of view for 1, considering their connectivity, the Zn(II) centers are viewed to be six-connected nodes, and m-BDC2-, the K(I) are simplified to linear connectors. Thus, the whole structure can be abstracted into a 6-connected framework with the Schl?fli symbol of {412.63}, and the long vertex symbol is 4?4?4?4?4?4?4?4?4?4?4? 4?*?*?*[11](Fig. 3b).

Fig. 3. (a) View of the 3D framework from the-axis, (b) Topological view showing the equivalent 3D framework

3. 2 Spectral and thermal analyses

The powder X-ray diffraction pattern of 1 is given in Fig. 4 with the pattern simulated on the basis of single-crystal structures. The positions of diffraction peaks in both patterns correspond well, which indicate that the as-synthesized samples of 1 are pure. In the IR spectra of 1 (Fig. 5), a band at 3452 cm-1could be assigned to the stretching vibration (OH) of lattice water molecules. The characteristic absorption bands of the carboxylate groups are shown in the range of 1565～1635 cm-1for the asymmetric stretching vibrationas(COO–) and 1345～1430 cm–1for the symmetric stretching vibrations(COO–).The above results are all confirmed by single-crystal X-ray diffraction analy- sis. In order to examine the stability of the frame- work, thermal gravimetric analyses (TGA) for 1 were carried out in nitrogen gas from 30 to 800 ℃ (Fig. 6). TG curve of 1 shows the first weight loss of 3.76% from 56 to 167 ℃ due to the removal of two water molecules per formula unit (calcd. 3.62%), after which the framework is collapsed corres- ponding to the decomposition of the organic ligands from 325 ℃.

Fig. 4. XRD powder patterns

Fig. 5. IR spectra of 1

Fig. 6. View of the TGA curve of complex 1

3. 3 Luminescent properties

Luminescence complexes are of great current interest because of their various applications in chemical sensors, photochemistry, and electrolu- minescent display[12]. It is well-known that metal coor- dination frameworks with a10configuration pos- sess excellent luminescence property[13]. Herein, the luminescent property measurement of complex 1 was carried out in the solid state at room temperature. As depicted in Fig. 7, an intense blue fluorescent emission is observed at 438 nm (exmax= 355 nm) in the emission spectrum at ambient temperature. Compared with the free m-H2BDC ligand, whose maximum emission peak was found at 385 nm (exmax= 327 nm), this can be assigned to the→* transitions[14]. Thus, the luminescence of complex 1 possibly originates from intra-ligand transitions[15]. The emission lifetime of 1 at ambient temperature is 1.2007 ns, indicating that the emission is fluore- scent[16]. The quantum yield of 1, measured under the excitation wave length of 355 nm at room temperature, is 56.02%. This observation suggests that complex 1 may be a good candidate for po- tential photoactive materials[17].

3. 4 NLO property

Complex 1is in acentric space group and may have potential applications as NLO-active mate- rials[18]. Approximate estimations were carried out on a pulsed Q-switched Nd:YAG laser produced by the powder sample to confirm its acentricity as well as to evaluate the potential application as second- order NLO materials. Results show that it displays the modest powder SHG efficiency of about 0.7 times of that produced by a potassium dihydrogen phosphate (KDP) powder.

3. 5 Ferroelectric property

Complex 1 crystallizes in the noncentrosymmetric space group21, which falls in the 10 point groups (1,2,C,2v,4,4v,3,3v,6,6v) re- quired for ferroelectric materials[19]. Its ferroelectric property was examined. The hysteresis loops of the electric polarization were obtained for 1, as shown in Fig. 8. At room temperature, the remanent polariza- tion (r) of 1 is ca. 0.0161C/cm2with a coercive ?eld (c) of ca. 441.15 V/cm. The saturation spontaneous polarization (s) of 1 is ca. 0.0185C/cm2. The Ps values of 1 are much smaller than that of the typical ferroelectric KDP (s: 5.0C/cm2)[20].

Fig. 7. Luminescent spectra at room temperature

Fig. 8. Electric hysteresis loops of 1

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5 March 2014;

7 May 2014 (CCDC 988991)

① This work was supported by the National Basic Research Program of China (973 Program, 2012CB821702), the National Natural Science Foundation of China (21233009 and 21173221) and the State Key Laboratory of Structural Chemistry

. E-mail: swdu@fjirsm.ac.cn