WANG Zhen FAN Hui-To WANG Hui-Fng ZHAO Qing LIU Sui-Jun

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Dual Functional Methoxyterephthalic Acid Terbium Complex with Luminescence and Magnetim①

WANG Zhena, bFAN Hui-TaoaWANG Hui-FangaZHAO Qianga②LIU Sui-Junc②

a(473061)b(()050035)c(341000)

A dual functional coordination polymer, namely, {[Tb(mat)1.5H2O]·2.5H2O}(1, H2mta = 2-methoxyterephthalic acid), was synthesized under solvothermal condition and characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, X-ray powder diffraction analysis (PXRD), and thermogravimetric analysis (TGA). Complex 1 exhibits a 3framework based on infinite rod-shaped secondary building units (SBUs). Furthermore, the solid-state luminescent property and magnetic properties of the complex were investigated at room temperature; the results show that the complex exhibits excellent luminescent properties in green luminescence and weak antiferromagentic behavior.

crystal structure, terbium(III), thermogravimetric analysis, magnetic properties, fluorescence spectrum;

1 INTRODUCTION

Because of the excellent optical, electrical and magnetic properties[1-3], lanthanide metal ions as a kind of special inorganic ions have been investi- gated by many researchers. Among all of these properties, the luminescent properties of lanthanide metal ions have been widely studied, and the molecular devices, electroluminescent devices and applications have become the focus of attention[4, 5]. In order to improve luminescent properties of lanthanide ions, lanthanide complexes have been encapsulated into inorganic-organic hybrid mate- rials[6-9]. Due to the effective intramolecular energy transfer from the coordinating ligands to the central lanthanide ions, lanthanide complexes with organic ligands exhibit sharp and intense emission lines upon ultraviolet light irradiation[10, 11]. Therefore, lanthanide complexes become a fast growing field in coordination chemistry and so many excellent compounds have been reported[12-14]. Herein, we report a new coordination complex based on 2-methoxyterephthalic acid with Tb3+. Furthermore, the solid-state luminescent property and magnetic properties of the complex were investigated at room temperature.

2 EXPERIMENTAL

2. 1 Materials and measurements

All the starting materials for the synthesis were of reagent grade and used as received. The ligand 2-dimethoxyterephthalic acid was synthesized according to the literature method[15].

Elemental analyses (C, H, and N) were carried out on a Perkin-Elmer 240C analyzer. IR spectra in the 4000～400 cm-1range were measured on a TENSOR 27 OPUS FT-IR spectrometer using KBr disks dispersed with sample powders. Fluorescence spectra were recorded at room temperature on a Varian Cary Eclipse ?uorescence spectrometer. Magnetic data were collected by a Quantum Design SQUID-VSM magnetometer.

2. 2 Synthesis of complex 1

A mixture of H2mat (19.6 mg, 0.1 mmol) and Tb(NO3)3·6H2O (45.3 mg, 0.1 mmol) in 6 mL mixed solvent of acetonitrile and H2O (1:2, v/v) was sealed in a 23 mL Teflon-lined stainless-steel container and heated at 115℃for 3 days. After the container was cooled to room temperature at a rate of 8℃·h-1, block-shaped crystals suitable for X-ray analysis were obtained directly, washed with acetonitrile, and dried in air (Yield: 40% based on Tb). Anal. Calcd. for C13.5H16TbO11(%): C, 31.60; H, 3.14. Found (%): C, 31.98; H, 3.56. FT-IR (KBr, cm–1): 3368, 1617, 1527, 1456, 1421, 1302, 1253, 1101, 1025, 868, 814, 776, 605, 524, 486.

2. 3 X-ray data collection and structure determination

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

Symmetry codes: A: –, –+1, –+1; B:-1,,; C: –, –, –+1

3 RESULTS AND DISCUSSION

3. 1 Crystal structure description

Fig.1. (a) Coordination environment of Tb in complex1.Symmetry codes: A: –, –+1, –+1; B:–1,,; C: –, –, –+1; D:+1,,; E: –, –, –+2; (b) Rod-shaped secondary building unit of complex 1; (c) Three-dimensional network structure. Hydrogen atoms are omitted for clarity

Fig. 2. (a) PXRD pattern of complex 1; (b)TGA result of complex1

Before fluorescence test, powder diffraction experiments and thermogravimetric experiment of the complex were carried out. In order to detect the purity of the complex, PXRD has been tested and the experimental results are consistent with the theoretical values (Fig. 2(a)).

Thermogravimetric analysis of the complex revealed three steps of decomposition with the 45.16 wt% total weight loss during 27.43～750 ℃, as shown in Fig. 2(b). The first major weight loss of 13.52 wt% occurred at the temperature between 27.43 and 130.32 ℃, corresponding to the removal of water molecules. There is nearly no weight loss at the temperature between 130.32 and 460.31 ℃. The second weight loss of 31.64 wt% occurred at the temperature between 460.31～750 ℃ due to the decomposition of the complex skeleton. Further heating shows no noticeable weight loss between 750 and 800 ℃. Thermogravimetric analysis indicates the complex has good thermal stability.

3. 2 Fluorescence spectrum

The solid-state fluorescence spectra of H2mat and complex 1were recorded at room temperature on a Varian Cary Eclipse ?uorescenceunder an excitation of 320 nm. As shown in Fig. 3, H2mat itself shows an emission at around 450 nm, and complex 1has characteristic emission peaks of Tb at 622 nm (5D→73), 587 nm (54→74), 547 nm (54→75), 494 nm (54→76), and the biggest emission peak at 587 nm. That means the obtained complex is a kind of pure green luminous material. In fact, the complex exhibits excellent luminescent properties in green luminescence. Furthermore, the emission color change of complex 1 under UV-vis lamp of 254 nm was also investi- gated, as depicted in Fig. 3 (inset). Under UV-vis lamp of 254 nm the complex has a bright green fluorescence. The reasons for the enhancement of Tb luminescence in the complex are as follows. Firstly, organic ligands have a high UV absorption coefficient. After the complex of mat and metal Tb was formed, the energy absorbed by the mat will be efficiently transferred to the metal Tb and the results lead to a high fluorescence efficiency of Tb in the complex[20, 21]. Secondly, there is a direct relationship between fluorescence enhancement and coordination effect. The conjugation effect of the new system was enhanced after the coordination reaction, which effectively increases the rigidity of the ligand and reduces the loss of energy by radiationless decay[22, 23]. Thirdly, rare earth ions tend to be more coordination. Complex 1 not only has organic negative ion ligand mat that satisfies the charge flat street but also has water molecules to meet the multi coordination. Then a three-element complex of Tb was formed, which can reduce the polarity of the complex, thereby increasing the volatility to meet the need of light emission[24, 25].

Fig.3. Emission spectra H2mat and complex 1 under an excitation of 320 nm; (inset) The emission color of complex 1 under the UV-vis lamp of 254 nm

3. 3 Magnetic properties

The magnetism of 1 was studied by solid state magnetic susceptibility measurements in the 2～300 K range at 1 kOe dc field and the isothermal field-dependent magnetizations() at fields up to 70 kOe at 2～5 K. As shown in Fig. 4,the roomtemperaturemproduct estimated as 25.38cm3×mol-1×K is in relatively good agreement with the presence of two uncoupledTbIIIions. As the temperature decreases,mvalue stays nearly constant at high temperature and then fast decreases to 14.73 cm3×mol-1×K at 2 K. The above-mentioned results indicate the existence of weak antiferro- magentic interactions. Curie-Weiss fittingofm-1plots in the range of2～300 Kleads to= 25.74cm3·mol-1·K and= –4.13 K. The negativevalue for 1 also indicatesweakantiferromagnetic couplings between the neighboringTbIIIions within the chain.

Fig. 4. (a)mandm-1.curves for 1; (b)curves at 2～25 K for 1

For 1,increases quickly at very low field, and the increase of magnetization is very slow and linear in the high field region, which may be attributed to the anisotropy of the polycrystalline sample. The value ofreaches to 11.21 Nat 70 kOe and 2 K,beingfar from the theoretical saturated values of 18 Nanticipated for two independent TbIIIions.It can be explained by the fact that the depopulation of the Stark levels of the7F6ground state under the ligand-field perturbation produces a much smaller effective spin. The/(Fig.5) data at 2～5 K show non-superposition plots and a rapid increase of the magnetization at low ?elds, which eventually reaches the maximum value without any sign of saturation. The reason is most likely due to anisotropy and important crystal-?eld effect at the TbIIIion, which eliminates the degeneracy76ground state.Reduced magne- tization curves do not superimpose, further indi- cating the presence of a signi?cant magnetic anisotropy and/or low lying excited states.

In the literature,using terephthalic acid as a ligand, Reinekesynthesized the coordination polymers Tb(BDC)NO3·2DMF[26]and Tb2(BDC)3·(H2O)4[27]and found that Tb(BDC)NO3adsorbed CO2and NH3adsorbed by Tb2(BDC)3; Guosynthesized Tb3(BDC)4.5(DMF)2(H2O)3·(DMF)(H2O)and Ln3(BDC)4.5(DMF)2(H2O)3·(DMF) (C2H5OH)0.5-(H2O)0.5(Ln=Dy, Ho, Er)[28]four rare earth com- plexes. The adsorption isotherms of water on Tb3(BDC)4.5(DMF)2(H2O)3·(DMF)(H2O)demonstratethat guest molecules in the coordination polymer backbone can be removed and form permanent microporosity; Chensynthesized the coordina- tion polymer

Er2(BDC)3(DMF)2(H2O)2·H2O[29]in 2006 to study the thermogravimetry and fluorescence spectroscopy properties of the coordination polymer. The above study is only for the study of the single nature of the complex. By changing the noncoor- dinating group structure of the ligand, a new complex is obtained, which is a fluorescent and magnetic bifunctional complex. The study of this complex has a good guiding significance for the synthesis of multifunctional complexes.

4 CONCLUSION

In summary, a three-dimensional network struc- ture has been synthesizedby the solvothermal method based on Tb and multiple acid ligands (H2mat). The results show that the complex exhibits excellent luminescent properties in green lumine- scence and weak antiferromagentic behavior.

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29 December 2017;

27 March 2018 (CCDC 1582771)

① This work was supported by Youth Fund of Nanyang Normal University (QN2017046)

Zhao Qiang. E-mail: zhaoqiang0522@126.com; Liu Sui-Jun.E-mail: liusuijun147@163.com

10.14102/j.cnki.0254-5861.2011-1934