Hdi Adul Nd LI Wei FENG Guo-Qing LU Pei-Xing

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Synthesis, Structure, Photoluminescence and Thermal Expansion of a Rare Earth Formate Oxalate Framework①

Hadi Abdul Nadaa,bLI WeiaFENG Guo-QiangcLU Pei-Xianga②

a(430074)b()c(430205)

A new lanthanideformate oxalate framework[Eu(C2O4)(HCOO)]n(1) has been synthesizedhydrothermal method and characterized by single-crystal X-ray diffraction (SC-XRD). The framework crystallizes in the orthorhombicspace group, with7.0984(4),= 6.6442(3),= 10.6793(6) ?,= 503.68(4) ?3,= 3, C3HO6Eu,M= 285.00 g/mol,D= 3.7581 g/cm3,(000) = 520.3796,= 12.413 mm-1, the final= 0.0264 and= 0.0628 for797 observed reflections with> 2(). The photoluminescence (PL) measurements reveal the significant red emission of the framework is dominated by the (5D0→7F2) electronic transition at 614 nm. Further variable-temperature powder X-ray diffraction(VT-PXRD) indicates that framework1 shows slight negative thermal expansion (NTE) along the-axis, and positive thermal expansion (PTE) along theandaxes.

formate oxalate framework, hydrothermal method, photoluminescence, thermal expansion;

1 INTRODUCTION

Lanthanide-organic frameworks are emerging types of materials which have potential applications in the fields of optoelectronics like optical display panels and light-emitting diodes (LEDs) due to their outstanding optical properties[1].These hybrid frame- works are composed by organic ligands and metal ions within a homogeneous structure establishing an extensive range of emissive phenomena, for instance linker-based luminescence[2], metal-based emission[3]or antenna effects[4]. Both inorganic and organic moieties can provide the platforms to produce lumi-nescence, typically the origin of luminescence emis-sion widely observed in lanthanide MOFs via the so-called (antenna effect) which is often referred to as the ligand-to-metal charge-transfer (LMCT), since the strongly absorbing ligand acts as a receiving antenna for the bond metal ion. In addition,coope-rative functionalities of permanent porosity and luminescence property qualified luminescent MOFs as a very attractive new class of sensing materials[5, 6].

Trivalent lanthanide Ln(ΙΙΙ) ions produce attractive optical properties. Recently, the luminance studies of rare earths metals that are synthesized with various organic ligands have been conducted. Ln(II?) che- lates provide narrow band emissions and very pure luminescence using UV-VIS excitation[7]. Thissensitized luminescence of the lanthanide compounds is commonly used for optical materials, biological applications, electroluminescent devices, optical amplifier or laser and sensors[8, 9]. Luminescence ofeuropium (Eu3+) and terbium (Tb3+) compounds takes the trending due to the narrow f-f transitions,especially the compounds of Eu3+have attracted a lot of attention because of their good sensitization luminescence that can be carried out through the antenna effect as well as the line of emission spectra look mostly sharp as it depends on the crystal field around the metal ions. Furthermore a great attention has been paid to the luminescence of Eu3+ions owing to the ability of acting as phosphors and progress of materials[10]. It is well-known that rare earth orga-nic frameworks have attracted comprehensive inte-rest over the past few years because not only their interesting applications but also their luminescence which originated from intra 4f or 4f–5d transitions[11]. This specific luminescence is characterized byhigh luminescence quantum yield, narrow bandwidth, long life time, large Stokes shifts, and ligand depen-dent luminescence sensitization[12, 13].

Herein we show crystalline framework materials incorporated both inorganic and organic moieties are attracting candidates because of their wide chemical and structural varieties that propose opportunities for making many technologically relevant properties[14]. Notably, the dense hybrid systems incorporated with infinite inorganic connectivity, like metal-oxygen-metal (M-O-M) arrays, can present the topological characteristics desired for certain types of physical properties[15]. In the present work,a new lanthanide-organic framework[Eu(C2O4)(HCOO)]n(1), consti-tuted by three basic building blocks with inorganic M-O-M chains and two organic bridging ligands of oxalate (C2O4)2-and formate (HCOO-) ions, has been successfully preparedby hydrothermal method, and the crystal structure has been described in details. The PL experiment shows that framework 1 exhibits strong red emission spectrum from the characteristicluminescence of Eu3+. In addition, VT-PXRD experi- ment reveals that compound 1 displays NTE along theaxis, whereasand c axes display PTE, which can be ascribed to the aforementioned structure arrangement.

2 EXPERIMENTAL

2. 1 Synthesis of [Eu(C2O4)(HCOO)]n

All chemicals and solvents were of regent grade and used as received without further purification. We prepared the title compound in a Pyrex-lined steel bomb (autogenous pressure) at temperature around 200°C over a week[16]. Crystals were obtained by heating an aqueous suspension of Eu oxalate hydrate Eu2(C2O4)3·10H2O with 15 mmol of acid oxalic dihydrate in 10 mL of water for a week. By varying the heating time less or more than 7 days, it has been observed that no single crystals were developed. The final product was harvested by filtration, washed with water and ethanol, and then dried in air and the production rate was 53%.

2. 2 Structure determination

PXRD patterns for framework 1 were collected on a (Empyrean, PANalytical, 40 kV, 40 mA) diffracto- meter by using Cu(1.540598 ?) radiation. Single- crystal X-ray data of 1 were collected at room temperature using OXFORD Diffraction Rigaku. XtalABminTMDiffractometer. The elemental analysis (EA) was performed on an Elementar Vario Micro Cube ELIIIelemental analyzer.

Single-crystal X-ray diffraction data were collec-ted on an Oxford CCD diffractometer with graphite- monochromated Moradiation (= 0.71073 ?). The structure was solved by direct methods and successive Fourier difference syntheses, and refined by full-matrix least-squares procedure on2with anisotropic thermal parameters for all non-hydrogen atoms using programs SHELXS-97 and SHELXL- 97, respectively[17]. Hydrogen atoms were located by geometric calculations and refined using a riding mode. X-Seed was used as an interface for the SHELX programs[18].Data reduction and absorption corrections were applied and determined using the CrysAlisProprogram[19]. A total of 3707 reflections (3.61≤≤32.93°) were collected with 893 unique ones, of which 797 observed reflections with> 2() were observed and used for structural illustra-tion. At convergence, the final= 0.0264 and= 0.0628 (= 1/[2(F2) + (0.0506)2+ 0.9326], where= (F2+ 2F2)/3), goodness-of-fit = 1.0349, (D)max= 3.6774 and (D)min= –1.6523 e/?3. The selected bond lengths and bond angles are listed in Table 1.

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

Symmetry codes: #1: 0.5+, 0.5–, 0.5–; #2:,, 0.5–; #3: 0.5+, 0.5–,

2. 3 Variable temperature powder X-ray diffraction

VT-PXRD experiments were performed on a Bruker a D8-discover X-ray diffractometer, where Curadiation (1= 1.5406 and2= 1.5443 ?) at 40 kV, 30 mA, 2 mm divergence slit; 0.6 mm anti-scatter slit; 0.2 mm receiving slit; monochro- matic; 0.1 mm detector slit. The temperature was adjusted by a cryostat with temperature-adjusting error less than 1 K. The angular scanning range was 10～50o with the step of 0.01o and scanning rate of 1/step. The cell parameters of variable temperatures were refined by Rietveld method using GSAS software. The in-situ VT-PXRD patterns were collected over the range of 180～280 K, with a 20 K interval.

3 RESULTS AND DISCUSSION

3. 1 Structure description

The single-crystal X-ray diffraction study revealed that the framework of1 has asymmetric units, as illustrated in Fig. 1. The structure of compound 1 is a 3-D hybrid framework in orthorhombicspace group. The dimensionalities of its inorganic and organic connectivity are 1-D and 2-D, respectively. The 1-D inorganic chains contain 9-coordinated europium polyhedra (EuO9) and their faces partici- pate in forming infinite zigzag chains along the [100] direction. To form the 2-D organic connectivity, europium chains are bridged by formate ligands along the [010] direction to generate infinite 2-D layers, and the layers nearby are subsequently bri- dged by the oxalate ligands along the [001] direction to make a 3-D framework structure.

Finally, the structure for1 is built up from Eu atoms sharing all their oxygen atoms with oxalate and formate ligands, thus giving rise to a 3D frame- work structure (Fig. 1). The bond distances of Eu(1)– O(4) and O(1)–C(2) are 2.472(5) and 1.274(6) ?, respectively, and the bond angles are as follows: O(4)#5–C(3)–O(3) 125.5(6) and O(1)–C(2)–O(2)#6126.1(8)°. For comparison, in the isostructural com- pound Ce(C2O4)(HCOO) previously reported[16], Ce–O(4) = 2.503(3), O(1)–C(1) = 1.237(8) ? and O(1)–C(1)–O(2) = 127.3(6), O(4)–C(2)–O(3) = 125.7(4)°.

Fig. 1. Illustration of 1 with asymmetric unit(a); Framework structure of 1 normal to the (b) {100}, (c) {010} and (d) {001} planes, respectively; Color scheme: Eu, green; O, red; C, blue; hydrogen atoms have been deleted for clarityin (b～d).Symmetry codes: #4: –, –, –, #5:, 1+,, #6: –, –, –0.5+

3. 2 Photoluminescent property

The PXRD patterns shown in Fig. 2 confirm the purity of the as-synthesized compound 1. Then PL measurements were performed using pulsed laser He-Cd (325 nm) (horiba Jobin Yvon) on the pure sample at room temperature. Fig. 3 shows the normalized PL measurement spectra of 1 with an excitation wavelength of the pulsed ultraviolet He-Cd laser 325 nm at room temperature. Notably, 1 shows emission region 300～800 nm and the characteristic peak of Eu3+transition is observed. The resonance energy levels of europium ions along withvarious luminescence transitions are presented in Table 2[20, 21]. The PL emission of the Eu3+ions indi-cates emission lines in the visible region with a comparatively simple energy level structure.

Fig. 2. PXRD pattern of the as-synthesized [Eu(C2O4)(HCOO)]n. The black lineis the experimental pattern and the blue vertical markers indicate the allowed Bragg reflection

Fig. 3. PL spectrum from1

Table 2. Energy Levels and Luminescent Transition for Europium

Herein, the luminescence for Eu3+ions may rise from the5D0→7FJtransition, where (J = 0, 1, 2, 3, 4) levels are dependent on the triplet state energy of the donor, thus the energy is fundamentally trans-ferred to5D0corresponding to 17260 cm-1[7, 20]. As shown in Fig.3, 1 exhibits four sets of charac- teristic emission bands from Eu3+ions in the visible spectral region with different intensities. The emi- ssion peak is centered at 590 nm originating from the5D0→7F1transition. The most prominent emission is located at 614 nm and is dominated by electric dipole transition of Eu3+, which corresponds to the strong red luminescence emission output.In fact, the5D0→7F2transition of Eu3+ion originates from electric dipole (ED) and is sensitive to the coordination environment around the Eu3+ion, it denotes that the Eu3+ion possesses non-centrosymmetric environ- ment that agrees with the resulting single crystal diffraction pattern. The PL peaks at 651 and 696 nm can be assigned to5D0→7F3, and5D0→7F4, respectively. In contrast, the emission intensity from5D0→7F1transition depends on the crystal field dominated by magnetic dipole (MD), which is insensitive to the site symmetry, as shown in the emission spectrum in Fig. 3. And the intensity of the5D0→7F2transition is about 3 times stronger than that of the5D0→7F1transition, which indicates that there is no inversion center at the Eu3+sites.

3. 3 Thermal expansion study

Thermal expansion behaviour of framework 1 was studiedvariable-temperature X-ray diffraction (VT-XRD) experiments. Fig. 4(a) shows the measu- red temperature-dependent variations in the lattice parameters between 180 and 280 K with a 20 K interval. It’s obvious that axesandshow positive thermal expansion (PTE) about +0.07% and +0.09%, respectively, whereasaxis exhibits negative ther- mal expansion (NTE) about ?0.03%. The obtained average coefficients of thermal expansion along three orthogonal axesand the volumetric thermal expan- sion coefficients area= –2.0(5),a=8.2(14),a= 7.6(11) anda= 16.7(25) MK-1, respectively,linear fits using the PASCal software[22]. Furthermoer, Fig. 4(b) presents different direction thermal expan- sions of 1 based on the PASCal program calculation. Notably, the NTE magnitude of 1 is slightly smaller than the dense rare earth formate framework [NH2CHNH2][Er(HCOO)4] (b= ?7.1(3) MK?1)[23], the most well-known NTE material cubic zirconium tungstate, ZrW2O8(= ?9 MK?1)[24]and rigid MOF-5 (= ?9 MK?1)[25], but significantly lower than other protypical NTE MOFs, such as flexible silver(I) 2-methylimidazolate (α= ?24.5 MK?1)[26], [Zn2(fu-L)2dabco][27]and InD(BDC)2(BDC = 1,4-benzenedicarboxylate,α= ?3.5 MK?1)[28].

Fig. 4. (a) Evolution of lattice parameters,andof 1 dependent on temperature (the solid lines there are to guide the eye); (b)Different direction thermal expansions of 1 based on the PASCal program calculation. Note: the positive and negative thermal expansions are illustrated in red and blue grids, respectively

Overall, the NTE alongaxis could be explained by the following reasons. First of all, with the temperatue increase, the bond lengths of formateand oxalate ligands expand along the [010] and [001] directions, respectively (see Fig. 4(b)). Such varia- tions are more likely to induce the flexing ofEuO9polyhedra.However, as stated previously, these EuO9polyhedrasshare their faces to construct 1-D infinite zigzag chains along the [100] direction (see Fig. 4(a), (b)). The flexing and the stronger interaction of the polyhedral chains maybe restrict the expanding along [100], thus giving rise to NTE along the [100] direction.

4 CONCLUSION

In this work, a 3-D hybrid framework 1 wassuccessfully prepared through hydrothermal method and characterized by SC-PXRD. The photolumi- nescence experiment shows that 1 gave a highluminescence at 614 nm, which originated from the5D0→7F2transition of Eu3+ion, implying red emis- sion output of 1. Furthermore, VT-PXRD studyrevealed that1 shows a slight NTE along the-axis and PTE along the- and- axes, which indicates this material’s anisotropic nature in response to the temperature variation. To conclude, 1 could have potential applications in solid state lighting and field emission displays. emission displays.

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20 April 2017;

5 July 2017 (CCDC 1553856)

10.14102/j.cnki.0254-5861.2011-1683

① This work was supported by the National Natural Science Foundation of China (21571072)

②Prof. Lu Pei-Xiang. E-mail: lupeixiang@hust.edu.cn; Dr. Feng Guo-Qiang. E-mail: gqfeng627@hust.edu.cn