DING Yi-Fan LI Fei YANG Jin-Xia QIN Ye-Yan YAO Yuan-Gen②

a (College of Chemistry, Fuzhou University, Fuzhou 350116, China)

b (Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

ABSTRACT A new rare earth organic complex with a double interleaved structure, namely[Sm(BDC)1.5(DMF)(H2O)]n (H2BDC = 1,4-benzenedicarboxylic acid, DMF = N,N΄-dimethyl formamide) was synthesized. The crystal structure is of triclinic, space groupwith a = 8.6343(6), b = 10.1470(5), c = 11.2073(6), α = 65.495(5), β = 71.626(5), γ = 78.130(5)°, V = 844.70(9), C15H15NO8Sm, Mr = 487.64, Z = 2, Dc = 1.917 g/cm3, F(000) = 476, μ = 3.519 mm-1, R = 0.0380 and wR = 0.0864 for 3504 observed reflections (I > 2σ(I)). The structure has been determined by single-crystal X-ray diffraction analyses and displays a 2-fold interpenetrated 3D network with the classical pcu topology. The compound was analyzed by X-ray powder, infrared spectroscopy,thermogravimetric analysis and fluorescent spectroscopy. The fluorescent property makes it a good candidate for photoactive materials.

Keywords: samarium, fluorescent, earth organic compound; DOI: 10.14102/j.cnki.0254-5861.2011-3076

1 INTRODUCTION

Metal-organic framework materials have become a hot spot in the field of materials chemistry due to their good structural tailorability and easy functionalization[1-3]. At present, people have been able to use the crystal engineering of coordination polymers to control the structures of coordination polymers to a certain extent,and at the same time, they can also select functional central metal ions and organic ligands with functional groups to give the target coordination polymers. With functions such as optical, electrical, magnetic, handshaped resolution, catalysis, ferroelectricity, and secondorder nonlinear optical properties, aromatic polycarboxylate acids have been widely studied in the coordination field due to the diversity of their carboxyl group coordination[4-11]. Rigid carboxylic acid ligands containing oxygen atoms are one of the most common ligands for preparing functional metal compounds.Among them, terephthalic acid has two 180° carboxyl groups and one conjugated benzene ring. It can build a very characteristic structure with transition metals or rare earths[12-14].

The excitation and luminescence of rare earth elements are caused by the transition of rare earth 4f electrons between different energy levels. There are f-f transitions between different energy levels in the f configuration, and f-d transitions between the f and d configurations. The energy difference between the excited and ground states of 4f electrons in different rare earth ions is different, so their luminescence properties are very different[15-19]. In recent years, the rare earth complexes of dibasic aromatic acid have been continuously synthesized. Its luminous performance has also been studied in depth. With rare earth elements and terephthalic acid as the basic building units, people use solvothermal synthesis, reverse microemulsion method,etc. successively obtained a series of coordination polymers with the same crystal structure[20-26]. The fluorescence intensity of the Sm3+ions is relatively weak,and there is not much research on it. The reason is that no suitable ligands for Sm3+compoundes to emit light have yet been developed. However, the red light emitted by the complex with samarium metal as the core ion is the best match with the spectral properties required for plant photosynthesis. Because the characteristic coordination atom of rare earth ion is oxygen, it can react with many oxygen-containing ligands.

Based on above consideration, this paper utilized Sm3+as the central ion and terephthalic acid as the rigid ligand to synthesize a novel rare earth organic compound, namely[Sm(BDC)1.5(DMF)(H2O)]n(H2BDC = 1,4-benzenedicarboxylic acid, DMF = N,N′-dimethylformamide). Furthermore,luminescent property of this complex has also been investigated in detail.

2 EXPERIMENTAL

2. 1 Materials and methods

Common reagents were purchased from general commercial channels and used without further purification. All compounds were synthesized under solvothermal conditions. Elemental analyses (C, H, N) were measured on a Vario El-Cube elemental analyser. The powder X-ray diffraction (PXRD) data were collected on a Rigaku MiniFlex II diffractometer by CuKα radiation.Simulated PXRD patterns were derived from the Mercury Version 3.10.2 software using the X-ray single crystal diffraction data. Thermo-gravimetric analyses(TGA) were carried out on a NETSCHZ STA-449C thermal analyzer under a nitrogen atmosphere and at a rate of 10 °C·min-1, starting from room temperature.Emission, excitation spectra were measured on an Edinburgh FLS980 fluorescence spectrophotometer in the solid state.

2. 2 Synthesis of [Sm(BCD)1.5(DMF)(H2O)]n (1)

Reactions of Sm(NO3)3·6H2O (0.2 mol ), H2BDC (0.2 mol)in the DMF/H2O media (15 mL v/v = 2:1) under ultrasonic treatment 30 min. Then, the resultant colorless solution was reacted in a 100 °C oven for 3days to give colorless crystals of 1 in 30% yield (based on Sm(NO3)3·6H2O). Elemental analyses calcd. for C15H15NO8Sm (291.04): C, 36.95; H, 3.10;N, 2.87%. Found: C, 37.32; H, 3.1; N, 2.35%.

2. 3 Single crystal X-ray diffraction determination

X-ray diffraction data were collected on a Rigaku Oxford SuperNova Single Source diffractometer with an Eos detector and a MoKα radiation radiation (λ =0.71073 Å). CrysAlisPro Agilent Technologies software was used for collecting the frames of data, indexing the reflections, determining the lattice constants, absorption correction, and data reduction[27]. The structures were solved by the direct methods, successive Fourier difference syntheses, and refined by the full-matrix least-squares method on F2(SHELXTL-2014)[28]. All non-hydrogen atoms are refined with anisotropic thermal parameters. Hydrogen atoms bonded to the carbon atoms were assigned to calculated positions. The water and hydroxyl hydrogen atoms could not be located. The selected bond lengths and angles of 1 are listed in are listed in Table 1.

Table 1. Selected Bond Lengths and Bond Angles for 1

3 RESULTS AND DISCUSSION

3. 1 Synthesis and characterization

It can be seen from Fig.1a, the smallest asymmetric unit of complex 1 contains one Sm3+cation, three half BDC2-ligands, one coordinated water molecule, and one coordinated DMF molecule. The Sm3+center is ninecoordinated in a twisted double-hat pentagonal biconical geometry by seven O atoms from four BDC2-ligands,one O atom from DMF, and one O atom from water molecule, and presents a twisted double-hat pentagonal biconical geometry. For convenience, the BDC2-ligands containing O(1), O(3), and O(5) are designated as BDC-1, BDC-2, and BDC-3, respectively. The Sm-O bond lengths of Sm coordinated with the oxygen atoms are between 2.389(4)～2.521(3) Å, and these M-O bond lengths are all within the normal Sm-O bond length range[16].

As shown in Fig.1b, both BDC-1 and BDC-2 ligands adopt a μ2-η1:η1:η1:η1chelating mode and the BDC-3 ligands are in a μ4-η1:η2:η1:η2chelating-bridging fashion.

Two adjacent Sm3+cations are linked by two bridging carboxylate groups to yield a dinuclear [Sm2(CO2)2]cluster with a Sm···Sm separation of 4.33 Å. Each[Sm2(CO2)2] cluster links to its neighbouring clusters by the BDC-2 and BDC-3 ligands, resulting in a 2D layer(Fig.1b). Furthermore, these 2D layers are further pillared by the BDC-1 ligands to form a 3D pillar-layered framework (Fig.1c). From the topological view, the [Sm2(CO2)2] cluster can be simplified as a six-connected node (Fig.1d), the BDC2-ligands are taken as linkers, and the 3D structure can be classified as a classical pcu architecture (Fig.1e). Along the b axis,the large hexagonal channels within a single 3D network allow for the interpenetration of an identical net, which directly leads to the formation of a 2-fold interpenetrating final framework (Fig.1f). In addition, the DMF and water molecules coordinate the Sm3+cations to finish the coordination sphere of the metal ions and give a more stabilized 3D structure (Fig.2).

Fig.1. (a) Coordination environment of Sm1 in complex 1 (symmetry codes: a = 1-x, -y, 2-z; b = 2-x, -y, 1-z;c = -x, 1-y, 1-z; d = 1-x, -1-y, 2-z). (b) The 2D sheet formed by [Sm2(CO2)2] clusters and BDC ligands. framework of 1.(c) View of the single 3D open (d) Perspective view of the six-connected node in 1. (e) Schematic view of the pcu topology.(f) Schematic representation of the 2-fold interpenetrating networks

Fig.2. 2-fold interpenetrated 3D framework with water molecules and DMF filled in the channels in 1 (The 2-fold interpenetrated networks are presented in different colors for clarity)

3. 2 XRD and TG analysis

In order to confirm the phase purity of complex 1, the powder X-ray diffraction (PXRD) experiments were carried out. From the Fig.3, it can be seen that the X-ray diffraction peaks of the complex obtained in the experiment are basically consistent with the simulated spectrum, which indicates that the purity of complex 1 is high and the related characterization is reliable.

As can be seen from Fig.4, complex 1 can exist stably in the air and maintain a stable crystal state at room temperature which makes it potential candidate for practical applications. In order to explore the thermal stability of complex 1, we conducted thermo-gravimetric analysis in the temperature range of 30～800 °C. The initial mass loss of complex 1 is 141～252oC. This weight loss process is attributed to the departure of coordinated water molecules and DMF (theoretical weight loss is 18.1%,actual weight loss is 18.5%). Thereafter, a stable platform was maintained between 252～552oC, which indicated that the framework of the complex was stable at this temperature.The second weight loss occurs in the range of 552～667oC,corresponding to the decomposition of BDC ligands. The entire framework of the complex collapses, remaining participants Sm2O3.

Fig.3. Experimental and simulated XRD pattern of complex 1

Fig.4. TGA curve of complex 1

3. 3 Solid state fluorescence performance

Luminescent rare earth organic frameworks have received great attention due to their potential applications in chemical sensors, photochemistry and rare earth light transfer agents.In this work, we studied the solid-state luminescence of ligands and complex 1 at room temperature. Performance, as a result, the complex shows strong fluorescence. The solid-state excitation and emission spectra of H2BDC ligand at room temperature. H2BDC is excited at 332 nm. H2BDC shows a broad band peak at 385 nm, which may be due to π*→π or π*→n electronic transition (LLCT)[29]. Complex 1 solid state excitation and emission spectra were performed at room temperature. The excitation spectrum is obtained by monitoring the strongest emission wavelength of Sm3+at 643 nm. The excitation spectrum has narrow and sharp excitation peaks in several other places, which are attributed to the characteristic excitation spectrum of Sm3+ions. The indirect excitation of the ligand in complex 1 is dominant, indicating that the sensitization of Sm3+luminescence is mainly through the indirect energy transfer process from the ligand to the Sm3+ion. When excited at 297 nm, the emission spectrum of complex 1 shows the characteristic spectral lines of Sm3+,showing characteristic emission bands of4G5/2to6H5/2,4G5/2to6H7/2,4G5/2to6H9/2,4G5/2to6H11/2at 560, 595, 640 nm and 702 nm, respectively. In addition to the characteristic emission band of Sm3+ion, complex 1 also shows the emission band in the blue region, which is due to the energy conversion within the ligand, which indicates that the energy conversion from the ligand to Sm3+is not very effective.(Fig.5) It can be seen from Fig.5 that the ligand is in the orange region and the compound is in the blue region.

Obviously, the ligand has no effect on the luminescence of the complex 1. The complex has fluorescence emission peaks around 560, 595, 641 and 703 nm, which correspond to the4G5/2to6H5/2,4G5/2to6H7/2,4G5/2to6H9/2,4G5/2to6H11/2transitions of Sm3+, respectively. The emission spectrum of the complex is the characteristic spectrum of Sm(Ⅲ)perturbed by the ligand.

Fig.5. Emission spectrum of H2BCD at λex= 332 nm and Emission spectrum of 1 at λex = 297 nm. Inset: photograph of its CIE color coordinates

4 CONCLUSION

This paper uses terephthalic acid as the ligand and cheap rare earth Sm as the central metal to synthesize a doubleinterleaved 3D structure, and analyze its structure and performance. Furthermore, the photoluminescence measurements reveal that complex 1 is a good candidate for photoactive materials, owing to their strong luminescent emissions.