Introduction

The metal organic coordination polymers synthesized with optically active trivalent rare earth cations such as Sm(III), Eu(III), Tb(III), Dy(III) and aromatic multicarboxylic acids have received much attention due to their unique luminescent properties and potential applications in photoluminescent materials [1–4]. Especially for the Eu(III) and Tb(III) complexes, they have excellent photoluminescent properties in visible region when the organic ligands are proper. In recent years, the organic multicarboxylic acids containing phenyl group have been widely used as the organic linkers in the synthesis of coordination polymers because they have two or more carboxyl groups and are inclined to forming multidimensional structure [5–8]. The pyridinedi- and tri-carboxylic acids are also important organic ligands for constructing coordination polymers as the nitrogen atom can also coordinate with center metal ions besides the oxygen atoms of carboxyl groups. Much attention has been paid to this field [9–11]. Some complexes synthesized have good luminescent properties and attractive topological structure [12–15].

In this paper, we used the solution of terbium chloride or dysprosium chloride and 2,4-pyridinedicarboxylic acid as the reactants and synthesized two new lanthanide complexes under hydrothermal condition. Parts of the ligands were decomposed and oxalate anions were formed. The oxalate anions also coordinated with rare earth ions. Two complexes of Tb(III) and Dy(III) with two ligands were got accidentally. The photoluminescent properties were studied and the results showed that both the complexes emit strong characteristic fluorescence of Tb(IIII) or Dy(III) ion in visible region at the excitation of ultraviolet rays.

Experimental Section

Synthesis of the Ligand

2,4-pyridinedicarboxylic acid was synthesized by oxidization of 2,4-dimethylpyridine with potassium permanganate. Terbium chloride or dysprosium chloride solution was obtained by reacting hydrochloric acid with Tb4O7 (≥ 99.95 %) or Dy2O3 (≥ 99.99 %) purchased from Grirem Advanced Materials Co. Ltd. Beijing.

Preparation of {[RE(pda)(ox)0.5(H2O)4]·2H2O}n (RE = Tb (1) or Dy (2))

A mixture of 2,4-pyridinedicarboxylic acid (0.3 mmol), TbCl3 or DyCl3 water solution 5 mL (0.040 mol·L−1) and deionized water (10 mL) were placed in a 25 mL Teflon-lined bomb, which were heated to 180 °C for 72 h. After the mixture was cooled to room temperature at a rate of 3 °C/h, colorless block crystals were obtained in yields of 43 % for 1 and 48 % for 2. Elemental Anal. Calcd. for C8H15NO12Tb (%): C, 20.18; H, 3.18; N, 2.94. Found: C, 20.36; H, 3.45; N, 3.19. For C8H15NO12Dy (%): C, 20.03; H, 3.15; N, 2.92. Found: C, 20.09; H, 2.83; N, 3.07.

General Characterization

Elemental analyses were performed in a Perkin-Elmer 240 analyzer. The fluorescence spectrum was tested on a 970CRT fluorescence spectrophotometer at room temperature.

X-ray Diffraction Analysis

The crystallographic data of the complexes were collected with an Oxford Supernova X-ray single crystal diffractometer. The diffractometer is equipped with a graphite monochromator, and Mo Kα radiation (λ = 0.71073 Å) was used for experiment. The structure was solved by direct methods with SHELXTL-97 programs [16] and refined by full-matrix least-squares techniques against F 2. All non-hydrogen atoms were refined anisotropically and hydrogen atoms located and refined isotropically. The crystallographic data and structure refinement parameters for the complex were given in Table 1 and the selected bond lengths and bond angles were listed in Supporting information.

Table 1 Crystallographic data for complexes 1 and 2
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Crystallographic data for the two complexes have been deposited at the Cambridge Crystallographic data centre, CCDC Numbers are 947588 for 1 and 947589 for 2. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB21EZ, U. K.; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk).

Results and Discussion

Structure Description

Complexes 1 and 2 are isostructural and 1 is taken as an example to describe the structure in detail, as shown in Fig. 1. Each Tb(III) ion was coordinated by two pda anions, one ox anion and four water molecules. One pda anion connects to Tb(III) ion with N(1) atom and O(1) atom of 2-carboxyl group by chelating way, the other connects to Tb(III) ion with O(2) atom of 2-carboxyl group by monodentate way. Each ox anion connects two Tb(III) ions with oxygen atoms by chelating way. O(5) and O(6A) atoms chelate one Tb(III) ion and O(6) and O(5A) atoms chelate the other one. The four water molecules coordinated with Tb(III) ion by oxygen atoms of O(7), O(8), O(9) and O(10). The 4-carboxyl group of pda anion does not participate in coordination. There are nine atoms bonded with one Tb(III) ion and they form a distorted tricapped trigonal prism, as shown in Fig. 2. The bond lengths of Tb(11)-O(1), Tb(11)-O(2), Tb(11)-O(5), Tb(11)-O(6A), Tb(11)-O(7A), Tb(11)-O(8), Tb(11)-O(9) and Tb(11)-O(10) are 2.333(2), 2.455(3), 2.531(2), 2.436(2), 2.448(3), 2.384(3), 2.360(3) and 2.435(3) Å, respectively; and the Tb(11)-N(1) bond length is 2.684(3) Å. The bond angles of O-Tb(11)-O range between 64.62(8)º and 143.80(9)º and that of O-Tb(11)-N range between 62.22(8)º and 144.30(9)º. By the connection of Tb(III) ions, pda and ox anions, 2D layered framework is formed, as shown in Fig. 3. The 2D layers formed 3D network by the connection of hydrogen bonds. These hydrogen bonds are formed between lattice water and coordinating water molecules like O(7)-H(7A)···O(11), O(8)-H(8B)···O(11), O(10)-H(10A)···O(12) and O(12)-H(12A)···O(8); lattice water molecules and oxygen atoms of ox anions like O(11)-H(11B)···O(6) and O(12)-H(12B)···O(5); coordinating water molecules and oxygen atoms of pda anions like O(8)-H(8A)···O(3), O(9)-H(9A)···O(3) and O(9)-H(9B)···O(4), as shown in Fig. 4.

Fig. 1
figure 1

The ORTEP representation of 1 showing the coordination environment of Tb(III) ions with 30 % probability of thermal ellipsoids. All the hydrogen atoms were omitted for clarity

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Fig. 2
figure 2

The distorted tricapped trigonal prism formed by the coordination atoms (Oxygen atoms, red; nitrogen atom, blue; Tb(III) ion, green)

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Fig. 3
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The two dimensional structure formed with Tb(III) ions and ligands of pda and ox anions

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Fig. 4
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The three dimensional sturcture constructed by hydrogen bonds

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Photoluminescent Property

The fluorescence spectra for the two complexes were determined with solid samples at room temperature. The strongest peak was at about 305 nm in the excitation spectrum for 1, as shown in Fig. 5. The emission spectra was shown in Fig. 6. Complex 1 displayed four emission bands at 491, 546, 584 and 625 nm with excitation of 300 nm, and they were ascribed to 5D4→7Fj (j = 6, 5, 4 and 3) transition of Tb(III) ion [17–19]. The excitation spectrum of 2 is similar to that of 1 except the intensity of peak, as shown in Fig. 7. Emission spectrum of 2 was determined with excitation of 315 nm, as shown in Fig. 8. Two Emission bands at 482 and 575 nm were assigned to the transition of 4F9/2→6H15/2 and 4F9/2→6H13/2 of Dy(III) ion [20–22]. The strong luminescence of the two complexes should be attributed to high effective energy transfer from pda anions to Tb(III) or Dy(III) ions [23, 24]. The energy transfer mechanism for solid lanthanide complexes has been widely discussed to interpret the luminescent properties [25–27]. Efficient energy transfer occured if the triplet-state energy levels of the ligands matched with the excited states energy levels of lanthanide ions. The results showed the 5D4 excited state energy level of Tb(III) ion and 4F9/2 excited state energy state level of Dy(III) ion mateded with the triplet-state energy levels of pda anions.

Fig. 5
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Excitation spectrum of complex 1

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Fig. 6
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Emission spectrum of complex 1 with excitation at 300 nm

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Fig. 7
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Excitation spectrum of complex 2

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Fig. 8
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Emission spectrum of complex 2 with excitation at 315 nm

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In summary, we have synthesized Tb(III) and Dy(III) coordination compound by using 2,4-pyridinedicarboxylate and oxalate anions as the linker. The two complexes formed two-dimensional layer structure and are isomorphic. They both emit strong characteristic fluorescence of Tb(III) or Dy(III) ions with the excitation of UV-rays.