DOI:
10.1039/C4RA02953D
(Paper)
RSC Adv., 2014,
4, 27013-27021
Four 2D Ln–Cd heterometal–organic coordination polymers based on tetranuclear Ln–Cd oxo-cluster with highly selective luminescent sensing of organic molecules and metal cations†
Received
3rd April 2014
, Accepted 28th May 2014
First published on 29th May 2014
Abstract
Two series of novel lanthanide–cadmium (Ln–Cd) heterometal–organic coordination polymers, i.e., [Ln2Cd2(DTPA)2(H2O)4] (H2O)4 (I: Ln = Eu 1a; Gd 1b) and [Ln2Cd3(DTPA)2Cl2(H2O)6] (II: Ln = Eu 2a; Sm 2b) (H5DTPA = diethylenetriamine pentaacetic acid), have been synthesized using a hydrothermal method. Compounds I and II both possess 2D grid-layer structure based on distinct tetranuclear Ln–Cd oxo-cluster units as secondary building units (SBUs). Two enantiomeric forms, i.e., α and β, exist in [Cd(DTPA)]3− in compounds I. Interestingly, compound 1a displays not only a highly sensitive sensing of small molecular organic solvents, especially methanol, triethylamine and ethanol amine, but also the sensing of cations, especially Cr3+ and Fe3+ ions. Moreover, Cr3+, Fe3+, Co2+, Ni3+, Cu2+, Nd3+ cations quenched the emissions of the Eu3+ ions of compound 2a, which indicates the potential of 1a and 2a for sensing of metal cations.
Introduction
Lanthanide–organic coordination polymers are attracting great interest because of their unique luminescent and magnetic properties that are derived from the electronic configurations [Xe]4fn (n = 0–14) of lanthanide elements.1 Varieties of lanthanide–organic coordination polymers with fascinating structural diversities and potential applications have been synthesized and researched. In order to enrich the structures and promote the properties of lanthanide–organic coordination polymers, heterometallic ions, especially d-block ions, were introduced into these compounds.1f–h,2 The introduced heterometallic ions not only make the structures more plentiful but also cause the energy levels of these materials to be more controllable.3 To the best of our knowledge, the vast majority of these lanthanide-transition(f–d) heterometal–organic coordination polymers are focused on Ln–M (M = Cu,2d,2h,4 Co,2j,5 Ni,2j,5b,6 Fe,2j,5f,7 Ag,8 Mn2b,9), and these transitional metal ions contain interesting magnetic properties. Relatively few studies are focused on the luminescent properties of the compounds that we are interested in. Most lanthanide ions emit line-like and strong lights under stimulus. However, we all know that the f–f transitions of lanthanide ions (Ln3+ ions) are parity-forbidden, which results in very low absorption coefficients. Organic ligands with intense absorption properties are coordinated to lanthanide ions as energy donors to induce characteristic emissions of lanthanide ions; i.e., the energy is first absorbed by the organic ligands, and then it will be sent to lanthanide ions by intramolecular energy transfer, which induces emissions of lanthanide ions. This method is known as the antenna effect.10 There has been significant interest in development of luminescent lanthanide–organic coordination polymers for applications such as optical materials3a and organic light-emitting diodes (OLEDs).11 The application of luminescent sensors is a promising research field.1d,12 The intensities of the emission spectra of lanthanide ions could be enhanced or quenched by some organic molecules such as acetone and acetylacetone or cations such as Co2+ and Cu2+. Thus, lanthanide–organic coordination polymers are potential candidate materials for chemical sensing.
In contrast to other lanthanide-transition(f–d) heterometal–organic coordination polymers, the studies of lanthanide–cadmium (Ln–Cd) heterometal–organic coordination polymers are very few. Cd2+ ion itself not only emits luminescence by ligand–metal interactions, but also acts as a bridge of energy transfer between organic ligands and Ln3+ ions. Because of their novel luminescent properties, Ln–Cd heterometal–organic coordination polymers will be useful tools as chemical sensors. To the best of our knowledge, the reported Ln–Cd heterometal–organic coordination polymers are mainly zero-dimensional (0D),1c,2j,13 one-dimensional (1D)14 and three-dimensional (3D)1d,15 coordination structures. Our group reported the first 2D Ln–Cd heterometal-layered structure.12e According to the soft-hard acid-base theory, Ln3+ ions belong to hard acids and have strong affinities to coordinate to hard bases like the O atom, whereas the Cd2+ ion is a soft acid and prefers to bond to soft bases such as S and N atoms. The competitions between Ln3+ and Cd2+ in the reaction system make it difficult to synthesize Ln–Cd heterometal–organic coordination polymers. Thus, the ligands with N- and O-donors can be elaborately selected and employed to make heterometal–organic coordination polymers. Diethylenetriamine pentaacetic acid (H5DTPA) with N- and O-donors has been used to prepare metal–organic coordination polymers with single transition metal (TM) or Ln ions;16 however, this assembly in the Ln–TM–organic coordination polymers remains unexplored. We selected H5DTPA as a candidate for assembling heterometal polymers based on the following considerations: (1) it has five carboxylate groups as hard bases and three coordinated N donors as soft bases, which makes it possible to bond to both Ln3+ and Cd2+ ions simultaneously in the reaction system. (2) It is a chelating agent, which has strong affinities with the Ln3+ and Cd2+ ions. (3) The carboxylic acid groups and N donors of the ligand can adjust the pH value of the reaction system.
On the basis of the aforementioned points, our aim is to prepare novel Ln–Cd heterometal–organic coordination polymers with luminescent sensing. Herein, we report the syntheses, structures and luminescent sensing of two series of 2D Ln–Cd heterometal–organic coordination polymers: [Ln2Cd2(DTPA)2(H2O)4] (H2O)4 (I: Ln = Eu 1a; Gd 1b) and [Ln2Cd3(DTPA)2Cl2(H2O)6] (II: Ln = Eu 2a; Sm 2b), which are 2D layers based on different tetranuclear [Ln2Cd2] oxo-clusters with highly selective luminescent sensing.
Experimental section
Materials and physical measurements
Commercially available solvents and chemicals were used without further purification. The elemental analyses for C, H and N were performed with an Elementar Vario EL III elemental analyzer. IR spectra were measured as KBr pellets on a Perkin-Elmer Spectrum 2000 FT-IR in the range of 400–4000 cm−1. Thermogravimetric data were collected on a Mettler Toledo TGA/SDTA 851e analyzer in flowing nitrogen at a heating rate of 10 °C min−1. Luminescence measurements were conducted with an Edinburgh Instrument FS920 TCSPC luminescence spectrometer on the powder crystal material of the compounds. The 1a solution was prepared by introducing 1a powder (5 mg) into methanol, acetonitrile, acetone, n-butanol, ethanol, ammonia water, H2O, DMF, iso-propanol, triethylamine, pyridine, ethanediamine, 1,2-propane diamine, and ethanol amine (5.00 mL) at room temperature. Similarly, the powder of 1a and 2a was immersed in an aqueous solution containing different M(NO3)x (M = Ca2+, Sr2+, Li+, K+, Na+, Mg2+, La3+, Ho3+, Ce3+, Tb3+, Yb3+, Zn2+, Cd2+, Pb2+, Al3+, Sm3+, Gd3+, Nd3+, Pr3+, Dy3+, Cu2+, Co2+, Cr3+, Fe3+, Ni2+ and Er3+) and different concentrations of Co(NO3)2. Samples were prepared by introducing powder (5 mg) of 1a and 2a into an aqueous solution (5.00 mL) at room temperature. Before luminescence measurements, the suspensions were sonicated for 10 minutes using ultrasonic waves to ensure uniform dispersion.
Synthesis of compounds
[Ln2Cd2(DTPA)2(H2O)4](H2O)4 (I: Ln = Eu 1a; Gd 1b). A mixture of H5DTPA (0.3931 g, 0.999 mmol), Cd(Ac)2·2H2O (0.5330 g, 2.000 mmol), Ln2O3 (Eu2O3, 0.1768 g, 0.502 mmol for 1a; Gd2O3, 0.1846 g, 0.509 mmol for 1b), and H2O (10 mL) was placed in 23 mL Teflon-lined stainless steel vessels, heated to 170 °C for 7 days, and then cooled to room temperature. Colorless block crystals were obtained. Yield: 0.6390 g (88% based on Eu(III)) for 1a, 0.5173 g (69.5% based on Gd(III)) for 1b. Anal. calc. for C28N6O28H52Cd2Eu2 1a: C, 23.18%; H, 3.59%; N, 5.80%. Found: C, 23.20%; H, 3.57%; N, 5.95%; for C28N6O28H52Cd2Gd2 1b: C, 23.01%; H, 3.56%; N, 5.75%. Found: C, 23.15%; H, 3.65%; N, 5.60%. Selected IR peaks (cm−1): 3190(br), 1595(s), 1442(m), 1403(m), 1321(m), 1243(m), 1100(s), 989(m), 924(s), 849(m), 745(m), 719(m), 643(m), 595(w), 520(w), 448(w) (Fig. S1†).
[Ln2Cd3(DTPA)2Cl2(H2O)6] (II: Ln = Eu 2a; Sm 2b). A mixture of H5DTPA (0.3939 g, 1.001 mmol), Cd(Ac)2·2H2O (0.2670 g, 1.002 mmol), Ln2O3 (Eu2O3, 0.3510 g, 0.997 mmol for 2a; Sm2O3 0.3475 g, 0.997 mmol for 2b), and H2O (10 mL) was placed in a 23 mL Teflon-lined stainless steel vessel. The pH was adjusted to 2 by 0.1 M HCl, and then the solution was heated to 170 °C for 7 days, and then cooled to room temperature. Colorless block crystals were obtained. Yield: 0.1665 g (10.5% based on Eu(III)) for 2a, 0.1187 g (7.5% based on Sm(III)) for 2b. Anal. calc. for C28N6O26H48Cd3Eu2 2a: C, 21.04%; H, 3.01%; N, 5.26%. Found: C, 21.20%; H, 3.17%; N, 5.15%; for C28N6O26H48Cd3Sm2 2b: C, 21.09%; H, 3.01%; N, 5.27%. Found: C, 21.15%; H, 3.15%; N, 5.40%. Selected IR peaks (cm−1): 3295(s), 1580(s), 1445(m), 1406(s), 1325(m), 1270(m), 1083(s), 996(w), 937(s), 865(w), 729(m), 647(m), 582(w), 500(w), 445(w) (Fig. S1†).
X-ray crystallography
Suitable single crystals were selected and mounted on a glass fiber. All data of 1a, 1b and 2b were obtained on a Rigaku R-AXIS RAPID diffractometer with graphite-monochromated MoKα (λ = 0.71073 Å) radiation in the ω scanning mode at room temperature. Moreover, all data of 2a were obtained on a Rigaku Saturn 724 CCD diffractometer with graphite-monochromated MoKα (λ = 0.71073 Å) radiation in the ω scanning mode at room temperature. The structures were solved by direct methods and refined by the full-matrix least-squares method on F2 using the SHELXTL-97 program package.17 Some free water molecules were refined isotropically. Hydrogen atoms bonding O and C were generated geometrically (C–H = 0.93 Å, O–H = 0.85 Å) and refined with fixed isotropic displacement parameters. Note that hydrogen atoms of some free water molecules were not generated. The selected crystal parameters, data collection, and refinements are summarized in Table 1.
Table 1 Crystal data and structural refinement parameters for 1a, 1b, 2a and 2b
|
1a |
1b |
2a |
2b |
R1 = ∑||F0| − |Fc||/∑|F0|. wR2 = {∑[w(F02 − Fc)2]/∑[w(F02)2]}1/2. |
Formula |
C28N6O28H52Cd2Eu2 |
C28N6O28H52Cd2Gd2 |
C28N6O26H48Cl2Cd3Eu2 |
C28N6O26H48Cl2Cd3Sm2 |
fw |
1449.48 |
1460.06 |
1596.74 |
1593.52 |
Crystal system |
Triclinic |
Triclinic |
Triclinic |
Triclinic |
Space group |
P![[1 with combining macron]](https://www.rsc.org/images/entities/char_0031_0304.gif) |
P![[1 with combining macron]](https://www.rsc.org/images/entities/char_0031_0304.gif) |
P![[1 with combining macron]](https://www.rsc.org/images/entities/char_0031_0304.gif) |
P![[1 with combining macron]](https://www.rsc.org/images/entities/char_0031_0304.gif) |
a (Å) |
9.804(3) |
9.779(2) |
8.6545(17) |
8.6521(17) |
b (Å) |
10.912(4) |
10.868(2) |
9.0700(18) |
9.0758(18) |
c (Å) |
20.446(6) |
20.395(4) |
15.407(3) |
15.422(3) |
α (°) |
100.683(12) |
100.77(3) |
98.47(3) |
98.43(3) |
β (°) |
92.766(9) |
92.74(3) |
101.37(3) |
101.51(3) |
γ (°) |
91.801(11) |
91.79(3) |
103.33(3) |
103.40(3) |
V (Å3) |
2145.1(12) |
2125.2(7) |
1129.9(4) |
1130.5(4) |
Z |
2 |
2 |
1 |
1 |
Dc (g cm−3) |
2.244 |
2.282 |
2.347 |
2.341 |
Rint |
0.0317 |
0.0630 |
0.0310 |
0.0198 |
θ range/° |
2.99–25.00 |
2.99–27.47 |
3.19–27.49 |
3.13–27.47 |
F(000) |
1416 |
1420 |
770 |
768 |
GOOF on F2 |
1.01 |
0.95 |
1.11 |
1.22 |
R1a, wR2b (I > 2σ(I)) |
0.0269, 0.0666 |
0.0433, 0.1109 |
0.0298, 0.0661 |
0.0243, 0.0798 |
R1, wR2 (all data) |
0.0317, 0.0685 |
0.0630, 0.1237 |
0.0317, 0.0671 |
0.0277, 0.0864 |
Results and discussion
Description of crystal structures
As shown in Fig. S2,† the PXRD patterns of compounds I and II agree with those calculated from the structures, indicating that complexes 1a and 1b, 2a and 2b are isostructural.
[Ln2Cd2(DTPA)2(H2O)4](H2O)4 (I: Ln = Eu 1a; Gd 1b). According to single-crystal X-ray diffraction analyses and PXRD patterns, compounds 1a and 1b are isostructural, which both crystallize in triclinic space groups of P
. Thus, only the structure of 1a will be described in detail here. In the asymmetrical unit of 1a, there are two unique Cd2+ ions, two Eu3+ ions, two DTPA5− anions and four coordinated water molecules, respectively (Fig. 1). The coordination geometries of the two seven-coordinated Cd2+ ions can be viewed as slightly distorted pentagonal bipyramids: four OCOO– atoms and three N atoms from one DTPA5− anion (Fig. S3† and 2). Moreover, the two Eu3+ ions are both nine-coordinated with geometries of tricapped trigonal prisms: seven OCOO– atoms from four DTPA5− anions and two coordinated water molecules (Fig. S4†). Cd–O and Cd–N bond lengths are in the range of 2.344(3) Å–2.417(3) Å and 2.358(3) Å–2.461(3) Å, respectively. Moreover, Eu–O bond lengths are 2.392(3) Å–2.568(3) Å, which are comparable to the reported bond distances in a previous study.15 The two DTPA5− anions in the asymmetrical unit both chelate Cd2+ ions to form pentagonal bipyramids. Interestingly, in the [Cd(DTPA)]3− units, the absolute configurations around Cd2+ ion, have two enantiomeric forms, i.e. α and β (Fig. 2 and S5†). Two μ2-O atoms from one DTPA5− ligands bridge one Cd2+ and one Eu3+, thus forming an edge-sharing heterometal binuclear [EuCd(μ2-O)2] unit with an Eu⋯Cd distance of 3.985(7) Å. Two binuclear [EuCd (μ2-O)2] units are connected each other by μ2-O from DTPA5− ligands between Eu3+ and Cd2+ ions to generate a cradle-like tetranuclear [Eu2Cd2(μ2-O)6] cluster unit (Fig. 3a and b). Each tetranuclear [Eu2Cd2(μ2-O)6] cluster unit acts as a four-connected node and links four other [Eu2Cd2(μ2-O)6] cluster units by DTPA5− anions to form a (4,4) grid layer in the [101] plane (Fig. 3c). The 2-D layers stack in –AAA– mode along the a-axis, and free water molecules are located between the layers. Moreover, the O–H⋯O hydrogen bonds between free water molecules and OCOO– atoms of DTPA5− anions link the layers into a 3D supramolecular structure (Fig. 4a). From a topological point of view, the 3-D supramolecular framework of 1a is an eight-connected net (Fig. 4b). Because only [Eu2Cd2(μ2-O)6] units act as eight-connected nodes, the overall framework can be represented as a hex-topology with the point symbol of (36.418.53.6), and the extended point symbol is [3.3.3.3.3.3.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.52.52.52.66] (Fig. 4b).
 |
| Fig. 1 The coordination environments of Eu3+ and Cd2+ in 1a. Atoms having A, B or C in their labels are symmetrically generated. A: 1 − x, 1 − y, 1 − z; B: 1 − x, −y, 1 − z; C: −1 + x, y, 1 + z. Hydrogen atoms are omitted for clarity. Color codes: Eu, yellow; Cd, cyan; O, red; N, blue; C, grey. | |
 |
| Fig. 2 Two enantiomeric forms in [Cd(DTPA)]3− units of type I. (a) α, (b) β. | |
 |
| Fig. 3 (a) Ball-stick presentation of a cradle-like tetranuclear [Eu2Cd2(μ2-O)6] cluster unit. (b) Polyhedron presentation of tetranuclear [Eu2Cd2(μ2-O)6] in 1a. (c) The 2D grid layer of compound 1a viewed along the [201] direction. | |
 |
| Fig. 4 (a) The 3D supramolecular framework via hydrogen bonds of O–H⋯O for 1a viewed along b axis. The green dotted lines for hydrogen bonds of O–H⋯O. (b) The supramolecular net with hex-type topology after simplification. Key: orange, [Eu2Cd2(μ2-O)6] units, the purple and green lines represent DTPA5− anions and hydrogen bonds, respectively. | |
[Ln2Cd3(DTPA)2Cl2(H2O)6] (II: Ln = Eu 2a; Sm 2b). Compounds II were synthesized from the same raw material as I but with a different pH value. HCl solution was added to adjust pH = 2 in the syntheses of compounds II. Interestingly, in compounds II, Cl− anions are coordinated to Cd2+ ions; thus, the structures of II are different from I. Note that compounds 2a and 2b are isostructural and both belong to the triclinic space groups of P
. In the asymmetrical unit of 2a, there are one and a half unique Cd2+ ions, one Eu3+ ion, one DTPA5− anion, three coordinated water molecules and one Cl− anion (Fig. 5). The coordination geometries for Cd1 and Cd2 are both octahedrons (Fig. S6†). Cd1 lying in a symmetry inversion coordinates to two OCOO– atoms and four coordinated water molecules, whereas Cd2 coordinates to five OCOO– atoms from three DTPA5− anions and one Cl− anion. Cd–O bond lengths are in the range of 2.212(3) Å–2.256(3) Å, which is shorter than that in compound 1a. Cd–Cl bond length is 2.463(9) Å, and this is comparable to the reported Cd–Cl bond distances in a previous study.18 The Eu3+ ion in the structure is chelated by five OCOO– atoms, three N atoms from one DTPA5− anion, and one coordinated water molecule to form a tricapped trigonal prism (Fig. S7†). Eu–O and Eu–N bond lengths are in the range of 2.346(3) Å–2.450(3) Å and 2.615(3) Å–2.752(3) Å, respectively. The DTPA5− anion effectively cups the Eu3+ ion in a hydrophilic cleft, leaving the ninth coordination site available to coordinate water. Four carboxylate μ2-O atoms alternately connect Eu3+ ions and Cd(2) atoms to form a chair-like eight-membered tetranuclear [Eu2Cd2(μ2-O)4] cluster unit (Fig. 6a and b). The shortest Cd⋯Eu distance is 4.793(6) Å, which is longer than that in compound 1a. Each [Eu2Cd2(μ2-O)4] cluster unit is connected with two such neighboring cluster units by [Cd(1)O6] octahedrons, generating an interesting 1D necklace-like chain that runs along the [001] direction (Fig. 6). The interconnections of the Eu3+ and Cd2+ ions from two neighboring chains via carboxylate groups of DTPA5− ligands result in a 2-D grid layer in the bc-plane (Fig. 6c and S8†). Each [Eu2Cd2(μ2-O)4] cluster unit serves as a four-node, which links four other [Eu2Cd2(μ2-O)4] cluster units; thus, the 2D layer is a (4,4) net with sql-topology. The layers are connected by hydrogen bonds between coordinated water molecules and OCOO– atoms of DTPA5− anions to form a 3D supramolecular framework (Fig. 7a). From the topological point of view, the 3-D supramolecular framework of 2a is a (4,6)-connected net (Fig. 7b). [Eu2Cd2(μ2-O)4] units act as six-connected nodes and [Cd(1)O6] octahedra as four-connected nodes, and the overall framework can be represented as a fsc-topology with the point symbol of (44.610.8)(44.62), and the extended point symbol is [4.4.4.4.62.62.65.65.65.65.65.65.65.65.820] for [Eu2Cd2O4] units and [4.4.4.4.64.64] for [Cd(1)O6] octahedrons.
 |
| Fig. 5 The coordination environments of Eu3+ and Cd2+ in 2a. Atoms having A, B or C in their labels are symmetrically generated. A: −x, −y, 1 − z; B: −x, −y, −z; C: −x, 1 − y, −z. Hydrogen atoms are omitted for clarity. Color codes: Eu, yellow; Cd, cyan; O, red; N, blue; C, grey. | |
 |
| Fig. 6 (a) Ball-stick presentation of tetranuclear [Eu2Cd2(μ2-O)4] unit. (b) Polyhedron presentation of [Eu2Cd2(μ2-O)4] unit in 2a. (c) The 2D grid layer of compound 2a viewed along a-axis. Color codes: Eu, yellow; Cd1, purple; Cd2, cyan; O, red; N, blue; C, grey. | |
 |
| Fig. 7 (a) The 3D supramolecular framework via hydrogen bonds of O–H⋯O for 2a viewed along b axis. The green dotted lines are for hydrogen bonds of O–H⋯O. (b) The supramolecular net with fsc-type topology after simplification. Key: orange, [Eu2Cd2(μ2-O)4] units; cyan, [Cd(1)O6] units. The purple and green lines represent DTPA5− anions and hydrogen bonds, respectively. | |
IR spectra
As shown in Fig. S1,† the similarities between the IR spectra of 1a and 1b, 2a and 2b suggests that 1a and 1b, 2a and 2b are iso-structural, respectively. The broad bands at 3700–2800 cm−1 for 1a and 1b and those at 3700–2900 cm−1 for 2a and 2b correspond to the stretching bands of O–H of water molecules. The bands at 1595 cm−1 for 1a and 1b and those at 1580 cm−1 for 2a and 2b correspond to the stretching bands of C
O of COO−.15 The red shifts of the bands of C
O indicate the coordination bonds between the metal cations and carboxylate groups of DTPA5−.
Thermogravimetric analyses
The thermal stabilities of 1a and 1b, 2a and 2b were examined by the TGA analyses in a dry-air atmosphere (Fig. S9†). In the TGA curves of 1a and 1b, the weight losses of 9.30% (calcd: 9.93%) for 1a and 9.30% (calcd: 9.86%) for 1b were observed in the temperature range 30–310 °C, corresponding to the successive release of all coordinated and free water molecules. The decomposition of H5DTPA was observed from 310 °C to 800 °C. The residue obtained might be Eu2O3·2CdO (calcd/found: 42.00%/43.47%) for 1a or Gd2O3·2CdO (calcd/found: 42.42%/42.87%) for 1b. Similarly, in the TGA curves of 2a and 2b, the weight losses of 6.73% (calcd: 6.76%) for 2a and 6.33 (calcd: 6.78%) for 2b in the temperature range 50–255 °C can be assigned to the successive release of six coordinated water molecules. The decomposition of H5DTPA and the removal of HCl were observed from 255 °C to 800 °C. Thus, the residue obtained may be Eu2O3·3CdO (calcd/found: 46.17%/46.60%) for 2a or Sm2O3·3CdO (calcd/found: 46.06%/48.42%) for 2b (Fig. S9†).
Luminescent properties
Solid-state emission spectra. The room-temperature luminescent properties of the compounds 1a, 2a and 2b in the solid state were measured. Compounds 1a and 2a show characteristic emissions of the Eu3+ ion, in which bands at ∼580, ∼592, ∼617, ∼650, ∼700 nm correspond to 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, 5D0 → 7F4 transitions, respectively19a (Fig. 8a and b). The intensities of the 5D0 → 7F2 transitions (electric dipole) are stronger than that of the 5D0 → 7F1 transitions (magnetic dipole) of compounds 1a and 2a, which indicates the low symmetrical coordination environment of Eu3+ ions, and these results agree with the crystallographic analyses. In the emission spectra of compound 2b, when 2b is excited at 403 nm, it exhibits three bands at 563, 598 and 645 nm, corresponding to 4G5/2 → 6H5/2, 4G5/2 → 6H7/2 and 4G5/2 → 6H9/2 transitions of Sm3+ ions19b (Fig. 8c).
 |
| Fig. 8 Emission spectra of 1a (λex = 395 nm) (a), 2a (λex = 395 nm) (b), 2b (λex = 403 nm) (c). | |
Luminescent sensing
In order to investigate 1a for the sensing of small molecules, its suspension-state luminescences were obtained. The powder of 1a was dispersed into methanol, acetonitrile, acetone, n-butanol, ethanol, ammonia water, H2O, DMF, iso-propanol, triethylamine, pyridine, ethanediamine, 1,2-propane diamine and ethanol amine. As shown in Fig. 9 and S10,† its luminescent spectra are largely dependent on the solvent molecules. Methanol can significantly improve luminescent intensity of 1a with remarkable enhancement effects. As for acetonitrile, acetone, n-butanol, ethanol, ammonia water, H2O, DMF, and iso-propanol, the luminescence spectra do not show significant differences from each other. However, for solvents like triethylamine, pyridine, ethanediamine, 1,2-propane diamine, and ethanol amine, the emission spectra of 1a were covered by emissions of these nitrogenous compounds (Fig. S10†). Particularly, in triethylamine and ethanol amine, the emissions of Eu3+ exhibited complete quenching of the luminescent intensity. These results suggest that 1a could be a promising luminescent probe for methanol, triethylamine and ethanol amine. This may be due to the fact that these nitrogenous solvents absorb the excitation light of Eu3+ ions (λex = 395 nm) (the excitation spectra of Eu3+ ions fall in the UV-vis absorption spectra of these nitrogenous solvents). As for the complete quenching of triethylamine and ethanolamine, this may be attributed to the complete UV-vis absorption of the excitation light of the Eu3+ ions.12e
 |
| Fig. 9 (a) Emission spectra of 1a (λex = 395 nm) and (b) the 5D0 → 7F2 transition intensities of 1a dispersed into different solvents. | |
Similarly, the powder samples of 1a and 2a were dispersed into an aqueous solution containing different 0.1 M M(NO3)x (M = Ca2+, Sr2+, Li+, K+, Na+, Mg2+, La3+, Ho3+, Ce3+, Tb3+, Yb3+, Zn2+, Cd2+, Pb2+, Al3+, Sm3+, Gd3+, Nd3+, Pr3+, Dy3+, Cu2+, Co2+, Cr3+, Fe3+, Ni2+ and Er3+) and the emissions of Eu3+ ions were measured (Fig. 10 and S13†). For 1a, most cations show an enhancement effect of the luminescent intensities. However, for the Cr3+ and Fe3+ ions, particularly the Fe3+ ion, the intensity showed significant quenching effects. Moreover, the Cr3+, Fe3+, Co2+, Ni3+, Cu2+, Nd3+ ions quenched the emissions of the Eu3+ ion of 2a. As illustrated in Fig. S14,† the luminescent intensity of 2a is almost completely quenched in the solution of Co(NO3)2 at a concentration of 10−2 mol L−1. These results indicate the potential of 1a and 2a for the sensing of metal cations. We assumed that these coloured aqueous solution containing cations, such as Cr3+, Fe3+, Co2+, Ni3+, Cu2+, and Nd3+, had absorbed more or less the excitation light of Eu3+ ions (λex = 395 nm), which resulted in the fluorescence quenching.
 |
| Fig. 10 (a) Emission spectra of 1a (λex = 395 nm) and (b) the 5D0 → 7F2 transition intensities of 1a dispersed into different aqueous solutions containing different 0.1 M M(NO3)x. | |
Conclusions
Two series of Ln–Cd heterometal organic coordination polymers, i.e., Ln2Cd2(DTPA)2(H2O)4] (H2O)4 (Ln = Eu 1a; Gd 1b) and [Ln2Cd3(DTPA)2Cl2(H2O)6] (Ln = Eu 2a; Sm 2b), have been synthesized by the hydrothermal reactions of Ln2O3, Cd(Ac)2·2H2O and H5DTPA. Although the reaction temperatures and main reactants are the same, the different ratios of the reactants and the introduction of the Cl− anions changed the coordination environments of Ln3+ and Cd2+ ions, resulting in the formations of different tetranuclear [Ln2Cd2] oxo-cluster units. Compound 1a possesses 2D-layered grid networks based on [Eu2Cd2(μ2-O)6] cluster units. Two enantiomeric forms, i.e., α and β, exist in [Cd(DTPA)]3−. These 2D layers are linked via O–H⋯O hydrogen bonds to generate a 3D supramolecular architecture with a hex-type topology. In 2a, [Eu2Cd2(μ2-O)4] units are connected by DTPA3− and [Cd(1)O6] octahedra to form a 2D lattice layer, which are further held together by hydrogen bonds, resulting in a 3D supramolecular framework with fsc-topology net. Compounds 1a and 2a emit characteristic emissions of Eu3+ ion, and compound 2b emits those of the Sm3+ ion. More interestingly, organic molecules such as triethylamine and ethanol amine and cations such as Fe3+ ion quenched the emission of 1a, and Cr3+, Fe3+, Co2+, Ni3+, Cu2+, Nd3+ ions quenched that of 2a. Compounds 1a and 2a may be used as an important role in the applications of the luminescent sensing for the organic solvents and metal cations.
Acknowledgements
This work was supported by the NNSF of China (no. 21003020), the Natural Science Fund of Fujian Province (no. 2013J01041) and the Foundation of State Key Laboratory of Structural Chemistry (project no. 20130012).
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Footnote |
† Electronic supplementary information (ESI) available: Synthesis, table, supplementary 14 plots including structures, luminescent spectra, XPRD, TG and FT-IR. CCDC 975527 (for 1a), 975528 (for 1b), 975529 (for 2a), and 975530 (for 2b). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra02953d |
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