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Two types of lanthanide coordination polymers showing luminescence and magnetic properties
Polyhedron 167 (2019) 26–32
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Two types of lanthanide coordination polymers showing luminescence and magnetic properties Jia-Qiang Du a, Jun-Liang Dong a, Fei Xie a, Hai-Ming Lan a, Ru-Xia Yang a, Duo-Zhi Wang a,b,⇑ a b
School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, PR China Key Laboratory of Energy Materials Chemistry, Ministry of Education, Xinjiang University, Urumqi 830046, Xinjiang, PR China
a r t i c l e
i n f o
Article history: Received 15 February 2019 Accepted 5 April 2019 Available online 18 April 2019 Keywords: Ln-CPs Crystal structure Luminescence properties Magnetic properties Hydrogen bonding
a b s t r a c t Lanthanide coordination polymers (Ln-CPs) {[Ln(HL)2(H2O)3](H2O)4Cl}n (Ln = Dy, 1; Eu, 2; Sm, 3) and [Ln0.18(HL)0.18(CH3COO)0.36(H2O)0.54]n (Ln = La, 4; H2L = 2-(hydroxymethyl)-1H-benzo[d]imidazole-5-carboxylic acid) have been successfully assembled by Ln3+ ions and asymmetric carboxylic acid ligand under solvothermal conditions. The structures of the complexes 1–4 were determined by single crystal X-ray diffraction and further characterized by elemental analyses, IR spectroscopy, powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA). The structure analysis display that complexes 1–3 are isostructural three-dimensional (3D) supramolecular network. Complexes 1–4 were constructed by hydrogen bonding interactions between the coordinated water molecules and the O atoms of the hydroxy group or free chloride ions. Furthermore, luminescence properties of complexes 1–4 have been measured in the solid state at room temperature. Complex 2 exhibits a strong red luminescence upon excitation at 283 nm and its lifetimes is 33.554 ns. In addition, magnetic studies reveal that complex 1 has a weak antiferromagnetic interaction. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction In the past two decades, the rational design and synthesis approaches of lanthanide coordination polymers (Ln-CPs) have attracted enormous interests because of the unusual coordination modes and connectivity of lanthanide ions, which could promote the formation of high-dimensional structures with high thermal stability [1–3]. Ln-CPs are considered as one of the promising functional materials for utilization in the area of gas adsorption and separation [4], catalysis [5], luminescence [6], magnetism [7], guest species exchange or chemical sensor and so forth [8]. Since Ln-CPs possess a specific 4f electronic configuration of a lanthanide ions, they can show characteristic optical properties [9]. Compared with transition metal based CPs, Ln-CPs possess higher coordination numbers and more flexible coordination geometries, thus the subcategory of Ln-CPs deserves special attention. More importantly, lanthanide ions often endow the CPs with unique luminescent properties (characteristic sharp emission, high color purity, and long excited-state luminescence lifetimes) as a result of transitions within the partially-filled 4f shells of the trivalent lanthanide ions [10]. The lanthanide complexes usually show ⇑ Corresponding author at: School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, PR China. E-mail address: [email protected] (D.-Z. Wang). https://doi.org/10.1016/j.poly.2019.04.006 0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.
intense emission over narrow wavelength ranges and readily identifiable emission bands in both visible and NIR regions, which are potentially applicable as fluorescent probes [11]. Among luminescent materials, the search for new white light emission materials is an active field, which can be applied in lighting with the feature of long persistence, energy-efficient and environmental friendly advantages [12,13]. Ln-CPs can generate strong luminescent signals with visible emission colors and can be developed as a new type of promising chemical sensor [14]. In addition, Ln-CPs with magnetic properties, especially those with slow magnetic relaxation, have also received much attention [15]. Owing to the strong spin-orbit coupling, the magnetic structure of the lanthanides is more complex than that of the transition metal system [16,17]. Single-molecule magnets (SMMs) are one of the most popular molecular magnetic materials due to their tiny size and potential application. Although many Dy(III) based SMMs have been reported, it is still very challenging to obtain materials with real application value [18]. However, it is relatively difficult to construct rigid Ln-CPs, because structural diversity of CPs depends on many factors, such as metal salts, the coordination ability of organic ligands, counter anion and reaction conditions (pH, temperature and so on) [19,20]. Among them, organic ligands and adjusting temperature are important factors in constructing CPs with the excellent properties and unique architectures. The organic ligand consisting of
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carboxylate and benzimidazole has widely used in the synthesis of CPs, and a large number of networks have been reported [21]. From a synthetic chemistry perspective, the design of ligands containing distinct coordination donors and bonding modes is considered a vital factor. Inspired by the aforementioned considerations, in this contribution a rigid carboxylic acid ligand 2-(hydroxymethyl)-1Hbenzo[d]imidazole-5-carboxylic acid (H2L) was selected as the organic ligand. Based on hydrogen bonding interactions of Ln-CPs: {[Ln(HL)2(H2O)3](H2O)4Cl}n (Ln = Dy, 1; Eu, 2; Sm, 3) and [Ln0.18(HL)0.18(CH3COO)0.36(H2O)0.54]n (Ln = La, 4) have been successfully synthesized under solvothermal conditions, and characterized by powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). In addition, luminescence properties and lifetimes of complexes 1–4 were studied in detail. Complex 1 shows antiferromagnetic interaction at low temperature.
(w), 1788 (w), 1624 (m), 1593 (w), 1507 (s), 1403 (s), 1320 (s), 1287 (s), 1227 (s), 1072 (m), 1045 (m), 1019 (w), 947 (w), 895 (w), 835 (m), 780 (w), 751 (m), 698 (w).
2. Experimental
2.2.4. Synthesis of [La0.18(HL)0.18(CH3COO)0.36(H2O)0.54]n (4) A mixture of La(CH3COO)3xH2O (63.6 mg, 0.2 mmol), H2L (19.3 mg, 0.1 mmol) and CH3CN/H2O (1:1, v/v, 7 mL) were sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 160 °C for 12 h. Then the reaction system was cooled to room temperature at a rate of 5 °Ch 1. Colorless block crystals were obtained by filtering, washed with ethanol absolute and dried in air. Yield: 42% (based on H2L). Anal. Calc. for C2.36H3.27La0.18N0.36O1.82(%): C, 31.22; H, 3.63; N, 5.55. Found: C, 31.13; H, 3.52; N, 5.81. IR (KBr pellet, cm 1): 3566 (w), 3454 (m), 3268 (m), 3143 (m), 3070 (m), 2937 (w), 1788 (w), 1629 (w), 1588 (m), 1534 (s), 1486 (s), 1452 (s), 1410 (s), 1372 (s), 1349 (m), 1288 (m), 1244 (w), 1203 (w), 1124 (w), 1097 (w), 1045 (m), 1013 (m), 980 (w), 944 (w), 893 (w), 829 (m), 793 (m), 758 (w), 696 (m), 674 (m), 667 (m), 612 (w).
2.1. Materials and measurements Synthesis of H2L ligand was reported in our previous work [22]. All other reagents and solvents for syntheses were purchased from Adamas Reagent Co., Ltd and employed without further purification. FT-IR spectra were measured on a Bruker Equinox 55 FT-IR spectrometer with KBr pellets in the range of 4000–400 cm 1. Elemental analysis of C, H and N was carried out with a Thermo Flash EA 1112-NCHS-O analyzer. Powder X-ray diffraction (PXRD) was recorded on a Rigaku D/Max-2500 diffractometer at 40 kV, 100 mA for a Cu-target tube and a graphite monochromator. Thermogravimetric analysis (TGA) were carried out with a PerkinElmer TG-7 analyzer heated from 25 to 700 °C at a heating rate of 10 °C/ min in an argon atmosphere. Solid state luminescent spectra of ligand H2L and complexes 1–4 were measured by Hitachi F-4500 Fluorescence Spectrophotometer with a Xe arc lamp as the light source and bandwidths of 2.5 nm at room temperature. Fluorescence lifetime data were acquired on Fluorolog-3 spectrofluorimeter equipped with nanoled as the light source. Magnetic measurements were recorded on a Quantum Design MPMSXL-7 SQUID magnetometer on polycrystalline samples. 2.2. Preparation of complexes 1–4 2.2.1. Synthesis of {[Dy(HL)2(H2O)3](H2O)4Cl}n (1) A mixture of DyCl36H2O (59.1 mg, 0.2 mmol), H2L (19.3 mg, 0.1 mmol), NaOH (40 mg, 1.0 mmol) and CH3CN/CH3CH2OH/H2O (5:1:1, v/v/v, 5 mL) were sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 160 °C for 12 h. Then the reaction system was cooled to room temperature at a rate of 5 °Ch 1. Light yellow crystals were obtained by filtering, washed with ethanol absolute and dried in air. Yield: 37% (based on H2L). Anal. Calc. for C18H19ClDyN4O9 (%): C, 34.14; H, 3.02; N, 8.85. Found: C, 34.05; H, 2.91; N, 8.96. IR (KBr pellet, cm 1): 3252 (m), 2850 (m), 2715 (m), 1592 (w), 1515 (s), 1492 (m), 1444 (m), 1411 (s), 1325 (m), 1284 (m), 1224 (w), 949 (w), 782 (w), 752 (w). 2.2.2. Synthesis of {[Eu(HL)2(H2O)3]H2O}n (2) A mixture of EuCl36H2O (63.2 mg, 0.2 mmol), H2L (20.5 mg, 0.1 mmol), NaOH (8 mg, 0.2 mmol) and CH3CN/H2O (3:1, v/v, 5 mL) were sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 140 °C for 12 h. Then the reaction system was cooled to room temperature at a rate of 5 °Ch 1. Light yellow crystals were obtained by filtering, washed with ethanol absolute and dried in air. Yield: 42% (based on H2L). Anal. Calc. for C18H19.50ClEuN4O13(%): C, 43.29; H, 3.84; N, 11.22. Found: C, 43.22; H, 3.78; N, 11.32. IR (KBr pellet, cm 1): 3189 (m), 2840 (m), 2703 (m), 1910
2.2.3. Synthesis of {[Sm(HL)2(H2O)3](H2O)4}n (3) This complex 3 was synthesized by following a procedure similar to that for 2 but using Sm(NO3)36H2O (52 mg, 0.1 mmol) instead of EuCl36H2O. Yellow crystals were obtained by filtering, washed with ethanol absolute and dried in air. Yield: 35% (based on H2L). Anal. Calc. for C18H20N4 O10Sm(%): C, 35.86; H, 3.35; N, 9.30. Found: C, 35.79; H, 3.29; N, 9.88. IR (KBr pellet, cm 1): 3359 (m), 1904 (w), 1785 (w), 1647 (m), 1591 (m), 1551 (w), 1489 (s), 1443 (m), 1404 (s), 1321 (s), 1284 (s), 1224 (s), 1205 (m), 1126 (m), 1078 (m), 1026 (m), 1004 (m), 890 (w), 836 (w), 780 (s), 751 (w), 696 (m), 628 (w).
2.3. X-ray crystallography The crystallographic data collections for complexes 1–4 were carried out on a Bruker Smart Apex CCD area-detector diffractometer with graphite monochromated Mo Ka radiation (k = 0.71073 Å) using x-scan technique. The diffraction data were integrated by using the SAINT program [23], which was also used for the intensity corrections for the Lorentz and polarization effects. Semi-empirical absorption correction was applied by using the SADABS program [24]. The structures were solved by direct methods and all the non-hydrogen atoms were refined with anisotropic displacement parameters using the SHELXL crystallographic software package [25]. Most of Hydrogen atoms were assigned to calculate positions and refined with fixed geometry with respect to their carrier atoms, and the water hydrogen atoms were located from difference maps. A summary of their crystallographic data and structure refinement are provided in Table 1. Selected bond distances and angles of the title complexes are listed in Table S1 of Supplementary 1. Hydrogen bonds distances and angles for complexes 1–4 are listed in Table S2 of Supplementary 1. 3. Results and discussion 3.1. Syntheses and general characterization A series of experiments were carried out in order to get the optimal reaction temperature and solvents for single crystal growth. Under the solvothermal conditions, the ligand H2L reacted with DyCl36H2O, EuCl36H2O, Sm(NO3)36H2O to generate isostructural complexes 1–3, while La(CH3COO)3xH2O reacted to form complex 4. Complexes 1–4 have a 1D chain structure and further assembled to form 2D sheet and 3D supramolecular structure by hydrogenbonding interactions.
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Table 1 Crystal data and structure refinement summary for complexes 1–4.
Formula Formula weight Crystal system Space group Unit cell dimensions T (K) a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z D (g cm 3) m (mm 1) F(0 0 0) Measured reflections Observed reflections Ra/wRb a b
1
2
3
4
C19H21ClDyN4O13 711.35 monoclinic P21/n
C18H19.5ClEuN4O13 687.28 monoclinic P21/n
C18H20N4O10Sm 602.73 monoclinic P21/n
C2.36H3.27La0.18N0.36O1.82 91.13 triclinic P 1
100.00(10) 11.1548(5) 11.2053(5) 19.9100(7) 90.00(3) 80.864(3) 90.000(4) 2457.04(18) 4 1.923 3.225 1400.0 13,441 5811 0.0444/0.1079
100.00(10) 11.0784(3) 11.2950(3) 20.0009(5) 90 98.884(2) 90 2472.70(11) 4 1.846 2.717 1358.0 27,754 6277 0.0399/0.0951
100.00(10) 11.0583(5) 11.3009(5) 20.0104(8) 90 98.683(4) 90 2472.00(19) 4 1.620 2.431 1192.0 18,627 6074 0.0450/0.1006
296(2) 8.4830(4) 10.7190(6) 10.8585(6) 88.3280(10) 68.6330(10) 82.4540 911.34(8) 11 1.826 2.398 494 5878 3233 0.0242/0.0567
R = R(||F0| |FC||)/R|F0|. wR = [Rw(|F0|2 |FC|2)2/Rw(F20)]1/2.
In the FT-IR spectra of the synthetic complexes 1–4 show that the characteristic absorption peaks of OAH or NAH in free or coordinated water molecules and coordinated ligands are about 3300 cm 1: 3252 cm 1 (1); 3189 cm 1 (2); 3359 cm 1 (3); 3268 cm 1 (4). The characteristic absorption peak of carbonyl group in the H2L is 1680 cm 1 and 1313 cm 1, while the carbonyl groups are coordinated with the metal in the 1–4, the absorption bands of complexes 1–4 from 1600 cm 1 to 1390 cm 1 show the existence of the carboxylic groups: 1492 cm 1, 1411 cm 1 (1); 1624 cm 1, 1403 cm 1 (2); 1591 cm 1, 1404 cm 1 (3); 1588 cm 1, 1452 cm 1, 1410 cm 1 (4). The H2L ligand and complexes 1–4 exhibit the peaks in the 1420–1630 cm 1 region clearly confirmed the formation of the benzimidazole groups.
3.2. Crystal structure description 3.2.1. Structural description of {[Ln(HL)2(H2O)3](H2O)4Cl}n (Ln = Dy, 1; Eu, 2; Sm, 3) Herein, only the structure of complex 2 is described in detail because complexes 1, 2 and 3 are isomorphic. Single crystal X-ray analysis reveals that complex 2 crystallizes in the monoclinic crystal system with the P21/n space group and features a 1D linear chain structure built from HL linkers and [Eu] nodes. As shown in Fig. 1a, each asymmetric unit consists of one crystallographically independent Eu(III) ions, two incomplete deprotonated HL ligands, three coordinated water molecules, four lattice water molecules and one guest chloride ion. Nine coordination positions are occupied by a nitrogen atom (N3) from one H2L organic ligand, and the eight remaining coordination sites are occupied by the eight oxygen atoms: four oxygen atoms (O2, O3, O4, O5) from two carboxyl groups of two different H2L ligands another oxygen atom (O6) from one hydroxyl group of one H2L ligand, and three oxygen atoms (O7, O8, O9) from three distinct coordinated water molecules, respectively. The central Eu(III) atoms are located in a distorted capped square antiprism configuration (Fig. 1b). Around the Eu(III) center, the (O2, O4, O5, O7, O8, O9, N3) atoms comprise the basal plane, and the (O3, O6) atoms occupy the axial positions. The EuAN bond length is 2.522(3) Å, while the EuAO bond distance fall in the region 2.441(3)–2.531(3) Å. In addition, the coordination angles of OAEuAO and OAEuAN are in the range of 52.16(9)–120.10(10)° and 64.82(10)–89.58(11)°, respectively. As illustrated in Fig. 1c, each pair neighboring Eu(III) ions are bridged by one carboxylate
and one hydroxy groups from one H2L ligand to form an infinite 1D linear chain structure along a-axis, in which the Eu Eu distance is 10.6585(6) Å (Fig. 1c). The chains were linked by hydrogen bonds of O(1)AH(1) Cl(2) (Table S2 of Supplementary 1) to formed a 2D layer structure (Fig. 1d). The neighboring 2D structures are further connected by the hydrogen bonds of O(8)AH(8A) Cl(2) to give a 3D supermolecular network structure (Fig. 1e), which contains a larger 1D channel along the c axis (Fig. 1f). 3.2.2. Structural description of [La0.18(HL)0.18(CH3COO)0.36(H2O)0.54]n (Ln = La, 4) Single-crystal X-ray crystallographic analysis reveals that complex 4 crystallizes in the triclinic space group P 1 and shows a 1D chain structure. As depicted in Fig. 2a, each asymmetric unit comprises zero point one eight La(III) atom, zero point one eight HL ligand, zero point three six acetate ion and zero point five four coordination water molecule. In complex 4, the central La(III) ions ten-coordinated environment with two carboxylic O atoms (O1, O2) from one HL ligand, six O atoms (O7#1, O6#1, O6, O5#2, O5, O4 symmetry code: #1 x, y + 2, z + 1, #2 x + 1, y + 2, z + 1) from four acetate ion and two O atoms from two H2O molecules. The central La(III) ion is located in distorted double cap tetragonal prism coordination geometry [LaO10] (Fig. 2b). The outer cap structure of the two sides of the prism is occupied by two atoms (O6#1, O5). The LaAO bond lengths are in the range of 2.490(2)– 2.799(2) Å. The La(III) ion is bridged by the pairs of acetic molecules to generate a 1D chain structure (Fig. 2c). Another prominent structural feature of 2D is the presence of abundant hydrogen bonding interactions (O9AH4W O3#4 symmetry code: #4 x, y + 1, z 1) (Table S2 of Supplementary 1) between these chains (Fig. 2d). Each chains connect three adjacent chains through (O3AH10 O4#3 symmetry code: #3 x + 1, y + 1, z + 1) hydrogen bonds to give an interesting three-dimensional (3D) supramolecular framework (Fig. 2e). Likewise, this 3D framework shows large irregular channels along the a axis. In addition, the rest of hydrogen bonds further enhanced the stability of the crystal structure. 3.3. Powder X-ray diffraction and thermogravimetric analysis The purity of bulk materials for 1–4 was confirmed by comparison of their powder X-ray diffraction patterns (PXRD) with those calculated from single-crystal X-ray diffraction data. As shown in
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Fig. 1. View of (a) The asymmetric unit of complex 2; (b) Distorted tricapped trigonal prism coordination environment of Eu in complex 2; (c) The 1D chain of complex 2; (d) The 2D layer though the hydrogen bonds of O(1)AH(1) Cl(2); (e) The 3D supermolecular network structure though the hydrogen bonds of O(8)AH(8A) Cl(2); (f) The 3D supermolecular network structure in space-filling mode.
Supporting Information (Fig. S1 of Supplementary 2), the diffraction peaks of as-synthesized samples are almost in agreement with the simulated patterns calculated from the single-crystal diffraction data, demonstrating the good phase purity of the complexes 1–4. To examine the thermal stability of complexes 1–4, the thermal behaviors were carried out under an Argon atmosphere in the temperature range of 25–700 °C with a heating rate of 10 °Cmin 1, and the TG curves are given in (Fig. S2 of Supplementary 2). For complex 1, a thermal effect in the 40–265 °C range refers to the removal of three coordinated H2O molecules (exptl, 7.58%; calc. 8.53%), followed by decomposition starting from 306 °C. Complex 2 records its first weight loss of 25.53% in the range of 30– 183 °C, which should be attributed to the release of three coordinated H2O molecules, four lattice water molecules and one guest molecule (calc.23.50%). There is no further weight loss until at about 216.81 °C where there is a significant decrease of 58.18% from 216.81 to 670 °C due to the onset of framework decomposition. In complex 3, an initial weight loss of 26.39% was observed in the region 30–156.96 °C, which can be attributed to the loss of three coordinated H2O molecules, four lattice water molecules and one guest molecule (calc. 26.79%). With a further increase of temperature, the decomposition of coordination polymer occurred in the range of 236–700 °C. For complex 4, a weight loss of 11.60% was recorded in the range of 106–161 °C, corresponding to the
release of zero point five four coordinated water molecule (calc. 10.67%). Above 172 °C, the entire architecture began to collapse. 3.4. Luminescence properties Lanthanide complexes have shown excellent luminescence properties in terms of their high color purity with high quantum efficiency [26], so the solid-state luminescence properties of the complexes 1–4 and free H2L ligand were measured at room temperature (Fig. S3 of Supplementary 2). The free H2L ligand displays a fluorescence emission peaks at 372 nm (kex = 271 nm), which probably derived from p ? p* or n ? p* transitions [27,28]. With the excitation at 283 nm, complex 1 shows three major characteristic emission peaks at 318 nm, 389 nm and 408 nm. For complex 1, the luminescence emission spectra is dominated by ligand-based emission, which may be resulted from the intermolecular excimertype interactions between ligands and ligand-to-ligand charge transitions (LLCT). When excited at 283 nm, complex 2 shows characteristic emissions at 438 nm. Notably, the maximum peak at about 438 nm exhibits a slight red-shift compared with the H2L ligand, which may be attributed to the emission of ligand-tometal charge transfer (LMCT) originated from the coordination of the L2 ligand to Eu(III) ions. Under excitation at 283 nm, the spectrum of complex 3 exhibits three sharp bands at 361, 368 and 441 nm; these are assignable to transitions characteristic of Sm
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Fig. 2. View of (a) The asymmetric unit of complex 4; (b) Distorted double cap tetragonal prism coordination environment of La in complex 4; (c) The 1D chain of complex 4; (d) The 2D layer though the hydrogen bonds of O9AH4W O3#4; (e) The 3D supermolecular network structure though the hydrogen bonds of O3AH10 O4#3.
(III) ions and are consistent with prior literature reports [29]. For complex 4, the emission peak at 377 nm are found (kex = 290 nm), which is red-shifted around 5 nm as compared with the free H2L ligand. Such an emission is assigned to the intraligand p–p* transition of H2L. As the H2L ligand interactions upon coordination with the metal ion affects the HOMO and LUMO levels of the ligand to change the transition energy, the p–p* transition band in complex 4 shifts toward the low energy side (red shift). The different maximum emission positions may be related to the variety of coordination environments around metal ions, as luminescence behavior is closely associated with the metal ions and the ligands coordinated around them. Interestingly, in comparison to the intensity of the emission spectrum of free H2L ligand, the intensity of the luminescence spectrum of complexes 1, 3 and 4 have been found to be enhanced, which might originated from the ligands coordination to the metal centres. This phenomenon expressly shows the fact that the rigidity H2L ligand restrains its configuration in complex 1, 3 and 4 consequently reduces the degree of freedom (compared to that of free H2L ligand), thereby decreasing the loss of energy via radiationless decay [30]. Luminescence studies indicate that complexes 1, 3 and 4 are better fluorescent materials as compared to complex 2. 3.5. Luminescence lifetimes To understand the luminescence properties of complexes 1–4 and H2L ligand, we also performed time-resolved measurements of complexes 1–4 and ligand by using the time-correlated single photon counting (TCSPC) technique. As shown in Fig. S4 of Supple-
mentary 2, the decay curves for solid complexes 1–4 and H2L ligand exhibit triple exponential behavior, irrespective of the media. The fluorescence decay lifetimes of the complexes 1–4 and H2L ligand are quite distinct (s = 1.497 ns for H2L; s = 0.039 ns for complex 1; s = 33.554 ns for complex 2; s = 31.955 ns for complex 3; s = 4.215 ns for complex 4, respectively) (Table S3 of Supplementary 1). These long lifetimes may be attributed to the strong chelating coordination ability and steric architectonic stabilization of the H2L ligand, which offering an effective pathway of energy transfer. In a word, complexes 1–4 exhibits higher luminescence lifetimes in the sequence 1 < H2L < 4 < 3 < 2 at 298 K, which emphasizes the significant impact of the different sizes, structural models, and the latticed water molecules on luminescent properties [31]. Additionally, coordinated H2O exists in complexes 1–4, which favors high-energy vibrational quenching via OAH oscillators and is expected to exhibit a relatively shorter lifetimes [32]. 3.6. Magnetic properties The variable-temperature magnetic susceptibility measurements of complex 1 was investigated in the temperature range of 2–300 K with an applied magnetic field of 1000 Oe. The data obtained for complex 1 are represented in Fig. 3. The value of vMT of complex 1 is 24.62 cm3 K mol 1 at room temperature (300 K), which is coincident with a theoretical value of 14.17 cm3 K mol 1 for one magnetically isolated Dy3+ ion (6H15/2, S = 5/2, L = 5, g = 4/3) in the free-ion approximation. Upon cooling, vMT starts to slowly decrease to 50 K, and then sharply decreases to
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4), respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data to this article can be found online at https://doi.org/10.1016/j.poly.2019.04.006. References
Fig. 3. Magnetic property of complex 1 in the form of vMT and vM1 versus T plot. The red solid line represents the fitting results over the range of 50–300 K. (Color online.)
11.13 cm3 K mol 1, which is more likely to be ascribed to the thermal depopulation of the Stark sublevels, and the possible antiferromagnetic intramolecular Dy–Dy interactions or dipole–dipole interactions between the molecules [33]. Also, the temperature dependence of the reciprocal susceptibilities (vM1) of complex 1 obeys the Curie–Weiss law (vM = C/(T h)), the best-fit parameters for the Curie–Weiss model in the temperature region of 50–300 K give Cm = 25.09 cm3 K mol 1 and h = 3.30 K. The negative h value further supports the existence of antiferromagnetic exchange interactions between the Dy3+ ions. 4. Conclusion In summary, we have successfully synthesized four new Ln-CPs constructed from 2-(hydroxymethyl)-1H-benzo[d]imidazole-5carboxylic acid (H2L) under solvothermal conditions. The distinct metal centers and reaction conditions are key factors to construct diverse frameworks. 1D chain structure of complex 4 has further assemble to form 2D sheet and 3D supramolecular network by hydrogen bonding interactions between the coordinated water molecules and the O atoms of the hydroxy group. Complexes 1–3 are isomorphic and display 3D supramolecular network constructed by hydrogen bonding interactions. The X-ray powder diffraction (PXRD) investigated complexes 1–4, which demonstrates that they have high phase purity. Moreover, thermal stability shows that complexes 1–4 possessed high thermostability. Noteworthily, intense fluorescence emission and lifetimes of 1–4 and H2L ligand exhibit that complexes 1, 3 and 4 are better fluorescent materials as compared to complex 2. Magnetic studies shows that antiferromagnetic exchange couplings between neighboring Ln3+ ions that also exists in complex 1. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (No. 21361026). Appendix A. Supplementary data CCDC 1896903–1896906 contains the supplementary crystallographic data for complexes {[Ln(HL)2(H2O)3](H2O)4Cl}n (Ln = Dy, 1; Eu, 2; Sm, 3) and [Ln0.18(HL)0.18(CH3COO)0.36(H2O)0.54]n (Ln = La,
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Two types of lanthanide coordination polymers showing luminescence and magnetic properties Jia-Qiang Du a, Jun-Liang Dong a, Fei Xie a, Hai-Ming Lan a, Ru-Xia Yang a, Duo-Zhi Wang a,b,⇑ a b
School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, PR China Key Laboratory of Energy Materials Chemistry, Ministry of Education, Xinjiang University, Urumqi 830046, Xinjiang, PR China
a r t i c l e
i n f o
Article history: Received 15 February 2019 Accepted 5 April 2019 Available online 18 April 2019 Keywords: Ln-CPs Crystal structure Luminescence properties Magnetic properties Hydrogen bonding
a b s t r a c t Lanthanide coordination polymers (Ln-CPs) {[Ln(HL)2(H2O)3](H2O)4Cl}n (Ln = Dy, 1; Eu, 2; Sm, 3) and [Ln0.18(HL)0.18(CH3COO)0.36(H2O)0.54]n (Ln = La, 4; H2L = 2-(hydroxymethyl)-1H-benzo[d]imidazole-5-carboxylic acid) have been successfully assembled by Ln3+ ions and asymmetric carboxylic acid ligand under solvothermal conditions. The structures of the complexes 1–4 were determined by single crystal X-ray diffraction and further characterized by elemental analyses, IR spectroscopy, powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA). The structure analysis display that complexes 1–3 are isostructural three-dimensional (3D) supramolecular network. Complexes 1–4 were constructed by hydrogen bonding interactions between the coordinated water molecules and the O atoms of the hydroxy group or free chloride ions. Furthermore, luminescence properties of complexes 1–4 have been measured in the solid state at room temperature. Complex 2 exhibits a strong red luminescence upon excitation at 283 nm and its lifetimes is 33.554 ns. In addition, magnetic studies reveal that complex 1 has a weak antiferromagnetic interaction. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction In the past two decades, the rational design and synthesis approaches of lanthanide coordination polymers (Ln-CPs) have attracted enormous interests because of the unusual coordination modes and connectivity of lanthanide ions, which could promote the formation of high-dimensional structures with high thermal stability [1–3]. Ln-CPs are considered as one of the promising functional materials for utilization in the area of gas adsorption and separation [4], catalysis [5], luminescence [6], magnetism [7], guest species exchange or chemical sensor and so forth [8]. Since Ln-CPs possess a specific 4f electronic configuration of a lanthanide ions, they can show characteristic optical properties [9]. Compared with transition metal based CPs, Ln-CPs possess higher coordination numbers and more flexible coordination geometries, thus the subcategory of Ln-CPs deserves special attention. More importantly, lanthanide ions often endow the CPs with unique luminescent properties (characteristic sharp emission, high color purity, and long excited-state luminescence lifetimes) as a result of transitions within the partially-filled 4f shells of the trivalent lanthanide ions [10]. The lanthanide complexes usually show ⇑ Corresponding author at: School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, PR China. E-mail address: [email protected] (D.-Z. Wang). https://doi.org/10.1016/j.poly.2019.04.006 0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.
intense emission over narrow wavelength ranges and readily identifiable emission bands in both visible and NIR regions, which are potentially applicable as fluorescent probes [11]. Among luminescent materials, the search for new white light emission materials is an active field, which can be applied in lighting with the feature of long persistence, energy-efficient and environmental friendly advantages [12,13]. Ln-CPs can generate strong luminescent signals with visible emission colors and can be developed as a new type of promising chemical sensor [14]. In addition, Ln-CPs with magnetic properties, especially those with slow magnetic relaxation, have also received much attention [15]. Owing to the strong spin-orbit coupling, the magnetic structure of the lanthanides is more complex than that of the transition metal system [16,17]. Single-molecule magnets (SMMs) are one of the most popular molecular magnetic materials due to their tiny size and potential application. Although many Dy(III) based SMMs have been reported, it is still very challenging to obtain materials with real application value [18]. However, it is relatively difficult to construct rigid Ln-CPs, because structural diversity of CPs depends on many factors, such as metal salts, the coordination ability of organic ligands, counter anion and reaction conditions (pH, temperature and so on) [19,20]. Among them, organic ligands and adjusting temperature are important factors in constructing CPs with the excellent properties and unique architectures. The organic ligand consisting of
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carboxylate and benzimidazole has widely used in the synthesis of CPs, and a large number of networks have been reported [21]. From a synthetic chemistry perspective, the design of ligands containing distinct coordination donors and bonding modes is considered a vital factor. Inspired by the aforementioned considerations, in this contribution a rigid carboxylic acid ligand 2-(hydroxymethyl)-1Hbenzo[d]imidazole-5-carboxylic acid (H2L) was selected as the organic ligand. Based on hydrogen bonding interactions of Ln-CPs: {[Ln(HL)2(H2O)3](H2O)4Cl}n (Ln = Dy, 1; Eu, 2; Sm, 3) and [Ln0.18(HL)0.18(CH3COO)0.36(H2O)0.54]n (Ln = La, 4) have been successfully synthesized under solvothermal conditions, and characterized by powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). In addition, luminescence properties and lifetimes of complexes 1–4 were studied in detail. Complex 1 shows antiferromagnetic interaction at low temperature.
(w), 1788 (w), 1624 (m), 1593 (w), 1507 (s), 1403 (s), 1320 (s), 1287 (s), 1227 (s), 1072 (m), 1045 (m), 1019 (w), 947 (w), 895 (w), 835 (m), 780 (w), 751 (m), 698 (w).
2. Experimental
2.2.4. Synthesis of [La0.18(HL)0.18(CH3COO)0.36(H2O)0.54]n (4) A mixture of La(CH3COO)3xH2O (63.6 mg, 0.2 mmol), H2L (19.3 mg, 0.1 mmol) and CH3CN/H2O (1:1, v/v, 7 mL) were sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 160 °C for 12 h. Then the reaction system was cooled to room temperature at a rate of 5 °Ch 1. Colorless block crystals were obtained by filtering, washed with ethanol absolute and dried in air. Yield: 42% (based on H2L). Anal. Calc. for C2.36H3.27La0.18N0.36O1.82(%): C, 31.22; H, 3.63; N, 5.55. Found: C, 31.13; H, 3.52; N, 5.81. IR (KBr pellet, cm 1): 3566 (w), 3454 (m), 3268 (m), 3143 (m), 3070 (m), 2937 (w), 1788 (w), 1629 (w), 1588 (m), 1534 (s), 1486 (s), 1452 (s), 1410 (s), 1372 (s), 1349 (m), 1288 (m), 1244 (w), 1203 (w), 1124 (w), 1097 (w), 1045 (m), 1013 (m), 980 (w), 944 (w), 893 (w), 829 (m), 793 (m), 758 (w), 696 (m), 674 (m), 667 (m), 612 (w).
2.1. Materials and measurements Synthesis of H2L ligand was reported in our previous work [22]. All other reagents and solvents for syntheses were purchased from Adamas Reagent Co., Ltd and employed without further purification. FT-IR spectra were measured on a Bruker Equinox 55 FT-IR spectrometer with KBr pellets in the range of 4000–400 cm 1. Elemental analysis of C, H and N was carried out with a Thermo Flash EA 1112-NCHS-O analyzer. Powder X-ray diffraction (PXRD) was recorded on a Rigaku D/Max-2500 diffractometer at 40 kV, 100 mA for a Cu-target tube and a graphite monochromator. Thermogravimetric analysis (TGA) were carried out with a PerkinElmer TG-7 analyzer heated from 25 to 700 °C at a heating rate of 10 °C/ min in an argon atmosphere. Solid state luminescent spectra of ligand H2L and complexes 1–4 were measured by Hitachi F-4500 Fluorescence Spectrophotometer with a Xe arc lamp as the light source and bandwidths of 2.5 nm at room temperature. Fluorescence lifetime data were acquired on Fluorolog-3 spectrofluorimeter equipped with nanoled as the light source. Magnetic measurements were recorded on a Quantum Design MPMSXL-7 SQUID magnetometer on polycrystalline samples. 2.2. Preparation of complexes 1–4 2.2.1. Synthesis of {[Dy(HL)2(H2O)3](H2O)4Cl}n (1) A mixture of DyCl36H2O (59.1 mg, 0.2 mmol), H2L (19.3 mg, 0.1 mmol), NaOH (40 mg, 1.0 mmol) and CH3CN/CH3CH2OH/H2O (5:1:1, v/v/v, 5 mL) were sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 160 °C for 12 h. Then the reaction system was cooled to room temperature at a rate of 5 °Ch 1. Light yellow crystals were obtained by filtering, washed with ethanol absolute and dried in air. Yield: 37% (based on H2L). Anal. Calc. for C18H19ClDyN4O9 (%): C, 34.14; H, 3.02; N, 8.85. Found: C, 34.05; H, 2.91; N, 8.96. IR (KBr pellet, cm 1): 3252 (m), 2850 (m), 2715 (m), 1592 (w), 1515 (s), 1492 (m), 1444 (m), 1411 (s), 1325 (m), 1284 (m), 1224 (w), 949 (w), 782 (w), 752 (w). 2.2.2. Synthesis of {[Eu(HL)2(H2O)3]H2O}n (2) A mixture of EuCl36H2O (63.2 mg, 0.2 mmol), H2L (20.5 mg, 0.1 mmol), NaOH (8 mg, 0.2 mmol) and CH3CN/H2O (3:1, v/v, 5 mL) were sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 140 °C for 12 h. Then the reaction system was cooled to room temperature at a rate of 5 °Ch 1. Light yellow crystals were obtained by filtering, washed with ethanol absolute and dried in air. Yield: 42% (based on H2L). Anal. Calc. for C18H19.50ClEuN4O13(%): C, 43.29; H, 3.84; N, 11.22. Found: C, 43.22; H, 3.78; N, 11.32. IR (KBr pellet, cm 1): 3189 (m), 2840 (m), 2703 (m), 1910
2.2.3. Synthesis of {[Sm(HL)2(H2O)3](H2O)4}n (3) This complex 3 was synthesized by following a procedure similar to that for 2 but using Sm(NO3)36H2O (52 mg, 0.1 mmol) instead of EuCl36H2O. Yellow crystals were obtained by filtering, washed with ethanol absolute and dried in air. Yield: 35% (based on H2L). Anal. Calc. for C18H20N4 O10Sm(%): C, 35.86; H, 3.35; N, 9.30. Found: C, 35.79; H, 3.29; N, 9.88. IR (KBr pellet, cm 1): 3359 (m), 1904 (w), 1785 (w), 1647 (m), 1591 (m), 1551 (w), 1489 (s), 1443 (m), 1404 (s), 1321 (s), 1284 (s), 1224 (s), 1205 (m), 1126 (m), 1078 (m), 1026 (m), 1004 (m), 890 (w), 836 (w), 780 (s), 751 (w), 696 (m), 628 (w).
2.3. X-ray crystallography The crystallographic data collections for complexes 1–4 were carried out on a Bruker Smart Apex CCD area-detector diffractometer with graphite monochromated Mo Ka radiation (k = 0.71073 Å) using x-scan technique. The diffraction data were integrated by using the SAINT program [23], which was also used for the intensity corrections for the Lorentz and polarization effects. Semi-empirical absorption correction was applied by using the SADABS program [24]. The structures were solved by direct methods and all the non-hydrogen atoms were refined with anisotropic displacement parameters using the SHELXL crystallographic software package [25]. Most of Hydrogen atoms were assigned to calculate positions and refined with fixed geometry with respect to their carrier atoms, and the water hydrogen atoms were located from difference maps. A summary of their crystallographic data and structure refinement are provided in Table 1. Selected bond distances and angles of the title complexes are listed in Table S1 of Supplementary 1. Hydrogen bonds distances and angles for complexes 1–4 are listed in Table S2 of Supplementary 1. 3. Results and discussion 3.1. Syntheses and general characterization A series of experiments were carried out in order to get the optimal reaction temperature and solvents for single crystal growth. Under the solvothermal conditions, the ligand H2L reacted with DyCl36H2O, EuCl36H2O, Sm(NO3)36H2O to generate isostructural complexes 1–3, while La(CH3COO)3xH2O reacted to form complex 4. Complexes 1–4 have a 1D chain structure and further assembled to form 2D sheet and 3D supramolecular structure by hydrogenbonding interactions.
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Table 1 Crystal data and structure refinement summary for complexes 1–4.
Formula Formula weight Crystal system Space group Unit cell dimensions T (K) a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z D (g cm 3) m (mm 1) F(0 0 0) Measured reflections Observed reflections Ra/wRb a b
1
2
3
4
C19H21ClDyN4O13 711.35 monoclinic P21/n
C18H19.5ClEuN4O13 687.28 monoclinic P21/n
C18H20N4O10Sm 602.73 monoclinic P21/n
C2.36H3.27La0.18N0.36O1.82 91.13 triclinic P 1
100.00(10) 11.1548(5) 11.2053(5) 19.9100(7) 90.00(3) 80.864(3) 90.000(4) 2457.04(18) 4 1.923 3.225 1400.0 13,441 5811 0.0444/0.1079
100.00(10) 11.0784(3) 11.2950(3) 20.0009(5) 90 98.884(2) 90 2472.70(11) 4 1.846 2.717 1358.0 27,754 6277 0.0399/0.0951
100.00(10) 11.0583(5) 11.3009(5) 20.0104(8) 90 98.683(4) 90 2472.00(19) 4 1.620 2.431 1192.0 18,627 6074 0.0450/0.1006
296(2) 8.4830(4) 10.7190(6) 10.8585(6) 88.3280(10) 68.6330(10) 82.4540 911.34(8) 11 1.826 2.398 494 5878 3233 0.0242/0.0567
R = R(||F0| |FC||)/R|F0|. wR = [Rw(|F0|2 |FC|2)2/Rw(F20)]1/2.
In the FT-IR spectra of the synthetic complexes 1–4 show that the characteristic absorption peaks of OAH or NAH in free or coordinated water molecules and coordinated ligands are about 3300 cm 1: 3252 cm 1 (1); 3189 cm 1 (2); 3359 cm 1 (3); 3268 cm 1 (4). The characteristic absorption peak of carbonyl group in the H2L is 1680 cm 1 and 1313 cm 1, while the carbonyl groups are coordinated with the metal in the 1–4, the absorption bands of complexes 1–4 from 1600 cm 1 to 1390 cm 1 show the existence of the carboxylic groups: 1492 cm 1, 1411 cm 1 (1); 1624 cm 1, 1403 cm 1 (2); 1591 cm 1, 1404 cm 1 (3); 1588 cm 1, 1452 cm 1, 1410 cm 1 (4). The H2L ligand and complexes 1–4 exhibit the peaks in the 1420–1630 cm 1 region clearly confirmed the formation of the benzimidazole groups.
3.2. Crystal structure description 3.2.1. Structural description of {[Ln(HL)2(H2O)3](H2O)4Cl}n (Ln = Dy, 1; Eu, 2; Sm, 3) Herein, only the structure of complex 2 is described in detail because complexes 1, 2 and 3 are isomorphic. Single crystal X-ray analysis reveals that complex 2 crystallizes in the monoclinic crystal system with the P21/n space group and features a 1D linear chain structure built from HL linkers and [Eu] nodes. As shown in Fig. 1a, each asymmetric unit consists of one crystallographically independent Eu(III) ions, two incomplete deprotonated HL ligands, three coordinated water molecules, four lattice water molecules and one guest chloride ion. Nine coordination positions are occupied by a nitrogen atom (N3) from one H2L organic ligand, and the eight remaining coordination sites are occupied by the eight oxygen atoms: four oxygen atoms (O2, O3, O4, O5) from two carboxyl groups of two different H2L ligands another oxygen atom (O6) from one hydroxyl group of one H2L ligand, and three oxygen atoms (O7, O8, O9) from three distinct coordinated water molecules, respectively. The central Eu(III) atoms are located in a distorted capped square antiprism configuration (Fig. 1b). Around the Eu(III) center, the (O2, O4, O5, O7, O8, O9, N3) atoms comprise the basal plane, and the (O3, O6) atoms occupy the axial positions. The EuAN bond length is 2.522(3) Å, while the EuAO bond distance fall in the region 2.441(3)–2.531(3) Å. In addition, the coordination angles of OAEuAO and OAEuAN are in the range of 52.16(9)–120.10(10)° and 64.82(10)–89.58(11)°, respectively. As illustrated in Fig. 1c, each pair neighboring Eu(III) ions are bridged by one carboxylate
and one hydroxy groups from one H2L ligand to form an infinite 1D linear chain structure along a-axis, in which the Eu Eu distance is 10.6585(6) Å (Fig. 1c). The chains were linked by hydrogen bonds of O(1)AH(1) Cl(2) (Table S2 of Supplementary 1) to formed a 2D layer structure (Fig. 1d). The neighboring 2D structures are further connected by the hydrogen bonds of O(8)AH(8A) Cl(2) to give a 3D supermolecular network structure (Fig. 1e), which contains a larger 1D channel along the c axis (Fig. 1f). 3.2.2. Structural description of [La0.18(HL)0.18(CH3COO)0.36(H2O)0.54]n (Ln = La, 4) Single-crystal X-ray crystallographic analysis reveals that complex 4 crystallizes in the triclinic space group P 1 and shows a 1D chain structure. As depicted in Fig. 2a, each asymmetric unit comprises zero point one eight La(III) atom, zero point one eight HL ligand, zero point three six acetate ion and zero point five four coordination water molecule. In complex 4, the central La(III) ions ten-coordinated environment with two carboxylic O atoms (O1, O2) from one HL ligand, six O atoms (O7#1, O6#1, O6, O5#2, O5, O4 symmetry code: #1 x, y + 2, z + 1, #2 x + 1, y + 2, z + 1) from four acetate ion and two O atoms from two H2O molecules. The central La(III) ion is located in distorted double cap tetragonal prism coordination geometry [LaO10] (Fig. 2b). The outer cap structure of the two sides of the prism is occupied by two atoms (O6#1, O5). The LaAO bond lengths are in the range of 2.490(2)– 2.799(2) Å. The La(III) ion is bridged by the pairs of acetic molecules to generate a 1D chain structure (Fig. 2c). Another prominent structural feature of 2D is the presence of abundant hydrogen bonding interactions (O9AH4W O3#4 symmetry code: #4 x, y + 1, z 1) (Table S2 of Supplementary 1) between these chains (Fig. 2d). Each chains connect three adjacent chains through (O3AH10 O4#3 symmetry code: #3 x + 1, y + 1, z + 1) hydrogen bonds to give an interesting three-dimensional (3D) supramolecular framework (Fig. 2e). Likewise, this 3D framework shows large irregular channels along the a axis. In addition, the rest of hydrogen bonds further enhanced the stability of the crystal structure. 3.3. Powder X-ray diffraction and thermogravimetric analysis The purity of bulk materials for 1–4 was confirmed by comparison of their powder X-ray diffraction patterns (PXRD) with those calculated from single-crystal X-ray diffraction data. As shown in
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Fig. 1. View of (a) The asymmetric unit of complex 2; (b) Distorted tricapped trigonal prism coordination environment of Eu in complex 2; (c) The 1D chain of complex 2; (d) The 2D layer though the hydrogen bonds of O(1)AH(1) Cl(2); (e) The 3D supermolecular network structure though the hydrogen bonds of O(8)AH(8A) Cl(2); (f) The 3D supermolecular network structure in space-filling mode.
Supporting Information (Fig. S1 of Supplementary 2), the diffraction peaks of as-synthesized samples are almost in agreement with the simulated patterns calculated from the single-crystal diffraction data, demonstrating the good phase purity of the complexes 1–4. To examine the thermal stability of complexes 1–4, the thermal behaviors were carried out under an Argon atmosphere in the temperature range of 25–700 °C with a heating rate of 10 °Cmin 1, and the TG curves are given in (Fig. S2 of Supplementary 2). For complex 1, a thermal effect in the 40–265 °C range refers to the removal of three coordinated H2O molecules (exptl, 7.58%; calc. 8.53%), followed by decomposition starting from 306 °C. Complex 2 records its first weight loss of 25.53% in the range of 30– 183 °C, which should be attributed to the release of three coordinated H2O molecules, four lattice water molecules and one guest molecule (calc.23.50%). There is no further weight loss until at about 216.81 °C where there is a significant decrease of 58.18% from 216.81 to 670 °C due to the onset of framework decomposition. In complex 3, an initial weight loss of 26.39% was observed in the region 30–156.96 °C, which can be attributed to the loss of three coordinated H2O molecules, four lattice water molecules and one guest molecule (calc. 26.79%). With a further increase of temperature, the decomposition of coordination polymer occurred in the range of 236–700 °C. For complex 4, a weight loss of 11.60% was recorded in the range of 106–161 °C, corresponding to the
release of zero point five four coordinated water molecule (calc. 10.67%). Above 172 °C, the entire architecture began to collapse. 3.4. Luminescence properties Lanthanide complexes have shown excellent luminescence properties in terms of their high color purity with high quantum efficiency [26], so the solid-state luminescence properties of the complexes 1–4 and free H2L ligand were measured at room temperature (Fig. S3 of Supplementary 2). The free H2L ligand displays a fluorescence emission peaks at 372 nm (kex = 271 nm), which probably derived from p ? p* or n ? p* transitions [27,28]. With the excitation at 283 nm, complex 1 shows three major characteristic emission peaks at 318 nm, 389 nm and 408 nm. For complex 1, the luminescence emission spectra is dominated by ligand-based emission, which may be resulted from the intermolecular excimertype interactions between ligands and ligand-to-ligand charge transitions (LLCT). When excited at 283 nm, complex 2 shows characteristic emissions at 438 nm. Notably, the maximum peak at about 438 nm exhibits a slight red-shift compared with the H2L ligand, which may be attributed to the emission of ligand-tometal charge transfer (LMCT) originated from the coordination of the L2 ligand to Eu(III) ions. Under excitation at 283 nm, the spectrum of complex 3 exhibits three sharp bands at 361, 368 and 441 nm; these are assignable to transitions characteristic of Sm
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Fig. 2. View of (a) The asymmetric unit of complex 4; (b) Distorted double cap tetragonal prism coordination environment of La in complex 4; (c) The 1D chain of complex 4; (d) The 2D layer though the hydrogen bonds of O9AH4W O3#4; (e) The 3D supermolecular network structure though the hydrogen bonds of O3AH10 O4#3.
(III) ions and are consistent with prior literature reports [29]. For complex 4, the emission peak at 377 nm are found (kex = 290 nm), which is red-shifted around 5 nm as compared with the free H2L ligand. Such an emission is assigned to the intraligand p–p* transition of H2L. As the H2L ligand interactions upon coordination with the metal ion affects the HOMO and LUMO levels of the ligand to change the transition energy, the p–p* transition band in complex 4 shifts toward the low energy side (red shift). The different maximum emission positions may be related to the variety of coordination environments around metal ions, as luminescence behavior is closely associated with the metal ions and the ligands coordinated around them. Interestingly, in comparison to the intensity of the emission spectrum of free H2L ligand, the intensity of the luminescence spectrum of complexes 1, 3 and 4 have been found to be enhanced, which might originated from the ligands coordination to the metal centres. This phenomenon expressly shows the fact that the rigidity H2L ligand restrains its configuration in complex 1, 3 and 4 consequently reduces the degree of freedom (compared to that of free H2L ligand), thereby decreasing the loss of energy via radiationless decay [30]. Luminescence studies indicate that complexes 1, 3 and 4 are better fluorescent materials as compared to complex 2. 3.5. Luminescence lifetimes To understand the luminescence properties of complexes 1–4 and H2L ligand, we also performed time-resolved measurements of complexes 1–4 and ligand by using the time-correlated single photon counting (TCSPC) technique. As shown in Fig. S4 of Supple-
mentary 2, the decay curves for solid complexes 1–4 and H2L ligand exhibit triple exponential behavior, irrespective of the media. The fluorescence decay lifetimes of the complexes 1–4 and H2L ligand are quite distinct (s = 1.497 ns for H2L; s = 0.039 ns for complex 1; s = 33.554 ns for complex 2; s = 31.955 ns for complex 3; s = 4.215 ns for complex 4, respectively) (Table S3 of Supplementary 1). These long lifetimes may be attributed to the strong chelating coordination ability and steric architectonic stabilization of the H2L ligand, which offering an effective pathway of energy transfer. In a word, complexes 1–4 exhibits higher luminescence lifetimes in the sequence 1 < H2L < 4 < 3 < 2 at 298 K, which emphasizes the significant impact of the different sizes, structural models, and the latticed water molecules on luminescent properties [31]. Additionally, coordinated H2O exists in complexes 1–4, which favors high-energy vibrational quenching via OAH oscillators and is expected to exhibit a relatively shorter lifetimes [32]. 3.6. Magnetic properties The variable-temperature magnetic susceptibility measurements of complex 1 was investigated in the temperature range of 2–300 K with an applied magnetic field of 1000 Oe. The data obtained for complex 1 are represented in Fig. 3. The value of vMT of complex 1 is 24.62 cm3 K mol 1 at room temperature (300 K), which is coincident with a theoretical value of 14.17 cm3 K mol 1 for one magnetically isolated Dy3+ ion (6H15/2, S = 5/2, L = 5, g = 4/3) in the free-ion approximation. Upon cooling, vMT starts to slowly decrease to 50 K, and then sharply decreases to
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4), respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data to this article can be found online at https://doi.org/10.1016/j.poly.2019.04.006. References
Fig. 3. Magnetic property of complex 1 in the form of vMT and vM1 versus T plot. The red solid line represents the fitting results over the range of 50–300 K. (Color online.)
11.13 cm3 K mol 1, which is more likely to be ascribed to the thermal depopulation of the Stark sublevels, and the possible antiferromagnetic intramolecular Dy–Dy interactions or dipole–dipole interactions between the molecules [33]. Also, the temperature dependence of the reciprocal susceptibilities (vM1) of complex 1 obeys the Curie–Weiss law (vM = C/(T h)), the best-fit parameters for the Curie–Weiss model in the temperature region of 50–300 K give Cm = 25.09 cm3 K mol 1 and h = 3.30 K. The negative h value further supports the existence of antiferromagnetic exchange interactions between the Dy3+ ions. 4. Conclusion In summary, we have successfully synthesized four new Ln-CPs constructed from 2-(hydroxymethyl)-1H-benzo[d]imidazole-5carboxylic acid (H2L) under solvothermal conditions. The distinct metal centers and reaction conditions are key factors to construct diverse frameworks. 1D chain structure of complex 4 has further assemble to form 2D sheet and 3D supramolecular network by hydrogen bonding interactions between the coordinated water molecules and the O atoms of the hydroxy group. Complexes 1–3 are isomorphic and display 3D supramolecular network constructed by hydrogen bonding interactions. The X-ray powder diffraction (PXRD) investigated complexes 1–4, which demonstrates that they have high phase purity. Moreover, thermal stability shows that complexes 1–4 possessed high thermostability. Noteworthily, intense fluorescence emission and lifetimes of 1–4 and H2L ligand exhibit that complexes 1, 3 and 4 are better fluorescent materials as compared to complex 2. Magnetic studies shows that antiferromagnetic exchange couplings between neighboring Ln3+ ions that also exists in complex 1. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (No. 21361026). Appendix A. Supplementary data CCDC 1896903–1896906 contains the supplementary crystallographic data for complexes {[Ln(HL)2(H2O)3](H2O)4Cl}n (Ln = Dy, 1; Eu, 2; Sm, 3) and [Ln0.18(HL)0.18(CH3COO)0.36(H2O)0.54]n (Ln = La,
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