A series of lanthanide complexes with different N-donor ligands: synthesis, structures, thermal properties and luminescence behaviors

Abstract

Four novel lanthanide complexes, [Ln(2,4-DClBA)₃(terpy)(H₂O)]·H₂O (Ln = Eu(1), Tb(2)); [Ln(2,4-DClBA)₃(5,5′-DM-2,2′-bipy)(C₂H₅OH)]₂ (Ln = Eu(3), Tb(4); 2,4-DClBA: 2,4-dichlorobenzoate; terpy: 2,2′:6′,2′′-terpyridine; 5,5′-DM-2,2′-bipy: 5,5′-dimethyl-2,2′-bipyridine) have been synthesized via a conventional solution method at room temperature and structurally characterized by single crystal and powder X-ray diffraction. Complexes 1–2 exhibit mononuclear lanthanide architectures and each Ln³⁺ ion is nine-coordinated adopting a distorted monocapped square antiprismatic molecular geometry, while complexes 3–4 exhibit binuclear lanthanide architectures in which each Ln³⁺ ion is eight-coordinated adopting a distorted square antiprismatic molecular geometry. Mononuclear complexes 1–2 are stitched together via Cl–π and hydrogen bonding interactions to form the 1D, 2D, 3D supramolecular structures. While complexes 3–4 are packed together through Cl–Cl, π–π, and hydrogen bonding interactions to form 1D, 2D supramolecular structures. Luminescence investigation reveals that complexes 1, 3 and 2, 4 display strong red and green emission respectively, showing that terpy and 5,5′-DM-2,2′-bipy can act as sensitizing chromophores, but the former is more effective. The IR, TG-DTG, and the heat capacities of complexes 1–4 were also measured.

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Introduction

In recent years, the study of lanthanide complexes combined with aromatic-carboxylic acids has attracted more and more attention and many compounds featuring a diverse array of coordination types have been recognized.^1–3 On the other hand, a wide variety of applications including electroluminescent materials, non-linear optics, luminescent bioprobes,⁴ catalysis⁵ and ion exchange⁶ have been utilized as functional materials. However, assembly of the structures is related to many external factors, such as flexible coordination geometry of the lanthanide ion, molecular configuration of the ligands, and different experimental conditions etc.^7–9

Lanthanide luminescence is always a major research topic because of the unique nature of lanthanide ion emission. But direct excitation of lanthanide metal centers is impeded by small absorption cross-sections, low molar absorption coefficients and the Laporte forbidden f–f transitions.^10,11 So effective energy transfer from an adjacent strongly absorbing chromophore is usually used to stimulate luminescence of lanthanide ions, which is called “antenna effect”.^12,13 To meet the condition of efficient sensitization of the corresponding lanthanide ions, the triplet states energies of organic chromophores should be high or close to lanthanide ions.^14,15

In this paper, 2,4-dichlorobenzoic acid (2,4-DClHBA) is selected as the main ligand, and 2,2′:6′,2′′-terpyridine (terpy) and 5,5′-DM-2,2′-bipy (5,5′-dimethyl-2,2′-bipyridine) as auxiliary ligands to validly sensitize the lanthanide metal ions to assemble four novel hybrid compounds [Ln(2,4-DClBA)₃(terpy)(H₂O)]·H₂O (Ln = Eu(1), Tb(2)), [Ln(2,4-DClBA)₃(5,5′-DM-2,2′-bipy)(C₂H₅OH)]₂ (Ln = Eu(3), Tb(4); 2,4-DClBA = 2,4-dichlorobenzoate; terpy = 2,2′:6′,2′′-terpyridine; 5,5′-DM-2,2′-bipy = 5,5′-dimethyl-2,2′-bipyridine). The ligand 5,5′-DM-2,2′-bipy has a weaker binding affinity for the Ln³⁺ ions than the tridentate N-donor terpy, so the two ligands have two different coordination types. The difference with our previous reports^16–20 is that the structures of complexes 1 and 2 are mononuclear molecules. While the complexes 3 and 4 are binuclear, which may be due to the effect of larger space of auxiliary ligands. The thermal decomposition processes were studied by the TG/DTG technology, and the heat capacities of the complexes were measured using DSC. In addition, Powder Xrd, IR and luminescence spectrum were also studied.

Experimental

Materials and methods

LnCl₃·6H₂O were acquired by the reaction of Eu₂O₃ and Tb₄O₇ respectively (Ln = Eu, Tb, Beijing Lanthanide Innovation Technology Co., Ltd, 99.9%) and hydrochloric acid aqueous solution in the condition of water bath heating at a constant temperature of 353.15 K, followed by evaporation of the liquid. The other analytically pure chemicals were purchased and used without further purification.

Equipment and conditions of the experiment

The percentage of C, H, N in the corresponding complexes were analyzed via Vario-EL II element analyzer and the percentage of the lanthanide cations were obtained by EDTA titrimetric analysis. The data of single crystal X-ray diffraction were collected on a Smart-1000 diffractometer with graphite-monochromatic Mo Kα (λ = 0.71073 Å) for complexes 1–4 at 298(2) K. The structures were solved using the SHELXS-97 program (direct methods) and refined by the full-matrix least-squares on F² using the SHELXL-97 program. Powder X-ray diffraction identification was carried out by a Bruker D8 ADVANCE X-ray diffractometer in a scanning range of 5–50° (2θ) with graphite-monochromatic Cu Kα (λ = 1.54178 Å) at 298(2) K. IR spectra were measured on a Bruker TENSOR27 spectrometer with KBr medium pellets in the range of 4000–400 cm⁻¹. TG and DTG analyses were performed with a heating rate of 10 K min⁻¹ (simulated air atmosphere) using a NETZSCH STA 449 F3 instrument. The luminescence spectra were measured on an F-4600 Hitachi Spectrophotometer. The solid fluorescence quantum yields of the title complexes were measured using C9920-02G Hamamatsu test system, which was constituted by integrating sphere of 10-inch diameter and connected to a CCD detector. Heat capacities of the prepared complexes were carried out on a NETZSCH DSC 200 F3 (nitrogen atmosphere) in the temperature of 263.15–337.15 K using an indirect measurement method.

Synthesis

Synthesis of [Ln(2,4-DClBA)₃(terpy)(H₂O)]·H₂O (Ln = Eu(1), Tb(2))

Dissolve 2,4-DClHBA (0.6 mmol) and terpy (0.2 mmol) in ethanol (95%) and adjust the solution at the pH of 6–7 with the prepared NaOH solution (1 mol L⁻¹). Add the mixed ligands solution to LnCl₃·6H₂O (0.2 mmol) aqueous solution under stirring and deposit it for 12 hours. The powders of target complexes were obtained by filtration and the colorless platy single crystals were acquired through the method of solvent extraction at room temperature. Element analysis: calcd (%) for C₃₆H₂₄Cl₆N₃O₈Eu: C, 43.26; H, 2.44; N, 4.24; Eu, 15.33. Found: C, 43.27; H, 2.47; N, 4.15; Eu, 15.30. Calcd (%) for C₃₆H₂₄Cl₆N₃O₈Tb: C, 43.31; H, 2.42; N, 4.21; Tb, 15.92. Found: C, 43.72; H, 2.46; N, 4.06; Tb, 15.89.

Synthesis of [Ln(2,4-DClBA)₃(5,5′-DM-2,2′-bipy)(C₂H₅OH)]₂ (Ln = Eu(3), Tb(4))

The synthesis of complexes 3 and 4 were similar to the above description for 1 and 2 except that the auxiliary ligands of terpy was replaced by 5,5′-DM-2,2′-bipy and the single crystals of complexes 3–4 are colorless with the shape of needle-like. Element analysis: calcd (%) for C₇₀H₅₄Cl₁₂N₄O₁₄Eu₂: C, 44.14; H, 2.86; N, 2.94; Eu, 15.96. Found: C, 43.95; H, 2.79; N, 3.02; Eu, 15.90. Calcd (%) for C₇₀H₅₄Cl₁₂N₄O₁₄Tb₂: C, 43.82; H, 2.84; N, 2.92; Tb, 16.57. Found: C, 43.75; H, 2.80; N, 2.88; Tb, 16.55.

Results and discussion

Crystal structure

Crystallographic data of complexes 1–4 are shown in Table 1, indicating the four complexes in the same crystal system (triclinic) and space group (P1̄). But the structures are different: the complexes 1–2 with the mononuclear structures and the complexes 3–4 with another structure of binuclear. As a consequence, the crystal structures of 1 and 3 will be described here in detail.

Table 1 Crystallographic data for complexes 1–4

	1	2	3	4
Empirical formula	C36H24Cl6N3O8Eu	C36H24Cl6N3O8Tb	C70H54Cl12N4O14Eu2	C70H54Cl12N4O14Tb2
Formula weight	991.24	998.20	1904.49	1918.41
Temperature (K)	298(2)	298(2)	298(2)	298(2)
Wavelength (Å)	0.71073	0.71073	0.71073	0.71073
Crystal system	Triclinic	Triclinic	Triclinic	Triclinic
Space group	P1̄	P1̄	P1̄	P1̄
Unit cell dimensions
a (Å)	11.7033(9)	11.7424(8)	10.1249(10)	10.1571(8)
b (Å)	12.5536(11)	12.6760(12)	13.1148(12)	13.0730(11)
c (Å)	13.3565(13)	13.4305(13)	15.5331(13)	15.5692(13)
α (°)	83.730(2)	83.812(2)	79.359(2)	79.315(2)
β (°)	77.3790(10)	77.887(2)	72.2760(10)	72.1150(10)
γ (°)	74.2240(10)	74.1320(10)	79.733(2)	79.624(2)
Volume (Å^3)	1840.2(3)	1877.4(3)	1914.4(3)	1916.5(3)
Z, calculated density (mg m^−3)	2, 1.789	2, 1.766	1, 1.652	1, 1.662
Absorption coefficient (mm^−1)	2.197	2.367	2.105	2.312
F(000)	980	984	944	948
Crystal size (mm)	0.25 × 0.21 × 0.12	0.42 × 0.27 × 0.12	0.33 × 0.14 × 0.11	0.31 × 0.13 × 0.11
Theta range for data collection/deg	2.39 to 25.02	2.34 to 25.02	2.13 to 25.02	2.41 to 25.02
Limiting indices	−13 ≤ h ≤ 13	−11 ≤ h ≤ 13	−11 ≤ h ≤ 12	−11 ≤ h ≤ 12
	−14 ≤ k ≤ 14	−15 ≤ k ≤ 15	−15 ≤ k ≤ 15	−15 ≤ k ≤ 15
	−12 ≤ l ≤ 15	−11 ≤ l ≤ 15	−14 ≤ l ≤ 18	−12 ≤ l ≤ 18
Reflections collected/unique	9278/6341 [R(int) = 0.0700]	9494/6468 [R(int) = 0.0280]	9870/6650 [R(int) = 0.0550]	9847/6647 [R(int) = 0.0434]
Completeness to theta = 25.02	97.7%	98.0%	98.3%	98.3%
Max. and min. transmission	0.7784 and 0.6096	0.7643 and 0.4364	0.8014 and 0.5433	0.7851 and 0.5343
Data/restraints/parameters	6341/0/487	6468/0/487	6650/0/463	6647/0/463
Goodness-of-fit on F^2	1.076	1.062	1.056	1.007
Final R indices [I > 2σ(I)]	R1 = 0.0895	R1 = 0.0393	R1 = 0.0676	R1 = 0.0520
	wR2 = 0.2078	wR2 = 0.0828	wR2 = 0.1199	wR2 = 0.0787
R indices (all data)	R1 = 0.1647	R1 = 0.0557	R1 = 0.1030	R1 = 0.0801
	wR2 = 0.2428	wR2 = 0.0917	wR2 = 0.1286	wR2 = 0.0843
Largest diff. peak and hole (e Å^−3)	3.468 and −1.534	1.236 and −0.760	2.028 and −1.296	1.118 and −0.965

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[Ln(2,4-DClBA)₃(terpy)(H₂O)]·H₂O (Ln = Eu(1), Tb(2)). Unlike with the most synthesized binuclear complexes, the complex 1 is mononuclear and the Eu³⁺ ion is bound by a tridentate terpy, a coordinated water molecule and three 2,4-DClBA ligands adopting two different coordination modes: bidentate chelateing, monodentate (Fig. 1a). Each Eu³⁺ ion is nine-coordinated adopting a distorted monocapped square antiprismatic molecular geometry (Fig. 1b) and among them there are five oxygen atoms (O1, O2, O3, O4, O5) from three 2,4-DClBA ligands, one oxygen atom (O7) supported by the coordinated water, three nitrogen atoms (N1, N2, N3) provided by the tridentate terpy. The Eu–O bond lengths range from 2.309(10) Å to 2.620(9) Å. Average Eu–N bond distances are 2.556 Å (Table S1, ESI†).

Fig. 1 (a) Crystal structure of complex 1, all H atoms omitted for clarity. (b) Coordination geometry of Eu³⁺ ion.

The mononuclear molecules of complex 1 are stitched together via O–H⋯O hydrogen bonding interactions (Table 2) and two independent and moderate Cl–π interactions to form a supramolecular 1D chain along the z axis direction (Fig. 2a). The distance of the O–H⋯O hydrogen bonds between carboxylic oxygen from bidentate 2,4-DClBA ligands and uncoordinated water molecules of neighboring unit is 2.424 Å. There are interactions between the chlorine (Cl2) of one 2,4-DClBA ligand with the edge of a terpy unit (C35) on the adjacent unit at a distance of 3.368 Å, and then a second interaction from the chlorine (Cl4) of another 2,4-DClBA ligand to the edge of a different neighboring terpy unit (C23) at a distance of 3.546 Å. All of the halogen–π interactions^21,22 are effective because that the two kinds of distance are similar with 3.450 Å (sums of the van der Waals radii with of carbon with lone pair containing atom).¹²

Table 2 Selected hydrogen-bond lengths (Å) and angles (°) for complex 1 and 3^a

Complex	Hydrogen-bond	D–H	H⋯A	D⋯A	D–H⋯A
a Symmetry codes: (i) x, y, z + 1.
1	O8H8⋯O4i	0.848	2.242	3.017	151.82
	C25H25⋯O6	0.931	2.594	3.245	127.46
	C25H25⋯O8	0.930	2.514	3.298	142.19
	O8H8⋯C12	0.844	2.850	3.627	153.93
3	C13H13⋯O6	0.930	2.884	3.534	128.08

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Fig. 2 (a) 1D chain structure along the z axis. (b) 2D sheets connected by hydrogen bond interactions in the xz plane. (c) 3D structure interconnected by hydrogen bonds (uncoordinated water units have been omitted for clarity).

The chains are further packed into a 2D sheet in the xz plane (Fig. 2b) through two kinds of different C–H⋯O hydrogen bonding interactions.^23–25 One is between carboxylic oxygen from monodentate 2,4-DClBA ligands and C–H of the adjacent terpy ligands unit at a distance of 2.594 Å along x-axis, and anther C–H⋯O hydrogen bonds exists between uncoordinated water molecules and C–H of the neighboring terpy unit along the line of angle from z-axis and negative x-axis at the distance of 2.514 Å. In the crystallography, the distance of hydrogen bonding is the most significant data to decide whether the interaction is effective or not, and the above two types of hydrogen bonding interactions within the range of 3 Å are all valid.^26,27 There exist O–H⋯C hydrogen bonds between carbon from 2,4-DClBA ligands unit and O–H of the adjacent coordinated water unit at the distance of 2.850 Å to form 3D supramolecular network (Fig. 2c).

[Ln(2,4-DClBA)₃(5,5′-DM-2,2′-bipy)(C₂H₅OH)]₂ (Ln = Eu(3), Tb(4)). Complex 3 is binuclear molecule and the two Eu³⁺ ions with the distance of 4.4615(10) Å have the same coordination environment. Each Eu³⁺ ion is coordinated by one 5,5′-DM-2,2′-bipy, one ethanol molecule and three 2,4-DClBA ligands adopting two different coordination modes: bridging bidentate, monodentate (Fig. 3a). The Eu³⁺ ion is eight-coordinated and composed a distorted square antiprismatic molecular geometry (Fig. 3b) and there are five oxygen atoms (O1, O2, O3, O4, O5) from three 2,4-DClBA ligands, one oxygen atom (O7) supported by the coordinated ethanol molecules, two nitrogen atoms (N1, N2) provided by the 5,5′-DM-2,2′-bipy ligand. The Eu–O bonds are within the distance of 2.354(6) Å and 2.486(6) Å and the average Eu–N bond distance is 2.606 Å.

Fig. 3 (a) Crystal structure of complex 3, all H atoms omitted for clarity. (b) Coordination geometry of Eu³⁺ ion.

The binuclear units are stitched together via the weaker halogen–halogen interactions,^28,29 which is similar to halogen–π interaction,³⁰ and π–π stacking interactions^31,32 to form a supramolecular 1D chain along the y axis direction (Fig. 4a). The distance of the halogen–halogen interactions between Cl1 from bridging bidentate 2,4-DClBA ligands and Cl5 from monodentate 2,4-DClBA ligands of neighboring unit is 3.737 Å. The slightly offset π–π interactions are between the centroid of the moiety of 5,5′-DM-2,2′-bipy on one unit and the edge of the bipy ring on the neighboring unit with the relevant distances and angles: Cg⋯Cg = 3.885(2) Å; Cg_⊥⋯Cg_⊥ = 3.555(3) Å; β = 20.179°. The chains are further packed into a 2D sheet in the xy plane (Fig. 4b) through C–H⋯O hydrogen bonding interactions, oxygen from monodentate 2,4-DClBA ligands and C–H of the adjacent bipy ligands unit at a distance of 2.884 Å along x-axis. The hydrogen bonding interactions within the range of 3 Å are also efficient.

Fig. 4 (a) 1D chain structure along the y axis. (b) 2D sheets connected by hydrogen bond interactions in the xy plane.

Structural discussion

As mentioned above, complexes [Ln(2,4-DClBA)₃(terpy)(H₂O)]·H₂O (Ln = Eu(1), Tb(2)) exhibit the crystal structure of mononuclear with nine-coordinated, which is different from binuclear complexes [Ln(2,4-DClBA)₃(5,5′-DM-2,2′-bipy)(C₂H₅OH)]₂ (Ln = Eu(3), Tb(4)) with eight-coordinated. The difference of the crystal structure is due to the different N-donor ligands between complexes 1–2 and complexes 3–4. The tridentate terpy have a stronger blinding affinity for the Ln³⁺ ions than the bidentate N-donors methyl substituted derivative of 2,2′-bipyridine (5,5′-DM-2,2′-bipy)³⁰ and the steric hindrance of terpy is smaller than the auxiliary ligand 5,5′-DM-2,2′-bipy. At 2014, Liu JY et al. have reported the complex [Eu(2,4-DClBA)₃(phen)], which the complex is binuclear and each Eu³⁺ ion is eight-coordinated.³³ The main ligands 2,4-DClBA adopts two coordination modes: bidentate and bridging bidentate. That is also proved that the difference of crystal structure depends on the choice of auxiliary ligand. Not only the auxiliary ligands but also the main (carboxylic acid) ligands can influence the crystal structures. The complex1 [Eu(2,4-DClBA)₃(terpy)(H₂O)]·H₂O is mononuclear and each Eu³⁺ is nine-coordinated. The 2,4-DClBA adopts two coordination modes: bidentate and monodentate. Mononuclear complexes 1–2 are stitched together via Cl–π and hydrogen bonding interactions to form the 1D, 2D, 3D supramolecular structures, which the non-covalent interactions (Cl–π, Cl–Cl interactions) is significant if the distance separating the electron-rich atom from one atom of the six-membered ring is in the range of the sum of their van der Waals radii.¹² At 2015, Jun-Chen Wu, et al. discussed the structure of complex [Eu(2,3-DClBA)₃(terpy)(H₂O)]₂, which the complex is binuclear and each Eu³⁺ ion is nine-coordinated.³⁴ The 2,3-DClBA adopts three coordination modes: bidentate, bridging bidentate and monodentate. There is no non-covalent interactions between the complex [Eu(2,3-DClBA)₃(terpy)(H₂O)] and it may be due to the substitution position of chloride atom. The non-covalent interactions also influence the thermal stability of the complex. Because the [Eu(2,3-DClBA)₃(terpy)(H₂O)]₂ begin to decompose at the temperature of 356.15 K, but the molecular frame of complex 1 start to resolve at 393.15 K. Besides molecular configuration of ligands, solvent molecules also affect the assembly of structure. For the title complexes, the water and ethanol molecules are all coordinated with the Ln³⁺ ions, respectively. The coordinated water and ethanol molecules are not simply fulfill the coordination requirement because of the coordination number of rare earth ions depends on many factors, which is not fixed. But it is to be sure that the coordinated water and ethanol molecules contribute to the structural pattern. For complexes 1–2, the coordinated water facilitates intermolecular hydrogen bonding interactions between tectons and it is in favor of the formation of the 3D supramolecular structures.

Powder X-ray diffraction

The powder X-ray diffraction (PXRD) patterns have been carried out at room temperature. The experiment and simulated curves of complexes 1–4 are shown in Fig. S1.† The diffraction peaks (a, b) are almost in agreement with simulated data 1–2, showing that the structures of the powder of complexes 1–2 are similar with the pure crystal. The peaks of experiment and simulated curves at about 7.0° are all dominating and this could be due to preferred orientation of crystal growth. As shown in Fig. S1b,† the curves (c, d) are in agreement with simulated data 3–4, which is confirmed that the structure of the powder of complexes 3–4 are the same with the pure crystal. In addition, the peaks are dominating at the 2θ with about 6.0 and 9.0°, which is related to the preferred orientation question.

IR spectra

As we all known, the information of the chemical bonds and functional groups could be obtained intuitively in the infrared spectra, because of specific groups corresponding the proper position of the infrared absorption.^35,36 The detailed infrared spectra of two kinds of ligands and complexes 1–4 have been shown in the Fig. S2.† The stretching vibration peaks at 1470 and 1467 cm⁻¹ attributed to the C=N bonds in terpy (g) and 5,5′-DM-2,2′-bipy (f) take place blue shift to 1481 (complex 1), 1484 cm⁻¹ (complex 2) and 1484 (complex 3), 1485 cm⁻¹ (complex 4), which suggests that nitrogen atoms of the neutral ligands coordinate with the lanthanide ions. It is found that characteristic vibration frequencies ν_C=O (1698 cm⁻¹) of free carboxylic acid ligands for 2,4-DClHBA (e) disappear in the spectrum of the complexes 1–4 and appear two another new absorption peak at 1588, 1594 cm⁻¹ and 1582, 1585 cm⁻¹, which are attributed to the asymmetric stretching vibrations (ν_asCOO⁻) and the symmetric stretching vibrations (ν_sCOO⁻) of the carboxylic group at 1405, 1406 cm⁻¹ and 1413, 1416 cm⁻¹ in the complexes, indicating coordination between 2,4-DClBA and Ln³⁺ ions via Ln–O. At the same time, the appearance of absorption peaks at about 524, 527 cm⁻¹ and 510, 512 cm⁻¹ are also attributed to the coordination of 2,4-DClBA and Ln³⁺ ions.

Thermal decomposition processes

The thermal decomposition processes of complexes 1–4 were determined at a heating rate of 10 K min⁻¹ under a simulated air atmosphere and the processes are related to the crystal structure of complex. There exist two types of crystal structures for complexes 1–4, which the complexes 1–2 are similar with mononuclear structure and complexes 3–4 are the same with binuclear structure. So the thermal decomposition processes of complexes 1–2 and 3–4 are different with each other and the thermal behaviors of complex 1 and complex 3 would be described in detail. The TG and DTG curves of complexes 1–4 are shown in Fig. 5 and the data of thermal analysis for the title complexes are listed in Table 3. As shown in Fig. 5, there are at least four thermal decomposition processes according to the DTG curve of complex 1. The first stage belongs to the release of one uncoordinated water between 377.15 K and 393.15 K approximately (Calcd: 1.82%, Obsd: 1.88%), the second stage at temperature of 393.15–680.15 K around accounting for the loss of one coordinated water and one terpy (Calcd: 25.35%, Obsd: 25.42%), the third and last belonging to the rest of three 2,4-DClBA ligands (Calcd: 55.08%, Obsd: 54.74%) at about 680.15–958.15 K. Up to ∼958.15 K, the complex is completely degraded into Eu₂O₃ with a total mass loss of 82.04% (calc. 82.25%). The thermal decomposition process of complex 3 divides into four steps according to the curve of DTG. The first step begins at ∼355.15 K and ends at ∼466.15 K with a mass loss of about 4.82% (Calcd: 4.84%), because of two coordinated ethanol molecules being resolved at that time. The second step takes place at the range of 466.15–697.15 K approximately with a mass lose of 19.30% (Calcd: 19.34%), on account of the decomposition of two 5,5′-DM-2,2′-bipy ligands. The third and last between ∼697.15 K and ∼1066.15 K correspond to the decomposition of six 2,4-DClBA ligands with the mass loss of 56.87% (Calcd: 57.34%), and the complex 3 is completely degraded into Eu₂O₃.

Fig. 5 TG-DTG curves of complexes 1–4 (complex 1 = a, complex 2 = b, complex 3 = c, complex 4 = d).

Table 3 Thermal decomposition data of the title complexes

Complexes	Step	Temperature range (K)	DTG (Tp, K)	Mass loss rate (%)		Probable expelled groups	Residue
				Calcd	Found
a Theoretical total weight loss of H2O.b Theoretical total weight loss of 2C2H5OH.c Theoretical total weight loss of H2O and terpy.d Theoretical total weight loss of 2(5,5′-DM-2,2′-bipy).e Theoretical total weight loss of 2,4-DClBA. Tp is the peak temperature of DTG.
1	I	377.15–393.15	390.33	1.82^a	1.88	H2O	[Eu(2,4-DClBA)3(terpy)(H2O)]
	II	393.15–680.15	509.95	25.32^c	25.42	H2O + terpy	[Eu(2,4-DClBA)3]
	III	680.15–774.15	752.75	55.08^e	34.16	x(2,4-DClBA)	[Eu(2,4-DClBA)3−x]
	IV	793.15–958.15	815.15		20.58	(3 − x) 2,4-DClBA	Eu2O3
2	I	380.15–399.15	391.45	1.81^a	1.80	H2O	[Tb(2,4-DClBA)3(terpy)(H2O)]
	II	399.15–688.15	509.55	25.17^c	25.15	H2O + terpy	[Tb(2,4-DClBA)3]
	III	688.15–763.15	743.75	54.30^e	38.42	x(2,4-DClBA)	[Tb(2,4-DClBA)3−x]
	IV	783.15–940.15	817.55		15.77	(3 − x) 2,4-DClBA	Tb4O7
3	I	355.15–466.15	382.95	4.84^b	4.82	2C2H5OH	[Eu(2,4-DClBA)3(5,5′-DM-2,2′-bipy)]2
	II	466.15–697.15	511.95	19.34^d	19.30	2(5,5′-DM-2,2′-bipy)	[Eu(2,4-DClBA)3]2
	III	697.15–783.15	759.95	57.34^e	35.35	x(2,4-DClBA)	[Eu(2,4-DClBA)6−x]
	IV	783.15–1066.15	813.45		21.52	(6 − x) 2,4-DClBA	Eu2O3
4	I	359.15–474.15	384.45	4.80^b	4.70	2C2H5OH	[Tb(2,4-DClBA)3(5,5′-DM-2,2′-bipy)]2
	II	474.15–718.15	512.45	19.20^d	19.17	2(5,5′-DM-2,2′-bipy)	[Tb(2,4-DClBA)3]2
	III	718.15–792.15	768.95	56.26^e	39.05	x(2,4-DClBA)	[Tb(2,4-DClBA)6−x]
	IV	792.15–1201.15	821.65		17.21	(6 − x) 2,4-DClBA	Tb4O7

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Heat capacities of the complexes

Heat capacity is overall nature of the materials and it can be expressed as the lattice, electronic, magnetic and other properties of different energy contribution combined. The heat capacities of the complexes 1–4 were obtained at a heating rate of 10 K min⁻¹ under the nitrogen atmosphere. It is important for the measured temperature of the molar heat capacities of complexes to be inferior to the temperature of decomposition. So the properties were testified at the temperature of survey, ranging from 263.15 K to 347.15 K. The average molar heat capacities of four complexes are listed in Table S2† and plotted in Fig. 6. As shown in Fig. 6 the molar heat capacities of complexes, all gradually increase with the augment of temperature, indicating the complexes possess good thermal stability within the range of temperature and there is no phase change or any other heat anomaly. While the C_p,m of complexes 1–2 are higher than the complexes 3–4 at the same temperature, which is maybe due to the different crystal structure. Within the measured temperature, the determination of heat capacity depends on the vibration of molecules and the more intense molecular vibration, the bigger heat capacity values is. The molecular vibration is related to the crystal structure, so the heat capacities of complexes 1–2 are similar, according to the same energy of molecular vibration and same crystal structure. There are different crystal structures between complexes 1–2 and complexes 3–4, which leads to the different heat capacity. The heat capacities of the complexes were fitted to the following polynomial equations by the method of least squares^37–39 at reduced temperature (x), which is calculated from the formula x = [T − (T_max + T_min)/2]/[(T_max − T_min)/2] (T_max = the highest temperature, T_min = the lowest temperature) and the correlation coefficient (R²) and standard deviation (SD) were also obtained. The smoothed heat capacities, enthalpy, entropy and Gibbs free energy relative to the reference temperature 298.15 K of the four complexes with an interval of 3 K (Table S3–S6†) were calculated according to the fitted polynomial equations and the following thermodynamic equations:

Fig. 6 Relationship of the molar heat capacities varying with temperature of complexes 1–4.

Complex 1 [Eu(2,4-DClBA)₃(terpy)(H₂O)]·H₂O, T = 263.15–337.15 K

C_p,m/J mol⁻¹ K⁻¹ = 754.02163 + 92.83805x + 8.68863x² − 22.19138x³ + 14.17503x⁴ + 34.85620x⁵ − 23.51122x⁶ − 14.85551x⁷ + 10.08865x⁸, R² = 1, SD = 0.11864

Complex 2 [Tb(2,4-DClBA)₃(terpy)(H₂O)]·H₂O, T = 263.15–337.15 K

C_p,m/J mol⁻¹ K⁻¹ = 782.45754 + 92.37852x + 13.97342x² − 25.65639x³ + 7.56629x⁴ + 65.53637x⁵ + 18.26601x⁶ − 35.37829x⁷ − 20.02932x⁸, R² = 0.99999, SD = 0.17294

Complex 3 [Eu(2,4-DClBA)₃(5,5′-DM-2,2′-bipy)(C₂H₅OH)]₂, T = 263.15–337.15 K

C_p,m/J mol⁻¹ K⁻¹ = 1540.25471 + 162.34793x + 38.63713x² + 12.29317x³ − 77.86584x⁴ − 41.23090x⁵ + 54.92960x⁶ + 27.55364x⁷ − 10.27386x⁸, R² = 0.99999, SD = 0.29275

Complex 4 [Tb(2,4-DClBA)₃(5,5′-DM-2,2′-bipy)(C₂H₅OH)]₂, T = 263.15–337.15 K

C_p,m/J mol⁻¹ K⁻¹ = 1660.08032 + 149.11880x + 25.01921x² + 22.05730x³ − 40.83 170x⁴ − 42.76729x⁵ + 15.08848x⁶ + 22.30919x⁷ + 2.55557x⁸, R² = 0.99999, SD = 0.25793

Luminescent behavior

The solid-state luminescent behaviors of complexes 1–4 have been investigated in detail at room temperature and the excitation and emission spectra of the title complexes are shown in Fig. 7. The excitation spectra of complexes 1 and 3 were recorded from 200–450 nm with the emission wavelength of 621 and 618 nm, respectively. As shown in Fig. 7a, there is a prominent broad band from 220 nm to 390 nm for complex 1 and a broad band from 220 nm to 380 nm for complex 3, which are all assigned to the absorption of the auxiliary ligands. Complexes 1 and 3 exhibit an intense characteristic red emission light under UV light, as can be seen from the CIE chromaticity diagrams in Fig. 7c and the detailed CIE chromaticity coordinate values of the above complexes in Table S7.† They all show the high purity color and could be considered applying in the optical materials.⁴⁰ The emission spectrums of complexes 1 and 3 were obtained with excitation wavelength at 330 nm and showed four different characteristic peaks at 596, 621, 653, 695 nm and 582, 618, 654, 705 nm, respectively in the range of 550–750 nm. These are associated with the 4f–4f transitions of ⁵D₀ excited state to its low-lying ⁷F_J (J = 1, 2, 3, and 4) levels of Eu³⁺ ions. The hypersensitive transitions (⁵D₀ → ⁷F₂) at 621 nm for complex 1 and 618 nm for complex 3 dominate the whole emission spectra, resulting in the characteristic red luminescence of europium complexes and the emissions of Eu³⁺ ions usually employed as a sensitive probe to investigate the local environment around cations.⁴¹ Due to the low symmetry of the Eu³⁺ metal center in both complexes 1 and 3, the transition of ⁵D₀ → ⁷F₂ are much more intense than ⁵D₀ → ⁷F₁ transition at the spectra of both complexes. The total fluorescent quantum yields are 0.477 (λ_ex = 330 nm) and 0.474 (λ_ex = 330 nm) for complexes 1 and 3 respectively, which are almost the same with each other, and the maximum quantum yield is observed for complexes 1 and 3 are 0.235 and 0.233, which are much higher than the related complexes.⁴² By comparing the excitation and emission spectrums and fluorescence quantum yields of both complexes 1 and 3, it may be come to the conclusion that the emissions of Eu³⁺ ions sensitized by the ligands of terpy and 5,5′-DM-2,2′-bipy.

Fig. 7 (a) Excitation spectra (left) and emission spectra (right) for complexes 1 and 3. (b) Excitation spectra (left) and emission spectra (right) for complexes 2 and 4. (c) CIE chromaticity diagram of the title complexes.

The excitation spectra of complexes 2 and 4 were recorded from 200–400 nm with the emission wavelength of 548 nm. This reveals a broad band from 200 nm to 370 nm for complex 2 and a broad band from 200 nm to 350 nm for complex 4, which are also assigned to the absorption of the auxiliary ligands. Complexes 2 and 4 exhibit an intense characteristic green emission light under UV light. The CIE chromaticity diagrams and detailed CIE chromaticity coordinate values of the complexes 2 and 4 can be seen in Fig. 7c and Table S7,† which indicates the complexes with high purity color. The emission spectrums of complexes 2 and 4 (the Tb³⁺ complexes) were received after excitation at 310 nm. As shown in Fig. 7b, the spectral bands of the terbium complexes 2 and 4 at 491, 548, 588, 624 nm and 492, 548, 589, 624 nm, attributing to the ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄, and ⁵D₄ → ⁷F₃ electronic transitions of the Tb³⁺ ions respectively. There are the strongest emission peaks at 548 nm, corresponding the transition of ⁵D₄ → ⁷F_5, and which thus result the green luminescence of Tb³⁺ ions. The total fluorescent quantum yields are 0.531 (λ_ex = 310 nm) and 0.510 (λ_ex = 310 nm) for complexes 2 and 4 respectively and the maximum quantum yield is observed for complexes 2 and 4 are 0.274 and 0.159 respectively. The higher fluorescence quantum yields of complex 2 may indicates the much efficient energy transfer from terpy ligands to Tb³⁺ can take place.

Both the Eu(III) and Tb(III) complexes had excellent luminescence and the Tb complexes were even stronger than Eu complexes, according to the experimental results of emission spectra and fluorescence quantum yield. Therefore, the title complexes had potential application as luminescent materials.

Conclusions

In summary, the synthesis and characterization of the title complexes have been successfully reported. These complexes 1–2 were isostructural and nine-coordinated, while the complexes 3–4 were eight-coordinated. The crystal structures of the complexes 1–2 have performed the mononuclear, which is totally different from the binuclear structure of the complexes 3–4. Complexes 1 and 3, complexes 2 and 4 show the characteristic emission of Eu³⁺ and Tb³⁺ with strong red and green emission respectively, and they all show the high purity color. The thermogravimetric analysis of complexes 1–4 can not demonstrate their good thermal stabilities because of the existence of uncoordinated water molecule and coordinated solvent molecules. The average molar heat capacity value of the complexes gradually increased with the increase of temperature, indicating the complexes have good thermal stability within the range of temperature and their heat capacities fitted to a polynomial equation by the method of least squares at reduced temperature x (x = [T − (T_max + T_min)/2]/[(T_max − T_min)/2]). Finally, the derived thermodynamic functions (H_T − H_298.15), (S_T − S_298.15) and (G_T − G_298.15) of the complexes relative to the standard reference temperature 298.15 K were also obtained.

Acknowledgements

The research work was supported by the National Natura Science Foundation of China (no. 21073053, 21473049) and the Natural Science Foundation of Hebei Province (no. B2012205022).

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Footnote

† Electronic supplementary information (ESI) available: The powder X-ray diffraction of complexes 1–4 were shown in Fig. S1 and the IR spectra of title complexes and ligands were shown in Fig. S2. The selected bond lengths (Å) of the title complexes were shown in Table S1. Experimental molar heat capacities of complexes 1–4 have been performed in Table S2 and smoothed molar heat capacities and thermodynamic functions of complexes 1–4 were shown in Table S3–S6, respectively. The CIE coordinates of the title complexes were listed in Table S7. CCDC 1448340–1448343. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra11393a

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