English

• 0.   Introduction

  With its unique electronic structure, lanthanides have significant application prospects in various disciplines^[1-3]. The synthetic approach utilizing lanthanide elements in combination with aromatic carboxylate-based ligands presents a novel conceptual framework. The introduction of ligands not only significantly improves the catalytic performance of the complexes, but also fine-regulates their optical properties^[4-6]. In addition, the nitrogen atom contained in the nitrogen-containing ligands can form a stable coordination environment with metal ions, making the metal ions extremely stable^[7-9]. Lanthanide elements have a strong emission band due to the f-f transition effect, but this emission band is often restricted by parity rules. Therefore, scientists have developed a strategy that allows ligands with conjugated structures to coordinate with lanthanide elements, which not only effectively reduces the coordination interference of the solution but also promotes the efficient energy transfer between the ligands and lanthanum ion, thus significantly enhancing the luminous effect^[10-13]. Complexes synthesized by the combination of lanthanides and ligands have an important position and application value in the fields of photosensitive emission^[14-16] and catalytic reaction^[17-18].

  The combination of thermal analysis and mass spectrometry constitutes a cutting-edge, comprehensive analysis strategy, which cleverly integrates the respective advantages of thermal analysis technologies, such as thermogravimetric (TG) analysis and mass spectrometry (MS), showing a unique analytical performance^[19-20]. This technology can synchronously and precisely track the mass changes of the sample during heat treatment and the specific components of the escaped gas, so as to deeply analyze the structural properties and dynamic evolution of the material. This comprehensive approach plays an irreplaceable role in the core areas of material pyrolysis exploration^[21-22] and catalytic oxidation processes^[23-24].

  In this article, 4,4′-dimethyl-2,2′-bipyridine (4,4′-DM-2,2′-bipy) and 3,4-diethoxybenzoic acid (3,4-DEOHBA) were selected as ligands for the synthesis of innovative complexes, namly [Ln(3,4-DEOBA)₃(4,4′-DM-2,2′-bipy)]₂·2C₂H₅OH [Ln=Dy (1), Eu (2), Tb (3), Sm (4), Ho (5), Gd (6)]. Then, these complexes were analyzed comprehensively and carefully by various advanced characterization methods. Advanced techniques TG-DSC/FTIR/MS (DSC=differential scanning calorimetry) were used to study the detailed decomposition, with a comprehensive discussion of the small molecules lost during each decomposition step. In addition, because of the luminescence of complexes 1-4, the results obtained from the tests clearly reveal the specific luminescence patterns of Dy³⁺, Eu³⁺, Tb³⁺, and Sm³⁺ ions.

  1.   Experimental

  1.1   Reagents and instruments

  3,4-DEOHBA with 98% purity was purchased from Ark Pharm, and 4,4′-DM-2,2′-bipy and lanthanide nitrates with 99% purity were purchased from Innochem. The Vario EL Ⅲ element analyzer was utilized to measure element content. The HORIBA LabRAM Soleit Raman spectrometer was employed for measuring Raman spectra, while the BRUKER TENSOR27 Fourier Transform infrared spectrometer was used to measure infrared spectra. For powder X-ray diffraction (PXRD) measurement, a Bruker D8 Advance X-ray diffractometer was applied, with data collected in a 2θ range of 5°-50° using a Cu Kα radiator (λ=0.154 18 nm) at 298 K, a working voltage of 40 kV, and a current of 40 mA. Additionally, the STA 8000 synchronous thermal analyzer, the spectrum3 FTR spectrometer, and the Clarus SQ8T mass spectrometer were jointly used to measure the decomposition process. Finally, the FS5 fluorescence spectrometer was employed to test luminescence.

  1.2   Preparation of complexes

  0.2 mmol of lanthanide nitrate was dissolved in 3 mL of water to form a lanthanide nitrate solution. Then, 0.6 mmol of 3,4-DEOHBA and 0.2 mmol of 4,4′-DM-2,2′-bipy were placed in a small beaker, and 6 mL of 95% ethanol was added. The mixture was stirred magnetically, and the solid was dissolved to obtain the ligand solution. The pH of the solution was adjusted to 5.8-6.4 with 1 mol·L^-1 NaOH solution. The above ligand solution was slowly added to the lanthanide nitrate solution, and the mixture was stirred at room temperature for 6 h, and then the reaction solution was left to stand for 12 h. After extraction and filtration, six types of single crystals were grown after about 7 d.

  1.3   Determination of crystal structure

  Single crystals without obvious crystal defects were selected and determined under the Smart-1000 (Bruker AXS, Germany) single crystal X-ray diffractometer. Mo Kα rays (λ=0.071 073 nm) that had been monochromatized with graphite were used as the incident light source. The crystal diffraction data were measured and collected at room temperature. The structure was analyzed by the direct method using the SHELXS-2018 program^[25-26], and the crystal structure was obtained by fine-tuning with the full matrix least square method on F². All the atoms in the structure except hydrogen have undergone anisotropic refinement.

  2.   Results and discussion

  2.1   Elemental analysis

  For complexes 1-6, the contents of C, H, and N elements are detailed in Table 1. After three parallel measurements, the average of these three measurements was taken and compared with the theoretical value, and the results were close to expectations.

  Table 1

  

  Table 1.  Elemental analysis data of complexes 1-6

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  Complex	wC / %			wH / %			wN / %
  	Found	Calcd.		Found	Calcd.		Found	Calcd.
  1	55.32	55.22		5.63	5.60		2.75	2.78
  2	55.90	55.93		5.69	5.73		2.77	2.74
  3	55.51	55.58		5.65	5.60		2.75	2.73
  4	55.98	55.86		5.70	5.65		2.78	2.86
  5	55.19	55.26		5.62	5.58		2.74	2.78
  6	55.60	55.55		5.66	5.63		2.76	2.79

  2.2   Crystal structure description

  The crystallographic data are shown in Table S1 (Supporting information). Important bond lengths are shown in Table S2. It can be seen from the determination that complexes 1-6 have an isomorphic structure and are all triclinic system. Herein, complex 3 is described in detail.

  Fig.1a shows the structural unit of complex 3, and Fig.1b shows the coordination environment of central Tb³⁺. As can be seen from Fig.1a, two 4,4′-DM-2,2′-bipy ligands and six 3,4-DEOBA^- ligands are directly connected to the two metal ions (Tb1 and Tb1#1). In addition to the directly connected ligands, two free ethanol molecules are also surrounded. There are eight atoms directly connected to the Tb1 ion: six oxygen atoms and two nitrogen atoms. Among them, the oxygen atoms are connected to the metal in the form of bidentate chelation (O9, O10) and bridging bidentate (O1, O2, O5, O6), and their shape is calculated as a twisted quadrilateral antiprism configuration^[27]. The distance between Tb1 and Tb1#1 is 0.423 79(9) nm. As shown in Table S4, the longest Tb—O bond is 0.242 6(6) nm, the shortest is 0.227 1(6) nm, and the average length of the six groups of Tb—O bonds is 0.233 5(6) nm. The Tb—N bond lengths are 0.254 8(7) and 0.254 4(7) nm, with an average of 0.254 6(7) nm. By comparison, it is found that the bond length between metal ions and oxygen atoms is short, and the bond energy is strong^[28].

  Figure 1

  

  Figure 1.  (a) Structural unit of complex 3; (b) coordination environment of the Tb³⁺ ion

  Ellipsoid probability: 50%; Symmetry code: #1:-x+1, -y+1, -z.

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  In the direction of a-axis, the single structural units are interconnected through C10—H10…O11 hydrogen bonds (C10…O11 0.342 7 nm) and extend into a 1D chain structure (Fig.2a). In addition, in the ac plane, these structural units are supported by C32—H32…O1 hydrogen bond (C32…O1 0.328 4 nm), and are further linked to form a 2D plane structure (Fig.2b).

  Figure 2

  

  Figure 2.  (a) One-dimensional chain structure of complex 3; (b) 2D layer structure

  Symmetry codes: #2: x-1, y, z; #3: x-1, y+1, z; #5:-x, 1-y, -z.

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  2.3   Infrared and Raman spectra

  As shown in Fig.3, IR and Raman spectra were obtained. Due to the similar structure of these complexes, their spectral characteristics also showed similarities^[29]. Combined with the data analysis in Table S3, it is evident that there was a vibration peak of ν_C=O at 1 691 cm^-1, but this vibration was not detected in the complex. In contrast, Ln—O bond vibrations in the complex occurred in a range of 422-416 cm^-1. In addition, at new wavenumbers of 1 539-1 529 cm^-1 and 1 432-1 429 cm^-1, carboxylic acid-based ν_as(COO^-) and ν_s(COO^-) were observed. The presence of these characteristic peaks indicates coordination between the metal and oxygen atoms^[30-31]. Further analysis of the Raman spectral data showed that the vibrational frequencies of ν_C=N and δ_C—H increased in the newly synthesized complex, which implies the enhanced stability of the complex^[32]. At the same time, the newly formed carboxylic acid group showed obvious vibration peaks of ν_as(COO^-) and ν_s(COO^-) at 1 561-1 555 cm^-1 and 1 429-1 421 cm^-1, respectively. In addition, the vibrations of the Ln—O bond and Ln—N bond appeared significantly at 429-424 cm^-1 and 278-274 cm^-1, respectively. These spectral features together confirm the successful synthesis of the complex^[33].

  Figure 3

  

  Figure 3.  (a) Infrared and (b) Raman spectra of the compounds

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  2.4   PXRD analysis

  The PXRD pattern obtained at room temperature is shown in Fig.4. Specifically, Fig.4a shows PXRD patterns for six complexes, while Fig.4b shows experimental and simulated PXRD patterns for two ligands and complex 3. First, it can be clearly observed from Fig.4a that the diffraction peaks of these four complexes exhibited a high degree of similarity, indicating that their structures are very similar^[34]. In Fig.4b, there was a significant difference in the diffraction peak positions between complex 3 and the two ligands, confirming that we have successfully synthesized a new substance that is completely different from the ligands. In addition, the experimental PXRD pattern of complex 3 was compared with its fitting pattern, showing that the peak positions of the two curves are highly similar, which further verified that the synthesized complex was not only pure in composition but also homogeneous in structure^[35].

  Figure 4

  

  Figure 4.  PXRD patterns of (a) complexes 1-6 and (b) the ligands and complex 3

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  2.5   Thermal analysis

  The decomposition processes of the six complexes were similar, thus taking complex 3 as an example. The decomposition curve is shown in Fig.5, the 3D-IR superposition diagram of the escaped gas is shown in Fig.6a, and the corresponding 2D diagram is shown in Fig.6b. Table 2 shows the decomposition data of complexes 1-6 in detail. The correlation spectra of other complexes are shown in Fig.S1-S5.

  Figure 5

  

  Figure 5.  Thermal decomposition curve of complex 3

  DTG=differential thermal gravimetry.

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  Figure 6

  

  Figure 6.  (a) Three-dimensional infrared spectra of escaping gases and (b) 2D infrared spectra of escaping gases for complex 3

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  Table 2

  

  Table 2.  Thermal decomposition data of complexes 1-6

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  Complex	Step	Temperature range / K	Tp in DTG / K	Weight loss rate / %		Probable expelled groups	Intermediate and residue
  				Found	Calcd.
  1	Ⅰ	332.20-593.71	416.90	23.10	4.51a	2C2H5OH+2(4,4′-DM-2,2′-bipy)	Dy2(3,4-DEOBA)6
  			447.09		18.05b
  	Ⅱ	593.71-1 009.50	683.26	64.24	59.16c	6(3,4-DEOBA)	Dy2O3
  			777.86
  	Total			81.34	81.72d
  2	Ⅰ	332.01-604.47	450.19	24.60	4.55a	2C2H5OH+2(4,4′-DM-2,2′-bipy)	Eu2(3,4-DEOBA)6
  					18.24b
  	Ⅱ	604.47-1 005.36	680.28	58.66	59.78c	6(3,4-DEOBA)	Eu2O3
  			780.53
  	Total			83.26	82.57d
  3	Ⅰ	338.29-720.29	472.07	59.53	4.52a	2C2H5OH+2(4,4′-DM-2,2′-bipy)+x(3,4-DEOBA)	Tb2(3,4-DEOBA)6-x
  			630.09		18.12b
  	Ⅱ	720.29-1 008.29	815.75	21.03	58.97c	(6-x)(3,4-DEOBA)	1/2Tb4O7
  	Total			80.56	81.61d
  4	Ⅰ	337.10-588.29	468.29	22.69	4.56a	2C2H5OH+2(4,4′-DM-2,2′-bipy)
  				18.27b			Sm2(3,4-DEOBA)6
  	Ⅱ	588.29-1 003.20	678.57	59.74	59.87c	6(3,4-DEOBA)	Sm2O3
  			778.96
  	Total			82.43	82.70d
  5	Ⅰ	335.28-608.84	443.29	21.85	4.50a	2C2H5OH+2(4,4′-DM-2,2′-bipy)	Ho2(3,4-DEOBA)6
  					18.01b
  	Ⅱ	608.84-1 003.36	658.36	57.51	59.02c	6(3,4-DEOBA)	Ho2O3
  			738.41
  	Total			79.36	81.53d
  6	Ⅰ	336.25-727.78	498.14	57.79	4.53a	2C2H5OH+2(4,4′-DM-2,2′-bipy)+x(3,4-DEOBA)	Gd2(3,4-DEOBA)6-x
  			698.18		18.15b
  	Ⅱ	727.78-1 008.15	808.20	23.71	59.46c	(6-x)(3,4-DEOBA)	Gd2O3
  	Total			81.50	82.14d
  a Two ethanol molecules are theoretically removed; b Two 4,4′-DM-2,2′-bipy molecules are theoretically removed; c Six 3,4-DEOBA- ions are theoretically removed; d Total weight loss.

  First of all, by observing the TG curve in Fig.5, it is clear that the decomposition process was divided into two consecutive stages. The first step of decomposition occurred within a range of 338.29-720.29 K, corresponding to DTG peak temperatures of 472.07 and 630.09 K, respectively. In the DSC curve, an upward heat absorption peak was followed by a downward heat release peak. The intermediate was Tb₂(3,4-DEOBA)_6-x, and the experimental weight loss was 59.53%, which was analyzed to be the loss of two ethanol molecules, two 4,4′-DM-2,2′-bipy ligands, and x 3,4-DEOBA^- ions. Subsequently, the second step decomposition occurred in a temperature range of 720.29-1 008.29 K, and the weight loss ratio was 21.03%. At 818.26 K, the DSC curve showed a downward heat release peak. The gases produced during heating were analyzed in depth, and the 2D spectrum was obtained by analyzing the 3D accumulation diagram in Fig.6a, as shown in Fig.6b. When the temperature reached 443.54 K, the characteristic vibration modes of ethanol molecules were revealed, including significant vibrations ν_s(C—H) at 2 896 cm^-1 and ν_as(C—H) at 2 930 cm^-1. At the same time, there was also the ν_O—H of 3 480-3 630 cm^-1 belonging to ethanol. There were also vibrations associated with the decomposition of 4,4′-DM-2,2′-bipy ligands, such as ν_C=N at 1 702-1 542 cm^-1, ν_C—C at 1 492 cm^-1, and carbon dioxide stretching vibrations from the decomposition of 3,4-DEOBA^-. It was located at 2 398-2 358 cm^-1, and ν_C=O at 1 705 cm^-1. In the 2D spectrum of 850.54 K, the stretching vibration of carbon dioxide (2 390-2 262 cm^-1), the ν_C=O of the carboxyl component at 1 712 cm^-1, and the ν_C—O of 1 270 cm^-1 were again observed, indicating that the decomposition of the acidic ligands also occurs at this stage. The total weight loss was close to 80.56% (Calcd. 81.61%), and the product was 1/2Tb₄O₇.

  2.6   Mass spectrometry

  The combination of TG-MS and TG-FTIR was conducive to the detection of ion fragments generated during the decomposition process of the complexes, aiming at better analyzing the decomposition law of the complexes^[36-37]. In view of the similar thermal decomposition behavior of complexes 1-6, complex 3 was selected as a representative sample for detailed analysis. As shown in Fig.S4a, the thermal decomposition mass spectra of complex 3 clearly showed its thermal decomposition process, while the standard mass spectra of neutral and acidic ligands can be seen in Fig.S4b and S4c, respectively.

  The results of a mass spectrometry analysis (Fig.S4a) showed a series of characteristic mass-charge ratios: 184 (C₁₂H₁₂N₂), 169 (C₁₁H₉N₂⁺), and 155 (C₁₀H₆N₂⁺). By comparing it with the standard mass spectrum of neutral ligands (Fig.S4b), it was determined that m/z=184 corresponds to 4,4′-DM-2,2′-bipy, and the change from m/z=184 to m/z=169 indicates that 4,4′-DM-2,2′-bipy loses one methyl group. The change from m/z=169 to m/z=155 further indicates the loss of a methylene group, confirming the progressive decomposition of the neutral ligand. In addition, several significant mass-charge ratios were observed in Fig.S4a: 210 (C₁₁H₁₄O₄), 182 (C₉H₉O₄⁺), 154 (C₇H₄O₄⁺), 137 (C₇H₃O₃⁺). Combined with the standard mass spectrogram of the acidic ligand (Fig.S4c), it can be inferred that the change from m/z=210 to m/z=182 results from the loss of one ethyl group of 3,4-DEOHBA, and that the change from m/z=182 to m/z=154 results from the loss of another ethyl group. The m/z change from 154 to 137 corresponds to the loss of a hydroxyl group; at the same time, there was CO₂ produced by the decomposition of acidic ligands^[38], revealing the progressive decomposition mechanism of the acidic ligand.

  2.7   Fluorescent properties

  The excitation spectrum of complex 1 (Fig.S5a) had a wide absorption peak between 250 and 400nm, with a peak at 321 nm, due to the π→π* transition. This wavelength was the corresponding excitation wavelength, and the emission spectrum (Fig.S5b) was obtained. There were two distinct peaks in the emission spectrum at 482nm (⁴F_9/2→⁶H_15/2) and 574 nm (⁴F_9/2→⁶H_13/2). The peak at 574 nm was more intense, which is why it emits yellow light^[39].

  The spectral characteristics of complex 2 are shown in Fig.7, where the black line represents the excitation spectrum and the other is the emission spectrum. In the emission spectrum, the peaks were located at 580, 590, 615, 651, and 703 nm. These peaks correspond to different energy level transitions, specifically ⁵D₀ to ⁷F₀, ⁷F₁, ⁷F₂, ⁷F₃, and ⁷F₄ states. It can be seen that the ⁵D₀→⁷F₂ transition showed a stronger peak^[40], which is the source of red light emitted by the europium complex. In addition, through the color coordinate analysis, the color coordinate of the complex was (0.661 5, 0.338 3).

  Figure 7

  

  Figure 7.  Fluorescence spectrum and color coordinate of complex 2

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  In the excitation spectrum of complex 3, the wide peak was located at 345 nm (Fig.8), and the strongest peak of the emission spectrum was located at 545 nm (⁵D₄→⁷F₅). In addition, there were three weak peaks located at 621 nm (⁵D₄→⁷F₃), 583nm (⁵D₄→⁷F₄), and 489 nm (⁵D₄→⁷F₆). Therefore, the green light of the complex is mainly due to the peak at 545 nm (⁵D₄→⁷F₅)^[41]. Substituting into the color coordinates gave the coordinate of (0.308 2, 0.594 5).

  Figure 8

  

  Figure 8.  Fluorescence spectrum and color coordinate of complex 3

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  As shown in Fig.S6a, complex 4 had a wide peak at 333 nm, and the peak of 333 nm was used as the excitation spectrum to obtain the corresponding emission spectrum. In Fig.S6b, three obvious peaks were observed. The transitions are ⁴G_5/2→⁶H_5/2 (563 nm), ⁴G_5/2→⁶H_7/2 (601 nm), and ⁴G_5/2→⁶H_9/2 (643 nm). The peak at 643 nm was more intense, so the light of complex 4 is orange-red^[39-40]. The emission data for complexes 1 and 4 were substituted into color coordinates (0.301 6, 0.330 0) and (0.489 9, 0.400 9), respectively (Fig.S7).

  Due to the strong luminescence of complexes 2 and 3, their respective life curves were obtained, as shown in Fig.9a and 9b. The fitting equation was I(t)=B₁exp(-t/τ₁)+B₂exp(-t/τ₂), where B₁ and B₂ are the coefficients; I(t) is the fluorescence intensity; τ₁ and τ₂ are the decay times. Then, through the lifetime equation τ=(B₁τ₁²+B₂τ₂²)/(B₁τ₁+B₂τ₂), the calculated results were 0.807 and 0.845 ms for complexes 2 and 3, respectively.

  Figure 9

  

  Figure 9.  Fluorescence decay curves of complexes (a) 2 and (b) 3

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  3.   Conclusions

  In this work, six lanthanide complexes were synthesized: [Ln(3,4-DEOBA)₃(4,4′-DM-2,2′-bipy)]₂·2C₂H₅OH, where Ln=Dy (1), Eu (2), Tb (3), Sm (4), Ho (5), Gd (6); 3,4-DEOBA^-=3,4-diethoxybenzoate; 4,4′-DM-2,2′-bipy=4,4′-dimethyl-2,2′-bipyridine. The single-crystal X-ray diffraction analysis results show that complexes 1-6 are triclinic crystal systems in the binuclear structure. For the fluorescence test of complex 1-4, the intensity values of transmitting data measured by experiments were normalized into the color coordinates, corresponding to yellow, red, green, and orange-red regions, respectively, so as to verify their respective luminous colors. Due to the strong fluorescence properties of complexes 2 and 3, their fluorescence lifetimes were further determined to be 0.807 and 0.845 ms, respectively. Through the analysis of the TG-DTG-DSC curve, it was found that the complex began to decompose in a range of 332-338 K, and the residue after high-temperature heating was the corresponding lanthanide oxide. The characteristics of the escaped gas were measured and compared with the infrared accumulation map and the molecular fragment mass spectrometry. The results are in agreement with the previous analysis.

  
  Acknowledgements: The research work is supported by the National Natural Science Foundation of China (Grant No. 22273015). Conflicts of interest: The authors declare no conflicts of interest.
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  (a) Structural unit of complex 3; (b) coordination environment of the Tb³⁺ ion

  Ellipsoid probability: 50%; Symmetry code: #1:-x+1, -y+1, -z.

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  Figure 2  (a) One-dimensional chain structure of complex 3; (b) 2D layer structure

  Symmetry codes: #2: x-1, y, z; #3: x-1, y+1, z; #5:-x, 1-y, -z.

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  Figure 3  (a) Infrared and (b) Raman spectra of the compounds

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  Figure 4  PXRD patterns of (a) complexes 1-6 and (b) the ligands and complex 3

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  Figure 5  Thermal decomposition curve of complex 3

  DTG=differential thermal gravimetry.

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  Figure 6  (a) Three-dimensional infrared spectra of escaping gases and (b) 2D infrared spectra of escaping gases for complex 3

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  Figure 7  Fluorescence spectrum and color coordinate of complex 2

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  Figure 8  Fluorescence spectrum and color coordinate of complex 3

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  Figure 9  Fluorescence decay curves of complexes (a) 2 and (b) 3

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  Table 1.  Elemental analysis data of complexes 1-6

  Complex	wC / %			wH / %			wN / %
  	Found	Calcd.		Found	Calcd.		Found	Calcd.
  1	55.32	55.22		5.63	5.60		2.75	2.78
  2	55.90	55.93		5.69	5.73		2.77	2.74
  3	55.51	55.58		5.65	5.60		2.75	2.73
  4	55.98	55.86		5.70	5.65		2.78	2.86
  5	55.19	55.26		5.62	5.58		2.74	2.78
  6	55.60	55.55		5.66	5.63		2.76	2.79

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  Table 2.  Thermal decomposition data of complexes 1-6

  Complex	Step	Temperature range / K	Tp in DTG / K	Weight loss rate / %		Probable expelled groups	Intermediate and residue
  				Found	Calcd.
  1	Ⅰ	332.20-593.71	416.90	23.10	4.51a	2C2H5OH+2(4,4′-DM-2,2′-bipy)	Dy2(3,4-DEOBA)6
  			447.09		18.05b
  	Ⅱ	593.71-1 009.50	683.26	64.24	59.16c	6(3,4-DEOBA)	Dy2O3
  			777.86
  	Total			81.34	81.72d
  2	Ⅰ	332.01-604.47	450.19	24.60	4.55a	2C2H5OH+2(4,4′-DM-2,2′-bipy)	Eu2(3,4-DEOBA)6
  					18.24b
  	Ⅱ	604.47-1 005.36	680.28	58.66	59.78c	6(3,4-DEOBA)	Eu2O3
  			780.53
  	Total			83.26	82.57d
  3	Ⅰ	338.29-720.29	472.07	59.53	4.52a	2C2H5OH+2(4,4′-DM-2,2′-bipy)+x(3,4-DEOBA)	Tb2(3,4-DEOBA)6-x
  			630.09		18.12b
  	Ⅱ	720.29-1 008.29	815.75	21.03	58.97c	(6-x)(3,4-DEOBA)	1/2Tb4O7
  	Total			80.56	81.61d
  4	Ⅰ	337.10-588.29	468.29	22.69	4.56a	2C2H5OH+2(4,4′-DM-2,2′-bipy)
  				18.27b			Sm2(3,4-DEOBA)6
  	Ⅱ	588.29-1 003.20	678.57	59.74	59.87c	6(3,4-DEOBA)	Sm2O3
  			778.96
  	Total			82.43	82.70d
  5	Ⅰ	335.28-608.84	443.29	21.85	4.50a	2C2H5OH+2(4,4′-DM-2,2′-bipy)	Ho2(3,4-DEOBA)6
  					18.01b
  	Ⅱ	608.84-1 003.36	658.36	57.51	59.02c	6(3,4-DEOBA)	Ho2O3
  			738.41
  	Total			79.36	81.53d
  6	Ⅰ	336.25-727.78	498.14	57.79	4.53a	2C2H5OH+2(4,4′-DM-2,2′-bipy)+x(3,4-DEOBA)	Gd2(3,4-DEOBA)6-x
  			698.18		18.15b
  	Ⅱ	727.78-1 008.15	808.20	23.71	59.46c	(6-x)(3,4-DEOBA)	Gd2O3
  	Total			81.50	82.14d
  a Two ethanol molecules are theoretically removed; b Two 4,4′-DM-2,2′-bipy molecules are theoretically removed; c Six 3,4-DEOBA- ions are theoretically removed; d Total weight loss.

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