Research article Received: 19 October 2014,
Revised: 30 January 2015,
Accepted: 26 February 2015
Published online in Wiley Online Library: 31 March 2015
(wileyonlinelibrary.com) DOI 10.1002/bio.2906
Synthesis, characterization and luminescent properties of europium complexes with 2,4,6-tris-(2-pyridyl)-s-triazine as highly efficient sensitizers Jie Kang, Ying-Nan Chen, Ai-Ling Wang, Hai-Yan Li, Yan-Rong Qu, Hai-Xia Zhang, Hai-Bin Chu* and Yong-Liang Zhao* ABSTRACT: Using 2,4,6-tris-(2-pyridyl)-s-triazine (TPTZ) as a neutral ligand, and p-hydroxybenzoic acid, terephthalic acid and nitrate as anion ligands, five novel europium complexes have been synthesized. These complexes were characterized using elemental analysis, rare earth coordination titrations, UV/vis absorption spectroscopy and infrared spectroscopy. Luminescence spectra, luminescence lifetime and quantum efficiency were investigated and the mechanism discussed in depth. The results show that the complexes have excellent emission intensities, long emission lifetimes and high quantum efficiencies. The superior luminescent properties of the complexes may be because the triplet energy level of the ligands matches well with the lowest excitation state energy level of Eu3+. Moreover, changing the ratio of the ligands and metal ions leads to different luminescent properties. Among the complexes, Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O shows the strongest luminescence intensity, longest emission lifetime and highest quantum efficiency. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: europium complex; 2,4,6-tris-(2-pyridyl)-s-triazine; aromatic carboxylic acid; luminescence property
Introduction
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Rare earth complexes are attracting considerable attention on account of their rapidly growing number of applications, which include the development of fluorescence materials (1,2), electroluminescence devices (3,4) and fluorescence probes and labels in a variety of biological systems (5–7). These application rely on the outstanding luminescence properties of rare-earth complexes (8). Europium complex is a class of luminescent material with an excellent narrow-band light-emitting performance (9–11). However, there are some restrictions to the practical application of europium compounds, essentially because of their weak luminescence intensity, high cost, short lifetime and low quantum efficiency (12,13). In order to overcome these drawbacks, ternary europium complexes have attracted considerable attention because of their unusual optical properties, for example, their enhanced emission intensities and long fluorescence lifetime (14–16). This is because the second organic ligand serves as an energy donor and enhances the fluorescence intensities of rare earth complexes. This is named the ’synergistic effect’ (17). Furthermore, introduction of the second organic ligand also can fulfil the coordination numbers of rare earth complexes due to its excellent coordination ability with rare earth ions and its ability to sensitize the luminescence of rare earth ions (16). Thus, europium complexes could be incorporated with some aromatic carboxylic acid and the hetero nitrogen ligands (18,19). 2,4,6-Tris(2-pyridyl)-1,3,5-triazine (TPTZ) is an almost planar molecule with an extended conjugated π system, which has three different ’coordination sites’ (major, middle and minor) in this molecule (20–23). The coordination form of TPTZ partly depends upon the size of the metal ions. TPTZ generally acts as a tridentate or
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binary ligand that coordinates with lanthanide ions and forms 1 : 1, 2 : 1 or 1 : 2 metal/ligand complexes (24–29). The suitability of the energy gap between the lowest excited state energy of the rare earth ions and the triplet level of the ligands is critical for efficient energy transfer (30,31). The lowest excited state energy of europium ion is 17,250 cm1 (18) and the triplet energy levels of TPTZ, p-hydroxybenzoic acid and terephthalic acid are 21,277, 17,241 and 25,160 cm1, respectively (32,33). The triplet state energy level of the ligands matches well with the lowest excitation state energy level of Eu3+. Furthermore, as a neutral ligand, TPTZ could lead to coordination saturation of Eu3+ ions, increase the steric hindrance of the compounds, and reduce the number of water molecules and solvent-quenching effect (34). Therefore, we selected TPTZ to coordinate with Eu3+. Carboxylates are widely used as ligands in rare earth complexes because of their diverse coordination modes, stronger bonding ability and efficient energy transfer to rare earth ions. In addition, the triplet energy level of NO–3 (24,814 cm–1) (31) is higher than the lowest exited state 5D0 of Eu3+. Thus, we also selected p-hydroxybenzoic acid, terephthalic acid and nitrate as anion ligands to form the complexes.
* Correspondence to: H.-B. Chu and Y.-L. Zhao, College of Chemistry and Chemical Engineering, InnerMongolia University, Huhhot, 010021, China. E-mail: [email protected]; [email protected] College of Chemistry and Chemical Engineering, Inner Mongolia University, Huhhot, China
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Synthesis, characterization and luminescent properties of Eu complexes
Experimental Materials and general methods The purity of lanthanide oxide Eu 2 O 3 was 99.99%. TPTZ, p-hydroxybenzoic acid, terephthalic acid, nitric acid, anhydrous ethanol and other reagents were all of analytical grade. Elemental analysis of C, H, N was performed with a Vario EL Cube elemental analyser. The europium contents of the complexes were determined by EDTA titration with xylenol orange as the indicator. UV/vis absorption spectra were recorded on a TU-1901 doublebeam UV/vis spectrophotometer, with dimethylsulfoxide (DMSO) as the solvent. Infrared (IR) spectra were recorded on a Nicolet Nexus 670 FT-IR spectrometer using the KBr pellet technique in the scope of 4000–400 cm–1. Luminescence spectra and luminescence lifetimes were recorded on an Edinburgh Instruments FLS-920 spectrometer at room temperature. The entrance and emission slit widths were 1.5 and 3 nm, respectively. Preparation of europium nitrate An ethanol solution of Eu(NO3)3 was prepared by reacting Eu2O3 and a dilute solution of nitric acid. The solution was heated, dissolved, evaporated and then dissolved in anhydrous ethanol. Preparation of complexes (1) Eu(TPTZ)(NO3)3(C2H5OH), (2) Eu(TPTZ)2(NO3)3(C2H5OH) and (3) Eu2(TPTZ)(NO3)6(C2H5OH) TPTZ (0.1872 g, 0.6 mmol) was first dissolved in 40 mL of ethanol solution with stirring at 60°C. Then NH3·H2O (1 : 4, v/v) was added to the solution until the pH was 6. Under stirring, 6 mL of Eu(NO3)3 (0.1 mol/L) ethanol solution was added to the above solution. The pH was readjusted to between 6 and 7 with NH3·H2O. After stirring at 60°C for 3 h, the solution was left at room temperature overnight. The mixture was separated by filtering and washing with ethanol and acetone. The product Eu(TPTZ)(NO3)3(C2H5OH) was obtained after drying at 50°C for 24 h. The procedure for the synthesis of (2) Eu(TPTZ)2(NO3)3(C2H5OH) and (3) Eu2(TPTZ)(NO3)6(C2H5OH) was similar to that of Eu(TPTZ)(NO3)3(C2H5OH), except that the molar ratios of TPTZ and Eu(NO3)3 were 2 : 1 and 1 : 2, respectively.
After stirring at 60°C for 2 h, the solution was left at room temperature overnight. The mixture was separated by filtering and washing three times with ethanol. After drying at 50°C for 24 h, the product of complex Eu(TPTZ)(C7H5O3)3(C2H5OH) was obtained. The preparation Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH) was similar to that of Eu(TPTZ)(C7H5O3)3(C2H5OH) except that the molar ratio of Eu(NO3)3 : TPTZ : C8H4O4 was 2 : 2 : 1.
Results and discussion Composition analysis and properties of the complexes The results of the elemental analyses of C, H, N and rare earth titration of the complexes are listed in Table 1. The results show that the compositions of the complexes are Eu(TPTZ)(NO3)3 (C2H5OH)·H2O, Eu(TPTZ)2(NO3)3(C2H5OH)·H2O, Eu2(TPTZ)(NO3)6 (C2H5OH)·2H2O, Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O and Eu2(TPTZ)2 (C8H4O4)(NO3)4(C2H5OH)·H2O, respectively. The products are all white powders and are stable in the air. They are slightly soluble in DMSO and dimethylformamide (DMF), but insoluble in water, ethanol, acetone and ether. UV absorption spectra The UV/vis spectra for the dissociative ligands and their corresponding complexes are shown in Fig. 1. As seen from the spectra, the ligands TPTZ, p-hydroxybenzoic acid and terephthalic acid have strong absorption peaks at 282, 285 and 272 nm, respectively. After coordination with Eu3+, the absorption peaks shift to 278 nm. This shows that a more extensive π–π* conjugating system is formed due to the coordination (25,35). This indicates
Preparation of complexes (4) Eu(TPTZ)(C7H5O3)3(C2H5OH) and (5) Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH) p-Hydroxybenzoic acid (0.1657 g, 1.2 mmol) and TPTZ (0.1248 g, 0.4 mmol) were dissolved in ethanol with stirring at 60°C. Four millilitres of Eu(NO3)3 (0.1 mol/L) ethanol solution was added to the above solution. The molar ratio of Eu3+ : C7H5O3H : TPTZ is 1 : 3 : 1. The pH of the solution was adjusted to 6.4–6.7 with NH3·H2O.
Figure 1. UV absorption spectra of the ligands and some complexes.
Table 1. Composition analysis of europium complexes Complex Eu(TPTZ)(NO3)3(C2H5OH)·H2O Eu(TPTZ)2(NO3)3(C2H5OH)·H2O Eu2(TPTZ)(NO3)6(C2H5OH )·2H2O Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O
C% 33.33 (33.63) 44.30 (44.45) 22.10 (22.44) 51.05 (51.42) 39.00 (39.33)
H% 2.92 (2.82) 3.08 (3.14) 2.25 (2.07) 4.15 (3.89) 2.65 (2.58)
N% 17.55 (17.65) 20.26 (20.46) 15.30 (15.70) 8.70 (8.770) 15.84 (15.95)
Eu % 21.21 (21.27) 14.35 (14.80) 28.21 (28.39) 15.47 (15.87) 21.43 (21.63)
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Theoretical values are given in parentheses.
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J. Kang et al. that the ligands are all coordinated to Eu3+. Moreover, the λmax of the complexes is similar, which may be because the π–π* transition energy of TPTZ is much stronger than that of p-hydroxybenzoic acid and terephthalic acid.
and v3 (doubly degenerate in-plane bending) at 691 and 742 cm–1 in the spectra of the complexes, which means that the nitrate ions coordinate with the lanthanide ions with bidentate structure (42).
Infrared spectra
Luminescence spectra
As shown in Fig. 2, the ring stretching of both the pyridine and triazine rings of TPTZ is positioned at 1371 cm–1 (36), and shifts toward the high wavenumber by 6–15 cm–1 in the complexes. This reveals that the lanthanide ions coordinate with the nitrogen atoms of the rings. The characteristic pyridine ring absorption band (1529 cm–1) that is ascribed to υ(C=N) and υ(C=C) (37–39) moves toward the low wavenumber by 3–4 cm–1 in the complexes, which also suggests that TPTZ coordinates to the lanthanide ions. As can be seen from Fig. 2, the bands of free carboxylic ions from p-hydroxybenzoic acid and terephthalic acid change significantly after coordination. The υ(C=O) peak of p-hydroxybenzoic acid at 1676 cm–1 disappears and two peaks at 1609 cm–1 (υas(COO- )) and 1493 cm–1 (υs(COO- )) appear in the complex. Also, the peak of terephthalic acid at 1684 cm–1 disappears and two peaks at 1605 cm–1 (υas(COO- )) and 1484cm–1 (υs(COO- )) appear in the complex. These changes indicate that the ligands coordinate with lanthanide ions by bidentate carboxylate groups (40,41). The coordinated nitrate ions show v1 (out-of-plane deformation) at 789 cm–1, v2 (doubly degenerate stretching) at 1234 and 1451 cm–1
The excitation wavelengths of the complexes were determined within the range of 220–450 nm at room temperature. The excitation spectra of the complexes were all broadband absorption, and the strongest peaks of Eu(TPTZ)(NO3)3(C2H5OH)· H2O, Eu(TPTZ)2(NO3)3(C2H5OH)·H2O, Eu2(TPTZ)(NO3)6(C2H5OH)· 2H2O and Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O were 362, 363, 360 and 365 nm, respectively. The emission spectra of the complexes were determined at the most efficacious excitation wavelength, and the emission spectra data of the complexes are listed in Table 2. All the europium complexes emit the characteristic luminescence of Eu3+. The luminescence emission peak at 614–616 nm is electric dipole transition (5D0→7F2), and is the strongest emission peak among the five characteristic peaks. As shown in Table 2 and Figs. 3 and 4, different complexes have varying fluorescence intensity. The intensity order of the complexes is Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH) > Eu2(TPTZ)(NO3)6(C2H5OH) > Eu(TPTZ)2(NO3)3(C2H5OH) > Eu(TPTZ)(C7H5O3)3(C2H5OH) > Eu(TPTZ)(NO3)3(C2H5OH), and the europium complex with terephthalic acid exhibits the strongest emission among the five europium complexes.
Figure 2. IR spectra of ligands and complexes. (a) TPTZ, (b) p-hydroxybenzoic acid, (c) terephthalic acid, (d) Eu(TPTZ)(NO3)3(C2H5OH)·H2O, (e) Eu(TPTZ)2(NO3)3(C2H5OH)· H2O, (f ) Eu2(TPTZ)(NO3)6(C2H5OH)·2H2O, ( g) Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O, (h) Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O.
Figure 3. Emission spectra of the complexes. (a) Eu(TPTZ)(NO3)3(C2H5OH)·H2O, (b) Eu(TPTZ)2(NO3)3(C2H5OH)·H2O, (c) Eu2(TPTZ)(NO3)6(C2H5OH)·2H2O, (d) Eu(TPTZ)(C7H5O3)3 (C2H5OH)·2H2O.
Table 2. Luminescent emission spectra data of the complexes D0→7F0
λ (nm)
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Eu(TPTZ)(NO3)3(C2H5OH)·H2O Eu(TPTZ)2(NO3)3(C2H5OH)·H2O Eu2(TPTZ)(NO3)6(C2H5OH)·2H2O Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O
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D0→7F1
5
Complex
579 579 580 580 580
D0→7F2
5
I (a.u.)
λ (nm) 2
5.950 × 10 5.080 × 102 4.126 × 103 2.040 × 102 1.700 × 104
593 591 592 590 590
I (a.u.)
λ (nm) 3
2.977 × 10 4.800 × 103 2.244 × 104 3.210 × 103 1.342 × 105
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D0→7F3
5
615 614 616 616 616
5
I (a.u.) 3
9.478 × 10 2.214 × 104 7.001 × 104 1.646 × 104 1.111 × 106
λ (nm)
I (a.u.)
649 649 649 651 650
3.000 × 102 6.120 × 102 9.770 × 102 2.990 × 102 2.677 × 104
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Synthesis, characterization and luminescent properties of Eu complexes Table 3. Luminescent life data of the europium complexes Complex
τ (ms)
Eu(TPTZ)(NO3)3(C2H5OH)·H2O Eu(TPTZ)2(NO3)3(C2H5OH)·H2O Eu2(TPTZ)(NO3)6(C2H5OH)·2H2O Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O
0.5479 0.4156 0.6316 0.6413 1.0486
η¼
Figure 4. Emission (1) and excitation (2) spectra of Eu2(TPTZ)2(C8H4O4)(NO3)4 (C2H5OH)·H2O.
Luminescence lifetimes and quantum efficiencies The luminescence lifetimes of the europium complexes were determined, the luminescence decay curves of the complexes are shown in Fig. 5 and the data are given in Table 3. The decay profile is well reproduced by a single exponential ln(I(t)/I0) = –k/t = –t/τ (where I0 is the initial luminescence intensity and τ is the luminescence lifetime), indicating that all Eu3+ ions occupy the same average coordination environment (34,36). In the europium complexes, the lifetime of Eu(TPTZ)2(NO3)3(C2H5OH)·H2O (0.4156 ms) is the shortest and that of Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O (1.0486 ms) is the longest. Assuming that only nonradiative and radiative processes are essentially involved in depopulation of the 5D0 state and according to the emission spectra and lifetimes of Eu3+ (τ, 5D0), the emission quantum yield (η, Table 4) can be defined as follows (19,43):
Ar Ar þ Anr
(1)
Ar and Anr are the radiative and nonradiative transition rates, respectively. Ar can be calculated from: Ar ¼ ∑ A0J ¼ A01 þ A02 þ A03 þ A04
(2)
Where A0J is the radiation rate for the 5D0→7FJ ( J = 0–4) transition of Eu3+ ions. A0J can be calculated as (44–46): A0J ¼ A01 ðI0J =I01 Þðν01 =ν0J Þ
(3)
A01 is Einstein spontaneous emission coefficient of the 5D0→7F1 transition. A01 is ~ 50 s1, and can be considered as a reference for the whole spectra. I0J is the integrated intensity of the 5D0→7FJ transition ( J = 0-4) with ν0J (ν0J = 1/λJ) energy centres. The lifetime (τ), radiative (Arad) and nonradiative (Anrad) transition rates are related via the following equation (18,34,47,48): Atot ¼ 1=τ ¼ Ar þ Anr
(4)
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Figure 5. The luminescence decay and fit curves of (a) Eu(TPTZ)(NO3)3(C2H5OH)·H2O, (b) Eu(TPTZ)2(NO3)3(C2H5OH)·H2O, (c) Eu2(TPTZ)(NO3)6(C2H5OH)·2H2O, (d) Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O, and (e) Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O.
J. Kang et al. Table 4. Luminescent quantum efficiency data of the europium complexes Complex ν00 (cm–1) ν01 (cm–1) ν02 (cm–1) ν03 (cm–1) I01 (a.u.) I02 (a.u.) I02/ I01 τ (ms) 1/τ Ar Anr η (%)
Eu(TPTZ)(NO3)3 (C2H5OH)·H2O 17,271 16,863 16,234 15,408 2,977 9,113 3.061 0.5479 1.825 224.3 1,601 12.29
Eu(TPTZ)2(NO3)3 (C2H5OH)·H2O
Eu2(TPTZ)(NO3)6 (C2H5OH)·2H2O
17,065 16,920 16,287 15,408 4,800 22,140 4.613 0.4156 2.406 304.6 2,101 12.66
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It can be seen that the quantum efficiencies (η) of the complexes mainly depend on the values of two factors: lifetime (τ) and I02/I01. From Tables 2 and 4, Eu(TPTZ)(NO3)3(C2H5OH)·H2O has the weakest luminescence intensity and the lowest quantum efficiency (12.29%) among the complexes. Complex Eu2(TPTZ)2 (C8H4O4)(NO3)4(C2H5OH)·H2O has the strongest luminescence intensity and the highest quantum efficiency (47.71%). This shows that the radiative transition rate (Ar) of Eu2(TPTZ)2(C8H4O4) (NO3)4(C2H5OH)·H2O is the highest and the nonradiative transition rate (Anr) is the lowest among the europium complexes. According to the energy transfer and intramolecular energy mechanism (49,50), the energy difference (ΔE) between the triplet state energy level of the ligand and the lowest excited state energy level of the lanthanide ion is one of the most important factors influencing the luminescence properties of lanthanide complexes. If the energy difference is too large, the energy-transfer rate constant will decrease due to diminution of the overlap between the energy donor (ligands) and acceptor (lanthanide ions). By contrast, if the energy difference is too small, the energy can back-transfer from the lanthanide ions to the triplet state of the ligands. In the luminescence theory of lanthanide complexes, the requirement for efficient intramolecular energy transfer is that the energy difference lies within the range 2000–5000 cm–1 (51). The triplet state energy level of TPTZ is ~ 21,277 cm–1 and the lowest excited energy level of Eu3+ is 17,250 cm–1, the energy difference (ΔE) between them is 4024 cm–1, within the required range of 2000–5000 cm–1. Therefore, the complexes of TPTZ have stronger luminescence intensity. The three europium complexes (Eu(TPTZ)(NO3)3(C2H5OH)·H2O, Eu(TPTZ)2(NO3)3(C2H5OH)·H2O and Eu2(TPTZ)(NO3)6(C2H5OH)· 2H2O) coordinated with the same types of ligands (TPTZ and NO-3), but at different ratios of Eu3+ : TPTZ, might show different fluorescence properties. As the concentration of TPTZ or the luminous centre ion increases, the luminous intensity is enhanced. However, within this range, there is no concentration quenching (33). The fluorescence intensity of Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O (4) is greater than that of Eu(TPTZ)(NO3)3(C2H5OH)·H2O (1). It can be deduced that although the triplet energy level of phydroxybenzoic acid (17,241 cm–1) is lower than the lowest exited state 5D0 of Eu3+ (17,250 cm–1), the aromatic ring of phydroxybenzoic acid can form π–π* packing with the pyridine and triazine rings of TPTZ, which can vary the energy level for
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17,241 16,892 16,234 15,385 22,440 70,010 3.120 0.6316 1.583 225.0 1,358 14.22
Eu(TPTZ)(C7H5O3)3 (C2H5OH)·2H2O 17,241 16,835 16,181 15,361 17,450 4197 4.158 0.6413 1.559 272.6 1,286 17.48
Eu2(TPTZ)2(C8H4O4) (NO3)4(C2H5OH)·H2O 17,241 16,863 16,207 15,385 134,200 1,111,000 6.819 1.0486 0.9536 419.1 534.5 47.71
absorbing and transferring light. Thus, the fluorescence intensity of complex (4) can be greatly enhanced. The triplet energy level of NO–3 (24,814 cm–1) is higher than the lowest exited state 5D0 of Eu3+, however, the fluorescence intensity of the complex (1) is lower than that of complex (4). Therefore, it is assumed that NO–3 is only coordinated to rare earth ions and the absorbance and energy transfer abilities are poor. For complex (5), the triplet energy level of terephthalic acid (25,160 cm–1) is higher than the lowest exited state 5D0 of Eu3+. The energy difference is > 5000 cm–1, but the complex has the strongest luminescence intensity; it may be that terephthalic acid can also pack with the pyridine and triazine rings of TPTZ to adjust the energy level to a more suitable match with the lowest exited state 5D0 of Eu3+. As a whole, the five complexes present superior luminescent properties. The intensity order of the complexes is Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH) > Eu2(TPTZ)(NO3)6(C2H5OH) > Eu(TPTZ)2(NO3)3(C2H5OH) > Eu(TPTZ)(C7H5O3)3(C2H5OH) > Eu(TPTZ)(NO3)3(C2H5OH), and the europium complexes with terephthalic acid exhibit the strongest emissions among the five europium complexes. This might result from the following. First, the triplet energy level of TPTZ matches well with the lowest excited level of Eu3+ and has outstanding capacity to absorb and transfer energy. Second, p-hydroxybenzoic acid and terephthalic acid can form π–π* packing with the pyridine and triazine rings of TPTZ, which can vary the energy level to absorb and transfer energy. Third, NO–3 is just coordinated to rare earth ions and the ability to absorb and transfer energy is poor (34). There may be other reasons for the above-mentioned phenomena (52–55) and further investigations are needed.
Conclusions Five ternary europium complexes have been synthesized using p-hydroxybenzoic acid, terephthalic acid and nitrate as anion ligands, and TPTZ as the neutral ligand. The compositions of these complexes are conjectured to be Eu(TPTZ)(NO3)3 (C2H5OH)·H2O, Eu(TPTZ)2(NO3)3(C2H5OH)·H2O, Eu2(TPTZ)(NO3)6 (C2H5OH)·2H2O, Eu(TPTZ)(C7H5O3)3-(C2H5OH)·2H2O and Eu2(TPTZ)2 (C8H4O4)(NO3)4(C2H5OH)·H2O. All the complexes show outstanding fluorescence. As the concentration of TPTZ or luminous centre ion increases, the fluorescence intensity for complexes with the same ligands was enhanced, and there was no concentration quenching effect. In addition, these complexes have longer emission
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Luminescence 2015; 30: 1360–1366
Synthesis, characterization and luminescent properties of Eu complexes lifetimes and higher emission quantum efficiencies. Based on the triplet energy level of all the ligands and intramolecular theory, aromatic ligands with such low or high triplet energy levels (e.g., p-hydroxybenzoic acid and terephthalic acid) can couple with other appropriate aromatic ligands to form highly luminous lanthanide complexes, improve radiative transition rates (Ar) and decrease nonradiative transition rates (Anr). As a result, the quantum efficiency is optimized. Acknowledgements The research work is supported by the National Natural Science Foundation of China (21161013), Natural Science Foundation of Inner Mongolia (2011MS0202), and the Opening Foundation ] for Significant Fundamental Research of Inner Mongolia (2010KF03).
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Copyright © 2015 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 1360–1366
Revised: 30 January 2015,
Accepted: 26 February 2015
Published online in Wiley Online Library: 31 March 2015
(wileyonlinelibrary.com) DOI 10.1002/bio.2906
Synthesis, characterization and luminescent properties of europium complexes with 2,4,6-tris-(2-pyridyl)-s-triazine as highly efficient sensitizers Jie Kang, Ying-Nan Chen, Ai-Ling Wang, Hai-Yan Li, Yan-Rong Qu, Hai-Xia Zhang, Hai-Bin Chu* and Yong-Liang Zhao* ABSTRACT: Using 2,4,6-tris-(2-pyridyl)-s-triazine (TPTZ) as a neutral ligand, and p-hydroxybenzoic acid, terephthalic acid and nitrate as anion ligands, five novel europium complexes have been synthesized. These complexes were characterized using elemental analysis, rare earth coordination titrations, UV/vis absorption spectroscopy and infrared spectroscopy. Luminescence spectra, luminescence lifetime and quantum efficiency were investigated and the mechanism discussed in depth. The results show that the complexes have excellent emission intensities, long emission lifetimes and high quantum efficiencies. The superior luminescent properties of the complexes may be because the triplet energy level of the ligands matches well with the lowest excitation state energy level of Eu3+. Moreover, changing the ratio of the ligands and metal ions leads to different luminescent properties. Among the complexes, Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O shows the strongest luminescence intensity, longest emission lifetime and highest quantum efficiency. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: europium complex; 2,4,6-tris-(2-pyridyl)-s-triazine; aromatic carboxylic acid; luminescence property
Introduction
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Rare earth complexes are attracting considerable attention on account of their rapidly growing number of applications, which include the development of fluorescence materials (1,2), electroluminescence devices (3,4) and fluorescence probes and labels in a variety of biological systems (5–7). These application rely on the outstanding luminescence properties of rare-earth complexes (8). Europium complex is a class of luminescent material with an excellent narrow-band light-emitting performance (9–11). However, there are some restrictions to the practical application of europium compounds, essentially because of their weak luminescence intensity, high cost, short lifetime and low quantum efficiency (12,13). In order to overcome these drawbacks, ternary europium complexes have attracted considerable attention because of their unusual optical properties, for example, their enhanced emission intensities and long fluorescence lifetime (14–16). This is because the second organic ligand serves as an energy donor and enhances the fluorescence intensities of rare earth complexes. This is named the ’synergistic effect’ (17). Furthermore, introduction of the second organic ligand also can fulfil the coordination numbers of rare earth complexes due to its excellent coordination ability with rare earth ions and its ability to sensitize the luminescence of rare earth ions (16). Thus, europium complexes could be incorporated with some aromatic carboxylic acid and the hetero nitrogen ligands (18,19). 2,4,6-Tris(2-pyridyl)-1,3,5-triazine (TPTZ) is an almost planar molecule with an extended conjugated π system, which has three different ’coordination sites’ (major, middle and minor) in this molecule (20–23). The coordination form of TPTZ partly depends upon the size of the metal ions. TPTZ generally acts as a tridentate or
Luminescence 2015; 30: 1360–1366
binary ligand that coordinates with lanthanide ions and forms 1 : 1, 2 : 1 or 1 : 2 metal/ligand complexes (24–29). The suitability of the energy gap between the lowest excited state energy of the rare earth ions and the triplet level of the ligands is critical for efficient energy transfer (30,31). The lowest excited state energy of europium ion is 17,250 cm1 (18) and the triplet energy levels of TPTZ, p-hydroxybenzoic acid and terephthalic acid are 21,277, 17,241 and 25,160 cm1, respectively (32,33). The triplet state energy level of the ligands matches well with the lowest excitation state energy level of Eu3+. Furthermore, as a neutral ligand, TPTZ could lead to coordination saturation of Eu3+ ions, increase the steric hindrance of the compounds, and reduce the number of water molecules and solvent-quenching effect (34). Therefore, we selected TPTZ to coordinate with Eu3+. Carboxylates are widely used as ligands in rare earth complexes because of their diverse coordination modes, stronger bonding ability and efficient energy transfer to rare earth ions. In addition, the triplet energy level of NO–3 (24,814 cm–1) (31) is higher than the lowest exited state 5D0 of Eu3+. Thus, we also selected p-hydroxybenzoic acid, terephthalic acid and nitrate as anion ligands to form the complexes.
* Correspondence to: H.-B. Chu and Y.-L. Zhao, College of Chemistry and Chemical Engineering, InnerMongolia University, Huhhot, 010021, China. E-mail: [email protected]; [email protected] College of Chemistry and Chemical Engineering, Inner Mongolia University, Huhhot, China
Copyright © 2015 John Wiley & Sons, Ltd.
Synthesis, characterization and luminescent properties of Eu complexes
Experimental Materials and general methods The purity of lanthanide oxide Eu 2 O 3 was 99.99%. TPTZ, p-hydroxybenzoic acid, terephthalic acid, nitric acid, anhydrous ethanol and other reagents were all of analytical grade. Elemental analysis of C, H, N was performed with a Vario EL Cube elemental analyser. The europium contents of the complexes were determined by EDTA titration with xylenol orange as the indicator. UV/vis absorption spectra were recorded on a TU-1901 doublebeam UV/vis spectrophotometer, with dimethylsulfoxide (DMSO) as the solvent. Infrared (IR) spectra were recorded on a Nicolet Nexus 670 FT-IR spectrometer using the KBr pellet technique in the scope of 4000–400 cm–1. Luminescence spectra and luminescence lifetimes were recorded on an Edinburgh Instruments FLS-920 spectrometer at room temperature. The entrance and emission slit widths were 1.5 and 3 nm, respectively. Preparation of europium nitrate An ethanol solution of Eu(NO3)3 was prepared by reacting Eu2O3 and a dilute solution of nitric acid. The solution was heated, dissolved, evaporated and then dissolved in anhydrous ethanol. Preparation of complexes (1) Eu(TPTZ)(NO3)3(C2H5OH), (2) Eu(TPTZ)2(NO3)3(C2H5OH) and (3) Eu2(TPTZ)(NO3)6(C2H5OH) TPTZ (0.1872 g, 0.6 mmol) was first dissolved in 40 mL of ethanol solution with stirring at 60°C. Then NH3·H2O (1 : 4, v/v) was added to the solution until the pH was 6. Under stirring, 6 mL of Eu(NO3)3 (0.1 mol/L) ethanol solution was added to the above solution. The pH was readjusted to between 6 and 7 with NH3·H2O. After stirring at 60°C for 3 h, the solution was left at room temperature overnight. The mixture was separated by filtering and washing with ethanol and acetone. The product Eu(TPTZ)(NO3)3(C2H5OH) was obtained after drying at 50°C for 24 h. The procedure for the synthesis of (2) Eu(TPTZ)2(NO3)3(C2H5OH) and (3) Eu2(TPTZ)(NO3)6(C2H5OH) was similar to that of Eu(TPTZ)(NO3)3(C2H5OH), except that the molar ratios of TPTZ and Eu(NO3)3 were 2 : 1 and 1 : 2, respectively.
After stirring at 60°C for 2 h, the solution was left at room temperature overnight. The mixture was separated by filtering and washing three times with ethanol. After drying at 50°C for 24 h, the product of complex Eu(TPTZ)(C7H5O3)3(C2H5OH) was obtained. The preparation Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH) was similar to that of Eu(TPTZ)(C7H5O3)3(C2H5OH) except that the molar ratio of Eu(NO3)3 : TPTZ : C8H4O4 was 2 : 2 : 1.
Results and discussion Composition analysis and properties of the complexes The results of the elemental analyses of C, H, N and rare earth titration of the complexes are listed in Table 1. The results show that the compositions of the complexes are Eu(TPTZ)(NO3)3 (C2H5OH)·H2O, Eu(TPTZ)2(NO3)3(C2H5OH)·H2O, Eu2(TPTZ)(NO3)6 (C2H5OH)·2H2O, Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O and Eu2(TPTZ)2 (C8H4O4)(NO3)4(C2H5OH)·H2O, respectively. The products are all white powders and are stable in the air. They are slightly soluble in DMSO and dimethylformamide (DMF), but insoluble in water, ethanol, acetone and ether. UV absorption spectra The UV/vis spectra for the dissociative ligands and their corresponding complexes are shown in Fig. 1. As seen from the spectra, the ligands TPTZ, p-hydroxybenzoic acid and terephthalic acid have strong absorption peaks at 282, 285 and 272 nm, respectively. After coordination with Eu3+, the absorption peaks shift to 278 nm. This shows that a more extensive π–π* conjugating system is formed due to the coordination (25,35). This indicates
Preparation of complexes (4) Eu(TPTZ)(C7H5O3)3(C2H5OH) and (5) Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH) p-Hydroxybenzoic acid (0.1657 g, 1.2 mmol) and TPTZ (0.1248 g, 0.4 mmol) were dissolved in ethanol with stirring at 60°C. Four millilitres of Eu(NO3)3 (0.1 mol/L) ethanol solution was added to the above solution. The molar ratio of Eu3+ : C7H5O3H : TPTZ is 1 : 3 : 1. The pH of the solution was adjusted to 6.4–6.7 with NH3·H2O.
Figure 1. UV absorption spectra of the ligands and some complexes.
Table 1. Composition analysis of europium complexes Complex Eu(TPTZ)(NO3)3(C2H5OH)·H2O Eu(TPTZ)2(NO3)3(C2H5OH)·H2O Eu2(TPTZ)(NO3)6(C2H5OH )·2H2O Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O
C% 33.33 (33.63) 44.30 (44.45) 22.10 (22.44) 51.05 (51.42) 39.00 (39.33)
H% 2.92 (2.82) 3.08 (3.14) 2.25 (2.07) 4.15 (3.89) 2.65 (2.58)
N% 17.55 (17.65) 20.26 (20.46) 15.30 (15.70) 8.70 (8.770) 15.84 (15.95)
Eu % 21.21 (21.27) 14.35 (14.80) 28.21 (28.39) 15.47 (15.87) 21.43 (21.63)
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Theoretical values are given in parentheses.
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J. Kang et al. that the ligands are all coordinated to Eu3+. Moreover, the λmax of the complexes is similar, which may be because the π–π* transition energy of TPTZ is much stronger than that of p-hydroxybenzoic acid and terephthalic acid.
and v3 (doubly degenerate in-plane bending) at 691 and 742 cm–1 in the spectra of the complexes, which means that the nitrate ions coordinate with the lanthanide ions with bidentate structure (42).
Infrared spectra
Luminescence spectra
As shown in Fig. 2, the ring stretching of both the pyridine and triazine rings of TPTZ is positioned at 1371 cm–1 (36), and shifts toward the high wavenumber by 6–15 cm–1 in the complexes. This reveals that the lanthanide ions coordinate with the nitrogen atoms of the rings. The characteristic pyridine ring absorption band (1529 cm–1) that is ascribed to υ(C=N) and υ(C=C) (37–39) moves toward the low wavenumber by 3–4 cm–1 in the complexes, which also suggests that TPTZ coordinates to the lanthanide ions. As can be seen from Fig. 2, the bands of free carboxylic ions from p-hydroxybenzoic acid and terephthalic acid change significantly after coordination. The υ(C=O) peak of p-hydroxybenzoic acid at 1676 cm–1 disappears and two peaks at 1609 cm–1 (υas(COO- )) and 1493 cm–1 (υs(COO- )) appear in the complex. Also, the peak of terephthalic acid at 1684 cm–1 disappears and two peaks at 1605 cm–1 (υas(COO- )) and 1484cm–1 (υs(COO- )) appear in the complex. These changes indicate that the ligands coordinate with lanthanide ions by bidentate carboxylate groups (40,41). The coordinated nitrate ions show v1 (out-of-plane deformation) at 789 cm–1, v2 (doubly degenerate stretching) at 1234 and 1451 cm–1
The excitation wavelengths of the complexes were determined within the range of 220–450 nm at room temperature. The excitation spectra of the complexes were all broadband absorption, and the strongest peaks of Eu(TPTZ)(NO3)3(C2H5OH)· H2O, Eu(TPTZ)2(NO3)3(C2H5OH)·H2O, Eu2(TPTZ)(NO3)6(C2H5OH)· 2H2O and Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O were 362, 363, 360 and 365 nm, respectively. The emission spectra of the complexes were determined at the most efficacious excitation wavelength, and the emission spectra data of the complexes are listed in Table 2. All the europium complexes emit the characteristic luminescence of Eu3+. The luminescence emission peak at 614–616 nm is electric dipole transition (5D0→7F2), and is the strongest emission peak among the five characteristic peaks. As shown in Table 2 and Figs. 3 and 4, different complexes have varying fluorescence intensity. The intensity order of the complexes is Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH) > Eu2(TPTZ)(NO3)6(C2H5OH) > Eu(TPTZ)2(NO3)3(C2H5OH) > Eu(TPTZ)(C7H5O3)3(C2H5OH) > Eu(TPTZ)(NO3)3(C2H5OH), and the europium complex with terephthalic acid exhibits the strongest emission among the five europium complexes.
Figure 2. IR spectra of ligands and complexes. (a) TPTZ, (b) p-hydroxybenzoic acid, (c) terephthalic acid, (d) Eu(TPTZ)(NO3)3(C2H5OH)·H2O, (e) Eu(TPTZ)2(NO3)3(C2H5OH)· H2O, (f ) Eu2(TPTZ)(NO3)6(C2H5OH)·2H2O, ( g) Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O, (h) Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O.
Figure 3. Emission spectra of the complexes. (a) Eu(TPTZ)(NO3)3(C2H5OH)·H2O, (b) Eu(TPTZ)2(NO3)3(C2H5OH)·H2O, (c) Eu2(TPTZ)(NO3)6(C2H5OH)·2H2O, (d) Eu(TPTZ)(C7H5O3)3 (C2H5OH)·2H2O.
Table 2. Luminescent emission spectra data of the complexes D0→7F0
λ (nm)
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Eu(TPTZ)(NO3)3(C2H5OH)·H2O Eu(TPTZ)2(NO3)3(C2H5OH)·H2O Eu2(TPTZ)(NO3)6(C2H5OH)·2H2O Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O
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D0→7F1
5
Complex
579 579 580 580 580
D0→7F2
5
I (a.u.)
λ (nm) 2
5.950 × 10 5.080 × 102 4.126 × 103 2.040 × 102 1.700 × 104
593 591 592 590 590
I (a.u.)
λ (nm) 3
2.977 × 10 4.800 × 103 2.244 × 104 3.210 × 103 1.342 × 105
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D0→7F3
5
615 614 616 616 616
5
I (a.u.) 3
9.478 × 10 2.214 × 104 7.001 × 104 1.646 × 104 1.111 × 106
λ (nm)
I (a.u.)
649 649 649 651 650
3.000 × 102 6.120 × 102 9.770 × 102 2.990 × 102 2.677 × 104
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Synthesis, characterization and luminescent properties of Eu complexes Table 3. Luminescent life data of the europium complexes Complex
τ (ms)
Eu(TPTZ)(NO3)3(C2H5OH)·H2O Eu(TPTZ)2(NO3)3(C2H5OH)·H2O Eu2(TPTZ)(NO3)6(C2H5OH)·2H2O Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O
0.5479 0.4156 0.6316 0.6413 1.0486
η¼
Figure 4. Emission (1) and excitation (2) spectra of Eu2(TPTZ)2(C8H4O4)(NO3)4 (C2H5OH)·H2O.
Luminescence lifetimes and quantum efficiencies The luminescence lifetimes of the europium complexes were determined, the luminescence decay curves of the complexes are shown in Fig. 5 and the data are given in Table 3. The decay profile is well reproduced by a single exponential ln(I(t)/I0) = –k/t = –t/τ (where I0 is the initial luminescence intensity and τ is the luminescence lifetime), indicating that all Eu3+ ions occupy the same average coordination environment (34,36). In the europium complexes, the lifetime of Eu(TPTZ)2(NO3)3(C2H5OH)·H2O (0.4156 ms) is the shortest and that of Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O (1.0486 ms) is the longest. Assuming that only nonradiative and radiative processes are essentially involved in depopulation of the 5D0 state and according to the emission spectra and lifetimes of Eu3+ (τ, 5D0), the emission quantum yield (η, Table 4) can be defined as follows (19,43):
Ar Ar þ Anr
(1)
Ar and Anr are the radiative and nonradiative transition rates, respectively. Ar can be calculated from: Ar ¼ ∑ A0J ¼ A01 þ A02 þ A03 þ A04
(2)
Where A0J is the radiation rate for the 5D0→7FJ ( J = 0–4) transition of Eu3+ ions. A0J can be calculated as (44–46): A0J ¼ A01 ðI0J =I01 Þðν01 =ν0J Þ
(3)
A01 is Einstein spontaneous emission coefficient of the 5D0→7F1 transition. A01 is ~ 50 s1, and can be considered as a reference for the whole spectra. I0J is the integrated intensity of the 5D0→7FJ transition ( J = 0-4) with ν0J (ν0J = 1/λJ) energy centres. The lifetime (τ), radiative (Arad) and nonradiative (Anrad) transition rates are related via the following equation (18,34,47,48): Atot ¼ 1=τ ¼ Ar þ Anr
(4)
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Figure 5. The luminescence decay and fit curves of (a) Eu(TPTZ)(NO3)3(C2H5OH)·H2O, (b) Eu(TPTZ)2(NO3)3(C2H5OH)·H2O, (c) Eu2(TPTZ)(NO3)6(C2H5OH)·2H2O, (d) Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O, and (e) Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH)·H2O.
J. Kang et al. Table 4. Luminescent quantum efficiency data of the europium complexes Complex ν00 (cm–1) ν01 (cm–1) ν02 (cm–1) ν03 (cm–1) I01 (a.u.) I02 (a.u.) I02/ I01 τ (ms) 1/τ Ar Anr η (%)
Eu(TPTZ)(NO3)3 (C2H5OH)·H2O 17,271 16,863 16,234 15,408 2,977 9,113 3.061 0.5479 1.825 224.3 1,601 12.29
Eu(TPTZ)2(NO3)3 (C2H5OH)·H2O
Eu2(TPTZ)(NO3)6 (C2H5OH)·2H2O
17,065 16,920 16,287 15,408 4,800 22,140 4.613 0.4156 2.406 304.6 2,101 12.66
1364
It can be seen that the quantum efficiencies (η) of the complexes mainly depend on the values of two factors: lifetime (τ) and I02/I01. From Tables 2 and 4, Eu(TPTZ)(NO3)3(C2H5OH)·H2O has the weakest luminescence intensity and the lowest quantum efficiency (12.29%) among the complexes. Complex Eu2(TPTZ)2 (C8H4O4)(NO3)4(C2H5OH)·H2O has the strongest luminescence intensity and the highest quantum efficiency (47.71%). This shows that the radiative transition rate (Ar) of Eu2(TPTZ)2(C8H4O4) (NO3)4(C2H5OH)·H2O is the highest and the nonradiative transition rate (Anr) is the lowest among the europium complexes. According to the energy transfer and intramolecular energy mechanism (49,50), the energy difference (ΔE) between the triplet state energy level of the ligand and the lowest excited state energy level of the lanthanide ion is one of the most important factors influencing the luminescence properties of lanthanide complexes. If the energy difference is too large, the energy-transfer rate constant will decrease due to diminution of the overlap between the energy donor (ligands) and acceptor (lanthanide ions). By contrast, if the energy difference is too small, the energy can back-transfer from the lanthanide ions to the triplet state of the ligands. In the luminescence theory of lanthanide complexes, the requirement for efficient intramolecular energy transfer is that the energy difference lies within the range 2000–5000 cm–1 (51). The triplet state energy level of TPTZ is ~ 21,277 cm–1 and the lowest excited energy level of Eu3+ is 17,250 cm–1, the energy difference (ΔE) between them is 4024 cm–1, within the required range of 2000–5000 cm–1. Therefore, the complexes of TPTZ have stronger luminescence intensity. The three europium complexes (Eu(TPTZ)(NO3)3(C2H5OH)·H2O, Eu(TPTZ)2(NO3)3(C2H5OH)·H2O and Eu2(TPTZ)(NO3)6(C2H5OH)· 2H2O) coordinated with the same types of ligands (TPTZ and NO-3), but at different ratios of Eu3+ : TPTZ, might show different fluorescence properties. As the concentration of TPTZ or the luminous centre ion increases, the luminous intensity is enhanced. However, within this range, there is no concentration quenching (33). The fluorescence intensity of Eu(TPTZ)(C7H5O3)3(C2H5OH)·2H2O (4) is greater than that of Eu(TPTZ)(NO3)3(C2H5OH)·H2O (1). It can be deduced that although the triplet energy level of phydroxybenzoic acid (17,241 cm–1) is lower than the lowest exited state 5D0 of Eu3+ (17,250 cm–1), the aromatic ring of phydroxybenzoic acid can form π–π* packing with the pyridine and triazine rings of TPTZ, which can vary the energy level for
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17,241 16,892 16,234 15,385 22,440 70,010 3.120 0.6316 1.583 225.0 1,358 14.22
Eu(TPTZ)(C7H5O3)3 (C2H5OH)·2H2O 17,241 16,835 16,181 15,361 17,450 4197 4.158 0.6413 1.559 272.6 1,286 17.48
Eu2(TPTZ)2(C8H4O4) (NO3)4(C2H5OH)·H2O 17,241 16,863 16,207 15,385 134,200 1,111,000 6.819 1.0486 0.9536 419.1 534.5 47.71
absorbing and transferring light. Thus, the fluorescence intensity of complex (4) can be greatly enhanced. The triplet energy level of NO–3 (24,814 cm–1) is higher than the lowest exited state 5D0 of Eu3+, however, the fluorescence intensity of the complex (1) is lower than that of complex (4). Therefore, it is assumed that NO–3 is only coordinated to rare earth ions and the absorbance and energy transfer abilities are poor. For complex (5), the triplet energy level of terephthalic acid (25,160 cm–1) is higher than the lowest exited state 5D0 of Eu3+. The energy difference is > 5000 cm–1, but the complex has the strongest luminescence intensity; it may be that terephthalic acid can also pack with the pyridine and triazine rings of TPTZ to adjust the energy level to a more suitable match with the lowest exited state 5D0 of Eu3+. As a whole, the five complexes present superior luminescent properties. The intensity order of the complexes is Eu2(TPTZ)2(C8H4O4)(NO3)4(C2H5OH) > Eu2(TPTZ)(NO3)6(C2H5OH) > Eu(TPTZ)2(NO3)3(C2H5OH) > Eu(TPTZ)(C7H5O3)3(C2H5OH) > Eu(TPTZ)(NO3)3(C2H5OH), and the europium complexes with terephthalic acid exhibit the strongest emissions among the five europium complexes. This might result from the following. First, the triplet energy level of TPTZ matches well with the lowest excited level of Eu3+ and has outstanding capacity to absorb and transfer energy. Second, p-hydroxybenzoic acid and terephthalic acid can form π–π* packing with the pyridine and triazine rings of TPTZ, which can vary the energy level to absorb and transfer energy. Third, NO–3 is just coordinated to rare earth ions and the ability to absorb and transfer energy is poor (34). There may be other reasons for the above-mentioned phenomena (52–55) and further investigations are needed.
Conclusions Five ternary europium complexes have been synthesized using p-hydroxybenzoic acid, terephthalic acid and nitrate as anion ligands, and TPTZ as the neutral ligand. The compositions of these complexes are conjectured to be Eu(TPTZ)(NO3)3 (C2H5OH)·H2O, Eu(TPTZ)2(NO3)3(C2H5OH)·H2O, Eu2(TPTZ)(NO3)6 (C2H5OH)·2H2O, Eu(TPTZ)(C7H5O3)3-(C2H5OH)·2H2O and Eu2(TPTZ)2 (C8H4O4)(NO3)4(C2H5OH)·H2O. All the complexes show outstanding fluorescence. As the concentration of TPTZ or luminous centre ion increases, the fluorescence intensity for complexes with the same ligands was enhanced, and there was no concentration quenching effect. In addition, these complexes have longer emission
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Luminescence 2015; 30: 1360–1366
Synthesis, characterization and luminescent properties of Eu complexes lifetimes and higher emission quantum efficiencies. Based on the triplet energy level of all the ligands and intramolecular theory, aromatic ligands with such low or high triplet energy levels (e.g., p-hydroxybenzoic acid and terephthalic acid) can couple with other appropriate aromatic ligands to form highly luminous lanthanide complexes, improve radiative transition rates (Ar) and decrease nonradiative transition rates (Anr). As a result, the quantum efficiency is optimized. Acknowledgements The research work is supported by the National Natural Science Foundation of China (21161013), Natural Science Foundation of Inner Mongolia (2011MS0202), and the Opening Foundation ] for Significant Fundamental Research of Inner Mongolia (2010KF03).
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