J. Cent. South Univ. Technol. (2007)01-0062-06

DOI: 10.1007/s11771-007-0013-4               

Syntheses and applications of Eu(III) complexes of 2-thienyltrifluoroacetonate, terephthalic acid and phenanthroline as light conversion agents

ZHAO Xue-hui(赵学辉)^1,2, HUANG Ke-long(黄可龙)¹, JIAO Fei-peng(焦飞鹏)¹,

YANG You-ping(杨幼平)¹, LI Zhao-jian(李朝建)¹, LIU Zhi-guo(刘志国)², HU Shun-qin(胡舜钦)²

(1. School of Chemistry and Chemical Engineering, Central South University,Changsha 410083, China;

2. Key Laboratory of Green Packaging and Application Biological Nanotechnology of Hunan Province,Hunan University of Technology, Zhuzhou 412008, China)

Abstract: A series of europium(Ⅲ) complexes of 2-thienyltrifluoroacetonate (HTTA), terephthalic acid (TPA) and phenanthroline (Phen) were synthesized. The new complexes Eu(TPA)(TTA)Phen and Eu₂(TPA)(TTA)₄Phen₂ were characterized by elemental analysis, IR spectrum, scanning electron microscope and thermal stability analysis. The results show that the thermal stability of the Eu(Ⅲ) complexes increases in the following order: the mononuclear complex Eu(TTA)₃Phen, the binuclear complex Eu₂(TPA)(TTA)₄Phen₂, the chain polynuclear complex Eu(TPA)(TTA)Phen. And the formation of the binuclear/polynuclear structure of the new complexes appears to be responsible for the enhancement of their thermal and optical stability. In addition, The fluorescence excitation spectra of these new complexes show more broad excitation bands than that of the complex Eu(TTA)₃Phen corresponding to their formation. The enhancement of Eu³⁺ fluorescence in the new complexes can be observed by the addition of Gd³⁺. The bright red luminescent plastics can be obtained when the complex EuGd(TPA)(TTA)₄Phen₂ is added above 0.5% (mass fraction).

Key words: fluorescent properties; europium; terephthalic acid; 2-thienyltrifluoroacetonate

1 Introduction

The design of complexes of trivalent rare earth ions is an important subject in the field of supramolecular and coordination polymers, because of the possibility to obtain the complexes having good luminescent properties. An important aspect of this research field is how to optimize the luminescent properties and thermal, optical stability of complexes by a suitable selection of ligands. Europium(Ⅲ) complexes with β-diketones ligands have good luminescent properties^[1-2], which have been found important application in light conversion events. As light conversion materials, ultraviolet light is firstly absorbed by β-diketones ligands, then the absorbed energy is transferred to Eu³⁺ ions and make them send out their characteristic light. β-diketones ligands have a broad absorption band in the region of near ultra-violet, and the energy level of the triplet state matches well with the emission energy level of rare earth europium(Ⅲ), so the energy transfer of β-diketones ligands to europium(Ⅲ) ions is very efficient. These result in the great increase in luminescent intensity of europium(Ⅲ) ion^[3-6]. However, the europium(Ⅲ) complexes with β-diketones ligands for practical application in light conversion events are very limited due to their poor thermal, optical stability and low mechanical resistances. In recent years, although both the solubility and fluorescent intensity of the Eu(Ⅲ)- carboxylates were weaker than those of Eu(Ⅲ)-β- diketones, they were used as light conversion agents still, due to their good stability^[7-8]. The reason why Eu(Ⅲ)- carboxylates have a good stability is that they can form binuclear complex or one-, two- and three-dimensional coordination polymers^[9].

FU et al^[10-13] studied the syntheses and luminescent properties of europium(Ⅲ) mononuclear complexes with β-diketones ligands (e.g. 2-thienyltrifluoro acetonate, dibenzoylmethide) and phenanthroline(Phen) or trioctylphosphine oxide. However, the studies on syntheses and luminescent properties of europium(Ⅲ) complexes of 2-thienyltrifluoroacetonate with terephtha- lic acid (TPA) and phenanthroline have not been reported yet.

The aim of this study is to study the synthesis and fluorescence properties of the new binuclear complex Eu₂(TPA)(TTA)₄(Phen)₂ and chain polynuclear complex Eu(TPA)(TTA)Phen, using bridging ligand TPA to link europium(III) ions to form binuclear or polynuclear. And their thermal and optical stability were investigated. In addition, IR spectrum, scanning electron microscope and fluorescence spectra of these complexes were also studied.

2 Experimental

2.1 Reagents and apparatus

99.99% Eu₂O₃ and 99.98% Gd₂O₃ were purchased from Jiangxi South Rare Earth Metals Institute in China. 2-thienyltrifluoroacetonate (HTTA), terephthalic acid (TPA), phenanthroline (Phen) and other reagents were all analytical grade and used without further purification.

The elemental analyses of C, H and N were performed on an American Perkin-Elmer 2400 Ⅱ CHNSLO elemental analyzer. The mass fraction of   Eu(Ⅲ) was determined by complexometric titration with EDTA. The infrared (IR) spectra were measured on a  Nicolet-550 spectrophotometer (American Perkin- Elmer) using KBr pellets in the range of 4 000-400 cm^-1 at room temperature. The scanning electronic microscope (SEM) was obtained on a microscope JSM-5600LV along with the gold sputtering technique. The differential thermoanalysis-thermogravimetric (DTA-TG) were performed on a SHDT-40 thermoanalyticmeter using aluminum crucibles with about 18.4 mg and 18.0 mg samples respectively, under dynamic synthetic air atmosphere (40 mL/min) and heating rate of 10 ℃/min at 30-600 ℃. An SPEX FL-2T2 spectrofluorometer was used to record the fluorescence excitation and emission spectra of the complexes using a 450 W Xenon lamp as excitation source. This apparatus was controlled by a DM3000F spectroscopic computer.

2.2 Synthesis of complexes

The complex Eu₂(TPA)(TTA)₄Phen₂ was prepared in the following steps. In the first place, standard solution (0.1 mol/L) of Eu³⁺ (or Gd³⁺) was prepared by dissolving Eu₂O₃ (or Gd₂O₃) in hot hydrochloric acid, evaporating up to syrup and diluting with ethanol to a desired volume. HTTA, TPA and Phen were dissolved in ethanol with molar ratio of 4?1?2. Subsequently EuCl₃ and HTTA solutions were mixed with molar ratio of 1?2, adjusting pH value to 5.0, stirred and refluxed for 40 min keeping temperature in water-bath. Then according to molar composition of formula Eu₂(TPA)(TTA)₄Phen₂, Phen and TPA solutions were added dropwise, keeping pH value 6.5, stirred and refluxed until the appearance of an orange precipitate. The solid product was filtered, washed and recrystallized in ethanol.

The Eu₂(TPA)₃Phen₂ and Eu(TPA)(TTA)Phen complexes were prepared by the similar process as Eu₂(TPA)(TTA)₄Phen₂, except that the products were a pale yellow precipitate.

The complexes Eu_2(1-x)Gd_2x(TPA)(TTA)₄Phen₂ and Eu_1-xGd_x(TPA)(TTA)Phen were prepared by the similar process as Eu₂(TPA)(TTA)₄Phen₂, except that the mixture solution of EuCl₃ and GdCl₃ was used instead of the EuCl₃. The products obtained were an orange or a pale yellow precipitate.

The complex Eu(TTA)₃Phen was synthesized by the similar process as described in Ref[14]. The product obtained was an orange precipitate.

3 Results and discussion

3. 1 Composition of complexes

The mass fraction of Eu(Ⅲ) was determined by complexometric titration with EDTA. The elemental analysis data of C, H, N and Eu(Ⅲ) for complexes are listed in Table 1, which are similar to the calculated values.

3. 2 Characterization of complexes

    Table 2 shows IR spectra data of complexes. The presence of carboxylate groups is definitely confirmed by both asymmetric stretching bands at 1 552-1 569 cm^-1 and symmetric stretching bands at 1 397-        1 405 cm^-1 respectively in Eu(III) complexes. The separations (Δυ=υ_as-υ_s) between υ_as (COO) peaks and  υ_s(COO) peaks are in the range of 155-164 cm^{-1 in Eu(}Ⅲ) complexes, which is attributed to the bidentate chelating, bidentate bridging or tridentate chelating-bridging coordination modes of carboxylate groups with Eu³⁺, since the separations (Δυ=υ_as-υ_s) in

Table 1 Elemental analysis data of complexes

Table 2 IR spectra data of complexes

Eu(Ⅲ) complexes are lower than that in Na₂TPA (Δυ= 168 cm^-1). Furthermore, owing to the great steric hindrance of the complexes Eu(TPA)(TTA)Phen and Eu₂(TPA)(TTA)₄Phen₂, both bidentate bridging coordination and tridentate chelating-bridging coordination of carboxylate groups with rare earth ions become more difficult than the bidentate chelating coordination does. Thus, the coordination mode of carboxylate groups with rare earth ions is mainly the bidentate chelating coordination mode in the complexes Eu(TPA)(TTA)Phen and Eu₂(TPA)(TTA)₄Phen₂, and their chemical structures proposed are shown in Fig.1. In addition, the IR spectra also show a displacement of υ(C==O) stretching from about 1 680 cm^-1, in free TTA ligand, to approximately  1 603 cm^{-1 in the complexes, and a displacement of υ(}C==N) stretching from 1 596 cm^-1, in free Phen ligand, to approximately 1 559 cm^{-1 in the complexes, indicating that Eu(}Ⅲ) ion is coordinated by the oxygen or nitrogen atoms^[15].

The SEM images of complexes Eu(TPA)(TTA)Phen and Eu₂(TPA)(TTA)₄Phen₂ are shown in Fig.2. For the

Fig.1 Chemical structures of complexes

(a) Eu₂(TPA)(TTA)₄ Phen₂; (b) Eu(TPA)(TTA)Phen

binuclear complex Eu₂(TPA)(TTA)₄Phen₂, the formation of the agglomerated structure can be obtained by intermolecular force, one of them behaving like a supermolecule recovering the other, and for the polynuclear complex Eu(TPA)(TTA)Phen, the needle/ stripe shaped structure can be obtained by chain conglomeration.

Fig.2 SEM images of complexes

(a) Eu₂(TPA)(TTA)₄Phen₂; (b) Eu(TPA)(TTA)Phen

The DTA and TG curves of Eu(TPA)(TTA)Phen and Eu₂(TPA)(TTA)₄Phen₂ are shown in Fig.3. From DTA curves it can be seen that Eu₂(TPA)(TTA)₄Phen₂     (Fig.3(a)) complex melts at about 210 ℃ and Eu(TPA)(TTA)Phen (Fig.3(b)) complex melts at about  229 ℃, both with no decomposition before the melting point. The thermal decomposition temperature of Eu(TPA)(TTA)Phen is higher than that of Eu₂(TPA)- (TTA)₄Phen₂. In addition, the temperature of the thermal decomposition of the mononuclear complex Eu(TTA)₃Phen^[14] is at 260-360 ℃. So the thermal stability for Eu(Ⅲ) complexes decreases in the following order: the chain polynuclear complex Eu(TPA)(TTA)Phen＞the binuclear complex Eu₂(TPA)(TTA)₄Phen₂＞the mononuclear complex Eu(TTA)₃Phen. The formation of the binuclear/ polynuclear structure of the complexes appears to be responsible for the enhancement of the thermal stability of the complexes. Moreover, the TG curves do not present a little water loss at 50-200 ℃, indicating that two new complexes are in anhydrous form. This is corroborated by IR spectra and elemental analysis results.

Fig.3 DTA and TG curves of complexes

(a) Eu₂(TPA)(TTA)₄Phen₂; (b) Eu(TPA)(TTA)Phen

3.3 Fluorescent properties of complexes

The excitation spectra of Eu(Ⅲ) complexes, recorded in the spectral range of 220-450 nm by monitoring the emission at the hypersensitive ⁵D₀→⁷F₂ transition, are shown in Fig. 4. The excitation spectrum of Eu₂(TPA)₃ Phen₂ consists of a broad band ranging from 230 to 350 nm with the maximum excitation wavelengths at 328 nm, corresponding to the absorber of ligand TPA. The excitation spectrum of Eu(TTA)₃Phen shows a strong broad band ranging from 275 to 425 nm with the maximum excitation wavelengths at 388 nm. However, the data of Eu(TPA)(TTA)Phen and Eu₂(TPA)(TTA)₄Phen₂ complexes exhibit more broad bands with maximum excitation peaks respectively at 376 nm for Eu(TPA)(TTA)Phen and at 382 nm for    Eu₂(TPA)(TTA)₄Phen₂. In addition, in contrast to Eu(TTA)₃Phen, the excitation band of the TTA in Eu₂(TPA)(TTA)₄Phen₂ shows a peak position blue shift, and it is same case for Eu(TPA)(TTA)Phen. Furthermore, compared with Eu₂(TPA)(TTA)₄Phen₂, the excitation band of Eu(TPA)(TTA)Phen also shows that a peak position blue shift occurs. These facts show the presence of the different orientation and coordination environ- ments for TPA and TTA ligands in different complexes, corresponding to the formation of Eu(Ⅲ) complexes.

The emission spectra of Eu(Ⅲ) complexes recorded in the range of 560-710 nm with the maximum excitation wavelengths at room temperature, are shown in Fig.5. It can be seen from Fig.5 that five typical    Eu(Ⅲ) emission bands appear at about 582, 593, 615, 654 and 704 nm, corresponding to ⁵D₀→⁷F₀, ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄, respectively. Of all the fluorescent emissions for Eu(III) complexes, the relative fluorescence intensity of ⁵D₀→⁷F₂ is the strongest. In addition, it can be also seen from Fig.5 that the ratios of the relative intensity ⁵D₀→⁷F₂ and ⁵D₀→⁷F₁ in the complexes Eu(TPA)(TTA)Phen and Eu₂(TPA)(TTA)₄- Phen₂ are higher than that of Eu₂(TPA)₃Phen₂. And the symmetry of the new complexes changes much less, i.e. the forced electric dipole transition (⁵D₀→⁷F₂) of Eu³⁺ in the new complexes is strengthened, and the color purity of the assembly luminescence is significantly improved.

Table 3 shows the relative fluorescence intensity for the interesting Eu(III) complexes studied, and their fluorescence intensities (⁵D₀→⁷F₂) decrease in the following order: EuGd(TPA)(TTA)₄Phen₂＞Eu(TTA)₃- Phen＞Eu_0.5Gd_0.5(TPA)(TTA)Phen＞Eu₂(TPA)(TTA)₄- Phen₂＞Eu(TPA)(TTA)Phen＞Eu(TPA)₃Phen. The results demonstrate that the fluorescence intensity of Eu(Ⅲ) complexes is influenced by the structure of complexes and the coordination environments of ligands. The fluorescence enhancement of EuGd(TPA)(TTA)₄- Phen₂ complex may be concerned with the formation of the binuclear structure.

Fig. 4 Excitation spectra of complexes

1—Eu₂(TPA)₃Phen₂; 2—Eu(TPA)(TTA)Phen;3—Eu₂(TPA)(TTA)₄Phen₂; 4—Eu(TTA)₃Phen

Fig.5 Emission spectra of complexes

1—Eu₂(TPA)₃Phen₂; 2—Eu(TPA)(TTA)Phen;3—Eu₂(TPA)(TTA)₄Phen₂

3.4 Application in light conversion plastics

The complexes EuGd(TPA)(TTA)₄Phen₂ and Eu_0.5Gd_0.5(TPA)(TTA)Phen, which have strong emission intensity and good thermal stability, were used as an additive for high pressure polyethylene, and red fluorescence plastics under ultraviolet excitation was obtained. Using the fluorescence plastics, light conversion agriculture film can be prepared. This film can converse ultraviolet light to red light, which can accelerate the process of photosynthesis and metastasis of plants, and improve the out-put and quality of agricultural products. In order to investigate the lumines-cent properties of the film, the relationship between the content of Eu(III) complexes in film and the fluorescence intensity of the film (0.1 mm in thickness) using 365 nm ultraviolet light are shown in Fig.6. It can be seen that the fluorescence intensity of the film increases with the increase of the content of Eu(III) complexes. When the content of Eu(III) complexes is above 0.5%(mass fraction), a bright red fluorescence plastic film can be obtained.

Fig.6 Relationship between luminescence intensity of film and mass fraction of Eu(Ⅲ) complexes(w)

1—EuGd(TPA)(TTA)₄Phen₂;2—Eu_0.5Gd_0.5(TPA)(TTA)Phen

In order to investigate the optical stability of Eu(III) complexes, some optical stability tests were carried out using 360 nm ultraviolet light. The results are shown in Fig.7. It can be seen that the optical stability of the Eu(III) complexes decreases in the following order: the polynuclear complex Eu_0.5Gd_0.5(TPA)(TTA)Phen＞the binuclear complex EuGd(TPA)(TTA)₄Phen₂＞the mononuclear complex Eu(TTA)₃Phen, and the optical stability of Eu(III) complexes in film is higher than that of the corresponding pure Eu(III) complexes.  

Table 3  Fluorescence spectra data of complexes

Fig.7 Relationship between luminescence intensity and ultraviolet irradiation time with different Eu(Ⅲ) complexes

1—Eu(TTA)₃Phen; 2—EuGd(TPA)(TTA)₄Phen₂;3—Eu_0.5Gd_0.5(TPA)(TTA)Phen;4—Film of EuGd (TPA)(TTA)₄Phen₂;5—Film of Eu_0.5Gd_0.5 (TPA)(TTA)Phen_.

4 Conclusions

1) A series of new light conversion complexes Eu₂(TPA)(TTA)₄Phen_2, Eu_2(1-x)Gd_2x(TPA)(TTA)₄Phen₂ and Eu(TPA)(TTA)Phen and Eu_1-xGd_x(TPA)(TTA)Phen, showing strong red fluorescence and the good thermal and optical stability, are synthesized.

2) The thermal and optical stability for Eu(III) complexes decreases in the following order: Eu(TPA)(TTA)Phen＞Eu₂(TPA)(TTA)₄Phen₂＞Eu- (TTA)₃Phen. The formation of the binuclear/ polynuclear structure of the complexes appears to be responsible for the enhancement of the thermal and optical stability of the complexes.

3) The relative fluorescence intensity for Eu(III) complexes decreases in the following order: EuGd(TPA)(TTA)₄Phen₂＞Eu(TTA)₃Phen＞Eu_0.5Gd_0.5- (TPA)(TTA)Phen＞Eu₂(TPA)(TTA)₄Phen₂＞Eu(TPA)- (TTA)Phen＞Eu(TPA)₃Phen

4) The light conversion agriculture film with good luminescent properties can be prepared by using the complex EuGd(TPA)(TTA)₄Phen₂, which has strong fluorescence together with the good thermal and optical stability, as the additive of high pressure polyethylene.

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(Edited by YNAG You-ping)

Foundation item: Project(20576142) supported by the National Natural Science Foundation of China; Project (05B075) supported by the Foundation of Hunan Provincial Education Department for Young Scholars

Received date: 2006-05-28; Accepted date: 2006-07-29

Corresponding author: HUANG Ke-long, Professor; Tel: +86-731-8879850; E-mail: klhuang@mail.csu.edu.cn