Syntheses and Co-Fluorescence of Complexes of Eu ( III ) / Gd ( III ) with Thienyltrifluoroacetonate , Terephthalic Acid and Phenanthroline

A series of complexes of europium (III)/gadolinium (III) with 2-thienyltrifluoroacetonate (HTTA), terephthalic acid (TPA) and phenanthroline (Phen) were synthesized by coprecipitation. The resulting complexes including Eu2(TPA)(TTA)4Phen2, Eu1.4Gd0.6(TPA)(TTA)4Phen2, Eu1.0Gd1.0(TPA)(TTA)4Phen2 and Eu0.8Gd1.2(TPA)(TTA)4Phen2 were characterized by elemental analysis, IR spectroscopy and thermal stability analysis. The results of analysis indicate that the complexes obtained have similar binuclear structure with each other. The thermal stability analysis indicates that the complexes Eu2(TPA)(TTA)4Phen2 and Eu1.0Gd1.0(TPA)(TTA)4Phen2 possess good thermal stability, which melt at ~241 ̊C and decompose at ~370 ̊C 430 ̊C corresponding to the formation of the complexes. The fluorescence spectra of Eu2(1-x)Gd2x(TPA)(TTA)4Phen2 (x = 0 1) complex powders and their doped silica gels were studied. The co-fluorescence effect of Gd3+ ions in complex powders is different from that of their doped silica gels. The optimum concentration of Gd3+ for complex powders and their doped silica gels is 0.5 and 0.3 (molar fraction), respectively. The co-fluorescence distinction of Gd3+ ions for complex powders and their doped silica gels is preferably interpreted from the proposed binuclear structure together with monomolecular compositions of the complexes for the first time. Both intermolecular energy transfer and intra molecular energy transfer in cross binuclear monomolecular EuGd(TPA)(TTA)4Phen2 are thought to be responsible for the co-fluorescence effect of the complex powders; yet only the latter is thought to be responsible for the co-fluorescence effect in silica gels, for the complex molecules in this case are isolated from each other. Corresponding author.


Introduction
The fluorescence enhancement of the trivalent rare earth complexes still enjoys a growing interest due to their important application in time-resolved fluoroimmunoassays, electro-optical devices and amorphous luminescent materials [1]- [4].Generally, the fluorescence enhancement can be achieved through ligand sensitization.In this process, ultraviolet light is firstly absorbed by organic Ligands.Then, the absorbed energy is transferred to rare earth ions and makes them send out their characteristic light.Organic ligands have usually a broad absorption band in the region of near ultra-violet.If the energy level of the triplet state of organic ligands matches well with the emission energy level of rare earth ions, the energy transfer of ligands to rare earth ions is very efficient.This will result in the great increase in fluorescence intensity of rare earth ions [5].For this purpose, various ligands have been used, the more popular ones being the β-diketones [6]- [8], such as 2-thienyltrifluoroacetonate, benzoylacetone and dibenzoylmethane.The aromatic carboxylic acids as an excellent ligand are also popular [9]- [11], such as trimesic acid, terephthalic acid and pyromellitic acid.The fluorescence of rare earth complexes can also be further enhanced by the use of synergistic agents, such as trioctylphosphine oxide, phenanthroline, organic phosphates and sulphoxides.
Another method to increase the fluorescence of the trivalent rare earth complexes is through the use of certain lanthanide ions such as La 3+ , Gd 3+ and Tb 3+ [12]- [15].In the presence of these ions, the fluorescence enhancement of some rare earth complexes can be obtained.This process is referred to as co-fluorescence, which is extensively studied in the mononuclear complexes of Europiun (III) ions with β-diketones ligands, such as Eu(TTA) 3 Phen [12], Eu(Dbm) 3 Phen [13] and Eu(TTA) 3 TPPO 2 .As far as the mononuclear complex is concerned, the co-fluorescence can be found in a micellar environment [16] [17].In an actual solution, there is no co-fluorescence because the long distance between the chelates makes intermolecular energy transfer impossible.In solids, especially in coprecipitates, the distance between the complex molecules can be short enough to incur an intermolecular energy transfer.However, for the binuclear or polynuclear complexes their co-fluorescence properties are different from those of mononuclear complexes.Therefore, it is very significant to study the cofluorescence effect of the binuclear or polynuclear complexes in solids.Such studies can be helpful in better understanding of the co-fluorescence mechanism.
In this paper, in order to investigate the co-fluorescence properties of binuclear complexes, the bridging ligand TPA is used to link rare earth ions to form the new binuclear complexes Eu 2(1-x) Gd 2x (TPA)(TTA) 4 Phen 2 (x = 0 -1).And the co-fluorescence mechanism of Gd 3+ in complex powders and their doped silica gels is preferably interpreted from the binuclear structure together with monomolecular composition of the complexes for the first time.In addition, IR absorption spectra and thermal stability of the above mentioned complexes were also studied.C, H, N analysis was performed on an American Perkin-Elmer 2400 II CHNSLO elemental analyzer.The percentages of rare earth ions were determined by complexometric titration with EDTA.The infrared spectra were measured at room temperature on Nicolet-550 spectrophotometer (American Perkin-Elmer) using KBr pellets in the spectral range of 4000 -400 cm −1 .Differential thermoanalysis (DTA) was performed in a SHDT-40 thermoanalyticmeter using aluminum crucibles with ~18.40 mg of sample, under dynamic synthetic air atmosphere (40 mL•min −1 ) and heating rate of 10˚C•min −1 in the temperature range of 30˚C -600˚C.The thermogravimetric (TG) curves were recorded with a thermobalance model SHDT-40 in the temperature interval of 30˚C -600˚C, using platinum aluminum with ~18.0 mg of sample, under dynamic synthetic air atmosphere (40 mL•min −1 ) and heating rate of 10˚C•min −1 .A Fluorolog FL3-L 1 (American JOBIN YVON) Spectrometer was used to record excitation and emission spectra of the complex powders and their doped silica gels.The bandwidth of monochromators was set at 2.5 nm for both excitation and emission.

Synthesis of Complexes
The complex Eu 2 (TPA)(TTA) 4 Phen 2 was prepared in the following steps.In the first step, standard solution of europium (III) (1.0 × 10 −1 mol•L −1 ) was prepared by dissolving Eu 2 O 3 in hot hydrochloric acid, evaporating up to syrup and diluting with ethanol to a desired volume.HTTA, TPA and Phen were dissolved separately in ethanol with molar ratio of 4:1:2.Subsequently EuCl 3 and HTTA solutions were mixed with molar ratio of 1:2, adjusting pH vales to 5.0, stirred and refluxed for 40 min keeping temperature in water-bath.Then according to molar composition of formula Eu 2 (TPA)(TTA) 4 Phen 2 , Phen and TPA solutions were added dropwise, keeping pH vales 6.5, stirred and refluxed until the appearance of an orange precipitate.The solid product was filtered, washed and recrystallized in ethanol.
The complexes Eu 2(1−x) Gd 2x (TPA)(TTA) 4 Phen 2 were prepared by the similar process as Eu 2 (TPA)(TTA) 4 Phen 2 , except that the mixture solution of EuCl 3 and GdCl 3 was used instead of the EuCl 3 .The products obtained are an orange precipitate.

Incorporation of the Complexes into Silica Gels
Silica gels were prepared by hydrolysis and condensation of tetraethoxysilane Si(OC 2 H 5 ) 4 (TEOS).TEOS and H 2 O were used according to molar ratio of 1:4.3 with the proper amount of dimethylformamide (DMF) as solvent.The DMF solution of the complexes was subsequently added to the silica precursor solution (n complex : n TEOS = 1:20).Then the mixed solution was refluxed in a 70˚C water bath until gelation occurred.The resulting gels were dried at 100˚C for two weeks for measurement purposes.

Composition of Complexes
The rare earth percentages were determined by complexometric titration with EDTA.Analytical data of C, H, N and rare earth ions percentages (found/calculated) for the complex Eu 2 (TPA)(TTA) 4

Characterization of Complexes
Table 1 shows some results of IR spectra.The presence of carboxylate groups in the Eu (III) complexes was definitely confirmed by both the asymmetric stretching bands at 1552 -1558 cm −1 and the symmetric stretching at 1397 -1401 cm −1 .The separations (∆ = υ as − υ s ) between υ as (coo) peaks and υ s (coo) pears are in the range of 155 -159 cm −1 in the Eu (III) complexes, which are attributed to the bidentate chelating, bidentate bridging and tridentate chelating-bridging coordination modes of carboxylate groups with rare earth ions, since the separations (∆ = υ as − υ s ) in the Eu (III) complexes are lower than that in Na 2 TPA (Δ = 168 cm −1 ) [19] [20].Furthermore, owing to the great steric hindrance of the complexes Eu 2(1−x) Gd 2x (TPA)(TTA) 4 Phen 2 , the bidentate bridging and tridentate chelating-bridging coordination of carboxylate groups with rare earth ions becomes more difficult than the bidentate chelating coordination does.Thus, the coordination mode of carboxylate groups of TPA with rare earth ions is mainly the bidentate chelating coordination mode in the complexes, and the proposed chemical structure of the complexes is binuclear structure.The IR spectra also show a displacement of υ (C=O) stretching from ~1680 cm −1 , in free TTA ligand, to ~1605 cm −1 in the complexes, and a displacement of υ (C=N) stretching from ~1596 cm −1 , in free Phen ligand, to ~1559 cm −1 in the complexes, indicating that rare earth ions are coordinated by the oxygen or nitrogen atoms [21].The DTA and TG curves of Eu 2 (TPA)(TTA) 4 Phen 2 and Eu 1.0 Gd 1.0 (TPA)(TTA) 4 Phen 2 are shown in Figure 1 in the temperature range from 50˚C to 600˚C.It can be seen that the DTA and TG curves of the complex Eu 2 (TPA)(TTA) 4 Phen 2 (Figure 1(A)) present similar profile to that of the complexes Eu 1.0 Gd 1.0 (TPA)(TTA) 4 Phen 2 (Figure 1(B)).Both Eu 2 (TPA)(TTA) 4 Phen 2 and Eu 1.0 Gd 1.0 (TPA)(TTA) 4 Phen 2 possess good thermal stability, which melt at ~241˚C and decompose at ~370˚C -430˚C, with no decomposition before the melting point.These indicate that both have similar chemical structure corresponding to the formation of the new complexes.Moreover, in the interval 50˚C -200˚C, the TG curves of the complexes do not present any event relative to water loss, which indicates that the new complexes are in anhydrous form.This is corroborated by elemental analysis.
The fluorescence excitation and emission spectra of the solid state complexes were performed in a Fluorolog FL3-L 1 Spectrometer.
Figure 2 shows the excitation spectra of the Eu 2 (TPA)(TTA) 4 Phen 2 , Eu 1.4 Gd 0.6 (TPA)(TTA) 4 Phen 2 and Eu 0.8 Gd 1.2 (TPA)(TTA) 4 Phen 2 complex powders recorded in the spectral range of 220 -450 nm by monitoring the emission at the hypersensitive 5 D 0 → 7 F 2 transition.These complex excitation spectra show a strong broad band ranging from 250 to 425 nm with the optimum excitation wavelength at ~382 nm, and they are entirely different from the excitation spectrum of Eu 3+ (without the organic ligands) [22].These indicate that the ligands have transferred the energy absorbed to the Eu 3+ ion, leading to the fluorescence enhancement of Eu 3+ .
The fluorescence emission spectra of the complexes Eu 2(1−x) Gd 2x (TPA)(TTA) 4 Phen 2 (x = 0 -1) are all similar except the relative intensity.Figure 3 shows only the emission spectra of Eu 2 (TPA)(TTA) 4 Phen 2 , Eu 1.4 Gd 0.6 (TPA)(TTA) 4 Phen 2 and Eu 0.8 Gd 1.2 (TPA)(TTA) 4 Phen 2 recorded in the range of 560-710 nm with the optimum excitation wavelength at room temperature.It can be seen that five typical Eu (III) emission bands appear at ~582, ~593, ~615, ~654, ~704 nm, corresponding to 5 D 0 → 7 F 0 , 5 D 0 → 7 F 1 , 5 D 0 → 7 F 2 , 5 D 0 → 7 F 3 , and 5 D 0 → 7 F 4 , respectively.The characteristic emission peaks of Eu 3+ ions do not change with the addition of cofluorescence Gd 3+ ions.Compared with the powdered complexes, the silica gels doped with these complexes show weaker split lines of the Eu 3+ ion.The phenomenon can be accounted for in the following way.For the powdered complexes, Eu 3+ ions with surrounding environment of different ligand groups is in a site without a center of inversion, so the emission peaks split into more lines under the ligand field.However, silica gel is a kind of noncrystalline substance with a porous macrostructure.And the complex molecules fixed in the pores are highly ordered.The weaker split lines of Eu 3+ are observed because the symmetry of Eu 3+ ion in the powder is lower than that in silica gel.
In Figure 3(A), the relative emission intensity of the complex powder of Eu 2 (TPA)(TTA) 4 Phen 2 is weaker than that of Eu 0.8 Gd 1.2 (TPA)(TTA) 4 Phen 2 .While for the silica gels doped with these complexes, in Figure 3(B), the reverse is true.But compared with the silica gels doped with Eu 1.4 Gd 0.6 (TPA)(TTA) 4 Phen 2 , the relative emission intensity of the silica gels with Eu 2 (TPA)(TTA) 4 Phen 2 remain weaker.The intermolecular energy transfer cannot explain perfectly these changes of the fluorescence intensity.

Energy Transfer Processes
To investigate the co-fluorescence effect of Gd 3+ for the complex powders and their doped silica gels, We would assume that co-fluorescence Gd 3+ ions have no influence on the fluorescence intensity of Eu 3+ in the complexes, the relative emission intensity value I calc of the complex powders and their doped silica gels was calculated according to the molar fraction of Eu 3+ in different co-fluorescence complexes, and the I exp is the relative emission intensity value for experiment, so the ratios of I exp and I calc can be gotten.The results are also listed in Table 2. Table 2. Fluorescence spectra peak positions and relative intensities of the complex powders and their doped silica gels, where L = (TPA)(TTA) 4 Phen 2 .
Complexes λex/nm complex powders λem/nm (relative intensity) in silica gel λem/nm (relative intensity) As is well known that the energy transfer for the complexes from the ligand to the lanthanide ions can take place via intra molecular energy transfer mode, and the energy transfer can also take place via intermolecular transfer mode.In this study, we think that the co-fluorescence distinction of Gd 3+ ions for complex powders and their doped silica gels can be preferably interpreted from the proposed binuclear structure together with monomolecular compositions of the complexes.Since the coordination mode of carboxylate groups of TPA with rare earth ions is mainly the bidentate chelating coordination mode in the complexes.The proposed binuclear structure may be given in Schemes 1(a)-(c).For a certain x value in Eu 2(1−x) Gd 2x (TPA)(TTA) 4 Phen 2 , the complex powders were prepared by coprecipitation, so the complexes are composed of monomolecular EuGd(TPA)(TTA) 4 Phen 2 , Eu 2 (TPA)(TTA) 4 Phen 2 and Gd 2 (TPA)(TTA) 4 Phen 2 .In addition, the chemical structure of the mononuclear complex Eu(TTA) 3 Phen is also given in Scheme 1(d) [8].
Figure 4 shows the relationship between the percentages of every monomolecular compositions and the content of Gd 3+ ions in the complexes.It can be seen from Figure 4 firstly, that with the increase of x value, i.e., the contents of Gd 3+ ions, the percentages of monomolecular EuGd(TPA)(TTA) 4 Phen 2 and Gd 2 (TPA)(TTA) 4 Phen 2 increase.Since energy can only be transferred from a molecule to other molecules at short distances, and the complex powders were prepared by coprecipitation, the short distance between Gd 2 (TPA)(TTA) 4 Phen 2 molecules and Eu 2 (TPA)(TTA) 4 Phen 2 molecules in the coprecipitate makes the intermolecular effective energy transfer be possible.In addition, intra molecular energy transfer in cross binuclear monomolecular EuGd(TPA)(TTA) 4 Phen 2 increases.They result in the increase of the emission intensity of the complex powders.When x value is 0.5, the percentage of monomolecular EuGd(TPA)(TTA) 4 Phen 2 reaches maximum, the intra molecular energy transfer reaches also maximum.Then with a further increase of x values the intra molecular energy transfer decreases, on the other hand, intermolecular energy transfer is thought to be concerned with the molar ratios of Eu 2 (TPA)(TTA) 4 Phen 2 and Gd 2 (TPA)(TTA) 4 Phen 2 .So the optimum concentration of Gd 3+ in the complex powders Eu 2(1−x) Gd 2x (TPA)(TTA) 4 Phen 2 is ~0.5 (molar fraction).
In the silica gel doped with these complexes, the complex molecules are trapped in the pores and isolated from each other.The long distance between complex molecules in the silica gel makes the intermolecular energy transfer impossible.Yet intra molecular energy transfer in cross binuclear monomolecular EuGd(TPA)(TTA) 4 Phen 2 is not affect by silica gel.So with the increase of the contents of Gd 3+ ions, the percentage of the EuGd(TPA)(TTA) 4 Phen 2 increases, the fluorescence intensity of the silica gel doped with these complexes increases.But when the contents of Gd 3+ ions attain to certain value, because of the decrease of Eu 2 (TPA)(TTA) 4 Phen 2 , the fluorescence intensity of the silica gel doped with these complexes decrease.The optimum concentration of Gd 3+ in silica gel doped is ~0.3 (molar fraction).These interpretations from the binuclear structure together with monomolecular composition of the complexes are consistent with the results of the experiment.

Conclusions
A series of complexes of europium (III)/gadolinium (III) with 2-thienyltrifluoroacetonate, terephthalic acid and Phenanthroline, showing strong red fluorescence and good thermal stability have been synthesized.Compositions of these complexes are revealed to be Eu 2(1−x) Gd 2x (TPA)(TTA) 4 Phen 2 .
The cofluorescence properties and the mixed complexes Eu 2(1−x) Gd 2x (TPA)(TTA) 4 Phen 2 (x = 0 -1) are thought to be, for a certain x value in Eu 2(1−x) Gd 2x (TPA)(TTA) 4 Phen 2 , composed of EuGd(TPA)(TTA) 4 Phen 2 , Eu 2 (TPA)(TTA) 4 Phen 2 and Gd 2 (TPA)(TTA) 4 Phen 2 .The fluorescence enhancement of the Eu (III) complexes is observed by the addition of relative cheap Gd 3+ ions.The optimum concentration of Gd 3+ is 0.5 (molar fraction).The mechanism of the fluorescence enhancement is preferably interpreted from the proposed binuclear structure together with the percentages of every chemical composition in the Eu (III) complexes.Both intermolecular energy transfer mode and intra molecular energy transfer mode are thought to be responsible for the fluorescence enhancement of the complex powders; yet only intra molecular energy transfer is thought to be responsible for the fluorescence enhancement the silica gels doped with these complexes, for the complex molecules are isolated from each other in this case.Both intermolecular energy transfer between monomolecular Gd 2 (TPA)(TTA) 4 Phen 2 and monomolecular Eu 2 (TPA)(TTA) 4 Phen 2 and intra molecular energy transfer in cross binuclear monomolecular EuGd(TPA)(TTA) 4 Phen 2 are thought to be responsible for the co-fluorescence of the complex powders; yet only intra molecular energy transfer in cross binuclear monomolecular EuGd(TPA)(TTA) 4 Phen 2 is thought to be responsible for the co-fluorescence in silica gels, for the complex molecules in this case are isolated from each other.