RESEARCH ARTICLE

Synthesis and optical properties of europium pentafluoropropionate 1,10-phenanthroline complex and its silica glass

  • Jiangbo SHE 1 ,
  • Dongdong LI , 2
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  • 1. State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences (CAS), Xi’an 710119, China
  • 2. Department of Electronic and Information Engineering, Xi’an Institute of Post & Telecommunications, Xi’an 710121, China

Received date: 19 Jun 2011

Accepted date: 24 Jul 2011

Published date: 05 Dec 2011

Copyright

2014 Higher Education Press and Springer-Verlag Berlin Heidelberg

Abstract

Europium pentafluoropropionate 1,10-phenanthroline complex, Eu(C2F5COO)3·Phen (Phen= 1,10-phenanthroline), were synthesized and characterized by elemental analysis, Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL) spectroscopy and thermogravimetric analysis (TA). At the same time, its silica glass was synthesized by in situ processes. The excitation spectra of the complex demonstrate that the energy collected by “antenna ligands” was transferred to Eu3+ ions efficiently. The room-temperature PL spectra of the complexes are composed of the typical Eu3+ ions red emission, due to transitions between 5D07FJ(J = 0→4). The decomposition temperature of the complex was 290°C, which indicates the host complex is quite stable to heat. Then the affection of anneal temperatures on PL properties of SiO2 glass were studied. The PL intensity of the SiO2 glass annealed at 160°C was higher than other annealed temperatures for 24 h in air.

Cite this article

Jiangbo SHE , Dongdong LI . Synthesis and optical properties of europium pentafluoropropionate 1,10-phenanthroline complex and its silica glass[J]. Frontiers of Optoelectronics, 2011 , 4(4) : 382 -386 . DOI: 10.1007/s12200-011-0179-0

Introduction

The design and synthesis of luminescence complexes have attracted considerable attention due to their potential application as molecular photonic devices [1], fluorescent sensors [2,3], and luminescent chemical probes [4,5].
Silica glasses obtained by the sol-gel method have been intensely investigated during the last decade [6,7]. Compared with conventional glasses technique, this method has many advantages, such as low temperature process, high concentration of rare-earth doped, pure host matrix and organic molecule easily introduced. But the sol-gel method can produce a lot of hydroxyl groups in silica gel, which cause the radiationless transition process of rare-earth ions. In order to decrease the nonradiative energy loss and obtain high luminescence, organic ligands have been used. In the silica glasses, the organic ligand absorbs energy and then transfers efficiently to the metal ion [8]. Moreover, these ligands can also be acted as the second assistant ligands to replace water or other small molecules in coordination sphere [9]. It has been reported in my previous study that pentafluoropropionate as the first liagnd not only can increase the luminescence properties, but also reduce the radiationless transition process via vibrational absorption of C–H and O–H bonds [10,11]. In this study, we presented in situ preparation processes and luminescence behaviors of europium pentafluoropropionate 1,10-phenanthroline complex in the sol-gel derived host material. It was found that the decomposition temperature of the complex was 290°C, which indicates the host complex is quite stable to heat. According to the optical spectrum, Eu(C2F5COO)3·Phen encapsulated in the SiO2 glass will be a promising inorganic–organic hybridized material.

Experiments

Materials and measurement

All analytical reagents were obtained from commercial sources and used directly without further purification. Elemental analysis (C, H, N) was determined with a German Vario EL III instrument. The infrared spectra were recorded as KBr pellet on a Nicolet 170SX FT-IR spectrometer. Excitation and emission spectra were obtained at room temperature using a spectrofluorometer FL900 (Edinburgh Instruments) with a Xe lamp as a light source fitted with a Hamamatsu photomultiplier. The TG-DTG measurements were done with a Thermoanalyzer Systems Q1000DSC+LNCS+FACS Q600SDT of TA company.

Sythesis of Eu(C2F5COO)3·Phen complex

First, 2.11 g (6 mmol) Eu2O3 was dissolved in 4.10 g (36 mmol) C2F5COOH with 20 mL H2O. The solution was refluxed for 4 h under stirring. Then, it was concentrated by vacuum rotary evaporation. A white precipitate, Eu (C2F5COO)3·2H2O, was obtained. 1, 10-Phenanthroline·H2O 0.594 g (3 mmol) and Eu(C2F5COO)3·2H2O 2.031 g (3 mmol) were mixed in 40 mL tetrahydrofuran and stirred for 2 h at room temperature. The products, Eu(C2F5COO)3·Phen was achieved after dried in vacuum under 120°C for 12 h. Elemental analysis calcd. for Eu(C2F5COO)3·Phen: C 30.7, H 0.98, N 3.41; found: C 30.80, H 1.04, N 3.39%. IR (KBr, cm-1): 1728(s, vas(COO)), 1664(vs, vas(COO)), 1523(m, v(C=N)), 1435(m, vs(COO)), 1327(m, vs(COO)), 1219(vs, v(C-F)), 844(s, v(C-H)), 729(vs, v(C-F)).

Sythesis of the complex in silica matrix

The solution used to prepare the silica xerogel consisted of 1 mol tetraethyl orthosilicate (TEOS), 4 mol ethanol and 4 mol distilled water. A small amount of HCl was added to the solution to promote hydrolysis. 0.01 mol Phen and 0.01 mol Eu(C2F5COO)3·2H2O were added. The pH value was controlled within the acid region (about 2-3). The mixed solution was vigorously stirred at room temperature for 1 h. The resulting sol was subsequently kept in a sealed container at 40°C until the onset of gelation. Aging and drying were allowed to proceed under ambient conditions over a period of several weeks.

Results and discussion

Thermogravimetric analyses of the complex

Fig.1 TGA and DTA curves of the complex

Full size|PPT slide

Thermogravimetric analysis (TGA) curve recorded in N2 atmosphere from 30°C to 600°C for Eu(C2F5COO)3·Phen was shown in Fig. 1. It indicates that the decomposition processes of Eu(C2F5COO)3·Phen can be divided into two stages. The decomposition temperature of the complex Eu(C2F5COO)3·Phen was found to be 290°C. For the complex Eu(C2F5COO)3·Phen, the first stage is 290°C–338°C, which was corresponded to the loss of 1mol Phen and 2 mol pentafluoropropionate anions. This degradation can be explained that the Eu–N bonds are less stable and easy to be broken down. The second stage of temperature was 335°C–550°C corresponded to the loss of 1 mol pentafluoropropionate anions. The decomposed stage has 75.5% mass loss, and the mass of the residue is 24.5% according with the theoretical mass summation (25.6%) of EuF3. The result of powder X-ray diffraction experiment proved that the major ingredient of the residue was EuF3. The thermal analysis indicates that the complex is quite stable to heat.

Photoluminescence (PL) spectroscopy

Excitation and emission spectra of the complex were recorded at room temperature. It emitted intense red fluorescence when exposed under ultraviolet light. The excitation spectrum of Eu(C2F5COO)3·Phen and Eu(C2F5COO)3·Phen in silica glass was shown in Fig. 2, which were recorded by monitoring the 5D07F2 emission band at 612 nm. Each of the excitation spectra consists of a very broad band instead of a characteristic narrow band of Eu3+ (395 nm), which indicates that the europium complex has been synthesized by an in situ technique via silica gel process. Compared to that of pure complex, the maximum excitation wavelength of the complex in silica gel shows a small red-shift changing from 332 to 344 nm, which implies that different compositions may exist in the two samples. For pure complex, the surrounding environment of the Eu3+ is homogeneous, so the excitation spectrum consists of asymmetric and broad band ranging from 250 to 400 nm. However, silica gel is a non-crystalline substance with a porous microstructure, so the excitation spectrum becomes an asymmetric band. The bands at 395, 415 and 465 nm are sharp f–f transitions characteristic of the Eu3+ ion, corresponding to those from the ground 7F0 level to the 5L6, 5D3 and 5D2 multiplet terms, respectively.
Fig.2 Room temperature excitation spectra (λem = 612 nm) of the complex (solid line) and the silica glass (dash line)

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The emission spectra of Eu(C2F5COO)3·Phen was shown in Fig. 3, which composed of narrow and well-resolved characteristic emission peaks of Eu3+ arising from the transition 5D07F0 (580 nm), 5D07F1 (592 nm), 5D07F2 (613 and 619 nm), respectively. A relevant feature that may be noted for the complex is the high intensity of the 5D07F2 transition, relative to the 5D07F1 lines, indicating that the Eu3+ ion coordinated in a local site without an inversion center. Further, the emission spectra of the complex was chacterized only one peak for 5D07F0 transition, suggesting the presence of a single chemical environment around the Eu3+ ion and also showing that the Eu3+ ion occupies a low-symmetry site. The lifetime value (τobs) of the 5D0 level was determined from the luminescence decay profile for Eu(C2F5COO)3·Phen at room temperature. And the lifetime value of the complex was fitting by bi-exponential function with the fast and slow decay lifetimes 2970 and 820 μs. The data presented suggested that two kind of symmetrical sites of Eu3+ ions exist in this composite. The lifetime value is longer than many fluorinated europium complexes [12,13]. Considering the long lifetime, the multiphonon relaxation by coupling to O–H and C–H vibrations are reduced.
Fig.3 Room temperature emission spectra (λem = 329 nm) of Eu(C2F5COO)3·Phen. Inset is the decay curve monitored at 612 nm

Full size|PPT slide

But the emission intensity of the silica glass was weaker than that of the complex. Synthesis of the complex by an in situ technique via silica gel process can introduce some of H2O molecules or OH groups to the europium ion, thus the emission intensity was quenched. In addition, the emission intensity of silica glass increased with the annealing temperatures and approached its maximum at 160°C, and then it decreased with the annealing temperature increase successively. The changes of the emission intensity can be explained as follows: as the annealing temperature increases, the concentration of hydroxyl groups (OH) presented in the glasses will decrease. Therefore the multiphonon relaxation of the 5D0 level is less effective and the higher emission intensity is expected. On the other hand, the decrease of intensity observed in samples annealed at higher temperatures is likely to be due to a lower interaction between the europium and the organic ligand, which can not protect the metal ion from interactions with –OH groups. The overall efficiency of this energy transfer process decreases as the temperature increases, due to the decrease of the stability of the complex.

Judd-Ofelt analysisof Eu(C2F5COO)3·Phen

Judd-Ofelt analysis is one of the most successful theories for evaluating the potential radiative properties of rare-earth-doped materials [14]. Interaction parameters of ligand fields are given by the Judd-Ofelt parameters, Ωt€(t = 2,4,6). To estimate the strength of crystal field in the complex, we calculated the Judd–Ofelt parameter Ω2 which is more sensitive to the symmetry and sequence of ligand fields. According to the Judd-Ofelt theory [15], the spontaneous emission probability of an electric dipole transition between initial J manifold |(S, L) J>to terminal manifold |(S′, L′) J′>is given by
Aed=A[(S,L)J;(S,L)J]=64π4e2ν33h(2J+1)n(n2+2)29Sed,Sedλ=2,4,6Ωλ|(S,L)JU(λ)(S,L)J|2,
where h is the Plank’s constant, m is the mass of electron, c is the velocity of light, n is the refractive index of medium, ν is the wavenumber of the transition, J is the total angular momentum of ground state, and the ||U(λ)||2 are the squared reduced matrix elements of the rank λ = 2,4,6. The squared reduced matrix elements D05U(2)F27=0.0032 and 5D0U(4)7F4=0.0023 in Eq. (1) were taken from Ref. [16].
It is well-known that the 5D07F1 of Eu3+ ion is a magnetic dipole transition. This transition is independent of the environment and can be used as a reference. The experimental radiative rate of the spontaneous emission, Arad, can be determined by summing of the all spontaneous contributions. For this purpose, each spontaneous emission coefficient A0J for each 5D07FJ transition is obtained from [17]:
A0J=A01I0JI01ν01ν0J,
where I01 and I0J are the integrated intensities of the 5D07F1 and 5D07FJ transitions (J = 2 and 4) with ν01 and ν0J being the respective energy barycenters of these transitions.
The experimental intensity parameters, Ω2 and Ω4, can be estimated from the emission spectra (Fig. 3) based on the 5D07F2 and 5D07F4 electronic transitions of Eu3+ ion according to the following equation [18]:
IJ(υ)dυImd(υ)dυ=AJAmd=e2SmdυJ3υmd3(n2+2)29n2ΩtU(t)2.
The 5D07F1 is the only transition which does not have electric dipole contribution and can be theoretically determined: Smd = 9.6×10-42 esu2·cm2 [19]. Using Eq. (3), the Judd–Ofelt parameters, Ω2 and Ω4, can be determined. The Ω6 intensity parameter was not determined because the 5D07F6 transition could not be experimentally detected. To estimate the capability of this europium complex for being a lasing medium, the stimulated emission cross-section (σ) value was determined. The σ value is one of the most important factors for laser amplification. With the value of the radiative transition rate calculated above and the corresponding emission spectrum, the σ is calculated by [20]:
σp(λp)=λp48πcn2ΔλeffA[(S,L)J;(S,L)J],
where c, λp, ∆λeff, n and A are the speed of light, the wavelength of the oscillation peak, the line width of fluorescence peak (FWHM), refractive index of the matrix and the radiative transition rate, respectively. With the measured lifetime τobs, together with the above calculated radiative lifetime τrad, the luminescence quantum yield η can be calculated:
η=τobsτrad.
Judd–Ofelt parameters (Ωt), the radiative decay rates (Arad), nonradiative decay rates (Anr), and measured lifetime (τobs), quantum yield (η) and the stimulated emission cross-section (σ) of 5D07F2 transition are presented in Table 1. It is worth mentioning that the value for Ω2 is relatively small (4.42×10-20 cm2). This is a clear indication of a reduced degree of covalence involving the metal-ligand coordination bond and also of a slightly polarizable chemical environment for the lanthanide center. It is well known that the emission cross-section (σ) of 5D07F2 fluorescence transition of Eu3+ is one of the most important parameters for laser design. Here, the emission cross-section (σ) of 5D07F2 fluorescence transition is 20.1×10-22 cm2, which is larger than some europium doped materials [21,22].
Tab.1 Judd–Ofelt parameters (Ωt), radiative (Arad) and nonradiative decay rate (Anr), 5D0 lifetime (τobs), quantum yield (η) for europium doped materials
materialΩ2/(10-20 cm2)Ω4/(10-20 cm2)Arad/(s-1)Anr/(s-1)τobs/μsη/%σ/(10-22 cm2)Refs.
this work4.422.07170140322754.820.1
Eu(hfa)3(BIPHEPO)48.111004.64[21]
Eu3+:L5FBE5.644.44188224012.3[22]

Conclusions

In conclusion, Eu(C2F5COO)3·Phen complex and its silica glass were synthesized and investigated. The thermal analysis indicates that the complex was quite stable to heat. An intense red light emission of the composite materials was observed under ultra-violet light irradiation, indicating an efficient energy transfer between the ligand (Phen) and the Eu3+ ion. The 5D0 lifetime and quantum yield of the complex are 2970 μs, 820 μs and 54.8% which reflect that the multiphonon relaxations by coupling to O—H and C—H vibrations are reduced. The value of the stimulated emission cross-section obtained in this work is 20.1×10-22 cm2, which is larger than some europium doped materials in the literature. The emission intensities of the composites markedly depended on the annealing temperature. The highest value of the relative emission intensity was obtained for the silica glass annealed at 160°C.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 61108061) and Chinese Academy of Science, Special Grant for Postgraduate Research, Innovation and Practice (NO. 200835).
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