Supporting Material
Electrospinning of Eu2(BTP)3(Phen)2/PMMA Composite Nanofibers with High Luminescence and Thermal Properties
Huiquan Gu·Yanjun Hou·Feng Xu·Shuhong Wang
Huiquan Gu·Feng Xu·Yanjun Hou(*)
Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China
e-mail:
Shuhong Wang(*)
Key Laboratory of Functional Inorganic Material Chemistry (School of Chemical Engineering and Material, Heilongjiang University), Ministry of Education, Harbin 150080, P.R. China
e-mail:
Judd-Ofelt analysis of neat Eu2(BTP)3Phen2 complex and Eu2(BTP)3Phen2/PMMA composite nanofibers:
The Judd-Ofelt theory is a widely accepted tool for exploring f→f inner shell electronic transitions in Ln3+ complexes. Interaction parameters of ligand fields are given by the Judd-Ofelt parameters Ωλ (λ = 2, 4, and 6). Usually Judd–Ofelt intensity parameters are obtained from absorption data. However, in the case of Eu3+, the pure magnetic dipolar character displayed by the 5D0→7F1 transition made possible the determination of the intensity parameters from emission spectra [1]. This transition did not depend on the local ligand field seen by Eu3+ ions, thus might be used as a reference for the whole spectrum, A(5D0→7F1)≈50 s-1 [2]. In addition, the electric dipolar 5D0→7F2, 4, 6 transitions were only U(2), U(4), and U(6) reduced matrix elements, respectively, which allowed for evaluation of the experimental Judd–Ofelt intensity Ω2,4,6 parameters directly from emission data. In the present work the Ω6 intensity parameter was not determined since the 5D0→7F6 transition could not be experimentally detected (Fig. 5), which suggested that Ω6 was not important in this study. The experimental Ω2 and Ω4 intensity parameters were determined from the emission spectra given in Fig. 5 by using the 5D0→7F2 and 5D0→7F4 electronic transitions, respectively. The emission intensity I0J, taken as the area S of the emission curves, for the 5D0→7F1, 2, 4 transitions, is expressed by the following equation [3, 4]:
(1)
where i and j represent the initial (5D0) and final (7F0-4) levels, respectively, ħωi→j is the transition energy, N(5D0) the population of the 5D0 emitting level, and ARAD, the radiative tranision rate, is given by the following equation [3,4]:
(2)
where e is the electronic charge, ω is the angular frequency of the transition, ħ is Planck’s constant over 2π, c is the velocity of light, χ is the Lorentz local field correction term, 7FJ‖U(λ)‖5D02 are the squared reduced matrix element with values being 0.0032 and 0.0023 for J = 2 and 4, respectively [5].
The experimental radiative rate, ARAD, can be evaluated from the relative intensities of the 5D0→7F0–4 bands using the following equation [6]:
(3)
where I01 and I0J are the integrated intensities of the 5D0→7F1 and 5D0→7FJ transitions (J = 2 and 4) with ν01 and ν0J being their energy barycenters, respectively. As mentioned above, the 5D0→7F1 transition does not depend on the local ligand field seen by Eu3+ ions and thus may be used as a reference for the whole spectrum, A01 ≈ 50 s-1. From the experimental values and the above equations, the values of Ω2, Ω4, and ARAD for the neat complex and the composite nanofibers with Eu2(BTP)3Phen2 content of 11, 13, and 15 wt.-% could be obtained, and the values were listed in Table 1.
As described in the literature [4], the lifetime of the excited state 5D0 level (τobs), the radiative rate (ARAD), and the non-radiative transition rate (ANR) are related as shown below:
(4)
From the experimental lifetime values (τobs) and radiative transition rate ARAD, the ANR contributions of the 5D0 emission of Eu3+ are determined. The quantum efficiency (Ф), which fundamentally determines the luminescent properties of the complex, is defined as:
(5)
Fluorescence dynamics and discussion of the surrounding media on the radiative lifetime of Eu3+ ion
The fluorescence decay curves of the excited Eu3+ ions at 5D0 state under 335 nm excitation were measured on an FLS920 instrument (Edinburgh) at room temperature, as shown in Fig. 7. It can be seen that the decay curves were all well-described in terms of a single-exponential function within the experimental errors, and the obtained lifetime values (τobs) were shown in Table 1. The exponential lifetimes for the 5D0 states of the neat complex and composite nanofibers with Eu2(BTP)3Phen2 content of 11, 13, and 15 wt % were obtained to be 906, 561, 556, and 514 μs, respectively. It can be seen that the τobs for the 5D0 excited state of Eu3+ ions in the composite fibers became shorter than that in the neat europium complex. As described in the literature [7, 8], the refractive index surrounding Eu3+ ions and the interaction parameters (Ωλ) of ligand field in the composites both have influences on the radiative lifetime, which can be written as
(6)
where λ0 is the wavelength in vacuum, and n is the refractive index of the material, f(ED) is the oscillator strength for the electronic dipole transition, e is the electronic charge, ω is the angular frequency of the transition, ħ is Planck’s constant over 2π, c is the velocity of light. According to the above equation, the significantly higher Ωλ values (see Table 1) of the composite fibers will induce the shorter radiative lifetime comparing to that of neat complex. While as demonstrated by Zhang and coworkers [7], the decreased refractive index of the surrounding media (PMMA) leads to the decrease of effective refractive index of the composite fibers, which can further induce the increase of the radiative lifetime in the composite. These two factors compete with each other to determine the final radiative lifetime. The experimental radiative lifetimes listed in Table 1 indicated that the ligand field interaction parameters Ωλ played major roles in determining the final radiative lifetime comparing to the refractive index n.
The sensitization luminescence and quenching mechanism of the Eu2(BTP)3Phen2/PMMA composite fibers
The sensitization luminescence of rare earth organic complex generally consists of an initial strong absorption of ultraviolet energy that excites the ligand from the ground state (S0) to the excited singlet (S1) state, this is followed by an energy migration via intersystem crossing from the S1 state to a ligand triplet (T) state. The energy is then nonradiatively transferred from the lowest triplet state (T1) of the ligand to a resonance state of a coordinated lanthanide ion, which in turn undergoes a multi-photon relaxation and subsequent emission in the visible region [9]. During this sensitization process, part of the absorbed energy may radiate in the form of ligand phosphorescence and part of them loss in the form of heat. The intramolecular energy migration efficiency from organic ligands to the central rare earth ions is the most important factor influencing the luminescent properties of the complex.
In the Eu2(BTP)3Phen2, it is possible for the Eu3+ ions to form aggregates on a microscopic level, which could result in a nonhomogeneous distribution of the Eu3+ ions. Such aggregates, regarded as multiplets, appear as a result of dipole-dipole attractions between different quadruplets with asymmetric distribution of charges [10], which ensures an enhanced electrostatic attraction between them. As a result, the distance among Eu3+ ions becomes considerably shorter than the average inter-ion distance at random distribution [11]. This can markedly increase the attractions between the europium ions. As the energy transfer between the lanthanide ions is a nonradiative process, this would account for the luminescent quenching of the Eu3+, especially at high content of Eu2(BTP)3Phen2.
References
[1] Braga, S. S.; Sá Ferreira, R. A.; Gonçalves, I. S.; Pillinger, M.; Rocha, J.; Teixeira-Dias, J. J. C.; Carlos, L. D. J. Phys. Chem. B 2002, 106, 11430.
[2] Hazenkamp, M. F.; Blasse, G. Chem. Mater. 1990, 2, 105.
[3] Malta, O. L.; Couto dos Santos, M. A.; Thompson, L. C.; Ito, N. K. J. Lumin. 1996, 69, 77.
[4] Malta, O. L.; Brito, H. F.; Menezes, J. F. S.; Gonçalves e Silva, F. R.; Alves Jr, S.; Farias Jr, F. S.; de Andrade A. V. M. J. Lumin. 1997, 75, 255.
[5] Carnall, W. T.; Crosswhite, H. M. Argonne National Laboratory Report, unnumbered, 1977.
[6] De Sá G. F.; Malta, O. L.; Mello Donegá, C.; Simas, A. M.; Longo, R. L.; Santa-Cruz, P. A.; Silva, Jr E. F. Coord. Chem. Rev. 2000, 196, 165.
[7] Zhang, H.; Song, H. W.; Yu, H. Q.; Li, S. W.; Bai, X.; Pan, G. H.; Dai, Q. L.; Wang, T.; Li, W. L.; Lu, S. Z.; Ren, X. G.; Zhao, H. F.; Kong, X. G. Appl. Phys. Lett. 2007, 90, 103103.
[8] R. S. Meltzer, S. Feofilov, and B. M. Tissue, Phys. Rev. B 1999, 60, R14012.
[9] Li, Q.; Li, T.; Wu, J. G. J. Phys. Chem. B 2001, 105, 12293.
[10] Smirnov, V. A.; Sukhadolski, G. A.; Philippova, O. E.; Khokhlov, A. R. J. Phys. Chem. B 1999, 103, 7621.
[11] Kirby, A. F.; Foster, D.; Richardson, F. S. Chem. Phys. Lett. 1983, 95, 507.
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