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PMMA systems
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Journal of Luminescence 127 (2007) 307–315 www.elsevier.com/locate/jlumin
Influences of compositions and ligands on photoluminescent properties of Eu(III) ions in composite europium complex/PMMA systems Hong-Guo Liua,c, Xu-Sheng Fengc, Kiwan Jangb, Sangsu Kimb, Tae-Jin Wona, Shengyun Cuia,d, Yong-Ill Leea, a
Department of Chemistry, Changwon National University, Changwon 641-773, Republic of Korea b Department of Physics, Changwon National University, Changwon 641-773, Republic of Korea c Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan 250100, China d Department of Chemistry, Yanbian University, Yanji 133002, China Received 6 June 2006; received in revised form 24 December 2006; accepted 9 January 2007 Available online 17 January 2007
Abstract Three kinds of europium complexes; Eu(phen)2Cl3(H2O)2, Eu(DN-bpy)phenCl3(H2O)2 and Eu(DB-bpy)phenCl3(H2O)2 (phen: 1,10phenanthroline, DN-bpy: 4,40 -Dinonyl-2,20 -dipyridyl, DB-bpy: 4,40 -Di-tert-butyl-2,20 -dipyridyl) were prepared and then incorporated into polymethyl methacrylate (PMMA) matrix with different molar ratios of CQO groups/Eu3+ ions. The final solid composites were formed by a self-assembly process among Eu3+ ion, the ligands and PMMA during the solvent evaporation process, and then the ligands re-coordinate to Eu(III). It was found that the ligands affect not only the emission properties of the pure complexes, but also the miscibility of the complexes and PMMA. More than one kind of symmetric sites of Eu3+ ions were formed in the composites due to the coordination of CQO in PMMA to Eu3+ ions. The micro-environments of Eu(III) in the composites were changed with the compositions and the ligands, leading to the change in the crystalline structure, and consequently, the emission characteristics. r 2007 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Europium complex; PMMA composite; Heterocyclic ligand
1. Introduction It is well known that europium complexes show strong and narrow red luminescence due to ‘‘antenna effect’’ of ligands and 4f-4f electron transition of Eu(III). Over the past decade, an interest has grown in several europium complexes for practical applications for laser development, phosphor materials, electro-optical devices, optical communication amplifiers, new generation of optical storage materials based on hole-burning technique, and probes to detect the local environments in biological systems. In general, pure europium complexes do not have good thermal and mechanical stabilities and also show poor processing ability. In addition, pure complexes can absorb moisture from the air and form clusters, leading to luminescence quenching. These factors make the pure Corresponding author. Tel.: +82 55 279 7437; fax: +82 55 279 7439.
E-mail address: [email protected] (Y.-I. Lee). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.01.001
complexes just have extensive promising photophysical applications but are limited to practical uses. In order to overcome these shortcomings, europium complexes are usually incorporated into organic, inorganic or organic/ inorganic hybrid matrices. Zeolites [1], mesoporous materials [2–4], sol–gel silica or organically modified silicates (ORMOSIL) [5–11] and polymers are the most common hosts and have been investigated in recent years. The proper incorporation of europium ligands into organic polymers represents an ideal and extremely versatile approach to generate such hybrid materials. Two main methods have been utilized to dope europium complexes into polymers. One uses the polymerization of monomers that contain Eu3+ ions or achieves with the aid of reactions between Eu3+ ions or europium complexes with polymers that have ligands in their chains [12–20]. On the other method, the composites can be prepared just by blending the polymers and the europium complexes in appropriate solvents [21–25] or melting the two components
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together [26]. It has been observed that the photoluminescent (PL) efficiency of the PVK-co-MMA-co-Eu(BA)2 (MMA)phen film [13] is higher than that of both Eu-complex neat film and Eu-complex/polymer blending film [24], indicating the advantages of the materials made by chemical reactions over those by blending. Although typically more homogeneous composite systems could be obtained using the polymerization/reaction method than the simply mixing method, well-dispersed systems could also be anticipated in some simply mixed systems if they have some special supramolecular interactions between the complex molecules and the polymer, such as the coordination of oxygen atoms to Eu3+ ions in the mixed systems of europium picrate/poly(oxyethylene) [22] and Eu3+-b-diketonates with the epoxy resin [20–27]. Moreover, it is much easier to tune the composition, structure and properties of the doped systems formed by the blending method than by the chemical reaction method. In addition, these kinds of interactions can be helpful to disperse Eu3+ ions or complexes into sol–gel nanohybrids [28–31]. The europium complexes with new ligands have been synthesized continuously [32–34]. However, the traditional ‘‘older’’ complexes show distinct new properties and applications in various matrices [35]. The spectroscopic characteristics of heterocyclic europium complexes including Eu(phen)2Cl3 and Eu(bpy)2Cl3 have been studied extensively [36–39]. Phen and bpy are typical light-harvesting ligands that can absorb near ultraviolet and transfer the energy to Eu3+ ions effectively. The composite materials containing this kind of complexes would have potential photophysical applications. To our knowledge, most of the work on the composite materials containing this kind of complexes has been focused mainly on silica [40–44], ORMOSIL [45] and silica/polymer hybrids [46,47] due to the good miscibility of the complexes and the matrices. Polymethyl methacrylate (PMMA) has been considered to form a quasi-cross-linked structure through strong dipole–dipole interactions which prevent it from crystallizing, and is regarded as transparent ‘‘glass’’ [48,49]. Various europium-b-diketonates have been doped in PMMA to form amorphous composites due to good miscibility of these components, and showed the potentials for applications in plastic optical fiber lasers and amplifiers [50]. It is rare to study the composite systems formed by europiumheterocyclic ligand complexes with such polymers like PMMA, because of the difficulties of mixing these components together. In this paper, three kinds of heterocyclic europium complexes synthesized, Eu(phen)2Cl3(H2O)2, Eu(DN-bpy)phenCl3(H2O)2 and Eu(DB-bpy)phenCl3(H2O)2 (shown in Fig. 1) were incorporated into PMMA by dissolving the corresponding materials in mixed CHCl3/C2H5OH solvents. Solid crystalline composites were formed well after evaporating the solvents. The influences of different host polymers on composite structures and PL properties of Eu(III) in Eu(phen)2Cl3(H2O)2 and Eu(DB-bpy)phenCl3 (H2O)2 were reported in our previous papers [51,52].
A significant focus of this work has been to investigate and discuss the influences of compositions and ligands on PL of Eu(III) ions in the composite systems deeply. 2. Experimental 2.1. Reagents and samples Eu2O3 (99.95%), phen (99+%), DN-bpy (97%), DB-bpy (98%) and PMMA (Mw15,000) were purchased from Aldrich Chemical Company, Inc. The complexes Eu(phen)2 Cl3(H2O)2, Eu(DN-bpy)phenCl3(H2O)2 and Eu(DB-bpy)phenCl3(H2O)2 were synthesized according to the methods in Ref. [44], and dried in air at room temperature. The elemental analysis results: Eu(phen)2Cl3(H2O)2: C: 43.0% (44.0%); H: 3.30% (3.08%); N: 8.59% (8.56%); Eu(DNbpy)phenCl3(H2O)2: C: 55.0% (54.4%); H: 6.91% (6.39%); N: 6.35% (6.34%); Eu(DB-bpy)phenCl3(H2O)2: C: 48.3% (48.5%); H: 5.39% (4.88%); N: 7.51% (7.54%). The values in parentheses are calculated ones. 2.2. Sample preparation and characterization Mixed systems of the complexes in PMMA were prepared by dissolving these components with certain molar ratios of CQO/Eu into ethanol/chloroform. The samples for PL spectroscopy and lifetime measurements
N EuCl3(H2O)2 N 2
N
N Cl3(H2O)2
Eu N
N
N
N Cl3(H2O)2
Eu N
N
Fig. 1. Molecular structures of the europium complexes.
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were made by casting the solutions onto clean glass slides. The solvents were allowed to evaporate in air at room temperature. PL spectra were obtained by using a PC 2000 spectroscope (Ocean Optics Inc.) with the excitation at 325 nm using a He–Cd laser (Omnichrome, LC-500). Decay curves were obtained by monitoring the 612–614 nm emission by using 300 MHz-digital oscilloscope (LeCroy 9310, Switzerland) and triple grating monochromator (Spectra Pro-750 ARC Actron, Research Corporation, MA) under the excitation at 355 nm dye laser (Spectron SL 4000B/G, Spectron laser systems, UK) at room temperature. All emission intensities were normalized with the spectral responses of PMT and optical setup in this work. The wavelengths of obtained spectra were also calibrated using a Hg(Ar) spectral calibration lamp (Model 6035, Oriel Co.) UV–vis spectra were recorded using a UV–vis spectrophotometer (Shimadzu, Japan). Pure chloroform was used as a reference, and the samples were diluted for the measurements with the spreading solutions mentioned above. Powder X-ray diffraction (XRD) patterns were obtained on a Philips X’Pert MPD/PW3040 X-ray diffractometer with Cu Ka irradiation at 293 K. 3. Results and discussion 3.1. PL spectra and XRD patterns of pure complexes The PL spectra of the pure complexes with different ligands are illustrated in Fig. 2. Upon excitation at 325 nm, PL spectra of these complexes exhibit characteristic ligandsensitized emission of Eu3+. Five emission bands corresponding to 5D0-7F0,1,2,3,4 can be clearly distinguished. The emission spectrum of Eu(phen)2Cl3(H2O)2 is identical to the reported one by Jin et al. [42]. Two peaks appear at 16,923 and 16,798 cm1 in 5D0-7F1 band; and four peaks appear at 16,345, 16,268, 16,184 and 16,080 cm1 in 5D0-7F2 band. 5
Relative Intensity
5 D -7 F 0 1
D0-7F2 Eu(phen)2Cl3(H2O)2
5 D -7 F 0 0
5 D -7 F 0 4
5 D -7 F 0 3
Eu(DB-bpy)phenCl3(H2O)2
16000 Wavenumber
15000
The emission spectrum of Eu(DN-bpy)phenCl3(H2O)2 is very similar to that of Eu(phen)2Cl3(H2O)2. This indicates that the replacement of a phen by a DN-bpy does not affect the first coordination sphere of Eu3+ and the point group of the complex. However, the emission spectrum of Eu(DBbpy)phenCl3(H2O)2 shows obvious different characteristics from those of the former two complexes. One peak at 16,869 cm1 and a shoulder at 16,952 cm1 appear in 5 D0-7F1 band, and two peaks at 16,305 and 16,184 cm1 appear in 5D0-7F2 band. The profile of the curve varies greatly, especially for 5D0-7F2 transition that is called as the ‘‘hypersensitive transition’’ [53], because it is very sensitive to the environments around Eu3+ ions, specifically to the symmetry of the coordination sphere. In addition, it can be seen that 5D0-7F4 band changes slightly, too. According to Judd–Ofelt theory [54–57], the f–f transitions of Eu3+ are parity forbidden and consist mainly of weak magnetic dipole (MD) and induced electric dipole (ED) transitions. The probabilities of MD transitions are independent of the chemical environment of the ion. The 5 D0-7F1 emission is a pure MD transition. The so-called ‘‘hypersensitive’’ 5D0-7F2 transition of Eu3+ ion is an ED transition allowed one and depends on the local electric field or the local symmetry. The intensity ratio of 5D0-7F2 transition and 5D0-7F1 transition is a good measure of the nature and symmetry of the first coordination sphere. The I (5D0-7F2)/I(5D0-7F1) ratios for the three complexes were calculated to be 3.55, 3.11 and 2.11, respectively, indicating the similar micro-environments of Eu(III) in Eu(phen)2Cl3(H2O)2 and Eu(DN-bpy)phenCl3(H2O)2, and different from Eu(DB-bpy)phenCl3(H2O)2. The coordination symmetry of Eu(III) in Eu(DB-bpy)phenCl3(H2O)2 is higher than in the other two complexes. Fig. 3 shows the XRD patterns of the pure complexes. There is obvious difference in the number and relative intensities in the diffraction peaks of the complexes. The former two complexes give similar patterns that are composed of a lot of diffraction peaks, whereas the XRD pattern of Eu(DB-bpy)phenCl3(H2O)2 seems very simple, just containing several peaks, indicating the higher symmetry of the crystal. This is consistent with the PL spectra. From these results, it is clearly seen that the ligands have great influence on the crystalline structure of the complexes, and in consequence, the emission properties. 3.2. Emission properties of the doped systems of Eu(phen)2Cl3(H2O)2 in PMMA
Eu(DN-bpy)phenCl3(H2O)2
17000
309
14000
(cm-1)
Fig. 2. Emission spectra of the pure complexes (293 K, lex: 325 nm).
As previously reported, the profiles of the emission curves of Eu(III) in the doped systems change regularly with the molar ratios of CQO/Eu [52]. The spectral features were summarized in Table 1. According to the emission spectral characteristics of 5D0-7F1 and 5D0-7F2 bands and intensity ratios of I(5D0-7F2)/I(5D0-7F1), three kinds of composites can be classified, i.e. the composites with the molar ratios greater than, close to
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310
and less than 50, respectively. These three kinds of composites show distinct luminescent properties. The luminescent decay curves of Eu3+ ions in the composites can be fitted by bi-exponential functions. The fitting data including lifetime values of 5D0 levels of Eu3+ ions and the corresponding relative weightings for each species are illustrated in Table 2. Although two kinds of luminescent lifetimes may be associated with 5D0 and 5D1
levels resulting from the superposition between the 5 D0-7F2/5D1-7F4 and 5D0-7F1/5D1-7F3 transitions in some cases including the composite systems of EuBr3/ poly(ethylene oxide) [58], the lifetimes obtained here should be ascribed to more than one kind of symmetrical site of Eu3+ in the composite systems according to the broader 5 D0-7F0 transition bands (FWHM: 60–70 cm1). This can be also related to heterogeneities in the charge distribution around the Eu3+ ions in most cases like Eu3+ in some glassy hosts [59,60], in some coordination complexes [61,62], and even in so called ‘‘pure complex’’ such as the
Eu(phen)2Cl3(H2O)2
Table 2 Fitting data of decay curves of Eu(phen)2Cl3(H2O)2/PMMA composites
Eu(DN-bpy)phenCl3(H2O)2
Eu(DB-bpy)phenCl3(H2O)2
10
20
30 2θ (°)
40
50
Fig. 3. X-ray diffraction patterns of the pure complex powders.
Molar ratio of CQO/Eu
t1 (ms)
RW (%)
t2 (ms)
RW (%)
125 100 83 67 56 50 45 40 36 33 20 10 Pure complex
35 32 68 54 65 55 61 88 102 114 153 176 175
80 75 50 56 32 26 25 48 48 47 70 93 96
608 441 360 370 299 295 302 390 400 391 428 1136 1153
20 25 50 44 68 74 75 52 52 53 30 7 4
Table 1 Emission spectral data for Eu(phen)2Cl3(H2O)2/PMMA composite systems CQO/Eu
5
D0-7F0 n1 (cm1)
5
D0-7F0 Dn1 (cm1)
5
D0-7F0 n2 (cm1)
5
D0-7F0 Dn2 (cm1)
5
D0-7F1 (cm1)
5
1000 500
17,252 17,248
122 126
17,279 17,274
95 100
333
17,246
128
17,274
100
250
17,247
127
17,275
99
200
17,248
126
17,276
98
100
17,246
128
17,273
101
50 33
17,249 17,253
125 121
17,276 17,281
98 93
25
17,257
117
17,284
90
20
17,254
120
17,280
94
10
17,251
123
17,277
97
5
17,253
121
17,280
94
2
17,253
121
17,280
94
Pure
17,254
120
17,281
93
17,013, 16906 16,915, 19,815 16,915, 16,810 16,915, 16,815 16,912, 16,810 16,918, 16,821 16,900 16,926, 16,815 16,929, 16,810 16,926, 16,798 16,926, 16,798 16,926, 16,798 16,926, 16,795 16,923, 16,798
D0-7F2 (cm1)
I(5D0-7F0)/ I(5D0-7F2)
I(5D0-7F2)/ I(5D0-7F1)
16,305 16,321, 16,197
0.0125 0.0141
45 4.73
16,319, 16,194
0.0143
4.78
16,319, 16,197
0.0137
4.98
16,313, 16,194
0.0135
4.77
16,319, 16,194
0.0135
4.89
16,316, 16,184 16,345, 16,265 16,189, 16,082 16,351, 16,263 16,189, 16,085 16,348, 16,363 16,189, 16,082 16,348, 16,263 16,189, 16,082 16,348, 16,263 16,189, 16,080 16,348,16,263 16,184, 16,080 16,345, 16,263 16,184, 16,080
0.0202 0.0122
3.46 3.86
0.0101
3.77
0.0105
3.66
0.0115
3.59
0.0118
3.56
0.0112
3.61
0.0116
3.55
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500 450
C=O/Eu: 50 Lorentz fit
400 Intensity
350 300 250 200 150 100 576
578 580 Wavelength (nm)
582
Fig. 4. Lorenzian decomposition of the 5D0-7F0 emission band of the composite formed by Eu(phen)2Cl3(H2O)2 and PMMA with the molar ratio of CQO/Eu of 50.
complex of Eu3+ with picrate ions due to the different number of coordinate water molecules [22]. For the first coordination sphere around Eu3+, the transition between 5D0 and 7F0 levels can give valuable information. Both the ground (7F0) and excited (5D0) states of Eu3+ are non-degenerate, therefore, only single emission transition can be observed for each unique Eu3+ environment in principle. Both the 7F0-5D0 transition band in the excitation spectrum and 5D0-7F0 transition band in the emission spectrum have been used to determine the number of the symmetric sites of Eu3+ by using multi-peak fitting technique and to get some information on the first coordination sphere around Eu3+ ions by relating to the nephelauxetic effect [63–65]. The 7F0-5D0 or 5D0-7F0 band can be resolved into individual peaks by the use of the Lorentzian function according to the method described by McNemar and Horrocks Jr. [66] to provide the most accurate description for solid compounds. The 5D0-7F0 emission band was decomposed into two peaks shown in Fig. 4. The detail data are summarized in Table 1. Two kinds of symmetrical sites can be induced based on the fitting results of the decay curves. The energy of 5D0-7F0 is usually related to the nephelauxetic effect [63,67,68] that concerns the influence of covalency contributions of the first coordination sphere on the reduction of the Eu3+ attractive potential relative to their free ion values. It was shown that the observed nephelauxetic effect was due to a decrease in the effective nuclear charge of Eu3+ ion upon binding to a negatively charged ligand [67]. The difference between the observed n value (cm1) of 5D0-7F0 emission peak of Eu3+ in a complex and n0, the n value of the gaseous Eu3+ ion with no coordinating atoms can be related by n n 0 ¼ n1 d1 þ n2 d2 þ þ nj dj ,
(1)
where dj is the parameter for atom type j, which measures the tendency of a particular atom to bond covalently to the
311
Eu3+ ions, and nj is the number of atoms of type j in the first coordination shell [63,68,69]. The n0 was chosen as 17,374 cm1 [68]. This means that the ability to produce a nephelauxetic effect in Eu3+ complexes and to reduce its 5 D0-7F0 energy should be related to the Eu3+ first coordination sphere, i.e. the local environments around Eu3+ ions. The Dn value depends not only on the sorts of the coordinative atoms but also on the coordination number [69]. The two Dn values for the pure Eu-phen complex are 120 and 93 cm1. The relative weighting of the second one is very low. As can be seen in Table 1, the two average Dn values for the composites with lower CQO/Eu molar ratios are nearly 121 and 94 cm1, while for the samples with higher molar ratios are about 126 and 99 cm1. This reflects that the composites with lower CQO/Eu molar ratios have quite different first coordination spheres around Eu3+ ions compared to that of higher molar ratios, which is consistent with the variation of the emission spectra. Jin et al. [42] have studied the doped systems of europium complex with phen in silica and found that two kinds of point group symmetries exist, which are hydrous Eu(phen)2Cl3(H2O)2 and anhydrous Eu(phen)2Cl3 complexes. The hydrous one gives an intense 5D0-7F2 peak at 620 nm (16,129 cm1), and the anhydrous one gives an intense peak at 615 nm (16,260 cm1) and a weak one at 622 nm (16,077 cm1). The authors assigned the 615 nm peak to Eu(phen)2Cl3 and the 622 nm one to Eu(phen)2Cl3(H2O)n that was considered to be formed on the surface of the anhydrous complex particle because of strong hygroscopic ability of the anhydrous complex. In the PL spectra of the doped systems [52], one intense peak with lmax at 16,189–16,186 and three weak peaks at 16,348–16,345, 16,265–16,263 and 16,077 cm1 were appeared for the composite systems of Eu/phen complex with lower CQO/Eu molar ratios in PMMA and the pure complex. This implies that Eu(phen)2Cl3(H2O)2 should be the main component of the complex. The two main species in the composites with low molar ratios should be the hydrous complex and a new species, corresponding to shorter and longer lifetimes listed in Table 2, and to the average Dn values of 121 and 94 cm1 in Table 1, respectively. The intensity ratio of I614.8/I617.8 in the PL spectra increases with increasing the molar ratios of CQO/Eu over 50, indicating that the relative amounts of the new species increase gradually with increasing PMMA contents, in line with the relative weightings listed in Table 2. The new species has a longer lifetime than the hydrous one. It is possible that the coordination of H2O molecules were replaced by CQO groups and the coordination number was also changed due to the effect of the host. The 5D0-7F2 transition bands reflecting the local environments around Eu3+ ions were quite different between the samples with higher molar ratios and those with lower molar ratios, which can be seen clearly in the Dn values of 5D0-7F0 transition peaks. The formations of the
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new crystal structures in the samples with higher CQO/Eu molar ratios was also found in XRD results and agree with the above results [52]. The two peaks in 5D0-7F2 emission bands should correspond to different species. The variation of the relative intensity of these two peaks indicates the change of the relative weightings of these species. According to the d values for various functional groups [69], the difference between the d values of the oxygen atoms in carbonyl group and in water molecule is ca. 5–6 cm1. Although it is difficult to deduce the exact coordination sphere around Eu3+ ions in the species with higher molar ratios, it is possible that the CQO group coordinates to Eu3+ ion, in other words, one H2O is replaced by a CQO group. The interactions between CQO groups and lanthanide ions are very popular in solid-doped systems such as in the composite systems of Nd3+/PMMA [70], EuL3(H2O)2 in polyimide (L: pyridine carboxylic acid) [71] and in the ureasil containing Eu(CF3SO3)3 [63]. FTIR spectra of these systems supply related informations on the coordinate interaction [52]. Another interesting feature of the 5D0-7F0 transition band lies in their intensity. The 5D0-7F0 transition cannot be accounted for by either the MD mechanism or the Judd–Ofelt theory. A more detailed analysis has indicated that this transition ‘‘borrows’’ intensity from the 5D0-7F2 transition through higher order perturbations by the ligand field [72–74] i.e. becomes allowed only through ‘‘J-mixing’’ [75]. The intensity ratio between 5D0-7F0 and 5D0-7F2 reflects the degree of J-mixing effect [63]. As can be seen from Table 1, the ratios for the samples with higher and lower CQO/Eu molar ratios are ca. 0.014 and 0.012, respectively. While, that of the composite with the molar ratio of 50 is found to be 0.020 which is obviously larger than the others, indicating the higher symmetry around Eu3+ ion. The ratio of I(5D0-7F2)/(5D0-7F1) for the composite with the molar ratio of 50 shows the lowest value among the composite systems studied, that is in good agreement with the above results. In addition, the luminescent lifetimes, the XRD patterns, the TEM micrographs and the emission bands also show the unique properties of this sample. 3.3. Emission properties of the doped systems of Eu(DNbpy)phenCl3(H2O)2 and Eu(DB-bpy)phenCl3(H2O)2 in PMMA The emission properties of Eu(DB-bpy)phenCl3(H2O)2/ PMMA composites were reported in the previous paper [51]. The PL spectra of Eu(DN-bpy)phenCl3(H2O)2/ PMMA composites were illustrated in Fig. 5. It can be seen that the emission properties of Eu(III) in the composites change with the concentrations, and the composites can be classified into several categories according to the PL spectra. The luminescent decay curves of 5D0 level for these systems were recorded. These curves should be fitted by bi- or tri-exponential functions, indicating more than one kinds of species of Eu(III) exist. By
5
12000 10000 Relative Intensity
312
D0-7F2
5D -7F 0 1
C=O/Eu
5 D -7 F 0 0
5D -7F 0 3
5D -7F 0 4
1000
8000
200 100
6000
50 20 10 5 2
4000 2000
pure complex
0 17000
16000
15000
14000
Wavenumber (cm-1) Fig. 5. Emission spectra of Eu(DN-bpy)phenCl3(H2O)2/PMMA composites with various molar ratios of CQO/Eu and the pure complex (293 K, lex: 325 nm).
analyzing the life times of 5D0 level and the 5D0-7F0 transition bands, we can get the same conclusion as that from analyzing the Eu(phen)2Cl3(H2O)2 that the microenvironment around Eu(III) changes with the composition, leading to the variation of structure and properties of these composites. As discussed in Section 3.1, the ligands have a great influence on the PL behaviors of the pure complexes. Here we focus on the influence of ligands on the emission properties of the composites. The composites with same molar ratios show different PL behaviors for the three series of doped systems. All the three samples with the molar ratio of 1000 give broader 5D0-7F2 emission bands. However, the emission band of Eu(phen)2Cl3(H2O)2-doped sample is an asymmetric one with smaller FWHM of 235 cm1 that is different from those of the other two samples that are more broader and symmetric bands with FWHMs of 260 and 254 cm1, respectively. The final structure depends largely on the interaction between the Eu3+ ions, ligands and PMMA during the solvent evaporation process. DN-bpy and DB-bpy have more interaction with PMMA than phen due to the alkyl substituents which help the Eu-ligand complex to incorporate into the host. There are abundant methyl groups on PMMA chains. The complex with DB-bpy can be dispersed in PMMA easier than that with DN-bpy due to the tert-butyl substituents, which can be seen from the PL spectra of the samples with the molar ratio of 200. With decreasing molar ratios, the PL spectra approach to those of pure complexes. The composites show similar PL behaviors to that of the corresponding pure complex when the molar ratios of CQO/Eu reach to 33 for Eu/phen complex and 10 for Eu/DN-bpy/phen complex. However, the samples containing Eu/DB-bpy/phen complex show very different spectra from that of pure Eu(DBbpy)phenCl3(H2O)2 even when the molar ratio reaches to 2.
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This should be attributed to different interactions between the components, especially, between phen, DN-bpy, DBbpy with PMMA, too.
a
4. Conclusion PMMA, as a matrix to luminescent europium complexes, has great effects on the PL properties of Eu3+ ions due to the strong coordination interaction between Eu3+ and CQO group. The interaction between the ligands and PMMA results in the formation of different microenvironments around Eu3+ ions that change with the compositions of the composites. This can be attributed to the great variation of the PL behaviors of the composites. In
0.8 phen
3.4. UV–vis spectra of the spreading solutions
C=O/Eu: 100 C=O/Eu: 10 C=O/Eu: 2 Eu(phen)2Cl3(H2O)2
Absorbance
0.6
0.4
0.2
0.0 260
280
1.0
320
340
DN-bpy/phen 1:1 Eu(DN-bpy)phenCl3(H2O)2/PMMA Eu/C=O: 1/100 Eu(DN-bpy)phenCl3(H2O)2
0.8
Absorbance
300 Wavelength (nm)
b
0.6
0.4
0.2
0.0 260
280
300 320 Wavelength (nm)
340
360
c 1.0
DB-bpy/phen 1:1 Eu(DB-bpy)phenCl3(H2O)2/PMMA Eu/C=O 1:100
0.8 Absorbance
Fig. 6 shows the UV–vis spectra of the composite systems, the ligands and the pure complex in CHCl3/ C2H5OH solutions. As seen in Fig. 6a, one peak of lmax at 265.6 nm and one shoulder at 279 nm appeared in the spectrum of pure phen. While one peak at 266.6 nm and two shoulders at 275 and 295 nm were observed in the spectrum of pure complex, presumably the complex formation. However, the composite systems show the similar spectra as that of pure phen, even in the solution with the molar ratio CQO/Eu of 2, suggesting that the complex molecules disassociate in the solutions by adding PMMA. This is a clear evidence for the interaction between the complex and PMMA in solution. The oxygen atom of the carbonyl group in PMMA is a kind of hard base and Eu3+ ion is also regarded as a hard acid. The coordination ability of ‘‘O’’ to Eu3+ ion is stronger than that of ‘‘N’’. The final structures of the casting films can be considered as the results of a self-assembly process during the solvent evaporation. Phen can coordinate to Eu3+ ion again with evaporating the solvent because of the ‘‘chelating effect’’. It is difficult to measure UV–vis spectra of the solid composites because opaque solid was formed due to the crystalline. Nevertheless, it is reasonable for phen coordinate to Eu(III) in the solid state. In a previous paper [52], we investigated the composite structures of Eu(phen)2Cl3 (H2O)2 doped in PMMA and PVP. It was found from the FTIR spectra of the composites that phen re-coordinate to Eu(III) after evaporating the solvent in the mixed systems with PMMA, while phen cannot re-coordinate to Eu(III) in the mixed systems with PVP. Figs. 6b and c show the UV–vis spectra of the composites of the other two complexes in PMMA. From the spectra, the same conclusion can be drawn as that from Eu(phen)2Cl3(H2O)2/PMMA composites that the complexes disassociate in the mixed spreading solutions due to the coordination effect of CQO to Eu3+, and the heterocyclic ligands re-coordinate to the metal ions during evaporation of the solvent, leading to the formation of the composites with more than one kind of species of Eu(III) due to the rudimental coordinated CQO groups.
313
Eu(DB-bpy)phenCl3(H2O)2
0.6
0.4
0.2
0.0 260
280
300 320 Wavelength (nm)
340
360
Fig. 6. UV–vis spectra of the composite Eu(phen)2Cl3(H2O)2/PMMA (a), Eu(DN-bpy)phenCl3(H2O)2/PMMA (b) and Eu(DN-bpy)phenCl3(H2O)2/ PMMA (c), the pure ligand and pure complex in CHCl3/C2H5OH solution.
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addition, slightly modification on the ligands can provoke great changes of the PL properties, too, due to the different miscibility of the complex in the host.
Acknowledgments The authors are indebted to the financial support of the Korea Research Foundation (Grant no. KRF 2000-005Y00071) and Education Ministry of China.
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Journal of Luminescence 127 (2007) 307–315 www.elsevier.com/locate/jlumin
Influences of compositions and ligands on photoluminescent properties of Eu(III) ions in composite europium complex/PMMA systems Hong-Guo Liua,c, Xu-Sheng Fengc, Kiwan Jangb, Sangsu Kimb, Tae-Jin Wona, Shengyun Cuia,d, Yong-Ill Leea, a
Department of Chemistry, Changwon National University, Changwon 641-773, Republic of Korea b Department of Physics, Changwon National University, Changwon 641-773, Republic of Korea c Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan 250100, China d Department of Chemistry, Yanbian University, Yanji 133002, China Received 6 June 2006; received in revised form 24 December 2006; accepted 9 January 2007 Available online 17 January 2007
Abstract Three kinds of europium complexes; Eu(phen)2Cl3(H2O)2, Eu(DN-bpy)phenCl3(H2O)2 and Eu(DB-bpy)phenCl3(H2O)2 (phen: 1,10phenanthroline, DN-bpy: 4,40 -Dinonyl-2,20 -dipyridyl, DB-bpy: 4,40 -Di-tert-butyl-2,20 -dipyridyl) were prepared and then incorporated into polymethyl methacrylate (PMMA) matrix with different molar ratios of CQO groups/Eu3+ ions. The final solid composites were formed by a self-assembly process among Eu3+ ion, the ligands and PMMA during the solvent evaporation process, and then the ligands re-coordinate to Eu(III). It was found that the ligands affect not only the emission properties of the pure complexes, but also the miscibility of the complexes and PMMA. More than one kind of symmetric sites of Eu3+ ions were formed in the composites due to the coordination of CQO in PMMA to Eu3+ ions. The micro-environments of Eu(III) in the composites were changed with the compositions and the ligands, leading to the change in the crystalline structure, and consequently, the emission characteristics. r 2007 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Europium complex; PMMA composite; Heterocyclic ligand
1. Introduction It is well known that europium complexes show strong and narrow red luminescence due to ‘‘antenna effect’’ of ligands and 4f-4f electron transition of Eu(III). Over the past decade, an interest has grown in several europium complexes for practical applications for laser development, phosphor materials, electro-optical devices, optical communication amplifiers, new generation of optical storage materials based on hole-burning technique, and probes to detect the local environments in biological systems. In general, pure europium complexes do not have good thermal and mechanical stabilities and also show poor processing ability. In addition, pure complexes can absorb moisture from the air and form clusters, leading to luminescence quenching. These factors make the pure Corresponding author. Tel.: +82 55 279 7437; fax: +82 55 279 7439.
E-mail address: [email protected] (Y.-I. Lee). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.01.001
complexes just have extensive promising photophysical applications but are limited to practical uses. In order to overcome these shortcomings, europium complexes are usually incorporated into organic, inorganic or organic/ inorganic hybrid matrices. Zeolites [1], mesoporous materials [2–4], sol–gel silica or organically modified silicates (ORMOSIL) [5–11] and polymers are the most common hosts and have been investigated in recent years. The proper incorporation of europium ligands into organic polymers represents an ideal and extremely versatile approach to generate such hybrid materials. Two main methods have been utilized to dope europium complexes into polymers. One uses the polymerization of monomers that contain Eu3+ ions or achieves with the aid of reactions between Eu3+ ions or europium complexes with polymers that have ligands in their chains [12–20]. On the other method, the composites can be prepared just by blending the polymers and the europium complexes in appropriate solvents [21–25] or melting the two components
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together [26]. It has been observed that the photoluminescent (PL) efficiency of the PVK-co-MMA-co-Eu(BA)2 (MMA)phen film [13] is higher than that of both Eu-complex neat film and Eu-complex/polymer blending film [24], indicating the advantages of the materials made by chemical reactions over those by blending. Although typically more homogeneous composite systems could be obtained using the polymerization/reaction method than the simply mixing method, well-dispersed systems could also be anticipated in some simply mixed systems if they have some special supramolecular interactions between the complex molecules and the polymer, such as the coordination of oxygen atoms to Eu3+ ions in the mixed systems of europium picrate/poly(oxyethylene) [22] and Eu3+-b-diketonates with the epoxy resin [20–27]. Moreover, it is much easier to tune the composition, structure and properties of the doped systems formed by the blending method than by the chemical reaction method. In addition, these kinds of interactions can be helpful to disperse Eu3+ ions or complexes into sol–gel nanohybrids [28–31]. The europium complexes with new ligands have been synthesized continuously [32–34]. However, the traditional ‘‘older’’ complexes show distinct new properties and applications in various matrices [35]. The spectroscopic characteristics of heterocyclic europium complexes including Eu(phen)2Cl3 and Eu(bpy)2Cl3 have been studied extensively [36–39]. Phen and bpy are typical light-harvesting ligands that can absorb near ultraviolet and transfer the energy to Eu3+ ions effectively. The composite materials containing this kind of complexes would have potential photophysical applications. To our knowledge, most of the work on the composite materials containing this kind of complexes has been focused mainly on silica [40–44], ORMOSIL [45] and silica/polymer hybrids [46,47] due to the good miscibility of the complexes and the matrices. Polymethyl methacrylate (PMMA) has been considered to form a quasi-cross-linked structure through strong dipole–dipole interactions which prevent it from crystallizing, and is regarded as transparent ‘‘glass’’ [48,49]. Various europium-b-diketonates have been doped in PMMA to form amorphous composites due to good miscibility of these components, and showed the potentials for applications in plastic optical fiber lasers and amplifiers [50]. It is rare to study the composite systems formed by europiumheterocyclic ligand complexes with such polymers like PMMA, because of the difficulties of mixing these components together. In this paper, three kinds of heterocyclic europium complexes synthesized, Eu(phen)2Cl3(H2O)2, Eu(DN-bpy)phenCl3(H2O)2 and Eu(DB-bpy)phenCl3(H2O)2 (shown in Fig. 1) were incorporated into PMMA by dissolving the corresponding materials in mixed CHCl3/C2H5OH solvents. Solid crystalline composites were formed well after evaporating the solvents. The influences of different host polymers on composite structures and PL properties of Eu(III) in Eu(phen)2Cl3(H2O)2 and Eu(DB-bpy)phenCl3 (H2O)2 were reported in our previous papers [51,52].
A significant focus of this work has been to investigate and discuss the influences of compositions and ligands on PL of Eu(III) ions in the composite systems deeply. 2. Experimental 2.1. Reagents and samples Eu2O3 (99.95%), phen (99+%), DN-bpy (97%), DB-bpy (98%) and PMMA (Mw15,000) were purchased from Aldrich Chemical Company, Inc. The complexes Eu(phen)2 Cl3(H2O)2, Eu(DN-bpy)phenCl3(H2O)2 and Eu(DB-bpy)phenCl3(H2O)2 were synthesized according to the methods in Ref. [44], and dried in air at room temperature. The elemental analysis results: Eu(phen)2Cl3(H2O)2: C: 43.0% (44.0%); H: 3.30% (3.08%); N: 8.59% (8.56%); Eu(DNbpy)phenCl3(H2O)2: C: 55.0% (54.4%); H: 6.91% (6.39%); N: 6.35% (6.34%); Eu(DB-bpy)phenCl3(H2O)2: C: 48.3% (48.5%); H: 5.39% (4.88%); N: 7.51% (7.54%). The values in parentheses are calculated ones. 2.2. Sample preparation and characterization Mixed systems of the complexes in PMMA were prepared by dissolving these components with certain molar ratios of CQO/Eu into ethanol/chloroform. The samples for PL spectroscopy and lifetime measurements
N EuCl3(H2O)2 N 2
N
N Cl3(H2O)2
Eu N
N
N
N Cl3(H2O)2
Eu N
N
Fig. 1. Molecular structures of the europium complexes.
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were made by casting the solutions onto clean glass slides. The solvents were allowed to evaporate in air at room temperature. PL spectra were obtained by using a PC 2000 spectroscope (Ocean Optics Inc.) with the excitation at 325 nm using a He–Cd laser (Omnichrome, LC-500). Decay curves were obtained by monitoring the 612–614 nm emission by using 300 MHz-digital oscilloscope (LeCroy 9310, Switzerland) and triple grating monochromator (Spectra Pro-750 ARC Actron, Research Corporation, MA) under the excitation at 355 nm dye laser (Spectron SL 4000B/G, Spectron laser systems, UK) at room temperature. All emission intensities were normalized with the spectral responses of PMT and optical setup in this work. The wavelengths of obtained spectra were also calibrated using a Hg(Ar) spectral calibration lamp (Model 6035, Oriel Co.) UV–vis spectra were recorded using a UV–vis spectrophotometer (Shimadzu, Japan). Pure chloroform was used as a reference, and the samples were diluted for the measurements with the spreading solutions mentioned above. Powder X-ray diffraction (XRD) patterns were obtained on a Philips X’Pert MPD/PW3040 X-ray diffractometer with Cu Ka irradiation at 293 K. 3. Results and discussion 3.1. PL spectra and XRD patterns of pure complexes The PL spectra of the pure complexes with different ligands are illustrated in Fig. 2. Upon excitation at 325 nm, PL spectra of these complexes exhibit characteristic ligandsensitized emission of Eu3+. Five emission bands corresponding to 5D0-7F0,1,2,3,4 can be clearly distinguished. The emission spectrum of Eu(phen)2Cl3(H2O)2 is identical to the reported one by Jin et al. [42]. Two peaks appear at 16,923 and 16,798 cm1 in 5D0-7F1 band; and four peaks appear at 16,345, 16,268, 16,184 and 16,080 cm1 in 5D0-7F2 band. 5
Relative Intensity
5 D -7 F 0 1
D0-7F2 Eu(phen)2Cl3(H2O)2
5 D -7 F 0 0
5 D -7 F 0 4
5 D -7 F 0 3
Eu(DB-bpy)phenCl3(H2O)2
16000 Wavenumber
15000
The emission spectrum of Eu(DN-bpy)phenCl3(H2O)2 is very similar to that of Eu(phen)2Cl3(H2O)2. This indicates that the replacement of a phen by a DN-bpy does not affect the first coordination sphere of Eu3+ and the point group of the complex. However, the emission spectrum of Eu(DBbpy)phenCl3(H2O)2 shows obvious different characteristics from those of the former two complexes. One peak at 16,869 cm1 and a shoulder at 16,952 cm1 appear in 5 D0-7F1 band, and two peaks at 16,305 and 16,184 cm1 appear in 5D0-7F2 band. The profile of the curve varies greatly, especially for 5D0-7F2 transition that is called as the ‘‘hypersensitive transition’’ [53], because it is very sensitive to the environments around Eu3+ ions, specifically to the symmetry of the coordination sphere. In addition, it can be seen that 5D0-7F4 band changes slightly, too. According to Judd–Ofelt theory [54–57], the f–f transitions of Eu3+ are parity forbidden and consist mainly of weak magnetic dipole (MD) and induced electric dipole (ED) transitions. The probabilities of MD transitions are independent of the chemical environment of the ion. The 5 D0-7F1 emission is a pure MD transition. The so-called ‘‘hypersensitive’’ 5D0-7F2 transition of Eu3+ ion is an ED transition allowed one and depends on the local electric field or the local symmetry. The intensity ratio of 5D0-7F2 transition and 5D0-7F1 transition is a good measure of the nature and symmetry of the first coordination sphere. The I (5D0-7F2)/I(5D0-7F1) ratios for the three complexes were calculated to be 3.55, 3.11 and 2.11, respectively, indicating the similar micro-environments of Eu(III) in Eu(phen)2Cl3(H2O)2 and Eu(DN-bpy)phenCl3(H2O)2, and different from Eu(DB-bpy)phenCl3(H2O)2. The coordination symmetry of Eu(III) in Eu(DB-bpy)phenCl3(H2O)2 is higher than in the other two complexes. Fig. 3 shows the XRD patterns of the pure complexes. There is obvious difference in the number and relative intensities in the diffraction peaks of the complexes. The former two complexes give similar patterns that are composed of a lot of diffraction peaks, whereas the XRD pattern of Eu(DB-bpy)phenCl3(H2O)2 seems very simple, just containing several peaks, indicating the higher symmetry of the crystal. This is consistent with the PL spectra. From these results, it is clearly seen that the ligands have great influence on the crystalline structure of the complexes, and in consequence, the emission properties. 3.2. Emission properties of the doped systems of Eu(phen)2Cl3(H2O)2 in PMMA
Eu(DN-bpy)phenCl3(H2O)2
17000
309
14000
(cm-1)
Fig. 2. Emission spectra of the pure complexes (293 K, lex: 325 nm).
As previously reported, the profiles of the emission curves of Eu(III) in the doped systems change regularly with the molar ratios of CQO/Eu [52]. The spectral features were summarized in Table 1. According to the emission spectral characteristics of 5D0-7F1 and 5D0-7F2 bands and intensity ratios of I(5D0-7F2)/I(5D0-7F1), three kinds of composites can be classified, i.e. the composites with the molar ratios greater than, close to
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310
and less than 50, respectively. These three kinds of composites show distinct luminescent properties. The luminescent decay curves of Eu3+ ions in the composites can be fitted by bi-exponential functions. The fitting data including lifetime values of 5D0 levels of Eu3+ ions and the corresponding relative weightings for each species are illustrated in Table 2. Although two kinds of luminescent lifetimes may be associated with 5D0 and 5D1
levels resulting from the superposition between the 5 D0-7F2/5D1-7F4 and 5D0-7F1/5D1-7F3 transitions in some cases including the composite systems of EuBr3/ poly(ethylene oxide) [58], the lifetimes obtained here should be ascribed to more than one kind of symmetrical site of Eu3+ in the composite systems according to the broader 5 D0-7F0 transition bands (FWHM: 60–70 cm1). This can be also related to heterogeneities in the charge distribution around the Eu3+ ions in most cases like Eu3+ in some glassy hosts [59,60], in some coordination complexes [61,62], and even in so called ‘‘pure complex’’ such as the
Eu(phen)2Cl3(H2O)2
Table 2 Fitting data of decay curves of Eu(phen)2Cl3(H2O)2/PMMA composites
Eu(DN-bpy)phenCl3(H2O)2
Eu(DB-bpy)phenCl3(H2O)2
10
20
30 2θ (°)
40
50
Fig. 3. X-ray diffraction patterns of the pure complex powders.
Molar ratio of CQO/Eu
t1 (ms)
RW (%)
t2 (ms)
RW (%)
125 100 83 67 56 50 45 40 36 33 20 10 Pure complex
35 32 68 54 65 55 61 88 102 114 153 176 175
80 75 50 56 32 26 25 48 48 47 70 93 96
608 441 360 370 299 295 302 390 400 391 428 1136 1153
20 25 50 44 68 74 75 52 52 53 30 7 4
Table 1 Emission spectral data for Eu(phen)2Cl3(H2O)2/PMMA composite systems CQO/Eu
5
D0-7F0 n1 (cm1)
5
D0-7F0 Dn1 (cm1)
5
D0-7F0 n2 (cm1)
5
D0-7F0 Dn2 (cm1)
5
D0-7F1 (cm1)
5
1000 500
17,252 17,248
122 126
17,279 17,274
95 100
333
17,246
128
17,274
100
250
17,247
127
17,275
99
200
17,248
126
17,276
98
100
17,246
128
17,273
101
50 33
17,249 17,253
125 121
17,276 17,281
98 93
25
17,257
117
17,284
90
20
17,254
120
17,280
94
10
17,251
123
17,277
97
5
17,253
121
17,280
94
2
17,253
121
17,280
94
Pure
17,254
120
17,281
93
17,013, 16906 16,915, 19,815 16,915, 16,810 16,915, 16,815 16,912, 16,810 16,918, 16,821 16,900 16,926, 16,815 16,929, 16,810 16,926, 16,798 16,926, 16,798 16,926, 16,798 16,926, 16,795 16,923, 16,798
D0-7F2 (cm1)
I(5D0-7F0)/ I(5D0-7F2)
I(5D0-7F2)/ I(5D0-7F1)
16,305 16,321, 16,197
0.0125 0.0141
45 4.73
16,319, 16,194
0.0143
4.78
16,319, 16,197
0.0137
4.98
16,313, 16,194
0.0135
4.77
16,319, 16,194
0.0135
4.89
16,316, 16,184 16,345, 16,265 16,189, 16,082 16,351, 16,263 16,189, 16,085 16,348, 16,363 16,189, 16,082 16,348, 16,263 16,189, 16,082 16,348, 16,263 16,189, 16,080 16,348,16,263 16,184, 16,080 16,345, 16,263 16,184, 16,080
0.0202 0.0122
3.46 3.86
0.0101
3.77
0.0105
3.66
0.0115
3.59
0.0118
3.56
0.0112
3.61
0.0116
3.55
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500 450
C=O/Eu: 50 Lorentz fit
400 Intensity
350 300 250 200 150 100 576
578 580 Wavelength (nm)
582
Fig. 4. Lorenzian decomposition of the 5D0-7F0 emission band of the composite formed by Eu(phen)2Cl3(H2O)2 and PMMA with the molar ratio of CQO/Eu of 50.
complex of Eu3+ with picrate ions due to the different number of coordinate water molecules [22]. For the first coordination sphere around Eu3+, the transition between 5D0 and 7F0 levels can give valuable information. Both the ground (7F0) and excited (5D0) states of Eu3+ are non-degenerate, therefore, only single emission transition can be observed for each unique Eu3+ environment in principle. Both the 7F0-5D0 transition band in the excitation spectrum and 5D0-7F0 transition band in the emission spectrum have been used to determine the number of the symmetric sites of Eu3+ by using multi-peak fitting technique and to get some information on the first coordination sphere around Eu3+ ions by relating to the nephelauxetic effect [63–65]. The 7F0-5D0 or 5D0-7F0 band can be resolved into individual peaks by the use of the Lorentzian function according to the method described by McNemar and Horrocks Jr. [66] to provide the most accurate description for solid compounds. The 5D0-7F0 emission band was decomposed into two peaks shown in Fig. 4. The detail data are summarized in Table 1. Two kinds of symmetrical sites can be induced based on the fitting results of the decay curves. The energy of 5D0-7F0 is usually related to the nephelauxetic effect [63,67,68] that concerns the influence of covalency contributions of the first coordination sphere on the reduction of the Eu3+ attractive potential relative to their free ion values. It was shown that the observed nephelauxetic effect was due to a decrease in the effective nuclear charge of Eu3+ ion upon binding to a negatively charged ligand [67]. The difference between the observed n value (cm1) of 5D0-7F0 emission peak of Eu3+ in a complex and n0, the n value of the gaseous Eu3+ ion with no coordinating atoms can be related by n n 0 ¼ n1 d1 þ n2 d2 þ þ nj dj ,
(1)
where dj is the parameter for atom type j, which measures the tendency of a particular atom to bond covalently to the
311
Eu3+ ions, and nj is the number of atoms of type j in the first coordination shell [63,68,69]. The n0 was chosen as 17,374 cm1 [68]. This means that the ability to produce a nephelauxetic effect in Eu3+ complexes and to reduce its 5 D0-7F0 energy should be related to the Eu3+ first coordination sphere, i.e. the local environments around Eu3+ ions. The Dn value depends not only on the sorts of the coordinative atoms but also on the coordination number [69]. The two Dn values for the pure Eu-phen complex are 120 and 93 cm1. The relative weighting of the second one is very low. As can be seen in Table 1, the two average Dn values for the composites with lower CQO/Eu molar ratios are nearly 121 and 94 cm1, while for the samples with higher molar ratios are about 126 and 99 cm1. This reflects that the composites with lower CQO/Eu molar ratios have quite different first coordination spheres around Eu3+ ions compared to that of higher molar ratios, which is consistent with the variation of the emission spectra. Jin et al. [42] have studied the doped systems of europium complex with phen in silica and found that two kinds of point group symmetries exist, which are hydrous Eu(phen)2Cl3(H2O)2 and anhydrous Eu(phen)2Cl3 complexes. The hydrous one gives an intense 5D0-7F2 peak at 620 nm (16,129 cm1), and the anhydrous one gives an intense peak at 615 nm (16,260 cm1) and a weak one at 622 nm (16,077 cm1). The authors assigned the 615 nm peak to Eu(phen)2Cl3 and the 622 nm one to Eu(phen)2Cl3(H2O)n that was considered to be formed on the surface of the anhydrous complex particle because of strong hygroscopic ability of the anhydrous complex. In the PL spectra of the doped systems [52], one intense peak with lmax at 16,189–16,186 and three weak peaks at 16,348–16,345, 16,265–16,263 and 16,077 cm1 were appeared for the composite systems of Eu/phen complex with lower CQO/Eu molar ratios in PMMA and the pure complex. This implies that Eu(phen)2Cl3(H2O)2 should be the main component of the complex. The two main species in the composites with low molar ratios should be the hydrous complex and a new species, corresponding to shorter and longer lifetimes listed in Table 2, and to the average Dn values of 121 and 94 cm1 in Table 1, respectively. The intensity ratio of I614.8/I617.8 in the PL spectra increases with increasing the molar ratios of CQO/Eu over 50, indicating that the relative amounts of the new species increase gradually with increasing PMMA contents, in line with the relative weightings listed in Table 2. The new species has a longer lifetime than the hydrous one. It is possible that the coordination of H2O molecules were replaced by CQO groups and the coordination number was also changed due to the effect of the host. The 5D0-7F2 transition bands reflecting the local environments around Eu3+ ions were quite different between the samples with higher molar ratios and those with lower molar ratios, which can be seen clearly in the Dn values of 5D0-7F0 transition peaks. The formations of the
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new crystal structures in the samples with higher CQO/Eu molar ratios was also found in XRD results and agree with the above results [52]. The two peaks in 5D0-7F2 emission bands should correspond to different species. The variation of the relative intensity of these two peaks indicates the change of the relative weightings of these species. According to the d values for various functional groups [69], the difference between the d values of the oxygen atoms in carbonyl group and in water molecule is ca. 5–6 cm1. Although it is difficult to deduce the exact coordination sphere around Eu3+ ions in the species with higher molar ratios, it is possible that the CQO group coordinates to Eu3+ ion, in other words, one H2O is replaced by a CQO group. The interactions between CQO groups and lanthanide ions are very popular in solid-doped systems such as in the composite systems of Nd3+/PMMA [70], EuL3(H2O)2 in polyimide (L: pyridine carboxylic acid) [71] and in the ureasil containing Eu(CF3SO3)3 [63]. FTIR spectra of these systems supply related informations on the coordinate interaction [52]. Another interesting feature of the 5D0-7F0 transition band lies in their intensity. The 5D0-7F0 transition cannot be accounted for by either the MD mechanism or the Judd–Ofelt theory. A more detailed analysis has indicated that this transition ‘‘borrows’’ intensity from the 5D0-7F2 transition through higher order perturbations by the ligand field [72–74] i.e. becomes allowed only through ‘‘J-mixing’’ [75]. The intensity ratio between 5D0-7F0 and 5D0-7F2 reflects the degree of J-mixing effect [63]. As can be seen from Table 1, the ratios for the samples with higher and lower CQO/Eu molar ratios are ca. 0.014 and 0.012, respectively. While, that of the composite with the molar ratio of 50 is found to be 0.020 which is obviously larger than the others, indicating the higher symmetry around Eu3+ ion. The ratio of I(5D0-7F2)/(5D0-7F1) for the composite with the molar ratio of 50 shows the lowest value among the composite systems studied, that is in good agreement with the above results. In addition, the luminescent lifetimes, the XRD patterns, the TEM micrographs and the emission bands also show the unique properties of this sample. 3.3. Emission properties of the doped systems of Eu(DNbpy)phenCl3(H2O)2 and Eu(DB-bpy)phenCl3(H2O)2 in PMMA The emission properties of Eu(DB-bpy)phenCl3(H2O)2/ PMMA composites were reported in the previous paper [51]. The PL spectra of Eu(DN-bpy)phenCl3(H2O)2/ PMMA composites were illustrated in Fig. 5. It can be seen that the emission properties of Eu(III) in the composites change with the concentrations, and the composites can be classified into several categories according to the PL spectra. The luminescent decay curves of 5D0 level for these systems were recorded. These curves should be fitted by bi- or tri-exponential functions, indicating more than one kinds of species of Eu(III) exist. By
5
12000 10000 Relative Intensity
312
D0-7F2
5D -7F 0 1
C=O/Eu
5 D -7 F 0 0
5D -7F 0 3
5D -7F 0 4
1000
8000
200 100
6000
50 20 10 5 2
4000 2000
pure complex
0 17000
16000
15000
14000
Wavenumber (cm-1) Fig. 5. Emission spectra of Eu(DN-bpy)phenCl3(H2O)2/PMMA composites with various molar ratios of CQO/Eu and the pure complex (293 K, lex: 325 nm).
analyzing the life times of 5D0 level and the 5D0-7F0 transition bands, we can get the same conclusion as that from analyzing the Eu(phen)2Cl3(H2O)2 that the microenvironment around Eu(III) changes with the composition, leading to the variation of structure and properties of these composites. As discussed in Section 3.1, the ligands have a great influence on the PL behaviors of the pure complexes. Here we focus on the influence of ligands on the emission properties of the composites. The composites with same molar ratios show different PL behaviors for the three series of doped systems. All the three samples with the molar ratio of 1000 give broader 5D0-7F2 emission bands. However, the emission band of Eu(phen)2Cl3(H2O)2-doped sample is an asymmetric one with smaller FWHM of 235 cm1 that is different from those of the other two samples that are more broader and symmetric bands with FWHMs of 260 and 254 cm1, respectively. The final structure depends largely on the interaction between the Eu3+ ions, ligands and PMMA during the solvent evaporation process. DN-bpy and DB-bpy have more interaction with PMMA than phen due to the alkyl substituents which help the Eu-ligand complex to incorporate into the host. There are abundant methyl groups on PMMA chains. The complex with DB-bpy can be dispersed in PMMA easier than that with DN-bpy due to the tert-butyl substituents, which can be seen from the PL spectra of the samples with the molar ratio of 200. With decreasing molar ratios, the PL spectra approach to those of pure complexes. The composites show similar PL behaviors to that of the corresponding pure complex when the molar ratios of CQO/Eu reach to 33 for Eu/phen complex and 10 for Eu/DN-bpy/phen complex. However, the samples containing Eu/DB-bpy/phen complex show very different spectra from that of pure Eu(DBbpy)phenCl3(H2O)2 even when the molar ratio reaches to 2.
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This should be attributed to different interactions between the components, especially, between phen, DN-bpy, DBbpy with PMMA, too.
a
4. Conclusion PMMA, as a matrix to luminescent europium complexes, has great effects on the PL properties of Eu3+ ions due to the strong coordination interaction between Eu3+ and CQO group. The interaction between the ligands and PMMA results in the formation of different microenvironments around Eu3+ ions that change with the compositions of the composites. This can be attributed to the great variation of the PL behaviors of the composites. In
0.8 phen
3.4. UV–vis spectra of the spreading solutions
C=O/Eu: 100 C=O/Eu: 10 C=O/Eu: 2 Eu(phen)2Cl3(H2O)2
Absorbance
0.6
0.4
0.2
0.0 260
280
1.0
320
340
DN-bpy/phen 1:1 Eu(DN-bpy)phenCl3(H2O)2/PMMA Eu/C=O: 1/100 Eu(DN-bpy)phenCl3(H2O)2
0.8
Absorbance
300 Wavelength (nm)
b
0.6
0.4
0.2
0.0 260
280
300 320 Wavelength (nm)
340
360
c 1.0
DB-bpy/phen 1:1 Eu(DB-bpy)phenCl3(H2O)2/PMMA Eu/C=O 1:100
0.8 Absorbance
Fig. 6 shows the UV–vis spectra of the composite systems, the ligands and the pure complex in CHCl3/ C2H5OH solutions. As seen in Fig. 6a, one peak of lmax at 265.6 nm and one shoulder at 279 nm appeared in the spectrum of pure phen. While one peak at 266.6 nm and two shoulders at 275 and 295 nm were observed in the spectrum of pure complex, presumably the complex formation. However, the composite systems show the similar spectra as that of pure phen, even in the solution with the molar ratio CQO/Eu of 2, suggesting that the complex molecules disassociate in the solutions by adding PMMA. This is a clear evidence for the interaction between the complex and PMMA in solution. The oxygen atom of the carbonyl group in PMMA is a kind of hard base and Eu3+ ion is also regarded as a hard acid. The coordination ability of ‘‘O’’ to Eu3+ ion is stronger than that of ‘‘N’’. The final structures of the casting films can be considered as the results of a self-assembly process during the solvent evaporation. Phen can coordinate to Eu3+ ion again with evaporating the solvent because of the ‘‘chelating effect’’. It is difficult to measure UV–vis spectra of the solid composites because opaque solid was formed due to the crystalline. Nevertheless, it is reasonable for phen coordinate to Eu(III) in the solid state. In a previous paper [52], we investigated the composite structures of Eu(phen)2Cl3 (H2O)2 doped in PMMA and PVP. It was found from the FTIR spectra of the composites that phen re-coordinate to Eu(III) after evaporating the solvent in the mixed systems with PMMA, while phen cannot re-coordinate to Eu(III) in the mixed systems with PVP. Figs. 6b and c show the UV–vis spectra of the composites of the other two complexes in PMMA. From the spectra, the same conclusion can be drawn as that from Eu(phen)2Cl3(H2O)2/PMMA composites that the complexes disassociate in the mixed spreading solutions due to the coordination effect of CQO to Eu3+, and the heterocyclic ligands re-coordinate to the metal ions during evaporation of the solvent, leading to the formation of the composites with more than one kind of species of Eu(III) due to the rudimental coordinated CQO groups.
313
Eu(DB-bpy)phenCl3(H2O)2
0.6
0.4
0.2
0.0 260
280
300 320 Wavelength (nm)
340
360
Fig. 6. UV–vis spectra of the composite Eu(phen)2Cl3(H2O)2/PMMA (a), Eu(DN-bpy)phenCl3(H2O)2/PMMA (b) and Eu(DN-bpy)phenCl3(H2O)2/ PMMA (c), the pure ligand and pure complex in CHCl3/C2H5OH solution.
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addition, slightly modification on the ligands can provoke great changes of the PL properties, too, due to the different miscibility of the complex in the host.
Acknowledgments The authors are indebted to the financial support of the Korea Research Foundation (Grant no. KRF 2000-005Y00071) and Education Ministry of China.
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