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Photoluminescence enhancement of Sm3+ in the Sm3+–salicylic acid–o-phenanthroline ternary composite
Journal of Alloys and Compounds 392 (2005) 96–99
Photoluminescence enhancement of Sm3+ in the Sm3+–salicylic acid–o-phenanthroline ternary composite Cun-jin Xua,b , Hui Yanga,∗ , Fei Xiea , Xing-zhong Guoa a
Center of Nanometer Science and Technology, Zhejiang Unversity, Hangzhou 310027, PR China b Department of Chemistry, Hangzhou Normal College, Hangzhou 310012, PR China
Received 16 June 2004; received in revised form 1 September 2004; accepted 10 September 2004 Available online 11 November 2004
Abstract The ternary Sm(III) complex with the salicylic acid (HSal) and o-phenanthroline (Phen) has been prepared. The photoluminescence intensity of the complex Sm(Sal)3 (Phen) is enhanced by more than two hundred times with respect to that of the binary complex Sm(Sal)3 (H2 O)2 . It is confirmed by Sm(III) content titration and the spectra of IR, UV–vis and fluorescence that the second ligand, Phen, also coordinates with the Sm3+ ion. The second ligand Phen shows enhancement effect on the fluorescence of the complex. These observations are discussed in detail and the results are rationalized. © 2004 Elsevier B.V. All rights reserved. Keywords: Salicylic acid; 1,10-Phenanthroline; Fluorescence enhancement; Sm3+ composite
1. Introduction There has been a growing interest in the study of the enhancement effect of fluorescence of the lanthanides since the lanthanides have low absorptivities and poor quantum yields. The fluorescence enhancement has generally been achieved through ligand sensitization [1–3]. In this process, an organic ligand is first excited by light absorption, followed by energy transfer from the ligand to the excited levels of a lanthanide. The photons are then emitted by the decay from the excited levels to its ground level. If the intramolecular energy transfer from the ligand to the lanthanide is efficient, the upper emitting level of the lanthanide is more efficiently pumped by this technique than by a direct excitation, resulting in an enhanced lanthanide fluorescence. It has been reported that aromatic carboxylic acids also serve as excellent ligands to sensitize the fluorescence of lanthanides efficiently [4–7]. Among the rare earth ions, those compounds with trivalent europium and terbium ions were the most thoroughly investigated, owing to their intrinsic electronic spectroscopic properties. However, the Eu3+ ion has attracted more atten∗
Corresponding author. Tel.: +86 571 87951408; fax: +86 571 87953054. E-mail address: [email protected] (H. Yang).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.09.031
tion than the Tb3+ ion probably due to the facilities in the interpretation of its energy level structure, considering that the former has a non-degenerate emitting level (5 D0 ) [8], while the Tb3+ ion has an emitting state (5 D4 ) multi-degenerate. On the other hand, the compounds containing the Sm3+ ion have been less studied than those with Eu3+ and Tb3+ ions, taking into consideration that the trivalent samarium ion in general presents low luminescence intensity. In this paper, we have initiated studies of the photoluminescence properties of the Sm(Sal)3 (H2 O)2 and the Sm(Sal)3 (Phen) composites for the first time. The luminescence intensity of the latter novel composite is more than two hundred times as high as that of the composite Sm(Sal)3 (H2 O)2 . It is believed that the second ligand may displace the coordinating H2 O and increase the molecular rigidity to attain this high efficiency.
2. Experimental 2.1. Apparatus and reagents The purity of Sm2 O3 exceeding 99.9% was used, and all other chemicals were analytical reagent grade. Element analysis of carbon, hydrogen and nitrogen was performed with
C.-j. Xu et al. / Journal of Alloys and Compounds 392 (2005) 96–99
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a Flash-EA1112 elemental analyzer. Infrared spectra were measured at room temperature in 400–4000 cm−1 region using a Bruker Tensor 27 FT–IR spectrophotometer with KBr pellet technique. UV spectra were obtained by a Unico UV2102 PC spectrophotometer. The excitation and emission spectra were recorded in a Hitachi-F4500 spectrofluorometer. 2.2. Preparation of complexes The binary composite Sm(Sal)3 (H2 O)2 was synthesized using a method similar to the literature [9]. The ternary complex Sm(Sal)3 (Phen) was synthesized as follows: salicylic acid (1.5 mmol) and o-phenanthroline (0.5 mmol) were dissolved in 95% C2 H5 OH respectively. The pH value of the salicylic acid C2 H5 OH solution was adjusted to the range of 6–7 with NaOH solution (1 mol L−1 ). The C2 H5 OH solutions of two ligands were mixed and then the mixture was added dropwise to the ethanolic SmCl3 (0.5 mmol) solution, which was stirred at temperature 80 ◦ C until the formation of the pale-yellow precipitate. The solid products were recrystallized from acetone and dried under vacuum over anhydrous calcium chloride in the desiccator at room temperature.
3. Results and discussion 3.1. Compositions of title complexes Lanthanide ions were analyzed by complexometeric titration with EDTA. The elemental analysis data of C, H, N and rare earth are listed in Table 1. From the Table 1, it can be seen that the composition of the complexes is Sm(Sal)3 (H2 O)2 and Sm(Sal)3 (Phen), respectively.
Fig. 1. IR spectra of Sm(Sal)3 (Phen) (a), HSal (b), NaSal (c) and Phen (d).
of the hydroxyl group to metal ion is occurring through the oxygen [11]. The C N and C H stretching vibration peaks appearing at 1589, 739 and 853 cm−1 in o-phenanthroline, respectively shift to about 1561, 729 and 847 cm−1 in the complex, which indicates that the chemical bonds are formed between rare earth ion and nitrogen atoms of o-phenanthroline. 3.3. UV spectra The UV absorption spectra of the ligands and the complexes in ethanol solution are given in Figs. 2 and 3, respectively. It can be seen that the absorption peaks of o-phenanthroline and salicylic acid in ultraviolet region appearing at 229.2 ± 0.2, 264.1 ± 0.2 nm and 236.3 ± 0.2, 305.3 ± 0.2 nm, respectively, are assigned to the band of * transitions of aromatic ring, which belongs to K band [12]. According to the graphs given below, the UV absorption of the complex is dominated by the absorption of Phen for the fact that HSal only presents a shoulder at 305.3 ± 0.2 nm while Phen still shows its characteristic absorption peak at
3.2. IR absorption spectra Fig. 1 gives IR transmission spectra (400–4000 cm−1 ) of Sm(Sal)3 (Phen), HSal, NaSal and Phen. The characteristic absorption peaks of –COOH, appearing at 2860.0, 2594.9, 1444.2 cm−1 in HSal, disappear in the complex Sm(Sal)3 (Phen). The asymmetric stretching vibrations and the symmetric stretching vibrations of carboxylate group in sodium salicylate, appearing at 1582.2 and 1377.4 cm−1 , are shifted to 1597.2 and 1389.2 cm−1 on formation of the complex, which indicates that for the complex, carboxylic group may belong to mondendate coordination [10]. The displacement of the νAr OH stretching from ∼1296 cm−1 in NaSal to ∼1222 cm−1 in the complex suggesting that the coordination
Fig. 2. UV absorption spectra of Phen (a) and HSal (b).
Table 1 Data of elemental analysis (%) of title complexesa Complexes
C
H
N
Sm
Sm(Sal)3 (H2 O)2 42.03 (42.19) 1.09 (1.07) 0.00 (0.00) 24.92 (25.11) Sm(Sal)3 (Phen) 53.28 (53.37) 3.00 (3.10) 3.52 (3.77) 19.89 (20.27) a
Numbers in brackets are calculated values.
Fig. 3. UV absorption spectra of Sm(Sal)3 (Phen) (c) and Sm(Sal)3 (H2 O)2 (d).
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Phen are present in the coordination sphere and may let the Sm(Sal)3 (Phen) composite be efficiently excited by UV light. As shown in the figures, the emission spectra of the complexes consist of narrow bands by the 4 G5/2 → 6 HJ transitions (where J = 5/2, 7/2 and 9/2) of the Sm3+ ion. The emission bands of the complex Sm(Sal)3 (H2 O)2 are shifted slightly and there is a different distribution of band intensities compared to that of Sm(Sal)3 (Phen). Table 2 shows the luminescence intensity of the two complexes. From Table 2, it is observed that the second ligand Phen can sensitize the luminescence of Sm3+ ion efficiently and the luminescence intensity of Sm(Sal)3 (Phen) is more than two hundred times as high as that of Sm(Sal)3 (H2 O)2 through the introduction of the second ligand Phen into the coordination sphere of Sm3+ by releasing precoordinated water molecules. This is due to the reasons of both the presence of O H vibrations in the complex Sm(Sal)3 (H2 O)2 , which contribute to non-radiative deactivation of the 4 G5/2 emitting state of the Sm3+ ion [13] and the increase in molecular rigidity through the introduction of Phen in the complex Sm(Sal)3 (Phen) [10]. In Sm(Sal)3 (H2 O)2 , the hydroxyl group in H2 O takes part in the coordination and destroys the planar molecular structure of the complex. Therefore, the efficiency of energy transfer from the excited ligand to Sm3+ is lowered, and the luminescence of this complex becomes weaker. On the other hand, the introduction of Phen, an excellent light-harvesting center, into the coordination sphere may result in a more asymmetric coordination geometry than that of the Sm(Sal)3 (H2 O)2 composite. The formation of an unique asymmetric environment surrounding the Sm3+ ion leads to an increased transition probability for the fluorescence, and results in greatly enhanced fluorescence of the Sm3+ ion [14]. The fact that the second ligand Phen sensitizes luminescence of Sm3+ ion can also be explained by the idea given by Sato and Wada [15], who indicated that the fluorescence yield would be decreased either because of the thermal deactivation process while the energy gap was small, or due to the diminution in energy overlap while the energy gap was large. Accordingly, the fluorescence yield shows a maximum for a proper energy gap between T (L) and 4 G5/2 (Sm3+ ) levels, as shown in Fig. 6. From the Fig. 6, we can see that the triplet state of Phen matches the excitation energy of Sm3+ ion. When the excitation energy is absorbed by Phen, the energy is transferred to the 4 G5/2 energy level of the Sm3+ ion, and the photons are emitted when the ions relax from the 4 G5/2 level to the 6 HJ (where J = 5/2, 7/2 and 9/2) level, which accounts for the bands observed. Furthermore, another
Fig. 4. Excitation and emission spectra of the complex Sm(Sal)3 (Phen) (a) λem = 600 nm and (b) λex = 350 nm.
Fig. 5. Excitation and emission spectra of the complex Sm(Sal)3 (H2 O)2 (c) λem = 600 nm and (d) λex = 350 nm.
264.1 ± 0.2 nm, which indicates that the fluorescence properties of the ternary complex are mainly determined by the energy transfer from Phen to the Sm3+ ion. This is different from that of the binary complex Sm(Sal)3 (H2 O)2 whose energy transfer is from HSal to the rare earth ion. As shown in the figures, no obvious changes of wavelength and band shape are found between the spectra of the complexes and that of the ligands except slight red shift, which confirms that the UV spectra of the complexes reflect an essentially absorption of the ligands. The red shift of the band can be explained by the fact that the nitrogen atoms of Phen in Sm(Sal)3 (Phen) contribute few electrons to the formation of the coordination bond with Sm3+ ion. This means that both HSal and Phen took part in the formation of the complex Sm(Sal)3 (Phen), not HSal alone nor Phen alone. 3.4. Fluorescence spectra The excitation and emission spectra of the complexes Sm(Sal)3 (Phen) and Sm(Sal)3 (H2 O)2 are shown in Figs. 4 and 5, respectively. As seen from the figures, the excitation spectrum of the Sm(Sal)3 (Phen) composite is different from that of Sm(Sal)3 (H2 O)2 . The latter has a small and narrow band centered at 209 nm while the former possesses a strong and wide band centered around 350 nm and two weak absorption bands near 380–390 nm due to the 4 D1/2 and 4G 3+ 11/2 levels of Sm ion. The excitation spectra demonstrate 3+ that the Sm ion can be well excited in a relatively broad wavelength range, and help to confirm that both HSal and Table 2 Luminescence intensity of the complexes Complexes
Sm(Sal)3 (H2 O)2 Sm(Sal)3 (Phen)
4G
5/2
→ 6 H5/2
4G
5/2
→ 6 H7/2
4G
5/2
→ 6 H9/2
λ (nm)
Intensity
λ (nm)
Intensity
λ (nm)
Intensity
560.2 565.4
1.6 154.2
596.2 601.8
1.4 265.0
640.4 644.6
0.6 110.7
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Acknowledgements The authors thank the Science Planning Project of Zhejiang Province (No. 2003G10006), the Science and Technology Developmental Plan of Hangzhou City (No. 2003131E04), and the National High Technology Research Developmental Plan (No. 2003AA302760).
Fig. 6. Energy level diagram relating the triplet level (T) of the ligands and some energy levels of Sm3+ ion.
energy transfer process may exist simultaneously that the excitation energy is first absorbed by HSal, then transferred to Phen in the coordination sphere, finally to the Sm3+ ion, as described similarly in ternary system reported in the literature [16]. Thus high efficiency of intramolecular energy transfer, and the more intense fluorescence are obtained.
4. Conclusions A novel high photoluminescence ternary complex Sm(Sal)3 (Phen) has been successfully prepared. The elemental analysis, IR, UV–vis and fluorescence spectra confirmed the formation of the complex. The luminescence intensity of Sm(Sal)3 (Phen) is over two hundred times as high as that of Sm(Sal)3 (H2 O)2 through the introduction of the second ligand Phen into the coordination sphere of Sm3+ . The high luminescence intensity in Sm(Sal)3 (Phen) indicates the efficient luminescence quenching due to OH oscillators in the inner coordination sphere of Sm3+ ion in the hydrated compound and the good sensitization of Phen for the luminescence of Sm3+ ion. The second ligand Phen acting as the light-harvesting center is involved in the highly efficient energy transfer process.
References [1] J.H. Yang, G.Y. Zhu, B. Wu, Anal. Chim. Acta. 198 (1987) 287. [2] G. Zhu, Z. Si, J. Yang, J. Ding, Anal. Chim. Acta 231 (1990) 157. [3] L.M. Perry, J.D. Winefordner, Anal. Chim. Acta 237 (1990) 273. [4] S. Peter, B.S. Panigrahi, K.S. Viswanathan, C.K. Mathews, Anal. Chim. Acta 260 (1992) 135. [5] B.S. Panigrahi, S. Peter, K.S. Viswanathan, C.K. Mathews, Anal. Chim. Acta 282 (1993) 117. [6] B.S. Panigrahi, S. Peter, K.S. Viswanathan, C.K. Mathews, Spectrochim. Acta, Part A 51A (1995) 2289. [7] Y.L. Zhao, F.Y. Zhao, J. Rare Earths 18 (4) (2000) 318. [8] H.F. Brito, V.R.L. Constantino, M.A. Bizeto, J. Alloys Comp. 311 (2) (2000) 159. [9] Y.H. Yang, Q. Cai, J.W. Meng, Sh.G. Hou, J. Lumin. 12 (2) (1991) 151. [10] Y.S. Yang, M.L. Gong, H.Y. Lei, J. Alloys Compd. 207/208 (1994) 112. [11] Y.H. Wan, X.J. Li, R.D. Yang, J. Rare Earths 13 (1) (1992) 4. [12] S.H. Hong, The Application of Spectral Analysis in Organic Chemistry, Science Publishing House, Beijing, 1981. [13] S. Salama, F.S. Richardson, J. Chem. Phys. 84 (1980) 512. [14] J.-C.G. B¨unzli, G.R. Choppin, Lathanide Probes in Life, Chemical and Earth Science, Elsevier, Amsterdam, 1989, p. 219. [15] S. Sato, M. Wada, Bull. Soc. Jpn. 43 (1970) 1961. [16] J.M. Hu, G.Q. Chen, Y.N. Zeng, Chin. J. Lumin. 11 (4) (1990) 300.
Photoluminescence enhancement of Sm3+ in the Sm3+–salicylic acid–o-phenanthroline ternary composite Cun-jin Xua,b , Hui Yanga,∗ , Fei Xiea , Xing-zhong Guoa a
Center of Nanometer Science and Technology, Zhejiang Unversity, Hangzhou 310027, PR China b Department of Chemistry, Hangzhou Normal College, Hangzhou 310012, PR China
Received 16 June 2004; received in revised form 1 September 2004; accepted 10 September 2004 Available online 11 November 2004
Abstract The ternary Sm(III) complex with the salicylic acid (HSal) and o-phenanthroline (Phen) has been prepared. The photoluminescence intensity of the complex Sm(Sal)3 (Phen) is enhanced by more than two hundred times with respect to that of the binary complex Sm(Sal)3 (H2 O)2 . It is confirmed by Sm(III) content titration and the spectra of IR, UV–vis and fluorescence that the second ligand, Phen, also coordinates with the Sm3+ ion. The second ligand Phen shows enhancement effect on the fluorescence of the complex. These observations are discussed in detail and the results are rationalized. © 2004 Elsevier B.V. All rights reserved. Keywords: Salicylic acid; 1,10-Phenanthroline; Fluorescence enhancement; Sm3+ composite
1. Introduction There has been a growing interest in the study of the enhancement effect of fluorescence of the lanthanides since the lanthanides have low absorptivities and poor quantum yields. The fluorescence enhancement has generally been achieved through ligand sensitization [1–3]. In this process, an organic ligand is first excited by light absorption, followed by energy transfer from the ligand to the excited levels of a lanthanide. The photons are then emitted by the decay from the excited levels to its ground level. If the intramolecular energy transfer from the ligand to the lanthanide is efficient, the upper emitting level of the lanthanide is more efficiently pumped by this technique than by a direct excitation, resulting in an enhanced lanthanide fluorescence. It has been reported that aromatic carboxylic acids also serve as excellent ligands to sensitize the fluorescence of lanthanides efficiently [4–7]. Among the rare earth ions, those compounds with trivalent europium and terbium ions were the most thoroughly investigated, owing to their intrinsic electronic spectroscopic properties. However, the Eu3+ ion has attracted more atten∗
Corresponding author. Tel.: +86 571 87951408; fax: +86 571 87953054. E-mail address: [email protected] (H. Yang).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.09.031
tion than the Tb3+ ion probably due to the facilities in the interpretation of its energy level structure, considering that the former has a non-degenerate emitting level (5 D0 ) [8], while the Tb3+ ion has an emitting state (5 D4 ) multi-degenerate. On the other hand, the compounds containing the Sm3+ ion have been less studied than those with Eu3+ and Tb3+ ions, taking into consideration that the trivalent samarium ion in general presents low luminescence intensity. In this paper, we have initiated studies of the photoluminescence properties of the Sm(Sal)3 (H2 O)2 and the Sm(Sal)3 (Phen) composites for the first time. The luminescence intensity of the latter novel composite is more than two hundred times as high as that of the composite Sm(Sal)3 (H2 O)2 . It is believed that the second ligand may displace the coordinating H2 O and increase the molecular rigidity to attain this high efficiency.
2. Experimental 2.1. Apparatus and reagents The purity of Sm2 O3 exceeding 99.9% was used, and all other chemicals were analytical reagent grade. Element analysis of carbon, hydrogen and nitrogen was performed with
C.-j. Xu et al. / Journal of Alloys and Compounds 392 (2005) 96–99
97
a Flash-EA1112 elemental analyzer. Infrared spectra were measured at room temperature in 400–4000 cm−1 region using a Bruker Tensor 27 FT–IR spectrophotometer with KBr pellet technique. UV spectra were obtained by a Unico UV2102 PC spectrophotometer. The excitation and emission spectra were recorded in a Hitachi-F4500 spectrofluorometer. 2.2. Preparation of complexes The binary composite Sm(Sal)3 (H2 O)2 was synthesized using a method similar to the literature [9]. The ternary complex Sm(Sal)3 (Phen) was synthesized as follows: salicylic acid (1.5 mmol) and o-phenanthroline (0.5 mmol) were dissolved in 95% C2 H5 OH respectively. The pH value of the salicylic acid C2 H5 OH solution was adjusted to the range of 6–7 with NaOH solution (1 mol L−1 ). The C2 H5 OH solutions of two ligands were mixed and then the mixture was added dropwise to the ethanolic SmCl3 (0.5 mmol) solution, which was stirred at temperature 80 ◦ C until the formation of the pale-yellow precipitate. The solid products were recrystallized from acetone and dried under vacuum over anhydrous calcium chloride in the desiccator at room temperature.
3. Results and discussion 3.1. Compositions of title complexes Lanthanide ions were analyzed by complexometeric titration with EDTA. The elemental analysis data of C, H, N and rare earth are listed in Table 1. From the Table 1, it can be seen that the composition of the complexes is Sm(Sal)3 (H2 O)2 and Sm(Sal)3 (Phen), respectively.
Fig. 1. IR spectra of Sm(Sal)3 (Phen) (a), HSal (b), NaSal (c) and Phen (d).
of the hydroxyl group to metal ion is occurring through the oxygen [11]. The C N and C H stretching vibration peaks appearing at 1589, 739 and 853 cm−1 in o-phenanthroline, respectively shift to about 1561, 729 and 847 cm−1 in the complex, which indicates that the chemical bonds are formed between rare earth ion and nitrogen atoms of o-phenanthroline. 3.3. UV spectra The UV absorption spectra of the ligands and the complexes in ethanol solution are given in Figs. 2 and 3, respectively. It can be seen that the absorption peaks of o-phenanthroline and salicylic acid in ultraviolet region appearing at 229.2 ± 0.2, 264.1 ± 0.2 nm and 236.3 ± 0.2, 305.3 ± 0.2 nm, respectively, are assigned to the band of * transitions of aromatic ring, which belongs to K band [12]. According to the graphs given below, the UV absorption of the complex is dominated by the absorption of Phen for the fact that HSal only presents a shoulder at 305.3 ± 0.2 nm while Phen still shows its characteristic absorption peak at
3.2. IR absorption spectra Fig. 1 gives IR transmission spectra (400–4000 cm−1 ) of Sm(Sal)3 (Phen), HSal, NaSal and Phen. The characteristic absorption peaks of –COOH, appearing at 2860.0, 2594.9, 1444.2 cm−1 in HSal, disappear in the complex Sm(Sal)3 (Phen). The asymmetric stretching vibrations and the symmetric stretching vibrations of carboxylate group in sodium salicylate, appearing at 1582.2 and 1377.4 cm−1 , are shifted to 1597.2 and 1389.2 cm−1 on formation of the complex, which indicates that for the complex, carboxylic group may belong to mondendate coordination [10]. The displacement of the νAr OH stretching from ∼1296 cm−1 in NaSal to ∼1222 cm−1 in the complex suggesting that the coordination
Fig. 2. UV absorption spectra of Phen (a) and HSal (b).
Table 1 Data of elemental analysis (%) of title complexesa Complexes
C
H
N
Sm
Sm(Sal)3 (H2 O)2 42.03 (42.19) 1.09 (1.07) 0.00 (0.00) 24.92 (25.11) Sm(Sal)3 (Phen) 53.28 (53.37) 3.00 (3.10) 3.52 (3.77) 19.89 (20.27) a
Numbers in brackets are calculated values.
Fig. 3. UV absorption spectra of Sm(Sal)3 (Phen) (c) and Sm(Sal)3 (H2 O)2 (d).
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Phen are present in the coordination sphere and may let the Sm(Sal)3 (Phen) composite be efficiently excited by UV light. As shown in the figures, the emission spectra of the complexes consist of narrow bands by the 4 G5/2 → 6 HJ transitions (where J = 5/2, 7/2 and 9/2) of the Sm3+ ion. The emission bands of the complex Sm(Sal)3 (H2 O)2 are shifted slightly and there is a different distribution of band intensities compared to that of Sm(Sal)3 (Phen). Table 2 shows the luminescence intensity of the two complexes. From Table 2, it is observed that the second ligand Phen can sensitize the luminescence of Sm3+ ion efficiently and the luminescence intensity of Sm(Sal)3 (Phen) is more than two hundred times as high as that of Sm(Sal)3 (H2 O)2 through the introduction of the second ligand Phen into the coordination sphere of Sm3+ by releasing precoordinated water molecules. This is due to the reasons of both the presence of O H vibrations in the complex Sm(Sal)3 (H2 O)2 , which contribute to non-radiative deactivation of the 4 G5/2 emitting state of the Sm3+ ion [13] and the increase in molecular rigidity through the introduction of Phen in the complex Sm(Sal)3 (Phen) [10]. In Sm(Sal)3 (H2 O)2 , the hydroxyl group in H2 O takes part in the coordination and destroys the planar molecular structure of the complex. Therefore, the efficiency of energy transfer from the excited ligand to Sm3+ is lowered, and the luminescence of this complex becomes weaker. On the other hand, the introduction of Phen, an excellent light-harvesting center, into the coordination sphere may result in a more asymmetric coordination geometry than that of the Sm(Sal)3 (H2 O)2 composite. The formation of an unique asymmetric environment surrounding the Sm3+ ion leads to an increased transition probability for the fluorescence, and results in greatly enhanced fluorescence of the Sm3+ ion [14]. The fact that the second ligand Phen sensitizes luminescence of Sm3+ ion can also be explained by the idea given by Sato and Wada [15], who indicated that the fluorescence yield would be decreased either because of the thermal deactivation process while the energy gap was small, or due to the diminution in energy overlap while the energy gap was large. Accordingly, the fluorescence yield shows a maximum for a proper energy gap between T (L) and 4 G5/2 (Sm3+ ) levels, as shown in Fig. 6. From the Fig. 6, we can see that the triplet state of Phen matches the excitation energy of Sm3+ ion. When the excitation energy is absorbed by Phen, the energy is transferred to the 4 G5/2 energy level of the Sm3+ ion, and the photons are emitted when the ions relax from the 4 G5/2 level to the 6 HJ (where J = 5/2, 7/2 and 9/2) level, which accounts for the bands observed. Furthermore, another
Fig. 4. Excitation and emission spectra of the complex Sm(Sal)3 (Phen) (a) λem = 600 nm and (b) λex = 350 nm.
Fig. 5. Excitation and emission spectra of the complex Sm(Sal)3 (H2 O)2 (c) λem = 600 nm and (d) λex = 350 nm.
264.1 ± 0.2 nm, which indicates that the fluorescence properties of the ternary complex are mainly determined by the energy transfer from Phen to the Sm3+ ion. This is different from that of the binary complex Sm(Sal)3 (H2 O)2 whose energy transfer is from HSal to the rare earth ion. As shown in the figures, no obvious changes of wavelength and band shape are found between the spectra of the complexes and that of the ligands except slight red shift, which confirms that the UV spectra of the complexes reflect an essentially absorption of the ligands. The red shift of the band can be explained by the fact that the nitrogen atoms of Phen in Sm(Sal)3 (Phen) contribute few electrons to the formation of the coordination bond with Sm3+ ion. This means that both HSal and Phen took part in the formation of the complex Sm(Sal)3 (Phen), not HSal alone nor Phen alone. 3.4. Fluorescence spectra The excitation and emission spectra of the complexes Sm(Sal)3 (Phen) and Sm(Sal)3 (H2 O)2 are shown in Figs. 4 and 5, respectively. As seen from the figures, the excitation spectrum of the Sm(Sal)3 (Phen) composite is different from that of Sm(Sal)3 (H2 O)2 . The latter has a small and narrow band centered at 209 nm while the former possesses a strong and wide band centered around 350 nm and two weak absorption bands near 380–390 nm due to the 4 D1/2 and 4G 3+ 11/2 levels of Sm ion. The excitation spectra demonstrate 3+ that the Sm ion can be well excited in a relatively broad wavelength range, and help to confirm that both HSal and Table 2 Luminescence intensity of the complexes Complexes
Sm(Sal)3 (H2 O)2 Sm(Sal)3 (Phen)
4G
5/2
→ 6 H5/2
4G
5/2
→ 6 H7/2
4G
5/2
→ 6 H9/2
λ (nm)
Intensity
λ (nm)
Intensity
λ (nm)
Intensity
560.2 565.4
1.6 154.2
596.2 601.8
1.4 265.0
640.4 644.6
0.6 110.7
C.-j. Xu et al. / Journal of Alloys and Compounds 392 (2005) 96–99
99
Acknowledgements The authors thank the Science Planning Project of Zhejiang Province (No. 2003G10006), the Science and Technology Developmental Plan of Hangzhou City (No. 2003131E04), and the National High Technology Research Developmental Plan (No. 2003AA302760).
Fig. 6. Energy level diagram relating the triplet level (T) of the ligands and some energy levels of Sm3+ ion.
energy transfer process may exist simultaneously that the excitation energy is first absorbed by HSal, then transferred to Phen in the coordination sphere, finally to the Sm3+ ion, as described similarly in ternary system reported in the literature [16]. Thus high efficiency of intramolecular energy transfer, and the more intense fluorescence are obtained.
4. Conclusions A novel high photoluminescence ternary complex Sm(Sal)3 (Phen) has been successfully prepared. The elemental analysis, IR, UV–vis and fluorescence spectra confirmed the formation of the complex. The luminescence intensity of Sm(Sal)3 (Phen) is over two hundred times as high as that of Sm(Sal)3 (H2 O)2 through the introduction of the second ligand Phen into the coordination sphere of Sm3+ . The high luminescence intensity in Sm(Sal)3 (Phen) indicates the efficient luminescence quenching due to OH oscillators in the inner coordination sphere of Sm3+ ion in the hydrated compound and the good sensitization of Phen for the luminescence of Sm3+ ion. The second ligand Phen acting as the light-harvesting center is involved in the highly efficient energy transfer process.
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