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Fluorescence enhancement of samarium complex co-doped with terbium complex in a poly(methyl methacrylate) matrix
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 317–319
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Fluorescence enhancement of samarium complex co-doped with terbium complex in a poly(methyl methacrylate) matrix Hongfang Jiu a,, Lixin Zhang b, Guode Liu a, Tao Fan b a b
Department of Chemistry, North University of China, Taiyuan 030051, PR China Department of Chemical Engineering, North University of China, Taiyuan 030051, PR China
a r t i c l e in fo
abstract
Article history: Received 13 July 2008 Received in revised form 12 October 2008 Accepted 28 October 2008 Available online 11 November 2008
The fluorescence property of Sm(DBM)3phen- (DBM—dibenzoylmethide, phen—1,10-phenanthroline) and Tb(DBM)3phen-co-doped poly(methyl methacrylate) (PMMA) was investigated. The excitation, emission spectra and fluorescence lifetime of the co-doped samples were examined. In the co-doped samples, the luminescence intensities of Sm3+ enhance with an increase of the Tb(DBM)3phen content and with a decrease of the Sm(DBM)3phen content. The reason for the fluorescence enhancement effect in the co-doped polymer is the intermolecular energy transfer. To give a vivid picture for this co-doped system, a model for the fluorescence enhancement of Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA is presented. & 2008 Elsevier B.V. All rights reserved.
Keywords: Polymers and organics Energy transfer Fluorescence enhancement
1. Introduction Lanthanide organic complexes have received great interest due to their intense emission peaks (half-maximum width o10 nm) in the visible and near-infrared region under UV excitation. As lightconversion units, these complexes were used in a variety of areas such as fluoroimmunoassays [1,2], energy-harvesting devices [3], optical signal amplification [4,5], etc. The luminescence of rare-earth ions stems from the intra-4f transitions, which in principle are forbidden transitions, resulting in relatively low emission efficiency. An effective approach to increase the luminescent efficiency is to modify the complexes with different kinds of ligands that have broad and intense absorption bands [6–8], so that the absorbed photon energy has a large chance to excite the rare-earth ions. The other method to resolve this problem is to mix the complexes with different lanthanide ions [9–10]. It was reported that adding certain nonfluorescing lanthanide ions, such as La3+, Gd3+, Tb3+ and Y3+, could significantly enhance the photoluminescence of the chelates of Eu3+, Tb3+ and Sm3+. This type of fluorescence enhancement is actually an intrinsic fluorescence phenomenon referred to as the ‘‘co-fluorescence’’ effect [11–13]. The effect can be found in coprecipitates, chelate suspensions and Langmuir–Blodgett films [14–16]. The incorporation of lanthanide complexes in polymer matrix has attracted much attention because such a composite possesses
advantages of the luminescence characteristics of lanthanide ions and the excellent mechanical properties of plastics such as light weight, good transparency, impact resistance, low-temperature processability and so on. Poly(methyl methacrylate) (PMMA) has often been used as a polymer matrix for lanthanide complexes. A systematic study on the co-fluorescence effect caused by codoped of Tb(DBM)3phen (DBM—dibenzoylmethide, phen—1,10phenanthroline) and Eu(DBM)3phen(Sm(DBM)3phen) in PMMA has been performed by the combinatorial method. In our previous work, large collections of compounds were synthesized and screened in a materials library simultaneously for a particular physical or chemical property [17–19]. Moreover, a detailed study on absorption and luminescence spectra of Eu(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA is presented to understand the energy transfer between complexes [20]. However, the understanding of fluorescence enhancement of Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA still needs more work. In this paper, a detailed study on excitation and emission spectra of Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA is presented to reveal the mechanism in the co-doped system. Under this circumstance, a series of Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA were prepared with different Sm/Tb contents, and the analysis of their excitation and emission was carried out.
2. Experimental Corresponding author. Tel.: +86 351 3921414; fax: +86 351 3921414.
E-mail address: [email protected] (H. Jiu). 0022-2313/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.10.015
Lanthanide complexes (Ln(DBM)3phen, Ln3+ ¼ Sm3+, Tb3+) were synthesized according to the procedure reported before
ARTICLE IN PRESS 318
H. Jiu et al. / Journal of Luminescence 129 (2009) 317–319
[21]. The central ion (Ln3+) is bound to three ligands of DBM ions. Phen acts as a synergic shielding ligand that can reduce the rate of nonradiative decays and enhance the luminescence intensity of the complex strongly [22]. The final products were recrystallized in acetone/petroleum ether (2:1). Narrow-dispersed PMMA (Mwo350,000) was purchased from Acros Chemical company and used as received. The rare-earth complexes and PMMA were dissolved in cyclopentanone solvent at a concentration of 5 g/L. To study the fluorescence-enhancing mechanism, individual films of Sm0.05:Tbx (x ¼ 0, 0.1, 0.2, 0.4, 0.6 and 0.8) were prepared at a spin speed of 2500 revolutions per minute (rpm) and the film thickness was about 20 nm. X is defined as the weight ratio of the rare-earth complexes to PMMA. The photoluminescence measurement was performed on a FLUOROLOG-3-TAU steady-state/lifetime spectrofluorophotometer.
3. Results and discussion Fig. 1 shows the excitation spectra of the co-doped samples monitored at 648 nm. It can be seen that there is a broad strong UV band from 310 to 440 nm for DBM. The excitation intensity increases with an increase of DBM content in the co-doped samples. The emission spectra of the co-doped samples were recorded from 375 to 675 nm under excitation at 350 nm as shown in Fig. 2. The whole spectra are composed of three parts. Firstly, the broad band emission centered at 438 nm is ascribed to the DBM ligand. Secondly, the emission centered at 489 and 545 nm can be attributed to 5D4-7F6 and 5D4-7F5 transitions of Tb3+, respectively. Finally, there are three emission peaks, which are centered at 560, 600 and 648 nm, and can be attributed to the 4G5/2-6H5/2, 4 G5/2-6H7/2 and 4G5/2-6H9/2 transitions of Sm3+, respectively. The intensity of the peaks assigned to DBM and Tb3+ decreased with an increase of the Tb(DBM)3phen content (see the left-hand side of Fig. 2). Conversely, the intensity assigned to Sm3+ enhances with a decrease of Sm(DBM)3phen in the co-doped PMMA (see the right-hand side of Fig. 2). To study the fluorescence enhancement mechanism in the codoped system, the fluorescence decay curves and fluorescence
Fig. 2. Emission spectra of co-doped samples. (1) Sm0.05Tb0, (2) Sm0.05Tb0.1, (3) Sm0.05Tb0.2, (4) Sm0.05Tb0.4, (5) Sm0.05Tb0.6 and (6) Sm0.05Tb0.8.
Fig. 3. Relationship between the fluorescence lifetimes of Sm3+ and the content of Tb(DBM)3phen.
Fig. 1. Excitation spectra of co-doped samples monitored at 648 nm. (1) Sm0.05Tb0, (2) Sm0.05Tb0.1, (3) Sm0.05Tb0.2, (4) Sm0.05Tb0.4; (5) Sm0.05Tb0.6 and (6) Sm0.05Tb0.8.
lifetimes of Sm3+ are measured. The relationship between the fluorescence lifetimes of Sm3+ calculated by the exponential equation and the content of the Tb(DBM)3phen is shown in Fig. 3. The lifetime of Sm3+ shows almost no changes whether Tb3+ is codoped or not. The above result suggests that no new complex is formed between Sm(DBM)3phen and Tb(DBM)3phen in PMMA, and intermolecular energy transfer occurs between Sm(DBM)3phen and Tb(DBM)3phen [23,24]. According to the Dexter’s theory [25], the suitability of the energy gap between the resonance level of Ln3+ and the triplet state of ligand is a critical factor for efficient energy transfer. If the energy gap is too big, the energy transfer rate constant decreases due to the diminution in the overlap between donor and acceptor. If the energy gap is too small, the efficiency of energy transfer decreases because of the thermal deexcitation process [26]. In the co-doped system, the singlet state of DBM undergoes a nonradiative transition to the triplet state, whose energy level is 20,520 cm1. This is close to the excited state energy level of Sm3+
ARTICLE IN PRESS H. Jiu et al. / Journal of Luminescence 129 (2009) 317–319
S1
S1
T
T 5D
4G 5/2
Ex
4
Ex Em
Em
S0 Ligand
Sm3+
Tb3+
S0 Ligand
Fig. 4. Model for relaxation and energy transfer processes of Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA.
(17,950 cm1 for 4G5/2 level) [27] and therefore an efficient intramolecular energy transfer can take place from DBM towards Sm3+(see the left-hand side of Fig. 4). After that, Sm3+ undergoes radiative transitions, which result in characteristic line-type emissions. The intramolecular energy transfer can explain the characteristic emissions of Sm3+ in the fluorescence spectra but not the changes of fluorescence intensity of Sm3+ with the addition of Tb3+ when Tb(DBM)3phen and Sm(DBM)3phen are co-doped in PMMA. It has been known that the triplet state energy level of DBM is 20,520 cm1, which is much closer to the resonance energy level of Tb3+ (20,400 cm1 for 5D4 level). The energy gap between DBM and Tb3+ is too small to effectively incur an intramolecular energy transfer because of the thermal deexcitation process. This is the reason that the characteristic emission intensity of Tb3+ is weak in the emission spectra. The dependence of luminescent intensity of Sm3+ on the content of Tb3+ complex is relevant to a triplet–triplet intermolecular energy transfer from DBM in Tb(DBM)3phen to that in Sm(DBM)3phen in PMMA [16]. Besides, another important factor that accounts for the fluorescence enhancing of Sm3+ luminescence is the energy transfer between Sm3+ and Tb3+. As schematically shown in Fig. 4, the resonance energy level of Sm3+ (4G5/2, 17,950 cm1) is lower than that of Tb3+ (5D4, 20,400 cm1). It is possible that Sm3+ in the mixed complex diverts a large portion of the energy from the 5D4 level of Tb3+, thus promoting the luminescence enhancing of Sm3+. The model for energy transfer processes is presented in Fig. 4 based on Refs. [28,29]. When these two complexes were co-doped into PMMA, intermolecular energy transfer would be included, which could result in fluorescence enhancement. According to the theories of Foster [30] and Dexter, energy can be transferred to molecules at short distances by an intermolecular energy transfer. The efficiency of the intermolecular energy transfer is dependent on close approach or contact of the donor to the acceptor. In the electrostatic model of energy transfer, its efficiency depends on a certain power of the reverse of the distance between donor and acceptor. Obviously, the efficiency of luminescence enhancement in Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA is also strongly dependent on the distance between the two kinds of complexes.
319
4. Summary Co-doped with Sm(DBM)3phen and Tb(DBM)3phen in PMMA, the luminescence intensity of Sm3+ increases with the decrease of Sm3+ content and the increase of the ligand and Tb3+ content. The fluorescence lifetime results show that no new complex is formed between Sm(DBM)3phen and Tb(DBM)3phen in the co-doped system. The reason of the fluorescence enhancement of Sm3+ is the two parts of intermolecular energy transfer, including from DBM ligand between Tb(DBM)3phen and Sm(DBM)3phen and from ions between Sm3+ and Tb3+. The efficiency of the luminescence enhancement in Sm(DBM)3phen and Tb(DBM)3phen-co-doped PMMA is strongly dependent on a certain power of the reverse of the distance between two kinds of complexes. As mentioned above, it is clearly seen that adding Tb(DBM)3phen to Sm(DBM)3phen is an efficient method to enhance luminescence intensity of a co-doped system. Based on this realization, we can enhance the luminescence of Sm(DBM)3phen by adding sensitization complexes.
Acknowledgements We appreciate the financial support from the ShanXi (20072194 and 20072185) Provincial Youth Technology Research Foundation of China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
I. Hemmila¨, Anal. Chem. 57 (1985) 1676. I. Hemmila¨, J. Alloys Compd. 225 (1995) 480. G.F. de Sa´, A.S. Jr, B.J.P. da Silva, E.F. da Silva Jr., Opt. Mater. 11 (1998) 23. L.H. Slooff, A. Polman, M.P.O. Wolbers, F.C.J.M. van Veggel, D.N. Reinhoudt, J.W. Hofstraat, J. Appl. Phys. 83 (1998) 497. H. Liang, Q. Zhang, Z. Zheng, H. Ming, Z. Li, J. Xu, B. Chen, H. Zhao, Opt. Lett. 29 (2004) 477. S. Maji, K.S. Viswanathan, J. Lumin. 128 (2008) 1255. F.G. Luciano, T.D.O. Kleber, R.N. Cla´udio, C.S.F. Paulo, J.D.B. Marcos, P.R. Ana, E.D.Z. Maria, A.S. Osvaldo, J. Lumin. 128 (2008) 1339. I.S. Muhammad, K. Eny, S. Bahruddin, A. Rohana, S.M. Abdussalam, M.Y. Bohari, J. Lumin. 126 (2007) 871. P.A. Tanner, Y.L. Liu, M. Chua, M.F. Reid, J. Alloys Compd. 207 (1994) 83. Q. Li, T. Li, J. Wu, J. Phys. Chem. B 105 (2001) 12293. M.J. Adams, J.G. Highfield, G.F. Kirkbright, Anal. Chem. 52 (1980) 1260. J.H. Yang, G.Y. Zhu, H. Wang, Anal. Chim. Acta 198 (1987) 287. Y.X. Ci, Z.H. Lan, Analyst 113 (1988) 1453. Y.X. Ci, Z.H. Lan, Anal. Lett. 21 (1988) 1499. S. Peter, B.S. Panigrahi, K.S. Viswanathan, C.K. Mathews, Anal. Chim. Acta 260 (1992) 135. G.L. Zhong, Y.H. Wang, C.K. Wang, K.Z. Yang, Thin Solid Films 385 (2001) 234. J.J. Ding, H.F. Jiu, J. Bao, J.C. Lu, W.R. Gui, Q.J. Zhang, C. Gao, J. Comb. Chem. 7 (2005) 69. H.F. Jiu, J.J. Ding, Y.Y. Sun, J. Bao, C. Gao, Q.J. Zhang, J. Non-Cryst. Solids 352 (2006) 197. H.F. Jiu, J.J. Ding, J. Bao, Q.J. Zhang, C. Gao, Spectrochim. Acta A 61 (2005) 3150. Y.H. Luo, Q. Yan, S. Wu, W.X. Wu, Q.J. Zhang, J. Photochem. Photobiol. A 191 (2007) 91. L.R. Melby, N.J. Rose, E. Abramson, J.C. Caris, J. Am. Chem. Soc. 86 (1964) 5117. J. Erostya´k, A. Buza´dy, A. Kasza´s, L. Kozma, I. Hornya´k, J. Lumin. 72–74 (1997) 570. M.Q. Wang, G.D. Qian, S.Z. Lu, Mater. Sci. Eng. B 55 (1998) 119. T. Berthoud, P. Decambox, B. Kirsch, P. Mauchien, C. Moulin, Anal. Chem. Acta 200 (1989) 235. D.L. Dexter, J. Chem. Phys. 21 (1953) 836. W.R. Dawson, J. Chem. Phys. 45 (1966) 2410. P.R. Biju, G. Jose, V. Thomas, V.P.N. Nampoori, N.V. Unnikrishnan, Opt. Mater. 24 (2004) 671. Y. Yang, Q. Su, G. Zhao, J. Mol. Struct. 525 (2000) 47. Y.S. Yang, M.L. Gong, Y.Y. Li, H.Y. Lei, S.L. Wu, J. Alloys Compd. 207 (1994) 112. T. Forster, Ann. Phys. 2 (1948) 55.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Fluorescence enhancement of samarium complex co-doped with terbium complex in a poly(methyl methacrylate) matrix Hongfang Jiu a,, Lixin Zhang b, Guode Liu a, Tao Fan b a b
Department of Chemistry, North University of China, Taiyuan 030051, PR China Department of Chemical Engineering, North University of China, Taiyuan 030051, PR China
a r t i c l e in fo
abstract
Article history: Received 13 July 2008 Received in revised form 12 October 2008 Accepted 28 October 2008 Available online 11 November 2008
The fluorescence property of Sm(DBM)3phen- (DBM—dibenzoylmethide, phen—1,10-phenanthroline) and Tb(DBM)3phen-co-doped poly(methyl methacrylate) (PMMA) was investigated. The excitation, emission spectra and fluorescence lifetime of the co-doped samples were examined. In the co-doped samples, the luminescence intensities of Sm3+ enhance with an increase of the Tb(DBM)3phen content and with a decrease of the Sm(DBM)3phen content. The reason for the fluorescence enhancement effect in the co-doped polymer is the intermolecular energy transfer. To give a vivid picture for this co-doped system, a model for the fluorescence enhancement of Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA is presented. & 2008 Elsevier B.V. All rights reserved.
Keywords: Polymers and organics Energy transfer Fluorescence enhancement
1. Introduction Lanthanide organic complexes have received great interest due to their intense emission peaks (half-maximum width o10 nm) in the visible and near-infrared region under UV excitation. As lightconversion units, these complexes were used in a variety of areas such as fluoroimmunoassays [1,2], energy-harvesting devices [3], optical signal amplification [4,5], etc. The luminescence of rare-earth ions stems from the intra-4f transitions, which in principle are forbidden transitions, resulting in relatively low emission efficiency. An effective approach to increase the luminescent efficiency is to modify the complexes with different kinds of ligands that have broad and intense absorption bands [6–8], so that the absorbed photon energy has a large chance to excite the rare-earth ions. The other method to resolve this problem is to mix the complexes with different lanthanide ions [9–10]. It was reported that adding certain nonfluorescing lanthanide ions, such as La3+, Gd3+, Tb3+ and Y3+, could significantly enhance the photoluminescence of the chelates of Eu3+, Tb3+ and Sm3+. This type of fluorescence enhancement is actually an intrinsic fluorescence phenomenon referred to as the ‘‘co-fluorescence’’ effect [11–13]. The effect can be found in coprecipitates, chelate suspensions and Langmuir–Blodgett films [14–16]. The incorporation of lanthanide complexes in polymer matrix has attracted much attention because such a composite possesses
advantages of the luminescence characteristics of lanthanide ions and the excellent mechanical properties of plastics such as light weight, good transparency, impact resistance, low-temperature processability and so on. Poly(methyl methacrylate) (PMMA) has often been used as a polymer matrix for lanthanide complexes. A systematic study on the co-fluorescence effect caused by codoped of Tb(DBM)3phen (DBM—dibenzoylmethide, phen—1,10phenanthroline) and Eu(DBM)3phen(Sm(DBM)3phen) in PMMA has been performed by the combinatorial method. In our previous work, large collections of compounds were synthesized and screened in a materials library simultaneously for a particular physical or chemical property [17–19]. Moreover, a detailed study on absorption and luminescence spectra of Eu(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA is presented to understand the energy transfer between complexes [20]. However, the understanding of fluorescence enhancement of Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA still needs more work. In this paper, a detailed study on excitation and emission spectra of Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA is presented to reveal the mechanism in the co-doped system. Under this circumstance, a series of Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA were prepared with different Sm/Tb contents, and the analysis of their excitation and emission was carried out.
2. Experimental Corresponding author. Tel.: +86 351 3921414; fax: +86 351 3921414.
E-mail address: [email protected] (H. Jiu). 0022-2313/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.10.015
Lanthanide complexes (Ln(DBM)3phen, Ln3+ ¼ Sm3+, Tb3+) were synthesized according to the procedure reported before
ARTICLE IN PRESS 318
H. Jiu et al. / Journal of Luminescence 129 (2009) 317–319
[21]. The central ion (Ln3+) is bound to three ligands of DBM ions. Phen acts as a synergic shielding ligand that can reduce the rate of nonradiative decays and enhance the luminescence intensity of the complex strongly [22]. The final products were recrystallized in acetone/petroleum ether (2:1). Narrow-dispersed PMMA (Mwo350,000) was purchased from Acros Chemical company and used as received. The rare-earth complexes and PMMA were dissolved in cyclopentanone solvent at a concentration of 5 g/L. To study the fluorescence-enhancing mechanism, individual films of Sm0.05:Tbx (x ¼ 0, 0.1, 0.2, 0.4, 0.6 and 0.8) were prepared at a spin speed of 2500 revolutions per minute (rpm) and the film thickness was about 20 nm. X is defined as the weight ratio of the rare-earth complexes to PMMA. The photoluminescence measurement was performed on a FLUOROLOG-3-TAU steady-state/lifetime spectrofluorophotometer.
3. Results and discussion Fig. 1 shows the excitation spectra of the co-doped samples monitored at 648 nm. It can be seen that there is a broad strong UV band from 310 to 440 nm for DBM. The excitation intensity increases with an increase of DBM content in the co-doped samples. The emission spectra of the co-doped samples were recorded from 375 to 675 nm under excitation at 350 nm as shown in Fig. 2. The whole spectra are composed of three parts. Firstly, the broad band emission centered at 438 nm is ascribed to the DBM ligand. Secondly, the emission centered at 489 and 545 nm can be attributed to 5D4-7F6 and 5D4-7F5 transitions of Tb3+, respectively. Finally, there are three emission peaks, which are centered at 560, 600 and 648 nm, and can be attributed to the 4G5/2-6H5/2, 4 G5/2-6H7/2 and 4G5/2-6H9/2 transitions of Sm3+, respectively. The intensity of the peaks assigned to DBM and Tb3+ decreased with an increase of the Tb(DBM)3phen content (see the left-hand side of Fig. 2). Conversely, the intensity assigned to Sm3+ enhances with a decrease of Sm(DBM)3phen in the co-doped PMMA (see the right-hand side of Fig. 2). To study the fluorescence enhancement mechanism in the codoped system, the fluorescence decay curves and fluorescence
Fig. 2. Emission spectra of co-doped samples. (1) Sm0.05Tb0, (2) Sm0.05Tb0.1, (3) Sm0.05Tb0.2, (4) Sm0.05Tb0.4, (5) Sm0.05Tb0.6 and (6) Sm0.05Tb0.8.
Fig. 3. Relationship between the fluorescence lifetimes of Sm3+ and the content of Tb(DBM)3phen.
Fig. 1. Excitation spectra of co-doped samples monitored at 648 nm. (1) Sm0.05Tb0, (2) Sm0.05Tb0.1, (3) Sm0.05Tb0.2, (4) Sm0.05Tb0.4; (5) Sm0.05Tb0.6 and (6) Sm0.05Tb0.8.
lifetimes of Sm3+ are measured. The relationship between the fluorescence lifetimes of Sm3+ calculated by the exponential equation and the content of the Tb(DBM)3phen is shown in Fig. 3. The lifetime of Sm3+ shows almost no changes whether Tb3+ is codoped or not. The above result suggests that no new complex is formed between Sm(DBM)3phen and Tb(DBM)3phen in PMMA, and intermolecular energy transfer occurs between Sm(DBM)3phen and Tb(DBM)3phen [23,24]. According to the Dexter’s theory [25], the suitability of the energy gap between the resonance level of Ln3+ and the triplet state of ligand is a critical factor for efficient energy transfer. If the energy gap is too big, the energy transfer rate constant decreases due to the diminution in the overlap between donor and acceptor. If the energy gap is too small, the efficiency of energy transfer decreases because of the thermal deexcitation process [26]. In the co-doped system, the singlet state of DBM undergoes a nonradiative transition to the triplet state, whose energy level is 20,520 cm1. This is close to the excited state energy level of Sm3+
ARTICLE IN PRESS H. Jiu et al. / Journal of Luminescence 129 (2009) 317–319
S1
S1
T
T 5D
4G 5/2
Ex
4
Ex Em
Em
S0 Ligand
Sm3+
Tb3+
S0 Ligand
Fig. 4. Model for relaxation and energy transfer processes of Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA.
(17,950 cm1 for 4G5/2 level) [27] and therefore an efficient intramolecular energy transfer can take place from DBM towards Sm3+(see the left-hand side of Fig. 4). After that, Sm3+ undergoes radiative transitions, which result in characteristic line-type emissions. The intramolecular energy transfer can explain the characteristic emissions of Sm3+ in the fluorescence spectra but not the changes of fluorescence intensity of Sm3+ with the addition of Tb3+ when Tb(DBM)3phen and Sm(DBM)3phen are co-doped in PMMA. It has been known that the triplet state energy level of DBM is 20,520 cm1, which is much closer to the resonance energy level of Tb3+ (20,400 cm1 for 5D4 level). The energy gap between DBM and Tb3+ is too small to effectively incur an intramolecular energy transfer because of the thermal deexcitation process. This is the reason that the characteristic emission intensity of Tb3+ is weak in the emission spectra. The dependence of luminescent intensity of Sm3+ on the content of Tb3+ complex is relevant to a triplet–triplet intermolecular energy transfer from DBM in Tb(DBM)3phen to that in Sm(DBM)3phen in PMMA [16]. Besides, another important factor that accounts for the fluorescence enhancing of Sm3+ luminescence is the energy transfer between Sm3+ and Tb3+. As schematically shown in Fig. 4, the resonance energy level of Sm3+ (4G5/2, 17,950 cm1) is lower than that of Tb3+ (5D4, 20,400 cm1). It is possible that Sm3+ in the mixed complex diverts a large portion of the energy from the 5D4 level of Tb3+, thus promoting the luminescence enhancing of Sm3+. The model for energy transfer processes is presented in Fig. 4 based on Refs. [28,29]. When these two complexes were co-doped into PMMA, intermolecular energy transfer would be included, which could result in fluorescence enhancement. According to the theories of Foster [30] and Dexter, energy can be transferred to molecules at short distances by an intermolecular energy transfer. The efficiency of the intermolecular energy transfer is dependent on close approach or contact of the donor to the acceptor. In the electrostatic model of energy transfer, its efficiency depends on a certain power of the reverse of the distance between donor and acceptor. Obviously, the efficiency of luminescence enhancement in Sm(DBM)3phen- and Tb(DBM)3phen-co-doped PMMA is also strongly dependent on the distance between the two kinds of complexes.
319
4. Summary Co-doped with Sm(DBM)3phen and Tb(DBM)3phen in PMMA, the luminescence intensity of Sm3+ increases with the decrease of Sm3+ content and the increase of the ligand and Tb3+ content. The fluorescence lifetime results show that no new complex is formed between Sm(DBM)3phen and Tb(DBM)3phen in the co-doped system. The reason of the fluorescence enhancement of Sm3+ is the two parts of intermolecular energy transfer, including from DBM ligand between Tb(DBM)3phen and Sm(DBM)3phen and from ions between Sm3+ and Tb3+. The efficiency of the luminescence enhancement in Sm(DBM)3phen and Tb(DBM)3phen-co-doped PMMA is strongly dependent on a certain power of the reverse of the distance between two kinds of complexes. As mentioned above, it is clearly seen that adding Tb(DBM)3phen to Sm(DBM)3phen is an efficient method to enhance luminescence intensity of a co-doped system. Based on this realization, we can enhance the luminescence of Sm(DBM)3phen by adding sensitization complexes.
Acknowledgements We appreciate the financial support from the ShanXi (20072194 and 20072185) Provincial Youth Technology Research Foundation of China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
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