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Langmuir 1998, 14, 5502-5506
Luminescence Enhancement Effect of Y(TTA)3Phen on Europium(III) and Intermolecular Energy Transfer in Langmuir-Blodgett Films Guo-Lun Zhong† and Kong-Zhang Yang* Institute of Colloid and Interface Chemistry, Shandong University, Jinan 250100, People’s Republic of China Received October 27, 1997. In Final Form: April 3, 1998 The luminescent enhancement effect of Y(TTA)3Phen (TTA ) thenoyltrifluoroacetone; Phen ) 1,10phenanthroline) on Eu(III) in Langmuir-Blodgett (LB) films of rare earth complexes mixing with arachidic acid (AA) in a molar ratio of 1:1 and the intermolecular energy transfer from Y(TTA)3Phen to Eu(TTA)3Phen in the films were investigated in this paper. The compositions of studied films follow the formula of [x% Eu(TTA)3Phen + (100 - x)% Y(TTA)3Phen]:AA ) 1:1 (x ) 0, 0.05, 0.1, 0.2, 0.3, 0.5, 30, 50, 80, and 100; in molar percentage). Luminescence intensities of Eu(III) in the films, compared with those that are regulated to no Y(TTA)3Phen, were enhanced in varying degrees. They also reveal that the lower the molar percentage of Eu(TTA)3Phen that exists in the films, the higher the efficiency of luminescent enhancement that is observed. Meanwhile, the monolayer and multilayer films were characterized by Brewster angle microscopy (BAM), electron diffraction (ED), and UV-vis spectra. The experimental results of BAM and ED show that a crystallization, which corresponds to the plateau in the π-A isotherm, occurs along with compressing the monolayers. UV-vis spectra indicate that the complexes are transferred onto the substrate successfully. The research on the intermolecular energy transfer proves that the transfer occurs between molecules of Y(TTA)3Phen and Eu(TTA)3Phen within the distance of 30 Å and belongs to a short-range exchange energy transfer involving the triplet of the ligand.
Introduction Because of the high luminescent efficiency and long fluorescent lifetime of Eu(III)-β-diketone complexes, their intrinsic and enhanced luminescence have been paid more attention for the past 50 years.1-4 The enhanced luminescence is caused mainly by intra- or intermolecular energy transfer. The intramolecular energy transfer from ligands of a complex to the 5D0 and 5D1 levels of Eu(III) has been reported extensively.5-7 However, studies on the intermolecular energy transfer between rare earth complexes in Langmuir-Blodgett (LB) films were very few, and the mechanism of the process has not been confirmed clearly. Generally this phenomenon was studied in a micella system, and the process was considered as a triplet energy transfer or exciton migration.8-10 Additionally, the LB technique has been a powerful tool for studying molecular aggregation, energy transfer, and electron transfer, for it can control the arrangement of molecules effectively and regulate the composition of film artificially.11-15 When rare earth ions with optical char† Present address: Department of Chemistry, Hangzhou Teachers College, Hangzhou 310012, People’s Republic of China.
(1) Weissman, S. I. J. Chem. Phys. 1942, 10, 214. (2) Halverson, F.; Brinen, J. S.; Leto, J. R. J. Chem. Phys. 1964, 41, 2752. (3) Ci, Y. X.; Lan, Z. H. Analyst 1988, 113, 1453. (4) Li, W. L.; Yu, G.; Zhao, X. J. Alloys Compd. 1994, 206, 195. (5) Sato, S.; Wada, M. Bull. Chem. Soc. Jpn. 1970, 43, 1955. (6) Crosby, G. A.; Whan, R. E.; Alire, R. M. J. Chem. Phys. 1961, 34, 743. (7) Freeman, J. J.; Crosby, G. A. J. Phys. Chem. 1963, 67, 2717. (8) Li, W. L.; Li, W. L.; Yang, Y. H.; Wei, G. D.; Guan, J. S.; Tang, M. D. J. Chin. Rare Earch Soc. 1989, 7, 27. (9) Yang, J. H.; Zhu, G. Y.; Wu, B. Anal. Chim. Acta 1987, 198, 287. (10) Ci, Y. X.; Zhang, H. L. Anal. Lett. 1988, 21, 1499. (11) Kuhn, H. Pure Appl. Chem. 1981, 53, 2105. (12) Kuhn, H.; Mobius, D. Angew. Chem., Int. Ed. Engl. 1971, 10, 620. (13) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1990, 94, 516.
acteristics are incorporated into LB films, it would be beneficial in fabricating the functional molecular devices and studying the energy transfer. Huang et al. reported nonlinear optical LB films of rare earth complexes.16,17 In our previous work, the luminescent films of rare earthβ-diketone complexes were studied,18,19 and the enhanced luminescence of Eu(III) in films of Eu(TTA)3Phen with RE(TTA)3Phen (TTA ) thenoyltrifluoroacetone; Phen ) 1,10-phenanthroline; RE ) La,Gd, Tb, and Y) were discussed.20,21 As a systematic research, the luminescence enhancement effect of Y(TTA)3Phen on Eu(III) in LB films was investigated in this paper. Brewster angle microscopy (BAM), electron diffaction (ED), and UV-vis spectra were used for characterizing the monolayers and multilayer. The intermolecular energy transfer was also approached, and its mechanism is proposed tentatively. Experimental Section Apparatus. A British NIMA 2000 fully computer-controlled alternate round trough was used for drawing surface pressurearea (π-A) isotherms and depositing LB films. Luminescence intensities of the films were determined on a Hitachi 850 (14) Ito, S.; Ohmori, S.; Yamamoto, M. Macromolecules 1992, 25, 185. (15) Hisada, K.; Ito, S.; Yamamoto, M. Langmuir 1995, 11, 996. (16) Li, H.; Huang, C. H.; Zhao, X. S.; Xie, X. M.; Xu, L. G.; Li, T. K. Langmuir 1994, 10, 3794. (17) Wang, K. Z.; Huang, C. H.; Zhou, D. J.; Xu, G. X.; Xu, Y.; Liu, Y. Q.; Zhu, D. B.; Zhao, X. S.; Xie, X. M. Solid State Commun. 1995, 93, 189. (18) Qian, D. J.; Nakahara, H.; Fukuda, K.; Yang, K. Z. Langmuir 1995, 11, 4491. (19) Qian, D. J.; Nakahara, H.; Fukuda, K.; Yang, K. Z. Chem. Lett. 1995, 175. (20) Zhong, G. L.; Zhang, R. J.; Yang, K. Z.; Zhu, G. Y. Chin. Chem. Lett. 1996, 7, 273. (21) Zhong, G. L.; Feng, Y.; Yang, K. Z.; Zhu, G. Y. Chem. Lett. 1996, 775.
S0743-7463(97)01164-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/21/1998
Y(TTA)3Phen Luminescent Enhancement on Eu(III)
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Figure 2. π-A isotherms of LB 2 (a), LB 7, (b) and LB 9 (c). Figure 1. Molecular structure of the complexes. Table 1. Compositions of Studied Films LB films 1 2 3 4 5 6 7 8 9 10
film-forming materials Eu(TTA)3Phen:AA ) 1:1 Y(TTA)3Phen:AA ) 1:1 (0.05% Eu(TTA)3Phen + 99.95% Y(TTA)3Phen):AA ) 1:1 (0.1% Eu(TTA)3Phen + 99.9% Y(TTA)3Phen):AA ) 1:1 (0.2% Eu(TTA)3Phen + 99.8% Y(TTA)3Phen):AA ) 1:1 (0.3% Eu(TTA)3Phen + 99.7% Y(TTA)3Phen):AA ) 1:1 (0.5% Eu(TTA)3Phen + 99.5% Y(TTA)3Phen):AA ) 1:1 (30.0% Eu(TTA)3Phen + 70.0% Y(TTA)3Phen):AA ) 1:1 (50.0% Eu(TTA)3Phen + 50.0% Y(TTA)3Phen):AA ) 1:1 (80.0% Eu(TTA)3Phen + 20.0% Y(TTA)3Phen):AA ) 1:1
subphase a b c c c c c c c c
a The composite subphase was saturated with Eu(TTA) Phen, 3 TTA, and Phen. b The composite subphase was saturated with Y(TTA)3Phen, TTA, and Phen. c The composite subphase was saturated with Eu(TTA)3Phen, Y(TTA)3Phen, TTA, and Phen.
fluorescence spectrophotometer. UV-vis spectra were recorded on a Shimadzu UV-240 spectrometer. Powder X-ray diffractograms were recorded using a Rigaku D/Max-γA diffractometer with Cu KR radiation. Materials. Rare earth oxides (99.99%) were purchased from the Yue Long Chemical Plant (Shanghai, People’s Republic of China) and TTA from Fluka Chemical Corp.; Phen, AA, and other reagents were used without further purification. Syntheses of Eu(TTA)3Phen and Y(TTA)3Phen were carried out according to the literature.22 Figure 1 shows the molecular structure of the complexes. Powder X-ray diffractograms of Eu(TTA)3Phen and Y(TTA)3Phen indicate that the crystal structures of two complexes are similar. Film Formation and Deposition. Generally these complexes cannot form stable monolayers by themselves. It is necessary to mix them with AA and spread on a composite subphase that was saturated with film-forming material(s), TTA, and Phen to inhibit dissolution and dissociation of the complexes at the surface.23 The films and subphases are listed in Table 1. The corresponding chloroform solution of film-forming material with a given concentration was spread onto a subphase. A surface pressure of 20 mN/m was chosen for depositing at a speed of 10 mm/min. Hydrophobic glass substrates were used for fabricating LB films of a Y-type model. Transfer ratios were around unity.
Results and Discussion π-A Isotherms. The stable monolayers of studied films were obtained. π-A isotherms of these materials are similar fundamentally; three of them are shown in Figure 2. Each isotherm displays two regions of rapid increase in surface pressure. One appears between 0.3 and 8.4 mN/m. Then, the surface pressure changes slowly until the pressure amounts to about 9.5 mN/m. After (22) Bauer, H.; Blanc, J.; Rosss, D. L. J. Am. Chem. Soc. 1964, 86, 5125. (23) Gao, X.; Liu, H. G.; Zhang, R. J.; Yang, K. Z. Thin Solid Films 1996, 284-285, 39.
that another region of rapid increase appears. The monolayer collapses about 50 mN/m. From the isotherms an average area of 65 Å2/molecule for the complex can be calculated at the pressure of 20 mN/m.21 If the molecule of the complex is treated as a sphere, it would have a diameter of about 9 Å. Characterization of Films. Monolayers of LB 7 and 9 were observed by BAM in situ. These monolayers changed into rigid ones gradually with an increase in the surface pressure and collapsed at the pressure of 50 mN/ m. Then the barrier of the trough was opened. We could see that the rigid films were composed of even particles floating on the aqueous subphase. Figure 3 shows the images of BAM for LB 7 at different pressure. They indicate that the compressing process of the monolayer results in an irreversible phase change. The monolayer of LB 9 was transferred onto a Formvarcoated Cu grid for the experiment of ED. Figure 4 is the ED pattern at the surface pressure of 9 mN/m. Hexagonal diffraction spots illustrate that there were microcrystal particles in the monolayer. The plateau in π-A isotherms corresponds to a crystallization. The same process occurs in other monolayers. UV-vis spectra of TTA and Phen in CHCl3, a 10-layer LB 7 on a quartz substrate, and a chloroform spreading solution of LB 7 are shown in Figure 5. The π f π* electronic transfer of the thiophene ring with a peak at 268 nm remains unchanged either in TTA, spreading solution, or LB film.24 The absorption band of the π f π* electronic transfer of the carbonyl group at 320 nm in TTA red shifts to 335 nm in the spreading solution, which indicates that a coordination reaction has taken place on the oxygen atom in the carbonyl group to rare earth ions. In addition, one absorption band of π f π* electronic transfer of Phen at 235 nm is similar in Phen and the spreading solution. However, another band at 263 nm red shifts to 288 nm in the spreading solution and the LB film. This is the evidence of the coordination of the nitrogen atom in the heterocycle of Phen to rare earth ions. Moreover, the new peaks at 366 and 430 nm appear in the spectrum of the LB film, which may result from a highly ordered arrangement of molecules in the film.17 By the above-mentioned spectra of the 10-layer LB 7 and its chloroform spreading solution, it is clear that the complexes have been transferred onto the substrate successfully. In this kind of film the hydrogen bond between the -CF3 group and the -COOH group plays an important role in stabilizing the film.25 Luminescent Properties of LB Films. The excitation and emission spectra for the 10-layer LB 7 are shown in Figure 6. By the excitation spectrum, a wavelength of (24) Wang, K. Z.; Huang, C. H.; Yao, G. Q.; Xu, G. X.; Cui, D. F.; Fan, Y. Chem. J. Chin. Univ. 1993, 14, 150. (25) Qian, D. J.; Yang, K. Z. Acta Phys.-Chim. Sin. 1993, 9, 148.
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Figure 4. ED pattern for LB 9 at the pressure of 9 mN/m.
Figure 5. UV-vis spectra: (1) TTA in CHCl3; (2) Phen in CHCl3; (3) spreading solution of LB 7; (4) LB 7 (10-layer). (The small peaks at about 200 nm are caused by instrumental noise.)
Figure 3. Images of BAM for LB 7: (A) at 0 mN/m; (B) at 9 mN/m; (C) after collapsing.
350 nm is chosen for the excitation wavelength. The emission peaks of Eu(III) at 593, 613, and 642 nm are assigned to the transitions from the 5D0 level to the 7F1, 7 F2, and 7F3 levels, respectively. The strongest emission peak at 613 nm is selected for the following experiments. Figure 7 shows a linear relationship of LB 7 between the luminescence intensity and the number of layers. It means that the films are vertically homogeneous. Luminescent Enhancement Effect in LB Films. To prove the luminescent enhancement effect existing in LB films, 10-layer LB films 3-10 were fabricated and the luminescence intensities of these films were determined, respectively. To clarify the luminescence enhancement of the films, the luminescence intensity of 10-layer LB 1 is regarded as a criterion. When there is no Y(TTA)3Phen, the luminescence intensities of LB films 3-10 can be
Figure 6. Excitation (1) and emission (2) spectra of 10-layer LB 7.
calculated by eq 1. Meanwhile, F is defined as a factor of luminescent enhancement and expresses the efficiency of luminescent enhancement. It is a ratio of the luminescence intensity of the LB film that contains Y(TTA)3Phen
Y(TTA)3Phen Luminescent Enhancement on Eu(III)
Langmuir, Vol. 14, No. 19, 1998 5505
Figure 7. Plot of luminescence intensity against the number of layers of LB 7.
to that which does not and can be calculated from eq 2.
Ix0 ) Ix1xEu%
(1)
F ) IxLB/Ix0
(2)
where, Ix0 is the calculated luminescence intensity for LB films 3-10, when there is no Y(TTA)3Phen and the molar percentage of Eu(TTA)3Phen is x; Ix1 is the luminescence intensity of LB 1; xEu% is a molar percentage of Eu(TTA)3Phen in a mixture of complexes; IxLB is the measured luminescence intensity for LB films 3-10, respectively. The calculated values of F for different LB films are listed in Table 2. As seen from F, the lower molar percentage of Eu(TTA)3Phen exists in the films; the higher efficiency of luminescent enhancement is observed. It is evidence that the efficiency is dependent on the concentration of Eu(TTA)3Phen or Y(TTA)3Phen, but the different values of F for LB films 3-7 indicate that the energy migration occurs between the molecules of Y(TTA)3Phen in the condition of a large number of Y(TTA)3Phen. Otherwise, under the conditions of this work, we would obtain the same F. Additionally, it also can be seen, from the values of x1/xLB and Ix1/IxLB, for example, the ratio of x1/x3 is 2000, but the luminescence intensity of LB 1 is merely 142.8 times stronger than that of LB 3. These results indicate strongly that the luminescence intensities of Eu(III) in LB films are enhanced by Y(TTA)3Phen. In this work, LB films 3-7 were made for studying the luminescent enhancement effect in films being of obvious efficiency of enhancement and LB 2 with subphase c for determining the blank value of luminescence intensity. However, the purpose of fabricating LB films 8-10 was to obtain films that have a stronger luminescence intensity than LB 1 by the effect. The result is not so satisfactory, being of a lower efficiency of energy transfer from Y(TTA)3Phen to Eu(TTA)3Phen.21 Only in the case of LB 10 is the luminescence intensity as strong as that of LB 1. To do this, one must substitute Eu(III) by Y(III), a more common element; the other is to make the dispersion of luminous points more even than the film of a single one.23 This has been proven by the fluorescence microscopy (FM) pattern of monolayer LB 9 on a solid substrate, but we failed to take a photograph because of the weak luminescence intensity.
Figure 8. Schematic illustration of the film structures for studying energy transfer: A, the layer of energy acceptors [0.5% Eu(TTA)3Phen + 99.5% AA]:AA ) 1:1; D, the layer of energy donorsLB 2.
Intermolecular Energy Transfer in Film. Figure 8 illustrates the schemes of film fabricated for studying the intermolecular energy transfer. The first two layers of AA in the Y-type model are used for eliminating the influence of the substrates. They were deposited using a triple-distilled water subphase. With determination of the luminescence intensities of these films, the outcome of intermolecular energy transfer can be obtained. The results show that the luminescence intensity of a(2) is 4.4 times stronger than that of a(1), but b(2) is the same as b(1). We reported that the luminescence intensity of LB 7 is 40 times stronger than that of a(1).21 This difference means that the energy transfer occurs mainly between the molecules of the same layer. In Y-type film, only between the layers of the head to head state of AA can Y(TTA)3Phen convey a little energy to Eu(TTA)3Phen. However, in the state of tail to tail Eu(TTA)3Phen cannot get any energy from Y(TTA)3Phen. It also can conclude that the distance for the energy transfer from Y(TTA)3Phen to Eu(TTA)3Phen is less than 30 Å. Therefore, energy transfer can be described as a short-range exchange energy transfer via the triplet of ligand TTA, because resonance energy transfer would occur at the distance of 50-100 Å.12 Mechanism on the Enhanced Luminescence. In LB film molecules arrange orderly. When a beam of ultraviolet light irradiates the films, the ligand TTA in the complexes absorbs energy and lies in its excited singlet since in the case of our experiments Phen does not absorb any energy (Figure 5). Then an intersystem crossing makes it relax to its triplet. This level is higher than the resonance level of the 5D0 level of europium. Thus, energy can be transferred from the triplet of TTA to the 5D0 level of Eu(III) by a radiationless decay. This is an intramo-
Table 2. Parameters of LB Films Ix x1/xLBb Ix1/IxLBc F
LB 1
LB 2a
LB 3
LB 4
LB 5
LB 6
LB 7
LB 8
LB 9
LB 10
100
0
0.7 2000 142 14.0
1.2 1000 83.3 12.0
2.2 500 45.4 11.0
3.2 333 31.2 10.7
4.2 200 23.8 8.4
72 3.3 1.4 2.4
84 2 1.2 1.7
100 1.2 1.0 1.2
a Using subphase c. b x /x c 1 LB is the ratio of molar percentage of Eu(TTA)3Phen in LB 1 to that in LB films 3-10, respectively. Ix1/IxLB is the ratio of fluorescence intensity of 10-layer LB 1 to that of 10-layer LB films 3-10, respectively.
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Conclusions
(TTA)3Phen. The monolayers were observed by BAM and ED. The results show that a crystalline process takes place when the surface pressure amounts to about 9 mN/ m. UV-vis spectra indicate that the complexes are transferred onto the solid substrate. The luminescence intensities of assembled films reveal that the energy transfer occurs mainly in the intralayer and belongs to a short-range exchange energy transfer. Therefore, the LB technique can be used for studying the intermolecular energy transfer of rare earth complexes. It will advance the development of theoretical research and provide the basic data for fabricating luminescent molecular devices. The further investigation on the energy transfer between molecules of rare earth complexes will be reported elsewhere.
We have investigated the luminescent enhancement effect in LB films of mixed rare earth complexes. The luminescence intensity of Eu(III) can be enhanced by a triplet energy transfer from Y(TTA)3Phen to Eu-
Acknowledgment. The authors thank the National Natural Science Foundation of China and the National Fundamental Research Key Project for support of this project.
lecular energy transfer and results in the luminescence of Eu(III).6 When the distance between molecules of Eu(TTA)3Phen and Y(TTA)3Phen is very close, energy in ligand TTA of Y(TTA)3Phen in film can be transferred to the ligand TTA of Eu(TTA)3Phen because yttrium has no suitable resonance levels to accept energy for its empty 4d orbit. That is a triplet-triplet intermolecular energy transfer.26 Then another intramolecular energy transfer occurs and makes the luminescence intensity of Eu(III) be enhanced greatly. When the magnitude of Eu(TTA)3Phen is very few, that is to say, very much Y(TTA)3Phen, the effect is obvious. At that time the energy migration may occur among molecules of Y(TTA)3Phen.
(26) Ermolaev, V. L. Sov. Phys., Dokl. 1967, 6, 600.
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