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Fluorescence Character of Rare Earth Complex with High Efficient Green Light in Ordered Molecular Films Ren-Jie Zhang and Kong-Zhang Yang* Institute of Colloid and Interface Chemistry, Shandong University, Jinan 250100, People’s Republic of China Received February 14, 1997. In Final Form: October 3, 1997X By using a composite subphase, the atypical amphiphilic Tb(acac)3Phen (where acac denotes acetylacetone and Phen denotes 1,10-phenanthroline) molecules without long hydrophobic chains were deposited onto solid substrates. The results of UV-vis spectra and fluorescence emission spectra show the Tb(acac)3Phen/ arachidic acid (1:1 molar ratio) Langmuir-Blodgett (LB) film to be homogeneous in direction normal to the substrate. A periodic layer structure of the LB film is demonstrated by low-angle X-ray diffraction results, the average layer spacing being 5.40 nm. The LB film containing Tb(acac)3Phen can emit strong green fluorescence, and the fluorescence signal can be detected from a single layer. The wavelengths of fluorescence emission are not altered in the ordered molecular film; however, the fluorescence intensities of some emission peaks are increased.
1. Introduction As highly functional compounds, luminous rare earth β-diketone complexes have aroused a wide range of attention in the world, since they can be used as laser materials, and they have vast application vistas in the areas of molecular photoelectronics, molecular optics, and molecular switching techniques. Previously, rare earth complexes with different luminous ions were synthesized,1-3 and different complex structures and their influences on fluorescence behavior were studied.4 More recently, with the development of new experimental methods and an increasing demand for new functional materials, rare earth complexes with highly efficient fluorescence and excellent nonlinear optical properties were also synthesized. Consequently, many intriguing results were obtained by using the Langmuir-Blodgett (LB) technique to assemble organized molecular films of these compounds. Our previous results demonstrated that fluorescence efficiency of Eu(TTA)3Phen was very high in LB films; fluorescence was visible by the naked eye from a single layer.5,6 Huang et al. showed that LB films of rare earth complexes with hemicyanine or azobenzene groups exhibited a large second-order molecular hyperpolarizability.7,8 However, in the area of assembling LB films of rare earth complexes, two points need to be further investigated. (i) Many functional rare earth molecules lack typical amphiphilic characteristics. It is difficult to obtain organized molecular films by the traditional methods of the LB film technique; consequently, their properties in ordered molecular films cannot be studied. (ii) Syntheses and spectroscopic properties were described for some Eu, Sm, and Tb β-diketone complexes in published papers; however, most of the spectra presented belong to * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Melby, L. R.; Rose, N. J.; Abramson, E.; Caris, J. C. J. Am. Chem. Soc. 1964, 86, 5117. (2) Crosby, G. A.; Whan, R. E.; Alire, R. M. J. Chem. Phys. 1961, 34, 743. (3) Whan, R. E.; Crosby, G. A. J. Mol. Spectrosc. 1962, 8, 315. (4) Bauer, H.; Blanc, J.; Ross, D. L. J. Am. Chem. Soc. 1964, 86, 5125. (5) Zhang, R.-J.; Liu, H.-G.; Yang, K.-Z.; Si, Z-K.; Zhu, G.-Y.; Zhang, H.-W. Thin Solid Films 1997, 295, 228. (6) Zhang, R.-J.; Liu, H.-G.; Zhang, C.-R.; Yang, K.-Z.; Zhu, G.-Y.; Zhang, H.-W. Thin Solid Films 1997, 302, 223. (7) Li, H.; Huang, C.-H.; Zhao, X.-S.; Xie, X.-M.; Xu, L.-G.; Li, T.-K. Langmuir 1994, 10, 3794. (8) Zhou, D.-J.; Wang, K.-Z.; Huang, C.-H.; Xu, G.-X.; Xu, L.-G.; Li, T.-K. Solid State Commun. 1995, 93, 167.
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Eu complexes, which emit red light under ultraviolet excitation. Few fluorescence spectra of terbium complexes have been reported. There are only two papers relating to LB films of terbium complexes.9,10 In the present paper, the above two points are addressed. On the basis of our previous work, LB films of Tb(acac)3Phen, which is not typically amphiphilic, were assembled. In papers published elsewhere, we reported that stable Eu(TTA)3Phen/AA (1:1) monolayers at air/ liquid interface,11 as well as high-quality Eu(TTA)3Phen/ AA (1:1) LB films, which can give off strong red light, were obtained.5,6 Tb(acac)3Phen is similar to Eu(TTA)3Phen in structure and is of interest for its green fluorescence. Can the terbium complex be assembled into LB films by the method used for assembling films of Eu(TTA)3Phen? Furthermore, what fluorescence characteristics are observed when Tb(acac)3Phen is assembled into ordered molecular films? We report the related experimental results in this paper. The present work further proves that functional LB films of atypical amphiphilic molecules, without hydrophobic chains, can be assembled using appropriate methods. The LB films obtained fluorescenced at 555 nm; i.e., ultrathin, ordered films that emitted green light were assembled. In LB films, Tb(acac)3Phen molecules oriented with higher order than those in solution or cast film and exhibited increased probability of some transitions. Thus, fluorescence can be detected for LB films of a single layer. 2. Experimental Details 2.1. Materials and Subphases. Tb(acac)3Phen was synthesized following the procedure described by Melby et al.;1 its melting point is 240-242 °C. Figure 1 shows the molecular structure of Tb(acac)3Phen. The commercially available arachidic acid (AA) was a spectroscopic pure reagent. Chloroform was an analytical pure reagent and used without further purification. Pure water for the subphase was first deionized and then doubly distilled (pH, 6.4; resistivity, 8 × 105 Ω‚cm). A solution of 10 mL of acetylacetone (acac), an excess amount of Tb(acac)3Phen, and 1,10-phenanthroline (Phen) in 5 L of water was used as subphase, which we will call the composite subphase. 2.2. Procedures and Apparatus. Monolayers at the air/ liquid interface and assembled LB films were obtained by means (9) Li, Q.; Zhou, D.-J.; Yao, G.-Q.; Huang, C.-H.; Umetani, S.; Matsui, M. Chem. J. Chin. Univ. 1996, 17, 1016. (10) Qian, D.-J.; Yang, K.-Z. Acta Phys.-Chim. Sin. 1993, 9, 148. (11) Gao, X.; Liu, H.-G.; Zhang, R.-J.; Yang, K.-Z. Thin Solid Films 1996, 284-285, 39.
© 1997 American Chemical Society
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Figure 1. Molecular structure of rare earth complex Tb(acac)3Phen.
Figure 2. π-A isotherms of (a) Tb(acac)3Phen and (b) Tb(acac)3Phen/arachidic acid (1:1 molar ratio) on composite subphase. of a British NIMA 2000 circular trough. Monolayers of the various compounds were obtained by dropping their chloroform solution onto the composite subphase. The solvent was allowed to evaporate 10 min prior to compressing; then π-A isotherms (surface pressure versus area per molecule) were recorded. LB films were assembled by a vertical deposition method. At room temperature of 20 ( 1 °C, the experiments were performed under a surface pressure of 20 mN‚m-1 at a speed of 5 mm‚min-1. During the deposition processes, the fabricated LB films were air-dried for 6 min every other layer. The literature method12 described by Honig et al. was adopted to treat the substrates. Hydrophobic glass substrates were used for low-angle X-ray diffraction and fluorescence spectroscopy, and optically polished quartz plates were used for UV-vis spectroscopy. Monolayers were observed in situ under a BAM-II microscope (China); the results were videotaped and photographed. The UV-vis absorption spectra of LB films were collected on a Shimadzu UV-240 spectrophotometer (Japan) with a blank quartz plate as reference. Low-angle X-ray diffraction spectra of the LB films were performed using a Rigaku D/max rB X-ray diffractometer (Japan). The fluorescence spectra of the LB films were recorded on a Hitachi 850 fluorescence spectrophotometer (Japan).
3. Results and Discussion 3.1. Monolayer Behaviors and LB Film Fabrication. Figure 2 shows the π-A isotherms of Tb(acac)3Phen and Tb(acac)3Phen/AA (1:1) monolayers on the composite subphase. The maximum surface pressure of the Tb(acac)3Phen monolayer was much lower than that of the Tb(acac)3Phen/AA (1:1) monolayer, which means that the compressibility of the former monolayer was not as great as that of the latter monolayer. The shape of π-A isotherms of the two monolayers did not vary with time and compression speeds. The above results were completely reproducible. The main reason for the formation (12) Honig, E. P.; Hengst, J. H. Th.; Engelson, D. Den. J. Colloid Interface Sci. 1973, 45, 92.
Figure 3. Brewster angle microscope micrographs of (a) Tb(acac)3Phen and (b) Tb(acac)3Phen/arachidic acid (1:1) monolayer at 5 mN‚m-1 on composite subphase.
of stable monolayers is that dissolution and dissociation of Tb(acac)3Phen from monolayer are all effectively inhibited by components in the composite subphase.5,6,11 However, this monolayer inclined to collapse when transferred to solid substrates. When Tb(acac)3Phen was mixed with AA, film formation was improved. Figure 2 illustrates that the surface pressure of 20 mN‚m-1 corresponds to a solid state monolayer in which molecules packed closely. Consequently, this surface pressure was selected for assembling LB films. During the transfer of the monolayer to solid substrates at a speed of 5 mm‚min-1, the transfer ratio was high, ranging from 0.85 to 1.05. It is well-known that many developed techniques have been widely used for characterizing structure of monolayers, e.g., the Brewster angle microscope13 and the fluorescence microscope.14 Figure 3 shows the Brewster angle microscope micrographs of Tb(acac)3Phen monolayer and Tb(acac)3Phen/AA (1:1) monolayer at 5 mN‚m-1. Under compression, large aggregates could be seen in the Tb(acac)3Phen monolayer. In the Tb(acac)3Phen/AA (1: 1) monolayer, due to the dispersive effect of the AA molecules, the monolayer was more homogeneous than the Tb(acac)3Phen monolayer; however, aggregates could still be observed. The formation of several layers of Tb(acac)3Phen aggregates within the monolayers contributes to the small limiting areas in the π-A isotherms, e.g., nearly 0.35 nm2 for the Tb(acac)3Phen/AA (1:1) monolayer. Another possible reason for this small limiting area is that a portion of the molecules of Tb(acac)3Phen complex is squeezed out from the monolayer and attached beneath the monolayer of AA. Work is in progress to make the reason more clear. 3.2. Structural Characterization of LB Films. Figure 4 shows the X-ray diffraction spectrum of 30 layers (13) Gehlert, U.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. Langmuir 1996, 12, 4892. (14) Thoma, M.; Pfohl, T.; Mo¨hwald, H. Langmuir 1995, 11, 2881.
Fluorescence Character of Rare Earth Complex
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Figure 4. Low-angle X-ray diffraction spectrum of 30 layer Tb(acac)3Phen/arachidic acid (1:1) LB film, dotted line (×0.005), in the range of 5-15°.
Figure 6. Plot of absorbance at 272.8 nm of Tb(acac)3Phen/ arachidic acid (1:1) LB films against number of layers.
Figure 5. UV-vis spectra of (a) solution (0.010 mg/mL) (×6), (b) cast film (×1), and (c) 18 layers LB films (×1) of Tb(acac)3Phen/arachidic acid (1:1).
of Tb(acac)3Phen/AA (1:1) LB films. In the spectrum, diffraction peaks appeared at 1.56°, 3.12°, 4.70°, 6.28°, and 7.88° (2θ). The average layer spacing was calculated to be 5.40 nm according to the Bragg diffraction formula. The above results show that the obtained LB films containing Tb(acac)3Phen have a periodic layer structure. The LB film was of the Y type; the layer spacing of nearly twice the length of AA chain showed that AA molecules oriented nearly perpendicular to the solid substrate. By combination of the results of the π-A isotherm, a Brewster angle microscope micrograph of the Tb(acac)3Phen/AA (1:1) monolayer, and X-ray diffraction spectrum of the Tb(acac)3Phen/AA (1:1) LB film, a clear structure of the LB film can be depicted as follows. The ordered LB film has a periodic layer structure; the Tb(acac)3Phen molecules aggregate to several layers in each deposited layer, whereas the AA molecules orient nearly perpendicular to the solid substrate. UV-vis spectra of Tb(acac)3Phen/AA (1:1) in solution, cast film, and 18-layer LB film are shown in Figure 5. Strong absorption bands corresponding to the π-π* transition of the ligands of Tb(acac)3Phen can be observed around 226, 271, and 298 nm. However, the peak near 270 nm is observed at 264.5 nm in the spectrum of the
Figure 7. Fluorescence excitation spectra of (a) solution (0.56 mg/mL) (×1), (b) cast film (×2), and (c) 18 layers LB film (×1) of Tb(acac)3Phen/arachidic acid (1:1).
solution, at 270 nm in that of the cast film, and at 272.8 nm in that of the LB film. It can be deduced that this peak shifts to longer wavelengths as the order of the Tb(acac)3Phen molecules increases. Aggregates of Tb(acac)3Phen molecules were detected as mentioned above; however, no obvious effects of aggregates of Tb(acac)3Phen on UV-vis spectra were observed; i.e., a large wavelength shift (larger than 10 nm) was not observed when the UVvis spectra of the Tb(acac)3Phen/AA (1:1) in solution and LB film were compared. This result is similar to that for study on other 1,10-phenanthroline rare earth complexes in LB films, e.g., Eu(TTA)3Phen/AA (1:1) LB film (TTA ) 2-thenoyltrifluoroacetone).5,6 A peak exhibiting a more clear difference between solution, cast film, and LB film appeared at 348 nm. In solution and cast film, this peak was not obvious. In LB film, the peak was very intense, becoming one of the main absorption peaks in the UVvis spectrum. The intense peak at 348 nm can be due to the strong interaction between the close-packing rare earth complex molecules in the ordered film, which increases
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Figure 8. Fluorescence emission spectra of (a) solution (0.56 mg/mL), (b) cast film, and (c) 18 layers LB film of Tb(acac)3Phen/ arachidic acid (1:1), dotted line (×0.1), in the range of 450-524 nm and 570.5-800 nm.
the probabilities of electron transition of the ligand (Phen) of the complex. It is well-known that, in solution, transitions between energy levels are affected by polarity of solvent and components in it, resulting in red shift or blue shift of absorption bands. The present experimental result for the UV-vis absorption spectra of Tb(acac)3Phen/ AA (1:1) in solution, cast film, and LB film suggests that shifts of absorption peaks could take place due to the interaction of close-packing molecules in a highly ordered film. In addition, transition probabilities between energy levels could also be changed, resulting in stronger or weaker absorption bands. Figure 6 shows a plot of absorbance of the LB films against a number of layers. A good linear relationship can be seen, which suggests that the LB films were homogeneously deposited. 3.3. Fluorescence Characterization of the LB Film. Fluorescence excitation spectra of Tb(acac)3Phen/ AA (1:1) in solution, cast film, and LB film are shown in Figure 7, which were obtained by measuring the emission at 550 nm and scanning the excitation over the range of 250-350 nm. The maximum excitation wavelength in
the LB film was at 273.5 nm, while that in the solution was at 319.0 nm. This result seems to be counterintuitive; however, it was completely reproducible. In addition, our experimental result excluded the occurrence of Tb(acac)3Phen exciplexes or excimers in solution. When the concentration of Tb(acac)3Phen/AA (1:1) solution in chloroform was changed, the excitation and emission wavelengths in the fluorescence spectra did not change. With correspondence to the UV-vis spectra of the Tb(acac)3Phen/AA (1:1) in solution and LB film, we deduce that light of 319.0 nm can excite the Tb(acac)3Phen molecules in solution while light of 264.5 nm cannot, being due to the complete energy loss during the energy transfer process from the ligands to the central ion of the Tb(acac)3Phen. In LB film, due to the ordered molecular packing, the light of 273.5 nm can be transferred to the central ion. A deeper investigation on the difference between energy transfer process in the solution and that in the LB film of Tb(acac)3Phen/AA (1:1) is ongoing. Fluorescence emission spectra of the Tb(acac)3Phen/ AA (1:1) LB film, cast film, and solution ranging from 450 to 800 nm are shown in Figure 8, the excitation wavelength
Fluorescence Character of Rare Earth Complex
Figure 9. Plot of fluorescence intensity at 555 nm of Tb(acac)3Phen/arachidic acid (1:1) LB films against number of layers.
being 273.5, 273.5, and 319.0 nm, respectively. Figure 8 illustrates that the main fluorescence emission peaks of the LB film appeared at 488.5, 555, 582.5, 617, 644, 664, and 674 nm, which were the same as those of Tb(acac)3Phen/AA (1:1) solution and cast film. These fluorescence emission peaks correspond to transitions from 5 D4 to 7F6, 7F5, 7F4, 7F3, 7F2, 7F1, and 7F0 energy levels of terbium(III) ion, respectively. The most intense peak was at 555 nm, belonging to the wavelength in the green light range. Fluorescence efficiency of the LB film containing Tb(acac)3Phen was very high and the fluorescence signal of a single layer of LB film could be detected by the fluorescence spectrophotometer. On analyzing our experimental results in fluorescence emission, we think that the reason that Tb(acac)3Phen/ AA (1:1) LB films emit strong fluorescence could be attributed to the following points. Firstly, the appropriate matching of energy levels between the ligand acac and the central ion Tb3+ would enable Tb(acac)3Phen to emit strong fluorescence. The triplet state of acetylacetone is 25 300 cm-1,15 and the 5D4 energy level of Tb3+ is about 20 410 cm-1.16 Thus, the ligand absorbs ultraviolet photons and the molecule is excited. After energy is transferred to the triplet by intersystem crossing, effective energy transfer between the ligand acac and the central ion Tb3+ takes place. Secondly, the synergistic effect of the ligand Phen can increase the fluorescence efficiency of the Tb(acac)3Phen. Thirdly, the ordered packing of Tb(acac)3Phen molecules in LB film also contributes to high fluorescence emission. Obvious changes of some emission peaks can be observed for the LB film compared to those in cast film and solution. These peaks are at 555,
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617, 690, and 582.5 nm. The fluorescence emission at 582.5 nm is less intense in the LB film, while those at 555, 617, and 690 nm are more intense; the most changed peak is at 555 nm. The above result suggests that transitions between resonance energy levels of rare earth ion can be altered when the molecules become oriented in different forms. Bauer et al. studied complexes formed by Eu(III) and TFTBD (4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione) or PBD (1-phenyl-1,3-butanedione) and found that transition probability between energy levels might be changed when the symmetry of molecular structure is different. Some transitions could increase, while others could decrease.4 In this paper also, one of the reasons that the Tb(acac)3Phen/AA (1:1) LB film emits strong fluorescence could be attributed to the ordered packing of Tb(acac)3Phen molecules. In the LB film, as the complex molecules exist in a highly ordered fashion, the interaction between molecules can be relatively enhanced. Thus, the symmetry of the Tb(acac)3Phen molecule might be slightly changed, consequently transition probability corresponding to 555 nm was increased. Qian et al. also observed similar phenomena in the study on LB films of other rare earth complexes.17 Figure 9 shows the linear relationship between fluorescence intensity at 555 nm of Tb(acac)3Phen/AA (1:1) LB film and number of layers. Both this result and that of UV-vis measurement (Figure 6) suggest that the Tb(acac)3Phen/AA (1:1) LB films are homogeneous in the direction normal to the substrate. 4. Conclusion By using the composite subphase, the LB film containing Tb(acac)3Phen was obtained. The LB film was homogeneous in the direction normal to the substrate. Strong green light was emitted by the LB film when it was excited under ultraviolet irradiation. Compared with solution and cast film, rare earth complex molecules were more ordered in the LB film, leading to the change of transition probability between molecular energy levels. In UV-vis spectra, the peak at 348.5 nm was enhanced. In fluorescence emission spectra, the peaks at 555, 617, and 690 nm were enhanced, while the peak at 582.5 nm was weakened. The present result provides an experimental base both for preparing ultrathin photoelectric devices and for further study of fluorescence scheme of green light emitting rare earth ion in surroundings of different molecular order. Acknowledgment. The authors are grateful for the financial support of the Climbing Program (a Fundamental Research Key Project) and the NNSFC. LA970156L (15) Sager, W. F.; Filipescu, N.; Serafin, F. A. J. Phys. Chem. 1965, 69, 1092. (16) Edelstein, N. M. Lanthanide and Actinide Chemistry and Spectroscopy; American Chemical Society: Washington, DC, 1980; p 354. (17) Qian, D.-J.; Nakahara, H.; Fukuda, K.; Yang, K.-Z. Langmuir 1995, 11, 4491.
