Monolayers of Europium Complexes with Different Long Chains and β

Emmanuel Terazzi , Amir Zaïm , Bernard Bocquet , Johan Varin , Laure Guénée , Thibault Dutronc , Jean-François Lemonnier , Sébastien Floquet , Em...
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Langmuir 1997, 13, 5925-5932

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Monolayers of Europium Complexes with Different Long Chains and β-Diketonate Ligands and Their Emission Properties in Langmuir-Blodgett Films Dong-Jin Qian,† Kong-Zhang Yang,*,† Hiroo Nakahara,‡ and Kiyoshige Fukuda‡ Key Laboratory for Colloid and Interface Chemistry of State Education Commission, Institute of Colloid and Interface Chemistry, Shandong University, Jinan 250100, China, and Faculty of Science, Saitama University, Urawa 338, Japan Received May 2, 1997. In Final Form: August 7, 1997X Monolayer morphology of some amphiphilic europium(III) complexes with different long chains and head groups has been investigated and directly observed at the air/water interface by Brewster angle microscopy. The domain structure and collapse processes for the monolayers of Eu complexes were closely related to the length and numbers of long chains and to the hydrophilicity of head groups of these complexes. Monolayer assembles of these complexes were fabricated by the LB method and a horizontal lifting technique. The emissions from both the higher excited singlet state 5D1 of the europium ion and the symmetric forbidden transition 5D0 f 7F0 are enhanced in LB films in comparison with those in solutions and solid powders. Stronger effects on europium ion emissions were revealed for the complexes with asymmetric β-diketonate ligands in LB films than for the complexes with symmetric ones. It is suggested that the nonradiative energy transfer 5D1 Df 5D0 of europium complexes is decreased and also the closely packed arrangement has some effects on the splitting of Eu3+ ground states in LB films, especially for those with low symmetry of the ligands around central ions.

Introduction The luminescent behaviors of lanthanoid complexes have been widely studied due to their sharp line and long life emissions, which are caused by an inter- and/or intramolecular energy transfer from the ligands to the metal ions followed by luminescence emission,1,2 and to various applications.3,4 Since the nature of the ligands and the compounds surrounding lanthanoid complexes strongly influence the photophysical properties, such as the relative intensities between each emission level of metal ions, it may be possible to tune them to the desired purpose. These studies can be performed by the chemical synthesis and the assembling of the complexes in a suitable array.5,6 Many papers have been focused on the synthesis of the ligands used as an antenna and on the synthesis of the lanthanoid complexes; others, on the effects of the surrounding species on the emissions of the metal ions.7,8 Strong ligand absorption, high energy transfer efficiency, and close arrangement structures of the complexes all enhance the luminescence of lanthanoid ions.9,10 Much attention has recently been attracted to the assembling of the lanthanoid complexes by physical and chemical methods. Some ways based on LangmuirBlodgett (LB) films have been used to deposit many lanthanoid complexes as ordered and ultrathin films. †

Shandong University. Saitama University. X Abstract published in Advance ACS Abstracts, September 15, 1997. ‡

(1) Buono-Core, G. E.; Li, H.; Marciniak, B. Coord. Chem. Rev. 1990, 99, 55. (2) Matsuda, Y.; Makishima, S.; Shinoya, S. Bull. Chem. Soc. Jpn. 1968, 41, 1513. (3) Weber, M. J. Handbook of Laser Science and Technology; CRC Press Inc.: Boca Raton, FL, 1987; Vol. 1, 397. (4) Dejersey, J.; Martin, R. B. Biochemistry 1980, 19, 1127. (5) Sabbatini, N.; Guardigli, M.; Lehn, J.-M. Coord. Chem. Rev. 1993, 123, 201. (6) Bard, A. J. Integrated Chemical Systems; John Wiley & Sons, Inc.: New York, 1994. (7) Alexander, V. Chem. Rev. 1995, 95, 273. (8) Gmelin Handbuch der Anordisches Chemie, 8th ed.; SpringerVerlag: New York, 1981; Part D3, Vol. D3, p 66. (9) Horrocks, W. D., Jr.; Albin, N. Prog. Inorg. Chem. 1981, 131, 1. (10) Alpha, B.; Lehn, J.-M.; Mathis, G. Angew. Chem. 1987, 99, 259.

S0743-7463(97)00451-4 CCC: $14.00

Osvaldo and his co-workers deposited some compounds as LB films that reacted with the europium ion and ligands, concluding that the films fabricated gave off strong fluorescence emission.11 Huang et al. introduced europium complex anions into some organic molecules with nonlinear optical properties and found that the Eu complex anions not only optimize the formation of the multilayers but also enhance the nonlinear optical efficiency of the organic molecules.12,13 Takada and his co-workers14 fabricated a film by spin coating PMMA containing 5% Eu complex and compared its mechanoluminescence, photoluminescence, and electroluminescence spectra. Since the electroluminescent devices based on lanthanoid complexes may have possible applications in large-area light-emitting displays, many europium complexes were deposited as thin layers, and a sharp red luminescence was obtained at the forward bias of a given voltage.15-17 To well understand the structure and properties of monolayer assemblies of the amphiphilic molecules, it is necessary to describe their Langmuir films. The twodimensional morphology, phase transition, and collapse processes for an insoluble monolayer at the air-water interface have been investigated by π-A isotherms, surface potential,18 and some other techniques, such as optical dark field microscopy19,20 and fluorescence microscopy.21 A stimulated study on describing the textures of the monolayers, formation of two-dimensional crystals, (11) Osvaldo, S. A.; Ieda, R. L. V.; Claudia, L. M.; Elizabete, D. Z. J. Lumin. 1994, 60-61, 112. (12) Huang, C. H.; Wang, K. Z.; Xu, G. X. J. Phys. Chem. 1995, 99, 14397. (13) Zhou, D. J.; Huang, C. H.; Wang, K. Z.; Xu, G. H.; Zhao, X. S.; Xie, X. M.; Xu, L. G.; Li, T. K. Langmuir 1994, 10, 1910. (14) Takada, N.; Sugiyama, J.; Minami, N.; Hieda, S. Mol. Cryst. Liq. Cryst. 1997, 295, 71. (15) Kido, J.; Nagai, K.; Okamoto, Y. J. Alloys Compounds 1993, 192, 30. (16) Kido, J.; Hayase, H.; Hongawa, K.; Nagai, K.; Okuyama, K. Appl. Phys. Lett. 1994, 65, 2124. (17) Sano, T.; Fujita, M.; Fujii, T.; Hamada, Y.; Shibata, K.; Kuroki, K. Jpn. J. Appl. Phys. 1995, 34, 1883. (18) Gains, G. L., Jr. Insoluble monolayer at liquid-gas interface; Interscience: New York, 1966. (19) Zocher, H.; Stiebel, F. Z. Phys. Chem. (Leipzig) 1930, A147, 401. (20) Langmuir, I.; Schaeffer, V. J. J. Am. Chem. Soc. 1937, 59, 2400. (21) Knobler, C. M. Science 1990, 249, 870.

© 1997 American Chemical Society

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and surface compressive processes in situ has been made in recent years based on the direct observation of the monolayers by Brewster angle microscopy (BAM).22 Many properties for molecules in monolayers have been described by BAM in the past few years.23-25 Previously, the authors26-28 studied the monolayer assemblies of some samarium, europium, and terbium complexes with the β-diketonate ligand thienyltrifluoroacetone and concluded that the closely packed structures of the complexes have some influences on the emissions of the lanthanoid ions, especially for the europium and samarium complexes. We have also found an enhancement or quenching of the emission of europium complexes by adding energy donors or acceptors in LB films.29,30 These studies could theoretically reveal the influence of a wellordered ultrathin structure on the deactivation process of the excited Eu3+ ion and provide an alternative way for the “light conversion device”,31 especially for the purpose of ultrathin optical devices. In the present work, monolayers of some newly synthesized europium complexes with different β-diketone ligands (symmetrical and asymmetrical ones) and long chains and their mixed monolayers with other materials have been investigated. The influence of the symmetry of the different β-diketone ligands on the fluorescence emissions of the Eu ion was examined, and the fluorescence characteristics of these complexes in LB films have been discussed and compared with those in chloroform solutions and solid powders. Experimental Section Europium oxide (99.99%) was purchased from Yue Long Chemical Plant (Shanghai). β-Diketone ligands hexafluoroacetone (HFA), thienyltrifluoroacetone (TTA), dibenzoylmethane (DBM), and 4-benzoyl-3-methyl-1-phenyl-5-pyrazolone (BMPP) and long chain compounds 1-hexadecylpyridinium bromide [(C16)1Br], N,N-dimethyldimyristylammonium bromide [(C14)2Br], N,N-dimethyldipalmitylammonium bromide [(C16)2Br], and N,N-distearyldimethylammonuim bromide [(C18)2Br] were from Tokyo Kasei Co., Ltd. Tetrahexadecylammonium bromide [(C16)4Br] was purchased from Aldrich Chemical Co. All these reagents were used as received. Arachidic acid (AA), octadecane (C18), and methyl stearate (C18Me) were purchased from Tokyo Kasei Co., Ltd. and recrystallized from ethanol solution; their purities were checked by liquid chromatography. Europium complexes (Cn)mEu(β-dik)4 shown in Figure 1, where (Cn)m refers to the long alkyl chains and β-dik is β-diketonate ligands, were synthesized by Bauer et al.’s method28,32 and purified by recrystallization from methanol solution. The Eu complexes with ligands HFA, TTA, and DBM are pale yellow powders, and those with the ligand BMPP are pale green. All these complexes are easily soluble in common organic solvents but insoluble in water. The purities for these complexes were checked by liquid chromatography and UV-vis spectroscopy. The results of elemental analysis are listed in Table 1, which are well consistent with the calculated values. Monolayers of europium complexes were spread from a chloroform solution onto the surface of pure water with a (22) Honig, D.; Mobius, D. J. Phys. Chem. 1991, 95, 4590. (23) Litvin, A. L.; Samuelson, L. A.; Charych, D. H.; Spevak, W.; Kaplan, D. L. J. Phys. Chem. 1995, 99, 492. (24) Gutberlet, T.; Vollhardt, D. J. Colloid Interface Sci. 1995, 173, 429. (25) Angelova, A.; Van der Auweraer, M.; Ionov, R.; Vollhardt, D.; De Schryver, F. C. Langmuir 1995, 11, 3167. (26) Qian, D. J.; Yang, K. Z. Acta Phys.-Chim. Sin. 1993, 9, 148. (27) Qian, D. J.; Nakahara, H.; Fukuda, K.; Yang, K. Z. Chem. Lett. 1995, 175. (28) Qian, D. J.; Nakahara, H.; Fukuda, K.; Yang, K. Z. Langmuir 1995, 11, 4494. (29) Zhong, G. L.; Feng, Y.; Yang, K. Z. Chem. Lett. 1996, 775. (30) Qian, D. J.; Nakahara, H.; Fukuda, K.; Yang, K. Z. Mol. Cryst. Liq. Cryst. 1997, 294, 213. (31) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89. (32) Bauer, H.; Blanc, J.; Ross, D. L. J. Am. Chem. Soc. 1964, 86, 5125.

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Figure 1. Complexes used in this work. Table 1. Results of Elemental Analyses found (calcd) Eu complexes

C

H

N

(C16)1Eu(HFA)4 (C18)2Eu(TTA)4 (C18)2Eu(DBM)4 (C16)4Eu(DBM)4 (C18)2Eu(BMPP)4 (C16)2Eu(BMPP)4 (C14)2Eu(BMPP)4

38.73 (38.36) 53.03 (52.95) 73.89 (73.75) 75.98 (75.96) 70.38 (70.25) 69.80 (69.76) 69.03 (69.24)

3.18 (3.22) 6.02 (6.09) 7.68 (7.83) 9.10 (9.05) 7.35 (7.34) 7.12 (7.12) 6.85 (6.88)

1.37 (1.09) 0.93 (0.88) 0.97 (0.88) 0.71 (0.71) 6.92 (6.96) 7.13 (7.18) 7.31 (7.42)

resistivity of 18 MΩ cm produced by a miliQpure purification system. The surface pressure-area (π-A) isotherms were measured by a Lauda film balance (model FW-1). The temperature of the subphase could be controlled by a water circulation system. The monolayers of the complexes or of their mixtures with AA, C18, and C18Me were transferred onto glass and quartz plates precoated with cadmium stearate monolayers, using the LB (KSV 2200) and the horizontal lifting methods. UV-vis absorption and fluorescence spectra were measured by spectrophotometers of Hitachi model 340 and MFP-3, respectively. A PTFE-coated rectangle stainless steel trough with a total surface area of 3.5 × 100 cm2 was used when the morphology and collapse processes of the monolayers at the air/water interface were observed. The monolayer compression rate was 5-7 cm2/ min. The trough was equipped with two barriers for monolayer compression, and the surface pressure of a monolayer was monitored by a Wilhelmy-type film balance. The morphology and the collapse processes of the monolayers of the complexes on the water surface were obtained with a Brewster angle microscope (BAM1, Nanofilm Technologic GmbH, FRG).

Results and Discussion Surface Pressure versus Area Isotherms. Figure 2 shows the π-A isotherms for the monolayers of the complexes (C16)1Eu(HFA)4, (C18)2Eu(TTA)4, (C18)2Eu(DBM)4, (C18)2Eu(BMPP)4, and (C16)4Eu(DBM)4 on the airwater interface at 20 °C. The molecular areas extrapolated from the linear part of the expanded region (Aπf0) were found to be about 108, 97, and 152 Å2 for the complexes (C18)2Eu(TTA)4, (C18)2Eu(DBM)4, and (C18)2Eu(BMPP)4, respectively, indicating that the molecular areas of these complexes in the monolayers corresponded to their closely packed large head groups.28 The molecular area of the complex (C16)4Eu(DBM)4 was about 201 Å2, which seems to be dominated by the four long chains in addition to the loosely packed head group. These results are confirmed by the monolayer behaviors of the compounds (C18)2Br28 and (C16)4Br; the π-A isotherm of the latter compound is similar to that of the complex (C16)4Eu(DBM)4.

Monolayers of Europium Complexes

Figure 2. π-A isotherms for the monolayers of the complexes (Cn)mEu(β-dik)4 on the water surface at 20 °C.

Figure 3. π-A isotherms for the monolayers of the complex (C18)2Eu(DBM)4 on the water surface at different temperatures.

A phase transition was observed for the monolayers of the complex (C18)2Eu(DBM)4 under about 12-15 mN/m at temperatures of the subphase below 20 °C, which almost disappeared with the increased temperatures, as seen in Figure 3. This indicates that the monolayer of (C18)2Eu(DBM)4 is expanded under low surface pressures, leading to a condensed monolayer if the surface pressure increased at low temperatures of the subphase. When the temperatures are above 25 °C, the curve indicates the typically expanded monolayer.33 For the monolayers of the other complexes investigated, the π-A isotherms show the condensed monolayers at subphase temperatures between 5 and 30 °C. A difference on the effect of the temperatures of the subphase on the π-A isotherms of the complexes is that when the temperatures increased, the collapse surface pressures for the monolayers of the complex (C18)2Eu(TTA)4 became higher28 while those of the complexes (Cn)2Eu(BMPP)4 were decreased. This may contribute to the stronger hydrophilicity of BMPP and the more loosely packed arrangement for the hydrophobic part of the molecules of the complexes (Cn)2Eu(BMPP)4 on the water surface. Although the complex (C16)1Eu(HFA)4 with only one long chain is difficult to form a stable monolayer on a pure water surface, as seen in Figure 2, its mixtures with AA could form condensed monolayers. The mixing effect of these complexes with AA, C18, or C18Me was also investigated. A condensed effect was observed in comparison with the calculated curves from the molecular area of each component for ideal mixing in the mixed monolayers of the complexes (C18)2Eu(TTA)4, (C18)2Eu(DBM)4, and (Cn)2(33) Adam, N. K.; Jessop, G. Proc. R. Soc. (London) 1926, A112, 362.

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Figure 4. π-A isotherms for the monolayers of the complex (C18)2Eu(BMPP)4 on the water surface at different temperatures.

Eu(BMPP)4 with AA,28 except for the mixed monolayers of (C16)4Eu(DBM)4. These can be attributed to the fact that for the complexes with less than two long chains, the large hydrophilic head groups dominate the monolayer behaviors, while for the complex (C16)4Eu(DBM)4 the molecular area is governed mainly by the four long chains. As a result, the AA molecules may partly insert into the matrix of the complexes (Cn)mEu(β-dik)4 (m < 2) but those mixed with (C16)4Eu(DBM)4 form a homogeneous monolayer or separate into two phases.18 Figure 4 shows the π-A isotherms for the monolayers of europium complexes (Cn)2Eu(BMPP)4 (n ) 14, 16, 18) on the water surface at 20 °C, which indicate very similar curves with the molecular areas falling into the range 150-154 Å2. The collapse surface pressures are about 23, 25, and 26 mN/m for the monolayers of the complexes (C14)2Eu(BMPP)4, (C16)2Eu(BMPP)4, and (C18)2Eu(BMPP)4, respectively. These results also reflect the effect of the structure of the molecules on their monolayer behaviors (large hydrophilic head group and small long chains). Morphology of the Monolayers of Europium Complexes. Figure 5 shows the pictures for the monolayers of the complexes (C18)2Eu(DBM)4 and (C16)4Eu(DBM)4 on the pure water surface in the gas-liquid expanded (GLE) region at 15 °C. A nearly homogeneous monolayer of (C18)2Eu(DBM)4 with some small domains was formed, while for the monolayer of the complex (C16)4Eu(DBM)4, “blocklike” domains irregularly floated on the water surface. This difference is suggested to relate to the structures of two molecules. Smaller hydrophobic long chains of (C18)2Eu(DBM)4 lead to a closely packed arrangement of the larger head group; reversibly, a smaller hydrophilic group of (C16)4Eu(DBM)4 leads to a close arrangement of the larger hydrophobic long chains. Molecular interactions among the molecules of (C18)2Eu(DBM)4 in monolayers are dominant by Eu(DBM)4- anions, resulting in a regularly packed arrangement of the complex. While the four chains of the complex (C16)4Eu(DBM)4 strongly interact with each other, forming the large aggregates. Different monolayer morphologies were observed from the pictures for the monolayers of europium complexes (C18)2Eu(BMPP)4, (C16)2Eu(BMPP)4, and (C14)2Eu(BMPP)4 in the G-LE region at 20 °C, as shown in Figure 6. When the complex (C18)2Eu(BMPP)4 was spread on the water surface, the domains of a beltlike structure with small holes were formed, while a thin homogeneous monolayer was observed for (C14)2Eu(BMPP)4 in the same case. On the other hand, although a belt domain structure was formed for the complex (C16)2Eu(BMPP)4 on the water surface, it appeared wider and less polar in comparison

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Figure 5. Pictures for the monolayers of the complexes on the water surface by BAM at 15 °C: (a) (C18)2Eu(DBM)4; (b) (C16)4Eu(DBM)4.

with that of (C18)2Eu(BMPP)4. From this direct observation in situ, it confirms that the length of the hydrophobic chains of molecules strongly influences the morphology of their monolayers, that is, the longer the long chains of a complex the higher the density of liquid monolayers formed. We have discussed the effect of the length and numbers of long chains of europium(III) complexes on the morphology of their monolayers; the hydrophilic groups of Eu complexes also influence the monolayer structures (compare Figure 5a with 6a). The complex (C18)2Eu(BMPP)4 with stronger hydrophilic head group Eu(BMPP)4- anions forms larger beltlike domains, while the complex (C18)2Eu(DBM)4 forms fine domains. It is also suggested that complexes in the monolayers of G-LE region are strongly interacted to each other with both long chains and head groups to form aggregates. Further, larger domains could be formed for the monolayers of the complexes with stronger hydrophilic head groups. Collapse Processes for the Monolayers of Europium Complexes. Figure 7 shows the pictures of the complexes (C18)2Eu(DBM)4 and (C16)4Eu(DBM)4 on the pure water surface at their collapse pressure observed by BAM at 15 °C. When the monolayer of the complex (C18)2Eu(DBM)4 was compressed over a critical point about 15 mN/m, which corresponds to the phase transition point in its π-A isotherm, some small nuclei appeared on the morphology of the monolayer, and with the surface pressure increased the nuclei became more numerous; finally, as the surface pressure was increased near the collapse pressure of the monolayer of (C18)2Eu(DBM)4 large numbers of nuclei appeared. For the monolayer of the complex (C16)4Eu(DBM)4 on water surface, many domains suddenly appeared in the morphology of the monolayer when the surface pressure was over its collapse surface pressure (about 30 mN/m). It can be suggested that the

Figure 6. Pictures for the monolayers of the complexes (Cn)2Eu(BMPP)4 on the water surface by BAM at 20 °C: (a) (C18)2Eu(BMPP)4; (b) (C16)2Eu(BMPP)4; (c) (C14)2Eu(BMPP)4.

collapse mechanism for the former complex with two long chains is a fine accumulation process,34,35 while that for the latter complex with four long chains is a domain formation process. The different collapse processes for the monolayers of the complexes (C18)2Eu(DBM)4 and (C16)4Eu(DBM)4 can be ascribed to the numbers of long chains. Four long chains of the complex (C16)4Eu(DBM)4 facilitate a loosely packed structure of the monolayer (especially for the hydrophilic part of the molecules) as indicated above, and so form the blocklike domains when the monolayers are collapsed. For the complex (C18)2Eu(DBM)4 with two long chains, a closely packed arrangement is formed in the hydrophilic part of the molecules and loosely packed hydrophobic part, resulting in disorientation or accumulation of the molecules in the monolayer when it is compressed at higher surface pressures. Although there are almost no obvious differences in the π-A isotherms between the monolayers of the complexes (C18)2Eu(BMPP)4, (C16)2Eu(BMPP)4, and (C14)2Eu (BMPP)4, the collapse processes for their monolayers are very different. Figure 8 shows a set of pictures of these three (34) Neuman, R. P. J. Colloid Interface Sci. 1976, 56, 505. (35) Neuman, R. P. J. Colloid Interface Sci. 1975, 53, 161.

Monolayers of Europium Complexes

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Figure 7. BAM pictures for the collapse processes of the monolayers of the complexes: (a) (C18)2Eu(DBM)4; (b) (C16)4Eu(DBM)4.

complexes on the pure water surface at the collapse pressures observed by BAM at 20 °C. When the surface pressure for the monolayer of the complex (C18)2Eu(BMPP)4 was near its collapse pressure (about 26 mN/m), a straight ridgelike collapse of the film was observed in a catastrophic way, which fits with the collapse processes proposed by some groups.36,37 With further compression, the number of ridges increased and some of them broadened, and finally, many ridges appeared and the morphology of monolayers became a little obscure, indicating the formation of multilayers. For the monolayer of the complex (C14)2Eu(BMPP)4 on the water surface, however, a different collapse process was observed. When the surface pressure was up to its collapse pressure, large numbers of fine dots were observed in the morphology of the monolayer; with further compression, the fine dots became more and more, and the picture of the monolayer became bright, suggesting the formation of the multilayers. This collapse process is a fine accumulation process as many other workers observed.34,35 The collapse process for the monolayer of the complex (C16)2Eu(BMPP)4 is different for both (C18)2Eu(BMPP)4 and (C14)2Eu(BMPP)4. It was found that as the surface pressure was near the collapse pressure, a ridgelike collapse process was first observed, although it was not as clear as that in the case of the complex (C18)2Eu(BMPP)4. When the monolayer was further compressed, the number of ridges increased and some of them broadened, but this change was also not as clear as that for the monolayer of the complex (C18)2Eu(BMPP)4 in the same case. When the monolayer was further compressed, the ridgelike structure disappeared slowly and a fine accumulation (36) Ries, H. E., Jr.; Kimball, W. A. Nature 1958, 181, 901. (37) Lu, Z.; Nakahara, H. Chem. Lett. 1994, 2005.

Figure 8. BAM pictures for the collapse processes of the monolayers of the complexes (Cn)2Eu(BMPP)4: (a) (C18)2Eu(BMPP)4; (b) (C16)2Eu(BMPP)4; (c) (C14)2Eu(BMPP)4.

collapse process was observed along the ridge, which is much like that in the case of the complex (C14)2Eu(BMPP)4. The two-dimensional to three-dimensional transformation resembles a fracture of a polycrystalline solid monolayer, which is also observed from the monolayers of some europium complexes without long chains on a composite subphase surface.38 The different brightness of the segments in the morphology results from a different azimuthal tilt angle within each segment,39 which indicates that the chain length of europium complexes strongly influences their accumulation state in monolayers, though there is only a little difference in the π-A isotherms. The monolayer of (C18)2Eu(BMPP)4 has a ridgelike cartographic collapse process, indicating this complex can form a brittle monolayer, while the monolayer of (C14)2Eu(BMPP)4 collapses with a fine dot formation process, which suggests that the complex forms a “soft” monolayer. Deposition of the Monolayer Assemblies of the Complexes. It was difficult for the monolayers of some complexes alone to be deposited as multilayers, so their (38) Gao, X.; Liu, H. G.; Zhang, R. J.; Yang, K. Z. Thin Solid Films 1996, 284-285, 39. (39) Henon, S.; Meunier, J. J. Chem. Phys. 1993, 98, 9148.

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Figure 9. Absorption spectra of the complex (C18)2Eu(DBM)4 in monolayer assembly (s) and ethanol solution (- - -).

mixed multilayers with other film-forming materials were fabricated. The mixed monolayers of (C18)2Eu(TTA)4 with AA in the molar ratio of 1:16 (or more) could be deposited onto hydrophobidized quartz plates by the LB method under 30 mN/m, and the monolayers of (C18)2Eu(TTA)4 mixed with C18 (molar ratio 1:4) could be deposited by the horizontal lifting technique at 20 mN/m. Although the monolayer of (C16)1Eu(HFA)4 alone was unstable, the mixed monolayer with AA (molar ratio 1:4) was deposited by the LB method at 30 mN/m. The monolayer assemblies of the complex (C18)2Eu(DBM)4, which was difficult to obtain by the LB method were deposited by the horizontal lifting technique under 14 mN/m at temperatures of the subphase below 15 °C. The monolayers for the complex (C16)4Eu(DBM)4 or its mixtures with C18Me (molar ratio 1:2) were deposited by the horizontal lifting and LB methods. The monolayer assemblies for the complexes (Cn)2Eu(BMPP)4 were obtained by the LB method under 20 mN/m at the temperatures of the subphase below 10 °C. Different methods for the deposition of the monolayers of europium complexes are, therefore, mainly dependent on both the β-diketone ligands and long chains. It is known that the hydrophilicity for the head groups of the complexes increased in the following order Eu(DBM)4- < Eu(TTA)4< Eu(BMPP)4-. The traditional LB method is very useful for assembling (Cn)2Eu(BMPP)4 with stronger hydrophilicity as multilayers, which may relate to the stronger interactions between two layers. On the other hand, the horizontal lifting technique is a valuable way for the deposition of the monolayers of molecules with weak hydrophilicity. UV and Fluorescence Spectra of the Complexes in Monolayer Assemblies. The predominant feature of the UV spectra of the complexes under this investigation is the strong band occurring from 280 to 380 nm due to the π-π* type transition of the ligands associated with the conjugated transitions. As an example, the absorption spectrum of the complex (C18)2Eu(DBM)4 in the monolayer assembly is shown in Figure 9 together with that in methanol solution. The π-π* band of the complex (C18)2Eu(DBM)4 appears at 349 nm in the solution, while it is shifted to a longer wavelength at about 368 nm in the monolayer assembly. A similar red shift of the π-π* transition was also observed in the multilayers of the other complexes. Many workers have explained these phenomena as to form a J-aggregate in monolayer assemblies,40,41 while the absorptive peaks of a J-aggregate (40) Kuroda, S.; Sugi, M.; Iizima, S. Thin Solid Films 1985, 133, 189. (41) Nakano, A.; Shimizu, S.; Takahashi, T.; Nakahara, H.; Fukuda, K. Thin Solid Films 1988, 160, 303.

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were found to be sharper than those in solution.42 Some workers suggested an effect of high pressure on the absorption of β-diketone ligands, which also occurs in LB films; they concluded that the high pressure on the complexes also makes the absorption appear at a little lower energy than that in solution.43 So the red shift of the π-π* band of the complexes in monolayer assemblies could be attributed to the closely packed arrangement of the molecules. The emission spectra for the complexes (C16)1Eu(HFA)4, (C18)2Eu(TTA)4, (C18)2Eu(DBM)4, and (C16)4Eu(DBM)4 in the monolayer assemblies and chloroform solutions are shown in Figure 10. The details of the emission wavelengths together with their assignments for these complexes in monolayer assemblies, CHCl3 solutions, and solid powders are listed in Table 2. The only emission transitions observed in the fluorescence spectra are from two resonance levels of 5D1 and 5D0 to the lower levels 7F0,1,2,3,4 of the ground multiplet. As with many europium complexes in the literature,1,2 the emissions in solutions and solid states are mainly the transitions 5D0 f 7Fn (n ) 1-4), and the hypersensitive transition is 5D0 f 7F2 (at about 613 nm).44 Hypersensitive emission is expected to be more sensitive to the ionic environment in terms of shifts in peak position as well as the relative intensity. The emissions from 5D1 f 7Fn were very weak for these complexes in chloroform solutions and solid powders (mixed with KBr). In the LB films of theses complexes, however, besides the transitions 5D0 f 7Fn, the emissions 5D1 f 7Fn could be obviously observed; that is, the emission probability from the excited level 5D1 compared with the lowest excited state 5D0 became higher in LB films than in solutions. The effects of the molecular arrangement on the luminescence of these complexes in different systems were found to increase in the following order: solutions < solid powders < monolayer assemblies. These results can be explained by considering the energy transfer processes in the europium complexes. Absorption of energy by the ground state singlet (S0) of the β-diketone ligand results in an excited state singlet (S1), which then goes through an intersystem crossover to give the excited triplet state T of the β-diketone ligand, which is transferred to the Eu ion and deactivated by fluorescence emission. The energy from the triplet state (T) is considered to reach the 5D0 state via nonradiative deactivation of the 5D1 level when the triplet level is above the 5D1 state in europium complexes. In this case the 5D0 state is populated primarily by nonradiative transfer from the 5D1 state.45 As the ligand triplet state of the complexes is above the 5D1 state of the europium ion,46 it is suggested that the energy reaches 5D mainly from the 5D state via nonradiative transfer 0 1 (5D1 Df 5D0) for these complexes. Recently, Hu et al.47 prepared some colloids of Eu(DBM)3 and the ultrafine particles of Eu(DBM)3; they found the transitions 5D1 f 7Fn could be observed at room temperature, which was suggested to be due to the competition of the emissions of the complex in colloid and ultrathin particles with the decay process and the quenching by temperature.48 Kirby and Richardson49 reported that (42) Ando, E.; Miyazaki, J.; Morimoto, K.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985, 133, 21. (43) Hayes, A. V.; Drickamer, H. G. J. Chem. Phys. 1982, 76, 114. (44) Kirby, A. K.; Palmer, R. A. Inorg. Chem. 1981, 20, 4219. (45) Watson, W. M.; Zerger, R. P.; Yardley, J. T.; Stucky, G. D. Inorg. Chem. 1975, 14, 2675. (46) Sager, W. F.; Filipescu, N.; Serafin, F. A. J. Phys. Chem. 1965, 69, 1092. (47) Hu, L.; Wang, B.; Li, J.; Li, X.; Li, H. Acta Phys.-Chim. 1997, 13, 56. (48) Sato, S.; Wada, M. Bull. Chem. Soc. Jpn. 1970, 43, 1955.

Monolayers of Europium Complexes

Langmuir, Vol. 13, No. 22, 1997 5931

Figure 10. Emission spectra for the complexes (Cn)mEu(β-dik)4 in monolayer assemblies (s) and in chloroform solutions (- - -). Table 2. Emission Wavelengths (nm) of the Complexes (Cn)mEu(β-dik)4 in Monolayer Assemblies, CHCl3 Solutions, and Solid Powders at Room Temperature (C16)1Eu(THF)4 LB 537 574 594 617 654 693 a

CHCl3 sol

(C18)2Eu(TTA)4

crysta

537

537

594 614 653 699

593 612 653 700

LB 529 593 570 590-592 615, 625 668 711

(C18)2Eu(DBM)4

CHCl3 sol

crysta

LB

538 580 594 613 654 702

524 534-539 580 589, 595 612, 614 657 696, 706

527 543 585 594 614 652 702

CHCl3 sol 537 593 614 652 699

(C16)4Eu(DBM)4

crysta

LB

538 581 593 613 655 703

520 536 583 594 615 657 703

CHCl3 sol

crysta

538

536

594 614 652 702

594 613 652 702

assignments 5D

1

5D

1

f 7F0 f 7F1 5D f 7F 0 0 5D f 7F 0 1 5D f 7F 0 2 5D f 7F 0 3 5D f 7F 0 4

Mixed with KBr.

although the emissions 5D1 f 7Fn could be observed in the spectra at all temperatures down to 30 K, they were considerably weaker than 5D0 f 7Fn emissions. The ratio of emission intensities of 5D1 f 7Fn vs 5D0 f 7Fn transitions decreases with the increased temperature. All these results indicate that there exists a thermal deactivating pathway between 5D1 and 5D0, and if it is decreased, the emissions from 5D1 would be increased. Therefore, the enhanced emission probabilities from higher excited states of the europium ion can be ascribed to the fact that the complexes in the monolayer assemblies are of highly ordered and closely packed structure, leading to a decrease in the thermal deactivation of the excited states. Another important result of this study is the influences of the symmetry of β-diketone ligands on the emissions of the complexes. By comparison of the fluorescence spectra of the complex (C18)2Eu(DBM)4 with that of the complex (C18)2Eu(TTA)4, it is found that the emission probability of 5D0 f 7F0 is lower in the monolayer assemblies of (C18)2Eu(DBM)4 than that in the monolayer assemblies of (C18)2Eu(TTA)4 mixed with AA. The splitting of 5D0 f 7F1 and 5D0 f 7F2 occur for the latter complex in monolayer assemblies, while no splitting occurs for the (49) Kirby, A. F.; Richardson, F. S. J. Phys. Chem. 1983, 87, 2544.

former. The emission character of the complexes (C16)4Eu(DBM)4 and (C16)1Eu(HFA)4 was similar to that of the complex (C18)2Eu(DBM)4. It has been concluded that the transitions of 5D0 f 7F1,2 that terminate on the levels with J ) 1,2 are unsplit in a field of Oh symmetry but split into two levels under the perturbing field of the ligands with a less symmetrical arrangement.50,51 Since the complex (C18)2Eu(TTA)4 coordinates with an asymmetrical β-diketone ligand TTA, a splitting was observed in the monolayer assemblies, while for (C18)2Eu(DBM)4 coordinated with a symmetrical ligand DBM no splitting occurred in any system investigated. Considering that 5D0 f 7F0 is strictly forbidden in a regular octahedral field, the emission from 5 D0 f 7F0 in LB films suggests the influence of the closely packed structure on the symmetry of the ligands surrounding the metal ions. The differences in the emission spectra for these complexes in multilayers suggest that the emissions of the complexes with asymmetrically substituted ligands are influenced stronger than those of the complexes with symmetric ones. (50) Filipescu, N.; Sager, W. F.; Serafin, F. A. J. Phys. Chem. 1964, 68, 3324. (51) Cunha, M. C. F.; Brito, H. F.; Zinner, L. B.; Vicentini, G. Coord. Chem. Rev. 1992, 119, 1.

5932 Langmuir, Vol. 13, No. 22, 1997

Conclusions Europium complexes with more than two long chains and β-diketone ligands form stable monolayers at the air/ water interface. The morphology of the monolayers of the complexes was directly observed by Brewster angle microscopy. Three kinds of collapsed models were demonstrated: straight ridgelike catastrophe, ultrafine particle accumulation, and large domain formation. It is found that both the morphology and the collapsed processes of the monolayers of europium complexes are closely related to the length and the numbers of hydrophobic chains and to the head groups of these complexes. The fluorescence emission characters of europium complexes with different β-diketonate ligands in ordered ultrathin films indicated that emissions from the excited

Qian et al.

state 5D1 and the transition 5D0 f 7F0 became higher in LB films than in solutions and solid powders, especially for the complexes with asymmetrically β-diketonate ligands. The results are attributed to the decrease of the nonradiative energy transfer between the excited states of the Eu ion and to the crystal splitting effects of the ligands to the central ion in monolayer assemblies. Acknowledgment. D.Q. expresses his gratitude to the Ministry of Education, Science, and Culture of Japan for the financial support of his stay at Saitama University. This work was partly supported by State Education Commission, the National Fundamental Research Key Project, and an NNSFC Grant of China. LA9704516