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Component-Controllable Mixed Monolayer and Langmuir-Blodgett Films of Ru(dpphen)32+ and Arachidic Acid Lu Weixing, Guo Weihua, Zhou Huilin, and He Pingsheng* Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, Anhui, China Received December 14, 1999. In Final Form: February 23, 2000 The miscibility and phase behavior of mixed monolayers of Ru(dpphen)32+ (dpphen ) 4,7-diphenyl1,10-phenanthroline) (abbreviated as Ru(II)) and arachidic acid (AA) with different molar proportions on pure water have been investigated. Ru(II) is water insoluble and acts as a surface counterion. The miscibility of two components was confirmed by collapse pressure of the monolayer and excess free energy of mixing. It was found that Ru(II):AA ) 1:2 is the best mixture ratio for the stable and condensed mixed monolayer on the pure water subphase, and any surplus Ru(II) or AA is not favorable for the mixed monolayer. The mixed Langmuir-Blodgett (LB) films with the optimum ratio of Ru(II):AA ) 1:2 were deposited and characterized by UV-visible, photoinduced emission spectra, and low-angle X-ray diffraction. Both absorption and fluorescence spectra have shown that the LB films have considerable optical properties in the visible region. The possible structures of the monolayer and LB film are proposed based on the X-ray diffraction data.
Introduction Ruthenium(II) complexes exhibit attractive photophysical and electrochemical properties for many fundamental and applied studies due to their variety of luminescence properties with long excited state reactivity and relatively high chemical and thermal stability. Their intense absorption and consequent emission in the visible spectrum can be tailored by synthetically modifying the substitution pattern of the ligands. Ruthenium(II) complexes also have a large molar absorption coefficient in the visible-light region, and the excited state performs oxidative and reductive electron transfer to various acceptors and donors. Such unique features of the ruthenium complex make them suitable as optical sensors,1-4 photocatalysts,5 photoelectrodes for solar cells,6 and nonlinear optical materials.7 The key problem in application of ruthenium complexes is to entrap them in solid matrixes homogeneously, particularly in thin films. Any heterogeneity on a microscopic scale results in nonsingle exponential fluorescence decays and poorly understood devices. Therefore the matrix materials and the dispensing methods have great effects on the optical characteristics of the films. To date, sol-gel and spreading coating techniques have been generally used to prepare the thin solid films, but these methods are not ideal for forming uniform films. Murray’s group was successful at fabricating a solid-state diode* To whom correspondence should be addressed. (1) Trettnak, W.; Gruber, W.; Franz Reininger, F.; Klimant, I. Sens. Actuators 1995, B29, 219. (2) Holst, G. A.; Koster, T.; Voges, E.; Lubbers, D. W. Sens. Actuators 1995, B29, 231. (3) Brook, T. E.; Narayanaswamy, R. Thin Solid Films 1997, B3839, 195. (4) Lu, W. X.; Wang, H. B. J. Transducer Technol. 1998, 17 (1) 34. (in Chinese) (5) Slama-Schwok, A.; Avnir, D.; Ottolenghi, M. J. Am. Ceram. Soc. 1991, 113, 3984. (6) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (7) Zyess, J.; Dhenaut, C.; Chauvan, T.; Ledoux, I., Chem. Phys. Lett. 1993, 206, 409.
like chemiluminescence device by placing Ru(II) complexes on an interdigitated array of platinum electrodes.8,9 But the preparation process is too complicated. Rubner’s group has explored a different approach to lightemitting electrochemical devices using various luminescent Ru(II) complexes and self-assembly techniques.10 The Langmuir-Blodgett (LB) technique is one of the most promising candidates to fabricate ultrathin film and controllable molecular array for efficient energy and electron transfer.11,12 The incorporation of a photofunctional group into LB films has been tried by various methods. Whitten’s group modified Ru(bpy)32+ by replacing a single 2,2′-bipyridine unit with the dioctadecyl or bis(dihydrocholesteryl) ester and used the modified Ru(bpy)32+ to form LB films.13 Miyashita’s group have proposed a copolymerization method to incorporate various functional chromophores into polymer LB films as a comonomer of amphiphilic N-dodecylacrylamide (PDDA), which has an excellent ability to form a condensed stable monolayer,14 and have succeeded in the preparation of stable polymer LB films containing Ru(bpy)32+ (p(DDARu) and p(t PA-Ru)).15 They also reported that Ru(dpphen)32+ without a long alkyl chain substituent could be dispersed molecularly into a barium stearate monolayer matrix with a molecular area of 1.2 nm2/molecule. The condensed mixed monolayer could be transferred onto a (8) Maness, K. M.; Terrill, R. H.; Meyer, T. J.; Murray, R. W.; Wightman, R. M. J. Am. Chem. Soc. 1996, 118, 10609. (9) Maness, K. M.; Masui, H. M.; Wightman, R. M.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 398. (10) Lee, J. K.; Yoo, D. S.; Handy, E. S.; Rubner, M. F. Appl. Phys. Lett. 1996, 69 (12), 1698. (11) Tredgold, R. H. Order in Thin Organic Films; Cambridge University Press: Cambridge, 1994. (12) He Pingsheng Polymers and Organic Solid; Lianghe Shi, Daoben Zhu, Eds.; Science Press: Beijing, 1997; p 25. (13) Spritschnik, G.; Sprintschnik, H. W.; Kirsch, P. P.; Whitten, D. G. J. Am. Chem. Soc. 1976, 98, 2337. (14) Mizuta, Y.; Matsuda, M.; Miyashita, T. Macromolecules 1991, 24, 5459-5462. (15) Taniguchi, T.; Fukasawa, Y.; Miyashita, T. J. Phys. Chem. 1999, B103, 1920-1924.
10.1021/la991630l CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000
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Figure 1. Molecule structure of Ru(II).
Figure 2. π-A curves of mixtures of Ru(II)-AA with different ratios on pure water: 1, 1:2; 2, 1:1; 3, 5:1; 4, 6:1; 5, 7:1; 6, 8:1; 7, pure Ru(II).
quartz slide giving a uniform LB film with a yellow color due to the absorption by Ru(dpphen)32+. And the LB films were the same as those in homogeneous solutions.16 The shortcoming of the mentioned methods is that they would not be able to control the amount of Ru(dpphen)32+ in the monolayer on the subphase containing barium ions because any metallic ions in the subphase will combine with stearic acid to form metal soap molecules and may cause the phase separation in monolayers. In our previous paper,17 we have succeeded in fabricating arachidic acid LB films containing photofunctional molecules, tris(1,10-phenanthroline)ruthenium(II) chloride hexahydrate ([Ru(phen)3]2+), deposited by using [Ru(phen)3]2+ aqueous solution as a subphase without any other ions. But the most common way to construct functional LB films involves transfer of monolayers formed by mixing insoluble function molecules with amphiphilic molecules. However, it is not easy to obtain homogeneous composite LB films due to the phase separation of components in mixed monolayer, which give heterogeneous structures with small domains of each component. In this paper we report a component-controllable mixed monolayer of Ru(dpphen)32+-AA mixtures and LB films on the subphase of pure water, but the Ru(dpphen)32+ were used as surface ions in this case. The phase transitions of mixed monolayers in different molar proportions and resulting LB films were studied by π-A isotherms, UV-visible and photoinduced emission spectra, and low-angle X-ray diffraction in detail. The measurement of basic optical properties and structural characterizations of the LB films indicate that homogeneous composite Y-type LB films could be obtained by transferring monolayers with a mixture of Ru(II):AA ) 1:2 onto microscopic cover-glass slides with red-orange color.
Figure 3. π-A curves of mixtures of Ru(II)-AA with different ratios on pure water: 1, pure AA; 2, 1:9; 3, 1:7; 4, 1:6; 5, 1:4; 6, 1:3; 7, 1:2.
Experimental Section Materials. The molecular structure of tris(4,7-diphenyl1,10-phenanthroline)ruthenium(II) chloride pentahydrate, [Ru(dpphen)3]Cl2 (Ru(II)), used in this study is shown in Figure 1. It was synthesized by slightly modifying procedure of Watts and Crosby.18 Arachidic acid (spectral purity) was purchased and used without further purification. Monolayer Measurement and Deposition Procedure. All the monolayer measurements and LB films deposition were carried out with a homemade computer-controlled Langmuir trough with Whilhelmy Pt-plate kept in a clean room at 28 °C. The mixtures of the Ru(II) solution in chloroform with a (16) Murakata, T.; Miyashita, T.; Matsuda, M. J. Phys. Chem. 1988, 92, 6040. (17) Lu, W. X.; Zhou, H. L.; He, P. S.; Guo, W. H. Thin Solid Films, in press. (18) Watts, R. J.; Crosby, G. A. J. Am. Chem. Soc. 1971, 93, 3184.
concentration of 2.11 × 10-3 M and AA solution in the same solvent with the same concentration were used for the spreading solution. The subphase used was pure water doubly distilled by a quartz sub-boiling ultrapurified distiller. The spreading monolayers were compressed with a barrier rate of 8 mm/min after 30 min of the spreading. The LB films were deposited at a constant surface pressure of 30 mN/m with a deposition speed of 2 mm/min. The substrate used was a microscope cover-glass slide. Equipment. The UV-vis absorption spectra and photoinduced emission spectra were measured by a Shimadzu UV-240 UV-Visible spectrometer and a Hitachi 850 fluorescence spectrophotometer, respectively. The wavelength of excitation light for emission spectra measurement is 470 nm. The low-angle X-ray diffraction profile was recorded on a RIGAKU D/max-γA rotating anode X-ray diffractometer using the Cu KR line (λ ) 0.15418 nm) with DS ) 1°, SS ) (1/6)o, RS ) 0.15 mm, and RSm ) 0.3 mm. The tube current and voltage were 50 mA and 40 kV.
Result and Discussion Monolayer Behavior at the Air/Water Interface. The Ru(II) compound used here is water insoluble and will be able to form a monolayer on pure water (without any other counterion) because any other counterion in water may cause the aggregation of Ru(II) prior to their compression19 and phase separation of Ru(II) and AA. The surface pressure (π)-area (A) isotherms of Ru(II), AA, and Ru(II)-AA mixtures with various mixing ratios are shown in Figure 2 and Figure 3, respectively. Compared with pure Ru(II) and AA, the monolayers of Ru(II)-AA with different mixing ratios are expanded. The expansion (19) Miller, C. J.; McCord, P.; Bard, A. J. Langmuir 1991, 7, 2781.
Component-Controllable Mixed Monolayers
of the mixture monolayer increases with increasing molar fraction of the Ru(II) in mixtures and the largest expansion is reached when the ratio of Ru(II)-AA reaches 1:2 (Figure 2). Above the saturation ratio of Ru(II):AA ) 1:2, the expansion of monolayer decreases (Figure 3). Since the molecule size of AA is much smaller than that of Ru(II), the average molecule area of the mixed monolayer, at first thinking, should be smaller than that of pure Ru(II) monolayer at high pressure. However, an opposing result was obtained in the present experimental data. The result infers that Ru(II) molecules in pure Ru(II) monolayer can penetrate one another to form a condensed monolayer at high pressure. This should be due to a large space among the ligands of Ru(II). The AA long chain molecule stands within the Ru(II) monolayer and will prevent Ru(II) molecules around it from penetrating one another. Properties of the mixed monolayer with immiscible two components reflect the properties of the separating individual component. The collapse behaviors of Ru(II), AA, and Ru(II)-AA mixtures with various mixing ratios are also shown in Figure 2 and Figure 3. The collapse pressure of the mixture monolayer with different compositions is a good indication of miscibility of the components of the mixture.20 When the components in the mixed system are immiscible, the monolayer formed collapses at a surface pressure corresponding to the component that collapses at the lower surface pressure. Of the two components, the one which forms the comparatively less stable monolayer is squeezed out first, followed by the further compression of the other component. It is also seen from Figures 2 and 3 that the collapse pressures of the mixed monolayers are larger than that of pure Ru(II) but smaller than that of AA. If the isotherms are replotted with area per AA instead of mean area of all molecules as the X axis, the isotherms of a mixture monolayer with different ratios could not merge below the collapse pressure. All of the results mentioned above infer that the two components, Ru(II) and AA, are miscible and the Ru(II) would not be squeezed out of the monolayers under collapse pressure. There would not be any pure Ru(II) islands in the monolayer, otherwise the collapse should be observed around 35 mN/m corresponding to collapse pressure of pure Ru(II) monolayer. When the mixing ratio of Ru(II)AA is larger than 1:2, the collapse pressure will decrease with increasing of Ru(II) component down to the collapse pressure of pure Ru(II) monolayer, and there is no other collapse pressure observed (Figure 2). That fact indicates that only one miscible phase exists in this range of mixing ratios and AA are dispersed into the sea of Ru(II) homogeneously in this situation. On the other hand, when the mixing ratio of Ru(II)-AA is smaller than 1:2 (Figure 3), there is a plateau at the surface pressure around 40 mN/m. The region of the plateau decreases gradually until it disappears as the proportion of AA in the mixture increases to unity. There must exist two phases in the monolayer: one corresponds to the miscible Ru(II)-AA monolayer with good miscibility, the other corresponds to the surplus pure AA monolayer. The monolayer in the miscible area begins to collapse when the surface pressure is above 40 mN/m. The miscibility of the two components and the nature of interaction between molecules could also be examined by calculating the excess free energy of mixing. The integral for the excess free energy, ∆Gex, can be calculated by drawing a perpendicular line to the x axis from the (20) Gains, L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1976.
Langmuir, Vol. 16, No. 11, 2000 5139 Table 1. Excess Free Energy ∆Gex ((mN/m) nm2)
mole fraction of Ru(II) in mixture
π ) 10 mN/m
π ) 20 mN/m
π ) 30 mN/m
0 0.10 0.125 0.143 0.2 0.25 0.333 0.5 0.833 0.857 0.875 0.889 1
0 2.08 2.78 3.21 3.58 4.43 4.85 4.41 3.24 2.95 2.21 1.14 0
0 3.32 4.66 5.40 6.30 7.78 8.62 7.47 6.37 5.73 4.22 2.32 0
0 3.95 6.18 7.15 8.34 10.52 11.34 9.94 8.62 7.60 5.36 3.04 0
isotherm at the respective surface pressure using the equation21,22
∆Gex )
∫0π (A12 - χ1A1 - χ2A2) dπ
where A1, A2, and A12 are the molecular areas at surface pressure π for pure components Ru(II) and AA and for the mixture, respectively. χ1 and χ2 are the molar fraction of the components in the monolayer. If the mixture monolayers are immiscible (phase separated), the integral function (A12 - χ1A1 - χ2A2) and ∆Gex should be equal to zero. The nonzero ∆Gex infers that some kind of mixture takes place. The excess free energies for various compositions at three different surface pressures for three different monolayers are listed in Table 1. Excess free energy is positive at all mole fractions and different pressures. That gives further proof for the miscibility of Ru(II) and AA in a mixture monolayer on the pure water subphase. ∆Gex has a maximum at the ratio of Ru(II):AA ) 1:2 and at π ) 30 mN/m. The positive ∆Gex indicates that the excess free energy comes mainly from AA’s obstruction effect among Ru(II) molecules. The positive divalent charge on the Ru(II) in the monolayers can be neutralized by AA molecules located on both sides to form a “combined group”, thus enhancing the adhesion among molecules in the monolayer. The fact that excess free energy increases with increasing of the number of the “combined group” reveals the repulsive interaction that exists between the “combined group” and Ru(II) (or AA). It can also be seen from Figure 2 that the isotherms of the mixture monolayers with Ru(II):AA ratio of 1:2 and 1:1 have their plateaus at the surface pressure of ∼10 mN/m, indicating that a phase transition was taking place between expanded and condensed phases. Some kind of molecular rearrangement was taking place. The monolayer with a mixture of Ru(II):AA ) 1:2 has the highest compressibility at the surface pressure of 20-40 mN/m, suggesting that it would be the best ratio to form a stable monolayer. When the isotherm data in Figures 2 and 3 were differentiated with respect to area and dπ/dA (A being mean molecular area) were plotted as a function of A (Figures 4 and 5), the phase transition and compressibility were revealed prominently: the minimum of dπ/dA and the humps correspond to the compressibility and phase transition, respectively. The pure Ru(II) monolayer possesses a relatively higher compressibility at A = 1.8 nm2, and there is no phase transition until the collapse surface pressure (curve 7 in Figure 4). With increasing amount (21) Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; Interscience: New York, 1976; p 148. (22) Shembekar, V. R.; Dhanabalan, A.; Talwar, S. S.; Contractor, A. Q. Thin Solid Films 1999, 342, 270-276.
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Figure 4. Differentiated π-A curves of mixtures of Ru(II)AA in different ratios on pure water: 1, 1:2; 2, 1:1; 3, 5:1; 4, 6:1; 5, 7:1; 6, 8:1; 7, pure Ru(II).
Figure 5. Differentiated π-A curves of mixtures of Ru(II)AA in different ratios on pure water: 1, pure AA; 2, 1:9; 3, 1:7; 4, 1:6; 5, 1:4; 6, 1:3; 7, 1:2.
of AA component the compressibility decreases rapidly, and some humps appear (Figure 4). The monolayer with the Ru(II):AA ratio of 5:1 has the lowest compressibility (curve 3 in Figure 4). When the AA component is added up to the Ru(II):AA ratio of 1:2, the highest compressibility is reached among the mixed monolayers and the phase transition becomes clear (curve 1 in Figure 4). If the amount of AA component increases continuously to the Ru(II):AA ratio of 1:6, the compressibility will decrease to a lowest state and the humps will gradually become more vague (Figure 5). From then on, the more AA component the monolayers contain, the higher compressibility and the more vague the humps of the possess. According to above discussions, we come to the conclusion that Ru(II):AA ) 1:2 is the best mixture ratio for forming stable and condensed mixed monolayer, and any surplus Ru(II) or AA is not favorable for the mixed monolayer on pure water subphase. LB Film Formation. The homogeneous composite Y-type mixed LB films with the mixture of Ru(II):AA ) 1:2 were deposited onto microscope cover-glass slides at a constant surface pressure of 30 mN/m with a transfer ratio of unity. The dry time for every deposited layer is about 10 min. The proposed structure is shown in the right corner of Figure 6. The UV absorption spectra of Ru(II) in acetonitrile and in LB films with various numbers of deposited layers are shown in Figure 7. The absorption peaks at 435 and 470 nm are the characteristic absorption of Ru(II) due to metal to ligand charge transfer (MLCT) (dash dot line in Figure 7).23,24 Fortunately, there are two familiar absorption peaks (23) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85.
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Figure 6. X-ray diffraction profiles from an 21 layers of mixed LB film with mixing ratio of Ru(II):AA ) 1:2 on glass slide. The inserted scheme is the proposed structure of the mixed LB film.
Figure 7. UV-visible absorption and luminescence spectra of Ru(II) in acetonitrile solution (dash dot lines) and in mixed LB films (solid lines) with mixing ratio of Ru(II):AA ) 1:2 in different layers: 1, 41 layers; 2, 31 layers; 3, 21 layers and 4, 11 layers.
at 440 and 476 nm in Figure 7, indicating that Ru(II) ions did deposit with arachidic acid molecules together onto the substrate. Compared with absorption of Ru(II) in solution, the absorption of Ru(II) in LB films slightly shifted toward the longer wavelength region, suggesting that some interaction among Ru(II) and AA due to the close molecular packing in the LB films such as J aggregates.25 Moreover, the absorption at 476 nm of Ru(II) in the LB films increases with increasing number of deposited layers. A linear relationship between the absorption and the number of layers was obtained for LB films with less than 41 layers (line a in Figure 8). The good linearity indicates that the Ru(II) ions have incorporated with arachidic acid molecules and can be successively and regularly deposited layer by layer during the whole deposition process. The fluorescence spectra of the mixed LB films are shown in Figure 7. The fluorescence maximums are observed at 625 nm. It could be seen from Figure 7 that a blue shift from 635 to 625 nm had taken place when Ru(II) was dissolved in acetonitrile and incorporated in AA LB films, respectively. There are some papers26-28 (24) Lu, W. X.; Shi, J. J.; Wang, H. B. Spectrosc. Spectral Anal. 1999, 10, 5 (in Chinese). (25) Czikkely, V.; Forsterling, D. H.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207. (26) Wrighton, M.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998. (27) Giordano, P. J.; Wrighton, M. S. J. Am. Chem. Soc. 1979, 101, 2888. (28) McKiernan, J.; Pouxviel, J. C.; Dunn, B.; Zink, J. I. J. Phys. Chem. 1989, 93, 2129.
Component-Controllable Mixed Monolayers
Figure 8. UV-vis absorbency measured at 476 nm (a) and luminescence measured at 625 nm (b) versus number of layers for mixed LB films with mixing ratio of Ru(II):AA ) 1:2.
dealing with the mechanism of so-called “rigidochromism” originally termed by Wrighton and Morse.26 Ru(II) emits light of longer wavelengths in a fluid solution than in a rigid matrix. The mixed LB films containing Ru(II) still have fairly high luminescence efficiency as Figure 7 shown. The linear relationship between emission at 625 nm and the number of layers (up to 41 layers) indicates the successive and regular deposition of the mixed monolayer (line b in Figure 8). The series of luminescence spectra measured at various multilayers also infer that probability of a self-quenching process in AA-Ru(II) multilayers is quite small, and it is very important for applications. Figure 6 shows the diffraction profiles of a 21-layer mixed LB film with a mixture of Ru(II):AA ) 1:2 on a glass slide in the 2θ region of 1.2°-10°. The results also indicated that the preparation of the mixed films as described above was successful and the proposed structure shown in Figure 6 was proved. The appearance of several Bragg diffraction peaks in the range 2θ ) 1.2°-10° indicated the presence of a regular, periodic structure in the LB film. Since we concluded that Ru(II) are incorporated in the multilayers, the Ru(II) ions would be expected to act as efficient scattering centers and these Bragg reflections correspond only to the d-spacing of the bilayer distances. It is reasonable that the (003) peak disappeared from Figure 6. According to Bragg’s law, the extinction of (003) peak suggests that there must be two sets of multilayers arranged alternatively with the same d-spacing value in the mixed LB films (Figure 6). When the distance between the two sets of multilayers is equal to one-sixth of the d value, the reflection lights coming from the two set of multilayers will have phase differences of π, 3π, ... at the reflection angles of (003), (009), ..., leading to the extinction of (003), (009), ... peaks, respectively. The d-spacing in the LB films can be expressed as d ) ld00l. From this equation the average spacing in the mixed LB films on the glass slide could be calculated and is d ) 5.11 nm.
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Due to the presence of the bulky Ru(II) inserted between AA, the AA cannot be packed tightly and the long chains cannot fully extended as a result. The size of Ru(II) is about 1.7 nm which is not matched with the value of (d-spacing)/6. This infers that to some extent penetration of Ru(II) molecules between the two adjacent layers of Ru(II) molecules took place. The Bragg reflections of the Ru(II)-AA LB films greatly reduced in peak intensity and sharpness compared with that of normal fatty acid LB films. This is mainly because of the bulky size of Ru(II) and probably because of local failure of deposition of the mixed LB films. However, what is important is that the mixed LB films with mixing ratio of Ru(II):AA ) 1:2 are uniform enough to allow making optical devices. Conclusion The pure Ru(II) in the mixture Ru(II)-AA monolayer with different mixing ratios on pure water can penetrate one another to form a condensed monolayer at high pressure. The AA molecule with long chain strands in the Ru(II) monolayer will prevent Ru(II) molecules around it from penetrating one another and lead to an expanded monolayer. Collapse pressure and excess free energy of the mixed monolayers with different compositions are a good indication of miscibility of the components of the mixture system. The mixture with a Ru(II):AA ratio of 1:2 is the best ratio for the stable and condensed mixed monolayer, and any surplus Ru(II) or AA is not favorable for the mixed monolayer on pure water subphase. The UV absorption spectra and luminescence spectra measured at various multilayers infer that the Ru(II) ions have been incorporated with arachidic acid molecules and give very good optical properties in the visible light region and can be successively and regularly deposited layer by layer during the depositing process. The low-angle X-ray diffraction has given further proof that the mixed LB multilayers provide regular orientation of the molecules involved. We proposed, for the first time to our knowledge, that the metal complexes could be used as surface ions (not as traditional way of using metallic ions in subphase) to produce stable and uniform mixed LB films. This concept is important for future applications such as optical gas sensors. Threshold concentration of Ru(II) in mixing AAOTS-Ru(II) monolayer on pure water and depositing Ru(II)-controlled LB films for optical gas sensors are in progress in our lab. Acknowledgment. Professor R. H. Tredgold’s concern and help are gratefully acknowledged. This work is supported by the National Nature Science Foundation of China LA991630L
