Effect of Subphase pH and Metal Ion on the Molecular Aggregates of

A novel amphiphilic Ru(II) complex, Ru(L18)(tpy-PO3H)PF6, where L18 ) 2,6-bis(N-octadecylbenzimi- dazol-2-yl)pyridine and tpy-PO3H)2,2′;6′ ...
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Langmuir 2002, 18, 3528-3536

Effect of Subphase pH and Metal Ion on the Molecular Aggregates of Amphiphilic Ru Complexes Containing 2,2′:6′,2′′-Terpyridine-4′-phosphonic Acid at the Air-Water Interface Kezhi Wang,†,‡ Masa-aki Haga,*,† Md. Delower Hossain,† Hitoshi Shindo,† Keiichi Hasebe,† and Hideaki Monjushiro§ Integrated Inorganic Materials Chemistry Laboratory, Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan, and Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama-cho, 1-1 Toyonaka, Osaka 560, Japan Received September 4, 2001. In Final Form: February 7, 2002 A novel amphiphilic Ru(II) complex, Ru(L18)(tpy-PO3H)PF6, where L18 ) 2,6-bis(N-octadecylbenzimidazol-2-yl)pyridine and tpy-PO3H ) 2,2′;6′,2′′-terpyridine-4′-phosphonic acid, was synthesized. The complex, Ru(L18)(tpy-PO3H)PF6, at the air-water interface exhibits a molecular aggregation, which is strongly affected by the solution pH or metal coordination. Surface pressure-area isotherms of Ru(L18)(tpy-PO3H)PF6 show a smaller molecular occupied area at lower pH. Furthermore, in situ UV-vis absorption spectra of the Ru complex at the air-water interface also depend on the subphase pH, which is distinct from the absorption spectra for CH3CN/buffer or chloroform solution. As the subphase becomes acidic, the absorption band at 363 nm for the intraligand π-π* transitions is sharpened with a little red shift, which strongly reveals the formation of molecular aggregates at the air-water interface. The monolayers formed on different subphases can be successfully transferred onto hydrophilic and hydrophobic glass substrates. Low-angle X-ray diffractions and UV-vis and X-ray photoelectron spectroscopy measurements indicate a regular layered structure in the Langmuir-Blodgett (LB) films transferred on the solid substrate. A tape-shaped morphology from the atomic force microscopy measurement was observed for the LB monolayer on mica. When the subphase containing metal sulfate MSO4 (M ) Zn, Cd or Mn) is used, metal coordination at the air-water interface was observed. The X-ray photoelectron spectroscopy measurement demonstrates a 1:1 ratio of the Ru complex/metal, M (M ) Zn, Cd, and Mn), for the LB composite films, which is isostructural with well-characterized metal phosphonate layered solids such as Mn(O3PC6H5)H2O. The Ru(II/III) oxidative electrochemical response was observed at +0.92 V vs SCE for the monolayer LB film on the indium-tin oxide electrode, which is not varied on the metal coordination to the phosphonate group. Therefore, Ru(L18)(tpy-PO3H)PF6 is a good template at the air-water interface for the synthesis of redox-active layered inorganic composites.

Introduction There has been growing interest in ordered molecular architectures such as monolayers,1,2 one-dimensional rods and tubules,3 and two-dimensional aggregates on the surface. The ordered molecular materials in supramolecular entities have a potential use for the functional molecular devices such as chemical sensors, molecular switches, and electroluminescence. The rational syntheses of artificial nanoscale and mesoscale structures have the potential for the development of novel functional materials. The important features of these syntheses are the prefabrication of molecular subunits or components, which subsequently aggregate, either spontaneously or under special conditions into larger meso structures. In recent years, the self-assembly of surfactants into micelles, which * To whom correspondence should be addressed. E-mail: mhaga@ apchem.chem.chuo-u.ac.jp. † Chuo University. ‡ Current address: Chemistry Department, Beijing Normal University, Beijing 100875, People’s Republic of China. § Osaka University. (1) Roberts, G. G. Langmuir-Bldgett Films; Plenum Press: New York, 1990. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Harcourt Brace Jovanivich: Boston, MA, 1991. (3) Ghadiri, M. R.; Granja, J. R.; Milligan, R. A.; McRee, D. E.; Khazanovich, N. Nature 1993, 366, 324.

reveals several different aggregates such as spherical, lamellar, rod shapes, or cylindrical shapes, is used as a template for the silica-based inorganic composites. By using a similar strategy, interesting inorganic/surfactant composite materials have been reported.4,5 The air-water interface in a Langmuir trough provides a restricted reaction environment for the composite formation of amphiphilic molecules. Therefore, the Langmuir-Blodgett (LB) technique has been widely used to fabricate wellcontrolled lamellar structures from two-dimensional molecular composite films.6 Recently, the complex formation between monolayers of an amphiphilic ligand and transition metal ions in the subphase has been reported, in which the interfacial coordination reaction took place at the airwater interface.7-9 However, the use of coordination bonds for the fabrication of supramolecular assemblies at the interface has not been extensively studied so far. (4) MacKnight, W. J.; Ponomarenko, E. A.; Tirrell, D. A. Acc. Chem. Res. 1998, 31, 781-788. (5) Antonietti, M.; Goltner, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 910-928. (6) Rapaport, H.; Kuzmenko, I.; Berfeld, M.; Kjaer, K.; Als-Nielsen, J.; Popovitz-Biro, R.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. J. Phys. Chem. 2000, 104, 1399-1428. (7) Liu, M.; Kira, A.; Nakahara, H. Langmuir 1997, 13, 779-783. (8) Liu, M.; Kira, A.; Nakahara, H. Langmuir 1997, 13, 4807. (9) Nagel, J.; Oertel, U.; Friedel, P.; Komber, H.; Mobius, D. Langmuir 1997, 13, 4693-4698.

10.1021/la011386+ CCC: $22.00 © 2002 American Chemical Society Published on Web 03/30/2002

Aggregation of Amphiphilic Ru Complexes

Our present study is focused on the surface coordination chemistry between amphiphilic redox-active Ru complexes and metal ions in a subphase at the air-water interface for the synthesis of ordered inorganic-organic composites toward molecular electronic devices. We have recently reported a new method called “metal complex as a ligand at the air-water interface” in order to prepare supramolecular complexes at the air-water interface, in which N-heterocyclic group has been used as a coordinating site.10 We are now using the phosphonate group for this purpose. Metal phosphonates are well-known compounds with layered structure, and some of them have practical applications as hosts in intercalation compounds, in catalyst supports, and in sensing and separation.11-17 The metal-phosphonate binding interaction has also been successfully used to deposit monolayer and multilayer thin films on the solid surface by self-assembled and LB techniques.18-23 An interesting magnetic property has been observed in the LB films based on the coordination of amphiphilic organophosphonate with magnetic metal ions at the air-water interface.22,23 The new organic-inorganic films containing well-organized layers of polyoxoanions such as Keggin polyoxometalates [Xn+W12O40](8-n)- (Xn+ ) 2(H+), PV, SiIV, BIII, CoII) have been constructed toward the ultrathin magnetic films.24,25 A phosphonate group has a strong affinity for metal oxide surfaces, which is suitable for the immobilization of photochemical sensitizers onto TiO2 or SnO2 electrodes.26-28 In the present study, new amphiphilic Ru complexes containing both 2,6-bis(N-octadecylbenzimidazol-2-yl)pyridine and 2,2′:6′,2′′terpyridine-4′-phosphonic acid29 were synthesized, and the interfacial coordination ability of the Ru complex at the air-water interface was examined. This amphiphilic Ru complex has several advantages: First, two alkyl chains on 2,6-bis(benzimidazol-2-yl)pyridine can increase amphiphilic property of the metal complexes and stabilize the monolayer at the air-water interface. Second, molecular ordering of octahedral metal complexes at the two(10) Haga, M.; Kato, N.; Monjushiro, H.; Wang, K. Z.; Hossain, M. D. Supramol. Sci. 1998, 5, 337. (11) Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420. (12) Cao, G.; Lynch, V. M.; Yacullo, L. N. Chem. Mater. 1993, 5, 1000. (13) Cao, G.; Mallouk, T. E. Inorg. Chem. 1991, 30, 1434. (14) Cao, G.; Lee, H.; Lynch, V. M.; Mallouk, T. E. Inorg. Chem. 1988, 27, 2781. (15) Carling, S. G.; Day, P.; Visser, D.; Kremer, R. K. J. Solid State Chem. 1993, 106, 111. (16) Scott, K. J.; Zhang, Y.; Wang, R.; Clearfield, A. Chem. Mater. 1995, 7, 1095. (17) Drumel, S.; Janvier, P.; Barboux, P.; Bujoli-Doeuff, M.; Bujoli, B. Inorg. Chem. 1995, 34, 148. (18) Lee, H.; Kepley, L. J.; Hong, H.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (19) Vermeulen, L. A.; Snover, J. L.; Sapochak, L. S.; Thompson, M. E. J. Am. Chem. Soc. 1993, 115, 11767. (20) Katz, H. E.; Scheller, G.; Putvinski, T. E.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (21) Byrd, H.; Pike, J. K.; Ma, J.; Nagler, S. E.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 295. (22) Seip, C. T.; Byrd, H.; Talham, D. R. Inorg. Chem. 1996, 35, 3479. (23) Seip, C. T.; Granroth, G. E.; Meisel, M. W.; Talham, D. R. J. Am. Chem. Soc. 1997, 119, 7084. (24) Clemente-Leon, M.; Agricole, B.; Mingotaud, C.; Gomez-Garcia, C. J.; Coronado, E.; Delhaes, P. Angew. Chem., Int. Ed. Eng. 1997, 36, 1114. (25) Clemente-Leon, M.; Agricole, B.; Mingotaud, C.; Gomez-Garcia, C. J.; Coronado, E.; Delhaes, P. Langmuir 1997, 13, 2340. (26) Trammell, S. A.; Wimbish, J. C.; Odobel, F.; Gallagher, L. A.; Narula, P. M.; Meyer, T. J. J. Am. Chem. Soc. 1998, 120, 13248. (27) Zakeeruddin, S. M.; Nazeeruddin, M. K.; Pechy, P.; Rotzinger, F. P.; Humphry-Baker, R.; Kalyanasundaram, K.; Gratzel, M.; Shklover, V.; Halibach. Inorg. Chem. 1997, 36, 5937-5946. (28) Montalti, M.; Wadhwa, S.; Kim, W. Y.; Kipp, R. A.; Schmehl, R. H. Inorg. Chem. 2000, 39, 76-84. (29) Nazeeruddin, M. K.; Zakeeruddin, S. M.; Humphry-Baker, R.; Kaden, T. A.; Gratzel, M. Inorg. Chem. 2000, 39, 4542-4547.

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dimensional surface may be controlled by using this ligand because the C2v symmetry of this complex reduces possible geometrical isomers around the Ru ion compared to bidentate ligands such as 2,2′-bipyridine. Finally, the Ru complex possesses both electrochemical and photochemical activities.30,31 Experimental Section Materials. 1-Bromooctadecane (Nacalai) and ruthenium trichloride trihydrate (N.E.Chemcat, Tokyo) were used without further purification. Dichrolomethane and chloroform were purified by distillation over P2O5. The compounds 2,6-bis(benzimidazol-2-yl)pyridine,32 2,6-bis(N-octadecylbenzimidazol2-yl)pyridine (L18),10 Ru(L18)Cl3,10,33-35 and 2,2′:6′,2′′-terpyridine4′-diethylphosphonate36 were prepared according to the literature methods. CdSO4‚3/8H2O, MnSO4‚5H2O, ZnSO4‚7H2O, and dichloroethane were all Wako commercial product of high purity and were used without further purification. All other supplied chemicals were of standard reagent grade quality. Indium-tin oxide (ITO) coated glass plates, purchased from Central Glass Co (surface resistance 280 °C. 1H NMR (DMSO-d6), δ, ppm: d 8.83 (dd, 6H), 7.55 (t, 2H), 7.30 (t, 2H). Mass spectrum: m/z ) 313 (M+). Anal. Calcd for C15H12N3O3P: C, 57.51; H, 3.86; N, 13.41. Found: C, 57.09; H, 3.44; N, 13.26. Synthesis of the Amphiphilic Complex, [Ru(L18)(tpyPO3H)](PF6). The ligand 2,2′:6′,2′′-terpyridine-4′-phosphonic acid (0.10 g, 0.33 mmol) was dissolved in ethylene glycol (30 mL) by heating. Then [Ru(L18)Cl3] (0.33 g, 0.33 mmol) was added to the reaction mixture and the heating was continued at 100 °C for a further 8 h, during which time the color of the solution changed from red-brown to brown. Upon cooling to room temperature, a saturated aqueous solution of NH4PF6 was added dropwise to the resulting solution, leading to the formation of a red-brown precipitate. The precipitate was collected by filtration and dried in vacuo. The crude product was then purified by column chromatography using SP Sephadex LH-20 with CH3CN-CHCl3 mixture as an eluant. The desired complex was eluted with CH3CN/CHCl3 (2:3 v/v) as a brown-red band. The pure complex was collected by evaporation of the solvents. Yield: 0.28 g (61%). Anal. Calcd for C70H96N8O3P2F6Ru‚2H2O: C, 59.60; H, 7.15; N, 7.94. Found: C, 59.34; H, 7.23; N, 7.93. FAB mass spectrum: m/z ) 1375 (M+), 1230 ([M - PF6]+). 1H NMR (DMSO-d6), δ, ppm: 9.36 (d, 2H, H(1)), 8.88 (d, 2H, H(2)), 8.87 (d, 2H, H(3)), 8.57 (t, 1H, H(4)), 7.83 (t, 2H, H(5)), 7.81 (d, 2H, H(6)), 7.42 (d, 2H, H(7)), 7.25 (t, 2H, H(8)), 7.18 (t, 2H, H(9)), 6.91 (t, 2H, H(10)), 5.84 (d, 2H, H(11)), 4.9 (t, 4H), 1.96 (t, 4H), 1.37 (t, 4H), 1.21 (m, 56H), 0.82 (t, 6H). LB Film Preparation. All LB experiments were performed essentially as previously described.10 The surface pressure-area (30) Chu, B. W.; Yam, V. W. Inorg. Chem. 2001, 40, 3324. (31) Taniguchi, T.; Fukasawa, Y.; Miyashita, T. J. Phys. Chem. B 1999, 103, 1920. (32) Addison, A. W.; Burke, P. G. J. Heterocycl. Chem. 1981, 18, 803. (33) Sullivan, B. P.; Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1980, 19, 1404. (34) Leising, I. A.; Kubow, S. A.; Churchill, M. R.; Buttrey, L. A.; Ziller, J. W.; Takeuchi, K. J. Inorg. Chem. 1990, 29, 1306. (35) Nazeeruddin, M. K.; Muller, E.; Humphry-Baker, R.; Vlachopoulos, N.; Gratzel, M. J. Chem. Soc., Dalton Trans. 1997, 45714578. (36) Pechy, P.; Rotzinger, F. P.; Nazeeruddin, M. K.; Kohle, O.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gratzel, M. J. Chem. Soc., Chem. Commun. 1995, 65.

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Table 1. Deposition Conditions and X-ray Diffraction Data for the Ru LB Films entry LB filma

pH

target pressure (mN/m)

substrateb

2θ/(00n) (deg)

d (nm)

fwhmc (deg)

Ru1 Ru2 Ru3 Ru4 Ru-Zn Ru-Mn Ru-Cd Ru Ru-Mn Ru-Zn Ru-Cd

5.3-5.4 5.3-5.4 2.8-3.0 9.2-9.4 5.8-6.0 6.2-6.4 5.4-5.6 5.3-5.6 6.4-6.6 6.1-6.3 5.4-5.5

20 55 30 25 25 25 20 20 20 20 20

h h h h h h h b b b b

2.20 (001); 4.34 (002); 6.44 (003) 2.16 (001); 4.24 (002); 6.36 (003) 2.02 (001); 4.14 (002); 6.36 (003) 2.24 (001) 2.36 (001); 8.64 (004) 1.82 (001) 2.16 (001); 6.36 (003) 1.94 (001); 6.16 (003) 2.02 (001) 2.02 (001); 8.52 (004) 2.02 (001); 6.32 (003)

2.03 2.07 2.13 1.97 1.96 2.43 2.06 2.21 2.19 2.13 2.14

0.27 0.28 0.50 0.55 0.36 0.78 0.33 0.39 0.89 0.56 0.47

a The symbols in the entry column reveal the difference of the substrate properties. The normal character stands for the use of a hydrophilic glass as a substrate. The italic character stands for the use of a hydrophobic glass precoated with OTS. See the Experimental Section in the text and Scheme 1. b Key: h ) hydrophilic glass; b ) hydrophobic glass precoated with OTS. c Indicated by the strongest peak.

Scheme 1. Two LB Deposition Methods for the Incorporation of the Metal Ion onto the Hydrophilic (a) and Hydrophobic Substrates (b), Which Is Abbreviated with Roman and Italic Characters Such as Ru-M and Ru-M

isotherms were recorded at a compression speed of 0.26 nm2 molecule-1 s-1. Hydrophilic treatments of glass or quartz substrates were made as described before.37 The substrates were made hydrophobic by following a literature procedure.23 All LB deposition onto the substrate was made in Y-type mode. The experimental LB deposition conditions were summarized in Table 1 together with the entry name in the first row and X-ray diffraction data. The Ru LB composite films containing metal ions (Zn2+, Cd2+, Mn2+) were prepared in two different processes as shown in Scheme 1: One is to transfer the amphiphilic Ru complex from the metal-ion containing subphase at suitable pH onto a hydrophilic glass substrate, and resultant films are referred to as Ru-M (M ) Zn(II), Cd(II), and Mn(II)) films. The other process used in the present study consists of two steps: first, the films were transferred onto the hydrophobic glass substrate from metal ion free pure water subphase (pH 5.4), which is referred hereafter to Ru (italics) film. Then, the films, Ru, were soaked in metal ion containing aqueous solutions (1 × 10-2 mol dm3) of suitable pH for 20 h, thus the obtained films are referred to as Ru-M (37) Wang, K. Z.; Haga, M.; Monjushiro, H.; M., A.; Sasaki, Y. Inorg. Chem. 2000, 39, 4022.

(M ) Zn(II), Cd(II), and Mn(II)) films (see Table 1). For both Ru-M and Ru-M films, final treatments were done by careful soaking in ultrapure water for a few minutes for several times, followed by air-drying overnight prior to any physical measurements in order to remove physically adsorbed metal ions but not to disturb the organized structure of the films. Physical Measurements. UV-vis spectra are obtained on a Shimadzu 3200 or a Hitachi U-4000 UV spectrophotometer. Electrochemical data are collected through a BAS 100/B electrochemical analyzer in a conventional three-electrode cell. X-ray diffraction patterns are recorded on an X-ray powder diffractometer MXP3VA (MacScience, Japan). X-ray photoelectron spectra are measured on a VG Scientific ESCA Mark II photoelectron spectrometer with Mg KR radiation as an excitation source. Binding energies were calibrated using C1s binding energy of 285.0 eV as a reference. In Situ UV-vis Spectra of Monolayers at the Air-Water Interface. In situ UV-vis spectra of monolayers on the pure water subphase were recorded on a UV-vis spectrophotometer USP-500 (Unisoku Ltd.) with a Hamamatsu R2949 photomultiplier and a Y-type quartz optical fiber, according to the reported apparatus configuration.38 Unless otherwise mentioned, the incidence angle of the beam relative to the normal of the subphase surface is zero. For all measurements, a Ru complex free subphase is used as a reference. Atomic Force Microscopy (AFM) Measurements. The AFM measurements were performed with a NanoScope III from Digital Instruments with a 10 µm scanner in intermittent contact mode (“tapping mode”) at room temperature. The cantilevers used were made of silicon. The LB monolayer of [Ru(L18)(tpyPO3H)](PF6) with various subphase conditions at the air-water interface was transferred to a freshly cleaved mica. After being dried in the desiccator overnight, the surface morphology of the LB monolayer was measured with AFM.

Results and Discussion Preparation of Amphiphilic Ru Complex. A filmforming Ru complex, [Ru(L18)(tpy-PO3H)]PF6, where L ) 2,6-bis(N-octadecylbenziimidazolyl)pyridine and tpyPO3H ) monoprotonated-4-phosphonate-2,2′;6′,2′′-terpyridine, was synthesized according to Scheme 2 and was well characterized by 1H NMR, IR, UV-vis, and FAB-MS spectroscopies and elemental analyses. Since a proton on phosphonic acid moiety can be easily dissociated during the preparation,29 the complex was isolated as monoprotonated [Ru(L18)(tpy-PO3H)]PF6. However, the protonation state can be controlled by the addition of acid or base. The complex is soluble in dichloroethane, toluene, DMF, and DMSO, sparingly soluble in chloroform, and insoluble in water. Surface Pressure-Area (π-A) Isotherms of [Ru(L18)(tpy-PO3H)]PF6: pH Dependence. Surface pres(38) Ouyang, J.; Lever, A. B. P. J. Phys. Chem. 1991, 95, 5272.

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Figure 1. Surface pressure-area isotherms of the Ru complex, [Ru(L18)(tpy-PO3H)](PF6), on the subphase at three different pH values: pH ) 2.8, 5.4, and 9.3. Scheme 2. Synthetic Route

Figure 2. Compression-expansion isotherms of [Ru(L18)(tpyPO3H)](PF6) on pure water at reversal pressures of (a) 35 and (b) 65 mN m-1.

sure-area isotherms of the Ru complex on aqueous subphases at different pH (pH 2.85, 5.40, and 9.33) are shown in Figure 1. On the water subphase at pH 5.40, the surface pressure appears at a molecular area of 1.50 nm2, and subsequently the molecules go into liquid expanding phase at an onset surface pressure of around 10 mN m-1 and a solid condensed phase (10-40 mN m-1) to form a closely packed monolayer film. Further compression induces a phase transition at 50 mN m-1, and the molecules go into a second solid condensed phase. With the progress of compression, the monolayer film collapses at a surface pressure of 65 mN m-1. By linear extrapolation of the phase (10-46 mN m-1) to a zero surface pressure, a limiting molecular area was derived to be 1.30 nm2 molecule-1, close to the value which we previously reported for [Ru(L18)(tppz)]2+ complex (1.40 nm2 molecule-1) (tppz ) 2,3,5,6-tetrakis(2′-pyridyl)pyrazine). This indicates that the phase (10-40 mN m-1) corresponds to the formation of closely packed monolayer film on the air-water interface. The second solid condensed phase has a limiting molecular area of 0.65 nm2, which is exactly one-half that for the first phase (10-46 mN m-1). At least three mechanisms are possible for this second phase: first, bilayer formation during compression to small area;39-41 next, the formation of molecular aggregation at the airwater interface also leads to dramatic decrease in molecular occupied area;42 finally, a orientation change of the Ru complex along a C2 axis crossing the tpy-Ru-L18 direction from parallel to perpendicular to the water surface. To gain more insight into the character of these two condensed phases (10-46 and 50-65 mN m-1) and to investigate the stability of the monolayers, we recorded (39) Takeda, F.; Matsumoto, M.; Takenaka, T.; Fujiyoshi, Y. J. Colloid Interface Sci. 1981, 84, 220. (40) Takeda, F.; Matsumoto, M.; Takenaka, T.; Fujiyoshi, Y.; Uyeda, N. J. Colloid Interface Sci. 1983, 91, 267. (41) Wagner, J.; Michel, T.; Nitsch, W. Langmuir 1996, 12, 2807. (42) Chen, H.; Herkstroeter, W. G.; Perlstein, J.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 5138.

compression-expansion (CE) isotherms of the monolayers. When a reversal pressure was set to 35 mN m-1, even three CE cycles did not show any appreciable hysteresis (only first CE curve is shown in Figure 2a). Furthermore, when the monolayer was left on a water surface overnight, the area loss was less than 5%. This is a manifestation of extraordinary stability of the monolayer film on the water surface. In contrast, the molecular area of [Ru(bpy)2(4,4′-octadecyl-2′,2′-bipyridine)]2+ monolayer films decreased with time,43 some counteranions or/and some auxiliary film-forming materials such as stearic acid had to be added into subphase or/and mixed with the Ru complex in order to stabilize the films. Contrary to the CE behavior of the first phase, the second phase reveals an appreciable hysteresis upon expansion at a reversal pressure of 65 mN m-1 (Figure 2b). This implies that the second phase (50-65 mN m-1) is associated with a bilayer formation, which will be proved by AFM measurements as described later. On an acidic subphase (pH 2.85), the monolayers show a relatively long solid condensed region (10-50 mN m-1), which is immediately followed by monolayer collapse at ca. 53 mN m-1. The smallest limiting molecular area of 1.2 nm2 is obtained. On a basic subphase (pH 9.33), the Ru complex shows a more expanded π-A curve with a shorter solid condensed region (13-38 mN m-1) than that on the pure water subphase. A limiting molecular area was found to be 1.58 nm2, which is 0.18 nm2 larger than that on the pure water subphase. The experimental results were quite unexpected from the viewpoint of electrostatic repulsion, because the total charge of the Ru complex becomes zero after deprotonation of phosphonic acid group and the electrostatic repulsion between the Ru complex molecules would be reduced. These facts can be explained by assuming the additional attractive interactions such as hydrogen bonding through phosphonic acid headgroups under the acidic and neutral subphase conditions. On the basic subphase, the dissociation of proton on phosphonic acid headgroup destroys the hydrogen bonding and thus induces static repulsion between the localized negative charges on the phosphonate groups.44-47 Therefore, we propose that the Ru complex molecules at the acidic subphase condition will have both benzimidazole π-π and (43) Valenty, S. J.; Behnken, D. E.; Gaines, G. L. J. Inorg. Chem. 1979, 18, 2160.

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Scheme 3. Possible Molecular Orientation Modes for the Acidic and Basic Subphase Conditions at the Air-Water Interface

Figure 3. Surface pressure-area isotherms of the Ru complex, [Ru(L18)(tpy-PO3H)](PF6), on aqueous subphases containing 5 × 10-3 mol dm-3 metal sulfate, MSO4 (M ) Zn2+, Cd2+, Mn2+).

hydrogen bonding interactions, but at the basic condition these interactions will be destroyed as shown in Scheme 3. These proposed intermolecular interactions, particularly benzimidazole π-π interaction, are elucidated by in situ UV-vis spectral measurements described later. Another feature is that the monolayer collapses earlier at a surface pressure of 40 mN m-1 so that subsequent phase transfer almost disappeared. Recently, the pKa values of tpy-PO3H2 and its Ru complex, [Ru(tpyPO3H)(Me2bpy)(NCS)] (Me2bpy ) 4,4′-dimethyl-2,2′-bipyridine), have been reported as pKa1 < 2 and pKa2 ) 5.99 for the ligand and pKa ) 6.0 for the Ru complex, respectively. 29 The Ru complexes containing bipyridyl phosphonic acid, [Ru(bpy)2(bpppH2)] (bpy ) 2,2′-bipyridine and bpppH2 ) 4-(2,2′-bipyrid-4-yl)phenylphosphonic acid) revealed the dissociation with pKa1 ) ∼2, pKa2 ) 6.3). 28 From these data, the present amphiphilic Ru complex, [Ru(L18)(tpy-PO3H)] will have a similar pKa value (pKa ∼ 6) at the air-water interface, judging from the pH dependence of the π-A isotherms. Effect of Metal Ion in the Subphase. The π-A curves for the Ru complex on the metal ion containing subphase are shown in Figure 3. The limiting molecular areas were found to be ca. 1.3 nm2 for each case, almost identical to that obtained from pure water subphase, although the shape or behavior of the curve differs to some extent from the pure water subphase. This finding is contrary to Ru(L18)(tppz)-M (M ) Cu(II), Zn(II), and Cd(II)), Cu-stearic acid, and -behenic acid systems,10,44 where all expanded films were obtained. The expansions of the molecular area (44) Bettarini, S.; Bonosi, F.; Gabrielli, G.; G., M.; Puggelli, M. Thin Solid Films 1992, 210/211, 42. (45) Gabrielli, G.; Puggelli, M.; Carubia, G.; Pedocchi, L. Colloids Surf. 1989, 41, 1. (46) Binks, B. P. Adv. Colloid Interface Sci. 1991, 34, 343. (47) Aveyard, R.; Binks, B. P.; Carr, N.; Cross, A. W. Thin Solid Films 1990, 138, 361.

were attributable to large ion-ion interaction and probable structural change accompanying with the metal ion coordination. Particularly for the Ru(L18)(tppz) system we studied before, an intermolecular electrostatic repulsion provided a significant contribution to this expansion. On the other hand, metal ion coordination to the phosphonate headgroup in the present amphiphilic Ru(L18)(tpy-PO3H) complex can lead to the neutralization of the charge on the phosphonate group. Additionally, the phosphonate headgroup lies below the bulky Ru-terpyridine part and the parts are separated from each other to some extent in the film. These would greatly reduce static repulsion effect. To chemically incorporate metal ions into both Ru-M and Ru-M LB films, it is crucial for the aqueous subphases or solutions to be adjusted to suitable pH. As shown in Table 1, suitable pH for metal coordination greatly varies from one metal to another. This fact is similar to that observed for fatty acids. Among these three metal ions, Mn(II) is recognized to have the lowest coordination capability to the phosphonate group on the Ru complex because the highest pH is required for the Mn(II) coordination reaction at the interface. Among Mn(II), Zn(II), and Cd(II) ions, Cd(II) ion has largest stability for the complex formation with the Ru complex by considering the surface coordination of Cd(II) at the lowest subphase pH. This stability order does not follow the IrvingWilliams series48 because of zero crystal field stabilization energy afforded by d5 (Mn2+) and d10 (Zn2+ and Cd2+) configurations but follows an effective nuclear charge order of metal ions. In Situ UV-vis Spectroscopy of the Ru Complex Monolayers at the Air-Water Interface. The pHdependent UV-vis spectra of the Ru complex in CH3CN/ buffer (1:1 v/v) are shown in Figure 4, in which a broad metal-to-ligand charge transfer (MLCT) band centered at 488 nm and three intraligand π-π* peaks at 361, 342, and 322 nm revealed little change with the solution pH. This result is somehow surprising since distinct pHdependent UV-vis spectra have been reported in similar Ru complexes containing the phosphonato-polypyridyl ligands.28,29 In the present [Ru(L18)(tpy-PO3H)] complex, the MLCT transition involves the Ru (dπ) to bis(benzimidazolyl)pyridine (π*) localized one; therefore the deprotonation of the remote site of tpy-PO3H2 has little affect on the MLCT transition. On the other hand, in situ UV-vis spectra were greatly affected by the pH values of the subphase (see Supplemental Figure 1S). On the subphase at pH 2.8, a MLCT (48) Irving, H.; Williams, R. J. P. J. Chem. Soc. 1953, 3192.

Aggregation of Amphiphilic Ru Complexes

Langmuir, Vol. 18, No. 9, 2002 3533 Scheme 4. Proposed Molecular Packing for the Aggregates of [Ru(L18)(tpy-PO3H)](PF6) on the Acidic Subphase

Figure 4. The pH dependence of UV-vis spectra of the Ru complex, [Ru(L18)(tpy-PO3H)](PF6), in CH3CN/buffer (1:1 v/v) at various pH values: pH ) 2.00, 3.10, 4.30, 6.10, and 7.80.

band appears at ca. 490 nm, which is much weaker than that observed for the subphase at pH 5.4 and 9.5 and for the CH3CN/buffer solution. More surprisingly, a very strong peak at 363 nm and two weak peaks at 328 and 343 nm for the intraligand π-π* transitions were observed, which is distinct from that for the CH3CN/buffer solution. Therefore, these in situ UV-vis spectral changes of the monolayers are not due to electronic effect of proton dissociation from the phosphonate group of the Ru complex upon increasing pH but most probably are due to pHinduced change in molecular aggregation, accompanied by the variation of the strength of the dipolar transition after deprotonation of PO3H2 group. The almost absence of MLCT absorption band at normal incidence for in situ spectra of [Ru(L18)(tpy-PO3H)] implies that the MLCT transition moment is oriented mainly normal to the water surface, since a light electric field under the normal incidence angle is parallel to the water surface. Furthermore, the spectral change in the wavelength region of 300-380 nm strongly indicates the formation of molecular aggregates at the air-water interface preferentially on the acidic subphase. The molecular packing in the J-like molecular aggregates is schematically shown in Scheme 4, in which the benzimidazole π-π interactions exist. With an increase of the subphase pH from 2.8 to 9.5, the intraligand π-π* absorption bands are broadened, which indicates that the molecular interactions are weakened in the aggregates. The destruction of aggregates reflects the expansion of molecular area in the π-A isotherms on the basic subphase (see Figure 1 and Scheme 4). When the metal ion containing subphase was used, in situ UVvis spectra of the Ru complex monolayer show the characteristic absorption band at 363 nm, which was similar to those at the subphases pH 2.8 and 5.4 (see Supplemental Figure 2S). These results indicate the metal ion assists the formation of aggregate through the coordination. In situ UV-vis spectroscopy is known to be sensitive for characterizing the aggregation formation of monomers in the LB monolayer,38,42,49 substitution reactions,50 changes in molecular orientations,51,52 and metal ion coordination reactions of amphiphilics at the interface.7,8 (49) Haga, M.; Wang, K. Z.; Kato, N.; Monjushiro, H. Mol. Cryst. Liq. Cryst. 1999, 337, 89. (50) Wang, K. Z.; Haga, M. Mol. Cryst. Liq. Cryst. 2000, 342, 225.

Film Deposition of [Ru(L18)(tpy-PO3H)]PF6 on the Solid Substrate. The Y-type film deposition on hydrophilic glass or quartz substrates was performed by first rising the substrate preinserted in the subphase (upstroke) and then repeated deposition cycles of down- and upstroke of the substrates through the interface. The multilayer films on the hydrophobic substrates were carried out by first lowering the substrate through the film on the airwater interface. As a result, the first Ru complex monolayer was deposited at a tail-to-tail structure with respect to basal self-assembled OTS films. Subsequent rising of the substrate produced a head-to-head Ru complex bilayer. For each case, a medium transfer rate of 2 mm min-1 was used. A fast deposition speed for the upstroke was undesirable for drainage of the water from the film and to aid in crystallizing the Ru complex lattice. It has been reported that the continuous deposition of metal ion-octadecylphosphonate multilayers failed because the Langmuir monolayer becomes increasingly rigid with time. Instead, a tenuous procedure had to be adopted; namely, after a monolayer film on metal ions containing subphase was transferred onto the substrate, it had to be removed and a new monolayer was spread.23 This condition never took place for the present Ru complex, which showed an excellent film formation under various subphases such as acidic, basic, and nearly neutral pH or metal ions containing aqueous solutions. This will be apparent from UV-vis spectral results below. Obviously, the Ru complex is readily handled for the LB film transfer. UV-vis Spectroscopy of Transferred LB Films on Solid Substrates. The monolayers on different subphases can be successfully transferred onto hydrophilic and hydrophobic glass or quartz substrates. Each film deposi(51) Kuhn, H.; Mobius, D.; Buher, H. In Techniques of Chemistry, Physical Methods of Chemistry, Part III B.; Weissberger, A., Rossiter, B., Eds.; Wiley-Interscience: New York, 1972; Vol. I. (52) Bonosi, F.; Lely, G.; Ricciardi, M.; Romanelli, G. Langmuir 1993, 9, 268.

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monolayer on a subphase of pH 2.8 was inherited to the transferred film. Also, the spectra are independent of surface pressures used, indicating that there is no interlayer intermolecular interaction. Similarly, LB transfers from the metal ions containing subphases were also successfully monitored by UV-vis spectroscopy. It is important to note that the film Ru4 prepared from the subphase of pH 9.3 shows UV bands at 361, 340, and 324 nm with the 324 nm band as the strongest one, and they are similar to those in CHCl3 solution. This change somewhat inherits monolayer characteristic on corresponding subphases and may result from the destruction of aggregates in the film. In contrast, UV-vis spectra were hardly altered after the coordination reaction took place, indicative of the insensitivity of UV-vis spectra to the metal ion coordination in the films of Ru-M and Ru-M. Low-Angle X-ray Diffraction of LB Multilayer Films. Low-angle X-ray diffraction is a powerful tool to characterize ordered structure of LB films. Many experimental and theoretical results of X-ray diffraction from many kinds of LB films have been obtained.53-55 According to the Bragg equation Figure 5. UV-vis spectra of [Ru(L18)(tpy-PO3H)](PF6) LB films of varied numbers of layers deposited on a glass plate at a surface pressure of 20 mN m-1 with the subphases of (a) pH 5.40 and (b) pH 9.33. The plots of absorbance vs the number of layers are shown as an inset.

Figure 6. XPS wide-scan spectrum for 15-layer [Ru(L18)(tpyPO3H)](PF6) LB films on a glass plate transferred at 20 mN m-1, and angular-dependent high-resolution spectra of C 1s and Ru 3d5/2 and Cd 3d5/2 regions measured at detection angles of 80° (solid line) and 25° (dotted line).

tion process was easily monitored by the UV-vis spectra (Figure 5). As exemplified in the inset of Figure 6, a linear increase in absorbance at 364 nm with increasing number of deposited layers was observed for films Ru1, Ru2, and Ru3, which indicate that the films transferred on the solid surface quantitatively. Despite different deposition conditions for the LB films Ru1, Ru2, and Ru3 (see Table 1 for the abbreviations), the shapes of UV-vis spectra are almost the same for all the cases. However, the absorbance of the films transferred at a surface pressure of 50 mN m-1 is approximately doubled compared to that transferred at 20 mN m-1. The weakness of the MLCT band for the

2d00n sin θ00n ) λ D ) λd00l )

nλ 2 sin θ00n

where λ is the wavelength of the incident wave (Cu KR, 0.15418 nm), n stands for the diffraction number, θ00n the incidence angle, d00n the interplane spacing of diffraction index (00n), and D the identity period of the film. For a Y-type film, thickness per layer is one-half of D. The representative XRD pattern is shown in Supplemental Figure 3S. The data 2θ00n, assigned diffraction index (00n), calculated mean film thickness per layer d, and the full width at half-maximum (fwhm) for the strongest diffraction peak are summarized in Table 1. The fwhm is very sensitive to order degree of films. As shown in Table 1, the films prepared from an aqueous subphase of pH 5.4 at surface pressure of both 20 (film Ru1) and 55 mN m-1 (film Ru2) and of pH 2.85 (film Ru3) exhibit similar diffraction patterns, assignable to (001), (002), and (003) Bragg diffractions. The mean thickness per layer calculated is 2.10 nm, which is quite reasonable, compared to 2.12 nm observed for Ru complex congener [Ru(L18)(tppz)](PF6) LB film10 and 1.60 nm for LB film of [Pt(L18)Cl](PF6) with a flat configuration.37 However, the d value is ca. 0.3-0.4 nm smaller than those for Cd(II) octadecylphosphonate (d ) 2.43) and for Mn(II) octadecylphosphonate (d ) 2.41 nm)23 and stearate (2.52 nm) LB films.56 This fact is an indication that the alkyl chain in the film of Ru complex we studied tilted ca. 36° with respect to the film surface normal. Another striking feature for these three films is that the even-order diffraction (002) is seen in addition to the adjacent odd-order peaks (001) and (003), which is in agreement with the behavior of the cadmiumfatty acid systems. This behavior could be ascribed to the existence of an electron-deficient layer at the center of the Ru-Ru unit cell. However, for the film (Ru4) deposited at pH 9.3, only (001) diffraction was observed. Very sharp diffractions for films Ru1 and Ru2, e.g., an fwhm value (53) Pomerantz, M.; Segmuller, A. Thin Solid Films 1980, 68, 33. (54) Frieling, M. V.; Bradaczek, H.; Durfee, W. S. Thin Solid Films 1988, 159, 451. (55) Shiozawa, T.; Fukuda, K. Thin Solid Films 1989, 178, 421. (56) Peng, X.; S., G.; Chai, X.; Jiang, Y.; Li, T. J. Phys. Chem. 1992, 96, 3170.

Aggregation of Amphiphilic Ru Complexes

of ca. 0.28°, which lie among those for typical LB materials reported, demonstrated that the LB film had a highly ordered lamellar structure. However, less ordered films Ru3 and Ru4 were inferred based on their larger fwhm values. Highly ordered film Ru deposited on OTS precoated glass substrate was also confirmed with the presence of (001) and (003) diffractions and absence of (002) diffraction. A thickness per layer of 2.21 nm is slightly bigger than the Ru1-Ru4 films. When interface coordination reactions incorporating metal ions (Zn(II), Cd(II), and Mn(II)) into the films of both Ru-M and Ru-M took place, the diffraction patterns depend on the metal ions Mn(II), Zn(II), and Cd(II). However, it is noteworthy that the diffraction patterns of Ru-M are similar to Ru-M when M is fixed. For the Ru-Cd and Ru-Cd films, (001) and (003) diffractions were observed, while (002) is absent. As for the films of Ru-Mn and Ru-Mn, only broad (001) diffraction was observed. Interestingly, for the films of Ru-Zn and Ru-Zn, (001) and (004) diffractions appeared and relative intensity of the (004) diffraction is quite high. This feature is quite unusual compared with their metal ion free counterpart. It is interesting to note that spacings for Ru-M and Ru-M films are comparable to those for Ru and Ru multilayer films except for Ru-Mn film, which has spacing about 0.2 nm bigger than the Ru film does. Generally, the metal coordination of monolayers of fatty acids at the air-water interface resulted in an increased spacing, e.g., 0.5 nm for the Cd-behenic acid system.57 X-ray Photoelectron Spectroscopy (XPS) of the Ru Complex LB Films. Angle-resolved XPS provides a sensitive tool for depth profiling of interested elements in the films since the escape depth of photoelectrons depends on the angle of analyzed electron beams with respect to the film surface.10,58,59 The highly resolved XPS spectra were measured for Ru, Ru-M, and Ru-M LB films. X-ray photoelectron sepctroscopy (XPS) spectra for LB films prepared from pure water at both 25 and 55 mN m-1 show characteristic peaks at 281.3 eV for Ru3d5/2, 400.4 eV for N1s, and 531.3 eV for O1s. For the Ru-M and Ru-M LB films, new peaks corresponding to M2+ appear for M ) Cd of 405.5 eV for Cd3d5/2, Mn of 640.8 eV for Mn2p3/2, and Zn of 1022.8 eV for Zn2p3/2, respectively, which indicate that the coordination reaction between the Ru complex and the metal ion occurred both at the air-water interface and within the Ru complex interlayers. The relative intensity ratios of the characteristic peaks for M (Mn ) Zn, Cd, and Mn) and Ru increases with increasing the collecting angle of photoelectrons measured from the surface normal. Angle-resolved C1s, Ru3d5/2, and Zn2p3/2 XPS spectra for a 15-layer Ru-Zn film are shown in Figure 6, relative intensity ratio of Zn2p3/2 to Ru3d5/2 at a lower angle of 25° of the analyzed electron beam relative to the film surface, is clearly greater than that at a higher angle of 80°. At least the presence of islands and bare surfaces having the size of a few score of molecules can be safely denied.60 Similar trends were also observed for Ru-Mn and Ru-Cd films. In general, the intensity of the XPS signal, I, depends on the collecting angle θ as I ∝ exp(d/L cos q), where d is the depth from the sample surface where the element of interest is present and L is attenuation length of photoelectrons for the corresponding (57) Belbeoch, B.; Roulliay, M.; Tournarie, M. J. Chim. Phys. (Paris) 1985, 82, 701. (58) Wang, K. Z.; Huang, C. H.; Xu, G. X.; Xu, Y.; Liu, Y. Q.; Zhao, X. S.; Xie, X. M.; Wu, N. Z. Chem. Mater. 1994, 6, 1986. (59) Akhter, S.; Lee, H.; Hong, H.-G.; Mallouk, T. E.; White, J. M. J. Vac. Sci. Technol., A 1989, 7, 1608. (60) Ohnishi, T.; Ishitani, A.; Ishida, H.; Yamamoto, N.; Tsubomura, H. J. Phys. Chem. 1989, 82, 1978.

Langmuir, Vol. 18, No. 9, 2002 3535

Figure 7. Cyclic voltammograms of [Ru(L18)(tpy-PO3H)](PF6) monolayer on ITO electrode at scan rates of 20, 50, 100, and 200 mV s-1.

signal. Therefore, this angular dependence reveals that the metal ion, M (Zn, Cd, and Mn), is located deeper than the Ru ion. This fact strongly supports the metal coordination with the phosphonate group of the Ru complex in the highly organized films. Furthermore, an approximately 1:1 molar ratio of Ru:M was derived for Ru-M and Ru-M films after the correction for XPS sensitivity factors of the interested elements was made, which is in contrast to the 1:2 ratio of metal ion to carboxylic acid for fatty acid salt LB films such as Cd-behenic acid system.57 This fact is in accordance with structurally characterized layered compounds M(O3PC6H5)‚H2O (M ) Mn, Zn), Mn(O3PCnH2n+1)‚H2O.13 Therefore, a similar structure to the above model complexes is tentatively proposed for the LB films of the Ru complex we studied, namely, the structure consists of layers of M (M ) Zn, Cd, Mn) ions, which are roughly coplanar, octahedral coordinated by five phosphonate oxygen and one water of hydration. The alkyl chains are pointing out of the metal ion layers, approximately perpendicular to the metal ion plane, and make van der Waals contact between layers.22,23 Electrochemistry of LB Monolayer on ITO Electrode. Figure 7 shows a typical cyclic voltammogram of LB monolayer of [Ru(L18)(tpy-PO3H)] complex transferred onto ITO electrode in 0.1 M Na2SO4. This voltammetric wave is assigned to the redox reaction of immobilized Ru(II/III) couple, compared to the oxidation potential of this Ru complex in solution. For this film, the characteristic surface formal potential of the Ru(III/II) couple appeared at +0.92 V vs SCE in aqueous 0.1 M Na2SO4 solution and its redox peak width at half-maximum are ca. 130 mV, significantly larger than the Nernstian width of 90 mV. The peak current of the cyclic voltammogram response is proportional to the scan rate in the range from 20 to 200 mV s-1, indicative of the redox reaction of the surface immobilized species. In this voltammogram, the significant asymmetric shape of the voltammogram was observed. The surface coverage of immobilized Ru(II) complex molecules was determined by integrating the oxidation current peak assuming a one-electron redox mechanism. The value of surface coverage obtained from the voltammogram in Figure 7 falls in 1.1 × 10-10 mol/cm2. The resulting surface concentration is consistent with the value 1.28 × 10-10 mol/cm2 estimated from the π-A isotherms. The metal coordination to phosphonate group in [Ru(L18)(tpy-PO3H)] does not affect the Ru(III/II) oxidation potential. Atomic Force Microscopic Measurements of the Ru Complex Monolayer on Mica Substrate. To examine surface morphology of the transferred LB films, we measured AFM images of LB monolayer films of [Ru(L18)(tpy-PO3H)]PF6 transferred onto mica at different

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imidazole groups in L18 ligand plays an important role for the formation of domain structure. With the transferred surface pressure set at 50 mN m-1, the irregularly superposed stripe morphology was observed with the molecular height of 2.7 nm, which corresponds to the molecular length of [Ru(L18)(tpy-PO3H)](PF6). This result strongly indicates that above the surface pressure of 50 mN m-1 the molecular layers are riding on top of each other, which leads to the formation of bilayer structures at the interface (Figure 8b). Unfortunately, we have not obtained any informative AFM images of the [Ru(L18)(tpy-PO3H)](PF6) complex upon metal coordination so far. Conclusion

Figure 8. AFM images (1 × 1 µm size) of the [Ru(L18)(tpyPO3H)](PF6) monolayer on the mica substrate at two surface pressures: (a) 20 and (b) 50 mN m-1.

surface pressures. Figure 8 shows the representative AFM images of the Ru films transferred at the different surface pressures of 20 and 50 mN m-1. Regular long stripe morphology was revealed at 20 mN m-1. The stripe shape became more densely gathered when the higher surface pressure was applied. The stripes are roughly 32 nm wide and 0.7 nm high (Figure 4S). Upon careful observation and magnification of the AFM images, we found that each stripe is assembled from many small nanometer-size globular domains. From this observation, we propose the mechanism of this stripe formation as follows: at first, small globular domains are formed and then the assembling of these globular domains takes place. The observed morphology in the present study is in sharp contrast to that for [Ru(L18)(tppz)](PF6)2, which reveals the circular domain structure (∼2 µm size).11 The change of the auxiliary ligand L on [Ru(L18)(L)] type complexes leads to the different morphology of molecular ensembles, even the intermolecular π-π interaction between benz-

This report has shown evidence that a new amphiphilic Ru complex, [Ru(L18)(tpy-PO3H)](PF6), is surprisingly stable on an aqueous subphase at pH 2.8-9.3 without any auxiliary counteranion in the subphase or additional amphiphile such as stearic acid for aiding in stable film formation. Subsequent film transfer invariably shows reproducible and reasonable transfer ratios close to unity at a readily handled way. In situ UV-vis spectra of [Ru(L18)(tpy-PO3H)](PF6) indicate the formation of molecular aggregates through the intramolecular interaction between Ru-L18 moieties, which is supported by AFM measurements. The layer number-dependent UV-vis spectroscopy and regular X-ray diffraction pattern strongly indicate the formation of a lamellar organized structure. Some functional metal ions with paramagnetic properties could be introduced into the film by coordination with the phosphonate group to form a two-dimensional polymeric structure as did their model phosphonate compounds reported. Recently, the Ru complex was shown to bind a specifically TiOx ultrafine nanosheet from the subphase. Also, the Ti(OBu)3 compound bonded through the phosphonate group of the Ru compound appears to polymerize on the pure water subphase. In other words, the redox active amphiphilic Ru complexes with a functional phosphonate group would have potential application as a molecular unit toward photoelectronic materials.30,31 Acknowledgment. M.H. gratefully acknowledges financial support from the Ministry of Education, Science, Culture and Sports for a Grant-in-Aid for Scientific Research (No. 12440188), for a Scientific Research on Priority Area of “Metal-assembled complexes” (10149101) and “Dynamic Control of Strongly Correlated Soft Materials” (413/13031), and also support from the Promotion and Mutual Aid Corporation for Private Schools of Japan. Support from the grant for JSPS Fellows to K.Z.W. is also acknowledged. M.D.H. also acknowledge the Ministry of Education for a Japanese Government (Monbusho) Scholarship. Supporting Information Available: In situ UV-vis absorption spectra of [Ru(L18)(tpy-PO3H)][PF6], XRD pattern of [Ru(L18)(tpy-PO3H)][PF6] Langmuir-Blodgett films, and crosssection analysis of AFM images in Figure 8. This material is available free of charge via the Internet at http://pubs.acs.org. LA011386+