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-octadecylbenzimi- dazol-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 supra- molecular 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 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 well- controlled lamellar structures from two-dimensional mo- lecular composite films. 6 Recently, the complex formation between monolayers of an amphiphilic ligand and transi- tion metal ions in the subphase has been reported, in which the interfacial coordination reaction took place at the air- water 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. * 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. (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. 3528 Langmuir 2002, 18, 3528-3536 10.1021/la011386+ CCC: $22.00 © 2002 American Chemical Society Published on Web 03/30/2002