In-trough cyclic voltammetric studies of Langmuir-Blodgett monolayers of a surfactant derivative of tris(2,2'-bipyridine)ruthenium(2+) at an indium-tin oxide electrode

Langmuir-Blodgett (L-B) monolayers of a surfactant derivative of ruthenium tris(bipyridine) complex, Ru(bpy)2(bpy-C19)2+, where bpy = 2,2'-bipyridine and C19 = CH2NHOC(CH2),,CH3, confined to the surface of an indium-tin oxide electrode have been studied by cyclic voltammetry (CV) carried out in a Langmuir trough. The surfactant monolayer spread on an electrolyte subphase solution was brought to contact with the electrode by either vertical dipping under a surface pressure or horizontal touching at arbitrary surface concentrations. In the horizontal touch experiment, the surface tension was used to sustain contact of the subphase solution with the electrode, which was held at a position slightly above the air/water interface. In this arrangement the exact electrode alignment with the surfactant layer is not important. The data obtained from in-trough CV measurements reveal that the spread surfactant molecules at submonolayer concentrations were highly aggregated at the air/water interface rather than distributed homogeneously as a two-dimensional gaseous state, and a major cause for the incomplete electrochemical recovery of deposited L-B monolayers frequently observed is the relaxation of the compressed surfactant molecules toward the limiting molecular area in an ionic aqueous solution. The CV measurements performed also provided information concerning electron transfer in the organized monolayer assemblies, the physical and chemical changes of the spread surfactant layer at the air/water interface, and the mass transfer of solution species across the L-B monolayer films. In a previous paper we reported for the first time the observation of the electrogenerated chemiluminescence (ECL) from an organized monomolecular layer of a surfactant derivative of Ru(bpy)32+ consisting of a single stearamidomethylene chain linked to one bipyridine unit a t the 4-position (abbreviated Ru(bpy),(bpy-C19)2+) on the surface of indium-doped tin oxide (ITO), Pt, and Au electrodes.' The surfactant monolayer was coated on the substrate electrodes by the L-B method and the ECL was generated in an electrochemical cell containing an aqueous oxalate solution by applying a positive potential to the electrode modified by the monolayer of R~(bpy)~(bpy-C~?+) .~ The ECL observation revealed that a Ru(bpy)2(bpy-Clg)z+ monolayer confined to a conductive surface by the L-B technique was excitable through an electrochemical route and that the radiative decay of the excited states could compete with the nonradiative ones. While this concept of immobilizing luminescent molecules on electrode ( I ) Zhang, X.; Bard, A. J . J . Phyys. Chem. 1988, 92, 5566. (2) For the proposed mechanism of this ECL system, see: (a) Chang, M.; Saji, T.; Bard, A. J. J . Am. Chem. SOC. 1977, 99, 5399. (b) Rubinstein, I.; Bard, A. J. J . Am. Chem. SOC. 1981, 103, 512. 0002-7863/89/1511-8098$01.50/0 surfaces led to the development of a new class of ECL systems that can be useful for a variety of purpo~es,~ it also motivated us to characterize these organized monolayers at electrodes by electrochemical techniques. Although monolayer assemblies of Ru(bpy)32+ derivatives formed by L-B and other methods have been the subject of extensive photochemical and spectroscopic ~ t u d i e s , ~ their electrochemical studies have been limited.5 In this paper we describe the application of cyclic voltammetry(CV) carried out in a Langmuir trough to the characterization of the L-B monolayer films of R~(bpy)~(bpy-C,#+. There have been a number of reported studies about the electrochemical behavior of organized assemblies of surfactant molecules on electrode In particular, two previous reports have (3) Zhang, X.; Bard, A. J. U. S. Patent, in preparation. (4) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, and references ( 5 ) Daifuku, H.; Aoki, K.; Tokuda, K.; Matsuda, H. J . Elecrroanal. Chem. ( 6 ) (a) Fujihira, M.; Araki, T. (a) Chem. L e f f . 1986, 921. (b) Fujihira, therein.