Langmuir 1994,10, 343-344
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Incorporation of Metal Complex from the Subphase into a Fatty Acid Langmuir-Blodgett Film M. Keith DeArmond’ and Hussein Samha Chemistry and Biochemistry Department, New Mexico State University, Las Cruces, New Mexico 88003
Ondrej Dvorak The J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Dojeskova 3, Prague 8,182 23 Czechoslovakia Received February 2, 1993. In Final Form: October 29, 1993
Simple metallic ions as Cd2+and Ba2+when incorporated into Langmuir-Blodgett (LB) films by an ion-exchange mechanism have a substantial influence on film stability and rigidity.l A similar technique has been used for a preparation of fatty acid films with a subphase phthalocyanine cobalt(I1) comple~.~J The successful exploitation of the ion-exchange-from-bathtechnique for incorporation of tris(2,2’-bipyridyl)ruthenium(II) cation into a stearic acid film is described in this paper. An alternate layer Langmuir-Blodgett trough from NIMA, Model TKB, was used for the Langmuir-Blodgett film preparation. The 1.0 X 10-3 M aqueous solution of tris(2,2’-bipyridyl)ruthenium(II) chloride was used as a subphase. A chloroform solution of stearic acid (1.5 mg/ mL) was spread on the subphase surface with a microsyringe. Glass or quartz slides were used as substrates for the film deposition. Before the deposition, slides were cleaned in hot piranha mixture (concentrated sulfuric acid and hydrogen peroxide 4:l (v/v)) and washed with deionized water. Transparent doped indium tin oxide (ITO) electrodes (Delta Technology) were used as substrates for the film electrochemistry. These substrates were washed with 2-propanol and deionized water. The films were deposited by the vertical dipping technique under surface tension control of 35 mN/m with a slide speed of 25 mm/ min and were soaked in deionized pure water after being produced. The bath temperature was maintained at 20 “C. A modified absorption spectrophotometer, Cary-14,was used to obtain absorption spectra. An LS-100 luminescence spectrometer from Photon TechnologyIntemational (PTI LS-100) was used for the emission spectra measurements. Electrochemistry was done using an electrochemical analyzer (BAS 100A)from Bioanalytical Systems with 0.001 M NaC104 as supporting electrolyte. Both solutions were separated by an asbestos diaphragm. A platinum flag was used as an auxiliary electrode. The pressure-area (r-A) isotherms of stearic acid LB M aqueous solution of tris(2,2’films on a 1.0 X bipyridyl)ruthenium(II) ion and on pure water subphase are shown in Figure 1. Typically, the isotherm obtained on the surface of the metal complex aqueous subphase resembles that obtained on the pure water subphase. In each of those isotherms, the three phase changes (gas, liquid, and solid) are distinguishable. Both films collapse at almost the same pressure. However, the area per stearic acid molecule obtained when the metal complex is dissolved in the subphase is larger-but reproducible-(23.0-24.0 A2) than the area obtained on (1) Roberta, G., Ed. Langmuir-Blodgett Films; Plenum Press: New
York, 1990.
(2) Palacin, S.; Barraud, A. J. Chem. SOC., Chem. Commun. 1989,45. (3) Palacin, S.; Ruadel, A.; Barraud, A. J. Phys. Chem. 1989,93,7195.
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Figure 1. PA isotherm of Langmuir-Blodgett film of stearic acid (A) on pure water and (B)on 0.001 M aqueous solution of [tris(2,2’-bipyridyl)ruthenium(II) chloride.
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Area/Molecule [i*] Figure 2. F A isotherms for Langmuir-Blodgett film of stearic acid on 0.001 M aqueous solution of CdC12.
the pure water subphase (20.0 A2). This is due to the interaction of the metal complex with the stearic acid LB monolayer. Comparing the isotherm of stearic acid obtained on the surface of tris(2,2’-bipyridyl)ruthenium(II)ion aqueous subphase with that obtained on the surface of 1.0 X 103 M CdCl2 aqueous subphase (Figure 21, one can see the difference in the shape of the isotherm of the film. With Cd2+ ion-paired stearic acid LB film, the compressed monolayer goes from the gas phase to the solid phase (Figure 2) while in the case of tris(2,2’-bipyridyl)ruthenium(I1) ion in the subphase the isotherm obtained shows the liquid phase change (Figure 1). This behavior can be explained by the large size of the metal complex ion which produces a less efficiently packed LB film of stearic acid than the ion-paired film with Cd2+ ions. Thus the ionpair interaction likely is smaller in the case of the ioncomplex film. The collapse of the film produced on the metal complex subphase occurs at a surface tension of 50 mN/m. The transferred films look homogeneous without visible defects up to 100layers, but some defects are visible for films with more than 100 layers. The absorption spectrum of a Langmuir-Blodgett film with incorporated ruthenium(I1) complexes is compared with the solution absorption spectrum of the complex in
0743-746319412410-0343$04.50/0 0 1994 American Chemical Society
344 Langmuir, Vol. 10, No. 1, 1994
Notes
1.3 &U/+io E [Volt]
Figure 5. Voltammogramof the oxidation of tris(2,2‘-bipyridyl)ruthenium(I1) compolex embedded in a monolayer of the stearic acid Langmuir-Blodgett film. The polarization rate is 100 mV/ a, and the working electrode area is 11.75 cm*. 200
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Figure 3. Absorption spectra of tris(2,2’-bipyridyl)ruthenium-
(11) complex in (A) acetonitrile solution and (B) stearic acid LB films (149 layers) on a quartz substrate.
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Figure 4. Emission spectraof tris(2,2’-bipyridyl)ruthenium(II)
complex in aqueous solution (-) and in stearic acid LB film (98 The slide was oriented at a 20° layers) on a glass slide (-). angle to the 450-nm excitation beam and 100 scans were done. (Intensity is normalized.)
Figure 3. A 20-nm red shift of the LB film absorption peaks from the solution is clearly visible. Emission spectra of dissolved complex and complex imbedded in the LB film are compared in Figure 4. The luminescencemaximum is blue shifted by 18nm compared to the aqueous solution value. The cyclic voltammogram of the tris(2,2’-bipyridyl)ruthenium(I1) complex/stearic acid Langmuir-Blodgett film is shown in Figure 5. The absorption and emission spectra provide persuasive evidence that the tris(2,2’-bipyridyl)ruthenium(II) cation is incorporated in the stearic acid Langmuir-Blodgett films by an ion exchange. The intercalation of the complex moleculebetween the alkyl chains of stearic acid molecules is improbable since there is only a small difference in the area per molecule of the stearic acid LB film from pure subphase water and the film for the subphase containing the metal complex. The complex molecules are likely localized in the carboxylic group region and beneath the stearic acid layer. The film could then be stabilized by an ion-pair intera~tion.~ The shift of absorption and emission peak maxima relative to the solution data supports this
interpretation. The interaction between this complex cation [Ru(bpy)3I2+and the fatty acid carboxylic groups may be stronger in the case of the ion exchange from the bath since the carboxylic groups are readily accessible to the dissolved [Ru(bpy)J2+ complex in the subphase. By comparison of the emission intensity obtained from a fluid solution of a multilayer LB film, after it was dissolved, with a standard emission intensity/concentration curve, ([Ru(bpy)312+emission),thequantity of the metal complex was determined. Stearic acid in the LB films has been determined by assuming that the area per SA is 20 A2 and that the close packed SA occupies the film area. The ratio of stearic acid to the ruthenium complex in a 48-layer film produced on 1.0 X 106 M aqueous subphase was found to be (1f 6)/1assuming the complex molecules are residing beneath the stearic acid in the transferred films. The electrochemistry of [Ru(bpy)3I2+ in aqueous solution6 shows that the oxidation potential is about 355 mV more positive than that of the film, but reliable quantitative data could not be obtained. Ultimately, the location (beneath or within the fatty acid) of the metal complex requires structural data that we will hope to acquire from atomic force microscopy (AFM) measurements. These data would also verify the ion-pair description of this film. The subphase film method offers some advantages comparing with traditionalmethods:’ (1)The alkyl chains of the fatty acid matrix can freely interact, since there are no structure-breaking molecules between them. Thus more stable and rigid films are formed with fewer defects. (2) The incorporation into the LB film of water soluble molecules is possible. The only condition is that sufficient ion-pairing attraction occurs between ionized carboxylic groups of stearic acid and complex cations. However, this ion-exchange interaction limits the possibility of preparation of such films only to cationic complexes. Moreover, the dissolution of the complex in the subphase requires a larger quantity of the complex than in the case of the standard surface technique.’ We have extended this subphase procedure to other watersoluble Ru(I1) complexes with stearic acid and will subsequently report these data.
Acknowledgment. Support from the Army Research Office (ARO), Research Triangle Park (RTP), NC, is gratefully acknowledged. (4) Murakata, T.; Miyashita, T.;Matauda,M. J. Phys. Chem. 1988,92,
6040. (5) Sutin, N.;Creutz, C . Adu. Chem. Ser. 1978, No. 168,l.
