Langmuir 1994,10, 4157-4163
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Incorporation of Metal Complexes from the Subphase into Fatty Acid Langmuir-Blodgett Films Hussein Samha and M. Keith D e h o n d * Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003 Received March 28, 1994. In Final Form: August 12, 1994@ The technique of producing fatty acid Langmuir-Blodgett (LB) films incorporating metal complexes from the subphase (subphasefilms) by an ion-pairing mechanism is explored. Tris(4,4‘,5,5’-tetramethyl2,2’-bipyridine)ruthenium(II), [Ru(tm-bpy)31z+, tris(1,10-phenanthroline)ruthenium(II),[Ru(phen)3I2+,y d tris(4,7-diphenyl-l,lO-phenanthroline)ruthenium(II), [Ru(dpphen)3I2+,ionic complexes are dissolved in the subphase solutions. The pressure-area (JC-A)isotherms obtained for stearic acid on the surface of metal complex subphases indicate the formation of stable subphase LB films capable of being transferred onto a solid substrate. The absorption and the emission of the ruthenium complexes in the transferred films are red-shifted with respect to the acetonitrile solutions of the metal complexes. The metal complex molecules are located beneath the film layer and/or in the head group regions of the stearic acid molecules as indicated from the n-A isotherms, the area per stearic acid unit calculated, and the shift in the absorption and the emission spectra. The surface LB films of the metal compledstearic acid produced on the surface of the water subphase are compared with the subphase films and found to be equilibrating with the subphase films in a short time. The composition of the subphase films is also determined from HPLC and luminescence measurements.
Introduction Generally, cations from the subphase undergo ion pairing with fatty acids in Langmuir-Blodgett (LB)films. Specifically, cations such as Ca2+,Ba2+,and Cd2+interact with the fatty acid head groups by an ion-exchange mechanism, producing stable and rigid LB films.’ A similar technique has been used for a preparation of fatty acid films with a subphase phthalocyanine cobalt(I1) comple~.~ We , ~r e p ~ r t e dthe ~ , ~successful exploitation of this mechanism for incoproration of tris(2,2’-bipyridyl)ruthenium(I1) cation from the subphase into a stearic acid LB film. In these ion-pair films, as well as other mixed films of metal compledfatty acid, a n advantage of the LB technique is the ability to vary the film concentration of the chromophore species. However, a problem for these hydrophobic metal complexes is that some are water soluble, and consequently, the nominal (initial) concentration is likely greater than the final film concentration. Since we ultimately hope to “locate”, using the scanning probe method, the metal complex in the film (beneath the fatty acid layer in the LB film or intercalating in the film), the area occupied by the film components is a vital piece of information. Therefore, we have utilized HPLC and metal complex luminescence to determine the ratio of the components. In this paper, the subphase technique of producing fatty acid LB films incorporating metal complexes from the subphase by a n ion-pairing mechanism is explored. For this purpose, three ruthenium complexes, tris(4,4’,5,5’tetramethyl-2,2’-bipyridine)ruthenium(II) perchlorate, mu(tm-bpy)31[C104]2,tris(1,lO-phenanthroline)ruthenium(II) perchlorate, [Ru(phen)31[C1O41~, and tris(4,7-diphenyl-l,10-phenanthroline)ruthenium( 11)chloride, [Ru(dpphenhl@
Abstract pubIished in Advance ACS Abstracts, September 15,
1994. (1)Roberts, G.Langmuir-Blodgett Films; Plenum Press: New York and London, 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. (4)DeArmond, M, K.; Samha, H.; Dvorak, 0. Langmuir 1994,10, 343. (5)DeArmond, A. H.; Dvorak, 0.; DeArmond, M. K. Thin Solid Films t999,282,115.
Clz, are chosen on the basis of the variation in the molecular sizes and the hydrophobicity.
Experimental Section Materials. Tris(4,4‘,5,5’-tetramethyL2,2’-bipyridine)ruthenium(I1) perchlorate, [Ru(tm-bpy)3l[ClO~12,tris(1,lO-phenanthroline)ruthenium(II)perchlorate, [Ru(phen)~I[C10412, and tris(4,7-diphenyl-l,lO-phenanthroline)rututhenium(II) chloride, [Ru(dpphen)&lz, were available from a previous study and were synthesizedusinga standard procedure. The stearic acid (99+%) was purchased from Aldrich. The subphasewater was deionized water with a resistivity of 18 MR/cm. Chloroform (99.9%), methanol,dichloromethane,and acetonitrilewere ofhigh-purity HPLC grade and were Aldrich products. The quartz plates used were obtained from Delta Technologies, Ltd., and were of dimensions 2.5 x 7.5 cm2. Instrumentation. ANIMA alternate layer trough of 500 x 2 cm2area is used for the preparation of the LB films. The U V vis absorption spectra were measured on a Cary 14 spectrophotometer with an updated operating and data acquisitionsystem. The film emission measurements were performed with an LS100 luminescencesystemfrom Photon Technology International (PTI).High-performance liquid chromatographic(HPLC)analysis of stearic acid in the LB films was carried out on a Rabbit-HP instrument from RAININ Instrument Co., Inc., attached to a Rainin pump. A reverse phase (2-18 column manufactured by D Y ” is used. Methods. Aqueous solution subphases of concentration3.50 x M [Ru(tm-bpy)3I2+, 7.64x M [Ru(phen)3I2+, and 1.70 x lo-’ M [Ru(dpphen)sl2+ were prepared by dissolvingthe metal complex in about 50 mL of acetonitrile, and then the whole solution is diluted with water to 2 L. An established general pro~edure~-~ was followed for the production of surface films of metal compledstearicacid on the surface of pure water (the metal complex and stearic acid were dissolved in chloroform and then the solution mixture is spread on the surface of the water subphase). The subphase LB films were prepared by spreading a chloroform solution of stearic acid (8.5 x 10-4M) on the surface of the aqueous solution (subphase)of the metal c~mplex.~,~ The (6)Samha, H.A.;Martinez, T. J.; DeArmond, M. K. Inorg. Chem. 1993,32,2583. (7) Samha, H.; DeArmond, M. K. Coord. Chem. Rev. 1991,111,73. (8)Samha, H.;Martinez, T. J.; DeArmond, M. K. Langmuir 1992, 8, 2001.
(9)Des Enfants, R. E.; Martinez,T. J.;DeArmond, M. K.;LangmuirBlodgett Films of Transition Metal Complexes;ACS Symposium Series; American Chemical Society: Washington, DC, 1992;Vol. 499,p 46.
0743-746319412410-4157$04.50/00 1994 American Chemical Society
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Area [ c m ] Area [cm 2 ] Area [ c m 2 ] Figure 1. Pressure-area (n-A) isotherms for [Ru(tm-bpy)3I2+/stearic acid surface LB film with molar ratio 1 : l O performed on a pure water subphase at 20 "C and a barrier speed of 50 cm2/minafter the solution mixture was spread on the surface of the subphase for (A) 5 min, (B) 60 min, and (C) 120 min. Table 1. Composition Measurements of Subphase LB Films of Stearic Acid Incorporating Ru(I1) Metal Complexes from the Subphase (Slide Area = 12.5 cm2) R ~ ( I I complex ) in subphase
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none and 18-crown-6ether of 10:1mole ratio, and then it is heated at 60-70 "C for 20 min. Then, the solution is injected in to the HPLC column, and the data obtained are compared with a standard calibration curve, of the peak area of (ANSA) versus the mole ratio of (ANSA).This calibration curve is prepared using standard AA, as an internal standard, and SA solutions. The eluent used in the HPLC was 85% methanol and 15% dichloromethane.
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pressure-area (n-A) isotherms were obtained by compressing the spread monolayer of the stearic acid on the surface of the subphase at a barrier speed of 50 cm2/minat 20 "C. Multilayer LB films of stearic acid were deposited by using the vertical dip mode of the NIMA trough. The deposited film is then soaked in pure water for about 30 min and dried in a stream of dry nitrogen. The UV-vis spectra were recorded using the Cary 14 spectrophotometer with solution concentrationsof -1.0 x M in acetonitrile solvent. The absorbances of multilayer films were measured on quartz slides. The emission spectra for multilayer films of the metal complexeswere obtainedusing a PTI(LS-100) spectrophotometer. The excitationwavelength was 450 nm, and 50-100 scans were done for each spectrum. The meta1:stearic acid ratio is determined in the films after being transferred on solid substrates (quartz slides)by using an HPLC techniquelofor the determination of fatty acid concentration and luminescence measurements for the determination of the metal complex concentration. For the determination of the amount of the metal complex in the LB films, the multilayer LB film of 50 layers is dissolved in 5.0 mL of chloroform and then the luminescence intensity is measured using the PTI Luminescence Spectrometer. By comparisonofthe emissionintensity of the solution with a standard emission intensitylconcentration calibration curve, the number of moles of the metal complex is determined (Table 1). For the HPLC measurements, the solution of the 50 layers film is used after preparation by adding to it 45 pL of (excess) KOH (3.4 x loV4g/mL, methanol solution) to produce the salt of the acid and 50 pL of arachidic acid (AA)(5.074 x M, chloroform solution)as an internal standard. Subsequently,the solution is evaporated to dryness, the residue is dissolved in 1.0 mL of an acetonitrile solution mixture of 2,4-dibromoacetophe(10) Zamir, I.; Grushka, E.; Cividalli, G. J. Chromatogr. 1991,565, 424.
Results and Discussion In the current study,ion-paired LB films of stearic acidmetal complex from aqueous solutions of ruthenium complexes that dissolve in the subphase (subphase films) have been investigated and compared with LB films of stearic acid on pure water and with surface films of metal compledstearic acid prepared on the water subphase. [Ru(tm-bpy)sI2+.The [ R ~ ( t m - b p y ) ~complex l~+ ion is used to produce subphase LB films. This compound is relatively more soluble in water (less hydrophobic) than [Ru(phen)3I2+. Production and characterization ofsurface LB films of [Ru(tm-bpy)312+have been reported by our l a b o r a t ~ r y .In ~ the current study, the properties of the subphase LB films of [Ru(tm-bpy)3I2+are discussed and compared with the surface LB films of Ru(tm-bpy)312+/ stearic acid from the previous study.g Figure 1A represents the n-A isotherm of asurface film of [Ru(tm-bpy)3lZ+/ SA of nominal molar ratio of 1 : l O produced on the surface of the pure water subphase. In this isotherm, the liquid region consists of two segments with different slopes. The expanded-liquid phase (segment a) results from the ruthenium molecules pushing from the subphase surface into the region beneath the surface as a result of the film compression pressure. Segment b represents the normal liquid phase of the isotherm. In comparison, the n-A isotherm of the subphase film of [Ru(tm-bpy)#+ shows that the expanded-liquid phase is not distinguishable as in the n-A isotherm for the surface film, but rather it overlapped with the liquid phase (Figure 2). Moreover, the liquid phase in the isotherm of Figure 2 overlaps with the solid phase due to the ion pairing between the metal complex ions from the subphase and the stearate groups of the stearic acid from the film on the subphase surface. The area per stearic acid unit in the compressed film of stearic acid on the surface of the [Ru(tm-bpy)3I2+ subphase is larger (23-24A2) than normal (Figure 2) due
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0 Metal Complex Stearic Acid Figure 3. Schematic illustration of the migration of the metal complexmolecules from (A)the surfaceofthe subphase (surface film)to (B)beneath the stearic acid monolayer (subphasefilm). to the ion-pair interaction between the stearic acid and the metal complex ions. The surface films equilibrate with the subphase ionpairing films as the x-A isotherms indicate. Figure 1B represents the x-A isotherm of a surface film of [Ru(tmbpy)31+2/SAof molar ratio 1:lO compressed after being spread and relaxed on the surface of water for 60 min. The isotherm shows a normal one-liquid-phase region which is sigrdicantly different from the liquid phase region of the surface film compressed after 5 min of being spread (Figure 1A). The same isotherm shape is obtained even when the spread monolayer is compressed after being relaxed for 120 min. This indicates that the surface film is equilibrated with the subphase film within 30-60 min. The collapse pressure of the film seems to be decreasing from A to C until it becomes as normal as the collapse pressure of pure stearic acid (55-65 mN/m, Figure 11, which indicates the migration of the complex molecules from the surface to beneath stearic acid layer (Figure 3). Comparison of the absorption spectrum of the surface films of [R~(tm-bpy)31+~/SA with the absorption spectrum ofthe solution ofthe metal complex indicates a small shift, which suggests a weak interaction between the metal complex and the stearic acid.g However, the ion-pair interaction is much stronger (as evidenced by the spectral shift)when the metal complex is dissolved in the subphase (subphase film). The absorption of [R~(tm-bpy)31+~ in the subphase LB films (Figure 4B) is shifted to the red compared with the absorption in acetonitrile solution (Figure 4A). The shift in the x-x* band a t 280 nm is measured to be about 20 nm. The emission measurements
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Figure 4. Absorbance of [Ru(tm-bpy)3I2+ in (A) acetonitrile solution and (B)145 layers of stearic acid LB film incorporating [Ru(tm-bpy)3I2+ from 3.50 x loT6M aqueoussubphase (subphase films)deposited on a quartz plate at 30 mN/m surface pressure and a dipping speed of 20 mm/min.
for [R~(tm-bpy)31+~ show a 7.4nm red shift for the emission ofthe subphase ion-pair films compared with the emission from the acetonitrile solution of the metal complex (Figure 5), while the emission ofthe surface LB films is not The ratio of metal comp1ex:stearicacid in the subphase LB films is reported in Table 1. The metal complex amount is estimated from the emission intensity obtained from the fluid solution of a multilayer subphase LB film after it was dissolved. The stearic acid amount is estimated on the basis of the HPLC measurements obtained from the same solution of the corresponding multilayer LB film. The average [R~(tm-bpy)~l~+:SA mole ratio of subphase LB films of SA prepared on the surface of the [Ru(tmbpy)3I2+aqueous subphase is found to be (1:60f6)(Table 1). This ratio represents a n average of seven measurements obtained from different multilayer subphase LB M aqueous films produced on the surface of a 3.50 x subphase of [Ru(tm-bpy)3I2+. [Ru(phen)#+. Figure 6 represents the x-A isotherm of a compressed surface film of [Ru(phen)J2+/SA with molar ratio of 1:lO. The average area occupied by each ruthenium molecule in the cmopressed surface films was calculated by extrapolating the linear region of the
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Figure 5. Room-temperatureemission of [Ru(tm-bpy)312+ in acetonitrile solution (-), in 59 layers of subphase stearic acid LB film prepared on the surface of 1.0 x M [Ru(tm-bpy)3I2+ aqueous subphase (---I, and in 69 layers of [Ru(tm-bpy)312+/ stearic acid surface LB film with a molar ratio of 1:lO prepared on the surface of pure water after being relaxed for 90 min (-1. All films are deposited on quartz plates at 30 mN/m surface pressure and a dipping speed of 20 mdmin at 20 "C. lex = 450 nm.
Figure 7. Pressure-area (x-A)isotherm for stearic acid performed on 7.65 x low6M [Ru(phen)#+ aqueous subphase at 20 "C and a barrier speed of 50 cm2/min.
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Figure 6. Pressure-area (n-A)isotherm for [Ru(phen)3I2+/ stearic acid with molar ratio 1 : l O performed on a pure water subphase at 20 "C and a barrier speed of 50 cm2/min. isotherm of the LB films to zero surface pressure, which gives the total area occupied by both molecules, [Ru(phen)#+ and stearic acid. On the basis of the molar ratio and the molar concentration of each compound in the mother mixed solution and the assumption that the area occupied by each stearic acid molecule is 20 k ,the avera e molecular area of [Ru(phen)3I2+is found to be 100 Typically, the isotherm of the surface monolayer film is a three-phase isotherm (gas-liquid-solid) which resembles the isotherm of pure stearic acid film prepared on the surface ofthe pure water subphase. Figure 7 shows the n-A isotherm of the stearic acid on a n aqueous subphase of [Ru(phen)312+with a concentration of 7.64x M. The major difference between Figure 6 and Figure 7 is the shape of the isotherm with a n expanded liquid phase and a n overlapping ofthe liquid and the solid regions occurring in Figure 7 . This overlap results from the ion pairing ofthe metal complex with the stearate head groups of stearic acid. This type of isotherm, by analogy to that of a cadmium stearate LB film1 produced on the surface of a n aqueous subphase of CdClp, indicates the rigidity of the monolayer formed on the surface ofthe subphase. The
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Figure 8. Absorbance of [Ru(phen)3I2+in (A) acetonitrile solution,(B) 120layers of [Ru(phen)3I2+/stearic acid surface LB film with a molar ratio of 1 : l O deposited on a quartz plate at 30 mN/m surface pressure and a dipping speed of 20 mdmin, and (C) 149 layers of subphase stearic acid LB film prepared on the surfaceof 7.64 x M [Ru(phen)3I2+ aqueous subphase and deposited on a quartz plate at 30 mN/m surface pressure and a dipping speed of 20 mdmin. area per stearic acid molecule in the ion-pairing subphase LB film on the surface of the [Ru(phen)3lZ+aqueous subphase is found to be 22-25 A2, which is significantly larger than the area reported per stearic acid molecule from the pure water subphase.' This increase in the area per stearic acid molecule is caused by the metal complex countercation. This suggests that the stearic acid (matrix) molecules in the film are not closely packed, with the result that some defects in the film are visible in the multilayer (after 100 layer) LB films. The film collapses in the range between 55 and 65 mN/m, a typical collapse pressure measured for stearic acid LB films. The absorption spectra of multilayer LB films of stearic acid incorporating [ R ~ ( p h e n ) ~complex l~+ ion are shown in Figure 8. The absorption of the multilayer subphase film is red-shifted compared with the absorption of the acetonitrile solution of the metal complex. The absorption maximum of the surface films of [Ru(phen)3lZ+/SA having
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Figure 9. Room-temperatureemission of [Ru(phen)#+ from a subphase aqueous solution of 7.64x lod6M (-) and of 120 layers of [Ru(phen)3I2+/stearic acid surfaceLB film with a molar ratio of 1:lO (-1 deposited on a quartz plate at 30 mN/m surface pressure and a dipping speed of 20 mm/min. de, = 450 nm.
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Figure 10. Room-temperatureemission of [Ru(phen)312+ from a subphase aqueous solution of 7.64 x M (-) and of 120 layers of stearic acid subphase LB film prepared on the surface of 7.64x M [Ru(phen)3I2+ aqueous subphase (-1 deposited on a quartz plate at 30 mN/m surface pressure and a dipping speed of 20 "/min. , 1 = 450 nm. the ratio of 1:lO is slightly red-shifted (Figure BB), while the shift in the absorption is larger (-12 nm for the n-n* band a t 280 nm) in the case of the ion-pairing interaction between the subphase metal complex and the stearate group (subphase film) (Figure €0. Comparison of the emission spectra of both the surface and the subphase films with the emission spectrum ofthe subphase aqueous solution shows that the LB films emit at a longer wavelength (red shift in the maximum of the emission peak) than the solution (Figures 9 and 10, respectively). A red shift in the emission is also noted for the LB film spectrum when it is compared with an acetonitrile solution of the metal complex. Because the metal complex ions in the subphase film are more accessible to the stearic acid than those on the surface of the water subphase (the surfme films),the ion-pairinginteraction likely is stronger, and so the shift in the absorption and in the emission will be larger. The average [Ru(phen)3I2+:SAmole ratio of subphase LB films of SA prepared on the surface ofthe [Ru(phen)3I2+ aqueous subphase is found to be 1:27 f 2) (Table 1). [Ru(dpphen)s12+. The hydrophobic complex [Ru( d p ~ h e n ) ~behaves l~+ differently from the previous com-
Figure 11. Pressure-area (n-A) isotherm for stearic acid performed on (A)7.76 x M [Ru(dpphen)312+aqueous subphase and (B) 1.70 x lo-' M [R~(dpphen)~l~+ aqueous subphase, at 20 "C and a barrier speed of 50 cmz/min. plexes when it is dissolved in the subphase. Because it is very insoluble in water, this complex was first dissolved in a large amount of acetonitrile (19 mg of complex per 200 mL of acetonitrile) and then diluted with water to 2 L to give an aqueous solution of 7.76 x M. This solution gives a contaminated surface when it is used as a subphase on the trough. A positive surface pressure is obtained whenever the surface of the subphase is squeezed to a small area. Moreover, a chloroform solution of 1mg/ mL of stearic acid will not spread on the surface of this subphase when this solution is added dropwise to the surface. Ifthe stearic acid solution added onto the surface of 7.76 x M aqueous subphase of [Ru(dpphen)3I2+is squeezed, a n-A isotherm, which is missing the solid phase, is obtained, and there is no collapse point for the squeezed layer (Figure 11A). However, the isotherm on a more diluted subphase (1.70 x lO-'M) exhibits the solid phase (Figure llB), and it resembles the isotherm obtained the pure water for the surface film of [Ru(dpphen)312+/SAon subphase and the isotherms obtained for the same complex by the Japanese workers." This indicates that a monolayer of stearic acid forms on the surface of a diluted aqueous subphase of [Ru(dpphen)3I2+but not on the surface of a concentrated subphase. This is due to the formation of a homogeneous surface (no insoluble materials on the surface) for the dilute subphase that allows the stearic acid solution to spread on the surface of the subphase. The area per stearic acid is estimated from the linear part of the isotherm (after the plateau) to be 22-23 A'. Monolayers of subphase films of stearic acid incorporating [Ru(dpphen)3lZ+ ions were successfullytransferred at a surface pressure of 28 mNIm from the surface of the subphase onto the surface of quartz plates. The absorption spectrum of the transferred multilayer subphase LB film is compared with the spectrum of the metal complex dissolved in acetonitrile in Figure 12. A small red shift in the absorption maximum of the film is obtained (Table 2). The emission from the transferred film is slightly red-shifted (Table 2) with respect to the emission from the acetonitrile solution of the metal complex (Figure 13). The shift in the absorption and the emission, as explained above, indicates the ion-pair formation between the ruthenium complex ion and the stearate groups. The small shift in the absorption suggests (11)Murakata, T.; Miyashita, T.; Matsuda, M. J.Phys. Chem. 1988, 92, 6040.
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Figure 12. Absorbance of [Ru(dpphen)3I2+in acetonitrile solution (-1 and in 37 layers of stearic acid subphase LB film prepared on the surface of 1.60 x M [Ru(dpphen)3I2+ aqueous subphase (-1 deposited on a quartz plate at 28 mN/m surface pressure and a dipping speed of 20 mdmin. Table 2. LB Data and Difference in the Absorption and Emission Spectra of the Subphuue LB Films of Stearic Acid and the Acetonitrile Solution of the Metal Comdex Ru(I1)complex area er shift” in the n-n* shift” in the in subphase SA (&I absorption band (nm) emission (nm) ~
[Ru(tm-bpy)#+ 23-24 M) [Ru(phen)3I2+ 22-25 (9.17 10-5 M) [Ru(dpphen)3I2+ 22-23 (1.76 10-7 M)
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Figure 13. Room-temperatureemission of [Ru(dpphen)3I2+ in acetonitrilesolution(-1 and in 37 layers of stearicacid subphase
LB film prepared on the surface of 1.60 x 10-6M [Ru(dpphen)3I2+ aqueous subphase (-1 deposited on a quartz plate at 28 mN/m surface pressure and a dipping speed of 20 mdmin at 20 “C. ,lex = 450 nm.
a small interaction between the ions (metal complex and stearic acid) in this [Ru(dpphen)3I2+ subphase film. Unlike the strong interaction that occurs in the case of watersoluble4Ru(I1)complexes like [Ru(bpy)3I2+where the shift in the absorption and the emission spectra is large, the ion-pair interaction for this complex is weak. This can be attributed to two factors: first, the bulk of the [Ru(dpphen)3I2+metal complex ion hinders the interaction; and second, the hydrophobicity of the molecules decreases the solubility in water, so that the metal complexmolecules
concentrate a t the surface of the subphase where they are less accessible to the hydrophilic stearate head groups in the film. The average [Ru(dpphen)3I2+:SAmoleratio ofsubphase LB films is found to be (1:16 f 2) (Table 1). In general, the metal complex to stearic acid ratio obtained for the subphase LB films is affected by (1) the concentration of the metal complex in the subphase, where the metal complex to stearic acid ratio in the LB films is larger when concentrated subphases are used, and by (2) the hydrophobicity of the metal complex. Thus the composition study indicates that the more hydrophobic molecules, like [Ru(dpphen)3I2+,produce high ratio metal comp1ex:stearic acid LB films. When the molecules aggregate a t the surface of the subphase, a contaminated surface is produced, and the molecules adsorbed at the surface of the stearic acid LB films during the dipping process result in a high metal complex: stearic acid ratio in the films. A clean surface can be checked by compressing the surface ofthe subphase while the surface pressure is monitored. In any case, the amount of stearic acid is affected by the size of the metal complex molecules. In the case of large metal complex molecules such as [Ru(dpphen)3I2+,the amount of stearic acid in the LB film is less than that obtained in the case of smaller metal complex molecules such as [Ru(tm-bpy)3I2+(Table 1). This suggests that the molecules of the metal complex occupy some location altering the stearic acid spacing in the subphase LB film (Figure 3). This abnormal spacing was indicated by the n-A isotherms where the molecular area of stearic acid is estimated for all cases of the metal complex containing films larger than 20 k ,the area of a stearic acid molecule on the surface of a pure water subphase.l On the basis of the composition measurements of the transferred films, the percent area of the slide covered by the film (Table 1) is determined by calculating the total area occupied by the stearic acid molecules (on the basis of the experimental measurements, the overall average area per stearic acid was calculated to be 23 A2) and considering the experimental areasgJ1of the ruthenium molecules, [Ru(tm-bpy)3I2+,[Ru(phen)3I2+,[Ru(dpphen)3I2+, to be 90, 100, and 120 A2, respectively. In the case of a [Ru(tm-bpy)3I2+subphase film, the stearic acid covers about 95%ofthe surface ofthe slide and the metal complex covers about 5%,while in the case of [Ru(phen)3I2+,the percent coverage with stearic acid is 86%and 14%metal complex. However, in the case of [Ru(dpphen)3I2+,where the molecule is larger and more hydrophobic than the previous molecules, the percent coverage with stearic acid is less, 75%,and that of the metal complex is large, 25% (Table 1). The error in estimating the amount of stearic acid in the prepared subphase LB films does not exceed 5%,while the error was between 10 and 20%in most cases for the estimation of the metal complex. The composition of the sul-face films of Ru(II)/SA mixtures was also studied. The measurements indicate that the actual amount of the metal complex in the transferred LB films is less than the nominal amount in the solution mixture spread on the surface of the water subphase. The two possibilities for the metal complex loss are dissolution in the subphase andor adsorption on the sides of the dipper head and the slide stainless steel holder. Obviously, more molecules can be dissolved when the film remains longer on the surface of the subphase. For example, when a solution mixture of [Ru(dpphen)J2+/ stearic acid of 1:lO molar ratio is used to produce a multilayer surface LB film, analysis of the molar ratio gave values between 1:19and 1:22ofmetal comp1ex:stearic acid. In this case the solution mixture is added on the
Incorporation of Metal Complexes into LB Films surface ofthe subphase about once an hour until 50 layers of the film are completed.
Conclusion Two types of mixed LB films of metal compledfatty acid were produced, one a surface film, and the second, a su bphase film. Stearic acid LB films incorporatingmetal complexes can be produced when the metal complex is mixed with stearic acid and then the mixture is spread on the surface of the subphase (surfacefilm). Ion-pair LB films of metal complex-stearate where the metal ions most likely reside in the head group region of the stearic
Langmuir, Vol. 10, No. 11, 1994 4163 acid for [Ru(dpphen)3l2+and beneath the stearic acid layer for [Ru(tm-bpy)9l2+and [Ru(phen)312+can be produced when the metal ions are dissolved in the subphase and the stearic acid solution is spread on the surface of the subphase (subphase film). The surface film appears to equilibrate with the subphase film within a short time. An ion-pairing interaction has a substantial effect on the spectroscopicproperties of the metal complex. A red shift in the absorption and the emission of the film metal complex from the solution is observed in all metal complexes. The metal complex to stearic acid ratio is estimated in the films from HPLC and luminescence measurements.
