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Spectroscopic Studies of Phthalocyanine Monolayers J. J. Burack, J. D. LeGrange,; J. L. Markham, and W. Rockward? AT&T Bell Laboratories, Box 900,Princeton, New Jersey 08540 Received August 20, 1991. In Final Form: October 31, 1991
We have deposited multilayer films of copper tetrakis(cumy1phenoxy)phthalocyaninein a 1:l mixture with octadecanol from a Langmuir monolayer. Using reflection spectroscopy from the monolayer at the gas-water interface, we observe that the phthalocyaninemolecules stack into multimeric structures as the monolayer is spread on the water surface and upon compression aggregatefurther. The monolayer cannot be expanded after compression,possibly because of the slow kinetics of dissociation of the aggregates, the formation of which is a highly favorable process. The compressed monolayer can be transferred to a solid support to form multilayer films in which the phthalocyanine rings are oriented out of the plane of the substrate. Such orientational order increases with number of layers.
Introduction Phthalocyanines, highly conjugated, symmetric diskshaped molecules, have been studied extensively in order to understand molecular spectroscopy in terms of molecular s t r ~ c t u r e l and - ~ to develop organic molecules for molecular electronics. These brilliant blue compounds, due to their relatively high temperature stability, twodimensional geometry, and nonlocalized charge distribution, are attractive candidates for gas sensors,4 photosensitizer~,~ organic conductors,6 and photochemical memories.' Recent work indicates that these molecules may also be useful for the fabrication of third-order nonlinear optical materials.8 Results of four wave mixing experiments on Langmuir-Blodgett monolayers of tetrakis(cumy1phenoxy)phthalocyanine, either by itself or mixed with octadecanol, demonstrate that there is a large third-order nonlinearity a t 605 nm, near the 620-nm resonance due to an intensity dependent saturation of the absorption? Such an effect may be enhanced by molecular aggregation. It has been shown in the exciton model that molecular aggregation could lead to triplet-state excitation enhancementg and therefore an intensity-dependent depletion of the ground state. The formation of dimers in a film of phthalocyanine could enhance the nonlinear response of the material. In addition to the nonlinearity of the molecular unit, which in the case of phthalocyanine may depend on the aggregation state of the molecules,the thirdorder susceptibility of a material, calculated by summing over the constituent molecules, depends on the packing density and orientational order of these molecules.1° + Current address: Department of Physics, Georgia Institute of Technology, Atlanta, GA 30332. (1)Schechtman, Barry H. Photoemission and Optical Spectra of Organic Solids: Phthalocyanines and Porphorins, Ph.D. Dissertation, Stanford University, 1969. (2)Moser, F. H.; Thomas, A. H. The Phthalocyanines; CRC Press, Inc.: Boca Raton. FL, 1983: Vol. I and 11. (3)Leznoff, C.'C.; Lever; A. B. P. Phthalocyanines Properties and Applications; VCH Publishers, Inc.: New York, 1989. (4)Snow, A. W.; Barger, W. R.; Klusty, M.; Wohltjen, H.; Jarvis, N. L. Langmuir 1986,2,513. (5)Kato, M.; Nishioka, Y.; Kaifu, K.; Kawamura, K.; Ohno, S. Appl. Phys. Lett. 1985,46, 196. (6) Schra", C. J.; Scaringe, R. P.; Stojakovic, D. R.; Hoffman, B. M.; Ibers, J. A.; Marks, T. J. J. Am. Chem. SOC.1980,102,6702. (7)Gutierrez, A. R.; Friedrich, J.; Haarer, D.; Wolfman, H. IEM J. Res. Deu. 1982,26, 198. (8)Prasad, P. N.; Casstevens, M. K.; Pfleger, J.; Logsdon, P. Proc. SPIE-Multifunctional Mater. 1988,878,106. (9) Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi,M.Pure Appl. Chem. 1965,11, 371.
The spectroscopy of phthalocyanines has been studied extensively in solution, in crystals, and in vapor-deposited The absorption spectrum of phthalocyanine in solution is dominated by strong bands occurring between 600 and 720 nm. The position and relative sizes of these bands depend on the degree of aggregation of the molec u l e ~ .The ~ ~ monomer exhibits a strong T-T* transition at 670 nm and a weaker band a t 605 nm. These bands appear in equal intensity for the dimer. Solutions of phthalocyanines typically exist in a monomer-dimer equilibrium, where dimerization is a favorable process. The equilibrium constant for dimerization depends on the metal atom in the center of the molecule, the substituents on the ring, and the solvent. The copper tetrakis(cumylphenoxy) compound in toluene, for example, forms dimers and trimers over the concentration range of 3 X to 2 X M.15 Dimerization is particularly favorable in solvents where phthalocyanines are less soluble.l6 Increasing the concentration of phthalocyanine in solution leads to the formation of higher order aggregates." The work described here represents the first study of the aggregation of phthalocyanines as observed by changes in the reflection spectrum from an air-water interface. The spectroscopy of dyes at an interface is important for the understanding of the collective properties of molecules in a two-dimensionalsystem.18 In a two-dimensionalLangmuir monolayer there is no solvent present, and we can investigate the aggregation of molecules upon spreading and compressing the monolayer. The stacking of phthalocyanines is relevant to both the fabrication of nonlinear materials and gas sensor applications utilizing the conductivity of thin films of phthalo~yanines.'~ The linear absorption spectrum of phthalocyanine in a multilayer structure differs from that in solution. The experiments described here, measurements of both a monolayer at the air-water interface and deposited films of (10)LeGrange, J. D.; Kuzyk, M. G.; Singer, K. D. Mol. Cryst. Liq. Cryst. 1987,15Ob,567. (11)Griffiths, C. H.; Walker, M. S.; Goldstein, P. Mol. Cryst. Lip. Cryst. 1976,33, 149. (12)Abkowitz, M.; Monahan, A. R. J. Chem. Phys. 1973,58,2281. (13)Lucia, E. A.; Verderame, F. D. J. Chem. Phys. 1968,48, 2674. (14)Allcock, H.R.; Neenan, T. X. Macromolecules 1986,19,1495. (15)Snow, A. W.; Jarvis, N. L. J. Am. Chem. SOC.1984,106,4706. (16)Monahan, A. R.; Brado, J. A.; DeLuca, A. F. J.Phys. Chem. 1972, 76,446. (17)Harriman, A.; Richoux, M. J.Chem. SOC.,Faraday Trans.2 1980, 7fi. .- 7 I G l R
(18)Orrit, M.; Mobius, D.; Lehmann, U.; Meyer, H. J. Chem. Phys. 1986,85,4966. (19)Pace, M. D.; Barger, W. R.; Snow, A. W. Langmuir 1989,5,973.
0743-7463/92/2408-0613$03.00/00 1992 American Chemical Society
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\V/BIFURCATED LIGHT GUIDE
Figure 1. Molecular structure of copper tetrakis(cumy1phenoxy)phthalocyanine.
varying thickness, were carried out in order to understand at which point in the process of depositing a LangmuirBlodgett film formation of an aggregated structure occurs and how that structure is affected upon transfer of the organic layer from the aqueous to solid surface.
Experimental Methods [Tetrakis(cumylphenoxy)phthalocyanine]copper, shown in Figure 1, was prepared by a modification of the procedure of Snow and J a r ~ i s . * ~ K2CO3 (0.65g, 0.005 mol) was addedto a solution of cumylpheno1 (0.6 g, 0.003 mol) and 4-nitrophthalonitrile (0.5 g, 0.003 mol) in dry dimethyl sulfoxide (5 mL) and the mixture was stirred overnight at room temperature under nitrogen. The undissolved solid was removed by suction filtration and washed with CH2C12. The combined filtrates were added with stirring to water (15 mL) and neutralized with 0.1 N HC1. The aqueous mixture was washed with three portions of CHzClz. The combined CHzClz layers were washed with 5% aqueous Na2C03,then washed with HzO, dried over MgS04, filtered and concentrated to obtain a thick oil. After chromatography (silica, xylenes/5% MeOH), 4(cumylphenoxy)-4-phthalonitrile(0.86 g, 86 % ) was obtained as a yellow oil which was further purified by chromatography (silica, CHZC12) before use in the next step. IR peaks of the oil dried on a salt plate were observed at 3072, 3040, 3024, 2960, 2928, 2864,2224,1568,1552,1488,1472,1296,1264,1232,1192,1152, 1072,1008,944,816,752 and 688 cm-l. 4-(Cumylphenoxy)-4-phthalonitrile(0.2 g, 0.59 mmol) and CuCl(O.12 g, 1.2 mmol) were dissolved in quinoline (2 mL) and stirred under nitrogen at 180 "C for 19 h. The reaction mixture was cooled to room temperature, slurried with methanol, and filtered. The collected solid was washed with toluene until the filtrate was colorless. The combined organic fractions were concentrated in vacuo. After chromatography (alumina I, methyl ethyl ketone), the blue oil was dissolvedin CH2C12. The solution was washed with three portions of 0.1 N HCl, then washed with one portion each of saturated aqueous NaHC03and H20, dried over MgS04,filtered, and concentrated to give crude [tetrakis(cumylphenoxy)phthalocyanine]copper (0.27 g) as a blue solid. The solid was dissolved in the minimum quantity of CH2C12, and dropped into methanol (100 mL) with stirring. The mixture was filtered into an extraction thimble and extracted overnight with methanol in a Soxhlet apparatus. The product was then extracted with CHzClzuntil the condensed solvent was colorless. Removal of the solvent afforded [tetrakis(cumylphenoxy)phthalocyaninelcopper (0.14g, 67% ). The purity of the final product has been verified by spectroscopic measurements on the followinginstruments: Nicolet 7199 FTIR, Bruker AC 250 NMR, and Perkin-Elmer 330 UV-vis. UV280,340, vis peaks of the compound in chloroform occur at ,A, 620, and 680 nm. The IR peaks of the sample dried on a salt plate are at 2980,1589,1495,1454,1393,1333,1225,1171,1083, and 738 cm-l. Proton NMR of the compound in deuterated chloroform exhibits broad singlet peaks at 6 1.6 and 7.25. Films were spread from a mixture of the phthalocyanine and octadecanol (Aldrich), where the concentration of each was 0.3 mM. This mixture was prepared in a volumetric flask, using chloroform stabilized with 0.75 5% ethanol (Kodak Spectra ACS Grade) as the solvent. Use of chloroform stabilized withamylene resulted in poor monolayer transfer. Before a monolayer was spread on the water surface, the solution was sonicated for a
3 WAY TRANSLATION
Figure 2. Schematic of apparatus for measuring reflection spectra from air-water interface. minimum of 30 min, followed by filtration through 0.5-rm filters to breakup and remove large aggregates. Monolayerswere spread at room temperature in a Teflon Langmuir-Blodgett trough (KSV Instruments) on a subphase of MilliQ water near neutral pH. The film was compressed at 5 (dyn/ cm)/min to measure isotherms and at 1 (dyn/cm)/min for spectroscopic measurements and film deposition. The barrier could move only in the direction of decreasing area in all experiments. Moving the barrier to larger areas resulted in a monolayer which could not be transferred to a solid support. For layer depositionthe monolayer was left to anneal at the deposition pressure for 30 min before beginning film transfer. The substrates, fused silica slides, were dipped in and out of the subphase at a speed of 5 mm/min, with a 20-min drying period after each cycle. Transfer ratios for deposition were unity for both the up and down stroke, resulting in a centrosymmetric film structure. Phthalocyanine multilayers could be deposited on fused silica, or thermally grown silica on silicon, or on bare silica optical fiber. Substrates were degreased in trichloroethylene at 80 "C for 10 min, followed by rinses in acetone, methanol, and water, all at room temperature. The slides were then soaked in a mixture of ammonium hydroxide, water, and hydrogen peroxide at 80 "C for 20 min, rinsed in flowing water, and dried with a nitrogen gun. Any organic residue was removed in an oxygen plasma (Plasma Preen), applied for 20 min prior to film deposition. This plasma clean was critical to obtaining reliable monolayer transfer, possibly due to the removal of hydrophilic OH groups resulting in a more hydrophobic surface. Good deposition also occurs if the glaas is exposed to chloroform vapor for severalhours, thereby creating a hydrophobic surface.20 In another study of phthalocyanine Langmuir-Blodgett films, it was reported that monolayer transfer was poor if very hydrophilic surfaces were used.21 Absorption spectra of the Langmuir monolayer as a function of surface pressure have been measured by a diode array spectrometer (Oriel), consisting of a monochrometer and diode array detector. Figure 2 depicts a schematic of the experimental setup. White light from a quartz halogen lamp is transmitted through a fiber bundle positioned over the water surface by an xyz stage. The incident light is transmitted across the air-water interface in two passes by a mirror on the bottom of the trough, collected by the fiber bundle which is bifurcated, and transmitted into the detection optics. The position of the fiber bundle above the water surface is adjusted to maximize the absorption signal from the monolayer. This height varies due to changes in the height of the water meniscus contacting the walls of the trough as the surface tension of the water is lowered by increasing the surface pressure of the monolayer. The spectra are corrected by subtracting the background of the clean water surface with the fiber bundle positioned at the same height. Absorption spectra from deposited Langmuir-Blodgett films were measured by a Perkin-Elmer 330 spectrometer. Polarizers transmitting horizontal 2 are placed in front of the detectors in the sample and reference compartments of the double beam spectrometer. Linear dichroism is obtained by measuring the horizontal componentof the transmitted light as the sample plane is rotated with respect to the incident light (which is normal to the film plane at OO). This measurement gives the degree of ~
(20) Barger, W. R.;Snow, A. W.; Wohltjen, H.; Jarvis,N. L.Thin Solid Films 1985, 133, 197. (21) Kalina, D. W.; Crane, S. W. Thin Solid Films 1985, 134, 109.
Langmuir, Vol. 8, No. 2, 1992 615
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(1')PER MOLECULE
Figure 3. Surface pressure-surface area isotherm measured at room temperature for monolayer spread from a 0.3 mM solution of copper tetrakis(cumy1phenoxy)phthalocyanine and octadecanol in a 1:l mole ratio. Isotherms are shown for compression (dotted curve) and expansion (solid curve). The areas per molecule shown are an average over the octadecanol and phthalocyanine molecules. anisotropy about the axis normal to the film, i.e. degree of orientational order in the transition momenta, and does not tell anything about in-plane anisotropy.
Results and Discussion Langmuir monolayers are formed at a gas-water interface by amphiphilic molecules. Phthalocyanine monolayers, which can be transferred to solid supports, are fabricated by adding substituents to the moleculesto make them more soluble. Such monolayers have been studied using soluble phthalocyanines substituted with alkyl, alkoxy,and cumylphenoxygroups.s22 We chose the latter due to the relative simplicity of the synthesis procedure,15 its reported large nonlinear response,8 and its potential for forming multilayer films up to hundreds of layers thick. A plot of the surface pressure-surface area isotherm is shown in Figure 3. This isotherm is for a monolayer spread from a filtered solution. The break in slope, observed during compression (dotted curve) at 8 dyn/cm, may be due to a change in the state of aggregation of the phthalocyanines. A large hysteresis occurs between the compression and expansion isotherms suggesting that this change in structure is not reversible on the time scale of the expansion. A positive surface pressure of several dynes per centimeter is observed upon expansion, and subsequent compressions exhibit the same isotherm as measured in the first expansion of the monolayer. Isotherms of monolayers spread from both filtered and unfiltered solutions exhibit these features, but only monolayers spread from filtered solutions could be transferred to a substrate. Furthermore, a monolayer which has been expanded cannot be transferred to a substrate. If the barrier is even allowed to expand in response to a feedback loop to maintain constant pressure, the monolayer is not transferable. We propose that the kink we observe in the isotherm is caused by the formation of higher order aggregates which are known to form in phthalocyanine concentrations a t higher concentration^.'^ These multimeric aggregates are known to be stable, and the kinetics of dissociation could be slow compared to the motion of the barrier. The monolayer, therefore, is not in equilibrium as the barrier is expanded; the monolayer structure is broken up into islands of aggregates which are pushed closer together upon subsequent compressions. This is not the same behavior, however, as the formation of large aggregates from the monomers and dimers initially spread (22) Baker, S.;Petty, M. C.; Roberts, G. G.; Twigg, M. V. Thin Solid Film 1983,99,53.
on the water surface during the first compression, the process which may cause the kink observed in the isotherm. The average area per molecule calculated from the isotherm is 74A2for a solution which has not been filtered. Assuming an area per molecule of 22 A2 for octadecanoP and a 1:l molar ratio at 0.3 mM, each phthalocyanine molecule occupies an area of 126 A2. The isotherms for the filtered and unfiltered solutions are similar except that the totalarea of the monolayer used to calculate area per molecule decreases by a factor of 1.6 for the filtered solution indicating that less material has been spread on the water surface. If we assume that only large phthalocyanine aggregates have been filtered out and that the area per phthalocyanine is 126 A2 (calculated from the isotherm of an unfiltered solution), the concentration of phthalocyanines is 0.55 that of the octadecanol. This is probably a crude assumption since removal of larger aggregates should affect the average area per phthalocyanine. The concentration of phthalocyanine, corrected in this manner, has been used to obtain the average area per molecule values on the abscissa of the isotherm shown in Figure 3. The area per phthalocyanine, 126 A2, is smaller than the 400A2 expected for a single molecule where the plane of the molecule is parallel to the air-water interface,2O suggesting that the molecules form multimeric stacks and that these stacks are tilted with respect to the water surface. The number of molecules in a stack depends on the orientation of the stack and therefore must be determined independently. In previous work on mixtures of the same compound with octadecanol the area per molecule obtained from the isotherm is 34A2(avalue which appears to include the octadecanol molecule^).^^,^^ This area is significantly smaller than the result reported here. We have found that the isotherm and deposition of a phthalocyanine monolayer are sensitive to small differences in concentration and solvent. The distribution of aggregates on the water surface after spreading may depend on these and other factors. It has been noted previously that films which are too "rigid" cannot be deposited, and selection of a solvent which evaporates slowly gives a less rigid This observation is consistent with the results reported here. A rigid film may be one which contains larger aggregates, formed upon evaporation of the solvent at a faster rate than the solution can spread on the water surface. Monolayers deposited from chloroform with amylene cannot be transferred, but solutions in chloroform stabilized with ethanol can be transferred. This could be because the ethanol (boilingpoint 78 "C) evaporates slower than the amylene with a boiling point of around 36 "C. Furthermore, more polar solvents limit aggregation due to electrostatic screening effects,l6qz6another possible explanation for the dependence of deposition on solvent choice. This picture of aggregation upon spreading and the role of aggregation in monolayer transfer support our observations that monolayers spread from concentrated solutions p0.3 mM) or from unfiltered solutions cannot be transferred to a substrate. Variation in areas per molecule obtained from isotherms may be due to variation in aggregate sizes, initially and after compression. The results, however, all suggest that the phthalocyanine (23) Barger, W.;Dote, J.; Klusty, M.; Mowery, R.; Price, R.; Snow, A. Thin Solid F i l m 1988,159, 369. (24) Snow, A. W.; Barger, W. R. Phthalocyanine Films and Chemical Sensors. In Leznoff, C. C.; Lever, A. B. P. Phthalocyanines Properties and Applications; VCH Publishers, Inc.; New York, 1989; p 341. (25) Baker, S.Znt. Symp. Future Electron Devices-Bioelectron. Mol. Electron. Devices (Tokyo) 1985, 53. (26) Monahan, A. R.;Brado, J., and DeLuca, A. F., J . Phys. Chem. 1972,76, 1994.
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616 Langmuir, Vol. 8, No. 2, 1992 0.6
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Figure 6. Spectrum measured in transmission from a monolayer deposited at 10 dyn/cm on a fused silica slide. 600
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Figure 4. Solution spectrum for 0.3mM solution of copper tetrakis(cumy1phenoxy)phthalocyanine in chloroform.
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Figure 5. Spectrumof a monolayer of copper tetrakis(cumy1phenoxy)phthalocyanine/octadecanolmixture measured in reflection from gas-liquid interface.
molecules exist with the molecule plane at some angle to the water surface and with some degree of stacking between molecules. Collapse of the aggregated monolayer occurs a t 25 dyn/ cm to a state, perhaps multimeric, of higher aggregation. At surface pressures greater than this collapse pressure, it is not possible to transfer the layer to a solid surface. Film deposition is also poor for monolayers spread from solutions of concentration greater than 0.3 mM or from solutions which are not filtered. A comparison of absorption spectra from the phthalocyanine/octadecanol solution in chloroform and the monolayer spread from that solution indicates that the degree of aggregation changes upon spreading the monolayer at the air-water interface. The solution spectrum, plotted in Figure 4, exhibits a large peak at 680 nm and a small peak a t 610 nm. This spectrum is characteristic of phthalocyanine monomers in solution. Upon spreading the solution on the water surface, even at 0 dyn/cm, aggregation has occurred. This is seen in the spectrum plotted in Figure 5, where there is an intense, broad absorption peak a t 615 nm, and the 680 nm absorption appears as a shoulder. The broad peak at 615 nm, between the monomer peak at 680 and its satellite peak at 610 nm, is characteristic of multimer formation between phthalocyanine molecules. A possible explanation for the formation of aggregates upon spreading the monolayer on the water surface is that droplets of volume on the order of several microliters are applied to the surface; this small quantity of solvent evaporates quickly before complete
spreading of the solution has occurred; locally there is a very high concentration of phthalocyanines and aggregates form. As the monolayer is compressed, these aggregates are pushed together and, as suggested by the kink in the isotherm, larger aggregates or stacks may form. These larger aggregates, however, are not apparent from the spectra at the air-water interface. With the monolayer compressed from 0 to 20 dyn/cm there is a 5% increase in intensity but the relative peak heights at 615 and 680 nm remain the same. This is consistent with a previous study correlating solution spectra for tetrakis(cumy1phenoxy)phthalocyanines containing different metal atoms such that the degree of association between monomers varies with vapor pressure osmometry.15 The spectra exhibit a broadening of the peak at 610 nm with increasing size of the aggregate. The spectrum of the monolayer, however, does not exhibit such an effect as the 615-nm peak is the major peak; it is broad even at 0 dyn/cm and the 680-nm peak appears as a slight shoulder. Previous work comparing the spectrum of the melt of a solid phthalocyanine film to that of dimers in solution indicates that those spectra are similar.16 This result suggests that higher order aggregates could form between phthalocyanine dimers or trimers as the monolayer is compressed but that these aggregates may not cause changes in the spectrum. The increase in intensity with compression reflects the increasing concentration of molecules as the total area is decreased. The intensity does not scale linearly with concentration expressed as area per molecule, a result which is consistent with the observation that spectra of phthalocyanines in solution as a function of concentration do not obey Beer's law.16 The spectrum of the monolayer transferred to a solid support, as shown in Figure 6, exhibits the same peaks as measured from the water-supported monolayer (Figure 5) and does not vary significantly with the surface pressure at deposition. A spectrum of the film on the water surface measured at pressures greater than 25 dyn/cm, the collapse pressure, is similar to the spectra acquired at lower pressure. The intensity of the peak at 615 nm, however, is roughly 2 times larger. This suggests that a multilayer structure forms after collapse consistent with our observation that the film cannot be transferred to a substrate at these pressures. The orientation of the molecules deposited on a substrate can be deduced from linear dichroism measurements. As the sample is rotated away from normal incidence of the probing beam, the component of electric field normal to the plane of the film increases. Figure 7 exhibits spectra of a 13-layer film as a function of the angle between the incidence light and the vector normal to the film. The peak at 615 nm increases in intensity as the sample is
Langmuir, Vol. 8, No. 2, 1992 617
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Figure 7. Absorption spectra measured in transmission from a 13-layerfilm at varying angle between sample normal and incident light (- - -,0";-, 20"; ., 40"; X, 60").
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rotated away from normal incidence. Because the transition moment correspondingto the 615-nm absorption is in the plane of the molecule, this dichroic effect suggests that the phthalocyanine planes are oriented out of the plane of the substrate surface, as is consistent with the area per molecule derived from the pressure-area isotherms. This result is also in agreement with a tilt angle of 70 to 80' between the phthalocyanine planes in a deposited monolayer and substrate surface, measured by electron spin resonance for metal-free (cumy1phenoxy)phthalocyanine in a film doped with a copper phthalocyanine probe.27 The spectra do not exhibit dichroism in the peak at 680 nm, suggesting that the phthalocyanine monomers contributing to that absorption are randomly oriented in the film. The dichroism in the 615-nm peak changes with film thickness. The absorption as a function of 0, normalized by the absorption measured at normal incidence, is plotted for films of varying thickness in Figure 8. The increase in absorption as the sample is rotated from 0 to 60°changes from less than 10% for a monolayer to about 33% for a 19-layer film, suggesting that the substrate surface is modified by the first layers and subsequent monolayers are transferred with a greater degree of orientation. The effect appears to saturate near 20 layers. The enhanced dichroism of the thicker films is also depicted in Figure 9 where the absorptions at B = 0 and B = 60' are plotted as a function of number of layers, and the difference between them increases with thickness. At both 0 and 60° the data scale roughly with number of layers as expected for uniform deposition. The increase in orientational order with thickness has also been observed by nonlinear optical measurements of this phthalocyanine.8 Using a range of spectroscopic and diffraction techniques, it has been shown in other materials forming LangmuirBlodgett films that the structure of the first monolayer differs from subsequent layer~.~8929 (27) Pace, M. D.; Barger, W. R.; Snow,A. 75, 73.
W.J. Magn. Reson. 1987,
5
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NUMBER OF LAYERS
Figure 9. Intensity (arbitrary units) of 615-nm peak as function of number of layers for 0 (circles) and 60" (squares) incidence.
Conclusions The results described in this paper show that the process of spreading a monolayer of phthalocyanine molecules at a liquid-vapor interface results in a layer of phthalocyanine stacks with the planes of the molecules tilted at some angle to the water surface. A kink is observed in the isotherm a t 8 dyn/cm, suggesting a structural change. The compressed structure, either the formation of larger aggregates or association between aggregates, is stable and the kinetics of dissociation appear to be slow on the time scale of expansion of the film. The aggregated structure at pressures below the collapse pressure is not a multilayer structure as it can be transferred as a Langmuir monolayer and the intensity of the spectrum does not increase substantially until collapse of the film. The transfer of the monolayer depends on the distribution of aggregates in the film. This distribution appears to depend on the concentration of the solution, choice of solvent, and filtration of the solution before spreading it (28) Amdt, T.; Bubeck, C. Thin Solid Films, 1988, 159,443. (29) Dierker, S. B.; Murray, C. A.; LeGrange, J. D.; Schlotter, N. E. Chem. Phys. Lett. 1987,137,453.
618 Langmuir, Vol. 8,No. 2, 1992 on the water surface. The film can be transferred to solid supports which are hydrophobic in nature. The first layers deposited modify the surface to be more hydrophobic so that subsequent layers are transferred with a higher degree of orientational order. This suggests that appropriate treatment of the substrate surface can be used to control the orientational order of a multilayer film, which may be useful to applications of phthalocyanines to device protection and nonlinear optics. The behavior at the liquidvapor interface demonstratesthat the stacking of molecules
Burack et al.
in a monolayer occurs in the spreadingprocess, even before compression to higher surface pressure.
Acknowledgment. We thank T. X. Neenan for a sample of 4-nitrophthalonitrile. We also acknowledge A. Snow and W. Barger for numerous helpful discussions on monolayer and multilayer preparation. Registry No. [Tetrakis(cumylphenoxy)phthalocyanine]copper, 121013-62-5;octanol, 112-92-5.
