Langmuir 1998, 14, 5267-5273
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Alcohol Chain Length and Mole Fraction Dependence of the Stability of Tetrakis(cumylphenoxy)phthalocyanine and Alcohol Langmuir Films Shenda M. Baker,* Joseph Danzer, Heather Desaire, Grace Credo, and Rebecca Flitton Department of Chemistry, Harvey Mudd College, Claremont, California 91711 Received November 10, 1997. In Final Form: June 22, 1998 While it has been known for about 10 years that octadecanol stabilizes Langmuir-Blodgett films of copper tetrakis(cumylphenoxy)phthalocyanine films, the mechanism for this stabilization is not fully understood. To understand more completely the molecular interactions responsible for making octadecanol a good transfer promoter, we studied mixed films’ isotherms of tetrakis(cumylphenoxy)phthalocyanine and various saturated alcohols ranging in length from 13 to 24 carbons. By varying mole fraction and alcohol chain length in the various mixed films, a model of interaction was deduced based on pressureversus-area isotherms obtained from a Langmuir trough. In this model, the aliphatic chains of the alcohols act as a stabilizing, hydrophobic support around phthalocyanine mounds of a preferred stack height. As film pressure increases, the mounds are forced above the alcohol sea, removing favorable ether-water interactions. Depending on chain length, a single or double collapse is observed, suggesting an adjustable physical barrier to forcing the stacks above the alcohols.
Introduction Phthalocyanine (Pc) thin films, their derivatives, and multicomponent films are used for applications ranging from chemical sensors, molecular metals, conducting polymers, fuel cell catalysts and sulfur effluent pollutant control to optical storage devices.1 Many of the desired characteristics of these devices depend on the film morphology, which affects the electronic properties. The physical properties of the film, including the stacking and orientation of the Pc, depend on the chemical properties of the Pc derivative, the technique of film preparation, the film-substrate interaction, and the copolymers or transfer promoters used to facilitate production of highquality films. The Langmuir-Blodgett (LB) technique, which often utilizes derivatized Pc’s for solubility, produces controllable morphologies on large areas. Specific molecular architectures and film properties can be achieved by symmetric or asymmetric substitutions or by different metal complexation. Selectively oriented multilayers of precisely controlled thickness can be reproducibly prepared by customizing the hydrophobic and hydrophilic regions of the molecule by derivatization of the phthalocyanine rings.1 Although the electronic and optical properties of many of these films have been studied extensively, the nature of the interaction of Pc films with the substrate, water, or layer interfaces is less well understood. Because underivatized phthalocyanines or those with added nonpolar groups tend to form irregular or poorly transferred films, an amphiphilic transfer promoter such as octadecanol is typically added to facilitate reproducible film transfer and to stabilize and homogenize films deposited by the Langmuir-Blodgett technique.2 Different metals, substituents, transfer promoters, or solvents have been shown to produce different orientations of the plane of the phthalocyanine relative to the surface. (1) Leznoff, C. C., Lever, A. B. P., Eds. Phthalocyanines; VCH Publishers: New York, 1989. (2) Hann, R. A. In Langmuir Blodgett Films; Roberts, G., Ed.; Plenum Press: Oxford, U.K., 1990; p 17.
For example, Fryer et al.3 assert that copper tertbutyl substituted phthalocyanines [CuPcBu4] lie with the plane of the molecule inclined but close to normal to the surface, which is consistent with previous work done by Lloyd et al.4 However, the lead analogue lies parallel to the surface.5 Without an amphiphilic promoter, islands of crystals within a disordered structure are observed for both metal chelated PcBu4’s. The tetrakis(cumylphenoxy) derivative of the phthalocyanine [PcCp4] with octadecanol as the transfer promoter has been used in films extensively as chemiresistors and has been widely examined. It has been suggested that the cumylphenoxy side groups increase the phthalocyanine’s solubility in polar solvents6 and provide better adhesion to hydrophilic solid substrates.7 Barger et al.6 demonstrated the need for the transfer promoter for a series of ether linked substituents on the Pc ring and were the first to suggest that the rings were not flat on the surface. Because of a small apparent mean molecular area (MMA) of the PcCp4, they proposed a stacking height of 8-10 PcCp4’s in fairly irregularly oriented mounds. Differential scanning calorimetry8 suggests significant phase separation between the octadecanol and the PcCp4 for a Langmuir-Blodgett film that behaves as a 2-D colloidal dispersion of 50-500 nm PcCp4 disklike aggregates in octadecanol. Furthermore, transmission electron microscopy (TEM) images of the mixed films confirm that the CuPcCp4 stacks into small, pancake-like mounds.8 Even at mole ratios of 5:1 octadecanol:PcCp4, domains of microcrystals of the Pc were observed by (3) Fryer, J. R.; McConnell, C. M.; Hann, R. A.; Eyres, B. L.; Gupta, S. K. Philos. Mag. B 1990, 61, 843. (4) Lloyd, J. P.; Pearson, C.; Petty, M. C. Thin Solid Films 1988, 160, 431. (5) Nichogi, N.; Waragai, K.; Taomoto, A.; Saito, Y.; Asakawa, S. Thin Solid Films 1989, 179, 297. (6) Barger, W. R.; Snow, A. W.; Wohltjen, H.; Jarvis, N. L. Thin Solid Films 1985, 133, 197. (7) Burack, J. J.; LeGrange, J. D.; Markham, J. L.; Rocaward, W. Langmuir 1992, 8, 613. (8) Barger, W.; Dote, J.; Klusty, M.; Mowery, R.; Price, R.; Snow, A. Thin Solid Films 1988, 159, 369.
S0743-7463(97)01217-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/08/1998
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Nichogi et al.,5 suggesting that dilution does not remove aggregation. A number of techniques have been used to determine the stacking direction of the PcCp4 within the mounds and to find a stacking height through the mean molecular area. Using pressure vs area arguments from a Langmuir trough study, Barger et al.6 assert that CuPcCp4, in a ratio of 1:1 with octadecanol, align in cofacial stacks of eight PcCp4’s with the plane of the Pc parallel to the surface and the stack axis inclined from perpendicular to the surface. Other LB studies suggest that the PcCp4’s are both sitting at an angle to the surface9 and stacking on top of each other.10 This orientation agrees with Fryer’s assertion that the copper can associate with a nitrogen in the Pc beneath (in the copper tetrakis (tert-butyl) substituted Pc’s).3 And while EPR results11 suggest that a number of the CuPcCp4’s may have their planes aligned 80 ( 10° relative to the surface, the orientation of the crystals laterally is not uniform. X-ray crystallography and freeze fracture data of the PcCp4 and octadecanol films have been interpreted as a model of eight phthalocyanines stacking on eight octadecanol poles, with the phthalocyanines parallel to the surface of the water in a well-organized crystal structure.10 IR absorbancereflectance spectroscopy (IRRAS)8 also suggests that in these mixed films the octadecanols are slightly tilted from perpendicular to the surface and the PcCp4’s are not flat on the surface. An optical second-harmonic generation analysis of nickel tetrakis(cumylphenoxy)phthalocyanine and octadecanol LB films showed that the phthalocyanine microcrystallites are tilted 25.5° with respect to the surface normal and the projection of the stack axis onto the film surface was along the deposition direction.12 Finally, in the similarly large, but lacking hydrophilic ether oxygens, CuPcTb4 system with icosanoic acid, electron spin resonance (ESR)13 studies also confirm an angular average of 80 ( 10° from parallel to the surface. The interaction between the transfer promoter and the PcCp4 is not well-understood. The octadecanol, being amphiphilic, provides adhesion to hydrophilic substrates and presumably also provides some mechanical support for the aggregates of PcCp4 molecules. The structure of the aggregate mounds likely contains some phthalocyanines stacked cofacially, but the amounts and orientations of the cofacial stacks within the films remain in question. To gain further understanding of the alcohol phthalocyanine interactions, we have studied mixed films of tetrakis(cumylphenoxy)phthalocyanine and various saturated alcohols (with chain lengths of 13-24 carbons long). By varying the alcohol chain length, the mole fraction, and the temperature, we have deduced a model of interaction based on data from pressure-versus-area isotherms of the films. Experimental Section Trough and Film Preparation. All glassware was washed in aqua regia (1:3 HNO3 to HCl) for at least 30 min, rinsed at least 10 times with 18.2 MΩ/cm Millipore water, oven dried, and rinsed twice with reagent-grade chloroform and twice with HPLC grade chloroform. Alcohols and tetrakis(cumylphenoxy)phthalocyanine, shown in Figure 1, were purchased from Sigma-Aldrich. Solutions of (9) Pace, M. D.; Barger, W. R.; Snow, A. W. Langmuir 1989, 5, 973. (10) Suzuki, A.; Awanoo, H.; Hikosaka, M.; Ohigashi, H. Thin Solid Films. 1992, 216, 283. (11) Pace, M. D.; Barger, W. R.; Snow, A. W. J. Magn. Reson. 1987, 75, 73. (12) Neuman, R. D.; Shah, P.; Akki, U. Opt. Lett. 1992, 17, 798. (13) Cook, M. J.; Danial, M. F.; Dunn, A. J.; Gold, A. A.; Thomson, A. J. J. Chem. Soc., Chem. Commun. 1986, 863.
Baker et al.
Figure 1. Molecular structure of tetrakis(cumlyphenoxy)phthalocyanine. the alcohols (tridecanol, tetradecanol, pentadecanol, octadecanol, eicosanol, docosanol, tricosanol, and tetracosanol) and PcCp4 were made by weighing out the components (10-20 mg) on a microbalance and preparing ∼5 × 10-4 M solutions in Aldrich HPLC grade chloroform. Solutions prepared from freshly distilled chloroform gave identical results. Solutions were stored in 10 mL of acid-cleaned, Millipore-rinsed volumetric flasks at room temperature for less than 1 week. Alcohol:PcCp4 solutions were made prior to each run. Separate solutions of alcohol and phthalocyanine were mixed, and this solution was applied to the trough using a carefully rinsed 500 µL syringe. Unless otherwise stated, the alcohol and phthalocyanine solutions were combined within 5 min of their application to the trough. All films were made on a computer controlled KSV 5000 Langmuir-Blodgett trough with an effective area of 150 mm × 700 mm filled with Millipore water. The surface pressure was measured by a ∼1 cm2 platinum foil Wilhelmy plate, flamed before each use. Cleaning the trough required several steps. First, one barrier was swept several times completely across the surface, removing surfactants that may have collected. The barriers were then cleaned with a chloroform-wetted Kimwipe; then the surface of the water was swept about 10 more times. Finally, the cleanliness of the surface was checked by allowing the barriers to move to their maximum compression position. A clean trough showed no net increase in pressure with compression and pressure fluctuation below (0.05mN/m. After the final trough cleaning, the solution was immediately applied to the trough. Drop size was approximately 4 µL/drop. The drops were spread evenly over the surface of the trough. The average application time was about 3 min. The chloroform was allowed to evaporate for 10 min. Pressure vs area isotherms were taken at a constant temperature of 25.0 ( 0.3 °C unless otherwise noted. The barrier speed for compression was a constant 50 mm/min. Experimental Considerations and Reproducibility. A number of experimental variables had a significant effect on the reproducibility of the isotherms. Our standard laboratory procedure was optimized so individual researchers all obtained identical isotherms. The main constraints are discussed below: (1) Drop Size and Placement of Drops. Both small (5 µL) drops were checked. Drops were carefully touched to the surface or dropped from about 1 in. Neither drop size nor placement affected the mean molecular area (MMA) or the stability (final pressure) of the isotherm as long as the drop remained at the surface and the solution was well-mixed prior to application. (2) Concentration. Concentrations of the alcohol and phthalocyanine solutions were varied from 1 × 10-5 to 2 × 10-3 M with no effect on the MMA or shape of the isotherm of mixed alcohol and PcCp4 films. (3) Barrier Speed. Various barrier speeds were checkeds from 10 to 150 mm/min. Isotherms collected at or below 50 mm/
Stability of PcCp4 and Alcohol Langmuir Films min showed no effect related to barrier speed. Isotherms collected at 90 mm/min or greater showed signs of the film not being at equilibrium. Therefore, a barrier speed of 50 mm/min was used for all of the isotherms. Reproducible isotherms of PcCp4 without alcohol were not obtained, and varying the concentration did not cause any trends in PcCp4’s MMA. PcCp4 isotherms without alcohol gave MMA’s ranging from about 55 to 80 Å2. Although this value differs significantly from the literature for copper centered PcCp4 (Barger et al.6 and Suzuki et al.10 measured MMA’s of approximately 35 Å2), this result indicates that the phthalocyanine associations without transfer promoter are very sensitive to external conditions such as solution concentration, rate of compression, temperature, and the absence of a copper center.7 Mixed films (alcohol:PcCp4) produced very reproducible isothermssoften the isotherms of alcohol to PcCp4 ratios greater than 2:1 superimpose exactly and were rarely different by more than 2%. One-to-one ratios were slightly less reproducible (
