J. Phys. Chem. 1994,98, 8985-8991
8985
Effect of Stearic Acid on Molecular Orientation in Metal-Free 2,9,16,23-Tetra-tert-butyltetrabenzotriazaporphineLangmuir-Blodgett Films Yansong Fu, Michael Forman, C. C. Leznoff, and A. B. P. Lever' Department of Chemistry, York University, North York (Toronto), Ontario M 3 J I P3, Canada Received: February 28, 1994; In Final Form: June IO, 1994" Langmuir-Blodgett films of 2,9,16,23-tetra-tert-butyltetrabenzotriazaporphine(TBTAPCls, Cls refers to a pentadecyl hydrocarbon long chain), codeposited with stearic acid (SteA), were studied. The TBTAPCls molecules, forming a monolayer in a pure state, can only assemble themselves on the water surface in an edge-touching fashion. However, the admixture of stearic acid substantially affects the molecular orientation, as indicated by the consistent increase in the area per TBTAPCls molecule with increasing stearic acid concentration. The transferability and dichroic behavior of the films with different orientations were investigated. The data are interpreted in terms of the T B T A P C I ~molecules changing their orientation from an essentially vertical pattern to a horizontal pattern when a sufficient amount of stearic acid was present. A cyclic voltammogram of the pure TBTAPCls LB film was obtained with an indium-doped Sn02 (ITO) electrode.
Introduction The extraordinary interest taken in the Langmuir-Blodgett (LB) technique'" during the past decade or so is, to a large extent, attributable to the employment of well-known compounds such as the porphyrins7-ll and phthalocyanines ( P C ) . ~ ~The -~~ optical and electrochemical properties of these macrocyclic compounds make them especially valuable when assembled into well-organized LB films, in areas such as molecular electronics,25-28 optical devices29-32 and chemical gas sens0rs.33-~~ The low solubility of metallophthalocyanines in organic solvents can be obviated by placing alkyl substituents peripherally on the macrocyclic rings or axially to the central metals. From the viewpoint of LB film fabrication, substitution is used to provide the parent molecules with the necessary amphipathicity to enable self-assembly of a monolayer on subphase surfaces. The organization and orientation of the monolayer molecules can also be controlled by adding certain substituents in specificways, and therefore the desirable structure of an LB film can be obtained.38 While the mixing of long-chain hydrocarbon species, such as octadecane,39 octadecan01,@.~1 stearic a ~ i d , 4or~ arichidic ,~~ acidIs with, e.g., porphyrins and phthalocyanines, has received credit for improving the film quality and facilitating film transfer to various substrates, much less attention has been paid to the effect of the long-chain hydrocarbons on the orientation of the parent molecules in the film.11-44-48 In the current studies, the influence of stearic acid (SteA) on the molecular orientation of tetra-tert-butyl tetrabenzotriazaporphine (TBTAPCls) LB films is investigated. We show that the area occupied by one tetrabenzotriazaporphine molecule changes in a consistent manner upon altering the concentration of stearic acid in the TBTAPCls/SteA mixtures. A model is proposed that the TBTAPCls molecules in a monolayer on the water subphase change their orientation from an essentially vertical pattern, when forming a monolayer in the pure state, to an approximately flat-sitting pattern when a certain amount of stearic acid is present in the monolayer. This conclusion is supported by area-per-molecule, transferability and dichroism experiments. Some preliminary electrochemical data of the TBTAPClS LB film are also presented. This appears to be the first example where mixing a phthalocyanine species with a film forming species such as a fatty acid alone causes a change in orientation of the phthalocyanine molecule. Previously, there has been one report11of theorientation of a copper porphinederivativebeing controlled in a triple mixture by adding a small amount of hexatriacontaneto a copper porphine/
* Abstract published in Advance ACS Abstracts, August 1 ,
1994.
icosanoic acid mixture. The icosanoic acid alone does not cause a change in orientation.
Experimental Section Materials. The synthesis of the metal-free 2,9,16,23-tetratert-butyltetrabenzotriazaporphine abbreviated (TBTAPCls), with its methylene bridge substituted by a hexadecyl group, has been described (formal name 2,9,16,23-tetrakis(2,2-dimethylethyl)-27-pentadecyl-29H,31H-tetrabenzo[b,g,l,q] [5,10,15]tria~aporphine]).~gAldrich HPLC-grade toluene was glass distilled and used as a spreading solvent. The subphase was water purified by double distillation over KMn04 followed by passage through a Barnstead organic removal cartridge and two Bamstead mixed-resin ultrapure cartridges. The stearic acid was recrystallized from absolute ethanol and dried in a vacuum oven at 50 OC for 24 h. The NaC104 electrolyte was used as supplied. Monolayer Formation and Isotherm Measurement. The LB trough was made from Teflon, built at York University, according to a described design.'O The system was controlled with an Apple 11+ microcomputer, using a local program, as described in our previous paper.& The concentration of the toluene solution of TBTAPCI5, was usually 8 X 10-4 M. In general, after spreading the solution, 5-1 0 min were allowed for the toluene to evaporate, the monolayer was then compressed at a speed of ca. 1.5 A2/ (molecule min), and a surface pressure ( T ) versus area per molecule (A) isotherm recorded. Glass and quartz slides were first immersed in boiling CH2Cl2 or CHC13 for about 5 minutes and rinsed with acetone followed by water (double distilled). After sonication in a 1 M NaOH aqueous solution for about 5 min, the slides were rinsed with water followed by acetone for drying, yielding hydrophilic surfaces. A hydrophobic surface was obtained by immersing the above slide in a 5% (vol) Me2C12Si/CC14 (Me = methyl) solution for a few minutes and rinsing it with acetone. For SnO2 film-coated glass slides ( I T 0 electrode), a slightly basic Decon solution was used instead of the NaOH solution. The monolayer was transferred onto a substrate by dipping-lifting cycles of the substrate through the air-water interface at 5 mm/min, at either high deposition pressure (around 20 mN/m) or low pressure (around 6 mN/m). PhysicalMeasurements. Electronicspectra were recorded with a CARY 2400 UV-vis-NIR spectrometer for solutions, and a Guided Wave Inc. Model 100 spectrum analyzer for LB films. Dichroic data were collected with a Hitachi Perkin-Elmer 340 microprocessor spectrometer with polaroid films as polarizers.
0022-3654/94/2098-8985$04.50/0 0 1994 American Chemical Society
8986
The Journal of Physical Chemistry, Vol. 98, No. 36, 1994 40
1
E z E
\
6
2
20
e P *3
2
10
VI
n
20
40
60
80
100
Ave. A r e a / M o l e c u l e , A'
Figure 1. Isotherms of surface pressure versus average area per molecule for the TBTAPC*s/SteA mixed monolayers with the molar ratios (TBTAPCl$:SteA,from left to right) being 1:19,1:9,1:4, 1:2, 1:1, 2:1, and 4:l. The inset shows the isotherms for the pure SteA (left) and TBTAPCls (right) species.
The cyclic voltammograms of the LB films were obtained with either a Princeton Applied Research (PARC) Model 173 potentiostat/galvanostat,controlled by a PARC 175 universal programmer, or a PINE Instruments RDE3 potentiostat. A normal three-electrode cell was used for the electrochemical studies with the I T 0 slide, a Pt wire and SCE as working, counter, and reference electrodes, respectively. The NaC104 aqueous solution was degassed by Ar-bubbling for at least 45 min before any measurements were taken.
Results and Discussion Formation of Monolayers on the Water Surface. Pure TBTAPC15 Monolayer. The isotherms of surface pressure ( T ) versus area per molecule ( A , P A isotherm) for the pure SteA and TBTAPCls species are shown in the inset of Figure 1. A typical three-phase isotherm is observed for SteA, with the solid phase very compact showing only about 1 A2/molecule compressibility (the area where solid phase forms minus the area where themonolayer collapses). On theother hand, the monolayer formed by TBTAPCl5 is very compressible, and about 50 Az/ molecule is lost from the point of film formation to the point of film collapse. From the inset of Figure 1, the area per molecule (APM) of the TBTAPCls monolayer at zero surface pressure is determined to be about 76 f 5 A2. In some experiments, an APM as low as 55 A2 was obtained. The TBTAPCls molecule looks like an ordinary 2,9,16,23-tetra-tert-butyl metal-freephthalocyanine with a long-chain hydrocarbon in place of one of the four aza nitrogens:
r" R: -CH3
C(CH3)S
: (CH$I&Hj
Molecular Structure of TBTAPC,,
Fu et ai. The observed APM for the TBTAPCls species is much smaller than the normal size of a phthalocyanine ring, about 200 AZas1 Therefore,the TBTAPC15molecules in the monolayer are believed to have oriented themselves at an angle with respect to the water surface in an edge-touching fashion, the same molecular orientation pattern observed with an LB film of Ag11TNPc(-2)." It is also conceivable that the molecules are sitting flat on the surface, but two or three molecules may be overlapping together, forminga multilayer. Thus, a triple layer with this pattern would show an APM of about 70 A2. In the studies of TBTAPCls in organic solution, Tse et al.49 reported the UV-vis spectra of TBTAPC15in DCB at ca. 5 X 10-4 M, which is about the same concentration as being used for making spreading solutions in the current study. The spectra showed a well-defined Soret band and double-peak Q-band, indicating that theTBTAPC15molecules therein were not aggregated. During the spreading, the TBTAPCls molecules initially orient themselves primarily with their rings parallel to the water surface. After the solvent evaporation it is unlikely that some macrocyclic planes would climb onto others to form a horizontally oriented multilayer, considering the steric influence of the four tert-butyl and one C15 hydrocarbon substituents, protruding into the air. Instead, during the monolayer compression, the alkyl substituents from different molecules begin to interact at a certain point. This interaction will become stronger with compression and eventually lift one side of the molecule (the side containing the C15 long chain) from the water surface, resulting in a monolayer with tilted molecules, where strong aggregationoccurs. Therefore, the films with this tilted molecular orientation will show a broad, blue-shifted Q-band compared to the solution spectrum (see next section). In other words, it is the surface pressure, generated by compression, which inhibits a flat orientation of the macrocyclic ring. TBTAPC,f/SteAMixed Monolayers. Quite different behavior is observed in the mixed films. Stearic acid, a typical amphiphilic species, was mixed in various ratios with the TBTAPC15spreading solution. Figure 1 shows the T-A isotherms ( A is the average area per molecule) for a series of mixed TBTAPCls/SteA binary monolayers. By extrapolating the first rising portion of the isotherms (low surface pressure part) back to zero pressure, the average APM shows, as expected, an increase with increasing molar fraction of TBTAPC15 in the TBTAPC15/SteA mixtures from 0.05 to 0.8. As the monomolecular areas of TBTAPC15 and SteA, when forming their pure monolayers, are known (inset of Figure l), the predicted average APM values for the above mixtures can be calculated. Comparison of the observed average APM (from Figure 1) with the calculated values provides information, such as the miscibility and interaction of the two components within the monolayers, and the change in molecular orientation (uide inf r a ). We have previously reported" that silver(I1) 2,9,16,23tetraneopentoxyphthalocyanine is immiscible with stearic acid and the resultant binary LB film shows a total area equal to the sum of the two component's areas with various compositions. In other cases, Petty et al.5z and Lee et al.53 reported that bis(phthalocyaninat0)ytterbium(II1) and N-docosyl-"-methylviologen are indeed miscible with stearic and arachidic acids, respectively, resulting in a negative deviation of the experimental average APM from calculated values. Shown in Figure 2 is a plot of the observed APM for the various mixtures (Figure 1) against molar fraction of TBTAPCls, along with the calculated54 average APM based upon the APM values of the individual molecules, 20 A2 for SteA and 76 A1 for TBTAPC15, respectively. Contrary to the studies mentioned above, the observed APM values deviate positively from the calculated areas. No such kind of behavior has been reported before, to our knowledge. This observation is reproducible and
Molecular Orientation in Langmuir-Blodgett Films
The Journal of Physical Chemistry, Vol. 98, No. 36, 1994 8987 35 30
-
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TBTAPC,, : SteA = 1 : 2
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Figure 2. Graph of the observed average area per molecule (APM, filled circle) and the calculated average APM (open square, based on the individual APM values of 20 A* for SteA and 76 A2 for TBTAPC15) for the various mixtures (as in Figure 1) versus the molar fraction of TBTAPC15. Theinset: correctedAPMvalues for theTBTAPCl5species in the mixed monolayers against the molar fraction of SteA. See the text for discussion of possible experimental error. cannot be explained by expelling interaction between the two components. If T B T A P C I ~and SteA are immiscible, the experimental average APM data should fall in the calculated straight line, as was the case for AgIITNPc(-Z)/Stea mixtures.ub If the two species are miscible, a negative deviation should be observed, as mentioned in the previous paragraph. Therefore it is possible that the TBTAPC15 molecules orient on the water surface differently when forming a monolayer in a pure state than they do when SteA is present in the monolayer. After mixing the long-chain SteA species into the monolayer, we suppose that the TBTAPC15-TBTAPC15 interactions are reduced and the macrocyclic ring tends to sit horizontally on the water surface. Further it is probable that the C15 side chain associates itself with the SteA molecules providing an additional mechanism to separate the TBTAPC15molecules. With this model, thelarger the amount of SteA present, the closer to horizontal will the TBTAPCls molecules sit, and eventually a completely flat orientation will be obtained when enough SteA is mixed in. Indeed, this model is consistent with our data (vide infra). Due to the high amphipathicity, stearic acid molecules can assemble very well on an aqueous subphase and the resultant solid-phase monolayer shows a total compressibility of only about 1 A2/molecule (Figure 1, inset). Therefore, to determine the APM value of the TBTAPCl5 species in each of the TBTAPC15/ SteA mixed films in Figure 1, it can be reasonably assumed that, in any of the mixed monolayers, the subtotal area occupied by the SteA species always equals the product of the number of the SteA molecules multiplied by the SteA monomolecular area (20 A2), which is constant from one mixture to another. Thus, the APM values for TBTAPCls in the different mixtures are obtained by subtracting the subtotal area occupied by stearic acid from the total area of the monolayer and then dividing the remaining area over the number of the TBTAPCIs molecules. The result is shown by the inset of Figure 2 (the APM values of TBTAPCl5 versus the molar fraction of stearic acid) and consistent with the model proposed above. Clearly, the inset of Figure 2 shows a concordant increase in APM of TBTAPC15 with increasing relative concentration of SteA in the mixtures, indicative of an orientation change of the TBTAPC15 molecules. The small APM value of TBTAPC15 at small SteA content represents the tilted orientation while the increased APM at high SteA suggests a transition to a flat
I
.
Figure 3. average areaimoleale for the mixture of TBTAPCIS/SteA = 1:2, the same as that in Figure 1, compared with a calculated isotherm (dotted curve) which is obtained mathematically by summing the two pure isotherms from the inset of Figure 1, based on the 1:2 molar ratio. orientation. When the amount of SteA in mixture is largeenough, e.g., 90%, theTBTAPC15 molecules apparently sit approximately horizontally on the water surface, showing an APM value of around 200 A2, about the planar size of the triazaporphine ring. The possible errors in the APM values of the TBTAPC15 species (inset of Figure 2) vary from one mixture to another, with relatively much larger errors found in the mixtures of very high SteA concentration. For example, with the 1:19 (TBTAPC15/SteA) mixture, the inset of Figure 2 shows an area per TBTAPCl5 molecule of about 220 A2, while our data from seven separate experiments scattered in a range between 180 and 260 A2, showing an error of f 4 0 A2. The 1:9 mixture showed an error of about f 1 5 A2, while the remaining mixtures had errors of f5-10 Az. In thecaseof very high SteAconcentrations, some SteA molecules could be pushed onto the flatly oriented triazaporphine rings upon monolayer compression, invalidating the assumption made aboveconcerningthe SteA subtotal area and resulting in a smaller APM for TBTAPCIS. The reason for those larger APM values is not very clear; certainly the real size of the triazaporphine ring plus the four tert-butyl substituents is larger than 220 A2. Another feature of the isotherms in Figure 1 is the second rise of surface pressure before the monolayers collapse, i.e., a phasetransformation behavior during the compression process. Richardson et al.55 have reported a similar observation with LB films of n-doxylstearic acid derivatives and attributed it to changes in film structure and molecular orientation. In this case, the phase transformation likely occurs when the TBTAPC15 molecules change their orientation from a horizontal to a vertical pattern. It should be made clear that the different orientations of the TBTAPC15 molecules in differently mixed monolayers, as discussed above, occur only in the low surface pressure compression region since only in this region do the measured average APM values show the positive deviations from the calculated data. After the "phase-transformation", Le., in the high surface pressure compression region, the TBTAPCl5 molecules orient themselves vertically on the water surface as is the case with the pure TBTAPCIs monolayer. This is demonstrated in Figure 3, using the mixture of TBTAPC15/SteA = 1:2 as an example, where the experimental isotherm (solid curve) is compared with a calculated one (dotted curve) which is obtained by taking a mathematical summation of the two pure isotherms from the inset of Figure 1. Figure 3 reveals that in the low surface pressure region the experimental isotherm deviates positively from the calculated data, while in the high-pressure region the two curves merge together. This is consistent with above discussion that at high surface pressure the TBTAPC15molecules orient themselves in
Fu et al.
8988 The Journal of Physical Chemistry, Vol. 98, No. 36, 1994 0.20
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TABLE 1: Transferability and Spectroscopic Data of Various TBTAPCdSteA Mixed LB Films on Glass Slides no.* transfer TBTAPClsOI of absorbancec at depostn transfer pressure, SteA Cycles 630nm rati@J typh mN/m 9: 1 4: 1 2: 1
0.10
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