Mixed Thin Films of a Cationic Amphiphilic Porphyrin and n-Alkanes

Benjamin M. D. O'Driscoll, Jeremy L. Ruggles, and Ian R. Gentle*. Department of Chemistry, The University of Queensland, Brisbane,. Queensland 4072 ...
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Mixed Thin Films of a Cationic Amphiphilic Porphyrin and n-Alkanes Benjamin M. D. O’Driscoll, Jeremy L. Ruggles, and Ian R. Gentle* Department of Chemistry, The University of Queensland, Brisbane, Queensland 4072, Australia Received January 5, 2004. In Final Form: April 29, 2004 Langmuir and Langmuir-Blodgett (LB) films of a cationic amphiphilic porphyrin mixed with n-alkanes octadecane and hexatriacontane were prepared and characterized, to examine the influence of the alkanes on film structure and stability. While the structure present in these films was controlled primarily by the porphyrin, the addition of the alkanes resulted in significant changes to both the phase behavior of the Langmuir films and the molecular arrangement of the LB films. These changes, as well as the observed chain length effects, are explained in terms of the intermolecular interactions present in the films.

Introduction Thin films of porphyrins and related compounds are of interest in a number of areas, including photovoltaics and chemical sensing, due to the unique properties of the chromophore.1-4 Of particular importance for these applications is the ability to control the orientation of the porphyrin in the film. In two previous studies, we examined Langmuir and Langmuir-Blodgett (LB) films of the amphiphilic porphyrin tetrakis(4-octadecylpyridinium)porphyrinato zinc(II) bromide (ZnTOPyP, Figure 1), using surface pressurearea (π-A) isotherms and X-ray reflectometry.5,6 At the air/water interface, it was found that the orientation of the porphyrin ring changed from parallel to the interface to an edge-on arrangement as the surface pressure increased.5 This difference was not however maintained upon transfer of the films to a solid support using the LB technique, with the porphyrin forming an interdigitated bilayer regardless of the surface pressure at which transfer was performed.6 With a view to both stabilizing the Langmuir films and controlling the structure of the LB films of ZnTOPyP, we report the preparation and characterization of Langmuir and LB films of ZnTOPyP mixed with either octadecane (OD) or hexatriacontane (HTC). The majority of previous studies of mixed films, where one component is a porphyrin or phthalocyanine derivative, have involved a surface-active second component, such as arachidic acid.7-9 However, in the small number of alkane mixed films reported, a wide range of effects have been observed, including stabilization of the film by the alkane and changes in either aggregation state or orientation of the chromophore.10-13 Additionally, the * Corresponding author. Telephone: +61 7 3365 4800. Fax: +61 7 3365 4299. E-mail: [email protected]. (1) Feng, X. S.; Kang, S. Z.; Liu, H. G.; Mu, J. Thin Solid Films 1999, 352, 223-227. (2) Pedrosa, J. M.; Dooling, C. M.; Richardson, T. H.; Hyde, R. K.; Hunter, C. A.; Martin, M. T.; Camacho, L. Mater. Sci. Eng., C 2002, 22, 433-438. (3) Benitez, I. O.; Bujoli, B.; Camus, L. J.; Lee, C. M.; Odobel, F.; Talham, D. R. J. Am. Chem. Soc. 2002, 124, 4363-4370. (4) Simonneaux, G.; Le Maux, P. Coord. Chem. Rev. 2002, 228, 4360. (5) O’Driscoll, B. M. D.; Ruggles, J. L.; Foran, G. J.; Lin, B.; Gentle, I. R. J. Porphyrins Phthalocyanines 2002, 6, 806-811. (6) O’Driscoll, B. M. D.; Ruggles, J. L.; Gentle, I. R. Aust. J. Chem. 2003, 56, 1059-1063.

Figure 1. The structure of tetrakis(octadecyl-4-pyridinium)porphinatozinc(II) bromide.

behavior of the mixed films showed a dependence on the length of the alkane chain used,13 and so, through control of the film composition and alkane chain length, it may be possible to gain further control over the arrangement of the porphyrin in the films. The Langmuir and LB films reported here were studied using surface pressure-area (π-A) isotherms, neutron reflectometry, UV-visible spectroscopy, and X-ray reflectivity. Materials and Methods OD and HTC (Sigma) and their fully deuterated analogues (CDN Isotopes) were used without further purification. The synthesis of ZnTOPyP has been reported previously.5,14 In this paper, the mixed films are referred to in terms of the alkane to porphyrin molar ratio. The Langmuir films were formed by dropwise spreading of solutions of the porphyrin (with or without alkane) (7) Gru¨niger, H.; Mo¨bius, D.; Meyer, H. J. Chem. Phys. 1983, 79, 3701-3710. (8) Goncalves da Silva, A. M.; Viseu, M. I.; Malathi, A.; Antunes, P.; Costa, S. M. B. Langmuir 2000, 16, 1196-1204. (9) Zhang, Y. J.; Li, L. S.; Jin, J.; Jiang, S.; Zhao, Y.; Li, T. J. Langmuir 1999, 15, 2183-2187. (10) Li, H.; Chen, T.; Wang, H.; Zhou, Q.; Xu, H.; Zhao, B. Ganguang Kexue Yu Guang Huaxue 1996, 14, 97-100. (11) Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.-I.; Sugi, M.; King, L. G.; Crossley, M. J. J. Phys. Chem. 1993, 97, 12862-12869. (12) Ohta, N.; Nakamura, M.; Yamazaki, I.; Shimomura, M.; Ijiro, K. Langmuir 1998, 14, 6226-6230. (13) Matsuzawa, Y.; Seki, T.; Ichimura, K. Langmuir 1998, 14, 683689. (14) Okuno, Y.; Ford, W. E.; Calvin, M. Synthesis 1980, 537-539.

10.1021/la049969h CCC: $27.50 © 2004 American Chemical Society Published on Web 06/24/2004

Porphyrin/Alkane Mixed Langmuir and LB Films

Figure 2. π-A isotherms of ZnTOPyP with OD: (solid line) ZnTOPyP, (dashed line) 8:1 OD/ZnTOPyP, and (dotted line) 20:1 OD/ZnTOPyP. Inset: Area-composition plot for the OD/ ZnTOPyP mixed system measured at 10 mN m-1.

dissolved in chloroform (Aldrich, A.C.S. spectrophotometric grade 99.8%), unless otherwise stated. The π-Α isotherms were measured in a poly(tetrafluoroethylene) trough (NIMA Technology, Coventry, U.K.), with Wilhelmy plates made from Whatman Chr 1 filter paper. Milli-Q water (18.2 MΩ cm resistivity, Millipore Ltd.) was used as the subphase, and both the π-Α isotherms and reflectivity profiles were collected at a temperature of 2324 °C. LB films were transferred from the Langmuir monolayers at 25 mN m-1, using the vertical dipping technique with a dipping speed of 25 mm min-1. For the X-ray reflectometry measurements, 11 pass and 54 pass films were transferred to 75 × 25 mm polished silicon wafers (Umicore Semiconductor Processing Co., Boston, MA), while 11 pass films were transferred onto 30 × 15 mm double-sided quartz substrates (Herbert A. Groiss and Sons, Melbourne, Australia) for the UV-vis spectrophotometry measurements. The neutron reflectometry experiments were performed on the SURF reflectometer at the ISIS pulsed neutron source, Rutherford Appleton Laboratory, Chilton, U.K. Measurements were performed at an incident angle of 1.5°, and a D2O subphase was used to normalize the profiles. Data were collected between 0.049 and 0.612 Å-1 in QZ ()4π sin θZ) on air-contrast-matched water (ACMW) and D2O. The X-ray reflectometry measurements were performed on a Bruker AXS GmbH D8 Advance X-ray diffractometer using Cu KR (1.541 Å) radiation with a Go¨bels mirror parallel beam attachment. The vertical size of the beam was adjusted during measurement so that the footprint of the beam was not larger than the substrate used. Data were collected up to 0.42 Å-1 in QZ. Reflectivity data were modeled using the Parratt32 program (HahnMeitner Institute).15 UV-vis spectra were collected on a Perkin-Elmer Lambda 2 spectrometer. Results π-A Isotherms. In the mixed films of ZnTOPyP and n-alkanes, the length of the alkane chain has a significant effect on the surface pressure-area isotherms of the Langmuir films (Figures 2 and 3). The underlying shape of the isotherms is similar to that of the pure porphyrin.5,16,17 As with pure ZnTOPyP, the OD/ZnTOPyP films show the presence of two phases, which have previously been attributed to the porphyrin ring lying parallel and (15) Parratt, L. G. Phys. Rev. 1954, 95, 359-369. (16) Ruaudel-Teixier, A.; Barraud, A.; Belbeoch, B.; Roulliay, M. Thin Solid Films 1983, 99, 33-40. (17) Lesieur, P.; Vandevyver, M.; Ruaudel-Teixier, A.; Barraud, A. Thin Solid Films 1988, 159, 315-322.

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Figure 3. π-A isotherms of ZnTOPyP with HTC: (solid line) ZnTOPyP, (dashed line) 4:1 HTC/ZnTOPyP, (dashed-dotted line) 8:1 HTC/ZnTOPyP, (dotted line) 12:1 HTC/ ZnTOPyP, and (dashed-double-dotted line) 16:1 HTC/ZnTOPyP. Inset: area-composition plot for the HTC/ZnTOPyP mixed system measured at 10 mN m-1.

tilted to the surface, respectively.5,8 In the HTC/ZnTOPyP films, the presence of the alkane gives rise to an additional phase, dependent on the concentration of alkane in the film. In the OD/ZnTOPyP isotherms, there is a plateau in the molecular areas at 10 mN m-1 for the films with alkane/ porphyrin ratios between 2:1 and 16:1 (see the inset in Figure 2). This is consistent with a filling-in type mechanism, as first proposed by Gru¨niger et al.,7 where the second component preferentially locates itself directly over the porphyrin ring, which lies parallel to the air/water interface. In the simplest sense, it would therefore be expected that if the total cross-sectional area of the second component was less than the total area occupied by the porphyrin then the average molecular area per porphyrin for the mixed film would be the same as that found in the pure porphyrin film. The presence of such a mechanism is reasonable as the interaction between the alkane and the porphyrin ring will be more favorable than that with the water, and so the alkane will preferentially occupy the porphyrin cavity. As is to be expected for this type of mechanism, there is a point at which the second component can no longer be incorporated into the porphyrin cavity. From molecular model calculations, it was found that the area of the porphyrin ring is ∼250 Å2.18 The maximum number of alkane chains that can be located in this area in a fully condensed manner is ∼12.5. Excluding the four chains that are part of the porphyrin molecule, the number of free alkane molecules needed to give a full cavity will then be ∼8.5. For the OD/ZnTOPyP films, the plateau region exists up to molar ratios of 16:1, indicating that there is some alkane located outside the porphyrin cavity, which is possible as at low surface pressures the measured porphyrin molecular area is greater than 250 Å2. However, as can be seen by the small but significant increase in the porphyrin molecular area as alkane is added to the films (inset, Figure 2), there is a relatively strong initial interaction between the alkane and the porphyrin, most likely with the porphyrin alkyl chains, which increases the distance between adjacent porphyrin molecules. The presence of such a rise is contrary to the behavior expected for a simple filling-in mechanism. The constant lift-off point seen in the isotherms (Figure 3) of the HTC/ZnTOPyP system indicates that the alkane occupies no area at the air/water interface. This is consistent with a filling-in mechanism, as opposed to the (18) Porteu, F.; Palacin, S.; Ruaudel-Teixier, A.; Barraud, A. J. Phys. Chem. 1991, 95, 7438-7447.

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Figure 4. SLD profiles of HTC/ZnTOPyP films at 35 mN m-1: (solid line) 12:1, (dashed line) 8:1, and (dotted line) 4:1.

lift-off behavior of the OD/ZnTOPyP films, and points to a type of filling-in mechanism being present in these films as well. However, the composition-area plot for the HTC/ ZnTOPyP films (inset, Figure 3) varies significantly from both that of the OD/ZnTOPyP system and that expected for a simple filling-in mechanism, suggesting that the manner in which the two alkanes interact with the porphyrin is different. In both the OD/ZnTOPyP and HTC/ZnTOPyP films, the molecular areas of the films are, under many circumstances, smaller than would be expected for a fully condensed alkane layer. For example, in the 16:1 OD/ ZnTOPyP film there are 20 alkane/alkyl chains to be accommodated (16 octadecane alkane chains and 4 alkyl chains bound to the porphyrin ring). Assuming an area of ∼19 Å2 per chain, the predicted area occupied by chains of ∼380 Å2 is much larger than the molecular area at the end of the first phase region. This suggests that there is a forcing out of the alkane from the monolayer. In the HTC/ZnTOPyP system, the tendency of the monolayer to squeeze out some of the alkane appears to be dependent on the ratio of alkane in the film. For the 4:1 and 8:1 films, the molecular area at which the second phase transition occurs is consistent with the molecular area of a complete alkane layer. For the 12:1 and 16:1 films, the molecular area of this transition is much smaller than the predicted molecular area of the alkane layer and seems to approach a limit at ∼230 Å2 porphyrin-1. The isotherm for the 16:1 OD/ZnTOPyP film, when measured at 5 °C, shows significant differences from the isotherm measured at 23 °C (Supporting Information). At lift-off, the molecular area at 5 °C is equal to that calculated for a fully condensed alkane layer, and the steep rise in the surface pressure indicates the presence of a rigid film. This result suggests that changing the temperature of the subphase can influence the squeeze-out of alkane in the OD/ZnTOPyP system. An analogous experiment in the HTC/ZnTOPyP system produced an expansion of the film (50 Å2) in the first phase region but did not induce the formation of a solid phase (results not shown). Neutron Reflectometry Measurements of Langmuir Films. In all of the profiles, the reflectivity was found to drop to the level of the incoherent background when QZ g 0.25 Å-1 (Supporting Information), which limits the ability of this technique to resolve fine structure in the films. For the HTC/ZnTOPyP films (ratios of 4:1, 8:1, and 12: 1), measurements were taken in the first phase region at 10 mN m-1 and in the third phase region at 35 mN m-1. At both surface pressures, there was a gradual increase in the scattering length density (SLD) of the films as the ratio of alkane was increased (Figure 4). However, despite the change in the density of the alkane, the thickness of

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Figure 5. SLD profiles of 8:1 HTC/ZnTOPyP films on D2O (solid line) and ACMW (dashed line) subphases at 35 mN m-1.

Figure 6. SLD profiles of a 8:1 HTC/ZnTOPyP film at 10 mN m-1 (dotted line), 35 mN m-1 (dashed line) and 50 mN m-1 (solid line).

the alkane layers is approximately constant for all ratios and is >80% of the length of the extended alkane. It is probable therefore that the alkane is oriented at or near to perpendicular to the subphase regardless of the alkane density or surface pressure. Additional measurements were also carried out on the 8:1 HTC/ZnTOPyP film at ∼50 mN m-1, just prior to collapse of the film, and at 35 mN m-1 on a D2O subphase. Substitution of ACMW for a D2O subphase, while retaining deuterium-labeled alkane, causes a dramatic change in the SLD of the porphyrin layers, which indicates that the porphyrin layer contains a significant amount of water (Figure 5). This is consistent with the high solubility of short-chained tetra(4-alkylpyridinium)porphyrins in water.19 Solution of the SLDs of the ACMW and D2O films simultaneously shows that approximately 50% of the observed SLD comes from the porphyrin while the remainder is from the subphase. This in turn gives a SLD for the porphyrin of ∼1.9 × 10-6 Å-2, which is consistent with the molecular formula of the porphyrin. Similarly, in the 8:1 HTC/ZnTOPyP film measured at 50 mN m-1, where the molecular area is equal to that of a fully condensed alkane layer (8 alkanes plus 2 alkyl chains due to tilting of the porphyrin ring), the SLD of the alkane layer is close to that expected for a condensed alkane layer (Figure 6). It can also be seen in Figure 6 that as the surface pressure is increased there is a thickening (with a corresponding increase in the SLD) of the porphyrin layer as the molecular area decreases, in line with the tilting of the porphyrin ring in the second phase region. The OD/ZnTOPyP (ratios of 1:1 and 2:1) and pure ZnTOPyP films were measured in both the first (10 mN m-1) and second (160 Å2 molecule-1) phase regions on ACMW. However, the low SLD and thickness of the films (19) Schneider, H. J.; Wang, M. J. Org. Chem. 1994, 59, 7464-7472.

Porphyrin/Alkane Mixed Langmuir and LB Films

Figure 7. UV-vis spectra of LB films of (solid line) ZnTOPyP (ref 6), (dotted line) 16:1 OD/ZnTOPyP, (dashed line) 16:1 HTC/ ZnTOPyP, and (dashed-dotted line) 4:1 HTC/ZnTOPyP.

made it impossible to fit the data for the pure porphyrin film and the OD/ZnTOPyP films in the first phase region. Like the HTC/ZnTOPyP films, the thickness of the alkane in the OD/ZnTOPyP films is approximately equal to the length of the alkane, indicating a perpendicular orientation. LB Film Formation. LB films of the alkane/porphyrin mixed systems were transferred from the monolayer films in the first phase region at 25 mN m-1. As has been noted previously20 for porphyrin films, the transfer ratios were not unity throughout the process. When a large number (>20 layers) was transferred, the behavior became distinctly x-type with close to ideal transfer ratios, but for lower numbers of layers, the behavior was approximately y-type with less ideal transfer ratios (Supporting Information). Visual inspection and comparison of the drainage times indicated that the 4:1 HTC/ZnTOPyP film was the highest quality film prepared. The 16:1 HTC/ZnTOPyP film was found to drain quickly but in a patchy fashion, with some water trapped in the film. The 16:1 film also had a marbled appearance (whereas the appearance of the 4:1 film was homogeneous), and this may due to the phase separation of some of the alkane from the porphyrin. UV-Vis Spectra of the LB Films. The UV-vis spectra of the 11 pass films show no significant shift in the energy of the Soret band from that observed in the pure porphyrin LB film (Figure 7).6 The environment (the relative orientation and distance of the adjacent porphyrins) around each porphyrin ring in the mixed films is therefore the same as that found in the pure porphyrin film. The increased absorbance of the 4:1 HTC/ZnTOPyP LB film can be attributed to the higher quality of this film, as noted above. X-ray Reflectometry Measurements of the LB Films. The normalized reflectivity profiles of the LB films of the OD/ZnTOPyP and HTC/ZnTOPyP films are shown in Figure 8. For the OD/ZnTOPyP films, the interlayer spacing was determined to be identical to that found in the pure ZnTOPyP film, suggesting that the same structure is present in the two films. For the HTC/ZnTOPyP films, two repeat spacings were observed. The lesser of these spacings (found only in the 4:1 film) also corresponds to the spacing observed in the pure porphyrin film, and so, as with the 16:1 OD/ZnTOPyP film, the same structure is likely to be present. The second structure gave a spacing of 48 Å, which is close to the length of a fully extended HTC alkane, suggesting that either an interdigitated or x-type structure is present. (20) Qian, X.; Tai, Z.; Sun, X.; Xiao, S.; Wu, H.; Lu, Z.; Wei, Y. Thin Solid Films 1996, 284-285, 432-435.

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Figure 8. Fresnel normalized X-ray reflectivity profiles of 11 pass LB films of (solid line) 16:1 HTC/ZnTOPyP, (dashed line) 16:1 OD/ZnTOPyP, (dashed-dotted line) 4:1 HTC/ZnTOPyP, and (dotted line) ZnTOPyP (ref 6).

Importantly, the absence of a second peak or a shoulder in the UV-vis spectrum means that the porphyrin rings will be arranged in the same manner in both structures. Discussion Langmuir Films. OD/ZnTOPyP Mixed Films. The hydrophobic nature of alkanes means that they are not intrinsically surface active themselves. Consequently, when the alkanes are mixed with the surface-active porphyrin, the strengths of the hydrophobic/hydrophobic interactions are of primary importance in determining how the alkane/porphyrin mixed films will behave. The two main attractive interactions that can be formed are the alkane/alkane and the alkane/alkyl chain van der Waals interactions (the alkyl chain/alkyl chain interaction will most likely be absent due to spatial separation). A third interaction, the alkane/porphyrin interaction, will also be present, and while important in the preferential filling-in of the porphyrin cavity, it will only be small in comparison to the other forces. For the OD/ZnTOPyP mixed films, the lengths of the porphyrin alkyl chain and the alkane are approximately equal, so there will be little difference in the magnitude of the two primary van der Waals interactions. Subsequently, for the lower ratio films (e8) in the first phase, the alkanes will preferentially associate with the alkyl chains, as this will minimize the interaction between each alkane and the subphase. This initial interaction is most probably the cause of the increase in the area per porphyrin observed at low alkane/porphyrin ratios (see the inset in Figure 2). As the alkane/porphyrin ratio increases, the alkane will tend to be incorporated into the porphyrin cavity in accordance with the filling-in mechanism, leading to the plateau observed in the composition-area plot. However, when the area of the alkane layer becomes greater than the total porphyrin molecular area (including the alkaneinduced expansion), then the area per porphyrin will increase (inset of Figure 2). For these higher ratio films, the observed increase in molecular area at 10 mN m-1 is substantially smaller than the increase predicted if the alkane is incorporated into the alkane layer. Hence some of the alkane must be squeezed out of (located above) the alkane layer (Figure 9). For the films with alkane/ porphyrin ratios between 8 and 16, this squeezing-out will occur midway through the first phase; however, no phase transition accompanies this process, which suggests that the alkane is in a relatively fluid state. For OD, the chain-melting temperature of the bulk material (28-30 °C) is close to the temperature at which

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Figure 10. Arrangement of the primary structure observed in the HTC/ZnTOPyP mixed films.

Figure 9. Schematic illustrating the squeezing-out of alkane from the (A) OD/ZnTOPyP films and (B) HTC/ZnTOPyP mixed films.

the isotherms were measured (23 °C). However, the confinement of the alkane at the air/water interface will hinder the molecules from adopting the most stable conformation in the solid state and lower the temperature of the chain-melting transition. This combined with the mixing of the alkanes with the alkyl chains, which will be slightly shorter than the alkane due to their attachment to the porphyrin, makes it highly likely that the alkane will be present in a fluidlike state. This hypothesis is confirmed by the presence of the solid phase at low pressure in the isotherm for the 16:1 OD/ZnTOPyP film measured at 5 °C. The extrapolated area of this phase is 380 Å2 molecule-1, which is exactly the area expected for a condensed layer of 20 alkane/alkyl chains (16 alkanes and 4 alkyl chains). HTC/ZnTOPyP Mixed Films. Due to the additional length of the HTC molecule, the alkane/alkane interaction in these films will be stronger than the alkane/alkyl chain interaction, which will make the alkanes more likely to associate with each other, leading to a more conventional filling-in behavior in the first instance. However, this interaction appears to interfere in the compression of the film, resulting in a gradual increase of the porphyrin molecular area with increasing alkane concentration in the first phase region. The second phase transition in the low-ratio films (e8) occurs at molecular areas equivalent to the calculated area of the alkane chains present, indicating the formation of a fully condensed alkane layer. It is known that beyond the first phase transition the porphyrin ring begins to tilt, removing some of the alkyl chains from the alkane layer (Supporting Information). Subsequently, in the 4:1 HTC/ZnTOPyP film, for example, approximately half of the alkyl chains will have been removed from the alkane layer, and so the molecular area at which a condensed alkane layer forms will be ∼120 Å2 (4 alkanes and 2 alkyl chains), as is observed in the π-A isotherms. For HTC/ZnTOPyP films with ratios higher than 8, the molecular area at which the second phase transition begins is less than the area of a fully condensed alkane layer and appears to approach a limit at ∼230 Å2 molecule-1. Consequently, squeezing-out of the alkane must also be occurring in these films. The absence of a phase transition at the area at which a fully condensed alkane layer is formed indicates, as with the OD/ZnTOPyP system, that

there is no energy barrier to the squeezing-out of some of the alkane from the film. For the HTC/ZnTOPyP system, the length of the alkane chain is approximately twice that of the alkyl chain; therefore, when an alkane molecule is squeezed out it is still capable of forming significant alkane/alkane interactions, while avoiding the unfavorable alkane/subphase interaction (Figure 9). In the higher ratio films, the area at which a condensed alkane layer is formed and squeeze-out begins (20 × 19 Å2 ) 380 Å2 for the 16:1 film) is in the first rather than the second phase. In this phase, with the porphyrin ring parallel to the subphase, the alkanes located in the porphyrin cavity will experience the favorable alkane/ porphyrin interaction and so it is likely that the squeezedout alkane will come from the alkane located outside the porphyrin cavity. Subsequently, at an area of ∼230 Å2 per porphyrin (the start of the third phase) the only alkane not squeezed out will be located in the porphyrin cavity, maximizing the resistance of the film against further squeezing-out and leading to the increase in surface pressure observed after the second phase transition. Further compression of the monolayer will lead to a tilting of the porphyrin ring and loss of the alkane/porphyrin interaction. LB Films. For the OD/ZnTOPyP system, the similarities between X-ray reflectivity profiles and UV-vis spectra of the 16:1 and the pure porphyrin film indicate the presence of almost identical structures, that is, an interdigitated bilayer.6 In the pure film, it was observed that the density of alkyl chains in the interdigitated region was relatively low. The alkane may be incorporated into this region, most likely oriented approximately perpendicular to the porphyrin ring. Because of the interdigitation of two monolayers, it is probable that some of the alkane will be excluded from the porphyrin-containing regions of the film; this would require a reorganization of the film structure after transfer. The increased intensity of the diffraction peaks of the OD/ZnTOPyP LB films over that of the pure films points to a better-defined layered structure. This is consistent with the increased density of the film in the interdigitated region and also agrees with the conclusions drawn from an analogous hexadecane/phthalocyanine system.10 The primary structure (48 Å) in the 16:1 and 4:1 HTC/ ZnTOPyP films has repeat spacing that is close to the length of the alkane, suggesting that the arrangement of the alkane is approximately perpendicular to the plane of the porphyrin ring (Figure 10). In this structure, because

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of the additional length of the alkane, the alkyl chains will be stacked rather than interdigitated. While in the 16:1 film there will be an excess of alkane and exclusion of some of the alkane, giving rise to the marbled appearance of the film, in the 4:1 film there will be slightly less than the 12.5 alkane/alkyl chains required to fully fill the porphyrin cavity. Thus the presence of a second structure in the 4:1 film may arise from an uneven distribution of the alkane in the film, with the secondary structure containing substantially less than the full complement of alkane. As with the 16:1 OD/ZnTOPyP film, the repeat spacing of this second (37 Å) structure is very similar to that of the pure porphyrin film, and the absorption spectra of both films are also similar. It is likely to correspond to an interdigitated structure, although the alkane incorporated into this structure will probably be disordered.

In the LB films, the incorporation of the alkane resulted in an increase in the quality of the films produced. Moreover, the structure(s) present in the films were found to change with the alkane length and concentration, and so control over the properties of the porphyrin-containing films was achieved through the addition of alkanes to the film.

Conclusions

Supporting Information Available: The π-A isotherm of the 16:1 OD/ZnTOPyP film at 5 °C, sample neutron reflectivity profiles (with the modeled fits) for OD/ZnTOPyP and HTC/ZnTOPyP, SLD plots of the OD/ZnTOPyP films, transfer ratio plots for the 4:1 HTC/ZnTOPyP film, and a schematic illustrating the removal of alkyl chains from the alkane layer upon tilting of the porphyrin. This material is available free of charge via the Internet at http://pubs.acs.org.

π-A isotherms of OD/ZnTOPyP and HTC/ZnTOPyP systems showed that the behavior of the mixed films was principally directed by the porphyrin but was also dependent on the length and concentration of the added alkane used. For the OD/ZnTOPyP system, the behavior of the monolayer was also observed to be dependent on temperature, and this was attributed to the alkane being present in a fluid state at ambient temperatures.

Acknowledgment. Travel grants through the Australian Government Access to Major Facilities Program, part of the Innovation Access Program, are gratefully acknowledged, as is funding through an Australian Research Council LIEF grant enabling access to the ISIS Facility. The authors acknowledge the assistance of Dr. Stephen Holt, ISIS, and Ms. Lauren MacDonald in performing these experiments.

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