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Langmuir-Blodgett Mixed Films of Titanyl(IV) Pthalocyanine and Arachidic Acid. Molecular Orientation and Film Structure T. Del Can˜o and R. Aroca* Materials and Surface Science Group, School of Physical Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada
J. A. De Saja and M. L. Rodriguez-Mendez Condensed Matters Department & Inorganic Chemistry Department, Faculty of Sciences, University of Valladolid, Prado de la Magdalena s/n 47011 Valladolid, Spain Received October 2, 2002. In Final Form: January 23, 2003 Langmuir and Langmuir-Blodgett (LB) films of neat titanyl(IV) phthalocyanine (TiOPc) and mixed (TiOPc)-arachidic acid (AA) films were studied. The surface pressure-area isotherms (π-A) were recorded, and an unusual expansion of the surface area per molecule was detected for mixed TiOPc-AA Langmuir monolayers. The expansion is attributed to the TiOPc molecular orientation in the neat films being different from that in the mixed films with arachidic acid. The molecular orientation in LB films was monitored using reflection-absorption infrared spectroscopy (RAIRS). It was found that the TiOPc molecules are tilted with respect to the substrate in the neat film, changing to a preferential face-on orientation in the TiOPc-AA mixed films. Complementary information about molecular organization and film structure was obtained using UV-visible absorption and micro-Raman imaging.
1. Introduction Titanyl(IV) phthalocyanine (TiOPc) is one of the most promising organic materials to be used in thin film devices.1 Its nonlinear optical properties, high photoconductivity, high carrier generation efficiency, and chemical stability confer upon this material great potential for applications as a main component in photoreceptors, sensors, solar cells, and electroluminescent devices.2-4 The TiOPc presents several polymorphic forms,5 which have been carefully examined.6,7 It has also been reported that the highest photovoltaic conversion efficiency is achieved in highly ordered TiOPc films.8 High vacuum TiOPc evaporated films usually exhibit a certain crystallinity, which is dependent on the preparation conditions, and further changes in film morphology can be induced by solvent vapor annealing.8-10 The molecular orientation of evaporated films has been studied by different techniques, such as infrared spectroscopy,11,12 Penning ioniza* Corresponding author. (1) Shirota, Y. J. Mater. Chem. 2000, 10, 1-25. (2) Law, K. Y. Chem. Rev. 1993, 93, 449. (3) Ohaku, K.; Nakano, H.; Kawara, T.; Yokota, S.; Takenouchi, O.; Aizawara, M. Electrophotography 1986, 25, 258. (4) Tsuzuki, T.; Shirota, Y.; Rostalski, J.; Meissner, D. Sol. Energy Mater. Sol. Cells 2000, 61, 1-8. (5) Hiller, W.; Strahle, J.; Kobel, W.; Hanack, M. Z. Kristallogr. 1982, 159, 173. (6) Nalwa, H. S.; Hari, S.; Saito, T.; Kakuta, A.; Atsusshi, I. T. J. Phys. Chem. 1993, 73, 10515. (7) Saito, T.; Sisk, W.; Kobayashi, T.; Suzuki, S.; Iwayanagi, T. J. Phys. Chem. 1993, 97, 8026-8031. (8) Tsuzuki, T.; Kuwabara, Y.; Noma, K.; Shirota, Y. Jpn. J. Appl. Phys. 1996, 35, L447-l450. (9) Adams, D. M.; Kerimo, J.; Olson, E. J. C.; Zaban, A.; Gregg, B. A.; Barbara, P. F. J. Am. Chem. Soc. 1997, 119, 10608-10619. (10) Conboy, J. C.; Olson, E. J. C.; Adams, D. C.; Kerimo, J.; Zaban, A.; Gregg, B. A.; Barbara, P. F. J. Phys. Chem. B 1998, 102, 4516-4525. (11) Aroca, R.; Thedchanamoorty, A. Chem. Mater. 1995, 7, 69-74. (12) Yonehara, H.; Etori, H.; Engel, M. K.; Tsushima, M.; Ikeda, N.; Ohno, T.; Pac, C. Chem. Mater. 2001, 13, 1015.
tion electron spectroscopy (PIES), and ultraviolet photoelectron spectroscopy (UPS).13 The Langmuir-Blodgett technique is a powerful tool to obtain highly organized thin films.14,15 Several efforts have been made to fabricate LB films of unsubstituted phthalocyanines.16-19 However, these phthalocyanine derivatives are likely to form aggregates at the air-water interface lacking preferential orientation.20 Organized LB films can be obtained by the attachment of hydrophilic susbstituents to direct the molecule-water interaction.21 A different strategy is to mix the nonconventional material with fatty acids such as stearic or arachidic acid. Since the pioneering work of Kuhn and Mo¨bius,22-24 many different organic materials, including phthalocyanines,25 have been studied using fatty acids as matrixes. There are only a handful of studies of the molecular orientation and optical properties of TiOPc LB films.26,27 Ogawa and co-workers27 reported that transferring several (13) Kera, S.; Abduaini, A.; Aoki, M.; Okudaira, K. K.; Ueno, N.; Harada, Y.; Shirota, Y.; Tsuzuki, T. Thin Solid Films 1998, 327-329, 278-282. (14) Ulman, A. An Introduction to Ultrthin Organic Films; Academic Press: San Diego, CA, 1991. (15) Petty, M. C. Langmuir-Blodgett Films. An Introduction; Cambridge University Press: Cambridge, 1996. (16) Battisti, D.; Tomilova, L.; Aroca, R. Chem. Mater. 1992, 4, 1323. (17) Jones, R.; Hunter, R. A.; Davidson, K. Thin Solid Films 1994, 250, 249. (18) Gan, L.; Liang, B.; Lu, Z.; Wei, Q. Supramol. Sci. 1998, 5, 583. (19) Ogawa, K.; Yonehara, H.; Pac, C. Langmuir 1994, 10, 2068. (20) Palacin, S. Adv. Colloid Interface Sci. 2000, 87, 165. (21) Fouriaux, S.; Armand, F.; Araspin, O.; Ruaudel-Teixier, A.; Maya, E. M.; Vazquez, P.; Torres, T. J. Phys. Chem. 1996, 100, 16984-16988. (22) Kuhn, H. Pure Appl. Chem. 1965, 11, 345. (23) Khun, H.; Mo¨bius, D.; Bu¨cher, H. In Physical Methods of Chemistry; Weissberger, A., Rossiter, B., Eds.; Wiley: New York, 1972; Vol. I, Part 3B, p 577. (24) Mo¨bius, D.; Khun, H. J. Appl. Phys. 1988, 64, 5138-5141. (25) Snow, A. W., Barger, W. R., Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publishers: Weinheim (Germany), 1989; p 343. (26) Ogawa, K.; Yonehara, H.; Pac, C. Mol. Cryst. Liq. Cryst. 1995, 258, 315.
10.1021/la020825h CCC: $25.00 © 2003 American Chemical Society Published on Web 04/03/2003
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Figure 1. Molecular structure of the titanyl(IV) phthalocyanine (TiOPc).
monolayers leads to the formation of LB films with a partial loss of molecular orientation. However, a highly ordered R-TiOPc is formed if the LB film is exposed to dichloromethane vapor. We discuss here the fabrication of highly ordered TiOPc LB films by mixing it with arachidic acid. In the first part, the surface pressurearea isotherms (π-A) and characterization of neat TiOPc and TiOPc-AA (1:1, 1:4 molecular ratios) LB multilayers are presented. It is then shown that the neat LB film presents certain preferential orientation; however, the TiOPc molecules show a much higher degree of organization when mixed with arachidic acid (AA). 2. Experimental Section 2.1. Materials. The highly purified TiOPc, whose structure is shown in Figure 1, was kindly provided by Dr. J. Duff from the Xerox Research Center of Canada. AA was purchased from Aldrich. 2.2. Film Preparation. TiOPc has very low solubility in organic solvents, and it has been reported that strong acids (such as TFA) are used to dissolve TiOPc.28 Notably, the use of strong acids may lead to the TiOPc oxidation.26 The solubility required to apply the LB technique (less than 10-4 M) can be achieved with dichloromethane and chloroform as solvents and by dissolving the aggregates using an ultrasonic sonicator for approximately 1 h. For LB multilayers fabrication, solutions of TiOPc and TiOPc-AA mixtures with different molecular ratios were prepared in chloroform. Neat and mixed LB monolayers and multilayers were fabricated using a KSV 5000 LB system. The solutions (8 × 10-5 M) were spread onto ultrapure water (Millipor, MilliQ), maintaining the temperature of the water bath at 19 °C. Spreading requires extra care due to the high density of the chloroform. After evaporation of the solvent (approximately 20 min), the floating films were compressed at a speed of 7 mm/ min. The films were then transferred at a speed of 10 mm/min to Corning 7059 glass slides, and glass slides coated with gold when a target pressure of 25 mN/m was reached. Z-deposition was used to transfer the neat and mixed LB films; the substrate was immersed into the water before spreading the solution and lifted after compressing the monolayer. Neat and TiOPc-AA (1:1, 1:4 molar ratios) mixed LB films of four layers were fabricated. The gold films (50 nm mass thickness) were fabricated using a Balzers vacuum evaporator equipped with a rotary vacuum pump, which functioned as a precursor to the diffusion pump. The deposition rate was 2 Å s-1, and the background pressure was 10-5 Torr. The TiOPc was also dispersed in KBr powders to produce pellets (bulk) that were used as references in molecular orientation studies. 2.3. Film Characterization. The UV-visible spectra of the solution, the neat and mixed LB films, and the bulk were recorded with a Varian Cary 50 Scan UV-visible spectrometer. (27) Ogawa, K.; Yao, J.; Yonehara, H.; Pac, C. J. Mater. Chem. 1996, 6, 143-148. (28) Duff, J.; Mayo, J. D.; Hsiao, C. K.; Hor, A. M.; Bluhm, G. K.; Hamer, G. K.; Kazmaier, P. M. Eur. Pat. Appl. EP460565, 1991.
Figure 2. Surface pressure-area isotherms in mN/m of neat TiOPc (solid line), 1:1 mixed TiOPc-AA (dotted line), and 1:4 mixed TiOPc-AA (dashed line). Langmuir films on water subphase. The Raman and resonance Raman spectra were obtained with a Renishaw Research Raman Microscope System RM2000 equipped with a Leica microscope (DMLM series). Two different lasers lines were used (514.5 and 633 nm).The spectrograph was equipped with a 1200 g/mm grating with additional angle-tuned band-pass filter optics. Raman and fluorescence spectra were recorded with an ∼4 cm-1 resolution using a 50× microscope objective. Global Raman images were recorded with the same equipment, selecting the 1512 cm-1 pyrrole stretching mode of the phthalocyanine as the characteristic peak. The total area mapped was 40 µm2. Reflection-absorption infrared spectroscopy (RAIRS) and transmission infrared spectra were recorded using a BOMEM DA3 FT-infrared spectrometer equipped with a MCT detector, with resolutions of 1 cm-1 for the bulk (256 scans) and 4 cm-1 for the neat and mixed films (3000 scans). The sample chamber was evacuated until 0.3 Torr. The RAIRS measurements were recorded at an incident angle of 80°.
3. Results and Discussion 3.1. Langmuir Monolayers. The Langmuir neat and mixed monolayers of TiOPc were prepared by spreading the solution on the water subphase with a microsyringe. Reproducible π-A isotherms obtained for neat TiOPc and mixed TiOPc-AA Langmuir films are shown in Figure 2. In the isotherm a stable solid phase is clearly defined for the neat TiOPc Langmuir monolayer, and the transfer pressure of 25 mN/min for LB fabrication was selected within the solid phase. The collapse point, which is the phase change observed when the value dπ/dA decreases, gives the collapse pressure at ∼45 mN/m. The area per molecule extrapolated to π ) 0 for the neat Langmuir film is 71 Å2, ∼10 Å2 larger than the previous value reported by Ogawa and co-workers.27 This area per molecule value requires that the TiOPc molecules be tilted on the water subphase, most likely, a consequence of the high molecular stacking. The 1:1 mixed floating monolayer isotherm follows closely the isotherm of the neat monolayer; however, an increase in the area per molecule was observed, giving an extrapolated area per molecule of 77 Å2. Since the AA monolayer consistently produces a smaller area per molecule (∼22 Å2), a reduced area per molecule would be expected for the mixed monolayer. In the present case the reduction of the area per molecule
Films of Titanyl(IV) Pthalocyanine and Arachidic Acid
Figure 3. Electronic absorption spectra of the 3 × 10-5 M solution before film spreading, of the bulk, of the four-layer LB film of neat TiOPc, and of the four-layer mixed TiOPc-AA (1:4) LB film.
would hold for well mixed or phase separated components. However, in this particular case, the presence of fatty acid seems to prevent the stacking of the TiOPc molecules, permitting a flat organization on the water surface. The increasing of the area per molecule in mixed Langmuir films with AA has also been observed in Langmuir monolayers of materials presenting a high degree of stacking.29,30 The unexpected increase in area per molecule in the mixed film continues up to 1:4 (TiOPc-AA) molecular ratio, providing the largest mixed monolayer expansion, as seen in Figure 2. The area per molecule of the 1:4 mixed Langmuir monolayers was found to be 82 Å2. Following the interpretation put forward by Da Cruz et al.,30 the fatty acid seems to fill in the space available between the macrocycles, preventing molecular stacking and facilitating a planar orientation of the Pc’s. The assumption that TiOPc molecules might be lying flat on the water subphase is supported by the results extracted from the RAIRS analysis of the LB films (vide infra). 3.2. Electronic Absorption. The phthalocyanines absorption spectrum presents two broad bands: the Q-band and the B or Soret band that can be attributed to electronic transitions in the outer phthalocyanine ring.18 The Q-band is normally assigned to the low energy HOMO-LUMO π-π* transition. The absorption spectra of the solution, the four-layer neat LB film, the four-layer mixed 1:4 TiOPc-AA film, and the bulk are shown in Figure 3. The solution absorption spectrum shows the structure of the characteristic Q-band (692, 659, and 623 nm) of the phthalocyanine chromophore. The bulk absorption spectrum exhibits a broad band with a main peak centered at 720 nm with two shoulders at 640 and 830 nm. This spectrum is considered to be a typical example for randomly orientated TiOPc in the solid phase and is commonly seen in amorphous TiOPc evaporated films.7,8 The absorption spectrum of the four-layer neat LB film resembles that of the bulk with the difference that in this case the maximum is centered at 830 nm. The 1:4 TiOPcAA mixed peak exhibits a high intensity absorption band centered at 850 nm assigned to the charge-transfer exciton band, in addition to two characteristic bands centered at (29) Lu, W.; Guo, W.; Zhou, H.; He, P. Langmuir 2000, 16, 5137. (30) Da Cruz, F.; Armand, F.; Albouy, P.-A.; Nierlich, M.; RuaudelTeixier, A. Langmuir 1999, 15, 3653.
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Figure 4. RAIRS spectra of the neat TiOPc and 1:4 mixed TiOPc-AA LB films. The reference spectrum of the pellet is also shown.
640 and 730 nm. This spectrum is similar in profile to that of the R-TiOPc form (phase II).6,7 The latter electronic absorption spectrum is usually observed in high vacuum evaporated films and LB films after exposure to vapor annealing.8-10,27 It can be concluded that the differences observed in the UV absorption profiles exhibited by the neat and mixed LB films are related to distinctive molecular organization in each one of the films. 3.3. Infrared Vibrational Analysis. Infrared vibrational analysis is a powerful tool to determine the molecular alignment in thin solid films.31 Transmission and reflection-absorption IR techniques have been used to explore the arrangements of a number of LB monolayers systems.32-34 The degree of the molecular orientation in neat TiOPc and mixed TiOPc-AA films can be extracted from the data obtained using transmission and reflection-absorption infrared spectroscopy (RAIRS). RAIRS profits from the perpendicular polarization of the electrical field at the nodal point on the reflecting surface plane. Therefore, the intensity of the vibrational modes with dynamic dipole components perpendicular to the surface will be selectively enhanced and vibrational modes with the dynamic dipole parallel to the surface will be selectively de-enhanced (surface selection rule). For instance, for the Pc macrocycle, the relative intensities of the in-plane and out-of-plane molecular vibrations can help to establish the molecular orientation over the substrate.35 The TiOPc pellets where the material is dispersed in a KBr matrix, that is, random spatial distribution of dynamic dipoles, provide a reference spectrum for molecular orientation studies. The TiOPc pellet transmission spectrum, neat TiOPc LB RAIRS spectra, and 1:4 TiOPc-AA mixed LB RAIRS spectra are shown in Figure 4. The relative intensities and assignments of the main vibrational modes are summarized in Table 1. The bulk spectrum is characterized by the high intensity of the 1334 cm-1 C-N and 1118 C-H in-plane modes and the 730 cm-1 C-H out-of-plane mode char(31) Debe, M. K. Prog. Surf. Sci. 1987, 24, 1. (32) Rabolt, J.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946-952. (33) Rabolt, J.; Jurich, M.; Swalen, J. D. Appl. Spectrosc. 1985, 39, 269. (34) Mo¨bius, D. In Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum Press: New York, 1990; pp 223-272. (35) Born, N.; Wolf, E. Principles of Optics, 5th ed.; Pergamon Series: Oxford, 1975.
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Table 1. Main Vibrational Modes and Intensities Observed in the RAIRS spectra of the Neat TiOPc LB Film and the 1:4 TiOPc-AA Mixed LB Filma rel int with respect to the 730 cm-1 C-H out-of-plane mode ν RAIRS neat RAIRS mixed (cm-1) pellet TiOPC 1:4 TiOPc-AA 1334 1118 1066 963 893 750 730
1.25 1.12 0.90 0.31 0.50 0.67 1.00
0.67 0.61 0.51 0.42 0.26 0.31 1.00
0.14 0.19 0.17 0.44 0.09 0.18 1.00
assignmentb CsN str ip mode CsH bend. ip mode CsH bend. ip mode TidO str oop mode ring deform. ring deform. CsH wag. oop mode
a The transmission (pellet) is the reference. b ip, in-plane; oop, out-of-plane.
acterizing a random distribution of the dipoles in the powder. In the four-layer neat TiOPc LB film RAIRS spectrum, the 730 cm-1 C-H wagging out of plane becomes the strongest peak, and correspondingly a considerable increase in the relative intensity of the 963 cm-1 TidO stretching out-of-plane mode is observed relative to the intensity measured for the in-plane modes. In the 1:4 TiOPc-AA mixed film, the relative intensity of the in-plane modes decreases more drastically, indicating that the molecules on average have organized with a preferential nearly face-on orientation over the substrate. This molecular arrangement confirms the explanation for the expansion in the limiting area observed in the surface pressure-area isotherm of the mixed film. The relative intensity of the 1334 cm-1 C-N in-plane mode, the strongest peak in the bulk, is just 0.14 with respect to the 730 cm-1 out-of-plane mode. In summary, the face-on molecular orientation extracted from the infrared spectra is consistent with the extrapolated area per molecule obtained from the 1:4 TiOPc-AA isotherm and supports the assumption that the AA brings about the face-on molecular organization of the TiOPc molecules in mixed films. From this qualitative analysis of the transmission and reflection spectroscopic data, it can also be extracted that the molecules in the neat film show a tilted molecular orientation. The tilted orientation is implied at the transfer surface pressure of 25 mN/m, and it seems to be preserved during the transfer of the monolayer to reflecting substrates. It should be pointed out that slight differences in the tilt angle for the molecule on the subphase and that on the substrate could take place due to the tendency of the molecules to incline during deposition.36,37 3.4. Raman and Resonance Raman Scattering on Smooth Gold Surfaces. Raman Imaging. The Raman and resonance Raman scattering spectra of the neat and mixed films deposited on gold were recorded. The spectra of the 1:4 TiOPc-AA LB film deposited on smooth gold, excited in resonance (633 nm) and preresonance (514.5 nm), are shown in Figure 5. The micro-Raman spectra obtained for all LB films are identical, and we have selected the mixed LB to illustrate the results. The fundamental vibrational modes observed are all TiOPc characteristic vibrations and are summarized in Table 2. The Raman bands have been assigned using the FT-Raman spectra of the TiOPc solid and the FT-Raman and Raman spectra of other phthalocyanine derivatives.38,39 The Raman (36) Chollet, P. A.; Messier, J.; Rosilio, C. J. Chem. Phys. 1976, 64, 1042-1050. (37) Kazjar, F.; Messier, J. Chem. Phys. 1981, 63, 123-133. (38) Jennings, C. A.; Aroca, R.; Kovacs, G. J.; Saio, C. H. J. Raman Spectrosc. 1996, 27, 867-872. (39) Aroca, R.; Pieczonka, N.; Kam, A. P. J. Porphyrins Phthalocyanines 2001, 5, 25-32.
Figure 5. Comparison between the Raman (LL ) 514.5 nm) and resonance Raman (LL ) 633 nm) spectra of the 1:4 TiOPcAA mixed film on smooth gold. Table 2. Characteristic Fundamental Vibrations Observed in Raman and Resonance Raman Spectra of the 1:4 TiOPc-AA Mixed Film Raman (514.5 nm)
resonance Raman (633 nm)
ν (cm-1)
int
fmwh ν (cm-1)
1512 1459 1429 1337 1300 1195 1144 1104 834 749 678 587 483
100 39 37 57 13 5 12 41 30
14 14 5 8 11 22 13 7 9
62 31 6
7 7 18
1509 1449 1431 1337 1302 1192 1141 1104 835 749 678 589 483
int
fmwh
assignment
90 16 30 56 23 17 24 16 24 31 100 21 25
13 7 7 14 10 14 10 5 9 6 5 7 7
pyrrole str isoindole str isoindole str isoindole str C-H bend. C-H bend. pyrrole str C-H bend. ring str ring str macrocycle breath. benzene radial ring deform.
spectrum, excited with the 514 nm laser line, is clearly dominated by the pyrrole stretching modes, observed at 1512 cm-1. However, when the exciting line (633 nm) approaches resonance with the electronic absorption (Qband), the relative intensities of the ring modes, such the 678 cm-1 macrocycle breathing, are clearly enhanced. Since the spectra of all films are identical, micro-Raman global imaging can be used to study phase separation in mixed thin solid films.40,41 Global imaging provides a direct method to probe the scattered-light distribution, for a particular vibrational frequency, over a wide field (ca. 40 µm2) of the sample. A large number of global images were recorded on the surface of TiOPc-AA multilayers with 1:1 and 1:4 molecular ratios using the 514.5 nm laser line and selecting the Raman scattered light of the fundamental vibrational mode at 1512 cm-1. Representative 2D and 3D global images for the 1:1 TiOPc-AA and 1:4 TiOPc-AA LB films on gold are shown in Figure 6. The Raman imaging confirms the assumption that, in these films, the TiOPc molecules are organized in reduced size domains (red spots in Figure 6). For the 1:4 TiOPc-AA mixed multilayer, bright spots of even larger size are observed, indicating that there is an expansion of the TiOPc domains. The same domain expansion can be seen in the 3-D images, showing a greater relative intensity (40) Markwort, L.; Kip, B. J. Appl. Polym. Sci. 1996, 61, 231. (41) Aroca, R.; Constantino, C. J. L. Langmuir 2000, 16, 5245.
Films of Titanyl(IV) Pthalocyanine and Arachidic Acid
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Figure 6. Typical 2-D and 3-D view of the global images recorded for the 1:1 and 1:4 TiOPc-AA mixed LB films. The fundamental vibration selected is the 1512 cm-1 mode, and the total area mapped is 40 µm2.
(brighter coloration) in larger areas, in agreement with the isotherm expansion of the floating monolayer. The results obtained here also agree with our previous work where a five-layer mixed LB film with 75/25% YbPc2/ stearic acid (SA) (relative content by weight) displays a smaller and more homogeneous distribution of aggregates compared with that of a five-layer mixed film of 25/75% YbPc2/SA.42 4. Conclusions An expansion in the surface pressure-area isotherms of mixed TiOPc-AA with respect to the π-A isotherm of neat TiOPc is reported. The spectroscopic data collected for neat and mixed LB films support the assumption that (42) Gaffo, L.; Constantino, C. J. L.; Moreira, W. C.; Aroca, R. F.; Oliveira, O. N., Jr. Langmuir 2002, 18, 3561.
mixing with fatty acid prevents stacking, inducing the flat organization of the TiOPc molecules on the water subphase. The transmission and RAIRS data for neat LB multilayers indicate a titled edge-on orientation over the substrate. Notably, a preferred face-on molecular orientation is found in highly ordered multilayers with introduction of AA up to a 1:4 ratio (Pc-AA). The electronic absorption spectra also give complementary evidence of the PC expansion in mixed films. Acknowledgment. Financial assistance from the Natural Science and Engineering Research Council of Canada is gratefully acknowledged. T.D.C. acknowledges a F.P.U fellowship from the Spanish Ministry of Education, MEC, Spain. LA020825H
