Preparation and Polymorphism of Thin Films of Unsubstituted Cobalt

Received March 6, 1998. In Final Form: June 15, 1998. A new method for preparing thin films of unsubstituted cobalt phthalocyanine (CoPc) is reported...
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Langmuir 1998, 14, 5188-5194

Preparation and Polymorphism of Thin Films of Unsubstituted Cobalt Phthalocyanine B. Bo¨ttger and U. Schindewolf Institut fu¨ r Physikalische Chemie und Elektrochemie III, Universita¨ t Karlsruhe, D-76131 Karlsruhe, Germany

D. Mo¨bius Max-Planck-Institut fu¨ r Biophysikalische Chemie, D-37018 Go¨ ttingen, Germany

J. L. A Ä vila, M. T. Martı´n, and R. Rodrı´guez-Amaro* Departamento de Quı´mica Fı´sica y Termodina´ mica Aplicada, Facultad de Ciencias, Universidad de Co´ rdoba, Avda. San Alberto Magno s/n, E-14004 Co´ rdoba, Spain Received March 6, 1998. In Final Form: June 15, 1998 A new method for preparing thin films of unsubstituted cobalt phthalocyanine (CoPc) is reported. CoPc was dissolved in mixed media consisting of trifluoroacetic or trichloroacetic acid and trichloromethane and spread onto different aqueous subphases. In this way, thin solid films were formed on the surface that exhibit no surface pressure and break readily under mechanical stress. Films were transferred to hydrophobic glass plates by using a horizontal lifting method. Changing the subphase pH led to three different film phases (I-III) that were characterized essentially by electronic spectroscopy of monolayers transferred on solid substrates and Brewster angle microscopy of monolayers floating on the subphase as well. The narrow Q-band absorptions exhibited by phases II and III reveal increased ordering in the film structure. The strong red-shift in the Q-band for phase II reveals lateral stacking of the molecules, which would be expected for trivalent rather than for divalent metallophthalocyanines. Polarized electronic absorption spectra recorded under oblique incidence were consistent with flat-lying molecules in all cases. Heating transferred films of phase II or III produced an additional film phase, IV, that shows marked in-plane anisotropy.

1. Introduction Metallophthalocyanines are interesting compounds with attractive catalytic properties that enable their use as electrochemical catalysts,1-3 electrochemical sensors,4,5 gas sensors,6,7 and photovoltaic devices,8,9 as well as in optical data storage10 and electrochromic displays.11,12 These applications entail the preparation of well-defined phthalocyanine films. While monomolecular films of substituted amphiphilic phthalocyanines can readily be obtained using the method of Langmuir-Blodgett,5-7,13-16 those of unsubstituted phthalocyanines are more difficult to prepare owing to * Corresponding author. E-mail: [email protected]. Phone: +34-957-218618. Fax: +34-957-218606. (1) Zagal, J. H. Coord. Chem. Rev. 1992, 119, 89. (2) Kapusta, S.; Hackerman, N. J. Electrochem. Soc. 1984, 131, 1511. (3) Kusuda, K.; Ishihara, R.; Yamaguchi, H.; Izumi, I. Electrochim. Acta 1986, 31, 657. (4) Hart, J. P.; Hartley, I. C. Analyst 1994, 119, 259. (5) Sauer, T.; Caseri, W.; Wegner, G.; Vogel A.; Hoffmann, B. J. Phys. D: Appl. Phys. 1990, 23, 79. (6) Wang, H.; Lando, J. B. Langmuir 1994, 10, 790. (7) Zhu, D. G.; Petty, M. C. Sens. Actuators, B 1990, 2, 265. (8) Kume, T.; Hayashi, S.; Ohkuma, H.; Yamamoto, K. Jpn. J. Appl. Phys., Part 1 1995, 34, 6448. (9) Mchale, G.; Newton, M. I.; Hooper, P. D.; Willis, M. R. Opt. Mater. 1996, 6, 89. (10) Seto, J.; Tamura, S.; Asai, N.; Kishii, N.; Kijima, Y. Pure Appl. Chem. 1996, 68, 1429. (11) Toshima, N.; Tominaga, T. Bull. Chem. Soc. Jpn. 1996, 69, 2111. (12) Green, J. M.; Faulkner, L. R. J. Am. Chem. Soc. 1983, 105, 2950. (13) Cook, M. J. J. Mater. Chem. 1996, 6, 677. (14) Rikukawa, M.; Rubner, M. F. Langmuir 1994, 10, 519. (15) Granito, C.; Goldenberg, L. M.; Bryce, M. R.; Monkman, A. P.; Troisi, L.; Pasimeni, L.; Petty, M. C. Langmuir 1996, 12, 472.

their nonamphiphilic character. In addition, unsubstituted phthalocyanines are insoluble in almost all usual solvents. However, their high thermal stability and simple structure make them ideal candidates for catalytic applications. For these reasons, films of unsubstituted phthalocyanines are usually prepared by using methods such as precipitation from concentrated sulfuric acid by dilution,2 drying of suspensions onto an electrode surface,3 incorporation into polymers, or other supporting materials,4,5,17 and vacuum vapor deposition,11,12,18-20 all of which usually produce poorly ordered structures. By contrast, vacuum (16) Chai, X. D.; Tian, K.; Chen, H. J.; Tang, X. Y.; Li, T. J.; Zhu, Z. Q.; Mo¨bius, D. Thin Solid Films 1989, 178, 221. (17) Komorsky-Lovric, S. J. Electroanal. Chem. 1995, 397, 211. (18) Jansen, R.; Beck, F. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1531. (19) Pasinszki, T.; Aoki, M.; Masuda, S.; Harada, Y.; Ueno, N.; Hoshi, H.; Maruyama, Y. J. Phys. Chem. 1995, 99, 12858. (20) Kobayashi, T.; Fujiyoshi, Y.; Iwatsu F.; Uyeda, N. Acta Crystallogr. 1981, A37, 692. (21) Kamiya, K.; Momose, M.; Kitamura, A.; Harada, Y.; Ueno, N.; Hasegawa, S.; Miyazaki, T.; Inokuchi, H.; Narioka, S.; Ishii, H.; Seki, K. J. Electron Spectrosc. Relat. Phenom. 1995, 76, 213. (22) Aoki, M.; Masuda, S.; Einaga, Y.; Kamiya, K.; Kitamura, A.; Momose, M.; Ueno, N.; Harada, Y.; Miyazaki, T.; Hasegawa, S.; Inokuchi, H.; Seki, K. J. Electron Spectrosc. Relat. Phenom. 1995 , 76, 259. (23) Hipps, K. W.; Lu, X.; Wang, D. X.; Mazur, U. J. Phys. Chem. 1996, 100, 11207. (24) Chau, L. K.; England, C. D.; Chen, S.; Armstrong, N. R. J. Phys. Chem. 1993, 97, 2699. (25) Schmidt, A.; Schlaf, R.; Louder, D.; Chau, L.; Chen, S.; Fritz, T.; Lawrence, M. F.; Parkinson, B. A.; Armstrong, N. R. Chem. Mater. 1995, 7, 2127.

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Thin Films of Unsubstituted Cobalt Phthalocyanine

deposition on a highly ordered support such as MoS21-25 tends to produce well-defined film structures. Organic molecular beam deposition (OMBE) provides various film phases of tri- and tetravalent metallophthalocyanines such as AlPcCl, InPcCl, TiOPc, and VOPc, some of these phases exhibit a highly red-shifted Q-band in its electronic spectrum.26,27 This red shift originates from lateral stacking of the molecules in the film structure, which has been reported for cyanine aggregates28-30 and is usually described by exciton coupling.24,30,31 On the other hand, bivalent metallophthalocyanines prefer a coplanar central stacking owing to missing axial ligands and provide a blue-shifted Q-band.14,15,24,32 Recently, Ogawa et al. reported the formation of thin films of CuPc32 and TiOPc33 by spreading solutions in trifluoroacetic acid (TFAA)-dichloromethane (DCM) or trichloroacetic acid (TCAA)-DCM mixtures onto a water surface and compressing them with a Langmuir film balance. CuPc exhibited a blue shift in its Q-band relative to the solution spectrum, while TiOPc produced two phases with a slightly red-shifted and a strongly red-shifted Q-band, respectively. In this work, we prepared thin ordered films of unsubstituted CoPc, which is of special catalytic interest,3,4 in order to carry out a subsequent preparation of similar films on indium tin oxide (ITO) electrodes. This would allow the electrochemical study and application of such films as modified electrodes. CoPc was dissolved in TFAA-trichloromethane (TCM) and TCAA-TCM mixed media and spread onto different aqueous subphases. Film formation under these conditions is a dynamic process that produces rigid films without any surface pressure. Altering the subphase pH led to different phases that were characterized spectroscopically. 2. Experimental Section CoPc was purchased from Fluka and used without further purification. All other chemicals were p.a. grade reagents and supplied by Fluka or Panreac. Ultrapure water was obtained from a Millipore “Milli-Q”-system. Freshly prepared solutions of 1 × 10-3 M CoPc in trichloromethane (TCM), modified by addition of trifluoroacetic acid (TFAA) or trichloroacetic acid (TCAA), were used for spreading. In the case of TFAA solutions, the phthalocyanine was first dissolved in the pure acid and then mixed with TCM to obtain the spreading solution (TFAA/TCM 1:4). On the other hand, prior to the addition of CoPc, 1 M TCAA in TCM was dissolved. The subphase consisted of NaOH at different concentrations or of pure water and was held in a flat rectangular polyethylene vessel of 90 cm2 surface placed on a vibration-free support. The subphase temperature was about 20 °C. By means of a microsyringe, the spreading solution was carefully applied to the center of the subphase until a visible island was formed. In this manner, a closed, uniform phthalocyanine film built up around the island that breaks readily under mechanical stress. Hydrophobic glass slides were used for spectroscopic examination. Hydrophobization was accomplished by immersion into a solution of dichlorodimethylsilane in TCM, as described elsewhere.34 Films were transferred horizontally by contacting (26) Yamashita, A.; Maruno, T.; Hayashi, T. J. Phys. Chem. 1994, 98, 12695. (27) Yamashita, A.; Maruno, T.; Hayashi, T. J. Phys. Chem. 1993, 97, 4567. (28) Da¨hne, L. J. Am. Chem. Soc. 1975, 117, 12855. (29) Nu¨esch, F.; Moser, J. E.; Shklover, V.; Gra¨tzel, M. J. Am. Chem. Soc. 1996, 118, 5420. (30) Czikkely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207. (31) Czikkely, V.; Dreizler, G.; Fo¨rsterling, H. D.; Kuhn, H.; Sondermann, J.; Tillmann, P.; Wiegand, J. Z. Naturforsch. 1969, 24a, 1821. (32) Ogawa, K.; Yonehara, H.; Pac, C. Langmuir 1994, 10, 2068. (33) Ogawa, K.; Yao, J.; Yonehara, H.; Pac, C. J. Mater. Chem. 1996, 6, 143.

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Figure 1. Absorption spectra for 5 × 10-5 M CoPc in (a) THF (solid line) and dioxane (dotted line) and (b) concentrated sulfuric acid (solid line) and TFAA (dotted line). the subphase with the glass support (Scha¨fer method).35 Under these conditions, the high fragility of the CoPc films precludes a double transfer. The slides were subsequently used to record spectra or stored in a dry atmosphere at room temperature. Spectroscopic examination was conducted on a Perkin-Elmer Lambda 3B UV/VIS spectrophotometer (2 nm resolution) equipped with a support for the glass slides and a polarization filter. The path length of the cuvette used in the measurement of solution spectra was 1 cm. Some spectra were obtained by using a highly sensitive film spectrophotometer. This and the Brewster angle microscope (BAM) used for in situ film imaging have been described elsewhere.36,37

3. Results and Discussion 3.1. Properties of CoPc Solutions. Most unsubstituted phthalocyanines are virtually insoluble in nearly every commonplace solvent owing to their strong intermolecular interactions. CoPc is slightly soluble in tetrahydrofurane (THF) and dioxane. Figure 1a shows typical absorption spectra obtained in these media. The Soret or B-band and the Q-band, typical of the neutral phthalocyanine ring, are clearly visible at about 340 and 660 nm, respectively, in both cases. A smaller absorption band at about 595 nm is also visible that has been shown to be a vibrational component of the Q-band.38-40 (34) Honig, E. P.; Hengst, J. H. T.; Den Engelsen, D. J. Colloid Interface Sci. 1973, 45, 93. (35) Langmuir, I.; Scha¨fer, V. J. Am. Chem. Soc. 1938, 60, 1351. (36) Kuhn, H.; Mo¨bius, D.; Bu¨cher, H. In Physical Methods of Chemistry; Weissberger, A., Rossiter, B., Eds.; Wiley: New York, 1972; Vol. 1, Part III B, p 577. (37) Ho¨nig, D.; Overbeck, G. A.; Mo¨bius, D. Adv. Mater. 1992, 4, 413. (38) Lever, A. B. P.; Pickens, S. R.; Minor, P. C.; Licoccia, S.; Ramaswamy, B. S.; Magnell, K. J. Am. Chem. Soc. 1981, 103, 6800. (39) Ough, E. A.; Stillman, M. J. Inorg. Chem. 1994, 33, 573. (40) Mack, J.; Stillman, M. J. J. Phys. Chem. 1995, 99, 7935.

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Figure 2. Absorption spectra for 5 × 10-5 M CoPc in (curve A) 1 M TCAA-TCM; (curve B) 20 vol % TFAA-TCM; (curve C) 1 M TCAA-TCM after neutralization with concentrated NaOH.

Figure 3. Absorption spectra for 5 × 10-5 M CoPc in 20 vol % TFAA-TCM immediately after preparation and after different times. T ) 20 °C.

Strongly acidic media, where the phthalocyanine forms protonated species, markedly increase its solubility. Figure 1b shows spectra for CoPc in concentrated sulfuric acid (solid line) and in TFAA (dotted line). Protonation of the meso-bridging aza-nitrogen atoms of the phthalocyanine causes red-shifts in the B- and Q-bands. This red-shift is considerably stronger in sulfuric acid than in the weaker TFAA, consistent with a higher degree of protonation in the former. The number of protons transferred can be determined by comparison with electronic spectra of previous protonation studies on CoPc derivatives.41 In this way, we determined that four protons were transferred in sulfuric acid and two protons were transferred in TFAA. In the latter, the spectrum exhibits a split Q-band due to the decreased symmetry resulting from incomplete protonation. Also it is noticed an increasing of absorbance in comparison with the neutral phthalocyanine that can be related to a different value of the extinction coefficient . Dissolving TFAA or TCAA in chlorinated hydrocarbons provides mixed solvents for unsubstituted phthalocyanines that spread at the water-air interface and are therefore suitable for film preparation.32,33 Figure 2 shows spectra for 5 × 10-5 M solution of CoPc in a solvent of 1 M TCAA in TCM (curve A) and 20 vol % TFAA in TCM (curve B), respectively. In both cases, the Q-band is split due to the decreased symmetry. The red-shift in the TFAA mixture is stronger owing to a double protonation. By contrast, the TCAA-TCM mixture exhibits a single protonation, determined as for TFAA.41 In any case, this interpretation should be taken as tentative, because acid species with variable “depths of protonation”, especially in organic media, can be formed.41 Treating the above solutions with saturated aqueous NaOH results in a partial precipitation of CoPc. The remaining dissolved phthalocyanine is completely deprotonated, as shown by the corresponding spectrum (Figure 2, curve C). The red-shift in the Q-band disappears and the spectrum resembles that in THF or dioxane (Figure 1B). Soon after preparation, CoPc solutions in TFAA-TCM or TCAA-TCM mixtures change from a green to a purple color, the change being accelerated by the presence of water traces (Figure 3). In the course of this process, a species with absorption maxima at 323, 526, and 850 nm is formed as the signal for the original protonated CoPc decreases.

A similar behavior has been observed by Ogawa et al.33 for TiOPc dissolved in TFAA-dichloromethane mixtures that they ascribed to the formation of a radical cation (TiOPc•+). The corresponding radical cation for MgPc has been formed by chemical oxidation and characterized spectroscopically and by electron paramagnetic resonance.42 Because of the similarity with our results, we shall assume the formation of CoPc•+. Molecular orbital calculations for ZnPc•+ also confirm this assignment.40 A comparison of the behavior of CoPc with that of other unsubstituted metallophthalocyanines in TFAA-TCM has shown that the rate at which the radical cation is formed depends strongly on the central metal ion. While phthalocyanines with hardly oxidizable metal ions such as ZnPc and CuPc or H2Pc do not show any instability in these acid media, CoPc and, especially, FePc exhibit rapid radical cation formation. Probably, the oxidation of the phthalocyanine ring is facilitated by the intermediate oxidation of the central metal ion. By addition of water to the solution of CoPc dissolved in FAA-TCM, the CoPc radical forms instantaneously. This fact is different from that observed for TiOPc in TFAA-dichloromethane,33 whose radical formed more slowly in the presence of water. Figure 4 shows the spectrum for CoPc in pure TFAA before (curve A) and after addition of different amounts of water (10% curve B and 20% curve C). The absorption peaks at 393, 680, and 713 nm decrease and eventually disappear at a water content of 20%. The solution color changes from green to purple as commented above and the characteristic radical cation absorption peaks at 503 and 670 nm appear. Upon dilution of solution TFAA + 20% water with methanol in the ratio 1:1 (curve D), the color changes to blue and the spectrum of the unprotonated neutral form is recovered, which is in accordance with the well-defined isosbestic points noted in Figure 4. This phenomenon demonstrates that the radical cation formation is reversible and does not destroy the phthalocyanine ring. 3.2. CoPc Film Formation. The formation of thin films at the air-water interface is a dynamic process in which the rate of neutralization of the spreading solution as its expands on the surface seems to be an important factor. Consequently, the subphase pH has a crucial effect on the properties of the films obtained. In the spreading solution the protonated form as well as the radical cation

(41) Bernstein, P. A.; Lever, A. B. P. Inorg. Chim. Acta 1992, 198200, 543.

(42) Ough, E.; Gasyna, Z.; Stillman, M. J. Inorg. Chem. 1991, 30, 2301.

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Figure 4. Absorption spectra for about 1 × 10-4 M CoPc in (curve A) pure TFAA, (curve B) TFAA + 10% water, (curve C) TFAA + 20% water, and (curve D) (TFAA + 20% water)/ methanol, in ratio of 1:1. Absorption values are corrected for the varying CoPc concentration.

of CoPc are complexed by trifluoroacetate or trichloroacetate anions, which interfere within the aggregation process. As a result, lateral aggregation can be favored under certain conditions, as rather expected for phthalocyanines with tri- and tetravalent metal ions. Using NaOH as a subphase additive, three pH ranges were distinguished, corresponding to different film phases: pH >12.5, phase I (+II, +III); pH 11.5-12.5, phase II; pH 7-11.5, phase III. Phases I and II can be formed from TFAA-TCM and TCAA-TCM solvent mixtures, whereas phase III can only be obtained from the former. The use of TCAA-TCM in this pH range leads to nonuniform films. 3.2.1. pH 12.5-14. Phase I (+II, +III). In strongly alkaline subphases, e.g., 0.1 M NaOH, the rate of neutralization is so high that the spreading solution during expansion already exhibits the typical blue color of the neutral CoPc species. The film aggregates as a closed island without covering all available space. From the estimated film surface and the volume of solution, the effective area per molecule can be estimated to be 0.30 nm2. Figure 5a shows the electronic spectra for a film on a hydrophobic glass slide recorded under normal (0°, solid line) and oblique incidence (45°) with different light polarization (s, dashed line; p, dotted line). The Q-band is relatively broad and exhibits three well-defined maxima at 628, 670, and 780 nm. While the blue-shifted main peak at 628 nm is characteristic of phase I films, the additional bands represent portions of phases II and III, which are present in variable proportions. The variation of the absorption intensity with polarization and the incidence angle theoretically allows one to determine the average tilt angle of the molecular planes to the normal of the glass surface. Orrit et al.43 derived the ratio between s and p absorption for monomolecular films isotropic around the surface normal at different incidence angles and for different molecular orientations. From Figure 5a we obtained a ∆Ts/∆Tp ratio of 1.43 at 628 nm, 1.47 at 670 nm, and 1.61 at 780 nm, respectively, at a 45° incidence angle. These ratios are consistent with the theoretical value of 1.50 for planar orientation, so we can assume such an orientation for the three components. Taking into account an effective film area of 0.30 nm2, we

should have a film thickness of five to six monolayers for an estimated area about 1.60 nm2 for a planar CoPc molecule. Phase I films were also examined by Brewster angle microscopy (BAM). Figure 5b shows the BAM image for a film floating on the surface of the alkaline subphase under polarized light. The visible area is about 1 mm2. Wavy structures of different reflection intensity can be distinguished that correspond to different film thicknesses. Other zones, e.g., the black regions near the center of spot, change in intensity as the position angle of the analyzer is altered and thus represent anisotropic regions that might have been formed by folding of the film structure leading to inclined layers. In conclusion, in this pH range we have a mixed film consisting of phases I, II, and III. Because of the strongly alkaline subphase, precipitation of cofacially aggregated CoPc molecules seems to predominate and leads to a blueshifted Q-band main component at about 628 nm (phase I). Additionally, strong phase II formation (see below), which probably takes place at the liquid-liquid interface before phase I precipitates, is observed. Phase II must thus lie underneath phase I on the subphase surface and therefore on top of it after the film is transferred to a solid. Finally, phase III is formed from phase II as shown below. We also studied this CoPc film phase electrochemically for catalytical activity toward silver deposition, and the results have been published elsewhere.44 3.2.2. pH 11.5-12.5. Phase II. Lowering the subphase pH to about 12 dramatically changes the conditions for CoPc aggregation. Instead of forming a closed island

(43) Orrit, M. J.; Mo¨bius, D.; Lehmann, U.; Meyer, H. J. Chem. Phys. 1986, 85, 4966.

(44) Bo¨ttger, B.; Schindewolf, U.; Avila, J. L.; Rodrı´guez-Amaro, R. J. Electroanal. Chem. 1997, 432, 161.

Figure 5. (a) Electronic spectra for a film obtained by spreading 1 mM CoPc in 1 M TCAA/TCM on 0.1 M NaOH, recorded at different incidence angles and polarization (b) Brewster angle microscopic (BAM) image of the film on the subphase surface. The visible area is about 1 mm2.

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Figure 6. (a) Electronic spectra for a film obtained by spreading 1 mM CoPc in 1 M TCAA/TCM on 0.01 M NaOH aqueous subphase, recorded at different incidence angles and polarization (b) BAM image of the film on the subphase surface.

the film spreads throughout the complete available surface. If sufficient spreading solution is applied, a closed, uniform film with a more compact central region due to CoPc excess is obtained. As with strongly alkaline subphases, these crystalline films exhibit no surface pressure and break easily. The effective film surface per CoPc molecule is about 0.80 nm2. Figure 6a shows the absorption spectra for a film made by spreading 1 mM CoPc in 1 M TCAA-TCM on 0.01 M aqueous NaOH (pH 12) and transferring it to a hydrophobic glass plate. A large red-shift in maximum of the Q-band to about 800-810 nm can be seen, which is typical of phase II films. The bandwidth is very small for solidstate absorption in the Q-band region and thus suggests film order at the molecular lever. The peak at 675 nm is due to phase III, which is formed to a lesser extent. The absorption ratio between s- and p-polarization at a 45° incidence angle is 1.50 for λ ) 675 nm, which is exactly the predicted value for planar orientation.43 On the basis of this result and an observed effective film area of 0.80 nm2, we propose a flat bilayer structure. The strong redshift must be caused by lateral aggregation of the molecules between the two layers. A BAM image was also recorded for a phase II film (see Figure 6b). This film phase is considerably more uniform than phase I and exhibits a lower overall reflected intensity. No anisotropy was detected. The bright spots with interference rings represent small solid particles that could have been present in the spreading solution, possibly due to an incomplete dissolution of CoPc in the solvent mixture. The red-shifted phase II films obtained by using TCAATCM mixtures as spreading solvent as in Figure 6 were quite stable. However, TFAA-TCM mixtures produced highly unstable phase II films. Figure 7a shows changes

Bo¨ ttger et al.

Figure 7. Electronic spectra for (a) a 1-layer film and (b) a 10-layer film obtained by spreading 1 mM CoPc in 20% TFAA/ TCM on 0.01 M NaOH, immediately after preparation and after different times in dry air (20 °C) and (b) showing the phase transition on heating at 120 °C for 24 h under polarized light, parallel (|) and normal (⊥) to the film expansion direction.

in the spectrum of the latter with time after the film was transferred. A rapid transformation into a phase III phase with an absorption maximum at 673 nm is observed. Figure 7a also reveals that the shoulder at 630 nm, visible in all phase II spectra, decreases simultaneously with the peak at 806 nm. It thus seems to be due to phase II. A new shoulder appears at about 610 nm as phase III is formed. Figure 7b represents the absorption of a 10-layer film of phase II made by transferring the individual layers without interruption. As in Figure 7a, spectra were recorded immediately after transfer and with increasing time intervals after deposition in dry air. We also observe a transformation into phase III, but unlike with one layer, the process stops at a low proportion. It seems to involve the outermost layer only and might result from air oxidation. Although 10 layers were transferred to the glass support, the overall shape of the spectrum remained unchanged and only a very slight red-shift was observed. This could suggest that the individual bilayers could be separated by large ligands (e.g. trifluoro- or trichloroacetate ions) as shown in Chart 1, where the TFAA molecules are complexed in the bilayer system (the structures of CoPc and TFAA have been added). If the slide is heated to 120 °C, the absorption spectrum changes completely to a new phase IV with blue-shifted maximum at 610 nm and a smaller peak at 681 nm. Figure 7b also shows spectra of the film heated for 24 h, obtained with polarization parallel (|) and normal (⊥) to the organic phase expansion direction while spreading. The new

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Chart 1. Structure of a Flat Bilayer of CoPc Molecules Complexed with TFAA, Horizontally Transferred on Hydrophobic Glassa

a The molecular structures of the two components are also shown.

phase exhibits significant in-plane anisotropy. The intensity of both peaks changes in the same manner with polarization. It thus seems likely that the peak at 681 nm is due to phase IV rather than to traces of phase III. 3.2.3. pH 7-11.5. Phase III. At low NaOH concentrations or using pure water as subphase, deprotonation in the spreading solution is too slow to develop to a significant extent before the solvent has evaporated. Instead, the radical cation is rapidly formed, as evident from its purple color in the moment of the organic phase expansion. Similar to pH 12, after the whole surface is covered, a dense island is formed at the center, surrounded by the uniform film. The characteristic absorption of phase III lies at 670675 nm (Figure 8a). A shoulder at about 620 nm can also be seen that varies in intensity but never disappears completely. As also shown in Figure 8a, the spectrum changes with time while stored in dry air. The main absorption band at 669 nm raises markedly in parallel to a small red-shift to 673 nm. Simultaneously, the broad absorption band between about 450 and 575 nm decreases, as does the shoulder at 620 nm. This suggests a slow increase in film order at the molecular level that is still persistent even after a week. If several layers are transferred, the absorption spectrum essentially preserves its shape and only a slight red-shift in the main absorption band (from 675 nm for one layer to 682 nm for five layers) is observed. The individual layers must thus be separated by trifluoroacetate ligands, as in phase II (see Chart 1). With more layers, the shift is slightly reduced and the absorption per layer markedly decreased as a result of decreased film ordering (Figure 8b). This suggests that trifluoroacetate ligands are not sufficiently stiff to form a rigid threedimensional lattice. This has so far precluded obtaining information from X-ray diffraction. Figure 9 shows spectra for a 10-layer film of phase III recorded at normal (solid line) and oblique incidence (45°) using polarized light. The absorption ratio between 45°s (dashed line) and 45°p (dotted line) is about 1.35. Taking into account that the theoretical ratio43 is lower than 1.5 owing to the high overall absorption of 10 layers, we could assume a planar or near-planar molecular orientation. The planar orientation and the observed effective film area of about 0.80 nm2, as in phase II films, are consistent

Figure 8. Electronic spectra for phase III, obtained by spreading 1 mM CoPc in 20% TFAA/TCM on pure water: (a) only one film after different times in dry air (20 °C); (b) a variable number of films (prior to transferring the next film, the slide was always stored in dry air at 20 °C for 24 h).

Figure 9. Absorption spectra for a 10-layer film of phase III obtained as in Figure 1, recorded with polarized light under different incidence angles.

with a bilayer assembly separated by trifluoroacetate ligands. The structure must be very similar to that of phase II films (Chart 1) considering the easy, reversible transformation of phase II into phase III; i.e., phase III can to some extent be converted into phase II by immersion in 1 M NaOH. The main difference seems to be the relative position of the two monomolecular layers within the bilayer assembly, which, in phase III films, causes only a small red-shift in the Q-band. To confirm the value obtained for the effective area of the CoPc in the film, the superficial concentration Γ was calculated for phase III using as a reference the integral Q-band absorption of the THF solution in Figure 1.There-

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fore it is necessary to assume an unchanged integral extinction coefficient  ) ∫(λ) dλ in the Q-band of the aggregated films. Although this generally may not be true, it seems more likely in phase III because its Q-band absorption maximum in nearly identical to that of the unprotonated solution species in THF and the optical characteristics of the individual molecules may be similar. Γ was obtained from the relation:45

Γ ) ∆T/(2.303forient)

(1)

where forient is an orientational factor, which is 1.5 for planar orientation46 of the phthalocyanine molecules and ∆T is, in this work, the integral Q-band absorption of the film samples. We obtained superficial concentrations between 0.77 and 1.25 molecules/nm2 for phase III, corresponding to an effective film area of 0.8-1.3 nm2 per molecule. Well-defined films with a sharp absorption band show values close to the lowest limit (0.8 nm2), in agreement with the effective area obtained above from the amount of spreading solution and the total film area. Heating phase III films results in the formation of phase IV, as previously shown for phase II (Figure 7b), giving rise to two bands at 615 and 685 nm. Also, strong inplane anisotropy was observed, even though phase III films seem to be completely isotropic. This anisotropy must have existed previously in phase III, but was invisible, probably owing to the planar orientation of the molecules. 4. Conclusions The novel method for preparing thin CoPc films is rather different from that of Langmuir-Blodgett, which is confined to amphiphilic molecules forming a fluid, equilibrated surface phase. With our method, aggregated films (45) Martı´n, M. T.; Prieto, I.; Camacho, L.; Mo¨bius, D. Langmuir 1996, 12, 6554. (46) Gru¨niger, H.; Mo¨bius, D.; Meyer, H. J. Chem. Phys. 1983, 79, 3701.

with no surface pressure build up in a dynamic process. This is strongly influenced by the subphase basicity, which ultimately determines the film structure. However, further studies are necessary to prove the hypothetical model (Chart 1), e.g., using infrared spectroscopy and other ligands. Four different film phases were observed (phases I-IV). Phase I was a nonhomogeneous mixture containing parts of phases II and III and exhibiting a nonuniform thickness of about 5-6 planar molecular layers. This phase is formed at subphase pH values above about 12.5, where the high neutralization rate favors the formation of cofacial aggregates. A blue-shifted Q-band at 628 nm is observed as a result. The bands at about 670 and 780 nm are due to phases III and II, respectively. On the other hand, phases II and III, which are formed over the pH range 11.5-12 and 7-11.5, respectively, are considerably more homogeneous and exhibit a significant arrangement at the molecular level. We believe that they consist of planar bilayers covered by trifluoroacetate (or trichloroacetate) ligands that prevent direct contact with the next bilayer in multilayer assemblies. Phases II and III differ in aggregation between the two monomolecular layers. Aggregation must be lateral in phase II, as it causes a strong red-shift in the Q-band from 658 nm (in THF) to about 800-810 nm. Phase III exhibits an intermediate type of aggregation, because it only causes a slight red-shift to about 676 nm. Phases II and III are unstable at 120 °C. On heating, both transform into phase IV, which exhibits a split and blue-shifted Q-band, 610 and 681 nm, respectively. Phase IV films show marked in-plane anisotropy. These results are promising in regard to the design of modified electrodes, e.g., ITO, for the subsequent use in catalytic processes. Acknowledgment. The authors wish to express their gratitude to DFG (Deutsche Forschungsgemeinschaft) and Spain’s DGICyT for financial support of this research in the framework of Projects PB94-0446 and PB94-0448. LA9802769