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Langmuir 1994,10, 2068-2070
Monolayers and Multilayers of Unsubstituted Copper Phthalocyanine Kimiya Ogawa,* Hisatomo Yonehara, and Chyongjin Pac Kawamura Znstitute of Chemical Research, Sakado, Sakura, Chiba 285, J a p a n Received March 2, 1994. Zn Final Form: May 19,1994@ Monolayersofunsubstituted copper phthalocyanine(CuPc)were prepared on a water surface by spreading a solution of CuPc in dichloromethane containing trifluoroacetic acid, and were deposited as multilayer films by means of a horizontal lifting method. A preferential orientation of CuPc molecules in the films was indicated from polarized absorption spectra and X-ray diffraction patterns. It was shown that a CuPc monolayer on the water surface consists of two-dimensional microcrystals.
Introduction Langmuir-Blodgett (LB) films of phthalocyanine (Pc) compounds have been reported so far in large numbers.’ For those studies, substituted Pc compounds were usually employed as sample materials, because of their good solubility in organic solvents such as chloroform and benzene. On the other hand, most unsubstituted PCSare practically insoluble in the common spreading solvents, and it is difficult very often to prepare spreading solutions of those Pcs for monolayer formation. Although a few groups reported LB films of some unsubstituted PCSwhich are exceptionally soluble in certain solvents, it still seems difficult to obtain real monolayer systems (not being threedimensional microcrystals) and good quality multilayer^.^-^ It is known that some unsubstituted Pcs are soluble in appropriate mixtures of strong organic acids and organic solvent^.^^^ However, no effort has been made, to our knowledge, to prepare LB films of various unsubstituted Pcs using such mixed solvents. The present paper deals with monolayers and multilayers of unsubstituted copper phthalocyanine (CuPc) prepared from a solution of CuPc in a mixed solvent of trifluoroacetic acid (TFAA) and dichloromethane (DCM). Experimental Section Crude CuPc (Dainippon Ink & Chemicals, Inc.)was sublimed twice for purification and was dissolved in a mixture of TFAA and DCM [1:10 (v/v)l to prepare spreading solutions. Concentrations of CuPc in the sample solutions were of the order of 2 x 10-4 M (mol dm-3). The sample solutions were spread onto doubly distilledpure water to make monolayers of CuPc. Surface pressure-area isotherms were recorded with a commercial LB trough system (KSV-5000LB). Multilayers of CuPc were deposited at a surface pressure of 9.5 mN/m by means of a modified horizontal liftingmethod.* Quartz plates were used as substrates after their surfaces had been made hydrophobic with dimethyldichlorosilane. In order to prove that the monolayer of CuPc on the water surface is a real monolayer system and is not composed of three-dimensional microcrystals, “combined and Abstract published in Advance A C S Abstracts, July 1, 1994. (1) Ulman, A. A n Introduction to Ultrathin Organic Films, From Langmuir-Blodgett to Self-Assembly;Academic Press: San Diego, 1991; pp 167-176. (2) Baker, S.; Petty, M. C.; Roberts, G. G.; Twigg, M. V. Thin Solid Films 1983, 99, 53. (3)Clavijo, R. E.; Battisti, D.; Aroca, R.; Kovacs, G . J.; Jennings, C. A. Langmuir 1992, 8, 113. (4)Kanezaki, E.; Wada, Y.; Egami, Y. Arab. J. Sci. Eng. 1990, 15, 375. ( 5 ) George, R. D.; McMillan, P. F.; Burrows, V. A,; Hervig, R. Thin Solid FiZms 1991, 203, 303. (6) Duff, J. M.; Mayo, J. D.; Hsiao, C-K.; Hor, A-M.; Bluhm, T. L.; Hamer, G. K.; Kazmaier, P. M. Eur. Pat. Appl. E P 460 565, 1991. (7)Ono, H.; Otsuka, S.; Hiroi, M. Jpn. Kokai Tokkyo Koho JP 02 269 776 [90 269 7761, 1990. (8)Fukuda, K.; Nakahara, H.; Kato, T. J. Colloid Interface Sci. 1976, 54, 430. @
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0743-7463/94/2410-2068$04.50/0
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Figure 1. Visible and near-infrared absorption spectra of a CuPc solution in (a) T F M C M (1:lO) mixed solvent and (b) 1-chloronaphthalene.
“alternate”multilayers of CuPc and copper tetrakis[(hexyloxy)carbonyllphthalocyanine( C G C ~ P Cwere ) ~ prepared. The CGCuPc sample was synthesized in our previous study, and is known to form highly ordered LB In the present study,monolayers of C6CuPc were prepared from chloroform solution, and deposited at a surface pressure of 15 mN/m in a way similar to that for CuPc. Visible and near-infrared absorption spectra for the sample solutions and for the multilayers were measured on a multichannel photodetector MCPD-1000 (Otsuka Electronics) system. X-ray diffractions(Cu Ka ray) were measured with an RAD-IIA diffractometer (Rigaku Co.).
Results and Discussion Figure 1 shows visible and near-infrared absorption spectra of CuPc in the spreading solvent (ca. 2 x Mj and in 1-chloronaphthalene (saturated), which are quite different in shape, probably due to the protonation of CuPc upon dissolution in the mixed solvent.1° Dilution of the spreading solution with DCM to 11400 concentration resulted in changes from spectrum a to one similar to spectrum b. Therefore, the difference in the spectra is not due to chemical decomposition of CuPc upon dissolution in the spreading solvent. A shoulder around 530 nm and two small peaks at 805 and 841 nm seem to arise from a small quantity of a n oxidized (cation radical) species of CUPC.“ A surface pressure-area isotherm (20 “C) for the monolayer of CuPc is shown in Figure 2. The limiting area for the isotherm was 56 A2,a fairly reasonable value for the so-called “edge-on”-style molecular orientation of CuPc in the monolayer. A cycled compression-decom(9) Ogawa, K; Yonehara, H.; Maekawa, E. Thin Solid Films 1992, 2101211, 535.
(10) Bernstein, P. A.; Lever, A. B. P. I n o g . Chim. Acta 1992,198-
200, 543.
(11)Homborg, V. H. 2.Anorg. Allg. Chem. 1983, 507, 35.
0 1994 American Chemical Society
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Langmuir, Vol. 10,No. 7,1994 2069
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Figure 2. Surfacepressure-area isotherm (20 "C) for a CuPc
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Figure 4. X-ray diffraction patterns for (A) a neat C6CuPc film, (B) a neat CuPc film, (C) a combined film, and (D) an alternate film.
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Figure 3. Polarized visible and near-infrared absorption spectra for a 60-layerLB film of CuPc at various incident angles of the light beam (p-polarized).
pression (0-9.5 mN/m) experiment revealed large hysteresis on the isotherm, which is presumably due to the rigidness of the monolayer and to very slow relaxation of surface pressure on decompression. However, we confirmed that the monolayer is fairly stable in spite of the lack of amphiphilic nature in the CuPc molecule: The decrease of surface area a t a constant ,urface pressure of 9.5 mN/m was only 3.8% in the first 10 min after a compression, and was 1.2% in the next 10 min. Figure 3 shows polarized visible and near-infrared absorption spectra for a deposited multilayer of CuPc (60 layers) a t various incident angles of the p-polarized light beam with respect to the film plane. While visible absorption spectra of solid thin films of Pcs are known to vary remarkably depending on their crystal form, the solidline spectrum in Figure 3 is characteristic of a-form CuPc: The two absorption maxima a t 626 and 694 nm correspond well with those (626 and 691 nm) reported for a sublimed thin film of a-CuPc but substantially differ .from the absorption maxima (644 and 725 nm) of a sublimed p-CuPc film.12 An important feature in Figure 3 is a clear dependence of the spectra on the incident angles, accompanied by the appearance of a shoulder peak around 570 nm a t higher incident angles, indicating certain preferential orientation of CuPc molecules in the film.13 The 570-nm shoulder can be correlated with a shoulder peak at 580 nm of the sublimed a-CuPc film.12 Moreover, X-ray diffraction patterns of the multilayer of CuPc (60 layers) exhibited only one diffraction peak a t 28 = 6.54" (d = 13.5 A), a s seen in Figure 4B. The lack of other diffraction peaks a t the higher 28 region indicates that a specific lattice plane, presumably the (200) plane _ _ _ ~ __ (12) Lucia, E. A.; Verderame, F. D. J. Chem. Phys. 1968,48,2674. (13) Ogawa,K.; Kinoshita, S.;Yonehara, H.; Nakahara,H.; Fukuda, K. J. Chem. SOC.,Chem. Commun. 1989, 477.
of a-CuPc, is preferentially oriented parallel to the film plane. It can be pointed out that the d spacing observed here is slightly larger than that reported for a-CuPc.14 From a numerical comparison of d spacings, it may be assigned as a peak from y-CuPc.15 However, since the CuPc monolayer on the water surface consists of twodimensional microcrystals (vide infra), small differences in the position of the diffraction peak from threedimensionally-grown crystals may well occur. Moreover, it is reported that a-CuPc and y-CuPc are practically identical except for their particle sizes.16 At any rate, it can be concluded that the planes of CuPc molecules are oriented nearly perpendicularly to the plane of the substrate (i.e., so-called edge-on orientation). This is consistent with the limiting area obtained from the surface pressure-area isotherm. Since the CuPc molecule is not amphiphilic, it is necessary to determine whether or not the monolayer formed on the water surface is a real monomolecular layer. In order to confirm this, we prepared the combined multilayer of 60 C6CuPc monolayers deposited on top of 60 CuPc monolayers and also the alternate multilayer in which each monolayer of C6CuPc and CuPc was deposited alternately, up to 120 layers total. In Figure 4 are shown X-ray diffraction patterns of the combined and alternate films together with that of a neat C6CuPc multilayer (60 layers). No diffraction peak was observed a t 28 higher than 10" in all cases. An important observation is that the alternate multilayer shows only one diffraction peak a t 5.0", but none at 6.5" (from neat CuPc) nor a t 4.3" (from neat C6CuPc) (Figure 4D). On the other hand, the two peaks from the neat Pcs are correspondingly seen in Figure 4C for the combined multilayer. If the (mono)layer of CuPc on the water surface were composed of threedimensional microcrystals, the same diffraction peak as that for the neat CuPc (Figure 4B) should be observed in the chart for the alternate multilayer, because the diffraction of X-ray should occur from lattice plane(s) within each microcrystal. This is again true for the monolayer of C6CuPc. Therefore, it is strongly suggested that the monolayer of CuPc (and C6CuPc) on the water surface is not composed of three-dimensional microcrystals, but is a real monolayer system (i.e., two-dimensional
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(14) Suito, E.; Uyeda, N. Kolloid 2.2.Polym. 1963, 193, 97. (15) Lever, A. B. P. Adu. Inorg. Chem. Radiochem. 1966, 7 , 27. (16)Assour, J. M. J. Phys. Chem. 1965, 69, 2295.
2070 Langmuir, Vol. 10,No. 7,1994
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Figure 5. Molecular arrangementmodels for (A)a neat CGCUPc film, (B)a neat CuPc film, (C) a combined film, and (D)an alternate film. Each bar with a circle at the center represents the side view of the Pc ring.
microcrystals). From interplanar spacings calculated from
Letters the 28 values, models of molecular arrangements for these films can be drawn coherently as illustrated in Figure 5.17 In conclusion, real monolayer systems and multilayers of CuPc with a specific molecular orientation were successfully prepared. In a preliminary study, it was confirmed that the present method can be applied to the fabrication of multilayers (LB films, in a broad sense) of several other unsubstituted Pcs by appropriate selection of the organic acid and organic solvent. We believe this is a convenient technique to fabricate monolayer assemblies of Pcs with higher thermal and chemical resistance and with higher photoelectric performance than many of the LB films of substituted PCS.Furthermore, the present study would exemplify that careful attention to the spreading solvent may help to develop a fruitful system on fundamental research of monolayers.
Acknowledgment. The authors wish to thank Mr. H. Maki, Analysis Group in Central Research Laboratories of Dainippon Ink & Chemicals, Inc., for his cooperation in the X-ray diffraction measurements. (17)The accuracy of the spacings is 10.5 A. Experimental errors may arise from slight shifts of the spacings (mostly “shrinkage”) on standing.
