Langmuir 1994,10, 4265-4269
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Langmuir-Blodgett Film Formation of a Series of Peripherally Octasubstituted Metallophthalocyanines M. Burghard,* M. Schmelzer, and S. Roth Max-Planck-Institut fur Festkorperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany
P. Haisch and M. Hanack Universitat Tubingen, Lehrstuhl f i r Organische Chemie 11, Auf der Morgenstelle 18, 72076 Tiibingen, Germany Received June 13, 1994. In Final Form: August 26, 1994@ The behavior at the airlwater interface of peripherally octa(penty1oxy)-substituted, nonamphiphilic metallophthalocyanines containing nickel, palladium, and platinum as central metals, respectively, has been studied. For all three, the same value of the limiting area per molecule has been found. Langmuirlayer stability turned out to be strongly dependent on the initial area per molecule given to the molecules upon spreading before compression. Provided that this area was chosen to be appropriately large, LB multilayers could be formed from all three phthalocyanines. As a representative example, LB films of the palladium phthalocyanine were studied by polarization-dependentU V l v i s and FTIR spectroscopies as well as small-angle X-ray diffraction. These investigations revealed edge-on orientation of the molecules in well-ordered monolayers.
Introduction Interest in phthalocyanines has existed for many decades because of their considerable stability as well as interesting optical and electrical properties.' A great variety of phthalocyanines are accessible through the choice of different central atoms and/or substitution patterns at the periphery.2 In recent years there have been some strategies to make phthalocyanines useful for Langmuir-Blodgett (LB) film f ~ r m a t i o n .Such ~ an ordered assembly is desired, e.g., for the goal of observing well-defined charge transport properties in appropriate solid state devices. In the case of vertical electron transport in a sandwich structure, it seems to be of advantage not to have a large distance between the phthalocyanine units in adjacent layers as might result from the presence of insulating long aliphatic chains. There have been studies on photoelectric ac characteristic^,^ and dark conductivit9-l0 of (metal/ phthalocyanine LB fildmetall-type sandwich structures. Complexities introduced by asymmetric electrodes, oxide layers, and indeed the problems of nondestructive con-
* Author to whom correspondence should be directed. Abstract published inAduunce ACSAbstructs, October 1,1994. (1)Leznoff, C. C., Lever, A. B. P., Eds. Phthalocyanines-Properties and Applications; VCH Publishers: New York, 1993;Vol. 3. (2)Schultz, H.; Lehmann, H.; Rein, M.; Hanack, M. In Structure and Bonding, 74;Buchler, J. W., Ed.; Springer: Berlin, 1991. (3)Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (4)Yoneygama, M.;Sugi, M.; Saito, M.; Ikegami, K.; Kuroda, S.; Izima, S. Jpn. J . Appl. Phys. 1986,25,961. (5)Hua, Y. J.; Jiang, D. P.; Shu, Z. Y.; Petty, M. C.; Roberts, G. G.; Ahmad, M. M. Thin Solid Films 1990,192,383. (6)(a)Donovan,K J.;Paradiso, R.; Scott, K.; Sudiwala, R. V.; Wilson, E. G.; Bonnett, R.; Wilkins, R. F.; Batzel, D. A.; Clark, T. R.; Kennedy, 253. (b) Donovan, K. J.; Scott, M. E. Thin Solid Films 1992,210/211, K.; Sudiwala, R. V.; Wilson, E. G.; Bonnett, R.; Wilkins, R. F.; Paradiso, R.; Clark, T. R.; Batzel, D. A.; Kenney, M. E. Thin Solid Films 1993, 232,110. (7)Mukhopadhyay,S.;Ray, A. K.; Hogarth, C. A. J . Mater. Sci. (Mater. Electron.) 1990,1, 110. (8)Silinsh, E.A.; Muzikante, I. J.;Tanre, L. F.; Shlihta, G. A. J . Mol. Electron. 1991,7,127. (9)Roberts, G. G.; Petty, M. C.; Baker, S.; Fowler, M. T.; Thomas, N. J. Thin Solid Films 1986,132,113. (10)Shutt, J.D.; Batzel, D. A.; Sudiwala, R. V.; Rickert, S.E.; Kenney, M. E. Langmuir 1988,4,1240. @
tacting must be considered in the development of devices based on such structures. Of course, in general before experiments on charge transport are begun, the film structure should be investigated to a reasonable degree in order to find out whether the structural integrity of the film is sufficient to justify covering it with a metal electrode. The first step toward phthalocyanine-based LB films is the attachment of substituents to the ring system to achieve sufficient solubility in nonpolar solvents. The LB film formation of tetrasubstituted" (as isomer mixtures) and octasubstituted12phthalocyanines has been reported, the latter also in the form ofrigid rod p01ymers.l~Although these substituted molecules are still nonamphiphilic,they were shown to form LB films, albeit with different degrees of order. In the monomer case, the ability to form a monolayer may be regarded as a result of the strong n-n interaction between neighboring rings together with the substituents providingfilm fluidity. The result is a stack of molecules lying on the water subphase with the stack axis in the film plane and each ring parallel or with some angle to the surface normal ("edge-on" configuration). Nevertheless, there is the risk that a (partial) multilayer ("slipped stack" configuration) is formed upon spreading,14 because the molecules do not interact strongly enough with the water subphase. This shows up in values of the limiting area per molecule (obtained from pressure-area isotherms) which are too small compared to the minimal values expected from the molecular dimensions. Furthermore, the limiting area per molecule together with the shape of the isotherm as well as film stability often turns out to be dependent on the central metal ion in an otherwise identical phthalocyanine,15J6 a behavior which (11)Hann, R. A.; Gupta, S. K.; Fryer, J . R.; Eyres, B. L. Thin Solid Films 1985,134,35. (12)Cook, M. J. J . Mater. Sci. (Mater. Electron.) 1994,5, 117. (13)Sauer, T.; Amdt, T.; Batchelder, D. N.; Kalachev, A. A.; Wegner, G . Thin Solid Films 1990.187. 357. (14)Barger, W. R.; Snow, A. W.; Wohltjen, H.; Jarvis, N. Thin Solid Films 1985,133,197. (15)Gobernado-Mitre,M. I.; Aroca, R.; DeSaja, J. A. Langmuir 1993, 9,2185. (16)Hann, R. A. In Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum Press: New York, 1990;p 63.
0 1994 American Chemical Society 0743-7463/94/2410-4265$04.50/0
Burghard et al.
4266 Langmuir, Vol. 10, No. 11, 1994
Scheme 1. Synthetic Route to 2,3,9,10,16,17,23,24-0cta(pentyloxy)-Substituted Metallophthalocyanines 6a-c.
4
P+/w
R= C& 60-C
cannot be explained purely on the basis of molecular shape but might be caused by different tendencies for aggregation. In an alternative approach, an asymmetric phthalocyanine ring system containing both hydrophilic groups and alkyl chain substituents can be created, now representing the classical amphiphilic LB film molecule. In this case, the interaction with the water subphase is enhanced, leading to well-organizedfilms in some cases.17J8 The goal of our research is to find high-quality LB film forming phthalocyanine molecules with only short alkyl side chains for use in solid state devices. In this respect, we wish to report in the following the behavior a t the aidwater interface, and the ability to form LB films, of three different nonamphiphilic 2,3,9,10,16,17,23,24-octa(pentyloxy)-substituted phthalocyanines (6a-c, Scheme 1) containing nickel, palladium, and platinum, respectively. Hence, this investigation covers the metal ions of the nickel triad (group 10)in the periodic table. None of (17)McKeown, N.B.;Cook, M. J.;Thomson,A. J.; Harrison, K. J.; Daniel, M. F.; Richardson, R. M.; Roser, S. J. Thin Solid Films 1988, 159, 469. (18)Cook, M.J.;McKeom, N. B.; Simmons,J. M.; Thomson, A. J.; Daniel, M. F.; Harrison, K. J.;Richarson, R. M.; Roser, S. J. J . Mater. Chem. 1991,I, 121.
M= Ni, Pd, Pt
the three compounds shows any liquid crystalline phase due to their relatively short pentyloxy substituents.
Experimental Section Materials. Scheme 1shows the synthetic route for all three compounds startingwith catechol (1). Refluxing the soluble 1,3diimino-5,6-bis(pentyloxy)-1,3-dihydroisoindolenine (6)in ( N J dimethy1amino)ethanol with the metal salts [Ni(OAc)2*4H20, Pd(acac)z, and PtClz] gave the corresponding metallophthalocyanines 6a-c in good yields. They were characterized by lH, I3C NMR, UVJvis, and IR spectroscopy and further by field desorption mass spectroscopy (FD-MS) and elemental analysis. Synthesis of the nickel and platinum species (f3a,c)has been reported in the l i t e r a t ~ r e .2,3,9,10,16,17,23,24-Octakis~~~~~ ((penty1oxy)phthalocyaninato)palladium was synthesized as follows. A mixture of 1.27 g (4 mmol) of 1,3-diimino-5,6-bis(pentyloxy)-l,3-dihydroisoindolenineand 0.334 g (1.1"01) of Pd(acac)z was refluxed in 6 mL of (NJ-dimethy1amino)ethanol for 6 h. Upon cooling, methanol was added and the precipitate was centrifuged. It was washed with acetone, methanol, and again with acetone. Further purification was achieved by column chromatography (4203; CHC13). Yield: 487 mg (37.2%). El(19)Hanack, M.; GUl, A.; Hirsch, A.; Mandal, B. IC;Subramanian, L. R.; Witke, E. Mol. C y s t . Liq. Cryst. 1990,187, 365. (20) Hanack, M.; Haisch, P.; Lehmann, H.; Subramanian, L. R. Synthesis 1993,387.
Langmuir-Blodgett Film Formation 40
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Langmuir, Vol. 10, No. 11, 1994 4267
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Figure 1. Pressure-area isotherm of palladium phthalocyanine 6b at 10 "C on pure water (curve 1);curve 2 follows upon expansion after compression to 22.5 mN/m. emental analysis calculated for C72H9&08Pd: C, 66.11; H, 7.39; N, 8.56. Found: C, 65.53; H, 7.67; N, 8.46. lH NMR (CDCl3): [ppml 1.14-1.20 (t, 24H, 8 x CH3, J = 7.16 Hz), 1.64-1.73 (m, 16H, 8 x CH2CH3),1.78-1.88 (brm, 16H, 8 x OCH2CH2CH2), 2.15-2.28 (brm, 16H, 8 x OCHZCH~), 4.40-4.50 (brt, 16H, 8 x OCHz), 7.87 (s, 8H, aromatic). 13CNMR [ppmll4.3,23.0,28.8, 29.7, 69.5, 104.8, 129.8, 138.0, 151.0. IR (KBr): [cm-ll 3082, 2957,2932,2860,2363,2338,1610,1512,1462,1420,1389,1360, 1283,1200,1117,1063,847,745. MS (FD): 1307.6 [amu]. W/ v i s (CHCl3): [nm] 663, 636, 598, 418, 321 (sh), 295. Methods. Monolayers at the aidwater interface were prepared by spreading 1.5 mM chloroform (Fluka, for W spectroscopy) solutions onto a Lauda FW1 Langmuir film balance equipped with a FilmliR FL-1. The temperature of the subphase was kept constant at 10 "C for all operations which were done under yellow light conditions (clean room class 1000)in a laminar flow box (class 100). The subphase consisted of pure water (resistivity 18 MS2 cm) purified in a Milli-Q-System (Millipore). A compression speed of 5 A2 molecule-l min-l was chosen for studying the isotherms. Monolayers were transferred at a constant surface pressure of 15 mN/m with a dipping speed of 20 m d m i n for the downstroke and 10 m d m i n for the upstroke (Y-type deposition). All substrates were initially cleaned by boiling in a mixture of H20/H202/NH40H (5/1/1, v/v/v) for 15 min. For W/vis absorption experiments, Suprasil glass slides (12 mm width) were used which were rendered hydrophobic by immersion in a 50% (v/v) chloroform solution of hexamethyldisilazane for 30 min at 40 "C. The optical absorption spectra were recorded with a Perkin Elmer Lambda 2 spectrometer, using a polarizer ranging from 315 to 900 nm for the polarized spectra. As substrates for the FTIR measurements in transmission geometry, Si(111)single crystals were taken. Immediately after the cleaning procedure, they were etched in 25% (w/w) hydrofluoric acid for 1min and rinsed with pure water. For the FTIR experiment in the grazing incidence geometry (84" to the surface normal), we used a 100 nm thick gold film evaporated on a Si(ll1) surface coated with 10 nm of chromium. FTIR spectra were recorded on a Bruker 113v spectrometer equipped with a u;ire-grid polarizer. Small angle X-ray diffractionwas measured with a commercialSiemens D 500 diffractometer in the Bragg-Brentano geometry using the Cu Ka line (2 = 1.54 tf) and a Si(ll1)wafer (treated the same as described above) as substrate.
Results and Discussion Monolayer Behavior. The surface pressure-area isotherm at 10 "C for the palladium compound 6b on pure water is shown in Figure 1(curve 1)as a representative example. Expanding the film after compression to 22.5 mN/m resulted in curve 2. In each further compression/ expansion cycle only curve 2 is followed, demonstrating
4
Figure 2. Molecular model of phthalocyanines 6a-c obtained by the Hyperchem computer program.
some irreversible aggregation effects during first compression. In the case of the nickel (6a)and platinum (6c) species, the isotherms have a similar shape and the same behavior upon repeated compressiodexpansion cycles was found. For all three, the limiting area per molecule turned out to be 90 f4 A2molecule-l. A slight dependency of the area per molecule on the spreading conditions, e.g. the concentration of the spreading solution and the speed of barrier compression, has been observed as it might be expected for molecules prone to stacking interactions. Chloroform solutions with higher concentrations than 1.5 mM should not be used because they give rise to reduced reproducibility. The effective ionic radii of Ni2+,Pd2+ and Pt2+are 0.69,0.86, and 0.80 A,respectively, while fo; the thickness of the ring n-system a value of 3.4 A can be assumed21so that the result of about the same limiting area per molecule for the three phthalocyanines is not unexpected. Figure 2 shows a molecular model of the phthalocyanines calculated with the Hyperchem computer program. If one accepts the given molecular dimensions, the area for a molecule standing perpendicular (edge-on) is calculated to be 25 A x 3.4 A = 85 Hi2. Assuming a tilt angle of arccos(85 A2/90 A2) = 19" with respect to the surface normal leads to the value of 90 Hi2 molecule-l. Such a small tilt angle is in accordance with the observation of in-plane dichroism by UV/vis and FTIR spectroscopy (see below) as this can only be seen if the molecules are not lying flat on the substrate. The main result is that the limiting area per molecule is compatible with the assumption of a true monolayer on the water subphase. Langmuir film stability, as expressed by the value of the collapse pressure, was strongly dependent on the initial water surface area onto which the molecules were spread. For example, for the nickel phthalocyanine (6a), the collapse pressure was determined to be 24 mNlm in the case of an initial area of 240 Hi2 molecule-l, whereas ncollapse = 30 mN/m for the initial area of 600 A2 molecule-l. Compounds 6b and 6c behaved essentially the same. These results clearly reflect aggregation effects directly upon contact with the water surface and correspond to the observation of multimeric structures consisting of phthalocyanine molecules at the aidwater interface by reflection spectroscopy.22The less stable Langmuir films could not be easily deposited onto solid support and yielded inhomogeneous LB films with patches visible to the naked eye. On the other hand, within the concentration limits, the Langmuir layers were transferred a t 10 "C and a surface pressure of ~t = 15 mN/m onto a variety of substrates including silicon, quartz, gold, and silver, resulting in visually uniform LB films. The transfer ratios were 1.0 for the upstroke and slightly smaller for the downstroke (0.95). (21) Simon, J.; Andre, J.-J. Molecular Semiconductors; Springer: Berlin, 1985; Chapter 3. (22)Burack, J. J.; LeGrange, J. D.; Markham, J. L.; Rockward, W. Langmuir 1992,8,613.
Burghard et al.
4268 Langmuir, Vol. 10, No. 11, 1994 I
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As the LB films of all three materials seem to have similar properties, we have chosen the palladium one (6b) for discussion of the structural investigations. Langmuir-Blodgett Films. In Figure 3, the comparison between the solution (3.4pM in chloroform) W/ vis spectrum (solid line), and a 10 monolayer LB film W/ vis spectrum (dashed line) ofthe palladium phthalocyanine 6b is shown. The solution spectrum exhibits the Q-band of a nonaggregated phthalocyanine in the region between 500 and 700 nm.23 The absorptions at d = 663,636,and 598 nm are attributed to the Qo-0, QI-0, and Q2-0 transitions, respectively. In the LB film spectrum, the Q-band is significantly broadened and blue-shifted ( A = 602 nm), while the Soret-bandZ3( A = 285 nm) is almost unaffected. This effect is characteristic of cofacially stacked phthalocyanine units due to excitonic interact i o n ~ . ~Smooth * layer-by-layer growth of the LB film is confirmed by the linear dependence of absorbance on the number of transferred monolayers demonstrated by Figure 4. The corresponding regression lines are plotted for the Soret-band (d = 285 nm) and the Q-band (d = 602 nm). (23) Sayer, P.; Gouterman,M.; Connell, C. R.Acc. Chem. Res. 1982, 15, 73. (24)Fujiki, M.;Tabei, H.; Kurihara, T. J . Phys. Chem. 1988,92, 1281.
Figure 5. Polarized UV/vis spectra of palladium phthalocyanine 6b LB film (10 monolayer) with electrical field vector perpendicular to (dotted curve) and in (solid curve) dipping direction.
Polarized W/vis spectra (Figure 5; 10 monolayer LB film of 6b) with the light beam perpendicular to the LB film plane and the electrical field vector parallel (solid curve) and perpendicular (dotted curve) to the dipping direction indicate an in-plane dichroism of the Q-band, the transition moment of which lies in the plane of the phthalocyanine ring.25 This suggests a flow orientation of the stack axis in dipping direction during film transfer. The value of the ratioA60~~(l)A602~(I I) is calculated to be 2.2. Dichroic ratios on the order of 2 have also been found for other LB systems of discotic molecules.26 However, it is not easy to unravel the contribution of flow effectsduring barrier movement upon film compression and during the dipping process to the observed in-plane orientation. Transferring the monolayers at a higher surface pressure (n= 20 "/m) did not detectably change the dichroic ratio (f0.1), whereas the use of a lower dipping speed of 5 m d m i n for both up- and downstrokes decreased it from 2.2to 1.2.Interestingly, Langmuir layers of6b have also been horizontally2' transferred under the same conditions used for vertical dipping, and in this case, the dichroic ratio was even higher (3.2). Further investigations on the multilayer structure of the LB films were performed by small-angle X-ray diffraction. The small-angle X-ray scattering curve of a 20 monolayer palladium phthalocyanine 6b LB film is shown in Figure 6. Only the first-order Bragg reflexion could be observed at 28 = 4.15",resultingin a layer spacing of d = 21.3 A. The occurrence of well-defined Kiessig fringes is a further support for homogeneous covering of the substrate and a regular film structure.28 The value of d = 21.3 A is in accordance with the molecular dimensions in the model of Figure 2,provided only a small tilt angle of the ring system with respect to the surface normal is assumed. In addition, there might be some interpenetration of the alkoxy side chains of molecules in adjacent layers, which would also reduce the Pd-Pd distance. That the obtained d value corresponds to a monolayer unit cell demonstrates that the small difference (25) VanCott, T. C.; Janna, L. R.; Misener, G. C.;Williamson, B. E.; Schrimpf,A.; Boyle, M. E.; Schatz, P. N. J . Phys. Chem. 1989,93,2999. (26) Ogawa, R;Yonehara, H.; Pac, C.; Maekawa, E. BUZZ. Chem. SOC.Jpn. 1993, 66, 1378. (27) Kawaguchi,T.;Nakahara,H.;Fukuda,K. Thin SolidFilms 1985, 133, 29. (28) Schaub, M.; Wenz, G.; Wegner, G.; Stein, A.; Klemm, D. Adu. Mater. 1993, 5, 919.
Langmuir-Blodgett Film Formation
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Langmuir, Vol. 10, No. 11, 1994 4269
12 monolayer 2 0 monolayer LB-film on Si ( 1 1 1 )
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in the transfer ratios for down- and upstrokes has no discernible influence on the orientation of the phthalocyanine rings in adjacent layers. Polarization-dependent FTIR measurements provided additional evidence for the edge-on configuration of the film molecules as well as an in-plane orientation of the ring systems. The grazing incidence FTIR spectrum of a palladium phthalocyanine 6b LB film consisting of 12 monolayers is shown in Figure 7, while the corresponding transmission spectrum for polarization perpendicular to the dipping direction is given in Figure 8 (12 monolayers on each side ofthe Si(ll1)substrate). When the intensity of, for example, the 1281 cm-l band (vc-0 stretch, aryl etherIz9in the latter spectrum is compared to the intensity of the same band in the transmission spectrum for polarization in the dipping direction (not shown), a dichroic ratio of about 2 is obtained. This agrees well with the results from polarized UV/vis spectra. In contrast to this, the intensities of the symmetrical and asymmetrical CH2 (2863 and 2938 cm-l, respectively) and the symmetrical and asymmetrical CH3 stretch vibrations (2874 and 2959 cm-l, respectively) are almost identical in these two spectra, demonstrating random orientation of the alkoxy side chains in the film plane. The position of the asym(29)Colthup, N. B.;Daly, L. H.; Wiberley, S . E. Introduction to Infrared and Raman Spectroscopy; Academic Press: San Diego, 1990; Chapter 10.
1200
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1500 2800 3000 3200
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Figure 8. Transmission FTIR spectrum of 12 monolayer 6b (on each side of the substrate) LB film for polarization perpendicular to dipping direction.
metric CHZstretching mode is known to be sensitive to changes in the local order.30 It is observed a t a relatively high wavenumber of 2938 cm-l (also compared to the KBr disk, 2932 cm-'), from which again a low degree ofordering of these side chains can be deduced. Vibrational modes with transition dipole moments lying in the phthalocyanine ring plane can be found in the region between 1000 and 1600 cm-l, e.g. the YC-c (pc ring) stretch31 a t 1420 cm-'. In the case of the ring systems lying flat on the substrate, these modes should be observed only in the transmission spectrum (regardless of the polarization direction because of their symmetrical square structure). In the grazing incidence spectrum, according to selectionrules, onlyvibrations with a transition moment perpendicular to the metal surface can be excited.32 Comparison of the grazing incidence and transmission FTIR spectra indicates that these modes occurred in both with similar relative intensities. Consequently, the flat orientation can be ruled out. This is further confirmed by the fact that the intensity of the 745 cm-' band due to the C-H out-of-plane bending of the phthalocyanine ring3is significantly stronger in the transmission spectrum than in the grazing incidence spectrum.
Conclusions The introduction of eight pentyloxy substituents to the peripheral positions of the phthalocyanine ring enabled it to form well-organized LB films in the case of Ni2+, Pd2+, and Pt2+ as central metal ions. The edge-on orientation of these molecules in a true monolayer has been demonstrated. Studies on the incorporation of these films into thin metalized sandwich structures are underway. Acknowledgment. We gratefully acknowledge R. N. Kraemer and Prof. N. Karl, University of Stuttgart, for their help in small-angle X-ray scattering experiments, and we thank J. Rauschnabel for his kind help in this work. This work is supported by Sonderforschungsbereich 329 and the ESPRIT network NEOME. (30) Snyder, R. G.; Strauss, H. L.; Ellinger, C. A. J . Phys. Chem. 1982,86,5145. (31) Fukui,M.;Katayama,N.;Ozaki,Y.;Araki,T.; Iriyama,K.Chem. Phys. Lett. 1991,177,247. (32)Umemura, J.; Kamata, T.;Kawai, T.; Takenaka, T.J . Phys. Chem. 1990,94,62. (33)Nakahara, H.;Fukuda, K.; Kitahara, K.; Nishi, H. Thin Solid Films 1989,178,361.
