Langmuir−Blodgett Layers from Schiff Base Copper(II) Complexes

Langmuir 1997, 13, 4693-4698

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Langmuir-Blodgett Layers from Schiff Base Copper(II) Complexes Ju¨rgen Nagel,† Ulrich Oertel,*,† Peter Friedel,† Hartmut Komber,† and Dietmar Mo¨bius*,‡ Institut fu¨ r Polymerforschung Dresden, Hohe Strasse 6, 01069 Dresden, Germany, and Max-Planck-Institut fu¨ r biophysikalische Chemie, P.O. Box 2841, 37018 Go¨ ttingen, Germany Received December 27, 1996. In Final Form: June 3, 1997X Metal complexes of salicylideneamines exhibit very interesting properties and have been well characterized since the late 1920s. Amphiphilic salicylideneamines have been synthesized in order to introduce such complexes in Langmuir-Blodgett multilayers. They do not hydrolyze under the applied experimental conditions and form stable monolayers. On subphases containing transition metal ions, these compounds form square-planar metal complexes by an interfacial reaction. Monolayers of these complexes have been transferred onto solid substrates to prepare Langmuir-Blodgett multilayers. Spectroscopic measurements with polarized light indicate an orientation of the complex plane nearly parallel to the substrate surface. In addition, monolayer experiments with presynthesized amphiphilic metal complexes have been carried out. Available hydroxyl groups in such complexes may be exploited to construct aggregates due to the ability of the molecules to form intermolecular bonds. Monolayers of these aggregates have also been used for the preparation of multilayers.

1. Introduction The Langmuir-Blodgett (LB) technique of monolayer transfer has often been used for the construction of highly ordered ultrathin films with special properties owing to their supramolecular structure.1,2 Many different functionalities can be introduced in such films. Examples are sensor groups3 or moieties with nonlinear optical properties.4,5 Further, LB layers may be used for modeling of biological or interfacial processes.6,7 Different kinds of amphiphiles are available for the design of such monolayer assemblies. Generally, polymeric amphiphiles are preferred in comparison to low-molecular-weight substances due to their enhanced stability. Metal complexes are compounds with fascinating chemical, optical, electrical, thermal, and electro-optical properties. Therefore, LB layers of some types of metal complexes are well characterized, for example phthalocyanine8-10 and porphyrin11-15 complexes and complexes containing ruthenium16,17 or europium.18 Since the initial works of Schiff19 and Pfeiffer,20 complexes of Schiff bases, for

instance salicylideneamines, have been well characterized, and they exhibit very interesting properties. For example, the low-molecular-weight amphiphile (1,3-bis((2,5-dihydroxybenzylidene)amino)-2-hexadecylpropanyl)cobalt(II) has been used as an oxygen carrier in LB multilayers.21 The aim of this work is to investigate the monolayer behavior of Schiff base metal complexes and the possibilities to construct LB multilayers with these compounds as well as to establish the basis for the application of coordination polymers starting from bifunctional salicylideneamines. Because of their different polarity, 2,5-dihydroxy-Nhexadecylbenzylideneamine (GA16) and N-hexadecylsalicylideneamine (SA16) were used as ligands; see structure 1. Two different ways to form monolayers of

* Correspondence authors. † Institut fu ¨ r Polymerforschung Dresden. ‡ Max-Planck-Institut fu ¨ r biophysikalische Chemie. X Abstract published in Advance ACS Abstracts, July 15, 1997. (1) Kuhn, H. Pure Appl. Chem. 1965, 11, 345. (2) Mo¨bius, D. In Physical Methods of Chemistry, 2nd ed.; Rossiter, B. W., Baetzold, R. C., Eds.; John Wiley & Sons Inc.: New York, 1993; Vol. IXB, p 375. (3) Budach, W.; Ahuja, R. C.; Mo¨bius, D. Langmuir 1993, 9, 3093. (4) Blankenburg, R.; Meller, P.; Ringsdorf, H.; Salesse, C. Biochemistry 1989, 28, 8214. (5) Laschewsky, A.; Ringsdorf, H.; Schmidt, G.; Schneider, J. J. Am. Chem. Soc. 1987, 109, 788. (6) Young, M. C. J.; Tredgold, R. H.; Hodge, P. Spec. Publ.sR. Soc. Chem. 1989, No. 69, pp 354-60. (7) Tredgold, R. H.; Allen, R. A.; Hodge, P.; Khoshdel, E. J. Phys. D.sAppl. Phys. 1987, 20, 1385. (8) Naito, K.; Miura, A.; Azuma, M. Thin Solid Films 1992, 210/211, 527. (9) Lu, A. D.; Jiang, D. P.; Li, Y. O.; Liu, W. N.; Pang, X. M.; Fau, Y.; Chen, W. G.; Li, T. O.; Dong, X. J.; Zhen, Z. K. Thin Solid Films 1992, 210/211, 606. (10) Chai, X. D.; Tian, K.; Chen, H. J.; Tang, X. Y.; Li, T. O.; Zhu, Z. Q.; Motsuo, D. Thin Solid Films 1989, 178, 221. (11) Hopf, F. R.; Mo¨bius, D.; Whitten, D. G. J. Am. Chem. Soc. 1976, 98, 1584. (12) Mo¨bius, D. Mol. Cryst. Liq. Cryst. 1983, 96, 319. (13) Mo¨bius, D. Z. Phys. Chem. N. F. 1987, 154, 121. (14) Loschek, R.; Mo¨bius, D. Chem. Phys. Lett. 1988, 151, 176.

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Schiff base metal complexes have been used: (i) The metal complex was synthesized in solution, isolated, dissolved in toluene, and spread on a pure water subphase. The monolayer prepared by this method will be referred to as a surface film in analogy to ref 22. (ii) A solution of the (15) Sun, L. Y.; Gu, C.; Wen, K.; Chao, X.; Li, T. O.; Hu, G. Y.; Sun, I. Y. Thin Solid Films 1992, 210/211, 486. (16) Poulter, M. W.; Roberts, G. G.; Castello, Z. F.; Davies, S. G.; Edwards, A. J. Thin Solid Films 1992, 210/211, 427. (17) Seefeld, K.-P.; Mo¨bius, D.; Kuhn, H. Helv. Chim. Acta 1977, 60, 2608. (18) Bu¨cher, H.; Drexhage, K. H.; Fleck, M.; Kuhn, H.; Mo¨bius, D.; Scha¨fer, F. P.; Sondermann, J.; Sperling, W.; Tillmann, P.; Wiegand, J. Mol. Cryst. 1967, 2, 199. (19) Schiff, H. Justus Liebigs Ann. Chem. 1869, 150, 193. (20) Pfeiffer, P.; Buchholz, E.; Bauer, O. J. Prakt. Chem. 1931, 129, 163. (21) Fukuda, A.; Hanabusa, K.; Shirai, H. Jpn. Kokai Tokkyo Koho 62258386 (87258386). (22) Samha, H.; DeArmond, M. K. Langmuir 1994, 10, 4157.

© 1997 American Chemical Society

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appropriate ligand in toluene was spread on an aqueous subphase containing metal ions. In this case the complex formation takes place as an interfacial reaction between the ligand monolayer and metal ions in the subphase. Such a monolayer will be described as a subphase film. 2. Experimental Section Materials. Salicylaldehyde and 2,5-dihydroxybenzaldehyde were purchased from Fluka and Aldrich, respectively. Hexadecylamine was a product of Fluka. N-Hexadecylsalicylideneamine (SA16). Hexadecylamine (0.05 mol) was dissolved in 100 mL of ethanol with heating up to the boiling point. Then, 0.05 mol of salicylaldehyde in 100 mL of ethanol was added slowly to the solution of hexadecylamine, so that the mixture was constantly boiling. The solution was colored yellow. The mixture was heated for a half-hour. After cooling to room temperature, a yellow solid was filtered off, recrystallized from ethanol, and dried at 60 °C. The product is soluble in most organic solvents. The NMR spectrum agrees with the proposed structure. Elemental anal. Found (calcd): 79.82 C (79.93), 11.52 H (11.38), 4.06 N (4.05). Mp: 34 °C. Bis(N-hexadecylsalicylideneaminato)copper(II) (Cu(SA16)2). Hexadecylamine (0.05 mol) was dissolved in 100 mL of heated ethanol, and 0.05 mol of salicylaldehyde was added slowly to avoid precipitation. Then, a solution of 0.025 mol of copper(II) acetate and 5 g of sodium acetate in 50 mL of water was added to the boiling mixture. During this addition a solid precipitated. After cooling, the precipitate was filtered off and recrystallized from methanol. The yellow-green powder is soluble in organic solvents, e.g., toluene. Mp: 73 °C. Elemental anal. Found (calcd): 72.44 C (73.39), 9.95 H (10.19), 3.41 N (3.72). 2,5-Dihydroxy-N-hexadecylbenzylideneamine (GA16). The synthesis was performed according to that of SA16. The product is an orange powder. Mp: 96.5 °C. Elemental anal. Found (calcd): 76.76 C (76.39), 11.07 H (10.88), 3.94 N (3.87). The NMR spectrum agrees with the given structure. Bis(2,5-dihydroxy-N-hexadecylbenzylideneaminato)copper(II) (Cu(GA16)2). GA16 (0.1 mol) was dissolved in 200 mL of hot ethanol. A mixture of 0.05 mol of copper(II) acetate and 30 mL of water was added dropwise to the boiling solution. After complete addition, the reaction mixture was refluxed for a halfhour, then cooled to room temperature and filtered. The residue was recrystallized from ethanol. The product forms a brown powder, mp 115 °C. Methods. The water used for the monolayer experiments was purified using a Milli-Q Plus system (Millipore, 18.2 MΩ/ cm). Due to the amount of metal salt added, the pH varies between 5 and 5.5, respectively. The monolayer and deposition experiments were carried out on a Lauda Filmwaage FW 2 (Lauda Dr. Wobser GmbH, Germany) with a Langmuir system and on a KSV 3000 trough with symmetrical compression and a Wilhelmy plate (KSV Instruments, Finland). In most cases, the complexes were prepared directly at the air-water interface by spreading the ligands on subphases containing metal ions (subphase film). Otherwise, the metal complex was spread on a pure water subphase (surface film). The concentration of the spreading solutions was about 1 mg/mL. The isotherms were recorded at 20 °C. The monolayer compression speeds were 76 cm2 min-1 (KSV 3000) and 92 cm2 min-1 (FW 2), respectively. Monolayer depositions from the various subphases were carried out with a dipping speed between 2 and 10 mm/min and with an upper delay of 30 min at 20 °C. The pressure was kept constant at 25 mN/m. For a qualitative discussion of the isotherms, the term collapse area will be used. In this work the collapse area is determined in agreement with a procedure given in ref 23 by extrapolation of the linear part of the isotherm in the solid-analogous region to π ) 0. Transparent supports for spectral investigations were synthetic quartz plates Suprasil 2 (Heraeus Quarzglas GmbH, Germany) and glass plates. The quartz supports were purified (23) Davis, F.; Hodge, P.; Towns, C. R.; Ali-Adib, Z. Macromolecules 1991, 24, 5695.

Nagel et al. in a mixture of H2SO4 and potassium peroxodisulfate, and the glass plates were purified with sulfuric acid containing potassium dichromate. UV/vis spectra were recorded on a Lambda 2 spectrophotometer (Perkin-Elmer), if not stated otherwise. 1H NMR spectra were recorded on a Bruker AMX 300 NMR spectrometer operating at 300.13 MHz. The solvent (DMSO-d6) was used as lock and internal standard (2.50 ppm vs TMS). X-ray investigations under grazing incidence were carried out on a θ/2θ apparatus at the Institute of Physical Chemistry, University of Mainz. The light reflection at the air-water interface is modified by the presence of a monolayer. Reflection spectra are equivalent to absorption spectra and were measured with a fiber optic spectrometer.24,25 The difference in reflectivity ∆R between the monolayer-covered water surface and a clean water surface is plotted vs wavelength. Absorption spectra of monolayer systems on transparent supports were measured with an improved version of the singlebeam instrument described earlier.26 Here, the absorption is given as difference in transmittance ∆T between a reference section of the plate without the absorbing material and of the section with the investigated monolayer system, normalized to the transmittance of the reference section.26 The average orientation of the transition moments is determined by using plane-polarized light under oblique incidence. Molecular Modeling. The calculations were carried out by means of an IBM RISC system 6000 with the operating system AIX 3.2 (IBM Corporation). The optimization of the molecular geometry was performed using classical mechanical methods of CERIUS2 1.5 (Molecular Simulations Inc., Cambridge, Great Britain) with Universal Force Field.27 The calculation of the transition moment was performed by configuration interaction (CI) of the ab initio program package GAMESS28 with the basis set STO-6G. The electronic excitation level was 2, and four empty molecular orbitals (MO) were included.

3. Results and Discussion Molecular Considerations. Transition Moment. The orientation of the GA16 molecule in monolayer systems on the solid substrate has to be determined by polarized light. This requires the knowledge of the direction of the transition moment. This was obtained by molecular calculations of 2,5-dihydroxy-N-ethylbenzylideneamine instead of GA16 because the former is easier to calculate and the length of the alkyl chain has little influence on the transition moment. As shown by the calculation, the transition moment is directed nearly parallel to the CdN bond and within the plane of the benzene and the chelate ring (see structure 2). The normal component is small.

The transition energy was calculated to be 1.6 eV or 773 nm, respectively. Large torsions around the bond between the phenyl C and the azomethine C atoms lead to (24) Gru¨ninger, H.; Mo¨bius, D.; Meyer, H. J. Chem. Phys. 1983, 79, 3701. (25) Orrit, M.; Mo¨bius, D.; Lehmann, U.; Meyer, H. J. Chem. Phys. 1986, 85, 4966. (26) Kuhn, H.; Mo¨bius, D.; Bu¨cher, H. In Physical Methods of Chemistry; Weissberger, A., Rossiter, R., Eds.; John Wiley & Sons Inc.: New York, 1972; Vol. 7, p 577. (27) Rappe, A. K.; Casewit, C. J.; Kolwell, K. S.; Godhard, W. A.; Smith, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (28) Schmidt, M. W. QPCE Bull. 1990, 10, 52.

LB Layers from Schiff Base Cu(II) Complexes

Figure 1. UV/vis spectra of GA16 and Cu(GA16)2: (a) GA16 in ethanol (room temperature, 2 × 10-4 M, absorption spectrum); (b) Cu(GA16)2 in ethanol (2 × 10-4 M, room temperature, prepared by adding an excess of Cu(ac)2 to a solution of GA16, absorption spectrum); (c) monolayer of Cu(GA16)2 on pure water surface, pressure 20 mN/m (reflection spectrum).

considerable changes in the transition energy but not in the direction of the transition moment. As compared with the measured absorption (345 nm, see below) the calculated wavelength is much too large. This may be due to the limitation of the capacity of the computer which requires the use of a low basis set and a low number of free MO in the CI calculation. However, the molecular geometry is certainly correct, and the calculated values of the direction of the transition moment and the energy of the transition may be considered as good estimations. Geometry. The area of the headgroup plane of SA16 is about 0.5 nm2, whereas the value for GA16 is about 0.53 nm2. The length of the straight alkyl chain including the C-N bond is calculated to be 2.1 nm. A computer fitting excluding external influences on the molecule gives an angle between the alkyl chain and the benzene ring of 146°. Assuming that the benzene ring lies parallel to the substrate plane and including a van der Waals radius of 0.12 nm, the double layer thickness for a Y-type LB film is calculated to be 2.59 nm. If aggregates are formed as shown in structure 4 for a ligand dimer, the smallest distance between the planes of two molecules Cu(GA16)2 is about 0.2 nm. Therefore, thickness for a Y-type layer including formation of such aggregates is then about 2.8 nm.

GA16. First, the complex formation of GA16 with Cu2+ and the possibility to form a stable monolayer of this complex at the water surface have been investigated. The UV/vis spectrum (Figure 1, curve a) of GA16 in ethanol solution shows two characteristic transitions, an n-π*transition at 430 nm and a π-π* transition at 345 nm. With addition of an excess of Cu(ac)2 the spectrum of the complex arises with the characteristic complex band around 400 nm (dependent on the solvent, see Figure 1, curve b). The

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Figure 2. Surface pressure-area isotherms of GA16 at subphases containing Cu(ac)2; 25 °C, Lauda FW 2, parameter Cu2+ concentration: (a) pure water; (b) 10-7 M; (c) 10-6 M; (d) 10-5 M; (e) 10-4 M; (f) 10-3 M.

band at this position is composed of a ligand transition and a charge transfer from the ligand to the copper ion.29 Further, a tail in the long-wavelength region is seen, which is assigned to a broad d-d transition of the planar coordinated copper ion. The same spectrum is observed with the isolated Cu(GA16)2 (see structure 1) in a fresh toluene solution. The reflection spectrum of a monolayer of Cu(GA16)2 on pure water (see Figure 1, curve c) shows that the complex is present at the surface and no hydrolysis has occurred during the experiment at a pH of 5.5. Here the complex absorption is detected at 400 nm. In further experiments, the complex formation was carried out as an interfacial reaction between the spread amphiphilic GA16 monolayer and Cu2+ ions in the aqueous subphase. Therefore, surface pressure-area (π/A) isotherms of GA16 on Cu(ac)2 solutions of different concentrations have been recorded (Figure 2). It is possible to obtain π/A isotherms from GA16 on pure water, too. This is remarkable, because the azomethine bond normally hydrolyzes in the presence of water. The hydrolysis results in the formation of the aldehyde and hexadecylamine. Both compounds are not amphiphilic under the applied pH. However, GA16 and SA16 form monolayers, and the characteristic reflection spectra of both of the chelating ligands on water are detectable (see Figure 3). In the 1H NMR spectrum of GA16 one of the phenolic protons is observed at 12.74 ppm. The significant downfield shift of 2.6 ppm in comparison to the parent aldehyde is characteristic for an intramolecular hydrogen bond. Obviously, a six-membered ring is formed including the 2-hydroxyl group and the azomethine nitrogen. The same effect is observed for SA16 (δ(OH) ) 13.67 ppm). This explanation is consistent with infrared spectroscopic investigations of other salicylidenearylamines,30 in which the O-H vibrations form a broad band at 3000 cm-1, indicating a N‚‚‚H‚‚‚O chelation. This may be the reason for the chemical stability of the azomethine bond against hydrolysis in the salicylideneamines under study. From earlier systematic investigations31 of a variety of p-alkylphenols, p-azoalkylphenols, p-alkylanilines, and p-alkylanisoles it is known that these compounds form stable monolayers, if the hydrophobic chain consists of at least 16 carbon atoms. The obtained collapse areas were (29) Patel, M. N.; Patil, S. H. J. Macromol. Sci. 1981, A16, 1429. (30) Poddar, B. K. Ph.D. Thesis 1965, TH Hannover. (31) Giles, C. H.; Neustadter, E. L. J. Chem. Soc. 1952, 918, 3806.

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Figure 3. Reflection spectra of monolayers of the ligands on the water surface: normal incidence, compressed to the solidanalogous state (SA16, A ) 0.38 nm2; GA16, A ) 0.23 nm2).

nearly 0.24 nm2 in all cases with only little effects of the substituents at the aromatic ring. With X-ray scattering measurements of stacks of benzene, the dimensions of the molecule were determined.32 From these values was calculated an area for the plane of the side view of benzene of about 0.24 nm2. Therefore, an orientation of the headgroup of the alkylphenols normal to the water surface was proposed. From the π/A isotherm of GA16 (Figure 2) is obtained a collapse area of 0.23 nm2, which suggests a similar arrangement as in the case of the p-alkylphenols. However, for GA16 another orientation is conceivable in which the phenyl ring is oriented parallel to the water surface and two molecules form a sandwich (see structure 3). Spectroscopic studies have been carried out to elucidate this question. The isotherms of GA16 are dependent on the concentration of copper(II) ions in the subphase (Figure 2). Due to the interfacial reaction between the GA16 with Cu2+ the isotherms are shifted toward higher areas. With this result it is evident that a structural change of the GA16 molecule has occurred due to the complex formation. The maximum collapse area of 0.39 nm2 is observed at 10-5 M. Further increase of the copper concentration leads to a small decrease of the collapse area. Assuming the formation of a 2:1 complex, Cu(GA16)2, the collapse area of the complex is about 0.78 nm2. An area of 0.95 nm2 is expected for an arrangement in which the plane of the complex is oriented parallel to the water surface. Therefore, it may be supposed that the complex is oriented not exactly parallel to the water surface. An unexpected behavior is observed for the presynthesized complex Cu(GA16)2. The complex was dissolved in toluene and stored for a few days at room temperature prior to the isotherm measurement. Surprisingly, a collapse area of 0.41 nm2 per complex was observed; see curve a in Figure 4 (please note: in contrast to Figure 2 the area/molecule was calculated here for the complex and not for a single ligand molecule). The compressed monolayer is stable for many hours. Considering the complex stoichiometry, this value is about the half of the area observed in the experiment with a subphase film of Cu(GA16)2 (see Figure 2, curve e, 0.78 nm2). For comparison, a freshly prepared solution of Cu(GA16)2 was used. In contrast to the results with the aged solution a (32) (a) Adam, N. K. Proc. R. Soc. London, Ser. A 1928, 119, 628. (b) Adam, N. K. Proc. R. Soc. London, Ser. A 1923, 103, 676.

Nagel et al.

Figure 4. Pressure-area isotherms of Cu(GA16)2 on pure water; 20 °C, Lauda FW 2, in dependence on the history of the spreading toluene solution: (a) stored for 2 days at room temperature; (b) applied immediately after preparation.

Figure 5. UV/vis spectra of Cu(GA16)2 in toluene solution in dependence on the time after preparation; [Cu(GA16)2] ) 10-4 M. The time is (1) immediately after preparation and after (2) 6 h, (3) 24 h, (4) 47 h, and (5) 170 h.

collapse area of 0.83 nm2 was observed (Figure 4, curve b). This corresponds to the value of 0.95 nm2 estimated from the simple molecular model. To get additional information, the UV/vis spectrum of the Cu(GA16)2 in toluene was measured (Figure 5) with increasing age of the solution: The intensity of the characteristic peak at 400 nm decreases with time, and a broad unspecific absorption (tail) at higher wavelengths arises. Two isosbestic points are detectable, which means that an equilibrium between two forms is shifted. Within 7 days the reaction is completed. The spectral change as well as the differences in the isotherms may be attributed to an aggregation process. In the complex Cu(GA16)2 two hydroxyl groups are available which may be the reason for the aggregation process. An explanation of the results may be given in the following way: If GA16 is spread at the surface of a subphase containing Cu2+ ions, the amphiphiles react with copper ions to the complex Cu(GA16)2. The observed maximum value for the molecular area is 0.78 nm2, which is significantly smaller than the value of 0.95 nm2 estimated from a simple model. The deviation of the experimental value from the

LB Layers from Schiff Base Cu(II) Complexes

Figure 6. Surface pressure-area isotherms of SA16 at subphases containing Cu(ac)2 and of Cu(SA16)2 on pure water; 25 °C, Lauda FW 2, parameter Cu2+ concentration: (a) pure water; (b) 10-6 M; (c) 10-2 M; (d) surface film of Cu(SA16)2.

calculated one and the small decrease of the collapse area at Cu2+ concentrations higher than 10-5 M may be attributed to the reaction of Cu(GA16)2 with surplus copper or the competition of Cu2+ for GA16 under the possible formation of Cu(GA16)1+ in both cases. These processes may be accompanied by orientational changes. A solution of Cu(GA16)2 (prepared in a polar medium) in toluene contains in the beginning monomeric Cu(GA16)2. Due to the less polar nature of the solvent an aggregation process of Cu(GA16)2 takes place in toluene solution. After the process has been finished, a collapse area of one-half of the initial value is observed. We assume that aggregates as shown in structure 4 have been formed. In these aggregates the molecules overlap each other partly. The reason for that aggregation may be coordinative bonds to a copper ion or hydrogen bonds. SA16. To examine the hypothesis above, the behavior of the amphiphilic ligand SA16 without the 5-hydroxyl groups was exploited. The isotherms of SA16 (Figure 6) exhibit a low collapse pressure of about 10 mN/m. Compressed monolayers are not stable enough for a monolayer transfer. The collapse area is about 0.5 nm2 on a pure water subphase, which is higher than in the case of GA16. In the presence of Cu2+ ions subphase monolayers with collapse areas of about 0.45 nm are formed. With respect to the 2:1 stoichiometry the collapse area of the complex is about 0.9 nm2, in agreement with the value estimated from the molecular model. The π/A isotherms for Cu2+ concentrations between 10-7 and 10-2 M are nearly independent of the Cu2+ concentration in the subphase. However, the color change due to complex formation in the monolayer is visible with the naked eye. As in the former experiments, a presynthesized Cu(SA16)2 has been spread from a toluene solution to prepare a surface film. Similar to the results with freshly prepared solutions of Cu(GA16)2, a collapse area of 0.9 nm2 is observed. This is the same value as determined from the experiments using the interfacial reaction between Cu2+ and SA16. In contrast to the result with Cu(GA16)2, the age of the spreading solution has no influence on the isotherm. Obviously, both the subphase film and the surface film have the same molecular structure. No aggregate formation is observed for the surface film of Cu(SA16)2. Consequently, the phenolic hydroxyl group must be responsible for the aggregation process in the case of Cu(GA16)2, supporting the considerations above.

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Figure 7. Transmission spectra of a GA16 monolayer on a glass support: incidence angle ) 45°; parallel (p-pol.) and perpendicular (s-pol.) polarization.

Orientation of the Headgroups on Transferred Monolayers. The collapse area taken from the isotherm gives some clues concerning the orientation of the complex. With the additional information derived from the molecular model it seems reasonable that the complex of Cu2+ with SA16 is oriented parallel to the water surface. This is valid for the surface film as well as for the subphase film. In the case of GA16 and Cu(GA16)2 additional information is required. Therefore, monolayers were transferred onto a glass plate and investigated with polarized light at an incidence angle of 45°. The ratio of the transmission ∆T measured with s- and p-polarized light, respectively, leads to an orientation parameter P, from which the average angle between the transition moment of the chromophore and the surface normal of the support is calculated. The method is described in detail in ref 25. The ∆T spectra for GA16 are shown in Figure 7, whereby

∆T ) 1 - (T(glass with monolayer)/T(glass)) The spectra are similar to the reflection spectrum on the water surface; see Figure 3. From the ratio ∆Tp:∆Ts at 350 nm the orientation parameter is obtained to be P ) 0.05. This value corresponds to an angle of about 77° between the transition moment and the surface normal. Because the transition moment is in the plane of the benzene ring and in the direction of the main axis of the molecule (see structure 2), this result supports the assumption of an orientation of GA16 nearly parallel to the water surface. Probably, two molecules GA16 form a sandwich structure as proposed in structure 3. Then, the collapse area may be estimated to be 0.25 nm2. This value agrees well with that derived from the isotherm measurements. LB Layers of Cu(GA16)2. There are some differences between the collapse areas for surface and subphase films of Cu(GA16)2 and those estimated from molecular models. Therefore, a transferred monolayer of Cu(GA16) 2 was prepared starting from a subphase film. With the ratio of ∆Ts and ∆Tp of the spectra in Figure 8 the orientation parameter is estimated to be P ) 0.1. The calculated angle to the surface normal is then about 70°. Therefore, this complex has also an average orientation nearly parallel to the substrate plane. In the case of the surface film an average angle of 75° was determined. Multilayers of Cu(GA16)2 complexes can be built up by starting with a subphase film as well as by starting with

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Figure 8. Transmission spectra of a monolayer of Cu(GA16)2 as a subphase film on glass: incidence angle ) 45°; parallel (p-pol.) and perpendicular (s-pol.) polarization.

Nagel et al.

Figure 10. UV/vis spectra of Y-type multilayers of Cu(GA16)2 on a quartz support starting from a surface or a subphase film; 20 °C, pH 5.5, 6 monolayers: (a) surface film of an aged solution; (b) subphase film, [Cu2+] ) 10-3 M.

Small-angle X-ray scattering measurements of Cu(GA16)2 as surface film, spread from an aged toluene solution, yield a scattering layer distance of 2.66 nm for a typical Y-type coating. This value agrees quite well with the calculated value of 2.8 nm (see discussion in the molecular considerations part). The experimental value indicates that the average tilt angle is smaller than 34° as calculated for the ligand dimer. 4. Conclusions

Figure 9. Absorbance of surface films of Cu(GA16)2 from aged toluene solution in relation to the layer number.

a surface film. The complexes form stable monolayers, and the average transfer ratio is 1 for the surface and 0.9 for the subphase film at transfer speeds between 2 and 10 mm/min. As shown in Figure 9 the absorbance of the multilayer at the complex band at 400 nm is proportional to the layer number, indicating that the transfer is reproducible. Therefore, the multilayer absorption is suitable for the following quantitative considerations. Figure 10 shows the absorption spectra of two Y-type multilayers of Cu(GA16)2. In the first case a surface film (prepared with an aged spreading solution of Cu(GA16)2 in toluene, curve a) was transferred, and in the second case, a subphase film. In both cases the characteristic complex spectrum is visible. But though the layer number is the same in both experiments, the absorption intensity of the surface film at 400 nm is higher by a factor of 1.8. This result supports the former assumption of dimer formation of the complex in toluene, as shown in structure 4. The fact that we do not observe a ratio of 2.0 may be explained by a decreased absorbance at 400 nm due to the aggregation process, as shown above for the experiment in solution.

We have shown that amphiphilic salicylideneamines do not hydrolyze in contact with water under the conditions used and form stable monolayers. The monolayer of SA16 is not stable enough for preparing LB layers. Substitution with a hydroxyl group of the salicylidene component leads to the formation of dimers and causes an increased stability of the monolayer at the water surface. The reason may be hydrogen bonding via the hydroxyl groups. Both compounds, SA16 and GA16, react in monolayers on the water surface with copper ions dissolved in the aqueous subphase under formation of the corresponding complexes. An alternative method for preparing monolayers of the complexes is the use of presynthesized metal complexes. For Cu(SA16)2 and freshly prepared solutions of Cu(GA16)2 both procedures lead to the same monolayer of monomeric complexes. In contrast, monolayers prepared with an aged solution of Cu(GA16)2 exhibit a reduced collapse area, indicating the formation of aggregates due to intermolecular coordinative or hydrogen bonds. The compressed monolayer may be transferred with transfer ratios close to 1. Acknowledgment. Financial support by the Deutsche Forschungsgesellschaft is gratefully acknowledged. We thank Mrs. U. Ho¨hnel (Mainz) for the X-ray measurements. For kindly support and helpful discussions we thank Dr. R. C. Ahuja (Go¨ttingen) and Prof. H. Ringsdorf (Mainz). For a lot of experimental work we thank Mrs. B. Pilch and Mrs. B. Ha¨nel. LA9621269