Langmuir 1997, 13, 4923-4925
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Notes Langmuir-Blodgett Films of Schiff Base Complexes of Copper(II) S. Shyamala Sundari, Aruna Dhathathreyan, M. Kanthimathi, and Balachandran Unni Nair* Chemical Laboratory, Central Leather Research Institute, Madras 600 020, India Received February 6, 1997. In Final Form: June 13, 1997
Introduction Recently the deposition of metal complexes as ordered ultrathin films by the Langmuir-Blodgett method has gained attention1-3 because of their possible application as biosensors and molecular devices.4-6 The valuable properties of metallophthalocyanins and metalloporphyrins including optical and electrical conductivity, spectroscopy, thermal and chemical stability, and diverse chemistry have led to a significant interest in their application in optical and microelectronic devices.7,8 The transfer of Langmuir films of functionalized metals onto solid support by the LB transfer technique may lead to chemically sensitive surfaces for potential analytical applications.9 Recently transition metal organometallic and coordination complexes have emerged as building blocks for second-order nonlinear optical materials.10 Schiff base ligands form stable complexes with a variety of transition metal ions in diverse oxidation states. As a result these complexes show interesting magnetic, spectroscopic, and catalytic properties. It is rather surprising that not much attention has been paid in investigating their thin film-forming abilities. Properties of thin films of Schiff base complexes can be changed at will by changing the metal ion and by changing the oxidation state of a particular metal ion. We have recently synthesized manganese(III) and chromium(III) complexes of a long chain Schiff base ligand.11,12 Long chain Schiff base ligands are particularly attractive for casting ultrathin films because of their increased hydrophobicities compared to those of the common short chain Schiff base ligands. In this Note we describe the monolayer-forming characteristics of the copper(II) complex of the Schiff base L. The surface pressure-molecular area (π-A) isotherm has been determined. The UV-vis spectra of LangmuirBlodgett films of the compound transferred onto a quartz substrate have been compared with the solution spectra. (1) Qian, D. J.; Nakahara, H.; Fukuda, K.; Yang, K. Z. Langmuir 1995, 11, 4491. (2) Liu, Y. Q.; Shigehara, K.; Hara, M.; Yamada, A. J. Am. Chem. Soc. 1991, 113, 440. (3) Zhou, D. J.; Huang, C. H.; Wang, K. A.; Xu, G. H.; Zhao, X. S.; Xie, X. M.; Xu, L. G.; Li, T. K. Langmuir 1994, 10, 1910. (4) Facci, P.; Erokhin,V.; Tronin, A.; Nicoteni, C. J. Phys. Chem. 1994, 98, 13323. (5) Dubrovsky, T.; Tronin, A.; Nicoline, C. Thin Solid Films 1995, 257, 130. (6) Nicolini, C. Biosens. Bioelectron 1995, 10, 105. (7) Wang, H. Y.; Mann, J. Y., Jr.; Lando, J. B.; Clark, T. R.; Eenney, M. E. Langmuir 1995, 11, 4549. (8) Azumi, R.; Matsumoto, M. Langmuir 1995, 11, 4495. (9) Parazak, D. P.; Khan, A. R.; D’Souza, V. T.; Stine, K. J. Langmuir 1996, 12, 4046. (10) Burland, D. M. Chem. Rev. 1994, 94, 1. (11) Rajan, R.; Nair, B. U. Ind. J. Chem. 1996, 35A, 233. (12) Kanthimathi, M.; Nair, B. U. Inorg. Chim. Acta Submitted.
S0743-7463(97)00125-X CCC: $14.00
(CH2)n N O–
N –O
L (n = 2, salen; 6, salhex)
Experimental Methods Compound Preparation. The copper(II) complex of L (n ) 2, salen) was prepared by the literature method.13 The Schiff base L (n ) 6, salhex) was prepared as described previously.11 The copper(II) complex of this ligand was prepared by reacting copper(II) acetate with the ligand in methanol. The analytical (Found: C, 61.82; H, 5.94; N, 7.53; Cu, 16.14. Calcd for CuC20H22N2O2: C, 62.25; H, 5.71; N, 7.26; Cu, 16.47) and spectroscopic results show that the isolated product is a monomeric copper(II) complex of salhex. Surface Pressure Isotherm. Surface pressure (π) versus area per molecule (A) isotherms were measured at 293 K by using a Nima (Model 611) Langmuir-Blodgett trough. Surfaces were prepared from triply distilled water. The monolayers were spread from solutions in chloroform. A compression rate of 0.002 nm2/(molecule min) was used for all the experiments Langmuir-Blodgett Film Preparation. LangmuirBlodgett films were prepared using a Nima trough, Model 611. The LB films (showing Y type transfer) were transferred at π ) 15 mN/m. The transfer ratio was around 0.9. The substrates used for deposition were 40 mm × 22 mm microscope cover glasses cleaned with chromic acid, sonicated in 1 M NaOH in triply distilled water, and then rinsed with triply distilled water. Electronic spectral measurements for LB films were carried out using a model 160 A Shimadzu spectrophotometer. Second-Harmonic Generation (SHG) Experiments. The SHG experiments were performed with an Nd:YAG laser (l ) 1064 nm) having a 66 mJ/10 ns pulse at 10 Hz and a focused spot size of 0.15 cm (approximately 370 mW/cm2). All detectable second-harmonic light (532 nm) was filtered from the fundamental beam. SHG from the sample was detected with an energy meter having a 1 pJ sensitivity. A box car and an averaging digital oscilloscope were used to improve the poor signal to noise ratio for the measurements. The background SHG from the glass surfaces was subtracted from the total SHG observed, and the SHG from the LB film was referenced to the SHG from a Y-cut quartz crystal. The LB films of Cu(salhex) were transferred to quartz glass slides for the above measurement in Fresnel geometry or in total internal reflection geometry in order to raise the signal level.14
Results and Discussion The monolayer-forming capability of Cu(salen) was investigated, and it was found that it does not form a monolayer on the air-water interface. Recently, mixed layers of ruthenium complexes and stearic acid were studied for their interesting optical properties in LB films.15 Hence, the π-A isotherm of Cu(salen) mixed with stearic acid (1:1 molar ratio) was investigated. The copper(II) complex of the Schiff base ligand salhex on the other hand formed a stable monolayer. The π-A isotherms of the Cu(salen)-stearic acid and Cu(salhex) are shown in Figures 1 and 2, respectively. As evident from Figure 1, the collapse pressure of the Cu(salen)-stearic (13) Pubsky, J. V.; Sokol, A. Collect. Czech. Chem. Commun. 1931, 3, 548. (14) Marowsky, G.; Gierulski, A.; Reider, G. A.; Schmidt, A. J. Appl. Phys. 1984, B34, 69. (15) Balasubramanian K. K.; Cammarata, V.; Wu, Q. Langmuir 1995, 11, 1658.
© 1997 American Chemical Society
4924 Langmuir, Vol. 13, No. 18, 1997
Figure 1. Surface area-pressure isotherm for Cu(salen)stearic acid: compression rate, 0.002 nm2/(molecule min); T, 293 K.
Notes
Figure 3. UV-vis spectra of (a, top) chloroform solution of Cu(salhex) and (b, bottom) Cu(salhex) film transferred on a quartz plate (30 layers). Table 1. SHG Observed with a Cu(salhex) Film no. of layers
type of layers
SHG (% of quartz)
film age (h)
15 20 40
Y Y Z
0.02 0.08 0.09
72 48 12
acid monolayer is in the region of 55.5 mN/m, and the π-A isotherm at the air-water interface shows three phase changes, as expected for homogeneous films. The high collapse pressure in this case is purely due to the stearic acid component. In recent studies on interactions of bivalent cations with octadecanoic acid, it has been shown that the ions, particularly Cu2+ and Zn2+, have a condensing effect on the π-A curves of the fatty acids.16 But since the Schiff base ligand employed here to complex the Cu2+ ion is a tetradentate chelate, no free Cu2+ ions are expected to be present in the stearic acid-salen mixed system. Hence, the slight compression observed in the π-A curve compared to that of stearic acid can only be due to Cu(salen). Assuming the area of stearic acid as 0.20 nm2, the area of Cu(salen) has been estimated from the π-A isotherm as 0.21 ( 0.01 nm2, which is in good agreement with the computed area of 0.25 nm2 based on the square planar geometry of the Cu(salen) complex. The transfer ratio for the Cu(salen)-stearic acid system in the case of LB film formation was around 0.40 and was very inhomogeneous. Hence, no attempts were made to transfer these mixed films. The π-A isotherm of the Cu(salhex) complex is markedly different from that of the
Cu(salen)-stearic acid system. It shows a highly steep isotherm in the liquid-expanded state. The gas-liquid coexistence region is very short in Cu(salhex) compared to the Cu(salen)-stearic acid system. The collapse pressure of Cu(salhex) is only 28 mN/m. The area estimated from the π-A isotherm of Cu(salhex) is 0.21 ( 0.02 nm2. The molecular model predicts the smallest projected mean molecular area to be 0.249 nm2. It can be seen from Figure 2 that an increase in surface pressure is observed at less than 0.21 nm2 and that collapse is found after compression to about 0.11 nm2. Such behavior has previously been reported for stilbene derivatives and other aromatic molecules.17,18 One possible reason for such behavior is that in Schiff bases like salhex the packing arrangement may be dominated by the planar π systems. When these films are compressed and held at constant (π ) 15 mN/m), the film area decreases less than 5% in 1 h which shows that the films are stable at this surface pressure. The film formed by Cu(salhex) could be transferred to a quartz plate with a transfer ratio of 0.85. The UV-vis spectrum of the transferred film (30 layers, Y-type) as well as Cu(salhex) in solution (CHCl3-hexane 4:1) is shown in Figure 3. Between 240 are 600 nm three intense bands are observed for both the LB film and the Cu(II) complex in solution. The solution spectra also show the weak d-d transition at 643 nm (not shown in Figure 3). The optical transition at 366 nm in solution is a charge transfer band, and this has been used for a nonlinear optics (NLO) study. The red shift corresponding to the low-energy band at 366 nm in solution and 374 nm in the LB film may arise due to H-aggregate formation in the film. In the Langmuir film, spreading, reorganization, and compression may lead to such aggregates, especially in the case of rigid structures. This clearly demonstrates that copper(II) complexes of long chain Schiff bases are capable not only of forming stable Langmuir-Blodgett films but also of retaining their optical properties in the film.
(16) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1996, 12, 1027 and references therein.
(17) Shnidman, Y.; Ulman, A.; Eilers, J. E. Langmuir 1993, 9, 1071. (18) Ulman, A.; Scaninge, R. P. Langmuir 1992, 8, 894.
Figure 2. Surface area-pressure isotherm for Cu(salhex).
Notes
Figure 4. Dependence of SHG intensity on the film thickness.
Second-harmonic generation (SHG) in organic systems is governed by the conditions of a non-centrosymmetric system.19 Therefore studies incorporating surface effects have gained in scientific interest for SHG. Compounds without long alkyl chains such as rigid rod oligo-pcarboranes and diphosphonic acids have been known to organize in Langmuir films and form stable multilayers.20,21 Second-harmonic generation in Cu(salhex) (19) Shen, Y. R. The Principles of Non-Linear Optics; Wiley: New York, 1984.
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films has been investigated in this study. The results of the SHG experiments are shown in Table 1. Although SHG was observed with all freshly prepared multilayer films, the SHG intensity was not stable over a period of days. Figure 4 shows the variation of second-harmonic signal intensity with the film thickness. The square law (SHG intensity should increase as the square of the film thickness) is not strictly obeyed, indicating that the directional alignment of the molecules is not entirely reproducible from one layer to the next. However, the upward trend of the signal as the thickness increases shows that a reasonable extent of polar alignment is maintained throughout the layers. The observation that the Cu(II) complex of the long chain Schiff base ligand salhex forms stable LB films is of significance because it opens up a lot of possibilities for synthesizing longer chain Schiff base ligands and their metal complexes with different optical and magnetic properties, which may show improved film characteristics. Acknowledgment. We acknowledge the help of Ms. K. Deepa in analytical work and of Mr. K. Venkatesh in the computation work. LA970125Q (20) Muller, J.; Base, K.; Magner, T. F.; Mich, J. J. Am. Chem. Soc. 1992, 114, 9721. (21) Katz, H. E.; Schullin, M. L.; Chidsey, C. E. D.; Putvinski, T. H.; Hutton R. S. Chem. Mater. 1991, 3, 699.
