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Spontaneous Formation of Vesicles and Monolayer Membranes in Organic Solvents by Long-Chained Bisschiff Base or Its Organometallic Complexes
Scheme 1
Xuefen Wang, Yingzhong Shen, Yi Pan, andYingqiu Liang* Institute of Mesocopic Solid State Chemistry and Department of Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China
Conventionally, molecular design of “amphiphiles” was carried out by assuming the use of “water” as the solvent. This is because the hydrophobic force is considered to be indispensable for efficient assembly of amphiphiles. If the general molecular design principles of aqueous vesicle membranes could be modified so as to encompass aprotic and or nonaqueous media, a novel rich field of molecular assembly would emerge. Actually, some works about vesicles formation in nonaqueous polar solvent systems have been reported, which are mainly on some phospholipids1,2 and other double-chained amphiphiles.3 On the other hand, bilayer vesicles may be formed in aprotic, organic media either as reversed bilayer4,5 or as fluorocarbon bilayers,6-9 such as from Ca2+-complexed phosphate amphiphiles,4 peptide-lipids,5 and fluorocarbon amphiphiles7-9 comprised of solvophilic and solvophobic moieties. In addition, Weiss et al.10,11 found a family of gelators of organic fluids whose aggregate structures rely upon weak attractive intermolecular interactions (dipolar and van der Waals forces). Recently, Ihara and Hanabusa et al.12,13 reported that some lipid analogues without special solvophobic parts such as perfluoroalkyl groups but with several amide groups per molecule can form regular, stable gel assemblies in aromatic media. Organoaluminum, gallium, and indium complexes have been under investigation for many years due to their rich structural information and wide applications in material science.14,15 And the complex with flexible alkyl chain substituted may have good processibility and mechanical properties. As an extension of these studies, here, we wish to report novel vesicles and monolayer membranes formed * Corresponding author. (1) Mcintosh, T. J.; Magid, A. D.; Simon, S. A. Biochemistry 1989, 28, 7. (2) Mcdaniel, R. V.; Mcintosh, T. J.; Simon, S. A. Biochim. Biophys. Acta 1983, 731, 97. (3) Kimizuka, N.; Wakiyama, T.; Miyauchi, H.; Yoshimi, T.; Kunitake, T. J. Am. Chem. Soc. 1996, 118, 5808. (4) Kim, J. M.; Kunitake, T. Chem. Lett. 1989, 959. (5) Norihiro, Y.; Katsuhiko, A.; Masanobu, N.; Kazuhiro, M.; Emiko, K. J. Am. Chem. Soc. 1998, 120, 12192. (6) Ishikawa, Y.; Kuwahara, H.; Kunitake, T. J. Am. Chem. Soc. 1994, 116, 5579. (7) Ishikawa, Y.; Kuwahara, H.; Kunitake, T. J. Am. Chem. Soc. 1989, 111, 8530. (8) Ishikawa, Y.; Kuwahara, H.; Kunitake, T. Chem. Lett. 1989, 1737. (9) Kuwahara, H.; Hamada, M.; Kunitake, T. J. Am. Chem. Soc. 1993, 115, 3002. (10) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (11) Furman, I.; Weiss, R. G. Langmuir 1993, 9, 2084. (12) Hanabusa, K.; Tanaka, R.; Suzuki, M.; Kimura, M.; Shirai, H. Adv. Mater. 1997, 9, 1095. (13) Ihara, H.; Hachisako, H.; Hirayama, C.; Yamada, K. J. Chem. Soc., Chem. Commun. 1992, 1244. (14) Oliver, J. P.; Kumar, R. Polyhedron 1990, 9, 409. (15) Cowley, A. H.; Jones, R. A. Angew. Chem., Int. Ed. Engl. 1989, 28, 1208.
in organic solvents spontaneously from long-chained bisschiff base L or its organometallic complexes L(MR2)2. Ligand L was synthesized by condensation of 4-dodecyloxysalicylaldehyde and ethylenediamine. The isolated pale yellow solid was purified by recrystallization from C2H5OH/CH2Cl2 and dried in a vacuum. FT-IR(KBr): 3455, 2956, 2920, 2850, 1628, 1574, 1473, 1229, 1172, 1146 cm-1. Anal. Calcd for C44H66N2O4: C, 75.53; H, 9.78; N, 4.70. Found: C, 75.47; H, 10.06; N, 4.50. The reaction of Me3M (M ) Ga, In) with the ligand L proceeded smoothly in benzene in a 3:1 ratio affording the corresponding 2:1 complexes16 as shown in Scheme 1. The reactions were performed in a glovebox under purified nitrogen. The complexes were isolated as white solids in high yields. IR and MS spectroscopies confirmed the proposed structure. Anal. Calcd for L(GaMe2)2: C, 63.33; H, 8.94; N, 3.36. Found: C, 63.19; H, 8.87; N, 3.25. Anal. Calcd for L(InMe2)2: C, 57.15; H, 8.07; N, 3.03. Found: C, 57.07; H, 7.99; N, 3.29. Although gallium and indium alkyls are extremely moisture and oxygen sensitive, the complexes obtained are fairly stable on exposure to air. Free ligand L and its complex L(MR2)2 without special solvophobic parts but with two Schiff-base groups as the rigid segments a molecule can be self-assembled in low polarity organic solvents, such as chloroform, benzene, toluene, and so on. They are readily dispersed in these solvents by sonication to give transparent or translucent solution. These dispersions were viscous and prone to foam by shaking, indicating the surface-active properties of these compounds. The fine structure of the aggregates can be elucidated most directly by electron microscopy (JEOL Model JEM-200CX). The transmission electron micrographs of free and complexed L in CHCl3 are illustrated in Figure 1. The dispersion of ligand L displays typical vesicular morphologies (Figure 1A). Complex L(GaMe2)2 in CHCl3 exhibits linearly connected vesicles. Suh17 found similar aggregates formed by coordinatively polymerized amphiphiles in formamide. As to complex L(InMe2)2, the formed aggregates show hexagonally perforated membrane structure. Eisenberg18 recently observed the formation of the perforated bilayers by block copolymers in dilute solution. Furthermore, a hexagonally perforated layered phase has been observed in block copolymers in bulk,19 and a randomly perforated lamellar (16) Chong, K. S.; Rettig, S. J.; Storr, A.; Trotter, J. Can. J. Chem. 1977, 55, 2540. (17) Suh, J.; Shim, H.; Shin, S. Langmuir 1996, 12, 2323. (18) Zhang, L.; Eisenberg, A. Macromolecules 1999, 32, 2239. (19) Hamley, I. W.; Koppi, K.; Rosedale, J. H.; Bates, F. S.; Almdal, K.; Mortensen, K. Macromolecules 1996, 29, 4764.
10.1021/la0001427 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/16/2000
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Figure 2. 1H NMR spectrum of complex L(InMe2)2 in CDCl3.
Figure 3. Schematic illustration of monolayer structure in organic solvent.
Figure 1. Transmission electron micrographs of assemblies of free or complexed L in CHCl3: (A) L (poststained by 2% ethanol uranyl acetate); (B) L(GaMe2)2; (C) L(InMe2)2 (without stain).
phase has been observed in lyotropics.20 We believe that our perforated structure in solution is morphologically similar to those formed in other systems. In addition, vesicles or other ordered aggregates also can be observed especially in their benzene or toluene solutions by electron microscopy. The cast film from chloroform solution was examined by small-angle X-ray diffraction (Rigaku model D/maxRA). The diffraction pattern displayed periodic peaks (2θ values) at 3.39, 6.77, 10.16, and 13.55 for L and 3.88, 7.79, 11.72, and 15.67 for L(GaMe2)2. It is clear that regular multimonolayers parallel the film plane. NMR spectroscopy is a powerful nondestructive technique for studying membrane systems,21,22 and the (20) Funari, S. S.; Holmes, M. C.; Tiddy, G. J. T. J. Phys. Chem. 1994, 98, 3015.
restricted molecular motion in synthetic ordered lipid membranes is readily reflected by the line broadening phenomenon in the 1H NMR spectra.23-25 The 1H NMR spectra of L and L(MR2)2 in CDCl3 show a broadening to some degree. As an example, the 1H NMR (Avance 300) spectrum of L(InMe2)2 in CDCl3 is shown in Figure 2. As to the middle rigid segment, the signals of Ga-methyl (-0.5 to 0.5 ppm) and N-methylene (5 ppm or so) protons are almost indiscernible, and those of two salicylideneaniline (6-9 ppm) are broad. In contrast, the peaks attributed to protons of the alkyl chains are strong and sharp. The integrated intensity ratio of the middle rigid segment (part A) protons and the alkyl chain (part B) protons in NMR is 0.13, which is lower than 0.48, the calculated ratio of proton number in middle rigid segment and in alkyl chain part. These results imply that they densely stack with each other at the middle rigid segment (two free or Ga, or In-complexed salicylideneaniline units) and that the alkyl chains are in fluid state. In conclusion, all the available data points to the formation of typical liquid-crystalline monolayer membranes in organic solvents of low polarity, as illustrated in Figure 3 (In-complex as an example). The rigid segments (21) Seiter, C. H. A.; Chan, S. I. J. Am. Chem. Soc. 1973, 95, 7541. (22) Itho, N.; Komatsu, H.; Handa, T.; Miyajima, K. J. Colloid Interface Sci. 1995, 174, 148. (23) Kunitake, T.; Okahata, Y. J. Am. Chem. Soc. 1977, 99, 3860. (24) Nagamura, T.; Mikara, S.; Okahata, Y.; Kunitake, T.; Matsuo, T. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 1093. (25) Okahata, Y.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 550.
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(two free or complexed salicylideneaniline units) establish the ordered stacking backbone of the molecule, and the alkyl chains act as the solvophilic parts in organic media to keep the monolayer surface stable. If interpeptide hydrogen bonding is crucial for molecular assembly in
Notes
organic media in polyamides without special solvophobic groups,12,13 the strong stacking of long rigid segments also makes the organized molecular assembly possible. LA0001427
