Langmuir 2000, 16, 3773-3776
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Vesicle Formation and Its Fractal Distribution by Bola-Amphiphilic [60]Fullerene Masahito Sano,* Kei Oishi, Tsutomu Ishi-i, and Seiji Shinkai* Chemotransfiguration Project - JST, 2432 Aikawa, Kurume, Fukuoka 839-0861, Japan Received November 30, 1999. In Final Form: January 26, 2000 A novel amphiphilic [60]fullerene derivative with two ammonium headgroups is synthesized, and its self-organization characteristics in water in the scale ranging from nanometer to micrometer are reported. At the molecular scale, the bola-amphiphilic [60]fullerene forms spherical vesicles. These vesicles, in turn, are placed within a thin wall producing a foamlike network in the scale-up to a few micrometers. TEM and light scattering measurements demonstrate that the mesoscopic-scale structure is self-similar and fractal with the dimension D ) 1.40. The novel aggregation modes result from the hydrophobic interaction produced by the [60]fullerene moieties exposed to water molecules by the disordered alkyl tails.
Controlling aggregation structures of a family of fullerenes and nanotubes in solution is of great importance for development of functional materials.1,2 It is, however, difficult to control structures even with the most wellstudied [60]fullerene, because of its high tendency for random aggregation in nearly all solvents.3 This has led researchers to look for alternative methods other than direct association. For instance, by mixing them with suitable surfactants, unmodified [60]fullerene molecules are incorporated into the hydrophobic core of micelles and can be dispersed as small clusters in water.4-8 Attaching polymer chains covalently to [60]fullerene results in dissolution of the derivative in the good solvent of the polymer, forming micellelike structures.9 Modification of [60]fullerene to be amphiphilic is also an important step toward controlled aggregations through self-organization. Previously, we have reported an interfacial behavior of the [60]fullerene derivative with an ammonium hydrophilic group attached via a single-chain spacer.10 The ammonium head with a short spacer was chosen so that, without the [60]fullerene moiety, it dissolves in water completely. It was shown that this hydrophilic group produces no significant improvement of solubility and does not lead to self-organized structures in solution, but formation of spread monolayers at the air-water interface was demonstrated. It is therefore expected that controlling aggregation structures in solution requires stronger hydrophilicity. Additionally, due to the much larger size of the [60]fullerene moiety compared with the single chain tail, intermolecular close packing left too much spaces between the neighboring (1) Chen, J.; Hamon, M. A.; Hu, H. Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (2) Dresselhaus, M. S. Annu. Rev. Mater. Sci. 1997, 27, 1. (3) Ruoff, R. S.; Tse, D. S.; Malhotra, R.; Lorents, D. C. J. Phys. Chem. 1993, 97, 3379. (4) Hungerbu¨hler, H.; Guldi, D. M.; Asmus, K.-D. J. Am. Chem. Soc. 1993, 115, 3386. (5) (a) Beeby, A.; Eastoe, J.; Heenan, R. K. J. Chem. Soc., Chem. Commun. 1994, 173. (b) Eastoe, J.; Crooks, E. R.; Beeby, A.; Heenan, R. K. Chem. Phys. Lett. 1995, 245, 571. (6) Bensasson, R. V.; Bienvenue, E.; Dellinger, M.; Leach, S.; Seta, P. J. Phys. Chem. 1994, 98, 3492. (7) Williams, R. M.; Crielaard, W.; Hellingwerf, K. J.; Verhoeven, J. W. Recl. Trav. Chim. Pays-Bas 1996, 115, 72. (8) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903. (9) Okamura, H.; Ide, N.; Minoda, M.; Komatsu, K.; Fukuda, T. Macromolecules 1998, 31, 1859. (10) Oh-ishi, K.; Okamura, J.; Ishi-i, T.; Sano, M.; Shinkai, S. Langmuir 1999, 15, 2224.
tails, resulting in poor orientation of the alkyl chains and the ammonium heads. These considerations suggest that a double-chain framework, with each having an ammonium headgroup, is more suited for self-organization as it offers stronger hydrophilicity and steric repulsion to counterbalance the [60]fullerene moiety and, at the same time, improves packing around the spacer chains. This molecular design is made possible because we have a fair understanding of molecular structures, intermolecular interactions, and thermodynamic conditions necessary for successful self-assembly of amphiphilic molecules.11,12 On the other hand, we know very little about how the self-assembled objects themselves organize in solution. The interactions between the large objects are often numerous and complicated.13 It is also our present interest to investigate if the effect of extremely hydrophobic [60]fullerene groups extends far beyond the molecular scales.14
The aggregation structures reported in this paper are two-folds. At the molecular scale, vesicle formation by the newly synthesized compound 1 in water is described. Then, over the scale ranging from a few tens of nanometers to over a micrometer, we report the existence of mesoscopicscale coherency in the distribution of these vesicles: the structure is self-similar and fractal. We comment that the hydrophobic interaction plays an important role in forming such large-scale structures. (11) For bola-amphiphiles, see, for example: (a) Okahata, Y.; Kunitake, T. J. Am. Chem. Soc. 1979, 101, 5231-5234. (b) Fuhrhop, J.-H.; Fritsch, D. Acc. Chem. Res. 1986, 19, 130-137. (c) Nagarajan, R. Chem. Eng. Comm. 1987, 55, 251-273. (12) (a) Lehn, J.-M. Supramolecular Chemistry, Concepts and Perspectives, VCH: Weinheim, Germany 1995. (b) Kunitake, T. Compr. Supramol. Chem. 1996, 9, 351-406. (13) Israelachvili, J. Intermolecular and Surface Forces; Academic: London, 1992. (14) Incidentally, Buckminster Fuller discussed synergy, meaning behavior of whole systems unpredicted by the behavior of their parts taken separately. Buckminster Fuller, R.; Applewhite, E. J. Synergetics: Explorations in the Geometry of Thinking; Macmillan: New York, 1979.
10.1021/la991550h CCC: $19.00 © 2000 American Chemical Society Published on Web 03/11/2000
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Langmuir, Vol. 16, No. 8, 2000 Scheme 1
Experimental Section Synthesis. Compound 1 was synthesized according to Scheme 1 and fully identified by IR, 1H NMR, 13C NMR, mass spectral evidence, and elemental analysis. All melting points are uncorrected. IR spectra were recorded on a SHIMADZU FT-IR 8100 M spectrophotometer and measured as KBr pellets. 1H and 13C NMR spectra were determined in CDCl3 or DMSO-d6 with a BRUKER ARX300 apparatus. Mass spectra were measured on a HITACHI M-2500 mass spectrometer. Column chromatography was carried out on silica gel (Wako C-300). Compound 2 was prepared according to the previously described method.15 1,9-Dihydro-64,65-dihydroxy-1.9-(methano[1,2]benzenomethano)[60]fullerene(3). To a solution of 2 (31 mg, 0.035 mmol) in dry toluene (15 mL) was added boron tribromide (0.2 mL, 2.3 mmol) at 0 °C under a nitrogen atmosphere. The mixture was stirred at room temperature overnight. After the reaction mixture was quenched by addition of cold water, it was extracted with ethyl acetate. The organic phase was washed with water, dried over anhydrous magnesium sulfate, and evaporated in vacuo to dryness. The residue was purified by reprecipitation from tetrahydrofuran/hexane to give 3 in 94% yield (28 mg, 0.033 mmol) as a brown solid: mp >400 °C; IR (KBr) νmax 3360, 2922, 1500, 1456, 1428, 1150, 1117, 768, 727, 695, 527; 1H NMR (DMSOd6, 22 °C) δ 7.21 (s, 2 H, ArH), 5.41 (s, 2 H, OH), 4.74-4.70 (m, 2 H, CH2), 4.32-4.27 (m, 2 H, CH2); MS (negative SIMS, NBA) m/z 856 (M-). Anal. Calcd for (C68H8O2‚2[C4H8O]): C, 91.19; H, 2.42. Found: C, 90.98; H, 2.34. 1,9-Dihydro-64,65-bis(11-bromoundecanoyloxyl)-1,9(methano[1,2]benzenomethano)[60]fullerene (4). To a solution of 3 (60 mg, 0.058 mmol), DMAP (2.1 mg, 0.017 mmol) and 11-bromoundecanoic acid (46 mg, 0.17 mmol) in dry CH2Cl2 (30 mL) was added DCC (35 mg, 0.17 mmol) at 0 °C under a nitrogen atmosphere. The mixture was stirred at room temperature for 20 h. The reaction mixture was filtered off to remove dicyclohexylurea, and the filtrate was evaporated in vacuo to dryness. The residue was purified by silica gel column chromatography eluting with toluene/hexane (3/1 v/v) to give 4 in 38% yield (30 mg, 0.022 mmol). An analytical sample was obtained by reprecipitation from dichloromethane/hexane as a brown solid: mp >400 °C; IR (KBr) νmax 2923, 2849, 1767 (νCO), 1497, 1456, 1428, (15) Diederich, F.; Jonas, U.; Gramlich, V.; Herrmann, A.; Ringsdorf, H.; Thilgen, C. Helv. Chem. Acta 1993, 76, 2445.
Sano et al. 1288, 1165, 1100, 527; 1H NMR (CDCl3, 22 °C) δ 7.55 (s, 2 H, ArH), 4.96-4.67 (m, 2 H, CH2), 4.52-4.32 (m, 2 H, CH2), 3.41 (t, J ) 6.8 Hz, 4 H, CH2Br), 2.62 (t, J ) 7.5 Hz, 4 H, COCH2), 1.90-1.75 (m, 8 H), 1.58-1.25 (m, 24 H); MS (negative SIMS, NBA) m/z 1350 (M -). Anal. Calcd for C90H46Br2O4: C, 80.00; H, 3.43. Found: C, 79.57; H, 3.68. 1,9-Dihydro-64,65-bis(11-trimethylammoniumundecanoyloxyl)-1,9-(methano[1,2]benzenomethano)[60]fullerene Dibromide (1). To a solution of 4 (25 mg, 0.019 mmol) in dry toluene was added excess trimethylamine at 0 °C. The mixture was stirred at room temperature for 7 days. The precipitate was collected by filtration and washed with toluene to give 1 in 79% yield (23 mg, 0.015 mmol) as a brown solid: mp > 400 °C; IR (KBr) νmax 2923, 2851, 1763 (νCO), 1100, 572; 1H NMR (DMSO-d6, 130 °C) δ 7.67 (s, 2 H, ArH), 4.77 (s, 4 H, CH2), 3.36-3.30 (m, 4 H, CH2N+), 2.91 (s, 18H, N+(CH3)3), 2.63 (t, J ) 7.3 Hz, 4 H, COCH2), 1.79-1.67 (m, 8 H, CH2), 1.48-1.32 (m, 24 H, CH2); 13C NMR (DMSO-d6, 130 °C) δ 169.89 (CO), 156.36, 146.60, 145.42, 145.16, 144.78, 144.37, 144.28, 143.68, 142.05, 141.53, 141.18, 141.01, 140.84, 140.53, 138.96, 135.85, 134.65 (C), 122.20 (CH), 121.61 (C), 65.52(CH2N+), 65.16 (fullerene sp3C), 52.12 (N+CH3), 52.08 (ArCH2), 32.85 (COCH2), 28.1-27.5 (CH2); MS (positive SIMS, NBA) m/z 1308 [(M - 2Br)+], 654 [(M 2Br)2+]. Anal. Calcd for C96H64Br2N2O4: C, 78.47; H, 4.39; N, 1.91. Found: C, 78.28; H, 4.31; N, 1.89. Sample Preparation and Characterization. Compound 1 was dispersed in water by an ultrasonicator (20 kHz, 600 W maximum power). Usually, 10 mM dispersion was used for the measurements. The samples for transmission electron microscopy (TEM, Hitachi H7100) were made by staining with uranyl acetate before they were picked up on carbon grids. The spectra of X-ray diffraction (XRD, Mac Science MXP18) were taken on a cast film of the amphiphile dispersion with the 2θ mode using Cu KR line. For the light scattering measurements (DLS-7000, Otsuka Electronics), the dispersion was diluted slightly after ultrasonication at 10 mM and was ultracentrifuged lightly.
Results and Discussion Vesicle Formation. When 1 was ultrasonicated in water at concentrations higher than 10.0 mM, the solution turned clear black. TEM reveals the presence of spherical vesicles, 20-50 nm in diameter (Figure 1d). XRD gives a broad peak at 4.0 nm (Figure 2). On the other hand, when 1 was ultrasonicated at concentrations less than 3.0 mM, cloudy brown dispersions resulted. With this sample, TEM shows only irregularly shaped solid aggregates. Thus, the compound 1 forms spherical vesicles with the critical aggregation concentration between 3.0 and 10.0 mM. This value is comparable to the critical micelle concentrations of other bola-form amphiphiles.11 CPK models indicate the extended molecular length of 3.2 nm when two ammonium chains extend in the same direction and 3.5 nm when they are in the opposite direction to make them as far apart as possible. The observed period is longer than these and is about twice the 2.2 nm principal period measured in the crystalline aggregates isolated from methanol.16 Thus, the vesicles are most likely made of bilayers with a head([60]fullerene)to-head conformation. The bilayer structure is also reasonable based on geometric packing considerations. Taking the radial distance of two ammonium heads11c (0.34 nm × 2) to be the radius of hydrophilic area, the radius of the [60]fullerene moiety,17 0.55 nm, suggests a nearly cylindrical, cone-shape packing unit which favors bilayer vesicles. The same consideration also indicates the large excess volume around the double alkyl chains even when both the ammonium groups and the [60]fullerene moiety pack closely. It is likely that the chains have many links (16) Oishi, K.; Ishi-i, T.; Sano, M.; Shinkai, S. Chem. Lett. 1999, 1089. (17) Kra¨tschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354-358.
Vesicle Formation and Its Fractal Distribution
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Figure 1. TEM micrographs of dispersed vesicles at various magnifications.
Figure 2. XRD spectrum of a cast film of amphiphilic [60]fullerene in water.
deviating from trans conformation and are surrounded by many water molecules. This explains the broad XRD peak without higher order peaks. Mesoscopic Structure. A sequence of TEM images shown in Figure 1 is taken at different magnifications. At large scales, most vesicles are not dispersed randomly, but are concentrated in thin walls that are connected threedimensionally. The connected foamlike structure (although the mixture does not foam) produces the large voids whose sizes range from a hundred nanometers to a few micrometers. Only a few vesicles are visible within the voids. At low magnification, only the voids are visible. Upon further magnification, the smaller voids in the previous image now appear to be the large voids, and essentially the same type of image re-appears. The process continues until the large vesicles appear as the smaller
voids. From TEM observations, the self-similar feature18 is recognizable from a few micrometers to several tens of nanometers. Since the samples used for TEM observation are dried, there is a possibility that the large-scale structure is formed during evaporation of water. To see if a fractal structure really exists in water, static light scattering was performed.19 Due to the chemical instability of the [60]fullerene compounds in solid states in general, there is always a small fraction of molecules that remain undispersed. Because the self-similar feature is larger than the size of filter pores commonly used in sample preparations, the solution was ultracentrifuged slightly to filter out the heavy solid aggregates. The scattering intensity I(q) of a fractal object at a scattering vector q is given by19
I(q) ∼ q-D for 1/R , q , 1/r
(1)
where D is a fractal dimension, R is the average radius of the fractal object and r is the size of the individual particles the object is made of. As depicted in Figure 3, a straight line can be drawn in the double-logarithmic plot of I(q) against q over a range equivalent to approximately 400-1500 nm in real space. The upper bound (18) Mandelbrot, B. B. The Fractal Geometry of Nature; W. H. Freeman: New York, 1983. (19) Vicsek, T. Fractal Growth Phenomena; World Scientific: Singapore, 1992; pp 74-78.
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Figure 3. Light scattering data presented in the scaling form with the intensity I(q) as a function of the scattering vector q.
agrees with the size of large voids seen by TEM. The lower bound is most likely due to the strong scattering from the residual solid aggregates that have not been filtered by the ultra-centrifugation. The slope gives the fractal dimension D ) 1.40. Unmodified [60]fullerene alone aggregates in benzene by reaction-limited cluster aggregation and has D ) 2.10.20 For another well-known limiting case of diffusion-limited cluster aggregation, D = 1.8. The present fractal does not belong to either of these limiting cases. This indicates that the fractal structure different from the previously known [60]fullerene aggregation exist in water. From the TEM micrographs, we see that vesicles of various sizes are placed in the space not occupied by others, in such a way that neighboring vesicles touch each other. The present self-similar structure is made of tangent vesicles packed within the thin walls. Among generally known inter-vesicle forces that are commonly present, electrostatic double layer, van der Waals, and undulation forces are long range.13 In the present salt-free condition, there is obviously strong electrostatic repulsion. It is not yet clear if van der Waals attraction alone is sufficient to form these tangent vesicles. On the other hand, we note that the vesicles made of the [60]fullerene derivative have formed a fractal structure. [60]Fullerene is known to possess strong hydrophobicity against water and lyophobicity against most organic solvents.3 Also, it can be packed rigidly enough to develop an exceedingly high surface pressure as a monolayer.21 The present study shows that the molecular packing within a vesicle is quite diffuse. These facts suggest that the alkyl tails can fluctuate sufficiently enough to expose the [60]fullerene moiety to water while the vesicle is stabilized by the [60]fullerene interlayer (Figure 4). Then, there will be an additional attractive force owing to hydrophobic interaction. Since the attraction can be much stronger than van der Waals force at small separations,13 the hydrophobic force may (20) Ying, Q.; Marecek, J.; Chu, B. J. Chem. Phys. 1994, 101, 26652672. (21) Obeng, Y. S.; Bard, A. J. J. Am. Chem. Soc. 1991, 113, 62796280.
Figure 4. Schematic drawing showing the contacting regions of two vesicles. The double chains are disordered to expose the [60]fullerene moieties to water molecules, producing the hydrophobic force.
be the main driving force for the formation of tangent vesicles. Given the long-range attraction, the tangent vesicles do not continue to coalesce randomly to result in a large aggregate. Instead, they are confined within a thin wall, producing voids. One of the explanations is that the foamlike structure is energetically most favorable, because the long-range, strong electrostatic repulsion is still effective over the whole collection. Another possibility is that the small air bubbles formed during ultrasonication position the vesicles at the bubble surfaces. Once the vesicles are positioned tangential to each other, a foamlike frame is sustained even after the disappearance of the air bubbles. We speculate that the stability of the mesoscopic structure rests on the energetic reason while the formation requires the ultrasonication, since the observed structure could not be formed by thermal heating without ultrasonication. On the other hand, many materials that form vesicles by ultrasonication do not usually produce the foamlike structure. Thus, the ultrasonication is not the primary cause, but is a mean to give the system the initial condition necessary to create the long-range coherency. Conclusion Attaching two hydrophilic chains to the [60]fullerene moiety forms the spherical vesicles. The mesoscopic-scale distribution of vesicles has geometrical coherency, characterized by self-similarity and fractality. Fundamental structural units are identified as tangent vesicles. The hydrophobic interaction associated with [60]fullerene is proposed as a main source of attractive forces. This possibility needs to be checked by diffraction or surface force experiments. It is interesting to investigate the cases where hydrophobic forces become important, such as stressed vesicles by alkyl chain-based amphiphilic compounds. LA991550H
