Micelle Formation in Langmuir Films of C60 Derivatives - Langmuir

Langmuir and Langmuir−Blodgett films of three new amphiphilic [60]fullerene derivatives have been investigated. The molecular design of these compou...
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Micelle Formation in Langmuir Films of C60 Derivatives Jean-Louis Gallani,* Delphine Felder, Daniel Guillon, Benoıˆt Heinrich, and Jean-Franc¸ ois Nierengarten IPCMS-GMO, 23 rue du Loess, 67037 Strasbourg Cedex, France Received November 2, 2001. In Final Form: December 28, 2001

Langmuir and Langmuir-Blodgett films of three new amphiphilic [60]fullerene derivatives have been investigated. The molecular design of these compounds was chosen to control the position of the C60 core relative to the water and to strongly influence the resulting physical properties. The first compound (MonoC8) is a methanofullerene, whereas the two other compounds (BisC8 and BisC12) are C60 cis-2-bis adducts. All of these compounds form stable Langmuir films. Even though the isotherms show some hysteresis, it was possible to transfer the films onto solid substrates, with the Langmuir-Blodgett method for two compounds and the Langmuir-Schaeffer technique for the last one. Grazing-incidence X-ray analysis indicates that LB films have a low roughness, i.e., are of good quality. MonoC8 forms monolayers, whereas the structure of the films of the two other compounds is more complex, with a micellar substructure within the film. These results show how sensitive the molecular design is to the molecular arrangement and to the conformation of the molecules in the films.

Introduction Because of their unique structure and peculiar electronic properties, [60]fullerene molecules are particularly enticing entities. Among other properties, superconductivity1 and magnetism2 of fullerene-based compounds have been reported. Fullerenes are also interesting materials for the fabrication of photoelectric devices,3 and supramolecular fullerene chemistry4 is an exponentially growing field. Because most C60 applications now require high-quality thin films, the elaboration of fullerene-based films is of great importance. This has proven difficult to achieve because of the tendency of C60 to form aggregates. Uniform films of C60 have been obtained through electrostatic selfassembly,5 chemical self-assembly,6 and epitaxy,7 but these techniques have limitations, and none allows for a wide choice in the type of deposited molecules. The Langmuir-Blodgett (LB) technique offers a nice alternative for the deposition of organic molecules. It has been possible to form films of pure C608 on water that could not be transferred onto solid substrates and that most often were multilayered.9 Functionalized fullerenes readily form Langmuir films, but aggregation remains a problem in many cases, and deposition is not always possible.10 Only very recently has the synthesis of fullerene (1) (a) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S. H.; Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R. Nature 1991, 350, 600. (b) Scho¨n, J. H.; Kloc, C.; Batlogg, B. Nature 2000, 408, 549. (2) Allemand, P. M.; Khemani, K. C.; Koch, A.; Wudl, F.; Holczer, K.; Donovan, S.; Gru¨ner, G.; Thompson, J. D. Science 1991, 253, 301. (3) (a) Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F.; Liu, S.-G.; Echegoyen, L.; Barigeletti, F.; Armaroli, N.; Ouali, L.; Krasnikov, V.; Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 7467. (b) Enger, O.; Nuesch, F.; Fibbioli, M.; Echegoyen, L.; Pretsch, E.; Diederich, F. J. Mater. Chem. 2000, 10, 2231. (4) Diederich, F.; Go´mez-Lopez, M. Chem. Soc. Rev. 1999, 28, 263 and references therein. (5) (a) Liu, Y.; Wang, Y.; Lu, H.; Claus, R. Phys. Chem. B 1999, 103, 12, 2035. (b) Luo, C.; Guldi, D. M.; Maggini, M.; Menna, E.; Mondini, S.; Kotov, N. A.; Prato, M. Angew. Chem., Int. Ed. 2000, 39, 3905. (6) (a) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993, 115, 1193. (b) Zhang, J. Z.; Geselbracht, M. J.; Ellis, A. B. J. Am. Chem. Soc. 1993, 115, 7789. (7) Kroto, H. W.; Fisher, J. E.; Cox, D. E. Fullerenes; Pergamon Press: New York, 1993. (8) Obeng, Y. S.; Bard, A. J. J. Am. Chem. Soc. 1991, 113, 6279.

derivatives with good spreading and deposition characteristics been reported.11 In a previous work,12 we reported the synthesis and characterization of fullerene amphiphiles such as G1Ac (shown in Figure 1). To prevent aggregation, the C60 moieties were embedded in boatlike amphiphilic structures, which proved very efficient in terms of film-forming properties.13 However, because of the modest collapse pressures observed with G1Ac, we thought it useful to increase the hydrophilicity of the molecule14 by attaching a carboxylic acid group at the end of each of the four surrounding alkyl chains. This led to the design of MonoC8, BisC8, and BisC12, each of which consists of four alkyl chains bearing terminal -COOH groups grafted onto a C60 core. In this paper, we now report the study of this new type of C60-based amphiphiles, including their characterization in terms of film-forming properties. Interestingly, at the air-water interface, compound (9) (a) Castillo, R.; Ramos, S.; Ruiz-Garcia, J. J. Phys. Chem. 1996, 100, 15235 and references therein. (b) Goldenberg, L. M.; Williams, G.; Bryce, R. M.; Monkman, A. P.; Petty, M. C.; Hirsch, A.; Soi, A. Chem. Commun. 1993, 17, 1310. (c) Maggini, M.; Karisson, A.; Pasimeni, L.; Scorrano, G.; Prato, M.; Valli, L. Tetrahedron Lett. 1994, 35, 2985. (d) Hawker, C. J.; Saville, P. M.; White, J. W. J. Org. Chem. 1994, 59, 3503. (e) Maggini, M.; Pasimeni, L.; Prato, M.; Scorrano, G.; Valli, L. Langmuir 1994, 10, 4164. (f) Ravaine, S.; Le Pecq, F.; Mingotaud, C.; Delhaes, P.; Hummelen, J. C.; Wudl, F.; Patterson, L. K. J. Phys. Chem. 1995, 99, 9551. (g) Zhu, C. C.; Xu, Y.; Liu, Y. Q.; Zhu, D. B. J. Org. Chem. 1997, 62, 1996. (10) missing footnote. (11) Cardullo, F.; Diederich, F.; Echegoyen, L.; Habicher, T.; Jayaraman, N.; Leblanc, R. M.; Stoddart, J. F.; Wang, S. Langmuir 1998, 14, 1955. (12) (a) Nierengarten, J. F.; Schall, C.; Nicoud, J. F.; Heinrich, B.; Guillon, D. Tetrahedron Lett. 1998, 39, 5747. (b) Felder, D.; Nierengarten, J.-F.; Chuard, T.; Deschenaux, R. Helv. Chim. Acta 2001, 84, 1119. (c) Nierengarten, J.-F.; Eckert, J.-F.; Rio, Y.; Carreon, Ma, D. P.; Gallani, J.-L.; Guillon, D. J. Am. Chem. Soc. 2001, 123, 9743. (d) Carreon, Ma. d. P.; Felder, D.; Gallani, J.-L.; Guillon, D.; Gutierrez, M.; Heinrich, B.; Luccisano, M.; Nierengarten, J.-F.; Schall, C. Helv. Chim. Acta 2002, 85, 288-319. (13) Felder, D.; Gallani, J.-L.; Guillon, D.; Heinrich, B.; Nicoud, J.F.; Nierengarten, J.-F. Angew. Chem., Int. Ed. 2000, 39, 1, 201. (14) Amphiphilic molecules have a hydrophilic part linked to a hydrophobic part. If the hydrophilic part dominates, then the molecule becomes soluble, whereas if the hydrophobic part dominates, the film is not stable (alkane chains do not form Langmuir films). There is a need for a balance between the two moieties.

10.1021/la011637e CCC: $22.00 © 2002 American Chemical Society Published on Web 03/01/2002

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Figure 1. Chemical structures of the compounds.

MonoC8 forms monolayers in which the molecules adopt an extended conformation. In contrast, the films obtained from the fullerene cis-2-bis adducts BisC8 and BisC12 are more complex, with a micellar substructure within the films. Experimental Section Experimental details regarding the synthesis and complete chemical characterization of BisC8 and BisC12 can be found in the Supporting Information. The synthesis of G1Ac has been published elsewhere.12 Langmuir and Langmuir-Blodgett Films. Data were collected with a KSV LB5000 system using a Teflon trough and symmetrical hydrophilic barriers. The trough was set in a Plexiglas enclosure so as to be protected from drafts and dust, and the temperature was controlled to within (0.1 °C. All isotherms were taken at 20 °C unless otherwise specified. The ultrapure water (F ) 18.2 MΩ cm) used for the subphase was obtained from a Milli-RO3Plus/Milli-Q185 ultra purification system from Millipore. Surface pressure was measured by means of a platinum Wilhelmy plate. Solutions at ∼1 mg/mL concentration were prepared using chloroform (analysis grade from Carlo Erba). Usually, 50 µL of these solutions was spread on the water surface using a microsyringe. As MonoC8 was poorly soluble in chloroform, a mixture of dimethyl sulfoxide and chloroform (1:10 v/v) was used as the solvent instead.15 Films were left to equilibrate for 20 min before any measurements were started. The monolayers were compressed at typical speeds (15) Being water-soluble, DMSO could lead to the loss of molecules in the subphase and alteration of the values of the molecular area A. The fact that we obtain values for A that are similar to that of BisC8 indicates that this is not the case.

of 2 Å2 molecule-1 min-1. The subphase consisted of a 5.0 × 10-4 M solution of cobalt acetate, with its pH adjusted to 5.30 ( 0.05 with hydrochloric acid. LB films of MonoC8 and BisC12 were obtained by transfer on glass slides or hydrophilic silicon(100) wafers at a surface pressure of 25 mN/m. Transfers started from below the surface, with a typical emersion speed of 1 mm/min (except for BisC8; see below). We usually operated at a rather high temperature, around 50 °C. Prior to transfer, the glass substrates were cleaned using the following procedure: The plates were immersed in a solution of KOH in ethanol, rinsed several times with hot water, then treated by a hot sulfochromic solution, and then rinsed 10 times with ultrapure water. The silicon wafers were rendered hydrophilic by treatment in an oxidizing mixture of H2SO4 and H2O2 (1:1 v/v), followed by several rinsings in water. Subsequent silanization with octadecyltrichlorosilane made the substrates hydrophobic. All treatments were done in an ultrasonic bath. Grazing-Incidence X-ray Analysis (GIXA). The grazingincidence X-ray studies of LB films were performed on a spectrometer equipped with a nickel beta filter, a programmable divergence slit (1/32°), a parallel-plate collimator, a flat Ge monochromator and a proportional Xe detector. The Cu KR line (wavelength ) 1.542 Å) was used. All measurements were recorded immediately after the transfer, but the LB films were usually stable, at least for several days.

Results and Discussion Isotherms. The chemical structures of the compounds are depicted in Figure 1. The pressure-area isotherms of MonoC8, BisC8, and BisC12 are given in Figure 2, together with that of G1Ac, for which stable and homogeneous films were observed.12 All three compounds exhibit similar behaviors that are almost independent of temperature

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Figure 2. Pressure-area isotherms of MonoC8, BisC8, and BisC12. The isotherm for G1Ac C8 is also shown for comparison.

between 20 and 60 °C: the surface pressure departs from 0 at molecular areas of around 140 Å2 and then increases gently until the film collapses at pressures between 30 mN/m for MonoC8 and 40 mN/m for BisC12. This behavior indicates a liquidlike molecular organization of the film, with long-range in-plane interactions. BisC8 and MonoC8 show only the so-called liquid condensed phase,16 whereas BisC12 can exhibit a liquid expanded phase between 140 and 75 Å2. Although the shapes of the isotherms and the values of the compressibility suggest nonsolid behavior, the films are actually very rigid. Indeed, in the case of BisC8, we noticed that, at the end of the compression, the Wilhelmy platinum plate was being pushed off its position by the film. For all three compounds, the films were stable in time. The molecular areas extrapolated at zero pressure are A0 ≈ 110 ( 5 Å2 for MonoC8, A0 ≈ 106 ( 5 Å2 for BisC8, and A0 ≈ 85 ( 4 Å2 for BisC12. In view of the molecular geometry, the final area of BisC12 is particularly small. As a single all-trans alkyl chain has a cross section17 of ∼22-25 Å2, one would expect a value of at least A0 ≈ 100 Å2 for all of the compounds, which is also the molecular area of a C60 sphere.8 The smaller value observed for BisC12 indicates either that defects are present or that the film is bilayered, at least partially, or that the molecules do not actually form a film in the usual sense (see discussion hereafter). Last, it is worth noting that the collapse pressures of MonoC8, BisC8, and BisC12 are at least twice as large as those of our previously published compounds,13 indicating a stronger film cohesion and better anchoring on the water surface. This is certainly due to the four carboxylic groups, at least partially, but the following discussion on the molecular organization will also shed some more light on this point. The isotherms of the three compounds show very limited reversibility, even if the surface pressure is kept below the collapse value. As an example, the hysteresis curve of BisC8 is shown in Figure 3. BisC12 and MonoC8 exhibit very similar behaviors (see Supporting Information). This is usually indicative of the occurrence of irreversible aggregation, and is consistent with our visual observation of a high film rigidity. As compared with G1Ac, the observation of smaller molecular areas for the MonoC8, BisC8, and BisC12, together with the lack of reversibility, is a clear signature of a different molecular conformation. In the G1Ac molecules, the C60 was embedded in a boatlike structure, (16) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons Inc.: New York, 1990. (17) Ulman, A. Ultrathin Organic Films; Academic Press Inc.: New York, 1991.

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Figure 3. Hysteresis of the isotherm for BisC8. The isotherm of G1Ac C8 is also shown for comparison. Arrows pointing upward (downward) indicate compression (decompression).

Figure 4. X-ray reflectivity spectrum of a monolayer LB film of MonoC8 on silicon (not all points are shown). The line is a fit to the data points.

with the alkyl chains screening the C60-C60 interactions (π-π interactions), preventing aggregation of neighboring molecules. For the new type of molecules described here, which have the carboxylic group attached at the very end of the alkyl chain, intramolecular repulsive forces between the fullerene and the chains18 act with the hydrophobic forces to induce a fully stretched conformation of the molecules. In this case, because the molecular area of the four alkyl chain matches that of the C60, the packing is more dense, as deduced from the final molecular area. Because of this close packing, π-π interactions between two adjacent molecules can occur, leading to aggregation and lack of reversibility. Still, in view of the results of the X-ray analysis, another hypothesis will be discussed below. Also see the Supporting Information for BAM images and surface potential measurements of MonoC8, BisC8, and BisC12. Transfers. It proved difficult to obtain LB films with MonoC8. We eventually managed to transfer a MonoC8 film onto hydrophilic silicon, with a transfer ratio (TR) of 0.75 ( 0.10, using a slow dipping speed and high subphase temperature. Grazing-incidence X-ray analysis (Figure 4) reveals the presence of a monolayer of thickness dm ) 25 ( 2 Å. The presence of only two Kiessig fringes is consistent with the low TR. The transfer took place at a surface pressure of 25 mN/m. At this surface pressure, the molecular area is A ) 92 Å2. Assuming a density of 1 g/cm3 for the compound gives an estimate for the (18) Fullerene does not mix with alkanes.

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Figure 6. Model for the structure of the Langmuir film of BisC8.

Figure 5. X-ray reflectivity spectrum of a LB film of BisC8 on silicon (not all points are shown). The line is a fit to the data points.

thickness of the Langmuir film of de ) 1.66 × Mw/A ≈ 30 Å, where Mw is the molecular weight, in reasonable agreement with the measured value. This value of the monolayer thickness confirms that the molecules are in an extended conformation. This conformation minimizes the energy, as the carboxylic groups, alkyl chains, and C60 core are each in a different plane. Some molecular rearrangement taking place during the transfer cannot be excluded, especially because TR < 1, but given the final molecular area of the molecules in the Langmuir film, we believe that this is not the case and that the value of TR is due to the imperfect film. Despite many attempts, BisC8 could not be transferred onto solid substrates with the usual dipping technique. The Langmuir film behaves like a brittle solid and transfers not continuously but in chunks. As a result, the roughness of the LB films is very high and does not allow for X-ray reflectivity measurements. This all indicates aggregation of the molecules in the Langmuir film. Still, we were more successful with the Langmuir-Schaeffer method,17 in which a piece of the Langmuir film was lifted horizontally. The surface pressure was 30 mN/m. X-ray measurements indicated that the film thickness was d ≈ 37 ( 2 Å, with a roughness of R ≈ 2 Å, although the fit was not quite perfect19 (Figure 5). As the molecule has a full length of ∼27 Å in an extended conformation, it becomes obvious that the measured film thickness does not correspond to a simple monolayer, as observed in the case of MonoC8. In a previous paper,20 we demonstrated that MonoC8 could form micelles: when disolved in water (pH 13) or polar solvents, the C60 cores cluster together and form spherical particles with a coating of aliphatic chains. From small-angle X-ray scattering (SAXS) measurements, we were able to deduce that each cluster consisted on average of 30 molecules and that the diameter of the assembly was about 52 Å. We believe that the Langmuir film of BisC8 is actually made of such clusters, i.e., inverted micelles floating on the water. If one supposes that such clusters are spherical, each one would occupy an area of ca. 2200 Å2 on the water. Given a packing coefficient of (19) Contrary to our approach for the G1Ac series, the film thickness was deduced from the sole position of the Kiessig fringes. As the structure of the films is more complex (see text), an efficient box model of the electronic density could not be applied. We used instead an average electronic density; hence, the lower quality of the fits. (20) Felder, D.; Guillon, D.; Le´vy, R.; Mathis, A.; Nicoud, J.-F.; Nierengarten, J.-F.; Rehspringer, J.-L.; Schell, J. J. Mater. Chem. 2000, 10, 887. The product is disolved in a 0.1 M NaOH solution at a concentration of 5 wt %.

Figure 7. X-ray reflectivity spectrum of a LB film of BisC12 on silicon (not all points are shown). The line is a fit to the data points.

0.7 and a supposed aggregation number of 30 molecules/ cluster, one then finds a molecular area of ca. 105 Å2, which is perfectly consistent with the measured value of 106 Å2. Of course, we have no direct evidence that the micelles are regular, and the actual configuration is very probably more similar to the one depicted in Figure 6. The average thickness should be more or less constant, as the LB film is flat enough to give Kiessig fringes in the X-ray spectra, but the lateral dimension of the micelles might vary. Moreover, these aggregates could interact via the alkyl chains and even link together because of H bonds forming between carboxylic end groups from adjacent micelles. This would then explain the lack of reversibility of the isotherms and the rigidity of the Langmuir film. As for the different behavior of MonoC8, it is probably because the molecular shape of MonoC8 is only marginally favorable for efficient spherical packing.21 The best Langmuir-Blodgett films were obtained with BisC12. The transfer ratio was 1.0 ( 0.1, indicating the effective transfer of a monolayer, but the X-ray reflectivity pattern (Figure 7) gives an overall layer thickness of ∼48 Å, as deduced from the position of the Kiessig fringes, which is incompatible with the size of a single molecule even fully extended (full length ≈ 32 Å). The increase in film thickness is nevertheless consistent with the values observed for the previous compound, as the chains are four carbons (i.e., ∼5 Å) longer. Here again, to unify the observations, we must conclude that the molecules form aggregates at the water surface. In this case, this hypothesis is supported by the fact that we have observed the formation of inverted micelles when BisC12 is dissolved in water.22 We therefore believe that the Langmuir film also has a structure similar to the one depicted in Figure 6: the C60 cores stick together, with the aliphatic (21) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 2000, and references therein. (22) Results of small-angle X-ray scattering are to be published. The compound was disolved in a 0.1 M NaOH aqueous solution at 5% concentration. Also, UV-vis spectra (in water) showed some broadening of the peaks, which is another indication of a certain degree of clustering.

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chains extending outward and H bonds between neighboring clusters being responsible for the lack of reversibility of the isotherms. Clustering of amphiphilic C60 derivatives has been reported and studied. Some experiments have provided evidence of aggregation of a small number (7) of molecules in aqueous solutions,23 others the formation of large bilayer vesicles24 made of 104 molecules. These observations were made on water solutions of the compounds. The spontaneous formation of bilayer-based superstructures in cast films of fullerene-based amphiphiles has also been observed.25 To the best of our knowledge, the formation of inverted micelles on a water surface has not yet been observed for fullerene-based molecules.26 It is now worth trying to determine the exact process of micelle formation at the surface of the trough. Several models for surfactant aggregation21 have been proposed, on the basis of cooperative self-association,27 free-energy considerations,28 or a combination of thermodynamics and molecular models.29 The formation of various adsorbed structures, in particular spheres, on hydrophilic substrates has been reported.30 The micellization process is complex, and calculation of the free energy must include many terms, including hydrophobic free energy, interfacial free energy, packing free energy (because of the loss of configurational degrees of freedom of the molecules in the micelles), and interactions (steric and electrostatic) between micelles. Such calculations are beyond the scope of this paper. For our compound, the formation of inverted micelles could take place in dichloromethane or chloroform, but it could also occur on the water surface, in a twodimensional diffusion-limited aggregation process. When the film is compressed, some critical micelle surface concentration31 is exceeded, and the molecules start to aggregate. Let us mention that, when globular micelles form, they are usually narrowly dispersed in size.29a This would explain why our films are flat enough to give Kiessig fringes when probed by X-ray reflectivity. What is the driving force behind clustering? The formation of a film according to the original plan would yield the structure depicted in Figure 8a: the carboxylic groups would dip in the water and deprotonate, the alkyl chains would extend outward, and the highly hydrophobic C60 would sit far away from the water surface. This conformation was considered the most favorable, because the CO2H groups would be in the water, the alkyl chain would be more or less packed together, and the C60 cores (23) Jeng, U.; Lin, T. S.; Tsao, C. S.; Lee, C. H.; Canteenwala, T.; Wang, L. Y.; Chiang, L. Y.; Han, C. C. J. Phys. Chem. B 1999, 103, 1059. (24) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944. (25) Nakashima, N.; Ishii, T.; Shirakusa, M.; Nakanishi, T.; Murakami, H.; Sagara, T. Chem. Eur. J. 2001, 7, 1766. (26) The formation of microscopic threadlike structures has been reported: Guldi, D. M.; Maggini, M.; Mondini, S.; Gue´rin, F.; Fendler, J. H. Langmuir 2000, 16, 1311. Other reports on the formation of supramolecular structures (nanorods, vesicles, membranes) in aqueous solutions have also been published: (a) Cassell, A. M.; Asplund, C. L.; Tour, J. M. Angew. Chem., Int. Ed. Engl. 1999, 38, 16, 2403. (b) Sano, M.; Oishi, K.; Ishi-i, T.; Shinkai, S. Langmuir 2000, 16, 3773. (c) Brettreich, M.; Burghardt, S.; Bo¨ttcher, C.; Bayerl, T.; Bayerl, S.; Hirsh, A. Angew. Chem., Int. Ed. 2000, 39, 10, 1845 and ref 24. (27) Mukerjee, P. Adv. Colloid Interf. Sci. 1967, 1, 241. (28) Tanford, C. The Hydrophobic Effect, 2nd ed.; Wiley: New York, 1980. (29) (a) Nagarajan, R.; Ruckenstein, E. Langmuir 1991, 7, 2934. (b) Puvvada, S.; Blankschtein, D. J. Phys. Chem. 1992, 966, 5567. (30) (a) Manne, S.; Gaub, H. E. Science, 1995, 270, 1480. (b) Manne, S.; Schaffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382. (c) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685. (31) The usual definition for the critical micelle concentration (cmc) is for a volume concentration of surfactant.

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Figure 8. (a) Model of a Langmuir film where the molecules remain at the air-water interface in a monolayer. (b) Model of a Langmuir film with micelle-forming molecules, schematically showing clusters and H bonds between them.

would be at the outermost position, each species being confined in its own plane. However, if the molecules then form clusters (Figure 8b), energy is gained from two sources: strong π-π interactions between the C60’s, with packing of the alkyl chains around these cores, and H bonds between adjacent clusters. Segregation still occurs, as the fullerenes have minimal interactions with the alkyl chains and the benefit of efficient screening from water because of the fatty coating. Of course, this requires the molecular conformation to allow for such a packing, or the entropy loss would be too high. This is why MonoC8 forms regular Langmuir films, with the molecules in an extended conformation: the molecular area of the C60 (∼100 Å2) matches that of the four alkyl chains (∼4 × 25 Å2), yielding much better packing than would be obtained in a micelle. In contrast, for both BisC8 and BisC12, the chains are separated by pairs because of their cyclic structures, thus forcing them to extend outward. The match of the molecular areas is therefore not that good in the Langmuir film, and this favors micelle formation. It would be very interesting to further investigate the formation of micelles on the water surface, and it could also prove useful to clarify the influence of the spreading solvent, as micelles might already preform in the solution. Conclusion In our previous work12,13 concerning G1Ac, the C60 moiety was at the core of a boatlike amphiphilic structure, with the hydrophilic head and hydrophobic tails attached to it. The molecular design behind the present study on MonoC8 and BisCn was devised with two main goals: (1) to keep the C60 in an alkyl cage as far away as possible from the water and (2) to increase the anchoring on the water with four hydrophilic groups instead of one. In the case of MonoC8, the molecule adopts an extended conformation, as deduced from the molecular area, which is close to the cross section of four alkyl chains, and from

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X-ray thickness measurements on the LB films. In this conformation, the C60 cores of different molecules can come into close contact and aggregate, as evidenced by the poor reversibility of the isotherms. The fact that the fully extended conformation is preferred is, of course, the result of the tendency of C60 to be as far as possible from the water and alkyl chains. In another attempt, fullerene bis adducts were synthesized. Unexpectedly, this led to behavior completely different from that of MonoC8, as the molecules formed micelles on the water surface. It might seem unfortunate that the synthesis of an elaborated molecule such as BisC8 or BisC12 did not lead to a significant improvement compared to pure C60 in terms of the film-forming properties, but we believe, on the contrary, that the results presented here open the way to a new type of molecular film. One generally considers LB films to be like a field of grass, with more or less rodlike molecules all planted head down with no or little tail interactions, the archetype being films of stearic acid. However, if the tails have some attractivity (as do the C60’s in our case), one might obtain more complex structures, such as the micelles described in the present paper. With a proper design, it then becomes possible to induce a supramolecular organization within the film and to add extra cohesion forces with intermicellar H bonds. We are currently investigating these new LB films with inner substructures, especially in trying to characterize and better control the micelles’ shape. One of the ideas behind the synthesis of these compounds was to try using them as optical limiters. Because the nonlinear optical properties of C60 decrease when intermolecular interactions can take place, the molecular

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organization adopted in thin films by the molecules described here does not favor of such an application. However, the spontaneous formation of micelles opens perspectives to other applications, especially in the field of biology. Many laboratories are currently working on fullerene solubilization for medical use,32 and the formation of vesicles incorporating fullerenes in their membranes proves very useful.33 It has also recently been suggested34 that covering a surface with hydroxylterminated globular dendrimers (with the ability to form multiple H bonds) could be an approach to selective molecular gluing. This would also work for the molecules presented here, with the additional advantage of not having to synthesize the dendrimers as the globular structure would also self-assemble on the substrate. Acknowledgment. We thank A. Mathis and F. Schnell for their help with the SAXS preliminary measurements. Supporting Information Available: Synthesis details for BisC8 and BisC12, Brewster angle microscopy images for the three compounds, hysteresis curves (isotherms), and surface potential measurements. This material is available free of charge via the Internet at http://pubs.acs.org. LA011637E (32) (a) Nakamura, E.; Isobe, H.; Tomita, N.; Sawamura, M.; Jinno, S.; Okayama, H. Angew. Chem., Int. Ed. 2000, 39, 23, 4254. (b) Ungurenasu, C.; Airinei, A. J. Med. Chem. 2000, 43, 3186. (c) Tabata, Y.; Ikada, Y. Pure Appl. Chem. 1999, 71, 2047. (d) Jensen, A. W.; Wilson, S. R.; Schuster, D. I. Bioorg. Med. Chem. 1996, 4, 767. (33) The authors of ref 24 consider the use of spherical bilayer vesicles in biology and medicine. (34) Tsukruk, V. V. Adv. Mater. 2001, 13, 101.