Preparation and Film Formation Behavior of the Supramolecular

Department of Chemistry, The Hong Kong University of Science and Technology, Clear ... Langmuir films of the [Dy@C82−C8A] complex have been studied ...
0 downloads 0 Views 220KB Size
Download PDF
8488

Langmuir 2002, 18, 8488-8495

Preparation and Film Formation Behavior of the Supramolecular Complex of the Endohedral Metallofullerene Dy@C82 with Calix[8]arene Shangfeng Yang and Shihe Yang* Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received May 6, 2002. In Final Form: July 16, 2002 The first host-guest supramolecular complex of the endohedral metallofullerene Dy@C82 and p-tertbutylcalix[8]arene (C8A) (1:1) has been prepared in solution. Langmuir films of the [Dy@C82-C8A] complex have been studied systematically in comparison with films of the [C60-C8A] complex. The Langmuir film formation of both complexes was found to be sensitively dependent on the initial concentration of the spreading solution. This concentration dependence is explained in terms of conformational changes at different concentrations. While the formation of the complex [C60-C8A] from the mixture of C60/C8A was facile at the air/water interface, this was not the case for [Dy@C82-C8A] due to the slower complexation rate and the aggregation tendency of the endohedral metallofullerene molecules. Langmuir-Blodgett films of the [Dy@C82-C8A] and [C60-C8A] complexes have been characterized by UV-vis absorption spectroscopy.

Introduction The discovery of fullerene inclusion by calixarenes has opened the possibility of creating many fullerene supramolecular complexes.1,2 The half-bowl-shaped calixarenes (refs 3 and 4) are the third-generation supramolecular host compounds after γ-cyclodextrins (ref 5) and azacrown ethers.6 Since their cavity structure is characterized by a π-electron surface, host-guest complexes of calixarene-fullerene through π-π interactions have received special interest.3e,4 In the calixarene family, host molecules that have been found to form discrete complexes with C60 and/or C70 include calix[3]-, calix[4]-, calix[5]-, calix[6]-, and calix[8]arenes and their derivatives.1,2,7-11 * To whom correspondence E-mail: [email protected].

should

be

addressed.

(1) Atwood, J. L.; Koutsantonis, G. A.; Raston, C. L. Nature 1994, 368, 229. (2) Suzuki, T.; Nakashina, K.; Shinkai, S. Chem. Lett. 1994, 699. (3) (a) Gutsche, C. D. Calixarenes; The Royal Society of Chemistry: Cambridge, 1989. (b) Calixarenes Revisited; Monographs in Supramolecular Chemistry; Stoddart, J. F., Ed.; The Royal Society of Chemistry: Cambridge, 1998. (c) Vicens, J.; Bo¨hmer, V. Calixarenes: A Versatile Class of Macrocyclic Compounds; Topics in Inclusion Science; Davies, J. E. D., Ed.; Kluwer Academic: Dordrecht, The Netherlands, 1991. (d) Mandolini, L.; Ungaro, R. Calixarenes in Action; Imperial College Press: London, 2000. (e) Asfari, Z.; Bo¨hmer, V.; Harrowfield, J.; Vicens, J.; Saadioui, M. Calixarenes 2001; Kluwer Academic: Dordrecht, The Netherlands, 2001. (4) Shinkai, S.; Ikeda, A. Pure Appl. Chem. 1999, 71, 275 and references therein. (5) (a) Anderson, T.; Westman, G.; Stenhagen, G.; Sundahi, M.; Wennerstrom, O. Tetrahedron Lett. 1995, 36, 597. (b) Yoshida, Z.; Takekuma, H.; Matsubara, Y. Angew. Chem., Int. Ed. Engl. 1994, 33, 1597. (c) Kuroda, Y.; Nozawa, H.; Ogoshi, H. Chem. Lett. 1995, 47. (d) Takekuma, S.; Takekuma, H.; Matsumoto, T.; Yoshida, Z. Tetrahedron Lett. 2000, 14, 1043. (e) Samal, S.; Geckeler, K. E. Chem. Commun. 2000, 1101. (6) Diederich, F.; Effing, J.; Jonas, U.; Jullien, L.; Plesnivy, T.; Ringsdorf, H.; Thilgen, C.; Weinstien, D. Angew. Chem., Int. Ed. Engl. 1992, 31, 1599. (7) (a) Ikeda, A.; Nobukuni, S.; Udzu, H.; Zhong, Z. L.; Shinkai, S. E. J. Org. Chem. 2000, 3287. (b) Atwood, J. L.; Barbour, L. J.; Nichols, P. J.; Raston, C. L.; Sandoval, C. A. Chem.sEur. J. 1999, 5, 990. (c) Islam, S. D. M.; Fujitsuka, M.; Ito, O.; Ikeda, A.; Hatano, T.; Shinkai, S. Chem. Lett. 2000, 1, 78. (d) Hatano, T.; Ikeda, A.; Akiyama, T.; Yamada, S.; Sano, M.; Kanekiyo, Y.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 2000, 909.

Endohedral metallofullerenes represent a novel type of superatom molecules, which is characterized by a robust fullerene cage with metal atoms trapped in its hollow.12 The encapsulation of the metal atoms brings about many additional and desirable properties such as peculiar redox behavior, radioactivity, luminescence, paramagnetism, and enhanced nonlinear optical response.12 In light of the successful synthesis of the calixarene-fullerene host-guest complexes, it is tempting to extend this endeavor to the metallofullerenes. In this way, new functionalities will be designed into the welldefined supramolecular system, giving rise to a host of potential applications in molecular electronics, photonics, and sensors. Fundamentally, the host-guest chemistry of the calixarenes and the metallofullerenes is interesting because the metallofullerene carbon cages are modified from the corresponding empty fullerene cages by the excess electrons transferred from the encapsulated metal atoms. Moreover, the possibility of selective complexation between the calixarenes and the metallofullerenes evokes hope for more efficient isolation of the exotic metallofullerene molecules. During the past few years, the ability to prepare bulk quantities of endohedral metallofullerenes has made it possible to characterize their structures and physical (8) (a) Sun, D. Y.; Reed, C. A. Chem. Commun. 2000, 2391. (b) Wang, J. S.; Gutsche, C. D. J. Org. Chem. 2000, 65, 6273. (c) Mizyed, S.; Tremaine, P. R.; Georghiou, P. E. J. Chem. Soc., Perkin Trans. 2 2001, 3. (d) Barbour, L. J.; William, G.; Atwood, J. L. Chem. Commun. 1997, 1439. (e) Hughes, E.; Jordan, J. L.; Gullion, T. J. Phys. Chem. B 2000, 104, 691. (9) (a) Haino, T.; Yanase, M.; Fukazawa, Y. Angew. Chem., Int. Ed. 1998, 37, 997. (b) Isaacs, N. S.; Nichols, P. J.; Raston, C. L.; Sandova, C. A.; Young, D. J. Chem. Commun. 1997, 1839. (10) (a) Atwood, J. L.; Barbour, L. J.; Raston, C. L.; Sudria, I. B. N. Angew. Chem., Int. Ed. 1998, 37, 981. (b) Schlachter, I.; Ho¨weler, U.; Iwanek, W.; Urbaniack, M.; Mattay, J. Tetrahedron 1999, 55, 14931. (11) (a) Williams, R. M.; Zwier, J. M.; Verhoever, J. W. J. Am. Chem. Soc. 1994, 116, 6965. (b) Paci, B.; Amoretti, G.; Arduini, G.; Ruani, G.; Shinkai, S.; Suzuki, T.; Ugozzoli, F.; Caciuffo, R. Phys. Rev. B 1997, 55, 5566. (c) Suzuki, T.; Nakashina, K.; Shinkai, S. Tetrahedron Lett. 1995, 36, 249. (d) Ikeda, A.; Yoshimura, M.; Shinkai, S. Tetrahedron Lett. 1997, 38, 2140. (12) (a) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843 and references therein. (b) Yang, S. H. Trends Chem. Phys. 2001, 9, 31.

10.1021/la020422h CCC: $22.00 © 2002 American Chemical Society Published on Web 10/03/2002

Complex of Metallofullerene with Calix[8]arene

properties.13-17 In particular, the Langmuir-Blodgett (LB) technique was successfully applied to construct multilayer films of endohedral metallofullerenes in our laboratory.18 The film quality was improved by incorporating the metallofullerene or fullerene molecules into a matrix of amphiphilic compounds. Analogous to the empty fullerenes C60 and C70,19,20 the metallofullerene Dy@C82 can form stable mixed LB films with long-chain fatty acids such as arachidic acid (AA).21 Since amphiphilic calixarenes can form Langmuir films at the air/water interface,22 they have been considered for the dispersion of fullerenes and metallofullerenes. Indeed, good-quality LB films were prepared by incorporating the fullerene C60 into the matrix of calixarenes such as calix[4]resorcinolarene and calix[6]- and calix[8]arene.23-25 However, such films have not been prepared for metallofullerenes up till now. In this paper, we report in detail the first successful preparation of [Dy@C82-C8A], a host-guest supramolecular complex of p-tert-butylcalix[8]arene (C8A) and the endohedral metallofullerene Dy@C82. Furthermore, the film formation behavior of [Dy@C82-C8A] is systematically studied in comparison with that of [C60-C8A] using a series of spreading solutions with different concentrations. The LB techniques and UV-vis spectroscopy are combined for the study of the LB films of [Dy@C82-C8A] and [C60C8A]. Experimental Section Materials. High-purity Dy@C82 and C60 (>99.0%) were prepared in our laboratory by a combination of the standard dc arc-discharge method and an isolation method described (13) Johnson, R. D.; de Vries, M. S.; Salem, J.; Bethune, D. S.; Yannoni, C. S. Nature 1992, 355, 239. (14) Rosen, A.; Wastberg, B. J. Am. Chem. Soc. 1988, 110, 8701. (15) Laasonen, K.; Andreoni, W.; Parrinello, M. Science 1992, 258, 1916. (16) Guo, T.; Smalley, R. E.; Scuseria, G. J. Chem. Phys. 1993, 99, 352. (17) Kikuchi, K.; Suzuki, S.; Nakao, Y.; Nakahara, Y.; Wakabayashi, T.; Shiromaru, T.; Saito, K.; Ikemoto, I.; Achiba, Y. Chem. Phys. Lett. 1993, 216, 67. (18) (a) Huang, H. J.; Yang, S. H. J. Organomet. Chem. 2000, 592, 42. (b) Huang, H. J.; Yang, S. H. Film Formation Behavior of the Endohedral Metallofullerene Dy@C82. In Amorphous and Nanostructured Carbon; Robertson, J., Sullivan, J. P., Zhou, O., Allen, T. B., Coll, B. F., Eds.; Materials Research Society: Boston, 2000; Vol. 593, pp 63-68. (19) (a) Obeng, Y. S.; Bard, A. J. J. Am. Chem. Soc. 1991, 113, 6279. (b) Bulhoes, L. O.; Obeng, Y. S.; Bard, A. J. Chem. Mater. 1993, 5, 110. (c) Maliszewskyj, N. C.; Heiney, P. A.; Jones, D. R.; Strongin, R. M.; Cichy, M. A.; Smith, A. B., III Langmuir 1993, 9, 1439. (20) (a) Milliken, J.; Dominguez, D. D.; Nelson, H. H.; Barger, W. R. Chem. Mater. 1992, 4, 252. (b) Xiao, Y. F.; Yao, Z. Q.; Jin, D. S.; Yan, F. Y.; Xue, Q. J. J. Phys. Chem. 1993, 97, 7072. (c) Xu, Y.; Guo, J.; Long, C. F.; Li, Y. L.; Liu, Y. Q.; Yao, Y. X.; Zhu, D. B. Thin Solid Films 1994, 242, 45. (d) Luo, C. P.; Huang, C. H.; Gan, L. B.; Zhou, D. J.; Xia, W. S.; Zhuang, Q. K.; Zhao, Y. L.; Huang, Y. J. Phys. Chem. 1996, 100, 16685. (21) (a) Yang, S. F.; Yang, S. H. J. Phys. Chem. B 2001, 105, 9406. (b) Li, X. G.; Yang, S. F.; Yang, S. H.; Xu, Y.; Liu, Y. Q.; Zhu, D. B. Thin Solid Films 2002, 413, 231. (22) (a) Nabok, A. V.; Lavrik, N. V.; Kazantseva, Z. I.; Nesterenko, B. A.; Markovskiy, L. N.; Kalchenko, V. I.; Shivaniuk, A. N. Thin Solid Films 1995, 259, 244. (b) Dei, L.; Casnati, A.; Lo Nostro, P.; Baglioni, P. Langmuir 1995, 11, 1268. (c) Dei, L.; Casnati, A.; Lo Nostro, P.; Pochini, A.; Ungaro, R.; Baglioni, P. Langmuir 1996, 12, 1589. (d) AliAdib, Z.; Davis, F.; Hodge, P.; Stirling, C. J. M. Supramol. Sci. 1997, 4, 201. (e) Davis, F.; O’Toole, L.; Short, R.; Stirling, C. J. M. Langmuir 1996, 12, 1892. (23) Kazantseva, Z. L.; Lavrik, N. V.; Nabok, A. V.; Dimitriev, O. P.; Nesterenko, B. A.; Kalchenko, V. I.; Vysotsky, S. V.; Markovskiy, L. N.; Marchenko, A. A. Supramol. Sci. 1997, 4, 341. (24) Castillo, R.; Ramos, S.; Cruz, R.; Martinez, M.; Lara, F.; RuizGarcia, J. J. Phys. Chem. 1996, 100, 709. (25) (a) Lo Nostro, P.; Casnati, A.; Bossoletti, L.; Dei, L.; Baglioni, P. Colloids Surf., A 1996, 116, 203. (b) Dei, L.; Lo Nostro, P.; Capuzzi, G.; Baglioni, P. Langmuir 1998, 14, 4143.

Langmuir, Vol. 18, No. 22, 2002 8489 previously.26-28 p-tert-Butylcalix[8]arene (C8A) was purchased from Aldrich. Deionized water was purified by passing it through an EASYpure compact ultrapure water system (Barnstead Co., U.S.A.), and the purified water was used in all the experiments reported in this paper. Preparation of the Calixarene-Metallofullerene (or Fullerene) Complexes. The procedure for the synthesis of [Dy@C82-C8A] is similar to that of [C60-C8A].1,2 Dy@C82 and C8A were dissolved in toluene in a 1:1 stoichiometry with a concentration of 2.31 × 10-4 mol dm-3 (2.0 mL). A black precipitate ([Dy@C82-C8A]) was formed with a yield of 33% after the mixture was stirred for 72 h at room temperature. A 100% yield could be obtained after 9 days. For the C60-C8A system, the same procedure yielded a greenish solid ([C60-C8A]) with a yield of 100% although the reaction time was only 12 h. The complexes prepared were examined by n-butane DCI negative ion mass spectrometry (TSQ7000, Finnigan MATMS) and high-performance liquid chromatography (HPLC) analysis (5PBB column, Cosmosil, NACACAI Tesque, Japan; mobile phase, toluene; flow rate, 1.5 mL/min). We did not find any effects of the ambient light and oxygen on the complexation yields of the reactions. Preparation of the Supramolecular LB Films. A Langmuir minitrough (Applied Imaging, U.K.) was employed for the film fabrication. The air/water interface was thoroughly cleaned by a complete barrier movement to ensure a maximum surface pressure difference of 30 min), the supramolecular complexes at the air/water interface were compressed at a barrier speed of 1 cm/min, and the surface pressure-area (π-A) isotherm was recorded. For the series of spreading solutions, the number of moles of the complexes (N), which is determined by the concentration (C) and volume (V), was kept constant to facilitate comparison. LB films of the complexes were deposited from the air/water interface onto quartz plates (10 × 15 mm; Electronic Space Products International, U.S.A.) at a vertical dipping rate of ∼0.9 mm/min and a constant surface pressure (15 mN m-1). The quartz plates were hydrophilically treated beforehand by refluxing in 2-propanol for 24 h.29 UV-vis absorption spectra were recorded with a Milton Roy spectrometer (Spectronic 3000).

Results and Discussion Preparation and Characterization of the [Dy@C82C8A] and [C60-C8A] Complexes. According to Atwood (26) Ding, J. Q.; Yang, S. H. Chem. Mater. 1996, 8, 2824. (27) (a) Ding, J. Q.; Weng, L. T.; Yang, S. H. J. Phys. Chem. 1996, 100, 11120. (b) Ding, J. Q.; Yang, S. H. Angew. Chem., Int. Ed. Engl. 1996, 35, 2234. (28) (a) Huang, H. J.; Yang, S. H. J. Phys. Chem. B 1998, 102, 10196. (b) Gu, G.; Huang, H. J.; Yang, S. H.; Yu, P.; Fu, J.; Wong, G. K. L.; Wan, X.; Dong, J.; Du, Y. Chem. Phys. Lett. 1998, 289, 167. (29) (a) Petty, M. C. Langmuir-Blodgett Films: Characterization and Properties; Roberts, G. G., Ed.; Plenum Press: New York, 1990; p 167. (b) Petty, M. C. Langmuir-Blodgett Films: An Introduction; Cambridge University Press: Cambridge, 1996. (c) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991.

8490

Langmuir, Vol. 18, No. 22, 2002

Yang and Yang

Chart 1. Possible Conformations of Pure C8A (I), the Dy@C82/C8A Equimolar Mixture (II), [Dy@C82-C8A] (III), and [C60-C8A] (IV) in the LB Films Prepared at Different Spreading Solution Concentrationsa

Figure 1. UV-vis absorption spectra of the [Dy@C82-C8A] complex (A), pure Dy@C82 (B), C8A (C), and the Dy@C82/C8A equimolar mixture (D). The inset enlarges the spectral portion of 350-700 nm. a Perp ) Perpendicular Orientation; Para ) Parallel Orientation.

et al. and Shinkai et al., the [C60-C8A] complex (1:1) was easily prepared with a nearly 100% yield.1,2 We repeated the synthesis and recorded the UV-vis absorption spectrum of [C60-C8A] in CCl4, which is in good agreement with those reported in the literature.1,25b However, to form the [Dy@C82-C8A] complex, a much longer reaction period (>72 h) was needed. In addition, even after reaction for 72 h, the yield of the complex was only ∼33%. This may be related to the larger size of Dy@C82 (d ) 8.0 Å) (refs 12 and 30) than that of C60 (d ) 7.1 Å),2,31 which has to be accommodated by the cavity of C8A (d ) 8.6 Å based on the Corey-Pauling-Koltun (CPK) model calculation and the crystal structure).2 Alternatively, an entropy effect might be important, which is associated with the loss of the rotational freedom upon complexation with the calixarene. Since Dy@C82 has an elongated ball shape, it probably gets into the cavity along its long axis as shown in Chart 1(part III). As such, the entropy effect is expected to be more severe for Dy@C82 than for C60 due to its lower symmetry. Such an explanation was applied to the formation of [C70-C8A].3d,11d,31 The UV-vis spectrum of the [Dy@C82-C8A] complex in the CCl4 solution is shown in Figure 1 (curve A), and for comparison, also presented in the figure are the UVvis spectra of Dy@C82 (curve B), C8A (curve C), and an equimolar mixture of Dy@C82/C8A (curve D). Obviously, the characteristic absorption peaks of Dy@C82 and C8A of the equimolar mixture (292 nm for C8A; 263, 396, and 632 nm for Dy@C82) have disappeared in curve A of Figure 1. Instead, quite different UV-vis absorption features (265 and 276 nm) are observed, suggesting the formation of a complex of Dy@C82 and C8A. Figure 2I shows the HPLC profiles of [Dy@C82-C8A] (trace A), [C60-C8A] (trace B), pure C8A (trace C), and an equimolar mixture of Dy@C82/C8A (trace D). Only a prominent peak appears at the retention time (tret) of 1.9 min in trace A, which is close to that of pure C8A (trace C). However, the fraction of this peak displays the same UV-vis spectrum as curve A in Figure 1, and we therefore assign it to the complex [Dy@C82-C8A]. If the complex decomposition has occurred, the Dy@C82 peak should have appeared at tret ) 38.6 min (see trace D of Figure 2I). Such (30) This is a rough size along the short axes of M@C82. Along the elongation axes, it is ∼8.8 Å. See ref 12a. (31) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, 1996.

decomposition does occur for the [C60-C8A] complex, for which an intense peak at tret ) 6.3 min corresponding to C60 was observed in our HPLC analysis (trace B of Figure 2I). This shows that the [Dy@C82-C8A] complex, although more difficult to form than [C60-C8A], is actually more stable than [C60-C8A] in toluene once formed. The [Dy@C82-C8A] complex (in CCl4) was analyzed by n-butane negative ion DCI-MS as shown in Figure 2II. The MS peaks at m/z ) 1146.9 and 1296.7 are clearly associated with the complex constituents Dy@C82 and C8A, respectively, indicating the complex decomposition. The decomposition was presumably caused by the high temperature (200-1200 °C) in the source region of the mass spectrometer. A closer examination of the mass spectrum reveals a small peak at m/ z ) 2442.5, which can be assigned to the [Dy@C82-C8A] complex (1:1) (see the inset of Figure 2II). Unfortunately, the characteristic isotopic envelope of the complex has not been obtained because of the severe decomposition and consequently the low peak intensity. For the MS analysis of [C60-C8A], we failed to observe any signal at m/z ) ∼2017 corresponding to the complex. This is not surprising given the much more facile decomposition of [C60-C8A] than [Dy@C82-C8A] as demonstrated in the HPLC analysis above. The results presented above suggest that the barriers for both the formation and decomposition of [Dy@C82C8A] are higher than those of [C60-C8A]. According to our experience, [C60-C8A] is barely soluble and quite unstable against decomposition in most organic solvents. Even in CCl4, [C60-C8A] decomposes slowly after 2 weeks. On the other hand, [Dy@C82-C8A] in CCl4 is quite stable and does not change even after 3 months under ambient conditions. The higher barrier of the Dy@C82-C8A system may be explained by the size effect and the entropy effect as mentioned above, while the higher stability of [Dy@C82C8A] is likely due to the enhanced CH-π interaction between the electron-rich metallofullerene cage and the tert-butyl groups of the calixarene.32 Langmuir Films of [Dy@C82-C8A] and [C60-C8A]. Under our experimental conditions (298 K, CCl4 was used as spreading solvent), the π-A isotherms we obtained in this work were all reversible and reproducible with a distinctive condensed phase region, indicating the absence of aggregation on compression and expansion of the films (32) (a) Andreetti, G. D.; Ori, O.; Ugozzoli, F.; Alfieri, C.; Pochino, A.; Ungaro, R. J. Inclusion Phenom. 1988, 6, 523. (b) Ungaro, R.; Pochini, A.; Domiano, P.; Andreetti, G. D. J. Chem. Soc., Perkin Trans. 2 1985, 197. (c) Andreetti, G. D.; Pochini, A.; Ungaro, R. J. Chem. Soc., Perkin Trans. 2 1983, 1773.

Complex of Metallofullerene with Calix[8]arene

Langmuir, Vol. 18, No. 22, 2002 8491

Figure 2. (I) HPLC profiles for [Dy@C82-C8A] in CCl4 (A), [C60-C8A] in CCl4 (B), C8A in CCl4 (C), and the Dy@C82/C8A equimolar mixture in toluene (D). The peaks marked with an asterisk are due to CCl4. (II) DCI n-butane negative ion mass spectrum of the purified [Dy@C82-C8A] complex (in CCl4). The inset shows the blow-up view in the mass region of the complex. The peak marked with an asterisk is assigned to the minor impurity C88 in the original Dy@C82 solution.

below the collapse pressures. Such a reproducibility of the π-A isotherms in the compression-expansion cycles and the steep pressure rises in the condensed phase signify the reasonably good stability of the Langmuir monolayer films.29 A typical π-A isotherm of [Dy@C82-C8A] is shown in curve A of Figure 3, which was obtained with a spreading solution concentration of 2.0 × 10-5 mol dm-3 (0.5 mL). As shown in curve A, the isotherm displays a steep pressure increase in the condensed phase region. By extrapolating the condensed phase region of the π-A isotherm to zero pressure, the limiting area per molecule was determined to be 99 Å2/molecule, comparable with that of [C60-C8A] obtained at similar conditions.25b However, as shown in curves B-D of Figure 3, the π-A isotherms of the Langmuir monolayer films changed significantly with more diluted spreading solutions. Specifically, the isotherms show clearly extended liquid-

expanded to liquid-condensed phase regions and larger limiting molecular areas as the spreading solution was diluted. For comparison, Figure 3 also shows the π-A isotherm of an equimolar mixture of Dy@C82/C8A (8.2 × 10-6 mol dm-3, 1.2 mL) (curve E). The π-A isotherm exhibits more kinks and a smaller limiting molecular area (85 Å2/ molecule). In marked contrast to the case of [Dy@C82C8A], the π-A isotherm of the mixture was scarcely changed when the concentrations of the spreading solutions varied from 2.5 × 10-5 to 2.0 × 10-6 mol dm-3 (see the inset in Figure 5II). This indicates that the complex [Dy@C82-C8A] has not been formed although [C60-C8A] was reportedly produced using the same procedure.25b Instead, in the mixture Langmuir film, C8A perhaps assumes a perpendicular pleated conformation that supports a metallofullerene bilayer structure as shown in Chart 1 (part II).

8492

Langmuir, Vol. 18, No. 22, 2002

Figure 3. Surface pressure-area (π-A) isotherms of [Dy@C82C8A] (A-D) and the Dy@C82/C8A equimolar mixture (E) at the air-water interface (pH ) 6.5). The barrier compression speed was 2 cm min-1. The inset enlarges curves A and E near the phase transition regions. The π-A isotherm of pure C8A is also shown (F).

As a reference, the π-A isotherm of pure C8A is given as curve F in Figure 3 with a spreading solution concentration of 1.6 × 10-5 mol dm-3. The shape of the isotherm and the limiting molecular area (151 Å2/molecule) are quite consistent with the data from the literature.23-25 According to the CPK model, the ideal maximum area per molecule of pure C8A is ∼322 Å2/molecule when a pleatedloop conformation is adopted and oriented parallel to the air/water interface.33 We found that the limiting area per molecule does not approach this value until the spreading solutions are diluted to concentrations of