17673
J. Phys. Chem. 1995,99, 17673-17676
Stable Monolayers and Langmuir -Blodgett Films of Functionalized Fullerenes Dirk M. Guldi, Yongchi Tian, and Janos H. Fendler* Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100
Hartmut Hungerbiihler Bereich Physikalische Chemie, Hahn-Meitner Institut, 14109 Berlin, Germany
Klaus-Dieter Asmus Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received: June 6, 1995; In Final Foim: August 30, 1995@
CW,derivatized by the bis(ethyl), 4a, the bis(n-propyl), 4b, the bis(dodecyl), 4c, and the bis(triethyeleneglyco1 monomethyl ether), 4d, esters of malonic acid was spread on water surfaces in a Langmuir film balance. The formation of high quality and stable monolayers and Langmuir-Blodgett films, with transfer ratios close to unity, was demonstrated only from 4d by determinations of the surface pressure us surface area isotherms, absorption spectra, and the Brewster angle and atomic force microscopic images. Introduction The esthetically pleasing structure of fullerenes and their remarkable physical and chemical properties have excited the imagination of chemists who have developed versatile syntheses and explored potential applications ranging from drug delivery to advanced nanostructured devices.' The organization of fullerenes into stable, two-dimensional arrays and threedimensional networks is an essential requirement for device construction. Although some reports have appeared on the spreading of fullerenes on water surfaces in a Langmuir the structure and stability of the films formed and their layer-by-layer transfer to solid substrates have remained less than optimal. We have, therefore, derivatized fullerene by the bis(ethyl), 4a, the bis(n-propyl), 4b, the bis(dodecyl), 4c, and the bis(triethyeleneglyco1monomethyl ether), 4,esters of malonic acid (Figure l), and examined monolayer and Langmuir-Blodgett (LB) film formation from them. Determinations of the surface pressure (n)vs surface area (A) isotherms, the absorption spectra, and the Brewster angle microscopic (BAM) and atomic force microscopic (AFM) images have provided evidence for stable and high quality monolayer formation from 4d in a broad range of concentrations. Importantly, good LB films, with high transfer ratios, have been formed from 4d monolayers. Conversely, 4a, 4b, and 4c have not produced stable and well-characterized monolayers or LB films.
(1)
(2)
R = ethyl (Zn); R = n-propyl (Zb); R * n-dodccyl (2c); R = metbyleneglpolmonomcthylethcr(2d). Clean-up procedures: 2. and 2 b . distillation; 2c recrystallized in m e h o l ; 2d flash chromatography(SO,), CHCl,/acctone = 9 v/v. General yield%85-
-
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90%
+Br2
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R = ethyl (3.); R = n-propyl (3b); R = n-dodecyl (3c); R = mcthyleneglycolmonomethylether (3d). Clean-up procedures: 3n and 3b distillation; 3c flash chromatography(SO,), n-hexane/toluene = U l (v/v); 3d - flash chromatogtaphy (SO,). CHCl,iacetonc = 9/1 (vlv), Yields: 3b, 3c = 80.85%; 36 = 55%.
-
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R = ethyl (4a), R = n-propyl (4b), R = n-dodccyl (4c), R = methylmeglycolmonomethylether (4d) Clean-up procedures 4n and 4 b . flash chromatography (SIO,). n-haanc/tolucne = 1/1 (v/v), 4e flash chromatography(SO& n-hcxaaeitolucne = 1/1 (v/v), 4d .flash chromatography (SiO,), CH,Ch/acctone = 9/1 (v/v) Ylelds 4b = 45%. 4c = 40%, 4d = 35%
-
Figure 1. Scheme for the syntheses and punfications of fullerenes.
Experimental Section The general methodology used for the syntheses of 4a, 4b, 4c, and 4d is summarized in Figure 1. The method is illustrated for the preparation of 4d. 3d (500 mg, 1.05 mmol) was added to a solution of c 6 0 (410mg, 0.57mmol) in 180 mL of toluene. Under nitrogen, solid sodium hydride (180mg, 7.5 m o l ) was added to the homogeneous, magenta-colored solution. The suspension was stirred at room temperature for 2 h with TLC control (chloroform:acetone = 9:l). The main spot at Rf = 0.3 was isolated (besides unreacted c 6 0 and some polar side products). Water (5 mL) was added to the red-wine-colored @Abstractpublished in Advance ACS Abstracts, November 15, 1995.
reaction mxture, and the toluene layer was separated and dried over MgS04. After filtration and evaporation of the toluene, the crude product was chromatographed on SiOz. Elution with CH2Ch:acetone = 9:1 gave unreacted C ~ (150 O mg) and product (4,140 mg, 35% based on reacted &). 'H NMR (250MHz, C6D6, room temperature) 6 3.15 (s, 6H), 3.36(t, 4H),3.50(m, 12H),3.53 (t, 4H), 4.43 (t, 4H). I3C NMR (62.9MHz, C a b , room temperature) 6 163.53,145.96,145.59,145.49,145.37,
145.06,144.96,144.88,144.13, 143.33,143.22,142.44,142.20, 141.12,139.59,72.36,72.15,71.02,70.89, 68.89,66.47,58.71, 53.02. FAB-MSmlz 1115 [(M H)']. Calculated amounts of toluene solutions of 4a, 4c, and 4d (0.1-0.2mM) were spread evenly on water in a commercial
+
0022-36541992099-17673$09.00/00 1995 American Chemical Society
Letters
17674 J. Phys. Chem., Vol. 99, No. 50, 1995
0
50
IO0
I50
200
SURFACE AREA (A) A*/molecule Figure 2. Surface pressure (ll)vs surface area (A) isotherms of 4s (0).4b (m), 4c (v),and 4d (A).The arrows indicate the collapse area (at ll = 0)which corresponds to the area of a molecule of 4d floating on the water surface. Insert: Cartoon of a monolayer prepared from 4d showing the immersion of the side chain in water. Lauda Model P film balance. The surface of the water was cleaned by repeated compression, aspiration, and expansion cycles. The surface was deemed clean when the surface
pressure increase was less than 0.2 dynlcm upon compression to 1/20 of the original area and when this surface pressure increase remained the same subsequent to aging for several hours. Monolayers were transferred to solid substrates (quartz slides for absorption spectroscopy, mica for AFM) by using lifting rates of 0.1 c d m i n . Transfer ratios were determined to be between 0.90 and 1.2. Water was purified by using a Millipore Milli-Q filter system provided with a 0.22-pm filter at the outlet. Brewster angle microscopy (BAM) was carried out by means of a home-constructed system.I0 Monolayers were compressed in a rectangular (95 mm x 350 mm) Teflon trough which was equipped with a Wilhelmy sensor. The beam of a polarized argon laser (11 mW at 1 = 488 mm) was directed to the water surface by mirrors which were attached to a precision xyz rotation mount. The phase-polarized laser beam reached the monolayer surface at the Brewster-angle (ca. 54") at a spot size of 1.2 mm in diameter. The refracted beam was absorbed by a piece of black Teflon which was placed at the bottom of the trough. A lens (fl = 2.4 cm) was used to collect the reflected light and focus it onto a CCD camera (MTI CCD 72; sensitivity = 0.002 lux). All structures appeared to be compressed in the vertical direction. The images were videotaped, frame-grabbed, and printed. Absorption spectra were taken on a Hewlett-Packard 8452A diode array spectrophotometer. AFM measurements were performed on a Topometrix Explorer 2000 scanning force microscope using its 2 pm x 2
Figure 3. BAM images of 4c prior (a) and subsequent ( h ) IO compression. BAM images of 4d prior to compre*rion. at l l = 5 mNim ( e ) . and subsequent to compression to nearly the collapse pressure. n = 35 mN/m (d). BAM images were taken on the water rurf;lce as dcscrihed in the Experimental Section. The length of each image shown is 200 pm.
Letters
J. Phys. Chem., Vol. 99,No. 50, 1995 176'15
75 hi?
15u nm
75 An
15C m
1.0 nm
0.5 nm
O m 0 m
Figure 4. Typical two-dimensional AFM image (top) and height vs distance line scan (bottom) of 4d Langmuir films subsequent to their vansfer
to mica. See Experimental Section for details.
pm scanner and a Si3N4 tip with a spring constant of 36-44 Nlm, F. = 176 kHz. Images were taken in air at rmm temperature using the tip in the contact mode. Images were taken subsequent to achieving stability (ca. 30 min) on at least three different areas. Results and Discussion Cyclopropylationof the fullerene core with malonate derivatives was selected as the synthetic methodology. The advantage of this approach is that, in contrast to previous syntheses,"-" it has uniformly led to the formation of a single isomer. The n-A behavior of the functionalized fullerenes, 4a, 4b, 4c, and 4d, was found to depend markedly on the structure of the alkyl chain on the malonic acid moiety, as is evident from their n vs A isotherms (Figure 2). Obviously, 4a, 4b, and 4c formed stable surface layers on water with collapse pressures of -70 mNIm. High slopes in the n-A isotherms indicated
the presence of incompressible and condensed two-dimensional phases and good transferabilities. Extrapolation of the linear parts of the isotherms, due to the solid phase of the monolayer, to zero surface pressure led to the assessment of the surface area of a fullerene moiety to be 29 f 2.31 f 2, and 43 f 2 Az for 4a. 4b, and 4c, respectively. These values are markably smaller than those estimated by a hexagonal space-filling monolayer model, suggesting the formation of multilayers. In contrast, n-A isotherms of 4d displayed a liquidlike region below 15 mNlm and a condensed region above this pressure, indicating a phase transition. The value of A obtained for 4d by extrapolation from the condensed phase to n = 0 (96 A2/ molecule) is in a satisfactory agreement with that reported previously for fullerene (93 A2/molecule),' indicating the formation of a true monolayer in which the polar side chain of the fullerene is directed to and hydrated by water (see insert in Figure 2). It should he noted that the spreading behavior of 4d
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17676 J. Pkys. Ckem., Vol. 99, No. 50, 1995 06
by absorption spectrophotometry. The absorption spectra of LB films, prepared from 4d, are characterized by maxima at 338, 267, and 222 nm (Figure 5 ) , as reported for c 6 0 derivatives.' The insert displays a plot of absorbances at 338 nm vs the number of layers transferred for LB films prepared from 4c and 4d. The linear dependence indicates satisfactory stacking of the fullerenes in the LB films, but the higher slope for 4c is indicative of multilayer stacking.
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Balancing the hydrophobic and hydrophilic parts in fullerene derivatives has been demonstrated, in the present letter, to be of paramount importance for forming stable and true monolayers. Our ability to form high-quality monolayers and LB films from functionalized fullerenes permits the viable construction of devices and sensors from these fascinating molecules.
Figure 5. Absorption spectra of LB films prepared from 4d. The insert shows plots of absorbances at 338 n m LISnumber of layers transferred for LB films prepared from 4c (m) and 4d ( 0 ) .
remained independent of its concentration in the range 0.1 2.0 mM, although, at the highest concentration, the estimated area occupied by a fullerene moiety increased to 140 A2 due to the interactions of the fullerene molecules in the liquidlike phase. The smaller surface areas per molecule for 4a, 4b, and 4c are indicative of multilayer formation. Fruitful information on the spreading behavior of functionalized fullerenes was obtained by means of BAM. Thus, images taken immediately after placing a drop of 4c on the water surface show randomly shaped, relatively bright islands (Figure 3a). The bright islands could be pushed together by lateral compression without an apparent change in their brightness (compare parts a and b of Figure 3). These results indicate uneven packing and the absence of appreciable phase transition, in accord with the observed II vs A isotherm (Figure 2 ) for 4c. In contrast, quite different BAM images were observed upon compressing 4d (Figure 3c,d). Domains, formed at a very low surface pressure (Figure 3c), enlarged gradually with increasing surface pressure, and their brightness increased until the film collapsed (compare Figure 3c,d). Once again, these observations are in agreement with the observed phase transition for 4d (Figure 2). AFM images of films prepared from 4c and 4d, at 20 mN/m surface pressure, provided evidence for the thicknesses of these films. The thickness of the film formed from 4c (25 i 5 A) is clearly greater than that which would correspond to a monolayer (ca. 10 A). Conversely, the formation of monolayer films from 4d was unequivocally demonstrated by AFM; the thickness of the film formed (7 i 3 A, Figure 4) corresponds to the diameter of fullerene (ca. I O A). Monolayers prepared from 4c and 4d were transferred to solid substrates by the LB technique, and the transfer was monitored
Acknowledgment. Support of this work by a grant from the National Science Foundation is gratefully acknowledged. D.G. also wishes to thank the Humboldt Foundation (Germany) for support in the form of a Feodor H. Lynen Fellowship. Part of this work was supported by the Office of Basic Energy Sciences of the Department of Energy (this is contribution No. NDRL-3841 from the Notre Dame Radiation Laboratory). References and Notes (1) Kroto, W. H.: Fischer. J. E.: Cox, D. E. Fullerenes: Pergamon Press: Oxford, 1993. Billups, W. E.; Ciufolini, M. A. Buckminste~uirllerenes,. VCH Publishers: New York. 1993. Braun. T. Angew. Chem., Int. Ed. Engl. 1992, 31. 588. (2) Obeng, Y. S.; Bard, A. J. J. Am. Chem. SOC. 1991. 113, 6279. Jehoulet, C.: Obeng, Y. S . ; Kim, Y.-T.; Zhou. F.; Bard, A. J. J. Am. Chem. SOC. 1992. 114. 4231. Bulhoes, L. 0. S.; Obeng, Y. S.; Bard. A. J. Chem. Mater. 1993, 5. 110. (3) Maliszewskyj, N. C.; Heiney, P. A,; Jones, D. R.: Strongin, R. M.: Chichy. M . A,: Smith, A. B., I11 Langmuir 1993. 9, 1439. (4) Nakamura, T.; Tachibana. H.: Yumura, M.: Matsumoto. M.: Azumi. R.; Tanaka, M.; Kawabata, Y. Langmuir 1992. 8, 4. Nakamura, T.: Tachibana. H.; Yumura. M.: Matsumoto, M.; Tagaki. W. Synrh. Met. 1993. 55-57, 3131. (5) Haxker. C. J.: Saville. P. M.; White, J. W. J . Org, Chem. 1994, 59, 3503. (6) Zhou. D.: Gan, L.: Luo. C.: Tan. H.: Huang, C.; Liu. Z: Wu. 2.: Zhao. X.: Xia. X.: Z h a nca S.: Sun. F.: Xia. Z.: Zou. Y. Chem. Phvs. Lett. 1995, 235, 548 (7) Vaknin. D Wane. J Y . Uuhaus. R A Langmurr 1995, 11. 1435 (8) Maggini. M.: Kazsson, A.;'Pasimeni. L.: Scorrano, G.: Prato, M.: Valli. L. Tetrahedron Lett. 1994. 35. 2985. (9) Ravaine. S.; Le Pecq, F.: Mingotaud. C.; Delhaes. P.; Hummelen, J. C.: Wudl. F.: Patterson, L. K. J. Phys. Chem. 1995, 99, 9551. (10) Tian, Y . ; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98. 4913. (1 I ) Arias, T.. Echegoyen. L.. Wilson. S . R.; Lu. Q, J . Am. Chem. SOC. 1995, 117, 1322. (12) Isaacs. L.: Ehrsig, A,. Diederich, F. Helv. Chim. Acra 1993, 76. 1231. (13) Diederich. F.; Jonas, U.;Gramlich, V.; Henmann, A. Ringsdorf. H.: Thilgen, C. Helv. Chim. Acra 1993. 76. 2445.
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