Q Copyright 1993 American Chemical Society
JUNE 1993 VOLUME 9, NUMBER 6
Letters Langmuir Films of
c60, CSOO, and C6lH2
Nicholas C. Maliszewskyj and Paul A. Heiney' Department of Physics and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104
David R. Jones, Robert M. Strongin, Maria A. Cichy, and Amos B. Smith, 111' Department of Chemistry, Monell Chemical Senses Center, and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received January 27,1993. In Final Form: April 2,1993 We report here the fvet observation of Langmuir f h of the buckminsterfullerene epoxide CmO and the 6,Gbridged fulleroid C61H2,as well as confirmation of previously reported Langmuir film formation of CBO. The measured limiting areas per molecule are 96 6,94 4, and 94 6 A2for CmO,C6& and Cw, respectively.
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The discovery' of an efficient synthesis for C ~ has O led to intense research on the properties of buckminsterfullerene and ita derivatives.2 The initial observation by Obeng and Bard3 of stable Langmuir films of CK,has been challengedby severalgroup8,l-Bwho report that such films are unstable. To explore further the limits of the stability of Langmuir films of fullerenes, we have studied the spreading behavior of CWat the air-water interface, as well as that of two functionalized derivatives: C61H2, the parent 6,6-bridged fulleroid recently described by Wudl (1)K r g h m e r , W.;Lamb, L.D.;Foetiropouloe, K.; Huffman, D.R. Nature 1980 347,364. (2)Fmher, J. E.;Heiney, P. A; Smith, A. B., III. Acc. Chem. Rea. 1992,25,112and references cited therein. (3)0beng, Y. S.;Bard, A. J. J. Am. Chem. Soc. 1991, 113, 6279. Jehoulet,C.;Obene,Y.S.;Kim,Y.-T.;ZhouF.;Bard,A.J. J.Am.Chem. Soc. 1992,114,4~7. (4)Milliken,J.; Dominguez.,D. D.;Nelson,H. H.; m e r , W. R Chem. Mater. 1992, 4, 262. N h u r a , T.;Tachibana, H.; Yumya, M.; Mataumoto, M.; Azumi, R.;Tanakn, M.; Kawabata, Y.Langmurr 1992, 8. 4. Willinma. C.: Pearson.. C.:. Brvce. - . M.:. Pettev, - . M. C.; Thin Solid Alma 1992,24?9,lb. (6)GUO,J.; Xu, Y.;Li,Y.;Y w ,C.; Yao, Y.;Zhu,D.; Bai, C. Chem. Phy6. h t t . 1992,196,626. (6) Back, R.;Lennox, R B. J. Phya. Chem. 1992,W,8149.
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et ale: and CWO,the fmt fullerene epoxide.8 The polar oxygen substituent in CmO should greatly enhance the stability of the Langmuir films, while the methylene group present on the surface of C8lH2 should contribute little or nothing to Langmuir film formation. In this letter we report the first obaervationsof Langmuir and Langmuk Blodgett films of CmO and CaH2; we ale0 c o n f i i the observations by Obeng and Bard3 of stable f h of Cm CwO, C61H2, and CWwere synthesized as deecribed previously.7~8Small amounta ( 10-9 g) of dry crystalline material were weighed on a Cahn electrobalance to an accuracy of 10-6 g. The solute was then dissolved in 1-1.5 mL of HPLC grade carbon disulfide, which was then diluted with 8-9 mL of HPLC grade methylene chloride. The resulting solution spread well and was immiscible with water, and the solventwas quite volatile (boilingpoint -40 "C). Parent solutions were serially diluted to make daughter solutionsof concentrations1VM) or not depositedwith sufficientuniformity (with the above spreading solvents), isotherms like those in Figure lc,e,g were observed,accompaniedby visible patchy domains, as reported by other^.^,^ Films of C61H2 and Cm were also quite sensitive to vibration^;^ films of CmO were somewhat less so. Following Gaines’ treatment,1° the limiting molecular area A0 was determined to be 95.9 Hi2 for CmO, 94.2 A2 for C61H2, and 94.1A2for Cm. Assumingthat the substituents are either attracted toward the water in the case of CmO (Figure 2) or repelled from it, as in the case of C61H2, and that the projection of the moleculesonto the water surface forms a triangular lattice of circular objects of radius r, the calculated area per molecule is 2(d3)r2. This yields nearest-neighbor distances of 10.5 f 0.6 A for CmO, 10.4 f 0.4 A for C61H2, and 10.4 f 0.6 A for Cm. Such distances are in good agreementwith the nearest-neighbordistances of 10.02,10.03, and 10.03A, in the close-packed (111)planes of Cm, CaO, and C61H2, re~pective1y.l~-l3 Films of Cm and CmO were sufficiently stable that it was possible to hold these systems at constant pressure without any noticeable area loss. This, however, was not the case for CelH2 at ambient temperatures. Attempts to transfer monolayer films of CmO and C61H2 by vertical dipping onto substrates such as clean and alkylated glass, quartz, and silicon were unsuccessful. On drawing the substrate through the film at constant pressure, the apparent surface area transferred, as measured by deflection of the movable barrier, was negligible. Subsequent observations of light specularly reflected from the film surface revealed a hole in the film made by the substrate. An alternative method for transferring the surface film is the horizontal “touch”technique,lowhich however suffers from the disadvantagethat there is no direct determination of the amount of surface area actuallytransferred. A series of quartz slides was alkylated by the method of Sagiv,14 and film transfers of CmO were attempted by either the dipping or touch method. A Hitachi U-2000 spectrophotometer was used to measure the UV/vis absorption spectrum in transmission through the slides. Specimens prepared via the touch method displayed at least 3 times more intensity in the measured spectrum (Figure 3) than did those prepared by vertical dipping, indicating that much more material was transferred by the former technique. In summary, our measurements have shown that, contrary to some reports,4p5 it is possible to form mono(10) Gahes, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (11) Heiney, P. A.; Fischer, J. E.; McGhie, A. R.; Romanow, W. J.; Denenstein, A. M.; McCauley, J. P.; Jr.; Smith, A. B., 111; Cox, D. E.; Phys. Rev. Lett. 1991,66, 2911 and references cited therein. (12) Solid CmO is face-centeredcubic (fcc) at room temperature,with a = 14.18 A: Vaughan, G. B. M.; Heiney, P. A.; Cox, D. E.; McGhie, A. R.; Jones, D. R.; Strongin, R. M.; Cichy, M. A.; Smith, A. B., 111. Chem. Phys. 1992,168,185. (13) Solid CslHz is also cubic at room temperature, with a = 14.19 Heiney, P. A.; Vaughan, G. B. M.; Strongin, R. M.; Brard, L.; Smith, A. B., 111; Stephens, P. W.; Liu, D. Unpublished results. (14) Sagiv,J. J. Am. Chem. SOC.1980, 102,92.
Letters
Langmuir, VoZ. 9, No. 6, 1993 1441
Figure 2. Schematic structure of CaO at the air-water interface, showing potential hydrogen bonds to water.
as well as bulk structure and UV/vis absorption data, of all three materials studied are quite similar. This is to be expected given their structural similarity. The stiffness of the condensed film and sensitivity to vibration make production of LB films by the vertical dipping technique impractical; the horizontal touch technique was more successful in this regard.
2* 0.00 7 200
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500
400
Wavelenath “
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nm
600
700
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Figure 3. W/vis absorption spectrum of CaO on quartz.
molecular films of derivatives of Ca at the air-water interface provided that extreme care is taken to prevent multilayerformation. C W O seemsto form Langmuir films more easily than C a or CsrHs. The isotherm behaviors,
Acknowledgment. We thank J. K. Blasie for the use of his film balance, and S. Xu and J. A. Chupa for their technical assistance. We are grateful to Y. S. Obeng for his technical assistance and for many useful discussions. This work was supported by the National ScienceFoundation through Grants DMFt 89-01219and DMR 91-20668.
