Langmuir 1998, 14, 821-824
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Self-Assembled Fullerene-Derivative Monolayers on a Gold Substrate Using Phenanthroline-Au Interactions Olaf Domı´nguez and Luis Echegoyen* Department of Chemistry, University of Miami, Coral Gables, Florida 33124
Fred Cunha and Nongjian Tao Department of Physics, Florida International University, Miami, Florida 33199 Received August 22, 1997. In Final Form: December 2, 1997
Bromination of commercially available 5,6-dimethyl-1,10-phenanthroline with N-bromosuccinimide led to the formation of 5,6-bis(bromomethyl)-1,10-phenanthroline, a new compound, in 33% isolated yield. Conversion of the brominated compound to its corresponding o-quinodimethane intermediate was accomplished by reaction with tetrahexylammonium iodide. Reaction of this intermediate with C60 in refluxing toluene resulted in the formation of the final product, phenanthrolyl[60]fullerene, compound 1, in a 43% isolated yield. Spontaneous self-assembly of 1,10-phenanthroline on a Au(111) surface resulted in the formation of well-ordered monolayers. Addition of 1 to these monolayers resulted in the intercalation of the phenanthrolyl group directly into the stacks. Self-assembly from a solution of 1 containing small amounts of 1,10-phenanthroline resulted in the formation of a secondary layer of fullerene moieties. Since the fullerene diameter is approximately 1.0 nm and the phenanthroline-phenanthroline distances are about 0.33 nm (almost exactly 1/3), the fullerene packing is approximately commensurate with that of the phenanthrolines.
Fullerene as well as fullerene derivative monolayers have been prepared and characterized by a variety of techniques, and these were reviewed recently.1 After the large scale preparation method for fullerenes first appeared in 1990,2 reports of the formation of fullerene films via thermal evaporation and solution casting soon followed.3 The techniques of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been fundamental in the characterization of these films.3 Besides evaporation and casting methods, Langmuir trough techniques have also been employed to prepare monolayer films of fullerenes and of their amphiphilic derivatives.1,4 Moreover, well-ordered fullerene films have been obtained using self-assembly techniques, in which the molecules spontaneously adsorb on a variety of surfaces selected on the basis of their chemical complementarity.5 For example, thiol derivatives of C60 spontaneously adsorb on Au surfaces to yield well-ordered monolayers.5a A much less explored area in the field of self-assembly is the use of bipyridine (bpy) and pyridyl-like compounds (1) Mirkin, C. A.; Caldwell, W. B. Tetrahedron 1996, 52, 5113. (2) Kra¨tschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (3) Some representative (not exhaustive) publications are as follows: (a) Wilson, R. J.; Meijer, G.; Bethune, D. S.; Johnson, R. D.; Chambliss, D. D.; de Vries, M. S.; Hunziker, H. E.; Wendt, H. R. Nature 1990, 348, 621. (b) Wragg, J. L.; Chamberlain, J. E.; White, H. W.; Kra¨tschmer, W.; Huffman, D. R. Nature 1990, 348, 623. (c) Altman, E. I.; Colton, R. J. Surf. Sci. 1992, 279, 49. (d) Altman, E. I.; Colton, R. J. J. Phys. Rev. B 1993, 48, 18244. (e) Altman, E. I.; Colton, R. J. Surf. Sci. 1993, 295, 13. (f) Li, Y. Z.; Patrin, J. C.; Chander, M.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E. Science 1991, 252, 547. (g) Hamza, A. V.; Balooch, M. Chem. Phys. Lett. 1993, 201, 404. (h) Hamza, A. V.; Balooch, M. Chem. Phys. Lett. 1992, 198, 603. (4) For a comprenhensive reference, see: Jonas, U.; Cardullo, F.; Belik, P.; Diederich, F.; Gu¨gel, A.; Harth, E.; Herrmann, A.; Isaacs, L.; Mu¨llen, K.; Ringsdorf, H.; Thilgen, C.; Uhlmann, P.; Vasella, A.; Waldraff, C. A. A.; Walter, M. Chem. Eur. J. 1995, 1, 243 and references therein.
to form monolayers on Au surfaces.6 In these reports it has been found that pyridyl nitrogens adsorb strongly to Au surfaces, in much the same way as thiol sulfurs do. For example, recent solution STM studies have shown that very well-ordered stacks of bpy monolayers can be formed on a Au(111) surface.7 Presumably, both the interaction between the pyridyl nitrogens and the gold surface as well as the lateral π-π interactions between the bpy’s contribute to the significant stability of these monolayers. Most importantly, once these well-ordered monolayers are formed, they are very robust and remain intact even after rinsing with a water-electrolyte solution.7b They are thus comparable to the thiol-based self-assembled monolayers (SAMs) which are currently being exploited in many applications.8 Particularly noteworthy in the field of pyridyl-based monolayers is the work of Abrun˜a and co-workers.6c,d They have used pyridyl (5) (a) Shi, X.; Caldwell, W. B.; Chen, K.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 11598. (b) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945. (c) Tsukruk, V. V.; Lander, L. M.; Brittain, W. J. Langmuir 1994, 10, 996. (d) Li, D.; Swanson, B. I. Langmuir 1993, 9, 3341. (e) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993, 115, 1193. (f) Chupa, J. A.; Xu, S.; Fischetti, R. F.; Strongin, R. M.; McCauley, J. P., Jr.; Smith, A. B., III.; Blasie, J. K. J. Am. Chem. Soc. 1993, 115, 4383. (g) Arias, F.; Godı´nez, L. A.; Wilson, S. R.; Kaifer, A. E.; Echegoyen, L. J. Am. Chem. Soc. 1996, 118, 6086. (6) (a) Yang, D.; Bizzotto, D.; Lipkowski, J.; Pettinger, B.; Mirwald, S. J. Phys. Chem. 1994, 98, 7083. (b) Armstrong, F. A.; Hill, H. A. O.; Walton, N. J. Acc. Chem. Res. 1988, 21, 407. (c) Hudson, J. E.; Abrun˜a, H. D. J. Phys. Chem. 1996, 100, 1036. (d) Maskus, M.; Tirado, J.; Hudson, J.; Bretz, R.; Abrun˜a, H. D. In Physical Supramlecular Chemistry; NATO ASI Series; Echegoyen L., Kaifer, A. E., Eds.; Kluwer Academic Publishers: Vol. C485, 1996; pp 337-353. (7) (a) Cunha, F.; Tao, N. J. Phys. Rev. Lett. 1995, 75, 2376. (b) Cunha, F.; Tao, N. J.; Wang, X. W.; Jin, Q.; Duong, B.; D’Agnese, J. Langmuir 1996, 12, 6410. (8) (a) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1994, 141, 2018. (b) Abbott, N. L.; Rolison, D. R.; Whitesides, G. M. Langmuir 1994, 10, 2672. (c) Mrksich, M.; Whitesides, G. M. Trends Biotechnol. 1995, 13, 228. (d) Ullman, A. Chem. Rev. 1996, 96, 1533.
S0743-7463(97)00949-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/24/1998
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Domı´nguez et al. Scheme 1
nitrogen tethers to adsorb transition-metal complexes directly onto Au(111) and Pt(111) surfaces. Here we report the first solution STM evidence for the formation of a well-ordered 1,10-phenanthroline (phen) monolayer on a Au(111) substrate. We also report the first evidence of the incorporation of a novel phenanthrolyl[60]fullerene derivative (1) into these monolayers and suggest that the stability of the resulting layers derives from the phenAu interactions, the phen-phen π-π interactions, and from the fullerene-fullerene interactions. The methods presented here may represent a useful alternative for the preparation of stable and well-ordered fullerene layers on gold. The preparation of compound 1 was recently reported in a preliminary communication.9 Briefly, it was synthesized by reacting 5,6-bis(bromomethyl)-1,10-phenanthroline with C60 in refluxing toluene, in the presence of tetrahexylammonium iodide, Scheme 1.10 The reaction, which proceeds via an o-quinodimethane intermediate, resulted in a 42.8% isolated yield of 1. The precursor 5,6-bis(bromomethyl)-1,10-phenanthroline, an unreported compound, was prepared in 33% yield from 5,6-dimethyl1,10-phenanthroline and N-bromosuccinimide (NBS) in CHCl3 as described in more detail in ref 9. Au(111) substrates were grown epitaxially on mica under ultrahigh vacuum (UHV) conditions11 and then were briefly flame-annealed before each experiment. The 23 (9) Arias, F.; Boulas, P.; Zuo, Y.; Domı´nguez, O.; Go´mez-Kaifer, M.; Echegoyen, L. In Fullerenes: Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical Society, Inc.: Pennington, NJ, 1996; Vol. 3, pp 165-176. (10) Phenanthrolyl[60]fullerene (1): A solution of 5,6-bis(bromomethyl)-1,10-phenanthroline (8.7 mg, 0.024 mmol) and C60 (81.9 mg, 0.114 mmol) in 125 mL of toluene was prepared and purged with nitrogen for 15 min. Then tetrahexylammonium iodide (THAI in Scheme 1, 62.8 mg, 0.130 mmol) dissolved in 20 mL of toluene was added, and the mixture was heated to reflux and allowed to react for 16 h under a nitrogen atmosphere. The solution was then filtered and dried. The solid was washed with ethanol and redissolved in carbon disulfide to proceed with column chromatography (Silica Gel). Unreacted C60 was recovered via elution with toluene, whereas pure compound 1 was eluted with a 9:1 mixture of chloroform-methanol to afford a 42.8% isolated yield. 1H NMR (400 MHz, CDCl3:CS2 (1:1), 295 K) δ 5.13 (s broad, 4H), 7.85 (dd, 2H), 8.86 (dd, 2H), and 9.31 (dd, 2H) ppm; measurement at 243 K: δ 4.89 (d, 2H) and 5.34 (d, 2H) ppm; UV-vis (THF) 259, 289, 304, 407, 434, 706 nm; MS (FAB) 926.78 (M+) and 720.31(C60+). (11) DeRose, J. A.; Lampher, D. B.; Lindsay, S. M.; Tao, N. J. J. Vac. Sci. Tecnol. 1993, 11, 776.
× x3 reconstruction was reproducibly observed at negative electrode charges.12 The STM experiments were carried out on a Pico-SPM (Molecular Imaging Co.) or on a Nanoscope III instrument (Digital Instrument Inc.). While all measurements reported herein were performed in a 0.1 M NaClO4 solution to reduce contamination, images obtained in air were very similar. A homemade Teflon cell, which was cleaned in 70% H2SO4 + 30% H2O2, was used for the STM measurements. The STM tips were electrochemically etched Pt0.8Ir0.2 wires which were coated with Apiezon wax. These tips give a typical leakage current of less than a few picoamperes. The STM images were obtained with a typical tunneling current of 150 pA and a bias voltage between 100 and 250 mV. Figure 1a is an image of a phen monolayer selfassembled on Au(111) by dipping a Au(111) substrate either in a 1.0 mM phen + 0.1 M NaClO4 solution or in a saturated phen solution in tetrahydrofuran (THF). Similar to the previously studied heterocyclic compounds adenine,13 guanine,13 2,2′-bpy, and 4,4′-bpy,7 phen molecules stand vertically with the nitrogen atoms facing the substrate and self-assemble into polymer-like chains in which individual molecules (elongated blobs) stack with a repeat distance of 0.38 ( 0.01 nm along a chain (see Figure 1b). The image also shows that each molecule is tilted from the chain axis by 65 ( 5°, leading to an intermolecular distance of 0.33 ( 0.01 nm (Figure 1b), which corresponds well to the distance between two perfectly π-stacked aromatic molecules.14 The separation between two adjacent chains (see Figure 1b) is about 1.07 ( 0.02 nm, remarkably close to the value reported for the nearest neighbor distance in crystalline C60.15 By exposing the self-assembled phen monolayer to a saturated solution of 1 in THF for several minutes and then rinsing with 0.1 M NaClO4, it was possible to intercalate 1 within the phen chains. Figure 2 shows a STM image of the phen monolayer with embedded 1 (bright (12) (a) Tao, N. J.; Linsday, S. M. J. Appl. Phys. 1991, 70, 5141. (b) Gao, X.; Weaver, M. J. J. Chem. Phys. 1991, 95, 6993. (c) Wang, J.; Davenport, A. J.; Isaacs, H. S.; Ocko, B. M. Science 1992, 255, 1416. (13) Tao, N. J.; DeRose, J. A.; Lindsay, S. M. J. Phys. Chem. 1993, 97, 910. (14) Hunter, C. A.; Sanders, K. M. J. Am. Chem. Soc. 1990, 112, 5225 and references therein. (15) Dietz, P.; Fostiropoulos, K.; Kra¨tschmer, W.; Hansma, P. K. Appl. Phys. Lett. 1992, 60, 62.
Fullerene-Derivative SAMs on Gold
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Figure 2. 2. STM image showing that the C60 derivative molecules intercalate into the phen chains after exposing a phen monolayer to a saturated solution of 1 in THF. Features smaller than 0.1 nm have been removed from the image with a Fourier filter.
Figure 1. 1. (a) STM image of a phen monolayer self-assembled on Au(111) from 0.1 M NaClO4. Features smaller than 0.1 nm have been removed from the image with a Fourier filter. (b) and (c) show a proposed model for the molecular packing based on the STM image. The filled circles in (c) represent nitrogen atoms which face the Au surface.
dots). The apparent diameter and apparent height of an isolated fullerene derivative from the STM image are about 1.0 and 0.3 nm, respectively, which are similar to the results obtained for an isolated C60.16 Noticeably important from Figure 2 is that no fullerene derivative molecule is located between the phen chains, which suggests that each one of them is intercalated into the chains (see Figure 3). This phenomenon is energetically favorable since it (16) Lang, H. P.; Thommen-Geiser, V.; Frommer, J.; Zahab, A.; Bernier, P.; Guntherodt, H. J. Europhys. Lett. 1992, 18, 29.
Figure 3. 3. The intercalation of 1 can be understood as the phenanthrolyl group of the C60 derivative forms p-stacks with the phen molecules. The intercalation does not seriously disturb the molecular packing of phen since the diameter of a C60 molecule corresponds roughly to three phen molecules along a chain and also to the distance between to adjacent chains.
allows the phen groups of 1 to participate in the π-stacking interactions of the phen monolayer. We have varied the exposition time of the phen monolayer to the fullerene derivative solution from several minutes to 5 h and observed no large increase in the coverage of the C60 derivative. This suggests that the few molecules of 1 that
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mM) that also contained 2 mM phen. Figure 4 is an STM image of this sample, which shows a much higher coverage of 1 than that evident in Figure 2. The tendency of the fullerene derivative molecules to stack is also visible as outlined in the figure. Along a chain, the center-to-center distance between two adjacent C60 derivative molecules is about 1.1 nm, which corresponds roughly to the distance of three stacked phen molecules (see Figure 3). Significantly, in the absence of phen, pure compound 1 forms a disordered layer on Au(111) from a saturated THF solution, and it is not stable enough to resist the scanning tip. Continuously scanning over a small area of this layer triggers a diminution in the coverage of 1 as the fullerene derivative molecules are swept away by the tip. Zooming out to a larger area clearly shows a window created by the scanning tip in the C60 derivative monolayer. This observation supports the suggestion that the π-π interaction between phen and 1 stabilizes the adsorption of the latter onto the Au surface via π-stacking. Conditions to improve the level of coverage of the Au surface with 1 are currently under investigation. Figure 4. 4. STM image showing that a higher coverage of the C60 derivative can be obtained by exposing a Au(111) surface to a saturated solution of 1 in THF containing a small amount of phenanthroline. The tendency of 1 to follow the phenanthroline chains is highlighted.
do incorporate do so by selecting defect sites on the phen monolayer lattice until saturation is reached. To increase the coverage of 1, we have dipped the Au substrate into a saturated solution of 1 in THF (∼10-2
Acknowledgment. We gratefully acknowledge the generous financial support from AFSOR (F49620-96-10346) and NIH (GM-08205) to N.J.T., and from the NSF Chemistry Division (CHE-9313018) to L.E. In addition, O.D. thanks Dr. Richard A. Sachleben for his helpful advice, and Sandra Mendoza, Marielle Go´mez-Kaifer, and Francisco Arias for their cooperation. LA9709498
