Langmuir 2005, 21, 919-923
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Supramolecular Structures of Coronene and Alkane Acids at the Au(111)-Solution Interface: A Scanning Tunneling Microscopy Study Brett J. Gyarfas, Bryan Wiggins, Monica Zosel, and K. W. Hipps* Department of Chemistry and Materials Science Program, Washington State University, Pullman, Washington 99164-4630 Received September 11, 2004. In Final Form: October 29, 2004 Scanning tunneling microscopy (STM) is utilized to study the solution-solid interface formed between Au(111) and solutions of coronene in hexanoic, heptanoic, and octanoic acids. In all three cases adsorbed coronene is observed and lays flat on the metal surface. Heptanoic and hexanoic acid solutions produce a hexagonal symmetry monolayer. For the heptanoic and hexanoic cases, dipole-image dipole interactions and H bonding stabilize a surface structure in which 12 acid molecules surround each coronene and produce a coronene spacing of 1.45 nm. In the case of octanoic acid as solvent, the incorporation of the solvent into the monolayer is not as strongly favored. The coronene spacing can range from close-packed (1.2 nm) with no solvent presumed present in the monolayer, to 1.50 nm with up to 12 solvent molecules surrounding each coronene. The close-packed regions have hexagonal symmetry, as do those with the largest (1.5 nm) spacing. Heptanoic acid solutions give the clearest STM images and are associated with the most stable two-component monolayer. The present paper demonstrates that non-covalent interactions at the solution-metal interface can lead to complex multicomponent monolayer structures.
Introduction Supramolecular chemistry is chemistry that uses molecules rather than atoms as building blocks. Weak intermolecular forces, not covalent bonds, are used to assemble by design large structures from tailored molecules.1,2 Since the pioneering work of Lehn, Cram, and Pedersen,3 there has been a steadily increasing interest in the development and application of supramolecular chemistry. In its beginning, focus was on molecular recognition, which is the selective binding of a guest by a host using non-covalent interactions.4 As the field grew, the rational design of molecular crystals using supramolecular interactions became an area of interest.5-7 Hydrogen bonding is the most common supramolecular interaction; however, other interactions including halogenhalogen,8 halogen-nitrogen,9,10 halogen-oxygen,7 electrostatic interactions, 11-15 and weak electron donoracceptor complexation16 have been used to organize molecules within a crystal. * To whom correspondence E-mail:
[email protected].
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(1) Lehn, J. M. Supramolecular Chemistry: Concept and Perspectives; VCH: Weinheim, Germany, 1995. (2) Lehn, J. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4763. (3) 1987 Noble Prize in Chemistry. (4) Gale, P. A. Philos. Trans. R. Soc. London, Ser. A 2000, 358, 431. (5) Reddy, D. S.; Craig, D. C.; Desiraju, G. M. J. Am. Chem. Soc. 1996, 118, 8, 4090. (6) Gillard, R.; Stoddard, J.; White, A.; Williams, B.; Williams, D. J. Org. Chem. 1996, 61, 4504. (7) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311. (8) Scmhidt, G. M. Pure Appl. Chem. 1971, 27, 647. (9) Reddy, D. S.; Ovchinnikov, Y.; Shishkin, Y.; Struchkov, Y.; Desiraju, G. J. Am. Chem. Soc. 1996, 118, 4085, and references therein. (10) Xu, K.; Ho, D.; Pascal, R. J. Org. Chem. 1995, 60, 7186. (11) Coates, G.; Dunn, A.; Henling, A.; Dougherty, D.; Grubbs, R. H. Angew. Chem., Int. Ed. Engl. 1997, 36, 248. (12) Coates, G.; Dunn, A.; Henling, L.; Ziller, J.; Lobkovsky, E.; Grubbs, R. H. J. Am. Chem. Soc. 1998, 120, 3641. (13) Gillard, R.; Stoddard, J.; White, A.; Williams, B.; Williams, D. J. Org. Chem. 1996, 61, 4504. (14) Williams, J. H. Acc. Chem. Res. 1993, 26, 593. (15) Hunter, C.; Lu, X.; Kapteijn, G.; Koten, G. J. Chem. Soc., Faraday Trans. 1995, 91, 2009.
Most recently, the desire to understand the solutionsolid interface,17-21 and the design of nanostructures by bottom-up methods,22-26 has driven the field of supramolecular chemistry from the three-dimensional (3D) realm to the two-dimensional (2D). The discovery and application of the scanning tunneling microscope has made this evolution to 2D supramolecular studies possible. The design strategies discovered for 3D supramolecular chemistry can be applied to the adsorbed state. Moreover, there is a particular relevance of the surface state to supramolecular synthesis with physisorbed molecules, because many of the weaker interactions used in generating synthons are probably distorted or destroyed in fluid solution and in chemisorption. The weak lateral forces exerted by the surface upon physisorbed molecules, and the image charges that occur in metal substrates, allow these weaker intermolecular forces to play a significant role in the formation of long-range order in the adsorbed phase. While some of the supramolecular synthons developed for crystal synthesis may be inappropriate for generating surface structures because of the planar template effect of the substrate, others may have their stability enhanced by the reduction in entropy, the steric constraints imposed by the surface, and the electrostatic (16) Bosch, E.; Radford, R.; Barnes, C. Org. Lett. 2001, 3, 881. (17) De Feyter, S.; De Schryver, F. C. Chem. Soc. Rev. 2003, 32, 139. (18) Yablon, D. G.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 2000, 104, 7627. (19) Yablon, D.; Guo, J.; Knapp, D.; Fang, H.; Flynn, G. W. J. Phys. Chem. B 2001, 105, 4313. (20) Cai, Y.; Bernasek, S. L. J. Am. Chem. Soc. 2003, 125, 1655. (21) Yoshimoto, S.; Suto, K.; Itaya, K.; Kobayashi, N. Chem. Commun. 2003, 2174. (22) Whitesides, G. M.; Boncheva, M. Proc. Natl. Aacad. Sci. U.S.A. 2002, 99, 4769. (23) Griessl, S. J. H.; Lackinger, M.; Jamitzky, F.; Markert, T.; Hietschol, M.; Heckl, W. M. J. Phys. Chem. B 2004, 108, 11556. (24) Hipps, K. W.; Scudiero, L.; Barlow, D. E.; Cooke, M. P. J. Am. Chem. Soc. 2002, 124, 2126. (25) Scudiero, L.; Hipps, K. W.; Barlow, D. E. J. Phys. Chem. B 2003, 107, 2903. (26) Lei, S.; Wang, C.; Wan, L.; Bai, C. J. Phys. Chem. B 2004, 108, 1173.
10.1021/la047726j CCC: $30.25 © 2005 American Chemical Society Published on Web 12/30/2004
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environment provided in the case of a metallic substrate. We restrict our consideration to physisorbed molecules to ensure that the lateral intermolecular interactions can play a significant role in coadsorbate ordering. Scanning tunneling microscopy (STM) is the only technique that can provide detailed sub-nanometer structural analysis at the solid-solution interface in real time. Through its application, a view is provided of the elegant architectures that occur in what one might think was the relativelydisorderedinterfacebetweensolidandsolution.17-21,23,26 STM has also been key to characterizing 2D supramolecular structures resulting from vapor deposition on solid surfaces.24,25,27,28 In the present study we will use STM to probe the interfacial layer that results when Au(111) comes into contact with a solution of coronene in various alkane acids. Our interest in this system was generated by the rich spectrum of potential weak interactions. The alkane acid might interact with the gold through dipole-induced dipole interactions, leading to a standing configuration, or dispersion type interactions might produce a surface covered by alkane acids in the striped array (lamellae) normally seen on graphite. Coronene is known to physisorb on graphite from the vapor in a close-packed hexagonal structure with the coronene lying flat on the surface and having a lattice spacing of about 1.1 nm.29 STM studies of coronene adsorbed Au and Ag reported a coronene spacing of about 1.2 nm.30-32 Another possibility is that coronene may adsorb onto lamellae of alkane acids (similar to phthalocyanine and tridedycelamine (TDA) on graphite33). Finally, the potential for hydrogen bonding between coronene and the carboxyl must be considered. What we actually find is a competition between several of these factors. Experimental Section Hexanoic acid (C6O2H12), heptanoic acid (C7O2H14), and octanoic acid (C8O2H16), all g99%, were used as supplied by Sigma-Aldrich. Coronene was also provided by Aldrich and was labeled as sublimed and 99% purity. Epitaxial Au(111) films with well-defined terraces and single atomic steps were prepared on mica by previously described methods.34-37 These films were 0.1-0.2 µm thick and had a mean single grain diameter of about 0.3 µm. The gold films were hydrogen flame annealed just prior to use. Solutions were prepared by adding 0.30 mg of coronene to 10 mL of the alkane acid in a 10 mL volumetric flask. This concentration gave coronene adsorption but was not so high as to cause crystallization problems. Future work should explore the effects of concentration on the structures observed. A small magnetic stirring bar was placed in the flask and the solution stirred under low heat for 2-3 h until all the coronene had completely dissolved. A new piece of gold was used for each day’s experiments and for each type of solution. A total of 5-10 samples of each acid type was studied. (27) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. M. Single Mol. 2002, 3, 25. (28) Barth, J. V.; Weckesser, J.; Lin, N.; Dmitriev, A.; Kern, K. Appl. Phys. A 2003, 76, 645. (29) (a) Lackinger, M.; Griessl, S.; Heckl, W. M.; Hietschold, M. Anal. Bioanal. Chem. 2002, 374, 685. (b) Walzer, K.; Sternberg, M.; Hietschold, M. Surf. Sci. 1998, 415, 376. (30) Uemura, S.; Sakata, M.; Taniguchi, I.; Hirayama, C.; Kunitake, M. Thin Solid Films 2002, 409, 206. (31) Yoshimoto, S.; Narita, R.; Itaya, K. Chem. Lett. 2002, 356. (32) Lackinger, M.; Griessl, S.; Heckl, W. M.; Hietschold, M. J. Phys. Chem. B 2002, 106, 4482. (33) Lei, S.; Wang, C.; Wan, L.; Bai, C. J. Phys. Chem. B 2004, 108, 1173. (34) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391. (35) Lu, X.; Hipps, K. W.; Wang, X.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197. (36) Barlow, D.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 2444. (37) Scudiero, L.; Barlow, D.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 11899.
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Figure 1. Constant-current STM images of two different samples of coronene in octanoic acid on Au(111). (A) Bias voltage, -0.40 V; set point, 500 pA; coronene spacing, 1.47 nm. (B) Bias voltage, -0.70 V; set point, 300 pA; coronene spacing, 1.24 nm. The gray scale extends over 0.12 nm. The STM head used was produced by Digital Instruments (now Veeco Metrology). A Digital Instruments Nanoscope E controller was used to acquire the reported data. STM image analysis was performed with the SPIP38 commercial software package. Constant-current images are reported, and any filtering is indicated in the appropriate figure caption. Both etched and cut Pt0.8Ir0.2 tips were used. In-plane spacing measured by STM was calibrated using the graphite lattice. In-plane measurements have a precision of less than (0.04 nm and an average absolute error of
