Detecting Molecular Chirality by Scanning Tunneling Microscopy DAVID M. WALBA* AND FORREST STEVENS Department of Chemistry and Biochemistry and Optoelectronic Computing Systems Center, Campus Box 215, University of Colorado, Boulder, Colorado 80309-0215 NOEL A. CLARK AND DANIEL C. PARKS Department of Physics and Optoelectronic Computing Systems Center, Campus Box 390, University of Colorado, Boulder, Colorado 80309-0390 Received July 22, 1996 Introduction A highly interesting but challenging goal in the field of single-molecule detection by scanning probe micros- copy (SPM) is the direct observation of molecular chirality. In this regard, to our knowledge the direct detection of chirality at individual tetrahedral stereo- centers in organic molecules has yet to be achieved. Aside from obvious intrinsic interest in such a feat, development of techniques for reliable establishment of absolute configuration by SPM has potential practi- cal implications as well. An excellent system for probe microscopy of rela- tively complex organic “small” molecules involves imaging of liquid crystals (LCs) on graphite. 1 This approach has a long history in the scanning tunneling microscopy (STM) field, and is currently an active research area. In the LC on graphite system a small droplet of LC sample melted into the isotropic phase is applied to a freshly cleaved (0001) surface of highly oriented pyrolytic graphite. The STM tip is engaged while the substrate is held at a temperature where the organic thin film is liquid crystalline. Scanning sometimes produces images which are interpreted as deriving from a monolayer of molecules strongly physisorbed to the surface in a two-dimensional (2-D) crystal. This 2-D crystal, while composed of molecules which form liquid crystal phases, is NOT liquid crystalline. The bulk LC overlayer is apparently not directly observed in the experiment. These images are thought to result from a combina- tion of topographical and electronic factors producing intramolecular contrast in the STM experiment. Con- ventional wisdom holds that the 2-D crystals are usually heteroepitaxial with the graphite substrate, the packing mode being dominated by the well-known relatively strong interaction of the R hydrogens of all- anti aliphatic chains with the graphite. Furthermore, it is thought that primarily due to electronic factors aromatic rings appear bright in the STM experiment while aliphatic chains appear dark. More specifically, when imaging is done in constant current mode the tip-surface separation required to maintain constant current is apparently larger when the tip is over aromatic moieties than when the tip is over aliphatic molecular fragments. While many LCs are chiral, the vast majority of LC/ STM studies have involved achiral materials, in particular simple alkyl- and alkoxycyanobiphenyls. Our own work focuses on chiral smectic (i.e., layered) ferroelectric liquid crystals (FLCs), 2 where we have been exploring STM as part of an approach for understanding the FLC/solid surface interactions re- sponsible for FLC alignment. In the course of this work many chiral FLC systems have been studied by STM, affording images of chiral molecules and chiral 2-D crystals at molecular, and in some cases near atomic, resolution. A discussion of some highlights of this work, focusing on stereochemical issues, fol- lows. Near Atomic Resolution Imaging of a Chiral Epoxide During the course of early work aimed at obtaining fast-switching FLCs for electrooptic applications, many chiral epoxy ether phenylbenzoates were prepared. 3 The Sharpless epoxide 4-[(R,R)-(2,3-epoxyhexyl)oxy]- phenyl 4-[(S)-(3,7-dimethyloctyl)oxy]benzoate (com- pound 1, Figure 1), possessing three tetrahedral stereocenters and stable LC phases close to room (1) (a) Smith, D. P. E.; Ho ¨rber, H.; Gerber, C.; Binnig, G. Science 1989, 245, 43-45. (b) Smith, D. P. E.; Hoerber, J. K. H.; Binnig, G.; Nejoh, H. Nature (London) 1990, 344, 641-644. (2) Walba, D. M. In Ferroelectric Liquid Crystals: A Unique State of Matter; in Advances in the Synthesis and Reactivity of Solids; Mallouk, T. E., Ed.; JAI Press Ltd.: Greenwich, CT, 1991; Vol. 1, pp 173-235. (3) Walba, D. M.; Vohra, R. T.; Clark, N. A.; Handschy, M. A.; Xue, J.; Parmar, D. S.; Lagerwall, S. T.; Skarp, K. J. Am. Chem. Soc. 1986, 108, 7424-7425. David M. Walba was born in Oakland, CA, in 1949. He received his B.S. degree in Chemistry from the University of California, Berkeley, in 1971, and his Ph.D with Robert E. Ireland at Caltech in 1975. After a postdoctoral associateship with Donald J. Cram at the University of California, Los Angeles, he joined the faculty of the Department of Chemistry at the University of Colorado in 1977. His research interests revolve around organic stereochemistry, with a current emphasis on organic photonic materials, including in particular ferroelectric liquid crystals. Noel A. Clark was born in Cleveland, OH, in 1940. He received his Ph.D. degree in physics with George B. Benedek at the Massachusetts Institute of Technology in 1970, and held the positions of Research Fellow and Assistant Professor of Applied Physics at Harvard University before moving to the Department of Physics at the University of Colorado in 1977. Research in his group is directed toward understanding and using the properties of condensed phases, and ranges from experiments on the fundamental physics of phase transitions, such as melting, to the development of liquid crystal electrooptic light valves. Forrest Stevens was born in Walnut Springs, CA, in 1969. He received his B.S degree in Chemistry from Whitman College in 1991, and his Ph.D. with David M. Walba at the University of Colorado at Boulder in 1996. He is currently a faculty intern with Thomas P. Beebe at the University of Utah. Daniel C. Parks was born in Lexington, NE, in 1960. He received his Ph.D. in physics at the University of Colorado with Noel A. Clark. He held the position of research associate at Macdonal Laboratory, Kansas State University, between 1992 and 1995, and is currently an NRC researcher at the National Institute of Standards and Technology in Gaithersburg, MD. 591 Acc. Chem. Res. 1996, 29, 591-597 S0001-4842(95)00251-2 CCC: $12.00 © 1996 American Chemical Society