Invited Paper SS-ThM3
2012 AVS Peter Mark Award Lecture: Effects of Chirality in Electron Tunneling, Molecular Excitation and Rotation

Thursday, November 1, 2012, 8:40 am, Room 22

Industrially, the selective conversion of prochiral reagents to chiral products is a crucial step in the production of a variety of asymmetric pharmaceuticals. While this feat is accomplished using either chiral catalysts or crystallization, many external influences have been shown to be capable of inducing such symmetry breaking including circularly polarized light, spin-polarized electrons, and combinations of unpolarized light and magnetic fields. Pioneering studies have made great strides towards explaining these various interactions, however many of the fundamental mechanisms by which chirality is transferred at the molecular-level are not yet fully understood. It is also a great challenge to design experimental setups with which to study these phenomena in a quantitative and reproducible manner. We report a simple thioether system in which symmetry breaking can be both induced and measured in situ at the single-molecule level. We demonstrate that electrical excitation of a prochiral molecule on an achiral surface produces large enantiomeric excesses in the chiral adsorbed state of up to 40%, whereas thermal annealing produces racemic mixtures as expected. These effects arise from a previously unreported phenomenon that standard polycrystalline metal scanning probe tips can possess intrinsic chirality.

Thioethers also constitute a simple, robust system with which to study molecular rotation as a function of temperature, electron energy, applied fields, and proximity of neighboring molecules. In order for molecules to be used as components in molecular machines, methods are required to couple individual molecules to external energy sources and to selectively excite motion in a given direction. Studying the rotation of molecules bound to surfaces offers the advantage that a single layer can be assembled, monitored and manipulated using the tools of surface science. We report that a butyl methyl sulfide (BuSMe) molecule adsorbed on a copper surface can be operated as a single-molecule electric motor. Electrons from a scanning tunneling microscope are used to drive directional motion of the BuSMe molecule in a two terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular-scale in real time. The direction and rate of the rotation are related to the chiralities of the molecule and the tip of the microscope (which serves as the electrode), which again illustrates the importance of the symmetry of the metal contacts in atomic-scale electrical devices.