Expression of chirality and visualization of stereogenic centers by scanning tunneling microscopy

Physisorbed monolayer films of both enantiomers of a chiral terephthalic acid derivative, 2,5-bis[[10-(2-methylbutoxy)decyl]oxy]terephthalic acid, have been characterized on graphite (HOPG) at the solution-substrate interface using scanning tunneling microscopy (STM). The monolayers, which are imaged with submolecular resolution, are enantiomorphous. The adlayer consists of a lamellar structure in which two distinct lamellar widths can be identified. The lamellar width can be directly related to the length of that part of the molecule which is adsorbed on the graphite surface. In the narrow lamellae, the 2-methylbutoxy groups point away from the surface, giving rise to specific features in the STM image. These features can be described as discontinuous, elongated spots and appear also at the positions of the stereogenic centers. Because of the presence of the methyl group at the stereogenic center, the tunneling current is locally modulated, enabling the visualization of the stereogenic center.