In his famous experiment Pasteur manually separated enantiomorphous ammonium sodium tartrate crystals and observed opposite optical activity for their aqueous solutions. His insight that enantiomorphism is based on molecular structure laid the foundation of modern structural organic chemistry. Two aspects in his experiment allowed the manual separation: i) Handedness was transferred from molecular structure into the macroscopic shape of the crystal (hemihedrism), and ii) the enantiomers precipitated into a conglomerate of homochiral crystals allowing manual separation. Up to date, both processes are still poorly understood. We are neither able to predict the shape of a crystal based on the molecular structure nor do we understand why racemic mixtures crystallize sometimes into conglomerates. A promising approach to answer these questions is studying two-dimensional (2D) crystallization phenomena on well-defined surfaces via scanning tunneling microscopy (STM). Here we present studies on different two-dimensional chiral systems and discuss their tendency to undergo enantiomeric separation. A special surface enantiomorphism is observed via STM after adsorption of the enantiomers of a helical aromatic hydrocarbon on Cu(111). Instead of crystallization into homochiral 2D domains on the surface, racemic mirror domains are observed. In this situation, a small excess of one enantiomer is sufficient to create domains possessing single handedness throughout the entire surface layer. [1] The induction of homochirality by chiral doping has also been observed for succinic acid [2] and achiral (R,S)-tartaric acid. Our findings are explained by cooperative interactions between many chiral units, similar to the mechanism of chiral amplification observed in helical polymers coined as "Sergeant and Soldiers" principle. The homochiral crystal templates have implications of this 2D chiral amplification for bulk crystallization and separation of enantiomers will be discussed.

The work was supported by the Schweizerischer Nationalfonds.

[1] R. Fasel, M. Parschau, K.-H. Ernst, Nature 2006 439, 449.

[2] M. Parschau, S. Romer, K.-H. Ernst, J Am Chem Soc 2004 126, 15398