Two-Dimensional Crystallization of Enantiopure and Racemic Heptahelicene on Ag(111) and Au(111)

Abstract

Insight into recognition among helical molecules is highly relevant for understanding chiral separation or biomolecular activity. The two-dimensional self-assembly of enantiopure and racemic heptahelicene has been studied on the (111) surfaces of silver and gold by means of scanning tunneling microscopy. As found earlier for racemic heptahelicene on Cu(111), the racemate forms zigzag rows with alternating enantiomers. In contrast to Cu(111), no enantiomorphism in the form of oblique alignment of molecular adlattice vectors with respect to substrate lattice vectors is observed. That is, the zigzag rows run parallel to the high-symmetry directions of the substrate. Because of the larger interatomic distances, stress builds up in the racemic molecular layer on Ag(111), which leads to relaxation into different structures after every third or fourth zigzag row. For enantiopure (M)-heptahelicene, identical structures at monolayer saturation coverage are observed, showing a pronounced transmission of chirality via azimuthal alignment of adjacent molecules.