Molecular Self-Assembly of Enantiopure Heptahelicene-2-Carboxylic Acid on Calcite (101̅4)

Abstract

Chirality can have a decisive influence on the molecular structure formation upon self-assembly on surfaces. In this paper, we study the structures formed by enantiopure (M)-heptahelicene-2-carboxylic acid ((M)-[7]HCA) on the calcite (101̅4) cleavage plane under ultrahigh vacuum conditions. Previous noncontact atomic force microscopy studies have revealed that the racemic mixture of (M)-[7]HCA and (P)-[7]HCA (1:1) self-assembles into well-defined molecular double rows that are oriented along the calcite [011̅0] direction. Here, we investigate the enantiopure (M)-[7]HCA compound, resulting in distinctly different molecular structures upon deposition onto calcite (101̅4). In sharp contrast to the racemate, the (M)-[7]HCA enantiomer forms molecular islands with a (2 × 3) superstructure. Comparison of the results presented here for the enantiopure compound with the results previously obtained from the racemate indicates that heterochiral recognition is responsible for the formation of the unidirectional double row structures formed by the racemate.