Coverage and Enantiomeric Excess Dependent Enantiomorphism in Two-Dimensional Molecular Crystals

The question of racemate versus conglomerate stability is tremendously important for chiral resolution via crystallization methods. Scanning tunneling microscopic studies with molecular resolution on two-dimensional (2D) model systems contribute at large to solve problems on complex crystallization phenomena. The dependence of lattice polymorphism and enantiomorphism on coverage and enantiomeric excess has been investigated for monolayers of enantiopure and racemic heptahelicene on a Cu(111) surface in ultrahigh vacuum. The densest packing is achieved in a homochiral lattice structure. This should favor 2D conglomerate formation for the racemate, since a higher coverage leads to an overall lower energy. However, only heterochiral structures are observed. Small enantiomeric excess (ee) induces lattice homochirality by suppressing one enantiomorph. At larger ee, we observe crystals disordered at the molecular level. Long-range ordered homochiral structures are suppressed due to small chiral impurities. The preference of a 2D racemic compound formation, the induction of lattice homochirality at small ee, and the solid solution formation at larger ee are discussed in the light of energetic, entropic, and kinetics effects.