Chiral phase transition in two-dimensional supramolecular assemblies of prochiral molecules.

The self-assembly of the rodlike two-dimensional chiral molecule 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid on the Cu(100) surface has been investigated by scanning tunneling microscopy. Upon adsorption at T>or=300 K, the molecules are deprotonated and assemble in parquet patterns when the coverage remains below a critical value. Corresponding high-resolution data reveal that the ordering implies mesoscopic chiral resolution as a result of chiroselective interactions (i.e., two domains comprise exclusively one enantiomer). When the critical coverage is exceeded, an abrupt transition to a single racemic phase is observed with a different lateral molecular coupling scheme. The shifting of the subtle balance between the weak lateral coupling, substrate bonding, and the packing requirements encountered with the increased molecular coverage is suggested to be the driving force for this homochiral-to-heterochiral phase transition.