Two-Dimensional Chiral Aperiodic Topological Self-Assembly

Surfaces functionalized with helicenes – helically-shaped polyaromatic hydrocarbons – are of interest for chiroptical electronic devices or for electron spin filtering.
We present a unique transmission of chirality from single molecules of a tetrahelicene derivative into two-dimensional self-assembled monolayers on a silver surface in ultrahigh vacuum. The relatively high molecular flexibility allows adaptation of handedness during crystal growth which is governed rather by entropy than by enthalpy. The 2D crystal is a solid solution with domains of opposite enantiomeric excess and supramolecular handedness, as revealed by sub-molecular resolution scanning tunneling microscopy (STM). The layer is dominated by motifs, such as nodes of different topology, two-armed and three-armed spiral arrangement and an enantiospecific lateral offset of oligomeric triangles. Differences from previously reported aperiodic molecular assemblies, such as Penrose tiling, are discussed. Our findings demonstrate that two-dimensional molecular self-assembly can be governed by topological constraints, leading to aperiodic tiling driven solely by intermolecular forces.