An aperiodic chiral tiling by topological molecular self-assembly

Abstract Crystallization remains an important method for separating chiral molecules into their two mirror-image isomers, yet the molecular recognition during aggregation into solids is still not fully understood. In particular, there are no definitive rules to predict whether a pair of chiral partners will crystallize together into racemic crystals or as separate counterparts. A more manageable version of this phenomenon is crystallization in two dimensions, such as the formation of surface structures by adsorbed molecules. The relatively simple spatial arrangement of these systems, coupled with the submolecular resolution provided by modern scanning probe technologies, makes it easier to study the effects of specific chiral interactions. Here, we report spiral topological patterns in the two-dimensional aggregation of chiral molecules on a silver crystal surface. The molecules tessellate the plane in chiral nodes and supramolecular triangles. Spiral annihilation is achieved only through the chiral triangular tiling of triads, with N±1 molecules at the edges and a specific strategy of chiral offsetting of triangles. Due to a high degree of molecular flexibility, entropy is maximized by switching molecular handedness in conjunction with the number of nodes controlled by triangle size. Our results demonstrate that two-dimensional self-assembly can be governed by topological constraints and that an aperiodic tiling of the plane can be realized solely by natural forces without human intervention.