Studying the self-assembly of chiral molecules in two dimensions offers insights into the fundamentals of crystallization. Using scanning tunneling microscopy, we examine an uncommon aggregation of polyaromatic chiral molecules on a silver surface. Dense packing is achieved through a chiral triangular tiling of triads, with N and N ± 1 molecules at the edges. The triangles feature a random distribution of mirror-isomers, with a significant excess of one isomer. Chirality at the domain boundaries causes a lateral shift, producing three distinct topological defects where six triangles converge. These defects partially contribute to the formation of supramolecular spirals. The observation of different equal-density arrangements suggests that entropy maximization must play a crucial role. Despite the potential for regular patterns, all observed tiling is aperiodic. 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 induced by intermolecular forces.

Department of Chemistry University of Zürich Zürich Switzerland

Empa Swiss Federal Laboratories for Materials Science and Technology Dübendorf Switzerland

Institute of Physics Swiss Federal Institute of Technology Lausanne Station 3 CH 1015 Lausanne Switzerland

Laboratory for 10 ray Nanoscience and Technologies Paul Scherrer Institut CH 5232 Villigen PSI Switzerland

Nanosurf Lab Institute of Physics of the Czech Academy of Sciences Prague Czech Republic

Univ Angers CNRS MOLTECH Anjou SFR MATRIX F 49000 Angers France

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An aperiodic chiral tiling by topological molecular self-assembly