Selective assembly on a surface of supramolecular aggregates with controlled size and shape

The realization of molecule-based miniature devices with advanced functions requires the development of new and efficient approaches for combining mol ecular building blocks into desired functional structures, ideally with the se structures supported on suitable substrates(1-4). Supramolecular aggrega tion occurs spontaneously and can lead to controlled structures if selectiv e and directional non-covalent interactions are exploited. But such selecti ve supramolecular assembly has yielded almost exclusively crystals or disso lved structures(5); the self-assembly of absorbed molecules into larger str uctures(6-8), in contrast, has not yet been directed by controlling selecti ve intermolecular interactions. Here we report the formation of surface-sup ported supramolecular structures whose size and aggregation pattern are rat ionally controlled by tuning the non-covalent interactions between individu al absorbed molecules. Using low-temperature scanning tunnelling microscopy , we show that substituted porphyrin molecules adsorbed on a gold surface f orm monomers, trimers, tetramers or extended wire-like structures. We rnd t hat each structure corresponds in a predictable fashion to the geometric an d chemical nature of the porphyrin substituents that mediate the interactio ns between individual adsorbed molecules. Our findings suggest that careful placement of functional groups that are able to participate in directed no ncovalent interactions will allow the rational design and construction of a wide range of supramolecular architectures absorbed to surfaces.