|
Self-assembly offers unique possibilities for the construction of nanometre scale structures on surfaces. Previously, by means of non-covalent interactions (primarily hydrogen bonding and metal coordination) a diverse range of two-dimensional assemblies have been fabricated, from isolated rows to extended networks. Here, by means of scanning tunnelling microscopy (STM), we report the creation of adsorbed systems that display complex self-assembly into localised structures. Firstly, inspired by the solution state coordination chemistry of metalloporphyrins, we have examined the adsorption of a zinc porphyrin on the Ag(100) surface, and its subsequent chemical manipulation via metal-ligand interactions. Through such means we describe the capping/uncapping by a ligand (DABCO) of a metalloporphyrin (zinc tetra-[3,5-di-tert-butylphenyl]porphyrin) adsorbed on the Ag(100) surface. The behaviour closely parallels that of the free zinc porphyrins interacting with DABCO in solution, pointing to the feasibility of devising porphyrin-functionalised surfaces in which the intrinsic chemistry of the porphyrin is preserved. Furthermore, by insertion of a bifunctional ligand (4-methoxypyridine) that acts as an axle, one end tethered to the Ag(100) surface, the other bonded to the metal centre of the zinc porphyrin, an entire population of porphyrin molecules can be induced to spin - the fabrication of molecular "pinwheels" of ~ 2.5 nanometres in diameter. The transition from static to rotating porphyrins is chemically driven by means of self-assembly through the addition or removal of the ligand. Secondly, we report the discovery of self-assembled discrete tetramers of styrene and styrene derivatives on the Ag(100) surface. Chirality defects, whose probability depends on monomer structure, can occur during tetramer growth leading to the formation of more complex structures. The phenomenon appears to be unprecedented and is due to metal-mediated coupling of the molecular π electron systems, an effect for which a general theoretical explanation is provided. This points the way to the ab initio design of finite self-assembled systems based on monomers that lack inversion symmetry. |