Conformational identification of individual adsorbed molecules with the STM

The structure and conformation of a molecule determine its chemical and physical properties. Molecular conformation at interfaces is of particular importance in organic thin films1: in organic optoelectronic devices2,3, for example, charge carrier injection is influenced by interfacial properties4. Here we present a real-space conformational analysis of individual porphyrin molecules using scanning tunnelling microscopy5,6. Porphyrins have been used as model systems to study charge transfer7 and in vivo photoactivation of drug precursors8, and have also been used in organic light-emitting diodes9. We find that changes in the porphyrins' conformation occur predominantly by rotations around the bonds to four tertiary butyl appendages, which differ on different metal substrates. On corrugated gold (110) surfaces, we identify two different conformations as the precursory (metastable) and final states of adsorption. This kind of conformational adaptation to a surface may be general for adsorbed organic molecules, and might have important consequences for the technological applications of organic thin films.