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 films(1): in organic optoelectronic devices(2,3) for example, charge carrier injection is influenced by interfacial properties(4). Here we present a real-space conformational analysis of individual porphyrin molecules using scanning tunnelling microscopy(5,6). Porphyrins have been used as model systems to study charge transfer(7) and in vivo photoactivation of drug precursors(8), and have also been used in organic light-emitting diodes(9). 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.