CONFORMATIONAL IDENTIFICATION OF INDIVIDUAL ADSORBED MOLECULES WITH THE STM

The structure and conformation of a molecule determine its chemical an d physical properties. Molecular conformation at interfaces is of part icular importance in organic thin films(1): in organic optoelectronic devices(2,3) for example, charge carrier injection is influenced by in terfacial properties(4). Here we present a real-space conformational a nalysis of individual porphyrin molecules using scanning tunnelling mi croscopy(5,6). Porphyrins have been used as model systems to study cha rge 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 rotati ons around the bonds to four tertiary butyl appendages, which differ o n different metal substrates. On corrugated gold(110) surfaces, we ide ntify two different conformations as the precursory (metastable) and f inal 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.