Dynamics of porphyrin adsorption on highly oriented pyrolytic graphite monitored by scanning tunnelling microscopy at the liquid/solid interface

Scanning tunnelling microscopy (STM) at solid/tetradecane interface is used to study the dynamics of zinc(II)-octaethylporphyrin (ZnOEP) and zinc(II)-meso-tetradodecylporphyrin (ZnTDP) adsorption on the surface of highly oriented pyrolytic graphite (HOPG). ZnOEP exhibits a polymorphic behaviour, with a first metastable phase remaining for the first 2 h. This initial α-phase is then converted into a β-phase, where all porphyrins lie parallel to the substrate with a hexagonal arrangement, in agreement with previous reports. At variance with this behaviour, no metastable phases are found during ZnTDP adsorption under the same conditions. We consider that this different behaviour is due to the combination of a stronger interaction and lower mobility of ZnTDP on the HOPG surface.