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

► STM images of Zn(II)-octaethylporphyrin monolayer were obtained in real time. ► Polymorphic behaviour of zinc(II)-octaethylporphyrin (ZnOEP) was identified. ► A phenomenological expression was proposed to explain the ZnOEP phase transition. ► STM images of Zn(II)-meso-tetradodecylporphyrin monolayer were obtained. ► The effect of the porphyrins alkyl substituents was discussed.