Polymorphism of porphyrin molecules on Ag(111) and how to weave a rigid monolayer

The molecular arrangement and the internal conformation of Co(II)-5,10,15,20-tetrakis-(3,5-di-tert-butylphenyl)porphyrin (Co(II)-TTBPP) on Ag(111) was studied by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. Interestingly, four different molecular arrangements were found using different routes to prepare the porphyrin films. After the preparation of a film with a coverage slightly below one morrolayer by thermal sublimation, three clearly distinguishable molecular arrangements could be observed. Alternatively, a monolayer was prepared by deposition of a multilayer followed by subsequent annealing to desorb the excess multilayer molecules. Thereby, an additional, very rigid herringbone structure formed via a thermally activated process, covering most of the surface. The four ordered phases go along with different conformations of the individual molecules. On the basis of STM images with submolecular resolution, models for the lateral arrangements of the different polymorphs including the internal conformation of the individual molecules are proposed. Thereby, as an extension to previous works, the rotation of the upper tert-butyl groups of Co-TTBPP was taken into account. The derived molecular arrangements in combination with the intramolecular conformations give insight into the intermolecular interactions within and the nature of the different molecular phases.