The self-assembly properties of two cyano (CN) functionalized porphyrin isomers on Cu(111) were studied at different coverages by means of scanning tunneling microscopy (STM). Both isomers have two voluminous 3,5-bis(tert-butyl)phenyl substituents and two rod-like 4-cyanobiphenyl substituents in cis and trans configurations. For coverages up to one monolayer, the cis porphyrins were found to form a variety of oligomeric clusters ranging from dimers to hexamers, held together by cyanophenyl directed anti-parallel dipole-dipole interactions, hydrogen bonding and metal complexation. Furthermore, small changes in preparation conditions were also found to have a profound effect on the size distribution of the clusters. This variety and size distribution of the oligomers demonstrates how the delicate balance between the strength of absorbate-adsorbate and adsorbate-substrate interactions has a major effect on self-assembly.

 

Previous work from Yokoyama et al.1, using a similar molecule with shorter cyanophenyl binding groups formed exclusively tetrameric structures on less reactive Au(111) substrates. In this study high resolution STM images showed the presence of structures ranging from dimeric to hexameric, as well as chainlike structures. On the basis of this data we conclude that on the more reactive Cu(111), the very strong adsorbate-substrate interactions present are the dominating influence, leading to the multimorphism of binding motifs and oligomers observed.

 

[1] T. Yokoyama, S. Yokoyama, T. Kamikado, Y. Okuno, S. Mashiko, Nature 2001, 413, 619-621.