Modification of supramolecular binding motifs induced by substrate registry: formation of self-assembled macrocycles...

The self-assembly properties of two Zn(II) porphyrin isomers on Cu(111) are studied at different coverage by means of scanning tunneling microscopy (STM). Both isomers are substituted in their meso-positions by two voluminous 3,5-di(tert-butyl)phenyl and two rod-like 4'-cyanobiphenyl groups, respectively. In the trans-isomer, the two 4'-cyanobiphenyl groups are opposite to each other, whereas they are located at right angle in the cis-isomer. For coverage up to one monolayer, the cis-substituted porphyrins self-assemble to form oligomeric macrocycles held together by antiparallel CNCN dipolar interactions and CNH-C(sp(2)) hydrogen bonding. Cyclic trimers and tetramers occur most frequently but everything from cyclic dimers to hexamers can be observed. Upon annealing of the samples at temperatures >150 degrees C, dimeric macrocyclic structures are observed, in which the two porphyrins are bridged by Cu atoms, originating from the surface, under formation of two CNCuNC coordination bonds. The trans-isomer builds up linear chains on Cu(111) at low coverage, whereas for higher coverage the molecules assemble in a periodic, densely packed structure. Both cis- and trans-bis(4'-cyanobiphenyl)-substituted Zn(II) porphyrins behave very differently on Cu(111) compared to similar porphyrins in literature on less reactive surfaces such as Au(111) and Ag(111). On the latter surfaces, there is no signal visible between molecular orientation and the crystal directions of the substrate, whereas on Cu(111), very strong adsorbate-substrate interactions have a dominating influence on all observed structures. This strong porphyrin-substrate interaction enables a much broader variety of structures, including also less favorable intermolecular bonding motifs and geometries.