Bimolecular two-dimensional organization of porphyrins on Au(111): site selective, metal ion-dependent adsorption of tetraphenylporphyrin.

Adlayers consisting of two components selected from the group of cobalt(II) tetraphenylporphyrin (CoTPP), copper(II) tetraphenylporphyrin (CuTPP), and zinc(II) tetraphenylporphyrin (ZnTPP) were prepared by immersing Au(111) substrate in a benzene solution containing those molecules. The bimolecular adlayers thus prepared were investigated in 0.1 M HClO4 by electrochemical scanning tunneling microscopy (EC-STM). The mixed adlayers consisting of CoTPP and CuTPP formed structurally ordered but compositionally disordered arrays on Au(111). The ratio of CoTPP to CuTPP molecules in the mixed adlayer was proportional to the ratio of CoTPP to CuTPP molecules in the solution phase. Accordingly, the composition of CoTPP and CuTPP in the adlayer on the Au(111) surface was independent of absolute concentrations of these species in the solution and immersion time. In contrast, the structural feature of the mixed adlayer consisting of CoTPP and ZnTPP was similar to that of the mixed adlayer of CoTPP and CuTPP when these adlayers were prepared in solutions containing those mixtures at a total concentration of 100 microM, whereas when the total concentration was lower, adsorption was site-selective depending on the coordinated metal ion. This finding indicates that the herringbone structure of reconstructed Au(111) served as a template for the bimolecular assembly of CoTPP and ZnTPP. The characteristic phase separation of CoTPP and ZnTPP molecules assisted by reconstructed Au(111) surface can be controlled by the subtle balance between kinetics and thermodynamics.