Bromine-passivated Au(111) as a platform for the formation of organic self-assembled monolayers under electrochemical conditions

Self-assembled monolayers of 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) were formed by equilibrium adsorption from a perchloric acid solution onto Au(111) using an interposed adlayer of bromide. The passivating bromide adlayer was generated by addition of 150 μM KBr to the electrochemical cell and allowed monolayer ordering at positive potentials where a disordered TPyP monolayer would be found on a bare Au(111) surface. The TPyP monolayers were characterized in situ with electrochemical scanning tunneling microscopy (EC-STM) and cyclic voltammetry. They were successfully observed at working electrode potentials between 0.0 and +1.3 V vs Ag/AgCl. This wide potential window of usability for the bromide adlayer extends to potentials more positive than what has been achieved for similar observations using iodide-modified Au(111). Within the TPyP monolayers, isolated domains with differing geometries could be distinguished, suggesting dynamic monolayer rearrangements. These results demonstrate that the presence of a passivating bromide adlayer is conducive to the formation of highly ordered organic monolayers. Indeed, bromide is not only one of the few anions that are suitable for this purpose, but it may be superior to the more frequently used iodide.