Self-Assembled Layers of Bisbenzamidines on Gold

The adsorption of bisbenzamidines on gold was studied by ellipsometry, surface plasmon spectroscopy (SPS), reflection absorption infrared spectroscopy (RAIR), and electrochemical techniques. The results indicate that the long-chain bolaamphiphiles form layers of nearly perpendicularly oriented molecules. In contrast to the corresponding layers of amidines formed on gold surfaces modified with monolayers (SAMs) of mercaptoalkanoic acids, the layers on gold could not be displaced by acid. Moreover, as on the acid-functionalized SAMs, the longer chain amidines showed a tendency for bilayer formation, whereas in contrast to the behavior on the carboxylic acid SAMs also shorter amidines formed layers stable to rinsing. The characterization indicated a crystalline order of longer amidines and a pronounced influence of the amidine chain length and time of adsorption on the amidine orientation on the surface. Cyclic voltammetry and ac impedance measurements showed that the longer chain amidines formed partially insulating layers with double-layer capacitances approaching those obtained for thiol SAMs. These positively charged layers furthermore exhibited a partial blocking effect toward a negatively charged redox couple, thus carrying charges of opposite sign to the monolayer functional groups.