Self-assembled layers of bisbenzamidines on gold

The adsorption of bisbenzamidines on gold was studied by ellipsometry, surf ace 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 molec ules. In contrast to the corresponding layers of amidines formed on gold su rfaces modified with monolayers (SAMs) of mercaptoalkanoic acids, the layer s on gold could not be displaced by acid. Moreover, as on the acid-function alized SAMs, the longer chain amidines showed a tendency for bilayer format ion, whereas in contrast to the behavior on the carboxylic acid SAMs also s horter amidines formed layers stable to rinsing. The characterization indic ated a crystalline order of longer amidines and a pronounced influence of t he amidine chain length and time of adsorption on the amidine orientation o n the surface. Cyclic voltammetry and ac impedance measurements showed that the longer chain amidines formed partially insulating layers with double-l ayer capacitances approaching those obtained for thiol SAMs. These positive ly 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.