Contribution of intermolecular interactions to electron transfer through monolayers of alkanethiols containing amide groups

Organothiol molecules with one or two amide moieties replacing the methylene groups in the alkyl chain were synthesized and self-assembled in mixed monolayers on the gold electrode. Structures of the monolayer films were studied by STM and electrochemical methods. Reflectance infrared spectroscopy confirmed the formation of intermolecular hydrogen bonds between the amide groups. In the case of diamides, the extent of hydrogen bonding in the external and internal planes of the monolayer were compared and both the odd and the even-membered molecules were considered. Impedance spectroscopy and chronoamperometry were employed to measure the electron transfer rate across mixed monolayers containing less than 10% of electroactive ferrocene organothiol with one or two amide groups in the chain. In the present paper, we focus on the amide location effects on kinetics of mediated electron transfer. Our results indicate that the distance between the planes where amide moieties are located and the electrode surface is of crucial importance. Deeply "buried" (i.e., located close to the electrode surface) amide groups were found to be responsible for the significant increase of electronic coupling comparing to simple n-alkanethiol monolayers. The effect of external amides was much less pronounced, This observation was explained as due to the different extent of hydrogen bonding in the internal and external plane of amide groups.