A hysteric current/voltage response of redox-active ruthenium complex molecules in self-assembled monolayers

We review on the current-voltage response of redox-active ruthenium complex molecules in self-assembled monolayers. In single molecular junctions with STM, the threshold voltage to the high conductance state in the molecular junctions of RuII complexes was consistent with the electronic energy gap between the Fermi level of the gold substrate and the lowest ligand-centered redox state of the metal complex molecule. As an active redox center leading to conductance switching in the molecule, the lowest ligand-centered redox state of RuII complexes was suggested to trap an electron injected from the gold substrate. In molecular monolayer devices of RuII complexes with conducting polymer (PEDOT:PSS), the hydrophilic terminal thiol group of the alkylthiolate-tethered RuII terpyridine complexes can interact with the hydrophilic sulfonic acid groups of PEDOT:PSS, preventing the penetration of soft and metal top electrodes into the SAMs to result in high yields. As the number of alkyl chains or as alkyl chain length increases, the device yield of molecular monolayer memory will improve and its retention time will increase, guiding a new strategy for the development of volatile to non-volatile memory.