Two-dimensional assembly and local redox-activity of molecular hybrid structures in an electrochemical environment

The self-assembly and redox-properties of two viologen derivatives, N-hexyl-N'-(6-thiohexyl)-4,4'-bipyridinium bromide (HS-6V6-H) and N,N'-bis(6-thiohexyl)-4,4'-bipyridinium bromide (HS-6V6-SH), immobilized on Au(lll)-(1 x 1) macro-electrodes were investigated by cyclic voltammetry, surface enhanced infrared spectroscopy (SEIRAS) and in situ scanning tunneling microscopy (STM). Depending on the assembly conditions one could distinguish three different types of adlayers for both viologens: a low coverage disordered and an ordered "striped" phase of flat oriented molecules as well as a high coverage monolayer composed of tilted viologen moieties. Both molecules, HS-6V6-H and HS-6V6-SH, were successfully immobilized on Au(poly) nano-electrodes, which gave a well-defined redox-response in the lower pA-current range. An in situ STM configuration was employed to explore electron transport properties of single molecule junctions Au(T)/HS-6V6-SH(HS-6V6-H)/Au(S). The observed sigmoidal potential dependence, measured at variable substrate potential E(S) and at constant bias voltage (E(T) - E(S)), was attributed to electronic structure changes of the viologen moiety during the one-electron reduction/re-oxidation process V2+ < -- > V+*. Tunneling experiments in asymmetric, STM-based junctions Au(T)-S-6V6-H/Au(S) revealed current (i(T))-voltage (E(T)) curves with a maximum located at the equilibrium potential of the redox-process V2+ < -- > V+*. The experimental i(T)--E(T) characteristics of the HS-6V6-H-modified tunneling junction were tentatively attributed to a sequential two-step electron transfer mechanism.