Diode-like electron transfer across nanostructured films containing a redox ligand

Novel redox active dithiols containing the viologen (4,4'-bipyridinium group) have been synthesised. Monolayers and multilayers consisting of alternating layers of the dithiol and gold nanoparticles have been self-assembled on a gold substrate and visualised using SPM. Such materials have been investigated for their electroactivity and their ability to mediate electron transfer to an external redox couple, [Ru(NH3)(6)](3+/2+), in aqueous solution. Viologen moieties in these multilayers are addressable electrochemically provided they are within 6-8 layers of the external surface and it is shown that electron transfer through the deeper layers is very facile as a consequence of the presence of partially filled viologen pi-orbitals. When the viologen containing dithiol is present in the outer layer, a diode effect is observed; electron transfer from the multilayer to the external couple takes place, but the reverse electron transport is inhibited.