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 π-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.