Hydroquinone-Benzoquinone Redox Couple as a Versatile Element
for Molecular Electronics

The possibility of controlling electron transport
in a single molecule bridged between two metal electrodes
represents the ultimate goal of molecular electronics.
Molecular electronics aims also at introducing specific
properties for the electron transport features both by
controlling the structural details of the junction and by
exploiting new chemical functionalities. Here we show that, in
a molecular junction, where electrodes are represented by a
gold substrate and the tip of a scanning tunneling microscope
in electrochemical environment, the use of a single molecular
species makes it possible to obtain different features for the
tunneling current according to the structural details of the junction. In particular, molecules endowed with redox properties brought about by a hydroquinone/benzoquinone redox couple can show both transistor-like and negative differential resistance (NDR) effects. We discuss the mechanistic processes that might describe the different behavior in light of theories of electron transfer between metal electrodes and redox molecules. The results show, on the one hand, the great potential and flexibility that molecular electronics offer and, on the other hand, the need of controlling as much as possible the details of the tunneling junction in order to obtain reproducible results.