Mechanism of Electrochemical Charge Transport in Individual Transition Metal Complexes

We used electrochemical scanning tunneling microscopy (STM) and spectroscopy (STS) to
elucidate the mechanism of electron transport through individual pyridyl-based Os complexes. Our tunneling
data obtained by two-dimensional electrochemical STS and STM imaging lead us to the conclusion that
electron transport occurs by thermally activated hopping. The conductance enhancement around the redox
potential of the complex, which is reminiscent of switching and transistor characterics in electronics, is
reflected both in the STM imaging contrast and directly in the tunneling current. The latter shows a biphasic
distance dependence, in line with a two-step electron hopping process. Under conditions where the substrate/
molecule electron transfer (ET) step is dominant in determining the overall tunneling current, we determined
the conductance of an individual Os complex to be 9 nS (Vbias ) 0.1 V). We use theoretical approaches to
connect the single-molecule conductance with electrochemical kinetics data obtained from monolayer
experiments. While the latter leave some controversy regarding the degree of electronic coupling, our results
suggest that electron transport occurs in the adiabatic limit of strong electronic coupling. Remarkably, and
in contrast to established ET theory, the redox-mediated tunneling current remains strongly distance
dependent due to the electronic coupling, even in the adiabatic limit. We exploit this feature and apply it to
electrochemical single-molecule conductance data. In this way, we attempt to paint a unified picture of
electrochemical charge transport at the single-molecule and monolayer levels.