Observing Donor-to-Acceptor Electron-Transfer Rates and the Marcus Inverted Parabola in Molecular Junctions.

A recurring theme in molecular electronics is the relationship between the intramolecular electron transfer rate in a donor-bridge-acceptor system and the conductance at low bias in the corresponding electrode-bridge-electrode junction. The similarities between through-bridge donor-to-acceptor electron tunneling and through-bridge electrode-to-electrode Landauer transport led to the suggestion of an approximate linear relationship between the rate and the conductance for any given bridge. A large body of work indicates that the two quantities are not necessarily linearly related, showing different behaviors as a function of temperature, voltage and bridge length. Apart from Landauer tunneling, incoherent hopping can be an important mechanism in molecular junctions. We propose a donor-bridge-acceptor molecular junction, functioning in the incoherent hopping regime, that is suited for establishing direct correlations between the electrode-to-electrode current and the intramolecular donor-to-acceptor electron transfer rate. We suggest that this type of junction may be used to observe the Marcus-inverted-parabola dependence of the intramolecular rate on energy gap by varying the bias voltage. The realization of such an experiment would enable meaningful comparisons between solution-phase electron transfer rates and molecular-junction currents for the same molecule.