Understanding electron transfer in single-molecule junctions

Electron-transfer reactions are ubiquitous in chemistry, however, there are still gaps in the fundamental understanding of electron transfer at the molecular level, particularly the degree to which the nuclear dynamics that accompany the process straddle the quantum-classical boundary. We use graphene-based single-molecule transistors [1] to study the mechanism of electron transfer over a wide range of temperatures – from 3 K to 77 K – at the level of an individual molecule. Charge transport through molecular junctions is often described either as a purely coherent or a purely classical phenomenon, and described using the Landauer formalism or Marcus theory, respectively. In our experiments, however, observe a simultaneous breakdown of quantum coherent Landauer and semi-classical Marcus theory. We propose a theoretical model based on generalised quantum master equation [2], where we derive an expression for current through a molecular junction modelled as a single electronic level coupled to a collection of thermalised vibrational modes, and demonstrate that it quantitatively describes the experimental data. We show that nuclear tunnelling enhances the rates of low-energy electron transfer, and demonstrate that the rates are sensitive to both the outer and inner-sphere environmental interactions. We find that the nuclear dynamics accompanying electron transfer must be treated quantum mechanically as the quantitative validity of Marcus theory is expected to occur at temperatures exceeding 298 K [3].

[1] Limburg et al., Adv. Funct. Mater. 1803629 (2018)
[2] Sowa et al., J. Chem. Phys. 149, 154112 (2018)
[3] Thomas et al., arXiv:1812.07562 (2018)