Charge-state dependent vibrational relaxation in a single-molecule junction

The outcome of an electron-transfer process is determined by quantum-mechanical interplay between electronic and vibrational degrees of freedom.  Conventional descriptions of electron transfer typically assume a thermalized nuclear environment and, although it is established that nonequilibrium vibrational dynamics can have a profound impact on charge-transfer rates, studying this experimentally is a challenge.  Single-molecule junctions offer a promising platform for investigating these effects as the electron-transfer driving force is controlled, coupling to individual modes can be resolved, and measurements can be performed in the steady-state limited.

In this study (published in Physical Review Letters), we report on electron transport through a porphyrin dimer molecule coupled to graphene electrodes, and show that nonequilibrium vibrational dynamics permit sequential tunnelling in the Coulomb-blockade regime.  We model the dynamics to show that the blockade is lifted by rapid sequential transport via a nonequilibrium vibrational disruption of low-energy modes.

The modes are likely related to torsional molecular motions of the porphyrin dimer.  The lower bound for the vibrational relaxation time is 8 ns and depends on the molecular charge state.