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

TL;DR

This study investigates how vibrational relaxation times in a single-molecule junction depend on charge state, revealing that slow vibrational dissipation enables non-equilibrium phonon effects during electron transport.

Contribution

It provides the first measurement of charge-state dependent vibrational relaxation times in a single-molecule junction, highlighting the role of vibrational dissipation in transport phenomena.

Findings

Vibrational relaxation time is at least 8 ns.

Sequential tunneling involves non-equilibrium vibrational states.

Relaxation times vary with molecular charge state.

Abstract

The interplay between nuclear and electronic degrees of freedom strongly influences molecular charge transport. Herein, we report on transport through a porphyrin dimer molecule, weakly coupled to graphene electrodes, that displays sequential tunneling within the Coulomb-blockade regime. The sequential transport is initiated by current-induced phonon absorption and proceeds by rapid sequential transport via a non-equilibrium vibrational distribution. We demonstrate this is possible only when the vibrational dissipation is slow relative to sequential tunneling rates, and obtain a lower bound for the vibrational relaxation time of 8 ns, a value that is dependent on the molecular charge state.