Toward a First-Principles Evaluation of Transport Mechanisms in Molecular Wires

Understanding charge transport
through molecular wires is important
for nanoscale electronics and biochemistry. Our goal is to establish
a simple first-principles protocol for predicting the charge transport
mechanism in such wires, in particular the crossover from coherent
tunneling for short wires to incoherent hopping for longer wires.
This protocol is based on a combination of density functional theory
with a polarizable continuum model introduced by Kaupp et al. for
mixed-valence molecules, which we had previously found to work well
for length-dependent charge delocalization in such systems. We combine
this protocol with a new charge delocalization measure tailored for
molecular wires, and we show that it can predict the tunneling-to-hopping
transition length with a maximum error of one subunit in five sets
of molecular wires studied experimentally in molecular junctions at
room temperature. This suggests that the protocol is also well suited
for estimating the extent of hopping sites as relevant, for example,
for the intermediate tunneling-hopping regime in DNA.