Marcus Theory and Long-Range Activationless Transport in Molecular Junctions.

Intrachain transport in molecular junctions (MJs) longer than 5 nm has been modeled within the theoretical framework of Marcus theory. We show that in oligo(bisthienylbenzene)-based MJs, electronic transport involves polarons, localized on three monomers that are close to 4 nm in length. They hop and tunnel between adjacent localized sites with reorganization energies λ close to 400-600 meV and electronic coupling parameters H _ab close to λ/2. As a consequence, the activation energy for intrachain transport, given by the equation Δ G * = (λ/4)(1 - 2 H _ab /λ) ² , is close to zero, and transport along the chain is activationless, in agreement with experimental observation. On the contrary, similar calculations on conjugated oligonaphthalenefluoreneimine wires show that H _ab is much less than λ/2 and predict that the activation energies for intrachain hopping between adjacent sites, close to λ/4, are ∼115 meV. This work proposes a new perspective for understanding long-range activationless transport in MJs beyond the tunneling regime.