Nano Lett. (2018)

The conductance of a classical metallic cable is inversely proportional to its length. In contrast, molecular wires usually act as tunnelling barriers where the conductance decays exponentially with length. Algethami and co-workers now show theoretically that, in some cases, the conductance can actually increase with the length of the molecular wire.

Density functional theory-based simulation is used to explore the electronic transport through fused porphyrin oligomers. The molecules, which are known for their good electron transmission, are anchored to either graphene or gold electrodes. The researchers compare the conductance of different oligomers as a function of the Fermi level alignment. When using gold electrodes, the conductance can either increase or decrease with the length, depending on the anchoring group used to couple the molecule to the electrode. For graphene electrodes, however, the conductance increases with the length, independent of the molecule–electrode bonding. Algethami et al. attribute this to the strong coupling between the porphyrin units which yields a sizeable reduction of the gap between the highest occupied and the lowest unoccupied molecular orbital. The reduced transport gap overcompensates the decay of tunnelling conductance with length, at least up to six fused porphyrin units.