Uncovering transport properties of 4,4'-bipyridine/gold molecular nanobridges

Here, the fascinating connection between the chemical and the transport properties of recently fabricated 4,4'-bipyridine/gold nanobridges is addressed. By means of first-principles ab initio calculations, the remarkable reproducibility of the 4,4'-bipyridine conductance properties is explained as the combined result of (i) the bonding of the molecule to the metallic leads through hybridization between the 4,4'-bipyridine highest occupied molecular orbitals and lowest unoccupied molecular orbitals (LUMOs) with s and d orbitals at low-coordination gold atoms, (ii) the limited number of molecule-lead arrangements due to gold-hydrogen steric repulsions, and (iii) the electron transmission through a LUMO-derived resonance, whose positioning with respect to the Fermi level determines which of the above arrangements yields nonnegligible conductance. Structural and electronic interpretations to the stepped dependence reported for the electronic transport of 4,4'-bipyridine as a function of the distance between the gold tips are also given.