Theoretical investigation on molecular rectification on the basis of asymmetric substitution and proton transfer reaction.

A series of linear conjugated molecular wires (diphenylacetylene connected by double or triple bonds) asymmetrically substituted by various functional groups was investigated by using density functional theory combined with nonequilibrium Green's function method. The transportation behaviors of these models did not show obvious rectification, inferring that the simple asymmetric substitution of the conjugation chain was insufficient to improve the molecular rectification. We proposed that the molecular transportation can be modulated by proton transfer between the adjacent dissociable groups on the molecular wire. The theoretical calculations showed that the rectification ratio increased about six times at 1.0 V after proton transfer. This behavior was interpreted by means of transmission spectra and spatial distribution of molecular orbitals; the alignment of molecular orbitals to the Fermi level promoted by proton transfer is also responsible for the rectification.