Orbital control of single-molecule conductance perturbed by π-accepting anchor groups: Cyanide and isocyanide

Electron transport properties through benzene molecules disubstituted with π-accepting cyanide and isocyanide anchor groups at their para and meta positions are investigated on the basis of a qualitative orbital analysis at the Hückel molecular orbital level of theory. The applicability of a previously derived orbital symmetry rule for electron transport is extended to the systems perturbed by the π-accepting anchor groups, where the HOMO-LUMO symmetry in the molecular orbital energies relative to the Fermi level is removed. The conservation of the HOMO-LUMO symmetry in the spatial distribution of the molecular orbitals between the unperturbed benzene molecule and the perturbed molecules with the anchor groups rationalizes symmetry-allowed electron transport through the para isomers. On the other hand, destructive interferences between the nearly 2-fold degenerate frontier orbitals constructed from the 2-fold degenerate orbitals of the unperturbed benzene molecule and the anchor groups lead to symmetry-forbidden electron transport through the meta isomers. The qualitative orbital thinking is supported by more quantitative density functional theory (DFT) calculations combined with the nonequilibrium Green's function (NEGF) method. The orbital analysis is a powerful tool for the understanding and rational design of molecular devices composed of π-conjugated hydrocarbons and those perturbed by the π-accepting anchor groups.