Tuning Conductance in Aromatic Molecules: Constructive and Counteractive Substituent Effects

Destructive quantum interference in aromatic hydrocarbons can be tuned using chemical substituents; however, classical chemical intuition is not enough to explain the effects on electron transport. Using Huckel theory and density functional theory calculations, in combination with the Landauer-Buttiker approach for charge transport, novel substituent effects are demonstrated. For a 1,3-linked benzene, an electron acceptor in position 2 is shown to have the same effect on the antiresonance energy as an electron donor in position 4 and vice versa. Substituents in position 5 have no effect on the antiresonance energy. The effects appear to be additive, such that a donor in position 2 will counteract a donor in position 4, leading to cancellation of the substituent effect. Counter- and nonactive substituent positions exist for all aromatic hydrocarbons and can be predicted using a diagrammatic approach. This insight should be useful when substituents are to be used for tuning destructive quantum interference features in the transmission relative to the Fermi energy of the electrodes.