Understanding and manipulating quantum interference (QI) effects in single molecule junction conductance can enable the design of molecular-scale devices. Here we demonstrate QI between σ and π molecular orbitals in an ∼4 Å molecule, pyrazine, bridging source and drain electrodes. Using single molecule conductance measurements, first-principles analysis, and electronic transport calculations, we show that this phenomenon leads to distinct patterns of electron transport in nanoscale junctions, such as destructive interference through the para position of a six-membered ring. These QI effects can be tuned to allow conductance switching using environmental pH control. Our work lays out a conceptual framework for engineering QI features in short molecular systems through synthetic and external manipulation that tunes the energies and symmetries of the σ and π channels.
Keywords: electron transport; electronic structure; molecular switches; nanoscale contacts; quantum interference; single-molecule conductance.