Manipulating Quantum Interference in Single Molecule Junctions via Chemical Substitution and Environmental Control

Understanding and manipulating quantum interference (QI) effects through chemical design and external control is important for the development of molecular-scale devices. QI results from electrons tunneling through multiple molecular orbitals (MOs) each with a distinct phase with respect to the electrode attachment points on the molecule. In this work, we use Scanning Tunneling Microscope Break Junction (STMBJ) single molecule conductance measurements and electronic transport calculations to explore the effects of chemical composition and substitution in pyrazine-based molecular junctions. We find that QI effects dominate observed transport characteristics of these molecules, resulting in distinct conductance peaks through the meta but not the para pathway. Critically, destructive interference through the para position of the pyrazine ring can be manipulated via substitution at the meta position. By varying the environment surrounding the junction, we can control the charge of the chemical substituents and manipulate the alignment of individual MOs with the Fermi energy to tune QI through the para channel in pyrazine-based junctions from destructive to constructive. This work demonstrates environmental control of molecular QI, opening the door to <i>in situ</i> tuning of QI effects in single molecule junctions.