Promotion and Suppression of Single-Molecule Conductance by Quantum Interference in Macrocyclic Circuits

In order to rival the performance of silicon-integrated systems, molecular-scale circuits must be designed to enable the construction of different electronic components. A common strategy is to manipulate the quantum interference (QI) between frontier orbitals close to the electrode Fermi energy. Aligning the energies of these molecular orbitals, however, remains a formidable challenge. Herein, we demonstrate a modular design of single-molecule circuits, which enable the construction of basic electronic components — namely, conductors or insulators — based on one tetracationic cyclophane platform. We demonstrate that the electron transport in cyclophane circuits is mediated by QI between channels formed from two lowest unoccupied molecular orbitals (LUMOs), while their highest occupied molecular orbitals (HOMOs) play no significant role. We further reveal that energy differences between these two LUMO channels induce constructive interference, leading to high conductance. By contrast, the phase differences between these LUMO channels result in destructive interference and a suppression in the overall conductance.