Manipulation of quantum interference by charge accumulation in conjugated structures†

Modulating charge accumulation in molecular structures to switch the quantum interference effect (QIE) presents a promising approach for manipulating conductance in molecular devices without altering the molecular structure, which is crucial for real molecular device applications. However, understanding the QIE switch by charging is still a challenge. In this study, we used carbon ring structures to induce and finely modulate the charge accumulation in cross-conjugated diphenylpenta-1,4-dien-3-one molecular junctions. Our measurements show that an approximate one-order magnitude conductance suppression was detected in the highly charged structure compared to the moderately charged structures by using the scanning tunneling microscopy break junction (STM-BJ) technique. The theoretical calculations revealed that the energy of anti-resonance in destructive QI was governed by the ratio of orbital coefficients between HOMO−1 and HOMO. Specifically, a higher ratio shifted the anti-resonance toward the HOMO, whereas a lower ratio maintained the anti-resonance around the Fermi energy. The charge accumulation within the conjugated structure decreased the coefficient ratio, thereby switching the QIE from constructive to destructive when the number of ring members reached seven. This study not only provides a chance to understand the mechanism of QIE switching by charge but also paves the way for utilizing the charge accumulation to manipulate the conductance switch in molecular devices.