Beyond the Dip: Silent and Non-Silent Fano Resonances in Quantum Systems

This study presents a theoretical investigation of quantum interference effects on charge transport in a series of bithiophene-bridged molecular derivatives, each functionalized with terminal thiol anchor groups. Using a combined density functional theory and nonequilibrium Green's function (DFT-NEGF) approach, we demonstrate that the electrical conductance is primarily governed by the amplitude of the frontier molecular orbitals (FMOs) at the anchoring sites, as predicted by the orbital product rule. Furthermore, we show that introducing pendant atoms (oxygen) onto a carbazole core creates localized electronic states that interfere with the delocalized backbone, generating distinct Fano resonances in the transmission spectrum. Derivative 3, engineered with two oxygen atoms, exhibits a complex quantum interference landscape featuring a pronounced second Fano resonance and a suppressed, "silent" resonance. Crucially, we reveal that the Fano resonance associated with the highly localized HOMO is extremely sensitive to minute fluctuations in the molecule-electrode binding configuration, rendering it experimentally silent. In contrast, a hypothetical symmetric junction achieves perfect unitary transmission due to quantum interference enforced by spatial symmetry. This work provides a detailed blueprint for designing molecular-scale quantum interference devices and highlights the critical role of structural fluctuations in determining measurable conductance.