Molecular Conductance Orbital Origins of Anti-resonance Peaks in Six-Carbon Polyene Molecular Junctions

In single-molecule electronics, electron transport through a molecular junction is governed by quantum effects owing to the molecular dimensions being comparable to the electron de Broglie wavelength. In this study, we systematically investigate the quantum transport properties of a polyene containing six carbon atoms with different bond orders and connection sites using molecular conductance orbitals (MCOs) within the Hückel model in the wide-band approximation. The transmission spectra exhibit anti-resonance peaks characteristic of destructive quantum interference (DQI), which remain largely unaffected by the bond order within the nearest-neighbor approximation. Under specific configurations, however, bond order affects the number and location of the anti-resonance peaks. The interaction between the two closest conducting orbitals frequently dominates the DQI. In particular, when the molecule is connected through the central two carbon atoms, the nearest-neighbor and second-nearest-neighbor conducting orbitals have the most significant effects on DQI away from the Fermi level. Furthermore, for certain connected sites, none of the conductive orbitals can be ignored. The dominant conductive mechanism is elucidated using MCO phases, MCO-electrode coupling strengths, MCO energy-level alignments, quantum interference maps, and projected transmissions. These findings advance the understanding of DQIs in conjugated molecular junctions and provide a theoretical foundation for fine-tuning quantum interference and developing high-performance molecular devices.