Conformational Control of σ-Interference Effects in the Conductance of Permethylated Oligosilanes

As silicon-based integrated circuits continue to shrink, their molecular characteristics become more pronounced. However, the structure–property relationship of silicon-based molecular junctions remains to be elucidated. Here, an intuitive explanation of the effects of backbone dihedral angles on transport properties in permethylated oligosilanes is presented employing the Ladder C model Hamiltonian combined with nonequilibrium Green’s function formalism. Backbone dihedral angles modulate quantum interference (QI), resulting in the change of the transmission coefficient at the Fermi energy (E_F) by up to 6 orders of magnitude in Si₄Me₁₀. Because the types of QI (constructive or destructive) between molecular conductance orbitals (MCOs) are unchanged, the relative magnitudes of contributions from QI are critical. This quantitative aspect of QI is often neglected in previous theoretical studies. Small backbone dihedral angles lead to localized MCOs near E_F and delocalized MCOs further away from E_F. As a result, the constructive QI between the MCOs near E_F is suppressed, while the destructive QI between other MCOs is enhanced. This insight opens an avenue to harness QI to realize ultrainsulating molecular devices.