Quantum Interference in a Molecular Analog of the Crystalline Silicon Unit Cell

This manuscript describes the emergence of destructive σ-quantum interference (σ-DQI) in sila-adamantane, a molecule whose cluster core is isostructural with the crystalline silicon unit cell. To reveal these σ-DQI effects, we take a bridge-cutting approach where we conceptually pare sila-adamantane down to its bicyclic Si[3.3.1] and linear oligosilane forms. Scanning tunneling microscopy break-junction (STM-BJ) measurements reveal that conductance in single-molecule junctions of the tricyclic sila-adamantane is 2.7-times lower than their bicyclic Si[3.3.1] analog. The only structural difference between their cluster cores is a remote dimethylsilylene bridge that is present in sila-adamantane yet absent in Si[3.3.1]. Density functional theory calculations reveal that this dimethylsilylene enforces C₃ symmetry at the sila-diamondoid bridgeheads, allowing each electrode to couple into the three cluster bridge dimensions equally. Though each bridge alignment is sterically equivalent, they have profound electronic differences: when electrodes align with the long branches of sila-adamantane, strong σ-DQI interactions occur between frontier molecular orbitals that suppress electronic transmission across the molecular junction. We exploit these alignment-dependent σ-DQI effects to create new forms of stereoelectronic conductance switches, where a reversible mechanical stimulus controls which pathway through the diamondoid framework the electrodes align through. This represents the first example of dynamic modulation of σ-DQI and enables us to achieve switching ratios (average on/off ∼5.6) higher than previously reported σ-stereoelectronic switches. These studies reveal how the innate dimensionality and symmetry of crystalline silicon influence charge transport at its most fundamental level, and how these principles can be harnessed to control quantum interference in single-molecule electronics.