Substituent Control of σ‑Interference Effects in the Transmission of Saturated Molecules

The single-molecule
conductance of saturated molecules can potentially
be fully suppressed by destructive quantum interference in their σ-system.
However, only few molecules with σ-interference have been identified,
and the structure–property relationship remains to be elucidated.
Here, we explore the role of substituents in modulating the electronic
transmission of saturated molecules. In functionalized bicyclo[2.2.2]­octanes,
the transmission is suppressed by σ-interference when fluorine
substituents are applied. For bicyclo[2.2.2]­octasilane and -octagermanes,
the transmission is suppressed when carbon-based substituents are
used, and such molecules are likely to be highly insulating. For the
carbon-based substituents, we find a strong correlation between the
appropriate Hammett constants and the transmission. The substituent
effect enables systematic optimization of the insulating properties
of saturated molecular cores.