  The electrical conductance of molecular junctions may strongly depend on the
temperature, and weakly on molecular length, under two distinct mechanisms:
phase-coherent resonant conduction, with charges proceeding via delocalized
molecular orbitals, and incoherent thermally-assisted multi-step hopping. While
in the case of coherent conduction the temperature dependence arises from the
broadening of the Fermi distribution in the metal electrodes, in the latter
case it corresponds to electron-vibration interaction effects on the junction.
With the objective to distill the thermally-activated hopping component, thus
expose intrinsic electron-vibration interaction phenomena on the junction, we
suggest the design of molecular junctions with "spacers", extended anchoring
groups that act to filter out phase-coherent resonant electrons. Specifically,
we study the electrical conductance of fixed-gap and variable-gap junctions
that include a tunneling block, with spacers at the boundaries. Using numerical
simulations and analytical considerations, we demonstrate that in our design,
resonant conduction is suppressed. As a result, the electrical conductance is
dominated by two (rather than three) mechanisms: superexchange (deep
tunneling), and multi-step thermally-induced hopping. We further exemplify our
analysis on DNA junctions with an A:T block serving as a tunneling barrier.
Here, we show that the electrical conductance is insensitive to the number of
G:C base-pairs at the boundaries. This indicates that the tunneling-to-hopping
crossover revealed in such sequences truly corresponds to the properties of the
A:T barrier.
