Single-Molecule Conductance of Viologen–Cucurbit[8]uril Host–Guest Complexes

The
local molecular environment is a critical factor which should
be taken into account when measuring single-molecule electrical properties
in condensed media or in the design of future molecular electronic
or single molecule sensing devices. Supramolecular interactions can
be used to control the local environment in molecular assemblies and
have been used to create microenvironments, for instance, for chemical
reactions. Here, we use supramolecular interactions to create microenvironments
which influence the electrical conductance of single molecule wires.
Cucurbit­[8]­uril (CB[8]) with a large hydrophobic cavity was used to
host the viologen (bipyridinium) molecular wires forming a 1:1 supramolecular
complex. Significant increases in the viologen wire single molecule
conductances are observed when it is threaded into CB[8] due to large
changes of the molecular microenvironment. The results were interpreted
within the framework of a Marcus-type model for electron transfer
as arising from a reduction in outer-sphere reorganization energy
when the viologen is confined within the hydrophobic CB[8] cavity.