  We studied charge transport phenomena through a core substituted
naphthalenediimide (NDI) single-molecule junctions using the electrochemical
STM-based break junction technique in combination with DFT calculations. The
conductance switch among three well-defined states is acquired by
electrochemically controlling the redox state of the pendent diimide unit of
the molecule in ionic liquid, and the conductance difference is more than one
order of magnitude between di-anion states and neutral state. The potential
dependent charge transport characteristics of the NDI molecules are confirmed
by DFT calculations accounting for electrochemical double-layer effects on the
conductance of the NDI junctions. This work suggests that the integration of
redox unit in the pendent position with strong coupling to molecular backbone
can significantly tune the charge transport of the single-molecule device by
controlling different redox states.
