Synthesis and Single-Molecule Conductances of Neutral and Cationic Indenofluorene-Extended Tetrathiafulvalenes: Kondo Effect Molecules

Development of molecules
that can switch between redox states with
paired and unpaired electrons is important for molecular electronics
and spintronics. In this work, a selection of redox-active indenofluorene-extended
tetrathiafulvalenes (IF-TTFs) with thioacetate end groups was prepared
from a readily obtainable dibromo-functionalized IF-TTF building block
using palladium-catalyzed cross-coupling reactions, such as the Suzuki
reaction. The end groups served as electrode anchoring groups for
single-molecule conductance studies, and the molecules were subjected
to mechanically controlled break-junction measurements with gold contacts
and to low-bias charge transport measurements in gated three-terminal
electromigration junctions. The neutral molecules showed clear conductance
signatures, and somewhat surprisingly, we found that a meta–meta anchoring configuration gave a higher conductance than a para–meta configuration. We explain this behavior
by “through-space” coupling between the gold electrode
and the phenyl on which the anchoring group is attached. Upon charging
the molecule in a gated junction, we found reproducibly a Kondo effect
(zero-bias conductance) attributed to a net spin. Ready generation
of radical cations was supported by cyclic voltammetry measurements,
revealing stepwise formation of radical cation and dication species
in solution. The first oxidation event was accompanied by association
reactions as the appearance of the first oxidation peak was strongly
concentration dependent.