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

Abstract

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.

Indenofluorene‐Extended Tetrathiafulvalene Scaffolds for Dye‐Sensitized Solar Cells

“…The product 10 was isolated in quantitative yield, but its poor solubility prevented NMR spectroscopic characterizations. Two dithiafulvene units were finally introduced by subjecting 10 to a twofold Horner–Wadsworth‐Emmons reaction upon treatment with the known phosphonate 11 , deprotonated with sodium hexamethyldisilazide (NaHMDS), furnishing IF‐TTF 12 in an excellent yield of 73 %.…”

Synthesis of Electro-active Compounds Suitable for Adsorption on Metal Surfaces

“…Preparation of the bis(acetythio) extended TTF compounds 127-129 starts from the dibromide 126. 76 Treatment with potassium thioacetate gives 127 while Suzuki reaction with the 3-or 4-(tert-butylthio)phenylboronic acid, followed by treatment with BBr 3 and acetyl chloride gives 128 and 129. The unsymmetrical hybrid compound with one meta and one para-substituted ring was also prepared.…”

“…The unsymmetrical hybrid compound with one meta and one para-substituted ring was also prepared. 76 The preparation of both 125 and 128/129 relies on a late-stage conversion of the relatively inert S-tert-butyl function 130 into the more reactive S-acetyl 131 (Scheme 33) and the utility of this method was further enhanced by the recent report that it could be achieved under milder conditions using bismuth triflate in place of boron tribromide. 77…”

Fulvalene‐Based Polycyclic Aromatic Hydrocarbon Ladder‐Type Structures: Synthesis and Properties

“…Benzannelation of dithiafulvalene at its cyclopentadiene ring provides compound 2 [7] that has, for example, been incorporated as a structural unit in either planar indenofluorene ( IF ) extended TTFs [8] of general structure 3 ( IF‐TTF ) or in concave truxene structures that can act as hosts for fullerenes [9] . Suitably functionalized IF‐TTFs have been explored as molecular wires in molecular electronics, [10] as semiconducting materials upon generation of radical cations by electrocrystallization, [11a] as tectons for redox‐controlled self‐assembly owing to remarkably strong associations between neutral and radical cation species (mixed valence complexes) and between radical cation species (π‐dimer complexes), [11] and as donor units in dye‐sensitized solar cells [12] …”