Isotropically conducting tetraaryl osmium(IV), silane, and methane molecular wire junctions

Structural motifs based on tetraphenylmethane have drawn substantial interest as components of molecular electronic circuits, self-assembled monolayers, and three-dimensional polymers. When appropriately functionalized these tetratopic nodes may serve as a distinct class of isotropic electrical conductors, where the transport pathway is independent of connectivity. Though the functionality of these species is currently limited by the conjugation-breaking sp3-hybridized carbon or silicon central atoms, we reasoned this could be greatly improved through exchange for a tetravalent transition metal ion; enhancing electronic coupling between the 𝜋-conjugated substituents and imparting reversible redox properties. To evaluate this hypothesis, we probe single-molecule junctions comprising oligoaryl wires with tetrahedral osmium(IV), silane, or methane centers. We find that charge transport through junctions formed from the intact organometallic species in a non-polar, inert solvent is more efficient over larger distances relative to the all-organic wires. Notably, the conductance of junctions comprising an osmium(IV) wire can also be electrochemically modulated by a factor of 47, to values 80× higher than for a silane analogue, by opening the bias window asymmetrically about the electrode Fermi level. Our experimental results are supported by first principles calculations, which predict the osmium(IV) wires can be substantially more conductive than their all-organic counterparts while also identifying specific challenges associated with modelling frozen tetraaryl compound geometries. Together, this work highlights the promising potential of transition metal tetraaryl complexes as isotropically conducting building blocks for functional circuits and extended materials.