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. However, the broad utility of such motifs is limited, for example, by the broken conjugation though the central, sp3-hybridized, carbon or silicon atom(s). To enrich their functionality, we reason the central atom could be exchanged for a transition metal to improve electronic coupling between the 𝜋-conjugated substituents and impart reversible redox properties. In evaluating this hypothesis, here we probe single-molecule junctions comprising oligoaryl wires with tetrahedral osmium(IV), silane, or methane centers. Surprisingly, we find that transport through junctions formed from the intact molecules in a non-polar, inert solvent appears largely independent of the central atom identity. In contrast, the conductance of junctions comprising an osmium(IV) wire can be modulated in an electrochemical environment 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 (EF). These measurements also indicate that such compounds can undergo in situ reactions that result in junctions comprising their dissociated oligoaryl arms, linked by chemisorbed Au-C(sp2) contacts. Our experimental results are supported by first principles calculations, which predict the osmium(IV) wires are substantially more conductive than the organic analogues due to their delocalized and well-coupled frontier orbitals with energies close to EF. This work highlights the promising potential of transition metal tetraaryl complexes as isotropic building blocks for functional circuits and extended materials, while opening avenues for further studies to enhance the connection between experimental observations and theoretical calculations.