Enhanced electronic coupling in tetraaryl molecular junctions with osmium(IV) centers

Abstract

Structural motifs based on tetraphenylmethane are widely used in molecular electronic circuits, self-assembled monolayers, and porous frameworks, but their performance in conductive systems is often limited by redox-inactive, sp³-hybridized central atoms that interrupt π-conjugation. Here, we show that replacing the group 14 central atom with a tetravalent transition metal provides a design strategy to enhance electronic coupling and enable bias-dependent control of charge transport. We demonstrate this by measuring the single-molecule conductance of oligoaryl wires incorporating tetrahedral osmium(IV), silicon, or carbon centres using scanning tunnelling microscope-based break junction measurements. In non-polar solvents, junctions comprising osmium(IV) complexes exhibit a significantly reduced conductance decay with length compared to their organic analogues. In polar media, their conductance can be electrochemically modulated to values up to 80× higher than those of a silane analogue. Combined electrochemical and spectroscopic studies, supported by first-principles calculations, indicate that osmium(IV) wires exhibit more delocalized frontier orbitals and smaller HOMO-LUMO gaps, leading to well-coupled HOMO-derived transmission resonances near the electrode Fermi level. Together, these results establish transition metal tetraaryl complexes as promising building blocks for molecular circuits and extended materials.