Au-Thiolate Interfacial Coordination: The Key to Determining the Spin State of a Blatter Radical When Incorporated into Gold-Molecule-Gold Junctions

The retention of the open-shell character of radicals when they are contacted to metallic electrodes in a junction is the primary prerequisite for their applications in high-performance molecular electronic, spintronic, and thermoelectric devices. Based on first-principles quantum transport calculations, we have investigated the spin state and the electronic transport properties of a series of single-molecule junctions incorporating a Blatter radical (BR) with thiol linkers and having diverse Au-S interfacial configurations. Our calculations suggest that the presence of Au-S bonding with a larger coordination number is conducive to maintaining the open-shell nature of the BR within a junction. If one treats the BR and the bonded Au adatoms as the central molecule, it is the energy of the singly occupied molecular orbital (SOMO) of the BR in such a unit (the so-called BR SOMO) that mainly determines the spin state of the molecule in the junction. Notably, the coupling strength between the BR SOMO and the electronic continuum states of the gold electrodes appears to play only a secondary role. These two factors together determine the electron population on the BR SOMO and thus the spin state of the BR within a junction. Our findings fill in gaps in understanding the influence of the metal-radical interfacial configurations on the open-shell character of a radical when bound in a junction. This knowledge facilitates the implementation of future radical-based multifunctional molecular devices.