Correlating electronic and magnetic coupling in large magnetic molecules via scanning tunneling microscopy

In an effort to improve the spin coupling in single-molecule magnets, we rationally designed a new building-block molecule with significantly enhanced spin coupling compared to a previously established molecule. We relate this to a stabilization of aromaticity in the central connecting carbon ring, promoting the spin-polarization mechanism. This correlation between magnetic and electronic properties is supported by bulk measurements as well as submolecularly resolved scanning tunneling microscopy and spectroscopy experiments, where we found distinct differences in the local density of states distribution of the two molecules, especially at the central carbon ring. While the established molecule exhibits localized, spatially decoupled and even switchable states, the improved building block exhibits symmetric local density of states delocalized over the entire molecule, also revealing that this main characteristic electronic property is preserved upon adsorption on a metal surface. Due to their planar geometry, these molecules can serve as model systems for scanning-probe based studies of molecular magnetism.