Frontier Orbital Degeneracy: A new Concept for Tailoring the Magnetic State in Organic Semiconductor Adsorbates

Kondo resonances in molecular adsorbates are an important building block for applications in the field of molecular spintronics. In our work, we introduce the novel concept of using frontier orbital degeneracy for tailoring the magnetic state, which is demonstrated for the case of the organic semiconductor 1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile (HATCN, C₁₈N₁₂) on Ag(111). Low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/STS) measurements reveal the existence of two types of adsorbed HATCN molecules with distinctly different appearances and magnetic states, as evident from the presence or absence of an Abrikosov-Suhl-Kondo resonance. Our DFT results show that HATCN on Ag(111) supports two almost isoenergetic states, both with one excess electron transferred from the Ag surface, but with magnetic moments of either 0 or 0.65 µ_B. Therefore, even though all molecules undergo charge transfer of one electron from the Ag substrate, they exist in two different molecular magnetic states that resemble a free doublet or an entangled spin state. We explain how the origin of this behavior lies in the twofold degeneracy of the lowest unoccupied molecular orbitals of gas phase HATCN, lifted upon adsorption and charge-transfer from Ag(111). These results provide a deeper insight into the Kondo effect in such interfacial systems between molecular spins and functional surfaces by considering the spin density distribution and associated magnetic moments for all supported configurations in the radical species instead of only connecting the effect to the extra electronic charge. Our combined STM and DFT study therefore introduces a new pathway to tailoring the magnetic state of single molecular adsorbates on functional surfaces, with significant potential for spintronics and quantum information science.