  Kondo resonances in molecular adsorbates are an important building block for
applications in the field of molecular spintronics. Here, 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 uB. 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). Our combined STM and
DFT study introduces a new pathway to tailoring the magnetic state of molecular
adsorbates on surfaces, with significant potential for spintronics and quantum
information science.
