  Beginning with the early work of Clar et al. in 1955, zethrenes and their
laterally-extended homologues, super-zethrenes, have been intensively studied
in the solution phase, and are widely investigated as optical and charge
transport materials. Super-zethrenes are also considered to exhibit an
open-shell ground state. Zethrenes may thus serve as model compounds to
investigate nanoscale pi-magnetism. However, their synthesis is extremely
challenging due to their high reactivity. We report here a combined in-solution
and on-surface synthesis of the hitherto largest zethrene homologue -
super-nonazethrene - on Au(111). Using single-molecule scanning tunneling
microscopy and spectroscopy, we show that super-nonazethrene exhibits an
open-shell singlet ground state featuring a large spin polarization-driven
electronic gap of 1 eV. We obtain real-space maps of the frontier molecular
orbitals, and find that they correspond to singly occupied molecular orbitals.
In consistence with the emergence of an open-shell ground state,
high-resolution tunneling spectroscopy reveals inelastic singlet-triplet spin
excitations in super-nonazethrene, characterized by a strong intramolecular
magnetic exchange coupling of 51 meV. Further insights are gained by mean-field
and many-body perturbation theory calculations. Given the paucity of zethrene
chemistry on surfaces, our results therefore provide unprecedented access to
large open-shell zethrene compounds amenable to scanning probe measurements,
with potential application in molecular spintronics.
