  Open-shell benzenoid polycyclic aromatic hydrocarbons, known as magnetic
nanographenes, exhibit unconventional p-magnetism arising from topological
frustration or strong electronic-electron (e-e) interaction. Imprinting
multiple strongly entangled spins into polyradical nanographenes creates a
major paradigm shift in realizing non-trivial collective quantum behaviors and
exotic quantum phases in organic quantum materials. However, conventional
design approaches are limited by a single magnetic origin, which can restrict
the number of correlated spins or the type of magnetic ordering in open-shell
nanographenes. Here, we present a novel design strategy combing topological
frustration and e-e interactions to fabricate the largest fully-fused
open-shell nanographene reported to date, a 'butterfly'-shaped tetraradical on
Au(111). We employed bond-resolved scanning tunneling microscopy and spin
excitation spectroscopy to unambiguously resolve the molecular backbone and
reveal the strongly correlated open-shell character, respectively. This
nanographene contains four unpaired electrons with both ferromagnetic and
anti-ferromagnetic interactions, harboring a many-body singlet ground state and
strong multi-spin entanglement, which can be well described by many-body
calculations. Furthermore, we demonstrate that the nickelocene magnetic probe
can sense highly-correlated spin states in nanographene. The ability to imprint
and characterize many-body strongly correlated spins in polyradical
nanographenes not only presents exciting opportunities for realizing
non-trivial quantum magnetism and phases in organic materials but also paves
the way toward high-density ultrafast spintronic devices and quantum
information technologies.
