Open-shell nanographenes exhibit unconventional π-magnetism arising from topological frustration or strong electron-electron interaction. However, conventional design approaches are typically limited to 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 design strategy that combines topological frustration and electron-electron interactions to fabricate a large fully fused 'butterfly'-shaped tetraradical nanographene on Au(111). We employ bond-resolved scanning tunnelling microscopy and spin-excitation spectroscopy to 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, harbouring a many-body singlet ground state and strong multi-spin entanglement, which is well described by many-body calculations. Furthermore, we study the magnetic properties and spin states in the nanographene using a nickelocene magnetic probe. The ability to imprint and characterize many-body strongly correlated spins in polyradical nanographenes paves the way for future advancements in quantum information technologies.

Center of Single Molecule Sciences Frontiers Science Center for New Organic Matter College of Electronic Information and Optical Engineering Nankai University Tianjin China

Department of Chemistry National University of Singapore Singapore Singapore

Department of Theoretical Chemistry J Heyrovsky Institute of Physical Chemistry Czech Academy of Sciences Prague Czech Republic

Institute for Functional Intelligent Materials National University of Singapore Singapore Singapore

Institute of Physics Czech Academy of Sciences Prague Czech Republic

National University of Singapore Research Institute Suzhou China

Regional Centre of Advanced Technologies and Materials Czech Advanced Technology and Research Institute Palacký University Olomouc Olomouc Czech Republic

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