Triangulenes are prototypical examples of open-shell nanographenes. Their magnetic properties, arising from the presence of unpaired π electrons, can be extensively tuned by modifying their size and shape or by introducing heteroatoms. Different triangulene derivatives have been designed and synthesized in recent years thanks to the development of on-surface synthesis strategies. Triangulene-based nanostructures with polyradical character, hosting several interacting spin units, can be challenging to fabricate but are particularly interesting for potential applications in carbon-based spintronics. Here, we combine pristine and N-doped triangulenes into a more complex nanographene, TTAT, predicted to possess three unpaired π electrons delocalized along the zigzag periphery. We generate the molecule on a Au(111) surface and detect direct fingerprints of multiradical coupling and high-spin state using scanning tunneling microscopy and spectroscopy. With the support of theoretical calculations, we show that its three radical units are localized at distinct parts of the molecule and couple via symmetric ferromagnetic interactions, which result in a S = 3/2 ground state, thus demonstrating the realization of a molecular ferromagnetic Heisenberg spin trimer.

Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares and Departamento de Química Orgánica Universidade de Santiago de Compostela Santiago de Compostela 15782 Spain

CIC NanoGUNE BRTA Donostia San Sebastián 20018 Spain

Czech Advanced Technology and Research Institute Palacký University Olomouc Olomouc 77900 Czech Republic

Donostia International Physics Center Donostia San Sebastián 20018 Spain

Ikerbasque Basque Foundation for Science Bilbao 48013 Spain

Institute of Physics Czech Academy of Sciences Prague 16200 Czech Republic

Materials Physics Center Donostia San Sebastián E 20018 Spain

Oportunius Galician Innovation Agency Santiago de Compostela 15702 Spain

Theory of Condensed Matter Cavendish Laboratory University of Cambridge Cambridge CB3 0HE U K

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