The design of open-shell carbon-based nanomaterials is at the vanguard of materials science, steered by their beneficial magnetic properties like weaker spin-orbit coupling than that of transition metal atoms and larger spin delocalization, which are of potential relevance for future spintronics and quantum technologies. A key parameter in magnetic materials is the magnetic exchange coupling (MEC) between unpaired spins, which should be large enough to allow device operation at practical temperatures. In this work, we theoretically and experimentally explore three distinct families of nanographenes (NGs) (A, B, and C) featuring majority zigzag peripheries. Through many-body calculations, we identify a transition from a closed-shell ground state to an open-shell ground state upon an increase of the molecular size. Our predictions indicate that the largest MEC for open-shell NGs occurs in proximity to the transition between closed-shell and open-shell states. Such predictions are corroborated by the on-surface syntheses and structural, electronic, and magnetic characterizations of three NGs (A[3,5], B[4,5], and C[4,3]), which are the smallest open-shell systems in their respective chemical families and are thus located the closest to the transition boundary. Notably, two of the NGs (B[4,5] and C[4,3]) feature record values of MEC (close to 200 meV) measured on the Au(111) surface. Our strategy for maximizing the MEC provides perspectives for designing carbon nanomaterials with robust magnetic ground states.

Departamento de Física de la Materia Condensada Universidad Autónoma de Madrid 28049 Madrid Spain

Department of Chemistry Biochemistry and Pharmaceutical Sciences University of Bern 3012 Bern Switzerland

Empa Swiss Federal Laboratories for Materials Science and Technology 8600 Dübendorf Switzerland

IBM Research─Zurich Säumerstrasse 4 8803 Rüschlikon Switzerland

IMDEA Nanoscience C Faraday 9 Campus de Cantoblanco 28049 Madrid Spain

Institute of Physics of the Czech Academy of Science 162 00 Praha Czech Republic

Instituto de Ciencia de Materiales de Madrid CSIC Cantoblanco 28049 Madrid Spain

Max Planck Institute for Polymer Research 55128 Mainz Germany

Regional Centre of Advanced Technologies and Materials Palacký University Olomouc CZ 771 46 Olomouc Czech Republic

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