Large magnetic exchange coupling in rhombus-shaped nanographenes with zigzag periphery

Nanographenes with zigzag edges are predicted to manifest non-trivial π-magnetism resulting from the interplay of concurrent electronic effects, such as hybridization of localized frontier states and Coulomb repulsion between valence electrons. This provides a chemically tunable platform to explore quantum magnetism at the nanoscale and opens avenues towards organic spintronics. The magnetic stability in nanographenes is thus far greatly limited by the weak magnetic exchange coupling, which remains below the room-temperature thermal energy. Here, we report the synthesis of large rhombus-shaped nanographenes with zigzag peripheries on gold and copper surfaces. Single-molecule scanning probe measurements show an emergent magnetic spin singlet ground state with increasing nanographene size. The magnetic exchange coupling in the largest nanographene (C70H22, containing five benzenoid rings along each edge), determined by inelastic electron tunnelling spectroscopy, exceeds 100 meV or 1,160 K, which outclasses most inorganic nanomaterials and survives on a metal electrode.This work was supported by the Swiss National Science Foundation (grant nos. 200020-182015 and IZLCZ2-170184), NCCR MARVEL funded by the Swiss National Science Foundation (grant no. 51NF40-182892), the European Union’s Horizon 2020 research and innovation programme (grant no. 785219, Graphene Flagship Core 2), the Office of Naval Research (N00014-18-1-2708), the Max Planck Society, Ministry of Science and Innovation of Spain (grant nos. PID2019-106114GB-I00 and PID2019-109539GB), Generalitat Valenciana and Fondo Social Europeo (grant no. ACIF/2018/175), MINECO-Spain (grant no. MAT2016-78625) and Portuguese FCT (grant no. UTAPEXPL/ NTec/0046/2017). Computational support from the Swiss Supercomputing Center (CSCS) under project ID s904 is gratefully acknowledged