The chemical versatility of carbon imparts manifold properties to organic compounds, wherein magnetism remains one of the most desirable but also elusive 1. Polycyclic aromatic hydrocarbons, also referred to as nanographenes, show a critical dependence of electronic structure on the topologies of the edges and the π-electron network, which makes them model systems to engineer unconventional properties including magnetism. In 1972, Erich Clar envisioned a bowtie-shaped nanographene C38H18 2,3 , where topological frustration in the π-electron network renders it impossible to assign a classical Kekulé structure without leaving unpaired electrons, driving the system into a magnetically non-trivial ground state 4. Here, we report the experimental realisation and in-depth characterisation of this emblematic nanographene known as Clar's goblet. Scanning tunneling microscopy and spin excitation spectroscopy of individual molecules on a gold surface reveal a robust antiferromagnetic order with an exchange coupling of 23 meV, exceeding the Landauer limit of minimum energy dissipation at room temperature 5. Through atomic manipulation, we realise switching of magnetic ground states in molecules with quenched spins. Our results provide direct evidence of carbon magnetism in a hitherto unrealised class of nanographenes 6 , and prove a long-predicted paradigm where topological frustration entails unconventional magnetism, with implications for room-temperature carbon-based spintronics 7,8 .

The electronic and magnetic properties of nanographenes strongly depend on their size, shape and topology. While many nanographenes present a closed-shell electronic structure, certain molecular topologies may lead to an open-shell structure.

Fractionalization is a phenomenon in which strong interactions in a quantum system drive the emergence of excitations with quantum numbers that are absent in the building blocks.Outstanding examples are excitations with charge e/3 in the fractional quantum Hall effect 1,2 , solitons in one-dimensional conducting polymers 3,4 and Majorana states in topological superconductors 5 . Fractionalization is also predicted to manifest itself in lowdimensional quantum magnets, such as one-dimensional antiferromagnetic S = 1 chains.

Nanographenes with zigzag edges are predicted to manifest non-trivial π-magnetism resulting from the interplay of concurring 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 toward organic spintronics. The magnetic stability in nanographenes is thus far 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 periphery 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 (C 70 H 22 , containing five benzenoid rings along each edge), determined by inelastic electron tunneling spectroscopy, exceeds 100 meV or 1160 K, which outclasses most inorganic nanomaterials and survives on a metal electrode.Magnetism in solids is usually associated to d-or f-block elements. However, since the isolation of graphene, the field of carbon magnetism has gained increased traction 1 . Though

Triangular zigzag nanographenes, such as triangulene and its π‐extended homologues, have received widespread attention as organic nanomagnets for molecular spintronics, and may serve as building blocks for high‐spin networks with long‐range magnetic order, which are of immense fundamental and technological relevance. As a first step towards these lines, we present the on‐surface synthesis and a proof‐of‐principle experimental study of magnetism in covalently bonded triangulene dimers. On‐surface reactions of rationally designed precursor molecules on Au(111) lead to the selective formation of triangulene dimers in which the triangulene units are either directly connected through their minority sublattice atoms, or are separated via a 1,4‐phenylene spacer. The chemical structures of the dimers have been characterized by bond‐resolved scanning tunneling microscopy. Scanning tunneling spectroscopy and inelastic electron tunneling spectroscopy measurements reveal collective singlet–triplet spin excitations in the dimers, demonstrating efficient intertriangulene magnetic coupling.