A route toward the on-surface synthesis of organic ferromagnetic quantum spin chains

Engineering a sublattice imbalance is an intuitive way to induce high-spin ground states in bipartite polycyclic conjugated hydrocarbons. Such high-spin molecules can be employed as building blocks of quantum spin chains, which are outstanding platforms to study many-body physics and fundamental models in quantum magnetism [1].

In contrast to antiferromagnetism, demonstration of ferromagnetic coupling between polycyclic conjugated hydrocarbons has been scarce. Here, we demonstrate the on-surface synthesis of short ferromagnetic spin chains based on dibenzotriangulene, a polycyclic conjugated hydrocarbon with a triplet ground state. We achieve a direct majority-minority sublattice coupling between adjacent units, which leads to a global sublattice imbalance in the chains and therefore a ferromagnetic ground state with a strong intermolecular ferromagnetic exchange. By means of scanning probe measurements and quantum chemistry calculations, we confirm quintet and septet ground states in dimers and trimers, respectively.

[1] S. Mishra et al., Nature 598, 287 (2021).