Antiferromagnetic Chains in a Monolayer of Molecular Qubits Assembled on Graphene

The neutral Cu²⁺ complex [Cu(dttt)₂], in which dttt^- is the 1,3,2-dithiazole-4-thione-5-thiolate ligand, is a promising molecular spin qubit where a hydrogen-free and sulfur-rich scaffold has been designed to enhance the spin coherence. In bulk, the structural organization induces strong intermolecular antiferromagnetic exchange couplings up to about 100 cm^-1, mediated by van der Waals interactions and propagated along 1D chains of molecules within the crystal structure. Here, the deposition by sublimation in ultra-high vacuum conditions of [Cu(dttt)₂] on a graphene surface is studied, focusing on investigating the topology and magnetism of ultrathin films. These deposits are characterized by combining X-ray photoelectron spectroscopy and scanning tunneling microscopy; the latter indicates an ordered chain-like arrangement of the assembled monolayer. Synchrotron-based X-ray absorption techniques flanked by density functional theory and wavefunction-based simulations confirm the molecular ordering. These reveal that the magnetic coupling observed in bulk is also present at the monolayer level, highlighting the persistence of a 1D antiferromagnetic intermolecular coupling of about 50 cm^-1 with a non-negligible contribution coming from a through-surface exchange path.