Engineering Ferrimagnetic Interactions in Molecular Quantum Systems

Achieving long-range ferrimagnetic order in purely organic systems remains a major challenge in molecular magnetism. Here, we report the synthesis and characterization of heterospin-coupling motifs, formed by covalently linking spin- $1/2$ and spin-1 triangular nanographenes. A combined solution-phase and on-surface synthetic strategy yields three distinct compounds, whose structures are elucidated by bond-resolved scanning probe microscopy. Starting from a spin- $1/2$ -spin-1 dimer as the elemental ferrimagnetic unit, we employ inelastic electron tunneling spectroscopy to resolve low-energy magnetic excitations and extract the parameters of the Heisenberg Hamiltonian. Extension to trimeric architectures results in two distinct spin configurations, with compensated ( $S=0$ ) and uncompensated ( $S=3/2$ ) ferrimagnetic ground states. The Heisenberg model accurately describes all magnetic transitions, offering direct insight into increasingly complex spin Hamiltonians. These findings establish a molecular platform for designing heterospin systems with robust exchange interactions, providing access to higher-dimensional spin states beyond the two-level qubit paradigm.