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‐ 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‐–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 () and uncompensated () 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.