Redox-Controlled Access to Spin States from S = 0 to S = 3/2 in a 1,2-Dicarbonyl Scaffold Enabled by a Naphthoquinone-Fused N-Heterocyclic Carbene

Abstract Singlet carbenes have emerged as powerful stabilizing ligands for persistent organic radicals; however, their application toward controllable high-spin organic systems remains largely unexplored. Herein, we report an air- and thermally stable 1,2-dicarbonyl radical cation supported by a redox-active naphthoquinone-fused carbene as a modular platform for accessing exchange-coupled spin manifolds spanning from diamagnetic S = 0 to quartet S = 3/2 states within a single organic scaffold. The radical cation exhibits three reversible redox events within the −2 to +1 V window, enabling sequential one-electron oxidation and reduction processes that systematically modulate spin multiplicity. Molecular orbital analysis reveals that electron-withdrawing naphthoquinone substituents stabilize the LUMO and LUMO+1, resulting in inversion of the SUMO–LUMO and SUMO–LUMO+1 energy ordering. This unusual orbital arrangement facilitates stepwise electron uptake into spatially distinct radical sites and enables systematic access to multiple spin states. All four redox states were isolated and comprehensively characterized, while the open-shell states were interrogated by multifrequency cw- and pulse-EPR spectroscopy, ENDOR, and density functional theory calculations. Spectroscopic analyses reveal that formation of higher-spin states preserves local radical character while enabling moderate ferromagnetic exchange coupling across expanded spin manifolds. These findings establish a molecular design strategy for controllable high-spin organic radicals within a single redox-active molecular framework.