A Stable Thioxanthene Dioxide Radical Motif for Modular Access to High-Spin Diradical and Fully-Fused Tetraradicaloid Scaffold.

Open-shell π-conjugated systems are of fundamental interest due to their fascinating electronic and magnetic properties, yet their rational construction remains challenging. Herein, we report a cyano-terminated thioxanthene dioxide radical (MR) as a robust building block for constructing complex di-/tetraradical architectures. The MR motif features a sulfone-fused, quasi-planar core that, synergized by electron-withdrawing para-cyano groups, provides significant thermodynamic stabilization to yield an exceptional ambient half-life time (t1/2) of 81 days. Leveraging this scaffold, we achieved the modular synthesis of an m-phenylene-bridged diradical (DR) and a fully-fused tetraradicaloid (TR) via a high-efficiency radical-mediated oxidative cyclization protocol. While DR represents a rare air-stable triplet-ground-state diradical (ΔES-T = 0.13 kcal/mol), the tetraradicaloid TR possesses a singlet ground state stabilized by global aromaticity across its π-core, accompanied by an unconventional energy hierarchy featuring a quintet manifold as the lowest thermally accessible excited state. Notably, TR demonstrates excellent room-temperature spin coherence, with spin-lattice relaxation/spin dephasing time (T1/T2) of 85.47/1.17 µs, suggesting its good potential as robust spin qubits under ambient conditions. These results establish MR as a versatile radical motif for developing complex multi-spin systems with emergent magnetic/quantum information functionalities.