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 ( t 1/2 ) 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 (Δ E S‐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 ( T 1 /T 2 ) 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.