Stabilizing Indeno[1,2‐ <i>a</i> ]Fluorene: The Only Ferromagnetic Diradical of Indenofluorene Isomers

ABSTRACT Among five indenofluorene regioisomers, only indeno[1,2‐ a ]fluorene is predicted to be a triplet diradical but has eluded unambiguous structural identification due to inherent instability. Here, we report the solution synthesis of a stabilized indeno[1,2‐ a ]fluorene derivative showing long persistence in ambient air (half‐life of ca. 29 h in solution and around 30 days in the solid state), achieved by simultaneously introducing more steric‐hindering groups and strongly electron‐withdrawing substituents. Single‐crystal X‐ray diffraction confirms its planar polycyclic framework, with a substantial contribution from the benzenoid diradical resonance. Magnetic characterizations by continuous‐wave/pulsed EPR spectroscopy and SQUID magnetometry experimentally prove a triplet ground state with strong ferromagnetic coupling (Δ E S‐T ≥ 2.2 kcal/mol; J / k B ≥ 550 K). Meanwhile, the compactly distributed spin population endows the diradical with an unusually large and significantly rhombic zero‐field splitting ( D = –757 MHz, E = 102 MHz). Together with long spin relaxation times ( T 1 = 19.3 ms and T m = 3.09 µs at 10 K), such properties provide a promising basis for future low‐field clock‐transition and coherent spin‐manipulation studies. As importantly, IF‐NO 2 demonstrates how molecular design can effectively tune the persistence and magnetic properties of high‐spin organic radicals, offering a platform for further exploration in the molecular quantum and spintronic fields.