Stabilizing Indeno[1,2-a]Fluorene: The Only Ferromagnetic Diradical of Indenofluorene Isomers

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 (ΔE_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 (T₁ = 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-NO₂ 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.