A Persistent Open-Shell <i>m</i> -QDM-Type Diindenoanthracene Diradicaloid with a Large Diradical Character

Indenofluorenes (IFs) have received great interest owing to their intrinsic antiaromaticity, open-shell diradical character, and narrow band gaps. Herein, we report the synthesis and comprehensive characterization of a hitherto unknown <i>m</i>-QDM-type IFs-based diradicaloid derived from a diindenoanthracene framework. The relationships among the structure, diradical character, and chemical and physical properties of this novel diradicaloid are systematically investigated. This diradicaloid exhibits pronounced open-shell character with a large diradical character (<i>y</i>₀ = 0.87), attributed to its unique fusion pattern and extended <i>m</i>-QDM-type structure. Despite its high reactivity, we successfully obtained a crystalline derivative, <b>2b</b>, by introducing a bulky 9-mesitylanthracene substituent to protect the spin-bearing centers. Remarkably, <b>2b</b> is stable in the solid state under ambient conditions but gradually decomposes to a ketone in solution with a long half-life of approximately 33 days. Additionally, <b>2b</b> displays low-energy absorption beyond 1300 nm, a feature arising from its antiaromatic and π-extended electronic structure. The narrow singlet-triplet energy gap (Δ<i>E</i>_S-T = -1.9 kcal mol^-1 from VT-EPR and -1.4 kcal mol^-1 from SQUID) of <b>2b</b> enables its thermally accessible triplet excited state, resulting in its paramagnetic behavior with moderate magnetic susceptibility (0.59 cm³ K mol^-1) at room temperature. This study offers valuable insights into the structure-property relationships governing the diradical character, stability, and reactivity of diradicaloids, providing guidance for the rational design and synthesis of new diradicaloid and polyradicaloid systems.