A Persistent Open-Shell <i>m</i>‑QDM-TypeDiindenoanthracene 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.