Crystalline 90°-Twisted Carbon-Centered Non-Kekulé Diradical Derived from N-Heterocyclic Carbene

Diradicals, molecules with two unpaired electrons, are crucial for understanding chemical reaction mechanisms, bond formation, and bond dissociation. Beyond their fundamental importance, they are increasingly studied for potential applications in batteries, luminescent materials, solar cells, and organic spintronic devices. As an essential property, determining the electronic ground state-singlet or triplet-of diradicals is critical for understanding their reactivity, physical properties, and spin-spin interactions. Tetramethyleneethane (TME), a prototypical disjoint non-Kekulé diradical, has long attracted attention; however, its ground state remains controversial because it is highly unstable. Here, by appending four N-heterocyclic carbene-functionalized p-phenylene units to an olefin skeleton, we prepared a TME analog that exhibits high stability in both solution and solid state at room temperature. Experimental characterization and quantum chemical calculations reveal that the isolated diradical adopts a triplet ground state, a 90°-twisted conformation with D_2d symmetry, and a central C─C bond length of 1.498(5) Å. Preliminary studies further demonstrate the unique reactivity of this carbon-centered diradical. This strategy offers a general approach for the rational design and isolation of other highly reactive diradicals, facilitating deeper insights into their electronic structures and properties.