A Magnetically Bistable Rigid Carbene─2,3-Benzofluorenylidene

Spin-crossover systems that can exist in two distinct spin states are rare among organic molecules lacking metal centers. The spin-bistable carbene 2,3-benzofluorenylidene was synthesized as a mixture of its singlet (S) and triplet (T) states at cryogenic temperatures, conditions under which intersystem crossing (ISC) between the two states is efficiently suppressed. Inert matrices (Ne, Ar, Xe, N₂) strongly influence the relative population of the two spin states: the T state dominates in Ar and Xe, whereas the S state is favored in Ne and N₂. Once formed, the S/T ratio cannot be altered by matrix annealing or irradiation. Ab initio calculations attribute this pronounced matrix dependence to spin-selective solvation effects, which modulate the ISC rate of the carbene across a broad temperature range. The remarkable spin bistability of 2,3-benzofluorenylidene enabled a direct comparison of the reactivity of the amphiphilic S state with that of the radical-like T state toward small molecules such as H₂, D₂, H₂O, O₂, and CO, uncovering striking spin-selective reactivity.