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.