A Thermally Populated, Perpendicularly Twisted Alkene Triplet Diradical

Variable‐temperature NMR and ESR spectroscopic studies reveal that bis(dibenzo[a,i]fluorenylidene) 1 possesses a singlet ground state, 1 (S 0 ), while the 90° twisted triplet 1 (T 1 ) is populated to a small extent already at room temperature. Analysis of the increasing amount of paramagnetic 1 (T 1 ) at temperatures between 300 and 500 K yields the exchange interaction J ex / h   c =3351 cm −1 and a singlet–triplet energy splitting of 9.6 kcal mol −1 , which is in excellent agreement with calculations (9.3 kcal mol −1 at the UKS BP86/B3LYP/revPBE level of theory). In contrast, the zero‐field splitting parameter D is very small (calculated value −0.018 cm −1 ) and unmeasurable.