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(S0), while the 90° twisted triplet 1(T1) is populated to a small extent already at room temperature. Analysis of the increasing amount of paramagnetic 1(T1) at temperatures between 300 and 500 K yields the exchange interaction Jex/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. Twist of fate: Bis(dibenzo[a,i ]fluorenylidene) (see scheme) has a closed‐shell singlet ground state, but the amount of the 90° twisted, antiferromagnetic triplet diradical increases in the temperature range 300–500 K. The exchange coupling Jex/h c was determined to be 3551 cm−1, which corresponds to a singlet–triplet energy splitting of 9.6 kcal mol−1.