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.