Spectroscopy and reactivity of Kekule hydrocarbons with very small singlet-triplet gaps

Two Kekule hydrocarbons, 2,2-dimethyl-2H-benzo [cd] fluoranthene (1) and it s benzannellated analogue 2,2-dimethyl-2H-dibenzo[cd,k]fluoranthene (2), we re generated photochemically from two different photoprecursors each and in vestigated spectroscopically in cryogenic matrices by UV-vis, fluorescence, and EPR and in solution using ns flash photolysis and chemical trapping ex periments. Hydrocarbon 1 is a groundstate singlet species, whereas compound 2 has a triplet ground state, the first such neutral Kekule hydrocarbon. T his difference, which is supported by density functional calculations, has profound influence on the spectroscopy and reactivity of the two compounds. Using the results of the spectroscopic measurements, trapping experiments, and density functional calculations, the singlet-tripler gap for 1 is esti mated to be 2.3-2.8 kcal mol(-1), with the singlet the ground state, and 0. 8-1.3 kcal mol(-1) for 2, in favor of the triplet.