1,4-Perinaphthadiyl. Singlet- and Triplet-State Reactivity of a Conjugated Hydrocarbon Biradical

Article abstract of DOI:10.1021/ja00216a027

1,4-Perinaphthadiyl (b) was generated from the azo precursor 1 by thermal elimination of nitrogen as well as by direct and sensitized photolysis.The biradical was persistent but light sensitive at low temperature in the triplet ground state (³b) and was characterized by optical and ESR spectroscopy; kinetic and spectrographic flash photolysis was used to monitor ³b at room temperature as a transient intermediate yielding naphthocyclobutane (2), divinylnaphthalene (3), and dimers (9,10).The lifetime of ³b was reduced from τ=0.2 ms in degassed solution to 0.2 μs in air-saturated solution, and as a result, peroxides (4, 6) were formed at the expense of the hydrocarbon products.However, the dependence of the peroxide yield on oxygen pressure required that a second intermediate also interacted with oxygen.This was identified as the singlet state of b undergoing catalyzed intersystem-crossing (ISC) upon diffusional encounter with triplet oxygen; its lifetime at 300 K was estimated as 1 ns, and the energy difference between ¹b and ³b as 5 kcal/mol.The efficiency of spontaneous ISC from ¹b to ³b was found to increase with decreasing temperature, and the yields of the different final products (2-4, 6, 9, 10) were strongly influenced by the conditions chosen for the formation of the biradical.The chemical properties of b are strikingly different from those of the related 1,3-perinaphthadiyls c and d due to the much shorter intrinsic lifetime of ¹c and ¹d.