Intersystem crossing arylnitrenes

Huron and Platz recently smdied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1-2 ps. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in aryl-nitrenes and is comparable to a carbenic process. As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19. 

The dimerization of phenylnitrene to form azobenzene is a characteristic reaction of the triplet state of an arylnitrene. Thus, some intermediate formed along the reaction coordinate must undergo intersystem crossing to the triplet manifold. 

Assuming that singlet nitrene reacts with alkanes at near diffusion controlled rates allowed deduction of a rate constant of singlet-to-triplet nitrene intersystem crossing (ISC) of 2-8 X 10 s . This ISC rate is slower than in carbenes, but significantly faster than with arylnitrenes, which are discussed in a subsequent section. 

An o-cyano group has little influence on kisc, but two o-cyano groups slightly accelerate intersystem crossing. ° ° Singlet arylnitrenes with electron-withdrawing groups in the para-position have little influence on the rate constant of ISC. 

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