Laser flash photolysis of azocumenes. Direct observation of stepwise decomposition.

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The mechanism of the decomposition of α,α′-azocumene and 2,2′-di-p-tolyl-2,2′-azopropane was studied by means of laser flash photolysis (λ=347 nm) and the optical absorption of 1-methyl-1-phenylethyl (α-cumyl) and α-cumyldiazenyl radicals was observed. The α-cumyl radicals were formed in two modes, a fast mode occurring during the flash and a slow mode after the flash. The ratio of the optical densities pertaining to the two modes depended on the absorbed dose rate. The higher the absorbed dose per flash, the more dominant was the fast mode. The transient band at λ=285 nm which decayed simultaneously with the grow- in of the α-cumyl radicals was assigned to α-cumyldiazenyl radicals. The experimental results strongly suggested the occurrence of the stepwise decomposition mechanism. The dose rate dependent kinetics of the slow formation process was explained in terms of the competition between the unimolecular decomposition and the dimerization of α-cumyldiazenyl radicals. The rate constants of the decomposition and the dimerization were estimated to be 1.1×105 s−1 and 5×109 M−1 s−1, respectively.
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