Irradiation of 2,5-diphenyltetrazole (2a) in an ether/pentane/ ethanol-glass at 77 K with UV-light of wavelength 280 or 290 nm yields diphenylnitrilimine (1b) by extrusion of nitrogen. The UV-spectrum of this 1,3-dipolar intermediate is shown in fig. 1. Between 160 and 175K, 1b undergoes a dimerization to give 1,2-bis(phenylazo)-1,2-diphenyl-ethylene (15). 2,4,5-Triphenyl-1,2,3-triazole (16), the main product of the photolysis of 2a at room temperature, is not formed on heating the solution of 15 to 300K.
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MEIER et al. (1980) studied this question.