Laser flash photolysis study of ( E)-1-(2′-arylidene-1′-phenylhydrazinyl)-1,2-dibenzoylalkenes: Mechanistic aspects of photochemical ring closure

Abstract

Several ( E)-1-(2′-arylidene-1′-phenylhydrazinyl)-1,2-dibenzoylethylenes, varying from one another in the nature of the p-substituent on the 2-benzylidene group, have been studied by nanosecond laser flash photolysis and pulse radiolysis. The transient absorption spectral changes from direct laser excitation (λ ex = 337.1 nm or λ ex = 355 nm) as well as under triplet sensitization by camphorquinone (λ ex = 485 nm) are explained in terms of the formation of zwitterionic intermediates in the course of intramolecular photocyclization (pentadienyl anion mode) and, in part by cis → trans isomerization about the ethylenic double bond. From the bleaching of ground state absorption, the lower limits for photochemical loss are estimated to be 0.01 – 0.1 (in benzene and in methanol). The triplets of the dibenzoylethylene substrates and the triplet excited states of zwitterionic intermediates are not observed on the nanosecond time scale, presumably because of their short-lived character.