Photocyclization of aryl halides. Part 4. 5-(2-halogenophenyl)-1,3-diphenyl-Δ2-pyrazolines. Predissociation and electron transfer between intramolecular but separate chromophores

Abstract

Photoreaction of 5-(2-iodophenyl)-1,3-diphenyl-Δ²-pyrazoline proceeds by simple bond homolysis from the S₁° state to give 1,3,5-triphenylpyrazoline and 2-phenylpyrazolo[1,5-f]phenanthridine in a ratio which depends on the hydrogen atom donor ability of the hydrocarbon solvent. The quantum yield for decomposition of the iodo-compound depends on the viscosity of the hydrocarbon solvent and on the temperature. The corresponding chloro- and bromo-compounds as well as the 3-iodo- and 4-iodophenyl compounds are photostable. Consideration of these facts and the complementary fluoresce data suggests a mechanism of interchromophoric predissociation to an nσ* state. However, examples are given where there is a smaller electron donor–acceptor energy gap between the two chromophores so that photoreaction occurs by an electron transfer pathway.