Thermochromism and photochromism of aryl-substituted acyclic azines III: Investigations on the mechanism of photochemical isomerization

Abstract

We studied the reaction mechanism of the unsensitized as well as the sensitized photochemical E— Z and Z— E isomerizations of benzophenone-9-anthraldehyde azine. The photochemistry of this compound follows a unitary E ⇌ Z reaction. For qualitative investigations we used benzophenone-9-acridinealdehyde azine, 9-anthranyl-2-pyridinealdehyde azine, 9-anthranyl-3-quinolinealdehyde azine, 9-anthranyl-9-acridinealdehyde azine and 9-acridinealdehyde azine. The experimental results are consistent with the following mechanism for the photoisomerization of azines. In the photochemical E— Z isomerization the S 1 state of the E isomer is reactive; sensitization of the triplet state does not induce the reaction. A singlet mechanism is also very probable for the Z— E isomerization. The most effective photophysical processes competing with isomerization are intersystem crossing in the E isomer and radiationless internal conversion in the Z isomer.