Photodeprotection Reaction Mechanisms of Caged Species Utilizing a Photochromism Function

Acetoxy-1,2,2-tri­(aryl)­ethanone (1) is a novel and visual release-and-report system that contains the photochromic diarylethylene function attached to the photocage dimethoxybenzoin platform. However, the mechanism of 1 cyclization and a subsequent deprotection remains unclear. Here, we use femtosecond and nanosecond transient absorption spectroscopies in combination with density functional theory computations to study the detailed reaction mechanism. The photodeprotection proceeds with competition between pathways initiated by two different configurations of the singlet excited state of 1 (labeled as 1 1 LE and 1 1 CT); the stepwise elimination after cyclization of 1 1 LE constitutes the predominant pathway, whereas the concerted removal of acetic acid after cyclization of 1 1 CT is the minor pathway. These results contribute to a detailed photodeprotection mechanism of 1 and provide new insights into the effect of geometric configurations of intermediates on the photodeprotection pathways. This new information can help in the further development of this type of the photolabile protecting group (PPG) for the protection of biorelevant molecules and in the design of an improved and versatile release-and-report PPG.