Revisiting the Photodegradation of the o -Nitrobenzyl Group via Elucidation of the Stability and Reactivity of o -Nitrosobenzaldehyde

The ortho -nitrobenzyl ( o NB) group is among the most widely used photodegradable units, serving in diverse fields, including its use as a protective group in organic synthesis, as a stimulus in photoresponsive and photodegradable polymer materials, and in caged molecules in biology. Despite its extensive application, however, little is known about the fate of the photodegraded fragments of the o NB group and their potential impact on the system in which they are used. Here, we isolated o -nitrosobenzaldehyde ( 2 ), a proposed primary photoadduct of the o NB group, and investigated its stability and reactivity. Compound 2 rapidly decomposed under both thermal and photochemical conditions, forming oligomers bearing azo- and amide-linkages. The structure of a trimer containing both linkages was unambiguously determined by single-crystal X-ray diffraction. We further found that dienes effectively trapped the nitroso group in 2 by the hetero-Diels–Alder reaction, thereby minimizing undesired oligomerization. This trapping strategy, in combination with a microflow reaction system, significantly enhances the photodegradation efficiency of the o NB-containing polymers. These findings shed new light on the photochemistry of o NB groups and provide a rational basis for the design and implementation of o NB groups in advanced functional systems.