For DiICztBuDPA4 no photodegradation is observed, and this is because no reaction takes place either with norbornadiene or O2 (Figure ). Instead, the intensity of the absorption spectrum after the reaction is slightly higher than before irradiation, which can be attributed to the poor solubility of this PC in the solution and where the concentration of dissolved PC slightly increases after stirring for 30 min. There is significant photodegradation observed for 4CzIPN under N2, and the profile is similar to that in air (Figure ). The absorption band at λabs = 377 nm and the shoulder at λabs = 450 nm disappear while a more red-shifted, less intense band at λabs = 520 nm appears. These results imply that the substrate reacts with 4CzIPN and that the [2+2] cycloaddition is likely radical stepwise as opposed to a concerted mechanism (Table , Entry 12 and 13).