The photostability of the PCs was tested under the reaction conditions under N2 and in air (Figure ). The absorption spectra of DiICztBu4, DiICzMes4, and DiICztBuCz4 show only a minimal decrease in intensity under N2, while there are greater changes with oxygen present that suggests more photodegradation, which is consistent with the less efficient performance when the reaction is carried out under air (Figure ). Interestingly, the small degree of photodegradation of the MR-TADF PCs in the [2+2] cycloaddition in air contrasts with the significant photodegradation in the E/Z isomerization with diisopropyl fumarate in air and the associated absence of product formation, despite both substrates having the same reported ET.