The reaction progression with DiICztBuDPA4 was assessed only under N2 as this reaction is completely quenched when oxygen is present. As a result of the ostensibly poorer spectral overlap due to its lower ET DiICztBuDPA4 photocatalyzes the reaction significantly slower than the other three PCs of this family, where after 60 min the E/Z ratio is 60/40. The slower rate of isomerization may explain why, for this PC, oxygen acts as a more effective competitive quencher. Given that there is no significant difference in the PL under N2 or in air, we can rule out this parameter as governing the reaction rates (Table ). Photostability studies of DiICztBuDPA4 reveal a more significant change after the reaction both in air and under N2 than using the other PCs (Figure ). This implies that there may be an O2-induced photochemical degradation of DiICztBuDPA4, which would explain why the E/Z isomerization reaction does not proceed in air with this PC. Under N2, there is an observed change in the absorption spectrum of DiICztBuDPA4 after 24 h of reaction; however, this spectral change is different to the one observed in air. Under N2, the SRCT and at λabs = 485 nm remains, yet there is a slight red-shift in its onset compared to efore irradiation (new λabs = 491 nm). It is not clear what effect, if any, this spectral change has on the performance of the PC in the E/Z isomerization of cinnamyl acetate (Figure ) nor were we able to detect or isolate any photodegradation products.