The isomerization reaction with 4CzIPN is significantly slower than with DiICztBu4, DiICzMes4, and DiICztBuCz4, only producing a 39/61 E/Z ratio after 60 min (Figure ). Thus, despite similar thermodynamic driving forces (i.e., similar ET), the origin of the higher yields of the Z isomer and faster reaction kinetics could be due to either the higher molar absorptivity of these three MR-TADF compounds at the photoexcitation wavelength and/or an increased spectral overlap of the phosphorescence of the MR-TADF emitters with the spin-forbidden absorption of the sub (Eq. 1). 4CzIPN is mostly photostable; however, a slight decrease in the absorbance of the CT band is observed in air, suggesting that some degradation is taking place if the reaction is carried out when oxygen is present (Figure ). Further, there is a significant dependency of the kinetics of the reaction with respect to the presence/absence of O2. After 5 min, the E/Z ratios are 80/20 and 94/6 under N2 and air, respectively. These results reveal that 4CzIPN photocatalyzes this reaction more slowly than DiICztBu4, DiICzMes4, and DiICztBuCz4 and that its triplet excited state is also more effectively quenched by O2. We next investigated the E/Z isomerization of diisopropyl fumarate, which has a higher ET of 2.7 eV, to produce the corresponding maleate (ET = 3.1 eV) (Table ). Despite the ETs of the PCs being lower than 2.7 eV ( To understand the divergence in behavior of these two isomerization reactions in the presence of O2, we interrogated the reaction rates of diisopropyl fumarate under N2 (Figure ). If the rates are slower compared to those with cinnamyl acetate, then the lower E/Z ratios may be explained by competitive O2 quenching of the T1 state to that of the fumarate. Unlike the reaction with cinnamyl acetate, the reaction rates under N2 differ between DiICzMes4, DiICztBu4, and DiICztBuCz4 but still are all faster than 4CzIPN. The fastest conversion to the Z isomer uses DiICzMes4, yielding an E/Z ratio of 22/79 ± 3 after 60 min (13:87 after 24 h, Table , Entry 6), while DiICztBu4 only produces an E/Z ratio of 60/41 ± 1 after 60 min, yet affords the same E/Z ratio after 24 h as DiICzMes4 (13:87 after 24 h, Table , Entry 4). The reaction reaches its steady state in E/Z ratio after 2 h for DiICzMes4 (E/Z ratio of 12/88), while for DiICztBu4 it takes 4 h to reach the 13/87 E/Z ratio. These rates are slower than those with cinnamyl acetate, where the reaction was completed after 30 min. Given that both DiICzMes4 and DiICztBu4 are stable during the reaction when conducted under N2, photodegradation of the PC can be excluded as the origin of the slower reaction kinetics observed for DiICztBu4. As the phosphorescence of both DiICztBu4 and DiICzMes4 are the same, the spectral overlap between the phosphorescence spectra of these two PCs and the spin-forbidden absorption spectrum of the sub will therefore be similar. A plausible conclusion is that the tert-butyl groups are bulkier than mesityl groups and impede the collisional interaction more, thus adversely affecting the reaction kinetics with DiICztBu4. We observed that these two PCs do photodegrade when the reaction is carried out under air. Thus, there is a divergence in the photochemical outcome wherein the kinetics of the E/Z isomerization of cinnamyl acetate outcompetes photodegradation while this is not the case with the E/Z isomerization of diisopropyl fumarate (Figures S45a&b). Surprisingly, while the initial rate of isomerization using DiICztBuCz4 is comparatively as fast as using DiICzMes4, there is a significant off-cycle photodegradation both in air and N2 that effectively caps the E/Z ratio at only 66/34 ± 4 (Figures S45c).