The E/Z isomerization of diisopropyl fumarate studies demonstrate that three of these PCs can catalyze PEnT reactions with substrates having ET of 2.7 eV. Thus, we next explored the intramolecular [2+2] cycloaddition of norbornadiene (ET = 2.7 eV ) to quadricyclane (Table ). Schmid et al. reported a 99% yield after one hour using an anionic Ir complex having an ET of 2.99 eV (Table , Entry 14). The reaction does not take place in the absence of a PC (Table , Entries 1 and 2). Initial tests using DiICztBu4 showed that quadricyclane is obtained in an 87% yield after two hours under N2 (Table , Entry 4). Running this reaction on a shorter time scale revealed that the reaction is already completed after 30 min. In contrast to the E/Z isomerization of fumarate, DiICzMes4 and DiICztBuCz4 perform as well as DiICztBu4, yielding 85, 88, and 89%, respectively, after 30 min (Table , Entries 5, 7, and 9). For all three PCs, the reaction is slightly quenched in the presence of oxygen, producing yields of 64, 71, and 75% after 30 min when using DiICztBu4, DiICzMes4, and DiICztBuCz4, respectively ( ). These results not only demonstrate the value of three of these MR-TADF PCs to photocatalyze a demanding PEnT reaction but also that ET as a parameter is too coarse when cross-comparing different reactions using substrates with similar ET. a Norbornadiene (0.2 mmol) and PC (1 mol%) in DCM (2 mL). Yields were determined by 1 H NMR spectroscopy using 1,3,5-trimethoxybenzene as the internal standard. b from ref. in CD3CN exc = 415 nm after 1 h, norbornadiene (0.05 mmol), PC (0.3 mol%), CD3CN (0.6 mL).