We began by studying the E/Z isomerization of an alkene, E-cinnamyl acetate, with a T1 energy similar to those of the MR-TADF emitters; notably, the structurally similar E-methyl cinnamate has a ET = 2.38 eV (Table ). The geometric isomerization does not take place in the absence of an irradiated PC ( Figure shows the reaction progression for the E/Z isomerization of cinnamyl acetate in air at short times, while for DiICztBuDPA4 the reaction under N2 is shown. The conversion saturates after 30 min for DiICztBu4, DiICzMes4 and DiICztBuCz4, with these PCs having similar reaction rates. There is no correlation observed between ∆EST and either the E/Z ratio or reaction rate for this transformation. The absence of any substantive change in the post reaction absorption spectra of DiICztBu4, DiICzMes4, and DiICztBuCz4 revealed that these compounds are photochemically stable to O2 under the reaction conditions (Figure ).