The quenching behavior was analyzed by titration series with each annihilator species, and the obtained TET rates were calculated from Equation S2. The resulting kTET are found in Table (see Figure for Stern-Volmer plots). As expected kTET are typically higher for the annihilators with lower-lying triplets (see Table for triplet energies and Figure for the phosphorescence of PPF), but fortunately endothermic TET from 4CzBN is also possible, yielding kTET on the order of 10 8 M -1 s -1 to the high-triplet energy annihilator PPD. We note that using phosphorescence spectra of rotationally flexible molecules typically underestimates the triplet energy, so the energy commonly referenced for TP (2.53 eV) is therefore likely underestimated. We choose instead a value of 2.62 eV which was obtained from quenching experiments, and which better correlates with the relatively slow TET (kTET = 4.1 ´ 10 8 M -1 s -1 ) observed from 4CzBN to TP.