TIPS-Naph was synthesized in accordance with a literature procedure, and during experiments a fluorescent contamination, which has not been reported previously, was discovered. As detailed in Section 2.5 of the Supporting Information, the removal of this contamination by additional cycles of recrystallization lead to a substantial increase in FUC,g. This could potentially explain why we see a much higher FUC,g (16.8%) compared to other studies using TIPS-Naph (FUC,g » 10%) in which FTET is reported to be close to unity. Our group has previously investigated the locked t-stilbene compound 5,10dihydroindeno[2,1-a]indene (I2), a highly fluorescent compound that unfortunately suffers from very low solubility in toluene. 2PI was chosen as a potentially more soluble equivalent to I2, and the solubility was indeed much higher. When samples containing 10 mM 2PI were used for TTA-UC, however, some light scattering was evident in the absorption. Additionally, the UC signal increased strongly over time during 405 nm excitation, reaching a maximum value after approximately 30 minutes (Figure ). The measured FUC,g for 10 mM 2PI was low (1.0%), which is a lower estimate given that the extended laser exposure not only causes the UC emission signal to increase but also the sensitizer to degrade. Upon lowering the 2PI concentration to 1 mM the scattering decreased significantly, indicating that the observed behavior was due to 2PI not being fully solvated at 10 mM. At 1 mM FUC,g went up to 4.4% and no signal increase was observed over time (Figure ). Probing Triplet Kinetics Using Time-Resolved Emission. To understand the differences in FUC,g between the annihilators we examined the kinetics of the UC samples. There are several important rate constants and parameters needed to properly evaluate TTA-UC systems, e.g., the annihilator triplet excited state lifetime, the triplet-triplet annihilation rate constant (kTTA), and the excitation threshold intensity (Ith). In the following section, we show that these can be determined from the same series of time-resolved UC emission measurements, thus circumventing the need for more challenging transient absorption measurements altogether.