The kTTA rates were determined from the same measurements as detailed in the Supporting Information, and the obtained rates are presented in Table . Interestingly, TIPS-Naph shows the lowest kTTA of the annihilators investigated here (6.2 ´ 10 8 M -1 s -1 ), PPO and PPF show similar rates of around 1.75 ´ 10 9 M -1 s -1 , while e.g., TP has an almost twice as high rate constant of 3.3 ´ 10 9 M -1 s -1 . We note that the measured value of kTTA for PPO is approximately three times lower than that reported previously. These results indicate that while the rate of the TTA event itself obviously affects the UC efficiency, it is the annihilator triplet lifetime that preferentially dictates the outcome. This is hardly surprising but worth reiterating, and great care should be given when evaluating especially the triplet lifetime of the annihilator. d Threshold excitation intensity evaluated at b = 0.5. e Rate constant for triplet-triplet annihilation (´10 9 ). f Maximum b value as defined by Equation , estimated at a laser fluence of 18 W cm -2 . g Spin-statistical factor, calculated from Eq. 1 using FTET = 0.89 and FTTA = bmax/2.