To fully assign the origin of the delayed emission, we next recorded the change in delayed lifetimes with temperature. The delayed fluorescence observed in 6NICz and 6,10NICz is noticeably faster at elevated temperatures (Figure and), and is assigned as TADF. As all five emitters exhibit negligible solvatochromism (Figure ), this process can be assigned as SRCT emission, which is characteristic of MR-TADF. The ΔEST of 6NICz and 6,10NICz in 1 wt% doped films in PMMA, measured from the difference in the onset of the steady-state PL and time-gated emission (1-10 ms) at 77 K (Figure , is 0.46 and 0.36 eV, respectively, which explains the very long delayed fluorescence lifetimes observed in this medium. The ΔEST of 4NICz, 5NICz and 7NICz is 0.59, 0.59 and 0.65 eV, respectively, which are too large for efficient TADF. We note that the ΔEST of 5NICz is likely to be slightly underestimated, due to the broadening of the phosphorescence spectrum at low temperature, which makes measuring the true onset difficult. Despite this, the trend in ΔEST largely matches that predicted by computations, with 6NICz and 6,10NICz having the smallest ΔEST, followed by 4NICz and 5NICz and then 7NICz having the largest ΔEST.