The The S1 and T1 energies were determined from the onsets of the steady-state PL and delayed emission (gate time: 1-9 ms) spectra in 2-MeTHF glass at 77 K, respectively (Figure ); the EST value is the difference in energy between these two (Figure , Table ). DiICztBu4, DiICzMes4 and DiICztBuCz4 have very similar triplet energies (ET = 2.58, 2.57, and 2.55 eV, respectively) and as their emission profiles are similar, so too will be their spectral overlap with the sub and thus the kDET should be comparable across these three PCs. ). DiICztBu4 and DiICzMes4 have previously been reported as MR-TADF emitters, and given the similar photophysical properties of DiICztBuCz4 and DiICztBuDPA4 to these two, the high molar absorptivity of the low-energy absorption band, the minimal positive PL solvatochromism, the narrowband emission and the small ∆EST, they can also be classified as MR-TADF emitters ( λexc = 400 nm for DiICztBuDPA4, λexc = 380 nm for 4CzIPN. Room temperature ET were estimated following our previously reported methodology, where ET(RT) = ES(RT -∆EST(LT), with ES(RT) being the onset of the steady-state PL in the respective solvents and ∆EST(LT) being measured in 2-MeTHF at 77 K.