To determine the DEST in these compounds, we have measured the steady-state, prompt and delayed emission spectra in 2-MeTHF glass at 77 K (Figure ) T-tBuCz-AQ showed a prompt emission at 597 nm while the delayed emission (gated time = 1 ms) peaks at 594 nm. The S1 and T1 energies, determined from the onsets of prompt and delayed emission, are 2.26 and 2.28 eV, respectively, and ∆EST is -20 meV. This apparent small inverted gap must result from the prompt and delayed emission occurring from different conformer species in the glass matrix; further, it is possible that 77 K is not sufficiently cold to deconvolute the phosphorescence spectrum from the residual delayed emission. Compounds T-MeOCz-AQ, C-tBuCz-AQ and C-MeOCz-AQ showed steady state emission at 640, 635, and 680 nm, respectively, yet no delayed emission was detected, likely due to the too weak emission of these compounds. The corresponding broad and structureless emission stems from the CT nature of the S1 state (Figure ). By contrast, tBuCz-PA showed a structured prompt emission peaking at 590 nm, which is also the same emission observed in the time-gated PL (Figure ), meaning that the emission is only occurring from the anthrone acceptor moiety (Figure ). Compound DMAC-PA showed very weak steady state emission while no delayed emission was detected. Next, we investigated the photophysical properties of the crystals. The crystals of T-tBuCz-AQ emit at λPL of 658 nm and have a ΦPL of 5% while the crystals of the other compounds in this study are not luminescent. The emission of the crystals of T-tBuCz-AQ is red-shifted compared to the 10 wt% doped film in PMMA (λPL = 640 nm) and blue-shifted with respect to the powder (λPL = 670 nm) (Figure ). The crystals also showed a delayed emission, with a τd of 0.5 µs. The TADF behavior was confirmed by variable temperature time-resolved PL measurements (Figure ). a ΦPL was recorded under air/ N2 atmosphere using an integrating sphere (λexc = 450 nm), b prompt lifetime (τp) was recorded using time correlated single photon counting (TCSPC) (λexc = 375 nm) and delayed lifetime (τd) was recorded using TCSPC for T-tBuCz-AQ, T-MeOCz-AQ and C-tBuCz-AQ and microsecond flash lamp for tBuCz-PA (λexc = 450 nm).