We had previously studied the photophysics of 4CzPz in 10 wt% doped films in PPT as this doping concentration was found to maximize the ΦPL at 75% (λPL of 487 nm). We thus, decided to maintain this doping concentration and host for 4tCzPz and 4αCbPz. 4tCzPz and 4αCbPz emit at λPL of 495 nm and 475 nm, and have ΦPL of 73% and 65%, respectively. These values are similar to those obtained in dilute toluene. The ΔEST values of 4tCzPz, 4CzPz, and 4αCbPz progressively increase from 0.22 eV to 0.23 eV, and 0.30 eV, respectively, determined from the difference in onsets of the prompt fluorescence and phosphorescence spectra. To probe the TADF behavior in the solid state, we have also measured timeresolved PL for 10 wt% doped films of 4tCzPz, 4CzPz, and 4αCbPz in PPT. In contrast to the behavior in toluene solutions, all three compounds show significant delayed PL in the PPT film (Figure ; PL spectra at different time points are shown in Figure ). Though the emission extends beyond the time range accessible on our experimental setup, the integrated PL reveals that at least 30% of the total PL fraction stems from delayed fluorescence (Figure ). A mono-exponential fit to the prompt PL reveals a tp of around 13 ns for all three materials. As with the toluene measurements, the delayed PL possesses a more complex, multi-exponential decay kinetics, so no fitting was attempted. We nonetheless note that in line with the measured ΔEST values, 4tCzPz shows the fastest delayed emission and 4αCbPz the slowest.