The profiles match closely to the calculated absorption spectra predicted by TDA-DFT (Figure ). 4tCzPz, 4CzPz, and 4αCbPz show a low energy CT/LE absorption band (reflected in the structured nature of this band), with a high molar absorptivity (ε) of 2.4 × 10 4 M -1 cm -1 (440 nm), 2.4 × 10 4 M -1 cm -1 (425 nm), and 2.3 × 10 4 M -1 cm -1 (422 nm), respectively. The similar ε values also match the predicted high oscillator strength (f ~ 0.40) for the three compounds. 4tCzPz, 4CzPz, and 4αCbPz also show strong absorption bands around 340 nm and 320 nm, which according to TDA-DFT are ascribed to the locally excited (LE) π → π* transitions over the whole molecule and donor moieties, respectively. The steady-state PL spectra for 4tCzPz, 4CzPz, and 4αCbPz in toluene are structureless, with peak maxima, λPL, of 475 nm, 470 nm, and 462 nm, respectively, which also match the trends in the S1 energies predicted by TDA-DFT calculations. 4tCzPz, 4CzPz, and 4αCbPz possess moderately high photoluminescence quantum yield (ΦPL) of 63%, 66%, and 75% in aerated toluene, which increased to 69%, 71%, and 79% in degassed toluene, respectively. The high ΦPL values together with the intense low-energy ground-state absorption bands indicate that the three emitters have potential as high gain and low-threshold materials for organic laser applications. The oxygen sensitivity of the FPL of the three emitters indicates the presence of accessible triplet excited states. Both 4tCzPz and 4αCbPz exhibit structured emission in low-polarity solvents like methylcyclohexane, and broad and unstructured spectra in high-polarity solvents like ethyl acetate and acetonitrile (Figure and), which is also observed in 4CzPz. The time-resolved PL of the materials was measured in dilute toluene solution, the PL decay kinetics are shown in Figure and the PL spectra at different time points are shown in Figure . 4tCzPz, 4CzPz, and 4αCbPz show a clear prompt decay component with a mono-exponential lifetime, tp, of 6.2, 6.4, and 5.8 ns, respectively. By contrast, all three emitters exhibit a very weak delayed PL component on microsecond timescales with complex multi-exponential kinetics. By integrating the total PL, we find that the delayed fluorescence accounts for less than 1% of the total emission (Figure ), indicating that the TADF process is not efficient in toluene solution. emitters were estimated from the onset of the prompt fluorescence and phosphorescence spectra, respectively. For 4CzPz and 4tCzPz the S1 and T1 energies are 2.94/2.91 eV and 2.66/2.63 eV, respectively, with ΔEST of 0.28 eV for both emitters. The S1 and T1 energies for 4αCbPz are destabilized to 3.04 eV and 2.71 eV, respectively, with slightly enlarged ΔEST of 0.33 eV. The S1 energies of three emitters in 2-MeTHF match those predicted by the TDA-DFT calculations, while the T1 energies are moderately destabilized (~0.20 eV), which explains the smaller experimentally determined ΔEST values determined in 2-MeTHF compared to the gas phase calculations.