Finally, turning to the excited state kinetics, we note that all three compounds display the typical biphasic prompt-delayed behavior of TADF materials. 4CzPz and 4αCbPz both possess long delayed lifetimes that extend beyond the 100 μs time range of our TA measurement, though fits to the second component nominally give a lifetime of 51.0 μs and 72.9 μs, respectively. By contrast, 4tCzPz possesses a shorter second component of 11.3 μs, consistent with the reduced ΔEST in this material. However, it is worth noting that none of the emitters show significant delayed fluorescence in toluene solutions. This indicates that the triplet exciton recombination pathway is likely dominated by non-radiative decay. Considering the photophysical properties and promising TA measurements of the molecules, we then evaluated their light amplification properties in ASE measurements. The 4tCzPz and 4αCbPz blend films (4 wt% in mCBP host) were spin-coated on glass substrates. The thickness of the films ranged between 175 and 190 nm, confirmed by ellipsometer, which is far above the cut-off thickness (91 nm). The samples consisted of glass/blended film/air waveguides. The samples were then pumped by a nitrogen laser (337 nm, 0.5 ns pulse width) and the emission was collected from the edge. The size of the excitation beam was 5.5 mm by 0.12 mm. Further details regarding sample fabrication and ASE measurement can be found in the Electronic Supporting Information. Figure shows the emission spectrum from the edge of the 4tCzPz waveguide sample under varying pump energy density. At low pump energy density, the emission spectrum is broad, with a FWHM of about 50 nm. We note the emission peak under low pump energy density is redshifted compared with the PL spectrum in solution, which can be ascribed to self-absorption of the waveguided light. A clear spectral line narrowing is observed with increasing pump energy density due to the amplification of the waveguided light by the stimulated emission process. The FWHM of the emission spectrum at 73.8 µJ/cm 2 dropped to 14 nm with a peak emission at 491 nm. To determine the ASE threshold, the peak intensity and FWHM of the edge emission spectrum of the sample as a function of the pump density was then plotted in Figure . At low pump energy density, the emission intensity increased linearly before an abrupt change in the slope of the emission at 41.0 µJ/cm 2 , indicating the onset of ASE. We identified this change as the threshold pump density for ASE. Similar analyses were conducted for the 4αCbPz and 4CzPz samples. We note that the 4CzPz sample (4 wt% in mCBP) was fabricated by thermal evaporation due to the low solubility of the 4CzPz molecule in chlorobenzene. The 4αCbPz sample shows an ASE threshold of 44.9 µJ/cm 2 and ASE peak at 474 nm, and the 4CzPz sample shows a threshold of increased doping concentration from 2 to 6 wt%, while all 4αCbPz samples show similar FPL (~70%), regardless of the doping concentration. The samples were then pumped by an OPO system (420 nm, 4 ns pulse width) and the emission was collected from the edge. The size of the excitation beam was 4 mm by 0.14 mm. Considering the low absorption (5 to 17%) at the excitation wavelength, the ASE threshold of polystyrene samples is quoted as absorbed pump energy density. For 4tCzPz (Figure ), at low doping concentration (2 and 4 wt%) the samples show comparable ASE thresholds of about 20 µJ/cm 2 . The threshold then increases to 25.7 µJ/cm 2 when the doping concentration is increased to 6 wt%. A different trend was observed for the 4αCbPz samples. As shown in Figure , the ASE threshold decreases with increasing doping concentration. We attribute this to a higher gain at higher doping concentration in the matrix. For both 4tCzPz and 4αCbPz samples, the ASE peaks (Figure and 9d) red-shift with increasing doping concentration, which we believe is due to self-absorption in the waveguide. The results of the ASE threshold and emission peak study are summarized in the Supporting Information. We were not able to increase the doping concentration further in the spin-coated films due to the limited solubility of these TADF emitters.