The photophysical properties in mCP, acting as a host matrix that possesses a sufficiently high triplet energy (ET = 3.0 eV) to confine excitons on the emitter, were then investigated. We first studied the steady-state PL behavior in doped film at various doping concentrations (Figure ). As the doping concentration increases from 2 to 10 wt%, there is an around 10 nm red-shift of the PL in PhDPA-DiKTa and 16 nm for MeODPA-DiKTa, which may be due to increasing contribution from aggregates at these higher doping concentrations. The highest ΦPL values were found to be for the 5 wt% doped films at 69% for PhDPA-DiKTa and 27% for MeODPA-DiKTa. Thus, further photophysical measurements were conducted in the 5 wt% doped films in mCP. MeODPA-DiKTa emits at λPL of 655 nm (FWHW = 71 nm), while PhDPA-DiKTa emits at λPL of 617 nm (FWHM = 57 nm), behavior that is analogous to that observed in toluene solution (Figures 3a). due to the energy gap law. The corresponding estimated kRISC rate for those compounds in these 5 wt% doped films in mCP are 1.72´10 3 and 4.92´10 3 s -1 , respectively. The energy levels of the S1 and T1 states were estimated from the onsets of the corresponding prompt fluorescence and phosphorescence spectra measured at 77 K (Figure ). Both compounds have the same smaller DEST of 0.16 eV than those in frozen toluene (ΔEST = 0.20 eV for PhDPA-DiKTa and 0.23 eV for MeODPA-DiKTa in toluene at 77 K), and that of 3DPA-DiKTa (DEST= 0.20 eV). Temperaturedependent time-resolved PL studies reveal the characteristic increase in the delayed emission with increasing temperature of TADF materials (Figure ).