Consistent with these decay kinetics, the experimental ΔEST of AZB-TRZ in 1 wt% doped zeonex films (0.01 eV, from the onsets of relaxed PF emission at RT and 80 K PH, corresponding to a 1 CT-3 LE gap), is substantially smaller than that of AZB-Ph-TRZ (0.25 eV). These ΔEST values qualitatively reproduce the ordering of the DFT calculated DEST gaps, although are substantially larger for AZB-Ph-TRZ. The observed kRISC (5.7±0.2 × 10 6 s -1 , from kinetic decay fitting ) for AZB-TRZ indicates its potential as an emitter in OLEDs (Table ). Similar trends in ΔEST were found in mCP and DPEPO films, having gaps ≥0.15 eV for AZB-Ph-TRZ and ≤0.02 eV for AZB-TRZ in both hosts (Table ). This results in a similarly large kRISC of 4.9±0.3 × 10 6 s -1 for AZB-TRZ in the 10 wt% doped mCP film. Finally, the FPL of the films was found to vary within experimental error, in the moderate range of 30-40% for each material in the two OLED hosts (Table ). ). Reflecting its stronger DF and faster kRISC, AZB-TRZ in mCP achieves a higher EQEmax of 10.5% and reduced efficiency roll-off compared to the device using DPEPO as the host. This EQEmax compares favorably to the equivalent film FPL (34% in mCP), indicating highly efficient exciton harvesting in the emissive layer and an IQE near 100% based on unassisted optical outcoupling. Increasing the emitter doping to 20 wt% from 10 wt% makes minimal difference to the electroluminescence (EL) spectrum (Figure ); however, though the EQEmax is attenuated (6.3%) yet higher luminances nearing 10,000 cd m -2 can be attained. The device with AZB-TRZ in DPEPO exhibits very similar EL and the same performance trends as in mCP, although with generally lower EQEmax (8.9%) and worse efficiency roll-off, limiting maximum brightness to just below 1000 cd m -2 (Figure ). These results in different hosts are consistent with their similar FPLs and worse DF emission intensity and kinetics in DPEPO.