Next, the compounds under investigation were co-doped with the high triplet-energy host bis [2-(diphenylphosphino)phenyl]ether oxide (DPEPO). Note that as PTZ-TPI showed no ICT emission and thus is not a candidate TADF material, no further investigation to its photophysical properties in DPEPO was pursued. A doping concentration of 6 wt% was found to be an optimal (Table and Figure shows the prompt fluorescence and phosphorescence spectra of TPI, DMAC-TPI, PXZ-TPI and PTZ-TPI in toluene glass at 77 K. S1 and T1 energies of TPI were determined at 4.00 eV and 2.75 eV, respectively, and the singlet-triplet energy splitting (ΔEST) at 1.25 eV (Table ). Prompt emission and phosphorescence of DMAC-TPI resembled those of TPI, with only slightly altered S1 and T1 levels. The prompt emission peak of PXZ-TPI resembles that of TPI at an energy of 3.97 eV. A complex emission profile is observed in the phosphorescence spectrum of PXZ-TPI. Superimposing PXZ-TPI phosphorescence with those of TPI and PXZ allowed us to ascribe individual bands to each of the chromophores (Figure ). The blue bands at 2.79 eV are associated with phosphorescence of PXZ, while the red onset matches that of TPI. The increase in triplet energy compared to TPI, however, was not sufficient to reduce the ΔEST to enable TADF in PXZ-TPI (1.19 eV). We note that the gas phase TDA-DFT computed DEST values for DMAC-TPI and PXZ-TPI (0.38 and 0.19 eV, respectively) are much smaller than the experimentally determined single-triplet gaps, possibly because the calculation predicts CT singlet excited states, whereas we observe LE-type behaviour for the prompt emission (Figure ).