The S1/T1/DEST energies of 2GtBuCzCO2HDCzB and tBuCzCO2HDCzB are 2.61/2.47/0.14 eV and 2.56/2.43/0.13 eV, respectively (Figure ), which are similar to those of DtBuCzB (2.57/2.44/0.13 eV in toluene at 77 K, Figure ). The similar energies of these low-lying excited states indicate that the pedant tBuCzCO2H and 2GtBuCzCO2H moieties do not materially affect the electronics of the emissive MR-TADF core in 2GtBuCzCO2HDCzB and tBuCzCO2HDCzB. This is due to the higher excited-state energies of the tBuCzCO2H and 2GtBuCzCO2H, which was confirmed by an analysis of the excited-state energies of the related model compounds tBuCzCN and tBuCzCN (Figure ). Notably, delayed emission is detected in toluene for 2GtBuCzCO2HDCzB and tBuCzCO2HDCzB, with td of 38 and 34 µs, respectively, which is absent in DtBuCzB (Figure ). the emission intensity changes observed for 2GtBuCzCN or tBuCzCN in their binary and ternary films, the FRET efficiencies were calculated to be 85 and 33%, respectively, which are consistent with the degree of spectral overlap (Figure ), indicating a higher FRET efficiency for the 2GtBuCzCN/DtBuCzB system. In the more concentrated 10:10 wt% doped films of 2GtBuCzCN/DtBuCzB and tBuCzCN/DtBuCzB in PMMA, the FRET efficiencies increased to 100 and 99% (Figures and), respectively. Given the short distance between the peripheral 2GtBuCzCN/DCzCN moieties and the central DtBuCzB emissive group, the FRET efficiencies in 2GtBuCzCO2HDCzB and tBuCzCO2HDCzB are quantitative, reflective in the single observed narrowband emission at at λPL of 490 (FWHM of 23 nm) and 495 nm (FWHM of 29 nm), respectively. A quantitative energy transfer was also observed found in the 10:10 wt% doped films of tBuCzCO2H/DtBuCzB in PMMA (Figure ). The PL spectra are narrower compared to that of DtBuCzB (λPL of 488 nm FWHM of 37 nm) as