However, the increased oscillator strength caused by the enhanced conjugation of the donors results in a higher FPL in these two compounds compared to TCZTRZ(Me), but at a cost of larger ΔEST, indicating that the methyl group is not sufficiently bulky to induce a highly twisted structure that can result in the required separation of the FMOs. Despite the red-shifted PL spectra, the OLEDs still exhibited blue emission with lEL at 476 and 460 nm and EQEmax of 20% and 13.3% for the devices with TRZ-CF and TRZ-CzF, respectively. Strengthening the donor by incorporating a distal carbazole served to red-shift the emission in TRZ-DCF (FPL: 84%; τd: 5.43 µs; 20 wt% in DPEPO) while the other photophysical properties remain unchanged. Compared to the OLED with TRZ-CF [λEL: 476 nm; CIE = (0.17, 0. The use of a phenyl substituent on the arylene bridge was explored by Lee et al. to modulate the conformation of the emitter. A cross-comparison of the photophysical properties of FPL. However, the donor group adopts a less twisted conformation that those of TCZTRZ(Me), TCZTRZ(Me2p) and TCZTRZ(Me2o), all of which contain methyl substituents. This suggests that the phenyl group is effectively less bulky than the methyl substituent. High carbazole are not TADF emitters, while similar analogs TRZ-CF and TRZ-CzF with different donor structures to the above work demonstrated TADF. The FPL was not completely dependent on the position of substituent groups on the bridge, for example, as evidenced by the large deviation between TRZCz-CF3-1 (FPL 48%) and TRZBuCz-CF3-1 (FPL 99%), the latter of which also contains a more strongly electron-donating 3,6-ditertbutylcarbazole donor.