Optimization of carbazole-pyrimidine linking pattern for achieving efficient TADF
High fluorescence quantum yield is essential for achieving high OLED efficiency. Due to typically strong charge-transfer (CT) character, thermally activated delayed fluorescence (TADF) compounds usually have lower prompt fluorescence efficiency, therefore the boost of total emission yield (ΦPL) mostly relies on enhancement of delayed fluorescence efficiency. In this paper, we present less frequent approach for high ΦPL by maximising both prompt and delayed emission yields of carbazole-pyrimidine compounds. Strong decoupling of HOMO and LUMO and subsequent emergence of TADF was achieved by introducing four carbazole units at both meta positions of phenyl unit, though ΦPL was low due to the prominent nonradiative triplet decay rate. Crowding the relatively loose carbazole units together at para and both meta positions, conversely, was shown to diminish the nonradiative decay, enhance the radiative decay rate and subsequently boost the fluorescence yield. Efficient blue electroluminescence with nearly 20% yield was demonstrated for the optimized compound.