Molecular engineering on all ortho-linked carbazole/oxadiazole hybrids toward highly-efficient thermally activated delayed fluorescence materials in OLEDs

The highest efficiency thermally activated delayed fluorescence (TADF) emitters in OLEDs are mostly based on twisted donor/acceptor (D/A) type organic molecules. Herein, we report the rational molecular design on twisted all ortho-linked carbazole/oxadiazole (Cz/OXD) hybrids with tunable D-A interactions by adjusting the numbers of donor/acceptor units and electron-donating abilities. Singlet-triplet energy bandgaps (ΔE_ST) are facilely tuned from~0.4, 0.15 to~0 eV in D-A, D-A-D to A-D-A type compounds. This variation correlates well with triplet-excited-state frontier orbital spatial separation efficiency. NonTADF feature with solid state photoluminescence quantum yield (PLQY)<10% is observed in D-A type 2CzOXD and D-A-D type 4CzOXD. Owing to the extremely low ΔE_ST for efficient reverse intersystem crossing, strong TADF with PLQY of 71%-92% is achieved in A-D-A type 4CzDOXD and 4tCzDOXD. High external quantum efficiency from 19.4% to 22.6% is achieved in A-D-A typed 4CzDOXD and 4tCzDOXD.