Pyridine-functionalized carbazole donor and benzophenone acceptor design for thermally activated delayed fluorescence emitters in blue organic light emitting diodes

In this paper we report a new molecular design approach for blue emitting thermally activated delayed fluorescence (TADF) molecules. The two novel TADF emitters, (4-(3,6-di(pyridin-3-yl)-9H-carbazol-9- yl)phenyl)(phenyl)methanone (3PyCzBP), and (4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl)(phenyl)methanone (4PyCzBP) possess a pyridine-functionalized carbazole donor and a benzophenone acceptor. Both compounds shows broad charge-transfer emission in DCM with a λ_max at 497 nm and a photoluminescence quantum yield, ΦPL of 56% for 3PyCzBP and a λmax at 477 nm and a ΦPL of 52% for 4PyCzBP. The ΦPL decreased to 18% and 10%, respectively for 3PyCzBP and 4PyCzBP in the presence of O2 confirming that triplet states involved in emission. The PMMA doped (10 wt%) films show blue-shifted emission with λ_max at 450 and 449 nm for 3PyCzBP and 4PyCzBP, respectively. The maximum ΦPL of 23.4% is achieved for these compounds in PMMA doped film. Difference in energy between the singlet and triplet excited states (ΔE_ST) is very small at 0.06 eV and 0.07 eV for 3PyCzBP and 4PyCzBP, respectively. Multilayer organic light emitting diode devices fabricated using these molecules as emitters show that the maximum efficiency (EQE_max) of the blue devices is 5.0%.