Highly efficient thermally activated delayed fluorescence emitters enabled by double charge transfer pathways via ortho -linked triarylboron/carbazole hybrids

Two thermally activated delayed fluorescence (TADF) emitters with double charge transfer pathways, o B-2Cz and o B-2 t Cz, were designed and synthesized. The phenyl linker allowed the charge to transfer from donors to acceptors along with the bond direction within the molecule; however, the ortho -linked D–A–D configuration with bulky steric hindrance resulted in large D–A dihedral angles, rigid structure and cofacial interactions with small D–A spatial distances, which are favorable for minimizing the lowest singlet and the lowest triplet energy gap (Δ E ST ) and improving the photoluminescent efficiencies. o B-2Cz and o B-2 t Cz were employed as emissive guest in the organic light emitting diode (OLED) devices and the maximum external quantum efficiencies (EQEs) of the devices reached 28.1% and 27.5%, respectively.