Asymmetric thermally activated delayed fluorescence (TADF) emitters with 5,9-dioxa-13: B -boranaphtho[3,2,1- de] anthracene (OBA) as the acceptor and highly efficient blue-emitting OLEDs

A series of new organic emitters was prepared by introducing various donors (Ds) to the meta-position of the boron atom in the central phenyl ring in the acceptor (A) of 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (OBA). Moreover, their thermal, photophysical, electrochemical and electroluminescent (EL) properties were characterized in detail. The photophysical results revealed that these OBA-based molecules adopted a D-A configuration and exhibited efficient thermally activated delayed fluorescence (TADF) properties with a reverse inter-system crossing constant (k_RISC) in the order of 10⁵ s^-1. It has been shown that introducing the -Br group to the OBA acceptor could increase k_RISC by 2 to 3 times. Importantly, the deep-blue TADF emitter OBA-O showed a high photoluminescent quantum yield (PLQY) of 0.84, while its blue analog OBA-BrO with the -Br group exhibited an even higher PLQY of 0.92 in the doped film state. Benefitting from their high PLQYs as blue-emitting TADF emitters, the doped organic light-emitting diodes (OLEDs) based on these OBA-based TADF emitters exhibited attractive electroluminescent (EL) performances. The blue-emitting OLEDs with CIE (0.17, 0.17) showed the maximum external quantum efficiency (η_ext) of 17.8%, current efficiency (η_L) of 33.2 cd A^-1 and power efficiency (η_P) of 34.2 lm W^-1, while that for the bluish-green device were 22.5%, 49.2 cd A^-1 and 56.9 lm W^-1, respectively. Thus, the great potential of these TADF emitter-based OBA acceptors was clearly demonstrated by their EL data. In addition, these asymmetric OBA-based molecules will enrich the structural diversity of this type of TADF emitter.