Organic Emitters with a Rigid 9-Phenyl-9-phosphafluorene Oxide Moiety as the Acceptor and Their Thermally Activated Delayed Fluorescence Behavior

With the 9-phenyl-9-phosphafluorene oxide (PhFlOP) moiety as the acceptor (A) and various donors (D), a series of new organic emitters have been synthesized with a D-A-D configuration. Their photophysical and electrochemical behaviors and electroluminescent (EL) performances have been characterized in detail. The photophysical results have indicated that the PhFlOP-based emitters with acridyl, phenoxazyl, and phenothiazyl as donors show efficient, thermally activated delayed fluorescence (TADF) behavior, especially for the TADF emitter with the phenoxazyl donor possessing an exceptionally high rate constant of reverse intersystem crossing (k_RISC) of 6.2 × 10⁵ s^-1. It has also been found that their TADF behavior can be greatly affected by the substitution position of the donors. Different from the reported aryl phosphine oxide (APO) acceptors in TADF emitters, the PhFlOP moiety adopts a highly rigid configuration to guarantee a photoluminescent quantum yield as high as 0.80 in the 4,4′-N,N′-dicarbazolebiphenyl film, representing the top-ranking emission ability for the TADF emitters with APO-type acceptors. Benefitting from their advanced TADF performances, the doped organic light-emitting diodes/devices based on these PhFlOP-based TADF emitters can achieve exceptional EL performances with the maximum external quantum efficiency (η_ext) of 23.3%, current efficiency (η_L) of 83.7 cd A^-1, and power efficiency (η_P) of 59.1 lm W^-1. These encouraging EL data show the great potential of the PhFlOP moiety in developing highly efficient TADF emitters.