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 (kRISC) 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 (ηext) of 23.3%, current
efficiency (ηL) of 83.7 cd A–1,
and power efficiency (ηP) of 59.1 lm W–1. These encouraging EL data show the great potential of the PhFlOP
moiety in developing highly efficient TADF emitters.