Fine Tuning of Donor-Acceptor Structures in Fused-Carbazole Containing Thermally Activated Delayed Fluorescence Materials towards High-Efficiency OLEDs

Conjugated fused-ring structures have attracted extensive attention due to their high molecular rigidity to restrain excited-state relaxation and non-radiative decay, and further to enhance the luminance efficiency for emissive materials. Herein, we develop a series of donor-acceptor type thermally activated delayed fluorescence (TADF) emitters by introducing fused-ring 5H-benzofuro[3,2-c]carbazole (32BFCz) as electron donating unit. Through optimizing the numbers and structure of donor and acceptor moieties, three compounds named 32BFCzA, mCF(3)BFCzOXD and dCF(3)BFCzOXD are designed, which are composed by mono-32BFCz/trifluoromethylpicolinonitrile, penta-BFCz/3-(trifluoromethyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene and penta-32BFCz/3,5-bis(trifluoromethyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene as donor/acceptor groups, respectively. They all exhibited green emission with peak value ranging from 532 to 540 nm. Through rationally tuning the donor/acceptor properties to reduce the distance between positive and negative charges, the ground state dipole moments decline from 8.93, 6.18 to 5.30 Debye, which further induced sequentially reduced singlet-triplet energy splits (Delta E(ST)s), increased photoluminescence quantum yields (PLQYs), enhanced reverse intersystem crossing rate (k(RISC)s) and diminished delayed fluorescence lifetime from 32BFCzA, mCF(3)BFCzOXD to dCF(3)BFCzOXD. Therefore, gradually increased external quantum efficiency (EQE) is accordingly achieved when employing them as emitters for TADF OLEDs. dCF(3)BFCzOXD with the mostly optimized donor/acceptor structures demonstrates the highest efficiency with maximum EQE to 21.4%. Our work provides guidelines on the design of high-efficiency D-A type TADF materials.