Organic light-emitting diodes (OLEDs) have advantages such as low driving voltage, high brightness, high color purity, flexible display capabilities, lightweight construction, and fast response times, which position them as promising next-generation display materials. Thermally activated delayed fluorescence (TADF) materials can achieve 100% exciton utilization through the reverse intersystem crossing (RICS) process and are known as the third-generation luminescent materials. TADF molecules achieve a small singlet-triplet energy gap (ΔE_ST) by separating the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) through a donor-acceptor (D-A) twisted structure, thereby enhancing the RISC rate. Currently, many donor molecules have been developed, while acceptor molecules have not been fully explored. In this study, 4-cyanopyrimidine was used as the acceptor, and phenoxazine, diphenylamine, carbazole, and acridine were used as donors. The donors and acceptors were connected through a benzene ring bridge by the Suzuki-Miyaura and Buchwald-Hartwig coupling reactions, and four D-A type TADF materials, PhXz-CN, Ta-CN, PhCz-CN, and PhAd-CN, were synthesized. The four prepared TADF molecules all have a twisted D-A molecular structure, with the HOMO and LUMO located on the donor and acceptor fragments, respectively, achieving spatial separation of HOMO and LUMO. Therefore, the four TADF molecules exhibit small singlet-triplet energy gaps (<0.3 eV). As the electron-donating ability of the donors decreases in sequence, the maximum emission spectra of the fluorescent materials PhXz-CN, PhAd-CN, Ta-CN, and PhCz-CN gradually blue-shift from 513 nm to 502, 496, and 440 nm. When Ta-CN and PhCz-CN are used as guest materials and doped in the host materials 4,4′-di(N-carbazolyl)-1,1′-biphenyl (CBP) and 1,3-bis(carbazol-9-yl) benzene (mCP), the maximum external quantum efficiencies (EQE) of the organic light-emitting diode (OLED) devices are 6.85% and 3.04%, respectively, achieving green and blue emissions. 4-cyanopyrimidine has excellent electron-withdrawing ability and can be used to prepare TADF materials with common donor molecules. Moreover, the emission wavelength can be precisely controlled by changing the electron-donating ability of the donors.

Key words: 4-Cyanopyrimidine acceptor, D-A structure, Energy level regulation, TADF, OLEDs