Synthesis And Characterization Of D-A Type Thermally Activated Delayed Fluorescence Materials Based On Xanthone

The triplet excitons of the thermally activated delayed fluorescence?TADF?materials can up-convert to the singlet level by the reverse intersystem crossing?RISC?process,emitting delayed fluorescence.TADF materials have attracted widespread attention,because they can reach 100%of theoretical internal quantum efficiency?IQE?.Moreover,in contrast to phosphorescent materials,heavy metal-free TADF materials are easier to realize the ultimate EL efficiency.Compared with blue and green TADF materials,red TADF materials,which is one of the three primary colors,have been developed more slowly.This is mainly due to the exponential growth of non-radiative transitions of molecules as the emission wavelength moves toward the long wavelength direction,which brings great challenge to the design of red TADF molecules.Therefore,it is highly desirable to develop a novel red TADF materials with high efficiencies and better stability.Xanthone?XOCN?is often used as an acceptor unit in TADF materials,because of its strong electron-withdrawing ability,planar rigid molecular structure,larger intersystem crossing probability,and smaller?E_STT characteristics.However,there are few reports on the use of a group which substituted by malononitrile,as an acceptor unit for the construction of TADF materials.1.Based on the D-A structure,a series of D-A type TADF materials carrying a malononitrile substituted xanthone unit with strong electron withdrawing ability as an acceptor,and diphenylamine?DPA?,9,9-dimethylacridin?DMAC?,phenoxazine?PXZ?,and phenothiazine?PTZ?as different donors,namely DPAXOCN,DMACXOCN,PXZXOCN and PTZXOCN,were designed and synthesized.With the enhancement of electron-donating ability of the donor unit?DPA<DMAC<PXZ<PTZ?,the intensities of the absorption peaks that caused by the intramolecular charge transfer?ICT?state transitions of the compounds DPAXOCN,DMACXOCN,PXZXOCN and PTZXOCN are gradually weakened.The compounds DPAXOCN,DMACXOCN and PXZXOCN have red emission,and their maximum emission peaks are at 617 nm,672 nm and 709 nm,respectively.The fluorescence quantum yields of compounds are 4.82%for DPAXOCN,1.58%for DMACXOCN,2.69%for PXZXOCN and 3.51%for PTZXOCN.The compounds give small?EST values,0.0077 eV for DMACXOCN,0.0052 eV for PXZXOCN,and 0.1226 eV for PTZXOCN.2.Based on the D-A-D structure,two D-A-D type symmetrical TADF materials2PXZXOCN and 2PXZXO,with phenoxazine?PXZ?as the donor and malononitrile substituted and unsubstituted xanthone?XOCN?as the acceptors,were designed and synthesized.The materials were identified using nuclear magnetic resonance spectroscopy and mass spectrometry.Moreover,the thermal stability,photophysics and electrochemical properties have been investigated..The results show that both compounds have good thermal stability.The maximum emission peaks of2PXZXOCN and 2PXZXO are at 626 nm and 772 nm,and their fluorescence quantum yields are 3.05%and 4.65%,respectively.Transient fluorescence spectra show that 2PXZXO has obvious TADF characteristic,and its delayed fluorescence lifetime is only 2?s.In addition,both compounds have obvious AIE characters,which is of great significance in overcoming the ACQ effect.