Study On Synthesis And Performance Of Orange-Red Photothermally Activated Delayed Fluorescence Material

Thermally activated delayed fluorescence(TADF)materials have attracted much attention as the third generation of light-emitting materials for organic light-emitting diodes(OLEDs)due to their advantages of high device performance,low cost,and environmental friendliness compared to the first generation of conventional fluorescent and second-generation phosphorescent materials.At present,the development of red light TADF OLEDs is much less common than that of blue and green light TADF OLEDs,and red light TADF materials are also relatively scarce.Therefore,the design and synthesis of novel red light TADF materials are important topics in this research field.In this paper,we designed and synthesized a series of structurally novel red light TADF materials with dibenzo[a,c]phenazine(DPPZ)as acceptor,characterized the structures of the compounds by NMR spectroscopy and mass spectrometry,and investigated their photophysical,electrochemical,thermal stability,and electroluminescent properties.And it is supported by the following findings:(1)Two TADF materials with D-A-D structure,4PXZ-DPPZ and 4TPA-DPPZ,have been designed and synthesized based on DPPZ as acceptor,and phenoxazine(PXZ)and triphenylamine(TPA)as the electron donor.The electron-donating ability of the donor unit PXZ is stronger than that of TPA,and therefore,the fluorescence emission wavelength(?_FL)of 4PXZ-DPPZ in toluene was 673 nm,where the?_FL of 4TPA-DPPZ is only 550 nm.4PXZ-DPPZ has a highly distorted molecular structure,which allows its highest molecular occupied orbital(HOMO)and lowest molecular unoccupied orbital(LUMO)to be well separated,resulting in a smaller?E_ST(0.18 e V),while the HOMO and LUMO of 4TPA-DPPZ overlapped more,resulting in the?E_ST being larger,reaching 0.37 e V.But since the fluorescence radiative rate of 4TPA-DPPZ is much smaller than that of 4TPA-DPPZ and the nonradiative transition rates of the two are close,its fluorescence quantum yield(PLQY,10%)is much smaller than that of 4TPA-DPPZ(58%).The electroluminescence(EL)wavelength of 4TPA-DPPZ based doped device was 550 nm with Commission Internationale d'Eclairage(CIE)chromatic coordinates of(0.43,0.56)belonging to yellow light emission with a maximum external quantum efficiency(EQE_max)of 10%and a maximum luminance(L_max)up to 16790 cd/m².While the 4PXZ-DPPZ based doped device with an EL wavelength of 662 nm and a CIE color coordinate of(0.64,0.35)is very close to the standard red emission,its EQE_max is smaller,only 1.5%,and its L_max is 783 cd/m².(2)To obtain efficient red light TADF materials,rigid 9,10-Dihydro-9,9-dimethylacridine(DMAC)was selected as donor in this chapter,and two TADF materials DPPZ-3DMAC and DPPZ-4DMAC were designed and synthesized by changing the linkage position and number of donors.The results show that by directly connecting three DMAC donor units to the DPPZ acceptor unit,a strong intramolecular charge transfer(ICT)will be generated in the molecule.Therefore,the?_FL of DPPZ-3DMAC in toluene is 612 nm,and then a DMAC donor is introduced on DPPZ,ICT is enhanced,the emission wavelength is red-shifted by 55 nm,and the emission wavelength reaches 667 nm.The?E_STs of DPPZ-3DMAC and DPPZ-4DMAC are small,0.16 and 0.13 e V,respectively;both materials show obvious delayed fluorescence characteristics,with delayed fluorescence lifetimes of 4.29and 1.47?s,respectively.Doped OLED based on DPPZ-3DMAC,EQE_max is 5.2%,EL wavelength is 622 nm,CIE color coordinate is(0.57,0.42);doped device based on DPPZ-4DMAC,EQE_max is 3.0%,EL wavelength is 644 nm,The CIE color coordinate is(0.58,0.41).TADF materials DPPZ-3DMAC and DPPZ-4DMAC both obtain red light emissions,but the device performance needs to be further improved.(3)To obtain more efficient red light TADF materials,DMAC with the strong electron-donating ability and rigid structure was still selected as the donor in this chapter,but two new TADF materials 3DMAC-DPPZ-Br and 3DMAC-DPPZ-CN were synthesized by introducing bromine atom and cyano group as co acceptor at 12-position of acceptor DPPZ.Both materials achieved highly distorted molecular structures,which allowed their homo and LUMO to be well separated with minor overlap.Because the electron-withdrawing ability of the cyano group is stronger than that of the bromine atom,and DPPZ-CN has a larger conjugated system than DPPZ-Br.Therefore,the?E_ST of 3DMAC-DPPZ-CN(0.02e V)is smaller than the?E_ST of 3DMAC-DPPZ-Br(0.04 e V).At the same time,in toluene solvent,the?_FL of 3DMAC-DPPZ-CN is 619 nm,which is 7 nm red-shifted from the?_FL of3DMAC-DPPZ-Br(612 nm).Both 3DMAC-DPPZ-Br and 3DMAC-DPPZ-CN achieved higher PLQY,83%and 92%,respectively.The doped OLEDs based on 3DMAC-DPPZ-Br and 3DMAC-DPPZ-CN have achieved good electroluminescence performance,EQE_maxreached 18.97%and 22.43%,respectively,EL wavelengths were 596 nm and 586 nm,CIE color coordinates were respectively(0.55,0.46)and(0.51,0.47),both achieve high-efficiency orange-red light emission,and both have lower open-circuit voltages of 2.7 and2.8 V,respectively.But compared with 3DMAC-DPPZ-Br,3DMAC-DPPZ-CN has better device performance.