Design And Synthesis Of Efficient Blue And Red Thermally Activated Delayed Fluorescence Emitters And Their Application On Organic Light-Emitting Diodes

In this work,we mainly focus on the frontier research fields of organic optoelectronic materials-organic light-emitting diodes(OLEDs)based on thermally activated delayed fluorescence(TADF)mechanism.In view of the critical scientific issues in blue and red TADF emitters,a series of highly efficient blue and red TADF emitters were designed and synthesized,and their physical properties and electroluminescence properties were systematically studied,and some breakthroughs have been made as listed below:(1)Amine-free electron-donating moieties dibenzothiophene(DBT)and dibenzofuran(DBF)as secondary donors are introduced to design efficient blue TADF materials.Four novel blue TADF emitters DBTCz-Trz,DBFCz-Trz,BDBTCz-Trz and BDBFCz-Trz are developed.The secondary donors extend the delocalization of the highest occupied molecular orbital(HOMO)effectively,resulting in obvious TADF characteristics as well as high photoluminescence quantum yield(PLQY)for all four emitters.The OLEDs based on DBTCz-Trz,DBFCz-Trz,BDBTCz-Trz and BDBFCz-Trz all exhibit stable blue emission,and their maximum external quantum efficiencies(EQEs)are as high as 21.7%,21.6%,23.4%and 25.1%,respectively.These results prove the great potential of amine-free donors in extending the frontier molecular orbital distributions and TADF molecular design.(2)To address the key scientific issues of strong non-radiative transition and excitons quenching of red TADF emitters,two novel red/near-infrared(NIR)TADF emitters BPPZ-PXZ and mDPBPZ-PXZ are designed and synthesized.BPPZ-PXZ employs the large planar rigid structure to suppress energy loss.While mDPBPZ-PXZ releases two pyridine substituents out of the planar rigid structure as an extra steric hindrance to suppress intermolecular ?-? interactions.Two TADF emitters show different properties in host-guest and non-doped systems.BPPZ-PXZ shows excellent exciton utilization with a PLQY of 100±0.8%and realizes a superior EQE of 25.2%in the optimized doped OLED,while a low EQE of 2.5%is realized in its non-doped OLED.Meanwhile the optimized doped and non-doped OLEDs based on mDPBPZ-PXZ exhibit maximum EQEs of 21.7%and 5.2%,respectively.These results demonstrate the importance of adjustment between molecular rigidity and steric hindrance in highly efficient red TADF molecular design.