Synthesis Of Highly Efficient Thermally Activated Delayed Fluorescent Emitters And Their Applications On Organic Light-emitting Diodes

In this work,we mainly focus on the design of novel thermally activated delayed fluorescent?TADF?emitters and the key points of TADF emitters,and some breakthroughs have been made as listed below:?1?A series of novel TADF electron-acceptor segments based on coumarin were designed,by linking with the large steric hindrance electron-donating segment of9,9-dimethyl-9,10-dihydroacridine,three novel TADF emitters were successfully prepared?ACPPO,ACMCO and ACBCO?.These emitters showed evident TADF characteristics.The Organic light-emitting diodes?OLEDs?based on these compounds exhibited sky blue to green emitting with CIE of?0.24,0.44?,?0.18,0.24?and?0.21,0.40?,respectively.The maximum external quantum efficiencies?EQEs?of these devices were 9.6%,9.8%and 15.1%,respectively.These results are much higher than the theoretical limit of the traditional fluorescent OLEDs and exhibit the bright future in applications in TADF emitters based on coumarin derivatives.?2?Two coumarin-based TADF emitters,3-methyl-6-?10H-phenoxazin-10-yl?-1H-isochromen-1-one?PXZMCO?and9-?10H-phenoxazin-10-yl?-6H-benzo[c]chromen-6-one?PXZBCO?have been report,both achieved relatively PLQYs and extremely small?E_STs.OLEDs based on these two TADF emitters respectively demonstrate high EQEs of 17.8%for PXZMCO and 19.6%for PXZBCO,which are the highest performances among the reported coumarin derivative-based OLEDs.More importantly,the EQEs of two devices remained 10.3%and 12.9%at 10000 cd m^-2,respectively,showing relatively low efficiency roll-offs at high brightness.These results reveal that the TADF emitters with high PLQYs and small?E_STs can effectively reduce the efficiency roll-off in the devices.?3?Two electron-acceptor segments with different steric hindrance have been designed,with them two TADF molecules PXZ-PPO and PXZ-BOO have been synthesized.Theoretical calculations and experimental results proved that PXZ-PPO have two conformational isomerization:nearly planar and nearly orthogonal conformations,in which the nearly planar conformation shown no TADF characteristics,while nearly orthogonal conformation achieved small?E_ST,leaded to efficient RISC.Due to evident exsitance of the nearly planar conformation,which was harmful to the device performance,thus only obtained 9.6%of maximum EQE.In PXZ-BOO,due to large steric hindrance between D segment and A segment,the influence of narly planar conformation can hardly be observed.The near-orthogonal conformation of PXZ-BOO exhibited TADF characteristics,and OLED based on PXZ-BOO obtained the maximum EQE of 18.1%,which was nearly two folds of that based on PXZ-PPO.?4?By introducing rigid and aromatic planar constituent groups,which can lead a large steric hindrance between electron-donor and electron-acceptor segments,a novel red TADF emitter 12,15-di?10H-phenoxazin-10-yl?dibenzo[a,c]dipyrido[3,2-h:2',3'-j]phenazine?DPXZ-BPPZ?is developed.By simultaneously realizing efficient fluorescent radiation transition and reverse intersystem crossing process,DPXZ-BPPZ successfully possesses an extremely high photoluminescence quantum efficiency of 97%under oxygen-free condition.The OLED based on DPXZ-BPPZ shows red emission with a peak at 612 nm and a CIE coordinate of?0.60,0.40?as well as high maximum forward-viewing efficiencies of 20.2%?external quantum efficiency?,30.3 cd A^-1?current efficiency?and 31.7 lm W^-1?power efficiency?,respectively.The prepared OLED exhibits the best performance among the reported red TADF-OLEDs.These results prove that DPXZ-BPPZ is an ideal candidate for red TADF emitters and the designing approach is valuable for highly efficient red TADF emitters.?5?A novel electron-donor segment 13,13-dimethyl-5-phenyl-11,13-dihydro-5H-indolo[2,3-b]acridine?IDAC?with suitable steric hindrance and large planar rigid structures is newly developed.TADF emitters based on IDAC could achieve small singlet-triplet energy splittings(?E_STs)and high photoluminance quantum yields?PLQYs?simultaneously.And two novel IDAC-based TADF emitters,6-?13,13-dimethyl-5-phenyl-5,13-dihydro-11H-indolo[2,3-b]acridin-11-yl?-3-methyl-1H-isochromen-1-one?IDAC-MCO?and 11-?4-?4,6-diphenyl-1,3,5-triazin-2-yl?-phenyl?-13,13-dimethyl-5-phenyl-11,13-dihydro-5H-indolo[2,3-b]acridine?IDAC-Trz?,have been designed and synthesized,both achieve efficient TADF characteristics with small?E_STs of 0.09 and 0.02 eV as well as high PLQYs of 81%and 90%.And IDAC-MCO-and IDAC-Trz-based OLEDs obtain respectively high EQEs of 18.0%and 20.5%with remarkable low efficiency roll-offs.These results reveal that IDAC segment may act as an ideal electron-donor to build efficient TADF emitters.