Design, Synthesis And Optoelectronic Properties Of Donor-Acceptor Type Thermally Activated Delayed Fluorescent Materials

In flat-panel displays,display technology is a very critical part that directly affects people’s senses and experience.Compared with LCD technology,organic light-emitting diode（OLEDs）have the advantages of self-emission,wide viewing angle,low power consumption,and short response time,that can replace LCD in the future.OLEDs products have gradually started commercial and become popular in recent years.In the past year development,stable and highly efficient materials are available.So far,traditional fluorescent materials have been used for blue OLEDs in commercial products,and phosphorescent materials have been used for green and red OLEDs.But there has some defect both in traditional fluorescent materials and phosphorescent materials.Firstly,traditional fluorescent materials are limited by the spin statistics in electroluminescence,that can only use 25%singlet excitons,so it is difficult to break through the efficiency.Secondly,phosphorescent can harvest triplet exciton and achieved 100%internal quantum efficiency,but phosphorescence materials contain rare metals,such as Ir（III）and Pt（II）,the cost of phosphorescent materials is generally high.It brings a lot of difficulties to commercial.Therefore,it is very urgent to develop materials that can replace traditional fluorescent materials and phosphorescence materials.Thermally activated delayed fluorescence（TADF）materials,can convert triplets to singlets via reversed intersystem crossing（RISC）.The internal quantum efficiency of the device can reach 100%.Furthermore,TADF materials based on pure organic materials that not contain rare and precious metals,that are low-cost and have great potential for commercial applications.In the past year development,the design strategy of TADF materials gradually has been enriched and improved.However,such materials still have defect.Based on the donor-acceptor type TADF materials often through steric effects to achieve strong twisting between donors and acceptor.Although this strategy can well separate HOMO and LUMO and achieve smallΔE_ST,but it will cause low the oscillator strength and photoluminescence quantum yields,which is very unfavorable to materials.In addition,based on the donor-acceptor type TADF materials cause the rigidity of molercular insufficient,which also have low the oscillator strength and photoluminescence quantum yields and affects the color purity.Therefore,it is very meaningful to improve the performance of materials in these two aspects.The main content of this article consists of the following three parts.1.In chapter II,the boron-containing bismesitylboryl（（Mes）₂B）acceptor was used to combine with the triazine/sulfonyl acceptor for D-A-A type TADF materials,PX-TRZ-B and PX-SF-B.Compared to common analogs TADF materials of single acceptor,such tandem double-acceptor structure not only reduce the overlap between the HOMO and the LUMO,but also achieve high photoluminescence quantum yields.Due to tinyΔE_ST and short lifetime,which can suppress the efficiency roll-off in OLEDs device.Based on PX-TRZ-B and PX-SF-B devices show excellent performance with high external quantum efficiencies of 19.2%and 24.8%,respectively.Two devices show very low efficiency roll-off of 2.1%and 6.5%at a practical luminance of 1000 cd m^-2.It is method to develop high-performance TADF materials by using the boron empty orbital extension acceptor unit.2.In chapter III,we based on chapter II works,we replace the the triazine/sulfonyl acceptor with carbonyl,and adjust the luminous color of the material.Donor utilized phenoxazine and phenothiazine,we synthesis two D-A-A type TADF materials PX-BP-B and PT-BP-B.Two compounds have very smallΔE_ST and High photoluminescence quantum yields.Based on PX-BP-B and PT-BP-B devices show excellent performance with high external quantum efficiencies of 21.6%and 15.9%.Due to PT-BP-B devices electroluminescence peak located at 580 nm and the FWHM is very suitable for making white OLEDs,combined with the classic blue material FIrpic,which exhibits a high color rendering index and color temperature.CIE（0.31,0.31）very close to the white light standard by the National Television Standards Committee（NTSC）.3.In chapter IV,we combine the weak donor carbazole with carbonyl group to construct a TADF acceptor with a rigid fused ring.Due to carbazole has a weak electron-donating ability,it can weaken the carbonyl acceptor ability.We utilize this novel TADF acceptor,to combined it with an electron donor diphenylacridine and synthesize two blue TADF materials,DPAC-CzBP1 and DPAC-CzBP2.Due to the rigidity of molecular,the fluorescence spectra are narrowly,and the FWHM of the two compounds in toluene solution(10^-5 M)are only 63 nm and 60 nm.The two compounds also have high photoluminescence quantum yields,the quantum yields in dope film are 0.74 and 0.45,respectively.Based on DPAC-CzBP1 and DPAC-CzBP2devices have highly efficient,and EQE_max of two devices are 20.4%and 10.7%,respectively.Due to the rigidity of molecular,electroluminescence spectra are narrowly and FWHM are 85 nm and 75 nm.It shows that building a rigid TADF acceptor is a means to narrow the luminescence spectrum and improve color purity.