Synthesis And Properties Of Quinoxaline Derived Organic Luminescent Materials

For nearly three decades,organic light-emitting diodes(OLEDs)offer a bright future in flexible display and solid state lighting due to their outstanding features of energy conservation,light weight and wide viewing angles.Thermally activated delayed fluorescence(TADF)emitters are currently attracting intensive interests in the area of OLEDs because they can utilize both singlet and triplet excitons to achieve 100%internal quantum efficiency(IQE)in theory.To achieve highly efficient TADF emitters,the most basic principle of a small energy difference between single and triplet(?EST)is necessary to be satisfied,which can be beneficial to the triplet exciton up-convert to singlet state through reverse intersystem crossing(RISC)by thermal activation.Compared with the high-efficiency blue and green TADF-based OLEDs,efficient orange/red TADF-based OLEDs are still very limited.On the other hand,TADF OLEDs often suffer from serious rolling-off that originates from the concentration accumulation of the long lifetime of the excited triplet states.Aiming at above problems,in this thesis,through in-depth analysis of the mechanism and rational molecular design,we developed a series of performance outstanding TADF materials,and harvested high efficiecy and low roll-offs of organic light emitting fluorescence devices.In chapter 1,we briefly introduced the basic situation of OLEDs,including the development process of OLEDs,luminous mechanism,device types and related parameters.Then,we introduced the development of TADF materials,including red,green and blue TADF materials.Finally,the research strategy and main contents of this thesis are described.In chapter 2,through an asymmetric structure with a phenoxazine as an electron-donor and a fluorine-substituted qunioxaline as an electron-acceptor,we designed and synthesised a new orange red TADF emitter,FDQPXZ.We successfullyrealize a small ?EST and a high fluorescence radiative rate(?rS)for long-wavelengthTADF emitters.Using FDQPXZ as the emitter,the vacuum-deposited orange-red OLED achieves a maximum current efficiency(CE)of 35.3 cd A-1,a maximum power efficiency(PE)of 36.5 lm W-1,a maximum EQE of 13.9%,and extremely low efficiency roll-off.On the other hand,for the solution-processed emitting layer,the device also acquires 9.0%of EQE.In chapter 3,to develop new orange/red TADF emitters and explore theradiative transition ways of molecular excited states,we synthesised a series of D-A-D type quinoxaline-based compounds.Through adjusting the electron-donating abilities of donor units as well as the dihedral angles between the donor and acceptor,the value of ?EST and the nature of T1 can be systematically tuned.Their photoluminescence(PL)spectra exhibit a wide range from green to orange,and the emission type from local excited-state(LE)to charge-transfer state(CT)transition have been successfully tuned.In chapter 4,based on the common point of designing TADF molecules and room temperature phosphorescence(RTP)molecules,we developed a new quinoxaline/phenoxazine hybrid emitter(DBQPXZ),and observed the compound is simultaneous TADF and RTP,moreover,we observed a threshold temperature of 170 K,and below 170 K,it is the phosphorescence radiation as the dominant process,while above which the reverse intersystem crossing process(TADF)gradually become the predominant status.Utilizing DBQPXZ as the emitter,the vacuum-deposited OLED achieves a maximum CE of 54.1 cd A-1,a maximum PE of 59.0 lm W-1,a maximum EQE of 16.8%.In chapter 5,in order to alleviate the phenomenon of excessive efficiency roll-offs for TADF-based OLEDs,we introduce fluorine atom into quinoxaline system owing to its strong intermolecular electron coupling capability and weak electron-withdrawing ability which are conducive to construct the aggregation-induced emission(AIE)-TADF molecules with the unchanged PL spectra.We designed and synthesized two new AIE-TADF compounds,namely SFDBQPXZ and DFDBQPXZ.The AIE-TADF-based doped OLEDs exhibit the best performances with a maximum CE of 78.3 cd A-1,a PEmax of 91.1 lm W-1,and a maximum EQE of 23.5%,in addition,the non-doped OLEDs show orange emission,and achieve a CEmax of 24.3 cd A-1,a PEmax of 22.5 lm W-1 and a maximum EQE of 10.1%.In chapter 6,four TADF compounds SBDBQ-DMAC,DBQ-3DMAC,SBDBQ-PXZ and DBQ-3PXZ are synthesized by introducing different donor units into quinoxaline system.All compounds show excellent thermal stability and very small ?EST with an order of 0.24,0.04,0.07 and 0.04 eV.Using DBQ-3DMAC as green emitter in OLED,the device achieves a CEmax of 80.3 cd A-1,a PEmax of 64.1 lm W-1 and a maximum EQE of 22.4%.It is worth mentioning that,the device efficiency roll-off ratio is only 3.6%at the brightness of 100 cd m-2.Using DBQ-3PXZ as orange emitter in OLED,the device exhibits perfect device performance with a CEmax of 36.1 cd A-1,,a PEmax of 28.1 lm W-1 and a maximum EQE of 14.1%,especially,the device efficiency roll-off ratio is only 1.4%and 21.3%at the brightness of 100 and 1000 cd m-2,repectively.In chapter 7,we designed and synthesized six asymmetric structure compounds based on the development of quinoxaline system,namely,SBPQ-BAZ,SBPQ-tBuCz,SBPQ-DtBuCz,SBPQ-DPAC,SBPQ-DMAC and SBPQ-PXZ.These compounds achieve the emission for full-color display and furthermore,we obtained two compounds with TADF properties,SBPQ-DPAC and SBPQ-DMAC.The TADF-based fluorescence OLEDs using SBPQ-DPAC as the emitter shows a CEmax of 45.1 cd A-1,a PEmax of 35.4 lm W-1 and a maximum EQE of 13.6%.It is particularly worth mentioning that,the device efficiency roll-off ratio is only 0.7%and 15.4%at the brightness of 100 and 1000 cd m-2,repectively.Such ultra-low efficiency decay is very rare,indicating that the radiation transition process of excitons in guest molecules is very effective.