Synthesis And Properties Of Red Fluorescence Materials Based On Quinoxaline Derivatives Acceptors

Organic light emitting diodes（OLEDs）technique was proposed by Chinese American sciensist Ching W.Tang in 1987,and it attracted much attention from scientific research and industry.OLEDs have been widely used in flat panel displays,solid-lighting,and medical fields.The core technology of OLEDs is organic light emitting materials,and the organic light emitting materials have experienced three phases.The traditional materials can only used the 25%singlet for lighting and the maximum external quantum efficiency（EQE）is as low as 5%,such as Alq₃;phosphor materials can ultilized the triplet for lighting directly,resulted by the heavy atom induced spin-orbit coupling,such as Ir or Pt complexes;the triplet of thermally actived delayed fluorescence（TADF）materials can be ultilized by the reverse intersystem crossing（RISC）process,owing to the small energy splitting of the singlet and triplet(ΔE_st),then the forwarded singlet radiate as delayed fluorescence achieving 100%exciton ultilization.TADF materials have made great progress from 2012 when the TADF mechanism was proposed by Adachi and coworks from Kyushu university.Many blue and green TADF materials achieve breakthrough on efficiency,but the red TADF materials developed relatively slowly restricted by the energy gap rules,and the reported red TADF materials suffer from severe efficiency roll-off at high luminance.Under this circumstance,this thesis is contred on red materials employing quinoxaline derivants as acceptors.A series of red materials have been synthesized,their properties have been explored and OLED devices have been fabricated.Several high-performance red materials have been obtained and the designing strategy has been put forward.1.In chapter II,D-A and D-π-A red materials were synthesized with 2,3-diphenylquinoxaline（DPQ）as acceptor and diphenylamine（DPA）,9,9-dimethyl-9,10-dihydroacridine（DMAC）,phenothiazine（PXZ）applied as donors.These compounds showed strong thermostability with melting point over 200℃and thermal decomposition temperature over 350℃.D-A type compound DPA-DPQ showed largerΔEst owing to the severe electron cloud overlap of the highest occupied molecular orbital（HOMO）and the lowest unoccupied molecular orbital（LUMO）,and it didn’t show TADF properties.A large twisted angle was obtained between DMAC unit and DPQ acceptor,and DMAC-DPQ exhibited smallerΔE_st and distinct TADF property.Although the overlap of HOMO-LUMO was decreaed by phenyl bridge insertion and smallerΔE_st values are prospective for PXZ-Ph-DPQ,the strength of charge transfer was also weaked resulting high energy of ¹CT and the energy level of ³CT is higher than that of ³LE simultaneously.As a result,PXZ-Ph-DPQ exhibited largerΔE_st and its phosphorescence spectrum showed obvious refined vibration peak attributaed to local excited state.Eventually,device with DMAC-DPQ as the emitter showed maximum EQE of 6.8%,and the efficiency roll-off at 100 and 1000 cd m^-2 were 7.6%and 38.8%,respectively.2.In chapter III,phenanthro[9,10-b]quinoxaline（DBP）was employed as the acceptor,and the rigidity of the acceptor was enhance compared with DPQ.Higher photoluminescence（PL）quantum efficiency（PLQY）was expected.DAP and PXZ were employed as the donors for fabricating“cross-shaped”and“linetype”D-A compounds.Similar with DPA-DPQ,DPA-DBP also showed largerΔE_st and non-TADF property.The electron-donating ability is strong,which usually results distribution of HOMO on electron acceptor units,and large overlap of HOMO-LUMO is obtained.So DPA is not a clever choice as TADF donor.“Linetype”PXZ-DBP showed higher PLQY and nearly zeroΔE_st.Transient decay curves and transient decay curves measurments at different temperatures identified the property of TADF.Devices employing PXZ-DBP as the emitter exhibited maximum EQE of 11.5～10.9%,and the emission peaks were located at 624～636 nm when the doping concentrations of 5～10%.The undoped device showed an emission peak at 664 nm with a maximum EQE of 2.1%.The efficiency roll-offs of the all devices were less than 40%.3.In chapter IV,cyanogroups were introduced to quinoxaline unit to order to enhance the PLQY of the compounds.2,3-Dicarbonitrile quinoxaline was employed as the acceptor,triphenylamine（TPA）and 9,9-diphenyl-9,10-dihydroacridine（DPAC）were used as the donors for D-A and D-π-A compounds.DPAC-DCNQ exhibited dual emissions in dilute solution（toluene）,doped films,and electroluminescence（EL）spectra.This was resulted from the different conformations of the donor DPAC explained by the theoretical calculation.For TPA-DCNQ,the dihedral angle between the phenyl bridge and the DCNQ core was small,resulting in large overlap of HOMO-LUMO and largeΔE_st.The dihedral angle of DPAC and the phenyl bridge was large,resulting separated HOMO-LUMO and smallΔE_st of DPAC-Ph-DCNQ.The introduction of the cyanogroups enhanced the electron-withdrawing ability of quinoxaline,and compouns showed deep red and near-infrared emission.Devices with DPAC-Ph-DCNQ as the emitter exhibited 620～692 nm emission peaks and the maximum EQEs were 13.4～3.2%.The undoped device showed 736 nm emission and0.9%EQE.Aditionally,a exciplex cohost TCTA:PIM-TRZ was employed as the host for fabricating device based on TPA-DCNQ,the maximum EQE was 8.9%,indicating the utilization of triplet.4.In chapter V,TPA was employed as donor and dicyano-substituted pyrazine/quinoxaline were employed as acceptors for dual acceptors and dual D-A units compounds.The compounds TPA-2DCNPZ and 2TPA-2DCNPZ comprised of dicyano-substituted pyrazine as the acceptors exhibited similar PLQYs,and the PLQYs were far lower than that of TPA-2DCNQ comprised of dicyano-substituted quinoxaline as the acceptors,suggesting the rigid extendedπ-conjugated acceptor plays an important role in PLQY.The dual acceptors of compound 2TPA-2DCNPZ were conjugated,and the distribution of LUMO was extended,resulting in a more separated HOMO-LUMO and smallerΔE_st compared with TPA-2DCNPZ.Eventually,TPA-2DCNQ showed the best device performance owing to the highest PLQYs in doped films,TPA-2DCNPZ showed the worst device performance beacese of the lowest PLQY and a largeΔE_st.The results indicated that rigid extendedπ-conjugated acceptor of red TADF materials could improve the PLQY of the materials and constrcting compounds comprised of dual D-A units is also an effective approach to high performance red TADF materials.In summary,we have designed and synthesized a series of red materials employing quinoxaline derivants as acceptors.Acridine and phenothiazine donors are more applicable for TADF materials,because the twisty conformations and smallerΔE_st are obtained more easily compared with those equipped with diphenylamine as the donor.The“linetype”D-A compounds are in favour of radiation of charge transfer excited state than the“cross-shaped”materials.Highly efficient red TADF mateirals have been obtained by the introduction cyanogroups to quinoxaline and adjuasting different donors,as well as adjusting the distance of donor and acceptor by inserting phenyl bridge between the donor and acceptor.Extending the rigidπ-conjugated acceptor is benefit for improving the PLQY of the compounds.The compounds comprised of dual D-A units are also in favor of the separation of HOMO-LUMO and smallΔE_st values are obtained more easily for high-performance TADF materials.The results of this thesis put forwared referential significance for developing highly efficient red TADF materials.