Investigations On Excited State Regulation And Photophysical Properties Of Multifunctional Luminescent Materials Based On Quinoxaline

Organic Light Emitting Diodes（OLEDs）have huge application prospects in lighting and displaying fields due to their advantages of low-power,self-luminescence,fast response,wide viewing angle,high resolution and flexible displays.Due to the spin-forbidden effect of traditional organic fluorescent materials,the limit of maximum external quantum efficiency（EQE）is 5%and the theoretical exciton utilization efficiency（EUE）is 25%because of the spin-forbidden,which cannot be satisfied the demand for industrialization.However,compared with phosphorescent materials and polymers,organic fluorescent materials have incomparable advantages,such as the determination of molecular structure,easy modification and simple purification.Therefore,it is an important challenge to make the 75%triplet excitons in organic small molecule fluorescent materials participate in the luminescence process,to enhance the luminescence efficiency and improve stability of OLEDs.Presently,there are three main ways to resolve above dilemma:the cold excitons for thermally activated delayed fluorescent（TADF）materials,the hot excitons for hybrid localized electron-charge transfer excited state（HLCT）materials,and triplet-triplet annihilation（TTA）materials.Regardless of the luminescence mechanism,the regulation of the excited state properties is achieved through the regulation of the molecular structure.Therefore,we aim to investigate the excite-state characteristic of emitting materials by adjusting the electron donating ability with a donor-acceptor structure and introducing bridged groups between D and A.The behaviors of different aggregation states and photophysical properties were studied in details.（1）.The effects of the donor group on the natures of excited state were studied.Three molecules with symmetric D-A-D and asymmetric D-A-D’structures were designed and synthesized using quinoxaline as an acceptor,triphenylamine as the first donor,and 9,9-dimethylacridine as the auxiliary donor,namely as 4,4’-（quinoxaline-5,8-diyl）bis（N,N-diphenylaniline）（TQT）,5,8-bis（9,9-dimethylacridin-10（9H）-yl）quinoxaline（DQD）and 4-（8-（9,9-dimethylacridin-10（9H）-yl）quinoxalin-5-yl）-N,N-diphenylaniline（TQD）.The symmetrical TQT exhibits traditional fluorescent characteristic according to the experimental and calculation results.Due to the increasing electron-donating ability,the charge-transfer state（CT）is enhanced.DQD and TQD presented typical TADF properties;Three different crystals of TQT have obtained by solvent diffusion method,TQT-G(λ_PL=530 nm),TQT-Y(λ_PL=575nm)and TQT-O(λ_PL)=584 nm),in which the photoluminescence quantum yield（PLQY）of TQT-G and TQT-Y reached 81.2%and 92.7%respectively.The switches of seven different emission colors were realized in respond to the external stimuli changing from 514 to 584 nm.Compounds of DQD and TQD exhibited aggregation-induced emission（AIE）and can realize the four-color switches responding to the external stimuli.The devices were fabricated using three materials as emitters.The doped device based TQT with 20 wt%doping concentration in4,4’-bis（9H-carbazol-9-yl）biphenyl（CBP）displayed excellent performances,with the maximum brightness of 14790 cd/m²,and the maximum EQE of 5.06%.Non-doped devices based on DQD achieved a maximum brightness of 1481 cd/m² and a maximum EQE of 4.51%.（2）.To suppress non-radiative transition in D-A structure and improve radiative rate,πbridged units of benzene and methylbenzene were introduced to limit the vibration of flexible groups.To adjust the emission color,the auxiliary ligand was replaced by phenothiazine（PTZ）.three molecules with symmetry D-π-A-π-D and asymmetric D-A-π-D’were designed and synthesized,donate as 5,8-bis（4-（9,9-dimethylacridin-10（9H）-yl）phenyl）quinoxaline（DPQPD）,5,8-bis（2-methyl-4-（10H-phenothiazin-10-yl）phenyl）quinoxaline（2PMQx）,and an4-（8-（4-（10H-phenothiazin-10-yl）phenyl）quinoxalin-5-yl）-N,N-diphenylaniline（TQPP）.The photophysical properties,excited state properties,aggregated state luminescence behaviors and EL performance of these three materials were characterized.The experimental results show that DPQPD and TQPP are traditional fluorescent molecules with AEE properties.The doped devices based on TQPP achieves a maximum brightness of 8558 cd/m² and a maximum EQE of 2.57%.The symmetrical structure of 2PMe Qx is a TADF material with polymorphism:M1(λ_PL=593 nm)and M2(λ_PL)=622 nm).The doped device based 2PMQx with doping concentration of 20 wt%in bis[2-（diphenylphosphino）phenyl]ether oxide（DPEPO）exhibited a maximum brightness of 1984 cd/m²,and the EQE of 5.19%.DPQPD,TQPP and 2PMe Qx also exhibited reversible mechanochromic due to their distorted molecular conformation and efficient fluorescence emission in solid state.