Synthesis And Electroluminescence Of D-A Type Panchromatic Fluorophors Containing Aromatic-fused-pyrazine As Acceptors

Pure organic luminescent materials(including traditional fluorescent materials and thermally activated delayed fluorescence(TADF)materials)are attractive as emitters in organic light-emitting diodes(OLED)due to merits of low costs and wide source of raw materials.TADF emitters are especially desired due to extra advantage of high emissive efficiencies.Generally the TADF molecules have typical donor-acceptor(D-A)structures,and development of novel and excellent acceptor units has always been one of the most challenging tasks nowadays.This dissertation is focus on developing novel D-A type fluorescent and delayed fluorescence materials with quinoxaline and pyrido[x,y-b]pyrazine as acceptor frameworks.The molecules were designed by tuning the intramolecular charge transfer extent,steric conformatioins,and so on,with the aims(1)to tune the emission color covering the whole visible light range,(2)to change the emission mechanism from traditional fluorescence to TADF,(3)to achieve high-performance single-color and white OLEDs using the obtained materials.The main research contents are as follows:(1)D-A-D type blue fluorescent materials with quinoxaline(Q)and pyrido[x,y-b]pyrazine(PP or iPP)as acceptors:A series of D-A-D type compounds,namely Q-NEtCz,FQ-NEtCz,TFMQ-NEtCz,iPP-NEtCz,and PP-NEtCz were designed and prepared with Q or PP or iPP as a acceptor skeleton and the N-ethylcarbazole as donor group.These compounds emit normal fluorescence with color in blue to bluish-green region(455 nm-493 nm),and the maximum external quantum efficiency(EQE)of their OLED did not exceed 5%.Further structure modification by inserting phenylene bridge between Q and Cz and tuning the D-A linking style realized isomers o-QCz,m-QCz and p-QCz.Unfortunately they still emit the traditional fluorescence as well,and their EOE ranged from 0.8%to 2.5%.These demonstrated that the combination of Q and PP or iPP acceptors with Cz donor was impossible to realize delayed fluorescence.However,by fine-tuning the torsional angle between the acceptor and donnor,m-QCz reveals a standard blue emission with wavelength peak at 432 nm and CIE coordinates of(0.16,0.09).(2)Fluorescent and TADF materials with PP/iPP acceptors and triphenylamine(TPA)donor:In order to obtain TADF materials with improved performance,triphenylamine(TPA)was set as electron donor and linked in combination with PP/iPP acceptors onto the phenylene bridge to form four isomers p-PPTPA,p-iPPTPA,m-PPTPA,and m-iPPTPA.Among them,two para-linked isomers p-PPTPA and p-iPPTPA only emit normal fluorescence,realizing maximum EQE below 5%in their doped OLED.In contrast,by increasing steric hindrance between D and A groups through meta-linkage,the conjugated degree between D and A decreases moderately in m-PPTPA and m-iPPTPA,resulting in tiny energy splittings(△EST)of 0.01 eV and 0.03 eV and long fluorescence lifetimes in the orders of microseconds,confirming their TADF feature.The maximum EQE of OLED with m-PPTPA and m-iPPTPA as doped emitters reached 9.0%.It is demonstrated in this chapter that selecting suitable donor groups with strong electron donating ability and decreasing the molecular planarity can help to realize TADF character.(3)Fluorescent and TADF materials covering the panchromatic range:a series of fluorescent and TADF compounds,namely p-QCz,Q-DMAC,iPP-DMAC,PP-DMAC,iPP-PXZ,and PP-PXZ were designed and prepared by using Q,PP and iPP as acceptors,carbazole(Cz),dimethyl acridine(DMAC)and phenoxazine(PXZ)as donors.By tuning the intramolecular charge transfer extent via different D and A pairs,the fluorescence or TADF color of these compounds are continuously tuned from pure blue via green and yellow to orange-red.At the same time,by tuning the electron donating or withdrawing strength of D or A groups and the steric molecular conformations,the emission mechanism was tuned from normal fluorescence for p-QCz to TADF for all other five compounds.The p-QCz based blue OLED exhibited an EQE of 1.2%.All other TADF devices realized much higher EQE of 12.9%,16.7%,14.1%,12.3%and 13.0%,respectively.(4)Full-TADF WOLED with newly developed orange TADF emitters:By incorporating CF3 and CN onto quinoxaline,the electron withdrawing strength of the generated acceptors,TFM-QP and CN-QP,was further enhanced.These modified acceptors were used in combination with PXZ donor to design novel compounds TFM-QP and CN-QP.Small energy difference(0.03 eV-0.04 eV)and long fluorescence life(1.6 μs-5.0 μs)confirmed the TADF properties of these two molecules.Due to further enhanced electron-withdrawing ability for the acceptor,its luminous wavelength shifted to over 600 nm.At the same time,these two compounds have aggregation induced luminescence(AIE)properties,which are beneficial to the luminescence performance of solid state devices.Their doped orange OLED exhibited high efficiencies of 14.4%and 9.7%,respectively.Full-TADF WOLED were fabricated by doping TFM-QP or CN-QP in sky-blue TADF emitter DMAC-DPS and exhibited EQE of 18.1%and 20.2%(48.3 cd A-1 and 48.2 cd A-1).As far as we know,these efficiencies are among the highest values for full-TADF WOLED ever reported so far.The present work provides a new method to realize low-cost and high-efficiency hybrid white light emission with high color rending index(CRI).