Design,Synthesis And Application Of Novel Phosphorescence Host Materials

Organic light-emitting diode(OLED)has received extensive attention because of its excellent technology and application characteristics.The development of OLED materials and device fabrication process has made great progress in the past three decades.The device efficiency,luminance and lifetime have nearly met the requirements of commercial applications.OLED as an electroluminescent device,the luminescent material is the core component of it.The properties of luminescent material could determine the device performance to a large extent.Therefore,the development of high performance luminescent material has been one of the most popular research fields.The first-generation of conventional fluorescent material can only use 25%of the singlet exciton to achieve luminescence,and the maximum internal quantum efficiency(IQE)of the device is generally limited to 25%.The second-generation phosphorescent material breaks the restriction of traditional luminescence mechanism and makes the radiation transition process of the triplet excitons possible,thus greatly improving the exciton utilization and the device can achieve nearly 100%IQE.The third-generation luminescent material is thermally activated delayed fluorescence(TADF)material.The small single-triplet energy difference(△EST)increases the probability of the triplet excitons returning to the singlet state through reverse intersystem crossing(RISC)process,and increases the utilization of triplet excitons.Theoretically,the maximum IQE ot the device can reach 100%.It is worth noting that although TADF-OLED devices can achieve high efficiency,the color purity,efficiency roll-off and lifetime issues of the devices need to be further improved.Phosphorescent emitters are still considered as the most promising luminescent materials.In general,in order to avoid the luminescence quenching caused by excessive concentration of luminescent materials.the emitters are usually doped into suitable host materials.Therefore,high performance host material is also very important for the realization of high efficiency OLEDs.Consequently,in this work,we mainly focus on the development of high performance phosphorescent host materials.Our work is mainly carried out in the following aspects:1.In chapter 2.one polycyclic aromatic hydrocarbon compound,3,6,11,14-tetraphenyldibenzo[g,p]chrysene(TPDBC),was designed and synthesized.The thermal stability,basic photophysical properties,electrochemical behavior,charge-transport behavior and electroluminescent properties of TPDBC were fully studied,and it exhibited excellent thermal stability,suitable triplet energy and highest occupied molecular orbital(HOMO)/lowest unoccupied molecular orbital(LUMO)for red phosphorescent organic light-emitting diode(PHOLED).As a result,red PHOLED was fabricated with a maximum external quantum efficiency(EQE)of 14.4%,which represented the first report of a dibenzo[g,p]chrysene motif as the building block for host materials.2.In chapter 3,we showed that the novel dispiro and propellane motifs were successfully used as robust molecular platfonns for the construction of host materials(TPA,Cz,SF,and SO).The propellane derivatives with the three functional groups arranged in the staggered conformation were studied for the first time as the host for organic light-emitting diodes.The green and red PHOLEDs hosted by these dispiro and propellane derivatives exhibited excellent electroluminescence performance.Particularly,the devices hosted by the propcllane-type SF achieved a maximum efficiency of 47.3 cd A-1/97.6 cd A-1,40.2 lm W-1/77.8 lm W1 and 26.6%/27.0%for the red and green devices,respectively.It is vorth nothing that the red OLED based on TPA showed a maximum EQE of 24.0%.3.In chapter 4,a novel spiro-type material SDBSO was designed and synthesized The introduction of spirobi[dibenzo[b,e][1,4]azasiline]makes the material retain high triplet energy over 3.10 eV.In addition,thanks to the highly steric and rigid structure.and spiro-linkage,SDBSO exhibited good thermal stability and suitable frontier molecular orbital(FMO)energy levels.The RGB(blue,green,and red)three-color PHOLEDs with SDBSO as the host material were fabricated and the maximum external quantum efficiencies were achieved over 20%for green and red devices with relative flat roll-off.4.In chapter 5.three novel donor-acceptor type organic semiconductors POITPA,PO2TPA,and 2POTPA composed of arylamine donor and arylphosphine oxide acceptor were designed and synthesized through ortho-linking strategy.By varying the donor/acceptor ratio in these compounds,the effects of donor-acceptor connection pattern on the thermal stability,photophysical and optoelectronic properties of these materials were studied systematically.POITPA,PO2TPA,and 2POTPA exhibited high triplet energy due to the ortho-substituted fashion.As a result they were applied in blue,green,and red PHOLEDs as host materials.PO1TPA exhibited best electroluminescent performance for green device with EQE over 20%.5.In chapter 6,two novel carbazole derivatives CNPhCz and DCNPhCz were designed and prepared as host materials for PHOLEDs.CNPhCz and DCNPhCz were synthesized via ortho-linkage strategy.The influences of different number of electron-withdrawing groups in these host materials on the thermal stability,basic photophysical properties,electrochemical behavior,charge-transport behavior and OLED device performance were studied systematically.The results indicated that the one with less electron-withdrawing cyanobenzene group substituted material(CNPhCz)gave better green PHOLEDs device performance with a maximum EQE of 24.4%.6.In chapter 7,three novel 9.10-dihydroacridine derivatives,MeAcPhCN,PhAcPhCN.and MeAcPyCN,were prepared by the attachment of[1,1’-biphenyl]-4-carbonitrile or 5-phenylpicolinonitrile to the 4-position of 9,10-dihydroacridine.This special linking strategy will limit the conjugation length,and maintain the triplet energy for these compounds.Consequently.red PHOLEDs were fabricated with these three host materials.As a result,the device hosted by MeAcPhCN showed good device performance with maximum EQE of 20.5%.7.In chapter 8,for a better understanding of the connection between molecular structures and properties,four molecules(1,2,3,and 4)based on different spiro-systems,spiro[cyclopropane-1,9’-fluorene],spiro[cyclopentane-1,9’-fluorene],1’,3’-dihydrospiro[fluorene-9,2’-indene]and spirobifluorene,were designed and prepared.The influences of different spiro units on photophysical properties and electroluminescence performance were fully studied.The experimental results showed that photophysical properties,electrochemical behavior,and frontier molecular orbital distributions of these different spirofluorenes were mostly identical because the πbackbones were less affected by the different cycles at the C-9 position of fluorene.However.green PHOLED based on spirobifluorene derivative 4 exhibited the highest electroluminescence performance among these four hosts,the maximum EQE was achieved 18.8%.8.In chapter 9.two novel heterofluorene motifs consisting of the group 14 elements Si and Ge were developed as robust molecular platforms for the construction of OLED materials.These two compounds exhibited similar photophysical properties,thermal stability and electrochemical behavior except the charge transport abilities.The red OLED hosted by 9-silafluorene derivative(DPS)with Ir(MDQ)2(acac)as the emitter exhibited the state-of-the-art current efficiency,power efficiency,and EQE with maxima of 50.7 cd A-1,44.7 lm W-1,nd 28.3%espectively.In addition,the maximum EQE of 9-germafluorene derivative(DPG)-based red device can reach 20%.In general,we systematically studied various systems of new phosphorescent host materials.The relationship between material structure and device performance has been probed initially,and some effects have been obtained.These results might provide some useful references for developing high efficient host materials.