Design,Synthesis And Optoelectronic Properties Of Blue Organic Light-emitting Materials Based On Crossover Structural Features

Organic light-emitting diodes(OLEDs)are widely used in the field of fullcolor flat-panel displays due to their light weight,fast response,self-luminescence and wide angle of view.The emitting layer(EML)is the core of the entire device,its selection plays a decisive role in the external quantum efficiency of the device.Compared to the mature red and green light-emitting materials,blue-emitting materials have been the slowest to develop due to their own optical band gap,limited conjugate structure,and Aggregation-caused quenching(ACQ)in the solid state.Stable and efficient dark blue materials are even rarer.Therefore,the development of new and efficient organic blue light-emitting materials and devices plays an important role in promoting the development of OLED industry.Molecules with electron donor-acceptor(D-A)type structures are often used to design organic electroluminescent materials to enhance their own carrier transport capabilities.However,the disadvantage is that the long-axis unidirectional blue D-A or D-π-A molecules usually have more severe intramolecular charge transfer(ICT)effects,which is not conducive to obtaining high purity and deep blue light.In addition,long-axis unidirectional molecules are prone to aggregation-induced bursting(ACQ)effects in the solid state,making it difficult to achieve high device efficiencies.In this thesis,starting from the core idea of designing "novel high-efficiency blue and deep-blue light-emitting materials",and considering the effects of molecular bipolarity,spatial resistance and the choice of donor on the molecular properties as well,we designed and synthesized blue light-emitting materials with crossed structures,in which phenyl acts as a bridge group and the donor and acceptor units are distributed laterally.We fully characterized their photoelectric properties.The main contents of this thesis are as follows:We synthesized TPA-CN,a blue luminescent molecule with crossover structure,using phenyl as a bridge group and cyano-benzene and triphenylamine as electron acceptor and donor respectively.The molecule has a distorted spatial configuration and high potential resistance,which effectively inhibits ACQ.It still exhibits high photoluminescence quantum yield(PLQY)in solid films.Experimental results show that TPA-CN has good thermochemical and electrochemical stability.Frontier molecular orbitals(FMO)electron clouds are also crisscrossed.Among them,the highest occupied molecular orbital(HOMO)is distributed on the transverse axis of the donor group,and the lowest unoccupied molecular orbital(LUMO)is distributed on the longitudinal axis of the acceptor group,HOMO and LUMO overlap on the intermediate bridging benzene ring,and the excited states of the molecules exhibit hybridized local and charge transfer(HLCT).TPA-CN shows a high and balanced carrier mobility in the electroluminescence process,and the TPA-CN doped device achieves a maximum external quantum efficiency(EQE)of 10.2%,corresponding to CIE coordinates of(0.144,0.201),and an internal quantum efficiency(IQE)of nearly 100%.2.Based on the previous chapter,we tried to shift the emission color of the material toward the deep blue light through molecular design.By replacing the triphenylamine with the naphthalene group,which has a weaker electron-giving ability and greater structural rigidity,while keeping the cyano-phenyl group as the acceptor and the bridging group as benzene unchanged,we successfully synthesized the deep-blue emission molecule NAP-CN.NAP-CN also has a twisted crossover structure and exhibits high PLQY in both solution and thin film states.Compared with TPA-CN,the introduction of weak donor naphthalene in NAP-CN lowers the HOMO energy level of the whole molecule and also weakens the intramolecular ICT effect,which allows NAP-CN to exhibit excellent deep blue light emission.The electroluminescence peak of the NAP-CN-based non-doped device is 436 nm with CIE coordinates of(0.158,0.057),which fully meets the blue light standard(y ≤ 0.06)set by the European Broadcasting Union(EBU).Although the EQE of the device does not exceed the theoretical upper limit of 5%,the performance of the device is better than that of the reported non-doped devices with CIE coordinates of y less than 0.06.