Design, Tunation And Application For The Hybrid Local And Charge-Transfer Excited State

Quality and innovation are the themes of China's industry in the thirteenth-fiveyear-plan period.In the area of organic light emitting diode(OLED),the significantly growing demands provide us golden opportunities for developing quality and great challenges for leading innovation.The development of next-generation OLED emissive material is hence the focus of the research area.The traditional OLED emissive materials are phosphorescent materials that could directly utilize the electro-triplet emissive excitons and realize a near 100% electroexciton utilization by incorporating noble atoms.However,phosphorescent materials suffer from the high cost,complexities in realizing massive synthesis,difficulties in the doping technology of and potential pollution.By contrast,fluorescent materials outperform in the limitations of phosphorescent materials.The only issue,we have to address before fluorescent materials become the next-generation OLED emissive materials applied in the industry,is that the spin statics rule in electroluminescence is initially broken.The ultilizable singlet exciton only accounts for 25% of the total electro-exciton.Many efforts have been taken to solve this problem,such as the TripletTriplet Annihilation(TTA),Thermally-Activated Delayed Fluorescence(TADF),Hybrid Locally-emissive and Charge-Transfer(HLCT),the “hot CT-exciton” and the doublet emissive strategy.The HLCT and “hot CT-exciton” strategy could harvest the maximized OLED efficiency from a compatible coexistence between high photoluminence efficiency and high electro-exciton utilization,corresponding to a golden combination between the locally-emissive(LE)component and the chargetransfer(CT)component of the excited state.Thus,it is possible to realize the highlyefficient non-doped OLED by using HLCT materials,and it is necessary to figure out the specific strategies for constructing efficient HLCT state for OLED.In this dissertation,I focus on the structure property relationship and rational molecular design for highly efficient HLCT state.The material system of this thesis is a series of cyano-substituted multiphenyl-aniline-phenanthroimidazole HLCT material.According to the topology of their intrinsic LE state species CT state species,they can be further divided into two groups: one is the “parallel-type”and the other is the “vertical-type”.The research contents of this dissertation are summarized as follows:(1)Firstly,the “parallel-type” HLCT material system is built up by cyanosubstitution on phenanthroimidazole ring.The “parallel-type” HLCT states partially breakthrough the spin-statistics bottleneck,but further effort should still be paid to optimize the colour purity and maximize the exciton utilization.(2)Then,a “vertical-type” HLCT material TPMCN is designed and synthesized by introducing cyano-substitution to the side phenyl group of phenanthroimidazole ring to further enlarge the electro-exciton utilization and improve the color purity.The exciton utilization of TPMCN reaches 85% due to an obviously enhanced CT character,but on the other hand,its instinct emission efficiency is seriously affected.(3)Furthermore,an optimized strategy for constructing efficient HLCT state “equivalent hybridization of LE and CT” is discovered in the blue-emissive “verticaltype” material TBPMCN.TBPMCN demonstrates satisfied non-doped OLED performance of a maximum current efficiency(CE)of 10.5 cd ·A-1,a maximum power efficiency(PE)of 5.5 lm W-1,and a maximum EQE of 7.8% with an electroexciton utilization of 98%,which is among the best results in non-doped blue emissive OLED that have ever been reported as far as we know.(4)The evidence that HLCT state is benefit to enhancing not only the electroexciton utilization but also the photoluminecscence is further detected by means of high-pressure experiment.A unique emission enhancement(RHIEE)phenomenon is found in this crystal under high pressure,which can be assigned to the change of excited state property from a CT-dominated state to an HLCT state,as a result of the rehybridization of nitrogen atom(sp~3 to sp~2).