Design,Synthesis And Device Performance Research Of Imidazole-based Luminescent Materials Of Excited State Regulation

Due to the advantages of active light emission,lightness and thinness,low power consumption,bright colors,and foldability,organic light-emitting diodes（OLEDs）technology have drawn intensive attentions from both academia and industry in recent years.At present,OLEDs as a display technology have been applied in various products such as mobile phones,tablets and wearable devices such as watches.As the core material of OLED,organic luminescent materials occupy a major position in OLED devices.In the emitting layer of OLED,organic luminescent materials are used in aggregate.However,traditional luminescent materials generally have a large planarπ-yoke structure and face severe aggregation-caused quenching（ACQ）effects,which greatly limits their practical applications.In terms of this,aggregation-induced emission（AIE）has been demonstrated can fundamentally solve the ACQ problem.It is convenient to control the emission and performance of the luminescent molecules by simple molecular designing to realize the high-efficiency electroluminescence efficiency in the OLED.In this paper,based on the imidazole derivatives with good carrier transport ability,styryl units with AIE effect and carbonyl ketone units were introduced into the molecule.Based on the principle of excited state regulation,a series of AIE molecules were synthesized,their molecular structures were determined by characterization methods such as NMR and mass spectrometry,and then their photophysical properties and applications in OLEDs were studied.The main research contents of the paper are as follows:（1）Using donor triphenylamine and bipolar phenanthrimidazole with good carrier transport ability as structural units,tris/tetrastyrene groups of different benzene rings were introduced and two AIE emitters were obtained.Through the study and analysis of the photophysical properties of two emitters,the molecular conjugation can be controlled and the intra-/inter-molecular charge transfer can be oriented to adjust the excited state of the molecule by adjusting the number of benzene ring substitutions of the vinyl group.In the solvents with different concentrations and different polarities,the directional adjustment from blue light emission to warm white light emission can be achieved by controlling the ratio of charge transfer within and between molecules of TPE-TPAPPI.In addition,due to the high PLQY and excellent thermal stability in aggregate state,Tri PE-TPAPPI and TPE-TPAPPI were used as the emitting layer of non-doped OLED devices.The non-doped OLED device based on TPE-TPAPPI existed the external quantum efficiency（4.01%）,which is closed to the theoretical maximum with a low turn-on voltage（2.7 V）.At the same time,the efficiency roll-off is only 4.75%even at the high brightness of 1000 cd m^-2,which is better than Tri PE-TPAPPI(EQE_max=3.05%,efficiency roll-off is 9.80%).It has been shown that potential applications of the AIE luminescent material with intermolecular charge transfer mechanism,which is better than a single intramolecular charge transfer mechanism after excited state regulation in low-roll-off OLED devices.（2）Using the self-developed pyridoimidazole group,two new orange-red AIDF luminescent materials were successfully synthesized through cyano group modification at different positions.Through steady-state and time-resolved spectroscopy and quantum chemical simulations,the charge transfer characteristics of the two molecules were systematically studied,which are conducive to long-wave emission and TADF properties.In addition,it is proved that the lifetime of TADF in the doped film can be shortened to less than1μs without reducing the inherentΦ_PL value after cyano modification.In addition,the excited state properties of TADF molecules can be controlled by changing the position of cyano group modification,and the RISC process in TADF molecules can be accelerated by enhancing the SOC effect between the singlet state and the triplet state.Transient absorption measurements were also performed to demonstrate the faster single-triplet energy transfer process in BDCN-PXZ.The non-doped device based on BDCN-PXZ shows a red emission peak at 606 nm,with the small EQE roll-off（5.6%）at 1000 cd m^-2.It is worth noting that the doped device F using5 wt%BDCN-PXZ:CBP as the emitting layer shows high CE,PE and EQE,respectively42.50 cd A^-1 and 27.81 lm W^-1 and 13.75%,the maximum brightness is 22081 cd m^-2.Moreover,it is worth noting that theη_ext（20 wt%BDCN-PXZ:CBP）of the device still maintains as high as 10.62%and 9.70%at the high brightness of 1000 and 5000 cd m^-2,showing an small efficiency roll-off（respectively 2.8%and 11.2%）.The research results demonstrate that AIE+TADF materials have a huge application potential in realizing non-doped OLED devices with low efficiency roll-off.