Synthesis And Electro-optical Propetry Investigation Of Bipolar Host Materials With High Triplet Energy Levele Attached Phenyl Bridge

Organic electroluminescent devices(OLEDs)have been gradually achieved commercialization due to their numerous advantages,such as light and thin,flexible and foldable,self-luminous,wide viewing angle,bright colors and high contrast.As the third-generation star organic electroluminescent materials,thermally activated delayed fluorescence(TADF)materials have aroused great interest of the scientific researchers,driven by the realization of 100%internal quantum efficiency through reverse intersystem crossing(RISC)from the nonradiative triplet excitons to radiative singlet excitons for efficient electroluminescence.In order to avoid a series of side effects caused by long-lived triplet excitons and achieve better device efficiency,a host-guest doped system was usually adopted,and the host materials played a solid solvent in the light-emitting layer.In addition,host materials played many critical roles in reducing concentration quenching,triplet-triplet exciton annihilation and so on.Consequently,developing host materials with higher performance is as equally important as the TADF emitters towards achieving excellent device efficiency.Besides,among the three primary colors of red,green,and blue,the efficiency and stability of the blue devices need to be further improved.Compared with the devices using vacuum evaporation method,the solution-processed devices have attracted wide attention because of its low cost,high preparation efficiency,and simple process.This paper was devoted to develop the host materials applied to the solution-processed blue TADF devices.The main researches were shown as follows:1.The triplet energy level of the host materials have been regulated through the reasonable molecular design.Two new host materials 3CzPh CN and 3CzPh CN-O-Czwere synthesized by inserting a conjugated benzene bridge between the central benzene ring and the electron-withdrawing cyano-group of the compound 3CzBN.The result of quantum chemical theoretical calculations showed that the LUMO orbitals of the molecules were mainly distributed on the cyano-group and the benzene ring connected to them,which showed that the extra benzene bridge enabled the HOMO and LUMO orbitals of the molecules to be better separated.The molecules had good thermal stabilities,with regard to the thermal decomposition temperatures of 375°C and 396°C,and the glass transition temperatures of 155°C and 96°C for 3CzPh CN and 3CzPh CN-O-Cz,respectively.The fluorescence emission peaks of these host materials were 407 nm and 415 nm in toluene solution,which had blue shifts of 24 nm and 16 nm for3CzPh CN and 3CzPh CN-O-Czcompared to 3CzBN.Besides,the triplet energy levels of the two molecules could be calculated for 3.05 e V and 3.01 e V by the onset of the phosphorescence spectroscopy in 77 K.In a word,the design concept of the molecules had been confirmed by the series of experimental results.The introduction of the benzene bridge extended the skeleton of the electron-withdrawing group,redistributed the electron cloud,weakened the intramolecular charge transfer interaction,and improved the triplet energy level of the host materials.Finally,the solution-processed blue TADF devices were conducted with 3CzPh CN and 3CzPh CN-O-Czas host materials,5TCzBN and 4TCzBN as the guests.The maximum current efficiency of 17.94 cd A^-1,the maximum power efficiency of 11.27 lm W^-1,the maximum external quantum efficiency of 7.73%were achieved.2.Three conjugated benzene bridges were introduced between the electron donating group carbazole and the central benzene ring of 3CzBN,and two new host materials 3Ph CzCN and3Ph CzCN-O-Czwere synthesized using a one-step C-N coupling reaction under high temperature and strong alkaline conditions.In addition,compared with the host materials3CzPh CN and 3CzPh CN-O-Cz,the effects on the properties and electroluminescence performance towards different positions and different numbers of benzene bridges were systematically studied.Besides,the molecule 3Ph CzCN had greater rigidity and poorer solubility,and the encapsulation of the peripheral carbazole greatly improved the ability of the film-forming property.The fluorescence emission peaks in toluene solution were 412 nm and422 nm,which had blue shifts of 19 nm and 9 nm for 3Ph CzCN and 3Ph CzCN-O-Cz,respectively.It was proved that the benzene ring extended the donor skeleton and weakened the intramolecular D-A interaction to a certain degree.The consequence of cyclic voltammetry test showed that the HOMO orbitals of the host materials have been improved in varying degrees.Furthermore,the triplet energy levels of both molecules were 2.85 e V calculated by the highest-energy vibronic peak of the phosphorescence spectrum.Compared with the host materials of3CzPh CN and 3CzPh CN-O-Cz,the triplet energy levels were reduced to a certain extent,because the three benzene bridges not only served as an intramolecular charge transfer inhibitor but also acted as the weak electron-donating group.Finally,the solution-processed blue TADF devices were conducted with 3Ph CzCN-O-Czas the host and 5TCzBN and 4TCzBN as the guest.Correspondingly,the maximum current efficiency of 2.43 cd A^-1,the maximum power efficiency of 1.09 lm W^-1,and the maximum external quantum efficiency of 1.05%were achieved.It was further demonstrated that effectively weakening the acceptor unit could better regulate the triplet energy level of the host materials and attain excellent device performance.