Design,Synthesis And Device Of Heterocyclic Thermally Activated Delayed Fluorescence Materials

Rich in photoelectric properties for heterocyclic organic small molecules make them widely used in many kinds of optoelectronic devices.In recent years,heterocyclic structures have acted as main donor and acceptor units to construct thermally activated delayed fluminescence(TADF)molecules.A molecule with heterocyclic moiety can achieve small energy gap(?EST)between S1 and T1 by rational molecule design,which can be less than 0.3 eV.TADF materials can fully utilize all triplet excitons theoretically by reverse intersystem crossing of the triplet excitons from T1 to S1 for emission,so their devices have already broken the theoretical limit of 25%of the internal quantum efficiency for fluorescent materials,which are comparable to these for phosphorescent materials.In this paper,two kinds of heterocyclic molecules have been studied by quantum chemistry method.1.Eleven kinds of thio-heterocyclic intramolecular charge transfer(ICT)compounds have been designed and their geometrical structures and excited states have been calculated.Among them,two compounds have been synthesized,characterized their structures and determined their photophysical properties.The experimental results are also compared with those from theoretical calculations.2.Design and calculations of a series of nitrogen oxide heterocyclic ICT compounds that containing same acceptor unit.Those compounds realize full-color emission by the modification of the donor units,etc.The details are as follows:1.Eleven ICT compounds(a-k)are designed using 5-phenyl-1,3-bis(p-methylphenylsulfonyl)benzene as acceptor,and their ground state geometrical structures are optimized by density functional theory(DFT).Based on these optimizations of ground state structures,the energy levels of the excited states are calculated with the optimal Hartree-Fock(HF)exchange method.The calculated results show that most compounds have high singlet energy levels,so they tend to emit blue lights.Their theoretical values of ?max are in the range of 385.5?512.5 nm and ?EBT within 0.0389?0.4996 eV.Therefore,most molecules have?EBT less than 0.3 eV(except compound a and c),indicating that most of them are potential blue TADF materials.This result also indicates that ?max and ?EBT can be adjusted by changing electron push-pull capacity of donor or acceptor,or incorporation of benzene ring as a bridge between D-A.2.The compounds a and b were synthesized by Buchwald-Hartwig reaction.Their ?max in toluene solution are 449 nm and 503 nm,respectively,which indicated that compound a is blue light emitter and compound b is green light emitter.Their Eg are 3.94 eV and 3.88 eV based on UV-Vis absorption spectroscopy.Their HOMOs are-5.19 eV and-5.20 eV by electrochemical measurements.Their ?f and ?d values in toluene solution are 0.009 and 0.056,0.026 and 0.115,respectively,which show the characteristics of TADF materials due to the quantum efficiency increased to some extent after oxygen elimination.Their Td was 420 ? and 261 ?,respectively,which suggests that compound a has good thermal stability.The device adopted compound b as emitting layer is configured as:glass/ITO/a-NPB(40 nm)/compound b(11 nm)/TmPyPB(37 nm)/Alq3(30 nm)/LiF(3 nm)/Al(174 nm)with starting voltage of 11.8 V.It emits green light,which is similar to its emitting color in toluene solution under UV irradiation.3.Four kinds of ICT compounds 1A-1D are designed with four different electron-donating capacities donors linked on 4-and 8-position of benzo-oxazole(BBO)central benzene ring(part of acceptor).Quantum calculations show that they exhibit emission covering the entire visible light scope from blue to red.The respective ?max and ?EST of 1A-1D are 437.6 nm and 0.5269 eV,474.5 nm and 0.038 eV,567.0 nm and 0.0921 eV,666.7 nm and 0.0235 eV,respectively.In order to achieve a highly efficient blue TADF,larger ?EST of 1A and the longer ?max of the 1B are need to be fine-tuned.After fine-tuning,?EST of compound 2F decreases to 0.1529 eV,and its?max increases to 462.2 nm.3C,3G,3M and 3N are also tuned,and their ?max are in the range of 446.4?462.1 nm,and their ?EST are within 0.1297?0.2064 eV.Interestingly,high vibrator strengths of 3G and 3N of 0.0037 and 0.0033,respectively,suggesting they are potentially efficient blue TADF materials.