Substitution Effect On The Light-emitting Properties Of Thermally Activated Delayed Fluorescence Molecules With Blue Emission: Theoretical Study And Molecular Design

Organic light-emitting diodes(OLED)are widely used in solid-state lighting and information display fields,due to their ultra-thinness,flexibility,and wide viewing angle.Thermally activated delayed fluorescence(TADF)materials often have a small energy gap between the first singlet excited state(S₁)and the first triplet excited state(T₁),which is conducive to the up-conversion of triplet excitons into singlet excitons,achieve nearly 100%exciton utilization rate.Therefore,OELD devices with TADF properties have been favored by major research groups in recent years.As one of the three primary colors,blue light is an important part of realizing full-color luminescence.However,blue light materials have many problems,such as impure chromaticity,low device efficiency,and low thermal stability.In order to develop a blue TADF luminescent material with high stability and high efficiency,a detailed theoretical study is carried out on the blue TADF luminescent molecule,and it is found that the addition of substituent groups on the donor group and the acceptor group would affect its ability to gain and lose electrons and molecular configuration,which can effectively improve the stability and efficiency of blue TADF molecular luminescence.Through theoretical research on blue TADF molecules with substitution effect,it provides theoretical support for its practical application in devices.In order to further verify the influence of the substitution effect on the blue TADF molecules,the positions and types of the substitution groups are changed in this study,revealing the enhanced luminescence mechanism of substitution.(1)Theoretical study on the influence of tert-butyl substitution on the excited state properties of blue thermally activated delayed fluorescence molecules.The work selected four molecules for research,they use perfluorobiphenyl(PFBP)units as acceptors and 9,9-dimethyl-9,10-dihydro-pyridine(DMAC)units as donors,in the donor-acceptor(D-A)type molecules and the donor-acceptor-donor(D-A-D)type molecules,the donor group is treated with and without the addition of tert-butyl group,then get the four molecules to be studied.The polarized continuum model(PCM)and the combined quantum mechanics and molecular mechanics(QM/MM)methods take into account the influence of toluene and solid on the surrounding environment,respectively.First of all,we found that the substitution of tert-butyl in the donor unit can effectively reduce the energy difference and increase the spin-orbit coupling(SOC)constant,and further improve the intersystem crossover(ISC)and reverse intersystem crossover(RISC)rate.Second,the solid phase limits the geometric change between the first singlet excited state and the ground state(S₀),and the root mean square displacement(RMSD)is reduced.Moreover,our calculated Huang-Rhys factor and reorganization energy are both reduced in the solid phase,which indicates that the non-radiative energy consumption process of S₁ is hindered by the enhanced intermolecular interaction in the rigid environment and brings about aggregation-induced emission.At the same time,due to the enhanced ISC and RISC processes,molecules with a donor-acceptor-donor configuration have more effective luminescence properties than D-A molecules.Therefore,the tert-butyl substituted D-A-D type molecule has outstanding TADF characteristics.In addition,our research provides theoretical perspectives for AIE and TADF mechanisms,and proposes effective TADF molecular design strategies that can promote the development of OLED.(2)Theoretical exploration and molecular design of the influence of methyl substitution on the luminescence properties of blue thermally activated delayed fluorescence molecules.In this work,the effects of positional effects(ortho,meta and para)and methyl substitution effects on the photophysical properties of nine blue TADF molecules are studied.Quantum mechanics and molecular mechanics method and thermal vibration correlation function(TVCF)method are used to study the solid environmental effects and excited state dynamics.The results show that,due to the weak electron donating ability of the methyl group,the introduction of methyl groups in the acceptor unit will bring about a significant emission blue shift,but it will be weaken its TADF properties.On the contrary,the methyl substitution on the donor unit will promote the process of RISC and radiation decay,which can significantly improve the luminous efficiency of blue TADF molecules.As for the change of the D-A position,it can be found that the counterpoint connection is beneficial to enhance the reverse intersystem crossing and radiation decay process.Therefore,the TADF properties can be improved by the methyl substitution in the donor unit,and deep blue light emission can be achieved by the substitution effect in the acceptor unit.In addition,counterpoint connection can be seen as a smart strategy for constructing effective blue TADF molecules.There are five chapters in this thesis.The first chapter is an introduction.It introduces the development process of organic diodes and the development history of luminescent materials.It focuses on the changes of thermally activated delayed fluorescence and blue thermally activated delayed fluorescence,and then systematically discusses the replacement.The development direction of the group in different dimensions.The second chapter specifically elaborates the theoretical methods used,including density functional theory(DFT),time-dependent density functional theory(TD-DFT),quantum mechanics and molecular mechanics methods,as well as radiation rate and non-radiation rate calculation method.Chapter 3 studies the luminescence mechanism of tert-butyl substitution in blue TADF molecules.Chapter 4 studies the changes in photophysical properties caused by the gain and loss of electrons in blue TADF molecules by methyl substitution.Chapter 5 summarizes the work done and looks forward to the future work arrangements.