Theoretical Study On Luminescent Mechanism Of Multifunctional Thermally Activated Delayed Fluorescence Molecules

Organic light-emitting diodes?OLEDs?have attracted great attention recently,since thermally activated delayed fluorescence?TADF?materials which can achieve nearly 100%exciton utilization were reported.Significant progress of TADF molecular materials has been achieved during the past few years,and some molecules even exhibit multifunctional properties such as the aggregation-induced emission?AIE?,mechanochromic luminescence?MCL?and circularly polarized luminescence?CPL?.These materials show more advantages in application.AIE can effectively solve the fluorescence quenching problem of TADF molecules in solid phase.MCL materials make use of external stimuli to provide the possibility of achieving control the color of luminescence,and they show the potential of wide application in memory devices,sensors and data storages.CPL materials can save energy on OLED displays and allow for high contrast3D images.However,the number of multifunctional light-emitting molecules is still quite limited,and the luminescence mechanisms are not clear enough.Therefore,it is of great significance to theoretically study the luminescence mechanism of multifunctional molecules.In this paper,we use the combined quantum mechanics and molecular mechanics?QM/MM?method to study TADF properties in the aggregation state.The mechanisms of AIE and aggregation induced delayed fluorescence?AIDF?were revealed by comparing with the light-emitting properties of TADF molecules in solvent.The bicolor switching mechanism of the molecule in solid phase is also studied based on first-principles calculations.This paper mainly includes the following three aspects:?1?Perspective for AIDF mechanism of DMF-BP-DMAC.In this work,excited state properties of DMF-BP-DMAC are theoretically studied in tetrahydrofuran?THF?and solid phase.Due to the increase of transition dipole moment and decrease of the energy difference between the first single excited state?S₁?and the ground state?S₀?,the radiative rate is increased by about 2orders of magnitude in solid phase.The energy dissipation of the non-radiative process from S₁ to S₀ is mainly contributed by low-frequency vibrational modes in THF,and they can be effectively suppressed in aggregation,which may lead to slow non-radiation process in solid phase.Both factors would induce enhanced luminescence efficiency of DMF-BP-DMAC in solid phase.Meanwhile,the small energy gap between S₁ and triplet excited states?T_n?results in high reverse intersystem crossing?RISC?rates in both THF and solid phase.Therefore,TADF is confirmed in both phases.Aggregation significantly influences both the ISC and RISC processes and more RISC channels are involved in solid state.The enhanced delayed fluorescence should be induced both by the enhanced fluorescent efficiency and ISC efficiency.Our calculation provides a reasonable explanation for experimental measurements and helps one to better understand the luminescence mechanism of AIDF molecules.?2?Theoretical study on the light-emitting mechanism of PTZ-AQ with TADF,AIE and MCL properties.Based on three different crystal structures,we built the ONIOM models and studied the properties of excited states with different aggregation states using QM/MM method.The influence of the stacking modes on the luminescence properties of the PTZ-AQ is studied.Relative mall energy gaps between S₁ and T_n and large spin-orbit coupling?SOC?are found in all three aggregation states,thus TADF properties should be confirmed.Based on the potential energy surface?PES?scan,the stable configurations of PTZ-AQ in THF can be found and their luminescence properties were calculated.It is found that the molecule had a smaller fluorescence rate and a larger nor-radiation rate than that in aggregation,so the AIE property is confirmed.By scanning the PES in the aggregation states,the influence of the molecular configurations on both the energy and emission wavelengths was studied.It is indicated that the change of molecular configurations could lead to the change of emission colors.?3?Bicolor switching mechanism of Cz-AQ in solid phase.Several kinds of models are studied for the Cz-AQ molecule in aggregation state.Our results indicate that the emission of Cz-AQ in solid phase does not come from monomers but dimers.The emission colors have a close relationship with intermolecular positions of two molecules in dimers.There is no need of conversion of lattice types for color switching,and slight variation of relative position of molecules in dimers will induce the emission color change between red and yellow.Our theoretical study not only gives a reasonable explanation of experimental results but also provides some insights on the design of new type multifunctional light-emitting molecules.This thesis includes six chapters.The first chapter is the introduction where the development of OLEDs and the progress of light-emitting materials are introduced.In addition,some multifunctional TADF molecules are also summarized.In the second chapter,the theoretical methods used in the research,including the density functional theory?DFT?,the time-dependent density functional theory?TD-DFT?,QM/MM,as well as theoretical methods for calculating radiation and non-radiation rates,are briefly described.From the third chapter to the fifth chapter,the main research work carried out based on the theory methods mentioned above is shown in detail.In chapter 3,the AIDF mechanism of DMF-BP-DMAC is studied.In chapter 4,the light-emitting mechanism of PTZ-AQ with TADF,AIE and MCL properties would be revealed.The bicolor switching mechanism of Cz-AQ in solid phase is revealed in Chapter 5.In the last chapter,a summary of the work mentioned in this thesis and the prospect of our future work are presented.