Theoretical Study On Dual Emission Mechanism Of Thermally Activated Delayed Fluorescence Molecules

Thermally activated delayed fluorescence（TADF）materials show important position in the light-emitting layer materials of organic light emitting diodes（OLEDs）because of their advantages such as low cost,high exciton utilization efficiency and low pollution.In recent years,TADF materials with dual emission have developed gradually,which shows great application prospect in white organic light emitting diodes（WOLEDs）,mechanochromic luminescence（MCL）etc.However,the numbers of TADF molecules with dual emission are quite limited.One of the reasons is the lack in understanding the mechanism which would favor the understanding the structure-property relationship and favor the molecular design.In this paper,the luminescence properties of a series of TADF molecules with dual emission are studied based on first-principles calculations,and the luminescence mechanism is revealed from a theoretical perspective.The polarizable continuum model（PCM）and the combined quantum mechanics and molecular mechanics（QM/MM）method are used to simulate the molecular environment respectively.Based on the study of the geometric structures,electronic structures and the dynamics of excited state,the dual emission mechanisms are revealed.The main conclusions are as follows:（1）Intermolecular hydrogen bond induced dual emission of TADF molecules.In this work,SOBF-OMe with intermolecular hydrogen bond was selected as the main research object,and are also compared with SOBF-H without intermolecular hydrogen bond.Therein,the TADF is observed for the SOBF-OMe in crystalline state,while no TADF is observed for it in solvent and for SOBF-H in crystalline state.By calculating the emission wavelength,it is found that the wavelength for single molecule of SOBF-OMe in aggregation state is consistent with the prompt fluorescence and phosphorescence wavelength detected in experiment.It is also indicated that TADF could not be generated in single SOBF-OMe molecules.By calculating the excited state properties and decay rate of SOBF-OMe single molecule in toluene and aggregation state,it is found that intermolecular interaction could limit the motion of the group units in aggregation when they are excited and reduceΔE_ST.Nevertheless,the energy gap is still too large to realize the up-conversion,which also proves that a single SOBF-OMe molecule cannot produce TADF emission peak.Different dimers are selected for calculation in aggregation state.It is found that the wavelength of dimers with intermolecular hydrogen bond is consistent with the wavelength of TADF,while dimers without intermolecular hydrogen bond in SOBF-OMe and dimers of SOBF-H cannot obtain TADF emission.These results indicate that TADF emission was induced by intermolecular hydrogen bonds.（2）Dual emission of TADF molecules with dual conformation.CPz P and CPz PO are selected as the research objects in this work.CPz PO has dual emission peaks in benzene solution and crystal state,and shows TADF characteristics in crystal state.CPz P has dual emission peaks in benzene solution and only TADF emission peaks in crystal state.Combined with our research experience,we can find that both molecules have two stable conformations（quasi-equatorial conformation and quasi-axial conformation）,and one component white light emission can be realized by color complementarity.It is found that CPz PO prefers to exists in quasi-axial conformation in ground state.According to the potential energy surface of the ground state,it is speculated that the two conformations can coexist in the ground state,and the emission peak from the excited states of the quasi-equator conformation can be directly excited or transformed from the quasi-axial conformation in the excited state to realize dual conformational emission.CPz P has similar conformational transformation characteristics.In addition,the excitation energies and emission wavelengths of monomers and dimers with intermolecular hydrogen bonds for CPz P and CPz PO in aggregation state are calculated respectively,which denies the influence of intermolecular hydrogen bonds on dual emission.This paper consists of five chapters.The first chapter is the introduction,which discusses the development,device structure,and luminescence principle of organic light-emitting diodes.The process of luminescent layer materials was briefly described,and the mechanisms of TADF materials with dual emission are summaried.In Chapter 2,the theoretical methods used in this thesis,including Density Functional Theory（DFT）,Time-Dependent Density Functional Theory（TD-DFT）,QM/MM method,and theoretical methods for calculating radiation transition rate and non-radiation transition rate are briefly described.In Chapter 3 and 4,the intermolecular hydrogen bond induced dual emission and the dual conformation induced dual emission are theoretically proved respectively.The last chapter summarizes the work in the thesis and provides some prospects for the ongoing work.