Theoretical Study On The Light-emitting Mechanism Of Thermally Activated Delayed Fluorescence Molecules In Aggregation

Recently,thermally activated delayed fluorescence?TADF?molecules have attracted great attention and have been widely applied in organic light-emitting diodes?OLEDs?.Since TADF molecules have small energy gap between the first singlet excited state?S₁?and the first triplet excited state?T₁?,triplet excitons can effectively up-convert to singlet excitons.Consequently nearly 100%exciton usage efficiency can be obtained when TADF moelcules are used as the electroluminescent materials,and the quantum efficiency of OLEDs can be significantly enhanced.Great progress has been achieved for TADF molecular materials,but their properties are still far from meeting the requirement of practical application.Theoretical study on the light-emitting mechanism of TADF molecules will help on better undertand the structure-property relationship,which would also provide reference for the design of electroluminescent molecules.Since TADF moelcules in OLEDs are usually in aggregation,theoretical study of light-emitting mechanism of TADF molecules in aggregation would help one undertand the light-emitting properties of TADF moelcules in practical devices and make clear the influecen of aggregation,which would provide useful information for the enhancement of light-emitting properties of TADF molecules.In this thesis,both the properties in ground state and excited states of TADF moelcules in aggregation are studied using the combined quantum mechanics and molecular mechanics?QM/MM?method.Based on the calculation of electronics structures and the analysis of vibrations,the decay rates for excited states of TADF molecules are calculated using the thermally vibrational correlation function method and other methods related.By comparing with the light-emitting properties of TADF molecules in solvent and analyzing the intermolecular interaction,the mechanisms of aggregation induced enhanced emission?AIEE?and aggregation caused quenching?ACQ?as well as the isomeric effect of TADF molecules in aggregation are revealed.The main contents and conclusions are as followes:?1?Mechanism of AIEE for the AI-CZ molecule:In this thesis,both the ground state and excited state of the AI-CZ moelcule are studied.It is found that the geometric change between S₀ and S₁ is quite small in aggregation due to intermolecular?-?and CH-?interactions.The energy gap between S₁ and T₁ is broadened and transition propertis of exicted states are changed in aggregation in comparison with that in solvent.The Huang-Ryhs factors and reorganization energy are both decreased in aggregation due to the suppression of the out-of-plane vibration of the donors and rotation of acceptors.The nonradiative rate is decreased and the light-emitting efficiency is enhanced.?2?Mechanism of ACQ for the TPA-QCN molecule:In this thesis,the light-emitting properties of TPA-QCN in aggregation are studied based on the QM/MM calculations.Our calculation found that the energy of S₁ in aggregation is lower and the energy gap between S₁and T₁ is decreased than that in solvent.The reverse intersystem crossing?RISC?rate is enhanced.By analyzing the Huang-Ryhs factors and reorganization energy,we found that intermolecular interaction can suppress the rotation of the donors and the conjugated bridge to some extent.However,the intermolecular H-bond interaction between the Nitrogen atom and Hydrogen atom can induce significant C-H stretching vibration and contribution to reorganization energy,thus the nonradiative rate is increased.In addition,theoretical study also found that the radiative rate is also decreased in aggregation.In conclusion,ACQ is induced by both the decreased radiative rate and the incrased nonradiative rate of TADF moelcules in aggregation.?3?Isometric effect of TADF molecules?PXZT and PXZ-TRZ?in aggregation:In this thesis,the light-emitting properties of the isomers?PXZT and PXZ-TRZ?are studied.Our calculation indicates that there are two conformations?A and B?for both molecules in solvent and conformer B is lower in energy than conformer A.Based on the calculation of excitation energy and decay rates of excited states,we predict that the RISC should be realized from T₁of conformer A to S₁ of conformer B.In addition,we found that the stacking patterns for both molecules are quite similar,and the intermolecular interaction energy values are also quite close with each other.Thus the variation in light-emitting properties of two molecules should mainly come from different intrinsic properties of two molecules.The RISC rate of PXZT is much smaller than the ISC rate,while the RISC rate for PXZ-TRZ is close to and even larger than the ISC rate,which may be the reason that PXZT shows phosphorescence at room temperature,but there is no room-temperature phosphorescence observed in PXZ-TRZ aggregation.The thesis is divided into six chapters as follows.The first chapter briefly introduces the development process,structure and working mechanism of organic light-emitting diodes,and systematically introduces the research progress and existing problems of thermally activated delayed fluorescent materials.The second chapter briefly describes the theoretical methods used in the research,including DFT,TD-DFT,QM/MM,and theoretical methods for calculating radiation and no radiation rate.Based on the abovementioned methods,our investations and corresponding results are presented in chapter three to chapter five.In the third chapter,the mechanism of AI-CZ molecular aggregation induced enhanced emission was studied.The mechanism of aggregation caused quenching by TPA-QCN molecular was studied in the fourth chapter.Besides,in the fifth chapter,we study the light-emitting properties of the isomers PXZT and PXZ-TRZ in the aggregated state.The conclusions of the whole thesis and outlook for our works are illuminated in the last chapter.