Theoretical Research On Luminescent Property Of Through Space Charge Transfer-based Thermally Activated Delayed Fluorescence Molecules

Organic light emitting diodes（OLEDs）have become the mainstream choice for the screen of electronic product,benefiting from their advantages of self-luminous,low consumption,light weight,fast response and flexibility.Among the emission layer materials of OLED,thermally activated delayed fluorescence（TADF）has attracted much attention because of its high exciton utilization and environmental friendliness.In order to take TADF materials into practical applications,it is necessary for TADF materials to achieve full-color emission.At present,TADF molecules are facing great challenges in blue emission.Fortunately,in recent years,TADF molecules based on through space charge transfer（TSCT）have achieved high efficiency in blue,even sky-blue emission,and this attracts great attention of researchers.Simultaneously,based on wise molecular designs and device syntheses of researchers,TSCT-based TADF molecules also show unique advantages in achieving tunable emission with extremely high efficiency.More and more efficient TSCT-based TADF materials with different emission wavelength have been reported.In order to design more promising TSCT-based TADF molecules,it is necessary to figure out the advantages of TSCT for TADF molecules.Simultaneously,revealing the solid effect and substitution effect on the luminescent properties of TSCT-based TADF molecules are also highly desired.However,up to now,the theoretical researches about these issues are still very rare and lack of systematicness.Therefore,theoretical and systematical studies on the photophysical properties of TSCT-based TADF molecules are highly required.In this thesis,the solvent and solid phase environments of molecules are simulated by using the polarizable continuum model（PCM）and the combined quantum mechanics and molecular mechanics（QM/MM）method,respectively.Based on the density functional theory（DFT）and time-dependent density functional theory（TD-DFT）,we have fully optimized the electronic structure and geometric structure of the studied molecules.Based on the optimized structure,we have calculated photophysical properties of molecules,such as the root mean square displacement（RMSD）,the adiabatic energy gap(ΔE_ST)between the lowest singlet excited state（S₁）and lowest triplet excited state（T₁）,spin orbit coupling（SOC）constant,donor electron donating capability,natural transition orbital（NTO）,TSCT ratio,the transition dipole moment（TDM）vector contribution of fragments,intramolecular and intermolecular interaction,reorganization energy and excited state decay rate,which could reveal the relationships between the structures and luminescenct properties of TSCT-based TADF molecules.Our works illustrate the internal mechanism of the advantages of TSCT on TADF molecules,and theoretically study the solid effect and substitution effect on the luminescent properties of TSCT-based TADF molecules.Wise molecular design strategies we proposed could provide guidance for the development of high-efficiency TSCT-based TADF materials.The main contents and conclusions of this thesis are concluded as follows:（1）The solid effects from environment on the luminescent properties of TSCT-based TADF molecules are investigated.In this work,we theoretically study the photophysical properties of TSCT-based TADF molecule S-CNDF-S-t Cz in solvent and solid phase,and we discuss the excited state dynamics and energy consumption process for the molecule in detail.The results show that the geometric structure of the molecule in solid phase is smaller than that in solvent,but its molecular reorganization energy in solid phase is relatively large.This is because the geometric structure changes of the acceptor part for molecule in the solid phase are restricted,which may result in the reduction of the reorganization energy contributed from bond angles.At the same time,the geometric changes of the donor unit of the molecule from the chloroform to the solid phase are remarkable,resulting in the increase of the reorganization energy from the contribution of bond length and dihedral angle.In addition,we also study the different multi-channel reverse intersystem crossing（RISC）processes for the molecule in solvent and in the solid phase,respectively.Benefiting from RISC processes,TADF molecules can improve the utilization of triplet state excitons.Furthermore,based on the calculated radiative decay rate and non-radiative dacay rate,we can obtain the fluorescence efficiency of the molecule.Due to the tight molecular environment in the solid phase reducing the non-radiative consumption,the fluorescence efficiency we calculated（29.2%）for molecule in the solid phase is significantly high,which is consistent with the experimental result（31%）.At the same time,the distance between the donor and the acceptor in the solid phase is reduced by the intermolecular van der Waals interaction,which brings enhanced TSCT feature for molecule in the solid phase.Therefore,we demonstrate that TSCT could light up multi-channel RISC to improve the utilization of triplet excitons,and thus lead to high TADF efficiency.The internal mechanisms of the solid effect on TSCT-based TADF molecule,which could improve the flourescence efficiency,are also revealed.Our calculations give reasonable explanations for the previous experimental results and could promote the development of efficient TSCT-based TADF emitters.（2）The substitution effects of numbers and positions on the luminescent properties of TSCT-based TADF molecules are analyzed.Based on the molecules S-CNDF-S-t Cz,S-CNDF-D-t Cz and T-CNDF-T-t Cz,a series of TSCT-based TADF molecules with ortho-,meta-or para-substitution are designed,these twelve molecules possess different numbers and positions of substitutional groups.In this work,we analyze the photophysical properties for four molecules reported in the experiment and twelve designed molecules.We find that molecules with ortho-substitution show small geometric changes and obvious intramolecular van der Waals interaction due to their close donor-acceptor（D-A）distances,which could bring the reduction of non-radiative consumption and large TSCT ratio.Furthermore,the increased TSCT transition density facilitate these molecules to obtain increased TDM,and thus fast radiative decay.In addition,by calculating the excited state properties and SOC of sixteen molecules,we find that molecules with more substitutional groups have smallerΔE_ST.This design strategy is consistent with the design strategy of TADF dendrimer.In addition,we also highlight some special properties of TSCT-based TADF molecules,such as steric hindrance,secondary acceptor and order-controlled D-A structure.Furthermore,we sumerize the influence from the substitution of positions and numbers on photophysical properties for TSCT-based TADF molecules.Results show that,the ortho-substituted molecules possess smaller large TSCT ratio and geometric structure changes which could facilitate them to reach high fluorescence efficiency.Meanwhile,molecules with more substitutional groups show smallerΔE_ST which could benefit them to achieve fast RISC process,and then high TADF efficiency.Our research provides a theoretical perspective for the internal mechanism of the influence of substitution effect from numbers and positions on TSCT-based TADF molecules,which could guide the molecular design of efficient TSCT-based TADF molecules.（3）The substitution effect of donor-type on TSCT-based TADF molecules are rescreahed.In order to reveal the substitution effect of donor-type on the luminescent properties of TSCT-based TADF molecules,the photophysical properties of nine TSCT-based TADF molecules（including a molecule with dual configurations）are studied theoretically.Results show that molecules with carbazole-type donor possess large oscillator strength and transition dipole moment,which is benefit to obtain fast radiative decay.Molecules with phenazine-type donor possess small molecular geometric structure changes,strong electron donating capability and smallΔE_ST.These properties are conducive to obtain effective RISC process,and these molecules have the opportunity to become promising TADF molecules.In addition,we also determine the dual configurations for the molecule 2CTF（2CTF2.1 and 2CTF2.2）.2CTF2.1 has large TSCT ratio and fast fluorescence decay process,which can achieve high luminescent efficiency.While for 2CTF2.2 with a small TSCT ratio,we determine its effective RISC process,which can achieve high exciton utilization.Therefore,molecule 2CTF may be ultilized as a self-doped TADF material with significantly high TADF efficiency.Our researches provide internal perspective for experimental results,and wise molecular design strategy based on the substitution effect of donor-type for efficient TSCT-based TADF molecules are proposed.This thesis is composed by six chapters.The first chapter introduces the development of OLED,the four generations of the emission layer and the resent researching progress of TSCT-based TADF materials,which we mainly concentrate in this thesis.The second chapter introduces the theoretical methods used in this thesis,including the DFT,TD-DFT,QM/MM method,excited state decay theory,calculation methods of intramolecular and intermolecular interactions and so on.In the third chapter,the solid effect on the luminescent properties of the TSCT-based TADF molecules are studied.In the fourth chapter,the substitution effects of the numbers and positions on the luminescent properties for TSCT-based TADF are studied.In the fifth chapter,the substitution effect of donor-type on TSCT-based TADF are studied.The sixth chapter contains the key conclusions of this thesis,as well as the challenges,opportunities and the future development of this field.