Impact Of Substitution Effect And Aggregation Induced Effect On Luminescence Properties Of Thermally Activated Delayed Fluorescence Molecules

Thermally activated delayed fluorescence（TADF）materials have broad application prospects in the field of organic light emitting diodes（OLEDs）due to their high exciton utilization efficiency,environmental friendliness and low cost.In order to achieve efficient delayed fluorescence emission,a common strategy is to construct highly twisted donor-acceptor（D-A）type and donor-acceptor-donor（D-A-D）type molecules with intramolecular charge transfer（ICT）characteristics.Many studies have demonstrated that the selection of the TADF molecule’s core functional group,acting as both donor and acceptor,can impact the molecular conformation,influencing photophysical properties of the TADF molecule and OLED device performance.However,the study of the effect of the donor and acceptor on the luminescence properties of TADF molecules is still not comprehensive and systematic enough.In addition,since TADF molecules are mostly applied in the form of aggregates in OLED devices,it is necessary to study the luminescence mechanism of TADF molecules in the aggregated state.Therefore,the study of the substitution effects of the donor and acceptor,as well as the aggregation induced emission（AIE）effects,is of great significance for the design of molecules with different functions and the preparation of efficient devices.This thesis investigates the effect of donor and acceptor substitution,as well as aggregation induced effects,on the luminescence properties of TADF molecules in both tetrahydrofuran solution and solid phase environments.The polarizable continuum model（PCM）method is used to simulate the liquid phase environment of tetrahydrofuran solution,while the combined quantum mechanics and molecular mechanics（QM/MM）method is used to simulate the solid phase environment of molecules in the aggregated state.By calculating the energy level structure,frontier molecular orbitals,spin orbital coupling（SOC）constants of excited states and decay rates of excited states,the effects of donor and acceptor substitution on the luminescence performance of D-A type and D-A-D type TADF molecules are analyzed systematically.The study reveals the impact of donor and acceptor substitution and AIE on the photophysical properties of the molecules.The main research content and conclusions are as follows:（1）Study on the mechanism of different substitution types and substitution positions of donors on the excited state properties of blue TADF molecule.The work uses quinoline as the acceptor and DMAC,PXZ,PTZ,DPA and Cz as the donor groups,with the donors substituted at ortho,meta and para positions.A total of 15 molecules with D-A connections are studied to examine their photophysical properties in tetrahydrofuran.The results show that on one hand the substitution effects of DPA and Cz provide a weak electron donating abilities,which is beneficial for improving the luminescence performance of blue TADF molecules.On the other hand,for different substitution positions,molecules with meta substitution have a larger?E_ST,which is unfavorable for the design of blue TADF molecules.However,the large SOC constants between the excited states break the barrier of large?E_ST,promoting intersystem crossing and reverse intersystem crossing processes.The computational results show that o-DPA-QL,m-DPA-QL,p-DPA-QL,o-Cz-QL and m-Cz-QL are five efficient D-A type blue TADF molecules.This work provides a rational explanation for experimental results and provides a design strategy for the development of efficient blue TADF molecules with D-A type connections.（2）Investigations into the mechanisms underlying the effect of distinct acceptor substituents on the luminescent characteristics of AIE-TADF molecules.In this work,the latest synthesized TADF molecules（3CPy M-PXZ and CBM-PXZ）with AIE properties by Professor Qi Zhengjian are selected as research objects,and their photophysical properties are investigated in both tetrahydrofuran solution and solid phase environments.We find that both molecules belong to the D1-A-D2 type molecule,with larger bond lengths L2 and dihedral anglesθ4,effectively separating the distribution of highest occupied molecular orbital（HOMO）and lowest unoccupied molecular orbital（LUMO）and obtaining a smaller energy gap.Both molecules have a twisted molecular configuration and significant intermolecular C-H···πinteractions,so the motion of the molecules is restricted in the solid phase,reducing the Huang-Rhys factor and recombination energy,hindering the decay process of non-radiative energy,and revealing the mechanism of AIE.In addition,the acceptor group（benzoylpyridine）of 3CPy M-PXZ has strong electron-withdrawing ability,resulting in the redshift of 3CPy M-PXZ’s emission(λ_em=536.67 nm)relative to CBM-PXZ(λ_em=520.74 nm).Furthermore,compared with CBM-PXZ,3CPy M-PXZ has larger K_r,K_ISC and K_RISC,and smaller K_nr in the aggregated state.Therefore,the luminescent properties of 3CPy M-PXZ are more prominent than CBM-PXZ in the aggregated state.This study theoretically elucidates the mechanisms of AIE and TADF,provides a reasonable explanation for the experimental results and contributes to the development of new efficient TADF emitters.