Regulation Mechanism Study Of Aggregation On The Delayed Fluorescence And Phosphorescence Emission In Organic Molecules

Thermally Activated Delayed Fluorescence（TADF）material is a new type of organic light-emitting molecular material,whose exciton utilization efficiency in the organic light-emitting diode（OLED）can reach 100%,so it has attracted wide attention.In recent years,it has been found that aggregation modes have significant impact on the generation of delayed fluorescence and phosphorescence,the emission color and the light-emitting efficiency.The effect of aggregation on the delayed fluorescence and phosphorescent emission of organic molecules has been carried out in experimental work,but theoretical studies on the regulation mechanism of delayed fluorescence and phosphorescent emission by aggregation have rarely reported.To this end,based on the combined quantum mechanics and molecular mechanics（QM/MM）method,we systematically study the geometry,electronic structure,excited states and excited-state dynamics of organic molecules under different aggregation modes.The influence of aggregation modes on the generation of TADF and super long organic phosphorescent（UOP）or TADF and room temperature phosphorescent（RTP）dual emisssion were studied,and the intrinsic mechanism was explained.Meanwhile,emission spectra with several peaks are explained,which shows that impurity could contribute to emission.The competition mechanism between TADF and RTP is also revealed.This work would provide theoretical reference for one to understand the influence of aggregation modes on molecular luminescence properties.The main research contents are described as follows:（1）Regulation mechanism of aggregation on TADF and ultra-long organophosphores.This work selected the chiral molecule 5-（9H-carbazole-9-yl）-2-（1,2,3,4-tetrahydronaphthalene-1-yl）isoindoline-1,3-dione（lmNCz）with two-crystal structures（SC_p crystal and SC_b crystal）as our study subject.Based on the ONIOM（our Own N-layer Integrated molecular Orbital molecular Mechanics）model,the energy difference(ΔE_S1T1)of the first signlet excited state and first triple excited state as well as the spin-orbit coupling constant（SOC）of（R）-lmNCz-SC_b were calculated based on QM/MM calculation.Large SOC and smallΔE_S1T1 were found to be responsible for the production of TADF in SC_p crystals.Dimer was found to have a similar phosphorescent emission wavelength with the（R）-lmNCz-SC_p monomer.The Complex composed of（R）-lmNCz and（R）-lmNBd（impurity molecule）contributes to generate phosphorescent emission peaks at about600nm,while the phosphorescent emission peak at about 650nm is generated by（R）-lmNBd.This suggests that the impurity can also contribute to the emission in the molecular crystal.The study clarified why SC_p crystals have TADF and multiple emission peaks of UOP,which helps one to understand the experimental results as well as the novel luminescent molecule design.（2）Regulation mechanism of aggregation on TADF and RTP dual emission.This work is based on the study of 2CZBZL with five crystal structures.It is found that crst-A,crst-C,crst-D and crst-E have similar stacking mode,they all have small S₁-T₂ energy gap,which promotes the excitons convertion from S₁ to high-energy excited state T₂,and then realize phosphorescent emission.On the other hand,molecules in crst-B is in loose aggregation,The energy gap of S₁-T₂is large,and the existence of transition barrier makes it difficult for the excitons to convert from S₁ to T₂,which inhibits the RTP process and promotes the TADF process.Moreover,the molecules in crst-B is more planar than that in other crystals,which increases the HOMO（Highest Occupied Molecular Orbital）-LUMO（Lowest Unoccupied Molecular Orbital）overlap and achieve redshift emission.We also simulated the configuration changes of 2CZBZL molecules in the crystal environment and performed a potential energy surface scan.Finally,we predicted the crystal structure of 2CZBZL and obtained the other possible configurations.Our work elucidates the relationship between the different packing modes and the TADF and RTP dual-emission luminescence properties,This article contains five chapters.The first chapter is the introduction,where the development,structure and practical application of OLEDs are decripbed.Following that,the development process of luminescent materials is simply introduced.Finally,the luminous properties of organic molecules in different aggregation methods such as H-aggregation,J-aggregation,X-aggregation,edge stacking and face-to-face stacking are illustrated.In Chapter 2,the related theories and computational methods used during the research process in this paper are listed.In the third and fourth chapters,the regulation mechanism of aggregation modes on the delayed fluorescence and ultra-long phosphorescent as well as the delayed fluorescence and room temperature phosphorescent dual-emission is systematically studied and discussed.In Chapter five,a summary of the previous work done and an outlook are provided.