Studies On Excitation State Conversion Mechanism Of Organic Phosphorescent Materials

Organic light-emitting diodes(OLED)have many potential advantages that haveattracted intensive attention in the field of display,such as light weight,simple manufacturing process and high resolution.Universally,the important part of OLED is the light-emitting materials of the central light-emitting layer,including fluorescent materials and phosphorescent materials,which directly affect the overall performance of the devices.This article mainly explores the organic phosphorescent materials.The geometric structure,orbital type,energy difference of single and triple excited state,optical performance and excited state transition of all phosphorescent molecules are studied based on density functional theory(DFT),time-dependent density functional theory(TDDFT),quantum mechanics/molecular mechanics methods(QM/MM),and molecular dynamics(MD).This article deeply explore the molecular structures and relative performance of the central luminescent materials,and the luminescence mechanism of organic phosphorescent materials from the perspective of theory and computational chemistry.So as to provide feasible ideas and reliable theoretical basis for the synthesis and design of new phosphorescent materials in the future.The main contents and results are described as follows:1.Unveiling the relationship between the phosphorescent quantum yield and structural modification to construct high-performance Pt(II)complexObtain efficient phosphorescent materials is difficult on account of the bond dissociation of the auxiliary ligand for the cyclometalated complex and the distortion of geometric structure during T₁?S₀transition process.Therefore,it is necessary to develop a simple and convenient strategy to adjust the structural deformation of the cyclometalated complex.In this chapter,taking Pt[(pmic)(acac)]molecule as a theoretical model,eight kinds of molecules are designed through introducing electron-donating and electron-withdrawing substituent groups.The reasons for the changes in molecular configuration and whether high-efficiency phosphorescent materials can be obtained with this method are studied.The electronic structure,emission properties and non-radiative deactivation process are calculated on the basis of DFT and TDDFT,to reveal the relationship between molecular geometry and phosphorescence quantum efficiency.The results show that the introduction of electron-withdrawing groups can effectively suppress structural distortion,thereby realizing high-performance phosphorescent OLED materials.Therefore,substituent-modified metal complexes with electron-withdrawing properties can provide another strategy for designing organic phosphorescent molecules for light-emitting materials.2.How the intermolecular interactions influence the luminescence properties of pure organic room temperature phosphorescent materialsSince organic light-emitting molecules are usually in aggregation state,the differentstacked structures and interaction between molecules have an important impact on their light-emitting properties.Here,the luminescence properties and transition properties of RTP in solid phase are studied by DFT,QM/MM and molecular field decomposition principle.The results indicate that the surface stacking between CPM molecules is more orderly under solid-phase conditions.However,the stacking structure has changed of CMOPM molecules,which promotes the increase of the non-radiation inactivation process,and the non-radiation rate is about 10 times that of the CPM.At the same time,the energy gap between the single and triplet states of CPM is smaller,which is more conducive to the occurrence of the intersystem crossing process.Theoretical simulations indicate that the methoxy substitutes show different intermolecular stacking,thereby affecting the intermolecular interactions.Our work has important guiding significance for the experimental synthesis of more efficient pure organic room temperature phosphorescent materials.3.Unveiling the mechanisms of organic room temperature phosphorescence in various surrounding environments:a computational studyRTP from pure organic molecules always depends on aggregation morphology.Itremains a formidable challenge to reveal the intrinsic mechanism of morphology-dependent RTP.In this chapter,we comparatively investigate the photophysical properties of pure organic room temperature phosphorescent molecules in diverse environments(solution,crystal,and amorphous).From solution to amorphous to crystal phase,the excited-state decay rates for the molecule indicate that the AIE phenomenon of molecules is mainly induced by the increased phosphorescence rates.However,the increased nonradiative decay rate k_nr of T₁?S₀ from the solution to the crystal phase could be attributed to be the different electron coupling in the crystal phase.Meantime,the theoretical results also show that the small energy gap between the lowest singlet excited state(S₁)and triplet excited state(T₁)and low reorganization energy can help enhance intersystem crossing to facilitate to a more competitive radiative process from T₁state to ground state(S₀).Additionally,the stronger intermolecular?-?interaction can cause high phosphorescence quantum efficiency in the crystalline phase.Our study presents a rational explanation for the aggregation induced RTP,which is beneficial for the design of new organic RTP materials in the future.4.Construction of high-efficiency and long-lived room temperature phosphorescent materials with methylene-effectIt is a huge challenge to achieve high-efficiency and long-lived RTP in pure organicmolecules due to the weak spin-orbit coupling(SOC)and the rapid deactivation of triplet excitons.Therefore,it is necessary to systematically elucidate the mechanism of obtaining high efficiency and long life of room temperature phosphorescence.In this chapter,we investigate two pure organic room temperature phosphorescent materials with relatively large differences in phosphorescence quantum efficiency of donor-methylene-acceptor molecular.The results show that RTP material appended with methylene is more conducive to obtaining higher phosphorous quantum efficiency in crystalline environment.More importantly,our calculation unveil the molecular rationale behind the better quantum efficiency performance as the introduction of methylene group.The calculation results show that the introduction of methylene group in the crystalline phase can inhibit the structural deformation of molecules during the excited state transition process,and can also give them stronger interactions between molecules.In addition,the heavy atom effect in the donor-acceptor configuration is more conducive to the formation of?-X(X=Br)interactions to accelerate the occurrence of the intersystem crossing process,and to obtain a higher intersystem crossing rate.Therefore,the donor-methylene-acceptor configuration molecule is expected to improve the room temperature phosphorescence quantum efficiency,and the addition of heavy atoms is more conducive to extending the room temperature phosphorescence lifetime,which opens up a new way for the rational design of efficient and long-lifetime room temperature phosphorescent materials.