The Effect Of Molecular Interaction On The Luminescence Properties Of Thermally Activated Delayed Fluorescence Molecules

With the continuous development of electroluminescent materials,organic light-emitting diode(OLED)materials are widely used in lighting,display and other fields due to the advantages of energy saving,low cost,stable light emission,and high efficiency.The first-generation luminescent materials are traditional fluorescent materials,and their exciton utilization rate can only reach 25% according to the spin quantum theory.In order to solve the problem of low luminous efficiency,researchers have developed the second-generation phosphorescent materials,in which the spin-orbit coupling strength between singlet and triplet states can be enhanced by doping heavy metals.The internal quantum efficiency of OLEDs made of phosphorescent materials can reach 100%.Although the performance of phosphorescent materials is good,phosphorescent materials face the challenge of limited materials,high cost and environmental pollution.Thermally activated delayed fluorescence(TADF)materials are a new type of low-cost and high-efficiency organic light-emitting materials,known as the third generation of organic light-emitting materials.TADF material solves the problems of low exciton utilization rate of organic fluorescent materials,high cost and environmental pollution caused by phosphorescent materials,and has a wide range of application prospects.In recent years,TADF materials with aggregation-induced luminescence(AIE)or circularly polarized luminescence(CPL)characteristics have become a research hotspot in the field of organic luminescence due to their high practical application value.In this thesis,we have systematically studied the luminescence mechanism of several typical TADF molecular materials recently reported in experiments based on the first-principles method.The geometric structure,energy level structure,charge transfer and the luminous efficiency of the molecules in the solution environment and the solid phase environment are analyzed based on the polarized continuum model(PCM)and quantum mechanics/molecular mechanics(QM/MM)method,respectively.Our theoretical results reveals the microscopic luminescence mechanism of TADF molecular materials.The specific work is as follows.(1)Study on the effect of imidazophenididine group on the luminescence properties of TADF molecules.In the field of research and development of OLED materials,the most important and difficult subject is the research and development of efficient and stable pure blue materials.In the experiment,three blue light molecules,Ac-BIP,PXZ-BIP and PXZ-IP,were newly synthesized and reported.Among them,the external quantum efficiency of the PXZ-BIP material can reach 21%,achieving a major breakthrough in the research of blue molecular materials.In this thesis,we used the polarized continuity model(PCM)to study the luminescence properties of three molecules in toluene solution.The research results show that the relative intensity of the electron donor and electron acceptor has a great influence on the molecular luminescence mechanism;the radiation rates of the three molecules are close,but the nonradiation decay rate of the PXZ-BIP molecule is three times lower than that of the other two molecules.In addition,the reverse intersystem crossing rate of PXZ-BIP molecules is nearly 3 orders of magnitude higher than the intersystem crossing efficiency.The triplet excitons are effectively converted into singlet excitons,which promotes instantaneous fluorescence and delayed fluorescence emission.(2)Study on the aggregation-induced luminescence mechanism of chiral molecules.We have systematically studied the geometric structure,electronic structure and luminescence mechanism of the two circularly polarized enantiomer molecules(g-BNMe2-Cp and m-BNMe2-Cp)in toluene solution and solid phase.The research results show that g-BNMe2-Cp molecules have the characteristics of aggregation-induced luminescence and chiral properties,while m-BNMe2-Cp molecules have relatively high luminous efficiency.The nonradiative attenuation hindered in solid phase is the main reason why g-BNMe2-Cp molecules exhibit aggregation-induced luminescence characteristics.The luminescence mechanisms of the molecules g-BNMe2-Cp and m-BNMe2-Cp are different.The radiation rate and non-radiation rate of m-BNMe2-Cp molecules in toluene solution and solid phase environment are almost the same.For m-BNMe2-Cp molecules in solid phase,the two-step process of up-conversion followed by the reverse intersystem corssing participates in the effective conversion of triplet excitons to singlet excitons,which becomes the main factor affecting the luminous efficiency of the molecule m-BNMe2-Cp.Studies have shown that the molecular environments have an important influence on the excited state dynamics of the molecules.(3)Study on the mechanism of excited state molecular proton transfer luminescence.We selected the newly synthesized 8HPIP molecule and its derivatives as the research object,and systematically analyzed the electronic structure and charge transfer mechanism of the four molecules.By comparing the hydrogen bonds in the molecular configurations of the ground state and the excited state,combined with potential energy surface scanning,reduced density gradient and electron density isosurface analysis as well as NTO analysis,the transfer path and the occurrence probability of excited state proton transfer are revealed.When the benzene ring acts as a proton donor in the molecule,the interaction between the benzene ring proton donor and the proton acceptor carrying the methyl group can effectively promote proton transfer.For one of the 8HPIP derivative in solution,the inherent twisted structure makes the the intramolecular proton donor far away from the transfer position,and the probability of intramolecular proton transfer is small.However,in solid phase,the molecular structure changes due to environmental effects,and the distance between the proton donor and the proton acceptor in the adjacent molecule becomes closer,which induces proton transfer between molecules in an excited state,and leads to the light emission.This work has certain reference significance for the design and research of pressure induced light-emitting diodes based on excited state proton transfer.