Luminescence Mechanism And Performance Study Of High Efficiencies Long Lifetimes Organic Molecular Materials

In recent years,organic light-emitting diodes(OLEDs)have had many attractive properties for lighting and display,such as being lightweight,ultra-thin and flexible.The crucial part of OLEDs is the luminous layer.The materials of the luminous layer have undergone continuous innovation and development,from the first generation of traditional fluorescent materials with low efficiency to the second generation of phosphorescent materials using heavy metals,and then to the current new organic luminous materials.The new generation of luminescent materials mainly include thermally activated delayed fluorescence(TADF)materials and organic room temperature phosphorescent(RTP)materials.Both TADF and RTP can theoretically achieve 100%exciton utilization.The difference is that TADF can achieve exciton conversion from triplet state to singlet state by reverse intersystem crossing depending on small energy difference between singlet state and triplet state;RTP promotes intersystem crossing by utilizing the strong spin-orbit coupling effect between singlet and triplet states.In this paper,the luminescence mechanism of TADF and RTP in solvents,crystals and disordered films has been studied by using the polarizable continuum model(PCM),quantum mechanics and molecular mechanics(QM/MM)and molecular dynamics(MD)methods,to reveal the molecular stacking pattern on the luminescence properties and design in theory out of the performance more excellent luminescence molecules,for the synthetic provides a new train of thought.The main research contents of this paper include the following aspects:(1)The role of intermolecular interaction on the luminescence properties of thermally activated delayed fluorescent moleculesThe intermolecular interactions play a significant role in defining the nature of the excited states and dynamics,further influence the performance of TADF.In order to explain intermolecular interactions on the luminescence properties of TADF molecules,the Independent Gradient Model(IGM)is used to visualize the intermolecular interactions and quantitatively calculated through the molecular force field energy decomposition method.Herein,the excited state dynamics of two novel TADF molecules BPPZ-PXZ and m DPBPZ-PXZ in toluene and solid phase are studied by and the combined QM/MM method respectively.The charge carrier mobilities of two compounds are investigated by Marcus equation and Monte Carlo simulations.We shed light on the relationship among molecular structures,intermolecular interactions and TADF as well as charge transfer properties.(2)The effect of solid phase environment on luminescence properties of NIR-TADF moleculesFluorescent emitters with long exciton lifetime and high luminescence efficiency show promising application in organic light emitting diodes(OLEDs)especially for these with aggregation induced emission(AIE)feature.Based on the AIE-TADF molecule NZ₂TPA reported in the experiment,we designed a new near-infrared TADF molecule--NO₂TPA with higher efficiency and longer life span.The structure of the thin film was simulated by molecular dynamics(MD)and the solid phase environment was simulated by QM/MM method.Compared with the reported NZ₂TPA,a novel near infrared TADF molecule NO₂TPA with higher efficiency and longer life span has been proposed theoretically,with improved life span and efficiency.We also calculated the luminescence properties under meteorological conditions,further proving the aggregation induction phenomenon of NZ₂TPA and NO₂TPA.This work not only explores the influence of the relative excited state properties of solid,but also provides a new idea for the exploration of efficient and long-life near-infrared(3)The effect of host and guest doping on luminescence properties of thermally activated delayed fluorescent moleculesIn order to obtain TADF molecules with both high excitons utilize ation and high luminescence efficiency,we take a new design strategy that the TADF molecule is used as a triplet energy harvesting source and the traditional efficient fluorescence molecule is used as end emitter.Thus,five new TADF-host:fluorescence-guest emitting systems are theoretically proposed.In these systems,the TADF-host does not emit light but passes all excitons to fluorescence-guest through the F?rster energy transfer processfor light-emitting.We have studied the photophysical properties and charge transfer properties of five molecules.In addition,the intersystem crossing was calculated using the Marcus equation.We know that a small S₁-T₁ gap plays a dominate role in achieving high ISC and RISC rates rather than small reorganization energy and large spin-orbit coupling coefficient.Furthermore,the charge transport types are identified for achieving efficient charge mobility and balanced charge transfer which are important in constructing OLEDs.(4)The effect of molecular packing modes on the properties of organic room temperature phosphorescenceOrganic room-temperature phosphorescence molecules have attracted much attention due to their applications in security,biology,sensing and OLED.However,the basic mechanism of RTP remains unclear.We have studied the excited state properties of CZs-CN with RTP features in three crystals by using the method of QM/MM.In addition,the photophysical properties of CZs-CN in solvents were studied by using the PCM.The results show that the aggregation induced luminescence of CZs-CN is mainly caused by the increase of the phosphorescent rate during the aggregation process,which is different from other mechanisms that reduce the radiation-free decay rate due to the restriction of the intramolecular motion mechanism,and the phosphorescent properties of CZs-CN are closely related to the crystal structure of the molecule.This work not only explains the experimental results,but also provides a new mechanism of aggregation induced RTP.(5)The effect of the length and number of alkyl chains on the phosphorescent properties of organic room temperatureIn order to obtain RTP molecules with both long lifetime and high efficiency,the photophysical properties of a series of long alkyl chain molecules have been systematically studied.The phosphorescent properties of these molecules with the absolute quantum yield(?_RTP)of 57.1%and lifetime(?)of 160 ms in solvent-free liquid is obtained were studied by the PCM and the newly proposed optimally-tuned range-separated(RS)functional method.Moreover,theoretical results show that the energy gap between the lowest triplet excited state and ground state could be reduced and the non-radiative energy consumption process is restricted through modulating the length and number of alkyl chain in organic RTP molecules.This work sheds light on the relationship among molecular structure,lifetime and efficiency,which could provide an important prototype to explore high-efficiency RTP by pure organic solvent-free liquid systems.This paper has eight chapters.The first chapter is the overview,which briefly introduces the development process,structure and working mechanism of organic light-emitting diodes,and systematically introduces the research progress and existing problems of thermally activated delayed fluorescent materials and organic room temperature phosphorescence materials.The second chapter describes the theoretical methods used in this study,including PCM regulation,QM/MM method and MD method.The third chapter to the seventh chapter is based on the above theoretical methods carried out research work.In the third chapter,the luminescence properties of AIE-TADF molecules in solvent and solid phase were studied by PCM and QM/MM methods,and the intermolecular interaction in solid phase was calculated by IGM method.In chapter four,the effect of solid-state environment on the properties of near-infrared TADF is further studied and a novel near-infrared TADF molecule with better performance is designed.In chapter five,a new design idea of TADF-host and high efficiency fluorescence-guest collocation is proposed,and five new types of high efficiency TADF molecules are obtained.In chapter six,explores RTP molecules by molecular stacking,and describes AIE-RTP phenomenon.In chapter seven,the luminescence efficiency and lifetime of RTP molecules are studied by adjusting the length and number of alkane chains.In the last chapter,we summarize the whole work of this thesis and provide a prospect on future work.