Theoretical Study On Luminescence Properties Of Novel Thermal Activated Delayed Fluorescence Molecules

In recent years,organic light-emitting diode（OLED）has been widely used in electronic products,the commercial field,transportation field,industrial control field,medical field,and so on due to its obvious advantages of lightness,ultra-thin,good flexibility,and strong flexibility.As the core part of OLED,luminous layer materials have experienced three stages:traditional fluorescent materials,phosphorescent materials,and thermal activated delayed fluorescence（TADF）materials.Among them,the third-generation OLED light-emitting materials developed in recent years,thermal activated delayed fluorescence materials,have low cost and no pollution,and their exciton utilization rate can be up to 100%in theory.Their luminous mechanism and molecular engineering design have become a research hotspot in the field of photoelectric functional materials and devices.In this paper,the luminescence mechanism of three different TADF materials is studied systematically based on the method of first principles calculation.The polarized continuum model（PCM）is used to simulate the solution environment of molecules.The properties of molecules in solid phase environment are studied by the combination of quantum mechanics and molecular mechanics（QM/MM）.（1）The luminescence mechanism of D-A green fluorescent emitters with aggregation-induced luminescence properties was studied.The excited state properties and luminescence properties of two green organic luminescent molecules PPZPPI and PPZTPI in toluene solution and solid phase environment were studied by PCM and QM/MM methods respectively.For PPZPPI,the increase of luminescence performance in a solid environment is mainly attributed to the increase of radiation rate from S₁to S₀with the increase of oscillator intensity.Because of the steric hindrance effect caused by tert-butyl substitution,the solid state environment has a greater influence on the energy level structure and excited state dynamics of molecular PPZTPI.From toluene solution to solid phase environment,the total intersystem channeling rate of molecular PPZTPI decreases by 2 orders of magnitude,while the antisystem channeling rate increases by nearly 4 orders of magnitude.The inhibition of radiation attenuation,the decrease of the intersystem channeling rate,and the increase of the antisystem channeling rate play a synergistic role in enhancing the delayed fluorescence emission efficiency of PPZTPI.Our calculations explain the experimental results reasonably,and contribute to a better understanding of the luminescence mechanism of AIE-TADF molecules.Using small organic molecular groups to modify the structure of TADF molecules can realize the regulation of its excited state dynamics and luminescence properties.（2）The effect of the position of the intramolecular group on the luminescence properties of AIE-TADF was studied without introducing the active group.The effect of carbazole substitution position on the properties of AIE-TADF was studied by using Cz Ph2AQ,Cz Ph3AQ,and Cz Ph4AQ reported recently.The results show that all three types of molecules exhibit AIE-TADF phenomenon in the solid environment,and the thermal activation delay fluorescence efficiency of Cz Ph2AQ with carbazole ortho-substitution is one order of magnitude higher than that of Cz Ph3AQ and Cz Ph4AQ,which is mainly due to the high molecular distortion caused by carbazole ortho-substitution.The result is a large steric hindrance effect between molecules,which inhibits the vibration and rotation of molecules to a large extent.The synergistic effect of lower non-radiative transition rate and higher radiative transition rate can increase the instantaneous fluorescence efficiency to 26.2%and the thermal activation delayed fluorescence efficiency to 17.4%.（3）The luminescence mechanism of TADF molecular materials with excited state intramolecular proton transfer was studied.The self-absorption effect of molecules is a major problem limiting OLED from laboratory research and development,and industrial production to market application.In this paper,imidazopyridine’s seven-member ring molecules 8HPIP-1,8HPIP-2,and 8HPIP-3 with the high spatial structure were studied,based on hydroxyl configuration before proton transfer and amino configuration after proton transfer,respectively.The mechanism of ESIPT-TADF luminescence in tetrahydrofuran solution and the solid film was studied systematically.Based on the preliminary results of potential energy surface scanning,it is proved that these three types of molecules have the characteristics of intramolecular proton transfer in excited states.The results showed that 8HPIP-1,8HPIP-2,and 8HPIP-3 exhibited the AIE-TADF phenomenon induced by the proton transfer of excited molecules,and the ESIPT-AIE-TADF phenomenon of 8HPIP-2 was the most obvious.In the follow-up work,we will further apply the method of potential energy surface scanning to determine the energy barrier that molecules need to cross in the proton transfer process,judge the difficulty of proton transfer,and then determine the specific path of the excited state proton transfer of the three types of molecules,and further reveal the ESIPT-AIE-TADF luminescence mechanism from the microscopic perspective.