Theoretical Study On The Structure And Excited State Properties Of Typical TADF Molecules 3DPyM-pDTC/2DPyM-mDTC

Organic light-emitting diodes（OLEDs）have become a hot spot for scientific research and commercialization because of their abundant materials,low cost,low energy consumption,flexible and large screen displays,and other advantages,widely used in the display and solid-state lighting.The upper limit of the electroluminescence internal quantum efficiency of early organic light-emitting materials was 25%.The emergence of phosphorescent materials broke through this limit,but the doping of noble metal materials increased the fabrication cost of devices and brought environmental pollution problems.In 2012,Adachi designed a new pure organic molecule with a single-triplet energy difference of less than 0.1 e V,whose triplet excitons can overcome the energy difference by absorbing ambient heat.This molecule is known as thermally activated delayed fluorescence（TADF）,and its internal quantum efficiency is theoretically up to 100%,which has attracted the interest of many researchers at home and abroad.In this work,two typical TADF molecules,3DPy M-p DTC and 2DPyM-mDTC,are theoretically investigated.Their structures,absorption and emission spectra,TADF processes and mechanisms are analyzed and discussed based on density functional theory（DFT）and time-dependent density functional theory（TDDFT）.The paper first compares the effects of exchange-correlated functionals containing different Hartree-Fock exchange components on the calculated results of the geometric conformation of the ground and excited states and the electronic structure of the excited states of the molecule.Furthermore,theoretical calculations of the absorption and fluorescence spectra are carried out using different solvent models.Finally,the luminescence process of the molecule in the solid-state is simulated.The main topics are as follows:1.Using the DFT/TDDFT method,the structures of the ground and excited states of the3DPy M-p DTC molecule were optimized using different basis sets and functionals.The electronic structures of the excited states were analyzed by natural transition orbital（NTO）,and it was found that the properties of the excited states of the molecule were closely related to the selected basis sets and functionals.The combination of the PBE0 functional and the 6-311G*can accurately describe the charge-transfer properties of the molecule and provide insight into the selection of functionals and basis sets for the study of TADF molecules.2.For the 3DPy M-p DTC molecule,the apparent red-shift of the emission spectrum with increasing solvent polarity cannot be described correctly by using an implicit solvent model.Only by employing an explicit solvent model,theoretical calculations can reproduce the red-shift of the absorption and fluorescence emission spectra of the molecule in polar solvents.Based on the crystal structure of 3DPy M-p DTC,the emission spectrum of 3DPy M-p DTC in thin films was calculated by combining quantum mechanics/molecular mechanics（QM/MM）and the theoretical emission wavelength of 463 nm agrees well with the experimental result of464 nm.3.The analysis of the S₁ and T₁ states of the 3DPy M-p DTC molecule under solid-phase conditions reveals that the T₁ state is dominated by local excitation.The difference with the S₁excitation component leads to a large spin-orbit coupling of the S₁-T₁ of this molecule,which facilitates the TADF process.Based on the Marcus theory and Einstein’s radiation rate equation,the rates of radiation,intersystem crossing,and reverse intersystem crossing of this molecule in thin films were calculated,and the theoretical results are consistent with the experimental data.4.For the flexible 2DPyM-mDTC molecule,the ten lowest energy isomers were selected by global molecular conformational search.The effect of molecular conformation on excitation energy and electron distribution was investigated by using time-dependent density functional theory.The energy difference of S₁-T₁ was calculated by the optimal HF%method,and it was found that the Model2 energy difference is more favorable to the molecular reverse intersystem crossing.