Theoretical Simulations And Design Of Efficient Organic Light Emitting Materials

Organic light emitting diodes（OLEDs） exhibit very promising applications in flat panel displays and solid-state lighting,due to their advantages of low cost,fast response,high efficiency,and light weight.Thermally activated delayed fluorescence（TADF） materials with the 100%exciton utilization,known as"third-generation"organic electroluminescent materials,have recently received increasing attention from researchers.Based on the density functional theory（DFT）and molecular dynamics simulation methods,we quantitatively analyze the photophysical process of organic light-emitting molecules,simulate and discuss the light-emitting properties of TADF molecules in the aggregated states,and provide guidance for the design of novel TADF molecules.The main research contents are as follows:（1）The complete photophysical processes and internal quantum efficiencies of two dimethylacridine-based blue-color TADF molecules（AC-OSO and ACRXTN）are systematically calculated.Theoretical calculations prove that the higher internal quantum efficiencies of these two molecules are attributed to the higher reverse intersystem crossing（RISC）rates(e.g.K_RISC=4.98×10^8-1)from T₁ to S₁ and the suppressed non-radiative decay rates(e.g.K_S^nr=1.68×10^6-1).It was found that the fluorescence nonradiative rates of S₁-S₀ and RISC rates of these two molecules are mainly affected by the twisting vibration of donor dimethylacridine（DMAC）and acceptor moiety.The theoretical calculations can well explain the high fluorescence efficiency observed in the experiments and provide theoretical support to design highly efficient TADF based OLED materials.（2）The TADF molecule with aggregation-induced emission(AIE properties(the 4,4′-(6-（9,9-dimethylacridine-10（9H）-yl）quinoline-2,3-Dibenzonitrile（DMAC-CNQ）were investigated using the coupled quantum mechanics and molecular mechanics(QM/MM method,and its photophysical properties in gas,crystal and amorphous states were analyzed.The simulation indicate that the molecular stacking of DMAC-CNQ in the aggregated states hinders the geometric torsion of the molecules,and thus suppresses the fluorescence nonradiative rates and increases the transition dipole moments（TDM）and RISC rates.Specifically,in the crystal and amorphous states,the geometric torsion of the donor DMAC moiety is restricted to reduce the fluorescence nonradiative rates,while the torsion of the acceptor dibenzonitrile（CNQ）moiety is restricted to increase the RISC rates.The calculated results are in good agreement with the experimental phenomena,and the simulation results show that the fluorescence quantum efficiencies of DMAC-CNQ molecules in gas,crystal and amorphous states are 0.01%,67%and 26%,respectively.The TADF mechanism with AIE properties is revealed through theoretical studies,which will facilitate the design of efficient TADF molecules.(3 The photophysical properties of two newly reported aggregation-induced TADF molecules:[4-（9,9-dimethyl-9,10-dihydroacridine） phenyl](pyridin-4-yl-methanone（Pyb-dmac） and[（4–9,9-diphenyl-9,10-dihydroacridine） phenyl](pyridin-4-yl-methanone(Pyb-dpac are theoretically investigated by using the multiscale methods.In the simulations,the molecular stacking in the solid phase restricts the molecular geometric torsion,which reduces the reorganization energy.These aggregation effects are beneficial to suppress the non-radiative rates from S₁ to S₀ and increase the RISC rates,hence effectively promote the TADF process.The calculated fluorescence quantum efficiency of Pyb-dmac in the crystal phase is 42%,which is in good agreement with the experimental result.Simulation studies demonstrate the luminescence mechanism of TADF molecules with aggregation-induced luminescence properties,and then quantify the experimental phenomenon accordingly.In addition,24 new TADF molecules were designed,and they are similar to two molecules(Pyb-dmac and Pyb-dpac.By analyzing the singlet-triplet energy gap(ΔE_ST),transition dipole moment（TDM） and the root mean squared displacement（RMSD）,two molecules（Pyb F-dmac and OSOF-dmac） were screened out,and their photophysical processes in toluene solvent and amorphous states were successfully simulated.The simulation results show that their luminescence quantum efficiencies in toluene solvent and amorphous states are as high as 45%,57%and 93.6%,95%,respectively.