Theoretical Research And Design Of Thermally Activated Delayed Fluorescence Materials Based On Small Organic Molecules

Organic light-emitting diodes（OLEDs）based on heat-activated delayed fluorescence（TADF）materials show many advantages in the field of display and lighting,such as rich materials,thinness and good luminescence effect,and are known as"third generation"electroluminescent materials.TADF materials can achieve 100%exciton utilization by reverse intersystem crossing（RISC）processes,which has received extensive attention from industry and academia.Based on first principles,first-order perturbation,Fermi’s golden rule and thermal vibration correlation function（TVCF）theory,this paper accurately simulates the luminescence performance of TADF molecules then designs and studies the luminescence process of new and efficient TADF molecules.This work theoretically explains the luminescence process of TADF and effectively guides the development of novel TADF molecules.The specific content of the research work is as follows:1.We comprehensively quantified and simulated the photophysical parameters of two TADF molecules QAD-Cz and QAD-2Cz based on the C=O/N（QAD）framework,and the calculated photophysical parameters were in good agreement with the experimental values.Based on QAD receptors,we have newly designed three new TADF molecules,QADPh-2,6-2Cz,QAD-TPA and QAD-DPTZ,with luminescent colors ranging from blue to red.Their photoluminescence quantum yield（PLQY）are as high as 90%,and reverse intersystem crossing rate(K_RISC)are greatly improved.The emission wavelength of QAD-DPTZ is 638 nm,and K_RISCis as high as 10⁷ s^-1.In addition,our study shows that QAD-DPTZ molecules also exhibit excellent luminescence performance in amorphous（aggregated state）.Our newly designed molecule broadens the emission spectrum range of QAD-based TADF molecules,and provides a new feasible idea for designing more TADF materials with high PLQY and K_RISC.2.At present,theoretical research work on doped films has rarely been reported.In this work,based on quantum mechanics/molecular mechanics（QM/MM）methods,we theoretically simulate the luminescence properties of the TADF molecule TCO-DMAC embedded in the triptene backbone and the reference group Ph CO-DNAC in thin films.We calculated that the photoluminescence quantum yield（PLQY）of TCO-DMAC was 97.6%and the reverse intersystem crossing rate(K_RISC)was 2.63×10⁶ s^-1,which were more consistent with the experimental values(92%and 1.33×10⁶ s^-1,respectively).Our study shows that embedding the triptene backbone can increase the rigidity of the molecule,thereby effectively reducing non-radiative processes.In addition,based on the triptene backbone,we introduced sulfur（S）atoms to newly design STCO-DMAC and calculate its luminescence properties in thin films.The introduction of the S atom increases the spin-orbit coupling coefficient and K_RISC of STCO-DMAC.Our work provides ideas for theoretical study of the luminescence properties of TADF materials in thin films,and at the same time provides theoretical possibilities for the development of new high-performance TADF materials.3.Most TADF molecules exhibit ggregation-caused quenching（ACQ）,which reduces luminescence performance.The emergence of TADF materials with aggregation-induced emission（AIE）properties may effectively solve this shortcoming.In this work,we theoretically investigated the luminescence properties of FDMAC-CNQ with AIE properties in toluene solvents and crystals.We calculated the photoluminescence quantum yield（PLQY）of FDMAC-CNQ in crystals and toluene are 66.4%and 0.3%,respectively.Our results show that the transition dipole moment and reverse intersystem crossing rate of the aggregated FDMAC-CNQ molecule are significantly increased,and the aggregated state limits the vibration of the molecule and effectively reduces the non-radiative process,so FDMAC-CNQ emits light in the crystal state but does not emit light in toluene,showing obvious AIE characteristics.Our work theoretically elucidates the phenomenon of AIE and can provide theoretical support for solving ACQ.