Theoretical Study And Molecular Design On The Thermally Activated Delayed Fluorescence Materials With Phenothiazine And Phenoxazine

Organic light-emitting diodes（OLEDs）have attracted much attention due to their great potential applications in the field of flat-panel displays and solid-state lighting.Compared to traditional fluorescent materials and phosphorescent materials,thermally activated delayed fluorescence（TADF）materials have been widely investigated by researchers and become one of the most ideal luminescent materials due to their advantages of high exciton utilization,wide color gamut,low cost and low pollution.The narrow gap of red TADF materials would enhance non-radiation processes,resulting to the low luminescence efficiency.Therefore,there is still a high demand for highly efficient and stable red TADF materials,which is worth further extensive research.Based on this,the TADF molecules containing phenothiazine and phenoxazine donors were investigated in this paper by the density functional theory（DFT）and time-dependent density functional theory（TD-DFT）methods,and molecular design and property prediction were carried out.Then the relationship between molecular structure and photophysical properties was systematically revealed,providing meaningful theoretical guidance and suggestions for designing highly efficient red TADF materials.The main research contents of this paper are as follows:（1）A series of D-A structured PTZ-TRZ type TADF molecules with the linkage of the electron-donating group（phenothiazine,PTZ）and electron-withdrawing groups（2,4,6-triphenyl-1,3,5-triazine,TRZ）were designed by using quantum chemical calculation method.In order to make the emission wavelength red-shift,the design strategy was adopted by introducing N atoms or cyano（-CN）groups on the phenyl ring of the TRZ acceptor to design new red TADF molecules.The geometric and electronic structures,spectroscopic and excited state properties of the designed PTZ-TRZ type TADF molecules were theoretically investigated,and the luminescent properties of the designed materials were predicted.The computational results show that the introduction of N atoms on the TRZ acceptor can effectively decrease the LUMO energy level of the molecule,reduce the HOMO-LUMO energy gap,and make the emission wavelength red-shift.The introduction of-CN unit can further reduce the LUMO energy level of the molecule,and the emission wavelength is red-shifted again to achieve pure red emission.In addition,it is found that the position of the introduced N atom has a significant influence on the energy level difference and intersystem crossing process.Furthermore,the calculated radiative and reverse intersystem crossing rates are larger than 10⁵ s^-1,indicating that the designed molecules are potentially efficient red TADF molecules.This work revealed the relationship between the molecular structure and photophysical properties of the PTZ-TRZ based TADF molecules,and provided theoretical guidance for the design of PTZ-TRZ based red TADF molecules.（2）The photophysical properties of TADF molecules the linkage of the electron-donating group（phenoxazine,PXZ）and bowl-shaped electron-withdrawing groups（phosphoryl centerπ-conjugated heterocycles（PCCH）units and their derivatives）were investigated theoretically.The design strategy of changing the bridging atoms（oxygen,sulfur,sulfone group）or the core（P=O,P=S,P=Se）of the bowl-shaped PCCH-based acceptor was used to design the new efficient TADF molecules.By analyzing the geometric and electronic structures,excited state properties,radiation and non-radiation process,and reverse intersystem crossing processes of the bowl-shaped acceptor molecules,the photophysical properties of the newly designed bowl-shaped acceptor TADF molecules were predicted theoretically.It is found that the change of bridging atoms has a great effect on the excited state properties of these molecules,and when the core is P=Se,there is a large spin-orbit coupling constant between the lowest singlet state（S₁）and the lowest triplet state（T₁）states due to the heavy atom effect of Se,which is beneficial to the reverse intersystem crossing from T₁ to S₁ states.Furthermore,the designed molecules have large reverse intersystem crossing and fluorescence radiation rates,indicating that the investigated molecules are all potential TADF molecules.This work provides feasible theoretical guidance for the application of bowl-shaped acceptors to TADF molecules.