Theoretical Study On Thermally Activated Delayed Fluorescence Properties Of Diphenyl Sulfoxide Derivatives

Fluorescent materials based on the principle of thermally activated delayed fluorescence(TADF)developed with 100%exciton utilization,20%?30%of the external quantum efficiency known as the "third generation" of organic electroluminescent materials.However,the research on the luminescence mechanism of TADF molecules is not deep enough.In this thesis,density functional theory(DFT)and time-dependent density functional theory(TDDFT)are used to compare two types of diphenyl sulfoxides with structurally similar and significantly different luminous efficiency.The TADF parameters of the molecules,in-depth exploration of the reasons for determining the luminous efficiency of TADF molecules;and on this basis,respectively,change the donor and acceptor units,design a series of new and efficient TADF molecules.Furthermore,in order to solve the problem of efficiency roll-off under high current densities of TADF molecules,we explored the effects of spin-orbit coupling on the reverse intersystem crossing(RISC)process of TADF molecules.Heavy atoms As and P were introduced,and their effects on the electronic properties of TADF molecules,spin-orbit coupling(SOC)and other TADF parameters were analyzed to provide a theoretical basis for experimental synthesis.The main research contents of the paper are as follows:1.Quantitative measures were used to calculate the parameters closely related to the luminescence properties of TADF,and the structurally similar 9,9'-(sulfonylbis(4,1 phenylene))bis(3,6-di,tert-buty1-9H-carbazole)was analyzed.(Al-D1)and 8-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)dibenzo[b,d]thiophene 5,5-dioxide(A2-D1)have significantly different luminous efficiencies s reason.The calculation results show that compared with A2-D1,although Al-D1 has a larger?DST,there is also a more dense Tn level distribution,a stronger SOC,and a lower minimum energy crossing point(MECP),making the Ti excitons of Al-D1 easier to transition to S1 in RISC mode,thereby increasing its delayed fluorescence quantum yield((?DF).In addition,the slightly larger radiation rate(kr)and the lower non-radiation rate(knr)of Al-Dl give it a higher instantaneous fluorescence photon quantum yield(?PF).Therefore,the total luminous efficiency of Al-D1 is higher,and the TADF characteristics are also more pronounced.In summary,when designing TADF molecules,the above factors should be fully considered,rather than considering ?EST and ?DF.In addition,we also designed a series of TADF molecules and characterized the relevant TADF parameters by quantitative calculations.The calculation results show that A1-D2,A2-D2,and A4-D2 have excellent performance and are comparable to high-efficiency Al-D1.It is noteworthy that A1-D3,its TADF characteristics in all aspects exceed Al-Dl,indicating that it is likely to have higher EQE than Al-D1.2.Although the external quantum efficiency(EQE)of OLED devices made of TADF molecules has reached 20%to 30%,there are still some problems that need to be solved,such as the efficiency roll-off problem under high current.In order to solve this problem,it is very important to explore the influence of spin-orbit coupling on the RISC process of TADF molecules.By introducing the heavy atoms As and P in the above-explored molecules,their effects on the electronic properties of the TADF molecules,SOC strength and other TADF parameters are analyzed,and it is expected to provide a theoretical reference for the development of new and highly efficient TADF molecules.The results show that the introduction of As and P is beneficial to enhance the molecular rigidity,reduce the relaxation loss,inhibit the non-radiative transition process,and increase the ?PF of the molecule.The introduction of heavy atoms does not affect ?EST,although the orbital overlap between HOMO and LUMO is enhanced.Still within a reasonable range;in addition,it simultaneously enhances the SOC of the T1?S1 and T1+S0 processes and the latter increases more than the former,which may imply that the introduction of heavy atoms makes it possible for molecules to be converted from TADF molecules to room temperature phosphorescence.molecular.Although the luminescence mechanism is different,this does not affect the efficient luminescence efficiency of the molecule.