Theoretical Study Of Luminescent Properties Of Organic Thermally Activated Delayed Fluorescent Materials

Thermally activated delayed fluorescence(TADF)materials have attracted much attention in science and industry due to their high stability and exciton utilization,and become the third generation of OLED materials.Under the condition of electroluminescence,TADF can break through the exciton utilization rate of 25%of spin statistics.The key lies in the up-conversion of low-energy T₁ state to high-energy S₁state,that is,reverse intersystem crossing(r ISC).As OLED devices working in solid state,designing high-efficiency TADF materials in solid state has become a research hotspot.Based on this,the purpose of this paper is to seek molecular descriptors to characterize the effective occurrence of ISC and r ISC,to clarify the luminescence mechanism of aggregation induced delayed fluorescence(AIDF)system,and to provide theoretical basis for rational design of efficient solid-state TADF materials.It is generally believed that a very small energy gap(?E_ST)between singlet and triplet is a necessary and sufficient condition for r ISC to occur.They increase the charge separation degree of donor(D)and acceptor(A)units,reduce?E_ST to a great extent,and even synthesize molecules with an energy gap of 0.However,the actual situation shows that?E_ST can't effectively distinguish whether TADF occurs or not.Therefore,we select11 D-A type systems with similar?E_ST value,and through a series of quantum chemical calculations,we find that there is a good linear relationship between intersystem crossing(ISC),r ISC rate and corresponding activation energy?G.Therefore,we propose that the activation energy?G can be used as a criterion for the occurrence of TADF.This provides effective molecular descriptors for efficient molecular design and machine learning of TADF materials.AIDF materials combine the advantages of aggregation-induced emission(AIE)and TADF,which can prevent the efficiency roll-off of TADF devices and show excellent organic luminescence properties.Based on the first principles calculation and the thermal vibration correlation function(TVCF),we systematically study the rate changing of various photophysical processes of a series of AIDF molecules from solution to solid phase.The results show that aggregation suppresses the relaxation of molecular structure in the internal conversion(IC)process,reduces the IC rate by 3?5 orders of magnitude from solution to solid,then promotes the fluorescence emission.In contrast,aggregation intensifies the structural relaxation in the ISC/r ISC process,increases the activation energy?G of the r ISC process,but also increases the spin orbit coupling(?),resulting in a small change in the ISC/r ISC rate,which can effectively occur at 10⁶⁷s^-1.Therefore,AIDF systems don't emit light in solution,but show strong TADF in solid state.More importantly,we find that the structural relaxation of C=O double bond is suppressed in the IC process from solution to solid,while the D-A torsional structural relaxation increases in the ISC/r ISC process.Thus,during the relationship between structure and properties,the luminescent properties can be controlled by changing the molecular structure in the future molecular design.