Theoretical Study On Luminescence Mechanism Of Several Types Of Thermally Activated Delayed Fluorescence Materials

As a new generation of display technology,organic light-emitting diodes（OLEDs）devices have received extensive attention because of their unique properties.The development and application of luminescent materials have always been an important direction in OLEDs research.In recent years,thermally activated delayed fluorescence（TADF）materials can significantly improve the luminous efficiency of the device by breaking through the limitations of the classical statistical law of excitons,which has become a hot topic in the research of OLEDs luminescent materials.In this paper,the luminescence mechanisms of several types of TADF materials were studied by theoretical calculations.Density functional theory（DFT）plays an important role in predicting the ground state properties of TADF materials and their properties of excited states were calculated using time-dependent density functional theory（TD-DFT）.In addition,the structure-property relationship and the luminescence mechanism of TADF materials were studied,and quantitative methods were used to predict their reverse intersystem crossing rates.Based on this,several novel TADF molecules were designed,and their TADF properties and luminescence mechanism were studied,which provided a theoretical basis for the synthesis of novel and efficient TADF materials.The main work includes the following three aspects:1.The purpose is to gain insight into the effects of different substituents on the luminescence properties of TADF molecules.Based upon two thermally activated delayed fluorescence（TADF）material molecules2,9-di（9H-carbazol-9-yl）benzo[d]benzo[4,5]imidazo[2,1-b]thiazole（A₁）and3,9-di（9H-carbazol-9-yl）benzo-[d]benzo[4,5]imidazo[2,1-b]thiazole（A₂）,compounds A₃-A₆ have been by replacing the carbazol group with the bis（4-biphenyl）amine one（A₃ and A₄）and introducing the electron-withdrawing-CF₃ group into the acceptor unit of A₃ and A₄（A₅ and A₆）.According to the calculation results of B3LYP/6-31G*method,there is a relatively large energy differences（approximate0.5 eV）between the S₁ and T₁ states(ΔE_ST)for the six targeted compounds.Using the TD-DFT method to study the excited state properties of the research object shows that the fluorescence rates of all six compounds are very large,in 10⁷-10⁸ orders of magnitude.In order to explain the highly-efficient TADF behavior observed in compounds A₁ and A₂,the“triplet reservoir”mechanism has been proposed.From the calculated data ofΔE_ST and fluorescence rate,it can be seen that the position of the substituent had little effect onΔE_ST and the fluorescence rate.According to the present calculations,it is a reasonable assumption that the newly designed compounds A₃-A₆ could be considered as the potential TADF emitters.2.It is very important to accurately predict the reverse intersystem crossing rate(k_RISC)for the development of new high-efficiency thermal activation delay fluorescence（TADF）materials.In this contribution,the k_RISC rates from the T₁ to S₁ states were evaluated for five D-π-A-type pyrimidine-based TADF emitters by using the semiclassical Marcus theory expression.Both the singlet-triplet energy differences(?E_ST)and spin-orbit coupling（V）between the S₁ and T₁ states were investigated by performing the density functional theory（DFT）and its time-dependent version（TD-DFT）calculations.In addition,their fluorescence emission wavelengths(λ_fluo)were also calculated at the TD-DFT level.The results show that:（1）The predicted k_RISC rates andλ_fluo values are found to reproduce well the available experimental findings.（2）The k_RISC rates of molecules possessing the unsymmetrical pyrimidine core are calculated to be slightly larger than those of the symmetrical pyrimidine analogues.（3）Introducing two tert-butyl groups into the donor moiety of the symmetric pyrimidine acceptor is also an effective method for increasing k_RISCISC when designing TADF emitters.（4）A more remarkable charge-transfer（CT）contribution to the T₁ state can achieve a smaller?E_ST,leading to a more efficient RISC process,and consequently a shorter delayed fluorescence lifetime as observed experimentally.3.To explore the influence ofπ-conjugated expansion in electron-donor（D）units on the luminescence ofTADFmaterials.Basedononeexperimentallyreportedmolecule[4-[9′H-（9,3′:6′,9″-Tercarbazol）-9′-yl]phenyl]-（pyridin-4-yl）methanone（BPy-p3C）,we strategically designed five new TADF molecules C₁-C₅,bearing an isoindole-dione core as the electron-acceptor（A）unit and dinitrocarbazolyl,carbazolyl,diphenylcarbazolyl,dicarbazolyl and tercarbazolyl groups as the D units,respectively.A main motivation is to make clear the influence ofπ-conjugated expansion in electron-donor（D）units on the luminescence of TADF molecules.The V andΔE_ST,between the S₁ and T₁states were calculated by density functional theory（DFT）and time-dependent DFT（TD-DFT）methods,and the k_RISC rates were estimated by using the semiclassical Marcus theory expression.The results indicate that on going from C₁ to C₅,the?E_ST value gradually decreases,and the k_RISC rate gradually increases.The k_RISC rates of C₄ and C₅ are calculated to be in 10⁵-10⁶ range,and especially,C₅ is found to exhibit the largest k_RISC value among five molecules considered in this work,as high as 1.22×10⁶ s^-1.According to the calculations,it is therefore expected that our theoretically designed molecules C₄ and C₅ may have the potential application as efficient TADF emitters.