| Organic light-emitting diodes?OLEDs?have attracted immense attention due to their broad applications in displays and solid-state lighting.In the last decades,OLEDs devices have made great progress.However,up to now,the biggest problem in the application with these devices is their low luminous efficiency.It is known that the luminescent layer is an important part of OLEDs,and the luminous efficiency is an important parameter in determining the performance of the device.Therefore,the development of efficient luminescent materials is the most effective way to improve the luminous efficiency of OLEDs.Compared with the highly-efficient phosphorescent materials,the thermally activated delayed fluorescence?TADF?materials have the ability of collecting singlet and triplet excitons through a reverse intersystem crossing process,and the maximum internal quantum efficiency?IQE?can be achieved100%theoretically.Moreover,TADF materials have been developed based on the pure organic compounds,and have become a research hotspot,due to their advantages such as low cost,easy synthesis and green environmental protection.At present,a large number of highly-efficient TADF-OLEDs devices have been reported,but there is still a lack of detailed theoretical study on their luminescent mechanism.In this paper,density functional theory?DFT?and time-dependent density functional theory?TD-DFT?were used to study the geometric structures and photophysical properties of several types of purely-organic TADF materials,and to analyze the relationship between their structures and properties,thus providing a theoretical guide for designing and developing new TADF emitters.The main research contents are as follows:1.According to one experimentally reported TADF emitter?AcDPA-2TP?,two new molecules?AcDPA-2PP and AcDPA-TPP?have been designed theoretically to probe into the effect of different acceptor units on their TADF properties.Geometries of the investigated molecules were optimized by the DFT and TD-DFT methods.The energy difference(?EST)between S1-T1 states,absorption and emission spectra were calculated,and the excited state properties were analyzed.The results show that they all have small energy differences;the emission spectra were all shown in the green range.In addition,the rates of reverse intersystem crossing(kRISC)of the three targeted molecules were calculated by the semiclassical Marcus rate expression.The present results demonstrate that the kRISC rate of AcDPA-2PP is estimated to be 5.56×106 s-1,about twice larger than that of AcDPA-2TP(2.63×106 s-1),and AcDPA-TPP is found to exhibit the largest kRISC value among the three molecules.Considering that AcDPA-2TP has been observed to be an efficient TADF emitter,our newly designed two molecules AcDPA-2PP and AcDPA-TPP are also expected to be potential green-light TADF materials.2.The development of efficient deep-blue TADF emitters is especially important for OLEDs as displays and lighting sources.Based on one experimentally reported blue-light TADF molecule DCZ-TTR,two new molecules?DCZ1-TTR and DCZ2-TTR?have been designed to investigate the effect of the change of relative position in two carbazole groups on their TADF properties.It is found that the absorption and emission spectra simulated using the BMK functional can reproduce the available experimental data very well.The fluorescence emissions of DCZ1-TTR and DCZ2-TTR are predicted to show clear blue-shifting in cyclohexane with respect to their analogue DCZ-TTR.Especially,the emission wavelength of DCZ2-TTR is estimated to be 435 nm,in the deep-blue light range.According to the Marcus rate theory,the rates of reverse intersystem crossing of DCZ1-TTR and DCZ2-TTR are 1-2 orders of magnitude larger than that of DCZ-TTR,which is more favorable for the occurrence of delayed fluorescence.This clearly that our newly designed two molecules DCZ1-TTR and DCZ2-TTR can be also expected to be potential blue-light or even deep-blue-light TADF emitters.This may be an effective strategy for realizing deep-blue emission by simply varying the relative position of two carbazole groups in TADF molecules.3.A reliable quantitative description of fluorescence efficiency is crucial for designing and developing new-type TADF emitters.In this work,we computed the conversion and decay rates of a novel TADF-active molecule,2-?9H-carbazol-9-yl?-thianthrene-5,5,10,10-tetraoxide?CZ-TTR?,involving the lowest singlet excited S1 state and triplet excited T1 state.The efficiency of total fluorescence containing prompt fluorescence and delayed fluorescence was obtained correspondingly.The present results reveal that the rate of reverse intersystem crossing from T1 to S1 is calculated to be 9.41×105 s-1,which can completely compete with the radiative and non-radiative decay rates of the S1 state(3.47×105 s-11 and1.04×105 s-1)at 300 K temperature,thus resulting in an occurrence of delayed fluorescence.Additionally,the total fluorescence efficiency is estimated to be 54.5%,which reproduces very well the photoluminescence quantum yield of 56.7%observed experimentally.It is also found that the dominant charge transfer characters in the S1 and T1 states produce a small energy difference between the two states,and consequently an efficient reverse intersystem crossing process and a high fluorescence efficiency. |
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