Theoretical Studies And Molecular Design On The Thermally Activated Delayed Fluorescence Materials In OLEDs Materials

Organic Light-Emitting Devices（OLEDs）have attracted a lot of attention because of their high efficiency,fast response,full color luminescence and low cost.The development of OLEDs materials has gone through two stages: the first generation is based on three（8-hydroxyquinoline）aluminum（Alq3）which is the representative of the fluorescent materials,theoretically the rate of exciton utilization can’t be more than 25%;The second-generation of OLEDs material is phosphorescent material among which iridium complex is the most representative,theoretically the rate of exciton utilization can reach up to 100%.However,as the lack of blue phosphorescent materials,the high cost and other reasons,it seems to be very difficult for phosphorescent materials to extensively apply and comprehensively commercialize.There are three ways to achieve high luminous efficiency and exciton utilization: The triplet-triplet annihilatio（TTA）,the thermally-activated delayed fluorescence（TADF）and the hybridized local charge transfer（HLCT）.At present,a lot of TADF materials have been synthesized in the experiment.The exciton utilization of the device has reached a very high level.However,there are few theoretical studies on the molecules of TADF at present,which makes the molecular design unfounded and thus leads to synthesis blindness.In this paper,we choose TADF molecules as the main research objects.A series of classical TADF molecules have been calculated and analyzed by means of quantum chemical calculation.The best way to calculate the TADF molecule is to be analyzed and the factors that affect the △E_S1T1 of the TADF molecules are to be summarized,which will provide the basis for the design of the highly efficient TADF molecule.The specific researches are as follows:Firstly,the classical TADF molecules are calculated by three kinds of functional methods to find out the most accurate calculation method: the first category is pure functional methods PBE and SVWN;the second is hybrid function: BLYP,B3 LYP,PBE0,BMK,BHLLYP,M06-2X and M06HF;the third is long-range correction function ωB97X.We choose the classical 6-31+G（d,p）as the basic set.TADF molecules（ACRFLCN,Spiro-CN,PXZ-TRZ,CzT,PhCzTAZ,PXZ-OXD,2PXZ-OXD,PXZ-TAZ and 2PXZ-TAZ）are selected for the high exciton utilization.The ten functional methods are used to calculate and analyze those nine TADF molecules,which include the ground state,excited state,absorption emission spectrum,transition characteristic and energy level distribution with the support of the Gaussian 09 software package.Finally,we come up with a conclusion:The hybrid function M062 X and BMK,the long-range correction function ωB97X are well reproducible for the calculation of TADF molecules.The calculation results of ωB97X are the closest to the experimental values,thus ωB97X is more suitable for the calculation of TADF molecules.Second,on the basis of the first part we continue to further analze the impact of △E_S1T1 factors.The optimal method of long-range correction function ωB97X is used,and the 6-31+G（d,p）basic set is selected to fing out the effects of △E_S1T1 factor by analyzing the changes of D-A strength,the different sites of D-A substitution and different number of substitutions of TADF molecules.It is concluded that there is a great influence on △E_S1T1 in terms of D-A strength,substitution site and substitution number.In order to design high exciton utilization,low △E_S1T1 TADF molecules we must take a full consideration of the factors that affect △E_S1T1 in the design of efficient TADF molecules.