Theoretical Design And Luminescent Properties Of D-A-D Type Based Thermal Activation Delayed Fluorescence Molecules

As the main component in organic light-emitting diodes(OLED),the light-emitting properties of organic light-emitting materials play an important role for the performance of OLEDs.The first generation of light-emitting materials is the traditional fluorescent material.Since the traditional fluorescent materials use singlet excitons,the quantum efficiency of such materials can not exceed 25% according to the quantum statistics law that single excitons only accounts for 25%.The second-generation light-emitting material is phosphorescent material,in which the spin-orbit coupling between the singlet and triplet excitons are enhanced by adding heavy mental atoms,and the singlet excitons can convert to the triplet excitons through the intersystem crossing process.Both singlet excitons and triplet excitons are used to radiation,and the internal quantum efficiency can reach up to 100%.However,heavy metals are quite expensive and easy to pollute the environment,phosphorescent materials are still not the ideal light-emitting materials.Recently,the third-generation OLED light-emitting materials,which are based on the reverse intersystem crossing(RISC)mechanism are proposed.Triplet excitons can up-convert to the singlet excitons at high temperature when the energy gaps between them are narrow enough,and they are called as thermally activated delayed fluorescence(TADF)molecules.The cost of such materials is low,and the internal quantum efficiency can also reach up to 100%.For most of TADF molecules,they are composed of donors(D)and acceptors(A).In this thesis,the factors that influence the energy gap between the first singlet excited state(S1)and the first triplet excited(T1)as well as the light-emitting efficiency of D-A-D type based TADF molecules were studied.The main contents are as follows.(1)The factors that affect the S1-T1 energy gap of TADF molecules are studied.The phenyl(pyridin-4-yl)methanone was chosen as the acceptor group.It is combined with the donors previously reported to form seven molecules.Molecules composed of different donors and acceptors are theoretically designed as potential TADF emitters,and their S1-T1 energy gap was studied using the optimal Hartree-Fork method.It was found that the S1-T1 energy gap is in reverse proportional to the electron-donating ability.Stronger electron-donating ability of donors will induce smaller highest occupied molecular orbital(HOMO)-lowest unoccupied molecular orbital(LUMO)overlap and also a smaller S-T energy gap.Based on our calculation results,three molecules are proposed to have great potential to be used as TADF emitters in OLEDs.(2)Influence of intramolecular interaction on the luminescent properties of TADF molecule named DCBPY are theoretically studied by using the density functional theory(DFT)and time-dependent density functional theory(TD-DFT).Four conformations(named as A,B,C,and D)of the molecules can be found by relax scanning.It is found that there significant intramolecular interaction in conformer C and D,which is quite different from conformer A and B.The energy of conformer C and D is lower than that of A and B.The molecular orbital,the energy structures and the photophysical properties of conformer A and B are similar with each other.All these properties of conformer C and D are also quite similar with each other.The nonradiative rates for conformer A and B are larger than conformer C and D,which indicates that molecules with intramolecular interaction between D and A could have small nonradiative rate.Based on the analysis of the energy structures and the excited-state dynamics of four conformers,we conclude that the prompt fluorescence is mainly contributed by the conformer C and D.The mechanism of the delayed fluorescence is thought as two components.One is by the two-step process which contributed by the up-conversion of lower triplet states to higher triplets and the intersystem process from higher triplet excited states to S1 of conformer C and D.The other one is thought as the synergistic effect of conformer A and B which act as exciton collectors.Then conformer A and B are converted to conformer C and D in S1,and the delayed fluorescence is observed.(3)The effect of intermolecular interaction on the luminescent properties of TADF molecules was studied.Since TADF molecules in OLEDs are in aggregation states,intermolecular interaction may play important role in their light-emitting properties.The light-emitting properties of AI-Cz molecule in solid state is studied using combined quantum mechanics and molecular mechanics(QM/MM).The photophysical properties of the AI-Cz molecule in solvent and in solid state are compared.It is found that aggregation may induce the change of the energy levels and the transition properties of excited states.In solid state,the intersystem crossing channels are decreased in comparing with that in solvent.The transition property of the S1 state changes from a CT state to a hybrid local excited and charge transfer(HLCT)state,which also induces the increase of the radiative rate.The nonradiative rate can also be suppressed in solid state.Both of them can induce emission enhancement.