Theoretical Study On The Energy Gap And Luminescent Mechanism Of Several Kinds Of Thermally Activated Delayed Fluorescence Molecular Materials

As the important component of organic light-emitting diodes?OLEDs?,the progress of organic light-emitting materials represents the development of OLED.Thermally activated delayed fluorescence?TADF?molecules,which break through the limit of exciton usage efficiency of traditional fluorescent materials and realize 100%exciton usage efficiency,enhance the quantum efficiency of OLEDs significantly,thus they are thought as the new-generation electroluminescent materials.The properties of TADF is that the energy gap between the first singlet excited state?S₁?and the first triplet excited state?T₁?is small.T₁ can up-convert to S₁ by reverse intersystem crossing?RISC?process with the thermal assist,and fluorescence can be found.Due to the RISC process is slower than normal fluorescence,the fluorescence generated by RISC is called as delayed fluorescence.Recently,TADF molecules have attracted great attention of researchers and industry at home and abroad,and have achieved significant development.Up to now,more than 400 TADF molecules have been reported with most composed of donors?D?and acceptors?A?.TADF molecules can be designed by effectively changing the species of D and A as well as their connection manner.Phosphine oxide based TADF molecules,the Triarylboron-based TADF molecules and U-type TADF molecules are some TADF molecules with distinctive character,and are found to have great potential application in OLEDs.However,both their species and amounts are still quite limited.It would provide important reference for one understanding the geometry-property relationship and the light-emitting phenomenon,and also help the design of the new type TADF molecules.Since S₁-T₁ energy gap is one important parameter of TADF molecules,the relationship between the geometry of Phosphine oxide based and Triarylboron-based?TAB?TADF molecules are studied.The influence of the species and amounts of donors as well as the connection manners on the photophysical properties of TADF molecules are revealed.In addition,the influence of intramolecular and intermolecular interaction on the light-emitting mechanism of U-type TADF molecules are investigated.The main contents and conclusions are as follows:?1?Relationship between geometry of twelve Phosphine oxide based molecules and the S₁-T₁ energy gap was studied using the optimal Hartree-Fork?OHF?method.The influence of the species and amounts of donors as well as the connection manners on the photophysical properties of TADF molecules are systematically studied.It is found that DMAC group can make the dihedral angle between D and A tend to 90°and a small S₁-T₁ energy gap.The energy gap values for all the twelve molecules are smaller than 0.15 eV,and indicates that they are all potential TADF molecules.Increasing the amounts of donors can effectively decrease the energy of the highest occupation molecular orbital?HOMO?and the lowest unoccupied molecular orbital?LUMO?,while it did not change the emission colors.Our results agree well with experiment.The S₁-T₁ energy gap becomes smaller with the increase of donors.Para-position is more helpful in decreasing the S₁-T₁ energy gap than the meta-position.The electron donating ability of the donors are little influenced by the species and amounts of donors.Opposite with the results of our former study,the S₁-T₁ energy gap becomes larger with the electron donating ability of donors increasing.?2?Relationship between geometry of a serial of Triarylboron-based molecules and the S₁-T₁ energy gap are studied using the Optimally“Tuned”Range-Separated Density Functional Theory.By analyzing the geometry,the electron donating ability,the transition property of excited state,the oscillator strength,absorption and emission wavelengths,we found that electron donating ability of TAB based molecule are:Cz<DPA<DMAC<PXZ.Weak donors will induce blue emission,while strong donors will result in red emission.For Mes2B based molecules,the emission wavelengths become longer with para-,meta-and ortho-positions,and the colors are close with each other with para-and meta positions.Stronger donors tend make S₁ and T₁ charge-transfer?CT?states and small S₁-T₁ energy gap.While weaker donors make the contribution of local-excitation?LE?increased and larger S₁-T₁ energy gap.However,stronger donors will also induce weak oscillator strength.In conclusion,the TAB molecules with Cz group are good candidates of TADF molecules,who possess both strong oscillator strength and large S₁-T₁ energy gap.Nevertheless,the second triplet excited states close to S₁ in energy can assist the up-conversion process and realized high exciton usage efficiency.?3?The light-emitting properties of B-oCz and B-oTC in both gas and solid phases are studied using the first-principles calculations and combined quantum mechanics and molecular mechanics?QM/MM?.It is found that the variation of intramolecular interaction has little influence on the light-emitting properties.However,the variation of donors significantly change the stacking manners of molecules in solid phase,thus different nonradiative paths are involved for two molecules in solid phase.For B-oCz,only the modes with high-frequency contribute to the reorganization energy.For B-oTC,vibration modes from both high-frequency and low-frequency have significant contribution to the reorganization energy.The intermolecular interaction of B-oTC in solid phase,the vibration with low frequencies can be effectively suppressed and thus high fluorescent efficiency can be obtained.Consequently,and aggregation induced emission enhancement phenomenon is expected for B-oTC.While the aggregation caused quenching property is predicted for B-oCz.