Molecular Design Of Organic Thermally Activated Delayed Fluorescence Materials

Organic thermally activated delayed fluorescence(TADF) material has become one of the most attractive research areas of organic electroluminescent, due to its merit of achieving 100% internal quantum efficiency(IQE) without using precious metals. Compared to fluorescent and phosphorescent material, organic TADF material has the advantages of low price, high luminous efficiency, etc., and is considered as the third-generation organic electroluminescence material. In this dissertation, a series of organic molecules with TADF properties are designed. The ground and excited state geometric structure, frontier molecular orbital, absorption and emission spectra, oscillator strength and the lowest singlet-triplet excited state energy gap(ΔEST) are studied in detail based on density functional theory(DFT) to clarify the relationship between molecular structure and photophysical properties of the designed TADF molecules. Specifically, the works include following parts:Part I: Through selecting 9-H-thioxanthene-9-one-10, 10-dioxide(TXO) as the electron acceptor(A), Diphenylamine, Carbazole, 9, 9-dimethyl-9, 10-dihydro-acridine, Phenoxazine as electron donors(D) and Benzene as the bridge(Ph), sixteen thioxanthones-based TADF molecules are designed. Based on the connection between acceptor and donor, they are classified as D-A, D-Ph-A, D-A-D, and D-Ph-A-Ph-D types. The optimal Hartree-Fock exchange method is used to evaluate the influence of the donors, Ph bridge, and dual donor on the TADF properties of these molecules. The results show that the steric hindrance of the electron donor is the decisive factor determining ΔEST for D-A type molecules, the greater steric hindrance, the smaller ΔEST. The introduction of Ph bridge reduces the orbital overlap between HOMO and LUMO, but not necessarily the ΔEST, and ΔEST is also insensitive to the introduction of dual donor. Among all the molecules, there are seven molecules which possess small ΔEST and significant optimal absorption and emission oscillator strength(f VA(OHF) and f VE(OHF)), indicating that they may be used as efficient TADF materials. In particularly, the emission wavelength(λem) and ΔEST of the 1d molecule designed by this work is almost identical with that of the molecule 1b which was experimentally synthesized TADF material with efficient yellow emission. However, the f VA(OHF) and f VE(OHF) of the 1d molecule are much greater than 1b molecule(an increase of 0.0323 and 0.0168, respectively), thus 1d molecule may be a more efficient yellow TADF molecule.Part II: Eight quinoxaline-based molecules are designed based on N3, N3, N6, N6-tetraphenyl-9-(4-(quinoxalin-6-yl)phenyl)-9H-carbazole-3, 6-diamine(DACQ) through inducing electron-donating and electron-withdrawing substituents and adjusting the connection between electron donor(D) and electron acceptor(A). The TADF properties of the designed molecules are predicted by the same method to clarify the influence of substituents and the connection position of D and A. The results show that the introducing of electron-donating substituents(-OCH3,-CH3) into A and changing the connection position of D and A can not reduce the ΔEST of DACQ. However, the introduced electron-withdrawing substituents(-2F,-4F, and-CN) into A will significantly decrease ΔEST along with a red shift of λem. Among them, for molecule 7, the λem, f VA(OHF), f VE(OHF) and ΔEST are 590 nm, 0.1737, 0.0954, and 0.093 e V, respectively, indicating it may be an efficient yellow TADF molecule. For molecules 8 and 9, λem are 660 and 680 nm, respectively, ΔEST are 0.096 and 0.092 e V, respectively. Both are red TADF molecules. Compared with the molecule 8, molecule 9 posess higher f VA(OHF) and f VE(OHF)(0.2231 and 0.1061), so the molecule 9 may be a more efficient red TADF molecule.