Design,Synthesis And Theoretical Studies Of Thermally Activated Delayed Fluorescence Materials

Thermally activated delayed fluorescence(TADF)materials,similar to phosphorescent materials,can make use of triplet excitons,so that the internal quantum efficiency of their devices under electric excitation can break through the theoretical limit of 25%for fluorescent materials.TADF materials can be designed with all-organic molecular structures from abundant organic structure resources and get key advantage of low cost due to avoiding using noble metals,such as Ir(?),Pt(?)and Os(?).The properties of the devices based on TADF materials can be comparable with those based on phosphorescent materials.Basically,TADF materials requires an energy gap between singlet and triplet energy levels(?EST)less than 0.3 eV,preferably less than 0.1 eV.In this thesis,fourteen intramolecular charge transfer(ICT)compounds based on 10-pheny1-10H-phenothiazine-5,5-dioxide,phenoxathiine-10,10-dioxide and lOH-pheno thiazine-5,5-dioxide were calculated using Gaussian 09 package,and their properties of ground states and excited states were investigated.Eight compounds were synthesized and characterized.Among them,five compounds show salient features of TADF materials on the basis of experimental data.1.Fourteen ICT compounds based on 10-phenyl-10H-phenothiazine-5,5-dioxide and 10H-phenothiazine-5,5-dioxide were designed with DMAC or PXZ as donors.Their ground state geometries were optimized utilizing density functional theory(DFT).On the basis of the optimized ground state molecular structures,their excited state vertical excitation calculations were performed with the optimal HF exchange method based on the time depend density functional theory(TD-DFT).According to the result,most of these compounds with high triplet energy levels and high bandgaps tend to emit blue light.The maximum theoretical emission wavelengths(?max)of compound 1f?1g?1h?2c and 2d are 461 nm?442 nm?486 nm?418 nm and 462 nm,and corresponding ?EST are predicted to be 0.303 eV,0.088 eV,0.165 eV,0.230 eV and 0.295 eV,successively,indicating that they are potential blue light emitting TADF materials.2.Four compounds(1e,1f,1g and 1h)were synthesized through Buchwald-Hartwig cross-coupling reactions.Their ?max are 468 nm,508 nm,475 nm and 530 nm in toluene,and corresponding photoluminescence quantum efficiency(?f and ?d)were 0.040 and 0.088,0.022 and 0.110,0.036 and 0.090,0.029 and 0.123 before and after bubbling nitrogen in toluene,respectively.Their fluorescence lifetime(?f)ranges from 5.97 ns to 11.47 ns in toluene,and the delayed fluorescence lifetime(?d)for 1f,1g and 1h are 1.16 us,1.03 us and 0.69 us,respectively,however,le don't show delayed fluorescence lifetime.According to the time-resolved photoluminescence and phosphorescence spectra of 1e,1f,1g and 1h doped in PMMA films at 77 K,their ?EST are obtained to be 0.181 eV?0.116 eV?0.130 eV and 0.089 eV,respectively.These experimental results show that 1f?1g and 1h are potential TADF materials.HOMO levels for 1e?1f?1g and 1h are determined to be-5.34 eV?-5.13 eV?-5.37 eV and-5.14 eV by cyclic voltammetry,respectively.3.Four compounds(2c,2d,2e and 2f)were synthesized by Suzuki cross-coupling reactions and Buchwald-Hartwig cross-coupling reactions.Their photophysical properties were investigated systematically.The results show that their ?max and ?d of these compounds in toluene are 455 nm and 0.083,511 nm and 0.098,422 nm and 0.367,459 nm and 0.105,respectively.Transient photoluminescence decay curves for 5%2c and 2d doped in PMMA films demonstrate that their delayed fluorescence lifetimes are 5.78 ?s and 20.00 ?s,respectively.According to the time-resolved photoluminescence and phosphorescence spectra of 2c and 2d doped in PMMA films at 77 K,their ?EST are determined to be 0.203 eV and 0.177 eV,respectively,so both compounds are expected to be potential TADF materials.All of the four compounds show good thermal stability with decomposition temperatures between 424?466 ? according to the thermal gravimetric analysis.