摘要/Abstract
以1，3，5-三苯酰基苯（TBP）作为电子受体，1，8-二甲基咔唑（DmCz）和1，3，6，8-四甲基咔唑（TmCz）分别作为电子给体，合成了两种新的热活化延迟荧光材料TBP-DmCz和TBP-TmCz.热重和差热测试结果表明，这两种材料都具有高热稳定性.理论计算显示，材料的最高电子占据轨道和最低电子未占轨道分别分布在咔唑和1，3，5-三苯酰基苯结构单元上，两种分子轨道几乎没有重叠，具有热活化延迟荧光材料分子轨道的典型特征.分子轨道能级测算结果显示，增加咔唑结构单元上甲基的数量，能明显升高材料的最高电子占据轨道能级.TBP-DmCz和TBP-TmCz的最低激发单重态和最低激发三重态间能级差（ΔEST）都非常小，分别为0.05和0.01 eV.在甲苯溶液中，两种材料均表现出了明显的分子内电荷转移跃迁吸收，TBP-DmCz和TBP-TmCz的发光峰分别出现在488和502 nm.用TBP-DmCz和TBP-TmCz作为掺杂客体材料制备出了两种高性能电致发光器件，器件的最大外量子效率分别达到了13.6%和18.3%.
关键词: 电致发光, 发光材料, 热活化延迟荧光, 1,3,5-三苯酰基苯, 咔唑
Two thermally activated delayed fluorescence (TADF) materials TBP-DmCz and TBP-TmCz were successfully synthesized using 1,3,5-tribenzoylbenzene (TBP) as electron-acceptor, 1,8-dimethylcarbazole (DmCz) and 1,3,6,8-tetra-methylcarbazole (TmCz) as electron-donor, respectively. Thermal gravimetric analysis show that the thermal decomposition temperatures (Td) are 479℃ for TBP-DmCz and 484℃ for TBP-TmCz and no glass transition was found for both materials during the differential scanning calorimetry investigations. The highest occupied molecular orbitals (HOMO) are confined on the carbazole unit, while the lowest unoccupied molecular orbitals (LUMO) are located on the 1,3,5-tribenzoylbenzene unit, and there is almost no overlap between HOMO and LUMO, which is the typical molecular orbital character of TADF materials. Meanwhile, TBP-DmCz and TBP-TmCz possess degenerated HOMO and LUMO, which would promote the radiative transitions as the transitions could take place from all degenerated LUMOs to HOMOs. The HOMO level of TBP-TmCz is obviously higher than that of TBP-DmCz due to increasing the number of methyl groups at the electron-donor carbazole, and the LUMO levels of TBP-DmCz and TBP-TmCz only show a small difference because these materials have the same electron-acceptor 1,3,5-tribenzoylbenzene. In toluene solution, these materials have very similar absorption spectra and exhibit absorption bands assigned to intramolecular charge-transfer transition. The spectral peaks are located at 488 nm for TBP-DmCz and 502 nm for TBP-TmCz, respectively, in toluene solution at room temperature. According to the fluorescence and phosphorescence spectra of these materials in 1,3-bis(N-carbazolyl)benzene (mCP) film at 77 K, the energy gaps between the lowest singlet and triplet (ΔEST) of TBP-DmCz and TBP-TmCz are calculated to be 0.05 and 0.01 eV, respectively. The fluorescence decay behaviors at different temperatures (100, 200 and 300 K) proved that emissions of TBP-DmCz and TBP-TmCz contain TADF component. The electroluminescence devices with TBP-DmCz and TBP-TmCz as the emitters show high efficiency and low efficiency roll-off. The maximum external quantum efficiencies of devices based on TBP-DmCz and TBP-TmCz are 13.6% and 18.3%, respectively.
Key words: electroluminescence, light-emitting material, thermally activated delayed fluorescence, 1,3,5-tribenzoylbenzene, carbazole
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