关键词: 蓝光, 热激活延迟荧光, 吩噻嗪, 最低单重态-最低三重态的能级差, 光物理性质, 含时密度泛函理论

Based on 10-methyl-10H-phenothiazine-5,5-dioxide unit as an acceptor and 9,10-dihydro-9,9-dimethyl-acridine (DMAC) or phenoxazine (PXZ) as donors, four intramolecular charge transfer (ICT) compounds were designed and synthesized by Buchwald-Hartwig cross coupling reactions. Their ground-state geometries were optimized with density functional theory (DFT) at B3LYP level and vertical transition energies were calculated using the optimal Hartree-Fock (HF) exchange method. Resulting compounds were characterized by fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, liquid chromatography-mass (LC-MS) spectrometry and hybrid quadrupole time-of-flight mass spectrometry (QTOF). Their photophysical properties were systematically investigated. The results show that compound 2a has maximum emission wavelength (λ_max) of 422 nm in toluene and its oxygen-free one has photoluminescence quantum yield (PLQY) of 0.367. Theλ_max and PLQY for 1a and 1b are 455 nm, 0.083 and 511 nm, 0.098, respectively. Transient photoluminescence decay curves for 5 wt% 1a and 1b doped in PMMA films indicate that their delayed fluorescence lifetimes are 5.78 and 20.00 μs, respectively. According to the time-resolved fluorescence and phosphorescence spectra of 1a and 1b doped in PMMA films at 77 K, their energy gaps (ΔE_ST) between the lowest singlet state (S₁) and the lowest triplet excited state (T₁) are determined to be 0.203 and 0.177 eV, respectively, which are consistent with the theoretical values 0.232 and 0.295 eV calculated with time-dependent density functional theory (TD-DFT). As a result, 1a and 1b are expected to be potential thermally activated delayed fluorescence (TADF) materials. The electrochemical properties of the four compounds in dichloromethane solution were determined through cyclic voltammetry using TBAPF₆ as supporting electrolyte and ferrocene as internal standard. The HOMO energy levels of 1a, 1b, 2a and 2b are estimated to be -5.369, -5.111, -5.412 and -5.075 eV, respectively. The thermal properties of the four compounds were investigated by thermal gravimetric analysis under nitrogen atmosphere at a heating rate of 10 ℃·min^-1 and all compounds show good thermal stability with decomposition temperatures above 420 ℃.

Key words: blue light, thermally activated delayed fluorescence (TADF), phenothiazine, energy gap between thelowest singlet state and thelowest triplet state, photophysical properties, time-dependent density functional theory (TD-DFT)