Woo Seung-Je, Kim Jang-Joo
Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea.
J Phys Chem A. 2021 Feb 11;125(5):1234-1242. doi: 10.1021/acs.jpca.0c11322. Epub 2021 Jan 31.
Reverse intersystem crossing (RISC) rate of a thermally activated delayed fluorescence (TADF) molecule is sensitive to the energy alignment of the singlet charge-transfer state (CT), triplet charge-transfer state (CT), and locally excited triplet state (LE). However, the energy distribution of the charge-transfer states originating from the conformational distribution of TADF molecules in a solid matrix inevitably generated during the preparation of a solid sample due to the rotatable donor-acceptor linkage is rarely considered. Moreover, the investigation of the energy distribution of the CT state is both theoretically and experimentally difficult due to the triplet instabilities of time-dependent density functional (TD-DFT) calculations and difficulties in phosphorescence measurements, respectively. As a result, the relationships between conformational distribution, configurations of excited state transition orbitals, and excited state energies/dynamics have not been clearly explained. In this work, we determined the energy distribution of CT states of the TADF emitter TPSA in frozen toluene at 77 K by the measurement of time-resolved spectra in the full time range (1 ns to 30 s) of emission including prompt fluorescence, TADF, CT phosphorescence, and LE phosphorescence. We obtained the energy band of CT states where CT and CT states are distributed in the range of 2.85-3.00 and 2.64-2.96 eV, respectively. We tested various global hybrid and long-range corrected functionals for the TD-DFT calculation of CT energy of TPSA and found that only the M11 functional shows consistent results without triplet instability. We performed TD-DFT with the M11* functional optimized for a robust dihedral angle scan of CT states without triplet instability and reproduced the energy band structure obtained from the experiment. Through TD-DFT and experimental investigations, it is estimated that the dihedral angles of donor-acceptor (θ) and acceptor-linker (θ) of TPSA in frozen toluene lie within the range 70° ≤ θ ≤ 90° and 0° ≤ θ ≤ 30° respectively. Our results show that the dihedral angle distribution must be considered for further investigation of the photophysics of TADF molecules and the development of stable and efficient TADF emitters.
热激活延迟荧光（TADF）分子的反向系间窜越（RISC）速率对单重态电荷转移态（CT）、三重态电荷转移态（CT）和局域激发三重态（LE）的能量排列敏感。然而，由于可旋转的供体-受体连接，在固体样品制备过程中不可避免地会产生源于TADF分子在固体基质中构象分布的电荷转移态的能量分布，这一点很少被考虑。此外，由于含时密度泛函（TD-DFT）计算的三重态不稳定性以及磷光测量的困难，CT态能量分布的研究在理论和实验上都很困难。因此，构象分布、激发态跃迁轨道构型与激发态能量/动力学之间的关系尚未得到清晰解释。在这项工作中，我们通过测量发射的全时间范围（1 ns至30 s）内的时间分辨光谱，包括即时荧光、TADF、CT磷光和LE磷光，确定了TADF发射体TPSA在77 K冷冻甲苯中的CT态能量分布。我们获得了CT态的能带，其中CT和CT态分别分布在2.85 - 3.00和2.64 - 2.96 eV范围内。我们测试了用于TPSA的CT能量TD-DFT计算的各种全局杂化和长程校正泛函，发现只有M11泛函显示出一致的结果且无三重态不稳定性。我们使用针对CT态的稳健二面角扫描进行了无三重态不稳定性优化的M11*泛函的TD-DFT计算，并重现了从实验获得的能带结构。通过TD-DFT和实验研究，估计冷冻甲苯中TPSA的供体-受体二面角（θ）和受体-连接体二面角（θ）分别在70°≤θ≤90°和0°≤θ≤30°范围内。我们的结果表明，为了进一步研究TADF分子的光物理性质以及开发稳定高效的TADF发射体，必须考虑二面角分布。
