Zhang Kai, Fan Jianzhong, Wang Chuan-Kui, Lin Lili
Shandong Province Key Laboratory of Medical Physics and Image Processing Technology, School of Physics and Electronics, Shandong Normal University, 250014 Jinan, China.
Phys Chem Chem Phys. 2021 Oct 6;23(38):21883-21892. doi: 10.1039/d1cp03144a.
Modulating the relationship between molecular structures and luminescence properties as well as charge transfer properties of deep-red thermally activated delayed fluorescence (TADF) emitters has always been a great challenge, especially in the solid state. In this work, the light-emitting properties of a T-shaped molecule (TPA-DPPZ) with donors at the -position are theoretically investigated in the solid state with the combined quantum mechanics/molecular mechanics (QM/MM) method and the thermal vibration correlation function (TVCF) theory. In comparison with a Y-shaped molecule (TPA-DPPZ with donors at the -position), TPA-DPPZ acquires a reduced HOMO-LUMO energy gap and red-shifted emission. In addition, it is found that the transition dipole moment is enhanced and the radiative rate is increased. The stacking pattern of TPA-DPPZ can effectively suppress the out-of-plane wagging vibration of donors, leading to the reduction of reorganization energy and inhibiting the loss of non-radiative energy in the excited state compared with TPA-DPPZ. Besides, a larger spin-orbit coupling constant and a smaller energy gap between T and S (Δ = 0.1 eV) are found in TPA-DPPZ, and thus a superior TADF emission is obtained. Moreover, the charge transport properties are studied using kinetic Monte Carlo simulations. The calculated mobilities for the electrons and holes of TPA-DPPZ are all larger than those of TPA-DPPZ, which is due to close packing modes in the TPA-DPPZ crystal. Balanced charge transport properties are found, which is helpful for generation of excitons and light emission. The calculation results shed light on the relationship between the molecular structures and light-emitting properties of TADF emitters, which would be helpful for developing efficient non-doped deep-red TADF devices by using T-shaped molecular design.
调控深红色热激活延迟荧光（TADF）发光体的分子结构与发光特性以及电荷转移特性之间的关系一直是一项巨大的挑战，尤其是在固态情况下。在这项工作中，利用量子力学/分子力学（QM/MM）方法和热振动相关函数（TVCF）理论，对一种在对位带有供体的T形分子（TPA-DPPZ）在固态下的发光特性进行了理论研究。与一种在间位带有供体的Y形分子（TPA-DPPZ）相比，TPA-DPPZ的最高占据分子轨道（HOMO）与最低未占据分子轨道（LUMO）的能隙减小，发射峰发生红移。此外，发现其跃迁偶极矩增强，辐射速率增加。与TPA-DPPZ相比，TPA-DPPZ的堆积模式能有效抑制供体的面外摇摆振动，导致重组能降低，并抑制激发态下非辐射能量的损失。此外，在TPA-DPPZ中发现了更大的自旋-轨道耦合常数以及T态和S态之间更小的能隙（Δ = 0.1 eV），因此获得了优异的TADF发射。此外，使用动力学蒙特卡罗模拟研究了电荷传输特性。计算得出TPA-DPPZ的电子和空穴迁移率均大于TPA-DPPZ的迁移率，这是由于TPA-DPPZ晶体中的紧密堆积模式。发现了平衡的电荷传输特性，这有助于激子的产生和发光。计算结果揭示了TADF发光体的分子结构与发光特性之间的关系，这将有助于通过T形分子设计开发高效的非掺杂深红色TADF器件。
