Liu Huanling, Zhang Kai, Zou Haipei, Mu Qingfang, Wang Xiaorui, Song Yuzhi, Lin Lili, Wang Chuan-Kui, Fan Jianzhong
Shandong Province Key Laboratory of Medical Physics and Image Processing Technology, Institute of Materials and Clean Energy, School of Physics and Electronics, Shandong Normal University, Jinan 250014, China.
Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates (South China University of Technology), Guangzhou 510640, China.
Phys Chem Chem Phys. 2022 Jul 21;24(28):17140-17154. doi: 10.1039/d2cp01574a.
Orange and red thermally activated delayed fluorescence (TADF) emitters have shown promising applications in organic light emitting diodes (OLEDs) and the bio-medical field. However, both the species and amounts of orange and red molecules are far from meeting the requirement for practical applications; this is due to the lack of systematic studies on the relationship between molecular structures and luminescence properties. Herein, the excited state dynamic processes and photophysical properties of six donor-acceptor (D-A) type orange-red TADF molecules, which possess the same acceptor, are theoretically studied in toluene by using the polarizable continuum model (PCM). Based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations coupled with the thermal vibration correlation function (TVCF) method, the adiabatic singlet-triplet energy gaps, natural transition orbital properties, reorganization energies, hole and electron distributions, and the radiative and non-radiative as well as the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes are theoretically analyzed. The results indicate that remarkable geometric changes between the lowest singlet excited state (S) and the ground state (S) are mainly caused by the rotation of the donor unit for NAI-R2, NAI-R3 and NAI-DPAC, and the reorganization energy is mainly contributed by the dihedral angle. However, for NAI-DMAC, BTDMAc-NAI and BFDMAc-NAI, remarkable geometric changes are found in the acceptor unit with large contribution of reorganization energy by bond length. These variations bring different non-radiative energy consumption processes. Moreover, small energy gaps between S and the lowest triplet excited state (T) are determined for all studied molecules and an efficient RISC process is detected. Furthermore, enhanced conjugacy in the donor unit and remarkable intramolecular interactions are determined for BTDMAc-NAI and BFDMAc-NAI, which is helpful to promote the up-conversion process. Our investigations give reasonable explanations for previous experimental measurements and the relationship between basic structures and luminescence properties is revealed, which could facilitate the development of new efficient TADF emitters.
橙色和红色热激活延迟荧光（TADF）发光体在有机发光二极管（OLED）和生物医学领域已展现出广阔的应用前景。然而，橙色和红色分子的种类和数量都远远无法满足实际应用的需求；这是由于缺乏对分子结构与发光性质之间关系的系统研究。在此，利用极化连续介质模型（PCM）在甲苯中对六个具有相同受体的供体-受体（D-A）型橙红色TADF分子的激发态动力学过程和光物理性质进行了理论研究。基于密度泛函理论（DFT）和含时密度泛函理论（TD-DFT）计算，并结合热振动相关函数（TVCF）方法，对绝热单重态-三重态能隙、自然跃迁轨道性质、重组能、空穴和电子分布以及辐射和非辐射以及系间窜越（ISC）和反向系间窜越（RISC）过程进行了理论分析。结果表明，对于NAI-R2、NAI-R3和NAI-DPAC，最低单重激发态（S）和基态（S）之间显著的几何变化主要由供体单元的旋转引起，且重组能主要由二面角贡献。然而，对于NAI-DMAC、BTDMAc-NAI和BFDMAc-NAI，在受体单元中发现了显著的几何变化，且重组能主要由键长贡献。这些变化带来了不同的非辐射能量消耗过程。此外，所有研究分子的S和最低三重激发态（T）之间的能隙都较小，并检测到了有效的RISC过程。此外，对于BTDMAc-NAI和BFDMAc-NAI，确定了供体单元中增强的共轭性和显著的分子内相互作用，这有助于促进上转换过程。我们的研究为先前的实验测量提供了合理的解释，并揭示了基本结构与发光性质之间的关系，这有助于新型高效TADF发光体的开发。
