有机发光二极管（OLED）在固态照明、屏显材料等诸多领域具有广阔的应用前景，研发高效、低耗、廉价、多彩的发光材料是当前的热点领域之一。受限于发光效率和制造成本问题，人们亟需寻找新型OLED发光材料来替代传统的第一代荧光和第二代磷光材料。第三代基于热活化延迟荧光（TADF）的OLED材料，凭借其高发光效率、低成本、发光性质易调控等优点受到人们的青睐。截至目前，实验人员已经合成开发了许多性能优异的TADF材料，但其激发态性质和发光机理仍有一些不清楚、甚至争议的地方。受限于现阶段的技术水平，实验研究仍难以解析错综复杂的微观本质。计算模拟则在微观水平揭示发光机制和预测发光性能等方面发挥着不可替代的作用。本论文采用高精度电子结构理论方法，结合量子力学/分子力学（QM/MM）及辐射和非辐射速率常数等计算方法，针对几个具有TADF性质的有机和有机金属配合物分子体系，开展了系统的理论研究，解释了实验现象，揭示了微观发光机制。主要研究内容如下：

在有机TADF材料中，选择合适的电子给体和受体，并对官能团进行适当的修饰可调控其发光性能。亚芳基二酰亚胺具有缺电子的特性，可以通过引入合适的电子给体产生电荷转移（CT）态，同时可以提供刚性的π共轭主链，有利于抑制振动猝灭过程。均苯四甲酸二酰亚胺（PmDI）是其中最简单的同系物，实验人员利用PmDI和咔唑（Cz）合成了CzPhPmDI分子并于聚甲基丙烯酸甲酯（PMMA）薄膜中观察到了TADF现象，但其发光机制尚不清楚。近期，我们在分子动力学模拟的基础上构建了包含CzPhPmDI的PMMA薄膜模型，在QM/MM组合方法框架下，采用密度泛函理论（DFT）、含时密度泛函理论（TD-DFT）和多态完全活化空间二阶微扰理论（MS-CASPT2）方法，系统地研究了CzPhPmDI在PMMA薄膜环境中的TADF发光机理。几何结构分析发现PMMA薄膜的空间位阻使得CzPhPmDI分子在激发态弛豫过程中结构并未发生明显变化。由于S₁-T₂能量差较大，T₂并不参与TADF过程，在此基础上我们提出了三态（S₀，S₁和T₁）TADF机理。室温下，S₁态激子既可以发射瞬时荧光也可以通过系间窜跃（ISC）跃迁至T₁，而T₁态激子的磷光速率非常慢，不能和反向系间窜跃过程（rISC）竞争，TADF得以发生。此外，rISC的速率常数在低温时变得非常小，使得rISC通道关闭，TADF现象消逝。通过Huang-Rhys因子的分析，我们发现对ISC和rISC过程贡献大的振动模式主要来自分子的各个官能团之间的低频扭转运动。该工作合理地解释了PMMA薄膜中CzPhPmDI分子的TADF现象，为设计发光性能优异的有机TADF材料提供一些理论依据（参见第3章）。

含硼有机分子可被用于制造蓝光器件，三配位的B原子凭借最外层的空p轨道可以接受来自其它基团的电子，有利于产生CT态。实验课题组基于咔唑和二芳基硼（BM）合成了系列蓝光TADF有机分子，但其发光机制尚不清楚。近期，我们选取三个CzBM分子作为研究对象，采用DFT和TD-DFT方法结合极化连续介质模型（PCM）优化了环己烷溶液中的基态和激发态的几何结构，并使用密度泛函/多参考组态相互作用（DFT/MRCI）组合方法进行单点能量校正，系统地研究了该类化合物的发光机理。计算结果表明，分子激发前后的结构变化主要来自B-N键的伸长以及Cz和BM基团的扭转。实验上主要的吸收和发射峰归因于S₀-S₁态的激发和去激发电子跃迁，并表现出电荷转移的特征。该特征有助于减小S₁-T₁态的能量差（?E_ST）。根据计算的辐射和非辐射速率常数，室温下三种分子的ISC和荧光速率相当，而rISC速率比磷光速率大3个数量级以上，表明瞬时荧光和延迟荧光均有可能发生。随温度的降低，rISC速率急剧下降而ISC过程仍十分高效，说明rISC过程对温度更加敏感。该工作从理论角度解释了含硼有机TADF分子的发光机理，为设计合成新型的含硼蓝光TADF材料提供了一些理论参考（参见第4章）。

与有机分子不同，有机金属配合物是通过金属与不同配体通过配位键连接，在激发时金属与配体之间可以发生电荷转移过程。此外，借助过渡金属原子的参与可以加强旋-轨耦合（SOC）效应，有利于激子在单重态和三重态之间跃迁。二配位卡宾-金属-胺类（CMA）配合物凭借优异的发光性能成为有机金属配合物TADF材料的重要研究对象。我们以一种典型的CMA-Au(I)配合物为例，采用DFT和TD-DFT方法结合PCM模型研究其光物理性质。计算的吸收和发射光谱数据与实验值吻合，最低激发态同时表现出配体到配体的电荷转移（LLCT）以及配体到金属的电荷转移（LMCT）；HOMO和LUMO之间较小的空间重叠可以减小?E_ST，Au(I)配合物大小合适的SOC效应有利于TADF现象的发生。300 K时，rISC过程优先于磷光发射，导致TADF现象；但是，随着温度降低，rISC通道逐渐关闭，TADF消失。该工作不仅帮助理解CMA-Au(I)配合物的发光机理，也为设计新型CMA-TADF材料提供一些有用的机理信息（参见第5章）。

Organic light-emitting diodes (OLEDs) have broad application prospects in solid-state lighting, screen display and so on. Realizing high-efficiency, low-cost, cheap, and full-color light-emitting materials is one of the current research hotspots. Due to the limitation of luminescence efficiency and manufacturing cost, there is an urgent need to develop new OLED materials to replace the traditional first and second generation of fluorescence and phosphorescence materials. Based on thermally activated delayed fluorescence (TADF), the third-generation OLED has grown in popularity for the high luminescence efficiency, low synthesis cost, tunable luminescence properties. Until now, experimenters have reported varieties of TADF materials with excellent performance, but their excited state properties and luminescence mechanism remains elusive and even controversial. However, limited by the experimental technology, revealing the intricate microscopic nature experimentally is difficult. The computational simulation plays an irreplaceable role in exploring the microscopic mechanism and predicting luminescence properties. Herein, we employed high-level electronic structure methods, combined with the methods of quantum mechanics/molecular mechanics (QM/MM), radiative and non-radiative rates and so on, to provide explanations of experimental phenomena and reveal the luminescence mechanisms of organic compounds and organometallic complexes with TADF properties. A brief introduction will be given below:

For organic TADF materials, the luminescence properties can be tuned by selecting appropriate electron donors and acceptors and modifying the functional groups. Arylene Diimides with electron-deficient characteristic can both exhibit charge transfer (CT) process by introducing proper electron donors and provide a rigid π-conjugated main chain that contributes to suppress the vibrational quenching process. Pyromellitic diimide (PmDI) is the simplest homologue among them. Experimenters synthesized a CzPhPmDI compound by introducing PmDI and Carbazole (Cz) and observed TADF emission in poly(methyl methacrylate) (PMMA) film. However, the inherent mechanism of CzPhPmDI is still elusive. Recently, we modeled the CzPhPmDI-PMMA film based on molecular dynamics simulations, and explored the TADF mechanism by density functional theory (DFT), time-dependent density functional theory (TD-DFT) and multistate complete active space second-order perturbation theory (MS-CASPT2) methods under the quantum mechanics/molecular mechanics (QM/MM) framework. The geometry analysis indicates the insignificantly change during the excited state relaxation due to the steric hindrance introduced by PMMA film. T₂ is absent in the TADF process because of the large energy difference between S₁ and T₂. On this basis, we proposed a TADF mechanism involving three states (S₀, S₁ and T₁). At room temperature, the S₁ excitons can either emit prompt fluorescence or hop to T₁ via intersystem crossing (ISC), while the T₁ excitons exhibit a slow phosphorescence rate which losses out in competition with the reverse intersystem crossing (rISC), hence the TADF is realized. In addition, the rISC channel is gradually crashed with the temperature decreases, indicating the demise of TADF. The analysis of Huang-Rhys factors shows that the vibrational modes which contributes most to the ISC and rISC processes are mainly from the torsional motion of functional groups in low frequency region. This work offers reasonable interpretation on TADF phenomenon of CzPhPmDI in PMMA film and provides a theoretical basis for designing organic TADF materials with superior luminescence performances (Chapter 3).

Boron-based organic compounds could be used to fabricate blue emitters. Three-coordinated B atoms contribute to generate charge transfer states because the empty p orbitals in the outermost layer can accept electrons from other groups. Recently, a series of blue-emission TADF compounds based on boron dimesityl (BM) and Carbazole were reported experimentally, but the luminescence mechanism remains unclear. Taking the CzBM-1 to CzBM-3 as examples, we employed DFT and TD-DFT methods combined with polarized continuum model (PCM) to optimize the geometry configuration in cyclohexane and applyed density functional theory/multireference configuration interaction (DFT/MRCI) method to refine single-point energies of optimized structures to systematically explore the luminescence mechanism. The results show that the structural changes after excitation are mainly from the elongation of the B-N bond and the torsion between Cz and BM groups. The absorption and emission spectra are attributed to the excitation and de-excitation between S₀ and S₁, and exhibit charge transfer characteristics which benefit to minimize the energy difference (ΔE_ST) between S₁ and T₁. According to the radiative and nonradiative decay rates, the ISC and fluorescence rates at room temperature are comparable, while the rISC rates are more than 3 orders of magnitude faster than that of phosphorescence, indicating the competitive emission of prompt fluorescence and delayed fluorescence. With the temperature decreases, the rISC process gradually calms down while ISC remains efficient, demonstrating the sensitivity to temperature of rISC. This work explains the luminescence mechanism of boron-based organic TADF compounds from the perspective of theoretical calculation, which provides some theoretical reference for the further design and synthesis of high performance boron-based TADF emitters (Chapter 4).

Different from organic compounds, organometallic complexes are coordinated by metal and different ligands, and easy to achieve a charge transfer process between metal and ligands when excited. In addition, the participation of transition metal atoms strengthens the spin-orbit coupling (SOC) effect, which is conducive to the transition of excitons between singlet and triplet states. Two-coordinate carbene-metal-amides (CMA) complexes have become significant research objects of organometallic complexes of TADF materials for their excellent luminescence properties. Taking a typical CMA-Au(I) complex as an example, we employed DFT and TD-DFT methods combined with PCM to explore photophysical properties. The calculated absorption and emission spectra are in agreement with the experimental data, and the lowest excited state exhibits both ligand-to-ligand charge transfer (LLCT) and ligand-to-metal charge transfer (LMCT). Smaller spatial overlap between HOMO and LUMO can minimize the ΔE_ST, combined with proper SOC of Au(I) complex, which are beneficial to TADF emission. At 300 K, the rISC process takes precedence over the phosphorescence emission, leading to the TADF. However, the rISC channel gradually closes and the TADF disappears as the temperature decreases. Our work can help to understand the luminescence mechanism of CMA-Au(I) complex and provide useful mechanism information for the subsequent design and synthesis of new CMA-based TADF materials (Chapter 5).