过渡金属配合物具有丰富的光电性质，被广泛应用于液晶显示、光传感、光催化、生物成像、光动力治疗等诸多研究领域，例如，光伏器件领域和第三代基于热活化延迟荧光(TADF)的有机发光二极管领域。为了理性设计基于过渡金属配合物的高性能器件材料，揭示光电转换及发光过程的微观机制极其重要。不同于有机分子，过渡金属配合物的激发态电子结构极其复杂，不仅涉及金属中心d-d电子激发态，而且有机配体和金属原子之间存在许多不同类型的电荷转移态。更为复杂的是，这些激发态的电子结构及能级顺序深受有机配体和金属中心之间相互作用的影响。因此，过渡金属配合物的光物理研究对实验和理论都是极具挑战的前沿课题之一，其中理论与计算模拟起着重要且与实验研究互补的作用。在本文中，我们采用激发态电子结构计算方法及辐射和无辐射跃迁速率常数理论方法，对几类过渡金属配合物的激发态电子和能量转移与热活化延迟荧光展开了系统的理论研究，其主要内容如下：

以锌酞菁（ZnPc）和富勒烯（C₆₀）为电子给体和受体的有机光伏器件表现出了优异的能量转换效率，但是ZnPc和C₆₀之间电子和能量转移微观机理仍不清楚。围绕这一问题，我们在论文的第三章中系统研究了ZnPc和C₆₀给受体之间空间取向和化学键链接方式对ZnPc-C₆₀复合物激发态性质及其动力学过程的影响。(1) 我们首先研究了空间取向对ZnPc-C₆₀复合物的激发态电子转移过程的影响。基于构建的face-on和edge-on两种不同空间取向的复合结构，我们对它们进行了激发态电子结构计算和非绝热动力学模拟，发现虽然它们的吸收峰都来自ZnPc上的局域激发（LE），但不同的空间取向显著影响了ZnPc上的LE态与ZnPc和C₆₀之间的电子转移（CT）态的能级顺序，使得face-on空间取向易于发生激发态电子转移，而edge-on空间取向不利于该过程自发发生。随后动力学模拟也证实了这一观点。(2) 随后，我们继续探索了化学键链接方式对ZnPc-C₆₀复合物的激发态能量转移过程的影响。我们找到了两种键连方式不同的ZnPc-C₆₀复合物，即5-6和6-6键连构型。在前者中，ZnPc的两个氧原子与C₆₀的一个五边形和一个六边形共用的两个碳原子；但是，在后者中，ZnPc的两个氧原子与C₆₀的两个六边形环共用的两个碳原子成键。电子结构计算表明5-6构型与6-6构型的吸收峰都来自ZnPc于LE电子激发态，但5-6与6-6键连方式的不同改变了ZnPc的LE激发态与ZnPc和C₆₀之间的电子CT态之间的能级顺序反转，导致了迥异的能量转移动力学行为。非绝热动力学模拟揭示5-6构型的ZnPc-C₆₀复合物呈现了超快的激发态能量转移过程，但是该过程并未在6-6构型中出现。上述研究结果表明，无论是空间取向还是化学键链接方式都显著影响ZnPc-C₆₀复合物的激发态性质及其动力学行为。

含重原子的有机金属复合物通常是良好的磷光材料。但是，近年来实验课题组陆续发现一些含贵金属原子的有机金属复合物也具有优异的TADF发光性能，但其微观发光机制则不清楚。围绕这一问题，我们在论文的第四章中开展了相关的理论研究。(1)李及其合作者合成了含有两个吡啶咔唑配体的Pd配合物，发现了TADF与室温磷光（RTP）的双发射性质。我们采用DFT和TDDFT方法，对实验测定的荧光及磷光峰进行了归属，发现除了荧光发射的S₁态及磷光发射的T₁态以外，T₂态也参与了TADF过程。最后，结合辐射与无辐射速率常数，我们提出了S₀、S₁、T₁和T₂态参与的四态TADF模型，提出了直接和间接的上转换通道，合理解释了实验现象，并阐述了TADF和RTP双发射的微观本质。(2) 实验课题组利用双二苯基膦基苯dppBz和过卤苯基C₆Cl₅作为有机配体制备了一种Au（I）复合物，其晶体在300 K时显示TADF。我们采用集成了DFT、TDDFT和QM/MM方法的计算方案，系统研究了该复合物在气相和晶体中的激发态性质，发现气相和晶体环境中的复合物激发态结构及其相关性质具有显著差别，揭示了晶体环境效应的调控作用。此外，我们针对气相与晶相中的复合物体系分别建立了包含S₀、S₁和T₁的三态TADF模型，通过对比发现只有准确考虑了环境效应，才能合理解释实验现象。(3) Kozhevnikov课题组最近报道了第一例含Ir原子的TADF金属复合物。为了理解其微观机制，我们采用DFT和TDDFT方法优化了基态及激发态的极小能量结构，并用高精度的多参考MS-CASPT2理论方法进行了能量矫正，指认了吸收和发射峰的电子态来源。然后，我们计算了考虑Duschinsky效应的无辐射速率常数，提出了包含S₀、S₁、T₁的三态TADF模型，合理解释了实验现象。计算结果同时表明，在77 K时，该复合物可能存在TADF与磷光的双发射性质，但由于荧光与磷光峰位置重叠，导致在实验上无法分辨出两个发射峰

    Transition metal complexes have rich photoelectric properties and are widely used in liquid crystal display, light sensing, photocatalysis, biological imaging, photodynamic therapy and many other research fields, such as photovoltaic devices and the third generation of organic lightemitting diodes based on thermally activation delayed fluorescence (TADF). In order to rationally design high performance device materials based on transition metal complexes, it is very important to reveal the microscopic mechanism of photoelectric conversion and luminescence. Unlike organic molecules, the excited electronic structures of transition metal complexes are extremely complex, involving not only the d-d electron excited states in the metal center, but also many different types of charge transfer states between organic ligands and metal atoms. Further complicating matters, the electronic structure and energy level order of these excited states are greatly influenced by the interaction between the organic ligand and the metal center. Therefore, the photophical study of transition metal complexes is one of the most challenging topics for both experimental and theoretical research, in which theoretical simulation plays an important and complementary role with experimental research. In this paper, we have systematically studied the excited state electron and energy transfer and thermal activation delayed fluorescence of several transition metal complexes using the calculation method of excited state electron structure and the theoretical method of radiative and non-radiative transition rate constant. The main contents are as follows: 
    Organic photovoltaic devices using zinc phthalocyanine (ZnPc) and fullerene (C60) as electron donors and acceptors show excellent energy conversion efficiency, but the microscopic mechanism of electron and energy transfer between ZnPc and C60 is still unclear. To solve this problem, we systematically studied the effects of spatial orientation and chemical bonding mode between ZnPc and C60 donor receptor on the excited state properties and kinetics of ZnPc-C60 complex. (1) We first studied the effect of spatial orientation on the excited state electron transfer process of ZnPc-C60 complex. Based on the constructed face-on and edge-on composite structures with different spatial orientations, we carried out the excited state electronic structure calculation and non-adiabatic dynamics simulation for them, and found that although their absorption peaks were derived from the local excitation (LE) on ZnPc, however, different spatial orientations significantly affect the energy level order of the electron transfer (CT) states between LE state and ZnPc and C60, making face-on spatial orientations prone to the excited electron transfer, while edge-on spatial orientations are not conducive to the spontaneous occurrence of this process. Subsequent dynamics simulations also confirmed this view. (2) Subsequently, we continued to explore the influence of chemical bond bonding mode on the excited state energy transfer process of ZnPc-C60 complex. We found two ZnPc-C60 complexes with different bonding patterns, namely 5-6 and 6-6 bonding configurations. In the former, two oxygen atoms of ZnPc share two carbon atoms with a pentagon and a hexagon of C60. In the latter, however, the two oxygen atoms of ZnPc bond with the two carbon atoms shared by the two hexagonal rings of C60. The electron structure calculation shows that the absorption peaks of 5-6 and 6-6 configurations are derived from the electron excited state of ZnPc at LE. However, the different bond modes of 5-6 and 6-6 change the energy level order inversion between the LE excited state of ZnPc and the electron CT states between ZnPc and C60, resulting in different energy transfer dynamics. The adiabatic dynamics simulation revealed that the ZnPc-C60 complex in 5-6 configuration showed an ultra-fast excited state energy transfer process, but this process did not occur in 6-6 configuration. These results indicate that both spatial orientation and chemical bonding mode significantly affect the excited state properties and kinetic behavior of ZnPc-C60 complex. 
    Organometallic complexes containing heavy atoms are generally good phosphorescent materials. However, in recent years, some organometallic complexes containing noble metal atoms have also been found to have excellent TADF luminescence properties, but their microscopic luminescence mechanism is not clear. Around this problem, we have carried out relevant theoretical research. (1) Li et al. synthesized a Pd complex bearing two pyridyl-carbazole ligands which showed dual emission properties of TADF and room temperature phosphorescence (RTP). We used DFT and TDDFT methods to assign the fluorescence and phosphorescence peaks measured experimentally, and found that in addition to S1 state of fluorescence emission and T1 state of phosphorescence emission, T2 state also participated in TADF process. Finally, combined with radiative and non-radiative rate constants, we propose a four-state TADF model in which S0, S1, T1 and T2 states are involved. Direct and indirect up-conversion channels are proposed to explain the experimental phenomena reasonably, and the microscopic nature of TADF and RTP dual emission is explained. (2) The experimental group prepared an Au (I) complex using diphenyl-phosphonyl benzene dppBz and perhalogenated phenyl C6Cl5 as organic coordination system, and its crystal showed TADF at 300 K. We systematically studied the excited state properties of the complex in the gas phase and crystal by using DFT, TDDFT and QM/MM methods, and found that the excited state structures and related properties of the complex in the gas phase and crystal environment are significantly different, revealing the regulatory role of the environmental effects of the crystal. In addition, we established three-state TADF models including S0, S1 and T1 for the complex systems in gas phase and crystal phase respectively. Through comparison, we found that only by taking environmental effects into account accurately can the experimental phenomenon be explained reasonably. (3) Kozhevnikov's group recently reported the first case of TADF metal complex containing Ir atoms. In order to understand the microscopic mechanism, we use DFT and TDDFT methods to optimize the minimal energy structure of the ground state and excited state, and use high-precision multi-reference MS-CASPT2 theoretical method to correct the energy and identify the source of the absorption and emission peaks. Then, we calculate the radiation-free rate constant considering Duschinsky effect, and propose a threestate TADF model including S0, S1 and T1 to explain the experimental phenomenon reasonably. The results also show that TADF and phosphorescence may have dual emission properties at 77 K, but the overlap of fluorescence and phosphorescence peaks makes it impossible to distinguish the two emission peaks experimentally.