蒽的[4+4]光二聚反应是最早为人所知的经典光化学反应之一，因其底物多样、操作简便、具有双稳态性质、产物性质独特，不仅在有机合成备受青睐，更使得该反应同时能够在物质、信息、能源三大方面扮演重要角色，故而其在大科学时代下对于推动科技进步与社会发展方面极具潜力。对于蒽[4+4]光二聚反应而言，目前的关键问题在于如何有效实现反应的调控。与此同时，镧系化学的发展使得镧系离子越发频繁地应用于化学反应的调控。三价镧系离子的[Xe]4fⁿ电子构型，能够产生丰富的电子能级，且内层的4f电子被5s，5p轨道所屏蔽，因此具有独特的配位效应和光学性质，能够对有机分子的激发态行为产生重要影响从而实现对光化学反应的调控。然而相关的研究工作由于缺乏理论的指导频频陷入困境。精确描绘出分子的激发态行为从而深刻理解光化学反应中激发态过程的微观本质，是有效实现精准的化学反应调控的基础，也是对于理论和实验研究而言极具挑战性的工作。针对以上难题，我们拓展和应用多组态微扰理论(CASSCF//IRC//CASPT2)，结合能量一致的相对论从头算赝势，通过轨道定域化与组态优选，成功揭示了对蒽及其衍生物——镧系蒽基膦配合物Ln^III(depma)(NO₃)₃(hmpa)₂的[4+4]光二聚反应路径和镧系离子对该反应的调控机制，主要研究内容及科学发现如下：

（1）蒽在波长为354nm的光照下布局到¹ππ*的Franck-Condon区域，随后能够形成激基缔合物并迅速弛豫至极小，而后通过系间窜跃过程到达³ππ*态，通过分步的方式分别在³ππ*态和基态上先后成键，完成无垒的[4+4]光二聚反应。单态协同加成的模式不利于该反应的进行。配合物体系发生[4+4]光二聚反应的路径与单纯蒽分子大体上一致，区别在于两次成键过程中先后需要越过11.8 kcal·mol^-1和2.7 kcal·mol^-1的能垒。能垒的产生主要源于蒽基上连接的大型取代基对蒽的弯折产生的限制作用，并为镧系金属离子淬灭反应提供了可能性。

（2）镧系离子的加入首先弥合了配体单-三态交叉点处的能隙，从而促进系间窜跃的发生。其次，在17250-15000 cm^-1的范围内存在电子能级的镧系离子，会在单-三态交叉点处产生多个与金属中心的4f电子激发态有关的交叉，同时因配体三态上能垒的阻碍，使得体系能够在金属中心强烈的旋轨耦合作用下发生快速的由^2S+3(³ππ*/M₀)态到^2S+1(S₀/M*)态的系间窜跃过程导致配体失活，从而淬灭[4+4]光二聚反应。以上结果为蒽[4+4]光二聚的调控提供理论依据，并为镧系离子调节分子激发态行为提供了有益的理论启示。

The [4＋4] photodimerizations of anthracene is one of the earliest known classic photochemical reactions. Because of its broad substrate scope, user-friendly operation, bistable characteristics and unique product properties, it is not only favored in organic synthesis, but also enables it to play an important role in matter, information, and energy. Therefore, it has great potential for promoting scientific and technological progress as well as social development in Big Science Times. The current key issue is how to effectively control the reaction. At the same time, lanthanide ions were used to regulate chemical reactions more frequently with the development of lanthanide chemistry. The trivalent lanthanide ions have [Xe]4fⁿ electron configuration which can generate abundant electron energy levels, and the inner 4f electrons are shielded from the influence of the environment by the higher energy 5s and 5p orbitals. As a result, they exhibit unique coordination abilities and excited state properties which can have an important influence on the excited state behavior of organic molecules to realize the regulation of photochemical reactions. However, due to the lack of theoretical guidance, the related research work is frequently in trouble. It is not only a challenging task for experimental and theoretical studies but also the basis for effective control of chemical reactions to deeply understand the microscopic nature of photochemical reactions by depicting the excited state behavior of molecules accurately. To solve these problems, The multi-configuration perturbation theory (CASSCF//IRC//CASPT2) approach associated with relativistic energy-consistent ab initio pseudopotentials was employed to figure out the reaction path of the [4＋4] photodimerizations of anthracene and its derivatives, which is Ln^III(depma)(NO₃)₃(hmpa)₂, as well as the regulation mechanism of lanthanide ion on this reaction by using the strategies of orbitals localization and selecting the optimal configuration in the active space. The main contributions of the present thesis can be summarized as follows:

(1) Anthracenes can form excimer in ¹ππ* single state upon photoexcitation at λ=354 nm. The excimer undergoes a rapid relaxation to ¹ππ* minimum, and then reaches ³ππ* triplet state through intersystem crossing (ISC). The [4+4] photocycloaddition pathway is stepwise, forming two σ-bonds in the ³ππ * state and the ground state respectively. Furthermore, this process is almost barrierless. This reaction does not tend to occur upon singlet through concerted process. The path of the [4+4] photodimerization reaction of the complexes system is roughly the same as that of the anthracenes. It is the main difference between the two that an energy barrier of 11.8 kcal·mol^-1 at ³ππ* and 2.7 kcal·mol^-1 at S₀ must to be overcome successively for complexes system which is in stark contrast to anthracenes. The barriers are due to that the large substituents connected to the anthracene group limit the bending of the planar structure of anthracene moieties, which provide a possible way for the trivalent lanthanide ions to quench the excited anthracene moieties.

(2) In the first place, the lanthanide ions close the energy gap between single state and triple state at crossing point, thereby promoting the occurrence of intersystem crossing. In the next place, if there are electronic energy levels of lanthanide ions in the range of 17250-15000 cm^-1, several nearly degenerate states which are correlated with the excited state of the 4f electron in lanthanide ions center will be generated at single-triplet-crossing region of ligand. With the barrier on triplet state of ligand, the system can undergo rapid intersystem crossing from the ^2S+3(³ππ*/M₀) state to the ^2S+1(S₀/M*) state by the strong spin-orbital coupling of the lanthanide ions. This process result in ligand come back to ground state and inactivation, and the [4＋4] photodimerizations be quenched by this way. These computational contributions provide the useful theoretical insights for the regulation of anthracene [4+4] photodimerization and for lanthanide ions to regulate the excited state behavior of organic molecules.