自从分子光化学作为一门学科出现以来，羰基化合物就一直是探索分子光物理和光化学基本过程和普适规律的典型性分子。在过去的几十年间，羰基化合物的光物理和光化学研究得到了实验和理论学家的广泛且深入的研究，对丰富分子光化学的基础知识起到了极其重要的作用。不同于母体羰基化合物，羰基氧被硫或者硒原子简单取代后形成的硫代或者硒代羰基化合物，表现出了许多独特的光物理性质和激发态行为，比如明显红移的吸收和发射光谱、超快的系间窜跃速率、较高的三重态量子产率等。但是，这些奇特的实验现象背后的物理机制和光动力学差异性的来源仍然不清楚。高精度的理论计算模拟在这个方面有着独到的作用。到目前为止，关于硫和硒代羰基化合物的理论研究仍然较少，且主要围绕光谱性质进行研究，很少探索光诱导的激发态弛豫途径及光物理机理，激发态性质和光动力学的差异性来源更是知之甚少。在本论文中，申请人采用多参考的激发态电子结构方法，结合量子力学/分子力学组合方法，系统研究了几个硫代和硒代的羰基化合物的激发态性质、极小能量结构、锥形交叉结构、激发态弛豫路径等，在此基础上探索了相关的光物理机理。最后，通过对比，揭示了硫和硒代导致光物理性质差异的根源:

 1. 本工作采用高精度的多态完全活化空间二阶微扰理论（MS-CASPT2）分别研究了含有和不含分子内氢键（C=S···H-O） 的两组芳香族硫酮分子的五个最低能量电子态（S₀, S₁,S₂, T₁, T₂）。MS-CASPT2 计算结果表明芳香硫酮的光物理性质受到激发态氢键相互作用的显著影响。 S₂(¹ππ*)态和 S₁(¹nπ*)态之间分子内氢键作用的差异导致了在 S₂ 态极小点附近两个电子态形成圆锥交叉， 因此， 来自 S₂ 态的荧光发射无法与超快的 S₂ → S₁ 内转换过程相竞争。强烈的 S₂ 态荧光是大多数硫代羰基化合物的特征，但是具有分子内氢键的芳香硫酮则没有此类荧光。从 S₀ 到 S₁（T₁）态，氢键相互作用明显减弱，从而 S₁（T₁）态相对能量显著增加，因此导致 S₁、 T₂ 和 T₁ 三个电子态是近简并的，在本工作中，首次确定了这三个电子态的极小能量交叉点，这是多种芳香硫酮的共同特征。在极小能量三态交叉处， T₂态可作为一个“桥梁”， 使得正向 S₁ → T₁ 系间窜跃和反向 T₁ → S₁ 系间窜跃过程在一个数十到数百皮秒的时间尺度上发生，这一现象得到了实验结果的支持。重要的是，超快的正向和反向系间窜跃过程可以从芳香硫酮的 S₁ 态产生热活化延迟荧光（TADF），具有作为有机发光二极管发射体的潜在应用价值。 同时，本工作报道的新型 TADF 机理涉及极小能量三态交叉结构，有助于改进纯有机 TADF 材料的设计原理。

2. 本工作采用量子力学/分子力学组合（QM/MM） 方法，首次报道了光激发后硒取代鸟嘌呤（6SeG） 在 DNA 环境中的激发态性质与衰退路径。 在 QM(MS-CASPT2)/MM 级别下， 6SeG 的 S₁ 态和 S₂态分别被预测为光谱暗态和光谱明态，在S₂ 态下有两个低能量的三重态。对于 DNA 中激发态的几何结构优化，由于 C=Se 键相对于周围鸟嘌呤和胸腺嘧啶核酸碱基的取向不同，得到了三种稳定的构型（“U”、“D”和“C”）。本工作在 U， D， C 三

种构型下分别讨论了 6SeG 的激发态弛豫机制。 从 U 构型开始，光激发到 S₂ 态后弛豫到 S₂态极小点， 可以越过一个较小的能垒，在 S2/S1 圆锥交叉点处快速衰退到 S₁  态。在 S₁ 态极小点附近经历系间窜跃过程后布居到 T₁ 态， 较大的旋-轨耦合常数表明 S₁(¹nπ*) → T₁(³ππ*)的跃迁是个有效的过程， 最后系统将克服小的能垒到达 T1/S0 交叉结构并失活回到基态。D 构型中发现了与 U 构型类似的激发态弛豫路径，没有定性的改变。 然而，由于在S₂ 态势能面上较大的能垒， C 构型的衰退通道是低效的。 将 QM 区从单个 6SeG 扩展到包含 6SeG与胞嘧啶的碱基对，极小点结构、锥形交叉点和极小能量交叉点结构在这两个 QM 区上是相似的，没有影响电子态的顺序，因此得到了相似的弛豫机制。但由于分子间氢键相互作用，纳入胞嘧啶的 QM 区的 S₂(¹pp*)态能量降低，而 S₁(¹np*)态升高，导致 S2/S1 态圆锥交叉结构能量下降了约 9 kcal/mol， 这会显著加速 S₂ → S₁  态内转换过程。 本工作对了解化学修饰的核酸碱基在 DNA 环境中的激发态性质与弛豫机制有着重要作用。

3. 本工作采用 CASPT2//CASSCF 方法探索了 UV-B 紫外线过滤器 4-甲基亚苄基樟脑（4MBC）的光保护机制。 在 Franck-Condon 区， V(¹ππ*)态是光谱明态， 从 V(¹ππ*)态开始，有两个非辐射跃迁过程可以有效衰退到能量最低的 ³ππ*态。在第一种情况下， V(¹ππ*)态在Franck-Condon 区通过振动弛豫失活到 V′(¹ππ*)态， V′(1ππ*)态通过在 ¹ππ*/¹nπ*圆锥交叉失活到 ¹nπ*态。 然后， ¹nπ*态通过一个有效的 ¹nπ* → ³ππ*系间窜跃过程跃迁到 ³ππ*态。 在第二种情况下， V(¹ππ*)态可以沿光异构化反应坐标进行非绝热弛豫。 在此过程中，一个¹ππ*/3nπ*交叉结构有助于 ¹ππ*系统衰退到 ³nπ*态，在 ³nπ*/³ππ*圆锥形交叉点通过内转换进一步衰减到 ³ππ*态。一旦形成 ³ππ*态，一条几乎无障碍的弛豫路径使 ³ππ*系统通过 ³ππ*/S₀交叉点跃迁到 S₀ 态。 相比于肉桂酸甲酯等典型 UV-B 过滤器，它们通过分子内 C=C 双键光异构化过程内转换失活到基态（¹ππ* → S₀）， 4MBC 的光致异构化路径表明其最低 1ππ*与 S₀ 态没有相交趋势， 相反， 系间窜跃是 4MBC 主要的激发态失活方式（¹ππ* → ³ππ* →S₀）。本工作揭示了 4MBC 过滤器新颖的光保护机制，为实验上有效的防晒霜分子设计提供了理论参考。

Since molecular photochemistry emerged as a discipline, carbonyl compounds have always been used as model molecules to explore the fundamental processes and general rules in the discipline. Over the past few decades, the photophysical and photochemical studies of carbonyl compounds have been extensively studied by both experimental and theoretical scientists, which has played an crucial role in enriching the basic knowledge of molecular photochemistry. Thiocarbonyl or seleno-carbonyl compounds, where the carbonyl oxygen is replaced by sulfur or selenium atom. Results indicated that thio-carbonyl and seleno-carbonyl compounds possess unique photophysical properties that are different from properties of their parent compounds. For example, the red-shifted absorption and emission spectra, the ultrafast intersystem crossing rates, the higher triplet quantum yield and so on. However, the physical mechanism behind these experimental phenomena and the origin of the photodynamic differences remain unclear. In this regard, high-level theoretical simulation is of great significance. Few theoretical studies on the photophysical and photochemical properties of these molecules have been reported up to now. In this thesis, the applicant employes multi-reference electronic structure methods, combined with quantum mechanics/molecular mechanics method to investigate the excited state properties, minima, crossing points, and photophysical processes of a series of thio- and seleno-carbonyl compounds. Finally, origin of differences in photophysical properties caused by the substitution of sulfur and selenium are revealed:

1. High-accuracy multistate complete active space second-order perturbation theory (MS-CASPT2) method was used to investigate the lowest-lying five electronic states (S₀, S₁,S₂, T₁, T₂) of two groups of aromatic thioketones, The members of one group of compounds were intramolecularly hydrogen-bonded (H-bonded), while the members of the other group of compounds did not exhibit intramolecular H-bonding (C=S···H-O). The results obtained using the MS-CASPT2 method revealed that the photophysical properties of the aromatic thioketones were significantly influenced by the excited-state dependent hydrogen bond (H-bond) interactions. The essential differences of the intramolecular H-bonds betwen the S₂ and S₁ states give rise to the conical intersection of the two states near the S₂ minimum. Consequently, the emission from the S₂ state cannot compete with the ultrafast S₂ → S₁ internal conversion. Most thiocarbonyl compounds exhibit an intense S₂ fluorescence, but the fluorescence is absent in the intramolecularly H-bonded aromatic thioketones investigated herein. A significant reduction in the strengths of the H-bond interactions is observed from S₀ to S₁ (T₁), resulting in a significant increase in the relative energies of the S₁ and T₁ states. As a result, the S₁, T₂, and T₁ states are quasi-degenerate. The minimum-energy intersection of the three states, a common feature of a wide variety of aromatic thioketones, has been determined in this seminal work. The T₂ state acts as a bridge and enables forward S₁ → T₁ and reverse T₁ → S₁ intersystem crossing processes (FISC and R-ISC, respectively) to occur on a timescale of dozens to hundreds of picoseconds, which is supported by numerous experimental findings. Importantly, the ultrafast F-ISC and R-ISC processes result in an efficient generation of thermally activated delayed fluorescence (TADF) from the S₁ state of the aromatic thioketones under study. These aromatic thioketones can be potentially used as emitters of organic light-emitting diodes. Analysis of the new TADF mechanism reported herein for a series of aromatic thioketones can help improve the design principle of pure organic TADF materials.

2. The excited-state properties and relaxation mechanisms after light irradiation of 6- selenoguanine (6SeG) in DNA have been investigated using a quantum mechanics/molecular mechanics (QM/MM) approach. The S₁(¹nπ*) and S₂(¹ππ*) states are predicted to be the spectroscopically dark and bright states, respectively. Two triplet states, T₁(³ππ*) and T₂(³ππ*), are found energetically below the S₂ state. For each excited state geometry optimization in DNA, three kind of structures (“up”, “down”, and “central”) are optimized which differ from each other by the orientation of the C=Se group with respect to the surrounding guanine and thymine nucleobases. After irradiation to the S₂ state, 6SeG evolves to the S₂ minimum, near to a S2/S1 conical intersection that allows for internal conversion to the S₁ state. Linear interpolation in internal coordinates indicate that the “central” orientation is less favorable since extra energy is needed to surmount the high barrier in order to reach the S2/S1 conical intersection. From the S₁ state, 6SeG can further decay to the T1(³ππ*) state via intersystem crossing, where it will be trapped due to the existence of a sizable energy barrier between the T₁ minimum and the T1/S0 crossing point. Two models are considered in the QM region, which extend from 6SeG to the 6SeG-Cyt base pair, the high-level MS-CASPT2//CASSCF method is adopted for both models. Extending the QM region to include the 6SeG-Cyt base pair slightly stabilizes the S₂ state and destabilizes the S₁, due to hydrogen-bonding interactions, but it does not affect the order of the states. In addition, the presence of intermolecular hydrogen bonding induces a steeper S₂ potential energy surface, that it is expected to accelerate the S₂ → S₁ internal conversion. This work plays an important role in understanding the excited state properties and relaxation mechanism of chemically modified nucleic acid bases in the DNA environment.

3. 4-Methylbenzylidene camphor (4MBC) is a commonly used ultraviolet (UV) filter in commercial sunscreens to protect skin from UV-B radiation. Herein, we have employed high-level CASPT2 and CASSCF methods to investigate the photoprotection mechanism of 4MBC. We have found that the V(¹ ππ*) state is populated with large probability in the Franck-Condon region. Starting from this state, there are two efficient nonradiative relaxation processes to populate the ³ππ* state. In the first one, the V(¹ππ*) state decays to the V′(¹ππ*) state by vibrational relaxation. The resultant V′(¹ππ*) state further jumps to the ¹nπ* state by internal conversion at the ¹ππ*/¹nπ* conical intersection. Then, the ¹nπ* state hops to the ³ππ* state through an efficient ¹nππ* → ³ππ* intersystem crossing process. In the second one, the V(¹pp*) state can diabatically relax along the photoisomerization reaction coordinate. In this process, a ¹ππ*/³np* crossing point helps the ¹ππ* system decay to the ³nπ* state, which further decays to the ³ππ* state through internal conversion at the ³nπ*/³ππ* conical intersection. Once the ³ππ* state is formed, a nearly barrierless relaxation path drives the ³ππ* system to hop to the S₀ state via the ³ππ*/S₀ crossing point. In contrast to typical UV-B filters such as methyl cinnamate, which are inactivated to the ground state via intramolecular C=C double bond photoisomerization (¹ππ* → S₀), 4MBC is a novel photoprotection mechanism which is mainly through intersystem crossing inactivation to the ground state (¹ππ* → ³ππ* → S₀). This work not only rationalizes recent experimental observations, but also enriches the photoprotection mechanism of the UV filter.