单线态裂分（Singlet Fission，SF）是发生在两个相互作用分子之间的量子相干现象，在光激发下，其中一个分子首先被提升到高能激发单重态，然后邻近的基态生色团能够与其共享激发能，进而转化为两个低能三重态激子。在过去的20年，众多的科研工作者应用多种技术手段研究这一新奇的量子倍增现象并将其广泛应用在发光晶体管、光动力治疗、OLED、光伏电池等领域。尽管在裂分材料的设计合成和器件化应用方面取得了重大进展，但单线态裂分机制研究尚处在起步阶段，同时也缺乏切实可行的调控策略。为此，本论文运用超快时间分辨光谱和高精度激发态计算方法相结合的策略，研究了一系列并苯类分子的吸收发射光谱、激发态弛豫以及裂分的动力学过程，检测并确认了¹(T₁T₁)、⁵(T₁T₁)和³(T₁S₁)等中间态，同时分析了其在裂分过程中的关键作用，提出了创新型的单线态裂分理论模型。主要科学贡献总结如下：
（1）运用稳态吸收光谱观测到并五苯两个强吸收带（峰值：676和580 nm），分别对应于布居在双体中两个不同分子的¹ππ^*明态，进一步的理论分析证实裂分前驱明态S_SF(¹ππ^*)波函数源于两个单体布居¹ππ^*态波函数的线性组合具有量子态叠加性质。激发态能量分析表明，S_SF(¹ππ^*)态与由两个三重态强耦合形成多重度为1的¹(T₁T₁)态在Franck-Condon区能量简并，使其在639 nm的光激发下可能直接布居¹(T₁T₁)态实现了S_SF→¹(T₁T₁)的快速裂分，但因¹(T₁T₁)态的暗态本质导致直接布居的几率大大减小。
（2）除了上述在FC区的直接布居外，在S_SF(¹ππ^*)态的弛豫过程中还发现了与¹(T₁T₁)态的圆锥交叉点CI[S_SF(¹ππ^*)/¹(T₁T₁)]调控的裂分通道，借助高效的S_SF→CI[S_SF(¹ππ^*)/¹(T₁T₁)]→¹(T₁T₁)弛豫途径，从而增加光子利用效率，提升裂分量子产率。飞秒瞬态吸收光谱实验观测到第一个新物种的信号在400 fs出现，通过高精度的理论计算证实该物种的电子组态为强耦合的三重对态¹(T₁T₁)，理论计算和实验证据表明S_SF→¹(T₁T₁)转化发生在飞秒至皮秒时间尺度内。
（3）并五苯双体沿着¹(T₁T₁)态的势能面弛豫至极小的过程中，发现了¹(T₁T₁)与弱耦合三重态对的五重态⁵(T₁T₁)之间的交叉点，即SQC[¹(T₁T₁)/⁵(T₁T₁)]。由于从¹(T₁T₁)→⁵(T₁T₁)要求两个电子发生自旋翻转，很难同时进行。因此，我们首次发现需要通过一个具有中转作用的³(T₁S₁)态，从而使得¹(T₁T₁)→³(T₁S₁)→⁵(T₁T₁)的两次自旋翻转的系间窜跃成为可能，得到弱耦合的⁵(T₁T₁)态，裂分为两个独立的三重态。

Singlet fission presents the phenomenon of quantum coherence in organic semiconductors, where the interacted two molecules first populate in the high-energy singlet state of one molecule and then allows the neighbouring ground-state chromophore to share its excitation energy, converting into two low-energy triplet excitons. Over the past two decades, numerous researchers have applied various techniques to study this novel Quantum Multiplication phenomenon, and its applications have extended to diverse areas such as light-emitting transistors, photodynamic therapy, OLEDs, and photovoltaic cells. Despite significant progress in the design, synthesis, and device application of SF materials, the mechanism of singlet fission is still in its infancy, and there is a lack of feasible control strategies. In this paper, a strategy combining ultrafast time-resolved spectroscopy and high-precision excited-state calculation methods was employed to investigate the absorption and emission spectra, excited-state dynamics, and fission kinetics of a series of Acenes materials. By detecting and confirming intermediate states, including ¹(T₁T₁),⁵(T₁T₁) and ³(T₁S₁), their crucial roles in the SF process were analyzed, and this led to the proposal of a novel theoretical model and mechanism for singlet fission. The main scientific contributions of this work can be summarized as follows: 
(1) Two strong absorption bands (peaking at 676 nm and 580 nm) were observed via steady-state absorption spectroscopy for pentacene dimer, which correspond to the ¹ππ^* excited states populated within the dimer's different molecules. Subsequent theoretical analysis has provided conclusive evidence that the precursor state, characterized by a S_SF(¹ππ^*) wavefunction, results from a linear combination of the ¹ππ^* excited states of the two individual monomers. Furthermore, this precursor state exhibits the fundamental properties of quantum state superposition. The energy analysis of the excited states revealed that S_SF(¹ππ^*) and the ¹(T₁T₁) state, which is formed by strong coupling of two triplet excitons with a multiplicity of 1, makes it possible to directly population the ¹(T₁T₁) state at 639 nm to achieve the rapid splitting of S_SF→¹(T₁T₁). However, due to the dark state nature of ¹(T₁T₁) state, the probability of direct population is greatly reduced.
(2) In addition to direct population in the FC region, a splitting channel has been discovered in the relaxation process of the S_SF(¹ππ^*) state, which is regulated by the conical intersection CI[S_SF(1ππ*)/1(T1T1)] between the S_SF(¹ππ^*) and ¹(T₁T₁) states. By utilizing the efficient S_SF→CI[S_SF(¹ππ^*)/¹(T₁T₁)]→¹(T₁T₁) relaxation pathway, the photon utilization efficiency can be increased, and the splitting quantum yield can be improved. The first signal of a new species was observed in the femtosecond transient absorption spectra experiment at 400 fs, and high-precision theoretical calculations confirmed that the electronic configuration of this species is a strongly coupled triplet pair state ¹(T₁T₁). Theoretical calculations and experimental evidence suggest that the S_SF→¹(T₁T₁) conversion occurs on a time scale ranging from femtoseconds to picoseconds.
(3) During the relaxation process of pentacene dimers along the potential energy surface of the ¹(T₁T₁) state, a crossing point between the ¹(T₁T₁) state and the weakly coupled triplet pair ⁵(T₁T₁) state namely SQC[¹(T₁T₁)/⁵(T₁T₁)] has been discovered. However, the ¹(T₁T₁)→⁵(T₁T₁) transition requires simultaneous spin-flip of two electrons, making it difficult to occur. Therefore, we have firstly identified the necessity of a mediating ³(T₁S₁) state to enable the two spin flips through a sequential ¹(T₁T₁)→³(T₁S₁)→⁵(T₁T₁) intersystem crossing, resulting in the formation of the weakly coupled ⁵(T₁T₁) state, which splits into two independent triplet states.