Singlet fission is the spin-allowed conversion of a photogenerated singlet exciton into two triplet excitons in organic semiconductors, which could enable single-junction photovoltaic cells to break the Shockley-Queisser limit. The conversion of singlets to free triplets is mediated by an intermediate correlated triplet pair (TT) state, but an understanding of how the formation and dissociation of these states depend on energetics and morphology is lacking. In this study, we probe the dynamics of TT states in a model endothermic fission system, TIPS-Tc nanoparticles, which show a mixture of crystalline and disordered regions. We observe the formation of different TT states, with varying yield and different rates of singlet decay, depending on the excitation energy. An emissive TT state is observed to grow in over 1 ns when excited at 480 nm, in contrast to excitation at lower energies where this emissive TT state is not observed. This suggests that the pathway for singlet fission in these nanoparticles is strongly influenced by the initial sub-100 fs relaxation of the photoexcited state away from the Franck-Condon point, with multiple possible TT states. On nanosecond time scales, the TT states are converted to free triplets, which suggests that TT states might diffuse into the disordered regions of the nanoparticles where their breakup to free triplets is favored. The free triplets then decay on μs time scales, despite the confined nature of the system. Our results provide important insights into the mechanism of endothermic singlet fission and the design of nanostructures to harness singlet fission.

中文翻译：

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阐明吸热单线态裂变系统中激发能相关的相关三重对形成途径

单线态裂变是有机半导体中光生单线态激子在自旋允许下转化为两个三线态激子，这可以使单结光伏电池突破肖克利-奎瑟极限。单线态向自由三线态的转化是由中间相关三线态对 (TT) 状态介导的，但缺乏对这些状态的形成和解离如何依赖于能量和形态的理解。在这项研究中，我们探索了模型吸热裂变系统 TIPS-Tc 纳米粒子中 TT 状态的动力学，该系统显示出结晶和无序区域的混合。我们观察到不同 TT 状态的形成，具有不同的产量和不同的单线态衰变率，这取决于激发能量。当在 480 nm 激发时，观察到发射 TT 状态在 1 ns 内增长，与未观察到这种发射 TT 状态的较低能量激发相反。这表明这些纳米粒子中单线态裂变的途径受到远离 Franck-Condon 点的光激发态的初始亚 100 fs 弛豫的强烈影响，具有多种可能的 TT 态。在纳秒时间尺度上，TT 状态转换为自由三重态，这表明 TT 态可能扩散到纳米粒子的无序区域，在那里它们有利于分解为自由三重态。尽管系统的受限性质，自由三元组然后在 μs 时间尺度上衰减。我们的研究结果提供了对吸热单线态裂变机制和纳米结构设计以利用单线态裂变的重要见解。这表明这些纳米粒子中单线态裂变的途径受到远离 Franck-Condon 点的光激发态的初始亚 100 fs 弛豫的强烈影响，具有多种可能的 TT 态。在纳秒时间尺度上，TT 状态转换为自由三重态，这表明 TT 态可能扩散到纳米粒子的无序区域，在那里它们有利于分解为自由三重态。尽管系统的受限性质，自由三元组然后在 μs 时间尺度上衰减。我们的研究结果提供了对吸热单线态裂变机制和纳米结构设计以利用单线态裂变的重要见解。这表明这些纳米粒子中单线态裂变的途径受到远离 Franck-Condon 点的光激发态的初始亚 100 fs 弛豫的强烈影响，具有多种可能的 TT 态。在纳秒时间尺度上，TT 状态转换为自由三重态，这表明 TT 态可能扩散到纳米粒子的无序区域，在那里它们有利于分解为自由三重态。尽管系统的受限性质，自由三元组然后在 μs 时间尺度上衰减。我们的研究结果提供了对吸热单线态裂变机制和纳米结构设计以利用单线态裂变的重要见解。