ABSTRACT

A multitude of ultrafast photoinduced reactions in organic semiconductors are governed by the close interplay between nuclear and electronic degrees of freedom. From biological light-harvesting and photoprotection to organic solar cells, the critical electronic dynamics are often precisely synchronized with and driven by nuclear motions, in a breakdown of the Born-Oppenheimer approximation. Ultrafast time-domain Raman methods exploit impulsive excitation to generate nuclear wavepackets and track their coherent evolution through these reaction pathways in real time. This tool of vibrational coherence has recently been applied to study singlet fission, a carrier multiplication process with the potential to boost solar cell efficiencies which has been under intense mechanistic investigation for the past decade. In this review, we present the essential features of the spectroscopic techniques and discuss how they have been used to elaborate a new perspective on the singlet fission mechanism. It is now established that ultrafast triplet-pair formation is driven by vibronic coupling, whether fission is exothermic or endothermic, and thus that full understanding of singlet fission requires explicit consideration of nuclear dynamics. Despite broad qualitative agreement between different vibrational coherence methods, differences in the detailed observations and interpretation raise important questions and pose new challenges for future research.

摘要

有机半导体中的许多超快光致反应受核自由度和电子自由度之间紧密相互作用的支配。从生物的光收集和光保护到有机太阳能电池，关键的电子动力学通常与核运动精确地同步，并由Born-Oppenheimer近似分解。超快时域拉曼方法利用脉冲激励来生成核波包，并通过这些反应路径实时跟踪它们的相干演化。这种振动相干的工具最近已用于研究单重态裂变，这是一种载流子倍增过程，具有提高太阳能电池效率的潜力，在过去的十年中一直在进行深入的机械研究。在这篇评论中，我们介绍了光谱技术的基本特征，并讨论了如何利用它们来阐明单重态裂变机理的新观点。现在已经确定，无论裂变是放热的还是吸热的，超快三重态对的形成都是由振动耦合引起的，因此，对单线态裂变的充分理解需要明确考虑核动力学。尽管不同的振动相干方法之间在质量上有着广泛的共识，但是在详细的观测和解释上的差异提出了重要的问题，并为未来的研究提出了新的挑战。裂变是放热的还是吸热的，因此对单重态裂变的充分理解需要明确考虑核动力学。尽管不同的振动相干方法之间在质量上有着广泛的共识，但是在详细的观测和解释上的差异提出了重要的问题，并为未来的研究提出了新的挑战。裂变是放热的还是吸热的，因此对单重态裂变的充分理解需要明确考虑核动力学。尽管不同的振动相干方法之间在质量上有着广泛的共识，但是在详细的观测和解释上的差异提出了重要的问题，并为未来的研究提出了新的挑战。