Intermolecular Vibrations Drive Ultrafast Singlet Fission
Hong-Guang Duan, Ajay Jha 1, Xin Li, Vandana Tiwari, Hanyang Ye, Pabitra K. Nayak, Xiao-Lei Zhu, Zheng Li, Todd J. Martinez, Michael Thorwart, R. J. Dwayne Miller
Singlet fission is a spin-allowed exciton multiplication process in organic semiconductors that converts one spin-singlet exciton to two triplet excitons. It offers the potential to enhance solar energy conversion by circumventing the Shockley-Queisser limit on efficiency. Recently, the mechanism of the primary singlet fission process in pentacene and its derivatives have been extensively investigated, however, the nature of the primary ultrafast process in singlet fission is still a matter of debate. Here, we study the singlet fission process in a pentacene film by employing a combination of transient-grating (TG) and two-dimensional (2D) electronic spectroscopy complemented by quantum chemical and nonadiabatic dynamics calculations. The high sensitivity of heterodyne detected TG spectroscopy enabled us to capture the vibrational coherence and to show that it mediates the transition from the singlet excited electronic state to the triplet-pair state. This coherent process is further examined by 2D electronic spectroscopy. Detailed analysis of the experimental data reveals that significant vibronic couplings of a few key modes in the low- and high-frequency region connect the excited singlet and triplet-pair states. Based on quantum chemical calculations, we identify these key intermolecular rocking modes along the longitudinal molecular axis between the pentacene molecules. They play the essential role of an electronic bridge between the singlet and triplet-pair states. Along with high-frequency local vibrations acting as tuning modes, these rocking motions drive the ultrafast dynamics at the multidimensional conical intersection in the singlet fission process.
Coherent vs Thermally Activated Singlet Exciton Fission in Acene Derivatives From First Principles Quantum Dynamics Simulations: Molecular Packing Makes the Difference
Theory of Singlet Fission in Polyenes, Acene Crystals and Covalently Linked Acene Dimers
我们报告四重配置相互作用的计算扩展Pariser $-$帕尔$-$ PHilople哈密顿量的激发态的聚集体的多烯，结晶并苯，并苯分子的共价连接的二聚体。我们确定精确的能量排序和分析集团波函数，以达到在这些不同的家庭材料的单重态裂变的全面的物理理解。我们的计算方法使我们能够保留一个非常大的数量的基础状态，从而获得正确的相对能量排序的一个电子$-$一个空穴Frenkel和电荷转移激子与分子内和分子间的两个电子$-$两个空穴三重态$-$三重态激发态。我们表明，从能量排序，它是可以理解的发生在多烯和并苯晶体中的单线态裂变，以及它的共价连接的并苯二聚体几乎完全不存在。与并苯晶体一样，多烯中的单线态裂变是一种多发色团现象，众所周知的2-$^1$ A $_g^-$不起直接作用。分子间电荷转移是并苯和多烯单重态裂变的基本过程，但由于聚合态激发态的性质和排列顺序的细微差别，并苯和多烯的单重态裂变机制略有不同。因此，我们能够给出定性的物理推理的慢单线态裂变的多烯，相对于结晶并五苯。我们的工作也提供了新的见解复杂的激子动力学并四苯晶体，这一直是很难理解的理论。我们的大规模多体计算为我们提供了理解不同类别的$π$共轭材料之间的单线态裂变产额和速率的定性差异的能力。
