A Direct Mechanism of Ultrafast Intramolecular Singlet Fission in Pentacene Dimers.
A Direct Mechanism of Ultrafast Intramolecular Singlet Fission in Pentacene Dimers.
复制标题
DOI：
10.1021/acscentsci.6b00063
复制
发表时间：
2016-05-25
影响因子：
18.2
通讯作者：
Ananth N
中科院分区：
文献类型：
--
作者：
Fuemmeler EG;Sanders SN;Pun AB;Kumarasamy E;Zeng T;Miyata K;Steigerwald ML;Zhu XY;Sfeir MY;Campos LM;Ananth N
由于可能超过太阳能转换效率的肖克利-奎瑟极限，人们对经历单重态裂变(SF)的材料的兴趣已经被催化。在传统材料中，SF的机制是一个分子间过程(XSF)，它由电荷转移(CT)态介导，并敏感地依赖于晶体堆积或分子碰撞。相比之下，最近报道的共价偶联五苯类化合物在单个分子中吸收的每个光子产生2个∼三联体：这是分子内单态裂变的标志。然而，ISF的发病机制尚不清楚。在这里，使用多参考电子结构计算和瞬时吸收光谱，我们建立了ISF可以通过与CT态无关的直接耦合机制发生。结果表明，尽管直接耦合很弱，但共价二聚体分子内模的振动耦合引起的电子态能量的近简并允许关联的三重态和局域激子态之间的强混合。我们确定了一种特定的、高频的分子内振动模式，该模式通过避免交叉来帮助从LE态到ME态的直接布居转移。
Interest in materials that undergo singlet fission (SF) has been catalyzed by the potential to exceed the Shockley–Queisser limit of solar power conversion efficiency. In conventional materials, the mechanism of SF is an intermolecular process (xSF), which is mediated by charge transfer (CT) states and depends sensitively on crystal packing or molecular collisions. In contrast, recently reported covalently coupled pentacenes yield ∼2 triplets per photon absorbed in individual molecules: the hallmark of intramolecular singlet fission (iSF). However, the mechanism of iSF is unclear. Here, using multireference electronic structure calculations and transient absorption spectroscopy, we establish that iSF can occur via a direct coupling mechanism that is independent of CT states. We show that a near-degeneracy in electronic state energies induced by vibronic coupling to intramolecular modes of the covalent dimer allows for strong mixing between the correlated triplet pair state and the local excitonic state, despite weak direct coupling. We identify a specific, high-frequency, intramolecular vibrational mode that assists direct population transfer from the LE state to the ME state through an avoided crossing.