Exciton Correlations in Intramolecular Singlet Fission
Exciton Correlations in Intramolecular Singlet Fission
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DOI：
10.1021/jacs.6b00657
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发表时间：
2016-06-15
影响因子：
15
通讯作者：
Sfeir, Matthew Y.
中科院分区：
文献类型：
--
作者：
Sanders, Samuel N.;Kumarasamy, Elango;Sfeir, Matthew Y.
我们合成了一系列具有可变长度共轭桥的不对称并五苯并四苯异二聚体，可进行快速有效的分子内单线裂变（iSF）。这些化合物具有不同的单线态和三线态能量，这使我们能够研究激子在 iSF 过程中的空间动力学，包括激子相关性在促进三线态对生成和重组中的重要作用。我们证明了共轭拨号器中的主要光激发是离域单峰，可以实现快速高效的 iSF。然而，在这些不对称二聚体中，随着桥长度的增加，单线态变得更加集中在较低能量单元上，相对于类似的对称二聚体减慢了 iSF 速度。我们解析了通过 iSF 产生的不等价三联体的重组动力学，并发现它们主要通过协同过程衰减。通过识别不同的衰变“通道，包括通过三重态三重态湮灭延迟荧光，我们可以分离与相关三重态对和不相关三重态相对应的瞬态物种。尽管我们的实验和理论证明单个三重态是高度局部化的并且无法跨缀合连接体运输，但三重态对的重组仍快速进行。在此类化合物中，不相关“三重态”的形成速率和产量随着桥长度的增加而增加。总体而言，这些“受约束的不对称系统提供了一个独特的平台来分离和研究单线裂变所必需的瞬态物质，否则很难在对称二聚体或凝聚相中观察到这些物质。
We have synthesized a series of asymmetric pentacene tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photo excitations in conjugated dialers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay "channels, including delayed fluorescence via triplet triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated "triplets increases with bridge length. Overall, these "constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases.