Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, 560012, India.
Department of Chemical Sciences, Tata Institute of Fundamental Research, Mumbai, 400005, India.
Nat Commun. 2019 Jan 3;10(1):33. doi: 10.1038/s41467-018-07736-3.
The formation of two triplet excitons at the cost of one photon via singlet exciton fission in organic semiconductors can potentially enhance the photocurrent in photovoltaic devices. However, the role of spin density distribution in driving this photophysical process has been unclear until now. Here we present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. We synthetically modulate the spin density distribution in a series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Using femtosecond transient absorption spectroscopy, we find that efficient iSEF is only observed for the phenyl-derivative in ~2.4 ps while absent in the other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character.
通过有机半导体中的单线态激子裂变以一个光子的代价形成两个三重态激子，有可能会增强光伏器件中的光电流。然而，到目前为止，自旋密度分布在驱动这一光物理过程中的作用还不清楚。在这里，我们展示了电子自旋密度分布在促进π桥联并五苯二聚体中有效的分子内单线态激子裂变（iSEF）中的重要性。我们使用苯并、噻吩基和硒基二酮吡咯并吡咯（DPP）衍生物作为π桥，合成性地调节了一系列并五苯二聚体中的自旋密度分布。通过飞秒瞬态吸收光谱，我们发现仅在苯-DPP 桥中观察到有效 iSEF，在其他两个二聚体中则不存在。电子结构计算表明，苯-DPP 桥将 α-和 β-自旋密度局域在不同的末端并五苯上。在光激发后，自旋交换机制能够使具有固有三重态对特征的单线态发生 iSEF。
