Department of Physical Chemistry, University of Geneva, CH-1211 Geneva, Switzerland.
Department of Organic Chemistry, University of Geneva, CH-1211 Geneva, Switzerland.
J Am Chem Soc. 2021 Feb 10;143(5):2361-2371. doi: 10.1021/jacs.0c12384. Epub 2021 Jan 29.
Singlet fission (SF), i.e., the splitting of a high-energy exciton into two lower-energy triplet excitons, has the potential to increase the efficiency for harvesting spectrally broad light. The path from the photopopulated singlet state to free triplets is complicated by competing processes that decrease the overall SF efficiency. A detailed understanding of the whole cascade and the nature of the photoexcited singlet state is still a major challenge. Here, we introduce a pentacene dimer with a flexible crown ether spacer enabling a control of the interchromophore coupling upon solvent-induced self-aggregation as well as cation binding. The systematic change of solvent polarity and viscosity and excitation wavelength, as well as the available conformational phase space, allows us to draw a coherent picture of the whole SF cascade from the femtosecond to microsecond time scales. High coupling leads to ultrafast SF (<2 ps), independent of the solvent polarity, and to highly coupled correlated triplet pairs. The absence of a polarity effect indicates that the solvent coordinate does not play a significant role and that SF is driven by intramolecular modes. Low coupling results in much slower SF (∼500 ps), which depends on viscosity, and leads to weakly coupled correlated triplet pairs. These two triplet pairs could be spectrally distinguished and their contribution to the overall SF efficiency, i.e., to the population of free triplets, could be determined. Our results reveal how the overall SF efficiency can be increased by conformational restrictions and control of the structural fluctuation dynamics.
单线态裂变 (SF)，即高能激子分裂成两个低能三重态激子，具有提高光谱宽带光收集效率的潜力。从光激发的单线态到自由三重态的途径受到降低整体 SF 效率的竞争过程的复杂化。对整个级联过程和光激发单线态的性质的详细了解仍然是一个主要挑战。在这里，我们引入了一个带有柔性冠醚间隔物的并五苯二聚体，能够在溶剂诱导自组装以及阳离子结合时控制发色团之间的耦合。溶剂极性和粘度以及激发波长的系统变化以及可用的构象相空间使我们能够从飞秒到微秒时间尺度上描绘整个 SF 级联的连贯图像。高耦合导致超快 SF(<2 ps)，与溶剂极性无关，并导致高度耦合的相关三重态对。没有极性效应表明溶剂坐标没有起到重要作用，并且 SF 是由分子内模式驱动的。低耦合导致慢得多的 SF(∼500 ps)，这取决于粘度，并导致弱耦合的相关三重态对。这两个三重态对可以在光谱上区分开来，并且可以确定它们对整体 SF 效率的贡献，即对自由三重态的填充。我们的结果揭示了如何通过构象限制和控制结构波动动力学来提高整体 SF 效率。
