Cavendish Laboratory, University of Cambridge , J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom.
Department of Chemistry and Chemical Biology, Cornell University , Ithaca, New York 14850, United States.
J Am Chem Soc. 2017 Sep 13;139(36):12488-12494. doi: 10.1021/jacs.7b05204. Epub 2017 Sep 1.
We have designed a series of pentacene dimers separated by homoconjugated or nonconjugated bridges that exhibit fast and efficient intramolecular singlet exciton fission (iSF). These materials are distinctive among reported iSF compounds because they exist in the unexplored regime of close spatial proximity but weak electronic coupling between the singlet exciton and triplet pair states. Using transient absorption spectroscopy to investigate photophysics in these molecules, we find that homoconjugated dimers display desirable excited-state dynamics, with significantly reduced recombination rates as compared to conjugated dimers with similar singlet fission rates. In addition, unlike conjugated dimers, the time constants for singlet fission are relatively insensitive to the interplanar angle between chromophores, since rotation about σ bonds negligibly affects the orbital overlap within the π-bonding network. In the nonconjugated dimer, where the iSF occurs with a time constant >10 ns, comparable to the fluorescence lifetime, we used electron spin resonance spectroscopy to unequivocally establish the formation of triplet-triplet multiexcitons and uncoupled triplet excitons through singlet fission. Together, these studies enable us to articulate the role of the conjugation motif in iSF.
我们设计了一系列通过同共轭或非共轭桥连接的并五苯二聚体，这些二聚体表现出快速且高效的分子内单重态激子裂变（iSF）。与报道的 iSF 化合物相比，这些材料具有独特之处，因为它们处于单重态激子和三重态对态之间空间接近但电子耦合较弱的未探索区域。我们使用瞬态吸收光谱研究这些分子中的光物理性质，发现同共轭二聚体表现出理想的激发态动力学，与具有相似单重态裂变速率的共轭二聚体相比，重组率显著降低。此外，与共轭二聚体不同，单重态裂变的时间常数相对不敏感于生色团之间的平面间角，因为σ 键的旋转对π键合网络内的轨道重叠影响可以忽略不计。在 iSF 时间常数>10 ns 的非共轭二聚体中，与荧光寿命相当，我们使用电子自旋共振光谱明确地证明了三重态-三重态多激子和通过单重态裂变形成的未偶联三重态激子的形成。这些研究使我们能够阐明共轭模式在 iSF 中的作用。
