Department of Chemistry, Columbia University, New York, New York 10027, United States.
Department of Physics, Graduate Center, City University of New York, New York, New York 10016, United States.
J Phys Chem A. 2020 Nov 12;124(45):9392-9399. doi: 10.1021/acs.jpca.0c08427. Epub 2020 Nov 3.
A major benefit of intramolecular singlet fission (iSF) materials, in which through-bond interactions mediate triplet pair formation, is the ability to control the triplet formation dynamics through molecular engineering. One common design strategy is the use of molecular bridges to mediate interchromophore interactions, decreasing electronic coupling by increasing chromophore-chromophore separation. Here, we report how the judicious choice of aromatic bridges can enhance chromophore-chromophore electronic coupling. This molecular engineering strategy takes advantage of "bridge resonance", in which the frontier orbital energies are nearly degenerate with those of the covalently linked singlet fission chromophores, resulting in fast iSF even at large interchromophore separations. Using transient absorption spectroscopy, we investigate this bridge resonance effect in a series of pentacene and tetracene-bridged dimers, and we find that the rate of triplet formation is enhanced as the bridge orbitals approach resonance. This work highlights the important role of molecular connectivity in controlling the rate of iSF through chemical bonds and establishes critical design principles for future use of iSF materials in optoelectronic devices.
分子内单重态裂变（iSF）材料的一个主要优点是，通过键相互作用介导三重态对的形成，从而能够通过分子工程控制三重态形成动力学。一种常见的设计策略是使用分子桥来介导发色团之间的相互作用，通过增加发色团-发色团之间的距离来减小电子耦合。在这里，我们报告了如何明智地选择芳族桥来增强发色团-发色团之间的电子耦合。这种分子工程策略利用了“桥共振”，其中前线轨道能量与共价连接的 iSF 发色团的能量几乎简并，从而即使在较大的发色团间分离下也能实现快速 iSF。我们使用瞬态吸收光谱研究了一系列五并苯和四并苯桥联二聚体中的这种桥共振效应，发现随着桥轨道接近共振，三重态形成的速率增强。这项工作强调了分子连接性在通过化学键控制 iSF 速率方面的重要作用，并为未来在光电设备中使用 iSF 材料建立了关键的设计原则。
