Lin Chenjian, Kim Taeyeon, Schultz Jonathan D, Young Ryan M, Wasielewski Michael R
Department of Chemistry and Institute for Sustainability and Energy at Northwestern, Northwestern University, Evanston, IL, USA.
Nat Chem. 2022 Jul;14(7):786-793. doi: 10.1038/s41557-022-00927-y. Epub 2022 Apr 25.
Understanding the photophysics and photochemistry of molecular π-stacked chromophores is important for utilizing them as functional photonic materials. However, these investigations have been mostly limited to covalent molecular dimers, which can only approximate the electronic and vibronic interactions present in the higher oligomers typical of functional organic materials. Here we show that a comparison of the excited-state dynamics of a covalent slip-stacked perylenediimide dimer (2) and trimer (3) provides fundamental insights into electronic state mixing and symmetry-breaking charge separation (SB-CS) beyond the dimer limit. We find that coherent vibronic coupling to high-frequency modes facilitates ultrafast state mixing between the Frenkel exciton (FE) and charge-transfer (CT) states. Subsequently, solvent fluctuations and interchromophore low-frequency vibrations promote CT character in the coherent FE/CT mixed state. The coherent FE/CT mixed state persists in 2, but, in 3, low-frequency vibronic coupling collapses the coherence, resulting in ultrafast SB-CS between the distal perylenediimide units.
了解分子π堆积发色团的光物理和光化学性质对于将它们用作功能性光子材料至关重要。然而，这些研究大多局限于共价分子二聚体，而共价分子二聚体只能近似于功能性有机材料中典型的高级低聚物中存在的电子和振动相互作用。在此，我们表明，对共价滑移堆积的苝二酰亚胺二聚体（2）和三聚体（3）的激发态动力学进行比较，可以深入了解超出二聚体极限的电子态混合和对称破缺电荷分离（SB-CS）。我们发现，与高频模式的相干振动耦合促进了弗伦克尔激子（FE）和电荷转移（CT）态之间的超快态混合。随后，溶剂涨落和发色团间的低频振动在相干的FE/CT混合态中促进CT特性。相干的FE/CT混合态在2中持续存在，但在3中，低频振动耦合破坏了相干性，导致远端苝二酰亚胺单元之间的超快SB-CS。
