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的光物理学和光化学堆叠的发色团对于将它们用作功能性光子材料很重要。然而，这些研究大多局限于共价分子二聚体，它只能近似于功能有机材料典型的高级低聚物中存在的电子和振动相互作用。在这里，我们展示了共价滑动堆叠苝二亚胺二聚体 (2) 和三聚体 (3) 的激发态动力学的比较，为超越二聚体极限的电子态混合和对称破缺电荷分离 (SB-CS) 提供了基本见解. 我们发现与高频模式的相干振动耦合促进了 Frenkel 激子 (FE) 和电荷转移 (CT) 状态之间的超快状态混合。随后，溶剂波动和发色团间低频振动促进了相干 FE/CT 混合状态下的 CT 特性。相干 FE/CT 混合状态在 2 中持续存在，但在 3 中，低频振动耦合破坏了相干性，导致远端苝二亚胺单元之间的超快 SB-CS。