We investigate the role of preaggregation among photocatalyst and substrate in the paradigmatic molecular dye rhodamine 6G by means of fluorescence correlation experiments and quantum-mechanical modeling. By varying the substrate concentration, we experimentally confirm earlier conclusions that consecutive photoelectron transfer between rhodamine 6G and a model substrate molecule is not diffusion limited, raising puzzling questions regarding the mechanisms underlying its established photocatalytic activity. By theoretically exploring alternative explanations for ultrafast excited-state charge transfer, it is indeed found that preaggregation between photocatalyst and substrate is required to reach charge-transfer rates similar to those experimentally observed. Electrostatic as well as dispersive interactions are found to be most important for the molecular attraction involved in preaggregation. We show that tuning these contributions by chemical design alters the binding energies of photocatalyst–substrate assemblies. This suggests that reaction rates can be adjusted by adapting the composition of the species involved in preaggregation, which appears as an appealing concept in photocatalysis.

中文翻译：

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预聚集在单分子光氧化还原催化中的作用

我们通过荧光相关实验和量子力学建模研究了典型分子染料罗丹明 6G 中光催化剂和底物之间预聚集的作用。通过改变底物浓度，我们通过实验证实了早期的结论，即罗丹明 6G 和模型底物分子之间的连续光电子转移不受扩散限制，这提出了关于其已建立的光催化活性的机制的令人费解的问题。通过从理论上探索超快激发态电荷转移的替代解释，确实发现光催化剂和基材之间需要预聚集才能达到与实验观察到的类似的电荷转移速率。发现静电和色散相互作用对于预聚集中涉及的分子吸引力最为重要。我们表明，通过化学设计调整这些贡献可以改变光催化剂-基底组件的结合能。这表明可以通过调整参与预聚集的物种的组成来调整反应速率，这在光催化中似乎是一个有吸引力的概念。