Understanding Anomalous Cage-Escape Dynamics in Photoredox Processes Driven by a Fe(III) N‑Heterocyclic Carbene Complex

Solvent cage-escape dynamics of bimolecular photoredox products in solution has been investigated computationally through a combination of molecular dynamics simulations and quantum chemical calculations. The present work focuses on the photoinduced oxidation of the organic electron donor dimethylaniline (DMA) by a Fe(III) N-heterocyclic carbene photosensitizer (Fe(III)NHC+) in two different solvents, serving as an example of current interest due to their relevance for the development of earth-abundant photocatalytic systems. Calculated solvent cage-escape yields of radical-cation and neutral photoproducts (DMA•+ and Fe(II)NHC, respectively) by molecular dynamics simulations reveal more favorable solvation in acetonitrile than in dichloromethane following the initial photoinduced charge-separation. These results agree with basic expectations from solvent polarity considerations but give an opposite trend compared to experimentally reported cage-escape yields. Alternative cage-escape mechanisms were therefore considered computationally to account for the anomalous experimental cage-escape yields. Both quantum chemical calculations and molecular dynamics simulations support the formation of radical-cation dimers ([(DMA)2]•+), allowing for more efficient charge migration involving the radical-cations as the polarity of the solvent is decreased. The results further demonstrate the ability of the counterion (PF6–) to stabilize the photoproducts through radical-cation–anion pairing, suggesting that these bimolecular interactions can also play an important role to preferentially promote photoproduct formation in less polar solvents. Both radical-cation dimer formation and radical-cation–counterion interactions are therefore proposed to provide additional pathways that help to explain the experimental observations of anomalous solvation dependence of the cage-escape dynamics in the investigated system. The broader implications of the bimolecular cage-escape processes on photocatalytic reaction dynamics are also considered based on our findings about light-induced intermolecular interactions.本研究通过结合分子动力学模拟（molecular dynamics simulations）与量子化学计算（quantum chemical calculations），对溶液中双分子光氧化还原产物（bimolecular photoredox products）的溶剂笼逃逸动力学（solvent cage-escape dynamics）开展了计算层面的系统探究。本研究聚焦于两种不同溶剂中，三价铁氮杂环卡宾光敏剂（Fe(III) N-heterocyclic carbene photosensitizer, Fe(III)NHC+）对有机电子供体二甲基苯胺（dimethylaniline, DMA）的光诱导氧化（photoinduced oxidation）反应，该体系因与地球储量丰富的光催化体系（earth-abundant photocatalytic systems）的开发密切相关而受到当前学界的广泛关注。通过分子动力学模拟计算得到的自由基阳离子（radical-cation）与中性光产物（neutral photoproducts，分别为DMA•+与Fe(II)NHC）的溶剂笼逃逸产率（solvent cage-escape yields）显示，在初始光诱导电荷分离（photoinduced charge-separation）后，乙腈（acetonitrile）中的溶剂化作用优于二氯甲烷（dichloromethane）。该结果符合基于溶剂极性（solvent polarity）考量的基本预期，但与实验报道的溶剂笼逃逸产率趋势恰好相反。因此，本研究通过计算模拟考察了多种替代溶剂笼逃逸机制，以解释该异常的实验溶剂笼逃逸产率。量子化学计算与分子动力学模拟均证实，自由基阳离子二聚体（radical-cation dimers, [(DMA)2]•+）的形成可促进电荷迁移（charge migration）过程——当溶剂极性降低时，由自由基阳离子参与的电荷迁移效率会显著提升。研究结果进一步表明，抗衡离子（counterion, PF6–，六氟磷酸根）可通过自由基阳离子-阴离子配对（radical-cation–anion pairing）作用稳定光产物，提示这类双分子相互作用同样可在低极性溶剂中优先促进光产物的生成，发挥关键作用。综上，本研究提出自由基阳离子二聚体形成与自由基阳离子-抗衡离子相互作用这两种额外途径，可用于解释所研究体系中溶剂笼逃逸动力学的溶剂依赖关系异常的实验观测结果。此外，本研究还基于光诱导分子间相互作用的相关发现，探讨了双分子溶剂笼逃逸过程对光催化反应动力学（photocatalytic reaction dynamics）的更广泛启示意义。