Sharma Sudhanshu, Yadav Dipti, Pandey Siddharth
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India.
J Phys Chem B. 2024 Aug 29;128(34):8194-8206. doi: 10.1021/acs.jpcb.4c03947. Epub 2024 Aug 20.
Due to the inherent high electronegativity of fluorine, perfluorocarbons have the potential to exhibit unusual characteristics. Fluorous solvents, in this context, may afford an anomalous solubilizing behavior compared to their hydrocarbon analogues. Addition of perfluorodecalin (PFD) to -hexane results in unusual fluorescence quenching of polycyclic aromatic hydrocarbons (PAHs) by the quencher nitromethane. As more viscous PFD is added to less viscous -hexane, the dynamic viscosity (η) of the media increases. The bimolecular quenching rate constants () of the PAHs, instead of decreasing, increase as PFD is added to -hexane until an equimolar mixture composition is obtained; exhibits an expected decrease only in the PFD-rich region of the mixture. The expected decrease in in the hydrocarbon analogue decalin added to -hexane is observed across all compositions. It is proposed that highly electronegative fluorines on PFD stabilize the partial positive charge (δ) that develops on excited PAHs during electron/charge transfer to the quencher nitromethane, facilitating quenching in the process. In the PFD-rich region, however, increased η starts to dominate the quenching, resulting in the expected decrease in . A monotonic decrease in is observed in the PFD-added -hexane system for fluoranthene quenching by triethylamine (TEA) as TEA acts as the electron/charge donor and a partial negative charge (δ) develops on excited fluoranthene during the quenching process. No such stabilization by PFD is observed when nitrobenzene is employed as the quencher. This is attributed to the significantly higher quenching ( > 10 M) of PAH fluorescence by nitrobenzene due to the presence of aromatic π-π interactions between PAH and nitrobenzene, further facilitated by the higher electron affinity of nitrobenzene as compared to nitromethane. The role of fluorous media in facilitating electron/charge transfer processes is clearly established.
由于氟具有固有的高电负性，全氟碳化合物有可能表现出异常特性。在这种情况下，含氟溶剂与其碳氢化合物类似物相比，可能呈现出反常的增溶行为。向正己烷中添加全氟萘烷（PFD）会导致猝灭剂硝基甲烷对多环芳烃（PAHs）产生异常的荧光猝灭。随着将更多粘性的PFD添加到粘性较小的正己烷中，介质的动态粘度（η）增加。当向正己烷中添加PFD时，PAHs的双分子猝灭速率常数（）不是降低，而是增加，直到获得等摩尔混合物组成；仅在混合物中富含PFD的区域才呈现出预期的降低。在向正己烷中添加碳氢化合物类似物萘烷的情况下，在所有组成中都观察到了预期的降低。据推测，PFD上的高电负性氟会稳定激发态PAHs在向猝灭剂硝基甲烷进行电子/电荷转移过程中产生的部分正电荷（δ），从而在此过程中促进猝灭。然而，在富含PFD的区域，η的增加开始主导猝灭，导致预期的降低。在通过三乙胺（TEA）猝灭荧蒽的添加了PFD的正己烷体系中，观察到随着TEA作为电子/电荷供体，并且在猝灭过程中激发态荧蒽上会产生部分负电荷（δ），呈现出单调降低。当使用硝基苯作为猝灭剂时，未观察到PFD的这种稳定作用。这归因于由于PAH与硝基苯之间存在芳香族π-π相互作用，以及硝基苯与硝基甲烷相比具有更高的电子亲和力，导致硝基苯对PAH荧光的猝灭作用显著更高（> 10 M）。含氟介质在促进电子/电荷转移过程中的作用已得到明确证实。
