Anomalous Fluorescence Quenching in Fluorous Solvent-Added\nMedia

Due to the inherent high electronegativity of fluorine,\nperfluorocarbons\nhave the potential to exhibit unusual characteristics. Fluorous solvents,\nin this context, may afford an anomalous solubilizing behavior compared\nto their hydrocarbon analogues. Addition of perfluorodecalin (PFD)\nto <i>n</i>-hexane results in unusual fluorescence quenching\nof polycyclic aromatic hydrocarbons (PAHs) by the quencher nitromethane.\nAs more viscous PFD is added to less viscous <i>n</i>-hexane,\nthe dynamic viscosity (η) of the media increases. The bimolecular\nquenching rate constants (<i>k</i><sub>q</sub>) of the PAHs,\ninstead of decreasing, increase as PFD is added to <i>n</i>-hexane until an equimolar mixture composition is obtained; <i>k</i><sub>q</sub> exhibits an expected decrease only in the\nPFD-rich region of the mixture. The expected decrease in <i>k</i><sub>q</sub> in the hydrocarbon analogue decalin added to <i>n</i>-hexane is observed across all compositions. It is proposed\nthat highly electronegative fluorines on PFD stabilize the partial\npositive charge (δ<sup>+</sup>) that develops on excited PAHs\nduring electron/charge transfer to the quencher nitromethane, facilitating\nquenching in the process. In the PFD-rich region, however, increased\nη starts to dominate the quenching, resulting in the expected\ndecrease in <i>k</i><sub>q</sub>. A monotonic decrease in <i>k</i><sub>q</sub> is observed in the PFD-added <i>n</i>-hexane system for fluoranthene quenching by triethylamine (TEA)\nas TEA acts as the electron/charge donor and a partial negative charge\n(δ<sup>–</sup>) develops on excited fluoranthene during\nthe quenching process. No such stabilization by PFD is observed when\nnitrobenzene is employed as the quencher. This is attributed to the\nsignificantly higher quenching (<i>K</i><sub>S</sub> >\n10<sup>4</sup> M<sup>–1</sup>) of PAH fluorescence by nitrobenzene\ndue to the presence of aromatic π–π interactions\nbetween PAH and nitrobenzene, further facilitated by the higher electron\naffinity of nitrobenzene as compared to nitromethane. The role of\nfluorous media in facilitating electron/charge transfer processes\nis clearly established.