Kumar Manish, Pandey Siddharth
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India.
J Phys Chem B. 2024 Aug 22;128(33):8040-8053. doi: 10.1021/acs.jpcb.4c03854. Epub 2024 Aug 11.
Fluorous solvents may offer distinctly different solvation environments to a solute compared to their hydrocarbon analogues due to the inherently high electronegativity associated with fluorine. Solute solvation within -perfluorooctane (PFO) is compared with that in -octane using the well-established polycyclic aromatic hydrocarbon (PAH) fluorescence probe pyrene in the temperature range of 288 to 318 K. Both density () and dynamic viscosity () of PFO are considerably higher than those of -octane. UV-vis molecular absorbance, fluorescence emission/excitation, and excited-state emission intensity decay reveal the cybotactic region of pyrene to be more nonpolar in PFO than that in -octane. Bimolecular quenching rate constants () for the pyrene-nitrobenzene fluorophore-quencher pair adhere to the Stokes-Einstein formulation; however, they are considerably higher than the estimated rate constants for the diffusion-controlled process (). This is due to the high electron affinity of nitrobenzene leading to aromatic π-π interactions between pyrene and nitrobenzene. For a nonaromatic low electron affinity quencher, such as nitromethane, while < in -octane, > in PFO. This is due to the fact that highly electronegative fluorines on PFO stabilize the partial positive charge (δ) that develops on excited pyrene during electron/charge transfer to the quencher nitromethane, facilitating quenching in the process. Exciplex formation between pyrene and triethylamine (TEA) is more favored in PFO as opposed to -octane although > The developing charge on the exciplex is stabilized by the electronegative fluorines of the PFO. The pyrene-TEA exciplex appears to form exclusively in the excited state of pyrene, and the kinetics of exciplex formation is in the subnanosecond regime. On the contrary, the efficiency of exciplex formation between pyrene and ,-dimethylaniline (DMA) is comparable in PFO and -octane, and the kinetics is slower in comparison to that of the pyrene-TEA exciplex. Certain ground-state heterogeneity is detected for the pyrene-DMA system in PFO due to the low solubilizing ability of the fluorous solvent. Highly electronegative fluorines on perfluorohydrocarbon solvents are found to offer unusual and unique solvation characteristics.
由于氟具有固有的高电负性，与它们的烃类类似物相比，含氟溶剂可能为溶质提供截然不同的溶剂化环境。在288至318K的温度范围内，使用成熟的多环芳烃（PAH）荧光探针芘，将全氟辛烷（PFO）中的溶质溶剂化与正辛烷中的溶质溶剂化进行比较。PFO的密度（ρ）和动态粘度（η）都明显高于正辛烷。紫外可见分子吸收、荧光发射/激发以及激发态发射强度衰减表明，芘在PFO中的次配位区域比在正辛烷中更具非极性。芘 - 硝基苯荧光团 - 猝灭剂对的双分子猝灭速率常数（kq）符合斯托克斯 - 爱因斯坦公式；然而，它们远高于扩散控制过程的估计速率常数（kd）。这是由于硝基苯具有高电子亲和力，导致芘与硝基苯之间形成芳香π - π相互作用。对于非芳香性低电子亲和力猝灭剂，如硝基甲烷，虽然在正辛烷中kq < kd，但在PFO中kq > kd。这是因为PFO上的高电负性氟使芘在向猝灭剂硝基甲烷进行电子/电荷转移过程中激发态时产生稳定的部分正电荷（δ），从而促进了猝灭过程。与正辛烷相比，芘与三乙胺（TEA）之间的激基复合物形成在PFO中更有利，尽管kd > kq。激基复合物上发展的电荷通过PFO的电负性氟得以稳定。芘 - TEA激基复合物似乎仅在芘的激发态形成，且激基复合物形成的动力学处于亚纳秒范围。相反，芘与N,N - 二甲基苯胺（DMA）之间激基复合物形成的效率在PFO和正辛烷中相当，并且与芘 - TEA激基复合物相比动力学较慢。由于含氟溶剂的低溶解能力，在PFO中检测到芘 - DMA体系存在一定的基态不均匀性。发现全氟烃溶剂上的高电负性氟提供了异常且独特的溶剂化特性。
