Yadav Anita, Kurur Narayanan D, Pandey Siddharth
Department of Chemistry, Indian Institute of Technology Delhi , Hauz Khas, New Delhi 110016, India.
J Phys Chem B. 2015 Oct 22;119(42):13367-78. doi: 10.1021/acs.jpcb.5b07282. Epub 2015 Oct 12.
Mixtures of ionic liquid with polyethylene glycol (PEG) have shown interesting features as solubilizing media. Intramolecular excimer formation dynamics of 1,3-bis-(1-pyrenyl)propane [1Py(3)1Py] is investigated within mixtures of a common and popular ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) with PEGs of average molecular weight (MW) 200 (PEG200), average MW 400 (PEG400), number-average MW Mn 570-630 (PEG600), and number-average MW Mn 950-1050 (PEG1000) over the complete composition range at a 10° interval in the temperature range 10-90 °C. Irrespective of the composition of the medium and the temperature, excited-state intensity decay of the excimer fluorescence best fits to a three-exponential decay function, suggesting the presence of one excited-state monomer and two kinetically distinguishable excimers where both excimers are populated simultaneously by the excited monomer with no interconversion between the two excimers. In neat PEGs for temperatures ≤ 50 °C, intensity decay data of monomer fluorescence best fits to a single-exponential decay function, which implies the dissociation of both excimers back to the monomer to be insignificant. As the temperature is increased, the fits become closer to a double-exponential decay function, implying dissociation of one of the excimers to become significant. In neat [bmim][PF6], while a double-exponential decay function is required to fit the monomer excited-state intensity decay data at lower temperatures, three exponentials are required to satisfactorily fit the data at higher temperatures, suggesting both excimers significantly dissociate back to the monomer at higher temperatures within the ionic liquid. Within long-chain PEG-containing ([bmim][PF6] + PEG) mixtures, PEG as opposed to [bmim][PF6] controls the excimer formation dynamics by supposedly wrapping around the excimer, thus hindering dissociation back to the monomer. The overall rate constant of the excimer formation within ([bmim][PF6] + PEG) mixtures is found to scale better with the microviscosity rather than the bulk viscosity of the mixtures.
离子液体与聚乙二醇（PEG）的混合物作为增溶介质展现出了有趣的特性。在常见且常用的离子液体1-丁基-3-甲基咪唑六氟磷酸盐（[bmim][PF6]）与平均分子量（MW）为200（PEG200）、平均MW为400（PEG400）、数均MW Mn为570 - 630（PEG600）以及数均MW Mn为950 - 1050（PEG1000）的PEG的混合物中，在10 - 90 °C温度范围内以10°间隔研究了1,3-双-(1-芘基)丙烷[1Py(3)1Py]的分子内准分子形成动力学。无论介质组成和温度如何，准分子荧光的激发态强度衰减最适合用三指数衰减函数来拟合，这表明存在一个激发态单体和两个动力学上可区分的准分子，其中两个准分子由激发单体同时填充，且两个准分子之间不存在相互转化。在温度≤50 °C的纯PEG中，单体荧光的强度衰减数据最适合用单指数衰减函数来拟合，这意味着两个准分子解离回单体的情况不显著。随着温度升高，拟合更接近双指数衰减函数，这意味着其中一个准分子的解离变得显著。在纯[bmim][PF6]中，虽然在较低温度下需要用双指数衰减函数来拟合单体激发态强度衰减数据，但在较高温度下需要三个指数才能令人满意地拟合数据，这表明在离子液体中较高温度下两个准分子都显著解离回单体。在含长链PEG的（[bmim][PF6] + PEG）混合物中，与[bmim][PF6]相反，PEG通过假定围绕准分子缠绕来控制准分子形成动力学，从而阻碍解离回单体。发现（[bmim][PF6] + PEG）混合物中准分子形成的总速率常数与混合物的微观粘度而非本体粘度的相关性更好。
