Yadav Anita, Pandey Siddharth
Department of Chemistry, Indian Institute of Technology Delhi , Hauz Khas, New Delhi 110016, India.
J Phys Chem B. 2017 Dec 7;121(48):10922-10933. doi: 10.1021/acs.jpcb.7b08329. Epub 2017 Nov 21.
Ionic liquids, being composed of ions alone, may offer alternative pathways for molecular aggregation. These pathways could be controlled by the chemical structure of the cation and the anion of the ionic liquids. Intramolecular excimer formation dynamics of a bifluorophoric probe, 1,3-bis(1-pyrenyl)decane [1Py(10)1Py], where the fluorophoric pyrene moieties are separated by a long decyl chain, is investigated in seven different ionic liquids in 10-90 °C temperature range. The long alkyl separator allows for ample interaction with the solubilizing milieu prior to the formation of the excimer. The ionic liquids are composed of two sets, one having four ionic liquids of 1-butyl-3-methylimidazolium cation ([bmim]) with different anions and the other having four ionic liquids of bis(trifluoromethylsulfonyl)imide anion ([TfN]) with different cations. The excimer-to-monomer emission intensity ratio (I/I) is found to increase with increasing temperature in sigmoidal fashion. Chemical structure of the ionic liquid controls the excimer formation efficiency, as I/I values within ionic liquids with the same viscosities are found to be significantly different. The excited-state intensity decay kinetics of 1Py(10)1Py in ionic liquids do not adhere to a simplistic Birk's scheme, where only one excimer conformer forms after excitation. The apparent rate constants of excimer formation (k) in highly viscous ionic liquids are an order of magnitude lower than those reported in organic solvents. In general, the higher the viscosity of the ionic liquid, the more sensitive is the k to the temperature with higher activation energy, E. The trend in E is found to be similar to that for activation energy of the viscous flow (E). Stokes-Einstein relationship is not followed in [bmim] ionic liquids; however, with the exception of [choline][TfN], it is found to be followed in [TfN] ionic liquids suggesting the cyclization dynamics of 1Py(10)1Py to be diffusion-controlled and to depend on the viscosity of the ionic liquid irrespective of the identity of the cation. The dependence of ionic liquid structure on cyclization dynamics to form intramolecular excimer is amply highlighted.
离子液体仅由离子组成，可能为分子聚集提供替代途径。这些途径可由离子液体的阳离子和阴离子的化学结构控制。在10 - 90°C温度范围内，在七种不同的离子液体中研究了双荧光探针1,3 - 双（1 - 芘基）癸烷[1Py(10)1Py]的分子内激基缔合物形成动力学，其中芘荧光团部分由长的癸基链隔开。长的烷基间隔基使得在激基缔合物形成之前能与增溶环境充分相互作用。离子液体分为两组，一组有四种1 - 丁基 - 3 - 甲基咪唑鎓阳离子（[bmim]）与不同阴离子组成的离子液体，另一组有四种双（三氟甲基磺酰）亚胺阴离子（[TfN]）与不同阳离子组成的离子液体。发现激基缔合物与单体发射强度比（I/I）随温度升高呈S形增加。离子液体的化学结构控制激基缔合物的形成效率，因为发现具有相同粘度的离子液体中的I/I值有显著差异。1Py(10)1Py在离子液体中的激发态强度衰减动力学并不遵循简单的伯克方案，即在激发后只形成一种激基缔合物构象。在高粘度离子液体中激基缔合物形成的表观速率常数（k）比在有机溶剂中报道的低一个数量级。一般来说，离子液体的粘度越高，k对温度就越敏感，活化能E越高。发现E的趋势与粘性流动的活化能（E）相似。在[bmim]离子液体中不遵循斯托克斯 - 爱因斯坦关系；然而，除了[胆碱][TfN]外，发现在[TfN]离子液体中遵循该关系，这表明1Py(10)1Py的环化动力学是扩散控制的，并且取决于离子液体的粘度，而与阳离子的身份无关。离子液体结构对形成分子内激基缔合物的环化动力学的依赖性得到了充分强调。
