Dynamic Solvent Control of a Reaction in Ionic Deep Eutectic Solvents: Time-Resolved Fluorescence Measurements of Reactive and Nonreactive Dynamics in (Choline Chloride + Urea) Melts

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• 作者：Anuradha Das ; Ranjit Biswas
• 刊名：Journal of Physical Chemistry B
• 出版年：2015
• 出版时间：August 6, 2015
• 年：2015
• 卷：119
• 期：31
• 页码：10102-10113
• 全文大小：736K
• ISSN：1520-5207

文摘

Dynamic fluorescence anisotropy and Stokes shift measurements of [f choline chloride + (1 鈥?f) urea)] deep eutectic solvents at f = 0.33 and 0.40 have been carried out using a dipolar solute, coumarin 153 (C153), in the temperature range 298 鈮?T 鈮?333 K. Subsequently, measured time-dependent solvent response is utilized to investigate the dynamic solvent control on the measured rates of photoexcited intramolecular charge transfer (ICT) reactions of two molecules, 4-(1-azetidinyl)benzonitrile (P4C) and 4-(1-pyrrolidinyl)benzonitrile (P5C), occurring in these media. Measured average reaction time scales (鉄ㄏ?sub>rxn鉄? exhibit the following dependence on average solvation times scales (鉄ㄏ?sub>s鉄?: 鉄ㄏ?sub>rxn鉄?鈭?鉄ㄏ?sub>s鉄?sup>伪 with 伪 = 0.5 and 0.35 for P4C and P5C, respectively. Such a strong dynamic solvent control of 鉄ㄏ?sub>rxn鉄? particularly for P4C, is different from earlier observations with these ICT molecules in conventional molecular solvents. Excitation wavelength-dependent fluorescence emissions of C153 and trans-2-[4-(dimethylamino)styryl]-benzothiazole (DMASBT), which differ widely in average fluorescence lifetimes (鉄ㄏ?sub>life鉄?, suggest the presence of substantial spatial heterogeneity in these systems. Dynamic heterogeneity is reflected via the following fractional viscosity (畏) dependences of 鉄ㄏ?sub>s鉄?and 鉄ㄏ?sub>r鉄?(鉄ㄏ?sub>r鉄?being solute鈥檚 average rotation time): 鉄ㄏ?sub>x鉄?鈭?(畏/T)^p with 0.7 鈮?p 鈮?0.9. Different correlations between 鉄ㄏ?sub>s鉄?and 鉄ㄏ?sub>r鉄?emerge at different temperature regimes, indicating variable frictional coupling at low and high temperatures. Estimated dynamic Stokes shifts in these media vary between 鈭?200 and 鈭?600 cm^鈥?, more than 50% of which possess a time scale much faster than the temporal resolution (鈭?5 ps) employed in these measurements. Estimated activation energy for 畏 is closer to that for 鉄ㄏ?sub>r鉄?than that for 鉄ㄏ?sub>s鉄? suggesting 鉄ㄏ?sub>s鉄?being more decoupled from 畏 than 鉄ㄏ?sub>r鉄?