Toward understanding solute-solvent interaction in room-temperature mono- and dicationic ionic liquids: a combined...

Rotational relaxation dynamics of nonpolar perylene, dipolar coumarin 153, and a negatively charged probe, sodium 8-methoxypyrene-1,3,6-sulfonate (MPTS), have been investigated in a dicationic ionic liquid, 1,6-bis-(3-methylimidazolium-1-yl)hexane bis-(trifluoromethylsulfonyl)amide ([C6(MIm)2][NTf2]2), and a structurally similar monocationic ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C6MIm][NTf2]), to have a comprehensive and a quantitative understanding on the solute-solvent interaction in these media. Analysis of the rotational relaxation dynamics data by Stokes-Einstein-Debye (SED) hydrodynamic theory reveals that perylene rotation is found to be the fastest compared to the other two probes and shows slip to sub-slip behavior, coumarin 153 rotation lies between the stick and slip boundary, and MPTS shows a superstick behavior in [C6MIm][NTf2]. Interestingly, MPTS exhibits a normal SED hydrodynamics in dicationic [C6(MIm)2][NTf2]2, in spite of the fact that dicationic ionic liquid contains two cationic sites bearing acidic hydrogen (C2-H) which may be available to form stronger interaction with the negatively charged MPTS. The difference in the rotational diffusion behavior of these three probes is a reflection of their location in different distinct environments of these ILs. Superstick behavior of MPTS in monocationic IL has been attributed to its specific hydrogen bonding interaction with the corresponding imidazolium cation. The relatively faster rotational behavior of MPTS in dicationic IL has been explained by resorting to mass spectrometry. Mass spectral analysis demonstrates that positively charged (imidazolium) sites in dicationic IL are strongly associated with negatively charged bis-(trifluoromethylsulfonyl)amide anion (NTf2(-)), which in turn makes it difficult for imidazolim cation to have stronger hydrogen bonding interaction with bulkier negatively charged molecule MPTS.