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Differences in the Behavior of Dicationic and Monocationic Ionic Liquids as Revealed by Time Resolved-Fluorescence, NMR and Fluorescence Correlation Spectroscopy (FCS) Study


With an aim to understand the behavior in terms of intermolecular interactions, structure and dynamics of dicationic and monocationic ionic liquids (ILs), two imidazolium-based dicationic ionic liquids (DILs), 1,8-bis-(3-methylimidazolium-1-yl)octane bis-(trifluoromethylsulfonyl)amide ([C8(mim)2][NTf2]2), 1,9-bis-(3-methylimidazolium-1-yl)nonane bis-(trifluoromethylsulfonyl)amide ([C9(mim)2][NTf2]2) and one monocationic ionic liquid (MIL), 1-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide ([C4(mim)][NTf2]), has been investigated through combined fluorescence, electron paramagnetic resonance (EPR), NMR and fluorescence correlation spectroscopy (FCS). The DILs have been synthesized by following a standard synthethic protocol and subsequently charecterized by different analytical techniques. Steady state absorption, emission and EPR spectroscopic data reveals that DILs are less polar as compared to MIL. The polarity of DILs and MIL are found to be close to acetonitrile and short chain alcohol respectively. The excitation wavelength dependent emission data reveals that DILs are more micro-heterogeneous in nature than MIL. The rotational diffussion of two organic solutes, perylene and 8-methoxypyrene-1,3,6-sulfonate (MPTS), have been examined in DILs and MIL. The rotational diffusion data for perylene and MPTS have been analyzed in light of Stokes-Einstein-Debye (SED) hydrodynamic theory. The rotation of perylene in DILs has been observed to be relatively faster than that in MIL, and it goes beyond the limit predicted by SED theory. In order to explain the rotational motion of perylene in DILs, the data has been analyzed further by invoking quasi-hydrodynamic theory. The observed rotational behavior of perylene has been explained by considering the fact that perylene is located in the nonpolar region of ILs, and larger solvent molecule (DILs) induces lower friction to the rotating solute. Interestingly, unlike perylene, rotations of MPTS in both ILs are observed to be much hindered indicating relatively stronger MPTS-IL interaction than perylene-IL interaction. More interestingly, rotation of MPTS is observesd to be faster in DILs than that in MIL despite the fact that DILs is more viscous than MIL. Relatively faster rotation of MPTS in DILs has been explained by resorting to NMR and FCS studies. Outcome of NMR and FCS studies have revealed that DIL in the experimental condition exists in folded form and because of this structural restriction of DIL it becomes difficult for bulky MPTS to make stronger hydrogen bonding interaction with DIL, which eventually makes rotation of MPTS in DIL faster. Essentially outcome of all these studies have demonstrated that the behavior of DILs are quite different than the usual MIL.

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Publication details

The article was received on 26 Dec 2017, accepted on 12 Feb 2018 and first published on 12 Feb 2018

Article type: Paper
DOI: 10.1039/C7CP08630J
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Differences in the Behavior of Dicationic and Monocationic Ionic Liquids as Revealed by Time Resolved-Fluorescence, NMR and Fluorescence Correlation Spectroscopy (FCS) Study

    D. Majhi, S. Seth and M. Sarkar, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C7CP08630J

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