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. 2021 Sep 26;26(19):5828.
doi: 10.3390/molecules26195828.

Validation of Structural Grounds for Anomalous Molecular Mobility in Ionic Liquid Glasses

Mikhail Yu Ivanov  1 , Sergey A Prikhod'ko  2 , Olga D Bakulina  1 , Alexey S Kiryutin  1 , Nicolay Yu Adonin  2 , Matvey V Fedin  1
Affiliations

Validation of Structural Grounds for Anomalous Molecular Mobility in Ionic Liquid Glasses

Mikhail Yu Ivanov et al. Molecules. .

Abstract

Ionic liquid (IL) glasses have recently drawn much interest as unusual media with unique physicochemical properties. In particular, anomalous suppression of molecular mobility in imidazolium IL glasses vs. increasing temperature was evidenced by pulse Electron Paramagnetic Resonance (EPR) spectroscopy. Although such behavior has been proven to originate from dynamics of alkyl chains of IL cations, the role of electron spin relaxation induced by surrounding protons still remains unclear. In this work we synthesized two deuterated imidazolium-based ILs to reduce electron-nuclear couplings between radical probe and alkyl chains of IL, and investigated molecular mobility in these glasses. The obtained trends were found closely similar for deuterated and protonated analogs, thus excluding the relaxation-induced artifacts and reliably demonstrating structural grounds of the observed anomalies in heterogeneous IL glasses.

Keywords: deuteration effect; glasses; ionic liquids; molecular mobility; nanostructure; spin probe.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Temperature dependence of spin echo dephasing rates, 1/T2, for N1 dissolved in ionic liquid [Bmim]BF4 (black squares, adapted from Ref. [56]) and for MTSSL in water/glycerol (1:1) mixture (green circles, adapted from Ref. [63]) measured at the maximum of echo-detected spectrum, position I in Figure 2a below.
Scheme 1
Scheme 1
Chemical structures of studied protonated and deuterated ILs and spin probe N1.
Figure 2
Figure 2
(a) Illustrative echo-detected spectrum with indicated field positions I and II where T2 was measured, B is the magnetic field. (b–g) Temperature dependence of T2−1 recorded in corresponding spectrum position for protonated and deutered ILs listed in Scheme 1. (h–j) Temperature dependence of the motional parameter L for nitroxide radical N1 in ILs, see text for details. Vertical dashed lines show the corresponding Tg values for each IL: Tg = 188 K for [Bmim]BF4 [56], Tg = 223 K for [Bmim]Br [77] and Tg = 197 K for [BuPy]BF4 [56].
Figure 3
Figure 3
Temperature dependence of T2−1 recorded in spectral positions I (a) and II (b) of N1 for protonated and deuterated ethanol. (c) Temperature dependence of the motional parameter L for nitroxide radical N1 in protonated and deuterated ethanol. The vertical dashed line shows the glass transition temperature for ethanol Tg = 97 K.
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