EPR Study of Rotational Diffusion in Viscous Ionic Liquids: Analysis by a Fractional Stokes–Einstein–Debye Law

Miyake, Yusuke; Hidemori, Takehiro; Akai, Nobuyuki; Kawai, Akio; Shibuya, Kazuhiko; Koguchi, Shinichi; Kitazume, Tomoya

doi:10.1246/cl.2009.124

Cited by 32 publications

(34 citation statements)

References 12 publications

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“…Rotational (as well as translational) dynamics of dilute solutes have also been measured using ESR techniques on stable radical probes by several groups. − Nearly all such studies have employed the TEMPO probe (2,2,6,6-tetramethylpiperidine-1-yloxyl) or its derivatives. For example, Strehmel and co-workers studied rotation of eight differently functionalized variants of TEMPO in a variety of ionic liquids .…”

Section: Introductionmentioning

confidence: 99%

Rotational Dynamics in Ionic Liquids from NMR Relaxation Experiments and Simulations: Benzene and 1-Ethyl-3-Methylimidazolium

et al. 2016

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Temperature-dependent (2)H longitudinal spin relaxation times (T1) of dilute benzene-d6 in 1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) and two deuterated variants of the 1-ethyl-3-methylimidazolium cation (Im21(+)-d1 and Im21(+)-d6) in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Im21][Tf2N]), measured at multiple Larmor frequencies, were used to probe rotational dynamics in ionic liquids. Rotational correlation times significantly faster than predicted by slip hydrodynamic calculations were observed for both solutes. Molecular dynamics simulations of these systems enabled extraction of more information about the rotational dynamics from the NMR data than rotation times alone. The multifrequency (2)H T1(T) data could be fit to within uncertainties over a broad region about the T1 minimum using models of the relevant rotational time correlation functions and their viscosity/temperature dependence derived from simulation. Such simulation-guided fitting provided confidence in the semiquantitative accuracy of the simulation models and enabled interpretation of NMR measurements to higher viscosities than previously possible. Simulations of the benzene system were therefore used to explore the nature of solute rotation in ionic liquids and how it might differ from rotation in conventional solvents. Whereas "spinning" about the C6 axis of benzene senses similarly weak solvent friction in both types of solvents, "tumbling" (rotations about in-plane axes) differs significantly in conventional solvents and ionic liquids. In the sluggish environment provided by ionic liquids, orientational caging and the presence of rare but influential large-amplitude (180°) jumps about in-plane axes lead to rotations being markedly nondiffusive, especially below room temperature.

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Section: Introductionmentioning

confidence: 99%

Rotational Dynamics in Ionic Liquids from NMR Relaxation Experiments and Simulations: Benzene and 1-Ethyl-3-Methylimidazolium

et al. 2016

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“…The activation energy for the probe’s rotational tumbling in each solvent were reasonably close to those of the ILs viscous flow. 26 On the other side, Miyake et al 23 have observed that the rotational correlation time of proxyl radicals in [C 4 mim][BF 4 ] and [C 4 mim][PF 6 ] obeys a fractional SED law, which they compared with the rotation of TEMPO in supercooled liquids.…”

Section: Introductionmentioning

confidence: 99%

Study of Nanostructural Organization of Ionic Liquids by Electron Paramagnetic Resonance Spectroscopy

Merunka

Perić

2015

J. Phys. Chem. B

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The X-band electron paramagnetic resonance spectroscopy (EPR) of a stable, spherical nitroxide spin probe, perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDTO) has been used to study the nanostructural organization of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids (ILs) with alkyl chain lengths from two to eight carbons. By employing nonlinear least-squares fitting of the EPR spectra, we have obtained values of the rotational correlation time and hyperfine coupling splitting of pDTO to high precision. The rotational correlation time of pDTO in ILs and squalane, a viscous alkane, can be fit very well to a power law functionality with a singular temperature, which often describes a number of physical quantities measured in supercooled liquids. The viscosity of the ILs and squalane, taken from the literature, can also be fit to the same power law expression, which means that the rotational correlation times and the ionic liquid viscosities have similar functional dependence on temperature. The apparent activation energy of both the rotational correlation time of pDTO and the viscous flow of ILs and squalane increases with decreasing temperature; in other words, they exhibit strong non-Arrhenius behavior. The rotational correlation time of pDTO as a function of η/T, where η is the shear viscosity and T is the temperature, is well described by the Stokes-Einstein-Debye (SED) law, while the hydrodynamic probe radii are solvent dependent and are smaller than the geometric radius of the probe. The temperature dependence of hyperfine coupling splitting is the same in all four ionic liquids. The value of the hyperfine coupling splitting starts decreasing with increasing alkyl chain length in the ionic liquids in which the number of carbons in the alkyl chain is greater than four. This decrease together with the decrease in the hydrodynamic radius of the probe indicates a possible existence of nonpolar nanodomains.

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“…2 These characteristic features appeal to many researchers, interested in making new solvents with unique properties. [3][4][5][6][7][8][9][10][11][12][13] Various novel ILs that have paramagnetic and/or photochromic properties have been synthesized. [5][6][7][8][9] Recently, a new type of ILs having photochromic activity has been developed.…”

Section: Introductionmentioning

confidence: 99%

“…[3][4][5][6][7][8][9][10][11][12][13] Various novel ILs that have paramagnetic and/or photochromic properties have been synthesized. [5][6][7][8][9] Recently, a new type of ILs having photochromic activity has been developed. [10][11][12][13] These ILs contain a photofunctional substituent, such as a phenylazo group or a stilbene moiety, and following photoirradiation undergo a photochromic reaction, which leads to a change in the solvent properties.…”

Section: Introductionmentioning

confidence: 99%

Isomerization dynamics of the 2-phenylazo-1,3-dimethylimidazolium cation photoexcited to the S2 (π, π*) state as studied by transient absorption spectroscopy in the time domain of 10−13 to 103 seconds

Ida

Cohen

Asaka

et al. 2011

Phys. Chem. Chem. Phys.

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Photoinduced isomerization of a novel photochromic cation, [2PA-Mmim](+) (2-phenylazo-1,3-dimethylimidazolium cation), was studied by optical spectroscopic methods. The UV-Vis absorption spectra of the [2PA-Mmim](+) cation show two prominent bands starting around 410 and 520 nm, corresponding to the S(0)-S(2) (π, π*) and S(0)-S(1) (n, π*) transitions, respectively. The photoisomerization mechanism is studied by femtosecond time-resolved transient absorption experiments performed after S(0)-S(2) (π, π*) excitation in several solvents with different viscosity, including ionic liquids. The transient absorption signals at two representative wavelengths were fitted by bi-exponential functions, which yield four decay components. The photoisomerization mechanism is discussed in light of the relaxation schemes available for azobenzene. Only one of the components depends on the solvent viscosity and it changes from 1.2 ps (dichloromethane, 0.4 cP) to 5.6 ps ([Bmim][BF(4)], 93 cP). This component is assigned to a molecule at the S(1) state, which is responsible for the "rotational" isomerization. The weak dependence on the solvent viscosity of this component is explained in terms of local change in the viscosity as a result of local heating due to excess energy released at S(2)-S(1) internal conversion. The other three components of ∼0.4, 1.0 and 10 ps are attributed to relaxation processes of the molecule at S(2), S(1) and S(0) states, respectively. The quantum yields for the forward E-Z photoisomerization are ∼0.15 after S(2) excitation. The backward Z-E isomerization is slow with a lifetime of 1 hour and an activation energy of 91 kJ mol(-1) through an "inversion" mechanism.

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EPR Study of Rotational Diffusion in Viscous Ionic Liquids: Analysis by a Fractional Stokes–Einstein–Debye Law

Cited by 32 publications

References 12 publications

Rotational Dynamics in Ionic Liquids from NMR Relaxation Experiments and Simulations: Benzene and 1-Ethyl-3-Methylimidazolium

Rotational Dynamics in Ionic Liquids from NMR Relaxation Experiments and Simulations: Benzene and 1-Ethyl-3-Methylimidazolium

Study of Nanostructural Organization of Ionic Liquids by Electron Paramagnetic Resonance Spectroscopy

Isomerization dynamics of the 2-phenylazo-1,3-dimethylimidazolium cation photoexcited to the S2 (π, π*) state as studied by transient absorption spectroscopy in the time domain of 10−13 to 103 seconds

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