Dynamics in Organic Ionic Liquids in Distinct Regions Using Charged and Uncharged Orientational Relaxation Probes

The dynamics of a series of small molecule probes with increasing alkyl chain length: water, methanol, and ethanol, diluted to low concentration in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, was investigated with 2D infrared vibrational echo (2D IR) spectroscopy and polarization resolved pump-probe (PP) experiments on the deuterated hydroxyl (O-D) stretching mode of each of the solutes. The long timescale spectral diffusion observed by 2D IR, capturing complete loss of vibrational frequency correlation through structural fluctuation of the medium, shows a clear but not dramatic slowing as the probe alkyl chain length is increased: 23 ps for water, 28 ps for methanol, and 34 ps for ethanol. Although in each case, only a single population of hydroxyl oscillators contributes to the infrared line shapes, the isotropic pump-probe decays (normally caused by population relaxation) are markedly nonexponential at short times. The early time features correspond to the timescales of the fast spectral diffusion measured with 2D IR. These fast isotropic pump-probe decays are produced by unequal pumping of the OD absorption band to a nonequilibrium frequency dependent population distribution caused by significant non-Condon effects. Orientational correlation functions for these three systems, obtained from pump-probe anisotropy decays, display several periods of restricted angular motion (wobbling-in-a-cone) followed by complete orientational randomization. The cone half-angles, which characterize the angular potential, become larger as the experimental frequency moves to the blue. These results indicate weakening of the angular potential with decreasing hydrogen bond strength. The slowest components of the orientational anisotropy decays are frequency-independent and correspond to the complete orientational randomization of the solute molecule. These components slow appreciably with increasing chain length: 25 ps for water, 42 ps for methanol, and 88 ps for ethanol. The shape and volume of the probe, therefore, impact reorientation far more severely than they do spectral diffusion at long times, though these two processes occur on similar timescales at earlier times.

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“…It should be noted that the non-hydrodynamic behavior is in contrast to the hydrodynamic behavior displayed by large solutes in RTILs. 63,64…”

Section: Orientational Relaxationmentioning

confidence: 99%

Dynamics of water, methanol, and ethanol in a room temperature ionic liquid

Kramer

Giammanco

Fayer

2015

The Journal of Chemical Physics

230

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“…Nevertheless, viscosities of the ionic liquids have also been altered by increasing the length of the alkyl chain on one of the ions in quite a few studies. [17][18][19][20][21][22][23][24][25]39 In a majority of instances, it has been observed that the experimentally measured reorientation times did not comply with the trends predicted by the Stokes− Einstein−Debye (SED) hydrodynamic theory. 43,44 In other words, τ r versus η/T plots displayed a significant degree of nonlinearity, and also τ r values at a given η/T, especially for nonpolar and dipolar solutes, are found to be lower in ionic liquids with longer alkyl chains.…”

Section: Introductionmentioning

confidence: 99%

Influence of the Organized Structure of 1-Alkyl-3-methylimidazolium Tetrafluoroborates on the Rotational Diffusion of Structurally Similar Nondipolar Solutes

Gangamallaiah

Dutt

2014

J. Phys. Chem. B

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To understand how the organized structure of the ionic liquids influences the location and mobility of nondipolar solutes, rotational diffusion of 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) has been examined in 1-alkyl-3-methylimidazolium (alkyl = ethyl, butyl, hexyl, and octyl) tetrafluoroborates. Both the solutes are structurally similar-the sole difference being the two NCH3 groups of DMDPP are replaced by two NH groups in DPP. The rotational diffusion of DPP is found to be significantly slower than DMDPP due to specific interactions between the NH groups of the solute and the anion of the ionic liquid. It has been observed that for a given viscosity and temperature, the rotational diffusion of DMDPP becomes progressively faster with an increase in the length of the alkyl chain on the imidazolium cation. DMDPP resides in the nonpolar domains of these ionic liquids whose sizes increase with an increase in the length of the alkyl chain, and as a result it experiences microviscosity that is lower than the bulk viscosity. However, an increase in the length of the alkyl chain has no apparent effect on the rotational diffusion of DPP because specific interactions with tetrafluoroborate necessitate the solute to be located in the vicinity of the anion. The results of this work exemplify that despite having similar size and shape, the rotational diffusion of DMDPP and DPP is quite contrasting as their sites of solubilization and the nature of interactions with the surroundings are vastly different owing to subtle variations in their chemical structures.

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“…In the context of this heterogeneous liquid structure, highly selective solvation is possible. (36) The resulting diversity of local environments can have a direct impact on the dynamics of processes where solvation environment is important. The effect of environment has been observed in the context of a photoisomerization.…”

Section: Introductionmentioning

confidence: 99%

Dynamics and structure of room temperature ionic liquids

Fayer

2014

Chemical Physics Letters

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Room temperature ionic liquids (RTIL) are intrinsically interesting because they simultaneously have properties that are similar to organic liquids and liquid salts. In addition, RTILs are increasingly being considered for and used in technological applications. RTILs are usually composed of an organic cation and an inorganic anion. The organic cation, such as imidazolium, has alkyl chains of various lengths. The disorder in the liquid produced by the presence of the alkyl groups lowers the temperature for crystallization below room temperature and can also result in supercooling and glass formation rather than crystallization. The presence of the alkyl moieties also results in a segregation of the liquid into ionic and organic regions. In this article, experiments are presented that address the relationship between RTIL dynamics and structure. Time resolved fluorescence anisotropy measurements were employed to study the local environments in the organic and ionic regions of RTILs using a nonpolar chromophore that locates in the organic regions and an ionic chromophore that locates in the ionic regions. In the alkyl regions, the in plane and out of plane orientational friction coefficients change in different manners as the alkyl chains get longer. Both friction coefficients converge toward those of a long chain length hydrocarbon as the RTIL chains increase in length, which demonstrates that for sufficiently long alkyl chains the RTIL organic regions have properties similar to a hydrocarbon. However, putting Li + in the ionic regions changes the friction coefficients in the alkyl regions, which

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Dynamics in Organic Ionic Liquids in Distinct Regions Using Charged and Uncharged Orientational Relaxation Probes

Cited by 117 publications

References 44 publications

Dynamics of water, methanol, and ethanol in a room temperature ionic liquid

Dynamics of water, methanol, and ethanol in a room temperature ionic liquid

Influence of the Organized Structure of 1-Alkyl-3-methylimidazolium Tetrafluoroborates on the Rotational Diffusion of Structurally Similar Nondipolar Solutes

Dynamics and structure of room temperature ionic liquids

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