Fluorescence correlation spectroscopy evidence for structural heterogeneity in ionic liquids

Abstract

In this work, we provide new experimental evidence for chain length-dependent self-aggregation in room temperature ionic liquids (RTILs) using fluorescence correlation spectroscopy (FCS). In studying a homologous series of N-alkyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide, [C(n)MPy][Tf(2)N] RTILs of varying alkyl chain length (n = 3, 4, 6, 8, and 10), biphasic rhodamine 6G solute diffusion dynamics were observed; both the fast and slow diffusion coefficients decreased with increasing alkyl chain length, with the relative contribution from slower diffusion increasing for longer-chain [C(n)MPy][Tf(2)N]. We propose that the biphasic diffusion dynamics originate from self-aggregation of the nonpolar alkyl chains in the cationic [C(n)MPy](+).

The influence of mesoscopic confinement on the dynamics of imidazolium-based room temperature ionic liquids in polyether sulfone membranes

“…4,5 An important attribute of ionic liquids, which has attracted considerable interest, is their ability to form nanodomains that involve the segregation of local ionic and apolar regions. [6][7][8] Since nanodomains' formation was first noticed via molecular dynamics simulations, [6][7][8] the picture of segregation has been used to explain a variety of experimental data in X-ray scattering, 9,10 dielectric spectroscopy, 11,12 neutron magnetic resonance (NMR) spectroscopy, 10,13,14 fluorescence spectroscopy, [15][16][17] optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments, [18][19][20] and two-dimensional infrared (2D IR) spectroscopy. 21,22 It is important to clarify that not all RTILs have the capability of forming these nanodomains.…”

Complementing Crystallography with Ultralow‐Frequency Raman Spectroscopy: Structural Insights into Nitrile‐Functionalized Ionic Liquids

“…They found biphasic diffusion dynamics in the ILs, indicating self-aggregation through the alkyl chains. [49] Patra and Samanta also showed, on the basis of their FCS and fluorescence lifetime data, that there are two distinct microscopice nvironments in some imidazolium ILs. [15] Although we believe that LFR spectroscopy is more suitable for structurale lucidation, because it can interrogate aw ider variety of ILs (whether liquid or solid at room temperature)w ithout the need for fluorescent probes, our conclusion presented above could be tested by the fluorescencemethod in future studies.…”

“…[11,15,[46][47][48][49] Shaw et al used fluorescence correlation spectroscopy (FCS) to study N-alkyl-N-methylpyrrolidiniumN Tf 2 with different alkyl chain lengths. [11,15,[46][47][48][49] Shaw et al used fluorescence correlation spectroscopy (FCS) to study N-alkyl-N-methylpyrrolidiniumN Tf 2 with different alkyl chain lengths.…”

“…Many pieces of evidence for structural heterogeneity in ILs have been obtainedf rom fluorescences tudies. [11,15,[46][47][48][49] Shaw et al used fluorescence correlation spectroscopy (FCS) to study N-alkyl-N-methylpyrrolidiniumN Tf 2 with different alkyl chain lengths. They found biphasic diffusion dynamics in the ILs, indicating self-aggregation through the alkyl chains.…”

Structure and Dynamics at Ionic Liquid/Electrode Interfaces

“…Thus, the authors concluded that the difference of the diffusion coefficients originates from subtle packing effects and not from inter-molecular interactions. Applying dye rhodamine 6G as tracer solute, a fast and a slow diffusion process were observed in pyrrolidinium TFSI-based ILs [60]. The respective fluorescence correlation spectroscopy study comprised ILs of various alkyl chain lengths.…”