Effect of Water, Methanol, and Acetonitrile on Solvent Relaxation and Rotational Relaxation of Coumarin 153 in Neat 1-Hexyl-3-methylimidazolium Hexafluorophosphate

Abstract

The dynamics of solvent relaxation in ionic liquid (IL)-water, IL-methanol, and IL-acetonitrile mixtures have been investigated using steady state and picosecond time-resolved fluorescence spectroscopy. We have used Coumarin 153 (C-153) and 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF(6)]) as fluorescence probe and IL, respectively. The steady-state emission spectra showed that the gradual addition of cosolvents increases the polarity of the mixtures. In neat [hmim][PF(6)] and all IL-cosolvent mixtures, solvation occurs in two well-separated time regimes within the time resolution of our instrument. A substantial portion of the solvation has been missed due to the limited time resolution of our instrument. The gradual addition of cosolvents decreases the viscosity of the medium and consequently solvation time also decreases. The decrease in solvation time is more pronounced on addition of acetonitrile compared to water and methanol. The rotational relaxation time of the probe is also decreasing with gradual addition of the cosolvents. The decrease in viscosity of the solution is responsible for the decrease in the rotational relaxation time of the probe molecule.

Spectroscopic Evidence for Unusual Microviscosity in Imidazolium Ionic Liquid and Tetraethylene Glycol Dimethyl Ether Cosolvent Mixtures

“…C153 is considered to be an ideal probe because the dynamics probed by C153 have been found to be primarily related to nonspecific solute−solvent interactions in many RTIL systems. 17,52,53,58 BPP is a particularly sensitive probe of microviscosity especially in the heterogeneous environment such as ionic liquid systems. 55−57,59−62 As a result, the microviscosity enhancement effects in the [bmim] [PF 6 ] and TEGDME hybrid system were observed by probing the microviscosity through the two complementary experimental methods.…”

Polarity Behaviour and Specific Interactions of Imidazolium-Based Ionic Liquids in Ethylene Glycol

“…On the other hand, solute-solvent interactions can affect the probesolvent interactions. [21] Preferential solvation may arise whenever the bulk mole fraction solvent composition is different from the solvation microsphere solvent composition. The response of the spectroscopic probe is dependent on the composition in the solvation microsphere, and therefore it is able to measure the eventual preferential solvation phenomena occurring in the solvent mixtures.…”

The role of specific interactions on dynamical processes in a room temperature ionic liquid

“…[14][15][16][17][18][19] The findings of these investigations reveal that solute rotation is sluggish compared to that in conventional solvents and the observed slow reorientation times have been ascribed to the high viscosity of the ionic liquids. Baker et al 14 have measured the reorientation times of a large non-polar solute BTBP (N,N-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicaboximide) in ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim + ][PF -6 ]) as a function of temperature and concluded that a single layer of [bmim + ] or ] associates with the solute.…”

“…Rotational diffusion and solvation dynamics has been investigated in mixtures of ionic liquid with polar solvents such as water, methanol and acetonitrile. 18,19,29 These studies reveal that solvation as well as rotational dynamics becomes faster with an increase in the amount of polar solvent which has been attributed to decrease in the viscosity of the ionic liquid. Addition of non-polar solvents such as toluene and 1,4-dioxane has been found to produce a similar effect on the dynamics.…”