MD Study of Stokes Shifts in Ionic Liquids: Temperature Dependence

Abstract

Effects of temperature on Stokes shifts, solvation structure, and dynamics in ionic liquids EMI(+)Tf2N(-), EMI(+)PF6(-), and BMI(+)PF6(-) (EMI(+) = 1-ethyl-3-methylimidazolium, BMI(+) = 1-butyl-3-methylimidazolium, Tf2N(-) = bis(trifluoromethylsulfonyl)imide, and PF6(-) = hexafluorophosphate) are investigated via molecular dynamics (MD) computer simulations in the temperature range 350 K ≤ T ≤ 500 K. Two different types of solutes are considered: a simple model diatomic solute and realistic coumarin 153, both of which are characterized by more polar S1 and less polar S0 states. In all three ionic liquids studied, the Stokes shift tends to decrease with increasing temperature. For coumarin 153, as T increases, the Franck-Condon energy for steady-state absorption decreases, whereas that for steady-state emission increases. Our findings indicate that the effective polarity of ionic liquids decreases as T increases. Their solvation dynamics are characterized by an ultrafast initial decay in the subpicosecond time scale, followed by slow dissipative relaxation, regardless of temperature. For both solutes, the solvent frequency that quantifies initial ultrafast dynamics shows little temperature dependence. By contrast, the long-time dissipative dynamics become significantly faster with rising T. Variations of solvation structure with temperature and their connection to Stokes shift and solvation dynamics are briefly examined.

Spectroscopic and MD Study of Dynamic and Structural Heterogeneities in Ionic Liquids

“…To gain insight into the experimental results, MD simulations of C153 were performed. We simulated 200 different trajectories, each for 50 ns, and analyzed the change in the Franck–Condon energy of the probe that is induced by solvation ,,− Here V g 0 ( r ) and V e 0 ( r ) are the ground- and excited-state electronic energies of the isolated probe, V g s ( r , R ) and V e s ( r , R ) are the corresponding energies in solution, and r and R are the solute and solvent coordinates, respectively. Z is a collective (solvent) coordinate, widely employed to describe solvation effects .…”

Dielectric Relaxation of the Ionic Liquid 1-Ethyl-3-methylimidazolium Ethyl Sulfate: Microwave and Far-IR Properties

“…One of the possible reasons for this discrepancy is the difference in temperature, i.e., 298 K for OKE vs 350 K for FIR. However, according to previous simulation studies of imidazolium-based ILs, 15,54 frequencies of resonance modes in dielectric loss and timeresolved fluorescence spectroscopies are not that sensitive to temperature though their intensities, especially in the former, are. Another reason is the difference in selection rules as mentioned above.…”

Solvation dynamics in polar solvents and imidazolium ionic liquids: failure of linear response approximations

“…31 ) is frequently used to justify the use of linear response theory in solvation dynamics of ionic liquids. 30 , 37 , 39 – 43 In this work of Kim and coworkers a set of 400 nonequilibrium simulations extending to 2 ps is used to describe the solvation dynamics of artificial solutes in two imidazolium ionic liquids, and compared to equilibrium simulations invoking linear response theory. 32 Although the authors argue that linear response holds reasonable well in their system, the agreement between the Stokes shift from their equilibrium and nonequilibrium simulations seems to be unsatisfactory, and the integral timescales differ considerably.…”