Studies on the Solvation Dynamics of Coumarin 153 in 1‐Ethyl‐3‐Methylimidazolium Alkylsulfate Ionic Liquids: Dependence on Alkyl Chain Length

Abstract

Steady-state and time-resolved fluorescence behavior of coumarin 153 (C153) is investigated in a series of 1-ethyl-3-methylimidazolium alkylsulfate ([C(2)mim][C(n)OSO(3)]) ionic liquids differing only in the length of the linear alkyl chain (n = 4, 6, and 8) in the anion. The aim of the present study is to understand the role of alkyl chain length in solute rotation and solvation dynamics of C153 in these ionic liquids. The blueshift observed in the steady-state absorption and emission maxima of C153 on going from the C(4)OSO(3) to the C(8)OSO(3) system indicates increasing nonpolar character of the microenvironment of the solute with increasing length of the alkyl side chain of the anion of the ionic liquids. The average solvation time is also found to increase on changing the substituent from butyl to octyl, and this is attributed to the increase in the bulk viscosity of the ILs. A steady blueshift of the time-zero maximum of the fluorescence spectrum with increasing alkyl chain length also indicates that the probe molecule experiences a less polar environment in the early part of the dynamics. Rotational dynamics of C153 are also analyzed by using the Stokes-Einstein-Debye (SED), Gierer-Wirtz (GW), and Dote-Kivelson-Schwartz (DKS) theories. Analyses of the results seem to suggest decoupling of the rotational motion of the probe from solvent viscosity.

How Does Addition of Lithium Salt Influence the Structure and Dynamics of Choline Chloride-Based Deep Eutectic Solvents?

“…For example, upon going from ethaline to glyceline, the rise in viscosity is nearly 10 times, whereas the ⟨τ s ⟩ increases by a factor of ∼3 (Table ). This feature can be observed more clearly when ⟨τ s ⟩ is fitted through an empirical relation , as follows where p denotes the dependence of the (η) to the ⟨τ s ⟩. From the fitted plot (Figure ), the value of p is found to be ∼0.65, indicating substantial nonlinearity of the (η) with ⟨τ s ⟩.…”

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

“…Nonpolar fluorophores sometimes exhibit times below slip predictions, ,, and molecules with charged functional groups may have rotation times greater than stick predictions. ,, Within a single solvent, rotation times typically conform to τ rot ∝ (η/ T ) p , and in most cases, p ≅ 1, as expected from hydrodynamics. However, much smaller powers are sometimes found. ,− For example, in one study where pressure rather than temperature was varied, values of p as small as 0.55 and 0.39 were reported . In a few cases, departures from such a dependence, suggestive of a decoupling of rotation from viscosity at higher viscosities, have been reported. , …”

“…In most cases, rotation times are found to fall between the limiting hydrodynamic predictions of stick and slip boundary conditions, but exceptions are also observed. Nonpolar fluorophores sometimes exhibit times below slip predictions, 18,28,16 and molecules with charged functional groups may have rotation times greater than stick predictions. 19,21,26 Within a single solvent, rotation times typically conform to τ rot ∝ (η/T) p , and in most cases, p ≅ 1, as expected from hydrodynamics.…”

“…To date, the method most often applied to measure rotational dynamics in ionic liquids has been time-dependent fluorescence anisotropy of aromatic fluorophores. − Early studies ,, showed that rotational correlation functions in ionic liquids are often nonexponential but with correlation times τ rot that conform to the hydrodynamic expectation τ rot ∝ η/ T , where η is the solution viscosity and T the temperature. These early studies also reported rotation times to be consistent with extrapolations of the times observed in conventional solvents to the higher viscosities prevailing in ionic liquids.…”

“…These early studies also reported rotation times to be consistent with extrapolations of the times observed in conventional solvents to the higher viscosities prevailing in ionic liquids. Since then, a large number of solute + ionic liquid systems have been studied using fluorescence anisotropy, particularly by the groups of Dutt − and Sarkar, − and a variety of behaviors have been reported. In most cases, rotation times are found to fall between the limiting hydrodynamic predictions of stick and slip boundary conditions, but exceptions are also observed.…”