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

Steady-state and time-resolved fluorescence behavior of coumarin 153 (C153) is investigated in a series of 1-ethyl-3-methylimidazolium alkylsulfate ([C2mim][CnOSO3]) 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 C4OSO3 to the C8OSO3 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 StokesEinsteinDebye (SED), GiererWirtz (GW), and DoteKivelsonSchwartz (DKS) theories. Analyses of the results seem to suggest decoupling of the rotational motion of the probe from solvent viscosity.