Solvation dynamics and rotation of coumarin 153 in alkylphosphonium ionic liquids

Solvation and rotational dynamics of coumarin 153 (C153) were measured in a series of phosphonium ionic liquids consisting of the trihexyl(tetradecyl)phosphonium cation and the anions Br-, Cl-, dicyanamide(-), bis(trifluoromethylsulfonyl)imide(-), and BF4-. None of these liquids display a prominent ultrafast solvation component comparable to the one observed in imidizalium ionic liquids. The solvation dynamics observed on the nanosecond time scale are also 5-fold slower than those in imidazolium liquids of comparable viscosities. Both rotation and solvation are highly nonexponential functions of time and can be represented by stretched-exponential decays with exponents near 0.5. Characteristic times of these processes track solvent viscosity. A number of similarities between the dynamics observed in these ionic liquids and supercooled glass-forming liquids are noted.