Molecular Solvation Dynamics in Phosphonium Ionic Liquids

Abstract

The goal of this research is to determine the solvation dynamics in four environmentally-friendly, “green”, phosphonium ionic liquids (PILs) + cosolvent binary mixtures. Rose Bengal is a prototypical fluorescent molecule known for its spectral sensitivity and is used to probe the IL mixtures. Neat ILs and methanol (MeOH) solvents were used to form an array of IL mixtures in which rose Bengal was dissolved. Solvation of rose Bengal was determined using steady-state and time-resolved fluorescence spectroscopy. The solute emission intensity is quenched most effectively at a mole fraction of ~0.1 PIL suggesting that the solvent-solute interactions are most unique in this range of mole fraction. Similarly, the lifetime data shows a minimum value at ~0.1 mol fraction PIL, also implying quenching of the probe at this solution composition. Rose Bengal is better solvated, more relaxed, at MeOH-rich mole fractions. Solvation of rose Bengal occurs at a faster rate in solutions of lower mole fraction PIL.