Molecular Solvation in Phosphonium Ionic Liquids

Abstract

The goal of this research is to understand the solvation dynamics of coumarin 153 (C153) in an environmentally-friendly room temperature phosphonium ionic liquid (RTPIL) solvent. With virtually no vapor pressure, ILs are attracting attention as potential “green” replacements for conventional volatile organic solvents. ILs are also known for chemical stability, non-flammability and recycling potential. C153 is a prototypical fluorescent molecule known for its spectral sensitivity when in solution making it ideal for these studies. Neat trihexyltetradecyl phosphonium chloride (PIL-Cl) and methanol (MeOH) solvents were used to form an array of PIL-Cl mixtures spanning the complete range of mol fraction, in which C153 was dissolved. Solvation of C153 was determined using steady-state and time-resolved fluorescence spectroscopy. The C153 steady-state data shows a systematic blue shift as PIL-Cl is added to solution. The system is at net higher energy at high mol fraction PIL-Cl implying that C153/PIL-Cl interactions are less favorable compared to C153/MeOH. The solute emission intensity is quenched most effectively at a mol fraction of ~0.03 PIL-Cl suggesting that the solvent-solute interactions are most unique in this range of mol fraction. Similarly, the lifetime data show a minimum value at ~0.03 mol fraction PIL-Cl, also implying quenching of the probe at this relative solution composition. C153 is better solvated, more relaxed, at MeOH-rich mol fractions. Solvation dynamics are characterized by time-resolved Stokes shift measurements. The time-resolved center of gravity and associated solvation correlation function, C(t), show that solvation of C153 occurs at a faster rate in solutions of lower mol fraction PIL-Cl. The solvation times correlate to solvent viscosity. PILs showed slower solvation due to much larger viscosities than MeOH.