Polymer Molecular Weight-Dependent Unusual Fluorescence Probe Behavior within 1-Butyl-3-methylimidazolium Hexafluorophosphate + Poly(ethylene glycol)

Abstract

Poly(ethylene glycols) (PEGs) and room-temperature ionic liquids (ILs) are both projected as possible alternatives to volatile organic compounds (VOCs). Their potential usage in chemical applications, however, is often hampered by their limited and, in some cases, undesired individual physicochemical properties. Properties of mixtures of PEG with a common IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) are assessed via responses of three fluorescence probes: pyrene (Py) and pyrene-1-carboxaldehyde (PyCHO) are the dipolarity sensing probes and 1,3-bis-(1-pyrenyl) propane (BPP) is the probe for microfluidity. All three probes demonstrate anomalous fluorescence behavior within the mixture of [bmim][PF6] with four different PEGs of average molecular weight (MW) 200, 400, 600, and 1500 g.mol(-1), respectively, across complete composition range. Cybotactic region dipolarity of the probe Py within the mixtures is observed to be higher than that expected from ideal additive behavior. PyCHO lowest energy fluorescence maxima implying the static dielectric constant around the cybotactic region shows values within the mixtures to be even higher than that in neat PEG, the component having higher static dielectric constant of the two, clearly indicating the milieu to have anomalously high dipolarity. "Hyperpolarity" inherent to the PEG+[bmim][PF6] mixture is confirmed. Intramolecular excimer-to-monomer fluorescence intensity ratio of BPP indicates the microfluidity within the mixture to be even lower than that within neat [bmim][PF6], the component with lowest microfluidity. Presence of strong solvent-solvent interactions within the mixture is proposed to be the major reason for the anomalous fluorescence probe responses. Specifically, extensive hydrogen-bonded network involving termini hydroxyls of PEGs and PF6- as well as ethoxy/hydroxyl oxygens of PEGs and the C2-H of bmim+ is proposed to be responsible for the unusual outcomes. Fluorescence probe responses are shown to be adequately predicted by a four-parameter simplified combined nearly ideal binary solvent/Redlich-Kister (CNIBS/R-K) model. Unusually altered physicochemical properties are demonstrated to be the key feature of the "hybrid green" PEG+IL systems.