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J. Phys. Chem. B 2009, 113, 7606–7614
Polymer Molecular Weight-Dependent Unusual Fluorescence Probe Behavior within 1-Butyl-3-methylimidazolium Hexafluorophosphate + Poly(ethylene glycol) Abhra Sarkar, Shruti Trivedi, and Siddharth Pandey* Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India ReceiVed: February 13, 2009; ReVised Manuscript ReceiVed: March 19, 2009
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. Introduction One of the longest-standing problems facing the chemical industries and academia is continued reliance on huge volumes of toxic, hazardous, flammable, and environmentally damaging organic solvents. It is of major concern among researchers and educators that the organic solvents used in many of the synthetic processes simply evaporate into the atmosphere with detrimental ecological effects and considerable impact on human health. These volatile organic solvents are generally expensive to purchase, problematic to separate/remove from the products, difficult to recycle/reuse, and impractical to dispose of without incurring substantial costs and/or adversely affecting the environment and/or personnel. The environment calls on the entire chemicals-related research edifice to define long-term strategic goals for environmentally responsible chemistry and to be patient and persistent in achieving them.1 A pollution prevention approach is much more sustainable than the “endof-pipe” approach embodied in many waste-management methods and represents one of the most profound and vital technological challenges of the 21st century. As a result, there is a clear economic, environmental, and political motivation to replace and/or minimize the use of traditional organic solvents. Among many alternatives to the traditional organic solvents that * To whom correspondence should be addressed. E-mail: sipandey@ chemistry.iitd.ac.in. Phone: +91-11-26596503. Fax: +91-11-26581102.
have been proposed in the current chemical literature poly(ethylene glycols) (PEGs) and room temperature ionic liquids (ILs) may have been the more interesting candidates with high potential as environmentally friendly solvents. While the advantages and disadvantages of PEGs as solvents of benign nature are more or less established, the same may not be said about ILs. Nonetheless the property that renders them the most attention perhaps is their extremely low vapor pressure. As a result, these solvents are employed extensively in chemistry and related disciplines.1-15 Although abundant chemical literature exists on applications of both ILs and PEGs as solvent media, there are certain drawbacks concerning these solvents that limit their potential as environmentally benign solvents. Apart from undesired physicochemical properties depending on a specific application, one of the most crucial drawbacks is the limited solubility of many common solutes in these solvents.1-16 It is interesting to note however that many solutes that are insoluble or sparingly soluble in ILs are soluble in PEGs and vice versa. Though the physicochemical properties of ILs and PEGs are proposed to be tunable by altering cation/anion combination in ILs and by varying the molecular mass of PEGs, respectively; nearly not enough modifications in key physicochemical properties can be achieved by doing so. Consequently, cosolvent-modified PEG and IL systems are becoming a topic of active research where “green” cosolvents, such as water, ethanol, supercritical CO2,
10.1021/jp901338x CCC: $40.75 2009 American Chemical Society Published on Web 05/05/2009
Unusual Fluorescence Probe Behavior etc., are of obvious importance. Addition of a cosolvent may result in favorable physicochemical properties of the system including enhanced solute solubility in certain cases. In this context, we have earlier reported our investigations on the effects of added water,17-19 ethanol,20 aqueous-ethanol,4d tetraethylene glycol,5i propylene carbonate,5j and other ionic liquids4f on different ILs. Miscibility of water and ethanol in a common and popular “hydrophobic” IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) is significantly low (∼2.1 and ∼9.7 wt %, respectively, at ambient conditions),3e,g,h,5a,f,21 which, in turn, makes it difficult to modify the physicochemical properties of this common IL to any desired extent. Investigations on the addition of water and ethanol to PEGs also imply the changes in the physicochemical properties of these systems to be fairly limited.5d Though ILs and PEGs differ considerably in many of their important properties, we found that surprisingly IL [bmim][PF6] and PEGs with average molecular weight e1500 are completely miscible with each other. The solvent system composed of ILs + PEGs provides an exciting “hybrid green” system that may afford interesting and favorably modified physicochemical properties. Because of its enhanced and possibly unusual features, this system may end up having enormous potential in many chemical applications. Subsequently, we started a detailed investigation of the properties of and solvation within IL + PEG systems. In a preliminary investigation, we observed highly unusual solvatochromic absorbance probe behavior within systems composed of IL [bmim][PF6] + PEGs of different average molecular weight at ambient conditions hinting at elevated polarity afforded by these mixtures.22 This investigation was restricted to few molecular electronic absorbance probes that only furnished limited information on dipolarity/polarizability, H-bond donating (HBD) acidity, and H-bond accepting (HBA) basicity of these systems. Though important as far as preliminary investigation is concerned, limited sensitivity and capabilities associated with electronic absorbance probes usually render such studies restricted in scope with limited understanding. In order to acquire insight to the solute-solvent and/or solvent-solvent interactions within [bmim][PF6] + PEG mixtures, we have used highly sensitive fluorescence dipolarity and microfluidity probes. In the present work we have stressed upon the several-fold motivation concerning this hybrid “green” solvent system mainly to compare it and its physicochemical features to neat and cosolvent-modified ILs and to combine ILs and PEGs as tunable, environmentally friendly media for a variety of chemical applications. Experimental Section Reagents and Supplies. IL [bmim][PF6] was purchased from Merck (ultra pure, halide content
