PERSPECTIVE pubs.acs.org/JPCL
Solvation Dynamics in Ionic Liquids: What We Have Learned from the Dynamic Fluorescence Stokes Shift Studies Anunay Samanta* School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
ABSTRACT Ionic liquids have generated tremendous excitement as a new class of solvent systems having the potential to serve as an environmentally friendly alternative to the traditionally used volatile organic compounds and have emerged as specialty media or materials for many advanced processes and devices. Solvent reorganization in ionic liquids is a complex process, which differs significantly from that in conventional polar molecular solvents. This Perspective highlights the present understanding of the mechanism and time scale of the dynamics of solvation in ionic liquids obtained from the dynamic fluorescence Stokes shift studies on dipolar molecules taking the related theoretical results into consideration.
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found conspicuous when the large difference in the molecular structures of the various solvent systems is taken into consideration. Reichardt could identify the role of the hydrogenbonding interaction of the C(2) hydrogen in determining the microscopic polarity of a large collection of imidazolium ILs by noting that the C(2)-alkylated imidazolium ILs are substantially less polar than those comprising the C(2) hydrogen.17 This implies that a narrow range of polarity of a large number of imidazolium ILs is the result of the leveling effect of the C(2) hydrogen atom. The solute-solvent hydrogen-bonding interaction energy in C(2) H-containing imidazolium ILs is overwhelming larger than that due to the other weaker interactions.
he low-melting salts that are liquid in ambient conditions are termed room-temperature ionic liquids (ILs). These substances, which, because of their negligible vapor pressure, attracted the attention of researchers worldwide as possible green alternatives to the commonly used volatile organic compounds, have emerged in recent years as specialty media or materials for several advanced processes and devices, many of which are only possible through the use of ILs.1-3 These ILs usually consist of an organic cation and an organic or inorganic anion. Some commonly used cationic and anionic constituents of the ILs are shown in Chart 1. Among the most frequently used ILs, those based on the substituted imidazolium cations with BF4-, PF6-, and (CF3SO2)2N- are the most popular ones. Utilization of a new solvent system in applications requires not only knowledge of its bulk properties but also an understanding of these properties from a molecular-level picture of the intermolecular interactions, structure, and dynamics of the substances. This explains why the past decade has seen a boom in the number of experimental and theoretical studies on ILs addressing this issue.4-11 The scope of this Perspectve is to highlight the current understanding of the mechanism of solvent reorganization dynamics in ILs. Even though we recognize the importance of the mixed solvents comprising the IL as one of the components and we are aware that a large number of studies have addressed solvent reorganization dynamics in these mixed solvents, we avoid any discussion on them because of the limited space available to this Perspective.12,13 Early steady-state absorption and fluorescence studies on dipolar solutes indicated that polarity of the imidazolium ILs, as indicated by the microscopic solvent polarity parameter (such as the ET(30) or ENT value)14 and determined by the local interaction between the solvatochromic probe molecule and the solvent, is comparable to that of the short-chain aliphatic alcohols.15,16 A rather small variation of the solvent polarity among the members of a large family of imidazolium ILs is
r 2010 American Chemical Society
A narrow range of polarity of a large number of imidazolium ILs is the result of the leveling effect of the C(2) hydrogen atom. The solute-solvent hydrogen bonding interaction energy in C(2) H-containing imidazolium ILs is overwhelmingly larger than that due to the other interactions. Dielectric constant is another (bulk) property often used as a measure of the polarity of a solvent. The dielectric constants of the imidazolium ILs, with the exception of one system, lie Received Date: February 27, 2010 Accepted Date: April 23, 2010 Published on Web Date: April 28, 2010
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DOI: 10.1021/jz100273b |J. Phys. Chem. Lett. 2010, 1, 1557–1562
PERSPECTIVE pubs.acs.org/JPCL
Karmakar and Samanta were the first to initiate the TDFSS studies in ILs to understand the mechanism of solvation dynamics.18-20 That common dipolar molecules such as coumarin 153, 4-aminophthalimide, and 6-propionyl-2dimethylaminonaphthalene (PRODAN) show wavelengthdependent fluorescence decay profiles and TDFSS in the ps-ns time scale (Figure 1) indicated that solvation dynamics in ILs is slow when compared to that of the common molecular solvents such as acetonitrile, methanol, or water. These early studies not only suggested that slow dynamics is a common feature in viscous ILs but also indicated that the time-resolvable component of the dynamics was biexponential in nature, with the average solvation time dependent on the viscosity of the media and the probe molecule.18-20 Interestingly, even though the dynamics was found to be slow, it was noted that nearly 50% of the spectral relaxation dynamics was quite fast compared to the time resolution (∼25 ps) of the experimental time-correlated single-photon counting (TCSPC) setup. These observations were later corroborated independently by Sarkar21 and Maroncelli,22-25 and studies were also extended to a wider range of ILs.26-29 Attempts were made to time resolve the ultrafast component, believed to be
