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Importance of Ionic Liquid Solvation Dynamics to Their Applications in Advanced Devices and Systems
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One of the other major differences between ILs and conventional solvents is that the viscosities of ILs are higher, ranging between that of ethylene glycol on the low end up to that of honey and beyond. Since the viscosity is related to the other dynamical processes of ILs, solvational, rotational, and translational processes take much (even up to 1000) longer in ILs than in other solvents. Samanta's Perspective3 covers these aspects in detail. Typical of glassy materials, ILs show a multicomponent solvation response ranging from picoseconds to nanoseconds.8 Moreover, this response is inhomogeneously broadened by the heterogeneous structure of ILs, potentially resulting in heterogeneous dynamics depending on the distribution of the probe system across various local environments. This has been manifested experimentally in terms of excitation-wavelength-dependent emission9 and electrontransfer dynamics,10 that is, “red edge effects”. Margulis11 has performed simulations demonstrating the effects of static and dynamic heterogeneity on electron transfer in ILs indicating that wavelength selection could be used to control the course of an electron-transfer reaction. Several researchers have begun to investigate electrontransfer reactions in ILs.10-16 In certain cases the electron transfer and IL solvent dynamical time scales are wellseparated, and the system can be treated by classical Marcus-Hush theory. This is particularly true if electron transfer is the slower process. If electron transfer is very fast (etens of picoseconds), it still would coincide with the inertial part of the dynamical response; however, it could be gated by the slower diffusional part of the response. A possible example of this is the work of Murray on electron transport in ferrocene-tethered imidazolium ILs, where ion atmosphere relaxation is invoked to explain the nearly equal diffusion coefficients and diffusion activation barriers for the electron and the ions.14 If the electron transfer and IL solvation processes overlap in time, the electron-transfer driving force and reorganization energy effectively vary as time progresses.15 In that case, the interpretation of the temperature dependence of electron transfer can be particularly complicated since the solvation process will get faster as the temperature increases, thus changing the effective electron-transfer energetics (and effective dielectric constant) with temperature as well.16 Therein lies the significance of solvation and solvation dynamics to the practical deployment of ILs in advanced systems for the production, storage, and efficient use of energy. Many of these applications involve electron-transfer reactions,2 such as electrochemical conversion of solar energy (dye-sensitized solar cells, quantum dots, solar-to-fuels), hydrogen production, fuel cells, electrochromic displays, electromechanical actuators and energy harvesters, batteries, supercapacitors, sensors, and electrorefining and
t has been approximately 10 years since the field of ionic liquid (IL) research appeared on the radar screens of the general community of chemists. During this time, interest and research activity in ILs have grown tremendously and show no sign of slowing down. Early research in ILs was led by electrochemists and synthetic chemists, and generally, the only physical data then available for ILs were conductivities, viscosities, and phase behavior. Slowly at first, and then with increasing vigor, physical chemists have risen to meet the challenges of understanding these fascinating and diverse materials that provide a path to new science and technologies.1,2 ILs have a wide variety of useful properties that can be tailored for almost any purpose by substitution, functionalization, and/or mixing of their constituent anions and cations. This tunability is one of the key factors in the usefulness of ILs for physical chemistry research (and real-world applications as well) because a set of ILs can be selected (or designed and created if necessary) to vary the specific property of interest. These properties include controlling the solubility of polar and nonpolar solutes, miscibility/immiscibility with water, organic or fluorous solvents, melting point and liquidus range, viscosity, conductivity, diffusion rates, electrochemical window, and vapor pressure, to name a few. Often, the properties of ILs lie at the edge or beyond those of conventional solvents, increasing both their potential utility and their curiosity factor. As described in the Perspective by Samanta,3 an early step that spectroscopists took was to characterize IL polarities by using a variety of solvatochromic probe molecules. They generally found polarities comparable to acetonitrile or methanol, with some exceptions and variations depending on the probe used. Subsequently, dielectric spectroscopy measurements4 estimated the static dielectric constants of several ILs to be roughly half of the values implied by most of the solvatochromic probes. Acontroversy might have ensued but for the contemporary publication of highly revelatory molecular dynamics simulations by Lopes and Padua5 depicting nanoscale organization in ILs, depending on the length of the alkyl chains attached to the cations. The segregation of the IL into local polar and nonpolar domains may account for the varied polarities obtained by different reporter molecules and their relationship to the bulk dielectric spectroscopy results. Recent work by Fayer on the fluorescence anisotropy decay of charged and uncharged fluorophores shows how probe molecules can interact with different parts of the IL.6 Thus, ILs are not homogeneous solvation environments. The existence of structural organization in ILs, with domain scales varying as a function of alkyl chain length, was demonstrated by Triolo and co-workers shortly thereafter.7 However, the exact nature of the ordering, and how it varies with chain length and cation and anion types and functionalization, is a yet-unsolved problem within the larger context of ongoing IL structural studies being pursued by many groups across the globe.
r 2010 American Chemical Society
Received Date: April 26, 2010 Accepted Date: April 26, 2010 Published on Web Date: May 20, 2010
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DOI: 10.1021/jz100532k |J. Phys. Chem. Lett. 2010, 1, 1629–1630
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plating of metals, including spent nuclear fuel recycling. Judicious choice of an IL with the correct dynamical properties for a given application will permit control of the electron-transfer reaction in the desired direction with optimum efficiency. A choice made without understanding the relationship between IL dynamics and electron transfer will likely lead to poor results. The problems that need to be solved are hard ones since several things may be changing at the same time. Nevertheless, the attractive properties of ILs for use in practical devices make them worth the challenge.
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James Wishart
Stereoselective Fluorescence Quenching by Photoinduced Electron Transfer in a Naproxen Dyad. J. Phys. Chem. B 2009, 113, 10825–10829. Wang, W.; Balasubramanian, R.; Murray, R. W. Electron Transport and Counterion Relaxation Dynamics in Neat Ferrocenated Imidazolium Ionic Liquids. J. Phys. Chem. C 2008, 112, 18207–18216. Shim, Y.; Kim, H. J. Adiabatic Electron Transfer in a RoomTemperature Ionic Liquid: Reaction Dynamics and Kinetics. J. Phys. Chem. B 2009, 113, 12964–12972. Lockard, J. V.; Wasielewski, M. R. Intramolecular Electron Transfer within a Covalent, Fixed-Distance Donor-Acceptor Molecule in an Ionic Liquid. J. Phys. Chem. B 2007, 111, 11638–11641.
Chemistry Department, Brookhaven National Laboratory Upton, New York 11973 ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under Contract # DE-AC02-98CH10886.
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Wishart, J. F.; Castner, E. W. The Physical Chemistry of Ionic Liquids. J. Phys. Chem. B 2007, 111, 4639–4640. (2) Wishart, J. F. Energy Applications of Ionic Liquids. Energy Environ. Sci. 2009, 2, 956–961. (3) Samanta, A. Solvation Dynamics in Ionic Liquids: What We Have Learned from the Dynamic Fluorescence Stokes Shift Studies. J. Phys. Chem. Lett. 2010, 1, 1557–1562. (4) Wakai, C.; Oleinikova, A.; Ott, M.; Weingartner, H. How Polar Are Ionic Liquids? Determination of the Static Dielectric Constant of an Imidazolium-Based Ionic Liquid by Microwave Dielectric Spectroscopy. J. Phys. Chem. B 2005, 109, 17028– 17030. (5) Lopes, J. N. A. C.; Padua, A. A. H. Nanostructural Organization in Ionic Liquids. J. Phys. Chem. B 2006, 110, 3330–3335. (6) Fruchey, K.; Fayer, M. D. Dynamics in Organic Ionic Liquids in Distinct Regions Using Charged and Uncharged Orientational Relaxation Probes. J. Phys. Chem. B 2010, 114, 2840–2845. (7) Triolo, A.; Russina, O.; Bleif, H. J.; Di Cola, E. Nanoscale Segregation in Room Temperature Ionic Liquids. J. Phys. Chem. B 2007, 111, 4641–4644. (8) Arzhantsev, S.; Jin, H.; Baker, G. A.; Maroncelli, M. Measurements of the Complete Solvation Response in Ionic Liquids. J. Phys. Chem. B 2007, 111, 4978–4989. (9) Samanta, A. Dynamic Stokes Shift and Excitation Wavelength Dependent Fluorescence of Dipolar Molecules in Room Temperature Ionic Liquids. J. Phys. Chem. B 2006, 110, 13704–13716. (10) Jin, H.; Li, X.; Maroncelli, M. Heterogeneous Solute Dynamics in Room Temperature Ionic Liquids. J. Phys. Chem. B 2007, 111, 13473–13478. (11) Annapureddy, H. V. R.; Margulis, C. J. Controlling the Outcome of Electron Transfer Reactions in Ionic Liquids. J. Phys. Chem. B 2009, 113, 12005–12012. (12) Vieira, R. C.; Falvey, D. E. Photoinduced Electron-Transfer Reactions in Two Room-Temperature Ionic Liquids: 1-Butyl3-methylimidazolium Hexafluorophosphate and 1-Octyl-3methylimidazolium Hexafluorophosphate. J. Phys. Chem. B 2007, 111, 5023–5029. (13) Bose, S.; Wijeratne, A. B.; Thite, A.; Kraus, G. A.; Armstrong, D. W.; Petrich, J. W. Influence of Chiral Ionic Liquids on
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