Measurements of the Complete Solvation Response in Ionic Liquids ² Sergei Arzhantsev, ‡ Hui Jin, ‡ Gary A. Baker, § and Mark Maroncelli* ,‡ Department of Chemistry, The PennsylVania State UniVersity, 104 Chemistry Building, UniVersity Park, PennsylVania 16802, and Chemical Sciences DiVision, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6110 ReceiVed: NoVember 4, 2006; In Final Form: January 2, 2007 Dynamic Stokes shift measurements of the solvatochromic probe trans-4-dimethylamino-4′-cyanostilbene were used to measure the solvation response of five imidazolium and one pyrrolidinium ionic liquid at 25 °C. The Kerr-gated emission and time-correlated single-photon-counting techniques were used to measure spectral dynamics occurring over the time ranges of 100 fs-200 ps and 50 ps-5 ns, respectively, and a combination of data sets from these two techniques enabled observation of the complete solvation response. Observed response functions were found to be biphasic, consisting of a sub-picosecond component of modest (10-20%) amplitude and a dominant slower component relaxing over times of a few picoseconds to several nanoseconds. The faster component could be correlated to inertial characteristics of the constituent ions, and the slower component to solvent viscosity. Dielectric continuum calculations of the sort previously used to predict solvation dynamics in dipolar liquids were shown to work poorly for predicting the response in these ionic liquids. 1. Introduction Widespread interest in the potential use of room-temperature ionic liquids as solvents in a variety of contexts has engendered study of the nature of solvation in these distinctive media. 1,2 In addition to many reports addressing the equilibrium “polarity” and solvating abilities of ionic liquids, 2-5 a number of recent papers have explored dynamical aspects of solvation in these solvents. Such studies include measurements of ultrafast po- larizability dynamics 6-14 and dielectric dispersion in neat ionic liquids 15-20 as well as studies concerned with the time-dependent response to solute-induced perturbations. 19-47 The latter dynam- ics, especially the response of a solvent to an electrical perturbation of a solute, commonly known as “polar solvation dynamics”, 21 is the topic of the present paper. Characterizing such dynamics is of interest both for the new perspectives it provides on fundamental aspects of liquid-phase dynamics and also for its relevance to understanding how ultrafast chemical reactions, especially charge-transfer processes, couple to an ionic-liquid environment. Measurements of polar solvation dynamics in ionic liquids actually began with the work of Huppert and co-workers, 22-25 prior to all of the recent interest in these solvents. These authors measured Stokes shift dynamics of several probe solutes in a number of tetraalkyammonium fused salts whose melting points (105-170 °C) place them on the border of what would now be considered “room-temperature” ionic liquids. The first measure- ments on modern ionic liquids, based on the imidazolium cation, were made by Karmakar and Samanta in 2002. 26,27 Subsequent work by Samanta and co-workers 28-31 as well as contributions by our group 32-37 and those of Sarkar, 38-42 Petrich, 43-46 Richert, 19,20 and others 47-49 have resulted in the accumulation of a sizable database on solvation dynamics in ionic liquids. Samanta has recently reviewed much of this work 50,51 so that here we will only highlight some of the emerging trends. Nearly all measurements to date have employed time- correlated single-photon-counting (TCSPC) or other time- resolved emission techniques having time resolutions in the 25- 100 ps range. Although there can be considerable variability in the quantitative results reported by different groups for the same ionic liquid 35,43 (especially in the case of highly viscous liquids 50 ) the general features are consistent in all cases. In picosecond experiments the solvation energy is observed to relax in a nonexponential manner, typically over times of tens of picoseconds to several nanoseconds. Some authors chose to fit this relaxation using biexponential functions of time and interpret these times in terms of two distinct types of solvent motion 26-31 whereas we 32-34 and others 19,20,35 prefer to employ a stretched- exponential representation and view the nonexponential relax- ation as resulting from a single but complex dynamical process that resembles the dynamics occurring in supercooled liquids. Independent of how one chooses to fit the data, a clear correlation exists between the integral solvation times measured in such experiments and solvent viscosity. This relationship to viscosity is obvious in measurements performed in a single ionic liquid as a function of temperature, 19,20,32,33,35 but it is also often found that approximately the same relationship between solva- tion times and viscosity holds across a number of different ionic liquids. 31,33,34,36,44 A variety of probe solutes have been employed to measure solvation dynamics, the most popular among these being coumarin 153 (C153). When measurements are conducted in a single laboratory it is generally found that different probes yield comparable solvation times, 50,51 but in a few cases variations of factors of 2-3 have been reported. 31,35 Despite the fact that integral solvation times observed in the aforementioned experiments are often in the nanosecond range, it was recognized early on that a substantial portion of the solvent relaxation is often missed. 32 In many ionic liquids, ² Part of the special issue “Physical Chemistry of Ionic Liquids”. * Author to whom correspondence should be addressed. E-mail: maroncelli@psu.edu. ‡ The Pennsylvania State University. § Oak Ridge National Laboratory. 4978 J. Phys. Chem. B 2007, 111, 4978-4989 10.1021/jp067273m CCC: $37.00 © 2007 American Chemical Society Published on Web 02/24/2007