Article pubs.acs.org/JPCB
Rotational Dynamics of Imidazolium-Based Ionic Liquids: Do the Nature of the Anion and the Length of the Alkyl Chain Influence the Dynamics? Sugosh R. Prabhu and G. B. Dutt* Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India S Supporting Information *
ABSTRACT: The rotational dynamics of 1-alkyl-3-methylimidazolium-based ionic liquids has been investigated by monitoring their inherent fluorescence with the intent to unravel the characteristics of the emitting species. For this purpose, temperaturedependent fluorescence anisotropies of 1-alkyl-3-methylimidazolium (alkyl = ethyl and hexyl) ionic liquids with anions such as tris(pentafluoroethyl)trifluorophosphate ([FAP]), bis(trifluoromethylsulfonyl)imide ([Tf2N]), tetrafluoroborate ([BF4]), and hexafluorophosphate ([PF6]) have been measured. It has been observed that the reorientation times (τr) of the ionic liquids with an ethyl chain scale linearly with viscosity and were found to be independent of the nature of the anion. The experimentally measured τr values are a factor of 3 longer than the ones calculated for 1ethyl-3-methylimidazolium cation using the Stokes−Einstein−Debye (SED) hydrodynamic theory with stick boundary condition, which suggests that the emitting species is not the imidazolium moiety but some kind of associated species. The reorientation times of ionic liquids with a hexyl chain, in contrast, follow the trend τr[FAP] > τr[Tf2N] = τr[BF4] > τr[PF6] at a given viscosity (η) and temperature (T). The ability of the ionic liquids with longer alkyl chains to form the organized structure appears to be responsible for the observed behavior considering the fact that significant deviations from linearity have been noticed in the τr versus η/T plots for strongly associating anions [BF4] and [PF6], especially at ambient temperatures.
1. INTRODUCTION There has been a tremendous surge in the research activities dealing with ionic liquids during the past decade because of their utility and novelty.1−10 The novel physicochemical properties of ionic liquids arise as a consequence of numerous interactions that prevail between the constituent cations and anions.7−9 The most commonly employed ionic liquids contain alkyl substituted imidazolium, pyridinium, pyrrolidinium, phosphonium, piperidinium, pyrazolium, sulfonium, thiazolium, and ammonium cations in conjunction with anions such as chloride, bromide, nitrate, acetate, hexafluorophosphate, tetrafluoroborate, bis(trifluoromethylsulfonyl)imide, alkylsulfate, alkylsulfonate, tosylate, and dicyanimide.6 Among these systems, the physicochemical properties of 1-alkyl-3-methylimidazolium-based ionic liquids with various anions have been extensively investigated. Numerous photochemical studies have also been performed in these imidazolium ionic liquids using optical spectroscopy.11−14 However, studies carried out by Samanta and co-workers15,16 suggest that optical transparency of these ionic liquids is debatable. According to their findings,15,16 all imidazolium ionic liquids display significant absorption in the ultraviolet region and a long absorption tail that extends into the visible region. It has been proposed that the imidazolium moiety and its associated structures are responsible for the optical absorption characteristics of these ionic liquids. Furthermore, these liquids exhibit fluorescence when excited in the wavelength range 250−450 © XXXX American Chemical Society
nm and the emission spectra, depending on the wavelength of excitation, extend from 300 to 700 nm.16 Apart from steadystate spectral measurements, fluorescence lifetimes of these imidazolium liquids have also been reported.15,16 The fluorescence decay profiles are best described by triexponential functions with the major component having a lifetime of about 500 ps and two minor components of ∼3 and ∼10 ns each. It has been noticed that these decay times are more or less independent of the wavelength of excitation as well as the wavelength at which the emission is monitored. Essentially, it is evident from these studies that 1-alkyl-3-methylimidazoliumbased ionic liquids are inherently fluorescent and the fluorescence is not due to impurities present in them. Instead, the emission of imidazolium ionic liquids arises as a consequence of cation−anion and cation−cation pairs, which have been termed as associated species.16 However, some researchers have expressed reservations against this inherent fluorescence and according to their findings the absorption and fluorescence characteristics of the imidazolium-based ionic liquids arise as a consequence of impurities present in them, which are difficult to eliminate.17,18 Despite these reservations, more reports have appeared in the literature19−25 dealing with the inherent fluorescence of ionic liquids subsequent to the Received: August 7, 2014 Revised: October 10, 2014
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dx.doi.org/10.1021/jp5079778 | J. Phys. Chem. B XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry B
Article
study. It can be noticed from the figure that two sets of cations in conjunction with four different anions have been employed in the present work, and the reasons for choosing them are as follows. The anions [FAP] and [Tf2N] are weakly associating ions, whereas [BF4] and [PF6] are strongly associating anions. Thus, the results of this study will enable us to comprehend the role of weakly and strongly associating anions on the rotational dynamics of the imidazolium ionic liquids. Moreover, since the emission arises from associated species, investigating the rotational dynamics of 1-alkyl-3-methylimidazolium ionic liquids with different anions is likely to provide insight into the nature of the emitting species. In addition to the variation of anions, anisotropy measurements carried out with ethyl and hexyl derivatives of the imidazolium ionic liquids will shed light on how their rotational dynamics is influenced by the structural heterogeneity of the medium, since the length of the alkyl chain on one of the constituent ions governs the formation of the organized structure.
studies of Samanta and co-workers.15,16 Fluorescence spectral measurements and lifetimes of imidazolium, morpholinium, and pyrrolidinium ionic liquids have been investigated by varying the length of the alkyl chain, the nature of the anion, and also as a function of the excitation and emission wavelengths.19−21 Even though one-to-one correlation between the lifetime data and structural organization of the ionic liquids is not always possible, all of these studies point to the existence of associated structures, which are responsible for the fluorescence behavior of the ionic liquids. More recently, fluorescence correlation spectroscopy studies carried out by Kim and co-workers22,23 suggest that long-wavelength fluorescence of imidazolium-based ionic liquids emanates from the aggregates of these liquids. In view of the existing reports in the literature, it would be imperative to get a better appreciation of the characteristics of the emitting species by investigating their rotational dynamics and finding out how the dynamics, in turn, is influenced by the nature of the anion and the length of the alkyl chain. The present work is one such endeavor toward that direction. Numerous studies are available in the literature that deal with the rotational dynamics of organic solutes dissolved in ionic liquids, and these studies provide information about solute− ionic liquid interactions.26−57 In this study, however, rotational dynamics of neat 1-alkyl-3-methylimidazolium ionic liquids has been examined by monitoring their inherent fluorescence. For this purpose, fluorescence anisotropy decays of 1-ethyl-3methylimidazolium ([EMIM]) and 1-hexyl-3-methylimidazolium ([HMIM]) ionic liquids with anions tris(pentafluoroethyl)trifluorophosphate ([FAP]), bis(trifluoromethylsulfonyl)imide ([Tf2N]), tetrafluoroborate ([BF4]), and hexafluorophosphate ([PF6]) have been measured as a function of temperature. Figure 1 gives the molecular structures of the cations and the anions of the ionic liquids used in this
2. EXPERIMENTAL SECTION The ionic liquids [EMIM][FAP] and [HMIM][FAP] were purchased from Merck, while the rest of the ionic liquids used in this work are from io-li-tec. The stated purity of the ionic liquids is >99% with
