Article pubs.acs.org/JPCB
Toward Understanding Solute−Solvent Interaction in RoomTemperature Mono- and Dicationic Ionic Liquids: A Combined Fluorescence Spectroscopy and Mass Spectrometry Analysis Prabhat Kumar Sahu, Sudhir Kumar Das, and Moloy Sarkar* School of Chemical Sciences, National Institute of Science Education and Research, Bhubaneswar 751005, India
ABSTRACT: Rotational relaxation dynamics of nonpolar perylene, dipolar coumarin 153, and a negatively charged probe, sodium 8-methoxypyrene-1,3,6-sulfonate (MPTS), have been investigated in a dicationic ionic liquid, 1,6-bis-(3methylimidazolium-1-yl)hexane bis-(trifluoromethylsulfonyl)amide ([C6(MIm)2][NTf2]2), and a structurally similar monocationic ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C6MIm][NTf2]), to have a comprehensive and a quantitative understanding on the solute−solvent interaction in these media. Analysis of the rotational relaxation dynamics data by Stokes−Einstein−Debye (SED) hydrodynamic theory reveals that perylene rotation is found to be the fastest compared to the other two probes and shows slip to sub-slip behavior, coumarin 153 rotation lies between the stick and slip boundary, and MPTS shows a superstick behavior in [C6MIm][NTf2]. Interestingly, MPTS exhibits a normal SED hydrodynamics in dicationic [C6(MIm)2][NTf2]2, in spite of the fact that dicationic ionic liquid contains two cationic sites bearing acidic hydrogen (C2−H) which may be available to form stronger interaction with the negatively charged MPTS. The difference in the rotational diffusion behavior of these three probes is a reflection of their location in different distinct environments of these ILs. Superstick behavior of MPTS in monocationic IL has been attributed to its specific hydrogen bonding interaction with the corresponding imidazolium cation. The relatively faster rotational behavior of MPTS in dicationic IL has been explained by resorting to mass spectrometry. Mass spectral analysis demonstrates that positively charged (imidazolium) sites in dicationic IL are strongly associated with negatively charged bis-(trifluoromethylsulfonyl)amide anion (NTf2−), which in turn makes it difficult for imidazolim cation to have stronger hydrogen bonding interaction with bulkier negatively charged molecule MPTS.
1. INTRODUCTION Room temperature ionic liquids (RTILs) have gained enormous popularity because of their widespread applications in versatile fields.1−10 This has been possible by virtue of interesting physicochemical properties that they possess. In recent times, the dicationic ionic liquids with their unique properties such as higher thermal stability, shear viscosity, surface tensions, etc., show great promise for future technological applications.11−17 It has been observed that intermolecular interactions have profound influence on many physicochemical properties of liquids and solution.18 It is, therefore, important to have an in-depth understanding of the various interactions that exist between the constituents (generally an organic cation and an inorganic or organic anion) of the RTILs and also their interaction with added © 2014 American Chemical Society
solutes so that they can be used to their full potential. To understand the relationships among the structures, intermolecular interactions, and dynamics in ionic liquids (ILs), many theoretical and experimental studies have been carried out.19−70 In this regard, studies on dynamics of solvation44−55 and rotational relaxation50−70 of solutes have been widely exploited as the properties of a medium can be well understood by the investigation of the interaction of a solute with the solvent. However, all of these44−70 studies have been carried out in monocationic ionic liquids. Studies on rotational relaxation dynamics in dicationic ionic liquids are elusive. Received: January 8, 2014 Revised: January 26, 2014 Published: January 29, 2014 1907
dx.doi.org/10.1021/jp500218r | J. Phys. Chem. B 2014, 118, 1907−1915
The Journal of Physical Chemistry B
Article
hexafluorophosphate ([PF6−]) anions.64 The difference in the rotational behavior of R110 in these ILs has been explained on the basis of the difference in their structural organization. Here, we would like to stress that rotational diffusion of organic apolar and dipolar solutes has been studied by only exploiting monocationic ionic liquids. Only very recently, few studies based on femtosecond Raman-induced Kerr effect spectroscopy and molecular dynamics (MD) simulation studies have been carried out on dicationic ionic liquids. Shirota and Ishida15 have studied both dicationic ([Cn(MIm)2][NTf2]2, where n = 6, 10, and 12; MIm = N-methylimidazolium) and corresponding monocationic ILs ([CnMIm][NTf2], where n = 3, 5, and 6) through femtosecond Raman-induced Kerr effect spectroscopy. The difference in the low-frequency Kerr spectral profiles in these two classes of RTILs has been attributed to the difference in their microstructures. They have suggested that dynamical heterogeneous behavior of dicationic IL correlates strongly with the structural variations. A recent study by Li et al.17 has shown that, for a short-range alkyl chain, the cations in dicationic and monocationic ionic liquids exhibit very similar structural nanoorganization and heterogeneity. Thus, it is evident from the aforementioned discussions that no studies based on the rotational relaxation dynamics by exploiting dicationic ionic liquid have been explored. It may be mentioned in this context that the density, viscosity, surface tension, glass transition temperature, melting point, and thermal stability of the dicationic ILs are quite higher than the monocationic analogues11−13 and hence are extremely promising for several industrial applications. Keeping the above facts in mind, we have investigated the rotational dynamics of perylene, coumarin 153 (C153), and sodium 8-methoxypyrene-1,3,6-sulfonate (MPTS) in a dicationic IL, 1,6-bis-(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide ([C6(MIm)2][NTf2]2), and a structurally similar monocationic analogue, 1-hexyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)amide ([C6MIm][NTf2]), through time-resolved fluorescence anisotropy study. The structures of the ILs and the probes are shown in Chart 1.
Studies on rotational relaxation dynamics of organic solutes in a given medium are found to be extremely useful, as it is capable of providing a great deal of information about the nature of interactions between solute and constituents of RTILs and thus microenvironments of solutes in such media.50−70 In this regard, the earlier work by Maroncelli and co-workers52,53 has demonstrated that the higher viscosity of the RTILs is the main factor responsible for retardation of the rotational relaxation time of organic solutes in these media. Subsequently, other researchers including ourselves have found that specific solute−solvent interaction has a profound role in affecting rotational motion of organic solutes in ionic liquids.55−69 In recent times, the effects of the alkyl chain length of one of the constituents of the RTILs on the rotational dynamics of several neutral and charged solutes have been studied extensively. Fruchey and Fayer studied the rotational dynamics of negatively charged (sodium 8-methoxypyrene-1,3,6-sulfonate, MPTS) and neutral (perylene) molecules in a series of N-alkylN-methylimidazolium ionic liquids.59 While MPTS showed superstick behavior attributed to its strong hydrogen bonding interaction with the imidazolium cation, perylene was found to follow slip to sub-slip hydrodynamics with increasing alkyl chain length. Das and Sarkar in their recent studies on rotational dynamics of coumarin 153 and 4-aminophthalimide (AP) in several 1-ethyl-3-methylimidazolium alkylsulfate ionic liquids56 have shown that the rotational dynamics of C153 become faster with an increase in alkyl chain length, as the larger size of the solvent molecule offers lower friction to the rotating solute. In the case of AP, superstick behavior has been observed. The superstick behavior has been attributed to the strong solute−solvent hydrogen bonding interaction. This observation demonstrated that two distinct rotational environments for two different solutes exist in these RTILs.56 The important contributions have been made by Dutt and his coworkers60−68 in terms of explaining the suitability of hydrodynamic theories in explaining the rotational diffusion of organic solutes in RTILs. In that context, they have studied the rotational dynamics of a charged probe rhodamine 110 (R110) and a neutral probe 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrol[3,4-c]pyrrole (DMDPP) in different N-alkyl-N-methylimidazolium ionic liquids containing tris(pentafluoroethyl)trifluorophosphate (FAP) anion.60 They have not observed any influence of the alkyl chain length of the cationic moiety on the rotational dynamics of DMDPP. However, rotational dynamics of R110 have been found to obey stick boundary conditions with an increase in alkyl chain length of the ionic liquids due to specific hydrogen bonding interaction between the solute and RTILs. In a separate work, Karve and Dutt have studied the rotational dynamics of the same probes (DMDPP and R110) in a variety of RTILs comprising piperidinium, pyrrolidinium, and morpholinium cations.61 The authors have observed a faster rotational diffusion of the positively charged solute R110 in highly viscous morpholinium IL. They have attributed this observation to the strong cation−anion association in morpholinium IL. Recently, Samanta and coworkers57 have demonstrated that the rotational environments for three different probe molecules AP, 6-propionyl-2dimethylaminonaphthalene (PRODAN), and a nonpolar solute, anthracene, are different in a series of N-alkyl-Nmethylmorpholinium ionic liquids. Very recently, Gangamallaiah and Dutt have studied the rotational dynamics of the charged R110 molecules in several 1-alkyl-3-methylimidazolium ionic liquids containing tetrafluoroborate ([BF4−]) and
Chart 1. Structures of the RTILs and the Fluorescent Probes
Nonpolar perylene and dipolar coumarin have been chosen due to their preference to nonpolar and polar environment, respectively.51,59 MPTS has been chosen due to its ability to participate in specific interaction with a hydrogen bond donor.59 In the present report, we resort to time-resolved fluorescence anisotropy and mass spectrometry study so that it entails a significant step forward in our understanding on the 1908
dx.doi.org/10.1021/jp500218r | J. Phys. Chem. B 2014, 118, 1907−1915
The Journal of Physical Chemistry B
Article
(Edinburgh, OB920). The samples were excited at 375 and 405 nm using different picosecond laser diodes (EPL). For lifetime measurements, the signals were collected at magic angle (54.7°) using a Hamamatsu microchannel plate photomultiplier tube (R3809U-50). The lamp profile was recorded by a scatterer (dilute Ludox solution in water) in place of the sample. The instrument response functions (fwhm) of our setup were ∼75 ps for 375 nm and 95 ps for 405 nm picosecond diode lasers, respectively. Decay curves were analyzed by a nonlinear least-squares iteration procedure using F900 decay analysis software. The qualities of the fit were judged by the chi square (χ2) values, and weighted deviations were obtained by fitting. The same setup was used for anisotropy measurements. The emission intensities at parallel (I∥) and perpendicular (I⊥) polarizations were collected alternatively until a peak difference between parallel (I∥) and perpendicular (I⊥) decay (at t = 0) of ∼5000 was reached. For G-factor calculation, the same procedure was adopted, but with five cycles and horizontal polarization of the exciting laser beam. The same software was also used to analyze the anisotropy data. The temperature was maintained by circulating water through the cell holder using a Quantum, North West (TC 125) temperature controller. The viscosities of the RTIL were measured by an LVDV-III Ultra Brookfield Cone and Plate viscometer (1% accuracy and 0.2% repeatability). High resolution mass spectra (HR-MS) of the RTILs were recorded by a BRUKER ESI-MICROTOF-Q-2 spectrometer. Methanol solvent was used for mass spectral measurements. 2.4. Method. We have investigated the rotational diffusion behavior of C153 in both dicationic and monocationic ILs. Time-resolved fluorescence anisotropy (r(t)) is estimated by the following equation
intermolecular solvent−solvent and solute−solvent interaction in RTILs.
2. EXPERIMENTAL SECTION 2.1. Materials. Coumarin 153 (C153) (laser grade, Exciton), MPTS (Fluka, Sigma-Aldrich), and perylene (Fluka, Sigma-Aldrich) were used as received. The RTIL [C6MIm][NTf2] was obtained from Merck, Germany (⟩99% purity), and used as received. The water and halide contents of the ILs were
