Understanding StructureâProperty Correlation in Monocationic and

Article pubs.acs.org/JPCB

Understanding Structure−Property Correlation in Monocationic and Dicationic Ionic Liquids through Combined Fluorescence and PulsedField Gradient (PFG) and Relaxation NMR Experiments Prabhat Kumar Sahu, Arindam Ghosh,* and Moloy Sarkar* School of Chemical Sciences, National Institute of Science Education and Research, Bhubaneswar 751005, India S Supporting Information *

ABSTRACT: Steady state, time-resolved fluorescence and NMR experiments are carried out to gain deeper insights into the structure−property correlation in structurally similar monocationic and dicationic room-temperature ionic liquids (RTILs). The excitation wavelength dependent fluorescence response of fl u o r o p h o r e i n 1- m et h y - 3 -p r o p y l l im id a z o l i um b i s(trifluoromethylsulfonyl)amide [C3MIm][NTf2] is found to be different from that of 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide [C6(MIm)2][NTf2]2 and 1hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [C6MIm][NTf2]. The outcomes of the present solvent dynamics study in [C3MIm][NTf2] when compared with those in [C6(MIm)2][NTf2]2 and in [C6MIm][NTf2] from our previous studies (Phys. Chem. Chem. Phys. 2014, 16, 1291812928) indicate the involvement of dipolar rotation of imidazolium cation during solvation. To correlate the findings of solvation dynamics study with the dipolar rotation of the imidazolium ring, pulsedfield gradient (PFG)-NMR technique for translational diffusion coefficient measurement and 1H as well as 19F spin−lattice relaxation measurements are employed. NMR investigation reveals that an ultrafast component of solvation can be related to the dipolar rotation of imidazolium cation; hence, the role of dipolar rotation of cations in governing the dynamics of solvation in ILs cannot be ignored. Analysis of the rotational relaxation dynamics data by the Stokes−Einstein−Debye hydrodynamic theory unveils distinctive features of solute−solvent interaction in [C3MIm][NTf2] and [C6(MIm)2][NTf2]2.

1. INTRODUCTION Room-temperature ionic liquids (RTILs) continue to gather considerable attention from academia and industry mainly because of their interesting physicochemical properties such as low vapor pressure, liquidity over a wide range of temperature, high conductivity, high thermal stability, large electrochemical window, etc.1−5 During the past decade, monocation-based ILs (MILs) have been exploited extensively to satisfy academic and industrial interests. However, dication-based ILs (DILs) have not gained considerable attention until recently, when DILs comprising imidazolium,6−10 ammonium, and pyridinium11−16 dication and mononegative inorganic or organic anions have been designed and developed. Interestingly, it has been observed that DILs exhibit physicochemical properties significantly different from those of the traditional MILs, such as higher thermal stability, higher shear viscosity, higher surface tensions, and larger liquid density.6−16 With the structural variation in both DILs and MILs in mind, it can also be realized that more cation and anion combinations are possible in DILs than in MILs.10 The greater structural variability in DILs is expected to result in diverse physiochemical properties of DILs, compared to MILs, making them even more tunable and versatile.10 Since dicationic-based ionic liquids show great promise for future applications, at this stage, it is now extremely © XXXX American Chemical Society

important to have proper understanding of the structure− property correlation in DILs so that they can be effectively utilized for several industrial and academic purposes. It is widely known that intra- and intermolecular interactions among solute and solvent molecules play a significant role in shaping physicochemical properties of liquids and solutions.17 Hence, investigation of these aspects with variation in structures of the concerned molecules would help in gaining deeper insights into the structure−property dynamics correlations among such systems. It may be noted here that solute reactivity in a particular solvent is intricately related to dynamical behavior of the solvent.18 Moreover, solute−solvent interaction is known to influence the rotational behavior of solute molecules.19−30 Due to these reasons, studies on solute and solvent dynamics by exploiting time-resolved fluorescence spectroscopy have been extensively exploited over the past few years, to successfully understand the behavior of medium. However, it is noteworthy that, predominantly in ionic liquid studies, the solute and solvent dynamics are carried out by taking MILs as target.17−64 Literature reports employing DILs Received: July 30, 2015 Revised: October 8, 2015

A

DOI: 10.1021/acs.jpcb.5b07357 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

involved in neat RTILs.74−85 Moreover, these NMR studies are expected to be helpful in connecting the molecular motion to the components of solvation. It may also be noted that the comparison of photophysical properties of DIL with an alkyl chain linker, with an MIL having an alkyl chain of half the length of the DIL linker, and an MIL having the same alkyl chain length as the DIL linker, would be helpful to understand the behavior of these ILs with the variation of their respective structures. In view of the above facts, we have carried out fluorescence and NMR studies by employing few fluorescent organic solutes and neat MIL and DIL. Specifically, solvation dynamics of C153 and rotational relaxation dynamics of C153, MPTS, and perylene in [C3MIm][NTf2] through steady state and timeresolved fluorescence spectroscopy has been studied. Excitation wavelength dependent fluorescence behavior in ([C3MIm][NTf2]) has also been studied. The results are compared with those in the DIL [C6(MIm)2][NTf2]2 as well as another MIL [C6MIm][NTf2] reported previously.72,73 The translational diffusion and reorientational dynamics of the cations of the neat RTILs by employing the PFG-NMR technique have been investigated in detail in the present work. The structures of the ILs and the probe are shown in Scheme 1.

as the medium for such investigations are extremely rare, both in theory65−69 and experiments.70,71 Recently, Shirota and Ishida71 have investigated the interionic vibrations among ILs by using femtosecond optical-heterodyne-detected Ramaninduced Kerr effect spectroscopy (OHD-RIKES). This study has revealed a difference in the low-frequency Kerr spectra between the monocationic 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide ([C3MIm][NTf2]) and the dicationic IL, 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide ([C6(MIm)2][NTf2]2). Segregation in the microstructures of DILs having longer alkyl chain is believed to be responsible for this behavior. The authors have also pointed out that the difference of Kerr spectra of the ILs arises due to the distinct angular momentum and relaxation behavior of the two cationic species [C6(MIm)2]2+ and [C3MIm]+.71 Molecular dynamics (MD) simulation studies on a series of germinal DILs by Bodo et al.68 have demonstrated many common characteristics with corresponding MILs. Cummings and co-workers69 have compared the structural nano-organization and heterogeneity in various mono- and dicationic ILs through MD simulation. They have observed that, for short alkyl chain, the cations in DILs and MILs exhibit very similar structural nano-organization and heterogeneity, whereas significant difference in structural heterogeneity is observed for medium- and long-chain DILs and MILs. As is realized at this stage, the studies on DILs are very limited, and no experimental studies by exploiting fluorescence on DILs have been carried out. With an aim to understand the nature of solute−solvent interactions in dicationic ILs we have investigated steady state and timeresolved fluorescence studies in DIL, 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide ([C6(MIm)2][NTf2]2), as well as in a structurally similar MIL, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C6MIm][NTf2]), and several interesting results have been obtained.72,73 Similar microheterogeneity of [C6(MIm)2][NTf2]2 and [C6MIm][NTf2] is observed through excitation wavelength dependent fluorescence measurement. More interestingly, no ultrafast component of solvation in DIL has been observed, even though the same is observed for MIL.73 It should be noted that the experimental results that are obtained from excitation wavelength dependent fluorescence measurements regarding the microheterogeneous behavior of MILs and DIL in the present case is different from the results obtained by MD simulation studies.69 It is to be noted that solvation dynamics in ILs is dominated by translational motion of the ions along with other collective motions, and the role of reorientational motion of the constituent ions of ILs was thought to be negligible.32 However, Biswas and co-workers61 with the help of a semimolecular theory have shown that reorientational dynamics of the dipolar imidazolium cation can play an important role in governing the dynamics of solvation. Though in a previous paper we have pointed out that there might be a connection between the dipolar rotation of the imidazolium cation and ultrafast component of solvation,57−61 no evidence was provided on DILs. The above discussion suggests that a more comprehensive study performed by exploiting additional spectroscopic techniques such as NMR along with the fluorescence study would be helpful to understand the kinship among structure, property, and dynamics in MILs and DILs. It may be noted that recently pulsed-field gradient (PFG)-NMR techniques have proven to be excellent tools to understand the internal diffusion processes

Scheme 1. Structures of the RTILs and Molecular Fluorescent Probes

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. Synthesis and characterization of dicationic ionic liquid [C6(MIm)2][NTf2]2 were reported previously.72 The RTILs were taken in different longnecked quartz cuvettes with proper precaution to avoid moisture absorption. Requisite amounts of different probes were added to the RTILs to prepare the solution. The cuvettes were properly sealed with septum and parafilm to maintain dry conditions. 2.2. Instrumentation. The absorption and fluorescence spectra were measured using a PerkinElmer (Lambda-750) spectrophotometer and a PerkinElmer LS 55 spectrofluorimeter, respectively. Time-resolved fluorescence measurements B


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boundary condition, C is less than unity, and is applicable to the condition where the size of the solute molecule is smaller or comparable to that of the solvent molecule. In SED theory shapes of the solute molecules are usually considered as either symmetric or asymmetric ellipsoids. For nonspherical molecules, f is greater than unity, and the magnitude of the deviation from unity in the value of f describes the degree of the nonspherical nature of the rotating solute. For Cslip calculation, we have used the probe properties that are available in literature.22,43 Details of the calculation have been described in our earlier publications.72 The rotational correlation times, τc’s, for each resolved NMR resonance peak of the cations of [C3Mim][NTf2], [C6Mim][NTf2], and [C6(MIm)2][NTf2]2 were calculated from 1H spin−lattice relaxation (T1) data. 1H T1 data were acquired using an inversion recovery method87 and were fit to the Bloembergen−Purcell−Pound (BPP) relationship88 (eq 3) to obtain τc

were carried out using a time-correlated single-photon counting (TCSPC) spectrometer (Edinburgh, OB920) with 405 nm picoseconds diode laser (EPL) as excitation source. The signals were collected at the magic angle (54.7°) using a Hamamatsu microchannel plate photomultiplier tube (R3809U-50) detector. The instrument response function (IRF) of our setup was obtained from the full width half maxima (fwhm) of the excitation laser pulse and was recorded using scatterer (dilute Ludox solution in water) in place of the sample. The instrument response function (IRF) was ∼98 ps for 405 nm picoseconds diode laser. Decay curves were analyzed by nonlinear least-squares iteration methods using the F900 decay analysis program. The qualities of the fit were judged by the chi square (χ2) values, and weighted deviations were obtained by fitting. For time-resolved fluorescence anisotropy measurements the same TCSPC setup was used. The emission intensities at parallel (I∥) and perpendicular (I⊥) polarizations were collected alternatively until the peak difference between parallel (I∥) and perpendicular (I⊥) decay (at t = 0) ∼ 5000 was reached. For G-factor calculation, the same procedure was adopted, but with 5 cycles and horizontal polarization of the exciting laser beam. Quantum, North West (TC 125), temperature controller was used to maintain the temperature of the cell. The viscosities of the RTIL were measured by LVDV-III Ultra Brookfield Cone and Plate viscometer (1% accuracy and 0.2% repeatability). NMR experiments were carried out using a 9.4 T Bruker Avance NMR spectrometer at Larmor frequencies of 400.1 MHz for 1H and 376.5 MHz for 19F. Stimulated echo bipolar pulse-gradient pulse (stebpgp) sequence is applied for the determination of translational diffusion coefficients, D, at different temperature (298−338 K). The echo heights were recorded at 16 equal intervals with variation of gradient pulse strength from 2% to 95% of the maximum gradient pulse strength (50 G/cm). The echo heights were fit to the equation S(g ) = S(0)exp[−Dγ 2δ 2g 2(Δ − δ /3)]

⎡ ⎤ τc 4τc 1 6 γ 4h2 ⎢ ⎥ = + + I ( I 1) T1 5 b6 1 + 4ω0 2τc 2 ⎦ ⎣ 1 + ω0 2τc 2

where γ is the gyromagnetic ratio of the proton (2π·42.576 MHz/Tesla), ℏ is the reduced Planck’s constant, b (=∑r) is the summed distance between protons (and the sum runs over all protons that are dipolar coupled to the proton evaluated), and I is the nuclear spin number of the proton (=1/2). ω0 = 2πν0, where ν0 is 400.1 MHz at 9.4 T, the proton observation frequency, and τc is the rotational correlation time. Equation 2 is derived under the isotropic motion approximation and applies in the extreme narrowing limit (T1 = T2). The only other unknown quantity in eq 3, other than τc, is b. To calculate b for a proton, T1 measurement was repeated for varying temperature, so that T1 can be measured for different τc. It can be easily shown from eq 3 that T1 passes through a minimum where ω0τc becomes equal to 0.616, which corresponds to τc of 2.45 × 10−10 s for 400.1 MHz proton frequency. The distance term, b, can from there be calculated using the minimum value of T1.87 As the distance term involves only near neighbor protons (within 5 Å), it is assumed that b remains unaltered with change in temperature. The summed distance b was therefore used to calculate τc from T1 data taken at all temperatures.

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where S(g) and S(0) are the echo height at the gradient strength g and 0, respectively, γ is the gyromagnetic ratio of the proton, δ is gradient pulse length, and Δ is the duration between the two gradient pulses. 2.3. Method. Detailed analysis of the time-resolved decay profiles and estimation of solvation and rotational relaxation times from the analysis have been mentioned previously.72,73 We have analyzed our results on rotational dynamics of C153 employing Stokes−Einstein−Debye (SED) hydrodynamic theory.86 According to this theory, reorientation time (τr) of a noninteracting solute in a solvent continuum of viscosity η is given by

τr SED =

ηVfC kT

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3. RESULTS AND DISCUSSION 3.1. Steady State Spectral Behavior of C153 and 2Amino-7-nitrofluorene (ANF) in RTILs. Figure 1 displays the absorption and emission spectra of C153 dissolved in [C3MIm][NTf2] and [C6(MIm)2][NTf2]2. Note that both absorption and emission spectra overlap with each other. The very close emission maximum for C153 in [C3MIm][NTf2] and [C6(MIm)2][NTf2]2 indicates that the polarities of the ionic liquids are very similar. The polarity of [C6(MIm)2][NTf2]2 is close to that of dichloromethane.73 The absorption and emission maxima of C153 are found not be affected by temperature from 293 to 333 K. Further, we have carried out excitation wavelength dependent fluorescence measurements to explore the spatial heterogeneity of the ionic liquids. It may be noted here that ionic liquids are microheterogeneous.31,32,40 The microheterogeneous nature of RTILs31,32,40 can be investigated by exploiting the excitation wavelength (λexc) dependent fluorescence measurements. If there is a distribution of energeti-

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In the above relation, V is the van der Waals volume of the solute molecule, and C is the boundary condition parameter, which expresses the measure of coupling between the solute and the solvent; f is the shape factor which accounts for the nonspherical shapes of the solute molecules. k is the Boltzmann constant, and T is absolute temperature. The two extreme boundary conditions are stick and slip according to SED hydrodynamic theory.86 According to the stick boundary condition, C is unity and is applicable to solute molecules larger in size than solvent molecules. According to the slip C


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the longer-wavelength region of the absorption band is known as the “red-edge effect” (REE).91,92 In the present study we have monitored the excitation wavelength dependent fluorescence behavior of 2-amino-7-nitrofluorene (ANF) in ILs. ANF has a shorter excited state lifetime (τ) ∼ 100 ps, and thus, it is expected that spectral behavior of ANF in the RTILs would be more sensitive toward the microheterogeneous nature of the ILs. Apart from a few theoretical studies,65−69 experimental data on the microheterogeneous behavior of dicationic ionic liquid is very rare.71,73 A steady red-shift in the fluorescence maximum of ANF has been observed with a change in the excitation wavelength in both monocationic IL [C3MIm][NTf2] and its dicationic counterpart [C6(MIm)2][NTf2]2. However, a significant difference in the magnitude of the total shift between monocationic and dicationic ionic liquid has been observed. The shift in fluorescence maxima in [C3MIm][NTf2] is found to be about half (∼125 cm−1) of the shift observed in [C6(MIm)2][NTf2]2 (∼251 cm−1) (Figure 2). Our observation indicates that [C6(MIm)2][NTf2]2 is more microheterogeneous than its monocationic counterpart [C3MIm][NTf2]. The present observation is interesting in a sense that quite different results were obtained in terms of heterogeneity for monocationic and dication ionic liquids. Cummings and co-workers,69 through molecular dynamics (MD) simulation, have shown that monocationic ILs have a higher heterogeneity order parameter (HOP) than their dicationic counterparts. They have found that dicationic and monocationic ILs with short-range alkyl chain (such as [C6(Mim)2][BF4]2 and monocationic [C3Mim][BF4]) exhibit very similar structural nano-organization. Another MD simulation study by Ishida and Shiorta,66 on dicationic and monocationic ionic liquids having different alkyl chain lengths, reveals that the length of the apolar alkyl side chain has strong influence on the heterogeneous behavior. From the above discussion it is clear that the effect of alkyl chain length on structural organization of mono- and dicationic ILs is different. Hence, further investigation exploiting both experimental and theoretical techniques is required to have an in-depth understanding of the subtle difference in the microheterogeneous behavior of the monocationic and dicationic ionic liquids. 3.2. Solvation Dynamics Study. It has been observed from several recent studies that dicationic ILs are superior than their monocationic counterparts in terms of physicochemical properties.6−16 Since reactivity of solute is closely related to solvation process,18 studies on dynamics of solvation would be helpful to understand and distinguish (if any) the dynamical behavior of both monocationic and dicationic ionic liquids. To study this phenomenon, the emission decay profiles of C153 at magic angle (54.70°) are collected at several wavelengths (5− 10 nm intervals) across the emission spectra by exciting the sample at λexc = 405 nm at different temperatures. The occurrence of the solvation process is indicated by the observation of faster decay at shorter monitoring wavelengths and a rise with usual decay at longer monitoring wavelength.93 The emission maximum at each time ν̅(t) was obtained by fitting the spectrum to a log-normal line-shape function which is given below

Figure 1. Combined normalized absorption (solid black line) and emission spectrum (solid red line) of C153 in [C3MIm][NTf2]. Absorption and emission spectra of C153 in [C6(MIm)2][NTf2]2 are shown with dotted blue and dotted green lines, respectively. The spectra were normalized with respect to the corresponding peak maxima. Spectrum corresponding to [C6(MIm)2][NTf2]2 is reproduced from our earlier study.73 λexc = 405 nm.

Figure 2. Excitation wavelength dependent fluorescence response of ANF in [C3MIm][NTf2], [C6(MIm)2][NTf2]2 at 298 K. Data for [C6(MIm)2][NTf2]2 has been taken from our previous work.72

Figure 3. Variation of fwhm (full width at half maxima) of TRES with time for [C3Mim][NTf2] and [C6(MIm) 2][NTf2]2. Data for [C6(MIm)2][NTf2]2 were obtained from our previous report.73

cally different solvated species in the ground state, and energy transfer between them is slow, then instead of obeying Kasha’s rule89,90 a gradual shift (red) of fluorescence maximum with a change in the excitation wavelength is expected. Therefore, this study can give at least a qualitative idea about the microheterogeneous nature of the medium. The red-shift of the fluorescence maximum of the dipolar solutes when excited at

I = h exp[− ln 2{ln(1 + α)/γ }2 ]

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for α > −1 = 0 and I = 0 for α ≤ −1 where α = 2γ(ν̅ − ν̅peak)/Δ, ν̅peak is the wavenumber corresponding to the peak height, Δ is the full width at half-maximum (fwhm), h is the D


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Table 1. Solvation Relaxation Parameters of C153 in [C3MIm][NTf2] and [C6(MIm)2][NTf2]2 at λexc = 405 nm at Different Temperatures biexponential fita visc (cP)/ T (K)

a1

τ1 (ns)

a2

τ2 (ns)

55/293 44/298 36/303 30/308 25/313 13/333

0.93 0.92 0.95 0.95 0.98 0.51

0.45 0.32 0.31 0.25 0.22 0.21

0.07 0.08 0.05 0.05 0.02 0.49

2.15 1.48 1.58 1.30 2.55 0.21

827/293 585/298 425/303 317/308 235/313 92/333

0.64 0.63 0.67 0.69 0.72 0.64

0.37 0.30 0.28 0.21 0.22 0.12

0.36 0.37 0.33 0.31 0.28 0.36

3.89 2.73 2.32 1.73 1.64 0.82

⟨τs⟩ (ns)

stretched exponential fita −1

Δνobs (10 cm ) 3

[C3MIm][NTf2] 0.46 1.11 0.41 1.19 0.37 1.08 0.30 1.13 0.26 1.04 0.21 0.94 [C6(MIm)2][NTf2]2b 1.64 1.64 1.20 1.65 0.95 1.61 0.68 1.71 0.60 1.53 0.38 0.95

−1

Δνest (10 cm )

fobs

β

τsol (ns)

⟨τst⟩ (ns)

1.38 1.37 1.36 1.36 1.33 1.33

0.80 0.86 0.79 0.83 0.78 0.71

0.95 0.96 0.95 0.96 1.01 1.05

0.37 0.36 0.33 0.27 0.25 0.22

0.38 0.37 0.34 0.28 0.24 0.21

1.43 1.42 1.40 1.43 1.43 1.40

1.15 1.16 1.15 1.19 1.07 0.67

0.57 0.57 0.58 0.59 0.64 0.65

0.94 0.74 0.59 0.42 0.40 0.28

1.53 1.19 0.92 0.64 0.57 0.38

3

Details of biexponential and stretched exponential fitting procedure has been reported previously.73 bData were taken from our previous report.73 Δνobs is the observed dynamic shift calculated time-resolved solvation data. Δνest is the difference between ν(∞) from the fits and the time-zero frequency estimated according to the methods of ref 93 and fobs = Δνobs/Δνest. Experimental error ±5%. a

Figure 4. (a) Temperature dependence of diffusion coefficient of cations of the ionic liquids. (b) Diffusion coefficient plotted against T/η. The dotted lines are linear fits of the data points.

Figure 5. Temperature dependence of 1H T1 (a) and correlation time (τc) (b) of protons of the cation [C3Mim]+.

peak, and γ is a measure of the asymmetry of the band shape. Solvent correlation function (C(t)) is calculated from the peak frequencies obtained from the log-normal fitting of timeresolved emission spectra (TRES)

C(t ) =

E

ν ̅ (t ) − ν ̅ (∞ ) ν ̅ (0) − ν ̅ (∞)

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Figure 6. Temperature dependence of measured 1H T1 (a) and correlation time (τc) (b) of protons of the cation [C6Mim]+.

Figure 7. Temperature dependence of 1H T1 (a) and correlation time (τc) (b) of protons of the cation [C6(Mim)2]2+.

Table 2. Spin−Lattice Relaxation Time (T1) for Cations and Anions of RTILs T1 (s) of cation RTIL

T (K)

proton 2

proton 4

T1 (s) of anion

[C3MIm][NTf2]

298 308 298 308 298 308

1.25 1.43 1.08 1.19 0.84 0.91

0.48 0.50 0.41 0.46 0.48 0.46

0.60 0.65 0.64 0.71 0.55 0.60

[C6MIm][NTf2] [C6(MIm)2][NTf2]2

where ν̅(∞), ν̅(0), and ν̅(t) are the peak frequencies at times infinity (∞), zero (t = 0), and t, respectively. The plot of C(t) against t (time) is fitted by a biexponential function as given below C(t ) = a1e

−t / τ1

+ a2e

−t / τ2

Figure 8. Time-resolved fluorescence anisotropy decay (TRFAD) for C153 in [C3Mim][NTf2] at 293 K. Solid line in the figure represents the biexponential fit to the data points.

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where τ1 and τ2 are the solvent relaxation time and a1 and a2 are normalized preexponential factors. The average solvation times were calculated from the following relationship ⟨τav⟩ = a1τ1 + a 2τ2

Table 3. van der Waals Volumes, Shape Factor, and Slip Boundary Condition Parameters

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We had also fitted C(t) by the stretched exponential function shown below. C(t ) = exp( −(t /tsolv)β )

system

van der Waals volume (Å3)

f

Cslip

Perylene C153 MPTS

225 243 343

1.76 1.5 1.33

0.085 0.18 0.11

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Table 4. Rotational Relaxation Parameters of MPTS, Perylene, and C153 in [C6(MIm)2][NTf2]2 and [C3Mim][NTf2] at Different Temperatures [C6(MIm)2][NTf2]2

[C3Mim][NTf2]

system

T (K)

viscosity (cP)

r0a

⟨τr⟩ (ns)

Crot(av)b

viscosity (cP)

r0a

⟨τr⟩ (ns)

Crot(av)b

perylene

293 298 303 308 313 318 323 328 333 293 298 303 308 313 318 323 328 333 293 298 303 308 313 318 323 328 333

827 585 425 317 235 183 144 115 92 827 585 425 317 235 183 144 115 92 827 585 425 317 235 183 144 115 92

0.30 0.36 0.35 0.33 0.34 0.30 0.28 0.27 0.25 0.32 0.31 0.32 0.32 0.32 0.33 0.33 0.32 0.34 0.30 0.28 0.30 0.30 0.30 0.30 0.30 0.30 0.30

4.30 3.80 3.30 3.00 2.50 2.40 2.00 1.87 1.57 14.98 12.32 10.49 7.99 6.64 5.48 4.61 3.79 2.93 36.0 27.4 22.2 18.7 16.5 13.0 11.4 10.0 8.4

0.13

55 44 36 30 25 21 18 16 13 55 44 36 30 25 21 18 16 13 55 44 36 30 25 21 18 16 13

0.29 0.29 0.28 0.27 0.27 0.28 0.25 0.21 0.20 0.38 0.35 0.36 0.34 0.34 0.33 0.32 0.31 0.33 0.37 0.37 0.36 0.31 0.36 0.35 0.37 0.35 0.35

0.95 0.80 0.70 0.58 0.50 0.45 0.41 0.38 0.34 2.65 2.24 1.92 1.70 1.46 1.33 1.23 1.10 0.91 6.85 5.93 4.69 4.08 3.56 3.12 2.66 2.33 2.03

0.23

C153

MPTSc

0.32

0.64

0.31

1.33

a r0 is the initial anisotropy. bCrot(av) is temperature averaged rotational coupling constant. cMPTS rotational relaxation time is calculated from single-exponential decay function.

Here 0 < β ≤ 1: τ ⟨τst⟩ = solv Γ(β −1) β

decreases. The observation can be attributed to the gradual lowering of bulk viscosity of the medium with rise in temperature.32 It has been observed that in dicationic IL the predominant contribution comes from weighted slow components to the average solvation response. The fact that weighted slow components contribute predominantly to the average solvation response has already been observed for monocationic imidazolium-based ionic liquids.32 We would like to note here that one of the interesting outcomes from the time dependent fluorescence Stokes shift measurements is the observation of the missing solvation component. The ultrafast component of solvation is generally missed due to finite time-resolution of the experimental setup.32 It has been observed that generally imidazoliumbased MILs exhibit a significant amount of missing solvation component.34 Interestingly, in the present case, one can clearly see (Table 1) that there is no ultrafast component of solvation in the case of the imidazolium-based dicationic ionic liquid, whereas an ultrafast solvation component of ∼20% is observed in monocationic-based IL, [C3Mim][NTf2]. More interestingly, at higher temperature (∼333 K), ∼32% missing solvation component was also observed for dicationic IL. This study clearly points out that even though the present monocationic IL and its dicationic counterpart are structurally similar their dynamical behavior is quite different. In our previous work73 the aspect of absence of ultrafast solvation component for dicationic ionic IL has been discussed in detail. In that study we had also tried to demonstrate that there is a connection

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where Γ is the gamma function and τst is the average solvation time considering C(t) was a stretched exponential function. Time-resolved emission spectra (TRES) of C153 in [C3MIm][NTf2] are constructed from the fitted decay profiles (Figure S1), which show a progressive red-shift of the fluorescence maximum with time. Plots of the spectral shift correlation function, C(t), versus time are provided in Figure S2. In all the cases, time dependent shifts of the emission maximum to lower energy have been observed. This indicates that with time the excited state of dipolar molecule is stabilized by solvent molecules. The variation of full width at half maxima (fwhm) of TRES with time for both [C3Mim][NTf2] and [C6(MIm)2][NTf2]2 is shown in Figure 3. It is noticeable from Figure 3 that fwhm decreases with time in both cases. It indicates the stabilization of the excited state of the solute by the solvent. However, the variation in the rate of decrease of fwhm with time (Figure 3) for MIL and DIL can be ascribed to the difference in the local solvation environment for minocationic and dication ILs. Local solvation environments are known to have a significant influence on the fwhm of the time-resolved emission spectrum.94,95 The detailed solvation parameters at different temperatures are collected in Table 1. It is noticeable from Table 1 that with the increase in temperature the average solvation time (⟨τs⟩) G


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length of alkyl side chains plays an important role in governing the reorientational motion of imidazolium cations. Moreover, since ionic liquids are composed of oppositely charged ions, it is generally believed that solvent response in ILs is primarily dominated by translational and other collective motion of the constituent ions of ILs.32 In this context theoretical calculation on solvation dynamics in ILs through dielectric spectra, by Petrich and co-workers,55 is worth mentioning. The authors have obtained very good agreement between the theoretically calculated solvation relaxation functions and those obtained from fluorescence upconversion spectroscopy. This study has suggested that translational motion of ions may not be the only predominant factor in solvation dynamics in ILs in the subnanosecond time range. Kashyap and Biswas60 through theoretical calculations have observed that solvation dynamics in RTILs is biphasic. The authors have used experimental dielectric relaxation data as input. Their calculations also showed that the solvent fast modes such as liberation and intermolecular vibrations may or may not couple completely to the polar solvation dynamics in RTILs studied here. Recent reports by Biswas and co-workers61,62 have shown that ionic liquids are fairly large and polarizable and hence a significant portion of solvent stabilization of dipolar solutes may also come from reorientational motion of the imidazolium cation. They have shown that, for imidazolium IL, contribution of dipole− dipole interaction to the total dynamic Stokes shift can be ∼40%, suggesting that a significant portion of solvent dynamics can come from reorientational motion of the imidazolium ion.61 The outcome of all these studies helped us to conjecture that perhaps there is a strong connection between the nature of cation and the ultrafast component of solvation dynamics in ionic liquids. It may be noted here that there are no reports available that focused on getting an idea about the reorientational motion of both dicationic and monocationic IL. It is expected that knowledge about the molecular rotational behavior of both monocationic and dicationic imidazolium moieties would be helpful to understand the correlation between the ultrafast solvation component and dipolar rotation of the imidazolium cation in a much better fashion. Note that recently PFG NMR techniques are found to be very useful in providing insight into the reorientational motion of cations and anions in RTILs.74−85 In view of this, in the present study, we have also studied the translational diffusion and reorientational dynamics of the cations of the ILs by exploiting PFG and relaxation NMR measurements (vide infra). 3.3. NMR Studies. 3.3.1. Translational Dynamics. Measurement of self-diffusion coefficient using PFG-NMR is a powerful noninvasive technique to estimate the effective size and molecular weight of a molecular species through measurement of translational diffusion, in a given set of conditions and with the assumption that the molecule is spherical. The famous Stokes−Einstein formula for translational diffusion coefficient (D) of spherical solute molecules that are large compared to the solvent molecules relates D with radius of solute molecule a as D = (kT)/(6πηa), where k is the Boltzmann constant, T is absolute temperature, and η is the coefficient of viscosity of the diffusion medium. The radius a, in turn, relates to the mass, M, of a spherical solute molecule as M = 4πa3ρ/3, with ρ being the density. This makes D dependent on M as D ∝ M−(1/3). Here we would like to mention that diffusion of ions in ILs and other confined media has been investigated through many experimental and simulation studies.96−104 It has been shown that motion of ions in ILs is

Figure 9. Log−log plots of ⟨τr⟩ vs η/T for (a) perylene, (b) C153, and (c) MPTS. The solid black line is the linear fit to the data points. Blue, red, and green dotted lines represent the boundary condition obtained from Gierer−Wirtz (GW) quasihydrodynamic theory95 in [C3Mim][NTf2], [C6Mim][NTf2], and [C6(MIm)2][NTf2]2, respectively.

between the nature of the cation and ultrafast component of solvation dynamics in ionic liquids based on some indirect inputs that were obtained from the studies on imidazoliumbased MILs. Recently, studies such as multinuclear NMR74−85 and quasielastic neutron scattering (QENS)96 indicated that the H


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Figure 10. Variation rotational relaxation time with temperature reduced viscosity in (a) [C6(MIm)2][NTf2]2 and (b) [C3Mim][NTf2]. Rotational relaxation times were normalized with respect to the maximum value.

while that of [C3MIm]+ was found to be the largest at all temperatures. This is expected as per their relative masses. Figure 4b shows D as a function of T/η and as expected from the Stokes−Einstein formula shows linear relationships for all three molecules. The slope of the D−T/η curve is given by k/ (6πa), and hence a steeper slope indicates smaller hydrodynamic radius. It is interesting to note that the slope was found to be the steepest for [C6(MIm)2]2+ (0.70 J K−1 m−1), in spite of the fact that it is the bulkiest cation. A possible explanation for the small hydrodynamic radius for [C6(MIm)2]2+ could be that it remains in the experimental condition in a folded form making a U-shape. 3.3.2. Self-Reorientation of the Cations and Anions of the RTILs. 1H NMR. Equation 3 is derived assuming isotropic motion, a condition satisfied only by spherically shaped molecules, and they are completely characterized by a single correlation time. However, the actual movement of molecules in solution can be quite complex. Moreover, shapes of many organic molecules show significant departures from spherical shape. Along with molecular reorientation, molecules may have various modes of internal motion such as rotation of methyl group, segmental motion of the alkyl side chain, isomerism, and conformational ring inversions, etc. These motions influence the dipole−dipole interaction between coupled sites and thereby spin−lattice relaxation. Hence, the τc in eq 3 is understood as an effective correlation time representing both overall reorientation and local internal motions.

Table 5. Activation Energies for Viscous Flow of the ILs and Rotational Relaxation for Perylene in the ILs RTILs

Eη/kJ mol−1

Er/kJ mol−1 for perylene

[C3Mim][NTf2] [C6Mim][NTf2] [C6(MIm)2][NTf2]2

28.50 31.26 44.25

20.82 26.00 20.00

jumplike.96−104 Apart from mass, shape and size of the cation/ anion, strength of cation−anion interaction, and the nanostructural organization in the ILs are some of the factors which influence the diffusion of ions in the ILs.104 Very recently, relaxation and diffusion properties of anions and cations in the neat RTIL, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imid ([Bmim][Tf2N]), have been investigated by Mattea and co-workers99 through PFG-NMR experiments. The authors have shown that the rotational and translational correlation times show different dependences on temperature which indicate the decoupling of translational and rotational dynamics of the ions. In the present case we have investigated the translational diffusion of cations in the ILs [C3Mim][NTf2], [C6Mim][NTf2], and [C6(MIm)2][NTf2]2 as there is variation in the shape and size of the cations in these ILs in terms of alkyl chain length and alkyl bridge. Figure 4a shows the temperature dependence of translational diffusion coefficients of the [C6(MIm)2]2+, [C6MIm]+, and [C3MIm]+ in neat RTILs. Diffusion coefficient of [C6(MIm)2]2+ was found to be smallest

Figure 11. Arrhenius plots of viscous flow of [C6(MIm)2][NTf2]2 (left panel) and rotational relaxation of MPTS in [C6(MIm)2][NTf2]2 (right). I


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times. This clearly indicates presence of local motions. At lower temperatures, the local motions are not dominant, and all protons show similar relaxation times contributed primarily by global rotation of the molecules. As temperature rises, different local motions set in and cause different T1 values for different protons. We can say that the overall molecular reorientational motion of the heavy dication [C6(Mim)2]2+ is slower than those of comparatively lighter monocationic [C3Mim]+ and [C6(Mim)2]2+, which is expected. In a recent study through molecular dynamic (MD) simulations by Pal and Biswas,105 it has been shown that the slow nanosecond solvation component in ILs originates from the dynamic heterogeneity (DH) of these media. Maximum cation jump length was shown to be around ∼50% of the ion diameter. The authors have shown that the DH time scale correlates well with the slow solvation rate. In this connection we would like to mention that the fact that we have not observed any missing component of solvation in the DIL can be due to the slower molecular reorientation of the DIL. 19 F NMR. We have also tried to observe the change in reorientational motion of the anions in the RTILs through 19F relaxation experiment. Here it should be noted that the anion is the same [NTf2]− in all the RTILs. We measured T1 at two different temperatures, 298 and 308 K. The reason for measuring T1 at a temperature higher than 298 K is because at higher temperature viscosity decreases which allows free reorientation of the anion; hence, any change in reorientational motion of the anions in different RTILs can be better observed at higher temperature. We have tried to compare the T1 values with those of the protons of two different positions of the cations of the RTILs: one is the imidazolium ring proton (proton 2), and the other is N−CH2 protons (proton 4). The results are presented in Table 2. A closer look at Table 2 reveals a significant difference in the variation of T1 of the protons of the cations and anions of the mono- and dicationic IL as well as within monocationic ILs. For example, between [C3MIm][NTf2] and [C6(MIm)2][NTf2]2, T1 for imidazolium ring proton (proton 2) varies by ∼33% (at 298 K) and ∼40% (at 308 K) whereas T1 for anions varies only by ∼8% at both temperature. Similarly, between [C6MIm][NTf2] and [C6(MIm)2][NTf2]2, T1 for proton 2 varies by ∼23% whereas T1 for anions varies by ∼15% at both temperatures. This shows that the reorientational motion of the cations is affected significantly, as compared to the anions on going from monocationic to dicationic IL. This is expected as the structural variation between the mono- and dicationic ILs is only in the cation of the ILs and not in the anion. Even among MILs, variation in T1 for proton 2 is more (15−17%) as compared to that of the anion (6−8%) at two temperatures. Variation in the T1 for N−CH2 protons (proton 4) among the RTILs is very small (within 15%). This can be attributed to the negligible change in the environment (in particular dipolar coupling) of the N−CH2 protons in the RTILs. In general we have observed a significant difference in reorientational motion among the cation of the RTILs whereas the same is not observed in the case of anions. The above discussion points out that the difference in solvation dynamics between the MILs and the DIL can be attributed to the difference in rerientational motion of the cation rather than the anions. 3.4. Time-Resolved Fluorescence Anisotropy Study. Solute−solvent and solvent−solvent interactions are known to play an important role in determining physicochemical properties of liquid and solution.17 Since study of molecular

Figures 5−7 show the temperature dependence of measured H spin−lattice relaxation times (T1) and effective rotational correlation time (τc) calculated for the individual proton resonances of [C 3 Mim][NTf 2 ], [C 6 Mim][NTf 2 ], and [C6(MIm)2][NTf2]2, respectively. The calculated τc represents overall correlation time of certain parts of the molecule and has contributions from not only global rotation but also from additional relaxation inducing processes like fast local motions such as internal rotation around a three symmetrical axis in CH3, segmental motions of alkyl chains, and conformational change of imidazolium ring. Hence, τc can be represented as 1

1 1 1 = + τc τ0 τs

(10)

Here τ0 and τs are the correlation times for the isotropic reorientation and local intramolecular motions, respectively. It can be seen from Figures 5a, 6a, and 7a that the imidazolium ring protons show two distinct characteristics across the molecules. First, they do not show a T1 minimum in the chosen temperature range (227−358 K), and second, their T1 values are the largest among all protons in the molecule. Such behavior has been previously observed for other imidazolium ionic liquids.75,77−79,81−84 The fact that the imidazolium protons do not show a minimum indicates fast motion of the ring causing very small correlation time. A ring flipping might be a possibility. With decrease in temperature the overall correlation time increases as all motions, global and local, become slower. However, if certain portions of the molecule exhibit fast motion (hence small correlation time) relative to other parts of the molecule, the correlation time does not increase enough to reach the condition ω0τc = 0.616, so that a minimum in T1 is observed. At higher temperature (lower value of 1000/T) a proton shows a minimum in T1; the more rigid part is the portion of the molecule where the proton is located. For all other protons showing T1 minimum, it is the molecular reorientational motion which contributes predominantly for the corresponding portion of the molecule. It is clear from Figures 5a and 6a that, as we go toward the terminal of the alkyl chain, τc becomes faster. For example τc values of terminal CH3 protons of [C3Mim]+(0.05 ns) and [C6Mim]+ (0.03 ns) are the fastest among all. This can be explained by the flapping motion of the end of a free chain. On the other hand N−CH2 protons show slower τc (0.66 ns in [C3Mim]+ and 0.71 ns in [C6Mim]+) as they are adjacent to the bulky imidazolium unit. Also in the dication [C6(Mim)2]2+, τc for N−CH2 protons is the slowest at lower temperature (1.33 ns at 298 K). However, with increase in temperature, τc for N−CH2 protons changes more as compared to the bridging alkyl protons (Figure 7b). At 358 K τc for N−CH2 protons becomes faster (0.041 ns) compared to the bridging alkyl protons (0.1 ns for proton 6 and 0.09 ns for proton 7) (Figure 7b). This is because the two imidazolium units in [C6(Mim)2]2+ restrict the flapping motion of alkyl bridge protons unlike the free alkyl chain in [C3Mim]+ and [C6(Mim)2]2+. At higher temperature probably the molecule exhibits a rapid interconversion between the folded U-shape and an unfolded linear shape and the again folded U-shape in the opposite direction. Such motion is possible as the bulky imidazolium rings are connected through a flexible alkyl bridge. Figures 5a, 6a, and 7a show an overall pattern that at lower temperature all protons have similar T1 and with increase in temperature the T1 values of protons in different portion of the molecules exhibit different relaxation J


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In these equations, Vs and Vp are the van der Waals volume of the solvent and solute, respectively. V s values for [C6(MIm)2][NTf2]2, [C6Mim][NTf2], and [C3Mim][NTf2] are 335.5, 201.9, and 171.2 cm3/mol, respectively.10 CGW is found to be 0.12 for perylene in the DIL, which is close to the average Crot value (Table 4). From Figure 9a we can clearly observe that the rotational relaxation behavior of perylene follows different hydrodynamics at different η/T in DIL. At lower η/T, rotational dynamics of perylene is well within GW quasihydrodynamics boundary and SED slip boundary. At higher η/T (>0.2) it departs from both SED and GW boundary, and in the range 0.1−0.2 of η/T, rotational dynamics of perylene lies within GW and SED slip boundary. This indicates that none of the hydrodynamics theory (SED and GW) is successful in explaining the rotational dynamics of perylene in DIL. This decoupling of rotational motion of perylene with medium viscosity can be rationalized in terms of microheterogeneous behavior of the IL.19 This data also demonstrates that solute−solvent interaction is similar in MILs but is different in DIL. Interestingly, for MPTS, rotational relaxation dynamics shows a different behavior in [C3Mim][NTf2] (Figure 9c). We have earlier observed superstick behavior for MPTS in [C6Mim][NTf2].72 However, the Crot, which measures the solute−solvent interaction,86 value is observed to be slightly less (1.33) for [C3Mim][NTf2] than that has been observed in [C6Mim][NTf2] (1.87). This shows that solute−solvent association is stronger in [C6Mim][NTf2] than in [C3Mim][NTf2]. Similar results have also been observed by Fruchey and Fayer22 earlier in different RTILs with varying alkyl chain length. The reason behind such observation can be explained by considering the difference in rotator volume in [C6Mim][NTf2] and [C3Mim][NTf2]. This inference originates from the outcome of the “solventberg” model107 which assumes attachment of solvent molecules of non-negligible size to the rotating solute, causing increase in effective rotator volume. Rotational dynamics of MPTS in the dicationic IL has already been explained by us through consideration of stronger cation− anion interaction.72 It has been observed that imidazolium cation sites in dicationic IL are strongly associated with the counteranion bis(trifluoromethylsulfonyl)amide (NTf2−). Due to this, imidazolium cation cannot form a stronger hydrogen bonding interaction with bulkier MPTS, and thereby solute (MPTS)−solvent (dicationic IL) interaction is reduced. This causes relatively faster rotation of MPTS in dicationic IL.72 Essentially, rotational relaxation behavior of 3 solutes in three ionic liquids demonstrates that the solutes locate themselves in distinct environment of the media. This data also depicts that the behavior of the DIL is quite different from that of the MILs having equal or half-hydrocarbon chain with respect to spacer chain length of the dicationic ionic liquid. We have also attempted to analyze our data from the log−log plots of normalized rotational relaxation time versus η/T (Figure 10). Upon looking at the variation of τr with η/T in DIL (Figure 10a), one can clearly see that the behavior of perylene is quite different than the other two probes C153 and MPTS, whereas in MIL, rotational relaxation behavior of perylene is observed to be similar to those of C153 and MPTS (Figure 10b). This indicates that location of dipolar C153 and charged MPTS are very similar, in both ILs, whereas perylene locates itself in a different environment during rotation. The data indicates that, in DIL, perylene experiences a more nonpolar environment

rotation in liquids helps us to understand intermolecular interaction, in the present work we have also carried out the time-resolved fluorescence anisotropy study in both monocationic and dicationic ionic liquids basically to understand how the structural variation within mono- and dicationic systems influences the intermolecular interactions. This is important as the outcome of this study would be helpful to understand structure−property correlation within ionic liquids systems. We have analyzed the rotational diffusion behavior of two neutral molecules perylene and C153, and one anionic species, MPTS, in monocationic IL, and also compared the data with those of the dicationic counterpart. Data corresponding to the abovementioned three probes in the dicationic ionic liquid were earlier studied by us.72 Representative time-resolved fluorescence anisotropy decay profile of C153 in [C3Mim][NTf2] is shown in Figure 8. Rotational relaxation time (τr) is calculated by single or biexponential tail-fitting of the fluorescence anisotropy decay profiles as per the relation τr = a1e−t / τ1 + a 2e−t / τ2

(11)

Table 3 lists the probe properties of all the three probes that are used in this study. Rotational relaxation parameters of the three probes in both dicationic and monocationic ionic liquids are collected in Table 4. As can be seen from Table 4, the average rotational relaxation time (⟨τr⟩) decreases with increase in temperature. The lowering of average rotational time upon increasing the temperature is caused due to the lowering of the medium viscosity upon increase in temperature. Further, the viscosity dependence of average rotational relaxation time has been rigorously analyzed in light of Stokes− Einstein−Debye (SED) equation.87 The data is analyzed by plotting log⟨τr⟩ versus log(η/T) along with the stick and slip boundary condition (Figure 9). From Figure 9a,b it seems that the rotational diffusion behavior of perylene and C153 is more or less similar in all three ionic liquids and lies between stick and slip boundary conditions. However, Figure 9a demonstrates that the rotational relaxation behavior of perylene is quite different in the DIL as compared to the MILs and shows departure from normal hydrodynamic behavior at higher η/T values in the DIL. The rotational relaxation behavior of perylene in dicationic IL has also been analyzed by invoking quasihydrodynamic theory (Gierer−Wirtz theory).106 GW theory takes into account both solute and solvent size. The theory considers solvent as concentric shells of spherical particles around the solute molecule. The friction coefficient in GW theory is expressed as CGW = σC0

(12)

where σ is the sticking factor. For a solvent volume Vs and probe volume Vp −1 ⎡ ⎛ V ⎞1/3 ⎤ ⎢ ⎥ s σ = ⎢1 + 6⎜⎜ ⎟⎟ C0 ⎥ ⎝ Vp ⎠ ⎣ ⎦

(13)

⎡ 1/3 ⎢ Vs 6 Vp ⎢ 1 C0 = ⎢ + 1/3 ⎤4 ⎡ ⎡ Vs ⎢ 1 + 2 Vs ⎥ ⎢1 + 4 V p Vp ⎢⎣ ⎢⎣ ⎦ ⎣

( ) ( )

⎤−1 ⎥ ⎥ ⎥ 3 1/3 ⎤ ⎥ ⎥ ⎥ ⎦ ⎦

( )

(14) K


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work entails a significant step forward in understanding the structure−property relationship in monocationic and dicationic ILs.

that the other two probes. The presence of a longer alkyl hydrophobic bridge in DIL might be responsible for the nonpolar environment faced by perylene. The variation in the hydrophobic chain length in DIL and MIL is probably responsible for the observed difference in the reorientation motion of perylene in DIL and MIL. We have also compared the activation energies of rotational motion (Er) of perylene in the ILs with those of the viscous flow (Eη) of the ILs, calculated from the respective Arrhenius plots (Table 5). Representative Arrhenius plots for the viscous flow of [C6(MIm)2][NTf2]2 and rotational relaxation of perylene in [C6(MIm)2][NTf2]2 are shown in Figure 11. We have observed that Er is considerably lower than Eη in DIL (∼24 kJ/mol). However, in MILs Er is lower by only 5−8 kJ/mol (Table 5). These results indicate that the microviscosity experienced by the probe in the DIL is significantly lower than the bulk viscosity. The present result is consistent with the recent report by Compton and co-workers.108 In this report rotational diffusion of a stable free radical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) has been studied in imidazolium, ammonium, pyrolidinium, and phosphonium ILs through ESR spectroscopy. It has been shown that Er for TEMPO in trialkylphosphonium RTILs is 5−10 kJ/mol less than Eη of the trialkylphosphonium RTILs. The authors have suggested that TEMPO rotates in zones of lower microviscosity than the bulk viscosity in these trialkylphosphonium RTILs. Hence, the observed faster rotational relaxation of perylene in the DIL can be attributed the low microviscosity of the DIL.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b07357. Time-resolved emission spectra (TRES) of C153 in [C 3 MIm][NTf 2 ] and plots of the spectral shift correlation function, C(t), versus time (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +91-674-2304050. Phone: +91-674-2304037. E-mail: [email protected]. *Fax: +91-674-2304050. Phone: +91-674-2304037. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Department of Science and Technology (DST), Government of India. P.K.S thanks National Institute of Science Education and Research (NISER), Bhubaneswar, for a fellowship.

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4. CONCLUSIONS In the present report, we have employed steady state and timeresolved fluorescence as well as NMR techniques to understand the structure−property correlation in MILs and one DIL. The study unravels several fascinating distinctions between dicationic and monocationic ionic liquids. Excitation wavelength dependent steady state fluorescence response of ANF reveals a significant difference in the magnitude of the total shift in the fluorescence maximum for three-membered monocationic and six-membered dicationic IL which is indicative of the difference in microheterogeneous behavior of the concerned ILs. The present excitation wavelength dependent fluorescence study also demonstrates that the effect of alkyl chain length on structural organization of mono- and dicationic ILs is different. NMR experiments have been carried out in order to examine correlation between the reorientational motion of the imidazolium cation and dynamics of solvation. PFG-NMR experiments have revealed distinctive features of translational diffusion of cations in the ionic liquids. 1H spin− lattice relaxation experiments demonstrate a dissimilar reorientational dynamics of the cations of monocationic and dicationic ILs. Thus, from the outcomes of the NMR experiments it can be inferred that the role of reorientational motion of the imidazolium cation and consequently the contribution from dipole−dipole interaction influencing the ultrafast component of solvation in particular and solvation dynamics in general in ionic liquids deserves serious consideration. Further, rotational dynamics of C153 and perylene was found to be similar for [C3MIm][NTf2] and [C6MIm][NTf2]. However, rotational dynamics of MPTS shows a stronger interaction of MPTS with [C6MIm][NTf2] than with [C3MIm][NTf2] which is in accordance with the earlier report.22 This shows that the structure−property correlation in MILs and DIL is quite different. The present

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