Ultrafast Torsional Relaxation of Thioflavin-T in Tris(pentafluoroethyl

Oct 12, 2015 - In this work, the torsional relaxation dynamics of Thioflavin-T (ThT), an ultrafast .... Free volume dependence of an ionic molecular r...
3 downloads 3 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCB

Ultrafast Torsional Relaxation of Thioflavin‑T in Tris(pentafluoroethyl)trifluorophosphate (FAP) Anion-Based Ionic Liquids Prabhat K. Singh,* Aruna K. Mora, and Sukhendu Nath Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India S Supporting Information *

ABSTRACT: Ultrafast spectroscopy on solutes, whose dynamics is very sensitive to the friction in its local environment, has strong potential to report on the microenvironment existing in complex fluids such as ionic liquids. In this work, the torsional relaxation dynamics of Thioflavin-T (ThT), an ultrafast molecular rotor, is investigated in two fluoroalkylphosphate ([FAP])-based ionic liquids, namely, 1et hyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM][FAP]) and 1-(2-hydroxyethyl)3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([OHEMIM][FAP]), using ultrafast fluorescence up-conversion spectroscopy. The emission quantum yield and the excited-state fluorescence lifetime measurement suggest that the torsional relaxation of Thioflavin-T, in this class of ionic liquids, is guided by the viscosity of the medium. The temporal profile of the dynamic Stokes’ shift of ThT, measured from time-resolved emission spectrum (TRES), displays a multiexponential behavior in both ionic liquids. The long time dynamics of the Stokes’ shift is reasonably slower for the hydroxyethyl derivative as compared to the ethyl derivative, which is in accordance with their measured shear viscosity. However, the short time dynamics of Stokes’ shift is very similar in both the ionic liquids, and seems to be independent of the measured shear viscosity of the ionic liquid. We rationalize these observations in terms of different locations of ThT in these ionic liquids. These results suggest that despite having a higher bulk viscosity in the ionic liquid, they can provide unique microenvironment in their complex structure, where the reaction can be faster than expected from their measured shear viscosity.

■

INTRODUCTION One of the most popular components of the modern day liquid chemistry are ionic liquids (ILs) that are extensively used in several fields of science because of their tunable and unique physicochemical properties such as viscosity, density, conductivity, hydrophobicity, chemical and thermal stability, etc.1−4 The most attractive feature of ILs is the potential to fine-tune their physicochemical properties by the various combinations of cations and anions, and in some cases by rational functionalization of either of the ions. These functionalized analogues of the ionic liquids are often termed as task-specific ILs (TSILs).5 The simplicity to create these TSILs by systematic chemical modifications of the constituent ions takes these systems far beyond the promise of designer solvents, and leads to a large variety of actual applications such as drug delivery,6 fuel cell electrolytes,7 solar cells,8 high energy materials,9 and many other applications.10,11 In recent years, investigations on the hydroxyl-functionalized ILs (also termed as one variety of TSILs) have been initiated due to their impressive capacity of reversible capturing of greenhouse gases.12−14 ILs with a hydroxyl functionalized cation provide interesting modifications to the classical imidazolium-based ILs, with some useful properties, such as solvation ability, polarity, etc., such that they can take over © XXXX American Chemical Society

traditional alcohols in specific applications. For example, the ease of synthesis of rhodium nanoparticles in hydroxyl ILs as compared to those in non-functionalized ILs, provides a highly stable and efficient catalytic system for biphasic hydrogenation reactions.15 Other applications include, but are not limited to, the enhancement in hydrophilicity and enzyme activity,16,17 increase in the enantioselectivity of the reaction,18 better solubility of inorganic salts.19,20 Thus, the purpose of attaching functional groups, to the cationic component of ILs, is to endow the ILs with additional interactions which leads to their specific applications. However, in the presence of a strongly coordinating anion, the ability of the solute to interact with the functional group is masked to a large extent.21 In such cases, an anion with weak coordination ability is desirable. In this regard, the tris(pentafluoroethyl) trifluorophosphate anion, [FAP]−, based ILs are unique because, owing to very weak coordinating ability, the [FAP] anion not only protects the desirable properties of the cation from being masked on interaction with otherwise strongly coordinating anion systems, but in some cases it even promotes Received: September 16, 2015 Revised: October 11, 2015

A

DOI: 10.1021/acs.jpcb.5b09028 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B Scheme 1. Chemical Structure of Thioflavin-T and Ionic Liquids Used in the Present Study

efficient nonradiative deactivation process.30 When this twisting motion is affected by the viscosity of its local environment, it influences its fluorescence yield.31,32 ThT is very weakly emissive in low viscosity solvents, and it is now established that a nonradiative torsional relaxation takes place in ThT around its central C−C bond (see molecular structure in Scheme 1), upon electronic excitation to a barrier-less potential energy surface.33 Since such a large amplitude torsional motion is operative in the excited state of ThT, the rate of this excited state reaction is strongly influenced by the viscosity of the medium.34 Owing to the extreme sensitivity of the emission quantum yield and the excited state lifetime toward the microscopic friction, ThT is projected as a probe for the microviscosity in several biological and chemical environments.35−40 Since the torsional relaxation dynamics of ThT is very sensitive to the friction in its immediate surroundings, it is expected to provide important insights into the microenvironment existing in these ionic liquids. In addition, we also wanted to investigate how the hydroxy functionalization of the imidazolium cation, in combination with the [FAP] anion influences the solute dynamics, when compared to their nonhydroxylated analogue. Thus, the two ionic liquids investigated were 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM][FAP]) and 1(2-hydroxyethyl)-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([OHEMIM][FAP]) (see Scheme 1 for chemical structures). The time-dependent evolution of the emission spectral features has been analyzed in detail, which includes peak frequency and integrated area under the emission spectra, and the obtained results have been compared for the hydroxylated and the nonhydroxylated derivatives.

certain desirable properties of the cation. For example, the presence of [FAP] anion increases the hydrogen-bond acidity of the hydroxyl functionalized cation, when compared to the presence of other basic anions, for example, the NTf2− anion.21 Apart from this, the [FAP]-based ILs, as such, are interesting in their own right due to their impressive solvent properties, which includes ultrahigh hydrophobicity, wide electrochemical window, and good hydrolytic stability.22,23 It is reported that [FAP]-based ILs are probably the most hydrophobic and it takes up water about 10 times lower as compared to ionic liquids with hexafluorophosphate anion.22 Because of their excellent solvent properties, as mentioned above, [FAP]-based ILs find several advantages in CO2 solubilization,24 battery application,25 electrosynthesis, etc.21,26 In addition, [FAP] anion-based ILs display very low viscosity due to very weak cation−anion interaction prevailing in these ionic liquids. It should be noted that the high viscosity of the conventional imidazolium-based IL is considered as a problem for efficient chemical reactions. Thus, the low viscosity imparted by the [FAP]-based ILs can be advantageous from the viewpoint of efficiency of the chemical reactions carried out in these ionic liquid medium. All these applications certainly imply that the [FAP] anions transmit unique characteristics to their cationic counterparts in ILs. Thus, a molecular-level understanding of the microscopic interactions in these ILs is essential to predict their properties, design TSILs, and to exploit the immense potential of this class of ionic liquids in several applications. Although there are numerous studies which detail the physicochemical properties of the [FAP]-based IL, dynamical studies in these systems are rather limited.27−29 To the best of our knowledge, there is no attempt to characterize the interaction between solute molecules and [FAP]-based ILs by powerful ultrafast spectroscopic technique. The dynamical studies in these systems are important because the dynamics of the solute is affected by the local environment of the solute and is intricately related to its physicochemical properties. In ̀ this work, we aim to investigate the torsional relaxation of an ultrafast molecular rotor, Thioflavin-T (ThT), in [FAP]based ILs, using the femtosecond resolved fluorescence upconversion technique. Thioflavin-T belongs to the family of ultrafast molecular rotors, which represent a class of compounds that has the ability to twist around a single bond in their excited state, and this twisting motion constitutes an

■

EXPERIMENTAL SECTION Thioflavin T (ThT), in the form of chloride salt, was obtained from Sigma-Aldrich and was purified according to the method reported in the literature.33,36 The two ionic liquids, namely, [EMIM][FAP]) and [OHEMIM][FAP] were obtained from Iolitec, Germany. The purity of these ILs is >99% with less than 100 ppm halide content. The water content of the ILs was determined using a Metrohm model 831 KF coulometer which uses the coulometric method for the Karl Fischer titration. The water content was found to be ∼85 ppm for [EMIM][FAP] B

DOI: 10.1021/acs.jpcb.5b09028 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B 2⎤ ⎡ ⎧1 ⎛ 2b(ν − νP) ⎞⎫ ⎥ I(ν) = a exp⎢ −ln(2)⎨ ln⎜1 + ⎟⎬ ⎢⎣ ⎠⎭ ⎥⎦ w ⎩b ⎝

and ∼95 ppm for [OHEMIM][FAP]. Samples were prepared in a closed chamber under a dry nitrogen flow atmosphere, and the quartz cuvettes containing the samples were sealed during the measurements. Ground state absorption measurements were carried out in JASCO spectrophotometer (model V650). Steady-state fluorescence measurements were performed in a Hitachi spectrofluorimeter (model F-4500). The emission spectra were corrected, using quinine sulfate solution, for the wavelength-dependent instrument response.41 The spectra in the frequency domain, I(ν), ̅ were obtained from the measured spectra in the wavelength domain, I(λ), by using the following equation.

I (ν ̅ ) = λ 2 I (λ )

2b(ν − νP) > −1 w 2b(ν − νP) = 0 if ≤ −1 w if

where a is the amplitude, νp is the peak frequency, b is the asymmetry parameter, and w is the width parameter. The decay traces were fitted with a multiexponential function of the following form, I(t ) = I(0) ∑ αi exp( −t /τi)

τ =

■

∑ Aiτi

where Ai = αi /∑ αi

(5)

RESULTS AND DISCUSSION Steady-State Emission Measurements. Steady-state emission measurements were carried out for ThT in both ionic liquids, ([EMIM][FAP]) and ([OHEMIM][FAP]) at an optically matched condition at the excitation wavelength (λexc = 410 nm) and the results are presented in Figure 1. It is evident

IsA r ns 2 IrA snr 2

(4)

The mean fluorescence lifetime is calculated according to the equation,43

(1)

The emission quantum yield (Φ) of ThT in ILs was determined by comparing the integrated fluorescence intensities of the ThT in ILs with that of the Coumarin 481 dye in acetonitrile (Φ = 0.0842) using the following equation,43 Φs = Φr

(3)

(2)

where the subscripts r and s refer to the reference (Coumarin 481) and sample (ThT in IL), respectively; I represents the integrated area under the emission band; n is the refractive index of the solvent, and A represents the absorbance of the solution at the excitation wavelength (λexc = 410 nm). The refractive indices of the ionic liquids were measured with a Mettler Toledo RE 50 refractometer using 583.9 nm of light. The refractive indices are found to be 1.3688 for [EMIM][FAP] and 1.3769 for [OHEMIM][FAP] at 25 °C. Time-resolved fluorescence measurements were carried out using a femtosecond fluorescence upconversion instrument (FOG 100, CDP Inc. Russia) which was described in detail earlier.44 A laser pulse at 410 nm (50 fs) is used for sample excitation. The instrument response function (IRF) was measured through the cross-correlation of the excitation laser pulse (410 nm) and the gate beam (820 nm), by overlapping both beams into a BBO crystal, with the gate beam reaching the BBO crystal after passing through a delay rail. The crosscorrelation intensity, as a function of time delay between the excitation and the gate pulse, was found to have a Gaussian profile with fwhm of 230 fs, which is a measure of IRF. To remove any contribution of rotational reorientation of the solute on the measured decay trace, the time-resolved data were collected at magic angle polarization. Samples were taken in a rotating sample cell with quartz window of path length 0.4 mm to avoid the photodegradation of the sample, and the reproducibility of the measurements were tested by collecting the decay traces for 2−3 times. Time-resolved emission spectra were reconstructed following the method proposed by Maroncelli and Fleming45 and is also detailed in our earlier publication.39,46,47 Each reconstructed spectrum was fitted using the log-normal function of the following form.45

Figure 1. Steady-state emission spectra of Thioflavin-T in [EMIM][FAP] (dashed blue line) and [OHEMIM][FAP] (solid red line) (λexc = 410 nm). Inset: Normalized emission spectra of ThT in [EMIM][FAP] (dashed blue line) and [OHEMIM][FAP] (solid red line).

from Figure 1 that ThT shows higher emission intensity in [OHEMIM][FAP] as compared to that in [EMIM][FAP]. We also determined the emission quantum yield of ThT in these two ionic liquids, using the comparison method detailed in the Experimental Section, and emission quantum yield was found to be higher by a factor of ∼2 for [OHEMIM][FAP] (Φ = 2.32 × 10−2) as compared to that for [EMIM][FAP] (Φ = 1.15 × 10−2). The emission quantum yield of ThT is strongly sensitive to the viscosity of the surrounding environment.34,36,38,48 It is reported that the emission quantum yield of ThT varies linearly with the viscosity of the medium.34,48 Thus, the relatively higher quantum yield of ThT in [OHEMIM][FAP] is in line with its higher viscosity; that is, η = 205 cP at 25 °C as compared to that for [EMIM][FAP] (η = 60 cP at 25 °C).27 Thus, this data suggest that the large amplitude torsional motion of ThT fragements, which controls the emission C

DOI: 10.1021/acs.jpcb.5b09028 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

reported in literature.33,34,53 Such nonexponential decay kinetics is believed to arise due to the torsional relaxation on the barrierless excited state potential energy surface where the relaxation rate is dependent on the twist angle.33,34,54 Similar nonexponential decay kinetics is noted for other molecules belonging to the molecular rotor family.55−57 However, such nonexponential kinetics for other fluorescent probe molecules (not belonging to the molecular rotor family) has been reported in the ionic liquid medium and has been rationalized in terms of heterogeneity of the ionic liquid environment.58−60 Thus, despite that the decay traces for ThT are inherently nonexponential in nature in homogeneous solvents, we do not discount the possibility of contribution of the heterogeneous environment of the ionic liquid on the observed nonexponentiality of decay kinetics of ThT. Because of this complexity, we adopt the average excited-state lifetime, to recognize the effect of different microenvironment of ionic liquid on the excited state torsional dynamics of ThT. A similar approach has been previously undertaken for molecules undergoing torsional relaxation in the excited state.55,57,61 For this purpose, both the decay traces are fitted with a multiexponential (triexponential) function and the average lifetime is calculated using eq 5 (see Experimental Section). The average excited state lifetime for ThT is estimated to be ∼32 ps for [EMIM][FAP] and ∼92 ps for [OHEMIM][FAP]. The viscosity of [OHEMIM][FAP] (η = 205 cP) is higher as compared to that of [EMIM][FAP] (η = 60 cP) due to the higher association of the hydroxyl group in the former. Thus, the present result suggests that the higher viscosity of [OHEMIM][FAP] provides a larger frictional force toward the torsional motion of ThT in its excited state and leads to a longer excited state lifetime in [OHEMIM][FAP] as compared to that in [EMIM][FAP]. This result is in agreement with the result on the emission quantum yield of ThT as described in the previous section. To gain a better understanding of the excited state dynamics of ThT in the ionic liquid, the fluorescence decay traces were measured for ThT in [EMIM][FAP] and [OHEMIM][FAP] at different emission wavelengths, ranging the whole emission spectrum. Figure 3 displays the fluorescence decay traces at some representative wavelengths. It is obvious from Figure 3 that the fluorescence decays are strongly dependent on the monitoring emission wavelength for both ILs. On the high energy (blue) side of the emission spectrum, the transient decay traces display, on an average, faster decay time which gradually becomes slower as the monitoring wavelength is moved toward the low energy (red) side of the emission spectrum. However, on comparison of the lower and upper panel of Figure 3, it is observed that the fluorescence decay in [OHEMIM][FAP] is quite slower as compared to that in [EMIM][FAP], at all the emission wavelengths. Similar features in the decay traces, regarding the wavelength dependence, for ThT has been observed earlier in the time-resolved fluorescence up-conversion measurements in homogeneous solvents and in confined nanocavities.33,34,37,39 Such a wavelength dependence has been assigned to the torsional relaxation of ThT in the excited state, which takes place via twisting of the dimethyl anilino group around its central C−C bond. This torsional relaxation causes the excited state to evolve from an initially populated high energy state, with strong emission character (LE state) toward the lower energy state, with weak emission character (TICT state, Scheme 2).33,34 The rate of such torsional process is strongly influenced by the friction

quantum yield of ThT, is more hindered in the more viscous [OHEMIM][FAP] as compared to in the [EMIM][FAP]. The inset of Figure 1 displays the normalized emission spectra for ThT in both ILs. As it is evident that the emission spectrum of ThT is very similar in both ionic liquids, a very small broadening is observed for the emission spectra of ThT in [OHEMIM][FAP] as compared to that of [EMIM][FAP]. Although, it is expected that the hydroxyl derivative might display a higher polarity compared to the nonhydroxyl derivative, and may cause a differential shift in the emission maximum of ThT in these two ionic liquids, this data suggest that ThT does not sense a drastically different polarity in these ionic liquids. However, it should be noted that ThT displays a very weak solvatochromism in normal conventional solvents.33,49 This fact might limit the information on the different polarities existing in these ionic liquids. It should be noted that we have not observed any significant emission from the neat ILs (see Figure S1 and S2, Supporting Information). The optical density of the neat ILs was negligible at the excitation wavelength, 410 nm. For neat ILs, in a 1 cm cell, at 410 nm, absorbance was 0.006 for [EMIM][FAP] and 0.002 for [OHEMIM][FAP], whereas the absorbance of the ThT was maintained at ∼0.2. Thus, our measurements are free from the contribution of any signal from the neat ILs. Time-Resolved Fluorescence Measurements. The excited-state lifetime of Thioflavin-T decreases severely as the rigidity of its immediate environment decreases.36,39,40 This phenomenon has been assigned to the twisting motion of the dimethyl anilino group around the central C−C bond in the excited state of ThT, which causes a severe loss of the oscillator strength, leading to the twisted intramolecular charge transfer (TICT) state.33,34,36 Therefore, the excited-state lifetime has been extensively used as a sensor of the local viscosity in reverse micelles,36,38,50 DNA,40,51,52 supramolecular cavitands.37,39 Thus, to understand the effect of the local environment of these ionic liquids on the torsional motion of ThT, we carried out fluorescence transient measurements for ThT in [EMIM][FAP] and [OHEMIM][FAP]. The transient decay traces for ThT in two ionic liquids are presented in Figure 2. It is obvious

Figure 2. Fluorescence transients at 490 nm for ThT in [OHEMIM][FAP] (blue) and [EMIM][FAP] (red). The dotted line shows the instrument response function (IRF).

from the figure that the decay trace for ThT in [OHEMIM][FAP] is slower as compared to that in [EMIM][FAP]. The decay traces for the fluorescence transients, for both ionic liquids, are found to follow nonexponential kinetics. The nonexponential decay kinetics for the torsional relaxation process for ThT in conventional organic solvents is already D

DOI: 10.1021/acs.jpcb.5b09028 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 3. Transient fluorescence decay of Thioflavin-T in [EMIM][FAP] (upper panel) and [OHEMIM] [FAP] (lower panel) at different emission wavelengths: (1) 440 nm, (2) 470 nm, (3) 520 nm, and (4) 600 nm. The dotted line shows the instrument response function (IRF).

Figure 4. Time-resolved emission spectra (TRES) of Thioflavin-T in [EMIM][FAP] (upper panel) [OHEMIM][FAP] (lower panel) at different times. The circles represent the experimental data points and the solid lines represent the log-normal fit (cf. eq 3) to the data points.

Scheme 2. Qualitative Potential Energy Surface for ThTa

that the emission spectra gradually shift toward the lower frequency with time resulting in a dynamic Stokes’ shift. The dynamic Stokes’ shift observed for ThT in nonionic solvents is assigned to an intramolecular torsional relaxation in the excited state.33,34 However, the solvent relaxation process is often times introduced to explain the dynamic Stokes’ shift of several fluorescent dyes bearing strong solvatochromism in polar solvents,33,34,62 but ThT displays very weak solvatochromism and its excited state population drastically decreases within the experimental time window. This was suggested as the reason to preclude any major involvement of solvent relaxation in the observed excited state dynamics of ThT in nonionic solvents.33,34 For other molecules which experience different types of intramolecular conformational relaxation such as bond-twisting, cis−trans isomerization, etc., a similar dynamic Stokes’ shift has been proclaimed in the literature in the nonionic solvents.55−57,61,63−67 Thus, on the basis of the literature reports on other similar chemical systems, as well as earlier literature reports on ThT in different chemical environments,33,34,37,39 it may be inferred that the dynamic Stokes’ shift for ThT in the present ILs has a predominant contribution from the intramolecular torsional relaxation process in the excited state of ThT. This intramolecular torsional relaxation process lowers the energy of the excited electronic state, from the high energy LE state to the low energy TICT state, and results in the dynamic Stokes’ shift. It is also obvious from Figure 4, that for ThT in [EMIM][FAP] and [OHEMIM][FAP], along with the spectral shift, an extensive decrease in the emission intensity also takes place. The change in the integrated area under the emission curve with time has been shown in Figure 5. For example, the intensity was found to decrease by ∼77% for [EMIM][FAP] and by 56% for [OHEMIM][FAP] within 50 ps following photoexcitation. These large changes in the integrated area under the emission spectra for ThT are reported in different

a

On photo-excitation, the dimethyl anilino group of ThT undergoes a torsional relaxation around central C−C bond, on the barrierless potential energy surface, to form a TICT state from the initially formed LE state. The solid downward arrows represent emission from different points on the potential energy surface. The curved arrow represents the transition from the non-emissive TICT state to the twisted ground state.

provided by the local environment.34,36,48 So the slower decay traces for ThT in [OHEMIM][FAP] as compared to that in [EMIM][FAP] for all the emission wavelengths is a manifestation of slower torsional relaxation of ThT in [OHEMIM][FAP] as compared to that in [EMIM][FAP]. Time-dependent emission spectra provide the detailed insights into the excited state dynamics and, for this purpose, the wavelength-dependent fluorescence decays, collected for ThT in both ILs, were transformed to time-resolved emission spectra (TRES) using the methodology mentioned in the Experimental Section. The TRES thus constructed for ThT in two ILs are presented in Figure 4. The resulting timedependent emission spectra are well represented by the lognormal function (cf. Figure 4). It is quite obvious from Figure 4 E

DOI: 10.1021/acs.jpcb.5b09028 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

features (structural and dynamic heterogeneity) of ionic liquids as compared to conventional solvents. For [EMIM][FAP] the fitted parameters are 0.3 ps (10%), 3.3 ps (10%), and 420 ps (80%), whereas for [OHEMIM][FAP], the fitted parameters are 0.3 ps (20%), 91.3 ps (16%), and 2300 ps (64%). The long time dynamics of the Stokes’ shift is reasonably slower for the hydroxyethyl derivative (η = 205 cP at 25 °C) as compared tothat for the ethyl derivative (η = 60 cP at 25 °C); however, interestingly, the faster component of the Stokes’ shift dynamics was found to be very similar ∼0.3 ps for both the ionic liquids. Since the Stokes’ shift dynamics of ThT is a manifestation of torsional relaxation in its excited state, which involves large amplitude motion, these Stokes’s shift data indicate that the diffusive motion of ThT fragments at longer time scale is guided by viscosity in the IL medium. However, in the early time scale, the torsional relaxation of ThT is largely decoupled from the viscosity in the case of the hydroxyethyl derivative. It is possible that although the overall friction of the hydroxyl functionalized ionic liquid can be large owing to its large viscosity, it can provide a unique microenvironment, depending on the location of the solute, where local effects can permit friction to be much smaller than expected from the bulk viscosity. At this point, we would like to briefly discuss the structure of the ILs used in the present study. It is known that, for the [FAP] anion-based IL, the cation−anion interaction is very weak due to the very weak basicity and bulky nature of the [FAP] anion.21 Such weak interaction between cation and anion leads to very low viscosity for the [FAP] anion-based IL when compared to the strongly basic anion attached with the same cation.21,27 However, when the cation is functionalized with the hydroxyl group, for example, 1-(2-hydroxyethyl)-3methylimidazolium cation, formation of different types of Hbonds can be expected. Thus, by replacing one hydrogen atom from the terminal methyl group with a hydroxyl group, a large amount of hydrogen bonds is formed between hydroxyl groups as well as between the hydroxyl group and the cationic headgroup, and this leads to a stronger association between the cations.68 It has been revealed from the molecular dynamics simulations that the major cation−cation interaction between the 1-(2-hydroxyethyl)-3-methylimidazolium cations is through intermolecular hydrogen bonding between O−H sites.69 The stronger cation−cation interaction for the hydroxyl-functionalized imidazolium cation is also facilitated by the voluminous nature of the [FAP] anion which leads to a large decline in the probability of finding the anion in the plane of the hydroxylfunctionalized imidazolium cation.69 This decreased probability of finding the anion in the plane of the imidazolium ring leads to a weaker cation−anion interaction which is replaced by intracationic and intercationic interaction.69 This intercationic interaction leads to stronger association between the cations and hence higher viscosity. Although the hydroxyl-functionalized ILs offer an overall larger friction, owing to cation−cation association facilitated by hydrogen bonding, it might be possible that local effects, such as translational motion of the ions, can permit friction to be much smaller than expected from the bulk viscosity. It has been also suggested for ILs that a larger scale liquid structure reorganizes on a longer time scale, whereas ions inside the local structure can move very rapidly,70,71 which may lead to effective friction in the shorter time scale, decoupled from bulk viscosity. Similar observations have been made for multiexponential rotational correlation functions of solutes in ILs, in which the

Figure 5. Variation in area under the emission curve (normalized to 1 at 50 fs) with time for [EMIM][FAP] (▽) and [OHEMIM][FAP] (○). The solid line is the fit to the triexponential function.

media and are attributed to the ultrafast nonradiative relaxation process occurring in its excited state.33,34,37,39 The changes in the integrated area could be fitted with a triexponential function for both ionic liquids. For [EMIM][FAP] the fitted parameters are 0.3 ps (21.3%), 4.8 ps (26.1%), and 65.6 ps (52.6%), whereas for [OHEMIM][FAP], the fitted parameters are 1.6 ps (14.2%), 18 ps (24%), and 189.2 ps (61.8%). Thus, the average time constant for the area decay is calculated to be 35.8 ps for [EMIM][FAP] and 121.3 ps for [OHEMIM][FAP]. The higher value for ThT in [OHEMIM][FAP] is consistent with its average excited state lifetimes which was also found to be higher by a factor of ∼3 for [OHEMIM][FAP] in comparison to [EMIM][FAP]. Thus, this data further confirm that the torsional relaxation of ThT is comparatively more retarded in [OHEMIM][FAP] in comparison to [EMIM][FAP]. It should be noted that, as the rate of the radiative transition is dependent on the emission frequency (kr α ν3), and since the emission frequency is found to be time dependent in the present case, so the radiative decay rate, kr, will also be time dependent. Thus, in the calculation of integrated area for each time, the emission intensities were divided by ν3 to correct for their frequency dependence. Figure 6 shows the changes in the emission peak frequency with time for both the ILs. The temporal profile for the

Figure 6. Variation in peak frequency with time for Thioflavin-T in [EMIM][FAP] (○) and [OHEMIM][FAP] (▽). The solid line is the fit to the experimental data by three exponential function.

dynamic Stokes’ shift could be well described by a triexponential function in both the ILs. This is in contrast to the homogeneous nonionic solvents, where the Stokes’ shift dynamics of ThT was found to follow a monoexponential behavior.33,34 This indicates that the Stokes’ shift dynamics of ThT in ionic liquid has the reminiscence of unique structural F

DOI: 10.1021/acs.jpcb.5b09028 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

[OHEMIM][FAP] as compared to that of the [EMIM][FAP]. Detailed analysis of the time-resolved emission spectral features reveal that the temporal profile of the dynamic Stokes’ shift of ThT displays multiexponential behavior in both the ionic liquids. Although the long time behavior of dynamic Stokes’ shift also suggest that the torsional relaxation of ThT is more hindered in the high viscosity of hydroxyl functionalized ionic liquid, interestingly, the short time behavior, of dynamic Stokes’ shift is found to be decoupled from the bulk viscosity. The short time dynamics has been assigned to the ThT molecules associated with the [FAP] anion and is rationalized in terms of possible fast motion of the anions. The long time behavior is assigned to ThT molecules associated with the cationic part of the ionic liquid, which in the case of hydroxyl-functionalized imidazolium cation, is found to impede the torsional dynamics of ThT more efficiently, due to highly associative nature of the cation. These data indicate that although [OHEMIM][FAP] displays higher bulk viscosity compared to [EMIM][FAP], these ILs can provide a unique microenvironment, depending on the location of the solute, where the reaction could be faster than that expected from the bulk viscosity. However, as mentioned in the introduction, the experimental data in the ultrafast time regime of this class of ionic liquids (TSILs) are limited, so more studies should be carried out on solutes which are sensitive to microscopic friction, using ultrafast spectroscopy, so that a better understanding of the different local environments in this new class of prospective ionic liquids can be achieved.

shorter time constant was assigned to solute molecules experiencing microviscosity that is an order of magnitude lower than measured bulk viscosity.58 Given the similarity in the faster decay component for both the ionic liquids, it might be reasonable to assume that the faster decay time constant may be associated with the anionic part of the ionic liquid because the anion ([FAP]) is common for both the ILs. It has been shown for some imidazolium-based ILs, through molecular dynamics simulation, that the fast fluctuation of the anion above and below the plane of the imidazolium ring and the translational motion of the ions take place on this time scale.72,73 Thus, we assign the faster component of the dynamic Stokes’ shift to the rapid fluctuations and translational motion of the anions, whereas the slower components are assigned to be associated with the cationic part of the ionic liquid. Since the hydroxyl-functionalized cations are strongly associative in nature, due to hydrogen-bonding interactions among them, they impart a relatively stronger friction toward the torsional relaxation of ThT as compared to [EMIM][FAP]. It should be noted that the slower time constant of the Stokes’ shift dynamics is longer than the slow time constant of the population relaxation dynamics. A similar behavior was observed earlier for another molecular rotor, Auramine O, in highly heterogeneous water pools of reverse micelles.61,74 On the basis of the detailed modeling of the time-resolved emission spectra in terms of diffusive rotational motion, using the generalized Smoluchowski equation, this behavior was attributed to the time-dependent nature of the diffusion coefficient, which implies that the motion along the reaction coordinate experiences frequencydependent friction.74 It should be noted that a useful description of the frequency-dependent friction is provided by the solvation time correlation function. Thus, it was suggested that solvation also plays a role in the relaxation dynamics of Auramine O. Thus, the slower time constant of the Stokes’ shift dynamics for Auramine O was attributed to the contribution from the slower solvation time constants which was measured earlier using C153 dye in the water pools of reverse micelles.61,74 In regard to the present system, the solvation in ionic liquids, in general, is slow and disperse. In particular, in FAP-based ILs, dynamic Stokes’ shift measurements of the well-known solvatochromic probe C153 have been performed earlier and reveal solvation time constants extending in the nanosecond time range.75 The solvation time was found to be slower with the increasing viscosity of the ionic liquid, which reflects the slower reorganization of the surrounding solvent molecule. Thus, it seems plausible that the slower time constant of the Stokes’ shift dynamics of ThT in the present ILs is also affected by solvent relaxation dynamics, which is expected to be slower in more viscous [OHEMIM][FAP] owing to the highly associative nature of the hydroxyl-functionalized cation as compared to that in less viscous [EMIM][FAP].

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b09028. The ground state absorption spectra and steady-state emission spectra of the neat ionic liquids (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the support and encouragement from their host institute. We would like to acknowledge Mr. Sugosh Prabhu for his help with Karl Fischer measurements.

■

REFERENCES

(1) Hubbard, C. D.; Illner, P.; Eldik, R. V. Understanding Chemical Reaction Mechanisms in Ionic Liquids: Successes and Challenges. Chem. Soc. Rev. 2011, 40, 272−290. (2) Torimoto, T.; Tsuda, T.; Okazaki, K.; Kuwabata, S. New Frontiers in Materials Science Opened by Ionic Liquids. Adv. Mater. 2010, 22, 1196−1221. (3) Dupont, J. From Molten Salts to Ionic Liquids: A “Nano” Journey. Acc. Chem. Res. 2011, 44, 1223−1231. (4) Castner, E. W., Jr.; Margulis, C. J.; Maroncelli, M.; Wishart, J. F. Ionic Liquids: Structure and Photochemical Reactions. Annu. Rev. Phys. Chem. 2011, 62, 85−105. (5) Giernoth, R. Task-Specific Ionic Liquids. Angew. Chem., Int. Ed. 2010, 49, 2834−2839. (6) Wojnarowska, Z.; Paluch, M.; Grzybowski, A.; Adrjanowicz, K.; Grzybowska, K.; Kaminski, K.; Wlodarczyk, P.; Pionteck, J. Study of

■

CONCLUSIONS We have investigated the excited-state torsional relaxation dynamics of ThT in two tris(pentafluoroethyl) trifluorophosphate [FAP] anion-based ionic liquid, one of which includes hydroxyl functionalization in the cationic part. The emission quantum yield and the excited-state fluorescence lifetime measurements indicated that the torsional relaxation in the excited state of ThT is more hindered in the hydroxyl functionalized analogue owing to higher viscosity of the G

DOI: 10.1021/acs.jpcb.5b09028 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B Molecular Dynamics of Pharmaceutically Important Protic Ionic Liquid-Verapamil Hydrochloride. I. Test of Thermodynamic Scaling. J. Chem. Phys. 2009, 131, 104505. (7) Ohno, H. Electrochemical Aspects of Ionic Liquids; John Wiley & Sons: New York, 2005. (8) MacFarlane, D. R.; Forsyth, M.; Howlett, P. C.; Pringle, J. M.; Sun, J.; Annat, G.; Neil, W.; Izgorodina, E. I. Ionic Liquids in Electrochemical Devices and Processes: Managing Interfacial Electrochemistry. Acc. Chem. Res. 2007, 40, 1165−1173. (9) Hough, W. L.; Rogers, R. D. Ionic Liquids Then and Now: From Solvents to Materials to Active Pharmaceutical Ingredients. Bull. Chem. Soc. Jpn. 2007, 80, 2262−2269. (10) Freemantle, M. Designer Solvents: Ionic Liquids May Boost Clean Technology Development. Chem. Eng. News 1998, 76, 32−37. (11) Castner, E. W., Jr.; Wishart, J. F. Spotlight on Ionic Liquids. J. Chem. Phys. 2010, 132, 120901. (12) Jalili, A. H.; Rahmati-Rostami, M.; Ghotbi, C.; Hosseini-Jenab, M.; Ahmadi, A. N. Solubility of H2S in Ionic Liquids [bmim][PF6], [bmim][BF4], and [bmim][Tf2N]. J. Chem. Eng. Data 2009, 54, 1844−1849. (13) Shokouhi, M.; Adibi, M.; Jalili, A. H.; Hosseini-Jenab, M.; Mehdizadeh, A. Solubility and Diffusion of H2S and CO2 in the Ionic Liquid 1-(2-Hydroxyethyl)-3-methylimidazolium Tetrafluoroborate. J. Chem. Eng. Data 2010, 55, 1663−1668. (14) Jalili, A. H.; Mahdizadeh, A.; Shokouhi, M.; Sakhaeinia, H.; Taghikhani, V. Solubility of CO2 in 1-(2-hydroxyethyl)-3-methylimidazolium Ionic Liquids with Different Anions. J. Chem. Thermodyn. 2010, 42, 787−791. (15) Yang, X.; Yan, N.; Fei, Z. F.; Crespo-Quesada, R. M.; Laurenczy, G.; Kiwi-Minsker, L.; Kou, Y.; Li, Y. D.; Dyson, P. J. Biphasic Hydrogenation over PVP Stabilized Rh Nanoparticles in Hydroxyl Functionalized Ionic Liquids. Inorg. Chem. 2008, 47, 7444−7446. (16) Holbrey, J. D.; Turner, M. B.; Reichert, W. M.; Rogers, R. D. New Ionic Liquids Containing an Appended Hydroxyl Functionality from the Atom-efficient, One-pot Reaction of 1-Methylimidazole and Acid with Propylene Oxide. Green Chem. 2003, 5, 731−736. (17) Zhang, S.; Qi, X.; Ma, X.; Lu, L.; Deng, Y. Hydroxyl Ionic Liquids: The Differentiating Effect of Hydroxyl on Polarity due to Ionic Hydrogen Bonds between Hydroxyl and Anions. J. Phys. Chem. B 2010, 114, 3912−3920. (18) Wallert, S.; Drauz, K.; Grayson, I.; Groger, H.; Maria, P. D. d.; Bolm, C. Ionic Liquids as Additives in the Pig Liver Esterase (PLE) Catalysed Synthesis of Chiral Disubstituted Malonates. Green Chem. 2005, 7, 602−605. (19) Sun, J.; Zhang, S.; Cheng, W.; Ren, J. Hydroxyl-functionalized Ionic Liquid: A Novel Efficient Catalyst for Chemical Fixation of CO2 to Cyclic Carbonate. Tetrahedron Lett. 2008, 49, 3588−3591. (20) Branco, L. C.; Rosa, J. N.; Ramos, J. J. M.; Afonso, C. A. M. Preparation and Characterization of New Room Temperature Ionic Liquids. Chem. - Eur. J. 2002, 8, 3671−3677. (21) Zhao, Q.; Eichhorn, J.; Pitner, W. R.; Anderson, J. L. Using the Solvation Parameter Model to Characterize Functionalized Ionic Liquids Containing the Tris(pentafluoroethyl)trifluorophosphate (FAP) Anion. Anal. Bioanal. Chem. 2009, 395, 225−234. (22) O’Mahony, A. M.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. Effect of Water on the Electrochemical Window and Potential Limits of Room-Temperature Ionic Liquids. J. Chem. Eng. Data 2008, 53, 2884−2891. (23) Ignat’ev, N. V.; Welz-Biermann, U.; Kucheryna, A.; Bissky, G.; Willner, H. New Ionic Liquids with Tris(perfluoroalkyl)trifluorophosphate (FAP) Anions. J. Fluorine Chem. 2005, 126, 1150−1159. (24) Muldoon, M. J.; Aki, S. N.; Anderson, J. L.; Dixon, J. K.; Brennecke, J. F. Improving Carbon Dioxide Solubility in Ionic Liquids. J. Phys. Chem. B 2007, 111, 9001−9009. (25) Endres, F.; MacFarlane, D.; Abbott, A. Electro Deposition from Ionic Liquids; Wiley-VCH: NewYork, 2008.

(26) Abedin, S. Z.; Borissenko, N.; Endres, F. Electrodeposition of Nanoscale Silicon in a Room Temperature Ionic Liquid. Electrochem. Commun. 2004, 6, 422−426. (27) Dutt, G. B. Influence of Specific Interactions on the Rotational Dynamics of Charged and Neutral Solutes in Ionic Liquids Containing Tris(pentafluoroethyl)trifluorophosphate (FAP) Anion. J. Phys. Chem. B 2010, 114, 8971−8977. (28) Karve, L.; Dutt, G. B. Rotational Diffusion of Neutral and Charged Solutes in Ionic Liquids: Is Solute Reorientation Influenced by the Nature of the Cation? J. Phys. Chem. B 2011, 115, 725−729. (29) Das, S. K.; Sarkar, M. Solvation and Rotational Relaxation of Coumarin 153 and 4-Aminophthalimide in a New Hydrophobic Ionic Liquid: Role of N−H. . .F Interaction on Solvation Dynamics. Chem. Phys. Lett. 2011, 515, 23−28. (30) Haidekker, M. A.; Nipper, M.; Mustafic, A.; Lichlyter, D.; Dakanali, M.; Theodorakis, E. A. Dyes with Segmental Mobility: Molecular Rotors. In Advanced Fluorescence Reporters in Chemistry and Biology I. Fundamentals and Molecular Design; Demchenko, A. P., Ed.; Springer-Verlag: Berlin, 2010; Vol. I, pp 267−308. (31) Haidekker, M. A.; Theodorakis, E. A. Molecular Rotors Fluorescent Biosensors for Viscosity and Flow. Org. Biomol. Chem. 2007, 5, 1669−1678. (32) Haidekker, M. A.; Theodorakis, E. A. Environment-Sensitive Behavior of Fluorescent Molecular Rotors. J. Biol. Eng. 2010, 4, 11. (33) Singh, P. K.; Kumbhakar, M.; Pal, H.; Nath, S. Ultrafast Bond Twisting Dynamics in Amyloid Fibril Sensor. J. Phys. Chem. B 2010, 114, 2541−2546. (34) Singh, P. K.; Kumbhakar, M.; Pal, H.; Nath, S. Viscosity Effect on the Ultrafast Bond Twisting Dynamics in an Amyloid Fibril Sensor: Thioflavin-T. J. Phys. Chem. B 2010, 114, 5920−5927. (35) Amdursky, N.; Erez, Y.; Huppert, D. Molecular Rotors: What Lies Behind the High Sensitivity of the Thioflavin-T Fluorescent Marker. Acc. Chem. Res. 2012, 45, 1548−1557. (36) Singh, P. K.; Kumbhakar, M.; Pal, H.; Nath, S. Ultrafast Torsional Dynamics of Protein Binding Dye Thioflavin-T in Nanoconfined Water Pool. J. Phys. Chem. B 2009, 113, 8532−8538. (37) Singh, P. K.; Kumbhakar, M.; Pal, H.; Nath, S. Confined Ultrafast Torsional Dynamics of Thioflavin-T in a Nanocavity. Phys. Chem. Chem. Phys. 2011, 13, 8008−8014. (38) Singh, P. K.; Nath, S. Ultrafast Torsional Dynamics in Nanoconfined Water Pool: Comparison between Neutral and Charged Reverse Micelles. J. Photochem. Photobiol., A 2012, 248, 42−49. (39) Singh, P. K.; Murudkar, S.; Mora, A. K.; Nath, S. Ultrafast Torsional Dynamics of Thioflavin-T in an Anionic Cyclodextrin Cavity. J. Photochem. Photobiol., A 2015, 298, 40−48. (40) Singh, P. K.; Nath, S. Molecular Recognition Controlled Delivery of a Small Molecule from a Nanocarrier to Natural DNA. J. Phys. Chem. B 2013, 117, 10370−10375. (41) Velapoldi, R. A.; Mielenz, K. D. Standard Reference Materials: A Fluorescence Standard Reference Material: Quinine Sulfate Dihydrate. Natl. Bur. Stand. (US) Spec. Publ. 1980, 260−264. (42) Nad, S.; Kumbhakar, M.; Pal, H. Photophysical Properties of Coumarin-152 and Coumarin-481 Dyes: Unusual Behavior in Nonpolar and in Higher Polarity Solvents. J. Phys. Chem. A 2003, 107, 4808−4816. (43) Lakowicz, J. R. Principle of Fluorescence Spectroscopy; Plenum Press: New York, 2006. (44) Singh, P. K.; Nath, S.; Bhasikuttan, A. C.; Kumbhakar, M.; Mohanty, J.; Sarkar, S. K.; Mukherjee, T.; Pal, H. Effect of Donor Orientation on Ultrafast Intermolecular Electron Transfer in Coumarin-Amine Systems. J. Chem. Phys. 2008, 129, 114504. (45) Maroncelli, M.; Fleming, G. R. Picosecond Solvation Dynamics of Coumarin 153: The Importance of Molecular Aspects of Solvation. J. Chem. Phys. 1987, 86, 6221−6239. (46) Singh, P. K.; Mora, A. K.; Murudkar, S.; Nath, S. Dynamics Under Confinement: Torsional Dynamics of Auramine O in a Nanocavity. RSC Adv. 2014, 4, 34992−35002. (47) Singh, P. K.; Nath, S.; Kumbhakar, M.; Bhasikuttan, A. C.; Pal, H. Quantitative Distinction between Competing Intramolecular Bond H

DOI: 10.1021/acs.jpcb.5b09028 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B Twisting and Solvent Relaxation Dynamics: An Ultrafast Study. J. Phys. Chem. A 2008, 112, 5598−5603. (48) Stsiapura, V. I.; Maskevich, A. A.; Kuzmitsky, V. A.; Uversky, V. N.; Kuznetsova, I. M.; Turoverov, K. K. Thioflavin T As a Molecular Rotor: Fluorescent Properties of Thioflavin T in Solvents with Different Viscosity. J. Phys. Chem. B 2008, 112, 15893−15902. (49) Maskevich, A. A.; Stsiapura, V. I.; Kuzmitsky, V. A.; Kuznetsova, I. M.; Povarova, O. I.; Uversky, V. N.; Turoverov, K. K. Spectral Properties of Thioflavin-T in Solvents with Different Dielectric Properties and in a Fibril-Incorporated Form. J. Proteome Res. 2007, 6, 1392−1401. (50) Singh, P. K.; Kumbhakar, M.; Pal, H.; Nath, S. A Nano-confined Charged Layer Defies the Principle of Electrostatic Interaction. Chem. Commun. 2011, 47, 6912−6914. (51) Murudkar, S.; Mora, A. K.; Singh, P. K.; Nath, S. Ultrafast Molecular Rotor: An Efficient Sensor for Premelting of Natural DNA. Chem. Commun. 2012, 48, 5301−5303. (52) Singh, P. K.; Sujana, J.; Mora, A. K.; Nath, S. Probing the DNA− Ionic Liquid Interaction Using an Ultrafast Molecular Rotor. J. Photochem. Photobiol., A 2012, 246, 16−22. (53) Amdursky, N.; Gepshtein, R.; Erez, Y.; Huppert, D. Temperature Dependence of the Fluorescence Properties of Thioflavin-T in Propanol, a Glass-Forming Liquid. J. Phys. Chem. A 2011, 115, 2540− 2548. (54) Erez, Y.; Liu, Y.; Amdursky, N.; Huppert, D. Modeling the Nonradiative Decay Rate of Electronically Excited Thioflavin T. J. Phys. Chem. A 2011, 115, 8479−8487. (55) Kondo, M.; Heisler, I. A.; Meech, S. R. Reactive Dynamics in Micelles: Auramine O in Solution and Adsorbed on Regular Micelles. J. Phys. Chem. B 2010, 114, 12859−12865. (56) Kondo, M.; Heisler, I. A.; Meech, S. R. Ultrafast Reaction Dynamics in Nanoscale Water Droplets Confined by Ionic Surfactants. Faraday Discuss. 2010, 145, 185−203. (57) Kondo, M.; Heisler, I. A.; Meech, S. R. Ultrafast Reaction Dynamics of Auramine O in a Cyclodextrin Nanocavity. J. Mol. Liq. 2012, 176, 17−21. (58) Funston, A. M.; Fadeeva, T. A.; Wishart, J. F.; Castner, E. W. Fluorescence Probing of Temperature-Dependent Dynamics and Friction in Ionic Liquid Local Environments. J. Phys. Chem. B 2007, 111, 4963−4977. (59) Jin, H.; Baker, G. A.; Arzhantsev, S.; Dong, J.; Maroncelli, M. Solvation and Rotational Dynamics of Coumarin 153 in Ionic Liquids: Comparisons to Conventional Solvents. J. Phys. Chem. B 2007, 111, 7291−7302. (60) Jin, H.; Li, X.; Maroncelli, M. Heterogeneous Solute Dynamics in Room Temperature Ionic Liquids. J. Phys. Chem. B 2007, 111, 13473−13478. (61) Kondo, M.; Heisler, I. A.; Conyard, J.; Rivett, J. P. H.; Meech, S. R. Reactive Dynamics in Confined Liquids: Interfacial Charge Effects on Ultrafast Torsional Dynamics in Water Nanodroplets. J. Phys. Chem. B 2009, 113, 1632−1639. (62) Levinger, N. E. Water in Confinement. Science 2002, 298, 1722−1723. (63) Chosrowjan, H.; Mataga, N.; Nakashima, N.; Imamaoto, Y.; Tokunaga, F. Femtosecond-picosecond Fluorescence Studies on Excited State Dynamics of Photoactive Yellow Protein from Ectothiorhodospira Halophila. Chem. Phys. Lett. 1997, 270, 267−272. (64) Changenet, P.; Zhang, H.; van der Meer, M. J.; Hellingwerf, K. J.; Glasbeek, M. Subpicosecond Fluorescence Upconversion Measurements of Primary Events in Yellow Proteins. Chem. Phys. Lett. 1998, 282, 276−282. (65) Aberg, U.; Akesson, E.; Alvarez, J. L.; Fedchenia, I.; Sundstrom, V. Femtosecond Spectral Evolution Monitoring the Bond-twisting Event in Barrierless Isomerization in Solution. Chem. Phys. 1994, 183, 269−288. (66) Du, M.; Fleming, G. R. Femtosecond Time-resolved Fluorescence Spectroscopy of Bacteriorhodopsin: Direct Observation of Excited State Dynamics in the Primary Step of the Proton Pump Cycle. Biophys. Chem. 1993, 48, 101−111.

(67) Yartsev, A.; Alvarez, J. L.; Aberg, U.; Sundstrom, V. Overdamped Wavepacket Motion Along a Barrierless Potential Energy Surface in Excited State Isomerization. Chem. Phys. Lett. 1995, 243, 281−289. (68) Wei, K.; Deng, L.; Wang, Y.; Ou-Yang, Z.; Wang, G. Effect of Side-Chain Length on Structural and Dynamic Properties of Ionic Liquids with Hydroxyl Cationic Tails. J. Phys. Chem. B 2014, 118, 3642−3649. (69) Fakhraee, M.; Zandkarimi, B.; Salari, H.; Gholami, M. R. Hydroxyl-Functionalized 1-(2-Hydroxyethyl)-3-methyl Imidazolium Ionic Liquids: Thermodynamic and Structural Properties using Molecular Dynamics Simulations and ab Initio Calculations. J. Phys. Chem. B 2014, 118, 14410−14428. (70) Del Popolo, M. G.; Voth, G. A. On the Structure and Dynamics of Ionic Liquids. J. Phys. Chem. B 2004, 108, 1744−1752. (71) Nagasawa, Y.; Miyasaka, H. Ultrafast Solvation Dynamics and Charge Transfer Reactions in Room Temperature Ionic Liquids. Phys. Chem. Chem. Phys. 2014, 16, 13008−13026. (72) Zahn, S.; Uhlig, F.; Thar, J.; Spickermann, C.; Kirchner, B. Intermolecular Forces in an Ionic Liquid ([Mmim][Cl]) versus Those in a Typical Salt (NaCl). Angew. Chem., Int. Ed. 2008, 47, 3639−3641. (73) Liu, H.; Sale, K. L.; Holmes, B. M.; Simmons, B. A.; Singh, S. Understanding the Interactions of Cellulose with Ionic Liquids: A Molecular Dynamics Study. J. Phys. Chem. B 2010, 114, 4293−4301. (74) Heisler, I. A.; Kondo, M.; Meech, S. R. Reactive Dynamics in Confined Liquids: Ultrafast Torsional Dynamics of Auramine O in Nanoconfined Water in Aerosol OT Reverse Micelles. J. Phys. Chem. B 2009, 113, 1623−1631. (75) Das, S. K.; Sahu, P. K.; Sarkar, M. Diffusion−Viscosity Decoupling in Solute Rotation and Solvent Relaxation of Coumarin153 in Ionic Liquids Containing Fluoroalkylphosphate (FAP) Anion: A Thermophysical and Photophysical Study. J. Phys. Chem. B 2013, 117, 636−647.

I

DOI: 10.1021/acs.jpcb.5b09028 J. Phys. Chem. B XXXX, XXX, XXX−XXX