Investigation the Influence of Alkyl Side Chain Length

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Apr 26, 2015 - DOI: 10.1039/b000000x. Fluorescence response of coumarin153 (C153) have been investigated in a series of 1-alkyl-3-methylimidazolium.

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Investigation the Influence of Alkyl Side Chain Length on the Fluorescence Response of C153 in a Series of Room Temperature Ionic Liquids Sudhir Kumar Das a,b, Debashis Majhia, Prabhat Kumar Sahua and Moloy Sarkara* 5

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Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Fluorescence response of coumarin153 (C153) have been investigated in a series of 1-alkyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate room temperature ionic liquids (RTILs) with systematic variation of alkyl chain length (ethyl, butyl and hexyl ) to examine the effect of alkyl side chain lengths on cationic moiety on solute and solvent relaxation dynamics. Physicochemical properties associated with the present RTILs are estimated at different temperatures. While the viscosity values are observed to be increased with increase in alkyl chain length, density values decrease with the increase in the length of alkyl side chain. Steady state fluorescence measurements reveal that C153 experiences more nonpolar microenvironments with increase in the alkyl chain length. Interestingly, time resolved study has demonstrated that length of alkyl side chain have noticeable influence in governing the solvation dynamics in these media. It has been observed that average solvent relaxation time estimated for these RTILs can be better correlated when both the size of the alkyl chains and bulk viscosity of the respective RTILs are considered. Quite interestingly, apart from the viscosity effect, negligible influence of alkyl chain length has been observed for rotational diffusion of C153.

1. Introduction

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Room temperature ionic liquids (RTILs) have attracted considerable interest from academia and industries in recent times.1-3 This has happened due to the fact that these materials possess very interesting physicochemical properties such as extremely low vapor pressure, high conductivity, high viscosity, high thermally stability etc.1-3Moreover, as they are generally consist of an organic cation and an organic or inorganic anion, it would allow one to appropriately combine the cations and anions, and develop RTILs keeping the end use in mind. Among all such possibilities, doing variation in the alkyl chain length in the constituents of RTILs could be interesting in a sense that this chain length variation can alter structural features and physicochemical properties of the RTILs in a systematic manner.4-10

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Few studies have been carried out to find structure-property correlation in these liquids.4-18It is pertinent to mention in this context that solvation dynamics, which is intricately related to the rate of a chemical reaction19, is one of the fast processes that have been extensively studied in room temperature ionic liquids by many research groups16, 20-32as well as by us14, 17, 33-36. Again studies on solute rotation11-18has also been investigated to obtain the idea about the interaction of ionic liquids with dissolved solute.1-3Understanding interaction of dissolved solute and the time scale of solvent organization in such reaction media is very important for designing a solvent for carrying a specific reaction for a desired product. From the recent study, it is found that variation in alkyl chain length in any of the constituents of ionic liquids can influence both solute rotation and solvent relaxation.11-18Fruchey and Fayer11observed that motion of perylene becomes faster with increasing alkyl chain length of the

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cationic moiety and this has been explained by invoking quashihydrodynamictheory.37 Solute and solvent dynamics by employing fluorescent organic solutes in a series of ionic liquids with the variation of alkyl chain length on the anionic moiety have been investigated by us. 13,14 We have proposed that diffusion-viscosity decoupling plays an important role during the solute rotation and solvent relaxation in ionic liquids. 13,14In fact, it has been observed that the decoupling of rotational diffusion of solute with the medium viscosity increases with increase in the alkyl side chain length. However, exception to this trend has also been reported by Dutt and coworkers.15They have observed that length of alkyl chain length has no bearing on the solute rotation of a nonpolar solute, 9-phenyl anthracene in fluoroalkylphosphate anion containing ILs. It may be mentioned in this context that the diffusion-viscosity decoupling has also been observed in eutectic melts by Biswas and coworkers.38-41They have rationalized the observation by considering the micro-heterogeneous nature of the medium. Recently, Gangamallaiah et al17have studied the rotation of a nonpolar solute, 9-phenylanthracene (PA), in a series of 1alkyl-3-methylimidazoliumionic liquids having different anions. They have shown that in case of ILs with bis(trifluoromethylsulfonyl)imide and tris(pentafluoroethyl)trifluorophosphate anions, no significant variation of solute-solvent coupling constants(Cobs) with an increase in the alkyl side chain lengths on the imidazolium moiety has been observed. Interestingly, ILs having tetrafluoroborate and hexafluorophosphate anions, Cobs value decreases with an increase in alkyl chain length. This has been explained on the basis of the organized structures of the ionic liquids. Above discussions reveal that that the studies on both solute and solvent dynamics by employing similar set of RTILs with a systematic variation of the alkyl side chain length on the [journal], [year], [vol], 00–00 |1

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Chart 1: Structural information of RTILs and C153. 5

far as the understanding the kinship between structure and dynamical behavior of these liquids are concerned.

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Keeping this in mind, we have carried out steady state and timeresolved fluorescence studies on coumarin153 in a series of 1alkyl-3-methylimidazolium fluoroalkylphosphate ionic liquids which are abbreviated as [Cnmim][FAP] (n= 2, 4, 6). These ionic liquids are purposefully chosen so that a systematic variation of alkyl chain length on the cationic moiety is maintained. Moreover, they are hydrophobic due to the presence of FAP anion. In the present RTILs the halide and water contents are also known to be less than 100 ppm.42 Thermophysical properties such as viscosities, densities are also estimated in these RTILs to get an idea about the variations of these properties with a change in the alkyl chain length on the cationic moiety. We have taken C153 as fluorescence probe due to its suitable photophysical properties.42 The molecular structural information of ILs and C153 are provided in chart 1.

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2. Experimental section 2.1. Materials 25

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Laser grade C153 was purchased from Exciton, USA.[Cnmim][FAP] (Chart1) was obtained from Merck, Germany (>99% purity) and used as received. 1, 3-bis (1pyrenyl) propane (BPP) was purchased from Invitrogen. [Cnmim][FAP] have been taken in different long-necked quartz cuvette and requisite amount of C153 was added to prepare the solution so that the optical density does not exceed 0.3. Proper precaution was maintained to avoid moisture absorption by this media during transferring the solute into the cuvette. The longnecked quartz cuvette was sealed with septum and parafilm to avoid moisture absorption form the environment. 2.2. Instrumentation The absorption and fluorescence spectra were collected using spectrophotometer (Cary 100 Bio) and spectrofluorimeter (Perkin Elmer LS 55) respectively. The fluorescence spectra were corrected for the spectral sensitivity of the instrument and samples were excited at 405 nm. Time-resolved fluorescence measurements were carried out using a time-correlated singlephoton counting (TCSPC) spectrometer (Edinburgh, OB920) using a 405nm picosecond pulse diode laser (EPL), and the

signals were collected at the magic angle (54.7°) using a Hamamatsu microchannel plate photomultiplier tube (R3809U50). The instrument response function (IRF) was recorded by scatterer (dilute ludox solution in water) in place of the sample. IRF of our TCSPC setup is 98 ps for 405 nm picoseconds pulse diode laser. Nonlinear least-squares iteration procedure was used for decay analysis using F900 decay analysis software. The qualities of the fit were determined by judgments of the chi square ( 2) values and by visual inspection of the residuals which are obtained by fitting. The same TCSPC instrument setup was used for anisotropy decay measurements. The emission intensities at parallel (III) and perpendicular (I ) polarizations were collected alternatively until a peak difference between parallel (III) and perpendicular (I ) decay (at t = 0) ~5000was reached. For Gfactor calculation, the same procedure was employed, but with 5 cycles and horizontal polarization of the exciting laser beam. The same software was also used to analyze the anisotropy decay profiles. The temperature was controlled by circulating water through the cell holder using a Quantum, North West (TC 125) temperature controller with 0.2 C accuracy bands. LVDV-III Ultra Brookfield Cone and Plate viscometer (1% accuracy and 0.2% repeatability) was used for viscosity measurement of the IL. A round cone plate was used with a round spindle (CP52) with a frequency 100 sec-1. An Anton Paar (DMA 5000) density meter was used to measure the densities of the ILs.

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2.3. Method The time-resolved fluorescence decay profiles were collected at 5/10 nm intervals across the entire steady-state emission spectra at magic angle (54.7 ). The total number of measurements was 25-30 in each case. Each decay curve was fitted by triexponential decay function after deconvoluting the instrument response function and the quality of the fit was judged by 2 values and from the visual inspection of the residuals curve which were obtained from fitting of the lifetime decay profile. The time resolved fluorescence decay profile was analyzed for constructing the time resolved emission spectra (TRES) using the standard method which is available in literature 44. The peak frequencies obtained from the log-normal fitting of TRES were then used to construct the decay of solvent response function (C (t)) which is given below C (t )

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(t ) (0)

( ) ( )

(1)

where, ( ), (0) and (t) are the peak frequencies at times infinity ( ), zero (t =0) and t respectively. Then the solvent response function was fitted by the following biexponential function. (2)

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Where, 1 and 2 are the solvent relaxation time and a 1 and a2 are normalized preexponential factors. After having the value of 1, 2, a1, and a2, the average solvation time were calculated by using the following relation. av

a1

2

a2

2

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constituents of RTILs could be an interesting research theme as

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solv st

(

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) (5)

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where is the gamma function and st is the average solvation time considering C(t) is a stretched exponential function.The rotational dynamics of photo excited probe is very sensitive to microenvironment of the probe and local viscosity of the medium. We have carried out the time resolved fluorescence anisotropy decay (TRAFD) of C153 in all the present ILs in order to know the effect of alkyl chain length on the rotation of probe molecule. Time-resolved fluorescence anisotropy decay (r(t)) is depicted by the following equation

r (t )

where,

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G I VV (t ) I VH (t ) GI VV (t ) 2 I VH (t )

G

(6)

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I HH (t ) I HV (t )

where G is the instrument correction factor for the detector sensitivity to the polarization of the emission, it is 0.7 for our TCSPC set up at the wavelength of detection. IHH(t) and IHV(t) is the intensity of fluorescence decays when the excitation and the emission polarizer are polarized at horizontal-horizontal and horizontal-vertical alignment respectively. Again IVV(t) and IVH(t) are the intensity of fluorescence decays when excitation and emission polarizer are polarized at vertical-vertical and verticalhorizontal alignment respectively.

3. Result and Discussion

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Fig. 1 Viscosities as a function of temperature for [Cnmim][FAP] (n= 2, 4 and 6) room temperature ionic liquids.

3.1. Thermophysical properties of ILs Physicochemical properties of ILs, depend on the constituents of ILs, and for that they are known as “designer solvent’. 45 In order to know the effect of alkyl chain on the cationic moiety in controlling the physicochemical properties of the ILs, thermophysical studies on the present ILs have been carried out. Moreover, the knowledge about the physicochemical properties of ILs is also expected to be helpful in explaining the new experimental observation such as fluorescence response of the dipolar probe in a proper manner. The measured viscosities and densities of the present ionic liquids at different temperatures are provided in the supplementary information (Tables S1 and S2). The variation of viscosities and densities of the present ILs at different temperatures are also shown in Figs. 1 and 2. From Fig. 1, it is observed that the viscosity decreases in the order of [C6mim][FAP] > [C4mim][FAP] > [C2mim][FAP]. The increase in viscosity values with increase in alkyl chain lengths possibly arises due to increase in van der Waals interaction with increase in size of the side chain of the cation. It is also observed that with increase in the alkyl chain length in the cationic moiety of ILs, This journal is © The Royal Society of Chemistry [year]

Fig. 2 Densities as a function of temperature for [Cnmim][FAP] (n= 2, 4 55

and 6) room temperature ionic liquids. The data presentations are provided in the same figure.

densities decreases due to increase in molecular volume of the concerned ILs (Fig. 2).The experimental viscosity η, density ρ, were fitted by the least-squares method using the following equation 46, 47 60

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logη/cP=A0+(A1/T)

(7)

ρ/(g.cm-3)=A2+A3T

(8)

where, η and ρ denote the viscosity and density of the ionic liquids and A0, A1, A2 and A3 are the correlation coefficients and T is the temperature in Kelvin. The correlation coefficients were determined by a least squares fitting method using equations 7 and 8. All the fitting parameters in relation to the measurements of density and viscosity are provided in the supplementary

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We have also fitted C(t) to the stretched exponential function which is shown below C(t) = exp(-(t/ solv) ) (4) where 0 1

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T/K 293 298 303 308 313 318 328

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Fig. 3 Steady state absorption spectra of C153 in [C nmim][FAP] (n = 2, 4 and 6) room temperature ionic liquids. Spectra are normalized at their corresponding peak maxima.

[C2mim][FAP] 6.89 6.91 6.94 6.96 6.98 7.01 7.03

α × 104 (K-1) [C4mim][FAP] 6.89 6.91 6.94 6.96 6.98 6.99 7.03

[C6mim][FAP] 6.92 6.95 6.98 7.00 7.02 7.05 7.07

Table 2 Absorption and emission maxima and steady state Stokes shifts (∆γ) of C153 in [Cnmim][FAP] (n = 2, 4 and 6) room temperature ionic liquids at 298 K.

RTILs [C2mim][FAP] [C4mim][FAP] [C6mim][FAP] a

abs(nm)

422 421 420

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flu.(nm)

524 518 514

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∆γ (cm-1) 4613 4498 4354

experimental error 2nm

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Fig. 4 Steady state emission spectra of C153 in [C nmim][FAP] (n = 2, 4 and 6) room temperature ionic liquids. All emission spectra are normalized at their corresponding peak maxima. Emission spectra of neat room temperature ionic liquids are also shown in the same figure. 10

information. The experimental density values are again used to estimate thermal expansion coefficient (α) of the two ILs by using following equation.46

(9) 15

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where, A2, A3 are the fitting parameters from equation 8 and αp and T are the thermal expansion coefficient and absolute temperature respectively. It may be mentioned that the thermal expansion coefficient (αP) is also known as volume expansivity. 47 The expansion coefficients for all the ILs are collected in Table 1. It can be observed from Table 1 that the coefficients of thermal expansion for all the 1-alkyl-3-mehtylimidazolium ILs do not change with increasing alkyl chain length, and also remain almost unaffected with change in temperature. This observation illustrates that there is no significant changes in volume expansivity for all the 1-alkyl-3-methylimiazolium cationcontaining ILs. The observed thermal expansion coefficient values in the present case are found to be similar to those reported for imidazolium, pyridinium, phosphonium, and ammonium based ILs, (4.8×10−4 to 6.5×10−4) K−1.49,50

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Fig. 5 Representative emission wavelength dependent decay profile for

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C153 in [C4mim][FAP] room temperature ionic liquid at 298K temperature. Circles denoted the experimental data points and solid line represents the fit to the data points. Instrument response function (IRF) is also shown in the same figure. The goodness of the fit parameters ( 2) in these two wave length are 1.12 and 1.06 respectively.

3.2. Steady-state studies Representative absorption and emission spectra of C153 are given in Figs. 3 and 4. Emission spectra of neat ILs are also shown in Fig. 4. While the absorption maxima of C153 in the 1-alkyl-3methylimidazoliumcation containing ILs almost remain unaffected (Fig.3), emission maxima exhibit the blue shift of the spectrum with increase in the alkyl chain length (Fig.4). The observation indicates that static dielectric constant or average polarity of the medium decreases with an increase in the length of This journal is © The Royal Society of Chemistry [year]

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Table 1 Thermal Expansion Coefficient Values of Presented Ionic Liquids as a Function of Temperature Using Equation 9.

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Fig. 6(a) Time resolved emission spectra (TRES) of C153 in [C4 mim][FAP] at 298 K at different time span at λexc. = 405 nm. The time intervals are indicated by the corresponding symbols. Solid lines represent lognormal fitting to the experimental data points. All spectra are normalized at their corresponding peak maxima. (b) Full width half maxima (FWHM) of time resolved emission spectra (TRES) of C153 at different ti me interval after excitation of the probe molecule in [C4 mim][FAP] room temperature ionic liquids at 298K temperature. Symbols denote the experimental data points at corresponding time intervals. 60

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Fig. 7 Spectral correlation function, C(t), versus time plot of C153 in [Cnmim][FAP] (n=2, 4 and 6) room temperature ionic liquids at 298K temperature. Symbols are denoting the experimental data points, and solid lines represent the biexponential fit to the experimental data points. 45

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alkyl side chain on the cation. It may be noted here that Weingӓ rtner and his co-workers51 though microwave dielectric spectroscopy have earlier shown that static dielectric constant decreases with an increasing in the alkyl side chain length of the cation in imidazolim-based ILs.We have also carried out exaction wavelength dependenct emission behavior of C153 in these ILs, but we have not found any change in the emission maximum of C153 in these ILs with the variation of excitation wavelength. It may be mentioned here that excitation wavelength dependendent fluorescence behaviour of a dipolar molecules in ILs have been rationalized by considering the distribution of energetically different solvated species in the ground state and a slower rate of their excited-state relaxation processes. 10 The observation in the present case is not surprising as C153 has fairly large excited This journal is © The Royal Society of Chemistry [year]

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state lifetime (3-6 ns),43 and hence fluorescence response of C153 in the RTILs is expected not to be that much responsive to those short-lived “transient” domains over which medium particles are correlated. Excitation wavelength dependence emission of C153 is provided in supporting information as Figs. S1 and S2. The relevant data corresponding to steady state absorption and emission spectra are provided in Table 2. 3.3. Time resolved studies 3.3.1. Solvation dynamics As discussed in the experimental section, the magic angle fluorescence decays of C153 are collected at 5-10 nm wavelength intervals covering the entire range of emission spectra. Faster decay response has been observed when the monitoring wave length are at the blue end of the emission spectrum, where as decays those are collected at the longer wave lengths exhibits a rise time with usual decay. The observation reflects the standard signature of stabilization of the photoexcited probe molecule via solvation. Representative wavelength dependent decay profiles are shown in Fig. 5. Time-resolved emission spectra (TRES) are constructed according to the procedure given by Fleming and Maroncelli44, and are shown in Fig. 6 (a) for 1-butyl-3methylimidazolium cation containing IL at different time intervals at 298 K temperature. A representative plot for full width half maxima (FWHM) of time-resolved emission spectra (TRES) of C153 at different time interval after excitation of the probe molecule in [C 4mim][FAP] is shown in Fig. 6 (b). A time-dependent shift of the emission spectra toward the lower energy confirms solvent-mediated relaxation of the excited state of the fluorophore. The gradual decrease in FWHM values of the TRES with time also indicates the stabilization of excited state through solvation (Fig 6 (b)). The spectral shift dynamicwhich is represented by the time-dependent decay of the solvation correlation function C(t) is shown in Fig. 7. The solvent relaxation parameters are collected in Table 3. Journal Name, [year], [vol], 00–00 |5

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RTILs

Temp.(K)

Vis.(cp)

[C2mim][FAP]

293 298 303 293 298 303 293 298 303

73.1 58.5 47.6 93.4 72.3 57.4 112.7 86.7 67.9

[C4mim][FAP]

[C6mim][FAP]

a

Biexponentialfita

a1 0.31 0.27 0.24 0.26 0.35 0.28 0.39 0.32 0.38

τ1(ns) 0.74 0.61 0.54 1.24 0.86 0.78 1.44 1.21 0.98

a2 0.69 0.73 0.76 0.74 0.65 0.72 0.61 0.68 0.62

τ2(ns) 0.15 0.11 0.10 0.39 0.27 0.26 0.39 0.37 0.24

Stretched exponential fitb τav.(ns) 0.33 0.24 0.20 0.61 0.48 0.40 0.80 0.64 0.52

solv.(ns)

0.75 0.71 0.77 0.87 0.85 0.88 0.81 0.85 0.78

0.257 0.174 0.154 0.538 0.415 0.359 0.683 0.535 0.420

Obs. shift(cm-1) st(ns)

0.306 0.217 0.179 0.577 0.451 0.381 0.767 0.582 0.485

1108 1173 1108 926 930 895 993 988 1028

according to equation 3 and baccording to equation 5. Experimental error is 5 .

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Fig. 8(a) Correlation among average solvation time, < s>with η/T in the present ionic liquids.(b)Correlation among average solvation time, < s> with η/T in the present ionic liquids and cation radius (R +) of the ionic liquids. The cation radius are calculated using the relation R+= (3v/4 )1/3, where v is the volume of cations obtained from Edward’s increment method. At all temperatures, the solvation dynamics are well described by often observed for many ILs, is observed to be not as good in the biphasic decays, with an initial faster component preceded by a present case (Fig. 8a). As can be seen from the figure, marked slower response. From Table 3, it is evident that for a particular deviations are observed in the linear correlation when all of these IL the average solvation time decreases with increase in ILs are compared. Since average salvation time is largely temperature. This observation happens due to the lowering of the 65 dependent on the bulk viscosity of the medium and decoupling of viscosity of the medium upon increase in temperature. of solvation time from medium viscosity is also observed in ILs Interestingly, when we looked at the variation of average due to structural variations within the ILs, 36 apart from the solvation time with bulk viscosity of the ILs, the average viscosity (η) of the medium, the size of the individual ions is solvation time of a particular IL appears to follow a linear expected to influence significantly the solvent response. In this correlation with the viscosity of the medium (Fig. 8 (a)). 70 context we would also like to mention that the Maroncelli and However, upon more careful look at all the ILs one can also see coworkers26 have demonstrated that for ILs having larger cation that the linear correlation is not so good (Fig. 8 (a)). For (phosphonium) average solvation time can depend on the size of example, average solvation time for [C2mim][FAP] at 298K and the cations, and it may follow the following relation [C4mim][FAP] at 303K are found to be 0.24 ns, 0.40 ns < s> ( /T)pR+q where p=1, q=4 and R+ is the radius of cation. respectively, whereas the concerned ILs are isoviscous under the 75 To see whether the size of the alkyl side chain is playing any role experimental condition. The linear correlation between the in relation to the linear correlation that are observed while fitting average salvation time and bulk viscosity of the medium that average solvation time and viscosity, we have calculated the radii 6|Journal Name, [year], [vol], 00–00

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Table 3 Relaxation Parameters of Solvation and Observed Shift for C153 in [C nmim][FAP] room temperature ionic liquid.

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Fig. 10 log−log plots of rotational relaxation time of C153 vs η/T in

Fig. 9 Time resolved fluorescence anisotropy decay (TRFAD) for C153 5

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in [C nmim][FAP] (n=2, 4 and 6) room temperature ionic liquids at 298K. Symbols are denote the experimental data points and the Solid lines in the same figure represent the biexponential fit to the experimental data points.

of all the cations by the Edward’s increment method52. The radii decreases in the order of log versus η/T could not provide [C6mim][FAP](~3.53Å)>[C4mim][FAP](~3.30Å)>[C2mim][FAP ](3.03Å). To elucidate how the cation size might affect the observed solvation times, analyses have been performed based on the methods of Maroncelli and co-workers26. When the plot of substantial improvement, we have taken into account the size of the cation as all the ILs contains similar anion. This is consistent with the reports of Jin et al. 26 As can been seen, when the average solvation time is plotted against the product of viscosity (at different temperature) and the radius of cation a better linear correlation (Fig. 8(b)), as compared to < s>vs η/T plot (Fig 8a) has been observed. It has been observed that the solvation time varies as (η/T) R4+, where R+ is the radius of the cation. Fig. 8(b) shows the dependence of solvation time with the viscosity and the radius of cations for the [Cnmim][FAP] (n=2, 4, 6) ILs. It indicates that beside the bulk viscosity of the solvent, average solvation time of probe molecule depends on the size of the cation of the ILs, and alkyl chain length strongly influences the solvent response. This perhaps also indicates that the contribution of the reorientational motion of ions to the overall relaxation process cannot be neglected. The total expected Stoke’s shift, estimated by the procedure of Fee and Maroncelli, is 20407 cm-1 for [Cnmim][FAP].53 This suggests that more than 15-20% of the dynamics is missed due to the limited time resolution of our TCSPC setup. In this context, it should be mentioned that ultrafast component may be associated with small-amplitude motions of the planar and polarizable imidazolium cations in the vicinity of the probe molecule as this component has not been observed in nonplanar ammonium and phosphonium ILs.23 We would also like to take a note here that recent investigations have revealed that the fast component of the solvent response function in imidazolium-based ILs is originated from rapid orientational relaxation involving the dipolar species, This journal is © The Royal Society of Chemistry [year]

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[Cnmim][FAP] (n=2, 4 and 6) room temperature ionic liquids with slip and stick boundary condition parameters. Data representations are clearly explained inside the figure.

whereas relaxation of the ion dynamic structure factor through ion translation produces the slow components. 30 50

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3.3.2. Rotational dynamics The time-resolved fluorescence anisotropy decay profiles for all the present ILs is shown in Fig. 9. The rotational relaxation parameters of C153 in different ILs are shown in the Table 4. Even though time resolved anisotropy decay profiles are well fitted by the biexponential function, the average reorientationtime obtained from such analysis are found not to be very differentfrom those which are obtained from single exponential fit to the experimental data points. Fit parameters to rotational anisotropy decays are provided in the supporting information (Table S5). It can be seen from Table S5 that average rotation time of C153 under isoviscous condition in [C2mim][FAP], [C4mim][FAP] are found to be 2.67 ns, 2.89 ns respectively which indicates, there is no bearing of alkyl chain length on solute rotation. A decrease in rotational relaxation time of C153 has been observed upon gradual increase in temperature due to the lowering of the viscosity of the medium (Table S5). From the Table S5, it can be also seen that there are two distinct regions, in one domain rotation time is faster and in other domain it is hindered. In this context, it is pertinent to mention that ionic liquids with alkyl chain C4 or longer are known to be spatially heterogeneous in nature having distinctly different polar and nonpolar domains.34-38, 45 Nonpolar domain is believed to be formed mainly by aggregation of a nonpolar alkyl chains, and polar domain forms because of the cations and anions. 6, 54-58In light of this, one can also reasonably assume that in case of the present ionic liquids, alkyl chain would be responsible to form a nonpolar domain and the imidazoliumcations and FAP anion would give rise to polar domain. In polar domain, rotation of C153 is expected to be slow as compared to the same in nonpolar domain due to the fact that polar domains are known to be more

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Fig. 11 Steady state emission spectra normalized to the peak of the 5

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monomer fluorescence band [IM(417nm) = 1.00) for BPP in ionic liquids[Cnmim][FAP] (n = 2 and 6) and ethanol; excitation wavelength: 330 nm.

structured than nonpolar domain. 59 In the present case, it can be seen from Table S5 that the faster component is almost constant at room temperature in all the present ILs. By looking at this data one may try to think that all present ILs form very similar nonpolar domains even though they differ by their length of alkyl side chain. However, previous study have shown that ionic liquids having different alkyl chain length can form different nonpolar domains.4This implies that present data cannot capture the micro-heterogeneous behaviour of ILs that may arise due to the presence of different alkyl chain length on the cationic moiety of the present ILs. To get a closer look on whether the change in the alkyl chain length on cationic moiety affects the solutesolvent coupling, we have further analyzed the experimentally measured rotation time with the help of well-known StokesEinstein-Debye (SED) hydrodynamic theory. According to this theory, the rotational movement of a medium sized solute molecule in a solvent continuum is implicit to occur by small step diffusion and its rotation time is correlated to the bulk viscosity of the solvent and temperature by the following relation SED r

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VfC k T

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(10)

where, k is the Boltzmann constant and T is the absolute temperature. V is the van der Waals volume of the solute molecule; f is the shape factor and C is the boundary condition parameter, which expresses the measure of coupling between the solute and solvent. The two extreme margin conditions are stick and slip according to SED hydrodynamic theory. 60 When the rotating solute molecule is larger in size than solvent molecule, C is close to unity and it denotes the stick boundary condition. In the case of a similar or smaller solute molecule than the molecule of the medium, C is less than unity. The shapes of the solute molecules are usually considered either to be symmetric or asymmetric ellipsoids in SED theory. For nonspherical molecules, f is greater than unity and the extent of deviation from unity in the value of f describes the degree of nonspherical nature 8|Journal Name, [year], [vol], 00–00

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DOI: 10.1039/C4RA16864J

of the rotating solute molecule. For the calculation of slip boundary condition (Cslip), C153 is considered as asymmetric ellipsoid to determine rotational time. For calculation of C slip, we have taken the probe properties which are already available in literature. 24 Details of the procedure have been described in our earlier publications.13 The van der Waals volume, shape factor, and calculated slip boundary condition parameter for C153 are 243 Å3, 1.5, and 0.18 respectively.13, 24 The slip and stick boundary limit, which have been assigned with the help of SED hydrodynamic theory are shown in the Fig. 10 with the experimentally measured reorientation times of C153. With the help of equation 10, experimentally observed rotational coupling constants values are calculated and provided in table S5 in the supporting information. Data that are collected in Table S5 also reveal that at a particular temperature solute-solvent coupling constant (Cobs) values are similar for all the ILs.24 The observation indicates that solute-solvent interaction are very similar for all the present ILs even though they possess different alkyl side chain length. The observation is in line with the earlier report by Dutt and coworkers.12, 15, 17 While alkyl chain length does affect both solute rotation and solvation but hydrodynamic analyses suggest near-independence of solute-solvent coupling parameter, CObs. This is an interesting observation. Cobs is related to solute-solvent short-range structure involving arrangement of solvent particles in the solute's immediate vicinity, and hydrodynamic description, being based on continuum model of solvent description, is perhaps unable to reflect the variation due to the alkyl chain length variation. Generally, Cobs values are calculated from hydrodynamic description for solute rotation in such viscous media. Thus, the present discrepancy perhaps arises due to the use of hydrodynamic expression which cannot account for nonhydrodynamic moves, such as jumps. 61 Solute relaxation dynamics in highly viscous media (super cooled systems, deep eutectics etc.) often find that solute-centred dynamics are occurring through stochastic Brownian moves but via nonhydrodynamic modes such as jumps.61 In such a case, still using hydrodynamic expressions, one bound to get results which may either be inconsistent or bury the subtle effects of environmental complexity through cancellation. One of these or both are probably contributing to the near independence of Cobs values which is calculated based on SED hydrodynamic analyses.61 The plot of r versus η/T indicates that the τr values of C153 lie between the slip and stick boundary condition of the SED theory and this theory is quite successful in explaining the solute rotation of C153 in the present ILs. Following relationship are obtained when, we fitted the log-log plot r vs. /T by the log(< r>)=a+plog( /T) relation. For all the ILs, p values are found to be near to unity. The linearity in rotation time of C153 in these ILs can be explained by taking in to consideration of the viscosity of the medium which increases with the increasing alkyl chain length in the cationic moiety of these ILs. C153 in [C2mim][FAP] log(< r>)=(1.0676 0.0196)+(0.9258 0.0242)log( /T) R=0.9990

N=5,

C153 in [C4mim][FAP] This journal is © The Royal Society of Chemistry [year]

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log(< r>)=(1.1044 0.033)+(0.8729 0.0452)log( /T) R=0.9960

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DOI: 10.1039/C4RA16864J

M. and P. K. S. are thankful to NISER, Bhubaneswar for research fellowship.

N=5, 60

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C153 in [C6mim][FAP] log(< r>)=(1.0837 0.204)+(0.857 0.0309)log( /T) N=5, R=0.9981 In order to get further idea about the microviscosity properties related to these ILs, we have analyzed the steady state fluorescence behavior of microviscosity probe, 1, 3-bis (1pyrenyl) propane (BPP) in two extreme cases (ethyl and hexyl systems) as well as in ethanol. BPP is well studied fluorescence probe, which can be used to get idea about the microviscosity of the medium by comparing the intensity ratio of excimer emission band (~ 400-500 nm)and monomer emission band (~300 nm).6265 When viscosity of solvent is very low, two pyrene moiety of BPP easily come closer to each other to form an intra molecular excimer. As a consequence, the emission spectrum of BPP exhibits usual structured monomer emission band at lower wavelength region along with broad structure less excimer emission band at higher wavelength (Fig. 11). As the microviscosity of the surrounding environment increases, the extent of excimer formation decreases, and as a result corresponding reduction in the intensity of the excimer band found to be observed. It can be seen from Fig. 11 that emission intensity corresponding to the excimer peak of BPP in these ionic liquids is observed to be significantly reduced than that observed in ethanol. Among the ILs, the observation of much less emission intensity of excimer band in hexyl analogue as compared to that in the ethyl indicates that microviscosity is increased from ethyl (n = 2 ) to hexyl (n = 6). Thus, overall increase in average rotational time with increase in alkyl chain length of ILs has been observed due to the increase in the microviscosity of the medium surrounding the solute molecule.

Conclusions

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In the present report, fluorescence response of C153 has been investigated in a series of fuloroalkylpohsphate anion containing ionic liquids with the systematic variation of alkyl chain length on the cationic moiety. Average solvent relaxation time found to increase with increase in cationic alkyl chain length. It has been observed from the solvent relaxation data that size of the cation also plays important role in influencing solvent relaxation behavior in ionic liquids. However, in case of rotational diffusion of C153, other than the viscosity effect no significant influence of the alkyl chain length on the cationic moiety has been observed. The increase in average rotational relaxation time of C153 with an increase in alkyl chain length has been interpreted by considering the increase in microviscosity values of the respective medium.

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This work was supported by the Council of Scientific and Industrial Research (CSIR), New Delhi, Government of India. S. K. D. is thankful to National Institute of Science Education and Research (NISER), Bhubaneswar for post doctoral fellowship. D. This journal is © The Royal Society of Chemistry [year]

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School of Chemical Sciences, National Institute of Science Education and Research, Bhubaneswar751005, India. Fax: +91-674-2304050; Tel: +91-674-2304037; E-mail: [email protected] b present address:Department of Mechanical Sciences and Engineering Graduate School of Engineering, Tokyo Institute of Technology, 2-12-1I1-15 Ookayama, Meguro-ku,Tokyo 152-8552 Japan. † Electronic Supplementary Information (ESI) available: Measured viscosities and densities of the present ionic liquids at different temperature and fitting parametes to the densities and viscosities are provided. See DOI: 10.1039/b000000x/

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Fluorescence response of coumarin153 (C153) have been investigated in a series of 1-alkyl-3methylimidazolium tris(pentafluoroethyl)trifluorophosphate room temperature ionic liquids (RTILs) with systematic variation of alkyl chain length (ethyl, butyl and hexyl ) to examine the effect of alkyl side chain lengths on cationic moiety on solute and solvent relaxation dynamics. Physicochemical properties associated with the present RTILs are estimated at different temperatures. While the viscosity values are observed to be increased with increase in alkyl chainlength, density values decrease with the increase in the length of alkyl side chain. Steady state

fluorescence

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reveal

that

C153

experiences

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nonpolar

microenvironments with increase in the alkyl chain length. Interestingly, time resolved study has demonstrated that length of alkyl side chain have noticeable influence in governing the solvation dynamics in these media. It has been observed that average solvent relaxation time estimated for these RTILs can be better correlated when both the size of the alkyl chains and bulk viscosity of the respective RTILs are considered. Quite interestingly, apart from the viscosity effect, negligible influence of alkyl chain length has been observed for rotational diffusion of C153.

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Graphical Abstract:

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