Picosecond Solvation Dynamics of Coumarin 153 in Bis ( 1-methyl-1 H-imidazol-3-ium-3-yl ) dihydroborate Cation Containing Room Temperature Ionic Liquid and Ionic Liquid-DMF Mixtures

Steady state and time-resolved fluorescence behavior of coumarin153(C153) in bis(1-methyl-1H-imidazol-3ium-3-yl)dihydroborate cation containing room temperature ionic liquid and its mixture with dimethylformamide (DMF) has been investigated. Density functional calculations on the present ionic liquid have been carried out to have ground state structural information of this system. C-H N and C-H O hydrogen bonding interactions between cationic and anionic moiety of the present ionic liquid has been observed. Steady state absorption and emission spectral profiles of C153 are found not to be influenced by the polar cosolvent. Time-resolved fluorescence anisotropy experiments show that the rotational motion of the probe becomes faster in presence of DMF. During time dependent dynamic Stokes shift measurements in ionic liquid-DMF mixtures, the average solvation time is found to decrease with the addition of DMF to the ionic liquid. The decrease in both average solvation and rotational time of probe molecule upon gradual addition of polar organic co-solvent is attributed to the lowering of bulk viscosity of the medium.


INTRODUCTION
In recent years, room-temperature ionic liquids (RTILs) have drawn great attention due to its versatile applications in the fields of chemical, material and biological sciences [1,2].Curiosity about their structure -property correlations have also motivated many researchers to carry out experimental and theoretical studies on these systems [3][4][5][6][7][8][9][10][11][12][13].However, many of the studies have been carried out by taking neat room temperature ionic liquid as the medium.Interestingly, over the past few years, mixed ionic liquid-cosolvent systems are found to be extremely useful in several new applications, such as organic synthesis [14], catalysis [15], polymerization processes [16], extraction of solutes from solvent [17,18] etc. Keeping in mind the uses of RTIL-cosolvent systems in new potential applications, it is of paramount importance to have thorough understanding of the roles of cosolvents that they play in governing the properties of RTILs.Some studies have been carried out to understand the physicochemical properties of the RTILs-cosolvents mixtures [19][20][21][22][23][24][25][26].It has been found out that viscosity [19], polarity [20][21][22], solvation [23,24] and electrochemical [25,26] behavior of the ionic liquid undergo perceptible changes in presence of cosolvents.Since solute reactivity is intricately related to solvation dynamics, studies on dynamics of solvation by monitoring the time-dependent fluorescence Stokes shift (TDFSS) of a dipolar solute in the given medium [6,[8][9][10][11] have been carried out to understand the dynamical features of IL-cosolvent mixtures.In this regard Paul et al. [6] have studied the effect of nonpolar toluene and dioxane on the solvation dynamics and solute rotation in 1-butyl-3methylimidazolium hexafluorophate.The composition dependence of Stokes shift dynamics of a fluorescent dye molecule in IL-cosolvent mixtures has been investigated theoretically by Ranjit Biswas et al. [12].The theory demonstrates the dynamic Stokes shift as a sum of contributions from the dye-RTIL and the dyepolar cosolvent interactions.The theory also suggests significant contribution from solute-cation dipole-dipole interaction in solvation energy relaxation.Moreover, Stokes shift dynamics in ([Bmim][BF4]+ dichloromethane) binary mixtures predicts a very weak nonlinear composition dependence.
In this context, it may be mentioned that Sarkar and coworkers' [8][9][10][11] have studied the effects of polar solvents such as, acetonitrile, water and methanol on the dynamics of solvation of ammonium and imadazolim-based ionic liquids.These studies have demonstrated that addition of cosolvents in RTILs affects both the solvent relaxation as well as solute rotation.Very recently, we have also investigated the solvation and rotational relaxation behavior of a dipolar probe in ultra hydrophobic 1-(2-Methoxyethyl)-1methylpyrrolidinium tris(pentafluorethyl)trifluorophosphate and its mixture with toluene [13].It has been observed that toluene can effectively penetrate into the ionic liquid-rich cybotactic region of the probe.This has been explained by considering favorable noncovalent interaction between ionic liquid and toluene.One general observation of the TDFSS studies is that the addition of a cosolvent to ionic liquid decreases the average solvation time and rotational time of the probe due to the lowering of viscosity of the media.Dimethylformamide (DMF) is an interesting solvent with a strong electron-pair donating and accepting ability.It has been used to dissolve a wide range of organic and inorganic compounds which in turn has also made this solvent to be used in variety of industrial processes [27][28][29].Moreover, it has been widely used in developing solvent reactivity relationships [30][31][32].In view of this, it is expected that the RTIL-DMF will serve as an excellent solvent system for several applications.Very recently, Anil Kumar and coworkers have studied the thermophysical properties of binary mixtures of DMF with ammonium and imidazolium based ionic liquids [33].However, to the best of our knowledge there is no report of solvent and rotational relaxation studies by taking DMF as cosolvent for RTILs.We would also like to take a note here that, RTIL-cosolvent systems are known to be micro-heterogeneous [34][35][36][37].In this regard, it would also be an interesting study to find a relationship between average solvation and rotational time and medium viscosity.Keeping all these in mind, we have investigated the solvation and rotational relaxation of Coumarin153 (C153) in Bis(1-methyl-1H-imidazol-3-ium-3yl)dihydroborate bis(trifluoromethylsulfonyl)amide and RTIL-DMF mixtures.The present RTIL is an interesting one in a sense that two methylimidazolium cationic moieties are bridged by a borohydride group and the cationic charge is compensated by NTf 2 -.In all previous cases, solvation and rotational dynamics were studied using monocationic imidazolium moiety.The probe C153 used in the present study is a well-known solvatochromic probe suitable for dynamic Stokes shift measurements [38].The molecular diagram of RTIL and C153 are shown in Figure 1.

Materials
C153 (laser grade, Exciton) was used as received.The ionic liquid was obtained from Frontier Scientific Chemicals, USA (95% purity) and purity of IL was confirmed from 1 H NMR spectrum.The IL was kept in high vacuum at 60°-70°C for one day to remove any moisture present in this solvent.The IL was taken in a long-necked quartz cuvette and requisite amount of fluorophore was added to prepare the solution.Proper precaution was taken to avoid moisture absorption by the media during transferring the solute into the cuvette.During measurements, the concentration of the dye molecule (C153) was kept low so as to prevent dye aggregation.

Instrumentation
The viscosity of the RTIL was measured by LVDV-III Ultra Brookfield Cone and Plate viscometer (1% accuracy and 0.2% repeatability). 1 H NMR spectra were obtained on a Spectrometer Bruker AVANCE 400 NMR Spectrometer at ambient temperature using tetramethylsilane (TMS) as an internal standard.(dilute ludox solution in water) in place of the sample.The temperature was controlled by the Quantum Northwest (TC125) instrument.Decay curves were analyzed by nonlinear least-squares iteration procedure using F900 decay analysis software.The quality of the fit was judged by the chi square ( 2) values and weighted deviation was obtained by fitting.The time-resolved fluorescence anisotropy measurement was done by using two polarizer by placing one of them in the excitation beam path and the other one in front of the detector.An alternate collection of the fluorescence intensity in parallel and perpendicular polarization (with respect to the vertically polarized excitation laser beam) for equal interval of time had been carried out until the count difference between the two polarizations (at t = 0) is ~5000.For G-factor calculation, the same procedure was adopted, but with 5 cycles and horizontal polarization of the exciting laser beam.

Method
The time resolved emission decay profile were measured at 5/10 nm intervals around the whole emission spectra.The total number of measurements was 25-35 in each case.In all cases, when decay was monitored at a shorter wavelength, a faster decay was observed, and at a longer wavelength, we observed a decay profile that consists clear rise with usual decay.Each decay curve was fitted to a triexponential decay function with an iterative reconvolution program (F900) maintaining the 2 between 1 and 1.2 as a measure of goodness of fit.The time resolved emission spectra (TRES) were constructed according to a procedure described earlier [13].
The emission maximum at each time (t) was obtained by fitting the spectrum to a log-normal lineshape function as given below (1) for > -1 and I = 0 for -1 where = 2 ( peak )/ , peak was the wavenumber corresponding to the peak, h was the peak height, was the full width at half maximum (FWHM) and was a measure of the asymmetry of the band shape.Optimizing these four parameters by nonlinear least squares iteration, the best fitted curve was obtained.The peak frequencies were obtained from this lognormal fitting of TRES.The peak frequencies were then used to construct the solvent correlation function (C (t)).
where (0) was the peak frequency at time t = 0, just after the electronic excitation and (t) was the peak frequency at time t = t.Again, ( ) was the peak frequencies at t = when solvent molecules were in the equilibrium position around the photoexcited probe molecule.The time dependence of the calculated C(t) was fitted by a biexponential function of the form where 1 and 2 are the solvent relaxation times and a 1 and a 2 were normalized pre-exponential factors.
After having the values of 1 , 2 and a 1, a 2, the average solvation time was calculated by using the following relation We have also fitted C(t) by the stretched exponential function as given below.

C(t) = exp(-(t/ solv ) )
( Where 0< 1 where was the gamma function and st was the average solvation time considering C(t) was a stretched exponential function.

Optimized Structure
To know the structural features of [BIMIMDBA][TF 2 N] we have optimized the structure of this IL in the ground state with B3LYP functional [39,40] and 6-31++G (d, p) basis set using Gaussian 3.0 program [41].The optimized structure of [BIMIMDBA][TF 2 N] is shown in Figure 2. The van der Waals criterion for hydrogen bond formation is that for the formation of X H Z bonds, the distance between X Z should be less than the sum of the X H covalent bond and the van der Waals radii of H and Z [42].The van der Waals radii of O, N and H are 1.52 Å, 1.55 Å and 1.2 Å respectively.In the optimized structure, the distance between O33-H17, O34-H4 and N28-H17 are measured to be 2.614 Å, 2.172 Å and 2.451Å respectively (Figure 2).Moreover, the bond angles C14-H17-N28, C14-H17-O33 and C5-H4-O34 are found to be 179.4°,122.3 o and 141.3 o respectively.In the present case, these calculated data of both the distance and the angle suggests the presence of strong C-H N hydrogen bonding interactions between the cationic and the anionic fragment of the RTIL.The data also depicts a weak C-H O interaction between the cationic and the anionic moiety.The structural parameters as obtained from the gas phase optimized structure of the present ionic liquid are collected in Table 1.

Steady State Behavior
The absorption and emission spectra of C153 are recorded both in neat RTIL and RTIL-DMF mixture.
The absorption and emission maxima of C153 in neat ionic liquid and its binary mixture with DMF are shown in Table 2.The representative steady state emission spectra of C153 in neat RTIL and RTIL-DMF binary mixtures are also shown in Figure 3.In the present case, no significant emission from the ionic liquid has been observed; the fact has enabled us to use the present ionic liquid for photophysical studies.Interestingly, the position of the absorption and emission maximum of C153 remain unchanged after the addition of organic cosolvent to RTIL.

Time-Resolved Studies
The emission decay profile of C153 has been measured at several wavelengths (5-10 nm intervals) across the emission spectra by exciting the sample at 405 nm.The decay profiles are found to be strongly dependent on the monitoring wavelengths (Figure 4).A faster decay is observed when the process is monitored at shorter wavelengths.However, at longer wavelengths, we observed a decay profile that consists of a clear rise with usual decay.This particular observation carries the typical signature of solvation process [38].The time-resolved emission spectra (TRES) of the system, constructed from the fitted decay profiles, show progressive red shift of the fluorescence maximum with time.Representative TRES of C153 is shown in Figure 5. Time-dependent shifts of the emission maximum to lower energy have been observed in all cases.This indicates that the excited states of dipolar molecules are stabilized by solvent molecules with time.From Table 3, it is evident that the average solvation time of C153 decreases with the increase in wt.percentage of DMF in RTIL.For example, the average solvation time decreases from 600 ps in neat ionic liquid to 240 ps in RTIL-DMF mixture (12 wt%).This lowering of average solvation time with gradual addition of DMF to neat ionic liquid could be attributed to gradual lowering of bulk viscosity of the media.To get a better understanding on the dependence of average solvation time on bulk viscosity of the probe molecule in the different RTIL-DMF composition, we have plotted the average solvation time of C153 against the bulk viscosity values of RTIL-DMF mixtures (Figure 7).Almost a linear decrease in average solvation time with bulk viscosity has been observed.In this context we would like to note that in neat DMF the average solvation and rotational time are found to be 2 ps and 47 ps respectively [43,44].The faster dynamics is attributed to the much low viscosity of DMF.Where G = I HH (t) where G is the correction factor for detector sensitivity to the polarization of the emission.G is 0. As can be seen from Table 4, the average rotational time of C153 which is higher in neat ionic liquid gradually decreases with gradual addition of DMF to RTIL.For example, at 293 K, the average rotational time of C153 decreases from 6.52 ns in neat ionic liquid to 2.60 ns in RTIL-DMF (12wt %) mixture.The faster rotation of C153 with decrease in viscosity of the RTIL-DMF mixture is also evident when the average rotational time of C153 is plotted against the viscosity of the media.The log-log plot of average rotational relaxation time ( r ) against viscosity with respect to temperature is shown in Figure 9.It is clear from the plot that rotational relaxation dynamics of C153 can be well explained within the broad limits of SED hydrodynamic theory.It is also evident from the plot that the average rotational relaxation time decreases with lowering of the bulk viscosity of the media.We have also fitted the < r > and /T data for C153 to the function, r = A( /T) P .In these expressions (equations 8, 9 and 10), the r values are in nanoseconds and /T are in cPK 1 , and N and R denote the number of data points and regression coefficient, respectively.Prominent nonlinear composition dependence has not been observed in the present case (equations 8, 9 and 10).
C153 in RTIL+ 12(wt%)DMF < r > = (32.9689± 13.62) ( /T)    We have also estimated the rotational coupling constants (C rot ) of C153 at different RTIL-DMF composition.According to the stick hydrodynamic theory, the rotational time constant ( stk ) of a nonspherical solute of volume V, rotating along the axis of the ellipsoid in a medium of viscosity at temperature T is given by where f stk accounts for nonspherical shape of the solute and k B is the Boltzmann constant.Using the literature value of V(243Å 3) and f stk (1.5), we computed the stk values for RTIL and RTIL-DMF mixture [7].From the computed stk values, the rotational coupling constants, C rot , which is defined as C rot = rot/ stk , is calculated.As can be seen from Table 4, the C rot value varies from 0.48 to 0.62.The calculated C rot values, which are measure of the extent of departure from normal hydrodynamic behavior of a solute due to specific interaction, are found to be very similar to that obtained for C153 in conventional solvents [44].The calculated C rot values also indicate no involvement of specific solute-solvent interaction in the present case.The observation demonstrates that other than the loweing of viscosity of the medium, DMF does not seem to play any role towards the rotational dynamics of C153 in RTIL-DMF mixtures.

CONCLUSION
We have studied the steady state and time resolved fluorescence behavior of C153 in a new diimidazolium cation based ionic liquid and its mixture with organic polar cosolvent DMF to determine the role of DMF that it plays in governing the solvation and rotational relaxation of C153 in these media.The present ionic liquid has negligible emission behavior and so is well suited for fluorescence studies.Steady state spectral behavior of C153 is found not to be affected by these polar cosolvents.However, in time-resolved fluorescence studies the influence of DMF is clearly observed.The addition of DMF to RTIL decreases the average solvation time due to a lowering of viscosity of the media.This also results in the faster rotational dynamics of C153.

Figure 3 :
Figure 3: Normalised steady state emission spectrum of C153 in neat RTIL+ DMF mixtures.Emission spectrum of neat RTIL is also shown in the same figure.exc.= 405 nm.

Figure 7 :
Figure 7: Average solvation time against bulk viscosity at 293 K.
8 at the wavelength of detection.I HH (t) and I HV (t) are the intensities of fluorescence decays when the excitation and the emission polarizers are polarized at horizontalhorizontal and horizontal-vertical alignments respectively.Again I VV (t) and I VH (t) are the intensities of fluorescence decays when excitation and emission polarizers are polarized at vertical-vertical and vertical-horizontal alignments respectively.The rotational relaxation parameters of C153 in neat RTIL and in RTIL-DMF at different temperatures are collected in Table 4.The anisotropy decay profiles are fitted to a bi-exponential function.Decay of time-resolved anisotropy (r(t)) of C153 in RTIL+6%(wt.)DMF at various temperatures are shown in Figure 8.

b
Crot is rotational coupling constant calculated according to equation 11.Temperature averaged Crot values are tabulated.

Figure 9 :
Figure 9: The log-log plots of < r> Vs. ( /T) showing the stick and slip boundary conditions with solid and dashed line respectively.

Table 4 : Rotational Relaxation Parameters of C153 in RTIL-DMF Binary Mixtures at Different Temperatures ( exc=405 nm)
a r0 is the initial anisotropy.