Solvation dynamics and red-edge effect of two electrically charged solutes in an imidazolium ionic liquid

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1 Indian Journal of Chemistry Vol 49A, May-June 2010, pp Solvation dynamics and red-edge effect of two electrically charged solutes in an imidazolium ionic liquid Dinesh Chandra Khara & Anunay Samanta* School of Chemistry, University of Hyderabad, Hyderabad , India Received 3 March 2010; accepted 31 March 2010 Steady state and time-resolved fluorescence behavior of two electrically charged solutes, viz., negatively charged 1-anilinonaphthalene-8-sulfonate (ANS) and positively charged ethidium bromide (EB), have been studied in an imidazolium ionic liquid. It is found that EB shows a much less pronounced excitation wavelength dependent fluorescence behavior (red-edge effect) as compared to ANS. The solvation dynamics of these probes in [bmim][bf 4 ] is found to be similar to those observed with neutral dipolar probes. A higher solvation time of EB as compared to ANS is possibly a reflection of hydrogen bonding interaction of the former system with the anionic component of the ionic liquid. Keywords: Ionic liquids, Solvation dynamics, Time-resolved studies, Fluorescence, Red-edge effect, Fluorescent probes Room temperature ionic liquids (ILs) are being studied actively because of their potential as green alternatives to the conventional organic solvents. 1-9 As characterization of the physicochemical properties is essential for utilization of this new class of solvents in various applications, several groups are engaged in elucidating the physicochemical properties of the ILs from both theoretical and experimental point of view These include several photophysical studies as well. As the ionic liquids are highly polar solvents, 10,19 a large number of photophysical studies in these media deal with the nature and mechanism of solvent stabilization of dipolar solutes in neat ILs and mixed solvents comprising IL and molecular solvents Since the early works of Karmakar and Samanta, several groups have studied the solvation dynamics in different ILs using dipolar fluorescent probe molecules to understand the mechanism and dynamics of solvation following an instantaneous separation of charge in the dipolar probe molecule These studies have indicated that the time-resolvable component of the dynamics is slow and biphasic (or non-exponential) in nature and is dependent on the viscosity of the medium. Of the two components, the fast component has a lifetime of a few hundred picoseconds, and the slow component has a lifetime of a few nanoseconds. The first component was attributed to the translational motion of the anion and the slower component to the collective motion of both cation and anion. These interpretations were based primarily on the literature available at that time on high temperature solvation dynamics data on molten salts. 39,40 In order to obtain an insight into the solvation process in ILs, simulation studies have been performed by several groups. 15,16 Shim et al. 16 attributed the fast component of the dynamics to the translational motion of the anion and the slow component to the overall diffusional motion of the cation and anion. On the other hand, Kobark and Znamenskiy 15 assigned the ultrafast component of the dynamics to the collective cation-anion motion. While some of these studies involving the dipolar probe molecules revealed a probe dependence of the dynamics, the origin of such dependence was not clear. 24,27 In one of their studies on solvation and rotational dynamics in IL, Ito et al. 33 used an ionic solute, namely, 10-methyl- 9-phenylacridinium ion, along with a few other neutral dipolar probes. The ionic probe molecule in this study appears to have been chosen without the specific intent of finding out whether solvation dynamics is dependent on the charge on the probe molecule. While a significant variation of the solvation time with the probe molecule was noted in this study, no particular significance was however attached to the data obtained with the ionic probe molecule. Sarkar and coworkers 29,41 have studied the solvation dynamics in mixed solvents comprising IL and conventional solvents.

2 KHARA & SAMANTA: SOLVATION DYNAMICS OF CHARGED SOLUTES IN IONIC LIQUIDS 715 Recently, femtosecond Kerr-gate spectroscopy and upconversion techniques have also been employed to elucidate the nature of various ultrafast relaxation processes and their exact time-scales in imidazolium and other ILs. 38,42-45 Except for the study of Ito et al. 33 we are not aware of other studies on solvation dynamics in ILs that involved ionic probes even though a number of studies on rotational dynamics of such probes are available. 46,47 This is why we have employed two charged probe molecules, viz., anionic, 1-anilinonaphthalene-8-sulfonate (ANS) and cationic, ethidium bromide (EB), to study the dynamics of the solvent relaxation process in an imidazolium ionic liquid, [bmim][bf 4 ]. Both EB and ANS meet the essential criteria of the fluorescent probe for study of solvation dynamics. Moreover, ANS has already been employed as a common probe for the solvation dynamics studies, whereas EB is a highly fluorescent molecule having high fluorescence lifetime and has been used to probe binding with DNA Materials and Methods ANS was procured from Molecular Probes and was recrystallized from ethanol. EB was obtained from Aldrich and recrystallized from methanol. It showed a single spot on the TLC plate (4:1:1 butanol:acetic acid:water). Advanced Material Research grade [bmim][bf 4 ] used in this work was obtained from Kanto Chemicals (Japan). The solvent was dried for several hours under high vacuum prior to use. Instrumentation The absorption and steady-state fluorescence spectra of the systems were recorded on a UV-visible spectrophotometer (Cary100, Varian) and a spectrofluorimeter (FluoroLog-3, Horiba Jobin Yvon), respectively. The fluorescence spectra were corrected for the instrumental response. Timeresolved fluorescence measurements were carried out using a time-correlated single-photon counting (TCSPC) spectrometer (5000, IBH). Diode lasers (λ exc = 375 nm, 10 MHz and 439 nm, 1 MHz) were used as the excitation sources, and an MCP photomultiplier (Hamamatsu R3809U-50) was used as the detector (response time 40 ps). The instrument response function of the experimental setup was limited by the fwhm of the exciting laser pulse and was 55 ps for 375 nm excitation and 100 ps for 439 nm excitation. The lamp profile was recorded by placing a scatterer (dilute solution of Ludox in water) in place of the sample. Decay curves were analyzed by nonlinear least-squares iteration procedure using IBH DAS6 (Version 2.2) decay analysis software. The quality of the fit was measured by the χ 2 values and the weighted deviation of the fitting. Procedure The time-resolved emission decay profiles were measured at 5/10 nm interval across the entire steadystate emission spectra. The wavelength selection was made by a monochromator with a bandpass of 2-6 nm. The total number of measurements was in each case. Each decay curve was fitted to a triexponential decay function with an iterative reconvolution program (IBH). This procedure increased the effective time resolution of the experiment to ~40-50 ps. The time-resolved emission spectra (TRES) were constructed according to a procedure described earlier. 20 The TRES at various times were fitted to log-normal function, to obtain the peak maxima at different times (Eq. 1), I = h exp[ ln 2{ln(1+α)/γ} 2 ] for α > 1 (1) = 0 for α 1 2γ(ν ν peak ) where α = ν peak = wave number corresponding to the peak, h = peak height, = fullwidth at half-maxima and γ corresponds to the asymmetry of the band-shape. All experiments were performed at 23 C.

3 716 INDIAN J CHEM, SEC A, MAY-JUNE 2010 Results and Discussion Steady-state behavior Steady state fluorescence and fluorescence excitation spectra of ANS and EB in [bmim][bf 4 ] are shown in Fig. 1. The spectral behavior of the systems is consistent with that observed in conventional polar solvents 48,51 except that both systems exhibit excitation wavelength dependent fluorescence behavior in ionic liquids, which is depicted in Fig. 2. Interestingly, the excitation wavelength dependence of EB fluorescence is found much less pronounced compared to ANS, as reported earlier by Mandal et al. 52 and reconfirmed in this study. In this context, it is to be noted that the excitation wavelength dependent fluorescence behavior of dipolar molecule in ionic liquids is observed when the probe molecules are excited at the long wavelength edge of the first absorption band. A red-shift of the emission maximum of the dipolar systems, as observed with increase in Fig. 1 Fluorescence excitation and emission spectra of ANS and EB in [bmim][bf 4 ]. [a, ANS; b, EB. The excitation wavelengths are 375 and 439 nm for ANS and EB, respectively]. excitation wavelength in ionic liquids 23,27,52 and other viscous solvents, goes against the Kasha s rule. This red edge effect (REE), as it is commonly called, is the consequence of (i) inhomogeneous broadening of the absorption spectrum of the dipolar system resulting from the presence of a distribution of energetically different molecules, and, (ii) slow excited-state relaxation process. The excitation wavelength dependent shift of the emission peak of ANS and EB has been found to be ~35 and ~5 nm, respectively. That EB exhibits much less pronounced REE compared to ANS, can perhaps be explained taking into account the charge delocalization in the two molecular systems and their fluorescence lifetimes. A better charge delocalization in EB makes it a less effective system from electrostatic consideration as compared to ANS. A higher lifetime (~13.75 ns for EB and ~6.05 ns for ANS ) of EB also makes it not a good system to exhibit REE. However, as EB is well known for its hydrogen bonding interaction with H-bond acceptor, 48 one can expect strong association of EB with [BF 4 ] - ion of the ionic liquid and this association perhaps gives rise to small REE observed for the system. Time-resolved behavior Wavelength-dependent decay profiles, which are a typical signature of slow solvation dynamics, have been observed for both the systems. Representative wavelength-dependent decay behavior is illustrated in Fig. 3. When monitored at the shorter wavelength region, only monotonous decay is observed, while at the longer wavelengths, the time profiles consist of a slow rise followed by the decay. The time-resolved emission spectra (TRES) have been constructed by fitting the individual decay curves to a multi-exponential function followed by Fig. 2 The max λ em versus λ ex plots for ANS and EB in [bmim][bf 4 ]. [a, ANS; b, EB].

4 KHARA & SAMANTA: SOLVATION DYNAMICS OF CHARGED SOLUTES IN IONIC LIQUIDS 717 normalization of the decay traces by steady-state spectra, by a procedure described earlier. 20 The TRES for EB and ANS at different time intervals are shown in Fig. 4. In both cases, a time-dependent solvent mediated relaxation of the excited state is observable. The total observed shift of the time-dependent emission peak calculated from the difference between the frequencies of the measured spectra at zero time, t(0), and infinite time, t( ), is found to 1397 and 864 cm -1 for ANS and EB, respectively. It is evident that due to the finite time resolution of our experimental setup (40-50 ps), we could not observe the ultrafast portion of the dynamics, if any, in our measurements. An estimate of this missed component of the dynamics is obtained following the procedure of Fee and Maroncelli. 55 In this procedure, the exact time zero frequency, ν cal (0), is calculated from the steady-state absorption, fluorescence, ν (em), using Eq. (2), ν cal(0) ν p (abs) ν np (abs) ν np (em) ν (abs), and (2) where the subscripts, p and np, refer to the peak frequencies (in cm -1 ) in polar and nonpolar solvent, respectively. The extent of the missing component is then determined by the value of ( νcal (0) ν (0) )/( νcal (0) ν ( ) ). We have calculated the missing component for ANS using cyclohexane as the nonpolar solvent, and the literature data on the steady-state absorption and emission. 56 In the present case, the estimated value of the missing component of the dynamics with ANS is found to be as high as ~60 %. As EB is highly insoluble in Fig. 3 Wavelength-dependent decay profiles of ANS and EB in [bmim][bf 4 ]. [a, ANS at (1) 450 nm, (2) 490 nm, (3) 550 nm, (4) 610 nm; (b) EB at (1) 550 nm, (2) 590 nm (3) 630 nm, (4) 670 nm. The lamp profiles are shown as dashed line]. Fig. 4 TRES of ANS and EB in [bmim][bf 4 ]. [a, ANS at (1) ( ) 0 ps, (2) ( ) 50 ps, (3) ( ) 250 ps, (4) ( ) 500 ps, (5) ( ) 2.0 ns; (b) EB at (1) ( ) 0 ps, (2) ( ) 100 ps, (3) ( ) 250 ps, (4) ( ) 500 ps, (5) ( ) 2.0 ns. All spectra are normalized at the corresponding peak maxima].

5 718 INDIAN J CHEM, SEC A, MAY-JUNE 2010 Fig. 5 Decay of the spectral shift correlation function, C(t), of ANS ( ) and EB ( ) in [bmim][bf 4 ]. [In (a) the solid lines represent the biexponential fit to the data; (b) the solid lines represent the stretched-exponential fit to the data. R 2 denotes the correlation coefficient]. Probe nonpolar solvents such as hexane and cyclohexane, the spectral data of the system could not be collected in these solvents and hence, the missing component for this system could not be estimated. The time constant for the observable part of the solvation dynamics has been calculated from the peak frequencies at various times obtained from the lognormal fits to the TRES. A spectral shift correlation function, C(t), is defined in terms of the peak frequencies at various times as Eq. (3), C(t) = [ ( ) ν ( ) Table 1 Solvent relaxation parameters of the two probes in [bmim][bf 4 ] Decay parameters from biexponential fit a ν t ]/[ ν ( 0) ν ( ) ] (3) where ν (t), ν (0), and ν ( ) are the peak frequencies (in cm -1 ) at times, t, 0, and following laser excitation of the probe molecule. The calculated C(t) values at different times are then plotted against time and fitted to a biexponential function of the form, C(t) = α 1 exp(-t/τ 1 ) + α 2 exp(-t/τ 2 ), where τ 1 and τ 2 are the solvent relaxation time and α 1 and α 2 are the normalized pre-exponential factors. The C(t) versus time plots for both the probe molecules are shown in Fig. 5(a) along with the best biexponential fits to the data. The fitting parameters suggest the fits to be quite good. The average solvation Decay parameters from stretched exponential fit c τ 1 (ns) (a 1 ) τ 2 (ns) (a 2 ) τ av b (ns) β τ solv d (ns) time for ANS is estimated to be 0.78 ns with the short and long time constant as 0.28 ns and 1.45 ns, respectively. On the other hand, the average solvation time for EB is 1.63 ns with the short and long time constant of 0.51 ns and 2.55 ns respectively. In this context, we also examined the correlation of the Stokes shift data with time according to the stretched exponential function given below, as often followed by Maroncelli and his co-workers 31 to obtain the average solvation time Eq. (4), ν t) = ν ( ) + ν exp( ( t / τ ) ( 0 β C t) = exp( ( t / τ ) ) (4) ( 0 where ν = ν ( 0) ν ( ), and 0 < β 1, and the average time of solvation is given by Eq. (5), 1 τ0 1 τ solv = ν ( t) ν ( ) dt = Γ( β ) ν 0 β β Observed shift [ ν ( 0) ν ( ) ] (cm -1 ) ANS 0.28 (0.57) 1.45 (0.43) EB 0.51 (0.45) 2.55 (0.55) a Using C(t) = a 1 exp(-t/τ 1 ) + a 2 exp(-t/τ 2 ). b Average solvation time, τ av = a 1 τ 1 + a 2 τ 2, where, a 1 + a 2 = 1, experimental error ±5%. The numbers in the parenthesis indicate the weighted amplitude. c Using Eq. (4). d Using Eq. (5). ) (5) where Γ is the gamma function. Representative stretched exponential fits to our data according to this equation are shown in Fig. 5(b) and the average solvation times obtained from this analysis are given in Table 1. It is

6 KHARA & SAMANTA: SOLVATION DYNAMICS OF CHARGED SOLUTES IN IONIC LIQUIDS 719 clearly evident that the stretched exponential fits to the data are not satisfactory for both the probes. Hence, it can be concluded that the time-resolvable part of the solvation dynamics of both ANS and EB in [bmim][bf 4 ] is biphasic in nature. While some variation of the average solvation time with probe molecule is not uncommon, nearly 2-fold variation of the average solvation time, as observed in this case, with the two probes, is somewhat unexpected and perhaps deserves some attention. In a recent work, Paul and Samanta 25 have observed probe dependence of the solvation time due to specific hydrogen bonding interaction of the probe with one of the constituting ions of the ionic liquid. Since EB is well known for its hydrogen bonding interaction 48 and we have speculated that this interaction is possibly responsible for small REE of the system, the large difference of the solvation times obtained with the two probe molecules is perhaps due to the influence of the hydrogen bonding interaction. Conclusions We have studied the excitation wavelength dependent fluorescence behavior and solvation dynamics of two charged probe molecules, viz., anionic ANS and cationic EB, in [bmim][bf 4 ]. The anionic probe, ANS, exhibits much more pronounced red-edge effect than the cationic molecule, EB. The study of solvent relaxation subsequent to photoexcitation of the systems reveals a slower solvation dynamics for EB compared to ANS. Based on the literature on the photophysical behavior of the systems, it is speculated that this slowness of the solvation process is possibly due to hydrogen bonding interaction between the NH 2 groups of EB and the BF 4 - ion of the IL. Interestingly, no specific influence of the charged nature of the probe molecules could be detected from the solvation dynamics study. Acknowledgement This work was supported by the Ramanna Fellowship of the Department of Science and Technology, New Delhi. DCK thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, for a fellowship. References 1 Weingrätner H, Angew Chem Int Ed, 47 (2008) Tzschucke C C, Markert C, Bannwarth W, Roller S, Hebel A & Haag R, Angew Chem Int Ed, 41 (2002) Plaquevent J C, Levillain J, Guillen F, Malhiac C & Gaumont A C, Chem Rev, 108 (2008) Pinkert A, Marsh K N, Pang S & Staiger M P, Chem Rev, 109 (2009) Rantwijk F V & Sheldon R A, Chem Rev, 107 (2007) Endres F & Abedin S Z E, Phys Chem Chem Phys, 8 (2006) Martins M A P, Frizzo C P, Moreira D N, Zanatta N & Bonacorso H G, Chem Rev, 108 (2008) Greaves T L & Drummond C J, Chem Rev, 108 (2008) Welton T, Chem Rev, 99 (1999) Aki S N V K, Brennecke J F & Samanta A, Chem Commun, (2001) Bhattacharya B & Samanta A, J Phys Chem B, 112 (2008) Paul A & Samanta A, J Phys Chem B, 111 (2007) Sarkar S, Pramanik R, Seth D, Setua P & Sarkar N, Chem Phys Lett, 47 (2009) Wakai C, Oleinikova A, Ott M & Weingartner H, J Phys Chem B, 109 (2005) Znamenskiy V & Kobrak M N, J Phys Chem B, 108 (2004) Shim Y, Duan J, Choi M Y & Kim H J, J Chem Phys, 119 (2003) Marquis S, Ferrer B, Alvaro M, Garcıa H & Roth H D, J Phys Chem B, 110 (2006) Edward W, Castner J, Wishart J F & Shirota H, Acc Chem Res, 40 (2007) Reichardt C, Green chem, 7 (2005) Karmakar R & Samanta A, J Phys Chem A, 106 (2002) Karmakar R & Samanta A, J Phys Chem A, 106 (2002) Karmakar R & Samanta A, J Phys Chem A, 107 (2003) Mandal P K, Paul A & Samanta A, J Photochem Photobiol A: Chem, 182 (2006) Mandal P K, Saha S, Karmakar R & Samanta A, Curr Sci, 90 (2006) Paul A & Samanta A, J Phys Chem B, 111 (2007) Paul A & Samanta A, J Phys Chem B, 112 (2008) Samanta A, J Phys Chem B, 110 (2006) Chakrabarty D, Chakraborty A, Seth D, Hazra P & Sarkar N, Chem Phys Lett, 397 (2004) Chakrabarty D, Chakraborty A, Seth D & Sarkar N, J Phys Chem A, 109 (2005) Chakrabarty D, Hazra P, Chakraborty A, Seth D & Sarkar N, Chem Phys Lett, 381 (2003) Ingram J A, Moog R S, Ito N, Biswas R & Maroncelli M, J Phys Chem B, 107 (2003) Ito N, Arzhantsev S, Heitz M & Maroncelli M, J Phys Chem B, 108 (2004) Ito N, Arzhantsev S & Maroncelli M, Chem Phys Lett, 396 (2004) Headley L S, Mukherjee P, Anderson J L, Ding R, Halder M, Armstrong D W, Song X & Petrich J W, J Phys Chem A, 110 (2006) Mukherjee P, Crank J A, Sharma P S, Wijeratne A B, Adhikary R, Bose S, Armstrong D W & Petrich J W, J Phys Chem B, 112 (2008) Adhikari A, Sahu K, Dey S, Ghosh S, Mandal U & Bhattacharyya K, J Phys Chem B, 111 (2007) Funston A M, Fadeeva T A, Wishart J F & Castner Jr E W, J Phys Chem B, 111 (2007) Lang B, Angulo G & Vauthey E, J Phys Chem A, 110 (2006) 7028.

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