J. Phys. Chem. B 2007, 111, 4781-4787
4781
Dynamics of Solvent and Rotational Relaxation of Coumarin-153 in Room-Temperature Ionic Liquid 1-Butyl-3-methyl Imidazolium Tetrafluoroborate Confined in Poly(oxyethylene glycol) Ethers Containing Micelles† Debabrata Seth, Anjan Chakraborty, Palash Setua, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, WB, India ReceiVed: October 30, 2006; In Final Form: March 23, 2007
We have investigated solvent and rotational relaxation of coumarin 153 (C-153) in room-temperature ionic liquid (RTILs) 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4]) and the ionic liquid confined in alkyl poly(oxyethylene glycol) ethers containing micelles. We have used octaethylene glycol monotetradecyl ether (C14E8) and octaethylene glycol monododecyl ether (C12E8) as surfactants. In the [bmim][BF4]-C14E8 micelle, we have observed only a 22% increase in solvation time compared to neat [bmim][BF4], whereas in the [bmim][BF4]-C12E8 system, we have observed ∼57% increase in average solvation time due to micelle formation. However, the slowing down in solvation time on going from neat RTIL to RTIL-confined micelles is much smaller compared to that on going from water to water confined micellar aggregates. The 22-57% increase in solvation time is attributed to the slowing down of collective motions of cations and anions in micelles. The rotational relaxation times become faster in both the micelles compare to neat [bmim][BF4].
1. Introduction Room-temperature ionic liquids (RTILs), a class of neoteric solvents, have been used as a “green substitute” for volatile organic solvents1-3 and large scale industrial applications3-4 due to their negligible vapor pressure, wide electrochemical windows, and wide liquefying temperature range (-96 to ∼300 °C). They can be extensively used for organic chemical reactions5 and have some electrochemical applications.6 RTILs also have applications in analytical chemistry.7 RTILs are not always green.8a Recently, Earle et al.8b showed that RTILs can be distilled at high temperature and low pressure. Moreover, halide containing RTILs are very unstable to moisture and hydrolyzed to produce volatile, harmful, corrosive HF, POF3, etc. Baker et al.8c showed that ionic liquid anions can be hydrolyzed. Several physical, photophysical, and ultrafast spectroscopic studies were investigated in these RTILs.9-16 Aki et al.9a determined the polarity of the imidazolium- and pyridinium-based RTILs using UV-vis absorption and fluorescence spectroscopy. Muldoon et al.9b determined the polarity of the RTILs using solvatochromic probes. There were many theoretical and experimental studies of solvation dynamics in neat RTILs.10-12 Solvation dynamics in RTILs-polar solvent mixture were also investigated.13,14 Recently Samanta et al.15a observed the heterogeneity in RTILs, and Margulis et al.15b also observed the red edge effect in RTILs using molecular dynamics simulation. Due to unique features of RTILs, ionic liquids containing micelles and microemulsions have drawn attention in recent years. Several groups prepared and characterized RTILs containing micelles and microemulsions.17-19 Recently Patrascu et al.19 prepared and characterized RTILs and alkyl poly(oxyethylene glycol) ethers containing micelles by surface tension, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) measurements. †
Part of the special issue “Physical Chemistry of Ionic Liquids”. * To whom correspondence should be addressed. E-mail: nilmoni@ chem.iitkgp.ernet.in. Fax: 91-3222-255303.
SCHEME 1: Structure of C12E8, C14E8, C-153, [bmim][BF4]
In this article, we have investigated solvent and rotational relaxation dynamics of 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4]) in [bmim][BF4]-C14E8, [bmim][BF4]C12E8 micelles using C-153 as a probe molecule. Recently, we have studied solvent relaxation of RTILs in RTILs containing micelles and microemulsions.20 The aim of this work is to choose two surfactants of different chain length and to verify whether there is any chain length dependence on solvation dynamics in RTILs containing micelles. The structures of C-153, [bmim][BF4], and alkyl poly(oxyethylene glycol) ethers are shown in Scheme 1. 2. Experimental Section Coumarin 153 (C-153) (laser grade, Exciton) was used as received. 1-Butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4]) was obtained from Acros chemicals (98% purity) and purified using the literature procedure.10c The RTIL was dried in vaccum for ∼24 h at 70-80 °C before use. We used
10.1021/jp067122j CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007
4782 J. Phys. Chem. B, Vol. 111, No. 18, 2007 two alkyl poly(oxyethylene glycol) ethers, namely octaethylene glycol monotetradecyl ether (C14E8) and octaethylene glycol monododecyl ether (C12E8). C14E8 and C12E8 were purchased from Sigma and used as received. The values of the CMCs of C14E8 and C12E8 micelles in [bmim][BF4] are 27 and 93 mM, respectively.19 For all experiments, the concentration of C-153 was kept at ∼1 × 10-5 M. C-153 is initially dissolved in methanol and transfered to a vial. The RTIL was added to the vial under nitrogen atmosphere in a glovebox and stirred for 10-20 min after removing the methanol under vaccum. Then the solutions were transferred to a quartz cuvette in a glovebox and sealed with septum and Parafilm. Then requisite amounts of surfactants were added to the cuvette containing RTIL, mixed thoroughly, and allowed to equilibrate for 6 h. The absorption and fluorescence spectra were measured using a Shimadzu (model no:UV-1601) spectrophotometer and a SpexFluorolog-3 (model no. FL3-11) spectrofluorimeter. The fluorescence spectra were corrected for the spectral sensitivity of the instrument. For steady-state experiments, all samples were excited at 408 nm. The time-resolved fluorescence spectroscopy setup was described in detail in our earlier publication.21 Briefly, the samples were excited at 408 nm using a picosecond diode laser (IBH), and the signals were collected at magic angle (54.7 °) polarization using a Hamamatsu microchannel plate photomultiplier tube (3809U). The instrument response function of our setup is ∼90 ps. The same setup was used for anisotropy measurements. For the anisotropy decays, we used a motorized polarizer in the emission side. The emission intensities at parallel (I|) and perpendicular (I⊥) polarizations were collected alternatively until a certain peak difference between parallel (I|) and perpendicular (I⊥) decays was reached. The peak differences depended on the tail matching of the parallel (I|) and perpendicular (I⊥) decays. The analysis of the time-resolved data was done using IBH DAS, version 6, decay analysis software. The same software was also used to analyze the anisotropy data. All experiments were carried out at 298 K. The temperature was maintained as a constant (298 K) by circulating water through the cell holder using a Neslab Thermostat (RTE7).
Seth et al.
Figure 1. (a) Absorption spectra of C-153 in (i) neat [bmim][BF4] (dotted line), (ii) [bmim][BF4]-C14E8 micelle at four times cmc (solid line), and (iii) [bmim][BF4]-C12E8 micelle (dash line) at four times the CMC. (b) Emission spectra of C-153 in (i) neat [bmim][BF4], (ii) [bmim][BF4]-C14E8 micelle at four times cmc (iii) [bmim][BF4]-C12E8 micelle at four times the CMC.
3. Results 3.1. Steady-State Studies. The absorption spectra of C-153 in neat [bmim][BF4] and micelles are shown in Figure 1a. These absorption spectra were constructed by subtracting the absorption spectra of neat [bmim][BF4] from the absorption spectra of respective C-153 containing solutions. The absorption spectra of neat [bmim][BF4] and C-153 doped [bmim][BF4] are shown in the Supporting Information (Figure SI 1). The emission peak of C-153 in neat [bmim][BF4] is at 528 nm. On addition of C14E8 and C12E8 to [bmim][BF4], the emission peak has been blue-shifted to 525 and 521 nm with an increase in emission intensity (Figure 1b). Thus, the microenvironment experienced by C-153 in the micelles is different from neat RTILs, and the polarity experienced by C-153 is less than neat RTILs in these RTILs containing micelles compared to neat RTILs. 3.2. Time-Resolved Studies. 3.2.1. SolVation Dynamics. In all RTILs containing micelles, we have observed a dynamic Stokes’ shift in the emission spectra of C-153. In all micelles, the fluorescence transient of C-153 is markedly dependent on the emission wavelength. At the red edge of the emission spectra, we have observed a decay profile consisting of a clear rise followed by usual decay, and at the short wavelength, a fast decay is observed (Figure 2). All decay profiles were best fitted by a triexponential function. The time-resolved emission spectrum (TRES) has been constructed following the procedure
Figure 2. Fluorescence decay of C-153 in [bmim][BF4]-C12E8 micelle at four times the CMC at (i) instrument response function, (ii) 470, (iii) 540, and (iv) 630 nm.
of Fleming and Maroncelli22 and described in our earlier publication.23 Each time-resolved emission spectrum was fitted by a log-normal function to extract the peak frequencies. These peak frequencies were then used to construct the decay of solvation correlation function (C(t)), which is defined as
C(t) )
ν(t) - ν(∞) ν(0) - ν(∞)
(1)
where ν(0), ν(t), and ν(∞) are the peak frequencies at time zero, t, and infinity, respectively. The representative TRES of C-153 in [bmim][BF4]-C14E8 micelle is shown in Figure 3. The decay of C(t) was fitted by biexponential function (Figure 4)
C(t) ) a1 e-t/τ1 + a2 e-t/τ2
(2)
where τ1 and τ2 are the two solvation times with amplitudes of
Dynamics of Coumarin-153
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4783 3.2.2. Time-ResolVed Fluorescence Anisotropy Studies. Timeresolved fluorescence anisotropy (r(t)) is calculated using the following equation:
r(t) )
Figure 3. Time-resolved emission spectra (TRES) of C-153 in [bmim][BF4]-C14E8 micelle at four times the CMC at (i) 0 (9), (ii) 200 (O), (iii) 1000 (2), and (iv) 4000 (3) ps.
I|(t) - GI⊥(t) I|(t) + 2GI⊥(t)
(3)
where G is the correction factor for detector sensitivity to the polarization direction of the emission and I|(t) and I⊥(t) are the fluorescence decays polarized parallel and perpendicular to the polarization of the excitation light, respectively. The G factor for our setup is 0.6. The rotational relaxation time of C-153 in all micelles is fitted to a biexponential function. In all micelles, the rotational relaxation time of C-153 decreases compared to neat RTILs (Figure 5). The anisotropy decay parameters are listed in Table 2. 4. Discussion
Figure 4. (a) Decay of the solvation correlation function (C(t)) (normalized) of C-153 in (i) [bmim][BF4] (1), (ii) [bmim][BF4]-C14E8 micelle at four times the CMC (O), and (iii) [bmim][BF4]-C12E8 micelle at four times the CMC (9). In the inset, ln(C(t)) vs time plot for C-153 in [bmim][BF4] (1), [bmim][BF4]-C14E8 micelle at four times the CMC (O), and [bmim][BF4]-C12E8 micelle at four times the CMC (9). (b) Decay of the solvation correlation function (C(t)) (excluding the missing component) of C-153 in (i) [bmim][BF4] (1), (ii) [bmim][BF4]-C14E8 micelle at four times the CMC (O), and (iii) [bmim][BF4]-C12E8 micelle at four times the CMC (9).
a1 and a2, respectively. Maroncelli et al.11 used stretched exponential function to fit the decay of C(t), but in our case, the best fitting was observed by using a biexponential function (eq 2). We had observed earlier that stretched exponential fitting is not good for fitting the solvent correlation function in RTIL containing micelles.20b,c Samanta et al.10 also reported similar observation. The average solvation time of C-153 in [bmim][BF4] is 510 ps with time constant 200 ps (71%) and 1280 ps (29%). The solvation time in [bmim][BF4] is very close to the value reported by Chowdhury et al.11f The average solvation times of C-153 in C14E8-[bmim][BF4] and C12E8-[bmim][BF4] micelle at four times of the CMC are respectively 620 and 800 ps. The decay parameters of C(t) are summarized in Table 1.
It is revealed from steady-state results that C-153 is located in a position in the micelle where polarity sensed by the C-153 molecule is less than the bulk [bmim][BF4], and probably its position in the micelle is in the interface of RTILs and surfactant. We have observed the average solvation time of C-153 in neat [bmim][BF4] is 0.510 ns at 298 K with components of 0.200 ns (71%) and 1.28 ns (29%). Chowdhury et al.11f observed that the average solvation time of C-153 in [bmim][BF4] is 0.460 ns at 293 K with components of 60 ps (62%) and 1.1 ns (38%). Samanta et al.10a,c reported that the average solvation time of C-153 in [bmim][BF4] is 2.13 ns. Thus, the solvation times reported by various groups are different. This is due to the fact that the qualities of the RTILs used by different groups are different and the presence of a small amount of impurities such as water and chloride ion vastly changes the viscosity of the RTILs. Recently, Maroncelli et al.11d reported that different results reported by different groups are due to different methods/ setups used by different groups to represent the data. With addition of surfactant in [bmim][BF4], the average solvation time increases to a small extent. The average solvation time of C-153 in [bmim][BF4]-C14E8 micelles at four times of the CMC is 0.620 ns with components 0.220 ns (72%) and 1.65 ns (28%); that is, we have observed only a 22% increase in solvation time with formation of micelles in [bmim][BF4]. In the case of [bmim][BF4]-C12E8 micelles at four times of the CMC, the average solvation time of C-153 is 0.800 ns with components of 0.270 ns (76%) and 2.47 ns (24%); that is, we have observed only an ∼57% increase in the average solvation time due to micelle formation. In this context, it is desirable to make comparison between the changes in solvation time on going from pure water to water containing micelles and that from pure RTIL to RTIL containing micelles. Before any further discussion, let us summarize the features of solvation dynamics in water containing micelles and pure water.24-25 It is reported by Jimenez et al.25a that solvent relaxation of C-343 in water consists of an initial decay of 55 fs (50%), attributed to the librational motion. Solvent relaxation of C-102 in water is bimodal with time constants of
