J. Phys. Chem. 1996, 100, 10523-10527
10523
Solvation Dynamics of Coumarin 480 in Reverse Micelles. Slow Relaxation of Water Molecules Nilmoni Sarkar, Kaustuv Das, Anindya Datta, Swati Das, and Kankan Bhattacharyya* Department of Physical Chemistry, Indian Association for the CultiVation of Science, JadaVpur, Calcutta 700 032, India ReceiVed: December 11, 1995; In Final Form: March 15, 1996X
The picosecond time-resolved Stokes shift of the laser dye coumarin 480 (I) is studied in Aerosol OT-heptane reverse micelles at various concentrations of water. On addition of water to a n-heptane solution of I containing 0.09 M AOT, the emission spectrum of I exhibits a prominent shoulder at 480 nm which is ascribed to the dye molecules in the water pool of the reverse micelles. In n-heptane, the fluorescence decays of I at short and long wavelengths are identical, which rules out any time-resolved Stokes shift. On addition of water to the AOT/heptane system, the decay at long wavelengths exhibit a distinct growth on the nanosecond time scale. This and the time-dependent Stokes shift indicate that the water molecules in the water pool of the AOT reverse micelles undergo slow relaxation on the nanosecond time scale.
1. Introduction
CHART 1: Coumarin 480, I
Recent progress in the field of solvation dynamics in aqueous solutions has revealed many interesting features.1-9 Because of the obvious importance of the water molecules in many natural processes, there have been numerous studies on the relaxation behavior of water molecules under ambient and special conditions.10 At room temperature the dielectric relaxation of water is very fast. Fleming et al. observed that in aqueous solution the solvation dynamics of coumarin 343 is described by three components of 126 fs, 880 fs, and 38 ps,2a while for coumarin 480 (I, Chart 1), the solvation time is less than 310 fs.9 In the presence of γ-cyclodextrin, however, the solvation process of the coumarin dyes in aqueous solutions is slowed by several orders of magnitude.9 Bagchi et al. have attributed the retardation of the solvation process in aqueous cyclodextrin solution to the freezing of the translational modes of the water molecules inside the cyclodextrin cavity.5d That the dielectric relaxation time of the water molecules bound to biological systems is significantly slower than that of the free water molecules has earlier been demonstrated in the dielectric relaxation studies of living tissues and various other biological materials in the frequency range 107-1010 Hz by Mashimo et al.11 They observed that these systems exhibit broadly two relaxation times, one around 20 GHz (corresponding to a few picoseconds) and the other around 100 MHz (corresponding to 10 ns). The faster relaxation has been ascribed to the free water molecules, and the slower component, to the bound water molecules. The highly structured water molecules in the reverse micelles or the water-in-oil microemulsions are interesting models of the water molecules present in biological systems.12-28 The reverse micelles are basically nanometer-sized water droplets surrounded by a layer of surfactant molecules (such as AOT) dispersed in a nonpolar or weakly polar organic solvent.12 In such aggregates the polar head groups of the surfactant molecules point inward, toward the water pool, and the alkyl chains point outward, toward the bulk organic solvent. Apart from liquid solutions at room temperature the reverse micelles have recently been studied in supercritical ethane20 and in ordinary solvents at subzero temperatures.25 The size of the water pool increases X
Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)03658-6 CCC: $12.00
as the ratio of the number of water molecules to that of the surfactant, w0, increases, and for AOT in n-heptane, the radius of the water pool (rw, in Å) is close to 2w0.15b At low w0, the water molecules in the pool remain very close to and are very strongly attracted by the polar head groups of the surfactants and hence their mobility is very low. Recent MD simulation and NMR studies indicate that at w0 ) 2 the mobility of the water molecules in the water pool is slower than that in pure water by at least 2 orders of magnitude.25-26 At large w0, the size of the water pool increases, and in such large water pools, the mobility of the water molecules is relatively high particularly for those near the central region of the water pool. Several workers have estimated the effective dielectric constant of the AOT reverse micelles using a fluorescence probe.22-23 Though using fluorescence several ultrafast processes, namely quenching, rotational relaxation, and proton transfer processes, have been studied in reverse micelles,16-29 there has been little study on the time-dependent Stokes shift in reverse micelles. Since only the latter gives direct information about the relaxation time of the solvent water molecules, we have decided to study the time-dependent Stokes shift of coumarin 480 in reverse micelles to understand how the structured water molecules with reduced mobility relax around an instantaneously created dipole inside the water pool in reverse micelles. 2. Experimental Section The single photon counting setup and the laser system are described elsewhere.30 AOT (dioctyl sulfosuccinate, sodium salt, Sigma) was purified by standard procedures24 and dried in vacuo for 10-12 h before use. n-Heptane (Aldrich) was freshly distilled. Coumarin 480 (Exciton) was used as received. The solutions were prepared following the literature procedure.19 Briefly, to a stock solution of I in 0.09 M AOT in n-heptane was added a requisite amount of water using a microliter syringe © 1996 American Chemical Society
10524 J. Phys. Chem., Vol. 100, No. 25, 1996
Figure 1. Emission spectra of 5 × 10-5 M coumarin 480 in n-heptane, λex ) 310 nm: (a) 0 M AOT; (b-e) 0.09 M AOT; (b) w0 ) 0; (c) w0 ) 4; (d) w0 ) 8; (e) w0 ) 32.
and the mixture was sonicated. The concentration of the dye was 5 × 10-5 M, and the wavelength of excitation for steady state and time-resolved emission was 310 nm. The fluorescence decays were recorded at intervals of 15 nm using a Corning 3-74 filter (to cut off scattered light) and a slitwidth less than 5 nm and were fitted to single-exponential or biexponential decays using global lifetime analysis software (PTI). The chi squares of the fits were in the range 1-1.2, and the DurbinWatson parameters, in the range 1.75-2. Reconstruction of the time-resolved spectra was done by following the procedure described by Maroncelli and Fleming.2c Briefly, the fluorescence intensities were normalized using the steady state emission spectrum. Then using the parameters of the best fit to the fluorescence decays time-resolved spectra at different time t, were generated by fitting the data to a log-normal function to get the peak frequency ν(t). The peak frequency at infinite time ν(∞) refers to the peak frequency at very long time when the spectrum no longer exhibits any time-dependent Stokes shift. All of the measurements were done at room temperature (296 K). 3. Results and Discussion 3.1. Steady State Spectra. In n-heptane, coumarin 480 exhibits an intense emission maximum at around 410 nm (Figure 1) with the absorption and excitation maxima at 360 nm (Figure 2a). The excitation spectrum in n-heptane is independent of the wavelength of emission. On addition of 0.09 M AOT, the intensity of the main emission peak decreases slightly (Figure 1) while the absorption spectrum exhibits a very slight increase in absorbance at long wavelengths (370-420 nm) (Figure 2b). On addition of water to the n-heptane/AOT solution, the absorption, emission, and excitation spectra, however, change considerably. On addition of water the emission spectrum exhibits a shoulder at around 480 nm. The shoulder becomes more and more prominent up to a water to surfactant ratio, w0 ≈ 10. For w0 >10 the emission spectrum (Figure 1) remains more or less the same. As shown in Figure 2b the excitation spectrum of I monitored at 480 nm is markedly red shifted relative to that monitored at 410 nm. The excitation spectrum monitored at 410 nm exhibits a peak at around 360 nm which is close to the absorption maximum and exhibits very little intensity above 390 nm. Evidently the excitation spectrum of I in the AOT/heptane/water system at 410 nm is almost identical
Sarkar et al.
Figure 2. (a) Coumarin 480 in n-heptane: (i) absorption spectrum. Excitation spectra monitored at 410 nm (ii) and at 480 nm (iii). (b) Coumarin 480 in n-heptane + 0.09 M AOT, w0 ) 32: (i) absorption spectrum; (ii and iii) excitation spectra monitored at 410 and 480 nm respectively; (iv) absorption spectrum in 0.09 M AOT, w0 ) 0.
with that observed for I in n-heptane in the absence of AOT or water (Figure 2a). The excitation spectrum of I in 0.09 M AOT at w0 ) 32 monitored at 480 nm, however, exhibits a peak at 390 nm. In homogeneous solvents coumarin 480 (I) exhibits a marked red shift of the absorption and the emission spectra on going from hydrocarbon solvents to aqueous medium. The absorption maximum of I shifts from 361 nm in hydrocarbon solvents to 396 nm in water while the emission maximum shifts from 407 to 489 nm.29 The marked difference in the excitation spectra in AOT/n-heptane/water reverse micelles monitored at the red (480 nm) and the blue end (410 nm) indicates that in the reverse micelle there are broadly two kinds of molecules. The first kind of molecules which absorb at around 360 nm and emit at 410 nm, are very similar to the coumarin molecules in n-heptane, and hence are assigned to the dye molecules in the bulk heptane. Molecules of the second kind absorb and emit at much longer wavelengths and obviously experience a much polar and protic environment. We ascribe the second set of the dye molecules to those in the water pool of the reverse micelles. The emission maxima of these molecules are around 480 nm and the excitation maximum at 390 nm, which is close to the reported emission and excitation maxima of coumarin 480 in alcohol,29 and are shorter than the corresponding values in water. Thus the steady state spectra indicate that the microenvironment of the coumarin 480 molecules in the water pool of the reverse micelles is less polar than water and is similar to alcohol. It may be mentioned that Guha Ray and Sengupta29 have also reported a polarity of the reverse micelles close to that of alcohol and Durocher et al.22 have proposed that the effective dielectric constant of 0.09 M AOT in n-heptane at w0 ) 32 is significantly lower than that of ordinary water. 3.2. Time-Resolved Studies. (a) Fluorescence Decays in AOT/n-Heptane/Water. In n-heptane, the fluorescence decays of I are found to be independent of wavelength, the lifetime being 2.6 ns (Figure 3a). This indicates that I exhibits no timedependent Stokes shift in n-heptane. In the presence of 0.09 M AOT in n-heptane, the decay at the red end (490 nm) is longer (τf ) 4.9 ns) than that at the blue end (400 nm, τf ) 2.6 ns). The increase in lifetime on addition of AOT may be attributed to the traces of water invariably present27 in AOT because the lifetime of I increases from 2.6 ns in cyclohexane to 5.9 ns in water.29
Solvation Dynamics of Coumarin 480 in Reverse Micelles
J. Phys. Chem., Vol. 100, No. 25, 1996 10525
Figure 3. Fluorescence decays of coumarin 480 in n-heptane: (a) without AOT, w0 ) 0, at (i) 400 and (ii) 470 nm; (b) in 0.09 M AOT, w0 ) 0, at (i) 400 and (ii) 490 nm; (c) in 0.09 M AOT, w0 ) 4, at (i) 400, (ii) 460, and (iii) 505 nm.
On addition of water in 0.09 M AOT in n-heptane more significant differences have been observed between the decay at the red end and the blue end of the emission spectrum (Figure 3c). The decay at the red end exhibits a distinct growth on the nanosecond time scale. For instance, at w0 ) 4, the decay at 505 nm is fitted to a biexponential with a growth component of 1.1 ns and a decay component of 7 ns while the decay at 400 nm is single exponential with a 2.6 ns lifetime (Figure 3c). This suggests that the energy of the guest dipole decreases with time due to solvation so that the solvated species emitting at the longer wavelength is produced in the nanosecond time scale.
(b) Dynamic Stokes Shift in AOT/n-Heptane/Water Microemulsion. From the decays recorded at different wavelengths the time-resolved emission spectra of coumarin 480 in 0.09 M AOT in n-heptane are constructed at different times at various water content. The time-resolved emission spectra (Figure 4) indicate that at short time the emission spectrum resembles that in n-heptane with a peak at 410 nm (Figure 4a) and is obviously dominated by those dye molecules in the bulk n-heptane. At long times (t ) 15 ns), the spectrum consists of one peak at 480 nm which is obviously due to the dye molecules in the relatively polar water pool (Figure 4c). At intermediate times
10526 J. Phys. Chem., Vol. 100, No. 25, 1996
Sarkar et al.
Figure 6. Decays of the response function, C(t), of coumarin 480 in n-heptane + 0.09 M AOT: (a) w0 ) 4 and (b) w0 ) 32. The points denote the actual values of C(t), and the solid line denotes the best fit to a single-exponential decay (a) and biexponential decay (b).
TABLE 1: Solvation Time and Frequencies of Coumarin 480 (I) in 0.09 M AOT in n-Heptane
Figure 4. Time-resolved emission spectra of coumarin 480 in 0.09 M AOT, w0 ) 32, at (a) 0 (O), (b) 5000 (b), and (c) 15 000 (]) ps.
Figure 5. Heptane-subtracted time-resolved emission spectra of coumarin 480 in 0.09 M AOT, w0 ) 32, at (a) 0, (b) 500, (c) 1500, (d) 5000, and (e) 15 000 ps.
the spectra consist of two peaks corresponding to the two sets of molecules (Figure 4b). Obviously the dye molecules in bulk n-heptane do not exhibit a time-resolved Stokes shift. To extract the rate constant of solvation of the dye molecule in the water pool from the timeresolved spectra, the contribution of the dye molecules in bulk n-heptane is subtracted using the decay observed in n-heptane and using the steady state emission intensities in the water/AOT/ heptane system. The time-resolved spectra so obtained exhibit a time-dependent Stokes shift (Figure 5). In order to extract the time constant of the solvation dynamics within the AOT reverse micelles, the reconstructed response function C(t) defined as
C(t) )
ν(t) - ν(∞) ν(0) - ν(∞)
w0
ν(0) (cm-1)
ν(∞) (cm-1)
a1
τ1 (ns)
a2
τ2 (ns)
4 32
22 350 22 130
20 135 20 190
1.0 0.50
8.0 1.7
0.50
12.0
was calculated using the peak frequencies of the time-resolved emission spectra, where ν(∞), ν(t), and ν(0) refer to the peak frequencies at times infinity, t, and zero. The decay characteristics of C(t) at w0 ) 4 and 32 are summarized in Table 1. The variation of C(t) as a function of time is shown in Figure 6. The main species causing solvation of the instantaneously created dipole, in this case, are the water molecules present in the water pool of the reverse micelle. At low water content (w0 ) 4) the decay of C(t) is single exponential with a 8.0 ns time constant (Table 1). At high water content (w0 ) 32) the decay of C(t) is distinctly much faster and contains a component of 1.7 ns and another of 12 ns (Figure 6 and Table 1). The decrease in the relaxation time from 8 ns at w0 ) 4 (rw ) 8 Å) to 1.7 ns in the bigger water pool at w0 ) 32 (rw ) 64 Å) indicates that with a rise in the water content the mobility of the water molecules in the water pool increases. The slower component at w0 ) 32 may be attributed to those water molecules near the ionic head group of the surfactant and the faster component to the water molecules in the central region of the water pool. However, even the faster component of 1.7 ns at w0 ) 32 is still significantly slower than the subpicosecond solvation time observed for I by Fleming et al.2 in water. This clearly suggests that all the water molecules in the reverse micelles are much slower than the water molecules in ordinary bulk water. Very recently using phase fluorimetry Bright et al.28 reported that, for a biological fluorophore in a reverse micelle, the relaxation times are 6 ns at w0 ) 2.8 and 2 ns for w0 ) 13.9, which are close to the values obtained in the present study. In the continuum picture the observed solvation time should be equal to the longitudinal relaxation time τL, given by
τL ) (∞/0)τD where ∞ and 0 are respectively the dielectric constant at infinite and zero (static) frequency, and τD, the dielectric relaxation time. To a first approximation, one can assume that the dielectric relaxation time, τD for the water pool of the reverse micelle is same (i.e., 10 ns) as the dielectric relaxation time of the water
Solvation Dynamics of Coumarin 480 in Reverse Micelles molecules bound to biological systems as determined by Mashimo et al.11 The observed solvation time 8 ns, then, corresponds to ∞/0 ) 0.8 at w0 ) 4. However, since the dielectric relaxation times and the frequency dependence of the dielectric constant of the water molecules present in the water pool of the reverse micelles are not known, it is difficult to justify or extend such an analysis any further. One can only say that the observed acceleration of the rate of decay of C(t) with increase in w0 supports the general view that the size and the dielectric constant of the water pool and the mobility of the water molecules in the reverse micelles increase with w0.12,19,22,24 In the present study done at room temperature and using a single photon counting (SPC) apparatus of resolution ∼50 ps, we might be missing a portion of the initial response ocurring in the time scale of e20 ps as discussed by Fee and Maroncelli.6d It is, however, not possible to apply the method prescribed by Fee and Maroncelli6d to the present problem due to lack of knowledge about the shape of the absorption spectrum of the probe solute (I) in the polar solvent (i.e., the water pool in this case). As shown in Figure 2b the absorption spectrum of the few probe molecules in the water pool appears as a weak tail against the very strong peak for the probe molecules in the bulk heptane medium. The differences in the intensities of the two kinds of spectra are so great that it is not possible to extract the absorption spectrum in the water pool with any great certainity. Nevertheless, the growth observed on the nanosecond time scale for the fluorescence decays at the long wavelengths unequivocally indicates the presence of a component of solvation in the nanosecond time scale. In summary, we conclude that in the water pool of the reverse micelles the relaxation of the water molecules definitely contains a component in the nanosecond time scale but with the limited time resolution of our setup we cannot rule out the further presence of a very fast component on the time scale of e20 ps. 4. Conclusion The present work shows that coumarin 480 exhibits a timedependent Stokes shift in the water pool of reverse micelles. The solvation times observed in reverse micelles are significantly slower than that observed in ordinary aqueous solutions. This indicates that the relaxation of the highly structured water molecules inside the water pool of the reverse micelle is several orders of magnitude slower than that in ordinary water. It is observed that the decay becomes faster with an increase in w0, which suggests an increase in the mobility of the water molecules in the water pool with an increase in w0. Acknowledgment. Thanks are due to SERC, Department of Science and Technology, and Council of Scientific and Industrial Research (CSIR), Government of India, for genereous reserach grants. N.S., K.D., S.D., and A.D. thank CSIR for providing fellowships. K.B. thanks Professor B. Bagchi for stimulating and helpful discussions and Dr. D. Nath for his help in instrumentation. References and Notes (1) Rossky, P. J.; Simon, J. D. Nature 1994, 370, 263. (2) (a) Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. Nature 1994, 369, 471. (b) Maroncelli, M.; MacInnis, J.; Fleming, G. R.
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