J. Phys. Chem. 1996, 100, 15483-15486
15483
Solvation Dynamics of Coumarin 480 in Micelles Nilmoni Sarkar, Anindya Datta, Swati Das, and Kankan Bhattacharyya* Department of Physical Chemistry, Indian Association for the CultiVation of Science, JadaVpur, Calcutta 700 032, India ReceiVed: February 29, 1996X
The picosecond time-resolved Stokes shift of the laser dye, coumarin 480 (I) is studied in neutral (Triton X-100, TX), cationic (cetyltrimethylammonium bromide, CTAB), and anionic (sodium dodecyl sulfate, SDS) micelles. Above critical micellar concentration (cmc) for all three micelles I exhibits wavelength dependent fluorescence decays and a distinct growth at the long wavelengths. The time dependent Stokes shift studies indicate that the water molecules in the Stern layer of the micelles relax on the time scale 180-550 ps, which is slower than the subpicosecond relaxation dynamics observed in ordinary bulk water.
1. Introduction
SCHEME 1. (a, top) Coumarin 480, I. (b, bottom) Structure of micelles.
Recent success in elucidating the early events of the solvation dynamics in homogeneous solutions has encouraged many researchers to explore microheterogeneous organized assemblies.1-10 Since the most important biological processes occur in organized and restricted environments such as membrane interfaces and hydrophobic pockets of proteins, there is considerable interest in understanding how the organized assemblies affect the ultrafast processes. Very recently several groups have addressed this question. Fleming et al. studied the time dependent Stokes shift of coumarin 480 (I, Scheme 1a) in γ-cyclodextrin (γ-CDx) and observed that the solvation dynamics of the laser dye, I, is significantly slowed down inside the hydrophobic cavity of γ-CDx.2d They showed that while in ordinary bulk water the solvation occurs in the sub-picosecond time scale (310 fs), in γ-CDx the solvation dynamics exhibits a component in the nanosecond time scale. Robinson et al. observed intramolecular electron transfer on the nanosecond time scale for anilinonaphthalene sulfonate in reverse micelles.8b Nanosecond solvation dynamics in reverse micelle is also reported by Bright et al. using phase fluorimetry for the solvation of a biological fluorophore covalently attached to albumin.9 The time-resolved emission studies in organized media are consistent with the recent dielectric relaxation and NMR studies.12 In these studies12 it is found that while the free water molecules in ordinary water relax on the time scale of a few picosecond, the water molecules bound to living tissues and other biological materials exhibit broadly two relaxation times, one around 10 ps and the other around 10 ns. More recently Bagchi et al. have ascribed 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.3d,e Apart from solvation, the rate of another interesting ultrafast process, twisted intramolecular charge transfer, is also slowed down considerably in different organized media.11 Inspired by these studies, we have recently initiated a systematic investigation on the solvation dynamics of coumarin 480 (I) in several organized assemblies. We have shown that in the solid host, zeolite 13X, solvation of I occurs on the nanosecond time scale.10a We also found that in the water pool of a reverse micelle the water molecules relax on the nanosecond time scale and the rate of relaxation increases as the water content increases.10b In the present work we wish to report on how the X
Abstract published in AdVance ACS Abstracts, August 15, 1996.
S0022-3654(96)00630-2 CCC: $12.00
solvation dynamics is affected when the probe solute, I, is transferred from the bulk water to the micellar aggregates. It is well-known that the surfactants (TX, CTAB, and SDS) form nearly spherical micellar aggregates when their concentrations exceed a certain critical value known as the critical micellar concentration (cmc). Such aggregates contain 100-150 surfactant molecules each and have a radius on the order of around 50 Å for TX and CTAB and 30 Å for SDS. Recent small angle x-ray, neutron scattering, NMR, and fluorescence studies have revealed detailed information about the structure of these micelles (Scheme 1b).13-15 According to these studies the core of the micelles is essentially “dry” and contains the hydrocarbon chains only. At the periphery of the micelles there is a “wet” shell of thickness 6-9 Å comprising the polar head group, the counterion (in the case of ionic micelles), and a considerable amount of water. This shell, known as the Stern layer, is quite polar although it is obviously not as polar as bulk water. In the micellar system the probe solute experiences broadly three different environments, the highly polar bulk aqueous phase, the hydrocarbon core, and the Stern layer. In our earlier work we observed that in aliphatic hydrocarbon solvents (e.g. n-heptane) coumarin 480 (I) does not exhibit wavelength dependent fluorescence decay and hence does not show time dependent Stokes shift.10b Therefore, the probe dye (I) is © 1996 American Chemical Society
15484 J. Phys. Chem., Vol. 100, No. 38, 1996
Sarkar et al.
Figure 1. Emission spectrum of coumarin 480 in (a) water (-‚-); (b) 3.5 mM CTAB (‚‚‚); (c) 32 mM SDS (---) and (d) 2 mM TX-100 (s).
expected to exhibit negligible time dependent solvent shift while it is in the aliphatic hydrocarbon core of the micelles. However, it may be pointed out that in nonpolar aromatic hydrocarbon solvents (e.g. butylbenzene) some other probe molecules have been reported to display non-negligible time dependent Stokes shift.7 In the bulk aqueous phase the solvation would be over by 1 ps since the time constant of solvation is determined to be 310 fs.2d The question that remains to be answered is what happens to the solvation dynamics of I in the Stern layer. 2. Experimental Section The single-photon-counting setup and the laser system are described elsewhere.11 Coumarin 480 (Exciton) and Triton X-100 (Aldrich) were used as received. Other surfactants were purified by recrystallization. The wavelength of excitation for steady state and time-resolved studies is 310 nm. The fluorescence decays were recorded at intervals of 15 nm using a Corning 3-74 filter (to cutoff scattered light) and a less than 5 nm slitwidth and were fitted to single- or biexponential decays using global lifetime analysis software (PTI). Reconstruction of the time-resolved spectra was done 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 times, t, were generated by fitting the data to a log-normal function to get the peak frequency υ(t). The peak frequency at infinite time υ(∞) refers to the peak frequency at very long time when the spectrum no longer exhibits any time dependent Stokes shift. 3. Results and Discussion 3.1. Steady State Spectra. In aqueous solution coumarin 480 (I) exhibits an intense emission maximum at around 490 nm (Figure 1).16 With increase in the surfactant concentration the emission spectra of I in aqueous solutions remain more or less unchanged up to the cmc. Above the cmc though the fluorescence quantum yields remain unchanged; the emission spectra exhibit a distinct blue shift to 470 nm in the case of TX-100 and 475 nm in CTAB and SDS, which indicates that
Figure 2. (a, top) Fluorescence decays of coumarin 480 in 3.5 mM CTAB at (i) 430, (ii) 455, and (iii) 500 nm. (System response has been shifted for clarity.) (b, bottom). Fluorescence decays of coumarin 480 in 2.0 mM TX-100 at (i) 420, (ii) 450, and (iii) 480 nm. (System response has been shifted for clarity.)
the probes are transferred from the polar aqueous phase to the less polar micellar region. In hydrocarbon media (cyclohexane or n-heptane) the emission maximum of I is observed to be at 410 nm.16 The steady state emission spectra of I in the presence of micelles exhibit very low intensity at 410 nm, which indicates very few probe molecules reside in the hydrocarbon-like micellar core. The position of the emission peak of I in the presence of the micelles is close to the emission maximum of I in alcohol (473 nm).16 However, it must be emphasized that at the micellar boundaries the dipole moment (or the charge distribution) of the excited coumarin molecule experiences a medium that is heterogeneous on the length scale of the variation of the dipolar electric field. Thus although the steady state emission peak of I in the micellar media is close to that in homogeneous alcohol medium (473 nm),16 it is debatable whether one can say that the average polarity experienced by the probe molecule in the micellar media is similar to that of alcohol or whether one can, at all, talk of an average polarity for the microheterogeneous micellar media. It is, perhaps, more justified to say that in the micellar media some parts of the probe molecules experience a polarity much lower than that of bulk water. 3.2. Time-Resolved Studies. It is observed that in the presence of the surfactants above the cmc the decays at the red end differ significantly from those at the blue end (Figure 2). The decay at the red end exhibits a distinct growth. For instance the decay at 500 nm for CTAB is fitted to a biexponential with a growth component of 1.1 ns and a decay component of 5.2 ns, while the decay at 430 nm is single exponential with a 1.2 ns lifetime (Figure 2a). This wavelength dependent dynamics indicates a time dependent Stokes shift of the emission spectra. In this case, the energy of the guest dipole decreases with time due to solvation producing the solvated species emitting at the longer wavelength, which is reflected in the growth observed at the red end of the emission spectra. To get detailed information on the solvation dynamics, the time-resolved emission spectra of coumarin 480 in aqueous
Solvation Dynamics of Coumarin 480 in Micelles
J. Phys. Chem., Vol. 100, No. 38, 1996 15485
TABLE 1: Emission Characteristics of Coumarin 480 in Aqueous Micelles surfactant CTAB TX-100 SDS a
conc (mM) 3.5 2.0 32.0
CMC (mM) 0.9 0.2 8.0
max λem (nm)
υ(0) - υ(∞) (cm-1)
490 475 470 475
a
1340 490 1035 445
decay characteristics of C(t) a1
τ1 (ps)
a2
τ2 (ps)
0.26a
