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Solvation dynamics of Coumarin 153 in aqueous andnon-aqueous reverse micelles
Partha Hazra, Debdeep Chakrabarty, Nilmoni Sarkar *
Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, WB, India
Received 29 November 2002; in final form 17 February 2003
Abstract
The solvation dynamics of water, methanol and acetonitrile in n-heptane/AOT/water, n-heptane/AOT/methanol and
n-heptane/AOT/acetonitrile reverse micelles have been investigated by picosecond time-resolved emission spectroscopy
using rigid and neutral probe, Coumarin 153 (C-153). We have observed a substantially slow dynamics (order of few
nanosecond) in all the three reverse micelles compared to pure solvents. The w dependency of solvation time is observed
in case of water and methanol reverse micelles. However, a very little w dependency is observed in case of acetonitrile
reverse micelles. Moreover, fluorescence anisotropy measurements are used to characterize rotational motion of C-153
in all the three reverse micelles.
� 2003 Elsevier Science B.V. All rights reserved.
1. Introduction
In recent years, several groups have investigated
the dynamics of solvation in polar solvents as wellas in biological and restricted environment using
time-resolved fluorescence as a tool [1–9]. The re-
verse micelle is an elegant example for biomem-
branes among the different organized assemblies.
In general, surfactant molecules in a non-polar
solvent self aggregates to form reverse micelle.
Depending upon the use of co-solvents (water or
polar organic solvents), the reverse micelles aretermed as aqueous or non-aqueous reverse micelles
and the chemistry occurring in them is partly
guided by these co-solvents. The size of the reverse
micelle is controlled by w ðw ¼ ½water or polarsolvent�=½AOT�Þ, can be increased by increasingthe number of water or polar solvent molecules to
that of surfactant. In recent years, there are several
studies to probe the dynamics of water or polar
solvent molecules confined in the reverse micelles
[3,4,8,21–24,26–28,31,35]. The observed dynamics
of the confined solvent molecules in the reverse
micelles is bimodal in nature, comprising of
two components, with one component of theorder of sub-picosecond time scale and the other
component from picosecond to nanosecond time
scale. The primary requirement for solvation dy-
namics study is the use of solvatochromic fluoro-
phores that exhibit large frequency shift in their
Chemical Physics Letters 371 (2003) 553–562
www.elsevier.com/locate/cplett
* Corresponding author. Fax: +91-3222-255303.
E-mail address: [email protected] (N. Sarkar).
0009-2614/03/$ - see front matter � 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00304-X
fluorescence spectra as a function of solvent po-
larity. The 7-aminocoumarins are used as efficient
probe for such purpose. Among them Coumarin
153 (Scheme 1) is a nearly ideal solvation probe for
a variety of reasons. Firstly, the ground state of
Coumarin 153 is polar with a measured dipolemoment of 6:55� 0:01 D [10]. The S0 ! S1 tran-
sition leads to the S1 dipole moment between 14.2
D and 16.0 D, depending on solvents [11]. Fur-
thermore, its S1 state is well separated from other
excited singlet states and there is no report of oc-
currence of any excited state reaction in most
solvents. Due to these characteristics, there are lots
of studies of solvation dynamics using this probe[12–20].
Maroncelli and Fleming [12] examined solva-
tion dynamics in polar liquids using C-153 as a
probe. The observed dynamics measured using
time correlated single photon counting (TCSPC)
technique deviates from that of expectations based
on continuum theory of solvation. By far the most
systematic measurements of solvation in a varietyof liquids using C-153 and other Coumarin probes
were done by Barbara and co-workers [13,14] us-
ing fluorescence up conversion technique. Rea-
sonably good correlation between observed and
predicted solvation time from continuum solvation
model was observed, neglecting the inertial part of
solvent�s dielectric response. Recently, Maroncelli
and co-workers [15] measured sub-picosecondpolar solvation dynamics in 24 common solvents
using C-153 as a probe by fluorescence up con-
version technique with a better time resolution. In
their experimental setup they detected the inertial
part of solvation and found that the observed
dynamics can be semi-quantitatively predicted
from simple theory based on dielectric response of
the pure solvent. They also investigated rotational
dynamics of C-153 in 35 common solvents and
eight solvent mixtures at room temperature [16].
They explore in this paper how the dielectric fric-
tion of solvents influences its rotational motion.Hara et al. [17] observed pressure effect on solva-
tion dynamics in TX-100 micelles using the C-153
as a probe and found that solvation time becomes
shorter with increasing pressure. Recently, Jarzeba
and co-workers [18] examined preferential solva-
tion of C-153 in toluene–acetonitrile and toluene–
methanol solvent mixtures.
There are also some studies of solvation dy-namics in molten salts and ionic liquids using the
same C-153 as a probe. Huppert and co-workers
[19] observed biphasic solvation of C-153 in mol-
ten tetraalkylammonium salts. They explained the
biphasic solvation dynamics in terms of two sep-
arate contributions those of cation and anion,
which occur on two different time scales. Very re-
cently, Samanta and Karmakar [20] observed bi-phasic solvation dynamics of the same probe in
room temperature ionic liquids. They proposed
that this biphasic solvation is due to the results of
relatively fast initial response of anion and a slow
collective diffusion motion of both cation and
anion.
In summary, there are lots of studies of solva-
tion dynamics in pure solvents [12–15], solventmixtures [16,18], molten salts [19] and ionic liquids
[20] using C-153 as a probe. But there is no such
study in aqueous and non-aqueous reverse micelles
using this probe. In this Letter we have reported
our present study to analyse solvation dynamics in
water, methanol and acetonitrile reverse micelles
using C-153 as a probe. We have chosen C-153 as a
probe since there is an appreciable change in di-pole moment of the molecule from ground state to
the excited state [10,11] and this molecule is rigid
in structure. So, the twisted intramolecular charge
transfer (TICT) is not possible in this molecule,
which is common to other Coumarin derivatives.
Moreover, the solvation dynamics and the an-
isotropy data are available in pure solvents using
this probe [13–15] and it will be helpful for us tocompare our solvation and anisotropy data with
that of pure solutions.Scheme 1. Structure of C-153.
554 P. Hazra et al. / Chemical Physics Letters 371 (2003) 553–562
2. Experimental
C-153 (laser grade from Exciton) was used as
received. AOT (dioctylsulfosuccinate, sodium salt,
Aldrich) was purified by standard procedure [21].n-heptane, methanol and acetonitrile of spectro-
scopic grade (Spectrochem) were freshly distilled
over calcium hydride (Spectrochem) before use.
The concentration of C-153 maintained in all the
measurements is 4� 105 M and that of AOT is
0.09 M. The preparation of solution is described
elsewhere [24].
For absorption and fluorescence measurementswe have used Shimadzu absorption spectropho-
tometer (model no: UV 1601) and Spex-fluorolog-3
spectrofluorimeter (model no: FL3-11), respec-
tively. The fluorescence spectra were corrected for
wavelength sensitivity of the detection system and
were obtained as a photon number intensity spec-
trum. The experimental setup for picosecond
TCSPC is as follows. Briefly, a picosecond diodelaser at 408 nm (IBH, UK, NanoLED-07, s/n
03931) was used as a light source. The fluorescence
signal was detected in magic angle (54.7�) polari-zation using Hamamatsu MCP PMT (3809U). The
typical system response of this laser system is 75ps. The decays were analysed using IBH DAS-6
decay analysis software. The same software was
also used for anisotropy analysis. The temperaturewas kept 298� 1 K for all measurements.
3. Results and discussions
3.1. Steady state absorption spectra
C-153 in n-heptane shows two strong absorp-tion peaks, one at 392 nm and another at around
410 nm, though the peak at 392 nm has maximum
absorbance (Fig. 1a). On addition of 0.09 M AOT,
absorbance slightly increases, though the peak
position remains unchanged (Fig. 1a). In case of
methanol/acetonitrile reverse micelles, the ab-
sorption spectra exhibit a small red shift with a
slight decrease in absorbance on addition ofmethanol/acetonitrile to the 0.09 M AOT con-
taining C-153 in n-heptane and also the spectra
become broader at high w value. The representa-
tive absorption spectra in case of methanol reverse
micelles are shown in Fig. 1a. However, absorp-
tion spectra in case of water reverse micelles are
almost unchanged on addition of water to 0.09 M
AOT containing C-153 in n-heptane. Another in-teresting feature for all the reverse micelles is that
as we move from w ¼ 0 to higher w value, a
Fig. 1. (a) Absorption spectra of C-153 in pure methanol and n-
heptane/AOT/methanol reverse micelle. Solid lines for pure
methanol, dash lines for pure n-heptane, dot lines for w ¼ 0,
dash dot lines for w ¼ 2 and dash dot dot lines for w ¼ 6. (b)
Emission and excitation spectra of C-153 in pure n-heptane,
pure methanol and methanol reverse micelles. Solid lines for
emission spectra ðkex ¼ 410 nmÞ of C-153 in (i) pure n-heptane,
(ii) w ¼ 0, (iii) w ¼ 2 (methanol), (iv) w ¼ 6 (methanol) and (v)
pure methanol. Dash lines and dot lines for excitation spectrum
of C-153 in pure n-heptane and pure methanol, respectively.
Dash dot lines and dash dot dot lines for excitation spectra of
methanol reverse micelle (w ¼ 6) monitored at 450 and 580 nm,
respectively.
P. Hazra et al. / Chemical Physics Letters 371 (2003) 553–562 555
shoulder is gradually appearing at P 430 nm
(Fig. 1a).
For all the reverse micelles, the excitation
spectra monitored at blue and red end of the
emission spectra are different. The excitation
spectra in case of methanol reverse micelle areshown in Fig. 1b. The difference in excitation
spectra at the blue and red end of the emission
spectra indicates that there are broadly two kinds
of probe molecules partitioned in all the reverse
micelles. The first kinds of molecules are assigned
to the dye molecules residing in bulk n-heptane.
The dye molecules those are residing in the core of
the reverse micelles are assumed to be the othertype. The excitation spectra of pure n-heptane and
pure methanol are also shown in Fig. 1b to dis-
tinguish it from the excitation spectra of methanol
reverse micelle.
3.2. Steady state emission spectra
On excitation, C-153 in n-heptane at 410 nmshows a strong emission peak at 450 nm (Fig. 2a
and Table 1). With the addition of 0.09 M AOT to
this solution, the emission spectrum is slightly red
shifted with the decrease in intensity (Fig. 2a).
With further addition of water to this solution the
intensity decreases but the peak position of the
emission spectrum is more or less unaffected (Fig.
2a).On addition of methanol/acetonitrile to the so-
lution containing C-153 in n-heptane and 0.09 M
AOT ðkex ¼ 410 nmÞ, the emission maximum is
gradually red shifted with decrease in its intensity
(Fig. 1b). The emission maxima are at 508 and 484
nm of w ¼ 6 methanol and w ¼ 4 acetonitrile re-
verse micelles, respectively (Table 1). The shift is
more in case of methanol reverse micelles thanacetonitrile reverse micelles. This may be due to
the higher static polarity experienced in methanol
reverse micelles compared to acetonitrile reverse
micelles. The emission spectrum of pure methanol
is shown in Fig. 1b for clarity. The spectra of re-
verse micelles are clearly different from that of
pure methanol.
The emission spectra of C-153 ðkex ¼ 430 nmÞin water reverse micelle at different w values are
shown in Fig. 2b. Since on addition of water, a
shoulder is gradually appearing at the red end side
of the absorption spectrum, it indicates the probe
molecules are gradually encapsulating in the water
pool of reverse micelles. On excitation, C-153 in
water reverse micelles at 430 nm, show distinct
peak at the red end side of the emission spectra atdifferent w values of water reverse micelles. The
spectral feature is clearly different from the emis-
sion spectra taken at 410 nm excitation. It indi-
cates that a certain percentage of C-153 molecules
are definitely incorporated in the reverse micelles.
Fig. 2. (a) Steady state emission spectra of C-153 in n-heptane/
AOT/water reverse micelle ðkex ¼ 410 nmÞ. Solid lines for pure
n-heptane, dash lines for w ¼ 0, dot lines for w ¼ 4 and dash
dot lines for w ¼ 32. (b) Steady state emission spectra of C-153
in pure water and n-heptane/AOT/water ðkex ¼ 430 nmÞ reversemicelle. Solid lines for C-153 in (i) n-heptane and in (ii) pure
water. Dash lines, dot lines, dash dot lines, dash dot dot lines
represent emission spectra of C-153 in w ¼ 0, w ¼ 4, w ¼ 16
and w ¼ 32 of water reverse micelles, respectively.
556 P. Hazra et al. / Chemical Physics Letters 371 (2003) 553–562
In case of methanol and acetonitrile reverse mi-celles there is no significant change in emission
spectra as we shift the excitation wavelength from
410 to 430 nm.
3.3. Time-resolved studies
3.3.1. Time-resolved fluorescence anisotropy mea-
surements
Before going to discuss detail result of our sol-
vation dynamics measurements in water, metha-
nol, and acetonitrile reverse micelles, we would
like to give idea about the location of the probe,
which predicts the dynamics. The location of the
probe can be determined more accurately by time-
resolved fluorescence anisotropy measurements.Time resolved fluorescence anisotropy, rðtÞ was
calculated using the following equation,
rðtÞ ¼ IkðtÞ GI?ðtÞIkðtÞ þ 2GI?ðtÞ
; ð1Þ
where G is the correction factor for detector sen-
sitivity to the polarization direction of the emis-
sion. IkðtÞ and I?ðtÞ are fluorescence decays
polarized parallel and perpendicular to the polar-
ization of the excitation light, respectively.
Fig. 3 shows a representative anisotropy decay
of C-153 in case of acetonitrile reverse micelles. Allthe results of anisotropy measurements are listed
in Table 2. For C-153 in n-heptane, the fluores-
cence depolarization occurs within 70 ps. The ro-
tational motions of C-153 in different reverse
Table 1
Steady state absorption and emission maxima of C-153 in
aqueous, non-aqueous reverse micelles and in pure solvents
(a) Reverse micelles
Polar solvent wa kabsðmaxÞ(nm)
kemðmaxÞ(nm)
– 0 392 453
Water 4 393 451
Water 32 393 451
Methanol 2 394 480
Methanol 6 395 508
Acetonitrile 2 395 476
Acetonitrile 4 397 484
(b) Pure solvents
Solvent
Water 430 540
Methanol 423 532
Acetonitrile 418 521
n-Heptane 392 450
aw ¼ ½polar solvent�=½AOT�.
Fig. 3. Decays of fluorescence anisotropy ðrðtÞÞ of C-153 in
pure n-heptane and acetonitrile reverse micelle. ðjÞ for pure n-heptane and (�) for w ¼ 2 ðCH3CNÞ.
Table 2
Rotational relaxation time ðsrÞ of C-153 in pure n-heptane, aqueous and non-aqueous reverse micelles
Polar solvent w r0 a1r s1r (ps) a2r s2r (ps)
n-Heptane – 0.40 0.40 70 0 0
Water 4 0.36 0.25 80 0.11 846
Water 32 0.31 0.22 70 0.09 800
Methanol 2 0.35 0.29 70 0.06 685
Methanol 6 0.32 0.27 80 0.05 600
Acetonitrile 2 0.32 0.27 70 0.05 1040
Acetonitrile 4 0.35 0.30 60 0.05 780
P. Hazra et al. / Chemical Physics Letters 371 (2003) 553–562 557
micelles occur at much slower rate. For a partic-
ular reverse micelle, as w value increases, the
contribution of the fast component increases and
that of slower component decreases. This indicates
that the rotational motion of C-153 in the largest
micelles, e.g., at w ¼ 32 is faster than in the smallermicelles and the probe molecule experience a de-
creasingly restricted environment with increasing
micellar size. This is also true for other reverse
micelles. The rotational correlation time of C-153
in pure acetonitrile and pure methanol are 22 and
35 ps, respectively [16], whereas fluorescence de-
polarization C-153 in the reverse micelles occurs
within 600 ps to 1.04 ns. It is revealed from Table 2that a 70 ps component is contributed to the
rotational correlation time in all the reverse mi-
celles and it is almost same as that of n-heptane.
The fast component in the anisotropy measure-
ments may arise due to rotational motion of the
probe molecules, those are residing either in the
pool of the reverse micelles or in the bulk n-hep-
tane. Moreover, the contribution of the slowcomponent is gradually decreasing as we move
from lower to higher w values. It confirms that
some probe molecules are intimately associated
with the micelles, probably resides at the micellar
interface near the AOT head groups. So, for all
reverse micelles we cannot rule out the possibility
of residing some probe molecules in bulk heptane.
Excitation spectra of all the reverse micelles alsosupport this conjecture.
3.3.2. Solvation dynamics
The time-resolved emission spectra (TRES)
have been constructed following the procedure
given by Fleming and Maroncelli [12]. In all the
reverse micelles, we have observed a shift in TRES.
The relative shifts ðDmÞ in the TRES for different
reverse micelles are shown in Table 3. The TRES
are shown in Fig. 4. The solvation dynamics is
defined by the decay of the solvent correlationfunction, CðtÞ as,
CðtÞ ¼ mðtÞ mð1Þmð0Þ mð1Þ ; ð2Þ
where mð0Þ, mðtÞ, mð1Þ are the peak frequencies at
time 0, t and 1, respectively.
To extract the solvation time in reverse micelles,we have subtracted the contribution of the dye
molecules in pure n-heptane as follows. We have
multiplied the emission spectrum of the probe in n-
heptane by the ratio of peak (at 450 nm) intensity
of the corresponding reverse micelles to n-heptane.
Subsequently, this spectrum is subtracted from the
corresponding spectrum of the reverse micelles.
Bhattacharyya and co-workers [26] earlier usedthis method and we have also used this method in
our earlier publications [23,24]. To construct the
TRES we have taken the steady state emission
spectrum in water reverse micelles at 430 nm ex-
citation because in this case only the probe mole-
cules that are entrapped in the reverse micelles are
excited. Due to the unavailability of the picosec-
ond laser diode at this wavelength we had to excitethe sample at 408 nm for time-resolved studies.
The subtraction of the contribution of the n-hep-
tane is important to construct the TRES in reverse
micelles. A representative TRES for w ¼ 32 of
water reverse micelle with and without subtraction
of n-heptane contribution is shown in Fig. 4a. It is
Table 3
Decay characteristics of C-153 in aqueous and non-aqueous reverse micelles
Polar solvent m (cm1)a w a1 s1 (ns) a2 s2 (ns) hssib (ns) Missing component
Water 1656 4 0 0 1.00 8.40 8.40 0.60
Water 757 32 0.48 0.933 0.52 10.24 5.77 0.80
Methanol 2984 2 0.57 2.171 0.43 17.90 8.93 0.05
Methanol 2958 6 0.72 0.956 0.28 14.42 4.72 0.11
Acetonitrile 2178 2 0.34 0.452 0.66 20.73 13.83 0.14
Acetonitrile 2069 4 0.37 0.412 0.63 19.35 12.34 0.20
aDm ¼ m0 m1.b hssi ¼ a1s1 þ a2s2.
558 P. Hazra et al. / Chemical Physics Letters 371 (2003) 553–562
clearly seen from the Fig. 4a that at t ¼ 0 in the
non-subtracted spectra both the contribution of
the probe in n-heptane and in the reverse micelle
are present. At t ¼ 10 ns, the contribution of the
probe in the reverse micelle is prominent with asubsequent red shift in the spectrum. Whereas in
the subtracted spectrum only the peak resembles
with reverse micelle comes in the TRES, indicating
suitability of the method to extract the dynamics in
reverse micelles. Due to the peak shift in case of
methanol and acetonitrile reverse micelles, the
above method is not applied in case of these two
reverse micelles. For these reverse micelles, we
have directly subtracted the n-heptane emission
spectrum from the corresponding reverse micelle�sspectrum. A representative TRES of w ¼ 2 of
acetonitrile reverse micelle is given in Fig. 4b. Thedecay of the solvent correlation function C(t) for
C-153 in n-heptane/AOT/water/methanol/acetoni-
trile reverse micelles are shown in Fig. 5 and the
results are listed in Table 3. The results demon-
strated that for all the reverse micelles, the solva-
tion dynamics are slowing down remarkably
Fig. 5. Decay of the solvent correlation function (CðtÞ) of
C-153 (a) (i) at w ¼ 4 ðdÞ, (ii) at w ¼ 32 ðNÞ of water reversemicelles, (b) (i) at w ¼ 2 ðjÞ, (ii) at w ¼ 6 ð.Þ of methanol
reverse micelles and (iii) at w ¼ 2 ðNÞ, (iv) at w ¼ 4 ð�Þ of
acetonitrile reverse micelles.
Fig. 4. TRES of C-153 in AOT reverse micelles. (a) Water
reverse micelle (w ¼ 32) at (i) 0 ðjÞ, (ii) 10000 ð.Þ ps using non-subtracted steady state data and (iii) 10 000 ð�Þ ps using hep-
tane subtracted steady state data. (b) Acetonitrile reverse
micelle (w ¼ 2) at (i) 0 ðjÞ, (ii) 500 ðdÞ, (iii) 5000 ðNÞ, (iv)10 000 ð�Þ ps using heptane subtracted steady state data. Solid
lines are fitted curves for experimental points.
P. Hazra et al. / Chemical Physics Letters 371 (2003) 553–562 559
compared to pure solvents. Solvation times of pure
methanol and pure acetonitrile are 5.0 ps and 260
fs, respectively, using C-153 as a probe [15].
However, there is no reported result of solvation
dynamics of pure water using this probe. Vajda
et al. [25] determined solvation time of pure waterusing a similar type of probe, C-480 and they re-
ported 310 fs solvation time in case of pure water.
The decrease of relaxation time from 8.40 ns at
w ¼ 4 to 5.77 ns at w ¼ 32 (Table 3) indicates that
with increase in water content the mobility of the
water molecules in the water pool of the reverse
micelle increases. The slower component at w ¼ 32
may be attributed to those water molecules (boundwater) near the ionic head group of the surfactant
and the water molecules (free water) residing at the
central region of the water pool contribute to the
faster component of solvation. However, even
the faster component of 0.933 ns at w ¼ 32 (Table
3) is still significantly slower than the sub-pico-
second solvation time observed for C-480 by
Fleming and co-workers [25] in water. This clearlyindicates that the dynamics of water molecules in
the reverse micelle are much slower than the water
molecules in ordinary bulk water.
The solvation dynamics in non-aqueous reverse
micelles show a non-exponential feature. This is
because non-aqueous polar solvent in reverse mi-
celles would also consist of some different kinds of
molecules. In case of methanol reverse micelles, thesolvation time and the corresponding relative am-
plitudes are as follows: for w ¼ 2, s1 ¼ 2:171 ns,
a1 ¼ 0:57 and s2 ¼ 17:90 ns, a2 ¼ 0:43 and w ¼ 6,
s1 ¼ 0:956 ns, a1 ¼ 0:72 and s2 ¼ 14:42 ns,
a2 ¼ 0:28 (Table 3). This reveals that with increase
in w value, the relative contribution of the fast
component is gradually increasing with subsequent
decrease in the slow component of the solvationtime. The decrease in solvation time with increase
in w value suggests that the probe is approaching
towards the core of the reverse micelle from the
interface of the reverse micelle because methanol
molecules are less tightly bound in the core of the
reverse micelle.
We have also observed w dependency of solva-
tion time in case of water and methanol reversemicelles, but there is little w dependency in case of
acetonitrile reverse micelle. Bhattacharyya and co-
workers [3,4,26] also observed similar w depen-
dency of solvation dynamics in case of water re-
verse micelle. Das et al. [27] found that the
solvation time of heavy water in reverse micelle is
1.2 times more than that of ordinary water in re-
verse micelle. This confirms that the solvationdynamics of water in reverse micelle is dependent
on hydrogen bonding network. Shirota et al. [28]
also observed w dependency of solvation time in
case of methanol reverse micelle and w indepen-
dency in case of acetonitrile reverse micelle. Ac-
cording to them, the different features of solvation
dynamics in two reverse micelles would be attrib-
uted to the role of hydrogen bonds in methanoland its absence in acetonitrile.
The w dependency of solvation dynamics in
case of water and methanol reverse micelles can be
explained with the help of experiments executed by
Moran et al. [29] and Li et al. [30]. By IR mea-
surements they showed that three or four water
molecules bridge between the SO3 and Naþ ions in
AOT reverse micelles. Basically, the water mole-cules are strongly hydrogen bonded to the wall of
the reverse micelles through Naþ ions in AOT re-
verse micelle. The microscopic feature of n-hep-
tane/AOT/methanol micelle should be similar to
that of aqueous reverse micelle. At low w value,
water/methanol molecules are strongly hydrogen
bonded to the wall of the reverse micelles and there
are only a few �free water/methanol� molecules.Consequently, the �bound water/methanol� mole-cules contribute more to the solvation dynamics.
So, we get a slower dynamics at low w value. At
high w value, the �free water/methanol� contributemore to the solvation dynamics. Hence, we ob-
serve rather fast dynamics at high w value of water
and methanol reverse micelles. The time resolution
of our experimental setup is 75 ps. So it is pos-sible for us to estimate lifetime upto 70 ps with
reliability after deconvolution. But in these sys-
tems there is a finite probability that the probe
molecules residing in the �free water/ methanol/
acetonitrile� should have ultrafast component
down to sub-picosecond time scale, which is not
detected in our setup. We have estimated the
missing components in these systems using themethod of Fee and Maroncelli, which we have
applied in our earlier system [21]. The estimated
560 P. Hazra et al. / Chemical Physics Letters 371 (2003) 553–562
missing component of these systems is shown in
Table 3. It is revealed from Table 3 that as we
increase the amount of polar solvent, the per-
centage of missing component is gradually in-
creasing and in case of water reverse micelle the
percentage of missing component is much highercompared to methanol/acetonitrile reverse micelle.
It indicates that the dynamics in water reverse
micelles is much faster compared to other system.
This is also supported by various other experi-
ments [1,5,6]. The w dependency of solvation dy-
namics in case of water and methanol reverse
micelles can be explained with the help of multi-
shell continuum model proposed for reverse mi-celles and proteins by Bagchi and co-workers [31].
In this model, they assume a dynamic exchange
between the �free� and �bound� water molecules.
The energetics of the exchange depends on the
strength and number of hydrogen bonds among
the bound water molecules and bio-molecules. As
the strength of hydrogen bond increases, the rel-
ative population of the �bound� water moleculesincreases. Consequently, the relative population of
the slow component also increases. This can also
explain the existence of solvation dynamics in
relatively two different time scales. We can suc-
cessfully use this model in the case of water and
methanol reverse micelles to explain the w depen-
dency of solvation dynamics.
The solvation dynamics of acetonitrile in dif-ferent w value show almost same solvation time,
e.g., at w ¼ 2 the average solvation time is 13.83 ns
whereas at w ¼ 4 it is 12.34 ns. In our previous
works [22–24] we also observed a little w depen-
dency in case of acetonitrile reverse micelle. Shi-
rota and Horie [28] also observed w independency
in case of acetonitrile reverse micelle. In contrast
to methanol molecule, acetonitrile molecule in re-verse micelle cannot bridge between SO
3 and Naþ
ion in AOT reverse micelle due to the lack of hy-
drogen bonding network in acetonitrile solvent. In
acetonitrile, only weak dipole dipole interactions
are present. Due to the lack of hydrogen bonding
and the presence of weak intermolecular force of
attraction, little w dependency is found. However,
we could not explain the slow dynamics of aceto-nitrile reverse micelles by the lack of hydrogen
bonding network. But the Naþ present in AOT
could affect such dynamics. Huppert and co-
workers [32,33] observed that solvation dynamics
in molten salts are biphasic and occur on pico-
second to nano-second time scale. Chapman and
Maroncelli [34] verified that at the same Naþ ion
concentration, the dynamics in the NaClO4=CH3CN is very slow compared to NaClO4=CH3OH. Riter et al. [35] also verified the effect of
counterion on solvation dynamics in AOT reverse
micelle. From this discussion we can conclude that
ion probe association is responsible for such a slow
dynamics in case of acetonitrile reverse micelle.
The present result in aqueous reverse micelle is
very close to that of Bhattacharyya and co-work-ers [26]. They have used Coumarin 480 (C-480) as
a probe during their measurement whereas we
have used C-153 as a probe for the present study.
Since the only difference between C-153 and C-480
is the replacement of trifluoromethyl substitution
in the former with a methyl group in the latter, one
can expect similar results. The present results de-
viate to some extent from our previous results dueto use of different types of probe in solvation dy-
namics measurements. In our earlier study [21–24]
we have used different flexible and neutral Cou-
marins as solvation probe. Here we have used rigid
and neutral probe for measurement. Shirota and
Horie [28] reported solvation dynamics measure-
ments in methanol and acetonitrile reverse micelles
using rigid and anionic probe, Coumarin 343(C-343). The difference in results in our case and
Shirota and Horie [28] may be due to use of dif-
ferent types of probe as well as different detection
technique by the latter group. The reliability and
accuracy of our experimental results is better than
that of Shirota and Horie [28] because they used
Streak Camera as detector and the experimental
error is �15% reported by them. Using TCSPC inour experiment, the accuracy of our result is much
better and the error is �5%. From this comparison
it can be concluded that solvation dynamics is
dependent on probe to some extent.
4. Conclusion
We have examined solvation dynamics of water,
methanol and acetonitrile in AOT reverse micelles.
P. Hazra et al. / Chemical Physics Letters 371 (2003) 553–562 561
The solvation dynamics is retarded almost thou-
sand times in the water, methanol and acetonitrile
reverse micelles compared to pure solvents. The
solvation dynamics of water and methanol in their
corresponding reverse micelles strongly depend on
w values. However, the solvation dynamics of ace-tonitrile in reverse micelle is little dependent of wvalue. The different features of the solvation dy-
namics in water, methanol and acetonitrile reverse
micelles are explained by the presence and absence
of hydrogen bonding network. The very slow dy-
namics in case of acetonitrile reverse micelle may be
attributed to the ion-probe association. Moreover,
fluorescence anisotropy measurements indicatethat rotational motion of C-153 in different reverse
micelles occur at much slower rate.
Acknowledgements
NS is thankful to Department of Science and
Technology (DST) and Council of Scientific andIndustrial Research (CSIR), India for generous
research grants. P.H. and D.C. are thankful to
CSIR for research fellowships. We are thankful to
the anonymous referee for constructive criticism
and suggestions.
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