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: nilmoni@chem.iitkgp.ernet.in (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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