Solvation dynamics of Coumarin 152A in methanol andacetonitrile reverse micelles

Partha Hazra, Debdeep Chakrabarty, Nilmoni Sarkar *

Department of Chemistry, Indian Institute of Technology, Kharagpur, WB 721 302, India

Received 26 February 2002; in final form 19 April 2002

Abstract

In this Letter we have reported solvation dynamics of methanol and acetonitrile in n-heptane/AOT/methanol and n-

heptane/AOT/acetonitrile reverse micelles using Coumarin 152A (C-152A) as a probe. The solvation dynamics is re-

tarded almost several thousand times compared to pure solvents. In case of methanol reverse micelle, the solvation time

depends on w value. However, a very little w dependence of solvation dynamics is observed in case of acetonitrile

reverse micelle. The different features of solvation dynamics in methanol and acetonitrile reverse micelles are explained

on the basis of presence and absence of intermolecular hydrogen bonding network. � 2002 Elsevier Science B.V. All

rights reserved.

1. Introduction

There are several experimental and theoreticalstudies related to polar solvation dynamics in ho-mogeneous pure solution [1–12]. For all solventsthe solvation dynamics is contributed by a initialfast component and a subsequent contribution ofthe diffusive slow component. The first experi-mental study on the solvation dynamics of thewater molecule was performed by Barbara andcoworkers [1]. Fleming and coworkers [4,5]showed that the solvation time of pure water is lessthan 1 ps. The low frequency librational (rota-tional) motions of the water molecules and inter-molecular hydrogen bonding are responsible for

the ultrafast relaxation [4–9]. The solvation dy-namics of pure acetonitrile and pure alcohol havealso been investigated both by experimental tech-nique and also by simulation [2,3,10–12]. The re-sults indicated that methanol molecule relaxeswithin 1–10 ps, whereas acetonitrile molecule re-laxes in less than 1 ps.Water plays a crucial role in many natural

processes and the dynamics of water molecule inself organized molecular assemblies are quite dif-ferent from pure water [15]. Recently, there is atremendous interest to study the solvation dy-namics in organized media. Reverse micelle is onesuch elegant example of organized media wherewater molecules confined in nanocavity [16]. Thereare several studies related to solvation dynamics ofwater in reverse micelle. The solvation dynamics ofwater in several sizes of AOT reverse micelles wasinvestigated by Zhang and Bright [19]. Zhang and

7 June 2002

Chemical Physics Letters 358 (2002) 523–530

www.elsevier.com/locate/cplett

* Corresponding author. Fax: +91-3222-755303.

E-mail address: nilmoni@chem.iitkgp.ernet.in (N. Sarkar).

0009-2614/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0009-2614 (02 )00679-6

Bright [17,18] also investigated the solvation dy-namics in supercritical fluids. Bhattacharyya et al.[13,14,20] examined the solvation dynamics ofwater in AOT reverse micelle using Coumarin 480by time correlated single photon counting setup.Mittleman et al. [21] investigated the water motionin AOT reverse micelle by ultrafast time domaintetrahertz spectroscopy. Recently, Levinger andcoworkers [22] observed a subpicosecond compo-nent in solvation dynamics measurement oflecithin/water/cyclohexane reverse micelle by fluo-rescence up conversion technique. Recently, wehave examined solvation dynamics of water inwater/AOT/n-heptane reverse micelle [25]. All thesolvation dynamics result revealed that the ap-pearance of longer relaxation time in reverse mi-celle, which is not present in bulk water andrelaxation of solvent water dramatically retardedin the reverse micellar core compared to bulkwater. This retardation of dynamics of water in thereverse micelle is due to confinement of watermolecules and the disruption of hydrogen bondingnetwork in the nanocavity [13,14,18–25].In comparison to the water reverse micelles,

there are only a few studies on non-aqueous re-verse micelle where the polar phase is other thanwater. Levinger et al. [23] first determined thesolvation dynamics in formamide/AOT/isooctanereverse micelle. Shirota and Horie [26] measureddynamics of polar solvation in non-aqueous AOTreverse micelles, specifically methanol/AOT/n-heptane and acetonitrile/AOT/n-heptane usingCoumarin 343 as probe. They observed a slowercomponent of solvent relaxation attributed to therestricted movement of the solvent in the reversemicellar core. Very recently, we have observed [27]the solvation dynamics in methanol/AOT/n-heptane and acetonitrile/AOT/n-heptane reversemicelle using Coumarin 490 as a probe. Thoughdifferent techniques and different types of probesused by different groups for solvation dynamicsmeasurements, but it is clear that solvation dy-namics in the reverse micelle exhibits unusuallyslow components (from a few hundreds of pico-seconds to several nanoseconds). In this Letter wewould like to report the results of solvation dy-namics investigated on methanol/AOT/n-heptaneand acetonitrile/AOT/n-heptane reverse micelles

using another neutral probe, Coumarin 152A(Scheme 1) and also want to check whether there isany probe dependence on solvation dynamics ornot.

2. Experimental

Coumarin 152A (Laser grade from Exciton)was used as received. AOT (dioctyl sulfosuccinate,Sodium salt, Aldrich) was purified by standardprocedure [28] and dried under vacuo for 12 hbefore use. All the solvents were spectroscopicgrade (Spectrochem, India) and freshly distilledover Calcium hydride (Spectrochem). The solutionwas prepared using literature procedure [23,26].The concentration of C152A maintained in all themeasurements is 4� 10�5 M and that of AOT is0.09 M. For absorption and fluorescence mea-surement we have used Shimadzu absorptionspectrophotometer (model no: UV1601) and SpexFluorolog-3 spectrofluorimeter (model no: FL3-11), respectively. The fluorescence spectrum wascorrected for spectral sensitivity of the instrument.For time-resolved study we have used time corre-lated single photon counting technique (TCSPC)[25]. The time resolution of our experiment is� 40 ps. We have excited the sample at 413 nm forboth steady state as well as time-resolved study.Temperature kept for all the measurements is298� 1 K.

3. Results and discussions

3.1. Steady-state absorption spectra

C152A in n-heptane shows two strong absorp-tion peaks, one at 396 nm and another at 377 nmthough the peak at 377 nm has maximum absor-

Scheme 1.

524 P. Hazra et al. / Chemical Physics Letters 358 (2002) 523–530

bance (Fig. 1). After addition of AOT and co-solvents (either methanol or acetonitrile), the ab-sorbance at 396 nm gradually increases. Anotherinteresting feature for both the reverse micelles isthat as we move from w ¼ 0 to higher w value, ashoulder is gradually appearing at P410 nm (thew value is defined as w¼ [cosolvent]/[AOT]). Thisindicates that probe is gradually incorporating inthe reverse micellar core. The excitation spectra(Fig. 2) of C-152 at w ¼ 6 of methanol or w ¼ 4 ofacetonitrile at two emission wavelength (kem ¼ 550and 440 nm) are different. It indicates that the

probe molecule C-152A is residing at two differentenvironments in the reverse micelles.

3.2. Steady-state emission spectra

In n-heptane C152A on excitation at 413 nmshows a strong peak at 430 nm (Fig. 1). After

(a)

(b)

Fig. 1. (a) Steady-state absorption and emission spectra of C-

152A in n-heptane/AOT/methanol reverse micelle; solid lines

for pure n-heptane, dash lines for w ¼ 0, dot lines for w ¼ 2 anddash dot lines for w ¼ 6, dash dot dot lines for C-152A in puremethanol. (b) Steady-state absorption and emission spectra of

C-152A in n-heptane/AOT/acetonitrile reverse micelle; solid

lines for pure n-heptane, dash lines for w ¼ 0, dot lines forw ¼ 2, dash dot lines for w ¼ 4 and dash dot dot lines for C-152A in pure acetonitrile.

(b)

(a)

Fig. 2. (a) Difference in absorption spectra between C-152A in

n-heptane with that of (i) C-152A in n-heptane (solid lines), (ii)

C-152A in 0.09 M AOT, w ¼ 2 in methanol (dash lines), (iii) C-152A in 0.09 M AOT, w ¼ 6 in methanol (dot lines), (iv) exci-tation spectrum of C-152A in 0.09 M AOT in n-heptane at

w ¼ 6 in methanol at kem ¼ 440 nm (dash dot lines), (v) exci-tation spectrum of C-152A in 0.09 M AOT in n-heptane at

w ¼ 6 in methanol at kem ¼ 550 nm (dash dot dot lines). (b)

Difference in absorption spectra between C-152A in n-heptane

with that of (i) C-152A in n-heptane (solid lines), (ii) C-152A in

0.09 M AOT, w ¼ 2 in acetonitrile (dash lines), (iii) C-152A in0.09 M AOT, w ¼ 4 in acetonitrile (dot lines), (iv) excitationspectrum of C-152A in 0.09 M AOT in n-heptane at w ¼ 4 inacetonitrile at kem ¼ 440 nm (dash dot lines), (v) excitation

spectrum of C-152A in 0.09 M AOT in n-heptane at w ¼ 4 inacetonitrile at kem ¼ 550 nm (dash dot dot lines).

P. Hazra et al. / Chemical Physics Letters 358 (2002) 523–530 525

adding 0.09 M AOT, the peak shifted to 454 nm.On addition of acetonitrile or methanol to thissolution, the emission is gradually red shifted (thevalues are listed in the Table 1). The change ismuch larger for methanol reverse micelles thanacetonitrile reverse micelles. This is due to thehigher static polarity experienced in methanol re-verse micelles compared to acetonitrile reversemicelle.

3.3. Solvation dynamics

We have measured the decay of C-152A at 430,480 and 580 nm (Fig. 3) in w ¼ 2 of methanol re-verse micelle. From the Fig. 3, it is clearly seen thatthe decay at short and long wavelength are quitedifferent. The decay at long wavelength exhibits adistinct growth in the nanosecond timescale. Thetime resolved emission spectrum (TRES) is con-structed with the help of spectral reconstructionmethod of Maroncelli [29]. In both the reversemicelles we have observed a shift in TRES. Arepresentative TRES at w ¼ 6 of methanol reversemicelle is shown in Fig. 4. This indicates that theprobe molecule is gradually solvated with time.The solvation dynamics is quantitatively measuredwith the help of solvation time correlation func-tion CðtÞ. CðtÞ is defined as

CðtÞ ¼ mðtÞ � mð1Þmð0Þ � mð1Þ ; ð1Þ

where mð0Þ, mðtÞ and mð1Þ are the observed fre-quencies at time zero, t and infinity respectively.To find out the solvation time in methanol andacetonitrile reverse micelles we have excited thesample at 413 nm where the optical density of theprobe in n-heptane is negligibly small. Moreover,to ascertain the solvation dynamics contribution inmethanol and acetonitrile reverse micelles exclu-sively in the core of the reverse micelles we havesubtracted the contribution of n-heptane duringthe construction of TRES.Before going to discuss the detail results of our

solvation dynamics measurement in methanol and

Fig. 4. Time-resolved emission spectra of C-152A in AOT

reverse micelles (w ¼ 6) using methanol at (i) 0 (j), (ii) 200 (),(iii) 1000 (N), (iv) 3000 (.) and (v) 5000 (�) ps.

Table 1

Steady-state absorption and emission spectra of C-152A in non-

aqueous reverse micelles, pure solvents

Polar solvent wa kabs kem(max/nm) (max/nm)

(a) Reverse micelles

Acetonitrile 0 377 450

Acetonitrile 2 380 457

Acetonitrile 4 382 462

Methanol 2 377 465

Methanol 6 378 480

(b) Pure solvents

Methanol 403 510

Acetonitrile 396 501

n-heptane 377 430

aw¼ [polar solvent]/[AOT].

Fig. 3. Fluorescence decays of C-152A in n-heptane/AOT/

methanol reverse micelles in w ¼ 2 at (i) instrument responsefunction (IRF), (ii) 430 nm, (iii) 480 nm and (iv) 580 nm.

526 P. Hazra et al. / Chemical Physics Letters 358 (2002) 523–530

acetonitrile reverse micelles, we would like to givean idea about the location of the probe, whichpredicts the dynamics. The location of the probe

could be determined more accurately by time-resolved fluorescence anisotropy measurement.But due to some instrumentation problem, we didnot able to perform this measurement. But we cangive qualitative idea about the location of theprobe from the difference in absorbance spectra ofC-152A in n-heptane without AOT and with co-solvents (methanol or acetonitrile) containing 0.09M AOT. For both the reverse micelles, the differ-ence in absorbance spectra (Fig. 2) indicates thatthere is a negative absorbance at around 370 nmand positive absorbance at around 410 nm. Thenegative absorbance of C-152A on addition ofAOT and co-solvents at 370 nm, conclude thepopulation of the probe at 370 nm decreases. Onthe other hand the increase as well as appearanceof a distinct peak at 410 nm on addition of AOTand co-solvent suggest that the probe molecule C-152A migrates to reverse micellar core. Moreover,we have excited our sample at 413 nm where theabsorbance of C-152A in n-heptane is minimum,i.e., we are investigating probe molecules residingin the interior of the reverse micelles.The decay feature of CðtÞ is shown in Fig. 5.

The results of solvation dynamics are given inTable 2. The results demonstrated that for boththe reverse micelles, the solvation dynamics areslowing down remarkably compared to pure sol-vents. The bulk methanol and acetonitrile mole-cule relaxes in picosecond and sub-picosecondtimescale respectively [10–12].Beside the slow dynamics, methanol/AOT/n-

heptane reverse micelles indicate a w-dependence.In case of methanol reverse micelles the solvationtime and that the corresponding relative ampli-tudes are as follows: for w ¼ 2, s1 ¼ 1:377 ns,

(a)

(b)

Fig. 5. Decay of the solvent correlation function ðCðtÞÞ of C-152A (a) at w ¼ 2 ðNÞ and at w ¼ 6 ðjÞ of methanol reversemicelles, (b) at w ¼ 2 ðjÞ and at w ¼ 4 ð�Þ of acetonitrile re-verse micelles.

Table 2

Decay characteristics of C-152A in non-aqueous reverse micelles

Polar solvent Dma wb a1 s1 a2 s2 hssi c(ns) (ns) (ns)

Methanol 2007.79 2 0.710 1.377 0.290 13.96 5.02 ð�0:251ÞMethanol 1910.16 6 0.800 0.636 0.200 7.647 2.04 ð�0:102ÞAcetonitrile 1385.82 2 0.39 0.560 0.61 15.44 9.64 ð�0:482ÞAcetonitrile 1262.20 4 0.43 0.434 0.57 15.27 8.89 ð�0:445ÞaDm ¼ m0 � m1.bw ¼ [polar solvent]/[AOT].c hssi ¼ a1s1 þ a2s2.

P. Hazra et al. / Chemical Physics Letters 358 (2002) 523–530 527

a1 ¼ 0:710 and s2 ¼ 13:96 ns, a2 ¼ 0:29 and forw ¼ 6, s1 ¼ 0:636 ns, a1 ¼ 0:800 and s2 ¼ 7:65 ns,a2 ¼ 0:200. This data indicates that with the in-crease of w value the contribution of the fastcomponent of the solvation time is gradually in-creasing with a subsequent decrease in the corre-sponding solvation time value. The microscopicfeature of methanol/AOT/n-heptane reverse mi-celle is very similar to that of water reverse micelle[30]. Like water molecule, methanol molecule maybridge between the SO�

3 and Naþ by H-bonding

(Scheme 2). At low w value most of the methanolmolecules are strongly H-bonded to the SO�

3 andNaþ present at the interface of the reverse micellesand there are only a few ‘free methanol’ molecules.Consequently, the ‘bound methanol’ moleculescontribute more to the solvation dynamics at loww. At high w value, ‘free methanol’ moleculescontribute more to the solvation dynamics. So, weobserve rather fast dynamics at high w value incase of methanol reverse micelle.The solvation dynamics of acetonitrile in dif-

ferent w values show the almost same solvationtime, e.g., at w ¼ 2 the average solvation time is9.64 ns whereas at w ¼ 4 it is 8.89 ns. Shirota andHorie [26] also observed w independence in case ofacetonitrile reverse micelle. In our previous work[27] we also observed a little w dependence in caseof acetonitrile reverse micelle. In contrast to

methanol molecule in reverse micelle, acetonitrilemolecule in reverse micelle cannot bridge betweenthe SO�

3 and Naþ in AOT reverse micelle due to

lack of hydrogen bonding network in acetonitrilesolvent. In acetonitrile, only weak dipole–dipoleinteractions are present. Though it is much weakerthan hydrogen bonding. The absence of hydrogenbonding network is responsible for such a little wdependence in case of acetonitrile/AOT/n-heptanereverse micelle. However, the weak dipole–dipoleinteraction should not be responsible for such aslow dynamics observed in acetonitrile reversemicelle. But the Naþ present in the AOT couldaffect such dynamics because ionic solvation isgenerally slow [24,31–34]. Riter et al. [24] verifiedthe effect of counter ion in AOT reverse micelle onsolvation dynamics. They observed slow dynamicsin case of Naþ ions than that of NHþ

4 ion and thisis due to strong interaction of water with the Naþ

ion present in the interior. Huppert and coworkers[31,32] examined the solvation dynamics in moltensalts and found that solvation dynamics in thesemolten salts are biphasic and occur on the pico-second–nanosecond timescale. Chapman andMaroncelli [33] verified the role of ions in the slowdynamics by taking NaClO4=acetonitrile andNaClO4=methanol solution. They observed that atthe same Naþ concentration, the dynamics in theformer is very slow. In our case also Naþ presentin the AOT may take part in solvation, i.e, ionprobe association is responsible for such a slowdynamics in case of acetonitrile reverse micelle.We can compare our results with the previously

determined solvation dynamics results in methanoland acetonitrile reverse micelles. Shirota and Ho-rie [26] reported the solvation dynamics measure-ment in methanol and acetonitrile reverse micellesusing anionic Coumarin 343 (C-343) as a probe. Inour earlier publication we have used neutralCoumarin 490 (C-490), the smallest Coumarin inthe flexible Coumarin series as a probe to study thedynamics in methanol and acetonitrile reversemicelles [27]. The solvation dynamics results ofShirota and Horie [26] and our previously deter-mined results [27] are summarized in Table 3. Therelative difference in solvation time in our case andShirota and Horie may be due to use of differenttypes of probes as well as different detectionScheme 2.

528 P. Hazra et al. / Chemical Physics Letters 358 (2002) 523–530

techniques used by both groups. Shirota and Horie[26] used Coumarin 343 (C-343) as an ionic probein their experiment whereas we have used a neutralprobe in our measurement. Moreover, the reli-ability and accuracy of our experimental result isbetter than Shirota and Horie [26] because theyhave used Streak Camera as detector and the ex-perimental error is �15% reported by them. Wehave used time correlated single photon counting(TCSPC) detection technique in our measurementand the experimental error in TCSPC is within�5%. The error in the different experimentalmeasurements is tabulated in Tables 2 and 3. Fromthis comparison it can be concluded that solvationdynamics is dependent on probe. Further probedependence can be confirmed by comparing pre-sent results (Table 2) with that of previously de-termined solvation dynamics results (Table 3) byus [27]. The present results are also close to theprevious results in case of acetonitrile reverse mi-celle but for methanol reverse micelle it is littledifferent. So, consulting Tables 2 and 3 we canconclude probe dependence in solvation dynamicsmeasurements in this experiment.

4. Conclusion

In this work we have examined solvation dy-namics of methanol and acetonitrile in AOT re-verse micelle. Solvation dynamics of both the

liquids in reverse micelle have dramatically re-tarded compared to pure solvents. We have alsoobserved w dependence in case of methanol reversemicelle whereas in case of acetonitrile reverse mi-celle a very little w dependence is observed. Dif-ferent w dependence in these two reverse micellescan be explained on the basis of presence andabsence of hydrogen bonding network in these twosolvents. The very slow dynamics in case ofacetonitrile reverse micelle may be due to presenceof Naþ ion present in the AOT. We have alsoobserved probe dependence in solvation dynamicsmeasurements.

Acknowledgements

N.S. is thankful to Department of Science andTechnology (DST) and Council of Scientific andIndustrial Research (CSIR), India for generousresearch grants. All the picosecond time-resolvedexperiments were carried out in the NationalCentre for Ultrafast Processes (NCUFP) inChennai, India. The authors are indebted to Prof.P. Natarajan, the director and Prof. P. Rama-murthy of this national center for their co-opera-tion and encouragement. The authors are alsothankful to Ms. K. Indira Priyadarshini for herassistance in time-resolved experiments. P.H. andD.C. are thankful to CSIR for research fellow-ships.

Table 3

Decay characteristics of C-343 and C-490 in non-aqueous reverse micelles taken from [26,27]

Probe used Polar solvent wa a1 s1 a2 s2 hssib(ns) (ns) (ns)

C-343c Methanol 2 0.21 0.67 0.79 6.49 5.27 ð�0:790ÞC-343c Methanol 4 0.52 0.54 0.48 4.50 2.44 ð�0:366ÞC-343c Acetonitrile 2 0.33 0.28 0.67 11.30 7.66 ð�1:149ÞC-343c Acetonitrile 4 0.38 0.31 0.62 11.30 7.01ð�1:051ÞC-490d Methanol 2 0.48 0.935 0.52 10.34 5.82 ð�0:291ÞC-490d Methanol 6 0.66 0.610 0.34 6.94 2.76 ð�0:138ÞC-490d Acetonitrile 2 0.38 0.477 0.62 14.86 9.40 ð�0:470ÞC-490d Acetonitrile 4 0.42 0.394 0.58 14.77 8.73 ð�0:436Þaw¼ [polar solvent]/[AOT].b hssi ¼ a1s1 þ a2s2.c [26].d [27].

P. Hazra et al. / Chemical Physics Letters 358 (2002) 523–530 529

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