Photoinduced intermolecular electron transfer from aromatic amines to coumarin dyes in sodium dodecyl sulphate micellar solutions

Photoinduced intermolecular electron transfer from aromatic amines tocoumarin dyes in sodium dodecyl sulphate micellar solutionsManoj Kumbhakar, Sukhendu Nath, Haridas Pal, Avinash V. Sapre, and Tulsi Mukherjee Citation: J. Chem. Phys. 119, 388 (2003); doi: 10.1063/1.1578059 View online: http://dx.doi.org/10.1063/1.1578059 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v119/i1 Published by the AIP Publishing LLC. Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors

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JOURNAL OF CHEMICAL PHYSICS VOLUME 119, NUMBER 1 1 JULY 2003

Photoinduced intermolecular electron transfer from aromatic aminesto coumarin dyes in sodium dodecyl sulphate micellar solutions

Manoj Kumbhakar, Sukhendu Nath, Haridas Pal,a) Avinash V. Sapre,and Tulsi MukherjeeRadiation Chemistry & Chemical Dynamics Division, Bhabha Atomic Research Centre,Mumbai 400 085, India

~Received 13 June 2002; accepted 7 April 2003!

Photoinduced intermolecular electron transfer interaction between coumarin dyes and aromaticamines has been investigated in sodium dodecyl sulphate micellar solutions using steady-state andtime-resolved fluorescence quenching measurements. Steady-state fluorescence quenching of thecoumarin dyes by the amine quenchers always shows a positive deviation from linear Stern–Volmerrelationship, which arises due to the localized high quencher concentrations at the micellar Sternlayer. In time-resolved fluorescence measurements, the analysis of the fluorescence decaysfollowing a micellar quenching kinetics model assuming a unified quenching constant (kq8) perquencher occupancy does not give satisfactory results, especially for the higher quencherconcentrations used. The observed fluorescence decays are, however, seen to fit reasonably wellfollowing a bi-exponential analysis for all the quencher concentrations used. The averagefluorescence lifetimes of the coumarin dyes in the micellar solution as estimated from thebi-exponential decay analysis are seen to undergo a systematic reduction with the effective meanquencher concentrations. The bimolecular quenching constants (kq) thus estimated are seen to bemuch smaller than those reported in the homogeneous solutions~e.g., in acetonitrile!, indicating thatthe electron transfer in the micellar media is inherently inefficient. Correlation of the observedkq

values in the micellar solutions with the free-energy changes (DG0) for electron transfer reactionsshow an inversion in the observed rates as predicted by Marcus’ outer sphere electron transfer theoryat exergonicities more that;0.65 eV. To the best of our knowledge this is the first report on theMarcus inverted region observed for the electron transfer reactions in micellar solution. ©2003American Institute of Physics.@DOI: 10.1063/1.1578059#

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I. INTRODUCTION

Electron transfer~ET! is one of the most fundamentaprocesses occurring in chemistry and biology.1–9 In mostchemical systems, transfer of an electron from a ground-sdonor to a ground-state acceptor is energetically unfavoraHowever, photoexcitation of either the acceptor or the dooften makes the ET reaction energetically favorable and tthe reaction occurs with a reasonable rate. In the lastdecades such photoinduced ET~PET! reactions in homogeneous solutions have been investigated extensively to unstand different aspects of ET dynamics and mechanism.1–17

Though PET processes in the homogeneous solutionsextensively studied, reports on such processes in the onized media, e.g., in micellar solutions, are not that extsive. In an organized media, the local environment wherereactant molecules reside is drastically different than thathe bulk homogeneous solutions.18–21Thus the dynamics andthe mechanism of the reactions in organized media arepected to differ largely as compared to those in homogenesolutions.18–23 Reactions in the organized media are alsodirect relevance to biology, medicinal chemistry, host-guchemistry, catalysis, molecular electronics, and many oth

a!Author to whom correspondence should be addressed. Fax: 91-22-25151. Electronic mail: [email protected]

3880021-9606/2003/119(1)/388/12/$20.00

Downloaded 17 Sep 2013 to 128.118.88.48. This article is copyrighted as indicated in the abstract.

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It is thus interesting to investigate such processes in onized media to understand how the confined geometry ctrols the mechanism and dynamics of such reactions.

Similar to other chemical reactions, the ET also involvthe crossing of the free-energy surface of the reactant toproduct at the transition state. The classical theory ofouter-sphere ET reactions, as developed by Marcus,presses the ET rate constant,kET , in its simplified form, as5

kET5n expS 2~DG01l!2

4lkBT D , ~1!

wheren is the frequency of motion in the reactant potentwell, DG0 is the free-energy change for the ET reaction,kB

is the Boltzmann constant,T is the absolute temperature, anl is the total reorganization energy, given asl5(ls1l i),ls being the solvent reorganization energy andl i being theintramolecular reorganization energy. The most interestfeature of the Marcus ET theory is the expected inversionthe ET rates as the exergonicity of the reaction (2DG0)exceedsl. Though the prediction was made by Marcus bain 1956, experimental evidence for the Marcus invertedgion could only be realized in the mid-eighties.24–26 Atpresent, there are several experimental demonstrations oMarcus inverted region for the forward electron transfer pcess in systems where the donors and acceptors are0-

© 2003 American Institute of Physics

Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

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389J. Chem. Phys., Vol. 119, No. 1, 1 July 2003 Photoinduced electron transfer in micellar solution

nected by chemical bonds,~i.e., intramolecular ET!.24–28An-other class of ET studies where Marcus inverted regionbeen observed is the charge recombination~CR! reactions inthe contact ion pair~CIP! states.29–32 For intermolecular ETreactions, so far, the Marcus inverted region could notobserved experimentally. This is mainly because of thefusion of the reactants, a rate determining step (kd), requiredto bring the reactants together for ET to take place. Thuthe intermolecular ET reactions under diffusive conditiothe diffusion process imposes an upper limit on the obserreaction rate and consequently masks the observation oexpected Marcus inverted region in the ET reactions.5,8,9

ET processes in organized media, e.g., in micellar sotions, could be the good candidates to show the Marcusverted region. In an organized media, the reactants arefined in a small volume. The environment of the locaconfined region is quite different than that in the bulk sotion. Further, the reactants are supposed to get entangledin the macromolecular chains of the surfactant moleculesthus the mobility of the reactants in micelles are expectedbe largely reduced.23 Thus the diffusion of reactants, whicis mainly responsible for masking the inverted region inintermolecular ET reactions in homogeneous solution, canlargely prevented by the rigid micelle structures. The retion dynamics and mechanism in micellar solutions are texpected to differ largely as compared to those in the hogeneous solutions.18–23

The other reason for not observing the Marcus inverregion in the intermolecular ET reactions in the homogneous solutions is the lack of availability of suitable donoacceptor pairs, which can offer a large exergonic(2DG0) for the reactions. Inversion in the ET rate (kET) canbe observed only when the exergonicity of the reactionceeds the reorganization energyl. Further, in the Marcusinverted region, the diffusional rate of the reactants inhomogeneous solution limits the observed reaction ratelong askET.kd . Thus to observe inversion inkET , the ex-ergonicity of the reaction to be made very large so thatkET,kd situation prevails. To observe Marcus invertedgion in the intermolecular ET reactions in homogeneouslutions, it is thus understood that the reactions should hvery high driving force and low reorganization energy. Itreported in the literature33–40 that the solvent motion islargely retarded in the confined medium, like cycldextrine,35,36microemulsion,37,38micelle,39,40etc. Because ofthis retardation of the solvent motion in the organized medthe role of solvent reorganization to the observed ET ramay be much less effective as compared to that in the bhomogeneous solution.41–45 Thus, for ET reactions in theorganized media, the effective reorganization energy cobe much lower than in homogeneous solution and conquently the Marcus inverted region could be achieved at rtively less exergonicities compared to that expected in amogeneous solution.

Coumarin dyes are good electron acceptors in theircited singlet (S1) states.41–50 For the last couple of yearsPET from amine donors to coumarin dyes have been invtigated quite extensively by different groups including ou


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in the homogeneous solutions.41–45,47–50In the present workwe have investigated the ET processes between the ex(S1) coumarin dyes and the aromatic amine donors indium dodecyl sulphate~SDS! micellar solutions using bothsteady-state ~SS! and time-resolved ~TR! fluorescencequenching measurements. Our main aim in this workbeen to see if in the micellar solutions the ET interactiobetween coumarin dyes and amine quenchers show anyference in comparison to the observations made earlier inhomogeneous solutions.48–50The aim has also been to findMarcus inverted region can be observed under the conditwhere ET reactions occur under restricted geometry.

II. MATERIALS AND METHODS

Laser grade coumarin dyes were obtained from Exciand used as received. Aromatic amines were obtained fSpectrochem~India! or Qualigens Fine Chemicals~India!and were purified by vacuum distillation just before use.SDS sample was obtained from Sigma and purified bycrystallization from methanol-water solvent mixture. Thchemical structures of the acceptors~coumarin dyes! and thedonors~aromatic amines! used in this study are given in Fig1.

Ground-state absorption measurements were madea Shimadzu UV-vis spectrophotometer, model UV-160Steady-state fluorescence measurements were carriedwith a Hitachi spectrofluorimeter, model F-4010. Timresolved fluorescence measurements were carried out us

FIG. 1. Chemical structures of the acceptors~coumarin dyes! and donors~aromatic amines! used in the present study. The abbreviations useddifferent acceptors and donors are also indicated.


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390 J. Chem. Phys., Vol. 119, No. 1, 1 July 2003 Kumbhakar et al.

time-correlated single photon counting~TCSPC! ~Refs. 51and 52! spectrometer from Edinburgh Instruments, Umodel 199. For the present experiments a hydrogen ficoaxial flash lamp having a pulse width of about 1.2 nsfull width at half maximum~FWHM! was used as the exctation source and a Philips XP-2020Q photpmultiplier tuwas used for the fluorescence detection. The fluorescedecays were analyzed by re-convolution procedure51,52usinga proper instrument response function, as obtained by sututing the sample cell with a light scatterer. The fluorescedecays were analyzed following either a reported micequenching kinetic scheme discussed in Sec. III B, or folloing exponential functions, expressed in general as

I ~ t !5( Ai exp~2t/t i !, ~2!

where t i is the fluorescence lifetime andAi is the pre-exponential factor for thei th component of the decay. Thquality of the fits were judged by the reduced chi-squ(x2) values and the distribution of the weighted residuamong the data channels.51,52For good fits thex2 value wereclose to unity and the weighted residuals were distriburandomly among the data channels.51,52

The reduction potentials of the coumarin dy$E(A/A2)% in SDS micellar solutions were measured by cclic voltammetric ~CV! method using a Eco-ChemiPotentiostat/Galvanostat-20, withGPES software. Solutionsof the coumarin dyes in micellar solutions containing 0mol dm23 potassium chloride as the supporting electrolwere first de-aerated by purging high-purity N2 gas for about30 min. CV measurements were then carried out usdropping mercury as the working electrode, graphiteas the counter electrode, and silver-silver chlor$Ag/AgCl/Cl2(saturated)% as the reference electrode. Thvalues thus obtained were then normalized with respecthe saturated calomel electrode~SCE!53,54 to be used in theestimation of the free-energy changes for ET reactions inpresent systems.

III. RESULTS AND DISCUSSION

A. Quenching studies using steady-statefluorescence measurements

All the coumarin dyes used in the present work aremost insoluble in water.55 Thus, in saturated aqueous soltions, the concentrations of the coumarin dyes were emated to be only in the range of;531027 mol dm23, usingspectrophotometric measurements. In SDS micellar stions, however, the dyes are reasonably soluble. Forpresent experiments the total coumarin concentrations inSDS micellar solutions were kept in the range of ab(10– 20)31026 mol dm23. For all the experiments the totaSDS concentration were kept constant at about31023 mol dm23, well above its critical micellar concentration ~CMC! of 8.031023 mol dm23.18 At this SDS concen-tration, the effective micelle concentration is estimated toabout 1.1631023 mol dm23, assuming the average aggregtion number for SDS micelle is 62.18 Thus the coumarinconcentration was always 50–100 times less than the e


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tive micelle concentration in the solution. Since the distribtion coefficients for the coumarin dyes in the micellar athe aqueous phases are reported to be very high, in the rof ;8000 dm3 mol21,56 under present experimental condtions it is reasonable to consider that almost all the dye mecules reside in the micellar phase and only;1–2% of themicelles are occupied by the coumarin molecules.

As the coumarin dyes are relatively polar molecules~di-pole moments;6 D!,57 it is expected that the dye moleculewill reside mostly at the micellar Stern layer,18–23 which isalso supported by the results reported in the literature.58,59

This is further supported by a comparison of the fluorescemaxima (n̄max) of the coumarin dyes in SDS micellar solutions with those measured in ethanol-water solvent mixtuwith different co-solvent compositions, indicating the dieletric constant~«! for the local environment of the coumaridyes in SDS micellar solutions is of the order of about 44.the literature« values for the SDS micellar Stern layer areported to be of the order of;32.18,60Though the reason fothe higher« value estimated from the present measuremeis not very clear, one possible reason could be the effecintermolecular hydrogen bonding between coumarin dand the water molecules,61–69 present in the micellar Sternlayer. It has been observed that the fluorescence spectthe coumarin dyes undergo an extra redshift of about 10nm due to intermolecular hydrogen bonding with solvemolecules,57,61 which can result in the overestimation of th« value for the micellar Stern layer using fluorescence sptral shifts. Since our estimate of«544 appears to be unusually higher than the average«;32 reported in theliterature,18,60 for energetic calculations and other correltions in the rest of the paper we will use an« value of 32 forthe SDS micellar Stern layer.18,60Amines used in the presenstudy are also quite insoluble in water and are supposemainly reside at the micellar Stern layer.22,70–72

From SS measurements, it is seen that the fluoresceof the coumarin dyes in SDS micellar solutions is quenchby the added aromatic amines in the solutions. Typical flrescence spectra of C153 in SDS micellar solution inabsence and presence of different concentrations of DMare shown in Fig. 2. It is indicated from Fig. 2 that, thouthe fluorescence intensity of the dye is reduced substantin the presence of the amine quenchers, the shape offluorescence spectra does not change even in the presenthe highest concentration of the amine used. Similar reswere also obtained for the other coumarin-amine systemSDS micellar solutions. These results thus indicate that this no exciplex formation during the interaction of the excitcoumarin dyes with the amine quenchers.73–75 From theground-state absorption studies also it is observed thatabsorption spectra of the coumarin dyes remain unchaneven in the presence of the highest concentration ofamines used. It is thus indicated that no ground-state cplex formation between the coumarin dyes and the amdonors occurs in the micellar solutions. Typical ground-stabsorption spectra for C153 in SDS solution in the abseand presence of about 23.631023 mol dm23 total concentra-tion of DMAN are shown in the inset of Fig. 2.

To have an estimate of the quenching process in


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present systems, SS fluorescence quenching results wererelated following the Stern-Volmer~SV! relationship,73–75

I 0

I511KSV@Q#eff , ~3!

where I 0 and I are the fluorescence intensities for the comarin dyes in the absence and presence of the quenc(Q[aromatic amines), respectively,KSV is the SV constant,and @Q#eff is the effective concentration of the aminquenchers in the micellar Stern layer. Since almost all ofaromatic amines reside in the micellar Stern layer,22,70–72theeffective amine concentration in the micellar phase was emated, based on the following considerations. At SDS ccentrations used in the present work, effectively sphermicelles are formed.18–23The average diameter of these mcelles is reported to be about 60 Å, with the nonpolar cdiameter of about 42 Å.23 Thus the thickness of the micellaStern layer is taken to be 9 Å, which gives the volume ofmicellar Stern layer to be about 7.433104 Å 3 per micelle or44.75 dm3 per mole of the micelle. Considering such a voume and assuming that all the amine molecules reside inmicellar Stern layer, the effective amine concentration inmicellar Stern layer is estimated as

@Q#eff5Nag@Q# t

44.75$@SDS# t2CMC%, ~4!

whereNag is the average aggregation number for SDS mcelle (Nag;62),18 @SDS# t is the total SDS concentratioused (8031023 mol dm23), CMC is the critical micellarconcentration of SDS micelle (;831023 mol dm23),18 and@Q# t is the total amine concentration used in the solution

Typical I 0 /I vs @Q#eff plots ~SV plots! for some of thecoumarin–amine systems obtained from the SS fluorescquenching measurements are shown in Fig. 3, which shopositive deviation from SV linearity. These results thus incate that as the amine concentrations are increased thergradual increases in the static quenching of the excited cmarin dyes in the micellar solutions.73–75As the ground-stateabsorption studies indicate, up to the concentration ra

FIG. 2. Quenching of C153 fluorescence by DMAN in SDS micelle. Fspectra 1–6 @DMAN # were: 0, 1.75, 3.93, 7.65, 12.87, and 2331023 mol dm23, respectively. Inset: Effect of DMAN on the C153 absorption spectra in SDS micellar solution. Spectrum with solid line is inabsence of DMAN and that with dashed line is in the presence of 231023 mol dm23 of DMAN.


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used for@Q#eff in Fig. 3, there is no indication of the groundstate complex formation between the coumarin dyes andamine quenchers. Thus the static quenching as indicatethe positive deviations in Fig. 3 seems to be arising duethe higher local concentration of the amine quenchers inmicellar Stern layer, which causes some of the coumaramine pairs to be in close contact. Such coumarin dyesundergo instantaneous quenching on excitation by the ammolecules in close contact, resulting in a positive deviatfrom the SV linearity.

Since a positive deviation in theI 0 /I vs @Q#eff plots isobserved in the micellar solutions for all the donor-acceppairs studied in the present work,KSV values for SS quenching were estimated from the initial slopes of the SV ploThe kq values for SS quenching were then calculated frtheseKSV values, knowing thet0 values of the coumarindyes in the absence of the quenchers~Sec. III B!. The kq

values thus estimated for different coumarin–amine systeare listed in Table I.

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FIG. 3. TheI 0 /I vs @Q#eff plots as obtained from the steady-state fluorecence quenching of C151, C153, and C481 dyes in SDS solutionsDMAN. The plots are seen to gradually undergo a positive deviation frlinearity with an increase in the amine concentration.

TABLE I. List of the bimolecular fluorescence quenching constants (kq) forthe coumarin–amine systems in SDS micellar solutions as obtainedsteady-state~SS! and time-resolved~TR! fluorescence quenching studies.

Coumarins t0 /ns Amines

kq/109 M21 s21

SS TR

C151 5.86 DMAN 2.285 1.253DEAN 1.800 1.310DMPT 2.575 1.392

C152 1.02 DMAN 2.784 2.262DEAN 3.944 2.173DMPT 6.612 2.355

C481 0.59 DMAN 8.236 3.561DEAN 6.844 3.376DMPT 4.060 2.784

C522 4.68 DMAN 2.018 1.339DEAN 1.682 1.339DMPT 2.459 1.509

C153 3.56 DMAN 1.601 0.986DEAN 1.531 0.998DMPT 2.192 1.242


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B. Quenching studies using time-resolvedfluorescence measurements

The fluorescence decays of the coumarin dyes in Smicelles were measured in the presence of different contrations of amines to understand the dynamics of interacin the present systems. It is seen that the fluorescence deof the dyes gradually become faster as the amine concetion is increased in the solution. In the absence of the amithe fluorescence decays for all the coumarin dyes are sefit well with a single-exponential function. The fluorescenlifetimes (t0) for coumarin dyes in SDS micellar solutionsthe absence of the quenchers thus estimated are listeTable I. In the presence of the amine quenchers, the fluocence decays are seen to become nonsingle exponentimicellar solutions, the quenching kinetic scheme has bwell discussed in the work of Tachiya.76–78According to thisscheme, the quenchers are assumed to migrate betweemicellar and the aqueous phases, and the excited fluphores are modeled to get quenched with a unified quencconstantkq8 per quencher occupancy in the micelle.76–78Fol-lowing this kinetic scheme, Tachiya derived Eq.~5! for thetime-dependent fluorescence quenching process in the mlar solution,76–78

I ~ t !5B expF2S k01n̄~kq8!k2

kq81k2D t

2n̄~kq8!2

~kq81k2!2 $12exp@2~kq81k2!t#%G , ~5!

wherek2 represents the rate constant for the exit procesthe quencher molecules from the micellar phase, andk0 isthe fluorescence decay rate constant in the absence oquenchers (k05t0

21), andn̄ is the average occupancy number of the quenchers in the micelles. The fluorescence deof the coumarin dyes in the presence of different concentions of amine quencher were tried to fit with Eq.~5!. Thefitting was found to be not satisfactorily in most cases athe fitted parameters obtained from such fittings are also sto be often unreasonable. Thus it is indicated that forpresent system, kinetics represented by Eq.~5!, is not appli-cable. The possible reason for this can be as follows. Forpresent systems, since the solubility of both fluoropho~coumarin dyes! and quenchers~aromatic amines! are verylow in aqueous media, it is expected that both the reactpreferentially exist in the micellar phase. Thus the exist rconstant of the quencher from the micelles is expected toquite low. For the micellar solutions, the exit of the reactaare reported to occur mostly in the microsecond tiscale.76,78 Since thet0 values of the coumarin dyes investgated in the present work are all in the nanosecond range~cf.Table I!, it is expected that within the lifetime of the fluorophores, there will be hardly any entry or exit of the quencers. Under this situation, Eq.~5! can be modified asfollows:76

I ~ t !'B exp„2k0t2n̄$12exp@2~kq8!t#%… ~6!

and the average occupancy numbern̄ can given by Eq.~7!,76


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where@Q# t and @M # t are the total quencher and the miceconcentrations, respectively. The most important featureEq. ~6! is that at very long time theI (t) must show a decaycomponent with time constantk0 .76–78 Since thet0 for thecoumarin dyes are in the nanosecond range, if the quencprocess also occurs in the similar time scale, this long-tik0 component can, however, be obscured.

For the present coumarin–amine systems, the obsefluorescence decays in the presence of different quenconcentrations were fitted following Eq.~6!. Typical fluores-cence decay curves for C522-DMAN systems in SDS miclar solutions along with the corresponding fitted curves otained following Eq. ~6! are shown in Fig. 4~A!. Thedistributions of the weighted residuals among the data chnels for each of these decay analyses are shown in the lopanel of Fig. 4~A!. It is seen that, though in the lowequencher concentrations decays fit reasonably, at higquencher concentrations, the fits are very poor. Thus forpresent systems the results obtained from the analysis odecays following Eq.~6! in the presence of lower quencheconcentrations are only considered for further consideratiand are listed in Table II.

Since the fluorescence decays of the coumarin dyethe presence of high amine concentrations do not fit withmicellar kinetic@Eq. ~6!# the decays were analyzed followin

FIG. 4. Fluorescence decay curves of C522 in SDS micellar solutions inabsence and presence of different concentrations of DMAN. For decurves~1!–~4! n̄ were 0, 2.04, 7.29, and 12.76, respectively. The curveLrepresents the instrument response function. The curve~1! was fitted with asingle-exponential function, givingt54.68 ns andx251.02. ~A! Decaycurves in presence of amines were fitted with micellar kinetics@Eq. ~6!#,giving the fitting parameters as~2! kq853.283107 s21, x251.08; ~3! kq853.593107 s21, x251.56, and~4! kq853.793107 s21, x253.01, respec-tively. ~B! Decay curves in presence of amines were fitted withexponential function giving the fitting parameters as~2! t153.84 ns~96.2%!, t250.53 ns ~3.8%!, x251.03, ~3! t152.47 ns ~88.8%!, t2

50.49 ns ~11.2%!, x251.11, and ~4! t151.78 ns ~78.2%!, t250.29 ns~21.8%!, x251.19, respectively. The lower panel of the figure showsdistribution of the weighted residuals among the data channels for cosponding above fitted data.


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multiexponential function@cf. Eq. ~2!#. It was seen that forall the amine concentrations used, the decays fit well witbi-exponential function. Typical bi-exponential analysisthe fluorescence decays of C522 in the presence of diffeconcentrations of DMAN in SDS micellar solutions ashown in Fig. 4~B!. The distributions of the weighted residuals of these decay analyses are shown at the lower panFig. 4~B!. Comparing Figs. 4~A! and ~B! it is indicated thatthe decays fit better with bi-exponential function than tmicellar kinetic Eq.~6!.

At this point it is interesting to consider why the kinetEq. ~6! does not give a good fit for the observed decay inpresent systems. In this model it is assumed that ET trantakes place when the reactants comes in contact throughfusion and thus the overall reaction rate is assumed to bkq8multiplied by the quencher occupancy number, i.e.,n̄kq8 .However, if the diffusion of the reactants is not fast enouthe overall rate constantn̄kq8 will be an oversimplification.Under this slow diffusion or nondiffusive condition, thoverall rate will depend on the mutual separation betwquenchers and fluorophores in the micelle and the quencconstant will be a function of the relative positions of tquenchers with respect to the fluorophore. Assumingsuch a situation prevails for the present system, as willdiscussed further in Sec. III C, consideration of the unifiedkq8per quencher occupancy seems to be an oversimplificaand is the cause for Eq.~6! not holding good for the presensystem.

Since the fluorescence decays in the presence ofquencher fitted well with bi-exponential function, in ordercorrelate the interaction strengths of different coumaramine systems, the average fluorescence lifetime of themarin dyes have been considered. By definition the averlifetime t is given by Eq.~8!:

t'*0

`tI ~ t !dt

*0`I ~ t !dt

. ~8!

TABLE II. List of the unified quenching constant$kq8 @cf. Eqs.~5! and~6!#and bimolecular quenching constantkq9 @cf. Eq. ~11!# for the coumarin–aromatic amine systems in SDS micellar solutions as obtained fromanalysis of the fluorescence decays following micellar kinetic Eq.~6!.

Coumarins Amines n̄ kq8/107 s21 kq9/109 M21 s21

C151 DMAN 0.94 3.57 1.598DEAN 1.48 3.84 1.718DMPT 1.88 4.17 1.866

C152 DMAN 1.79 5.27 2.359DEAN 1.35 4.94 2.211DMPT 2.10 5.41 2.421

C481 DMAN 0.94 9.52 4.260DEAN 1.35 10.01 4.479DMPT 2.31 6.55 2.931

C522 DMAN 1.97 3.57 1.598DEAN 0.76 3.85 1.723DMPT 1.05 4.12 1.844

C153 DMAN 0.94 2.17 0.971DEAN 1.06 2.32 1.038DMPT 1.504 2.898 1.297


afnt

of

eferif-

,

nng

ate

on

he

–u-ge

Since theI (t) is seen to fit well with a bi-exponential function @cf. Eq. ~2!#, Eq. ~8! can be solved to obtain followingequation for the average fluorescence lifetimet:

t51

100~A1t11A2t2!, ~9!

wheret1 andt2 are respective lifetimes of the fast and slocomponents of the fluorescence decays andA1 and A2 aretheir respective percentage contributions.79 It is seen that forall the coumarin–amine pairs the average fluorescencetimes ~t! of the coumarin dyes thus estimated gradually dcrease with an increase in the amine concentration in Smicellar solution.

To have an estimate of the quenching constant, theduction in the average fluorescence lifetime~t! of the cou-marin dyes in SDS micellar solutions with effective amiconcentrations were correlated following the SV relations@Eq. ~10!#.73–75Interestingly, it is seen that unlike the caseSS quenching,t0 /t vs @Q#eff plots obtained from TRquenching measurements follow the linear SV relation:73–75

t0

t511KSV@Q#eff511kqt0@Q#eff . ~10!

Typical t0 /t vs @Q#eff plots for some of the coumarin–aminsystems obtained in TR quenching measurements are shin Fig. 5. Thekq values obtained from the slopes of the linet0 /t vs @Q#eff plots for different coumarin–amine systemare listed in Table I. It is seen from this table that for aparticular coumarin–amine system, thekq values obtainedfrom TR measurements are always lower than those obtafrom SS measurements. The apparently higherkq values ob-tained from the SS measurements are obviously due tocontribution from the static quenching, which is not reflectin the measuredt values, as the fluorescence decays are msured with a nanosecond TR instrument. Thus we infer tkq values obtained from TR measurements represent theteraction kinetics arising due to the dynamic quenchingthe excited dye molecules with the amine quenchers inmicellar Stern layer. In further analysis of our quenchiresults thus we will be dealing only with thekq values ob-tained from TR measurements. At this point it should

e

FIG. 5. Thet0 /t vs @Q#eff plots as obtained from the time-resolved fluorescence quenching of C151, C153, and C481 dyes in SDS solutionDMAN following Eq. ~10!. The plots are seen to be linear as expected frthe Stern–Volmer relation@cf. Eq. ~10!#.


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mentioned that thekq (M21 s21) defined in Eq.~10! is dif-ferent than thekq8 (s21) defined in Eqs.~5! and ~7!. As kq8represents the quenching constant per quencher occupanthe micelle, assuming that all the quenchers are in the milar Stern layer, an equivalent bimolecular quenching conskq9 (M21 s21) can be obtained by using the following reltion:

kq95kq8344.75, ~11!

where the factor 44.75 comes to consider the molar voluof the SDS micellar Stern layer. Thekq9 values thus obtainedare listed in Table II. Comparing thekq ~TR! ~cf. Table I! andkq9 ~cf. Table II! values it is seen that both the quenchiconstants are almost in a similar range. Thus it is indicathat the bi-exponential analysis of the fluorescence decfollowed by SV analysis of the average lifetimes resultsthe satisfactory estimation of the micellar quenching kinics.

Regarding the mechanism for the observed quenchprocess in the present systems, a number of alternate pbilities exist. Among these, ET from amine donors to texcited coumarin dyes seems to be the most likely mecnism, as the coumarin dyes are good electron acceptorsthe amines are good electron donors.41–50 Further, for thesimilar coumarin–amine systems in the homogeneous stions, the presence of the ET mechanism has been confirin our earlier works using picosecond laser flash photoly~LFP! and pulse radiolysis~PR! experiments.48–50 Drawingan analogy with these earlier results, the quenching proin SDS micellar solutions is attributed to ET interactionthe excited coumarin dyes with the amine donors. An alnative mechanism like singlet-singlet energy transfer frexcited coumarins to the ground-state amines is ruled outhe S1 state energies (E00) of the amine quenchers80 aremuch higher than those of the coumarin dyes~cf. Table III!.Similarly a mechanism involving proton-transfer/hydrogbonding interactions of the coumarin dyes with the amquenchers can also be easily ruled out from the followconsiderations:~i! The tertiary amines used as the quench

TABLE III. Redox potential,E00 values, andDG0 values for the coumarin–amine systems studied in SDS micellar solution.

CoumarinaE(A/A2)/V

vs SCE E00 /eV AminesE(D/D1)/V

vs SCE DG0/eV

C151 21.396 2.815 DMAN 0.595 20.893DEAN 0.555 20.931DMPT 0.496 20.991

C152 21.445 2.650 DMAN 20.676DEAN 20.714DMPT 20.774

C481 21.500 2.634 DMAN 20.604DEAN 20.641DMPT 20.701

C522 21.480 2.581 DMAN 20.570DEAN 20.608DMPT 20.668

C153 21.538 2.488 DMAN 20.419DEAN 20.456DMPT 20.516


y inl-nt

e

dys

-

gsi-

a-nd

u-edis

ss

r-

as

egs

in the present work cannot act as proton/hydrogen-bondnors to the coumarin dyes, as there is no polar hydrogethese molecules.~ii ! Considering the amines to act as protohydrogen-bond acceptors, since none of the coumarin dexcept C151 have a polar hydrogen available~e.g., the hy-drogens of the 7-NH2 group!, fluorescence quenching wanot expected for these dyes except for C151. From the abconsiderations it is clear that the quenching of the excicoumarin dyes by amine quenchers in SDS micellar sotions occurs due to ET mechanism.

At this point it is interesting to compare present resuin SDS micellar solutions with those obtained earlier for tsimilar coumarin–aromatic amine systems in homogenesolution using acetonitrile as the solvent.48–50 It is seen thatin SDS micellar solutions, the observedkq values are muchlower than those observed for similar coumarin–amine stems in the homogeneous solutions~in acetonitrile!. Thusin acetonitrile the maximumkq values were observed to bin the diffusion-controlled limit of ;1.531010 dm3

mol21 s21,48–50where as in SDS solutions the maximumkq

values are of the order of;3.63109 dm3 mol21 s21. It isthus indicated that the quenching process in the micellarlutions is less efficient than in the homogeneous solutionsmay be mentioned that similar observations have also bmade in the literature for the other donor-acceptor systemmicellar solutions and explained on the basis of the laraverage donor-acceptor separation in the micelles than inmogeneous solutions.23 It is supposed that the reactants anormally entangled by the macromolecular chains of the sfactant molecules, and thus the donor-acceptor separatiohigher inside the micelles than in homogeneous solution,pecially for the dynamic part of the quenching as estimafrom TR measurements.

C. Correlation of the quenching constants with thefree-energy changes of ET reactions

Feasibility of PET from a ground-state donor~amines! toan excited-state acceptor~coumarin dyes! is mainly dictatedby the standard free-energy change (DG0) for the ET reac-tion. In general theDG0 for a PET reaction between a dono~D! and an acceptor~A! can be expressed by the followinrelation:5,81

DG05IPD2EAA2DgS02DgS

e2e2

«SR2E00, ~12!

where IPD is the ionization potential of the donor, EAA is theelectron affinity of the acceptor,DgS

0 andDgSe are the solva-

tion energy of the produced free ions due to the orientatioand electronic polarization of the solvent, respectivee2/«SR is the Coulomb energy to bring the two ions atseparation distance ofR ~interaction distance!, «S is the staticdielectric constant of the reaction medium, andE00 is theexcitation energy of the fluorophore in theS1 state. Equation~12! can also be presented in terms of the redox potentialdonor$E(D/D1)% and acceptor$E(A/A2)%, as the latter arerelated to the ionization potential and electron affinity of tdonor and acceptor, respectively, by Eq.~13!,5

E~D/D1!2E~A/A2!5IPD2EAA2DgS02DgS

e . ~13!


tionx

te

ezedset wo

ine

-te

Tnethinrep

th

temitefutrosub

-

s

th.7rte

heor. Iinndas

ree

theatolu-der

theS

rva-i-

co-

s-


Thus considering Eq.~13!, Eq. ~12! can be rewritten as

DG05E~D/D1!2E~A/A2!2e2

«SR2E00 ~14!

which is the well-known Rehm–Weller equation.82

For the present coumarin dyes the reduction poten$E(A/A2)% values were estimated in SDS micellar solutiusing CV measurements and are listed in Table III. The odation potential values$E(D/D1)% of the amine donors inSDS micellar solutions were estimated from the reporE(D/D1) values in acetonitrile solutions,42,83,84 and arelisted in Table III. E00 values of the coumarin dyes werobtained from the intersecting wavelengths of the normalifluorescence and excitation spectra and are also listeTable III. The«S value for the Stern layer of SDS micellewas considered to be;32.18,60 Since the encounter distancR cannot be accurately estimated, in the present contexassumed it to be equal to the sum of the radii of the donand acceptors used. The radii of the donors (r A) and accep-tors (r D) for the molecules studied were estimated usEdward’s volume addition method, assuming these to befective spheres.85 The DG0 values thus estimated for different donor-acceptor pairs in SDS micellar solutions are lisin Table III.

Figures 6~A!–~C! show the plot of ln(kq) vs DG0 forcoumarin-DMAN, coumarin-DEAN, and coumarin-DMPsystems, respectively, in SDS micellar solutions, as obtaifrom TR results. In these correlations, instead of plottingdata points for all the coumarin–amine systems togethersingle plot, we preferred to distinguish ET systems withspect to the different amines used, to have a more clearsentation of the data points in ln(kq) vs DG0 plots. Consid-ering all the coumarin–amine systems together, thoughoverall trend for the ln(kq) vs DG0 plot appeared to remainsimilar to those seen in Figs. 6~A!–~C!, the data pointsshowed large fluctuations. It was thus indicated that ET raare also largely dependent on the amine donors used. Itbe mentioned that in the ET interactions between exccoumarin dyes with different amine donors under nondifsive conditions, it has already been shown that the eleccoupling matrix elementVel and consequently ET ratelargely depend on the donor amines and their sstituents.41–44,47

From Figs. 6~A!–~C!, it is indicated that there is an inversion in the ET rate as the exergonicity (2DG0) of thereaction exceeds about 0.6–0.7 eV. Though the analysithe fluorescence decays following micellar kinetic Eq.~6!was not very successful, yet thekq9 values as obtained fromsuch analysis followed by Eq.~11! were also plotted vsDG0

and showed similar inversion in ET rates as in Fig. 6, asexergonicity (2DG0) of the reaction exceeds about 0.6–0eV. Present ET systems thus appear to display the inveregion in the ET rates as predicted by Marcus outer sphET theory.1–17 It may be noted that such an inversion in tln(kq) vs DG0 plots is so far not reported in the literature fany of the intermolecular ET systems in micellar solutionsmay also be mentioned here that for the similar coumaramine systems in homogeneous acetonitrile solutions udiffusive conditions, no such Marcus’ inverted region w


al

i-

d

din

ers

gf-

d

dea-re-

e

sayd-n

-

of

e

edre

t–er

observed.48–50 In fact, in acetonitrile solution, the ln(kq) vsDG0 plots for the similar coumarin–amine systems weseen to get leveled off at the diffusion-controlled limit for thbimolecular reaction rate at the higher exergonicities ofreactions.48–50 It is thus evident from the present results ththe coumarin–aromatic amine systems in SDS micellar stions behave differently than in homogeneous solution undiffusive condition.

The appearance of the Marcus inverted region inln(kq) vs DG0 plots for the coumarin–amine systems in SDmicellar solutions appears to be a very interesting obsetion. Due to the microheterogeneity, the PET reaction in mcellar media is a complex phenomenon. Fayer and

FIG. 6. The ln(kq) vs DG0 plots for ~A! coumarin-DMAN, ~B! coumarin-DEAN, and~C! coumarin-DMPT systems in SDS micellar solutions. A ditinct inversion in the ET rates are observed as the exergonicity (2DG0) ofthe reactions exceed a value of about 0.6–0.7 eV.


thon

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inx-et-itfon-innar

tot

gh

b-

o

e

e

heifi-hehsoheTane

ve

stet

eaausto

ac-long

T re-

ex-us

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is

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workers modeled the micellar ET processes asbimolecular reactions with the reactants diffusing freelythe two-dimensional micellar surface.22,86–91 Based on thismodel, it is difficult to rationalize the Marcus inverted rgion, as is observed for the coumarin–amine systems in Smicellar solution~cf. Fig. 6!. If the reactants were freelydiffusing, then it is expected that the bimolecular quenchprocess will effectively be diffusion controlled at higher eergonicity (2DG0) region. Thus, instead of displaying thMarcus inverted region, thekq values should have been geting leveled off at the bimolecular diffusion-controlled lim(kd) at higher exergonicity, as are generally observedmost bimolecular reactions under diffusive coditions.48–50,73–75If, however, it is argued that the ET ratesthe micellar solutions are much slower than the diffusiorates of the reactants, then one can apparently account foobserved Marcus inverted region for the present system5

Such a situation, however, seems to be unlikely becausemolecular diffusion inside the micelle is very unlikely texceed the time scales of the present ET reactions. Forcoumarin–amine systems in SDS micellar solution, the hiest kq values obtained are of the order of;3.63109 dm3 mol21 s21 from TR measurements and;8.23109 dm3 mol21 s21 from SS measurements. Thus, to oserve the Marcus inverted region, thekd value in the SDSmicellar solution should be at least of the order;1010 dm3 mol21 s21. Such a highkd value is unlikely inthe micellar phase, as the microviscosity~h! in the micellarStern layer (h;10– 30 cP)18,40,90 is much higher than theviscosity of most of the bulk solvents~e.g.,h for acetonitrileis ;0.345 cP!.92 It should be mentioned that for the samcoumarin–amine systems thekd value in acetonitrile solutionwas found to be of the order of 1.531010

dm3 mol21 s21.48–50 Comparing the h values insidemicelles18,40,90 and in bulk acetonitrile solution,92 one canexpect that thekd values in the former can be only of thorder of ;53108 dm3 mol21 s21 ~assuming h;10 cP),much lower than the maximumkq values observed for thepresent systems. In fact, in the work of Sobolevaet al.,93 akd value for SDS micellar solution is estimated to be;33108 dm3 mol21 s21. Present results thus indicate that tmutual diffusion of the reactants might not play any signcant role in determining the quenching kinetics for tpresent systems. We thus feel that the observed quenckinetics in the coumarin–amine systems in SDS micellarlutions is mainly determined by the spatial distribution of tdonor molecules around the excited acceptors. Recentlyernieret al.90 have also indicated that the radial distributiofunction g(r ) plays the dominant role in determining thobserved ET kinetics in the micellar solutions.

In the absence of diffusion of the reactants, the obserET rate will be determined by the effective distributiong(r )of the amine quenchers around the excited coumarin dyethe micellar Stern layer. In the present context, it is expecthat the average donor-acceptor distance, specially fordynamic part of the interaction as measured from TR msurements~excluding the static part indicated from SS mesurements!, will be much higher than in the homogeneosolutions,23,48–50as there is no diffusion of the reactants


e

S

g

r

lthes.he

he-

f

ing-

v-

d

ind

he-

-

bring them closer. Further, in the micellar solution, the retant molecules are supposed to be entangled by thechain surfactant molecules,23 which will also effectively in-crease the average donor-acceptor separation for the Eaction in comparison to those in homogeneous solutions.48–50

Hence the observed ET rates in micellar systems arepected to be much lower than in homogeneosolutions.48–50

From the conventional ET theories,1–17 it is evident thatthe rate of the ET reactions becomes maximum at2DG0

5l, wherel is the total reorganization energy. It is indcated from Figs. 6 that for the present ET systems in Smicellar solutions the effectivel could be in the range o;0.6–0.7 eV, based on the conventional ET theory. If thisso, the total reorganization energyl for the ET reactions inthe present systems appear to be substantially smaller cpared to that observed in homogeneous acetonitrile solu(l5;1 eV).48–50 Since the polarity at the micellar Sterlayer is not very different than in acetonitrile, such a lowlfor the present systems seems to be quite unusual.

To have an estimate for the solvent reorganizationergy ls for the present systems, we adopted the approgiven by Fayer and co-workers22,90,91 for the micellar solu-tions. Thus the micelles were viewed as three region stems, with low polarity micellar core having dielectric properties similar to hydrocarbons, the medium polarity spherishell around the core~Stern layer! with dielectric constant;32,18,60 and aqueous phase around the Stern layer hadielectric properties similar to water.22,90,91Thus thels forthe SDS micellar systems were calculated following Faand co-workers,22,90,91assuming the core diameter to be;42Å,23 and the Stern layer of thickness to be;9 Å.23 The ls

value thus estimated for the donor-acceptor close conta;0.94 eV, which is supposed to be the lower limit, aspresent systems the donor-acceptor separation is supposbe more than the contact distance.23

Consideringls>0.94 eV, it is apparent from Fig. 6 thafor the present coumarin–amine systems in SDS micesolution the inversion in the ET rates occurs at much lowexergonicity than expected. These results thus indicatethe solvent reorganization dynamics might not be ablecompete with the ET dynamics, as is the situation prevailsthe two-dimensional ET~2DET! model.41–45,94Though theET reactions in the micellar media is much more compthan the ET reactions investigated in coumarin-neat amsystems under nondiffusive conditions where ET ratesmuch faster than the solvation rates,41–45within the approxi-mation that the dielectric properties remains very simithroughout the Stern layer,22,86–91one can assume that an Escheme similar to the hybrid 2DET model as discussedcoumarin-neat amine systems41–45,94–97is also applicable forthe present coumarin–amine systems in SDS micellar stion. Following the 2DET model, the free-energy surfacesthe reactant and product states can be represented byisoenergy contours~circular trajectories! drawn on the two-dimensional plane, with solvent coordinateX as the abscissaand the nuclear coordinateq as the ordinate, as shown in Fig7.41–45,94In this figure, the line C–C indicates the projectioof the reactant and product potential-energy surfa


-

laag-lle

ent

enoncae

rm

g

de

are

teseo theleanionon-

ed

ere

s

he

nte

cityer-bea-

toon,to. 6.ilar

lu-ions inbli-

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ho-or-ETin

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twtw

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oin

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crossings.94 For simplicity, the participation of the highfrequency vibrational modes95–97 is not considered in thepresent context as we are not going for any explicit calcution of the ET rates. Qualitatively, following Fig. 7, forgiven solvent configurationX, the ET can take place alonthe nuclear coordinateq, even if the solvent diffusional motion is completely frozen, as indicated by the arrows parato the q axis.41–45,94 Thus, unlike the conventional ETtheories,1–17 in the 2DET model it is not essential for threactants to cross the potential-energy barrier only at poiEalong the solvent axisX ~cf. Fig. 7! for the ET reaction totake place. Consequently, for the situations of slow solvdiffusional motions, the role of the solvent reorganizatitowards the free energy of activation for the ET reactionsbe less effective than expected from the classical ET thries, where the reactant is considered to be always in theequilibrium along the solvent coordinate.1–17

Following the 2DET model, the ET rate alongq coordi-nate for a given solvent coordinateX is given by the follow-ing equation:41–45,94–97

k~X!52p

\Vel

2 ~4pl ikBT!21/2expS 2DG* ~X!

kBT D , ~15!

where,Vel is the electronic coupling matrix element,l i is theintramolecular reorganization energy,kB is the Boltzmannconstant,T is the absolute temperature, andDG* is the freeenergy of activation, which is given by the followinequation:41–45,94–97

DG* ~X!5@ls~122X!1DG01l i #

2

4l i. ~16!

FIG. 7. Isoenergy contours of the potential-energy surfaces drawn in adimensional plane for the reactant and product states in relation to thedimensional ET model,X representing the solvent coordinate andq repre-senting the nuclear coordinate. The normalizedX,q coordinates for thereactant and product state potential-energy minima are customarily coered as~0,0! and ~1,1!, respectively. The line C–C represents the transitstate curve corresponding to the crossing of the reactant and productpotential-energy surfaces. In this model electron transfer can occur alonq coordinate for any solvent configuration~X!, as are shown by arrowsparallel toq coordinate. The point E corresponds to the single crossing pof the reactant and product state potential-energy surfaces along theX axisand represents the unique transition state following the conventionaltron transfer theory~Refs. 1–17!.


-

l

t

no-al

Since we are not going for any explicitk(X) calculation, theterm corresponding to the high-frequency vibrational mohas been omitted in Eq.~16! for simplification.41–45,97

In Fig. 7, the reactant and product energy surfacespresented based on the normalizedX andq coordinates andconsequently the minima of the reactant (D¯A* ) and prod-uct (D1

¯A2) surfaces are represented by the coordina~0, 0! and~1, 1!, respectively. For the photoinduced ET in thpresent systems, since the reactant state corresponds texcited state (S1) of the coumarin dye, and since the dipomoment of the excited coumarin dye is much higher ththat in the ground state, immediately after photoexcitatthe reactant state is placed in a nonequilibrium solvent cfiguration, which is being determined by theX coordinate ofthe ground-state potential-energy minimum, i.e., (Xg). Thevalue of Xg can roughly be estimated from the observdifferences in the Stokes shifts~Dn! for the coumarin dyesbetween the micellar solution and a nonpolar solvent whthe dielectric solvation is expected to be negligible. ThusDnandXg can be related as in Eq.~17!,41

2lsXg25Dn. ~17!

The average value ofDn for the coumarin dyes studied ifound to be;1200 cm21. Thus, using als value of;0.94eV, the Xg value is estimated to be about 0.281 for tpresent systems.

Based on Eq.~16!, the contribution ofls towards thefree energy of activation for the initially prepared reactastate following photoexcitation is expected to b(1 – 2Xg)ls , which is estimated to be;0.41 eV. This ap-pears to be somewhat lower than the effective exergoniof ;0.6–0.7 eV at which the ET rates start showing invsion ~cf. Fig. 6!. The apparent difference is supposed toarising due to the contribution of the intramolecular reorgnization energyl i @cf. Eq. ~16!#.

Thus considering the ET process in micellar solutionsbe mostly governed by the intramolecular reorganizatione can account for the effectively lower exergonicityobserve the Marcus inverted region as indicated from FigPresently we are investigating the ET processes in the simcoumarin–amine systems in different other micellar sotions to confirm the presence of the Marcus inverted regand also to get more insight about the observed resultthese systems. These results will be reported in future pucations.

IV. CONCLUSION

Intermolecular ET from aromatic amine to excited comarin dyes has been investigated in SDS micellar solutiby fluorescence quenching measurements. It is seen thaobserved bimolecular quenching rates (kq) in micellar solu-tions are always much lower than those observed in themogeneous acetonitrile solutions for the same donacceptor pairs. These results thus indicate that theprocesses in micellar solutions are inherently slower thanhomogeneous solutions. Correlation of the observedkq val-ues with the free-energy changes (DG0) for the ET reactionsin SDS micellar solutions clearly indicate the presence ofMarcus inverted region in the ET rates at exergonicit

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greater than;0.6–0.7 eV. It is interesting to note that sucan inversion in the ET rates has so far not been reportedany of the intermolecular ET systems, both in homogenesolutions as well as in micellar solutions. Consideringobserved ET rates with present systems and the expediffusional rates of the reactants in the micelles, it is incated that the diffusional kinetic scheme of fluorescequenching is not applicable for the present ET reactioConsidering the polarity of the micellar Stern layer wherereactant molecules reside, the appearance of the Marcuversion in the ET rates at a lower exergonicity of;0.6–0.7eV appears to be unlikely based on the conventionaltheory, as the solvent reorganization energyls for thepresent systems appears to> 0.9 eV. Present results havbeen rationalized based on the 2DET model, where thereactions are considered to occur along the intramoleccoordinate even in the presence of the nonequilibriumvent reorganization.

ACKNOWLEDGMENTS

The authors are thankful to A. K. Satpati and Dr. R.Sundaresan of Analytical Chemistry Division, BARC, ftheir kind help in the cyclic voltammetric measuremenThanks are also due to Smt. S. Nad of RC&CD Divisionher help during the present study.

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Photoinduced intermolecular electron transfer from aromatic amines to coumarin dyes in sodium dodecyl sulphate micellar solutions