Ultrafast Electron Transfer Dynamics in Micellar Media Using Surfactant as the Intrinsic Electron Acceptor

Ultrafast Electron Transfer Dynamics in Micellar Media Using Surfactant as the IntrinsicElectron Acceptor

Manoj Kumbhakar,*,† Prabhat Kumar Singh,† Ashis Kumar Satpati,‡ Sukhendu Nath,† andHaridas Pal†

Radiation & Photochemistry DiVision, and Analytical Chemistry DiVision, Bhabha Atomic Research Centre,Mumbai 400 085, India

ReceiVed: March 12, 2010; ReVised Manuscript ReceiVed: June 9, 2010

Ultrafast photoinduced intermolecular electron transfer (ET) dynamics involving 7-aminocoumarin derivativesas electron donor and pyridinium moiety of surfactant molecules in cetylpyridinium chloride (CPC) micelleas electron acceptor has been investigated to understand the role of separation and orientation of reactants onmicellar ET reactions. Unlike in noninteracting micelles (like Triton-X-100, sodium dodecyl sulfate,dodecyltrimethylammonium bromide, etc.), where surfactant-separated donor-acceptor pairs are understoodto give the ultrafast ET component with the shortest time constant in the range of ∼4 ps, in CPC micelleswith pyridinium moiety as the intrinsic acceptor the ultrafast ET component is found to be in the subpicosecondtime scale (of around 240 fs). This time scale is very similar to the values reported in the cases of ultrafastET reactions involving coumarin dyes in electron-donating solvents. The ultrafast ET times in CPC micellesare significantly faster than the diffusive solvation dynamics in the micellar media. Correlation of the observedET rates in the present cases with the free-energy changes of the reactions shows the inverse-bell-shapedcorrelation, predicted by Marcus ET theory. Interestingly, the onset of the Marcus inversion appears at arelatively lower exergonicity, which is attributed to the nonequilibrium solvent configuration during the ultrafastET reaction, as envisaged from two-dimensional ET (2DET) model. Along with the ultrafast ET component,there are also slower ET components in these systems, which are attributed to those close-contactdonor-acceptor populations in the micelles that have relatively weaker electronic coupling due to improperorientation of the interacting donor-acceptor pairs. The present results suggest that, along with the shiftingof Marcus inversion at lower exergonicity, the ET rates can also be maximized in a micellar media by usingsurfactant molecule as an intrinsic reactant.

Introduction

Tuning chemical reactivity is an important aspect of researchin chemical sciences.1-3 Understanding various factors thatgovern the reaction rates and reaction mechanisms is ofparamount importance to find ways to tune chemical reactions.In recent years considerable efforts have been made to explorethe details of electron transfer (ET) processes in variousorganized assemblies, such as micelles,4-27 reverse micelles,28,29

polymer-surfactant aggregates,30-32 etc. The main aim of suchstudies is to reveal the controlling factors that determine theET dynamics in such systems, in order to control the dynamicsand mechanism of ET reactions.

Theoretically, the rate constant ket for outer-sphere ET reactionis expressed as2,3,33-37

where ∆G0 is the free energy change of the reaction, λs is thesolvent reorganization energy, Vel is the electronic couplingbetween the reactant and product states, and τs is the solvent

relaxation time. From eq 1 it is seen that in the adiabatic limit(4πVel

2τs/pλs . 1),38-40 the ET rate constant is inversely relatedto τs and is expressed as

At room temperature, the value of 16πkBT is about 1.3 eV. Sincethe value of λs is normally either close to or less than thisvalue,35,36,38-41 the maximum value for ket can at the most beequal to τs

-1 for the barrierless ET reactions. Though for numberof ET systems the dependence of ket on τs has been demonstratedexperimentally by many authors, Barbara and co-workers42-46

and Yoshihara and co-workers37,41,47-54 have demonstrated thatthe above is not generally true for any of the ET reactions. Theseauthors have demonstrated that for a number of intramolecularET systems and also for intermolecular close-contact donor-acceptor systems (e.g., fluorophores in electron-donating sol-vents) the ET rates often become faster (ket ≈ 1013 s-1) thanthe predicted τs

-1 value. To explain the ultrafast nature of suchET dynamics, both intramolecular vibrational motions and thediffusive solvent motions were independently considered in adifferent fashion than in the conventional ET theories.55-57 Oflate, there have been several similar reports on ultrafast ET aswell.22,58-71 However, similar studies in confined organizedassemblies like micelles, reverse micelles, etc. (a mimic to

* To whom correspondence should be addressed. E-mail: [email protected], [email protected]. Fax: 91-22-25505151/25519613.

† Radiation and Photochemistry Division.‡ Analytical Chemistry Division.

ket )1

1 + 4πVel2τs/pλs

{2πp

Vel2

√4πλskBTexp(- (∆G0 + λs)

2

kBT )}(1)

ket )1τs� λs

16πkBTexp(- (∆G0 + λs)

2

4λskBT ) (2)

J. Phys. Chem. B 2010, 114, 10057–10065 10057

10.1021/jp102258y 2010 American Chemical SocietyPublished on Web 07/19/2010

cellular membranes) are very rare.10,16,17 In fact, in organizedassemblies, the demonstration of an ultrafast electron transfercomponent faster than even the few picoseconds of solvationtimes in such media is still to be documented very unambigu-ously. In the present study, our aim is to design a close-contactdonor-acceptor situation in a micellar media such that thesituation similar to that of a dye in a reacting solvent mediumcan be emulated to explore different aspects of ultrafast ETreactions in a microheterogeneous media.

Unlike in fast relaxing polar homogeneous solvents, thesluggish solvation dynamics in organized assemblies favors toreduce the contribution of the solvent reorganization energytoward the activation barrier of the ET reaction.7 Therefore, theMarcus inversion for the ET rates in organized assemblies shiftstoward a relatively lower exergonicity (-∆G0) than that in ahomogeneous medium of similar polarity. Thus, it helps us toobserve the bell-shaped Marcus correlation curve for intermo-lecular ET reactions easily in microheterogeneous media,without increasing the reaction exergonicity (-∆G0) to anunreasonably high value.7 Additionally, in organized assemblies,the tethering of the reactants within the surfactant chains causes alarge retardation in the rate of reactant diffusion, making the ETreactions to occur effectively under nondiffusive condition.7-11,15

In one of our recent studies, it was observed that, forintermolecular ET reactions in different noninteracting micelles,the fastest ET time corresponding to the maxima of the Marcuscorrelation curves is only in the range of about 4 ps.10 Notethat for intermolecular ET in homogeneous media undernondiffusive condition, i.e., when the electron acceptor dyesare dissolved directly in neat electron-donating solvents (cf.Chart 1a), the fastest ET component is found to be as short asabout 200 fs.52,53,72,73 Though both in micelles and in electron-donating solvents the intermolecular ET takes place undernondiffusive condition, the unusually higher value for the fastestET time in micelles is supposedly due to the intervention ofsurfactant chains between the reacting donor-acceptor pairs,10,11

causing a larger separation between the reacting pairs (cf. Chart1b) and consequently a lower electronic coupling. In the presentstudy, we use a micelle that emulates the situation similar tothat of the reacting solvent media such that intervention ofsurfactant chains between the reacting pairs can be avoided andhence ultrafast ET time as short as that in electron-donatingsolvents can be realized. Our aim is also to see if such anultrafast ET also follows the Marcus inversion behavior asobserved earlier in other micellar media.

In the present study, to achieve close-contact donor-acceptorconfiguration, we have selected cetylpyridinium chloride (CPC,CH3-(CH2)15-C5H4N+ Cl-) as the surfactant such that thepyridinium moiety of the surfactant in the micelle acts as theelectron acceptor, while a number of coumarin derivatives areused as the electron donors. The coumarin concentrations usedin this study are very low (<5%) compared to the micelleconcentration used in the solution. With this experimentalcondition, hardly any micelle will have more than one coumarinmolecule in it and accordingly the question of a change in themicellar structure does not arise. The schematic of the presentET system can be shown as in Chart 1c. Since the Stern layerthickness of CPC micelle (∼5 Å; approximately similar to thediameter of the pyridinium moiety74) is quite similar to the sizeof the coumarin fluorophores used, it is expected that in thesesystems the donor-acceptor pairs will have a close-contactconfiguration (cf. Chart 1c). The coumarin dyes used in thepresent study are listed in Chart 2 along with their structures.

Experimental Section

Ground-state absorption spectra were recorded using aJASCO model V530 spectrophotometer (Tokyo, Japan). Steady-state fluorescence spectra were recorded using a HITACHImodel F-4010 spectrofluorimeter (Tokyo, Japan).

Ultrafast fluorescence decays were recorded using the fluo-rescence up-conversion measurements. Briefly, in the femto-second fluorescence up-conversion setup (FOG 100, CDP Inc.,Russia) the coumarin dyes in CPC micelles were excited closeto their absorption maxima (∼390-425 nm) using the secondharmonic (SH) of a mode locked Ti:sapphire laser (CDP Inc.,Russia, 50 fs, 82.2 MHz repetition rate), pumped by a 5W DPSSlaser. The SH was generated in a type-I BBO angle-tuned phase-matched nonlinear crystal with 0.5 mm thickness. Optical delaybetween the excitation and the gate pulses was varied using adelay rail (6.6 fs per steps) at the path of the gate pulses. Theup-converted signal (in a 0.5 mm Type-I BBO crystal) corre-sponding to the emission maxima of the dyes (∼490-535 nm)was measured with a photon counter after passing through a

CHART 1: (a) Close-Contact Configuration forFluorophore in Electron-Donating Solvent and (b)Donor-Acceptor Systems in Noninteracting Micelles, and(c) Donor-Acceptor Configuration in CPC Micellea

a Green and orange colors indicate electron donor and acceptormoiety, respectively. The close-contact configuration (a) for fluorophorein electron-donating solvent shows an ultrafast ET component of ∼200fs. For similar donor-acceptor systems in noninteracting micelles (b),the fastest ET component is of ∼4 ps only. Yellow and blue shadesindicate micelle and bulk water, respectively. Surfactants are representedby the circular head with wavy tail. In the close-contact donor-acceptorconfiguration in CPC micelle (c), the surfactant head group acts asone of the reactants for intermolecular ET process and it emulates thesituation of dye in reacting solvent media.

CHART 2: Chemical Structures of the Coumarin DyesUsed in the Present Study

10058 J. Phys. Chem. B, Vol. 114, No. 31, 2010 Kumbhakar et al.

proper band-pass filter and a double monochromator. For eachof the decays, at least three scans were taken to check thereproducibility of the decays. In all these measurements, thesamples were taken in a rotating cell (0.4 mm path length) tohave better heat dissipation and thus to avoid the photodegra-dation of the dyes. For the measurements of the fluorescencedecays, the polarization of the excitation beam was set to magicangle (54.7°) with respect to the horizontally polarized gate lightpulses using the Berek wave plate arrangement (CDP Inc.,Russia).75 A cross correlation of the fundamental and the SHdisplayed a full width at half-maximum (fwhm) of ∼210 fs forthe instrument response function (IRF). Femtosecond transientswere fitted by convolution analysis using a Gaussian shape forthe IRF.

The nanosecond fluorescence decays were measured using adiode laser (374 and 408 nm, <100 ps, 1 MHz) based time-correlated single photon counting (TCSPC) setup (IBH, UK).In the present work, a MCP-PMT detector (IBH, UK) was usedfor the fluorescence decay measurements. The instrumentresponse function for this setup is ∼130 ps at fwhm. All theexperiments were carried out at ambient temperature,22 ( 1 °C.

Laser-grade coumarin dyes were obtained from Exciton, USA.CPC was obtained from E. Merck (Germany) and used asreceived. Nanopure water, having a conductivity of <0.1 µScm-1, was obtained by using a Millipore Gradiant A10 systemand used for the preparation of the micellar solutions. Theconcentration of the CPC micelle used in the present study isabout 3 mM. The micelle concentration was calculated by usingthe following relation, [micelle] ) ([surfactant] - cmc)Nagg

-1,where cmc is the critical micellar concentration (cmc ∼ 8.0 ×10-4 mol dm-3) and Nagg is the micellar aggregation number(∼ 80 for CPC micelle).76 The concentration of the micelleswas intentionally kept reasonably low (∼3 mM) such that thesituation of monodispersity of the spherical micelles is ap-plicable. The concentration of dyes used is typically ∼50 µM,which is much lower than the micelle concentration used inthis study. Ground-state absorption and steady-state fluorescencespectra of all the dyes in CPC micelle are shown in Figure 1and their respective maxima are listed in Table 1.

Redox potentials of the coumarin dyes (ED/D+) and thepyridinium moiety (EA+/A) of CPC were measured usingpotentiostat/galvanostat Autolab 100 at ambient temperature withthree-electrode geometry of cyclic voltammetric measurements.Glassy carbon was used as the electrode material for themeasurements. In CPC micelle, the coumarin oxidation peakswere completely masked by the broad oxidation peak ofpyridinium moiety. In the present study, we thus carried outthe cyclic voltammetric measurements in acetonitrile (ACN)media (cf. Table 1) and suitable corrections for the polaritydifferences between CPC and ACN were made in the calculationof the free energy changes of the ET reactions. For the presentcoumarin-amine systems, the ∆G0 values for the ET reactionswere thus calculated using the following relation77

where E00 is the excitation energy of the coumarins from theS0 state to the S1 state, e is the charge of an electron, and εS

and εR are the static dielectric constants of the CPC reaction

medium and ACN, respectively. R0, RD, and RA are the centerto center distances between the interacting coumarin andpyridinium moiety, radius of coumarin, and radius of pyridiniummoiety, respectively. E00 values of the coumarins were obtainedfrom the intersecting wavelengths of the normalized (peakintensities) fluorescence and excitation spectra and are listed inTable 1. The radii of the pyridinium moiety and coumarins wereestimated using Edward’s volume addition method,74 and R0

was considered to be equal to the sum of the radii of thepyridinium moiety and the coumarin involved. The εs value forthe Stern layer of CPC micelle is considered as ∼30.22 The ∆G0

values estimated for different coumarin-CPC pairs are listedin Table 1.

Results and Discussion

Fluorescence decays of the coumarin dyes in CPC micellarsolutions were recorded at their respective emission maximausing femtosecond fluorescence up-conversion measurements.Representative decays of C151, C120, C102, and C1 dyes inCPC micelle are shown in Figure 2. It is observed that thefluorescence decays are nonexponential in nature for all thecoumarin dyes used and in all the cases there is a strongcontribution of an ultrafast decay component.

A common feature of ultrafast ET dynamics using solventas one of the reactants is its strong nonexponential behaviorand in such cases the observed fluorescence decays are in generalanalyzed by using a multiexponential function.53,73 The fluo-rescence decays of coumarins in the present cases are alsoclearly nonexponential in nature, quite similar to those observedfor the cases of the dyes in electron-donating solvents. The

∆G0 ) ED/D+ - EA+/A - E00 - e2

εSR0-

e2

2 ( 1εR

- 1εS

)( 1RD

- 1RA

) (3)

Figure 1. Steady-state absorption (a) and fluorescence spectra (b) ofall the dyes in CPC micelles.

Ultrafast ET in Electron-Accepting Micellar Media J. Phys. Chem. B, Vol. 114, No. 31, 2010 10059

present observations are also qualitatively very similar to thoseobserved earlier by us for coumarin-amine systems in nonin-teracting neutral [TX100, (CH3)3C-CH2-C(CH3)2-(C6H4)-(OCH2CH2)10-OH], cationic [DTAB, CH3(CH2)11N(CH3)3

+Br-],and anionic [SDS, CH3(CH2)11SO4

-Na+] micelles. In the presentcases, the observed fluorescence decays were analyzed satis-factorily by using a triexponential function following a recon-vulation procedure using the proper instrument response func-tion. Table 2 lists the time constants (τi) obtained for differentcoumarins in CPC micelle along with their relative contributions(ai). It is interesting to note that in CPC micelle a very fastdecay component with time constant (τ1) as short as 240 fs is

observed for C120 dye, which is very similar to the fastest decaytime constant observed in systems with coumarins dissolved inelectron-donating amine solvents (∼200 fs).52,53,72,73 In the lattersystems, coumarin dyes (acts as acceptors) are always in contactwith the amine molecules in their first solvation shell and thusin a suitable donor-acceptor pair, for which the exergonicityof ET reaction is close to the barrierless situation, and ET occurswith an unusually high rate, showing a time constant as shortas ∼200 fs. In CPC micelles, coumarin dyes act as the electrondonors while the pyridinium moiety of CPC acts as theacceptor.22 Thus, in the present systems, in the Stern layer ofCPC micelle, the electron donor molecules (dyes) are surrounded

TABLE 1: Various Energetic and ET Parameters of Coumarin Dyes in CPC Micelles

systems λabsmax(nm) λflu

max (nm) ED/D+(V)a EA+/A(V)b RD (Å) RA (Å)b E00(eV) ∆G0(eV)

C102 395 475 0.80 -1.12 3.82 2.60 2.79 -0.94C2 370 445 1.08 3.54 2.95 -0.82C339 388 465 1.01 3.58 2.86 -0.81C1 384 443 1.05 3.75 2.87 -0.77C120 357 441 1.30 3.31 3.00 -0.67C153 435 533 0.95 3.88 2.51 -0.51C522 423 520 1.06 3.76 2.58 -0.48C307 407 500 1.19 3.72 2.65 -0.41C152 409 514 1.17 3.62 2.63 -0.41C500 404 505 1.22 3.62 2.67 -0.40C481 413 512 1.17 3.82 2.61 -0.39C151 396 492 1.35 3.40 2.73 -0.34

a In acetonitrile solvent. b Of pyridinium moiety.

Figure 2. Initial part of the fluorescence decay traces (O) measured in the up-conversion setup and their corresponding triexponential fits (line)following a convolution procedure using the instrument response function shown for C102 (a), C1 (b), C120 (c), and C151 (d) dyes. Insets showthe corresponding full decay traces measured in these studies.


by the electron-accepting head groups (pyridinium moiety) ofthe CPC molecules such that the donors and acceptors are inphysical contact. Thus, the ultrafast ET rates observed in thepresent systems are always faster than those observed previouslyin noninteracting micelles, like TX-100, DTAB, and SDS. Inthe noninteracting micelles, it is found that the fastest ETcomponent observed at the barrierless situation is ∼4 ps, whichis much higher than the fastest ET time observed (∼200 fs) forcoumarins in electron-donating solvents. This we inferred asdue to the intervention of surfactant chains between theinteracting donor-acceptor pairs, even though ET in all themicelles was indicated to take place effectively under nondif-fusive condition, similar to that of the dye in electron-donatingsolvents. In the present case of CPC micelle, the fastest ETcomponent (∼240 fs) observed is much faster than the corre-sponding component in noninteracting micelle. We envisage thatit is due to the contribution of the close-contact donor-acceptorpairs in the present cases, a situation that was not possible toachieve in normal micelles with the range of the quencherconcentrations used. Use of extremely high concentration ofquenchers in normal micelles was not justified as it would causea change in the micellar structure.

In the present systems, ET from excited coumarins to thepyridinium moiety seems to be the most likely mechanism, asthe pyridinium moiety is a very good electron acceptor andcoumarins can act as good electron donors. This is alsosupported from the correlation of the measured quenchingkinetics with the estimated energetic of the ET reactions, asdiscussed later. An alternative mechanism like singlet-singletenergy transfer from excited coumarins to the ground-statepyridinium moiety is ruled out as the S1 state energy (E00 > 4eV) of the pyridinium moiety is much higher than those of thecoumarin dyes. Similarly, a mechanism involving proton-transfer/hydrogen-bonding interactions of the coumarin dyeswith the pyridinium quencher can also be discarded from thefollowing considerations. (i) The pyridinium moiety used as thequencher in the present work cannot act as the proton/hydrogen-bond donors to the coumarin dyes, as there is no polar hydrogenpresent in this moiety. (ii) Considering proton transfer as thequenching mechanism, C151 and C120 should show similarquenching dynamics as both of the two dyes have similar polarhydrogens available, e.g., the hydrogens of the 7-NH2 group.From the above considerations, it is inferred that the quenchingof the excited coumarin dyes by the pyridinium moiety of CPCoccurs due to ET mechanism.

Before going for the further analysis of the ultrafast resultsin the present systems, we would like to understand why for

some of the coumarin dyes in CPC micelle the ET kinetics aresignificantly slow. The reason for the above becomes quite clearwhen we compare the redox potentials of the 4-CF3 and 4-CH3

series of the coumarin dyes (redox potentials are listed in Table1). As the pyridinium moiety of CPC is a strong oxidizing agent,with this acceptor even the 4-CF3 series of coumarins alsoparticipate in the ET reactions as the electron donors, albeitwith a much lower reaction rate than that of the 4-CH3 seriesof the coumarin dyes. As expected from the redox potentials,the driving force (-∆G0) for the ET reactions involving 4-CH3

series of coumarin dyes would be much higher than the 4-CF3

series of the dyes and accordingly the latter dyes show a muchslower ET rates than the former on reacting with the electron-accepting pyridinium moiety of the CPC micelle.

Recent solvation dynamics studies by Bhattacharyya and co-workers in different micellar media using up-conversion tech-nique have indicated that the fastest solvation component inthese systems is of the order of a few picoseconds (e.g., 3.5 psfor cationic CTAB, 2.3 ps for anionic SDS, and 2.7 ps fornonionic TX-100 micelles).16,17 Besides this ultrafast solvationcomponent of a few picoseconds, other slow solvation compo-nents of hundreds of picoseconds to nanoseconds are alsoreported for micellar media. Further, it has been reported thatall these solvation time constants vary significantly dependingupon the relative size of the probe and Corona region, the degreeof micellar hydration, and the specific interactions present inthe micellar media.78-80 Solvation dynamics in the CPC micelleis not reported from up-conversion measurements possiblybecause of the very low fluorescence efficiency of the probedyes at the blue and red edge of their emission spectra (due tofluorescence quenching). However, drawing an analogy withthe literature reports in different micelles and specifically withthat in cationic micelles, it can be expected that the fastestsolvation time in the CPC micelle could also be in the range ofa few picoseconds with other slow components in the subnano-second region.78,79 Therefore, the fastest component of the ETdynamics observed especially for the 4-CH3 series of thecoumarin dyes in the CPC micelle (τet < 1 ps) suggests that theET reactions in these cases occur faster than the solvationdynamics in this micelle. Thus, it is evident that the conventionalET theory, where solvent reorganization is considered to bemuch faster than the ET reaction, is not adequate to explainthe ultrafast ET results in the present systems. To account forthe ultrafast nature of the ET dynamics, the 2-dimensional ETmodel (2DET),55 where the fast dynamics along the intramo-lecular vibrational coordinate is treated separately from the slowdynamics along the solvent relaxation axis, is invoked (con-

TABLE 2: Fluorescence Decay Parameters of Coumarin Dyes in CPC Micelles

coumarin inCPC micelle

decay time constants, ps decay time const fromTCSPC (ns)aτ1 (a1)a τ2 (a2)a τ3 (a3)a

C102 0.74 (0.28) 6.90 (0.33) 65.8 (0.39) 0.38 (43.1), 0.68 (56.9)C2 0.55 (0.49) 7.14 (0.29) 65.9 (0.22) 0.55 (55.3), 0.87 (44.7)C339 0.50 (0.36) 8.59 (0.33) 65.8 (0.31) 0.42C1 0.40 (0.33) 9.09 (0.34) 77.5 (0.33) 0.14C120 0.24 (0.51) 6.65 (0.27) 76.3 (0.22) 0.56 (24.3), 1.53 (75.7)C153 2.35 (0.15) 55.01 (0.17) 469.7 (0.68) 0.22 (26.7), 0.65 (73.3)C522 5.60 (0.05) 70.37 (0.16) 535.2 (0.79) 0.23 (17.2), 0.73 (82.8)C307 16.02 (0.40) 344.0 (0.19) 548.5 (0.41) 0.21 (18.1), 0.74 (81.9)C152 11.67 (0.17) 142.82 (0.15) 381.4 (0.68) 0.41C500 10.44 (0.11) 96.62 (0.14) 757.7 (0.75) 0.36 (20.4), 1.36 (79.6)C481 6.94 (0.03) 98.16 (0.14) 398.0 (0.83) 0.31 (71.3), 0.62 (28.7)C151 11.0 (0.03) 253 (0.88) 631.2 (0.09) 0.53 (65.5), 1.19 (34.5)

a Amplitudes are listed within parentheses.


ceptually presented in Chart 3). Such a 2DET model has beenextensively used to comprehend and explain the ET kineticsfaster than the solvation dynamics even in homogeneous media.However, for the ET components slower than the solventrelaxation dynamics, namely second and third ET componentsfor 4-CH3 coumarins and all the components of the ET timesfor 4-CF3 dyes in the CPC micelle, the ultrafast solventreorganization dynamics should affect the ET rate quitesubstantially. Note here that the slow solvation times of450-740 ps reported in cationic micelles78,79 are either sloweror comparable to the observed second and third ET components,involving especially the 4-CF3 dyes. Additionally, the fastinertial solvent reorganization, which occurs in less than 100fs for normal water81 and may also have some contribution inthe micellar media, plays some role in modulating the ultrafastET process, though its characterization is difficult from thepresent study.

The reason for the observation of an ultrafast subpicosecondET component along with other few picosecond componentsin the present contact donor-acceptor configuration in CPCmicelle needs to be scrutinized further. It is seen from Table 2that the τ2 component for 4-CH3 series of the dyes is about10-25 times slower than the τ1 component (<1 ps). More thanorders of magnitude difference in the first two ET time constantsfor these dyes cannot be simply rationalized on the basis of thedifferences in the donor-acceptor separations, as in the presentsystems the donor-acceptors are in close-contact configurationand within the time scale of the τ2 component there will behardly any change in the donor-acceptor separation. Thus, wefeel that theτ2 componentcouldberelated to thosedonor-acceptorclose-contact pairs that are not suitably oriented to give highelectronic coupling (Vel) and consequently relatively slower ETdynamics. This proposition is further substantiated from theresults of the theoretical calculations by Castner and co-workers,72 where the relative orientations of the donors and

acceptors are shown to play a crucial role in modulating the Vel

value and accordingly the ET rate even for the close-contactdonor-acceptor configuration. It has been highlighted from theirstudies that Vel is in fact a dynamic variable in such systems(due to fluctuations in the donor-acceptor orientation), thoughthis aspect is difficult to realize from the present experimentalresults.

The ultrafast ET process occurring in the picosecond tosubpicosecond time domain is commonly observed in a reactivemedium, as the present case, and can be attributed to the reactantsubpopulation that have donor-acceptor configurations veryfavorable to give high Vel. It has been proposed that in the caseof fluorophores in electron-donating solvents there are largenumber of donor solvent molecules present in the first solvationshell of an excited fluorophore and more than one orientationsof these close-contact solvent molecules can cause high Vel andthus can lead to ultrafast ET process.72 Accordingly, the presenceof the nonexponential ET dynamics in such systems is inferredto be due to the distribution of the coupling matrix element,Vel, among the different donor-acceptor orientations. Recently,the aspects of orientation dependence of Vel and the influenceof translational and rotational motions on the ultrafast ETreactions in solutions have been critically looked into bydifferent groups employing various theoretical tools.61 In thepresent case, the inherent heterogeneity for the pyridiniummoieties surrounding a coumarin molecule in the Stern layerof CPC micelle might also be the cause for the observednonexponentiality in the ET dynamics.

Among the different mechanistic models suggested to ratio-nalize the nonexponential ET kinetics for the fluorophores inelectron-donating solvents (i.e., amine solvents), the one withlargedifferences inVel due to thedifferences in thedonor-acceptororientations72,82 appears to be more reasonable than the onewhere large differences in the interacting donor-acceptorseparations51-53,73 is considered. The other explanation83-85

where both charge transfer (CT) complex formation andphotoinduced charge separation (CS) are considered for close-contact and well-separated donor-acceptor pairs, respectively,is generally not supported because no CT emission band isobserved for coumarins in electron-donating solvents. For thecoumarin dyes in CPC micelles also there is no indication ofany CT complex formation. For the coumarin dyes in CPCmicelle, the translational diffusion of the reactants prior to ETis highly improbable as the diffusion of the reactants in themicelle (longitudinal diffusion coefficient, DL of coumarin inmicelle is ∼10-6 cm2 s-1)15 would be much slower comparedto the observed ET rates (τ1

-1 and τ2-1).10 Accordingly, the role

of translational diffusion prior to ET, as considered by Vauthyand co-workers,58,59,86,87 to explain the nonexponential natureof ultrafast ET seems to be not applicable for the ET reactionsin CPC micelle. Moreover, variations in the spatial separationfor the donor-acceptor pairs in the Stern layer of CPC micelleconsisting of the electron-accepting pyridine head groups is alsoconsidered to be not that large because the thickness of the Sternlayer of CPC micelle is quite similar to that of the coumarindyes. Therefore, we feel that the fastest ET time (τ1) observedfor different coumarin dyes in CPC micelle is mainly due tothe face-to-face orientation of the interacting coumarin-pyridin-ium pairs which can give very high Vel value. The relativelyslower second component of the ET time (τ2) in these systemsis possibly due to those donor-acceptor pairs that are notoriented properly face-to-face to give high Vel. Thus, for thesedonor-acceptor pairs, Vel will be significantly lower andaccordingly should undergo a slower ET reaction. For the very

CHART 3: Isoenergy Contours of the Potential EnergySurfaces Drawn in a Two-Dimensional Plane for theReactant and Product States in Relation to theTwo-Dimensional ET Modela

a X represents the solvent coordinate and q represents the nuclearcoordinate. The normalized X,q coordinates for the reactant and productstate potential energy minima are customarily considered as (0,0) and(1,1), respectively. The line C-C represents the transition-state curvecorresponding to the crossing of the reactant and product state potentialenergy surfaces. In this model, electron transfer can occur along the qcoordinate for any solvent configuration (X), as shown by arrows parallelto the q coordinate. The point E corresponds to the single crossingpoint of the reactant and product state potential energy surfaces alongthe X axis and represents the unique transition state following theconventional electron-transfer theory.


slow ET component (τ3), however, solvent relaxation as wellas the rotational and translational diffusion of the reactants mightjointly contribute in modulating the ET reaction. In the presentcontext, it is also interesting to note that, within the frameworkof the 2DET model, ET occurs along the q coordinate (cf. Chart3) with different efficiencies depending on the position of thereactants along the X coordinate (by the diffusive motion alongthe solvent coordinate). Hence, the overall ET rate is the sumof all the reaction rates over the entire X coordinates and thusshould show inherent nonexponential behavior.34 The latter couldalso be one of the reasons for the observed nonexponential decaybehavior of the dyes in micellar media.

It is interesting at this point to correlate the observed ET rateswith the ∆G0 values, especially because in recent times it hasbeen shown that the micellar media provide a conduciveenvironment for the observation of Marcus inversion region forintramolecular ET reactions.7,9,11,15 Moreover, it has been shownthat the inversion region of the Marcus correlation curve canbe suitably tuned by changing the characteristics of the micellarmedia.10,88 For different coumarin dyes in CPC micelle, thecorrelation of the fastest ET rates, i.e., the inverse of the firstcomponent of the ET times (ket ≈ τ1

-1) with the ∆G0 values ofthe ET reactions, is shown in Figure 3. An inverse bell-shapedcorrelation is clearly evident from Figure 3. The appearance ofthe barrierless region at an exergonicity (-∆G0) of about -0.7eV is in accordance with the results reported in other nonin-teracting micelles like SDS, TX-100, etc., where coumarins andamines are used as the electron acceptors and donors, respec-tively. Such an exergonicity value of ∼0.7 eV for the onset ofthe Marcus inversion certainly indicates that the solventreorganization energy contributes only partially toward theactivation barrier for the ET reaction, as discussed in our earlierwork, and is rationalized by using the 2DET model. Accordingto 2DET theory (cf. Chart 3a), the free energy of activation∆G*(X) for the ET reaction is a function of the solvationcoordinate X and is expressed as43,51-55,73,89,90

where X is the normalized solvent coordinate and λi is theintramolecular reorganization energy. In photoinduced ETreaction, as is the present case, it is likely that the initial Xcoordinate for the reactant state, say Xg, where the system is

initially produced immediately after photoexcitation, is quiteaway from the equilibrium X coordinate (X ) 0) for the reactant.Under this situation, if the solvent relaxation process isconsidered to be substantially slow, as is the case for the presentultrafast ET component in CPC micelle, only the (1 - 2Xg)fraction of λs should contribute to ∆G*(X).

The value of Xg can roughly be estimated from the observeddifferences in the Stokes shifts (∆(∆ν)) for the coumarin dyesbetween the micellar solution and a nonpolar solvent (e.g.,hexane) where the dielectric solvation is expected to benegligible. The ∆ν and Xg values are related by this equation53

The average value of ∆(∆ν) for the coumarin dyes studied isfound to be ∼1400 cm-1. Thus, using a λs value of ∼1.3 eV(λs ≈ e2(1/(2RD) + 1/(2RA) - 1/(RDA))(1/nr

2 - 1/εs); εs ) 30and refractive index (nr) is assumed to be similar to that of water,1.33),3 the Xg value is estimated to be about 0.26 for the presentsystems. Based on eq 4, the contribution of λs toward the freeenergy of activation for the initially prepared reactant statefollowing photoexcitation is expected to be (1 - 2Xg)λs, whichis estimated to be ∼0.62 eV. This appears to be somewhat lowerthan the effective exergonicity of ∼0.7 eV at which the ET ratesstart showing inversion (cf. Figure 3). The apparent differenceis supposed to be arising due to the contribution of theintramolecular reorganization energy λi (cf. eq 4), for which adirect estimation is not possible. From the above consideration,it is apparent that the ultrafast component (τ1) of the ET processin CPC micelle is mainly governed by the partial solventreorganization and the solvent reorganization does not contributemuch to modulate this ultrafast ET component. Thus, one cansuitably account for the effectively lower exergonicity asobserved for the appearance of the Marcus inverted region, asindicated in Figure 3. It is interesting to note that almost similarbehavior is also observed for the correlation of the relativelyslower second and third ET components (i.e., τ2

-1 and τ3-1) in

the present case with the ∆G0 values, as are shown in Figure 3.In these cases, however, the appearance of the Marcus inversionis dictated to be gradually shifted toward higher exergonicitywith the slowing down of the ET rates. This is in accordancewith the fact that at longer times due to solvent reorganization,the initial nonequilibrated reactant state along the solventcoordinate (i.e., Xg) will move appreciably toward the solventequilibrated reactant state (i.e., the normalized X,q coordinateof 0,0). Therefore, with increase in ET times, the contributionof solvent reorganization, i.e., (1 - 2Xg)λs, will increase (dueto a decrease in Xg value). As a consequence of this increasingcontribution of solvent reorganization energy toward the activa-tion energy barrier, the appearance of Marcus inversion willshift toward higher exergonicity for longer ET times (orconversely with slower ET rates). Moreover, as the curvatureof Marcus correlation also depends on the value of reorganiza-tion energy, it is possible that the curvature of Marcus correlationwill differ for k1, k2, and k3 plots, as evident in Figure 3.

At this juncture, it is worth mentioning that theoreticalsimulation of ultrafast ET rates under nonequilibrium solventconfiguration in homogeneous media has been extensivelyanalyzed by Yoshihara and co-workers37,41,47-54 based on thetheoretical formulation of 2DET model given by Sumi, Nadler,and Marcus,55,91 which was later modified by Jortner and Bixonto incorporate the high-frequency vibrational modes.39,40,89,90,92,93

Although many ET parameters are not known, but within the

Figure 3. Plot of ln(ki) vs ∆G0 for coumarin-CPC systems. The ETrates are k1 ()τ1

-1), k2 ()τ2-1), and k3 ()τ3

-1), represented by blue,black, and red spheres, respectively. The solid lines are just guide forthe eyes.

∆G*(X) )[λs(1 - 2X) + ∆G0(X) + λi]

2

4λi(4)

∆(∆ν) ) 2λsXg2 (5)


limitations of these ET parameters, the simulated ET rates wereobserved to correlate at least semiquantitatively with theexperimentally observed ET rates of coumarins in electron-donating amine solutions.37,41,47-54 In micellar media, however,proper theoretical evaluation of ultrafast ET rates under non-equilibrium solvent configuration following 2DET is extremelydifficult due to the inherent heterogeneity of the probe microen-vironment, inhomogeneity in the relative orientation of thedonor-acceptor systems, unavailability of solvation parametersin CPC micelle, and the restriction on the reactant confinementwithin the Stern layer of the micelle. Hence, a simulation ofthe ET rates following 2DET model is beyond the scope of thepresent paper and has not been attempted.

On comparison of the present results with those reportedearlier in noninteracting micelles,10 it is seen that the fastestET rate at the barrierless condition is about 17 times higher inCPC micelle (τ1 ) 240 fs) than in other micelles (τ1 ) 4.1 ps).It is thus clearly indicated that in the present micelle, wherethe surfactant headgroup acts as one of the reactant, it is possibleto enhance the ET rate by more than 1 order of magnitude. Thepresent results are also in accordance with our earlier propositionthat the absence of subpicosecond ET component in thenoninteracting micellar media was due to the intervention ofthe surfactant chains between the interacting donor-acceptorpairs when the donors and acceptors are used as the isolatedreactants for the ET reaction. In CPC micelle, as one of thereactant becomes the integral part of the micelle, i.e., thepyridinium moiety of the CPC molecule, the donor-acceptorpairs can easily have their close-contact configuration and thusshow an ultrafast ET process occurring in the subpicosecondtime domain. Hence, this finding is very similar to thoseobserved earlier for coumarin acceptors in electron-donatingamine solvents. It is thus concluded from the present resultsthat even in micellar media the close-contact donor-acceptorconfiguration can be suitably imitated by using surfactant asone of the reactants and accordingly an ultrafast intermolecularET reaction faster than the diffusive solvation dynamics can beeasily realized, quite similar to the cases of fluorophores in thereactive solvent media. The present results further substantiatethe fact that, unlike in homogeneous solution, in micellar mediathe Marcus inversion behavior for the bimolecular ET reactionscan be easily observed due to the nondiffusive nature of theET reactants and the partial contribution of the solventreorganization energy to the activation barrier of the ET process.

Conclusions

Photoinduced intermolecular ET has been investigated in CPCmicellar media, where the pyridinium moiety of the surfactantitself acts as an electron acceptor and different coumarin dyeswere used as electron donors. Under such condition of interact-ing micellar media, we could demonstrate the fastest ET timeof 240 fs, quite similar to that reported for coumarins in electron-donating amine solvents. This ultrafast time constant has beenattributed to the electron donor-acceptor pairs that are in closecontact with proper orientation for large electronic coupling.The correlation of this ultrafast ET rates with the free energychange of the reaction also demonstrates the inverse-bell-shapedMarcus correlation. A shift of the appearance of Marcusinversion region toward higher exergonicity along with thereduction in the curvature has been observed for the graduallyslower ET components, although the ET mechanism is proposedto be the same for all the cases (k1, k2, and k3), i.e., betweenclose-contact donor-acceptor pairs, albeit with differentdonor-acceptor orientations. The shift in the appearance of

Marcus inversion for k1, k2, and k3 components has beenrationalized on the basis of varying contributions of solventreorganization energy depending on the relative propensity ofET and solvation rates. All these results have been criticallyanalyzed and explained following the 2DET model. The presentresults convincingly demonstrate that with suitable selection ofmicellar media we can not only shift the appearance of inversionin ET rates to lower exergonicity but also maximize the ETrates.

Acknowledgment. The authors are thankful to Dr. S. K.Sarkar and Dr. T. Mukherjee for their constant encouragementand support during the course of this work.

References and Notes

(1) Bolton, J. R.; Mataga, N.; McLendon, G. L. Electron Transfer inInorganic, Organic and Biological Systems; American Chemical Society:Washington, DC, 1991.

(2) Jortner, J.; Bixon, M. Electron Transfer From Isolated Moleculesto Biomolecules; Jortner, J., Bixon, M., Eds.; Advances in Chemical Physics;Wiley: New York, 1999.

(3) Kavarnos, G. J. Fundamentals of Photoinduced Electron Transfer;VCH Publishers: New York, 1993.

(4) Swallen, S. F.; Weidemaier, K.; Fayer, M. D. J. Phys. Chem. 1995,99, 1856.

(5) Swallen, S. F.; Weidemaier, K.; Tavernier, H. L.; Fayer, M. D. J.Phys. Chem. 1996, 100, 8106.

(6) Pal, S. K.; Mandal, D.; Sukul, D.; Bhattacharyya, K. Chem. Phys.1999, 249, 63.

(7) Kumbhakar, M.; Nath, S.; Pal, H.; Sapre, A. V.; Mukherjee, T.J. Chem. Phys. 2003, 119, 388.

(8) Kumbhakar, M.; Nath, S.; Mukherjee, T.; Pal, H. J. Chem. Phys.2005, 122, 084512.


(10) Kumbhakar, M.; Singh, P. K.; Nath, S.; Bhasikuttan, A. C.; Pal,H. J. Phys. Chem. B 2008, 112, 6646.


(12) Kumbhakar, M.; Mukherjee, T.; Pal, H. Chem. Phys. Lett. 2005,410, 94.

(13) Kumbhakar, M.; Nath, S.; Rath, M. C.; Mukherjee, T.; Pal, H.Photochem. Photobiol. 2004, 79, 1.

(14) Kumbhakar, M.; Nath, S.; Mukherjee, T.; Pal, H. J. Photochem.Photobiol. A: Chem. 2006, 182, 7.

(15) Satpati, A. K.; Kumbhakar, M.; Nath, S.; Pal, H. J. Phys. Chem. B2007, 111, 7550.

(16) Ghosh, S.; Sahu, K.; Mondal, S. K.; Sen, P.; Bhattacharyya, K.J. Chem. Phys. 2006, 125, 54509.

(17) Ghosh, S.; Mondal, S. K.; Sahu, K.; Bhattacharyya, K. J. Chem.Phys. 2007, 126, 204708.

(18) Mandal, U.; Ghosh, S.; Dey, S.; Adhikari, A.; Bhattacharyya, K.J. Chem. Phys. 2008, 128, 164505.

(19) Mukherjee, T. K.; Mishra, P. P.; Datta, A. Chem. Phys. Lett. 2005,407, 119.

(20) Chakraborty, A.; Chakrabarty, D.; Hazra, P.; Seth, D.; Sarkar, N.Chem. Phys. Lett. 2003, 382, 508.

(21) Chakraborty, A.; Chakrabarty, D.; Hazra, P.; Seth, D.; Sarkar, N.Chem. Phys. Lett. 2004, 387, 517.

(22) Singh, A. K.; Mondal, J. A.; Ramakrishna, G.; Ghosh, H. N.;Bandyopadhyay, T.; Palit, D. K. J. Phys. Chem. B 2005, 109, 4014.

(23) Cang, H.; Brace, D. D.; Fayer, M. D. J. Phys. Chem. B 2001, 105,10007.

(24) Quitevis, E. L.; Marcus, A. H.; Fayer, M. D. J. Phys. Chem. 1993,97, 5762.

(25) Tavernier, H. L.; Laine, F.; Fayer, M. D. J. Phys. Chem. A 2001,105, 8944.

(26) Weidemaier, K.; Fayer, M. D. J. Phys. Chem. 1996, 100, 3767.(27) Weidemaier, K.; Tavernier, H. L.; Fayer, M. D. J. Phys. Chem. B

1997, 101, 9352.(28) Choudhury, S. D.; Kumbhakar, M.; Nath, S.; Sarkar, S. K.;

Mukherjee, T.; Pal, H. J. Phys. Chem. B 2007, 111, 8842.(29) Chakraborty, A.; Seth, D.; Chakrabarty, D.; Hazra, P.; Sarkar, N.

Chem. Phys. Lett. 2005, 405, 18.(30) Chakraborty, A.; Seth, D.; Setua, P.; Sarkar, N. J. Chem. Phys.

2008, 128, 204510.(31) Chakraborty, A.; Seth, D.; Seth, P.; Sarkar, N. J. Phys. Chem. B

2006, 110, 16607.


(32) Chakraborty, A.; Seth, D.; Setua, P.; Sarkar, N. J. Chem. Phys.2006, 124, 074512.

(33) Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996,100, 13148.

(34) Heitele, H. Angew. Chem., Int. Ed. 1993, 32, 359.(35) Marcus, R. A. J. Chem. Phys. 1956, 24, 966.(36) Marcus, R. A. J. Chem. Phys. 1956, 24, 979.(37) Yoshihara, K.; Tominaga, K.; Nagasawa, Y. Bull. Chem. Soc. Jpn.

1995, 68, 696.(38) Bixon, M.; Jortner, J. Chem. Phys. 1993, 176, 467.(39) Rips, I.; Jortner, J. J. Chem. Phys. 1987, 87, 2090.(40) Rips, I.; Jortner, J. J. Chem. Phys. 1987, 87, 6513.(41) Kobayashi, T.; Takagi, Y.; Kandori, H.; Kemnitz, K.; Yoshihara,

K. Chem. Phys. Lett. 1991, 180, 416.(42) Kang, T. J.; Jarzeba, W.; Barbara, P. F.; Fonseca, T. Chem. Phys.

1990, 149, 81.(43) Walker, G. C.; Akesson, E.; Johnson, A. E.; Levinger, N. E.;

Barbara, P. F. J. Phys. Chem. 1992, 96, 3728.(44) Akesson, E.; Walker, G. C.; Barbara, P. F. J. Chem. Phys. 1991,

95, 4188.(45) Akesson, E.; Johnson, A. E.; Levinger, N. E.; Walker, G. C.;

DuBruil, T. P.; Barbara, P. F. J. Chem. Phys. 1992, 96, 7859.(46) Kahlow, M. A.; Kang, T. J.; Barbara, P. F. J. Phys. Chem. 1987,

91, 6452.(47) Kandori, H.; Kemnitz, K.; Yoshihara, K. J. Phys. Chem. 1992, 96,

8042.(48) Yartsev, A.; Nagasawa, Y.; Douhal, A.; Yoshihara, K. Chem. Phys.

Lett. 1993, 207, 546.(49) Nagasawa, Y.; Yartsev, A. P.; Tominaga, K.; Johnson, A. E.;

Yoshihara, K. J. Am. Chem. Soc. 1993, 115, 7922.(50) Nagasawa, Y.; Yartsev, A. P.; Tominaga, K.; Johnson, A. E.;

Yoshihara, K. J. Chem. Phys. 1994, 101, 5717.(51) Nagasawa, Y.; Yartsev, A. P.; Tominaga, K.; Bisht, P. B.; Johnson,

A. E.; Yoshihara, K. J. Phys. Chem. 1995, 99, 653.(52) Pal, H.; Nagasawa, Y.; Tominaga, K.; Yoshihara, K. J. Phys. Chem.

1996, 100, 11964.(53) Pal, H.; Shirota, H.; Tominaga, K.; Yoshihara, K. J. Chem. Phys.

1999, 110, 11454.(54) Shirota, H.; Pal, H.; Tominaga, K.; Yoshihara, K. J. Phys. Chem.

A 1998, 102, 3089.(55) Sumi, H.; Marcus, R. A. J. Chem. Phys. 1986, 84, 4894.(56) Agmon, N.; Hopfield, J. J. J. Phys. Chem. 1983, 78, 6947.(57) Bagchi, B.; Fleming, G. R.; Oxtoby, D. W. J. Chem. Phys. 1983,

78, 7375.(58) Morandeira, A.; Furstenberg, A.; Gumy, J.-C.; Vauthey, E. J. Phys.

Chem. A 2001, 107, 5375.(59) Morandeira, A.; Furstenberg, A.; Vauthey, E. J. Phys. Chem. A

2004, 108, 8190.(60) Schmidhammer, U.; Megerle, U.; Lochbrunner, S.; Riedle, E.;

Karpiuk, J. J. Phys. Chem. A 2008, 112, 8487.(61) Scherer, P. O. J. J. Phys. Chem. A 2000, 104, 6301.(62) Xu, R.-X.; Chen, Y.; Cui, P.; Ke, H.-W.; Yan, Y. J. Phys. Chem.

A 2007, 111, 9618.

(63) Bizjak, T.; Karpiuk, J.; Lochbrunner, S.; Riedle, E. J. Phys. Chem.A 2004, 108, 10763.

(64) Baigar, E.; Gilch, P.; Zinth, W.; Stockl, M.; Harter, P.; Feilitzsch,T. v.; Michel-Beyerle, M. E. Chem. Phys. Lett. 2002, 352, 176.

(65) Son, D. H.; Kambhampati, P.; Kee, T. W.; Barbara, P. F. J. Phys.Chem. A 2002, 106, 4591.

(66) Mataga, N.; Chosrowjan, H.; Taniguchi, S.; Shibata, Y.; Yoshida,N.; Osuka, A.; Kikuzawa, T.; Okada, T. J. Phys. Chem. A 2002, 106, 12191.

(67) Giaimo, J. M.; Gusev, A. V.; Wasielewski, M. R. J. Am. Chem.Soc. 2002, 124, 8530.

(68) Kovalenko, S. A.; Lustres, J. L. P.; Ernsting, N. P.; Rettig, W. J.Phys. Chem. A 2003, 107, 10228.

(69) Xu, Q.-H.; Scholes, G. D.; Yang, M.; Fleming, G. R. J. Phys. Chem.A 1999, 103, 10348.

(70) Zhang, L.; Kao, Y.-T.; Qiu, W.; Wang, L.; Zhong, D. J. Phys. Chem.B 2006, 110, 18097.

(71) Lockard, J. V.; Wasielewski, M. R. J. Phys. Chem. B 2007, 111,11638.

(72) Castner, E. W., Jr.; Kennedy, D.; Cave, R. J. J. Phys. Chem. A2000, 104, 2869.

(73) Shirota, H.; Pal, H.; Tominaga, K.; Yoshihara, K. Chem. Phys. 1998,236, 355.

(74) Edward, J. T. J. Chem. Educ. 1970, 47, 261.(75) Hecht, E. Optics, 2nd ed.; Addison-Wesley Publishing Co.: Reading,

MA, 1987.(76) Kalyansundaram, K. Photochemistry in Microheterogeneous Sys-

tems; Academic Press: Orlando, FL, 1987.(77) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259.(78) Tamoto, Y.; Segawa, H.; Shirota, H. Langmuir 2005, 21, 3757.(79) Sarkar, N.; Datta, A.; Das, S.; Bhattacharyya, K. J. Phys. Chem.

1996, 100, 15483.(80) Kumbhakar, M.; Goel, T.; Mukherjee, T.; Pal, H. J. Phys. Chem.

B 2004, 108, 19246.(81) Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. Nature

1994, 369, 471.(82) Issa, J. B.; Salameh, A. S.; Castner, E. W., Jr.; Wishart, J. F.; Isied,

S. S. J. Phys. Chem. B 2007, 111, 6878.(83) Murata, S.; Tachiya, M. J. Phys. Chem. A 2007, 111, 9240.(84) Iwai, S.; Murata, S.; Katoh, R.; Tachiya, M.; Kikuchi, K.; Takahashi,

Y. J. Chem. Phys. 2000, 112, 7111.(85) Iwai, S.; Murata, S.; Tachiya, M. J. Chem. Phys. 1998, 109, 5963.(86) Vauthey, E. J. Phys. Chem. A 2001, 105, 340.(87) Morandeira, A.; Furstenberg, A.; Gumy, J.-C.; Vauthey, E. J. Phys.

Chem. A 2003, 107, 5375.(88) Satpati, A. K.; Kumbhakar, M.; Nath, S.; Pal, H. J. Photochem.

Photobiol. A: Chem. 2008, 200, 270.(89) Jortner, J.; Bixon, M. J. Chem. Phys. 1988, 88, 167.(90) Bixon, M.; Jortner, J. Chem. Phys. 1993, 176, 467.(91) Nadler, W.; Marcus, R. A. J. Chem. Phys. 1987, 86, 3906.(92) Rips, I.; Jortner, J. J. Chem. Phys. 1988, 88, 818.(93) Bixon, M.; Jortner, J.; Cortes, J.; Heitele, H.; Michel-Beyerle, M. E.

J. Phys. Chem. 1994, 98, 7289.

JP102258Y


Documents

Ultrafast Electron Transfer Dynamics in Micellar Media Using Surfactant as the Intrinsic Electron Acceptor