Ultrafast Photoinduced Electron Transfer from Peroxide Dianion

Ultrafast photoinduced electron transfer from dimethylaniline to coumarin dyes in sodium dodecyl sulfate and triton X-100 micelles Subhadip Ghosh, Sudip Kumar Mondal, Kalyanasis Sahu, and Kankan Bhattacharyya Citation: J. Chem. Phys. 126, 204708 (2007); doi: 10.1063/1.2733667 View online: http://dx.doi.org/10.1063/1.2733667 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v126/i20 Published by the AIP Publishing LLC.

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THE JOURNAL OF CHEMICAL PHYSICS 126, 204708 共2007兲

Ultrafast photoinduced electron transfer from dimethylaniline to coumarin dyes in sodium dodecyl sulfate and triton X-100 micelles Subhadip Ghosh, Sudip Kumar Mondal, Kalyanasis Sahu, and Kankan Bhattacharyyaa兲 Physical Chemistry Department, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

共Received 29 November 2006; accepted 30 March 2007; published online 29 May 2007兲 The primary steps of photoinduced electron transfer 共PET兲 from N , N-dimethylaniline 共DMA兲 to five coumarin dyes are studied in an anionic micelle 关sodium dodecyl sulfate 共SDS兲兴 and a neutral micelle 关triton X-100 共TX-100兲兴 using femtosecond upconversion. The rate of PET in micelle is found to be highly nonexponential. In both the micelles, PET displays components much faster 共⬃10 ps兲 than the slow components 共180– 2900 ps兲 of solvation dynamics. The ultrafast components of electron transfer exhibit a bell-shaped dependence on the free energy change. This is similar to Marcus inversion. The rates of PET in TX-100 and SDS micelle are, in general, faster than those in cetyltrimethylammonium bromide 共CTAB兲 micelle. In the SDS and TX-100 micelle, the Marcus inversion occurs at −⌬G0 ⬃0.7 eV which is lower than that 共⬃1.2 eV兲 in CTAB micelle. Possible causes of variation of PET in different micelles are discussed. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2733667兴 I. INTRODUCTION

Electron transfer 共ET兲 plays a very important role in many chemical and biological processes.1–4 According to the Marcus Theory, the rate of ET 共kET兲 exhibits a quadratic dependence on the free energy change ⌬G0 as1–4 kET =

冋

册

V2el 4␲2 共⌬G0 + ␭兲2 exp − , h 冑2␲␭skBT 4␭kBT

共1兲

where Vel is the electronic matrix element and ␭ is the sum of solvent 共␭s兲 and nuclear reorganization energies 共␭I兲.1–4 This leads to a bell-shaped dependence of kET on ⌬G0. For −⌬G0 ⬍ ␭ 共normal region兲, the rate of ET increases with the increase in −⌬G0. However, when −⌬G0 ⬎ ␭ 共inverted region兲, the rate of ET decreases with the increase in −⌬G0. The Marcus theory is based on the assumption that the solvation is faster than electron transfer rate and hence, at each point along the reaction coordinate equilibrium is established.1–4 Zusman5 showed that the rate of ET is inversely proportional to the solvation time ␶L, kET =

1 ␶L

冑

冋

册

共⌬G0 + ␭兲2 ␭s exp − . 16␲kBT 4␭kBT

共2兲

Yoshihara and co-workers observed that in neat donor 共amines as solvents兲 the ET rate is faster than the solvation time.6–11 Interestingly, they also reported a bell-shaped dependence on ⌬G0 which is reminiscent of the Marcus inverted region.6–11 Walker et al. showed that the ultrafast ET in betaine dyes is completely decoupled from solvation.12 In an attempt to correlate the highly nonexponential ET process with the highly nonexponential solvation dynamics, Bagchi and co-workers developed a non-Markovian model.13,14 They Author to whom correspondence should be addressed. Fax: ⫹91-33-24732805; Electronic mail: [email protected]

a兲

0021-9606/2007/126共20兲/204708/11/$23.00

noted that in most solvents solvation dynamics consists of an ultrafast subpicosecond inertial component and a slower component in tens of picosecond. They showed that 30%– 40% of the solvent energy relaxation is sufficient to bring about ultrafast ET.13,14 It is suggested that ET is strongly influenced by factors other than the solvent relaxation time.12–21 Ovchinnikova15 and later Zusman16 considered the role of fast classical vibrational modes in the stochastic ET theory. Sumi and Marcus17 proposed a two-dimensional ET 共2D-ET兲 model which involves a solvent polarization coordinate 共X兲 and a low frequency classical vibrational coordinate 共Q兲. According to this model, the relaxation along Q is much faster than that along X and the effect of Q is included using a position dependent rate constant, k共X兲.5,13–18,21 Barbara and Olson considered a classical low frequency vibration, a classical solvent coordinate 共X兲, and a high frequency quantum mode.19 For a very fast ET process 共faster than solvation兲, there is a partial contribution of ␭s and this gives rise to Marcus inversion at a lower exergonicity. In the case of slow ET 共slower than solvation兲, there is always a full contribution of ␭s which leads to an inversion at a higher exergonicity. ET in confined environments, e.g., micelle,22–25 vesicles,26 proteins,27,28 and DNA 共Refs. 29–32兲 differ from those in bulk liquids due to prolonged lifetime of the charge transferred state in the former environments. Further, in a micelle, the donor 关e.g., dimethylaniline 共DMA兲兴 and the acceptor stay very close to each other. Hence, photoinduced electron transfer 共PET兲 in a micelle is expected to be almost as fast as that in neat DMA. This expectation coupled with the observed slow solvation dynamics makes a micelle an ideal environment to study PET processes faster than solvation dynamics. Several groups have earlier studied PET in micelle using a picosecond33–40 and femtosecond41,42 setup. Obviously, the

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SCHEME 1. Structures of coumarine dyes: 共A兲 C151, 共B兲 C152, 共C兲 C481, 共D兲 C153, and 共E兲 C480.

picosecond studies missed the ultrafast component of ET which occurs in 艋10 ps time scale and hence, reported rate constants which are slower by an order of magnitude than that obtained in a femtosecond setup. Previously, we studied ultrafast ET in a cationic micelle, cetyltrimethylammonium bromide 共CTAB兲, and in a nanocavity of a hydroxypropyl ␥-cyclodextrin using femtosecond upconversion.41,42 In the present work, we report on ultrafast ET in an anionic 关sodium dodecyl sulfate 共SDS兲兴 and a neutral 关triton X-100 共TX-100兲兴 micelle and compare the results with those in CTAB micelle. II. EXPERIMENT

Laser grade coumarin dyes C151, C152, C481, C153, and C480 were purchased from Exciton, Inc. 共Scheme 1兲 and were used without further purification. SDS and N , N-dimethylaniline 共DMA兲 were obtained from Aldrich and TX-100 共Nacalai Tesque兲 used as received. The steady state absorption and emission spectra were recorded in a Shimadzu UV-2401 spectrophotometer and a Spex FluoroMax-3 spectrofluorimeter, respectively. The oxidation and reduction potentials in the micellar media exhibit a general shift from those in acetontrile.8–11,38–45 The oxidation and reduction potentials of DMA and coumarin in micellar medium were estimated from the reported values in acetonitrile with a suitable correction for the micellar medium.38–45 We used a shift factor of 0.16 and 0.13 V from the potentials in acetonitrile, respectively, for SDS and TX-100 micelle.8,10,38–45

In our femtosecond upconversion setup 共FOG 100, CDP兲 the sample was excited at 405 nm. The sample was excited using the second harmonic of a mode-locked Ti: sapphire laser 共Tsunami, Spectra Physics with 80 MHz repetition rate兲 pumped by 5 W Millennia 共Spectra Physics兲. The fundamental beam was frequency doubled in a nonlinear crystal 共1 mm beta barium borate 共BBO兲, ␪ = 25°, ␾ = 90°兲. The fluorescence emitted from the sample was upconverted in a nonlinear crystal 共0.5 mm BBO, ␪ = 38°, ␾ = 90°兲 using a gate pulse of the fundamental beam. The upconverted light is dispersed in a monochromator and detected using photon counting electronics. A cross-correlation function obtained using the Raman scattering from ethanol displayed a full width at half maximum 共FWHM兲 of the excitation pulse which is 350 fs. The femtosecond fluorescence transients were fitted using a Gaussian shape for the excitation pulse. To fit the femtosecond data one needs to know the long decay components which were detected using a picosecond setup. In a picosecond setup, the samples were excited at 405 nm using a picosecond diode laser 共IBH Nanoled-07兲 in an IBH Fluorocube apparatus. The emission was collected at a magic angle polarization using a Hamamatsu microchannel plate 共MCP兲 photomultiplier 共5000U-09兲. The time correlated single photon counting setup consists of an Ortec 9327 CFD and a Tennelec TC 863 TAC. The data are collected with a PCA3 card 共Oxford兲 as a multichannel analyzer. The typical FWHM of the system response using a liquid scatterer is about 90 ps. The fluorescence decays were deconvoluted using IBH DAS6 software. In the present study, the concentrations of SDS and TX-100 were kept at 80 and 140 mM, respectively. The aggregation 共Nag兲 numbers of SDS and TX-100 are ⬃62 and 100,46–48 and the critical micellar concentrations 共CMCs兲 are ⬃8 and 0.24 mM.46–48 We used 80 mM SDS and 140 mM TX-100, so the concentrations of the micelle are calculated to be ⬃1.16 and 1.39 mM, respectively. The coumarin 共electron acceptor兲 concentrations were kept ⬃40 ␮M which is much less than that of micellar concentration. Bulk amine 共electron donor兲 concentrations were varied over ranges up to 50 and 100 mM for SDS and TX-100 micelle, respectively. Note that for 80 mM SDS the maximum achievable DMA concentration is 50 mM. The average radii of SDS and TX-100 micelles are reported to be 30 and 50 Å, with nonpolar core radii of about 21 and 25 Å,46–48 respectively. Thus the corresponding thickness of micellar Stern and palisade layer are calculated to be 9 and 25 Å, which gives the volume of the micellar Stern and palisade layer of ⬃7.43 ⫻ 104 and 45.79⫻ 104 Å3 per micelle. The electron donor 共DMA兲 is distributed between the water and Stern 共or palisade兲 layers of the micelles and the solubility of DMA in water at room temperature is ⬃8 mM.35 Hence, the effective concentration of DMA in the Stern 共or palisade兲 layer was estimated as 关Q兴eff =

Nag关Q − 0.008兴 , A兵关micelle兴 − CMC其

共3兲

where Nag is the aggregation number of the micelle 共Nag = 62 and 100 for SDS and TX-100 micelles兲,46–48 关micelle兴 is the total surfactant concentration, CMC is the

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Photoinduced electron transfer

critical micellar concentration, A is the volume of the Stern 共or palisade兲 layer in dm3 / mole of the micelle, and Q is the total bulk DMA concentration in molar unit. Since the available volume for the solutes inside the micelle is restricted by the micellar wall, the effective concentration of the amine in the Stern 共or palisade兲 layer of the micelle is much higher than the bulk concentration. Thus 50 and 100 mM bulk concentrations of DMA correspond to a local concentrations of ⬃810 and 260 mM in 80 mM SDS and in 140 mM TX-100, respectively. The time resolved emission spectra 共TRES兲 are constructed using the steady state intensity and the fluorescence decay parameters following the method of Maroncelli and Fleming.49 The solvation dynamics is described by the decay of the solvent response function C共t兲, defined as C共t兲 =

␯共t兲 − ␯共⬁兲 , ␯共0兲 − ␯共⬁兲

共4兲

where ␯共0兲, ␯共t兲, and ␯共⬁兲 are the emission maximum frequencies at time zero, t, and infinity. In order to study fluorescence anisotropy decay, the analyzer was rotated at regular intervals to get perpendicular 共I⬜兲 and parallel 共I储兲 components. Then the anisotropy function r共t兲 was calculated using the formula I储共t兲 − GI⬜共t兲 . r共t兲 = I储共t兲 + 2GI⬜共t兲

共5兲

共6兲

where E00 is the energy difference between S0 and S1 states of the electron acceptor coumarin dyes and may be obtained from the intersection wavelength of the fluorescence spectra and the excitation spectra.50 E共D / D+兲 and E共C / C−兲 denote the oxidation and reduction potentials of the electron donor 共DMA兲 and electron acceptor 共coumarin dyes兲 molecules, respectively. The last term 共EIPS兲 of Eq. 共6兲 denotes the ion pair stabilization energy in the medium and is given by50 EIPS =

e2 , ␧ sr 0

III. RESULTS A. Steady state absorption and emission studies

The G value of the setup was determined using a probe whose rotational relaxation is very fast, e.g., coumarin 153 in methanol, and the G value was found to be 1.5. ⌬G0 for a photoinduced electron transfer reaction between an electron donor 共D兲 and an electron acceptor 共A兲 is given by the Rehm-Weller equation.50 ⌬G0 = E共D/D+兲 − E共C/C−兲 − E00 − EIPS ,

FIG. 1. Emission spectra of C152 and C480 in 80 mM SDS 关共A兲 and 共B兲兴 and in 140 mM TX-100 关共C兲 and 共D兲兴 at bulk DMA concentrations: 共i兲 0 mM, 共ii兲 15 mM, 共iii兲 30 mM, 共iv兲 50 mM, and 共v兲 100 mM.

共7兲

where e is the electronic charge 共=1.602⫻ 10−19 C兲. The dielectric constants 共␧s兲 of the SDS and TX-100 micelle are found to be 32 and 26.37–42 In Eq. 共7兲, r0 is the interaction distance between the donor and the acceptor. In order to calculate r0, we have estimated the molecular volume 共V兲 of the donor and the acceptor using a simple MM2 calculation. Assuming a spherical shape the radius 共r兲 of the donor 共and acceptor兲 is calculated from the relation r = 共3V / 4␲兲1/3. The sum of radii of the donor and the acceptor is r0 and is thus found to be ⬃7 Å.

In both SDS and TX-100 micelles the absorption spectra of the coumarin dyes remain unaffected on addition of DMA. This indicates that there is no interaction or complex formation involving the donor and the acceptor in the ground state. The emission maxima of the coumarin dyes in a micelle are found to be markedly blue shifted 共e.g., for C480, 470 nm in TX-100 and 475 nm in SDS兲 from that in bulk water 共489 nm for C480兲. This suggests that the dyes are located in the hydrated Stern 共or palisade兲 layer of the micelle which is less polar compared to bulk water. The observed emission maximum of a coumarin dye in a micelle is very close to the reported51 emission maxima of the dye in ethanol 共e.g., 473 nm for C480兲. Thus, it is inferred that the microenvironment of the dye in the micellar resembles that of ethanol. With the addition of DMA 共electron donor兲, a drastic quenching of fluorescence intensity is observed for all the coumarin dyes. The dramatic decrease is ascribed to electron transfer from DMA to the coumarin dyes. Figure 1 shows the effect of addition of DMA on the emission spectra of C152 and C480 in SDS and TX-100 micelles, respectively. It is readily seen that at maximum DMA concentration 共100 and 50 mM, respectively in TX-100 and SDS兲, in each micelle the extent of quenching of steady state emission intensity is maximum for C151 and minimum for C480. For instance, in TX-100 micelle, C480 shows only approximately four fold decrease of emission intensity on addition of DMA, whereas C151 exhibits an ⬃20-fold quenching in the same micelle. For the same coumarin dye, the extent of quenching of steady state intensity is found to be larger in SDS 共e.g., 38 times for C151兲 compared to that 共20 times兲 in TX-100 micelle.

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J. Chem. Phys. 126, 204708 共2007兲

Ghosh et al. TABLE I. Fluorescence decay parameters of coumarin dyes in 80 mM SDS.

Acceptor

␭em 共nm兲 450

C151

520

480 C152

560

490 C481

550

500 C153

590

440 C480

530

关DMA兴 共mM兲

␶1 共a1兲a 共ps兲

␶2 共a2兲a 共ps兲

␶3 共a3兲a 共ps兲

0 50 0 50

1.7 共0.31兲 0.9 共0.44兲 1.2 共−0.37兲 8.4 共0.32兲

30 共0.16兲 10 共0.30兲 168 共−0.04兲 76 共0.50兲

3700 共0.53兲 170 共0.26兲 5770 共1.41兲 3200 共0.18兲

0 50 0 50

2.0 共0.28兲 1.0 共0.47兲 3 共−0.25兲 9.2 共0.33兲

30 共0.21兲 10 共0.33兲 47 共−0.07兲 54 共0.36兲

620 100 970 345

共0.51兲 共0.20兲 共1.32兲 共0.31兲

0 50 0 50

1.8 共0.26兲 1.2 共0.43兲 0.75 共−0.42兲 15 共0.41兲

34 共0.19兲 12 共0.33兲 8 共−0.14兲 80 共0.28兲

460 100 600 275

共0.55兲 共0.24兲 共1.56兲 共0.31兲

0 50 0 50

2.0 共0.31兲 1.6 共0.32兲 2.5 共−0.37兲 15 共0.07兲

50 共0.27兲 16 共0.37兲 248 共−0.51兲 82 共0.49兲

1600 共0.42兲 155 共0.31兲 3600 共1.88兲 940 共0.44兲

0 50 0 50

4.8 共0.19兲 3.3 共0.14兲 2.4 共−0.58兲 2.5 共−0.40兲

180 共0.26兲 13 共0.25兲 120 共−0.43兲 375 共1.13兲

1900 共0.55兲 200 共0.61兲 6000 共2.01兲 1000 共0.27兲

a

±10%.

B. Ultrafast electron transfer dynamics in SDS and TX-100 micelles

In both SDS and TX-100 micelles, addition of DMA causes a dramatic decrease in the fluorescence lifetime of

coumarin dyes 共Tables I and II兲. For all the coumarin dyes except C480, the rise at the red end 共which originates from solvation dynamics, Sec. III C and Table III兲 vanishes on addition of DMA. Figures 2 and 3 show the femtosecond

TABLE II. Fluorescence decay parameters of coumarin dyes in 140 mM TX-100.

Acceptor

␭em 共nm兲 450

C151

530

460 C152

550

450 C481

540

480 C153

570

440 C480

525

关DMA兴 共mM兲

␶1 共a1兲a 共ps兲

␶2 共a2兲a 共ps兲

␶3 共a3兲a 共ps兲

0 100 0 100

8.2 共0.13兲 2.5 共0.32兲 3.6 共−0.28兲 4.8 共0.14兲

186 共0.10兲 22 共0.29兲 1540 共−1.5兲 53 共0.38兲

2450 共0.78兲 336 共0.39兲 5600 共2.78兲 2000 共0.48兲

0 100 0 100

4.2 共0.19兲 1.2 共0.40兲 5 共−0.26兲 5 共0.18兲

60 共0.27兲 13 共0.34兲 138 共−0.33兲 60 共0.34兲

988 共0.54兲 162 共0.26兲 2200 共1.59兲 1100 共0.48兲

0 100 0 100

3.6 共0.16兲 1.5 共0.33兲 5.4 共−0.21兲 5.5 共0.18兲

64 共0.20兲 12 共0.31兲 200 共−0.16兲 72 共0.41兲

1310 共0.64兲 150 共0.36兲 2600 共1.37兲 1000 共0.41兲

0 100 0 100

5.4 共0.12兲 2.7 共0.21兲 4.7 共−0.25兲 ¯

80 共0.19兲 37 共0.31兲 1300 共−2.5兲 150 共0.31兲

1700 共0.69兲 390 共0.48兲 4800 共3.75兲 1600 共0.69兲

0 100 0 100

7.2 共0.08兲 37 共0.12兲 18 共−0.60兲 4.5 共−0.38兲

250 共0.24兲 200 共0.15兲 2100 共−1.5兲 80 共−0.7兲

3900 共0.68兲 820 共0.73兲 7800 共3.1兲 2900 共2.08兲

a

±10%.

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TABLE III. Decay parameters of C共t兲 of C480 in SDS, TX-100, and CTAB at different bulk DMA concentrations.

System

SDS micelle

TX100 micelle

Bulk 关DMA兴 mM

Total solvent shift detected ⌬␯ 共cm−1兲

0

670

50

220

0

1164

100

380

0

850

50

1500

␶2 共a3兲 共ps兲

1.5 共39%兲 2.3 共65%兲

180 共61%兲 45 共35%兲

2.7 共9%兲 1 共64%兲

100 共5%兲 100 共36%兲

2900 共86%兲 ¯

15.2

0.3 共36%兲 3.5 共51%兲

10 共26%兲 100 共49%兲

280 共38%兲 ¯

125

c

CTAB

a

␶3 共a4兲 共ps兲

␶C481 ET 共=1 / k481 ET 兲 共ps兲b

␶1 共a2兲 共ps兲 a

a

¯ ¯

24.5

a

±10%. Electron transfer time for C481 共␶C481 ET 兲 in different micelles. c Taken from Ref. 41. b

transients of C151, C152, C481, and C480 in SDS micelle as a function of DMA concentration at the blue and the red ends, respectively. Figure 4 shows the picosecond decays of C152 and C480 in a SDS micelle as a function of DMA concentration. The absence of rise component at the red end at a high DMA concentration indicates that ET is faster than solvation dynamics. Thus, the rise is masked by ultrafast decay of the quenched emission for all the coumarin dyes except C480 共Fig. 3兲. In contrast to these four dyes, C480 continues to display a rise at the red end, even at a very high DMA concentration 共Fig. 3兲. In SDS micelle, at 50 mM bulk DMA concentration, C480 exhibits an ultrafast rise time of 2.5 ps which is roughly equal to the ultrafast component of solvation 共2.3 ps, Table III兲. In TX-100 micelle, the rise

components of 4.5 and 80 ps are in the order of solvation time in the presence of 100 mM bulk DMA concentration 共1 and 100 ps, Table III兲. This suggests that C480 exhibits much slower ET in both micelles and for this dye, the slow component of solvation precedes ET.41,42 In order to determine the rate constant of the electron transfer process 共kET兲 we used the equation37–42

FIG. 2. Femtosecond transients at blue end of 共A兲 C151 共␭em = 450 nm兲, 共B兲 C152 共␭em = 480 nm兲, 共C兲 C481 共␭em = 490 nm兲, and 共D兲 C480 共␭em = 440 nm兲 in 80 mM SDS with bulk DMA concentrations: 共i兲 0 mM, 共ii兲 15 mM, 共iii兲 30 mM, and 共iv兲 50 mM 共␭ex = 405 nm兲.

FIG. 3. Femtosecond transients at red end of 共A兲 C151 共␭em = 520 nm兲, 共B兲 C152 共␭em = 560 nm兲, 共C兲 C481 共␭em = 550 nm兲, and 共D兲 C480 共␭em = 530 nm兲 in 80 mM SDS with bulk DMA concentrations: 共i兲 0 mM and 共ii兲 50 mM 共␭ex = 405 nm兲.

1 1 = + k 关Q兴 , ␶ ␶0 ET eff

共8兲

where ␶ and ␶0 denote lifetimes of the donor 共coumarin兲 in the presence and in the absence of the quencher Q 共DMA兲 and 关Q兴eff denotes the local or effective concentration of the

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J. Chem. Phys. 126, 204708 共2007兲

Ghosh et al.

end even at high DMA concentration, Fig. 3兲, we used the slow component of decay at the blue end as ␶ at all amine concentrations. The rate constants of ET for the different systems are listed in Tables IV and V. The rate constant is found to be very different for different coumarin dyes even in the same micelle. For both the micelles, C152 shows fastest electron transfer while C480 exhibits the slowest electron transfer. As shown in Table IV, the rate constant of ET in SDS micelle varies about 8.5 times from 4.76⫻ 1010 s−1 M −1 for C152 to 0.56⫻ 1010 s−1 M −1 for C480. For TX-100 micelle, ET rate varies about 18.5 times from 7.47⫻ 1010 s−1 M −1 for C152 to 0.4⫻ 1010 s−1 M −1 for C480 共Table V兲. The magnitude of the rate constant of ET in SDS and TX-100 micelles are found to be much larger than that in CTAB micelle. Previously,41 we reported that in CTAB micelle, the rate constant of ET for C481 is 8 ⫻ 109 s−1 M −1 which is approximately five times and approximately eight times slower than that in SDS and TX-100 micelles, respectively. The rate of ET detected in the present work using a femtosecond setup is much higher than those reported earlier37–39 using a picosecond setup which obviously missed the ultrafast initial component of the fluorescence transients.

FIG. 4. Picosecond transients of C152 at 共A兲 ␭em = 480 nm, 共B兲 ␭em = 560 nm, and C480 at 共C兲 ␭em = 440 nm 共D兲 ␭em = 530 nm in the presence of 80 mM SDS with bulk DMA concentrations: 共i兲 0 mM 共ii兲, 15 mM, 共iii兲 30 mM, and 共iv兲 50 mM 共␭ex = 405 nm兲.

quencher in the Stern layer of the micelle. We assumed that in the absence of the amine the faster components of decay at the blue end are due to the solvation dynamics for all the coumarin dyes. Therefore, we used the slow component of the decay at the blue end as ␶0. In the presence of DMA, we used the average lifetime of the fluorescence decays of C151, C152, C481, and C153 as ␶. For C480, since solvation is faster than ET 共indicated by the rise component at the red

C. Solvation dynamics in SDS and TX-100 micelles

We have studied the solvation dynamics in SDS and TX-100 micelles using C480 as the solvation probe. In the absence of DMA, all the coumarin dyes exhibit wavelength dependent emission decays with a rise component at the red

TABLE IV. Electron transfer parameters of coumarin dyes in SDS micelle at different DMA concentrations.

Probe

C151

C152

C481

C153

C480

关DMA兴 共mM兲

关DMA兴effa 共M兲

0 15 30 50

¯ 0.135 0.423 0.808

0 15 30 50

¯ 0.135 0.423 0.808

0 15 30 50

¯ 0.135 0.423 0.808

0 15 30 50

¯ 0.135 0.423 0.808

0 15 30 50

¯ 0.135 0.423 0.808

⌬G0 共eV兲

−0.89

−0.67

−0.60

−0.42

−0.31

具␶典b 共ps兲 3700c 302 103 47 620c 100 57 24 457c 100 57 28 1600c 170 96 54 1900c 569c 353c 200c

kET 共s−1 M −1兲

ln共kET兲

2.51⫻ 1010

23.95

4.76⫻ 1010

24.58

4.07⫻ 1010

24.43

2.27⫻ 1010

23.84

0.56⫻ 1010

22.44

关DMA兴eff denotes the local concentration of DMA in the Stern layer of SDS micelle. 具␶典 = ⌺ai␶i. c Longest component of decay. a

b

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Photoinduced electron transfer

TABLE V. Electron transfer parameters of coumarin dyes in TX-100 micelle at different bulk DMA concentrations.

Probe

C151

C152

C481

C153

C480

关DMA兴 共mM兲

⌬G0 共eV兲

关DMA兴effa 共M兲

0 15 30 50 100

¯ 0.018 0.057 0.109 0.259

0 15 30 50 100

¯ 0.018 0.057 0.109 0.259

0 15 30 50 100

¯ 0.018 0.057 0.109 0.259

0 15 30 50 100

¯ 0.018 0.057 0.109 0.259

0 15 30 50 100

¯ 0.018 0.057 0.109 0.259

具␶典b 共ps兲

kET 共s−1 M −1兲

ln共kET兲

2.57⫻ 1010

23.97

7.47⫻ 1010

25.04

6.55⫻ 1010

24.90

1.63⫻ 1010

23.51

0.40⫻ 1010

22.12

c

−0.82

−0.75

−0.72

−0.50

−0.32

2450 1115 571 348 138 988c 308 194 144 47

1310c 312 190 117 58 1700c 793 595 395 200 3900c 2100c 1900c 1300c 820c

关DMA兴eff denotes the local concentration of DMA in the palisade layer of TX-100 micelle. 具␶典 = ⌺ai␶i. c Longest component of decay. a

b

end 共Fig. 3兲. The rise is attributed to solvation dynamics. Figure 5 shows the TRES of C480 in a TX-100 micelle in the presence of DMA. Figure 6 shows decay of C共t兲 for C480 in SDS and TX-100 micelle in the presence of DMA. It is observed that in the absence of DMA the solvation dynamics in SDS micelle exhibits two components—1.5 and 180 ps 共Table III兲. In the presence of 50 mM DMA, the components of solvation dynamics in a SDS micelle are found to be 2.3 and 45 ps 共Table III, Fig. 6兲, respectively. In TX-100 micelle,

in the absence of DMA exhibits three components of solvation dynamics—2.7, 100, and 2900 ps 共Table III兲. In 100 mM DMA, solvation in TX-100 micelle is found to be faster with components of 1 and 100 ps 共Fig. 6兲. This suggests that both in the presence and in the absence of DMA there is a component of solvation which is in a time scale ⬍10 ps. The ultrafast component of solvation obviously facilitates the ultrafast component of electron transfer which occurs in ⬃10 ps time scale.

FIG. 5. Time resolved emission spectra of C480 in 140 mM TX-100 in the presence of 100 mM DMA 共␭ex = 405 nm兲 at 0 ps 共䊏兲, 1 ps 共䊊兲, 10 ps 共䉱兲, and 350 ps 共䉮兲.

FIG. 6. Solvent response function C共t兲 of C480 in the presence of DMA in 共A兲 80 mM SDS 共䊊兲 and 共B兲 140 mM TX-100 共䉭兲.

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Ghosh et al.

IV. DISCUSSION

In this work, we have detected the ultrafast 共few picoseconds兲 part of the photoinduced intermolecular electron transfer process between DMA and coumarin dyes in SDS and TX-100 micelles. It is observed that the PET process is faster in the case of SDS and TX-100 micelles compared to that reported previously for a CTAB micelle.41 We first attempt to rationalize the differences in the ET rates for the three micelles. As discussed earlier, the rate of ET depends on solvation. The parameters of solvation dynamics, i.e., decay of C共t兲, for the three micelles are summarized in Table III. In the presence of DMA, the solvation dynamics in CTAB is described by two components of 3.5 ps 共51%兲 and 100 ps 共49%兲 with average solvation time of ⬃52 ps. For SDS and TX-100, the average solvation times are 17 and ⬃36 ps, respectively. Thus the solvation time in the three micelles is of the order SDS⬍ TX-100⬍ CTAB. Structure and dynamics in cationic 共C10TAB兲 共Ref. 52兲 and anionic53–55 micelles have been investigated by computer simulations. These studies indicate water molecules penetrate the cationic head group and reveal slower solvation dynamics near a cationic micelle compared to an anionic micelle.52 One possible reason for slower dynamics near a cationic micelle could be immobilization of the heavier end of water 共oxygen atom兲 by Coulombic attraction of the cationic head group.52–54 The immobilization of water near a cationic micelle is manifested in longer residence time.52 It seems that the slowest solvation dynamics in CTAB may be responsible for the slowest ET rate. Table III also contains the ET time for C481 which displays almost fastest rate of ET. It is evident that for SDS and TX-100 micelles, the ET time is faster than the solvation time in the presence of DMA. However, about 65% of the solvation occurs within the time constant of ET in these two micelles 共Table III兲. For CTAB, the time constant of ET is longer than the solvation time 共Table III兲. Thus solvation is complete before ET occurs in CTAB micelle. Since, in a micelle the ultrafast part of solvation is present even in the case of ultrafast ET it is reasonable to search for the Marcus inverted region in a micelle.13,14 The spontaneity of the electron transfer process from a ground state electron donor to an excited state electron acceptor is determined by the standard free energy change 共⌬G0兲 of the PET reaction. Tables IV and V show the ⌬G0 values for different donor-acceptor systems in SDS and TX-100 micelles. We used the Stern-Volmer plots to determine the electron transfer rates 共Fig. 7兲. The ⌬G0 dependence of ET rate of coumarin dyes in a CTAB,41 SDS, and TX-100 micelle are shown in Fig. 8. It is apparent that the plot of ET rate against ⌬G0 共Fig. 8兲 is bell shaped and very clearly indicates a Marcus-type inverted region. It should be further noted that the shape of the curve in Fig. 8 is asymmetric. The asymmetry may be ascribed to the inverted regime where ET becomes irreversible.17 It is interesting to note that in CTAB micelle where solvation is complete within the ET time exhibits Marcus inversion at a higher exergonicity 共−1.2 eV兲 in comparison to

FIG. 7. Stern-Volmer plots for C151 共䉭兲, C481 共䉮兲, and C480 共〫兲 in 共A兲 80 mM SDS and 共B兲 140 mM TX-100 at different local DMA concentrations.

SDS and TX-100 micelles 共−0.7 eV兲. In the present case, the ET corresponds to a situation where solvation is incomplete, i.e., the solvent is nonequilibrated. In such a condition, many groups have used the 2D-ET model.5,8,13–18,21,56,57 In this model the free energy of activation depends on the solvent coordinate 共X兲 as58–60

k共X兲 =

冉

冊

2␲ 2 − ⌬G ⴱ 共X兲 . Vel共4␲␭IkBT兲−1/2 exp ប k BT

共9兲

Here, ⌬G* is the free energy of activation and ␭1 is the intramolecular reorganization energy. In the case of ET faster than solvation the effective solvent reorganization energy ␭eff is less than that 共␭s兲 when solvation is complete. ␭eff is related to 共␭s兲 as8,17,56–60

FIG. 8. Plot of ln共kET兲 against −⌬G0 for coumarin-DMA system in CTAB 共䊏兲, SDS 共쎲兲, and TX-100 共䊊兲 micelles.

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FIG. 9. Fluorescence anisotropy decay 共␭ex = 405 nm兲 of 共A兲 C152 共␭em = 480 nm兲 and 共B兲 C480 共␭em = 440 nm兲 in 80 mM SDS. The points denote the actual values of anisotropy and the solid lines denote the best fit to the experimental data.

␭eff = ␭s共1 − 2X2g兲,

共10兲

where Xg denotes nonequilibrated solvation coordinate. For the coumarin dyes-DMA system ␭s ⬃ 0.93 eV and ␭1 is 0.3 eV.22,23,34 We have estimated the amount of reorganization energy lost 共2␭sX2g兲 because of incomplete solvation from the comparison of the time constant of ET and solvation dynamics 共Table III兲. For CTAB, solvation is complete within the time scale of ET 共Table III兲 and hence ␭eff ⬃ ␭s. Thus for CTAB micelle, ␭eff + ␭I = 1.23 and thus in this case one should observe Marcus inversion at −⌬G0 ⬃ 1.23 eV. This is very similar to the observed inversion at 1.2 eV 共Fig. 8兲. For SDS and TX-100 only about 65% of solvation is complete within ET time 共Table III兲. Therefore, for these two micelles, ␭eff ⬃ 0.65␭s = 0.60 eV. Thus for these two micelles one should observe Marcus inversion at −⌬G0 = ␭eff + ␭I ⬃ 0.9 eV. This is close to the observed inversion at 0.7 eV 共Fig. 8兲.

The slowest ET rate in CTAB may also originate from relatively larger donor-acceptor distance 共R兲. According to recent simulations,52 the bromide ions are located near the cationic head group of C10TAB micelle. The existence of the bromide counterion and consequent repulsion may result in a larger donor-acceptor separation.61 The rate of ET depends strongly on the electronic coupling matrix Vel in Eq. 共1兲 that decreases exponentially with the increase in R as exp共−␤R兲, where ␤ is in the order of 1 Å−1.1,17,22,35 For SDS micelle, the smaller counterion 共Na+兲 leads to lower steric repulsion and also holds the donor and acceptor at close distance by cation-␲ interaction. In TX-100 micelle there is no counterion to separate the donor and the acceptor and very large thick palisade layer 共⬃25 Å兲 共Refs. 46–48兲 accommodates both the donor and the acceptor at a close distance. Several groups have considered the role of diffusion on PET in micelle.22,23,25,33,34 The diffusion coefficient of coumarin dyes in SDS and TX-100 micelles may be obtained using fluorescence anisotropy decay. Figure 9 shows anisotropy decay of C152 and C480 in SDS micelle in the absence of DMA. For example, the fluorescence anisotropy decay was analyzed in terms of wobbling in cone model.62–66 Using wobbling in cone model the values of the average translational diffusion coefficient 共Dt兲 of the coumarin dyes in SDS and TX-100 micelles are found to be 2.11⫻ 10−9 and 1.50 ⫻ 10−9 m2 s−1, respectively 共Table VI兲. One can easily calculate the diffusion distance 共冑2Dt␶兲 of coumarin dyes along the micellar surface.67 The time scales of ultrafast PET in SDS micelle are 47, 24, 28, 54, and 200 ps for C151, C152, C481, C153, and C480 共Table IV兲, respectively, at the highest DMA concentration 共50 mM兲 which correspond to diffusion lengths of 4.4, 3.2, 3.4, 4.8, and 9.2 Å, respectively. One should note that all these distances are less than the size of the acceptor or donor. The observed variation in the rate of PET does not seem to originate from rotational diffusion of the acceptors 共coumarin dyes兲. Finally, we consider the effect of distribution of D-A distance on PET in a micelle. The NMR and electronic spectra suggest that the donor 共DMA兲 molecules reside near the head group of the micelles.34 Thus the donors are present at

TABLE VI. Fluorescence anisotropy decay parameters of coumarin dyes in 80 mM SDS and 140 mM TX-100.

Probe

Micelle

r0

␶1a 共a1兲 共ps兲

␶2a 共a2兲 共ps兲

Dt ⫻ 109 共m2 s−1兲

Dw ⫻ 10−7 共s−1兲

C151

SDS TX-100

0.26 0.32

250 共0.16兲 790 共0.37兲

1340 共0.36兲 3365 共0.63兲

1.07 1.21

47.8 7.71

C152

SDS TX-100

0.29 0.30

150 共0.41兲 300 共0.14兲

590 共0.17兲 2350 共0.86兲

2.49 1.74

43.7 8.16

C481

SDS TX-100

0.39 0.29

50 共0.35兲 830 共0.17兲

470 共0.65兲 3490 共0.83兲

3.14 1.17

132 3.15

C153

SDS TX-100

0.30 0.37

180 共0.46兲 330 共0.23兲

730 共0.54兲 2800 共0.77兲

2.0 1.46

41.8 12.58

C480

SDS TX-100

0.33 0.35

240 共0.64兲 390 共0.32兲

825 共0.36兲 2100 共0.68兲

1.77 1.96

43.4 13.98

a

±10%.

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204708-10

the periphery of the micelles. Note, under the condition of our studies there are more than ten donor 共DMA兲 molecules per micelle while there is at most one acceptor per micelle. Because of the presence of many donors in one micelle, the acceptor molecule always finds one donor molecule in its vicinity. It seems that ET is dominated by the donor closest to the acceptor. Thus the rate constant of ET does not change with increase in donor concentration. This gives rise to the linearity of the Stern-Volmer plots 共Fig. 7兲. In order to check the possible D-A distance dependence of ET, we have excited our sample at various excitation wavelengths 共␭ex = 375– 435 nm兲. The absorption and emission maxima of the coumarin dyes strongly depend on solvent polarity.51 Thus by varying the ␭ex, one may excite the acceptor 共coumarin dye兲 in different regions of the micelle. This method has recently been successfully used to study location dependent solvation dynamics65,66 and energy transfer67 in micelles. Excitation of the acceptor in different regions of the micelle in principle may affect the D-A distances. However, we did not find any variation of rate of ET with ␭ex. This also suggests that the effective D-A distances does not change with ␭ex. V. CONCLUSION

This work demonstrates that ultrafast ET process in a micelle depends on the nature of micelle. The ET rate is found to be in the order TX-100⬎ SDS⬎ CTAB. This is explained in terms of the donor-acceptor distance which is highest for CTAB due to steric repulsion from bromide counterion and lowest for TX-100 with no counterion. A Marcus inverted region is recovered for all the micelles. The free energy change corresponding to inversion point is found to be different for different micelles. For CTAB, ET is slower and a major portion of solvation is complete in the time scale of ET and hence the inversion point is observed at a large −⌬G0. For SDS and TX-100, where ET is very fast and only a fraction of solvation is complete in the time scale of ET inversion occurs at a lower −⌬G0. The role of diffusion of donor and acceptor is found to be minor except in the case of C480. ACKNOWLEDGMENTS

Thanks are due to Department of Science and Technology, India 共Project No. IR/I1/CF-01/2002兲 and Council of Scientific and Industrial Research 共CSIR兲 for generous research grants. Three of the authors 共S.G., S.K.M., and K.S.兲 thank CSIR. Another author 共K.B.兲 thanks Professor B. Bagchi for many illuminating discussions. R. A. Marcus, Rev. Mod. Phys. 65, 599 共1993兲. R. A. Marcus, Adv. Chem. Phys. 106, 1 共1999兲. 3 E. A. McArthur and K. B. Eisenthal, J. Am. Chem. Soc. 128, 1068 共2006兲. 4 B. Bagchi and N. Gayathri, Adv. Chem. Phys. 107, 1 共1999兲. 5 L. D. Zusman, Chem. Phys. 49, 295 共1980兲. 6 K. Yoshihara, Adv. Chem. Phys. 107, 371 共1999兲. 7 T. Kobayashi, Y. Takagi, H. Kandori, K. Kemnitz, and K. Yoshihara, Chem. Phys. Lett. 180, 416 共1991兲. 8 H. Pal, Y. Nagasawa, K. Tominaga, and K. Yoshihara, J. Phys. Chem. 100, 11964 共1996兲. 1 2

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Photoinduced electron transfer

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