J. Phys. Chem. B 2010, 114, 10057–10065
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Ultrafast Electron Transfer Dynamics in Micellar Media Using Surfactant as the Intrinsic Electron Acceptor Manoj Kumbhakar,*,† Prabhat Kumar Singh,† Ashis Kumar Satpati,‡ Sukhendu Nath,† and Haridas 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 derivatives as electron donor and pyridinium moiety of surfactant molecules in cetylpyridinium chloride (CPC) micelle as electron acceptor has been investigated to understand the role of separation and orientation of reactants on micellar ET reactions. Unlike in noninteracting micelles (like Triton-X-100, sodium dodecyl sulfate, dodecyltrimethylammonium bromide, etc.), where surfactant-separated donor-acceptor pairs are understood to give the ultrafast ET component with the shortest time constant in the range of ∼4 ps, in CPC micelles with pyridinium moiety as the intrinsic acceptor the ultrafast ET component is found to be in the subpicosecond time scale (of around 240 fs). This time scale is very similar to the values reported in the cases of ultrafast ET reactions involving coumarin dyes in electron-donating solvents. The ultrafast ET times in CPC micelles are significantly faster than the diffusive solvation dynamics in the micellar media. Correlation of the observed ET rates in the present cases with the free-energy changes of the reactions shows the inverse-bell-shaped correlation, predicted by Marcus ET theory. Interestingly, the onset of the Marcus inversion appears at a relatively lower exergonicity, which is attributed to the nonequilibrium solvent configuration during the ultrafast ET 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-contact donor-acceptor populations in the micelles that have relatively weaker electronic coupling due to improper orientation of the interacting donor-acceptor pairs. The present results suggest that, along with the shifting of Marcus inversion at lower exergonicity, the ET rates can also be maximized in a micellar media by using surfactant molecule as an intrinsic reactant. Introduction Tuning chemical reactivity is an important aspect of research in chemical sciences.1-3 Understanding various factors that govern the reaction rates and reaction mechanisms is of paramount importance to find ways to tune chemical reactions. In recent years considerable efforts have been made to explore the details of electron transfer (ET) processes in various organized assemblies, such as micelles,4-27 reverse micelles,28,29 polymer-surfactant aggregates,30-32 etc. The main aim of such studies is to reveal the controlling factors that determine the ET dynamics in such systems, in order to control the dynamics and mechanism of ET reactions. Theoretically, the rate constant ket for outer-sphere ET reaction is expressed as2,3,33-37 ket )
{
(
2 (∆G0 + λs)2 2π Vel 1 exp kBT 1 + 4πVel2τs /pλs p √4πλskBT
)}
(1)
where ∆G0 is the free energy change of the reaction, λs is the solvent reorganization energy, Vel is the electronic coupling between the reactant and product states, and τs is the solvent * To whom correspondence should be addressed. E-mail: manojk@ barc.gov.in,
[email protected]. Fax: 91-22-25505151/25519613. † Radiation and Photochemistry Division. ‡ Analytical Chemistry Division.
relaxation time. From eq 1 it is seen that in the adiabatic limit (4πVel2τs/pλs . 1),38-40 the ET rate constant is inversely related to τs and is expressed as
ket )
1 τs
(
(∆G0 + λs)2 λs exp 16πkBT 4λskBT
)
(2)
At room temperature, the value of 16πkBT is about 1.3 eV. Since the value of λs is normally either close to or less than this value,35,36,38-41 the maximum value for ket can at the most be equal to τs-1 for the barrierless ET reactions. Though for number of ET systems the dependence of ket on τs has been demonstrated experimentally by many authors, Barbara and co-workers42-46 and Yoshihara and co-workers37,41,47-54 have demonstrated that the above is not generally true for any of the ET reactions. These authors have demonstrated that for a number of intramolecular ET systems and also for intermolecular close-contact donoracceptor systems (e.g., fluorophores in electron-donating solvents) the ET rates often become faster (ket ≈ 1013 s-1) than the predicted τs-1 value. To explain the ultrafast nature of such ET dynamics, both intramolecular vibrational motions and the diffusive solvent motions were independently considered in a different fashion than in the conventional ET theories.55-57 Of late, there have been several similar reports on ultrafast ET as well.22,58-71 However, similar studies in confined organized assemblies like micelles, reverse micelles, etc. (a mimic to
10.1021/jp102258y 2010 American Chemical Society Published on Web 07/19/2010
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J. Phys. Chem. B, Vol. 114, No. 31, 2010
CHART 1: (a) Close-Contact Configuration for Fluorophore in Electron-Donating Solvent and (b) Donor-Acceptor Systems in Noninteracting Micelles, and (c) Donor-Acceptor Configuration in CPC Micellea
Kumbhakar et al. CHART 2: Chemical Structures of the Coumarin Dyes Used in the Present Study
a
Green and orange colors indicate electron donor and acceptor moiety, respectively. The close-contact configuration (a) for fluorophore in electron-donating solvent shows an ultrafast ET component of ∼200 fs. For similar donor-acceptor systems in noninteracting micelles (b), the fastest ET component is of ∼4 ps only. Yellow and blue shades indicate micelle and bulk water, respectively. Surfactants are represented by the circular head with wavy tail. In the close-contact donor-acceptor configuration in CPC micelle (c), the surfactant head group acts as one of the reactants for intermolecular ET process and it emulates the situation of dye in reacting solvent media.
cellular membranes) are very rare.10,16,17 In fact, in organized assemblies, the demonstration of an ultrafast electron transfer component faster than even the few picoseconds of solvation times in such media is still to be documented very unambiguously. In the present study, our aim is to design a close-contact donor-acceptor situation in a micellar media such that the situation similar to that of a dye in a reacting solvent medium can be emulated to explore different aspects of ultrafast ET reactions in a microheterogeneous media. Unlike in fast relaxing polar homogeneous solvents, the sluggish solvation dynamics in organized assemblies favors to reduce the contribution of the solvent reorganization energy toward the activation barrier of the ET reaction.7 Therefore, the Marcus inversion for the ET rates in organized assemblies shifts toward a relatively lower exergonicity (-∆G0) than that in a homogeneous medium of similar polarity. Thus, it helps us to observe the bell-shaped Marcus correlation curve for intermolecular ET reactions easily in microheterogeneous media, without increasing the reaction exergonicity (-∆G0) to an unreasonably high value.7 Additionally, in organized assemblies, the tethering of the reactants within the surfactant chains causes a large retardation in the rate of reactant diffusion, making the ET reactions to occur effectively under nondiffusive condition.7-11,15 In one of our recent studies, it was observed that, for intermolecular ET reactions in different noninteracting micelles, the fastest ET time corresponding to the maxima of the Marcus correlation curves is only in the range of about 4 ps.10 Note that for intermolecular ET in homogeneous media under nondiffusive condition, i.e., when the electron acceptor dyes are dissolved directly in neat electron-donating solvents (cf. Chart 1a), the fastest ET component is found to be as short as about 200 fs.52,53,72,73 Though both in micelles and in electrondonating solvents the intermolecular ET takes place under nondiffusive condition, the unusually higher value for the fastest ET time in micelles is supposedly due to the intervention of surfactant chains between the reacting donor-acceptor pairs,10,11 causing a larger separation between the reacting pairs (cf. Chart 1b) and consequently a lower electronic coupling. In the present study, we use a micelle that emulates the situation similar to that of the reacting solvent media such that intervention of surfactant chains between the reacting pairs can be avoided and hence ultrafast ET time as short as that in electron-donating solvents can be realized. Our aim is also to see if such an ultrafast ET also follows the Marcus inversion behavior as observed earlier in other micellar media.
In the present study, to achieve close-contact donor-acceptor configuration, we have selected cetylpyridinium chloride (CPC, CH3-(CH2)15-C5H4N+ Cl-) as the surfactant such that the pyridinium moiety of the surfactant in the micelle acts as the electron acceptor, while a number of coumarin derivatives are used as the electron donors. The coumarin concentrations used in this study are very low (
