6646
J. Phys. Chem. B 2008, 112, 6646–6652
Ultrafast Bimolecular Electron Transfer Dynamics in Micellar Media Manoj Kumbhakar,* Prabhat K. Singh, Sukhendu Nath, Achikanath C. Bhasikuttan, and Haridas Pal* Radiation and Photochemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400 085, India ReceiVed: January 25, 2008; ReVised Manuscript ReceiVed: March 7, 2008
Ultrafast photoinduced bimolecular electron transfer (ET) dynamics between 7-aminocoumarin derivatives and N,N-dimethylaniline (DMAN) has been studied in neutral (TX100), cationic (DTAB) and anionic (SDS) micellar media. A very fast decay time constant (τfast) shorter than ∼10 ps has been observed for the coumarins in the presence of DMAN in all of the three micellar media. In this time scale, reactants in the micellar phase undergo ET interactions without involving diffusion or reorientation of the reactants and thus can be envisaged as equivalent to nondiffusive bimolecular ET reaction. The fastest ET rates estimated as the inverse of the shortest lifetime components of the fluorescence decay (ket = τfast-1) nicely follow the predicted Marcus inversion behavior with reaction exergonicity (-∆G°), irrespective of the nature of micelles considered. Onset of inversion in ET rates occur at ∼0.61 eV lower exergonicity in SDS and TX100 micelles compared with that in DTAB micelle and are rationalized following two-dimensional ET (2DET) theory. These differences suggest the possibility of tuning Marcus inversion by proper selection of micelles. Interestingly, ET rates (k′et) obtained from the conventional Stern-Volmer analysis of the relatively longer time constants of the fluorescence decays also exhibit similar Marcus correlation with ∆G°, showing clear inversion behavior. Fitting of Marcus correlation curves for ket and k′et indicate two largely different values for the electronic coupling parameters. In micellar media, as the interacting donor–acceptor molecules are on an average expected to be separated by an intervening surfactant chain and the reorientation rate of the reactants is quite slow, it is predicted that the ultrafast ET (ket) component arises because of the surfactant separated donor–acceptor pairs that are orientated perfectly to give the maximum electronic coupling. The slower ET (k′et) is predicted to arise because of those pairs where the donor–acceptor orientations are not very suitable but good enough to give a sizable electronic coupling. Introduction In the past few decades, electron transfer (ET) studies have received tremendous attention in order to understand various aspects that govern the ET dynamics and mechanism. ET is the most fundamental reaction that occurs in chemistry and biology and has diverse technological applications. From theoretical and experimental studies, it is realized that the important factors that control the dynamics of ET reactions are the free energy change of the reaction (∆G°) and the activation free energy (∆G*).1–14 According to Marcus ET theory,15,16 the ET rate ket, is related to ∆G° by the following quadratic equation
{
2 2π Vel (∆G° + λ)2 ket ) exp p 4πλ k T 4λkBT √ sB
}
(1)
where Vel is the electronic coupling element, kB is the Boltzmann constant, T is the absolute temperature, and λ is the total reorganization energy, given as λ ) (λs + λi), with λs being the solvent reorganization energy and λi being the intramolecular reorganization energy. By following this equation, in the region -∆G° < λ, ket increases with an increase in exergonicity (-∆G°) of the reaction (normal region) until the barrierless situation is reached at -∆G° ) λ, where the ET reaction proceeds with the maximum rate. However, the most interesting feature of this equation is the inversion in the ET rates with * Authors to whom correspondence should be addressed. E-mail:
[email protected] and
[email protected] (M.K.) and hpal@ barc.gov.in (H.P.). Fax: 91-22-25505151/25519613.
exergonicity at -∆G° > λ, known as the Marcus inversion region. This inversion region has tremendous implications in applied areas; as in most ET systems both charge separation and back electron transfer reaction often occur simultaneously.3 This can be easily perceived if we consider the reverse ET reaction, which produces back the ground-state reactants and occurs at higher exergonicity than that of the related forward ET reaction.2,3 In designing an efficient photoinduced charge separation system, it is quite expected that the forward ET should be carried out at a much higher rate than the reverse ET rate. This can ideally be possible if the exergonicity of the forward ET reaction is quite close to the barrierless situation such that the exergonicity of the reverse ET invariably appears at the inversion region. Thus, by taking advantage of this relationship, it might be possible to control or modulate ET reactions; a classic example is the primary charge separation events in photosynthesis that occur in nature with maximum efficiency. Although, the inversion region in the Marcus correlation curve has been easily demonstrated experimentally for many intramolecular ET reactions17–19 and in many charge recombination processes,20–23 such behavior is understood to be very difficult to observe for bimolecular ET reactions.24–26 The two important factors that hinder the observation of the Marcus inversion region in bimolecular ET reactions are the diffusion of the reactants that limits the maximum possible rate for a bimolecular reaction and the nonavailability of suitable donor–acceptor series that can allow it to reach a very high exergonicity region for the ET reaction.2,3,5,27 These limiting conditions have been circumvented to a large extent by carrying out ET reactions in
10.1021/jp800752d CCC: $40.75 2008 American Chemical Society Published on Web 05/06/2008
Ultrafast Bimolecular Electron Transfer Dynamics organized assemblies like micelles,28 which closely resemble the structure of biological membranes.29 It has been demonstrated for the first time by our group that the organized media are quite conducive for the observation of Marcus inversion behavior for bimolecular ET reactions.28 After the initial report from our group, many ET studies on bimolecular ET reactions in confined and organized assemblies have been reported in the literature, mostly following measurements in the nanosecond time domain, with an aim to understand various aspects of ET dynamics in such media, especially the inversion behavior at higher exergonicity region.28,30–42 However, there are still many finer aspects of the bimolecular ET reactions in organized media that need to be unveiled at the fast and ultrafast time scales. One of the interesting findings in micellar ET reactions using steady-state fluorescence measurements is the positive deviation in the observed Stern-Volmer plots.28,30–38,43 In absence of any ground-state complex formation between the donors (i.e., aniline derivatives) and the acceptors (i.e., 7-aminocoumarin derivatives), as indicated from the unchanged absorption spectra of the acceptors in the absence and presence of the donors, instantaneous or static quenching has been cited as the reason for the observed positive deviations in these systems.28,30–35 Because of limited time resolution in the previous nanosecond measurements, however, it was not possible to explore this ultrafast ET kinetics. Though, Bhattacharyya and co-workers41,42 have recently carried out some such ultrafast fluorescence upconversion measurements, they did not carry out explicit analysis to understand the characteristics of the ultrafast ET components, rather they applied the simple classical Stern-Volmer (SV) analysis to extract only an average picture for the ET kinetics. Therefore, the important features of the ultrafast ET components in micellar media hitherto remain unexplored. In some recent literatures, there are some dubious reports regarding the possible influence of reactant diffusion on the observed inversion behavior for ET reactions in micellar media.37,38 For ET reactions occurring in the nanosecond to subnanosecond time scales, the possibility of any strong influence of reactant diffusion on ET kinetics in micelles is quite unlikely, even if the effect can not be excluded completely. However, the ambiguity regarding the origin of inversion in ket values at higher exergonicity, whether it is the reflection of the changes in the activation free energy as expected from Marcus ET theory or that of the changes in the diffusion characteristics of the donor–acceptor pairs, needs to be clarified undoubtedly. The observed static quenching for the micellar ET reactions can simply be visualized as arising from those fluorophores (acceptor) that have a pre-existence of at least one quencher (donor) within its sphere of action and thus should be associated with an ultrafast (subnanosecond) fluorescence decay component.44 In the short subnanosecond timescales, reactants should basically remain static. So that the reaction can effectively occur as equivalent to an intramolecular reaction with donors and acceptors distributed in space, they are separated by the intervening solvent (water) and surfactant chains.34,45 Therefore, the ET rates corresponding to the ultrafast quenching component in micellar media should definitely resolve the ambiguity about the origin of Marcus inversion behavior. Motivation for this study is also to understand the nature of the donor–acceptor pairs that are responsible for ET reactions at different time scales. Role of the spatial orientation of reactants, the electronic coupling parameter, and the nature of the micellar environment on different ET components are also the aspects of investigation in the present study. In the present work, we carry out the ultrafast fluorescence quenching studies (by ET) for different
J. Phys. Chem. B, Vol. 112, No. 21, 2008 6647 SCHEME 1: Chemical Structures of Coumarin Acceptors and DMAN Donor
coumarin derivatives (cf. Scheme 1) using N,N-dimethylaniline (DMAN) as the donor in neutral [TX100, (CH3)3CH2C(CH3)2-(C6H4)-(OCH2CH2)10-OH], cationic [DTAB, CH3(CH2)11N(CH3)3+Br-], and anionic [SDS, CH3(CH2)11SO4-Na+] micellar media to answer some of the above queries related to intermolecular ET reactions in micellar media. Experimental Section Steady-state absorption spectra were recorded using a JASCO (Tokyo, Japan) model V530 spectrophotometer. Fluorescence spectra were recorded using a HITACHI (Tokyo, Japan) model F-4010 spectrofluorimeter. In our femtosecond fluorescence up-conversion setup (FOG 100, CDP), acceptors were excited, and signals were monitored close to the absorption (∼390–425 nm) and emission maxima (∼490–535 nm) of the dye, respectively. The second harmonic (SH) of a mode locked Ti:sapphire laser (CDP Corp., Moscow, Russia, 82.2 MHz repetition rate) pumped by a 5 W DPSS laser was used for excitation. The SH was generated in a type I BBO angle-tuned phase-matched nonlinear crystal with 1 mm thickness. Optical delay between the excitation and the gate pulse was varied using a delay rail (6.6 fs per steps) at the path of the gate pulses. The up-converted signal was measured with a photon counter after passing through a proper band-pass filter and a double monochromator. For each of the decays, at least three scans were taken to average the data points and also to see the reproducibility of the decays measured. In all of these measurements, the samples were taken in a rotating cell (0.4 mm path length) to have a better heat dissipation and thus to avoid the photodegradation of the dye. To check this further, the absorption spectra of the sample were recorded before and after the fluorescence up-conversion measurements and were found to be quite similar. For the measurements of fluorescence transients, polarization of the SH beam was set to the magic angle (54.7°) with respect to the observed horizontally polarized fluorescence signal using Berek wave plate arrangement46 (CDP Corp., Moscow, Russia). The fluorescence signal from the sample was up-converted in another nonlinear BBO crystal (thickness 0.5 mm) using the Ti:sapphire fundamental beam as the gate pulse. A cross correlation of fundamental and the SH displayed a full width at half-maxima (fwhm) of ∼210 fs for the instrument response function (IRF). Femtosecond transients were fitted by convolution analysis using a Gaussian shape for the IRF. In case of fluorescence anisotropy measurements,
6648 J. Phys. Chem. B, Vol. 112, No. 21, 2008
Kumbhakar et al.
samples were excited with vertically and horizontally polarized SH pulses using different angles for Berek waveplate. In the present setup, since the horizontally polarized fluorescence is mixed with the horizontally polarized gate pulses in the upconversion crystal, the measured sum frequency signal for horizontally polarized excitation measures I| and that for vertically polarized excitation measures I⊥. Accordingly, fluorescence anisotropy decay r(t) was calculated as r(t) ) (I| I⊥)/(I| + 2I⊥).44 In the present study, the long fluorescence lifetimes (nanosecond) were measured using a diode laser (408 nm,
