Ultrafast intermolecular electron transfer dynamics: Perylene in electron-accepting micellar medium

The dynamics of ultrafast photoinduced intermolecular electron transfer (ET) from the excited singlet (S1) state of perylene (Pe) to an electron-accepting cationic surfactant molecule, N-cetylpyridinium chloride (CPC), in aqueous micellar solutions has been investigated using the femtosecond transient absorption spectroscopic technique with temporal resolution of 120 fs. The Pe molecule is localized at or near the micellar surface, where it coexists with the pyridinium moieties (headgroups of the micelle) of the surfactant molecule. Following photoexcitation of Pe, an electron is transferred to the neat and geometrically restricted headgroup of the micelle. Dynamics of the forward ET process as well as the geminate recombination or back ET (BET) process have been followed by monitoring the temporal evolution of the S1 state of Pe and the cation radical of Pe (Pe*+), respectively. The multiexponential forward ET process indicates that the ET dynamics is highly correlated with the spatial distributions of the micellar headgroups around a donor Pe molecule and thus dependent on the donor-acceptor distance. The distance-dependent ET and BET rates have been calculated following the method of Weidemaier and Fayer (J. Chem. Phys. 1995, 102, 3820) to get the best fit parameters for the multiexponetial temporal profiles for the S1 state of Pe as well as Pe*+. Because the acceptor is a constituent of the neat micellar medium, their confinement on the surface of the microheterogeneous medium provides a very large concentration such that, even though the forward transfer rate is 0.06 ps(-1) at the distance of closest approach, the ET process is complete within a 200-ps time domain. If the concepts of distribution of ET distances are utilized, the possible role of material diffusion on the kinetics of forward ET is ruled out. This is an experimental study to show, for the first time, the ultrafast distance-dependent light-induced ET dynamics following both the excited state of the donor and the cation radical formed in an ET process using the transient absorption spectroscopic technique in a self-reactive restrictive environment.     

Ultrafast excited state dynamics of the perylene radical cation generated upon bimolecular photoinduced electron transfer reaction

The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe(*+), generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump-pump-probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe(*+) was found to depend on the time delay between the first and second pump pulses, Deltat(12), i.e., on the "age" of the ion. At short Deltat(12), the GSR dynamics is biphasic, and at Deltat(12) larger than about 500 ps, it becomes exponential with a time constant around 3 ps. With trans-1,2-dicyanoethylene as acceptor, the free ion yield is essentially zero and the GSR dynamics of Pe(*+) remains biphasic independently of Deltat(12). The change of dynamics observed with 1,4-dicyanobenzene is ascribed to the transition from paired to free solvated ion, because in the pair, the excited ion has an additional decay channel to the ground state, i.e., charge recombination followed by charge separation. The rate constants deduced from the analysis of these GSR dynamics are all fully consistent with this hypothesis.     

Kinetics and mechanism of bimolecular electron transfer reaction in quinone-amine systems in micellar solution

Photoinduced electron transfer (ET) reactions between anthraquinone derivatives and aromatic amines have been investigated in sodium dodecyl sulphate (SDS) micellar solutions. Significant static quenching of the quinone fluorescence due to high amine concentration in the micellar phase has been observed in steady-state measurements. The bimolecular rate constants for the dynamic quenching in the present systems k(q) (TR), as estimated from the time-resolved measurements, have been correlated with the free energy changes DeltaG(0) for the ET reactions. Interestingly it is seen that the k(q) (TR) vs DeltaG(0) plot displays an inversion behavior with maximum k(q) (TR) at around 0.7 eV, a trend similar to that predicted in Marcus ET theory. Like the present results, Marcus inversion in the k(q) (TR) values was also observed earlier in coumarin-amine systems in SDS and TX-100 micellar solutions, with maximum k(q) (TR) at around the same exergonicity. These results thus suggest that Marcus inversion in bimolecular ET reaction is a general phenomenon in micellar media. Present observations have been rationalized on the basis of the two-dimensional ET (2DET) theory, which seems to be more suitable for micellar ET reactions than the conventional ET theory. For the quinone-amine systems, it is interestingly seen that k(q) (TR) vs DeltaG(0) plot is somewhat wider in comparison to that of the coumarin-amine systems, even though the maxima in the k(q) (TR) vs DeltaG(0) plots appear at almost similar exergonicity for both the acceptor-donor systems. These observations have been rationalized on the basis of the differences in the reaction windows along the solvation axis, as envisaged within the framework of the 2DET theory, and arise due to the differences in the locations of the quinones and coumarin dyes in the micellar phase.     

Femtodynamics reflected in nanoseconds: bimolecular free electron transfer in nonpolar media

The (free) electron transfer (FET) from electron donor molecules to parent solvent radical cations of alkanes and alkyl chlorides exhibits mechanistic peculiarities that are conditioned by the low polarity of these solvents. Because of the negligible solvation of ions in such systems and the almost complete lack of an activation barrier, the electron jump takes place at the very first encounter of the reactants and, as such, in extremely short times of 相似文献   

Effect of micellar environment on Marcus correlation curves for photoinduced bimolecular electron transfer reactions

Photoinduced electron transfer (ET) between coumarin dyes and aromatic amine has been investigated in two cationic micelles, namely, cetyltrimethyl ammonium bromide (CTAB) and dodecyltrimethyl ammonium bromide (DTAB), and the results have been compared with those observed earlier in sodium dodecyl sulphate (SDS) and triton-X-100 (TX-100) micelles for similar donor-acceptor pairs. Due to a reasonably high effective concentration of the amines in the micellar Stern layer, the steady-state fluorescence results show significant static quenching. In the time-resolved (TR) measurements with subnanosecond time resolution, contribution from static quenching is avoided. Correlations of the dynamic quenching constants (k(q) (TR)), as estimated from the TR measurements, show the typical bell-shaped curves with the free-energy changes (DeltaG(0)) of the ET reactions, as predicted by the Marcus outersphere ET theory. Comparing present results with those obtained earlier for similar coumarin-amine systems in SDS and TX-100 micelles, it is seen that the inversion in the present micelles occurs at an exergonicity (-DeltaG(0)> approximately 1.2-1.3 eV) much higher than that observed in SDS and TX-100 micelles (-DeltaG(0)> approximately 0.7 eV), which has been rationalized based on the relative propensities of the ET and solvation rates in different micelles. In CTAB and DTAB micelles, the k(q) (TR) values are lower than the solvation rates, which result in the full contribution of the solvent reorganization energy (lambda(s)) towards the activation barrier for the ET reaction. Contrary to this, in SDS and TX-100 micelles, k(q) (TR) values are either higher or comparable with the solvation rates, causing only a partial contribution of lambda(s) in these cases. Thus, Marcus inversion in present cationic micelles is inferred to be the true inversion, whereas that in the anionic SDS and neutral TX-100 micelles are understood to be the apparent inversion, as envisaged from two-dimensional ET theory.     

Ultrafast energy delocalization and electron transfer dynamics in 2-aminopurine-containing trinucleotides

The fate of electronically excited states in DNA base stacks is of tremendous importance for subsequent photochemical damage reactions in the genome. In this study we present a femtosecond broadband pump-probe study on the adenine isomer 2-aminopurine (Ap) incorporated into trinucleotides. After selective excitation of Ap we can monitor energy delocalization between neighboring Ap moieties as well as excited state electron transfer, depending on the sequence of the trinucleotide. Our results establish the time scale for intrastand excimer formation and reveal the lifetime of the excimer state.     

Ultrafast electron dynamics at ice-metal interfaces: competition between heterogeneous electron transfer and solvation

Microscopic insight into heterogeneous electron transfer requires an understanding of the participating donor and acceptor states and of their respective interaction. In the regime of strong electronic coupling, two limits have been discussed where either the states overlap directly or the states are separated by a potential barrier. In both situations, the transfer probability is determined by the magnitude of the wave function overlap, whereby in the case of the potential barrier, its width and height are rate limiting. In our study, we observe a dynamical crossover between these two regimes by investigating the electron-transfer dynamics of localized, solvated electrons at ice-metal interfaces. Employing femtosecond time-resolved two-photon photoelectron spectroscopy, we analyze the population dynamics of excess electrons in the ice layer, which experience the competing processes of transfer to the metal electrode and energetic stabilization in the ice by molecular reorientation. Comparing the dynamics of D(2)O on Cu(111) and Ru(001), we observe an early regime at t < 300 fs, where the transfer time is determined by wave-function overlap with the metal and a second regime (t > 300 fs), where the transfer proceeds nearly independent of the substrate. The assignment of these two regimes to the established mechanisms of electron transfer is backed by an empirical model calculation that reproduces the experimental data in an excellent manner.     

Photoinduced bimolecular electron transfer kinetics in small unilamellar vesicles

Photoinduced electron transfer (ET) from N,N-dimethylaniline to some coumarin derivatives has been studied in small unilamellar vesicles (SUVs) of the phospholipid, DL-alpha-dimyristoyl-phosphatidylcholine, using steady-state and time-resolved fluorescence quenching, both below and above the phase transition temperature of the vesicles. The primary interest was to examine whether Marcus inversion [H. Sumi and R. A. Marcus, J. Chem. Phys. 84, 4894 (1986)] could be observed for the present ET systems in these organized assemblies. The influence of the topology of SUVs on the photophysical properties of the reactants and consequently on their ET kinetics has also been investigated. Absorption and fluorescence spectral data of the coumarins in SUVs and the variation of their fluorescence decays with temperature indicate that the dyes are localized in the bilayer of the SUVs. Time-resolved area normalized emission spectra analysis, however, reveals that the dyes are distributed in two different microenvironments in the SUVs, which we attribute to the two leaflets of the bilayer, one toward bulk water and the other toward the inner water pool. The microenvironments in the two leaflets are, however, not indicated to be that significantly different. Time-resolved anisotropy decays were biexponential for all the dyes in SUVs, and this has been interpreted in terms of the compound motion model according to which the dye molecules can experience a fast wobbling-in-cone type of motion as well as a slow overall rotating motion of the cone containing the molecule. The expected bimolecular diffusion-controlled rates in SUVs, as estimated by comparing the microviscosities in SUVs (determined from rotational correlation times) and that in acetonitrile solution, are much slower than the observed fluorescence quenching rates, suggesting that reactant diffusion (translational) does not play any role in the quenching kinetics in the present systems. Accordingly, clear inversions are observed in the correlation of the fluorescence quenching rate constants k(q) with the free energy change, DeltaG(0) of the reactions. However, the coumarin dyes, C152 and C481 (cf. Scheme 1), show unusually high k(q) values and high activation barriers, which is not expected from Marcus ET theory. This unusual behavior is explained on the basis of participation of the twisted intramolecular charge transfer states of these two dyes in the ET kinetics.     

Ultrafast energy transfer and structural dynamics in DNA

Ultrafast structural dynamics concomitant to excitation energy transfer in DNA has been studied using a pair of pyrene-labeled DNA bases. The temporal evolution of the femtosecond pump-probe spectra reveals the existence of two electronic coupling pathways, through-base stack and through-space, which lead to excitation energy transfer and excimer formation even when the labeled DNA bases are separated by one AT base pair. The electronic coupling which mediates through-base stack energy transfer is so strong that a new absorption band arises in the excited-state absorption spectrum within 300 fs. From the analysis of time-dependent spectral shifts due to through-space excimer formation, the local structural dynamics and flexibility of DNA are characterized on the picosecond and nanosecond time scale.     

Ultrafast dynamics of resonance energy transfer in cryptochrome

In this communication, we report the ultrafast dynamics of resonance energy transfer in a blue-light photoreceptor, Vibrio cholerae cryptochrome. The transfer was observed to occur in 60 ps. We also studied the local rigidity and solvation around the binding site of the photoantenna molecule. The results for the first time show energy transfer in cryptochrome suggesting some mechanistic similarities between photolyase that repairs damaged DNA and cryptochrome that mediates blue-light signaling.     

Ultrafast proton transfer dynamics of hydroxystilbene photoacids

The effects of 4-cyano and 3-cyano substituents on the spectroscopic properties and photoacidity of 3- and 4-hydroxystilbene have been investigated. In nonpolar solvents, the 3-hydroxycyanostilbenes have much longer singlet lifetimes and larger fluorescence quantum yields than do the 4-hydroxycyanostilbenes. The longer lifetimes of 3-hydroxystilbene and its cyano derivatives are attributed to a "meta effect" on the stilbene torsional barrier, similar to that previously observed for the aminostilbenes. The cyano substituent causes a marked increase in both ground state and excited-state acidity of the hydroxystilbenes in aqueous solution. The dynamics of excited-state proton transfer in methanol-water solution have been investigated by means of femtosecond time-resolved transient absorption spectroscopy. Assignment of the transient absorption spectra is facilitated by comparison to the spectra of the corresponding potassium salts of the conjugate bases and the methyl ethers, which do not undergo excited-state proton transfer. The 4-cyanohydroxystilbenes undergo excited-state proton transfer with rate constants of 5 x 10(11) s(-1). These rate constants are comparable to the fastest that have been reported to date for a hydroxyaromatic photoacid and approach the theoretical limit for water-mediated proton transfer. The isotope effect for proton transfer in deuterated methanol-water is 1.3 +/- 0.2, similar to the isotope effect for the dielectric response of water. The barrier for excited state double bond torsion of the conjugate bases is small for 4-cyano-4-hydroxystilbene but large for 4-cyano-3-hydroxystilbene. Thus the "meta effect" is observed for the singlet states of both the neutral and conjugate base.     

Photoinduced electron transfer between quinones and amines in micellar media: Tuning the Marcus inversion region

Electron transfer (ET) rates between quinone acceptors and amine donors in micellar media show Marcus inversion behavior on correlating with the free energy changes of the ET reactions. The onset of Marcus inversion in these systems is seen to be tuned by about 0.25 eV by changing the type of the micelle. The results are rationalized on the basis of two-dimensional ET theory where ET occurs along intramolecular coordinate with non-equilibrium configuration along solvation coordinate. Maximum ET rates are seen to vary by about one order of magnitude in different micelles, and are attributed to the micelle-dependent changes in the separations of the interacting quinone–amine pairs. Tunings of Marcus inversion and ET rates by changing micellar microenvironments have been observed and suggested to have useful implications in different applied areas.     

Ultrafast photoinduced electron transfer in directly linked porphyrin-ferrocene dyads

The ultrafast electron transfer occurring upon Soret excitation of three new porphyrin-ferrocene (XP-Fc) dyads has been studied by femtosecond up-conversion and pump-probe techniques. In the XP-Fc dyads (XP-Fcs) designed in this study, the ferrocene moiety is covalently bonded to the meso positions of 3,5-di-tert-butylphenyl zinc porphyrin (BPZnP-Fc), pentafluorophenyl zinc porphyrin (FPZnP-Fc), and 3,5-di-tert-butylphenyl free-base porphyrin (BPH2P-Fc). Charge separation and recombination in the XP-Fcs were confirmed by transient absorption spectra, and the lifetimes of the charge-separated states were estimated from the decay rate of the porphyrin radical anion band to be approximately 20 ps. The charge-separation rates of the XP-Fcs were found to be >10(13) s-1 from the S2 state and 6.3x10(12) s-1 from the S1 state. Charge separation from the S2 state was particularly efficient for BPZnP-Fc, whereas the main reaction pathway was from the S1 state for BPH2P-Fc. Charge separation from the S2 and S1 states occurred at virtually the same rate in benzene and tetrahydrofuran and was much faster than their solvation times. Analysis of these results using semiquantum Marcus theory indicates that the magnitude of the electronic-tunneling matrix element is rather large and far outside the range of nonadiabatic approximation. The pump-probe data show the presence of vibrational coherence during the reactions, suggesting that wavepacket dynamics on the adiabatic potential energy surface might regulate the ultrafast reactions.     

Ultrafast dynamics for electron photodetachment from aqueous hydroxide

Charge-transfer-to-solvent reactions of hydroxide induced by 200 nm monophotonic or 337 and 389 nm biphotonic excitation of this anion in aqueous solution have been studied by means of pump-probe ultrafast laser spectroscopy. Transient absorption kinetics of the hydrated electron, e(aq) (-), have been observed, from a few hundred femtoseconds out to 600 ps, and studied as function of hydroxide concentration and temperature. The geminate decay kinetics are bimodal, with a fast exponential component ( approximately 13 ps) and a slower power "tail" due to the diffusional escape of the electrons. For the biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8 times higher than for the monophotonic 200 nm excitation (31% versus 17.5% at 25 degrees C, respectively), due to the broadening of the electron distribution. The biphotonic electron detachment is very inefficient; the corresponding absorption coefficient at 400 nm is <4 cm TW(-1) M(-1) (assuming unity quantum efficiency for the photodetachment). For [OH(-)] between 10 mM and 10 M, almost no concentration dependence of the time profiles of solvated electron kinetics was observed. At higher temperature, the escape fraction of the electrons increases with a slope of 3x10(-3) K(-1) and the recombination and diffusion-controlled dissociation of the close pairs become faster. Activation energies of 8.3 and 22.3 kJ/mol for these two processes were obtained. The semianalytical theory of Shushin for diffusion controlled reactions in the central force field was used to model the geminate dynamics. The implications of these results for photoionization of water are discussed.     

Resonance energy transfer between lasing dyes in micellar media

The photophysical behaviour of rhodamine 6G and coumarin 1 has been studied in a range of aqueous micellar media. Resonance energy transfer is demonstrated in dilute (10^?5 M) solutions and the location of the dye molecules in the micelle deduced from fluorescence lifetimes. A highly efficient, energy transfer dye laser is reported for dyes in micellar matrices.     

Photoinduced bimolecular electron transfer investigated by femtosecond time-resolved infrared spectroscopy

Ultrafast infrared transient absorption spectroscopy is used to study the photoinduced bimolecular electron transfer reaction between perylene in the first singlet excited state and 1,4-dicyanobenzene in acetonitrile and dichloromethane. Following vibrational marker modes on both donor and acceptor sides in real time provides direct insight into the structural dynamics during the reaction. A band narrowing on a time scale of a few tens of picoseconds observed on the antisymmetric CN stretching vibration of the dicyanobenzene radical anion indicates that a substantial part of the excess energy is channeled into vibrational modes of the product, despite the fact that the reaction is weakly exergonic. An additional narrowing of the same band on a time scale of several hundreds of picoseconds observed in acetonitrile only is interpreted as a signature of the dissociation of the geminate ion pairs into free ions.     

Ultrafast charge-transfer-to-solvent dynamics of iodide in tetrahydrofuran. 2. Photoinduced electron transfer to counterions in solution

The excited states of atomic anions in liquids are bound only by the polarization of the surrounding solvent. Thus, the electron-detachment process following excitation to one of these solvent-bound states, known as charge-transfer-to-solvent (CTTS) states, provides a useful probe of solvent structure and dynamics. These transitions and subsequent relaxation dynamics also are influenced by other factors that alter the solution environment local to the CTTS anion, including the presence of cosolutes, cosolvents, and other ions. In this paper, we examine the ultrafast CTTS dynamics of iodide in liquid tetrahydrofuran (THF) with a particular focus on how the solvent dynamics and the CTTS electron-ejection process are altered in the presence of various counterions. In weakly polar solvents such as THF, iodide salts can be strongly ion-paired in solution; the steady-state UV-visible absorption spectroscopy of various iodide salts in liquid THF indicates that the degree of ion-pairing changes from strong to weak to none as the counterion is switched from Na+ to tetrabutylammonium (t-BA+) to crown-ether-complexed Na+, respectively. In our ultrafast experiments, we have excited the I- CTTS transition of these various iodide salts at 263 nm and probed the dynamics of the CTTS-detached electrons throughout the visible and near-IR. In the previous paper of this series (Bragg, A. E.; Schwartz, B. J. J. Phys. Chem. B 2008, 112, 483-494), we found that for "counterion-free" I- (obtained by complexing Na+ with a crown ether) the CTTS electrons were ejected approximately 6 nm from their partner iodine atoms, the result of significant nonadiabatic coupling between the CTTS excited state and extended electronic states supported by the naturally existing solvent cavities in liquid THF, which also serve as pre-existing electron traps. In contrast, for the highly ion-paired NaI/THF system, we find that approximately 90% of the CTTS electrons are "captured" by a nearby Na+ to form (Na+, e-)THF "tight-contact pairs" (TCPs), which are chemically and spectroscopically distinct from both solvated neutral sodium atoms and free solvated electrons. A simple kinetic model is able to reproduce the details of the electron capture process, with 63% of the electrons captured quickly in approximately 2.3 ps, 26% captured diffusively in approximately 63 ps, and the remaining 11% escaping out into the solution on subnanosecond time scales. We also find that the majority of the CTTS electrons are ejected to within 1 or 2 nm of the Na+. This demonstrates that the presence of the nearby cation biases the relocalization of CTTS-generated electrons from I- in THF, changing the nonadiabatic coupling to the extended, cavity-supported electronic states in THF to produce a much tighter distribution of electron-ejection distances. In the case of the more loosely ion-paired t-BA+-I-/THF system, we find that only 10-15% of the CTTS-ejected electrons associate with t-BA+ to form "loose-contact pairs" (LCPs), which are characterized by a much weaker interaction between the electron and cation than occurs in TCPs. The formation of (t-BA+, e-)THF LCPs is characterized by a Coulombically induced blue shift of the free eTHF- spectrum on a approximately 5-ps time scale. We argue that the weaker interaction between t-BA+ and the parent I- results in little change to the CTTS-ejection process, so that only those electrons that happen to localize in the vicinity of t-BA+ are captured to form LCPs. Finally, we interpret the correlation between electron capture yield and counterion-induced perturbation of the I- CTTS transition as arising from changes in the distribution of ion-pair separations with cation identity, and we discuss our results in the context of relevant solution conductivity measurements.     

Ultrafast back electron transfer processes in the photoexcited methylviologen-iodide charge transfer complexes

The ultrafast back electron transfer in the excited charge transfer complexes of the methylviologen with iodide ions has been investigated using femtosecond transient absorption spectroscopy. Methylviologen and iodide form two types of charge transfer complexes each characterized by a charge transfer band in the same spectral region. At low I^- concentrations mainly a 1:1 complex MV²⁺(I^-) is present while at high I^- concentrations both 1:1 and 1:2 complexes MV²⁺(I^-)₂ can be observed. Ultrashort laser pulses at 400 nm are used to excite both complexes in their charge transfer band. The observed transient absorption can be represented by a biexponential function with 1 ps and 20 ps time constants and attributed to the decay of the MV^+./I^. and MV^+./I₂ ^.- radical pair respectively. The excitation of the 1:1 complex leads to the formation of the MV^+./I^. radical pair while the excitation of the 1:2 complex leads to the formation of the MV^+./I^. and MV^+./I₂ ^.- radical pairs.