Photoinduced Forward Electron Transfer Research Articles

Nonequilibrium dynamics of photoinduced forward and backward electron transfer reactions.

The biological functions of photoenzymes are often triggered by photoinduced electron transfer (ET) reactions. An ultrafast backward ET (BET) reaction follows the initial photoinduced forward ET (FET), which dissipates the energy of absorbed photons and terminates the biological function in vain. Based upon our previous works, we reasoned that the dynamics of the BET is coupled with that of the FET and other local motions. In this work, the dynamics of the FET and BET is modeled as the master equation of the reduced density operator of a three-state system coupled with a classical harmonic reservoir. The coupling of the FET and BET is reflected in the time-evolution of the charge-transfer state's population, which is generated by a source, the reaction flux for the FET, and annihilated by a sink, the reaction flux for the BET. Surprisingly, numerical simulations show that when the BET is in the Marcus normal region, the BET can be accelerated by nonequilibrium local motions and becomes faster than what is predicted by the Marcus theory. The experimental confirmation of this novel dynamics would provide qualitative evidence for nonequilibrium effects on ultrafast ET dynamics. Additionally, the effects of quantum vibrational modes on the dynamics are discussed. This work can help understand the dynamical interactions between the chain of ultrafast reactions and the complex local environmental motions, revealing the physical nature underlying biological functions.

Free Energy Dependence of Photoinduced Electron Transfer in Octathiophene-Diimide Dyads

Donor-acceptor dyads consisting of octathiophene (T8) paired with three (di)imide acceptors (naphthalene diimide (NDI), benzene diimide (BDI), and naphthalimide (NI)) were synthesized and probed for their photoinduced forward electron transfer (ET) and charge recombination kinetics by using ultrafast transient absorption (TA) spectroscopy. The three acceptors have different electron affinities, leading to variation in the energy of the charge-separated state and the driving force (ΔG) for forward ET and charge recombination. Analysis of the TA spectra and kinetics allows assignment of rates for forward ET and charge recombination for each of the oligomers. Electrochemistry and photoluminescence spectroscopy are used to determine the ΔG values for the ET processes. For two of the oligomers (T8NDI and T8BDI), the rates for forward ET and charge recombination are very rapid (k > 3 × 1010 s-1). By contrast, for the third oligomer (T8NI), the rates for both processes are considerably slower (k < 5 × 109 s-1). Analysis of the rate/free energy correlation for the series of oligomers reveals generally good agreement with the Marcus semiclassical theory. In all of the oligomers, the ET reactions are nonadiabatic, in part, due to weak coupling caused by out-of-plane twisting of the phenylene spacer that lies between the T8 segment and the (di)imide acceptors. The rapid ET dynamics for T8NDI and T8BDI are explained as arising due to the processes occurring near the barrierless region (-ΔG ≈ λ) or slightly into the Marcus inverted region (-ΔG > λ). The slower dynamics for T8NI are explained as arising because the forward ET is weakly exothermic, whereas charge recombination is deep into the inverted region. This study is the first to produce experimental results that match a full Marcus bell-shaped curve with ET rates in the normal, barrrierless, and inverted regions in dyads based on a π-conjugated oligomer donor.

Photoinduced electron transfer of Rhodamine 6G/N,N-diethylaniline revealed by multiplex transient grating and transient absorption spectroscopies

The dynamics and spectroscopic characteristics of the ultrafast photoinduced electron transfer (ET) of Rhodamine 6G (Rh6G+) in N,N-diethylaniline (DEA) were studied using femtosecond time-resolved multiplex transient grating and transient absorption spectroscopies. The ultrafast photoinduced forward ET from DEA to the Rh6G+* cation radical excited state has a time constant of τFET = 219–318 fs. The much slower backward ET from the neutral radical Rh6G· to DEA+ with a time constant of τBET = 22.76–42.31 ps occurs in the inverted region. Intramolecular vibrational relaxation of the excited state takes place in τIVR = 2.18–6.91 ps.

Photoinduced electron transfer and geminate recombination in the group head region of micelles

A pump-probe spectroscopic study of photoinduced forward electron transfer and geminate recombination between donors and acceptors located in the head group regions of micelles is presented. The hole donor is octadecyl-rhodamine B (ODRB) and the hole acceptor is N,N-dimethyl-aniline (DMA). The experiments are conducted as a function of the DMA concentration in the dodecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide micelles. In spite of the fact that the absorptions of both the ODRB radical and ground state bleach spectrally overlap with the ODRB excited state absorption, a procedure that makes it possible to determine the geminate recombination dynamics is presented. These experiments are the first to measure the dynamics of geminate recombination in micelles, and the experiments have two orders of magnitude better time resolution than previous studies of forward transfer. The experimental data are compared to statistical mechanics theoretical calculations of both the forward transfer and the geminate recombination. The theory includes important aspects of the topology of the micelle and the diffusion of the donor-acceptors in the micelle head group region. A semiquantitative but nonquantitative agreement between theory and experiments is achieved.

Photoinduced electron transfer and geminate recombination in liquids on short time scales: Experiments and theory

The coupled processes of intermolecular photoinduced forward electron transfer and geminate recombination between the (hole) donor (Rhodamine 3B) and (hole) acceptors (N,N-dimethylaniline) are studied in three molecular liquids: acetonitrile, butyronitrile, and benzonitrile. Two color pump-probe experiments on time scales from approximately 100 fs to hundreds of picoseconds give information about the depletion of the donor excited state due to forward electron transfer and the survival kinetics of the radicals produced by forward electron transfer. The data are analyzed with a model presented previously that includes distance dependent forward and back electron transfer rates, donor and acceptor diffusion, solvent structure, and the hydrodynamic effect in a mean-field theory of through solvent electron transfer. The forward electron transfer is in the normal regime, and the Marcus equation for the distance dependence of the transfer rate is used. The forward electron transfer data for several concentrations in the three solvents are fitted to the theory with a single adjustable parameter, the electronic coupling matrix element Jf at contact. Within experimental error all concentrations in all three solvents are fitted with the same value of Jf. The geminate recombination (back transfer) is in the inverted region, and semiclassical treatment developed by Jortner [J. Chem. Phys. 64, 4860 (1976)] is used to describe the distance dependence of the back electron transfer. The data are fitted with the single adjustable parameter Jb. It is found that the value of Jb decreases as the solvent viscosity increases. Possible explanations are discussed.

Photoinduced electron transfer (ET) within some novel synthesized derivatives of phenanthrene acting as donors and 9-fluorenone/9-cyanoanthracene behaving as acceptors

With the help of both steady state and time-resolved measurements, possibilities of the occurrences of photoinduced electron transfer (ET) reactions within some novel synthesized derivatives of phenanthrene (1,1-dimethyl-9-methoxy-1,2,3,4-tetrahydrophenanthrene, 9MTHP; 1,1-dimethyl-6,9-dimethoxy-7-isopropyl-1,2,3,4-tetrahydrophenanthrene, 69DMTHP and 1,1-dimethyl-6,7,9-trimethoxy-1,2,3,4-tetrahydrophenanthrene, 679TMTHP) donors and 9-fluorenone/9-cyanoanthracene acceptors in acetonitrile fluid solution were studied at the ambient temperature (296 K). No ground state charge transfer complex was apparent from the observed electronic absorption spectra of the present reacting (D–A) systems. On singlet (S 1) excitation of acceptor moiety, low to moderate exothermic photoinduced ET reactions seem to be primarily responsible for the observed fluorescence quenching phenomena. Occurrence of such process within the excited acceptor and ground state donor was also confirmed from the measurement of transient absorption spectra at different delay times between the exciting and analyzing pulses. An attempt was made to test the Marcus theory both in the normal and inverted region for the photoinduced forward ET reactions within the donor and acceptor systems used in the present investigation.

Investigation of the Origin of the Sequence Selectivity for the 5-Halo-2‘-deoxyuridine Sensitization of DNA to Damage by UV-Irradiation

A contrathermodynamic sequence selectivity (5‘-deoxyadenosine > 5‘-deoxyguanosine) for UV-irradiation-induced strand damage in duplex DNA containing 5-bromo-2‘-deoxyuridine was reported several years ago (Saito, I.; Sugiyama, H. J. Am. Chem. Soc. 1990, 112, 6720.). In contrast, much smaller sequence selectivity was observed for similar duplexes containing 5-iodo-2‘-deoxyuridine. We investigated the mechanism of UV-irradiation-induced cleavage of duplex DNA containing 5-bromo-2‘-deoxyuridine (1, BrdU) and 5-iodo-2‘-deoxyuridine (2, IdU) under anaerobic conditions using a variety of structural probes. The preference for UV-induced cleavage in 5‘-dABrdU sequences is a confluence of at least three factors, photoinduced forward electron transfer, charge recombination, and electron migration within the DNA duplex. Our results also indicate that UV-irradiation of duplexes (32 nucleotides long) containing 5-iodo-2‘-deoxyuridine results in strand scission involving initial photoinduced single electron transfer. Th...

Magnetic field effect as a probe of distance-dependent electron transfer in systems undergoing free diffusion

A theoretical analysis is presented of the problem of how distance-dependent electron transfer in photoinduced forward electron transfer followed by geminate backward electron transfer in liquid solution is reflected in the viscosity dependence of the magnetic field effect (MFE) on the efficiency of free radical formation (φce) in such reactions. The stochastic Liouville equation formalism is employed to model the reaction behaviour of distance-distributed, triplet-born radical pairs (RPs) undergoing free diffusion, distance- and spin-dependent backward electron transfer, coherent and incoherent spin evolution in the ps time domain. In comparison with real systems the spin situation is simplified by reducing it to a two state (S, T 0) problem, yet it is parametrized in a way that allows sensible comparison of the results with those of recent experiments. It is predicted that the MFE on φce exhibits characteristic minima in the MFE versus viscosity curves, and it is verified in detail that this feature is peculiar to the diffusional model with distance-dependent electron transfer, i.e. cannot be reproduced with the simpler (‘exponential’) RP model employing distance-independent rate constants. Thus, the MFE versus viscosity curves are established as a genuine fingerprint of distance-dependent electron transfer. The theoretical results compare favourably with recent experimental results obtained with RuIII complex/methylviologen RPs.

Estimation of the Bimolecular Rate Constant for Exciplex Formation from the Analysis of Its Emission Spectrum

The bimolecular rate constant for the quenching of 1-methylnaphthalene by triethylamine was measured in series of solvents covering a large range of solvent polarity (e = 2−37). The formation of a fluorescent exciplex was observed in all the solvents studied. Analyzing the emission of the exciplex, it is possible to estimate the relevant parameters associated with the radiative back electron transfer process, namely, the solvent dependent Gibbs energy change, the solvent reorganization energy, and the internal reorganization energy. These parameters are used to calculate, according to Marcus theory, the rate constant for the photoinduced forward electron transfer reaction. A good correlation between calculated and experimental quenching rate constants is observed.

Photoinduced Electron Transfer and Geminate Recombination in Liquids

The coupled processes of intermolecular photoinduced forward electron transfer and geminate recombination between donors (rubrene) and acceptors (duroquinone) are studied in two molecular liquids: dibutyl phthalate and diethyl sebacate. Time-correlated single-photon counting and fluorescence yield measurements give information about the depletion of the donor excited state due to forward transfer, while pump−probe experiments give direct information about the radical survival kinetics. A straightforward procedure is presented for removing contributions from excited-state−excited-state absorption to the pump−probe data. The data are analyzed with a previously presented model that includes solvent structure and hydrodynamic effects in a detailed theory of through-solvent electron transfer. Models that neglect these effects are incapable of describing the data. When a detailed description of solvent effects is included in the theory, agreement with the experimental results is obtained. Forward electron transfer is well-described with a classical Marcus form of the rate equation, though the precise values of the rate parameters depend on the details of the solvents' radial distribution function. The additional experimental results presented here permit a more accurate determination of the forward transfer parameters than those presented previously.1 The geminate recombination (back transfer) data are highly inverted and cannot be analyzed with a classical Marcus expression. Good fits are instead obtained with an exponential distance dependence model of the rate constant and also with a more detailed semiclassical treatment suggested by Jortner.2 Analysis of the pump−probe data, however, suggests that the geminate recombination cannot be described with a single solvent dielectric constant. Rather, a time-dependent dielectric constant is required to properly account for diffusion occurring in a time-varying Coulomb potential. A model using a longitudinal dielectric relaxation time is presented. Additionally, previously reported theoretical results3 are rederived in a general form that permits important physical effects to be included more rigorously.

Role of Diffusion in Photoinduced Electron Transfer on a Micelle Surface: Theoretical and Monte Carlo Investigations

A detailed theoretical treatment of donor−acceptor photoinduced forward electron transfer and back transfer (geminate recombination) for molecules diffusing on a micelle surface is presented. Expressions are given for the time-dependent survival probabilities of both the excited-state donor and the charge-transfer state formed by forward electron transfer. Incorporation of diffusion has a pronounced effect on the kinetics of both the forward and the back transfer, and the amount of geminate recombination depends critically on the Coulombic potential between the ions. Ion spatial distributions as a function of time are presented and used to discuss the possibility of achieving long-term ion separation. The validity of the theory is demonstrated by comparison to Monte Carlo simulations of the problem, and the perfect agreement obtained confirms the accuracy of the theoretical derivation.