Outer‐sphere electron transfer in polar solvents: Quantum scaling of strongly interacting systems

Quantum effects in ultrafast electron transfers within cryptochromes

Cryptochromes and photolyases are flavoproteins that may undergo ultrafast charge separation upon electronic excitation of their flavin cofactors.

Reorganization energy of electron transfer processes in ionic fluids: a molecular Debye-Hückel approach

The reorganization energy of electron transfer processes in ionic fluids is studied under the linear response approximation using a molecule Debye-Hückel theory. Reorganization energies of some model reactants of electron transfer reactions in molten salts are obtained from molecular simulations and a molecule Debye-Hückel approach. Good agreements between simulation results and the results from our theoretical calculations using the same model Hamiltonian are found. Applications of our theory to electron transfer reactions in room temperature ionic liquids further demonstrate that our theoretical approach presents a reliable and accurate methodology for the estimation of reorganization energies of electron transfer reactions in ionic fluids.

EFFECT OF ENVIRONMENT ON SUPEREXCHANGE ELECTRON TRANSFER IN A MULTIPLE BRIDGED DONOR–ACCEPTOR SYSTEM

Electron transfer dynamics in donor–bridge–acceptor (DBA) systems under dissipative environment is investigated by using the quasi-adiabatic propagator path integral approach. The dephasing and relaxation rates are extracted from the Fourier transform of the time-dependent population difference of the donor and the acceptor states. Comparing the rates obtained from the DBA and the effective two state systems, respectively, we reveal the validity of the superexchange mechanism. It is found that the coupling of the environment to the bridges does not affect the superexchange rates for the bridges having high enough electronic energies, and the superexchange mechanism still works quite well even though the population on the bridges is over than 0.2 in a tested system.

DENSITY MATRIX ANALYSIS AND SIMULATION OF ELECTRON DYNAMICS IN MULTIPLE-BRIDGED DONOR/ACCEPTOR MOLECULES UNDER DISSIPATIVE ENVIRONMENTS

Feynman and Vernon path integral approach is adopted to investigate electron transfer dynamics in donor–bridge–acceptor molecules under dissipative environments. Especially, we focus on the solvent effect on the superexchange process of electron transfer. The results reveal that at high enough bridge energies or low enough temperature, electron can transfer with a superexchange mechanism no matter whether solvent is incorporated or not. However, the superexchange changes from coherent to incoherent limits when the dissipative strength increases, and electron transfer rates are much dependent on the dissipative strength.

Quantum effect of intramolecular high-frequency vibrational modes on diffusion-controlled electron transfer rate: From the weak to the strong electronic coupling regions

The Sumi-Marcus theory is extended by introducing two approaches to investigate electron transfer reactions from weak-to-strong electronic coupling regime. One of these approaches is the quantum R-matrix theory, useful for dealing with the intramolecular vibrational motions in the whole electronic coupling domain. The other is the split operator approach that is employed to solve the reaction-diffusion equation. The approaches are then applied to electron transfer in the Marcus inverted regime to investigate the nuclear tunneling effect on the long time rate and the survival probabilities. The numerical results illustrate that the adiabatic suppression obtained from the R-matrix approach is much smaller than that from the Landau-Zener theory whereas it cannot be predicted by the perturbation theory. The jointed effects of the electronic coupling and solvent relaxation time on the rates are also explored.

Photophysics of ANS. IV. Electron transfer quenching of ANS in alcoholic solvents and mixtures

ANS is observed to decay from the fluorescent state with distributed kinetics in nearly pure ethanol solvent, notwithstanding that in mixed ethanol/water solvents the decay is discrete and biexponential. The origin of this behavior is investigated. In particular, a theory of electron transfer theory in the adiabatic regime is adduced, with specific involvement of solvent cage structure in the form of the solvent-electron polaron wave function. Properties of various polarons for various solvent systems are predicted and, for the case of ethanol and cyclohexanol, employed to generate the observed Arrhenius-type decay parameters in a quantitative fashion.

Semiclassical Treatment of Thermally Activated Electron Transfer in the Intermediate to Strong Electronic Coupling Regime under the Fast Dielectric Relaxation

The generalized nonadiabatic transition-state theory (NA-TST) (Zhao, Y.; et al. J. Chem. Phys. 2004, 121, 8854) is used to study electron transfer with use of the Zhu-Nakamura (ZN) formulas of nonadiabatic transition in the case of fast dielectric relaxation. The rate constant is expressed as a product of the well-known Marcus formula and a coefficient which represents the correction due to the strong electronic coupling. In the case of general multidimensional systems, the Monte Carlo approach is utilized to evaluate the rate by taking into account the multidimensionality of the crossing seam surface. Numerical demonstration is made by using a model system of a collection of harmonic oscillators in the Marcus normal region. The results are naturally coincident with the perturbation theory in the weak electronic coupling limit; while in the intermediate to strong electronic coupling regime where the perturbation theory breaks down the present results are in good agreement with those from the quantum mechanical flux-flux correlation function within the model of effective one-dimensional mode.

Path-integral Monte Carlo simulations for electronic dynamics on molecular chains. II. Transport across impurities

Electron transfer (ET) across molecular chains including an impurity is studied based on a recently improved real-time path-integral Monte Carlo (PIMC) approach [L. Mühlbacher, J. Ankerhold, and C. Escher, J. Chem. Phys. 121 12696 (2004)]. The reduced electronic dynamics is studied for various bridge lengths and defect site energies. By determining intersite hopping rates from PIMC simulations up to moderate times, the relaxation process in the extreme long-time limit is captured within a sequential transfer model. The total transfer rate is extracted and shown to be enhanced for certain defect site energies. Super-exchange turns out to be relevant for extreme gap energies only and then gives rise to different dynamical signatures for high- and low-lying defects. Further, it is revealed that the entire bridge compound approaches a steady state on a much shorter time scale than that related to the total transfer. This allows for a simplified description of ET along donor-bridge-acceptor systems in the long-time range.

Path-integral Monte Carlo simulations for electronic dynamics on molecular chains. I. Sequential hopping and super exchange

An improved real-time quantum Monte Carlo procedure is presented and applied to describe the electronic transfer dynamics along molecular chains. The model consists of discrete electronic sites coupled to a thermal environment which is integrated out exactly within the path integral formulation. The approach is numerically exact and its results reduce to known analytical findings (Marcus theory, golden rule) in proper limits. Special attention is paid to the role of superexchange and sequential hopping at lower temperatures in symmetric donor-bridge-acceptor systems. In contrast to previous approximate studies, superexchange turns out to play a significant role only for extremely high-lying bridges where the transfer is basically frozen or for extremely low temperatures where for weaker dissipation a description in terms of rate constants is no longer feasible. For bridges with increasing length an algebraic decrease of the yield is found for short as well as for long bridges. The approach can be extended to electronic systems with more complicated topologies including impurities and in presence of external time-dependent forces.