Modeling Kinetics of Ultrafast Photoinduced Intramolecular Proton-Coupled Electron Transfer

Abstract

A model of photoinduced intramolecular proton-coupled electron transfer is derived. The model includes three states as follows: the ground, excited, and product states. The charge transfer is associated with both stages, photoexcitation and product formation. A larger part of the model parameters can be extracted from the stationary absorption and fluorescence spectra of a particular fluorophore. Two different reaction coordinates are associated with the two stages, which are not independent. The angle between the reaction coordinates strongly influences on the kinetics of ultrafast product formation. The stochastic multichannel approach is exploited for simulations of the kinetics. The simulations well reproduce the kinetics of ultrafast intramolecular protoncoupled electron transfer in 2-((2-(2-hydroxyphenyl)benzo[d]oxazol-6-yl)methylene)malononitrile in a few solvents. The transfer is shown to occur totally or partly, depending on the solvent, in the nonequilibrium regime. Analysis of the kinetics of the excited-state decay has uncovered a significant decrease in the magnitude of the reorganization energies of slow nuclear modes with increasing the solvent polarity. Such an unusual behavior of the total reorganization energy can be rationalized under the assumptions: (i) a slow intramolecular reorganization of a significant magnitude associates with the transition between excited and product states and (ii) intramolecular slow reorganization is accompanied by a change in the dipole moment of the fluorophore.

Nonstationary Theory of Excited State Charge Transfer Symmetry Breaking Driven by Polar Solvent

“…For the numerical solution of eq , the Brownian simulation method is used, which operates in terms of stochastic trajectories and was successfully used for simulation of electron transfer kinetics. ,− A set of the stochastic trajectories describe the motion of an ensemble of particles on the free energy surface G ( D m ). The free energy G ( D m ) is numerically found by minimization of the free energy functional G int [Ψ, P⃗ ( r⃗ )] with respect to the dissymmetry parameter D using the golden-section search, so far as the domain of the variable D is restricted and belongs the range −1 ≤ D ≤ 1 as well as the G int [Ψ, P⃗ ( r⃗ )] has only one minimum in this domain for a given value of D m .…”