A Unified Semiclassical Framework for Ultrafast Competitive Electron Transfer in Multiredox Molecular Systems

Abstract: Ultrafast multistage electron transfer (ET) in molecular systems with multiple redox centers is fundamental to photochemical energy conversion, including processes in natural photosynthesis, molecular optoelectronics, and organic photovoltaics. These systems often operate under nonequilibrium conditions, where solvent relaxation, intramolecular vibrations, and competing ET pathways jointly determine reaction kinetics and product yields. In this chapter, we present a unified semiclassical framework for modeling ultrafast, competitive ET in multiredox compounds embedded in polar environments with complex relaxation dynamics. The approach constructs diabatic free energy surfaces (FESs) in a multidimensional coordinate space that integrates both polarization and relaxation components of the environment within a unified representation. Electron dynamics are described using a stochastic point-transition method that captures the coupling between nonadiabatic quantum transitions and classical nuclear motion. The formalism generalizes and unifies several established semiclassical models - including the Najbar-Tachiya, Zusman-Beratan, and Sumi-Marcus approaches - and supports efficient simulation of multistage ET cascades. As an application, we investigate ultrafast charge separation in donor-acceptor-acceptor (D-A1-A2) triads, showing how hot charge shift to a secondary acceptor can suppress nonequilibrium charge recombination. Numerical simulations reveal how reorganization energies, vibrational coupling, molecular geometry, and bending angle collectively influence ET efficiency. The proposed framework offers a general and scalable tool for the rational design of photofunctional molecular systems.