Instantaneous Marcus Theory for Photoinduced Charge Transfer in Condensed Phase

ABSTRACT Simulating photoinduced charge transfer (CT) in the condensed phase is essential for understanding solar energy conversion. Traditional Marcus theory is limited by its assumption of a thermally equilibrated initial state, which is often invalid for photoinduced processes, where vertical excitation creates a nonequilibrium nuclear state. The subsequent structural relaxation requires a time‐dependent rate coefficient. This review focuses on Instantaneous Marcus Theory (IMT), an approach recently developed to capture these nonequilibrium effects. Derived as the classical limit of the nonequilibrium Fermi's golden rule (NE‐FGR), IMT provides a practical, Marcus‐like expression for the time‐dependent rate based on the dynamical average and variance of the donor‐acceptor energy gap. While the direct evaluation of IMT requires computationally expensive nonequilibrium molecular dynamics, the nonlinear‐response (NLR) formulation reformulates the theory in terms of efficient equilibrium molecular dynamics simulations. This framework has been extended to multistate systems, allowing the simulation of complex reaction networks through a set of coupled Pauli's master equations. We highlight the application of these methods to the carotenoid‐porphyrin‐fullerene molecular triad, a prototypical organic photovoltaic system, dissolved in organic solvent. For this system, IMT correctly predicts a transient enhancement of the CT rate by over an order of magnitude, a nonequilibrium effect missed by Marcus theory. The population dynamics from multistate IMT are in excellent agreement with results from all‐atom nonadiabatic semiclassical mapping dynamics and quantum NE‐FGR calculations. This work establishes the multistate NLR‐IMT method as a reliable and cost‐effective tool for simulating photoinduced CT dynamics in realistic condensed‐phase systems. This article is categorized under: Theoretical and Physical Chemistry > Reaction Dynamics and Kinetics Structure and Mechanism > Reaction Mechanisms and Catalysis Software > Simulation Methods