Simulation of solution phase electron transfer in a compact donor-acceptor dyad.

Area of Science:

• Computational Chemistry and Biophysics
• Photochemistry and Photophysics
• Renewable Energy Research

Background:

• Controlling charge separation (CS) and charge recombination (CR) is critical for efficient photon-to-current conversion in artificial photosynthetic systems.
• The ratio of CS to CR rates directly impacts the overall efficiency of light-harvesting and energy conversion processes.
• A deep mechanistic understanding of electron transfer (ET) and the environmental influences is necessary for rational design of improved systems.

Purpose of the Study:

• To develop and validate a novel Quantum Mechanics/Molecular Mechanics (QM/MM) protocol for simulating electron transfer processes.
• To characterize charge recombination (CR) kinetics in the formanilide-anthraquinone dyad (FAAQ) using the developed simulation protocol.
• To investigate the influence of the environment, including solvent polarization and molecular conformation (cis-trans isomerization), on CR dynamics.

Main Methods:

• Implementation of a QM/MM protocol utilizing constrained density functional theory (DFT) for on-the-fly diabatic electronic state calculations.
• Integration of a polarizable force field based on the Drude oscillator model to capture solvent orientational and electronic polarization.
• Molecular dynamics (MD) simulations to characterize CR in the FAQA dyad, analyzing electronic couplings, energy gaps, and solvent effects.

Main Results:

• The simulations accurately predict fast charge recombination of the charge-transfer excited state in the FAQA dyad, consistent with experimental findings.
• Computed electronic couplings exhibit dependence on the electronic state and are found to be weaker in solution compared to the gas phase.
• A strong correlation between vertical energy gaps and a collective solvent polarization coordinate was observed, highlighting the role of solvent reorganization.

Conclusions:

• The developed QM/MM protocol provides a unified approach to study electron transfer phenomena in light-harvesting systems.
• Environmental factors, particularly solvent polarization and molecular conformation, significantly influence charge recombination kinetics.
• The findings offer insights for the rational design of artificial photosynthetic architectures with enhanced photon-to-current conversion efficiencies.