Surface Hopping within an Exciton Picture. An Electrostatic Embedding Scheme.

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Spectroscopy

Background:

• Accurately simulating electronic excitation energy transfer (EET) in multichromophoric systems is crucial for understanding energy flow in light-harvesting complexes and molecular devices.
• Traditional methods often struggle with computational cost for ab initio nonadiabatic dynamics.

Purpose of the Study:

• To develop and implement an efficient exciton approach for ab initio nonadiabatic dynamics simulations of EET.
• To validate the exciton model by comparing its predictions to full time-dependent density functional theory (TDDFT) dynamics.

Main Methods:

• Developed a trajectory-based surface hopping formulation incorporating an exciton model.
• Employed a hybrid quantum mechanics/molecular mechanics (QM/MM) scheme with electrostatic embedding for potential energy surfaces and gradients.
• Applied the method to a BODIPY-tetrathiophene molecular dyad using TDDFT.

Main Results:

• The exciton model demonstrated excellent agreement with full TDDFT dynamics for the studied molecular dyad.
• Confirmed that Coulomb interaction terms dominate the couplings responsible for EET, negating the need for charge-transfer states in this context.
• Observed ultrafast coherent oscillations of excitation energy between the dyad's units within the first 50 femtoseconds.

Conclusions:

• The developed exciton approach provides a computationally efficient and accurate tool for simulating nonadiabatic dynamics and EET.
• The findings highlight the primary role of Coulombic interactions in driving EET in such systems.
• The observed coherent oscillations offer insights into the fundamental mechanisms of energy transfer at the ultrafast timescale.