Delocalization effects in singlet fission: Comparing models with two and three interacting molecules.

Area of Science:

• Photophysics
• Computational Chemistry
• Materials Science

Background:

• Singlet fission is a photophysical process where one high-energy exciton splits into two lower-energy excitons.
• Understanding singlet fission is crucial for developing advanced photovoltaic and optoelectronic devices.
• 2,5-bis(fluorene-9-ylidene)-2,5-dihydrothiophene (ThBF) is a promising material for singlet fission applications.

Purpose of the Study:

• To investigate the impact of excited-state delocalization on singlet fission dynamics in ThBF.
• To explore how extending the molecular model from a dimer to a trimer affects photophysical properties.
• To determine the singlet fission quantum yield for different molecular aggregation models.

Main Methods:

• Surface hopping simulations were employed to model the photodynamics.
• Quantum mechanics/molecular mechanics (QM/MM) schemes were utilized.
• Simulations considered two (dimer) and three (trimer) ThBF molecules in the quantum region, embedded in a molecular mechanics crystal environment.

Main Results:

• The trimer model exhibited faster time evolution of state populations compared to the dimer model.
• Significant differences were observed in the rise and decay times of intermediate charge transfer states.
• A singlet fission quantum yield of approximately 204% was predicted for the trimer model, exceeding the dimer model's yield (approximately 179%) and the theoretical 200% limit for dimers.

Conclusions:

• Extending excited-state delocalization beyond a dimer significantly influences singlet fission dynamics in ThBF.
• The trimer model provides a more accurate representation of the enhanced singlet fission process.
• Findings highlight the importance of considering larger molecular aggregates for optimizing singlet fission efficiency.