Rationalising Exciton Interactions in Aggregates Based on the Transition Density.

Area of Science:

• Solid-state organic materials science
• Photophysics and spectroscopy
• Quantum chemistry

Background:

• Molecular aggregation significantly influences spectroscopic properties of organic materials.
• Understanding excitonic coupling (H- or J-type) is key to controlling optical properties, determining if the aggregate's lowest excited state is optically bright.
• Current models often lack the scope to cover the full range of electronic interactions in aggregates.

Purpose of the Study:

• To develop a general and intuitive phenomenological model for estimating exciton coupling in molecular aggregates.
• To determine the factors governing H- vs. J-type coupling based on molecular structure and electronic properties.
• To provide a framework for designing molecular materials with tailored spectroscopic behaviors.

Main Methods:

• Utilized a supermolecular approach to analyze exciton splitting terms.
• Developed a model based on the shape of the monomer transition density.
• Applied the model to stacked anthracene and perylene-diimide dimer systems.

Main Results:

• The model accurately predicts the sign and magnitude of exciton coupling across various electronic coupling regimes (Frenkel excitons to charge-transfer states).
• Successfully explained both long-range coupling behavior and atom-scale oscillations in stacked systems.
• Demonstrated the model's applicability to real molecular systems.

Conclusions:

• The developed model offers a powerful tool for understanding and predicting excitonic coupling in solid-state organic materials.
• Provides deeper insight into intermolecular interactions beyond simple frontier orbital theories.
• Facilitates the rational design of advanced molecular materials with desired optical and electronic properties.