Excitonic Hamiltonian for Singlet Fission: Beyond a Dimer Model

Area of Science:

• Materials Science
• Computational Chemistry
• Physical Chemistry

Background:

• Singlet fission is crucial for advanced energy harvesting, but characterizing molecular aggregates involved is computationally challenging.
• Accurate ab initio calculations are difficult due to the high dimensionality and multiconfiguration nature of electronic states in these systems.

Purpose of the Study:

• To develop a computationally tractable model for studying singlet fission in large molecular aggregates.
• To enable accurate characterization of energy states and properties relevant to energy harvesting.

Main Methods:

• A spin-resolved tight-binding excitonic model was developed, parameterized using ab initio calculations on smaller molecular units.
• The model was applied to large aggregates, specifically the pentacene crystal, to calculate energetics, spectra, and density of states.

Main Results:

• The model successfully evaluated the spectra and density of states for the pentacene crystal.
• Analysis revealed the emergence of state bands and provided insights into the multireference character of eigenstates via participation ratios.

Conclusions:

• The developed coarse-grained excitonic model offers a feasible approach for understanding the structure of extended molecular aggregates.
• This method serves as a foundation for incorporating vibronic coupling and simulating singlet fission dynamics at quantum-classical or semiclassical levels.