Hidden Selection Rule for Exciton Coupling in Organic Crystals

Organic molecular crystals host unique exciton-exciton interactions, allowing the formation of multiexcitons through exciton fission and long-lived exciton energy carriers. However, a general understanding of how the crystal structure affects exciton fission is lacking, requiring computationally demanding calculations to classify each candidate material. Here, we present a DFT-derived, effective Hamiltonian approach to understand structural effects on exciton-exciton interactions in molecular crystals directly from their electronic band structure. We use this model to derive hidden selection rules on crystal pentacene and predict that the common bulk polymorph supports fast Coulomb-mediated singlet fission, with a coupling that is at least one order of magnitude larger than that of the thin-film polymorph, a result we confirm with explicit calculations based on many-body perturbation theory. Our approach can be used to understand a variety of hidden symmetries involving electronic and optical excitations in complex materials from DFT calculations, and provides design principles for the experimental and computational discovery of new materials with efficient non-radiative exciton decay rates.