Frenkel excitons are primary photoexcitations in molecular semiconductors and are unequivocally responsible for their optical properties. Exciton-exciton interactions are very weak in organic systems; consequently, there is scant evidence of bound biexcitons. The details of biexciton correlations determine, for example, exciton bimolecular annihilation, which is ubiquitous in organic semiconductors. Using a Frenkel exciton model, we relate the biexciton binding energy to the magnitude and the sign of the exciton-exciton interaction energy and the inter-site hopping energy, which are molecular parameters that can be quantified by quantum chemistry. Unexpectedly, biexcitons can be formed with both attractive and repulsive interactions. Moreover, background excitations created by broad-band lineshapes used ultrafast techniques can contribute to transient background dynamics which contribute to both the inhomogeneous lineshape. Using Ito calculus, we develop an analytical theory that accounts for transient many-body background dynamics that co-evolve with the non-linear spectral signals. Our work enables new opportunities for general insights into the many-body electronic structure in molecular excitonic systems such as organic semiconductor crystals, molecular aggregates, photosynthetic light-harvesting complexes, and DNA.

Frenkel biexcitons, and many-body contributions in organic semiconductors.

ACS Spring Meeting 2021

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