  Frenkel excitons, the primary photoexcitations in organic semiconductors that
are unequivocally responsible for the optical properties of this materials
class, are predicted to form \emph{bound} exciton pairs, i.e., biexcitons.
These are key intermediates, ubiquitous in many relevant photophysical
processes; for example, they determine the exciton bimolecular annihilation
dynamics in such systems. Deciphering the details of biexciton correlations is,
thus, of utmost importance to understand the optical processes in these
semiconductors. To date, however, due to their spectral ambiguity, there has
been only scant direct evidence of bound biexcitons, limiting the insights that
can be gained. Moreover, a quantum-mechanical basis describing biexciton
correlation/stability has so far been lacking. By employing nonlinear coherent
spectroscopy, we identify here bound biexcitons in a model polymeric
semiconductor. We find, unexpectedly, that excitons with \emph{interchain}
vibronic dispersion reveal \emph{intrachain} biexciton correlations and vice
versa. Moreover, using a Frenkel exciton model, we can relate the biexciton
binding energy to molecular parameters quantified by quantum chemistry,
including the magnitude and sign of the exciton-exciton interaction the
inter-site hopping energies. Therefore, our work promises a window towards
general insights into the many-body electronic structure in polymeric
semiconductors and beyond; e.g., other excitonic systems such as organic
semiconductor crystals, molecular aggregates, photosynthetic light-harvesting
complexes, or DNA.
