By using nonlinear coherent spectroscopy, we identify bound Frenkel biexcitons in a model polymeric semiconductor and find, unexpectedly, that excitons with interchain vibronic dispersion reveal intrachain biexciton correlations and vice versa. Moreover, we relate the biexciton binding energy to molecular parameters quantified by quantum chemistry, including the magnitude and sign of the exciton-exciton interaction the intersite hopping energies. This work shows how multidimensionnal spectroscopy can be used to gain insights into many-body physics despite the complexity of the material hosting them. This required a careful theoretical treatment of coherent dynamics to distinguish them from incoherent artefacts, which we provide in supplementary information.
