  Frenkel excitons are the primary photoexcitations in organic semiconductors
and are ultimately responsible for the optical properties of such materials.
They are also predicted to form \emph{bound} exciton pairs, termed biexcitons,
which are consequential intermediates in a wide range of photophysical
processes. Generally, we think of bound states as arising from an attractive
interaction. However, here we report on our recent theoretical analysis
predicting the formation of stable biexciton states in a conjugated polymer
material arising from both attractive and repulsive interactions. We show that
in J-aggregate systems, JJ-biexcitons can arise from repulsive dipolar
interactions with energies $E_{JJ}> 2E_J$ while in H-aggregates, HH-biexciton
states $E_{HH} < 2E_H$ corresponding to attractive dipole exciton/exciton
interactions. These predictions are corroborated by using ultrafast
double-quantum coherence spectroscopy on a PBTTT material that exhibits both J-
and H-like excitonic behavior.
