Concerning the stability of biexcitons in hybrid HJ aggregates of π-conjugated polymers

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 analysispredicting the formation of stable biexciton statesin a conjugated polymer material arising from both attractive and repulsive interactions.We show that in J-aggregate systems, 2J-biexcitonscan arise from repulsive dipolar interactions with energies $E_{2J}> 2E_J$ while in H-aggregates, 2H-biexciton states $E_{2H} < 2E_H$ 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.