Hybrid biexcitons in organic polymer aggregates: A case of Dr. Jekyl meeting Mr. Hyde. Eric R. Bittner ∗ Department of Chemistry, University of Houston, Houston, Texas 77204, United States Carlos Silva-Acu˜ na † School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta GA 30332, United States ‡ (Dated: August 4, 2021) 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 bound exci- ton 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 EJJ > 2EJ while in H-aggregates, HH-biexciton states EHH < 2EH 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. I. INTRODUCTION It is generally understood that the primary photoexci- tations in organic semiconducting materials are molecu- lar π −π ∗ electronic singlet states (S 1 ) termed Frenkel ex- citons [1]. While local in nature, at sufficiently high pack- ing densities, excitons can delocalized over several molec- ular units and sufficiently higher excitation densities, exciton-exciton interactions begin to dominate the op- tical properties of such materials [2]. Biexcitons, bound pairs of excitons, are consequential intermediates in a wide range of photophysical processes such as exciton dissociation into electrons (e − ) and holes (h + ) [3], S 0 +2 ω −−→ [2 S 1 ] ‡ −−→ 2e − +2h + (1) bimolecular annihilation [4], S 1 +S 1 −−→ [2 S 1 ] ‡ −−→ S 0 +S 0 (2) and singlet fission producing triplet (T 1 ) states [5] S 0 +2 ω −−→ [2 S 1 ] ‡ −−→ T 1 +T 1 . (3) Ref. 4 notes that bimolecular annihilation may be me- diated both by resonance energy transfer and diffusion- limited exciton-exciton scattering, but in either case we * ebittner@central.uh.edu † carlos.silva@gatech.edu ‡ School of Physics, Georgia Institute of Technology, 837 State Street, Atlanta GA 30332, United States; School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive NW, Atlanta GA 30332, United States; Departa- mento de F´ ısica Aplicada, Centro de Investigaci´ on y de Estudios Avanzados del Instituto Polit´ ecnico Nacional, 97310 M´ erida, Yu- cat´an,M´ exico invoke the key intermediate [2S 1 ] ‡ . Examples of this oc- cur in biological light harvesting complexes where multi- exciton interactions may play important roles [6] in the excitonic transport process,and biexcitons can be crucial in cascade quantum emitters as a source of entangled photons [7]. While ample theoretical work points towards the existence of biexcitons in organic solids [8–14], and in optical lattices [15], there has been only indirect evi- dence of the dynamic formation of two-quantum exciton bound states in polymeric semiconductors by incoherent, sequential ultrafast excitation [3–5, 16, 17]. Recently, we reported upon the the direct spec- troscopic observation of bound Frenkel biexcitons, i.e., bound two-exciton quasiparticles ([2 S 1 ] ‡ ), in a model polymeric semiconductor, [poly(2,5-bis(3- hexadecylthiophene-2-yl)thieno[3,2-b]thiophene)] [18] (PBTTT) using coherent two-dimensional ultrafast spectroscopy. [19] The chemical structure of PBTTT is given in Fig. 1(A). Molecular aggregation in the bulk gives rise to π- stacking interactions between localized ππ ∗ electronic ex- citations (excitons) that to a good approximation are de- termined by the relative orientation of the local transi- tion dipole moments between the ground and first sin- glet excited states. These interactions depend upon the local geometric arrangement of the molecular sites and give rise to exciton states that can be delocalized over multiple units. For example, for a two site system, the excitonic states can be written as ψ ± = 1 √ 2 (φ 1 ± φ 2 ) (4) with energies ǫ ± = ǫ o ± t where t is exciton transfer integral and ǫ o is the local site energy. Of the two possi- ble eigenstates, only the symmetric state ψ + carries os- cillator strength to the ground state. We shall assume arXiv:2108.01618v1 [cond-mat.mtrl-sci] 3 Aug 2021