Configuration Interaction in Frontier Molecular Orbital Basis for Screening the Spin-Correlated, Spatially Separated Triplet Pair State 1(T···T) Formation

In the theoretical screening of Singlet Fission rates in molecular aggregates, often the frontier molecular orbital model for dimers is employed. However, the dimer approach fails to account for recent experimental findings that suggest singlet fission progresses through a further intermediate state featuring two spatially separated, spin-correlated triplets, specifically a ¹(T···T) state. We address this limitation by generalizing the often used frontier molecular orbital model for singlet fission by incorporation of both separated Charge Transfer (C···T) and ¹(T···T) states as well as mixed triplet-charge transfer states, delivering analytic expressions for the diabatic matrix elements. Applying the methodology to the perylene diimide trimer, we examine the packing dependence of competing formation pathways of the ¹(T···T) state by evaluation of diabatic matrix elements.