Dynamics of Triplet-Triplet Formation Versus Yield of Free Triplets in Intramolecular Singlet Fission

Utilization of singlet fission (SF) in organic photovoltaics requires detailed knowledge of the triplet-triplet (TT) spin biexciton. We report many-body calculations of excited states, including TT, in 2 hypothetical phenylene-linked anthracene dimers. These systems are candidates for intra-molecular SF (iSF) as opposed to inter-molecular SF (xSF), but our results should apply to both types of systems. Although the molecules in question do not exist in nature, the results prove valuable due to the high accuracy of the calculations given our ability to retain a large active space of molecular orbitals, and they translate to larger systems. We obtain the unexpected result that slower TT formation may ultimately lead to greater yield of free triplets than systems in which TT formation is truly ultrafast. This finding is consistent with previous theoretical and experimental results obtained for real dimers that exhibit iSF. Finally, we present future avenues for identifying and characterizing systems that may be strong candidates for singlet fission.