Singlet Fission Involves an Interplay between Energetic Driving Force and Electronic Coupling in Perylenediimide Films

Due to its ability to offset thermalization losses in photoharvesting systems, singlet fission has become a topic of research interest. During singlet fission, a high energy spin-singlet state in an organic semiconductor divides its energy to form two lower energy spin-triplet excitations on neighboring chromophores. While key insights into mechanisms leading to singlet fission have been gained recently, developing photostable compounds that undergo quantitative singlet fission remains a key challenge. In this report, we explore triplet exciton production via singlet fission in films of perylenediimides, a class of compounds with a long history of use as industrial dyes and pigments due to their photostability. As singlet fission necessitates electron transfer between neighboring molecules, its rate and yield depend sensitively on their local arrangement. By adding different functional groups at their imide positions, we control how perylenediimides pack in the solid state. We find inducing a long axis displacement of ∼3 Å between neighboring perylenediimides gives a maximal triplet production yield of 178% with a fission rate of ∼245 ps despite the presence of an activation barrier of ∼190 meV. These findings disagree with Marcus theory predictions for the optimal perylenediimide geometry for singlet fission, but do agree with Redfield theory calculations that allow singlet fission to occur via a charge transfer-mediated superexchange mechanism. Unfortunately, triplets produced by singlet fission are found to decay over tens of nanoseconds. Our results highlight that singlet fission materials must be designed to not only produce triplet excitons but to also facilitate their extraction.