Exciton-polaritons are quasiparticles that consist of mixed photon and exciton character. They possess the potential to modify the properties of organic materials by manipulating their energetics under ambient conditions. In this study, we explore that potential using singlet fission as a model process. Singlet fission is a carrier multiplication process in organic molecules, where two individual excited triplets can be generated at the expense of one photoexcited singlet. This conversion is enabled by an intermediate triplet pair state. Despite the central role of triplet pair states in fission and the recent wide-ranging efforts in developing new library of fission materials, there remains little understanding of how to tune the energy of this state. The triplet pair states are generally non-emissive or “dark”, hence traditional optical spectroscopic techniques have permitted determination of the triplet-pair energy in only a small subset of materials. However, previous work has revealed that even high-spin triplet-pair states in TIPS-tetracene can be “harvested” by exciton-polaritons, thanks to the weak admixture of singlet character within the triplet-pair wavefunction. Building on this result, we apply this concept to microcavities of TIPS-pentacene, where singlet fission is known to be strongly exothermic though the triplet-pair energy is uncharacterized. By analyzing the temperature-dependence and lifetime of lower polariton emission as a function of detuning, we identify contributions to the polariton population from two distinct intra-reservoir states. The lower of these tracks the reported behavior of triplet pairs in transient absorption, permitting direct assignment of the triplet-pair energy from the polariton emission. This finding opens a new avenue of using exciton-polaritons to map the energetic structure of functionally important but poorly characterized dark electronic states and turning them on in the process towards a promising direction.

Mapping triplet pair state energies for singlet fission using exciton-polaritons

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.