Direct Exciton Harvesting from a Bound Triplet Pair

Singlet fission is commonly defined as the generation of two triplet excitons from a single absorbed photon, which has received considerable attention because of its potential to overcome the Shockley-Queisser limit on the power conversion efficiency (PCE) of solar cells. However, ambiguities within this definition arise due to the complexity of the various double triplet states that exist in SF chromophores and corresponding interconversion processes. To clarify this process, singlet fission is frequently depicted as sequential two-step conversion in which a singlet exciton decays into a bound triplet pair biexciton state which dissociates into two “free” triplet excitons. However, this model discounts the potential for direct harvesting from the coupled biexciton state. Using intramolecular SF dimers, we demonstrate that individual triplet excitons can be extracted directly from a bound triplet pair state prior to dissociation. These compounds have well-defined electronic interactions that allowing for detailed studies of multiexciton state and its dynamics. We show that due to the requirement for geminate triplet-triplet annihilation in these compounds, unique spectral and kinetic signatures can be used to quantify triplet pair harvesting yields. We achieve an internal quantum efficiency for triplet exciton transfer from the triplet pair of greater than 50%, limited only by the solubility of the compounds. The harvesting process is not dependent on the net multiplicity of the triplet pair state, suggesting that an explicit dissociation step is not a requirement for using triplet pairs to do chemical or electrical work.