Singlet Fission in Polypentacene

A recent surge of interest in singlet-fission (SF) dimers is inspired by their utility as model systems as well as by the inherent advantages of an intramolecular SF process. Here, we extend the scope of these materials to polypentacene, a model one-dimensional system designed to bridge studies of SF in constrained SF dimers and extended condensed phase systems. Using transient absorption spectroscopy, we studied a soluble polypentacene derivative and well-defined oligomers with 2-5 repeat units to elaborate the trend in triplet-pair dynamics with increasing lengths. We found that the SF rate constant increased monotonically with the number of repeat units. However, recombination in all cases was rapid (<1 ns) and was nearly independent of the number of repeat units. The transient spectra and constant triplet-pair lifetime indicated that correlated triplet pairs were formed on adjacent chromophores and that they did not diffuse apart before recombination.