Distinct Routes of Singlet Fission and Triplet Fusion: A Fluorescence Kinetic Study of Rubrene

Singlet fission of organic molecules has attracted recent attention owing to its potential advantages in organic photovoltaic and electroluminescence applications. Its microscopic mechanism however remains stymied. Large couplings from charge-transfer (CT) state mediation were invoked to explain the ultrafast singlet fission rate observed in crystalline polyacene, but its experimental confirmation is still lacking. The singlet fission and triplet fusion of amorphous rubrene were investigated with time-resolved photoluminescence spectroscopy at different temperatures to extract the rates of singlet fission, triplet fusion, and triplet hopping. On the basis of the Marcus electron-transfer model, the deduced electronic coupling constant of the singlet fission process was found to be larger than that of the triplet fusion process, indicating that the singlet fission process undertakes a CT-state-mediated channel while the triplet fission process assumes a direct channel. This study thus confers supporting evidence of the existence of the CT-state-mediated channel for singlet fission of rubrene and offers an experimental approach to study singlet fission dynamics. © 2019 American Chemical Society.