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Singlet fission (SF) is a multielectron process in which one singlet excitonS converts into a pair of triplet excitons T+T. SF is widely studied as it mayhelp overcome the Shockley-Queisser efficiency limit for semiconductorphotovoltaic cells. To elucidate and control the SF mechanism, great attentionhas been given to the identification of intermediate states in SF materials,which often appear elusive due to the complexity and fast timescales of the SFprocess. Here, we apply 10fs-1ms transient absorption techniques to high-purityrubrene single crystals to disentangle the intrinsic fission dynamics from theeffects of defects and grain boundaries and to identify reliably the fissionintermediates. We show that above-gap excitation directly generates a hybridvibronically assisted mixture of singlet state and triplet-pair multiexciton[S:TT], which rapidly (<100fs) and coherently branches into pure singlet ortriplet excitations. The relaxation of [S:TT] to S is followed by a relativelyslow and temperature-activated (48 meV activation energy) incoherent fissionprocess. The SF competing pathways and intermediates revealed here unify theobservations and models presented in previous studies of SF in rubrene andpropose alternative strategies for the development of SF-enhanced photovoltaicmaterials.

Interplay Between Mixed and Pure Exciton States Controls Singlet Fission in Rubrene Single Crystals