Competition between triplet pair formation and excimer-like recombination controls singlet fission yield

The ultimate goal for singlet fission is that each photo-excited singlet exciton, S-1, will result in two triplet excitons with unity yield. However, the singlet fission is now recognized to be complicated, involving bright/dark excited states of different spin multiplicity. Identifying the role of such states is vital to optimize singlet fission yield but difficult due to their elusive spectral signature. Here, we develop an experimental protocol based on a refined magneto-optical probe to access the fast time evolution of various excited states. In diphenylhexatriene crystal, the S-1 is found to undergo two competing processes-to form one of the two dark triplet pair intermediates having different exchange energies or to form a bright state, S-x, exhibiting excimer-like delayed photoluminescence. Our result provides a clear picture of a competition event in singlet fission, which is beneficial for the development and tailoring of singlet fission materials with high yield.