Quintet-triplet mixing determines the fate of the multiexciton state produced by singlet fission in a terrylenediimide dimer at room temperature.

Singlet fission (SF) is a photophysical process in which one of two adjacent organic molecules absorbs a single photon, resulting in rapid formation of a correlated triplet pair (TT) state whose spin dynamics influence the successful generation of uncorrelated triplets (T). Femtosecond transient visible and near-infrared absorption spectroscopy of a linear terrylene-3,4:11,12-bis(dicarboximide) dimer (TDI), in which the two TDI molecules are directly linked at one of their imide positions, reveals ultrafast formation of the (TT) state. The spin dynamics of the (TT) state and the processes leading to uncoupled triplets (T) were studied at room temperature for TDI aligned in 4-cyano-4'-pentylbiphenyl (5CB), a nematic liquid crystal. Time-resolved electron paramagnetic resonance spectroscopy shows that the (TT) state has mixed (TT) and (TT) character at room temperature. This mixing is magnetic field dependent, resulting in a maximum triplet yield at ∼200 mT. The accessibility of the (TT) state opens a pathway for triplet-triplet annihilation that produces a single uncorrelated T state. The presence of the (TT) state at room temperature and its relationship with the (TT) and (TT) states emphasize that understanding the relationship among different (TT) spin states is critical for ensuring high-yield T formation from singlet fission.