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 (T 1 T 1 ) state whose spin dynamics influence the successful generation of uncorrelated triplets (T 1 ). Femtosecond transient visible and near-infrared absorption spectroscopy of a linear terrylene-3,4:11,12-bis(dicarboximide) dimer (TDI 2 ), in which the two TDI molecules are directly linked at one of their imide positions, reveals ultrafast formation of the (T 1 T 1 ) state. The spin dynamics of the (T 1 T 1 ) state and the processes leading to uncoupled triplets (T 1 ) were studied at room temperature for TDI 2 aligned in 4-cyano-4′-pentylbiphenyl (5CB), a nematic liquid crystal. Time-resolved electron paramagnetic resonance spectroscopy shows that the (T 1 T 1 ) state has mixed 5 (T 1 T 1 ) and 3 (T 1 T 1 ) character at room temperature. This mixing is magnetic field dependent, resulting in a maximum triplet yield at ∼200 mT. The accessibility of the 3 (T 1 T 1 ) state opens a pathway for triplet–triplet annihilation that produces a single uncorrelated T 1 state. The presence of the 5 (T 1 T 1 ) state at room temperature and its relationship with the 1 (T 1 T 1 ) and 3 (T 1 T 1 ) states emphasize that understanding the relationship among different (T 1 T 1 ) spin states is critical for ensuring high-yield T 1 formation from singlet fission.