Weakly coupled triplet pair states probed by quantum beating in delayed fluorescence in tetracene crystals

The intermediate state of a correlated triplet pair [1(TT)] is pivotal for understanding the mechanism of the singlet fission process in molecule systems. Recently, a spatially separated state of a triplet pair [1(T⋯T)] has been proposed to drive the dissociation of exchange-coupled 1(TT) into free triplets. Here, we study the correlated triplet pairs by monitoring quantum beats in delayed fluorescence from tetracene crystals upon applying magnetic fields of different amplitudes and alignments. We argue that the triplet pairs probed by the quantum beat spectrum are weakly coupled, i.e., the spatially separated 1(T⋯T) state. The experimental data suggest the existence of a direct channel of S1 → 1(T⋯T) in addition to the widely accepted channel of S1 → 1(TT) → 1(T⋯T) for singlet fission. Our work suggests that the quantum beat spectrum is a useful tool to directly probe the 1(T⋯T) state which is valuable for elucidating the intrinsic mechanism of singlet fission.