Dynamics of the Triplet-pair State Reveals the Likely Coexistence of Coherent and Incoherent Singlet Fission in Crystalline Hexacene

The absorption of a photon usually creates a singlet exciton (S) in molecular systems, but in some cases S may split into two triplets (2×T) in a process called singlet fission. Singlet fission is believed to proceed through the correlated triplet-pair (TT) state. Here, we probe the (TT) state in crystalline hexacene using time-resolved photoemission and transient absorption spectroscopies. We find a distinctive (TT) state, which decays to 2×T with a time constant of 270 fs. However, the decay of S and the formation of (TT) occur on different timescales of 180 fs and <50 fs, respectively. Theoretical analysis suggests that, in addition to an incoherent S→(TT) rate process responsible for the 180 fs timescale, S may couple coherently to a vibronically excited (TT) on ultrafast timescales (<50 fs). The coexistence of coherent and incoherent singlet fission may also reconcile different experimental observations in other acenes.