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-1) in molecular systems, but in some cases S-1 may split into two triplets (2xT(1)) in a process called singlet fission. Singlet fission is believed to proceed through the correlated tripletpair (1)(TT) state. Here, we probe the (1)(TT) state in crystalline hexacene using time-resolved photoemission and transient absorption spectroscopies. We find a distinctive (1)(TT) state, which decays to 2xT(1) with a time constant of 270 fs. However, the decay of S-1 and the formation of (1)(TT) occur on different timescales of 180 fs and <50 fs, respectively. Theoretical analysis suggests that, in addition to an incoherent S-1 ->(1)(TT) rate process responsible for the 180 fs timescale, S-1 may couple coherently to a vibronically excited (1)(TT) on ultrafast timescales (< 50 fs). The coexistence of coherent and incoherent singlet fission may also reconcile different experimental observations in other acenes.