Dynamics of the triplet-pair state reveals the likely coexistence of coherent and incoherent singlet fission in crystalline hexacene

Abstract

The absorption of a photon usually creates a singlet exciton (S₁) in molecular systems, but in some cases S₁ may split into two triplets (2×T₁) in a process called singlet fission. Singlet fission is believed to proceed through the correlated triplet-pair 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 2×T₁ with a time constant of 270 fs. However, the decay of S₁ 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 S₁→¹(TT) rate process responsible for the 180 fs timescale, S₁ 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.