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 (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.

Vibronic exciton theory of singlet fission. I. Linear absorption and the anatomy of the correlated triplet pair state

“…Such a determination of diabatic energies is key to obtaining reliable results from dynamical calculations using a microscopic diabatic basis, especially with regards to the recent indications that resonances between vibrational modes and electronic splittings facilitate rapid singlet fission. 15,16 In this respect, it is interesting to note that a different diabatic splitting of 170 meV was applied in Ref. 26 to yield a near-resonance between the adiabatic states.…”

“…The aforementioned spectroscopic studies were mostly supported by calculations performed using phenomenological models, including a minimal number of electronic degrees of freedom. [14][15][16] Although such modeling is extremely helpful for interpreting the spectroscopic measurements, it does not provide fully microscopic insights at the molecular level. On the other hand, available high-level theoretical studies on fission materials 17,18,[21][22][23] are numerically constrained to identifying excited states in a time-independent framework for limited cluster sizes.…”

“…In such systems efficient fission is found to occur very rapidly, 13,14 and evidence exists that a near instantaneous population of crucial fisa) Electronic mail: r.tempelaar@gmail.com b) Electronic mail: drr2103@columbia.edu sion intermediates concomitantly occurs. [14][15][16] Early attempts to account for these observations relied on very strong electronic coupling between the initially excited singlet exciton and the correlated triplet pair state, 14 the fission intermediate which renders the overall process spin-allowed. However such sizable coupling seems inconsistent with most electronic structure calculations.…”

“…[17][18][19] Recent time-resolved spectroscopic studies, seeking to find alternative explanations, have pointed to the importance of vibronic coupling. 15,16,20 Transient absorption measurements on pentacene derivatives suggested that a conical intersection is the driving mechanism for fission. 20 Yet another scenario for pentacene was proposed based on two-dimensional electronic spectroscopy (2DES), in which high-frequency intramolecular vibrations generate a resonance between the singlet and correlated triplet pair states by matching their energy difference.…”

“…20 Yet another scenario for pentacene was proposed based on two-dimensional electronic spectroscopy (2DES), in which high-frequency intramolecular vibrations generate a resonance between the singlet and correlated triplet pair states by matching their energy difference. 15 Twophoton photoemission measurements on hexacene were interpreted similarly, 16 despite the very different energetics of this material.…”