In organic photovoltaics (OPVs), singlet fission (SF) converts a singlet exciton (S1) into a pair of triplets (T1), S1 → 1(T1T1) → 2T1, and doubles the electric current generated by incident high-energy photons. A pentacene-based OPV allows the exothermic and spontaneous SF reaction to happen and its solar conversion efficiency can exceed the Shockley–Queisser limit of 33.7%. SF undergoes via a mysterious, “multi-exciton (ME)” intermediate, for which the character is still under debate for the lack of strong supports from ultrafast quantum dynamics. Based on many popular hypotheses, ME has a large charge transfer (CT) character but is also strongly coupled to S1 and 1(T1T1). As a unimolecular reaction, intramolecular SF (ISF) occurring to covalently-bound pentacene dimers provides an excellent model for local excitons and simplifies the studies of direct quantum dynamics. In the present study, the ISF mechanism is investigated for three dimeric species, ortho-, meta-, and para-bis(6,13-bis(triisopropylsilylethynyl)pentacene)benzene, in three different environments: vacuum, implicit solvent, and explicit solvent. The real-time, non-adiabatic quantum mechanical/molecular mechanical (QM/MM) dynamics is propagated among multiple potential energy surfaces associated with the diabatic, SF-relevant states -- S1, 1(T1T1) and CT. The energy difference and non-adiabatic coupling between each pair of states fluctuate significantly with time and instantaneous molecular configurations. Condon and non-Condon ISF dynamics obtained from the present study are also quantitatively compared with solution-based experimental data. The results are expected to decipher the roles of ME and CT states in the ISF mechanism and propose a design strategy to maximize the ISF efficiency in the OPV materials.

Photochemical dynamics for intramolecular singlet fission in covalently-bound pentacene dimers

254th National Meeting (2017)

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