Singlet fission (SF) is a process in which a molecular chromophore in singlet excited state shares its excitation energy with an unexcited neighbour leading to formation of two triplet excitons through a spin-allowed process. However, short triplet lifetime results in complex device architectures for SF-boosted solar cells. In order to elucidate the fundamental mechanism of SF and the triplet-triplet recombination pathways, it is important to understand the intrinsic nature of the transiently formed correlated T-T pair states and its dissociation to the free triplet (T) states in different environments. Herein we probed the dissociation of T-T pair state and the free triplet exciton transport across a chiral environment analogous to the spin transport of isolated electrons which defines the CISS effect. In order to test this hypothesis, we synthesized two novel chiral diketopyrrolopyrrole DPP molecules and self-assembled them into two enantiomeric chiral nanoaggregates. Using femtosecond transient absorption spectroscopy, we demonstrate a distinct spectroscopic signature of T-T pair state for the first time for these novel DPP nanoaggregates. We find that the appearance of the T-T pair state occurs in ~300 fs while its decay in ~20 picoseconds, though did not show a significant effect of the chiral field. We will discuss our results in the context of how to design new chiral structures to strengthen the effect of spin-polarized transport of triplet excitons in aggregates.

Probing SF-induced TT pair generation in enantiomeric chiral diketopyrrolopyrrole aggregates

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