TT Pair in Singlet Fission
Singlet fission1-2 (SF) is a process in which a molecular chromophore in its singlet excited state shares its excitation energy with an unexcited neighbour leading to formation of two triplet excitons through a spin-allowed process. Due to the high singlet-to-triplet conversion quantum efficiencies up to 200%, SF chromophores have potential application in boosting solar cell efficiencies3. However molecular triplet excitons produced through SF usually have shorter triplet lifetimes due to exciton-exciton recombination and relaxation pathways, thereby resulting 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 highly important to understand the intrinsic nature of the intermediate correlated T-T pair states. Here, using femtosecond transient absorption and femtosecond stimulated Raman spectroscopy, we demonstrate optical as well as vibrational signature of TT pair state in SF across different kinds of SF chromophores namely carotenoids and diketopyrrolopyrrole derivatives. These spectroscopic signatures will help us to understand the origin of the TT pair and fate of it in SF, importance of the morphology of the aggregates to get distinct signatures of the TT pair compared to the free triplets and enhancement of the free triplet lifetime.4 A complete fundamental understanding of the T-T pair state will enable us to draw the proper structure-function relationship to get the next generation SF chromophores for the application in solar cell devises.
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2) Smith, M. B.; Michl, J., Recent advances in singlet fission. Annual review of physical chemistry 2013, 64, 361-386.
3) Rao, A.; Friend, R. H., Harnessing singlet exciton fission to break the Shockley-Queisser limit. Nature reviews materials 2017, 2, 1-12.
4) Kundu, A.; Dasgupta, J., Photogeneration of Long-Lived Triplet States through Singlet Fission in Lycopene H-Aggregates. J. Phys. Chem. Lett. 2021, 12, 5, 1468–1474