Direct observation of ultrafast singlet exciton fission in three dimensions

Arjun Ashoka, Nicolas Gauriot, Aswathy V. Girija, Nipun Sawhney, Alexander J. Sneyd, Kenji Watanabe, Takashi Taniguchi, Jooyoung Sung, Christoph Schnedermann () and Akshay Rao ()
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Arjun Ashoka: University of Cambridge
Nicolas Gauriot: University of Cambridge
Aswathy V. Girija: University of Cambridge
Nipun Sawhney: University of Cambridge
Alexander J. Sneyd: University of Cambridge
Kenji Watanabe: National Institute for Materials Science
Takashi Taniguchi: National Institute for Materials Science
Jooyoung Sung: DGIST
Christoph Schnedermann: University of Cambridge
Akshay Rao: University of Cambridge

Nature Communications, 2022, vol. 13, issue 1, 1-8

Abstract: Abstract We present quantitative ultrafast interferometric pump-probe microscopy capable of tracking of photoexcitations with sub-10 nm spatial precision in three dimensions with 15 fs temporal resolution, through retrieval of the full transient photoinduced complex refractive index. We use this methodology to study the spatiotemporal dynamics of the quantum coherent photophysical process of ultrafast singlet exciton fission. Measurements on microcrystalline pentacene films grown on glass (SiO2) and boron nitride (hBN) reveal a 25 nm, 70 fs expansion of the joint-density-of-states along the crystal a,c-axes accompanied by a 6 nm, 115 fs change in the exciton density along the crystal b-axis. We propose that photogenerated singlet excitons expand along the direction of maximal orbital π-overlap in the crystal a,c-plane to form correlated triplet pairs, which subsequently electronically decouples into free triplets along the crystal b-axis due to molecular sliding motion of neighbouring pentacene molecules. Our methodology lays the foundation for the study of three dimensional transport on ultrafast timescales.

Date: 2022
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DOI: 10.1038/s41467-022-33647-5

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