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Spatial separation of triplet excitons drives endothermic singlet fission

Abstract

Molecules that undergo singlet fission, converting singlet excitons into pairs of triplet excitons, have potential as photovoltaic materials. The possible advantages of endothermic singlet fission (enhanced use of photon energy and larger triplet energies for coupling with common absorbers) motivated us to assess the role of exciton delocalization in the activation of this process. Here we report the synthesis of a series of linear perylene oligomers that undergo endothermic singlet fission and have endothermicities in the range 5–10 kBT at room temperature in solution. We study these compounds using transient spectroscopy and modelling to unravel the singlet and triplet dynamics. We show that the minimal number of coupled chromophores needed to undergo endothermic singlet fission is three, which provides sufficient statistical space for triplet excitons to separate and avoid annihilation—and a subsequent fast return to the singlet state. Our data additionally suggest that torsional motion of chromophores about the molecular axis following triplet-pair separation contributes to the increase in entropy, thus lengthening the triplet lifetime in longer oligomers.

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Fig. 1: Absorption and emission spectra of dilute THF solutions of each oligomer compound.
Fig. 2: Femtosecond transient absorption spectra of dilute THF solutions of the oligomer compounds.
Fig. 3: Spectral overlap of the triplet features from transient absorption in dilute THF solution.
Fig. 4: Concentration kinetics of singlet and triplet species as derived from transient absorption datasets for 3-OPP and 4-OPP solutions.
Fig. 5: Kinetic model of endothermic SF.
Fig. 6: Spectra associated with femtosecond to nanosecond evolution from 1-OPP to 4-OPP in dilute THF solution.

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Data availability

The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.

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Acknowledgements

This work was authored by Alliance for Sustainable Energy, the manager and operator of the National Renewable Energy Laboratory for the US Department of Energy (DOE) under contract no. DE-AC36-08GO28308. The views expressed in the Article do not necessarily represent the views of the US Department of Energy nor the US Government. Funding was provided by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences. N.V.K. acknowledges the Laboratory Directed Research and Development programme at NREL for a Director’s Postdoctoral Fellowship. The matrix-assisted laser desorption/ionization mass spectrometry–Fourier transform mass spectrometry data were collected by L. Laurens at the NREL spin-resonance facility. We thank G. Dukovic, K. Vrouwenvelder and O. M. Pearce for access to the time-correlated single photon counting apparatus. Gel permeation chromatography data were provided by W. Braunecker.

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N.V.K. synthesized samples, performed steady-state and time-resolved spectroscopic studies, and contributed to experimental design. C.H.C. performed electronic structure calculations. J.C.J. contributed to overall experimental design and supervised the project. All authors discussed the results and commented on the manuscript.

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Correspondence to Justin C. Johnson.

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Synthesis and characterization of compounds, additional spectroscopic characterization, computational details, Supplementary Figs.1–31 and Tables 1–17.

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Korovina, N.V., Chang, C.H. & Johnson, J.C. Spatial separation of triplet excitons drives endothermic singlet fission. Nat. Chem. 12, 391–398 (2020). https://doi.org/10.1038/s41557-020-0422-7

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