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Discovery and characterization of an acridine radical photoreductant

Nature volume 580, pages 76–80 (2020)Cite this article

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Abstract

Photoinduced electron transfer (PET) is a phenomenon whereby the absorption of light by a chemical species provides an energetic driving force for an electron-transfer reaction1,2,3,4. This mechanism is relevant in many areas of chemistry, including the study of natural and artificial photosynthesis, photovoltaics and photosensitive materials. In recent years, research in the area of photoredox catalysis has enabled the use of PET for the catalytic generation of both neutral and charged organic free-radical species. These technologies have enabled previously inaccessible chemical transformations and have been widely used in both academic and industrial settings. Such reactions are often catalysed by visible-light-absorbing organic molecules or transition-metal complexes of ruthenium, iridium, chromium or copper5,6. Although various closed-shell organic molecules have been shown to behave as competent electron-transfer catalysts in photoredox reactions, there are only limited reports of PET reactions involving neutral organic radicals as excited-state donors or acceptors. This is unsurprising because the lifetimes of doublet excited states of neutral organic radicals are typically several orders of magnitude shorter than the singlet lifetimes of known transition-metal photoredox catalysts7,8,9,10,11. Here we document the discovery, characterization and reactivity of a neutral acridine radical with a maximum excited-state oxidation potential of −3.36 volts versus a saturated calomel electrode, which is similarly reducing to elemental lithium, making this radical one of the most potent chemical reductants reported12. Spectroscopic, computational and chemical studies indicate that the formation of a twisted intramolecular charge-transfer species enables the population of higher-energy doublet excited states, leading to the observed potent photoreducing behaviour. We demonstrate that this catalytically generated PET catalyst facilitates several chemical reactions that typically require alkali metal reductants and can be used in other organic transformations that require dissolving metal reductants.

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Fig. 1: Mechanistic studies of Mes-Acr radical.
Fig. 2
Fig. 3: Scope of reductive detosylation catalysed by Mes-Acr•.

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The data supporting the findings of this study are available within the paper and its Supplementary Information.

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Acknowledgements

This work was supported in part by the National Institutes of Health (NIGMS) Award number R01 GM120186 (D.A.N.). L.W. was supported by the National Natural Science Foundation of China (21801011) and the International Postdoctoral Exchange Fellowship Program (20180033). O.F.W. and A.M. were supported by the National Science Foundation under grant CHE-1763207. Photophysical measurements were performed in the AMPED EFRC Instrumentation Facility established by the Alliance for Molecular PhotoElectrode Design for Solar Fuels (AMPED), an Energy Frontier Research Center (EFRC) funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-SC0001011. B.D.D. acknowledges support for this project by the Department of Energy, Office of Basic Energy Sciences through the Chemical Sciences Geosciences and Biosciences Division, under grant number DE- SC0016501.

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Author notes

  1. These authors contributed equally: Ian A. MacKenzie, Leifeng Wang, Nicholas P. R. Onuska

Authors and Affiliations

  1. Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA

    Ian A. MacKenzie, Leifeng Wang, Nicholas P. R. Onuska, Olivia F. Williams, Andrew M. Moran & David A. Nicewicz

  2. Department of Physics, Kent State University, Kent, OH, USA

    Khadiza Begam

  3. Department of Chemistry and Biochemistry, Kent State University, Kent, OH, USA

    Barry D. Dunietz

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Contributions

I.A.M. and D.A.N. were responsible for the initial conception of the project. I.A.M., L.W. and N.P.R.O. devised and executed all experimental work. N.P.R.O., D.A.N., K.B., B.D.D., O.F.W. and I.A.M. assisted in the preparation and editing of the final manuscript. O.F.W. assisted in the collection and O.F.W. and A.M.M. performed analysis of transient absorption data. B.D.D. designed the computational approach, K.B. executed the calculations and K.B., B.D.D., N.P.R.O. and D.A.N. were responsible for the analysis of computations.

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Correspondence to David A. Nicewicz.

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MacKenzie, I.A., Wang, L., Onuska, N.P.R. et al. Discovery and characterization of an acridine radical photoreductant. Nature 580, 76–80 (2020). https://doi.org/10.1038/s41586-020-2131-1

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